Strengthening a medium-carbon steel to 2800 MPa by tailoring nanosized precipitates and the phase ratio

Abstract

TEM observations and atom probe topography (APT) studies provided an intriguing observation that the combination of ultra-high strength and good ductility can be achieved by changing the nature of precipitates and the ratio of microstructural constituents in a low-manganese- medium-carbon steel (1.76Mn-1.61Al-0.47Si-0.52Cu-0.56C, wt.%). The ultra-high tensile strength of 2,800 MPa (ultimate-tensile-stress) and good ductility (true strain at 0.18) were obtained by adopting inter-critical annealing at 820 °C for 180 s, quenching to 180 °C and holding for 300 s, followed by cooling to room temperature in air. This led to a multi-phase microstructure consisting of 67% martensite, 10% retained austenite and 23% ferrite (in volume fraction), and nanosized precipitates. Low holding temperature promoted the formation of rod-like ε-Fe3C precipitates, which significantly increased the work-hardening behavior of steel. In addition, a localized work-hardening effect was provided by deformation-induced transformation of fcc retained austenite (RA) into bcc martensite during tensile straining. It delayed the commencement of localized necking and led to uniform ductility.