Continuous cooling transformation of a microalloyed non－quenched and tempered steel for automobile

The continuous cooling transformation curve of supercooled austenite of a microalloyed non－quenched and tempered steel for automobile was measured, and the effect of cooling rate on phase transformation structure and microhardness was studied. The results show that the critical point A_c3 of the test steel was 838 ℃,and A_c1 was 732 ℃. When the cooling rate was less than 0.2 ℃/s, the continuous cooling transformation products of the test steel were ferrite, pearlite and bainite. When the cooling rate was 0.2 ℃/s, martensite appeared in the transformation product. When the cooling rate was 5 ℃/s, ferrite and pearlite disappeared, and the transformation products were bainite and martensite. With the increase of cooling rate, the content of martensite increased gradually, and the content of bainite decreased gradually or even disappeared completely. When the cooling rate increased to 20 ℃/s, the transformation products were martensite. With the increase of cooling rate, the microhardness of the test steel increased rapidly at first, and then the growth rate slowed down.