Post-fire mechanical and microstructural properties of dual-phase and martensitic steels

Advanced High Strength Steels (AHSS) are increasingly considered for structural applications due to their superior strength-to-weight ratios and formability. However, the complex multi-phase microstructures in AHSS, developed through specific manufacturing processes, render them highly sensitive to thermal exposures. Understanding their post-fire residual mechanical properties is critical for assessing structural integrity and determining post-fire strategies such as reuse, retrofitting, and replacement. This study experimentally investigates the residual mechanical behaviour and microstructural properties of dual-phase (DP) steel with grade of DP980 and martensitic (MS) steel with grade of MS1500. Steady-state tensile tests were conducted on cold-rolled coupons heated to temperatures ranging from 200 °C to 1000 °C, followed by natural air-cooling. Complementary Scanning Electron Microscopy (SEM) analyses were performed to correlate macroscopic mechanical response with microstructural changes. Test data indicate that DP and MS steels experienced significant strength deterioration, reaching a first minimum residual yield strength of 50% at 725 °C for DP and 32% at 800 °C for MS steels due to the tempering of the initial martensite phase. However, strengths started to recover after critical temperatures with DP steel recovering 36% of its original yield strength at 875 °C and a mild recovery of 5% for MS steel at 1000 °C. SEM analyses demonstrated that the recoveries are attributed to reaustenitisation and the subsequent formation of fresh martensite during the cooling phase. The predictive equations available in literature failed to precisely estimate the residual post-fire tensile properties of DP980 and MS1500 steels. Therefore, a set of new predictive equations are proposed using the experimental data generated in this study to facilitate the accurate post-fire assessment of AHSS structures.