Influence of die materials on the microstructural evolution of HSS sheets in hot stamping.

Hot stamping of High Strength Steels sheets is gaining more and more popularity, particularly in the automotive industry, due to the sound microstructures achieved at the end of the process. The significant improvement of the mechanical properties achieved in the process enables to reduce the initial sheet thickness in favour of cost and fuel consumption reduction. However, a martensitic microstructure implies significant drawbacks in final trimming and cutting operations, which becomes more difficult and expensive due to tools wear and high blanking forces. This paper aims at investigating the performances of non-metallic materials to be used in heated dies, in order to inhibit the martensite formation by locally reducing the sheet cooling rate. To analyze the influence of the main process parameters, a new experimental set-up was designed and developed in a laboratory environment that allow applying controlled pressure and temperatures to HSS metal sheets. An analytical model was set-up in order to evaluate the influence of process conditions on the cooling profiles in different areas of the specimen. Accordingly, experiments were carried out to investigate the material behaviour when cooled in the different conditions. The experimentally acquired temperatures were analyzed and evaluated together with hardness measurements of metal sheets in order to assess the feasibility of the proposed approach in producing microstructurally-tailored components