Roles of cooling rate of undercooled austenite on isothermal transformation kinetics, microstructure, and impact toughness of bainitic steel

Abstract

At present, the application of extra thick super bainite steel has attracted considerable attention. In this paper, medium carbon bainite steel is taken as the research object, and the effects of the cooling rate of undercooled austenite on the transformation kinetics, microstructure and mechanical properties of bainite steel are studied systematically. The results show that the incubation period of bainite shortens and the transformation kinetics accelerates with the decrease of the cooling rate of undercooled austenite. The segregation of solute atoms at a slow cooling rate shortens the incubation period. When bainite nucleates and grows by an autocatalytic method, the size of the untransformed austenite is large and the strain compatibility is strong, leading to low activation energy of its autocatalytic nucleation and accelerating the kinetics of bainite transformation. The size of block structure and the length of bainite sheaves decrease with the increase of cooling rate. The impact toughness of the sample with a cooling rate of 30 °C/s is twice as high as that of the sample with a cooling rate of 0.3 °C/s. As the smallest unit to control the toughness of bainite steel, block structure affects the impact toughness. The impact toughness of bainite increases with the decrease of its block size. The content of film-like retained austenite increases with the increase of cooling rate, which is also one of the factors of excellent impact toughness of bainite at high cooling rate.