Page 152 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

gi; - Section 5.2 Production of Iron and Steel
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Coke ovens
FIGURE 5.l Schematic illustration of a blast furnace.
Preheating the incoming air is necessary because the burning coke alone does not
produce sufficiently high temperatures for these reactions to occur.
The molten metal accumulates at the bottom of the blast furnace, while the
impurities float to the top of the metal. At intervals of four to five hours, the molten
metal is drawn off (rapped) into ladle cars, each holding as much as 145 metric tons
of molten iron. The molten metal at this stage is called pig iron or simply hot metal;
it has a typical composition of 4% C, 1.5% Si, 1% Mn, 0.04% S, 0.4% P, the rest
being iron. The word pig comes from the early practice of pouring the molten iron
into small sand molds arranged around a main channel; these closely packed molds
reminded early ironworkers of a litter of small pigs crowding against their mother
sow. The solidified metal (pig) is then used in making iron and steels.

5.2.3 Steelmaking

Steel was first produced in China and Japan about 600 to 800 A.D. The steelmaking
process is essentially one of refining the pig iron by reducing the percentages of man-
ganese, silicon, carbon, and other elements and by controlling the composition of
the output through the addition of various elements. The molten metal from the blast
furnace is transported into one of three types of furnaces: open-hearth, electric, or
basic-oxygen. The name “open-hearth” derives from the shallow hearth shape that is
open directly to the flames that melt the metal. Developed in the 1860s, the open-
hearth furnace has been essentially replaced by electric furnaces and by the basic-
oxygen process, because the latter two are more efficient and produce steels of better
quality.

Electric Furnace. The source of heat in this furnace is a continuous electric arc
that is formed between the electrodes and the charged metal (Figs. 5.2a and b).
Temperatures as high as 1925°C are generated in this type of furnace. There are usu-