Oxygen-free resistance heating with nitrogen and silane as an energy-efficient heating process for hot stamping

Abstract

Abstract Hot stamping is a process for the production of ultra-high-strength components used in the automotive industry for passenger protection. In this process, cut blanks are heated in a roller hearth furnace, which is operated with gas, to approx. 950 °C in 8 to 10 min and then formed and quenched in a water-cooled tool. This extends the yield strength of the components up to 1500 MPa. The long heating time is caused by the coating of aluminium and silicon (AlSi) with a thickness of 70 μm. During the 8 to 10 min, an intermetallic phase forms and ensures that the blank does not scale. Due to the long heating time and the energy carrier gas, the process is very energy inefficient and can only be controlled slowly. Resistance heating provides an energy-efficient alternative. Due to the direct current flow through the sheet, up to 68% of the energy can be saved compared to roller hearth furnace heating. The process is a high-speed heating process that can heat sheets to 950 °C in less than 10 seconds. The heating time is not sufficient for conventional coatings to bond sufficiently with the base material, which means that there are currently no suitable coatings for resistance rapid heating. A new approach is to suppress scale formation by reducing the oxygen during heating. By lowering the oxygen partial pressure to the XHV level, scale formation is not possible. At the same time, this absence of oxygen is an ideal condition for applying coatings. Due to the developed resistance heating device, sheets can be heated within a very short time without scale and without the need for fossil fuels.