Partial resistance tempering of hot-stamped components made of 22MnB5 for subsequent bending

Abstract

The manganese-boron alloy 22MnB5 is particularly used for structural and safety-relevant parts in the automotive industry. Parts made from this alloy are usually produced using the hot forming process. Here, the sheet is heated to over 950 °C using an industrial roller hearth furnace. The heated sheet is then simultaneously formed and quenched in a cooled tool with a temperature gradient of more than 27 K/s. This leads to the formation of a martensitic microstructure with a hardness value of over 450 HV10 and an elongation at break of less than 6%. The small strain potential of such components makes them difficult to form after hot-stamping. Due to the high temperature gradients of resistance heating, a sheet can be heat-treated locally without a large temperature transition zone. This can be used to locally soften already hot-stamped components for subsequent operations such as bending. Within the scope of this paper, resistance heating is used to soften a hot-stamped 22MnB5+AlSi sheet stripe of 3 mm width. The sheet could consequently be bent over an angle of 90° without cracking the substrate.