JP3749704B2 - High strength steel pipe with excellent buckling resistance and method for producing the same - Google Patents

Description

【００１０】
アーム間の距離を１０００ｍｍとし、試験体中央の曲げ外側部に歪みゲージを貼り、アームに荷重を負荷しながら歪みを測定した。最大荷重を示す歪みεｍを耐座屈特性として表し、残留オーステナイト量との関係を示したものが図１である。このように、残留オーステナイト量を増やすと耐座屈特性が高くなることを新たな知見として得た。
【００１１】
なお、残留オーステナイト量の測定は、鋼管の肉厚中央部より小片を採取し、ＭｏのＫα線を用いたＸ線解析によって行い、体積分率として算出した。具体的には、Ｘ線回折法によりフェライトの（２００）面、（２１１）面およびオーステナイトの（２００）面、（２２０）面、（３１１）面の積分反射強度を測定し、Ｊｏｕｒｎａｌ ｏｆ Ｔｈｅ ｉｒｏｎ ａｎｄ ｓｔｅｅｌ ｉｎｓｔｉｔｕｔｅ，２０６（１９６８）Ｐ６０に示された方法によって求めた。
【００１２】
また、本発明者は、残留オーステナイト量の制御には、オーステナイトのベイナイト変態およびマルテンサイト変態に及ぼす影響の大きい、圧延後の冷却が有効であると考えた。そこで、表２に示すスラブを１１００℃に再加熱し、再結晶温度域で粗圧延を行い、引き続き累積圧下率８０％の未再結晶圧延を行って１６ｍｍ厚に仕上げ、種々のパターンで冷却する小型圧延実験を行った。
【００１３】
【表２】

【００１４】
その結果、図２に示す二段冷却パターンが有効であることを見いだし、残留オーステナイトを含むベイナイトおよびマルテンサイトとフェライトとの複合組織を得る製造方法を明らかにした。
【００１５】
すなわち、圧延後、フェライト変態を抑制するためにＡｒ₃［℃］以上の温度から第一段冷却を開始し、十分なフェライトが生成する温度で冷却を終了して保持した後、残留オーステナイトのフェライトおよび炭化物への分解を抑制してベイナイト変態させるために第二段冷却し、その後放冷する。これにより、フェライトの平均粒径を微細化し、安定な残留オーステナイトを生成させることができる。
【００１６】
なお、第一段冷却を終了して十分なフェライトを生成させるためには、下記（１）式によって鋼中の化学成分から計算されるＴｓ［℃］よりも１００℃高い温度から５０℃低い温度で保持することが必要であることを見いだした。
Ｔｓ［℃］＝７８０−２７０×Ｃ−９０×Ｍｎ−３７×Ｎｉ−７０×Ｃｒ−８３×Ｍｏ ・・・ （１）
以下、成分含有量の規定した理由について述べる。
【００１７】
Ｃ量は、０．０２〜０．１５％以下に限定する。Ｃは高強度化には最も有効な元素であり、十分な強度を得るためには０．０２％以上は必要である。しかしながら、過度に多くなると、溶接性が悪くなることから上限を０．１５％以下とした。
【００１８】
Ｓｉは脱酸あるいは強度向上に有効であるとともに、安定な残留オーステナイトの生成に有効な元素である。その効果を得るためには０．１％以上必要であるが、２．０％以上含有すると溶接熱影響部の靭性が著しく劣化する。したがって、Ｓｉの添加量を０．１〜２．０％の範囲とした。
【００１９】
Ｍｎは強度上昇に有効な元素であり、十分な強度向上を得るためには０．５％以上の添加が必要である。しかしながら、２．５％よりも多く含有すると伸びが確保できなくなり、上限を２．５％以下とした。
【００２０】
ＰおよびＳは不純物であり、Ｐは粒界に偏析し、また、ＳはＭｎＳとして析出する。ＰおよびＳはそれぞれ、０．０２％超および０．００５％超を含有すると靭性を劣化させるので、ＰおよびＳの上限は、それぞれ０．０２％以下、０．００５％以下とする。
【００２１】
Ａｌは強力な脱酸元素であり、組織微細化にも寄与する。脱酸するためには、０．０１％以上必要である。ただし、０．１％超となると溶接熱影響部の靭性を劣化させるので、上限を０．１％とした。
【００２２】
Ｎは強化元素として有効であるが、０．０１％超を含有すると固溶Ｎ量が多くなり、伸びを著しく劣化させるので、上限を０．０１％以下とした。
【００２３】
Ｎｂ、Ｖ、Ｔｉは強化のため１種または２種以上含有する。これらは炭窒化物を形成し、析出強化として寄与する。しかしながら、それぞれ０．１％よりも多く含有すると粗大な析出物として存在し靭性を劣化させることから、それぞれの上限を０．１％以下とした。下限は、特に限定しないが、析出強化を発揮するためそれぞれ０．０１％以上が好ましい。
【００２４】
さらに、必要に応じて、Ｍｏ、Ｃｕ、Ｎｉ、Ｃｒ、Ｂ、Ｃａ、Ｍｇ、ＲＥＭの１種または２種以上を含有しても良い。
【００２５】
Ｍｏ、Ｃｕ、Ｎｉ、Ｃｒは焼き入れ性を高め、高強度化に寄与する。しかし、添加元素が多すぎると、経済性だけでなく、溶接熱影響部の靭性あるいは現地溶接性を劣化させるので、Ｍｏ、Ｃｕ、Ｎｉ、Ｃｒの上限を、それぞれ、１．０％、２．０％、２．０％、１．０％とすることが好ましい。強化の効果は得るためには、Ｍｏ、Ｃｕ、Ｎｉ、Ｃｒがそれぞれ０．０５％以上含有することが好ましい。
【００２６】
Ｂは少量で焼き入れ性を大幅に高め強化に寄与する。しかしながら、多量に添加すると、伸びの劣化、溶接熱影響部の硬化を招くので、上限を０．００５％以下とすることが好ましい。強化の効果を得るためには、０．０００１％以上含有することが好ましい。
【００２７】
Ｃａ、ＲＥＭは硫化物の形態を制御し、靭性の向上に寄与する。しかしながら過度に添加すると、大型の介在物として存在しかえって靭性を劣化させるので、それぞれの上限を０．０１％、０．０２％以下とすることが好ましい。硫化物の形態制御のためには、それぞれ０．０００１％以上含有することが好ましい。
【００２８】
Ｍｇは巨力な脱酸元素であり、微細な酸化物として分散した場合、溶接熱影響部の靭性向上に大きく寄与する。しかしながら、０．１％超を添加すると粗大な酸化物として存在しかえって靭性を劣化させるので、上限を０．１％とすることが好ましい。靭性向上の効果を得るためには、０．０００１％以上含有することが好ましい。
【００２９】
次にミクロ組織について説明する。
【００３０】
フェライトの平均結晶粒径の微細化によって靭性が向上するが、−２０℃におけるシャルピー吸収エネルギーを１５０Ｊ以上とするには、フェライトの平均結晶粒径を１０μｍ以下とする必要がある。フェライトの平均結晶粒径は微細であるほど好ましいが、現状の技術では１μｍ程度が限界である。また、フェライトの面積率は、７０％未満では硬質のベイナイトおよびマルテンサイト量が多くなるため耐座屈特性およびｎ値が低下し、９０％超では強度が低下する。したがって、フェライトの面積率を７０〜９０％の範囲とした。ミクロ組織において、面積率７０〜９０％のフェライトの残部は、残留オーステナイトを含むベイナイトおよびマルテンサイトである。
【００３１】
なお、フェライトの平均結晶粒径および面積率は以下の方法で求めることができる。鋼管の肉厚中心部より小片を切り出し、管軸方向に平行な断面を鏡面研磨後、エッチングし、現出した組織を観察し、フェライトとベイナイトおよびマルテンサイトを区別して、任意の１０視野におけるフェライトの平均結晶粒径をＪＩＳ Ｇ ０５５２に準じて切断法によって求め、平均値をフェライトの平均結晶粒径とする。フェライトの面積率は、画像解析装置を用いてフェライト粒の面積の和を求め、視野の面積で除した百分率として算出した。
【００３２】
耐座屈特性が顕著に向上するには、残留オーステナイトの体積分率は５％以上が必要である。しかしながら、１５％超になると靭性を損ねるため、残留オーステナイトの体積分率は、５〜１５％に限定した。なお、残留オーステナイトの体積分率は、ミクロ組織による測定が難しいため、Ｘ線解析により求めることとする。
【００３３】
本発明の鋼管は、その使用環境で求められる強度を満足するために、鋼管のサイズを肉厚１０ｍｍ以上、外径１００ｍｍ以上に限定する。なお現状の技術では、肉厚および外形をそれぞれ５０ｍｍ超および１２００ｍｍ超とすることは困難である。
【００３４】
ｎ値の向上とともに耐座屈特性が向上するが、この効果を得るにはｎ値を０．２以上とすることが好ましい。ｎ値の上限は規定しないが、現状の技術では０．２６超とすることは困難である。なお、ｎ値の測定は、鋼管の管軸方向を長手としてＪＩＳ Ｚ ２２０１に準じて１２Ｂ号または１２Ｃ号の弧状引張試験片を採取し、ＪＩＳ Ｚ ２２４１に準じて引張試験を行い、歪みが１％から８％の範囲の加工硬化率として求めることができる。
【００３５】
次に製造方法について述べる。
【００３６】
スラブの加熱温度は、熱間圧延でＮｂの炭窒化物を微細に析出させるために、Ｎｂが固溶する１０５０℃以上に限定する。上限は特に規定しないが、オーステナイト粒径の粗大化を抑制して靭性を向上させるためには、スラブの加熱温度の上限は１２５０℃が好ましい。
【００３７】
仕上げ圧延は、フェライトの平均結晶粒径を微細化するために再結晶温度未満の９００℃以下で行うことが必要である。また、圧延終了温度が、Ａｒ₃［℃］を下回ると圧延による加工歪みを導入されたフェライトが残存し、靭性を大きく劣化させる。したがって、圧延終了温度はＡｒ₃以上に限定する。さらに、仕上げ圧延の累積圧下量は、フェライトの平均結晶粒径を１０μｍ以下とするために６５％以上に限定する。上限は規定しないが、技術的な制限から９０％以下にすることが好ましい。
【００３８】
なお、Ａｒ₃は、鋼中の化学成分から、各元素の単位を質量％として、次式によって求められる。
Ａｒ₃［℃］＝９２１−３２５×Ｃ＋３３×Ｓｉ＋２８７×Ｐ＋４０×Ａｌ−９２×（Ｍｎ＋Ｍｏ＋Ｃｕ）−４６×（Ｃｒ＋Ｎｉ）
本発明において、圧延後の冷却はフェライトの平均結晶粒径の微細化および残留オーステナイト量の制御に最も重要なプロセスである。仕上げ圧延後、フェライト変態を抑制するために第一段冷却し、十分なフェライトを生成させるための中間保持を行い、残留オーステナイトのフェライトと炭化物への分解を抑制するために第二段冷却を行い、その後放冷する。
【００３９】
第一段冷却は、熱延後のフェライト変態を抑制するために、開始温度をＡｒ₃以上とし、冷却速度を５℃／ｓ以上とする。これは、開始温度がＡｒ₃未満、冷却速度が５℃／ｓ未満であると粗大なフェライト粒が生成し、靭性が低下するためである。第一段冷却は、Ａｒ₃よりも高い温度であれば良いため、上限を規定しない。したがって、仕上げ圧延の終了直後に開始しても良い。冷却速度の上限は規定しないが、現状の技術では３００℃／ｓを超えることは困難である。
【００４０】
さらに、十分なフェライト生成をさせるためには、鋼中の化学成分から各元素の単位を質量％として、下記（１）式によって求められるＴｓ［℃］よりも１００℃高い温度からＴＳよりも５０℃低い温度の範囲で保持する必要がある。その理由は、ＴＳ＋１００℃超の温度で保持するとフェライトの平均結晶粒径が粗大化して靭性が低下し、ＴＳ−５０℃よりも低い温度で保持するとベイナイト変態が起こり、フェライト面積率が減少して耐座屈特性が低下するためである。
【００４１】
Ｔｓ［℃］＝７８０−２７０×Ｃ−９０×Ｍｎ−３７×Ｎｉ−７０×Ｃｒ−８３×Ｍｏ ・・・ （１）
保持時間は、フェライトを十分に生成させ、面積率を７０％以上とするために、３０秒以上必要である。ただし、３００秒超になると生成したフェライトの平均結晶粒径が粗大化して靭性が低下するため、上限を３００秒に限定する。なお、この保持時間内に急激な温度低下が生じることは好ましくない。通常は、放冷すれば良いが、保熱炉、保熱カバーによって温度低下を抑制しても構わない。
【００４２】
この中間保持の後、第二段冷却を行うが、これは残留オーステナイト量を５〜１０％に制御して耐座屈特性を向上させるために２０℃／ｓ以上で行う必要がある。その理由は、冷却を２０℃／ｓ未満で行うとベイナイト中の残留オーステナイトがフェライトと炭化物に分解が多量に生成し、残留オーステナイト量が低下するためである。上限は特に規定しないが、現状の加速冷却装置の制限で、１００℃／ｓを超えることは困難である。
【００４３】
また、耐座屈特性の向上のため残留オーステナイトを生成させるには、第二段冷却の終了温度を３５０〜４５０℃の範囲内とすることが必要である。第二段冷却の終了温度が４５０℃超では、第二段冷却によって残留オーステナイトが炭化物とフェライトに分解して残留オーステナイト量が減少する。一方、第二段冷却を３５０℃未満まで行うとマルテンサイト変態が生じて残留オーステナイト量が減少する。したがって、第二段冷却の終了温度は、３５０℃〜４５０℃に限定する。
【００４４】
造管は冷間加工により成形し、端部をシーム溶接によって接合する製造プロセスで行う。冷間加工はプレス加工、ロール成形の何れでも良く、その組み合わせても構わないが、Ｃ成形、Ｕ成形およびＯ成形を行い、端部をシーム溶接して拡管するＵＯＥ方式による鋼管製造プロセスが好ましい。シーム溶接は、ＭＡＧアーク溶接、サブマージアーク溶接の何れでも良く、その組み合わせでも良い。
【００４５】
【実施例】
表３に示す成分の鋼を溶製し、表４に示す条件で熱間圧延して第一段冷却、中間保持および第ニ段冷却を行い、板厚１６ｍｍの鋼板を製造した。粗圧延は再結晶温度以上で、仕上げ圧延はＡｒ₃以上９００℃以下の範囲で行った。その鋼板を冷間成形し、内外面溶接を行った後、拡管を行い、外径６９０ｍｍ、肉厚１６ｍｍのＵＯＥ鋼管に造管した。
【００４６】
鋼管の肉厚中心部より小片を切り出し、管軸方向に平行な断面を鏡面研磨後、ナイタールによりエッチングして現出した組織を観察した。ミクロ組織は、フェライトと、残部が残留オーステナイトを含むベイナイトおよびマルテンサイトであった。フェライトと残留オーステナイトを含むベイナイトおよびマルテンサイトを区別して、任意の１０視野におけるフェライトの平均結晶粒径をＪＩＳ Ｇ０５５２に準じて切断法によって求め、平均値をフェライトの平均結晶粒径とした。フェライトの面積率は、画像解析装置を用いてフェライト粒の面積の和を求め、視野の面積で除した百分率として算出した。残留オーステナイトの体積分率は、Ｘ線解析により求めた。鋼管の溶接部から周方向に９０°、１８０°、２７０°の位置からＪＩＳ Ｚ２２０１に準じて１２Ｃ号円弧状引張試験片を採取し、ＪＩＳ Ｚ ２２４１に準じて引張特性を測定し、引張強度の平均値を求めた。管軸方向のｎ値の測定は、歪みが１〜８％における加工硬化率として行い、平均値を求めた。
【００４７】
各鋼材の耐座屈特性は、４点曲げ試験によって評価した。試験対のサイズは、肉厚１６ｍｍ、外径６９０ｍｍ、長さ３０００ｍｍとし、押しつけアーム部間の距離を１０００ｍｍとした。試験体中央の曲げ外側部に歪みゲージを貼って管軸方向の歪みを測定しながら、アーム部に荷重を負荷し、最大荷重までの歪みεｍを座屈歪みとして求め、耐座屈特性の指標とした。
【００４８】
靭性の測定は、鋼管の肉厚中央部から周方向を長手としてＪＩＳ Ｚ ２２０２に準じて２ｍｍＶノッチのシャルピー試験片を採取し、ＪＩＳ Ｚ ２２４２に準じて０℃でのシャルピー試験を行い、吸収エネルギーを測定した。
【００４９】
表５に得られた結果を示す。本発明に従って製造した鋼管は、体積分率で５％以上の残留オーステナイト量を有し、ｎ値が０．２以上である結果、従来鋼管と比較して座屈歪みが高いとともに、靭性も良好である。
【００５０】
一方、試験Ｎｏ．１４は仕上げ圧延の累積圧下量が少なく、試験Ｎｏ．１５および１６は、第一段冷却開始温度が低く、試験Ｎｏ．２２は中間保持の時間が長いため、何れもフェライトの平均結晶粒径が大きくなり靭性が低下している。試験Ｎｏ．１７および１８は、第一段冷却の冷却速度が遅いため、フェライトの平均結晶粒径が大きくなって靭性が低下し、フェライト面積率が多くなり引張強度がやや低めになっている。
【００５１】
試験Ｎｏ．１９および２０は中間保持の開始温度及び終了温度がともに本発明の範囲よりも低いため、残留オーステナイト量が減少し、耐座屈特性が低下している。試験Ｎｏ．２１は中間保持時間が短いため、フェライト面積率が少なく、座屈歪みが低下している。Ｎｏ．２３および２４は中間保持の終了温度が低いため、試験Ｎｏ．２５は第二段冷却速度が遅く、試験Ｎｏ．２６は第二段冷却の終了温度が低いため、何れも残留オーステナイト量が少なく、座屈歪みが低い。
【００５２】
【表３】

【００５３】
【表４】

【００５４】
【表５】

【図面の簡単な説明】
【図１】座屈歪みに及ぼす残留オーステナイト体積分率の影響を示す図。
【図２】本発明による二段冷却パターンを示す図。[0001]
[Technical field belonging to the invention]
The present invention relates to a high-strength steel pipe excellent in deformation characteristics such as local buckling characteristics and excellent in low-temperature toughness, particularly suitable for pipes such as line pipes or structural steel pipes such as steel pipe columns and steel pipe piles. Further, the present invention relates to a steel pipe having excellent buckling resistance in bending.
[0002]
[Prior art]
Up until now, structural steel pipes used for crude oil and natural gas transportation pipes or pillars and piles have mainly been required to have high strength and toughness. Therefore, there is a growing need for development of high-strength steel pipes with excellent deformation characteristics in addition to strength and toughness.
[0003]
Line pipes, steel pipe columns and steel pipe piles are destroyed by local buckling mainly due to compression or bending deformation when an earthquake or the like occurs. So far, it has been reported that lowering yield is effective for improving local buckling resistance. For example, Patent Document 1 discloses a steel pipe having a microstructure that is a hard second-phase composite structure containing ferrite and martensite or bainite, and has a high work hardening index (hereinafter, n value) and a method for manufacturing the same. ing. However, this method rolls in a low-temperature two-phase region, and since ferrite and a hard second phase are formed in layers, there is a problem that separation occurs and the toughness is greatly deteriorated.
[0004]
[Patent Document 1]
Japanese Patent Laid-Open No. 11-279700
[Problems to be solved by the invention]
The present invention is suitable for pipes such as line pipes, or structural steel pipes such as steel pipe columns and steel pipe piles, and is a high strength steel pipe excellent in buckling resistance by compression and bending and excellent in low temperature toughness, and a method for producing the same Is to provide.
[0006]
[Means for Solving the Problems]
The inventor conducted a detailed investigation on a structure control method capable of improving the n value while ensuring low temperature toughness. As a result, strict cooling control is performed under the steel sheet manufacturing conditions, and the microstructure is a composite structure of fine ferrite and bainite and martensite containing retained austenite, thereby improving the n value without impairing low-temperature toughness. As a result, the inventors have invented a high-strength steel pipe excellent in buckling resistance and a manufacturing method thereof. The gist of the present invention is as follows.
(1) By mass%, C: 0.02% to 0.15%, Si: 0.1% to 2.0%, Mn: 0.5% to 2.5%, Al: 0.01% to 0.1%, N: 0.01% or less, P: 0.02% or less, S: 0.005% or less, Nb: 0.1% or less, V: 0.1% or less, Ti: containing one or more of 0.1% or less, the balance being iron and inevitable impurities, the average crystal grain size of 10 μm or less, the area ratio of 70 to 90% ferrite and the balance residual austenite , Having a microstructure composed of bainite and martensite, the amount of retained austenite by X-ray analysis is 5 to 15% in volume fraction, the wall thickness is 10 mm or more, and the outer diameter is 100 mm or more. High strength steel pipe with excellent buckling resistance.
(2) By mass%, Mo: 1.0% or less, Cu: 2.0% or less, Ni: 2.0% or less, Cr: 1.0% or less A high-strength steel pipe excellent in buckling resistance as described in (1).
(3) The high-strength steel pipe excellent in buckling resistance according to (1) or (2), characterized by containing B: 0.005% or less by mass%.
(4) It is characterized by containing, by mass%, one or more of Ca: 0.01% or less, Mg: 0.1% or less, REM: 0.02% or less (1) A high-strength steel pipe excellent in buckling resistance according to any one of to (3).
(5) The seat resistance according to any one of (1) to (4), wherein the n value in the tube axis direction is 0.2 or more, and the Charpy absorbed energy at −20 ° C. is 150 J or more. High strength steel pipe with excellent bending characteristics.
(6) After heating the slab composed of the component according to any one of (1) to (4) to 1050 ° C. or higher, rough rolling is performed at a recrystallization temperature or higher, and subsequently, Ar ₃ or higher and 900 ° C. or lower. After performing finish rolling with a cumulative reduction amount of 65% or more, cooling at a temperature of Ar ₃ or more at 5 ° C./s or more, and holding for 30 to 300 seconds in the range of Ts-50 [° C.] to Ts + 100 [° C.] High strength with excellent buckling resistance, characterized by cooling the steel sheet to 350-450 ° C at a temperature of 20 ° C / s or more and allowing it to cool and then forming a hollow shape by cold forming and then performing seam welding. Steel pipe manufacturing method. However, the unit of each element is mass%,
Ts [° C.] = 780-270 × C-90 × Mn-37 × Ni-70 × Cr-83 × Mo (1)
[0007]
DETAILED DESCRIPTION OF THE INVENTION
Since the buckling is a local deformation at a strain concentrated portion, the present inventor has directed a method of suppressing the occurrence of the local deformation by significantly hardening the portion where the plastic deformation is caused by the strain concentration. For this purpose, it is necessary that the strain-concentrated portion is significantly hardened by plastic deformation, and it is considered effective to use the work-induced martensitic transformation of austenite left in the steel material.
[0008]
In order to clarify the influence of the retained austenite amount on the buckling resistance, the following experiment was conducted. A four-point bending test was performed using steel pipes having an outer diameter of 690 mm and a wall thickness of 19 mm as shown in Table 1 and different amounts of retained austenite.
[0009]
[Table 1]

[0010]
The distance between the arms was set to 1000 mm, a strain gauge was attached to the bending outer portion at the center of the specimen, and the strain was measured while applying a load to the arms. FIG. 1 shows the strain εm indicating the maximum load as a buckling resistance and shows the relationship with the amount of retained austenite. Thus, it has been obtained as a new finding that increasing the amount of retained austenite increases the buckling resistance.
[0011]
The amount of retained austenite was measured by taking a small piece from the thickness center of the steel pipe and performing X-ray analysis using Mo Kα rays to calculate the volume fraction. Specifically, the integral reflection intensity of ferrite (200) plane, (211) plane and austenite (200) plane, (220) plane, (311) plane is measured by X-ray diffraction, and Journal of The Iron. and steel Institute, 206 (1968) P60.
[0012]
Further, the present inventor considered that cooling after rolling, which has a great influence on the bainite transformation and martensite transformation of austenite, is effective in controlling the amount of retained austenite. Therefore, the slab shown in Table 2 is reheated to 1100 ° C., roughly rolled in the recrystallization temperature range, subsequently subjected to non-recrystallization rolling with a cumulative reduction of 80%, finished to a thickness of 16 mm, and cooled in various patterns. A small rolling experiment was conducted.
[0013]
[Table 2]

[0014]
As a result, the two-stage cooling pattern shown in FIG. 2 was found to be effective, and a manufacturing method for obtaining a bainite containing retained austenite and a composite structure of martensite and ferrite was clarified.
[0015]
That is, after rolling, first-stage cooling is started from a temperature of Ar ₃ [° C.] or higher in order to suppress ferrite transformation, and after completion of cooling and holding at a temperature at which sufficient ferrite is generated, residual austenite ferrite In order to suppress decomposition into carbides and to transform into bainite, the second stage cooling is performed, and then the mixture is allowed to cool. Thereby, the average particle diameter of ferrite can be refined and stable retained austenite can be generated.
[0016]
In addition, in order to complete | finish 1st stage cooling and to produce | generate sufficient ferrite, the temperature lower by 50 degreeC from the temperature 100 degreeC higher than Ts [degreeC] calculated from the chemical component in steel by the following (1) formula I found that it was necessary to hold on.
Ts [° C.] = 780-270 × C-90 × Mn-37 × Ni-70 × Cr-83 × Mo (1)
The reason why the content of the component is specified will be described below.
[0017]
The amount of C is limited to 0.02 to 0.15% or less. C is the most effective element for increasing the strength, and 0.02% or more is necessary to obtain sufficient strength. However, if the amount increases excessively, weldability deteriorates, so the upper limit was made 0.15% or less.
[0018]
Si is an element effective for deoxidation or strength improvement, and also effective for the formation of stable retained austenite. In order to obtain the effect, 0.1% or more is necessary. However, if the content is 2.0% or more, the toughness of the weld heat affected zone is remarkably deteriorated. Therefore, the amount of Si added is in the range of 0.1 to 2.0%.
[0019]
Mn is an element effective for increasing the strength, and 0.5% or more must be added to obtain a sufficient strength improvement. However, if the content exceeds 2.5%, the elongation cannot be secured, and the upper limit is set to 2.5% or less.
[0020]
P and S are impurities, P is segregated at the grain boundary, and S is precipitated as MnS. If P and S contain more than 0.02% and more than 0.005%, respectively, the toughness deteriorates, so the upper limits of P and S are made 0.02% or less and 0.005% or less, respectively.
[0021]
Al is a powerful deoxidizing element and contributes to the refinement of the structure. In order to deoxidize, 0.01% or more is necessary. However, if it exceeds 0.1%, the toughness of the weld heat affected zone is deteriorated, so the upper limit was made 0.1%.
[0022]
N is effective as a strengthening element, but if it exceeds 0.01%, the amount of dissolved N increases and the elongation is remarkably deteriorated, so the upper limit was made 0.01% or less.
[0023]
Nb, V, and Ti are contained in one or more kinds for strengthening. These form carbonitrides and contribute as precipitation strengthening. However, if each content exceeds 0.1%, it exists as coarse precipitates and deteriorates toughness. Therefore, the upper limit of each content is set to 0.1% or less. Although a minimum is not specifically limited, In order to exhibit precipitation strengthening, 0.01% or more is respectively preferable.
[0024]
Furthermore, you may contain 1 type, or 2 or more types, Mo, Cu, Ni, Cr, B, Ca, Mg, and REM as needed.
[0025]
Mo, Cu, Ni, and Cr increase the hardenability and contribute to high strength. However, if there are too many additional elements, not only the economic efficiency but also the toughness of the heat affected zone or the weldability of the field will be deteriorated, so the upper limit of Mo, Cu, Ni and Cr is 1.0% and 2. 0%, 2.0%, and 1.0% are preferable. In order to obtain the strengthening effect, it is preferable that each of Mo, Cu, Ni, and Cr contains 0.05% or more.
[0026]
B, in a small amount, greatly enhances hardenability and contributes to strengthening. However, when added in a large amount, deterioration of elongation and hardening of the heat affected zone are caused, so the upper limit is preferably made 0.005% or less. In order to acquire the effect of reinforcement | strengthening, it is preferable to contain 0.0001% or more.
[0027]
Ca and REM control the form of sulfide and contribute to the improvement of toughness. However, if added excessively, it exists as a large inclusion and deteriorates toughness. Therefore, it is preferable to set the upper limit to 0.01% or less and 0.02% or less, respectively. In order to control the form of the sulfide, each content is preferably 0.0001% or more.
[0028]
Mg is a powerful deoxidizing element and, when dispersed as a fine oxide, greatly contributes to improving the toughness of the weld heat affected zone. However, if over 0.1% is added, it will exist as a coarse oxide and deteriorate toughness, so the upper limit is preferably made 0.1%. In order to acquire the effect of a toughness improvement, it is preferable to contain 0.0001% or more.
[0029]
Next, the microstructure will be described.
[0030]
Although the toughness is improved by making the average crystal grain size of ferrite finer, the average crystal grain size of ferrite needs to be 10 μm or less in order to make Charpy absorbed energy at −20 ° C. 150 J or more. The average grain size of ferrite is preferably as fine as possible, but the current technology has a limit of about 1 μm. Further, if the area ratio of ferrite is less than 70%, the amount of hard bainite and martensite increases, so that the buckling resistance and n value decrease, and if it exceeds 90%, the strength decreases. Therefore, the area ratio of ferrite is set in the range of 70 to 90%. In the microstructure, the remainder of the ferrite having an area ratio of 70 to 90% is bainite and martensite containing retained austenite.
[0031]
The average crystal grain size and area ratio of ferrite can be obtained by the following method. A small piece is cut out from the thickness center of the steel pipe, the cross section parallel to the pipe axis direction is mirror-polished, etched, and the resulting structure is observed to distinguish ferrite from bainite and martensite. Is obtained by a cutting method according to JIS G 0552, and the average value is defined as the average crystal grain size of ferrite. The area ratio of the ferrite was calculated as a percentage obtained by calculating the sum of the areas of the ferrite grains using an image analyzer and dividing by the area of the visual field.
[0032]
In order to remarkably improve the buckling resistance, the volume fraction of retained austenite needs to be 5% or more. However, when it exceeds 15%, the toughness is impaired, so the volume fraction of retained austenite is limited to 5 to 15%. The volume fraction of retained austenite is determined by X-ray analysis because it is difficult to measure with a microstructure.
[0033]
The steel pipe of the present invention limits the size of the steel pipe to a thickness of 10 mm or more and an outer diameter of 100 mm or more in order to satisfy the strength required in its use environment. In the current technology, it is difficult to make the thickness and the outer shape more than 50 mm and more than 1200 mm, respectively.
[0034]
The buckling resistance is improved as the n value is improved. To obtain this effect, the n value is preferably 0.2 or more. Although the upper limit of the n value is not specified, it is difficult to make it more than 0.26 with the current technology. The n value was measured by taking a 12B or 12C arc-shaped tensile test piece according to JIS Z 2201 with the tube axis direction of the steel pipe as the longitudinal direction, performing a tensile test according to JIS Z 2241, and having a strain of 1 It can be determined as a work hardening rate in the range of 8% to 8%.
[0035]
Next, a manufacturing method will be described.
[0036]
The heating temperature of the slab is limited to 1050 ° C. or higher at which Nb is dissolved in order to finely precipitate Nb carbonitride by hot rolling. Although the upper limit is not particularly defined, the upper limit of the heating temperature of the slab is preferably 1250 ° C. in order to suppress the coarsening of the austenite grain size and improve the toughness.
[0037]
The finish rolling needs to be performed at 900 ° C. or less below the recrystallization temperature in order to refine the average crystal grain size of ferrite. On the other hand, when the rolling end temperature is lower than Ar ₃ [° C.], ferrite into which processing strain due to rolling is introduced remains, and the toughness is greatly deteriorated. Therefore, the rolling end temperature is limited to Ar ₃ or higher. Further, the cumulative reduction amount of the finish rolling is limited to 65% or more so that the average crystal grain size of ferrite is 10 μm or less. Although the upper limit is not specified, it is preferably 90% or less because of technical limitations.
[0038]
Ar ₃ is determined from the chemical components in steel by the following formula using the unit of each element as mass%.
Ar ₃ [° C.] = 921−325 × C + 33 × Si + 287 × P + 40 × Al−92 × (Mn + Mo + Cu) −46 × (Cr + Ni)
In the present invention, cooling after rolling is the most important process for refining the average grain size of ferrite and controlling the amount of retained austenite. After finish rolling, first stage cooling is performed to suppress ferrite transformation, intermediate holding is performed to generate sufficient ferrite, and second stage cooling is performed to suppress decomposition of residual austenite into ferrite and carbide. Then, let it cool.
[0039]
In the first stage cooling, in order to suppress the ferrite transformation after hot rolling, the starting temperature is set to Ar ₃ or higher, and the cooling rate is set to 5 ° C./s or higher. This is because if the starting temperature is less than Ar ₃ and the cooling rate is less than 5 ° C./s, coarse ferrite grains are generated and the toughness is lowered. Since the first stage cooling may be a temperature higher than Ar ₃ , no upper limit is defined. Therefore, you may start immediately after completion | finish of finish rolling. Although the upper limit of the cooling rate is not specified, it is difficult to exceed 300 ° C./s with the current technology.
[0040]
Furthermore, in order to generate sufficient ferrite, the unit of each element is defined as mass% from the chemical components in the steel, and the temperature is 50 ° C. higher than TS from 100 ° C. higher than Ts [° C.] determined by the following formula (1). It is necessary to keep the temperature in the range of a low temperature. The reason is that if held at a temperature exceeding TS + 100 ° C., the average crystal grain size of ferrite becomes coarse and the toughness decreases, and if held at a temperature lower than TS-50 ° C., bainite transformation occurs and the ferrite area ratio decreases. This is because the buckling resistance is deteriorated.
[0041]
Ts [° C.] = 780-270 × C-90 × Mn-37 × Ni-70 × Cr-83 × Mo (1)
The holding time is required to be 30 seconds or more in order to sufficiently generate ferrite and to make the area ratio 70% or more. However, if it exceeds 300 seconds, the average grain size of the generated ferrite becomes coarse and the toughness decreases, so the upper limit is limited to 300 seconds. In addition, it is not preferable that a sudden temperature drop occurs within this holding time. Usually, it may be allowed to cool, but the temperature drop may be suppressed by a heat insulation furnace or a heat insulation cover.
[0042]
After this intermediate holding, second-stage cooling is performed, and this needs to be performed at 20 ° C./s or more in order to improve the buckling resistance by controlling the amount of retained austenite to 5 to 10%. The reason is that when cooling is performed at less than 20 ° C./s, the retained austenite in bainite is decomposed in a large amount into ferrite and carbides, and the amount of retained austenite decreases. The upper limit is not particularly specified, but it is difficult to exceed 100 ° C./s due to the limitation of the current accelerated cooling apparatus.
[0043]
Further, in order to generate retained austenite for improving the buckling resistance, it is necessary to set the end temperature of the second stage cooling within the range of 350 to 450 ° C. When the end temperature of the second stage cooling exceeds 450 ° C., the retained austenite is decomposed into carbide and ferrite by the second stage cooling, and the amount of retained austenite decreases. On the other hand, when the second stage cooling is performed to less than 350 ° C., martensitic transformation occurs and the amount of retained austenite decreases. Therefore, the end temperature of the second stage cooling is limited to 350 ° C. to 450 ° C.
[0044]
Pipemaking is performed by a manufacturing process in which cold forming is performed and ends are joined by seam welding. The cold working may be either press working or roll forming, and any combination thereof may be used, but a steel pipe manufacturing process based on the UOE method in which C forming, U forming and O forming are performed and the end is seam welded to expand the tube is preferable. . Seam welding may be MAG arc welding, submerged arc welding, or a combination thereof.
[0045]
【Example】
Steels having the components shown in Table 3 were melted and hot-rolled under the conditions shown in Table 4 to perform first stage cooling, intermediate holding, and second stage cooling to produce a steel plate having a plate thickness of 16 mm. Rough rolling was performed at a recrystallization temperature or higher, and finish rolling was performed in a range of Ar _{3 to} 900 ° C. The steel sheet was cold-formed, welded on the inside and outside, and then expanded to form a UOE steel pipe having an outer diameter of 690 mm and a wall thickness of 16 mm.
[0046]
A small piece was cut out from the thickness center of the steel pipe, and the cross-section parallel to the pipe axis direction was mirror-polished and then etched with nital to observe the revealed structure. The microstructure was ferrite and bainite and martensite with the balance containing retained austenite. Differentiating ferrite from bainite and martensite containing retained austenite, the average crystal grain size of ferrite in any of 10 fields of view was determined by a cutting method according to JIS G0552, and the average value was defined as the average crystal grain size of ferrite. The area ratio of the ferrite was calculated as a percentage obtained by calculating the sum of the areas of the ferrite grains using an image analyzer and dividing by the area of the visual field. The volume fraction of retained austenite was determined by X-ray analysis. Sample No. 12C arc-shaped tensile test specimens according to JIS Z2201 from 90 °, 180 °, and 270 ° in the circumferential direction from the welded portion of the steel pipe, and measuring the tensile properties according to JIS Z 2241 The average value was obtained. The n value in the tube axis direction was measured as a work hardening rate when the strain was 1 to 8%, and an average value was obtained.
[0047]
The buckling resistance of each steel material was evaluated by a four-point bending test. The test pair had a thickness of 16 mm, an outer diameter of 690 mm, a length of 3000 mm, and a distance between the pressing arm portions of 1000 mm. While measuring strain in the tube axis direction by attaching a strain gauge to the bending outer part at the center of the specimen, load is applied to the arm and the strain εm up to the maximum load is obtained as the buckling strain. It was.
[0048]
The toughness is measured by taking a Charpy test piece of 2 mmV notch according to JIS Z 2202 with the circumferential direction as the longitudinal direction from the thickness center of the steel pipe, performing a Charpy test at 0 ° C. according to JIS Z 2242, and absorbing energy Was measured.
[0049]
Table 5 shows the results obtained. The steel pipe manufactured according to the present invention has a retained austenite amount of 5% or more in volume fraction and an n value of 0.2 or more. As a result, the buckling strain is higher than that of the conventional steel pipe and the toughness is also good. It is.
[0050]
On the other hand, test no. No. 14 has a small amount of cumulative reduction in finish rolling, and test No. Nos. 15 and 16 have low first stage cooling start temperatures. Since No. 22 has a long intermediate holding time, the average crystal grain size of ferrite increases and the toughness decreases. Test No. In Nos. 17 and 18, since the cooling rate of the first stage cooling is slow, the average crystal grain size of ferrite is increased, the toughness is lowered, the ferrite area ratio is increased, and the tensile strength is slightly lowered.
[0051]
Test No. Since No. 19 and 20 have both the intermediate holding start temperature and end temperature lower than the range of the present invention, the amount of retained austenite is reduced and the buckling resistance is lowered. Test No. Since No. 21 has a short intermediate holding time, the ferrite area ratio is small and the buckling strain is reduced. No. Nos. 23 and 24 have low end temperatures for intermediate holding, so No. 25 has a slow second stage cooling rate. Since No. 26 has a low end temperature of the second stage cooling, the amount of retained austenite is small and the buckling strain is low.
[0052]
[Table 3]

[0053]
[Table 4]

[0054]
[Table 5]

[Brief description of the drawings]
FIG. 1 is a graph showing the effect of retained austenite volume fraction on buckling strain.
FIG. 2 shows a two-stage cooling pattern according to the present invention.

Claims (6)

% By mass
C: 0.02% to 0.15%,
Si: 0.1% to 2.0%,
Mn: 0.5% to 2.5%
Al: 0.01% to 0.1%,
N: 0.01% or less,
P: 0.02% or less,
S: 0.005% or less,
In addition,
Nb: 0.1% or less,
V: 0.1% or less,
Ti: containing one or more of 0.1% or less, the balance being iron and inevitable impurities, the average crystal grain size of 10 μm or less, the area ratio of 70 to 90% ferrite and the balance residual austenite , Having a microstructure composed of bainite and martensite, the amount of retained austenite by X-ray analysis is 5 to 15% in volume fraction, the wall thickness is 10 mm or more, and the outer diameter is 100 mm or more. High strength steel pipe with excellent buckling resistance. It is characterized by further containing one or more of Mo: 1.0% or less, Cu: 2.0% or less, Ni: 2.0% or less, Cr: 1.0% or less. A high-strength steel pipe excellent in buckling resistance according to claim 1. The high-strength steel pipe excellent in buckling resistance according to claim 1 or 2, characterized by further containing, by mass%, B: 0.005% or less. The composition further comprises one or more of Ca: 0.01% or less, Mg: 0.1% or less, and REM: 0.02% or less in mass%. A high-strength steel pipe excellent in buckling resistance according to any one of the items. The n value in the tube axis direction is 0.2 or more, and the Charpy absorbed energy at -20 ° C is 150 J or more, which is excellent in buckling resistance according to any one of claims 1 to 4. High strength steel pipe. After heating the slab containing components according to above 1050 ° C. to claim 1, carried out rough rolling recrystallization temperature or higher, then subsequently, a cumulative reduction in the Ar ₃ [° C.] or higher 900 ° C. or less After performing finish rolling with an amount of 65% or more, cooling at a temperature of 5 ° C./s or higher from a temperature of Ar ₃ [° C.] or higher, and holding for 30 to 300 seconds in the range of Ts−50 [° C.] to Ts + 100 [° C.]. The steel sheet is cooled to a temperature of 350 to 450 ° C. at a temperature of 20 ° C./s or more and allowed to cool, and then formed into a hollow shape by cold forming and then subjected to seam welding. A manufacturing method for high strength steel pipes. However, the unit of each element is mass%,
Ts [° C.] = 780-270 × C-90 × Mn-37 × Ni-70 × Cr-83 × Mo (1)

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