采用DIL805L型淬火膨胀仪测定了780 MPa级低碳贝氏体钢的连续冷却转变（CCT）曲线，研究了冷却速度对该钢组织转变和硬度的影响。结果表明：780 MPa级低碳贝氏体钢在冷却速度小于5℃·s^-1时，转变产物为贝氏体；当冷却速度大于5℃·s^-1时，转变产物中开始出现马氏体组织，且随着冷却速度的增加，马氏体逐渐增多，贝氏体逐渐减少；随着冷却速度的增加，试验钢的显微硬度逐渐增大，在冷却速度为5℃·s^-1时，硬度值有明显大幅度的增加；透射电镜分析结果显示冷却速度为5℃·s^-1时，在贝氏体组织内，位错堆积，并在晶界处最先形成马氏体。

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.