Microalloying strategies for achieving high-temperature strength retention in lean-Mo fire-resistant steels

Fire-resistant steels are designed to retain sufficient strength at elevated temperatures and prevent sudden structural collapse during fire exposure. Although Mo is widely recognized as the most effective alloying element for enhancing high-temperature strength retention, the development of low-Mo steels with comparable fire-resistant performance is increasingly demanded due to the high cost and limited availability of Mo. In this study, the feasibility of achieving fire-resistant performance in lean-Mo steels containing only 0.15 wt.% Mo is systematically investigated using a phase-fraction-based framework. Seven steels with systematically varied alloying combinations were fabricated, and their fire-resistant performance was evaluated by tensile testing at room temperature (RT) and 600 o C. Microstructural characterization focused on quantitative phase-fraction analysis using electron backscatter diffraction. Notably, an appropriate alloy design enabled the formation of a high bainite fraction exceeding 83% even in 0.15 wt.% Mo. This increase in bainite fraction led to a substantial enhancement in yield strength (YS) from 290 to 565 MPa at RT and from 109 to 406 MPa at 600 o C, resulting in a YS ratio (600 o C/RT) of 0.674, thereby satisfying the commonly accepted fire-resistant steel criterion. Furthermore, the YS at both RT and 600 o C, as well as the yield-strength ratio (600 o C/RT), exhibited strong linear correlations with the bainite fraction. These results demonstrate that lean-Mo fire-resistant steels can be realized through rational microalloying strategies and confirm that the bainite fraction serves as a robust, physically meaningful, and experimentally accessible descriptor for evaluating and designing fire-resistant steels.