Effect of Slag Chemistry on the Desulfurization Kinetics in Secondary Refining Processes
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IN the production of bearing steel, it is important to ensure that the level of nonmetallic inclusions is as low as possible, especially for inclusions larger than 15 lm, since the inclusions can potentially initiate cracks under severe conditions.[1] It is well known that sulfur is easily precipitated at the grain boundary in the form of sulfide inclusions due to a low solubility in the austenite and ferrite matrix. Consequently, the mechanical properties of steel become degraded. During the secondary refining process, slag chemistry is very important since sulfur is removed via slag-metal reactions. Typically, the CaO-SiO2-Al2O3–based slags are used and MgO is added to prevent excessive corrosion of the MgO-C–type ladle lining. Moreover,
JIN GYU KANG, JAE HONG SHIN, and JOO HYUN PARK are with the Department of Materials Engineering, Hanyang University, Ansan 15588, Korea. Contact email: [email protected] YONGSUG CHUNG is with the Department of Advanced Materials Engineering, Korea Polytechnic University, Siheung 429-793, Korea. Manuscript submitted September 3, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
CaF2 is added to control slag properties, such as viscosity and melting temperature. Hino et al.[2] reported that when MgO is dissolved into the CaO-Al2O3 slag at a constant CaO/Al2O3 ratio, desulfurization of the slag becomes slightly more effective than that of the CaO-Al2O3 binary slag. He et al.[3] also found that the equilibrium S partition ratio between the metal and slag slightly increased by increasing the MgO content from 7 to 13 mass pct, which could be explained by a decrease in FeO activity and an increase in sulfide capacity for the desulfurization of ultra-low-carbon steel. One of the present authors reported that MgO could increase the activity of the free O2– ion (i.e., basicity) under conditions of relatively low CaO activity.[4] For the role of CaF2 with regard to the physicochemical properties of ladle slag, Kim and Park reported that the viscosity of low-silica calcium aluminosilicate melts at a fixed MgO content decreased through the addition of small quantities of CaF2 (approximately 5 mass pct), followed by a slight decrease during further CaF2 additions.[5] On the other hand, Ban-ya et al.[6] found that the activity coefficient of CaS (cCaS) decreased with an increase in CaF2 content in the CaOsat-Al2O3-CaF2 slag system.
II. LITERATURE REVIEW FOR DESULFURIZATION KINETICS Apart from thermodynamic studies, because desulfurization kinetics is also very important for the cost-effective design of secondary refining processes, many researchers have investigated the relationship between ladle stirring power and desulfurization rate in terms of mass-transfer coefficients.[7–16] It is well known that lime-saturated slag is very effective with regard to desulfurization efficiency. Thus, experimental slag compositions in previous studies have been relatively narrow, viz. close to the lime saturation phase boundary. For the desulfurization reaction at steelmaking temperatures, the chemical reactio
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