Thermodynamics of inclusion formation in Fe-Ti-C-N alloys
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I.
INTRODUCTION
MOST steels contain nonmetallic inclusions of some kind, at least in small quantities. Depending on the physical chemistry of steel, nonmetallic inclusions may be in the form of oxides, sulfides, oxysulfides, carbides, nitrides, carbonitrides, etc. In the case of titanium-containing high-strength low-alloy or stainless steels, particles of titanium carbide, nitride, and carbonitride are the main inclusions of concern. The presence of these inclusions is undesirable for many applications. They have a deleterious effect on toughness, ductility, and weldability. These titanium-base nonmetallic inclusions also affect the surface quality of the steel and can also cause nozzle clogging during continuous casting operation in the case of stainless steel. For example, the low ductility in these steels over a wide range of temperatures is due, in part, to strain-induced precipitation of particles of titanium carbide, nitride, or carbonitride that may precipitate at the grain boundaries. Therefore, the control of this kind of nonmetallic inclusion in steel has recently been an area of great interest, in particular, in the production of stainless and other alloys. Titanium-base inclusions in Fe-Ti-C-N alloys may form at steelmaking temperatures or may precipitate during the cooling of the steel. Titanium carbide and titanium nitride precipitated may react to form titanium carbonitride or titanium carbonitride may result directly from the reaction between titanium, nitrogen, and carbon in steel. In order to prevent the formation of these titanium-base inclusions, it is necessary to have information on the thermodynamics of the Fe-Ti-C-N system at the temperature of interest. The work on the thermodynamics of this system is limited. In relation to blast furnace burdens containing titanium bearings which cause poor operation, Delve et al. ttJ have investigated the solubility of titanium in carbon-saturated iron at 1773 and 1873 K under neutral or nitrogen atmospheres. They identified the precipitated phases as titanium carbide and titanium carbonitride. However, the conditions used in BAHRI OZTURK, Research Faculty, and R.J. FRUEHAN, Professor, are with the Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA 15213. Manuscript submitted December 8, 1989. METALLURGICAL TRANSACTIONS B
this study do not necessarily represent the case for steels where the concentration of carbon is much lower than that of the blast furnace. Turkdogan t21 recently considered nitride and carbonitride precipitation during continuous casting. However, in that analysis for titaniumbearing inclusions, pure titanium carbide and nitride were considered but not the carbonitride. Therefore, it is the purpose of this investigation to obtain the thermodynamics of the formation of titanium carbonitride in liquid iron-titanium-carbon-nitrogen alloys to predict under what condition it will form. II.
EXPERIMENTAL PROCEDURE
The metal-carbonitride equilibration technique was used to study the th
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