Grain refinement of dual phase steel maximizes deformation ability of martensite, leading to simultaneous enhancement of strength and ductility

Dual-phase (DP) steel, composed of soft ferrite and hard martensite, is well-known advanced high-strength steel (AHSS) because of its exceptional strength-ductility balance and low manufacturing cost. The present study found that microstructural refinement of DP steel enhanced not only its yield strength but also strain-hardening, leading to increasing both strength and ductility. Digital image correlation (DIC) analysis showed that strains were localized much more in soft ferrite than in hard martensite but the refinement of DP structure decreased the difference in average strains of ferrite and martensite, which avoided crack initiation in ferrite and led to large ductility. Consequently, the refinement of DP structure induced more plastic deformation in martensite through enhanced deformation constraints by the increase in ferrite/martensite interfaces. In-situ neutron diffraction experiment during tensile deformation quantitatively showed that higher phase stress was borne in hard martensite than in soft ferrite and microstructure refinement made martensite bear higher phase stress through the enhanced deformation constraint. Using strain/stress-partitioning results obtained by μ-DIC and in-situ neutron diffraction, individual stress-strain curves of ferrite and martensite could be successfully constructed for the first time. Individual stress-strain curves of ferrite and martensite reasonably explained the strength-ductility synergy in the fine-grained DP structure in terms of deformation constraint between two phases. Microstructure refinement in DP structures enhanced deformation constraint between two phases to maximize the deformation ability of martensite. The insight obtained in the present study could be applied to general heterostructured materials composed of soft and hard domains for overcoming the strength-ductility trade-off relationship.