JP3925291B2 - High-tensile welded steel pipe excellent in workability and material uniformity of welds, its manufacturing method, and steel strip for welded steel pipe material - Google Patents
High-tensile welded steel pipe excellent in workability and material uniformity of welds, its manufacturing method, and steel strip for welded steel pipe material Download PDFInfo
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Description
【０００１】
【発明の属する技術分野】
本発明は、引張強さ５９０〜１１８０ＭＰａの強度を有し、曲げ、液圧、拡管、縮管、およびこれらを複合した成形等に必要な加工性と、優れた溶接部の材質均一性とを兼備し、自動車、オートバイ等の構造部材に好適な高張力溶接鋼管およびその製造方法、ならびにその素材用鋼帯に関する。
【０００２】
【従来の技術】
車体の軽量化、高剛性化の観点から、サスペンションアーム、サスペンションメンバー、アクスルビーム、スタビライザー、フレーム、シャフト等の自動車構造部材への高張力溶接鋼管の適用が検討されており、これらに適した高張力鋼管が強く求められている。これまでに、このような自動車構造部材に適用される高張力溶接鋼管に関する技術が種々提案されている。
【０００３】
特開平１１−２７９６９７号公報には、Ｃ−Ｓｉ−Ｍｎ−Ｃｒを主成分とする鋼スラブを熱延後２５０℃以下で巻取り、フェライトと残部マルテンサイト及びベイナイトからなる複合組織を有することを特徴とする電縫鋼管に関する技術が開示され、特開平１１−２７９６９９号公報には、Ｃ−Ｓｉ−Ｍｎを主成分とする鋼スラブを熱延後６００℃以下で巻取るか、熱延後さらに酸洗、冷延、連続焼鈍した５〜１０％の準安定オーステナイトを含む複合組織を有することを特徴とする電縫鋼管に関する技術が開示されている。これらの技術によれば、引張強さ（ＴＳ）が５５０〜７８０ＭＰａの範囲で比較的良好な伸び（Ｅｌ）が得られるものの、延性を確保するために１％程度のＳｉを添加していることもあり、電縫溶接部の材質均一性に問題がある。
【０００４】
特開平５−３０２１２１号公報には、Ｃ−Ｓｉ−Ｍｎ−Ｎｂ−Ｍｏを主成分とする鋼スラブを熱延、冷延、電縫造管後、焼鈍することにより、ＴＳが８１０〜９２０ＭＰａの機械電縫鋼管を得る技術が開示されている。この技術では造管後の熱処理により歪時効のために電縫溶接部の材質均一性に問題がある。
【０００５】
特許第３２３５１６８号公報には、Ｃ−Ｓｉ−Ｍｎ−Ｎｂ−微量Ｔｉを主成分とする鋼スラブを熱延後６００〜２００℃で巻取り、電縫溶接することを特徴とする技術が開示され、特開平５−２７１８５９号公報には、Ｃ−Ｓｉ−Ｍｎ−Ｎｂ−微量Ｔｉ−Ｂを主成分とする鋼スラブを熱延後６００〜２００℃で巻取り電縫溶接することを特徴とする技術が開示され、特開平５−４３９８０号公報には、Ｃ−Ｓｉ−Ｍｎ−Ｎｂ−微量Ｔｉ−Ｍｏを主成分とする熱延鋼帯を電縫溶接することを特徴とする技術が開示されている。これら技術によれば、ＴＳが６８０〜９４０ＭＰａの範囲で比較的良好な電縫溶接部靱性が得られるものの、その加工性に問題がある。
【０００６】
【発明が解決しようとする課題】
以上のように従来の技術では、自動車等の構造部材に必要とされる加工性と溶接部の加工性とを兼備した高張力溶接鋼管が得られていない。特に、高張力鋼管を用いた自動車構造部材では、部品の信頼性という観点から溶接部の材質均一性が重要視される。
【０００７】
本発明はかかる事情に鑑みてなされたものであって、引張強さ５９０〜１１８０ＭＰａの強度を有し、曲げ、液圧、拡管、縮管、およびこれらを複合した成形等に必要な加工性と、優れた溶接部の材質均一性とを兼備した、自動車、オートバイ等の構造部材に好適な高張力溶接鋼管およびその製造方法、ならびに溶接鋼管素材用鋼帯を提供することを目的とする。
【０００８】
【課題を解決するための手段】
本発明者らは、強度、加工性、靱性といった相反する特性を同時に満たす溶接鋼管を得るために、溶接鋼管の化学成分、ミクロ組織、析出物存在状態を種々変化させて系統的な実験検討を行った。その結果、６０％以上の面分率を占めるフェライト組織中に、平均粒径が１０ｎｍ未満の極微細な炭化物を析出させることで、所望の強度、加工性、溶接部の材質均一性を同時に満たす溶接鋼管が得られることを見出した。
【０００９】
本発明はこのような知見に基づいて完成されたものであり、以下の（１）〜（４）を提供する。
【００１０】
（１） 重量％で、Ｃ：０．０３５〜０．０９５％、Ｍｎ：０．７５〜１．９５％、Ｍｏ：０．０１〜０．６５％、Ｔｉ：０．０１０〜０．１４５％、Ｐ：０．０３％以下、Ｓ：０．００７％以下、Ｎ：０．００６％以下、Ｏ：０．００４％以下、を含有し、さらに、Ｓｉ：０．００５〜０．７５％、Ａｌ：０．０１０〜０．１０％、Ｃｒ：０．０１〜０．２９％、Ｎｂ：０．００１〜０．０４０％、Ｖ：０．００１〜０．０５０％、Ｗ：０．００１〜０．５０％、Ｎｉ：０．０１〜０．５０％、Ｃｕ：０．０１〜０．５０％、Ｂ：０．０００１〜０．０００９％、Ｃａ：０．０００１〜０．００４０％、ＲＥＭ：０．０００１〜０．００４０％のうちの１種以上を含有し、残部がＦｅおよび不可避的不純物からなり、Ｔｉ、Ｍｏ、Ｎの重量％で表される以下の（１）式を満たし、平均粒径が１０ｎｍ未満の微細炭化物が析出したフェライト組織が組織面分率で６０〜１００％であることを特徴とする加工性と溶接部の材質均一性に優れた高張力溶接鋼管。
０．１５≦｛Ｔｉ−（４８／１４）Ｎ｝／Ｍｏ≦１ ‥‥（１）
【００１２】
（２） 上記（１）に記載の溶接鋼管を製造するにあたり、上記組成の鋼スラブを１１５０℃以上に加熱した後、仕上げ圧延温度を８５０℃以上とする熱間圧延を施し、仕上げ圧延後８秒間以内に７２０℃以下まで冷却し、６００℃超〜６５０℃未満で巻取って熱延鋼帯とし、酸洗、スリット後造管することを特徴とする加工性と靱性に優れる高張力溶接鋼管の製造方法。
【００１３】
（３） 上記（１）に記載の成分組成を有し、平均粒径が１０ｎｍ未満の微細炭化物が析出したフェライト組織が組織面分率で６０〜１００％であることを特徴とする加工性と溶接部の材質均一性に優れた高張力溶接鋼管素材用鋼帯。
【００１４】
【発明の実施の形態】
以下、本発明について具体的に説明する。
本発明の溶接鋼管の最も重要な点は、平均粒径１０ｎｍ未満の極微細な炭化物が析出したフェライト組織が組織面分率で６０〜１００％である点である。これにより所望の強度、加工性、溶接部の材質均一性を同時に有する溶接鋼管が得られる。このように溶接部の均一性、加工性を劣化させずに高強度化を図ることができるのは、炭化物を微細析出させることで、強度上昇に必要な析出物の最近接粒子間距離を得るために必要な元素量が少なくなることなどが要因の一つと考えられる。
【００１５】
図１に鋼管のフェライト分率、フェライト組織中の析出物の大きさ（平均粒径）と溶接部の材質均一性との関係を示す。溶接部の材質均一性は、鋼管長さ２０ｍ毎にｎ＝３０〜１００でサンプリングした鋼管を拡管試験、扁平試験、シャルピー衝撃試験に供し、得られた限界拡管率、限界扁平率、シャルピー吸収エネルギーから平均値および標準偏差σをそれぞれ求め、（平均値−２σ）の値を材質均一性の指標とした。（平均値−２σ）の値は平均値よりも低い値となる。なお、限界拡管率は、６０°円錐を管端に押込み、亀裂の入った時の外径ｄｂ（ｍｍ）と管の初期外形ｄ（ｍｍ）との比ｄｂ／ｄとして求め、限界扁平率は、シームを圧縮方向と９０°となるように扁平した際に亀裂の入った時の高さｈ（ｍｍ）と初期外径ｄ（ｍｍ）とより（ｄ−ｈ）／ｄとして求め、シャルピー吸収エネルギーは、溶接突合せ部より円周方向に切出して展開したＪＩＳ Ｚ２２０２規定のＶノッチ試験片の−２０℃での吸収エネルギーから求めた。図１の下段の丸プロット内の数字が限界拡管率ｄｂ／ｄの（平均値−２σ）の値を示し、図１の中段の四角プロット内の数字が限界扁平率（ｄ−ｈ）／ｄの（平均値−２σ）の値を示し、図１の上段の三角プロット内の数字が−２０℃での溶接部シャルピー吸収エネルギーの（平均値−２σ）の値を示す。なお、図１の中段のプロットのうち黒く塗りつぶしたものは密着しても亀裂が入らなかったものである。図１から、６０％以上の面分率を占めるフェライト組織中に平均粒径１０ｎｍ未満の極微細な炭化物を析出させることで、限界拡管率の（平均値−２σ）の値が１．２５以上、限界扁平率の（平均値−２σ）の値が０．７５以上、−２０℃でのシャルピー吸収エネルギーの（平均値−２σ）の値が９０Ｊ／ｃｍ２以上の良好な溶接部の材質均一性が得られることがわかる。そして、フェライト組織の組織面分率を９０％以上とすることで、、限界拡管率の（平均値−２σ）の値が１．３５以上、限界扁平率の（平均値−２σ）の値が０．９２以上、−２０℃でのシャルピー吸収エネルギーの（平均値−２σ）の値が１００Ｊ／ｃｍ２以上のさらに良好な溶接部の材質均一性が得られる。このような本発明の溶接鋼管における溶接部の材質均一性の優位性は、鋼管に曲げ、液圧、拡管、縮管などの成形加工を加えた後でも保たれる。なお、フェライト組織中の析出物の大きさは、鋼管から切出し、研磨した薄膜の４０万倍の透過型電子顕微鏡写真より計測し、その平均粒径を求めることによって把握した。
【００１６】
なお、本発明におけるフェライト組織の組織面分率とは、硬質組織、相である、パーライト組織、ベイナイト組織、マルテンサイト組織、残留オーステナイト相を除いた面分率のことで、ポリゴナルフェライト組織、擬ポリゴナルフェライト組織、アシキュラーフェライト組織を含み、その形態は問わない。
【００１７】
次に、組成について説明する。
本発明の溶接鋼管の組成は、重量％で、Ｃ：０．０３５〜０．０９５％、Ｍｎ：０．７５〜１．９５％、Ｍｏ：０．０１〜０．６５％、Ｔｉ：０．０１０〜０．１４５％、Ｐ：０．０３％以下、Ｓ：０．００７％以下、Ｎ：０．００６％以下、Ｏ：０．００４％以下であり、０．１５≦｛Ｔｉ−（４８／１４）Ｎ｝／Ｍｏ≦１を満たし、さらに、Ｓｉ：０．００５〜０．７５％、Ａｌ：０．０１０〜０．１０％、Ｃｒ：０．０１〜０．２９％、Ｎｂ：０．００１〜０．０４０％、Ｖ：０．００１〜０．０５０％、Ｗ：０．００１〜０．５０％、Ｎｉ：０．０１〜０．５０％、Ｃｕ：０．０１〜０．５０％、Ｂ：０．０００１〜０．０００９％、Ｃａ：０．０００１〜０．００４０％、ＲＥＭ：０．０００１〜０．００４０％のうちの１種以上を含有し、残部Ｆｅおよび不可避的不純物からなる。
【００１８】
これらの限定理由は以下の通りである。
Ｃ： Ｃは所望の強度、加工性、溶接部の材質均一性を得るために必要なフェライト組織中の微細炭化物を構成する必須元素である。しかし、その量が０．０３５％未満であると強度確保に必要な微細炭化物が得られず強度不足となり、一方、０．０９５％を超えると炭化物の析出挙動が変化し、加工性と溶接部靱性が低下する。したがって、Ｃ含有量を０．０３５〜０．０９５％とする。
【００１９】
Ｍｎ： Ｍｎは微細炭化物の成長速度を抑制し、平均粒径が１０ｎｍ未満の微細炭化物を形成させるための必須元素である。しかし、その量が０．７５％未満では平均粒径が１０ｎｍ未満の微細炭化物が十分に形成されないため所望の強度、加工性、溶接部の材質均一性が得られず、一方、１．９５％を超えるとフェライト組織の面分率が６０％未満となり所望の加工性が得られない。したがって、Ｍｎ含有量を０．７５〜１．９５％とする。
【００２０】
Ｍｏ： Ｍｏは所望の強度、加工性、溶接部の材質均一性を得るために必要なフェライト組織中の微細炭化物を生成させる必須元素である。しかし、その量が０．０１％未満であると、強度、加工性確保に必要な量の微細炭化物が得られず、強度、加工性不足となり、一方、０．６５％を超えると溶接部の材質均一性が低下する。したがって、Ｍｏ含有量を０．０１〜０．６５％とする。
【００２１】
Ｔｉ： ＴｉはＭｏと同様、所望の強度、加工性、溶接部の材質均一性を得るために必要なフェライト組織中の微細炭化物を生成させる必須元素である。しかし、その量が０．０１０％未満であると、強度、加工性確保に必要な量の微細炭化物が得られず、強度、加工性不足となり、一方、０．１４５％を超えると溶接部の材質均一性が低下する。したがって、Ｔｉ含有量を０．０１０〜０．１４５％とする。
【００２２】
｛Ｔｉ−（４８／１４）Ｎ｝／Ｍｏ： フェライト組織中の微細炭化物のサイズを１０ｎｍ未満の微細なものとするためには、Ｔｉ、Ｍｏ、Ｎの重量％で表される｛Ｔｉ−（４８／１４）Ｎ｝／Ｍｏの値が０．１５〜１の範囲内であることが必要である。これは、Ｔｉ原子とＭｏ原子が相互作用することにより、炭化物の粗大化が抑止されているためであると考えられる。｛Ｔｉ−（４８／１４）Ｎ｝／Ｍｏの値が０．１５未満であると、析出する炭化物サイズが大きくなり、強度加工性が低下し、一方、その値が１を超えると炭化物の析出挙動が変化し、加工性と溶接部の材質均一性とが低下する。
【００２３】
Ｐ，Ｓ，Ｎ，Ｏ： これらはいずれも加工性、溶接部の材質均一性を低下させる不純物元素であり、Ｐは０．０３％、Ｓは０．００７％、Ｎは０．００６％、Ｏは０．００４％を超えるとその悪影響が顕在化するため、その値をそれぞれの上限とする。
【００２４】
Ｓｉ： Ｓｉはフェライト組織の生成を促進し、所望のフェライト組織面分率を得るために添加することができる。その量が０．００５％未満ではその効果に乏しく、一方、０．７５％を超えると溶接部の材質均一性が低下するため、Ｓｉを添加する場合には、その含有量を０．００５〜０．７５％とする。
【００２５】
Ａｌ： Ａｌは製鋼時の脱酸元素であるとともに、適量の添加により溶接部の材質均一性向上に寄与するため、添加することができる。その量が０．０１０％未満ではその効果に乏しく、一方、０．１０％を超えると溶接部の材質均一性が劣化するため、Ａｌを添加する場合には、その含有量を０．０１０〜０．１０％とする。
【００２６】
Ｃｒ： ＣｒはＭｎの微細炭化物の成長速度抑制作用を補う働きがあるため、添加することができる。その量が０．０１％未満ではその効果に乏しく、一方、０．２９％を超えると溶接部の材質均一性が劣化するため、Ｃｒを添加する場合には、その含有量を０．０１〜０．２９％とする。
【００２７】
Ｎｂ，Ｖ，Ｗ： これらの元素は炭化物を形成することで強度を補完する有効な元素であるため添加することができる。いずれも０．００１％未満ではその効果に乏しく、Ｎｂは０．０４０％、Ｖは０．０５０％、Ｗは０．５０％を超えると加工性の劣化が著しくなる。したがって、これらを添加する場合には、Ｎｂ：０．００１〜０．０４０％、Ｖ：０．００１〜０．０５０％、Ｗ：０．００１〜０．５０％とする。
【００２８】
Ｎｉ，Ｃｕ： これらはＭｎの微細炭化物の成長速度抑制作用を補う働きがあるため、添加することができる。その量が０．０１％未満ではその効果に乏しく、一方、０．５０％を超えると加工性が劣化するため、Ｎｉ，Ｃｕを添加する場合には、その含有量をそれぞれ０．０１〜０．５０％とする。
【００２９】
Ｂ： Ｂは鋼管に焼き入れ性を付与するために添加することができる。その量が０．０００１％未満ではその効果に乏しく、０．０００９％を超えるとその効果が飽和するため、Ｂを添加する場合には、その含有量を０．０００１〜０．０００９％とする。
【００３０】
Ｃａ，ＲＥＭ： Ｃａ、ＲＥＭは硫化物の形態制御により加工性を一層高める働きがあるため添加することができる。いずれも０．０００１％未満ではその効果に乏しく、０．００４０％を超えてもその効果が飽和するので、これらを添加する場合には、それぞれ０．０００１〜０．００４０％とする。
【００３１】
次に、溶接鋼管の製造条件について説明する。
本発明では、上記組成の鋼スラブを１１５０℃以上に加熱した後、仕上げ圧延温度を８５０℃以上とする熱間圧延を施し、仕上げ圧延後８秒間以内に７２０℃以下まで冷却し、６００℃超〜６５０℃未満で巻取って熱延鋼帯とし、酸洗、スリット後造管する。
【００３２】
以下、これらの限定理由について説明する。
スラブ再加熱温度： 冷却された鋼スラブを再加熱後圧延する場合には、鋼中の析出物の多くを再固溶させ、微細炭化物をフェライト組織中に析出させるために、鋼スラブの再加熱温度を１１５０℃以上とする必要がある。
【００３３】
仕上げ圧延温度： 加工誘起析出による粗大な炭化物の析出を抑制するためには熱延仕上げ温度を８５０℃以上とする必要がある。
【００３４】
熱延ランナウト冷却条件： 炭化物の析出状態を制御し、平均粒径１０ｎｍ未満に微細析出させるためには熱延ランナウト冷却条件の制御が重要である。炭化物の成長を抑制し所望の微細析出物を得るには熱延仕上げ圧延終了後、８秒間以内に７２０℃まで冷却する必要がある。
【００３５】
巻取温度： 平均粒径が１０ｎｍ未満の微細炭化物が析出したフェライト組織を組織面分率で６０〜１００％とするには、熱延巻取温度を６００℃超〜６５０℃未満とする必要がある。６５０℃以上であると炭化物が成長するため強度が低下し、一方６００℃以下であるとフェライト組織分率が低下する。
【００３６】
鋼帯から溶接管への造管方法は特に限定されないが、ロールフォーミング、電縫溶接、サイザー等による形状矯正という手順で電縫溶接管とする場合には、加工性と靱性の確保のために、以下の式で定義される幅絞りを０．３〜１０％の範囲とすることが望ましい。
本発明の溶接鋼管には溶接部の良好な加工性、靱性の安全確保の観点から、さらにポストアニーリング、酸素濃度の雰囲気制御下でのシーム溶接等を行うことができる。また、本発明規定のミクロ組織、析出物状態を失しない範囲でシーム溶接前後工程での冷間加工、温間加工、熱間加工、熱処理、メッキ処理、表面潤滑処理を加えることができる。
【００３８】
【実施例】
（実施例１）
表１に示すＡ〜Ｔの２０種類の鋼スラブを約１２５０℃に再加熱後、仕上圧延温度約８９０℃、ランナウトでの７２０℃までの冷却時間約４秒、巻取温度約６２５℃の条件で板厚２．０ｍｍの熱延鋼帯とし、酸洗、スリッティング、ロール成形した後、溶接し、外径７０ｍｍの溶接鋼管とした。幅絞りは約４％とした。
【００３９】
これら鋼管のミクロ組織を観察し、析出物の平均粒径および組成を求めた。その結果を表２に示す。ミクロ組織は断面をナイタールエッチング後に走査型電子顕微鏡観察により評価し、析出物の平均粒径は、薄膜の透過型電子顕微鏡観察により評価した。
【００４０】
また、これら鋼管からＪＩＳ１１号試験片を切り出して引張試験を行い引張強度を求めるとともに、これら鋼管の曲げ加工特性、液圧加工特性、溶接部の材質均一性を求めた。その結果を表３に示す。
【００４１】
曲げ加工性は、プレッシャーダイと心金とを併用した回転引曲げによる限界曲げ半径（管中心軸の曲げ半径）ρ（ｍｍ）と管外径ｄ（ｍｍ）との比ρ／ｄで鋼管の強度ＴＳ（ＭＰａ）を割った値ＴＳ／（ρ／ｄ）（ＭＰａ）により評価した。値が大きいほど曲げ加工性は良好となる。さらに、複合加工特性を評価する目的で外径縮径率１０％の縮径加工後の限界曲げ半径ρ’より、このときの限界曲げ半径ρ’（ｍｍ）と管外径ｄ’（ｍｍ）との比ρ’／ｄ’の値も求めた。
【００４２】
液圧加工特性は液圧自由バルジ試験時の破断限界周長増加率により、変形部長さを２ｄ（ｄ：管外形）とし、軸圧縮「なし」および「あり」の２条件で評価した。軸圧縮力は、管体の応力比（軸方向応力／円周方向応力）＝Ｗ／（２πｒ２Ｐ）＝−０．５となる条件とした。ただし、Ｗ：圧縮応力、ｒ：肉厚中心半径、Ｐ：内圧である。
【００４３】
溶接部の材質均一性は、鋼管長さ２０ｍ毎にｎ＝３０〜１００でサンプリングした鋼管を拡管試験、扁平試験、シャルピー衝撃試験に供し、得られた限界拡管率、限界扁平率、シャルピー吸収エネルギーから平均値および標準偏差σをそれぞれ求め、（平均値−２σ）の値を材質均一性の指標とした。なお、限界拡管率は、６０°円錐を管端に押込み、亀裂の入った時の外径ｄｂ（ｍｍ）と管の初期外形ｄ（ｍｍ）との比ｄｂ／ｄとして求め、限界扁平率は、シームを圧縮方向と９０°となるように扁平した際に亀裂の入った時の高さｈ（ｍｍ）と初期外径ｄ（ｍｍ）とより（ｄ−ｈ）／ｄとして求め、シャルピー吸収エネルギーは、溶接突合せ部より円周方向に切出して展開したＪＩＳ Ｚ２２０２規定のＶノッチ試験片の−２０℃での吸収エネルギーから求めた。
【００４４】
表１のＮｏ．１〜１１は、成分組成が本発明の範囲内であるとともに、平均粒径１０ｎｍ未満の微細炭化物が析出したフェライト組織が組織面分率で６０〜１００％である本発明例であり、引張強度ＴＳが５９０〜１１８０ＭＰａの範囲で、ＴＳ／（ρ／ｄ）が４００ＭＰａ以上、縮径後の限界曲げ半径ρ’／ｄ’が２．８以下の優れた曲げ加工特性、軸圧縮なしでの周長増加率９％以上、軸圧縮ありで周長増加率１７％以上の優れた液圧加工特性を示し、限界拡管率の（平均値−２σ）の値が１．２５以上、限界扁平率の（平均値−２σ）の値が０．７５以上、−２０℃でのシャルピー吸収エネルギーの（平均値−２σ）の値が９０Ｊ／ｃｍ２以上の良好な溶接部の材質均一性を示した。
【００４５】
一方、本発明範囲からＣ、Ｍｎが低く外れた鋼Ｌ、ＮのＮｏ．１２，１４は、微細な析出物の量が不十分であり、また、Ｔｉ、Ｍｏが低く外れた鋼Ｑ、ＳのＮｏ．１７，１９は、炭化物の平均粒径が１０ｎｍ以上であり、いずれも強度が５９０ＭＰａ未満でかつＴＳ／（ρ／ｄ）が４００ＭＰａ未満と曲げ加工特性が低く、限界拡管率の（平均値−２σ）の値が１．２５未満、限界扁平率の（平均値−２σ）の値が０．７５未満、−２０℃でのシャルピー吸収エネルギーの（平均値−２σ）の値が９０Ｊ／ｃｍ２未満と溶接部の材質均一性も低かった。本発明の範囲からＣ、Ｍｎ、Ｏ、Ｔｉ、Ｍｏが高く外れた鋼Ｍ、Ｏ、Ｐ、Ｒ、ＴのＮｏ．１３，１５，１６，１８，２０は、ＴＳ／（ρ／ｄ）が４００ＭＰａ未満と曲げ加工特性が低く、軸圧縮なしでの周長増加率８％以下、軸圧縮ありで周長増加率１４％以下と液圧加工特性が低く、限界扁平率の（平均値−２σ）の値が０．７５未満、−２０℃でのシャルピー吸収エネルギーの（平均値−２σ）の値が９０Ｊ／ｃｍ２未満と溶接部の材質均一性も低かった。
【００４６】
【表１】
【表２】
【表３】
次に、鋼成分組成が本発明の範囲内にある鋼Ａ、鋼Ｇを表４に示す条件で熱間圧延して板厚２．０ｍｍの熱延鋼帯とし、酸洗、スリッティング、ロール成形した後、溶接し、外径７０ｍｍの溶接鋼管とした。幅絞りは約４％とした。得られた鋼管のミクロ組織と析出物の平均粒径および組成を表５に、引張強度、曲げ加工特性、液圧加工特性、溶接部の材質均一性を表６にそれぞれ示す。
【００５０】
熱延条件が本発明範囲内にあるＮｏ．２１，２６は、平均粒径１０ｎｍ未満の微細炭化物が析出したフェライト組織が組織面分率で６０〜１００％であり、引張強度ＴＳが５９０〜１１８０ＭＰａの範囲で、ＴＳ／（ρ／ｄ）が４００ＭＰａ以上の優れた曲げ加工特性、軸圧縮なしでの周長増加率９％以上、軸圧縮ありで周長増加率１７％以上の優れた液圧加工特性を示し、限界拡管率の（平均値−２σ）の値が１．２５以上、限界扁平率の（平均値−２σ）の値が０．７５以上、−２０℃でのシャルピー吸収エネルギーの（平均値−２σ）の値が９０Ｊ／ｃｍ２以上の良好な溶接部の材質均一性を示した。
【００５１】
一方、スラブ加熱温度が本発明範囲から外れたＮｏ．２７では、フェライト中の析出物粒径が２０ｎｍ以上と大きく、引張強度ＴＳが５９０ＭＰａ未満で、かつＴＳ／（ρ／ｄ）が４００ＭＰａ未満と曲げ加工特性が低かった。仕上圧延温度、仕上圧延後７２０℃までの冷却時間、巻取温度のいずれかが本発明の範囲から外れたＮｏ．２２，２３，２４，２５，２８，２９，３０，３１では、いずれもフェライト中の析出物粒径が２０ｎｍ以上と大きく、ＴＳ／（ρ／ｄ）が４００ＭＰａ未満と曲げ加工特性が低く、液圧加工特性、溶接部の材質均一性も低かった。
【００５２】
【表４】
【表５】
【表６】
本発明の範囲内の溶接鋼管は、型内での液圧加工特性にも優れ、曲げ、液圧、拡管、縮管などを複合した成形においても優れた加工性を示す。
【００５６】
【発明の効果】
以上説明したように、本発明によれば、引張強さ５９０〜１１８０ＭＰａの強度を有し、曲げ、液圧、拡管、縮管、およびこれらを複合した成形等に必要な加工性と、優れた溶接部の材質均一性とを兼備した高張力溶接鋼管を得ることができる。本発明の高張力溶接鋼管は、サスペンションアーム、サスペンションメンバー、アクスルビーム、スタビライザー、フレーム、シャフト等の閉断面自動車構造部材素材として必要な強度、加工性、溶接部の材質均一性を満たしており、これらの素材として極めて有効である。
【図面の簡単な説明】
【図１】鋼管のフェライト分率、フェライト組織中の析出物の大きさと溶接部の材質均一性の関係を示すグラフ。[0001]
BACKGROUND OF THE INVENTION
The present invention has a tensile strength of 590 to 1180 MPa, and has the workability necessary for bending, hydraulic pressure, pipe expansion, contraction, and molding that combines these, and excellent material uniformity of the weld. In addition, the present invention relates to a high-tensile welded steel pipe suitable for structural members such as automobiles and motorcycles, a manufacturing method thereof, and a steel strip for a material thereof.
[0002]
[Prior art]
From the viewpoint of reducing the weight and rigidity of the vehicle body, the application of high-tensile welded steel pipes to automotive structural members such as suspension arms, suspension members, axle beams, stabilizers, frames, and shafts is being investigated. There is a strong demand for tensile steel pipes. So far, various techniques relating to high-tensile welded steel pipes applied to such automobile structural members have been proposed.
[0003]
Japanese Patent Application Laid-Open No. 11-279597 discloses that a steel slab mainly composed of C—Si—Mn—Cr is rolled up at 250 ° C. or less after hot rolling, and has a composite structure composed of ferrite, the remaining martensite and bainite. A technique relating to a characteristic ERW steel pipe is disclosed, and in Japanese Patent Application Laid-Open No. 11-279699, a steel slab containing C-Si-Mn as a main component is wound at 600 ° C. or less after hot rolling, A technique related to an ERW steel pipe characterized by having a composite structure containing 5 to 10% metastable austenite that has been pickled, cold-rolled, and continuously annealed is disclosed. According to these techniques, relatively good elongation (El) is obtained when the tensile strength (TS) is in the range of 550 to 780 MPa, but about 1% Si is added to ensure ductility. There is also a problem in the material uniformity of the ERW weld.
[0004]
In JP-A-5-302121, a steel slab mainly composed of C-Si-Mn-Nb-Mo is hot-rolled, cold-rolled, and annealed after being welded, so that TS is 810 to 920 MPa. A technique for obtaining a mechanically welded steel pipe is disclosed. In this technique, there is a problem in the material uniformity of the ERW weld due to strain aging due to heat treatment after pipe forming.
[0005]
Japanese Patent No. 3235168 discloses a technique characterized in that a steel slab mainly composed of C—Si—Mn—Nb—a trace amount of Ti is wound up at 600 to 200 ° C. after hot rolling and electro-welded. JP-A-5-271859 is characterized in that a steel slab mainly composed of C—Si—Mn—Nb—a trace amount of Ti—B is wound by electro-welding at 600 to 200 ° C. after hot rolling. Japanese Patent Laid-Open No. 5-43980 discloses a technique characterized by electro-welding a hot-rolled steel strip mainly composed of C—Si—Mn—Nb—a trace amount of Ti—Mo. ing. According to these techniques, although relatively good ERW weld toughness can be obtained when TS is in the range of 680 to 940 MPa, there is a problem in its workability.
[0006]
[Problems to be solved by the invention]
As described above, in the conventional technique, a high-tensile welded steel pipe having both workability required for a structural member such as an automobile and workability of a welded part has not been obtained. In particular, in automobile structural members using high-tensile steel pipes, the material uniformity of the welded portion is regarded as important from the viewpoint of component reliability.
[0007]
The present invention has been made in view of such circumstances, has a tensile strength of 590 to 1180 MPa, and has the workability necessary for bending, hydraulic pressure, pipe expansion, contraction, and molding that combines these. Another object of the present invention is to provide a high-strength welded steel pipe suitable for structural members such as automobiles and motorcycles, a method for manufacturing the same, and a steel strip for a welded steel pipe material, which combines excellent material uniformity of the welded portion.
[0008]
[Means for Solving the Problems]
In order to obtain a welded steel pipe that simultaneously satisfies conflicting properties such as strength, workability, and toughness, the present inventors have made various systematic experimental studies by changing the chemical composition, microstructure, and precipitate existing state of the welded steel pipe in various ways. went. As a result, ultrafine carbide with an average particle size of less than 10 nm is precipitated in a ferrite structure occupying a surface area ratio of 60% or more, thereby simultaneously satisfying desired strength, workability, and material uniformity of the welded portion. It has been found that a welded steel pipe is obtained.
[0009]
The present invention has been completed based on such findings and provides the following (1) to (4).
[0010]
(1) By weight, C: 0.035 to 0.095%, Mn: 0.75 to 1.95%, Mo: 0.01 to 0.65%, Ti: 0.010 to 0.145% , P: 0.03% or less, S: 0.007% or less, N: 0.006% or less, O: 0.004% or less, and Si: 0.005 to 0.75%, Al: 0.010 to 0.10%, Cr: 0.01 to 0.29%, Nb: 0.001 to 0.040%, V: 0.001 to 0.050%, W: 0.001 0.50%, Ni: 0.01 to 0.50%, Cu: 0.01 to 0.50%, B: 0.0001 to 0.0009%, Ca: 0.0001 to 0.0040%, REM : Containing one or more of 0.0001 to 0.0040%, the balance consisting of Fe and inevitable impurities, and the weight percent of Ti, Mo, N The ferrite structure satisfying the following formula (1) represented by formula (1) and having fine carbides with an average particle size of less than 10 nm deposited is 60 to 100% in terms of the structure area fraction and the weldability High-tensile welded steel pipe with excellent material uniformity.
0.15 ≦ {Ti− (48/14) N} / Mo ≦ 1 (1)
[0012]
( 2 ) In manufacturing the welded steel pipe described in (1) above, after heating the steel slab having the above composition to 1150 ° C. or higher, it is hot-rolled to a finish rolling temperature of 850 ° C. or higher, and finished after rolling 8 High-tensile welded steel pipe with excellent workability and toughness, which is cooled to 720 ° C. or less within a second, wound at a temperature above 600 ° C. to less than 650 ° C. to form a hot-rolled steel strip, and pipe forming after pickling and slitting Manufacturing method.
[0013]
( 3 ) Workability characterized in that the ferrite structure having the component composition as described in (1) above and having fine carbides with an average particle size of less than 10 nm precipitated is 60 to 100% in terms of the texture area. Steel strip for high-tensile welded steel pipe material with excellent material uniformity in the weld zone.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be specifically described.
The most important point of the welded steel pipe of the present invention is that the ferrite structure in which ultrafine carbides having an average particle size of less than 10 nm are precipitated has a structure area fraction of 60 to 100%. As a result, a welded steel pipe having the desired strength, workability, and material uniformity of the welded portion can be obtained. In this way, the strength can be increased without degrading the uniformity and workability of the welded portion, and the distance between the nearest particles of the precipitate necessary for increasing the strength is obtained by finely precipitating the carbide. One of the factors is considered to be a decrease in the amount of elements required for this purpose.
[0015]
FIG. 1 shows the relationship between the ferrite fraction of a steel pipe, the size of precipitates in the ferrite structure (average particle size), and the material uniformity of the weld. The material uniformity of the welded part is obtained by subjecting a steel pipe sampled at n = 30 to 100 every 20 m of the steel pipe to a pipe expansion test, a flattening test, and a Charpy impact test. Then, the average value and the standard deviation σ were respectively obtained, and the value of (average value−2σ) was used as an index of material uniformity. The value of (average value−2σ) is lower than the average value. Incidentally, the limit pipe expansion ratio determines the 60 ° cone pipe end pusher, as a ratio d b / d between the outer diameter d b (mm) and an initial outline d (mm) of the tube when cracked, limits flat The rate is obtained as (d−h) / d from the height h (mm) and the initial outer diameter d (mm) when the seam is flattened so as to be 90 ° with the compression direction, and the initial outer diameter d (mm). The Charpy absorbed energy was determined from the absorbed energy at −20 ° C. of a V-notch test piece defined in JIS Z2202 that was cut out in the circumferential direction from the weld butt and developed. The numbers in the lower circle plot in FIG. 1 indicate the (average value −2σ) value of the limit tube expansion rate d b / d, and the numbers in the middle square plot in FIG. 1 indicate the limit flatness ratio (d−h) / 1 shows the value of (average value −2σ), and the numbers in the upper triangular plot of FIG. 1 indicate the value of (average value −2σ) of the Charpy absorbed energy at −20 ° C. In the middle plot of FIG. 1, the black-colored plots are those that did not crack even when they are in close contact. From FIG. 1, the value of (average value−2σ) of the critical tube expansion ratio is 1.25 or more by precipitating ultrafine carbides having an average particle diameter of less than 10 nm in a ferrite structure occupying a surface area ratio of 60% or more. The material of a good welded portion having a (average value-2σ) value of 0.75 or more of the critical flatness and a (average value-2σ) value of Charpy absorbed energy at −20 ° C. of 90 J / cm 2 or more. It can be seen that sex is obtained. And by making the structure area fraction of a
[0016]
Incidentally, the structure area fraction of the ferrite structure in the present invention is a hard structure, a phase, a pearlite structure, a bainite structure, a martensite structure, an area fraction excluding the retained austenite phase, a polygonal ferrite structure, A pseudo-polygonal ferrite structure and an acicular ferrite structure are included, and the form is not limited.
[0017]
Next, the composition will be described.
The composition of the welded steel pipe of the present invention is, by weight, C: 0.035 to 0.095%, Mn: 0.75 to 1.95%, Mo: 0.01 to 0.65%, Ti: 0.00. 0.10 to 0.145%, P: 0.03% or less, S: 0.007% or less, N: 0.006% or less, O: 0.004% or less, and 0.15 ≦ {Ti− (48 / 14) N} / Mo ≦ 1, and Si: 0.005 to 0.75%, Al: 0.010 to 0.10%, Cr: 0.01 to 0.29%, Nb: 0 0.001 to 0.040%, V: 0.001 to 0.050%, W: 0.001 to 0.50%, Ni: 0.01 to 0.50%, Cu: 0.01 to 0.50 %, B: 0.0001 to 0.0009%, Ca: 0.0001 to 0.0040%, REM: 0.0001 to 0.0040% Contained, the balance being Fe and unavoidable impurities.
[0018]
The reasons for these limitations are as follows.
C: C is an essential element constituting fine carbides in the ferrite structure necessary for obtaining desired strength, workability, and material uniformity of the welded portion. However, if the amount is less than 0.035%, the fine carbide necessary for securing the strength cannot be obtained, and the strength is insufficient. On the other hand, if the amount exceeds 0.095%, the precipitation behavior of the carbide changes, and the workability and the weld zone are reduced. Toughness decreases. Therefore, the C content is 0.035 to 0.095%.
[0019]
Mn: Mn is an essential element for suppressing the growth rate of fine carbides and forming fine carbides having an average particle size of less than 10 nm. However, if the amount is less than 0.75%, fine carbides having an average particle size of less than 10 nm are not sufficiently formed, so that desired strength, workability, and material uniformity of the welded portion cannot be obtained, while 1.95% If it exceeds 1, the surface fraction of the ferrite structure becomes less than 60%, and the desired workability cannot be obtained. Therefore, the Mn content is set to 0.75 to 1.95%.
[0020]
Mo: Mo is an essential element that generates fine carbides in a ferrite structure necessary for obtaining desired strength, workability, and material uniformity of a welded portion. However, if the amount is less than 0.01%, the amount of fine carbide necessary for ensuring strength and workability cannot be obtained, and the strength and workability are insufficient. On the other hand, if the amount exceeds 0.65%, Material uniformity decreases. Therefore, the Mo content is set to 0.01 to 0.65%.
[0021]
Ti: Like Mo, Ti is an essential element that generates fine carbides in a ferrite structure necessary for obtaining desired strength, workability, and material uniformity of a welded portion. However, if the amount is less than 0.010%, the amount of fine carbide necessary for securing strength and workability cannot be obtained, resulting in insufficient strength and workability. On the other hand, if it exceeds 0.145%, Material uniformity decreases. Therefore, the Ti content is set to 0.010 to 0.145%.
[0022]
{Ti- (48/14) N} / Mo: In order to make the size of the fine carbide in the ferrite structure as fine as less than 10 nm, {Ti- ( 48/14) The value of N} / Mo needs to be in the range of 0.15 to 1. This is considered to be because the coarsening of the carbide is suppressed by the interaction between Ti atoms and Mo atoms. If the value of {Ti- (48/14) N} / Mo is less than 0.15, the carbide size to be precipitated increases, and the strength workability decreases. On the other hand, if the value exceeds 1, the precipitation of carbides. The behavior changes, and the workability and the material uniformity of the weld are reduced.
[0023]
P, S, N, O: These are all impurity elements that reduce workability and material uniformity of the welded portion, P is 0.03%, S is 0.007%, N is 0.006%, If O exceeds 0.004%, the adverse effect becomes obvious, so the value is made the upper limit of each.
[0024]
Si: Si can be added to promote the formation of a ferrite structure and to obtain a desired ferrite structure surface fraction. If the amount is less than 0.005%, the effect is poor. On the other hand, if it exceeds 0.75%, the material uniformity of the welded portion is lowered. 0.75%.
[0025]
Al: Al is a deoxidizing element at the time of steel making, and can be added because it contributes to improving the material uniformity of the welded portion by adding an appropriate amount. If the amount is less than 0.010%, the effect is poor. On the other hand, if it exceeds 0.10%, the material uniformity of the welded portion deteriorates. 0.10%.
[0026]
Cr: Cr can be added because it has a function to supplement the growth rate suppressing effect of the fine carbide of Mn. If the amount is less than 0.01%, the effect is poor. On the other hand, if it exceeds 0.29%, the material uniformity of the welded portion deteriorates. Therefore, when Cr is added, the content is 0.01 to 0.29%.
[0027]
Nb, V, W: These elements can be added because they are effective elements that complement the strength by forming carbides. In any case, if less than 0.001%, the effect is poor, and if Nb is 0.040%, V is 0.050%, and W exceeds 0.50%, the workability deteriorates remarkably. Therefore, when adding these, it is set as Nb: 0.001-0.040%, V: 0.001-0.050%, W: 0.001-0.50%.
[0028]
Ni, Cu: Since these have a function to supplement the growth rate suppressing action of the fine carbide of Mn, they can be added. If the amount is less than 0.01%, the effect is poor. On the other hand, if it exceeds 0.50%, the workability deteriorates. Therefore, when adding Ni or Cu, the content is 0.01 to 0 respectively. .50%.
[0029]
B: B can be added to impart hardenability to the steel pipe. If the amount is less than 0.0001%, the effect is poor. If the amount exceeds 0.0009%, the effect is saturated. Therefore, when B is added, the content is 0.0001 to 0.0009%. .
[0030]
Ca, REM: Ca and REM can be added because they have a function of further improving the workability by controlling the form of sulfide. In any case, if less than 0.0001%, the effect is poor, and even if it exceeds 0.0040%, the effect is saturated. Therefore, when these are added, the content is made 0.0001 to 0.0040%, respectively.
[0031]
Next, manufacturing conditions for the welded steel pipe will be described.
In the present invention, after the steel slab having the above composition is heated to 1150 ° C. or higher, it is hot-rolled to a finish rolling temperature of 850 ° C. or higher, cooled to 720 ° C. or lower within 8 seconds after finish rolling, and exceeds 600 ° C. It is wound at a temperature below ˜650 ° C. to form a hot-rolled steel strip, and is piped after pickling and slitting.
[0032]
Hereinafter, these reasons for limitation will be described.
Slab reheating temperature: When the cooled steel slab is rolled after reheating, the steel slab is reheated to re-dissolve most of the precipitates in the steel and precipitate fine carbides in the ferrite structure. The temperature needs to be 1150 ° C. or higher.
[0033]
Finishing rolling temperature: In order to suppress the precipitation of coarse carbides due to processing-induced precipitation, the hot rolling finishing temperature needs to be 850 ° C or higher.
[0034]
Hot-rolled runout cooling conditions: Controlling the hot-rolled runout cooling conditions is important in order to control the precipitation state of carbides and finely precipitate the particles with an average particle size of less than 10 nm. In order to suppress the growth of carbide and obtain a desired fine precipitate, it is necessary to cool to 720 ° C. within 8 seconds after completion of hot rolling finish rolling.
[0035]
Winding temperature: In order to make the ferrite structure in which fine carbides having an average particle size of less than 10 nm are precipitated in a structure area fraction of 60 to 100%, the hot rolling coiling temperature needs to be more than 600 ° C. to less than 650 ° C. is there. If the temperature is 650 ° C. or higher, carbides grow and the strength is reduced. On the other hand, if the temperature is 600 ° C. or lower, the ferrite structure fraction is reduced.
[0036]
The method of forming the steel strip from the welded pipe is not particularly limited, but in order to ensure workability and toughness when making an ERW welded pipe by the procedure of roll forming, ERW welding, shape correction by sizer, etc. It is desirable that the width stop defined by the following formula is in the range of 0.3 to 10%.
The welded steel pipe of the present invention can be further subjected to post-annealing, seam welding under controlled atmosphere of oxygen concentration, and the like from the viewpoint of ensuring good workability of the welded portion and safety of toughness. In addition, cold working, warm working, hot working, heat treatment, plating treatment, and surface lubrication treatment can be added in the process before and after seam welding within the range not losing the microstructure and precipitate state defined in the present invention.
[0038]
【Example】
Example 1
20 types of steel slabs A to T shown in Table 1 were reheated to about 1250 ° C., then the finish rolling temperature was about 890 ° C., the cooling time to 720 ° C. was about 4 seconds, and the coiling temperature was about 625 ° C. Thus, a hot-rolled steel strip having a thickness of 2.0 mm was formed, pickled, slitted, and roll-formed, and then welded to obtain a welded steel pipe having an outer diameter of 70 mm. The width diaphragm was about 4%.
[0039]
The microstructure of these steel pipes was observed, and the average particle size and composition of the precipitates were determined. The results are shown in Table 2. The microstructure was evaluated by observation with a scanning electron microscope after the nital etching, and the average particle size of the precipitate was evaluated by observation with a transmission electron microscope of the thin film.
[0040]
In addition, JIS No. 11 test pieces were cut out from these steel pipes and subjected to a tensile test to determine the tensile strength, as well as the bending characteristics, hydraulic processing characteristics, and material uniformity of the welds of these steel pipes. The results are shown in Table 3.
[0041]
Bending workability is the ratio ρ / d between the limit bending radius (bending radius of the pipe center axis) ρ (mm) and the pipe outer diameter d (mm) by rotary pulling using both a pressure die and a mandrel. Evaluation was made by the value TS / (ρ / d) (MPa) obtained by dividing the strength TS (MPa). The larger the value, the better the bending workability. Further, for the purpose of evaluating the composite machining characteristics, the limit bending radius ρ ′ (mm) and the pipe outer diameter d ′ (mm) at this time are obtained from the limit bending radius ρ ′ after the diameter reduction processing with the outer diameter reduction ratio of 10%. The ratio ρ ′ / d ′ was also determined.
[0042]
The hydraulic processing characteristics were evaluated under the two conditions of axial compression “none” and “with”, with the deformed portion length being 2d (d: pipe outer shape) based on the rate of increase in fracture limit circumference during the hydraulic free bulge test. The axial compression force was a condition that the stress ratio of the tube (axial stress / circumferential stress) = W / (2πr 2 P) = − 0.5. However, W: compression stress, r: thickness center radius, P: internal pressure.
[0043]
The material uniformity of the welded part is obtained by subjecting a steel pipe sampled at n = 30 to 100 every 20 m of the steel pipe to a pipe expansion test, a flattening test, and a Charpy impact test. Then, the average value and the standard deviation σ were respectively obtained, and the value of (average value−2σ) was used as an index of material uniformity. Incidentally, the limit pipe expansion ratio determines the 60 ° cone pipe end pusher, as a ratio d b / d between the outer diameter d b (mm) and an initial outline d (mm) of the tube when cracked, limits flat The rate is obtained as (d−h) / d from the height h (mm) and the initial outer diameter d (mm) when the seam is flattened so as to be 90 ° with the compression direction, and the initial outer diameter d (mm). The Charpy absorbed energy was determined from the absorbed energy at −20 ° C. of a V-notch test piece defined in JIS Z2202 that was cut out in the circumferential direction from the weld butt and developed.
[0044]
No. in Table 1 1 to 11 are examples of the present invention in which the component composition is within the range of the present invention, and the ferrite structure in which fine carbides having an average particle size of less than 10 nm are precipitated is 60 to 100% in terms of the texture area. Excellent bending characteristics with a TS of 590 to 1180 MPa, TS / (ρ / d) of 400 MPa or more, and a limit bending radius ρ ′ / d ′ of 2.8 or less after the diameter reduction. Excellent hydraulic processing characteristics with a length increase rate of 9% or more, with axial compression and a peripheral length increase rate of 17% or more, a value of (average value -2σ) of the limit tube expansion rate of 1.25 or more, Good material uniformity of the welded part having a value of (average value-2σ) of 0.75 or more and a value of (average value-2σ) of Charpy absorbed energy at −20 ° C. of 90 J / cm 2 or more was shown.
[0045]
On the other hand, Nos. Of steels L and N in which C and Mn deviate low from the scope of the present invention. In Nos. 12 and 14, the amount of fine precipitates is insufficient, and Nos. Nos. 17 and 19 have an average particle size of carbide of 10 nm or more, both of which have a strength of less than 590 MPa and TS / (ρ / d) of less than 400 MPa, have low bending properties, and have an average tube expansion ratio of (average value −2σ ) Is less than 1.25, (average value-2σ) of the critical flatness is less than 0.75, and (average value-2σ) of Charpy absorbed energy at -20 ° C is less than 90 J / cm 2 The material uniformity of the weld was also low. Nos. Of steels M, O, P, R, and T in which C, Mn, O, Ti, and Mo are significantly out of the scope of the present invention. Nos. 13, 15, 16, 18, and 20 have a low bending work characteristic with TS / (ρ / d) of less than 400 MPa, a peripheral length increase rate of 8% or less without axial compression, and a peripheral length increase rate of 14 with axial compression. % Or less, the value of (average value-2σ) of the critical flatness is less than 0.75, and the value of (average value-2σ) of Charpy absorbed energy at -20 ° C is 90 J / cm 2. The material uniformity of the welded part was also low.
[0046]
[Table 1]
[Table 2]
[Table 3]
Next, steel A and steel G whose steel composition is within the scope of the present invention are hot-rolled under the conditions shown in Table 4 to form a hot-rolled steel strip having a thickness of 2.0 mm, pickling, slitting, roll After forming, it was welded to obtain a welded steel pipe having an outer diameter of 70 mm. The width diaphragm was about 4%. Table 5 shows the microstructure of the obtained steel pipe and the average particle size and composition of the precipitates, and Table 6 shows the tensile strength, bending work characteristics, hydraulic work characteristics, and material uniformity of the welded parts.
[0050]
No. in which the hot rolling conditions are within the scope of the present invention. In Nos. 21 and 26, the ferrite structure in which fine carbides having an average particle size of less than 10 nm are precipitated has a structure area fraction of 60 to 100%, the tensile strength TS is in the range of 590 to 1180 MPa, and TS / (ρ / d) is Excellent bending characteristics of 400MPa or higher, circumference increase rate of 9% or more without axial compression, excellent hydraulic machining characteristics of circumference increase rate of 17% or more with axial compression, -2σ) value is 1.25 or more, (average value -2σ) value of critical flatness is 0.75 or more, and (average value -2σ) value of Charpy absorbed energy at -20 ° C is 90 J / cm Two or more good weld uniformity was shown.
[0051]
On the other hand, the slab heating temperature deviated from the scope of the present invention. In No. 27, the precipitate grain size in the ferrite was as large as 20 nm or more, the tensile strength TS was less than 590 MPa, and TS / (ρ / d) was less than 400 MPa. Any one of the finish rolling temperature, the cooling time to 720 ° C. after the finish rolling, and the coiling temperature deviated from the scope of the present invention. In 22, 23, 24, 25, 28, 29, 30, and 31, the precipitate grain size in the ferrite is as large as 20 nm or more, TS / (ρ / d) is less than 400 MPa, and the bending property is low. The pressure processing characteristics and the material uniformity of the weld were also low.
[0052]
[Table 4]
[Table 5]
[Table 6]
The welded steel pipe within the scope of the present invention is also excellent in the hydraulic processing characteristics in the mold, and exhibits excellent workability even in molding in which bending, hydraulic pressure, pipe expansion, contraction pipe and the like are combined.
[0056]
【The invention's effect】
As described above, according to the present invention, it has a tensile strength of 590 to 1180 MPa, and has excellent workability necessary for bending, hydraulic pressure, pipe expansion, contraction, and molding that combines these. A high-tensile welded steel pipe that combines the material uniformity of the welded portion can be obtained. The high-tensile welded steel pipe of the present invention satisfies the strength, workability, and material uniformity of the welded portion necessary for a closed cross-section automotive structural material such as a suspension arm, suspension member, axle beam, stabilizer, frame, shaft, etc. It is extremely effective as these materials.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between the ferrite fraction of a steel pipe, the size of precipitates in the ferrite structure, and the material uniformity of the weld.
Claims (3)
Ｃ：０．０３５〜０．０９５％、
Ｍｎ：０．７５〜１．９５％、
Ｍｏ：０．０１〜０．６５％、
Ｔｉ：０．０１０〜０．１４５％、
Ｐ：０．０３％以下、
Ｓ：０．００７％以下、
Ｎ：０．００６％以下、
Ｏ：０．００４％以下、
を含有し、
さらに、
Ｓｉ：０．００５〜０．７５％、
Ａｌ：０．０１０〜０．１０％、
Ｃｒ：０．０１〜０．２９％、
Ｎｂ：０．００１〜０．０４０％、
Ｖ：０．００１〜０．０５０％、
Ｗ：０．００１〜０．５０％、
Ｎｉ：０．０１〜０．５０％、
Ｃｕ：０．０１〜０．５０％、
Ｂ：０．０００１〜０．０００９％、
Ｃａ：０．０００１〜０．００４０％、
ＲＥＭ：０．０００１〜０．００４０％
のうちの１種以上を含有し、
残部がＦｅおよび不可避的不純物からなり、
Ｔｉ、Ｍｏ、Ｎの重量％で表される以下の（１）式を満たし、
平均粒径が１０ｎｍ未満の微細炭化物が析出したフェライト組織が組織面分率で６０〜１００％であることを特徴とする加工性と溶接部の材質均一性に優れた高張力溶接鋼管。
０．１５≦｛Ｔｉ−（４８／１４）Ｎ｝／Ｍｏ≦１ ‥‥（１）% By weight
C: 0.035 to 0.095%,
Mn: 0.75 to 1.95%,
Mo: 0.01 to 0.65%,
Ti: 0.010 to 0.145%,
P: 0.03% or less,
S: 0.007% or less,
N: 0.006% or less,
O: 0.004% or less,
Containing
further,
Si: 0.005 to 0.75%,
Al: 0.010 to 0.10%,
Cr: 0.01 to 0.29%,
Nb: 0.001 to 0.040%,
V: 0.001 to 0.050%,
W: 0.001 to 0.50%,
Ni: 0.01 to 0.50%,
Cu: 0.01 to 0.50%,
B: 0.0001 to 0.0009%
Ca: 0.0001 to 0.0040%,
REM: 0.0001 to 0.0040%
Containing one or more of
The balance consists of Fe and inevitable impurities ,
Satisfying the following formula (1) expressed by weight% of Ti, Mo, N,
A high-strength welded steel pipe excellent in workability and material uniformity of a welded portion, characterized in that a ferrite structure in which fine carbides having an average particle size of less than 10 nm are precipitated has a structure surface fraction of 60 to 100%.
0.15 ≦ {Ti− (48/14) N} / Mo ≦ 1 (1)
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| JP2002126689A JP3925291B2 (en) | 2002-04-26 | 2002-04-26 | High-tensile welded steel pipe excellent in workability and material uniformity of welds, its manufacturing method, and steel strip for welded steel pipe material |
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| JP3925291B2 true JP3925291B2 (en) | 2007-06-06 |
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Cited By (1)
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| CN109642284A (en) * | 2016-10-03 | 2019-04-16 | 新日铁住金株式会社 | Torsion beam electric welded steel pipe |
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| JP4547951B2 (en) * | 2004-03-18 | 2010-09-22 | Ｊｆｅスチール株式会社 | Thick high-strength hot-rolled steel sheet excellent in workability and method for producing the same |
| JP4466619B2 (en) | 2006-07-05 | 2010-05-26 | Ｊｆｅスチール株式会社 | High tensile welded steel pipe for automobile structural members and method for manufacturing the same |
| JP5499559B2 (en) * | 2009-08-12 | 2014-05-21 | Ｊｆｅスチール株式会社 | High tensile steel material for automobile undercarriage members having excellent formability and torsional fatigue resistance and method for producing the same |
| CN105506503A (en) * | 2015-12-15 | 2016-04-20 | 苏州爱盟机械有限公司 | Front axle I beam for vehicle |
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Cited By (1)
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| CN109642284A (en) * | 2016-10-03 | 2019-04-16 | 新日铁住金株式会社 | Torsion beam electric welded steel pipe |
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