Evolution of Microstructure and Mechanical Properties of QP980/DP590 Laser Welded Joints after Simulated Fire

QP980/DP590 laser welded tailor-welded blanks were used for experiments. Different heating temperatures,cooling methods and holding time were set by heat treatment to simulate the process of automobile fire. The microstructure and mechanical properties of laser welded joints of dissimilar automotive steels after heating were investigated. The results show that after heating at 100-400 ℃, the microstructure has no obvious change, and the elongation increases slightly, when it rises to 15% at 400 ℃, it has exceeded the original sample. After heating at 500-700 ℃, the martensite transforms into tempered martensite, the strength and microhardness decrease, the yield strength at 700 ℃ has been reduced to 318 MPa, and the elongation continues to rise to 21% with the increase of temperature. After heating at 800-1000 ℃, the cooling method has a significant effect on the microstructure and properties. After water cooling, the volume fraction of martensite increases sharply, the strength and hardness of the joint increase significantly. At 900 ℃, the tensile strength is as high as1105 MPa, an increase of 85%, but the elongation decreases sharply, only 4% at 1000 ℃. Bainite is formed in the microstructure of the joint after air cooling. With the increase of heated temperature, the grains grow up in different degrees, and the strength and hardness of the test steel decrease. QP980/DP590 laser welding tailor blanks can still maintain the original level after heated at 100-400 ℃. After heating at 500-1000 ℃, the comprehensive mechanical properties show a downward trend, which will cause the premature fracture failure of vehicle structural parts, resulting in a decline in vehicle service safety.