A unified constitutive model for a low alloy steel during warm deformation considering phase differences

The warm flow behaviors of a low alloy steel were investigated by isothermal compression tests over a broad range of temperatures (873-1173 K) and strain rates (0.001-1 s−1). The compression strain-stress curves showed that the flow stress at 1023 K was lower than that at 1073 K under four studied strain rates, with an abnormal stress-temperature region emerging. It was found that this abnormal phenomenon appeared in the austenite-ferrite dual-phase temperature region, while ferrite occurred at low temperatures (873, 923 and 973 K) and austenite occurred at high temperatures (1123 and 1173 K), respectively. The constitutive relationship of each phase was constructed by a dislocation model coupling Arrhenius equations, which was verified by microstructure observations. Based on single phase's constitutive relationship, the abnormal stress-strain curves were manifested through a modified mixture law, in which a higher strain rate sensitivity of ferrite at intercritical temperatures was taken into consideration. In the end, a unified constitutive model which could adequately describe the warm flow behavior of the studied low alloy steel over the entire ranges of temperatures was developed.