Experimental study on mechanical properties of Q690D high strength steel during the cooling stage of fire

Previous studies on the mechanical properties of high strength steel (HSS) Q690 at fire were mainly focused on the heating stage and post-fire stage, while few studies have been conducted on those at the cooling stage of fire. In this paper, mechanical properties of Q690D steel at the cooling stage were experimentally studied and were compared with those at the heating stage. The results show that the differences in mechanical properties of Q690D steel between the cooling and heating stages are relatively small when the ultimate heating temperature (Tu) is within 400 °C, while the differences enlarge as Tu further increases, especially after Tu exceeds 800 °C. Moreover, the effects of both the cooling rate and holding duration at Tu on the ultimate strength of Q690D steel at the cooling stage are negligible, while these effects on Young's modulus, yield strength and ductility are not negligible and related to Tu. It is noted that different heating and cooling processes have an effect on the microstructure of the steel, especially after Tu exceeds 800 °C, which would influence the mechanical properties of the steel.