A series of simulated on-fire processing experiments on Q345 R steel plates was conducted, and the plates’ Brinell hardness, tensile strength, and impact energy were tested. Microstructure morphologies were systematically analyzed using a scanning electron microscope with the aim of investigating the effect of the steel’s microstructure on its performance. All examined performance parameters exhibited a substantial decrease in the cases of samples heat-treated at temperatures near 700°C. However, although the banded structure decreased with increasing treatment temperature and holding time, it had little effect on the performance decline in fact. Further analysis revealed that pearlite degeneration near 700°C, which was induced by the interaction of both subcritical annealing and conventional spherical annealing, was the primary reason for the degradation behavior. Consequently, some nonlinear mathematical models of different mechanical performances were established to facilitate processing adjustments. A series of simulated on-fire processing experiments on Q345 R steel plates was conducted, and the plates’ Brinell hardness, tensile strength, and impact energy were tested. Microstructure morphologies were systematically analyzed using a scanning electron microscope with the aim of investigating the effect of the steel’s microstructure on its performance. All examined performance parameters showed a substantial decrease in the cases of samples heat-treated at temperatures near 700 ° C. However, although the banded structure decreased with increasing treatment temperature and holding time, it had little effect on the performance decline in fact. Further analysis revealed that pearlite degeneration near 700 ° C, which was induced by the interaction of both interaction of both subcritical annealing and conventional ball annealing, was the primary reason for the degradation behavior. mechanical performances were established to facilitate pro cessing adjustments.