basics of steel rolling, Rolling,
mill installations , The
stages in hot rolling of steel , Cold
forming of steel
steel, melted and cast in the steel mills, still lacks the shape suitable for
the steel consumer while its technological properties as yet fail to meet
order to obtain the shapes, dimensions and properties demanded by the steel
user, the blooms and slabs need to undergo further stages of processing and
treatment. In fact, many of the
necessary production and treatment processes are carried out either
partly or completely in the steel mills.
many working and downstream processing stages can be classified as follows:
the properties of the metal (heat treatment).
most important stage of primary forming where the molten metal usually obtains
its primary shape, is
Forming of steel in the steel
generation of shapes, dimensions and properties
required by the steel consumer,
carried out by various processes (and combinations)
This has already been discussed. Forming is the conversion
of a given primary form into an intermediate or final form
of the working stages carried out in the steel mills belong to the forming
basics of steel rolling
solid body changes its shape under the influence of external forces. During temporary
or elastic deformation, the momentarily deformed body regains its original shape
once the external load has been removed. If , however, a certain deformation
takes place. This capability for permanent deformation is used in the forming
processes of the rolling mills.
consist of crystals with defined space lattice. Plastic deformation starts when
the external force is so severe that the crystals begin to slide in the
so-called sliding planes. With increasing plastic deformation, resistance to
further sliding grows within the space lattice, with the result that, due to the
distortion and destruction of the crystals, further sliding is hindered. The
changes in the physical and technological properties are classified under the
term “work hardening”.
deformed steel attempts to the most important process is rolling which
classifies as a pressure-forming technique. Other important forming processes
include forging, pressing and drawing.
for applying corrosion protection
coats are likewise largely performed in the steel mills.
shifting of atoms against each other, caused by sliding of preferential
planes (sliding system).
into the ordered condition of the crystals; mostly, however, these are strained
and so that this is not easily possible. However,
creation of material cohesion
example: casting of steel
transformation of an already existing shape into another
heated, the crystals reform anew; this process is termed
in plastic deformation is the deformation resistance of the steel. The
deformation resistance of metals is lower elevated forming temperatures.
Consequently, plastic deformation of steel is possible at high temperatures with
only small input of load and energy. Moreover, high-temperature forming leads to
an immediate recrystallization of
the structure, work hardening does not take place. This recrystallization at
high temperatures and work hardening at low temperatures are used to classify
the two basic methods of forming steel: hot forming at temperatures of about 800
to 1,150°C, cold forming at temperatures below recrystallization temperature.
these two types of metal forming are simply distinguished according to the
criterion of temperature. Cold forming is
performed without heating, hot forming is carried out at an elevated
rolling-stock temperature. The recrystallization temperature is no longer
relevant in this definition.
formation of crystals at elevated temperatures after previous forming
combined with structural distortion.
of the blank at high
temperatures; mostly plastic deformation of the steel above the
This entails continuos
formation of crystals, no work
hardening, low resistance to the
of the blank without heating;
mostly, plastic deformation
of the steel below the recrystallization temperature.
This entails continuously growing
distortion of the crystal structure, work
hardening, growing resistance to
deformation. In the case of excessive
deformation, fracture results.
processes are divided into:
push pull forming,
forming by bending,
rolling is a continuous or stepwise forming with the aid of several rotating
tools, the rolls. The roll act on
the metal primarily through pressure. Therefore
rolling is classified among the pressure-forming methods.
Depending upon the reciprocal movements of tool and workpiece, we
cross rolling, skew rolling.
longitudinal rolling, the rolling
stock is deformed along its longitudinal axis and thus vertically to the axes of
the rolls. Most flat and sectional products at the steel mills are produced by
cross rolling, the rolling stock is
turned around its own axis without movements in the direction of the axis.
This cross rolling (eg ring rolling) is of minor significance regarding
skew rolling, the rolling stock ist turned around its own axis as well as in the
direction of the longitudinal axis, and is formed at the same time.
The axial movement of the workpiece is caused by longitudinal feed due to
the skew position of the rolls. Skew
rolling is used for producing seamless tubes (blank piercing).
simplest arrangement for longitudinal rolling consists of two parallel
cylindrical rolls. “Roll
clearance” describes the distance between the two rolls.
“Roll gap” is the deformation zone.
The length and the area of the contacting zone between the rolling stock
and the rolls are termed “contact length” and “contact area”,
rolling action is initiated by the rolls gripping the rolling stock and drawing
it into the roll gap. The “bite
conditions” must be ensured in order to prevent the rolls slipping on the
rolling stock. Steel does not
compress, so the “continuity law” applies to the deformation process in the
roll gap. This states that at a
constant circumferential speed of the rolls the same amount of steel per unit of
time and at all levels passes through the roll gap.
As the height of the roll gap decreases, the rolling stock extends during
the process. This means that the
exit speed exceeds the entry speed. During
flat rolling, the rolling stock not only elongates but also spreads (free
the geometrical data we have the reduction of height, the roll diameter, the
radio between the width/height of the rolling stock and the radio between
width/contact length. Physical
parameters are the rolling stock temperature, the rolling speed, surface
roughness of the rolls and the rolling stock material, as well as the
products are produced with plain rolls. The
rolling stock elongates and spreads without hindrance.
Because of the stress during the rolling process, the geometry of the
roll gap and hence the cross section of the deformed rolling stock change.
These changes are described by:
roll spring (increase in the height of the roll gap
caused by elastic deformation of the roll stand components),
changes in the height of the roll gap across the roll
body length due to the elastic deflection of the rolls,
elastic roll flattening, caused by the influence of
to these alterations in the roll gap, which are dependent on the rolling force,
there are gradual changes in the surface of the rolls.
These changes are caused by wear. Because
of the heating of the roll bodies through friction and heat transfer, there is
an opposite change in the roll gap.
to the respective operating conditions, several roll contours are used to
counteract the mechanical and thermal influences in the roll gap.
Furthermore, complex electromechanical or hydraulic control systems for
dynamically adjusting the roll gap during the rolling process exist.
roll gap is adjusted by bending the work or back up rolls, by cooling down in
zones or by additionally shifting the rolls horizontally.
methods have been developed to achieve this, eg CVC .
In this process, the contour of the work rolls is shaped like an S.
Shifting of the rolls in opposite direction in a continuously changing camber.
Together with conventional bending of the work or back up rolls, the roll
gap can be ideally adjusted to the rolling stock or rolling schedule.
This means that the cross section and the shape of flat products can be
rolls are used for rolling sections and semi-finished products. The grooves are
concentric slits in the roll barrels of the matching set of rolls.
shape of these grooves and their arrangement to each other is selected under
consideration of flow characteristics so that the desired shape of the section
can be rolled in a defined number of passes. Depending on the arrangement of the
grooves in a production stage, we distinguish between blooming and finishing
to the shape of the section and the corresponding production stages, various
passes are required. Examples of basic passes are:
groove (almost rectangular or square, with rounded-off edges),
groove (with opening angles between 95 an 120°),
groove (diagonals running vertical or parallel to the axis of the roll)
groove (bow-type oval, leading oval or edging groove for rolling round bars),
oval (trapezoidal-shaped halves of the groove, with rounded-off edges for high
cross-section reduction, as breaking-down grooves for rolling merchant bars and
groove (finishing groove for round bars and wire).
series are used for rolling simple cross-sections in which similar pass forms of
decreasing cross section alternate. Examples of groove series are:
angle of the edge flanks of the grooves is termed the taper and serves for
releasing the rolling stock from the groove, as well as
for reworking worn grooves by dressing them.
rolling complex cross-sections, several grooves of differing shape and
cross-section are required.
position at which the groove is divided is called the pass closing; it can be
open or closed.
Rolled steel products
distinguish between two man groups
of rolled steel products
are the products of the continuous
casting plant, the blooming and slabbing as
well as the billet or the hot wide strip mills. As a rule, semis need some kind
of downstream hot forming. Normally, they are deformed in the steel plants into
finished products. Depending on dimensions and cross-sections, semis are
classified as follows (Euronorm 79 ):
semis (including wide strip for further rolling),
products are those whose hot forming has been completed in the steel or rolling
mills. From the point of view of the downstream stages in the metal-working
industry, these products might, of course, again become “semis”. Engineering
standards list about 70,000 different rolled shapes.
rolled steel products are classified according to the shape of their
in these product groups are shapes formed partly by rolling and partly by other
are the tubes/pipes, tyres or
shaped and coated sheet and plate.
products are subdivided into groups each
with differing individual profiles. These include :
steel includes I, H, U sections and wide-flanged beams . Also included in the
category of sectional steel are the
I and U sections with dissimilar or asymmetrical flanges, colliery arches, steel
for waggon construction, and similar sections. The most important yardstick for
classifying such steel is the overall height which should be at least 80mm.
Sectional steel is rolled in straight lengths.
cross-sections of steel bars are circular, square, rectangular, octagonal or
semi-circular. Sections shaped like an
L, a T or a Z and featuring additional bulges, also belong to the steel bar
category. The same applies to standard and non-standard
special sections as well as to I, H and U sections of less than 80 mmm
height. Bars are supplied straight as in the rolled condition or straightened.
a diameter measuring 6 to 28 mm, reinforcing steel is also a bar with a circular
cross-sections. Its surface is usually ribbed.
Unribbed reinforcing steel is of hardly any significance.
Reinforcing steel is used in its as-rolled (naturally hard)
or in a cold deformed state.
its hot state, wire rod is randomly coiled.
Its cross-section may be circular,
oval, square, rectangular, hexagonal, octagonal, semi-circular, etc. its surface
is mostly smooth. Thicknesses range
from 5 to 40 mm. Most wire rod
undergoes further treatment by cold drawing or cold rolling.
category covers all the parts
needed for building railway tracks.
are not only the sleepers, clamping plates, fish-plates, rail chairs, etc.
on the weight per metre of the rails and sleepers, a distinction is made between
light and heavy rail accessories materials.
Sheet piling sections
are hot-rolled finished products whose shape allows them to be joined together
by later shifting or to be clamped. They
are rammed into the ground to form bridge or permanent walls.
Flats steel products
are rectangular in cross-section.
width is much larger than the thickness. The
surface is mostly smooth but many also be patterned.
products include :
wide flat steel,
plates and sheets,
forms of flats in cut-to-length sheets or coils are electric sheet with special
magnetic properties, tin sheet, galvanized sheet, galvanized strip and coated
Wide flat steel
flat steel is hot rolled on all four sides and thus differs from sheet and
plate. The width is between 150 and 1,250 mm and thickness is over 3 mm.
Sheet and plate
sheets and plates have unfinished, trimmed or sheared edges.
Depending on the deformation temperature, we distinguish between hot-and
cold-rolled sheet and plate. Sheet
and plate is classified by thickness into:
plate (3 mm and more thick),
(0.3 mm thick)
sheet and tin sheet (less than 0.5 mm thick)
grades are defined by their use. Heavy
plate is supplied in the categories A
and B; sheet in drawing, deep drawing and special deep drawing grades.
Black/tin sheet is always cold rolled and generally tin coated.
strip is directly coiled after it has been rolled. However, it can also be supplied with trimmed edges or be
split from wider strip.
on width, we refer to narrow, medium and wide strip. In terms of deformation temperature, we distinguish between
hot and cold strip.
wide strip is a particularly important flat.
It is the starting product for marking cold-rolled sheet and welded pipe
and tube. As a finished product, it
is also used for a variety f purposes.
Pipe and tube
and tubes are hollow sections. Although
normally circular in cross-sections, other forms are also produced (special
hollow-section, special sections tube). Pipes
and tubes may be seamless or welded. We distinguish between hot-rolled,
hot pressed, extruded, hot-drawn, cold redrawn and cold-rerolled pipe and
tyres and solid wheels for transportation and technology of mechanism design are
also included among the items produced with the aid of special rolling
is rolled with the aid of two parallel rolls rotating in opposite direction.
The steel is fed between the rolls, the high pressure causing it to
stretch and constantly decrease in cross-section.
The extent of deformation is expressed in terms of the reduction in
height or change in cross-section.
all the equipment needed to roll the steel (furnaces, rolling train,
of roll stands needed to roll the steel, including the
production stages of handling, turn over, cutting
roll stands are the central elements of a rolling mill. They accommodate the rolls that deform the metal.
A normal roll stand consists of:
including rolls, chocks and bearings,
mechanism incl. Weight compensation.
facilities and components are:
for guiding the rolled stock,
roll stand has two roll housings which are usually made of cast
roll stands might also be welded or of lamellar
design. Roll housing are
manufactured in one piece (closed) or whit a removable top crosshead (open). The rolls are replaced through the side window or with the
crosshead removed. Roll housings
must be dimensioned both to absorb the high rolling forces and to permit a
slight expansion. The chocks
accommodate the roll bearings and are installed in
the windows of the roll housings. Their
height is adjustable. The roll
bearings guide the rolls, absorb the rolling forces and transfers these via the
down mechanism that can be operated mechanically, electomechanically or
the respective roll gap to be read. Mechanical, electric or hydraulic overload safety devices
protect against possible damage or destruction.
that the metal enters the roll gap in the proper position and at the envisaged
place and so that it leaves as straight as possible, a number of guiding
mechanisms are necessary. These
and exit guides.
and three-phase motors may be used for the rotary drive. Torque is transferred to spindles and rolls either via pinion
gears or contra-rotating drive
Roll stand types
stands are classified:
the number of rolls and/or
most important distinction criterion is the number of rolls
and their arrangement in the stand :
basic arrangement includes additional different designs. The four-high and
cluster mills use special back-up rolls addition to the thin work rolls.
mills (12 or 20-high stands)are only used for cold rolling where special grades
of strip of maximum dimensional accuracy are required.
In the case of the planetary mills, many small work rolls rotate like
planets around the heavy large-diameter back-up roll.
This design permits thickness reductions of
90% and more. The newly
developed CVC system (continuously variable crown) is used on two-, four-and
six-high stands. In their
final deforming stage, bare and wire -rod mills tend to use rolling
blocks of which several group to form a compact set. Rolls or disk
rolls of small diameter and compact arrangement permit the desired high
exit speed of about 120 m/s and more thus considerably increase throughput.
sets of rolling stands posibly two, four and six high, are increasingly being
used for the roughing trains of the
bar and wire-rod mills (fig.78). The
layout resembles that of the stands in the final stage of the bar and wire-rod mills.
The rolls are mostly arranged horizontally/vertically (under 45° and
reductions in cross-section are achieved with the aid of special rolling
stands (high deformation plants with rolling or continuous forging
systems). Such configurations are
sometimes used for deforming
continuously cast steel immediately after the casting stage.
rolling plants consist of one to two continuous forges, each with four to eight
hammers forging round bars from
rectangular billets , combined with 3-to 12 -high rolling blocks.
This permits an overall degree of deformation of 70 : 1, so that forge
rolling plants may be used as continuous roughing trains for bar and wire-rod
mills. The special advantage of
such configurations is the compression of the
as-cast structure of the continuously cast billets.
classifying rolling stands for semis and sections , the roll diameter is
referred to in some cases while for plate and wide-strip mills the usable length
(barrel length) of the rolls is stated. Latter
may be in metres or inches.
rolls are the instruments for changing the shape of the steel.
The forming part of the rolls is termed
the barrel. The basic shapes
have already been described.
are used as bearings for the rolls. They
are followed by wobblers in the case of driven rolls. Non-driven rolls are referred to as idle rolls.
on their function , rolls are classified as
steel is deformed between the work rolls. The function of the back-up rolls is
to prevent work-roll deflection. The
terms “work rolls” and
“back-up rolls” are used in the rolling of flat products.
the rolls directly influence the product (dimensional accuracy and surface
quality), their configuration, shape, quality and wear properties are vital
factors in the rolling process. Rolls
are forged or cast. Latter are made
from chilled cast iron or chromium cast iron.
Rolls made from sintered hard metal are primarily used as disk rolls in
high-speed wire-rod mills.
the rolls is of critical significance if damage is to be avoided.
Roll damage-apart from wear in the course of normal operation -can take
the form of spalling, shelling, heat cracks and roll damage.
rolling mill combines all the equipment and components needed for producing hot
or cold-rolled products.
generally divided into the following zones:
area (for heat supply prior to deformation),
no further heat supply and generally include the following areas:
trains are necessary as the total deformation cannot be achieved in one single
pass. So, many deformation stages
or roll passes are required in the various roll stands arranged to form a
rolling train. In cluding all their
accessories, rolling trains/mills are sometimes several 100 m in length.
stock passes through the rolling mill with the aid of extensive handling and
guiding equipment. The most
important are the roller tables, transfer equipment, manipulators for
turning-over and shifting. These
various items of equipment interlink the
various process stations with each other.
the metal is to be deformed by hot rolling, it must first be homogeneously
preheated to a defined temperature.
Ingots, slabs or semis are preheated to the required rolling temperature in the
rolling-mill furnaces (fig. 80.)
the stock passes through the rolling mill, it must frequently be reheated.
furnaces are divided into:
with batch charging;
pits for ingots and slabs;
furnaces for heat treating hot-rolled products;
furnaces for heat treating coils and wire-rod coils.
with continuous charging;
furnaces and continuous pusher-type furnaces for ingots, continuously cast slabs
hearth furnaces and walking-beam furnaces for roughed slabs, ingots,
continuously cast slabs and billets, as well as roller-hearth
furnaces for heat treating hot-rolled sheet and bar-shaped products;
annealing furnaces for heat treating cold-rolled sheet and plate.
trains are normally adapted to the products to be rolled.
Hence, there are very many different types of train.
Depending on the charging temperature of the stock , they are divided
trains are best described according to the rolled products.
Further designations in use are ingot-slab trains, hot-strip trains (for
narrow and medium wide strip), cold-strip trains (for narrow and medium wide
strip) and skin-pass trains.
further aspect in the classification of rolling trains are their different
parts-roughing trains (fig. 81 ) intermediate and finishing trains.
continuous rolling trains of high capacity are characterized by an advanced
degree of mechanization and automation. In
fact, with very little manual work involved they can be operated with only few
spread of continuous casting and efforts aimed at casting small cross-sections
have also impacted on the layout of the rolling mills. Frequently, the first deformation stage is now eliminated.
Also practised on a large scale nowadays
is the rolling of continuously cast products without intermediate
heating, in “one heat”, the direct forming of continuously cast products, as
well as special rolling techniques for substantially transforming the as-cast
finishing line completes the rolling process.
The is where the products are made ready for further treatment or delivery to customers.
and between the various deformation stages, some finishing
processes may additionally be required (eg
descaling ,heat treatment).
most important functions of the finishing shop are:
(shearing to length, slitting and cutting-to-length-line, trimming),
rolling ingots into blooms
rolling ingots into roughed slabs
rolling blooms) into billets
rolling blooms) into sections, rail accessories or sheet pilings
rolling blooms) or billets) into steel bars
rolling billets) into wire rod
wide strip train
rolling roughed slabs) into hot wide strip
wide strip train
rolling hot-rolled wide strip into cold wide strip
rolling slab ingots) or roughed slabs into plates
or continuously cast
(with the help of cluster-mill straightening machines),
protection (eg oiling),
and retrieving (sampling, quality control, dimensional control),
sequence and extent of these jobs will depend on the nature of the product.
stages in hot rolling of steel
steel production is accounting for a steadily increasing share of the production of flats. Particularly on the rise has been the production of hot wide
trend towards greater flexibility in the production and casting of steel and
towards smaller lot sizes his impacted on the dimensions of the rolling trains.
In future, instead of rolling trains with very large outputs, the
tendency will be to build smaller and more adaptable trains.
This applies to both flat and sectional steel products.
In the case of sectional steel mills, there is also a movement towards
rolling trains with a widely diversified, more universal range of products.
and sectional steel products are rolled nowadays with the deformation, the
controlled cooling and the processes within the microstructure all closely
coordinated. In this context one
refers to thermomechanical deformation. The
condition of the steel obtainable in this way is comparable to steel after
normalizing. So, such
thermomechanical rolling allows separate heat treatment stages to be dispensed
Rolling semi-finished products
the wide-spread use of continuously cast materials, the importance of the
rolling semi-finished products has declined sharply.
Rolling semi -finished products is still practised to a certain extent,
especially in Eastern Europe.
been heated to rolling temperature, ingots and slab ingots are rolled into
blooms (for sections) or roughed slabs (for flat products) in single-stand
blooming trains, slabbing trains or
ingot.slab trains. In modern mills,
these trains use a reversing two-high stand.
The blooming -train rolls are grooved, the slabbing -train ones are
plain. The semis rolled out by the
slabbing o blooming train undergo a cleaning process (usually flame scarfing )
to remove any surface defects. Blooms
are rolled into billet trains. Continuous
casting plants produce semis which can be immediately processed in the second
order to accelerate the process chain and save costs and energy, direct charging
is practised nowadays. This means
that the slabs are not completely cooled down after casting but are charged into
the furnaces of the rolling mill n a still hot condition. This process requires precise timing between the steelmarking
plant and the rolling mill, as well as defectless charging material.
Trains for producing sections
in sectional-steel rolling trains are characterized by high flexibility in
product ranges, narrow rolling tolerances and improved web centricity
(eg for I sections), long service -life of the roll grooves, high
efficiency, brief set-up times for production changes or roll changing, and
reduced investment outlay.
further improve the rolling technology, the stands are arranged in an HV
configuration. This means that the rolls in one stand are arranged in a
horizontal direction and in the following stand,
vertically. This technique
eliminates a “twisting” of the rolling strand (fig 82).
rolling mills tend to be laid out in a continuous line, with the rolling stands
positioned in a straight line one after the other. Such rolling mills also operated automatically. Because of
the complex sections, the grooving of the rolls is
This also a reason why a sectional-rolling mill may possibly produce only
one group of product, eg wide flange beams or rails.
and wire rod mills, nowadays normally arranged n a continuous pattern, share
many features in common. Combined
and wire rod mills for alternatively rolling either of these products, are
frequently in use. As the rolls are
grooved, only two-high rolling
stands are employed. Wire rod and
bars are increasingly also being rolled in rolling blocks and special rolling
stands. For improving rolling
quality, bars and wire rod are rolled in a single strand. This means that only one strand
passes through the stand at a given time; in multistrand rolling, several strands are deformed
simultaneously in one rolling stand.
Trains for producing flats
nowadays are primarily produced by two processes:
rolling heavy plate,
rolling hot wide strip.
either case, the preferred configuration is the four-high rolling stand.
Heavy plate, which cannot be coiled because of its thickness, is rolled
in plate mills. Such mills consist
of one or two stands and in exceptional cases can roll plate up to 5.5 m wide.
The stock is reversed through the stands while the roll gap becomes
narrower and narrower. Annealing,
shearing and straightening , in which the plates are uniformly “ironed” , complete the production stages.
Continuous and automated hot wide strip mills of high capacity have
become generally accepted for rolling hot wide strip which is coiled .
Such mills produce strip of high uniformity and surface quality as well
as good dimensional accuracy up to widths of more than 2 m. Hot wide strip mills are not suitable for thickness under 1.5
mm. As a consequence, thinner strip
must undergo a further cold rolling process.
A special problem in hot rolling wide strip is the temperature loss on
the roller table between the roughing and the finishing train.
This problem has been resolved with the aid of a coil box which coils the
transfer bar and largely prevents temperature loss.
Also obtainable by this method are superior surface quality, slighter
dimensional deviations and higher coil weights.
Another substantial technological problem is maintaining sheet thickness
across the entire width of the strip. In
order to influence thickness, the rolls are ground cambered or
S-shaped. The work rolls can
be bonded, based on thickness measurements, and/or shifted horizontally within
Cold forming of steel
are many applications for which the cross-sections, surface quality, dimensional
accuracies, strength properties and dimensions obtainable through hot
rolling , are inadequate. Cold
forming permits a smoother surface with higher dimensional accuracy and, because
of the work hardening , greater strength and any desired small dimensions are
obtainable. A combination of cold
forming and heat treatment permits specified technological properties to be
obtained. Cold forming, because of
the high deformation resistance at ambient temperatures, requires considerably
greater forces compared to hot forming. Prior
to each stage, scale must be carefully removed. This is done mechanically (eg shot blasting ) or chemically
with the aid of various pickling processes.
The most important cold-forming techniques are:
are many other possibilities of cold forming.
Examples are: upsetting, stamping, bending, punching, cold extruding,
deep drawing and bulging by explosive forming.
is cold rolled mainly for producing flat products such as deepdrawing sheet, tin
sheet and stainless sheet. Sectional
steel products and tubes are also cold rolled, albeit to a considerably lesser
extent. The most wide-spread
process is the cold rolling of strip. Single-rolled
sheets are rarely cold rolled nowadays. Strip
is cold rolled on two-high, four-high or cluster mills.
The use of reversing stands enables the strip to be immediately deformed
after each pass by the reversed direction of the rolls.
Cluster reversing stands, which with their small working rolls can
exercise a high pressure on the strip in the roll gap, are used for steels of
a high level of work hardening, eg stainless and electric.
Another possibility is to have four to six four-high rolling stands
mounted in sequence as a so-called cold-rolling tandem
mill. Such trains have the
advantage that the strip uncoiled from the payoff reel can be rolled to ist
desired final thickness in one pass. In
order to eliminate work hardening after cold rolling, heat treatment by
annealing is frequently applied. This
process leads to recrystallization. With
the aid of precisely controlled strip
tension between the stands, a highly developed instrumentation and automation
system, special measuring equipment for determining strip thickness and shape as
well as a variety of control elements for influencing the roll gap, flat
cold-rolled strip with thickness variances of only a
few microns combined with
high surface quality are obtained . Final
cold rerolling (“skin-pass
rolling”) in one or two four-high rolling stands with thickness reductions of
less than 3% improves the deformability of deep-drawing sheets-preventing
stretcher strains by surface work hardening-and guarantees good shape.
Final thicknesses obtainable by cold rolling are in the range of 0.15
mm. The maximum final rolling
speeds of the cold -rolling tandem
mills are up to 2,400 m/min. Final
rolling speeds of about 1,800 m/min are possible during the rerolling stare.
Cold forming of steel
of small dimensions unobtainable by hot rolling
dimensional accuracy than with hot forming
strength properties for a specific application
cold rolling of flat products
cold drawing of steel bars, tubes, wire rod
completion of deformation, steel products frequently do not posses the required
properties, mechanical ones and workability, for example.
Frequently, the material or microstructure conditions required for these
properties are not yet present or achieved.
So, the various grades of steel undergo special heat treatment.
Heat treatment is designed to deliver in each specific case the desired
properties. Alongside chemical
composition, heat treatment is of special importance for the properties of the
steel. Depending on the material
and the application, a wide variety of processes exist. The fundamental ones are:
treatment is normally performed after the essential deformation processes have
been completed. Additional
heat-treatment processes are:
thermochemical treatment, the chemical composition of the steel is selectively
changed by diffusing one or several elements into or out of the surface layer.
This process is designed for arriving at definite properties such as
scale resistance, corrosion resistance, or enhanced wear resistance.
Nitrogen, aluminium, silicon, boron and chromium are the elements mainly
used in these thermochemical processes. Besides
the separately carried out heat-treatment processes, controlled cooling applied
in connection with the deformation processes is of growing importance as it
allows additional downstream heat treatment to be dispensed with.
treatment is a hot-forming process in which both temperature and deformation are
controlled in order to achieve a specific condition of metal and hence certain
properties. Strip is frequently
heat treated in batch-type annealing furnaces (fig. 113).
For this purpose the strip must be coiled.
Continuous annealing installations are already in use for strip
(fig.114). The advantages are the following:
surface and flatness,
homogeneous mechanical strength
strength at lower quantities elements.
with the batch-type variety, continuous annealing plants are costly.
Many different processes have emerged for continuous heat treatment in
rolling mills. They differ
according to the methods used for cooling down (eg, cooling with a mixture of
nitrogen and hydrogen, water spraying and water -cooled rollers) and methods of
obtaining defined atmospheres. Generally,
two annealing and cooling cycles are performed
in the plant. This allows
the strength properties of the strip to be precisely controlled.
Annealing is heating the steel to defined temperatures, maintaining these over a lengthy period, followed by cooling. The lower temperature limit is close to the level at which the steel becomes red hot, about 600°C . Depending on the task and the temperature/time cycles employed, we distinguish between stress-relieving, soft, recrystallization annealing, normalizing and diffusion annealing. All the various annealing processes are designed to transform the material into a more homogeneous and stable in terms of its microstructure and its internal stress as well as to reduce the tensile strength. Such methods of heat treatment are particulary important in the working of steel.
is a technique of heat treatment designed to lead to a hard microstructure as a
result of fundamental changes in the lattice of the material.
Hardening is based on the following principle: above a certain elevated
temperature (austenitizing temperature), the steel changes its microstructure
fundamentally. If this microstructure (austenite) cools down rapidly to
about ambient temperature, what emerges is a different microstructure
(martensite), much harder than the “normal” one.
This hardening process can be preceded by annealing . Combined with
subsequent tempering, hardening results in a broad spectrum of obtainable mechanical properties.
hardening, which follows a different pattern, likewise results in substantial
hardness increases. Characteristic
properties are greater strength resulting from the precipitation of fine
hardening processes affect only the surface layers of the workpiece, eg case
hardening and induction hardening. Such
surface hardening processes produce hard surface layers while maintaining a
surface layers are also obtainable simply by selective diffusion of elements
such as carbon, nitrogen (nitriding) and boron (boriding).
is heating up following a preceding hardening to temperature at which austenite
does not develop but at which the hardness of the steel decreases and its
Quenching and tempering
followed by tempering at relatively high temperatures (up to about 700°C) is
termed quenching and tempering (QT). This
combined method of heat treatment produces good toughness combined with a
generally slight decrease of yield stress compared with tensile strength.
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