Abstract: The surface layer of narrow edge of ultra-low carbon steel slab was planed layer by layer, and the morphology, quantity, size, composition and space distribution characteristics of bubble defects carried in the 20 mm zone of surface layer with size more than 50 μm were analyzed by optical microscope, scanning electron microscope and energy spectrum. The results show that the bubble defects are caused by bubbles and alumina associated bubbles. Bubbles exist in the form of single and aggregation. A total of 122 bubble defects are found, and the number of bubble defects with the particle size between 50-100 μm is the largest, accounting for 60.66 % of the total. The bubble defects are unevenly distributed on the surface layer of the slab, more at both ends of narrow surface, and less at 40-60 mm away from narrow face on the inner arc of wide face. The average number density of bubble defects on the narrow face is greater than that on the wide face, and there are more bubble defects near 3 mm subcutaneously on the slab surface. After 11 mm subcutaneously, the size of bubble defects decreases with the increasing of subcutaneous distance.

Key words: ultra-low carbon steel, bubble defect, size distribution, space distribution