Evolution of Microstructure and Mechanical Properties of DP590 Structural Steel of Vehicle in Heat-Treatment Post-Fire Simulation

To investigate the post-fire change of the microstructure and properties of a vehicle body made of DP590 structural steel, this study simulated the post-fire process of a vehicle by considering heating processes with different heat treatment methods. The results reveal that, when the temperature was 200–700 °C, the strength decreased to varying degrees as the temperature increased, and the microstructure consisted of ferrites and martensites. With heating to 800 °C and air-cooling, the microstructure consisted of ferrites, bainites, and pearlites, and the strength and elongation were lower compared with the ordinal material. After water-cooling, the microstructure consisted of ferrites and martensites, and the strength was much higher compared with the original base material, but the elongation decreased. With heating to 900–1000 °C, the microstructure consisted of ferrites, bainites, and pearlites after air-cooling, and the strength and elongation decreased. After water-cooling, the microstructure transformed into bainites and martensites, with high strength but low elongation. Moreover, the high-temperature mechanical properties of DP590 steel were measured. When the temperature was lower than 400 °C, the strength increased and the elongation decreased. When the temperature was higher than 400 °C, the strength decreased sharply and the elongation increased.