Abstract: To decrease the surface defects on cold rolling sheets caused by inclusions, large inclusions in ultra-low carbon steel slab produced by bending vertical caster were characterized in details along the whole thickness direction. Metallographic samples with large section dimension of 70 mm×80 mm were specially prepared, and the inclusions were inspected by the automatic secondary electron microscopy ASPEX explorer. It was found many large inclusions still exist even T.O mass fraction in slab was reduced to 0.001 2 %. Most majority of them were alumina clusters within 20－200 μm and the others were the “bubble+Al2O3” type with big sizes in the range of 30－100 μm and some were larger than 100 μm. While the wellknown moldflux entrapped inclusions were not detected. These large inclusions were not only in the surface/subsurface but also located deeply in the slab, indicating an obvious quarterband accumulation especially for the “bubble+Al2O3” inclusions.

Key words: ultra-low carbon steel, inclusion, alumina, bubble, continuous casting slab