Microstructural evolution and strength-toughness behavior of fire-resistant steel under thermo-mechanical controlled processing

Fire-resistant steels are engineered to retain high strength at elevated temperatures, typically maintaining a yield strength (YS) ratio (600 °C/RT) above 0.67 to prevent sudden structural collapse during fire exposure. Although achieving such strength retention is critical, ensuring sufficient fracture toughness is equally essential for structural reliability. However, toughness behavior in fire-resistant steels has been largely overlooked, particularly in relation to thermomechanical processing. In this study, the influence of thermomechanical controlled processing (TMCP) on the microstructure and mechanical behavior of fire-resistant steel was investigated, with an emphasis on the strength-toughness trade-off. With increasing TMCP conducted below the non-recrystallization temperature (T nr ), the bainite fraction decreased markedly from 79 % to 27 %, whereas the ferrite fraction increased. The prior austenite grain size of the bainite was significantly refined, whereas the ferrite grain size remained nearly unchanged. This microstructural evolution led to a gradual reduction in yield strength (YS) at both room and elevated temperatures, decreasing the YS ratio (600 °C/RT) from 0.717 to 0.501. Meanwhile, the Charpy impact energy increased from 32.9 to 169.5 J, thereby demonstrating a clear trade-off between strength and toughness. Notably, the bainite fraction exhibited a strong linear correlation with the strength and YS ratio, whereas ferrite played a dominant role in enhancing toughness, with a complementary contribution from bainitic grain refinement. These findings demonstrate that the mechanical performance of fire-resistant steels can be effectively tuned through process optimization alone, thereby providing a practical strategy for designing steels with balanced strength and toughness.