Enhancing Strength-Ductility in High Manganese Steel Through Combining Deep Cryogenic Treatment and Aging

Abstract

Deep cryogenic treatment (DCT) provides an effective and convenient way to improve the overall mechanical properties of multiphase steels. However, in high-manganese austenitic steels, the transformation characteristics of austenite during DCT and its influence on the precipitation behavior and austenite transformation in the subsequent aging stage remain unclear. Here we report the use of additional deep cryogenic treatment prior to aging to adjust the deformation twin distribution and the size and mechanical stability of austenite grains in the high manganese steel. The results show that after DCT + aging the high manganese steel contains a large number of twins and austenite grains with different mechanical stability due to different carbon content and grain size. The austenite mechanical stability of the samples treated by DCT+aging is improved, and large scale austenite grains with lower carbon content induce a large number of deformation twins during deformation, while small scale austenite grains with higher carbon content do not undergo TWIP effect due to their higher mechanical stability. In addition, the DCT+ aging treatment induces a large number of nanoscale copper-rich phases on the surface of the matrix, which can be used as nucleation sites for austenite to refine the grains. In conclusion, a copper-containing high-manganese steel reinforced by different stability of austenite , twins and copper-rich phases (CRPs) was tailored via DCT and aging. The ultimate tensile strength (UTS) and elongation of the DCT + AGE sample is respectively increased by 208 MPa and 15.8%, compared to that of the base sample. ε-martensite and twins developed due to DCT and behaved like a hard obstacle for dislocation movement in the twinned microstructure. CRPs developed due to aging, pinning dislocations to enhance strength. A combination of different stability austenite-based deformation behavior, twins and CRPs may be a potential solution to achieve superior properties of high manganese steel.

Keywords: High manganese steels, phase transformation, Strength-ductility, TRIP, TWIP, Grain refinement