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Principles
Forming processes convert solidified steel into products useful for the fabricating and construction industries. The objectives are to obtain a desired shape, to improve cast steel’s physical properties (which are not suitable for most applications), and to produce a surface suitable for a specific use. During plastic forming, the large crystals in cast steel are converted into many small, long crystals, transforming the usually brittle cast into a ductile and tough steel. In order to accomplish this, it is often necessary to reduce the cross section of a cast structure to one-eighth or even less of its original.
The major forming processes are carried out hot, at about 1,200° C (2,200° F), because of steel’s low resistance to plastic deformation at this temperature. This requires the use of reheating furnaces of different designs. Cold forming is often applied as a secondary process for making special steel products such as sheet or wire.
There are a number of steel-forming processes—including forging, pressing, piercing, drawing, and extruding—but by far the most important one is rolling. In this process, the rolls, working always in pairs, are driven in opposite directions with the same peripheral velocity and are held at a specific distance from each other by heavy bearings and mill housings. The steel workpiece is pulled by friction into the roll gap, which is smaller than the cross section of the workpiece, so that both rolls exert a pressure and continuously form the piece until it leaves the roll gap with a smaller section and increased length. As shown in the , the reduction in cross section is calculated by subtracting the out-section (S2) from the in-section (S1) and then dividing by S1. Assuming the workpiece maintains its original volume as it is formed, the elongation (L2) divided by the original length (L1) equals S1 divided by S2. When rolling flat products, there is not much change in width, so that the thickness alone can be used to calculate reduction.
The basic principles of a rolling-mill design are shown in B in the . Two heavy bearings mounted on each side of a roll sit in chocks, which slide in a mill housing for adjusting the roll gap with a screw. The two housings are connected to each other and to the foundation, and the complete assembly is called a roll stand. There are also compact rolling units (C in the ), which do not have housings; often used in the tandem rolling of long products, they can be exchanged quickly for repair or for a change in the rolling program. Rolls are driven through spindles and couplings, either directly or via a gear, by one or several electric motors. Depending on the product rolled, there are stands that have two, three, four, and more rolls; accordingly, they are given the names two-high, three-high, four-high, six-high, cluster mill, and planetary mill (schematically shown in the ). For rolling strip, heavy backup rolls support the smaller work rolls, because thin rolls form flat material better than do large-diameter rolls.
In a rolling shop, stands are arranged according to three layout principles. One is called the open train (G in the ), in which the stands are arranged side by side, often driven by the same motor and linked by spindles. This arrangement is applied only to the rolling of long products, with guides or cross-transfers being used to move the workpiece from stand to stand. A tandem mill arrangement (H in the ) has one stand behind the other and is used for high-production rolling of almost all products. This continuous arrangement requires the construction of long rolling trains and buildings, but layouts can be shortened by a so-called semicontinuous mill, in which the workpiece is passed back and forth through a reversing mill before being sent through the rest of the line. When open-train and tandem arrangements are combined for rolling long products in more compact layouts, it is called a cross-country mill.
Slabs and blooms
Cast ingots, sometimes still hot, arrive at slabbing and blooming mills on railroad cars and are charged upright by a special crane into under-floor soaking pits. These are gas-fired rectangular chambers, about 5 metres deep, in which four to eight ingots are simultaneously heated to about 1,250° C (2,300° F). An ingot used for conversion into a slab can be 1.5 metres wide, 0.8 metre thick, and 2.5 metres high and can weigh 23 tons. The soaking pits are highly computerized for scheduling, firing rates, heating times (which can last 8 to 18 hours), and rolling programs.
After heating, a tiltable transfer buggy brings a hot ingot to a two-high reversing mill, which takes one pass after another, reversing the rolls and roller table each time the ingot has passed through. Because each pass reduces the slab by only about 50 millimetres, it may take 21 passes, including several edge passes with the slab standing upright on its edges, to obtain a slab measuring 0.2 metre thick, 1.5 metres wide, and 10 metres long.
The rolls usually have a diameter of about 1.2 metres; each is driven by one or two electric motors totaling 7,000 to 12,000 horsepower. The two roller tables, situated in front and in back of the stand, have movable manipulators that guide the slab into the rolls and turn it onto its edges when required. High-pressure water nozzles remove surface scale, and a crop-shear discards the ends and cuts the slab into proper length. Some slabbing mills place a pair of heavy vertical rolls next to the horizontal rolls for edge rolling; this avoids the time-consuming turning of the slab into an upright position. Such an arrangement is called a universal mill.
For making long products, blooms some 250 millimetres square are rolled from ingots in a similar fashion on the same type of mill.
Plates
Rolled from heavy slabs supplied by a slabbing mill or continuous caster or sometimes rolled directly from an ingot, plates vary greatly in dimensions. The largest mills can roll plates 200 millimetres thick, 5 metres wide, and 35 metres long. These three dimensions are determined by the slab or ingot weight as well as the rolling-mill size. Sometimes only a few plates of the same dimensions and quality specifications are ordered.
Most mills have two continuous, broadside push-through or walk-through furnaces, which heat the slabs to about 1,250° C. Sometimes two batch-type furnaces are also used for heating odd-sized or extra-heavy slabs and ingots. Before rolling, high-pressure water jets descale the slabs. Most plate mills are four-high mills, as shown in C in the , and are supplemented by vertical edge rolls. The work rolls and backup rolls of large mills have diameters of 1.2 and 2.4 metres, respectively, and a roll face length up to 6 metres. Their maximum total rolling force is often 10,000 tons, and their rolls are driven by an 8,000-kilowatt motor. Most mills have hydraulic roll adjustment, which transmits the roll pressure to a computer; the computer uses this and other rolling parameters, such as temperature and thickness of the plate at all locations, to control the rolling process by a mathematical model. This technology—actually a computerized art—permits not only the rolling of huge workpieces with high accuracy (e.g., to a thickness tolerance of 0.2 millimetre) but also the control of rectangularity, flatness, plan-view shape, yield, physical properties, and profile. Several plants are even capable of rolling plates with a tapered or stepped thickness. Sometimes plants use two rolling mills, a roughing stand and a finishing stand, to improve surface quality and increase production. Most plate mills also have elaborate equipment for leveling, cooling, shearing or milling of edges, heat-treating, and marking.
