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rolling mill:see steelsteel,
alloy of iron, carbon, and small proportions of other elements. Iron contains impurities in the form of silicon, phosphorus, sulfur, and manganese; steelmaking involves the removal of these impurities, known as slag, and the addition of desirable alloying elements.
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a machine for the pressure shaping of metal and other materials between rotating rolls (rolling). In a broader sense, a rolling mill is an automatic system or line of machines that performs both rolling and auxiliary operations: transport of the original billet from the stock to the heating furnaces and the mill rolls, transfer of the rolled material from one groove to another, turning, transport of the metal after rolling, cutting into sections, marking or stamping, trimming, packing, and conveyance to the stock of finished product.
Historical outline. It is not known when and where the first rolling mill was used. The rolling of nonferrous metals (lead, tin, copper, and alloys used in coinage) was undoubtedly practiced before the rolling of iron. The earliest document describing a machine used in rolling is a captioned drawing made by Leonardo da Vinci in 1495, which shows a device used to roll tin. Until approximately the end of the 17th century, rolling mills were manually operated. The first water-driven mills date to the 18th century. The industrial rolling of iron began in about the 18th century. In Russia, the rolling of iron became especially well-developed in the Ural Mountains. Rolling mills were used to produce roofing iron, to flatten wrought iron billets into strips or sheets, and to divide forged bands lengthwise into reduced sections with square or rectangular cross sections (so-called cutting mills).
Steam engines were first used to drive rolling mills in the late 18th century. Rolling became one of the three major components in the production cycle at metallurgical factories, gradually replacing the less efficient method of forging. During this period, the first industrial use was made of the rolling mill with grooved rolls, invented by H. Cort of Great Britain in 1783. In time, rolling mills were subdivided into cogging, sheet, and section mills. In the 1830’s and 1840’s, the rapid development of railroads in various countries led to the use of rolling in the manufacture of rails. In 1856 and 1857 the first mill for rolling large beams was set up in the Saar, Germany.
Design improvements and the specialization of mills led to the establishment of blooming and slab mills in the USA in the late 19th century. In 1867, H. Bedson constructed the first continuous wire mill in Great Britain. In 1885, the German brothers M. Mannesmann and R. Mannesmann invented a method of rotary-rolling seamless pipe in rolling mills with skew rolls. In 1886 in the USA, W. Edenborn and C. Morgan were the first to use a high-speed wire coiler with axial feed. The first flying shears were designed by W. Edwards and came into use in 1892 in the USA. In 1897 an electric motor was successfully used to drive a rolling mill in Germany. In 1906 a rolling mill with a reversible electric motor was put into service in Třinec, in what is now Czechoslovakia.
The principle of continuous hot rolling of sheets found its first practical application in 1892, when a semicontinuous mill was put into operation in Teplice, in present-day Czechoslovakia. The first continuous wide-band sheet mill was constructed in 1923 in the USA. Cold rolling of sheets was begun in the 1880’s, and the cold rolling of pipes was introduced in 1930 in the USA.
The first Soviet achievement in the manufacture of mills was the construction of two blooming mills at the Izhora Factory. In 1933 these two mills were placed in operation at the Makeevka and Dneprodzerzhinsk metallurgical factories. In the 1940’s, 1950’s, and 1960’s, the All-Union Scientific Research and Design Institute for Metallurgical Machine Building (VNIIMETMASh) designed a series of rolling mills for new technological processes that made it possible to roll many items previously manufactured by other, less efficient methods. The items included thin-walled flangeless pipes, tapered sheets, variable round sections, spheres, sleeves, coarse-pitch screws, and gilled pipes. Between 1959 and 1962, VNIIMETMASh and the Electric Steel Factory for Heavy Machine Building designed a number of fundamentally new pipe mills with infinite reduction of pipes, both by furnace and high-frequency welding. In addition, mills were designed for the continuous rolling of seamless pipe (mill model 30–102) with outputs by some ten times higher than those of other mills of the time (about 550,000 tons annually). During this period, the first mills designed by VNIIMETMASh, the Scientific Research Institute of the Automotive Industry, and the Gorky Automotive Plant for the rolling of cylindrical and conical wheels were put into operation.
In the 1960’s, the USSR, the USA. the Federal Republic of Germany, and Italy began work on foundry and rolling units that combine the processes of continuous casting and rolling in one unbroken line. These units are already widely used in the production of aluminum-alloy sheets, steel billets, and wire rod made from aluminum and copper alloys.
Classification and design. The major factor determining the design of a rolling mill is the mill’s function relative to the range of production or the technological process. Based on the range of production, rolling mills are divided into billet mills (including mills that roll slabs and blooms), sheet and band mills, section mills (including beam and wire mills), and mills that roll tubes and various parts (tires, wheels, axles). Rolling mills are also classified on the basis of their technological processes as casting and rolling units, mills for the reduction of ingots (including slab and blooming mills), reversing single-stand mills, tandem mills, multistand mills, continuous mills, and cold-rolling mills. The size classification of a rolling mill designed for rolling sheets or bands is determined by the roll body. The size classification of a rolling mill designed for billets and section metal is determined by the diameter of the rolls, and that of a tube mill is determined by the external diameter of the rolled tubes.
The apparatus for deforming metal between rotating rollers is called the main apparatus, whereas the apparatus for carrying out other operations is called the auxiliary apparatus. The main apparatus consists of a single line or of several main lines, each of which contains three types of devices: (1) one or several roll stands, consisting of the rolls, bearings, frames, adjusting gears, plates, and leads, (2) electric motors for turning the rolls, and (3) transmission devices from the motors to the rolls, consisting mainly of gear housings, spindles, and clutches. A reducer is sometimes placed between the gear housing and the electric motor. If each roller has its own electric motor, the transmission consists only of shafts. Mills with horizontal rolls are the most common and may be two-high, three-high, four-high, or multiroll. Stands with vertical rolls (edgers) are used to cog the metal on its side surfaces. Universal mills, in which the vertical rolls are placed near the horizontal rolls, are used to roll wide bands and H beams. In rotary-rolling, the rolls are placed askew in the roll stands at an angle to the feed. These mills are used for rolling tubes, axles, and spheres.
The number and arrangement of the roll stands of a rolling mill are determined by the function of the mill, the number of times the metal must pass between the rolls to produce a given section, and the assigned output. Eight types of rolling mills are distinguished on this basis. Single-stand mills include most blooming mills, slab mills, ball-rolling mills, and mills for the cold rolling of sheets, strips, and tubes. Mills with several roll stands are used when it is impossible to place the required number of grooves in one roll stand or when a high output is required. The most advanced multiple-stand mill is a continuous mill in which the metal is simultaneously rolled in several stands. Continuous mills are used in the hot rolling of billets, bands, section metal, wire, and tubes and in the cold rolling of sheets, sheet metal, and strips.
Rolling speeds vary greatly and depend mainly on the required output of the rolling mill, the range of rolled products, and the technological process. In cogging, billet, thick-sheet, and heavy mills, the speed of rolling is about 2–8 m/sec. The highest speeds are those attained by continuous mills when rolling section metal (10–20 m/sec), band metal (25–35 m/sec), and wire (50–70 m/sec) and when cold-rolling sheet metal (40 m/sec). Table 1 presents data on the output, drive power, and weight of the equipment of some of the most common rolling mills used in the USSR to produce hot-rolled steel.
Billet mills fall into two types, depending on whether cast ingots or continuously cast billets serve as the starting material. When cast ingots are used, the billet mill is also a cogging mill. Typical examples of these mills are slab mills, in which flat billets with large cross sections (slabs) are used, and blooming mills with an attached continuous billet mill, which are used if billets must be rolled for section or tube mills. Flying shears are sometimes installed after the last stand of these mills to cut the billets into segments of the required length. Alternatively, saws and racks are used for cutting, cooling, and inspection of the billets. When continuously cast billets are used, the billet mill is installed next to the machine for continuous casting in order to use the heat of the uncooled metal. Some billet mills are constructed in such a way that the cast billet proceeds from the ingot mold into the rolls of the continuous mill without cutting; in other words, an infinitely long billet is rolled that is divided by flying shears or saws into segments of the required length after leaving the mill rolls.
Sheet and band mills for hot rolling are designed to take plates 50–350 mm thick, sheets 3–50 mm thick, and reeled bands 1.2–20 mm thick. Thick-sheet mills usually consist of one or two two- or four-high stands, with roll bodies ranging in length from 3,500 to 5,500 mm. Additional stands with vertical rolls for cogging the side edges are sometimes attached to the front of the mill. Wide-band continuous or semicontinuous mills consisting of 10–15 four-high stands with 1,500–2,500 mm roll bodies and several stands with vertical rolls are the types most commonly used for rolling bands. All the rolled material is wound in reels weighing 15–50 tons. Because these mills are considerably more efficient than thick-sheet mills, they are also used for rolling sheets 4–20 mm thick; the sheets are made by unwinding the reels and then cutting the metal. Roll tables and a great deal of auxiliary equipment used in subsequent treatment of the rolled product and transport are installed on the side where the rolled metal exits the rolls. In thick-sheet mills, the auxiliary equipment includes straighteners, shears, and furnaces for heat treatment. In wide-band mills, the equipment includes coilers for rolling the bands into reels, a conveyor for transporting the reels, and equipment for unwinding the reels, straightening the metal, and cutting the metal into sheets.
Section mills vary widely in their features and in the arrangement of equipment. Universal mills for rolling wide-band beams usually consist of three or five stands arranged one behind another; two or three stands are universal stands with horizontal rolls about 1,350 mm in diameter, and one or two stands are two-high stands with rolls about 800 mm in diameter. Multistage rail and beam mills consist of two or several lines with three-high or two-high working stands and rolls about 800 mm in diameter. Multistage or semicontinuous mills for large sections consist of two or several lines with three- or two-high working stands and rolls about 650 mm in diameter. In addition, there are semicontinuous or noncontinuous multistage medium mills with two or three lines, mills for small sections, usually continuous or semicontinuous, continuous thin-band mills, and continuous wire mills.
Casting and rolling mills are the most efficient mills for producing wire made from aluminum and copper alloys. The wire is manufactured by a continuous process from liquid metal. After crystallization of an infinite ingot between the rim of the mill’s rotating wheel and the steel band covering the wheel, the rolling operation is carried out on a continuous mill. Output is 5–8 tons per hour.
Like sheet mills, section mills have various auxiliary devices set up along the flow of the rolled metal that perform all auxiliary technological and transport operations at the appropriate rate without manual labor, from introduction of the starting billet to conveyance to the stock of finished product.
Tube-rolling units usually consist of three mills. The first mill makes a hole in the billet or ingot by rotary rolling, the second stretches the pierced billet into a tube, and the third reduces the diameter of the rolled tube. The design of the units is determined mainly by the technological process of the second (drawing) mill. The most efficient mills for this operation are continuous mills. Other mills that are also used include two-high mills
|Table 1. Principal mills used to hot-roll steel|
|Type||Range of rolled products||Annual production (thousand tons)*||Total power of major drives (kW)*||Weight (tons)*|
|*Maximum †Minimum **Tons per hour|
|Two-high single-stand blooming mill, model 1000–1300||Blooms from 200 × 200 mm to 370 × 370 mm||6,000||13,600||5,500|
|Continuous wide-band sheet mill, model 2000||Bands 1.2–16 mm thick and up to 1,850 mm wide||6,000||120,000||40,000|
|Double thick-sheet mill, model 3600||Sheets and plates 5–200 mm thick and up to 3,200 mm wide||1.750||21,000||60,000|
|Continuous billet mill, model 900/700/500||Billets with cross sections of 80 × 80 mm to 200 × 200 mm||5,500||30,400||10,500|
|Three-high multistage rail and beam mill, model 800||Rails and beams from no. 24 to no. 60, channel bars from no. 20 to no. 40||1,700||9,800||22,000|
|Three-high multistage heavy mill, model 650||Round steel 70–220 mm in diameter, beams from no. 16 to no. 20||750||8,700||6,500|
|Semicontinuous heavy mill, model 600||Round steel 50–120 mm in diameter, beams from no. 10 to no. 20||1,600||34,400||18,000|
|Semicontinuous medium mill, model 350||Round steel 20–75 mm in diameter, beams and channel bars up to no. 10||1,000||16,000||7,200|
|Continuous small mill, model 250||Round steel 8–30 mm in diameter, angular sections from 20 × 20 mm to 40 × 40 mm||800||16,000||6,600|
|Continuous thin-band mill, model 300||Bands 2–8 mm thick and 120–460 mm wide||1,000 †||15,200||2,700|
|Continuous wire mill, model 150||Wire rod 5.5–12.5 mm in diameter||900||—||—|
|Automatic tube mill, model 400||Seamless pipe 140–426 mm in diameter||50–70**||12,000||8,000–12,000|
|Continuous tube mill, model 110||Seamless pipe 50–110 mm in diameter||50–80**||12,000||3,500–5,000|
with short mandrels, Pilger mills, and three-high thread-rolling mills.
Cold-rolling mills for steel and nonferrous metals include (1) sheet mills for piece work rolling, (2) wide-band sheet mills for reel rolling, (3) strip-rolling mills for rolling strips 1 mm to 4 mm thick and 20 to 600 mm wide, the strips then being coiled or reeled, (4) foil-rolling mills for rolling bands less than 0.1 mm thick, (5) flatting mills for reducing wire to thin strips, and (6) mills for cold rolling tubes. When bands are reel-rolled, winding and tensioning drums set on both sides of the stand are used to unwind the reels before the metal is fed into the rolls and to wind the metal on exiting the rolls. Continuous mills are the most productive type of sheet mills and are also more efficient in their use of coilers and other auxiliary equipment. In continuous mills, coilers are placed at the rear, with mechanisms for feeding and unwinding the reels and directing the metal into the rolls of the first stand located in front.
Rolling mills that produce billets for machine parts operate mainly on the principle of helical rolling and make tools (hobs, drills) and precision billets for parts used in machine building (round indented shafts, balls, screws, gilled tubes, gear wheels). These mills vary in their construction and are highly mechanized and automated.
Equipment of rolling mills. The major parts and mechanisms of rolling mills, despite the variety and difference in function, are often identical in design. The major elements of the working stand are the rolls, bearing assemblies, frame, spindles, clutches, leads, and mechanisms for setting the rolls.
The roll bearings operate at very high stresses, which in some mills reach 30–60 meganewtons (3,000–6,000 tons-force) per roll. Size is limited by the diameter of the rolls. The rolling or liquid-friction bearings are mounted in large housings called cushions, which are located in openings of the frame.
Since the frame of the working stand accepts all the forces produced during the rolling operation, it is very large, weighing 60–120 tons and more, and is made of cast steel with 0.25–0.35 percent carbon. It is installed on steel foundation plates bolted to a concrete or reinforced-concrete foundation. Section mills are making increased use of prestressed working stands, in which rigidity is increased by using special clamping mechanisms rather than by increasing the size of the frame.
Rotation is transmitted to the rolls by means of universal spindles with Hooke’s joints.
The auxiliary equipment of rolling mills performs various functions. Ingot buggies transfer the metal from the heating apparatus to the mill’s receiving roller conveyor, turning devices turn the ingot on the roller conveyor, roller conveyors or transport vehicles transport the metal in accordance with the technological process, manipulators move the metal along the roll for transfer to the proper groove, and tilters turn the metal relative to its longitudinal axis. Other devices cool and pickle the metal (coolers and picklers), unwind the reels (unwinders), wind the bands into reels or wire into coils (coilers), and cut the metal (shears and saws). In the finishing operation, straighteners and presses are used to straighten the metal, and other devices are used for temper rolling, marking, stacking, oiling, and packing.
The electrical equipment of rolling mills has high power outputs and large drive systems. The power output of a single electric motor is 6–7 megawatts (MW) or more, and total power outputs are 200–300 MW. The systems that control the electric drives are complex chiefly because of the need to regulate automatically most of the machines in the rolling mill at greatly varying rates.
The lubricating equipment of rolling mills provides for the continuous automatic feed of lubricant to all the operating parts. In the case of mills that roll nonferrous metals and cold-roll steel, the equipment also feeds industrial lubricants to the working surface of the rolls. The lubricating systems are usually located in special lower compartments.
The automatic equipment of large rolling mills consists of a series of linked local systems for controlling the entire technological process, from the time the starting material is brought to and taken from the stock to the time the rolled product is brought to the stock of finished product and loaded onto cars. Each local system has many and varied sensing devices to collect and transmit information on the technological process, including data on the temperature of the metal, the pressure of the metal on the roll, and the characteristics of the workpiece, especially the dimensions, position, and movement of the rolled section. All this information is fed into the computers of local systems for processing, after which commands are given for regulating the individual machines and mechanisms that are part of the given local system. The computers for the local systems also provide information to the computer that integrates the local systems, the central computer then making the necessary corrections in the operation of machines and mechanisms in segments of the rolling mill controlled by the other local systems. A major problem in automation and the one that is most crucial economically involves the process of automatically regulating the dimensions of rolled sections by means of automatic variations in the gap between rolls on the basis of continuous data provided by a section gauge. The resultant sharp increase in the precision of the section size leads to closer production tolerances, an increase in metal quality, and a reduction in the unit consumption of metal. This is especially true when thin sheets are being manufactured.
The successful resolution of the above problem has become possible through the use of computer technology. Conventional self-adjusting systems cannot regulate the space between the rolls because the speeds of rolling are too high—some 30–40 m/sec.
Efficiency and economy are also greatly aided by automation of the processes of quality controlling the finished product and applying protective coatings. Because of their continuous operation and their large-scale production of a standard product, rolling mills have all the necessary prerequisites to become one of the first completely automated industrial systems.
REFERENCESProkatnoe proizvodstvo: Spravochnik, vols. 1–2. Edited by E. S. Rokotian. Moscow, 1962.
Korolev, A. A. Prokatnye stany i oborudovanie prokatnykh tsekhov: Atlas. Moscow, 1963.
Korolev, A. A. Mekhanicheskoe oborudovanie prokatnykh tsekhov, 2nd ed. Moscow, 1965.
Spetsial’nye prokatnye stany. Edited by A. I. Tselikov. Moscow, 1971.
Tselikov, A. I., and V. I. Ziuzin. Sovremennoe razvitie prokatnykh stanov. Moscow, 1972.
Tribology in Iron and Steel Works. London, 1970.
A. I. TSELIKOV