Page 179 - Materials Chemistry, Second Edition

166 3 Metals

Figure 3.4. Schematic of a blast iron. Shown are: 1 – incoming pre-heated air from the Cowper stove;
2 – melting zone; 3 – reduction zone of FeO; 4 – reduction zone of Fe 2 O 3 ; 5 – pre-heating zone; 6 – feed
of iron ore, limestone, and coke; 7 – exhaust gases; 8 – column of ore, coke, and limestone; 9 – removal
of slag; 10 – tapping of molten pig iron; 11 – collection of waste gases. Image courtesy of Robert Blazek.

directly to the transfer ladle at 1,350–1,400 C (Figure 3.5b) in order to facilitate
removal of sulfur (Eq. 11):
ð11Þ CaO þ FeS ! CaS þ FeO
The resultant slag is raked from the ladle and the remaining molten iron is charged
into the BOF along with steel scraps (Figure 3.4c). A supersonic jet of oxygen is

used within the BOF to yield a temperature of 1,620–1,720 C. Under these extreme
oxidizing conditions, most of the remaining C, P, Si and Mn impurities are depleted
from the hot metal, resulting in a steel with a concentration of 0.03–0.07 wt.%
carbon. The entire oxidation process within the BOF lasts only 20–25 min
(Figure 3.6).
The next process of steelmaking consists of fine-tuning the composition of the
steel generated from the BOF. If low-carbon steel is desired, the molten steel is
degassed to yield a product concentration of <0.005 wt.% C (Figure 3.7). Alterna-
tively, desired dopants (to alter overall properties – see section 3.2) may be added
directly to the molten iron within the alloying ladle as a metal wire or powder.
Alloying is afforded by lowering arc furnace electrodes into the ladle; argon gas is
injected at the bottom of the ladle to prevent oxidation and induce stirring that
is required for homogenization of the steel.
Once the desired composition of the steel has been achieved, the processes of bulk
0
forming may begin. Long blocks of steel (ca. 30 ) are formed by slab casting,
whereby the molten steel is poured into a water-cooled copper mold via a tundish.
As shown in Figure 3.8, a series of rollers slowly draw the solidified steel downward