Effect of Austenitizing and Cooling Process on Microstructure Transformation of Low-Carbon Bainite Steel

Microstructure and grain size are important factors affecting the mechanical properties of low-carbon bainite steels. The influence of austenitization and cooling rate on the phase transformation of low-carbon bainite steel was investigated. The results show that the continuous cooling microstructure of low-carbon bainite steel was determined by pre-deformation, austenitization temperature, and cooling rate. The austenitizing process significantly affected the phase transformation of low-carbon bainite steel. When samples were heated in the range of 830–930 °C, their austenite grain sizes were relatively small with a maximum of 12.3 μm; when they were cooled at the cooling rate range of 1–30 °C/s, the microstructures consisted of mainly a large amount of ferrite and a small amount of retained austenite. When the austenitizing temperature increased to 1220 °C, the austenite grain size increased to 71.2 μm. The structure was mainly proeutectoid ferrite and granular bainite at the cooling rate of 1 °C/s. As the cooling rate increased, the amount of proeutectoid ferrite reduced, and the amount of bainite increased, resulting in a higher hardness. When the cooling rate was 10 °C/s, the proeutectoid ferrite disappeared completely, and the bainite was further refined.