Effects of Different Heat Treatment Processes on Microstructure and Properties of 0.2C1.9Mn0.9Si0.7Al Cold-rolled High Strength Steel

The present work designed three different heat treatment processes for the cold rolled 0.2C1.9Mn0.9Si0.7Al steel:quenching and austempering（QAT）, quenching and tempering（QT） and quenching partitioning（Q＆P）. The microstructure evolution and its relationship with mechanical properties were analyzed by electron probe microanalyzer（EPMA）, X-ray diffraction（XRD） and electronic universal testing machine. The results show that the microstructure of QAT sample at room temperature mainly consists of intercritical ferrite, bainite and martensite/austenite island（M/A）. The microstructure of QT sample is mainly composed of intercritical ferrite and tempered martensite. And Q＆P sample includes intercritical ferrite,tempered martensite, bainite and M/A island. When the annealing temperature is 800 ℃, the volume fraction of retained austenite is significantly increased, showing a good combination of strength and ductility. As the annealing temperature increases in Q＆P samples, the yield strength and ultimate tensile strength increase while the elongation decreases, which is due to the increase of tempered martensite and the decrease of retained austenite.