Coating Materials Under Oxygen-Free Silane Atmosphere for Hot Stamping

In the hot-stamping process, a sheet blank, usually a manganese boron steel like 22MnB5, is heated up to the austenitising temperature in a roller hearth furnace and then formed in a cooled forming tool. This leads to tensile strengths of about 1500 MPa in the formed parts. The components produced in this process are used in particular in the automotive body construction. The roller hearth furnaces used require high investment costs, much space, are maintenance-intensive and have a low efficiency. As an alternative to roller hearth furnaces, resistance heating offers significantly higher heating rates (>100 K/s) and consequently an energy-efficient, cost-effective process for heating electrically conductive materials. The sheet material 22MnB5 is conventionally coated with an AlSi layer, specially developed for hot stamping and the purpose of scale protection. Therefore, the coated blanks must be heated for several minutes to achieve a sufficient intermetallic phase. Within the framework of SFB 1368, an experimental chamber was developed in which an uncoated 22MnB5 sheet metal is simultaneously heated and coated without scale in a nitrogen-silane atmosphere by means of resistance heating. In the process, nitrogen displaces the regular atmosphere in the test chamber. Subsequently, silane reacts with the residual oxygen, resulting in an oxygen-free atmosphere. The Ni700 coating material used is nickel-based and is specially designed for the high heating rates of resistance heating. The investigations prove that for the production of hot-stamped components, coating in an oxygen-free silane atmosphere during resistance heating is possible.