Steels_ Metallurgy and Applications, Third Edition

84 Steels: Metallurgy and Applications

important to achieve good bendability whereas avoiding the carbides that would
be formed at the grain boundaries at the higher temperatures was necessary to give
good delayed fracture characteristics. Both factors were controlled by tempering
at temperatures below 240"C.
Ultra-high-strength steels containing a high proportion of bainite may clearly
be obtained by rapidly cooling a steel with a suitable composition to temperatures
close to 400"C, as has been used for the TRIP steels. As before, optimization
of the strength/elongation balance depends on retaining a sufficient proportion
of retained austenite. An example is illustrated in Figure 1.96, which shows
how the strength elongation and structure vary with annealing temperature for a
steel containing 0.4% carbon, 1.55% silicon and 0.8% manganese. The highest
values of elongation were obtained for low annealing temperatures for which the
carbon in the austenite would have been high which led to the highest proportion
of retained austenite. Quenching from the single-phase austenite region was,

Ac 9 m

100 ~ Retained austenite

8O

OL 6O

40

20

3O

.5

_o UJ lO

1200

A

O. a~ 1000

or) 800

(^1)!
Bainite! i
I
,, ,
I
I I
I I
' I Ferrite I
! I -!
I
I
! I I s
I I
,/
I i i |.
J: /Yield ',
-! / =r ss :
760 800 840 880
Annealing temperature (~
Figure 1.96 Variation of strength, elongation and structure with annealing temperature,
samples quenched to 400"C and heM for 5 rain (Matsumura et al. 145)