Rolling Production

the production of various finished and semifinished articles from steel and other metals by means of rolling, as well as additional treatment to improve the quality of the articles (heat treatment, pickling, and application of coatings). In industrialized countries, more than four-fifths of smelted steel is rolled. Rolling is usually done in metallurgical factories, although it is also done at machine-building plants. As a rule, and especially in ferrous metallurgy, rolling production is the final step in the production cycle.

The major types of rolled products are billets, sheets, sections, rolled tubes, and machine parts, such as wheels, rings, axles, drills, balls, and variable sections. The sum total of rolled items of various sizes is called the range of rolled products. Most rolled products are standardized in the USSR. Most are made from low-carbon steel, although some are made of alloy steel and steel with more than 0.4 percent carbon. Nonferrous metals are mainly rolled into sheets, bands, and wire; tubes and sections of nonferrous metals are produced primarily by pressing.

Steel. Two methods are used to manufacture rolled steel products in modern metallurgical factories. In the first method, ingots cast in molds are usually converted into the finished rolled product in two stages. The ingots are first heated and then rolled into billets in cogging mills. After inspection of the billets and removal of surface defects (laps, cracks), the billets are heated and the final product is rolled on specialized mills. The size and shape of the cross section of the billet depend on the billet’s intended use. When sheet and flat metal are being rolled, the billets (slabs) have a rectangular cross section that is 400–2,500 mm wide and 75–600 mm thick. The billets used in rolling section metal have square cross sections of approximately 60×60 mm to 400×400 mm. Billets with round cross sections 80–350 mm in diameter are used for seamless rolled tubes.

In the second method, which has been in use since about 1950, rolling of the original billet is replaced by continuous casting in special machines. After inspection and removal of defects as in the first method, the billet enters the mill for rolling of the final product. The use of continuously cast billets is eliminating the need for slabbing and blooming mills, improving the quality of the rolled product, and reducing losses caused by cutting off the ingot head, which amounts to 15–20 percent of killed-steel ingots.

The advantages of using continuously cast billets in the manufacture of rolled products become even greater when continuous casting and rolling are combined in a single continuous flow. Casting and rolling units have been constructed in which the ingot, on leaving the mold, is not cut but rather passes through a furnace—where the temperature is equalized throughout the cross section—and then enters the mill rolls. Crystallization occurs and an endless ingot is rolled; that is, there is continuous rolling from liquid metal. This process has found wide application in the rolling of nonferrous metals and is also used to produce small, high-quality steel billets (less than 150 × 150 mm in cross section). The major difficulty in developing this process lies in the low exit speed of the ingot from the mold (1–6 m/min), which does not permit use of continuous rolling mills to the full extent of production capacities.

Sheet metal is rolled from rolled or continuously cast slabs, with only sheets thicker than 50–100 mm manufactured directly from ingots or forged slabs. The following major operations are part of the technological process: (1) feeding of the slabs from the stock to the heating furnace, (2) heating, (3) feeding by roll table into the roll stand and rolling in several passes (the slabs are sometimes introduced sideways or at an angle in the first several passes to obtain sheets of the required width), (4) straightening on roller straighteners, (5) cooling in coolers, (6) inspection and markings, (7) cutting off of the lengthwise edges, (8) cutting off of the ends and cutting of the metal into sheets of a given length, (9) heat treatment and painting (sometimes), and (10) transfer to the stock of finished product.

Sheets 4–50 mm thick and plates up to 350 mm thick are rolled on plate or armor plate mills consisting of one or two roll stands. Sheets 1.2–20 mm thick are manufactured on much more efficient continuous mills, in which sheets are produced as strips more than 500 m long; on leaving the final roll stand, the strips are wound into a reel.

Sheets less than 1.5–3 mm thick are best cold-worked, and thus the sheets are usually further reduced in thickness in cold-rolling mills. After the reels are obtained from the continuous hot-rolling mill, they are transported to the cold-rolling unit, where scale is removed from the surface of the metal in a continuous pickling line. The ends are then cut off and resistance welding is done to ensure the complete continuity of the subsequent process. The pickled reels are unwound and cogged in several passes to the required thickness; the total reduction for low-carbon steel is 75–90 percent. The rolling operation is carried out in continuous mills consisting of four or six four-high stands or in single-stand reversing stands. After cold rolling, the band is annealed to eliminate cold work and then temper-rolled, straightened, cut into sheets, and packed.

The rolling of section metal first involves heating the metal to 1100°-1250°C. The hot billet is then fed into the roll stands and rolled in several passes in grooves; gradually the cross section of the original billet approaches the final cross section. The rolled product is cut by saws or shears into segments of the required length or wound into coils. The metal is cooled in coolers, straightened on roller straighteners, inspected, and transferred to the stock of finished product.

The number of passes is selected according to the size and shape of the cross section of the original billet and the final cross section. The number of passes averages 9 for rails, 9–13 for beams, 5–12 for angular and other sections (for example, Z-shaped sections), and 15–21 for wire. These operations are performed on specialized section mills, which are automatic flow systems consisting of a number of machines.

The hot rolling of tubes consists of three major and several auxiliary operations. The first operation, called piercing, is the creation of a hole in the billet or ingot, which results in the formation of a thick-walled tube called a shell. The second operation, called tube sizing, involves lengthening the pierced billet and reducing wall thickness to approximately the thickness required in the final tube. Both these operations are carried out after one heating but on different rolling mills, which are placed side by side and are part of a general system of tube-rolling machines. The first operation is performed in piercing mills by screw rolling between barrel-shaped or disk rollers on a short mandrel. The second operation is carried out in continuous, Pilger, automatic, and three-high screw rolling mills. The third operation involves reduction of the tubes after sizing. Reduction is done on sizing mills, after which the tubes are cooled, straightened, inspected, and cut into sections of a given length. Tubes less than 65–70 mm in diameter receive additional hot rolling in reduction mills. After hot rolling, the tubes are cold-rolled on specialized mills and drawn in order to reduce the thickness of the tube wall and diameter and to obtain better mechanical properties, a smooth surface, and precise dimensions.

Machine parts are rolled mainly in the manufacture of various solids of revolution and variable sections, including train wheels, axles, tires, roller bearing rings, gears, screws, and drills. Rolling is sometimes used in combination with forging or stamping to perform only one operation.

Nonferrous metals. Rolling has found its greatest use in the manufacture of sheets, strips, foils, and wire from aluminum, copper, magnesium, and zinc and from their alloys.

The industrial rolling of aluminum alloy sheets consists of the following major operations: (1) preliminary rolling of flat continuously cast ingots weighing 0.5–5 tons, with reduction of about 10 percent to smooth the surface of the ingots, (2) straightening on roller straighteners, (3) milling to obtain a pure and smooth surface, (4) application of aluminum sheets to both sides of the ingot, (5) heating, (6) hot rolling with cladding to a thickness of 4–12 mm, with subsequent reeling, and (7) annealing and cold rolling. After cold rolling, the reels are unwound and cut into sheets, which are then tempered, pickled, cold-rolled once again (to smooth the metal or to produce cold working), pickled, cut, and packed.

In the early 1960’s a new method was developed for rolling aluminum and aluminum alloy sheets. The novel feature of this process is the combination of continuous casting with rolling. Liquid aluminum passes through a guide into a gap formed between two horizontal rolls. The aluminum touches the rolls and crystallizes, with the band that is formed continuously exiting the mill rolls. The rolls may be arranged either vertically or horizontally; in the first case the aluminum enters from the side, whereas in the second it enters from below. This method is used to manufacture bands 8–12 mm thick and 1,000–1,600 mm wide, which are then wound onto reels. There are great economic advantages of using this technology instead of rolling bands from large ingots.

The billets used in rolling sheets and strips from copper and brass are flat ingots weighing 0.5–1 ton and measuring 100–150 mm in thickness. The billets are hot-rolled to a thickness of 10–15 mm. The rolled sheets are milled to remove surface defects and then cold-rolled, with intermediate annealings at 450°–800°C.

The most efficient method of producing wire rod from aluminum and copper alloys is continuous casting combined with rolling in a continuous mill.

Technical advances in rolling production are characterized primarily by improvement in the quality of the rolled products; this improvement allows the consumer to make more effective use of the metal. To this end, the rolled item is given the shape best facilitating reduction of mass, the production of economical rolled sections is expanding, and exactness in the dimensions of rolled products is improving. In addition, strength and other properties of the metal are being improved and effective protective coatings are being applied to the metal surface. At the same time, the cost of production losses is being reduced by introducing continuous processes (from liquid metal to the finished rolled product), increasing the rolling speed, and automating all the technological processes.