High temperature deformation behaviour and microstructure evolution in Cu containing High Strength Low Alloy (HSLA) steel

The hot deformation behaviour of a Cu containing high strength low alloy (HSLA) steel has been evaluated by carrying out hot compression tests over a temperature range of 900 degrees C to 1100 degrees C and constant true strain rates of 0.0003 to 1 s(-1). Although single peak flow curve is observed predominantly, a tendency to exhibit multiple peaks at low strain rates (0.0003 and/or 0.001 s(-1)) at temperature greater than 950 degrees C. The important parameters pertaining to dynamic recrystallisation (DRX), such as critical, peak, steady state stress and strain were ascertained, and the DRX state diagram depicting the variation of critical parameters governing DRX with Zener-Hollomon parameter Z was established. The volume fraction of recrystallised grains, computed from flow stress, as expected, follows a sigmoidal trend indicating higher kinetics at low strain rates for a given temperature. Reconstruction of prior austenite grain boundaries (PAGB) based on electron back scattered diffraction (EBSD) technique for a few select specimens clearly elucidated the effect of temperature and strain rate on the DRX behaviour of the steel. The DRX state diagram established here clearly demarcates the deformation space i.e. temperature-strain rate-stain space into regimes where the material exhibits full DRX, hardening/softening + DRX and only hardening (no DRX) behaviour. This would be useful for designing suitable forging sequence to achieve the desired microstructure and mechanical properties.