A novel double-step intercritical annealing (DSIA) process was conducted on Fe-0.16C-4.7Mn-1.6Al-0.2Si transformation induced plasticity (TRIP) steel. In a present study, an effect of different heating and cooling rates applied to the second intercritical annealing (IA) step on the microstructure was studied in detail by using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) techniques. The quantitative analysis of the retained austenite fractions for particular heat treatment variants was carried out via X-ray diffraction (XRD). The formation of complex microstructures containing a small fraction of ferrite, retained austenite and two types of martensite was observed regardless on the applied heating and cooling rates. It was found that the soft ferrite can be replaced by martensite through the incomplete stabilization of austenite with C and Mn during short second intercritical annealing step. The heating rate influences significantly the kinetics of austenite formation during the second heat treatment step. Low heating rates ensure more time for formation and homogenization of austenite during heating improving its thermal stability. Thus, the best balance of martensite and austenite fractions in combination with homogenous morphology of the microstructure was noted for the specimen heated at a rate of 3 °C/s, which was the lowest one in the reported experiment.

Medium-Mn automotive sheet steels require optimized heat-treatment processes to obtain benefits caused by strain-induced martensitic transformation of retained austenite (RA) during sheet metal forming or crash events. The intercritical annealing (IA) approach at different temperatures in a range of 640 °C to 800 °C is proposed in the study for a 5Mn hot-rolled medium-Mn sheet steel. The experiments were performed in terms of dilatometry. The analysis of the cooling curves allowed development of a new method for calculating the high-temperature phase equilibrium. The calculations were validated by modeling with JMatPro and experimentally verified by X-ray diffraction (XRD). The microstructure evolution was characterized using light optical microscopy and scanning electron microscopy (SEM), including electron backscatter diffraction (EBSD). The quantitative determination of the fraction, morphology, chemical composition, and stability of the RA was done. Mechanical properties were determined by hardness measurements. The research showed a substantial influence of the IA temperature on the RA fraction and chemical stability and properties of medium-Mn Al-alloyed steel. At temperatures of 680 °C and 700 °C, the largest fraction of over 35 pct of stable RA was obtained, which does not transform to martensite during cooling.