JP4072009B2

JP4072009B2 - Manufacturing method of UOE steel pipe with high crushing strength

Info

Publication number: JP4072009B2
Application number: JP2002192531A
Authority: JP
Inventors: 均朝日; 英司津留
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2002-07-01
Filing date: 2002-07-01
Publication date: 2008-04-02
Anticipated expiration: 2022-07-01
Also published as: JP2004035925A

Description

【０００１】
【発明の属する技術分野】
本発明は深海用ラインパイプ等に使用される圧潰特性に優れたＵＯＥ工程で製造する大径鋼管の製造法に関するものである。
【０００２】
【従来の技術】
近年、原油・天然ガスの長距離輸送方法としてラインパイプの重要性が高まっており、なかでも海洋を渡る海底ラインパイプは３０００ｍに及ぶ深度にまで達してきた。一般にパイプラインの設計では、まず流体輸送量より鋼管内径が決定され、続いて内圧負荷時の周方向応力を一定値に押さえるべく亀裂伝播特性、腐食減量を考慮し、肉厚、材質が決定されている。しかし、海底ラインパイプの深海化に伴って鋼管に負荷される水圧が高まり、従来はあまり重要視されなかった圧潰強度が問題になりつつある。圧潰強度は外径と肉厚の比に相関があり、鋼管の圧潰強度を高めることによって大径化及び薄肉化が可能になる。従って、圧潰強度が鋼管サイズを決定する主な設計因子になり始めている。
【０００３】
ところで、鋼管の圧潰強度は、油井管を対象としては古くから研究されており、統計的にも数多くの実験式が提案されてきた。その中で外径／肉厚比、降伏強度、真円度、偏肉度、残留応力がその主な支配因子とされた。これらの研究は材質が均質なシームレス鋼管について主に行われたものであるため、材料の異方性については多くを論じる必要はなかった。
【０００４】
しかし、長距離輸送に使用される幹線ラインパイプでは大径であるため、ＵＯＥ工程の製造法による鋼管が使用される。ＵＯＥ工程は、図１に示すようにＣ成形（プレス）、Ｕ成形（プレス）、Ｏ成形（プレス）、シーム溶接及び拡管する工程からなる。いずれの成形も冷間で行われるため、最終製品、すなわち鋼管は、加工硬化とバウシンガー効果との複合によって機械的性質に異方性を生じることになる。なお、バウシンガー効果とは材料に塑性歪を与えた後、それとは逆方向の降伏強度が低下する現象である。従って、周方向に引張方向の塑性歪を与えたＵＯＥ鋼管は、周方向の圧縮降伏強度、すなわち外圧負荷に対する降伏強度がバウシンガー効果によって低下する。
【０００５】
一方、軸方向の荷重負荷に対しては成形時の主歪みに荷重方向が直交するため、軸方向の引張及び圧縮負荷ではその応力挙動に差を生じにくい。また、周方向の荷重負荷が引張応力である場合、すなわち内圧負荷に対しては全厚引張試験から得られる値を基準に強度設計を行えば問題が生じることはない。
【０００６】
しかし、近年では深海用ラインパイプに適用し得るＵＯＥ鋼管の需要が高まり、外圧による鋼管の圧潰強度が問題になり始めた。圧潰は外圧により鋼管が潰れる現象であり、座屈の一つであるため、圧縮の降伏強度が圧潰強度を決定することとなる。従って、圧潰強度が要求されるラインパイプにＵＯＥ鋼管を適用する際には、バウシンガー効果による周方向の圧縮強度の低下が問題になる。
【０００７】
このような問題に対し、鋼管を拡管後ガスバーナーによって加熱する方法が特開平９−３５４５号公報に開示されている。しかし、このような加熱を行っても、全てのＵＯＥ鋼管の圧潰強度が２０％以上向上するわけではなく、加熱によるバウシンガー効果の回復が小さい鋼管も見られた。
【０００８】
【発明が解決しようとする課題】
本発明は、ＵＯＥ工程で造管後、加熱によって安定的に圧縮降伏強度が回復する、圧潰強度の高い鋼管の製造方法の提供を目的としている。
【０００９】
【課題を解決するための手段】
本発明者らは、バウシンガー効果により低下した圧潰強度が加熱によって回復する効果について、鋼管の素材である鋼板の化学成分及び圧延、加速冷却等の製造条件にまで遡り詳細に検討した。その結果、熱延後、３００℃以下の低温域まで冷却したＮｂ添加鋼の圧縮強度は、熱延後、空冷した鋼板又は５００〜６００℃まで水冷して冷却を停止した鋼板よりも高いことがわかった。さらに、ＵＯＥ工程によって鋼管を製造し、拡管後８０〜５５０℃の範囲に加熱すると、圧潰強度が拡管前と同等以上に回復することがわかった。さらに、この効果はＴｉ添加によって顕著になり、８０〜１５０℃未満という低温に加熱しても、バウシンガー効果が回復することがわかった。
【００１０】
本発明はこのような知見を基になされたものであり、その要旨は次のとおりである。
（１）質量％で、Ｃ：０．０３〜０．１５％、Ｓｉ：０．８％以下、Ｍｎ：０．３〜２．５％、Ｐ：０．０３％以下、Ｓ：０．０１％以下、Ｎｂ：０．０１〜０．３％、Ｔｉ：０．００５〜０．０３％、Ａｌ：０．１％以下、Ｎ：０．００１〜０．０１％を含有し、残部が鉄及び不可避的不純物からなる鋼片をオーステナイト域に再加熱後、再結晶域で粗圧延を行い、引き続き９００℃以下の未再結晶温度域で累積圧下率５０％以上の仕上げ圧延を行い、Ａ_r3点以上の温度から５〜４０℃／秒の冷却速度で３００℃以下まで冷却して製造した厚鋼板を、そのまま順にＣ成形、Ｕ成形、Ｏ成形し、厚鋼板の端部同士をシーム溶接後、拡管するＵＯＥ工程により造管した鋼管の、少なくとも外表面から肉厚中心までの範囲を８０〜５５０℃に加熱することを特徴とする、圧潰強度の高いＵＯＥ鋼管の製造方法。
（２）質量％で、さらに、Ｎｉ：１％以下、Ｍｏ：０．６％以下、Ｃｒ：１％以下、Ｃｕ：１％以下、Ｖ：０．３％以下、Ｂ：０．０００３〜０．００３％、Ｃａ：０．０１％以下、ＲＥＭ：０．０２％以下、Ｍｇ：０．００６％以下の１種又は２種以上を含有することを特徴とする、（１）に記載の圧潰強度の高いＵＯＥ鋼管の製造方法。
（３）ＵＯＥ工程により造管した鋼管の圧潰強度ａ[ＭＰａ]と、少なくとも外表面から肉厚中心までの範囲を８０〜５５０℃に加熱した後のＵＯＥ工程により造管した鋼管の圧潰強度ｂ[ＭＰａ]との比ｂ／ａが１．１０以上であることを特徴とする、（１）及び（２）に記載の圧潰強度の高いＵＯＥ鋼管の製造方法。
（４）加熱中又は加熱後の冷却中に塗覆装を行うことを特徴とする、（１）〜（３）のいずれか１項に記載の圧潰強度の高いＵＯＥ鋼管の製造方法。
である。
【００１１】
【発明の実施の形態】
本発明者らは、バウシンガー効果によって低下した鋼の降伏強度が熱処理によって回復するという現象に関して、加熱温度と鋼の化学成分及び組織との関係を詳細に調べた。まず、種々の成分を含有する鋼を実験室で熱延条件を変化させて製造し、これらの鋼板から引張試験片を加工して４％の引張歪みを付与した。次に、５０〜７００℃で１０分間加熱し、引張まま及び加熱後の引張試験片から圧縮試験片を採取して圧縮試験を実施した。これらの試験片の圧縮試験での０．２％耐力と引張り歪付与前の材料の圧縮試験での０．２％耐力の比（以下、圧縮強度比）を判断基準とした。
【００１２】
その結果、熱延後、３００℃以下の低温域まで冷却したＮｂ添加鋼の圧縮強度比は、熱延後空冷した鋼及び熱延後５００〜６００℃まで水冷して冷却を停止した鋼に比べて高く、さらに８０〜５５０℃での熱処理によって圧縮強度比が１．０を超えることがわかった。さらに、Ｎｂ−Ｔｉ添加鋼は、８０〜１５０℃未満に加熱すると効果が得られることが明らかになった。
【００１３】
これに対して、５００〜６００℃で冷却を停止したＮｂ−Ｔｉ無添加鋼では、８０〜１５０℃未満に加熱しても圧縮強度比は全く変化しなかった。さらに、１５０℃以上の温度に加熱しても、５００〜６００℃で冷却を停止したＮｂ−Ｔｉ無添加鋼のバウシンガー効果の回復は、３００℃以下で冷却を停止したＮｂ−Ｔｉ添加鋼よりも効果が小さいことがわかった。
【００１４】
この３００℃以下で巻取った鋼のミクロ組織を調査した結果、上部ベイナイトなどの低温変態生成相を含む組織を有していることが明らかになった。このような低温変態生成相が、バウシンガー効果による圧縮降伏強度の低下を抑制すると考えられる。さらに、拡管後の圧縮降伏応力が、１００℃程度に加熱することにより、拡管前の圧縮降伏強度と同等以上に上昇する理由は、バウシンガー効果を引き起こす転位周りの応力場が容易に変化することと、Ｃ等の固溶状態で存在している元素が転位に固着するためと推定している。
【００１５】
さらに、詳細に析出物の解析を行った結果、Ｔｉ添加鋼には微細なＴｉＮが析出していることがわかった。この微細なＴｉＮによって、スラブ再加熱時及びＨＡＺのオーステナイト粒の粗大化が抑制され、母材及びＨＡＺのミクロ組織が微細化したと考えられる。このような均質で微細なミクロ組織が得られた結果、粒内での応力の不均一が軽減されて残留応力が均一に分布し易くなり、Ｎｂ添加との相乗効果によって、バウシンガー効果により低下した圧潰強度が低温の熱処理によって容易に向上することを見出した。
【００１６】
このようにして製造した鋼板をＵＯＥ工程で鋼管とし、バウシンガー効果による圧縮降伏強度の低下について、鋼管の外表面側から内表面側までの肉厚方向において詳細に調査した。その結果、外表面側は筒状に成形する工程と拡管の工程での周方向の引張歪みを受けており、バウシンガー効果のために圧縮降伏強度が低下しているが、内面側はＵＯ工程での曲げ加工による圧縮の加工硬化が拡管後も残留し、圧縮降伏強度が低下しないことがわかった。
【００１７】
さらに、鋼管の外表面側から内表面側までの肉厚方向の表層部、肉厚中心部及び１／４肉厚部で、バウシンガー効果による圧縮降伏強度の低下に対する加熱の効果について検討を行った。その結果、内表面側を加熱する効果は小さいが、外表面側を加熱することによって圧潰強度が向上することがわかった。この加熱は、８０〜５５０℃の範囲が効果的であり、また８０〜２５０℃の範囲でも効果が大きく、８０〜１５０℃未満の低温でも効果が認められた。
【００１８】
次に、成分元素の限定理由を述べる。
【００１９】
Ｃ量は０．０３〜０．１５％に限定する。炭素は鋼の強度向上に極めて有効であり、目標とする強度を得るためには、最低０．０３％は必要である。しかし、Ｃ量が０．１５％よりも多いと母材、ＨＡＺの低温靱性や現地溶接性の著しい劣化を招くので、その上限を０．１５％とした。一様伸びはＣ量が多い方が高くなり、低温靭性や溶接性はＣ量が少ない方が良好であり、要求特性の水準によりバランスを考える必要がある。
【００２０】
Ｓｉは脱酸や強度向上のために添加する元素であるが、０．８％よりも多く添加するとＨＡＺ靱性及び現地溶接性が著しく劣化するので、Ｓｉ量の上限を０．８％とした。なお、鋼の脱酸はＡｌ及びＴｉでも可能であり、Ｓｉは必ずしも添加する必要はないが、通常、０．１％程度を含有する。
【００２１】
Ｍｎは本発明鋼の母相のミクロ組織をベイナイト主体の組織とし、優れた強度と低温靱性のバランスを確保する上で不可欠な元素であり、その下限は０．３％である。しかし、Ｍｎ量が２．５％よりも多いと、フェライトを分散して生成させることが困難になるので上限を２．５％とした。
【００２２】
また、本発明鋼では、必須の元素としてＮｂ：０．０１〜０．３％、Ｔｉ：０．００５〜０．０３％を含有する。
【００２３】
Ｎｂは制御圧延時にオーステナイトの再結晶を抑制して組織を微細化するだけでなく、焼入れ性増大にも寄与し、鋼を強靱化する。この効果は、Ｎｂ量が０．０１％未満では小さいため下限とする。しかし、Ｎｂ添加量が０．３％よりも多いと、ＨＡＺ靱性や現地溶接性に悪影響をもたらすので、その上限を０．３％とした。
【００２４】
Ｔｉ添加は微細なＴｉＮを形成して、母材及びＨＡＺのミクロ組織を微細化し、バウシンガー効果によって低下した圧潰強度が、８０〜５５０℃、とりわけ８０〜１５０℃未満に加熱することによって向上する効果を促進する。また、母材及びＨＡＺの低温靱性を改善する。この効果はＮｂとの複合添加により極めて顕著になる。この目的のために、Ｔｉ量は３．４Ｎ（各々質量％）以上添加することが好ましい。また、Ａｌ量が少ない時（たとえば０．００５％以下）、Ｔｉは酸化物を形成し、ＨＡＺにおいて粒内フェライト生成核として作用し、ＨＡＺ組織を微細化する効果も有する。このような効果を発現させるためには、最低０．００５％のＴｉ添加が必要である。しかし、Ｔｉ量が０．０３％よりも多いと、ＴｉＮの粗大化やＴｉＣによる析出硬化が生じ、低温靱性を劣化させるので、その上限を０．０３％に限定した。
【００２５】
Ａｌは通常脱酸材として鋼に含まれる元素で、組織の微細化にも効果を有する。しかし、Ａｌ量が０．１％を越えるとＡｌ系非金属介在物が増加して鋼の清浄度を害するので、上限を０．１％とした。また、脱酸はＴｉ及びＳｉでも可能であり、Ａｌは必ずしも添加する必要はないが、現状の技術では０．００１％程度を含有する。
【００２６】
ＮはＴｉＮを形成し、スラブ再加熱時及びＨＡＺのオーステナイト粒の粗大化を抑制して、母材及びＨＡＺの低温靱性を向上させる。このために必要な最小量は０．００１％である。しかし、Ｎ量が０．０１％よりも多すぎるとＴｉＮが増えすぎ、表面疵、靭性劣化等の弊害が生じるので、その上限は０．０１％に抑える必要がある。
【００２７】
さらに、本発明では、不純物元素であるＰ及びＳ量をそれぞれ０．０３％及び０．０１％以下とする。この主たる理由は母材及びＨＡＺの低温靱性をより一層向上させるためである。Ｐ量の低減は連続鋳造スラブの中心偏析を軽減するとともに、粒界破壊を防止して低温靱性を向上させる。また、Ｓ量の低減は熱間圧延で延伸化するＭｎＳを低減して延靱性を向上させる効果がある。両者共、少ない程望ましいが、特性とコストのバランスで決定する必要があり、通常、Ｐ及びＳは、それぞれ０．００１％以上及び０．０００１％以上を含有する。
【００２８】
次に、選択元素であるＮｉ、Ｍｏ、Ｃｒ、Ｃｕ、Ｖ、Ｃａ、ＲＥＭ及びＭｇを添加する目的について説明する。基本となる成分に、更にこれらの元素を添加する主たる目的は、本発明鋼の優れた特徴を損なうことなく、強度・靱性の一層の向上や製造可能な鋼材サイズの拡大を図るためである。
【００２９】
Ｎｉを添加する目的は、低炭素の本発明鋼を低温靱性や現地溶接性を劣化させることなく向上させるためであり、０．１％以上添加することが好ましい。Ｎｉの添加はＭｎ、Ｃｒ及びＭｏの添加と比較して、圧延組織中、特に連続鋳造鋼片の中心偏析帯中に低温靱性に有害な硬化組織を形成することが少ない。しかし、Ｎｉ量が１％よりも多すぎると、経済性だけでなく、ＨＡＺ靱性や現地溶接性を劣化させるので、その上限を１％とした。また、Ｎｉの添加は連続鋳造時及び熱間圧延時におけるＣｕ割れの防止にも有効である。この場合、ＮｉはＣｕ量の１／３以上添加する必要がある。
【００３０】
Ｍｏを添加する理由は、鋼の焼入れ性を向上させて高強度を得るためであり、０．１％以上を添加することが好ましい。また、ＭｏはＮｂと共存して制御圧延時にオーステナイトの再結晶を抑制し、オーステナイト組織の微細化にも効果がある。しかし、０．６％を超える過剰なＭｏの添加はＨＡＺ靱性及び現地溶接性を劣化させ、さらにフェライトを分散して生成させるのが困難になるので、その上限を０．６％とした。
【００３１】
Ｃｒは母材及び溶接部の強度を増加させるため、０．１％以上添加することが好ましいが、１％超を添加するとＨＡＺ靱性や現地溶接性を著しく劣化させる。このためＣｒ量の上限は１％とした。
【００３２】
Ｃｕは母材、溶接部の強度を増加させるため、０．１％以上添加することが好ましいが、１％よりも多く添加するとＨＡＺ靱性や現地溶接性を著しく劣化させる。このためＣｕ量の上限は１％とした。
【００３３】
ＶはＮｂとほぼ同様の効果を有するが、その効果はＮｂに比較して弱い。また、溶接部の軟化を抑制する効果も有する。上限としてＨＡＺ靱性、現地溶接性の点から０．３％まで許容できるが、特に０．０３〜０．０８％の添加が好ましい。
【００３４】
Ｂは、極微量の添加により鋼の焼入れ性を高める元素であるが、この効果はＢ量が０．０００３％未満では不十分であるため、Ｂ量の下限を０．０００３％とした。一方、Ｂを０．００３％よりも過剰に添加すると、Ｆｅ₂₃（Ｃ，Ｂ）₆等の脆性粒子の形成を促進し、低温靱性を劣化させるので、Ｂ量の上限を０．００３％とした。
【００３５】
Ｃａ及びＲＥＭは硫化物（ＭｎＳ）の形態を制御し、低温靱性を向上させる。この効果を得るにはＣａを０．００１％以上、ＲＥＭを０．００２％以上とすることが好ましい。Ｃａ量が０．０１％、ＲＥＭが０．０２％を越えて添加するとＣａＯ−ＣａＳ又はＲＥＭ−ＣａＳが大量に生成して大型クラスター、大型介在物となり、鋼の清浄度を害するだけでなく、現地溶接性にも悪影響を及ぼす。このためＣａ及びＲＥＭの添加量の上限を、それぞれ０．０１％及び０．０２％に制限した。なお超高強度ラインパイプでは、Ｓ量及びＯ量をそれぞれ０．００１％及び０．００２％以下に低減し、かつＥＳＳＰ＝（Ｃａ）〔１−１２４（Ｏ）〕／１．２５Ｓを０．５≦ＥＳＳＰ≦１０．０とすることが特に有効である。
【００３６】
Ｍｇは微細分散した酸化物を形成し、溶接熱影響部の粒粗大化を抑制して低温靭性を向上させるため、０．０００１％以上添加することが好ましい。しかし、０．００６％超を添加すると粗大酸化物を生成し、靭性を劣化させるため、０．００６％を上限とした。
【００３７】
次に製造方法について説明する。
【００３８】
まず、圧延加熱前にオーステナイト域に再加熱するが、Ｎｂが固溶する温度に昇温する必要がある。この再加熱は、Ｎｂが固溶し、さらに結晶粒が粗大化しない１０５０℃〜１２５０℃が好ましい範囲である。
【００３９】
再加熱後、再結晶温度域で粗圧延を行い、引き続き９００℃以下の未再結晶温度域で仕上げ圧延を行う。これは、ラインパイプに基本的に必要な低温靭性を高めるためである。なお、仕上げ圧延の終了温度がＡ_r3点未満では、冷却後、上部ベイナイト等の低温変態生成相を含み、且つＣやＮｂが固溶して存在する組織を得られないため、Ａ_r3点を仕上げ圧延の終了温度の下限とする。仕上げ圧延の累積圧下率は５０％以上とする。これは、ラインパイプに必要な低温靭性を確保するためである。仕上げ圧延の累積圧下率の上限は、再結晶圧延終了時の厚みと製品板厚の比で決まる。
【００４０】
仕上げ圧延後、Ａ_r3点以上の温度から３００℃以下まで冷却する。これは、上部ベイナイト等の低温変態生成相を含み、且つＣやＮｂが固溶して存在する組織を得るためである。冷却終了温度の下限は、特性上からは特には制限がないが、通常、５０〜１５０℃の範囲である。Ａ_r3点以上の温度から３００℃以下まで冷却する際の冷却速度は、５〜４０℃／秒とする。これは、上部ベイナイト等の低温変態生成相を含み、且つＣやＮｂが固溶して存在する組織を得るためである。
【００４１】
このようにして製造した鋼板を、そのまま順にＣ成形、Ｕ成形、Ｏ成形によって筒状に成形し、突合せ部を接合する。溶接はその後、真円度を高めるために拡管を行う。
【００４２】
圧潰強度を高めるには、少なくとも外表面側から肉厚中心までの範囲を８０〜５５０℃に加熱することが必要である。加熱温度は８０℃未満では、本発明鋼であってもバウシンガー効果により低下した圧縮強度の回復は殆ど起きない。一方、５５０℃を超える高温に加熱すると軟化が起こるために、却って圧縮降伏強度が低下する。従って、加熱温度は、８０〜５５０℃の範囲とするが、８０〜２５０℃の範囲でも効果が大きく、特に８０〜１５０℃未満の低温でも効果が認められた。内表面側については、この温度域での加熱では変化が殆どないので、加熱しなくても良い。
【００４３】
加熱温度での保持時間は、高温では熱処理温度に到達後、直ちに冷却しても良く、低温では６０００秒以下保持しても良い。好ましい範囲は、６０〜１８００秒である。
【００４４】
また、誘導加熱によって外表面を加熱する方法が効果的であるが、誘導加熱以外にも油槽、あるいはソルトバスによっても可能である。
【００４５】
このようにして製造したＵＯＥ鋼管の圧潰強度は、熱間圧延後の厚板の圧縮強度から計算される圧潰強度と同等以上であることが必要である。ＵＯＥ工程により造管した鋼管の圧潰強度ａ [ＭＰａ]と、少なくとも外表面から肉厚中心までの範囲を８０〜５５０℃に加熱した後の鋼管の圧潰強度ｂ[ＭＰａ]との比ｂ／ａが、１．１０以上であることは、熱間圧延後の厚板の圧縮強度から計算される圧潰強度と同等以上になることを意味するものである。
【００４６】
防食のために鋼管外面に塗覆装を行っても良い。海底ラインパイプでは主にプラスティックコーティングを行うが、密着強度を上げるために１５０〜２５０℃程度の温度で実施する必要がある。このコーティングの際に加熱されてもバウシンガー効果が回復するため、極めて効率が良い。
【００４７】
【実施例】
表１に示す化学成分を含有する鋼を転炉溶製して連続鋳造鋼片とし、表２に示した条件で熱間圧延した。仕上げ圧延の終了温度はいずれもＡ_r3点以上であった。これらの鋼板をそのまま順にＣ成形、Ｕ成形、Ｏ成形し、厚鋼板の端部同士をシーム溶接後、拡管するＵＯＥ工程で、表２に示した材質、外径及び肉厚の鋼管にした。これらの鋼管に高周波での移動過熱方式による加熱を施した。鋼管の外表面及び内表面の温度を熱電対によって測定した。肉厚中心の温度は外表面温度と内表面温度の平均値として計算した。表２に示した加熱温度は最外表面の温度であり、実質的な加熱時間は１８０秒程度であった。内表面の温度は特に制御しなかったが、外表面温度より３０℃程度低かった。従って、肉厚中心の温度は外表面温度より約１５℃低くなっている。
【００４８】
このようにして製造した鋼管を５ｍに切断し、圧力容器内に鋼管を設置して、鋼管に軸力が発生しないように水圧を負荷する単軸圧潰試験を行った。水圧を負荷して、急に水圧が低下し始めた圧力を圧潰強度とした。これらの鋼管の造管ままでの圧潰強度ａ[ＭＰａ]、加熱処理後の圧潰強度ｂ[ＭＰａ]及び両者の比ｂ／ａを表２に示した。
【００４９】
本発明法の実施例１〜９では、加熱により圧潰強度が１８〜２９％上昇し、高圧潰強度の鋼管が得られている。特に、１５０℃未満の低温に加熱しても圧潰強度が向上し、１５０℃以上でも実施例２と１１の比較からわかるように本発明の製造法で製造した鋼の方が圧潰強度の上昇が大きい。実施例１０、１２では、冷却停止温度が高いため、１４０℃の加熱では圧潰強度が向上せず、低い圧潰強度である。実施例１１は３００℃と加熱温度が高いが、冷却停止温度が高いため、圧潰強度が向上しない。実施例１４はＮｂ−Ｔｉを含有せず化学成分が本発明外であるため、圧潰強度が向上しない。
【００５０】
【表１】

【００５１】
【表２】

【００５２】
【発明の効果】
以上述べたように本発明によれば、鋼管の外表面および内表面に異なった加熱履歴を与えることで、ＵＯＥ工程で製造した鋼管に、より高い圧潰強度を付与することが可能であり、圧潰強度に優れたＵＯＥ鋼管を低コストで提供できる、これは、深海のような高い圧潰強度が要求される環境においても、天然ガス、原油等の輸送用ラインパイプ等に使用することができ、産業上、極めて貢献度が高いものである。
【図面の簡単な説明】
【図１】ＵＯＥ工程による鋼管製造プロセスの模式図。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for manufacturing a large-diameter steel pipe manufactured by a UOE process having excellent crushing characteristics used for a deep sea line pipe or the like.
[0002]
[Prior art]
In recent years, the importance of line pipes has increased as a long-distance transport method for crude oil and natural gas. Among them, submarine line pipes that cross the ocean have reached a depth of 3000 m. In general, in pipeline design, the inner diameter of the steel pipe is first determined from the amount of fluid transport, and then the wall thickness and material are determined in consideration of crack propagation characteristics and corrosion weight loss in order to keep the circumferential stress during internal pressure loading to a constant value. ing. However, as the seabed linepipe becomes deeper, the water pressure applied to the steel pipe is increased, and the crushing strength, which has not been regarded as important so far, is becoming a problem. The crushing strength has a correlation with the ratio between the outer diameter and the wall thickness. By increasing the crushing strength of the steel pipe, the diameter can be increased and the wall thickness can be reduced. Therefore, crushing strength is becoming the main design factor that determines the steel pipe size.
[0003]
By the way, the crushing strength of steel pipes has been studied for a long time for oil well pipes, and many empirical formulas have been proposed statistically. Among them, the outer diameter / thickness ratio, yield strength, roundness, thickness deviation, and residual stress were the main controlling factors. Since these studies were mainly conducted on seamless steel pipes with homogeneous material, it was not necessary to discuss much about material anisotropy.
[0004]
However, since the main line pipe used for long-distance transportation has a large diameter, a steel pipe produced by the manufacturing method of the UOE process is used. As shown in FIG. 1, the UOE process includes C forming (pressing), U forming (pressing), O forming (pressing), seam welding, and pipe expanding. Since both the moldings are performed cold, the final product, that is, the steel pipe, becomes anisotropic in mechanical properties by a combination of work hardening and the Bauschinger effect. The Bauschinger effect is a phenomenon in which the yield strength in the opposite direction is lowered after plastic strain is applied to a material. Therefore, in the UOE steel pipe in which the plastic strain in the tensile direction is given in the circumferential direction, the compressive yield strength in the circumferential direction, that is, the yield strength with respect to the external pressure load is lowered by the Bauschinger effect.
[0005]
On the other hand, since the load direction is orthogonal to the main strain during molding with respect to the load in the axial direction, a difference in stress behavior is hardly caused in the tensile and compression loads in the axial direction. Further, when the load in the circumferential direction is a tensile stress, that is, for the internal pressure load, there is no problem if the strength is designed based on the value obtained from the full thickness tensile test.
[0006]
However, in recent years, the demand for UOE steel pipes applicable to deep sea line pipes has increased, and the crushing strength of steel pipes due to external pressure has become a problem. Crushing is a phenomenon in which a steel pipe is crushed by external pressure, and is one of buckling, so the yield strength of compression determines the crushing strength. Therefore, when a UOE steel pipe is applied to a line pipe that requires crushing strength, a decrease in the circumferential compressive strength due to the Bausinger effect becomes a problem.
[0007]
In order to solve such a problem, Japanese Patent Laid-Open No. 9-3545 discloses a method of heating a steel pipe with a gas burner after expansion. However, even when such heating is performed, the crushing strength of all UOE steel pipes is not improved by 20% or more, and steel pipes with a small recovery of the Bausinger effect by heating were also observed.
[0008]
[Problems to be solved by the invention]
An object of the present invention is to provide a method of manufacturing a steel pipe with high crushing strength, in which the compressive yield strength is stably recovered by heating after pipe making in the UOE process.
[0009]
[Means for Solving the Problems]
The present inventors have studied in detail the effects of recovering the crushing strength, which has been reduced by the Bauschinger effect, by heating up to the chemical composition of the steel sheet as the material of the steel pipe and the production conditions such as rolling and accelerated cooling. As a result, after hot rolling, the compressive strength of the Nb-added steel cooled to a low temperature range of 300 ° C. or lower is higher than that of a steel plate that has been air-cooled after hot rolling or water cooled to 500 to 600 ° C. to stop cooling. all right. Furthermore, it was found that when a steel pipe is manufactured by the UOE process and heated to a temperature in the range of 80 to 550 ° C. after the pipe expansion, the crushing strength is restored to the same level or higher as before the pipe expansion. Furthermore, it was found that this effect becomes remarkable by the addition of Ti, and that the Bausinger effect is recovered even when heated to a low temperature of 80 to 150 ° C.
[0010]
The present invention has been made on the basis of such knowledge, and the gist thereof is as follows.
(1) By mass%, C: 0.03-0.15%, Si: 0.8% or less, Mn: 0.3-2.5%, P: 0.03% or less, S: 0.01 %: Nb: 0.01 to 0.3%, Ti: 0.005 to 0.03%, Al: 0.1% or less, N: 0.001 to 0.01%, the balance being iron and after reheating steel slabs consisting of unavoidable impurities in the austenite region, performs rough rolling in recrystallization region, subsequently subjected to finish rolling of 50% or more cumulative rolling reduction in the non-recrystallization temperature region of 900 ° C. or less, a _r3 Thick steel plates manufactured by cooling from a temperature above the point to 300 ° C. or less at a cooling rate of 5 to 40 ° C./second are sequentially C-formed, U-formed, and O-formed, and the ends of the thick steel plates are seam welded together. , Heating at least 80 to 550 ° C in the range from the outer surface to the thickness center of the steel pipe made by the UOE process Characterized Rukoto method of high UOE steel pipe crushing strength.
(2) By mass%, Ni: 1% or less, Mo: 0.6% or less, Cr: 1% or less, Cu: 1% or less, V: 0.3% or less, B: 0.0003-0 0.003%, Ca: 0.01% or less, REM: 0.02% or less, Mg: 0.006% or less, 1 or 2 or more types, The crushing as described in (1) characterized by the above-mentioned Manufacturing method of high strength UOE steel pipe.
(3) Crushing strength a [MPa] of the steel pipe made by the UOE process and crushing strength b of the steel pipe made by the UOE process after heating at least the range from the outer surface to the thickness center at 80 to 550 ° C. The method for producing a UOE steel pipe with high crushing strength according to (1) and (2), wherein the ratio b / a to [MPa] is 1.10 or more.
(4) The method for producing a UOE steel pipe with high crushing strength according to any one of (1) to (3), wherein coating is performed during heating or during cooling after heating.
It is.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
The present inventors have investigated in detail the relationship between the heating temperature and the chemical composition and structure of the steel with respect to the phenomenon that the yield strength of the steel, which has been lowered by the Bauschinger effect, is recovered by heat treatment. First, steels containing various components were produced in a laboratory by changing the hot rolling conditions, and tensile test pieces were processed from these steel plates to give a tensile strain of 4%. Next, it heated at 50-700 degreeC for 10 minute (s), the compression test piece was extract | collected from the tension test piece as it was and after the heating, and the compression test was implemented. The ratio of 0.2% proof stress in the compression test of these test pieces and 0.2% proof stress in the compression test of the material before applying tensile strain (hereinafter referred to as compressive strength ratio) was used as a criterion.
[0012]
As a result, the compressive strength ratio of the Nb-added steel cooled to a low temperature range of 300 ° C. or less after hot rolling is higher than that of steel cooled by air after hot rolling and steel cooled by water cooling to 500 to 600 ° C. after hot rolling. It was found that the compression strength ratio exceeded 1.0 by heat treatment at 80 to 550 ° C. Furthermore, it became clear that the Nb—Ti-added steel is effective when heated to less than 80 to 150 ° C.
[0013]
On the other hand, in the Nb—Ti additive-free steel whose cooling was stopped at 500 to 600 ° C., the compression strength ratio did not change at all even when heated to less than 80 to 150 ° C. Furthermore, even when heated to a temperature of 150 ° C. or higher, the recovery of the Bausinger effect of the Nb—Ti additive-free steel whose cooling was stopped at 500 to 600 ° C. is more than that of the Nb—Ti-added steel whose cooling was stopped at 300 ° C. or less. Was found to be less effective.
[0014]
As a result of investigating the microstructure of the steel wound at 300 ° C. or lower, it was found that it has a structure including a low-temperature transformation generation phase such as upper bainite. Such a low-temperature transformation generation phase is considered to suppress a decrease in compressive yield strength due to the Bauschinger effect. Furthermore, the reason why the compressive yield stress after pipe expansion rises to the same or higher level than the compressive yield strength before pipe expansion by heating to about 100 ° C is that the stress field around the dislocation that causes the Bauschinger effect changes easily. It is estimated that an element existing in a solid solution state such as C is fixed to the dislocation.
[0015]
Furthermore, as a result of detailed analysis of precipitates, it was found that fine TiN was precipitated in the Ti-added steel. It is considered that the fine TiN suppresses the coarsening of the austenite grains of the HAZ during reheating of the slab and the microstructure of the base material and the HAZ. As a result of obtaining such a homogeneous and fine microstructure, non-uniform stress in the grains is reduced, and the residual stress is easily distributed uniformly, and the synergistic effect with the addition of Nb reduces by the Bauschinger effect. It was found that the crushing strength improved easily by low-temperature heat treatment.
[0016]
The steel plate thus produced was used as a steel pipe in the UOE process, and the reduction in compressive yield strength due to the Bauschinger effect was investigated in detail in the thickness direction from the outer surface side to the inner surface side of the steel pipe. As a result, the outer surface side is subjected to a circumferential tensile strain in the tubular forming step and the tube expanding step, and the compressive yield strength is reduced due to the bausinger effect, but the inner surface side is the UO step It was found that the work hardening of the compression due to bending at the end of the tube remains even after tube expansion, and the compression yield strength does not decrease.
[0017]
Furthermore, the effect of heating on the reduction of the compressive yield strength due to the Bausinger effect was investigated at the surface layer part in the thickness direction from the outer surface side to the inner surface side of the steel pipe, the thickness center part, and the 1/4 thickness part. It was. As a result, it was found that although the effect of heating the inner surface side is small, the crushing strength is improved by heating the outer surface side. This heating is effective in the range of 80 to 550 ° C., and the effect is large even in the range of 80 to 250 ° C., and the effect is recognized even at a low temperature of 80 to less than 150 ° C.
[0018]
Next, the reasons for limiting the component elements will be described.
[0019]
The amount of C is limited to 0.03 to 0.15%. Carbon is extremely effective in improving the strength of steel, and at least 0.03% is necessary to obtain the target strength. However, if the C content is more than 0.15%, the base material, HAZ low temperature toughness and on-site weldability are significantly deteriorated, so the upper limit was made 0.15%. Uniform elongation is higher when the amount of C is larger, and low temperature toughness and weldability are better when the amount of C is smaller, and it is necessary to consider the balance depending on the level of required characteristics.
[0020]
Si is an element added for deoxidation and strength improvement, but if added more than 0.8%, HAZ toughness and on-site weldability deteriorate significantly, so the upper limit of Si content was set to 0.8%. Note that steel can be deoxidized with Al and Ti. Si does not necessarily need to be added, but usually contains about 0.1%.
[0021]
Mn is a bainite-based microstructure in the matrix of the steel of the present invention, and is an indispensable element for ensuring a balance between excellent strength and low-temperature toughness, and its lower limit is 0.3%. However, if the amount of Mn is more than 2.5%, it becomes difficult to disperse and generate ferrite, so the upper limit was made 2.5%.
[0022]
Moreover, in this invention steel, Nb: 0.01-0.3% and Ti: 0.005-0.03% are contained as an essential element.
[0023]
Nb not only suppresses recrystallization of austenite during controlled rolling and refines the structure, but also contributes to an increase in hardenability and strengthens the steel. Since this effect is small when the Nb content is less than 0.01%, the lower limit is set. However, if the Nb addition amount is more than 0.3%, the HAZ toughness and on-site weldability are adversely affected, so the upper limit was made 0.3%.
[0024]
Ti addition forms fine TiN, refines the microstructure of the base metal and HAZ, and the crushing strength reduced by the Bauschinger effect is improved by heating to 80 to 550 ° C., particularly less than 80 to 150 ° C. Promote the effect. It also improves the low temperature toughness of the base material and HAZ. This effect becomes very remarkable by the combined addition with Nb. For this purpose, the Ti amount is preferably added to 3.4 N (each by mass) or more. Further, when the amount of Al is small (for example, 0.005% or less), Ti forms an oxide, acts as an intragranular ferrite formation nucleus in the HAZ, and has an effect of refining the HAZ structure. In order to exhibit such an effect, at least 0.005% of Ti should be added. However, if the amount of Ti is more than 0.03%, TiN coarsening and precipitation hardening due to TiC occur and the low temperature toughness is deteriorated, so the upper limit was limited to 0.03%.
[0025]
Al is an element usually contained in steel as a deoxidizing material, and has an effect on refinement of the structure. However, if the Al content exceeds 0.1%, Al-based non-metallic inclusions increase to impair the cleanliness of the steel, so the upper limit was made 0.1%. Further, deoxidation is possible with Ti and Si, and Al is not necessarily added, but the present technology contains about 0.001%.
[0026]
N forms TiN and suppresses coarsening of the austenite grains of the HAZ during reheating of the slab and improves the low temperature toughness of the base material and the HAZ. The minimum amount required for this is 0.001%. However, if the amount of N is more than 0.01%, TiN will increase too much, causing adverse effects such as surface flaws and toughness deterioration, so the upper limit must be limited to 0.01%.
[0027]
Furthermore, in the present invention, the amounts of impurity elements P and S are set to 0.03% and 0.01% or less, respectively. The main reason is to further improve the low temperature toughness of the base material and the HAZ. The reduction of the amount of P reduces the center segregation of the continuously cast slab and prevents the grain boundary fracture, thereby improving the low temperature toughness. Further, the reduction of the amount of S has the effect of reducing the MnS stretched by hot rolling and improving the ductility. In both cases, the smaller the amount, the better. However, it is necessary to determine the balance between the characteristics and the cost. Usually, P and S contain 0.001% or more and 0.0001% or more, respectively.
[0028]
Next, the purpose of adding the selective elements Ni, Mo, Cr, Cu, V, Ca, REM and Mg will be described. The main purpose of adding these elements to the basic components is to further improve the strength and toughness and enlarge the size of the steel material that can be manufactured without impairing the excellent characteristics of the steel of the present invention.
[0029]
The purpose of adding Ni is to improve the low carbon steel of the present invention without deteriorating the low temperature toughness and on-site weldability, and it is preferable to add 0.1% or more. Compared with the addition of Mn, Cr and Mo, the addition of Ni rarely forms a hardened structure that is harmful to low temperature toughness in the rolled structure, particularly in the central segregation zone of the continuous cast steel slab. However, if the amount of Ni is more than 1%, not only economic efficiency but also HAZ toughness and on-site weldability are deteriorated, so the upper limit was made 1%. The addition of Ni is also effective for preventing Cu cracking during continuous casting and hot rolling. In this case, Ni needs to be added by 1/3 or more of the amount of Cu.
[0030]
The reason for adding Mo is to improve the hardenability of the steel to obtain high strength, and it is preferable to add 0.1% or more. In addition, Mo coexists with Nb, suppresses recrystallization of austenite during controlled rolling, and is effective in refining the austenite structure. However, addition of excess Mo exceeding 0.6% deteriorates the HAZ toughness and on-site weldability and further makes it difficult to disperse and form ferrite, so the upper limit was made 0.6%.
[0031]
Since Cr increases the strength of the base metal and the welded portion, it is preferable to add 0.1% or more. However, if over 1% is added, the HAZ toughness and on-site weldability are significantly deteriorated. For this reason, the upper limit of the Cr amount is set to 1%.
[0032]
Since Cu increases the strength of the base metal and the welded portion, it is preferable to add 0.1% or more. However, if it is added more than 1%, the HAZ toughness and on-site weldability are remarkably deteriorated. For this reason, the upper limit of the amount of Cu was made into 1%.
[0033]
V has almost the same effect as Nb, but the effect is weaker than Nb. Moreover, it has the effect which suppresses softening of a welding part. The upper limit is acceptable up to 0.3% from the viewpoint of HAZ toughness and field weldability, but addition of 0.03 to 0.08% is particularly preferable.
[0034]
B is an element that enhances the hardenability of the steel by adding a very small amount, but since this effect is insufficient when the B content is less than 0.0003%, the lower limit of the B content is set to 0.0003%. On the other hand, if B is added in excess of 0.003%, the formation of brittle particles such as Fe ₂₃ (C, B) ₆ is promoted and the low temperature toughness is deteriorated. did.
[0035]
Ca and REM control the form of sulfide (MnS) and improve low temperature toughness. In order to obtain this effect, Ca is preferably 0.001% or more and REM is 0.002% or more. When Ca content is 0.01% and REM exceeds 0.02%, CaO-CaS or REM-CaS is produced in large quantities to form large clusters and large inclusions, not only detracting from the cleanliness of steel, It also adversely affects on-site weldability. For this reason, the upper limit of the addition amount of Ca and REM was limited to 0.01% and 0.02%, respectively. In the ultra-high-strength line pipe, the S content and the O content are reduced to 0.001% and 0.002% or less, respectively, and ESSP = (Ca) [1-124 (O)] / 1.25S is reduced to 0.00. It is particularly effective to satisfy 5 ≦ ESSP ≦ 10.0.
[0036]
Mg is preferably added in an amount of 0.0001% or more in order to form a finely dispersed oxide and suppress the coarsening of the weld heat affected zone to improve the low temperature toughness. However, if over 0.006% is added, a coarse oxide is formed and the toughness is deteriorated, so 0.006% was made the upper limit.
[0037]
Next, a manufacturing method will be described.
[0038]
First, it is reheated to the austenite region before rolling and heating, but it is necessary to raise the temperature to a temperature at which Nb is dissolved. This reheating is preferably performed at 1050 ° C. to 1250 ° C. in which Nb is dissolved and crystal grains are not coarsened.
[0039]
After reheating, rough rolling is performed in the recrystallization temperature range, and finish rolling is subsequently performed in the non-recrystallization temperature range of 900 ° C. or lower. This is to increase the low temperature toughness basically required for the line pipe. In the below termination temperature A _r3 point finish rolling, cooled, it comprises a low-temperature transformation product phase such as upper bainite, and for C and Nb can not be obtained a tissue present in solid solution, A _r3 point the The lower limit of the finish rolling finish temperature. The cumulative rolling reduction of finish rolling is 50% or more. This is to ensure the low temperature toughness required for the line pipe. The upper limit of the finish rolling cumulative rolling reduction is determined by the ratio between the thickness at the end of recrystallization rolling and the product sheet thickness.
[0040]
After finish rolling, cooling is performed from a temperature of _Ar3 or higher to 300 ° C or lower. This is to obtain a structure containing a low-temperature transformation-generating phase such as upper bainite and in which C and Nb are dissolved. The lower limit of the cooling end temperature is not particularly limited in terms of characteristics, but is usually in the range of 50 to 150 ° C. The cooling rate when cooling from a temperature of _{Ar 3} point or higher to 300 ° C. or lower is 5 to 40 ° C./second. This is to obtain a structure containing a low-temperature transformation-generating phase such as upper bainite and in which C and Nb are dissolved.
[0041]
The steel plates thus manufactured are formed into a cylindrical shape by C forming, U forming, and O forming in this order, and the butt portions are joined. Welding is then expanded to increase roundness.
[0042]
In order to increase the crushing strength, it is necessary to heat at least the range from the outer surface side to the thickness center at 80 to 550 ° C. When the heating temperature is less than 80 ° C., even the steel of the present invention hardly recovers the compressive strength lowered due to the Bauschinger effect. On the other hand, when heated to a high temperature exceeding 550 ° C., softening occurs, so that the compressive yield strength decreases. Accordingly, the heating temperature is in the range of 80 to 550 ° C., but the effect is large even in the range of 80 to 250 ° C., and the effect is recognized even at a low temperature of 80 to 150 ° C. in particular. The inner surface side does not need to be heated because there is almost no change in heating in this temperature range.
[0043]
The holding time at the heating temperature may be cooled immediately after reaching the heat treatment temperature at a high temperature, or may be held for 6000 seconds or less at a low temperature. A preferred range is 60 to 1800 seconds.
[0044]
Moreover, although the method of heating an outer surface by induction heating is effective, it is also possible by an oil tank or a salt bath other than induction heating.
[0045]
The crushing strength of the UOE steel pipe manufactured in this way needs to be equal to or greater than the crushing strength calculated from the compressive strength of the thick plate after hot rolling. Ratio b / a of the crushing strength a [MPa] of the steel pipe formed by the UOE process and the crushing strength b [MPa] of the steel pipe after heating the range from at least the outer surface to the thickness center at 80 to 550 ° C. However, it is 1.10 or more means that it becomes equal to or more than the crushing strength calculated from the compressive strength of the thick plate after hot rolling.
[0046]
You may coat the outer surface of a steel pipe for corrosion prevention. The submarine line pipe mainly performs plastic coating, but it needs to be performed at a temperature of about 150 to 250 ° C. in order to increase the adhesion strength. Even if the coating is heated, the bausinger effect is restored, so that the efficiency is very good.
[0047]
【Example】
Steel containing the chemical components shown in Table 1 was melted into a converter to form a continuously cast steel slab, which was hot-rolled under the conditions shown in Table 2. The finishing temperature of finish rolling was _Ar3 or higher. These steel plates were formed into C tubes, U shapes, and O shapes in this order, and steel pipes having the materials, outer diameters, and wall thicknesses shown in Table 2 in the UOE process in which the ends of the thick steel plates were seam welded and then expanded. These steel pipes were heated by high-frequency moving superheating. The temperature of the outer surface and inner surface of the steel pipe was measured with a thermocouple. The temperature at the center of the wall thickness was calculated as the average value of the outer surface temperature and the inner surface temperature. The heating temperature shown in Table 2 was the temperature of the outermost surface, and the substantial heating time was about 180 seconds. Although the inner surface temperature was not particularly controlled, it was about 30 ° C. lower than the outer surface temperature. Accordingly, the temperature at the center of the wall thickness is about 15 ° C. lower than the outer surface temperature.
[0048]
The steel pipe manufactured in this way was cut into 5 m, a steel pipe was installed in the pressure vessel, and a uniaxial crushing test was performed in which water pressure was applied so that no axial force was generated in the steel pipe. The pressure at which water pressure was applied and the water pressure began to drop suddenly was taken as the crushing strength. Table 2 shows the crushing strength a [MPa] of these steel pipes as made, the crushing strength b [MPa] after the heat treatment, and the ratio b / a between them.
[0049]
In Examples 1 to 9 of the method of the present invention, the crushing strength is increased by 18 to 29% by heating, and a steel pipe having a high pressure crushing strength is obtained. In particular, even when heated to a low temperature of less than 150 ° C., the crushing strength is improved. As can be seen from the comparison between Examples 2 and 11 at 150 ° C. or higher, the steel produced by the production method of the present invention has an increased crushing strength. large. In Examples 10 and 12, since the cooling stop temperature is high, the crushing strength is not improved by heating at 140 ° C., and the crushing strength is low. In Example 11, the heating temperature is as high as 300 ° C., but the crushing strength is not improved because the cooling stop temperature is high. Since Example 14 does not contain Nb-Ti and the chemical component is outside the present invention, the crushing strength is not improved.
[0050]
[Table 1]

[0051]
[Table 2]

[0052]
【The invention's effect】
As described above, according to the present invention, by giving different heating histories to the outer surface and the inner surface of the steel pipe, it is possible to give a higher crushing strength to the steel pipe manufactured in the UOE process. UOE steel pipes with excellent strength can be provided at low cost. This can be used for line pipes for transportation of natural gas, crude oil, etc. even in environments where high crushing strength is required such as in the deep sea. In addition, the contribution is extremely high.
[Brief description of the drawings]
FIG. 1 is a schematic diagram of a steel pipe manufacturing process by a UOE process.

Claims

% By mass
C: 0.03-0.15%,
Si: 0.8% or less,
Mn: 0.3 to 2.5%
P: 0.03% or less,
S: 0.01% or less,
Nb: 0.01-0.3%
Ti: 0.005 to 0.03%,
Al: 0.1% or less,
N: 0.001 to 0.01%
The steel slab consisting of iron and unavoidable impurities is reheated to the austenite region, followed by rough rolling in the recrystallization region, followed by a cumulative reduction of 50% or more in the non-recrystallization temperature region of 900 ° C. or less. perform finish rolling, the steel plate was prepared and cooled from a _r3 point above temperature to 300 ° C. or less at a cooling rate of 5 to 40 ° C. / sec, C shaped, U shaped, and O molded directly in the order, the steel plate Production of UOE steel pipe with high crushing strength, characterized in that at least the range from the outer surface to the thickness center of the steel pipe formed by the UOE process of expanding the pipes after seam welding between the ends is heated to 80 to 550 ° C. Method.

In mass%,
Ni: 1% or less,
Mo: 0.6% or less,
Cr: 1% or less,
Cu: 1% or less,
V: 0.3% or less,
B: 0.0003 to 0.003%,
Ca: 0.01% or less,
REM: 0.02% or less,
Mg: 0.006% or less,
1 or 2 types or more of these are contained, The manufacturing method of the UOE steel pipe with high crushing strength of Claim 1 characterized by the above-mentioned.

Crushing strength a [MPa] of the steel pipe made by the UOE process, and crushing strength b [MPa] of the steel pipe made by the UOE process after heating at 80 to 550 ° C. at least in the range from the outer surface to the thickness center. The ratio b / a with respect to the above is 1.10 or more, and the method for producing a UOE steel pipe with high crushing strength according to claim 1 or 2.

The method for producing a UOE steel pipe having high crushing strength according to any one of claims 1 to 3, wherein coating is performed during the heating or cooling after the heating.