Insights into the relationship between γ/α transformation and strength-ductility of auto-steel with 980 MPa grade based on experiments and theoretical calculations

Two novel dual-phase steels with high ductility (DH steel) were designed to satisfy the rising demand and stringent requirements for automotive steel. In order to improve mechanical properties, water quenching and isothermal bainite treatment (IBT) were applied after intercritical annealing (IA) respectively. Three-dimensional atom probe (3DAP) observation and kinetic simulation showed that the intercritical austenite formation was mainly controlled by the partitioning of C during intercritical austenite and ferrite (γ/α) transformation. The steels after water quenching were characterized by martensite and ferrite, and the enrichment of C in the martensite could enhance the strength, while a higher ferrite content could improve plasticity. After IBT treatment, the steels exhibited superiorly comprehensive mechanical properties, exhibiting more than 1000 MPa ultimate tensile strength (UTS) and total elongation (TEL) of about 22%. The matrix microstructure composition of ferrite, bainite and retained austenite (RA) was affected by the IA transformation. The higher content of intercritical austenite led to higher levels of bainite and RA, but the stability of RA was low. The bainite could lead to higher strength of the sample, while the unstable RA transformed into martensite before reaching the yield stress, exhibiting weaker transformation-induced plasticity (TRIP) effect and lower plasticity.