JP4071995B2 - UOE steel pipe manufacturing method with excellent crushing strength - Google Patents
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- JP4071995B2 JP4071995B2 JP2002151008A JP2002151008A JP4071995B2 JP 4071995 B2 JP4071995 B2 JP 4071995B2 JP 2002151008 A JP2002151008 A JP 2002151008A JP 2002151008 A JP2002151008 A JP 2002151008A JP 4071995 B2 JP4071995 B2 JP 4071995B2
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【０００１】
【発明の属する技術分野】
本発明はラインパイプ等に使用される鋼管をＵＯＥ製造法で成形する方法において、圧潰特性を改善するための方法に関する。
【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におよ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、まず鋼管内径が流体輸送量より決定され、続いて肉厚、材質が、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、決定されている。しかし、深海化に伴って水圧が高まり、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となりはじめた。
【０００３】
ところで、鋼管の圧潰強度は油井管におい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方式の製造法による鋼管が使用される。ＵＯＥ方式による鋼管の製造工程は、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の複合により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、低下した圧縮降伏強度を熱処理によって回復させる方法が、特開平９−３５４５号公報及び特開平９−４９０２５号公報に開示されており、また、多くの研究論文に報告されている。これらの方法では、成形により低下した圧縮降伏強度は成形前の板材レベルにまで回復し、ＵＯＥ鋼管の圧潰強度はある程度改善される。
【０００８】
【発明が解決しようとする課題】
しかし、通電加熱や熱処理炉によって鋼管全体を加熱すると、ＵＯＥ方式による製造工程では鋼管の内表面から肉厚中心部（以下、内表面側）で起こる圧縮歪みによる加工硬化も失われることになる。従って、ＵＯＥ鋼管全体を均一に熱処理する圧潰強度の改善方法は、極めて有効であるとは言い難い。
【０００９】
本発明はＵＯＥ鋼管に、内表面側の冷間加工による加工硬化を残存させ、外表面から肉厚中心部（以下、外表面側）のバウシンガー効果を消滅させる熱処理を施すことで圧潰強度を改善することを目的としている。
【００１０】
【課題を解決するための手段】
このような課題を解決するために、本発明者は、まずＵＯＥ鋼管において圧潰強度の板厚方向による変化を明らかにするために、詳細な検討を行った。その結果、外表面側の圧縮降伏強度は成形前の鋼板よりも低下し、内表面側では上昇していることを見出した。この知見から、外表面が高温で内表面を低温とする熱処理によって、外表面側の圧縮降伏強度を鋼板と同等まで向上させ、かつ、内表面側の圧縮降伏強度を維持し、軸方向の強度の低下も抑制する熱処理方法について詳細に検討した。その結果、最適な熱処理条件を見出し、圧潰強度に優れたＵＯＥ鋼管の製造に成功した。
すなわち、本発明の要旨とするところは、以下の通りである。
（１） 鋼板を順にＣ成形、Ｕ成形、Ｏ成形し、鋼板の端部同士をシーム溶接後、拡管するＵＯＥ鋼管の製造方法において、外表面の温度Ｔ1及び外表面の加熱時間ｔ1が（１）式を満足し、かつ内表面の温度Ｔ2及び内表面の加熱時間ｔ2が（２）式を満足するように熱処理を施すことを特徴とする圧潰強度に優れたＵＯＥ鋼管の製造方法。
Ｔ1−５０３≧−３７．６×lnｔ1 ・・・（１）
ここでＴ1：外表面の加熱温度（℃）
ｔ1：外表面の加熱時間（ｓ）
Ｔ2−４０７＜−３１．３×lnｔ2 ・・・（２）
ここでＴ2：内表面の加熱温度（℃）
ｔ2：内表面の加熱時間（ｓ）
（２） 熱処理を誘導加熱により行うことを特徴とする（１）に記載の圧潰強度に優れたＵＯＥ鋼管の製造方法。
（３） 熱処理中又は熱処理後の冷却中に塗覆装を行うことを特徴とする（１）又は（２）に記載の圧潰強度に優れた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、先ず、深海用ラインパイプに使用される代表サイズのＵＯＥ鋼管、φ６６０×２５．４ｍｍ、Ｘ−６５について周方向の圧縮応力−歪み挙動を全断面について調査した。試験片は直径６ｍｍ、長さ１５ｍｍの円柱で、外表面より１／４全厚部、内表面より１／４全厚部及び肉厚中心部で周方向を長手として採取した。図２に、圧縮降伏強度増減率を板厚方向の試験片採取位置に対してプロットした。圧縮降伏強度増減率は、鋼管の圧縮降伏強度より成形前の鋼板の圧縮強度を減じ、それを成形前の鋼板の圧縮強度で除した値の百分率である。これより、外表面側では圧縮降伏強度が成形前の鋼板に対して約２０％以上も低下するが、逆に内面側では上昇していることがわかった。
【００１２】
これまでは、ＵＯＥ成形プロセスでは最終的には拡管時の引張歪に起因するバウシンガー効果によって、圧縮降伏強度の低下が板厚全体で起こるものと考えられていた。しかし、実際には圧縮降伏強度の低下は外表面側に限定され、内表面側では製管時の曲げ加工による圧縮の加工硬化が拡管後も残留し、圧縮降伏強度が上昇していることを見出した。
【００１３】
以上のことから、成形時に低下した外表面側の圧縮降伏強度を回復させ、かつ成形時に上昇した内表面側の圧縮降伏強度を維持する熱処理方法を指向し、検討を行った。
【００１４】
冷間歪みにより変化した鋼の機械的性質は熱処理により回復することがよく知られている。そこでラインパイプとして実績のあるＸ−６５、Ｘ−８０及びさらに高強度の８００ＭＰａ以上の強度を有する材料について、引張加工予歪みを与えた後に熱処理を施し、引張予歪みにより低下した圧縮降伏強度が素材の９０％にまで回復する加熱温度と加熱時間を調査した。その結果、最適な熱処理条件は、材質や１％以上の予歪みに大きく依存せず、図３の関係にあることがわかった。これを定式化すると１８０℃以上の温度に対し、式（１）で表される。
Ｔ1−５０３≧−３７．６×lnｔ1 ・・・（１）
ここでＴ1：外表面の加熱温度（℃）
ｔ1：外表面の加熱時間（ｓ）
なお、外表面の加熱温度Ｔ1は、相変態等の組織変化を生じさせないように、上限を７００℃以下とすることが好ましい。上限は生産性を考慮すると６０００ｓ以下にすることが好ましい。
【００１５】
さらに同様に圧縮予歪みについて検討し、加工硬化により増加した降伏強度を維持し得る熱処理条件を調査したところ、材質や１％以上の予歪みに大きく依存せず、図４の関係が得られ、式（２）で定式化できた。
Ｔ2−４０７＜−３１．３×lnｔ2 ・・・（２）
ここでＴ2：内表面の加熱温度（℃）
ｔ2：内表面の加熱時間（ｓ）
なお、内表面の加熱温度Ｔ2は低いほど良いが、実際には外表面からの熱伝導によって下限は１００℃程度になる。また、内表面の加熱時間ｔ2の下限は規定せず、加熱温度Ｔ2に到達後、直ちに冷却しても良い。上限は外表面からの熱伝導に依存するため、規定しない。
【００１６】
すなわち、図５に示すように外表面温度と内表面温度をそれぞれ別の温度−時間範囲に加熱することによって、加工硬化により上昇した内面側の圧縮降伏強度を維持し、バウシンガー効果により低下した外表面側の圧縮降伏強度を上昇させることに成功した。
【００１７】
バウシンガー効果による外面側の圧縮降伏強度低下率は拡管時の引張歪の大きさ及び肉厚によって変化する造管時の曲げ加工による引張歪みの大きさには依存しない。一方、内面側の圧縮降伏強度の上昇は、Ｏプレス時の圧縮歪みの大きさ及び肉厚によって変化する加工硬化の度合いに依存する。従って、肉厚が厚くなると外面側の圧縮降伏強度は変化せず、内面側の圧縮降伏強度が大きくなるため、肉厚が２５〜４０ｍｍ程度の深海用ラインパイプ鋼管においては、本発明による効果が極めて顕著になる。また、圧潰モードは外径／肉厚比で区別できることが知られており、薄肉材では圧潰強度が圧縮降伏強度に依存しない弾性圧潰を呈し、厚肉になるほど、圧縮降伏強度に依存する遷移圧潰、塑性圧潰、降伏圧潰へと推移する。一般的なラインパイプのサイズは比較的薄肉であるため、弾性圧潰領域にあったが、深海に適用されるサイズでは厚肉になるため、圧縮降伏強度が強く影響し始める。従って、本発明の熱処理による内面側の圧縮降伏強度の維持及び外面側の圧縮強度の回復が効果を発揮する。
【００１８】
また、外表面が高温で内表面が低温という温度勾配のある温度履歴を得るには、誘導加熱によって外表面を加熱する方法が極めて効果的であり、内外面に急激な温度分布を短時間で簡便に付けることができる。温度分布の調整は誘導加熱の浸透深さ、鋼管搬送速度を肉厚との関係で適宜、最適化することで得ることができる。誘導加熱以外にも本発明の温度分布を得る方法に油槽、あるいはソルトバスなど、外表面から熱伝達係数の大きい雰囲気で加熱する、又は、内表面から強制冷却することでも実現できる。
【００１９】
また、熱処理後の潜熱を利用して、コーティングを極めて効率良く行うことができる。海底ラインパイプでは主に耐食性を向上させるために鋼管の外表面にプラスティックコーティングを行う。このようなプラスティック等のコーティングは、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。
【００２０】
従って、従来のように鋼管全体を高温に加熱して圧潰強度を向上させた場合、特に肉厚が大きい場合には適正温度範囲になるまで長時間の待ち時間を必要とした。本発明では内表面と外表面の温度差が大きいために肉厚方向の熱伝導による外表面温度の低下が速く、短時間でコーティングの適正温度に到達する。これにより時間あたりのコーティング本数が増し、圧潰強度を向上させると同時に製造コスト削減を図ることができる。
【００２１】
コーティング剤の塗覆装を熱処理中に行う際には、内表面温度が本発明で定義する内表面の温度−時間条件を超える場合もあり得る。しかし、本発明の内表面の温度範囲は、外表面を本発明の温度範囲に保ったとき、伝熱上可能で、かつ内表面側の加工硬化が失われない範囲に規定している。従って、コーティング時の外表面及び内表面の温度−時間の関係が、伝熱上可能でないという理由で本発明範囲を満たさなくても、熱処理時の温度−時間の関係が（１）式及び（２）式を満たせば、本発明に含まれる。
【００２２】
【実施例】
材質がＸ−６５、Ｘ８０及びＸ１００であり、外径及び肉厚が、それぞれ６６０〜７１１ｍｍ及び２５〜３８ｍｍの範囲であるＵＯＥ鋼管を図１に示す方法により製造した。これに、表１に示した条件で熱処理を施した。なお、鋼管の外表面及び内表面の温度は熱電対によって測定した。これらの鋼管の圧潰強度を単軸圧潰試験によって測定した。単軸圧潰試験は、長さ５ｍの鋼管を圧潰試験体とし、圧力容器内に設置して鋼管に軸力が発生しないように水圧を負荷して行った。結果を表１に示す。製造Ｎｏ．１〜６及び９〜１２は誘導加熱による熱処理であり、製造Ｎｏ．７及び８は雰囲気加熱と強制冷却を併用した熱処理である。熱処理時間は、加熱温度が４００℃以上では、温度上昇が急峻であるため、外面温度については当該温度を上回った時間を意味し、内表面温度については当該温度を最高温度に−１０℃までの温度範囲にあった時間を意味する。
【００２３】
製造Ｎｏ．１３、１５、１７及び１８は熱処理を施していないため、それぞれ、同サイズかつ同材質で本発明の範囲内の製造Ｎｏ．１〜３、４〜８、９及び１０よりも圧潰強度が大幅に低下している。また、製造Ｎｏ．１４は、内表面の熱処理条件が本発明の範囲外であるため、外表面が同じ温度に加熱された本発明の範囲内である製造Ｎｏ．１より明らかに圧潰強度が低下していることがわかった。
【００２４】
また、製造Ｎｏ．１６は外表面及び内表面の熱処理条件が本発明の範囲外であるため、外面が同じ温度にまで加熱された本発明の範囲内の製造Ｎｏ．６より圧潰強度が明らかに低下している。製造Ｎｏ．１１は、材質がＸ−１００のＵＯＥ鋼管に本発明の範囲内の熱処理を施したもので、圧潰試験の結果、高い圧潰強度が得られた。
【００２５】
【表１】
【発明の効果】
以上述べたように本発明によれば、鋼管の外表面及び内表面に異なった加熱履歴を与えることで、ＵＯＥ方式で製造した鋼管に、より高い圧潰抵抗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抵抗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方式による鋼管製造プロセスの模式図。
【図２】肉厚断面の位置による圧縮降伏強度増減率の変化。
【図３】高圧潰強度を得るための鋼管の外表面における加熱温度と時間の関係。
【図４】高圧潰強度を得るための鋼管の内表面における加熱温度と時間の関係。
【図５】高圧潰強度を得るための外表面及び内表面温度の組み合わせ。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for improving crushing characteristics in a method of forming a steel pipe used for a line pipe or the like by a UOE manufacturing method.
[0002]
[Prior art]
In recent years, the importance of line pipes has increased as a long-distance transportation method for crude oil and natural gas. In particular, submarine line pipes that cross the ocean have reached a depth of 3000 m. Generally, in pipeline design, the inner diameter of the steel pipe is first determined from the amount of fluid transport, and then the thickness and material are determined taking into account crack propagation characteristics and corrosion weight loss to keep the circumferential stress at the time of internal pressure load constant. Has been. However, the water pressure increases with the deepening of the sea, and the crushing strength, which has not been regarded as important so far, is becoming a problem. The crushing strength has a correlation with the ratio between the outer diameter and the wall thickness. By increasing the crushing strength of the steel pipe, the diameter can be increased and the wall thickness can be reduced. Therefore, crushing strength has begun to become the main design factor that determines the steel pipe size.
[0003]
By the way, the crushing strength of steel pipes has been studied for a long time in oil well pipes, and many empirical formulas have been proposed statistically. Among them, the outer diameter / thickness ratio, yield strength, roundness, thickness deviation, and residual stress were the main controlling factors. Since these studies were mainly conducted on seamless steel pipes with homogeneous material, it was not necessary to discuss much about material anisotropy.
[0004]
However, since a main line pipe used for long-distance transportation has a large diameter, a steel pipe produced by a UOE manufacturing method is used. As shown in FIG. 1, the manufacturing process of a steel pipe by the UOE method includes C forming (pressing), U forming (pressing), O forming (pressing), seam welding, and pipe expanding steps. Since both the moldings are performed cold, the final product, that is, the steel pipe, has anisotropy in mechanical properties due to a combination of work hardening and the Bauschinger effect. The Bauschinger effect is a phenomenon in which the yield strength in the opposite direction is lowered after plastic strain is applied to a material. Therefore, in the UOE steel pipe in which the plastic strain in the tensile direction is given in the circumferential direction, the compressive yield strength in the circumferential direction, that is, the yield strength with respect to the external pressure load is lowered by the Bauschinger effect.
[0005]
On the other hand, since the load direction is orthogonal to the main strain during molding with respect to the load in the axial direction, a difference in stress behavior is hardly caused in the tensile and compression loads in the axial direction. Further, when the load in the circumferential direction is a tensile stress, that is, for the internal pressure load, there is no problem if the strength is designed based on the value obtained from the full thickness tensile test.
[0006]
However, in recent years, the demand for UOE steel pipes applicable to deep sea line pipes has increased, and the crushing strength of steel pipes due to external pressure has become a problem. Crushing is a phenomenon in which a steel pipe is crushed by external pressure, and is one of buckling, so the yield strength of compression determines the crushing strength. Therefore, when a UOE steel pipe is applied to a line pipe that requires crushing strength, a decrease in the circumferential compressive strength due to the Bausinger effect becomes a problem.
[0007]
In order to solve such a problem, methods for recovering the reduced compressive yield strength by heat treatment are disclosed in JP-A-9-3545 and JP-A-9-49025, and have been reported in many research papers. ing. In these methods, the compression yield strength reduced by forming is restored to the level of the plate material before forming, and the crushing strength of the UOE steel pipe is improved to some extent.
[0008]
[Problems to be solved by the invention]
However, when the entire steel pipe is heated by current heating or a heat treatment furnace, work hardening due to compressive strain occurring from the inner surface of the steel pipe to the thickness center portion (hereinafter referred to as the inner surface side) is lost in the UOE manufacturing process. Therefore, it is difficult to say that the crushing strength improving method for uniformly heat-treating the entire UOE steel pipe is extremely effective.
[0009]
In the present invention, the UOE steel pipe is subjected to a heat treatment that leaves work hardening by cold working on the inner surface side and extinguishes the Bausinger effect of the thickness center portion (hereinafter referred to as the outer surface side) from the outer surface. It aims to improve.
[0010]
[Means for Solving the Problems]
In order to solve such a problem, the present inventor first made a detailed examination in order to clarify the change of the crushing strength in the thickness direction in the UOE steel pipe. As a result, it was found that the compressive yield strength on the outer surface side was lower than that on the steel sheet before forming, and increased on the inner surface side. From this knowledge, the heat treatment that the outer surface is hot and the inner surface is lowered improves the compressive yield strength on the outer surface side to the same level as the steel sheet, and maintains the compressive yield strength on the inner surface side, and the axial strength. The heat treatment method that suppresses the decrease in the temperature was examined in detail. As a result, the optimum heat treatment conditions were found and a UOE steel pipe excellent in crushing strength was successfully produced.
That is, the gist of the present invention is as follows.
(1) In a method for manufacturing a UOE steel pipe, in which a steel sheet is C-shaped, U-shaped, O-shaped in order, and the ends of the steel sheet are seam welded, and then expanded, the outer surface temperature T 1 and the outer surface heating time t 1 are Production of a UOE steel pipe excellent in crushing strength, characterized in that the heat treatment is performed so that the equation (1) is satisfied and the inner surface temperature T 2 and the inner surface heating time t 2 satisfy the equation (2). Method.
T 1 −503 ≧ −37.6 × lnt 1 (1)
Where T 1 is the outer surface heating temperature (° C.)
t 1 : Heating time of outer surface (s)
T 2 −407 <−31.3 × lnt 2 (2)
Where T 2 : inner surface heating temperature (° C.)
t 2 : inner surface heating time (s)
(2) The method for producing a UOE steel pipe having excellent crushing strength according to (1), wherein the heat treatment is performed by induction heating.
(3) The method for producing a UOE steel pipe having excellent crushing strength according to (1) or (2), wherein coating is performed during heat treatment or during cooling after heat treatment.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
First, the present inventors investigated the compression stress-strain behavior in the circumferential direction with respect to the entire cross section of a typical size UOE steel pipe, φ660 × 25.4 mm, X-65 used for a deep sea line pipe. The test piece was a cylinder having a diameter of 6 mm and a length of 15 mm, and was collected from the outer surface at a quarter full thickness part, from the inner surface a quarter full thickness part and a wall thickness center part with the circumferential direction as the longitudinal direction. In FIG. 2, the rate of increase / decrease in compressive yield strength is plotted against the specimen collection position in the thickness direction. The compression yield strength increase / decrease rate is a percentage of a value obtained by subtracting the compressive strength of the steel sheet before forming from the compressive yield strength of the steel pipe and dividing it by the compressive strength of the steel sheet before forming. From this, it was found that the compressive yield strength decreased by about 20% or more with respect to the steel sheet before forming on the outer surface side, but conversely increased on the inner surface side.
[0012]
Until now, it was thought that in the UOE forming process, the compressive yield strength was reduced over the entire thickness due to the Bauschinger effect resulting from the tensile strain during tube expansion. However, in practice, the decrease in compressive yield strength is limited to the outer surface side, and on the inner surface side, the work hardening of compression due to bending during pipe making remains after pipe expansion, indicating that the compressive yield strength has increased. I found it.
[0013]
In view of the above, a heat treatment method for recovering the compressive yield strength on the outer surface side, which was reduced during molding, and maintaining the compressive yield strength on the inner surface side, which was increased during molding, was investigated and studied.
[0014]
It is well known that the mechanical properties of steel changed by cold strain are recovered by heat treatment. Therefore, with regard to X-65, X-80, which have a proven track record as a line pipe, and higher strength materials having a strength of 800 MPa or more, heat treatment is performed after applying tensile processing pre-strain, and the compressive yield strength decreased due to tensile pre-strain. The heating temperature and heating time for recovering to 90% of the material were investigated. As a result, it was found that the optimum heat treatment condition does not depend greatly on the material and the pre-strain of 1% or more and has the relationship shown in FIG. When this is formulated, it is expressed by the formula (1) for a temperature of 180 ° C. or higher.
T 1 −503 ≧ −37.6 × lnt 1 (1)
Where T 1 is the outer surface heating temperature (° C.)
t 1 : Heating time of outer surface (s)
The upper limit of the heating temperature T 1 of the outer surface is preferably set to 700 ° C. or lower so as not to cause structural changes such as phase transformation. In consideration of productivity, the upper limit is preferably 6000 s or less.
[0015]
Further, similarly, the compression pre-strain was examined, and the heat treatment conditions capable of maintaining the yield strength increased by work hardening were investigated. The relationship shown in FIG. 4 was obtained without depending greatly on the material and pre-strain of 1% or more. It could be formulated by formula (2).
T 2 −407 <−31.3 × lnt 2 (2)
Where T 2 : inner surface heating temperature (° C.)
t 2 : inner surface heating time (s)
The lower the heating temperature T 2 of the inner surface is, the better. However, the lower limit is actually about 100 ° C. due to heat conduction from the outer surface. In addition, the lower limit of the heating time t 2 on the inner surface is not defined, and it may be cooled immediately after reaching the heating temperature T 2 . The upper limit is not specified because it depends on heat conduction from the outer surface.
[0016]
That is, as shown in FIG. 5, by heating the outer surface temperature and the inner surface temperature to different temperature-time ranges, the compressive yield strength on the inner surface side increased by work hardening is maintained, and decreased by the Bauschinger effect. The compressive yield strength on the outer surface was successfully increased.
[0017]
The rate of decrease in compressive yield strength on the outer surface due to the Bauschinger effect does not depend on the magnitude of tensile strain during pipe expansion and the magnitude of tensile strain caused by bending during pipe making, which varies with the wall thickness. On the other hand, the increase in compressive yield strength on the inner surface side depends on the degree of work hardening that varies depending on the magnitude and thickness of compressive strain during O-pressing. Therefore, when the wall thickness is increased, the compressive yield strength on the outer surface side does not change, and the compressive yield strength on the inner surface side is increased. Therefore, the effect of the present invention is effective in a deep-pipe line pipe steel pipe having a wall thickness of about 25 to 40 mm. Become extremely prominent. In addition, it is known that the crushing mode can be distinguished by the outer diameter / thickness ratio, and the thin-walled material exhibits elastic crushing whose crushing strength does not depend on the compressive yield strength. Transition to plastic crushing and yield crushing. Since the size of a general line pipe is relatively thin, it was in the elastic crush area. However, since the size applied to the deep sea is thick, the compressive yield strength starts to have a strong influence. Therefore, the maintenance of the compressive yield strength on the inner surface side and the recovery of the compressive strength on the outer surface side by the heat treatment of the present invention are effective.
[0018]
In addition, in order to obtain a temperature history with a temperature gradient of high temperature on the outer surface and low temperature on the inner surface, the method of heating the outer surface by induction heating is extremely effective, and a rapid temperature distribution on the inner and outer surfaces can be achieved in a short time. It can be easily attached. Adjustment of the temperature distribution can be obtained by appropriately optimizing the penetration depth of induction heating and the steel pipe conveyance speed in relation to the wall thickness. In addition to induction heating, the method for obtaining the temperature distribution of the present invention can be realized by heating in an atmosphere having a large heat transfer coefficient from the outer surface, such as an oil bath or a salt bath, or by forced cooling from the inner surface.
[0019]
In addition, coating can be performed very efficiently using latent heat after heat treatment. In the submarine line pipe, plastic coating is applied to the outer surface of the steel pipe mainly to improve the corrosion resistance. Such a coating of plastic or the like needs to be performed at a temperature of about 150 to 250 ° C. in order to increase the adhesion strength.
[0020]
Therefore, when the entire steel pipe is heated to a high temperature to improve the crushing strength as in the prior art, a long waiting time is required until the temperature reaches an appropriate temperature range, particularly when the wall thickness is large. In the present invention, since the temperature difference between the inner surface and the outer surface is large, the outer surface temperature is rapidly decreased due to heat conduction in the thickness direction, and the appropriate temperature of the coating is reached in a short time. This increases the number of coatings per hour, improving the crushing strength and simultaneously reducing the manufacturing cost.
[0021]
When coating the coating agent during the heat treatment, the inner surface temperature may exceed the inner surface temperature-time condition defined in the present invention. However, the temperature range of the inner surface of the present invention is defined in such a range that heat transfer is possible and work hardening on the inner surface side is not lost when the outer surface is kept in the temperature range of the present invention. Therefore, even if the temperature-time relationship between the outer surface and the inner surface at the time of coating does not satisfy the scope of the present invention because heat transfer is not possible, the relationship between the temperature and time at the time of heat treatment is expressed by the equations (1) and ( If the formula (2) is satisfied, it is included in the present invention.
[0022]
【Example】
UOE steel pipes having materials of X-65, X80, and X100 and having outer diameters and thicknesses of 660 to 711 mm and 25 to 38 mm, respectively, were manufactured by the method shown in FIG. This was heat-treated under the conditions shown in Table 1. In addition, the temperature of the outer surface and inner surface of a steel pipe was measured with the thermocouple. The crushing strength of these steel pipes was measured by a uniaxial crushing test. In the uniaxial crushing test, a steel pipe having a length of 5 m was used as a crushing test body, which was installed in a pressure vessel and loaded with water pressure so that no axial force was generated in the steel pipe. The results are shown in Table 1. Production No. 1 to 6 and 9 to 12 are heat treatments by induction heating. 7 and 8 are heat treatments using both atmospheric heating and forced cooling. The heat treatment time means that when the heating temperature is 400 ° C. or higher, the temperature rises sharply. Therefore, the outer surface temperature means a time exceeding the temperature, and the inner surface temperature is the maximum temperature up to −10 ° C. It means the time in the temperature range.
[0023]
Production No. Since Nos. 13, 15, 17 and 18 were not subjected to heat treatment, they were produced with the same size and the same material as the production Nos. Within the scope of the present invention. The crushing strength is significantly lower than those of 1-3, 4-8, 9, and 10. In addition, production No. No. 14, because the heat treatment condition of the inner surface is outside the scope of the present invention, the production No. 14 in which the outer surface is within the scope of the present invention heated to the same temperature. 1 clearly shows that the crushing strength is lowered.
[0024]
In addition, production No. Since the heat treatment conditions of the outer surface and the inner surface are outside the scope of the present invention, the production No. 16 within the scope of the present invention in which the outer surface was heated to the same temperature. The crushing strength is clearly lower than 6. Production No. No. 11 was obtained by subjecting a UOE steel pipe made of X-100 to a heat treatment within the scope of the present invention. As a result of a crush test, a high crushing strength was obtained.
[0025]
[Table 1]
【The invention's effect】
As described above, according to the present invention, by giving different heating histories to the outer surface and the inner surface of the steel pipe, it is possible to give higher crush resistance to the steel pipe manufactured by the UOE method. UOE steel pipe with excellent strength can be provided at low cost. This can be used for line pipes for transportation of natural gas, crude oil, etc. even in environments where high crush resistance is required such as in the deep sea. In addition, the contribution is extremely high.
[Brief description of the drawings]
FIG. 1 is a schematic diagram of a steel pipe manufacturing process using a UOE method.
FIG. 2 shows the change in the rate of increase or decrease in compression yield strength depending on the position of the thick section.
FIG. 3 shows the relationship between heating temperature and time on the outer surface of a steel pipe for obtaining high pressure crushing strength.
FIG. 4 shows the relationship between heating temperature and time on the inner surface of a steel pipe for obtaining high pressure crushing strength.
FIG. 5 shows a combination of outer and inner surface temperatures to obtain high pressure crush strength.
Claims (3)
Ｔ1−５０３≧−３７．６×lnｔ1 ・・・（１）
ここでＴ1：外表面の加熱温度（℃）
ｔ1：外表面の加熱時間（ｓ）
Ｔ2−４０７＜−３１．３×lnｔ2 ・・・（２）
ここでＴ2：内表面の加熱温度（℃）
ｔ2：内表面の加熱時間（ｓ）In the manufacturing method of a UOE steel pipe in which steel plates are sequentially formed into C, U, and O, and the ends of the steel plates are seam welded, and then expanded, the outer surface temperature T 1 and the outer surface heating time t 1 are (1 ), And a heat treatment is performed so that the inner surface temperature T 2 and the inner surface heating time t 2 satisfy the following expression (2): .
T 1 −503 ≧ −37.6 × lnt 1 (1)
Where T 1 is the outer surface heating temperature (° C.)
t 1 : Heating time of outer surface (s)
T 2 −407 <−31.3 × lnt 2 (2)
Where T 2 : inner surface heating temperature (° C.)
t 2 : inner surface heating time (s)
Priority Applications (5)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP2002151008A JP4071995B2 (en) | 2002-05-24 | 2002-05-24 | UOE steel pipe manufacturing method with excellent crushing strength |
| EP03733045A EP1541252B1 (en) | 2002-05-24 | 2003-05-23 | Uoe steel pipe with excellent crash resistance, and method of manufacturing the uoe steel pipe |
| US10/515,543 US7892368B2 (en) | 2002-05-24 | 2003-05-23 | UOE steel pipe excellent in collapse strength and method of production thereof |
| PCT/JP2003/006486 WO2003099482A1 (en) | 2002-05-24 | 2003-05-23 | Uoe steel pipe with excellent crash resistance, and method of manufacturing the uoe steel pipe |
| US12/462,218 US7967926B2 (en) | 2002-05-24 | 2009-07-30 | UOE steel pipe excellent in collapse strength and method of production thereof |
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| JP2002151008A JP4071995B2 (en) | 2002-05-24 | 2002-05-24 | UOE steel pipe manufacturing method with excellent crushing strength |
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| US20120305122A1 (en) | 2009-11-25 | 2012-12-06 | Nobuyuki Ishikawa | Welded steel pipe for linepipe having high compressive strength and high fracture toughness and manufacturing method thereof |
| WO2011065579A1 (en) | 2009-11-25 | 2011-06-03 | Ｊｆｅスチール株式会社 | Welded steel pipe for linepipe with superior compressive strength, and process for producing same |
| CN102639734A (en) | 2009-11-25 | 2012-08-15 | 杰富意钢铁株式会社 | Welded steel pipe for linepipe with superior compressive strength and excellent sour resistance, and process for producing same |
| JP2012006069A (en) * | 2010-06-28 | 2012-01-12 | Jfe Steel Corp | Steel pipe excellent in crushing resistance |
| JP5966441B2 (en) * | 2012-03-01 | 2016-08-10 | Ｊｆｅスチール株式会社 | Welded steel pipe excellent in pressure crushing performance and internal pressure fracture resistance and manufacturing method thereof |
| US11401568B2 (en) | 2018-01-30 | 2022-08-02 | Jfe Steel Corporation | Steel material for line pipes, method for producing the same, and method for producing line pipe |
| US20210032732A1 (en) | 2018-01-30 | 2021-02-04 | Jfe Steel Corporation | Steel material for line pipes, method for producing the same, and method for producing line pipe |
| CN113646455B (en) | 2019-03-28 | 2023-06-27 | 杰富意钢铁株式会社 | Steel material for line pipe and method for producing same, and line pipe and method for producing same |
| EP4116453A4 (en) * | 2020-03-04 | 2023-03-22 | Nippon Steel Corporation | Steel pipe and steel sheet |
| CN114752750B (en) * | 2022-04-08 | 2024-01-26 | 马帅 | Weld joint treatment pry for field construction of oil and gas long-distance pipeline and treatment process thereof |
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