Recrystallization Controlling in a Cold-Rolled Medium Mn Steel and Its Effect on Mechanical Properties

Owing to the excellent combination of specific strength and ductility, medium Mn steels (MMSs) with Mn contents of 3%-12% (mass fraction) are considered the most promising candidates for the third-generation advanced high-strength steel. The combination of excellent strength-ductility is mainly attributed to the active transformation-induced plasticity effect of the metastable retained austenite during deformation. Therefore, producing a considerable amount of retained austenite with reasonable stabilities in the steel by various heat treatment schedules is always important. In this study, granular- and lamellar-structured retained austenites were developed in a cold-rolled 0.15C-5Mn MMS by introducing a technical process of precontrolling ferrite recrystallization in the annealing schedule. The microstructures of the annealed samples were analyzed using SEM, EBSD, and TEM. The results show that duplex microstructures comprising various amounts of recrystallized ferrite and fresh martensite can be obtained in the cold-rolled MMS when controlling the occurrence of recrystallization at different intercritical temperatures by a preannealing process. When this microstructure is used for the final austenite reverted transformation annealing, the resultant ultrafine duplex microstructure with recrystallized ferrite and two types of heterogeneous retained austenite, i. e., lamellar and granular, is produced. The heterogeneous-structured austenite shows more sensitivity to increasing strain, i. e., various mechanical stabilities, which enable an excellent strength-ductility combination and reduced Luders strain in the cold-rolled medium Mn steel.