Studies on accumulation of non-metallic inclusion in sub-surface of continuously cast slabs were carried out. Results obtained are as follows:
(1) Accumulation of alumina cluster in sub-surface of slabs occurs at the stagnant zone of the upward streams from the immersion nozzle in mold. The degree of the above-mentioned accumulation of alumina can be decreased by improving absorption capasity of mold flux.
(2) Accumulation of alumina cluster in the sub-surface of slabs is caused by entrapping in solidifing shell near the meniscus of alumina, which is carried up to the surface of molten steel in the mold and can not be absorbed enough in the mold flux.
(3) Argon gas bowing nozzle is effective in reduction of alumina cluster in sub-surface of slabs.
(4) Degree of surface defects, caused by fluctuation of surface level of molten steel, can be explained by the products of its fluctuation velocity and its fluctuation width.
