Enhancing strength-ductility combination in a novel Cu–Ni bearing Q&P steel by tailoring the characteristics of fresh martensite

The present work aims to study the effect of quenching temperature on phase transformation, microstructural characteristics and mechanical properties of a novel Cu–Ni bearing Q&P steel based on combining the experiments and modeling. In this study, a new constrained carbon equilibrium (CCE) model was proposed by considering the effect of intercritical ferrite and multi-element synergistic partitioning. The optimum quenching temperature and corresponding retained austenite (RA) fraction can be reflected in the new CCE model, which is in good agreement with XRD results. Furthermore, the significant role of fresh martensite in the tensile strength and fracture behavior were elucidated. As the volume fraction of fresh martensite increases from 9.8% to 57.8%, the strengthening contribution to the tensile strength increases from 13.7% to 68.8%. Meanwhile, the fracture mechanism transforms from ductile into quasi-cleavage fracture due to an increased amount of fresh martensite and the absence of tempered martensite. The superior mechanical properties, ultimate tensile strength of 1152 MPa, yield strength of 797 MPa, total elongation of 25%, and the product of strength and elongation (PSE) up to 28.8 GPa% were obtained in QP-200, which is largely due to the combined effect of the better deformation ability of a considerable amount of tempered martensite and the continuous TRIP effect provided by multiscale RA with different morphologies.