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 Fe t O-Al 2 O 3-TiO x 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 Fe t O-Al 2 O 3-TiO x , which shows good wettability both to liquid steel and to refractory. This would be a precursor of clog material inside the nozzle. KEY WORDS: Ti added ultra-low carbon steel; nozzle clogging; interfacial reaction; thermodynamics; Fe t O-Al 2 O 3-TiO x liquid oxide.

Phase equilibria of Fe-Al-Ti-O system is shortly reviewed, in particular for stability of oxide phases in equilibrium with liquid Fe alloy in dilute concentration. This system is relevant to Ultra Low Carbon steel with Ti alloying (Ti-ULC), often causing clogging of Submerged Entry Nozzle (SEN) during continuous casting. In order to predict stable oxides in the Ti-ULC steel, thermodynamic calculation using CALPHAD approach was employed. Previously available thermodynamic calculations were examined, and compared with available experimental data. A slight modification of thermodynamic modeling of previous investigation resulted in noticeable improvement for the prediction of stable phases by thermodynamic calculation. Significance of the present revision of the thermodynamic modeling is discussed in the view of nozzle clogging of Ti-ULC casting.

Nozzle clogging during continuous casting of Ti-ULC (Ultra Low C) steel was investigated by inspections of plant-used SENs and laboratory scale experiments using a rotating finger method. Various Al/Ti ratios in the steel and different kinds of nozzles (with or without CaO) were employed. Clog deposits found in the used SENs were composed of complicated oxide (CaO-Al 2 O 3-TiO x-…), Fe drops and chunks. Increasing Ti concentration increased amount of the deposit the metallic deposits. In the laboratory scale experiments, increasing Al/Ti ratios was effective to suppress the formation of the deposit and the oxidation loss of Ti in the steel. When a refractory without CaO was used, increasing Al/Ti ratio decreased the portion of Ti oxide in the deposit. Almost no Fe drops were observed, except for a thin layer of Fe on surface of the oxide deposit. When CaO-lined refractory was used, many numbers of Fe drops were found inside the deposit. Increasing Al/Ti ratio lowered Ti peak in the oxide deposit. The present results lend a strong support that Ti-ULC steel is oxidized by CO gas from nozzle refractory, forming Fe t O-containing liquid oxide and solid alumina. The liquid oxide is reduced to leave Fe drops and Al 2 O 3-TiO x layer. If CaO presents in the nozzle, then a complicated oxide of CaO-Al 2 O 3-TiO x forms, as was found in the SEN from the practical operation. KEY WORDS: Ti-ULC steel casting; nozzle clogging; Al/Ti ratio; Fe t O-Al 2 O 3-TiO x ; nozzle refractory.