The effect of warm deforming and reversal austenization on the microstructure and mechanical properties of a microalloyed steel

The influence of warm deforming and reversal austenization on the microstructure and mechanical properties of a microalloyed steel was elucidated. Grain refinement induced by warm deforming and reversal austenization was studied by thermal simulation experiments, and its effect on properties was assessed by thin-plate hot rolling experiments. The study suggested that austenite was refined to ~10 µm after reversal austenization, which was further refined to ~5 µm after reversal austenization and application of warm deforming. Warm deforming recrystallized the microstructure during the reheating process, when the reheating rate was less than 2 °C/s. While recrystallization was inhibited at reheating rates greater than 5 °C/s. The ultimate microstructure obtained from the refined austenite via combination of warm deforming and reversal austenization, comprised of fine-grained (4.7±3.2 µm) ferrite and pearlite with small colony size (1.3±0.6 µm). On the other hand, the fine-grained microstructure was characterized by reduced extent of coarse precipitation (9.3±3.4 nm), compared to the coarse-grained thermo-mechanical controlled processing (TMCP) plate with dispersed and fine precipitation (4.0±1.3 nm), which was responsible for lower strength (~70 MPa) of the fine-grained plate than the coarse-grained TMCP plate. However, the fine-grained plate had an excellent low temperature toughness resulting from fine grain size, coarse precipitation, and small pearlite colony size.