Use of pre-oxidation to improve reactive wetting of high manganese alloyed steel during hot-dip galvanizing

The present study discusses hot-dip galvanizing of a Fe-23% Mn-0.6% C-0.3% Si steel using a Zn-0.22%Al bath. The paper concentrates on reactive Zn wetting on top a covering external oxide layer occurring after in-line annealing. Annealing was performed by soaking at 800 degrees C/60 s in 5% H-2-N-2 at different dewpoints. In-line pre- oxidation at 600 degrees C/10 s in 1.8% O-2-N-2 was further performed and the impact on selective oxidation as well as reactive Zn wetting was examined. After conventional annealing Zn wetting turns to increase if a roughly globular MnO layer appears on the external steel surface and Si is internally oxidized. Reactive wetting including the formation of Fe2Al5 crystals occurs on top of the MnO layer, because metallic-bond Fe exists incorporated within this MnO layer (-> MnO center dot Fe-metall layer). The amount of metallic-bond Fe within the MnO center dot Fe-metall layer increases considerably if pre-oxidation is conducted. This results in an intensified Fe2Al5 formation on top of a MnO center dot Fe-metall which improves liquid Zn wetting. Brittle Fe-Zn intermetallics were absent in all trials. These results offer a new way for hot-dip galvanizing (high) Mn alloyed steels. The absence of Fe-Zn intermetallics and (partial) MnO reduction implies that the currently discussed model of aluminothermic MnO reduction during hot-dip galvanizing Mn alloyed steel seems not to be dominating in the present case of reactive Zn wetting on top of a covering MnO center dot Fe-metall layer. (C) 2011 Elsevier B.V. All rights reserved.