The synergistic effects of ultrafine grains and nano-size Cu-rich precipitates on the mechanical properties of DP steels

Simultaneous improvement in strength and ductility has always been a major concern for ferrite/martensite dual-phase (DP) steels. To this end, the synergistic effects of ultrafine grains and nano-size Cu-rich precipitates on the mechanical properties of DP steels were studied using an experimental method combined with first-principles calculations and crystal plasticity (CP) simulations. Ultrafine-grained DP steels consisting of soft ferrite, with an approximately 10% volume fraction of martensite and dispersed nano-sized Cu-rich precipitates, were obtained through warm rolling and subsequent aging heat treatment. Compared to warm deformation with a 50% reduction, that with a 75% reduction leads to finer grains and a higher tensile strength of 976 ± 15 MPa, accompanied by a small loss in elongation due to the aligned connection of the martensite phase, preferred grain orientation distribution, and relatively higher dislocation density. The optimal mechanical properties are a yield strength of 876 ± 13 MPa, tensile strength of 976 ± 15 MPa and total elongation of 15.2%. The CP simulation results indicate that the heterogeneous distribution of martensite leads to strong strain and stress partitioning, thus facilitating early damage nucleation. Additionally, the strengthening effect of precipitation was evaluated based on shearing mechanisms, and the results suggest that the precipitation of nano Cu-rich precipitates contributes to a strengthening of approximately 300 MPa in the steels.