Through-thickness microstructure and mechanical property evolution of a 460 MPa-grade QST H-section steel for seismic and fire-resistant applications

This study examines the through-thickness microstructure and mechanical property evolution of a 460 MPa-grade H-section steel developed for combined seismic and fire resistance. The steel was produced using the quenching and self-tempering (QST) process, which enables high strength in thick sections while reducing property gradients. Microstructural analysis revealed that the 24 mm-thick flange surface is dominated by tempered martensite, while the interior transitions to a multiphase mixture of ferrite, martensite–austenite (M/A) constituents and degenerated pearlite (DP), reflecting the cooling-rate gradient inherent to QST. Nanoindentation mapping confirmed phase-specific hardness, with M/A constituents hardest, DP intermediate, and ferrite softest. Tensile tests showed that the surface exhibits the highest strength but limited ductility, whereas the interior displays lower strength and improved strain hardening due to the higher DP fraction. After two thermal cycles at 600 °C, M/A constituents decomposed, and cementite spheroidization occurred in both tempered martensite and DP regions, accompanied by carbon diffusion and Cr-/Mn-rich carbide precipitation. These transformations increased yield strength while slightly reducing tensile strength and ductility. Overall, the findings elucidate the microstructure–property relationship across the flange thickness provide guidance for the design of QST-processed H-section steels for seismic and fire-resistant applications.