The Effect of Bi on the Selective Oxide Formation on CMnSi TRIP Steel
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INTRODUCTION
THE current vehicle lightweighting efforts of the automobile industry are, in part, made possible due to the increased use of high-strength steel. High- and ultrahigh-strength steels allow for the use of thinner steel sheet to reduce the weight of vehicles substantially. This leads to an increased fuel efficiency and a lower environmental impact. Transformation-induced plasticity (TRIP) steel is a high-strength steel with a high formability suitable for press forming. The use of galvanized TRIP steel has remained relatively limited due to its poor galvanizability. Al, Si, and Mn are typically added to the composition of TRIP steel to obtain the characteristic TRIP microstructure. These alloying elements are susceptible to selective external oxidation in industrial continuous galvanizing lines. They form surface oxides during the recrystallization annealing of the strip in the N2 + (5-10) vol pct H2 protective gas atmosphere. This layer prevents the formation of the Fe2Al5 xZnx inhibition layer at the Zn/Steel interface and results in poor galvanizability.[1–6] Several methods have been tested to address the issue. Figure 1 shows the proposed solutions for improving the galvanizability of TRIP steels. The first method is JONGHAN OH, Graduate Student, LAWRENCE CHO, Research Associate, and BRUNO C. DE COOMAN, Professor and Director, are with the Materials Design Laboratory, Graduate Institute of Ferrous Technology, Pohang University of Science and Technology (POSTECH), Pohang, 790-784 Republic of Korea. Contact e-mail: [email protected] MYUNGSOO KIM, Senior Principal Researcher, and KICHUL KANG, Senior Researcher, are with the Surface Treatment Research Group, POSCO Technical Research Laboratories, Gwangyang, Republic of Korea. Manuscript submitted April 14, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
the adjustment of the steel composition. When the Mn/ Si mass ratio is higher than 2, the selective oxidation of Si at the surface is suppressed, and a galvanized TRIP steel without bare spots can be produced.[7] The second method is based on the annealing furnace gas atmosphere dew point (DP) control, which results in internal oxidation rather than external oxidation.[8,9] A third method is a two-step procedure consisting of a surface oxidation followed by a reduction. In the oxidation stage, Fe oxide is formed on the surface. Si, Mn, and Al form oxides which are embedded in the Fe oxide. In the reduction stage, the Fe oxide is fully reduced to a thin Fe layer in which the selective oxides remain embedded.[4,10,11] The fourth method consists of the deposition of a thin layer of pure Fe, Ni, or Cu prior to annealing. The presence of this thin layer can suppress the oxide formation during annealing.[6,12–14] The fifth method is based on the reduction of the selective oxides. The surface oxides, which prevent the Fe–Al reaction during the dipping of the strip in the Zn bath can be reduced by reducing the oxygen partial pressure.[15,16] The sixth method consists of the addition of a surface-active elemen
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