Nitrogen solubility in liquid manganese and ferromanganese alloys
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MANGANESE and ferromanganese alloys are widely used as additives in steelmaking processes. The increased demand for ultra-low-carbon and -nitrogen steels requires the production of high-purity ferromanganese alloys. Recently, the oxygen refining of high-carbon ferromanganese is realized as an effective process for the production of low-carbon ferromanganese alloys.[1–5] In this process, the oxygen-inert gas mixture is injected so that carbon is oxidized in preference to manganese, because CO is diluted with inert gases such as argon and nitrogen. Nitrogen is preferred as an inert gas due to its lower cost. However, a substantial amount of nitrogen would dissolve in molten ferromanganese alloys when nitrogen is used as the inert gas. At present, the nitrogen specification on the low-carbon ferromanganese alloy is less than 500 ppm. There will be a trend toward more stringent nitrogen specifications in ferromanganese alloys in the steel industry, due to the increased production of ultra-low-carbon and -nitrogen steel grades. An understanding of the thermodynamics of nitrogen dissolution in liquid manganese alloys is very important in manufacturing low-nitrogen ferromanganese alloys. For example, in the decarburization of a high-carbon ferromanganese melt by an oxygen-nitrogen gas mixture, the nitrogen solubility in this system varies with nitrogen partial pressure as a function of carbon content and melt temperature. There are some thermodynamic data available for the EUI-JUN KIM, Graduate Student, and JONG-JIN PAK, Professor, are with the Division of Materials and Chemical Engineering, Hanyang University, Ansan 425 791, Korea. BYUNG-DON YOU, Professor, is with the Division of Materials Science and Engineering, Inha University, Inchon 402-751, Korea. Manuscript submitted March 29, 2000.
METALLURGICAL AND MATERIALS TRANSACTIONS B
nitrogen solubility in pure manganese[6–12] and manganeseiron melts.[10–16] However, there are no data available on the nitrogen solubility in manganese-carbon or manganese-ironcarbon melts. The nitrogen solubility data reported for a pure manganese melt[6–12] have been measured mostly at an atmospheric nitrogen pressure of 101.33 kPa. The nitrogen solubility in liquid manganese is relatively high, and it is not clear if the nitrogen dissolution follows Sievert’s law. Pure liquid metals of very high nitrogen solubility, such as niobium, exhibit a large deviation from Sievert’s law,[17] probably due to the interaction between nitrogen atoms in the melt. In order to determine the equilibrium constant for the nitrogen dissolution in liquid manganese, KN (⫽ fN (pct N)/PN1/22 ), the activity coefficient of nitrogen should be determined at various nitrogen contents. Such data can be obtained at reduced partial pressures of nitrogen by dilution with a noble gas. In the present study, the nitrogen solubility in pure liquid manganese was measured at nitrogen partial pressures from 3.04 to 101.33 kPa in the temperature range of 1623 to 1823 K. The effect of nitrogen partial pressure on the nitrogen di
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