Steels_ Metallurgy and Applications, Third Edition

68 Steels: Metallurgy and Applications

The details of the chemistry and the processing that must be used, however,

depend on the steelmaking capability and on the annealing method, as well as

on the strength and r value of the steel required.

Batch-annealed, bake-hardening steels
Most batch-annealed, bake-hardening steels are based on a conventional
aluminium-killed steel as indicated above, using hot-rolling conditions and
aluminium and nitrogen contents necessary to give high r values. With normal
annealing conditions with a top annealing temperature below 723~ and with
a carbon content above about 0.03%, the carbon that is taken into solution
at the annealing temperature reprecipitates almost completely during cooling
and the steel becomes non-strain ageing and non-bake hardening. With lower
carbon contents, reprecipitation becomes more difficult, a small amount of carbon
remains in solution and a useful degree of bake hardening is obtained. Figure 1.74
shows how the bake hardening varies with total carbon content for tight coil
annealing, but also shows that a higher bake-hardening index is obtained using
the faster cooling rates and higher annealing temperatures that may be obtained
from open-coil annealing. Such a method, however, is rarely used. It is common
practice, however, to utilize the higher cooling rates that may be obtained from
hydrogen annealing compared with the older HNX annealing to develop a higher
degree of bake hardening.
The yield stress of a steel prepared as above is comparable with that of
a conventional non-bake-hardening, batch-annealed, aluminium-killed steel, but

80 ..... 9 i 'r" w ' ,. , , ',. .'~']r

60

_g

,),...,,.

!

20 I" TCA """Q,,

,~

0 J t_ t f i I I I~ I. ,l I

0 0.02 0.04 0.06 0 0.3 0 0.04

Mn P (%)

Figure 1.74 Effect of chemical composition and batch-annealing furnace on bake-harde-

nability (base composition:

0.01% C-0.11% Mn-0.07% P-0.05% AI

OCA: Open coil annealing

soaked at 740~

cooled at 80~

TCA: Tight coil annealing

soaked at 710"C

cooled at 20*C/h)

(After Mizui et al. lit)