Simultaneous improvement of yield strength and ductility in medium Mn steel by combining gradient structure and TRIP effect

Medium Mn steel with multi-gradient microstructure was processed using a novel pre-torsion and intercritical annealing treatment. Uniaxial tensile tests showed that the gradient sample annealed at 680 degrees C for 10 min exhibited an excellent combination of strength and ductility. In comparison with the relatively homogenous counterpart, the yield strength and total elongation of the gradient structured sample simultaneously increased by approximately 49% and 68%, respectively, and a comparable ultimate tensile strength was obtained. Finally, the product of ultimate tensile strength and total elongation in the gradient sample increased from 35 GPa center dot% to 55 GPa center dot%. Microstructural observations showed that with the increase in tensile strain, a reverse gradient structure opposite to the initial gradient structure gradually formed, in which the austenite content and grain size gradually increased, whereas the dislocation density gradually decreased along the radial direction from the center to the surface. This reverse gradient structure could render persistent TRIP effect and heterogeneous deformation-induced (HDI) hardening at prolonged straining, thus improving ductility. The HDI strengthening primarily resulted in improved yield strength of the gradient structured medium Mn steel.

Automated summary

A medium Mn steel with multi-gradient microstructure was treated using a pre-torsion and intercritical annealing treatment. The gradient structured sample exhibited improved strength and ductility compared to the homogenous counterpart, with an increase in yield strength and total elongation by approximately 49% and 68% respectively. The formation of a reverse gradient structure during tensile strain led to improved ductility through persistent TRIP effect and HDI hardening.