Thermodynamic study of MnO-SiO 2 -Al 2 O 3 slag system: Liquidus lines and activities of MnO at 1823 K
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plex deoxidation is indispensable for the production of high-value steels such as tire-cord steel, in order to avoid the harmful effects of solid Al2O3 inclusions formed during Al deoxidation. In order to precisely control inclusions which form during Si/Mn deoxidation, inclusion engineering, based on the thermodynamic relations between inclusions and liquid steel, should be carried out during the secondary refining stage in the ladle and tundish. In this regard, the MnO-Al2O3-SiO2 ternary system is of great importance for inclusion control in Mn/Si-killed steels. However, most of the previous work[1,2,3] on this system covers only limited compositional ranges: for instance, saturation with mullite or alumina. In the present study, activities of MnO over a wide compositional range for the liquid MnO-SiO2-Al2O3 system at 1823 K were determined. The applicability of the regular-solution model and modified quasi-chemical model to the MnOSiO2-Al2O3 system at 1823 K was examined.
*DS700 is a trademark of Australian Oxytrol System Pty., Ltd., Victoria, Australia.
II. EXPERIMENTAL A MnO-SiO2-Al2O3 slag, together with platinum foil, was melted in a platinum crucible under a gaseous atmosphere at a given oxygen potential, and then the entire system was equilibrated by holding for long-enough time. The following reaction equilibrium was utilized to determine the activity of MnO: Mn(in Pt) ⫹ 1/2 O2(g) ⫽ MnO(in slag)
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DAE-HEE WOO, Professor, YOUN-BAE KANG, Graduate Assistant, and HAE-GEON LEE, Professor, are with the Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang, 790-784, Korea. Contact e-mail: [email protected] Manuscript submitted May 1, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS B
Master slags of MnO-SiO2-Al2O3 were prepared by mixing and melting reagent grades (purity: ⬎99.9 mass pct) of MnO, SiO2, and Al2O3. For each equilibrium experiment, 0.6 to 0.8 ⫻ 10⫺3 kg of slag was charged with a piece of platinum foil (purity: ⬎99.99 mass pct, and size: (0.1 ⫻ 10⫺3m) ⫻ (2 ⫻ 10⫺3m) ⫻ (7 ⫻ 10⫺3m) in a platinum crucible (purity: ⬎99.99 mass pct, and size: (10 ⫻ 10⫺3m i.d.) ⫻ (8 ⫻ 10⫺3m in height). The crucible assembly was then placed in a vertical resistance heating furnace (heating element: MoSi2, and furnace tube: alumina, (77 ⫻ 10⫺3m o.d.) ⫻ (70 ⫻ 10⫺3m i.d.) ⫻ (1.0 m long). The oxygen potential in the furnace was controlled by using a N2-H2-CO2 gas mixture flowing throughout the equilibrium experiment (flow rate: 600 ⫻ 10⫺6m3min⫺1). The experimental apparatus is presented schematically in Figure 1. The temperature in the system was measured using a thermocouple (Pt-6 pct Rh/ Pt-30 pct Rh). The oxygen partial pressure in the gas phase inside the furnace was determined by using a commercially available oxygen sensor, DS700.* The electrochemical prin-
ciple of the sensor is represented by the following cell expression: Pt, Air(PO2 ⫽ 0.209 atm)/ZrO2 (⫹ Y2O3)/N2 ⫺ CO2 ⫺ H2, Pt The time required for complete equilibration of the reaction system was determined by meas
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