Steels_ Metallurgy and Applications, Third Edition

Low-carbon strip steels 81

15

c

10

c

i

5

i,, , , ,, , , , , ,

I

0

~"~ t = 480 s

~.~. ~'"'-o t = 60 s

t=lOs

| ,, i I i i I

10 20 30

Nominal strain (%)

Figure 1.91 Effect of bainite transformation time (t) on change in volume fraction of
retained austenite with deformation (After ltami et al. 141)

0.4

:3

0.3

r

r

'~ 0.2

0.1

~Q

(.-.,,.

:,?,.,.

Dual phase t = 480 S

_ | | ...... 9

0.1 0.2

True strain

Figure 1.92 Effect of bainite transformation time (t) on instantaneous n value (After

Itami et al. 141 )

giving the highest carbon content in the retained austenite, the n value is retained

at a high level up to 0.2 true strain corresponding to 22% elongation. The effect

on properties is illustrated in Figure 1.93. This figure shows that the 8-minute

sample with the highest carbon content in the retained austenite has the highest

yield stress as well as the highest elongation. This is clearly a reflection of the

gradual transformation of austenite to ferrite over the complete strain range.