Effect of Slag Composition on the Concentration of Al 2 O 3 in the Inclusions in Si-Mn-killed Steel
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NTRODUCTION
NON-metallic inclusions in steel cause deterioration of mechanical properties as well as severe problems during steel processing.[1–3] The cleanliness of the steel is important in spring steel and tire cord steel since nonmetallic inclusions act as crack initiation sites when subjected to cyclic stress. However, no steel can be totally free from inclusions. For this reason, Si and Mn are generally used as deoxidizers in spring steel and tire cord steel in order to avoid harmful Al2O3-rich inclusions.[4,5] A number of studies have been carried out to identify techniques that achieve inclusions that are as harmless as possible.[4–18] The application of optimized slag to suppress the formation of harmful inclusions such as alumina and spinel is one such method used for various steel grades including Si-Mn-killed steel. For these reasons, the MnO-Al2O3-SiO2 ternary system has gained attention for several decades. Many studies have been performed to fundamentally understand this oxide system in terms of thermodynamics.[6–10] Thanks to previous work, if we determine the concentration range of Mn, Si, Al, and O in molten steel, we can predict the corresponding composition window for MnO, SiO2, and Al2O3 in the above inclusion system. Based on this background, several researchers suggested deoxidation for liquid inclusion in the MnO-Al2O3-SiO2 system.
Kang and Lee[9] reported that inclusions having a MnO/ SiO2 mass pct ratio near unity and Al2O3 content in the range of 10 to 20 mass pct have low liquidus temperatures [~1473 K (1200 C)] and suggested that a Mn/Si ratio of 2 to 5 meets these conditions when Mn + Si = 1.0 mass pct in the steel. Bertrand et al.[12] found that slag composition during secondary metallurgy must have a low basicity (CaO/SiO2), and Al content in the steel should be maintained under 0.0015 mass pct to avoid the formation of Al2O3. Chen et al.[5] studied the effect of top slag on the inclusion composition in tire cord steel and found that inclusion plasticization can be achieved by controlling the binary basicity of top slag (CaO/SiO2) around 1.0 and maintaining the Al2O3 content in top slag below 10 mass pct. However, an experimental study to identify conditions for obtaining liquid inclusion when the slag has relatively high basicity (CaO/SiO2) coupled with Al2O3 saturation, which corresponds to the alumina refractory lining, has not yet been conducted. Therefore, in the present study, the equilibria between CaO-Al2O3-SiO2-CaF2-MgO(-MnO) slag and Fe-1.5 mass pct Mn-0.5 mass pct Si-0.5 mass pct Cr melt was investigated at 1873 K (1600 C) in order to understand the effect of slag composition on the concentration of Al2O3 in the inclusions.
II. JUN SEOK PARK, Graduate Student, is with the School of Materials Science and Engineering, University of Ulsan, Ulsan 680-749, Korea. JOO HYUN PARK, Professor, is with the Department of Materials Engineering, Hanyang University, Ansan 426-791, Korea. Contact e-mail: [email protected] Manuscript submitted January 25, 2013. METALLURGICAL AND MATE
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