Microstructural characteristics and tensile behavior of medium manganese steels with different manganese additions

Manganese is normally considered as the most important alloying element in medium Mn steels. However, the influence of Mn additions on the microstructural characteristics and the ensuring mechanical properties has not been much studied. This was addressed in the present study by investigating two variants of medium Mn steels with compositions of 0.2C-7/10Mn-3Al (in wt%), subjected to intercritical annealing (IA) and tensile testing. Results showed that a higher Mn addition can effectively increase the fraction of retained austenite (RA) at ambient temperature. However, the mechanical stability of RA was reduced, due to the lower C concentration partitioned into austenite during IA treatments. The higher fraction and lower mechanical stability of RA in the 10Mn steel gave rise to an enhanced transformation-induced plasticity (TRIP) effect, which was the main contributor to its higher strain hardening rate and improved mechanical properties, confirmed by a dislocation density-based strain hardening model. The yielding behavior and the Portevin-Le Chatelier (PLC) effect of the investigated steels were also found to be altered with different austenite mechanical stability as a consequence of different Mn additions and IA temperatures. Higher deformation-induced martensite transformation (DIMT) kinetics, i.e. lower mechanical stability of austenite, resulted in a lower yield point elongation, and eventually changed the yielding to a continuous manner due to the formation of stress-induced martensite. The PLC effect only occurred in the intermediate austenite stability range, and the critical strain for the onset of jerky flow showed a first decrease and then an increasing trend with higher DIMT kinetics. The observed beneficial effect of Mn additions on mechanical property improvement in medium Mn steels offers a significant aspect for further alloy design of such steels.