Abstract: Fracture strain becomes critical for the local formability and crash performance of carbody components when the tensile strength exceeds 1000 MPa. Regrettably, high-strength quenching and partitioning (Q&P) steels and dual-phase (DP) steels always focus on improving the tensile ductility for stretch formability, while ignoring their limited fracture strain. In this work, we explored a novel strategy, i.e., developing a high fracture strain ferrite-martensite dual-phase steel (HFS-DP) maintaining good strength-ductility bal-ance by suppressing intense strain localization during deformation and enhancing martensite deformabil-ity via microstructure design including grain refinement, nano-precipitate hardening in soft ferrite phase, low-carbon and high fraction martensite. HFS-DP demonstrates a remarkable 26% and 47% improvement in tensile ductility and fracture strain, respectively, compared to commercial DP1180 steel with simi-lar ultimate tensile strength. Furthermore, HFS-DP also exhibits a substantial 39% improvement in frac-ture strain compared to retained austenite-involved commercial QP1180 steel. The detailed processes of strain partitioning, strain localization, and damage formation during deformation were revealed through in-situ scanning electron microscopy (SEM) observation combined with digital image correlation (DIC). The results indicate that the excellent coordinated deformation between ferrite and martensite, coupled with microstructure refinement, effectively suppresses intense strain localization. Moreover, the excellent martensite deformability resulting from the low carbon content also aids in retarding crack formation. This combination effectively suppresses damage initiation and development during deformation, therefore the fracture strain is significantly improved. This study not only contributes to a deeper understanding of the strain localization and damage process during tensile deformation of DP steels, but also provides a new perspective on designing ultrahigh strength steels with high ductility and fracture strain.

Key words: Dual-phase steel, Fracture strain, Grain refinement, Coordination deformation, Strain localization