Ductilizing martensite in lean steel via chemical heterogeneity

Martensitic steels typically exhibit high strength but limited ductility, necessitating complex heat treatments to balance strength and ductility via the transformation induced plasticity effect. Here we demonstrate that chemical heterogeneity within martensite alone, induced through flash annealing and cooling, enhances strength and ductility in a low alloyed steel without reliance on retained austenite. The chemical heterogeneity in the initial cold-rolled ferritic + pearlitic microstructure generated structural heterogeneity in the final martensite, i.e. soft Mn-depleted martensite (MDM) with multiple variants and hard Mn-enriched martensite (MEM) with a single variant. This heterogeneity drives geometrically necessary dislocation accumulation at MDM/MEM interfaces, leading to sustained back-stress hardening during deformation. Consequently, the heterogeneous sample achieved mechanical properties that surpasses its homogeneous counterparts despite minimal retained austenite (∼2 %). This work establishes that intrinsic chemically induced structural heterogeneity in martensite can enable exceptional strength-ductility synergy, offering a streamlined alternative for advanced high-strength steels.