Experimental study on the properties of Q960 ultra–high–strength steel after fire exposure

The Q960 ultra–high–strength steel (UHSS) has broad application prospects in engineering structures owing to its superior mechanical properties. Elevated temperatures caused by fire have lasting effects on the mechanical properties of structural steel. Therefore, to accurately evaluate the service performance of the UHSS structure after fire, the post–fire mechanical properties of Q960 UHSS were investigated experimentally in this study. Different exposure temperatures and cooling methods were used in the experiments. The changes in apparent features and microstructures of Q960 UHSS exposed to elevated temperatures were discussed. A mechanical test was also conducted to clarify the effects of exposure temperature and cooling method on mechanical properties of Q960 UHSS. The post–fire mechanical properties of mild steels (Q235, Q345, and Q420), high–strength steels (Q460, Q620, and Q690), UHSSs (Q960 and S960), AL 6082–T6 aluminum alloy, and bimetallic steels were investigated to comprehensively clarify the effects of exposure temperature and cooling method. A detailed discussion on the differences in stress–strain curves between the S960 and Q960 UHSSs after fire exposure was conducted. Based on the test results, the predictive equations for the key mechanical parameters of Q960 UHSS exposed to elevated temperatures were derived. The reliability analysis on the predictive model for key mechanical parameters was performed, where the resistance factor was suggested for Q960 UHSS after fire exposure. In the end, the predictive model was proposed to quantify the residual stress–strain curve of Q960 UHSS with different exposure temperatures and cooling methods.