72 Steels: Metallurgy and ApplicationsA5040302010/ ./ Solid solution-hardened
(IF Steel)" TRIP
_ / ".~?S~p~r~H~SS~_..j~.... -~"(Transf~176 plasticity)Solid sc m [~" i~=,.. ",- ,~ .t Dual phase
harden, (AI-K sleuwl ~ '~k,, ~IP'.~ ~ ~ ~. A~...~ ~ ~ "~ -~ (+ precipitation)precipitation hardened ~ ~. ~..~.
i
I I I,, I I i I I I -= I ,I I
400 600 800 1,000 1,200 1,400
Tensile strength (MPa)Figure 1.77 Strength ductility balance of high-strength steel (After Shimada et al. TM)
was taken in this type of steel. The steels are first annealed in the intercritical
region of the phase diagram to produce a structure of ferrite and austenite with
a relatively high carbon content in the austenite. On cooling through the inter-
critical region, the austenite may become further enriched with carbon and the
subsequent cooling rate must be sufficiently fast to avoid the formation of pearlite
or bainite. Sufficient martensite must then form to be able to eliminate the yield
point elongation. This critical cooling rate depends on the manganese content of
the steel, as illustrated in Figure 1.78, but is also influenced by the chromium
and molybdenum content. An equation was given 123 to calculate the equivalent
manganese content to take into account the effect of molybdenum and chromium
as follows:
[Mn]eq = [Mn] + 1.3[Cr] + 2.67[Mo]This equation shows that both chromium and molybdenum are more effective in
reducing the critical cooling rate than manganese. Other work has shown that the
critical cooling rate may depend on the silicon and phosphorus contents and on
the holding time at temperature. It is clear that the fairly rapid cooling should
continue at least to the Ms temperature if the formation of higher temperature
transformation products, such as bainite, is to be avoided.
It is found that the tensile strength of a classical, ferrite-martensite dual-
phase steel increases as the volume fraction of martensite increases, but that
the yield stress first decreases and then increases, as illustrated in Figure 1.79.
This is associated with the gradual removal of the yield point elongation at
low martensite volume fractions. For a given volume fraction of martensite,
Gladman j25 showed that a decrease in mean island diameter had little influence
on tensile strength but has a marked effect in increasing the uniform elongation.
Other workers, 126 however, observed an increase in tensile strength. The effect of
the martensite volume fraction on the yield stress depends on the relative strengths