Steels_ Metallurgy and Applications, Third Edition

Low-carbon strip steels 43

form during the hot-rolling sequence and influence the development of the hot-

rolled texture. 74 The work showed that at the slab reheat temperature, titanium

sulphide (TiS) forms and may transform into titanium carbo-sulphide (Ti4C2S2)

on cooling in the hot-rolling temperature range. This is accompanied by a much

greater drop in the solute carbon content than would be possible in the absence

of sulphur. It was considered 74 that the low-solute carbon content would promote

the development of a desirable (111) texture in the hot band which would, in turn,

lead to a more beneficial texture in the final cold-rolled and annealed product.

Batch-annealed, aluminium-killed steel

These steels are batch annealed using annealing temperatures close to 700~

The main elements which influence the properties are aluminium, nitrogen

and carbon, with a minor influence of manganese. The optimum nitrogen and

soluble aluminium contents are 0.005-0.01 wt% and 0.025-0.04 wt% respec-

tively, although acceptable r values may be obtained just outside these limits,

as illustrated in Figure 1.43. An increase in carbon content above and below

the range causes an increase in strength. Normally with carbon contents close

to 0.04%, the carbon that is taken into solution is reprecipitated completely

during cooling mainly by diffusion onto the undissolved carbides. The nitrogen

is combined as aluminium nitride and the steel becomes perfectly non-ageing

as indicated previously. At lower carbon contents, the number of undissolved

0.016

/

l 1.8 1.7 1.6 1.4

0.012

8 o.oos

Z

0.004

i _ i _ i ... i

0.02 0.04 0.06 0.08

Alumlnlum content (wt%)

Figure 1.43 Effect of finished sheet nitrogen and aluminium on the rm values of
batch-annealed, aluminium-killed steel (Gawne ~5)