| 摘要 |
PROBLEM TO BE SOLVED: To stably obtain a boron-added steel in high yield by beforehand charging a flux for reforming into molten steel for executing a vacuum degassing treatment, successively, controlling the ratio of molten steel weight to circulating flowing velocity so as to become a specified range after adding the boron as FeB and circulating the molten steel. SOLUTION: The molten steel obtd. by melting in a converter and reforming slag, is charged into the vacuum degassing apparatus, and the flux and FeB are charged and circulated under reduced pressure to execute the vacuum treatment, impurities are removed, and also, the hardness, toughness, hardability, or the like, are improved. At this time, the flux for refining of CaO-CaF2, or the like, is beforehand charged in this molten steel, and the molten steel is circulated and the boron-addition is executed by charging FeB into this molten steel. Further, after adding the boron, the circulation of molten steel is executed in the condition satisfying the molten steel wt. (ton)/the molten steel circulating velocity (ton/min) = 1-3. In this way, the boron in the molten steel is uniformized and stabilized to prevent the lowering in yield of the boron.
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