Electron Microscopic Study of Stress-assisted Martensitic Transformation in a Medium Mn Steel

Abstract

1. INTRODUCTION Medium manganese steel shows a dramatic change of yield strength depending on the constituent phase and fractions and their mechanical stability [1-3]. The dynamic variation of the tensile property in medium manganese steel was focused on the candidate of the third-generation advanced high-strength steel. The critical issue on achieving high strength will understand the yielding mechanism in the steel. Stress-assisted martensitic transformation (SAMT) and its role on tensile behavior are studied by in-situ and ex-situ transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) analyses. 2. EXPERIMENTAL An Fe-10Mn-0.2C based alloy was prepared by vacuum induction melting and subsequent hot-rolling process. Thin + lamellar structure forms after reheating and quenching of Mn-partitioned specimen. SAMT behavior of unstable  lamella was investigated by in-situ TEM and EBSD tensile experiments. The observed results are compared to the ex-situ result and the stress-strain curve of the tensile test and further discussed in view of dislocation pile-up for SAMT. 3. RESULTS AND DISCUSSION Premature yielding occurs when  is mechanically unstable. SAMT was observed in the specimen deformed about 1%. However,  remain even after tensile fracture when the steel was heat-treated at 200°C for 10 min(C-partitioning). Fig. 1 shows the microstructures after tensile fracture in the carbon-partitioned specimen. The deformation-induced martensitic transformation was observed. The martensitic transformation of prior  remains low angle grain boundary (GB) in  lamella. Interestingly, the low-angle GB is composed of a row of a[110] edge dislocations. The stability of  increases by C-partitioning treatment. The C-partitioning specimen shows the increased yield strength. We correlate  stability and yield strength of α′+γ dual-phase steel. SAMT controls the yield strength of the medium manganese steel. We investigated the evolution of microstructure during the tensile deformation. The change of microstructure is correlated with tensile behaviors. The role of SAMT on yielding and work-hardening is further discussed. REFERENCES 1. Y. -U. Heo et al.: Acta Mater., 77 (2014), 236. 2. Y. -U. Heo et al.; Metall. Mater.Trans. A, 47A (2016), 6005. 3. Y. -U. Heo et al.; Metall. Mater. Trans. A, 50A (2019), 151.