An innovative self-tempering process for martensite-bainite multiphase in 2 GPa press hardening steel and its strengthening-toughening mechanisms

To meet the growing demand for lightweight automotive materials, this study overcomes the conventional strength-toughness trade-off in 2 GPa-grade press-hardened steels by integrating martensite/bainite (M/B) multiphase microstructure design with modulation of the self-tempering effect. The optimized specimens demonstrate remarkable improvements, including a 7.03% increase in ultimate tensile strength (2056 MPa), a 124.05% improvement in uniform elongation, and a 45.99% boost in impact toughness compared to conventional steels. Multiscale mechanistic analyses reveal that the significant strength enhancement (yield strength increase of 402 MPa) results from the synergistic effects of bainite's plastic constraint and nano-sized ε-carbide precipitation, with Orowan strengthening particularly enhanced by the self-tempering treatment. The self-tempering treatment also regulates dislocation configurations and precipitate/matrix interfaces, reducing lattice distortion and local stress concentrations, which suppress microcrack initiation. Furthermore, the combined action of the TRIP (transformation-induced plasticity) effect from film-like retained austenite and crack deflection toughening by lower bainite efficiently dissipates crack propagation energy. These integrated mechanisms collectively achieve an exceptional balance of strength and toughness in the developed steel.