Thermodynamics of Nitrogen in Fe-Mn-Al-Si-C Alloy Melts
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I.
INTRODUCTION
RECENTLY, high alloy steels such as high Mn-Si steel, high Al and TWIP (TWining Induced Plasticity) steels have been developed as next generation advanced high strength steels for automotive applications. High amounts of alloying elements bring great challenges to steelmaking and casting processes; significant corrosion of refractories, new types of non-metallic inclusion formation, high solubility of gas species, and long solidification range. For example, TWIP steel shows an excellent combination of high strength, ductility, and formability over conventional steels by alloying a large amount of Mn (15 to 20 wt pct), Al (1 to 3 wt pct), Si (~0.3 wt pct), and C (0.4 to 0.6 wt pct).[1–3] However, Mn significantly increases the N solubility in liquid iron, while decreases the melting temperature of the steels down to near 1723 K (1450 C).[4] In addition, the aluminum nitride, AlN, can be easily formed by the strong affinity of Al with N in the high-Mn and high-Al alloyed liquid steels.[5] Although the AlN inclusion is considered as a detrimental phase for the surface quality as well as the mechanical property of the final steel product, the critical N content for the AlN formation during cooling and solidification of high alloyed steels is less predictable due to the lack of the thermodynamic information.
So far, the most commonly used thermodynamic model to describe the N solubility in molten steel is the Wagner interaction parameter formalism.[6] But this model is limited only for specific alloy composition such as low carbon steel. The other model used in the commercial steel databases such as FactSage FSStel database[7] and Thermo-Calc TCFE database[8] is the Bragg–Williams random mixing model with regular or subregular parameters. However, the available models and parameters are insufficient to reproduce the thermodynamic behavior of N in high alloy melts. The purpose of this study is to develop a new thermodynamic database accurately describing the thermodynamic behavior of N in molten steel. In order to keep a pace with the development of high alloy steels, the database has been developed to describe entire composition range of liquid alloys in Fe-Al-Mn-Si-C system. In the present study, all the available experimental data related to the N solubility in unary, binary, ternary, and high order liquid were reviewed. Using the Modified Quasichemical Model with binary interaction model parameters between N and alloying elements, all available experimental data in ternary and higher order liquid systems were accurately predicted.
II.
THERMODYNAMIC MODEL
A. Limitations of Previous Thermodynamic Models
MIN-KYU PAEK, Postdoctoral Fellow, and IN-HO JUNG, Associate Professor, are with the Department of Mining and Materials Engineering, McGill University, Montreal, QC H3A 0C5, Canada. Contact e-mail: [email protected] SAIKAT CHATTERJEE, Graduate Student, formerly with the Department of Mining and Materials Engineering, McGill University, is now with the Department of Materials Science and Engineering, Univer
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