Exploring the impact of intercritical annealing on microstructural evolution and mechanical performance in low alloy multiphase TRIP-assisted steels

Understanding the role of the constituent phase on mechanical properties is the key strategy for achieving advanced mechanical properties in multiphase TRIP-assisted steels. This work unravels the opposing role of ferrite on tensile ductility and impact toughness in the constituent phase fraction-controlled multiphase TRIP-assisted steel. The specimens were subjected to three different intercritical annealing (IA) temperatures (775 °C, 800 °C, and 825 °C) for 20 min, and then continuously followed the bainitic isothermal transformation (BIT) at 400 °C for 30 min. With an increase in the IA temperature, the bainite fraction and the stability of retained austenite increased, accompanied by a decrease in the ferrite fraction. In-situ EBSD observation revealed the crack initiation at the deformation-induced martensite/ferrite interface. The strain localization in ferrite, assisted by transforming neighboring austenite to martensite, promoted crack initiation during the tensile test. The tensile elongation decreased from about 28 % to 17 % due to the lower strain partitioning in ferrite originating in the reduced ferrite fraction and TRIP amounts as the IA temperature increased. However, the refined grain size and the reduced ferrite fraction by a higher IA treatment improve impact toughness by about 60 % at room temperature. This study on the imbalance between tensile elongation and impact toughness sheds light on the microstructure design for advanced mechanical properties in multiphase TRIP-assisted steels.