| 摘要 |
PURPOSE:To eliminate occurrence of defects due to impurities in the boundary and defective adhesion in inserting core in a mold with charged molten metal by charging the molten metal at a specified surface rising speed after forming a borofluoride layer on the surface of the core. CONSTITUTION:After removing the oxide film from the surface of a core 1, a coating layer containing borofluorides such as NH4BF4, KBF4 is formed to a thickness of 50-1,500mum. The core 1 is placed in a mold 4, and molten steel is charged into the mold from a sprue at a surface rising speed of 200- 1,000mm./min. Consequently, the coating layer comes into contact with the molten steel 10, and the borofluoride is thermally decomposed to produce BF3 gas. This gas excludes scum on the surface of the molten metal from near the surface of the core 1, and prevents the scum dragged in the interface between the core 1 and molten steel 10. Inside of the mold 4 is filled with nonoxidizing BF3 gas, and occurrence of defects due to impurities at the boundary caused by presence of oxide is restrained. When the surface rising speed is not within aforementioned range, the object cannot be attained due to previous decomposition of the borofluoride and insufficient vaporization of a part of the coating layer. |
