JP4071906B2

JP4071906B2 - Manufacturing method of steel pipe for high tension line pipe with excellent low temperature toughness

Info

Publication number: JP4071906B2
Application number: JP33302499A
Authority: JP
Inventors: 卓也原; 龍治植森; 博為広
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 1999-11-24
Filing date: 1999-11-24
Publication date: 2008-04-02
Anticipated expiration: 2019-11-24
Also published as: JP2001152248A

Description

【０００１】
【発明の属する技術分野】
本発明は、強度、低温靱性および溶接性の優れたラインパイプ用鋼管の製造方法に関わるものである。
【０００２】
【従来の技術】
近年、経済性、安全性等の面から溶接構造物（建築、圧力容器、造船、ラインパイプ）における、高張力鋼の使用は多岐にわたり、溶接性高張力鋼の需要は着実な増加を示している。溶接構造物に使用される鋼は当然のことながら高強度に加え、安全性、作業性の面から、高靱性と優れた溶接性を併せ持つことが要求されるが、これらの特性を満足する鋼の製造法として現在ではラインパイプ材の製造に広く使用されている制御圧延法（ＣＲ法）と圧延後焼き入れ焼き戻し処理を行う焼き入れ焼き戻し法（ＱＴ法）がよく知られている。しかし、前者の方法では圧延組織は一般的にフェライト・パーライトであり、得られる強度と板厚には自ら限界を生じている。この理由は、製造された鋼において強度・靭性に優れるアシキュラーフェライトもしくはベイナイト組織とするには冷却速度を著しく速めるかもしくは多量の合金添加を必要とするため、経済性等の点から実用化が困難であった。また、後者の方法では、再加熱工程が必要なためコスト高になると共に生産能力上の制約があった。
【０００３】
このため、現在ではこれらの方法を一歩進め、省エネルギ−、省資源（合金元素の削減）化を徹底した制御圧延・制御冷却法（ＴＭＣＰ法）の開発が進められている。この方法で製造した鋼はＣＲとＱＴ法の長所を併せ持ち低合金ないし特別な合金添加無しで優れた材質が得られるという特徴を有しているが、一方では、従来の制御冷却法で製造した鋼は次のような欠点を有している。
【０００４】
▲１▼ 圧延後急冷を行った場合、強度が高すぎるため延靱性回復のために焼き戻し処理が必須となる。
▲２▼ 溶接時の熱影響部（ＨＡＺ）の軟化が大きく、特に高降伏点、高張力鋼では溶接部の強度確保が困難である。
▲３▼ 板厚断面方向の組織が不均一で硬度差が大きい。
【０００５】
▲４▼ 冷却条件（冷却開始、停止温度及び速度）のコントロールが微妙で材質が不安定である。
これらの欠点を改善する方法として、例えば、特開昭６３−１７９０２０号公報あるいは特開昭６１−６７７１７号公報では、成分、圧下量、冷却速度、冷却停止温度を規定することによって、板厚断面硬度差を小さくすることが開示されている。しかしながら、これらの方法では、比較的薄い板厚の鋼板には適用できても極厚鋼板では板厚方向での冷却速度の制御が困難であり、板厚断面硬度差を小さくするという効果を得ることはできないという問題がある。
【０００６】
また、特開昭５８−７７５２８号公報には、ＮｂとＢの複合添加により板厚方向の組織をベイナイト均一組織とし、板厚方向の硬度差を小さくする方法が開示されている。しかしながら、この方法の場合、ベイナイト均一組織とするために冷却速度を１５〜４０℃／秒にする必要があるために、極厚鋼板での板厚方向の硬度差を均一にすることは難しいという問題があった。
【０００７】
【発明が解決しようとする課題】
本発明は、上記従来技術の問題点に鑑みて、板厚方向での材質のバラツキが少なく、かつ強度・低温靱性および溶接性の優れたＸ６５グレード（降伏応力が４４８ＭＰａ以上、引張り応力が５３０ＭＰａ以上）以上の機械的性質を有するラインパイプ用鋼管を製造することを目的とするものである。なお、引張り応力の上限は、本発明の実施例の鋼Ｎｏ．１４及び１６の７００ＭＰａに基づいて、７００ＭＰａ以下とする。
【０００８】
【課題を解決するための手段】
すなわち、本発明の要旨とするところは、以下の通りである。
（１）質量％で、
Ｃ：０．００５〜０．１２％、
Ｓｉ：０．０２〜０．５％、
Ｍｎ：０．６〜２．２％、
Ｐ：≦０．０１％、
Ｓ：≦０．００５％、
Ｍｏ：０．０５〜０．５％、
Ａｌ：≦０．０５％、
Ｔｉ：０．００５〜０．０３％、
Ｂ：０．０００５〜０．００３％、
Ｎ：≦０．００６％、
残部が鉄および不可避的不純物からなり、かつ０％≦Ｔｉ（％）−３．４Ｎ（％）≦０．０２％を満足する鋼片を１０００〜１２５０℃の温度に加熱し、９５０℃以下の全圧下量が４０％以上、かつ仕上温度が７００〜８５０℃となるように圧延を行い、圧延後０．１℃／秒以上４０℃／秒以下の冷却速度で冷却して得られた鋼板を冷間成形した後、溶接し、引張り応力５３０〜７００ＭＰａのＵＯＥ鋼管としたことを特徴とする低温靱性の優れた高張力ラインパイプ用鋼管の製造方法。
（２）質量％で、
Ｃ：０．００５〜０．１２％、
Ｓｉ：０．０２〜０．５％、
Ｍｎ：０．６〜２．２％、
Ｐ：≦０．０１％、
Ｓ：≦０．００５％、
Ｎｂ：０．０１〜０．１％、
Ｍｏ：０．０５〜０．５％、
Ａｌ：≦０．０５％、
Ｔｉ：０．００５〜０．０３％、
Ｂ：０．０００５〜０．００３％、
Ｎ：≦０．００６％、
残部が鉄および不可避的不純物からなり、かつ０％≦Ｔｉ（％）−３．４Ｎ（％）≦０．０２％を満足する鋼片を１０００〜１２５０℃の温度に加熱し、９５０℃以下の全圧下量が４０％以上、かつ仕上げ温度が７００〜８５０℃となるように圧延を行い、圧延後０．１℃／秒以上４０℃／秒以下の冷却速度で冷却して得られた鋼板を冷間成形した後、溶接し、引張り応力５３０〜７００ＭＰａのＵＯＥ鋼管としたことを特徴とする低温靱性の優れた高張力ラインパイプ用鋼管の製造方法。
（３）鋼片がさらに、質量％で、
Ｎｉ：０．１〜１％、
Ｃｒ：０．１〜１％、
Ｃｕ：０．１〜１．５％、
Ｖ：０．０１〜０．１％、
Ｃａ：０．０００５〜０．００５％、
ＲＥＭ：０．０００５〜０．００５％、
Ｍｇ：０．０００１〜０．００５％、
のうち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１または請求項２の何れかに記載の低温靱性の優れた高張力ラインパイプ用鋼管の製造方法。
【０００９】
【発明の実施の形態】
本発明者らは、上述した従来法の欠点を解決すべく制御圧延・制御冷却法（ＴＭＣＰ法）に適した鋼の成分系、加熱、圧延、冷却条件などの製造プロセスについて多数の実験と詳細な検討を実施した結果、Ｍｏ単独添加またはＭｏ及びＮｂの複合添加し、更に微量のＴｉ及びＢを添加した鋼を制御圧延、冷却することによって板厚方向において均一組織でかつ強度・低温靱性バランスが飛躍的に向上することを見いだした。
【００１０】
本発明者の研究によれば、Ｂは鋼の焼き入れ性向上元素としてよく知られているが、単にＢを添加することによって焼き入れ性を向上させるだけでは良好な強度・低温靱性は得られないことが判明している。そこで、本発明者は、微量のＢとＴｉの添加と共にＭｏ単独、またはＭｏ及びＮｂの複合添加した鋼に着目し、それを用いた制御圧延、冷却条件の詳細な検討を行った。
【００１１】
従来から、Ｔｉは鋼中のＮを固定し、Ｂの焼き入れ性向上効果を安定化させると共にＮとの結合でできた微細なＴｉＮは加熱圧延中のオーステナイト粒成長を抑制し、変態後のフェライト粒をも細粒化する効果があることが知られている。また、Ｎｂもよく知られているように低温域での圧延（約９５０℃以下）によってオーステナイト粒を未再結晶化させ圧延組織を細粒化させ、更に、固溶化あるいは炭窒化物の析出によって、鋼の強度を向上させる効果がある。Ｍｏも固溶化によって鋼の強度を向上させることが知られている。
【００１２】
しかしながら、本発明者らの詳細な検討の結果、上記成分を単独添加する際に従来知られていた効果の他に、Ｂの微量添加とＭｏ単独あるいはＭｏ及びＮｂの複合添加を行った場合に新しい現象が起きることを発見した。すなわち、Ｂの微量添加とＭｏ単独あるいはＭｏ及びＮｂの複合添加は、オーステナイトの未再結晶化開始温度（再結晶温度）が、５０℃以上も高くなると同時に、焼き入れ性が大幅に向上してＮｂ、Ｍｏ、Ｂのそれぞれの単独添加から予想される値に比べて強度・低温靱性バランスが極めて向上することがわかった。この効果は通常の熱処理または制御圧延の単独効果よりも大きい。
【００１３】
この微量Ｂ添加と、Ｍｏ単独あるいはＭｏ及びＮｂの複合添加において、強度・低温靱性バランスが向上する理由は以下のように考えられる。
Ｂ単独添加鋼の場合、Ｂはオーステナイト粒界に偏析しているもの以外に、Ｍ₂₃（ＣＢ）₆の粗大な析出物を生成する。しかしながら、ＢとＭｏ単独あるいはＭｏ及びＮｂの複合添加時は、Ｎｂの炭窒化物およびＮｂおよびＭｏのＣクラスターが微細に析出し、Ｎｂ、Ｍｏによるオーステナイト中でのＣ原子の拡散速度が減少し、Ｍ₂₃（ＣＢ）₆へのＣ原子を抑制する。このため、Ｂ原子の偏析が増加し、焼き入れ性が増大したものと考えられる。この微量ＴｉおよびＢの添加とＭｏの複合添加または微量Ｔｉ、Ｂ、Ｍｏ、Ｎｂの複合添加鋼を用いれば、冷却速度が０．１℃／秒以上４０℃／秒以下の範囲で板厚方向の硬度差が少なく、かつ均一なベイナイト組織を有する鋼が得られることが判明した。
【００１４】
本発明によれば、前述の従来の制御冷却法における▲１▼から▲４▼に記載した問題点は解決される。すなわち、▲１▼については、ミクロ組織がベイナイト（アシュキラーフェライト、ベイニテイックフェライト、上部ベイナイト、下部ベイナイトを含む）単相組織となるため、焼き戻し処理がなくても延靱性が良好である。▲２▼については、ＴｉとＢとＭｏ、あるいはＴｉとＢとＭｏとＮｂの複合添加の効果により、溶接部についても焼き入れ性が向上し、溶接部の強度確保が容易である。▲３▼については、ＴｉとＢとＭｏ、あるいはＴｉとＢとＭｏとＮｂの複合添加の効果により細粒化効果、焼き入れ性が増大するために冷却速度・厚みにかかわらず安定した硬さ分布を示す。さらに、９５０℃以下の低温未再結晶温度域で全圧下量４０％以上で圧延するため、表面ほど細粒オーステナイトとなり、焼き入れ性が低下して厚み方向の組織は均一となる。▲４▼については、オーステナイト粒の細粒化の徹底、焼き入れ性の安定確保により、比較的広範囲の加熱圧延冷却条件下で安定な強度／低温靱性バランスを示す。
【００１５】
本発明により製造した鋼は、従来の鋼材に比べ、低成分（低炭素当量）で優れた強度・低温靱性が得られるため、溶接時の硬化性、割れ感受性が低く、また、溶接部の靱性が極めて良好である。このため、本発明鋼は建築、圧力容器、造船、ラインパイプ等に適用可能である。
以下、本発明の成分の限定理由について述べる。
【００１６】
Ｃ：鋼における母材強度を向上させる基本的な元素として欠かせない元素であり、その有効な下限値として０．００５％以上の添加が必要であるが、０．１２％を越える過剰の添加では、鋼材の溶接性や靱性の低下を招くので、その上限を０．１２％とした。
Ｓｉ：Ｓｉは製鋼上脱酸元素として必要な元素であり、鋼中に０．０２％以上の添加が必要であるが、０．５％を越えると溶接部ならびにの靱性を低下させるのでそれを上限とする。
【００１７】
Ｍｎ：Ｍｎは、母材の強度および靱性の確保に必要な元素であるが、２．２％を越えると焼き入れ性が増加し、ベイナイトあるいは島状マルテンサイトが多量に生成し、母材ならびに溶接部の靱性を著しく阻害するが、一方、０．６％未満では、母材の強度確保が困難になるために、その範囲を０．６〜２．２％とする。
【００１８】
Ｐ：Ｐは鋼の靱性に影響を与える元素であり、０．０１％を越えて含有すると鋼材の母材だけでなく溶接部の靱性を著しく阻害するのでその含有される上限を０．０１％とした。
Ｓ：Ｓは０．００５０％を越えて過剰に添加されると粗大な硫化物の生成の原因となり、母材ならびに溶接部の靱性を劣化させるのでその含有される上限を０．００５％とした。
【００１９】
Ｍｏ：母材の強度・低温靱性をともに向上させる元素であるが、０．０５％未満では顕著な効果がなく、一方、０．５％を超えると焼き入れ性が増大し、母材、溶接部の靱性を劣化させるので、その添加量を０．０５〜０．５％とした。
Ａｌ：Ａｌは、通常脱酸材として添加されるが、０．０５％を越えると溶接部の靱性が劣化するために上限を０．０５％とした。
【００２０】
Ｔｉ：Ｔｉは、その添加量が少ない範囲（Ｔｉ：０．００５〜０．０３％）で微細なＴｉＮを形成し、圧延組織およびＨＡＺの細粒化、つまり、靱性向上に効果的である。この場合、ＮとＴｉは化学量論的に当量近傍が望ましく、０％≦Ｔｉ（％）−３．４Ｎ（％）≦０．０２％が良好である。また、本発明では、ＴｉはＮを固定、Ｂの焼き入れ性を保護する効果を併せ持つ。Ｔｉ添加量の上限は、微細なＴｉＮが鋼片中に通常の製法で得られ、また、ＴｉＣによる靱性劣化が起きない条件から０．０２５％とした。また、０．００５％未満ではＴｉＮの十分な効果が得られないので下限を０．００５％とした。
【００２１】
Ｂ：圧延中にオーステナイト粒界に偏析し、焼き入れ性を上げ、ベイナイト組織を生成しやすくするが、０．０００５％未満では顕著な焼き入れ性改善効果が無く、０．００３％超になるとＢＮやＢconstituent （硼化物）を多く生成するようになるために母材やＨＡＺの靱性を劣化させる。このため、下限を０．０００５％、上限を０．００３％とした。
【００２２】
Ｎ：溶鋼中に不可避的に混入し、鋼の靱性を劣化させる。特に多量のフリーＮはＨＡＺ部に島状マルテンサイトを発生させやすく、ＨＡＺ部を大幅に劣化させる。このＨＡＺ部靱性および母材靱性を改善する目的で前記したようにＴｉを添加するが、Ｎが０．００６％を越えると鋼中のＴｉＮサイズが大きくなり、ＴｉＮの効果が減少するためにＮの上限を０．００６％とした。
【００２３】
Ｎｂ：圧延組織の細粒化、焼き入れ性の向上と析出硬化のため含有させるもので強度・低温靱性を共に向上させる重要な元素である。制御圧延材では１．０％を越えて添加しても材質効果がなく、また、溶接性およびＨＡＺ靱性に有害であるために上限を０．１％に限定した。また、下限０．０１％は材質上の効果を有する最小値である。
【００２４】
ＴｉとＮ量を０％≦Ｔｉ（％）−３．４Ｎ（％）≦０．０２％と限定した理由は、ＴｉによってＮを十分に固定し、Ｂの焼き入れ性向上効果を発揮させるためであって、上限０．０２％は過剰のＴｉがＴｉＣを大量に形成して靱性を劣化させない条件から、また、下限０％はフリーＮが多くなってＢＮを形成し、焼き入れ性が低下しない条件から決定した。
【００２５】
本発明による鋼は上述した各成分を基本成分とするものであるが、更にＮｉ，Ｃｒ，Ｃｕ，Ｖ，Ｃａ，Ｒｅｍ，Ｍｇの１種または２種以上を複合添加することもできる。以下にそれらの成分の限定理由について述べる。
Ｎｉは、：ＨＡＺの硬化性および靱性に悪影響を与えることなく母材の強度・低温靱性を向上させる特性を持つが、０．１％未満ではその効果が無く、１．０％を越えるとＨＡＺの硬化性および靱性上好ましく無いため、下限を０．１％、上限を１．０％とした。Ｖは、Ｎｂとほぼ同様の効果をもつが、０．０１％以下では顕著な効果が無く、上限は０．１０％まで許容できる。Ｃｒは、母材の強度を高め、耐水素誘起割れ性にも効果を有するが、０．１％未満では顕著な効果が無く、１．０％を越えるとＨＡＺの硬化性を増大させ、低温靱性・溶接性の低下が大きくなり好ましくない。このため、下限を０．１％、上限を１．０％とした。Ｃｕは、Ｎｉとほぼ同等の効果を持つと共に、耐食性、耐水素誘起割れ性にも効果がある。しかし、０．１％未満ではＮｉ同様顕著な効果が無く、１．５％を越えるとＮｉを添加しても圧延中に割れが発生し、製造が難しくなる。このため、下限を０．１％、上限を１．５％とした。Ｃａ、ＲＥＭは、ＭｎＳを球状化させ、シャルピー吸収エネルギ−衝撃値を向上させる他、圧延によって、延伸化したＭｎＳと水素による内部欠管の発生防止を防止する。ＲＥＭの含有用については０．０００５％未満であると事実上効果が無く、また、０．００５％を越えて添加するとＲＥＭ−ＳまたはＲＥＭ−Ｏ−Ｓが大量に生成して大型介在物となり、鋼の低温靱性のみならず清浄度を害し、また溶接性についても悪影響を及ぼす。ＣａについてもＲＥＭと同様の効果をもち、その有効範囲は０．０００５〜０．００５％である。Ｍｇは、Ｔｉとの複合脱酸によって微細な酸化物が微細分散し、溶接部の粗大粒成長の防止、粒内フェライトが生成、ＭｎＳの球状化によってシャルピー吸収エネルギ−、延性脆性遷移温度が向上する。０．０００１％未満であると事実上効果が無く、また、０．００５％を越えて添加すると粗大なＭｇ酸化物、Ｍｇ硫化物が生成して大型介在物となり、鋼の低温靱性のみならず清浄度を害し、また溶接性についても悪影響を及ぼす。
【００２６】
次に、上述した成分を有する鋼板の製造条件について述べる。加熱温度を１０００〜１２５０℃に限定した理由は、加熱時のオーステナイト粒を小さく保ち圧延組織の細粒化をはかるためである。１２５０℃は加熱時のオーステナイト粒が極端に粗大化しない上限であって、加熱温度がこれを越えるとオーステナイト粒が粗大混粒化し、冷却後の上部ベイナイト組織も粗大化するため、鋼の靱性が著しく劣化する。一方、加熱温度があまりに低すぎると、Ｎｂ，Ｖなどの析出硬化元素が十分に固溶せず強度・低温靱性バランスが劣化するだけでなく、圧延終段の温度の下がりすぎのために、制御冷却による十分な材質向上効果が期待できない。このため、下限を１０００℃とする必要がある。
【００２７】
また、９００℃以下の未再結晶温度域での圧下量を４０％以上とし、仕上げ温度を７００〜８５０℃の範囲とした理由は、未再結晶温度での十分な圧延を加えることによってオーステナイト粒の細粒化・延伸化を徹底し、冷却後に生成する変態組織を細粒均一化するためである。このように細粒オーステナイトを十分延伸化することにより、圧延冷却後生成するフェライト、上部ベイナイト組織を十分細粒化すると、靱性が大幅に向上する。しかし、仕上げ温度が不適当であると良好な強度・低温靱性が得られない。仕上げ温度の下限を７００℃としたのは過度の変態点以下の（γ＋α）域圧延によって延靱性を劣化させないためである。また、仕上げ温度が７００℃未満では制御圧延による十分な強度上昇効果が期待できない。一方、仕上げ温度が余りにも高すぎると制御圧延によるオーステナイト粒の細粒化効果が期待できず靱性が低下する。このため上限を８５０℃とする必要がある。
【００２８】
圧延後の冷却条件については、良好な強度、低温靱性を得るために板厚方向に均一な変態組織が得られるように行わなければならない。このため、種々の実験を行った結果、圧延終了後から０．１℃／秒以上４０℃／秒以下の冷却速度で冷却すると板厚方向に均一な変態組織が得られることが判明した。この理由は０．１℃／秒未満ではベイナイト組織が生成しにくく、強度の向上が十分でない。また、４０℃／秒超では多量の島状マルテンサイトが生成し、延靱性を劣化させるからである。
【００２９】
【実施例】
次に、本発明の実施例について述べる。
転炉、連続鋳造工程で製造した種々の化学成分の鋳片を用い、製造プロセスを変えて板厚１６〜５０ｍｍの鋼板を製造した。これらの鋼板を冷間成形し、仮付け溶接、内外面溶接を行った後、拡管を行いＵＯＥ鋼管とした。その鋼管の母材および溶接部の機械的性質を表１および表２に示した。
【００３０】
【表１】

【００３１】
【表２】

【００３２】
本発明に従って製造した鋼管１〜２２はいずれも優れた母材、溶接部の特性を有している。これに対して、本発明によらない比較鋼は母材或いは溶接部の特性にいずれかが不満足で、溶接用鋼材としてのバランスに欠けている。比較鋼中、鋼２３，２４，２５では本発明の鋼の必須元素であるＭｏ、Ｂ、Ｔｉのいずれかが添加されていない。このため、鋼２３〜２５では、Ｔｉ、ＢとＭｏあるいはＴｉとＢとＭｏとＮｂの複合効果になっていないために母材強度が劣っている。本発明鋼では５３０ＭＰａ以上の引張り強度がでる。また、鋼２５ではＨＡＺ組織が粗くなり溶接部靱性も劣っている。鋼２６では加熱温度が低すぎるため、鋼２７では仕上げ温度が低すぎるために、鋼２８は９００℃以下の圧下量が十分なために強度がでない。鋼２９は冷却速度が遅すぎるために十分な強度達成されない。
【００３３】
【発明の効果】
本発明によって強度・低温靱性および溶接性の優れたラインパイプ用鋼管の製造が可能となった。[0001]
BACKGROUND OF THE INVENTION
The present invention, strength, those involved in the manufacturing method of low-temperature toughness and weldability of excellent line steel pipe pipe.
[0002]
[Prior art]
In recent years, the use of high-strength steel has been widespread in welded structures (buildings, pressure vessels, shipbuilding, line pipes) in terms of economy and safety, and the demand for weldable high-strength steel has shown a steady increase. Yes. Of course, steel used in welded structures is required to have both high toughness and excellent weldability from the aspects of safety and workability in addition to high strength. Steel that satisfies these characteristics As a manufacturing method, a controlled rolling method (CR method) widely used in the production of line pipe materials and a quenching and tempering method (QT method) for performing quenching and tempering after rolling are well known. However, in the former method, the rolled structure is generally ferrite pearlite, and the strength and the thickness obtained are limited by themselves. The reason for this is that, in order to obtain an acicular ferrite or bainite structure that is excellent in strength and toughness in the manufactured steel, the cooling rate is remarkably increased or a large amount of alloy addition is required. It was difficult. In the latter method, since a reheating step is required, the cost is increased and the production capacity is restricted.
[0003]
Therefore, at present, these methods are advanced one step, and development of a controlled rolling / controlled cooling method (TMCP method) in which energy saving and resource saving (reduction of alloying elements) are thoroughly promoted. The steel produced by this method has the advantages of CR and QT methods, and has the characteristics that an excellent material can be obtained without adding a low alloy or special alloy, but on the other hand, it was produced by the conventional controlled cooling method. Steel has the following disadvantages.
[0004]
{Circle around (1)} When rapid cooling is performed after rolling, the strength is too high, so that a tempering treatment is essential to recover the ductility.
{Circle around (2)} The heat affected zone (HAZ) during welding is greatly softened, and it is difficult to ensure the strength of the welded portion, particularly with a high yield point and high strength steel.
(3) The structure in the cross-sectional direction of the plate thickness is uneven and the hardness difference is large.
[0005]
(4) The control of the cooling conditions (cooling start, stop temperature and speed) is delicate and the material is unstable.
As a method for improving these drawbacks, for example, in Japanese Patent Laid-Open No. 63-179020 or Japanese Patent Laid-Open No. 61-67717, a thickness cross section is defined by defining a component, a reduction amount, a cooling rate, and a cooling stop temperature. It is disclosed to reduce the hardness difference. However, these methods can be applied to a steel sheet having a relatively small thickness, but it is difficult to control the cooling rate in the thickness direction with an extra-thick steel sheet, and the effect of reducing the thickness difference in thickness section is obtained. There is a problem that you can not.
[0006]
Japanese Laid-Open Patent Publication No. 58-77528 discloses a method of reducing the hardness difference in the plate thickness direction by making the structure in the plate thickness direction a bainite uniform structure by the combined addition of Nb and B. However, in this method, since it is necessary to set the cooling rate to 15 to 40 ° C./second in order to obtain a uniform bainite structure, it is difficult to make the difference in hardness in the thickness direction of the extra-thick steel plate uniform. There was a problem.
[0007]
[Problems to be solved by the invention]
In view of the above-mentioned problems of the prior art, the present invention has X65 grade (yield stress is 448 MPa or more, tensile stress is 530 MPa or more) with less material variation in the thickness direction and excellent strength, low temperature toughness and weldability. ) it is an object of the present invention to produce a Lula-pipe for steel pipe having a mechanical properties above. In addition, the upper limit of the tensile stress is the steel No. in the example of the present invention. Based on 14 and 16 of 700 MPa, the pressure is set to 700 MPa or less.
[0008]
[Means for Solving the Problems]
That is, the gist of the present invention is as follows.
(1) In mass% ,
C: 0.005-0.12%,
Si: 0.02 to 0.5%,
Mn: 0.6 to 2.2%
P: ≦ 0.01%
S: ≦ 0.005%,
Mo: 0.05-0.5%
Al: ≦ 0.05%,
Ti: 0.005 to 0.03%,
B: 0.0005-0.003%,
N: ≦ 0.006%,
The balance is composed of iron and inevitable impurities, and the steel slab satisfying 0% ≦ Ti (%)-3.4N (%) ≦ 0.02% is heated to a temperature of 1000 to 1250 ° C. A steel sheet obtained by rolling at a total reduction amount of 40% or more and a finishing temperature of 700 to 850 ° C. and cooling at a cooling rate of 0.1 ° C./second to 40 ° C./second after rolling. A method for producing a steel pipe for a high-tensile line pipe excellent in low-temperature toughness, characterized in that after cold forming, welding is performed to obtain a UOE steel pipe having a tensile stress of 530 to 700 MPa.
(2) In mass% ,
C: 0.005~0.12%,
Si: 0.02~0.5%,
Mn: 0.6~2.2%,
P: ≦ 0.01%,
S: ≦ 0.005%,
Nb: 0.01~0.1%,
Mo: 0.05~0.5%,
Al: ≦ 0.05%,
Ti: 0.005 to 0.03% ,
B: 0.0005 to 0.003% ,
N: ≦ 0.006%,
The balance is composed of iron and inevitable impurities, and the steel slab satisfying 0% ≦ Ti (%)-3.4N (%) ≦ 0.02% is heated to a temperature of 1000 to 1250 ° C. A steel sheet obtained by rolling so that the total reduction amount is 40% or more and the finishing temperature is 700 to 850 ° C., and is cooled at a cooling rate of 0.1 ° C./second to 40 ° C./second after rolling. A method for producing a steel pipe for a high-tensile line pipe excellent in low-temperature toughness, characterized in that after cold forming, welding is performed to obtain a UOE steel pipe having a tensile stress of 530 to 700 MPa.
(3) The steel slab is further mass% ,
Ni: 0.1 to 1%,
Cr: 0.1 to 1%,
Cu: 0.1 to 1.5%,
V: 0.01-0.1%
Ca: 0.0005 to 0.005%,
REM: 0.0005 to 0.005%,
Mg: 0.0001 to 0.005%,
One or manufacturing method of the excellent high tension line pipes for steel tubes low temperature toughness according to claim 1 or claim 2, characterized by containing two or more of.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
The inventors of the present invention have made numerous experiments and details on manufacturing processes such as steel component systems, heating, rolling, and cooling conditions suitable for the controlled rolling / controlled cooling method (TMCP method) in order to solve the above-described drawbacks of the conventional methods. As a result of carrying out various investigations, steel is added with Mo alone or combined with Mo and Nb, and steel with a small amount of Ti and B added is controlled rolled and cooled to achieve a uniform structure and balance between strength and low temperature toughness in the sheet thickness direction. Found a dramatic improvement.
[0010]
According to the inventor's research, B is well known as an element for improving the hardenability of steel. However, by simply adding B to improve the hardenability, good strength and low temperature toughness can be obtained. It turns out not. Therefore, the present inventor paid attention to steel added with a small amount of B and Ti and added with Mo alone or combined with Mo and Nb, and performed detailed examination of controlled rolling and cooling conditions using the steel.
[0011]
Conventionally, Ti fixes N in steel, stabilizes the effect of improving the hardenability of B, and fine TiN formed by bonding with N suppresses austenite grain growth during hot rolling, and after transformation It is known that there is an effect of refining ferrite grains. As is well known, Nb is not recrystallized by rolling in a low temperature region (about 950 ° C. or less) to refine the rolled structure, and further, by solid solution or carbonitride precipitation. It has the effect of improving the strength of steel. Mo is also known to improve the strength of steel by solid solution.
[0012]
However, as a result of detailed investigations by the present inventors, in addition to the effects conventionally known when the above components are added alone, a small amount of B and Mo alone or a combined addition of Mo and Nb are performed. I discovered a new phenomenon. That is, the addition of a small amount of B and Mo alone or the combined addition of Mo and Nb increases the austenite non-recrystallization start temperature (recrystallization temperature) by 50 ° C. or more and at the same time significantly improves the hardenability. It was found that the balance between strength and low temperature toughness was greatly improved as compared with the values expected from the individual addition of Nb, Mo and B. This effect is greater than the single effect of normal heat treatment or controlled rolling.
[0013]
The reason why the balance between strength and low-temperature toughness is improved in the addition of a small amount of B and the combined addition of Mo alone or Mo and Nb is considered as follows.
In the case of the steel containing B alone, B forms coarse precipitates of M ₂₃ (CB) _{6 in} addition to those segregated at the austenite grain boundaries. However, when B and Mo are added alone or Mo and Nb are combined, Nb carbonitrides and C clusters of Nb and Mo precipitate finely, and the diffusion rate of C atoms in austenite by Nb and Mo decreases. , Suppresses the C atom to M ₂₃ (CB) ₆ . For this reason, it is considered that segregation of B atoms increased and the hardenability increased. If this addition of trace amounts of Ti and B and combined addition of Mo or combined addition of trace amounts of Ti, B, Mo, and Nb is used, the cooling rate is in the range of 0.1 ° C / second to 40 ° C / second in the thickness direction. It was found that a steel having a uniform hardness and a uniform bainite structure can be obtained.
[0014]
According to the present invention, the problems described in (1) to (4) in the above-described conventional controlled cooling method are solved. That is, for (1), since the microstructure is a bainite (including ash ferrite, bainitic ferrite, upper bainite, and lower bainite) single phase structure, the ductility is good even without tempering treatment. . With regard to (2), the effect of combined addition of Ti, B, and Mo, or Ti, B, Mo, and Nb improves the hardenability of the welded portion and makes it easy to ensure the strength of the welded portion. Regarding (3), since the effect of grain refinement and hardenability is increased by the combined addition of Ti and B and Mo or Ti, B, Mo and Nb, the hardness is stable regardless of the cooling rate and thickness. Show the distribution. Furthermore, since rolling is performed at a low unrecrystallization temperature range of 950 ° C. or less with a total reduction of 40% or more, the surface becomes fine-grained austenite, the hardenability decreases, and the structure in the thickness direction becomes uniform. Regarding (4), a stable strength / low temperature toughness balance is exhibited under a relatively wide range of heating, rolling and cooling conditions by thoroughly reducing the austenite grains and ensuring stable hardenability.
[0015]
The steel produced according to the present invention has excellent strength and low temperature toughness with low components (low carbon equivalent) compared to conventional steel materials, so it has low curability and cracking susceptibility during welding, and the toughness of the weld zone. Is very good. For this reason, this invention steel is applicable to a building, a pressure vessel, shipbuilding, a line pipe, etc.
Hereinafter, the reasons for limiting the components of the present invention will be described.
[0016]
C: Element that is indispensable as a basic element for improving the strength of the base metal in steel, and an effective lower limit value of 0.005% or more is necessary, but excessive addition exceeding 0.12% Then, since the weldability and toughness of steel materials are reduced, the upper limit was made 0.12%.
Si: Si is an element necessary as a deoxidizing element in steelmaking, and it is necessary to add 0.02% or more to the steel. However, if it exceeds 0.5%, the toughness of the welded part and the steel deteriorates. The upper limit.
[0017]
Mn: Mn is an element necessary for ensuring the strength and toughness of the base material. However, if it exceeds 2.2%, the hardenability increases, and a large amount of bainite or island-like martensite is generated. Although the toughness of the welded portion is significantly inhibited, on the other hand, if it is less than 0.6%, it becomes difficult to ensure the strength of the base material, so the range is made 0.6 to 2.2%.
[0018]
P: P is an element that affects the toughness of steel, and if it exceeds 0.01%, not only the base material of the steel material but also the toughness of the welded portion is significantly inhibited. It was.
S: When S is added in excess of 0.0050%, coarse sulfides are formed and the toughness of the base metal and the welded portion is deteriorated. Therefore, the upper limit of the content is set to 0.005%. .
[0019]
Mo: An element that improves both the strength and low-temperature toughness of the base metal. If it is less than 0.05%, there is no significant effect. On the other hand, if it exceeds 0.5%, the hardenability increases, and the base metal and welding. Since the toughness of the part is deteriorated, the addition amount is set to 0.05 to 0.5%.
Al: Al is usually added as a deoxidizer, but if it exceeds 0.05%, the toughness of the welded portion deteriorates, so the upper limit was made 0.05%.
[0020]
Ti: Ti forms fine TiN in a range where the amount of addition is small (Ti: 0.005 to 0.03%), and is effective in reducing the rolling structure and HAZ, that is, improving toughness. In this case, N and Ti are desirably in the vicinity of an equivalent stoichiometric amount, and 0% ≦ Ti (%) − 3.4N (%) ≦ 0.02% is favorable. In the present invention, Ti also has the effect of fixing N and protecting the hardenability of B. The upper limit of the amount of Ti added is set to 0.025% from the condition that fine TiN is obtained in a steel slab by a normal manufacturing method and toughness deterioration due to TiC does not occur. Further, if it is less than 0.005%, a sufficient effect of TiN cannot be obtained, so the lower limit was made 0.005%.
[0021]
B: Segregates at austenite grain boundaries during rolling, improves hardenability and facilitates the formation of a bainite structure, but if it is less than 0.0005%, there is no significant effect of improving hardenability, and if it exceeds 0.003% Since a large amount of BN and Bconstituent (boride) is generated, the toughness of the base material and the HAZ is deteriorated. Therefore, the lower limit is set to 0.0005% and the upper limit is set to 0.003%.
[0022]
N: inevitably mixed in molten steel to deteriorate the toughness of the steel. In particular, a large amount of free N tends to generate island martensite in the HAZ part, and the HAZ part is greatly deteriorated. In order to improve the HAZ toughness and the base metal toughness, Ti is added as described above. However, when N exceeds 0.006%, the TiN size in the steel increases, and the effect of TiN decreases. The upper limit of 0.006%.
[0023]
Nb: It is an important element for improving both strength and low-temperature toughness because it is included for making the rolled structure finer, improving hardenability and precipitation hardening. In the case of controlled rolled material, even if added over 1.0%, there is no material effect, and since it is harmful to weldability and HAZ toughness, the upper limit was limited to 0.1%. The lower limit of 0.01% is the minimum value having an effect on the material.
[0024]
The reason for limiting the amount of Ti and N to 0% ≦ Ti (%) − 3.4N (%) ≦ 0.02% is to sufficiently fix N with Ti and to exert the effect of improving the hardenability of B. The upper limit of 0.02% is based on the condition that excessive Ti does not form a large amount of TiC and deteriorates the toughness, and the lower limit of 0% increases the free N and forms BN, resulting in reduced hardenability. Not determined from the conditions.
[0025]
The steel according to the present invention has the above-described components as basic components, but it is also possible to add one or more of Ni, Cr, Cu, V, Ca, Rem, and Mg in combination. The reasons for limiting these components are described below.
Ni has the characteristics of improving the strength and low-temperature toughness of the base material without adversely affecting the curability and toughness of HAZ. However, if it is less than 0.1%, it has no effect, and if it exceeds 1.0%, HAZ Therefore, the lower limit was set to 0.1% and the upper limit was set to 1.0%. V has substantially the same effect as Nb, but there is no remarkable effect below 0.01%, and the upper limit is allowable up to 0.10%. Cr increases the strength of the base metal and has an effect on resistance to hydrogen-induced cracking, but if it is less than 0.1%, there is no significant effect, and if it exceeds 1.0%, it increases the curability of the HAZ and lowers the temperature. It is not preferable because the deterioration of toughness and weldability is increased. Therefore, the lower limit is set to 0.1% and the upper limit is set to 1.0%. Cu has substantially the same effect as Ni, and is also effective in corrosion resistance and resistance to hydrogen-induced cracking. However, if it is less than 0.1%, there is no remarkable effect like Ni, and if it exceeds 1.5%, even if Ni is added, cracks are generated during rolling, making the production difficult. Therefore, the lower limit is set to 0.1% and the upper limit is set to 1.5%. Ca and REM spheroidize MnS to improve the Charpy absorbed energy-impact value, and also prevent the occurrence of internal breakage due to the extended MnS and hydrogen by rolling. When the content of REM is less than 0.0005%, there is practically no effect. When the content exceeds 0.005%, a large amount of REM-S or REM-O-S is generated and becomes a large inclusion. , Not only the low temperature toughness of steel, but also the cleanliness, and the weldability is also adversely affected. Ca has the same effect as REM, and its effective range is 0.0005 to 0.005%. Mg is finely dispersed with fine oxides by complex deoxidation with Ti, preventing coarse grain growth in welds, forming intragranular ferrite, and improving the Charpy absorbed energy and ductile brittle transition temperature by spheroidizing MnS. To do. If it is less than 0.0001%, there is practically no effect, and if added over 0.005%, coarse Mg oxides and Mg sulfides are formed and become large inclusions, not only low temperature toughness of steel It impairs cleanliness and adversely affects weldability.
[0026]
Next, manufacturing conditions for the steel sheet having the above-described components will be described. The reason for limiting the heating temperature to 1000 to 1250 ° C. is to keep the austenite grains during heating small and to refine the rolling structure. 1250 ° C is the upper limit at which the austenite grains during heating are not extremely coarsened, and when the heating temperature exceeds this, the austenite grains become coarsely mixed and the upper bainite structure after cooling also coarsens, so the toughness of the steel Deteriorates significantly. On the other hand, if the heating temperature is too low, precipitation hardening elements such as Nb and V do not dissolve sufficiently and the balance between strength and low temperature toughness is deteriorated, and the temperature at the final stage of rolling is too low. A sufficient material improvement effect by cooling cannot be expected. For this reason, it is necessary to make a minimum into 1000 ° C.
[0027]
Moreover, the reason why the reduction amount in the non-recrystallization temperature range of 900 ° C. or less is set to 40% or more and the finishing temperature is in the range of 700 to 850 ° C. is that austenite grains are obtained by applying sufficient rolling at the non-recrystallization temperature. This is because the transformation structure formed after cooling is thoroughly refined and stretched to make the transformed structure uniform. As described above, when the fine-grained austenite is sufficiently stretched to sufficiently refine the ferrite and upper bainite structure produced after rolling and cooling, the toughness is greatly improved. However, if the finishing temperature is inappropriate, good strength and low temperature toughness cannot be obtained. The reason why the lower limit of the finishing temperature is set to 700 ° C. is that the toughness is not deteriorated by (γ + α) region rolling below an excessive transformation point. Further, if the finishing temperature is less than 700 ° C., a sufficient strength increase effect by controlled rolling cannot be expected. On the other hand, if the finishing temperature is too high, the effect of refining austenite grains by controlled rolling cannot be expected and the toughness is lowered. For this reason, it is necessary to make an upper limit into 850 degreeC.
[0028]
As for the cooling conditions after rolling, in order to obtain good strength and low temperature toughness, it must be performed so that a uniform transformation structure can be obtained in the thickness direction. For this reason, as a result of various experiments, it was found that a uniform transformation structure can be obtained in the plate thickness direction by cooling at a cooling rate of 0.1 ° C./second or more and 40 ° C./second or less after the end of rolling. The reason for this is that if it is less than 0.1 ° C./second, a bainite structure is not easily formed, and the strength is not sufficiently improved. Further, if it exceeds 40 ° C./second, a large amount of island-like martensite is generated and the ductility is deteriorated.
[0029]
【Example】
Next, examples of the present invention will be described.
Steel plates having a thickness of 16 to 50 mm were manufactured by changing the manufacturing process using slabs of various chemical components manufactured in a converter and continuous casting process. These steel sheets were cold formed, tack welded and inner / outer surface welded, and then expanded to obtain a UOE steel pipe. Tables 1 and 2 show the mechanical properties of the base metal and the weld of the steel pipe.
[0030]
[Table 1]

[0031]
[Table 2]

[0032]
The steel pipes 1 to 22 manufactured according to the present invention all have excellent base metal and welded portion characteristics. On the other hand, the comparative steel not according to the present invention is unsatisfactory in the characteristics of the base metal or the welded portion, and lacks the balance as a steel material for welding. Among the comparative steels, steels 23, 24, and 25 do not contain any of Mo, B, and Ti, which are essential elements of the steel of the present invention. For this reason, in the steels 23-25, since it is not the combined effect of Ti, B, and Mo or Ti, B, Mo, and Nb, base material strength is inferior. The steel of the present invention has a tensile strength of 530 MPa or more. Moreover, in the steel 25, the HAZ structure becomes coarse and the weld zone toughness is also inferior. Since the heating temperature is too low for steel 26 and the finishing temperature is too low for steel 27, steel 28 is not strong because the amount of rolling below 900 ° C. is sufficient. Steel 29 does not achieve sufficient strength because the cooling rate is too slow.
[0033]
【The invention's effect】
Production of superior line pipe steel pipe strength and low temperature toughness and weldability by the present invention becomes possible.

Claims

% By mass
C: 0.005-0.12%,
Si: 0.02 to 0.5%,
Mn: 0.6 to 2.2%
P: ≦ 0.01%
S: ≦ 0.005%,
Mo: 0.05-0.5%
Al: ≦ 0.05%,
Ti: 0.005 to 0.03%,
B: 0.0005-0.003%,
N: ≦ 0.006%,
The balance is composed of iron and inevitable impurities, and the steel slab satisfying 0% ≦ Ti (%)-3.4N (%) ≦ 0.02% is heated to a temperature of 1000 to 1250 ° C. A steel sheet obtained by rolling at a total reduction amount of 40% or more and a finishing temperature of 700 to 850 ° C. and cooling at a cooling rate of 0.1 ° C./second to 40 ° C./second after rolling. A method for producing a steel pipe for a high-tensile line pipe excellent in low-temperature toughness, characterized in that after cold forming, welding is performed to obtain a UOE steel pipe having a tensile stress of 530 to 700 MPa.

% By mass
C: 0.005~0.12%,
Si: 0.02~0.5%,
Mn: 0.6~2.2%,
P: ≦ 0.01%,
S: ≦ 0.005%,
Nb: 0.01~0.1%,
Mo: 0.05~0.5%,
Al: ≦ 0.05%,
Ti: 0.005 to 0.03% ,
B: 0.0005 to 0.003% ,
N: ≦ 0.006%,
The balance is composed of iron and inevitable impurities, and the steel slab satisfying 0% ≦ Ti (%)-3.4N (%) ≦ 0.02% is heated to a temperature of 1000 to 1250 ° C. A steel sheet obtained by rolling so that the total reduction amount is 40% or more and the finishing temperature is 700 to 850 ° C., and is cooled at a cooling rate of 0.1 ° C./second to 40 ° C./second after rolling. A method for producing a steel pipe for a high-tensile line pipe excellent in low-temperature toughness, characterized in that after cold forming, welding is performed to obtain a UOE steel pipe having a tensile stress of 530 to 700 MPa.

The billet is further mass% ,
Ni: 0.1 to 1%,
Cr: 0.1 to 1%,
Cu: 0.1 to 1.5%,
V: 0.01-0.1%
Ca: 0.0005 to 0.005%,
REM: 0.0005 to 0.005%,
Mg: 0.0001 to 0.005%,
The manufacturing method of the steel pipe for high tension line pipes excellent in low-temperature toughness in any one of Claim 1 or 2 characterized by including 1 type, or 2 or more types.