Steels_ Metallurgy and Applications, Third Edition

Low-carbon strip steels 41

just above the stoichiometric composition for titanium steels, has become more
gradual and the highest r values in certain niobium-bearing steels may be obtained
with niobium contents close to 0.01% which is below the stoichiometric compo-
sition (Figure 1.38). A number of different combinations of niobium and titanium
are also, however, in common use. Figure 1.38 also shows that the relationships
between elongation and niobium or titanium content are similar to those relating
r value to niobium or titanium content and that coiling temperature also has a
marked effect on properties. There is, however, a strong relationship between hot
band grain size and mean r value with a finer grain size leading to higher values
(Figure 1.39). This is consistent with the effect of hot band grain size on final
texture mentioned in a previous section.
Slab reheat temperature has a marked influence on elongation and r value
with higher values coming from lower slab reheat temperatures, as illustrated in
Figure 1.40. A high cold reduction (Figure 1.41) and a high annealing tempera-
ture (Figure 1.42) also lead to higher r values, but the properties are relatively
insensitive to heating rate. This is the reason why ULC IF steels may be satis-
factorily processed by both batch and continuous annealing.
Recent work 73 has highlighted the importance of sulphur with respect to the
properties of steels containing titanium. This is because titanium carbo-sulphide
is more stable than titanium carbide, with the result that the former compound can

2.1

2.0

1.9

1.8

1.7

1.6

1.5

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t

A~k /

r451 A J

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I I

I I

Al

I

Coiling temp.: 993 K

Cold rolling: 80.0-80.3%

Annealing temp.-- 1,043 K

5 6 7 8

Grain size no.

Figure 1.39 Effect of grain size on the r values of IF steel sheets (After gino et al. 69)