76 Steels: Metallurgy and Applications500CL tO
~4o~
P.
C::300200 9 0 wt% Cr ~'
m 0.1 wt% Cr ,"~// 9 o,~l0.,4,"~p.y~'"11' Vieldstress ,,~10 100 1000
Cooling rate (~Figure 1.84 Variation of yieM and tensile strength with cooling rate for steels containing
1.2 wt% Mn and different chromium contents (After Irie et al. 123)
It is useful to note that dual-phase steels tend to possess high n values for their
strength at low degrees of strain in a tensile test, but that the n values reduce at
higher strains. In addition, dual-phase steels usually possess poor hole expansion
values. The hole expansion values may, however, be improved using a titanium
addition to provide precipitation strengthening to the ferdte matrix which reduces
the hardness difference between the two phases present.TRIP or multiphase steels
These steels are based on carbon-manganese compositions, but usually also
containing a high level of silicon or possibly aluminium to inhibit carbide precip-
itation. The early stages of annealing are similar to those of dual-phase steels, but
the main difference between the annealing cycles is that TRIP steels are cooled to
a temperature reasonably close to 400~ called the austempering temperature, to
develop the necessary transformation products. The details of the microstructural
changes that occur and their effects on final properties depend critically on the
precise chemistry and processing used. The information given in this section is,
therefore, intended to give a broad illustration of the microstructural changes that
may take place.
The steels are given an intercritical anneal on a continuous annealing line
since, as for dual-phase steels, the cooling rates possible with batch annealing