Steels_ Metallurgy and Applications, Third Edition

80 Steels: Metallurgy and Applications

When the austenite retained at room temperature is deformed, for example
during forming, there is a tendency for it to transform to martensite, as indicated
previously, and this is the process that leads to the high work-hardening rate
and consequently high values of uniform elongation. It is found that austenite
regions with different carbon contents have different stabilities in the presence of
the deformation. 136 Austenite with a very high carbon content may be so stable
that most of it remains untransformed even after high levels of strain. Alterna-
tively, austenite containing lower levels of carbon could transform completely
to martensite under the action of relatively small amounts of strain. Neither of
these two conditions give an optimum effect on formability. Thus the mechan-
ical stability is critical if the best ductility is to be obtained. 136 The austenite
should have an intermediate stability in order that it transforms gradually over
the complete strain range needed. 14~
An example of the effect of holding time in the bainitic region on the build-
up of carbon in the retained austenite is illustrated for a steel containing 0.14%
carbon, 1.94% silicon and 1.66% manganese in Figure 1.90. It is seen that holding
at 400"C for 10 seconds, 1 minute or 8 minutes leads to a progressive build-up
of carbon in the retained austenite. The effect on the stability of the austenite
during deformation is illustrated in Figure 1.91. This figure shows that much of
the austenite retained after the shorter holds transforms in a tensile test during the
first few per cent of strain, whereas the austenite retained after the 8-minute hold
transforms more gradually over 30% strain. The effect on the work-hardening
coefficient n throughout the test is illustrated in Figure 1.92, and compared with
the effect for a ferrite-martensite dual-phase steel. It is evident that the n value
decreases sharply at low strains for the dual-phase steel. The n values for the
two shorter holds are generally similar to those of the dual-phase steels, but
the reduction with increasing strain is at a lower rate. For the longest hold time,

1.5

n-

.c_

"E 1.0

8

0.5

' 9 "'I 9 9 9 9 " ~ "'i .... ="--1.=

Strain = 0%

CInYR

.... I ........ | ...... ,i

101 10 2 10 3

Bainite transformation time (s)

Figure 1.90 Effect of bainite transformation time at 400"C on C content in retained
austenite (After Itami et al. 141)