Blockage of Tundish nozzles during the continuous casting of aluminum-killed steel

A critical review of the literature has shown that the inclusions which participate in nozzle blockage are primarily deoxidation products and other pre-existing solid particles which are suspended in the molten seel. The review also revealed that the precipitation of inclusions at the nozzle surface due to temperature drop, ingress of oxygen, or chemical interaction between dissolved elements in the steel and less stable elements in the nozzle material are not capable of accounting for the well-known preferential inclusion deposition sites. Likewise, the three existing models for the transfer of suspended inclusions to the nozzle wall are shown to be unable to account for the blockage patterns observed in practice. Through a series of geometrically similar water modelling and steel casting experiments, this thesis has demonstrated that nozzle blockage is caused by the existence of separated flow. The overall flow patterns within nine different experimental nozzle configurations have been visualized by the hydrogen bubble technique and subsequently quantified by the laser Doppler velocimetry. These experiments show that the separation zone is a region of relatively low velocity and yet highly turbulent flow in the which flow reversals and stalls are common. Subjection of some of these water model nozzle designs to laboratory scale steel casting experiments has provided a direct correlation between separated flow and the inclusion deposition sites encountered in industrial casting nozzles. Possible inclusions transport mechanisms based on the unique characteristics of separated flow are proposed to provide an insight into the manner by which primary deoxidation products and other solid inclusions suspended in the flow may be deposited on the nozzle wall.;The most critical parameter in the development of nozzle blockage is the nozzle geometry. Abrupt changes in geometry must be avoided to ensure that the flow is streamlined along the entire nozzle and all flow separations are eliminated. Nozzle orientation is also important in achieving a successful cast sequence. It has been shown that a misalignment of only two degrees from the vertical can result in the development of a visibly large separation zone in a well designed nozzle. It is presumed that even smaller deviations from the vertical will produce the flow conditions which initiate nozzle blockage. Finally, it has been established that the surface roughness of the nozzle has a significant effect on the deposition of inclusions. The working surface of the refractory should therefore be as smooth as possible and the integrity of the surface should not deteriorate over the duration of the cast.