For the first time I could finish a print. It worked by reducing the extrusion rate after a few layers, so that the nozzle didn't crash in the part after some time.
I think the problems with the first few layers came from the buildplate, which got deformed due to the heat. The melting point of steel is at around 1450°C, so there is a lot of heat on the builtplate.
I'm planning to solve this issue by automatic bed leveling with an inductive poximity sensor which should get delivered later this week or early next week.