Steels_ Metallurgy and Applications, Third Edition

Low-carbon strip steels 53

1.6

1.5

._. 1.4

.~ so

IJj 48

~~, 330

~ ~ 320

210

, 200

~ 19o

180

o,., g~

s

o

o ..... 8" -'P-o -

o---o.....

"~',,o,o

044 wt% C

0.018 wt% C "~ %

700 750 800 850

Annealing temperature (~C)

Figure 1.56 Variation of mechanical properties with annealing temperature for
aluminium-killed steel annealed for 1.5 min after 71% cold reduction (After Ono et al. sT)

When the carbon in solution at the start of overageing is low, the carbon precip-
itates by diffusion to the ferrite grain boundaries. This is a long-range diffusion
process and takes place, therefore, relatively slowly. With higher solute carbon
contents at this stage, there is the possibility of the nucleation of new, fine precip-
itates within the grains. Reprecipitation then involves short range diffusion and
can proceed, therefore, more rapidly. The objective of the part of the annealing
cycle immediately prior to the overageing section is, therefore, to retain a high
level of carbon in solution in order to maximize the possibility of the nucleation
of a fine array of closely spaced carbides. This may be done by cooling rapidly
from a temperature close to the temperature (723~ of maximum solubility. In
practice, however, it is usual to commence the rapid cooling from a tempera-
ture well below 700~ to minimize the risk of strip distortion and to avoid the
formation of too many fine carbides.
It has been found that the carbides also tend to precipitate on fine manganese
sulphide precipitates. The provision of such particles by control of the manganese
content and the slab reheating temperature provides a means, therefore, of
promoting the overageing process.
The effect of cooling rate itself on final properties is illustrated in Figure 1.57.
This figure shows that there is a tendency for the yield stress to decrease as the
cooling rate increases to an optimum value of about 100~ but that higher
cooling rates lead to higher values of yield stress. Additionally, lower over-
ageing temperatures lead to a higher yield stress in spite of the short precipitation
diffusion distances due to the closely spaced carbides. The reason is clear from