Combined contribution of Cu-rich precipitates and retained austenite on mechanical properties of a novel low-carbon medium-Mn steel plate

Simultaneous improvement in yield strength, ductility and impact toughness has always been a challenge for structural steels. In the attempt to achieve this goal, we proposed a Cu-precipitation-strengthened low-carbon medium-Mn steel in the present study. Upon aging at 600 °C for 1 h, a considerable amount of Cu-rich precipitates with average radius of 6.3 nm was uniformly distributed in the tempered martensitic matrix. An ultra-high yield strength, therefore, was attained by the strong interaction of Cu-rich precipitates with dislocations. In addition to the precipitation strengthening effect, the introduction of Cu also dramatically promoted the dissolution of cementite particles and formation of retained austenite at triple junctions or other high-angle grain boundaries. Compositional analysis of cementite and retained austenite was conducted by atom probe tomography. Austenite formation removed Mn segregation from the grain boundaries, consumed brittle cementite particles, and hence eliminated intergranular fracture, leading to a remarkable improvement in toughness. Moreover, a volume fraction of 23.2% retained austenite with appropriate mechanical stability also enabled the steel to be more ductile. The optimal mechanical properties, yield strength of 1005 MPa, tensile strength of 1070 MPa, total elongation of 24.3% and room temperature impact energy of ~ 73 J were obtained, through the cumulative contribution of Cu-rich precipitates and retained austenite.