JP3832222B2 - Method for refining molten steel - Google Patents
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【０００１】
【発明の属する技術分野】
本発明は、真空脱ガス装置を用いて溶鋼の脱水素精錬を行う溶鋼の精錬方法に関するものである。
【０００２】
【従来の技術】
ラインパイプや造船向に代表されるような、過酷な使用目的の鋼板では、水素割れや水素脆化等の水素系欠陥を防止するために、鋼の溶製時の水素含有量 (以下、単に水素値という) を3.0ppm、好ましくは2.0ppm 以下を目標値とし、極力、水素値を下げることが望まれている。
【０００３】
この要望に対応するため、鋼の溶製時には、例えばRH、DHなどの真空脱ガス装置による二次精錬工程において脱水素処理を行っている。
さらに連続鋳造後のスラブや圧延後の鋼板では、鋳造後または圧延後に、スラブや鋼板を一定時間放置することによって脱水素を実施している。
【０００４】
しかし、特に大気中の湿度が高い時期には、二次精錬工程における合金等の添加物に付着する水分量が多くなり、溶鋼中の水素値が上昇したり、真空脱ガス装置の故障により真空度が上がらない、というような不具合が発生する場合がある。このような場合、要求される水素値を達成できなかったり、脱水素のための真空脱ガス処理時間を長くとる必要性が生じ、二次精錬の効率を阻害していた。
【０００５】
この脱水素処理の効率向上を目的として、従来から脱水素反応を促進させる方法がいくつか提案されている。
特許第2783029 号公報では、真空流滴脱ガス法を採用し、減圧下で流滴にArガスプラズマを照射することにより溶融金属の表面温度を上昇させ、脱水素および脱窒素を促進する方法が提案されている。
【０００６】
特公平７−65120 号公報においては、溶鋼中にガスを吹き込み、生成する気泡を利用して精錬を行う際に、羽口に超音波を印加することにより気泡の微細化を図り、脱酸速度を向上させる方法が提案されている。
【０００７】
特公平７−100813号公報には、脱炭時に減圧下の溶鋼表面に粉体を吹き込むことにより脱炭反応を促進させ、溶鋼内部よりCOガスを発生させ、ガス−メタル界面の領域を増大させて脱水素を促進する方法が提案されている。
【０００８】
しかしながら、特許第2783029 号公報、特公平７−65120 号公報、特公平７−100813号公報の提案する精錬技術を実際の製造装置に適用するためには、高価な設備投資およびランニングコストの上昇を伴うことから、大量生産を前提とした実操業においては、コストおよび設備保守の観点から必ずしも有効ではない。
【０００９】
ところで、特開平８−311529号公報では、脱炭、脱水素を同時に行う真空脱ガス精錬において、鋼中の酸素活量を酸素濃度で200ppm以上にする技術が開示されている。しかし、この方法は [Ｃ] が例えば20〜50ppm という極低炭素の領域における脱炭を主目的として脱ガスを促進するための手段であり、また還流ガスにＮ2 を使用したときの溶鋼へのＮ2 ピックアップを抑制するための手段であり、脱水素反応時間の短縮に関しては言及していない。
【００１０】
さらに、特開平６−33133 号公報では、鋼中酸素が界面活性元素であることに着目し、物理化学的なアプローチによって脱ガス反応を促進させる方法が提案されている。しかし、この方法は本質的には極低炭素鋼の脱炭に関する発明であり、脱水素反応には触れていない。しかも、鋼中酸素の補充のために酸素含有物質を脱ガス真空槽内に供給する方法を採用しており、コストがかかる。
【００１１】
【発明が解決しようとする課題】
前述の通り、従来にあっても、脱水素の効率向上を目的として、様々な方法が提案されているが、いずれの方法においても、大幅なコストアップは避けられない。
【００１２】
本発明の課題は、真空脱ガス装置に新たな設備投資をする必要がなく、かつ操業上のコストアップを伴わず、脱水素処理の効率が向上する溶鋼の精錬方法を提供することである。
【００１３】
【課題を解決するための手段】
本発明者らは溶鋼の脱水素処理の効率を向上させる方法に関し、溶鋼中の酸素活量に着目して熱力学的に検討した。
【００１４】
鋼の吸水素反応は、(1) 式に示す化学反応式により進行する。
【００１５】
【化１】
【００１６】
この化学反応式において、実際に鋼の吸水素反応を抑制する方法としては、水蒸気分圧を低下させること、および溶鋼中酸素活量を増加させることが考えられる。
【００１７】
しかし、前者の方法を実操業に適用することは、実際上不可能と考えられることから、本発明者らは、後者の溶鋼中の酸素活量を増加させることに着目し、更なる検討を行った。
【００１８】
一般的に、真空脱ガス装置における脱水素速度は、(2) 式で記述される。
【００１９】
【化２】
【００２０】
(2) 式より明らかなように、脱水素速度を向上させる手段として、脱水素限界濃度[H]e の値を小さくすることが考えられる。
[H]e が、(1) 式で示した化学反応式により決定されると仮定した場合には、(3) 式が得られる。
【００２１】
【化３】
【００２２】
【化４】
【００２３】
以上の熱力学的検討をもとに、溶鋼中の酸素活量が異なる条件下において水素濃度の連続測定を行った結果を図１に示す。
ここで [H]o は、脱水素処理前の水素濃度 (ppm)である。
【００２４】
この測定結果より、溶鋼中の酸素活量を高位にするほど、脱水素速度が向上することが確認された。換言すれば、酸素吹込みによる脱炭処理が先行する場合には、脱炭のための溶鋼への酸素吹き込みが終了した直後から直ちに真空脱ガス処理による脱水素を開始すれば、酸素活量の高い状態で脱水素を進行させることができる。
【００２５】
本発明においては溶鋼[C] 、成品[C] のいずれについても制限されないが、極低炭鋼([C]≦30ppm)を製造するには真空脱ガス装置において真空脱ガスに先立って酸素吹込みによる脱炭処理を行えばよい。しかし、一般には真空脱ガス装置での脱炭を行ってもそのような極低炭レベルまで脱炭することなく、成品[C] ≧100ppm程度の脱炭精錬が好ましく、その場合には転炉出鋼時に溶鋼[C] ≧300ppmとすればよい。
【００２６】
以上の検討より、本発明は上述の課題を解決するために次のような構成をとる。
（１）転炉における精錬と真空脱ガス装置による二次精錬とを行う溶鋼の精錬方法において、転炉出鋼後の溶鋼に真空脱ガス装置を使用して真空脱ガス処理を行うことで脱水素する二次精錬を行い、その際に酸素活量が０．０００３％以上かつ０．０１００％以下の溶鋼に真空脱ガス処理を行うことを特徴とする、溶鋼の精錬方法。
【００２７】
（２）転炉における精錬と真空脱ガス装置による二次精錬とを行う溶鋼の精錬方法において、転炉精錬により［Ｃ］≧１００ｐｐｍに脱炭処理を行い、転炉出鋼後の溶鋼に真空脱ガス装置を使用して真空脱ガス処理を行うことで脱水素する二次精錬を行い、その際に酸素活量が０．０００３％以上かつ０．０１００％以下の溶鋼に真空脱ガス処理を行うことを特徴とする、［Ｃ］≧１００ｐｐｍの溶鋼の精錬方法。
【００２８】
（３）前記二次精錬において、前記真空脱ガス処理の開始時点の溶鋼水素含有量と目標水素含有量の差を少なくとも２０％とすることを特徴とする、上記（１）または（２）に記載の溶鋼の精錬方法。
【００２９】
（４）前記二次精錬において、溶鋼水素含有量３ｐｐｍにまで脱水素する間、溶鋼の酸素活量を０．０００３％以上かつ０．０１００％以下とすることを特徴とする、上記（１）ないし（３）のいずれかに記載の溶鋼の精錬方法。
【００３０】
（５）脱水素処理終了後、溶鋼に脱酸剤を添加して脱酸を行うことを特徴とする、上記（１）ないし（４） のいずれかに記載の溶鋼の精錬方法。
【００３１】
（６）成品［Ｈ］≦３．０ｐｐｍを満足する上記（１）ないし（５）のいずれかに記載の精錬方法。
【００３２】
【発明の実施の形態】
ここで、本発明にかかる精錬方法を実施する場合について具体的に説明すると次の通りである。
【００３３】
本発明において転炉における精錬は従来法に従って行えばよく、そのこと自体に特に制限はない。例えば、上底吹き転炉を使い、溶銑を酸素吹錬によって通常Ｃ：0.03〜0.50質量％の所定濃度にまで脱炭処理を行えばよい。通常はＣ：0.04〜0.50質量％程度であるのが好ましい。一般には転炉における脱炭処理により[C] ≧100ppmにまですればよい。
【００３４】
転炉での精錬が終了してから取鍋に出鋼するが、必要により、出鋼流にＡｌなどの脱酸剤を添加することで、あるいは、Ｓｉ、Ｍｎ添加等の手段によって、溶鋼の脱酸を行う。後続の真空脱ガス処理を行う際の溶鋼の酸素活量が０．０００３（０．０００３％ , 以下同じ）以上、好ましくは０．０１００以下となる限り、脱酸の手段は特に制限なく、後述する脱水素中の酸素活量を考慮して決定すればよい。
【００３５】
なお、真空脱ガス処理に先立って真空脱ガス装置内で酸素の吹込みを行って極低炭素鋼 (Ｃ≦30ppm)のレベルにまで脱炭をする場合には、酸素活量の調整はその段階で行ってもよい。
【００３６】
取鍋への出鋼が終了してから真空脱ガス装置、例えば、RH脱ガス装置、DH真空脱ガス装置等を使い、取鍋内の溶鋼に真空脱ガス処理を行う。
ここに、本発明によれば、そのような真空脱ガス開始時の溶鋼の酸素活量を0.0003以上、そして好ましくは0.0100以下に調整する。
【００３７】
かかる酸素活量は、酸素含有率に大きく依存することから、本明細書においては便宜上酸素濃度でもって表わす。したがって酸素活量は取鍋からの出鋼時の脱酸の程度あるいは後述する酸素吹込みの程度を調整することで調節できる。
【００３８】
図２に真空脱ガス装置における脱水素処理中の水素値の推移を、図３に酸素活量の推移を本発明法と従来法とで概略比較して示した。
ここで、脱酸剤添加時点で水素値が上昇している原因は、脱酸剤の付着水によるものである。
【００３９】
本発明の実施態様では転炉もしくはさらに必要により真空脱ガス装置における脱炭のための酸素吹き込みが終了した後に、脱水素を主目的とした真空脱ガス処理を開始する。脱水素は真空脱ガス装置における真空度を上げることにより行う。通常、真空度を上げるためには時間がかかるが、便宜的に直ちに行えるとする。
【００４０】
なお、本明細書では10.0Torrより大きい場合を低真空、10.0Torr以下を高真空として区別する。
従来法では、脱炭のための酸素吹き込みが終了した後、できるだけ早く脱酸剤を添加していた。これは、溶鋼の酸素活量の高い状態が長時間継続されると、取鍋や真空脱ガス装置の耐火物が溶損されることを懸念したためである。このため、真空脱ガス処理中での溶鋼の酸素活量は低くなっていた。
【００４１】
しかしながら、本発明法において、まず、脱炭のための酸素吹き込みによって酸素活量を高位にし、脱酸剤の添加時期を遅らせることによって、酸素活量を高位に保持する。つまり、本発明では脱炭のための溶鋼への酸素吹き込み後、一定時間、脱酸剤の添加をしないことにより、溶鋼の酸素活量を高位に保持する。
【００４２】
本発明の好適態様では溶鋼の酸素活量を高位に保持する期間は、取鍋や真空脱ガス装置の耐火物の溶損も鑑みて、脱水素処理を主目的とした真空脱ガス処理の開始時点の溶鋼水素値と目標水素値の差の少なくとも20％まで脱水素する間、とする。
【００４３】
本発明によれば酸素活量を高位に保持することから耐火物の損傷が予想されたが、酸素活量が高いことから脱水素が速やかに行われ、予想外にも耐火物の溶損は実質上みられなかった。なお、従来にあっては耐火物溶損が予想されたので予め脱酸を行っていた。
【００４４】
本発明においても、脱水素が終了してからは、必要に応じて脱酸剤を添加し、真空脱ガス処理を終了してもよい。脱酸剤添加後は溶鋼の酸素活量が低くなるため、脱酸剤添加後の脱水素速度は従来と同じとなり、この段階では、脱水素の効果が小さくなる。
【００４５】
よって、最も望ましい脱酸剤の添加時期は、溶鋼水素値が例えば３ppm という目標水素値に到達したとき以降である。
ここに、脱水素処理は所定の酸素活量の溶鋼を真空下に保持し、必要により上昇管、下降管を経て循環させることにより行う。
【００４６】
本発明の好適態様では、脱酸剤添加時にも高真空を継続する。脱酸剤添加後に高真空状態にすることはCOガスの発生を促進し、溶鋼を攪拌する効果がある。このため、脱水素の目的とは別に、脱酸剤添加時に生成する介在物を凝集・合体させて浮上除去する目的で、脱酸剤添加後も高真空の状態を保持してもよい。
【００４７】
脱酸剤添加後は脱酸剤に付着した水分によって水素値が高くなるため、目標水素値は、本来の目標値から脱酸剤付着水分による水素値の増加分をあらかじめ差し引いた値とすることが好ましい。
【００４８】
脱酸剤付着水分による水素値の増加分はあくまで予想値であるため、脱酸剤添加後、水素値を測定して本来の目標値を達成していることを確認してから真空脱ガス処理を終了することが好ましい。
【００４９】
本発明の好適態様において、高真空中での酸素活量は図１の脱水素速度を考慮して0.0003以上とする。好ましくは0.0005以上、さらに好ましくは0.0010以上である。
【００５０】
脱水素処理における酸素活量が0.0003未満の場合には、真空脱ガス装置において気体もしくは固体の形態で強制的に酸素を供給するか、酸化物の添加により、溶鋼中の酸素活量を上昇させることも可能である。しかし、スラグ中の低級酸化物濃度 (FeO 、MnO)が上昇することにより溶鋼の清浄性が悪化することがあるため望ましくない。
【００５１】
これに対し、脱水素処理における溶鋼の酸素活量が0.0100を越えた場合には、脱水素反応には有利となる反面、溶鋼中のＣとＯが結合してCOガスとなり、脱水素反応と並行して脱炭反応が起こることが予想される。
【００５２】
従って、脱水素処理前の酸素活量に応じて事前に脱酸剤を添加することにより酸素活量を0.0100以下に調整した後、高真空下において脱水素処理を行うことが望ましい。
【００５３】
また、真空脱ガス処理実施中には、所望により、水素値のさらなる低減が求められているときには、溶鋼の自由界面に対してのプラズマ照射やArやＮ2 等の不活性ガスの吹き込み、さらには粉体の酸化剤や精錬用の粉体の吹き込みを併用してもよい。
【００５４】
【実施例】
真空脱ガス装置(RH)を使用し、270 〜280tの溶鋼において脱水素精錬を行った実施例を、本発明例と従来例とで比較して記述する。
【００５５】
かくして、本発明によれば好ましくは[H] ≦3.0ppm、[C] ≧100ppmの溶鋼が得られ、真空脱ガス装置での脱ガスを行えばさらに[C] を低減できる。なお、本例では[C] は転炉における脱炭処理を行って[C] ＝0.05〜0.10％として取鍋に出鋼した。
【００５６】
従来例では、RH真空脱ガス装置における、脱炭のための酸素吹き込みが終了した後に、装置の許す限り短い作業時間で脱酸剤を添加し、その後で真空脱水素処理を開始した。
【００５７】
真空脱水素処理の条件は、最大真空度：1.0Torr 以下、還流Arガス流量：1.3Nm3/min とした。
真空脱水素処理を開始してから一定時間間隔で溶鋼の水素値を測定し、例えば２ppm という目標水素値に到達した時点で真空脱水素処理を終了した。
【００５８】
本発明例では、脱炭のための酸素吹き込みが終了した後に、真空脱水素処理開始前に溶鋼の酸素活量を測定し、酸素活量が十分0.0003以上を確保できることを確認してから真空脱水素処理を開始した。
【００５９】
真空脱水素処理の条件は従来例と同じく、最大真空度：1.0Torr 以下、還流Arガス流量：1.3Nm3/min とした。真空脱水素処理を開始してから一定時間間隔で溶鋼の水素値を測定した。
【００６０】
本発明例の目標水素値は、従来例とは異なり、本来の目標値から、脱酸剤付着水分による水素値の増加分をあらかじめ差し引いた値とした。そして、その目標値まで水素値が低下してから脱酸剤を添加した。
【００６１】
その後、水素値が本来の目標値に到達していることを測定により確認して、真空脱ガス処理を終了した。
表１のNo.1〜６に本発明例、No.7〜12に従来例を示す。
【００６２】
まず、本発明例で脱酸剤としてAlを使用した場合をNo.1〜３、従来例も同じく脱酸剤としてAlを使用した場合をNo.7〜９に示す。処理前 [Ｈ] ＝5.8 〜6.0ppm、処理後 [Ｈ] ＝1.9 〜2.1ppm とほぼ同一条件であるのにもかかわらず、本発明例では処理時間が平均7.8 分短縮されていることがわかる。
【００６３】
次に、本発明例で脱酸剤としてSiを使用した場合をNo.4〜6 、従来例も同じく脱酸剤としてSiを使用した場合をNo.10 〜12に示す。脱酸剤としてSiを使用した場合でも処理時間を平均で2.5 分短縮することが可能である。
【００６４】
【表１】
【００６５】
これによると、目標水素値が低いほど本発明法による効果が大きく、また、本発明法は従来法と比較して平均5.8 分だけ脱水素処理時間が短縮された。
本発明法によって、真空脱ガス装置における脱水素処理時間の短縮が可能になり、図５に示すように、真空ポンプの稼働に必要な電力、蒸気、さらには還流Arガスの削減ができ、大幅な省エネルギー化が実現された。
【００６６】
【発明の効果】
本発明法によって、真空脱ガス装置における脱水素処理時間の短縮が可能となるばかりでなく、大気中の湿度が高い日や、真空脱ガス装置の真空度が上がりきらない場合でも、目標水素値の低い材質を安定して製造することが可能となり、さらにこれによって、連続鋳造によって製造したスラブ、圧延後の鋼板における、脱水素のための放置時間を短縮することができた。
【図面の簡単な説明】
【図１】脱水素速度に及ぼす溶鋼中酸素活量の影響を示すグラフである。
【図２】脱水素処理中における溶鋼中水素値推移の概略を示す図である。
【図３】脱水素処理中における酸素活量推移の概略を示す図である。
【図４】従来例と本発明例において、目標水素値別での脱水素処理時間を比較したグラフである。
【図５】従来例と本発明例において、エネルギー原単位を比較したグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a molten steel refining method for dehydrogenating a molten steel using a vacuum degassing apparatus.
[0002]
[Prior art]
In steel plates intended for severe use, such as those for line pipes and shipbuilding, the hydrogen content at the time of melting steel (hereinafter simply referred to as `` Hydrogen Defects '') is used to prevent hydrogen defects such as hydrogen cracking and hydrogen embrittlement. It is desired to reduce the hydrogen value as much as possible with a target value of 3.0 ppm, preferably 2.0 ppm or less.
[0003]
In order to meet this demand, when steel is melted, dehydrogenation is performed in a secondary refining process using a vacuum degasser such as RH or DH.
Furthermore, in the slab after continuous casting and the steel plate after rolling, dehydrogenation is performed by leaving the slab and steel plate for a certain period after casting or rolling.
[0004]
However, especially when the humidity in the atmosphere is high, the amount of moisture adhering to additives such as alloys in the secondary refining process increases, resulting in an increase in the hydrogen value in the molten steel or a vacuum due to a vacuum degassing device failure. There may be a problem that the degree does not increase. In such a case, the required hydrogen value could not be achieved, and it was necessary to take a long vacuum degassing treatment time for dehydrogenation, which hindered the efficiency of secondary refining.
[0005]
In order to improve the efficiency of this dehydrogenation process, several methods for promoting the dehydrogenation reaction have been proposed.
In Japanese Patent No. 2783029, there is a method of adopting a vacuum drop degassing method, and irradiating the drop with Ar gas plasma under reduced pressure to increase the surface temperature of the molten metal to promote dehydrogenation and denitrogenation. Proposed.
[0006]
In Japanese Examined Patent Publication No. 7-65120, gas is blown into molten steel, and when refining is performed using the generated bubbles, bubbles are refined by applying ultrasonic waves to the tuyere, and the deoxidation rate A method for improving the above has been proposed.
[0007]
Japanese Examined Patent Publication No. 7-100813 discloses that the decarburization reaction is promoted by blowing powder onto the molten steel surface under reduced pressure during decarburization, and CO gas is generated from the inside of the molten steel, thereby increasing the region of the gas-metal interface. A method for promoting dehydrogenation has been proposed.
[0008]
However, in order to apply the refining technology proposed in Japanese Patent Publication No. 2783029, Japanese Patent Publication No. 7-65120, and Japanese Patent Publication No. 7-1000081 to an actual manufacturing apparatus, it is necessary to increase expensive capital investment and running costs. Therefore, in actual operation on the premise of mass production, it is not always effective from the viewpoint of cost and equipment maintenance.
[0009]
By the way, Japanese Patent Laid-Open No. 8-311529 discloses a technique for increasing the oxygen activity in steel to 200 ppm or more in the vacuum degassing refining in which decarburization and dehydrogenation are performed simultaneously. However, this method is a means for promoting degassing mainly for decarburization in a very low carbon region of, for example, 20 to 50 ppm in [C], and also to molten steel when N 2 is used as the reflux gas. This is a means for suppressing the N 2 pickup and does not mention shortening of the dehydrogenation reaction time.
[0010]
Further, JP-A-6-33133 proposes a method for promoting the degassing reaction by a physicochemical approach, focusing on the fact that oxygen in steel is a surface active element. However, this method is essentially an invention related to decarburization of ultra-low carbon steel, and does not mention dehydrogenation reaction. In addition, a method of supplying an oxygen-containing substance into the degassing vacuum tank for replenishing oxygen in the steel is adopted, which is expensive.
[0011]
[Problems to be solved by the invention]
As described above, even in the past, various methods have been proposed for the purpose of improving the efficiency of dehydrogenation, but in any method, a significant increase in cost is inevitable.
[0012]
An object of the present invention is to provide a molten steel refining method that does not require a new capital investment in a vacuum degassing apparatus and that does not involve an increase in operation cost, and that improves the efficiency of dehydrogenation.
[0013]
[Means for Solving the Problems]
The present inventors have studied thermodynamically, focusing on the oxygen activity in molten steel, with respect to a method for improving the efficiency of dehydrogenation treatment of molten steel.
[0014]
The hydrogen absorption reaction of steel proceeds according to the chemical reaction equation shown in equation (1).
[0015]
[Chemical 1]
[0016]
In this chemical reaction formula, as a method of actually suppressing the hydrogen absorption reaction of steel, it is conceivable to reduce the water vapor partial pressure and increase the oxygen activity in molten steel.
[0017]
However, since it is considered impossible in practice to apply the former method to actual operation, the inventors focused on increasing the oxygen activity in the latter molten steel, and conducted further studies. went.
[0018]
In general, the dehydrogenation rate in a vacuum degasser is described by equation (2).
[0019]
[Chemical 2]
[0020]
As is clear from the equation (2), it is conceivable to reduce the value of the dehydrogenation limit concentration [H] e as a means for improving the dehydrogenation rate.
Assuming that [H] e is determined by the chemical reaction equation shown in equation (1), equation (3) is obtained.
[0021]
[Chemical 3]
[0022]
[Formula 4]
[0023]
FIG. 1 shows the result of continuous measurement of the hydrogen concentration under the conditions in which the oxygen activity in the molten steel is different based on the above thermodynamic examination.
Here, [H] o is the hydrogen concentration (ppm) before dehydrogenation.
[0024]
From this measurement result, it was confirmed that the higher the oxygen activity in the molten steel, the higher the dehydrogenation rate. In other words, when decarburization processing by oxygen blowing precedes, if dehydrogenation by vacuum degassing processing is started immediately after the completion of oxygen blowing into molten steel for decarburization, the oxygen activity is reduced. Dehydrogenation can proceed in a high state.
[0025]
In the present invention, neither the molten steel [C] nor the product [C] is limited. However, in order to produce extremely low carbon steel ([C] ≦ 30 ppm), oxygen blowing is performed prior to vacuum degassing in a vacuum degassing apparatus. The decarburization process may be performed. However, in general, decarburization and refining of the product [C] ≧ 100 ppm is preferable without decarburizing even to decarburization using a vacuum degassing apparatus, in which case the converter is used. The molten steel [C] ≧ 300 ppm may be set at the time of steel production.
[0026]
From the above examination, the present invention has the following configuration in order to solve the above-described problems.
(1) In the molten steel refining method that performs refining in the converter and secondary refining with the vacuum degassing device, dewatering is performed by performing vacuum degassing treatment on the molten steel after the converter steel is discharged using a vacuum degassing device. performed Motos Ru secondary refining, and performing vacuum degassing process to the molten steel oxygen activity is less 0.0003% or more and 0.0100% in the molten steel process of refining.
[0027]
(2) In a molten steel refining method that performs refining in a converter and secondary refining by a vacuum degassing device, the decarburization treatment is performed to [C] ≧ 100 ppm by converter refining, and the molten steel after the converter steel is vacuumed perform secondary refining you dehydrogenation by performing vacuum degassing process by using the degasser, a vacuum degassing treatment to the molten steel oxygen activity is less 0.0003% or more and 0.0100% in the A method for refining molten steel with [C] ≧ 100 ppm, characterized in that:
[0028]
(3) In the secondary refining, the difference between the molten steel hydrogen content and the target hydrogen content at the start of the vacuum degassing treatment is at least 20%. The molten steel refining method as described.
[0029]
(4) In the secondary refining, the oxygen activity of the molten steel is 0.0003 % or more and 0.0100% or less while dehydrogenating to a molten steel hydrogen content of 3 ppm, (1) Thru | or the refining method of the molten steel in any one of (3).
[0030]
( 5 ) The method for refining molten steel according to any one of (1) to ( 4 ) above, wherein after the dehydrogenation treatment is completed, a deoxidizer is added to the molten steel to perform deoxidation.
[0031]
( 6 ) The refining method according to any one of (1) to ( 5 ), wherein the product [H] ≦ 3.0 ppm is satisfied.
[0032]
DETAILED DESCRIPTION OF THE INVENTION
Here, it is as follows when the case where the refining method concerning this invention is implemented is demonstrated concretely.
[0033]
In the present invention, the refining in the converter may be performed according to the conventional method, and there is no particular limitation on the refining itself. For example, a decarburization process may be performed to a predetermined concentration of C: 0.03 to 0.50 mass% by oxygen blowing, using an upper bottom blowing converter. Usually, C is preferably about 0.04 to 0.50 mass%. In general, [C] ≧ 100 ppm should be achieved by decarburization in the converter.
[0034]
After the refining in the converter is finished, the steel is taken out into the ladle, but if necessary, by adding a deoxidizing agent such as Al to the outgoing steel flow, or by means such as Si and Mn addition, Deoxidize. As long as the oxygen activity of the molten steel in the subsequent vacuum degassing treatment is 0.0003 (0.0003% , the same shall apply hereinafter ) or more, preferably 0.0100 or less, the deoxidation means is not particularly limited and will be described later. It may be determined in consideration of the oxygen activity during dehydrogenation.
[0035]
In addition, when decarburizing to the level of ultra-low carbon steel (C ≦ 30ppm) by injecting oxygen in the vacuum degassing device prior to vacuum degassing treatment, adjustment of oxygen activity is It may be done in stages.
[0036]
After steeling out to the ladle is completed, vacuum degassing is performed on the molten steel in the ladle using a vacuum degassing apparatus such as an RH degassing apparatus or a DH vacuum degassing apparatus.
Here, according to the present invention, the oxygen activity of the molten steel at the start of such vacuum degassing is adjusted to 0.0003 or more, and preferably 0.0100 or less.
[0037]
Since this oxygen activity greatly depends on the oxygen content, it is expressed in this specification by the oxygen concentration for convenience. Therefore, the oxygen activity can be adjusted by adjusting the degree of deoxidation at the time of steel removal from the ladle or the degree of oxygen blowing described later.
[0038]
FIG. 2 shows the transition of the hydrogen value during the dehydrogenation process in the vacuum degassing apparatus, and FIG. 3 schematically shows the transition of the oxygen activity between the method of the present invention and the conventional method.
Here, the cause of the increase in the hydrogen value at the time of addition of the deoxidizer is due to water adhering to the deoxidizer.
[0039]
In the embodiment of the present invention, after the oxygen blowing for decarburization in the converter or, if necessary, the vacuum degassing apparatus is completed, the vacuum degassing process mainly for dehydrogenation is started. Dehydrogenation is performed by increasing the degree of vacuum in the vacuum degassing apparatus. Normally, it takes time to raise the degree of vacuum, but it is assumed that it can be performed immediately for convenience.
[0040]
In the present specification, a case where it is greater than 10.0 Torr is distinguished as a low vacuum, and a case where 10.0 Torr or less is designated as a high vacuum.
In the conventional method, the deoxidizer is added as soon as possible after the oxygen blowing for decarburization is completed. This is because the refractory material of the ladle and the vacuum degassing apparatus is concerned that the refractory of the ladle and the vacuum degassing apparatus will be melted down if the state of high oxygen activity of the molten steel is continued for a long time. For this reason, the oxygen activity of the molten steel during the vacuum degassing treatment was low.
[0041]
However, in the method of the present invention, first, the oxygen activity is increased by blowing oxygen for decarburization, and the oxygen activity is maintained at a higher level by delaying the addition time of the deoxidizer. That is, in the present invention, the oxygen activity of the molten steel is maintained at a high level by not adding a deoxidizer for a certain time after blowing oxygen into the molten steel for decarburization.
[0042]
In the preferred embodiment of the present invention, the period during which the oxygen activity of the molten steel is maintained at a high level is the start of the vacuum degassing process mainly for the dehydrogenation process in view of the melting damage of the refractories of the ladle and the vacuum degassing apparatus. During dehydrogenation to at least 20% of the difference between the current molten steel hydrogen value and the target hydrogen value.
[0043]
According to the present invention, damage to the refractory was expected because the oxygen activity was maintained at a high level, but dehydrogenation was performed quickly because the oxygen activity was high. Virtually not seen. Conventionally, since refractory melting was expected, deoxidation was performed in advance.
[0044]
Also in the present invention, after the dehydrogenation is completed, a deoxidizer may be added as necessary, and the vacuum degassing process may be terminated. Since the oxygen activity of the molten steel is lowered after the deoxidizer is added, the dehydrogenation rate after the deoxidizer is added is the same as before, and the dehydrogenation effect is reduced at this stage.
[0045]
Therefore, the most desirable addition time of the deoxidizer is after the molten steel hydrogen value reaches the target hydrogen value of 3 ppm, for example.
Here, the dehydrogenation treatment is performed by holding molten steel having a predetermined oxygen activity under vacuum and circulating it through an ascending pipe and a descending pipe as necessary.
[0046]
In a preferred embodiment of the present invention, the high vacuum is continued even when the deoxidizer is added. Making a high vacuum state after adding the deoxidizer promotes the generation of CO gas and has the effect of stirring the molten steel. For this reason, apart from the purpose of dehydrogenation, a high vacuum state may be maintained after the addition of the deoxidizer for the purpose of agglomerating and coalescing inclusions generated at the time of adding the deoxidizer to float and remove.
[0047]
Since the hydrogen value increases due to moisture adhering to the deoxidizer after the addition of the deoxidizer, the target hydrogen value should be a value obtained by subtracting in advance the increase in the hydrogen value due to the moisture adhering to the deoxidizer from the original target value. Is preferred.
[0048]
Since the increase in the hydrogen value due to the moisture attached to the deoxidizer is only an expected value, after adding the deoxidizer, measure the hydrogen value and confirm that the original target value has been achieved before vacuum degassing Is preferably terminated.
[0049]
In a preferred embodiment of the present invention, the oxygen activity in high vacuum is set to 0.0003 or more in consideration of the dehydrogenation rate in FIG. Preferably it is 0.0005 or more, More preferably, it is 0.0010 or more.
[0050]
When the oxygen activity in the dehydrogenation process is less than 0.0003, the oxygen activity in the molten steel is increased by forcibly supplying oxygen in the form of gas or solid in a vacuum degassing apparatus or by adding an oxide. It is also possible. However, it is not desirable because the cleanliness of the molten steel may be deteriorated by increasing the lower oxide concentration (FeO, MnO) in the slag.
[0051]
On the other hand, if the oxygen activity of the molten steel in the dehydrogenation process exceeds 0.0100, it is advantageous for the dehydrogenation reaction, but C and O in the molten steel are combined to form CO gas. A decarburization reaction is expected to occur in parallel.
[0052]
Accordingly, it is desirable to adjust the oxygen activity to 0.0100 or less by adding a deoxidizing agent in advance according to the oxygen activity before the dehydrogenation treatment, and then perform the dehydrogenation treatment under high vacuum.
[0053]
Further, during the vacuum degassing treatment, when it is desired to further reduce the hydrogen value, plasma irradiation or blowing of an inert gas such as Ar or N 2 to the free interface of the molten steel, May be used in combination with a powder oxidizing agent or a refining powder blowing.
[0054]
【Example】
An example in which dehydrogenation refining was performed on 270 to 280 t of molten steel using a vacuum degassing apparatus (RH) will be described in comparison with the present invention example and the conventional example.
[0055]
Thus, according to the present invention, molten steel with [H] ≦ 3.0 ppm and [C] ≧ 100 ppm is preferably obtained, and [C] can be further reduced by degassing with a vacuum degassing apparatus. In this example, [C] was decarburized in a converter, and [C] = 0.05 to 0.10% and steel was taken out in a ladle.
[0056]
In the conventional example, after the oxygen blowing for decarburization in the RH vacuum degassing apparatus was completed, the deoxidizer was added in a work time as short as the apparatus allowed, and then the vacuum dehydrogenation process was started.
[0057]
The conditions for the vacuum dehydrogenation were a maximum degree of vacuum: 1.0 Torr or less and a reflux Ar gas flow rate: 1.3 Nm 3 / min.
The hydrogen value of the molten steel was measured at regular intervals after the start of the vacuum dehydrogenation treatment, and the vacuum dehydrogenation treatment was terminated when the target hydrogen value of 2 ppm, for example, was reached.
[0058]
In the present invention example, after the oxygen blowing for decarburization is completed, the oxygen activity of the molten steel is measured before the start of the vacuum dehydrogenation treatment, and after confirming that the oxygen activity can be sufficiently ensured to be 0.0003 or more, vacuum dehydration is performed. Raw processing started.
[0059]
The conditions for the vacuum dehydrogenation were the same as in the conventional example, the maximum degree of vacuum: 1.0 Torr or less, and the reflux Ar gas flow rate: 1.3 Nm 3 / min. After starting the vacuum dehydrogenation treatment, the hydrogen value of the molten steel was measured at regular time intervals.
[0060]
Unlike the conventional example, the target hydrogen value of the example of the present invention was a value obtained by subtracting in advance the increase in the hydrogen value due to deoxidizer adhering moisture from the original target value. And after the hydrogen value fell to the target value, the deoxidizer was added.
[0061]
Thereafter, it was confirmed by measurement that the hydrogen value had reached the original target value, and the vacuum degassing process was terminated.
Nos. 1 to 6 in Table 1 show examples of the present invention, and Nos. 7 to 12 show conventional examples.
[0062]
First, the case where Al is used as a deoxidizer in the examples of the present invention is No. 1 to 3, and the case where Al is also used as a deoxidizer is also shown in Nos. 7 to 9 in the conventional example. It can be seen that the treatment time is shortened by an average of 7.8 minutes in the present invention example despite the almost identical conditions before treatment [H] = 5.8 to 6.0 ppm and after treatment [H] = 1.9 to 2.1 ppm. .
[0063]
Next, Nos. 4 to 6 show the case where Si is used as the deoxidizing agent in the examples of the present invention, and Nos. 10 to 12 show the case where Si is also used as the deoxidizing agent in the conventional example. Even when Si is used as a deoxidizer, the processing time can be reduced by an average of 2.5 minutes.
[0064]
[Table 1]
[0065]
According to this, the lower the target hydrogen value, the greater the effect of the method of the present invention, and the method of the present invention shortened the dehydrogenation processing time by an average of 5.8 minutes compared to the conventional method.
By the method of the present invention, it is possible to shorten the dehydrogenation processing time in the vacuum degassing apparatus, and as shown in FIG. 5, it is possible to reduce the power, steam, and reflux Ar gas necessary for the operation of the vacuum pump. Energy saving was realized.
[0066]
【The invention's effect】
The method of the present invention not only enables shortening of the dehydrogenation processing time in the vacuum degassing apparatus, but also the target hydrogen value even when the humidity in the atmosphere is high or the vacuum degree of the vacuum degassing apparatus cannot be fully increased. Thus, it was possible to stably produce a low-quality material, and further to reduce the standing time for dehydrogenation in a slab produced by continuous casting and a steel plate after rolling.
[Brief description of the drawings]
FIG. 1 is a graph showing the influence of oxygen activity in molten steel on the dehydrogenation rate.
FIG. 2 is a diagram showing an outline of transition of hydrogen value in molten steel during dehydrogenation treatment.
FIG. 3 is a diagram showing an outline of oxygen activity transition during dehydrogenation treatment.
FIG. 4 is a graph comparing dehydrogenation processing time for each target hydrogen value in a conventional example and an example of the present invention.
FIG. 5 is a graph comparing energy intensity in a conventional example and an example of the present invention.
Claims (6)
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