12th International Conference on Zinc & Zinc Alloy Coated Steel Sheet - GALVATECH 2021

The Press Hardened Steels (PHS) offer unique weight saving opportunities to the automotive market, allowing to reach extremely high mechanical properties (until 2000 MPa) on complex parts with a well-controlled geometry. Since 2001, pre-coated PHS are available. AlSi coated PHS are the mostly used worldwide with more than 75% of the hot stamping lines in the world dedicated to them. This predominance can be explained by the simplicity of the corresponding hot stamping process, the large process windows and the robustness of the product performance. Furthermore, Al based coating-alloy remains the best in class solution regarding perforating corrosion resistance. Nevertheless, Al-Si remains sensitive to the red rusting from the coating surface, especially in areas with a too thin E-coat or areas where E-coat is locally eliminated (like stone chipping). Zn based coatings can give an appropriate answer to that point, but the process-window is quite narrow due to its rapid evolution at austenitizing temperature (for Zn from 880°C to 910°C). Aiming at gathering Al-Si strengths (compatibility with direct Press Hardening Process, perforating corrosion resistance, no micro-cracks in the substrate…) with those from Zn based product (cosmetic corrosion and red rusting resistance) a new hot dip coating offer is being developed. Such new bath containing Al-Si-Zn-Mg has been developed with an innovative strategy consisting in taking profit of the austenitizing step to finish coating transformation, in particular regarding surface functionalization. Consequent improvement on corrosion resistance at painted and unpainted state will be highlighted. Moreover, product corrosion performances are constant in a wide range of press hardening conditions. Risk assessment and performance validation were done at lab scale. Industrial development is in progress. The challenge in the development of coatings dedicated to PHS is to satisfy the main requests of the final customers by keeping a solution working in a large industrial range of production for the Hot Stamper.