The mechanism of alumina buildup on immersion nozzles in continuous casting was studied by considering both inclusions suspended in the molten steel such as deoxidation products (suspended inclusions) and inclusions generated by chemical reaction in refractory and molten steel (reacted inclusions).
(1) Two kinds of generation mechanism of reacted inclusions are considered as follows.
a) alumina generated by the reaction at the interface between refractory and molten steel (interfacial reacted inclusions).
b) alumina generated by nucleation when [Al]2[O]3 exceeds critical supersaturation degree in the concentration film layer near the interface between the refractory and molten steel (nucleation reacted inclusions).
(2) The deposition rate of reacted inclusions is evaluated to be smaller than that of suspended inclusions. Therefore, inclusions deposited on the immersion nozzles may mainly originate from suspended inclusions.
(3) At the early stage of alumina buildup, the network alumina is formed by deposition of interfacial reacted inclusions, nucleation reacted inclusions and suspended inclusions. Inclusions in the concentration film layer nucleation reacted inclusions and suspended inclusions are forced to move to the refractory due to the interfacial tension gradient formed by the concentration gradient of silicon etc.
When the reaction in the refractory proceeds and oxygen feeding rate decreases, mainly suspended inclusions are deposited on the network alumina because the generation rate of reacted inclusions decreases.
