Effect of Niobium Content on Microstructure Evolution, Mechanical Properties, and Phase Transformation Kinetics of an Automotive Beam Steel Under Continuous Cooling Conditions

This study investigates the effects of niobium (Nb) content (0.02 wt% and 0.05 wt%) on an automotive beam steel microstructural evolution, mechanical properties, and phase transformation kinetics under continuous cooling via optical microscopy, electron backscatter diffraction (EBSD), tensile testing, thermal dilation analysis, and the Johnson–Mehl–Avrami (JMA) model, revealing Nb's significant regulatory mechanism on phase transformation pathways and kinetics: with increasing cooling rates (1–40 °C s −1 ), both steels transform from ferrite/pearlite to bainite (granular to lath), but 0.05Nb steel exhibits faster transformation and lower start temperature. EBSD shows 0.05Nb steel has a significantly higher proportion of high‐angle grain boundaries (HAGBs), with bainite laths refine to 0.25 μm at 40 °C s −1 (55% smaller than 0.02Nb steel's 0.55 μm). Mechanical tests reveal 0.05Nb steel at 40 °C s −1 achieves 1000 MPa yield strength (YS) and 1160 MPa tensile strength (TS), 32% and 29% higher than 0.02Nb steel (755, 900 MPa), with slight uniform elongation (UE) reduction. Kinetic analysis indicates Nb inhibits ferrite via solute drag, expands bainite region, and enhances solid solution, providing a basis for high‐strength, tough Nb‐microalloyed steel design.