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metalworking Metalworking is the process of shaping and reshaping metals to create useful objects, parts, assemblies, and large scale structures. As a term it covers a wide and diverse range of processes, skills, and tools for producing objects on every scal ...
, rolling is a
metal forming Forming, metal forming, is the metalworking process of fashioning metal parts and objects through mechanical deformation; the workpiece is reshaped without adding or removing material, and its mass remains unchanged. Forming operates on the mater ...
process in which
metal A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...
stock is passed through one or more pairs of rolls to reduce the thickness, to make the thickness uniform, and/or to impart a desired mechanical property. The concept is similar to the rolling of dough. Rolling is classified according to the temperature of the metal rolled. If the temperature of the metal is above its recrystallization temperature, then the process is known as hot rolling. If the temperature of the metal is below its recrystallization temperature, the process is known as cold rolling. In terms of usage, hot rolling processes more tonnage than any other manufacturing process, and cold rolling processes the most tonnage out of all
cold working In metallurgy, cold forming or cold working is any metalworking process in which metal is shaped below its recrystallization temperature, usually at the ambient temperature. Such processes are contrasted with hot working techniques like hot r ...
processes... Roll stands holding pairs of rolls are grouped together into rolling mills that can quickly process metal, typically
steel Steel is an alloy made up of iron with added carbon to improve its strength and fracture resistance compared to other forms of iron. Many other elements may be present or added. Stainless steels that are corrosion- and oxidation-resistant ty ...
, into products such as
structural steel Structural steel is a category of steel used for making construction materials in a variety of shapes. Many structural steel shapes take the form of an elongated beam having a profile of a specific cross section. Structural steel shapes, sizes, ...
(
I-beam An I-beam, also known as H-beam (for universal column, UC), w-beam (for "wide flange"), universal beam (UB), rolled steel joist (RSJ), or double-T (especially in Polish language, Polish, Bulgarian language, Bulgarian, Spanish language, Spanish ...
s, angle stock, channel stock),
bar stock Bar stock, also (colloquially) known as blank, slug or billet, is a common form of raw purified metal, used by industry to manufacture metal parts and products. Bar stock is available in a variety of extrusion shapes and lengths. The most common ...
, and
rails Rail or rails may refer to: Rail transport *Rail transport and related matters *Rail (rail transport) or railway lines, the running surface of a railway Arts and media Film * ''Rails'' (film), a 1929 Italian film by Mario Camerini * ''Rail'' ( ...
. Most
steel mill A steel mill or steelworks is an industrial plant for the manufacture of steel. It may be an integrated steel works carrying out all steps of steelmaking from smelting iron ore to rolled product, but may also be a plant where steel semi-finish ...
s have rolling mill divisions that convert the semi-finished casting products into finished products. There are many types of rolling processes, including ''ring rolling'', ''roll bending'', ''roll forming'', ''profile rolling'', and ''controlled rolling''.


Iron and steel

The invention of the rolling mill in Europe may be attributed to
Leonardo da Vinci Leonardo di ser Piero da Vinci (15 April 14522 May 1519) was an Italian polymath of the High Renaissance who was active as a painter, Drawing, draughtsman, engineer, scientist, theorist, sculptor, and architect. While his fame initially res ...
in his drawings. The earliest rolling mills in crude form but the same basic principles were found in Middle East and South Asia as early as 600 BCE. Earliest rolling mills were
slitting mill The slitting mill was a watermill for slitting bars of iron into rods. The rods then were passed to nailers who made the rods into nails, by giving them a point and head. The slitting mill was probably invented near Liège in what is now Belg ...
s, which were introduced from what is now
Belgium Belgium, ; french: Belgique ; german: Belgien officially the Kingdom of Belgium, is a country in Northwestern Europe. The country is bordered by the Netherlands to the north, Germany to the east, Luxembourg to the southeast, France to th ...
to
England England is a country that is part of the United Kingdom. It shares land borders with Wales to its west and Scotland to its north. The Irish Sea lies northwest and the Celtic Sea to the southwest. It is separated from continental Europe b ...
in 1590. These passed flat bars between rolls to form a plate of iron, which was then passed between grooved rolls (slitters) to produce rods of iron. The first experiments at rolling iron for tinplate took place about 1670. In 1697, Major John Hanbury erected a mill at Pontypool to roll 'Pontypool plates' – blackplate. Later this began to be rerolled and tinned to make
tinplate Tinplate consists of sheets of steel coated with a thin layer of tin to impede rusting. Before the advent of cheap milled steel, the backing metal was wrought iron. While once more widely used, the primary use of tinplate now is the manufacture ...
. The earlier production of plate iron in Europe had been in forges, not rolling mills. The slitting mill was adapted to producing hoops (for barrels) and iron with a half-round or other sections by means that were the subject of two patents of c. 1679. Some of the earliest literature on rolling mills can be traced back to the Swedish engineer
Christopher Polhem Christopher Polhammar (18 December 1661 – 30 August 1751) better known as Christopher Polhem (), which he took after his ennoblement in 1716, was a Swedish scientist, inventor and industrialist. He made significant contributions to the econom ...
in his ''Patriotista Testamente'' of 1761, where he mentions rolling mills for both plate and bar iron. He also explains how rolling mills can save on time and labor because a rolling mill can produce 10 to 20 or more bars at the same time. A patent was granted to Thomas Blockley of England in 1759 for the polishing and rolling of metals. Another patent was granted in 1766 to Richard Ford of England for the first tandem mill.. A tandem mill is one in which the metal is rolled in successive stands; Ford's tandem mill was for hot rolling of wire rods.


Other metals

Rolling mills for lead seem to have existed by the late 17th century. Copper and brass were also rolled by the late 18th century.


Modern rolling

Until well into the eighteenth century, rolling mills derived their power from
water wheels A water wheel is a machine for converting the energy of flowing or falling water into useful forms of power, often in a watermill. A water wheel consists of a wheel (usually constructed from wood or metal), with a number of blades or bucket ...
. The first recorded use of a
steam engine A steam engine is a heat engine that performs mechanical work using steam as its working fluid. The steam engine uses the force produced by steam pressure to push a piston back and forth inside a cylinder. This pushing force can be trans ...
directly driving a mill is attributed to John Wilkinson's Bradley Works where, in 1786, a Boulton and Watt engine was coupled to a slitting and rolling mill. The use of steam engines considerably enhanced the production capabilities of the mills, until this form of power was displaced by
electric motors An electric motor is an electrical machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a wire winding to generate forc ...
soon after 1900. Modern rolling practice can be attributed to the pioneering efforts of
Henry Cort Henry Cort (c. 1740 – 23 May 1800) was an English ironware producer although formerly a Navy pay agent. During the Industrial Revolution in England, Cort began refining iron from pig iron to wrought iron (or bar iron) using innovative producti ...
of Funtley Iron Mills, near Fareham in
Hampshire Hampshire (, ; abbreviated to Hants) is a ceremonial county, ceremonial and non-metropolitan county, non-metropolitan counties of England, county in western South East England on the coast of the English Channel. Home to two major English citi ...
, England. In 1783, a patent number was issued to Henry Cort for his use of grooved rolls for rolling iron bars. With this new design, mills were able to produce 15 times more output per day than with a hammer.. Although Cort was not the first to use grooved rolls, he was the first to combine the use of many of the best features of various ironmaking and shaping processes known at the time. Thus modern writers have called him "father of modern rolling". The first rail rolling mill was established by John Birkenshaw at
Bedlington Ironworks Bedlington Ironworks, in Blyth Dene, Northumberland, England, operated between 1736 and 1867. It is most remembered as the place where wrought iron rails were invented by John Birkinshaw in 1820, which triggered the railway age, with their fir ...
in
Northumberland Northumberland () is a county in Northern England, one of two counties in England which border with Scotland. Notable landmarks in the county include Alnwick Castle, Bamburgh Castle, Hadrian's Wall and Hexham Abbey. It is bordered by land on ...
, England, in 1820, where he produced fish-bellied wrought iron rails in lengths of 15 to 18 feet. With the advancement of technology in rolling mills, the size of rolling mills grew rapidly along with the size of the products being rolled. One example of this was at
The Great Exhibition The Great Exhibition of the Works of Industry of All Nations, also known as the Great Exhibition or the Crystal Palace Exhibition (in reference to the temporary structure in which it was held), was an international exhibition which took pl ...
in London in 1851, where a plate 20 feet long, 3 feet wide, and 7/16 of an inch thick, and weighing 1,125 pounds, was exhibited by the
Consett Iron Company The Consett Iron Company Ltd was an industrial business based in the Consett area of County Durham in the United Kingdom. The company owned coal mines and limestone quarries, and manufactured iron and steel. It was registered on 4 April 1864 a ...
. Further evolution of the rolling mill came with the introduction of three-high mills in 1853 used for rolling heavy sections.


Hot and cold rolling


Hot rolling

Hot rolling is a
metalworking Metalworking is the process of shaping and reshaping metals to create useful objects, parts, assemblies, and large scale structures. As a term it covers a wide and diverse range of processes, skills, and tools for producing objects on every scal ...
process that occurs above the
recrystallization temperature In materials science, recrystallization is a process by which deformed grains are replaced by a new set of defect-free grains that nucleate and grow until the original grains have been entirely consumed. Recrystallization is usually accompanied b ...
of the material. After the grains deform during processing, they recrystallize, which maintains an
equiaxed Equiaxed crystals are crystals that have axes Axes, plural of '' axe'' and of '' axis'', may refer to * ''Axes'' (album), a 2005 rock album by the British band Electrelane * a possibly still empty plot (graphics) See also * Axess (disambiguation ...
microstructure Microstructure is the very small scale structure of a material, defined as the structure of a prepared surface of material as revealed by an optical microscope above 25× magnification. The microstructure of a material (such as metals, polymers ...
and prevents the metal from
work hardening In materials science, work hardening, also known as strain hardening, is the strengthening of a metal or polymer by plastic deformation. Work hardening may be desirable, undesirable, or inconsequential, depending on the context. This strengt ...
. The starting material is usually large pieces of metal, like semi-finished casting products, such as ingots, slabs, blooms, and
billets A billet is a living-quarters to which a soldier is assigned to sleep. Historically, a billet was a private dwelling that was required to accept the soldier. Soldiers are generally billeted in barracks or garrisons when not on combat duty, al ...
. File:Ingot.jpg, Ingot lifted from soaking pit File:Slabs stack.jpg, Photo of cold slabs File:Steel blooms Hayange.jpg, Steel blooms on rail wagon File:Celsa Steel Works (30910245794).jpg, Billets on rail wagon If these products came from a
continuous casting Continuous casting, also called strand casting, is the process whereby molten metal is solidified into a "semifinished" billet, bloom, or slab for subsequent rolling in the finishing mills. Prior to the introduction of continuous casting in the ...
operation, the products are usually fed directly into the rolling mills at the proper temperature. In smaller operations, the material starts at room temperature and must be heated. This is done in a gas- or oil-fired
soaking pit Soaking may refer to: * Steeping * Bathing * Soaking (sexual practice) * A preparatory operation for tanning in the production of leather * A discontinued practice to put out a runner in baseball by hitting the runner with a ball thrown by a fie ...
for larger workpieces; for smaller workpieces,
induction heating Induction heating is the process of heating electrically conductive materials, namely metals or semi-conductors, by electromagnetic induction, through heat transfer passing through an induction coil that creates an electromagnetic field within th ...
is used. As the material is worked, the temperature must be monitored to make sure it remains above the recrystallization temperature. To maintain a
safety factor In engineering, a factor of safety (FoS), also known as (and used interchangeably with) safety factor (SF), expresses how much stronger a system is than it needs to be for an intended load. Safety factors are often calculated using detailed analy ...
a ''finishing temperature'' is defined above the recrystallization temperature; this is usually 50 to 100 °C (90 to 180 °F) above the recrystallization temperature. If the temperature does drop below this temperature the material must be re-heated prior to additional hot rolling.. Hot-rolled metals generally have little directionality in their mechanical properties or deformation-induced
residual stress In materials science and solid mechanics, residual stresses are Stress (physics), stresses that remain in a solid material after the original cause of the stresses has been removed. Residual stress may be desirable or undesirable. For example, l ...
es. However, in certain instances non-metallic inclusions will impart some directionality and workpieces less than thick often have some directional properties. Non-uniform cooling will induce a lot of residual stresses, which usually occurs in shapes that have a non-uniform cross-section, such as
I-beam An I-beam, also known as H-beam (for universal column, UC), w-beam (for "wide flange"), universal beam (UB), rolled steel joist (RSJ), or double-T (especially in Polish language, Polish, Bulgarian language, Bulgarian, Spanish language, Spanish ...
s. While the finished product is of good quality, the surface is covered in
mill scale Mill scale, often shortened to just scale, is the flaky surface of hot rolled steel, consisting of the mixed iron oxides iron(II) oxide (FeO), iron(III) oxide (), and iron(II,III) oxide (, magnetite). Mill scale is formed on the outer surfaces ...
, which is an
oxide An oxide () is a chemical compound that contains at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion of oxygen, an O2– (molecular) ion. with oxygen in the oxidation state of −2. Most of the E ...
that forms at high temperatures. It is usually removed via
pickling Pickling is the process of preserving or extending the shelf life of food by either anaerobic fermentation in brine or immersion in vinegar. The pickling procedure typically affects the food's texture and flavor. The resulting food is cal ...
or the smooth clean surface (SCS) process, which reveals a smooth surface. Dimensional tolerances are usually 2 to 5% of the overall dimension.. Hot-rolled mild steel seems to have a wider tolerance for the level of included carbon than does cold-rolled steel, and is, therefore, more difficult for a blacksmith to use. Also for similar metals, hot-rolled products seem to be less costly than cold-rolled ones. Hot rolling is used mainly to produce
sheet metal Sheet metal is metal formed into thin, flat pieces, usually by an industrial process. Sheet metal is one of the fundamental forms used in metalworking, and it can be cut and bent into a variety of shapes. Thicknesses can vary significantly; ex ...
or simple cross-sections, such as
rail tracks A railway track (British English and UIC terminology) or railroad track (American English), also known as permanent way or simply track, is the structure on a railway or railroad consisting of the rails, fasteners, railroad ties (sleepers, ...
. Other typical uses for hot-rolled metal: *Truck frames *Automotive clutch plates, wheels and wheel rims *Pipes and tubes *Water heaters *Agricultural equipment *Strappings *Stampings *Compressor shells *Metal buildings *Railroad hopper cars and railcar components *Doors and shelving *Discs *Guard rails for streets and highways


Shape rolling design

Rolling mills In metalworking, rolling is a metal forming process in which metal stock is passed through one or more pairs of rolls to reduce the thickness, to make the thickness uniform, and/or to impart a desired mechanical property. The concept is simil ...
are often divided into roughing, intermediate and finishing rolling cages. During shape rolling, an initial billet (round or square) with edge of diameter typically ranging between 100 and 140 mm is continuously deformed to produce a certain finished product with smaller cross section dimension and geometry. Starting from a given billet, different sequences can be adopted to produce a certain final product. However, since each rolling mill is significantly expensive (up to 2 million euros), a typical requirement is to reduce the number of rolling passes. Different approaches have been achieved, including empirical knowledge, employment of numerical models, and Artificial Intelligence techniques. Lambiase et al. validated a finite element model (FE) for predicting the final shape of a rolled bar in round-flat pass. One of the major concerns when designing rolling mills is to reduce the number of passes. A possible solution to such requirements is the slit pass, also called split pass, which divides an incoming bar in two or more subparts, thus virtually increasing the cross section reduction ratio per pass as reported by Lambiase. Another solution for reducing the number of passes in rolling mills is the employment of automated systems for Roll Pass Design as that proposed by Lambiase and Langella. subsequently, Lambiase further developed an Automated System based on
Artificial Intelligence Artificial intelligence (AI) is intelligence—perceiving, synthesizing, and inferring information—demonstrated by machines, as opposed to intelligence displayed by animals and humans. Example tasks in which this is done include speech re ...
and particularly an integrated system including an inferential engine based on
Genetic Algorithms In computer science and operations research, a genetic algorithm (GA) is a metaheuristic inspired by the process of natural selection that belongs to the larger class of evolutionary algorithms (EA). Genetic algorithms are commonly used to gene ...
a knowledge database based on an
Artificial Neural Network Artificial neural networks (ANNs), usually simply called neural networks (NNs) or neural nets, are computing systems inspired by the biological neural networks that constitute animal brains. An ANN is based on a collection of connected unit ...
trained by a parametric Finite element model and to optimize and automatically design rolling mills.


Cold rolling

Cold rolling occurs with the metal below its recrystallization temperature (usually at room temperature), which increases the
strength Strength may refer to: Physical strength *Physical strength, as in people or animals * Hysterical strength, extreme strength occurring when people are in life-and-death situations *Superhuman strength, great physical strength far above human c ...
via
strain hardening In materials science, work hardening, also known as strain hardening, is the strengthening of a metal or polymer by plastic deformation. Work hardening may be desirable, undesirable, or inconsequential, depending on the context. This strength ...
up to 20%. It also improves the surface finish and holds tighter
tolerances Engineering tolerance is the permissible limit or limits of variation in: # a physical dimension; # a measured value or physical property of a material, manufacturing, manufactured object, system, or service; # other measured values (such as t ...
. Commonly cold-rolled products include sheets, strips, bars, and rods; these products are usually smaller than the same products that are hot rolled. Because of the smaller size of the workpieces and their greater strength, as compared to hot rolled stock, four-high or cluster mills are used. Cold rolling cannot reduce the thickness of a workpiece as much as hot rolling in a single pass. Cold-rolled sheets and strips come in various conditions: ''full-hard'', ''half-hard'', ''quarter-hard'', and ''skin-rolled''. Full-hard rolling reduces the thickness by 50%, while the others involve less of a reduction. Cold rolled steel is then annealed to induce ductility in the cold rolled steel which is simply known as a ''Cold Rolled and Close Annealed''. Skin-rolling, also known as a ''skin-pass'', involves the least amount of reduction: 0.5–1%. It is used to produce a smooth surface, a uniform thickness, and reduce the
yield point In materials science and engineering, the yield point is the point on a stress-strain curve that indicates the limit of elastic behavior and the beginning of plastic behavior. Below the yield point, a material will deform elastically and wi ...
phenomenon (by preventing
Lüders band Lüders bands, is type of slip bands in metals or stretcher-strain marks which are formed due to localized bands of plastic deformation in metals experiencing tensile stresses, common to low-carbon steels and certain Al-Mg alloys. First reported b ...
s from forming in later processing). It locks dislocations at the surface and thereby reduces the possibility of formation of Lüders bands. To avoid the formation of Lüders bands it is necessary to create substantial density of unpinned dislocations in ferrite matrix. It is also used to break up the spangles in galvanized steel. Skin-rolled stock is usually used in subsequent cold-working processes where good ductility is required. Other shapes can be cold-rolled if the cross-section is relatively uniform and the transverse dimension is relatively small. Cold rolling shapes requires a series of shaping operations, usually along the lines of sizing, breakdown, roughing, semi-roughing, semi-finishing, and finishing. If processed by a blacksmith, the smoother, more consistent, and lower levels of carbon encapsulated in the steel makes it easier to process, but at the cost of being more expensive. Typical uses for cold-rolled steel include metal furniture, desks, filing cabinets, tables, chairs, motorcycle exhaust pipes, computer cabinets and hardware, home appliances and components, shelving, lighting fixtures, hinges, tubing, steel drums, lawn mowers, electronic cabinetry, water heaters, metal containers, fan blades, frying pans, wall and ceiling mount kits, and a variety of construction-related products.


Processes


Roll bending

Roll bending produces a cylindrical shaped product from plate or steel metals .


Roll forming

Roll forming, roll bending or plate rolling is a continuous bending operation in which a long strip of metal (typically coiled steel) is passed through consecutive sets of rolls, or stands, each performing only an incremental part of the bend, until the desired cross-section profile is obtained. Roll forming is ideal for producing parts with long lengths or in large quantities. There are three main processes: 4 rollers, 3 rollers and 2 rollers, each of which has as different advantages according to the desired specifications of the output plate.


Flat rolling

Flat rolling is the most basic form of rolling with the starting and ending material having a rectangular cross-section. The material is fed in between two ''rollers'', called ''working rolls'', that rotate in opposite directions. The gap between the two rolls is less than the thickness of the starting material, which causes it to
deform Deformation can refer to: * Deformation (engineering), changes in an object's shape or form due to the application of a force or forces. ** Deformation (physics), such changes considered and analyzed as displacements of continuum bodies. * De ...
. The decrease in material thickness causes the material to elongate. The
friction Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction: *Dry friction is a force that opposes the relative lateral motion of t ...
at the interface between the material and the rolls causes the material to be pushed through. The amount of deformation possible in a single pass is limited by the friction between the rolls; if the change in thickness is too great the rolls just slip over the material and do not draw it in. The final product is either sheet or plate, with the former being less than thick and the latter greater than; however, heavy plates tend to be formed using a
press Press may refer to: Media * Print media or news media, commonly called "the press" * Printing press, commonly called "the press" * Press (newspaper), a list of newspapers * Press TV, an Iranian television network People * Press (surname), a fam ...
, which is termed ''forging'', rather than rolling. Often the rolls are heated to assist in the workability of the metal. Lubrication is often used to keep the workpiece from sticking to the rolls. To fine-tune the process, the speed of the rolls and the temperature of the rollers are adjusted. For thin sheet metal with a thickness less than , the rolling is done in a '' cluster mill'' because the small thickness requires a small diameter rolls. To reduce the need for small rolls ''pack rolling'' is used, which rolls multiple sheets together to increase the effective starting thickness. As the foil sheets come through the rollers, they are trimmed and slitted with circular or razor-like
knives A knife ( : knives; from Old Norse 'knife, dirk') is a tool or weapon with a cutting edge or blade, usually attached to a handle or hilt. One of the earliest tools used by humanity, knives appeared at least 2.5 million years ago, as evidenced ...
. Trimming refers to the edges of the foil, while slitting involves cutting it into several sheets.
Aluminum foil Aluminium foil (or aluminum foil in North American English; often informally called tin foil) is aluminium prepared in thin metal leaves with a thickness less than ; thinner gauges down to are also commonly used. Standard household foil is typ ...
is the most commonly produced product via pack rolling. This is evident from the two different surface finishes; the shiny side is on the roll side and the dull side is against the other sheet of foil.


Ring rolling

Ring rolling is a specialized type of hot rolling that increases the diameter of a ring. The starting material is a thick-walled ring. This workpiece is placed between two rolls, an inner ''idler roll'' and a ''driven roll'', which presses the ring from the outside. As the rolling occurs the wall thickness decreases as the diameter increases. The rolls may be shaped to form various cross-sectional shapes. The resulting grain structure is circumferential, which gives better mechanical properties. Diameters can be as large as and face heights as tall as . Common applications include railway tyres, bearings,
gear A gear is a rotating circular machine part having cut teeth or, in the case of a cogwheel or gearwheel, inserted teeth (called ''cogs''), which mesh with another (compatible) toothed part to transmit (convert) torque and speed. The basic ...
s,
rocket A rocket (from it, rocchetto, , bobbin/spool) is a vehicle that uses jet propulsion to accelerate without using the surrounding air. A rocket engine produces thrust by reaction to exhaust expelled at high speed. Rocket engines work entirely fr ...
s,
turbine A turbine ( or ) (from the Greek , ''tyrbē'', or Latin ''turbo'', meaning vortex) is a rotary mechanical device that extracts energy from a fluid flow and converts it into useful work. The work produced by a turbine can be used for generating ...
s,
airplane An airplane or aeroplane (informally plane) is a fixed-wing aircraft that is propelled forward by thrust from a jet engine, propeller, or rocket engine. Airplanes come in a variety of sizes, shapes, and wing configurations. The broad spe ...
s,
pipes Pipe(s), PIPE(S) or piping may refer to: Objects * Pipe (fluid conveyance), a hollow cylinder following certain dimension rules ** Piping, the use of pipes in industry * Smoking pipe ** Tobacco pipe * Half-pipe and quarter pipe, semi-circula ...
, and pressure vessels..


Structural shape rolling


Controlled rolling

''Controlled rolling'' is a type of thermomechanical processing which integrates controlled deformation and
heat treating Heat treating (or heat treatment) is a group of industrial, thermal and metalworking processes used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are also ...
. The heat which brings the workpiece above the recrystallization temperature is also used to perform the heat treatments so that any subsequent heat treating is unnecessary. Types of heat treatments include the production of a fine grain structure; controlling the nature, size, and distribution of various transformation products (such as ferrite,
austenite Austenite, also known as gamma-phase iron (γ-Fe), is a metallic, non-magnetic allotrope of iron or a solid solution of iron with an alloying element. In plain-carbon steel, austenite exists above the critical eutectoid temperature of 1000 K ...
,
pearlite Pearlite is a two-phased, lamellar (or layered) structure composed of alternating layers of ferrite (87.5 wt%) and cementite (12.5 wt%) that occurs in some steels and cast irons. During slow cooling of an iron-carbon alloy, pearlite forms ...
,
bainite Bainite is a plate-like microstructure that forms in steels at temperatures of 125–550 °C (depending on alloy content). First described by E. S. Davenport and Edgar Bain, it is one of the products that may form when austenite (the face-c ...
, and
martensite Martensite is a very hard form of steel crystalline structure. It is named after German metallurgist Adolf Martens. By analogy the term can also refer to any crystal structure that is formed by diffusionless transformation. Properties M ...
in steel); inducing
precipitation hardening Precipitation hardening, also called age hardening or particle hardening, is a heat treatment technique used to increase the yield strength of malleable materials, including most structural alloys of aluminium, magnesium, nickel, titanium, and ...
; and, controlling the
toughness In materials science and metallurgy, toughness is the ability of a material to absorb energy and plastically deform without fracturing.


Forge rolling

Forge rolling is a longitudinal rolling process to reduce the cross-sectional area of heated bars or billets by leading them between two contrary rotating roll segments. The process is mainly used to provide optimized material distribution for subsequent die forging processes. Owing to this a better material utilization, lower process forces and better surface quality of parts can be achieved in die forging processes. Basically any forgeable metal can also be forge-rolled. Forge rolling is mainly used to preform long-scaled billets through targeted mass distribution for parts such as crankshafts, connection rods, steering knuckles and vehicle axles. Narrowest manufacturing tolerances can only partially be achieved by forge rolling. This is the main reason why forge rolling is rarely used for finishing, but mainly for preforming. Characteristics of forge rolling: * high productivity and high material utilization * good surface quality of forge-rolled workpieces * extended tool life-time * small tools and low tool costs * improved mechanical properties due to optimized grain flow compared to exclusively die forged workpieces


Mills

A ''rolling mill'', also known as a ''reduction mill'' or ''mill'', has a common construction independent of the specific type of rolling being performed:. *Work rolls *Backup rolls – are intended to provide rigid support required by the working rolls to prevent bending under the rolling load *Rolling balance system – to ensure that the upper work and back up rolls are maintained in proper position relative to lower rolls *Roll changing devices – use of an overhead crane and a unit designed to attach to the neck of the roll to be removed from or inserted into the mill. *Mill protection devices – to ensure that forces applied to the backup roll chocks are not of such a magnitude to fracture the roll necks or damage the mill housing *Roll cooling and lubrication systems *Pinions – gears to divide power between the two spindles, rotating them at the same speed but in different directions *Gearing – to establish desired rolling speed *Drive motors – rolling narrow foil product to thousands of horsepower *Electrical controls – constant and variable voltages applied to the motors *Coilers and uncoilers – to unroll and roll up coils of metal Slabs are the feed material for hot strip mills or plate mills and blooms are rolled to billets in a billet mill or large sections in a structural mill. The output from a strip mill is coiled and, subsequently, used as the feed for a cold rolling mill or used directly by fabricators. Billets, for re-rolling, are subsequently rolled in either a merchant, bar or rod mill. Merchant or bar mills produce a variety of shaped products such as angles, channels, beams, rounds (long or coiled) and hexagons.


Configurations

Mills are designed in different types of configurations, with the most basic being a ''two-high non-reversing'', which means there are two rolls that only turn in one direction. The ''two-high reversing'' mill has rolls that can rotate in both directions, but the disadvantage is that the rolls must be stopped, reversed, and then brought back up to rolling speed between each pass. To resolve this, the ''three-high'' mill was invented, which uses three rolls that rotate in one direction; the metal is fed through two of the rolls and then returned through the other pair. The disadvantage to this system is the workpiece must be lifted and lowered using an elevator. All of these mills are usually used for primary rolling and the roll diameters range from . To minimize the roll diameter a ''four-high'' or ''
cluster may refer to: Science and technology Astronomy * Cluster (spacecraft), constellation of four European Space Agency spacecraft * Asteroid cluster, a small asteroid family * Cluster II (spacecraft), a European Space Agency mission to study t ...
'' mill is used. A small roll diameter is advantageous because less roll is in contact with the material, which results in a lower force and power requirement. The problem with a small roll is a reduction of stiffness, which is overcome using ''backup rolls''. These backup rolls are larger and contact the back side of the smaller rolls. A four-high mill has four rolls, two small and two large. A cluster mill has more than four rolls, usually in three tiers. These types of mills are commonly used to hot roll wide plates, most cold rolling applications, and to roll foils. Historically mills were classified by the product produced: *
Blooming Bloom or blooming may refer to: Science and technology Biology * Bloom, one or more flowers on a flowering plant * Algal bloom, a rapid increase or accumulation in the population of algae in an aquatic system * Jellyfish bloom, a collective n ...
, cogging and slabbing mills, being the preparatory mills to rolling finished
rails Rail or rails may refer to: Rail transport *Rail transport and related matters *Rail (rail transport) or railway lines, the running surface of a railway Arts and media Film * ''Rails'' (film), a 1929 Italian film by Mario Camerini * ''Rail'' ( ...
, shapes or plates, respectively. If reversing, they are from 34 to 48 inches in diameter, and if three-high, from 28 to 42 inches in diameter. *
Billet mill Semi-finished casting products are intermediate castings produced in a steel mill that need further processing before being finished goods. There are four types: ''ingots'', ''blooms'', ''billets'', and ''slabs''. Ingot Ingots are large rough ...
s, three-high, rolls from 24 to 32 inches in diameter, used for the further reduction of blooms down to 1.5x1.5-inch billets, being the nubpreparatory mills for the bar and rod * Beam mills, three-high, rolls from 28 to 36 inches in diameter, for the production of heavy beams and channels 12 inches and over. * Rail mills with rolls from 26 to 40 inches in diameter. * Shape mills with rolls from 20 to 26 inches in diameter, for smaller sizes of beams and channels and other structural shapes. * Merchant bar mills with rolls from 16 to 20 inches in diameter. * Small merchant bar mills with finishing rolls from 8 to 16 inches in diameter, generally arranged with a larger size roughing stand. * Rod and
wire Overhead power cabling. The conductor consists of seven strands of steel (centre, high tensile strength), surrounded by four outer layers of aluminium (high conductivity). Sample diameter 40 mm A wire is a flexible strand of metal. Wire is c ...
mills with finishing rolls from 8 to 12 inches in diameter, always arranged with larger size roughing stands. * Hoop and cotton tie mills, similar to small merchant bar mills. * Armour plate mills with rolls from 44 to 50 inches in diameter and 140 to 180-inch body. * Plate mills with rolls from 28 to 44 inches in diameter. * Sheet mills with rolls from 20 to 32 inches in diameter. * Universal mills for the production of square-edged or so-called universal plates and various wide flanged shapes by a system of vertical and horizontal rolls.


Tandem mill

A tandem mill is a special type of modern rolling mill where rolling is done in one pass. In a traditional rolling mill rolling is done in several passes, but in tandem mill there are several ''stands'' (>=2 stands) and reductions take place successively. The number of stands ranges from 2 to 18. Tandem mills can be either of hot or cold rolling mill types. Cold rolling mills may be further divided into continuous or batch processing. A continuous mill has a looping tower which allows the mill to continue rolling slowly the strip in the tower, while a strip welder joins the tail of the current coil to the head of the next coil. At the exit end of the mill there is normally a flying shear (to cut the strip at or near the weld) followed by two coilers; one being unloaded while the other winds on the current coil. Looping towers are also used in other places; such as continuous annealing lines and continuous electrolytic tinning and continuous galvanising lines.


Defects


Thickness changes along length

In hot rolling, if the temperature of the workpiece is not uniform the flow of the material will occur more in the warmer parts and less in the cooler. If the temperature difference is great enough cracking and tearing can occur. The cooler sections are, among other things, a result of the supports in the re-heat furnace. When cold rolling, virtually all of the strip thickness variation is the result of the eccentricity and out-of-roundness of the Back-up Rolls from about Stand 3 of the Hot Strip Mill through to the Finished Product. The Back-up Roll eccentricity can be up to 100 μm in magnitude per stack. The eccentricity can be measured off-line by plotting the force variation against time with the Mill on creep, no strip present, and the Mill Stand below face. A modified Fourier analysis was employed by the 5 Stand Cold Mill at Bluescope Steel, Port Kembla from 1986 until that Cold Mill ceased production in 2009. Within each coil, the exit thickness deviation times 10 for every meter of strip was stored in a file. This file was analyzed separately for each frequency/wavelength from 5 m to 60 m in steps of 0.1 m. To improve the accuracy, care was taken to use a full multiple of each wavelength (100*). The calculate amplitudes were plotted against the wavelength, so that the spikes could be compared to the expected wavelengths created by the Backup Rolls of each Stand. If a Mill Stand is fitted with Hydraulic Pistons in series with, or instead of the electrically driven Mechanical Screws, then it is possible to eliminate the effect of that Stands Back-up Roll eccentricity. While rolling, the eccentricity of each Back-up Roll is determined by sampling the roll force and assigning it to the corresponding portion of each Back-up Roll’s rotational position. These recordings are then used to operate the Hydraulic Piston so as to neutralize the eccentricities.


Flatness and shape

In a flat metal workpiece, the flatness is a descriptive attribute characterizing the extent of the geometric deviation from a reference plane. The deviation from complete flatness is the direct result of the workpiece relaxation after hot or cold rolling, due to the internal stress pattern caused by the non-uniform transversal compressive action of the rolls and the uneven geometrical properties of the entry material. The transverse distribution of differential strain/elongation-induced stress with respect to the material's average applied stress is commonly referenced to as shape. Due to the strict relationship between shape and flatness, these terms can be used in an interchangeable manner. In the case of metal strips and sheets, the flatness reflects the differential fiber elongation across the width of the workpiece. This property must be subject to an accurate feedback-based control in order to guarantee the machinability of the metal sheets in the final transformation processes. Some technological details about the feedback control of flatness are given in.


Profile

Profile is made up of the measurements of crown and wedge. Crown is the thickness in the center as compared to the average thickness at the edges of the workpiece. Wedge is a measure of the thickness at one edge as opposed to the other edge. Both may be expressed as absolute measurements or as relative measurements. For instance, one could have 2 mil of crown (the center of the workpiece is 2 mil thicker than the edges), or one could have 2% crown (the center of the workpiece is 2% thicker than the edges). It is typically desirable to have some crown in the workpiece as this will cause the workpiece to tend to pull to the center of the mill, and thus will run with higher stability.


Flatness

Maintaining a uniform gap between the rolls is difficult because the rolls deflect under the load required to deform the workpiece. The deflection causes the workpiece to be thinner on the edges and thicker in the middle. This can be overcome by using a crowned roller (parabolic crown), however the crowned roller will only compensate for one set of conditions, specifically the material, temperature, and amount of deformation. Other methods of compensating for roll deformation include continual varying crown (CVC), pair cross rolling, and work roll bending. CVC was developed by SMS-Siemag AG and involves grinding a third order polynomial curve into the work rolls and then shifting the work rolls laterally, equally, and opposite to each other. The effect is that the rolls will have a gap between them that is parabolic in shape, and will vary with lateral shift, thus allowing for control of the crown of the rolls dynamically. Pair cross rolling involves using either flat or parabolically crowned rolls, but shifting the ends at an angle so that the gap between the edges of the rolls will increase or decrease, thus allowing for dynamic crown control. Work roll bending involves using hydraulic cylinders at the ends of the rolls to counteract roll deflection. Another way to overcome deflection issues is by decreasing the load on the rolls, which can be done by applying a longitudinal force; this is essentially
drawing Drawing is a form of visual art in which an artist uses instruments to mark paper or other two-dimensional surface. Drawing instruments include graphite pencils, pen and ink, various kinds of paints, inked brushes, colored pencils, crayons, ...
. Other method of decreasing roll deflection include increasing the
elastic modulus An elastic modulus (also known as modulus of elasticity) is the unit of measurement of an object's or substance's resistance to being deformed elastically (i.e., non-permanently) when a stress is applied to it. The elastic modulus of an object is ...
of the roll material and adding back-up supports to the rolls. The different classifications for flatness defects are: *Symmetrical edge wave - the edges on both sides of the workpiece are "wavy" due to the material at the edges being longer than the material in the center. *Asymmetrical edge wave - one edge is "wavy" due to the material at one side being longer than the other side. *Center buckle - The center of the strip is "wavy" due to the strip in the center being longer than the strip at the edges. *Quarter buckle - This is a rare defect where the fibers are elongated in the quarter regions (the portion of the strip between the center and the edge). This is normally attributed to using excessive roll bending force since the bending force may not compensate for the roll deflection across the entire length of the roll. It is important to note that one could have a flatness defect even with the workpiece having the same thickness across the width. Also, one could have fairly high crown or wedge, but still produce material that is flat. In order to produce flat material, the material must be reduced by the same percentage across the width. This is important because mass flow of the material must be preserved, and the more a material is reduced, the more it is elongated. If a material is elongated in the same manner across the width, then the flatness coming into the mill will be preserved at the exit of the mill.


Draught

The difference between the thickness of initial and rolled metal piece is called Draught. Thus if t_i is initial thickness and t_f is final thickness, then the draught is given by :d = t_i - t_f The maximum draught that can be achieved via rollers of radius with coefficient of static friction between the roller and the metal surface is given by :d_ = f^2R This is the case when the frictional force on the metal from inlet contact matches the negative force from the exit contact.


Surface defect types

There are six types of surface defects: ;Lap : This type of defect occurs when a corner or fin is folded over and rolled but not welded into the metal. They appear as seams across the surface of the metal. ;Mill-shearing : These defects occur as a feather-like lap. ;Rolled-in scale : This occurs when
mill scale Mill scale, often shortened to just scale, is the flaky surface of hot rolled steel, consisting of the mixed iron oxides iron(II) oxide (FeO), iron(III) oxide (), and iron(II,III) oxide (, magnetite). Mill scale is formed on the outer surfaces ...
is rolled into metal. ;Scabs : These are long patches of loose metal that have been rolled into the surface of the metal. ;Seams : They are open, broken lines that run along the length of the metal and caused by the presence of scale as well as due to pass roughness of Roughing mill. ;Slivers : Prominent surface ruptures.


Surface defect remediation

Many surface defects can be scarfed off the surface of semi-finished rolled products before further rolling. Methods of scarfing have included hand-chipping with chisels (18th and 19th centuries); powered chipping and grinding with air chisels and grinders; burning with an
oxy-fuel Oxy-fuel combustion is the process of burning a fuel using pure oxygen, or a mixture of oxygen and recirculated flue gas, instead of air. Since the nitrogen component of air is not heated, fuel consumption is reduced, and higher flame temperatures ...
torch, whose gas pressure blows away the metal or slag melted by the flame; and laser scarfing.


See also

* Bernard Lauth, invented and patented the process for cold rolling of iron *
John B. Tytus John Butler Tytus, Jr. (1875-1944) was the inventor of the first practical wide-strip continuous rolling process for manufacturing steel. This process greatly reduced the cost of manufacturing steel, and was first implemented in a new Armco plant in ...
, inventor of the first practical wide-strip continuous rolling process for manufacturing steel * Tadeusz Sendzimir, whose name has been given to revolutionary methods of processing steel and metals *
Electron beam texturing Electron beam texturing (EBT) is a technology used to apply roughness to the surface of rolling mill cylinders by impinging the surface of these cylinders with a modulated electron beam Cathode rays or electron beam (e-beam) are streams of e ...
, used to apply roughness to the surface of rolling mill cylinders *
Drawer slides roll forming machine A drawer slide roll forming machine is a cold roll forming machine used to manufacture drawer slides. They have similarities with roofing roll formed products, but require a higher performance and skills in profile forming. These machines are al ...
* Calender *
Roll bender A roll bender is a mechanical jig having three rollers used to bend a metal bar into a circular arc. The rollers freely rotate about three parallel axes, which are arranged with uniform horizontal spacing. Two outer rollers, usually immobile, c ...


Notes


Bibliography

* * * * Doege, E.; Behrens, B.-A.: ''Handbuch Umformtechnik: Grundlagen, Technologien, Maschinen'' (in German), 2nd Edition, Springer Verlag, 2010,


Further reading

* * * *Reed-Hill, Robert, et al. ''Physical Metallurgy Principles'', 3rd Edition, PWS publishing, Boston, 1991. . *Callister Jr., William D., ''Materials Science and Engineering - an Introduction'', 6th Edition, John Wiley & Sons, New York, NY, 2003. *Suhel khan pathan,IJSRDV5I70206 ''"Three Roller Rolling Machine"''(IJSRD/Vol 5/Issue 07/2017/270).


External links

{{DEFAULTSORT:Rolling (Metalworking) Metal forming