JP3918695B2

JP3918695B2 - Method for producing ultra-low sulfur steel

Info

Publication number: JP3918695B2
Application number: JP2002270877A
Authority: JP
Inventors: 尚志小川; 一之加藤; 伸和北川; 嘉久北野; 廣西川
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2002-09-18
Filing date: 2002-09-18
Publication date: 2007-05-23
Anticipated expiration: 2022-09-18
Also published as: JP2004107716A

Description

【０００１】
【発明の属する技術分野】
本発明は、極低硫鋼の製造方法に係わり、詳しくは、転炉出鋼後の溶鋼を二次精錬においてＶＯＤあるいはＬＦ装置を用いずに、取鍋を利用し、安価にハイテン等の極低硫鋼にする技術に関する。
【０００２】
【従来の技術】
一般に、炭素鋼で極低硫鋼（溶鋼の硫黄濃度で［Ｓ］≦１４ｐｐｍ）を製造するには、まず溶銑の段階で、ソーダ灰、金属Ｍｇ系あるいは石灰系の脱硫剤を用い、強力に予備脱硫を行ない、溶銑の硫黄濃度を２０〜５０ｐｐｍ程度に低下する。そして、該溶銑を転炉等にて脱炭精錬を行なった後、得られた溶鋼に対しさらなる精錬（二次精錬という）を行って脱硫し、最終目標の硫黄濃度にする。その二次精錬で行う脱硫には、下記のような方法が利用される。
▲１▼取鍋、蓋及び加熱用電極からなる所謂「ＬＦ装置」を用い、電気エネルギーによる昇熱とスラグ−メタル精錬とを行い脱硫する方法
▲２▼取鍋に保持した溶鋼に、ガス吹き込みノズルを介して精錬用フラックス（脱硫剤）を大気下で吹き込み脱硫する方法
▲３▼取鍋に保持した溶鋼をＲＨ真空脱ガス槽との間で環流させ、上方より脱硫剤を吹き付け、脱硫する方法
▲４▼取鍋に保持した溶鋼をＶＯＤ真空脱ガス槽内にセットし、強撹拌して脱硫する方法
ＬＦ装置を用いる方法は、電力エネルギーで精錬用フラックスを溶かし、溶鋼浴面を覆い、保温に有効な技術である。また、溶融し難い精錬用フラックスでも利用でき、スラグの硫黄保持能力（Ｓｃａｐａｃｉｔｙ）を高めることができるので、脱硫反応効率が高いという利点がある。しかしながら、ＬＦ装置を用いる場合は、電力エネルギーを使うために、製造コストが高くなるばかりでなく、溶製時間が長く、生産性も低いという問題点がある。
【０００３】
また、ＶＯＤ真空脱ガス槽を用いる方法は、撹拌力が大きいので、脱硫反応効率は大きいが、溶製時間が長く、処理コストが高いという問題があった。また、溶鋼の強撹拌により、取鍋の内張り耐火物の溶損が著しく大きくなるという問題も生じている。
【０００４】
さらに、ＬＦ装置やＶＯＤ真空脱ガス槽を用いずに、２ｐｐｍ以下の硫黄濃度を安定して溶製する方法もある（特許文献１参照）。しかしながら、その方法では、予め溶銑の予備処理段階で硫黄濃度を１０〜３５ｐｐｍに低下させる必要があり、溶銑予備処理に要する時間と費用が莫大になる。また、目標達成が不十分の時には、ＲＨ真空脱ガス槽を用いたさらなる脱硫処理、つまり二次精錬だけで２段階の脱硫処理が必要とされる。
【０００５】
【特許文献１】
特開平９−２１７１１０号公報（２頁、１欄の２行〜１９行）
【０００６】
【発明が解決しようとする課題】
本発明は、かかる事情に鑑み、硫黄濃度が１４ｐｐｍ以下の溶鋼を、従来より簡便、且つ安価に溶製可能な極低硫鋼の製造方法を提供することを目的としている。
【０００７】
【課題を解決するための手段】
発明者は、上記目的を達成するため鋭意研究を重ね、その成果を本発明に具現化した。
【０００８】
すなわち、本発明は、溶銑の予備脱硫処理、転炉での脱炭精錬を経て溶製した溶鋼を転炉から取鍋へ出鋼し、ＣａＯ系精錬用フラックス及びＡｌを添加し、その後常圧下で溶鋼浴面下への不活性ガスの吹き込み撹拌を行い極低硫鋼を製造するに際して、前記取鍋を蓋で大気遮断した後、該蓋と溶鋼浴面との間に形成されるフリーボード部を不活性ガスによっ置換し、その後に溶鋼浴面下への不活性ガスの吹き込み撹拌を開始し、さらに、フリーードヘも別途不活性ガスを供給することにより該フリーボード部の雰囲気中酸素濃度を１０容量％以下に保ちつつ、電極による溶鋼等の加熱を行うことなしに前記溶鋼浴面下への不活性ガスの吹き込み撹拌を行うことを特徴とする極低硫鋼の製造方法である。また、前記溶鋼の撹絆を、（１）式で示される撹拌動力密度の条件を満たすように行うのが好ましい。
【０００９】
ε＝７４２・（ＱＴ／Ｗ）・ｌｎ（１＋ｇρＨ／Ｐ）≧２００ ……（１）
Ｑ：不活性ガスの流量（ｍ³／ｓ）、Ｔ：不活性ガスの温度（Ｋ）、Ｗ：溶鋼の重量（ｔ）、Ｈ：不活性ガスの浮上距離（ｍ）、Ｐ：雰囲気の圧力（Ｐａ）、ｇ：重力加速度（ｍ／ｓ²）、ρ：溶鋼の密度（ｋｇ／ｍ³）
本発明によれば、溶銑予備処理を過酷に行わなくても、既存の取鍋、蓋、ガス吹き込みランスを利用し、簡便且つ安価に硫黄濃度が１４ｐｐｍ以下の極低硫鋼を安定して製造できるようになる。
【００１０】
【発明の実施の形態】
以下、発明をなすに至った経緯をまじえ、本発明の実施の形態を説明する。
【００１１】
まず、発明者は、操業費の嵩むＲＨ，ＶＯＤ等の真空脱ガス槽や前記ＬＦ装置を利用しなくても、極低硫黄濃度領域まで脱硫することの可能性を追求した。具体的には、脱炭後に転炉から出鋼された溶鋼を取鍋に受け、ＣａＯ系精錬フラックスとＡｌを添加した上で、ガス吹き込みランスを介して不活性ガスを吹き込む方法である。このような方式の溶鋼の脱硫方法は、前述の特許文献１記載の発明の一部をなす技術である。しかし、該特許文献１記載の発明は、鋼中硫黄濃度が３ｐｐｍ以下という従来にない超低硫鋼の製造を意図したものであり、溶銑の予備処理段階で硫黄濃度を１０〜３５ｐｐｍのレベルまで低減し、転炉出鋼後の取鍋内でのガス撹拌でさらに鋼中硫黄濃度を４ｐｐｍ以下に低減し、その上引き続くＲＨ処理でもフラックスを添加して脱硫するという過重な処理を行うものであった。本発明者は、この従来技術の取鍋内でのガス撹拌処理の部分だけを取り出して、溶鋼を１４ｐｐｍ以下の極低硫レベルまで脱硫処理することを考え、実験を行ったところ、重大な問題に直面した。すなわち、上記従来技術が意図する超低硫レベルほどではないにせよ、溶銑予備処理やＲＨ等の前後工程に過大な負荷をかけずに硫黄濃度を１４ｐｐｍ以下の極低硫領域まで脱硫しようとすれば、脱硫フラックスとして溶鋼トン当たり２０ｋｇを超える多量の原単位が必要になり、それによる溶鋼温度の降下が無視できないものとなることが判明した。それを防止するには、転炉の出鋼温度として１７００℃近い高温が必要となり、転炉の耐火物寿命を大きく損なうことが懸念された。そこで、発明者は、ガス吹き込み撹拌による脱硫処理でのフラックスの反応効率を高めようと考えた。
【００１２】
溶鋼の脱硫は、所謂「スラグ―メタル反応」で行われるが、その反応を促進するには、スラグ及びメタル（この場合、溶鋼）の撹拌が重要である。この撹拌を前記ガス吹きランスを介してのガス吹き込み撹拌で行う場合、大気下では、該大気に含まれる酸素によって溶鋼及びスラグが酸化され、脱硫反応を阻害する。そこで、発明者は、硫黄濃度を５〜１４ｐｐｍまでに低減するには、この大気酸化による脱硫効率の低下を防止しなければならないと考えた。そこで、図２に示すように、溶鋼１及びスラグ４を保持した取鍋２に蓋５を被せ、それ以上の大気の浸入を防止し、溶鋼等の浴面と蓋との間に形成されるフリーボード６に残存する大気を配管７から供給するアルゴン又は窒素等の不活性ガスで置換すると共に、撹拌中もフリーボード６の雰囲気に酸素が入らないように不活性ガスを流し続けると共に、ランス３を用いて溶鋼中への不活性ガスの吹き込み撹拌をする実験を行った。フリーボード６内の雰囲気ガスをガス採取管８によって採取し、酸素濃度測定装置９によってガス中の酸素濃度を測定した。
【００１３】
フリーボード６の雰囲気中酸素濃度を１０容量％以下とした場合をＢ、１０容量％より高い場合をＡとして、図１中にその結果を示す。
【００１４】
その結果、図１に示すように、フリーボード６内の雰囲気中酸素濃度を１０容量％以下にすれば、目的が達成できることがわかったので、この事実を要件に本発明を完成させたのである。
【００１５】
また、その実験では、ＬＦ装置に付帯されるような電極による溶鋼等の加熱を行わずに、しかも常圧下で５〜１４ｐｐｍまで脱硫することについても調査した。その結果、下記で算出される撹拌動力密度εが２００ワット／ｔ以上になるように、ガス吹き込み撹拌すれば良いことも見出し、この要件も加えた本発明も完成した（図１参照）。
【００１６】
ε＝７４２（ＱＴ／Ｗ）・ｌｎ（１＋ｇρＨ／Ｐ）≧２００ ……（１）
ここで、Ｑ：不活性ガスの流量（ｍ³／ｓ）、Ｔ：不活性ガスの温度（Ｋ）、Ｗ：溶鋼の重量（ｔ）、Ｈ：不活性ガスの浮上距離（ｍ）、Ｐ：雰囲気の圧力（Ｐａ）、ｇ：重力加速度（ｍ／ｓ²）、ρ：溶鋼の密度（ｋｇ／ｍ³）
このようにすることで、前述の従来技術での取鍋内溶鋼へのガス吹き込み撹拌による脱硫処理では、溶鋼の硫黄濃度を１ｐｐｍ低下するのにフラックスを溶鋼トン当たり約１ｋｇ必要であったのに比較して、本発明ではその半分以下のフラックス量ですむことがわかった。
【００１７】
このように、転炉出鋼後の溶鋼段階で少ないフラックス量で十分な脱硫が達成できることから、転炉装入前の溶銑段階での予備脱硫の負荷を低減することが可能となる。例えば、製品として要求される硫黄濃度が１０ｐｐｍの溶鋼を製造する場合、ガス吹き込み撹拌処理で１５ｋｇ／ｔのフラックスを使用して脱硫しようとすれば、脱硫量は３０ｐｐｍ以上であるので、転炉出鋼時の溶鋼中硫黄濃度（転炉内では、ほとんど脱硫は期待できないので、溶銑予備処理終了時点での溶銑の硫黄濃度と同じ）は、４０ｐｐｍ程度で良い。したがって、溶銑予備処理では、脱硫後の硫黄濃度を前記従来技術での値よりも高く、例えば３５ｐｐｍを超える硫黄濃度にとどめておくことが可能になり、溶銑予備処理の負荷を軽減できるのである。
【００１８】
【実施例】
高炉から出銑された溶銑を用い、以下の工程で脱燐、脱硫及び脱炭を行い、極低硫鋼を溶製した。
【００１９】
溶銑予備処理の脱燐（トピード・カー利用のインジェクション法）→溶銑予備処理の脱硫（機械撹拌による脱硫法）→転炉（脱炭）→ガス吹き込み撹拌による脱硫→ＲＨ脱ガス（成分調整）
なお、転炉出鋼後の溶鋼のガス吹き込み撹拌による脱硫は、ほとんどの場合、図２に示す装置構成により取鍋に蓋を被せて行ったが、被せずにも行った。つまり、本発明に係る極低硫鋼の製造方法を適用した場合及び本発明の要件を満たさない場合を実施した。フリーボードに供給する不活性ガス及びガス吹き込み撹拌に使用する不活性ガスとしてはいずれもアルゴンを用いた。実施の条件及び結果を表１に一括して示す。
【００２０】
【表１】

【００２１】
表１より、本発明によれば、フリーボードの雰囲気中酸素濃度を１０容量％以下にすることで、溶銑予備処理段階での脱硫を、溶銑中硫黄濃度が４０ｐｐｍに留めるように行っても、溶鋼中硫黄濃度を８〜１４ｐｐｍへ低減できることが明らかである。なお、得られた溶鋼の他の成分は、０．０８質量％Ｃ、０．７０質量％Ｓｉ、１．５０質量％Ｍｎ、０．０２質量％Ｐであった。
【００２２】
なお、表１において、ＦＢはフリーボードを、Ｂｕはガス吹き込み撹拌のことである。また、ガス吹き込み撹拌時の撹拌動力密度を２００Ｗ／ｔ以上とすることにより、溶鋼中硫黄濃度を１１ｐｐｍ以下へ低減できた。
【００２３】
【発明の効果】
以上述べたように、本発明により、溶銑予備処理を過酷に行わず、且つ高価な真空脱ガス処理をしなくても、既存の取鍋、蓋、ガス吹き込みランスを利用し、簡便且つ安価に硫黄濃度が１４ｐｐｍ以下の極低硫鋼を安定して製造できるようになる。
【図面の簡単な説明】
【図１】本発明の基礎とした「溶鋼の脱硫処理時のガス撹拌動力密度と連鋳片の硫黄濃度との関係」を示した図である。
【図２】本発明を実施する装置を示す縦断面図である。
【符号の説明】
１溶鋼
２取鍋
３ガス吹き込みランス
４スラグ
５蓋
６フリーボード
７フリーボードへの不活性ガス供給配管
８フリーボード内ガスの採取管
９酸素濃度測定装置[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing extremely low-sulfur steel. Specifically, the molten steel after the converter steel is used in secondary refining without using a VOD or LF apparatus, and a ladle is used at a low price such as high tensile steel. It relates to technology to make low sulfur steel.
[0002]
[Prior art]
Generally, in order to produce extremely low-sulfur steel (Sulfur concentration of molten steel [S] ≦ 14ppm) with carbon steel, first use soda ash, metallic Mg-based or lime-based desulfurizing agent at the hot metal stage to Pre-desulfurization is performed to reduce the sulfur concentration of the hot metal to about 20 to 50 ppm. Then, after the hot metal is decarburized and refined in a converter or the like, the obtained molten steel is further refined (referred to as secondary refining) to desulfurize to a final target sulfur concentration. The following methods are used for desulfurization performed in the secondary refining.
(1) A method of desulfurization by using a so-called “LF device” consisting of a ladle, a lid and a heating electrode to perform desulfurization by heating with electric energy and slag-metal refining (2) Gas is blown into the molten steel held in the ladle Method of desulfurization by blowing refining flux (desulfurizing agent) through the nozzle in the atmosphere (3) The molten steel held in the ladle is circulated between the RH vacuum degassing tank, and desulfurizing agent is sprayed from above to desulfurize. Method (4) Method in which the molten steel held in the ladle is set in a VOD vacuum degassing tank and desulfurized by vigorous stirring. The method using the LF apparatus melts the refining flux with electric power energy, covers the molten steel bath surface, This technology is effective for keeping warm. Further, it can be used even for a refining flux that is difficult to melt, and the sulfur retention capacity (S capacity) of the slag can be increased. However, when the LF device is used, there is a problem that not only the manufacturing cost becomes high because electric power energy is used, but also the melting time is long and the productivity is low.
[0003]
In addition, the method using a VOD vacuum degassing tank has a problem that the desulfurization reaction efficiency is large because the stirring force is large, but the melting time is long and the processing cost is high. Moreover, the problem that the melting loss of the lining refractory of a ladle becomes remarkably large by the strong stirring of molten steel also arises.
[0004]
Furthermore, there is also a method of stably melting a sulfur concentration of 2 ppm or less without using an LF device or a VOD vacuum degassing tank (see Patent Document 1). However, in this method, it is necessary to reduce the sulfur concentration to 10 to 35 ppm in advance in the hot metal pretreatment stage, and the time and cost required for the hot metal pretreatment are enormous. Further, when the achievement of the target is insufficient, a further desulfurization process using an RH vacuum degassing tank, that is, a two-stage desulfurization process is required only by secondary refining.
[0005]
[Patent Document 1]
Japanese Patent Laid-Open No. 9-217110 (page 2, line 2 to line 19)
[0006]
[Problems to be solved by the invention]
In view of such circumstances, an object of the present invention is to provide a method for producing an ultra-low-sulfur steel capable of melting a molten steel having a sulfur concentration of 14 ppm or less in a simpler and cheaper manner.
[0007]
[Means for Solving the Problems]
The inventor has intensively studied to achieve the above object, and the results have been embodied in the present invention.
[0008]
That is, the present invention is a method in which molten steel that has been melted through preliminary desulfurization treatment of hot metal and decarburization refining in a converter is discharged from the converter to a ladle, and then CaO-based refining flux and Al are added, and then under normal pressure In producing ultra-low sulfur steel by blowing and stirring an inert gas below the molten steel bath surface, the ladle is shut off to the atmosphere with a lid, and then free formed between the lid and the molten steel bath surface. The board portion is replaced with an inert gas, and then the stirring and stirring of the inert gas under the molten steel bath surface are started. Further, by supplying an inert gas separately to the freeboard , oxygen in the atmosphere of the free board portion An ultra-low-sulfur steel manufacturing method characterized by performing stirring and blowing an inert gas below the molten steel bath surface without heating the molten steel or the like with an electrode while maintaining the concentration at 10% by volume or less. . Also, the撹絆of the molten steel, preferably carried out so as to satisfy the condition of stirring power density indicated by the equation (1).
[0009]
ε = 742 · (QT / W) · ln (1 + gρH / P) ≧ 200 (1)
Q: flow rate of inert gas (m ³ / s), T: temperature of inert gas (K), W: weight of molten steel (t), H: flying distance of inert gas (m), P: atmosphere Pressure (Pa), g: Gravitational acceleration (m / s ² ), ρ: Density of molten steel (kg / m ³ )
According to the present invention, an ultra-low sulfur steel having a sulfur concentration of 14 ppm or less can be stably and easily produced using existing ladle, lid, and gas blowing lance even without severe hot metal pretreatment. become able to.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below based on the circumstances leading to the invention.
[0011]
First, the inventor sought the possibility of desulfurization to an extremely low sulfur concentration region without using vacuum degassing tanks such as RH and VOD, which have high operating costs, and the LF device. Specifically, the molten steel produced from the converter after decarburization is received in a ladle, CaO-based refining flux and Al are added, and then an inert gas is blown through a gas blowing lance. Such a method of desulfurizing molten steel is a technique that forms part of the invention described in Patent Document 1 described above. However, the invention described in Patent Document 1 is intended to produce an unprecedented ultra-low sulfur steel having a sulfur concentration in steel of 3 ppm or less, and the sulfur concentration is reduced to a level of 10 to 35 ppm in the hot metal pretreatment stage. Reduce the sulfur concentration in the steel to 4 ppm or less by gas agitation in the ladle after the converter steel, and also add excessive flux in the subsequent RH treatment to desulfurize. there were. The present inventor took out only a part of the gas stirring treatment in the ladle according to the prior art and conducted an experiment considering that the molten steel is desulfurized to an extremely low sulfur level of 14 ppm or less. Faced with. In other words, although not as much as the ultra-low sulfur level intended by the above-mentioned prior art, an attempt is made to desulfurize to a very low sulfur region where the sulfur concentration is 14 ppm or less without overloading the preceding and following processes such as hot metal pretreatment and RH. For example, a large amount of basic unit exceeding 20 kg per ton of molten steel is required as desulfurization flux, and it has been found that the drop in molten steel temperature cannot be ignored. In order to prevent this, a high temperature close to 1700 ° C. is required as the steel output temperature of the converter, and there is a concern that the refractory life of the converter may be greatly impaired. Therefore, the inventor considered to increase the reaction efficiency of the flux in the desulfurization process by gas blowing stirring.
[0012]
The desulfurization of molten steel is performed by a so-called “slag-metal reaction”, and stirring of slag and metal (in this case, molten steel) is important to promote the reaction. When this stirring is performed by gas blowing stirring through the gas blowing lance, in the atmosphere, the molten steel and slag are oxidized by oxygen contained in the atmosphere, thereby inhibiting the desulfurization reaction. In view of this, the inventor has thought that in order to reduce the sulfur concentration to 5 to 14 ppm, it is necessary to prevent a decrease in the desulfurization efficiency due to atmospheric oxidation. Therefore, as shown in FIG. 2, the ladle 2 holding the molten steel 1 and the slag 4 is covered with a lid 5 to prevent further intrusion of the atmosphere, and is formed between a bath surface of molten steel or the like and the lid. The atmosphere remaining in the freeboard 6 is replaced with an inert gas such as argon or nitrogen supplied from the pipe 7, and the inert gas is kept flowing so that oxygen does not enter the atmosphere of the freeboard 6 during stirring. No. 3 was used to perform an experiment in which an inert gas was blown and stirred into the molten steel. The atmospheric gas in the free board 6 was sampled by the gas sampling tube 8, and the oxygen concentration in the gas was measured by the oxygen concentration measuring device 9.
[0013]
The results are shown in FIG. 1, where B is the case where the oxygen concentration in the atmosphere of the free board 6 is 10% by volume or less and B is higher than 10% by volume.
[0014]
As a result, as shown in FIG. 1, it has been found that the object can be achieved if the oxygen concentration in the atmosphere in the free board 6 is 10% by volume or less. Therefore, the present invention has been completed based on this fact. .
[0015]
Moreover, in the experiment, it investigated also about desulfurization to 5-14 ppm under normal pressure, without heating the molten steel etc. by the electrode which is incidental to LF apparatus. As a result, it was found that the gas blowing stirring may be performed so that the stirring power density ε calculated below is 200 watts / t or more, and the present invention including this requirement was also completed (see FIG. 1).
[0016]
ε = 742 (QT / W) · ln (1 + gρH / P) ≧ 200 (1)
Where, Q: flow rate of inert gas (m ³ / s), T: temperature of inert gas (K), W: weight of molten steel (t), H: flying height of inert gas (m), P : Pressure of atmosphere (Pa), g: acceleration of gravity (m / s ² ), ρ: density of molten steel (kg / m ³ )
In this way, in the desulfurization process by the gas blowing and stirring to the molten steel in the ladle in the above-described prior art, about 1 kg of flux per ton of molten steel was required to reduce the sulfur concentration of molten steel by 1 ppm. In comparison, it has been found that the present invention requires less than half that amount of flux.
[0017]
Thus, since sufficient desulfurization can be achieved with a small amount of flux at the molten steel stage after the converter steel, it is possible to reduce the load of preliminary desulfurization at the hot metal stage before charging the converter. For example, when manufacturing molten steel having a sulfur concentration of 10 ppm required as a product, if desulfurization is performed using a 15 kg / t flux by gas blowing stirring treatment, the desulfurization amount is 30 ppm or more. The sulfur concentration in the molten steel at the time of steel (since almost no desulfurization is expected in the converter, it is the same as the sulfur concentration of the molten iron at the end of the hot metal pretreatment) may be about 40 ppm. Therefore, in the hot metal pretreatment, it is possible to keep the sulfur concentration after desulfurization higher than the value in the prior art, for example, exceeding 35 ppm, and the load of the hot metal pretreatment can be reduced.
[0018]
【Example】
Using the hot metal discharged from the blast furnace, dephosphorization, desulfurization and decarburization were carried out in the following steps to produce extremely low sulfur steel.
[0019]
Dephosphorization of hot metal pretreatment (injection method using topped car) → Desulfurization of hot metal pretreatment (desulfurization method by mechanical stirring) → Converter (decarburization) → Desulfurization by gas blowing stirring → RH degassing (component adjustment)
In most cases, desulfurization by gas blowing and stirring of molten steel after converter steel was performed with the ladle covered by the apparatus configuration shown in FIG. 2, but it was also performed without covering. That is, the case where the manufacturing method of the ultra-low sulfur steel according to the present invention was applied and the case where the requirements of the present invention were not satisfied were carried out. Argon was used for both the inert gas supplied to the freeboard and the inert gas used for gas blowing and stirring. The conditions and results of the implementation are collectively shown in Table 1.
[0020]
[Table 1]

[0021]
From Table 1, according to the present invention, the oxygen concentration in the atmosphere of the free board is 10% by volume or less, so that desulfurization in the hot metal pretreatment stage is performed so that the sulfur concentration in the hot metal remains at 40 ppm, It is clear that the sulfur concentration in molten steel can be reduced to 8-14 ppm. In addition, the other component of the obtained molten steel was 0.08 mass% C, 0.70 mass% Si, 1.50 mass% Mn, and 0.02 mass% P.
[0022]
In Table 1, FB is free board, and Bu is gas blowing and stirring. Moreover, the sulfur density | concentration in molten steel could be reduced to 11 ppm or less by making the stirring power density at the time of gas blowing stirring into 200 W / t or more.
[0023]
【The invention's effect】
As described above, according to the present invention, the existing ladle, lid, and gas blowing lance can be used simply and inexpensively without the hot metal preliminary treatment being severely performed and without expensive vacuum degassing treatment. An extremely low sulfur steel having a sulfur concentration of 14 ppm or less can be stably produced.
[Brief description of the drawings]
FIG. 1 is a view showing a “relationship between gas stirring power density during desulfurization treatment of molten steel and sulfur concentration in a continuous cast piece” as a basis of the present invention.
FIG. 2 is a longitudinal sectional view showing an apparatus for carrying out the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Molten steel 2 Ladle 3 Gas blowing lance 4 Slag 5 Lid 6 Free board 7 Inert gas supply piping 8 to free board Gas sampling pipe 9 in free board 9 Oxygen concentration measuring device

Claims

The molten steel that has been melted through preliminary desulfurization treatment of hot metal and decarburization refining in the converter is discharged from the converter to the ladle, CaO-based refining flux and Al are added, and then under the surface of the molten steel bath under normal pressure When producing ultra-low sulfur steel by stirring and blowing with inert gas,
After the ladle is shut off to the atmosphere with a lid, the free board portion formed between the lid and the molten steel bath surface is replaced with an inert gas, and then the inert gas is blown and stirred under the molten steel bath surface. Further, by supplying an inert gas separately to the freeboard , the molten steel bath is maintained without heating the molten steel or the like with the electrode while maintaining the oxygen concentration in the atmosphere of the freeboard portion at 10% by volume or less. A method for producing ultra-low sulfur steel, characterized by performing stirring and blowing an inert gas below the surface.

The method for producing an ultra-low-sulfur steel according to claim 1, wherein the stirring of the molten steel is performed so as to satisfy the condition of the stirring power density represented by the formula (1).
ε = 742 · (QT / W) · 1n (1 + gρH / P) ≧ 200 (1)
Q: Flow rate of inert gas (m ^３3 / S), T: temperature of the inert gas (K),
W: Weight of molten steel (t), H: Flying distance of inert gas (m), P: Pressure of atmosphere (Pa), g: Gravitational acceleration (m / s _２2 ), Ρ: Density of molten steel (kg / m ^３3 )