Selective oxidation and segregation in commercial steels and model alloys : (tools for improving the surface wettability by liquid Zn during hot dip galvanizing

To improve a wettability of annealed high strength steels and model alloys by Zn bath, two processes were applied to modify the surface: selective oxidation during annealing in 5% H2/N2 at 820°C at dew points -60, -30 and 0°C on HSS (DP 500, Ti-IF and TiNb-IF), and preoxidation in air at 700°C with subsequent annealing in 5% H2/N2 at dew point -30°C on model alloys (Fe-Al, Fe-Si and Fe-Mn).External oxidation of Al and Mn is observed at dew point -60°C. Nitrides, AlN on DP 500 and TiN on interstitial free steels, formed by the reaction of nitrogen from the steel with Al and Ti are detected as well. BN film/particle forms on DP 500 due to reaction of nitrogen adsorbed from the atmosphere with B from the steel. Surface of DP 500 shows almost no wettability by Zn bath thus suggesting no wettability of BN. Interstitial free steels exhibit good wettability since covered by small Al and Mn oxides only. With increasing dew point to -30°C a transition of Al oxidation from external to internal takes place where the maximum of Mn, Si and Cr external oxidation is detected. B is mostly present as oxide. Segregation and oxidation of B is found to suppress Si segregation. The wettability of the surfaces decreases on interstitial free steels at dew point -30°C while on DP 500 wettability increases compared to the one at dew point -60°C. The dominant process at dew point 0°C is external oxidation of P and formation of (Fe)Mn phosphate at the grain boundaries, while the grains are predominantly covered by MnO and some Cr2O3 and SiO2 on Ti-IF and DP 500 steels. The wettability behaviour of the steel surfaces annealed at dew point 0°C is much better than after annealing at dew point -30°C. The absence of Si on TiNb-IF at all three dew points can be understood by a site competition between segregating B and/or S and Si at low dew points, and P and/or C and Si at higher dew points.Preoxidation of model Fe-Al and Fe-Si alloys with less than 3 wt. % of Al(Si) leads to the formation of mostly Fe-oxides and partially Fe-Al(Si) oxides. During the subsequent annealing the Fe-oxides are reduced to metallic Fe and the surfaces show a better wettability in a Zn bath than only on annealed alloys. The best wetting is obtained on the Fe-1 wt. % Si, where a spongy metallic Fe forms after reduction in the annealing cycle. When the alloys contain 3 wt. % Si(Al) or more, a larger area of the surface is covered by non wetted Si and Al oxides and such surfaces show no improved wettability after the preoxidation and annealing. On the Fe-Mn alloys oxidized in air complex oxide layers are detected, mostly (Fe, Mn) oxides and Fe-oxides. Although not completely reduced to metallic Fe, these surfaces seem to give no significant problems in the wetting process up to 5 wt. % Mn.

Grantor dc:publisher

Publikationsserver der RWTH Aachen University

Year dc:date

2005

Author dc:creator

• Parezanovic, Ivana

Contributors dc:contributor

• Spiegel, Michael

• info:eu-repo/classification/ddc/660
• Technische Chemie

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

eng

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-123440%22

OAI identifier oai:identifier

oai:publications.rwth-aachen.de:61819