Achieving high strength and high ductility of dual-phase steel via alternating lamellar microstructure

Overcoming the strength-plasticity trade-off in GPa-grade dual-phase steel while simultaneously enhancing both properties poses a significant challenge. In this study, we propose an innovative strategy to break this inherent dilemma by designing an alternately arranged ferrite and martensite lamellar microstructure with appreciable deformation coordination in dual-phase steel. Compared to traditional dual-phase steel (DP steel) with a block microstructure and the same phase components, the newly developed heterogeneous lamellar dual-phase steel (L-DP steel) prepared by a cyclic intercritical quenching process demonstrates a remarkable 90 % increase in uniform elongation (from 9.1 +/- 0.4 % in DP steel to 17.3 +/- 0.3 % in L-DP steel). The ultimate tensile strength increases slightly from 1002 +/- 10 MPa (DP steel) to 1034 +/- 11 MPa (L-DP steel). The lamellar martensite and lamellar ferrite with a certain hardness difference lead to high back stress during plastic deformation, thereby ensuring the ultra-high tensile strength of the L-DP steel. The improved ductility of the L-DP steel is primarily attributed to the appreciable coordinated deformation ability of the lamellar microstructure and the considerable plastic deformation ability of lamellar martensite. These two outstanding capabilities allow this unique microstructure to elongate continuously along the tensile direction, facilitating persistent work hardening without premature cracking. Furthermore, the heterogeneous hardening triggered by the mutual constraints of the lamellar microstructure during the plastic deformation also contributes additional work hardening to enhance the ductility. The design of lamellar microstructure under a cyclic intercritical quenching process presents an efficient and feasible pathway for producing GPa-grade dual-phase steel with considerable ductility.