Entwicklung einer Ofenrollenbeschichtung für das Presshärten

Press hardening of MnB steels to high-strength body parts is a key technology in the automotive industry for improving passenger safety and reducing car weight. The dominant method in automotive production is direct press hardening, which comprises the austenitizing of pre-cut 22MnB5 sheets in a roller hearth furnace and immediate hot forming, with simultaneous hardening (martensite formation due to rapid cooling). A metallic AlSi coating protects the steel sheets from scaling. Since, however, it melts at 600°C, the coating is transferred onto the mullite conveyor rollers through the constant mechanical contact. Beside the formation of process-disturbing residues baked onto the rollers, this melt infiltrates the microstructure of the furnace rollers. The mullite phase of the roller material is dissolved, and a zonal microstructure is formed, leading to uncontrolled roller fractures and hence to production downtimes. The approach chosen for preventing this corrosive process and increasing plant availability of the roller-hearth furnace is a sol-gel composite coating using a transitory aluminum oxide sol as the binder phase. The suitability of various ceramic filler powders is tested in a variety of laboratory-scale tests imitating industrial service conditions. Different coatings made with three selected powder qualities which were identified as being suitable are then tested under realistic conditions in a pilot-plant scale roller hearth furnace. After three promising coating systems have been identi-fied, an actual production field test is carried out in order to determine the optimal coating system. In the first field test as well as in the subsequent test runs in different roller hearth furnaces, the application of this final coating causes a significant increase of roller life as well as a marked decrease in the spread of damage events. The coating minimises the number of baked-on residues and prevents, across a large number of conveyed sheets, the infiltration of the roller material.