Thermodynamics of phosphorus and sulfur in the BaO-MnO flux system between 1573 and 1673 K
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I.
INTRODUCTION
W I T H increasing demand for production of high manganese steels with durable characteristics against lowtemperature brittleness, dephosphorization of these steels or high-carbon ferromanganese is becoming very important. Since the dephosphorization ability of CaObearing fluxes is not large enough for Fe-Mn alloys, extremely basic fluxes are needed for this purpose. As both BaO and MnO have high melting temperatures, it has been believed that the BaO-MnO flux would not melt at hot metal temperatures. However, in the present study, the authors have found that the BaO-MnO flux may melt at these temperatures because of the presence of a small amount of carbon. Therefore, the thermodynamic properties of the BaO-MnO flux, such as phosphorus and manganese partition ratios between the BaO-MnO flux and Fe-Mn-C~atd alloy, were measured between 1573 and 1673 K in a CO atmosphere. In order to clarify the melting behavior of the BaO-MnO system, its carbon content was examined as functions of flux composition, temperature, and partial pressure of CO, and the dissolution mechanism of carbon is discussed. Furthermore, the effect of replacement of BaO in the flux by CaO on dephosphorization was investigated, since the cost of BaO is much higher than CaO for industrial application of the flux. The sulfide capacity of the BaO-MnO flux was also measured at 1573 K.
I1.
EXPERIMENTAL
A. Phosphorus and Manganese Partition Ratios
A chemical equilibration technique was employed for the measurement of phosphorus and manganese partitions between the BaO-MnO, BaO-MnO-BaF2, and BaOCaOsatd-MnO fluxes and molten Fe-Mn-C~atd. alloys at hot metal temperatures. Six grams of these fluxes were equilibrated with 2.5 g of molten Fe-Mn-Csatd containing 9 to 92 mass pct of Mn in a dense graphite crucible (12-ram ID, 22-ram OD, 60-mm height). The MnO content ranged from 3 to 31 mass pct for the BaO-MnO system and from 11 to 23 mass pct for the BaO-MnOBaFz system, and the ratio (mass pct BaO)/(mass pct BaF2) was kept constant as approximately 8. A MnO or a BaO pellet was used for some experiments to ensure MnO or BaO saturation of the flux. In the case of the BaO-CaO~,~d-MnO system, a CaO pellet was equilibrated with the flux to ensure the CaO saturation, and the MnO content of the flux changed from 15 to 40 mass pct. A graphite crucible with the sample put in a graphite holder (28-mm ID, 32-mm OD, 90-mm height) was set in an electric furnace. Temperature was controlled within -+ 1 K. Purified CO was flowed with the rate of 1.7 x 10 -7 m3/s. The oxygen partial pressure of the system was controlled by Reaction [1]. 1 c (s) + 2 o2 (g) = c o (g)
YOSHINORI WATANABE, formerly Graduate Student, Department of Metallurgy, The University of Tokyo, is with NKK, Kawasaki, Kanagawa, Japan. K A T S U H I R O K I T A M U R A , formerly Undergraduate Student, Department of Metallurgy, The University of Tokyo, is with Honda R&D Co. Ltd., Tocbigi, Japan. IVAN PETKOV RACHEV, formerly Graduate Student, Department of Metallurgy, The University of Tokyo
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