The long-term fire safety of steel structures necessitates fire-resistant (FR) steels with superior strength retention after prolonged exposure to elevated temperatures. This study investigates two FR steels with Mo contents of 0.25 wt.% and 0.50 wt.% (correspondingly adjusted Cr contents to maintain a nearly constant total Mo + Cr alloy level) after thermal exposure at 600 °C for up to 6 h. Both steels exhibit a polygonal ferrite (PF) and granular bainite (GB) microstructure, but with different phase fractions. Increasing Mo to 0.5 wt.% promotes bainite formation, resulting in a predominantly granular bainitic microstructure (88% GB and 12% PF). A multi-scale characterization reveals that the higher-Mo/lower-Cr design not only increases the fraction of GB but also dramatically enhances its thermal stability. Importantly, Mo suppresses the premature precipitation of coarse (Nb, Ti)(C, N) in the parent austenite, thereby retaining solutes for the subsequent precipitation of a high number density of fine carbides within the bainitic ferrite laths during thermal exposure. The combination of a high-density dislocation substructure within the stable GB and the strong precipitation strengthening from these dispersed nano-carbides is primarily responsible for the superior strength retention in the higher-Mo/lower-Cr FR3 steel. This steel maintains a high yield strength of 440 MPa after 6 h at 600 °C, which is 158 MPa higher than its 0.25 wt.% Mo counterpart and satisfies the international fire-resistance criterion (YS600 °C/YSRT > 2/3). This work provides a mechanistic basis for optimizing Mo–Cr alloy design to achieve exceptional long-term high-temperature performance in FR steels.
