Effect of Microstructure on High Temperature Strength of a Low‐Alloyed Fire‐Resistance Steel

Two distinct heat treatments are applied to a low‐alloyed fire‐resistance steel to develop either ≈55 vol% polygonal ferrite + ≈45 vol% granular bainite (PF + GB) or full granular bainite (GB) microstructures. Microstructural evolution at 600 °C for 3 and 6 h is characterized using scanning electron microscopy, transmission electron microscopy, energy‐dispersive X‐ray spectroscopy, and backscattering diffraction technologies. Results reveal superior thermal stability (i.e., degradation resistance at 600 °C) in full GB versus PF + GB microstructure. After 6 h exposure, the PF + GB microstructure exhibits heavy decomposition and partial recrystallization in the GB region, generating dislocation‐depleted equiaxed ferrite with coarse carbides. In full GB microstructure, moderate dislocation density and low carbon content provide abundant nucleation sites of precipitates while retarding coarsening kinetics, enabling the formation of finely dispersed carbides during thermal exposure. These fine carbides effectively inhibit subgrain boundary migration, thereby delaying recovery or recrystallization of the bainitic matrix. Consequently, full GB steel retains enhanced high‐temperature strength (402 MPa yield strength after exposure at 600 °C for 6 h), significantly outperforming PF + GB steel (only 282 MPa under identical conditions).