Bubbles were found in the middle of H 13 die steel forging plate. What caused them?

Bubbles are caused by poor degassing and deoxidation of wet raw materials in molten steel or steel spraying system, which are generally divided into subcutaneous bubbles and internal bubbles. (Preventive measures: the oxidation boiling reduction and deoxidation of molten steel should be good, the furnace and all refractory materials in contact with molten steel should be dry, the tapping trough, steamed bread, hand board and mold should be dry, the rust spots of the mold should be cleaned, and the oil should be coated thinly and evenly. )