JP3613016B2

JP3613016B2 - Adjustment method of nitrogen concentration in molten steel

Info

Publication number: JP3613016B2
Application number: JP21642898A
Authority: JP
Inventors: 健二大島; 浩樹西; 純一蓮沼
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 1998-07-15
Filing date: 1998-07-15
Publication date: 2005-01-26
Anticipated expiration: 2018-07-15
Also published as: JP2000034513A

Description

【０００１】
【発明の属する技術分野】
本発明は、溶鋼中窒素濃度の調整方法に係わり、特に、真空脱ガス装置内で脱ガス槽と取鍋の間で環流させた溶鋼に窒素ガスを吹き込み、脱ガス槽の圧力を適正に減圧しつつ、目標値への的中率を高める技術である。
【０００２】
【従来の技術】
一般に、転炉で酸素吹錬した溶鋼には、窒素、酸素及び水素等のガス成分が含まれており、該溶鋼をそのまま鋳造して得た鋼材にガス成分が多量に含まれていると、該鋼材の性質に悪影響を及ぼす。このため、従来より、転炉から出鋼した溶鋼を取鍋ごと真空脱ガス装置に装入し、脱ガス処理を行っている。例えば、真空脱ガス装置７としては、図２に示すような環流式のもの（通称、ＲＨ）が多用されている。それは、脱ガス槽１の下部に配設された上昇浸漬管２（以下、上昇管）と下降浸漬管３（以下、下降管）を取鍋４内の溶鋼５中に浸漬し、該上昇管２に吹き込んだガス６（以下、環流ガス６）でその部分にある溶鋼５の比重を低減し、減圧された脱ガス槽１内に取鍋４から溶鋼５を吸い上げ、下降管３を介して脱ガス槽１内の溶鋼５を取鍋４に重力で戻すことで、溶鋼５を取鍋４と脱ガス槽１との間で循環させる。その際、脱ガス槽８内の雰囲気ガスは、排気されるので、溶鋼５中に溶存する窒素、酸素、水素等のガス成分が装置外に除去される。
【０００３】
ところで、鋼材に含有される窒素は、その「靭性」を低下させることから、溶鋼段階で該窒素をできるだけ低減するのが好ましい。しかしながら、鋼種によっては、窒素は必ずしも有害であるとは限らず、例えば、特殊な合金元素を含ませた特殊鋼（ステンレス鋼等の高合金鋼）では、それを強化するための元素として窒素が注目されており、溶鋼段階で窒素含有量（以下、濃度ということが多い）を適切な値に調整することが試みられている。また、窒素は、鋼中にオ−ステナイト相を形成させたり、鋼の強度を高めるのに役立つばかりでなく、非金属介在物を少なくする。さらに、結晶を微細化して、鋼の「疲労強度」を高めるため、溶鋼５中に窒素を富化し、それを一定の値に調整して、機械構造用材料としての高窒素含有鋼を溶製することもある。
【０００４】
この溶鋼５中の窒素濃度をある値に調整するため、従来より多くの技術が提案されている。例えば、特開平２−９３０１６号公報は、転炉等で目標濃度以上に窒素ガスを吹込み加窒し、その後真空脱ガス装置７で窒素分圧を調整して、目標窒素濃度とする方法を開示している。しかしながら、この方法では、転炉で吹込む窒素が無駄になることが多く、実用的でない。そこで、前記した環流式真空脱ガス装置内で加窒するようになり、該装置７の上昇管２に吹き込む環流ガス６の種類を、脱ガス処理の途中で、窒素ガスからアルゴン・ガスに切り換えるようにして、吸窒と脱窒を行い、溶鋼５中の窒素を調整していた。しかしながら、この方法の実施に際しては、溶鋼中の窒素含有量を富化する窒素ガスの吹き込み時間だけが決められ、その後の脱窒のためのアルゴン・ガスの吹き込み時間は、溶鋼５の温度が所定の温度になるまでとされていた。従って、処理時間は、最初に予定していた時間と異なり、例えば、処理時間が予定よりオーバすると、図３から明らかなように、目標窒素濃度より低い値（記号Ａで示す）になってしまい、鋼中窒素濃度のばらつきが大きくなる。つまり、目標窒素濃度の的中率（調整精度）が悪いという問題があった。
【０００５】
また、特開平６−１７１１３号公報は、この的中率を高めるため、上記環流ガス６にアルゴン・ガスと窒素ガスとの混合ガスを用いる方法を提案している。その方法は、従来の操業データから、脱ガス前の溶鋼中窒素濃度と脱ガス終了後の濃度との差の値と、その混合比との関係を事前に定めておき、この関係を利用して操業するという実戦的な方法であった。しかしながら、この方法は、従来データの確保や整理に結構手間を要し、また得られた関係の質にも問題があって、実用されるまでに至らなかった。
さらに、特開平４−２８８１４号公報は、環流式真空脱ガス装置７の真空度を調整してステンレス溶鋼の平衡窒素濃度を制御し、ステンレス溶鋼中窒素濃度を調整する方法を開示している。この方法は、溶鋼の平衡窒素濃度が、目標窒素濃度となるように脱ガス槽内の真空度を求め、この真空度に維持することに着眼したものである。しかしながら、該公報には、溶鋼中窒素の平衡関係や速度式は明記されているが、この平衡関係で定まる窒素分圧Ｐ_Ｎ２と脱ガス槽の真空度Ｐとの関係については、単にＰ_Ｎ２＝ｆ（Ｐ）と記載されているだけである。これでは、具体的にその技術を実施できず、その効果の確認も難しい。
【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、真空脱ガス装置における溶鋼中の窒素濃度を、従来より精度良く調整可能な溶鋼中窒素濃度の調整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、従来技術の問題点を見直し、その克服に鋭意努力した。そして、確実に目標到達精度を高めるには、調整途上で溶鋼中窒素濃度を確認する必要性を痛感し、本発明を着想した。
すなわち、本発明は、真空脱ガス装置内に保持した溶鋼に窒素ガスを吹き込むと同時に、該真空脱ガス装置内を減圧して、溶鋼中窒素濃度を目標値に調整するに際して、前記調整の途上で、溶鋼中窒素濃度を迅速に測定し、該測定値をその時点での予定の時間と該溶鋼中窒素濃度との関係で定まる予定値と比較し、前記測定値と前記溶鋼中窒素濃度の予定値との偏差を解消させるように、前記真空脱ガス装置内の圧力を変更することを特徴とする溶鋼中窒素濃度の調整方法である。
【０００８】
また、本発明は、前記真空脱ガス装置が溶鋼環流式であることを特徴とする溶鋼中窒素濃度の調整方法でもある。
本発明によれば、真空脱ガス装置内の圧力を、該溶鋼の平衡窒素濃度を確実に達成できるように補正するので、従来よりも確実、且つ迅速に目標窒素濃度へ到達し、達成精度も向上するようになる。その結果、特殊鋼や窒素含有鋼が安定して溶製できるようになるばかりでなく、目標値外れの発生による経済的なデメリットも解消される。
【０００９】
【発明の実施の形態】
以下、発明をなすに至った経緯も含め、本発明の実施の形態を説明する。
まず、本発明では、溶鋼５への吹込みガス（真空脱ガス装置７が環流式の場合は、環流ガス６）に、窒素ガスだけを使用し、該装置７内の雰囲気を窒素雰囲気で、且つ減圧とする。
その減圧の程度は、溶鋼５中の窒素濃度が平衡値となる時の窒素分圧とする。それは、以下のようにして定められる。
溶鋼５のある温度（Ｔ，単位は絶対温度である）における窒素濃度（Ｎ、重量％）と窒素ガス（Ｎ₂ ）との関係は、（１）式で表わされる。
１／２Ｎ₂ ＝Ｎ・・・・（１）
この平衡関係は、（２）式となる。
ｌｏｇＫ＝ｌｏｇ（ａＮ／Ｐ _N2 ^1/2）＝−５１８／Ｔ −１．０６３・・（２）ここで、ａ_N は、溶鋼中の窒素の活量
Ｐ_N2は、窒素ガスの分圧
Ｋは，平衡定数
従って、（２）式から、
ｌｏｇＫ＝ｌｏｇａ_N −１／２・ｌｏｇＰ_N2 ・・・（３）
となり、平衡窒素濃度は、温度Ｔが一定の条件では、溶鋼の組成と窒素分圧が定まれば、求められる。つまり、脱ガス装置７内の圧力を適切な窒素分圧に設定できれば、いずれ溶鋼中の窒素濃度は平衡窒素濃度となるので、該平衡濃度を溶鋼の目標窒素濃度とするならば、目標窒素濃度に調整することは容易と考えられる。
【００１０】
すなわち、ａ_N は、下記式で表わされるので、
ｌｏｇａ_N ＝ｌｏｇｆ_N ＋ｌｏｇ［Ｎ］・・・・（４）
ここで、ｆ_N は活量係数
具体的に、炭素（Ｃ）が０．１３重量％、珪素（Ｓｉ）が０．０４８重量％、マンガン（Ｍｎ）が０．０２重量％、アルミニウム（Ａｌ）が０．００１重量％の濃度である溶鋼を、温度１５５０℃で処理する場合には、
ｌｏｇｆ_N ＝０．１６４，ｌｏｇＫ＝−１．３４７となる。
その結果、最終的に（５）式が導かれ、
ｌｏｇ［Ｎ］＝１／２・ｌｏｇＰ_N2 −１．５１１・・（５）
ＰＮ２さえ適切に設定すれば、溶鋼中窒素濃度Ｎを目標値に調整できることになる。
【００１１】
ところが、かかる平衡関係を利用した従来技術を用いても、実際には、調整精度が思ったほど良くならない。その理由は、真空脱ガス装置内の雰囲気は、適切な窒素分圧にすることが難しいことにある。つまり、脱ガス槽１には、操業中にリークによって空気が入り込み、その窒素が平衡時の窒素分圧Ｐ_Ｎ２の値に少なからず影響を与えるので、脱ガス槽１内の窒素分圧を平衡時の値に維持したつもりでも、実際は、もっと高い窒素分圧になっている可能性がある。そこで、発明者は、真空脱ガス装置７内の圧力を調整途中で変更し、確実に、且つ予定時間通りに目標平衡窒素濃度にすることを着想したのである。
そのためには、予定の時間と窒素濃度との増加（又は減少）関係を定めておく必要がある。発明者は、この関係を過去の操業データをコンピュータで整理して定め、本発明に具現化したのである。
【００１２】
従って、本発明では、目標（平衡）窒素濃度を例えば９３ｐｐｍとする場合、この９３ｐｐｍと平衡する窒素分圧に脱ガス槽内の圧力を設定して調整を開始する。その際、調整途上で溶鋼の窒素濃度を迅速に分析し、図４に示すように、その値（実線上の黒丸）を前記の関係（点線上の黒丸）に比較して、予定の増加量からの偏差を解消するように圧力の再設定を行うようにした。
なお、溶鋼中窒素濃度の迅速分析は、ヘレウス・エレクトナイト社が最近開発した装置（商品名：ＮＩＴＲＩＳ）を用いて行った。これによれば、従来の分析に比べて格段と早く、試料採取から分析結果の報告まで１分で済んだ。
【００１３】
【実施例】
Ｃが０．１７重量％、Ｓｉが０．２５重量％、Ｍｎが０．８０重量％、Ｐが０．０１５重量％、Ｓが０．０１７重量％、Ａｌが０．０４０重量％、Ｃｒが１．１０重量％、Ｎが４０ｐｐｍの溶鋼５を、図２の環流式真空脱ガス装置７を用いて処理し、溶鋼の目標窒素濃度を７０ｐｐｍに調整する試みを行った。なお、溶鋼の処理時温度は、１５５０℃とした。
その際、前記した平衡計算により、平衡時の窒素分圧が１５トールであったので、該脱ガス槽１内の圧力は、１５トールに設定した。なお、この１５トールは、排気手段であるブースターの蒸気量をコントロールして維持できる。そして、該槽１の上昇管２に、環流ガス６として窒素ガスを１５００Ｎリットル／ｍｉｎで吹込むと同時に、この１５トールを２０分間維持した。その時点で溶鋼から試料を取り分析したところ、予定増加量の６５ｐｐｍに対して実績値は、６０ｐｐｍであった。そこで、この偏差を取り戻すべく、脱ガス槽内の圧力を２１トールに減圧し、調整を続行した。その結果、予定の３０分に再度分析したところ、７１ｐｐｍでので、直ちに出鋼した。実に、１ｐｐｍしか目標を外れていない。
【００１４】
そこで、この本発明に係る窒素調整方法を種々の鋼種に多数チャージ適用し、その結果を従来のアルゴン・ガスと窒素ガスの切り換えを利用した調整方法を適用した場合と比較した。比較結果を、表１及び図１の度数分布に示す。ここで、図１の横軸は、（目標値−実績値）であり、縦軸はその発生度数率である。
表１及び図１より、本発明に係る調整方法は、従来のものに比べ、格段に優れていることが明らかである。
【００１５】
【表１】

【００１６】
なお、上記実施例では、真空脱ガス装置に所謂環流式のＲＨ真空脱ガス装置を使用したが、本発明は、それに限るものではなく、所謂ＶＯＤ方式、ＤＨ方式等にも適用できることは言うまでもない。
【００１７】
【発明の効果】
以上述べたように、本発明により、従来よりも確実に目標窒素濃度への達成精度が向上するようになる。その結果、特殊鋼や窒素含有鋼が安定して溶製できるようになるばかりでなく、目標外れの発生による経済的なデメリットも解消された。
【図面の簡単な説明】
【図１】本発明に係る方法及び従来法による溶鋼中窒素濃度の調整を比較した度数分布図である。
【図２】環流式真空脱ガス装置の働きを説明する図である。
【図３】従来の窒素及びアルゴン・ガスの吹込み時期切り換えによる窒素濃度調整方法を説明する図である。
【図４】本発明に係る溶鋼中窒素濃度の調整方法を説明する概念図である。
【符号の説明】
１脱ガス槽
２上昇浸漬管（上昇管）
３下降浸漬管（下降管）
４取鍋
５溶鋼
６環流ガス
７真空脱ガス装置[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for adjusting the concentration of nitrogen in molten steel, and in particular, nitrogen gas is blown into molten steel circulated between a degassing tank and a ladle in a vacuum degassing apparatus, and the pressure in the degassing tank is appropriately reduced. However, it is a technology that increases the hit rate to the target value.
[0002]
[Prior art]
In general, molten steel blown with oxygen in a converter contains gas components such as nitrogen, oxygen, and hydrogen, and if the steel material obtained by casting the molten steel as it is contains a large amount of gas components, This adversely affects the properties of the steel material. For this reason, conventionally, the molten steel discharged from the converter is charged together with the ladle into a vacuum degassing apparatus to perform degassing treatment. For example, as the vacuum degassing apparatus 7, a recirculation type apparatus (commonly called RH) as shown in FIG. 2 is frequently used. The ascending dip tube 2 (hereinafter referred to as “rising tube”) and the descending dip tube 3 (hereinafter referred to as “downward tube”) disposed in the lower part of the degassing tank 1 are immersed in the molten steel 5 in the ladle 4, The specific gravity of the molten steel 5 in that portion is reduced by the gas 6 blown into the gas 2 (hereinafter referred to as the circulating gas 6), and the molten steel 5 is sucked up from the ladle 4 into the degassing tank 1 that has been decompressed, The molten steel 5 in the degassing tank 1 is returned to the ladle 4 by gravity, whereby the molten steel 5 is circulated between the ladle 4 and the degassing tank 1. At that time, since the atmospheric gas in the degassing tank 8 is exhausted, gas components such as nitrogen, oxygen and hydrogen dissolved in the molten steel 5 are removed outside the apparatus.
[0003]
By the way, since nitrogen contained in a steel material lowers its “toughness”, it is preferable to reduce the nitrogen as much as possible in the molten steel stage. However, depending on the type of steel, nitrogen is not necessarily harmful. For example, in special steels containing special alloy elements (high alloy steels such as stainless steel), nitrogen is used as an element for strengthening it. Attention has been focused on, and attempts have been made to adjust the nitrogen content (hereinafter often referred to as concentration) to an appropriate value in the molten steel stage. Nitrogen not only helps to form an austenite phase in the steel and increases the strength of the steel, but also reduces non-metallic inclusions. Furthermore, in order to refine the crystals and increase the “fatigue strength” of the steel, the molten steel 5 is enriched with nitrogen, adjusted to a certain value, and the high nitrogen content steel as the material for machine structure is melted. Sometimes.
[0004]
In order to adjust the nitrogen concentration in the molten steel 5 to a certain value, more techniques have been proposed than before. For example, Japanese Patent Laid-Open No. 2-93016 discloses a method in which nitrogen gas is blown and nitrogend to a target concentration or higher in a converter or the like, and then the nitrogen partial pressure is adjusted by a vacuum degassing device 7 to obtain a target nitrogen concentration. Disclosure. However, this method is not practical because nitrogen blown in the converter is often wasted. Therefore, nitriding is performed in the above-described reflux vacuum degassing apparatus, and the type of the reflux gas 6 blown into the ascending pipe 2 of the apparatus 7 is switched from nitrogen gas to argon gas during the degassing process. Thus, nitrogen absorption and denitrification were performed to adjust nitrogen in the molten steel 5. However, in carrying out this method, only the nitrogen gas blowing time for enriching the nitrogen content in the molten steel is determined, and the argon gas blowing time for subsequent denitrification is determined by the temperature of the molten steel 5 being predetermined. It was supposed to be until the temperature. Therefore, the processing time is different from the originally scheduled time. For example, if the processing time exceeds the schedule, the processing time becomes a value (indicated by symbol A) lower than the target nitrogen concentration, as is apparent from FIG. , The variation in nitrogen concentration in steel increases. That is, there is a problem that the target ratio (adjustment accuracy) of the target nitrogen concentration is poor.
[0005]
Japanese Patent Application Laid-Open No. 6-17113 proposes a method of using a mixed gas of argon gas and nitrogen gas as the reflux gas 6 in order to increase the accuracy. The method uses the relationship between the conventional operation data and the relationship between the nitrogen concentration in the molten steel before degassing and the concentration after degassing and the mixing ratio in advance. It was a practical way to operate. However, this method has been quite troublesome in securing and organizing conventional data, and there is a problem in the quality of the obtained relationship, and it has not been put into practical use.
Further, Japanese Patent Laid-Open No. 4-28814 discloses a method for adjusting the nitrogen concentration in the molten stainless steel by adjusting the degree of vacuum of the reflux vacuum degassing device 7 to control the equilibrium nitrogen concentration of the molten stainless steel. This method focuses on obtaining the degree of vacuum in the degassing tank so that the equilibrium nitrogen concentration of the molten steel becomes the target nitrogen concentration and maintaining this degree of vacuum. However, in this publication, although the equilibrium relationship and speed equation of nitrogen in molten steel are specified, the relationship between the nitrogen partial pressure P _N2 determined by this equilibrium relationship and the vacuum degree P of the degassing tank is simply P _N2. = F (P) only. In this case, the technology cannot be specifically implemented, and it is difficult to confirm the effect.
[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 adjusting the nitrogen concentration in molten steel, which can adjust the nitrogen concentration in molten steel in a vacuum degassing apparatus with higher accuracy than before.
[0007]
[Means for Solving the Problems]
In order to achieve the above object, the inventor has reviewed the problems of the prior art and made an eager effort to overcome them. And in order to improve the target achievement accuracy reliably, the necessity of confirming the nitrogen concentration in the molten steel during the adjustment was felt and the present invention was conceived.
That is, according to the present invention, the nitrogen gas is blown into the molten steel held in the vacuum degassing apparatus, and at the same time, the inside of the vacuum degassing apparatus is decompressed to adjust the nitrogen concentration in the molten steel to the target value. in rapidly measure the concentration of nitrogen in molten steel, compared to the expected value determined by the relationship between the time and the solution steels nitrogen concentration expected at that time the measured value, the molten steel in nitrogen concentration and the measured value of so as to eliminate the deviation between the predetermined value, an adjustment method of the molten steel in the concentration of nitrogen and changing the pressure before Symbol vacuum degassing apparatus.
[0008]
The present invention is also a method for adjusting a nitrogen concentration in molten steel, characterized in that the vacuum degassing apparatus is a molten steel reflux type.
According to the present invention, the pressure in the vacuum degassing apparatus is corrected so that the equilibrium nitrogen concentration of the molten steel can be reliably achieved, so that the target nitrogen concentration is reached more reliably and quickly than in the past, and the achievement accuracy is also improved. To improve. As a result, not only can special steel and nitrogen-containing steel be melted in a stable manner, but also the economic demerits due to the occurrence of off-target values are eliminated.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below, including the background to the invention.
First, in the present invention, only nitrogen gas is used for the gas blown into the molten steel 5 (in the case where the vacuum degassing apparatus 7 is a recirculation type), the atmosphere in the apparatus 7 is a nitrogen atmosphere, The pressure is reduced.
The degree of the reduced pressure is the nitrogen partial pressure when the nitrogen concentration in the molten steel 5 reaches an equilibrium value. It is determined as follows.
The relationship between the nitrogen concentration ( N , wt%) and the nitrogen gas (N ₂ ) at a certain temperature (T, the unit is absolute temperature) of the molten steel 5 is expressed by equation (1).
1 / 2N ₂ = N (1)
This equilibrium relationship is expressed by equation (2).
logK = log (aN / P _N2 ^1/2 ) = − 518 / T −1.063 (2) where a _N is the activity P _N2 of nitrogen in the molten steel and K is the partial pressure K of nitrogen gas Is the equilibrium constant.
log K = log a _N −1 / 2 · log P _N2 (3)
Thus, the equilibrium nitrogen concentration can be obtained if the composition of the molten steel and the nitrogen partial pressure are determined under the condition where the temperature T is constant. That is, if the pressure in the degassing device 7 can be set to an appropriate nitrogen partial pressure, the nitrogen concentration in the molten steel will eventually become an equilibrium nitrogen concentration. Therefore, if the equilibrium concentration is the target nitrogen concentration of the molten steel, the target nitrogen concentration It is considered easy to adjust.
[0010]
That is, since a _N is expressed by the following formula,
log a _N = log f _N + log [ N ] (4)
Here, f _N is an activity coefficient, specifically, carbon (C) is 0.13% by weight, silicon (Si) is 0.048% by weight, manganese (Mn) is 0.02% by weight, aluminum (Al) When the molten steel having a concentration of 0.001% by weight is processed at a temperature of 1550 ° C.,
log f _N = 0.164, log K = −1.347.
As a result, equation (5) is finally derived,
log [ N ] = 1 / 2.log P _N2 -1.511 (5)
If even PN2 is set appropriately, the nitrogen concentration N in the molten steel can be adjusted to the target value.
[0011]
However, even if the conventional technique using such equilibrium relation is used, the adjustment accuracy is not improved as much as expected. The reason is that it is difficult for the atmosphere in the vacuum degassing apparatus to have an appropriate nitrogen partial pressure. That is, the degassing tank 1 enters the air by leakage during operation, since the nitrogen gives not a little effect on the value of the nitrogen partial pressure P _N2 at equilibrium, the equilibrium nitrogen partial pressure of the degassing tank 1 Even if you intend to keep it at the time, you may actually have a higher nitrogen partial pressure. Therefore, the inventor has conceived that the pressure in the vacuum degassing apparatus 7 is changed during the adjustment, and the target equilibrium nitrogen concentration is surely and as scheduled.
For that purpose, it is necessary to define an increase (or decrease) relationship between the scheduled time and the nitrogen concentration. The inventor determined this relationship by organizing past operation data with a computer and embodied it in the present invention.
[0012]
Therefore, in the present invention, when the target (equilibrium) nitrogen concentration is, for example, 93 ppm, the adjustment is started by setting the pressure in the degassing tank to the nitrogen partial pressure that balances with this 93 ppm. At that time, the nitrogen concentration of the molten steel is quickly analyzed during the adjustment process, and as shown in FIG. 4, the value (black circle on the solid line) is compared with the above relationship (black circle on the dotted line), and the expected increase The pressure was reset so as to eliminate the deviation from.
In addition, the rapid analysis of nitrogen concentration in molten steel was performed using an apparatus (trade name: NITRIS) recently developed by Heraeus Electnite. According to this, it was much faster than conventional analysis, and it took only 1 minute from sampling to reporting of the analysis result.
[0013]
【Example】
C is 0.17 wt%, Si is 0.25 wt%, Mn is 0.80 wt%, P is 0.015 wt%, S is 0.017 wt%, Al is 0.040 wt%, Cr is An attempt was made to adjust the target nitrogen concentration of the molten steel to 70 ppm by treating the molten steel 5 with 1.10 wt% and N of 40 ppm using the reflux vacuum degassing apparatus 7 of FIG. 2. In addition, the processing temperature of molten steel was 1550 degreeC.
At that time, since the nitrogen partial pressure at the time of equilibrium was 15 Torr according to the above-described equilibrium calculation, the pressure in the degassing tank 1 was set to 15 Torr. This 15 Torr can be maintained by controlling the amount of steam of the booster as exhaust means. Then, nitrogen gas was blown into the ascending pipe 2 of the tank 1 as a reflux gas 6 at 1500 Nl / min, and at the same time, 15 Torr was maintained for 20 minutes. When a sample was taken from the molten steel and analyzed at that time, the actual value was 60 ppm with respect to the expected increase of 65 ppm. Therefore, in order to recover this deviation, the pressure in the degassing tank was reduced to 21 Torr and adjustment was continued. As a result, when the analysis was made again at the scheduled 30 minutes, it was 71 ppm, so steel was immediately produced. Indeed, only 1 ppm is off the target.
[0014]
Thus, the nitrogen adjustment method according to the present invention was applied to various types of steel with many charges, and the result was compared with the case where the conventional adjustment method using switching between argon gas and nitrogen gas was applied. The comparison results are shown in Table 1 and the frequency distribution of FIG. Here, the horizontal axis of FIG. 1 is (target value-actual value), and the vertical axis is the occurrence frequency rate.
From Table 1 and FIG. 1, it is clear that the adjustment method according to the present invention is remarkably superior to the conventional one.
[0015]
[Table 1]

[0016]
In the above embodiment, a so-called reflux RH vacuum degassing apparatus is used as the vacuum degassing apparatus. However, the present invention is not limited to this, and it goes without saying that the present invention can also be applied to the so-called VOD system, DH system, and the like. .
[0017]
【The invention's effect】
As described above, according to the present invention, the achievement accuracy to the target nitrogen concentration can be improved more reliably than before. As a result, not only can special steel and nitrogen-containing steel be melted stably, but also the economic disadvantages due to the occurrence of off-target have been eliminated.
[Brief description of the drawings]
FIG. 1 is a frequency distribution diagram comparing adjustment of nitrogen concentration in molten steel by a method according to the present invention and a conventional method.
FIG. 2 is a diagram for explaining the operation of a reflux type vacuum degassing apparatus.
FIG. 3 is a diagram for explaining a conventional nitrogen concentration adjustment method by switching the blowing timing of nitrogen and argon gas.
FIG. 4 is a conceptual diagram illustrating a method for adjusting a nitrogen concentration in molten steel according to the present invention.
[Explanation of symbols]
1 Degassing tank 2 Rising dip pipe (rising pipe)
3 Down-dip pipe (down pipe)
4 Ladle 5 Molten steel 6 Circulating gas 7 Vacuum degassing device

Claims

When nitrogen gas is blown into the molten steel held in the vacuum degassing device, and the vacuum degassing device is depressurized to adjust the nitrogen concentration in the molten steel to the target value,
In the course of the adjustment, the nitrogen concentration in the molten steel is quickly measured, and the measured value is compared with a predetermined value determined by the relationship between the scheduled time at that time and the nitrogen concentration in the molten steel, and the measured value and the molten steel so as to eliminate the deviation between the expected value of the medium nitrogen concentration, before Symbol method of adjusting the nitrogen concentration in the molten steel and changes the pressure in the vacuum degasser.

2. The method for adjusting the nitrogen concentration in molten steel according to claim 1, wherein the vacuum degassing apparatus is a molten steel reflux type.