5th International High Manganese Steel Conference - HMNS 2022

Third generation Advanced High Strength Steels (AHSS), among which medium Mn steels, have potential applications for the automotive industry, due to their high balance between strength and ductility and their excellent forming properties. These good mechanical properties are possible thanks to the presence of a high amount of metastable retained austenite, which will transform into martensite during deformation, leading to an efficient TRIP (transformation induced plasticity) effect. Medium Mn steels microstructure usually consists of refined ferrite, retained austenite, and sometimes fresh martensite, and are obtained through intercritical annealing. Controlling the stability and the enrichment of the austenite phase during this thermal process is of vital importance for the steel to show an efficient TRIP effect.
The interplays between the different deformation mechanisms (dislocation gliding and strain-induced martensitic transformation) are not well understood in the literature. Moreover, the local gradient of alloying element composition exists inside grains of the same phase, which induces a gradient of austenite stability, and cementite precipitates can remain in the microstructure. Finally, mechanical instabilities, such as Lüders banding and the Portevin-Le Chatelier effect, are often reported in medium Mn steels, adding to the complexity of the deformation mechanism in such steels. Therefore, a local analysis in micro and nanoscale is necessary to understand the interactions between all those phenomena.
The contribution of this work is to characterize, via Transmission Electron Microscopy (TEM) and its associate analytical techniques (EDS, EELS) the stability of retained austenite, measuring the alloying element partitioning (Mn and C content) between retained austenite and the ferritic matrix. The characteristics of strain-induced martensite and deformation-induced defects (dislocation structures, twins…) are also inspected. Cold-rolled ferritic samples with composition 0.2C-4Mn-0.8Al-1.5Si were processed through different thermal treatments (intercritical annealing at various temperatures, with or without pre-austenitizing annealing) to provide the different medium Mn microstructures studied in this work.