Low-carbon strip steels 110 t " '" I! ......
0.04 wt% Nb. 0.09 wt% V
~12.'~ Sv ('f, mm'/mms~ 8 0--
170
~. 6 m~x......._m ~
i
s controlled rolled .......
i.. | ....
0 2 4 6 8 1'0 12
Cooling rote (~Figure 1.6 The change in the transformed microstructure with increasing cooling rate
after controlled rolling, for different austenite grain boundary surface area Sv per unit
volume for a 0.10 wt% C. 1.50 wt% Mn, 0.04 wt% Nb. 0.09 wt% V steel (Ouchi ~6)
about 500 MPa, depending on whether the steel is in the hot-rolled condition or
whether it has also been cold rolled and annealed. Thus, if a steel containing
about 1% manganese is cooled rapidly from an austenite state to a temperature
close to 450~ the ferrite and pearlite reactions are suppressed and the austenitr
will transform to a lower temperature transformation product called bainite.
Bainite consists of small packets of lath-like ferrite grains with low
misorientation between the grains and high-angle boundaries between the
packets. 9 Carbides are present at the lath and packet boundaries and there is
a variable dislocation density within the laths. Upper bainite formed at higher
temperatures in the bainitic range has a coarser structure than lower bainite
formed at lower temperatures and the finer lower bainite may also contain
carbides within the laths. 9 Acicular ferrite may be formed at temperatures between
the pearlitic and bainitic regions and may be considered to be a form of very
low-carbon upper bainite. The formation of both pearlite and bainite involve
the diffusion of carbon to form separate ferrite and carbide phases. Bainite may
form over a range of temperatures starting with the bainite start temperature (Bs)
and ending at the bainite finish temperature (BF). These temperatures are given
approximately by the following equations, 17 where the symbols in brackets refer
to weight percentages.
Bs(C ~ = 830- 270[C] - 90[Mn] - 37[Ni1 - 70[Or] - 83[Mo]
and BF(C ~ = Bs - 120[C]High-alloy contents and rapid cooling to below the bainite finish temperature
may lead to the formation of a separate structure called martensite by a
diffusionless transformation. In this transformation, a large number of atoms shear
cooperatively to form a fine plate-like structure with the carbon held substantially
in solution. Is Carbon, however, may be precipitated as carbide by tempering.
The crystal structure of martensite is tetragonal and the ratio of the c-axis of the