MSE 2022
Lecture
27.09.2022 (CEST)
Coating of manganese boron steel in oxygen-free atmosphere with nitrogen silane
LA

Lorenz Albracht (M.Sc.)

Leibniz Universität Hannover

Albracht, L. (Speaker)¹; Behrens, B.-A.¹; Farahmand, E.¹; Holländer, U.²; Hübner, S.¹; Langohr, A.²
¹Institute of Forming Technology and Machines, Garbsen; ²Institute of Materials Science, Garbsen
Vorschau
15 Min. Untertitel (CC)

Hot stamping is a process for producing ultra-high-strength components. Sheet metals from 22MnB5 with an aluminium-silicon (AlSi) coating are heated up to 950 °C in roller hearth furnaces for 4 to 8 min. Subsequently, the sheet is formed and at the same time hardened in a water-cooled tool. This method is able to produce geometries close to the final contour with tensile strengths of 1500 MPa which are typically used in crash relevant components such as A- or B-pillars. Resistance heating (also known as conductive heating) offers a sustainable heating method in comparison to roller hearth furnaces. By directly converting the electrical energy, sheets can be heated up to 90 % more energy-efficiently than the well-known roller hearth furnace heating and with a heating gradient greater than 100 K/s. The widely used and established AlSi coating, however, requires a much slower heating rate due to the slow formation of the intermetallic phase between the sheet and the AlSi coating during heating. Uncoated sheets, on the other hand, scale during resistance heating, requiring costly cleaning of the tool after hot forming, and the resulting component has no corrosion protection layer. Resistance heating in an oxygen-free, silane-doped nitrogen atmosphere represents a new sustainable process chain for uncoated manganese-boron steels; moreover, these process conditions enable an "in situ" coating process with powdered Ni-based brazing metals applied before or during heating process on the sheet surface. First investigations with 22MnB5 sheets, which were simultaneously heated and braze-coated in a developed experimental chamber, show promising results. The silane-doped nitrogen suppress scale formation during heating as the oxygen content is reduced to 10-21 vol.%. This virtually oxygen-free environment also allows for rapid wetting of the sheet with the melting braze powder during resistance heating, giving the sheet corrosion protection for subsequent transfer. This approach provides a sustainable alternative heating method for hot stamping.

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