JP6337681B2 - Vacuum refining method for molten steel - Google Patents

Vacuum refining method for molten steel Download PDF

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JP6337681B2
JP6337681B2 JP2014164416A JP2014164416A JP6337681B2 JP 6337681 B2 JP6337681 B2 JP 6337681B2 JP 2014164416 A JP2014164416 A JP 2014164416A JP 2014164416 A JP2014164416 A JP 2014164416A JP 6337681 B2 JP6337681 B2 JP 6337681B2
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molten steel
decarburization
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JP2016040400A (en
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敦 岡山
敦 岡山
隆之 西
隆之 西
秀平 笠原
秀平 笠原
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Nippon Steel Corp
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Description

本発明は、鋼材の製造段階において、溶製コストを抑えつつ脱炭末期の脱炭速度を向上させ、更には製品段階で製品性能を低下させる要因になり得るアルミナ系介在物の除去を促進させることが可能な環流型脱ガス装置における溶鋼の減圧精錬方法に関する。   The present invention improves the decarburization rate at the end of decarburization while suppressing the melting cost in the production stage of steel materials, and further promotes the removal of alumina inclusions that can be a factor in reducing product performance in the product stage. The present invention relates to a vacuum refining method for molten steel in a recirculation type degassing apparatus.

自動車用鋼板を始めとする高清浄極低炭素鋼は、環流型脱ガス装置を用いた脱炭処理、それに引き続き行われるＡｌ添加および環流処理によって製造されている。   Highly clean ultra-low carbon steels such as steel sheets for automobiles are manufactured by decarburization using a recirculation type degassing apparatus, followed by Al addition and recirculation.

脱炭処理では、未脱酸鋼を環流型脱ガス装置で減圧処理することで、Ｃ濃度が１０ｐｐｍ以下といった極低炭素領域まで脱炭する技術がある。これは、（Ａ）式に示す脱炭反応において、ＣＯ分圧を下げることで脱炭反応を促進させるものである。   In the decarburization treatment, there is a technique for decarburizing undeoxidized steel to a very low carbon region such as a C concentration of 10 ppm or less by subjecting the deoxidized steel to a reduced pressure treatment with a reflux degasser. This promotes the decarburization reaction by lowering the CO partial pressure in the decarburization reaction represented by the formula (A).

Ｃ＋Ｏ＝ＣＯ（ｇ） ・・・（Ａ）
脱炭反応は、非特許文献１で報告されているように、内部脱炭、表面脱炭、気泡脱炭に大別できる。内部脱炭は、溶鋼を減圧処理した際、溶鋼中のＣとＯの濃度積が雰囲気圧力とＣＯ生成臨界圧の和よりも高い領域からＣＯガスが生成する反応であり、脱炭初期から中期にかけて生じる。脱炭反応が進んだ脱炭末期ではＣとＯの濃度積が小さくなるため、内部脱炭が停滞し、脱炭速度は大きく低下する。この時、表面脱炭、気泡脱炭の寄与率が相対的に大きくなるため、表面脱炭、気泡脱炭を促進させることで脱炭時間の短縮が期待できる。 C + O = CO (g) (A)
As reported in Non-Patent Document 1, the decarburization reaction can be roughly divided into internal decarburization, surface decarburization, and bubble decarburization. Internal decarburization is a reaction in which CO gas is generated from a region where the concentration product of C and O in molten steel is higher than the sum of the atmospheric pressure and the CO production critical pressure when the molten steel is subjected to reduced pressure treatment. To occur. At the end of decarburization where the decarburization reaction has progressed, the concentration product of C and O becomes small, so that internal decarburization is stagnant and the decarburization rate is greatly reduced. At this time, since the contribution ratio of surface decarburization and bubble decarburization becomes relatively large, shortening of the decarburization time can be expected by promoting surface decarburization and bubble decarburization.

特許文献１では「溶鋼中炭素濃度が１００ｐｐｍ以下に低下した後、真空槽内溶鋼の浴深を静止状態に換算して２０ｃｍ以下としつつ処理すること」を特徴とする、ＲＨ法による極低炭素鋼の溶製方法が開示されている。この技術は、余分なＡｒガスを使わずに浴深を浅くして炭素濃度の低い時期に溶鋼のスプラッシュ量を著しく増加させ、脱炭のための気−液界面積を増やすことで、表面脱炭の促進を狙ったものである。しかしながら、この技術はスプラッシュを増加させることになるため、真空槽内に地金が付着し、操業阻害を生じさせてしまう頻度が増大してしまう。   In Patent Document 1, “After the carbon concentration in the molten steel is reduced to 100 ppm or less, the bath depth of the molten steel in the vacuum chamber is converted to a static state and processed to be 20 cm or less”, and extremely low carbon by the RH method A method for melting steel is disclosed. This technology reduces surface desorption by reducing the bath depth without using excess Ar gas, significantly increasing the amount of splash of molten steel when the carbon concentration is low, and increasing the gas-liquid interface area for decarburization. It is aimed at promoting charcoal. However, since this technique increases splash, the frequency at which the metal is attached to the vacuum chamber and the operation is hindered increases.

また、特許文献２では「溶鋼中の炭素濃度が１００ｐｐｍ以下の領域で真空脱炭処理の溶鋼に水素含有物質を添加する」ことを特徴とする、極低炭素溶鋼の製造方法が開示されている。この技術は、水素ガスボイリングを生じさせることによる脱炭反応の促進を狙ったものである。しかしながら、この技術は水素含有物質を添加するため、新たに脱Ｈ処理が必要となり、全体としての処理時間短縮にはならない。   Patent Document 2 discloses a method for producing ultra-low carbon molten steel, characterized in that “a hydrogen-containing material is added to molten steel subjected to vacuum decarburization in a region where the carbon concentration in molten steel is 100 ppm or less”. . This technology aims to accelerate the decarburization reaction by causing hydrogen gas boiling. However, since this technique adds a hydrogen-containing substance, a new de-H treatment is required, and the overall treatment time is not shortened.

さらに、特許文献３では「［Ｃ］＞２０ｐｐｍまでの脱炭反応の前半期に環流ガス量を３３〜４２Ｎｌ／ｍｉｎ．ｃｍとし、［Ｃ］≦２０ｐｐｍの脱炭反応の後半期に環流ガス量を８〜２５Ｎｌ／ｍｉｎ．ｃｍに低下させること」を特徴とする極低炭素鋼の溶製方法が開示されている。この技術は、極低炭素領域での環流速度を低下させることで、溶鋼中の微細ＣＯ気泡の真空槽内の滞留時間を延長させ、気泡脱炭の促進を狙ったものである。しかしながら、この技術では環流ガス量を低下させることにより環流ガス全体の存在割合が低下するため、処理時間短縮には繋がらない。   Further, in Patent Document 3, “the reflux gas amount is set to 33 to 42 Nl / min.cm in the first half of the decarburization reaction up to [C]> 20 ppm, and the reflux gas amount in the second half of the decarburization reaction of [C] ≦ 20 ppm. Is reduced to 8-25 Nl / min.cm ", a method for melting ultra-low carbon steel is disclosed. This technique aims to promote bubble decarburization by extending the residence time of fine CO bubbles in molten steel in the vacuum chamber by reducing the reflux velocity in the extremely low carbon region. However, this technique does not lead to shortening of the processing time because the ratio of the entire circulating gas is reduced by reducing the amount of the circulating gas.

一方、脱炭処理に続くＡｌ添加後に溶鋼を清浄化させる場合、従来よりスラグ中のＦｅＯ濃度を低減したり、スラグ中のＣａＯやＭｇＯ濃度を増加させることでスラグを固化させ、反応性を低下させる技術が開示されている。   On the other hand, when cleaning molten steel after Al addition following decarburization treatment, the FeO concentration in the slag is reduced or the slag is solidified by increasing the CaO or MgO concentration in the slag, reducing the reactivity. Techniques for making them disclosed are disclosed.

特許文献４では、「大気圧もしくはそれ以下の状態で溶融金属をそれに可溶なガスでバブリングして該溶融金属中にガスを溶解せしめ、その後急速に減圧して溶融金属中に微細ガス気泡を発生させると共に、この減圧で該溶融金属中に溶け残っているバブリングガスの脱ガスを合わせて行い、溶融金属中に浮遊する介在物をバブリングによるガス気泡および減圧により発生した微細ガス気泡にトラップせしめて、浮上後これを除去すること」を特徴とする溶融金属の減圧清浄化方法が開示されている。この技術は溶鋼中の浮上除去促進を狙ったものである。   In Patent Document 4, “a molten metal is bubbled with a gas soluble in the molten metal at atmospheric pressure or lower to dissolve the gas in the molten metal, and then the pressure is rapidly reduced to form fine gas bubbles in the molten metal. At the same time, the bubbling gas remaining in the molten metal is degassed at this reduced pressure, and the inclusions floating in the molten metal are trapped in the gas bubbles generated by bubbling and the fine gas bubbles generated by the reduced pressure. Then, a method for cleaning the molten metal under reduced pressure is disclosed, which is characterized by removing it after rising. This technology aims to promote floating removal in molten steel.

また、特許文献５では、「筒状槽内圧力を減圧して精錬を開始してから一定時間経過するまでは、該溶鋼への浸漬管の浸漬深さを通常の精錬時より浅くし、その後に通常精錬時の浸漬深さとして精錬すること」を特徴とするＲＨ脱ガス装置による溶鋼の精錬方法、ならびに、「溶鋼にアルミニウムを添加して脱酸を開始してから一定時間経過するまでは、前記浸漬管の浸漬深さを通常の精錬時より浅くし、その後に通常精錬時の浸漬深さとして精錬すること」を特徴とするＲＨ脱ガス装置による溶鋼の精錬方法が開示されている。この技術の前半は、精錬開始から一定時間経過するまで浸漬管の浸漬深さを浅くすることで、一端真空槽内に吸い込んだスラグを取鍋内に排出し易くすることを狙ったものである。また、後半は、Ａｌ添加により真空槽内に生成した脱酸生成物を速やかに排出することを狙ったものである。この技術は浸漬管の浸漬深さを変えることで溶鋼を清浄化するものであるが、主に真空槽内の取鍋スラグもしくは脱酸生成物からの汚染を防ぐことを狙った技術であり、清浄化の効率向上を指向するものではない。また、この技術は、脱炭精錬効率に関しては、「従来例と何ら遜色がない」と記載するに留まっている。   Further, in Patent Document 5, “until a certain time elapses after the pressure inside the cylindrical tank is reduced and the refining is started, the immersion depth of the dip tube in the molten steel is made shallower than that during normal refining, and thereafter To the refining method of the molten steel by the RH degassing apparatus, characterized by “refining as the immersion depth during normal refining”, and “until a certain time has elapsed since the start of deoxidation by adding aluminum to the molten steel Further, a method for refining molten steel using an RH degassing apparatus is disclosed, characterized in that the immersion depth of the dip tube is made shallower than that during normal refining, and then refining is performed as the immersion depth during normal refining. The first half of this technology aims to make it easier to discharge the slag sucked into the vacuum chamber into the pan by reducing the immersion depth of the dip tube until a certain time has elapsed since the start of refining. . The second half aims to quickly discharge the deoxidation product generated in the vacuum chamber by addition of Al. This technology is to purify the molten steel by changing the immersion depth of the dip tube, but it is mainly aimed at preventing contamination from ladle slag or deoxidation products in the vacuum chamber, It is not intended to improve the efficiency of cleaning. In addition, this technology merely describes that “decarburizing and refining efficiency is not inferior to the conventional example”.

特開平４−１４１５１２号公報JP-A-4-141512 特許昭６０−２１２０７号公報Japanese Patent No. 60-21207 特開平３−１９７６１４号公報Japanese Patent Laid-Open No. 3-197614 特開平２−０９９２６３号公報JP-A-2-099263 特開２００３−２６８４３９号公報JP 2003-268439 A

北村ら：鉄と鋼 ８０（１９９４）、２１３．Kitamura et al .: Iron and Steel 80 (1994), 213.

近年、転炉等の製鋼炉での処理時間の高速化、並びに、連続鋳造時の鋳造速度の高速化が進められており、生産の高効率化のためには二次精錬での高速処理化が必須である。また、二次精錬で高速処理化した場合であっても、成分や清浄度が常に安定していることが必要である。さらに、これらは真空槽および浸漬管の耐火物の溶損を抑えて安価に達成されることと両立する技術であることが必要である。   In recent years, the processing time in steelmaking furnaces such as converters has been increased, and the casting speed during continuous casting has been increased. To increase production efficiency, the processing speed has been increased in secondary refining. Is essential. Further, even when the high-speed processing is performed by secondary refining, it is necessary that the components and cleanliness are always stable. Furthermore, these are required to be techniques that are compatible with being achieved at a low cost by suppressing the melting loss of the refractories in the vacuum chamber and the dip tube.

本発明は上記した課題に鑑みてなされたものであり、その目的は、環流型脱ガス装置を用いて、真空槽および浸漬管の耐火物の溶損を抑えるとともに脱炭末期の脱炭速度を向上させ、更には脱炭処理に続くＡｌ添加後の溶鋼の清浄化処理時間を短縮する手法を安価に提供することである。   The present invention has been made in view of the above-described problems. The purpose of the present invention is to reduce the melting loss of the refractories in the vacuum tank and the dip tube and reduce the decarburization rate at the end of the decarburization using a reflux degassing apparatus. It is to provide an inexpensive method for improving the cleaning time of molten steel after addition of Al following decarburization.

本発明者らは、上記課題を解決するにあたり、環流処理中の浸漬管の浸漬深さに着目して検討を重ねた。その結果、脱炭開始直後は浸漬管の浸漬深さを従来より浅くした状態、つまり、真空槽内の湯面から真空槽の槽底までの溶鋼深さを従来より浅い状態にしても脱炭速度は従来と同様であり、浸漬管の耐火物保護の面では有利であることを知見した。また、脱炭がある程度進んだ後に浸漬管の浸漬深さを深くし、内部脱炭が停滞する脱炭末期の気泡浮上距離を確保し、気泡表面積を増大させることで、気泡脱炭が促進して脱炭末期の脱炭速度が向上することを知見した。   In order to solve the above-mentioned problems, the present inventors have repeatedly studied focusing on the immersion depth of the dip tube during the reflux treatment. As a result, immediately after the start of decarburization, decarburization is performed even when the immersion depth of the dip tube is shallower than before, that is, even when the molten steel depth from the molten metal surface in the vacuum chamber to the bottom of the vacuum chamber is shallower than before. The speed was the same as before, and it was found that it was advantageous in terms of refractory protection of the dip tube. In addition, after the decarburization has progressed to some extent, the immersion depth of the dip tube is increased to ensure the bubble floating distance at the end of decarburization where internal decarburization is stagnant, and by increasing the bubble surface area, bubble decarburization is promoted. It was found that the decarburization speed at the end of decarburization was improved.

さらに、脱炭処理に続くＡｌ添加後において、浸漬管の浸漬深さを浅くし、真空槽内の湯面から真空槽の槽底までの溶鋼深さを従来より浅くした状態で環流処理すると、真空槽内の溶鋼量が低下することになるため、単位体積当たりの攪拌動力密度が大きくなり、非金属介在物の凝集、浮上除去が促進され、清浄化時間を短縮できることを知見した。本発明者らは、脱炭末期に気泡脱炭が効率的に生じる真空槽内の湯面から真空槽の槽底までの溶鋼深さおよび処理時間、脱炭処理に続くＡｌ添加後に非金属介在物の凝集、浮上除去が効率的に生じる真空槽内の湯面から真空槽の槽底までの溶鋼深さを明らかにすることで、本発明を完成するに至った。本発明は、以下の通りである。   Furthermore, after the addition of Al following the decarburization treatment, the immersion depth of the dip tube is made shallower, and the reflux treatment is performed in a state where the molten steel depth from the hot water surface in the vacuum vessel to the bottom of the vacuum vessel is made shallower than before, Since the amount of molten steel in the vacuum chamber is reduced, the stirring power density per unit volume is increased, and the non-metallic inclusions are agglomerated and lifted and removed, and the cleaning time can be shortened. The inventors of the present invention have found that the molten steel depth and treatment time from the molten metal surface in the vacuum tank to the bottom of the vacuum tank where bubble decarburization occurs efficiently at the end of decarburization, and non-metallic intervention after the addition of Al following the decarburization process The present invention has been completed by clarifying the depth of molten steel from the surface of the vacuum tank to the bottom of the vacuum chamber where the agglomeration and levitating removal of the objects are efficiently performed. The present invention is as follows.

本発明は、以下の通りである。
（１）製鋼炉から取鍋に出鋼した溶鋼を環流型脱ガス装置で精錬するにあたり、脱炭開始時点から少なくとも５分間は、（１）式で求まる真空槽内の湯面から真空槽の槽底までの溶鋼深さＤが５０ｍｍ以上１００ｍｍ未満の範囲で脱炭処理を行うとともに、脱炭終了前の少なくとも５分間は、前記溶鋼深さＤが４００ｍｍ以上の範囲でＣ濃度を０．００４０質量％以下に脱炭処理を行うことを特徴とする、環流型脱ガス装置における溶鋼の減圧精錬方法。 The present invention is as follows.
(1) When refining the molten steel from the steelmaking furnace to the ladle using a recirculation type degasser, at least 5 minutes from the start of decarburization, from the surface of the vacuum tank obtained by equation (1), While decarburization is performed in the range where the molten steel depth D to the tank bottom is 50 mm or more and less than 100 mm, the C concentration is 0.0040 in the range where the molten steel depth D is 400 mm or more for at least 5 minutes before the end of decarburization. A depressurizing method for molten steel in a recirculation type degassing apparatus, wherein the decarburization treatment is performed to a mass% or less.

Ｄ＝（Ｐ０−Ｐ）／（ρ・ｇ）＋Ｈ−Ｌ ・・・（１）
Ｄ：真空槽内の湯面から真空槽の槽底までの溶鋼深さ（ｍ）、Ｐ０：大気圧（Ｐａ）、Ｐ：真空槽内の圧力（Ｐａ）、ρ：溶鋼密度（ｋｇ／ｍ３）、ｇ：重力加速度（ｍ／ｓ２）、Ｈ：浸漬管の浸漬深さ（ｍ）、Ｌ：浸漬管下端から真空槽内の槽底までの長さ（ｍ）
（２）前記脱炭処理に続くＡｌ添加後に前記浸漬管の浸漬深さＨを浅くする操作によって前記溶鋼深さＤが５０ｍｍ以上１００ｍｍ未満の範囲で環流処理を行うことを特徴とする、上記（１）に記載の環流型脱ガス装置における溶鋼の減圧精錬方法。 D = (P 0 −P) / (ρ · g) + H−L (1)
D: Molten steel depth (m) from the hot water surface in the vacuum chamber to the bottom of the vacuum chamber, P 0 : atmospheric pressure (Pa), P: pressure in the vacuum chamber (Pa), ρ: molten steel density (kg / m 3 ), g: gravitational acceleration (m / s 2 ), H: immersion depth (m) of dip tube, L: length from bottom of dip tube to bath bottom in vacuum chamber (m)
(2) The reflux treatment is performed in a range where the molten steel depth D is 50 mm or more and less than 100 mm by an operation of decreasing the immersion depth H of the dip tube after the addition of Al following the decarburization treatment. A method for refining molten steel under reduced pressure in the reflux degassing apparatus according to 1).

本発明によれば、浸漬管の浸漬深さを深くすることで生じる浸漬管の耐火物損耗コスト増大を最小限に抑えつつ、脱炭に要する処理時間を短縮し、更には従来と同じ溶鋼清浄度を短時間の処理で得られる。これらは、既存の設備やプロセスを大きく変更することなく溶製可能であることから製造コストの増大を抑制可能であり、本発明の社会的貢献度は非常に大きい。   According to the present invention, the processing time required for decarburization is shortened while minimizing the increase in the refractory wear cost of the dip tube caused by increasing the immersion depth of the dip tube, and further, the same molten steel cleaning as the conventional one is performed. The degree can be obtained in a short time. Since these can be melted without greatly changing existing facilities and processes, an increase in manufacturing cost can be suppressed, and the social contribution of the present invention is very large.

図１は、環流型脱ガス装置の概略図である。FIG. 1 is a schematic view of a reflux degassing apparatus. 図２は、比較例１の真空槽内の溶鋼深さＤおよび浸漬管の浸漬深さＨと処理時間の関係を示す図である。FIG. 2 is a diagram illustrating the relationship between the molten steel depth D in the vacuum chamber of Comparative Example 1 and the immersion depth H of the dip tube and the treatment time. 図３は、比較例２の真空槽内の溶鋼深さＤおよび浸漬管の浸漬深さＨと処理時間の関係を示す図である。FIG. 3 is a view showing the relationship between the molten steel depth D in the vacuum chamber of Comparative Example 2 and the immersion depth H of the dip tube and the treatment time. 図４は、比較例３の真空槽内の溶鋼深さＤおよび浸漬管の浸漬深さＨと処理時間の関係を示す図である。FIG. 4 is a diagram showing the relationship between the treatment depth and the molten steel depth D in the vacuum chamber of Comparative Example 3 and the immersion depth H of the dip tube. 図５は、発明例１の真空槽内の溶鋼深さＤおよび浸漬管の浸漬深さＨと処理時間の関係を示す図である。FIG. 5 is a diagram showing the relationship between the molten steel depth D in the vacuum chamber of Invention Example 1 and the immersion depth H of the dip tube and the treatment time. 図６は、発明例２の真空槽内の溶鋼深さＤおよび浸漬管の浸漬深さＨと処理時間の関係を示す図である。FIG. 6 is a graph showing the relationship between the molten steel depth D in the vacuum chamber of Invention Example 2 and the immersion depth H of the dip tube and the treatment time. 図７は、発明例３の真空槽内の溶鋼深さＤおよび浸漬管の浸漬深さＨと処理時間の関係を示す図である。FIG. 7 is a view showing the relationship between the molten steel depth D in the vacuum chamber of Invention Example 3, the immersion depth H of the dip tube, and the treatment time.

１．本発明における用語の定義
「製鋼炉」とは、転炉または電気炉を指し、製鋼炉から出鋼された「溶鋼」とは、脱硫、脱りんもしくは脱炭といった一次精錬処理が実施された状態であるものとする。 1. Definition of Terms in the Present Invention “Steelmaking furnace” refers to a converter or electric furnace, and “molten steel” produced from the steelmaking furnace is a state in which primary refining treatment such as desulfurization, dephosphorization, or decarburization has been performed. Suppose that

「環流型脱ガス装置」とは、図１に示すような真空槽１を有する溶鋼処理装置であって、代表的な装置としてＲＨがある。図１に示すように、真空槽１は浸漬管５および６と連設されている。浸漬管５および６の一部は取鍋９内の溶鋼８に浸漬されている。浸漬管５には還流ガス吹込み孔４が設けられている。還流ガス３は還流ガス吹込み孔４から浸漬管内の溶鋼に吹き込まれる。本発明において、Ｈは浸漬管の浸漬深さであり、浸漬管５および６の下端から取鍋内の溶鋼の表面までの高さを表す。Ｌは浸漬管５および６の下端から真空槽１内の槽底２までの長さを表す。Ｄは真空槽１内の湯面から真空槽１の槽底２までの溶鋼深さを表す。   The “circulating degassing apparatus” is a molten steel processing apparatus having a vacuum chamber 1 as shown in FIG. 1, and RH is a representative apparatus. As shown in FIG. 1, the vacuum chamber 1 is connected to the dip tubes 5 and 6. Part of the dip tubes 5 and 6 are immersed in the molten steel 8 in the ladle 9. The dip tube 5 is provided with a reflux gas blowing hole 4. The reflux gas 3 is blown into the molten steel in the dip tube through the reflux gas blowing hole 4. In the present invention, H is the immersion depth of the dip tube, and represents the height from the lower end of the dip tubes 5 and 6 to the surface of the molten steel in the ladle. L represents the length from the lower end of the dip tubes 5 and 6 to the tank bottom 2 in the vacuum tank 1. D represents the depth of molten steel from the hot water surface in the vacuum chamber 1 to the bottom 2 of the vacuum chamber 1.

「環流処理」とは、環流型脱ガス装置を用いて、取鍋に溶鋼を受鋼している状態で、真空槽内の圧力Ｐを低下させることで溶鋼を真空槽に吸い上げ、環流ガスを流すことで、溶鋼を取鍋と真空槽間で循環させる操作を指す。   The “circulation treatment” means that the molten steel is sucked into the vacuum tank by reducing the pressure P in the vacuum tank in a state where the molten steel is received in the ladle using the reflux type degassing device, and the reflux gas is By flowing, it refers to the operation of circulating molten steel between the ladle and the vacuum chamber.

「脱炭処理」とは、上記環流処理に関連し、真空槽内の圧力Ｐを低下させて（Ａ）式に示す脱炭反応を生じさせる処理を指す。（Ａ）式は主に溶鋼中の炭素と酸素の反応であるため、Ａｌ等で溶鋼を脱酸し、溶鋼中の酸素濃度が低減した状態では、脱炭反応は顕著に生じない。本発明においては、溶鋼中に多量のＣとＯが存在し、減圧処理することで（Ａ）式に示す反応が生じている、Ａｌ添加する前までの状態を指す。   The “decarburization process” refers to a process related to the above-described reflux process, in which the pressure P in the vacuum chamber is reduced to cause the decarburization reaction shown in the formula (A). Since the formula (A) is mainly a reaction between carbon and oxygen in the molten steel, the decarburization reaction does not remarkably occur when the molten steel is deoxidized with Al or the like and the oxygen concentration in the molten steel is reduced. In the present invention, a large amount of C and O is present in the molten steel, and the reaction shown in the formula (A) is caused by the reduced pressure treatment before the addition of Al.

Ｃ＋Ｏ＝ＣＯ（ｇ） ・・・（Ａ）
２．処理手順
２．（１） 脱炭開始時から少なくとも５分間：溶鋼深さＤ：５０ｍｍ以上１００ｍｍ未満
本発明において、溶鋼は製鋼炉から取鍋に出鋼された後、環流型脱ガス装置にて減圧処理される。この時、脱炭開始時から少なくとも５分間は、（１）式で求まる真空槽内の湯面から真空槽の槽底までの溶鋼深さＤが５０ｍｍ以上１００ｍｍ未満であることが必要である。 C + O = CO (g) (A)
2. Processing procedure (1) At least 5 minutes from the start of decarburization: Molten steel depth D: 50 mm or more and less than 100 mm In the present invention, the molten steel is removed from the steelmaking furnace into a ladle and then subjected to reduced pressure treatment in a circulating degasser. . At this time, for at least 5 minutes from the start of decarburization, it is necessary that the molten steel depth D from the molten metal surface in the vacuum tank to the tank bottom of the vacuum tank obtained by the equation (1) be 50 mm or more and less than 100 mm.

Ｄ＝（Ｐ０−Ｐ）／（ρ・ｇ）＋Ｈ−Ｌ ・・・（１）
Ｄは真空槽内の湯面から真空槽の槽底までの溶鋼深さ（ｍ）、Ｐ０は大気圧（Ｐａ）、Ｐは真空槽内の圧力（Ｐａ）、ρは溶鋼密度（ｋｇ／ｍ３）、ｇは重力加速度（ｍ／ｓ２）、Ｈは浸漬管の浸漬深さ（ｍ）、Ｌは浸漬管下端から真空槽内の槽底までの長さ（ｍ）を表す。 D = (P 0 −P) / (ρ · g) + H−L (1)
D is the depth of molten steel (m) from the molten metal surface in the vacuum chamber to the bottom of the vacuum chamber, P 0 is atmospheric pressure (Pa), P is the pressure (Pa) in the vacuum chamber, and ρ is the molten steel density (kg / kg). m 3 ) and g are gravitational accelerations (m / s 2 ), H is the immersion depth (m) of the dip tube, and L is the length (m) from the lower end of the dip tube to the bath bottom in the vacuum chamber.

環流型脱ガス装置で環流処理を行う場合、脱炭初期はＣＯ濃度積が大きく、真空槽内の溶鋼の深い部分から内部脱炭が生じることになるが、初期の脱炭速度は大きいため、脱炭が開始するとＣＯ濃度積は急激に減少し、真空槽内の溶鋼の深い部分からの脱炭は沈静化し、主に溶鋼表面から１００ｍｍ程度以下で反応が生じるようになる。このため、脱炭前半では環流が生じる溶鋼深さＤが確保されていれば、必要以上に真空槽内の溶鋼量を増加させても効果は無い。また、早い段階で浸漬管を深く浸漬すると、浸漬管の耐火物が溶損することで溶製コストが増加してしまう。したがって、溶鋼深さの上限は１００ｍｍ未満とする。   When performing reflux treatment with a reflux type degassing device, the CO concentration product is large at the beginning of decarburization, and internal decarburization occurs from a deep portion of the molten steel in the vacuum tank, but the initial decarburization rate is large, When decarburization starts, the CO concentration product decreases rapidly, decarburization from a deep portion of the molten steel in the vacuum chamber is calmed down, and a reaction mainly occurs at about 100 mm or less from the molten steel surface. For this reason, in the first half of the decarburization, if the molten steel depth D at which recirculation occurs is secured, there is no effect even if the amount of molten steel in the vacuum chamber is increased more than necessary. In addition, if the dip tube is deeply immersed at an early stage, the refractory material of the dip tube is melted and the melting cost increases. Therefore, the upper limit of the molten steel depth is less than 100 mm.

一方、脱炭時の還流をバラツキなく行うためには、溶鋼深さＤは５０ｍｍ以上必要となる。なお、本発明においては、溶鋼深さを５０ｍｍ程度に低減したとしても問題なく溶鋼が還流し、還流のバラツキは見られなかった。したがって、溶鋼深さＤを５０ｍｍ以上１００ｍｍ未満に維持する期間は、真空槽内の溶鋼中Ｃ濃度が０．００５０質量％までとすることが好ましい。このＣ濃度は、後述するように、排ガス濃度を用いたＣ濃度推定手法によって脱炭処理中に把握することができる。   On the other hand, in order to perform the reflux at the time of decarburization without variation, the molten steel depth D is required to be 50 mm or more. In the present invention, even when the depth of the molten steel was reduced to about 50 mm, the molten steel recirculated without any problem, and no variation in recirculation was observed. Therefore, it is preferable that the C concentration in the molten steel in the vacuum chamber is up to 0.0050 mass% during the period of maintaining the molten steel depth D at 50 mm or more and less than 100 mm. This C concentration can be grasped during the decarburization process by a C concentration estimation method using the exhaust gas concentration, as will be described later.

上記処理時間に関して例示すると、例えば脱炭時間が１０分間である場合、前半の５分間は溶鋼深さＤを５０ｍｍ以上１００ｍｍｍ未満とし、後半の５分間は後述のように溶鋼深さＤを４００ｍｍ以上とする必要がある。また、脱炭時間が２０分間である場合、少なくとも脱炭開始時から５分間は前述のように従来よりも溶鋼深さＤを浅くする。浸漬管耐火物の溶損低減効果を考慮すると、少なくとも５分間は必要である。この時間はこの効果を発揮するためにさらに長くても構わない。また、このような時間帯ではＣ濃度が０．００５０質量％以上であるため、Ｃ濃度は浴鋼深さＤを浅く維持する時間を決めるための目安となる。   For example, when the decarburization time is 10 minutes, the molten steel depth D is set to 50 mm or more and less than 100 mm for the first 5 minutes, and the molten steel depth D is set to 400 mm or more for the latter 5 minutes as described later. It is necessary to. Further, when the decarburization time is 20 minutes, the molten steel depth D is made shallower than before as described above for at least 5 minutes from the start of decarburization. Considering the effect of reducing the melting loss of the dip tube refractory, at least 5 minutes are necessary. This time may be longer to exhibit this effect. Further, since the C concentration is 0.0050 mass% or more in such a time zone, the C concentration is a guideline for determining the time for keeping the bath steel depth D shallow.

そして、脱炭終了前の少なくとも５分間は後述のように溶鋼深さＤを深くする。この時間は、製品のＣ濃度によって調整すれば良く、５分間以上であっても構わない。   And the molten steel depth D is made deep as mentioned later for at least 5 minutes before the end of decarburization. This time may be adjusted according to the C concentration of the product, and may be 5 minutes or more.

この時、真空槽内の圧力Ｐに応じて浸漬管の浸漬深さＨを調整し、溶鋼深さＤを浅くすることで、真空槽内の溶鋼の概ね全量を脱炭領域にできることに加え、内部脱炭が生じる際に槽底の耐火物がＣＯ気泡生成核としても作用することになる。また、環流中の真空槽内は上昇管側から溶鋼が常に供給されており、静止状態の溶鋼深さよりも実際の溶鋼深さは高くなる。このため、脱炭初期に溶鋼深さＤを１００ｍｍ未満にしても、溶鋼深さＤを１００ｍｍ以上と比較した場合に脱炭速度に大きな差異は生じない。このため、溶鋼深さＤを１００ｍｍ未満とすることで、脱炭効率を維持したままで浸漬管の溶損を低減できる。一方、溶鋼深さＤが５０ｍｍ未満になると下降管への溶鋼供給が不足し、溶鋼環流量が低下して操業を阻害する可能性があるため、溶鋼深さＤは５０ｍｍ以上であることが必要である。本発明では、溶鋼深さＤがこの範囲内であれば、処理中に真空槽内の溶鋼湯面が揺らいでも所望の特性が得られる。   At this time, in addition to adjusting the immersion depth H of the dip tube according to the pressure P in the vacuum chamber and making the molten steel depth D shallow, almost all of the molten steel in the vacuum chamber can be decarburized, When internal decarburization occurs, the refractory at the bottom of the tank also acts as a CO bubble generation nucleus. Moreover, the molten steel is always supplied from the riser side in the vacuum tank in the reflux, and the actual molten steel depth becomes higher than the molten steel depth in a stationary state. For this reason, even if the molten steel depth D is less than 100 mm in the initial stage of decarburization, there is no significant difference in the decarburization speed when the molten steel depth D is compared with 100 mm or more. For this reason, by making the molten steel depth D less than 100 mm, it is possible to reduce the melting loss of the dip tube while maintaining the decarburization efficiency. On the other hand, when the molten steel depth D is less than 50 mm, the molten steel supply to the downcomer pipe is insufficient, and the molten steel ring flow rate may be reduced to hinder the operation. Therefore, the molten steel depth D needs to be 50 mm or more. It is. In the present invention, if the molten steel depth D is within this range, desired characteristics can be obtained even if the molten steel surface in the vacuum chamber fluctuates during processing.

また、脱炭開始時から少なくとも５分間での環流ガス流量Ｑは、効率的に精錬するため、５．７Ｎｌ／（ｍｉｎ・ｔｏｎ）以上であることが望ましく、６．１Ｎｌ／（ｍｉｎ・ｔｏｎ）以上であることがより望ましく、６．５Ｎｌ／（ｍｉｎ・ｔｏｎ）以上であることが更に望ましい。   In addition, the reflux gas flow rate Q in at least 5 minutes from the start of decarburization is preferably 5.7 Nl / (min · ton) or more for efficient refining, and 6.1 Nl / (min · ton). More preferably, it is more preferably 6.5 Nl / (min · ton) or more.

２．（２） 脱炭終了前の少なくとも５分間：溶鋼深さＤ：４００ｍｍ以上
脱炭が進み、脱炭末期になると、内部脱炭が停滞し、溶鋼表面および気泡表面で生じる脱炭が主体となる。この段階では、真空槽内の溶鋼表面積を増加させる、もしくは、気泡表面積を増加させることにより、脱炭速度が向上できる。 2. (2) At least 5 minutes before the end of decarburization: Molten steel depth D: 400 mm or more When decarburization progresses and the end of decarburization is reached, internal decarburization stagnates and mainly decarburization occurs on the molten steel surface and bubble surface. . At this stage, the decarburization rate can be improved by increasing the surface area of the molten steel in the vacuum chamber or increasing the surface area of the bubbles.

環流型脱ガス装置で環流処理する場合、環流が生じている状態では、上昇管内は１ｍ／ｓ程度で溶鋼が上昇しており、真空槽内には多量の溶鋼が飛散しており、環流処理中は真空槽の幾何断面積以上の溶鋼表面積となると考えられる。しかしながら、飛散する溶鋼量や飛散する溶鋼粒の大きさを意図的に制御できないため、溶鋼表面積を増加させるのは困難である。また、スプラッシュの活用を考えた場合、真空槽内に地金が付着し、操業阻害を生じさせてしまう頻度が増大してしまう。   When recirculation treatment is performed with a recirculation type degassing device, in the state where recirculation occurs, the molten steel rises at about 1 m / s in the riser, and a large amount of molten steel is scattered in the vacuum chamber. The inside is considered to have a molten steel surface area greater than the geometric cross-sectional area of the vacuum chamber. However, it is difficult to increase the surface area of the molten steel because the amount of the molten steel scattered and the size of the molten steel grains scattered cannot be controlled intentionally. Moreover, when the utilization of splash is considered, the frequency that a bullion adheres in a vacuum chamber and causes operation obstruction increases.

特許文献１では、溶鋼深さＤを２０ｃｍ以下として処理することでスプラッシュ量を増大させ、表面脱炭を促進させている。その結果、表面脱炭が促進されることで脱炭末期の脱炭速度が向上する効果が得られている。脱炭末期の脱炭速度を向上させるには表面脱炭もしくは気泡脱炭を活用することになるが、特許文献１では表面脱炭に着目している。溶鋼深さＤを操作し、浅くした場合は表面脱炭が促進されるが、気泡脱炭が抑制され、深くした場合は気泡脱炭が促進される。特許文献１では、前者を選択しており、脱炭速度が向上する効果は得られると考えられるが、本来内部脱炭が停滞することでスプラッシュも抑制されている時期に意図的にスプラッシュを発生させることになるため、地金付きの影響が無視できなくなる。このことを考慮すると、脱炭末期に溶鋼表面積を増大させるよりも、後者のように気泡表面積を増大させた方が溶製コストを抑制しながら脱炭速度を向上できると考えられる。   In patent document 1, the amount of splash is increased by processing the molten steel depth D as 20 cm or less, and surface decarburization is promoted. As a result, the effect of improving the decarburization rate at the end of decarburization is obtained by promoting surface decarburization. In order to improve the decarburization speed at the end of decarburization, surface decarburization or bubble decarburization is used. However, Patent Document 1 focuses on surface decarburization. When the molten steel depth D is manipulated and made shallower, surface decarburization is promoted, but bubble decarburization is suppressed, and when it is made deeper, bubble decarburization is promoted. In Patent Document 1, the former is selected, and it is considered that the effect of improving the decarburization speed is obtained. However, the splash is intentionally generated at the time when the splash is also suppressed due to the stagnation of internal decarburization. As a result, the effects of bullion cannot be ignored. Considering this, it is considered that the decarburization rate can be improved while suppressing the melting cost by increasing the bubble surface area as in the latter case, rather than increasing the molten steel surface area at the end of decarburization.

この状況で、気泡脱炭を促進させるには、真空槽内の気泡表面積を増加させれば良いことになるが、環流ガス流量Ｑが一定とした場合、浸漬管の浸漬深さＨを変えて気泡上昇距離を確保すれば良いことになる。この操作では、環流ガス流量Ｑはそのままの状態で、気泡界面積のみを増加でき、スプラッシュの増加も生じない。しかしながら、浸漬深さＨを深くすると、耐火物の溶損が加速されるため、溶製コストが悪化する。この時、耐火物溶損に伴う溶製コストの悪化よりも、脱炭末期の脱炭速度を向上させたことによる溶製コスト改善の効果を得るには、溶鋼深さＤを４００ｍｍ以上確保することが必要である。溶鋼深さＤは５００ｍｍ以上であることが望ましい。   In this situation, in order to promote bubble decarburization, the bubble surface area in the vacuum chamber may be increased. However, when the reflux gas flow rate Q is constant, the immersion depth H of the dip tube is changed. It is sufficient to ensure the bubble rising distance. In this operation, while the reflux gas flow rate Q remains unchanged, only the bubble interfacial area can be increased, and no splash increases. However, when the immersion depth H is increased, melting loss of the refractory is accelerated, so that the melting cost is deteriorated. At this time, in order to obtain the effect of improving the melting cost by improving the decarburization speed at the end of the decarburization rather than the deterioration of the melting cost due to refractory melting, the molten steel depth D is secured to 400 mm or more. It is necessary. The molten steel depth D is desirably 500 mm or more.

溶鋼深さＤを調整するための時間は、通常１分以内である。
また、この時の真空槽内の圧力Ｐを精緻に制御することが望ましい。真空槽内の気泡挙動は真空槽内の圧力に大きく依存することに加え、本発明では溶鋼深さＤを４００ｍｍ以上確保することから、環流ガス吹込み孔での溶鋼静圧も通常よりも大きくなる。このため、溶鋼深さＤを４００ｍｍ以上とした時点で真空槽内の圧力Ｐが３００Ｐａよりも大きい状況である場合、環流ガスが溶鋼に吹き込まれた直後の気泡が合体して相対的に気泡表面積が減少してしまうことに加え、溶鋼表面近傍での気泡膨張が不足し、気泡表面積の増大効果が十分に得られない場合がある。このため、溶鋼深さＤを４００ｍｍ以上確保している間は終始真空槽内の圧力Ｐを３００Ｐａ以下とすることが好ましい。なお、真空槽内の圧力Ｐは１５０Ｐａ以下であることがより好ましい。 The time for adjusting the molten steel depth D is usually within 1 minute.
It is desirable to precisely control the pressure P in the vacuum chamber at this time. In addition to the fact that the bubble behavior in the vacuum chamber greatly depends on the pressure in the vacuum chamber, and in the present invention, the molten steel depth D is ensured to be 400 mm or more, the molten steel static pressure in the reflux gas injection hole is also larger than usual. Become. For this reason, when the pressure P in the vacuum chamber is larger than 300 Pa when the molten steel depth D is set to 400 mm or more, the bubbles immediately after the reflux gas is blown into the molten steel are merged to form a relative bubble surface area. In addition, the bubble expansion near the molten steel surface is insufficient, and the effect of increasing the bubble surface area may not be sufficiently obtained. For this reason, while ensuring the molten steel depth D of 400 mm or more, it is preferable to make the pressure P in a vacuum chamber into 300 Pa or less throughout. The pressure P in the vacuum chamber is more preferably 150 Pa or less.

さらに、脱炭末期は内部脱炭が沈静化していることから、気泡脱炭促進効果を十分得るために、脱炭終了前の短くとも５分間は溶鋼深さＤを４００ｍｍ以上確保することが必要である。この条件は、前記した真空槽内の溶鋼中Ｃ濃度が０．００５０質量％未満になった時点以降、脱炭終了時点まで維持することが好ましい。   Furthermore, since the internal decarburization is calmed at the end of decarburization, it is necessary to secure the molten steel depth D to 400 mm or more for at least 5 minutes before the end of decarburization in order to sufficiently obtain the effect of promoting the decarburization of bubbles. It is. This condition is preferably maintained from the time when the C concentration in the molten steel in the vacuum chamber is less than 0.0050 mass% to the end of decarburization.

本発明では、溶鋼深さＤや真空槽内の圧力Ｐがこれらの範囲内であれば、処理中に真空槽内の溶鋼湯面が揺らいでも所望の特性が得られる。   In the present invention, if the molten steel depth D and the pressure P in the vacuum chamber are within these ranges, desired characteristics can be obtained even if the molten steel surface in the vacuum chamber fluctuates during processing.

特許文献５では、脱炭末期に溶鋼深さＤを４１０〜４２０ｍｍとして真空脱炭処理をしているが、この発明は真空脱炭処理の直前に酸素吹錬して真空槽内の圧力が１００〜５０ｔｏｒｒである状態から真空脱炭処理している。一般的に真空槽内の圧力を５０ｔｏｒｒから２ｔｏｒｒ近傍まで低下させるには２分から３分を要する。特許文献５では、発明例、比較例を含めて溶鋼深さＤが４１０〜４２０ｍｍとして真空脱炭処理しているが、「脱炭精錬効率に関しては従来例と何ら遜色がない」となっている。そこで、本発明では、気泡脱炭促進効果を明確に得るには溶鋼深さＤを４００ｍｍ以上にすることに加え、真空槽内の圧力Ｐを低値制御することが望ましい。   In Patent Document 5, vacuum decarburization processing is performed with a molten steel depth D of 410 to 420 mm at the end of decarburization. However, in the present invention, oxygen is blown immediately before vacuum decarburization and the pressure in the vacuum chamber is 100. Vacuum decarburization is performed from a state of ˜50 torr. Generally, it takes 2 to 3 minutes to reduce the pressure in the vacuum chamber from 50 torr to around 2 torr. In Patent Document 5, vacuum decarburization processing is performed with a molten steel depth D of 410 to 420 mm including the invention example and the comparative example, but “the decarburization refining efficiency is no different from the conventional example”. . Therefore, in the present invention, in order to clearly obtain the effect of promoting bubble decarburization, in addition to setting the molten steel depth D to 400 mm or more, it is desirable to control the pressure P in the vacuum chamber to a low value.

また、環流ガス流量Ｑは、効率的に精錬するため、５．７Ｎｌ／（ｍｉｎ・ｔｏｎ）以上であることが望ましく、６．１Ｎｌ／（ｍｉｎ・ｔｏｎ）以上であることがより望ましく、６．５Ｎｌ／（ｍｉｎ・ｔｏｎ）以上であることが更に望ましい。   Further, the recirculation gas flow rate Q is preferably 5.7 Nl / (min · ton) or more, more preferably 6.1 Nl / (min · ton) or more, in order to efficiently refining. More preferably, it is 5 Nl / (min · ton) or more.

このような条件で脱炭処理を行うと、処理後には溶鋼中Ｃ濃度が０．００４０質量％以下に低減される。好ましくは０．００３０質量％以下である。つまり、本発明はこのような極低炭素鋼を製造するための溶鋼の減圧製錬方法であると言える。なお、本発明において、「脱炭終了時点」とは、Ａｌ添加時点のことを表し、脱炭濃度がこれらの値になった時点ではない。   When the decarburization process is performed under such conditions, the C concentration in the molten steel is reduced to 0.0040% by mass or less after the process. Preferably it is 0.0030 mass% or less. In other words, it can be said that the present invention is a method for vacuum smelting of molten steel for producing such an ultra-low carbon steel. In the present invention, the “decarburization end time” means the time when Al is added, and is not the time when the decarburization concentration reaches these values.

２．（３） Ａｌ添加後：溶鋼深さＤ：５０ｍｍ以上１００ｍｍ未満
脱炭が終了した段階で、溶鋼にＡｌを添加し溶鋼を脱酸するとともに、溶鋼を環流させることによる清浄化処理を行う。Ａｌ添加により、溶鋼中には大量のＡｌ２Ｏ３が生成することになるが、これらＡｌ２Ｏ３が製品段階まで残存すると、製品段階で欠陥を生じるため、環流型脱ガス装置での環流処理でこれら非金属介在物を除去する。この時、溶鋼深さＤは５０ｍｍ以上１００ｍｍ未満であることが望ましい。溶鋼深さＤを上記範囲として環流処理すると、真空槽内の溶鋼量は少なくなるため、溶鋼の単位体積あたりの攪拌動力密度は大きくなることに加え、湯面までの距離が短くなり、浮上に要する時間が短くなるため、溶鋼中の非金属介在物は凝集、浮上が促進される。このため、同じ清浄性を確保するために必要な環流処理時間は、溶鋼深さＤを１００ｍｍ以上とした場合と比較して短くできる。一方、溶鋼深さが５０ｍｍ未満では、脱炭処理と同様に下降管への溶鋼供給が不足し、溶鋼環流量が低下して操業を阻害する可能性がある。したがって、Ａｌ添加後の溶鋼深さは５０ｍｍ以上１００ｍｍ未満とすることが望ましい。 2. (3) After Al addition: Molten steel depth D: 50 mm or more and less than 100 mm At the stage where decarburization is finished, Al is added to the molten steel to deoxidize the molten steel, and cleaning treatment is performed by circulating the molten steel. A large amount of Al 2 O 3 is produced in the molten steel due to the addition of Al, but if these Al 2 O 3 remains in the product stage, defects occur in the product stage. These non-metallic inclusions are removed by treatment. At this time, the molten steel depth D is desirably 50 mm or more and less than 100 mm. When the molten steel depth D is in the above range, the amount of molten steel in the vacuum chamber is reduced, so that the stirring power density per unit volume of the molten steel is increased, and the distance to the molten metal surface is shortened, resulting in floating. Since the time required is shortened, the nonmetallic inclusions in the molten steel are promoted to agglomerate and float. For this reason, the recirculation | reflux processing time required in order to ensure the same cleanliness can be shortened compared with the case where the molten steel depth D is 100 mm or more. On the other hand, if the molten steel depth is less than 50 mm, the molten steel supply to the downcomer is insufficient as in the decarburization process, and the flow rate of the molten steel ring may be reduced to hinder the operation. Therefore, it is desirable that the molten steel depth after addition of Al is 50 mm or more and less than 100 mm.

なお、環流処理中は最後まで真空槽内の圧力Ｐを低くしておく必要はなく、環流が生じる程度の真空槽内の圧力Ｐであれば、真空槽内の圧力Ｐを高くしても良いが、この場合、真空槽内の圧力Ｐに応じて浸漬管の浸漬深さＨを変え、溶鋼深さＤを５０ｍｍ以上１００ｍｍ未満に調整すれば良い。溶鋼深さＤを調整するための時間は、通常１分以内である。   During the reflux treatment, it is not necessary to keep the pressure P in the vacuum chamber low until the end, and the pressure P in the vacuum chamber may be increased as long as the pressure P in the vacuum chamber is sufficient to cause reflux. However, in this case, the immersion depth H of the dip tube may be changed according to the pressure P in the vacuum chamber, and the molten steel depth D may be adjusted to 50 mm or more and less than 100 mm. The time for adjusting the molten steel depth D is usually within 1 minute.

溶鋼深さＤは、この範囲内であれば途中で真空槽内の溶鋼湯面が揺らいでも所望の特性が得られる。   If the molten steel depth D is within this range, desired characteristics can be obtained even if the molten steel surface in the vacuum chamber fluctuates midway.

３．溶製時の溶鋼中Ｃ濃度の推定方法
本発明において、浸漬管の浸漬深さＨを変更するタイミングを決めるため、溶鋼中のＣ濃度を知ることができると好都合である。溶鋼中のＣ濃度は、特許文献６に記載の方法を用い、排ガス情報および既知の操業条件から算出すれば良い。特許文献６に記載の方法は、排ガス情報に基づくＣ濃度の推定方法であるため、排ガスの分析遅れ等に伴う実際の溶鋼中Ｃ濃度との差違が生じるが、本発明では浸漬管の浸漬深さＨの変更タイミングを決める手段としての利用を想定しており、その用途であれば問題なく適用できる。 3. Method for Estimating C Concentration in Molten Steel During Melting In the present invention, it is advantageous if the C concentration in molten steel can be known in order to determine the timing for changing the immersion depth H of the dip tube. The C concentration in the molten steel may be calculated from exhaust gas information and known operating conditions using the method described in Patent Document 6. The method described in Patent Document 6 is an estimation method of C concentration based on exhaust gas information, and therefore there is a difference from the actual C concentration in molten steel due to analysis delay of exhaust gas. In the present invention, the immersion depth of the dip tube It is assumed to be used as a means for determining the change timing of the height H, and can be applied without any problem if it is used.

４．効果の確認方法
本発明では、脱炭速度向上効果を確認するため、脱炭処理末期に連続採取したサンプルの分析値をもとに、Ｃ濃度が０．００２０％になるまでに要した時間を算出した。同様に、Ａｌ添加後の環流処理末期に連続採取したサンプルの分析値をもとに、Ａｌ添加してから酸不溶性Ａｌ濃度が０．００２０％になるまでに要した時間を算出した。効果を比較するため、対象鋼種やＡｌ添加量は同じ条件とした。また、同じ処理条件で複数回の環流処理した後の耐火物の厚み測定結果から耐火物溶損速度および耐火物コストを算出し、処理時間短縮効果を含めた溶製コストを算出し、条件毎に比較した。 4). Method for confirming the effect In the present invention, in order to confirm the effect of improving the decarburization rate, the time required until the C concentration becomes 0.0020% based on the analytical value of the sample continuously collected at the end of the decarburization treatment is used. Calculated. Similarly, based on the analytical value of the sample continuously collected at the end of the reflux treatment after the addition of Al, the time required from the addition of Al until the acid-insoluble Al concentration reached 0.0020% was calculated. In order to compare the effects, the target steel type and Al addition amount were set to the same conditions. In addition, the refractory melting rate and refractory cost are calculated from the refractory thickness measurement results after multiple recirculation treatments under the same processing conditions, and the smelting cost including the effect of shortening the processing time is calculated. Compared to.

製鋼炉から出鋼した、Ｃ濃度が０．０３３％の溶鋼３５０ｔｏｎを環流型脱ガス装置まで搬送した後、減圧処理した。処理開始時の溶存酸素濃度はおよそ０．０６％であった。処理前半ではＣ濃度を０．００２０％以下まで脱炭処理し、引き続きＡｌを０．１９％相当添加して脱酸処理し、一定時間環流処理した後、処理を完了した。環流処理中の環流ガスはＡｒとし、ガス流量は５．７Ｎｌ／ｍｉｎとした。この時、浸漬管の浸漬深さＨを種々変更することで真空槽内の溶鋼深さＤを調整し、その効果を確認した。図２から図７に真空槽内の溶鋼深さＤ、浸漬管の浸漬深さＨと時間の関係を示す。この時、大気圧Ｐ０は１０１３２５Ｐａ、溶鋼密度ρは７０００ｋｇ／ｍ３、重力加速度ｇは９．８ｍ／ｓ２とし、（１）式から溶鋼深さＤを算出した。そして、以下に示す各条件にて脱炭処理および清浄化処理を行い、脱炭時間、清浄化時間およびコストを、比較例１を基準として求めた。 350 ton of molten steel having a C concentration of 0.033%, which was produced from a steelmaking furnace, was transported to a recirculation type degassing apparatus and then subjected to reduced pressure treatment. The dissolved oxygen concentration at the start of the treatment was approximately 0.06%. In the first half of the treatment, the carbon concentration was decarburized to 0.0020% or less, subsequently 0.19% equivalent of Al was added and deoxidized, and after a reflux treatment for a certain time, the treatment was completed. The reflux gas during the reflux treatment was Ar, and the gas flow rate was 5.7 Nl / min. At this time, the molten steel depth D in the vacuum chamber was adjusted by variously changing the immersion depth H of the dip tube, and the effect was confirmed. FIGS. 2 to 7 show the relationship between the molten steel depth D in the vacuum chamber, the immersion depth H of the dip tube, and the time. At this time, the atmospheric pressure P 0 was 101325 Pa, the molten steel density ρ was 7000 kg / m 3 , the gravitational acceleration g was 9.8 m / s 2, and the molten steel depth D was calculated from the equation (1). And the decarburization process and the cleaning process were performed on each condition shown below, and the decarburization time, the cleaning time, and cost were calculated | required on the basis of the comparative example 1. FIG.

・比較例１
従来法として、環流処理中は浸漬管の浸漬深さＨを一定とした条件で処理した。浸漬管を取鍋の溶鋼表面から３６０ｍｍ浸漬した状態から減圧処理を開始し、処理中は浸漬深さＨを変えなかった。減圧開始から８．５分経過した時点でＣ濃度が０．００５０％となり、１６．４分経過した時点でＣ濃度が０．００２０％となった。真空槽の槽底までの溶鋼深さＤは真空槽内の圧力低下とともに深くなり、５．０分時点での真空槽内の圧力Ｐは１３３３Ｐａ、溶鋼深さＤは２５８ｍｍ、８．５分時点での真空槽内の圧力Ｐは２００Ｐａ、溶鋼深さＤは２７４ｍｍ、１６．４分経過した時点での真空槽内の圧力Ｐは１３３Ｐａ、溶鋼深さＤは２７５ｍｍであった。その後、真空槽内の圧力Ｐは１３３Ｐａ、溶鋼深さＤは２７５ｍｍの状態でＡｌ添加し、その後は真空槽内の圧力Ｐと溶鋼深さＤを変えないまま、Ａｌ添加後１２．１分環流処理した時点で酸不溶性Ａｌ濃度は０．００２０％となった。この時の溶製コストをベースとする。 Comparative example 1
As a conventional method, during the reflux treatment, the dip tube was treated under the condition that the immersion depth H was constant. The pressure reduction treatment was started from a state where the dip tube was dipped 360 mm from the molten steel surface of the ladle, and the immersion depth H was not changed during the treatment. C concentration became 0.0050% when 8.5 minutes passed from the start of decompression, and C concentration became 0.0020% when 16.4 minutes passed. The molten steel depth D to the bottom of the vacuum chamber becomes deeper as the pressure in the vacuum chamber decreases, the pressure P in the vacuum chamber at 5.0 minutes is 1333 Pa, the molten steel depth D is 258 mm, at 8.5 minutes. The pressure P in the vacuum chamber was 200 Pa, the molten steel depth D was 274 mm, the pressure P in the vacuum chamber after 16.4 minutes was 133 Pa, and the molten steel depth D was 275 mm. Thereafter, Al was added in a state where the pressure P in the vacuum chamber was 133 Pa and the molten steel depth D was 275 mm. Thereafter, the pressure P and the molten steel depth D in the vacuum chamber were not changed. At the time of treatment, the acid-insoluble Al concentration was 0.0020%. Based on the melting cost at this time.

・比較例２
従来よりも浸漬管の浸漬深さＨを深くし、環流中は一定とした条件で処理した。浸漬管を取鍋の溶鋼表面から５００ｍｍ浸漬した状態から減圧処理を開始し、処理中は浸漬深さＨを変えなかった。Ｃ濃度が０．００５０％となったのは、減圧開始から８．４分と比較例１とほぼ同じであったが、Ｃ濃度が０．００２０％となったのは減圧開始から１５．１分となった。真空槽の槽底までの溶鋼深さＤは真空槽内の圧力低下とともに深くなり、５．０分時点での真空槽内の圧力Ｐは１３３３Ｐａ、溶鋼深さＤは３９８ｍｍ、８．４分時点での真空槽内の圧力Ｐは２００Ｐａ、溶鋼深さＤは４１３ｍｍ、１５．１分経過した時点での真空槽内の圧力Ｐは１３３Ｐａ、溶鋼深さＤは４１５ｍｍであった。その後、真空槽内の圧力Ｐは１３３Ｐａ、溶鋼深さＤは４１５ｍｍの状態でＡｌ添加し、その後は真空槽内の圧力Ｐと溶鋼深さＤを変えないまま、Ａｌ添加後１２．９分環流処理した時点で酸不溶性Ａｌ濃度は０．００２０％となった。この時、処理時間は比較例１よりも０．５分短縮されたが、浸漬管を深く浸漬したことで耐火物溶損が進み、比較例１の溶製コストを１．０とした溶製コスト指数は１．２１に悪化した。 Comparative example 2
The immersion depth H of the dip tube was made deeper than before, and the dip tube was processed under constant conditions during reflux. The pressure reduction treatment was started from a state where the dip tube was immersed 500 mm from the molten steel surface of the ladle, and the immersion depth H was not changed during the treatment. The C concentration was 0.0050%, which was almost the same as that of Comparative Example 1 at 8.4 minutes from the start of decompression. However, the C concentration was 0.0020% from 15.1 after the start of decompression. It became minutes. The molten steel depth D to the bottom of the vacuum chamber becomes deeper as the pressure in the vacuum chamber decreases, the pressure P in the vacuum chamber at 5.0 minutes is 1333 Pa, the molten steel depth D is 398 mm, at 8.4 minutes. The pressure P in the vacuum chamber at 200 Pa, the molten steel depth D was 413 mm, the pressure P in the vacuum chamber at the time when 15.1 minutes had passed was 133 Pa, and the molten steel depth D was 415 mm. Thereafter, Al was added in a state where the pressure P in the vacuum chamber was 133 Pa and the molten steel depth D was 415 mm. Thereafter, the pressure P and the molten steel depth D in the vacuum chamber were not changed, and 12.9 minutes after the addition of Al At the time of treatment, the acid-insoluble Al concentration was 0.0020%. At this time, the treatment time was shortened by 0.5 minutes compared with Comparative Example 1, but the refractory erosion progressed by deeply immersing the dip tube, so that the melting cost of Comparative Example 1 was 1.0. The cost index deteriorated to 1.21.

・比較例３
脱炭末期の浸漬管の浸漬管深さＤを深くし、環流中は従来法と同じ条件で処理した。浸漬管を取鍋の溶鋼表面から３６０ｍｍ浸漬した状態から減圧処理を開始した。Ｃ濃度が０．００５０％となったのは、減圧開始から８．５分と比較例１と同じであったが、Ｃ濃度が０．００５０％になるタイミングで浸漬管の浸漬深さＨを５００ｍｍとした結果、Ｃ濃度が０．００２０％となったのは減圧開始から１５．２分となった。真空槽の槽底までの溶鋼深さＤは真空槽内の圧力低下とともに深くなり、５．０分時点での真空槽内の圧力Ｐは１３３３Ｐａ、溶鋼深さＤは２５８ｍｍ、８．０分時点での真空槽内の圧力Ｐは２６７Ｐａ、溶鋼深さＤは２７３ｍｍであり、浸漬管の浸漬深さＨを変更し、８．５分時点で溶鋼深さＤは４１４ｍｍであり、１５．２分経過した時点での真空槽内の圧力Ｐは１３３Ｐａ、溶鋼深さＤは４１５ｍｍであった。その後、真空槽内の圧力Ｐは１３３Ｐａ、溶鋼深さＤは４１５ｍｍの状態でＡｌ添加し、浸漬管の浸漬深さＨを３６０ｍｍとし、真空槽内の圧力Ｐは１３３Ｐａ、溶鋼深さＤは２７５ｍｍの状態でＡｌ添加後１２．０分環流処理した時点で酸不溶性Ａｌ濃度は０．００２０％となった。この時、処理時間は比較例１よりも１．３分短縮されたが、処理時間短縮の効果よりも耐火物溶損の影響が大きく、溶製コスト指数は１．０６に悪化した。 Comparative example 3
The dip tube depth D of the dip tube at the end of decarburization was deepened, and the treatment was performed under the same conditions as in the conventional method during reflux. The pressure reduction treatment was started from a state where the dip tube was immersed 360 mm from the molten steel surface of the ladle. The C concentration became 0.0050%, which was 8.5 minutes from the start of decompression and the same as in Comparative Example 1. However, the immersion depth H of the dip tube was set at the timing when the C concentration became 0.0050%. As a result, the C concentration became 0.0020% after 15.2 minutes from the start of the decompression. The molten steel depth D to the bottom of the vacuum chamber becomes deeper as the pressure in the vacuum chamber decreases, the pressure P in the vacuum chamber at 5.0 minutes is 1333 Pa, the molten steel depth D is 258 mm, at 8.0 minutes. The pressure P in the vacuum chamber is 267 Pa, the molten steel depth D is 273 mm, the immersion depth H of the dip tube is changed, and at 8.5 minutes the molten steel depth D is 414 mm, 15.2 minutes The pressure P in the vacuum chamber at the time when it passed was 133 Pa, and the molten steel depth D was 415 mm. Thereafter, Al is added in a state where the pressure P in the vacuum chamber is 133 Pa and the molten steel depth D is 415 mm, the immersion depth H of the dip tube is 360 mm, the pressure P in the vacuum chamber is 133 Pa, and the molten steel depth D is 275 mm. In this state, the acid-insoluble Al concentration was 0.0020% when the reflux treatment was performed for 12.0 minutes after the addition of Al. At this time, the treatment time was shortened by 1.3 minutes compared with Comparative Example 1, but the influence of the refractory melt was greater than the effect of shortening the treatment time, and the melting cost index deteriorated to 1.06.

・発明例１
脱炭処理中に環流処理が停滞しない範囲で溶鋼深さＤを浅くし、脱炭末期の浸漬管の浸漬管深さを深くし、環流中は従来法と同じ条件で処理した。浸漬管を取鍋の溶鋼表面から５００ｍｍ浸漬した状態から減圧処理を開始し、真空槽内の圧力Ｐに合わせて浸漬管の浸漬深さＨを調整し、溶鋼深さＤを８３ｍｍとした。Ｃ濃度が０．００５０％となったのは、減圧開始から８．６分と比較例１とほぼ同じであり、Ｃ濃度が０．００５０％になるタイミングで浸漬管の浸漬深さＨを５００ｍｍとした結果、Ｃ濃度が０．００２０％となったのは減圧開始から１５．３分となった。真空槽の槽底までの溶鋼深さＤはＣ濃度が０．００５０％となるまで８３ｍｍであり、５．０分時点での真空槽内の圧力Ｐは１３３３Ｐａ、溶鋼深さＤは８３ｍｍ、８．０分時点での真空槽内の圧力Ｐは２６７Ｐａ、溶鋼深さＤは８３ｍｍであり、浸漬管の浸漬深さＨを変更し、８．６分時点で溶鋼深さＤは４１４ｍｍであり、１５．３分経過した時点での真空槽内の圧力Ｐは１３３Ｐａ、溶鋼深さＤは４１５ｍｍであった。その後、真空槽内の圧力Ｐは１３３Ｐａ、溶鋼深さＤは４１５ｍｍの状態でＡｌ添加し、浸漬管の浸漬深さＨを３６０ｍｍとし、真空槽内の圧力Ｐは１３３Ｐａ、溶鋼深さＤは２７５ｍｍの状態でＡｌ添加後１２．２分環流処理した時点で酸不溶性Ａｌ濃度は０．００２０％となった。この時、酸不溶性Ａｌ濃度が０．００２０％となるまでの処理時間は比較例１よりも１．０分短縮された事に加え、脱炭処理中の浸漬管の耐火物溶損が低減できたことで、溶製コスト指数は０．８６に改善した。 ・ Invention Example 1
The depth D of the molten steel was made shallow within the range in which the recirculation treatment did not stagnate during the decarburization treatment, and the dip tube depth of the dip tube at the end of decarburization was deepened. The pressure reduction treatment was started from a state in which the dip tube was immersed 500 mm from the surface of the molten steel in the ladle, the immersion depth H of the dip tube was adjusted according to the pressure P in the vacuum chamber, and the molten steel depth D was 83 mm. The C concentration became 0.0050%, which was 8.6 minutes from the start of decompression, which was almost the same as in Comparative Example 1. As a result, the C concentration became 0.0020% after 15.3 minutes from the start of decompression. The molten steel depth D to the bottom of the vacuum chamber is 83 mm until the C concentration reaches 0.0050%, the pressure P in the vacuum chamber at 5.0 minutes is 1333 Pa, the molten steel depth D is 83 mm, 8 The pressure P in the vacuum chamber at the time of 0 minutes is 267 Pa, the molten steel depth D is 83 mm, the immersion depth H of the dip tube is changed, and the molten steel depth D is 414 mm at the time of 8.6 minutes, The pressure P in the vacuum chamber at the time when 15.3 minutes passed was 133 Pa, and the molten steel depth D was 415 mm. Thereafter, Al is added in a state where the pressure P in the vacuum chamber is 133 Pa and the molten steel depth D is 415 mm, the immersion depth H of the dip tube is 360 mm, the pressure P in the vacuum chamber is 133 Pa, and the molten steel depth D is 275 mm. In this state, the acid-insoluble Al concentration was 0.0020% when the reflux treatment was performed for 12.2 minutes after the addition of Al. At this time, in addition to the fact that the treatment time until the acid-insoluble Al concentration reaches 0.0020% is shorter than that of Comparative Example 1, the refractory damage of the dip tube during the decarburization treatment can be reduced. As a result, the melt cost index was improved to 0.86.

・発明例２
脱炭処理中に環流処理が停滞しない範囲で溶鋼深さＤを浅くし、脱炭末期の浸漬管の浸漬管深さを深くし、環流中は従来法と同じ条件で処理した。浸漬管を取鍋の溶鋼表面から５００ｍｍ浸漬した状態から減圧処理を開始し、真空槽内の圧力Ｐに合わせて浸漬管の浸漬深さＨを調整し、溶鋼深さＤを８３ｍｍとした。Ｃ濃度が０．００５０％となったのは、減圧開始から８．５分と比較例１と同じであり、Ｃ濃度が０．００５０％になるタイミングで浸漬管の浸漬深さＨを５００ｍｍとした結果、Ｃ濃度が０．００２０％となったのは減圧開始から１５．２分となった。真空槽の槽底までの溶鋼深さＤはＣ濃度が０．００５０％となるまで８３ｍｍであり、５．０分時点での真空槽内の圧力Ｐは１３３３Ｐａ、溶鋼深さＤは８３ｍｍ、８．０分時点での真空槽内の圧力Ｐは２６７Ｐａ、溶鋼深さＤは８３ｍｍであり、浸漬管の浸漬深さＨを変更し、８．５分時点で溶鋼深さＤは４１４ｍｍであり、１５．２分経過した時点での真空槽内の圧力Ｐは１３３Ｐａ、溶鋼深さＤは４１５ｍｍであった。その後、真空槽内の圧力Ｐは１３３Ｐａ、溶鋼深さＤは４１５ｍｍの状態でＡｌ添加し、浸漬管の浸漬深さＨを２３０ｍｍとし、真空槽内の圧力Ｐは１３３Ｐａ、溶鋼深さＤは１４５ｍｍの状態でＡｌ添加後１２．１分環流処理した時点で酸不溶性Ａｌ濃度は０．００２０％となった。この時、酸不溶性Ａｌ濃度が０．００２０％となるまでの処理時間は比較例１よりも１．２分短縮された事に加え、脱炭処理中の浸漬管の耐火物溶損が低減できたことで、溶製コスト指数は０．８４に改善した。 -Invention Example 2
The depth D of the molten steel was made shallow within the range in which the recirculation treatment did not stagnate during the decarburization treatment, and the dip tube depth of the dip tube at the end of decarburization was deepened. The pressure reduction treatment was started from a state in which the dip tube was immersed 500 mm from the surface of the molten steel in the ladle, the immersion depth H of the dip tube was adjusted according to the pressure P in the vacuum chamber, and the molten steel depth D was 83 mm. The C concentration was 0.0050%, 8.5 minutes from the start of decompression, the same as in Comparative Example 1, and the immersion depth H of the dip tube was set to 500 mm at the timing when the C concentration became 0.0050%. As a result, the C concentration became 0.0020% after 15.2 minutes from the start of decompression. The molten steel depth D to the bottom of the vacuum chamber is 83 mm until the C concentration reaches 0.0050%, the pressure P in the vacuum chamber at 5.0 minutes is 1333 Pa, the molten steel depth D is 83 mm, 8 The pressure P in the vacuum chamber at 0.0 minutes is 267 Pa, the molten steel depth D is 83 mm, the immersion depth H of the dip tube is changed, and the molten steel depth D is 414 mm at 8.5 minutes, At the time when 15.2 minutes passed, the pressure P in the vacuum chamber was 133 Pa, and the molten steel depth D was 415 mm. Thereafter, Al is added in a state where the pressure P in the vacuum chamber is 133 Pa and the molten steel depth D is 415 mm, the immersion depth H of the dip tube is 230 mm, the pressure P in the vacuum chamber is 133 Pa, and the molten steel depth D is 145 mm. In this state, the acid-insoluble Al concentration was 0.0020% when the reflux treatment was performed for 12.1 minutes after the addition of Al. At this time, in addition to the fact that the treatment time until the acid-insoluble Al concentration becomes 0.0020% is shorter than that of Comparative Example 1 by 1.2 minutes, the refractory damage of the dip tube during the decarburization treatment can be reduced. As a result, the melting cost index was improved to 0.84.

・発明例３
脱炭処理中に環流処理が停滞しない範囲で溶鋼深さＤを浅くし、脱炭末期の浸漬管の浸漬管深さを深くし、環流中も環流処理が停滞しない範囲で溶鋼深さＤを浅くした条件で処理した。浸漬管を取鍋の溶鋼表面から５００ｍｍ浸漬した状態から減圧処理を開始し、真空槽内の圧力Ｐに合わせて浸漬管の浸漬深さＨを調整し、溶鋼深さＤを８３ｍｍとした。Ｃ濃度が０．００５０％となったのは、減圧開始から８．５分と比較例１と同じであり、Ｃ濃度が０．００５０％になるタイミングで浸漬管の浸漬深さＨを５００ｍｍとした結果、Ｃ濃度が０．００２０％となったのは減圧開始から１５．１分となった。真空槽の槽底までの溶鋼深さＤはＣ濃度が０．００５０％となるまで８３ｍｍであり、５．０分時点での真空槽内の圧力Ｐは１３３３Ｐａ、溶鋼深さＤは８３ｍｍ、８．０分時点での真空槽内の圧力Ｐは２６７Ｐａ、溶鋼深さＤは８３ｍｍであり、浸漬管の浸漬深さＨを変更し、８．５分時点で溶鋼深さＤは４１４ｍｍであり、１５．１分経過した時点での真空槽内の圧力Ｐは１３３Ｐａ、溶鋼深さＤは４１５ｍｍであった。その後、真空槽内の圧力Ｐは１３３Ｐａ、溶鋼深さＤは４１５ｍｍの状態でＡｌ添加し、真空槽内の圧力Ｐに合わせて浸漬管の浸漬深さＨを１６５ｍｍに調整して溶鋼深さＤを８０ｍｍとし、真空槽内の圧力Ｐは１３３Ｐａ、溶鋼深さＤは８０ｍｍの状態でＡｌ添加後１０．８分環流処理した時点で酸不溶性Ａｌ濃度は０．００２０％となった。この時、酸不溶性Ａｌ濃度が０．００２０％となるまでの処理時間は比較例１よりも２．６分短縮された事に加え、脱炭処理中、Ａｌ添加後の浸漬管の耐火物溶損が低減できたことで、溶製コスト指数は０．７９に改善した。 -Invention Example 3
The depth D of the molten steel is made shallow within the range where the recirculation treatment does not stagnate during the decarburization treatment, the dip tube depth of the dip tube at the end of the decarburization is deepened, and the molten steel depth D is set within the range where the recirculation treatment does not stagnate even during recirculation. Processed under shallow conditions. The pressure reduction treatment was started from a state in which the dip tube was immersed 500 mm from the surface of the molten steel in the ladle, the immersion depth H of the dip tube was adjusted according to the pressure P in the vacuum chamber, and the molten steel depth D was 83 mm. The C concentration was 0.0050%, 8.5 minutes from the start of decompression, the same as in Comparative Example 1, and the immersion depth H of the dip tube was set to 500 mm at the timing when the C concentration became 0.0050%. As a result, the C concentration became 0.0020% after 15.1 minutes from the start of decompression. The molten steel depth D to the bottom of the vacuum chamber is 83 mm until the C concentration reaches 0.0050%, the pressure P in the vacuum chamber at 5.0 minutes is 1333 Pa, the molten steel depth D is 83 mm, 8 The pressure P in the vacuum chamber at 0.0 minutes is 267 Pa, the molten steel depth D is 83 mm, the immersion depth H of the dip tube is changed, and the molten steel depth D is 414 mm at 8.5 minutes, The pressure P in the vacuum chamber when 15.1 minutes passed was 133 Pa, and the molten steel depth D was 415 mm. Thereafter, Al was added in a state where the pressure P in the vacuum chamber was 133 Pa and the molten steel depth D was 415 mm, and the immersion depth H of the dip tube was adjusted to 165 mm in accordance with the pressure P in the vacuum chamber. Was 80 mm, the pressure P in the vacuum chamber was 133 Pa, and the molten steel depth D was 80 mm. When the reflux treatment was performed for 10.8 minutes after the addition of Al, the acid-insoluble Al concentration was 0.0020%. At this time, in addition to the fact that the treatment time until the acid-insoluble Al concentration became 0.0020% was shortened by 2.6 minutes compared with Comparative Example 1, during the decarburization treatment, the refractory solution of the dip tube after the addition of Al Since the loss could be reduced, the melting cost index was improved to 0.79.

上記したように、脱炭処理中に環流処理が停滞しない範囲で溶鋼深さＤを浅くし、脱炭末期の溶鋼深さＤを深くすることによって、脱炭速度を向上させた上で溶製コストを低減できることが示された。さらに、Ａｌ添加後にも溶鋼深さＤを浅くすることによって、非金属介在物の凝集浮上を促進させた上で溶製コストを低減できることが示された。特に、溶鋼深さＤを１００ｍｍ未満に制御することで、溶鋼の清浄化効果が顕著に表れることが示された。   As described above, the molten steel depth D is reduced within a range in which the reflux treatment does not stagnate during the decarburization treatment, and the molten steel depth D at the end of the decarburization is deepened to improve the decarburization speed and then perform the melting. It has been shown that costs can be reduced. Furthermore, it has been shown that by reducing the molten steel depth D even after Al addition, the melting cost can be reduced while promoting the agglomeration and floating of nonmetallic inclusions. It was shown that the cleaning effect of molten steel appears notably by controlling the molten steel depth D to less than 100 mm especially.

１ 真空槽、２ 真空槽の槽底位置、３ 環流ガス、４ 環流ガス吹込み孔、５ 浸漬管（上昇管）、６ 浸漬管（下降管）、７ スラグ、８ 溶鋼、９ 取鍋   1 vacuum tank, 2 tank bottom position, 3 reflux gas, 4 reflux gas injection holes, 5 dip pipe (rising pipe), 6 dip pipe (down pipe), 7 slag, 8 molten steel, 9 ladle

Claims (2)

製鋼炉から取鍋に出鋼した溶鋼を環流型脱ガス装置で精錬するにあたり、
脱炭開始時点から少なくとも８．０分間は、（１）式で求まる真空槽内の湯面から真空槽の槽底までの溶鋼深さＤが５０ｍｍ以上１００ｍｍ未満の範囲で脱炭処理を行うとともに、脱炭終了前の少なくとも５分間は、前記溶鋼深さＤが４００ｍｍ以上の範囲でＣ濃度を０．００４０質量％以下に脱炭処理を行うことを特徴とする、環流型脱ガス装置における溶鋼の減圧精錬方法。
Ｄ＝（Ｐ０−Ｐ）／（ρ・ｇ）＋Ｈ−Ｌ ・・・（１）
Ｄ：真空槽内の湯面から真空槽の槽底までの溶鋼深さ（ｍ）、Ｐ０：大気圧（Ｐａ）、Ｐ：真空槽内の圧力（Ｐａ）、ρ：溶鋼密度（ｋｇ／ｍ３）、ｇ：重力加速度（ｍ／ｓ２）、Ｈ：浸漬管の浸漬深さ（ｍ）、Ｌ：浸漬管下端から真空槽内の槽底までの長さ（ｍ） In refining the molten steel from the steelmaking furnace to the ladle using a recirculation type degassing device,
For at least 8.0 minutes from the start of decarburization, decarburization treatment is performed in a range where the molten steel depth D from the hot water surface in the vacuum tank to the tank bottom of the vacuum tank obtained by equation (1) is 50 mm or more and less than 100 mm. The molten steel in the recirculation type degassing apparatus is characterized in that the decarburization treatment is performed to a C concentration of 0.0040% by mass or less in the range where the molten steel depth D is 400 mm or more for at least 5 minutes before the end of decarburization Vacuum refining method.
D = (P 0 −P) / (ρ · g) + H−L (1)
D: Molten steel depth (m) from the hot water surface in the vacuum chamber to the bottom of the vacuum chamber, P 0 : atmospheric pressure (Pa), P: pressure in the vacuum chamber (Pa), ρ: molten steel density (kg / m 3 ), g: gravitational acceleration (m / s 2 ), H: immersion depth (m) of dip tube, L: length from bottom of dip tube to bath bottom in vacuum chamber (m) 前記脱炭処理に続くＡｌ添加後に前記浸漬管の浸漬深さＨを浅くする操作によって前記溶鋼深さＤが５０ｍｍ以上１００ｍｍ未満の範囲で環流処理を行うことを特徴とする、請求項１に記載の環流型脱ガス装置における溶鋼の減圧精錬方法。   The reflux treatment is performed in a range where the molten steel depth D is 50 mm or more and less than 100 mm by an operation of decreasing the immersion depth H of the dip tube after the addition of Al following the decarburization treatment. Method for refining molten steel in a recirculation type degassing apparatus.
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JPH04141512A (en) * 1990-09-28 1992-05-15 Kawasaki Steel Corp Method for refining ultra low carbon steel by rh process
US20050109161A1 (en) * 2002-02-22 2005-05-26 Eric Perrin Method for deep decarburisation of steel melts
JP3891013B2 (en) * 2002-03-14 2007-03-07 Ｊｆｅスチール株式会社 Method of refining molten steel with RH degassing equipment
JP4062213B2 (en) * 2003-08-25 2008-03-19 Ｊｆｅスチール株式会社 Method for adjusting the composition of molten steel in an RH degasser
JP4062212B2 (en) * 2003-08-25 2008-03-19 Ｊｆｅスチール株式会社 Method for refining molten steel with RH degassing equipment

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