Today, the continuous casting process is used to produce semifinished products directly from liquid steel, thus eliminating the ingot molds and primary mills used previously. With continuous casting, the steel is poured from sequenced ladles to maintain a desired level in a tundish above an oscillating water-cooled copper mold (Fig. 1.15). The outer skin of the steel strand solidifies as it passes through the mold, and this action is further aided by water sprayed on the skin just after the strand exits the mold. The strand passes through sets of supporting rolls, curving rolls, and straightening rolls and is then rolled into slabs. The slabs
are cut to length from the moving strand and held for subsequent rolling into finished product.
Not only is the continuous casting process a more efficient method, but it also results in
improved quality through more consistent chemical composition and better surfaces on the
finished product.
Plates, produced from slabs or directly from ingots, are distinguished from sheet, strip,
and flat bars by size limitations in ASTM A6. Generally, plates are heavier, per linear foot, than these other products. Plates are formed with straight horizontal rolls and later trimmed (sheared or gas cut) on all edges.
Slabs usually are reheated in a furnace and descaled with high-pressure water sprays
before they are rolled into plates. The plastic slabs are gradually brought to desired dimensions by passage through a series of rollers. In the last rolling step, the plates pass through leveling, or flattening, rollers. Generally, the thinner the plate, the more flattening required.
After passing through the leveler, plates are cooled uniformly, then sheared or gas cut to desired length, while still hot.
Some of the plates may be heat treated, depending on grade of steel and intended use.
For carbon steel, the treatment may be annealing, normalizing, or stress relieving. Plates of HSLA or constructional alloy steels may be quenched and tempered. Some mills provide facilities for on-line heat treating or for thermomechanical processing (controlled rolling).
Other mills heat treat off-line.
Shapes are rolled from continuously cast beam blanks or from blooms that first are reheated to 2250F. Rolls gradually reduce the plastic blooms to the desired shapes and sizes.
The shapes then are cut to length for convenient handling, with a hot saw. After that, they are cooled uniformly. Next, they are straightened, in a roller straightener or in a gag press.
Finally, they are cut to desired length, usually by hot shearing, hot sawing, or cold sawing.
Also, column ends may be milled to close tolerances.
ASTM A6 requires that material for delivery shall be free from injurious defects and shall have a workmanlike finish. The specification permits manufacturers to condition plates and shapes for the removal of injurious surface imperfections or surface depressions by grinding, or chipping and grinding. . . . Except in alloy steels, small surface imperfections may be corrected by chipping or grinding, then depositing weld metal with low-hydrogen
electrodes. Conditioning also may be done on slabs before they are made into other products.
In addition to chipping and grinding, they may be scarfed to remove surface defects.
Hand chipping is done with a cold chisel in a pneumatic hammer. Machine chipping may be done with a planer or a milling machine.
Scarfing, by hand or machine, removes defects with an oxygen torch. This can create problems that do not arise with other conditioning methods. When the heat source is removed from the conditioned area, a quenching effect is produced by rapid extraction of heat from the hot area by the surrounding relatively cold areas. The rapid cooling hardens the steel, the amount depending on carbon content and hardenability of the steel. In low-carbon steels, the effect may be insignificant. In high-carbon and alloy steels, however, the effect may be severe. If preventive measures are not taken, the hardened area will crack. To prevent scarfing cracks, the steel should be preheated before scarfing to between 300 and 500F and, in some cases, postheated for stress relief. The hardened surface later can be removed by normalizing or annealing.
Internal structure and many properties of plates and shapes are determined largely by the chemistry of the steel, rolling practice, cooling conditions after rolling, and heat treatment, where used. Because the sections are rolled in a temperature range at which steel is austenitic (see Art. 1.20), internal structure is affected in several ways.
The final austenitic grain size is determined by the temperature of the steel during the last passes through the rolls (see Art. 1.21). In addition, inclusions are reoriented in the direction of rolling. As a result, ductility and bendability are much better in the longitudinal direction than in the transverse, and these properties are poorest in the thickness direction.
The cooling rate after rolling determines the distribution of ferrite and the grain size of the ferrite. Since air cooling is the usual practice, the final internal structure and, therefore, the properties of plates and shapes depend principally on the chemistry of the steel, section size, and heat treatment. By normalizing the steel and by use of steels made to fine-grain
practice (with grain-growth inhibitors, such as aluminum, vanadium, and titanium), grain size can be refined and properties consequently improved.
In addition to the preceding effects, rolling also may induce residual stresses in plates and shapes (see Art. 1.15). Still other effects are a consequence of the final thickness of the hot-rolled material.
Thicker material requires less rolling, the finish rolling temperature is higher, and the cooling rate is slower than for thin material. As a consequence, thin material has a superior microstructure. Furthermore, thicker material can have a more unfavorable state of stress because of stress raisers, such as tiny cracks and inclusions, and residual stresses.
Consequently, thin material develops higher tensile and yield strengths than thick material of the same steel chemistry. ASTM specifications for structural steels recognize this usually by setting lower yield points for thicker material. A36 steel, however, has the same yield point for all thicknesses. To achieve this, the chemistry is varied for plates and shapes and for thin and thick plates. Thicker plates contain more carbon and manganese to raise the yield point. This cannot be done for high-strength steels because of the adverse effect on notch toughness, ductility, and weldability.
Thin material generally has greater ductility and lower transition temperatures than thick material of the same steel. Since normalizing refines the grain structure, thick material improves relatively more with normalizing than does thin material. The improvement is even greater with silicon-aluminum-killed steels.
(W. T. Lankford, Jr., ed., The Making, Shaping and Treating of Steel, Association of Iron
and Steel Engineers, Pittsburgh, Pa.)
