Modeling Manganese Silicate Inclusion Composition Changes during Ladle Treatment Using FactSage Macros
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ACKGROUND
THE work presented here focused on changes in oxide inclusion composition during ladle treatment of steel which has been deoxidized with a combination of silicon and manganese, and which is in contact with MgO-saturated CaO-Al2O3-SiO2 slag. For such a system, several processes can contribute to changes in oxide inclusion composition, as summarized in Figure 1; the processes are chemical reactions between steel and slag, refractory, and oxide inclusions, slag–refractory reactions, and oxide inclusion flotation. While oxide inclusion flotation is not a chemical reaction, it does affect steel–inclusion reactions by changing the mass of oxide inclusions remaining in the steel. If such a system were to reach full equilibrium, the activities of all species in all phases would be equal. That implies that the oxide inclusion composition would approach the composition of the slag. For Si-Mn-killed steel, in contact with CaO-Al2O3-SiO2-MgO slag (as considered in this work), changes in oxide inclusion composition mainly occur because of reduction of SiO2 and MnO from the inclusions by Al (picked up from the slag) in the steel. Overall, these reactions are driven by the differences in the activities of Al2O3, SiO2 and MnO between the slag and the oxide inclusions. STEPHANO P.T. PIVA, Graduate Student, is with the Department of Materials Science and Engineering, Center for Iron and Steelmaking Research, Carnegie Mellon University, Pittsburgh, PA 15213, and also with the CAPES Foundation, Ministry of Education of Brazil, Brası´ lia, DF 70040-020, Brazil. DEEPOO KUMAR, Graduate Student, and P. CHRIS PISTORIUS, POSCO Professor, are with the Department of Materials Science and Engineering, Center for Iron and Steelmaking Research, Carnegie Mellon University. Contact e-mail: [email protected] Manuscript submitted December 31, 2015. Article published online July 26, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
The rate at which the composition of the oxide inclusions changes is limited (or partially limited) by the concentration of elements in liquid steel; liquid steel serves as the medium which transfers elements between the slag and oxide inclusions. For changes in the composition of oxide inclusions, mass transfer in steel and slag to and from the steel–slag interface is the main rate-determining step. In contrast, steel–inclusion reactions are expected not to be rate determining, because of the large product of the mass transfer coefficient and inclusion–steel contact area; see Table I. For small nonmetallic inclusions, static diffusion through the surrounding liquid steel is the dominant mass transfer mechanism; for static diffusion, the Sherwood number for spherical particles is 2, giving a mass transfer coefficient equal to D/r, where D is diffusivity in the liquid steel and r is the particle diameter. As shown in Table I, the product of mass transfer coefficient and reaction area (mA) is two orders of magnitude larger for steel–inclusion reactions than for steel–slag reactions (even in a strongly gas-stirred ladle, to which
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