MOLTEN STEEL SECONDARY REFINING METHOD AND SEEL PRODUCTION METHOD

To provide a molten steel secondary refining method enabling nitrogen concentration to stably reach low nitrogen concentration zone at high speed without causing an operation problem, and a steel production method.SOLUTION: A secondary refining method is provided in which an Al addition step of adding a metal Al-containing substance to molten steel to form Al-containing molten steel and a CaO addition step of adding a CaO-containing substance to molten steel are combined to form slag containing CaO and Al2O3, then an acid transfer treatment including a denitrification treatment in which oxygen-containing gas is blown to the Al-containing molten steel through the slag is performed, wherein based on the stirring power density ε(W/t) upon slag formation, an Al concentration [Al]i(mass%) in the molten steel immediately before the acid transfer treatment is set to be a value [Al]e or more calculated by the following formula (A), and the Al concentration [Al]f at the end of the acid transfer treatment is set to be 0.03 mass% or more. A steel production method is also provided in which the obtained molten steel is cast after composition adjustment. [Al]e=-0.072×ln(ε)+0.5822 (A).SELECTED DRAWING: Figure 1 【課題】操業上の問題を起こすことなく安定して高速で低窒素濃度域まで到達し得る溶鋼の二次精錬方法および鋼の製造方法を提案する。【解決手段】溶鋼に金属Ａｌ含有物質を添加しＡｌ含有溶鋼とするＡｌ添加ステップと、溶鋼にＣａＯ含有物質を添加するＣａＯ添加ステップと、を組み合わせてＣａＯおよびＡｌ２Ｏ３を含有するスラグを形成したのちに、スラグを貫通してＡｌ含有溶鋼に酸素含有ガスを吹き付けて脱窒処理を含む送酸処理を実施する溶鋼の二次精錬方法であって、スラグ形成時の攪拌動力密度ε（Ｗ／ｔ）に基づき、送酸処理直前の溶鋼中のＡｌ濃度［Ａｌ］ｉ（ｍａｓｓ％）を下記の式（Ａ）で計算される値［Ａｌ］ｅ以上とし、送酸処理終了時のＡｌ濃度［Ａｌ］ｆを０．０３ｍａｓｓ％以上とする方法である。得られた溶鋼に対し成分調整後鋳造する鋼の製造方法である。［Ａｌ］ｅ＝－０．０７２×ｌｎ（ε）＋０．５８２２ ・・・・（Ａ）【選択図】図１