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.

The comparison of the dynamic mechanical behavior and microstructure of two medium manganese sheet steels (3Mn–Al and 5Mn–Al) alloyed with aluminum is aimed. Mechanical properties under dynamic tensile loads are determined by means of rotary hammer dynamic tests at strain rates of 250 and 1000 s⁻¹ and analyzed together with the results of static tensile test. It is found that the results are significantly affected by the variations in Mn content in the range from 3 to 5 wt%. In both steels, the tensile strength increases with increasing strain rate, but the variation in the strain rate range has a moderate effect on mechanical behavior. The highest ultimate tensile strength of 1475 MPa is measured in the 5Mn–Al steel, whereas the 3Mn–Al steel is characterized by better total elongation due to a larger fraction of retained austenite and more pronounced transformation-induced plasticity effect. The results show that the mechanical properties of 3Mn steel are more strain rate sensitive than those of 5Mn steel. The microstructural features are characterized qualitatively and quantitatively by X-ray diffraction, scanning electron microscopy, and electron back-scattered diffraction techniques.

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.