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In order to elucidate possible mechanism causing nozzle clogging during continuous casting of Ti added Ultra Low C (Ti-ULC) steel, thermodynamic analysis and experimental validation were carried out with an emphasis on the chemical reaction between the liquid steel and nozzle refractory. It was pointed out that the reaction occurs between CO gas from the nozzle refractory and the liquid steel, at the interface between them. A series of thermodynamic calculations were carried out in order to predict related phase equilibria. It was found that Ti in the steel induces the formation of a liquid oxide composed of FetO–Al2O3–TiOx along with solid alumina. This was different to a case of Ti-free ULC where only solid alumina was stable. In order to verify the thermodynamic predictions, a series of experiments were conducted. A number of Fe–Al–Ti alloys were reacted with CO gas at 1 560°C in order to simulate the interfacial reaction. Surface and cross section of the alloy samples were analyzed using Scanning Electron Microscopy (SEM) with Energy Dispersive Spectrometry (EDS). The experimental results were in good agreement with the thermodynamic predictions. This finding provides an idea why nozzle clogging is 

deteriorated by addition of Ti in ULC steel. It is proposed that Ti is oxidized together with Fe and Al by CO gas from a nozzle, and forms a liquid oxide composed of FetO–Al2O3–TiOx, which shows good wettability both to liquid steel and to refractory. This would be a precursor of clog material inside the nozzle.

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KEY WORDS: Ti added ultra-low carbon steel; nozzle clogging; interfacial reaction; thermodynamics;
FetO–Al2O3–TiOx liquid oxide.