JP5151448B2 - Method of melting ultra-low sulfur ultra-low oxygen ultra-low nitrogen steel - Google Patents
Method of melting ultra-low sulfur ultra-low oxygen ultra-low nitrogen steel Download PDFInfo
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Description
本発明はラインパイプ、ガスタンク、鋼構造体等に用いられる極低硫極低酸素極低窒素厚板鋼の溶製方法に関し、詳しくは、減圧処理において短時間で鋼中酸素濃度、窒素濃度、硫黄濃度を極低濃度域まで低減する鋼の溶製方法に関する。 The present invention relates to a method for melting ultra-low sulfur ultra-low oxygen ultra-low nitrogen thick steel plate used for line pipes, gas tanks, steel structures, etc. The present invention relates to a steel melting method for reducing the sulfur concentration to an extremely low concentration range.
鋼中の硫黄（以下、「Ｓ」と記する。）、酸素(以下、「Ｏ」と記する。）、および窒素(以下、「Ｎ」と記する。）は、各種欠陥や溶接性の低下を招くため、従来からこれらの低減技術が多数開発されてきた。 Sulfur (hereinafter referred to as “S”), oxygen (hereinafter referred to as “O”), and nitrogen (hereinafter referred to as “N”) in steel are used for various defects and weldability. Many of these reduction techniques have been developed so far in order to cause a decrease.
しかし、近年では、要求性能が更に高まり、単なるＳ低減、Ｏ低減またはＮ低減のみでは不十分となり、Ｓ，Ｏ，Ｎを同時に低減することが求められるようになった。これに対応するには、従来のＳ，Ｏ，Ｎの各不純物を低減する技術の向上が必要となり、そのための技術がこれまでも提案されてきている（例えば特許文献１〜３）。
However, in recent years, the required performance has further increased, and simple S reduction, O reduction, or N reduction has become insufficient, and it has become necessary to simultaneously reduce S, O, and N. In order to cope with this, it is necessary to improve conventional techniques for reducing impurities of S, O, and N, and techniques for that purpose have been proposed (for example,
加えて、従来開発されたＳ，Ｏ，Ｎそれぞれの低減技術を用いることで、Ｓ，Ｏ，Ｎを同時に低減できると考えられてきたが、これが困難であることが解ってきた。
第一の理由は生産性の問題である。Ｓ，Ｏ，Ｎそれぞれを低減する処理を単純に組み合わせれば、処理時間が単に長くなるだけではなく、溶鋼温度降下の観点からそういった長時間処理は実質的に不可能である。
In addition, it has been considered that S, O, and N can be simultaneously reduced by using conventionally developed technologies for reducing S, O, and N, but it has been found that this is difficult.
The first reason is a productivity problem. If the treatments for reducing S, O, and N are simply combined, the treatment time is not only long, but such a long treatment is virtually impossible from the viewpoint of the temperature drop of the molten steel.
第二の理由は各元素の低減処理の化学反応が互いに干渉することである。例えばＳ、Ｏを低減すると溶鋼は吸窒しやすくなるためＮ低減が困難になる。また、介在物巻き込み抑制によるＯ低減のために溶鋼撹拌を弱めると、脱硫が滞りＳ低減が困難になる。 The second reason is that chemical reactions of the reduction treatment of each element interfere with each other. For example, when S and O are reduced, it becomes difficult to reduce N because the molten steel easily absorbs nitrogen. Moreover, if molten steel stirring is weakened in order to reduce O by suppressing inclusion inclusion, desulfurization is delayed and S reduction becomes difficult.
以上の理由から、工業的にはＳ，Ｏ，Ｎを同時に低減し、かつその低減量を従来よりも増加させることには課題が多かった。
本発明は、上記課題に鑑み、ＲＨ式真空脱ガス処理装置を用いた脱硫処理において工業的に容易にＳ，Ｏ，Ｎを同時に低減する鋼の溶製方法を提供することにある。 The present invention is to provide in view of the above problems, RH type vacuum degassing apparatus desulfurized odor Te industrial and easily S using, O, the melting method of the steel to reduce at the same time N.
１．ＲＨによる高効率脱硫時の高効率低Ｏ低Ｎ化技術
ＳおよびＯを低減すると溶鉄−気相界面における化学反応抵抗が小さくなるため、雰囲気中窒素分圧が高ければ吸窒が活発化するが、雰囲気中窒素分圧が十分に低ければ脱窒が活発化する。つまり、十分に雰囲気中窒素分圧を低くした状態で、Ｓ，Ｏを低減することで、Ｎも低減できることになる。十分に雰囲気中窒素分圧を低減できる工業的設備としてはＲＨ、ＶＯＤ、タンク脱ガスなどの真空脱ガス装置が一般的である。しかし、真空脱ガス装置を用いても、Ｓ，Ｏ，Ｎを同時に低減することは容易ではない。なぜなら、ＶＯＤやタンク脱ガス装置では、溶鋼表面にスラグが存在するためＳ低減は容易であるが、スラグの巻き込みによるＯ低減効果の低下、およびスラグによる溶鋼自由表面の減少とスラグ重量に起因する溶鋼表面静圧増加とによるＮ低減効果の低下が起こる。一方、ＲＨは真空槽内溶鋼表面にスラグが存在しないため、上記課題は生じないが、逆にＳ低減効果が低下する。
1. High-efficiency, low-O, low-N technology for high-efficiency desulfurization by RH If S and O are reduced, chemical reaction resistance at the molten iron-gas phase interface decreases, so if nitrogen partial pressure in the atmosphere is high, nitrogen absorption is activated If the nitrogen partial pressure in the atmosphere is sufficiently low, denitrification is activated. That is, N can also be reduced by reducing S and O in a state where the nitrogen partial pressure in the atmosphere is sufficiently lowered. As industrial equipment capable of sufficiently reducing the nitrogen partial pressure in the atmosphere, vacuum degassing apparatuses such as RH, VOD and tank degassing are common. However, even if a vacuum degassing apparatus is used, it is not easy to simultaneously reduce S, O, and N. This is because, in VOD and tank degassing equipment, S reduction is easy because slag exists on the surface of the molten steel. The N reduction effect is reduced due to an increase in the surface static pressure of the molten steel. On the other hand, since RH does not have slag on the surface of the molten steel in the vacuum chamber, the above problem does not occur, but the effect of reducing S is reduced.
以上の様に、低Ｓ，Ｏ，Ｎ溶鋼を得る原理は容易であるが、現状の工業的設備においてこれを実現することは容易ではない。
前述した工業的設備での各課題を検討すると、ＶＯＤやタンク脱ガス装置ではスラグによる脱窒阻害が発生するが、これを回避することは難しい。一方、ＲＨではスラグによる阻害が生じない。そこで、ＲＨにてＳ，Ｏ，Ｎを同時に効率よく極低濃度域まで低減できる技術を検討することとした。
As described above, the principle of obtaining low S, O, N molten steel is easy, but it is not easy to realize this in the current industrial equipment.
Examining the above-mentioned problems with industrial equipment, VOD and tank degassing equipment inhibit denitrification due to slag, but it is difficult to avoid this. On the other hand, RH does not inhibit slag. Therefore, it was decided to study a technology that can efficiently reduce S, O, and N simultaneously to an extremely low concentration region by RH.
ところで、ＲＨにおいて低Ｓ化を図る技術は従来から多数開発されており、前述の特許文献１〜３はそのような技術を開示している。これらの技術により、ＲＨにおいて極低Ｓ濃度までの高効率の脱硫が可能となっている。
By the way, many techniques for reducing S in RH have been developed, and the above-mentioned
しかしながら、近年要求されるのは、この高効率脱硫に加えて、低Ｏと低Ｎとを同時に図ることであり、従来技術では十分にこの要求に応えることができない。
そこで、本発明者らは従来のＲＨでの高効率脱硫に加えて、高効率低Ｏ低Ｎ化を図る技術を検討した。
However, in recent years, in addition to this high-efficiency desulfurization, it is necessary to simultaneously achieve low O and low N, and the prior art cannot fully meet this requirement.
Therefore, the present inventors examined a technique for achieving high efficiency, low O, and low N in addition to conventional high efficiency desulfurization with RH.
２．鋼成分
上記の方針に基づき検討を行うにあたって、まず、本発明の処理対象となる鉄以外の鋼成分を以下の理由により特定した。なお、本明細書において、鋼組成およびＲＥＭ濃度（詳細は後述。）における「％」は特にことわりがない場合は「質量％」を意味する。
2. Steel components In conducting the study based on the above policy, first, steel components other than iron to be treated in the present invention were specified for the following reasons. In the present specification, “%” in the steel composition and REM concentration (details will be described later) means “mass%” unless otherwise specified.
Ｃ:Ｃは減圧下で脱酸元素として作用する他に、Ｓ，Ｎの活量に影響する。このため、Ｃが０．００２％未満では低酸素化効果が不安定となり、０．４％を超えて高くなるとS,Nの活量が大きく変化し、反応機構が変化してしまう。そこで、Ｃは０．００２％以上０．４％以下とした。 C: C acts as a deoxidizing element under reduced pressure and affects the activities of S and N. For this reason, if C is less than 0.002%, the effect of reducing oxygen becomes unstable, and if it exceeds 0.4%, the activity of S and N changes greatly, and the reaction mechanism changes. Therefore, C is set to 0.002% or more and 0.4% or less.
Ｍｎ:Ｍｎも脱酸元素であり、各種鋼材特性を改善することから、必須元素である。従って、０．１％未満では脱酸が不安定になり、２％を超えて高くなるとＳの活量を低下させ、脱硫を困難とする。従って、Ｍｎ濃度は０．１％以上２％以下とした。 Mn: Mn is also a deoxidizing element and is an essential element because it improves various steel properties. Therefore, if it is less than 0.1%, deoxidation becomes unstable, and if it exceeds 2%, the activity of S is lowered and desulfurization is difficult. Therefore, the Mn concentration is set to 0.1% or more and 2% or less.
Ｓｉ:ＳｉもＭｎ同様脱酸安定に欠くことのできない元素であるが、０．００１％未満では脱酸が不安定となり、１％を超えて高くなるとＮ活量を増加させ脱窒を促進する。本発明では、そのような低Ｎ化が容易な成分系でない成分系において窒素を含む不純物を効率的に低減することを目的としているので、Ｓｉは１％以下とする。 Si: Si is an element indispensable for deoxidation stability as well as Mn. However, deoxidation is unstable if it is less than 0.001%, and if it exceeds 1%, N activity is increased to promote denitrification. . In the present invention, Si is set to 1% or less because the object is to efficiently reduce nitrogen-containing impurities in a component system that is not easy to reduce N.
Ａｌ:Ａｌは最も強い脱酸力を有する元素であるため、低Ｏ、低Ｓかつ低Ｎを実現するためには必須である。この脱酸効果を得るには０．００５％以上が必要である。一方、１％を超えて高くなると再び溶解酸素濃度が高くなって低Ｏを実現することが困難となるため、１％未満が必要である。 Al: Al is an element having the strongest deoxidizing power, and is essential for realizing low O, low S and low N. To obtain this deoxidation effect, 0.005% or more is necessary. On the other hand, if it exceeds 1%, the dissolved oxygen concentration becomes high again, and it becomes difficult to realize low O, so less than 1% is required.
Ｓ:Ｓは除去対象元素であるが、０．００５％を超えて高くなると、物質収支的に脱硫剤使用量が大幅に増加するため、コストが増加する。そこで、本発明では０．００５％以下の溶鋼を処理対象とする。 S: S is an element to be removed, but if it exceeds 0.005%, the amount of desulfurization agent used will increase significantly in terms of material balance, and the cost will increase. Therefore, in the present invention, 0.005% or less of molten steel is treated.
Ｎ:ＮもＳ同様除去対象元素であるが、０．００７％を超えて高くなると処理時間を短縮することが困難となるため、０．００７％以下の溶鋼を処理対象とした。 N: N is also an element to be removed as in S, but if it exceeds 0.007%, it becomes difficult to shorten the treatment time, so 0.007% or less of molten steel was treated.
Ｏ:Ｏも除去対象元素であるが、Ｓｉ，ＡｌおよびＭｎが上記の濃度範囲にあると、Ｏ濃度が０．００５％を超えて高い場合には、大量に非金属介在物（以下、「介在物」という。）が溶鋼中に存在することとなる。この状態で後述するＣａＯ系フラックスを用いると、ＣａＯフラックスが介在物と衝突することによって脱硫能が低下するため、Ｏ濃度は０．００５％以下とした。 O: O is also an element to be removed. However, when Si, Al, and Mn are in the above-mentioned concentration range, a large amount of non-metallic inclusions (hereinafter, “ "Inclusions") will be present in the molten steel. In this state, when a CaO-based flux described later is used, the desulfurization ability is lowered due to collision of the CaO flux with inclusions, so the O concentration is set to 0.005% or less.
上記の必須の成分のほか、脱硫、脱酸、および／または脱窒に影響を及ぼさない範囲で他の元素が含まれていてもよい。 In addition to the above essential components, other elements may be contained within a range not affecting desulfurization, deoxidation, and / or denitrification.
３．課題解決の基本方針
次に、上記の鋼組成を前提として、従来のＲＨ脱硫技術に低Ｏ化、低Ｎ化を付与可能な技術を具体的に検討した。
3. Basic Policy for Problem Solving Next, on the premise of the above steel composition, a technology capable of imparting low O and low N to the conventional RH desulfurization technology was specifically examined.
低Ｎ化を図るにはＳ，Ｏが低いことが有利であり、低Ｓを図るには低Ｏであることが有利である。すると、低Ｏ化を図ることが最も優先されることであり、さらにＳと親和力の強い元素を併用すれば効果が向上する可能性がある。すなわち、脱酸力を有し低Ｏ化を実現することができ、同時にＳと親和力のある元素を用いることが望ましい。また、本発明は真空処理を前提にしていることから、蒸気圧の低い元素であることが望ましい。 Low S and O are advantageous for achieving low N, and low O is advantageous for achieving low S. Then, it is the highest priority to achieve low O, and there is a possibility that the effect may be improved if elements having strong affinity with S are used together. That is, it is desirable to use an element that has deoxidizing power and can achieve low O, and at the same time has an affinity for S. Since the present invention is premised on vacuum processing, it is desirable that the element has a low vapor pressure.
以上のように考えると、この様な条件を満足する元素として、Ｌａ、Ｃｅ、Ｎｄ、Ｙ等の希土類元素（以下、「ＲＥＭ」と記する。）が知られている。つまり、これらＲＥＭを用いれば簡便な方法で低Ｓ低Ｏ低Ｎ鋼が得られると期待される。 Considering the above, rare earth elements (hereinafter referred to as “REM”) such as La, Ce, Nd, and Y are known as elements satisfying such conditions. That is, if these REMs are used, it is expected that low S, low O, low N steel can be obtained by a simple method.
しかし、ＲＥＭを用いる場合には、以下の問題があり、現実には容易に使用できない。
第一にＲＥＭとＯおよびＳとが反応した結果生じる介在物の比重が溶鋼比重に近いため、これらが浮上しない。従って、ＲＥＭを単純に添加すると溶解Ｓ濃度およびＯ濃度は低下するが、鋼中Ｓ濃度およびＯ濃度はあまり変化せず、むしろ清浄性が悪化する。
However, when REM is used, there are the following problems, which cannot be easily used in reality.
First, since the specific gravity of inclusions produced as a result of the reaction of REM with O and S is close to the specific gravity of the molten steel, they do not rise. Therefore, when REM is simply added, the dissolved S concentration and O concentration decrease, but the S concentration and O concentration in the steel do not change much, but rather the cleanliness deteriorates.
第二に、これらの介在物が存在すると鋳造時のノズル閉塞などの問題を誘発し、生産性を著しく低下させる。 Secondly, the presence of these inclusions induces problems such as nozzle clogging during casting and significantly reduces productivity.
第三にＲＥＭ自体が鋼材特性に影響する場合があり、これらが鋼中に含まれることが不適当である場合がある。 Thirdly, REM itself may affect steel properties, and it may be inappropriate to include these in steel.
すなわち、ＲＥＭを用いることで低Ｎ等の精錬効果が見込めるが、ＲＥＭのみで低Ｏ低Ｓ化を図ることで低Ｎ化を促進しようとしても、上記のような問題が発生し、現実には達成できないのである。この問題に対して、本発明者らは、ＣａＯを主体としたフラックスの脱硫能力とＲＥＭの能力を適正に組み合わせることで、低Ｏ低Ｓ低Ｎ化を同時に図ることが可能であると考えた。 In other words, refining effects such as low N can be expected by using REM, but the above problem occurs even if trying to promote low N by reducing O and S by using only REM. It cannot be achieved. In response to this problem, the present inventors considered that it is possible to simultaneously achieve low O, low S and low N by appropriately combining the desulfurization ability of the flux mainly composed of CaO and the ability of REM. .
ここで、本発明における“ＣａＯを主体としたフラックス”について説明する。ＣａＯとは酸化カルシウム化合物を指すが、ＣａＯとして工業的には炭酸カルシウムや水酸化カルシウム等の不純物が含まれる生石灰が用いられる場合が多い。したがって、本発明でいう”ＣａＯを主体としたフラックス”とはフラックス中酸化カルシウム(ＣａＯ)の純分が８０重量％以上であるフラックスを指す。なお、以下、この“ＣａＯを主体としたフラックス”を単に「ＣａＯフラックス」とも記述する。 Here, “flux mainly composed of CaO” in the present invention will be described. CaO refers to a calcium oxide compound, and as lime, quick lime containing impurities such as calcium carbonate and calcium hydroxide is often used industrially as CaO. Therefore, the “flux mainly composed of CaO” in the present invention refers to a flux in which the pure content of calcium oxide (CaO) in the flux is 80% by weight or more. Hereinafter, this “flux mainly composed of CaO” is also simply referred to as “CaO flux”.
ＲＥＭと溶鋼中Ｓとの反応を抑制し、酸素活量のみをＣａＯ上吹き初期から低減すれば、ＣａＯフラックス脱硫が促進され、結果、ＲＥＭによる低Ｏ化とＣａＯフラックスによる低Ｓ化によって、同時に脱窒も促進される。ただし、このとき、ＲＥＭの添加濃度が過剰であれば、ＲＥＭと溶鋼中Ｏとの反応によるＲＥＭ酸化物系介在物の生成、およびＲＥＭとＣａＯフラックスとの反応が進行することによる清浄性の悪化とＣａＯフラックス脱硫能の低下とを招く。一方、ＲＥＭの添加濃度が過小であれば、何ら効果が得られない。 If the reaction between REM and S in the molten steel is suppressed and only the oxygen activity is reduced from the initial stage of top blowing on CaO, CaO flux desulfurization is promoted. As a result, low O by REM and low S by CaO flux simultaneously. Denitrification is also promoted. However, at this time, if the concentration of REM added is excessive, the generation of REM oxide inclusions due to the reaction between REM and O in molten steel, and the deterioration of cleanliness due to the progress of the reaction between REM and CaO flux. And lowering the CaO flux desulfurization ability. On the other hand, if the addition concentration of REM is too small, no effect is obtained.
４．溶鋼を用いた調査
従って、ＲＥＭとＣａＯフラックス上吹き精錬とを用いて、低Ｓ低Ｏ低Ｎを同時に達成するとともに清浄性の維持向上を実現するには、ＲＥＭの添加濃度に適正範囲が存在する。溶鋼中反応は、ＲＥＭおよびＣａＯフラックスとＳおよびＯとの反応に加え、溶鋼からの脱窒と複数の反応とが同時進行する、競合反応速度過程となるため、熱力学的計算により、このＲＥＭの添加濃度の適正な範囲を明示することは不可能である。そこで、このＲＥＭの添加濃度の適正な範囲の明確化を目的として、溶鋼を用いて調査した。
4). Therefore, using REM and CaO flux top blowing refining, it is possible to achieve low S, low O, and low N at the same time and to maintain and improve cleanliness. To do. The reaction in the molten steel is a competitive reaction rate process in which denitrification from the molten steel and a plurality of reactions proceed simultaneously in addition to the reaction of REM and CaO flux with S and O. It is impossible to specify an appropriate range of the concentration of the addition. Therefore, for the purpose of clarifying the appropriate range of the REM addition concentration, investigation was made using molten steel.
調査は以下の方法で行った。溶鋼１５００ｋｇを１８７３Ｋに加熱し、雰囲気をＡｒ雰囲気として圧力を１３３〜９００Ｐａとした。その後、上記の範囲に溶鋼成分を調整し、ＲＥＭとしてＬａ，ＣｅおよびＮｄからなる群から選ばれる一種または二種以上を混合し、所定の量を溶鋼に一括で添加した。なお、ＲＥＭにはＬａ，Ｃｅ，Ｎｄ，Ｙ等があり、これらの物性が近いことから同様の効果が期待されるが、本発明ではランタノイドであるＬａ、Ｃｅ、Ｎｄを用いて調査を行った。 The survey was conducted as follows. 1500 kg of molten steel was heated to 1873K, the atmosphere was an Ar atmosphere, and the pressure was 133 to 900 Pa. Thereafter, the molten steel components were adjusted to the above range, and one or more selected from the group consisting of La, Ce and Nd were mixed as REM, and a predetermined amount was added to the molten steel all at once. Note that REM includes La, Ce, Nd, Y, and the like, and since these properties are close to each other, similar effects are expected. However, in the present invention, investigation was performed using lanthanoids La, Ce, and Nd. .
その後、速やかにＣａＯフラックスを上吹きランスからＡｒガスと共に吹き付けた。ＣａＯフラックスは１００メッシュアンダー、Ａｒガス流量は５００Ｎｌ/ｍｉｎ、ランスノズルはストレートとした。ＣａＯフラックス吹き付け速度は５００ｇ/ｍｉｎ、吹き付け時間は３０ｍｉｎで、総吹き付けＣａＯ量は１５ｋｇとした。ＣａＯフラックス吹き付け直前のＳ濃度は３０ｐｐｍ、Ｎ濃度は３０ｐｐｍである。 Thereafter, CaO flux was quickly sprayed together with Ar gas from the top blowing lance. The CaO flux was 100 mesh under, the Ar gas flow rate was 500 Nl / min, and the lance nozzle was straight. The CaO flux spraying speed was 500 g / min, the spraying time was 30 min, and the total sprayed CaO amount was 15 kg. The S concentration immediately before the CaO flux spraying is 30 ppm, and the N concentration is 30 ppm.
３０ｍｉｎの吹き付け処理後、溶鋼からサンプルを採取し、溶鋼中Ｏ濃度、Ｓ濃度およびＮ濃度を定量した。なお、溶鋼中Ｏ濃度とは溶鋼に溶解している溶解Ｏと酸化物系介在物として溶鋼中に存在するＯとの合計であり、溶鋼中Ｓ濃度とは溶鋼に溶解している溶解Ｓと硫化物系介在物として溶鋼中に存在するＳとの合計である。また、介在物個数を計測したが、介在物個数は最もＲＥＭの添加濃度を低くしたときの介在物個数を１として規格化した。なお、介在物個数は走査型電子顕微鏡を用い、酸化物と硫化物を区分して計測した。計測対象介在物は大きさ１μm以上、被検面積は７ｃｍ２である。 After spraying for 30 min, a sample was taken from the molten steel, and the O concentration, S concentration and N concentration in the molten steel were quantified. The O concentration in the molten steel is the sum of the dissolved O dissolved in the molten steel and the O present in the molten steel as oxide inclusions. The S concentration in the molten steel is the dissolved S dissolved in the molten steel. This is the total of S present in the molten steel as sulfide inclusions. In addition, the number of inclusions was measured, and the number of inclusions was normalized by setting the number of inclusions at 1 when the addition concentration of REM was the lowest. The number of inclusions was measured using a scanning electron microscope by classifying oxides and sulfides. The inclusion to be measured has a size of 1 μm or more, and the test area is 7 cm 2 .
３０ｍｉｎの吹き付け処理後のＲＥＭ濃度とＳ濃度とＮ濃度との関係を図１に示す。ここで、ＲＥＭ濃度とは、Ｌａ，ＣｅおよびＮｄからなる群から選ばれる一種を添加した場合はそれらの溶鋼に対する濃度（単位：質量％）をいい、Ｌａ，ＣｅおよびＮｄからなる群から選ばれる二種以上を混合して添加した場合にはそれらの合計の溶鋼に対する濃度（単位：質量％）をいう。 FIG. 1 shows the relationship among the REM concentration, the S concentration, and the N concentration after the 30 min spraying process. Here, the REM concentration refers to the concentration (unit: mass%) with respect to the molten steel when one selected from the group consisting of La, Ce and Nd is added, and is selected from the group consisting of La, Ce and Nd. When two or more kinds are mixed and added, it means the concentration (unit: mass%) with respect to the total molten steel.
図１から、ＲＥＭ濃度を０．０００５％以上とすると低Ｎ化効果が非常に大きくなることが導かれる。つまり、ＲＥＭを０．０００５％以上とすることで、低窒素鋼を溶製することが可能となる。 From FIG. 1, it is derived that the effect of reducing N is greatly increased when the REM concentration is 0.0005% or more. That is, low nitrogen steel can be melted by making REM 0.0005% or more.
一方、上記のようにＲＥＭ濃度を０．０００５％以上とすると低Ｓ化効果が大きくなるが、０．００８％を超えると溶鋼中Ｓ濃度が増加する。Ｏも同様の挙動を示す。この現象は以下のように理解できる。ＲＥＭ濃度を０．０００５％以上に高めると、ＲＥＭによって低Ｓ化に有害なＯが低減され、低Ｓ化が進行する。さらにＲＥＭ濃度を高めると、ＲＥＭとＯとの反応およびＲＥＭとＳとの反応が進行して溶解Ｏ濃度およびＳ濃度は低減されるが、ＲＥＭの酸化物系介在物および硫化物系介在物が形成される。 On the other hand, when the REM concentration is 0.0005% or more as described above, the effect of reducing S is increased, but when it exceeds 0.008%, the S concentration in molten steel increases. O also exhibits the same behavior. This phenomenon can be understood as follows. When the REM concentration is increased to 0.0005% or more, O harmful to lowering S is reduced by REM, and lowering of S proceeds. When the REM concentration is further increased, the reaction between REM and O and the reaction between REM and S proceed to reduce the dissolved O concentration and S concentration. However, REM oxide inclusions and sulfide inclusions are reduced. It is formed.
低Ｎ化に有害なのは溶解するＯおよびＳであるから、ＲＥＭ濃度を高めれば低Ｎ化効果は大きくなる。しかし、生成したＲＥＭ酸化物系介在物およびＲＥＭ硫化物系介在物は比重が大きいため溶鋼から浮上分離しにくく、また、ＲＥＭによって溶解するＳが過度に消費されるためＣａＯフラックスによる低Ｓ化が進行しにくい。したがって、ＲＥＭ濃度を一定値以上高めると、溶解Ｓ濃度は低下するものの、同時に酸化物系介在物と硫化物系介在物が多数生成するため溶鋼中のＯ濃度およびＳ濃度は高くなる。 Since it is the dissolved O and S that are harmful to the decrease in N, increasing the REM concentration increases the effect of decreasing N. However, since the generated REM oxide inclusions and REM sulfide inclusions have a large specific gravity, they are difficult to float and separate from the molten steel, and the S dissolved by the REM is excessively consumed. Difficult to progress. Therefore, when the REM concentration is increased by a certain value or more, the dissolved S concentration decreases, but at the same time, a large number of oxide inclusions and sulfide inclusions are generated, so the O concentration and S concentration in the molten steel increase.
図２に溶鋼中介在物個数指数とＲＥＭ濃度の関係を示す。ＲＥＭ濃度が０．００７％を超えると、介在物個数が増加する傾向があることが示されている。この実験事実は上記推定を裏付けている。 FIG. 2 shows the relationship between the inclusion index in molten steel and the REM concentration. It is shown that when the REM concentration exceeds 0.007%, the number of inclusions tends to increase. This experimental fact supports the above estimation.
以上の結果から、ＲＨのようにスラグを有さない減圧下溶鋼表面にＣａＯ系フラックスを吹き付けて低Ｓ化を図る処理においては、ＣａＯフラックス吹き付け前の溶鋼におけるＬａ，ＣｅおよびＮｄからなる群から選ばれる一種または二種以上の濃度を０．０００５％以上０．００８％以下とすることで、従来技術以上の低Ｓ化に加え、低Ｎ化低Ｏ化を同時に図ることができ、［Ｓ］≦１０ｐｐｍ，[Ｏ]≦１５ｐｐｍかつ［Ｎ］≦２５ｐｐｍなる極低硫極低酸素極低窒素鋼を溶製することが可能となることが明確化された。 From the above results, in the process of reducing the S by spraying CaO-based flux on the molten steel surface under reduced pressure, such as RH, from the group consisting of La, Ce and Nd in the molten steel before the CaO flux is sprayed. By setting the concentration of one or two or more selected to be 0.0005% or more and 0.008% or less, in addition to lowering S than in the prior art, lowering N and lowering O can be achieved simultaneously. ] ≦ 10 ppm, [O] ≦ 15 ppm, and [N] ≦ 25 ppm.
ところで、前述したように微量であってもＲＥＭが鋼材に含有されることを回避しなければならない場合には、以下のようにしてＲＥＭを除去すればよい。ＣａＯフラックス上吹き後には、Ｓ，Ｏ，Ｎの低減が終了しており、ＲＥＭの精錬上の目的は達成されており不要となる。また、Ｎは気相に離脱し、ＳはＣａＯフラックスと共に、介在物は単独で溶鋼から取鍋スラグ中に離脱する。従って、溶鋼中ＲＥＭを除去しても、再びＮ、Ｓ、Ｏ濃度が急激に増加することはない。 By the way, when it is necessary to avoid that REM is contained in the steel material even in a small amount as described above, REM may be removed as follows. After the CaO flux is blown up, the reduction of S, O, and N is completed, and the purpose of REM refining is achieved and is unnecessary. Further, N is released into the gas phase, S is released together with the CaO flux, and the inclusions are released from the molten steel alone into the ladle slag. Therefore, even if REM in molten steel is removed, the N, S, and O concentrations will not increase rapidly again.
溶鋼からのＲＥＭ除去は、その高い脱酸力を利用して、溶鋼に少量の酸素を添加すればよい。具体的には真空槽内溶鋼表面に酸素ガスを吹き付ける方法が最も簡便であり、吹き付ける酸素ガス量は溶鋼１ｔｏｎ当たり０．１〜０．４Ｎｍ３で十分である。ＲＥＭの平衡酸素活量は０．０００１〜０．０００３と非常に低く、また本発明で用いるＲＥＭ濃度も低いため、極少量の酸素付与でＲＥＭを完全に除去できる。 To remove REM from molten steel, a small amount of oxygen may be added to the molten steel using its high deoxidizing power. Specifically, the method of spraying oxygen gas on the surface of the molten steel in the vacuum chamber is the simplest, and the amount of oxygen gas sprayed is 0.1 to 0.4 Nm 3 per ton of molten steel. Since the equilibrium oxygen activity of REM is as very low as 0.0001 to 0.0003 and the REM concentration used in the present invention is also low, REM can be completely removed by applying a very small amount of oxygen.
前述した実験の後、異なる量の酸素ガス（０．０５、０．１、０．３、０．４および０．４５Ｎｍ３）を溶鋼に吹き付けてこの点について確認した。その結果、０．０５Ｎｍ３ではＲＥＭ濃度は０．０００１％となり一部残留した。一方、０．１Ｎｍ３以上ではＲＥＭ濃度は分析限界以下となったが、０．４５Ｎｍ３では酸素濃度が１７ｐｐｍとなり酸素濃度の若干の増加が認められた。したがって、上記の確認実験により、ＲＥＭを溶鋼中に残留させないためには、酸素を上吹きすればよいこと、好ましくは酸素ガス量を０．１〜０．４Ｎｍ３にすべきであるとの知見が得られた。 After this experiment, different amounts of oxygen gas (0.05, 0.1, 0.3, 0.4 and 0.45 Nm 3 ) were sprayed on the molten steel to confirm this point. As a result, at 0.05 Nm 3 , the REM concentration became 0.0001%, and a part remained. On the other hand, at 0.1 Nm 3 or more, the REM concentration was below the analysis limit, but at 0.45 Nm 3 , the oxygen concentration was 17 ppm, and a slight increase in oxygen concentration was observed. Therefore, according to the above confirmation experiment, in order to prevent REM from remaining in the molten steel, it is necessary to blow up oxygen, preferably the amount of oxygen gas should be 0.1 to 0.4 Nm 3 was gotten.
以上述べたように、効率よく同時Ｓ，Ｏ，Ｎの低減を図るにはＣａＯフラックス上吹き前の溶鋼のＲＥＭ濃度を０．０００５％以上０．００８％以下とすることが必要である。また、ＲＥＭを溶鋼中に残留させないためにはＣａＯフラックス上吹き後に酸素ガスを上吹きすればよい。 As described above, in order to efficiently reduce S, O, and N simultaneously, it is necessary that the REM concentration of the molten steel before blowing on the CaO flux be 0.0005% or more and 0.008% or less. In order not to leave REM in the molten steel, oxygen gas may be blown up after the CaO flux is blown up.
本発明は、以上の知見に基づいてなされたもので、その要旨は下記の通りである。
（１）質量％で、Ｃ：０．００２％以上０．４％以下、Ｍｎ:０．１％以上２％以下、Ｓｉ:０．００１％以上１％以下、Ｓ:０．００５％以下、Ａｌ:０．００５％以上１％以下、Ｎ:０．００７％以下、Ｏ:０．００５％以下なる溶鋼をＲＨ式真空脱ガス処理装置にて、フラックス中ＣａＯの純分が８０重量％以上であるフラックスを溶鋼表面に吹き付けて脱硫する処理において、前記フラックス吹き付け前の溶鋼にＬａ、ＣｅおよびＮｄからなる群から選ばれる一種または二種以上を添加し、これらの溶鋼に対する濃度を合計で０．０００５質量％以上０．００８質量％以下に調整した後、前記フラックスを溶鋼表面に吹き付けることを特徴とする［Ｓ］≦１０ｐｐｍ，[Ｏ]≦１５ｐｐｍかつ［Ｎ］≦２５ｐｐｍなる極低硫極低酸素極低窒素鋼の溶製方法。
The present invention has been made based on the above findings, and the gist thereof is as follows.
( 1 ) By mass%, C: 0.002% to 0.4%, Mn: 0.1% to 2%, Si: 0.001% to 1%, S: 0.005% or less, Al: 0.005% or more, 1% or less, N: 0.007% or less, O: 0.005% or less of molten steel with RH vacuum degassing treatment equipment, the pure content of CaO in the flux is 80% by weight or more In the treatment of desulfurization by spraying the flux on the surface of the molten steel, one or more selected from the group consisting of La, Ce and Nd is added to the molten steel before the flux is sprayed, and the total concentration of these molten steels is 0. [S] ≦ 10 ppm, [O] ≦ 15 ppm and [N] ≦ 25 ppm, characterized in that the flux is sprayed on the surface of the molten steel after being adjusted to .0005 mass% to 0.008 mass% Low oxygen ultra low nitrogen steel Method of melting.
（２）前記フラックスを吹き付けた後に、溶鋼表面に酸素ガスを吹き付けて、前記Ｌａ、ＣｅおよびＮｄからなる群から選ばれる一種または二種以上の合計濃度を０質量％にすることを特徴とする上記（１）記載の極低硫極低酸素極低窒素鋼の溶製方法。
(2) After spraying the flux, by blowing oxygen gas to the molten steel surface, said La, wherein to Rukoto one or two or more of the total concentration selected from the group consisting of Ce and
本発明により、極低酸素濃度、極低硫黄濃度、および極低窒素濃度の溶鋼を効率よく、しかも一工程で製造することができる。 According to the present invention, molten steel having an extremely low oxygen concentration, an extremely low sulfur concentration, and an extremely low nitrogen concentration can be efficiently produced in one step.
以下、本発明を詳細に説明する。
（１）ＲＨ処理前の工程
本発明を転炉とＲＨを用いて実施する場合を例に、最良の形態を説明する。転炉処理終了後に溶鋼を取鍋へ出鋼する。出鋼時にＳｉ，Ｍｎ等の合金を加えても良いし、ＣａＯ等の造滓剤を添加しても良い。また、出鋼時にスラグ中低級酸化物を低減することを目的にスラグ改質剤やＡｌを用いても良い。このとき、スラグ量は１０ｋｇ/ｔｏｎ以上となることが望ましい。これは、スラグ量が少ないと溶鋼表面の被覆効果が小さくなり、大気からの再酸化および吸窒を受けやすくなるためである。また、スラグ組成はスラグ中ＦｅＯとＭｎＯとの合計が３質量％以下であることが好ましく、更に好ましくは１．５質量％以下である。スラグ中ＦｅＯ，ＭｎＯ濃度が高いと精錬処理後から鋳込み終了にかけての再酸化による清浄性悪化が進行しやすくなるためである。
Hereinafter, the present invention will be described in detail.
(1) Process before RH treatment The best mode will be described by taking the case where the present invention is carried out using a converter and RH as an example. After the converter process is completed, the molten steel is discharged into a ladle. An alloy such as Si or Mn may be added at the time of steeling, or a faux-forming agent such as CaO may be added. Moreover, you may use a slag modifier and Al for the purpose of reducing a lower oxide in a slag at the time of steel production. At this time, the amount of slag is desirably 10 kg / ton or more. This is because when the amount of slag is small, the coating effect on the surface of the molten steel is reduced, and reoxidation and nitrogen absorption from the atmosphere are likely to occur. Moreover, it is preferable that the sum total of FeO and MnO in a slag is 3 mass% or less, and, more preferably, a slag composition is 1.5 mass% or less. This is because if the FeO and MnO concentrations in the slag are high, cleanliness deterioration due to re-oxidation from the refining process to the end of casting tends to proceed.
取鍋はＲＨへ移動するが、必要に応じて不活性ガス吹き込みなどの取鍋精錬装置を用いて予備処理を施しても良い。特に、出鋼時の吸窒を抑制する場合、出鋼時のＡｌ，Ｓｉの添加量を抑制することが効果的である。この場合、上記スラグ組成への制御が困難になるため、スラグ制御を行うことを目的に、ＲＨ処理前に不活性ガス吹き込みなどの取鍋精錬装置を用いて予備処理を行うと良い。ただし、この取鍋精錬装置を用いた処理を長時間行うとスラグ制御が容易になる一方、徐々に溶鋼中Ｎ濃度が増加すると共に処理時間が長くなり生産性が低下する。従って、取鍋精錬装置を用いる場合でもその処理時間は１０分以内が望ましい。 Although the ladle moves to RH, preliminary treatment may be performed using a ladle refining device such as blowing an inert gas as necessary. In particular, when suppressing nitrogen absorption during steel output, it is effective to suppress the amount of Al and Si added during steel output. In this case, since it becomes difficult to control the slag composition, pretreatment is preferably performed using a ladle refining device such as blowing an inert gas before the RH treatment for the purpose of slag control. However, when the treatment using the ladle refining apparatus is performed for a long time, the slag control becomes easy, while the N concentration in the molten steel gradually increases and the treatment time becomes longer and the productivity is lowered. Therefore, even when using a ladle refining apparatus, the processing time is preferably within 10 minutes.
（２）ＲＨでの処理順序
ＲＨへ取鍋を移送後、直ちに処理を開始する。ＲＨでの処理は、脱水素等の真空脱ガス、溶鋼温度調整、成分調整、そして本発明による同時低Ｏ低Ｎ低Ｓ化処理がある。これらの処理はどの順番で実施しても差し支えないが、好ましくは、温度調整、成分調整、本発明、真空脱ガスの順である。これは、以下の理由による。温度調整は溶鋼にＡｌを添加した後、溶鋼に酸素を供給してＡｌの酸化熱を利用して行われるが、この処理ではアルミナ系介在物が生成する。本発明により低Ｏ化が図れるが、この温度調整処理が本発明の後となると、清浄性がやや悪化する場合がある。また、成分調整では各種合金が添加されるが、この合金中不純物としてＮ，Ｏ，Ｓが含有される場合がある。これらの不純物によって微量のＮ，Ｏ，Ｓが溶鋼に持ち込まれることは、本発明が意図する極低Ｓ，Ｏ，Ｎ鋼にとって好ましいことではない。また、脱酸元素であるＡｌ、ＳｉはＲＥＭ添加前に調整しておくことが望ましい。
(2) Processing order in RH After the ladle is transferred to RH, processing is started immediately. The treatment with RH includes vacuum degassing such as dehydrogenation, molten steel temperature adjustment, component adjustment, and simultaneous low O low N low S treatment according to the present invention. These treatments may be carried out in any order, but are preferably the order of temperature adjustment, component adjustment, the present invention, and vacuum degassing. This is due to the following reason. The temperature is adjusted by adding Al to the molten steel, then supplying oxygen to the molten steel and utilizing the heat of oxidation of Al. In this treatment, alumina inclusions are generated. Although low O can be achieved by the present invention, if this temperature adjustment treatment is performed after the present invention, the cleanliness may be slightly deteriorated. In addition, various alloys are added for component adjustment, and N, O, and S may be contained as impurities in the alloy. It is not preferable for the extremely low S, O, N steel intended by the present invention that a small amount of N, O, S is brought into the molten steel by these impurities. Moreover, it is desirable to adjust Al and Si as deoxidizing elements before adding REM.
（３）ＣａＯフラックス上吹き量
次に本発明の具体的方法を説明する。転炉処理終了時のＳ濃度は迅速分析によってRH処理前に知見することができる。このＳ濃度を元にＣａＯフラックス上吹き量をはじめに決定する。ＣａＯフラックス上吹き総量Ｗ（ｋｇ/ｔｏｎ）は処理前溶鋼中Ｓ濃度［Ｓ］（ｐｐｍ）との関係で下記式(1)を満足することが望ましい。
(3) CaO flux top blowing amount Next, a specific method of the present invention will be described. The S concentration at the end of the converter process can be found before RH treatment by rapid analysis. Based on this S concentration, the CaO flux top blowing amount is first determined. It is desirable that the total amount W (kg / ton) of the CaO flux top blow satisfies the following formula (1) in relation to the S concentration [S] (ppm) in the molten steel before treatment.
Ｗ≧0.25×[S]＋3 ・・・ (1)
処理前Ｓ濃度が高ければ主脱硫剤であるＣａＯは増加するが、本発明はＲＥＭとＣａＯによってさらに低Ｎも図るため、Ｓ濃度が非常に低くても３ｋｇ/ｔｏｎ以上のＣａＯ上吹きが必要となる。なお、Ｗは１８ｋｇ/ｔｏｎ以下であることが望ましい。Ｗが多いほど効果は増加するが、１８ｋｇ/ｔｏｎを超えて高くなると効果が飽和するに加えて、スラグ量が多くなる。
W ≧ 0.25 × [S] +3 (1)
If the pre-treatment S concentration is high, the main desulfurization agent CaO will increase, but the present invention will further reduce the N by REM and CaO, so even if the S concentration is very low, it is necessary to blow over 3 kg / ton or more of CaO It becomes. Note that W is desirably 18 kg / ton or less. The effect increases as W increases, but when the value exceeds 18 kg / ton, the effect becomes saturated and the amount of slag increases.
粉体の供給速度は溶鋼１ｔｏｎあたりの速度で、０．５〜２．０ｋｇ/(ｔｏｎ・ｍｉｎ)が好ましく、更に好ましくは０．８〜１．５ｋｇ/(ｔｏｎ・ｍｉｎ)である。粉体供給速度が過度に遅いと総処理時間が長くなり、温度降下等の操業上の課題を生じる。一方、過度に早いと真空槽内溶鋼表面積に占めるフラックスの面積が増加し、脱窒有効界面積が減少する場合がある。 The supply rate of the powder is a rate per 1 ton of molten steel, preferably 0.5 to 2.0 kg / (ton · min), and more preferably 0.8 to 1.5 kg / (ton · min). When the powder supply rate is excessively slow, the total processing time becomes long, which causes operational problems such as temperature drop. On the other hand, if it is too early, the area of the flux occupying the molten steel surface area in the vacuum chamber may increase, and the effective denitrification interface area may decrease.
（４）ＲＥＭ添加方法と添加量
以上の様にＣａＯフラックス上吹き量と上吹き速度とを決定し、本発明による同時低Ｏ低Ｎ低Ｓ化処理に入る。
(4) REM addition method and addition amount As described above, the CaO flux upper blowing amount and the upper blowing speed are determined, and the simultaneous low O low N low S processing according to the present invention is started.
はじめにＲＥＭを添加する。ＲＥＭの添加時期はＣａＯフラックス上吹き前であればよいが、ＲＥＭを添加した後、長時間保持してしまうと、ＲＥＭが耐火物等と反応して、その濃度が低下する。このため、ＣａＯフラックス上吹き前のＲＥＭ濃度が不安定となる恐れがある。従って、ＲＥＭ添加はＣａＯフラックス添加直前が望ましく、正確にはＣａＯフラックス上吹き開始１．５〜３分前に添加することがさらに望ましい。これは、ＲＥＭの均一混合時間に相当し、ＣａＯフラックス上吹き時に溶鋼成分を安定させるためである。 First, REM is added. The REM may be added before the CaO flux is blown up. However, if the REM is added for a long time after the REM is added, the REM reacts with the refractory or the like and the concentration thereof decreases. For this reason, there exists a possibility that the REM density | concentration before CaO flux top blowing may become unstable. Therefore, REM is added immediately before the addition of CaO flux, and more precisely, it is more preferably added 1.5 to 3 minutes before the start of blowing on CaO flux. This is equivalent to the uniform mixing time of REM and is to stabilize the molten steel components when the CaO flux is blown up.
ＲＥＭ濃度は前述の０．０００５質量％以上に調整するが、ＲＥＭは真空槽内の溶鋼に添加すればよい。所定濃度に調整する方法として、実績歩留まりを用いればよいが、減圧添加時の通常歩留まりは６０〜７０％である。なお、窒素のみの低減を図る場合にはＲＥＭ濃度は０．０００５％以上に調整するが、窒素低減のみを図り、Ｓ、Ｏの低減が不要な場合でも、好ましくは、ＲＥＭ濃度は０．１５％以下であることが望ましい。ＲＥＭ濃度が０．１５％を超えて高くなると、Ｓ，Ｏの溶解度が高くなり、汚染等の外乱を受けやすくなるため、精錬反応が不安定化する場合がある。 The REM concentration is adjusted to 0.0005% by mass or more as described above, but REM may be added to the molten steel in the vacuum chamber. As a method of adjusting to a predetermined concentration, the actual yield may be used, but the normal yield at the time of adding reduced pressure is 60 to 70%. Note that when reducing only nitrogen, the REM concentration is adjusted to 0.0005% or more. However, even when only nitrogen reduction is performed and S and O reduction is unnecessary, the REM concentration is preferably 0.15. % Or less is desirable. When the REM concentration is higher than 0.15%, the solubility of S and O is increased, and the refining reaction may become unstable because it is susceptible to disturbance such as contamination.
添加するＲＥＭはＬａ，Ｃｅ，Ｎｄといった金属を単独または混合して添加しても良いし、ミッシュメタルなどのこれらの合金を用いても良い。また、Ｌａ，Ｃｅ，Ｎｄ等を混合させたものまたはこれらの合金を用いる場合には、その混合比は任意で良く、合計の濃度が上記の濃度を満足すればよい。ただし、濃度制御性をさらに向上させるにはＬａ，ＣｅおよびＮｄからなる群から選ばれる一種類の金属または一種類のＲＥＭとＡｌ，Ｓｉ等との合金を用いることが望ましい。なお、ランタノイドであるＬａ，ＣｅおよびＮｄはいずれも同等の効果を生じるが、Ｎｄを用いることが好ましい。これは、Ｎｄ化合物の比重がＬａ，Ｃｅより大きいため、溶鋼から浮上しにくく精錬効果をより高めることが期待されるためである。 As the REM to be added, a metal such as La, Ce, or Nd may be added alone or in combination, or an alloy such as misch metal may be used. Moreover, when using what mixed La, Ce, Nd etc., or these alloys, the mixing ratio may be arbitrary and a total density | concentration should just satisfy said density | concentration. However, in order to further improve the concentration controllability, it is desirable to use one kind of metal selected from the group consisting of La, Ce and Nd or one kind of REM and an alloy of Al, Si and the like. Although all the lanthanoids La, Ce, and Nd produce the same effect, it is preferable to use Nd. This is because the specific gravity of the Nd compound is larger than La and Ce, so that it is difficult to float from the molten steel and the refining effect is expected to be further enhanced.
ＲＥＭの添加は真空槽内から一括で添加すればよいが、このときの真空槽内雰囲気圧力はＲＨの環流を維持できる程度の圧力、すなわち１３ｋＰａ以下であればよく、好ましくは６．５ｋＰａ以下３．９ｋＰａ以上である。３．９ｋＰａ未満ではＲＥＭの飛散や蒸発によってＲＥＭを消耗する場合がある。６．５ｋＰａを超えて圧力が高いと環流速度が遅くなり、均一混合までの所要時間が長くなる場合がある。 REM may be added all at once from the inside of the vacuum chamber, but the atmosphere pressure in the vacuum chamber at this time may be a pressure that can maintain the RH reflux, that is, 13 kPa or less, and preferably 6.5 kPa or less 3 .9 kPa or more. If it is less than 3.9 kPa, REM may be consumed by scattering or evaporation of REM. If the pressure exceeds 6.5 kPa and the pressure is high, the reflux speed becomes slow, and the time required until uniform mixing may become longer.
また、ＲＥＭ濃度がＣａＯフラックス上吹き中に低下する場合は、ＣａＯフラックス上吹き前にＲＥＭを添加して前述の濃度範囲（０．０００５質量％以上）内に調整した後、ＣａＯフラックス上吹き中に再度ＲＥＭを添加してもよい。ただし、ＣａＯフラックス上吹き終了後のＲＥＭ濃度が上記濃度範囲を満足する必要がある。 In addition, when the REM concentration decreases during the top blowing of the CaO flux, after adding the REM before the top blowing of the CaO flux and adjusting it to the above-described concentration range (0.0005 mass% or more), REM may be added again. However, the REM concentration after finishing the CaO flux top blowing needs to satisfy the above concentration range.
（５）ＣａＯフラックス上吹き条件
ＲＥＭ濃度調整後にＣａＯフラックス上吹きを開始するが、このときの真空槽内雰囲気圧力は、３．９ｋＰａ以下が好ましく、さらに好ましくは０．４ｋＰａ以下である。雰囲気圧力が低いほど、ＣａＯフラックス粉の溶鋼への侵入深さが深くなると同時に、溶鋼の撹拌が強くなるため反応効率がより向上する。
(5) CaO flux top blowing condition After the REM concentration is adjusted, CaO flux top blowing is started. At this time, the atmospheric pressure in the vacuum chamber is preferably 3.9 kPa or less, more preferably 0.4 kPa or less. The lower the atmospheric pressure, the deeper the penetration depth of the CaO flux powder into the molten steel, and the stronger the stirring of the molten steel, the better the reaction efficiency.
ＲＥＭ添加後に上吹きするＣａＯフラックスは酸化カルシウム単体の他、ＣａＯ純分が８０％以上である生石灰やＣａＯ含有物でもよい。さらに効果を高めるためにＣａＯフラックスにＣａ、Ａｌ、Ｍｇの金属またはこれらの合金を混合しても良い。この金属Ｃａ，Ａｌ，Ｍｇ等の好ましい範囲はＣａＯ純分との重量比で１００：２〜１００：１０の範囲である。１００：２未満では効果が生じにくく、１００：１０を超えて高いとＮの活量が低下するため効果が小さくなる傾向がある。さらに、フラックスの融点を低下させることを目的にＡｌ２Ｏ３、ＣａＦ２などのＣａ以外の酸化物もしくはＣａのフッ化物などの化合物をＣａＯフラックスに混合しても良いが、本発明では高い精錬能力を十分発揮できるため、特にこれらを要さない。 The CaO flux blown up after the addition of REM may be calcium oxide alone, quick lime with a CaO pure content of 80% or more, or a CaO-containing material. In order to further enhance the effect, a CaO flux, a metal of Ca, Al, Mg, or an alloy thereof may be mixed. A preferable range of the metal Ca, Al, Mg, etc. is in a range of 100: 2 to 100: 10 in terms of a weight ratio with respect to the CaO pure content. If the ratio is less than 100: 2, the effect is hardly generated, and if it exceeds 100: 10, the activity of N tends to decrease, so the effect tends to be small. Furthermore, for the purpose of reducing the melting point of the flux, compounds other than Ca, such as Al 2 O 3 and CaF 2 , or compounds such as Ca fluoride may be mixed with the CaO flux. These are not particularly necessary because they can fully demonstrate their abilities.
ＣａＯフラックス上吹きは前述した速度、量が好ましいが、さらに上吹きランス下端と真空槽内の溶鋼表面との鉛直距離は１ｍ以上３ｍ以下が望ましい。１ｍ未満ではスプラッシュによるランス損耗が激しくなり、３ｍを超えて高いとフラックス粉の着地効率が低下する場合がある。また、キャリヤーガスはＡｒ等の不活性ガスで、流量は２Ｎｍ３／ｍｉｎ以上１０Ｎｍ３／ｍｉｎ以下であることが望ましい。２Ｎｍ３／ｍｉｎ未満ではフラックス粉の搬送が不安定となり、１０Ｎｍ３／ｍｉｎを超えて高くなるとスプラッシュ等が激しくなる。
ランスノズルの形状はラバール、ストレート、先細など如何なる形状でも良いが、粉体加速にはラバールノズルが望ましい。
The above-mentioned speed and amount are preferable for CaO flux top blowing, but the vertical distance between the bottom end of the top blowing lance and the molten steel surface in the vacuum chamber is preferably 1 m or more and 3 m or less. If it is less than 1 m, lance wear due to splash becomes severe, and if it exceeds 3 m, the landing efficiency of the flux powder may decrease. The carrier gas is an inert gas such as Ar, and the flow rate is preferably 2 Nm 3 / min to 10 Nm 3 / min. 2Nm conveyance of flux powder becomes unstable in less than 3 / min, the splash or the like becomes severe becomes higher beyond 10 Nm 3 / min.
The shape of the lance nozzle may be any shape such as laval, straight or tapered, but a laval nozzle is desirable for powder acceleration.
以上の方法により、ＲＨにてＲＥＭ添加後にＣａＯフラックス上吹きを行うことで本発明を実施する。本発明実施後に、必要に応じて脱ガス、成分最終調整、温度調整を行えばよい。 By the above method, this invention is implemented by performing CaO flux top blowing after REM addition by RH. After implementation of the present invention, degassing, final component adjustment, and temperature adjustment may be performed as necessary.
（６）その他好適条件
なお、ＲＨ処理中のスラグ中ＣａＯとＡｌ２Ｏ３の重量濃度比ＣａＯ／Ａｌ２Ｏ３は１．５以上であることが望ましく、更に望ましくは１．８以上３．５以下である。ＣａＯ／Ａｌ２Ｏ３比が１．５未満であると、復硫が起こりやすく、１．８を超えると復硫はほぼ抑制される。また、３を超えて高いとスラグ量が増加するに加えて、スラグが固化するなど操業性が低下する。
(6) Other suitable conditions The weight concentration ratio CaO / Al 2 O 3 of CaO to Al 2 O 3 in the slag during the RH treatment is preferably 1.5 or more, more preferably 1.8 or more. 5 or less. If the CaO / Al 2 O 3 ratio is less than 1.5, resulfurization easily occurs, and if it exceeds 1.8, the resulfurization is substantially suppressed. On the other hand, if the ratio is higher than 3, the slag amount is increased, and the operability is deteriorated such that the slag is solidified.
本発明により、溶鋼からの脱窒が促進されるが、本発明と同時にＲＨ吸窒防止技術を併用するとさらに効果が高まる。吸窒防止法としては、浸漬管構造・耐火物材質の適正化の他、ＲＨ下部槽と取鍋間の空間を不活性ガス等でパージする方法が知られている。 According to the present invention, denitrification from molten steel is promoted, but when the RH nitrogen absorption prevention technique is used in combination with the present invention, the effect is further enhanced. As a method for preventing nitrogen absorption, a method of purging the space between the RH lower tank and the ladle with an inert gas or the like is known in addition to optimization of the dip tube structure and the refractory material.
また、鋼中ＲＥＭ濃度を高めたい場合は本発明処理後にＲＥＭを再添加すれば良い。ＲＥＭは前述したようにＳ，Ｏと高い親和力を有するために、Ｓ，Ｏ濃度が高い状態でＲＥＭ濃度を高めると酸硫化物介在物が多数生成する。このため、本発明に従い、Ｓ，Ｏを十分低減した後、所定ＲＥＭ濃度まで高めることが有効である。 Moreover, what is necessary is just to re-add REM after this invention process, when raising the REM density | concentration in steel. Since REM has a high affinity with S and O as described above, a large number of oxysulfide inclusions are produced when the REM concentration is increased in a state where the S and O concentrations are high. For this reason, according to the present invention, it is effective to sufficiently reduce S and O, and then increase to a predetermined REM concentration.
なお、Ｃａ処理が必要な場合は、ＲＨ処理後から鋳造までの間にＣａ処理を実施すればよい。Ｃａ添加量をＣａ純分で０．１〜０．３ｋｇ／ｔｏｎとすることで、介在物はＣａ系介在物に制御することができ、鋳造性や耐食性を改善できる。Ｃａの添加方法は如何なる方法でも良いが、ＲＥＭ濃度を増加させる場合は、ＲＥＭ添加後にＣａ処理を行うことが望ましい。ＲＥＭ添加後にＣａ処理を行うことで、ＲＥＭ系介在物をＣａ系に制御することが可能となる。 In addition, what is necessary is just to implement Ca treatment between RH processing and casting, when Ca processing is required. By making the Ca addition amount 0.1 to 0.3 kg / ton in terms of pure Ca, inclusions can be controlled to Ca inclusions, and castability and corrosion resistance can be improved. Any method may be used for adding Ca, but when increasing the REM concentration, it is desirable to perform Ca treatment after the REM addition. By performing Ca treatment after REM addition, it becomes possible to control the REM inclusions to Ca.
また、その他製品特性確保を目的に必要に応じてＣｒ，Ｎｉ，Ｃｕ，Ｍｏ，Ｖ，ＢおよびＮｂからなる群から選ばれる一種または二種以上を任意に添加しても良い。好ましくは、Ｃｒ：２０％以下、Ｎｉ：１０％以下、Ｃｕ：０．１％以下、Ｍｏ：２％以下、Ｖ：２％以下、Ｂ：０．００３％以下、Ｎｂ：０．１％以下である。これらの元素は本発明が意図する精錬反応に影響しない。よって、強度、耐食性、溶接性等の製品特性を向上させることを目的にこれらの元素を任意に添加し、製品中濃度を調整しても良い。 Moreover, you may add arbitrarily 1 type, or 2 or more types chosen from the group which consists of Cr, Ni, Cu, Mo, V, B, and Nb as needed for the purpose of ensuring other product characteristics. Preferably, Cr: 20% or less, Ni: 10% or less, Cu: 0.1% or less, Mo: 2% or less, V: 2% or less, B: 0.003% or less, Nb: 0.1% or less It is. These elements do not affect the refining reaction intended by the present invention. Therefore, for the purpose of improving product properties such as strength, corrosion resistance, and weldability, these elements may be arbitrarily added to adjust the concentration in the product.
予め、必要に応じて溶銑脱硫および溶銑脱燐処理を行った溶銑を、２５０トン（ｔ）規模の上底吹き転炉に装入し、溶鉄中Ｃ含有率が０．０３〜０．２％になるまで粗脱炭吹錬を行い、終点温度を１６３０〜１６９０℃として粗脱炭溶鋼を取鍋に出鋼し、出鋼時に各種脱酸剤および合金を添加して取鍋内溶鋼成分を、Ｃ：０．０４〜０．０７％、Ｓｉ：０．１〜０．３％、Ｍｎ：０．５〜１．３％、Ｐ：０．００５〜０．０１３％、Ｓ：２０〜２４ｐｐｍ、ｓｏｌ．Ａｌ：０．００７〜０．０５％とした。さらに、出鋼時にＣａＯを添加し、スラグ中ＣａＯ/Ａｌ２Ｏ３重量比を２〜２．５、スラグ中ＦｅＯとＭｎＯとの合計濃度を５質量％以下に調整した。 The hot metal that has been subjected to hot metal desulfurization and hot metal dephosphorization treatment as needed is charged into a 250 ton (t) scale top-bottom converter, and the C content in the molten iron is 0.03 to 0.2%. Rough decarburization blowing is performed until the end temperature is 1630 to 1690 ° C., and the raw decarburized molten steel is taken out into the ladle. , C: 0.04 to 0.07%, Si: 0.1 to 0.3%, Mn: 0.5 to 1.3%, P: 0.005 to 0.013%, S: 20 to 24 ppm , Sol. Al: 0.007 to 0.05%. Furthermore, CaO was added at the time of steel production, and the CaO / Al 2 O 3 weight ratio in the slag was adjusted to 2 to 2.5, and the total concentration of FeO and MnO in the slag was adjusted to 5% by mass or less.
その後、取鍋をＲＨへ移送し、成分調整、温度調整を行った後、溶鋼にＲＥＭ（組成：金属Ｎｄ、金属Ｌａ、金属ＣｅまたはＮｄ，Ｌａ，Ｃｅを重量比で１：１：１とした混合物）を溶鋼に対して添加し、添加２分後に真空槽内圧力１３３ＰａでＣａＯ純分含有率が９３％の生石灰からなるフラックス（以下「生石灰フラックス」という。）上吹きを実施した。生石灰フラックス上吹き量は６ｋｇ／ｔｏｎ、生石灰フラックス供給速度は１ｋｇ／（ｔｏｎ・ｍｉｎ）、キャリヤーガスはＡｒで流量は４Ｎｍ３／ｍｉｎである。生石灰フラックス上吹き終了後、１０分間環流処理を行い、ＲＨ処理を終了した。 After that, the ladle is transferred to RH, the components are adjusted, and the temperature is adjusted. Then, REM (composition: metal Nd, metal La, metal Ce or Nd, La, Ce is used at a weight ratio of 1: 1: 1. 2 minutes after the addition, a flux (hereinafter referred to as “quick lime flux”) composed of quick lime having a CaO pure content of 93% and a CaO pure content of 93% was added. The quick lime flux top spray rate is 6 kg / ton, the quick lime flux supply rate is 1 kg / (ton · min), the carrier gas is Ar, and the flow rate is 4 Nm 3 / min. After finishing the quick-lime flux top blowing, a reflux treatment was performed for 10 minutes to complete the RH treatment.
ＲＥＭ添加後の溶鋼中成分とＲＨ処理終了後の成分を表１に示す。表１で試験番号１〜１４ではＲＥＭ添加を行い、試験番号１５，１６ではＲＥＭ添加を行わなかった。また、試験番号９，１０，１１では生石灰フラックス上吹き後に酸素ガスを４０Ｎｍ３／ｍｉｎの流量で５０Ｎｍ３を真空槽内溶鋼表面に上吹きランスを用いて吹き付け、その後、１０分間環流処理を行い、ＲＨ処理を終了した。なお、表中の濃度における”０”は分析下限濃度以下であったことを示す。
Table 1 shows the components in the molten steel after the addition of REM and the components after the completion of the RH treatment. In Table 1,
表１から、ＲＥＭ濃度を０．０００５％以上に高め、０．００８５〜０．０１１％とした番号１〜３ではＮ濃度を１９ｐｐｍまで低減できており、［Ｎ］≦２５ｐｐｍの鋼を溶製できた。 From Table 1, the REM concentration was increased to 0.0005% or more, and in Nos. 1 to 3 with 0.0085 to 0.011%, the N concentration could be reduced to 19 ppm, and steel with [N] ≦ 25 ppm was melted. did it.
さらに、ＲＥＭ濃度を０．０００５〜０．０８％の範囲とした番号４〜８では、［Ｎ］≦２５ｐｐｍとすると同時に［Ｓ］≦４ｐｐｍかつ［Ｏ］≦１２ｐｐｍとなり、請求項２を満足しない番号１〜３に比較して、Ｎに加えＳ，Ｏも十分に低減された。 Further, in Nos. 4 to 8 in which the REM concentration is in the range of 0.0005 to 0.08%, [S] ≦ 4 ppm and [O] ≦ 12 ppm at the same time as [N] ≦ 25 ppm, which does not satisfy claim 2 Compared with Nos. 1 to 3, S and O were sufficiently reduced in addition to N.
一方、ＲＥＭ濃度を０．０００５％以上に高め、生石灰フラックス上吹き後に酸素を上吹きした番号９ではＮ濃度が低減でき、ＲＥＭ濃度を０．０００５〜０．０８％の範囲とし、生石灰フラックス上吹き後に酸素を上吹きした番号１０，１１ではＮに加えＳ，Ｏも十分低減されている。 On the other hand, the REM concentration was increased to 0.0005% or more, and in No. 9 in which oxygen was blown up after the quick lime flux was blown, the N concentration could be reduced, and the REM concentration was in the range of 0.0005 to 0.08%. In numbers 10 and 11 in which oxygen was blown up after blowing, S and O were sufficiently reduced in addition to N.
しかし、ＲＥＭ濃度が不足し本発明を満足しない番号１２〜１４およびＲＥＭを添加しなかった番号１５，１６ではＮ，Ｓ，Ｏを低減することができなかった。 However, N, S, and O could not be reduced in Nos. 12 to 14 that do not satisfy the present invention due to insufficient REM concentration and Nos. 15 and 16 in which REM was not added.
以上のように、ＲＥＭ濃度を０．０００５％以上に高めることでＮ低減が促進され（番号１〜３）、ＲＥＭ濃度を０．０００５〜０．０８％の範囲とすることでＮと同時にＳ，Ｏの低減が促進される（番号４〜８）。さらに、ＲＥＭ濃度を０．０００５％以上に高め、生石灰フラックス上吹き後に酸素を上吹きすることでＮを低減すると同時にＲＥＭ濃度の低減が可能（番号９）で、ＲＥＭ濃度を０．０００５〜０．０８％の範囲とし、生石灰フラックス上吹き後に酸素を上吹きすることでＮ，Ｓ，Ｏを同時に低減し、ＲＥＭも低減できる（番号１０，１１）。 As described above, N reduction is promoted by increasing the REM concentration to 0.0005% or more (Nos. 1 to 3), and by setting the REM concentration to a range of 0.0005 to 0.08%, S and N are simultaneously added. , O is promoted (numbers 4 to 8). Further, the REM concentration is increased to 0.0005% or more, and N is reduced by blowing oxygen after the quick lime flux is blown. At the same time, the REM concentration can be reduced (No. 9). The range of 0.08% is set, and N, S, and O are simultaneously reduced by blowing oxygen up after quick lime flux blowing, and REM can also be reduced (Nos. 10 and 11).
しかし、番号１２〜１６からＲＥＭ濃度が０．０００５％未満の場合は、Ｎのみの低減すら困難であり，Ｎ，Ｓ，Ｏの同時低減は促進できないことが解った。 However, from numbers 12 to 16, it was found that when the REM concentration is less than 0.0005%, it is difficult to reduce only N, and simultaneous reduction of N, S, and O cannot be promoted.
Claims (2)
前記フラックス吹き付け前の溶鋼にＬａ、ＣｅおよびＮｄからなる群から選ばれる一種または二種以上を添加し、これらの溶鋼に対する濃度を合計で０．０００５質量％以上０．００８質量％以下に調整した後、前記フラックスを溶鋼表面に吹き付けることを特徴とする［Ｓ］≦１０ｐｐｍ，[Ｏ]≦１５ｐｐｍかつ［Ｎ］≦２５ｐｐｍなる極低硫極低酸素極低窒素鋼の溶製方法。 C: 0.002% to 0.4%, Mn: 0.1% to 2%, Si: 0.001% to 1%, S: 0.005% or less, Al: 0% by mass A flux in which the pure content of CaO in the flux is 80% by weight or more with a RH vacuum degassing apparatus for molten steel of 0.005% to 1%, N: 0.007% or less, O: 0.005% or less In the process of desulfurizing by spraying the surface of the molten steel,
One or more selected from the group consisting of La, Ce and Nd was added to the molten steel before the flux was sprayed, and the total concentration of these molten steel was adjusted to 0.0005 mass% or more and 0.008 mass% or less. Thereafter, the flux is sprayed on the surface of the molten steel, and the method for melting the ultra-low sulfur ultra-low oxygen ultra-low nitrogen steel of [S] ≦ 10 ppm, [O] ≦ 15 ppm and [N] ≦ 25 ppm.
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