Low reoxidation tundish metallurgy at Fundia Koverhar steel plant

Abstract

Production of Al killed steel grades at Koverhar began in 1992 after revamping the casting machine no. 1. The refractories and equipment of caster no. 1 were planned to prevent reoxidation. However, nozzle clogging problems with impaired inclusion cleanliness were occasionally observed. In order to clarify the rôle of reoxidation in continuous casting, a research project was carried out with the Helsinki University of Technology. The whole casting sequences of 3 steel grades were scrutinized. The changes of steel composition from ladle to mold as well as tundish slag composition were systematically investigated. The starts of casting sequences, ladle changes and cast endings were especially examined. The effects of rice husk ash as the tundish cover and the carryover of ladle slag to the tundish were studied as well. The research project clearly revealed different reoxidation phenomena during the casting process. Exchange reactions between the steel and the tundish slag caused reduction of silicon and manganese oxides from the slag to the steel and oxidation of aluminium, eventually titanium, from the steel, respectively. These reactions, however, did not impair steel cleanliness, as oxide inclusions produced by the reactions were formed in the vicinity of the metal‐slag interface, and thus inclusions were readily removed to the slag. Nitrogen pick‐up was observed especially during the cast starts, indicating atmospheric reoxidation. It had, however, neither any notable influence on steel cleanliness nor on castability. Any tendency to clogging was not observed during the campaigns.