Tailoring multi-stage microstructural evolution and mechanical responses in advanced high-strength steels across wide quenching Temperatures: Synergistic design of martensitic/bainitic carbon partitioning

This study systematically investigates the synergistic interplay between martensitic and bainitic carbon partitioning mechanisms in advanced high-strength steels (AHSS) across a broad quenching temperature (QT) range (25–400 °C). It can be found that the bainite transformation kinetics during isothermal holding are governed by QT, showing three stage-dependent acceleration linked to defect density of constituent phases. The retained austenite (RA) fractions follow QT-dependent carbon partitioning mechanisms: limited RA (≤3 vol%) arises solely from martensitic carbon partitioning at low QTs (≤100 °C), while intermediate QTs (150–200 °C) enable synergy between martensitic and bainitic carbon partitioning, with the dominant martensitic mechanism driving a rapid increase in RA content (up to ∼12 vol%). High QTs (≥225 °C) stabilize RA (∼12 vol%) via bainitic carbon partitioning constrained by the T₀-line limit. In addition, a modified constrained carbon equilibrium model was developed by integrating T₀-line or WBs theory to incorporate bainite formation, enabling high-precision prediction of austenite retention across the full QT range (25–400 °C). The mechanical responses also exhibit three-stage QT dependence, dictated by the characteristics of RA and dominant phase: ultrahigh strength with low ductility in M₁-dominated microstructures (QT ≤ 100 °C); excellent strength-ductility combination (UTS ∼1100 MPa, TEL ∼22 %) in intermediate QTs (150–200 °C) with microstructures incorporated RA (≤12.1 vol%) and minor bainite; and stabilize (UTS ∼1000 MPa, TEL ∼22 %) in bainite-dominated regimes (QT ≥ 225 °C) with stable phase fractions. This remarkable microstructural and mechanical consistency across QTs ≥275 °C provides unparalleled process flexibility for industrial AHSS production, eliminating stringent thermal control requirements.