Results
The flow curves obtained from the double-hit deformation at 1200 °C and
1150 °C and strain rates of 0.25 s-1 and 0.5
s-1 and the interpass time of 5s are displayed in Fig.
2. Note that this temperature is approximately 450 °C and 400 °C above
the para-equilibrium Ae3 temperature of the alloy. As
expected, the results show that the stress levels increase with strain
rate and temperature. After the first deformation, both flow curves show
a steady increase in stress and then gradually decrease after a peak
stress of 58.2 MPa and 63.8 MPa for strain rates of 0.25 and 0.50
s-1 (at 1200 °C), respectively. Similarly, at 1150 °C,
peak stresses of 74.1 MPa and 81.2 MPa were recorded followed by gradual
decrease in stress for strain rates of 0.25 and 0.50
s-1, respectively. During the second deformation, a
peak stress drop of about 15% (compared to the first deformation) can
be seen for both testing temperatures. These results reveal that dynamic
softening has taken place during the 5 s time interval
It is well recognized that during hot deformation of medium-carbon
low-alloy steels, in addition to DRX, the DT of austenite to ferrite
also occur. The kinetics of DT is much faster than DRX [14]. The
double differentiation technique, proposed by Poliak and Jonas [12,
15], was used to evaluate the critical strains and stresses for the
initiation of both DT and DRX. A dedicated script was developed in
MATLAB® for analyzing the flow curves using this method.