Controllable selection of martensitic variant enables concurrent enhancement of strength and ductility in a low-carbon steel

Abstract

A novel strategy through controlling the selection of martensitic variant in metallic materials is proposed to break through the trade-off between strength and ductility. Here we prepared a high-strength low-carbon steel by applying the two-stage mechanical processing combined with high-temperature large-reduction compression and the subsequent compression above the low critical point of austenitization. The steel has the lowest degree of static recrystallization and dynamic recrystallization after the two-stage mechanical processing, leading to the smallest prior austenite grains with an average diameter of 16.4 μm and the highest dislocation density of 10.21 × 1014 m−2. With the refinement of the prior austenite grains caused by the adjustment of the two-stage deformation process, the driving force required for martensitic transformation increases and variants selectivity becomes stronger, leading to a large number of fine martensitic laths and nanoscale twins after martensitic transformation. Consequently, the best combination of strength and ductility is obtained, with the yield strength of 871 MPa, tensile strength of 1054 MPa and the total elongation of 25%. The use of variant selection exploits the strengthening of both grain boundaries and dislocations, resulting in simultaneous enhancement of strength and ductility. These findings demonstrate how multiple deformation mechanisms can be deliberately activated via the controllable selection of martensitic variants.