JP3742615B2 - Method of melting high cleanliness steel - Google Patents

Method of melting high cleanliness steel Download PDF

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JP3742615B2
JP3742615B2 JP2002302775A JP2002302775A JP3742615B2 JP 3742615 B2 JP3742615 B2 JP 3742615B2 JP 2002302775 A JP2002302775 A JP 2002302775A JP 2002302775 A JP2002302775 A JP 2002302775A JP 3742615 B2 JP3742615 B2 JP 3742615B2
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molten steel
slag
added
cao
flux
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JP2004137550A (en
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拓男 三戸
祐 渡辺
和徳 吹上
進 務川
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Nippon Steel Corp
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    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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Description

【０００１】
【発明の属する技術分野】
本発明は、介在物及び窒素量の少ない溶鋼を安定して製造することができる高清浄度鋼の溶製方法に関するものである。
【０００２】
【従来の技術】
【特許文献１】
特許第３２３００６６号公報（請求項１）
【０００３】
Ａｌ2Ｏ3に代表される介在物は特に缶用鋼板のような薄肉製品では、破胴に代表される重大な製品欠陥の原因となる。このような介在物の生成要因は大きく分けて３つある。すなわち、溶鋼中のフリー酸素を、Ａｌ等の還元性元素で脱酸する際に生じるもの。転炉で吹錬された酸化スラグ中のＦｅＯやＭｎＯなどの酸化性酸化物と、脱酸後の溶鋼中のＡｌ等の還元性元素が、ＦｅＯ+Ａｌ→Ｆｅ+Ａｌ2Ｏ3等の化学反応をし生じるもの。さらに、スラグや耐火物が巻き込まれて介在物となるものである。このうちスラグ中酸化性酸化物と溶鋼中還元性元素との反応によって生じる介在物を抑制するため、従来からスラグ改質が行われている。
【０００４】
スラグ改質はスラグに改質剤を添加することにより、溶鋼中のＡｌとの反応を抑制しようとする方法である。例えば上記の特許文献１には、出鋼中にＣａＯあるいはＭｇＯを添加して取鍋上のスラグを高融点化するとともに、スラグ上にＡｌを散布するスラグ改質法が記載されている。この方法によれば、取鍋中のスラグを固化させてその反応性を低下させることができ、またスラグ上に散布するＡｌによってスラグ中のＦｅＯやＭｎＯなどの還元を行い、溶鋼中のＡｌとの反応を抑制することができる。
【０００５】
しかし上記の特許文献１の方法では、取鍋上のスラグが高融点となるために、取鍋全体にわたるスラグの均質な改質が困難となり、スラグ性状のばらつきが大きいという問題がある。また、スラグは大気にさらされた上面から凝固するため、スラグ下面の溶鋼との界面付近に、低融点の高酸素ポテンシャルのスラグが濃縮され、溶鋼の再酸化が生じて溶鋼中のトータル酸素（溶鋼中に溶解した酸素と介在物として存在する酸素合計）のばらつきが大きくなるという問題がある。さらに、溶鋼中のＡｌが溶鋼中のＦｅＯやＭｎＯと反応して生じた介在物が浮上しようとしても、表面にある固化したスラグに吸収されにくく、溶鋼中に浮遊したままとなり易いという問題がある。このため、特許文献１の方法では溶鋼の清浄性の指標である溶鋼中のトータル酸素量を２０ｐｐｍ以下にまで低下させることは容易ではなかった。
【０００６】
このほか、転炉から取鍋に出鋼する際に溶鋼中にＡｌを添加する方法では、大気中の窒素が溶鋼中に取り込まれる窒素ピックアップを避けることができず、溶鋼中の窒素濃度を十分に下げることができないという問題もあった。
【０００７】
【発明が解決しようとする課題】
本発明は上記した従来の問題点を解決し、従来のスラグ改質法とは異なるスラグ改質法を採用することにより、介在物及び窒素量の少ない溶鋼を安定して製造することができる高清浄度鋼の溶製方法を提供するためになされたものである。
【０００８】
【課題を解決するための手段】
上記の課題を解決するためになされた本発明の高清浄度鋼の溶製方法は、転炉から取鍋に出鋼される溶鋼にフラックスを添加する第１工程と、取鍋内の溶鋼に溶鋼中のＡｌ濃度が０．０８〜０．１７質量％となるようにＡｌを添加してガス撹拌を行う第２工程と、脱ガス装置にて撹拌を行う第３工程とからなることを特徴とするものである。なお、第１工程で添加されるフラックスが、４０〜６０質量％のＣａＯと、残部を占めるＡｌ２Ｏ３と不可避的不純物よりなり、かつスラグ中のＣａＯ/Ａｌ２Ｏ３濃度比が２〜４となるよう添加されることが好ましい。また、フラックスの粒径が５〜３０μｍであることが好ましく、第１工程においてフラックスの他にＣａＯ源を添加することもできる。
【０００９】
また、第２工程におけるガス吹き込み量を、０.０５〜０.１３Ｎｍ３/ｔ/ｈとすることが好ましい。更に、第３工程における処理時間を５分以上とすることが好ましく、第３工程における脱ガス装置の槽内圧力を３００ｔｏｒｒ以下とすることが好ましい。
【００１０】
本発明の高清浄度鋼の溶製方法によれば、取鍋内の溶鋼に溶鋼中のＡｌ濃度が０．０８〜０．１７質量％となるように直接Ａｌを添加してガス撹拌を行い、溶鋼に含有されるＦｅＯやＭｎＯをＡｌと反応させて還元することによって、スラグ中酸化性酸化物と溶鋼中還元性元素との反応によって生じる介在物を抑制することができる。また、この方法によれば、溶鋼中のＡｌによってスラグ還元を行うため、溶鋼との界面付近のスラグも改質することができ効果的である。しかも請求項２に示すようにスラグ中のＣａＯ/Ａｌ２Ｏ３濃度比が２〜４となるようにフラックスを添加してスラグを軟質化させ、発生したＡｌ２Ｏ３等の介在物をスラグに吸着させ易くする。これによって介在物の少ない溶鋼を安定して製造することができる。本発明のその他の効果については、以下に実施形態とともに説明する。
【００１１】
【発明の実施の形態】
図１は本発明の工程を示す説明図であり、先ず▲１▼のように転炉１による吹錬が行なわれる。この工程では特別なフラックス等の添加は行われず、従来技術と同様である。吹錬が終了した溶鋼は、▲２▼のように転炉１から取鍋２に出鋼されるが、この際に溶鋼にフラックス３が添加される（第１工程）。
【００１２】
フラックス３はＣａＯとＡｌ２Ｏ３と不可避的不純物よりなり、好ましくはプリメルトされたカルシウムアルミネートである。フラックス３は、取鍋２上のスラグ中のＣａＯ/Ａｌ２Ｏ３濃度比が２〜４となるように添加される。ここでカルシウムアルミネートを選択したのは、スラグ中のＣａＯ/Ａｌ２Ｏ３濃度比を直接コントロールするためである。フラックス３中のＣａＯ濃度を４０〜６０質量％としたのは、この範囲でそれ自体が低融点となり、「種スラグ」として働かせることができるからである。またスラグ中のＣａＯ/Ａｌ２Ｏ３濃度比を２〜４とすると、改質終了後のスラグ中のＣａＯ/Ａｌ２Ｏ３濃度比は０.９〜１.５となり、図２に示すようにスラグの融点が最も低下して軟質のスラグとすることができる。
【００１３】
なおこの第１工程で添加されるフラックス３は、５〜３０mmの粒径とすることが好ましい。５mmよりも細かくなると投入時に舞い上がり易くなり、３０mmを越えると溶解し難くなるためである。なお後記する実施例に示すように、フラックス３とともに生石灰等のＣａＯ源を添加することにより、スラグの組成を調整することもできる。以上の第１工程により、取鍋２上に低融点で軟質のスラグが形成される。
【００１４】
次に、▲３▼に示すように取鍋２内の溶鋼にＡｌを添加するとともに、底部に設置したノズル４からアルゴンガス等を吹き込み、溶鋼のガス撹拌を行う（第２工程）。このように溶鋼中に直接Ａｌを添加して攪拌することにより、溶鋼の一括脱酸を行なわせ、発生するアルミナ（介在物）を大径化させることができる。そして発生した介在物は上方に浮上し、軟質のスラグによって確実に捕捉される。
【００１５】
第２工程におけるＡｌの添加量は、溶鋼中のＡｌ濃度が０．０８％以上となるように決定することが好ましい。図３に示すように、溶鋼中のトータル酸素を安定して２０ｐｐｍ以下とするためには、Ａｌ濃度を０．０８％以上とする必要があるためである。これに対して従来は溶鋼中のＡｌ濃度を０．０２％以下とし、その結果として溶鋼中のトータル酸素が大きくばらついていたことが、図３に示されている。ただし溶鋼中のＡｌ濃度が０．１７％を越えると後工程で脱Ａｌ処理が必要となり、処理時間が長くなって連続鋳造に間に合わなくなるので、Ａｌ濃度の上限値は０．１７％とする。なお、3）に示すように取鍋２にＡｌとともに生石灰を添加し、スラグの組成を調整してもよい。
【００１６】
第２工程におけるガス吹き込み量は、０.０５〜０.１３Ｎｍ3/ｔ/ｈとすることが好ましい。ガス吹き込み量がこの範囲よりも少ないとスラグと溶鋼との間の反応性を確保しにくくなり、また介在物の凝集効果も不十分となる。逆にガス吹き込み量がこの範囲を越えると、空気の巻き込みが生じて溶鋼中への窒素ピックアップ量が増加するので好ましくない。この第２工程におけるガス攪拌は、スラグと溶鋼との間の反応性及び介在物の凝集効果を確保するため、３分間以上行なうことが好ましい。
【００１７】
上記した第２工程の終了後、取鍋２内の溶鋼は▲４▼に示すように、ＲＨ等の脱ガス装置５で撹拌される（第３工程）。脱ガス装置５の槽内圧力は３００ｔｏｒｒ以下とし、強攪拌を行なうことが好ましい。このようにして溶鋼中に生成された介在物を凝集させるとともに、浮上した介在物を低融点スラグに吸着させて溶鋼中から分離する。この攪拌時間は５分以上とすることが好ましい。図４のグラフに示すように、５分以上の攪拌により溶鋼中のトータル酸素を２０ｐｐｍ以下とすることができるからである。なお、上記の攪拌時間は第３工程において成分調整が完了した後の攪拌時間を意味するものである。
【００１８】
上記の工程により溶製された溶鋼はトータル酸素が２０ｐｐｍ以下、窒素量が２０ｐｐｍ以下の高清浄度鋼であり、介在物の含有量を従来よりも大幅に減少させることができる。このため、引き続き連続鋳造を行ない肉薄の缶用鋼板を製造した場合の不良発生率を、低下させることが可能となる。
【００１９】
【実施例】
（実施例１）転炉からの出鋼中に、ＣａＯ５０％、Ａｌ2Ｏ3４３％のフラックスを、溶鋼１トン当たり３.６ｋｇの比率で溶鋼に添加した。フラックスはプリメルト品であり、粒径が５〜２５mmの塊状体である。取鍋内の溶鋼に粒径２０mmのショットアルミと、粒径が５〜４０mmの塊状の生石灰とをそれぞれ溶鋼１トン当たり３.６ｋｇの比率で添加し、ガス吹き込み量を０.０６Ｎｍ3/ｔ/ｈとしたボトムバブリングを上記の添加後３分（合計１０分）行なった。この段階における溶鋼中のＡｌは０.１２５%である。次に０.５ｔｏｒｒで１５分のＲＨ処理を実施した。処理後のスラグ組成はトータルＦｅ：１.６%、ＭｎＯ：１.３%、ＣａＯ/Ａｌ2Ｏ3：０.９である。得られた高清浄度鋼中のトータル酸素は１６ｐｐｍ，窒素量は１８ｐｐｍであった。
【００２０】
（実施例２）第２の溶鋼に、実施例１と同一条件でフラックス添加、ショットアルミと生石灰との添加及びボトムバブリング、ＲＨ処理を実施した。溶鋼中のＡｌは０.１５２%、処理後のスラグ組成はトータルＦｅ：１.１%、ＭｎＯ：１.０%、ＣａＯ/Ａｌ2Ｏ3：１.０である。得られた高清浄度鋼中のトータル酸素は１１ｐｐｍ，窒素量は２０ｐｐｍであった。
（実施例３）第３の溶鋼に、実施例１と同一条件でフラックス添加、ショットアルミと生石灰との添加及びボトムバブリング、ＲＨ処理を実施した。溶鋼中のＡｌは０.１６９%、処理後のスラグ組成はトータルＦｅ：１.０%、ＭｎＯ：１.２%、ＣａＯ/Ａｌ2Ｏ3：１.０である。得られた高清浄度鋼中のトータル酸素は１０ｐｐｍ，窒素量は１６ｐｐｍであった。
【００２１】
（実施例４）第４の溶鋼に、実施例１と同一条件でフラックス添加、ショットアルミと生石灰との添加及びボトムバブリング、ＲＨ処理を実施した。なお、第２工程において、溶鋼１トン当たりフェロＭｎ１.４ｋｇの合金添加を行なった。溶鋼中のＡｌは０.１１４%、処理後のスラグ組成はトータルＦｅ：１.３%、ＭｎＯ：１.０%、ＣａＯ/Ａｌ2Ｏ3：０.９である。得られた高清浄度鋼中のトータル酸素は１４ｐｐｍ，窒素量は１７ｐｐｍであった。このように、各実施例では、トータル酸素、窒素ともに２０ｐｐｍ以下の安定した高清浄度鋼が得られた。
【００２２】
（比較例１）転炉中の溶鋼に粒径が５〜４０mmの塊状の生石灰を溶鋼１トン当たり３.６ｋｇの比率で添加したのち、出鋼した。取鍋内の溶鋼に粒径２０mmのショットアルミと、粒径が５〜４０mmの塊状の生石灰とをそれぞれ溶鋼１トン当たり３.６ｋｇの比率で添加しガス吹き込み量を０.０６Ｎｍ3/ｔ/ｈとしたボトムバブリングを添加後３分（合計１０分）行なった。溶鋼中のＡｌは０.００４%と低かった。次に６０ｔｏｒｒで１５分のＲＨ処理を実施し、この段階で溶鋼１トン当たりフェロＭｎ０.６ｋｇの合金添加を行なった。処理後のスラグ組成はトータルＦｅ：５.１%、ＭｎＯ：２.８%、ＣａＯ/Ａｌ2Ｏ3：１.７である。得られた高清浄度鋼中のトータル酸素は２１ｐｐｍ，窒素量は２４ｐｐｍであった。この比較例１は前記した特許文献１のスラグ改質に近い内容であるが、本発明に比較してトータル酸素のばらつきも大きくなる。
【００２３】
（比較例２）転炉から取鍋に出鋼された溶鋼に、プリメルトされていないAl粉とＣａＯ粉とからなるフラックスを溶鋼１トン当たり３.６ｋｇの比率で添加した。それらの粒径は１mm以下であり、その組成はＡｌ：４５%、Ａｌ2Ｏ3：２６%、ＣａＯ：１５%である。その他の条件は実施例１と同様とした。この場合、出鋼時のスラグ軟化がないので均質化が困難であり、処理後のスラグ組成はトータルＦｅ：４.２%、ＭｎＯ：３.９%、ＣａＯ/Ａｌ2Ｏ3：０.２である。得られた高清浄度鋼中のトータル酸素は２５ｐｐｍ，窒素量は２５ｐｐｍであった。
【００２４】
（比較例３）転炉中の溶鋼にプリメルトされていないAl粉とＣａＯ粉とからなるフラックスを溶鋼１トン当たり３.６ｋｇの比率で添加した。それらの粒径は１mm以下であり、その組成はＡｌ：４５%、Ａｌ2Ｏ3：２６%、ＣａＯ：１５%である。更に取鍋に出鋼された溶鋼にも、同じフラックスを同量添加したが、アルミショットの添加は行なわなかった。その他の条件は実施例１と同様とした。この場合、溶鋼中のＡｌは０.００４%と低くなり、処理後のスラグ組成はトータルＦｅ：１.６%、ＭｎＯ：６.６%、ＣａＯ/Ａｌ2Ｏ3：０.８である。得られた高清浄度鋼中のトータル酸素は２０ｐｐｍ，窒素量は３７ｐｐｍであった。Al粉の添加により溶鋼中の窒素量が大きく増加した。
【００２５】
【発明の効果】
以上に説明したように、本発明の高清浄度鋼の溶製方法によれば、介在物及び窒素量の少ない溶鋼を安定して製造することができ、缶用鋼板のような薄肉製品を製造する場合にも、製品不良率を大幅に低減できる効果がある。
【図面の簡単な説明】
【図１】本発明の工程説明図である。
【図２】ＣａＯ/Ａｌ2Ｏ3濃度比とスラグの軟化温度との関係を示すグラフである。
【図３】溶鋼中のＡｌ濃度とトータル酸素との関係を示すグラフである。
【図４】第３工程における攪拌時間と溶鋼中のトータル酸素との関係を示すグラフである。
【符号の説明】
１ 転炉
２ 取鍋
３ フラックス
４ ノズル
５ 脱ガス装置[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing high cleanliness steel that can stably produce molten steel with a small amount of inclusions and nitrogen.
[0002]
[Prior art]
[Patent Document 1]
Japanese Patent No. 3230066 (Claim 1)
[0003]
Inclusions typified by Al 2 O 3 cause serious product defects typified by broken bodies, especially in thin products such as steel plates for cans. There are three main factors for the formation of such inclusions. That is, it is generated when deoxidizing free oxygen in molten steel with a reducing element such as Al. Oxidizing oxides such as FeO and MnO in oxidized slag blown in the converter, and reducing elements such as Al in the molten steel after deoxidation are chemistry such as FeO + Al → Fe + Al 2 O 3 Something that reacts. Furthermore, slag and a refractory are involved and become inclusions. Of these, slag reforming has been conventionally performed in order to suppress inclusions generated by the reaction between the oxidizing oxide in slag and the reducing element in molten steel.
[0004]
Slag reforming is a method of suppressing reaction with Al in molten steel by adding a modifier to slag. For example, Patent Document 1 described above describes a slag reforming method in which CaO or MgO is added to the steel tapping to increase the melting point of the slag on the ladle and to spray Al on the slag. According to this method, the slag in the ladle can be solidified to reduce its reactivity, and FeO, MnO, etc. in the slag can be reduced by Al sprayed on the slag, and the Al in the molten steel can be reduced. This reaction can be suppressed.
[0005]
However, in the method of Patent Document 1 described above, since the slag on the ladle has a high melting point, it is difficult to uniformly reform the slag over the entire ladle, and there is a problem that slag properties vary greatly. Also, since slag solidifies from the upper surface exposed to the atmosphere, slag with a low melting point and high oxygen potential is concentrated near the interface with the molten steel on the lower surface of the slag, resulting in reoxidation of the molten steel and total oxygen in the molten steel ( There is a problem that variation in oxygen dissolved in the molten steel and oxygen present as inclusions increases. Furthermore, even if the inclusions generated by the reaction of Al in the molten steel with FeO or MnO in the molten steel try to float up, there is a problem that it is difficult to be absorbed by the solidified slag on the surface and remains floating in the molten steel. . For this reason, it is not easy to reduce the total oxygen content in molten steel, which is an index of cleanliness of molten steel, to 20 ppm or less by the method of Patent Document 1.
[0006]
In addition, in the method of adding Al to the molten steel when the steel is discharged from the converter to the ladle, a nitrogen pickup in which nitrogen in the atmosphere is taken into the molten steel cannot be avoided, and the nitrogen concentration in the molten steel is sufficient. There was also a problem that it could not be lowered.
[0007]
[Problems to be solved by the invention]
The present invention solves the above-described conventional problems, and adopts a slag reforming method different from the conventional slag reforming method, thereby enabling stable production of molten steel with a small amount of inclusions and nitrogen. It was made to provide a method for melting cleanliness steel.
[0008]
[Means for Solving the Problems]
In order to solve the above-mentioned problems, the method for melting high cleanliness steel according to the present invention includes a first step of adding flux to the molten steel discharged from the converter to the ladle, and the molten steel in the ladle. It is characterized by comprising a second step in which Al is added and gas stirring is performed so that the Al concentration in the molten steel becomes 0.08 to 0.17% by mass, and a third step in which stirring is performed by a degassing apparatus. It is what. The flux added in the first step is composed of 40 to 60 % by mass of CaO , Al 2 O 3 occupying the balance and unavoidable impurities, and the CaO / Al 2 O 3 concentration ratio in the slag is 2 to 2. It is preferable to be added to be 4. Moreover, it is preferable that the particle size of a flux is 5-30 micrometers, and it can also add a CaO source other than a flux in a 1st process.
[0009]
Moreover, it is preferable that the gas blowing amount in the second step is 0.05 to 0.13 Nm 3 / t / h. Furthermore, the treatment time in the third step is preferably 5 minutes or more, and the pressure in the tank of the degassing device in the third step is preferably 300 torr or less.
[0010]
According to the melting method of high cleanliness steel of the present invention, gas is agitated by directly adding Al to the molten steel in the ladle so that the Al concentration in the molten steel is 0.08 to 0.17% by mass. By reducing FeO and MnO contained in the molten steel by reacting with Al, inclusions produced by the reaction between the oxidizing oxide in the slag and the reducing element in the molten steel can be suppressed. Moreover, according to this method, since slag reduction is performed by Al in the molten steel, the slag near the interface with the molten steel can also be modified, which is effective. Moreover, as shown in claim 2, flux is added so that the CaO / Al 2 O 3 concentration ratio in the slag is 2 to 4, the slag is softened, and the generated inclusions such as Al 2 O 3 are slag. Make it easy to adsorb. As a result, molten steel with few inclusions can be stably produced. Other effects of the present invention will be described below together with embodiments.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is an explanatory view showing the steps of the present invention. First, blowing by the converter 1 is performed as shown in (1). In this process, no special flux or the like is added, and the process is the same as in the prior art. The molten steel that has been blown is discharged from the converter 1 to the ladle 2 as shown in (2). At this time, the flux 3 is added to the molten steel (first step).
[0012]
The flux 3 is made of CaO, Al 2 O 3 and unavoidable impurities, and is preferably premelted calcium aluminate. The flux 3 is added so that the CaO / Al 2 O 3 concentration ratio in the slag on the ladle 2 is 2-4. The reason why calcium aluminate was selected here is to directly control the CaO / Al 2 O 3 concentration ratio in the slag. The reason why the CaO concentration in the flux 3 is set to 40 to 60% by mass is that in this range, the CaO itself has a low melting point and can act as “seed slag”. Further, when the 2-4 the CaO / Al 2 O 3 concentration ratio in the slag, CaO / Al 2 O 3 concentration ratio in the slag after the modification completion becomes 0.9 to 1.5, as shown in FIG. 2 In addition, the melting point of the slag is lowered most, so that a soft slag can be obtained.
[0013]
In addition, it is preferable that the flux 3 added in the first step has a particle diameter of 5 to 30 mm. This is because if it is finer than 5 mm, it tends to rise when it is introduced, and if it exceeds 30 mm, it becomes difficult to dissolve. In addition, as shown in the Example mentioned later, the composition of slag can also be adjusted by adding CaO sources, such as quicklime, with the flux 3. FIG. By the above first step, soft slag having a low melting point is formed on the ladle 2.
[0014]
Next, as shown in (3), while adding Al to the molten steel in the ladle 2, argon gas or the like is blown from the nozzle 4 installed at the bottom to stir the molten steel (second step). In this way, by directly adding Al to the molten steel and stirring, the molten steel can be subjected to batch deoxidation and the generated alumina (inclusions) can be enlarged. The generated inclusions float upward and are reliably captured by the soft slag.
[0015]
The amount of Al added in the second step is preferably determined so that the Al concentration in the molten steel is 0.08% or more. As shown in FIG. 3, in order to stably bring the total oxygen in the molten steel to 20 ppm or less, the Al concentration needs to be 0.08% or more. On the other hand, FIG. 3 shows that the Al concentration in the molten steel is 0.02% or less and the total oxygen in the molten steel varies greatly as a result. However, if the Al concentration in the molten steel exceeds 0.17%, a de-Al treatment is required in the subsequent process, and the treatment time becomes long and the continuous casting cannot be made in time. Therefore, the upper limit of the Al concentration is set to 0.17% . In addition, as shown to 3), quick lime may be added to the ladle 2 with Al, and the composition of slag may be adjusted.
[0016]
The gas blowing rate in the second step is preferably 0.05 to 0.13 Nm 3 / t / h. If the gas blowing amount is less than this range, it becomes difficult to ensure the reactivity between the slag and the molten steel, and the effect of agglomerating inclusions becomes insufficient. On the other hand, if the amount of gas blown exceeds this range, air entrainment occurs and the amount of nitrogen pick-up into the molten steel increases, which is not preferable. The gas agitation in the second step is preferably performed for 3 minutes or more in order to ensure the reactivity between the slag and the molten steel and the effect of inclusion aggregation.
[0017]
After completion of the second step, the molten steel in the ladle 2 is stirred by a degassing device 5 such as RH as shown in (4) (third step). It is preferable that the pressure in the tank of the degassing device 5 is 300 torr or less and strong stirring is performed. In this way, inclusions generated in the molten steel are aggregated, and the floating inclusions are adsorbed on the low melting point slag and separated from the molten steel. The stirring time is preferably 5 minutes or longer. This is because, as shown in the graph of FIG. 4, the total oxygen in the molten steel can be reduced to 20 ppm or less by stirring for 5 minutes or more. In addition, said stirring time means the stirring time after completion of a component adjustment in a 3rd process.
[0018]
The molten steel produced by the above process is a high cleanliness steel having a total oxygen content of 20 ppm or less and a nitrogen content of 20 ppm or less, and can greatly reduce the inclusion content. For this reason, it becomes possible to reduce the defect occurrence rate when continuous casting is carried out to produce a thin steel plate for cans.
[0019]
【Example】
(Example 1) In the steel output from the converter, 50% CaO and 43% Al 2 O 3 flux were added to the molten steel at a rate of 3.6 kg per ton of molten steel. The flux is a pre-melt product and is a massive body having a particle size of 5 to 25 mm. To the molten steel in the ladle, shot aluminum with a particle size of 20 mm and massive quick lime with a particle size of 5 to 40 mm are added at a rate of 3.6 kg per ton of molten steel, and the gas blowing rate is 0.06 Nm 3 / t. The bottom bubbling was performed for 3 minutes (total 10 minutes) after the above addition. Al in the molten steel at this stage is 0.125%. Next, RH treatment for 15 minutes was performed at 0.5 torr. The slag composition after the treatment is total Fe: 1.6%, MnO: 1.3%, and CaO / Al 2 O 3 : 0.9. Total oxygen in the obtained high cleanliness steel was 16 ppm, and the nitrogen content was 18 ppm.
[0020]
(Example 2) The second molten steel was subjected to flux addition, shot aluminum and quicklime addition, bottom bubbling, and RH treatment under the same conditions as in Example 1. Al in the molten steel is 0.152%, and the slag composition after the treatment is total Fe: 1.1%, MnO: 1.0%, CaO / Al 2 O 3 : 1.0. Total oxygen in the obtained high cleanliness steel was 11 ppm, and the nitrogen content was 20 ppm.
(Example 3) Flux addition, addition of shot aluminum and quicklime, bottom bubbling, and RH treatment were performed on the third molten steel under the same conditions as in Example 1. Al in the molten steel is 0.169%, and the slag composition after the treatment is total Fe: 1.0%, MnO: 1.2%, CaO / Al 2 O 3 : 1.0. Total oxygen in the obtained high cleanliness steel was 10 ppm, and the nitrogen content was 16 ppm.
[0021]
(Example 4) Flux addition, shot aluminum and quicklime addition, bottom bubbling, and RH treatment were performed on the fourth molten steel under the same conditions as in Example 1. In the second step, 1.4 kg of ferro-Mn alloy was added per ton of molten steel. Al in the molten steel is 0.114%, and the slag composition after the treatment is total Fe: 1.3%, MnO: 1.0%, and CaO / Al 2 O 3 : 0.9. Total oxygen in the obtained high cleanliness steel was 14 ppm, and the nitrogen content was 17 ppm. Thus, in each Example, the stable high cleanliness steel whose total oxygen and nitrogen are both 20 ppm or less was obtained.
[0022]
(Comparative Example 1) Bulk molten lime having a particle size of 5 to 40 mm was added to the molten steel in the converter at a rate of 3.6 kg per ton of molten steel, and then steel was produced. To the molten steel in the ladle, shot aluminum with a particle size of 20 mm and massive quick lime with a particle size of 5 to 40 mm are added at a rate of 3.6 kg per ton of molten steel, and the gas blowing rate is 0.06 Nm 3 / t / 3 minutes after addition of the bottom bubbling set to h (total 10 minutes). Al in the molten steel was as low as 0.004%. Next, RH treatment was performed at 60 torr for 15 minutes, and at this stage, 0.6 kg of ferro-Mn alloy was added per ton of molten steel. The slag composition after the treatment is total Fe: 5.1%, MnO: 2.8%, and CaO / Al 2 O 3 : 1.7. Total oxygen in the high cleanliness steel obtained was 21 ppm, and the nitrogen content was 24 ppm. Although the comparative example 1 is similar to the slag reforming described in Patent Document 1, the variation in total oxygen is larger than that of the present invention.
[0023]
(Comparative example 2) The flux which consists of Al powder and CaO powder which are not pre-melted was added to the molten steel delivered to the ladle from the converter in the ratio of 3.6 kg per ton of molten steel. Their particle size is 1 mm or less, and the composition is Al: 45%, Al 2 O 3 : 26%, CaO: 15%. Other conditions were the same as in Example 1. In this case, it is difficult to homogenize because there is no slag softening at the time of steel production. The slag composition after the treatment is total Fe: 4.2%, MnO: 3.9%, CaO / Al 2 O 3 : 0.2 It is. Total oxygen in the obtained high cleanliness steel was 25 ppm, and the amount of nitrogen was 25 ppm.
[0024]
(Comparative example 3) The flux which consists of Al powder and CaO powder which are not pre-melted to the molten steel in a converter was added in the ratio of 3.6 kg per ton of molten steel. Their particle size is 1 mm or less, and the composition is Al: 45%, Al 2 O 3 : 26%, CaO: 15%. Furthermore, the same amount of the same flux was added to the molten steel delivered to the ladle, but aluminum shot was not added. Other conditions were the same as in Example 1. In this case, Al in the molten steel is as low as 0.004%, and the slag composition after the treatment is total Fe: 1.6%, MnO: 6.6%, CaO / Al 2 O 3 : 0.8. Total oxygen in the obtained high cleanliness steel was 20 ppm, and the nitrogen content was 37 ppm. Addition of Al powder greatly increased the amount of nitrogen in molten steel.
[0025]
【The invention's effect】
As described above, according to the method for melting high cleanliness steel of the present invention, molten steel with a small amount of inclusions and nitrogen can be stably produced, and a thin product such as a steel plate for cans is produced. In this case, the product defect rate can be greatly reduced.
[Brief description of the drawings]
FIG. 1 is a process explanatory diagram of the present invention.
FIG. 2 is a graph showing the relationship between the CaO / Al 2 O 3 concentration ratio and the softening temperature of slag.
FIG. 3 is a graph showing the relationship between Al concentration in molten steel and total oxygen.
FIG. 4 is a graph showing the relationship between the stirring time in the third step and the total oxygen in the molten steel.
[Explanation of symbols]
1 Converter 2 Ladle 3 Flux 4 Nozzle 5 Degasser

Claims (7)

転炉から取鍋に出鋼される溶鋼にフラックスを添加する第１工程と、取鍋内の溶鋼に溶鋼中のＡｌ濃度が０．０８〜０．１７質量％となるようにＡｌを添加してガス撹拌を行う第２工程と、脱ガス装置にて撹拌を行う第３工程とからなることを特徴とする高清浄度鋼の溶製方法。Al is added so that the Al concentration in the molten steel is 0.08 to 0.17% by mass in the first step of adding flux to the molten steel discharged from the converter to the ladle. A method for melting high cleanliness steel comprising a second step of performing gas stirring and a third step of stirring in a degassing apparatus. 第１工程で添加されるフラックスが、４０〜６０質量％のＣａＯと、残部を占めるＡｌ２Ｏ３と不可避的不純物よりなり、かつスラグ中のＣａＯ/Ａｌ２Ｏ３濃度比が２〜４となるよう添加される請求項１記載の高清浄度鋼の溶製方法。The flux added in the first step is composed of 40-60 mass% CaO, the remaining Al 2 O 3 and inevitable impurities, and the CaO / Al 2 O 3 concentration ratio in the slag is 2-4. The method for melting high-cleanliness steel according to claim 1, which is added so as to become. 第１工程で添加されるフラックスが、５〜３０mmの粒径のものである請求項１または２記載の高清浄度鋼の溶製方法。  The method for melting high cleanliness steel according to claim 1 or 2, wherein the flux added in the first step has a particle diameter of 5 to 30 mm. 第１工程において、フラックスの他にＣａＯ源を添加する請求項１〜３のいずれか記載の高清浄度鋼の溶製方法。  The method for melting high cleanliness steel according to any one of claims 1 to 3, wherein a CaO source is added in addition to the flux in the first step. 第２工程におけるガス吹き込み量を、０.０５〜０.１３Ｎｍ３/ｔ/ｈとする請求項１〜４のいずれか記載の高清浄度鋼の溶製方法。The method for melting high cleanliness steel according to any one of claims 1 to 4, wherein a gas blowing amount in the second step is set to 0.05 to 0.13 Nm 3 / t / h. 第３工程における処理時間を、５分以上とする請求項１〜５のいずれか記載の高清浄度鋼の溶製方法。  The method for melting high cleanliness steel according to any one of claims 1 to 5, wherein the treatment time in the third step is 5 minutes or more. 第３工程における脱ガス装置の槽内圧力を、３００ｔｏｒｒ以下とする請求項１〜６のいずれか記載の高清浄度鋼の溶製方法。  The in-tank pressure of the degassing apparatus in a 3rd process is 300 torr or less, The melting method of the high cleanliness steel in any one of Claims 1-6.
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