Combined effect of Cu partitioning and nano-size precipitates on improving strength-ductility balance of Cu bearing Q&P steel

Cu was intentionally added to the traditional Q&P steel for further improving comprehensive mechanical properties through Cu partitioning, nano-size precipitates and microstructural refinement. Substantial enrich-ment of C, Mn, Cu and Ni atoms in newly formed austenite during intercritical annealing effectively inhibited the formation of bainite during the partitioning stage. The final microstructure of Cu bearing Q&P steel (Cu-QP steel) was significantly refined because of the refined prior austenite grains. Moreover, the modified constrained carbon equilibrium (CCE) model considering intercritical ferrite, Cu partitioning and the refinement effect of grain size was newly developed to better predict retained austenite (RA) fractions at different annealing tem-peratures in Cu-QP steel and Cu free Q&P steel (base steel). Both experiments and modeling results indicated that Cu-QP steel exhibited stronger austenite retention ability. The discrepancy of solid solubility between alpha and gamma phases resulted in 14-18 nm Cu-rich precipitates randomly dispersed at the grain boundaries and in the interior of tempered martensite and intercritical ferrite after partitioning holding. The outstanding mechanical properties with ultra-high yield strength of 706-962 MPa, tensile strength of 1065-1139 MPa, and total elongation of 22-26% were obtained after the intercritical Q&P process. More volume fraction of tempered martensite and/or fresh martensite was formed in Cu-QP steel as a substitute for softer bainite, which remarkably improved the tensile strength. High density of nano-size Cu-rich precipitates and the refined microstructure enabled an appreciable increase of yield strength, while larger-fraction of ultrafine RA with higher stability contributed to better ductility.