Surface Selective Oxide Reduction During the Intercritical Annealing of Medium Mn Steel
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The use of advanced high- and ultrahigh-strength steel in the automotive industry makes it possible to meet strict emission-related environmental regulations by vehicle lightweighting. The reduced fuel consumption lowers the environmental impact of vehicles, and the increased use of advanced high-strength steel (AHSS) is therefore an unavoidable trend in automotive materials selection. Among the AHSS, the medium Mn steels have attracted a growing interest due to their excellent strength and ductility combination. Metallic coating systems are required to protect medium Mn steel from cosmetic corrosion and perforation corrosion. Zn and
KYOUNG RAE JO, LAWRENCE CHO, JONG HAN OH, and BRUNO C. DE COOMAN are with the Materials Design Laboratory, Graduate Institute of Ferrous Technology (GIFT), Pohang University of Science and Technology (POSTECH), 77 CheongamRo, Nam Gu, Pohang, 790-784, South Korea. Contact e-mail: [email protected] MYOUNG SOO KIM and KI CHEOL KANG are with the Technical Research Laboratories, POSCO, Gwangyang, 545-090, South Korea. Manuscript submitted January 9, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
Zn alloys coating are widely used on automotive steel grades as they provide an excellent cathodic protection. The key alloying elements in medium Mn steel include Mn, Si, and Al. These alloying additions are detrimental to the hot-dip coating of medium Mn steel. While the standard annealing furnace gas atmospheres in continuous annealing lines are reducing with respect to iron, they selectively oxidize alloying elements such as Mn, Si, and Al. The formation of a surface layer of MnO during the continuous annealing of medium Mn steel deteriorates the Zn coating quality, as it prevents the reactive wetting of the steel in the molten Zn which results in the formation of the Fe2Al5 xZnx inhibition layer. Selective oxidation makes it therefore challenging to use hot-dip galvanizing (HDG) as a coating method for medium Mn steel. Several methods have been proposed to prevent the deterioration of the Zn coating quality. The hot-dip galvanizing at low temperature, the deposition of a flash coating prior to HDG, electrogalvanizing prior to HDG, dew point control, and the combined pre-oxidation and reduction method have been suggested as methods to obtain strip surface conditions suitable for the HDG process.[1–10] The method proposed in the present report is potentially the simplest method in terms of industrial implementation, as it does not require modification of existing industrial production facilities. The results of the present study show that the direct reduction of the selective MnO oxide formed on medium Mn steel by highly reducing gas atmospheres can result in the effective suppression of the negative impact of these oxides. A Fe-0.1 wt pct C-6.0 wt pct Mn medium Mn steel was produced by vacuum induction melting, hot rolling, and cold rolling. Cold-rolled, full-hard medium Mn steel panels were cut into 20 mm 9 50 mm size samples and electropolished with 5 pct perchloric acid +95 pct acetic ac
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