Ultra-low cycle fatigue performance of Q690 high-strength steel after exposure to elevated temperatures

High-strength steel (HSS) can reduce steel consumption compared to ordinary steel and has extensive applications in the building industry. HSS members in building structures are inevitably subjected to fire. The elevated temperature caused by fire leads to significant changes in the service properties of HSS. Therefore, research on the service properties of HSS after a fire is of great significance when evaluating whether the HSS structure can continue to serve. A series of studies on the ultra-low cycle fatigue (ULCF) performance of Q690 HSS after fire was carried out to evaluate the ability of Q690 HSS to resist strong earthquakes after fire. Q690 HSS specimens were subjected to heat treatment at 600–900 °C, and ULCF tests with different strain amplitudes were carried out on the cooled specimens. The hysteretic curves, fatigue life, and total energy dissipation density of the Q690 HSS after the fire were obtained. On this basis, the stress and strain evolution laws under the ULCF load of Q690 HSS, with different fatigue strain amplitudes and exposed temperatures, were analysed. Subsequently, the metallographic structure and ULCF fracture behaviour of Q690 HSS after fire were discussed. Based on the test results, two sets of independent prediction equations were proposed to evaluate the ULCF performance of Q690 HSS after a fire, which was in good agreement with the test results.