Effect of Double Annealing on the Microstructure and Mechanical Properties of a Medium Manganese Steel

In this study, the evolution of microstructure and mechanical behavior in medium manganese steel subjected to double intercritical annealing was investigated. Austenite reversion transformation (ART) occurred during both single (730°C) and double intercritical annealing (700°C and 750°C). A complex ultrafine microstructure consisting of three phases—retained austenite, martensite, and ferrite—with fine carbides was formed after single annealing. Following double annealing at 700°C, the martensite generated during the first intercritical annealing reverted to austenite, resulting in the maximum stabilized austenite fraction and a final microstructure composed mainly of ferrite and retained austenite. When the second annealing temperature was increased to 750°C, a larger amount of austenite was formed; however, its reduced thermal stability caused partial transformation into martensite. Double intercritical annealing thus produced multiple populations of austenite and ferrite/martensite phases with varying thermal and mechanical stabilities, enhancing the overall mechanical performance. The strength was maximized after high‐temperature (750°C) double annealing, whereas elongation and ductility were optimized at the lower annealing temperature (700°C).