JP3608422B2 - High corrosion resistance steel pipe coil with excellent low cycle fatigue strength and method for producing the same - Google Patents
High corrosion resistance steel pipe coil with excellent low cycle fatigue strength and method for producing the same Download PDFInfo
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Description
【0001】
【発明の属する技術分野】
本発明は、コイル状に巻き取られた鋼管コイルに関し、より詳しくは長さの短い複数本の鋼管を順次突き合わせて円周溶接することで長尺鋼管となし、この長尺鋼管をコイル状に巻き取った鋼管コイルであるにも係わらず、その使用時に伸ばしと巻き戻しを繰り返し行った際にも良好な曲げ戻し強度を有する低サイクル疲労強度に優れた高耐食鋼管コイルとその製造方法に関する。
【0002】
【従来の技術】
コイル状に巻き取られた鋼管コイルは、現地での施工工数を少なくする目的で用いられ、主として油井プラントや化学工業プラントなどの配管材料として使用されている。
【0003】
具体的に説明すると、例えば、化学工業プラントにおいては、各種の装置間を連結する配管部材として用いられ、施工に際しては、その内部に高圧水や腐食性の強い高圧ガスや液体などを注入し、所定の配管形状に伸ばして使用される。この場合、その伸ばし工程は、通常、1回であり、その際、鋼管コイルが局部的に繰り返しの塑性歪みを受けた場合でも高々10回程度の繰り返し曲げであるので、低サイクル疲労強度を特に考慮する必要はない。
【0004】
これに対し、石油や天然ガスを生産する油井プラントに使用される鋼管コイルは、次のようにして用いられている油井管の代替え品として使用される。
【0005】
すなわち、油井管は長さの短い真直な鋼管である。このため、その使用時は複数本の鋼管をねじ継手を用いて機械的に接続して長尺鋼管となし、使用後は解体して別の油井プラントに移送し、上記のねじ継手による機械的接続を行って再使用するという操作を繰り返して使用される。
【0006】
従来、上記の油井プラントに使用される鋼管コイルとしては、複数の帯鋼を順次突き合わせ溶接接合して長尺帯鋼とした後溶接製管された長尺の溶接鋼管を巻き取ったものが主に用いられていた。そして、この場合、複数の帯鋼を順次突き合わせ溶接接合する際、溶接線の配置を工夫することにより、その突き合わせ接合部の信頼性の向上が図られている。
【0007】
しかし、溶接鋼管には鋼管本体の溶接部の信頼性に問題があるので、最近では長さは短いが溶接鋼管よりも信頼性の高い継目無鋼管を順次突き合わせ円周溶接して長尺鋼管にし、この長尺鋼管を巻き取った鋼管コイルが多用されるようになってきた(例えば、Society of Petroleum Engineering、SPE37645(1997)参照)。
【0008】
そして、上記の文献では、より優れた繰り返し曲げ強度を備えた継目無鋼管を素材とする鋼管コイルを得るためには、アーク溶接法、具体的にはTIG溶接法ではなく、拡散接合法で突き合わせ円周溶接を行うのがよいとしている。
【0009】
しかし、上記の文献に示される結果は、素材の継目無鋼管が低合金鋼製の場合の結果でしかなく、油井プラントにおいて用いられる多量のCrを含むステンレス鋼製の継目無鋼管の場合でも同じであるか否かについては何らの記載もない。
【0010】
【発明が解決しようとする課題】
本発明の目的は、ステンレス鋼製の長さの短い継目無鋼管または溶接鋼管を素材とする鋼管コイルを提供することにある。すなわち、その製造時における突き合わせ円周溶接を、拡散接合法に限らずTIG溶接法などのアーク溶接法で行うことが可能であるにもかかわらず、突き合わせ円周溶接部に貫通割れが発生するまでの繰り返し曲げ回数が250回以上という極めて良好な曲げ戻し強度を有する低サイクル疲労強度に優れた高耐食ステンレス鋼管コイルを提供することにある。
【0011】
また、別の目的は、その管内に次に述べる目的を持って挿入されことがある管部材や工具の挿入作業が円滑に行え、しかも管内面に付着した異物の除去も確実に行えるようにした鋼管コイルとその製造方法を提供することにある。
【0012】
すなわち、上記の管部材は、外径が鋼管コイルの内径よりも若干小さい管材で、巻き取られた長尺の鋼管コイルのうちの実際に伸ばすべきコイル部分に高圧水や腐食性の強い高圧ガスや液体などを直接供給するために挿入される。また、上記の工具は、外径が上記の管部材と同様の棒状や管状のもので、例えば油井プラントの場合のように真直に伸ばして使用され、その使用中に管内面に付着した異物を除去するために挿入される。
【0013】
【課題を解決するための手段】
本発明の要旨は、下記(1)と(2)の低サイクル疲労強度に優れた高耐食鋼管コイル、および(3)のその製造方法にある。
【0014】
(1)質量%で、10.5%以上のCrを含むステンレス鋼からなる複数本の鋼管が順次突き合わせ円周溶接され、この長尺鋼管がコイル状に巻き取られた鋼管コイルであって、前記突き合わせ円周溶接部における管軸方向の硬化部の長さWが母材部の管肉厚tの2倍以下であり、この硬化部の硬さと母材部の硬さとの硬度差がビッカース硬度で80以下である低サイクル疲労強度に優れた高耐食鋼管コイル。
【0015】
(2)突き合わせ円周溶接部における内面ビードの高さが0.3mm以下である上記(1)に記載の低サイクル疲労強度に優れた高耐食鋼管コイル。
【0016】
(3)上記の(2)に記載の低サイクル疲労強度に優れた高耐食鋼管コイルの製造方法であって、突き合わせ円周溶接部の溶接金属中に含まれるS、O(酸素)およびAlの量が、質量%で、S:0.005%以下、O:0.008%以下、Al:0.06%以下、かつSとO、OとAlの関係がそれぞれ下記の▲1▼式および▲2▼式を満たすように突き合わせ円周溶接する低サイクル疲労強度に優れた高耐食鋼管コイルの製造方法。
【0017】
0.004≦(O+2×S)≦0.012 ・・・ ▲1▼
O≧0.2×Al ・・・・・・・・・・・・・・ ▲2▼
ここで、▲1▼式および▲2▼式中の元素記号は、溶接金属中に含まれる各元素の含有量(質量%)を意味する。
【0018】
上記の本発明は、下記の知見に基づいて完成させた。すなわち、本発明者らは、ステンレス鋼製の長さの短い継目無鋼管または溶接鋼管を素材とする鋼管コイルの突き合わせ円周溶接部の繰り返し曲げ特性を向上させる方法について種々検討した結果、次のことを知見した。
【0019】
曲げ変形を受ける領域での硬さ分布の変動範囲を所定値以下、具体的には、突き合わせ円周溶接部における管軸方向の硬化部の長さWを母材部の管肉厚tの2倍以下とし、かつこの硬化部の硬さと母材部の硬さとの硬度差をビッカース硬度で80以下にすると、繰り返し曲げ寿命が向上する。
【0020】
すなわち、高耐食ステンレス鋼では、Crを多く添加した鋼を用いる必要があるが、その際の組織はマルテンサイト系、フェライト系、フェライトとオーステナイトの二相系およびオーステナイト系のいずれかになる。そして、マルテンサイト系の場合には、溶接熱サイクルにより焼入れ硬化が生じる。また、フェライト系の場合には、溶接熱によりフェライトの一部がマルテンサイト化して焼入れ硬化が生じたり析出硬化が生じる。さらに、二相系とオーステナイト系の場合には、多層溶接時における溶接熱歪みの重畳により加工硬化が生じる。その結果、いずれの場合も、突き合わせ円周溶接部での硬さの変動が大きなり、これに伴って突き合わせ円周溶接部、具体的には硬化部または硬化部と母材部との境界部分の繰り返し曲げ特性も大きくばらつく。
【0021】
しかし、素材鋼管の化学組成、溶接材料の化学組成、溶接条件、および溶接後における後熱処理の有無(有時はその条件)の如何にかかわらず、上記の硬さ要件を満たす突き合わせ円周溶接部であれば、該部の繰り返し曲げ時における低サイクル疲労強度が高くなることが確認された。
【0022】
また、突き合わせ円周溶接部の内面ビード高さが0.3mm以下でないと、前述した目的を持って管内に挿入される管部材や工具を円滑に挿入することができないだけでなく、母材部の管内面に付着している異物が十分に除去されないことが判明した。
【0023】
上記0.3mm以下の内面ビードは、溶接金属中に含まれるS、O(酸素)およびAlの含有量を、質量%で、それぞれ、0.005%以下、0.008%以下、0.06%以下にし、かつSとO、OとAlの関係がそれぞれ上記の▲1▼式および▲2▼式を満たすように調整する場合に確保される。その理由は次のとおりである。
【0024】
S、AlおよびOの含有量、並びにSとO、OとAlの関係が上記の条件を満たす場合に限って溶融池が安定するほか、十分な溶け込み深さが確保されるためである。
【0025】
【発明の実施の形態】
以下、本発明の鋼管コイルの構成要件とその限定理由につい説明する。
【0026】
素材鋼管の材質と種類:
本発明の鋼管コイルの素材鋼管の材質は、質量%で、10.5%以上のCrを含むステンレス鋼とする。これは、Crの含有量が10.5%未満のステンレス鋼では、炭酸ガスなどの腐食環境下での耐食性が確保できないためである。
【0027】
なお、上記のステンレス鋼とは、Crを除く元素のうち、Feの含有量が50質量%以上のステンレス鋼であり、その組織はマルテンサイト系、フェライト系、オーステナイト系、およびフェライト相とオーステナイト相との二相系のいずれであってもよい。具体的に例示すると、JISやAISIなどの各種の規格に規定される各種のステンレス鋼、およびこれらのステンレス鋼に適量のMo、Cu、W、Ti、Zrなどの合金元素を添加して耐食性を高めた高耐食ステンレス鋼などを挙げることができる。
【0028】
素材鋼管は継目無鋼管が好ましいが、溶接製管後に短く切断された溶接鋼管であってもよい。また、素材鋼管としての溶接鋼管は、母材部と溶接部の少なくとも機械的性質が実質的に均一に調整されたものであることが好ましいが、その溶接線が管軸と平行なものであれば、母材部と溶接部の機械的性質が異なるものであってもよい。これは、溶接線が管軸と平行な溶接部は、低サイクル疲労強度に何らの悪影響も及ぼさないためである。
【0029】
突き合わせ円周溶接部における管軸方向の硬化部長さと硬さ:
本発明の鋼管コイルは、突き合わせ円周溶接部における管軸方向の硬化部の長さWが母材部の管肉厚tの2倍以下{(W/t)≦2}であり、かつこの硬化部の硬さと母材部の硬さとの硬度差△Hがビッカース硬度で80以下(△Hv≦80)でなければならない。
【0030】
その理由は、上記の硬化部の長さWが母材部の管肉厚tの2倍超であったり、硬度差△Hvが80超であると、当該円周溶接部に繰り返し曲げを付与した際、曲げ歪みの分布の不均一が顕著になって母材部との境界付近に塑性歪みが集中し、少ない繰り返し曲げ回数で主として前記の境界付近に肉厚を貫通する割れが生じ、所望の繰り返し曲げ強度が確保できないためである。
【0031】
突き合わせ円周溶接の手段としては、アーク溶接法のほかに拡散接合法を用いてもよく、その際の溶接条件は上記の要件が満たされる条件を適宜選定すればよい。また、材料によっては、上記の要件を満たすために溶接終了後に適当な後熱処理を施してもよい。
【0032】
TIG溶接法を用いる場合の溶接条件の一例を示せば、溶接電流130〜200A、溶接電圧12〜16V、溶接速度15cm/minであり、この条件で溶接すれば、溶接のままで上記の要件を満たす突き合わせ円周溶接部を有する鋼管コイルが得られる。ただし、一部の材料については、上記の条件による溶接のままでは上記の要件を満たさないが、この場合には、例えば650〜750℃に5分間程度加熱保持する後熱処理を施せば、上記の要件を満たす突き合わせ円周溶接部を有する鋼管コイルが得られる。
【0033】
突き合わせ円周溶接部における内面ビードの高さ:
本発明の鋼管コイルは、その突き合わせ円周溶接部の内面ビードの高さが0.3mm以下であることが好ましい。その理由は次のとおりである。すなわち、前述したように、鋼管コイルの管内には、前述した目的を持って管部材や工具が挿入されることがある。しかし、突き合わせ円周溶接部の内面ビードの高さが0.3mm超であると、管部材や工具を円滑に挿入することができなくなるだけでなく、母材部の管内面にある異物を十分に除去できなくなるためである。
【0034】
突き合わせ円周溶接の手段がアーク溶接法の場合、高さが0.3mm以下の内面ビードを得るには、溶接金属中に含まれるS、O(酸素)およびAlの含有量を、質量%で、それぞれ、0.005%以下、0.008%以下、0.06%以下にし、かつSとO、OとAlの関係がそれぞれ上記の▲1▼式および▲2▼式を満たすように調整するのがよい。その理由は次のとおりである。
【0035】
すなわち、良好な内面ビードを形成させ、しかもその高さを低くするためには溶融池の安定化と十分な溶け込み深さの確保が欠かせない。しかし、溶接金属中のS含有量が0.005質量%超、またはO含有量が0.008質量%超であると、溶融池の対流が内向きになって著しく凸型の内面ビードが形成されるようになり、0.3mm以下の内面ビードが形成されなくなる。
【0036】
また、Al含有量が0.06質量%超であると、溶融池内のスラグ量が増加して溶融池が不安定になり、0.3mm以下の内面ビードが形成されなくなる。
【0037】
さらに、SとOの含有量が上記の上限値以下であっても、O値と2倍のS値の合計値が0.012質量%を超えると、溶融池後方部での熱膨張に呼応して生じる溶融池の揺動が十分に抑制されず、高さ0.3mm以下の安定した内面ビードが形成されなくなる。
【0038】
一方、SとOを低減しすぎて上記の合計値を0.004質量%未満にすると、溶接時の溶け込み深さが十分に確保できず、裏面ビードの未形成や融合不良などが生じやすくなり、却って溶接作業性が劣化する。
【0039】
また、Oによる溶け込みと溶融池の安定化効果を十分に発揮させるためには、溶融池でAlと結合せずに溶融池中に溶解しているOの存在が重要であるが、そのO量がAl含有量の1/5未満であると、溶融池中に溶解するO量が不足して十分な溶け込み深さが確保されない。
【0040】
以上のことから、本発明では、突き合わせ円周溶接の手段がアーク溶接法の場合における高さ0.3mm以下の内面ビードを得る必要がある場合には、上記のような条件とするのが望ましい。
【0041】
なお、突き合わせ円周溶接の手段に拡散接合法を用いる場合に、高さ0.3mm以下の内面ビードを形成させるには、素材鋼管の鋼種と加熱温度に応じて接合時における加圧力を適宜制御すればよい。
【0042】
【実施例】
(実施例1)
表1に示す化学組成を有する3種類のステンレス鋼からなり、外径80mm、肉厚6mm、および外径130mm、肉厚12mmで、長さがいずれも12mの継目無鋼管と、表2に示す化学組成を有する8種類の溶接材料を準備した。
【0043】
【表1】
【0044】
【表2】
【0045】
そして、準備した継目無鋼管の管端に、片側30度のV開先を設け、表2に示す溶接材料のうちのF1〜F4のいずれかを用い、管外面側からTIG溶接法にて突き合わせ円周溶接して長さ480mの長尺鋼管にした後、直径10m、幅3mのボビンに巻き取った。
【0046】
その際、突き合わせ円周溶接は、いずれも、溶接電流130A、溶接電圧12V、溶接速度15cm/minの条件で行った。また、これらのうち、一部の鋼管コイルについては、その円周溶接部に、650℃に5分間または670℃に5分間加熱保持する後熱処理を施した。
【0047】
なお、一部の鋼管コイルは、突き合わせ円周溶接に拡散接合法を用い、融点1050℃、厚さ50μmのインサート材(19%Cr−1.5%Si−7%B−残部Ni)を介在させ、加熱温度1200℃、加圧力15MPaの条件で製造した。
【0048】
得られた各鋼管コイルからは、長さ方向の中央部分に円周溶接部を有し、全体形状が円弧状の長さ15mの試験材を採取して下記の繰り返し曲げ試験に供し、試験材の円周溶接部、具体的には硬化部または硬化部と母材部との境界部分に肉厚を貫通する亀裂が発生するまでのサイクル数を調べることにより、その低サイクル疲労強度を調査した。その結果を表3に示した。
【0049】
繰り返し曲げ試験:
採取した試験材の片端を直径10mの円筒部材の表面に固定装着する一方、他方端部を把持して円筒部材の同一円周上の外面に完全に沿うように巻き付け、この状態から真直に伸ばすことを1サイクルとする操作を繰り返す。
【0050】
なお、試験は、管内に、塩素イオン濃度が6質量%、pHが4.5の水溶液と圧力が10MPaの炭酸ガスを封入して行った。
【0051】
【表3】
【0052】
表3に示す結果から明らかなように、本発明で規定する要件を満たす鋼管コイル(試番1〜12)は、サイクル数が250回に達しても、その円周溶接部には肉厚を貫通する亀裂は発生せず、良好な低サイクル疲労強度を有していた。
【0053】
これに対し、本発明で規定する要件を満たさない鋼管コイル(試番13〜17)は、最大でもサイクル数が170回に達し時点で、その円周溶接部に肉厚を貫通する亀裂が発生し、低サイクル疲労強度が劣っていた。
【0054】
(実施例2)
実施例1においてTIG溶接法で製造した鋼管コイルのうち、後熱処理を施さなかった試番1〜11の鋼管コイルと、その溶接条件を溶接電流200A、溶接電圧16V、溶接速度15cm/minにして製造した鋼管コイル、および溶接材料に表2中のG1〜G4を用いる一方、その溶接要条件を溶接電流130A、溶接電圧12V、溶接速度15cm/min、並びに上記と同じ溶接電流200A、溶接電圧16V、溶接速度15cm/minにして製造した鋼管コイルを準備した。
【0055】
そして、各鋼管コイルの溶接金属中のS、OおよびAlの含有量を調べるとともに、円周溶接部から試験片を採取して内面ビードの最大高さを測定し、その結果を表4に示した。
【0056】
【表4】
【0057】
表4に示す結果から明らかなように、溶接金属中のS、OおよびAlの含有量が望ましい条件の上限値以下であり、かつSとO、OとAlの関係がそれぞれ本発明で規定する▲1▼式および▲2▼式を満たす鋼管コイル(試番1〜11)は、いずれも内面ビードの高さが0.3mm以下であった。また、この0.3mm以下の内面ビードは、広い溶接条件で得られた。
【0058】
これに対し、溶接金属中のS、OおよびAlの含有量が望ましい条件の上限値以下であるが、SとO、OとAlの関係が▲1▼式および▲2▼式を満たさない鋼管コイル(試番18〜21)は、内面ビードの高さが、いずれも0.3mmよりも大きかった。したがって、内面ビードの高さを0.3mm以下にする必要がある場合には、溶接金属中に含まれるS、O(酸素)およびAlの含有量を、質量%で、それぞれ、0.005%以下、0.008%以下、0.06%以下にし、かつSとO、OとAlの関係がそれぞれ上記の▲1▼式および▲2▼式を満たすように調整するのがよいことがわかる。
【0059】
【発明の効果】
本発明の鋼管コイルは、円周溶接部の低サイクル疲労強度が優れるので、油井や化学工業プラントの配管部材に用いて有効であり、特に伸ばし時、その管内に腐食性の強いガスや液体が注入され、しかも伸ばしと巻き取りの回数が多い油井用の配管部材に用いて極めて有効ある。
【0060】
また、円周溶接部の内面ビード高さが0.3mm以下の鋼管コイルは、管内への管部材や工具挿入が円滑に行えるほか、管本体の内面に付着している異物を確実に除去することが可能である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a steel pipe coil wound in a coil shape, and more specifically, a plurality of short steel pipes are sequentially butted and circumferentially welded to form a long steel pipe, and the long steel pipe is formed into a coil shape. The present invention relates to a highly corrosion-resistant steel tube coil excellent in low cycle fatigue strength, which has a good bending back strength even when it is repeatedly wound and unwound, even though it is a wound steel tube coil, and a method for producing the same.
[0002]
[Prior art]
The steel pipe coil wound up in a coil shape is used for the purpose of reducing the number of on-site construction man-hours, and is mainly used as piping material for oil well plants and chemical industry plants.
[0003]
Specifically, for example, in a chemical industrial plant, it is used as a piping member that connects various devices, and in the construction, high-pressure water or highly corrosive high-pressure gas or liquid is injected into the inside, It is used by extending to a predetermined piping shape. In this case, the stretching process is usually one time. In this case, even when the steel pipe coil is locally subjected to repeated plastic strain, it is repeatedly bent at most about 10 times. There is no need to consider.
[0004]
On the other hand, steel pipe coils used in oil well plants that produce oil and natural gas are used as substitutes for oil well pipes used as follows.
[0005]
That is, the oil well pipe is a straight steel pipe having a short length. For this reason, during use, a plurality of steel pipes are mechanically connected using threaded joints to form a long steel pipe, and after use, they are disassembled and transferred to another oil well plant. It is used repeatedly by connecting and reusing it.
[0006]
Conventionally, steel pipe coils used in the above-mentioned oil well plants are mainly those obtained by winding a long welded steel pipe that has been welded and piped after a plurality of steel strips are sequentially butt welded and joined to form a long steel strip. It was used for. In this case, when a plurality of steel strips are successively butt welded and joined, the reliability of the butt joint is improved by devising the arrangement of the weld lines.
[0007]
However, since there is a problem with the reliability of the welded part of the steel pipe body, welded steel pipes have recently been welded into a long steel pipe by sequentially butting circumferentially welding seamless steel pipes that are shorter but more reliable than welded steel pipes. Steel pipe coils obtained by winding this long steel pipe have been frequently used (see, for example, Society of Petroleum Engineering, SPE 37645 (1997)).
[0008]
And in said literature, in order to obtain the steel pipe coil which uses the seamless steel pipe with the better repeated bending strength as a raw material, it is not matched by the arc welding method, specifically, the diffusion bonding method, not the TIG welding method. It is recommended to perform circumferential welding.
[0009]
However, the results shown in the above documents are only the results when the raw seamless steel pipe is made of low alloy steel, and the same is true for stainless steel seamless steel pipes containing a large amount of Cr used in oil well plants. There is no description about whether or not.
[0010]
[Problems to be solved by the invention]
An object of the present invention is to provide a steel pipe coil made of stainless steel and made of a short seamless steel pipe or a welded steel pipe. That is, the butt circumferential welding at the time of manufacture is not limited to the diffusion bonding method but can be performed by an arc welding method such as a TIG welding method, but until a through crack occurs in the butt circumferential welded portion. An object of the present invention is to provide a highly corrosion-resistant stainless steel tube coil excellent in low cycle fatigue strength having a very good bending return strength of 250 times or more.
[0011]
Another purpose is to smoothly insert tube members and tools that may be inserted into the pipe for the following purposes, and to reliably remove foreign substances adhering to the inner surface of the pipe. An object of the present invention is to provide a steel pipe coil and a manufacturing method thereof.
[0012]
That is, the above-mentioned pipe member is a pipe material whose outer diameter is slightly smaller than the inner diameter of the steel pipe coil, and high-pressure water or highly corrosive high-pressure gas is applied to the coil part to be actually stretched out of the wound long steel pipe coil. It is inserted to supply liquid or liquid directly. Further, the above tool is a rod or tube having the same outer diameter as that of the above pipe member, and is used by extending straight, as in the case of an oil well plant, for example, to remove foreign matter adhering to the inner surface of the pipe during use. Inserted to remove.
[0013]
[Means for Solving the Problems]
The gist of the present invention resides in the following (1) and (2) high corrosion-resistant steel pipe coils having excellent low cycle fatigue strength, and (3) the production method thereof.
[0014]
(1) A steel pipe coil in which a plurality of steel pipes made of stainless steel containing 10.5% or more of Cr in mass% are sequentially butted and circumferentially welded, and this long steel pipe is wound into a coil shape, The length W of the hardened portion in the tube axis direction in the butt circumference welded portion is not more than twice the tube thickness t of the base material portion, and the hardness difference between the hardness of the hardened portion and the base material portion is Vickers. A highly corrosion-resistant steel tube coil with a low cycle fatigue strength of 80 or less in hardness.
[0015]
(2) The high corrosion-resistant steel tube coil excellent in low cycle fatigue strength according to the above (1), wherein the height of the inner surface bead in the butt circumferential weld is 0.3 mm or less.
[0016]
(3) A method for producing a highly corrosion-resistant steel pipe coil having excellent low cycle fatigue strength as described in (2) above, wherein S, O (oxygen) and Al contained in the weld metal of the butt-circumferential weld The amount is mass%, S: 0.005% or less, O: 0.008% or less, Al: 0.06% or less, and the relationship between S and O, O and Al is the following formula (1) and (2) A method for producing a highly corrosion-resistant steel pipe coil having excellent low-cycle fatigue strength in which butt welding is performed so as to satisfy the formula (2).
[0017]
0.004 ≦ (O + 2 × S) ≦ 0.012 (1)
O ≧ 0.2 × Al ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ▲ 2 ▼
Here, the element symbols in the formulas (1) and (2) mean the content (% by mass) of each element contained in the weld metal.
[0018]
The present invention has been completed based on the following findings. That is, the present inventors have conducted various studies on methods for improving the repeated bending characteristics of a butt circumferential weld of a steel pipe coil made of a stainless steel short-length seamless steel pipe or welded steel pipe. I found out.
[0019]
The variation range of the hardness distribution in the region subjected to the bending deformation is equal to or less than a predetermined value. Specifically, the length W of the hardened portion in the tube axis direction in the butt circumferential welded portion is 2 of the tube thickness t of the base metal portion. When the hardness difference between the hardness of the hardened portion and the hardness of the base material portion is 80 or less in terms of Vickers hardness, the repeated bending life is improved.
[0020]
That is, in the high corrosion resistance stainless steel, it is necessary to use a steel to which a large amount of Cr is added. The structure at that time is any of martensite, ferrite, two-phase ferrite and austenite, and austenite. In the case of the martensite system, quench hardening occurs due to the welding heat cycle. In the case of a ferrite type, a part of the ferrite is martensite by welding heat to cause quenching hardening or precipitation hardening. Furthermore, in the case of a two-phase system and an austenite system, work hardening occurs due to superposition of welding thermal distortion during multi-layer welding. As a result, in any case, the variation in hardness at the butt circumference welded portion is large, and accordingly, the butt circumference welded portion, specifically, the hardened portion or the boundary portion between the hardened portion and the base material portion. The repetitive bending characteristics vary greatly.
[0021]
However, butt-circumferential welds satisfying the above hardness requirements regardless of the chemical composition of the raw steel pipe, the chemical composition of the welding material, the welding conditions, and the presence or absence of post-heat treatment after welding (if any) Then, it was confirmed that the low cycle fatigue strength at the time of repeated bending of the portion is increased.
[0022]
Also, if the inner bead height of the butt circumference welded part is not less than 0.3 mm, not only can the tube member and tool inserted into the pipe with the above-mentioned purpose be smoothly inserted, but also the base material part It was found that the foreign matter adhering to the inner surface of the tube was not sufficiently removed.
[0023]
The inner bead of 0.3 mm or less has a content of S, O (oxygen) and Al contained in the weld metal in mass%, 0.005% or less, 0.008% or less, 0.06, respectively. % And below, and the relationship between S and O, and O and Al is secured when adjusting so as to satisfy the above-mentioned formulas (1) and (2). The reason is as follows.
[0024]
This is because the molten pool is stabilized only when the contents of S, Al and O, and the relationship between S and O and O and Al satisfy the above conditions, and a sufficient penetration depth is secured.
[0025]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the structural requirements of the steel pipe coil of this invention and the reason for the limitation are demonstrated.
[0026]
Material and type of steel pipe:
The material of the material steel pipe of the steel pipe coil of the present invention is stainless steel containing 10.5% or more of Cr in mass%. This is because stainless steel having a Cr content of less than 10.5% cannot secure corrosion resistance in a corrosive environment such as carbon dioxide.
[0027]
In addition, said stainless steel is stainless steel whose content of Fe is 50 mass% or more among elements except Cr, and the structure is martensitic, ferritic, austenitic, and ferrite phase and austenitic phase. Or a two-phase system. Specifically, various stainless steels defined in various standards such as JIS and AISI, and an appropriate amount of alloying elements such as Mo, Cu, W, Ti, and Zr are added to these stainless steels for corrosion resistance. Examples include enhanced high corrosion resistance stainless steel.
[0028]
The material steel pipe is preferably a seamless steel pipe, but may be a welded steel pipe cut short after welding. In addition, a welded steel pipe as a raw steel pipe is preferably one in which at least the mechanical properties of the base metal part and the welded part are adjusted substantially uniformly, but the weld line is parallel to the pipe axis. For example, the base material portion and the welded portion may have different mechanical properties. This is because the weld where the weld line is parallel to the tube axis does not have any adverse effect on the low cycle fatigue strength.
[0029]
Hardened part length and hardness in the tube axis direction at the butt circumference welded part:
In the steel tube coil of the present invention, the length W of the hardened portion in the tube axis direction in the butt circumferential weld is not more than twice the tube thickness t of the base material portion {(W / t) ≦ 2}, and this The hardness difference ΔH between the hardness of the hardened portion and the hardness of the base material portion must be 80 or less (ΔHv ≦ 80) in terms of Vickers hardness.
[0030]
The reason is that if the length W of the hardened part is more than twice the tube thickness t of the base metal part or the hardness difference ΔHv is more than 80, the circumferential welded part is repeatedly bent. In this case, the uneven distribution of the bending strain becomes prominent, and the plastic strain concentrates near the boundary with the base material part, and cracks penetrating the wall thickness mainly occur near the boundary with a small number of repeated bendings. This is because the repeated bending strength cannot be ensured.
[0031]
As a means of butt circumference welding, a diffusion bonding method may be used in addition to the arc welding method, and the welding conditions at that time may be appropriately selected as long as the above requirements are satisfied. Depending on the material, an appropriate post-heat treatment may be performed after the end of welding in order to satisfy the above requirements.
[0032]
An example of welding conditions in the case of using the TIG welding method is a welding current of 130 to 200 A, a welding voltage of 12 to 16 V, and a welding speed of 15 cm / min. A steel pipe coil having a butt circumferential weld to fill is obtained. However, for some materials, the above-mentioned requirements are not satisfied with welding under the above-mentioned conditions. In this case, for example, if a heat treatment is performed by heating and holding at 650 to 750 ° C. for about 5 minutes, A steel pipe coil having a butt circumference weld that meets the requirements is obtained.
[0033]
Inner bead height at butt circumferential weld:
In the steel pipe coil of the present invention, the inner bead height of the butt circumferential weld is preferably 0.3 mm or less. The reason is as follows. That is, as described above, a pipe member or a tool may be inserted into the pipe of the steel pipe coil with the above-described purpose. However, if the height of the inner surface bead of the butt-circumferential weld is greater than 0.3 mm, not only can the tube member and tool not be smoothly inserted, but also foreign matter on the inner surface of the base material portion is sufficiently removed. This is because it cannot be removed.
[0034]
When the means of butt circumference welding is an arc welding method, in order to obtain an inner surface bead having a height of 0.3 mm or less, the contents of S, O (oxygen) and Al contained in the weld metal are expressed in mass%. , 0.005% or less, 0.008% or less, and 0.06% or less, respectively, and the relationship between S and O, O and Al is adjusted to satisfy the above formulas (1) and (2), respectively. It is good to do. The reason is as follows.
[0035]
That is, in order to form a good inner surface bead and reduce the height thereof, it is essential to stabilize the molten pool and ensure a sufficient penetration depth. However, if the S content in the weld metal is more than 0.005% by mass or the O content is more than 0.008% by mass, the convection of the molten pool becomes inward and a remarkably convex inner bead is formed. As a result, an inner bead of 0.3 mm or less is not formed.
[0036]
If the Al content is more than 0.06% by mass, the amount of slag in the molten pool increases, the molten pool becomes unstable, and an inner bead of 0.3 mm or less is not formed.
[0037]
Furthermore, even if the contents of S and O are not more than the above upper limit, if the total value of the O value and the double S value exceeds 0.012% by mass, it responds to the thermal expansion in the rear part of the molten pool. Thus, the fluctuation of the molten pool generated is not sufficiently suppressed, and a stable inner bead having a height of 0.3 mm or less is not formed.
[0038]
On the other hand, if S and O are reduced too much and the above total value is less than 0.004% by mass, the penetration depth during welding cannot be sufficiently secured, and the formation of backside beads or poor fusion tends to occur. On the contrary, welding workability deteriorates.
[0039]
Moreover, in order to fully exhibit the effect of melting by O and the stabilization of the molten pool, the presence of O dissolved in the molten pool without being combined with Al in the molten pool is important. Is less than 1/5 of the Al content, the amount of O dissolved in the molten pool is insufficient, and a sufficient penetration depth is not ensured.
[0040]
From the above, in the present invention, when it is necessary to obtain an inner surface bead having a height of 0.3 mm or less in the case of the arc welding method as the butt circumferential welding means, it is desirable to satisfy the above conditions. .
[0041]
In addition, when the diffusion bonding method is used as a means of butt-circumferential welding, in order to form an inner bead with a height of 0.3 mm or less, the pressurizing force at the time of joining is appropriately controlled according to the steel type and heating temperature of the material steel pipe do it.
[0042]
【Example】
Example 1
A seamless steel pipe made of three types of stainless steel having the chemical composition shown in Table 1 and having an outer diameter of 80 mm, a wall thickness of 6 mm, an outer diameter of 130 mm, a wall thickness of 12 mm, and a length of 12 m. Eight types of welding materials having chemical compositions were prepared.
[0043]
[Table 1]
[0044]
[Table 2]
[0045]
And the V groove of 30 degree | times on one side is provided in the pipe end of the prepared seamless steel pipe, and it butt | matches by the TIG welding method from the pipe outer surface side using either of F1-F4 of the welding materials shown in Table 2. After circumferential welding to form a long steel pipe having a length of 480 m, it was wound around a bobbin having a diameter of 10 m and a width of 3 m.
[0046]
At that time, butt circumferential welding was performed under the conditions of a welding current of 130 A, a welding voltage of 12 V, and a welding speed of 15 cm / min. Among these, some of the steel pipe coils were subjected to a heat treatment after being heated and held at 650 ° C. for 5 minutes or 670 ° C. for 5 minutes.
[0047]
Some steel pipe coils use diffusion bonding for butt-circumferential welding, and insert material (19% Cr-1.5% Si-7% B-balance Ni) with a melting point of 1050 ° C and a thickness of 50 µm And produced under conditions of a heating temperature of 1200 ° C. and a pressure of 15 MPa.
[0048]
From each of the obtained steel pipe coils, a test material having a circumferential welded portion at the center in the length direction and having an arc shape of a total length of 15 m was collected and subjected to the following repeated bending test. The low cycle fatigue strength was investigated by investigating the number of cycles until a crack that penetrates the wall thickness occurred in the circumferential welded part of the steel, specifically, the hardened part or the boundary part between the hardened part and the base metal part. . The results are shown in Table 3.
[0049]
Repeated bending test:
One end of the collected test material is fixedly attached to the surface of a cylindrical member having a diameter of 10 m, while the other end is grasped and wound so as to be completely along the outer surface on the same circumference of the cylindrical member, and straightened from this state. This operation is repeated for one cycle.
[0050]
The test was conducted by enclosing an aqueous solution having a chlorine ion concentration of 6 mass% and a pH of 4.5 and a carbon dioxide gas having a pressure of 10 MPa in a tube.
[0051]
[Table 3]
[0052]
As is apparent from the results shown in Table 3, the steel pipe coils (trial numbers 1 to 12) that satisfy the requirements defined in the present invention have a wall thickness at the circumferential weld even when the number of cycles reaches 250 times. There were no cracks penetrating and good low cycle fatigue strength.
[0053]
On the other hand, in steel pipe coils (trial numbers 13 to 17) that do not satisfy the requirements specified in the present invention, when the number of cycles reaches 170 times at the maximum, cracks that penetrate through the wall thickness occur in the circumferential welds. The low cycle fatigue strength was inferior.
[0054]
(Example 2)
Among the steel pipe coils manufactured by the TIG welding method in Example 1, the steel pipe coils of trial numbers 1 to 11 that were not subjected to post-heat treatment, and the welding conditions were a welding current of 200 A, a welding voltage of 16 V, and a welding speed of 15 cm / min. While the manufactured steel pipe coil and G1 to G4 in Table 2 are used for the welding material, the welding conditions are a welding current of 130A, a welding voltage of 12V, a welding speed of 15cm / min, and a welding current of 200A and a welding voltage of 16V as described above. A steel pipe coil manufactured at a welding speed of 15 cm / min was prepared.
[0055]
And while examining content of S, O, and Al in the weld metal of each steel pipe coil, the test piece was extract | collected from the circumferential weld part, the maximum height of an inner surface bead was measured, and the result is shown in Table 4. It was.
[0056]
[Table 4]
[0057]
As is apparent from the results shown in Table 4, the contents of S, O and Al in the weld metal are below the upper limit of desirable conditions, and the relationship between S and O, and O and Al are defined in the present invention. The steel pipe coils (trial numbers 1 to 11) satisfying the formulas (1) and (2) all had an inner bead height of 0.3 mm or less. Further, the inner bead of 0.3 mm or less was obtained under wide welding conditions.
[0058]
On the other hand, the steel pipe in which the contents of S, O and Al in the weld metal are below the upper limit of desirable conditions, but the relationship between S and O, and O and Al does not satisfy the formulas (1) and (2). Each of the coils (trial numbers 18 to 21) had an inner bead height of greater than 0.3 mm. Therefore, when the height of the inner surface bead needs to be 0.3 mm or less, the contents of S, O (oxygen) and Al contained in the weld metal are 0.005% by mass%, respectively. In the following, it can be seen that it is better to adjust to 0.008% or less, 0.06% or less, and so that the relationship between S and O, and O and Al satisfies the above formulas (1) and (2), respectively. .
[0059]
【The invention's effect】
The steel pipe coil of the present invention has excellent low cycle fatigue strength at the circumferential welded portion, so it is effective for use in piping members for oil wells and chemical industrial plants.Especially when it is stretched, a highly corrosive gas or liquid is present in the pipe. It is very effective when used for piping members for oil wells that are injected and that are frequently stretched and wound up.
[0060]
In addition, steel pipe coils with an inner bead height of 0.3 mm or less at the circumferential welded part can smoothly insert tube members and tools into the pipe, and also reliably remove foreign substances adhering to the inner surface of the pipe body. It is possible.
Claims (3)
0.004≦(O+2×S)≦0.012 ・・・ ▲1▼
O≧0.2×Al ・・・・・・・・・・・・・・ ▲2▼
ここで、▲1▼式および▲2▼式中の元素記号は、溶接金属中に含まれる各元素の含有量(質量%)を意味する。It is a manufacturing method of the high corrosion-resistant steel pipe coil excellent in the low cycle fatigue strength of Claim 2, Comprising: The quantity of S, O (oxygen) and Al contained in the weld metal of a butt | circumferential circumferential weld part is mass%. S: 0.005% or less, O: 0.008% or less, Al: 0.06% or less, and the relationship between S and O and O and Al is the following formulas (1) and (2), respectively. A method for producing a highly corrosion-resistant steel pipe coil excellent in low cycle fatigue strength, characterized by performing butt circumferential welding so as to satisfy.
0.004 ≦ (O + 2 × S) ≦ 0.012 (1)
O ≧ 0.2 × Al ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ▲ 2 ▼
Here, the element symbols in the formulas (1) and (2) mean the content (% by mass) of each element contained in the weld metal.
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