Hot formability of DIN 27MnCrB5 steel sheets under controlled thinning

Abstract Hot stamping has been widely studied and increasingly applied in the automotive industry. This process is characterized by its ability to stamp high strength steels, yielding products with high mechanical strength, thus reducing the weight of stamped components and therefore the vehicles weight. It also demands less energy because steel sheets can be heated by induction, more efficient than electric furnaces. With controlled thinning, it is possible to manufacture thinner stamped parts with high mechanical strength, therefore it is necessary to know the formability limits to prevent failure and achieve the largest possible thickness reduction. In this work the hot formability of DIN 27MnCrB5 steel sheets 4 mm thick, under thinning conditions was evaluated by numerical simulation with the finite element (FEM) software Forge2008. Tensile tests were carried out at 500, 600, 700, and 800 °C and with strain rates from 0.1 to 4 s −1 . With the results of tensile tests, it was possible to calculate Hensel–Spittel coefficients to model the steel sheet and simulate the hot Nakazima test to evaluate the highest dome which could be formed without failure risks caused by sheet thinning. Simulation results obtained with specimens 200 mm long and 125, 150 or 200 mm wide that were stamped at 930 °C, showed the radial position and dome height associated to plastic instability as well provided the thickness distribution along the specimen. The numerical results were compared to experimental tests and showed a good agreement in terms of failure initiation and localization. As a result, a new numerical and experimental strategy was elaborated to define the hot formability based on the plastic instability and necking localization as a function of stamping temperature and blank dimensions. This strategy proved to be useful to define the safe formability region and therefore the larger thickness reduction that can be done during hot stamping to reduce vehicular components weight and to avoid cracks and failures usually observed in components like clutch covers.