JP3549365B2

JP3549365B2 - Manufacturing method of steel for ERW steel pipe

Info

Publication number: JP3549365B2
Application number: JP14229897A
Authority: JP
Inventors: 英明山村; 明博宮坂; 良之上島
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 1997-05-30
Filing date: 1997-05-30
Publication date: 2004-08-04
Anticipated expiration: 2017-05-30
Also published as: JPH10330827A

Description

【０００１】
【発明の属する技術分野】
本発明は、電縫鋼管用鋼の製造方法に係わり、さらに詳しくはたとえば石油・天然ガス掘削、あるいは輸送用等の電縫鋼管において電縫溶接部での超音波探傷欠陥の少ない、割れ抵抗の高く、靱性の高い電縫鋼管用鋼の製造方法に関する。
【０００２】
【従来の技術】
近年生産される石油・天然ガス中には、硫化水素を含む場合が非常に多く、さらに海水、淡水などの水が共存する場合には鋼表面で起こる腐食に基づく減肉だけではなく、腐食によって鋼表面で発生した水素が鋼中に侵入することによって破壊を起こすことがあり、問題となっている。この破壊は高張力鋼に古くから認められる硫化物応力割れとは異なり、外部からの付加応力がなくとも発生が認められる。
【０００３】
この破壊は、環境中から侵入した水素が母材中に存在する圧延方向に長く伸びたＭｎＳなどのＡ系硫化物系介在物と地鉄との境界に集積してガス化し、そのガス圧によって発生するもので、前記ＭｎＳなどのＡ系硫化物系介在物が鋭い切り欠きとなり、これを割れの核として板面平行割れに成長し、この板面平行割れが板厚方向に連結されるものである。この種の割れを以下「水素膨れ割れ」とよぶ。
【０００４】
こうした水素膨れ割れに対する抵抗の高い鋼について従来から様々な研究がなされ、種々の鋼が提案されている。それらは例えば特公昭５７−１７０６５号公報あるいは特公昭５７−１６１８４号公報などにその代表例がみられるごとくＣｕやＣｏ添加による割れ防止、極低Ｓ化によるＭｎＳの減少、Ｃａあるいは希土類元素などの添加によるＳの固定などを利用するものであってこれらの技術によって現在までにかなり厳しい環境にまで耐え得る鋼が開発されている。
【０００５】
ところで、電縫鋼管はホットコイルなどの鋼板を成形し電縫溶接するものであって、いうまでもなく鋼板との決定的な相違は溶接部および溶接熱影響部が存在することである。この電縫溶接部ではＭｎＳなどの硫化物系介在物が存在しない場合でも電縫溶接部に水素膨れ割れを生じることがあり、しかも電縫溶接部の場合には板面垂直割れ型の水素膨れ割れであることが母材部と異なっており、母材の板面平行型水素膨れ割れと同等あるいはそれ以上に重大な欠陥である。しかも、この割れは従来の水素膨れ割れに対する対策鋼を使用した電縫鋼管であっても発生し、上述の様な技術では防止できない。
【０００６】
一方において近年石油・天然ガスが産出される地域はアラスカ、ロシア、北極海といった極寒地まで広がっており、こうした地域で使用されるラインパイプには母材および電縫溶接部の両方において低温靱性が優れていることが要求される。このとき産出流体に硫化水素を含む場合には低温靱性とともに耐サワー性も必要であることはいうまでもない。
【０００７】
電縫鋼管においては、溶接部の靱性が母材に比べて低下するため電縫溶接部も含めて靱性の優れた電縫鋼管についても従来から様々な研究がなされ、種々の方法および鋼管が提案されている。それらは、たとえば特開昭５４−１３６５１２号公報、特開昭５７−１４０８２３号公報、特公昭５８−５３７０７号公報あるいは特公昭５８ー５３７０８号公報等にその代表例がみられるごとく熱延工程の仕上げ温度および巻き取り温度の管理による素材の靱性向上、造管後冷却速度の制限による結晶粒度の制御、固溶Ｎの減少，ＮｂあるいはＶによる結晶粒の微細化などを利用するものであって、これらの技術によって現在までに靱性のかなり優れた電縫鋼管が開発されている。
【０００８】
しかしながらこれらの電縫鋼管は通常の環境で使用されるものであって、硫化水素や水を含んだいわゆるサワー環境で使用することを考慮したものではない。また、耐サワー電縫鋼管の電縫溶接部の電縫衝合部において靱性が母材に比べて著しく劣る場合がある。この場合上述の各種技術をもってしても改善されない。
【０００９】
このような、電縫鋼管の電縫溶接部の水素膨れ割れおよび靱性低下の原因は電縫衝合部およびその両側の熱影響部に存在する板状のＣａＯ，Ａｌ_２Ｏ_３を主成分とする酸化物系介在物であり、母材中に予め存在した球状に近いこの酸化物系介在物が電縫溶接時の熱影響によって鋼の融点近くにまで加熱されたうえスクイズロールによって両側から加圧されるために板状に変形して生成したものである。このような変形した介在物は電縫衝合部およびその両側の熱影響部を超音波探傷した際に特に欠陥となって検出される。また上述のＣａＯ，Ａｌ_２Ｏ_３を主成分とする酸化物系介在物は鋼板の圧延時に延ばされて電縫溶接部以外でも欠陥の原因となる。
【００１０】
そこで、特公昭６３−１６４６１号では、従来より主に脱酸を目的として添加されてきたＡｌを極力減少させ、Ｔｉを脱酸元素として使用することによって圧延時や電縫溶接時に変形しやすい介在物の生成を防止し、超音波探傷欠陥の少ない、母材および電縫溶接部の耐サワー性と靱性に優れた電縫鋼管用鋼を提案している。
【００１１】
【発明が解決しようとする課題】
今後、石油・天然ガス資源の枯渇が懸念され、その産出地域の環境は更に厳しくなり、また産出流体の硫化水素含有量も多くなることが予想される。こうした状況で使用されるラインパイプには母材および電縫溶接部の両方においてこれまで以上の低温靱性、耐サワー性が要求されるようになるが、これまでの技術では十分ではないことが判明した。
【００１２】
本発明は、上記のような従来の物の欠点、すなわち電縫衝合部靱性の低下および電縫溶接部の板面垂直型水素膨れ割れが溶接熱影響部に存在する板状の酸化物系介在物によって発生することを解決するためになされた物であって、溶鋼中の酸素を２５０ｐｐｍ以下に制限した溶鋼に１０〜７０％のＴｉを含むＴｉ合金を添加して脱酸することによって鋼中の酸化物系介在物の組成および径を制御する超音波探傷欠陥の発生の少ない耐サワー性と靱性の優れた電縫鋼管用鋼の製造方法を提供する物である。
【００１３】
【課題を解決するための手段】
すなわち、上記課題を解決するための本発明の要旨とするところは、転炉で溶製して出鋼した溶鋼を真空脱ガスおよび／もしくはＭｎ，Ｓｉによる脱酸を行って溶鋼中の酸素を２５０ｐｐｍ以下とし、次いで化学組成がＴｉ：１０〜７０重量％の成分と残部はＦｅ、Ｍｎ、Ｓｉのうち１種または２種以上および不可避的不純物とからなる合金を溶鋼に添加し、重量％でＴｉ：０．０１０〜０．２％、Ｃ：０．０１〜０．３５％、Ｓｉ：０．０２〜０．５％、Ｍｎ：０．１〜２．０％を含有し、かつＡｌ：０．００６％以下、Ｐ：０．０１５％以下、Ｓ：０．００８％以下に制限し、あるいはさらに（ａ）Ｃａ：０．０００５〜０．０２％、（ｂ）Ｃｕ：０．２〜０．６％、Ｎｉ：０．１〜１．０％、Ｃｒ：０．２〜３．０％の１種または２種以上、または（ｃ）Ｍｏ：０．１０〜１．０％、Ｎｂ：０．０１〜０．１５％、Ｖ：０．０１〜０．１５％の１種または２種以上の（ａ）、（ｂ）、（ｃ）いずれか〜３者を含有し、残部は鉄および不可避的不純物からなる鋼を連続鋳造することを特徴とする電縫鋼管用鋼の製造方法である。
【００１４】
【作用】
最初に本発明において各成分範囲を前述のごとく限定した理由を以下に述べる。
まずＣは、鋼の強度を最も安定して向上させる基本的な元素であるため、強度確保のためには０．０１％以上含有させることが必要であるが、０．３５％を越えると鋼の靱性に対しては好ましくない影響があるので、０．０１〜０．３５％とした。
【００１５】
次にＳｉは強度を向上させる元素であるので０．０２％以上含有すべきであるが、靱性確保のため上限の含有量を０．５％以下とすべきである。
また、Ｍｎは強度上必要な元素なので０．１％以上含有すべきであるが、溶接性および靱性確保のためには、上限含有量を２．０％とすべきである。
【００１６】
ＴｉはＡｌに代えて脱酸に使用する主要な元素であり、チタン酸化物を主成分とする複合介在物は電縫溶接時に著しく変形しにくいが、０．０１％未満ではチタン酸化物はＣａＯ、ＳｉＯ_２、ＭｎＯと複合した介在物を生成する。この介在物はＣａＯとＡｌ_２Ｏ_３を主成分とする酸化物系介在物と同様変形しやすく、電縫溶接時の熱影響によって鋼の融点近くにまで加熱されたうえスクイズロールによって両側から加圧され板状に変形して、電縫鋼管の電縫溶接部の水素膨れ割れおよび靱性低下を招く。一方、Ｔｉが０．２％を越えると靱性を低下させるために、Ｔｉは０．０１〜０．２％とすべきである。
【００１７】
一方、ＡｌはＣａおよびＯと結合して変形しやすい介在物を生成するために、０．００６％以下に制限すべきであり、少ないほど好ましい。
また、Ｐは母材の水素膨れ割れを伝播しやすくさせる元素であるので、０．０１５％以下とすべきである。
さらに、ＳはＭｎと結合して母材部の水素誘起割れの起点となるＭｎＳを作るので、母材部の耐サワー性確保のためには０．００８％以下に押さえなければならない。
【００１８】
以上が基本成分系であるが、本発明ではこのほかにそれぞれの用途に応じて（ａ）Ｃａ、（ｂ）Ｃｕ、Ｎｉ、Ｃｒの１種以上、（ｂ）Ｍｏ、Ｎｂ、Ｖの１種以上の（ａ）、（ｂ）、（ｃ）のいずれか〜３者を含有させることができる。
【００１９】
まず、Ｃａは鋼中のＳをＣａＳとして固定してＭｎＳの生成を防止することによって母材の耐サワー性向上に非常に有効な元素であり、母材の耐サワー性が特に要求される場合には０．０００５％以上含有することが必要であるが、０．０２％を越えるとＣａＳ−ＣａＯを主成分とする大型介在物を形成するので上限含有量は０．０２％とすべきである。
【００２０】
次に、Ｃｕ、ＮｉおよびＣｒは、いずれも母材の耐食性向上と鋼中への水素侵入量減少の効果を有する。
Ｃｕは０．２０％未満では効果がなく、０．６０％を越えると熱間加工性に悪影響を及ぼすので、０．２０〜０．６０％の範囲に限定する。
Ｎｉは０．１％未満では効果がなく、１．０％を越えると硫化物応力腐食割れを誘発するおそれがあるので、０．１〜１．０％の範囲に限定する。
Ｃｒは０．２％未満では効果がなく、３．０％を越えると鋼の靱性を低下させるので０．２〜３．０％の範囲に限定する。
【００２１】
さらにＭｏ、ＮｂおよびＶはいずれも鋼の強度を向上させる元素であって、Ｍｏは０．１０％以上、ＮｂおよびＶは０．０１％以上含有させることによって同等の強度向上効果を示すが、Ｍｏは１．０％、ＮｂおよびＶは０．１５％を越えて添加すると靱性を低下させるおそれがあるため、Ｍｏは０．１０〜１．０％、ＮｂおよびＶは０．０１〜０．１５％の範囲に限定した。
【００２２】
上述の各合金成分はそれぞれ単独に、あるいは併用しても、上記の制限範囲内において本発明が目的とする効果になんら支障を与えるものではない。
不純物のうちＮ量は０．０１０％を越えると溶接性に問題を生じるので好ましくないものであって、０．０１０％以下であれば鋼の材質に著しい影響を及ぼさないが、歪み時効の影響や円周溶接部の靱性なども考慮すると少ない程良い。
【００２３】
一方、Ｏ量はＣａの大部分が酸化物とならずにＳの固定に有効に利用されるには０．０１０％以下で少ない程良い。
Ｔｉで脱酸した鋼はチタン酸化物を含む介在物を含有する。このチタン酸化物が主成分である介在物は電縫溶接時に著しく変形しにくく、電縫鋼管の電縫溶接部の水素膨れ割れおよび靱性低下を防止することが可能である。
【００２４】
本発明者らはチタン酸化物を主成分とする組成の種々の大きさの介在物を分散させた鋼を実験室的に溶製、鋳造し、通常の方法で圧延を行って１１ｍｍ厚の鋼板とし、通常の工程によって電縫鋼管とした。これらの電縫管の電縫溶接部を含んで厚さ９ｍｍ、幅２０ｍｍ、長さ１００ｍｍの試験片を採取し、耐サワー性の評価に供した。また、母材からも同様な寸法、形状、採取方法の試験片を採取して耐サワー性の評価試験に供した。耐サワー性の評価試験としては上記の試験片をＨ_２Ｓを飽和させた５％ＮａＣｌ水溶液に０．５％ＣＨ_３ＣＯＯＨを添加した溶液（温度２５℃、ｐＨ２．８〜３．８）中に９８時間浸漬し割れを測定した。割れの発生の有無は電縫溶接部を含む試験片では試験片の断面について超音波探傷し、その後の断面の検鏡観察によって判定した。
【００２５】
こうして観察した割れ部の介在物厚み、幅、長さから変形を受ける前の鋳片内の介在物の大きさを算出した結果、割れ部の介在物はいずれも２００μｍより大きかったことが判った。欠陥が発生しなかった部分を切断し鋼中の介在物の大きさを測定したが、これには２００μｍ以下の介在物が検出された。
以上のことより、割れを発生させないためには介在物の大きさを２００μｍ以下とすることが必要であることが推測されたため、２００μｍ以下のチタン酸化物を主成分とする組成の介在物のみを分散させた鋼を実験室的に溶製、鋳造し、通常の方法で圧延を行って鋼板とし、通常の工程によって電縫鋼管とした。これらの電縫管の電縫溶接部の耐サワー性の評価試験を行い、超音波探傷して割れの発生を検査したところ割れは発生していなかった。したがって、チタン酸化物が主成分の粒径が２００μｍ以下の酸化物系介在物を鋼中に含有せしめることが有効である。
【００２６】
しかしながら、通常の製造方法で製造した場合には、チタン酸化物が主成分で粒径が２００μｍ以下の酸化物系介在物のみを鋼中に含有せしめること事は困難である。
そこで、本発明者らは種々の実験検討を行った結果、脱酸時の過飽和度を小さくすると生成する介在物の個数及び介在物径が小さくなることが判明した。過飽和度はＴｉと酸素の積で決まるので、過飽和度を小さくする方法として脱酸合金中のＴｉ含有量を低くすることと脱酸時の溶鋼中の酸素を低くすることが有効である。脱酸合金中のＴｉ含有量が高い場合には溶鋼中に添加した脱酸合金の周囲にＴｉ濃度の高い部分が生成して過飽和度が高くなるので、Ｔｉ含有量の低い脱酸合金を使用する。
【００２７】
さらに、酸素濃度と合金中Ｔｉ含有量を変化させた実験・検討を行い、酸素濃度および合金中Ｔｉ含有量が低くなるにしたがって介在物径は小さくなり、酸素を２５０ｐｐｍ以下とし、かつ、Ｔｉ含有量が７０％以下の合金で脱酸することで、最大でも２００μｍ以下の介在物となることを見いだした。Ｔｉ含有量が高くなると介在物径が大きくなるとともに、脱酸時にチタン酸化物の割合の高い介在物が生成し、それが溶鋼中に残存し混在する。Ｔｉ濃度が低すぎると合金量が多くなりすぎ、溶鋼温度の低下が起こって溶鋼の凝固や鋳造が困難になったり、添加に時間がかかり生産性に障害を与える。Ｔｉ含有量が高い場合には少量ずつ添加すると部分的に過飽和度の高い部分が少なくなり有効である。
【００２８】
また、ＴｉをＦｅやＳｉ、Ｍｎとの合金とすることで、Ｔｉの活量を下げるとともに部分的に濃度の高い領域を減少させるために、過飽和度が一層減少し、微小な介在物の生成を促進する。
【００２９】
【発明の実施の形態】
本発明の製造法についてさらに詳細に説明する。
まず、転炉で目標とする０．０１〜０．３５％のＣを含む溶鋼溶製する。この際、溶鋼中のＣが目標とするＣ濃度より高い場合には出鋼後に真空脱ガス装置等による脱炭処理を行い所定のＣ濃度まで低減し、目標とするＣ濃度より低い場合には出鋼後にＣを添加して所定のＣ濃度とする。
また、溶鋼を出鋼する際必要に応じてＦｅ−Ｍｎを投入してもよい。
【００３０】
次に、出鋼した溶鋼をＭｎ，Ｓｉおよび／または真空脱ガス処理による予備脱酸を行って溶鋼中の酸素を２５０ｐｐｍ以下とする。Ａｌは低い方がよいので、Ｆｅ−ＭｎやＦｅ−Ｓｉを添加してＭｎ，Ｓｉにより脱酸を行う。Ｍｎ，Ｓｉの添加量は脱酸時に添加するＴｉ合金中に含まれるＳｉやＭｎによって増加する量を考慮して添加する。また、ＭｎやＳｉは脱酸力が弱いので製品によっては目標範囲内では溶鋼中の酸素を２５０ｐｐｍ以下にすることが困難な場合があるので、その際には真空脱ガス処理により真空脱酸を行い酸素を下げる。この際、必要があればＣ源を溶鋼中に添加してもよい。
【００３１】
溶鋼中の酸素が２５０ｐｐｍより高くなると、Ｔｉ合金を多量に添加することが必要になり、後述するように脱酸時の過飽和度が大きくなりすぎ、Ｔｉ添加時に高融点粗大なチタン酸化物が多数生成し、さらにこれらが凝集して大きな介在物となる。
このようにして溶鋼中の酸素を２５０ｐｐｍ以下に調整した溶鋼に、化学組成がＴｉ：１０〜７０重量％の成分と残部はＦｅ、Ｍｎ、Ｓｉのうち１種または２種以上および不可避的不純物とからなる合金を添加して、Ｔｉを溶鋼成分として０．０１０〜０．２％含有させる。さらに、他の成分を調整するのに必要な合金を添加して、所定の成分とする。
【００３２】
このようにして溶製した溶鋼中にはチタン酸化物が主成分で最大でも２００μｍ以下の介在物のみを含有すし、この溶鋼は通常と同じ方法でタンディッシュを通して、連続鋳造機で鋳造することが可能である。さらに、この鋳片を通常と同じ方法で熱間圧延まま、あるいは熱間圧延直後の制御冷却工程、さらには圧延材を焼準、焼き戻しあるいは焼き入れ、焼き戻しする等通常の鋼材に使用される製造工程を適用して鋼板にした後、通常の工程によって電縫鋼管とする。さらに、この電縫鋼管の一部または全体に焼準、焼き戻しあるいは焼き入れ、焼き戻しする工程を適用しても良い。いずれの工程を適用または併用するかは高度、靱性などの特性確保の必要に応じて決定すればよい。
【００３３】
【実施例】
表１は本発明および比較する従来法の鋼の製造方法例である。２７０トン転炉で溶製した溶鋼を表１に示す方法で溶製し、表２に示す成分とした鋼を連続鋳造機で鋳造した鋳片を常法により１２．７ｍｍ厚の鋼板に熱間圧延した後、通常の工程によって外径４０６ｍｍの電縫鋼管とした。本発明法でも従来法と同様に問題なく製造が可能であった。
【００３４】
鋳片の一部を採取し、断面を調査し、介在物の組成、大きさ、形状を調査した。その結果を表１に示す。本発明法では鋳片内にはチタン酸化物を主体とする組成で、かつ、２００μｍ以下の大きさの球形の介在物が検出されており、Ａｌ２Ｏ３，ＣａＯを主成分とする介在物は検出されなかった。
鋼管の超音波探傷および耐サワー性の評価試験を行った結果を表１に併せて示す。本発明法をによって製造した鋼管では超音波探傷欠陥の発生が少なく、さらに電縫部および母材において水素膨れ割れは発生しておらず、かつ電縫部においても靱性の低下は非常に小さいのに対して、比較法を使用した鋼管では超音波探傷欠陥が多く、板面垂直型の水素膨れ割れが発生するとともに電縫部の靱性が著しく低下している。
なお、Ｂ１〜Ｂ３は本発明法と同じ方法を適用したものであるが、本発明の限定範囲を超えたものであり、Ｂ４、Ｂ５は従来法である。
【００３５】
【発明の効果】
本発明は、電縫鋼管部での欠陥が少なく、ｐＨが低く厳しい環境においても水素膨れ割れがなくかつ低温靱性の良好な耐サワー性に優れた高靱性電縫鋼管用鋼の製造方法を提供することを可能ならしめたものであり、産業の発展に貢献するところきわめて大なるものがある。
【００３６】
【表１】

【００３７】
【表２】

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for producing a steel for electric resistance welded steel pipe, and more particularly less ultrasonic flaw defect at the electric-resistance welded portion in ERW steel pipe, such as for example oil and natural gas drilling, or transportation, the cracking resistance The present invention relates to a method for producing high-toughness steel for electric resistance welded steel pipes.
[0002]
[Prior art]
Oil and natural gas produced in recent years contains hydrogen sulfide very often, and when water such as seawater or freshwater coexists, not only the thinning due to the corrosion that occurs on the steel surface, but also the corrosion Hydrogen generated on the steel surface may cause destruction by invading the steel, which is a problem. This fracture is different from sulfide stress cracking, which has long been observed in high-strength steels, and is observed even without externally applied stress.
[0003]
Hydrogen invading from the environment accumulates at the boundary between A-based sulfide-based inclusions such as MnS and the like, which extend long in the rolling direction, existing in the base material, and the base iron, and gasifies. A type of sulfide-based inclusions such as MnS, which are formed as sharp notches, grow into parallel cracks on the plate with these cracks as nuclei of cracks, and the parallel cracks on the plate are connected in the thickness direction. It is. This type of crack is hereinafter referred to as “hydrogen blister crack”.
[0004]
Various studies have hitherto been made on steel having high resistance to hydrogen blister cracking, and various steels have been proposed. As shown in, for example, Japanese Patent Publication No. 57-17065 or Japanese Patent Publication No. 57-16184, crack prevention by addition of Cu or Co, reduction of MnS due to extremely low S, reduction of Ca or rare earth elements, etc. Steels that use the fixation of S by addition or the like and which can withstand even a severe environment by these techniques have been developed.
[0005]
By the way, the ERW pipe is formed by forming a steel sheet such as a hot coil and performing ERW welding. Needless to say, a critical difference from the steel sheet is that a welded portion and a welding heat affected zone are present. In this ERW weld, hydrogen swelling cracks may occur in the ERW weld even when sulfide-based inclusions such as MnS are not present. The crack is different from that of the base metal part, and is a defect that is as serious as or more serious than the hydrogen swelling crack of the base material parallel to the plate surface. In addition, this cracking occurs even in an electric resistance welded steel pipe using a conventional countermeasure steel against hydrogen blister cracking, and cannot be prevented by the above-described technique.
[0006]
On the other hand, areas where oil and natural gas are produced in recent years have spread to arctic regions such as Alaska, Russia and the Arctic Ocean, and line pipes used in these regions have low-temperature toughness in both the base metal and ERW welds. It is required to be excellent. At this time, when hydrogen sulfide is contained in the produced fluid, it goes without saying that sour resistance as well as low-temperature toughness are required.
[0007]
Since the toughness of the welded portion of the ERW pipe is lower than that of the base metal, various studies have been conducted on ERW pipes with excellent toughness including the ERW weld, and various methods and steel pipes have been proposed. Have been. These are described in, for example, JP-A-54-136512, JP-A-57-140823, JP-B-58-53707 and JP-B-58-53708. It utilizes the improvement of the toughness of the material by controlling the finishing temperature and the winding temperature, the control of the crystal grain size by restricting the cooling rate after pipe making, the reduction of the solute N, and the refinement of the crystal grains by Nb or V. By these techniques, an electric resistance welded steel tube having considerably excellent toughness has been developed up to now.
[0008]
However, these electric resistance welded steel pipes are used in a normal environment, and are not intended for use in a so-called sour environment containing hydrogen sulfide and water. Further, the toughness of the electric resistance welded joint of the electric resistance welded portion of the sour electric resistance welded steel pipe may be significantly lower than that of the base metal. In this case, there is no improvement even with the various techniques described above.
[0009]
The cause of such hydrogen swelling cracking and toughness reduction in the ERW welded portion of the ERW steel pipe is mainly caused by plate-like CaO and Al ₂ O ₃ existing in the ERW joint portion and the heat-affected zone on both sides thereof. This oxide-based inclusion, which is pre-existing in the base metal and is close to a sphere, is heated to near the melting point of steel by the thermal effect of ERW and is applied from both sides by squeeze rolls. It is generated by being deformed into a plate shape due to being pressed. Such deformed inclusions are particularly detected as defects when ultrasonically flaw detection is performed on the electric seam joint portion and the heat affected zone on both sides thereof. In addition, the above-mentioned oxide inclusions mainly composed of CaO and Al ₂ O ₃ are elongated at the time of rolling the steel sheet, and cause defects other than the electric resistance welded portion.
[0010]
Therefore, Japanese Patent Publication No. 63-16461 discloses a method in which Al, which has been conventionally added mainly for deoxidation, is reduced as much as possible, and Ti is used as a deoxidizing element, thereby making it easy to deform during rolling or ERW welding. We have proposed a steel for ERW steel pipes that prevents the formation of materials and has few ultrasonic flaws, and has excellent sour resistance and toughness of the base metal and ERW welds.
[0011]
[Problems to be solved by the invention]
It is anticipated that oil and natural gas resources will be depleted in the future, the environment in the producing area will be more severe, and the hydrogen sulfide content of the produced fluid will also increase. Line pipes used in such situations will require higher low-temperature toughness and sour resistance in both the base metal and ERW welds, but it turns out that conventional technologies are not sufficient did.
[0012]
SUMMARY OF THE INVENTION The present invention provides a plate-like oxide system in which the above-mentioned drawbacks of the conventional products, namely, a reduction in toughness of an electric resistance welded joint and a hydrogen swelling crack perpendicular to the plate surface of an electric resistance weld are present in the weld heat affected zone. It is an object to solve the problem of being generated by inclusions, wherein a steel alloy is prepared by adding a Ti alloy containing 10 to 70% of Ti to molten steel in which oxygen in the molten steel is limited to 250 ppm or less and deoxidizing the steel. An object of the present invention is to provide a method for producing a steel for an electric resistance welded steel pipe excellent in sour resistance and toughness with less occurrence of ultrasonic inspection defects for controlling the composition and diameter of oxide inclusions in the steel.
[0013]
[Means for Solving the Problems]
That is, the gist of the present invention for solving the above-mentioned problem is that the molten steel produced and melted in a converter is subjected to vacuum degassing and / or deoxidation with Mn and Si to reduce oxygen in the molten steel. 250 ppm or less, then an alloy composed of a component having a chemical composition of Ti: 10 to 70% by weight and one or more of Fe, Mn, and Si and inevitable impurities is added to molten steel. Ti: 0.010 to 0.2%, C: 0.01 to 0.35%, Si: 0.02 to 0.5%, Mn: 0.1 to 2.0%, and Al: 0.006% or less, P: 0.015% or less, S: 0.008% or less, or (a) Ca: 0.0005 to 0.02%, (b) Cu: 0.2 to One or two of 0.6%, Ni: 0.1 to 1.0%, and Cr: 0.2 to 3.0% (C) Mo: 0.10 to 1.0%, Nb: 0.01 to 0.15%, V: 0.01 to 0.15%, one or more (a), (B) A method for producing steel for electric resistance welded steel, which comprises continuously casting steel containing any one to three members and the balance consisting of iron and unavoidable impurities.
[0014]
[Action]
First, the reasons for limiting the range of each component in the present invention as described above will be described below.
First, C is a basic element that most stably improves the strength of steel, so it is necessary to contain 0.01% or more in order to secure the strength. Since there is an unfavorable effect on the toughness of the steel, the content is set to 0.01 to 0.35%.
[0015]
Next, since Si is an element for improving strength, it should be contained at 0.02% or more, but the upper limit content should be 0.5% or less for securing toughness.
Since Mn is an element necessary for strength, Mn should be contained in an amount of 0.1% or more. However, in order to ensure weldability and toughness, the upper limit content should be 2.0%.
[0016]
Ti is a main element used for deoxidation in place of Al, and composite inclusions containing titanium oxide as a main component are hardly deformed significantly during electric resistance welding, but if less than 0.01%, titanium oxide becomes CaO , SiO ₂ and MnO to form inclusions. This inclusion is easily deformed similarly to oxide-based inclusions mainly composed of CaO and Al ₂ O _{3. The} inclusion is heated to a temperature close to the melting point of steel due to the heat effect during electric resistance welding, and is added from both sides by a squeeze roll. The steel plate is deformed into a plate shape by being pressed, and causes hydrogen swelling cracks and a decrease in toughness of the ERW welded portion of the ERW steel pipe. On the other hand, if Ti exceeds 0.2%, the toughness is reduced, so that Ti should be 0.01 to 0.2%.
[0017]
On the other hand, Al should be limited to 0.006% or less in order to form inclusions that are easily deformed by combining with Ca and O, and a smaller amount is more preferable.
Further, P is an element that facilitates the propagation of hydrogen swelling cracks in the base material, and therefore should be set to 0.015% or less.
Further, S combines with Mn to form MnS, which is a starting point of hydrogen-induced cracking of the base material, so that it must be suppressed to 0.008% or less in order to ensure the sour resistance of the base material.
[0018]
The above is the basic component system. In the present invention, in addition to this, one or more of (a) Ca, (b) Cu, Ni, and Cr, and (b) one of Mo, Nb, and V, according to each application. Any of the above (a), (b) and (c) can be contained.
[0019]
First, Ca is a very effective element for improving the sour resistance of the base metal by fixing S in the steel as CaS and preventing the generation of MnS, and when the sour resistance of the base material is particularly required. Must contain 0.0005% or more, but if it exceeds 0.02%, large inclusions containing CaS—CaO as a main component are formed, so the upper limit content should be 0.02%. is there.
[0020]
Next, Cu, Ni and Cr all have the effect of improving the corrosion resistance of the base material and reducing the amount of hydrogen penetrating into steel.
If Cu is less than 0.20%, there is no effect, and if it exceeds 0.60%, the hot workability is adversely affected. Therefore, Cu is limited to the range of 0.20 to 0.60%.
If Ni is less than 0.1%, there is no effect, and if it exceeds 1.0%, sulfide stress corrosion cracking may be induced, so it is limited to the range of 0.1 to 1.0%.
If Cr is less than 0.2%, there is no effect, and if it exceeds 3.0%, the toughness of the steel is reduced. Therefore, the Cr content is limited to the range of 0.2 to 3.0%.
[0021]
Further, Mo, Nb and V are all elements for improving the strength of steel, and when Mo is contained at 0.10% or more and Nb and V are contained at 0.01% or more, the same strength improving effect is obtained. If Mo is added in excess of 1.0% and Nb and V exceed 0.15%, the toughness may be reduced. Therefore, Mo is 0.10 to 1.0%, and Nb and V are 0.01 to 0. The range was limited to 15%.
[0022]
The above-mentioned alloy components, when used alone or in combination, do not adversely affect the effects intended by the present invention within the above-mentioned limits.
If the N content of the impurities exceeds 0.010%, it is not preferable because N causes a problem in weldability. If the N content is 0.010% or less, the steel material is not significantly affected. Considering the toughness of the girth and the circumferential weld, the smaller the better.
[0023]
On the other hand, the O amount is preferably as small as 0.010% or less so that most of Ca is effectively used for fixing S without becoming oxide.
Steel deoxidized with Ti contains inclusions containing titanium oxide. The inclusions containing titanium oxide as a main component are not significantly deformed during electric resistance welding, and can prevent hydrogen swelling cracks and a decrease in toughness of the electric resistance welded portion of the electric resistance welded steel pipe.
[0024]
The present inventors experimentally melted and cast a steel in which inclusions of various sizes having a composition containing titanium oxide as a main component were dispersed and rolled it by a usual method to obtain a steel sheet having a thickness of 11 mm. And an electric resistance welded steel pipe was formed by a normal process. Test pieces having a thickness of 9 mm, a width of 20 mm, and a length of 100 mm including the ERW welded portions of these ERW pipes were sampled and subjected to evaluation of sour resistance. In addition, test pieces of the same size, shape, and sampling method were sampled from the base material and subjected to an evaluation test for sour resistance. As the evaluation test for sour resistance, the above test piece was placed in a solution (temperature: 25 ° C., pH: 2.8 to 3.8) obtained by adding 0.5% CH ₃ COOH to a 5% NaCl aqueous solution saturated with H ₂ S. For 98 hours and the cracks were measured. The presence or absence of cracks was determined by ultrasonic inspection of the cross section of the test piece including the ERW welded part, and then by microscopic observation of the cross section.
[0025]
As a result of calculating the size of the inclusions in the slab before being deformed from the thickness, width and length of the inclusions of the cracks observed in this way, it was found that all of the inclusions in the cracks were larger than 200 μm. . The portion where no defect occurred was cut and the size of the inclusions in the steel was measured. Inclusions of 200 μm or less were detected.
From the above, it was presumed that the size of the inclusions was required to be 200 μm or less in order to prevent the occurrence of cracks. Therefore, only inclusions having a composition mainly containing titanium oxide of 200 μm or less were used. The dispersed steel was smelted and cast in a laboratory, rolled by a normal method to obtain a steel plate, and an electric resistance welded steel pipe was formed by a normal process. An evaluation test of the sour resistance of the ERW welded portions of these ERW pipes was performed, and the occurrence of cracks was inspected by ultrasonic flaw detection. As a result, no cracks were found. Therefore, it is effective to include in the steel an oxide-based inclusion whose main component is titanium oxide and whose particle size is 200 μm or less.
[0026]
However, when manufactured by a normal manufacturing method, it is difficult to include only oxide-based inclusions having titanium oxide as a main component and a particle size of 200 μm or less in steel.
Thus, the present inventors conducted various experimental studies and found that the number of inclusions and the diameter of the inclusions were reduced when the degree of supersaturation during deoxidation was reduced. Since the degree of supersaturation is determined by the product of Ti and oxygen, it is effective to reduce the degree of supersaturation by lowering the Ti content in the deoxidized alloy and lowering the oxygen in the molten steel during deoxidation. If the Ti content in the deoxidized alloy is high, a portion with a high Ti concentration is formed around the deoxidized alloy added to the molten steel and the degree of supersaturation increases, so use a deoxidized alloy with a low Ti content. I do.
[0027]
Furthermore, experiments and examinations were performed in which the oxygen concentration and the Ti content in the alloy were changed. As the oxygen concentration and the Ti content in the alloy decreased, the inclusion diameter became smaller, the oxygen was reduced to 250 ppm or less, and the Ti content was reduced. It has been found that deoxidation with an alloy having an amount of 70% or less results in inclusions having a maximum of 200 μm or less. Increasing the Ti content increases the size of the inclusions, and at the time of deoxidation, inclusions having a high proportion of titanium oxide are generated, which remain and are mixed in the molten steel. If the Ti concentration is too low, the amount of the alloy becomes too large, and the temperature of the molten steel decreases, which makes it difficult to solidify or cast the molten steel, and it takes a long time to add the molten steel, thereby impairing productivity. When the content of Ti is high, it is effective to add a small amount at a time, since a portion having a high degree of supersaturation partially decreases.
[0028]
Further, by using Ti as an alloy with Fe, Si, and Mn, the activity of Ti is reduced and a region having a high concentration is partially reduced, so that the degree of supersaturation is further reduced, and the generation of fine inclusions is reduced. To promote.
[0029]
BEST MODE FOR CARRYING OUT THE INVENTION
The production method of the present invention will be described in more detail.
First, molten steel containing 0.01 to 0.35% of C as a target is produced in a converter. At this time, if the C in the molten steel is higher than the target C concentration, the steel is subjected to decarburization treatment using a vacuum degassing device or the like after tapping to reduce the C concentration to a predetermined C concentration, and if the C concentration is lower than the target C concentration. After tapping, C is added to a predetermined C concentration.
Further, when tapping molten steel, Fe-Mn may be added as needed.
[0030]
Next, the molten steel that has been tapped is preliminarily deoxidized by Mn, Si and / or vacuum degassing to reduce the oxygen in the molten steel to 250 ppm or less. Since it is better that Al is low, Fe-Mn or Fe-Si is added and deoxidation is performed with Mn and Si. The addition amount of Mn and Si is added in consideration of the increase amount due to Si and Mn contained in the Ti alloy added at the time of deoxidation. In addition, since Mn and Si have low deoxidizing power, it may be difficult to reduce oxygen in molten steel to 250 ppm or less within a target range depending on a product. In this case, vacuum degassing is performed by vacuum degassing. And lower the oxygen. At this time, if necessary, a C source may be added to the molten steel.
[0031]
If the oxygen in the molten steel is higher than 250 ppm, it is necessary to add a large amount of Ti alloy, and as described later, the degree of supersaturation at the time of deoxidation becomes too large, and a large amount of titanium oxide having a high melting point and coarseness at the time of adding Ti. Formed, and these aggregate to form large inclusions.
In this way, in the molten steel in which the oxygen in the molten steel was adjusted to 250 ppm or less, the chemical composition of Ti: 10 to 70% by weight, and the balance was one or more of Fe, Mn, and Si, and unavoidable impurities. Is added, and Ti is contained in an amount of 0.010 to 0.2% as a molten steel component. Further, an alloy necessary for adjusting other components is added to obtain a predetermined component.
[0032]
The molten steel thus melted contains titanium oxide as a main component and only inclusions having a maximum size of 200 μm or less, and this molten steel can be cast by a continuous casting machine through a tundish in the same manner as usual. It is possible. Furthermore, this slab is used for ordinary steel materials, such as hot-rolled as it is, or in a controlled cooling step immediately after hot-rolling, and further normalizing, tempering or quenching and tempering the rolled material. After the steel sheet is formed by applying the manufacturing process described above, an ERW pipe is formed by a normal process. Further, a process of normalizing, tempering or quenching and tempering a part or the whole of the ERW steel pipe may be applied. Which of the steps is applied or used may be determined according to the necessity of securing properties such as high altitude and toughness.
[0033]
【Example】
Table 1 is an example of a method for producing steel according to the present invention and a conventional method to be compared. The molten steel melted in the 270 ton converter was smelted by the method shown in Table 1, and the steel slab having the components shown in Table 2 was cast by a continuous casting machine into a 12.7 mm thick steel sheet by a normal method. After rolling, an electric resistance welded steel pipe having an outer diameter of 406 mm was obtained by a usual process. According to the method of the present invention, similar to the conventional method, production was possible without any problem.
[0034]
A part of the slab was sampled, the cross section was investigated, and the composition, size and shape of the inclusions were investigated. Table 1 shows the results. In the method of the present invention, spherical inclusions having a composition mainly composed of titanium oxide and having a size of 200 μm or less are detected in the slab, and inclusions mainly composed of Al 2 O 3 and CaO are detected. Did not.
Table 1 also shows the results of the ultrasonic test and the evaluation test for sour resistance of the steel pipe. In the steel pipe manufactured by the method of the present invention, there are few occurrences of ultrasonic inspection defects, furthermore, hydrogen blister cracking does not occur in the ERW part and the base material, and the decrease in toughness is also very small in the ERW part. In addition, the steel pipe using the comparative method has many ultrasonic flaws, generates hydrogen swelling cracks perpendicular to the plate surface, and significantly reduces the toughness of the electric resistance welded portion.
Note that B1 to B3 apply the same method as the method of the present invention, but are outside the limited range of the present invention, and B4 and B5 are conventional methods.
[0035]
【The invention's effect】
The present invention provides a method for producing a high toughness ERW steel pipe which has few defects in the ERW pipe part, has no hydrogen swelling crack even in a low pH and severe environment, and has excellent low temperature toughness and excellent sour resistance. It is possible to do so, and there is an extremely large part that contributes to the development of industry.
[0036]
[Table 1]

[0037]
[Table 2]

Claims

Vacuum degassing and / or deoxidation with Mn and Si of the molten steel produced and melted in the converter is performed to reduce the oxygen in the molten steel to 250 ppm or less, and then the chemical composition of the steel is Ti: 10 to 70% by weight. The balance is made by adding an alloy comprising one or more of Fe, Mn, and Si and inevitable impurities to the molten steel. Ti: 0.010 to 0.2% by weight%, C: 0.01 to 0.2% 0.35%, Si: 0.02-0.5%, Mn: 0.1-2.0%, Al: 0.006% or less, P: 0.015% or less, S: 0 is limited to .008%, the remaining portion is electric-resistance method for producing a steel pipe steel, which comprises continuous casting a steel consisting of iron and unavoidable impurities.

Vacuum degassing and / or deoxidation with Mn and Si is performed on the molten steel produced and melted in the converter to reduce the oxygen in the molten steel to 250 ppm or less. As for the balance, an alloy consisting of one or more of Fe, Mn, and Si and inevitable impurities was added to the molten steel, and Ti: 0.010 to 0.2% by weight, C: 0.01 0.35%, Si: 0.02 to 0.5%, Mn: 0.1 to 2.0%, containing the following (a) and / or (b) and / or (c); S: 0.006% or less, P: 0.015% or less, S: 0.008% or less, with the balance being a continuous cast steel made of iron and unavoidable impurities. Manufacturing method.
(A) Ca: 0.0005 to 0.02%
(B) Any one or more of Cu: 0.2 to 0.6%, Ni: 0.1 to 1.0%, Cr: 0.2 to 3.0%
(C) Mo: 0.10 to 1.0%, Nb: 0.01 to 0.15%, V: 0.01 to 0.15%, any one or more of them