On the Transition of Internal to External Selective Oxidation on CMnSi TRIP Steel
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INTRODUCTION
THE interaction of the surface of steel with the gas atmosphere in the annealing furnace of a hot-dip galvanizing (HDG) line results in the reduction of iron oxide. The alloying elements, Mn, Si, Al, Ti and V, are subject to oxidation. This is due to the fact that the N2 + 5 to 10 pct H2 gas mixture contains water vapor. The H2O dissociation reaction H2O M H2 + 1/2 O2 determines the oxygen partial pressure (PO2 ), which is, albeit extremely low, still oxidizing toward Mn, Si, and Al. The amount of water vapor in the annealing furnace is represented by the dew point (DP) temperature. The furnace gas atmosphere typically has a low DP, resulting in the external selective oxidation of Mn and Si. Selective oxidation occurs when a solute atom is oxidized rather than the matrix element. The selective oxidation phenomenon occurring at the surface (external oxidation) and in the subsurface (internal oxidation) of most industrial steel grades has been investigated in detail for interstitial free steel, dual phase steel, and transformation-induced plasticity (TRIP) steel.[1–7] 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
LAWRENCE CHO and GEUN SU JUNG, Graduate Students, are with the Graduate Institute of Ferrous Metallurgy, Pohang University of Science and Technology, Pohang 790-784, South Korea. BRUNO C. DE COOMAN, Professor, is with the Graduate Institute of Ferrous Metallurgy, Pohang University of Science and Technology and also Director, with the Materials Design Laboratory, Pohang University of Science and Technology. Contact e-mail: decooman@ postech.ac.kr Manuscript submitted October 3, 2013. Article published online July 18, 2014 5158—VOLUME 45A, OCTOBER 2014
intercritically annealed strip by the molten Zn during the galvanizing process.[8–10] These oxides also act as barriers to the interdiffusion of Fe and Zn during galvannealing process.[11–14] Annealing in a high DP gas atmosphere has been shown to result in internal oxidation rather than external oxidation, and the control of the DP has been proposed as a method to improve the quality of Zn and Zn-alloy coatings on Si alloyed steels.[4–6] Several theories for internal oxidation have been proposed. Wagner[15] proposed a theory for the internal oxidation of binary alloys. The original theory was based on the assumption of a single oxidizing element forming a single oxide with a zero solubility product. Numerous improvements of the original model were proposed. Bo¨hm and Kahlweit[16] and Rapp[17] extended the model to account for a none-zero oxide solubility product. A model for multi-component alloys was proposed by Niu and Gesmundo.[18] Huin et al.[19] developed an internal oxidation model for multi-component ferrous alloys assuming a finite solubility product. Brunac et al.[20] then extended the model proposed by Huin et al.[19] to account for the possibilities that a large fraction of precipitates may act as diffusion barriers and that
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