Effect of Nb Microalloying on the Dynamic Recrystallization, Bainitic Microstructure, and Mechanical Properties of Hot-Rolled Bainitic Steels

Controlling prior austenite morphology during single-pass hot rolling is essential for improving the strength–ductility balance of strip casting high-strength steels. In this study, Fe–0.24C–1.5Si–1.8Mn steels with and without 0.03 wt.% Nb were deformed by 50% at 950–1100 °C and subsequently held at 350 °C for 60 min for bainitic transformation. The effects of deformation temperature and Nb addition on austenite dynamic recrystallization, bainitic microstructure, and mechanical properties were investigated. For the Nb-free steel, recrystallization occurred at all these investigated deformation temperatures. The addition of 0.03 wt.% Nb markedly suppressed austenite recrystallization during hot deformation, resulting in the retention of elongated deformed austenite after deformation at 950 °C. Compared with the bainitic microstructure formed from fine recrystallized austenite in the 0Nb steel (without Nb addition), that formed from elongated austenite in the 03Nb steel (with 0.03 wt.% Nb) deformed at 950 °C exhibited a smaller lath width and higher HAGB and dislocation densities. In addition, the volume fraction of retained austenite increased from 4.5% to 9.8%, while its average thickness decreased from 105.6 nm to 49.1 nm. Consequently, under identical deformation conditions (50% reduction at 950 °C), Nb addition increased the yield and tensile strengths from 1064 MPa and 1261 MPa to 1088 MPa and 1323 MPa, respectively, while the total elongation remained nearly unchanged (24.8% vs 24.6%). These findings provide new insights into the optimization of thermomechanical processing for strip casting bainitic steels.