JP2004035925A

JP2004035925A - Method for producing uoe steel pipe having high crushing strength

Info

Publication number: JP2004035925A
Application number: JP2002192531A
Authority: JP
Inventors: Hitoshi Asahi; 朝日　均; Eiji Tsuru; 津留　英司
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2002-07-01
Filing date: 2002-07-01
Publication date: 2004-02-05
Anticipated expiration: 2022-07-01
Also published as: JP4072009B2

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a steel pipe having high crushing strength by stably recovering the crushing strength by heating after making the pipe by a UOE process. <P>SOLUTION: After reheating a steel slab containing a prescribed amounts of C, Si, Mn, P, S, Nb, Ti, Al, N and if necessary, one or more kinds of Ni, Mo, Cr, Cu, V, B, Ca, REM, Mg and the balance Fe with inevitable impurities into an austenitic range, a rough-rolling is performed in the recrystallization range. Subsequently, a finish-rolling having ≥ 50% accumulative rolling-reduction ratio is performed in a non-recrystallization temperature range of ≤ 900°C, and a thick steel plate produced by cooling to ≤ 300°C at 5 to 40°C/sec cooling rate from the temperature of not lower than A<SB>r3</SB>point is C-formed, U-formed and O-formed in this order. Then, after seam-welding both end parts of the thick steel plate, the range at least from the outer surface to the center of the thickness of the steel pipe made by the UOE process for enlarging the diameter of the pipe is heated in a temperature range of 80 to 550°C. <P>COPYRIGHT: (C)2004,JPO

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、Ｆｅ_２３（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]
TECHNICAL FIELD 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 and used for a deep-sea line pipe or the like.
[0002]
[Prior art]
In recent years, the importance of line pipes as a long-distance transportation method for crude oil and natural gas has been increasing. In particular, submarine line pipes that cross the ocean have reached a depth of up to 3000 m. Generally, in the design of a pipeline, the inner diameter of a steel pipe is first determined from the fluid transport volume, and then the wall thickness and material are determined in consideration of crack propagation characteristics and corrosion loss so as to keep the circumferential stress under internal pressure load at a constant value. ing. However, the water pressure applied to the steel pipe is increasing with the deepening of the seabed line pipe, and the crushing strength, which has not been regarded as much important in the past, 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, it is possible to increase the diameter and reduce the wall thickness. Therefore, crushing strength is beginning to be a major design factor in determining steel pipe size.
[0003]
By the way, the crushing strength of steel pipes has been studied for oil well pipes for a long time, and many empirical formulas have been proposed statistically. Outer diameter / thickness ratio, yield strength, roundness, uneven thickness, and residual stress were the main controlling factors. Since these studies were mainly performed on seamless steel pipes with homogeneous materials, it was not necessary to discuss much about the anisotropy of the materials.
[0004]
However, since the main line pipe used for long-distance transportation has a large diameter, a steel pipe manufactured by the UOE process is used. The UOE step includes, as shown in FIG. 1, a step of forming C (press), forming U (press), forming O (press), seam welding, and expanding a pipe. Since both moldings are performed in a cold state, the final product, that is, the steel pipe, has anisotropic mechanical properties due to the combination of the work hardening and the Bauschinger effect. Note that the Bauschinger effect is a phenomenon in which after a plastic strain is applied to a material, the yield strength in a direction opposite to the plastic strain decreases. Therefore, in the UOE steel pipe to which the plastic strain is applied in the circumferential direction in the tensile direction, the compressive yield strength in the circumferential direction, that is, the yield strength against an external pressure load is reduced by the Bauschinger effect.
[0005]
On the other hand, the load direction is orthogonal to the main strain during molding with respect to the axial load, so that there is little difference in the stress behavior between the tensile and compressive loads in the axial direction. When the circumferential load is tensile stress, that is, when the strength is designed based on the value obtained from the full-thickness tensile test for the internal pressure load, no problem occurs.
[0006]
However, in recent years, 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 begun to become a problem. Crushing is a phenomenon in which a steel pipe is crushed by an external pressure and is one of buckling. Therefore, the yield strength of compression determines crushing strength. Therefore, when applying a UOE steel pipe to a line pipe requiring crushing strength, there is a problem that the compressive strength in the circumferential direction decreases due to the Bauschinger effect.
[0007]
To cope with such a problem, Japanese Patent Application Laid-Open No. 9-3545 discloses a method of heating a steel pipe with a gas burner after expanding the pipe. However, even if such heating is performed, the crushing strength of all UOE steel pipes is not improved by 20% or more, and some steel pipes whose recovery of the Bauschinger effect by heating is small are seen.
[0008]
[Problems to be solved by the invention]
An object of the present invention is to provide a method for producing a steel pipe having a high crushing strength, in which the compressive yield strength is stably recovered by heating after pipe formation in a UOE process.
[0009]
[Means for Solving the Problems]
The present inventors have studied in detail the effect that the crushing strength reduced by the Bauschinger effect is restored by heating, as far back as the chemical composition of the steel sheet as the material of the steel pipe and the manufacturing 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 sheet cooled by air or a steel sheet cooled by water cooling to 500 to 600 ° C. after hot rolling. all right. Furthermore, it was found that when a steel pipe was manufactured by the UOE process and heated to a temperature in the range of 80 to 550 ° C. after expansion, the crushing strength was restored to a level equal to or higher than that before expansion. Furthermore, it was found that this effect became remarkable by the addition of Ti, and that the Bauschinger effect was restored even when heating was performed at a low temperature of 80 to less than 150 ° C.
[0010]
The present invention has been made based on such findings, and the gist is as follows.
(1) In mass%, C: 0.03 to 0.15%, Si: 0.8% or less, Mn: 0.3 to 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 After reheating the steel slab comprising unavoidable impurities to the austenite region, rough rolling is performed in the recrystallization region, followed by finish rolling with a cumulative draft of 50% or more in the non-recrystallization temperature region of 900 ° C. or less, and _Ar3 The thick steel plate manufactured by cooling from the temperature above the point to 300 ° C. or lower at a cooling rate of 5 to 40 ° C./sec is subjected to C forming, U forming, and O forming in that order, and the ends of the thick steel plate are seam-welded. At least the range from the outer surface to the center of the wall thickness of the steel pipe formed by the UOE process is expanded to 80 to 550 ° C. A method for producing a UOE steel pipe having high crushing strength, characterized by heating.
(2) 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: 0.0003-0 0.001% or less, Ca: 0.01% or less, REM: 0.02% or less, and Mg: 0.006% or less. A method for manufacturing a high strength UOE steel pipe.
(3) The crushing strength a [MPa] of the steel pipe formed by the UOE step and the crushing strength b of the steel pipe formed by the UOE step after heating at least the range from the outer surface to the center of the wall to 80 to 550 ° C. The method for producing a UOE steel pipe having high crushing strength according to (1) or (2), wherein the ratio b / a to [MPa] is 1.10.
(4) The method for producing a UOE steel pipe having a high crushing strength according to any one of (1) to (3), wherein the coating and covering is performed during heating or during cooling after heating.
It is.
[0011]
BEST MODE FOR CARRYING OUT THE INVENTION
The present inventors have examined in detail the relationship between the heating temperature and the chemical composition and structure of the steel regarding the phenomenon that the yield strength of the steel reduced by the Bauschinger effect is restored by the heat treatment. First, steels containing various components were manufactured in a laboratory by changing hot rolling conditions, and tensile test pieces were processed from these steel sheets to give a tensile strain of 4%. Next, the specimen was heated at 50 to 700 ° C. for 10 minutes, and a compression test was performed by collecting a compression test specimen from the tensile test specimen as it was under tension and after heating. The ratio between the 0.2% proof stress of these test pieces in the compression test and the 0.2% proof stress of the material before the tensile strain was applied in the compression test (hereinafter, 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 lower than that of the steel that was air-cooled after hot rolling and the steel that was cooled by water cooling to 500 to 600 ° C. after hot rolling. It was found that the compressive strength ratio exceeded 1.0 by the heat treatment at 80 to 550 ° C. Furthermore, it became clear that the effect can be obtained when the Nb-Ti added steel is heated to 80 to less than 150C.
[0013]
On the other hand, in the case of Nb-Ti-free steel stopped cooling at 500 to 600 ° C, the compressive 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 more, the recovery of the Bauschinger effect of the Nb—Ti-free steel stopped cooling at 500 to 600 ° C. is better than that of the Nb—Ti added steel stopped cooled at 300 ° C. or less. It was also found that the effect was small.
[0014]
As a result of examining the microstructure of the steel wound at a temperature of 300 ° C. or lower, it was found that the steel had a structure including a low-temperature transformation generation phase such as upper bainite. It is considered that such a low-temperature transformation generation phase suppresses a decrease in compressive yield strength due to the Bauschinger effect. Furthermore, the reason why the compressive yield stress after pipe expansion is increased to about the same or higher 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 presumed that elements existing in a solid solution state such as C are fixed to dislocations.
[0015]
Furthermore, as a result of analyzing the precipitates in detail, it was found that fine TiN was precipitated in the Ti-added steel. It is considered that this fine TiN suppressed the coarsening of the austenite grains of the HAZ at the time of slab reheating and the microstructure of the base material and the HAZ. As a result of obtaining such a homogeneous and fine microstructure, unevenness of stress in the grains is reduced, and the residual stress is easily distributed evenly. The synergistic effect with the addition of Nb reduces the Bauschinger effect. It has been found that the obtained crushing strength is easily improved by a low-temperature heat treatment.
[0016]
The steel plate manufactured in this manner was formed into a steel pipe in the UOE process, and the reduction in compressive yield strength due to the Bauschinger effect was examined 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 is subjected to tensile strain in the circumferential direction in the process of forming into a tubular shape and in the process of expanding the tube, and the compressive yield strength is reduced due to the Bauschinger effect. It was found that the work hardening of the compression due to the bending process remained after the pipe expansion, and the compression yield strength did not decrease.
[0017]
Furthermore, the effect of heating on the reduction of the compressive yield strength due to the Bauschinger effect was examined at the surface layer, the center of the thickness, and the 1/4 thick part in the thickness direction from the outer surface side to the inner surface side of the steel pipe. Was. As a result, although the effect of heating the inner surface was small, it was found that the crushing strength was improved by heating the outer surface. 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 less than 80 to 150 ° C.
[0018]
Next, reasons for limiting the component elements will be described.
[0019]
C content is limited to 0.03 to 0.15%. Carbon is extremely effective in improving the strength of steel, and at least 0.03% is required to obtain a target strength. However, if the C content is more than 0.15%, the low-temperature toughness and the on-site weldability of the base material and HAZ are remarkably deteriorated, so the upper limit is made 0.15%. The uniform elongation is higher when the C content is larger, and the low temperature toughness and weldability are better when the C content is smaller, and it is necessary to consider the balance according to the required property level.
[0020]
Si is an element added for deoxidation and improvement of strength. However, if added in excess of 0.8%, HAZ toughness and on-site weldability are significantly deteriorated, so the upper limit of the amount of Si was set to 0.8%. Note that steel can be deoxidized with Al and Ti, and Si need not always be added, but usually contains about 0.1%.
[0021]
Mn is an element indispensable for securing the balance between excellent strength and low-temperature toughness by making the microstructure of the parent phase of the steel of the present invention a bainite-based structure, and its lower limit is 0.3%. However, if the Mn content is more than 2.5%, it becomes difficult to disperse and produce ferrite, so the upper limit was made 2.5%.
[0022]
The steel of the present invention contains Nb: 0.01 to 0.3% and Ti: 0.005 to 0.03% as essential elements.
[0023]
Nb not only suppresses the recrystallization of austenite during controlled rolling to refine 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 content is more than 0.3%, the HAZ toughness and on-site weldability are adversely affected, so the upper limit is set to 0.3%.
[0024]
The addition of Ti forms fine TiN, refines the microstructures of the base material and the HAZ, and the crushing strength reduced by the Bauschinger effect is improved by heating to 80 to 550 ° C, especially to 80 to less than 150 ° C. Promote the effect. It also improves the low-temperature toughness of the base material and HAZ. This effect becomes extremely remarkable by the complex addition with Nb. For this purpose, it is preferable to add Ti in an amount of 3.4 N (each by mass%) or more. When the amount of Al is small (for example, 0.005% or less), Ti forms an oxide, acts as an intragranular ferrite generation nucleus in the HAZ, and has an effect of making the HAZ structure finer. In order to exhibit such an effect, it is necessary to add at least 0.005% of Ti. However, if the amount of Ti is more than 0.03%, coarsening of TiN and precipitation hardening due to TiC occur, deteriorating low-temperature toughness. Therefore, the upper limit is limited to 0.03%.
[0025]
Al is an element usually contained in steel as a deoxidizing material, and also has an effect on refining the structure. However, if the amount of Al exceeds 0.1%, Al-based nonmetallic inclusions increase and impair the cleanliness of the steel, so the upper limit was made 0.1%. Deoxidation is also possible with Ti and Si, and it is not always necessary to add Al, but the current technology contains about 0.001%.
[0026]
N forms TiN and suppresses the coarsening of the austenite grains of the HAZ during slab reheating and improves the low-temperature toughness of the base metal and the HAZ. The minimum required for this is 0.001%. However, if the N content is more than 0.01%, the amount of TiN will increase too much and adverse effects such as surface flaws and toughness deterioration will occur. Therefore, the upper limit must be suppressed to 0.01%.
[0027]
Further, in the present invention, the amounts of P and S, which are impurity elements, are set to 0.03% and 0.01% or less, respectively. The main reason for this is to further improve the low-temperature toughness of the base material and the HAZ. The reduction of the P content reduces the center segregation of the continuously cast slab, and also prevents the intergranular fracture and improves the low-temperature toughness. Further, the reduction of the amount of S has the effect of reducing MnS to be stretched by hot rolling and improving ductility. Both are preferably as small as possible, but it is necessary to determine the balance between properties and costs. Usually, P and S contain 0.001% or more and 0.0001% or more, respectively.
[0028]
Next, the purpose of adding the selected 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 to expand 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 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 reduces the formation of a hardened structure that is detrimental to low-temperature toughness in the rolled structure, particularly in the central segregation zone of a continuously cast steel slab. However, if the Ni content is more than 1%, not only economic efficiency but also HAZ toughness and on-site weldability are deteriorated, so the upper limit is set to 1%. The addition of Ni is also effective in preventing Cu cracking during continuous casting and hot rolling. In this case, Ni needs to be added at least 1/3 of the Cu amount.
[0030]
The reason for adding Mo is to improve the hardenability of steel and obtain high strength, and it is preferable to add 0.1% or more. Further, Mo coexists with Nb to suppress recrystallization of austenite during controlled rolling, and is also effective in refining the austenite structure. However, the addition of excess Mo exceeding 0.6% deteriorates HAZ toughness and on-site weldability, and further makes it difficult to disperse and produce ferrite. Therefore, the upper limit was made 0.6%.
[0031]
Cr is preferably added in an amount of 0.1% or more to increase the strength of the base material and the welded portion. However, if it exceeds 1%, the HAZ toughness and on-site weldability are significantly deteriorated. Therefore, the upper limit of the amount of Cr is set to 1%.
[0032]
Cu is preferably added in an amount of 0.1% or more in order to increase the strength of the base material and the welded portion, but if added in excess of 1%, the HAZ toughness and on-site weldability are significantly deteriorated. Therefore, the upper limit of the amount of Cu is set to 1%.
[0033]
V has almost the same effect as Nb, but the effect is weaker than Nb. It also has the effect of suppressing softening of the weld. From the viewpoint of HAZ toughness and on-site weldability, an upper limit of 0.3% is allowable, but addition of 0.03 to 0.08% is particularly preferable.
[0034]
B is an element that enhances the hardenability of steel by adding a trace amount thereof, 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 sulfide (MnS) morphology and improve low temperature toughness. In order to obtain this effect, it is preferable that Ca is 0.001% or more and REM is 0.002% or more. If the Ca content exceeds 0.01% and the REM exceeds 0.02%, CaO-CaS or REM-CaS is generated in large amounts to form large clusters and large inclusions, which not only impairs the cleanliness of the steel, It also has an adverse effect on local weldability. For this reason, the upper limits of the added amounts of Ca and REM are limited to 0.01% and 0.02%, respectively. In the ultrahigh-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 set to 0.1. 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, suppress grain coarsening of the weld heat affected zone, and improve low temperature toughness. However, if more than 0.006% is added, a coarse oxide is generated and the toughness is deteriorated. Therefore, the upper limit is made 0.006%.
[0037]
Next, a manufacturing method will be described.
[0038]
First, reheating to the austenite region is performed 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 a temperature of 1050 ° C. to 1250 ° C. at which Nb forms a solid solution and crystal grains are not coarsened.
[0039]
After reheating, rough rolling is performed in a recrystallization temperature range, and then finish rolling is performed in a non-recrystallization temperature range of 900 ° C. or less. 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, 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 the finish rolling is 50% or more. This is to ensure the low-temperature toughness required for the line pipe. The upper limit of the cumulative rolling reduction in finish rolling is determined by the ratio of the thickness at the end of recrystallization rolling to the product sheet thickness.
[0040]
After the finish rolling, it is cooled from a temperature of _Ar 3 or more to 300 ° C. or less. This is for obtaining a structure containing a low-temperature transformation generation phase such as upper bainite and having C and Nb as a solid solution. 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 at the time of cooling from the temperature of _Ar 3 or more to 300 ° C. or less is 5 to 40 ° C./sec. This is for obtaining a structure containing a low-temperature transformation generation phase such as upper bainite and having C and Nb as a solid solution.
[0041]
The steel sheet manufactured in this manner is formed into a cylindrical shape by C forming, U forming, and O forming in that order, and the butted portions are joined. The weld 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 center of the wall thickness to 80 to 550 ° C. If the heating temperature is lower than 80 ° C., even with the steel of the present invention, the recovery of the compressive strength reduced by the Bauschinger effect hardly occurs. On the other hand, when heated to a high temperature exceeding 550 ° C., softening occurs, so that the compressive yield strength is rather lowered. 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 particularly recognized even at a low temperature of less than 80 to 150 ° C. On the inner surface side, heating in this temperature range hardly changes, so that it is not necessary to heat.
[0043]
The holding time at the heating temperature may be cooling immediately after reaching the heat treatment temperature at a high temperature, or may be held at 6000 seconds or less at a low temperature. A preferred range is 60 to 1800 seconds.
[0044]
In addition, a method of heating the outer surface by induction heating is effective, but it is also possible to use an oil tank or a salt bath in addition to induction heating.
[0045]
The crushing strength of the UOE steel pipe manufactured in this way needs to be equal to or more than the crushing strength calculated from the compressive strength of the thick plate after hot rolling. Ratio b / a between 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 at least the range from the outer surface to the center of the wall to 80 to 550 ° C. However, that 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]
The outer surface of the steel pipe may be coated and coated for corrosion protection. Submarine line pipes are mainly subjected to plastic coating, but must be performed at a temperature of about 150 to 250 ° C. in order to increase the adhesion strength. Even if heated during this coating, the Bauschinger effect is restored, so that the efficiency is extremely high.
[0047]
【Example】
A steel containing the chemical components shown in Table 1 was melted in a converter to form a continuously cast steel slab and hot rolled under the conditions shown in Table 2. The finish temperatures of the finish rolling were all _{Ar 3} points or more. These steel sheets were subjected to C-forming, U-forming, and O-forming in that order, and seam welding was performed on the ends of the thick steel sheets. Then, in a UOE step of expanding the pipes, steel pipes having the materials, outer diameters, and wall thicknesses shown in Table 2 were obtained. These steel pipes were heated by a high-frequency moving superheating method. The temperature of the outer surface and the inner surface of the steel pipe was measured with a thermocouple. The temperature at the center of the thickness was calculated as the average 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. The temperature of the inner surface was not particularly controlled, but was lower by about 30 ° C. than the outer surface temperature. Therefore, the temperature at the center of the thickness is lower by about 15 ° C. than the outer surface temperature.
[0048]
The steel pipe manufactured in this manner was cut into 5 m, the steel pipe was set in a pressure vessel, and a uniaxial crush test was performed in which a water pressure was applied so that no axial force was generated in the steel pipe. The water pressure was applied, and the pressure at which the water pressure began to drop suddenly was defined as the crushing strength. Table 2 shows the crushing strength a [MPa] of these steel pipes as made, the crushing strength b [MPa] after heat treatment, and the ratio b / a of both.
[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, the crushing strength is improved even when heated to a low temperature of less than 150 ° C., and even at 150 ° C. or more, the steel produced by the production method of the present invention has an increased crushing strength as can be seen from a comparison between Examples 2 and 11. large. In Examples 10 and 12, since the cooling stop temperature was high, the crushing strength was not improved by heating at 140 ° C., and the crushing strength was low. In Example 11, the heating temperature was as high as 300 ° C., but the crushing strength was not improved because the cooling stop temperature was high. In Example 14, the crushing strength was not improved because Nb-Ti was not contained and the chemical components were outside the present invention.
[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 impart higher crushing strength to the steel pipe manufactured in the UOE process, UOE steel pipes with excellent strength can be provided at low cost. It can be used for line pipes for transporting natural gas, crude oil, etc. even in an environment where high crushing strength is required such as deep sea. In addition, the contribution is extremely high.
[Brief description of the drawings]
FIG. 1 is a schematic view of a steel pipe manufacturing process by a UOE process.

Claims

In mass%,
C: 0.03-0.15%,
Si: 0.8% or less,
Mn: 0.3-2.5%,
P: 0.03% or less,
S: 0.01% or less,
Nb: 0.01 to 0.3%,
Ti: 0.005 to 0.03%,
Al: 0.1% or less,
N: 0.001 to 0.01%
After reheating a steel slab containing iron and unavoidable impurities to the austenite region, rough rolling is performed in the recrystallization region, and the cumulative reduction ratio is 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 center of the wall thickness of the steel pipe formed by the UOE process of expanding the pipe after seam welding the ends is heated to 80 to 550 ° C. Method.

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,
The method for producing a UOE steel pipe having high crushing strength according to claim 1, wherein the method comprises one or more of the following.

The crushing strength a [MPa] of the steel pipe formed by the UOE step and the crushing strength b [MPa] of the steel pipe formed by the UOE step after heating at least the range from the outer surface to the center of the wall to 80 to 550 ° C. 3. The method for producing a UOE steel pipe having a high crushing strength according to claim 1, wherein the ratio b / a of the UOE steel pipe is not less than 1.10.

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