Effect of thermomechanical processing on the retained austenite content in a Si-Mn transformation-induced-plasticity steel

The influence of hot deformation on the microstructure of a hot-rolled Si-Mn transformation-induced-plasticity (TRIP) steel was evaluated in an effort to better control retained austenite content. In this study, axial compressive strains varying in amounts from 0 to 60 pct were imposed in the austenite phase field, and effects on the formation of polygonal ferrite, bainite, and retained austenite were determined. In addition, modifications in simulated coiling temperature from 420 °C to 480 °C and cooling rates from the rolling temperature, between 10 °C/s and 35 °C/s, were assessed. Fast cooling rates, low coiling temperatures, and low degrees of hot deformation were generally found to decrease the amount of polygonal ferrite and increase retained austenite fraction. Unexpectedly, a sharp increase in polygonal ferrite content and decrease in retained austenite content occurred when the fastest cooling rate, 35 °C/s, was coupled with extensive hot deformation and high coiling temperatures. This effect is believed to be due to insufficient time for full recovery and recrystallization of the deformed austenite, even in the absence of intentional microalloying additions to control recrystallization kinetics. The resultant decrease in hardenability allowed the ferrite transformation to continue into the holding time at high (simulated) coiling temperatures.