Magnetic field-assisted heat treatment for controlling microstructure and synergistic enhancement of strength-plasticity in low-temperature bainitic steel

By isothermal transformation experiments under strong magnetic fields, combining the MTEX microstructure reconstruction technique based on orientation relationships during phase transformations, this study systematically reveals the effects that magnetic fields exert on microstructural evolution, variant selection, carbon redistribution, and mechanical properties in low-temperature bainitic steels. The results reveal that applying a magnetic field reduces the Gibbs free energy barrier of the austenite-to-bainite transformation, thereby accelerating bainite formation and promoting the preferential selection of specific variant pairs—particularly high-misorientation combinations such as V1/V2 and V1/V6—which in turn increases the density of high-angle grain boundaries. Simultaneously, the magnetic field enhances carbon segregation at bainite/austenite interfaces, leading to a more uniform and enriched carbon distribution within retained austenite. This redistribution improves the thermodynamic stability of retained austenite, drives its morphological evolution from blocky to film-like structures, and alters the activation pathway of the TRIP effect. Furthermore, magnetic field–induced carbon partitioning suppresses the rise of autocatalytic nucleation activation energy during transformation, further facilitating the formation of energetically favorable variants. Through the synergistic contributions of microstructural refinement, variant selectivity, and stabilization of carbon-enriched retained austenite, the steel exhibits markedly enhanced mechanical performance, including higher yield strength and a transition toward continuous yielding. This work provides a transformative approach for designing high-strength, ductile structural materials via non-contact magnetic field–assisted processing.