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

The development of galvanised high strength steels requires a better understanding of the gas – metal reactions during continuous annealing and their influence on steel – zinc reactivity during hot dip galvanising. Indeed, external selective oxidation, mainly formed during soaking, will hamper the formation of the inhibition layer, which is a key step for zinc wetting and further adequate coating adhesion. Therefore, the influence of the atmosphere of the different parts of the furnace on steel surface chemistry and by the way on zinc wetting and coating adhesion has been investigated by in situ surface analyses, wetting measurements and adhesion tests. This study is focussed on the behaviour of a TRIP-Si steel, which has been submitted to a Q&P annealing cycle and galvanized in GI and Zn-1.1%Al-1.1%Mg baths. The influences of an oxidation during heating and a higher dew point during soaking have been more particularly investigated. It can be concluded that if water vapour injection already enhances the coating quality, an additional improvement can be obtained with an oxidation step during heating, even if the process window is relatively narrow.