Correlation between discontinuous and continuous mechanical behavior and austenite microstructure in 0.06C-9Mn medium-manganese steel

Abstract

The microstructure-dependent mechanism underlying the phenomenon of high tensile performance accompanied by serrated flow remains unclear in medium-manganese (medium-Mn) steel. This is primarily due to the conventional dynamic strain aging (DSA) theory, which focuses on the effect of carbon atoms while largely neglecting the influence of austenite characteristics. In this study, 0.06C-9Mn steel with varying austenite microstructures was fabricated through different rolling and annealing processes. The correlation between discontinuous and continuous mechanical behavior and austenite characteristics was investigated through microstructural characterization, local and global strain and kinetics measurements, and analytical modeling. To elucidate the microstructure-dependent discontinuous and continuous mechanical behavior of medium-Mn steel, two key parameters were introduced: the effective carbon content for pinning mobile dislocations (X_E) and the intensity coefficient of the TRIP effect (T_E), both of which were influenced by grain size and initial dislocation density under a given austenite volume fraction. An increase in grain size and initial dislocation density of austenite resulted in a decrease in X_E and an increase in T_E, with serrated flow emerging once the X_E-T_E balance reached a critical state. This phenomenon may be attributed to the activation of the DSA mechanism, where a weakened dislocation pinning ability is counteracted by enhanced dislocation mobility driven by the TRIP effect. Furthermore, an increase in T_E contributed to improved tensile performance in medium-Mn steel, leading to high tensile strength accompanied by serrated flow. Additionally, the discontinuous stepwise kinetics of strain-induced α′-martensite (SIM_α′) transformation was accurately described using an analytical model based on strain surges or localization at the observation site as the PLC band propagated. These findings provide deeper insight into the mechanical behavior of medium-Mn steel and offer a pathway to achieving an optimized strength-elongation balance with minimized serrated flow.