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THE FORMING OF STEEL

 The basics of steel rolling, Rolling, Fundamentals, Flat rolling, Section rolling, Rolled steel products , Rolling mill installations , The stages in hot rolling of steel , Cold forming of steel , Thermomechanical Treatment

The 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 required demands.

In 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.

 The many working and downstream processing stages can be classified as follows:

-primary forming,

-forming,

-separating,

-joining,

-coating (surface protection),

-changing the properties of the metal (heat treatment).

The most important stage of primary forming where the molten metal usually obtains its primary shape, is

Casting. This has already been discussed. Forming is the conversion  of a given primary form into an intermediate or final form

Most of the working stages carried out in the steel mills belong to the forming category.

The basics of steel rolling

Deformation

Any 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.

Steel 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”.

The 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.

Processes for applying  corrosion protection coats are likewise largely performed in the steel mills.

Return into the ordered condition of the crystals; mostly, however, these are strained  and   so that this is not easily possible. However,

when heated, the crystals reform anew; this process is termed “recrystallization”.

Decisive 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.

Nowadays, 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.

Rolling

Forming processes are divided into:

-       pressure forming,

-       push pull forming,

-       tensile forming

-       forming by bending,

-       shear forming.

Steel 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 distinguish between:

-       longitudinal rolling,

-       cross rolling, skew rolling.

In 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 longitudinal rolling.

During 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 rolled-steel products.

During 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).

Fundamentals

The 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”, respectively.

The 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 spread).

Among 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 deformation resistance.

Flat rolling

Flat 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 deformation resistance.

Additional 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.

Corresponding 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.

The roll gap is adjusted by bending the work or back up rolls, by cooling down in zones or by additionally shifting the rolls horizontally.

Several 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 decisively improved.

Section rolling

Grooved 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. 

The 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 grooves.

According to the shape of the section and the corresponding production stages, various passes are required. Examples of basic passes are:

-box groove (almost rectangular or square, with rounded-off edges),

-diamond groove (with opening angles between 95 an 120°),

-square groove (diagonals running vertical or parallel to the axis of the roll)

-oval groove (bow-type oval, leading oval or edging groove for rolling round bars),

-Swedish 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 wire),

-round groove (finishing groove for round bars and wire).

Grooves series are used for rolling simple cross-sections in which similar pass forms of decreasing cross section alternate. Examples of groove series are:

-square-diamond-square,

-square-oval-square,

-square-Swedish oval-square,

-round-oval-round.

The 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.

For rolling complex cross-sections, several grooves of differing shape and cross-section are required.

The position at which the groove is divided is called the pass closing; it can be open or closed.

Rolled steel products

We distinguish  between two man groups of rolled steel products

-semi-finished products

-finished products.

Semi-finished products

Semis 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 ):

Square semis

-blooms,

-billets

Rectangular semis

-roughed slabs,

-rectangular billets,

-flat semis (including wide strip for further rolling),

-flat slabs

-sheet bars;

Semis for sections.

Finished products

Finished 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.

Finished rolled steel products are classified according to the shape of their cross-sections as

 -long products,

 -flat products.

Included in these product groups are shapes formed partly by rolling and partly by other forming processes.

These are the tubes/pipes, tyres  or shaped and coated sheet and plate.

Long products

Long products are subdivided into groups  each with differing individual profiles. These include :

-sectionals steel

 (incl.wide-flanged beams),

-steel bars

 (incl.reinforcing  steel),

-wire rod,

-rail accessories

-sheet piling sections.

Sectional steel

Sectional 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.

Steel bars

The 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.

Reinforcing steel

With 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.

Wire  rod

In 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.

Rail  accessories

This category  covers all the parts needed for building railway tracks.

Included are not only the sleepers, clamping plates, fish-plates, rail chairs, etc.

Depending on the weight per metre of the rails and sleepers, a distinction is made between light and heavy rail accessories materials.

Sheet piling sections

These 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

Flats are rectangular in cross-section.

The width is much larger than the thickness.  The surface is mostly smooth but many also be patterned.

Flat products include :

-       wide flat steel,

-       plates and sheets,

-       strip.

Special 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 sheet.

Wide flat steel

Wide 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

Cut-to-length 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:

-heavy plate (3 mm and more thick),

-sheet (0.3 mm thick)

-black sheet and tin sheet (less than 0.5 mm thick)

The 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

Finished strip is directly coiled after it has been rolled.  However, it can also be supplied with trimmed edges or be split from wider strip.

Depending on width, we refer to narrow, medium and wide strip.  In terms of deformation temperature, we distinguish between hot and cold strip.

Hot 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

Pipes 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  tube.

Tyres

Rings, tyres and solid wheels for transportation and technology of mechanism design are also included among the items produced with the aid of special rolling techniques.

Rolling mill installations

Steel 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.

Roll stands 

The roll stands are the central elements of a rolling mill.  They accommodate the rolls that deform the metal.  A normal roll stand consists of:

-roll housings,

-assembly including rolls, chocks and bearings,

-screw-down  mechanism incl. Weight compensation.

Additional facilities and components are:

-overload  safety devices,

-devices for guiding the rolled stock,

-drive systems.

A roll stand has two roll housings which are usually made of cast  steel.  Smaller  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 hydraulically.  An   indicator   allows  the respective roll gap to be  read.  Mechanical, electric or hydraulic overload safety devices protect against possible damage or destruction.

So 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 include:

-shifting manipulators,

-tilting devices,

-entry and exit guides.

D.C. 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 units.

Roll stand types

Roll stands are classified:

-by the number of rolls and/or

-the roll dimensions

The most important distinction criterion is the number of rolls  and their arrangement in the stand :

-two-high stands,

-three-high stands,

-four-high stands,

-six-high stands,

-cluster mills,

-planetary mills,

-rolling blocks,

-special rolling stands.

Each basic arrangement includes additional different designs.  The four-high  and cluster mills use special back-up rolls addition to the thin work rolls.

Cluster 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.

Compact 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 offset 90°).

Exceptional 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.

Forge 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.

In 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.

Rolling-mill rolls

The 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.

Necks 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.

Depending on their function , rolls are classified as

-work rolls and

-back-up rolls.

The 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.

As 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.

Cooling 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 mills

A rolling mill combines all the equipment and components needed for producing hot or cold-rolled products.

Hot-rolling mills

are generally divided into the following zones:

-furnace area (for heat supply prior to deformation),

-rolling train,

-finishing area.

Cold-rolling mills

require no further heat supply and generally include the following areas:

-pickling area,

-rolling train,

-heat treatment,

-finishing area.

Rolling 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.

The 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.

Rolling-mill furnaces

If 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.)

While the stock passes through the rolling mill, it must frequently be reheated.

Rolling-mill furnaces are divided into:

Furnaces with batch charging;

 -soaking pits for ingots and slabs;

-bogie-hearth furnaces for heat treating hot-rolled products;

-bell-type furnaces for heat treating coils and wire-rod coils.

Furnaces with continuous charging;

-pusher-type furnaces and continuous pusher-type furnaces for ingots, continuously cast slabs and billets;

-rotary 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;

-continuous annealing furnaces for heat treating cold-rolled  sheet and plate.

Rolling trains

Rolling 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 into:

-hot-rolling trains,

-cold-rolling trains.

Rolling 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.

A further aspect in the classification of rolling trains are their different parts-roughing trains (fig. 81 ) intermediate and finishing trains.

Present-.day 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 skilled workers.

The 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 structure.

Finishing line

The finishing line completes the rolling process.  The is where the products are made ready for  further treatment or delivery to customers.

Before and between the various deformation stages, some finishing  processes may additionally be required (eg  descaling ,heat treatment).

The most important functions of the finishing shop are:

-cutting (shearing to length, slitting and cutting-to-length-line, trimming),

-straightening (with the help of cluster-mill straightening machines),

-surface protection (eg oiling),

-stacking and retrieving (sampling, quality control, dimensional control),

-sorting, marking,

-collecting, bundling, packing.

The sequence and extent of these jobs will depend on the nature of the product.

The stages in hot rolling of steel

Rolled 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 strip.

The 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.

Flat 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 with.

Rolling semi-finished products

With 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.

Having 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 deformation stage.

In 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.

Rolling finished products

Trains for producing sections

Developments 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.

 To 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).

Sectional-steel 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

complicated.  This also a reason why a sectional-rolling mill may possibly produce only one group of product, eg wide flange beams or rails.

Bar 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

Flats  nowadays are primarily produced by two processes:

-         rolling heavy plate,

-         rolling hot wide strip.

In 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 defined limits.

Cold forming of steel

There 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:

-cold rolling,

-cold drawing.

There are many other possibilities of cold forming.  Examples are: upsetting, stamping, bending, punching, cold extruding, deep drawing and bulging by explosive forming.

Cold rolling

Steel 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.

Thermomechanical Treatment

On 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:

-annealing,

-pendening,

-demeoring,

-quenching and empering.

Heat treatment is normally performed after the essential deformation processes have been completed.  Additional heat-treatment processes are:

-thermochemical treatment,

-thermomechanical treatment,

-special heat-treatment processes.

In 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.

Thermochemical 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:

-reduced cycle times,

-improved surface and flatness,

-more homogeneous mechanical strength

-higher strength at lower quantities elements.

Compared 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

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.

Hardening

Hardening 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.

Age hardening, which follows a different pattern, likewise results in substantial hardness increases.  Characteristic properties are greater strength resulting from the precipitation of fine microstructure components.

Special 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 ductile core.

Hard surface layers are also obtainable simply by selective diffusion of elements such as carbon, nitrogen (nitriding) and boron (boriding).

Tempering

This 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 toughness increases.

Quenching and tempering

Hardening 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.

Artículos/Papers

Juan José Pineda Rodríguez, Cesar Garay Zavala, Luis Carlos Talamantes Arredondo

Descripción de Sistema de Flexionado de Rodillos ( "Roll Bending") para Molinos Laminadores

Manuel Suárez A., Horacio Domínguez L.