A mechanistic study of wetting and dewetting during hot dip galvanising of high strength steels
Understanding a selective oxidation of alloying elements, which occurs during the annealing of high strength steels, is a major task and also a powerful tool for improving wettability behaviour during the hot-dip galvanising. With increasing dew point of H2/N2 annealing atmosphere, a transition from external to internal oxidation can be achieved. At very low dew points maximum surface oxidation of Al is observed. Mn, Si and Cr external oxidation reaches a maximum at medium dew points (around - 30 °C) and the surface coverage by oxides is the highest. B segregates and oxidises on the surface even at very low dew points and BN formation takes place. It was shown that BN is almost not wetted by liquid Zn. At increased dew points (Mn, B) oxide forms and this shows the most negative influence on the wetting. If the dew point increases above - 10 °C, the B oxidation shifts to internal thus improving wettability. Preoxidation is also a way to avoid B oxides but also oxides of other alloying elements since mostly formation of Fe oxides at oxide/gas interface is expected. These Fe oxides can be reduced in a subsequent annealing cycle to metallic Fe. There is a critical concentration of the alloying element (3) 5 wt. % above which no significant improvement by preoxidation can be achieved. Nitriding of the surface by nitrogen or ammonium decreases oxidation since the sites for oxygen adsorption are occupied by nitrogen but these surfaces covered by nitrides are poorly wetted by Zn. Stirring in bath, increase of the strip/bath T and decrease of hydrogen content in a gas can improve wettability as well. Only in the case of some oxides pickling can be used successfully.
Bibliographic Reference: EUR 21993 EN (2006), 201 pp. Euro: 35
Availability: Katalogue Number: KI-NA-21993-EN-S The paper version can be ordered online and the PDF version downloaded at: http://bookshop.europa.eu
ISBN: ISBN: 92-79-01648-2
Record Number: 200618668 / Last updated on: 2006-07-20
Original language: en
Available languages: en