Improving the ductility of 2.6-GPa high-carbon martensitic steel by prior bainitic transformation

In this study, we designed a lean and time-efficient high-carbon bainite/martensite (B/M) steel with 2.64 GPa ultimate tensile strength and 9% uniform elongation. Prior lower bainite nucleated along prior austenite grain boundaries (PAGBs) suppressed premature intergranular fracture in martensitic constituent. Blocky retained austenite (RA) was observed adjacent to the bainite sheaves along PAGBs. During tensile deformation, significant work hardening behaviour was observed, primarily due to the transformation-induced-plasticity (TRIP) effect of blocky RA. A brittle-to-ductile transition was observed with increasing bainitic transformation time at the early stage. Overall, a low fraction of prior lower bainite can effectively enhance the ductility of the high-carbon martensite without sacrificing its ultrahigh strength. • Prior lower bainite along PAGB can enhance local deformability and suppress intergranular fracture. • Brittle-to-ductile transition attributes to the presence of lower bainite. • Design of a lean and time-efficient route to fabricate Bainite/martensite steels with 2.64 GPa ultimate tensile strengthens and 12% ductility.