"Iron and steel have played a leading role in the development of human civilization and their techniques. Together with its derivative, steel, iron has no real rival in its particular fields of application and has become a synonym of progress, being an essential element in mankind’s greatest technological achievements. It was at the origin of the industrial and scientific revolutions and at the heart of all the great discoveries which have marked the history of humanity from the manufacture of high quality swords in ancient times to today’s architectural wonders.
The present book covers different important aspects of steel processing with the casting method of steel for flat products, rolling of rails, wheels and rings, rolling of different steel products, production of fasteners, welded pipes, steel products for the building trade and many more.
The book is very useful for everybody who wants the thorough study on steel and steel products or wants to diversify in to this field.
Structural Changes in Steel During Hot Rolling
Structural Changes During Reheating
One of the consequences of
reheating process is grain coarsing. The control of grain coarsing behavior of
steels is an important step in the design of thermomechemical process striving
to achieve fine-grained products.
For microalloyed steels, the
reheating temperature should be high enough to provide solubility of stable
particles. If the stable particles remain undissolved, the beneficial
precipitation hardening effects cannot be obtained.
Addition of aluminum, niobium,
vanium, titanium, etc., produces abnormal type of grain growth (Fig. 1) which
involves the growth of very few grains in relatively unchanged fine-grain
matrix. The abnormal grain growth occurs at the temperatures, which are
significantly lower than the microalloying solution temperature. The temperature
that corresponds to commencing of the abnormal grain growth is sometimes
referred to as grain-coarsing temperature.
      The grain
size distribution has a complicated dependence on the reheating temperature as
depicted in Fig. 2 in application to Nb-V-microalloyed steel. When reheating
temperature is equal to 1200ºC (2192ºF), the maximum area fraction of the
steel microstructure corresponds to the grain size of approximately 0.12 mm
(0.0048/in.). When the reheating temperature is lowered to 1150ºC (2102ºF),
the grain size occupying the maximum area fraction is reduced to 0.06 mm (0.0024
in.). However, further decrease in reheating temperature to 1050ºC (1922ºF)
produces two pronounced peaks in distribution of the grain size, one of each is
at the grain size of about 0.18 mm (0.0072 in.) and the second one is at 0.022
mm (0.0009 in.).
Reheating temperature also
affects a formation of so-called deformation bands which play an important role
during subsequent grain restoration processes. As can be seen from Fig. 3, the
higher reheating temperature the smaller amount of deformation bands will be
formed and with less uniformity after the same reduction.
      While it does not
appear that the final average austenite grain size after deformation is strongly
dependent on the reheated grain size, it is likely that the distribution of the
grain sizes above average is much smaller when the reheating temperature is kept
below the grain-coarsing temperature.
Kinds of Grain Restoration Process
      Prior to the
start of hot rolling, the steel microstructure consists of coarse equiaxed
grains of austenite. During passing through the rolls, the austenite grains are
getting flattened and elongated on the average; each austenite grain undergoes a
dimensional change corresponding to that of the work piece as a whole. The
deformation bands may also be induced within the grains as illustrated in Fig.
4.
The three following kinds of
restoration process are associated with hot rolling:
1. Dynamic restoration process—This process starts and
completes during deformation.
2. Metadynamic restoration process—This process starts
during deformation and completes after deformation.
3. Static restoration process—This process starts and
completes after deformation.
Dynamic Restoration Process
      When steel is
deformed in the austenitic state at high temperature, the flow stress rises to a
maximum and then falls to a steady-state as shown in Fig. 5a.
Dynamic restoration process includes dynamic
recovery and dynamic recrystallization.
Dynamic recovery is a reduction of work-hardening
effects without motion of large-angle grain boundaries. It occurs in a range of
strain less than that for peak stress.
Dynamic recrystallization takes place in the range
of strain that corresponds to steady state of flow stress.
Role of dynamic
recrystallization of austenite in practical rolling of C-Mn steels is small. It
is due to the fact that a critical strain required for achieving the steady
state of the flow stress is very large, even at high temperatures. The grain
refinement of these steels is usually achieved by static recrystallization.
Static Restoration Process
      The
microstructures developed by dynamic restoration are not stable and at the
elevated temperatures are modified by metadynamic and static restoration
processes. The latter processes may include static recovery, static
recrystallization and metadynamic recrystallization as shown in Fig. 5b.
In hot rolling, static recrystallization may start
spontaneously. Nuclei of recrystallization take place preferentially at
elongated grain boundaries and interfaces of deformation bands.
Softening by static recovery and recrystallization
occurs at the rates which depend on the prior deformation conditions and the
holding temperature. The recrystallization curves generally follow an Avrami
equation of the form.Â
Fundamental Principles of the Metal Rolling Process
      The chief
departments of a metallurgical plant operating on a complete ore-to-finished
product cycle are the blast furnace, steel making and rolling departments (Fig.
1).
Almost all the steel that is
produced in the steel making department passes through the rolling department;
only a small portion is used for making castings and forgings. The rolling
process, in which the finished product is produced, is the concluding stage of
metallurgical production.
The finished product of such a
plant is rolled stock of various types, designed for various purposes, such as:
rails; beams; channels; angles; round, square or strip steel; special-purpose
shapes, plate the sheet; tubes, etc.
The initial material supplied to
the rolling mill is the ingot, which may be either square or rectangular in
cross-section. In certain cases round ingots are employed (in the production of
tubes, wheels and types).
       The rolling process in a modern metallurgical plant
comprises two stages: 1) rolling the ingot into the semi finished product and 2)
rolling the semi finished into the finished product.
       It is not expedient to roll small blooms or billets
in heavy blooming mills since this lowers the productive capacity and
considerably increases power consumption of the mills.
       A blooming mill can operate efficiently if it rolls
ingots into blooms of large cross-section, from 200 × 200 to 350 × 350 mm in
size. These blooms are subsequently rolled into billets of various size (to suit
the production schedules of the mills rolling the finished product) in billet
mills.
Billet mills are usually located
adjacent to the blooming mills. This arrangement enables small billets to be
rolled from heavy ingots in a single heating. This is obviously good practice
from the economical point of view.
The rolling of billets in two
mills has proved to be highly efficient. The larger the final cross-section of
the bloom, the higher the blooming mill output will be. On the other hand, the
smaller the cross-section of the billet supplied to the mill rolling the
finished product, the simpler the design of this mill will be and the higher its
productive capacity. Another factor is the higher size accuracy and quality of a
finished product rolled from small billets or blanks.
The
breaking-down department, producing semi-finished products, may contain only a
blooming mill or a blooming mill with a continuous billet mill. The preferable
arrangement depends on the production facilities of the section-rolling
department.
In modern metallurgical plants, the production of sheet
and plate also comprises two stages: 1) rolling ingots into slabs and 2) rolling
slabs into plate or sheet.
Advantages of this two-stage procedure over that
practiced in old metallurgical plants, where sheet and plate were rolled
directly from the ingot, are:
 (1) Output
of sheet and plate mills is increased because the billet they roll is of
comparatively small thickness and because the top and bottom of the ingot are
cut off after slabbing;
(2) The quality of the rolled plate and sheet is improved
since the slabbing mill reduces the ingot on all sides and the slabs may be
inspected after rolling so that defects may be removed.
       Slabs may be produced either in blooming or in
slabbing mills. The ever-increasing production of plate and sheet steel, in
conjunction with the development of continuous sheet and plate mills,
facilitated the widespread use of powerful slabbing mills, designed specially
for this purpose. The chief advantage of the slabbing mill, in comparison with
the blooming mill, is that the former has two vertical rolls in addition to its
horizontal rolls. This enables the width to be rolled without turning the ingots
on edge.
       The slabbing mill is the chief breaking-down or
primary mill in plants designed for the large-volume production of sheet and
plate. However, because of their narrower filed of application, slabbing mills
are much more seldom installed than blooming mills. In the majority of cases, it
is necessary for the primary mill to roll both blooms and slabs. Only a blooming
mill will serve this purpose.Â
The main requirements in rolling the finished product
are:
 (1) To
obtain a finished product of the specified size and shape at the highest
possible rate of production and the lowest cost;
(2) To obtain a finished product of the highest feasible
quality concerning, not only its physical and mechanical properties, but also
its surface condition.
These requirements may be met
only if the processing schedule for all operations in producing the given rolled
product is strictly followed.
       The number of operations comprising the rolling
process depends on the specifications stipulated for the shape accuracy,
physical and mechanical properties, surface condition and macro-and
microstructures of the rolled metal. The more exacting these specifications are,
the more complicated the rolling procedure will be and the more operations it
will comprise.
The chief operations in metal rolling production are:
(a)Â Â Â Â Â Â Â Â Â
Preparing the initial material for rolling;
(b)Â Â Â Â Â Â Â Â Â
Heating the initial material before rolling;
(c)Â Â Â Â Â Â Â Â Â
Rolling;
(d)Â Â Â Â Â Â Â Â Â
Finishing, including cutting, cooling, straightening, removing surface
defects etc.
       The preparation of the initial material for rolling
consists in the removal of various surface defects. This is a very important
operation, especially in rolling high-quality carbon and alloy steels, as it
ensures a high output of proper quality with minimum rejects.
       Strict observance of the prescribed conditions for
heating the metal before rolling, proper determination of the temperatures at
the beginning and end of the rolling process and determination of an optimum
draughting schedule are of vital importance and directly influence the quality
of the finished product.
       The prescribed procedure to be followed in cooling
the metal after rolling may be quite significant in many cases. If it is not
observed, the rolled product obtained may have defects such as flakes or cracks
or it may have unsatisfactory properties.
       It is necessary as well to observe the prescribed
conditions for all of the remaining finishing operations, which ensure a
finished product of the specified quality.
       Fig. 2 shows a flow diagram of rolled stock
production from the ingot to the finished product in modern rolling departments.
It represents the production of ordinary and quality carbon steel and alloy
steel stock.
       Proper control over the rolling process and quality
control of the finished product are of primary importance.
       One quality control procedure practised in modern
rolling mill departments is melt inspection on the basis of which the quality of
the steel is determined and the melt is assigned for rolling. The scope of this
inspection depends upon the requirements made to the grade of steel under
consideration.
       Melt inspection begins in the steel making
department where samples of the melt are taken to determine the average chemical
composition of the steel in each melt. Usually two samples are taken from each
ladle. The second sample serves to additionally check the chemical analysis.
This is done in cases when it is necessary to check the content of certain
elements or if such inspection is stipulated by special requirements of the
customer. In certain cases, the samples for this analysis are taken in the
rolling mill department from the billet or the finished product. In the last
years, many elements are determined by spectrographic analysis, which has become
one of the most widespread physical methods of determining the chemical
composition of metals and alloys.
       Further melt inspection may include: determining
the quality of the melt by its macro-and/or microstructure and longitudinal
fracture; determining the grain size of the steel; determining the mechanical
properties and hard-enability and other tests. For this purpose one or two
control ingots are selected. In the latter case, one ingot is selected from the
first tap and the second from the last tap. Control ingots of high-quality
steels are selected from each tap. These ingots are rolled into billets (and
sometimes into finished products) either separately or together with the ingots
of the whole melt.
       Samples for melt inspection tests are selected from
bars rolled from the steel just under the top of the control ingot. In certain
cases, samples are taken from bars rolled from definite sections of the ingot in
height, for example, from the top, middle and bottom sections.
       Macrostructure inspection of steel enables gas
holes (not permissible in killed steel) to be revealed as well as shrinkage
cavities, porosity, segregations, hairline cracks, flakes and other defects.
These defects are evaluated by standard scales.
       Longitudinal fracture enables the degree of slatiness and
the grain size to be determined; it reveals such defects as seams, shrinkage
cavities, porosity, inclusions, stone like appearance, naphthalene appearance
and other defects that can be seen by visual inspection.
       The microstructure inspection, the amount of
nonmetallic inclusions, grain size, depth of the decarburised layer and other
factors are determined.
       The second type of control is over the rolling
process. It should ensure proper heating procedure for the initial materials,
proper observation of the pass schedule for rolling the given section within the
specified tolerances and proper finishing of the rolled material.
       The marking on the initial material must be
carefully checked in the storehouse before charging it into the heating
facilities.
       The temperature of hot ingots should be measured
before placing them into the soaking pits. This will prevent thermal cracks.
Processing instructions usually list the minimum permissible temperature of the
ingot surface and the maximum soaking pit temperature. The heating temperatures
and flame temperatures are checked during heating by appropriate instruments.
       In rolling metal it is necessary, first of all, to
check the initial and final rolling temperatures, as well as the pass schedule.
The setting of the rolls is checked continuously by measurements of the rolled
sections; the condition of the roll grooves and roll gear is also checked
frequently. Lately much attention has been paid to determining the pressure on
the rolls and the torques applied in rolling by means of load cells and other
instruments. This enables the available power of the mills to be utilized more
correctly and fully.
       Mechanization and automation of rolling mills allow
the rolling speed to be considerably increased, particularly for continuous
mills, and enable more attained in cold rolling sheet steel in continuous mills,
for example, became possible only after instruments were developed for
contactless continuous gauging of the strip. Such instruments include radiation
strip thickness gauges based on the use of gamma or beta radiation. Radioactive
isotopes are utilized as the source of energy. By means of a strip thickness
control system, these instruments actuate the roll-adjusting and strip tension
devices. In the very latest installations, computers and television have also
been applied.Â
Steels for Magnetic Applications
1. Electrical
steels-metallurgy and properties
Introduction
      Â
Together with nickel, cobalt and a few other
elements, iron is one of the rare ferromagnetic metals. This property is
intimately related to the electronic structure of the iron atom and a detailed
explanation can only be given in terms of quantum mechanics. However, the
consequence is that the spins of certain electrons are aligned, resulting in an
overall magnetic moment, which is in the same direction throughout small areas
of the crystal structure. Iron and the ferrite steels are, therefore, composed
of small saturated magnetic domains, each corresponding to a microscopic magnet.
The magnetization is not always obvious on a macroscopic scale, due to the fact
that the magnetization directions of individual domains tend to compensate one
another. The magnetization directions of adjacent magnetic (Weiss) domains are
different, and often opposite, and they are separated by bound-aries called
Block walls (Fig. 1). When an external magnetic field is applied, the
magnetization directions tend to reorient and domains more favorably oriented
tend to grow at the expense of the others by movement of the Block walls, so
that the individual magnetic fields no longer cancel out. In magnetically soft
materials, the microstructure is such that displacement of the Block walls is
facilitated, enabling the metal to respond rapidly to external excitation, and
to transmit the magnetic flux with minimum power losses. On the contrary, in
magnetically hard materials (magnets), the aim is to conserve a strong
macroscopic magnetization (remanent induction) with a high coercive force or
coercivity (the external field necessary to overcome the remanent induction).
Figure 2 shows the typical shape of a hysteresis curve, illustrating the
fundamental parameters Bs (saturation induction). Br (remanent induction or
remanence) and He (coercivity). The magnetic permeability, which reflects the
ability of the metal to transmit the magnetic flux, corresponds to the slope of
the initial magnetization curve. A good electrical steel sheet must have a high
permeability and minimum coercivity, together with other properties, which will
be described in detail later.
Utilization and Property Requirements
       Transformer sheets are sold in the finished or
semi-finished conditions and are used in the form of lamination stacks, mainly
in electric motors, alternators and compressors, depending on their properties.
The stacks form the magnetic core of the apparatus concerned. The sheets must
satisfy several, sometimes contradictory, requirements, whose priorities depend
on the specific application, such as high magnetic permeability, low hysteresis
losses (i.e. low power consumption) and ease of cutting to shape. Electrical
steels come in two principal categories, the oriented and non-oriented grades.
       Grain-oriented sheets are obtained by a complex
processing cycle and give excellent results in terms of permeability and core
losses in certain conditions, particularly in unidirectional fields, such as in
transformers. The particular feature of these materials is their microstructure
consisting of very coarse grains, oriented with the cube edge parallel to the
rolling direction (110) ( the so-called Goss texture). The
crystallograbphic direction is a direction of easy magnetization, so
that when the sheet is magnetized in the rolling direction, its permeability
will be very high and its coercive force very low. Figure 3 shows the magnetic
anisotropy of iron and illustrates the advantage of having a
direction parallel to the exciton field. In contrast, because of their planer
anisotropy, grain-oriented sheets have much poorer performance in rotating
machinery, such as motors, where the excitation field rotates in the plane of
the sheet. They are difficult to blank because of their large grain size. In
France, they are manufactured by the Ugine S.A. company.
       Non-oriented sheets are produced by a much more
conventional process, and can be subdivided into two categories:
 (a)        Â
"Fully-processed" grades are delivered in the finished
condition and are alloyed with silicon. They are continuously annealed at high
temperature, and sometimes varnished. Although their magnetic properties in the
rolling direction are not as good as in grain-oriented sheets, they are much
less textured, making them better suited for use in rotary fields. They have
good blankability, which depends on the silicon content, the grain size, and
possibly on the coating.
 (b)        Â
"Semi-processed" grades are continuously annealed at a lower
temperature and are delivered after a significant skin-pass reduction (several
%). They have good blankability, but must be given a final annealing treatment
by the user to develop their magnetic properties. This treatment has several
effects. Firstly, it produces a coarse grain size, due to the critical strain
(1-5%) imparted by the skin-pass, leading to relatively few recrystallization
nuclei, enhancing the magnetic properties. Secondly, decarburization is promoted
by the use of a controlled atmosphere. Thirdly slight oxidation, producing a
bluish tinge, provides electrical insulation between the laminations in a stack,
limiting the formation of eddy-currents in alternating current machines. The
uses of semi-processed grades are similar to those for fully processed
materials, even though the clients are often different. Today's top semi
processed grades are roughly equivalent to mid-range fully-processed products.
Optimization of Magnetic Properties
       The remainder of this article will concentrate on
semi- and fully-processed non-oriented sheet.
       Among other characteristics, a good electric motor
must have a high maximum speed (or torque) and high efficiency. The magnetic
circuit design and the choice of metal have a decisive influence on these
properties. Power losses, which affect the efficiency, have two major sources,
resistive losses in the copper windings, and core losses due to induced
eddy-currents and magnetic hysteresis. The resistive losses are proportional to
the square of the induction current, and can be decreased if the coupling
between the stator and the rotor is increased, i.e. if he core has a high
permeability.
Hysteresis losses
The applied fields associated
with alternating currents are sinusoidal, so that the hysteresis cycle is
repeated 50 times per second at the normal mains frequency. The energy consumed
by this process alone is equal to the area inside the hysteresis loop. The
hysteresis is caused by internal magnetic friction associated with the movement
of the Block walls. To reduce the losses, it is necessary to lower the coercive
force, which correlates to high domain wall mobility. This can be done by
eliminating as far as possible all precipitate particles, particularly cementite,
requiring extremely low carbon contents. The "fully-processed" grades
are decarburized either in the melting shop or by strand annealing after cold
rolling. The "semi-processed" grades are decarburized by the user
during final annealing after cutting to shape.
The very low solubility of
carbon in ferrite at room temperature makes these steels highly sensitive to
magnetic aging, which can lead to increased power losses due to the natural
precipitation of fine carbide particles. The final carbon content must therefore
be less than or equal to 20 ppm. Figure 4 shows the percentage increase in power
losses with carbon content after simulated aging in a fully-processed grade.
Manganese sulfide and aluminum nitride particles are less detrimental, due to
their large size (induced by appropriate metallurgical control).
Another way of reducing
hysteresis losses is to decrease the coercive force by increasing the grain
size, since grain boundaries are effective obstacles to the movement of domain
walls. In non-oriented grades, cold rolling is performed in a single reduction
of about 70%. Fully-processed steels are continuously annealed at high
temperature to obtain coarse grain sizes of the order of 4 to 8 ASTM (20-80 um).
Semi-processed sheets are given a slightly smaller cold reduction, followed by
strand annealing at lower temperature and skin-pass following corresponding to a
strain of less than 10%. Critical strain recrystallization during the final high
temperature annealing treatment carried out by the customer gives typical grain
sizes of about 0 to ASTM (80-300 um), strongly diminishing hysteresis losses.
The skin-pass reduction enables the blankability to be optimized. The electric
motor manufacturers or blanking shops usually prefer YS/UTS ratios greater than
0.85 in order to improve productivity and reduce tool wear. The amount of cold
work required to obtain such ratios is always a few percent higher than the
critical recrystallization strain. Figure 5 shows a typical example of the
variation of YS/UTS ratio with skin-pass reduction. The accompanying micrographs
illustrate the as-annealed grain sizes. For low strains, recrystallizaion does
not occur and the grain size remains fine, while strains of a few percent lead
to coarse recrystallized grains.
The density of dislocations and
other mechanical defects is also important. Semi-annealed sheets are annealed
after blanking and therefore have very low dislocation densities. In contrast,
fully-processed sheets can contain local regions of high strain after blanking.
This is particularly true at notches in the stator, where the deformation is
most severe. Since the purpose of these notches is to transmit the magnetic flux
from the stator to the rotor, certain users perform a stress relieving treatment
at about 800ºC to produce local recovery or even recrystallization.
       Finally, another useful way to reduce hysteresis losses
to modify the texture, enhancing the density of (100) and (110) planes, which
significantly raises the low flux density permeability and sharply lowers the
coercivity. The texture can be modified by adapting certain stages of
processing, but the operations involved are relatively complex.
Eddy-current losses
The other important source of
core losses is the resistive heating associated with induced eddy-currents. The
latter are caused by interaction between the magnetic filed and the conduction
electrons of the metal. The losses due to this phenomenon are proportional to
the square of the frequency and the thickness of the sheet (the current loops
appear in the sheet section perpendicular to the magnetic flux, and create a
counter-field which opposes the induction field). Apart from reducing the
working frequency, which is rarely possible, there are three ways of decreasing
this effect.
• Decreasing the sheet thickness is common practice,
since magnetic cores are generally composed of stacks of thin sheets, less than
1 mm thick. This approach is limited by productivity considerations.
• Efficient inter-lamellar electrical insulation,
generally obtained by oxidation (bluing) in semi-processed grades, at the end of
the final annealing treatment performed by the user, and by varnishing for
fully-processed products. The latter technique is much more effective.
• The use of alloying elements such as silicon,
phosphorus, aluminum and manganese have the advantage of considerably increasing
the resistivity. This significantly decreases the intensity of the eddy-currents
and the associated power losses. Silicon can be incorporated in the steel in
appreciable amounts, up to 3.5%, with appropriately adapted processes. Beyond
this level, excessive brittleness prevents cold rolling. Furthermore, silicon
reduces the saturation induction of the steel, and hence its permeability at
medium and high flux densities. Steels and hence its permeability at medium and
high flux densities. Steels with more than 0.5% silicon have various
technological disadvantages. After continuous casting, the slabs must be
transferred hot to the rolling mills, due to their tendency to crack on cooling,
and silicon-containing sheets are difficult to weld. However, in spite of these
drawbacks, the functional benefits of silicon-containing electrical steels have
led to their widespread use, due to the critical importance of low power
consumption, especially in large motors.
2. Type of electrical steel.
Classification
Non-oriented electrical steel
sheets are covered by three French standards, depending on whether they are
semi-or fully-processed. Semiprocessed products are described by the C 28-925
and 28-926 standards, while NF C 28-900 treats fully-processed sheet. It all
cases, only the magnetic properties are guaranteed, particularly the total
specific losses at a given magnetization level and the induction level for a
given applied field, in both the transverse and rolling directions. For
semi-processed sheet, after a reference heat treatment, the metal must guarantee
a power consumption less than a specified value, together with a minimum
induction level for a given applied field. The reference heat treatment
corresponds to annealing for two
hours at 790ºC in a decarburizing atmosphere. The apparatus used for the
measurement is called in Epstein frame and is described by the NF C 28-911
standard.
The designations of different
steel grades according to these standards reflect the maximum power losses in
watts for one kilogram of the metal concerned and an induction of 1.5.T. Since
these losses depend on the thickness, the designation includes the standard
thicknesses (0.5 and 0.65 mm for the semi- and fully-processed produces. plus
0.35 mm for fully-processed materials). For example, the FeV 660-50 grade
corresponds to a material with a thickness of 50 hundredths of a millimeter, for
which the guaranteed total losses must not exceed 6.6 W/kg measured in a
standard Epstein frame. The difference between the two semi-processed product
standards NF C 28-925 and 28-926 concerns the amounts of alloying elements, the
first applying to unalloyed steels (< 0.5 wt. % Si) and the second to alloyed
grenades (according to the NF EN 10-020 standard). The designations are
differentiated by two letters. HD for unalloyed semi-processed grades, HE for
alloyed semi-processed materials and HA for the fully-processed sheets. The NF C
28-900 standard mentions certain requirements concerning magnetic aging, the
packing factor (volume increase on stacking) and magnetic anisotropy.
Steel Grades
Usidecoupe
Although not really electrical
steels, these XC grades are used for the manufacture of small motors for very
infrequent utilization (e.g. kitchen mixers, electric automobile window drives,
etc.), for which the efficiency is of little importance. Although their magnetic
properties are never guaranteed, their mechanical characteristics are severely
controlled. Their blanking capacity must be excellent and invariant.
Semi-processed grades
Four grades are presently
available, in different thicknesses. They cover all the requirements of the
domestic appliance and small industrial motor markets. The differences concern
the guaranteed maximum power losses, and are essentially related to the amount
of alloying additions in the steel. The latter include the four main elements
which increase the resistively, i.e., silicon, aluminum, phosphorus and
manganese. Table 1 gives the guaranteed magnetic properties of these grades,
compared to those required by the standards, together with their typical
mechanical characteristics.
       The trend in semi-processed steels is towards more
efficient decarburization during melting. This improves their intrinsic quality,
but most of all, simplifies the process for the end user, by eliminating the
need for decarburizing, increasing the productivity of the grain coarsening
furnace. Moreover, modern stacking techniques, such as the Fastec process,
become accessible to the semi-processed grades. Since assembly is performed
immediately after blanking, decarburizing is not possible. However, in these
applications, varnishing of the coils becomes essential. This development
enables the steelmaker to differentiate between semi- and fully-processed grades
at a much later stage in the manufacturing cycle, significantly simplifying
internal product management.
Fully-processed grades
A large number of fully
processed grades are available on the market. For a given thickness, it is
possible to choose from a wide range of different power loss levels. Various
types of varnish can be applied to the sheets. Table 2. illustrates the extreme
grades in terms of guaranteed maximum power losses for three standard
thicknesses, together with their typical mechanical characteristics.
The choice of power loss level
depends on the application concerned, varying greatly from a washing machine
motor to a nuclear power station turbine. Metallurgically speaking, the
variations are due mainly to differences in the silicon and aluminium contents
and in the annealing cyles employed.Â
Preparing and heating the initial materials
Preparations for rolling
The initial materials (ingots
and billets) are prepared to be rolled by removing various surface flaws. This
is called surface conditioning and is an important operation, especially in
rolling quality carbon and alloy steels intended for the manufacture of vital
components in various fields of industry. The cost of preparing the initial
materials will be justified in this case by the possibility of obtaining a
product of the specified quality and by the decrease in the amount of rejects.
As a rule, hot ingots are
charged into the soaking pits and, therefore, surface conditioning operations
are performed on the billets (however, flaws may also be removed from the
surfaces of the hot ingots).
Especially exacting requirements
are made to the surface of alloy and high-alloy steel ingots. Therefore, the
cast ingots may be completely cooled, surface conditioned and then reheated for
subsequent rolling (a softening heat treatment may be employed before removing
the flaws). In this case, surface flaws are removed both from the ingots and
from the billets.
Surface defects, subject to
removal, include: scabs, hair lines, cracks, rolling laps, nonmetallic
inclusions, scratches, etc. All of these flaws are revealed by inspection. If
all surface defects are to be removed, the billet is first pickled. Defects,
enclosed by scale and therefore hidden from ordinary inspection, may be detected
by acid pickling.
       Chipping operations to remove surface flaws,
performed by means of pneumatic hammers usually operating under an air pressure
of 5 to 5.5 atm, are still in wide use. This method has a low production
capacity, especially for alloy steels, and is a health hazard as far as the
operations are concerned.
Chipping requires the highest
labour input of all rolling operations and is therefore little suited to modern
rates of production. It is chiefly employed in the surface conditioning of
billets and sometimes as a supplementary operation for removing certain deep
flaws on ingots. These are flaws such as may remain, for example, after
rough-machining the whole surface of the ingot or after performing some other
allover cleaning operation.
Defects are usually chipped in a
direction along the length of the billet or ingot since transverse chipping may
caulk such defects as cracks. The sides of the chipped groove should slope
gently and its width-to-depth ratio must be such that no new cracks or other
defects will be produced on the surface of the finished product in subsequent
rolling.
Swing-frame grinders are also
used for surface conditioning operations. They are mounted on a special
suspension device allowing them to be easily swiveled about a vertical axis and
titled from the vertical position. Due to the high grinding speeds, the abrasive
wheels used are of aluminum oxide with a resinoid (bakelite) bond.
In contradiction to chipping
with pneumatic hammers, conditioning with a grinder is carried out in the
transverse direction (on the billet) since it is difficult to detect hair lines
or fine cracks, usually running along the billet, in longitudinal grinding.Â
Grinders have a low production
capacity in surface conditioning operations and the cost is higher than for
chipping. Consequently grinding finds application in removing a large amount of
slight defects and chiefly for conditioning high-alloy steel billets. Chipping
of the latter with pneumatic hammers is extremely difficult or even impossible.
In removing flaws by grinding from hard steel billets, the intensive heating of
the metal and subsequent cooling may lead to the formation of grinding cracks.
This can be avoided by taking lighter cuts and by selecting the grain size,
grade and peripheral speed of the grinding wheel to suit the steel being
processed. Grinding cracks are more liable to appear in conditioning hard
steels. That were rapidly cooled after rolling, producing high stresses.
Air-hardening steels should be conditioned in the as-annealed state to prevent
the formation of cracks.
A new surface conditioning
method, called flame scarifing, has come into use in the last years. It consists
in burning out the flaws in the surface by means of a torch with an oxyacetylene
flame.
Flame scarfing is performed either by hand or in
special machines.
As a rule, hand (torch) scarfing
is a localised operation in which certain definite defects are removed from the
surfaces of ingots and billets. Here the tip of the torch is directed to one end
of the defect and the metal is preheated to a temperature of 950°-1,000°C.
This takes several seconds. The torch is held at a angle of 75°-80° to the
billet surface. When the metal is heated to the burning point, the amount of
oxygen supplied to the torch is increased.
This oxygen removes a layer of
metal by oxidizing or burning the metal. As soon as the metal begins to burn,
the torch is titled to an angle of 25°-30° to the surface. Then the jet of
oxygen not only burns the metal but also blows the slag and liquid metal from
the surface being scarfed.
At present, almost all types of
steel are torch scarfed. All carbon and low-alloy steels may be torch scarfed
without difficulty. Stainless, heat-resistant and other steels, with high
chromium content, require the application of special fluxes and coatings, which
facilitate burning and form slag with a low melting point. Frequently a special
torch is used to scarf these steels. It differs from the ordinary torch in
having a supplementary injector, supplied through one hose with an oxygen-flux
mixture, and with additional “cutting†oxygen through a second hose.Â
First, the beginning of the cut
is heated, then the oxygen and flux delivery are turned on. When a sufficient
amount of molten slag has formed, the supplementary oxygen injector is turned on
and torch is moved along the line of scarfing.Â
Cracks, which may be formed in
flame scarfing, are due to the thermal stresses resulting from large temperature
differences along the cross-section of the billet and also to stresses
associated with the formation of a martensite or troosto-martensite structure
from the austenite. The lower the thermal conductivity and initial temperature
of the billet being scarfed, the higher its coefficient of thermal expansion and
the lower the ambient temperature, the higher the thermal stresses will be. The
higher the austenite stability and carbon content of the steel and the higher
the cooling rate after scarfing, the more susceptible the steel will be to crack
formation. Stresses due to scarfing may be reduced by preheating the billet or
by surface conditioning directly after rolling when the billet is still hot.
Torch scarfing is performed in a
direction along the billet. The scarfed groove should have sloping sides and
should be of a width at least five times its depth and of a length at least
three times the width.
The production capacity of hand
torch scarfing is several times higher than chipping with pneumatic hammers.
Highest output is achieved, however, in flame scarfing machines where surface
defects are removed by an allover burning of the surface layer with an
oxyacetylene flame.
Flame scarfing machines are
usually installed beyond the mill, for example, in the roller table line between
the blooming mill and the shear. Here hot blooms and slabs are scarfed directly
after they leave the mill. Here, the surface layer of metal is removed
simultaneously from all four sides of the bloom.
The same type of mechanised
allover surface conditioning is also practiced in the billet storage where
billets are treated either cold or preheated. In these installations, the torch
head is mounted on a truck traveling on rails along which the billets are
arranged.
Other surface conditioning methods, used for
billets and ingots, are: 1) milling, 2) planning and 3) turning.
Of these, turning is the most extensively used
operation for roughing ingots and billets.
Turning is applied in roughing
round ingots and billets of heat-resistant, stainless and other special grade
steels for the production of high-quality seamless tubing. Special lathes are
available at present for roughing such round ingots. These lathes are equipped
with special fixtures for rapidly setting up and centre-drilling the ingots. The
use of several tools clamped on a single carriage effectively reduces machining
time.
Alloy and high-alloy steel
ingots, intended for producing sections, sheet or plate, are also turned in
special cases. Such ingots have a square or rectangular cross-section and are
machined on special tracer-controlled lathes. These machine tools, called
multiple-cornered lathes, can accommodate ingots and billets of any
cross-section.
Before multiple-cornered lathes
came into use, square and rectangular ingots were roughed all over on
general-purpose and special planers. The low output and high cost have almost
excluded this method from general practice.
The surfaces of ingots and billets are milled both
for allover surface roughing and for removing separate defects. This operation
is performed on special milling machines.
Large surface defects, of a
width up to 50 mm and a depth up to 15 mm, difficult to cut out by pneumatic
hammer chipping, are usually removed by a local milling operation.
Ingots and billets of mild and
medium hard steels are machined without being previously annealed. Hard steels
are annealed before machining.Â
Bolt and NuT Manufacturing Technology
Introduction
       The metal-working industry employs a range of
different technologies. These include the casting of molten metal into moulds,
fusion by welding, cold and hot mechanical working, cutting and chemical
machining. technologies consist of a variety of distinct ‘techniques’; for
example, metal cutting technology includes amongst many other ‘techniques’
turning, milling and drilling. The manufacture of nearly all threaded fasteners
require the employment of methods or ‘techniques’ from more than one
technology.
       In later chapters, bolts and nuts are described as
being manufactured using a particular technology. It does not necessarily follow
that only one technology is employed in the full sequence of manufacturing
operations. Rather, the characterization refers only to the most important
technology that is used in the complete manufacturing sequence.
       The next section of this chapter provides an
introduction to the principles involved in the manufacturing of threaded
fasteners. It is followed by more detailed discussion of alternative,
technologies, the ranges of alternative ‘techniques’ within each technology
and the types of tools and machines that are used. The descriptions of the
principles and technologies are not comprehensive; the purpose of this chapter
is to give a reasonable idea about the nature of choice currently available and
to provide sufficient information to enable the reader unfamiliar with fastener
manufacturing to follow the discussion in later chapters of the study.
       The three main metal-working technologies used in
the manufacture of threaded fasteners are cold mechanical working or cold
forming, hot mechanical working or hot forging and metal; cutting. In mechanical
working, the processes used to shape metal include rolling, drawing, extrusion,
up-setting open die forging, closed die forging, and presswork. Although
mechanical working changes the shape of work pieces it does not change their
volumes substantially. The processes involve the plastic re-shaping or
deformation of either cold or heated work pieces by the external action of
special tools. Mechanical working strengthens components by drawing impurities
and grains into bands along the direction of working and closing minute
cavities.
Fundamentals of Mechanically working and cutting metals
(a) Cold Forming
       When metals are subjected to progressively larger
loads they first deform elastically. If the load is increased beyond a certain
point the metal becomes plastic. Curve A on the load extension diagram in figure
1 shows how the rate and degree of deformation can vary with the magnitude of
the applied load. Stress, which is the load per unit area, is plotted on the
vertical axis of the diagram while deformation, expressed in terms of strain is
shown on the horizontal axis. Beyond the transition point on curve A between
elastic and plastic deformation some deformation remains when the load is
removed. If the load continues to be raised the metal eventually breaks.
       Cold forming metal within its plastic range
decreases its plasticity and this phenomenon is called strain or work hardening.
Curves A and B in Figure provide a comprison of how mild steel behaves before
and after cold working. Work hardening is not only associated with an
appreciable decrease in plasticity but also with an increase in strength.
Advantage is sometimes taken of this to deliberately increase the strength of a
piece of metal by cold working it, but since plasticity decreases with cold
working there is a maximum limit to the amount of cold working that may be
undertaken. Advantages of cold forming over hot forging are that parts are free
of surface scale, may require less raw material since they can be formed closer
to final size, do not need to be pre-heated and require little cool-downtime
after working.
(b) Hot Forging
       For each metal there is a range of temperatures in
which the plasticity and resistance to work hardening is greatly increased. The
forging temperature range varies from metal to metal. For mild steel it is
around 1,200°C while for lead it is room temperature. The forgability of a
metal is its ability to flow into the required shape without cracking and offer
low resistance to the forces shaping it. Forging a metal to a given shape
requires considerably less power than cold working it, but the accuracy and
finish of the work piece are generally inferior.
(c) Metal Cutting
There are several ways of cutting metal: sawing,
abrasive cutting-off (grinding), shearing or cropping, and machining. These
methods have much in common both with each other and with the mechanical working
technologies; all involve working metal in the zone beyond the elastic limit.
Manufacturing Technologies
       In this section each of the main bolt and nut
manufacturing technologies employing the principles described in Section 2 is
discussed.
(a) Cold Forming of Bolts
       Four widely used methods of cold forming are shown
in the top row of Figure 2. These are upsetting etc. which involves a reduction
in length and increase in cross-sectional area of work pieces, forward and
backward extrusion which both have the effect of reducing the area and
increasing the length of work pieces and thread rolling. In thread rolling the
dies penetrate the surface of the blank to form the thread root, the displaced
material flowing outwards and upwards to form the crest of the thread. Cropping
or shearing, trimming and piercing are often carried out in association with
cold forming but they employ cutting rather than forming techniques, are shown
in the lower half of Figure 2. Most work pieces or blanks to be cold formed are
first cut off by cropping them from wire stock. Hexagonal bolt heads are usually
formed by trimming the periphery of cylindrical upset heads with a hollow
hexagonal punch. Trimming removes any cracks on the edge of the heads caused by
upsetting. Piercing involves the removal of a slug from the work piece to form a
hole and is used in the cold forming of nuts.
       One sequence of operations which can be used to
produce a bolt to its final shape together with an outline of the necessary
tooling is shown in the left-hand side of Figure 3. In this case the heading
machine cuts off and upsets the blanks in two blows. The additional secondary
operations required to finish the blanks are carried out on a single blow
trimming machine and thread rolling machine.
       In heading machines, the end of a coil of cold
drawn wire that has been precoated with a lubricant passes through straightening
and feed rollers which push the wire through the cut-off quill until it comes up
against a stop. The blank is sheared from the end of the wire and transferred to
the heading position by the cut-off mechanism. During the firsts stroke in the
heading station the punch pushes the blank into the die until it comes up
against the ejector pin and then commences the shaping of the head by upsetting
a cone. Between the first and second blow the first punch is moved aside and the
second punch takes its place. The second punch forms the head into a cylindrical
cheese and usually also embosses the top of the head of the blank with symbols
to permit identification of the manufacture, tensile strength and sometimes of
the tread forms. Once the second punch has withdrawn, the blank is forced from
the die by the ejector pin and falls clear of the die before the next blank is
inserted.
       Two blows are needed to form the cheese head
because there is a limit to the amount of metal that can be upset in a single
blow without buckling the portion of the blank that is not supported within the
die. In a single blow, the maximum amount that can be cold upset under control
is two and a quarter diameters but most single blow heading is within the range
of one to one and a half diameters. In two blow heading four and a half
diameters can be upset.
       There are other limitations to the lengths of
material that can be headed using the methods shown in Figure 3. Ejector pins
buckle if they are unsupported over a length of more than eight times their
diameter which limits the length diameter ratio of formed b1they anks contained
within a die. One way round this problem is to provide support to the ejector
pins by using a telescopic ejector mechanism. This makes it possible to remove
formed blanks with lengths contained within the dies up to twelve times as long
as their diameters. For even longer parts which are upset (but not extruded)
split dies are used which are made in two halves. The two halves are held
together during the upsetting operation and act in a similar way to normal
closed dies but are forced apart before ejection. With this arrangement blanks
are inserted from the back as opposed to the front of the dies and are cut off
by the back of the split dies before they close for the upsetting operation. The
length of the wire for the next blank to be upset ejects the previous part from
the die. The head of very long bolts can be cold formed by holding pre-cut
blanko in split dies during upsetting. The heads of most cold formed threaded
fasteners are upset using the closed rather than the split die arrangement. The
reason is that when bolts are made from wire the split die configuration
requires a different set of dies for each blank length whereas only the ejector
pin requires replacement when, blank lengths are altered using closed dies.
       After heading, the blanks are collected from under
the heading machine and transported to the trimming machine. The trimming
machine performs two distinct functions: first it gives the head a hexagonal
shape by trimming off material and second, it straightens the body and forward
extrudes the end of the body in preparation for thread rolling. The headed
blanks are usually deposited into the hopper of a trimming machine where they
are first automatically orientated correctly for delivery into a chute at the
lower end of which they are picked up and transferred to a point over the
trimming die centre by a pair of fingers. The punch pushes the blank through the
extrusion die as depicted in the left hand column of Figure3. Towards the end of
its forward stroke the punch forces the head of the blank through the hexagonal
trimming die. Once the punch is on the return stroke the blank is ejected from
the die by means of a spring behind the ejector pin.
       There are limitations to the amount of forward
extrusion that can be carried out with a single blow. If buckling of the portion
of the blank that is not supported in the die is to be avoided during extrusion,
the reduction in cross-sectional area resulting from a single operation must not
exceed 30 per cent.
       The diameter to be thread rolled could be reduced
by machining rather than extrusion, but this would waste material. Another
method of producing threads is to cut them but this also wastes material. Thread
rolling has other advantages, however, over thread cutting. Firstly, the cold
working action during rolling increases the strength of the threads since the
grain of the material follows the thread contours and secondly, compressive
stresses are imparted to the thread roots during rolling which offset the
tensile stresses produced by tightening nuts into bolts. In flat die thread
rolling, parts are threaded between a pair of flat dies, one reciprocating and
one stationary. The thread-shaped ridges on the working faces of the dies are
inclined at the angle necessary to produce a continuous thread on the blank. A
thread is rolled on one blank at a time while it rotates about its own moving
axis, during the forward stroke of the reciprocating die. Thread rolling can
take other forms. In planetary thread rolling, blanks are rolled between a
centrally located rotating die and a stationary concave die segment. The
circular die in the middle rotates continuously with the result that two or
three parts can be passing between the dies simultaneously. Such machines can
only roll relatively small diameter blanks. In another arrangement the blank is
squeezed between two cylindrical rotating dies, which have, thread form cut
round their circumferences.
       Thread rolling machines, like trimming machines,
are usually arranged so that the blanks are automatically fed down a chute from
a hopper although, in the case of very long blanks, it is necessary to hand-feed
both trimming and rolling machines. Some fastener manufacturers link their
heading, trimming and rolling machines by providing conveyors, which collect the
blanks from under one machine and deliver them to the hopper feed on the next
machine in the line. Although conveyors can reduce the amount of materials
handling required between the operations they have two disadvantages. First, all
the machines in a line linked by conveyor must be working on the same size of
blank (when the machines are not linked up this is unnecessary) and second,
overall machine utilization in a linked system can be low because when one
machine stops running for more than a short period the other machines linked to
it must also halt.
       The heading, trimming, extrusion and rolling
operations can be combined in a single specialized machine. The operations are
illustrated diagramatically on the right hand side of Figure3. The drawing shows
a machine having a bank of five stations, with a cut-off, four dies, and four
punches. Transfer fingers move each blank along the row of dies one die position
at a time between blows. Each blank receives a single blow from each punch. In
the case shown in Figure 3, wire with a diameter greater than the finished shank
diameter is used. By extruding the body if the bolt down to its finished shank
diameter in the first die the head can be upset in only one blow by the second
punch without buckling occurring. This is possible because the section of the
blank forming the head is less than two and a quarter times the diameter of the
original wire. compared to two blow heading there is less deformation in the
head for a given bolt size. The result is a better balance of properties between
head and shank with the result that normalizing of finished bolts may be
necessary. The portion of the shank that is to be thread rolled is extruded in
the third die and the head is trimmed in the fourth die. In some machines, the
trimmed blanks pass through the centre of the trimming die and enter a tube
through which they are delivered to either a pointing and then a thread rolling
station or directly to a thread rolling station. The pointing operation cuts a
chamfer round the tip of the body so that the start of the rolled thread is
even. In other machines, a trimming punch is used and parts are allowed to drop
into a collecting belt after ejection from the last die.
       There are several types of cold forming machines,
which represent intermediate stages between two blow heading machines and fully
integrated machines. Often when parts with large diameter heads and slender
shanks are required, two dies, three blow headers are used. Machines with four
or five stations in line but without pointing or thread rolling stations are
called either progressive or transfer headers. These machines are most often
used for cylindrical parts other than bolts which would otherwise have to be
machined from solid bar.
       In addition to the precoated solid lubricants on
the wire small quantity of oil is usually applied to the material stock to
lubricate forming operations. An oily rag is often tied round the wire ahead of
the feed rolls but oil can be sprayed into the tool zone where there is no risk
of large quantities of oil entering the die and causing work pieces to be
partially filled. The important characteristics of these oils are resistance to
high pressure and temperature and the minimum emission of smoke and toxic
vapours. Most cold forming wires are precoated with thin dry coatings.
(b) Cold Forming of Nuts
Figure 4. shows a cold nut
forming sequence and an outline of the tooling. Cold nut forming machines are
quite similar to short stroke progressive headers. The wire stock is fed through
feed rollers, the cut off quill and the cut off bush up against the length stop.
A cylindrical blank is cut off and inserted into a transfer gripper which
presents it to the first forming station. After forming in the first die the
blank is ejected and received by a second gripper for transfer to the next
forming station. This sequence is repeated until the blank leaves the last die
position finished formed. During two of the transfers in the sequence shown in
Figure 4 the blank is turned end on end through 180 degrees to enable the
punches to work on both ends of the blank. Because of the work hardening that
occurs during forming, the slug is punched out cleanly at the last station.
Nuts can also be formed in four stations cold nut
forming machines which accept wire with a hexagonal cross-section or
alternatively by cutting off and performing cylindrical blanks on one machine,
heat treating the blanks to remove the effect of the work hardening induced
during the initial forming and then finish forming the blanks on a second
machine.
After forming, nut blanks have
their thread cut on tapping machines.
(c) Hot Forging of Bolts
       Figure 5. shows two alternative operation sequences
for the production of hot forged bolts together with outlines of the necessary
tooling. Bolts are forged from hot rolled round bar of a diameter equal to that
of the finished bolt shanks.
       In the first process in Figure 5, a blank, or pin,
of sufficient length to form one bolt is cut off. Sawing is rarely used for this
operation since it takes longer than cropping. Equipment used for cropping
ranges from manually operated shears intended for cutting reinforcing bar on
building sites to fully automatic high speed cropping machines. Manually
controlled purpose-made semi-automatic cropping machines are available but
simple reciprocating mechanical presses can be equipped with cropping tools to
do the job in the same way. In such machines the bar stock is supported on the
lower shear knife and the head carrying the upper knife reciprocates
continuously while an operator pushes the bar in by hand against a stop during
the interval between strokes. Sometimes two pins can be cut per stroke by
loading bars in parallel. There is usually provision for stopping the head in
the raised position between strokes to allow time for the manipulation of heavy
bars. Automatic cropping machines are fed from stocks of bars held in magazines,
which can be replenished without interrupting production.
       Most hot forged bolts are thread cut because steel
in the hot rolled state tends to have an uneven and scaly finish, which is
unsuitable for extrusion and thread rolling. Slight chamfers are usually cut on
the ends of the shanks; these serve to remove metal distorted during cropping
which might prevent the shank entering the forging die, provide lead-ins for
thread cutting tools and also help start threads when bolts and nuts are
assembled. This pointing operation is similar to that carried out on some cold
formed bolts. A variety of purpose built pointing machines are available ranging
from magazine fed automatics to simple manually loaded machines. Sometimes old
lathes can be modified to do the job.
       Before forging, the end of the pin, which is to
form the bolt head, is heated. Simple pin heaters consist of open hearths
burning solid fuel with ledges round the top edges on which pins rest. The tips
of the pins are introduced through holes in the refectory bricks. More
sophisticated furnaces using oil, gas or electric induction heating are
available which automatically deliver heated pins at a predetermined rate.
Induction heating equipment is expensive to purchase but results in less surface
scale and allows precise control over temperature. Three unsupported diameters
of bar can be hot upset in a single blow if the end of the bar is square to its
longitudinal axis. However, because the ends of most cropped pins are not flat
and may be deflected sideways it is usual to employ two blows even for heads
requiring less than three diameters of material. The two blow hot heading method
shown in the left hand column of Figure 5 is similar to two blow cold heading in
that a cone and cheese are upset using two punches and a closed die which
supports the shank of the blank. On most two blow headers the die is mounted on
a saddle so that it can be moved clear of the punches for the insertion of long
pins. The operator can push the saddle back and forward but more often this is
done automatically. The operator uses tongs to load the heated pins into the
die. After heading, the ejector pin pushes the blank out so that its shank can
be grasped by tongs for complete removal. Due to the uneven surface of hot
rolled bar more clearance is provided between the bores of hot heading dies and
work pieces than in cold heading dies. Because of this clearance and because the
ejector pin does not have to push the bolt right out of the die, the length of
blank that can be hot headed in a closed die is not restricted as in cold
forming and this is a major advantage of hot over cold solid die heading. Water
is usually used to cool the hot heading tools.
       After hot heading a separate machine is used to hot
trim or strip the cheese to form a hexagonal head. The layout consists of an oil
fired pin heater, a hot heading machine and a hot stripping machine. This layout
enables a furnace man and one operator on each machine to head and strip bolts
in one heat. The rate of output of such teams of three men is usually governed
by the work pace of the heading machine operator.
       The sequence of operations on the right hand side
of Figure 5 can be used to forge bolt heads without trimming waste. This process
is called hot upsetting and is carried out on forging machines. Forging machines
squeeze rather than hammer the work into the required shape. Because scale and
surface defects are set cut away from round the head by stripping operation,
this method is usually employed for heading large bolts with wider absolute
tolerances than those on small bolts. The length of bar required to form the
head is measured by the operator pushing the heated end of the pin against the
first outside die. The gripper dies are closed to hold the shank before the
heading stroke of the outside die commences. If the finish of the head is
unsatisfactory, the bar can be rotated one sixth of a turn and the operation
repeated. The head is finish formed at the second station.
       Forging machines can be equipped so that the work
piece is manipulated mechanically, but on the smaller machines required for most
sizes of bolt, the operator usually inserts and removes the work with tongs.
Since the shanks of bolts protrude from the forging machine during upsetting the
process does not restrict the length of bolt that can be headed.
       Screw cutting which is the final operation required
to finish hot forged bolts is usually carried out on special purpose machines
using rotating tangential cutting dies arranged as shown at the bottom of Figure
5. One advantage of this particular type of thread cutting is that the dies can
be resharpened many times. The dies cut the thread in one pass and spring open
before the return stroke.Â
(d) Hot Forging of Nuts
       The material stock for hot forged steel nuts is
normally hot rolled bar. Figure 6 shows alternative ways of hot forging nut
blanks. In the sequence on the left of the figure the rectangular bar is
progressively fed into the semi-automatic nut press by an operator. Once the bar
has cooled below the forging temperature it is returned by the operator to the
furnace. Forging is accomplished by first forging vees into the top and bottom
faces of the bar to produce two sides of the hexagon of the first nut and two
sides of the second nut and then shearing the blank of the first nut off from
the parent bar. During shearing the cut off tool pushes the blank horizontally
into an enclosed die where the hole is then pierced before the finished blank is
ejected. Since forging in a hot nut press takes place to an enclosed die an
automatic adjustment is provided to compensate for surface scale and variations
in bar section. After hot pressing it is common for the end face of a nut to be
lightly machined to remove punch burrs. This operation, called frazing, is most
simply carried out using an end milling cutter mounted centrally in a vertical
rotating spindle above a nut clamping fixture.
       The automatic hot nut forging sequence on the right
hand side of Figure 6 has more in common with the cold nut forming process in
Figure 4 than with the hot nut pressing method. The bar heated can be either fed
from an automatic magazine or manually and the heat source can be a coal, oil or
gas fired furnace or an electric induction coil. From the heater bars pass
straight into the hot nut forging machine. Due to the ductility of the hot steel
only three forming stations are required.
       A third method of upsetting a hexagon from round
bar in a forging machine is not shown in Figure 6 since it is similar to that
used for hot upsetting bolt heads shown on the right hand side of Figure 5.
After upsetting a hexagonal nut blank on the end of a round bar, a hole is
pierced through the upset hexagon, by pushing the bar back, this leaves the
forged nut in the die. The metal pierced out of the nut remains attached to the
end of the bar and is incorporated into the next nut. Several nuts can be made
from a length of bar in one heat. Although this upsetting process wastes little
or no metal, it is relatively slow with the result and it is usually only used
for large nuts for which there is a relatively small demand.
(e) Machining of Bolts and Nuts from Hexagon Bar
       The bar stock for turned bolts and nuts is usually
cold drawn to a hexagonal section and machining is usually on automatic lathes
or operator controlled capstan or turret lathes. Both capstan and turret lathes
grip one end of the work piece in a chuck or collet leaving the space around the
other end of the work piece unrestricted so that a series of tools can be
positioned there in an indexible turret. This arrangement is convenient for
drilling and boring through the centre of a part and for facing its unsupported
end. Turret and capstan lathes can have additional tools mounted on cross slides
between the chuck and the turret.
       Figure 7 shows sequences of operations for turning
bolts and nut blanks from hexagonal bar. The chuck which rotates the bar is
opened when it is necessary to feed a new length of bar. The bar stock is fed
against a job stop mounted in the first turret station. The second turret
station holds a roller steady tool holder for turning the shank of the bolt.
Roller steady tool holders are designed so that the cutting thrust, which tends
to deflect the work piece away from the tool is balanced by rollers located on
the opposite side of the work piece. During the roller supported cutting
operation the turret is driven along the bed. The tool in the third turret
station is a screw cutting die head, which springs open automatically at the end
of the screw cutting run prior to a rapid return stroke. The back of the cross
slide carries a parting off tool which is also used for cutting an annular
groove prior to the head chamfering operation. A form cutter for chamfering is
amounted on the front of the cross slide. This illustration is based on the use
of three turret stations, but the turrets could be ‘double kitted’ by
loading an identical set of tools into the remaining stations to halve the
turret indexing time per piece.
       Nuts are turned using a similar tool set up to that
just described for bolts. The place of the roller box tool holder is taken by a
drill and a tool to chamfer the lip of the hole as the drill completes its cut.
       On manually operated lathes, the operator starts
and stops the spindle, changes gear to alter the spindle speeds and tool feed
rates between cuts, indexes the turret and feeds the bar stock. It may therefore
not be possible to take two cuts simultaneously by, for example, cutting the
chamfer on the corners of a nut while the hole is being drilled. But some
automatic lathes can take more than one cut at a time, which can result in
considerable savings in the time required to manufacture each piece. Another
advantage of the automatic lathe is its reduced dependence on operator skills;
this in turn reduces waste and inspection requirements. Multi-spindle automatics
are available which can work on several parts simultaneously by passing each
part to a succession of tools. The tool motions and feed rates on most
automatics are controlled by rotating cams but increasingly electrical sequence
controlled single spindle automatics are used which can be set up more quickly
than can automatics and require less attention from operators than turret
lathes.
       The internal threads on standard nuts are usually
not cut before the nut is parted from the material stock because of the need to
reverse the direction of spindle rotation to remove the thread cutting tap from
the blind hole.
       The threads on most nuts are cut by specialized
bent tap machines. Blanks are gravity fed down a channel from a magazine onto
the nose of a continuously rotating tap. The cutting flutes of the tap are
positioned in the middle of a stationary hollow hexagonal guide way, which
prevents the nuts from rotating as they move along the tap. The shank of the tap
beyond the threaded portion is bent so that it can be driven and the nuts are
thrown off radially. Another type of nut tapper has a number of vertical
spindles carrying straight taps. Nuts are placed by hand in a trough of
lubricant under each spindle and foot pedals lower the spinning taps into the
nuts. Threaded nuts collect on the taps and are periodically removed by hand.Â
Casting of Steel for Flat Products
Type of Cast Products
Cast products utilized for flat
rolling can be produced in the following forms:
1. Ingots—These are castings of simple shape.
Slab ingots range in weight from 9 to 36 metric tons (10 to 40 net tons). In
order to roll strip from the ingots, the latter are usually first rolled down to
the size of a slab with thickness range from 150 to 350 mm (6 to 14 in.). Then
they are further reduced in thickness at the roughing stands of hot strip mill
down to 25-65 mm (1.0-2.5 in.) with subsequent reduction to the desired hot
rolled thickness at the finishing mill. Final reduction in thickness may be done
by rolling at the cold mill.
2. Thick cast slabs—These
castings are usually from 150 to 350 mm (6 to 14 in.) in thickness. Utilization
of the thick cast slabs allows one to eliminate reduction at the slabbing mill.
3. Thin cast slabs—These
castings may be from 25 to 64 mm (1 to 2.5 in.) thick. Utilization of the thin
slabs allows the elimination of both the slabbing mill and the roughing mill.
4. Cast strip—The
thickness of the cast strip can be as thin as 1.3 mm (0.05 in.). It allows one
to eliminate entirely the hot rolling process.
Casting of Ingot
       After the steel making operation is completed, the
liquid steel is poured into a steel ladle. Additional alloying materials and
deoxidizers may be added during the tapping of heat. The steel is then poured or
teemed into a series of molds of the designed dimensions.
       The ingot molds are tall box-like containers made
of cast iron with the internal cavity that is usually tapered from the top to
the bottom of the mold. There are two principal types of molds:
        Â
(a)Â Â Â Â Â Â Â Â Â
Big-end-down molds.
        Â
(b)Â Â Â Â Â Â Â Â Â
Big-end-up molds.
       The inner wells of the molds may be plain sided,
cambered, corrugated, or fluted. The last two shapes of the wall promote faster
cooling and therefore minimize surface cracking during solidification.
       There are two methods of teeming the ingots:
        Â
(a)Â Â Â Â Â Â Â Â Â
Top-pouring method,
        Â
(b)Â Â Â Â Â Â Â Â Â
Bottom-pouring method.
       The use of the bottom-pouring method is found
especially beneficial for high quality steels.
Types of Ingots
Molten steel solidifies first at the regions close
to the mold walls, so the gases, chiefly oxygen, evolved from still-liquid
portions may be trapped to produce blowholes.
Depending on
the amount of gases released during solidification, the following types of
ingots are known:
1. Fully-killed ingot—It
is fully deoxidized and therefore it evolves no gas, its top is slightly
concave, and below the top there is a shrinkage cavity that is commonly called
pipe.
2. Semi-killed ingot—This
ingot is deoxidized less than fully-killed. As a result, a small amount of
carbon monoxide evolves producing a domed top. The blowhole formation in the
lower half of the ingot is prevented due to ferrostatic pressure.
3. Capped ingot—It is produced by pouring steel
into big-end-down bottle-top-molds in which the constructed top or mouth of the
mold facilitates the capping operation. The rimming action is allowed to begin
normally but is then terminated at the end by sealing the mold with a cast-iron
cap. In capped ingot, the gas bubbles in upper half are swept away due to the
strong rimming action. An ingot of this type does not have the interiors of its
blowholes exposed to oxidation during heating and soaking.
4. Rimmed ingot—This
type of ingots is usually tapped without addition of deoxidizers to the steel in
the furnace, and with only small additions to the molten steel in the ladle. The
evolution of gas produces a boiling action that is commonly known as rimming
action. Ingot No. 7 in Fig. 3 is a typical rimmed ingot in which gas evolution
was so strong that the formation of blowholes was confined to only lower part of
the ingot.
There are two types of design for the ingots.
        Â
(a)Â Â Â Â Â Â Â Â Â
Hot-topped ingots
        Â
(b)Â Â Â Â Â Â Â Â Â
Non hot-topped ingots.
The big-end-up, hot-topped killed steel ingots are
used in order to provide a complete freedom from pipe.
Methods of Continuous Casting of Thick Slabs
       A number of methods have been proposed for
continuous casting of steel. Below are some of the methods that have been
practically implemented.
Vertical or “stickâ€
casting—In this method, a straight mold, a vertical cooling chamber and a
flame cut-off are used. A tilting receiving mechanism transfers the continuously
cast slabs onto horizontal run-out table.
Vertical plus bending
casting—In this method the casting direction is smoothly changed from
vertical to horizontal as soon as the cast steel emerges from vertical cooling
chamber.
Semi-horizontal or curved mold casting—This
method allows one to simplify design and to substantially reduce dimensions of
the continuous casting machines.
Horizontal continuous casting—Schematic
representation of a typical horizontal casting machine is shown in Fig. 6. Some
horizontal casting machines provide continuous movement of strand with
oscillation of either both tundish and mold or mold only. However, the most
reliable operation was achieved by providing an intermittent strand movement as
shown Figs. 6c-6e.
Continuous Casting of Thick Slabs
       The most common method for continuous casting of
thick slabs is vertical plus bending casting. Below is a brief description of
the casting process that utilizes this method.
       In order to start the casting process, the dummy
bar is inserted in the mold so that its top closes the bottom of the mold. The
insertion of the dummy bar is made either from the top of the machine or through
entire machine in the bottom of the mold. Liquid steel is then poured at a
controlled rate from ladle into the tundish and then the metal flows through
nozzles in the bottom of tundish and fills the mold.
There are two methods of pouring the steel from
ladle to tundish and from tundish to mold:
(a)Â Â Â Â Â
Open stream casting
(b)Â Â Â Â Â
Close stream or shrouded casting.
       In an open stream casting the liquid metal flows
through the air and therefore it picks up oxygen and some nitrogen from the air.
It results in formation of undesired inclusion in the liquid steel. Shrouded
casting allows to avoid this problem. In this method, steel is protected from
contact with the air either by refractory tubes or by gas shrouding as shown in
Fig. 9.
       After the mold is filled, withdrawal of the dummy
bar is initiated. The gradually solidifying metal would follow the dummy bar
head. At certain position, the dummy bar head is mechanically disassociated from
solidified metal being cast and then the dummy bar is removed.
       Liquid steel starts to solidify in the water-cooled
mold and the solidification of steel continues progressively along its path. The
rate of solidification is controlled by secondary cooling water sprays. The
distance from the meniscus level in the mold to the point of complete
solidification is called metallurgical length. The point of complete
solidification is usually ahead of straightener. Electromagnetic stirring of
liquid steel during solidification may be implemented in order to improve steel
quality and increase casting rate.
       The mold is oscillated in a vertical direction in
order to prevent sticking of the solidified shell to the mold. Also, lubricants
such as oils or fluxes are used to reduce friction. Support rolls are installed
to guide the metal and to prevent bulging of the solidifying shell from internal
ferrostatic pressure. Cutting of the cast section is done after straightening
either by shears or by torches.
Table 1 shows main
characteristics of one of the continuous casting machines installed at the
Indiana Harbor Works for casting of thick slabs.
Slab Width Control
Desired slab width is usually achieved by using one
of the following three methods.
1. Slab slitting—This
method allows one to cast a small number of ‘master’ slab sizes with the
slab product being slit longitudinally in a separate operation using either
oxy-natural gas torches or rolling machines.
2. Adjustable mold width—This
method allows one to minimize the time required to replace a mold. Various
design for changing the mold widths are utilized. In the continuous casting
machines of earlier designs, the mold width adjustment can be made while the
previously cast slab is being removed from the machine. In the latest designs,
the mold taper can be changed during the actual casting operation.
3. Divided molds—According
to this method a divider installed in the mold permits the casting of two narrow
slabs simultaneously in a single strand machine.
The Rolling of Rails, Wheels and Rings
Introduction
       By definition, the standard rail is a section
symmetrical about its vertical axis, which consists of three areas : head, web
and base. The term "tee" is used to designate the general class of
rail designs which resemble an inverted letter T, and to distinguish those
rails, which are generally used in open-track construction, from girder and
girder-guard rails which are usually embedded in pavements. Crane rails differ
from standard rails in that they feature shorter, thicker webs, larger heads and
thicker bases to withstand heavy, concentrated loads. For railroad applications
rails are rolled to sections up to 155 pounds per yard although most rails made
today are 140 pounds per yard or less and are of the standard length of 39 feet.
       Normally, the rail section is formed from
rectangular blooms by a series of 10 passes. Roll passes must be carefully
designed and the rolling operation properly supervised in order to meet the
stringent dimensional and quality specifications. After rolling, the rails, with
their complete identification hot-stamped or rolled into them, are hotsawn so
that they cool to within 3/8 inch of the desired cold length. They are then
cambered (with the head on the convex side) so that they will be essentially
straight at ambient temperature.
       Many rails are controlled cooled, being cooled
normally on hot beds until their temperature falls to within the range 725 to
1000ºF. The rails are then charged into large insulated metal containers for a
minimum of 10 hours.
         Â
After cooling, rails are subjected to various
finishing operations (straightening and drilling for joint bolts), inspected,
the rail head chamfered in a grinding operations and the ends of the rail
hardened.
       Many rails are now heat-treated by a full
oil-quench and temper, which hardens the entire rail section, or subjected to an
induction-heating operation, which provides a surface hardening of the wearing
surface of the head. However, with the use of chromium and molybdenum as
alloying elements, rails are being conventionally produced with yield strength
of 200,000 psi and with a wear resistance equivalent to that of heat-treated
rails.
       This chapter reviews the early types of rails and
their production, examines the evolution of rail mills, describes some modern
rail-making facilities and discusses rail joints and their manufacture. In
addition, the production of railroad wheels and the hot rolling of rings are
also reviewed.
Early Types of Rails and Their Production
       The earliest type of metal rail used in the
eighteenth century consisted of a wooden base with flat strips of cast metal
about 4 inches wide, 1¼ inches thick and 5 feet long nailed to wooden
stringers, as illustrated in Figure 1 A. The cast metal straps were replaced by
rolled iron straps about 1820 but this simple design soon proved to be
inadequate. As a consequence, many improvements were soon developed, one of
which is shown in Figure 1 B. This particular type of rail was rolled for the
Amboy Division of the Pennsylvania Railroad as late as 1831.
       John Birkenshaw of the Bedligton Iron Works in
England produced in 1820 rails consisting of a head and a web but no base. Rails
such as these, laid on the ties in cast chairs, were used on the Stocketon
Darlington Railway in 1825. They were produced with rolls contoured as
illustrated in Figure 1 D. A more advanced design, shown in Figure 1 E, was used
on the Boston and Lowell Railroad in 1830 as well as in England.
       Although the preference in England was for the
bullhead rail shown in Figure 1 F, in the U.S.A., the tee rail soon became
popular. R. L. Stevens of the Camden and Amboy Railroad designed the first tee
rail in 1830, which was rolled in Great Britain in 1831.
       Another popular type of rail was the U-rail shown
in Figure 1 H, rolled by the Mount Savage Rolling Mill Company of Allegheny
County, Maryland, in 1844 and said to be the first shaped rail produced in the
U.S.A. Roll passes used for a similar type of rail made in England about 1855
are shown in Figure 2.
       Of the many different rails produced during the
last two hundred years, one interesting type was the hollow iron or closed-U
rail rolled at the Camkbria Iron Works. However, the demand for more metal in
the head of the rail for better wear resistance forced a final return in the
period 1858 to 1868 to the tee shape with wide thin flanges.
       With respect to the early rolling of rails, it is
probable that existing mills designed to roll bars were utilized with such
alterations as were necessary. Credit for rolling the first steel rail in 1857
is given to the Dowlais Plant in Wales while credit for the first steel rail in
the U.S.A. goes to Captain Ward's North Chicago Rolling Mills, where the first
50-pound Bessemer-steel rails were rolled experimentally in 1865 from blooms
made of hammered ingots at Wyandotte, Michigan.
       Rail production was initiated on two-high mills
with the bar pulled back over the top roll. To eliminate "idle"
passes, various mills of unique design were tried. These included mills with
oscillatory rolls, which provided rails of limited length, and the
"Double-Duo" mill featuring two pairs of work rolls in the same mill
stand, such as was used at the Dowlais Plant. Another mill, credited to Cabrols
Colamineur, was developed about 1850. It consisted of two trains of rolls set
almost side-by-side with each set rolling in opposite directions. After a bar
had emerged from one pair of rolls, it was transferred laterally by a hand buggy
for entry into the other pair.
       In 1866, a two-high mill utilizing, for the first
time, a reversing steam engine was developed by the Ransbottom Crewe Works of
the London and Northwestern Railway. About the same period, nonreversing engines
were used with gearing and clutches being employed to reverse the mills.
The Evolution for the Rail Mill
       The successful development of the three-high mill
in 1857 by John Fritz of the Cambria Iron Company of Johnstown, Pennsylvania,
led to its general use in rail mills. In fact, by 1866, many of the two-high
mills used for rolling rails had been converted to three-high units, commonly
called Fritz mills.
       The three-high mill produced rails from blooms or
piles of bars in 7 passes with the stand using hanging guides on the top roll.
Such guides were necessary for the back pass and their construction is
illustrated in the top drawing of Figure 3. However, in 1857, an English 3-high
rail mill was designed to roll rails in 5 passes and used resting guides
throughout, as shown in the bottom drawing of Figure 3. To avoid the use of
hanging guides and alternate live and dead holes in the mill, it was necessary
to turn the bar over between passes.
       Because of the considerable demand for rails, a
large number of mills were built in the U.S.A. primarily for rolling such
products. Sixty-nine mills were reported to have been rolling rails of various
weights in 1874, one being as far west as Laramie, Wyoming. Of these mills, one
rolled Bessemer rails exclusively, seven rolled iron and Bessemer, two rolled
steel-headed rails only, two rolled steel-headed and iron rails and one produced
cast steel and also rolled iron rails. Of the sixty-nine mills, thirty five made
heavy (65 to 140 IL/yd) and thirty four made light rails (60 1b/yd and lighter).
       The layout of a rail mill built in 1881 at South
Chicago is illustrated in Figure 4. Considered an excellent mill for its time,
it consisted of a 40-inch three-high blooming mill rolling 14½-inch square
ingots and a 26-inch two-high reversing finishing mill with a table equipped
with bevelled collars on the table rollers for turning the bars when needed. The
ingots were heated in four flat-hearth furnaces, each holding 12 ingots. The
ingots were charged and discharged from the furnaces by a hydraulic machine but
were delivered to the blooming train by a hand-operated buggy. The bloomer had
15 passes (eleven box and four roughing passes).
       After blooming and roughing, the workpiece was
conveyed on driven spools to the finishing rolls 120 feet ahead of the end of
the lifting table. Sixty feet ahead of the finishing mill was a shear where the
workpiece was cropped after one pass in the finishing mill. After cropping, the
piece was returned to the finishing mill where the rolling was completed using a
total of two roughing and five finishing passes.
       The finished rail was then conveyed to a single hot
saw equipped with movable gages. After sawing, the rail passed through a
cambering machine and then to the cooling beds, with the straightening of the
rails being accomplished by gag presses.
       In 1902, the mill was completely rebuilt on a more
elaborate basis as shown in Figure 5. In 1927, the three-high blooming mill was
replaced with a two-high reversing mill. The finishing mill, as it was
reconstructed, consisted for four three-high stands. The roughing and finishing
stands were driven by one engine and the second roughing and dummy were driven
by another. The blooming mill produced a bloom approximately 8 inches square and
the finishing mill converted the bloom to a rail in 9 passes direct from the
ingot (three in the roughing, one in the second roughing, one in the dummy stand
and four in the finishing stand). Using the three-high blooming mill and this
pass arrangement, a record tonnage of 1730 gross tons of 90-1b rails was rolled
in a 12-hour turn with approximately one hour delay.
       Prior to 1865, the stock used for rolling the
larger rails in the U.S.A. consisted of wrought-iron piles or hammered or rolled
blooms of puddled iron. In rolling tee rails from piles, considerable difficulty
was experienced in the flange passes because of unsatisfactory welding between
the layers of the piles. As mentioned in the preceding section, a few small
Bessemer-steel ingots were experimentally rolled into light rails at the works
of the North Chicago Rolling Mill Company in 1865. Two years later, the first
Bessemer-steel rails made to order for a railroad were produced on the 21-inch
three-high mill of the Cambria Iron Company of Johnstown, Pennsylvania. Although
iron rails were not completely superseded until much later, steel rails were in
great demand and several new mills were built to roll them. Yet, as the demand
for rails subsequently slackened, many of the rail mills went out of existence
or were converted to roll other products.
Modern Rail Mills
       By the mid 1900's, only eleven mills on the North
American continent were rolling large tee rails and, of these, only eight were
classified strictly as rail mills. Some of these had been built much earlier and
had been modified in the intervening years. Only three mills in the U.S.A.
rolled rails directly from the ingot without any reheating operation.
       In the 1950's the largest rail mill in the U.S.A.
was that of the U.S. Steel corporation in Gary, Indiana, the layout and
roll-pass design for which are shown in Figure 6. This mill began operation in
February, 1909 and rolled more than 880,000 tons of rails and other products
during 1951. This mill includes for 40-inch two-high stands in tandem (one
2000-hp, 214-rpm, 6600-v a-c motor driving stands 1 and 2 through gear drives
and a comparable motor driving stands 3 and 4) in which 24-inch square fluted
ingots of lengths ranging from 74½ inches to 89 inches are given one pass per
stand and are turned after each pass. The first four blooming passes are of the
diamond, diamond-square and box-pass design. The bloom then enters a three-high
40-inch blooming mill stand with five box passes, the final pass being slightly
tapered. Following this stand is the 10-inch by 10-inch electrically driven
bloom shear and the cross-country arrangement of seven stands in two groups of
three stands and one separate stand. These seven 28-inch stands are as follows:
a three-high rougher (with three passes) equipped with vertical lifting tables,
a two-high former stand, a dummy stand, a first edger stand, a second edger
stand, a leader stand (containing a head wheel when rolling rails) and a
finishing stand (using a base wheel for rail rolling). The rougher, second edger
and leader stands are in a train powered by a 6000-hp, 83.3-rpm, 6600-v a-c
motor through pinion gears. The former stand is a 28-inch single, two-high unit
powered by one 2000-hp, 58-rpm motor. The dummy, first edger and finisher stands
constitute the finishing train which is driven by a 6000-hp, 88-rpm, 6600-v a-c
motor. Transfer beds are located after the dummy stand for conveyance of the
workpieces to the second mill line and after the second edger stand for their
transfer to the third mill line.
       Data pertaining to the various passes used in
rolling rail section 11525 are presented in Table 1 and sketches illustrating
the passes of the leader and finisher stands are shown in Figure 7.
       At the end of 1969, only five mills in the U.S.A.
were producing railroad rails. These mills were far from standardized in layout.
Some had a large number of stands others only a few. Some used two-high stands
throughout while others used three-high units.
       Rails are formed by two general methods know as the
tongue-and-groove (or flat or slab-and-edging) and the diagonal (or angular)
methods. Some rail mills combine these two methods. The tongue-and-groove method
is illustrated by a roughing stand shown in Figure 8 and it is to be noted that
the axis of symmetry of the rail coincides with the pitch line and is parallel
to the train line of the rolls. The diagonal or angular method of rolling is
exemplified by the roughing stand shown in Figure 9. It differs from the
slabbing method in that the shaping of the rail is begun in the first pass in
the roughers and, instead of first compressing the bloom to a smaller size and
then forming the section partly through compression and partly by spreading, the
process is one of compression from beginning to end.
       One of the more recently installed large-section
mills that is used for the rolling of heavy rails has been described by Gino and
Gocho. A layout of this mill is shown in Figure 10. The facility uses 13-ton
ingots (that have been bloomed, hot-scarfed, sheared, cooled, spot-scarfed and
reheated) to roll rails as long as 50 meters. The pass sequence utilized for the
rolling of rails is shown in Figure 11 and it is seen that four stands are used,
these being a breakdown mill, two roughers and a finisher.
       To produce rails of the desired quality, the mill
stands feature high moduli (22 ton/mm), mill motors of adequate power and
excellent control characteristics, a high-pressure descaling system and
roll-pass lubrication (consumption about 2 litres/ton). Grain rolls are used in
the second rougher and finishing stands to minimize spalling, these being
specially manufactured to provide a barrel hardness of 50±3 shore, a barrel
strength of 7 to 8 kg/mm2 and a wobbler strength of 20 kg/mm2.
       Following the rolling operation, ordinary rails are
identified on the web by the use of a marking wheel (carrying type on its
periphery), end-finished and shipped. Rails to be head-hardened are transferred
by crane on to the quenching-furnace approach table without pretreatment, such
as prebending. Charging into the furnace is continuous at 360 mm/min, with the
aid of pinch rolls. The 2390-mm long quenching furnace has a temperature of 1150ºC
which heats the rails to about 820ºC. The rails are then water-quenched and
tempered in a 3460º-mm long furnace held at 820ºC which reheats the rails to
about 570ºC. Only the head is hardened and it should be noted that, while the
rail is passing through the two furnaces, a reducing atmpsphere is maintained in
both to prevent decarburization. The material in the rail below the head is
colled by water-cooled pipes and the rail, after heat-treatment, exhibits a
down-ward camber and Shore hardness of 50±3 in the top of the rail head. For
end hardening, the rail ends are rapidly heated to 800 to 1000ºC in three
minutes and then air-quenched at a rate of 3 to 10ºC/sec. to ensure a fine
pearlite texture with a Shore hardness of 48±3.
       The sizes of the sections rolled on the above mill
and the corresponding production rates are presented in Table 2.
       Another Japanese rail mill, built at the Yawata
Works of Nippon Steel Corporation was commissioned in 1970. It utilizes
reversing break-down and universal mill stands.
       The U.S.S.R. claims to be the world leader in rail
production rolling some 3.2 million tonnes in 1975. Open-train structural mills
built in post World War II years roll heavy rails (up to 75 kg/m) from 6-tonne
blooms at speeds up to 2.5 m/sec.
       The latest rail mill planned for the U.S.A. is that
to be built by U.S. Steel Corporation at Chicago. This facility will be fed by a
continuous caster.Â
Mill automation for the rolling of flat products
Automation of flying shear operation in a continuous hot-rolling mill.
       Upon leaving the last stand, the strip speed is
usually 2 to 5 per cent higher than the peripheral speed of the rolls due to
forward slip which is a function of many variable values (temperature,
coefficient of friction, strip thickness, etc.) that change during the rolling
of even a single strip.
       To obtain higher accuracy in cutting the strip into
measured lengths, the speed of the flying shear must coincide with the actual
travelling speed of the strip on the table and not with the peripheral speed of
the rolls. For this purpose, a special measuring roller is held against the
strip from which it is rotated without slipping. A tachogenerator TGR is mounted
on the roller shaft (Fig. 1.). Before rolling begins, the speed of the shear
drums conforms to the stand roll speed by mean of tachogeneratorÂ
TGS whose voltage is compared to the voltage of the shear generator G. As
the strip runs out of the rolls, the excitation of this generator is switched
over to the tachogenerator TGR of the measuring roller. After this, the speed of
the shear will exactly coincide with that of strip travel (by comparison of the
voltages of TGR and TGS which excite the rotary amplifier RA in the circuit of
the exciter GE of generator G).
       Still higher accuracy may be achieved with an
electronic counting circuit (Fig. 2). Here, the speed of the driven measuring
rollers MR corresponds to that of the driven feed rolls FR. A photoelectric
relay PR is mounted before the feed rolls at a definite distance from the shear.
This relay switches on shear SH upon the approach of a strip. The strip speed
and the distance from the photoelectric relay to the shear determine the length
of strip cut. The speed of the measuring rollers equals the strip speed; the
pulse generator PG on the roller shaft (a wheel with teeth and a coil) transmit
pulses to the electronic counting circuit ECC. Rotation of the roller pulse
generator through one division (one tooth) corresponds to a strip travel 50 mm.
Provision is made for obtaining 200 pulses when strip is cut into the maximum
lengths of 10 m. The pulses are counted by the counting circuit; after a
definite number of pulses, the counting circuit sends a command to operate the
shear. This number of pulses after which a cut is made is set up beforehand by
the operator.
Automation of coiler operation for hot strip
       When the strip leaves the last stand of a
continuous strip mill it is traveling at a speed from 6 to 10 m per sec along
table I; driven pinch rolls 2 and guide rolls 3 direct the strip to coiler drum
4. The strip is held against the drum by wrapping rollers 5. After 3 or 4 turns
of the strip on the drum, the wrapping rollers are withdrawn. The tightness of
the first turns is due to the fact that the peripheral speed of the drum is
slightly higher than that of the pinch rolls. Speeds are made to correspond in
the following manner.
       The reference winding RW is connected to the
armature of tachogenerator TG on the lat stand of the mill while the voltage
winding VW is connected to the armature of generator GPR which supplies the
pinch roll drive motor MPR. The characteristics of the drive windings are
adjusted so that upon idle rotation of the coiler drum its speed exceeds that of
the pinch rolls. Therefore, the strip is tensioned when it begins to wind on the
coiler to obtain several tight turns of the strip on the drum. This loads the
pinch roll motor MPR. Strip tension is regulated by the current winding CW and
also in the circuit of the coiling drum motor MCD.
Automation of strip measuring gauges for hot rolling
          Â
Measuring the strip width. It is desirable that the
slab width correspond to the given strip width at a definite thickness. This
reduces the amount of scrap trimmed from the side edges of the strip. If the
strip obtained from the slab is wider than required for subsequent trimming to
the finished sheet, it will be necessary to roll a narrower slab in the
broadside stand.
       A photoelectric width gauge is mounted beyond the
finishing stand of the mill, above the roll table, to continuously check the
strip width.
       The edges of the hot strip are projected through
optical lenses to the photoelectric heads PH. Upon changes in strip width, the
intensity of the light illuminating the photoelectric heads is changed
correspondingly. As a result, the indicating or registering instrument on the
control desk will show the variation (in mm) of strip width.
       Measuring strip thickness. One system of continuous
measurement of the thickness of moving hot-rolled strip is based on the
principle of X-ray absorption by the strip.
       The X-ray tube XT emits two perpendicular beams,
one upwards through wedge WI on the strip and the other, to the right on the
master wedge W2. The tube is supplied with alternating current and, therefore,
its beams pulsate, Head H2 is arranged behind wedge W2 and has a fluorescent
screen. The head contains a gas-discharge tube which also pulsates and serves as
a standard. If the brightness of the fluorescent screen upon pulsation of the
right-hand beam from the X-ray tube is equal to the standard brightness of the
gas-discharge tube, then the luminous flux incident to a photoelectric element
in head H2 does not have variable component and the potential at amplifier EA2
equals zero. If there is a difference in luminescence, then the voltage of
amplifier EA2 acts upon the sensitive electronic regulating device RD which
reduces the voltage on the high-voltage transformer TIIV as much as required to
equalize the brightness of the screen and of the tube. Since the tube serves as
a standard, the regulating device RD maintains constant emission capacity of the
X-ray tube XT. Head HI is similar to H2; if the strip thickness equals the
thickness of wedge W1, the potential at the electronic amplifier EA1 equals
zero. If potential appears at EAI, the reversible motor RM is switched on. This
motor will advance wedge W1 and thus equalize the brightness of the luminescence
on the screen and tube in head HI. The total thickness of the strip and wedge WI
will be equal to this thickness of wedge W2.
       Thus, the strip thickness equals the difference in
thicknesses of W2 and W1. The position of wedge W1 is transmitted by synchro-transmitter
ST to synchrorepeater, SR which has a scale graduated into units indicating the
deviation of the strip thickness from the specified value. The same type of
transmission is provided in the actuating mechanism of wedge W2; on the scale
STI, the operator sets the graduation beforehand corresponding to the nominal
thickness of the strip. If the hand on the other scale OI points to zero then
the strip thickness equals the specified value.
       Deviation of hand OI from its zero position
corresponds to deviation of strip thickness from the preset value. A recording
device can be connected to the synchrorepeater. SR for registering deviations in
strip thickness during the rolling process. The readings of the instruments are
stabilized by supplying the whole installation from a constant voltage regulator
CVR. The X-ray tube is mounted above the roll table in a rigid water-cooled
hood.
Automation of continuous pickle line operation
       Strip is pickled in a continuous line at a speed of
2-3 m per sec. To provide continuous operation, the tail end of each strip is
welded to the leading end to the next strip. This operation is done in a welder
where the ends of the strip are set by hand under the clamps of the machine to
obtain an even weld. The strips are welded without stopping the movement of the
strip through the baths. Looping pits are provided in the line to maintain
continuous strip travel. Several loops of the strip with a total length of 50 to
200 m are made available beforehand in the pit. During welding of the strips the
line is supplied from the slack in the pit. After making the weld, the entry
speed of the strip into the looping pit increases and the required slack is
restored.
       In new mills, the size of the loops in looping pits
2 is checked automatically by means of photocells and light sources. When the
number of loops of strip is reduced in the pit, the photocells are excited one
after another by the lamps. Pulses transmitted by the photocells reduce the
strip speed through pickling tanks I and increase the strip uncoiling speed.
       The strip speed through the tanks is controlled by a dancer
roll; the lever of this roll is linked to the slide of a rheostat which varies
the exciting current of motor M1 powering the pinch rolls PR.
Automation of strip thickness gauges for cold reduction
       The strip thickness is measured in cold reduction
in a continuous (or reversing) mill with the same type of X-ray gauge used for
hot rolling (this gauge was first applied to cold reduction and only later—to
hot rolling).
       Essential disadvantages of X-ray gauges are their
high cost and the necessity for having a complete outfit to generate the rays.
       Radiation thickness-measuring gauges that have come
into use in the last years, since they are more economical, employ gamma rays
(for thick strip) or beta rays (for thin strip), i.e., they employ radioactive
isotopes that emit these rays.
       Beta-ray gauges are substantially cheaper than
X-ray types while their accuracy is sufficient for this purpose (1 percent for
strip up to 0.5 mm thick).
       The principle of the radiation gauges is similar to
that of the X-ray gauge. An artificial radioactive emitter E of beta rays is
located under the strip S. Part of the rays passing through the strip enter the
ionizing chamber IC, arranged above the strip and produce an ionizing current.
This current is very small, however, and cannot be directly measured by the
instrument. Therefore, a high-ohmic measuring resistance MR is connected into
the current circuit. The voltage drop over this resistance is proportional to
the chamber current and, therefore, to the strip thickness. This voltage drop is
not directly measured but is compared to a preset value, i.e., the determined
value is the difference in voltages appearing when the strip thickness deviates
from that specified. The recording instrument RI is connected in parallel with
the indicating device ID, as well as the signalling device SD. The latter is
operated when the strip thickness variation exceeds the permissible value and it
signals the operator. If the isotope thallium 204 is employed, steel strip up to
0.15 mm thick can be measured while strontium 90 is suitable for strip up to 0.9
mm thick. The half – life of this second isotope is such that it can be used
for up to 40 years and, therefore, does not practically require replacement.
       In passing through the strip, the beam of beta rays
is weakened not only by the metal but by oil and water as well. Therefore, in
rolling it is necessary to see that the strip surfaces are clean at the place of
measurement (by wiping, etc.).Â
General Steelmaking Processes
Welding Material for Super Low Temperature Steels
       U.S. Patent 3,966,424; June 29, 1976; assigned to
Kobe Steel Ltd., Japan describe a nickel-base alloy welding material which can
give excellent strength and impact value characteristics to the weld zone in
welding of super low temperature steels. These and other objects as will
hereinafter become more readily apparent, have been attained by this welding
material comprising not more than 0.2% carbon, 5 to 12% manganese, not more than
30% chromium, 4 to 8% niobium, not more than 22% iron and not more than 1.5%
silicon, the balance being substantially nickel. The welding material is formed
by integrally combining a metal-forming material having the above composition
with a flux of the lime or lime-titania type.
       The welding material can be used with any known
welding method, such as manual welding, TIG welding or submerged arc welding.
The term “Welding material formed by integrally combining a metal-forming
material with a flux†includes a coated electrode for arc welding, composed of
a core wire covered with a flux. A composite wire composed of a metal casing
packed with alloy powder optionally together with a flux; or the like.
       The flux comprises, on the weight basis, 10 to 50%
calcium carbonate, 16 to 50% fluorspar, 2 to 20% magnesia clinker and up to 10%
rutile. An especially preferable flux is one containing magnesia clinker and
having a ratio of fluorspar to calcium carbonate of 1 to 1.5. Not more than 60%
of ingredients of such flux may be substituted by a deoxidizing agent, an
alloy-constituting element or the like.
       The process of preparing a welding material is
described briefly by reference to a welding rod. The components of the lime or
lime-titania coating material and the above alloy components are blended
together with water glass (an aqueous solution of a mixture of sodium silicate
and potassium silicate)—10 to 2% based on the total weight of the welding rod.
The assembly is dried at 200º to 250ºC. thus, the process is not different
from the conventional process of preparing welding rods.
Example: Welding Material
composition (1)—The chemical composition of the core wire (%) is:
C, 0.05: Mn, 10.2; Si, 0.42; P, 0.005; S, 0.006;
Cr, 18.5: Nb, 6.0; Fe, 11.5: and Ni, the balance.
       The blending ratios of the ingredients of the
coating material are calcium carbonate, 40%; fluorspar, 53%; rutile, 5%; and
ferrosilicon, 2% (silicon content equals 50%). The binder is an aqueous solution
of the mixture of sodium silicate and potassium silicate of SG 1.40. The
covering ratio of the coating material is 25% based on the total weight of the
welding rod.
       Welding Material Composition (2)—The chemical
composition of the core wire (%) is:
       C, 0.06; Mn, 1.2; Si, 0.55; P, 0.4; S, 0.006; Cr,
14.0; Nb, 1.5; Fe, 7.3; and Ni, the balance.
       The blending ratios of the ingredients of the
coating material are calcium carbonate, 28%; fluorspar, 31%; rutile, 2%;
magnesia clinker, 4%; metallic mangahjese, 15%; and ferroniobium, 20% (niobium
content equals 70%). The binder is an aqueous solution of the mixture of sodium
silicate and potassium silicate of SG 1.40. The covering ratio of the coating
material is 40% based on the total weight of the welding rod.
       Welding Material Composition (3)—The filler rod
composition (%) is:
       C, 0,10; Mn, 7.0; Si, 0.7; P, 0.05; S, 0.04; Cr, 14.0; Nb,
6.0; Fe, 10; and Ni, the balance.
       The above three welding materials [welding
materials of the compositions (1) and (2) are for arc welding using a coated
electrode, and the welding material of the filler rod composition (3) is for MIG
welding], and a commercially available nickel-base alloy welding rod were
subjected to tensile test, impact test and chemical analysis with respect to the
as-welded deposited metal. Further the tensile test and impact test were
conducted on the weld metal in the weld zone of 9% nickel steel.
       Test Results—A sample was collected and chemical
analysis was conducted to obtain the following results:
Welding rod (1):
C, 0.05; Mn, 9.17; Si; 0.40; P, 0.04, S, 0.06; Cr,
16.8; Nb, 5.40; and Fe, 10.94.
Welding rod (2):
C, 0.06; Mn, 5.88; Si, 0.33; P, 0.004, S, 0.05; Cr,
12.6; Nb, 4.71; and Fe, 8.66.
Welding rod (3):
C, 0.09; Mn, 6.80; Si, 0.65; P, 0.04; S, 0.06; Cr,
13.5; Nb, 5.8; and Fe, 9.7.
Commercially available Ni-base alloy welding rod:
C, 0.03; Mn, 6.82; Si, 0.52; P, 0.005; S, 0.004;
Cr, 13.5; Nb, 1.72; and Fe, 9.20.
       The welding was conducted and tensile test
specimens and impact test specimens were taken. The tensile test was conducted
at room temperature and the impact test at –196ºC. As is apparent from the
test results on welding rods shown in the following table, the deposited metal
exhibited a tensile strength and ductility comparable to those of 9% nickel
steel base, together with a sufficient low temperature toughness.
       9% Ni steel was subjected to groove welding, and
mechanical properties of the weld metal diluted with the base metal were
examined. Two sheets of 20 mm (thickness) × 250 mm (length) × 200 mm (width)
were welded in a butt joint to form a test specimen. The groove conditions were
as follows: a groove angle of 60º, a root face of 0.5 mm, and a root gap of 1
mm. The surface side was metal-welded, and then the back chipping was effected,
followed by one layer welding on the back.
       Tensile test specimens were taken in a direction
parallel to the welding direction. Impact test specimens were taken. The tensile
test was conducted at room temperature and the impact test was effected at
–196ºC. As is apparent from the test results shown in the table below, in
welding rods the weld metal exhibited a tensile strength comparable to that of
the base metal and a sufficient low temperature toughness.
Refining Steel by Blowing Oxygen Beneath the Surface
       U.S. Patent 3.960,547; June 1, 1976; assigned to
Youngstown Sheet and Tube Company describe a process for melting iron-bearing
material; adding the melt to another molten composition to modify the carbon
content of the composition; and further refining the resultant mix with oxygen.
       The method includes oxy-fuel melting a charge of
solid material, bearing iron, which melting produces a relatively low
carbon-containing composition. The low carbon composition is added to another
molten composition of relatively higher carbon content, such as that produced by
conventional blast furnace practice, to provide a molten mix. Unmolten
iron-bearing material is added to the molten mix in a refining vessel having
means for introducing essentially pure oxygen beneath the surface of the melt of
the refining vessel.
       Suitably, the high carbon melt may be blast furnace
iron at 2400º to 2500ºF comprising, by weight: 0.5 to 2.0% silicon, at least
2% carbon, 0.40 to 1.5% manganese, and the balance being essentially iron.
Preferably, the high carbon molten composition comprises: 1% silicon, 4% carbon,
0.5 to 1.0% manganese, and the balance essentially iron. Also preferably, a
composite molten mix is provided which is comprised of 40 to 75% low carbon
composition and 60 to 25% of the high carbon composition. The mix will usually
result in a composition being at a temperature of 2600ºF and comprising: 0.5 to
0.6% silicon, 1.8 to 2.0% carbon, 0.3 to 0.4% manganese, and the balance
essentially iron.
       In a typical and preferred process, sufficient
molten mix metal is provided to the vessel, where refining is to take place
without the addition of more heat, to constitute 85 to 95% of the total
anticipated work charge. The remaining 5 to 15% of the charge may be
advantageously comprised of cold unmolten scarp, and/or iron ore pellets, and/or
other iron bearing materials in solid form. After the charge is completed,
refining is conducted by introducing substantially pure oxygen beneath the
surface and blowing through the molten charge. Of course, if additional heat is
provided, such as by burners in the refining vessel, then the amount of unmolten
scrap may be increased.
       It will be noted that the total hot metal
(relatively high carbon content composition) input to the refining vessel is 22
to 54%, i.e., 25 to 60% total charge to mixing vessel × 90% total charge to
refining vessel. In contrast, conventional open hearth and BOF practices utilize
55 to 60% and 70% hot metal, respectively. It is also anticipated that higher
yields of usable steel are attainable through the use of the refining medium
below the surface of the molten bath, as opposed to blowing unto the surface.
One of the contributing factors is better utilization of the refining medium
attained by virtue of the more intimate contact with the bath. Another factor is
that there is less iron oxide emission loss than that encountered with the use
of oxygen lances and the resultant fuming.
Cold Reduced Aluminum Stabilized Steel having High Drawability
       U.S. Patent 3.959,020; May 25, 1976 assigned to
Nippon Kokan KK, Japan described a steel which is suitable for severe cold
forming. Al stabilized steel is subjected, after a first cold working step, to
decarburizing annealing as an intermediate heat treatment. The steel is then
successively passed through a second cold reducing step and then a final
softening annealing step. The resulting steel is capable of withstanding any
press-forming operation.
       Al stabilized steel which may be used consists of
0.03 to 0.15% C, 0.02 to 0.07% SolAl, and other elements, e.g., Fe, Mn, P, S, N,
present in ordinary quantities as in other Al stabilized steels. When continuous
hot rolling is used, the finishing temperature should be more than the Ar3
point, and the coiling should be carried out at less than 600ºC, so that
precipitation of AlN does not occur. In this case, thickness of more than 3.2 mm
will be desired as the finishing thickness of a hot rolled strip, because the
next two stages of cold reducing may be more readily carried out depending upon
the thickness.
       The first cold reducing is carried out at a
reduction rate of more than 30% and successively the steel is subjected to an
intermediate decarburizing annealing wherein the C content in the steel is
reduced to less than 0.01%, preferably to 0.002%. The reduction rate of the
second cold reducing stage is more than 30%, and preferably more than 50%. A
final annealing process is carried out to produce a steel having an r value
(Lankford value) or 2.2 to 2.3. Such a Lankford value shows that the steel is
capable of sustaining any severe press forming.
Sulfide Modification of Steel
       U.S. Patent 3,955,967; May 11, 1976; assigned to
The Algoma Steel Corporation, Limited, Canada describes sulfide modification of
molten steel in the ladle suitably during transportation of the molten steel
from the steel making furnace for casting. This treatment is effected by the
addition of agents, which usually have a high affinity for oxygen, particularly
rare earth metal silicides.
       The process comprises enclosing the addition agent
in a metal container, and fixedly suspending the container in the ladle to
submerge at least that portion of the container containing the addition agent so
as to melt the container and release the addition agent beneath the surface of
the molten steel in the ladle. The container has walls of selected thickness to
provide the required time delay in releasing the addition agent into the molten
steel to allow for the desired amount of deoxidation of the steel to have taken
place. While rare earth metal silicides are particularly preferred to completely
deoxidize molten steel for sulfide modification, other additives particularly
boron compounds, calcium metal, and alloys, e.g., calcium alloys and deoxidizers
may be used.
       The process also provides a device for use in the
treatment of molten steel in a ladle during transportation of the molten steel
from a steelmaking furnace for casting by the addition of agents which are
normally strong deoxidizing agents. The device comprises a yoke adapted to
extend across the open top of the ladle and at least one hollow tube closed at
one end for containing the addition agent.
       The tube is fixedly mounted in the yoke such that
when the yoke is in position on the ladle each tube is suspended vertically in
the ladle with at least that portion of the tube adjacent the closed end
containing the addition agent submerged in molten steel contained in the ladle.
Each tube is of preselected wall thickness to provide a selected time delay in
releasing the addition agents into the molten steel in the ladle after initial
contact of each tube with molten metal to allow for the desired amount of
deoxidation to take place.
       It is a critical feature that the container
containing the additive be fixedly suspended in the ladle with that portion
containing the additive disposed beneath the surface of the steel bath. This
ensures that the additive is released beneath the surface of the bath and not on
the surface of the bath and further that the distribution pattern of the
additive in the molten steel can be selected to best advantage.
       A heat was produced in a basic oxygen furnace of
nominal 100 ton capacity. It was desired to produce plate to meet a minimum
yield strength of 65,000 psi and notch toughness of minimum Charpy impact energy
(heat average based on 2/3 size transverse specimen at 0ºF) of 30 ft-lb. To
attain these minimum requirements it is necessary to desulfurize the steel to a
level of 0.012% sulfur and to modify the sulfide inclusion morphology by
treatment with rare earth elements. Prior to tapping the furnace into the ladle,
a yoke was assembled suspending two 10 foot long, 11 inch diameter steel pipes
each having a wall thickness of one-half inch. Before assembling, each pipe had
been filled with 65 lb of rare earth silicide which contained 30% of contained
rare earth elements. The space in the pipes above the top level of the rare
earth silicide was filled with lime. The wall thickness of the pipes had been so
designed that the steel pipes at their lower extremity will melt whereby the
rare earth silicides are released well below the level of molten steel in the
ladle during the pour from the furnace. Further more, the upper regions of the
pipes do not melt until a discrete time interval after completion of the tap
from the furnace and after the artificial slag cover has been formed.
       At the time of tapping the furnace into the ladle, the
following ladle additions were made: 340 lb aluminum, 3,800 lb ferromanganese,
330 lb ferrocolumbium, and 250 lb lime. In addition, 700 lb of fly ash was added
to the ladle at the end of the tap to provide the artificial slag cover. The
temperature of the steel after all additions were made was 2855ºF. Prior to
teeming the melt into ingot molds, the steel in the ladle was stirred with an
argon gas injector for 7 minutes to achieve a uniform distribution of the
additions made and to equalize the temperature for teeming at 2840ºF. The final
steel analysis was 0.14% C, 1.50% Mn, 0.01% S, 0.005% P, 0.15% Si, 0.06% Cb and
0.0% Al.
       From the analysis and properties attained it would
be obvious that good sulfide modification was effected and full value obtained
from the rare earth silicide addition. In an alternative process, the steel is
tipped into the ladle from the steel-making furnace together with the
deoxidizing agent and the device is subsequently lowered into the ladle so as to
submerge the tubes. The melting of the walls of the tubes release the rare earth
metal silicides into the steel beneath the surface of the slag and provide for a
distribution of the rare earth metal silicides in the steel. Good distribution
of the rare earth metal silicides and other additives having a strong affinity
for oxygen is assisted by argon stirring the ladle immediately prior to teeming
the steel into the ingot molds.
Steel Sheets having Excellent Enamelability
       U.S. Patent 3,950,191; April 13, 1976; assigned to
Kawasaki Steel Corporation, Japan describe a method for producing cold rolled
steel sheets having an excellent enamelability. The process comprises adding to
a molten steel containing not more than 0.020% of carbon, not more than 0.03% of
silicon, not more than 0.50% of manganese, not more than 0.01% of aluminum, not
more than 0.050% of oxygen, and having a nitrogen content of less than 0.01%,
boron with a range of 0.005 to 0.020% in such amounts that B(%) x O(%) is more
than 1 × 10–5, hot rolling the resulting steel, cold rolling, and
recrystallization annealing under a decarburization atmosphere.
       Example: The steels having the components as shown
in Table 1 were melted and slabbed and then hot rolled into a sheet having a
thickness of 2.8 mm at a finishing temperature of 860ºC and a coiling
temperature of 550ºC. The hot rolled sheet was pickled and then cold rolled by
a tandem roll into a sheet having a thickness of 0.8 mm, after which the
resulting sheet was annealed in a bell type annealing furnace at a uniform
temperature of 760ºC or subjected to decarburization annealing in an open
annealing furnace at a uniform temperature of 700ºC and then to a tempering
rolling of 1%. The resulting steel sheets are subjected to the enameling
treatment to obtain the results as shown in Table 2.
       As the conditions for the enameling treatment, such
pretreatment steps that fish scales and the other defects are liable to be
caused were selected and the immersion in Ni bath which is practically
inevitable was omitted and the frit for the high temperature firing was used.
       In Table 2, the adherence PEI (%) was determined by
means of Porcelain Enamel Institute adherence tester as follows. The sample was
subjected to a compression deformation and the glaze was forcedly exfoliated to
measure electrically the exposed area of the base metal and calculate the area
applied with the glaze on the deformed portion and the total area of the
deformed portion to read as PEI (%); no exfoliation is expressed by 100% and the
entirely exfoliated area is expressed by 0%. The method for measuring this index
is described in ASTM C-313. From the results of this test, it can be seen that
even if the steel is enameled under the conditions which are apt to cause the
defects, very stable results can be obtained.
Liquid Sintering with Titanium Alloys
         Â
Sintering ferrous materials have found wide
application as structural components in machines. The failure to use sintered
materials as main structural components stems from the porous character of such
components, which are inferior in mechanical properties to casting, or forged
materials having the same composition. To promote the use of sintered materials
in such components, various efforts have been made to increase the density of
the sintered material up to a value close to the theoretical density.
       Accordingly, U.S. Patent 3.950,165; April 13, 1976;
assigned to Mitsubishi Jukogyo KK, Japan describe a method of sintering ferrous
materials which comprises mixing an iron powder with an alloy of iron-titanium
and forming a liquid phase during the sintering. The alloy is prepared by mixing
selected amounts of iron and titanium powders to form an iron-titanium alloy
powder consisting of 14 to 46% by weight of tatanium and the balance iron, and
preferably 14 to 30% by weight of titanium. The sintering step is carried out as
a temperature at which the powdered mixture is always in the liquid phase during
sintering. Under these conditions oxidation of titanium is suppressed during
melting and the alloy is not too soft nor too difficult to pulverize and use for
manufacturing machine components such as piston rings, which have surprisingly
good properties.
Liquid-Solid Alloys for Casting
U.S. Patent 3.948,650; Apr. 6,
1976; assigned to Massachusetts Institute of Technology describe a metal
composition characterized by degenerate dendritic or nodular primary discrete
solid particles suspended in a secondary phase having a lower melting point than
the primary particles and which secondary phase can be solid or liquid. This
composition may be prepared by raising the temperature of a metal alloy to a
value at which the alloy is largely or completely in the molten state.
The melt then is subjected to
vigorous agitation and the temperature is reduced to increase the portion of the
mixture in solid degenerate dendrite or nodular form up to 65%, but usually up
to 50%, while continuing the agitation. At this juncture the temperature of the
liquid-solid composition can be stored and later it can be brought again to the
liquid-solid mixture state and then recast.
       The compositions can be formed form any metal alloy
system or pure metal regardless of its chemical composition. Even though pure
metals and eutectics melt at a single temperature, they can be employed to form
the composition since they can exist in liquid-solid equilibrium at the melting
point by controlling the net heat input or output to the melt so that, at the
melting point, the pure metal of eutectic contains sufficient heat to fuse only
a portion of the metal or eutectic liquid. This occurs since complete removal of
heat of fusion in a slurry employed in the casting process cannot be obtained
instantaneously due to the size of the casting normally used and the desired
composition is obtained by equating the thermal energy supplied, for example by
vigorous agitation and that removed by a cooler surrounding environment.Â
Varnishing and Printing of Packaging Steels
Introduction
         Â
The success of metal packaging in tinplate or
chromium plated steel is due to a large extent to the varnishing and decorative
printing operations which complete the corrosion protection already provided by
the electrolytic coatings. For this reason, varnishing is always used on
chromium plated sheet and is also employed in the majority of tinplate
applications. In the sheet-by-sheet varnishing process, most frequently used for
packaging materials, one coat of varnish or ink is applied at a time, followed
by baking in a convectively heated horizontal oven. It is a discontinuous
operation, in which the sheets are unstacked at the start of the line and
restacked at the end, as many times as there are coats of varnish on one or
other of the two faces. Continuous coil varnishing processes also exist, capable
of high speed coating of either one or both faces.
General Aspects of Organic Coatings Used for Varnishing and Printing
       The organic coatings (varnishes, pigmented coatings
or inks) are used for the internal protection and external decoration of the
packaging whenever justified by the technical or commercial requirements of the
intended application. In fact, these coatings serve three purposes:
–      Â
They protect the metal against chemical corrosion by the contents of the
packaging and against various external aggressions, such as atmospheric
corrosion, scratching, shock, forming, etc.;
–      Â
They decorate the outside of the packaging;
–      Â
They facilitate forming, for example by acting as a lubricant during
drawing.
Definition
       Varnishes are liquid preparations which, when
applied as a thin film on a substrate, are transformed by evaporation of their
volatile constituents and reaction of their binder resins to a solid sheet or
film which adheres to the metal surface.
Types of Organic Coating
       Varnishes are clear transparent substances, which
may sometimes be colored. Apart from a few special cases, such as colored
finishing varnishes (blue, green, etc.), the color of varnishes is due solely to
their base resin. For example, the golden tint of epoxy-phenolic varnishes is
due to the color of the phenolic resin. When varnishes are colored by white
titanium oxide, they are called “white pigmented lacquersâ€. This is the case
most frequently encountered in food packaging, but other colors, such as yellow,
black, etc., are often used for decorative purposes on the outside. The organic
coatings are classified according to their applications:
•        Â
Printing inks are used only for printing the outside of the packages.
•        Â
Primer coats are applied directly to the metal (usually tinplate) and
serve as a bond layer for the inks, films or varnish topcoats. They are used in
small quantities (3-6 g/m2).
•      Â
Finish coats or topcoats are applied in similar thicknesses (4-6 g/m2).
•        Â
Internal varnishes are specifically designed to resist mechanical forming
(drawing) and chemical attack. The thickness applied (5-8 g/m2) depends on the
degree of protection required.
•        Â
White, or more rarely, colored coatings are generally applied to the
outside, but are occasionally used on the inside. Their thicknesses vary from 15
to 20 g/m2 on average, depending on the degree of protection desired on the
inside or the opacity required on the outside.
Organic Coating Constituents
       The formulation of varnishes includes resins,
pigments, solvents and additives, such as surfactants, lubricants, catalysts and
filler materials.
The resins or binders
       The binders are composed of a non-crystalline solid
(natural or synthetic resin), which, after drying and hardening, forms the
essential component of the film. The resins belong to a limited number of
chemical families (the phenolic, epoxy, vinyl, polyester, acrylic and
aminoresins), but can be modified to a greater or lesser extent with other
resins, depending on the properties required. These different families and their
principal characteristic properties are described in Table 1.
The pigments
       The pigments are present in the form of very fine
crushed solid particles, of the order of a micron in size, dispersed in part of
the binder, in which they are insoluble. They are usually inorganic substances,
such as titanium and iron oxides or aluminum. The printing inks use other
pigments, which may be either inorganic or organic, in order to cover a much
wider range of colors.
The solvents
       The solvents are volatile liquids, which transform
the binder to a solution sufficiently fluid to facilitate application. Since the
resin must be soluble, the solvents employed vary with the type of binder, the
most common ones being alcohols, esters, ketones, aliphatic and aromatic
hydrocarbons and water. In fact, mixtures of two or more solvents are often
employed, since more than one resin must generally be dissolved. After
application, the solvents are allowed to evaporate from the varnish layer,
leading to uniform spreading and better wetting of the metal, and avoiding the
formation of a porous coating. The solvents also serve to adjust the viscosity
of the varnishes or lacquers, in order to compensate for evaporation during
recycling of the unused product.
The additives
       Although the additives represent minor constituents
of varnishes and films, they are by no means of secondary importance. In
particular, the surfactants lower the surface tension of the paint, enhancing
wetting and facilitating spreading.
The lubricants
       The lubricants are added to facilitate handling and
forming of the varnished sheets, particularly drawing operations.
The catalysts
       The catalysts enhance chemical reactions between
the different constituents of the varnish and accelerate cross-linking. They are
usually acids, amines or metal salts.
Application and curing of organic Coatings
Application with Roller Varnishing Machines
       The liquid mixture mentioned above, consisting of
macro-molecules and solvents, is generally applied on a flat surface. This can
be done on separate sheets or coiled strip with the aid of a roller-varnishing
machine. The latter has a number of rollers, which transfer an appropriate
quantity of varnish from a tray and apply it in a uniform controlled thickness
onto the surface of the metal as it passes through the machine. The varnish is
pumped from a container E into the tray in which the feed roller A rotates,
taking up a small amount of varnish, which is distributed over the transfer
roller B. The varnish is then passed from the transfer roller to the application
roller C which coats the surface of the metal sheet. The pressures between the
different rollers and their relative velocities can be adjusted to deposit a
precise quantity of varnish on the metal. The application roller is made of
steel and is coated either with hardened gelatine or polyurethane elastomer. The
varnish can either be applied to the whole surface or uncoated edges can be
left, as is necessary, for example for welded cans. The application roller then
has an appropriate profile so as to leave part of the sheet unvarnished.
       In coil coating operations, both faces can be
coated simultaneously, by means of a second application roller, which replaces
the counter-roller D, additional rollers being added to drive the strip. In
sheet-by-sheet varnishing, the tangential speed of the application roll is
generally equal to the sheet displacement velocity, whereas in coil coating, the
application rollers can move in the opposite direction to the strip, leading to
improved coating quality.
Curing
       After application, the organic coatings are baked
to obtain a dry film. The solvents are evaporated and the constituents react to
produce cross-linking of the polymer chains, forming a solid film firmly
attached to the metal. Baking or curing is generally performed in a
conveyor-chain tunnel oven. Conveyor ovens consists of a series of individual
metal frames or wickets mounted on a chain which is drawn slowly through the
heating chamber. The chain speed being synchronized with the varnishing machine,
so that each wickets receives a sheet after passing through the machine. The
furnaces are generally gas-fired, with either direct or indirect heating. The
baking cycle (temperature and time) depends on the type of varnish, and
specifications are usually given by the manufacturers. Higher temperatures allow
the use of shorter baking times, so that various combinations are possible.
Certain varnishes can be cured in extremely short times, of the order of a few
seconds (“flash†curing).
       Another curing technique is based on induction
heating, the heat being generated directly within the metal substrate. Such
treatments can be extremely rapid (a few seconds) and are well suited to high
varnishing speeds (>100 m/mn), such as those prevailing in coil-coating. In
such cases, flash curing is essential to maintain reasonable oven lengths.
Other Application Techniques
Varnishes can also be applied with spray guns, for
applications such as:
–      Â
Local side stripe lacquering of weld zones to protect the regions left
uncoated before the welding operation;
–      Â
For the internal protection of DWI (drawn and wall-ironed) beverage cans
and certain aerosol containers, where varnishing cannot be performed before
forming;
–      Â
For repair varnishing of the inside of DRD (draw and redraw) cans after
drawing.
Other application techniques
include electrostatic powder deposition, which is used for side stripe
lacquering and is being introduced in other areas, due to process improvements.
Several heating methods are employed for baking side stripe lacquers, including
hot air, induction, direct flame impingement, and infrared radiation. Curing
times vary from 2 to 30 seconds.
Inspection Methods
The protective action of the varnish is related to
three essential factors, namely adherence to the metal substrate, chemical
inertia and the absence of porosity. The film/substrate interface is severely
loaded during forming of the varnished tinplate to can components, and the
varnish must be able to follow the metal without delamination or flaking. Good
adhesion is obtained when surface bonding of the two mating components is
complete, but there are sometimes incompatibilities between the two materials.
Chemical inertia is generally assured if the varnish has been adequately cured,
since complete cross-linking eliminates the active groups capable of reacting
with ions from the can contents. The presence of porosity can considerably
impair the protective quality of the film, and must be carefully controlled.
Increasing film thickness decreases the tendency for porosity.Â
Phase Transformation in Steel
Phase Diagram
A phase diagram is a graphical
representation of the temperature, pressure and composition limits of phase
fields in an alloy system as they exist under conditions of complete
equilibrium. It is also known as equilibrium or constitutional diagram.
In a phase diagram, temperature
is plotted vertically and composition is plotted horizontally. Any point on the
diagram represents a definite composition of a constituent and its temperature,
each value being found by projecting to the proper reference axes. For
illustration, let us consider the changes that take place during cooling of an
alloy containing 50 percent element A and 50 percent element B. The alloy
remains homogeneous liquid solution until temperature drops to a value indicated
by the intersection of the liquidus line at c0. The crystals which form from
50-50 liquid consist of a solid solution, the composition of which is found on
the solidus line at c1, 80 percent element B and 20 percent element A.
       As the mass cools, the composition of the growing
crystals changes along the solidus line from c1 to c5, while the remaining
liquid alloy varies in composition along the liquidus line from c0 to c4.
       Figure 2 illustrates the iron-cementite phase
diagram, which is also known as iron-carbon phase diagram.
Constituents in Steels
       Plain carbon steels are generally defined as the
alloys of iron and carbon which contain up to 2.0% carbon. For the present, we
will neglect the effects of such elements as manganese which may be present in
most ordinary steels and regard steels as being simple iron-carbon alloys.
       Constituents in steels exist mainly as phases. They
include molten alloy, delta ferrite, austenite (gamma phase), ferrite (alpha
phase), cementite and graphite. Another constituent in steels is pearlite. It is
not a phase but an aggregate.
Austenite
       In iron-carbon alloys austenite is the solid
solution formed when carbon dissolves in face-centered cubic (gamma) iron in
amounts up to 2%. Its microstructure is usually large grained.
       Austenite is a difficult structure to retain at
room temperature unless a steel contains a large percentage of alloy, such as
manganese or nickel. Austenitic steel is characterized by high tensile strength
and unusually great ductility. The tensile strength is often around 125,000
pounds per square inch with elongation in two inches of 35 to 40 percent.
Ferrite
       In iron-carbon alloys ferrite is a very dilute
solid solution of carbon in a body-centered cubic (alpha) iron and containing at
the most only 0.02% carbon. Its microstructure appears as polyhedral grains.
       Ferrite is very ductile and soft and has a low
tensile strength but high elongation. Its tensile strength is about 40,000
pounds per square inch and an elongation in 2 inches of about 40%.
Graphite
Graphite, or graphitic carbon, is
a free carbon in steel or cast iron. The carbon is amorphous, having no
particular form.
       The metallographic appearance of graphite in a
low-carbon steel which has been subjected to a prolonged heating at a
temperature below that at which austenite is formed.
Cementite
      Â
Cementite, or iron carbide, is an interstitial
compound of iron and carbon containing 6.69% carbon. Its approximate chemical
formula is Fe3C. When it occurs as a phase in steel, the chemical composition
will be altered by the presence of manganese and other carbide-forming elements.
       In case of a slow cooled, relatively high-carbon
steel, microstructure of cementite appears as a brilliant white network around
the pearlite colonies or as some needles interspersed with the peralite. The
metallographic appearance of spheroidized cementite in a steel, which has been
heated to a temperature just below that at which austenite first forms.
       Cementite is a very hard compound. Its tensile
strength is about 5,000 pounds per square inch and an elongation in 2 inches is
equal to zero. Cementite is an unstable phase. Given sufficient time, cementite
decomposes into two complete equilibrium constituents, iron and graphite.
Eutectoid
The term ‘eutectoid’ is usually defined as:
1.     Â
An isothermal reversible reaction in which a solid solution is converted
into two or more intimately mixed solids on cooling, the number of solids formed
being the same as the number of components in the system.
2.     Â
An alloy having the composition indicated by the eutectoid point on an
equilibrium reaction.
3.     Â
An alloy structure of intermixed solid constituents formed by a
eutectoid.
Pearlite
       Pearlite is a lamellar aggregate of ferrite and
cementite. It is a result of the eutectoid reaction which takes place when a
plain carbon steel of approximately 0.08% carbon is cooled slowly from the
temperature range at which austenite is stable.
       Pearlite has lamellar micrographic structure known
as the eutectoid structure. It exerts maximum hardening power of any
constituent. It has a tensile strength of around 125,000 pounds per square inch
and an elongation in 2 inches of 10 percent.
Eutectic
The term ‘eutectic’ is usually defined as:
1.     Â
An isothermal reversible reaction in which a liquid solution is converted
into two or more intimately mixed solids on cooling, the number of solids formed
being the same as the number of components in the system.
2.     Â
An alloy having the composition indicated by the eutectic point on an
equilibrium diagram.
3.     Â
An alloy structure of intermixed solid constituents formed by an eutectic
reaction.
Ledeburite
Ledeburite is a eutectic of the
iron-carbon system, the constituents being an austenite and a cementite.
The eutectic contains 4.3% carbon.
This eutectic is a constituent of iron-carbon alloys containing more than 2.%
carbon and for this reason the dividing line between steels and cast iron is set
at 2.0% carbon.
Phases in Hypoeutectoid Steel
       Hypoeutectoid steels are those containing less than
the eutectoid percentage of carbon, which is about 0.80% in plain carbon steel.
       At some temperature above Ae3, steel containing
0.40% carbon is completely austenitic. On slow cooling below Ae3 the austenite
first rejects ferrite, which concentrates at grain boundaries. As the
temperature falls down to Ae1, the crystals of austenite shrink and their carbon
content increases to 0.80%. On cooling below Ae1, the austenite changes to
pearlite so that the final constituents in steels below Ae1 are ferrite and
pearlite as illustrated in Fig. 3.
Phases in Eutectoid Steel
       Eutectoid steel is a steel containing the eutectoid
percentage of carbon which is about 0.80% in plain carbon steels.
       The eutectoid steel will not begin to transform
from austenite on cooling until the critical temperature Ae1-3 is reached. Then
the transformation will begin and end at the same temperature (723ºC or 1333ºF).
The final structure will be entirely pearlite as shown in Fig. 4.
Phases in Hypereutectoid Steel
       Hypereutectoid steels are those containing more
than the eutectoid percentage of carbon, which is about 0.80% in plain carbon
steels.
       At some temperature above Aecm, a steel containing
1.2% carbon is completely austenitic. On slow cooling below Aecm the carbon will
precipitate as needle-shaped crystals of cementite around the austenite grain
boundaries. As a result the carbon content in an austenite will be gradually
reduced down to 0.80% at the temperature Ae1-3. Below this point the remaining
austenite will then transform to pearlite as shown in Fig. 5.
Phase Transformation Hysteresis
       The phase transformations do not occur at the same
temperature in heating as in cooling. The metal is rather reluctant to change
its physical state so that on heating, the Ac temperatures are somewhat higher
than equilibrium temperature Ae. Likewise, the Ar temperatures on cooling are
lower than equilibrium temperatures Ae. The difference in temperature between
the Ac and the Ar varies. In some cases it is as great as 24ºC, or 75ºF.
Supercooling or Austenite
       As it has been shown in this chapter that austenite
transforms to pearlite when it is cooled slowly below the Ar critical
temperature. When more rapidly cooled, however, this transformation is retarded.
The faster the cooling rate, the lower the temperature at which transformation
occurs resulting in a formation of the micro-constituents shown in Table 1.
Martensite
       Martensite is a metastable phase of steel formed by
a transformation of austenite below Ms temperature. It is an interstitial
supersaturated solid solution of carbon in iron having a body-centered
tetragonal lattice.
       Transformation to martensite occurs almost
instantly during cooling and the percentage of transformation is dependent only
on the temperature to which it is cooled. It is the hardest of the
transformation products of austenite. The microstructure of martensite is
acicular, or needlelike.
       This structure is formed when martensite is
reheated to a subcritical temperature after quenching.Â
Optimization and Modernization of Hot Strip Mills
Main Strategy in Optimization of Rolling Process
       In the process of rolling a uniformly preheated
slab in hot strip mill, its temperature changes due to the various types of the
heat transfer have been described earlier. The following three temperature
profiles are usually used for evaluating the temperature rundown of the
workpiece as well as a degree of uniformity of the temperature along its length
and width:
1.        Â
Temperature rundown of a selected portion (for example, a head end, tail
end, or a middle portion of the workpiece expressed) in relation to each rolling
pass.
2.        Â
Temperature variation along the workpiece length after the same rolling
pass.
3.        Â
Temperature variation across the workpiece width.
The temperature rundown in hot
strip mill is shown in general form in Fig. 1. The main parameters of the
temperature rundown include.
T0 = slab reheat furnace dropout temperature
TR = bar temperature after leaving roughing train
TF = bar temperature at finishing train entry
TE = strip temperature at finishing train exit.
The temperature variation along the workpiece
length is usually defined by the following parameters:
DT0 = absolute value of temperature differential between
slab head and tail ends.
DTF = absolute value of temperature differential between
transfer bar head and tail ends at finishing train entry.
DTE = absolute value of temperature differential between
strip head and tail ends at finishing train exit.
The temperature variation across the workpiece
length can be defined as a difference between the temperatures measured at the
middle and near the edge of the workpiece DTW.
The strategy of
controlling the workpiece temperature during hot rolling is twofold. Firstly, it
is necessary to maintain the optimum temperature of the rolled piece, which
allows one to obtain the desired properties of the rolled product with minimum
energy consumption, required production rate, and maximum yield. Secondly, it is
desirable to achieve a uniform workpiece temperature in both longitudinal and
transverse direction during each rolling pass which helps to improve quality of
the rolled product.
Metallurgical Requirements
       The boundary conditions for material temperature
during the rolling deformation process are defined by metallurgical
requirements.
To ensure the
homogeneity of the rolled product, all deformations in conventional hot rolling
process are usually made in the austenitic phase. For low-carbon steel, this
implies that the last deformation must occur at a strip temperature TE above the
phase transformation point between austenite and ferrite; for low-carbon steel
the optimum range for TE is 1550 to 1650ºF.
The second important
metallurgical requirement is that the slab temperature T0 be high enough to
ensure dissolution of intermetallic phases or compounds resulting from the
addition of alloying elements. From this point of view, the minimum value of T0
for low-carbon steel is approximately 2000ºF.
       The maximum value of T0 is usually limited because
of another metallurgical phenomenon related to excessive grain coarsening, which
can have a detrimental effect on the final product. The maximum value of T0 for
low-carbon steel is approximately 2400ºF.
       More detailed description of the metallurgical
requirements for rolling of different types of steels is given in the following
chapters.
Energy Consumption Requirements
       Energy consumption directly related to the hot rolling
process can be divided into three components:
 (a)        Â
Energy required for heating the slab in the reheat furnace.
 (b)        Â
Energy required for maintaining heat during transfer of the workpiece
between rolling mill stands.
 (c)        Â
Energy required for hot rolling of the workpiece.
Product Quality Requirements
       Temperature variation of the rolled material in
both the longitudinal and the transverse direction is a major obstacle in
maintaining the required strip gage, profile and shape tolerance.
       The most drastic variation in the longitudinal
direction occurs when the transfer bar enters the first finishing stand. Because
the head end of the bar is usually transferred from the last roughing stand to
the first finishing stand in less time than the tail end of the bar, the tail
end is subjected to heat radiation loss for a longer time than the head end. The
resulting temperature rundown increases with increasing slab weight. As will be
shown later, if no preventive measures are taken to reduce this rundown, the
temperature differential between head and tail end of the bar at the entry of
the finishing train DTF can be as much as 300ºF for a 1000-PIW coil.
       The adverse effect of this temperature differential
on strip shape is inversely proportional to the rolled material thickness.
       Rolled material temperature variation in the
transverse direction is mainly due to excessive radiation near the edges where
the surface-to-volume ratio increases substantially. If no measures are taken to
reduce edge cooling, the transverse temperature variation DTF can be as much as
180ºF.
Analysis of Temperature Conditions in Hot Strip Mill
         Â
Review of the foregoing requirements shows that
there is no universal definition of optimum temperature conditions for hot strip
mills. For example, a possible reduction in reheat furnace temperature due to
heat conservation on the transfer table might not be fully utilized because of
power limitations of the roughing train or, in another case, because of poor
surface quality of the slabs loaded into the furnace, which requires maintaining
the higher reheat furnace temperature needed to enhance the scaling process that
helps to improve the slab surface.
These facts suggest that the
optimum temperature conditions must be found for each hot strip mill on an
individual basis. However, the following common criteria can be applied for
objective evaluation of different solutions:
        Â
(a)Â Â Â Â Â Â Â Â Â
Reduction in mean temperature differential, MTD
        Â
(b)Â Â Â Â Â Â Â Â Â
Reduction in primary scale, m
        Â
(c)Â Â Â Â Â Â Â Â Â
Savings in fuel energy, Ef
        Â
(d)Â Â Â Â Â Â Â Â Â
Savings in electrical energy consumption, Ee
        Â
(e)Â Â Â Â Â Â Â Â Â
Total annual cost savings due to reheating and rolling optimization, St
        Â
(f)Â Â Â Â Â Â Â Â Â
Total additional capital cost, Ct
        Â
(g)Â Â Â Â Â Â Â Â Â
Payback time, PBT.Â
Low Carbon Constructional Alloy Steels
Low Temperature High Strength Tough Steel
       U.S. Patent 3,960,612; June 1, 1976; assigned to
Nippon Steel Corporation, Japan describe the preparation of steel for use as a
pressure container to be used at temperature below the ice point, or as a
structural material such as a pipeline in a cold environment capable of standing
high pressure and low temperature.
       The method comprises: (1) providing a steel
material as hot-rolled comprising 0.03 to 0.15% C, 0.05 to 0.40% Si, 0.2 to 2.0%
Mn, 1.0 to 4.5% Ni, 0.1 to 0.5% Mo, 0.005 to 0.050% Nb, not more than 0.02% N,
0.005 to 0.070% Al, and if necessary, one or more than one member of the group
consisting of V, Ti, Cr, Ca and Ce, the rest being iron and unavoidable
impurities; (2) quenching the material after heating at 660º to 750ºC; and
then (3) tempering after heating at 650ºC or less.
       The steel material in the form of plates, rods,
etc. having the above composition can be manufactured as follows. The molten
steel obtained by the use of a converter, electric furnace or other smelting
furnaces, and if necessary, a vacuum degassing apparatus is formed into a slab
through the steps of ingoting, blooming or continuous casting, and then
hot-rolled to the steel material. The steel material as hot-rolled can be any
type, but it is preferable that the crystal grain is larger than the crystal
grain size No. 5 of JIS and that the space factor is below 80%, and the smaller,
the better. In order to make the structure of the steel finer as hot-rolled, the
steel material used may be that which has been heated at 840º to 930ºC and
quenched.
       The steel material which can thus be manufactured
by hot rolling with or without the subsequent heat treatment is subjected to the
quench treatment of heating at 660º to 750ºC), followed by rapid cooling,
whereby extremely fine structure can be obtained, which results in the
enhancement of the low temperature toughness. This steel material is further
subjected to a temper treatment at 650ºC or less (preferably at least 400ºC,
whereby the strength and the toughness are enhanced and the cold workability and
the brittleness due to strain aging can be improved.
       In the heat treatment normalizing the steel
material after it has been hot rolled but before temper treatment may be
considered. The quenching treatment of this process, however, produces a
material of much finer structure and is thus of advantage.
Alloy Steel for Arctic Service
       The development of oil and gas fields in the Arctic
had encouraged a search for structural steels having good low-temperature
properties for such applications as line pipe, line-pipe fittings and critical
bridge members. The low-cost carbon and high strength, low-alloy steels
currently used for these applications in warmer environments do not have the
desired toughness at low temperatures in section thicknesses of about 1 to 2
inches. For such Arctic applications, it will be necessary that the structural
steel have a minimum yield strength of at least 60 ksi, and good impact
toughness down to temperatures as low as –80ºF.
       U.S. Patent 3,955,971; May 11, 1976; assigned to United
States Steel Corporation describes a low alloy steel ideally suited for Arctic
applications. This weldable, low-alloy steel is characterized in the quenched
condition by a ferritic-pearlitic-bainitic microstructure which in the tempered
condition has a minimum yield strength of 65 ksi in plate thicknesses to at
least 2 inches, and a Charpy V-notch 50% shear-transition temperature below
–80ºF and a Charpy V-notch energy absorption of at least 50 ft/lb in both the
longitudinal and transverse directions.
       In the quenched and tempered condition, at least in
thicker sections (i.e., 5/8-inch and greater) the above composition will render
a ferritic-pearlitic-bainitic micro-structure. Unlike the quenched and tempered
low-carbon constructional alloy steels, the above steel is not characterized by
high hardenability and is not martensitic in the quenched condition. Indeed,
lower yield strengths are achieved but low temperature toughness is improved.
The quenched and tempered low-carbon ultraservice steels can be similarly
distinguished in addition to containing considerably more carbon and total alloy
content.
       Example: An 80-ton commercial heat was produced in
an electric furnace, aiming for a content of 1% each of nickel and chromium and
0.30% molybdenum. The product composition was 0.09% C, 0.58% Mn, 0.007% P,
0.010% S, 0.31% Si, 1.05% Ni, 0.98% Cr, 0.30% Mo and 0.03% Al. Ingots from this
heat were processed to 5/8-, 1-and 2-inch-thick plates and to 24-inch-OD by
0.969-inch-wall seamless pipe (610 by 24.6 mm). The table 1 gives the test
results. It is significant to note that all products exceed a 65 ksi yield
strength and a transverse Charpy V-notch energy absorption of 50 ft/lb and 50%
shear-fracture appearance at –80ºF.
High Strength Cold Rolled Steel with High Press Formability
       Demands have been increasingly made for development
of a cold rolled steel sheet having still higher strength without substantially
lowering press-formability as compared with the conventional cold rolled steel
sheet for use in inside sheets and outside skins of a safety automobile.
Particularly, for parts such as member sides which are subjected to severe
stretching and bending and whose increased strength has a large effect on the
safety, demands are increasingly made for a cold rolled steel sheet which has
high tensile strength such as 45 to 90 kg/mm2, 35 to 75 kg/mm2 yield strength as
well as excellent ductility such as stretchability and yet shows a high F value
of drawability in certain applications.
       U.S. Patent 3,951,696; April 20, 1976; assigned to
Nippon Steel Corporation, Japan describe a method for producing a high-strength
cold rolled steel sheet having the above strength properties and yet having good
press-formability, particularly stretchability. The method comprises hot rolling
and cold rolling a low Si-Mn killed steel, heating the cold rolled steel sheet
with an average heating rate not lower than 3ºC per second, annealing the steel
sheet for 1 to 15 minutes at a temperature between 650ºC and the A3
transformation point and cooling of the steel sheet at an average cooling rate
between 0.5 and 30ºC per second down to 500ºC.
       The steel comprises 0.03 to 0.30% C, less than 0.7%
Si, 0.6 to 2.5% Mn, 0.01 to 0.20% sol Al, not more than 0.015% O with the
balance being Fe and unavoidable impurities.
       Example: Steel slabs were produced by melting in a
converter, by ordinary ingot-making and partly by a continuous casting (Steels
A2 and B2), and these slabs were subjected to hot rolling, cold rolling,
annealing and averaging to obtain cold rolled steel sheets of 1.0 mm thickness.
All of the products were subjected to skin-pass rolling of 1.0%. The chemical
compositions, production conditions, mechanical properties. F values and
secondary workability are shown in the table 2 and 3.
       As for the secondary workability test, the
following impact secondary workability test was conducted. A steel sheet disc of
80 to 160 mm diameter was drawn into a cup-like form with an appropriate drawing
ratio (primary working drawing ratio), and this cup-like test piece was immersed
in a vessel containing water and ice to lower the temperature of the test piece
fully.
       Then a conical punch was inserted into the cup-like
test piece on the thick steel plate and a steel lump of 20 kg weight was dropped
from a height of 3 m to the punch, to see if an embrittlement rupture
(longitudinal crack) was caused in the test piece. In this test, the largest
primary working drawing ratio (limit drawing ratio), which does not cause the
embrittlement crack, represents better impact secondary workability. The
secondary workability tends to lower in a steel sheet having higher strength. In
case of an ordinary mild rimmed steel the limit drawing ratio is 3.0 to 3.2. As
understood from the table, when the steel composition is worked into a cold
rolled steel sheet by the production steps including the continuous annealing
according to this process, it is possible to produce a high-strength cold rolled
steel sheet having a high yield ratio of 0.75 and yet excellent secondary
workability or drawability.
       Meanwhile, if the steel composition is subjected to
a box annealing at 700ºC, a high yield point cannot be obtained although
satisfactory drawability is obtained so that the utility of this process
directed to the inside sheets and outside sheets of safety automobilies is
remarkably limited.
       In case of a box annealing, the grain growth is
suppressed when the annealing is done at a low temperature (600ºC) and it is
possible to obtain a somewhat high yield point property, but remarkable results
as obtained by the rapid heating and the short-time annealing cannot be
expected.
Production of High Strength Cold Rolled Steel Sheet
       U.S. Patent 3,947,293; March 30, 1976; assigned to
Nippon Steel Corporation, Japan describe a method for producing a high-strength
cold rolled steel or strip. Steel comprising 0.05 to 0.15% of C; 0.02 to 0.30%
of Si; 0.10 to 1.5% of Mn; 0.02 to 0.07% of Al; and a total of 0.02 to 1.15% of
art least one of Nb, V, Ti and Zr; with the remainder being iron and unavoidable
impurities, is hot rolled whereafter
the hot rolled steel sheet or strip is coiled below 750ºC. The coiled sheet or
strip is then cold rolled whereafter the cold rolled steel sheet or strip is
subjected to annealing at 670º to 900ºC for 20 seconds to 10 minutes.
       The increase of strength by the continuous
annealing is considered to be due to the fact that the solid dissolved elements,
such as Nb, V, Ti and Zr, which have not completely precipitated during the hot
rolling, remain as partial precipitates during the continuous annealing. If the
holding time of the continuous annealing is excessively long, the precipitates
of the above elements become coarse above the A1 transformation temperature,
thus lowering the strength as in case of box annealing and causing economical
disadvantage.
       Then the cold rolled steel sheet or strip which has
been subjected to the short-time continuous annealing as above is rapidly cooled
so that much carbon in solid solution remains in the steel, particularly when
the coiling temperature is relatively low, to wit, not higher than about 550ºC.
       Material deterioration, commonly called quench
aging, and lowering ductility is thus caused. In order to avoid this problem, it
is desirable that an overaging treatment for one or ten minutes, preferably two
to five minutes at between 300º to 400ºC, preferably 300º to 350ºC, is
conducted during the cooling step after the continuous annealing to accelerate
the carbide precipitation, thereby avoiding the hardening effects peculiar to
continuously annealed materials.
       In this context it should be observed that the
carbides and nitrides of Nb, V, Ti, etc. do not precipitate completely with a
low temperature coiling at 550ºC or lower so that it is necessary to
precipitate them completely by overaging in the continuous annealing step. By
contrast, in case of a high temperature coiling between 550º and 750ºC, the
carbides and nitrides of Nb, V, Ti, etc. are precipitated completely so that the
overaging treatment is not necessary in the continuous annealing step.
       The material properties, particularly the balance
between yield point and total elongation of the high-strength cold rolled steel
sheet produced in the above manner are found to be better than those obtained by
box annealing. Also it is possible to control very strictly the annealing
temperature along the whole length of the coil so that nonuniformity of strength
and ductility due to the temperature difference encountered in box annealing can
be avoided.
       Example: A steel heat comprised of 0.12% C, 0.25%
Si, 1.33% Mn, 0.0113% P, 0.007% S, 0.05% Nb, 0.03% V, 0.026% Al, and 0.0042% N
was tapped and continuously hot rolled. In the hot rolling step, the steel was
hot rolled to a thickness of 3.2 mm and coiled at 490ºC. The thus obtained hot
rolled steel strip was cold rolled to a thickness of 0.8 mm by an ordinary
method, and thereafter subjected to a continuous annealing at 700ºC for one
minute and 750ºC for one minute and successively subjected to an overaging
treatment at 350ºC for five minutes. The results are shown below in comparison
with those of the box annealing.
Full Continuous Annealing Process
       U.S. Patent 3,936,324; February, 3, 1976 and K.
Uchida, K. Araki, H. Narita, S. Fukunaka and T. Kurihara; U.S. Patent 3,904,446;
September 9, 1975; both assigned to Nippon Kokan KK, Japan described a method of
making a high strength cold reduced steel having the most suitable mechanical
properties required as a safe countermeasure for an automobile, and more
particularly being easily pressable into a required shape and stepping up the
strength by a coating-baking treatment after the above presswork.
       A steel comprising 0.04 to 0.12% C, 0.50 to 2.00%
Si, and 0.10 to 1.60% Mn is passed through ordinary hot and cold rolling
processes and is subjected to a full continuous annealing process. The full
continuous annealing process is selected from the following processes depending
upon the intended use and the required strength level.
 (1)   Â
Rapid heating to 650º to 900ºC at 200ºC/min or faster ® holding for
10 to 120 seconds at the above temperature ® ordinary cooling ® coiling;
 (2)   Â
Rapid heating at mentioned above ® holding as mentioned above ®
quenching in jet water ® reheating to 300º to 500ºC × 10 10 300 seconds ®
ordinary cooling ® coiling;
 (3)   Â
Rapid heating to 700º to 900ºC at 200ºC/min or faster ® holding as
mentioned above ® quenching as mentioned above ® reheating to 180º to 300ºC
for 4 to 300 seconds ® ordinary cooling ® coiling.
High Strength Killed Wire Rods and Bars
       U.S. Patent 3,926;687; December 16, 1975; assigned
to Nippon Steel Corporation, Japan describe a method for producing a
high-strength steel wire rod having a structure of good workability by
controlling the temperature of the rolled steel material and also controlling
the cooling rate after the finish rolling. The steel material obtained is useful
for high-strength bolts, PC wire, metal networks, Umbrella ribs, spring washers
and springs.
       A wire rod as hot rolled is subjected only to
slight skin-pass drawing into a required size, and then to heading and threading
works, to obtain a bolt having 80 to 100 kg/mm2 of tensile strength without any
defect. Heat treatments such as spheroidizing annealing, quenching and tempering
can be omitted and thus a high level of economy is assured.
       Further, when applied to production of PC wires (prestressed
concrete wires) it is sufficient that the wire rod is subjected only to slight
skin-pass drawing and shape working including indent work for application in
prestressed concrete products. Thus the patenting heat treatment which is
conventionally done can be omitted. Yet a wire having high tensile strength and
very excellent spot weld-ability can be obtained.
       The wire contains 0.02 to 0.20% C, 0.03 to 0.90% Si
and 1.00 to 1.85% Mn together with one or more of not more than 0.05% Nb, not
more than 0.08% V, not more than 0.25% Ti, not more than 0.30% Zr, not more than
0.005% B and not more than 0.40% Cr, and contains Al in an amount as contained
in an ordinary killed-steel with the balance being iron and unavoidable
impurities.
       The process is carried out by heating a steel
having the above composition to at least 1150ºC, conducting intermediate
rolling and/or finish rolling at 700º to 1150ºC, controlling the cooling rate
from finish of the hot rolling to a coiling to 40º to 350ºC/sec, and
controlling the cooling rate from the coiling to gathering to 1º to 15ºC/sec.
Hot rolled steel wire rods and bars are obtained having excellent workability
and spot weldability and having a tensile strength not lower than 50 kg/mm2 and
a reduction of area not lower than 50%.
High Formability High Strength Steel
       There is an ever present and increasing demand for
high strength steels having good formability properties particularly drawings,
biaxial stretching and uniaxial bending properties required, for example, by the
automotive industry for auto-mobile bumper systems.
       U.S. Patent 3,926,686; December 16, 1975; assigned
to The Algoma Steel Corporation, Limited, Canada describe a high strength low
alloy steel strip having a minimum yield strength of 50,000 psi and good
formability properties. The steel consists, by weight, of 0.10% maximum carbon,
0.30 to 0.80% manganese, 0.01% maximum sulfur, 0.02 to 0.06% aluminum, 0.01 to
0.12% columbium, 0.06% maximum cerium, the balance being iron and incidental
impurities. Depending upon the composition of the steel, lower yield strengths
of 50,000 to 80,000 psi are attained. This composition when hot-rolled finished
at 1620º to 1700ºF and coiled or collected at 1150º to 1375ºF, exhibits a
unique relationship of strength and maximum formability at each of the yield
strength levels in the range.
       At each strength level, from 50,000 to 80,000 psi,
the final structure of the steel is composed principally of ferrite with very
limited amounts of pearlite. In conjunction with this and essential to the
improved formability properties is the controlled dispersion of the columbium as
columbium carbides of columbium carbonitrides. While pearlite is in grain
boundaries and as cementite (Fe3C) in the form of skeletal carbides, the
columbium has been found to have a dual form, of row precipitates in excess of
200 Angstrom units from which the initial ferrite grains have formed and
secondly, within the ferrite grain itself, as a finely dispersed carbonitride of
30 to 120 Angstrom units.
       It may be noted in the steel composition that the
level of both C and Mn is considerably lower than in known steels in the same
strength range, which provides several major factors contributing to the
improved formability properties. The low content of the steel composition, of
course, reduces the pearlite content directly. However, more important, the low
carbon content and low manganese values act to increase the austenite to ferrite
transformation temperature.
       It has been found that this increase or higher
austenite to ferrite transformation temperature controls the proportion of the
columbium that precipitates into coarse and fine dispersion in the composition
and distributes the available columbium into its dual form for the purposes of
grain refinement and precipitation strengthening. The coarse precipitation of
columbium occurs during the hot deformation or actual rolling of the steel up to
and including the final deformation, the collecting. This coarse precipitation
acts to retard the recrystallization of austenite, immediately after the final
deformation, until transformation to ferrite is started. Transformation from the
highly deformed austenite guarantees transformation to a fixed and constant
ferrite grain size. The ferrite grain size will vary according to the amount of
columbium present and the temperature at which the finishing rolling is carried
^ Top
Adhesives, Chemical, Drugs, Gums, Insecticides, Jute, Pe ...