Steels_ Metallurgy and Applications, Third Edition

166 Steels: Metallurgy and Applications

with aluminium prior to calcium treatment with an initial oxygen content in the
range 20-100 ppm. After calcium treatment, the oxygen content is reduced to
10-20 ppm. The authors report that the desulphurizing effect of calcium is deter-
mined largely by the amount of calcium added but, as illustrated in Figure 2.15,
the effect is influenced greatly by the type of ladle refractories employed. With
dolomite ladles, little reaction takes place between the molten steel and refracto-
ries and the low level of oxygen in the steel enhances the effect of calcium as a
desulphurizing agent. With dolomite ladles, the addition of 1 kg of calcium per
ton reduces the sulphur content from an initial level of 0.02% to a final level of
0.003%. However, the authors state that sulphur contents less than 0.001% can
be produced by this technique. As illustrated in Figure 2.15, magnesium is also
very effective as a desulphurizing agent. However, calcium is generally preferred
because it is cheaper and more controllable.
As indicated earlier, anisotropy of toughness and ductility is caused by the
elongation of inclusions into planar arrays and both manganese sulphides and
stringers of oxide are damaging in this respect. However, the problem is largely
eliminated if the inclusions are present as small, isolated, non-deformed particles.
Therefore major attention has been devoted to inclusion shape control in addition
to the reduction in the volume fraction of inclusions. The elements used as
inclusion modifiers are calcium, zirconium, tellurium and the rare earth metals.
In aluminium deoxidized steels, the inclusion population will generally include
elongated Type II manganese sulphides, alumina and some silicates. However,
after calcium treatment, the inclusions are restricted to calcium aluminate of the
type CaO.AI203. The sulphur in the steel is also associated with these inclusions,
either as calcium sulphide or as sulphur in solution. The calcium aluminate parti-
cles are globular in nature and tend to retain their shape on hot rolling. The

0 Fire-clay ladle

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99 , , , i, i ,

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4

Calcium or magnesium added (kg per ton)

Figure 2.15 Effect of calcium and magnesium on desulphurization. The desulphurizing
additions were blown to a depth of 2.7 m in a steel bath having a temperature of 1580" C
(After Pircher and Klapdar 17