Temperature-Dependent Microstructural Evolution of Al-Rich Medium-Mn Steel During Intercritical Annealing

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 degrees C to 800 degrees 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 degrees C and 700 degrees C, the largest fraction of over 35 pct of stable RA was obtained, which does not transform to martensite during cooling.

Automated summary

This study investigates the impact of heat-treatment processes on automotive medium-Mn sheet steels. A new method for calculating high-temperature phase equilibrium is developed and validated through experiments. The results show that the intercritical annealing temperature has a significant influence on the microstructure, chemical stability, and mechanical properties of the medium-Mn Al-alloyed steel.