12th International Conference on Zinc & Zinc Alloy Coated Steel Sheet - GALVATECH 2021

For galvanized press hardening steels, a low and homogeneous surface resistance is important to ensure good welding behavior for body in white assembly. Surface resistance and oxide composition was studied for steel grade 20MnB8 with different coatings (GI vs. GA) and different annealing times. The results were also compared to standard steel grade 22MnB5 coated with GI. Oxide composition was determined by grazing angle XRD measurements, SEM investigation and EDX measurements on cross section polished samples. Surface resistance was measured with an in-house built instrument at room temperature. Samples were clamped between to spot welding electrodes. 22MnB5 coated with GI shows the highest surface resistance compared to the other studied samples. Especially when annealed for longer furnace dwell times. The 20MnB8 coated with GI has lower resistance compared to 22MnB5. 20MnB8 with a galvannealed (GA) coating has the lowest surface resistance with the smallest variation even after annealing for longer times. It will be shown that the low surface resistance of 20MnB8 is attributed to formation of Hetaerolit (ZnMn2O4). Hetaerolit has a spinell like structure. The lower resistances of GA compared to GI is probably caused by the absence of aluminum resp. of the diffusion inhibiting Fe2Al5 – layer and/or slower coating transformation from GI to GA during the PHS process in comparison to the galvannealing process in the HDG-line. This enables manganese diffusion to the surface already in the early stages of annealing during press hardening. In case of 22MnB5+GI which has a inhibiting layer and a lower manganese content, the formation of ZnO is more pronounced. If an annealed sheet of 20MnB8+GA coating is annealed for a second time in an reducing atmosphere with λ<1, decomposition of Hetaerolit takes place and the surface resistance increases 10 to 20 times of the initial value. For excellent welding behavior, shot blasting is a measure to ensure welding performance also for long annealing times in the PHS furnace. Three different steel shots were compared in terms of impact energy, surface coverage in respect to resulting surface resistance. Shot blasting energies have been assessed with Almen-strips. Impact of a spinning plate shot blasting system in comparison to a conveyor belt shot blasting line, which is used in the production of press hardened parts, and a blast cabinet was also investigated. It is shown that the more the merrier is not generally effective.