Effect of Silicon Carbide on Reactions between Molten Steel and Fused Magnesia Silicon Carbide Composite Refractory

Transcription

1 Effect of Silicon Carbide on Reactions between Molten Steel and Fused Magnesia Silicon Carbide Composite Refractory Interactions between MgO SiC composite and liquid steel resulted in decomposition of SiC particles by the molten steel that damaged the integrity of the composite. Yaowu Wei, Nan Li and Changming Ke Interactions between refractories and liquid steel have attracted attention with the development of clean steelmaking. 1 3 Low-carbon (LC) steel and ultra-low-carbon (ULC) steel require low carbon content. However, carbon-containing refractories are a source of carbon to the molten steel. Steelmakers prefer to use low-carbon-content refractories as steelmaking vessel linings. However, their service life remains a concern. SiC is considered a component substitute for graphite, which is widely used in steelmaking refractories. A study by Li and Chi 4 emphasizes SiC as substitute for ordinary carbon materials in mold fluxes for continuous casting of ULC steel. The results show that the carbon pickup Fig. 1 Schematic of the experimental arrangement. for ULC steel is weak (10% compared with traditional carbon-containing material). Carbon pickup of interstitial-free steel from MgO SiC composites has been reported. 5 Results show that aluminum powder decreases SiC oxidation and accelerates deoxidization in the steel. This accelerates the process of carbon pickup of steel from the SiC. SiC possesses many interesting properties, such as high thermal conductivity, good wetting resistance to slag and liquid steel, and low recarburization compared with carbon. Therefore, attention has been directed to the research and manufacture of MgO SiC composites. Because of their high-temperature properties, MgO SiC composites can be used as a component under high-temperature conditions. The eutectic point of the MgO 2MgO system is 1860 C. Studies of interactions between refractories and molten steel reveal that the refractory influences the properties of the steel. 6,7 The study on the route of steel penetration into refractories also is important. This study has investigated the reaction between molten steel and MgO SiC composites. 9201

2 Table 1 Chemical Composition of Raw Materials Fused MgO SiC powder Component (mass%) (mass%) MgO CaO Fe SiC 97.9 LOI Fig.2 Illustration of one piece of MgO SiC composite after the experiment. The Experiment Fused MgO aggregates, MgO fine powder and SiC fine powder were used in this study (Table 1). The SiC content in the composites was 7 mass%. The chemical composition of interstitial free (IF) steel was determined (Table 2). The raw materials were mixed with phenol formaldehyde resin (4 mass% added). The mixtures were fabricated using cold isostatic pressing at 200 MPa. The fabricated refractory crucible was heated at 230 C for 24 h. IF steel specimens (110 mm diameter 75 mm length) were embedded in the crucibles and covered with mold flux (Table 3). MgO-based ramming materials filled the interspace between the crucible and furnace induction coil (Fig. 1). The MgO SiC crucible that contained IF steel and flux was heated at 1600 C for 3 h in the induction furnace. A fast-heating process was adopted in this study. It took only 2 h to heat from room temperature to 1600 C. Table 2 Chemical Composition of IF Steel Component Composition (mass%) Oxygen Carbon Silicon 0.03 Manganese 0.12 Titanium Sulfur Aluminum Nitrogen Table 3 Chemical Composition of Mold Flux Component Composition (mass%) MgO Al CaO 40.2 Fe CaF The boundary between molten steel and composite was investigated using X-ray diffractometry (XRD) analysis and electron probe microanalysis (EPMA) with energy-dispersive spectroscopy (EDS) analysis. The original layer of the composite also was studied. Composite Corroded by Liquid Steel The MgO SiC crucible was cut into several pieces after the experiment. The MgO SiC composite obviously was corroded by the liquid steel (Fig. 2). Scanning electron microscopy and EDAS analyses of the reaction layer between the MgO SiC composite and steel were conducted (Fig. 3). The results showed that steel did penetrate into the composite (white dots in Fig. 3(A)). Moreover, the reaction layer was composed of MgO particles (point 2 in Fig. 3(B)), MgO CaO oxide mixture (point 4) and Fe Si mixture (point 1). A MgO particle that contained a small amount of Al 2 also was observed (point 3). The silicon content of the steel in the matrix was much higher than that of the IF steel sample. Therefore, the silicon in the Fe Si mixture might have come from SiC in the composite. No SiC particles were found in the reaction layer. The SiC content in the reaction layer was small. Therefore, SiC might have transferred to the other constituent; e.g., SiC decomposed into the steel. The hot face of the composite was investigated using XRD analysis. MgO, 2MgSiO 4 and CaMgSiO 4 existed in the hot face of the composite (Fig. 4). The original layer of the MgO SiC composite was studied 9202

3 after it was heated. SiC particles existed in that layer (Fig. 5). Interface Reactions When SiC particles come in contact with molten steel, SiC directly dissolves into the steel: SiC(s) = [C] + [Si] G 1 = T J/mol (1) The [Si] and [C] later react with [O] in the steel melt: [C] + [O] = CO(g) G 2 = T J/mol (2) [Si] + 2[O] = G 3 = T J/mol (3) Therefore, when the SiC dissolves into the steel, interspace remains for further steel penetration. This study shows that significant porosity exists in the reaction layer (Fig. 2). Some is removed from the liquid steel and some reacts with MgO CaO in the composite matrix. CaO and might also result from impurities in the fused MgO powder. One of the CaO MgO oxide mixture formations is described as MgO(s) + CaO(s) + SiO2(s) CaMgSiO 4 (s) (4) When the heating time is increased, more CaO MgO oxide forms in the matrix. At the same time, the aluminum content oxidizes and then reacts with MgO to form a nonstoichiometric spinel: MgO(s) + Al 2 (s) MgO Al 2 (s) G 5 = T J/mol (5) MgO CaO oxides observed at room temperature might be liquid oxide mixtures at high temperature. 8 It also has been reported that MgO CaO oxides, such as CaMgSiO 4, decompose to MgO and liquid phase at temperatures <1492 C. 9 Once the liquid forms in the matrix of a MgO SiC composite, it flows into the interparticle pores and subsequently (A) (B) penetrates the particle boundaries in the compact. Finally, the particles are surrounded by the liquid phase. The impurity Al 2 dissolves into the oxide mixture under high temperatures, but only in a small amount. The low-melting-phase CaO MgO oxides become the passageway for steel penetration. Molten steel corrodes the MgO aggregates along the crystal boundary, and MgO particles enter into molten steel once it is surrounded by steel melt. Fig. 3 SEM and EDAS analysis of the composite steel reaction layer (hot face is on the left hand) at (A) low magnification and (B) higher magnification showing various specific particles. Iron Silicon Magnesium Calcium Oxygen Aluminum Point (mass%) (at.%) (mass%) (at.%) (mass%) (at.%) (mass%) (at.%) (mass%) (at.%) (mass%) (at.%)

4 Counts 2θ (deg) Fig. 4 XRD analysis of reaction layer between molten steel and composite. Fig. 5 Original layer of MgO SiC composite. Some SiC particles in the composites might oxidize during heating, and, then, film forms around the particle surface. 10 2MgO forms on the boundary between MgO and SiC particles, and its amount increases in the matrix with heating temperature and time. Refractory Integrity Damaged The molten steel damaged the integrity of MgO SiC composites under high temperature. SiC particles can be directly dissolved into molten steel. The interspace that remains after SiC decomposition acts as the route for steel penetration. MgO particles become isolated and dissolve into the molten steel. that forms around the SiC surface reacts with CaO and MgO to form a low-melting-point phase, such as CaMgSiO 4, that also damages the integrity of MgO SiC composites. About the Authors Yaowu Wei is Associate Professor at Hubei Province Key Lab of Refractories and Ceramics, Wuhan University of Science and Technology, Wuhan, People s Republic of China. His research focuses on refractories for clean steelmaking. The interactions between refractories and molten melt are a main subject in his study. Nan Li is Professor and Director for Ph.D. studies at Hubei Province Key Lab of Refractories and Ceramics. His research focuses on refractories used for metallurgical industry. He is a Chinese commissioner of UNITECR. Changming Ke is Professor at Hubei Province Key Lab of Refractories and Ceramics. His research focuses on refractories and refractories recycling. Correspondence regarding this article should be addressed to Yaowu Wei via at yaowuwei@hotmail.com or wangyure@public.wh.hb.cn. 9204

5 References 1 N. Li, Interactions between Refractories and Steel and Its Influence on the Property of Steel; pp Metallurgical Industrial Publishing, G.Z. Ruan and N. Li, Carbon Pickup of Interstitial Free Steel from Al 2 C Refractories, Ironmaking Steelmaking, 31 [4] (2004). 3 P. He, R. Chen, G.F. Fang, et al., Study of Carbon Pickup of IF Steel Caused by Carbon-Containing Refractories, Naihuo Cailiao, 39 [4] (2005). 4 N. Li and J. Chi, Study on SiC as the Substitute of the Mold Fluxes in Continuous Casting of ULC Steel, Journal of Shanguan University (Natural Science), 19 [3] (1988). 5 Y. Wei, N. Li, et al., Behavior of Al Powder on the Carburization of MgO SiC Material to Molten Steel, Naihuo Cailiao, 40 [5] (2006). 6 J. Potschke, K. Beimdiek and M. Ollig, Reaction between MgO C Bricks and Steel Melts ; pp in UNITE- CR 99 Proceedings. Edited by J. Kramss, German Refractory Association, Berlin, H. Soulard, J. Lehmann and M. Boher, Interaction Mechanism between MgO C and Al-Killed Steels; see Ref. 6, pp N. Li, L.H. Li and Y.W. Wei, Effect of Microsilica in MgO-Based Castables on Oxygen Content of Interstitial Free Steel, Br. Ceram. Trans., 103, (2003). 9 G. Yu Guocheng, Theory of Basic Refractories; p Shanghai Technology Publishing, Shanghai, Z. Chen, Chemical Thermodynamics of Refractories; p Metallurgical Industrial Publishing, Beijing,