Strategic niobium integration and thermomechanical processing in the advancement of novel CMnSiAlPMo TRIP-aided bainitic steel

This study examines the effects of niobium (Nb) addition and different thermomechanical controlled processing (TMCP) regimes on the flow stress behaviour and microstructure evolution of a newly developed CMnSiAlPMo TRIP-aided bainitic steel. TMCP tests were conducted with various hot deformation passes, followed by austempering at 400 °C for 10 min using a Gleeble 3800 thermomechanical simulator. Microstructures were analysed using scanning electron microscopy with electron backscattering diffraction and X-ray diffraction. Results showed that increasing the number of passes and reducing the final deformation temperature (FDT) enhanced the flow behaviour for both 0Nb and 0.05Nb alloys, with strain hardening being the dominant mechanism across all regimes. The four-pass regime with an FDT of 850 °C for the 0Nb alloy achieved the highest hardness (457 HV), attributed to grain refinement, which was more influential than the retained austenite fraction. For the 0.05Nb alloy, the two-pass regime at 1050 °C showed the highest hardness (428 HV), resulting from a lower retained austenite fraction. Additionally, Nb addition significantly refined the microstructure and increased the peak flow stress from 385 MPa to 421 MPa for the four-pass regime. The prior austenite grain size decreased from 23 to 12 μm in the single-pass regime, and the largest grain size in the cumulative grain size distribution (D90%) decreased from 8.45 to 7.49 μm.