Mechanical properties and constitutive model of fire-resistant steel at elevated temperature

Fire-resistant steel offers improved retention in strength and elastic modulus at high temperatures, enhancing the fire performance of steel structures. However, unified constitutive models for the high-temperature behavior of fire-resistant steel are still limited. This study aims to develop a broadly applicable high-temperature constitutive model based on experimental research and comparative analysis. Fifty high-temperature steady-state tensile tests were conducted on Q355FRC fire-resistant steel in accordance with Chinese and American testing standards, covering a temperature range from room temperature to 900 °C. The results show that Q355FRC fire-resistant steel does not exhibit a yield plateau at either room or elevated temperatures. Compared with conventional structural steel, Q355FRC fire-resistant steel shows improved high-temperature performance, retaining approximately 78 % of its elastic modulus and about two-thirds of its nominal yield strength even at 600 °C. The reduction coefficients specified in current design codes for conventional structural steel tend to be conservative for fire-resistant steel. To address this issue, predictive equations and a modified three-stage Ramberg–Osgood model are proposed. The generality of the proposed three-stage model is validated using experimental data from other studies. Simplified expressions for strain-hardening parameters are developed, enabling convenient estimation of the high-temperature mechanical properties for a broader range of fire-resistant steel grades. • Experimental study on high-temp tensile properties of Q355FRC steel up to 900 °C. • Predictive equations for elastic modulus and strength reduction at elevated temperatures. • A three-stage R–O model was proposed to better reflect strain hardening. • Formulas for strain hardening parameters were derived from test and published data.