Solubility of Nitrogen in High Manganese Steel (HMnS) Melts: Interaction Parameter between Mn and N
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h manganese steels (HMnS; 10 to 25 mass pct Mn) are of interest due to their good mechanical properties such as superior strength and good ductility.[1–3] The effects of nitrogen on the properties and microstructures of HMnS have been investigated by several authors.[4–6] Recently, the nitride inclusions, e.g., AlN formed during steelmaking and continuous casting processes, was shown to affect not only the surface qualities but also the microstructures of steel products.[7–9] Therefore, the nitrogen content in molten HMnS should be tightly controlled during manufacturing at high temperatures. However, reliable thermodynamic data for nitrogen solubility and the interaction parameter between Mn and N in HMnS melts are required in order to control the formation of AlN. Even though many authors have measured the solubility of nitrogen in molten Fe-Mn alloy melts, there have been relatively large discrepancies in the interaction JAE HONG SHIN, Graduate Student, JOO HYUN PARK, Assistant Professor, are with the School of Materials Science and Engineering, University of Ulsan, Ulsan 680-749, Korea. Contact e-mail: [email protected] JOONHO LEE, Associate Professor, is with the Department of Material Science and Engineering, Korea University, Seoul 136-713, Korea. DONG JOON MIN, POSCO Professor and Head of Department of Materials Science and Engineering, is with Yonsei University, Seoul 120-749, Korea. Manuscript submitted: June 8, 2011. Article published online October 4, 2011. METALLURGICAL AND MATERIALS TRANSACTIONS B
parameter between Mn and N, eMn N , for molten alloys, as shown in Table I.[10–23] That is, the value of eMn N ranges from about 0.04 to 0.01 at 1873 K (1600 °C) and the temperature dependency of eMn N for high Mn content is largely lacking. This inconsistency in thermodynamic information results in uncertainty in estimating the formation behavior of nitride inclusions in HMnS. Hence, we assessed the solubility of nitrogen under pN2 ¼ 1 atm using a N2 gas bubbling methodology from 1823 K to 1923 K (1550 °C to 1650 °C). We used our results to derive an expression for the interaction parameter between Mn and N as a function of temperature. Figure 1 shows a schematic diagram of the experimental apparatus. Experiments were carried out by equilibrating the N2 gas with Fe-Mn melt in a high frequency induction furnace with a quartz reaction tube. The experimental temperature range was 1823 K to 1923 K (1550 °C to 1650 °C), which was controlled within ± 2 K using a B-type (Pt-30 mass pct Rh/Pt6 mass pct Rh) thermocouple and a proportional integral differential controller. High purity electrolytic iron (3 N purity, 475 g) and manganese (4 N purity, 25 g) were initially melted in an alumina crucible (OD: 56 mm, ID: 50 mm, Ht: 96 mm) using a high frequency induction furnace under purified Ar-3 pct H2 atmosphere. After melting the Fe-5 mass pct Mn alloy, the gas was switched to purified N2 gas, and then an alumina lance for providing N2 gas was inserted into the molten alloy and kept 5 mm above the bottom of the cru
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