Effect of the cooling rate in the medium temperature zone on the phase transformation and microstructure of carbide-free bainitic steel

Controlling continuous cooling is considered an effective means of producing high-strength and high-toughness bainitic steel. A medium carbon bainite frog steel is designed. By combining a thermal dilatometer with in-situ high-temperature metallography, the influence of continuous cooling rate on the phase transformation of bainitic steel in the medium temperature range (500-365 °C) is studied. The microstructure and crystallographic orientation are characterized. The results indicate that with the increase of cooling rate in the temperature range for bainitic transformation, the initial temperature of bainitic transformation significantly decreases, and the transformation kinetics enhance, which causes the cooling rate for obtaining a maximum fraction of bainite significantly shifts to the left. As the cooling rate increases, the morphology of the bainite changes from a mixture of granular and coarse lath-like forms to fine lath-like forms. The volume fraction and average size of retained austenite as well as the size of bainite laths decrease with the increase of cooling rate. In addition, there are significant differences in the high-angle interface under different cooling rates, which is related to the variant selection mechanism of bainitic transformation at different cooling rates. The research results reveal the transformation process and microstructure evolution of bainite under different cooling rates in the medium temperature range, providing support for the microstructure control and performance optimization of bainite frog steel under continuous cooling control.