JP2004300463A - Method for producing low nitrogen chromium-containing steel - Google Patents
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【０００１】
【発明の属する技術分野】
本発明は、フェライト系ステンレス鋼などのクロム（Ｃｒ）含有鋼を、窒素（Ｎ）濃度を低くして製造するための低窒素クロム含有鋼の製造方法に関する。
【０００２】
【従来の技術】
従来から、ステンレス鋼の製造に際しては、窒素濃度を低減する技術が開発されている。たとえば１８ｍａｓｓ％程度のクロムを含有して１８クロム系と称されるＳＵＳ４３０で代表されるフェライト系ステンレス鋼は、耐応力腐食割れ性に優れている特徴を有しているけれども、窒素濃度が高くなると硬化して加工性が悪くなったり、溶接を行うようなときは、溶接部の耐食性が低下する。このため、窒素濃度の低減が要求されている。
【０００３】
ステンレス鋼の低窒素化技術としては、底吹き羽口を有する転炉で、溶銑中の炭素濃度が４ｍａｓｓ％以上になるまで加炭を行い、脱炭時に生じる一酸化炭素（ＣＯ）ガスを増やすことで、脱窒素を促進する方法が知られている。さらに、加炭後に生石灰（ＣａＯ）や蛍石（ＣａＦ２）を主剤とする高塩基度のフラックスを生成し、アルゴン（Ａｒ）ガスによる強攪拌を行う脱窒方法も併用することができる。また、転炉での吹錬後に脱酸処理を行わずにそのまま取鍋に出鋼して、その際、取鍋内に炭酸マグネシウム（ＭｇＣＯ３）物質を供給することで、出鋼時にその分解反応によって生じる炭酸ガスによるシール作用で大気雰囲気からの吸窒を防止する方法も併用することができる。これらの方法では、窒素濃度が０．００５０〜０．００８５ｍａｓｓ％となる低窒素ステンレス鋼が得られる旨が記載されている（たとえば、特許文献１参照）。
【０００４】
含クロム溶銑を底吹き羽口と上吹きランスとを備える容器内で、上吹きランスから脱炭用の酸素（Ｏ２）を上吹きするとともに、底吹き羽口から溶銑を強攪拌することによって脱炭吹錬を行う際に、脱炭用の酸素の希釈や底吹き攪拌用のガスに、鋼中の炭素（Ｃ）濃度が０．８ｍａｓｓ％に低下するまでは窒素ガスを使用し、その後はアルゴンを使用する方法も知られている。この方法では、脱炭吹錬後の窒素濃度は０．０１５〜０．０５ｍａｓｓ％程度である。さらに、アルゴンガスを底吹きしながら脱酸剤であるフェロシリコン（Ｆｅ−Ｓｉ）等を投入するアルゴンリンスを行えば、０．００５〜０．０１５ｍａｓｓ％程度まで低下することも知られている（たとえば、特許文献２および特許文献３参照）。
【０００５】
【特許文献１】
特公平３−４６５２７号公報
【特許文献２】
特許第２６７４０４１号公報
【特許文献３】
特許第２７５１９２４号公報
【０００６】
【発明が解決しようとする課題】
特許文献１で開示されているような、加炭による脱窒素の方法では、脱炭炉への負荷が大きくなり、酸素吹精時間の延長によるデメリットが生じる。さらに脱窒素のためだけにフラックスを添加することは、製造コストを増加させるデメリットとなってしまう。取鍋内に炭酸マグネシウムを投入する方法も、作業性や経済性の面からデメリットが生じる。
【０００７】
特許文献２や特許文献３で得られると記載されている脱炭吹錬後の窒素濃度は、０．０１５０ｍａｓｓ％であるので、アルゴンリンスを追加しないと、特許文献１と同程度の低窒素化を達成することはできない。アルゴンリンスの追加は、作業性や経済性の面からデメリットを生じさせると考えられる。
【０００８】
本発明の目的は、経済的に窒素濃度を低減することが可能な低窒素クロム含有鋼の製造方法を提供することである。
【０００９】
【課題を解決するための手段】
本発明は、予め定める成品用濃度範囲でクロムを含有するクロム含有鋼を製造する際に、
脱炭処理前の溶鋼または溶銑中のクロム濃度を、該成品用濃度範囲よりも低く、脱炭処理で窒素含有量の減少も可能となる濃度範囲に調整しておき、
脱炭中または脱炭後に、クロムの濃度が該成品用濃度範囲内となるように、成分調整用のクロム合金を投入して成分調整を行い、
該濃度範囲は、該成分調整用のクロム合金を投入する際の窒素のピックアップによる窒素含有量の増加分が該脱炭処理での窒素含有量の減少分よりも小さい範囲に、予め設定しておくことを特徴とする低窒素クロム含有鋼の製造方法である。
【００１０】
本発明に従えば、予め定める成品用濃度範囲でクロムを含有するクロム含有鋼を製造する際に、脱炭処理を行う溶鋼または溶銑中のクロム濃度が低ければ、窒素濃度も低くなることに着目し、脱炭処理を行う前の溶鋼または溶銑中のクロム濃度を、成品用濃度範囲よりも低い濃度範囲に留めておく。クロム濃度は、低い方が窒素濃度を低くすることができるけれども、脱炭処理後にクロム濃度を成品用濃度範囲に成分調整するために、脱炭処理後の溶鋼にクロム合金を投入する際に、大気からの窒素のピックアップで溶鋼中に導入される窒素濃度も増加してしまう。脱炭処理前の溶鋼または溶銑中のクロム濃度範囲を、成分調整用のクロム合金を投入する際の窒素のピックアップによる窒素含有量の増加分が該脱炭処理での窒素含有量の減少分よりも小さい範囲に予め設定するので、有効に窒素濃度の低減を図ることができる。成分調整用のクロム合金の投入は、投入量の増加はあっても、従来から行われている工程であり、余分な作業とはならない。成分調整で投入するクロム量の増加は、脱炭処理前の溶鋼または溶銑の段階で投入するクロム量の減少となるので、成分調整時のクロム合金の投入量を増加しても、製造コストの上昇を抑えることができる。
【００１１】
また本発明で、前記クロム含有鋼は、クロムを前記成品用濃度範囲として１０ｍａｓｓ％以上含有するクロム含有鋼であり、
前記脱炭処理前の溶鋼中のクロム濃度範囲を、該成品用濃度範囲から１ｍａｓｓ％以上で５ｍａｓｓ％以下の範囲で小さくなるように設定することを特徴とする。
【００１２】
本発明に従えば、１０ｍａｓｓ％以上のクロムを成品用濃度範囲として含有するクロム含有鋼を、脱炭処理前には成品用濃度範囲よりも１〜５ｍａｓｓ％小さい濃度範囲としておいて、脱炭処理を行い、窒素濃度の低減を図ることができる。成品用濃度範囲に対して不足するクロム濃度は、成分調整によって増加させることができる。成分調整では、最大で５ｍａｓｓ％程度のクロムを添加すればよいので、成分調整用のクロム合金の投入時に大気からのピックアップで溶鋼中に導入される窒素の量を少なくすることができる。
【００１３】
【発明の実施の形態】
図１は、本発明の実施の一形態として、転炉による脱炭処理を行う際に、窒素濃度の低減を図る操業を行って得られる結果を、転炉装入前のクロム濃度と成品窒素濃度との関係で示す。操業の対象となるクロム含有鋼製品は、成品クロム濃度が１８ｍａｓｓ％であるステンレス鋼である。矩形のプロットは、転炉装入前の窒素濃度を示し、菱形のプロットはクロム濃度を調整した成品の窒素濃度を示す。転炉装入前には、クロム濃度の減少と窒素濃度の減少とが対応しているけれども、成品での窒素濃度は、クロム濃度が１３ｍａｓｓ％以下となると増加するようになることが判る。この理由は、クロム濃度が小さくなると、成分調整で追加するクロム量が多くなり、多量のフェロクロム（Ｆｅ−Ｃｒ）などのクロム合金を溶鋼中に投入する際に、大気中の窒素をピックアップして、溶鋼中の窒素濃度が増加してしまうからであると推定される。
【００１４】
図１から、成品クロム濃度が１８％必要な場合に、転炉装入前のクロム濃度を、１３．０〜１７．０ｍａｓｓ％としておけば、菱形のプロットで示す成品中の窒素濃度が０．０１２ｍａｓｓ％以下になることも判る。さらに、転炉装入前のクロム濃度を、１４．０〜１６．０ｍａｓｓ％としておけば、成品中の窒素濃度が０．０１ｍａｓｓ％以下になることも判る。
【００１５】
このステンレス鋼製造用の溶銑は、電気炉で溶製している。電気炉の１チャージは、約７５ｔであり、溶銑の主成分は、次の表１に示す。
【００１６】
【表１】
図１にプロットした結果のうちの主要なデータを、転炉装入前のクロム濃度が１３．０〜１７．０ｍａｓｓ％の範囲を濃度範囲として、この濃度範囲内に入る発明例と、この濃度範囲内に入らない比較例とに分類して、次の表２に示す。
【００１８】
【表２】
図２は、本実施形態で成品の窒素濃度を測定するまでの概略的な工程を示す。ステップｓ１では、電気炉で主成分が前述の表１のようになる溶銑を溶製する。次にステップｓ２で、加炭や加炭後のフラックス添加を行わずに、転炉での脱炭および脱窒処理を行う。ただし、脱窒処理は、脱炭処理に伴って行われる。図１に示す転炉装入前の窒素濃度やクロム濃度は、この段階の脱炭処理前の溶銑に対する分析値である。
【００２０】
脱炭処理は、炭素濃度が０．０５〜０．３０ｍａｓｓ％となるまでの粗脱炭処理として行う。次にステップｓ３として、クロム濃度が１７．５〜１８．５ｍａｓｓ％の範囲となるように、成分の粗調整を行う。この成分調整は、脱炭中に並行して、または脱炭後に行う。次に、ステップｓ４では、未脱酸で溶鋼のみを取鍋に出鋼する。その後ステップｓ５で、ＶＯＤ（Ｖａｃｕｕｍ Ｏｘｙｇｅｎ Ｄｅｃａｂｕｒｉｚａｔｉｏｎ）などの取鍋真空脱炭法で炭素濃度が０．０２０ｍａｓｓ％以下になるまで脱炭を行う。次にステップｓ６で、成分温度調整を行い、クロム濃度を１８ｍａｓｓ％を中心とする成品濃度範囲に調整し、温度も連続鋳造に適切な範囲に調整して、ステップｓ７で連続鋳造を行う。図１に示す成品窒素の分析値は、ステップｓ８で連続鋳造装置のモールド内から溶鋼をサンプリングし、ステップｓ９で成品窒素濃度を分析して得られる。
【００２１】
図３は、本発明の実施の他の形態として、成品のクロム濃度が１４ｍａｓｓ％である場合について、転炉による脱炭処理を行う際に、窒素濃度の低減を図る操業を行って得られる結果を、転炉装入前のクロム濃度と成品窒素濃度との関係で示す。脱炭用濃度範囲として、転炉装入前のクロム濃度を９．０〜１３．０ｍａｓｓ％の範囲としておけば、成品窒素濃度が０．００８ｍａｓｓ％以下になることが判る。この脱炭用濃度範囲は、成品クロム濃度を１４．０ｍａｓｓ％とすると、１．０〜５．０ｍａｓｓ％低くなる範囲となる。
【００２２】
本実施形態で、成品窒素濃度の分析値を得る手順は、図２と同様である。すなわち、このステンレス鋼製造用の溶銑も、電気炉で溶製し、電気炉の１チャージは、約７５ｔである。ただし、図２のステップｓ３でのクロム成分の粗調整では、１３．５〜１４．５ｍａｓｓ％の範囲に調整している。溶銑の主成分は、次の表３に示す。
【００２３】
【表３】
以上で説明しているように、転炉装入前の溶銑のクロム濃度を、成品のクロム濃度よりも１．０〜５．０ｍａｓｓ％小さくなるように調整しておくことによって窒素濃度の低減が従来とほぼ同様な工程で可能となる。なお、脱炭は転炉で行うばかりではなく、ＡＯＤ（Ａｒｇｏｎ Ｏｘｙｇｅｎ Ｄｅｃａｒｂｕｒｉｚａｔｉｏｎ）のように、酸素およびアルゴンを底吹きする精練法など、他の方法で行う場合にも、本発明を適用することができる。
【００２５】
また、本発明は、クロムの成品濃度が１８ｍａｓｓ％や１４ｍａｓｓ％のフェライト系ステンレス鋼に限らず、１０ｍａｓｓ％程度以上の成品濃度でクロムを含むクロム含有鋼に適用することができる。クロム含有鋼を製造する際に、脱炭処理前の溶鋼または溶銑中のクロム濃度を、成品用濃度範囲よりも低く、脱炭処理で窒素含有量の減少も可能となる濃度範囲に調整しておき、脱炭中または脱炭後に、クロムの濃度が該成品用濃度範囲内となるように、成分調整用のクロム合金を投入して成分調整を行い、脱炭用濃度範囲は、成分調整用のクロム合金を投入する際の窒素のピックアップによる窒素含有量の増加分が該脱炭処理での窒素含有量の減少分よりも小さい範囲に、予め設定しておけばよい。
【００２６】
本発明は、脱炭処理を行う溶鋼または溶銑中のクロム濃度が低ければ、窒素濃度も低くなることに着目し、脱炭処理を行う前の溶鋼または溶銑中のクロム濃度を、成品用濃度範囲よりも低い濃度範囲に留めておく。ただしクロム濃度が低くなりすぎると、クロム濃度を成品用濃度範囲に高める成分調整の際に、多量のクロム合金を投入することが必要となり、大気からの窒素のピックアップで溶鋼中に導入される窒素濃度も増加してしまうことも見出し、濃度範囲を、成分調整用のクロム合金を投入する際の窒素のピックアップによる窒素含有量の増加分が該脱炭処理での窒素含有量の減少分よりも小さい範囲に予め設定するので、有効に窒素濃度の低減を図ることができる。成分調整用のクロム合金の投入は、投入量の増加はあっても、従来から行われている工程であり、余分な作業とはならない。成分調整で投入するクロム量の増加は、脱炭処理前の溶鋼または溶銑の段階で投入するクロム量の減少となるので、成分調整時のクロム合金の投入量を増加しても、製造コストの上昇を抑えることができる。
【００２７】
【発明の効果】
以上のように本発明によれば、脱炭処理を行う溶鋼または溶銑中のクロム濃度が低ければ、窒素濃度も低くなることに着目し、脱炭処理を行う前の溶鋼または溶銑中のクロム濃度を、成品用濃度範囲よりも低い濃度範囲にして窒素濃度を低減することができる。濃度範囲は、クロム濃度を成品用濃度範囲に成分調整するために、脱炭処理後の溶鋼にクロム合金を投入する際に、大気からの窒素のピックアップで溶鋼中に導入される窒素濃度が増加しないようにも、設定されるので、有効に窒素濃度の低減を図ることができる。成分調整用のクロム合金の投入は、従来から行われている工程であり、余分な作業とはならない。成分調整で投入するクロム量が増加しても、脱炭処理前の溶鋼または溶銑の段階で投入するクロム量は減少するので、製造コストの上昇を抑え、経済的に窒素濃度の低減を図ることができる。
【００２８】
また本発明によれば、１０ｍａｓｓ％以上のクロムを成品用濃度範囲として含有するクロム含有鋼を製造する際に、成品用濃度範囲よりも１〜５ｍａｓｓ％小さい濃度範囲のクロム濃度で脱炭処理を行い、窒素濃度の低減を図ることができる。脱炭後の成分調整では、最大で５ｍａｓｓ％程度のクロムを添加すればよいので、クロム合金などの投入に伴って大気からのピックアップで溶鋼中に導入される窒素の量を少なくすることができる。
【図面の簡単な説明】
【図１】本発明の実施の一形態として、転炉による脱炭処理を行う際に、成品クロム濃度１８ｍａｓｓ％を目的として窒素濃度の低減を図る操業を行って得られる結果を、転炉装入前のクロム濃度と成品窒素濃度との関係で示すグラフである。
【図２】図１の結果を得る実施形態で、成品の窒素濃度を測定するまでの概略的な工程図である。
【図３】本発明の実施の他の形態として、成品のクロム濃度が１４ｍａｓｓ％である場合について、転炉による脱炭処理を行う際に、窒素濃度の低減を図る操業を行って得られる結果を、転炉装入前のクロム濃度と成品窒素濃度との関係で示すグラフである。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for producing a low nitrogen chromium-containing steel for producing a chromium (Cr) -containing steel such as a ferritic stainless steel with a low nitrogen (N) concentration.
[0002]
[Prior art]
Conventionally, in the production of stainless steel, a technology for reducing the nitrogen concentration has been developed. For example, a ferritic stainless steel represented by SUS430, which contains about 18 mass% of chromium and is referred to as 18 chromium series, has a feature of being excellent in stress corrosion cracking resistance, but when the nitrogen concentration becomes high. When the workability is deteriorated due to hardening or when welding is performed, the corrosion resistance of the welded portion is reduced. For this reason, a reduction in the nitrogen concentration is required.
[0003]
As a technique for reducing the nitrogen content of stainless steel, in a converter with a bottom-blowing tuyere, carburizing is performed until the carbon concentration in the hot metal reaches 4 mass% or more, and carbon monoxide (CO) gas generated during decarburization is increased. Thus, a method for promoting denitrification is known. Further, a denitrification method in which a high basicity flux mainly composed of quick lime (CaO) or fluorite (CaF 2 ) is generated after carburization and strong stirring is performed with argon (Ar) gas can also be used. Also, after blowing in the converter, the steel is directly discharged to the ladle without performing the deoxidizing treatment, and at this time, magnesium carbonate (MgCO 3 ) is supplied into the ladle to dissolve the steel at the time of tapping. A method of preventing nitrogen absorption from the atmospheric atmosphere by a sealing effect of carbon dioxide gas generated by the reaction can also be used together. It is described that in these methods, a low-nitrogen stainless steel having a nitrogen concentration of 0.0050 to 0.0085 mass% is obtained (for example, see Patent Document 1).
[0004]
In a vessel provided with a bottom blow tuyere and a top blow lance, chromium-containing hot metal is blown up with oxygen (O 2 ) for decarburization from the top blow lance, and the hot metal is vigorously stirred from the bottom blow tuyere. When performing decarburization blowing, use nitrogen gas as the gas for diluting oxygen for decarburization and stirring for bottom blowing until the carbon (C) concentration in steel is reduced to 0.8 mass%. It is also known to use argon. In this method, the nitrogen concentration after decarburization blowing is about 0.015 to 0.05 mass%. Further, it is also known that if argon rinse is performed in which ferrosilicon (Fe-Si) or the like, which is a deoxidizing agent, is injected while the bottom of the argon gas is blown, the concentration is reduced to about 0.005 to 0.015 mass% ( For example, see
[0005]
[Patent Document 1]
Japanese Patent Publication No. 3-46527 [Patent Document 2]
Japanese Patent No. 2670441 [Patent Document 3]
Japanese Patent No. 2751924 [0006]
[Problems to be solved by the invention]
In the method of denitrification by carburization as disclosed in
[0007]
Since the nitrogen concentration after decarburization blowing described as obtained in
[0008]
An object of the present invention is to provide a method for producing a low-nitrogen chromium-containing steel capable of economically reducing the nitrogen concentration.
[0009]
[Means for Solving the Problems]
The present invention, when manufacturing a chromium-containing steel containing chromium in a predetermined product concentration range,
The chromium concentration in the molten steel or hot metal before the decarburization treatment is adjusted to a concentration range lower than the concentration range for the product and in which the nitrogen content can be reduced by the decarburization treatment,
During or after decarburization, a chromium alloy for component adjustment is added and component adjustment is performed so that the concentration of chromium is within the product concentration range.
The concentration range is set in advance to a range in which the increase in the nitrogen content due to the pick-up of nitrogen when the chromium alloy for component adjustment is introduced is smaller than the decrease in the nitrogen content in the decarburization treatment. This is a method for producing low nitrogen chromium-containing steel.
[0010]
According to the present invention, when producing a chromium-containing steel containing chromium in a predetermined product concentration range, if the chromium concentration in the molten steel or hot metal subjected to decarburization treatment is low, the nitrogen concentration is also reduced. Then, the chromium concentration in the molten steel or hot metal before the decarburization treatment is kept in a concentration range lower than the product concentration range. Although the lower the chromium concentration, the lower the nitrogen concentration, the lower the chromium concentration in the molten steel after the decarburization process, in order to adjust the chromium concentration to the product concentration range after the decarburization process, The pickup of nitrogen from the atmosphere also increases the concentration of nitrogen introduced into the molten steel. The range of the chromium concentration in the molten steel or hot metal before the decarburization treatment is defined as the increase in the nitrogen content due to the nitrogen pickup when the chromium alloy for component adjustment is charged is calculated from the decrease in the nitrogen content in the decarburization treatment. Is set in advance to a small range, so that the nitrogen concentration can be effectively reduced. The input of the chromium alloy for component adjustment is a conventionally performed step even though the input amount is increased, and does not require extra work. An increase in the amount of chromium to be added for component adjustment means a decrease in the amount of chromium to be input at the stage of molten steel or hot metal before decarburization treatment. The rise can be suppressed.
[0011]
Further, in the present invention, the chromium-containing steel is a chromium-containing steel containing chromium of 10 mass% or more as the concentration range for the product,
The chromium concentration range in the molten steel before the decarburization treatment is set to be smaller in the range from 1 mass% to 5 mass% from the product concentration range.
[0012]
According to the present invention, a chromium-containing steel containing 10 mass% or more of chromium as a concentration range for a product is set to a concentration range that is 1 to 5 mass% smaller than the concentration range for a product before the decarburization treatment. To reduce the nitrogen concentration. The chromium concentration that is insufficient for the product concentration range can be increased by adjusting the components. In the composition adjustment, chromium of at most about 5 mass% may be added, so that the amount of nitrogen introduced into the molten steel by the pickup from the atmosphere when the chromium alloy for composition adjustment is charged can be reduced.
[0013]
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1 shows, as an embodiment of the present invention, a result obtained by performing an operation for reducing the nitrogen concentration when performing a decarburization treatment by a converter, the chromium concentration and the product nitrogen before charging the converter. Shown in relation to concentration. The chromium-containing steel product to be operated is a stainless steel having a product chromium concentration of 18 mass%. The rectangular plot shows the nitrogen concentration before charging the converter, and the diamond plot shows the nitrogen concentration of the chromium-adjusted product. Before charging the converter, the decrease in the chromium concentration and the decrease in the nitrogen concentration correspond to each other, but it is understood that the nitrogen concentration in the product increases when the chromium concentration becomes 13 mass% or less. The reason is that when the chromium concentration decreases, the amount of chromium added by component adjustment increases, and when a large amount of chromium alloy such as ferrochrome (Fe-Cr) is put into molten steel, nitrogen in the atmosphere is picked up. It is presumed that this is because the nitrogen concentration in the molten steel increases.
[0014]
From FIG. 1, when the product chromium concentration is required to be 18%, if the chromium concentration before charging the converter is set to 13.0 to 17.0 mass%, the nitrogen concentration in the product indicated by the diamond-shaped plot is 0.1%. It can also be seen that it becomes 012 mass% or less. Further, if the chromium concentration before charging the converter is set to 14.0 to 16.0 mass%, it can be seen that the nitrogen concentration in the product becomes 0.01 mass% or less.
[0015]
The hot metal for producing stainless steel is produced in an electric furnace. One charge of the electric furnace is about 75 t, and the main components of the hot metal are shown in Table 1 below.
[0016]
[Table 1]
The main data of the results plotted in FIG. 1 are based on the results of the invention examples in which the chromium concentration before charging the converter falls within the range of 13.0 to 17.0 mass%, and the chromium concentration falling within the concentration range. The results are shown in the following Table 2 by being classified into comparative examples that do not fall within the range.
[0018]
[Table 2]
FIG. 2 shows schematic steps up to measurement of the nitrogen concentration of the product in the present embodiment. In step s1, molten iron whose main component is as shown in Table 1 is melted in an electric furnace. Next, in step s2, decarburization and denitrification in a converter are performed without performing carburizing or flux addition after carburizing. However, the denitrification treatment is performed along with the decarburization treatment. The nitrogen concentration and chromium concentration before charging the converter shown in FIG. 1 are analysis values for the hot metal before the decarburization treatment at this stage.
[0020]
The decarburization treatment is performed as a rough decarburization treatment until the carbon concentration becomes 0.05 to 0.30 mass%. Next, as step s3, the components are roughly adjusted so that the chromium concentration is in the range of 17.5 to 18.5 mass%. This component adjustment is performed during or after decarburization. Next, in step s4, only the molten steel that has not been deoxidized is tapped into a ladle. Thereafter, in step s5, decarburization is performed by a ladle vacuum decarburization method such as VOD (Vacuum Oxygen Decaburization) until the carbon concentration becomes 0.020 mass% or less. Next, in step s6, the component temperature is adjusted, the chromium concentration is adjusted to a product concentration range centering on 18 mass%, and the temperature is also adjusted to a range suitable for continuous casting, and continuous casting is performed in step s7. The analytical value of the product nitrogen shown in FIG. 1 is obtained by sampling the molten steel from inside the mold of the continuous casting apparatus in step s8 and analyzing the product nitrogen concentration in step s9.
[0021]
FIG. 3 shows a result obtained by performing an operation for reducing the nitrogen concentration when performing the decarburization treatment by the converter when the chromium concentration of the product is 14 mass% as another embodiment of the present invention. Is shown as a relationship between the chromium concentration and the product nitrogen concentration before charging the converter. If the chromium concentration before charging the converter is set in the range of 9.0 to 13.0 mass% as the decarburization concentration range, it can be seen that the product nitrogen concentration becomes 0.008 mass% or less. This concentration range for decarburization is a range where the concentration of the product chromium is reduced by 1.0 to 5.0 mass% when the chromium concentration is 14.0 mass%.
[0022]
In this embodiment, the procedure for obtaining the analytical value of the product nitrogen concentration is the same as that in FIG. That is, the hot metal for producing stainless steel is also produced in an electric furnace, and one charge of the electric furnace is about 75 t. However, in the coarse adjustment of the chromium component in step s3 in FIG. 2, the chromium component is adjusted to a range of 13.5 to 14.5 mass%. The main components of the hot metal are shown in Table 3 below.
[0023]
[Table 3]
As described above, the nitrogen concentration can be reduced by adjusting the chromium concentration of the hot metal before charging the converter so as to be 1.0 to 5.0 mass% smaller than the chromium concentration of the product. It becomes possible by a process substantially similar to the conventional one. The present invention can be applied not only to the case where decarburization is performed in a converter but also to the case where the decarburization is performed by another method such as a scouring method in which oxygen and argon are blown from the bottom, such as AOD (Argon Oxygen Decarburization). it can.
[0025]
Further, the present invention is not limited to ferrite-based stainless steel having a product concentration of chromium of 18 mass% or 14 mass%, and can be applied to a chromium-containing steel containing chromium at a product concentration of about 10 mass% or more. When manufacturing chromium-containing steel, adjust the chromium concentration in the molten steel or hot metal before the decarburization treatment to a concentration range lower than the concentration range for products, and in which the nitrogen content can be reduced by the decarburization treatment. During or after decarburization, a chromium alloy for component adjustment is added and component adjustment is performed so that the chromium concentration is within the product concentration range, and the decarburization concentration range is for component adjustment. The amount of increase in the nitrogen content due to the pickup of nitrogen when the chromium alloy is charged may be set in advance to a range smaller than the decrease in the nitrogen content in the decarburization treatment.
[0026]
The present invention focuses on the fact that the lower the chromium concentration in the molten steel or hot metal subjected to decarburization treatment, the lower the nitrogen concentration, and the chromium concentration in the molten steel or hot metal before decarburization treatment is reduced to a product concentration range. In the lower concentration range. However, if the chromium concentration is too low, it is necessary to introduce a large amount of chromium alloy when adjusting the composition to raise the chromium concentration to the product concentration range, and the nitrogen introduced into the molten steel by the pickup of nitrogen from the atmosphere. It was also found that the concentration also increased, and the concentration range was set such that the increase in the nitrogen content due to the pickup of nitrogen when the chromium alloy for component adjustment was charged was smaller than the decrease in the nitrogen content in the decarburization treatment. Since it is set in advance to a small range, the nitrogen concentration can be effectively reduced. The input of the chromium alloy for component adjustment is a conventionally performed step even though the input amount is increased, and does not require extra work. An increase in the amount of chromium to be added for component adjustment means a decrease in the amount of chromium to be input at the stage of molten steel or hot metal before decarburization treatment. The rise can be suppressed.
[0027]
【The invention's effect】
As described above, according to the present invention, focusing on the fact that if the chromium concentration in the molten steel or hot metal subjected to the decarburization treatment is low, the nitrogen concentration is also reduced, and the chromium concentration in the molten steel or the hot metal before the decarburization treatment is performed. Can be reduced to a concentration range lower than the product concentration range to reduce the nitrogen concentration. In order to adjust the chromium concentration to the product concentration range, when the chromium alloy is added to the molten steel after decarburization, the concentration of nitrogen introduced into the molten steel by the pickup of nitrogen from the atmosphere increases. Since the setting is made so as not to be performed, the nitrogen concentration can be effectively reduced. The introduction of the chromium alloy for component adjustment is a conventionally performed step, and does not require extra work. Even if the amount of chromium added for component adjustment increases, the amount of chromium added at the stage of molten steel or hot metal before the decarburization treatment decreases, so the increase in production costs is suppressed and the nitrogen concentration is economically reduced. Can be.
[0028]
According to the present invention, when producing a chromium-containing steel containing 10 mass% or more of chromium as a product concentration range, the decarburization treatment is performed at a chromium concentration of 1 to 5 mass% smaller than the product concentration range. As a result, the nitrogen concentration can be reduced. In the composition adjustment after decarburization, chromium of about 5 mass% at the maximum may be added, so that the amount of nitrogen introduced into the molten steel by pickup from the atmosphere with the introduction of a chromium alloy or the like can be reduced. .
[Brief description of the drawings]
FIG. 1 shows a result obtained by performing an operation for reducing a nitrogen concentration for the purpose of achieving a product chromium concentration of 18 mass% when performing a decarburization treatment by a converter as an embodiment of the present invention. It is a graph shown by the relationship between the chromium concentration before entry and the product nitrogen concentration.
FIG. 2 is a schematic flow chart of a process for obtaining a result of FIG. 1 until a nitrogen concentration of a product is measured.
FIG. 3 shows a result obtained by performing an operation for reducing the nitrogen concentration when performing decarburization treatment by a converter when the chromium concentration of the product is 14 mass% as another embodiment of the present invention. Is a graph showing the relationship between the chromium concentration and the product nitrogen concentration before charging the converter.
Claims (2)
脱炭処理前の溶鋼または溶銑中のクロム濃度を、該成品用濃度範囲よりも低く、脱炭処理で窒素含有量の減少も可能となる濃度範囲に調整しておき、
脱炭中または脱炭後に、クロムの濃度が該成品用濃度範囲内となるように、成分調整用のクロム合金を投入して成分調整を行い、
該濃度範囲は、該成分調整用のクロム合金を投入する際の窒素のピックアップによる窒素含有量の増加分が該脱炭処理での窒素含有量の減少分よりも小さい範囲に、予め設定しておくことを特徴とする低窒素クロム含有鋼の製造方法。When producing chromium-containing steel containing chromium in a predetermined product concentration range,
The chromium concentration in the molten steel or hot metal before the decarburization treatment is adjusted to a concentration range lower than the concentration range for the product and in which the nitrogen content can be reduced by the decarburization treatment,
During or after decarburization, a chromium alloy for component adjustment is added and component adjustment is performed so that the concentration of chromium is within the product concentration range.
The concentration range is set in advance to a range in which the increase in the nitrogen content due to the pick-up of nitrogen when the chromium alloy for component adjustment is introduced is smaller than the decrease in the nitrogen content in the decarburization treatment. A method for producing low nitrogen chromium-containing steel.
前記脱炭処理前の溶鋼中のクロム濃度範囲を、該成品用濃度範囲から１ｍａｓｓ％以上で５ｍａｓｓ％以下の範囲で小さくなるように設定することを特徴とする請求項１記載の低窒素クロム含有鋼の製造方法。The chromium-containing steel is a chromium-containing steel containing 10 mass% or more of chromium as the product concentration range,
2. The low nitrogen chromium content according to claim 1, wherein the chromium concentration range in the molten steel before the decarburization treatment is set to be smaller in a range from 1 mass% to 5 mass% from the product concentration range. Steel production method.
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