Nanobainite formation in high-Al medium-Mn steels: thermodynamic approach

Abstract

AbstractThe objective of this study was to analyze the thermodynamic feasibility of forming nanobainite in Al-alloyed medium-Mn steels through intercritical annealing (IA) and subsequent heat treatments. The research aimed to determine the influence of IA temperature and Mn content on the stability of austenite, the Ms temperature, and the resulting bainite plate thickness (BPT). Our findings indicate that the IA temperature range of 780–860 °C effectively decreased the Ms temperature, facilitating the formation of nanobainite. The results demonstrated that a higher Mn content increases an austenite fraction during IA, thus enhancing the potential for nanobainite formation. For the 3MnNb steel, the IA temperature of 860°C was sufficient to achieve bainitic plates thinner than 100 nm, whereas the 4MnNb steel required lower IA temperatures due to its higher Mn content. The transformation kinetics was found to be faster in 3MnNb steel, with a complete transformation time of 300 min, compared to approximately 600 min for the 4MnNb steel. Dilatometric analysis confirmed that the real austenite fractions were approximately 20% higher than the ones predicted by thermodynamic simulations, indicating potential limitations of the commercial software in accurate predicting the experimental conditions. The obtained results validate the proposed heat treatment strategy for achieving nanobainitic structures in medium-Mn steels.