Abstract:
Clogging events are among the biggest process deviations that affect steelmakers worldwide, negatively impacting continuous casting productivity, yield and quality. When the clogging occurs between the ladle and the tundish, the steel level in the tundish continuously decreases, making it hard to keep the sequence in production, leading to casting speed reduction, and, sometimes, if the problem cannot be solved, to an unplanned stoppage of the caster, or at least, to an increased metal return in the ladle. The aim of this work was to mitigate ladle nozzle clogging events in the ArcelorMittal Tubarão steelmaking shop with an industrial problem-solving approach. The problem was stratified in order to understand which steel grades had the higher frequency in the phenomenon, a computational thermodynamic software was used to evaluate what kind of non-metallic inclusions were present in the system, and statistical analysis compared process variables between heats with and without the problem. It was shown that the problem was highly concentrated on Al-Si killed steels without Ca treatment. It was found that additions of FeSi in the ladle during secondary metallurgical treatments produced small amounts of Ca in the steel, leading to the formation of high melting temperature non-metallic inclusions based in the CaO-Al₂O₃ system, which finally caused the majority events of ladle nozzle clogging during casting. The strategy for Si chemical adjustment of those steel grades was changed, and a new ferroalloy was adopted in the secondary refining stations. Those practices combined were enough to highly reduce the problem frequency.