Effect of cooling rate on mechanical properties and microstructure of 900 MPa grade cold rolled martensitic steel

Based on the continuous annealing production line with high hydrogen cooling process, and taking the 900 MPa grade cold rolled martensitic ultra-high strength steel as the research object, the transformation rule of the continuous cooling phase transformation zone and the effect of the rapid cooling process on the mechanical properties and microstructure of the steel were studied. The results showed that the continuous cooling transformation zone was composed of the pro-eutectoid ferrite transformation zone, bainite transformation zone and martensite transformation zone. With the increase of the cooling rate, the content of pro-eutectoid ferrite decreased gradually, while the content of bainite and martensite increased gradually. When the cooling rate was greater than 40 ℃/s, there was no pro-eutectoid ferrite transformation. When the cooling rate was greater than 80 ℃/s, it completely entered the martensite transformation zone. With the increase of the cooling rate of the rapid cooling process, the yield strength, tensile strength and yield ratio of the steel increased gradually, while the elongation decreased gradually. When the cooling rate reached 50 ℃/s, the yield strength, tensile strength and elongation of the steel met the mechanical property requirements of 900 MPa grade cold rolled martensitic ultra-high strength steel.