Металл пішіндіру процесі: Прокатка технологиясы

Metal forming process

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 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 processes. Roll stands holding pairs of rolls are grouped together into rolling mills that can quickly process metal, typically steel, into products such as structural steel (I beams, angle stock, channel stock), bar stock, and rails. Most steel mills 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.

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. The invention of the rolling mill in Europe may be attributed to Leonardo da Vinci in his drawings. Earliest rolling mills were slitting mills, which were introduced from what is now Belgium to England 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. 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 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.

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

Until well into the eighteenth century, rolling mills derived their power from water wheels. The first recorded use of a steam engine 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 soon after 1900. Modern rolling practice can be attributed to the pioneering efforts of Henry Cort of Funtley Iron Mills, near Fareham in Hampshire, 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 in Northumberland, England, in 1820, where he produced fish bellied wrought iron rails in lengths of 15 to 18 feet. Hot rolled metals generally have little directionality in their mechanical properties or deformation induced residual stresses. 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 beams. While the finished product is of good quality, the surface is covered in mill scale, which is an oxide that forms at high temperatures. It is usually removed via pickling or the smooth clean surface (SCS) process, which reveals a smooth surface. 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. Hot rolling is used mainly to produce sheet metal or simple cross sections, such as rail tracks.

Rolling mills 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 and particularly an integrated system including an inferential engine based on Genetic Algorithms a knowledge database based on an Artificial Neural Network trained by a parametric Finite element model and to optimize and automatically design rolling mills.

Cold rolling occurs with the metal below its recrystallization temperature (usually at room temperature), which increases the strength via strain hardening up to 20%. It also improves the surface finish and holds tighter tolerances. 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.

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 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. The decrease in material thickness causes the material to elongate. The friction 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. Aluminum foil 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 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, gears, rockets, turbines, airplanes, pipes, and pressure vessels.

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
Blooming, cogging and slabbing mills, being the preparatory mills to rolling finished rails, 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 mills, 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 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.

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.

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.

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.

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. 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 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.

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 torch, whose gas pressure blows away the metal or slag melted by the flame; and laser scarfing.