Effect of Torsion on Microstructure and Mechanical Properties of Medium Manganese Steel

In this study, a multi-gradient microstructure was introduced into medium-Mn steel through torsion following heat treatment at different annealing temperatures, and through investigations on the mechanical properties under two annealing temperatures, it has also been revealed that different annealing temperatures before torsion affect the stability of austenite after torsion, thereby leading to distinct variations in mechanical performance. The yield strengths of the studied steel after annealing at 600 °C and 620 °C were 762 MPa and 673 MPa, with total elongation of 47.4% and 44.1%, respectively. After 90° torsion, the yield strength of experimental steels increased to 834 MPa and 808 MPa, while the elongation decreased to 21.6% and 29.5%, respectively. The gradient distributions from the center to the edge were observed for the austenite volume fraction, average grain size, martensite volume fraction, GND density, and hardness. The comparative analysis of the two annealing temperatures indicates that the larger grain size in the 620—annealed sample leads to its lower yield strength, while its higher austenite volume fraction and moderate stability promote a more sustained TRIP effect during deformation, contributing to its enhanced elongation. This multi-gradient microstructure is responsible for the yield strength improvements of 72 MPa and 135 MPa in the torsioned samples annealed at 600 °C and 620 °C, respectively.