Below the slab skin, there are large bubbles with diameters and lengths of 1 mm and 10 mm or more growing in the direction of columnar crystals. These bubbles are called surface bubbles if they are exposed outside, and those that are not exposed are called subcutaneous bubbles. Smaller and denser holes than bubbles are called subcutaneous pinholes. In the heating furnace, the surface bubbles or the inner surface of the subcutaneous bubbles of the slab are oxidized to form a decarburized layer, which cannot be welded after rolling to form surface defects. Shallowly buried bubbles can be removed by grinding wheels, air shovels and flame cleaning. Deeply buried air bubbles are difficult to find and can cause cracks in the product. Insufficient deoxidation of molten steel is the main reason for the generation of bubbles. If enhanced deoxidation is used to reduce the oxygen content in steel, the aluminum content in molten steel will reach 0.01-0.015%, thereby eliminating bubbles. In addition, the gas content (especially hydrogen) in molten steel is also an important reason for the formation of bubbles. Therefore, all materials added to molten steel (such as ferroalloy, slag powder, etc.) should be dried, the ladle and tundish should be baked, the amount of lubricating oil should be appropriate, and the pouring should be protected by pouring, which has an obvious effect on reducing air bubbles.
