Under the skin of the billet there are large bubbles growing in the direction of columnar crystals with diameter and length of more than 1 mm and 10 mm respectively. These bubbles, such as those exposed to the outside, are called surface bubbles, those that are not exposed are called subcutaneous bubbles, and the dense holes smaller than the bubbles are called subcutaneous pinholes.
In the reheating furnace, the inner surface of the bubble or the bubble under the skin of the cast blank is oxidized to form a decarburized layer, which can not be welded after rolling and form surface defects. Shallow buried bubbles can be removed by grinding wheel, wind shovel and flame cleaning. Deep buried bubbles are difficult to detect and can cause cracks in the product.
Insufficient deoxidation of molten steel is the main reason for the formation of bubbles, such as the use of enhanced deoxidation to reduce the oxygen content in steel, will make the aluminum content in molten steel reach 0.01 ~ 0.015%, so as to eliminate bubbles. In addition, the gas content in molten steel (especially hydrogen) is also an important reason for the formation of bubbles. Therefore, all materials added to the molten steel (such as ferroalloy, slag powder, etc.) should be dry; the ladle and tundish should be baked, the amount of lubricating oil should be appropriate; the injection flow is protected by pouring, and this measures can reduce the bubbles.
