Effect of Heat Treatment Process on Microstructure and Properties of 780 MPa Grade Low Carbon Bainitic Steel

The continuous cooling transformation (CCT) curves of 780 MPa grade low carbon bainitic steel were determined by DIL805L quenching dilatometer to study the effect of cooling rates on the microstructure transformation and hardness of the steel. The results show that for 780 MPa grade low carbon bainitic steel, when the cooling rate is lower than 5℃·s^-1 the transformation product is bainite, and when the cooling rate is higher than 5℃·s^-1, martensite structure begins to appeare in the transformation product. With the increase of cooling rate, the martensite increases gradually and the bainite decreases gradually. With the increase of cooling rate, the microhardness of the test steel increases gradually, and the hardness increases obviously at the cooling rate of 5℃·s^-1. Transmission electron microscope (TEM) analysis results show that when the cooling rate is 5℃·s^-1, dislocation stacks in the bainite, and martensite first forms at the grain boundary.