Influence of Minor Alloying Elements on Selective Oxidation and Reactive Wetting of CMnSi TRIP Steel during Hot Dip Galv
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THE gas atmosphere in annealing furnaces of continuous hot dip galvanizing (HDG) lines reduces the iron oxides formed during cold rolling to elemental Fe. The alloying elements, Mn, Si, and Al are subject to selective oxidation since they have a high affinity for oxygen. This selective oxidation occurs at the steel surface in the low dew point (DP) annealing furnace gas atmosphere of HDG lines operated in standard conditions. The presence of film-forming surface oxides, in particular amorphous a-xMnOÆSiO2 (x < 0.9) and a-SiO2 oxides, leads to a deterioration of the wettability of the intercritically annealed strip by the molten Zn and prevents the formation of the Fe2Al5xZnx inhibition layer at the steel surface.[1–4] This selective oxidation phenomenon occurring at the surface and in the subsurface of most industrial steel grades has been investigated in detail for interstitial-free steel, dual phase steel, and transformation-induced plasticity (TRIP).[5–7] The four methods which have been proposed to improve the quality of the hot dip Zn coatings are LAWRENCE CHO, Graduate Student, is with Materials Design Laboratory, Graduate Institute of Ferrous Metallurgy, Pohang University of Science and Technology, Pohang 790-784, South Korea. MYUNG SOO KIM and YOUNG HA KIM, Research Staff Members, are with POSCO Technical Research Laboratories, Gwangyang 545-090, South Korea. BRUNO C. DE COOMAN, Professor, is with Graduate Institute of Ferrous Metallurgy, Pohang University of Science and Technology, and also Director, with Materials Design Laboratory, Pohang University of Science and Technology. Contact e-mail: [email protected] Manuscript submitted October 25, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
shown in Figure 1. The first method consists of annealing the strip in a high DP atmosphere, which results in internal oxidation rather than external oxidation.[1,2,8] The second method is a two step procedure consisting of an oxidation followed by a reduction.[9–12] In the oxidation stage, the surface is fully oxidized to a Feoxide layer. The Mn, Si, and Al, which diffuse to the interface between the ferrite and the Fe-oxide layer, form compound oxides. In the reduction stage, the Fe oxide is fully reduced to a pure Fe layer with embedded selective oxides. The third method involves the deposition of a thin layer of pure Fe, Ni, or Cu prior to the annealing stage in HDG lines.[13–15] The fourth method involves the addition of surface active elements such as Sn and Sb to the steel composition. These elements tend to segregate to grain boundaries and the surface during annealing. When these elements segregated on the surface, they suppress the surface oxidation by occupying the oxygen adsorption surface sites.[16–18] The fourth method, i.e., the addition of surface active elements, is potentially the easiest to implement industrially, as the other methods involve a modification to the parts of the industrial lines. However, the research on the use of this approach to the galvanizing industry is in the initial stages
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