In-Situ Observation of the Multi-phase Transition and Microstructure Evolution of 22MnB5 Steel

An experimental investigation was conducted to elucidate the impact of cooling rate on the microstructure evolution of boron-containing steel 22MnB5 by in-situ observation using the confocal laser scanning microscope (CLSM). The observations manifested distinct multi-phase formation of reconstructive grain boundary allotriomorphic ferrite (GBA) and pearlite (P) to sympathetic intergranular acicular ferrite (IAF) and bainite (B), as well as the displacive martensite (M) under different predefined cooling rates (1, 10, and 20 °C/s). Notably, as the cooling rate escalated from 1 to 10 or 20 °C/s, the starting and finishing phase transition temperature decreased significantly. For the 22MnB5 steel cooled at 1 °C/s, the solid phase transition sequence followed γ→GBA→P→IAF, while for the steel cooled at 10 and 20 °C/s, the transition sequence shifted to γ→B→M. Additionally, in the sample cooled at 20 °C/s, the bainite lath spacing reduced and more small size martensite appeared as the mechanical stabilization of austenite. Simultaneously, the dislocation density increased compared to the slow cooled samples, primarily due to the elevated nucleation rate from austenite to ferrite and larger internal stress induced by the enhancing cooling intensity.

Graphical Abstract