Microstructures and Formation of Tundish Clogging Deposits in Ti-Alloyed Al-Killed Steel

The continuous casting of Ti-alloyed Al-killed steels traditionally experiences clogging in the tundish, impeding stable operation. We studied the microstructures and mode of formation of such clogging deposits in tundish skulls recovered after long cast series. We sampled the steel skull along the whole tundish bottom from under the ladle shroud (impact zone) up to the tundish well and nozzle. The microstructures imply that the steel solidifies to the refractory (gunning mass) and remains partially solid through the entire cast. Thus, a “mushy zone” of steel loaded with solid delta ferrites is present along the margins of the steel during casting. In this mushy zone, the widely observed clusters of variably sintered alumina (NMI) accumulate, along with a substantial amount of gas bubbles acting as inclusion scavengers. The clustered alumina particles show a lognormal particle size distribution (PSD) in all locations, which becomes more pronounced if correcting for the observable sintering. Sintering of alumina size distributions linearizes a PSD, and further evolution, once the deposit is locked in place, occurs by Ostwald ripening, introducing skewness to the distributions. The observed PSDs contrast with the established log-linearity of PSDs of individual floating alumina NMI in the bulk steel, and suggest that the source of the alumina in the clogging deposits is not the passing bulk steel. However, ubiquitous sedimentary features show that the alumina particles are also not grown by reoxidation in situ. An upstream source separate from the bulk stream must be inferred, which is also required by the consistent sporadic presence of oxidic materials which cannot be derived from local sidewall reaction. In summary, the development of clogging deposits in this type of steel is an example of “slurry casting” of partially solidifying steel over thermal loss surfaces of refractories in the nozzle area.