JP4555512B2 - Slag reforming method - Google Patents
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Description
【０００１】
【発明の属する技術分野】
本発明は、高清浄鋼を得るためのスラグの改質方法に関するものである。
【０００２】
【従来の技術】
極低炭素鋼の製造においては、転炉で酸素を吹き付け０．０４％Ｃ程度まで脱炭処理した溶鋼を、さらに真空脱ガス処理して固溶酸素と炭素との反応によりＣ濃度を数十ＰＰＭ程度まで低減する。この脱炭処理により溶鋼中の酸素濃度は大きく増加するが、脱炭処理後にＡｌを添加して溶鋼を脱酸すると共に、真空脱ガス装置を用いて軽真空度で溶鋼を環流させるため、脱酸により生成した多数のアルミナ系介在物は除去される。しかしながら、転炉での脱炭処理に伴いスラグ中の酸素濃度（ＭｎＯ、ＦｅＯ等の低級酸化物の濃度）も増加するため、Ａｌ脱酸後にスラグ中の酸素が溶鋼中のＡｌと下記（１）式および（２）式で表される再酸化反応を起こし、溶鋼の清浄性を大きく低下させる。この再酸化反応は、Ｃ濃度を低下させるために多量の酸素を吹き付け、スラグ中の酸素濃度が高い極低炭素鋼で激しく、その対策は重要な課題となっている。
【０００３】
３ＭｎＯ＋２Ａｌ＝Ａｌ2Ｏ3＋３Ｍｎ …… （１）
３ＦｅＯ＋２Ａｌ＝Ａｌ2Ｏ3＋３Ｆｅ …… （２）
このため、例えば、特開昭５９−７０７１０号公報や特許２６９０３５０号公報では、転炉出鋼後の取鍋スラグにＡｌ又はＡｌ含有フラックスを添加し、また特開平６−１２２９１７号公報ではＡｌ又はＡｌ合金を含む複数本の棒状材料をスラグ中に浸漬させ、スラグ中のＭｎＯやＦｅＯの低級酸化物を還元するスラグ改質方法が実施され、スラグによる溶鋼再酸化の防止が図られている。
【０００４】
【発明が解決しようとする課題】
従来のスラグ改質方法では、スラグ表面にＡｌを添加する際に、取鍋底のポーラスプラグからＡｒガス等の不活性ガスを吹き込み、スラグを攪拌することにより、スラグ中のＭｎＯ、ＦｅＯを十分に還元しようとしている。このため、スラグがバブリングの攪拌により不必要に巻き込まれ、スラグ起因の介在物欠陥の原因となっていた。さらに、従来の方法では必ずしもスラグ全体に均一にＡｌを添加できていなかったため、スラグが部分的に未還元状態となり、スラグによる溶鋼の再酸化を十分に防止できなかった。また、スラグに添加したＡｌの一部は、溶鋼中に溶け込みＡｌ濃度が必要以上に高くなるため、溶鋼中Ａｌの再酸化速度が速くなり、溶鋼の清浄性が低下するといった問題もあった。
【０００５】
これらの問題に鑑み、本発明は、スラグの巻き込みや溶鋼成分の変化を生じることなく、スラグ中のＭｎＯやＦｅＯを確実に還元できるスラグの改質方法を提供することを課題とするものである。
【０００６】
【課題を解決するための手段】
上記課題を解決するために、本発明は以下の構成を要旨とする。即ち、（１）転炉で脱炭処理した溶鋼を取鍋内に出鋼した後、真空脱ガス処理を行うまでに、取鍋内の溶鋼上にあるスラグの酸素濃度を低減するスラグ改質法において、ＭｇＯ含有率を３０質量％以上、Ｓｉ、Ａｌ、Ｔｉ、Ｚｒの内１種類以上の還元材の総含有率を１０質量％以上としたスラグ改質材を、前記の取鍋内の溶鋼上にあるスラグに上方から添加して、スラグ中の低級酸化物を還元することを特徴とするスラグ改質方法である。また、（２）転炉で脱炭処理した溶鋼を取鍋内に出鋼した後、真空脱ガス処理を行うまでに、取鍋内の溶鋼上にあるスラグの酸素濃度を低減するスラグ改質法において、ＭｇＯ含有率を３０質量％以上、Ｓｉ、Ａｌ、Ｔｉ、Ｚｒの内１種類以上の還元材の総含有率を１０質量％以上とした平均粒径が０．５ｍｍから５０ｍｍのスラグ改質材を、前記の取鍋内の溶鋼上にあるスラグに上方から添加して、スラグ中の低級酸化物を還元することを特徴とするスラグ改質方法である。また、（３）転炉で脱炭処理した溶鋼を取鍋内に出鋼した後、真空脱ガス処理を行うまでに、取鍋内の溶鋼上にあるスラグの酸素濃度を低減するスラグ改質法において、ＭｇＯ含有率を３０質量％以上、Ｓｉ、Ａｌ、Ｔｉ、Ｚｒの内１種類以上の還元材の総含有率を１０質量％以上としたスラグ改質材を、前記の取鍋内の溶鋼上にあるスラグに上方から添加し、その後取鍋に耐火蓋を被せてスラグ中の低級酸化物を還元することを特徴とするスラグ改質方法である。また、（４）転炉で脱炭処理した溶鋼を取鍋内に出鋼した後、真空脱ガス処理を行うまでに、取鍋内の溶鋼上にあるスラグの酸素濃度を低減するスラグ改質法において、ＭｇＯ含有率を３０質量％以上、Ｓｉ、Ａｌ、Ｔｉ、Ｚｒの内１種類以上の還元材の総含有率を１０質量％以上とした平均粒径が０．５ｍｍから５０ｍｍのスラグ改質材を、前記の取鍋内の溶鋼上にあるスラグに上方から添加し、その後取鍋に耐火蓋を被せてスラグ中の低級酸化物を還元することを特徴とするスラグ改質方法である。
【０００７】
【発明の実施の形態】
図１に本発明の実施形態を示す。ＭｇＯと還元材として例えばＡｌからなるスラグ改質材１を、取鍋２内溶鋼３上のスラグ４に上方から添加すると、改質材１の温度の上昇と共に、取鍋２内のスラグ４中に徐々に溶解していく。同時に、改質材１中で下記（３）式の反応が起こり、発生したＭｇガス５がスラグ４中に連続的に供給される。
【０００８】
ＭｇＯ＋Ａｌ＝Ｍｇ（ガス）＋Ａｌ2Ｏ3 …… （３）
このため、Ｍｇガス５はスラグ４中のＭｎＯやＦｅＯを下記（４）式および（５）式により還元し、スラグの酸化度を低減することができる。
ＭｎＯ＋Ｍｇ（ガス）＝ＭｇＯ＋Ｍｎ …… （４）
ＦｅＯ＋Ｍｇ（ガス）＝ＭｇＯ＋Ｆｅ …… （５）
ＭｇはＡｌよりも還元力が強いため、スラグの還元はＡｌに比べて速やかに進行する。このため、不必要にスラグを攪拌する必要がなく、スラグの巻き込みによる介在物欠陥の発生を防止できる。また、改質材中で発生したＭｇガスの一部は直接気相側に移行し、取鍋内のスラグ表面全体にＭｇガス層を形成する。このため、Ｍｇガスはスラグ全面に供給され、スラグ全体に渡って均一に還元反応が進行する。この際、取鍋に耐火蓋６を被せると、Ｍｇガスが取鍋外に拡散することを抑制できるため、Ｍｇガスによるスラグ改質をより効率的に実施できる。さらに、本発明ではＭｇを金属としてスラグに添加するのではなく、Ｍｇガスとしてスラグ中、或いはスラグ表面に供給するため、溶鋼中に溶け込むことはなく、溶鋼成分を変化させるといった問題も生じない。
【０００９】
本発明に関する上記説明は、スラグ改質材中の還元材としてＡｌを例に行ったが、還元材はＡｌに限られたものではなく、ＭｇＯを十分に還元できるものであれば良く、本発明者らの実験的検討ではＳｉ、Ｔｉ、Ｚｒでも良いことが分かっている。また、スラグ改質材中のＭｇＯ源は、必ずしも純粋なＭｇＯに限られたものではなく、ＭｇＡｌ2Ｏ4等のようにＭｇＯを含む化合物であっても良い。
【００１０】
スラグ改質材のＭｇＯ含有率は３０質量％以上、還元材の含有率は１０質量％以上にする必要があり、何れもそれ未満ではスラグを改質するのに十分なＭｇガスを供給できないためである。また、スラグ改質材には、比重調整用の鉄粉を加えたり、ＭｇＯ以外の酸化物を含有させることも可能であり、その場合上記ＭｇＯ含有率と還元材含有率を満足していれば良い。本発明のスラグ改質材の平均粒径は、０．５ｍｍから５０ｍｍ程度が好ましい。改質材の平均粒径が０．５ｍｍ未満では改質材のスラグへの溶解速度が速くなり、（３）式のＭｇガス発生反応を行わせるのに十分な時間が確保できなくなるため、改質材の平均粒径が５０ｍｍ超では改質材の温度上昇が遅くなり（３）式のＭｇガス発生反応の速度が低下するためである。
【００１１】
本発明は、スラグからの再酸化が激しい極低炭素鋼でその効果が大きいが、必ずしもこれに限られたものではなく、その原理から考えて低炭素鋼でも十分適用できる。特に、Ａｌ合金を脱酸材として使用しないか、或いは使用しても僅かであるＴｉ添加低酸素鋼等ではスラグ改質によるＡｌの混入を防止できる点で、本発明のスラグ改質法は優れた効果を有している。
【００１２】
【実施例】
以下に、実施例及び比較例を挙げて、本発明について説明する。
（実施例１）
転炉で脱炭処理した溶鋼２８０ｔを取鍋内に出鋼し、スラグ改質場所まで移動させた。スラグ改質場所で取鍋スラグ１０００ｋｇに対し、６９質量％ＭｇＯ−３１質量％Ａｌの平均粒径３〜５ｍｍのスラグ改質材１５０ｋｇを添加した。これによりスラグ中のトータルＦｅは全体的に１０質量％から３質量％まで低下した。この溶鋼を真空脱ガス処理して脱酸した後、成分調整し、成分Ｃ：３０ｐｐｍ、Ｓｉ：０．０１５質量％、Ｍｎ：０．２５質量％、Ｐ：０．０２質量％、Ｓ：０．０１質量％の溶鋼を溶製した。その後、鋳造速度１．６ｍ／ｍｉｎで鋳造した。得られた鋳片のＡｌ濃度は０．０４質量％、全酸素濃度（Ｔ．Ｏ）は１５ｐｐｍ以下となり高清浄鋼が得られた。
（実施例２）
転炉で脱炭処理した溶鋼２８０ｔを取鍋内に出鋼し、スラグ改質場所まで移動させた。スラグ改質場所で取鍋スラグ１０００ｋｇに対し、３０質量％Ａｌ2Ｏ3−４４質量％ＭｇＯ−２６質量％Ｔｉの平均粒径１〜５ｍｍのスラグ改質材２２０ｋｇを添加した。これによりスラグ中のトータルＦｅは全体的に１０質量％から２質量％まで低下した。この溶鋼を真空脱ガス処理して脱酸した後、成分調整し、成分Ｃ：３０ｐｐｍ、Ｓｉ：０．０１５質量％、Ｍｎ：０．２５質量％、Ｐ：０．０２質量％、Ｓ：０．０１質量％の溶鋼を溶製した。その後、鋳造速度１．６ｍ／ｍｉｎで鋳造した。得られた鋳片のＡｌ濃度は０．０４質量％、全酸素濃度（Ｔ．Ｏ）は１４ｐｐｍ以下となり高清浄鋼が得られた。
（比較例１）
転炉で脱炭処理した溶鋼２８０ｔを取鍋内に出鋼した。この溶鋼はスラグ改質せずに、真空脱ガス処理して成分調整し、成分Ｃ：３０ｐｐｍ、Ｓｉ：０．０１５質量％、Ｍｎ：０．２５質量％、Ｐ：０．０２質量％、Ｓ：０．０１質量％の溶鋼を溶製した。取鍋内のスラグ中トータルＦｅは１０質量％であった。その後、鋳造速度１．６ｍ／ｍｉｎで鋳造した。得られた鋳片のＡｌ濃度は０．０３５質量％、（Ｔ．Ｏ）は４０ｐｐｍ以上となり高清浄鋼が得られなかった。
（比較例２）
転炉で脱炭処理した溶鋼２８０ｔを取鍋内に出鋼し、スラグ改質場所まで移動させた。スラグ改質場所で取鍋スラグ１０００ｋｇに対し金属Ａｌを２００ｋｇ添加して、スラグ改質した。スラグ中のトータルＦｅは測定位置によるバラツキが大きく、改質後もトータルＦｅで８質量％程度と高い値を示す場所もあった。この溶鋼を真空脱ガス処理して脱酸・成分調整し、成分Ｃ：３０ｐｐｍ、Ｓｉ：０．０１５質量％、Ｍｎ：０．２５質量％、Ｐ：０．０２質量％、Ｓ：０．０１質量％の溶鋼を溶製した。その後、鋳造速度１．６ｍ／ｍｉｎで鋳造した。得られた鋳片のＡｌ濃度は０．０５５質量％、全酸素濃度（Ｔ．Ｏ）は３０ｐｐｍ以上となり高清浄鋼が得られなかった。
【００１３】
【発明の効果】
以上の如く、本発明のスラグ改質方法を適用することにより、スラグの巻き込みや溶鋼成分の変化を生じることなく、スラグ中の酸素を確実に還元できるため、鋳片の品質は向上し、歩留まりは格段に向上する。また、清浄性が向上するため、浸漬ノズルの閉塞も抑制されるため、非定常作業を省略することができ、操業性も大きく改善される。
【図面の簡単な説明】
【図１】本発明の実施形態を示す図。
【符号の説明】
１…スラグ改質材
２…取鍋
３…溶鋼
４…スラグ
５…Ｍｇガス
６…耐火蓋[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a slag reforming method for obtaining highly clean steel.
[0002]
[Prior art]
In the production of ultra-low carbon steel, molten steel that has been decarburized to about 0.04% C by blowing oxygen in a converter is further vacuum degassed, and the C concentration is increased to several tens by reaction between solid solution oxygen and carbon. Reduce to about PPM. Although the oxygen concentration in the molten steel is greatly increased by this decarburization treatment, Al is added after the decarburization treatment to deoxidize the molten steel, and the molten steel is circulated at a light vacuum using a vacuum degassing device. Many alumina inclusions produced by the acid are removed. However, since the oxygen concentration in the slag (concentration of lower oxides such as MnO and FeO) also increases with the decarburization treatment in the converter, the oxygen in the slag after Al deoxidation is reduced to the following (1 ) And the reoxidation reaction represented by the formula (2) are caused to greatly reduce the cleanliness of the molten steel. This re-oxidation reaction is intense in ultra-low carbon steel, in which a large amount of oxygen is blown to lower the C concentration and the oxygen concentration in the slag is high, and countermeasures are an important issue.
[0003]
3MnO + 2Al = Al 2 O 3 + 3Mn (1)
3FeO + 2Al = Al 2 O 3 + 3Fe (2)
For this reason, for example, in Japanese Patent Laid-Open No. 59-70710 and Japanese Patent No. 2690350, Al or Al-containing flux is added to the ladle slag after the steel from the converter, and in Japanese Patent Laid-Open No. 6-122917, Al or A slag reforming method in which a plurality of rod-like materials containing an Al alloy is immersed in slag and a lower oxide of MnO or FeO in the slag is reduced is performed, and prevention of reoxidation of molten steel by the slag is achieved.
[0004]
[Problems to be solved by the invention]
In the conventional slag reforming method, when adding Al to the slag surface, an inert gas such as Ar gas is blown from a porous plug at the bottom of the ladle and the slag is stirred, so that MnO and FeO in the slag are sufficiently obtained. Trying to reduce. For this reason, the slag was unnecessarily engulfed by bubbling agitation, causing inclusion defects due to the slag. Furthermore, in the conventional method, Al could not be uniformly added to the entire slag, so that the slag was partially unreduced and the reoxidation of the molten steel by the slag could not be sufficiently prevented. In addition, since a part of Al added to the slag melts into the molten steel and the Al concentration becomes higher than necessary, the reoxidation rate of Al in the molten steel is increased and the cleanliness of the molten steel is reduced.
[0005]
In view of these problems, an object of the present invention is to provide a slag reforming method that can reliably reduce MnO and FeO in slag without causing slag entrainment and changes in molten steel components. .
[0006]
[Means for Solving the Problems]
In order to solve the above problems, the present invention is summarized as following configuration. That is, (1) Slag reforming that reduces the oxygen concentration of slag on the molten steel in the ladle after the decarburized molten steel is put into the ladle and vacuum degassing is performed. In the method, a slag modifier having an MgO content of 30% by mass or more and a total content of one or more reducing materials of Si, Al, Ti, Zr of 10% by mass or more is contained in the ladle. The slag reforming method is characterized in that the lower oxide in the slag is reduced by adding to the slag on the molten steel from above . Further, (2) after the molten steel was decarburization in a converter furnace and tapped into the ladle, before performing vacuum degassing process, slag reforming to reduce the oxygen concentration in the slag in the molten steel in the ladle In this method, the slag modification with an average particle size of 0.5 mm to 50 mm with an MgO content of 30% by mass or more and a total content of one or more reducing materials of Si, Al, Ti, Zr of 10% by mass or more. It is a slag reforming method characterized by adding a material to slag on molten steel in the ladle from above to reduce lower oxides in the slag. (3) Slag reforming that reduces the oxygen concentration of the slag on the molten steel in the ladle after the decarburized molten steel is taken out into the ladle and vacuum degassing is performed. In the method, a slag modifier having an MgO content of 30% by mass or more and a total content of one or more reducing materials of Si, Al, Ti, Zr of 10% by mass or more is contained in the ladle. The slag reforming method is characterized in that a slag on molten steel is added from above, and a ladle is then put on a refractory cover to reduce lower oxides in the slag. (4) Slag reforming that reduces the oxygen concentration of the slag on the molten steel in the ladle after the decarburized molten steel is taken out in the converter and vacuum degassing is performed. In this method, the slag modification with an average particle size of 0.5 mm to 50 mm with an MgO content of 30% by mass or more and a total content of one or more reducing materials of Si, Al, Ti, Zr of 10% by mass or more. The material is added to the slag on the molten steel in the ladle from above , and then the ladle is covered with a refractory lid to reduce lower oxides in the slag. .
[0007]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows an embodiment of the present invention. When a slag modifier 1 made of, for example, Al as MgO and a reducing material is added from above to the slag 4 on the
[0008]
MgO + Al = Mg (gas) + Al 2 O 3 (3)
For this reason, the Mg gas 5 can reduce MnO and FeO in the slag 4 by the following formulas (4) and (5) and reduce the degree of oxidation of the slag.
MnO + Mg (gas) = MgO + Mn (4)
FeO + Mg (gas) = MgO + Fe (5)
Since Mg has a reducing power stronger than Al, slag reduction proceeds more rapidly than Al. For this reason, it is not necessary to stir the slag unnecessarily, and the occurrence of inclusion defects due to the slag entrainment can be prevented. Moreover, a part of Mg gas generated in the reforming material moves directly to the gas phase side, and forms an Mg gas layer on the entire slag surface in the ladle. For this reason, Mg gas is supplied to the whole surface of the slag, and the reduction reaction proceeds uniformly over the entire slag. At this time, if the ladle is covered with the
[0009]
In the above description of the present invention, Al is used as an example of the reducing material in the slag modifier. However, the reducing material is not limited to Al, and may be any material that can sufficiently reduce MgO. Their experimental study has shown that Si, Ti, and Zr may be used. The MgO source in the slag modifier is not necessarily limited to pure MgO, and may be a compound containing MgO, such as MgAl 2 O 4 .
[0010]
The MgO content of the slag modifying material must be 30% by mass or more, and the content of the reducing material must be 10% by mass or more. If both are less than that, sufficient Mg gas cannot be supplied to modify the slag. It is. In addition, iron powder for adjusting specific gravity can be added to the slag modifier, or oxides other than MgO can be contained, and in that case, if the above MgO content and reducing material content are satisfied good. The average particle size of the slag modifier of the present invention is preferably about 0.5 mm to 50 mm. If the average particle size of the modifying material is less than 0.5 mm, the dissolution rate of the modifying material in the slag increases, and it becomes impossible to secure sufficient time for the Mg gas generation reaction of formula (3) to be performed. This is because when the average particle diameter of the material exceeds 50 mm, the temperature of the reforming material rises slowly and the rate of the Mg gas generation reaction of formula (3) decreases.
[0011]
The present invention is extremely low carbon steel that is highly reoxidized from slag, and the effect is large. However, the present invention is not necessarily limited to this, and low carbon steel can be sufficiently applied in view of its principle. In particular, the slag reforming method of the present invention is superior in that it does not use an Al alloy as a deoxidizing material, or in the case of Ti-added low-oxygen steel, etc. It has the effect.
[0012]
【Example】
Hereinafter, the present invention will be described with reference to examples and comparative examples.
Example 1
280 t of molten steel decarburized in the converter was taken out into the ladle and moved to the slag reforming site. 150 kg of slag modifier having an average particle diameter of 3 to 5 mm of 69 mass% MgO-31 mass% Al was added to 1000 kg of ladle slag at the slag modification site. Thereby, the total Fe in the slag was reduced from 10% by mass to 3% by mass as a whole. The molten steel was degassed by vacuum degassing, and then the components were adjusted. Component C: 30 ppm, Si: 0.015 mass%, Mn: 0.25 mass%, P: 0.02 mass%, S: 0 .01% by mass of molten steel was produced. Thereafter, casting was performed at a casting speed of 1.6 m / min. The resulting cast slab had an Al concentration of 0.04% by mass and a total oxygen concentration (TO) of 15 ppm or less, and a highly clean steel was obtained.
(Example 2)
280 t of molten steel decarburized in the converter was taken out into the ladle and moved to the slag reforming site. 220 kg of slag modifier having an average particle diameter of 1 to 5 mm of 30% by mass Al 2 O 3 -44% by mass MgO-26% by mass Ti was added to 1000 kg of ladle slag at the slag reforming place. Thereby, the total Fe in the slag was reduced from 10% by mass to 2% by mass as a whole. The molten steel was degassed by vacuum degassing, and then the components were adjusted. Component C: 30 ppm, Si: 0.015 mass%, Mn: 0.25 mass%, P: 0.02 mass%, S: 0 .01% by mass of molten steel was produced. Thereafter, casting was performed at a casting speed of 1.6 m / min. The resulting cast slab had an Al concentration of 0.04% by mass and a total oxygen concentration (TO) of 14 ppm or less, and a highly clean steel was obtained.
(Comparative Example 1)
280 t of molten steel decarburized in the converter was taken out into the ladle. This molten steel was subjected to vacuum degassing treatment without slag reforming and component adjustment, component C: 30 ppm, Si: 0.015 mass%, Mn: 0.25 mass%, P: 0.02 mass%, S : 0.01% by mass of molten steel was produced. Total Fe in the slag in the ladle was 10% by mass. Thereafter, casting was performed at a casting speed of 1.6 m / min. The resulting slab had an Al concentration of 0.035% by mass, and (T.O) was 40 ppm or more, and a highly clean steel could not be obtained.
(Comparative Example 2)
280 t of molten steel decarburized in the converter was taken out into the ladle and moved to the slag reforming site. 200 kg of metal Al was added to 1000 kg of ladle slag at the slag reforming place to reform the slag. The total Fe in the slag varies greatly depending on the measurement position, and there were places where the total Fe was as high as about 8% by mass even after modification. This molten steel was subjected to vacuum degassing treatment for deoxidation and component adjustment. Component C: 30 ppm, Si: 0.015 mass%, Mn: 0.25 mass%, P: 0.02 mass%, S: 0.01 A mass% of molten steel was produced. Thereafter, casting was performed at a casting speed of 1.6 m / min. The obtained cast slab had an Al concentration of 0.055% by mass and a total oxygen concentration (TO) of 30 ppm or more, and a highly clean steel was not obtained.
[0013]
【The invention's effect】
As described above, by applying the slag reforming method of the present invention, oxygen in the slag can be reliably reduced without causing slag entrainment or changes in the molten steel components, so the quality of the slab is improved and the yield is increased. Is significantly improved. Further, since the cleanliness is improved, the immersion nozzle is also blocked, so that unsteady work can be omitted, and the operability is greatly improved.
[Brief description of the drawings]
FIG. 1 shows an embodiment of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Slag modifier 2 ...
Claims (4)
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| KR101617747B1 (en) | 2014-08-26 | 2016-05-03 | 주식회사 포스코 | Method for Refining Molten Steel by Converter |
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