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

A Fe-0.15C-5Mn-0.5Si-0.05Nb medium Mn steel annealed at 660℃ and 685℃ both exhibited inhomogeneous deformation with Lüders deformation and extremely high work hardening rates, but with different Lüders strain and work hardening behavior. In-situ neutron diffraction measurements during tensile test were performed to investigate changes in the phase stresses and in the contributed stresses to the strength of the constituent phases, and crystal orientation of austenite. The role of each constituent phase in the deformation and the effect of crystallographic orientation on austenite stability were discussed. Deformation induced martensite showed excellent phase stress and contributed to the strength approximately 1000 MPa, which is close to macroscopic tensile strength. Although austenite contributed less to the strength, but during Lüders deformation and work hardening stage, 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 in all crystallographic orientations during Lüders deformation, but there was a tendency for more <311> austenite grains parallel to the tensile direction to remain.