Increasing the performance of hot forming parts by resistance heating in XHV-adequate atmosphere

¹ Leibniz University Hannover, Institute of Forming Technology and Machines, An der Universität 2, 30823 Garbsen, Germany
² Leibniz University Hannover, Institute of Materials Science, An der Universität 2, 30823 Garbsen, Germany

Abstract

In hot stamping, manganese-boron steel is heated above the austenitisation temperature and subsequently cooled at a rate of 27 K/s using a water-cooled process, resulting in lightweight components with ultra-high strength that enhance passenger safety, such as A- or B-pillars. Industrial heating takes place in roller hearth furnaces, where the sheet metal is heated up to 950 °C within 6 to 10 minutes and the AlSi coating bonds with the base material to protect it from scaling. Resistance heating is an alternative heating method. The direct current flow through the sheet enables heating rates of more than 100 K/s and heats the sheet in less than 10 seconds. This rapid heating also improves the mechanical properties by reducing the growth of coarse grains. However, the AlSi coating is not designed for such rapid heating, as there is not enough time for the diffusion layer to form. However, an XHV (extreme high vacuum)-adequate protective atmosphere with nitrogen and silane offers an alternative for scale-free heating without coatings. This process atmosphere binds oxygen, effectively preventing scale formation.