Wetting Behavior of Zn-Al Liquid on Si-Containing Steel After Surface Oxidation and Reduction Treatment
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HOT-DIP zinc galvanization has been applied widely for rust-proofing steel sheet products in many applications, including in automobiles. In general, steel sheet material is pre-annealed for a few seconds at ~ 1123 K in a N2-H2 atmosphere of a low dew point for recrystallization treatment of the steel microstructure, then it is passed through a liquid zinc pot that is heated at ~ 723 K for a few seconds to be galvanized. For galvannealed (GA) steel sheet production, which is used most widely for automobile manufacturing, the galvanized steel sheet is annealed to control the Fe-Zn alloying behavior to optimize anticorrosion and mechanical properties.
MASANORI SUZUKI and TOSHIHIRO TANAKA are with the Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka, 565-0871, Japan. Contact e-mail: suzuki@ mat.eng.osaka-u.ac.jp YUSUKE FUSHIWAKI and YUSUKE OKUMURA are with the Steel Research Laboratories, JFE Steel Corporation, 1 Kokan-cho, Fukuyama, Hiroshima, 721-8510, Japan. Manuscript submitted June 3, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS B
High-strength steel sheets are expected to contribute energy savings and collision safety performance when applied in automobile body materials. Many kinds of high-strength steels contain Si and Mn as alloying elements for mechanical hardening. However, conventional hot-dip zinc galvanization of the Si, Mn-containing high-strength steel often shows non-wetting spots, which may reduce the anticorrosion properties of the galvanized steel sheets. The selective surface oxidation of elemental Si and Mn during pre-annealing has been believed to be the main reason for the non-wetting spot formation; because elemental Si and Mn can be oxidized even under a reducing atmosphere during pre-annealing of the steel sheet, they can form oxide phases on the annealed high-strength steel surface,[1–7] which often shows a bad wettability with liquid Zn-Al that is used for the galvanization. Many trials have been performed thus far to identify the non-wetting spot formation mechanism and to improve the galvanization of Si, Mn-containing high-strength steels.[8–10] Surface oxidation–reduction treatment has been proposed to address the above problem of the galvanization of Si, Mn-containing high-strength steels.[11–15] During this treatment, an iron-oxide layer with nanoscale thickness is formed on the steel surface, and it is reduced to a metallic iron layer. Although elemental Si and Mn in steel may still react with oxygen in the
atmosphere to form oxide phases, they are expected to be trapped as oxides under the reduced iron layer. Thus, it is expected that the galvanization of the Si, Mn-containing high-strength steel would be improved by a good wettability between the Zn-Al liquid and the reduced metallic iron layer on the Si, Mn-containing highstrength steel surface. Our previous studies have revealed that the surface oxidation–reduction treatment of some metals forms a three-dimensional interconnected porous microstructure in the reduced metal layer when its thic
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