The equilibrium titanium oxide phase was identified as Ti 3 O 5 (s) for iron melts containing 0.0012-0.25 mass% Ti at 1 873 K. The equilibrium constant of Ti deoxidation reaction for the formation of pure Ti 3 O 5 (s) were estimated as follows. The activity coefficient of Ti in liquid iron at infinite dilution, g°T i(s) was assessed as 0.011, 0.014 and 0.018 at 1 823 K, 1 873 K and 1 923 K, respectively.

The AlN solubility product in liquid iron containing manganese up to 22 mass% were measured by the metal-nitride-gas equilibration technique in the temperature range from 1 823 to 1 873 K. Manganese significantly increased the AlN solubility product in liquid iron primarily due to the large effect of manganese on the nitrogen solubility. Using the Wagner's formalism, the experimental results were thermodynamically analyzed to determine the thermodynamic interaction parameters between manganese and aluminum in high Mn-Al alloyed liquid steels as follows;(Mn ≤ 22 mass%, Al ≤ 1.8 mass%, 1 823-1 873 K) KEY WORDS: high manganese steel; AlN; aluminum; nitrogen; interaction parameter.

Fig. 7. Equilibrium relations between [%Ti] and [%N] in liquidiron at different temperatures.

a Fe-Al ϭϪ1 112/TϪ0.585 g°A l ϭϪ185.0/Tϩ0.165 KEY WORDS: liquid iron; aluminum; nitrogen; AlN; equilibrium constant; interaction parameters. ISIJ International, Vol. 47 (2007), No. 7, pp. 945-954 945 © 2007 ISIJ In the present study, thermodynamics of aluminum, nitrogen and AlN formation in liquid iron was investigated using the metal-nitride-gas equilibration technique in the temperature range of 1 873-1 973 K. The sampling method utilizing a high frequency induction furnace was used to determine the equilibrium relations of aluminum and nitrogen in iron melts containing up to 4.53 mass% Al at different nitrogen partial pressures. The effect of aluminum content on nitrogen solubility in liquid iron was measured. The solubility product of aluminum and nitrogen in liquid iron saturated with pure solid AlN was also measured as a function of temperature. Using Wagner's formalism, 9) the interaction parameters of aluminum and nitrogen and the equilibrium constant of AlN formation in liquid iron were determined and compared with previous investigations. In addition, thermodynamics of aluminum in liquid iron determined in the present study were examined by Darken's quadratic formalism. 15,16) Experimental ProceduresFive hundred grams of high purity electrolytic iron (99.95 mass% purity, 200 mass ppm O, Ͻ5 mass ppm N, 18 mass ppm C, Ͻ10 mass ppm Si), contained in an Al 2 O 3 crucible (OD: 56 mm, ID: 50 mm, H: 96 mm) was melted in the temperature range of 1 873-1 973 K by a 15 kW/30 kHz high frequency induction furnace as shown in Fig. 2. The alumina crucible was placed in a graphite crucible and a porous alumina crucible as outer crucibles. The reaction chamber consisted of a 96 mm OD quartz tube open at both ends and was connected to a gas delivery system.After melting the iron, the temperature of the melt was monitored by a Pt/Pt-13 mass%Rh thermocouple sheathed with an 8 mm OD alumina tube immersed in the melt. Any possible influence of high frequency noise on the temperature reading was avoided by grounding the circuit of the thermocouple. Preliminary trials confirmed that no significant noise was detected. The temperature fluctuation of iron melt could be controlled within 2 K during experiment by the PID controller of the induction furnace. The temperature reading of the PID controller was calibrated by the sourcing DC voltage calibrator for the thermal EMF of Rtype thermocouple.After the temperature of melt was reached to a desired value, an Ar-10%H 2 gas was blown onto the melt surface at a high flow rate of ϳ5 000 mL/min for 2 h to deoxidize the iron melt. The oxygen content in the melt after this procedure was in the range of 15-20 mass ppm. Then the gas was switched to a N 2 -3%H 2 gas (P N 2 ϭ0.97 atm) or a mixture of Ar-10%H 2 and N 2 gases (P N 2 ϭ0.2 atm). The flow rate of gas mixture was controlled by a mass flow controller in the range of 1 000 to 2 000 mL/min depending on nitrogen partial pressures in the gas.Strong agitation of melt by an induction furnace resulted in a fast attainment of equil...

Table 2. Number of bubbles generated at various gas flow rates-converted based on the calculation of gas flow rate divided by the volume of bubble.

The nitrogen solubilities in liquid manganese, manganese-iron, manganese-carbon, and manganeseiron-carbon alloys have been measured by the gas-liquid metal equilibration technique in the temperature range of 1623 to 1823 K. The equilibrium nitrogen content in pure liquid manganese at an atmospheric nitrogen pressure is high, and it does not follow Sievert's law, i.e., f N is not unity. The reduced nitrogen partial pressures by dilution with argon enabled us to obtain more reliable information on the thermodynamics of nitrogen in liquid manganese. The nitrogen dissolution follows Sievert's law at nitrogen contents below 1 wt pct. The standard free-energy change for the dissolution of nitrogen in pure liquid manganese has been determined as Ϫ67,222 ϩ 30.32T J/g atom, with the standard state of nitrogen taken as a 1 wt pct solution. Carbon and iron in manganese-rich melts decrease the nitrogen solubility significantly. The first-and second-order interaction parameters between nitrogen and other elements in manganese alloy melts have been determined. The activity coefficient of nitrogen in a ferromanganese alloy melt can be expressed as log f N ϭ 0.005 (pct N) ϩ 0.029 (pct N) 2 ϩ 0.015 (pct Fe) ϩ 0.09 (pct C) ϩ 0.013 (pct C) 2 where the interaction parameters are independent of temperature in the temperature range of 1623 to 1823 K.

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Thermodynamic behavior of nitrogen in the entire range of the Fe-Mn-Al-Si-C liquid solution was modeled based on the critical evaluation and optimization of all available experimental data. The Gibbs energy of liquid solution was described using the Modified Quasichemical Model (MQM) in the pair approximation, instead of classical Wagner Interaction Parameter Formalism, to consider the strong interactions between species in liquid state. In particular, the dissolution behavior of N and formation of AlN in the entire ternary and higher order liquids were accurately predicted from the MQM only with binary model parameters of N.