Superior strength-ductility-bendability combination and marginal oxidation in a 1000 MPa grade press-hardened steel
摘要
1000 MPa grade press-hardened steels are widely used in energy-absorbing domains of automotive parts. These press-hardened steels are often required to be protected by Al-Si coating to prevent severe oxidation during press hardening. In this study, a 1000 MPa grade press-hardened medium-Mn steel with marginal oxidation was designed using tailored additions of Al, Si, and Mn. Because the low Mn and appropriate Al content reduced the soaking time required for Mn diffusion during the heating process of the steel blank, a sufficient volume fraction (44.8 ± 0.6%) of retained austenite was achieved at 740 °C for a short soaking time of only 5 min. Following this process, an excellent combination of tensile strength of 1134 ± 1 MPa, total elongation of 37.3 ± 0.3%, and bending angle of 110 ± 3° was achieved in the press-hardened steel, which was ascribed to the transformation-induced plasticity effect provided by retained austenite during tensile deformation and three-point bending. Moreover, the thickness of the oxidation layer on the medium-Mn steel oxidized at 740 °C was only 101 ± 20 nm, which was much thinner than that on the commercial 1000 MPa grade press-hardened steel oxidized at 740 °C (3.6 ± 0.5 μm) or 930 °C (60.4 ± 2.4 μm). This work provides composition and processing design strategies for the development of oxidation-resistant high energy-absorbing press-hardened steel.