Role of Retained Austenite and Deformation-induced Martensite in 0.15C-5Mn Steel Monitored by <i>In-situ</i> Neutron Diffraction Measurement during Tensile Deformation

An Fe-0.15C-5Mn-0.5Si-0.05Nb medium-Mn steel annealed at 660°C and 685°C both exhibited inhomogeneous deformation with Lüders deformation and followed by an extremely high work hardening stage, but with different Lüders strain and work hardening behavior. In-situ neutron diffraction measurements during tensile tests were performed to investigate changes in the phase stresses, the stress contribution to the strength of the constituent phases, and the crystal orientation of austenite. The role of each constituent phase on the deformation behavior and the crystallographic orientation dependence of austenite stability were examined. Deformation-induced martensite showed excellent phase stress and contributed to the strength of approximately 1000 MPa at around tensile strength, which is close to macroscopic tensile strength. Although austenite contributed less to the strength, during Lüders deformation and work hardening stages, it continuously transformed to martensite as the deformation progressed, suggesting that it mainly contributed to the ductility of the steels through a transformation-induced plasticity effect. Austenite transformed to martensite during Lüders deformation almost regardless of its crystallographic orientations. However, there was a tendency for more 311 oriented austenite grains to remain parallel to the tensile direction.