The high temperature of fire has an impact on the mechanical properties of structural steel, hindering its use. Q345, Q460, Q550 and Q690 structural streels were chosen to investigate their mechanical behaviours after fire treatment at temperatures from 300 degrees C to 800 degrees C and air cooling. The mechanical parameters (yield stress, elastic modulus, yield plateau size, ultimate strength and necking strain) were analysed by considering the fire tem-perature, and the residual factors of the parameters for the Q345, Q460, Q550 and Q690 steels were compared with the results obtained at an ambient temperature. The results showed that the mechanical parameters for each steel began to reduce when the target temperature exceeded 600 degrees C, and the residual factor was a descending exponential function or a linear function of the target temperature. The predictive equations of residual factors in the literature, about Q460 and Q690 steels, were collected and analysed, and the new equations were predicted depending on all available experimental data. Finally, a stress-strain model incorporating fire temperature was proposed by a piecewise function to describe the stages of the stress-strain curve (elasticity, yielding and plasticity), and the model can represent the experimental results accurately.
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