JP3896031B2

JP3896031B2 - Manufacturing method of high strength UOE steel pipe

Info

Publication number: JP3896031B2
Application number: JP2002123742A
Authority: JP
Inventors: 裕森本; 茂大北; 秀樹宮崎; 達哉吉田; 邦夫小山; 功一品田
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2002-04-25
Filing date: 2002-04-25
Publication date: 2007-03-22
Anticipated expiration: 2022-04-25
Also published as: JP2003311321A

Description

【０００１】
【発明の属する技術分野】
本発明は、鋼管の製造方法に関し、詳しくは、天然ガスや原油などを長距離輸送するためのラインパイプ等に用いられる引っ張り強度が８００ＭＰａ以上で大径の高強度ＵＯＥ鋼管の製造方法に関するものである。
【０００２】
【従来の技術】
天然ガスや原油などを長距離輸送するためのラインパイプに多く用いられるＵＯＥ又はＵＯ鋼管と呼ばれる大径のシーム溶接鋼管は、一般に、図１（ａ）〜（ｉ）に示すように、（ａ）所定寸法の平鋼板１を用いて、（ｂ）その板幅調整、両幅端部の開先加工２を行い、（ｃ）両幅端部の曲げ加工３を行い、さらに、（ｄ）Ｕ形の成形４、（ｅ）Ｏ形の成形５により管状にプレス成形した後、（ｆ）突合わせ部をガスシ−ルドアーク溶接、レーザー溶接等により仮付け溶接６、（ｇ）サブマージアーク溶接などにより内面シーム溶接７および（ｈ）外面シーム溶接８を行い、その後、（ｉ）鋼管の真円度を高めるためにエキスパンダ−などにより拡管成形９を行う工程で製造される。最近のＵＯＥ鋼管の製造工場は、高生産性確保のために、これらの各工程が１本のライン線上に配置され連続的に加工できるよう工夫されている。
【０００３】
近年、ガス、石油の利用量の拡大、輸送コストの低減等を理由にした高圧輸送および輸送効率拡大のため、ラインパイプの大型化、薄肉・高強度化が進んでいる。このような背景で、現在、ラインパイプ材として引張強度７００ＭＰａ級の高強度ＵＯＥ鋼管の試作が開始され、８００ＭＰａまたは１０００ＭＰａ級の高強度ＵＯＥ鋼管の開発も進められている。
【０００４】
一般に、シーム溶接鋼管では、溶接ＨＡＺおよび溶接金属の引っ張り強度が母材の引っ張り強度と同等以上とし、シーム溶接鋼管の引張試験の際に母材部分で破断することが求められる。しかしながら、引張強度７００ＭＰａ級以上の高強度ＵＯＥ鋼管では、溶接金属の高強度化により溶接ＨＡＺに歪が集中して引張試験の際に溶接ＨＡＺの位置で破断する危険性が高くなる。
【０００５】
更に、引張強度８００ＭＰａ以上の高強度ＵＯＥ鋼管では、製造工程でシーム溶接部に横割れが発生するという問題が新たに発生してきた。
【０００６】
この横割れは、従来から溶接金属の拡散性水素による水素脆化、割れ感受性の増大、引張応力の付加の３つ要因により発生する水素割れであると言われている（例えば、溶接接合便覧：丸善（株）平成２年９月、Ｐ８８５）。
【０００７】
また、ＨＴ８０（引張強さ７８０ＭＰａ以上）の高強度鋼の低温割れに関して溶接金属に発生する横割れの防止が最も困難であるとの報告がなされており（「溶接学会誌」第４６巻（１９７７）第１２号、８７５〜８８０頁）、拡散性水素量と溶接ワイヤの組成から横割れ発生限界を予測する試みが行われている（「溶接学会誌」第４６巻（１９７７）第８号、５６１〜５６６頁）。
【０００８】
しかし、これらの報告では、特定の水素含有量で割れが発生する場合にはシーム溶接時に予熱温度または層間温度を高くすることにより割れ発生が防止できるとの見解を表明するにとどまっている。
【０００９】
このように溶接金属の横割れの対策としてシーム溶接時の予熱処理または後熱処理の実施や溶接材料の拡散性水素量の低減等が挙げられる。
【００１０】
しかし、溶接時の予熱処理または後熱処理は、ＵＯＥ鋼管の製造工程を繁雑化し、生産性の低下や製造コストの増加を招き好ましくない。
【００１１】
また、８００ＭＰａ以上の高強度鋼管では、鋼管引張試験時の母材破断を実現できる溶接金属の靱性を確保するためには、サブマージ溶接に高塩基度のフラックスを用いて溶接金属中の酸素量を低減する必要があり、一般に高塩基度フラックスは拡散性水素量が高いため、溶接材料の拡散性水素量の低減は、溶接金属の靱性確保の点から限界がある。
【００１２】
更に、特開２００１−７１１７６号公報では、溶接金属の化学組成を規定することにより溶接後冷却時のマルテンサイト変態温度を調整し溶接残留応力を緩和することで溶接金属の横割れを防止する方法が提案されている。
【００１３】
しかし、溶接金属のマルテンサイト変態点の低下のための合金元素の増加は、靱性低下や溶接高温割れを助長し、鋼管引張試験時の母材破断を実現するための溶接金属の強度と靱性のバランスおよび溶接性の確保の観点から鋼管の母材および溶接金属の成分設計の制約が大きくなり好ましくない。
【００１４】
従って、引張強度８００ＭＰａ以上の高強度ＵＯＥ鋼管を製造する際に発生する溶接金属の横割れを、生産性の低下や製造コストの増加、さらには母材および溶接金属の成分設計の制約も少なく、かつ、シーム溶接部の靱性などの鋼管の機械特性を維持できる方法が望まれている。
【００１５】
【発明が解決しようとする課題】
上記の従来技術の問題に鑑みて、本発明は、８００ＭＰａ以上の高強度ＵＯＥ鋼管を製造する際に、生産性の低下や製造コストの増加、さらには母材および溶接金属の成分設計の制約も少なく、かつ、シーム溶接部の靱性などの鋼管の機械特性を維持しつつ、高強度ＵＯＥ鋼管の製造時の溶接金属の横割れを防止し、さらには拡管成形時の拡管割れを防止できる高強度ＵＯＥ鋼管の製造方法を提供することを目的とする。
【００１６】
【課題を解決するための手段】
本発明は、溶接金属の横割れの挙動の解析結果から、横割れの発生は、シーム溶接完了から拡管成形開始までの時間と拡散性水素量、拡管成形時の拡管率、溶接金属の強度との関係で整理することができ、これらの関係を規定することにより抑制できるという知見に基になされたものである。
【００１７】
つまり、本発明の要旨とするところは、母材の引張強度が８００ＭＰａ以上、外径が４０６ｍｍ以上、かつ肉厚が１０ｍｍ以上の高強度ＵＯＥ鋼管の製造方法において、鋼板を管状に成形後、突合わせ部を仮付け溶接後、拡散性水素量が２０ｍｌ／１００ｇ以下の溶接フラックスを用い、シ−ム溶接金属の引張強度が１４００ＭＰａ以下、かつ母材の引張強度の０．８倍〜１．４倍の条件で鋼管の内面および外面をサブマージアーク溶接によるシーム溶接を行い、この溶接終了時から３０分以上経過した後に、拡管率が５％以下の条件で拡管成形することを特徴とする高強度ＵＯＥ鋼管の製造方法、である。
【００１８】
【発明の実施の形態】
本発明者らは、まず、８００ＭＰａ以上の高強度ＵＯＥ鋼管を製造する際に、溶接金属の横割れが発生するＵＯＥ鋼管のサイズおよび強度を調査した結果、以下のことがわかった。
【００１９】
（１）高強度ＵＯＥ鋼管のサイズについては、その肉厚が１０ｍｍ以上の場合に溶接金属の横割れが多く発生する。また、その外径が４０６ｍｍ以上の場合には、通常、板厚が１０ｍｍ以上となるため溶接金属の横割れが多く発生する。
【００２０】
なお、８００ＭＰａ以上の高強度ＵＯＥ鋼管では、設備能力によりＵ形およびＯ形のプレス成形または拡管成形が可能な鋼管の肉厚は規制を受け、現状設備能力上、製造可能な鋼管の肉厚の上限は約４０ｍｍである。
【００２１】
（２）ＵＯＥ鋼管の強度については、鋼管母材の引張強度が８００ＭＰａ以上で溶接金属の横割れが多く発生する。
【００２２】
なお、鋼管母材の引張強度が１２００ＭＰａを超えるような超高強度鋼管は、現状の製造工程で製造する場合に、成形性の低下、シ−ム溶接部の溶接ＨＡＺおよび溶接金属の靱性低下および拡管割れの発生などの問題が多くなり、現状の製造可能な鋼管の引張強度の上限は、約１２００ＭＰａである。
【００２３】
以上を踏まえて、本発明では、溶接金属の横割れが多く発生する溶接金属の母材の引張強度が８００ＭＰａ以上、外径が４０６ｍｍ以上、かつ肉厚が１０ｍｍ以上の高強度ＵＯＥ鋼管の製造方法を対象とした。
【００２４】
以下に本発明の高強度ＵＯＥ鋼管の製造方法における製造条件の限定理由について説明する。
【００２５】
本発明者らは、引っ張り強度が９５０ＭＰａの鋼板を管状にプレス成形後、突合せ部を引っ張り強度および拡散性水素量が異なる種々のフラックスとワイヤを用いてサブマージアーク溶接によるシーム溶接を行い、その後、シーム溶接後から拡管開始までの時間および拡管率を変えて拡管成形を行うことにより長さ１０ｍのＵＯＥ鋼管を製造し、溶接金属の横割れ発生状況を調査した。
フラックス中の拡散性水素量は、ＪＩＳＺ３１１８鋼溶接部の水素量測定方法に準じて、２号試験片を使用して当該フラックスを用いてサブマージアーク溶接直後の溶接金属中の拡散性水素量を、測定したものであり、溶着金属１００ｇ中の拡散性水素量である（以下、同様とする）。また、溶接金属の横割れの有無は、シーム溶接後７２時間経過後に溶接ビード表面を超音波探傷試験により全長にわたり調査し、１個でも横割れが発生していた場合は、横割れ有りと判断した（以下、同様とする）。
【００２６】
本発明者らの実験により、本発明が対象とする肉厚が約４０ｍｍ以下のＵＯＥ鋼管の製造条件では、シーム溶接後、約２５分経過すると溶接金属は２００℃から１００℃前後の温度に冷却されることが確認された。また、溶接金属中の拡散性水素は、１００℃を超える温度では拡散されて母材あるいは管外に放出されるが、１００℃前後を境に拡散性水素の拡散速度は急激に低下するため、１００℃以下の温度では溶接金属中の拡散性水素は移動されにくくなることがわかった。
【００２７】
一方、鋼管の拡管成形工程では、鋼管内部から押し広げられるために、母材およびシーム溶接部は鋼管円周方向に伸ばされ、逆に溶接線方向に縮もうとする。この際、シーム溶接部の溶接金属の変形能は母材に比べて変形能が大きいため、溶接金属が過大な塑性変形により収縮する際に母材に拘束される結果、溶接金属の溶接線方向に引張応力が生じることを確認している。
【００２８】
溶接金属の引っ張り強度が高い場合には、溶接金属の割れ感受性が高くなり鋼管の拡管成形後に横割れが多く発生する傾向にあることも確認した。
【００２９】
これらの知見から、本発明者らは、鋼管の拡管成形後に、シーム溶接線を横切って鋼管円周方向に発生する溶接金属の横割の発生は、特に、シーム溶接直後の溶接金属中の拡散性水素量、シーム溶接後から拡管成形開始までの時間に依存する拡散性水素の拡散速度、拡管成形時の拡管率、さらには溶接金属の引っ張り強度の条件に依存するものと考え、更に、これらの条件について詳細な検討を行った。
【００３０】
（フラックス中の拡散性水素量）
図２にシーム溶接後から拡管成形開始までの時間とシーム溶接時に用いるフラックス中の拡散性水素量中の拡散性水素量および溶接金属の横割れ発生状況との関係を示す。なお、シーム溶接得られる溶接金属の引っ張り強度は１０００ＭＰａ、拡管成形時の拡管率は３％で行った。
【００３１】
図２から、シーム溶接後から拡管成形開始までの時間が３０分以上で、かつシーム溶接時に用いるフラックス中の拡散性水素量中の拡散性水素量が２０ｍｌ／１００ｇ以下の条件で、溶接金属の横割れは発生しない。
【００３２】
シーム溶接時に用いるフラックス中の拡散性水素量中の拡散性水素量が２０ｍｌ／１００ｇ以下の場合には、シーム溶接後から拡管成形開始までの時間が３０分で溶接金属中の拡散性水素が十分に拡散されて母材あるいは管外に放出されるため、その時間が３０分以上での拡管成形時に溶接金属中に残存する拡散性水素量は少なく、拡管成形により溶接金属の溶接方向に引張応力が負荷されても水素脆化による横割れが発生しにくくなる。
【００３３】
一方、シーム溶接時に用いるフラックス中の拡散性水素量中の拡散性水素量が２０ｍｌ／１００ｇを超える場合には、シーム溶接後から拡管成形開始までの時間が３０分ではまだ溶接金属中の拡散性水素が十分に拡散されず残存し、かつ、その時間が３０分以上では溶接金属中の拡散性水素の拡散速度が極度に遅くなり拡散されにくくなるため、拡管成形により溶接金属の溶接方向に引張応力が負荷された場合に水素脆化による横割れが発生しやすくなる。このため、シーム溶接後から拡管成形開始までの時間が１２０分の場合でも、拡管成形後に横割れが生じている。
【００３４】
従って、本発明では、サブマージアーク溶接によるシーム溶接時に用いるフラックス中の拡散性水素量を２０ｍｌ／１００ｇ以下に規定する。なお、フラックス中の拡散性水素量は、ＪＩＳＺ３１１８鋼溶接部の水素量測定方法に準じて、２号試験片を使用して当該フラックスを用いてサブマージアーク溶接直後の溶接金属中の拡散性水素量を、測定したものであり、溶着金属１００ｇ中の拡散性水素量である。
【００３５】
（拡管成形時の拡管率）
図３にシーム溶接後から拡管成形開始までの時間と拡管成形時の拡管率および溶接金属の横割れ発生状況との関係を示す。なお、シーム溶接で得られる溶接金属の引っ張り強度は１０００ＭＰａ、シーム溶接に用いたフラックス中の拡散性水素量は１０ｍｌ／１００ｇで行った。
【００３６】
図２から、シーム溶接後から拡管成形開始までの時間が３０分以上で、かつ拡管成形時の拡管率が５％以下の条件で、溶接金属の横割れは発生しない。一方、拡管成形時の拡管率が５％を超える場合には、シーム溶接後から拡管成形開始までの時間にかかわらず過剰な応力集中により横割れが発生し、更に、シーム溶接部のビード形状によっては、拡管成形時に溶接ＨＡＺ（溶接止端部）位置での破断（拡管割れ）が生じる。
【００３７】
拡管成形時の拡管率が５％以下の場合には、拡管成形により溶接金属の溶接方向に負荷させる引張応力が小さいために水素脆化による横割れが発生しにくくなり、シーム溶接後から拡管成形開始までの時間が３０分以上の溶接金属中の拡散性水素が十分に拡散され、残存する拡散性水素量が少ない場合には、水素脆化の抑制との相乗作用により横割れは発生しなくなる。
【００３８】
一方、拡管成形時の拡管率が５％を超える場合には、拡管成形により溶接金属の溶接方向に負荷させる引張応力が大きいために水素脆化による横割れが発生しやすくなり、シーム溶接後から拡管成形開始までの時間が３０分以上の溶接金属中の拡散性水素が十分に拡散され、残存する拡散性水素量が少ない場合でも、溶接金属の横割れが発生しやすくなり、更に、塑性変形が大きすぎると、拡管割れ（溶接ＨＡＺ位置での破断）が発生する危険性が生じる。
【００３９】
従って、本発明では、拡管成形時の拡管率を５％以下に規定する。
【００４０】
なお、拡管成形時の拡管率の下限は、特に規定する必要はないが、母材の引張強度が８００ＭＰａ以上の高強度ＵＯＥ鋼管では、拡管率が過度に小さすぎると、鋼管の真円度が悪化し品質上好ましくないため、鋼管の品質の観点からその下限を０．３％とするのが好ましい。
【００４１】
（溶接金属の引っ張り強度）
図４にシーム溶接後から拡管成形開始までの時間とシーム溶接部の溶接金属の引っ張り強度および溶接金属の横割れ発生状況との関係を示す。なお、シーム溶接に用いたフラックス中の拡散性水素量は１０ｍｌ／１００ｇ、拡管成形時の拡管率は３％で行った。
【００４２】
図２から、シーム溶接後から拡管成形開始までの時間が３０分以上で、かつ溶接金属の引っ張り強度が１４００ＭＰａ以下の条件で、溶接金属の横割れは発生しない。一方、溶接金属の引っ張り強度が１４００ＭＰａを超える場合には、シーム溶接後から拡管成形開始までの時間にかかわらず横割れが発生する。
【００４３】
溶接金属の引っ張り強度が１４００ＭＰａ以下の場合には、溶接金属の高強度化により溶接金属の水素脆化に対する感受性が増加することによる影響は少ないため、シーム溶接後から拡管成形開始までの時間が３０分以上の溶接金属中の拡散性水素が十分に拡散され、残存する拡散性水素量が少ない場合には、水素脆化の抑制との相乗作用により横割れは発生しなくなる。
【００４４】
一方、溶接金属の引っ張り強度が１４００ＭＰａを超える場合には、強化元素の増加による溶接金属の水素脆化に対する感受性が大きくなり、シーム溶接後から拡管成形開始までの時間が３０分以上の溶接金属中の拡散性水素が十分に拡散され、残存する拡散性水素量が少ない場合でも、横割れが発生しやすくなる。従って、本発明では、シーム溶接の溶接金属の引っ張り強度の上限を１４００ＭＰａ以下に規定する。
【００４５】
（溶接金属の母材に対する引っ張り強度比）
本発明では、引張強度８００ＭＰａ以上の高強度ＵＯＥ鋼管を対象とするが、このような高強度ＵＯＥ鋼管では、溶接金属の横割れの発生と共に、拡管成形時に溶接ＨＡＺまたは溶接金属の位置で破断する拡管割れの発生が問題となる。
【００４６】
本発明では、溶接金属の横割れの発生を抑制すると共に、拡管割れの発生を防止するために、上述した溶接金属の引っ張り強度の上限を１４００ＭＰａ以下に規定すると共に、その引張強度を母材の引っ張り強度の０．８倍〜１．４倍に規定する。
【００４７】
溶接金属の引っ張り強度が母材の引っ張り強度の１．４倍を超えると、拡管時にシーム溶接部の溶接ＨＡＺ部に過剰に塑性歪が集中し拡管割れが発生しやすくなる。一方、溶接金属の引っ張り強度が母材引っ張り強度の０．８倍より低くなると、シーム溶接部の引っ張り強度が母材に比較して低すぎるため、拡管成形時にシーム溶接部の溶接金属位置で破断しやすくなる。従って、溶接金属の引張強度を母材の引っ張り強度の０．８倍〜１．４倍に規定する。
【００４８】
本発明では、上述した製造条件を満足させることにより、母材の引張強度が８００ＭＰａ以上、外径が４０６ｍｍ以上、かつ肉厚が１０ｍｍ以上の高強度ＵＯＥ鋼管の製造方法において、溶接金属の横割れ、および、拡管割れの発生を防止し、優れた品質の高強度ＵＯＥ鋼管を製造することができる。
【００４９】
本発明の高強度ＵＯ鋼管は、通常のＵＯ鋼管の製造工程を用いて製造することができる。
【００５０】
鋼板を管状にプレス成形した後、突合せ部のシーム溶接に先立って行われる仮付け溶接は、多様されているガスシールドアーク溶接でも、最近実用されつつあるレーザーなどのビーム溶接でも良い。
【００５１】
シーム溶接は、Ｘ開先に加工した突合せ部を、鋼管の内面側および外面側からそれぞれ１層の溶接を行うことで行われる。このシーム溶接は、溶接能率が高く、開先形状の許容範囲が広く、更に、仮付け溶接金属の再溶融が可能である、サブマージアーク溶接が好ましく、溶接能率向上の観点から多電極を用い、１電極目は溶込みの安定性からワイヤ＋極の直流電極が好ましい。電流、電圧条件は、内面溶接は仮付け溶接を突き抜けない条件で、外面溶接は完全に仮付け溶接を再溶融する条件が好ましい。また、溶接速度は板厚にもよるが、板厚２０ｍｍで１．５ｍ／ｍｉｎ程度であり、当然早いほど好ましい。
【００５２】
サブマージアーク溶接に用いられるフラックスは、溶接金属中の酸素濃度を低減し溶接金属の靱性を向上することができる中塩基度また高塩基度のフラックスが好ましい。
【００５３】
また、本発明の対象である引張強度が８００ＭＰａ以上の鋼管の母材成分は、特に規定する必要はなく、例えば、以下に示すＣ：０．０２〜０．１２％、Ｓｉ：０．３５％以下、Ｍn：０．５〜２．０％、Ｎｉ：０．０２〜４％、ＣｒおよぶＭｏの何れか１種または２種の合計量：０．１〜４％を含有する鋼で良い。
なお、ここで％は質量％を示す（以下、同様である）。
【００５４】
Ｃは、鋼中に添加することにより低コストで強度を向上することができ、多いほどコスト低減できるが、あまり多いとシーム溶接した際の溶接ＨＡＺ部に島状マルテンサイトと呼ばれる硬組織が生成し、靱性が低下するため好ましくなく、一方、少なすぎると焼入れ性不足で、強度、靱性の確保が難しい。そのため、Ｃ含有量は０．０２〜０．１２％にするのが好ましい。
【００５５】
Ｓｉは、溶接ＨＡＺに島状マルテンサイトを形成しやすい成分であり、あまり添加量が高いと溶接ＨＡＺ部の靱性低下を引起こすため、その含有量は、０．３５％以下とするのが好ましい。
【００５６】
Ｍｎは、焼入れ性を高め強度、靱性を確保するための成分であり、少ないと強度、靱性の確保が難しい。しかし、含有量が２．０％を超えると造塊割れの原因になる。そにため、Ｍｎ含有量は０．５〜２．０％にするのが好ましい。
【００５７】
ＮｉもＭｎと同様に、強度、靱性、特に靱性を確保するための成分であり、０．０２％以上の添加が好ましい。しかし、あまり高いと高価な元素のため、経済的でなくなる。そのため、Ｎｉ含有量は０．０２〜４％にするのが好ましい。
【００５８】
Ｃｒ、Ｍｏは何れも強度確保のための元素で、少ないと強度確保ができない。また、あまり高いと熱影響部が硬くなりすぎ、単に拡管成形のタイミングを調整しただけでは溶接割れを防止できない。そのため、ＣｒおよぶＭｏの何れか１種または２種の合計量を０．１〜４％にするのが好ましい。
【００５９】
Ｐ、Ｓは不可避的に混入する成分であり、靱性確保のため、Ｐは０．０４％以下、Ｓは０．０３％以下に制限するのが好ましい。
【００６０】
更に強度、靱性の向上等のために、Ａｌ、Ｔｉ、Ｎｂ、Ｖ、Ｂ、ＣａおよびＭｇのうちの何れか１種または２種以上を合計量で１％以下添加しても良い。
【００６１】
また、鋼管のシーム溶接部の溶接金属の成分も特に規制しないが、共金系のワイヤおよびフラックスの溶接材料により溶接金属の成分を母材成分とほぼ同じ成分系とするのが好ましい。しかし、ラインパイプの多くは溶接ままで使用され、母材のように圧延による組織制御により強度、靱性の確保等の操作ができないため、母材の機械的特性とのバランスを考慮して添加成分の含有量を調整するのが好ましい。
【００６２】
【実施例】
次に、実施例に基づき本発明を更に具体的に説明する。
【００６３】
表１に鋼管製造時に用いた鋼板の板厚、強度および化学成分を、表２にシーム溶接に用いたフラックスおよび溶接条件を、表３にシーム溶接に用いた溶接ワイヤの化学成分をそれぞれ示す。表４に表１〜３の鋼板、シーム溶接条件の何れかの条件を組み合わせて、シーム溶接および拡管条件が本発明範囲内（本発明例）および範囲外（比較例）でＵＯＥ鋼管を製造後、溶接金属の横割れおよび拡管割れの発生状況を示す。なお、表２に示した高塩基度−溶融型フラックスは、ＳｉＯ₂：１０％、Ａｌ₂Ｏ₃：２５％、ＣａＯ：１５％、ＣａＦ₂：３５％、その他成分：１５％からなる、粒度：８０以下のメシュのものを用いた。また、高塩基度−焼成型フラックスの基本成分系は溶融型フラックスと同じであり、これに合金材として、Ｆｅ、Ｓｉ、Ｍｎ、Ｎｉを合計量で５％程度添加し、水ガラスで顆粒状にして４８０℃で焼成したものを用いた。
【００６４】
また、表４におけるフラックス中の拡散性水素量の調整は、表２に示す密封保管された各フラックスをそのまま使用する他、密封保管された各フラックスを４日ほど開封放置して拡散性水素量を高くしたものを使用することで行った。
【００６５】
なお、表４におけるフラックス中の拡散性水素量は、ＪＩＳＺ３１１８鋼溶接部の水素量測定方法に準じて２号試験片を使用し、交流６２５Ａ、電圧３０Ｖ、溶接速度６０ｃｍ／ｍｉｎ、ワイヤ付出し３０ｍｍで溶接した後、拡散性水素量の測定はガスクロ法で行った。この際のワイヤは表３に示すＷＣ：径４．８ｍｍを使用し、フラックスは溶接試験に使用したと同じ条件のものを使用した。
【００６６】
溶接ワイヤは表３に示すＷＡ，ＷＢ，ＷＣの３種類を母材強度により、組み合わせを変えて使用した。
【００６７】
溶接金属の横割れ観察は、目視、ＰＴおよびＲＴ（ＪＩＳＺ３１０４高溶接部の放射線透過試験方法および透過写真の等級分類方法）でシーム溶接後７２時間経過した後に行った。
【００６８】
管の製造は８〜１０ｍ長さの鋼板を、まず管サイズに必要な幅に切断した後、端部をシ−ム溶接部がＸ開先になるように開先加工し、この後、Ｕプレス、Ｏプレスを行い、筒状に加工した。その後、仮付け溶接として、５７０ＭＰａ級ワイヤを使用して入熱６．５ｋＪ／ｃｍの炭酸ガス溶接により鋼管の外側から全線溶接した。この後、表３に示すワイヤを使用して、表２に示す条件で鋼管の内面、その後、外面からそれぞれ同一条件で１パスのサブマージアーク溶接によりシーム溶接を行った。
【００６９】
シーム溶接後の拡管成形は、内面から油圧で押し拡げる装置を使用して行い、拡管前後の管外周長さを巻き尺を使用して測定し、下記（１）式より拡管率を求めた。

【００７０】
実施番号１の発明例は、板厚１６ｍｍ、強度１０５０ＭＰａで外径９３０ｍｍの鋼管を、拡散性水素量４ｍｌ／１００ｇの高塩基度の溶融型フラックスを使用して３電極潜弧溶接し、４０分後に拡管率２％の拡管成形した。このときの溶接金属強度は１０７７ＭＰａ、溶接金属強度／母材強度は１．０３である。この場合、本発明の条件を満たしているため、溶接金属の横割れおよび拡管割れのない健全なＵＯＥ鋼管が製造できた。
【００７１】
実施番号２の発明例は、実施番号１の発明例と同じ成分で板厚２５ｍｍ、強度１０１０ＭＰａ、外径４５７ｍｍの鋼管を、拡散性水素量１８ｍｌ／１００ｇの高塩基度の溶融型フラックスを使用して３電極潜弧溶接し、４０分後に拡管率５％の拡管成形した。このときの溶接金属強度は１１４５ＭＰａ、溶接金属強度／母材強度は１．３５と溶接金属の強度は高い。しかし、拡管時期が３０分以降で、拡散性水素量、拡管率および溶接金属の強度も本発明の範囲内のため溶接金属の横割れおよび拡管割れのない健全なＵＯＥ鋼管が得られている。
【００７２】
実施番号３の発明例は、板厚１４ｍｍ、強度１１９０ＭＰａ、外径７６２ｍｍの鋼管を、拡散性水素量１０ｍｌ／１００ｇの高塩基度の溶融型フラックスを使用して３電極潜弧溶接し、３５分後に拡管率１％の拡管成形した。このときの溶接金属強度は１０３０ＭＰａ、溶接金属強度／母材強度０．８７である。この場合も、本発明の条件範囲内のため、溶接金属の横割れおよび拡管割れのない健全なＵＯＥ鋼管が得られている。
【００７３】
実施番号４の本発明例は、板厚３２ｍｍ、強度８３０ＭＰａ、外径９３０ｍｍの鋼管を、拡散性水素量５ｍｌ／１００ｇの高塩基度の焼結型フラックスを使用して４電極潜弧溶接し、３５分後に拡管率３％の拡管成形した。このときの溶接金属強度は８０５ＭＰａ、溶接金属強度／母材強度０．９５であったこの場合も、本発明の条件を満たしており、溶接金属の横割れおよび拡管割れのない、健全なＵＯＥ鋼管が得られている。
【００７４】
実施番号５の比較例は、実施番号１の発明例と同じ材質および寸法の鋼管を同じ溶接条件でシーム溶接したが、シ−ム溶接後２０分で拡管成形したため、溶接金属に横割れが発生した。
【００７５】
実施番号６の比較例は、実施例２の発明例と同じ材質および寸法の鋼管を同じ溶接条件でシ−ム溶接したが、シ−ム溶接後２５分で拡管成形したため、溶接金属に横割れが発生した。
【００７６】
実施番号７の比較例は、実施例２の発明例と同じ成分で板厚が２５ｍの鋼管で、高塩基度の溶融型フラックスで、３電極潜弧溶接をした。溶接金属中の拡散性水素量は１５ｍｌ／１００ｇである。また、シ−ム溶接後２０分で拡管率５％の拡管成形をした。これは溶接金属の強度が低いため割れは認められなかったが、溶接金属の強度が低く、溶接金属位置で拡管割れが発生した。
【００７７】
実施番号８の比較例は、板厚１４ｍｍで、強度１１９０ＭＰａ、外径７６２ｍｍの鋼管を、拡散性水素量２２ｍｌ／１００ｇの高塩基度の溶融型フラックスを使用して３電極で、溶接金属の強度１３４０ＭＰａの条件でサブマージアーク溶接した。拡管時期は４０分後で拡管率も本発明の範囲内であるが、このとき使用したフラックスが４日ほど開封放置したもので、拡散性水素量が過剰のため溶接金属に横割れが発生している。
【００７８】
実施番号９の比較例は、実施番号４の発明例と同じ材料で板厚３２ｍｍ、強度９３０ＭＰａで外径９３０ｍｍの鋼管を、高塩基度の焼結型フラックスを使用して４電極潜弧溶接し、溶接金属強度は８９０ＭＰａで、４０分後に拡管率２％の拡管成形した。拡管時期、溶接金属の強度および拡管率は本発明の条件を満たしている。また、溶接金属の強度も低く横割れに対しては有利であるが、このとき使用したフラックスが４日ほど開封放置したもので、拡散性水素量が２４ｍｌ／１００ｇと高く、そのため、溶接金属に横割れが認められた。
【００７９】
実施番号１０の比較例は、板厚１６ｍｍ、強度１０５０ＭＰａで外径９３０ｍｍの鋼管を、拡散性水素量５ｍｌ／１００ｇの高塩基度の溶融型フラックスを使用して３電極で、溶接金属強度１５００ＭＰａで溶接金属強度／母材強度１．４２の条件で潜弧溶接し、３５分後に拡管率５％の拡管成形した。拡管時期や拡散性水素量は本発明の条件範囲にはいっているが、溶接金属強度が過剰で溶接金属強度／母材強度も１．４２と高く溶接金属に横割れが発生した他、拡管成形時に溶接ＨＡＺ位置で拡管割れが発生した。
【００８０】
実施番号１１の比較例は、実施番号１の発明例と同じ条件の板厚１６ｍｍ、強度１０５０ＭＰａで外径９３０ｍｍの鋼管を、拡散性水素量４ｍｌ／１００ｇの高塩基度の溶融型フラックスを使用して３電極で、溶接金属強度は１０７９ＭＰａ条件で潜弧溶接し、４５分後に拡管率７％の拡管成形した。拡管率が過剰のため、溶接金属に微小な横割れが確認された他、拡管成形時に溶接ＨＡＺ位置で拡管割れが発生した。
【００８１】
【表１】

【００８２】
【表２】

【００８３】
【表３】

【００８４】
【表４】

【００８５】
【発明の効果】
本発明により、天然ガスや原油などを長距離輸送するためのラインパイプ等で用いられる引っ張り強度が８００ＭＰａ以上の大径の高強度ＵＯＥ鋼管の製造方法において、生産性の低下や製造コストの増加、さらには母材および溶接金属の成分設計の制約も少なく、かつ、シーム溶接部の靱性などの鋼管の機械特性を維持しつつ、シーム部の溶接金属の横割れ、さらには拡管成形時の拡管割れを防止することができる。
【００８６】
これにより、ガス、石油の利用量の拡大、輸送コストの低減等を理由にした高圧輸送および輸送効率拡大のためのラインパイプの大型化、薄肉・高強度化のニーズに応えられる引っ張り強度が８００N／ｍｍ²以上の高強度ＵＯＥ鋼管を高品質および高生産で製造することが期待できる。
【図面の簡単な説明】
【図１】一般的なＵＯＥ鋼管の製造工程（ａ）〜（ｉ）を示す図である。
【図２】シーム溶接後から拡管成形開始までの時間とシーム溶接時に用いるフラックス中の拡散性水素量および溶接金属の横割れ発生状況との関係を示す図である。
【図３】シーム溶接後から拡管成形開始までの時間と拡管成形時の拡管率および溶接金属の横割れ発生状況との関係を示す図である。
【図４】シーム溶接後から拡管成形開始までの時間とシーム溶接部の溶接金属の引っ張り強度および溶接金属の横割れ発生状況との関係を示す図である。
【符号の説明】
１鋼板
２板幅調整、開先加工
３端部曲げ加工
４Ｕ成形
５Ｏ成形
６仮付け溶接
７内面シーム溶接
８外面シーム溶接
９拡管成形[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method of manufacturing a steel pipe, and more particularly to a method of manufacturing a high-strength UOE steel pipe having a large tensile strength of 800 MPa or more and used for a line pipe for transporting natural gas or crude oil over a long distance. is there.
[0002]
[Prior art]
A large-diameter seam welded steel pipe called UOE or UO steel pipe, which is often used for line pipes for transporting natural gas or crude oil over a long distance, generally has (a) to (i) as shown in FIGS. ) Using a flat steel plate 1 having a predetermined size, (b) adjusting the plate width, performing groove processing 2 on both width end portions, (c) performing bending processing 3 on both width end portions, and (d) After U-shaped molding 4, (e) O-shaped molding 5 is press-formed into a tubular shape, (f) Butt joint 6 is tack welded 6 by gas shielded arc welding, laser welding, etc. (g) Submerged arc welding, etc. The inner surface seam welding 7 and (h) the outer surface seam welding 8 are performed by the following, and then (i) the tube expansion molding 9 is performed by an expander or the like in order to increase the roundness of the steel pipe. Recent manufacturing plants for UOE steel pipes have been devised so that these processes can be arranged on a single line and continuously processed in order to ensure high productivity.
[0003]
In recent years, line pipes have become larger, thinner and stronger in order to increase high-pressure transportation and transportation efficiency due to the expansion of gas and oil usage and transportation cost. Against this background, a trial production of a high strength UOE steel pipe having a tensile strength of 700 MPa class has been started as a line pipe material, and development of a high strength UOE steel pipe having an 800 MPa level or a 1000 MPa class has been promoted.
[0004]
Generally, in a seam welded steel pipe, it is required that the tensile strength of the welded HAZ and the weld metal is equal to or higher than the tensile strength of the base metal, and that the base metal part breaks during the tensile test of the seam welded steel pipe. However, in a high-strength UOE steel pipe having a tensile strength of 700 MPa or higher, the strain is concentrated on the weld HAZ due to the strengthening of the weld metal, and there is a high risk of fracture at the position of the weld HAZ during the tensile test.
[0005]
Furthermore, in a high-strength UOE steel pipe having a tensile strength of 800 MPa or more, there has been a new problem that a transverse crack is generated in the seam welded part in the manufacturing process.
[0006]
This transverse crack is conventionally said to be a hydrogen crack that occurs due to three factors: hydrogen embrittlement due to diffusible hydrogen in the weld metal, increased crack susceptibility, and the addition of tensile stress (for example, welding joint manual: Maruzen Co., Ltd. September 1990, P885).
[0007]
In addition, it has been reported that it is most difficult to prevent transverse cracks occurring in weld metal with respect to low temperature cracking of high strength steel of HT80 (tensile strength of 780 MPa or more) (Journal of the Japan Welding Society, Vol. 46 (1977)). ) No. 12, pages 875-880), an attempt has been made to predict the occurrence limit of transverse cracks from the amount of diffusible hydrogen and the composition of the welding wire (Journal of the Japan Welding Society, Vol. 46 (1977) No. 8, 561-566).
[0008]
However, these reports only express the opinion that if cracking occurs at a specific hydrogen content, cracking can be prevented by increasing the preheating temperature or interlayer temperature during seam welding.
[0009]
As described above, measures for transverse cracking of the weld metal include pre-heat treatment or post-heat treatment during seam welding and reduction of the amount of diffusible hydrogen in the welding material.
[0010]
However, pre-heat treatment or post-heat treatment at the time of welding is not preferable because it complicates the manufacturing process of the UOE steel pipe, resulting in a decrease in productivity and an increase in manufacturing cost.
[0011]
In addition, in high strength steel pipes of 800 MPa or more, in order to ensure the toughness of the weld metal that can realize the fracture of the base metal during the steel pipe tensile test, the oxygen content in the weld metal is reduced by using a high basicity flux for submerged welding. In general, a high basicity flux has a high amount of diffusible hydrogen, and thus there is a limit to reducing the amount of diffusible hydrogen in the welding material from the viewpoint of securing the toughness of the weld metal.
[0012]
Furthermore, in Japanese Patent Application Laid-Open No. 2001-71176, a method of preventing transverse cracking of the weld metal by adjusting the martensite transformation temperature during post-weld cooling by relaxing the welding residual stress by defining the chemical composition of the weld metal. Has been proposed.
[0013]
However, the increase in alloying elements to lower the martensitic transformation point of weld metal promotes toughness reduction and weld hot cracking, and the strength and toughness of weld metal to achieve base metal fracture during steel pipe tensile tests. From the viewpoint of ensuring balance and weldability, the restrictions on the component design of the base material of the steel pipe and the weld metal increase, which is not preferable.
[0014]
Therefore, transverse cracks in the weld metal that occur when producing a high-strength UOE steel pipe with a tensile strength of 800 MPa or more, less productivity, increased production costs, and further, there are few constraints on the base material and weld metal component design, And the method of maintaining the mechanical characteristics of steel pipes, such as the toughness of a seam weld part, is desired.
[0015]
[Problems to be solved by the invention]
In view of the above-mentioned problems of the prior art, the present invention has a decrease in productivity, an increase in manufacturing cost, and restrictions on the component design of the base material and the weld metal when manufacturing a high strength UOE steel pipe of 800 MPa or more. High strength that can prevent transverse cracking of weld metal during the manufacture of high-strength UOE steel pipes, and also prevent expansion cracks during pipe forming while maintaining the mechanical properties of steel pipes such as toughness of seam welds. It aims at providing the manufacturing method of a UOE steel pipe.
[0016]
[Means for Solving the Problems]
From the analysis results of the behavior of transverse cracks in the weld metal, the present invention indicates that the occurrence of transverse cracks is the time from the completion of seam welding to the start of tube expansion forming, the amount of diffusible hydrogen, the tube expansion rate during tube expansion forming, the strength of the weld metal, It is based on the knowledge that it can be organized by the relationship, and can be suppressed by defining these relationships.
[0017]
That is, the gist of the present invention is that in the method of manufacturing a high strength UOE steel pipe having a tensile strength of the base material of 800 MPa or more, an outer diameter of 406 mm or more, and a wall thickness of 10 mm or more, After tack welding the mating portion, a welding flux having a diffusible hydrogen amount of 20 ml / 100 g or less is used, the tensile strength of the seam weld metal is 1400 MPa or less, and 0.8 times to 1.4 times the tensile strength of the base metal. High strength, characterized by performing seam welding by submerged arc welding on the inner and outer surfaces of a steel pipe under double conditions, and after 30 minutes or more have elapsed from the end of this welding, the pipe expansion rate is 5% or less. It is a manufacturing method of a UOE steel pipe.
[0018]
DETAILED DESCRIPTION OF THE INVENTION
As a result of investigating the size and strength of a UOE steel pipe in which transverse cracking of the weld metal occurs when manufacturing a high strength UOE steel pipe of 800 MPa or more, the present inventors have found the following.
[0019]
(1) Regarding the size of the high-strength UOE steel pipe, when the wall thickness is 10 mm or more, transverse cracks of the weld metal often occur. In addition, when the outer diameter is 406 mm or more, the plate thickness is usually 10 mm or more, so that a lot of transverse cracks in the weld metal occur.
[0020]
For high-strength UOE steel pipes of 800 MPa or more, the thickness of steel pipes that can be U-shaped and O-shaped press-molded or expanded-formed by the equipment capacity is regulated. The upper limit is about 40 mm.
[0021]
(2) Regarding the strength of the UOE steel pipe, the weld metal has many transverse cracks when the tensile strength of the steel pipe base material is 800 MPa or more.
[0022]
It should be noted that an ultra-high-strength steel pipe having a steel pipe base material with a tensile strength exceeding 1200 MPa is reduced in formability, welded HAZ and weld metal toughness in seam welds, and manufactured in the current manufacturing process. Problems such as the occurrence of expanded pipe cracks increase, and the upper limit of the tensile strength of steel pipes that can be produced at present is about 1200 MPa.
[0023]
Based on the above, in the present invention, a method for producing a high-strength UOE steel pipe having a tensile strength of a weld metal base material in which a transverse crack of the weld metal frequently occurs is 800 MPa or more, an outer diameter is 406 mm or more, and a wall thickness is 10 mm or more. Targeted.
[0024]
Below, the reason for limitation of the manufacturing conditions in the manufacturing method of the high intensity | strength UOE steel pipe of this invention is demonstrated.
[0025]
The inventors of the present invention, after press forming a steel plate having a tensile strength of 950 MPa into a tubular shape, performs seam welding by submerged arc welding using various fluxes and wires having different tensile strength and diffusible hydrogen amount at the butt portion, and then A 10 m long UOE steel pipe was manufactured by changing the time from the seam welding to the start of pipe expansion and the pipe expansion ratio, and the occurrence of transverse cracks in the weld metal was investigated.
The amount of diffusible hydrogen in the flux is the amount of diffusible hydrogen in the weld metal immediately after submerged arc welding using the No. 2 test piece according to the method for measuring the amount of hydrogen in JIS Z3118 steel welds. The amount of diffusible hydrogen in 100 g of the deposited metal (hereinafter the same). In addition, the presence or absence of transverse cracks in the weld metal was investigated over the entire length of the weld bead surface by ultrasonic flaw detection 72 hours after seam welding. If even one transverse crack occurred, it was determined that there was a transverse crack. (Hereinafter the same).
[0026]
According to the experiments by the present inventors, under the manufacturing conditions of a UOE steel pipe having a wall thickness of about 40 mm or less, which is the subject of the present invention, the weld metal is cooled from 200 ° C. to around 100 ° C. after about 25 minutes have passed since seam welding. It was confirmed that In addition, diffusible hydrogen in the weld metal is diffused at a temperature exceeding 100 ° C. and released to the outside of the base material or the pipe, but the diffusion rate of diffusible hydrogen rapidly decreases around 100 ° C., It has been found that diffusible hydrogen in the weld metal becomes difficult to move at temperatures below 100 ° C.
[0027]
On the other hand, in the pipe expanding and forming process of the steel pipe, the base material and the seam welded portion are stretched in the circumferential direction of the steel pipe because they are pushed from the inside of the steel pipe, and conversely, they tend to shrink in the weld line direction. At this time, since the deformability of the weld metal in the seam weld zone is larger than that of the base metal, the weld metal is restrained by the base metal when shrinking due to excessive plastic deformation. It has been confirmed that a tensile stress is generated.
[0028]
It was also confirmed that when the tensile strength of the weld metal is high, the cracking sensitivity of the weld metal is high, and there is a tendency that a large number of transverse cracks occur after the steel pipe is expanded.
[0029]
From these findings, the present inventors have found that the occurrence of transverse cracking of the weld metal that occurs in the circumferential direction of the steel pipe across the seam weld line after the pipe expansion forming is particularly diffusive in the weld metal immediately after seam welding. It is considered that it depends on the amount of hydrogen, the diffusion rate of diffusible hydrogen depending on the time from seam welding to the start of tube expansion, the tube expansion rate at the time of tube expansion, and the tensile strength of the weld metal. Detailed examination was conducted on the conditions.
[0030]
(Amount of diffusible hydrogen in flux)
FIG. 2 shows the relationship between the time from seam welding to the start of tube expansion forming, the amount of diffusible hydrogen in the amount of diffusible hydrogen in the flux used during seam welding, and the occurrence of transverse cracks in the weld metal. In addition, the tensile strength of the weld metal obtained by seam welding was 1000 MPa, and the pipe expansion rate at the time of pipe expansion molding was 3%.
[0031]
FIG. 2 shows that the time from seam welding to the start of tube expansion forming is 30 minutes or more and the amount of diffusible hydrogen in the amount of diffusible hydrogen in the flux used during seam welding is 20 ml / 100 g or less. No transverse cracks occur.
[0032]
When the amount of diffusible hydrogen in the diffusible hydrogen in the flux used during seam welding is 20 ml / 100 g or less, the time from seam welding to the start of tube forming is 30 minutes, and diffusible hydrogen in the weld metal is sufficient. The amount of diffusible hydrogen remaining in the weld metal during pipe expansion molding in which the time is 30 minutes or longer is small and the tensile stress in the welding direction of the weld metal is reduced by pipe expansion molding. Even if a load is applied, transverse cracking due to hydrogen embrittlement is less likely to occur.
[0033]
On the other hand, when the amount of diffusible hydrogen in the amount of diffusible hydrogen in the flux used during seam welding exceeds 20 ml / 100 g, the diffusibility in the weld metal is still 30 minutes from the start of seam welding to the start of tube expansion forming. If hydrogen is not sufficiently diffused and remains for more than 30 minutes, the diffusion rate of diffusible hydrogen in the weld metal becomes extremely slow and difficult to diffuse. When stress is applied, lateral cracking due to hydrogen embrittlement tends to occur. For this reason, even when the time from seam welding to the start of tube expansion forming is 120 minutes, transverse cracks occur after tube expansion forming.
[0034]
Therefore, in the present invention, the amount of diffusible hydrogen in the flux used at the time of seam welding by submerged arc welding is regulated to 20 ml / 100 g or less. In addition, the amount of diffusible hydrogen in the flux is determined according to the method for measuring the amount of hydrogen in the JIS Z3118 steel weld zone, using a No. 2 test piece and using the flux, the diffusible hydrogen in the weld metal immediately after submerged arc welding. The amount is measured and is the amount of diffusible hydrogen in 100 g of the deposited metal.
[0035]
(Tube expansion ratio during pipe expansion molding)
FIG. 3 shows the relationship between the time from seam welding to the start of tube expansion forming, the tube expansion ratio during tube expansion forming, and the occurrence of transverse cracks in the weld metal. The tensile strength of the weld metal obtained by seam welding was 1000 MPa, and the amount of diffusible hydrogen in the flux used for seam welding was 10 ml / 100 g.
[0036]
From FIG. 2, transverse cracking of the weld metal does not occur under conditions where the time from seam welding to the start of tube expansion forming is 30 minutes or more and the tube expansion rate during tube expansion forming is 5% or less. On the other hand, if the tube expansion ratio during tube expansion molding exceeds 5%, transverse cracks occur due to excessive stress concentration regardless of the time from seam welding to the start of tube expansion molding, and further, due to the bead shape of the seam welded part. Breaks (expanded crack) at the weld HAZ (welded toe) position during tube expansion forming.
[0037]
When the expansion ratio during pipe expansion is 5% or less, the tensile stress applied in the welding direction of the weld metal by the pipe expansion molding is small, so that horizontal cracking due to hydrogen embrittlement is less likely to occur. When the diffusible hydrogen in the weld metal with a start time of 30 minutes or more is sufficiently diffused and the amount of remaining diffusible hydrogen is small, transverse cracks do not occur due to the synergistic effect of suppressing hydrogen embrittlement. .
[0038]
On the other hand, when the tube expansion ratio at the time of tube expansion exceeds 5%, the tensile stress applied in the welding direction of the weld metal by tube expansion molding is large, so that lateral cracking due to hydrogen embrittlement tends to occur, and after the seam welding Even if the diffusible hydrogen in the weld metal is 30 minutes or longer until the start of pipe expansion forming and the remaining diffusible hydrogen is small, transverse cracking of the weld metal is likely to occur, and plastic deformation occurs. If is too large, there is a risk of tube expansion cracking (breakage at the welded HAZ position).
[0039]
Therefore, in this invention, the pipe expansion rate at the time of pipe expansion molding is prescribed | regulated to 5% or less.
[0040]
The lower limit of the pipe expansion ratio at the time of pipe expansion molding does not need to be specified in particular. However, in a high strength UOE steel pipe whose base material has a tensile strength of 800 MPa or more, if the pipe expansion ratio is too small, the roundness of the steel pipe is reduced. Since it deteriorates and is not preferable in terms of quality, the lower limit is preferably set to 0.3% from the viewpoint of the quality of the steel pipe.
[0041]
(Tensile strength of weld metal)
FIG. 4 shows the relationship between the time from seam welding to the start of tube expansion forming, the tensile strength of the weld metal in the seam weld and the occurrence of transverse cracks in the weld metal. The amount of diffusible hydrogen in the flux used for seam welding was 10 ml / 100 g, and the tube expansion rate during tube expansion was 3%.
[0042]
From FIG. 2, the transverse crack of the weld metal does not occur when the time from seam welding to the start of tube expansion forming is 30 minutes or more and the tensile strength of the weld metal is 1400 MPa or less. On the other hand, when the tensile strength of the weld metal exceeds 1400 MPa, transverse cracks occur regardless of the time from seam welding to the start of tube expansion forming.
[0043]
When the tensile strength of the weld metal is 1400 MPa or less, there is little influence due to the increased sensitivity of the weld metal to hydrogen embrittlement by increasing the strength of the weld metal, so the time from seam welding to the start of pipe expansion forming is 30 When diffusible hydrogen in the weld metal of more than a minute is sufficiently diffused and the amount of remaining diffusible hydrogen is small, transverse cracks do not occur due to synergistic action with suppression of hydrogen embrittlement.
[0044]
On the other hand, when the tensile strength of the weld metal exceeds 1400 MPa, the sensitivity of the weld metal to hydrogen embrittlement due to an increase in the strengthening element increases, and the time from seam welding to the start of tube expansion forming is 30 minutes or longer. Even when the diffusible hydrogen is sufficiently diffused and the amount of remaining diffusible hydrogen is small, transverse cracks are likely to occur. Therefore, in this invention, the upper limit of the tensile strength of the weld metal of seam welding is prescribed | regulated to 1400 Mpa or less.
[0045]
(Tensile strength ratio of weld metal to base material)
In the present invention, a high-strength UOE steel pipe having a tensile strength of 800 MPa or more is targeted. However, in such a high-strength UOE steel pipe, a transverse crack of the weld metal is generated and fracture occurs at the position of the weld HAZ or the weld metal during pipe expansion forming. The occurrence of expansion cracks becomes a problem.
[0046]
In the present invention, in order to suppress the occurrence of transverse cracks in the weld metal and to prevent the occurrence of pipe expansion cracks, the upper limit of the tensile strength of the weld metal described above is specified to be 1400 MPa or less, and the tensile strength of the base metal is determined. It is defined as 0.8 to 1.4 times the tensile strength.
[0047]
When the tensile strength of the weld metal exceeds 1.4 times the tensile strength of the base metal, plastic strain is excessively concentrated in the welded HAZ portion of the seam welded portion at the time of pipe expansion, and a pipe expansion crack is likely to occur. On the other hand, if the tensile strength of the weld metal is lower than 0.8 times the tensile strength of the base metal, the tensile strength of the seam weld is too low compared to the base metal, so it breaks at the weld metal position of the seam weld during pipe expansion forming. It becomes easy to do. Therefore, the tensile strength of the weld metal is defined as 0.8 to 1.4 times the tensile strength of the base metal.
[0048]
In the present invention, by satisfying the manufacturing conditions described above, in the method for manufacturing a high strength UOE steel pipe having a base metal tensile strength of 800 MPa or more, an outer diameter of 406 mm or more, and a wall thickness of 10 mm or more, transverse cracking of the weld metal Further, the occurrence of expansion cracking can be prevented, and a high-strength UOE steel pipe with excellent quality can be manufactured.
[0049]
The high-strength UO steel pipe of the present invention can be manufactured using a normal UO steel pipe manufacturing process.
[0050]
After the steel plate is formed into a tubular shape, the tack welding performed prior to the seam welding of the butt portion may be a variety of gas shielded arc welding or beam welding such as laser that has recently been put into practical use.
[0051]
Seam welding is performed by welding one layer each from the inner surface side and the outer surface side of the steel pipe at the butt portion processed into the X groove. This seam welding has a high welding efficiency, a wide tolerance range of the groove shape, and is capable of remelting the tack weld metal, preferably a submerged arc welding, using multiple electrodes from the viewpoint of improving the welding efficiency, The first electrode is preferably a wire + electrode DC electrode in terms of penetration stability. The current and voltage conditions are preferably such that inner surface welding does not penetrate tack welding and outer surface welding completely remelts tack welding. Further, although the welding speed depends on the plate thickness, it is about 1.5 m / min at a plate thickness of 20 mm.
[0052]
The flux used for submerged arc welding is preferably a medium or high basicity flux that can reduce the oxygen concentration in the weld metal and improve the toughness of the weld metal.
[0053]
Further, the base material component of the steel pipe having a tensile strength of 800 MPa or more, which is the subject of the present invention, does not need to be specified in particular. For example, C: 0.02 to 0.12%, Si: 0.35% shown below Hereinafter, steel containing Mn: 0.5 to 2.0%, Ni: 0.02 to 4%, any one or two of Cr and Mo: 0.1 to 4% may be used.
Here,% indicates mass% (the same applies hereinafter).
[0054]
By adding C to steel, the strength can be improved at low cost, and the more it can be, the lower the cost can be. However, if it is too much, a hard structure called island martensite is generated in the welded HAZ part when seam welding is performed. However, it is not preferable because the toughness is lowered. On the other hand, if the amount is too small, the hardenability is insufficient and it is difficult to ensure the strength and toughness. Therefore, the C content is preferably 0.02 to 0.12%.
[0055]
Si is a component that easily forms island-like martensite in the welded HAZ, and if the addition amount is too high, it causes a reduction in the toughness of the welded HAZ part. Therefore, the content is preferably 0.35% or less. .
[0056]
Mn is a component for increasing hardenability and ensuring strength and toughness. If it is small, it is difficult to ensure strength and toughness. However, if the content exceeds 2.0%, it may cause ingot cracking. Therefore, the Mn content is preferably 0.5 to 2.0%.
[0057]
Ni, like Mn, is a component for securing strength, toughness, particularly toughness, and is preferably added in an amount of 0.02% or more. However, if it is too high, it is not economical because it is an expensive element. Therefore, the Ni content is preferably 0.02 to 4%.
[0058]
Cr and Mo are both elements for securing strength, and if the amount is small, strength cannot be secured. On the other hand, if it is too high, the heat-affected zone becomes too hard, and welding cracks cannot be prevented simply by adjusting the pipe expansion timing. Therefore, the total amount of either one or two of Cr and Mo is preferably 0.1 to 4%.
[0059]
P and S are components inevitably mixed. In order to ensure toughness, it is preferable to limit P to 0.04% or less and S to 0.03% or less.
[0060]
Furthermore, in order to improve the strength and toughness, any one or two or more of Al, Ti, Nb, V, B, Ca and Mg may be added in a total amount of 1% or less.
[0061]
Further, although the component of the weld metal in the seam welded portion of the steel pipe is not particularly restricted, it is preferable that the component of the weld metal is substantially the same as that of the base metal component by using a metal wire and flux welding material. However, many of the line pipes are used as welded, and as the base material cannot be operated to secure strength and toughness by controlling the structure by rolling, the additive component is taken into consideration with the balance with the mechanical properties of the base material. It is preferable to adjust the content of.
[0062]
【Example】
Next, the present invention will be described more specifically based on examples.
[0063]
Table 1 shows the plate thickness, strength, and chemical composition of the steel sheet used during the production of the steel pipe, Table 2 shows the flux and welding conditions used for seam welding, and Table 3 shows the chemical composition of the welding wire used for seam welding. After manufacturing UOE steel pipe with seam welding and pipe expansion conditions within the scope of the present invention (example of the present invention) and out of scope (comparative example) by combining Table 4 with any of the steel sheets and seam welding conditions of Tables 1-3 The occurrence of transverse cracks and pipe expansion cracks in weld metal is shown. The high basicity-melting flux shown in Table 2 is SiO. ₂ : 10%, Al ₂ O _Three : 25%, CaO: 15%, CaF ₂ : 35%, other components: 15%, mesh size: 80 or less mesh. In addition, the basic component system of the high basicity-firing type flux is the same as that of the melt type flux, and the total amount of Fe, Si, Mn and Ni is added to the alloy material as about 5%, and it is granulated with water glass. And baked at 480 ° C.
[0064]
In addition, the amount of diffusible hydrogen in the flux in Table 4 can be adjusted by using each of the sealed and stored fluxes shown in Table 2 as they are, or by leaving each sealed and stored flux unsealed for about 4 days. It was done by using the thing which made high.
[0065]
The amount of diffusible hydrogen in the flux in Table 4 uses a No. 2 test piece in accordance with the method for measuring the amount of hydrogen in the JIS Z3118 steel weld, AC 625 A, voltage 30 V, welding speed 60 cm / min, wire attached After welding at 30 mm, the amount of diffusible hydrogen was measured by the gas chromatography method. In this case, the wire used was WC: diameter 4.8 mm shown in Table 3, and the flux was the same as that used in the welding test.
[0066]
Three types of welding wires WA, WB, and WC shown in Table 3 were used in different combinations depending on the strength of the base material.
[0067]
Observation of transverse cracks in the weld metal was performed after 72 hours had passed after seam welding by visual inspection, PT and RT (radiation transmission test method for JIS Z3104 high weld zone and transmission photographic classification method).
[0068]
The tube was manufactured by first cutting a steel plate having a length of 8 to 10 m into a width necessary for the tube size, and then groove-growing the end so that the seam welded portion becomes an X groove. Pressing and O-pressing were performed to form a cylinder. Thereafter, full wire welding was performed from the outside of the steel pipe by carbon dioxide gas welding with a heat input of 6.5 kJ / cm using a 570 MPa class wire as tack welding. Thereafter, using the wires shown in Table 3, seam welding was performed by one-pass submerged arc welding under the same conditions from the inner surface of the steel pipe and then the outer surface under the conditions shown in Table 2.
[0069]
The pipe expansion after seam welding was performed using a device that expanded hydraulically from the inner surface, the pipe outer circumference before and after pipe expansion was measured using a tape measure, and the pipe expansion rate was determined from the following formula (1).

[0070]
Inventive example No. 1 is a steel pipe having a plate thickness of 16 mm, a strength of 1050 MPa, and an outer diameter of 930 mm, and a three-electrode submerged arc welding using a high basicity molten flux with a diffusible hydrogen content of 4 ml / 100 g. Later, the tube was expanded at a tube expansion rate of 2%. The weld metal strength at this time is 1077 MPa, and the weld metal strength / base material strength is 1.03. In this case, since the conditions of the present invention were satisfied, a sound UOE steel pipe free from transverse cracks and expanded cracks of the weld metal could be produced.
[0071]
Inventive example No. 2 uses a steel pipe with the same components as in inventive example No. 1 and a plate thickness of 25 mm, a strength of 1010 MPa, an outer diameter of 457 mm, and a high basicity molten flux with a diffusible hydrogen content of 18 ml / 100 g. Three electrode submerged arc welding was carried out, and after 40 minutes, the tube was formed at a tube expansion ratio of 5%. At this time, the weld metal strength is 1145 MPa, the weld metal strength / base material strength is 1.35, and the weld metal strength is high. However, since the pipe expansion time is 30 minutes or later, the amount of diffusible hydrogen, the pipe expansion ratio, and the strength of the weld metal are also within the scope of the present invention, so that a sound UOE steel pipe free from transverse cracks and pipe expansion cracks of the weld metal is obtained.
[0072]
Inventive example No. 3 is a 35-minute, three-electrode submerged arc welding of a steel pipe having a plate thickness of 14 mm, a strength of 1190 MPa, and an outer diameter of 762 mm, using a high basicity molten flux with a diffusible hydrogen content of 10 ml / 100 g. Later, tube expansion with a tube expansion ratio of 1% was performed. The weld metal strength at this time is 1030 MPa, and the weld metal strength / base material strength is 0.87. Also in this case, a sound UOE steel pipe free from weld metal transverse cracks and expanded pipe cracks is obtained because it falls within the condition range of the present invention.
[0073]
In the example of the present invention of Run No. 4, a steel tube having a plate thickness of 32 mm, a strength of 830 MPa, and an outer diameter of 930 mm was subjected to 4-electrode subarc welding using a high basicity sintered flux with a diffusible hydrogen content of 5 ml / 100 g. After 35 minutes, the tube was expanded at a tube expansion rate of 3%. The weld metal strength at this time was 805 MPa, and the weld metal strength / base metal strength was 0.95. Also in this case, the conditions of the present invention were satisfied, and a sound UOE steel pipe free from transverse cracks and expanded cracks of the weld metal Is obtained.
[0074]
In the comparative example of execution number 5, a steel pipe of the same material and size as the invention example of execution number 1 was seam welded under the same welding conditions, but because the pipe was expanded 20 minutes after seam welding, transverse cracks occurred in the weld metal. did.
[0075]
In the comparative example of execution number 6, a steel pipe having the same material and dimensions as the invention example of embodiment 2 was seam welded under the same welding conditions. There has occurred.
[0076]
The comparative example of execution number 7 was the same component as the invention example of Example 2, and the steel tube with a plate thickness of 25 m was subjected to the three-electrode subarc welding with a high basicity melting type flux. The amount of diffusible hydrogen in the weld metal is 15 ml / 100 g. Further, 20 minutes after the seam welding, the tube was formed with a tube expansion ratio of 5%. Since the weld metal was low in strength, no cracks were observed, but the weld metal was low in strength, and a pipe expansion crack occurred at the weld metal position.
[0077]
The comparative example of Run No. 8 is a steel pipe with a plate thickness of 14 mm, strength of 1190 MPa, outer diameter of 762 mm, three electrodes using a high basicity molten flux with a diffusible hydrogen content of 22 ml / 100 g, and the strength of the weld metal. Submerged arc welding was performed under the condition of 1340 MPa. The expansion time is 40 minutes later, and the expansion rate is within the scope of the present invention. However, the flux used at this time is left unsealed for about 4 days, and the amount of diffusible hydrogen is excessive, causing transverse cracks in the weld metal. ing.
[0078]
The comparative example of Run No. 9 is the same material as the Invention Example of Run No. 4, but a steel tube having a plate thickness of 32 mm, a strength of 930 MPa and an outer diameter of 930 mm is welded to a 4-electrode subarc using a high basicity sintered flux. The weld metal strength was 890 MPa, and after 40 minutes, the tube was formed with a tube expansion ratio of 2%. The pipe expansion time, the strength of the weld metal, and the pipe expansion rate satisfy the conditions of the present invention. In addition, although the strength of the weld metal is low and advantageous against transverse cracks, the flux used at this time is left open for about 4 days, and the amount of diffusible hydrogen is as high as 24 ml / 100 g. Lateral cracks were observed.
[0079]
The comparative example of execution number 10 is a steel pipe having a plate thickness of 16 mm, a strength of 1050 MPa and an outer diameter of 930 mm, three electrodes using a high basicity molten flux with a diffusible hydrogen content of 5 ml / 100 g, and a weld metal strength of 1500 MPa. Submerged arc welding was performed under the condition of weld metal strength / base material strength of 1.42, and after 35 minutes, the tube was formed at a tube expansion rate of 5%. The pipe expansion time and the amount of diffusible hydrogen are within the conditions of the present invention, but the weld metal strength is excessive and the weld metal strength / base metal strength is as high as 1.42. Sometimes pipe cracking occurred at the weld HAZ position.
[0080]
The comparative example of Run No. 11 uses a steel pipe with a plate thickness of 16 mm, a strength of 1050 MPa and an outer diameter of 930 mm under the same conditions as the Invention Example of Run No. 1, and a high basicity molten flux with a diffusible hydrogen content of 4 ml / 100 g. With three electrodes, the weld metal strength was 1039 MPa under submerged arc welding, and after 45 minutes, tube expansion with a tube expansion rate of 7% was performed. Since the pipe expansion rate was excessive, minute transverse cracks were confirmed in the weld metal, and pipe expansion cracks occurred at the weld HAZ position during pipe expansion molding.
[0081]
[Table 1]

[0082]
[Table 2]

[0083]
[Table 3]

[0084]
[Table 4]

[0085]
【The invention's effect】
According to the present invention, in a method for producing a large-diameter high-strength UOE steel pipe having a tensile strength of 800 MPa or more used in a line pipe or the like for transporting natural gas or crude oil over a long distance, the productivity is reduced and the production cost is increased. Furthermore, there are few restrictions on the component design of the base metal and weld metal, and while maintaining the mechanical properties of the steel pipe such as the toughness of the seam weld, the transverse crack of the weld metal in the seam part, and further the pipe crack at the time of pipe expansion forming Can be prevented.
[0086]
As a result, the tensile strength is 800N to meet the needs for high-pressure transportation and increased line efficiency, thinning and high strength for expanding high-pressure transportation and transportation efficiency due to the expansion of gas and oil usage and transportation costs. / Mm ² It can be expected that the above high-strength UOE steel pipe will be manufactured with high quality and high production.
[Brief description of the drawings]
FIG. 1 is a diagram showing manufacturing steps (a) to (i) of a general UOE steel pipe.
FIG. 2 is a diagram showing the relationship between the time from seam welding to the start of tube expansion forming, the amount of diffusible hydrogen in the flux used during seam welding, and the occurrence of transverse cracks in the weld metal.
FIG. 3 is a diagram showing the relationship between the time from seam welding to the start of tube expansion forming, the tube expansion rate during tube expansion forming, and the occurrence of transverse cracks in the weld metal.
FIG. 4 is a diagram showing the relationship between the time from seam welding to the start of tube expansion forming, the tensile strength of the weld metal in the seam weld, and the occurrence of transverse cracks in the weld metal.
[Explanation of symbols]
1 Steel plate
2 Plate width adjustment, groove processing
3 End bending
4 U molding
5 O molding
6 Tack welding
7 Internal seam welding
8 External seam welding
9 Tube expansion molding

Claims

In a manufacturing method of a high strength UOE steel pipe having a tensile strength of a base metal of 800 MPa or more, an outer diameter of 406 mm or more, and a wall thickness of 10 mm or more, a steel plate is formed into a tubular shape, a butt portion is tack-welded, diffusible hydrogen The inner and outer surfaces of the steel pipe are submerged under the conditions that the welding flux is 20 ml / 100 g or less, the tensile strength of the seam weld metal is 1400 MPa or less, and 0.8 to 1.4 times the tensile strength of the base metal. A method for producing a high-strength UOE steel pipe, characterized in that seam welding by arc welding is performed, and after 30 minutes or more have elapsed from the end of the welding, the pipe expansion rate is 5% or less.