Page 154 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Section 5 3 Casting of lngots

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Charging scrap into furnace Charging molten iron Addition of burnt lime
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Blowing with oxygen Tapping the furnace Pouring the slag
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FIGURE 5.3 Schematic illustrations showing charging, melting, and pouring of molten iron
in a basic-oxygen process.

5.3 Casting of lngots

Traditionally, the next step in the steelmaking process is the shaping of the molten
steel into a solid form (ingot) for such further processing as rolling it into shapes,
casting it into semifinished forms, or forging it. The molten metal is poured (teemed)
from the ladle into ingot molds, in which the metal solidifies. Molds usually are
made of cupola iron or blast-furnace iron with 3.5% carbon. They are tapered in
order to facilitate the removal of the solidified metal. The bottoms of the molds may
be closed or open; if they are open, the molds are placed on a flat surface. The
cooled ingots are removed (stripped) from the molds and lowered into soaking pits,
where they are reheated to a uniform temperature of about 1200°C for subsequent
processing by rolling. lngots may be square, rectangular, or round in cross section,
and their weights range from a few hundred kilograms to 36 metric tons.
Certain reactions take place during the solidification of an ingot that have an
important influence on the quality of the steel produced. For example, significant
amounts of oxygen and other gases can dissolve in the molten metal during steel-
making. Most of these gases are rejected during the solidification of the metal, be-
cause the solubility limit of the gases in the metal decreases sharply as its temperature
decreases (see Fig. 10.15 ). Rejected oxygen combines with carbon to form carbon
monoxide, which causes porosity in the solidified ingot.
Depending on the amount of gas evolved during solidification, three types of
steel ingots can be produced: killed, semi-killed, and rimmed.
I. Killed Steel. Killed steel is a fully deoxidized steel; that is, oxygen is removed
and the associated porosity is thus eliminated. In the deoxidation process, the
dissolved oxygen in the molten metal is made to react with elements such as