JP2667007C

JP2667007C -

Info

Publication number: JP2667007C
Authority: JP
Original assignee: 川崎製鉄株式会社
Publication date: 1999-09-13

Description

【発明の詳細な説明】［産業上の利用分野］この発明は、溶鋼の真空脱ガス・脱炭処理方法に関し、特に真空脱ガス処理中
における溶鋼の温度低下を防止し併せて脱炭反応の効果的に促進するようにした
溶鋼の真空脱ガス・脱炭処理方法に関するものである。［従来の技術］溶鋼を真空下において脱炭処理する方法としてはRH脱ガス法を利用した方法（特開昭52−5614号公報参照）、とくに高Cr綱等の脱炭において鋼浴浴面下の比較
的浅い位置に容器側壁より酸素ガスを吹き込む方法（特開昭51−140815号公報参
照）、気体酸素に加え、固体酸素を脱炭促進剤として添加する方法（特開昭47-1
7619号公報参照）、あるいはラバールノズル付ランスにて鋼浴上から上吹きする
方法（特開昭55−125220号公報参照）などが知られている。ところで上記の技術
は何れも脱炭促進には有利であるが、脱炭処理において最も問題となる溶鋼の温
度降下については何ら考慮されていなかった。そのため脱炭処理に当っては、予め転炉等で溶鋼温度を上げておき、処理時に
おける温度降下を補償する必要があるが、転炉等一次精錬鍋炉で溶鋼の温度を上
げると、精錬炉や受鋼鍋の耐火物が著しく損耗するという問題があった。一方、真空脱ガス処理において溶鋼を昇熱する方法としては、RH−OB法（鉄と
鋼、No.11、VOL64(1978)S635参照）が、またRH槽内又は取鍋内にAl，Si等の発熱
剤を添加し溶鋼内に酸素ガスを導く方法（特開昭53-81416号公報、同59-89708号
公報参照）が知られている。ここに従来、真空脱ガス処理中に溶鋼の温度低下を招くことなく脱炭反応を進
行させる場合、上記の如き従来技術を単に組合せた次のような方法がとられてい
た。１）まず未脱酸溶鋼を脱炭処理しその後Al，Si等の発熱剤を添加し、酸素を供給
することにより昇熱させる。２）予めAl，Si等の発熱剤を添加し酸素を供給して昇熱させ、Al，Siを全て燃焼
しつくしてから脱炭処理する。３）溶鋼中に含有する成分、たとえば高Cr鋼などでは、酸素の供給により、Crの
酸化を生じさせ、その反応熱で脱炭に必要な温度を補償する（特開昭55−125220
号公報参照）。［発明が解決しようとする課題］しかしながらこのような方法では以下に示すような問題があった。すなわちａ）１），２）の方法では、脱炭期と昇熱期にわけられるため処理時間が延長し
生産性を著しく阻害する。特に、１）の方法では高炭素鋼を溶製する際は、Al，Siを燃焼しつくさねばならぬため著しく時間がかかる。又、２）の方法も同様にAl，Siを燃焼しつくさね
ばならないため時間がかかる。ｂ）Al，Siを燃焼させているので、その結果溶鋼中にAl₂O₃およびSiO₂等非金属
介在物が生成し、品質上好ましくない。ｃ）Al，Si等、特別な発熱剤を用いているのでコストが高い。ｄ）鋼中成分、例えばCr等の燃焼熱を利用する方法ではCr等のロスが大で歩留の
劣化は避けられない。この発明は上述したような従来問題を解消し、溶鋼の脱ガス処理中、溶鋼の温
度低下を伴うことなく、脱炭処理についても有利に促進させることができる、新
規な方法を与えることがこの発明の目的である。［課題を解決するための手段］上記及び上記以外の目的を達成するために、本発明の第一の構成によれば、製
鋼炉で溶製された普通鋼の未脱酸溶鋼もしくは弱脱酸溶鋼の脱ガス・脱炭処理を
RH法またはDH法等を用いて行う真空脱ガス方法において、真空脱ガス処理を開始
し、減圧脱炭によって溶鋼から真空脱ガス処理槽内にCOガスが発生した後に、真
空脱ガス処理槽内における溶鋼の浴面から所定距離離隔して上方位置から酸素ガ
ス又は酸素含有ガスを溶鋼表面に吹付け、溶鋼の脱炭反応を進行させるとともに
、溶鋼表面近傍で脱ガス処理中に発生するCOガスを排ガス中のCO₂／ (CO＋CO₂)
比が約30％以上となるように燃焼させる処理を排ガス中の (CO＋CO₂)の割合が５
％未満になるまで行い、溶鋼温度の降下量を低減させることを特徴とする溶鋼の
真空脱ガス・脱炭処理方法が提供される。また、本発明の第二の構成によれば、製鋼炉で溶製された普通鋼の未脱酸溶鋼
もしくは弱脱酸溶鋼の脱ガス・脱炭処理をRH法又はDH法等を用いて行う真空脱ガ
ス方法において、真空脱ガス処理槽内の真空度が１Torr以上となっている時期に
、溶鋼の浴面における到達圧力Ｐが15以上かつ 950以下となる圧力で真空脱ガス
処理槽内の溶鋼の浴面上部から酸素ガス又は酸素含有ガスを吹込み、溶鋼の脱炭
反応を進行させるとともに、脱ガス処理中に発生するCOガスを燃焼させることを
特徴とする溶鋼の真空脱ガス・脱炭処理方法が提供される。ここでＰは下に示す式で定義される。 log₁₀Ｐ＝−0.808(LH)^0.7＋0.00191(PV)＋0.00388(D₂/D₁)²Ｑ＋2.970 LH；脱ガス処理槽内における溶鋼の静止浴面からの距離［単位ｍ］ PV；送酸終了時の脱ガス処理槽内の到達真空度［単位：Torr］ D₁；吹込みラバルノズルにおけるスロート径［単位mm］ D₂；吹込みランスチップの出口径［単位mm］（ストレートノズルの場合はD₁＝D₂となる）Ｑ；酸素ガス流量［Nm³/分］（酸素含有ガスの場合は酸素含有量に換算した流量）なお、上記した本発明の第一及び第二の構成において、脱ガス処理開始時の溶
鋼温度、溶鋼中の炭素量と処理終了時の目標温度、目標とする溶鋼中の炭素量と
から脱炭すべき脱炭量、許容される温度降下量を算出し、それらに応じて酸素ガ
ス又は酸素含有ガス供給高さ、酸素ガス又は酸素含有ガス供給量及び酸素ガス又
は酸素含有ガス供給時間を決めることが好ましい。また、COガスを燃焼させるた
めに酸素ガス又は酸素含有ガスを吹付けるランスと脱炭を促進するために酸素ガ
ス又は酸素含有ガスを吹付けるランスを共通の１本のランスとすることも出来、
また要すればCOガスを燃焼させるために酸素ガス又は酸素含有ガスを吹付けるラ
ンスと、脱炭を促進するために酸素ガス又は酸素含有ガスを吹付けるランスを個
別に設けることも可能である。なお、前者の場合、好ましくは酸素ガス又は酸素
含有ガスの吹付け位置が脱ガス処理槽内における溶鋼の静止浴面から 1.6〜4.5m
上方に離隔して配設する。また、後者の場合にはCOガスを燃焼させるために酸素
ガス又は酸素含有ガスを吹付ける位置が脱ガス処理槽内における溶鋼の静止浴面
から 1.6〜4.5m上方に離隔し、脱炭を促進するために酸素ガス又は酸素含有ガス
を吹付ける位置が脱ガス処理槽内における溶鋼の静止浴面から1.6m以下の距離に
離隔して配設することが望ましい。なお、脱ガス処理槽内の真空度は１〜200 Torrの範囲にコントロールすること
が望ましい。［作用］転炉等、製鋼炉で溶製された未脱酸溶鋼もしくは弱脱酸溶鋼を真空脱ガス処理
すると、溶鋼中でＣ＋Ｏ→CO↑の如く反応をおこし、COガスが処理槽内に発生する。この発明はこの発生COガスを処理槽に設置した上吹きランス等により脱炭反
応を阻害しないよう適切な条件下で酸素ガスあるいは酸素含有ガスを供給する。
そして、 CO ＋1/2O₂ → CO₂ という反応を生じさせ、この際の発生熱を溶鋼に着熱させることにより該溶鋼の
温度降下を防止しようとするものである。従って、この発明では例えば従来のRH−OB法の如きとは異なり、溶鋼内に直接
酸素を供給するのではなくその浴面に酸素を供給する必要がある。この酸素は、
一部は脱炭反応を促進させるものであって全て脱炭反応に使用されると溶鋼への
着熱が困難となるので、真空脱ガスの操業条件例えば、ランス高さ、真空度、酸
素流量、ランス形状等をコントロールし、酸素ジェットの湯面到達圧力をある適
正値にする必要がある。これにより脱炭を促進しつつ、湯面近傍で溶鋼より発生
するCOガスを燃焼させ効率よく湯面に着熱させることができる。ここに酸素供給
高さとは、処理槽内に吸上げられた溶鋼の静止浴面からランス先端部までの高さ
を意味する。まず、この発明において脱ガス処理中に酸素を吹き込む場合酸素供給高さ、真
空度、使用するランスの形状及び酸素流量等複合的な条件があり、これらの１つ
が変化するとその作用は大きく変化する。そこで、これらの条件の変化による作
用を吹込まれた酸素ジェットの中心軸（ランスの中心軸）の湯面への到達圧力Ｐ
（Torr）で判定することとした。ここで、Ｐは log₁₀Ｐ＝−0.808(LH)^0.7＋0.00191(PV)＋0.00388(D₂/D₁)²Ｑ＋2.970 で定義される。この酸素ジェットの中心軸の圧力Ｐは、種々の出口径とスロート
径をもったラバルノズルとストレートノズル、及び酸素供給高さ酸素流量及び真
空度を変化させて実測した圧力を、最も相関係数の高い条件で回帰した式である
。これを実操業の結果を入れて求めたＰと［Ｃ］＝40 ppmまでの脱炭速度定数及
び処理開始15分までの溶鋼温度降下量との関係を図−１に示す。図から脱炭速度
定数はＰの増加とともに増加する。これは湯面への到達圧力が高い方が酸素が溶
鋼内部まで供給されるため脱炭に対して有利なためである。一方溶鋼温度降下は
、Ｐが大きいと、前述の理由から２次燃焼が小さくなり、またＰが小さいと２次
燃焼した熱が溶鋼に着熱せず高温ガス体として排気にひかれてしまう。その結果、
温度降下としては大きくなり適正なＰがあることがわかる。以上の結果、脱炭と
着熱を両方ともに効果的におこなうためには、脱炭速度の下限 0.145（比較例の
平均値）から、第１図よりＰを15と決定した。また、Ｐの上限については、溶鋼
への最適着熱を適用例10を限界としてＰを 950と決定した。次に、この発明において、処理槽内の真空度を１〜200 Torrとしたのは、１To
rr未満では発生するCOガスが減少し酸素を供給しても十分な燃焼熱が得られない
。一方 200Torrを超えると、脱炭反応が十分に進行せず、そのため発生するCOガ
スも少なく酸素を供給しても十分な燃焼熱が得られない。従って酸素吹錬時の処
理槽内の真空度は、１Torr〜200 Torrとする必要がある。なおこの発明では、具体的に真空脱ガス処理開始後200Torr以下になった時点
で酸素吹錬を開始し、その後脱炭の促進により真空度は徐々に高くなるが１Torr
以下となったところで酸素の供給を停止する。次に、酸素供給高さであるが、後に詳述するように、酸素供給高さが1.6m未満
では酸素が鋼の脱炭に使用される比率が高くなり、脱炭には有利であるが、COガ
スを燃焼させるための酸素が著しく低下し溶鋼の温度降下を防止できない。一方
酸素供給高さが4.5mを超えると、COガスの燃焼領域が処理槽の上部となるため溶
鋼への着熱が著しく低下し溶鋼の温度低下を防止できない。従ってCOガスを効率
よく燃焼させ溶鋼へ着熱できるよう酸素供給高さを 1.6〜4.5mとする必要がある
。第２図は、Ｃ：0.056 ％、Si：0.02％、Mn：0.28％、Ｏ：358 ppm、温度1588
℃になる溶鋼の脱ガス処理（RH法）中に処理槽内へ酸素を供給した実験における
排ガス中のガス濃度と真空度の変化状況を調べた結果を示すグラフであり、また
第３図はＣ：0.035 ％、Si：Tr％、Mn：0.27％、Ｏ：411 ppm、温度1592℃にな
る溶鋼を酸素を供給せずに脱ガス処理した場合の同様の調査結果を示すグラフで
ある。第２図により、処理槽内へ酸素を供給した場合には、第２次燃焼率るとCOガスの発生がないためその燃焼はゼロであり、さらに処理時間の経過とともにCO＋CO₂濃度が減少し真空度１Torrで５％となる。これは第３図において示
した CO₂濃度とほぼ同じであり、酸素供給による溶鋼への着熱はほとんどないこ
とが明らかである。従って、脱ガス処理中、処理槽内は真空度を１Torrから 200
Torrの間において酸素を供給するのが最も効率が良いことがわかる。次に第４図は酸素供給高さと２次燃焼率（処理開始２分〜８分の平均）および
処理開始から15分までの溶鋼温度の降下状況を示すグラフである。第４図において、２次燃焼率は酸素供給高さに伴なって増大することが明らか
に示されるが、一方、溶鋼の温度降下は２次燃焼率が30％未満では、酸素供給を
しない場合と比べて大差なく、２次燃焼率が約30％以上となる場合にかなり減少
していることがわかる。従って溶鋼の温度降下を減少させる効果を充分に与える
ためには、約30％以上の２次燃焼率が必要である。酸素供給高さについてみてみると、第４図より、酸素供給高さが1.6m未満では
、２次燃焼率が酸素を供給しない場合とほぼ等しい。即ち酸素供給高さが1.6m未
満では酸素が鋼の脱炭に使用される比率が高くなり、脱炭には有利であるが、CO
ガスを燃焼させるための酸素が著しく低下し溶鋼の温度降下を防止できない。一
方酸素供給高さが4.5mを超える場合、２次燃焼率は高いが、COガスの燃焼領域が
処理槽の上部となるため溶鋼への着熱が著しく低下し溶鋼の温度降下を防止でき
ない。従ってCOガスを効率よく燃焼させ溶鋼へ着熱できるように酸素供給高さを
1.6〜4.5mとする必要がある。さらにこの発明において、処理槽内の真空度を１〜200 Torrとしたのは、１To
rr未満では発生するCOガスが減少し酸素を供給しても十分な燃焼熱が得られない
。一方 200Torrを超えると、脱炭反応が十分に進行せず、そのため発生するCOガ
スも少なく酸素を供給しても十分な燃焼熱が得られない。従って酸素吹錬時の処
理槽内の真空度は、１Torr〜200 Torrとする必要がある。なおこの発明では、具体的に真空脱ガス処理開始後 200Torr以下になった時点
で酸素吹錬を開始し、その後脱炭の促進により真空度は徐々に高くなるが１Torr
以下となったところで酸素の供給を停止する。なおRH方式では設備によってもまた処理中の浴面の変動によっても多少異なる
が、その静止浴面は一般的には処理槽内底面から 250〜500 mmくらいであり、とくにRH方式を適用する場合酸素供給高さの設定に当たっては上記のことを考慮す
れはよい。また、脱ガス処理においては、処理終了時に目標とする温度、溶鋼中炭素量に
適確に到達することが肝要である。本発明においては、脱ガス処理開始時の溶鋼
温度、溶鋼中の炭素量と処理終了時の目標温度、目標とする溶鋼中の炭素量とか
ら脱炭すべき脱炭量、許容される温度降下量を算出し、それらに応じて酸素ガス
又は酸素含有ガス供給高さ、酸素ガス又は酸素含有ガス供給量及び酸素ガス又は
酸素含有ガス供給時間を決定して、目標温度、炭素量に適確に到達させることを
可能にしている。即ち、式によりΔＣだけ脱炭するのに必要な酸素量を計算し
、式により２次燃焼に必要な酸素を算出する。ここで式中の２次燃焼率って決定される。、式より必要な酸素量Ｑ₀₂は式で表わされる。一方、温度降下防止能は
、式で表現できる。ここで式中の送酸速度Ｆ₀₂は式で表現できる。許容さ
れる温度降下量をΔＴとした場合、必要送酸時間ｔ₀₂は式を表わされることが
わかった。〜式で満たすように標準酸素供給高さL.H.s、送酸速度Ｆ₀₂を選
択することで、必要送酸時間ｔ₀₂を決定することができ、目標とする温度、溶鋼
中炭素量に適確に到達することが可能となる。 ΔＯ＝Ｗ₁・ΔＣ＋Ｗ₂（ΔＣ＞Ｏ）Ｑ₀₂＝Ｑ_02-I＋Ｑ_02-＋Ｑ´ Ｑ＝θ₁（L.H.s−θ₂）^θ3 ｔ₀₂＝｛ΔＴ＋dt_R−ｅ［Ｏ〕ｉ｝／ζ 但し、 ΔＣ：目標とする脱炭量（kg） ΔＯ：ΔＣだけ脱炭される間の溶鋼中含有酸素の減少量（Nm³）Ｑ_02-I：ΔＣだけ脱炭するのに必要な上吹き酸素量（Nm³）Ｗ₁：上吹き送酸によりΔＣだけ脱炭される間のΔＣと鋼中酸素減少量の比例関係を表わす比例定数（0〜2000）Ｗ₂：処理中に脱炭反応および脱炭反応以外の要因で減少する溶鋼中酸素量のうち、ΔＣに比例しない量を表わす定数（0〜10 Nm³）Ｑ_02-II：ΔＣだけ脱炭する間に２次燃焼に使われる上吹き酸素量（Nm³）Ｑ'：排ガス中に排出される上吹き酸素量（Nm³） θ₁,θ₂：排ガス中に排出される上吹き酸素量に対するランス高さの影響を表わす比例定数 θ₃：排ガス中に排出される上吹き酸素量に対するランス高さの影響を表わすべき乗数 L.H.s：酸素供給高さａ，ｂ：酸素供給高さに伴って変化する２次燃焼率の比例定数（−10〜10）Ｃ：酸素供給高さに伴って変化する２次燃焼率の定数値（０〜１）Ｘ：酸素供給高さと２次燃焼率の関数関係を表わすべき乗数（０〜10）Ｑ₀₂：必要な酸素供給量（Nm³）ｐ：昇熱能に対する酸素供給高さの影響を表わす定数（0.1〜10.0）ｑ：昇熱能に対する酸素供給高さの影響を表わすべき乗数（0.05〜10.0） ζ：温度降下防止能（℃/min）Ｆ₀₂：送酸速度の平均値 ξ：送酸速度と酸素供給高さで決まる温度降下防止能の比例定数（0.1〜20）ｔ₀₂：必要な送酸時間（min）ｔ_R：標準リムド処理時間（min）〔Ｏ〕δ：処理直前の溶鋼中フリー酸素濃度(ppm) ｄ：リムド処理中の温度降下速度（℃/min）ｅ：リムド処理前の溶鋼中フリー酸素儂度が温度変化に与える効果の度合を示す定数（０〜２） ΔＴ：温度降下量（℃）第５図にRH式真空脱ガス処理装置の模式を示し、図中１は取鍋、２は転炉等の
製錬炉で溶製された未脱酸溶鋼もしくは弱脱酸溶鋼３は真空排気系にダクト４を
介して接続されるRH式脱ガス槽、５は脱ガス槽３内に酸素を吹き込むランスそし
て６は溶鋼２を脱ガス槽３内に吸上げる役目を果す不活性ガス等の供給羽口であ
り、この発明では脱ガス処理中に発生するCOガスはランス５より吹き込まれる酸
素にて燃焼され、溶鋼２の温度降下を伴うことなしに脱ガス・脱炭反応が進行す
ることとなる。なお、上掲第６図ではランス５を脱ガス槽３の上方より挿入する形式として示
したが、酸素供給高さが上述した如き条件を満足するものであれば、脱ガス槽３
の側面より挿入し、溶鋼浴面へ向けて酸素を吹き込めるような羽口あるいはラン
スを設けてもよい。またこの発明では第６図に示すようにCOガスを燃焼させる専用のランス5bと、
脱炭を促進させる専用のランス5aを個別に設けることもできる。この場合ランス
5aは溶鋼浴面に、ランス5bは溶鋼浴面より1.6〜4.5m上方位置に配設することが
肝要である。［実施例］実施例１ 230Ton底吹き転炉で溶製したＣ：0.02〜0.05％になる溶鋼を上掲第４図に示す
上吹きランスを有する230Ton用RH式還流脱ガス装置を用い表−１に示す条件下に
脱ガス・脱炭処理し、処理中の溶鋼温度降下状況等について調査した。その結果を表−１に併せて示す。この発明に従って処理したとくにヒートNo.1〜９では発生COガスの２次燃焼に
より、処理中における溶鋼の温度降下量（ΔＴ）は平均25.3℃と非常に小さいの
に対し従来法では平均40.8℃と、その差は15.5℃であり、この発明が有効である
ことが確かめられた。なお、ヒートNo.10，11，12 については酸素供給高さを、最も効率の良い着熱
が実現できる 1.6〜4.5mの位置に設定しない場合であるが、従来法によって処理
したヒートNo.13 と比べ溶鋼の温度降下量（ΔＴ）が小さいことが明らかである
。実施例２第６図に示すような２本のランスを設置した230Ton用RH還流脱ガス装置を用い
表−２に示す条件にて溶鋼の脱ガス・脱炭処理を行い処理中における溶鋼温度の
降下量、脱炭速度について調査した。なお脱炭専用ランスは酸素供給高さを0.8mに、２次燃焼用ランスは 2.0〜3.0m
の範囲に設置し、供給酸素量はそれぞれ20 Nm³／分（Total 40 Nm³／分）とした
。その結果を表−２に併せて示す。表−２より、この発明によれば脱炭速度も速く処理中における温度降下も十分
防止できることが確認できた。なおこの発明においては、RH式の真空処理を例として述べたがDH式真空処理を
適用することもできる。［発明の効果］この発明は以下の効果をもたらし、大幅なコストダウンを実現できる。１）温度降下防止のみならず、酸素供給高さの変更により、脱炭速度も制御可能
であり、状況に応じて処理時間の短縮、Ｃの低下が可能である。２）転炉等一次精錬炉での出鋼温度を必要以上に高める必要がなく、出鋼Ｃの増
大を可能とするためスラグの酸化度が低減し、精錬炉、受鋼鍋の耐火物消耗が減
少する。 Description: FIELD OF THE INVENTION The present invention relates to a method for vacuum degassing and decarburization of molten steel, and more particularly, to preventing the temperature of molten steel from being lowered during vacuum degassing and simultaneously performing a decarburization reaction. The present invention relates to a method for vacuum degassing and decarburization of molten steel which is effectively promoted. [Related Art] As a method for decarburizing molten steel under vacuum, a method utilizing RH degassing method (see Japanese Patent Application Laid-Open No. 52-5614), particularly in the case of decarburizing high Cr steel, etc. A method in which oxygen gas is blown from a side wall of a container into a relatively shallow position below (see JP-A-51-140815), a method in which solid oxygen is added as a decarburizing accelerator in addition to gaseous oxygen (Japanese Patent Laid-Open No. 47-1)
There is known a method in which a lance with a Laval nozzle blows upward from a steel bath (see JP-A-55-125220). By the way, any of the above techniques is advantageous for promoting decarburization, but no consideration has been given to the temperature drop of molten steel, which is the most problematic in the decarburization treatment. Therefore, in the decarburization process, it is necessary to raise the temperature of the molten steel in a converter or the like in advance to compensate for the temperature drop during the process, but if the temperature of the molten steel is increased in a primary smelting pot furnace such as a converter, the refining furnace There is a problem that the refractory of the steel pan and the receiving pan is significantly worn. On the other hand, as a method for raising the temperature of molten steel in vacuum degassing, the RH-OB method (iron and steel, No. 11, see VOL64 (1978) S635) is also used. A method of introducing an oxygen gas into molten steel by adding a heating agent such as the above (see JP-A-53-81416 and JP-A-59-89708) is known. Heretofore, conventionally, when the decarburization reaction proceeds without causing the temperature of the molten steel to decrease during the vacuum degassing treatment, the following method has been taken simply by combining the conventional techniques as described above. 1) First, deoxidized molten steel is decarburized, and then a heating agent such as Al or Si is added, and the temperature is increased by supplying oxygen. 2) A heating agent such as Al or Si is added in advance, oxygen is supplied to raise the temperature, and Al and Si are completely burned and then decarburized. 3) In a component contained in molten steel, for example, in a high Cr steel, the supply of oxygen causes oxidation of Cr, and the reaction heat compensates for the temperature required for decarburization (Japanese Patent Application Laid-Open No. 55-125220).
Reference). [Problems to be Solved by the Invention] However, such a method has the following problems. That is, in the methods a) 1) and 2), the treatment time is prolonged because the method is divided into the decarburization period and the heating period, and productivity is significantly impaired. In particular, in the method 1), when melting high carbon steel, it takes much time since Al and Si must be completely burned. Also, the method 2) takes a long time since the Al and Si must be completely burned. b) Since Al and Si are burned, non-metallic inclusions such as Al ₂ O ₃ and SiO ₂ are generated in the molten steel, which is not preferable in quality. c) The cost is high because a special exothermic agent such as Al or Si is used. d) In a method utilizing the heat of combustion of a component in steel, for example, Cr or the like, loss of Cr or the like is large and deterioration of the yield is inevitable. The present invention solves the conventional problems as described above, and provides a novel method capable of advantageously promoting the decarburization treatment without reducing the temperature of the molten steel during the degassing treatment of the molten steel. It is an object of the invention. [Means for Solving the Problems] To achieve the above and other objects, according to the first configuration of the present invention, undeoxidized molten steel or weakly deoxidized ordinary steel melted in a steelmaking furnace. Degassing and decarburization of molten steel
In the vacuum degassing method using the RH method or the DH method, etc., the vacuum degassing process is started, and after the CO gas is generated from the molten steel by the vacuum degassing in the vacuum degassing bath, the vacuum degassing process is started. Oxygen gas or oxygen-containing gas is sprayed from above on the molten steel surface at a predetermined distance from the molten steel bath surface to promote the decarburization reaction of the molten steel and CO gas generated during degassing near the molten steel surface CO ₂ in exhaust gas / (CO + CO ₂ )
Combustion treatment so that the ratio becomes about 30% or more is performed when the ratio of (CO + CO ₂ ) in exhaust gas is 5
%, And a method for vacuum degassing / decarburizing molten steel, characterized in that the temperature is reduced to less than 0.1% to reduce the amount of temperature drop of the molten steel. Further, according to the second configuration of the present invention, the degassing and decarburizing treatment of undeoxidized molten steel or weakly deoxidized molten steel of ordinary steel melted in a steelmaking furnace is performed using the RH method or the DH method. In the vacuum degassing method, when the degree of vacuum in the vacuum degassing tank is 1 Torr or more, the ultimate pressure P on the molten steel bath surface is 15 or more and 950 or less. Oxygen gas or oxygen-containing gas is blown from the upper part of the bath surface of molten steel to advance the decarburization reaction of the molten steel and to burn CO gas generated during the degassing process. A charcoal treatment method is provided. Here, P is defined by the following equation. _{log 10 P = -0.808 (LH)} 0.7 +0.00191 (PV) +0.00388 (D 2 / D 1) 2 Q + 2.970 LH; distance from the stationary bath surface of the molten steel in the degassing tank [unit m] PV; Ultimate vacuum in degassing tank at the end of acid feeding [unit: Torr] D ₁ : Throat diameter at blowing Laval nozzle [unit mm] D ₂ : Outlet diameter of blowing lance tip [unit mm] (straight the D ₁ = D ₂ in the case of the nozzle) Q; oxygen gas flow rate [Nm ³ / min] (oxygen flow rate in terms of oxygen content in the case of containing gas) Incidentally, the first and second invention described above In the configuration of degassing, the temperature of molten steel at the start of degassing, the amount of carbon in molten steel and the target temperature at the end of the processing, the amount of decarburization to be decarburized from the target amount of carbon in molten steel, and the allowable temperature drop Is calculated, and the oxygen gas or oxygen-containing gas supply height, the oxygen gas or oxygen-containing gas supply amount, and the oxygen gas or acid It is preferable to determine the content gas supply time. Also, a lance that blows oxygen gas or oxygen-containing gas to burn CO gas and a lance that blows oxygen gas or oxygen-containing gas to promote decarburization can be a common single lance,
If necessary, a lance for blowing oxygen gas or oxygen-containing gas to burn CO gas and a lance for blowing oxygen gas or oxygen-containing gas to promote decarburization can be separately provided. In the case of the former, preferably, the spray position of the oxygen gas or the oxygen-containing gas is 1.6 to 4.5 m from the stationary bath surface of the molten steel in the degassing tank.
It is arranged to be separated upward. In the latter case, the position where the oxygen gas or oxygen-containing gas is blown to burn the CO gas is separated from the stationary bath surface of the molten steel in the degassing tank by 1.6 to 4.5 m to promote decarburization. In order to achieve this, it is desirable that the position where the oxygen gas or oxygen-containing gas is blown be disposed at a distance of 1.6 m or less from the stationary bath surface of the molten steel in the degassing tank. The degree of vacuum in the degassing tank is desirably controlled within a range of 1 to 200 Torr. [Operation] When undeoxidized or weakly deoxidized molten steel melted in a steelmaking furnace, such as a converter, is subjected to vacuum degassing, a reaction such as C + O → CO ↑ occurs in the molten steel, and CO gas enters the treatment tank. Occur. According to the present invention, the generated CO gas is supplied with an oxygen gas or an oxygen-containing gas under appropriate conditions so as not to hinder the decarburization reaction by an upper blowing lance or the like provided in the treatment tank.
Then, a reaction of CO + 1 / 2O ₂ → CO ₂ is caused, and the generated heat is heated to the molten steel to prevent a temperature drop of the molten steel. Therefore, in the present invention, for example, unlike the conventional RH-OB method, it is necessary to supply oxygen to the bath surface instead of directly supplying oxygen into the molten steel. This oxygen is
Some of them promote the decarburization reaction, and if all are used for the decarburization reaction, it becomes difficult to heat the molten steel, so the operating conditions of vacuum degassing, such as lance height, vacuum degree, and oxygen flow rate It is necessary to control the lance shape, etc., and make the pressure at which the oxygen jet reaches the molten metal surface a certain appropriate value. Thereby, while promoting decarburization, the CO gas generated from the molten steel in the vicinity of the molten metal surface can be burned to efficiently heat the molten metal surface. Here, the oxygen supply height means the height from the stationary bath surface of the molten steel sucked into the treatment tank to the tip of the lance. First, in the present invention, when oxygen is blown during the degassing process, there are complex conditions such as an oxygen supply height, a degree of vacuum, a shape of a lance to be used, and an oxygen flow rate, and when one of these changes, the action greatly changes. . Therefore, the ultimate pressure P of the central axis of the oxygen jet (the central axis of the lance) to the surface of the molten metal injected with the action due to the change of these conditions.
(Torr). Here, P is defined as log ₁₀ P = −0.808 (LH) ^0.7 + 0.00191 (PV) +0.00388 (D ₂ / D ₁ ) ² Q + 2.970. The pressure P of the central axis of the oxygen jet is determined by changing the Laval nozzle and the straight nozzle having various outlet diameters and the throat diameter, the oxygen supply height, the oxygen flow rate, and the pressure actually measured by changing the degree of vacuum. This is an equation regressed under high conditions. Fig. 1 shows the relationship between P obtained by taking the results of the actual operation into consideration, the decarburization rate constant up to [C] = 40 ppm, and the molten steel temperature drop amount up to 15 minutes after the start of the treatment. From the figure, the decarburization rate constant increases with increasing P. This is because the higher the pressure reaching the molten metal surface, the more oxygen is supplied to the inside of the molten steel, which is advantageous for decarburization. On the other hand, when P is large, the secondary combustion becomes small for the above-mentioned reason, and when P is small, the heat of the secondary combustion does not heat the molten steel but is drawn to the exhaust as a high-temperature gas body. as a result,
It can be seen that the temperature drop increases and there is an appropriate P. As a result, in order to effectively perform both decarburization and heat arrival, P was determined to be 15 from FIG. 1 from the lower limit of the decarburization rate of 0.145 (average value of the comparative example). Further, as for the upper limit of P, P was determined to be 950, with the optimum heat transfer to molten steel being the limit of Application Example 10. Next, in the present invention, the reason why the degree of vacuum in the processing tank is 1 to 200 Torr is that 1 To
If it is less than rr, the amount of generated CO gas decreases and sufficient heat of combustion cannot be obtained even if oxygen is supplied. On the other hand, when the pressure exceeds 200 Torr, the decarburization reaction does not proceed sufficiently, so that a small amount of CO gas is generated and sufficient combustion heat cannot be obtained even if oxygen is supplied. Therefore, the degree of vacuum in the processing tank during oxygen blowing needs to be 1 Torr to 200 Torr. In the present invention, oxygen blowing is started when the pressure becomes 200 Torr or less after the start of vacuum degassing, and the degree of vacuum is gradually increased by promoting decarburization.
When the following conditions are satisfied, the supply of oxygen is stopped. Next, regarding the oxygen supply height, as will be described in detail later, when the oxygen supply height is less than 1.6 m, the ratio of oxygen used for decarburizing steel increases, which is advantageous for decarburization. In addition, oxygen for burning CO gas is remarkably reduced, and the temperature drop of molten steel cannot be prevented. On the other hand, if the oxygen supply height exceeds 4.5 m, the CO gas combustion region is located at the upper part of the treatment tank, so that the heat applied to the molten steel is significantly reduced and the temperature of the molten steel cannot be prevented from lowering. Therefore, it is necessary to set the oxygen supply height to 1.6 to 4.5 m so that the CO gas can be efficiently burned and the molten steel can be heated. FIG. 2 shows C: 0.056%, Si: 0.02%, Mn: 0.28%, O: 358 ppm, temperature 1588.
FIG. 3 is a graph showing a result of examining a change state of a gas concentration in an exhaust gas and a degree of vacuum in an experiment in which oxygen was supplied into a treatment tank during a degassing treatment (RH method) of molten steel having a temperature of ℃. It is a graph which shows the same investigation result at the time of degassing the molten steel which becomes C: 0.035%, Si: Tr%, Mn: 0.27%, O: 411ppm, temperature 1592 degreeC, without supplying oxygen. According to FIG. 2, when oxygen is supplied into the processing tank, the secondary combustion rate Then, since there is no generation of CO gas, the combustion is zero, and the CO + CO ₂ concentration decreases as the processing time elapses, and reaches 5% at a degree of vacuum of 1 Torr. This is almost the same as the CO ₂ concentration shown in FIG. 3, and it is clear that the molten steel is hardly heated by oxygen supply. Therefore, during the degassing process, the degree of vacuum in the processing tank was increased from 1 Torr to 200
It can be seen that supplying oxygen during Torr is the most efficient. Next, FIG. 4 is a graph showing the oxygen supply height and the secondary combustion rate (average of 2 minutes to 8 minutes from the start of the treatment) and the temperature drop of the molten steel from the start of the treatment to 15 minutes. In FIG. 4, it is clearly shown that the secondary combustion rate increases with the oxygen supply height, while the temperature drop of the molten steel decreases when the secondary combustion rate is less than 30% without oxygen supply. It can be seen that there is not much difference as compared with the case where the secondary combustion rate is about 30% or more, and that it is considerably reduced. Therefore, in order to sufficiently provide the effect of reducing the temperature drop of molten steel, a secondary combustion rate of about 30% or more is required. Looking at the oxygen supply height, it can be seen from FIG. 4 that when the oxygen supply height is less than 1.6 m, the secondary combustion rate is almost the same as when no oxygen is supplied. That is, when the oxygen supply height is less than 1.6 m, the ratio of oxygen used for decarburizing steel increases, which is advantageous for decarburization.
Oxygen for burning the gas is remarkably reduced, and the temperature drop of the molten steel cannot be prevented. On the other hand, when the oxygen supply height exceeds 4.5 m, the secondary combustion rate is high, but since the CO gas combustion region is located in the upper part of the treatment tank, the heat applied to the molten steel is significantly reduced, and the temperature drop of the molten steel cannot be prevented. Therefore, increase the oxygen supply height so that CO gas can be burned efficiently and heat can be applied to molten steel.
It is necessary to be 1.6-4.5m. Further, in the present invention, the degree of vacuum in the processing tank is set to 1 to 200 Torr by 1 To
If it is less than rr, the amount of generated CO gas decreases and sufficient heat of combustion cannot be obtained even if oxygen is supplied. On the other hand, when the pressure exceeds 200 Torr, the decarburization reaction does not proceed sufficiently, so that a small amount of CO gas is generated and sufficient combustion heat cannot be obtained even if oxygen is supplied. Therefore, the degree of vacuum in the processing tank during oxygen blowing needs to be 1 Torr to 200 Torr. According to the present invention, oxygen blowing is started when the pressure becomes 200 Torr or less after the start of vacuum degassing, and the degree of vacuum is gradually increased by promoting decarburization.
When the following conditions are satisfied, the supply of oxygen is stopped. Although the RH method differs slightly depending on the equipment and the fluctuation of the bath surface during processing, the stationary bath surface is generally about 250 to 500 mm from the bottom of the processing tank, especially when the RH method is applied. In setting the oxygen supply height, it is good to consider the above. In the degassing process, it is important to properly reach the target temperature and the amount of carbon in the molten steel at the end of the process. In the present invention, the temperature of molten steel at the start of degassing treatment, the amount of carbon in molten steel and the target temperature at the end of treatment, the amount of decarburization to be decarbonized from the target amount of carbon in molten steel, the allowable temperature drop Calculate the oxygen gas or oxygen-containing gas supply height, the oxygen gas or oxygen-containing gas supply amount and the oxygen gas or oxygen-containing gas supply time accordingly, and calculate the target temperature and carbon amount accurately. To reach them. That is, the amount of oxygen required for decarburization by ΔC is calculated by the equation, and the oxygen required for the secondary combustion is calculated by the equation. Where the secondary combustion rate in the equation Is determined. From the formula, the necessary oxygen amount _Q02 is represented by the formula. On the other hand, the temperature drop prevention ability can be expressed by an equation. Here, the acid feed rate F ₀₂ in the equation can be expressed by the equation. Assuming that the allowable amount of temperature drop is ΔT, it was found that the required acid supply time t ₀₂ is represented by the following equation. By selecting the standard oxygen supply height LHs and the acid supply rate F ₀₂ so as to satisfy the formula, the required acid supply time t ₀₂ can be determined, and the target temperature and the carbon amount in the molten steel can be accurately determined. It is possible to reach. ΔO = W ₁ · ΔC + W ₂ (ΔC> O) Q ₀₂ = Q _02-I + Q _02- + Q ′ Q = θ ₁ (LHs−θ ₂ ) ^θ3 t ₀₂ = {ΔT + dt _R -e [O] i} / ζ where ΔC: target decarburization amount (kg) ΔO: reduction amount of oxygen contained in molten steel during decarburization by ΔC (Nm ³ ) Q _02-I : Amount of top-blown oxygen required for decarburization by ΔC (Nm ³ ) W ₁ : Proportion showing the proportional relationship between ΔC and the amount of oxygen reduction in steel during decarburization by ΔC by top-blown acid Constant (0 to 2000) W ₂ : A constant (0 to 10 Nm ³ ) Q _02- representing the amount of oxygen in molten steel that decreases during treatment due to decarburization and other factors other than decarburization, and is not proportional to ΔC. _II : Upper blowing oxygen amount (Nm ³ ) used for secondary combustion while decarburizing by ΔC Q ': Upper blowing oxygen amount (Nm ³ ) discharged in exhaust gas θ ₁ , θ ₂ : Emitted in exhaust gas Proportional to the effect of the lance height on the amount of top blown oxygen that is blown θ ₃ : A power representing the effect of the lance height on the amount of top blown oxygen discharged into the exhaust gas number LHs: Oxygen supply height a, b: Proportional constant of secondary combustion rate that varies with oxygen supply height (-10 to 10) C: Constant value of secondary combustion rate that varies with oxygen supply height (0-1) X: Exponent (0-10) representing the functional relationship between the oxygen supply height and the secondary combustion rate Q ₀₂ : Required oxygen supply amount (Nm ³ ) p: Effect of oxygen supply height on heat-up ability Q: Multiplier (0.05 to 10.0) representing the effect of the oxygen supply height on the heat-up ability 能: Temperature drop prevention ability (° C./min) F ₀₂ : Average value of acid feed rate ξ: Proportional constant of temperature drop prevention ability determined by acid supply rate and oxygen supply height (0.1 to 20) t ₀₂ : Required acid supply time (min) t _R : Standard rimmed treatment time (min) [O] δ: Immediately before treatment Free oxygen concentration in molten steel (ppm) d: Temperature drop rate during limed treatment (° C / min) e: Free oxygen concentration in molten steel before limed treatment varies with temperature Constant (0 to 2) indicating the degree of effect on gasification ΔT: temperature drop (° C) Fig. 5 shows a schematic diagram of an RH type vacuum degassing apparatus, where 1 is a ladle and 2 is a converter. The non-deoxidized molten steel or the weakly deoxidized molten steel 3 melted in the smelting furnace such as RH type degassing tank connected to the evacuation system via the duct 4, and 5 blows oxygen into the degassing tank 3. Lance 6 is a supply tuyere of an inert gas or the like serving to suck molten steel 2 into degassing tank 3. In the present invention, CO gas generated during degassing is supplied by oxygen blown from lance 5. The gas is burned, and the degassing / decarburizing reaction proceeds without the temperature drop of the molten steel 2. In FIG. 6, the lance 5 is shown as being inserted from above the degassing tank 3, but if the oxygen supply height satisfies the above conditions, the degassing tank 3 is inserted.
And a tuyere or lance through which oxygen can be blown into the molten steel bath surface. In the present invention, as shown in FIG. 6, a dedicated lance 5b for burning CO gas,
A dedicated lance 5a for promoting decarburization can be provided separately. In this case lance
It is important that 5a be placed on the molten steel bath surface and lance 5b be placed 1.6 to 4.5m above the molten steel bath surface. [Example] Example 1 C: 0.02 to 0.05% of molten steel melted in a 230-ton bottom-blowing converter was prepared using a RH-type reflux degassing apparatus for 230-ton with a top-blowing lance shown in FIG. Degassing and decarburizing treatments were performed under the conditions shown in Fig. 1, and the temperature drop of molten steel during the treatment was investigated. The results are shown in Table 1. In particular, in heat Nos. 1 to 9, due to the secondary combustion of the generated CO gas, the temperature drop (.DELTA.T) of the molten steel during the treatment is very small at an average of 25.3.degree. The difference was 15.5 ° C., which confirmed that the present invention was effective. For Heat Nos. 10, 11, and 12, the oxygen supply height is not set to a position of 1.6 to 4.5 m where the most efficient heat generation can be achieved. It is apparent that the temperature drop (ΔT) of the molten steel is smaller than that of the molten steel. Example 2 Degassing and decarburizing treatment of molten steel was performed under the conditions shown in Table 2 using a RH reflux degassing device for 230 Ton provided with two lances as shown in FIG. The amount of descent and the decarburization rate were investigated. The decarburization lance has an oxygen supply height of 0.8m and the secondary combustion lance has a 2.0-3.0m lance.
And the amount of supplied oxygen was 20 Nm ³ / min (Total 40 Nm ³ / min). The results are shown in Table-2. From Table 2, it was confirmed that according to the present invention, the decarburization rate was high and the temperature drop during the treatment was sufficiently prevented. In the present invention, RH type vacuum processing has been described as an example, but DH type vacuum processing can also be applied. [Effects of the Invention] The present invention has the following effects, and can realize significant cost reduction. 1) In addition to preventing the temperature drop, the decarburization rate can be controlled by changing the oxygen supply height, and the processing time and C can be reduced according to the situation. 2) There is no need to raise the tapping temperature in the primary refining furnace such as a converter more than necessary, and the degree of oxidation of slag is reduced to enable an increase in tapping C. Decrease.

【図面の簡単な説明】第１図は到達圧力Ｐと、［Ｃ］−４０ｐｐｍまでの脱炭速度定数及び溶鋼の温
度降下の関係を示すグラフ、第２図は酸素供給時における排ガス中のガス濃度と真空度の関係グラフ、第３図は脱ガス処理中のガス濃度の変化を示すグラフ、第４図は酸素供給高さと、溶鋼の温度降下および２次燃焼率の影響を示すグラ
フ、第５図、第６図は、RH還流脱ガス装置の模式図である。１…取鍋２…溶鋼３…脱ガス槽４…ダクト５…ランス６…羽口BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph showing the relationship between ultimate pressure P, decarburization rate constant up to [C] -40 ppm, and temperature drop of molten steel. FIG. 2 shows gas in exhaust gas during oxygen supply. 3 is a graph showing the change in gas concentration during the degassing process, FIG. 4 is a graph showing the effect of the oxygen supply height, the temperature drop of the molten steel and the secondary combustion rate, FIG. 5 and FIG. 6 are schematic diagrams of the RH reflux degassing device. 1 ladle 2 molten steel 3 degassing tank 4 duct 5 lance 6 tuyere

Claims

Claims (1) Degassing of undeoxidized molten steel or weakly deoxidized molten steel of ordinary steel melted in a steelmaking furnace
In the vacuum degassing method in which the decarburization treatment is performed using the RH method or the DH method, etc., the vacuum degassing process is started, and after the CO gas is generated from the molten steel in the vacuum degassing tank by decompression under reduced pressure, the vacuum Oxygen gas or oxygen-containing gas is sprayed onto the molten steel surface from above at a predetermined distance from the bath surface of the molten steel in the gas treatment tank to promote the decarburization reaction of the molten steel, and during degassing near the molten steel surface. The generated CO gas is converted into CO ₂ / (CO + CO
₂ ) Combustion is performed so that the ratio becomes 30% or more. (CO + CO ₂ ) ratio in exhaust gas is 5%.
A degassing / decarburizing method for molten steel, wherein the method is performed until the temperature of the molten steel becomes lower than the temperature of the molten steel. (2) Degassing of undeoxidized or weakly deoxidized molten steel of ordinary steel melted in a steelmaking furnace
In the vacuum degassing method in which the decarburization treatment is performed by using the RH method or the DH method, the ultimate pressure P of the molten steel on the bath surface is 15 or more when the degree of vacuum in the vacuum degassing tank is 1 Torr or more. In addition, oxygen gas or oxygen-containing gas is blown from the upper part of the molten steel bath surface in the vacuum degassing tank at a pressure of 950 or less to promote the decarburization reaction of the molten steel and to reduce the CO gas generated during the degassing process. Vacuum degassing of molten steel characterized by burning
Decarburization processing method. Here, P is defined by the following equation. _{log 10 P = -0.808 (LH)} 0.7 +0.00191 (PV) +0.00388 (D 2 / D 1) 2 Q + 2.970 LH; distance from the stationary bath surface of the molten steel in the degassing tank [unit m] PV; Ultimate vacuum in degassing tank at the end of acid feeding [unit: Torr] D ₁ : Throat diameter at blowing Laval nozzle [unit mm] D ₂ : Outlet diameter of blowing lance tip [unit mm] (straight If the nozzle becomes D ₁ = D ₂₎ Q; oxygen gas flow rate [Nm ³ / min] (flow rate in the case of the oxygen-containing gas in terms of oxygen content) (3) degassing at the start of the molten steel temperature, Calculate the amount of decarburization to be decarburized and the allowable temperature drop from the amount of carbon in the molten steel, the target temperature at the end of the treatment, and the target amount of carbon in the molten steel. The gas supply height, the supply amount of oxygen gas or oxygen-containing gas, and the supply time of oxygen gas or oxygen-containing gas are determined. 3. The method of claim 1 or claim 2. (4) A lance that blows oxygen gas or oxygen-containing gas to burn CO gas and a lance that blows oxygen gas or oxygen-containing gas to promote decarburization are common.
The method according to any one of claims 1 to 3, wherein the method is a book lance. (5) A lance for blowing oxygen gas or oxygen-containing gas for burning CO gas and a lance for blowing oxygen gas or oxygen-containing gas for promoting decarburization are provided separately. Item 4. The method according to any one of Items 1 to 3. (6) The method according to any one of claims 1 to 5, wherein the degree of vacuum in the degassing tank is controlled within a range of 1 to 200 Torr. (7) The spraying position of the oxygen gas or the oxygen-containing gas is separated from the stationary bath surface of the molten steel in the degassing tank by 1.6 to 4.5 m above, in any one of claims 1 to 4 and 6. The method described in. (8) The position where the oxygen gas or oxygen-containing gas is blown to burn the CO gas is separated from the stationary bath surface of the molten steel in the degassing tank by 1.6 to 4.5 m, and the oxygen is blown to promote decarburization. The position where the gas or oxygen-containing gas is blown is separated from the stationary bath surface of the molten steel in the degassing treatment tank by a distance of 1.6 m or less.
7. The method according to any of paragraphs 6 and 6.