Effect of Si Content on Oxide Formation on Surface of Molten Fe-Cr-C Alloy Bath During Oxygen Top Blowing
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IN decarburization of stainless steel in a converter, precise control of the temperature as a function of the C and Cr content is important. As the oxygen potential required to oxidize Cr is very close to that required to oxidize C, significant oxidation of Cr can occur during decarburization unless the operation is conducted under optimal conditions. Many researchers have reported the conditions necessary to suppress Cr oxidation in the low-C-content region on the basis of laboratory studies, industrial studies, and mathematical models.[1–16] However, in actual operation, oxidation of Cr occurs not only in the low-C region but also in the initial stage of decarburization owing to the low temperature.[2,10]
RYOSUKE MIHARA is with the JFE Steel Corporation, 1, Kawasakitori, Mizushima, Kurashiki, Okayama 7128511, Japan. XU GAO, SHIGERU UEDA, HIROYUKI SHIBATA, and SHIN-YA KITAMURA are with the Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1, Katahira, Aoba-ku, Sendai, Miyagi, 9808577, Japan. Contact email: [email protected] SUN-JOONG KIM is with the Department of Materials and Science, College of Engineering, Chosun University, 309 Pilmundaero, Dong-gu, Gwangju, 61452, Republic of Korea. MIN OH SEOK is with the Technical Research Laboratories, POSCO, Gwangyang, Jeollannam, 57807, Republic of Korea. Manuscript submitted March 15, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
Thermodynamic calculations clearly showed that if the Cr content in the melt increases before the temperature becomes sufficiently high, Cr will be preferentially oxidized even if the carbon content is high. This situation occurs when ferrochromium alloy is added to the dephosphorized hot metal at low temperature, or when the charging temperature of the Cr-containing hot metal made by smelting reduction is not high enough. Clarification of this phenomenon is important for understanding the optimal conditions for preferential decarburization in the initial stage, but current knowledge is limited. On the basis of this background, we constructed an original experimental setup to allow direct observation of oxide formation at the bath surface during decarburization by top-blown oxygen.[17–19] By using this experimental method, the oxide formation behavior of molten Fe-14 mass pct Cr-5 mass pct C alloy was investigated at various temperatures. Oxides having three morphologies were seen to form sequentially during decarburization. First, oxide particles were formed, followed by unstable oxide films and, finally, stable oxide films. We defined the critical C content at which oxide particles, unstable oxide films, and stable oxide films started to S form as CP, CU F ; and CF ; respectively. The value of CP was always in the high-C-content range, irrespective of S the temperature or initial Cr content, but CU F and CF decreased as the temperature increased. By comparing the equilibrium relations, we found that oxide particles and unstable oxide films were formed under
nonequilibrium conditions, whereas stable o
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