JP3770038B2

JP3770038B2 - Method for producing enamel steel

Info

Publication number: JP3770038B2
Application number: JP2000084219A
Authority: JP
Inventors: 光裕沼田; 貴司木村; 直広佐藤; 善彦樋口
Original assignee: Sumitomo Metal Industries Ltd
Current assignee: Nippon Steel Corp
Priority date: 2000-03-24
Filing date: 2000-03-24
Publication date: 2006-04-26
Anticipated expiration: 2020-03-24
Also published as: JP2001271179A

Description

【０００１】
【発明の属する技術分野】
本発明は、耐爪飛び性にすぐれ、かつ表面欠陥の少ないほうろう用鋼の製造方法に関する。
【０００２】
【従来の技術】
ほうろう製品は、鍋などの台所用品、建築材など幅広い用途に用いられるが、その用途から高い加工性や耐爪飛び性が要求される。
【０００３】
これらの特性確保を目的に、鋼中酸素濃度を高め、同時にＭｎ、Ｃｒ、Ｖなどを添加する技術が知られている。この技術は、鋼中介在物量を増大させ、爪飛び欠陥を防止すると同時に、加工性を良好にすることを目的としたものである。
【０００４】
なお、爪飛び欠陥とは、ほうろう製品製造後、すなわち、釉薬を焼成してから約６ヶ月までに鋼鈑と釉薬の境界に鋼鈑中の水素ガスが析出し、その圧力で釉薬層を弾き飛ばして起こる表面欠陥をいう
爪飛び欠陥を防止するには、鋼鈑中の介在物を多くすることにより、鋼板の水素の吸収能を大きくすることが重要であるとされている。
【０００５】
また、近年では、ほうろう製品はその用途の多様化から表面疵の低減も求められている。表面疵は介在物により発生するが、前述した爪飛び欠陥の防止のため介在物量を低減することができず、表面疵低減と爪飛び欠陥の防止との両立は困難とされてきた。
【０００６】
また、表面疵は、高酸素溶鋼との反応により耐火物の一部が損傷し溶鋼中に侵入した外来性大型介在物と、溶鋼に添加したＡｌなどで生成した大型非金属介在物（１０ミクロン以上）とによって発生し、このいずれの介在物を低減する場合でも溶鋼中の酸素濃度の低減が有効である。しかし、単純に酸素を低減してしまうと、微細な介在物量も減少し、耐爪飛び性が得られない。
【０００７】
例えば、特開平９−２７２９１４号公報には、表面疵低減と爪飛び欠陥の防止とを両立させる方法としてＡｌ添加量を適正化する方法が示され、Ａｌ添加量は少なければ少ないほどよいとされている。また、その他の技術においてもＡｌ濃度は上限が規制されており、可能な限りその濃度が低いことが望ましいとされている。
【０００８】
【発明が解決しようとする課題】
しかし、Ａｌ、Ｓｉといった脱酸力を有する物質の濃度を低濃度にすると、以下の問題を生じる。
【０００９】
Ｃ濃度が数十ppm と低く、かつＭｎなどの弱脱酸元素しかない溶鋼中では酸素濃度が不安定となり、溶鋼中に酸素を安定させる脱酸元素が存在しないことから、酸素は溶鋼中にわずかに存在する脱酸元素と反応し、Ｏ−Ｍｎ−Ｆｅ−Ａｌ系、Ｏ−Ｍｎ−Ｓｉ系、Ｏ−Ｍｎ−Ｍｇ系、Ｏ−Ｆｅ−Ｍｎ系など多種の介在物を同時に形成する。これらの介在物の組成、大きさも様々であり、このうち大径の介在物が表面欠陥の原因となる。
【００１０】
酸素濃度、介在物大きさが制御できなければ、安定した耐爪飛び性も得られない。
すなわち、従来技術にあるように、単純に脱酸元素濃度を低減させると溶鋼中酸素の反応を制御できないため、介在物制御が不安定となる。この結果、耐爪飛び性も不安定となる。
【００１１】
耐爪飛び性を向上させるには、介在物を微細な介在物として生成させればよく、表面欠陥を低減するには大径の介在物のみを低減すればよい。これには介在物の形態制御が必要となる。この介在物の形態制御が可能となれば、表面疵低減と爪飛び欠陥の防止との両立が可能なほうろう用鋼が製造可能となる。
【００１２】
本発明の目的は、表面疵低減と爪飛び欠陥の防止との両立が可能なほうろう用鋼の製造方法を提供することにある。
【００１３】
【課題を解決するための手段】
本発明者らは、ほうろう用鋼の表面疵低減と爪飛び欠陥の防止との両立を行うべく調査を行い以下の知見を得た。
【００１４】
（Ａ）ほうろう用鋼では、微細な介在物を多数必要とするため、耐爪飛び性確保には酸素濃度は高ければ高いほどよいが、質量ppm で９００ppm （以下、単にppm で表す）を越えて酸素濃度が高い場合、相応の脱酸剤を添加しなければならず、大径の介在物を生成し、介在物形態制御の精度が低下するため、耐爪飛び性が不安定となることが確認された。
【００１５】
一方、酸素濃度が４００ppm 未満の場合、脱酸剤濃度に関わらず、介在物個数が不十分であり、爪飛び欠陥が多発した。特に、爪飛び欠陥が発生しやすく鋼板への要求の厳しい１回掛けほうろう製品では欠陥が多発した。
【００１６】
但し、２回掛けほうろう用途の場合は、上塗りと下塗りでほうろう釉薬を使い分けており、下塗りでは爪飛び鋼板要因のほうろう欠陥が生じにくい釉薬を、上塗りでは外観を重視した釉薬が使用できるため、鋼板に求められるほうろう性は１回掛けよりも低く、鋼板の酸素量の下限は１００ppm まで適用可能である。
【００１７】
（Ｂ）次に、介在物の形態制御方法について検討した。
介在物の形態制御を行うには、酸素との反応安定性を確保するためにＡｌ、Ｓｉ、ＣａおよびＭｇなどの脱酸剤を効果的に用いることが重要であり、先ずＡｌ濃度と介在物の形態について調査した。
【００１８】
Ａｌ濃度が３ppm 未満では、溶鋼中の酸素濃度を低下できず、耐火物の激しい損傷と他種類の介在物生成により表面欠陥が多発する。
また、酸素濃度、介在物量ともに変動し易く安定しないため、爪飛びも安定して防止できない。
【００１９】
一方、Ａｌ濃度が４０ppm を越えて高くなると、Ａｌ−Ｏ系とＡｌ−Ｍｎ−Ｏ系の２種類の介在物が生成し、介在物形態制御性が悪化する。特に、Ａｌ−Ｍｎ−Ｏ系介在物は大粒化しやすいため、直接疵の原因になるほか、微細介在物量を減少させて耐爪飛び性も低下させる。
【００２０】
Ａｌ濃度が３ppm 以上４０ppm 以下の場合、Ｆｅ−Ｍｎ−Ｏ系を主体として微量のＡｌを含有した介在物となる。これはＡｌにより適正に酸素濃度が制御されたためであるが、Ｆｅ−Ｍｎ−Ｏ系を主体として微量のＡｌを含有した介在物は大粒化しにくいことが確認された。
【００２１】
（Ｃ）Ｓｉ、Ｃａ、Ｍｇに関しても同様の調査を行った結果、Ａｌ同様それぞれに高い効果が得られる濃度範囲があることが確認された。
Ｓｉの場合、その濃度が２０ppm 未満ではＳｉによる酸素制御が困難であり、Ｓｉ濃度が２４０ppm を越えて高くなるとＳｉ−Ｍｎ−Ｏ系介在物とＳｉ−Ｏ系介在物が生成し、このうちＳｉ−Ｏ系介在物によって表面疵が発生する。一方、Ｓｉ濃度が２０ppm 以上２４０ppm 以下の場合、介在物はＦｅ−Ｍｎ−Ｏを主体として微量のＳｉを含有する。この介在物も微細に分散しやすいことが確認された。
【００２２】
Ｃａ、Ｍｇも同様の機構であるが、Ｃａ、Ｍｇはともに酸素と非常に強い反応性を有する。従って、その濃度は１ppm と極めて低い状態でＡｌ、Ｓｉと同様の効果を発生させることが可能となる。また、ＣａとＭｇの脱酸特性は近いため、それぞれ単独で用いても、両者の合計濃度で用いてもよい。一方、Ｃａ、Ｍｇ濃度が合計で２５ppm 超となると、大粒のＣａ−Ｏ、Ｃａ−Ｏ−Ｓ、Ｃａ−Ｓ、Ｍｇ−Ｏ、Ｍｇ−Ｃａ−Ｏなどの介在物が生成する。
【００２３】
（Ｄ）次に、転炉、真空脱ガス装置を用いて、溶鋼中の酸素濃度と脱酸元素濃度を制御し介在物形態制御を効率よく行う方法を検討した。
通常、転炉で粗脱炭し、Ｃ濃度：１００〜９００ppm 、Ｍｎ濃度：１０００〜５０００ppm に調整した後、ＲＨ等の真空脱ガス装置で脱炭し、Ｃ濃度：３０ppm 以下とする。真空脱炭では脱炭速度を大きくするために、酸素濃度をより高くすることが効果的であることはよく知られている。
【００２４】
しかし、酸素濃度が過剰に高い場合、脱炭後も多量の酸素が残留し、これを目標の酸素濃度まで低減するには大量のＡｌ、Ｓｉなどの脱酸剤を要する。このため、表面疵の原因となる大粒の介在物が多量に生成する。
【００２５】
一方、酸素濃度が低い場合、脱炭反応が停滞する以外に、介在物に対しても悪影響が生じる。すなわち、酸素濃度が低い場合、Ｍｎに比較して強い脱酸力を有するＡｌは、２Ａｌ＋３Ｏ→Ａｌ₂Ｏ₃なる反応により、Ａｌ−Ｏ系介在物を生成し、Ｍｎ＋Ｏ→ＭｎＯという反応が進行せず、Ｍｎ系介在物が生成しないため、前記の微細なＦｅ−Ｍｎ−Ｏ系介在物が生成できない。
【００２６】
このような状態を回避するには、脱酸元素濃度と酸素濃度をバランスよく制御する必要があり、真空脱炭処理後のＭｎ質量濃度と酸素質量濃度の比（Ｍｎ／Ｏ）を２〜１９とし、真空脱炭処理後にＭｎ濃度を調整することが重要である。比（Ｍｎ／Ｏ）が２未満となるとＭｎ濃度が酸素濃度に対して低くなり、酸素濃度が高くなりすぎる。比（Ｍｎ／Ｏ）が１９を越えて高くなると酸素濃度に対してＭｎ濃度が高くなりすぎ、酸素濃度が低くなりすぎる。
【００２７】
（Ｅ）その後にＡｌ、Ｓｉ、Ｃａおよび／またはＭｇを少なくとも１種の濃度を適正範囲に調整し、酸素濃度を４００〜９００ppm に調整することにより、微細なＦｅ−Ｍｎ−Ｏ系介在物が大量に生成できる。
【００２８】
（Ｆ）次に、溶鋼中の酸素濃度の測定方法に関し、各種センサを用いて酸素濃度測定精度を調査したところ以下のことが明らかとなった。
ほうろう用鋼のように酸素濃度が４００〜９００ppm と高い場合、固体電解質酸素センサのセンサ部に、例えばアルミナ等の被覆が施されていると、測定値がマイナス側に最大約５００ppm 偏った誤差が発生し易くなり、正確な測定ができない。この誤差範囲では介在物形態制御に必要な各種脱酸元素濃度調整が困難である。
【００２９】
一方、センサ部に被覆を施さない場合、測定誤差は±２０ppm 以下となり、大幅に精度が向上することを見出した。
センサ部に被覆を施さない場合に測定誤差が低減する理由は被覆による溶鋼中の酸素濃度の変化が生じないためと推定できる。
【００３０】
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、下記のとおりである。
（１）真空脱炭処理後の溶鋼のＭｎ濃度（質量ｐｐｍ）とＯ濃度（質量ｐｐｍ）との比（Ｍｎ／Ｏ）を２〜１９とし、その後、Ａｌ：３〜４０ｐｐｍ、Ｓｉ：２０〜２４０ｐｐｍ、Ｃａおよび／またはＭｇの合計濃度：１〜２５ｐｐｍの少なくとも１種の濃度調整を行うことを特徴とするほうろう用鋼の製造方法。
（２）質量ｐｐｍでＣ：３０ｐｐｍ以下、Ｍｎ：７０〜１００００ｐｐｍおよびＯ：４００〜９００ｐｐｍの基本組成の高酸素極低炭素鋼であって、Ａｌ：３〜４０ｐｐｍ、Ｓｉ：２０〜２４０ｐｐｍ、Ｃａおよび／またはＭｇの合計濃度：１〜２５ｐｐｍの少なくとも１種をさらに含むほうろう用鋼を製造するに際し、真空脱炭処理後の溶鋼のＭｎ濃度（質量ｐｐｍ）とＯ濃度（質量ｐｐｍ）との比（Ｍｎ／Ｏ）を２〜１９とし、その後、Ａｌ：３〜４０ｐｐｍ、Ｓｉ：２０〜２４０ｐｐｍ、Ｃａおよび／またはＭｇの合計濃度：１〜２５ｐｐｍの少なくとも１種の濃度調整を行うことを特徴とするほうろう用鋼の製造方法。
（３）前記真空脱炭処理に際して行う溶鋼のＯ濃度の測定に使用する固体電解質酸素センサがセンサ部に被覆がなされていない酸素センサであることを特徴とする上記（１）または（２）に記載のほうろう用鋼の製造方法。
【００３１】
【発明の実施の形態】
転炉でＣ濃度を１００〜９００ppm とした後、ＲＨ真空脱ガス装置などの真空脱ガス装置を用いて、Ｃ濃度を３０ppm 以下まで脱炭する。Ｍｎ濃度は転炉操業時、出鋼時等に調整し、調整したＭｎ濃度と転炉終了時のＣ濃度から必要酸素を計算し、真空脱炭終了時の比（Ｍｎ／Ｏ）が２〜１９を満足するように酸素濃度を調整する。酸素濃度の調整は、転炉操業時、出鋼時、あるいは真空脱ガス装置での真空脱炭処理前などに酸素ガスまたはＦｅ₂Ｏ₃、ＦｅＯなどの固体酸化物を溶鋼に添加すればよい。
【００３２】
酸素濃度を調整完了後、真空脱炭を行う。真空脱炭後、脱酸剤のＡｌ、Ｓｉ、Ｃａおよび／またはＭｇを少なくとも１種を溶鋼に添加し、溶鋼中の濃度がＡｌ：３〜４０ppm 、Ｓｉ：２０〜２４０ppm 、Ｃａおよび／またはＭｇの合計濃度：１〜２５ppm の少なくとも１種を満足するように調整する。
【００３３】
上記Ａｌ、Ｓｉ、ＣａおよびＭｇの調整は、これらを脱酸剤として直接添加する方法、これらを含有する金属またはフラックスを添加する方法、あるいはスラグを用いる方法等で行うことができる。
【００３４】
このとき、真空脱炭後の前記比（Ｍｎ／Ｏ）が満足されていれば、酸素濃度は介在物制御効果により４００〜９００ppm となる。
この溶鋼中の酸素濃度は、酸素濃度の測定に使用する固体電解質酸素センサがセンサ部に被覆がなされていない酸素センサであることにより、精度良く溶鋼中の酸素濃度を測定できる。
【００３５】
脱酸剤添加により脱酸剤濃度調整した後、溶鋼の真空脱ガス処理を終了し、処理済溶鋼を連続鋳造機に供給し、スラブ等を鋳造する。
【００３６】
【実施例】
転炉でＣ濃度：１００〜３００ppm 、Ｍｎ濃度：３０００〜５０００ppm に調整し取鍋内へ出鋼した。取鍋をＲＨ真空脱ガス装置へ移動し、前記Ｃ濃度およびＭｎ濃度から真空脱炭後の比（Ｍｎ／Ｏ）が２〜１９となるようにＦｅ₂Ｏ₃の添加により酸素濃度を調整した。
【００３７】
比（Ｍｎ／Ｏ）を所定値に調整後、真空下で脱炭処理を行い、Ｃ濃度を３０ppm 以下とした。
真空脱炭後、脱酸剤のＡｌ、Ｓｉ、Ｃａおよび／またはＭｇの少なくとも１種を溶鋼に添加し、溶鋼中の酸素濃度を所定値に調整した。
【００３８】
ＲＨ処理後、連続鋳造機を用いて鋳造し、その後常法により冷延鋼板とした。得られた試作用鋼板に以下に示す条件でほうろう掛け試験を行った。
（１）酸洗条件［酸洗液組成（硫酸：１７質量％、鉄濃度：約５質量％、酸洗液温度：７８℃、酸洗時間：５分間］
（２）Ｎｉフラッシュめっき条件［めっき液：１２ｇ／ＬＮｉＳＯ₄・７Ｈ₂Ｏ、鉄濃度：約５質量％、ｐＨ：２．８、めっき液温度：８２℃、めっき時間：５分間］
（３）施釉条件［日本フェロー製の１５５３Ｂ釉薬をスプレーにより片面当たり１００μｍにて両面施釉］
（４）焼成条件［８２０℃で６分間］
（５）繰り返し焼成条件［上記の施釉と焼成の工程を再度実施］
（６）熱処理条件［２００℃で８分間保持にて爪飛び欠陥の発生を促進］
上記条件下で得られた鋼板の爪飛びと表面疵の有無を調べた。
【００３９】
表１に試験結果を示す。
【００４０】
【表１】

なお、表中の脱炭後の比（Ｍｎ／Ｏ）は、真空脱炭処理後（但し、脱酸処理前）のＭｎ濃度（質量ppm ）とＯ濃度（質量ppm ）との比（Ｍｎ／Ｏ）を表し、それ以外の濃度は、脱酸処理後の濃度を表す化学分析値である。
【００４１】
但し、Ｏ濃度は、（Ａ）表示の化学分析値と、（Ｂ）表示の真空処理終了時点でのオンライン分析（センサに被覆無し条件）および（Ｃ）表示のオンライン分析（センサにアルミナの被覆有り条件）との比較を行った。
【００４２】
また、化学分析値（Ａ）と、それぞれのオンライン分析値との差［（Ｂ）−（Ａ）または（Ｃ）−（Ａ））を試算し、その最大値、最小値およびバラツキ範囲を比較した。ここで、バラツキ範囲とは、バラツキ範囲＝最大値−最小値と定義される。
【００４３】
さらに、表中のＡｌ、Ｓｉ、Ｃａ＋Ｍｇ濃度における（○）および（×）の評価は、以下の通りである。
Ａｌ濃度が３〜４０ppm の適正範囲であれば（○）、それ以外は（×）とした。
【００４４】
Ｓｉ濃度が２０〜２４０ppm の適正範囲であれば（○）、それ以外は（×）とした。
Ｃａおよび／またはＭｇの合計濃度が１〜２５ppm の適正範囲であれば（○）、それ以外は（×）とした。
【００４５】
同表の試験番号１〜１３に示すように、真空脱炭後の比（Ｍｎ／Ｏ）が２〜１９であり、脱酸後のＡｌ、Ｓｉ濃度、Ｃａおよび／またはＭｇの合計濃度の内の少なくとも１種の脱酸剤の濃度が適正範囲であれば、爪飛びおよび表面疵が発生しなかった。
【００４６】
しかし、同表の試験番号１５〜２１に示すように、脱酸後のＡｌ、Ｓｉ濃度、Ｃａおよび／またはＭｇの合計濃度が全て適正範囲外の場合には、爪飛びおよび表面疵の発生を防止できなかった。
【００４７】
なお、試験番号１４では、表面疵を防止できたが爪飛びを防止できなかった。
また、（Ａ）化学分析による酸素濃度と、（Ｂ）オンライン分析（センサに被覆無し条件）および（Ｃ）オンライン分析（センサにアルミナの被覆有り条件）との比較を行った結果、（Ｃ）オンライン分析（センサにアルミナの被覆有り条件）のバラツキ範囲は４００ppm であったのに対して、（Ｂ）オンライン分析（センサに被覆無し条件）のバラツキ範囲は４３ppm と小さくなった。
【００４８】
【発明の効果】
本発明により、表面疵低減と爪飛び欠陥の防止との両立が可能なほうろう用鋼を製造できる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing an enamel steel having excellent nail flying resistance and few surface defects.
[0002]
[Prior art]
Enamel products are used in a wide range of applications such as pots and other kitchen utensils and building materials, and high workability and anti-nail resistance are required for these applications.
[0003]
For the purpose of securing these characteristics, a technique for increasing the oxygen concentration in steel and simultaneously adding Mn, Cr, V or the like is known. This technique aims to increase the amount of inclusions in the steel, prevent claw skipping defects, and improve workability.
[0004]
Note that the nail skipping defect means that after the enamel product is manufactured, that is, about 6 months after the glaze is fired, hydrogen gas in the steel is deposited at the boundary between the steel and the glaze, and the glaze layer is repelled by that pressure. In order to prevent nail skipping defects, which are surface defects caused by flying, it is important to increase the hydrogen absorption capacity of the steel sheet by increasing the inclusions in the steel sheet.
[0005]
In recent years, enamel products are also required to reduce surface wrinkles due to diversification of their uses. Although surface flaws are generated by inclusions, the amount of inclusions cannot be reduced to prevent the above-described claw skipping defect, and it has been difficult to achieve both reduction of surface flaws and prevention of claw skipping defects.
[0006]
In addition, the surface defects are large foreign metallic inclusions (10 micron) produced by exogenous large inclusions that have been partially damaged by the reaction with high oxygen molten steel and have entered the molten steel, and Al added to the molten steel. Even when reducing any of these inclusions, it is effective to reduce the oxygen concentration in the molten steel. However, if oxygen is simply reduced, the amount of fine inclusions is also reduced, and nail skipping resistance cannot be obtained.
[0007]
For example, Japanese Patent Application Laid-Open No. 9-272914 discloses a method for optimizing the amount of Al added as a method for achieving both surface flaw reduction and prevention of nail skipping defects. The smaller the amount of added Al, the better. ing. In other techniques, the upper limit of the Al concentration is regulated, and it is desirable that the concentration be as low as possible.
[0008]
[Problems to be solved by the invention]
However, when the concentration of a substance having a deoxidizing power such as Al or Si is made low, the following problems occur.
[0009]
In molten steel that has a low C concentration of tens of ppm and only weak deoxidation elements such as Mn, the oxygen concentration becomes unstable, and there is no deoxidation element that stabilizes oxygen in the molten steel. It reacts with a slightly present deoxidizing element to simultaneously form various inclusions such as O—Mn—Fe—Al, O—Mn—Si, O—Mn—Mg, and O—Fe—Mn. The composition and size of these inclusions vary, and among these, inclusions with a large diameter cause surface defects.
[0010]
If the oxygen concentration and the size of inclusions cannot be controlled, stable nail flying resistance cannot be obtained.
That is, as in the prior art, if the deoxidizing element concentration is simply reduced, the reaction of oxygen in the molten steel cannot be controlled, so that inclusion control becomes unstable. As a result, the nail flying resistance becomes unstable.
[0011]
In order to improve the nail flying resistance, the inclusions may be generated as fine inclusions, and in order to reduce surface defects, only the large-diameter inclusions need be reduced. This requires the inclusion shape control. If the inclusions can be controlled in shape, enamel steel capable of achieving both surface flaw reduction and prevention of claw skipping defects can be manufactured.
[0012]
An object of the present invention is to provide a method for producing enamel steel capable of achieving both reduction of surface flaws and prevention of claw skipping defects.
[0013]
[Means for Solving the Problems]
The inventors of the present invention have investigated to achieve both reduction of surface defects and prevention of claw skipping defects in enamel steel, and have obtained the following knowledge.
[0014]
(A) Since enamel steel requires a lot of fine inclusions, the higher the oxygen concentration, the better to ensure nail-flip resistance, but it exceeds 900 ppm in mass ppm (hereinafter simply expressed in ppm). When the oxygen concentration is high, a corresponding deoxidizer must be added, and large-diameter inclusions are generated, and the accuracy of inclusion form control is reduced, resulting in unstable nail flying resistance. Was confirmed.
[0015]
On the other hand, when the oxygen concentration was less than 400 ppm, the number of inclusions was insufficient regardless of the deoxidizer concentration, and nail skipping defects occurred frequently. In particular, the nail skipping defect was likely to occur, and the defect was frequently generated in the single enameled product which required the steel plate.
[0016]
However, in the case of double enamel application, enamel glaze is used separately for top coat and undercoat. The required enamelability is lower than the one-time, and the lower limit of the oxygen content of the steel sheet is applicable up to 100 ppm.
[0017]
(B) Next, the form control method of the inclusion was examined.
In order to control the form of inclusions, it is important to effectively use a deoxidizing agent such as Al, Si, Ca and Mg in order to ensure the reaction stability with oxygen. The form of was investigated.
[0018]
If the Al concentration is less than 3 ppm, the oxygen concentration in the molten steel cannot be lowered, and surface defects frequently occur due to severe damage to the refractory and the generation of other types of inclusions.
Further, since both the oxygen concentration and the amount of inclusions tend to fluctuate and are not stable, it is not possible to stably prevent nail skipping.
[0019]
On the other hand, when the Al concentration exceeds 40 ppm, two types of inclusions, Al—O and Al—Mn—O, are generated, and inclusion form controllability deteriorates. In particular, since Al—Mn—O-based inclusions are likely to become large in size, they directly cause wrinkles, and also reduce the amount of fine inclusions and reduce the nail flying resistance.
[0020]
When the Al concentration is 3 ppm or more and 40 ppm or less, the inclusion contains a trace amount of Al mainly composed of the Fe—Mn—O system. This is because the oxygen concentration was appropriately controlled by Al. However, it was confirmed that inclusions containing a small amount of Al mainly composed of Fe-Mn-O system are difficult to increase in size.
[0021]
(C) As a result of conducting the same investigation on Si, Ca, and Mg, it was confirmed that there was a concentration range in which a high effect was obtained similarly to Al.
In the case of Si, if the concentration is less than 20 ppm, it is difficult to control oxygen by Si, and if the Si concentration exceeds 240 ppm, Si—Mn—O inclusions and Si—O inclusions are produced. Surface flaws are generated by -O inclusions. On the other hand, when the Si concentration is 20 ppm or more and 240 ppm or less, the inclusion mainly contains Fe-Mn-O and contains a small amount of Si. It was confirmed that the inclusions are also easily dispersed finely.
[0022]
Ca and Mg have the same mechanism, but both Ca and Mg have very strong reactivity with oxygen. Therefore, the same effect as Al and Si can be generated with the concentration being as low as 1 ppm. Moreover, since the deoxidation characteristics of Ca and Mg are close to each other, they may be used alone or in their total concentration. On the other hand, when the Ca and Mg concentrations exceed 25 ppm in total, inclusions such as large grains of Ca—O, Ca—O—S, Ca—S, Mg—O, and Mg—Ca—O are generated.
[0023]
(D) Next, using a converter and a vacuum degassing device, a method for efficiently controlling the inclusion form control by controlling the oxygen concentration and deoxidizing element concentration in the molten steel was studied.
Usually, after rough decarburization with a converter and C concentration: 100-900 ppm and Mn concentration: 1000-5000 ppm, decarburization with a vacuum degassing apparatus such as RH, etc., C concentration: 30 ppm or less. It is well known that in vacuum decarburization, it is effective to increase the oxygen concentration in order to increase the decarburization rate.
[0024]
However, if the oxygen concentration is excessively high, a large amount of oxygen remains even after decarburization, and a large amount of deoxidizer such as Al or Si is required to reduce this to the target oxygen concentration. For this reason, a large amount of large inclusions that cause surface flaws are generated.
[0025]
On the other hand, when the oxygen concentration is low, besides the decarburization reaction stagnation, there is an adverse effect on inclusions. That is, when the oxygen concentration is low, Al having a stronger deoxidizing power than Mn generates Al—O inclusions by a reaction of 2Al + 3O → Al ₂ O _{3, and the} reaction of Mn + O → MnO proceeds. In addition, since the Mn-based inclusions are not generated, the fine Fe—Mn—O-based inclusions cannot be generated.
[0026]
In order to avoid such a state, it is necessary to control the deoxidation element concentration and the oxygen concentration in a well-balanced manner, and the ratio (Mn / O) of the Mn mass concentration to the oxygen mass concentration after the vacuum decarburization treatment is 2-19. It is important to adjust the Mn concentration after the vacuum decarburization treatment. When the ratio (Mn / O) is less than 2, the Mn concentration becomes lower than the oxygen concentration, and the oxygen concentration becomes too high. When the ratio (Mn / O) is higher than 19, the Mn concentration becomes too high with respect to the oxygen concentration, and the oxygen concentration becomes too low.
[0027]
(E) Thereafter, by adjusting the concentration of at least one of Al, Si, Ca and / or Mg within an appropriate range, and adjusting the oxygen concentration to 400 to 900 ppm, fine Fe—Mn—O-based inclusions can be obtained. Can be generated in large quantities.
[0028]
(F) Next, regarding the method for measuring the oxygen concentration in molten steel, the oxygen concentration measurement accuracy was investigated using various sensors, and the following was found.
When the oxygen concentration is as high as 400-900ppm, such as in enamel steel, if the sensor part of the solid electrolyte oxygen sensor is coated with, for example, alumina, an error in which the measured value is biased up to about 500ppm on the negative side will occur. It tends to occur and accurate measurement cannot be performed. Within this error range, it is difficult to adjust various deoxidation element concentrations necessary for inclusion form control.
[0029]
On the other hand, when the sensor part was not coated, the measurement error was ± 20 ppm or less, and it was found that the accuracy was greatly improved.
The reason why the measurement error is reduced when the sensor portion is not coated can be presumed that the change in oxygen concentration in the molten steel due to the coating does not occur.
[0030]
The present invention has been made based on the above findings, and the gist thereof is as follows.
(1) the ratio of the Mn concentration of the molten steel after the vacuum decarburization treatment (mass ppm) and O concentration (mass ppm) of (Mn / O) and 2 to 19, then, Al: 3~40ppm, Si: 20~ 240 ppm, total concentration of Ca and / or Mg: at least one concentration adjustment of 1 to 25 ppm is carried out.
(2) High oxygen ultra-low carbon steel having a basic composition of mass ppm, C: 30 ppm or less, Mn: 70-10000 ppm, and O: 400-900 ppm, Al: 3-40 ppm, Si: 20-240 ppm, Ca and // Mg concentration: When producing enamel steel further containing at least one of 1 to 25 ppm, ratio of Mn concentration (mass ppm) and O concentration (mass ppm) of molten steel after vacuum decarburization treatment ( Mn / O) is set to 2 to 19, and thereafter, Al: 3 to 40 ppm, Si: 20 to 240 ppm, total concentration of Ca and / or Mg: at least one concentration adjustment of 1 to 25 ppm is performed. A method for producing enamel steel.
(3) In the above (1) or (2), the solid electrolyte oxygen sensor used for measuring the O concentration of the molten steel performed in the vacuum decarburization treatment is an oxygen sensor in which the sensor portion is not coated The manufacturing method of the enamel steel described.
[0031]
DETAILED DESCRIPTION OF THE INVENTION
After the C concentration is adjusted to 100 to 900 ppm in the converter, the carbon concentration is decarburized to 30 ppm or less using a vacuum degassing apparatus such as an RH vacuum degassing apparatus. The Mn concentration is adjusted at the time of converter operation, at the time of steel output, the required oxygen is calculated from the adjusted Mn concentration and the C concentration at the end of the converter, and the ratio (Mn / O) at the end of vacuum decarburization is 2 to 2. The oxygen concentration is adjusted to satisfy 19. To adjust the oxygen concentration, oxygen gas or solid oxides such as Fe ₂ O ₃ and FeO may be added to the molten steel at the time of converter operation, at the time of steel output, or before vacuum decarburization with a vacuum degassing apparatus. .
[0032]
After adjusting the oxygen concentration, vacuum decarburization is performed. After vacuum decarburization, at least one of deoxidizers Al, Si, Ca and / or Mg is added to the molten steel, and the concentration in the molten steel is Al: 3 to 40 ppm, Si: 20 to 240 ppm, Ca and / or Mg. The total concentration is adjusted so as to satisfy at least one of 1 to 25 ppm.
[0033]
The Al, Si, Ca and Mg can be adjusted by a method of directly adding them as a deoxidizing agent, a method of adding a metal or flux containing them, a method of using slag, or the like.
[0034]
At this time, if the ratio (Mn / O) after vacuum decarburization is satisfied, the oxygen concentration is 400 to 900 ppm due to the inclusion control effect.
As for the oxygen concentration in the molten steel, the oxygen concentration in the molten steel can be accurately measured because the solid electrolyte oxygen sensor used for measuring the oxygen concentration is an oxygen sensor in which the sensor portion is not covered.
[0035]
After adjusting the concentration of the deoxidizer by adding the deoxidizer, the vacuum degassing treatment of the molten steel is finished, the treated molten steel is supplied to a continuous casting machine, and a slab or the like is cast.
[0036]
【Example】
In a converter, the C concentration was adjusted to 100 to 300 ppm and the Mn concentration was adjusted to 3000 to 5000 ppm, and the steel was put into the ladle. The ladle was moved to the RH vacuum degassing apparatus, and the oxygen concentration was adjusted by adding Fe ₂ O ₃ so that the ratio (Mn / O) after vacuum decarburization was ₂ to 19 from the C concentration and Mn concentration. .
[0037]
After adjusting the ratio (Mn / O) to a predetermined value, decarburization treatment was performed under vacuum to make the C concentration 30 ppm or less.
After vacuum decarburization, at least one of deoxidizers Al, Si, Ca and / or Mg was added to the molten steel, and the oxygen concentration in the molten steel was adjusted to a predetermined value.
[0038]
After the RH treatment, casting was performed using a continuous casting machine, and then cold-rolled steel sheets were formed by a conventional method. The obtained prototype steel plate was subjected to a brazing test under the following conditions.
(1) Pickling conditions [Pickling solution composition (sulfuric acid: 17% by mass, iron concentration: about 5% by mass, pickling solution temperature: 78 ° C., pickling time: 5 minutes]
(2) Ni flash plating conditions [plating solution: 12 g / L NiSO ₄ .7H ₂ O, iron concentration: about 5 mass%, pH: 2.8, plating solution temperature: 82 ° C., plating time: 5 minutes]
(3) Glazing conditions [Double-sided glazing at 100 μm per side by spraying 1553B glaze made by Nippon Fellow]
(4) Firing conditions [820 ° C. for 6 minutes]
(5) Repeated firing conditions [Repeat the above glazing and firing steps]
(6) Heat treatment conditions [Promoting generation of nail skipping defects by holding at 200 ° C. for 8 minutes]
The steel plates obtained under the above conditions were examined for the presence or absence of nail skipping and surface flaws.
[0039]
Table 1 shows the test results.
[0040]
[Table 1]

The ratio after decarburization (Mn / O) in the table is the ratio of Mn concentration (mass ppm) and O concentration (mass ppm) after vacuum decarburization (but before deoxidation) (Mn / O). O) and the other concentrations are chemical analysis values indicating the concentration after the deoxidation treatment.
[0041]
However, the O concentration is determined by (A) chemical analysis value, (B) on-line analysis at the end of the vacuum process (condition without coating on the sensor) and (C) on-line analysis (sensor coated with alumina) Comparison with existing conditions).
[0042]
Also, the difference [(B)-(A) or (C)-(A)) between the chemical analysis value (A) and each online analysis value is estimated, and the maximum value, minimum value, and variation range are compared. did. Here, the variation range is defined as variation range = maximum value−minimum value.
[0043]
Furthermore, the evaluation of (◯) and (×) in the Al, Si, and Ca + Mg concentrations in the table is as follows.
If the Al concentration is within the appropriate range of 3 to 40 ppm (◯), otherwise (×).
[0044]
If the Si concentration is in the proper range of 20 to 240 ppm (◯), otherwise (×).
If the total concentration of Ca and / or Mg is in the appropriate range of 1 to 25 ppm (◯), otherwise (×).
[0045]
As shown in Test Nos. 1 to 13 in the same table, the ratio (Mn / O) after vacuum decarburization is 2 to 19, and within the total concentration of Al, Si concentration, Ca and / or Mg after deoxidation When the concentration of the at least one deoxidizer was within an appropriate range, nail skipping and surface wrinkles did not occur.
[0046]
However, as shown in the test numbers 15 to 21 of the same table, when the total concentration of Al, Si concentration, Ca and / or Mg after deoxidation is all outside the proper range, the occurrence of nail jumping and surface wrinkles Could not prevent.
[0047]
In Test No. 14, it was possible to prevent surface wrinkling but not to prevent nail skipping.
In addition, as a result of comparison between (A) oxygen concentration by chemical analysis, (B) online analysis (condition without coating on the sensor) and (C) online analysis (condition with alumina coating on the sensor), (C) The variation range of on-line analysis (conditions with alumina coating on the sensor) was 400 ppm, whereas the variation range of (B) on-line analysis (conditions without coating on the sensor) was as small as 43 ppm.
[0048]
【The invention's effect】
According to the present invention, it is possible to manufacture an enamel steel capable of achieving both surface flaw reduction and prevention of nail skipping defects.

Claims

The ratio of the Mn concentration in the molten steel after the vacuum decarburization process (mass ppm) and O concentration (mass ppm) of (Mn / O) and 2 to 19, then, Al: 3~40ppm, Si: 20~240ppm , Ca And / or a total concentration of Mg: at least one concentration of 1 to 25 ppm is adjusted.

A high oxygen ultra-low carbon steel with a basic composition of C: 30 ppm or less, Mn: 70-10000 ppm and O: 400-900 ppm in mass ppm, Al: 3-40 ppm, Si: 20-240 ppm, Ca and / or Mg Concentration of: When producing enamel steel further containing at least one of 1 to 25 ppm, the ratio of Mn concentration (mass ppm) to O concentration (mass ppm) of the molten steel after vacuum decarburization treatment (Mn / O ) Is set to 2 to 19, and thereafter, at least one concentration adjustment of Al: 3 to 40 ppm, Si: 20 to 240 ppm, and Ca and / or Mg total concentration: 1 to 25 ppm is performed. Manufacturing method.

Of enamel steel according to claim 1 or 2, wherein the solid electrolyte oxygen sensor used to measure the O concentration of the molten steel to perform during the vacuum decarburization treatment is an oxygen sensor that is not made covering the sensor unit Production method.