Comparative investigation of phase transformation behavior as a function of annealing temperature and cooling rate of two Medium-Mn steels

Transformation-induced plasticity (TRIP)-assisted medium-Mn steels with a ferritic matrix containing considerable amounts of retained austenite are a promising candidate to fulfill the requirements for the third-generation of advanced high-strength steels (AHSS), which is currently under development. The influence of the intercritical annealing temperature and cooling rate on the final microstructure of a 0.1C3.5Mn and 0.1C5Mn steel, respectively, was elaborately investigated. Dilatometric experiments were carried out and additionally supported by microstructural observations. During soaking in the two-phase ferrite-austenite region and subsequent slow cooling the C and Mn concentration in the austenite increased and resulted in its chemical stabilization. The variation of the annealing temperature and cooling rate altered the amounts of ferrite, retained austenite, bainite, pearlite, and martensite in the final microstructure. Furthermore, two thermodynamical models for the prediction of the maximum retained austenite content and the optimal annealing temperature have been thoroughly evaluated in this work. It can be stated that the experimental data revealed a shift of the maximum retained austenite to higher annealing temperatures compared to the model calculations. As diffusional transformations were not considered in the applied models, slow cooling rates led to pronounced deviations between calculation and experiment, thereby showing the need for model adaptions. Two low-carbon medium-Mn steels with a lean Mn-content of 3.5 wt% and a commonly reported one of 5 wt%, respectively, are investigated by means of dilatometry. The microstructural evolution is thoroughly studied (SEM/EBSD/XRD) - focusing on the mechanisms of austenite stabilization.