Effects of Heat Treatment Parameters on Strength-toughness Combination of Low Carbon Bainitic Steel

Heat treatment of hot-rolled steel plate was carried out by different process parameters. The microstructural evolution of the experimental steel under different heat treatment processes was studied by high temperature confocal scanning microscope and scanning electron microscope, and the strong plasticity and toughness of the experimental steels under different process conditions were analyzed. The results show that the segregation of Mn slows down the transformation kinetics, leading to the formation of banded martensite. Especially, the bainitic transformation shows local feature for the case of refined prior austenite grain size. With the increase of austenization temperature and isothermal time, the microstructure becomes uniform and the banded martensite is avoided. However, the coalescence of bainite occurs for coarsened austenite grain size. The ductility and toughness of the bainitic steel is influenced by complex factors such as the bainite fraction, banded martensite and retained austenite, and hard brittle martensite in the bainite matrix is the main reason for toughness deterioration.