Elevated‐Temperature and Post‐Fire Mechanical–Microstructural Evolution of 350‐Grade Fire‐Resistant Structural Steel

Fire‐resistant steel (FRS) is rapidly transforming modern construction by significantly enhancing structural resilience during and after fire events. Unlike normal structural steel (NSS), FRS exhibits exceptional mechanical performance at elevated temperatures, yet its behaviour under extreme heat and subsequent cooling remains insufficiently understood. This study delivers a thorough experimental investigation of 350‐grade FRS subjected to high temperatures between 500°C and 1000°C, followed by cooling via air and water. Through detailed tensile testing at elevated temperatures and after cooling, key mechanical properties, including yield strength, ultimate strength, Young's modulus and ductility, are quantified to reveal critical performance trends. Results demonstrate that FRS retains much of its strength up to 500°C, contrasting sharply with NSS, but degrades significantly beyond 600°C. Interestingly, post‐fire tests highlight the pivotal influence of cooling conditions: specimens cooled in air exhibit strength losses beyond 800°C, whereas water‐cooled specimens show strength gains accompanied by reduced ductility. This dichotomy underscores the vital role of cooling regimes in shaping residual structural capacity. Complementary microstructural analyses using scanning electron microscopy (SEM) reveal phase transformations that directly correlate with these mechanical changes, offering new insights into the metallurgical mechanisms driving FRS behaviour after fire exposure. Collectively, these findings deepen our understanding of FRS post‐fire performance and pave the way for more resilient, fire‐safe structural designs.