Steels_ Metallurgy and Applications, Third Edition

74 Steels: Metallurgy and Applications

The chemical composition of the steel has a marked influence on the prop-
erties developed by influencing the volume fraction of martensite formed, with
increasing carbon, manganese and silicon all leading to an increase in strength. 129
Phosphorus and vanadium 13~ also increase strength: the phosphorus by solid
solution strengthening and the vanadium by increasing the martensite volume
fraction and by decreasing the ferrite grain size. Silicon also provides solid solu-
tion strengthening, increases the carbon content of the martensite, as mentioned
previously, and suppresses pearlite formation. 131
Variations in hot-rolling conditions may affect the strength of a cold-rolled
and annealed dual-phase steel by influencing the hot band microstructure, 132
but no specific microstructure was reported to give a superior strength/ductility
balance, as illustrated in Figure 1.80. A higher coiling temperature may lead to
a coarser grain size and a lower volume fraction of martensite. 133 In many cases,
however, hot-rolling conditions must be selected to give a low hot band strength
to minimize cold mill loads rather than to optimize product properties.
Annealing temperature is important for the processing of dual-phase steel
because it has a major influence in determining the volume fraction of marten-
site. Figure 1.81 shows, for a range of carbon, manganese and silicon contents,
how the yield stress and tensile strength vary with annealing temperature for gas
jet-cooled steels. The strength increases rapidly through the intercritical range
which leads to an increase in martensite volume fraction, but then increases
more slowly at higher temperatures. Sufficient time is needed at the annealing
temperature for the formation of sufficient austenite and data for a 0.06% carbon,
1.23% manganese steel (Figure 1.82) show that the volume fraction continues to
increase after holding for 10 minutes at 775~ The yield point elongation is,
however, eliminated after 1 minute at this temperature (Figure 1.83). The effect
of cooling rate is illustrated in Figure 1.84 for steels containing 1.2% manganese
and up to 0.5% chromium. The tensile strength increases with increasing cooling
rate, whereas the yield stress first decreases due to the removal of the yield point
elongation and then increases as a proof stress.

25

20

15

' " Ann. Cold red.

temp. 50% 80%

• F 750~ 0
  O~L = 800~. :

| , ,

7OO

| ,,, |

800 900

Tensile strength (MPa)

Figure 1.80 Tensile strength-elongation balance for dual-phase steels showing the

strength developed from different hot band structures and different cold reductions (After

Shirasawa and ThomsonZ32). F = Ferrite and carbide prior microstructure, B -- bainite

and M = martensite prior microstructure