Deformation Behavior of Austenite-Base High Mn Steels

Abstract

The demands for developing high performance high Mn steels have been expanded in recent years since they have excellent combination of high tensile strength and elongation with the relatively effective cost. However, the deformation behavior and strengthening mechanism has not been clearly understood yet since the microstructure can consist of multi-phases depending on the alloy composition. In this regard, the study on them is needed to maximize the mechanical properties and optimize the alloy composition. Moreover, the formation and deformation mechanism of ε-martensite has to be also understood, which plays as an intermediate phase in the whole phase transformation and affects the work hardening and final mechanical properties. Furthermore, the demand for cryogenic materials has been extensively expanded in recent years, for example LNG storage tank and cryogenic offshore structure. In this regard, there are many efforts to replace the commercial Ni steels with Mn steels due to the relatively low cost and the metallurgical similarities of Mn steels with Ni steels, and austenite-base high Mn steels are promising candidate. However, the low temperature deformation behavior of high Mn steels is not clear since the deformation behavior and microstructure evolution can be quite different with the various test temperatures. In this study, the deformation behavior and strengthening mechanism of Fe-17Mn-0.02C containing ε-martensite within austenite matrix have been examined via the microstructure analysis. Moreover, in-situ neutron diffraction study was conducted to investigate the deformation behavior in detail at 298 K and 77 K. Based on the analyses of change in phase fraction and lattice strain, it has been shown that the steel shows the deformation-induced phase transformation of austenite -> ε-martensite -> α’-martensite at both temperatures. However, the kinetics of such transformation varies with temperature, resulting in a higher and more persistent work hardening at 77 K than at 298 K.