JP4116817B2 - Manufacturing method of high strength steel pipes and steel sheets for steel pipes with excellent low temperature toughness and deformability - Google Patents
Manufacturing method of high strength steel pipes and steel sheets for steel pipes with excellent low temperature toughness and deformability Download PDFInfo
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Description
【０００１】
【発明の属する技術分野】
本発明は、米国石油協会（ＡＰＩ）規格でＸ６０〜Ｘ８０（降伏強度で約４１３〜５５１ＭＰａ）の強度と優れた溶接熱影響部（ＨＡＺ）靭性および変形能を有する鋼管に関するものである。
【０００２】
【従来の技術】
原油・天然ガスを長距離輸送するパイプラインに使用するラインパイプは、高圧による輸送効率の向上や薄肉化による現地での溶接効率向上のための高張力化が要求され、これまでにＡＰＩ規格でＸ８０までのラインパイプが実用化されている。一方、敷設域の寒冷地化に伴う高ＨＡＺ靭性化および地震発生時に十分な吸収エネルギーを確保するための高変形能も近年要求されつつあり、安全性の高い鋼管が望まれている。
【０００３】
低合金鋼のＨＡＺ靭性は、（１）結晶粒のサイズ、（２）高炭素島状マルテンサイト（Ｍ* ）、上部ベイナイト（Ｂｕ）などの硬化相の分散状態、（３）粒界脆化の有無、（４）元素のミクロ偏析など種々の冶金学的要因に支配される。なかでも、ＨＡＺの結晶粒のサイズは低温靭性に大きな影響を与えることが知られており、ＨＡＺ組織を微細化する数多くの技術が開発実用化されている。
【０００４】
例えば、ＴｉＮを微細に分散させ、４９０ＭＰａ級高張力鋼の大入熱溶接時のＨＡＺ靭性を改善する手段が開示されている（「鉄と鋼」（昭和５４年６月発行、第６５巻第８号１２３２頁）。しかし、これらの析出物は溶融線近傍においては１４００℃以上の高温にさらされるため大部分が粗大化或いは溶解し、ＨＡＺ組織が粗大化してＨＡＺ靭性が劣化するという欠点を有する。
【０００５】
この問題に対して、鋼中にＴｉ酸化物を微細分散させて、溶接時のＨＡＺにおいて粒内アシキュラーフェライト（以下ＩＧＦと呼ぶ）を生成させることにより溶融線近傍のＨＡＺ組織は微細化され、ＨＡＺ靭性が改善されることが特開昭６３−２１０２３５号公報、特開平１−１５３２１号公報などに開示されている。しかしながら、―５０〜−６０℃となるような寒冷地では十分に対応できず、ＨＡＺ靭性の改善が強く望まれている。
【０００６】
一方、変形能に関して、特開平１１−２７９７００号公報では、面積分率で１０〜５０％の下部ベイナイトを含有する対座屈特性に優れた鋼管、特開平１１−３４３５４２号公報では、平均アスペクト比が２〜１５である島状マルテンサイトを面積分率で２〜１５％含有する耐座屈特性に優れた鋼管が開示されている。これは鋼管母材における耐局部座屈性を向上させることを目的としたものであるが、高い変形能と良好なＨＡＺ靭性を同時に満足することを目的とした鋼管に関するものではない。
【０００７】
【発明が解決しようとする課題】
本発明は、良好なＨＡＺ靭性および優れた変形能を有するＸ６０〜Ｘ８０の高強度鋼管およびその製造方法を提供するものである。
【０００８】
【課題を解決するための手段】
本発明の要旨は、以下のとおりである。
（１）質量％で、Ｃ：０．０３〜０．１０％、Ｓｉ：０．６％以下、Ｍｎ：０．８〜２．０％、Ｐ：０．０１５％以下、Ｓ：０．００１２〜０．００５％、Ｎｂ：０．００５〜０．０５％、Ｔｉ：０．００５〜０．０３０％、Ａｌ：０．００１〜０．００５％、Ｍｇ：０．０００１〜０．００５０％、Ｎ：０．００１〜０．００６％、Ｏ：０．００１〜０．００６％を含有し、残部が鉄および不可避的不純物からなり、ＣＥ＝Ｃ＋Ｍｎ／６＋（Ｃｒ＋Ｍｏ＋Ｖ）／５＋（Ｎｉ＋Ｃｕ）／１５で定義されるＣＥ値が０．３０〜０．４５の範囲にあり、ＭｇとＡｌからなる酸化物を内包する０．０１〜０．５μｍのＴｉＮが１００００個／ｍｍ2以上含有し、かつ酸化物と硫化物が複合した形態で０．３質量％以上 のＭｎを含有する０．５〜１０μｍの粒子が１０個／ｍｍ2以上含有する母材と、
Ｃ：０．０３〜０．１０％、Ｓｉ：０．６％以下、Ｍｎ：１．０〜２．２％、Ｐ：０．０１５％以下、Ｓ：０．０１％以下、Ｎｂ：０．００５〜０．０５％、Ｔｉ：０．００５〜０．０３％、Ｂ：０．０００３〜０．００２％、Ａｌ：０．０５％以下、Ｎ：０．００１〜０．０１％、Ｏ：０．０１５〜０．０３０％を含有し、残部が鉄及び不可避的不純物からなり、かつ、Ｐ＝｛１．５（Ｏ−０．８９Ａｌ）＋３．４Ｎ｝−Ｔｉで定義されるＰ値が−０．０１０〜０．０１０の範囲であり、さらにＣＥ＝Ｃ＋Ｍｎ／６＋（Ｃｒ＋Ｍｏ＋Ｖ）／５＋（Ｎｉ＋Ｃｕ）／１５で定義されるＣＥ値が０．３５〜０．５０の範囲にある溶接金属部を有することを特徴とする低温靭性と変形能に優れた高強度鋼管。
【０００９】
（２）前記溶接金属部が、さらに質量％で、
Ｎｉ：０．１〜２．０％、 Ｃｕ：０．１〜１．０％、
Ｃｒ：０．１〜２．０％、 Ｍｏ：０．１〜２．０％、
Ｖ ：０．０１〜０．１％、 Ｃａ：０．００１〜０．００５％
のうち１種または２種以上を含有していることを特徴とする前記（１）に記載の低温靭性と変形能に優れた高強度鋼管。
【００１０】
（３）前記母材が、さらに質量％で、
Ｎｉ：０．１〜１．０％、 Ｃｕ：０．１〜１．２％、
Ｃｒ：０．１〜１．０％、 Ｍｏ：０．１〜１．０％、
Ｖ ：０．０１〜０．１％、 Ｃａ：０．０００５〜０．００５０％
の１種または２種以上を含有し、前記溶接金属部が、さらに質量％で、
Ｎｉ：０．１〜２．０％、 Ｃｕ：０．１〜１．０％、
Ｃｒ：０．１〜２．０％、 Ｍｏ：０．１〜２．０％、
Ｖ ：０．０１〜０．１％、 Ｃａ：０．００１〜０．００５％
のうち１種または２種以上を含有していることを特徴とする前記（１）に記載の低温靭性と変形能に優れた高強度鋼管。
【００１１】
（４）前記（１）〜（３）のいずれかに記載の鋼管において、さらに母材部の金属組織が粒径２０μｍ以下のフェライトを３０〜７０％含有することを特徴とする低温靭性と変形能に優れた高強度鋼管。
（５）前記（１）〜（３）のいずれかに記載の鋼管において、さらに溶接金属部における硬さが母材部における硬さの０．９５〜１．１５倍であることを特徴とする低温靭性と変形能に優れた高強度鋼管。
（６）前記（１）〜（３）のいずれかに記載の鋼管において、さらに母材部の金属組織が粒径２０μｍ以下のフェライトを３０〜７０％、溶接金属部における硬さが母材部における硬さの０．９５〜１．１５倍であることを特徴とする低温靭性と変形能に優れた高強度鋼管。
【００１２】
（７）質量％で、Ｃ：０．０３〜０．１０％、Ｓｉ：０．６％以下、Ｍｎ：０．８〜２．０％、Ｐ：０．０１５％以下、Ｓ：０．００１２〜０．００５％、Ｎｂ：０．００５〜０．０５％、Ｔｉ：０．００５〜０．０３０％、Ａｌ：０．００１〜０．００５％、Ｍｇ：０．０００１〜０．００５０％、Ｎ：０．００１〜０．００６％、Ｏ：０．００１〜０．００６％を含有し、残部が鉄および不可避的不純物からなり、ＣＥ＝Ｃ＋Ｍｎ／６＋（Ｃｒ＋Ｍｏ＋Ｖ）／５＋（Ｎｉ＋Ｃｕ）／１５で定義されるＣＥ値が０．３０〜０．４５の範囲にあり、ＭｇとＡｌからなる酸化物を内包する０．０１〜０．５μｍのＴｉＮが１００００個／ｍｍ2以上含有し、かつ酸化物と硫化物が複合した形態で０．３質量％以上 のＭｎを含有する０．５〜１０μｍの粒子が１０個／ｍｍ2以上含有する鋳片を９５０〜 １２００℃に加熱した後、９５０℃以下の圧下率を５０％以上とし、７００〜８５０℃の温度範囲で圧延を終了した後、６５０〜８００℃の温度範囲から２℃／秒以上の冷却速度で４５０℃以下の任意の温度まで冷却し、その後空冷することを特徴とする低温靭性と変形能に優れた鋼管用鋼板の製造法。
【００１３】
（８）鋳片がさらに質量％で、
Ｎｉ：０．１〜１．０％、 Ｃｕ：０．１〜１．０％、
Ｃｒ：０．１〜１．０％、 Ｍｏ：０．１〜１．０％、
Ｖ ：０．０１〜０．１％、 Ｃａ：０．０００５〜０．００５０％
の１種または２種以上を含有することを特徴とする前記（７）に記載の低温靭性と変形能に優れた鋼管用鋼板の製造法。
【００１４】
【発明の実施の形態】
以下に、本発明の高強度鋼管について詳細に説明する。
本発明の特徴は、低Ｃ―Ｎｂ−Ｔｉ系を基本にＭｇ，ＮおよびＯ量を厳格に制限し、かつＭｇとＡｌからなる酸化物を内包する微細な炭窒化物、および酸化物と硫化物からなる複合物とを含有させた母材部と、低Ｃ−Ｍｎ−Ｂ系の溶接金属部から構成される鋼管において、良好なＨＡＺ靭性と高い変形能を有する高強度鋼管にある。
【００１５】
低合金鋼の低温靱性は、（１）結晶粒のサイズ、（２）ＭＡや上部ベイナイト（Ｂｕ）などの硬化相の分散状態など種々の冶金学的要因に支配される。なかでもＨＡＺの結晶粒のサイズおよびＭＡは低温靱性に大きな影響を与えることが知られている。
【００１６】
高強度鋼管のＨＡＺにおいて、靭性に有害なＭＡが多量に生成するためにＨＡＺ靱性が劣化する傾向にある。靭性に有害なＭＡの悪影響を無くすためにはＨＡＺの結晶粒を徹底的に微細化しなければならない。そこで、ＨＡＺにおけるオーステナイト（γ）粒の粗大化を抑制する技術とともに、γ粒内からＩＧＦを生成させる技術との複合効果により、ＨＡＺの結晶粒を微細化し、ＨＡＺ靭性を著しく改善できることを見出し、本発明に至った。
【００１７】
すなわち、Ｍｇの添加によりＭｇとＡｌからなる酸化物を内包する微細なＴｉＮなどの炭窒化物を鋼中に生成させることにより、ＨＡＺにおけるγ粒の粗大化を抑制すること、およびＭｇ，Ｍｎ，Ｓを含む酸化物・析出物からＩＧＦを生成することにより結晶粒を微細化でき、ＨＡＺ靱性を向上させることが可能である。ＭｇとＡｌからなる酸化物を内包する微細なＴｉＮなどの炭窒化物およびＭｇ、Ｍｎ，Ｓを含む酸化物・析出物は高温でも化学的に安定で溶解しないため、γ粒の粗大化抑制効果およびＩＧＦの生成効果が維持される。
【００１８】
そこで、溶融線近傍の１４００℃以上に加熱されるＨＡＺにおいても、化学的に安定な微細な酸化物をピンニング粒子として用いること、および０．５μｍ以上の酸化物・硫化物をＩＧＦの生成核として用いることにより、ＨＡＺ組織を徹底的に微細化する方法を検討した。
【００１９】
この結果、まず、微量のＭｇとＡｌを含有させることにより、０．０１〜０．０５μｍの微細な（Ｍｇ，Ａｌ）酸化物が多量に生成することを見出した。０．０１〜０．５μｍのＴｉＮがこの微細な（Ｍｇ，Ａｌ）酸化物を核として複合析出するため、１４００℃以上の高温においても優れたγ粒のピンニング効果を維持できることを明らかにした。この時、鋼中に含有する０．０１〜０．５μｍのＴｉＮが１００００個／ｍｍ2 未満の場合には、γ粒の粗大化抑制効果が不十分となり、良好なＨＡＺ靱性を得ることができない。そこで、ＭｇとＡｌから成る酸化物を内包する０．０１〜０．５μｍのＴｉＮを１００００個／ｍｍ2 以上含有させる必要がある。
【００２０】
さらに、このＴｉＮを生成させるためには０．０００１％以上のＭｇを添加する必要がある。Ｍｇ添加量が多すぎるとＭｇ系酸化物が増加し、低温靱性を劣化させるので、その上限を０．００５０％に限定した。さらに、ＴｉＮの核となる微細な（Ｍｇ，Ａｌ）酸化物を生成させるためには、微量のＡｌを含有させる必要がある。しかしながら、Ａｌの添加により、粗大なアルミナのクラスターが生成し、低温靱性に悪影響を与える。このため、Ａｌの含有量を０．００１〜０．００５％に限定した。０．００１％以上のＡｌ量であれば、微細な（Ｍｇ，Ａｌ）酸化物を生成させることができる。
【００２１】
次に、ＩＧＦ生成の核となる酸化物・硫化物の必要な要件として、酸化物・硫化物の複合体の個数、サイズおよび組成を制御することにより、溶融線近傍のＨＡＺにおいてもＩＧＦが生成し、ＨＡＺ組織が微細化され、ＨＡＺ靭性が改善されることを見出した。
【００２２】
まず、ＩＧＦの生成核となる酸化物・硫化物の複合体の個数は、少なくとも１０個／ｍｍ2 以上必要である。ＩＧＦ変態核が１０個／ｍｍ2 未満ではＨＡＺ組織の微細化が不十分となり良好なＨＡＺ靭性は得られない。
また、ＩＧＦの変態核として機能するためには、０．５μｍ以上の大きさが必要である。０．５μｍ未満ではＩＧＦ変態核として十分に機能せず、ＨＡＺ組織の微細化効果が得られない。一方、１０μｍを超える酸化物・硫化物の複合体の場合、脆性破壊の発生点となるため、良好なＨＡＺ靭性が得られない。
【００２３】
さらに、ＩＧＦの変態核として機能するためには、０．３質量％以上のＭｎを含有する必要がある。本発明では、１４００℃以上の高温においてγ粒のピンニングに有効な微細な粒子を生成させるために、Ｍｎよりも脱酸力の強いＭｇ，Ａｌ，Ｔｉを含有するので、酸化物の中にＭｎを含有させることは難しい。そこで、Ｍｎを含む硫化物を酸化物上に複合析出させる必要がある。酸化物・硫化物の複合体におけるＭｎ量が０．３質量％未満の場合、十分なＩＧＦ生成機能が得られず、ＨＡＺ組織は微細化しない。
【００２４】
合金元素の添加量を適切にしないとＨＡＺ靭性は劣化する。そこで、ＨＡＺ靭性を大幅な劣化を招くことなく目標とする強度を得るために、合金元素の適正な添加量について検討した。ＣＥ値で定義される値を所定の範囲に限定することにより、十分な強度を確保することができる。また溶接金属中の合金元素添加量についても、ＣＥ値および値を所定の範囲に制御すれば、溶接金属の靭性を大きく損なうことなく、目標とする強度が得られる。
【００２５】
地震の多発する地域や永久凍土に敷設されるパイプラインにおいては、数％の歪がパイプラインに負荷されるといわれている。この場合、溶接金属部における硬さが母材部における硬さの０．９５〜１．１５倍であれば、延性亀裂の発生が防止できることを見出した。また、母材の一様伸びを増加させるためには２０μｍ以下のフェライトを３０〜７０％含有することが必要であることを見出した。また、鋼管用鋼板の製造法として、７００〜８５０℃の温度範囲で圧延を終了し、６５０〜８００℃の温度範囲から２℃／秒以上の冷却速度で４５０℃以下の任意の温度まで冷却し、その後空冷することにより、高強度と高一様伸びを両立する鋼板が得られることを見出し、本発明に至った。
【００２６】
すなわち本発明の特徴は、鋼管母材として、低Ｃ−Ｎｂ−Ｔｉ−Ｍｇ系成分を適用するに際し、目標とする強度を確保するために、合金元素添加量をＣＥ値で定義される適正な範囲に限定すること、および溶接金属として、靭性の劣化を損なうことなく目標とする強度を満足させるために、合金元素添加量をＣＥ値で定義される適正な範囲に限定すること、溶接金属の低温靭性を確保するために、合金元素添加量をＰ値で定義される適正な範囲に限定すること、さらに優れた変形能を確保するために、溶接金属部における硬さが母材部における硬さの０．９５〜１．１５倍にすること、大きな一様伸びを得るために、母材部の金属組織が粒径２０μｍ以下のフェライトを３０〜７０％含有することにある。
【００２７】
以下に、鋼管母材の成分限定理由について説明する。
Ｃは母材とＨＡＺの強度、靭性および高い一様伸びを確保するために、０．０３％以上の添加が必要である。しかし、０．１０％を超えると母材およびＨＡＺの靭性が低下すると共に、溶接性が劣化するので、０．１０％を上限値とした。
【００２８】
目標とするＸ６０〜Ｘ８０の強度を満足させるためには、合金元素の添加量の適正化が必要である。すなわち、ＣＥ＝Ｃ＋Ｍｎ／６＋（Ｃｒ＋Ｍｏ＋Ｖ）／５＋（Ｎｉ＋Ｃｕ）／１５で定義されるＣＥ値を０．３０〜０．４５の範囲にしなければならない。ＣＥ値が０．３０未満では目標とするＸ６０以上の強度が確保できない。また、ＣＥ値が０．４５を超えるとＭ* の生成が顕著となり、ＨＡＺ靭性が劣化する。このためＣＥ値の範囲を０．３０〜０．４５に限定した。
【００２９】
Ｓｉは脱酸や強度向上のため添加する元素であるが、多く添加すると現地溶接性、ＨＡＺ靭性を劣化させるので、上限を０．６％とした。鋼の脱酸はＴｉのみでも十分であり、Ｓｉは必ずしも添加する必要はない。
【００３０】
Ｍｎは強度、低温靭性を確保する上で不可欠な元素であり、その下限は０．８％である。しかし、Ｍｎが多すぎると鋼の焼入性が増加して現地溶接性、ＨＡＺ靭性を劣化させるだけでなく、連続鋳造鋼片の中心偏析を助長し、低温靭性も劣化させるので上限を２．０％とした。
【００３１】
本発明において、不可避的不純物であるＰ量を０．０１５％以下とする。この主たる理由は母材及びＨＡＺの低温靭性をより一層向上させるためである。Ｐ量の低減は連続鋳造スラブの中心偏析を低減させて、粒界破壊を防止し低温靭性を向上させる。
【００３２】
Ｓは本発明において重要な元素である。ＩＧＦ変態核として酸化物上に硫化物を複合析出させるためには０．００１２％以上含有しなければならない。しかし、Ｓが０．００５％を超えると母材およびＨＡＺの靭性が劣化するので、０．００５％を上限値とする。
【００３３】
Ｎｂは制御圧延時にγの再結晶を抑制して結晶粒を微細化するだけでなく、析出硬化や焼入性の増大にも寄与し、鋼を強靭化する作用を有し、本発明において必須の元素である。この効果を得るためには最低０．００５％のＮｂが必要である。しかしながら、Ｎｂ量が多すぎるとＨＡＺ靭性が劣化するので、その上限値を０．０５％に限定した。
【００３４】
Ｔｉは微細なＴｉＮを形成し、スラブ再加熱時及びＨＡＺのγ粒の粗大化を抑制して、ミクロ組織を微細化して、母材及びＨＡＺの低温靭性を改善し、本発明において必須の元素である。この効果を発揮させるためには、０．００５％以上の添加が必要である。また、多すぎるとＴｉＮの粗大化やＴｉＣによる析出硬化が生じ、低温靭性を劣化させるので、その上限値を０．０３％に限定した。
【００３５】
ＮはＴｉＮを形成し、スラブ再加熱時及びＨＡＺのγ粒の粗大化を抑制して母材、ＨＡＺの低温靭性を向上させる。このために必要な最小量は０．００１％である。しかし、Ｎ量が多すぎるとスラブ表面疵や固溶ＮによるＨＡＺ靭性の劣化の原因となるので、その上限値は０．００６％に抑える必要がある。
【００３６】
Ｏは、超微細な（Ｍｇ，Ａｌ）酸化物を形成して、ＨＡＺのγ粒の粗大化抑制効果を発揮すると同時に、０．５〜１０μｍのＭｇ含有酸化物を形成してＨＡＺにおいてＩＧＦ変態核として機能する。これらの機能を発揮させるためには、０．００１％以上のＯが必要である。Ｏが０．００１％未満の場合、１００００個／ｍｍ2 以上の超微細酸化物や１０個／ｍｍ2 以上の０．５〜１０μｍ酸化物を確保することが困難である。しかし、Ｏが０．００６％を超えると１０μｍを超える粗大な酸化物が生成し、母材やＨＡＺにおいて脆性破壊の発生点となるため、０．００６％を上限値とした。
【００３７】
次にＮｉ，Ｃｕ，Ｃｒ，Ｍｏ，Ｖ，Ｃａを添加する理由について説明する。
基本成分にこれらの元素を添加する主たる目的は、本発明鋼の特徴を損なうことなく、強度・低温靭性などの特性の向上をはかるためである。したがってその添加量は自ら制限されるべき性質のものである。
【００３８】
Ｎｉは溶接性、ＨＡＺ靭性に悪影響を及ぼすことなく母材の強度、低温靭性を向上させるが、０．１％未満では効果が薄く、１．０％超の添加は溶接性に好ましくないため、その上限値を１．０％とした。
【００３９】
ＣｕはＮｉとほぼ同様の効果を有すると共に耐食性、耐水素誘起割れ性などにも効果があり、０．１％以上の添加が必要である。しかし、過剰に添加すると析出硬化により母材、ＨＡＺ靭性劣化や熱間圧延時にＣｕ−クラックが発生するため、その上限値を１．２％とした。
【００４０】
Ｃｒは母材、溶接部の強度を増加させる効果があり、０．１％以上の添加が必要である。しかし、多すぎると現地溶接性やＨＡＺ靭性を著しく劣化させる。このためＣｒ量の上限は１．０％とした。
【００４１】
Ｍｏは母材及び溶接部の強度を上昇させる元素であるが、１．０％を超えるとＣｒと同様に母材、ＨＡＺ靭性及び溶接性を劣化させる。また、０．１％未満の添加ではその効果が薄い。
【００４２】
Ｖは、ほぼＮｂと同様の効果を有するが、その効果はＮｂに比較して格段に弱い。その効果を発揮させるためには０．０１％以上の添加が必要である。また、上限は現地溶接性、ＨＡＺ靭性の点から０．１％まで許容できる。
【００４３】
Ｃａは硫化物（ＭｎＳ）の携帯を制御し、低温靭性を向上（シャルピー試験における吸収エネルギーの増加など）させるほか、耐サワー性の向上にも著しい効果を発揮する。０．０００５％未満ではその効果が薄く、また０．００５％を超えて添加するとＣａＯ−ＣａＳが大量に生成してクラスター、大型介在物となり、鋼の清浄度を害するだけでなく、現地溶接性にも悪影響を及ぼす。このためＣａ添加量を０．０００５〜０．００５％に制限した。
【００４４】
一方、鋼管長手方向の溶接金属部の低温靱性は、（１）結晶粒のサイズ、（２）島状マルテンサイトなどの硬化相の分散状態など種々の冶金学的要因に支配される。とくに高強度化、厚肉化するほど合金元素の添加量は必然的に多くなり、組織は上部ベイナイト主体の組織となり、靭性は劣化しやすくなる。そこでＡｌ，Ｎ，酸素およびＴｉ量のバンランスを適正化することにより低温靱性を飛躍的に改善できる。
すなわちＰ＝｛１．５（Ｏ−０．８９Ａｌ）＋３．４Ｎ｝−Ｔｉで表される式において、Ｐ値が−０．０１０〜０．０１０％になるように各成分を適正化することにより、低温靱性が向上する。Ｐ値はＴｉ量の過不足を示したもので、Ｐ値が低い（マイナス）場合にはＴｉが過剰に添加されていることになり、ＴｉＣなど析出硬化により低温靱性が劣化する。一方Ｐ値が高い（プラス）場合にはＴｉ量が不足（または酸素量が過剰）しているために、低温靱性が劣化する。良好な低温靱性を得るためにはＰ値を−０．０１０〜０．０１０％にする必要がある。
【００４５】
次に溶接金属の成分限定理由について説明する。
溶接金属の高温割れを防止するために、Ｃ量は０．０３％以上必要である。０．０３％未満では溶接後、凝固する過程でδ凝固が起こり、高温割れが発生するためである。しかしながらＣ量が０．１０％を超えると、溶接金属の低温靭性が劣化するため、その上限値を０．１０％とした。
【００４６】
Ｓｉは脱酸や強度向上のため添加する元素であるが、多く添加すると低温靭性や現地溶接性を劣化させるので、上限を０．６％とした。
【００４７】
Ｍｎは強度、低温靭性を確保する上で不可欠な元素であり、その下限は１．０％である。しかし、Ｍｎが多すぎると鋼の焼入性が増加して低温靭性や現地溶接性を劣化させるので、上限を２．２％とした。
【００４８】
Ｎｂは鋼を強靭化する作用を有し、０．００５％以上必要である。しかし、Ｎｂを０．０５％超添加すると現地溶接性や低温靭性に悪影響をもたらすので、その上限を０．０５％とした。
【００４９】
Ｔｉ添加は微細なＴｉＮを形成し、低温靭性を改善する。このようなＴｉＮの効果を発現させるためには、最低０．００５％のＴｉ添加が必要である。しかし、Ｔｉ量が多すぎるとＴｉＮの粗大化やＴｉＣによる析出硬化が生じ、低温靭性が劣化するので、その上限は０．０３％に限定しなければならない。
【００５０】
Ｂは極微量で鋼の焼入性を飛躍的に高める元素である。このような効果を得るためには、Ｂは最低でも０．０００３％必要である。一方、過剰に添加すると、低温靭性を劣化させるだけでなく、かえってＢの焼入性向上効果を消失せしめることもあるので、その上限を０．００２％とした。
【００５１】
Ａｌは、通常脱酸元素として効果を有する。しかし、Ａｌ量が０．０５％を超えるとＡｌ系非金属介在物が増加して鋼の清浄度を害するので、上限を０．０５％とした。
【００５２】
ＮはＴｉＮを形成して低温靭性を向上させる。このために必要な最小量は０．００１％である。しかし、多すぎると低温靭性を劣化させるので、その上限は０．０１％に抑える必要がある。
【００５３】
Ｏは溶接金属中において酸化物を形成し、粒内変態フェライトの核として作用し、組織の微細化に効果がある。しかし、多すぎると溶接金属の低温靭性が劣化すると共に、スラグ巻きこみなどの溶接欠陥を起こす。このため、Ｏ量の下限を０．０１５％、上限を０．０３０％とした。
【００５４】
さらに本発明では、不純物元素であるＰ，Ｓ量をそれぞれ０．０１５％以下、０．００５％以下とする。この主たる理由は低温靭性をより一層向上させるためである。Ｐ量の低減は粒界破壊を防止し、低温靭性を向上させる。また、Ｓ量の低減はＭｎＳを低減して、延靭性を向上させる効果がある。
【００５５】
次にＮｉ，Ｃｕ，Ｃｒ，Ｍｏ，Ｖ，Ｃａを添加する理由について説明する。
基本となる成分にさらに、必要に応じてこれらの元素を添加する主たる目的は、本発明鋼の優れた特徴を損なうことなく、溶接金属の強度・低温靭性などの特性の向上をはかるためである。したがって、その添加量は自ら制限されるべき性質のものである。
【００５６】
Ｎｉを添加する目的は、低温靭性や現地溶接性を劣化させることなく、強度を上昇させるためである。しかし、添加量が多すぎると経済性だけでなく、低温靭性などを劣化させるので、その上限を２．０％、下限を０．１％とした。
【００５７】
ＣｕはＮｉと同様に低温靭性や現地溶接性を劣化させることなく、強度を上昇させる。しかし、過剰に添加すると低温靭性が劣化するので、その上限を１．０％とした。Ｃｕの下限０．１％は添加による材質上の効果が顕著になる最小値である。
【００５８】
Ｃｒは強度を増加させるが、多すぎると低温靭性や現地溶接性を著しく劣化させる。このため、Ｃｒ量の上限を２．０％、下限を０．１％とした。
【００５９】
Ｍｏを添加する理由は、鋼の焼入性を向上させるためである。この効果を得るためには、Ｍｏは最低０．１％必要である。しかし、過剰なＭｏ添加は低温靭性、現地溶接性を劣化させるので、その上限を２．０％とした。
【００６０】
Ｖは、ほぼＮｂと同様の効果を有するが、その効果はＮｂに比較して弱い。Ｖは歪誘起析出し、強度を上昇させる。下限は０．０１％、その上限は現地溶接性、低温靭性の観点から０．１％まで許容できる。
【００６１】
Ｃａは硫化物（ＭｎＳ）の形態を制御し、低温靭性を向上（シャルピー試験における吸収エネルギーの増加など）させる。しかし、Ｃａ量が０．００１％未満では実用上効果がなく、また０．００５％を超えて添加するとＣａＯ−ＣａＳが大量に発生して、溶接欠陥を発生させる。このためＣａ添加量を０．００１〜０．００５％に限定した。
【００６２】
さらに、溶接金属においても十分な強度を満足させるためには、合金元素添加量の適正化が必要である。すなわち、ＣＥ＝Ｃ＋Ｍｎ／６＋（Ｃｒ＋Ｍｏ＋Ｖ）／５＋（Ｎｉ＋Ｃｕ）／１５で定義されるＣＥ値を０．３５〜０．５０の範囲にしなければならない。ＣＥ値が０．３５未満では十分な溶接部強度を確保できない。また、ＣＥ値が０．５０を超えるとＭ* の生成が顕著となり、靭性が劣化する。このためＣＥ値の範囲を０．３５〜０．５０に限定した。
【００６３】
次に高い変形能を得るための限定理由について以下に述べる。
地震多発地域や永久凍土に敷設されるパイプラインにおいて、数％のひずみがパイプラインに負荷される場合、溶接金属部における硬さが母材部における硬さの０．９５〜１．１５倍にすることにより延性亀裂の発生が防止できる。母材部における硬さが０．９５倍未満の場合、溶接金属にひずみが集中し、延性亀裂が溶接金属部から発生する。一方、１．１５倍を超えるとＨＡＺにひずみが集中し、ＨＡＺから母材部の領域から延性亀裂が発生する。このため、その範囲を０．９５〜１．１５倍に限定した。
【００６４】
母材の一様伸びを増加させるためには２０μｍ以下のフェライトを３０〜７０％含有することが必要である。２０μｍを超えると母材の靭性が著しく低下するためである。フェライト分率が３０％未満の場合、一様伸びの向上効果が得られないためである。また、７０％を超えると十分な強度が得られないため、フェライト分率の含有量を３０〜７０％に限定した。
【００６５】
鋼管に使用する鋼板の製造法として、鋳片を９５０〜１２００℃に加熱した後、９５０℃以下での圧下率を５０％以上とし、７００〜８５０℃の温度範囲で圧延を終了した後、６５０〜８００℃の温度範囲から２℃／秒以上の冷却速度で４５０℃以下の任意の温度まで冷却する必要がある。
まず、再加熱温度を９５０〜１２００℃の範囲に限定する。再加熱温度はＮｂ析出物を固溶させ、圧延中の組織を微細化し、優れた低温靭性を得るために９５０℃以上としなければならない。しかし、再加熱温度が１２００℃を超えると、γ粒が著しく粗大化し、圧延によっても完全に微細化できないため、優れた低温靭性が得られない。このため再加熱温度の上限を１２００℃とした。
【００６６】
さらに９５０℃以下の累積圧下率を５０％以上、圧延終了温度を７００〜８５０℃としなければならない。これは、再結晶域圧延で微細化したγ粒を低温圧延によって延伸化し、結晶粒の徹底的な微細化をはかって低温靭性を改善するためである。累積圧下率が５０％未満ではγ組織の延伸化が不十分で、微細な結晶粒が得られない。また、圧延終了温度が８５０℃以上では、例えば累積圧下率が５０％以上でも微細な結晶粒は達成できない。また、圧延温度が低すぎると過度のγ／α２相域圧延となり、低温靭性が劣化するので、圧延終了温度の下限を７００℃とした。
【００６７】
圧延後、鋼板を加速冷却することが必須である。加速冷却は、低温靭性を損なわずに強度の増加及びミクロ組織の制御に基づく一様伸びの向上を可能にする。加速冷却の条件としては、圧延後６５０〜８００℃の温度範囲から冷却速度２℃／秒以上で４５０℃以下の任意の温度まで冷却し、その後空冷しなければならない。冷却を開始する温度が８００℃を超えると、一様伸びが低下する。また、冷却を開始する温度が６５０℃以下の場合、十分な強度が得られない。したがって、冷却を開始する温度範囲を６５０〜８００℃に限定した。また、冷却速度が小さすぎたり、冷却停止温度が高すぎると加速冷却の効果が十分に得られず、十分な強度を得ることができない。
【００６８】
本発明は厚板ミルに適用することが最も好ましいが、ホットコイルにも適用できる（この場合、圧延冷却後の鋼板は巻き取られ、冷却される）。また、この方法で製造した鋼板は低温靭性に優れているので、寒冷地におけるパイプラインのほか圧力容器などにも適用できる。
【００６９】
【実施例】
本発明の実施例について述べる。
転炉−連続鋳造法で種々の鋼成分の鋼片から製造された鋼板を用いて、鋼管を製造し、諸性質を調査した。鋼管溶接部の特性は内外面の１層のＳＡＷ（サブマージドアーク溶接）を実施した後、鋼板１／２ｔ部より採取したシャルピー試験片を用いて評価した。ノッチ位置は溶接金属中央及びＨＡＺ（内面溶接と外面溶接の溶接金属が交わる点から１ｍｍ）とした。
また、引張試験は直径１２．７ｍｍ、ゲージレングス５０．８ｍｍの丸棒引張試験片を使用した。
【００７０】
試験の条件、結果を表１〜表３に示す。表１（表１−１〜表１−５）に鋼管母材と溶接金属の化学成分を示し、表２（表２−１〜表２−２）に酸化物の個数、鋼板製造条件および組織を示し、表３（表３−１〜表３−２）に鋼管母材の機械的性質、鋼管溶接部の機械的性質を示した。
鋼 No.１〜１４が本発明鋼で、鋼 No.１５〜４３が比較鋼である。
表から明らかなように、本発明の鋼管は優れた強度（ＹＳ、ＴＳ）、一様伸び（ｕＥｌ）、低温靭性、溶接部靭性を有する。
【００７１】
これに対して比較鋼は、化学成分や具備すべき条件が適切でなく、いずれかの特性が劣る。
鋼１５はＣ量が少ないため、母材の一様伸びが劣る。鋼１６はＳ量が少ないため、ＨＡＺ靭性が劣る。鋼１７は母材のＡｌ量が少ないため、ＨＡＺ靭性が劣る。鋼１８は母材のＡｌ量が多いため、ＨＡＺ靭性が劣る。鋼１９は母材のＭｇ量が少ないため、ＨＡＺ靭性が劣る。鋼２０は母材のＭｇ量が多いため、母材の靭性が劣る。
【００７２】
鋼２１は母材のＣＥ値が低すぎるため、目標の強度を満足しない。鋼２２は母材のＣＥ値が高すぎるため、ＨＡＺ靭性が劣る。鋼２３は溶接金属のＣ量が少ないため、溶接金属の高温割れが発生する。鋼２４は溶接金属のＣ量が多すぎるため、溶接金属の低温靭性が劣る。鋼２５は溶接金属のＣＥ値が低すぎるため、溶接部の強度が低い。鋼２６は溶接金属のＣＥ値が高すぎるため、溶接金属の靭性が劣る。
【００７３】
鋼２７は溶接金属のＰ値が低すぎるため、溶接金属の靭性が劣る。鋼２８は溶接金属のＰ値が高すぎるため、溶接金属の靭性が劣る。鋼２９はＭｇとＡｌからなる酸化物を内包する０．０１〜０．５μｍのＴｉＮ、すなわちピン止め粒子の個数が少ないため、ＨＡＺ靭性が劣る。鋼３０は酸化物と硫化物が複合した形態で０．３質量％以上のＭｎを含有する０．５〜１０μｍの粒子、すなわちＩＧＦ変態核の個数が少ないため、ＨＡＺ靭性が劣る。
【００７４】
鋼３１は２０μｍ以下のフェライト分率が３０％未満であるために十分な一様伸びが得られない。鋼３２は２０μｍ以下のフェライト分率が７０％を超えるために十分な強度が得られない。鋼３３は溶接金属の硬さが母材の硬さの０．９５倍未満であるために、十分な耐延性亀裂特性が得られない。鋼３４は溶接金属の硬さが母材お方さの１．１５倍を超えるために、十分な耐延性亀裂特性が得られない。
【００７５】
鋼３５はスラブ再加熱温度が９５０℃以下であるために十分な強度が得られない。鋼３６はスラブ再加熱温度が１２００℃を超えるために優れた低温靭性が得られない。鋼３７は９５０℃以下の圧下量が５０％未満であるために良好な低温靭性が得られない。鋼３８は圧延終了温度が８５０℃を超えるために良好な低温靭性が得られない。鋼３９は圧延終了温度が７００℃未満であるために良好な低温靭性が得られない。鋼４０は冷却開始温度が８００℃を超えるために良好な一様伸びが得られない。鋼４１は冷却開始温度が６５０℃未満であるために十分な強度が得られない。鋼４２は冷却停止温度が４５０℃を超えるために十分な強度が得られない。鋼４３は冷却速度が小さいために十分な強度が得られない。
【００７６】
【表１】
【表２】
【表３】
【表４】
【表５】
【表６】
【表７】
【表８】
【表９】
【発明の効果】
本発明によるＨＡＺ靭性に優れ、高い変形能を有する高強度鋼管（ＡＰＩ規格Ｘ６０〜Ｘ８０）をパイプラインに採用することにより、パイプラインの安全性が著しく向上すると共に、輸送効率が飛躍的に改善される。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a steel pipe having a strength of X60 to X80 (yield strength of about 413 to 551 MPa) according to the American Petroleum Institute (API) standard and excellent weld heat affected zone (HAZ) toughness and deformability.
[0002]
[Prior art]
Line pipes used for pipelines for long-distance transportation of crude oil and natural gas are required to have high tension to improve transportation efficiency due to high pressure and to improve local welding efficiency due to thinning. Line pipes up to X80 have been put into practical use. On the other hand, the high HAZ toughness associated with the cold climate of the laying area and sufficient absorption energy when an earthquake occursNeIn recent years, a high deformability for securing luge has been demanded, and a steel pipe with high safety is desired.
[0003]
The HAZ toughness of low alloy steels is (1) grain size, (2) high carbon island martensite (M*), The dispersion state of a hardened phase such as upper bainite (Bu), (3) presence or absence of grain boundary embrittlement, and (4) micrometal segregation of elements. Among them, the size of the HAZ crystal grains is known to have a great influence on the low temperature toughness, and many techniques for refining the HAZ structure have been developed and put into practical use.
[0004]
For example, a means to finely disperse TiN and improve the HAZ toughness during high heat input welding of 490 MPa class high-strength steel is disclosed (“Iron and Steel” (published in June 1979, Vol. 65, No. However, since these precipitates are exposed to high temperatures of 1400 ° C. or higher near the melting line, most of them are coarsened or dissolved, and the HAZ structure is coarsened to deteriorate the HAZ toughness. Have.
[0005]
In response to this problem, the HAZ structure near the melting line is refined by finely dispersing Ti oxide in steel and generating intragranular acicular ferrite (hereinafter referred to as IGF) in the HAZ during welding. It is disclosed in Japanese Patent Laid-Open Nos. 63-210235 and 1-15321 that HAZ toughness is improved. However, in cold regions where the temperature is -50 to -60 ° C, it is not possible to cope with it sufficiently, and improvement of HAZ toughness is strongly desired.
[0006]
On the other hand, regarding deformability, Japanese Patent Application Laid-Open No. 11-279700 discloses a steel pipe excellent in anti-buckling characteristics containing 10 to 50% of lower bainite in area fraction, and Japanese Patent Application Laid-Open No. 11-343542 has an average aspect ratio. A steel pipe excellent in buckling resistance is disclosed, containing 2 to 15% of island-like martensite in an area fraction of 2 to 15. This is intended to improve the local buckling resistance of the steel pipe base material, but is not related to a steel pipe aimed at simultaneously satisfying high deformability and good HAZ toughness.
[0007]
[Problems to be solved by the invention]
The present invention provides an X60 to X80 high-strength steel pipe having good HAZ toughness and excellent deformability, and a method for producing the same.
[0008]
[Means for Solving the Problems]
The gist of the present invention is as follows.
(1) In mass%, C: 0.03-0.10%, Si: 0.6% or less, Mn: 0.8-2.0%, P: 0.015% or less, S:0.0012-0.005%, Nb: 0.005-0.05%, Ti: 0.005-0.030%, Al: 0.001-0.005%, Mg: 0.0001-0.0050%, N: 0.001 to 0.006%, O: 0.001 to 0.006%, with the balance being iron and inevitable impurities, CE = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 The CE value defined by is in the range of 0.30 to 0.45, and 0.01 to 0.5 μm of TiN containing an oxide composed of Mg and Al is 10,000 / mm.210 / mm of 0.5-10 μm particles containing 0.3% by mass or more of Mn in the form of a composite of oxide and sulfide.2A base material containing above,
C: 0.03-0.10%, Si: 0.6% or less, Mn: 1.0-2.2%, P: 0.015% or less, S: 0.01% or less, Nb: 0.0. 005 to 0.05%, Ti: 0.005 to 0.03%, B: 0.0003 to 0.002%, Al: 0.05% or less, N: 0.001 to 0.01%, O: A P value defined by P = {1.5 (O−0.89Al) + 3.4N} −Ti containing 0.015 to 0.030%, the balance being iron and inevitable impurities, A weld metal part having a CE value defined by CE = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 in the range of −0.010 to 0.010 and 0.35 to 0.50. A high-strength steel pipe excellent in low-temperature toughness and deformability characterized by having
[0009]
(2)The weld metal portion further% By mass
Ni: 0.1 to 2.0%, Cu: 0.1 to 1.0%
Cr: 0.1 to 2.0%, Mo: 0.1 to 2.0%,
V: 0.01-0.1% Ca: 0.001 to 0.005%
1 type or 2 types or more are containedIn (1) aboveHigh strength steel pipe with excellent low temperature toughness and deformability.
[0010]
(3)The base material further% By mass
Ni: 0.1 to 1.0%, Cu: 0.1 to 1.2%,
Cr: 0.1 to 1.0%, Mo: 0.1 to 1.0%,
V: 0.01 to 0.1%, Ca: 0.0005 to 0.0050%
Containing one or more ofThe weld metal portion further% By mass
Ni: 0.1 to 2.0%, Cu: 0.1 to 1.0%
Cr: 0.1 to 2.0%, Mo: 0.1 to 2.0%,
V: 0.01-0.1% Ca: 0.001 to 0.005%
1 type or 2 types or more are containedIn (1) aboveHigh strength steel pipe with excellent low temperature toughness and deformability.
[0011]
(4) Low temperature toughness and deformation characterized in that in the steel pipe according to any one of the above (1) to (3), the metal structure of the base metal part further contains 30 to 70% ferrite having a particle size of 20 μm or less. High-strength steel pipe with excellent performance.
(5) In the steel pipe according to any one of (1) to (3), the hardness in the weld metal part is 0.95 to 1.15 times the hardness in the base material part. High strength steel pipe with excellent low temperature toughness and deformability.
(6) In the steel pipe according to any one of (1) to (3), the metal structure of the base metal part is 30 to 70% of ferrite having a particle size of 20 μm or less, and the hardness in the weld metal part is the base metal part. A high-strength steel pipe excellent in low-temperature toughness and deformability characterized by being 0.95 to 1.15 times the hardness of the steel.
[0012]
(7) By mass%, C: 0.03 to 0.10%, Si: 0.6% or less, Mn: 0.8 to 2.0%, P: 0.015% or less, S:0.0012-0.005%, Nb: 0.005-0.05%, Ti: 0.005-0.030%, Al: 0.001-0.005%, Mg: 0.0001-0.0050%, N: 0.001 to 0.006%, O: 0.001 to 0.006%, with the balance being iron and inevitable impurities, CE = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 The CE value defined by is in the range of 0.30 to 0.45, and 0.01 to 0.5 μm of TiN containing an oxide composed of Mg and Al is 10,000 / mm.210 / mm of 0.5-10 μm particles containing 0.3% by mass or more of Mn in the form of a composite of oxide and sulfide.2After heating the cast slab containing 950 to 1200 ° C., the reduction ratio of 950 ° C. or less is set to 50% or more, and rolling is finished in a temperature range of 700 to 850 ° C., and then from a temperature range of 650 to 800 ° C. A method for producing a steel sheet for steel pipes having excellent low-temperature toughness and deformability, wherein the steel sheet is cooled to an arbitrary temperature of 450 ° C. or lower at a cooling rate of at least ° C./second and then air-cooled.
[0013]
(8) More slabsIn mass%,
Ni: 0.1 to 1.0%, Cu: 0.1 to 1.0%,
Cr: 0.1 to 1.0%, Mo: 0.1 to 1.0%,
V: 0.01 to 0.1%, Ca: 0.0005 to 0.0050%
The manufacturing method of the steel plate for steel pipes which was excellent in the low temperature toughness and deformability as described in said (7) characterized by including 1 type (s) or 2 or more types.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Below, the high-strength steel pipe of this invention is demonstrated in detail.
The feature of the present invention is that the amount of Mg, N and O is strictly limited based on the low C—Nb—Ti system, and fine carbonitride containing an oxide composed of Mg and Al, and oxide and sulfide. A steel pipe composed of a base metal part containing a composite composed of a material and a low C-Mn-B weld metal part is a high-strength steel pipe having good HAZ toughness and high deformability.
[0015]
Low temperature toughness of low alloy steel is governed by various metallurgical factors such as (1) grain size, (2) dispersion state of hardened phase such as MA and upper bainite (Bu). Among these, it is known that the HAZ crystal grain size and MA greatly affect low-temperature toughness.
[0016]
In HAZ of high-strength steel pipe, there is a tendency that HAZ toughness deteriorates because a large amount of MA harmful to toughness is generated. In order to eliminate the adverse effect of MA, which is harmful to toughness, HAZ crystal grains must be thoroughly refined. Therefore, it has been found that the HAZ toughness can be remarkably improved by refining HAZ crystal grains by a combined effect with the technique of suppressing the coarsening of austenite (γ) grains in HAZ and the technique of generating IGF from the γ grains. The present invention has been reached.
[0017]
That is, by adding fine MgN in the steel to form fine carbonitride such as TiN containing Mg and Al oxides in the steel, suppressing the coarsening of γ grains in the HAZ, and Mg, Mn, By generating IGF from an oxide / precipitate containing S, crystal grains can be refined and HAZ toughness can be improved. Fine carbonitrides such as TiN containing oxides composed of Mg and Al, and oxides / precipitates containing Mg, Mn, and S are chemically stable and do not dissolve even at high temperatures. And the production effect of IGF is maintained.
[0018]
Therefore, even in HAZ heated to 1400 ° C. or more near the melting line, chemically stable fine oxides are used as pinning particles, and oxides and sulfides of 0.5 μm or more are used as IGF nuclei. By using this method, a method for thoroughly refining the HAZ structure was studied.
[0019]
As a result, it was first found that a small amount of Mg (Al) oxide of 0.01 to 0.05 μm was produced in a large amount by containing a small amount of Mg and Al. It has been clarified that since 0.01 to 0.5 μm of TiN is complex-precipitated with this fine (Mg, Al) oxide as a nucleus, an excellent pinning effect of γ grains can be maintained even at a high temperature of 1400 ° C. or higher. At this time, 0.01 to 0.5 μm TiN contained in the steel is 10,000 pieces / mm.2If it is less than 1, the effect of suppressing the coarsening of γ grains becomes insufficient, and good HAZ toughness cannot be obtained. Therefore, 10000 pieces / mm of 0.01 to 0.5 μm TiN containing an oxide composed of Mg and Al.2It is necessary to contain above.
[0020]
Furthermore, in order to produce this TiN, it is necessary to add 0.0001% or more of Mg. If the amount of Mg added is too large, Mg-based oxides increase and low temperature toughness deteriorates, so the upper limit was limited to 0.0050%. Further, in order to generate a fine (Mg, Al) oxide serving as a nucleus of TiN, it is necessary to contain a trace amount of Al. However, the addition of Al produces coarse alumina clusters, which adversely affects low temperature toughness. For this reason, the content of Al is limited to 0.001 to 0.005%. If the amount of Al is 0.001% or more, a fine (Mg, Al) oxide can be generated.
[0021]
Next, as a necessary requirement for oxides and sulfides that form the core of IGF generation, IGF is also generated in the HAZ near the melting line by controlling the number, size, and composition of the oxide / sulfide complex. The HAZ structure was refined and the HAZ toughness was improved.
[0022]
First, the number of oxide / sulfide complexes that form IGF production nuclei is at least 10 / mm.2This is necessary. 10 IGF transformation nuclei / mm2If it is less than the range, the HAZ structure is not sufficiently refined and good HAZ toughness cannot be obtained.
Moreover, in order to function as a transformation nucleus of IGF, a size of 0.5 μm or more is necessary. If it is less than 0.5 μm, it does not function sufficiently as an IGF transformation nucleus, and the effect of refining the HAZ structure cannot be obtained. On the other hand, in the case of a composite of oxide and sulfide exceeding 10 μm, it becomes a point of occurrence of brittle fracture, so that good HAZ toughness cannot be obtained.
[0023]
Furthermore, in order to function as a transformation nucleus of IGF, it is necessary to contain 0.3% by mass or more of Mn. In the present invention, in order to generate fine particles effective for pinning γ grains at a high temperature of 1400 ° C. or higher, Mg, Al, Ti, which has a stronger deoxidizing power than Mn, is contained. It is difficult to contain. Therefore, it is necessary to complex precipitate sulfide containing Mn on the oxide. When the amount of Mn in the oxide / sulfide composite is less than 0.3% by mass, a sufficient IGF generation function cannot be obtained, and the HAZ structure is not refined.
[0024]
If the addition amount of the alloy element is not appropriate, the HAZ toughness deteriorates. Therefore, in order to obtain the target strength without causing significant deterioration of the HAZ toughness, an appropriate addition amount of the alloy element was examined. By limiting the value defined by the CE value to a predetermined range, sufficient strength can be ensured. Further, with respect to the alloy element addition amount in the weld metal, if the CE value and the value are controlled within a predetermined range, the target strength can be obtained without greatly impairing the toughness of the weld metal.
[0025]
It is said that several percent of strain is applied to pipelines in earthquake-prone areas and pipelines laid on permafrost. In this case, it was found that if the hardness in the weld metal part is 0.95 to 1.15 times the hardness in the base metal part, the occurrence of ductile cracks can be prevented. Moreover, in order to increase the uniform elongation of a base material, it discovered that it was necessary to contain 30-70% of ferrite of 20 micrometers or less. Moreover, as a manufacturing method of the steel plate for steel pipes, rolling is finished in a temperature range of 700 to 850 ° C., and is cooled from a temperature range of 650 to 800 ° C. to an arbitrary temperature of 450 ° C. or less at a cooling rate of 2 ° C./second or more. Then, it was found that a steel sheet having both high strength and high uniform elongation can be obtained by air cooling thereafter, and the present invention has been achieved.
[0026]
That is, the feature of the present invention is that when a low C—Nb—Ti—Mg-based component is applied as a steel pipe base material, the alloy element addition amount is an appropriate value defined by the CE value in order to ensure the target strength. In order to satisfy the target strength without impairing toughness as a weld metal, the alloy element addition amount should be limited to an appropriate range defined by the CE value. In order to ensure low temperature toughness, the alloy element addition amount should be limited to an appropriate range defined by the P value, and in order to ensure excellent deformability, the hardness in the weld metal portion should be the hardness in the base material portion. In order to obtain 0.95 to 1.15 times the thickness, and to obtain a large uniform elongation, the metal structure of the base material part is to contain 30 to 70% of ferrite having a particle diameter of 20 μm or less.
[0027]
The reason for limiting the components of the steel pipe base material will be described below.
C is required to be added in an amount of 0.03% or more in order to ensure the strength, toughness and high uniform elongation of the base material and the HAZ. However, if it exceeds 0.10%, the toughness of the base metal and the HAZ is lowered and the weldability is deteriorated, so 0.10% was made the upper limit.
[0028]
In order to satisfy the target strengths of X60 to X80, it is necessary to optimize the addition amount of the alloy element. That is, the CE value defined by CE = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 must be in the range of 0.30 to 0.45. If the CE value is less than 0.30, the target strength of X60 or more cannot be secured. If the CE value exceeds 0.45, M*Generation becomes remarkable, and the HAZ toughness deteriorates. For this reason, the range of the CE value was limited to 0.30 to 0.45.
[0029]
Si is an element added for deoxidation and strength improvement, but if added in large amounts, the field weldability and the HAZ toughness deteriorate, so the upper limit was made 0.6%. For the deoxidation of steel, Ti alone is sufficient, and Si does not necessarily have to be added.
[0030]
Mn is an indispensable element for securing strength and low temperature toughness, and its lower limit is 0.8%. However, if Mn is too much, not only the hardenability of the steel is increased and the on-site weldability and HAZ toughness are deteriorated, but also the center segregation of continuously cast steel pieces is promoted and the low temperature toughness is also deteriorated. 0%.
[0031]
In the present invention, the amount of P which is an inevitable impurity is set to 0.015% or less. The main reason is to further improve the low temperature toughness of the base material and the HAZ. The reduction of the P content reduces the center segregation of the continuously cast slab, prevents the grain boundary fracture and improves the low temperature toughness.
[0032]
S is an important element in the present invention. In order to make composite precipitation of sulfide on oxide as IGF transformation nucleus0.0012% Must be contained. However, if S exceeds 0.005%, the toughness of the base material and the HAZ deteriorates, so 0.005% is made the upper limit.
[0033]
Nb not only suppresses recrystallization of γ during controlled rolling and refines the crystal grains, but also contributes to increase in precipitation hardening and hardenability, and has the effect of strengthening steel and is essential in the present invention. Elements. To obtain this effect, a minimum of 0.005% Nb is required. However, if the amount of Nb is too large, the HAZ toughness deteriorates, so the upper limit was limited to 0.05%.
[0034]
Ti forms fine TiN, suppresses coarsening of γ grains of HAZ during reheating of the slab, refines the microstructure, improves the low temperature toughness of the base material and HAZ, and is an essential element in the present invention It is. In order to exhibit this effect, addition of 0.005% or more is necessary. On the other hand, if the amount is too large, TiN coarsening and precipitation hardening due to TiC occur and the low-temperature toughness is deteriorated, so the upper limit was limited to 0.03%.
[0035]
N forms TiN, and suppresses coarsening of γ grains of HAZ during reheating of the slab and improves the low temperature toughness of the base material and HAZ. The minimum amount required for this is 0.001%. However, if the amount of N is too large, the HAZ toughness may be deteriorated due to slab surface flaws or solute N, so the upper limit must be limited to 0.006%.
[0036]
O forms an ultrafine (Mg, Al) oxide and exhibits the effect of suppressing the coarsening of γ grains of HAZ, and at the same time forms an Mg-containing oxide of 0.5 to 10 μm to form an IGF transformation in HAZ. Functions as a nucleus. In order to exhibit these functions, 0.001% or more of O is necessary. When O is less than 0.001%, 10,000 pieces / mm2Above ultrafine oxide and 10 / mm2It is difficult to secure the above 0.5 to 10 μm oxide. However, if O exceeds 0.006%, a coarse oxide exceeding 10 μm is generated, which becomes a point of occurrence of brittle fracture in the base material and HAZ, so 0.006% was made the upper limit.
[0037]
Next, the reason for adding Ni, Cu, Cr, Mo, V, and Ca will be described.
The main purpose of adding these elements to the basic component is to improve properties such as strength and low temperature toughness without impairing the characteristics of the steel of the present invention. Therefore, the amount added is of a nature that should be restricted by itself.
[0038]
Ni improves the strength and low temperature toughness of the base metal without adversely affecting weldability and HAZ toughness, but the effect is low at less than 0.1%, and addition of more than 1.0% is not preferable for weldability. The upper limit was set to 1.0%.
[0039]
Cu has substantially the same effect as Ni, and is also effective in corrosion resistance, resistance to hydrogen-induced cracking, and the like, and it is necessary to add 0.1% or more. However, if added excessively, Cu-cracks are generated during precipitation hardening due to precipitation hardening and HAZ toughness, and the upper limit is set to 1.2%.
[0040]
Cr has the effect of increasing the strength of the base metal and the welded portion, and it is necessary to add 0.1% or more. However, if too much, field weldability and HAZ toughness are significantly deteriorated. For this reason, the upper limit of the Cr amount is set to 1.0%.
[0041]
Mo is an element that increases the strength of the base metal and the welded portion. However, if it exceeds 1.0%, the base metal, the HAZ toughness and the weldability are deteriorated similarly to Cr. Moreover, the effect is thin when added less than 0.1%.
[0042]
V has substantially the same effect as Nb, but the effect is much weaker than Nb. In order to exhibit the effect, addition of 0.01% or more is necessary. Further, the upper limit is allowable up to 0.1% from the viewpoint of on-site weldability and HAZ toughness.
[0043]
Ca controls the carrying of sulfide (MnS) and improves low temperature toughness (increased absorbed energy in the Charpy test, etc.), and also exhibits a remarkable effect in improving sour resistance. If less than 0.0005%, the effect is small, and if added over 0.005%, a large amount of CaO-CaS is formed to form clusters and large inclusions. It also has an adverse effect. For this reason, the amount of Ca added is limited to 0.0005 to 0.005%.
[0044]
On the other hand, the low temperature toughness of the weld metal part in the longitudinal direction of the steel pipe is governed by various metallurgical factors such as (1) the size of crystal grains and (2) the dispersion state of the hardened phase such as island martensite. In particular, as the strength and thickness are increased, the amount of alloy element added inevitably increases, the structure becomes a structure mainly composed of upper bainite, and the toughness tends to deteriorate. Therefore, the low temperature toughness can be dramatically improved by optimizing the balance of the amounts of Al, N, oxygen and Ti.
That is, in the formula represented by P = {1.5 (O−0.89Al) + 3.4N} −Ti, each component is optimized so that the P value becomes −0.010 to 0.010%. As a result, the low temperature toughness is improved. The P value indicates an excess or deficiency of the Ti amount. When the P value is low (minus), Ti is excessively added, and low temperature toughness deteriorates due to precipitation hardening such as TiC. On the other hand, when the P value is high (plus), the Ti amount is insufficient (or the oxygen amount is excessive), so that the low temperature toughness deteriorates. In order to obtain good low temperature toughness, the P value needs to be -0.010 to 0.010%.
[0045]
Next, the reasons for limiting the components of the weld metal will be described.
In order to prevent hot cracking of the weld metal, the C content needs to be 0.03% or more. If it is less than 0.03%, δ solidification occurs in the process of solidification after welding, and hot cracking occurs. However, if the C content exceeds 0.10%, the low temperature toughness of the weld metal deteriorates, so the upper limit was made 0.10%.
[0046]
Si is an element added for deoxidation and strength improvement, but if added in a large amount, the low temperature toughness and on-site weldability deteriorate, so the upper limit was made 0.6%.
[0047]
Mn is an element indispensable for securing strength and low temperature toughness, and its lower limit is 1.0%. However, if there is too much Mn, the hardenability of the steel increases and the low temperature toughness and on-site weldability deteriorate, so the upper limit was made 2.2%.
[0048]
Nb has the effect | action which strengthens steel and needs 0.005% or more. However, if Nb exceeds 0.05%, on-site weldability and low temperature toughness are adversely affected, so the upper limit was made 0.05%.
[0049]
Ti addition forms fine TiN and improves low temperature toughness. In order to exhibit such an effect of TiN, it is necessary to add at least 0.005% Ti. However, if the amount of Ti is too large, TiN coarsening and precipitation hardening due to TiC occur and the low temperature toughness deteriorates, so the upper limit must be limited to 0.03%.
[0050]
B is an element that greatly increases the hardenability of steel in a very small amount. In order to obtain such an effect, B must be at least 0.0003%. On the other hand, if added excessively, not only the low temperature toughness is deteriorated, but also the effect of improving the hardenability of B may be lost, so the upper limit was made 0.002%.
[0051]
Al usually has an effect as a deoxidizing element. However, if the Al content exceeds 0.05%, Al-based non-metallic inclusions increase to impair the cleanliness of the steel, so the upper limit was made 0.05%.
[0052]
N forms TiN and improves low temperature toughness. The minimum amount required for this is 0.001%. However, if the amount is too large, the low temperature toughness is deteriorated, so the upper limit must be suppressed to 0.01%.
[0053]
O forms an oxide in the weld metal, acts as a nucleus of intragranular transformed ferrite, and is effective in refining the structure. However, if the amount is too large, the low temperature toughness of the weld metal deteriorates and welding defects such as slag entrainment occur. For this reason, the lower limit of the amount of O is set to 0.015%, and the upper limit is set to 0.030%.
[0054]
Further, in the present invention, the amounts of impurity elements P and S are set to 0.015% or less and 0.005% or less, respectively. The main reason is to further improve the low temperature toughness. Reduction of the P content prevents grain boundary fracture and improves low temperature toughness. Moreover, reduction of the amount of S has the effect of reducing MnS and improving ductility.
[0055]
Next, the reason for adding Ni, Cu, Cr, Mo, V, and Ca will be described.
In addition to the basic components, the main purpose of adding these elements as necessary is to improve the properties such as strength and low temperature toughness of the weld metal without impairing the excellent characteristics of the steel of the present invention. . Therefore, the amount of addition is a property that should be restricted by itself.
[0056]
The purpose of adding Ni is to increase the strength without deteriorating the low temperature toughness and on-site weldability. However, if the addition amount is too large, not only the economy but also the low temperature toughness is deteriorated, so the upper limit was made 2.0% and the lower limit was made 0.1%.
[0057]
Cu, like Ni, increases strength without deteriorating low-temperature toughness and on-site weldability. However, if added in excess, the low temperature toughness deteriorates, so the upper limit was made 1.0%. The lower limit of 0.1% of Cu is the minimum value at which the effect on the material due to addition becomes remarkable.
[0058]
Cr increases the strength, but if it is too much, the low temperature toughness and on-site weldability deteriorate significantly. For this reason, the upper limit of Cr content was set to 2.0%, and the lower limit was set to 0.1%.
[0059]
The reason for adding Mo is to improve the hardenability of the steel. In order to obtain this effect, Mo needs to be at least 0.1%. However, excessive addition of Mo deteriorates low temperature toughness and on-site weldability, so the upper limit was made 2.0%.
[0060]
V has almost the same effect as Nb, but the effect is weaker than that of Nb. V causes strain-induced precipitation and increases the strength. The lower limit is 0.01%, and the upper limit is acceptable up to 0.1% from the viewpoint of on-site weldability and low temperature toughness.
[0061]
Ca controls the form of sulfide (MnS) and improves low-temperature toughness (such as an increase in absorbed energy in the Charpy test). However, if the amount of Ca is less than 0.001%, there is no practical effect, and if added over 0.005%, a large amount of CaO—CaS is generated and a weld defect is generated. For this reason, Ca addition amount was limited to 0.001 to 0.005%.
[0062]
Furthermore, in order to satisfy the sufficient strength even in the weld metal, it is necessary to optimize the addition amount of the alloy element. That is, the CE value defined by CE = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 must be in the range of 0.35 to 0.50. If the CE value is less than 0.35, sufficient weld strength cannot be ensured. If the CE value exceeds 0.50, M*Generation becomes remarkable and toughness deteriorates. For this reason, the range of the CE value was limited to 0.35 to 0.50.
[0063]
Next, the reason for limitation for obtaining the high deformability will be described below.
In pipelines laid in earthquake-prone areas or permafrost, when several percent strain is applied to the pipeline, the hardness of the weld metal part is 0.95 to 1.15 times the hardness of the base metal part. By doing so, the occurrence of ductile cracks can be prevented. When the hardness in the base metal part is less than 0.95 times, strain is concentrated on the weld metal, and ductile cracks are generated from the weld metal part. On the other hand, if it exceeds 1.15 times, strain concentrates in the HAZ, and a ductile crack is generated from the region of the base material portion from the HAZ. For this reason, the range was limited to 0.95 to 1.15 times.
[0064]
In order to increase the uniform elongation of the base material, it is necessary to contain 30 to 70% of ferrite of 20 μm or less. This is because if it exceeds 20 μm, the toughness of the base material is significantly reduced. This is because when the ferrite fraction is less than 30%, the effect of improving uniform elongation cannot be obtained. Moreover, since sufficient intensity | strength will not be obtained when it exceeds 70%, content of the ferrite fraction was limited to 30 to 70%.
[0065]
As a manufacturing method of a steel plate used for a steel pipe, after heating a slab to 950 to 1200 ° C., setting a reduction rate at 950 ° C. or less to 50% or more and finishing rolling in a temperature range of 700 to 850 ° C., 650 It is necessary to cool from a temperature range of ˜800 ° C. to an arbitrary temperature of 450 ° C. or less at a cooling rate of 2 ° C./second or more.
First, the reheating temperature is limited to a range of 950 to 1200 ° C. The reheating temperature must be 950 ° C. or higher in order to dissolve Nb precipitates, refine the structure during rolling, and obtain excellent low temperature toughness. However, when the reheating temperature exceeds 1200 ° C., the γ grains become extremely coarse and cannot be completely miniaturized even by rolling, so that excellent low temperature toughness cannot be obtained. For this reason, the upper limit of the reheating temperature was set to 1200 ° C.
[0066]
Furthermore, the cumulative rolling reduction at 950 ° C. or less must be 50% or more, and the rolling end temperature must be 700 to 850 ° C. This is because the γ grains refined by recrystallization zone rolling are elongated by low temperature rolling, and the crystal grains are thoroughly refined to improve low temperature toughness. If the cumulative rolling reduction is less than 50%, the γ structure is not sufficiently stretched and fine crystal grains cannot be obtained. Further, if the rolling end temperature is 850 ° C. or higher, fine crystal grains cannot be achieved even if the cumulative rolling reduction is 50% or higher, for example. Further, if the rolling temperature is too low, excessive γ / α2 phase rolling occurs and the low temperature toughness deteriorates, so the lower limit of the rolling end temperature was set to 700 ° C.
[0067]
After rolling, it is essential to cool the steel plate at an accelerated rate. Accelerated cooling allows for increased strength and improved uniform elongation based on microstructure control without compromising low temperature toughness. As conditions for accelerated cooling, the steel must be cooled from a temperature range of 650 to 800 ° C. to an arbitrary temperature of 450 ° C. or less at a cooling rate of 2 ° C./second after rolling, and then air-cooled. When the temperature at which cooling starts exceeds 800 ° C., the uniform elongation decreases. Moreover, when the temperature which starts cooling is 650 degrees C or less, sufficient intensity | strength is not obtained. Therefore, the temperature range at which cooling is started is limited to 650 to 800 ° C. Further, if the cooling rate is too low or the cooling stop temperature is too high, the effect of accelerated cooling cannot be obtained sufficiently, and sufficient strength cannot be obtained.
[0068]
The present invention is most preferably applied to a thick plate mill, but can also be applied to a hot coil (in this case, the steel sheet after rolling and cooling is wound and cooled). Moreover, since the steel plate manufactured by this method is excellent in low temperature toughness, it can be applied to a pressure vessel as well as a pipeline in a cold region.
[0069]
【Example】
Examples of the present invention will be described.
Steel tubes manufactured from steel pieces of various steel components by a converter-continuous casting method were used to manufacture steel pipes, and various properties were investigated. The characteristics of the steel pipe welded part were evaluated using a Charpy test piece taken from a 1/2 t part of the steel sheet after performing SAW (submerged arc welding) of one layer on the inner and outer surfaces. The notch positions were the center of the weld metal and HAZ (1 mm from the point where the weld metal of the inner surface welding and the outer surface welding intersected).
The tensile test used a round bar tensile test piece having a diameter of 12.7 mm and a gauge length of 50.8 mm.
[0070]
Tables 1 to 3 show the test conditions and results. Table 1 (Table 1-1 to Table 1-5) shows the chemical composition of the steel pipe base metal and the weld metal, and Table 2 (Table 2-1 to Table 2-2) shows the number of oxides, steel sheet production conditions and structure. Table 3 (Table 3-1 to Table 3-2) shows the mechanical properties of the steel pipe base material and the mechanical properties of the steel pipe weld.
Steel Nos. 1 to 14 are steels of the present invention, and steel Nos. 15 to 43 are comparative steels.
As is clear from the table, the steel pipe of the present invention has excellent strength (YS, TS), uniform elongation (uEl), low temperature toughness, and weld zone toughness.
[0071]
On the other hand, the comparative steel has inadequate chemical components and conditions to be provided, and any properties are inferior.
Since the steel 15 has a small amount of C, the uniform elongation of the base material is inferior. Since the steel 16 has a small amount of S, the HAZ toughness is inferior. Steel 17 is inferior in HAZ toughness because the amount of Al in the base material is small. Since the steel 18 has a large amount of Al in the base material, the HAZ toughness is inferior. Steel 19 is inferior in HAZ toughness because the amount of Mg in the base material is small. Since steel 20 has a large amount of Mg in the base material, the toughness of the base material is inferior.
[0072]
Steel 21 does not satisfy the target strength because the CE value of the base material is too low. Steel 22 is inferior in HAZ toughness because the CE value of the base material is too high. Since the steel 23 has a small amount of C in the weld metal, hot cracking of the weld metal occurs. Since the steel 24 has too much C amount of the weld metal, the low temperature toughness of the weld metal is inferior. Since the CE value of the weld metal is too low, the strength of the welded portion is low. Steel 26 has an inferior weld metal toughness because the CE value of the weld metal is too high.
[0073]
Steel 27 is inferior in toughness of the weld metal because the P value of the weld metal is too low. Steel 28 has poor weld metal toughness because the P value of the weld metal is too high. Steel 29 is inferior in HAZ toughness due to the small number of 0.01 to 0.5 μm TiN containing an oxide composed of Mg and Al, that is, pinning particles. Steel 30 is in the form of a composite of oxide and sulfide, and has a particle size of 0.5 to 10 μm containing 0.3% by mass or more of Mn, that is, the number of IGF transformation nuclei, so that HAZ toughness is inferior.
[0074]
Since the steel 31 has a ferrite fraction of 20 μm or less of less than 30%, sufficient uniform elongation cannot be obtained. In steel 32, the ferrite fraction of 20 μm or less exceeds 70%, so that sufficient strength cannot be obtained. Since the hardness of the weld metal of the steel 33 is less than 0.95 times the hardness of the base metal, sufficient ductile cracking characteristics cannot be obtained. Since the hardness of the weld metal of steel 34 exceeds 1.15 times that of the base metal, sufficient ductile cracking characteristics cannot be obtained.
[0075]
Since the steel 35 has a slab reheating temperature of 950 ° C. or less, sufficient strength cannot be obtained. Steel 36 cannot obtain excellent low temperature toughness because the slab reheating temperature exceeds 1200 ° C. Steel 37 cannot obtain good low temperature toughness because the amount of reduction at 950 ° C. or less is less than 50%. Since the rolling end temperature of the steel 38 exceeds 850 ° C., good low temperature toughness cannot be obtained. Since the rolling end temperature of steel 39 is less than 700 ° C., good low temperature toughness cannot be obtained. Since steel 40 has a cooling start temperature exceeding 800 ° C., good uniform elongation cannot be obtained. Since the cooling start temperature of the steel 41 is less than 650 ° C., sufficient strength cannot be obtained. Since the cooling stop temperature exceeds 450 ° C., the steel 42 cannot obtain sufficient strength. Since the steel 43 has a low cooling rate, sufficient strength cannot be obtained.
[0076]
[Table 1]
[Table 2]
[Table 3]
[Table 4]
[Table 5]
[Table 6]
[Table 7]
[Table 8]
[Table 9]
【The invention's effect】
By adopting high strength steel pipes (API standards X60 to X80) with excellent HAZ toughness and high deformability according to the present invention in pipelines, the safety of pipelines is significantly improved and the transport efficiency is dramatically improved. Is done.
Claims (8)
Ｃ ：０．０３〜０．１０％、
Ｓｉ：０．６％以下、
Ｍｎ：０．８〜２．０％、
Ｐ ：０．０１５％以下、
Ｓ ：０．００１２〜０．００５％、
Ｎｂ：０．００５〜０．０５％、
Ｔｉ：０．００５〜０．０３０％、
Ａｌ：０．００１〜０．００５％、
Ｍｇ：０．０００１〜０．００５０％、
Ｎ ：０．００１〜０．００６％、
Ｏ ：０．００１〜０．００６％
を含有し、残部が鉄および不可避的不純物からなり、
ＣＥ＝Ｃ＋Ｍｎ／６＋（Ｃｒ＋Ｍｏ＋Ｖ）／５＋（Ｎｉ＋Ｃｕ）／１５
で定義されるＣＥ値が０．３０〜０．４５の範囲にあり、ＭｇとＡｌからなる酸化物を内包する０．０１〜０．５μｍのＴｉＮが１００００個／ｍｍ２以上含有し、かつ酸化物と硫化物が複合した形態で０．３質量％以上のＭｎを含有する０．５〜１０μｍの粒子が１０個／ｍｍ２ 以上含有する母材と、
Ｃ ：０．０３〜０．１０％、
Ｓｉ：０．６％以下、
Ｍｎ：１．０〜２．２％、
Ｐ ：０．０１５％以下、
Ｓ ：０．０１％以下、
Ｎｂ：０．００５〜０．０５％、
Ｔｉ：０．００５〜０．０３％、
Ｂ ：０．０００３〜０．００２％、
Ａｌ：０．０５％以下、
Ｎ ：０．００１〜０．０１％、
Ｏ ：０．０１５〜０．０３０％
を含有し、残部が鉄及び不可避的不純物からなり、かつ
Ｐ＝｛１．５（Ｏ−０．８９Ａｌ）＋３．４Ｎ｝−Ｔｉ
で定義されるＰ値が−０．０１０〜０．０１０の範囲であり、さらに
ＣＥ＝Ｃ＋Ｍｎ／６＋（Ｃｒ＋Ｍｏ＋Ｖ）／５＋（Ｎｉ＋Ｃｕ）／１５
で定義されるＣＥ値が０．３５〜０．５０の範囲にある溶接金属部を有することを特徴とする低温靭性と変形能に優れた高強度鋼管。% By mass
C: 0.03-0.10%,
Si: 0.6% or less,
Mn: 0.8 to 2.0%,
P: 0.015% or less,
S: 0.0012 to 0.005%,
Nb: 0.005 to 0.05%,
Ti: 0.005 to 0.030%,
Al: 0.001 to 0.005%,
Mg: 0.0001 to 0.0050%,
N: 0.001 to 0.006%,
O: 0.001 to 0.006%
The balance consists of iron and inevitable impurities,
CE = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15
The CE value defined by is in the range of 0.30 to 0.45, contains 0.01 to 0.5 μm of TiN containing an oxide composed of Mg and Al, and contains 10000 / mm 2 or more. A base material containing 10 particles / mm 2 or more of 0.5 to 10 μm particles containing 0.3% by mass or more of Mn in a form in which the product and sulfide are combined,
C: 0.03-0.10%,
Si: 0.6% or less,
Mn: 1.0-2.2%
P: 0.015% or less,
S: 0.01% or less,
Nb: 0.005 to 0.05%,
Ti: 0.005 to 0.03%,
B: 0.0003 to 0.002%,
Al: 0.05% or less,
N: 0.001 to 0.01%,
O: 0.015-0.030%
And the balance consists of iron and inevitable impurities, and P = {1.5 (O−0.89Al) + 3.4N} −Ti
The P value defined by is in the range of −0.010 to 0.010, and CE = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15
A high-strength steel pipe excellent in low-temperature toughness and deformability, characterized by having a weld metal part with a CE value defined in the range of 0.35 to 0.50.
Ｎｉ：０．１〜２．０％、
Ｃｕ：０．１〜１．０％、
Ｃｒ：０．１〜２．０％、
Ｍｏ：０．１〜２．０％、
Ｖ ：０．０１〜０．１％、
Ｃａ：０．００１〜０．００５％
のうち１種または２種以上を含有していることを特徴とする請求項１に記載の低温靭性と変形能に優れた高強度鋼管。 The weld metal part is further in mass%,
Ni: 0.1 to 2.0%,
Cu: 0.1 to 1.0%
Cr: 0.1 to 2.0%,
Mo: 0.1 to 2.0%,
V: 0.01-0.1%
Ca: 0.001 to 0.005%
The high-strength steel pipe excellent in low temperature toughness and deformability according to claim 1, wherein one or more of them are contained.
Ｎｉ：０．１〜１．０％、
Ｃｕ：０．１〜１．２％、
Ｃｒ：０．１〜１．０％、
Ｍｏ：０．１〜１．０％、
Ｖ ：０．０１〜０．１％、
Ｃａ：０．０００５〜０．００５０％
の１種または２種以上を含有し、前記溶接金属部が、さらに質量％で、
Ｎｉ：０．１〜２．０％、
Ｃｕ：０．１〜１．０％、
Ｃｒ：０．１〜２．０％、
Ｍｏ：０．１〜２．０％、
Ｖ ：０．０１〜０．１％、
Ｃａ：０．００１〜０．００５％
のうち１種または２種以上を含有していることを特徴とする請求項１に記載の低温靭性と変形能に優れた高強度鋼管。 The base material is further mass%,
Ni: 0.1 to 1.0%,
Cu: 0.1 to 1.2%,
Cr: 0.1 to 1.0%,
Mo: 0.1 to 1.0%,
V: 0.01-0.1%
Ca: 0.0005 to 0.0050%
1 type or 2 types or more, and the weld metal part is further in % by mass,
Ni: 0.1 to 2.0%,
Cu: 0.1 to 1.0%
Cr: 0.1 to 2.0%,
Mo: 0.1 to 2.0%,
V: 0.01-0.1%
Ca: 0.001 to 0.005%
The high-strength steel pipe excellent in low temperature toughness and deformability according to claim 1, wherein one or more of them are contained.
Ｃ ：０．０３〜０．１０％、
Ｓｉ：０．６％以下、
Ｍｎ：０．８〜２．０％、
Ｐ ：０．０１５％以下、
Ｓ ：０．００１２〜０．００５％、
Ｎｂ：０．００５〜０．０５％、
Ｔｉ：０．００５〜０．０３０％、
Ａｌ：０．００１〜０．００５％、
Ｍｇ：０．０００１〜０．００５０％、
Ｎ ：０．００１〜０．００６％、
Ｏ ：０．００１〜０．００６％
を含有し、残部が鉄および不可避的不純物からなり、
ＣＥ＝Ｃ＋Ｍｎ／６＋（Ｃｒ＋Ｍｏ＋Ｖ）／５＋（Ｎｉ＋Ｃｕ）／１５
で定義されるＣＥ値が０．３０〜０．４５の範囲にあり、ＭｇとＡｌからなる酸化物を内包する０．０１〜０．５μｍのＴｉＮが１００００個／ｍｍ２以上含有し、かつ酸化物と硫化物が複合した形態で０．３質量％以上のＭｎを含有する０．５〜１０μｍの粒子が１０個／ｍｍ２ 以上含有する鋳片を９５０〜１２００℃に加熱した後、９５０℃以下の圧下率を５０％以上とし、７００〜８５０℃の温度範囲で圧延を終了した後、６５０〜８００℃の温度範囲から２℃／秒以上の冷却速度で４５０℃以下の任意の温度まで冷却し、その後空冷することを特徴とする低温靭性と変形能に優れた鋼管用鋼板の製造法。% By mass
C: 0.03-0.10%,
Si: 0.6% or less,
Mn: 0.8 to 2.0%,
P: 0.015% or less,
S: 0.0012 to 0.005%,
Nb: 0.005 to 0.05%,
Ti: 0.005 to 0.030%,
Al: 0.001 to 0.005%,
Mg: 0.0001 to 0.0050%,
N: 0.001 to 0.006%,
O: 0.001 to 0.006%
The balance consists of iron and inevitable impurities,
CE = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15
The CE value defined by is in the range of 0.30 to 0.45, contains 0.01 to 0.5 μm of TiN containing an oxide composed of Mg and Al, and contains 10000 / mm 2 or more. 950 ° C. after heating a cast slab containing 10 to 10 particles / mm 2 of 0.5 to 10 μm particles containing 0.3% by mass or more of Mn in a form in which the product and sulfide are combined After rolling in the temperature range of 700 to 850 ° C. with the following reduction rate of 50% or more, cooling is performed from the temperature range of 650 to 800 ° C. to an arbitrary temperature of 450 ° C. or less at a cooling rate of 2 ° C./second or more. A method for producing a steel plate for steel pipes, which is then excellent in low temperature toughness and deformability, characterized by air cooling.
Ｎｉ：０．１〜１．０％、
Ｃｕ：０．１〜１．０％、
Ｃｒ：０．１〜１．０％、
Ｍｏ：０．１〜１．０％、
Ｖ ：０．０１〜０．１％、
Ｃａ：０．０００５〜０．００５０％
の１種または２種以上を含有することを特徴とする請求項７に記載の低温靭性と変形能に優れた鋼管用鋼板の製造法。The slab is more mass% ,
Ni: 0.1 to 1.0%,
Cu: 0.1 to 1.0%
Cr: 0.1 to 1.0%,
Mo: 0.1 to 1.0%,
V: 0.01-0.1%
Ca: 0.0005 to 0.0050%
The manufacturing method of the steel plate for steel pipes excellent in the low temperature toughness and deformability of Claim 7 characterized by including 1 type (s) or 2 or more types of these.
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| CN110524138B (en) * | 2019-08-27 | 2021-09-10 | 西安理工大学 | X80 pipeline steel submerged arc welding wire with low cost and good low-temperature toughness |
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| JP4528089B2 (en) * | 2003-10-22 | 2010-08-18 | 新日本製鐵株式会社 | Large heat input butt welded joints for ship hulls with brittle fracture resistance |
| JP4696615B2 (en) | 2005-03-17 | 2011-06-08 | 住友金属工業株式会社 | High-tensile steel plate, welded steel pipe and manufacturing method thereof |
| JP4772486B2 (en) * | 2005-04-26 | 2011-09-14 | 新日本製鐵株式会社 | High strength steel pipe for low temperature |
| CN103320692B (en) * | 2013-06-19 | 2016-07-06 | 宝山钢铁股份有限公司 | Superhigh tenacity, superior weldability HT550 steel plate and manufacture method thereof |
| CN111790753A (en) * | 2020-05-28 | 2020-10-20 | 南京钢铁股份有限公司 | Wide and thin X60 steel grade pipeline steel of single-stand steckel mill and rolling method |
| CN113046630B (en) * | 2021-02-07 | 2022-05-17 | 首钢集团有限公司 | Thick pipeline steel and preparation method thereof |
| KR20230095234A (en) | 2021-12-22 | 2023-06-29 | 주식회사 포스코 | High strength steel plate having excellent toughness at low temperature and low yield ratio and method for manufacturing the same |
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