JP4007050B2 - High-tensile welded steel pipe excellent in workability and fatigue characteristics, its manufacturing method, and steel strip for welded steel pipe material - Google Patents
High-tensile welded steel pipe excellent in workability and fatigue characteristics, its manufacturing method, and steel strip for welded steel pipe material Download PDFInfo
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【０００１】
【発明の属する技術分野】
本発明は、引張強さ５９０ＭＰａ以上の強度を有し、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、優れた疲労特性とを兼備し、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、疲労特性に問題がある。
【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。しかし、この技術では疲労特性、特に成形加工後の疲労特性が十分に得られない問題がある。
【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、その加工性と疲労特性、特に加工後の疲労特性に問題がある。
【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疲労特性、特に部材成形加工後の疲労特性を兼備した高張力溶接鋼管が得られていない。
【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５９０ＭＰａ以上の強度を有し、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、優れた疲労特性とを兼備した、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、疲労特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、粒径１０ｎｍ以下で、原子比でＭｏ／（Ｔｉ＋Ｍｏ）＝０．３３〜０．７７である（Ｔｉ，Ｍｏ）複合炭化物を微細に析出させることで、所望の強度、加工性、疲労特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）〜（６）を提供する。
【００１０】
（１） 重量％で、Ｃ：０．０３５〜０．１８５％、Ｍｎ：０．７５〜１．９５％、Ｍｏ：０．０１〜０．４９％、Ｔｉ：０．０１０〜０．１４５％、Ａｌ：０．０１１〜０．１０％、Ｐ：０．０３％以下、Ｓ：０．００４％以下、Ｎ：０．００６％以下、Ｏ：０．００４％以下、残部がＦｅおよび不可避不純物からなり、粒径が１０ｎｍ以下で、原子比でＭｏ／（Ｔｉ＋Ｍｏ）＝０．３３〜０．７７である（Ｔｉ，Ｍｏ）複合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疲労特性に優れた高張力溶接鋼管。
【００１１】
（２） 上記（１）において、さらに、重量％で、Ｓｉ：０．００５〜１．５０％、Ｃｒ：０．０１〜０．２４％、Ｎｂ：０．００１〜０．０６０％、Ｖ：０．００１〜０．０５０％、Ｗ：０．００１〜０．５０％、Ｎｉ：０．０１〜０．５０％、Ｃｕ：０．０１〜０．２４％、Ｂ：０．０００１〜０．０００６％、Ｃａ：０．０００１〜０．００４０％、ＲＥＭ：０．０００１〜０．００４０％のうちの１種以上を含有することを特徴とする加工性と疲労特性に優れた高張力溶接鋼管。
【００１２】
（３） 上記（１）または（２）において、Ｔｉ、Ｍｏ、Ｎの重量％で表される以下の（１）式を満たすことを特徴とする加工性と疲労特性に優れた高張力溶接鋼管。
０．１５≦｛Ｔｉ−（４８／１４）Ｎ｝／Ｍｏ≦１ ‥‥（１）
【００１３】
（４） 上記（１）から（３）のいずれか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５秒間以内に７００℃以下まで冷却し、６００℃超〜６７５℃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疲労特性に優れた高張力溶接鋼管の製造方法。
【００１４】
（５） 上記（１）または（２）の成分組成を有し、粒径が１０ｎｍ以下で、原子比でＭｏ／（Ｔｉ＋Ｍｏ）＝０．３３〜０．７７である（Ｔｉ，Ｍｏ）複合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疲労特性に優れた高張力溶接鋼管素材用鋼帯。
【００１５】
（６） 上記（５）において、Ｔｉ、Ｍｏ、Ｎの重量％で表される以下の（１）式を満たすことを特徴とする加工性と疲労特性に優れた高張力溶接鋼管素材用鋼帯。
０．１５≦｛Ｔｉ−（４８／１４）Ｎ｝／Ｍｏ≦１ ‥‥（１）
【００１６】
【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、粒径１０ｎｍ以下で、原子比でＭｏ／（Ｔｉ＋Ｍｏ）＝０．３３〜０．７７である微細な（Ｔｉ，Ｍｏ）複合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、疲労特性を同時に有する溶接鋼管が得られる。このように疲労特性、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。
【００１７】
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と曲げ加工特性の関係を示す。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）と管外形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。なお、この時のプレッシャーダイ押し力は座屈、しわの発生しない最大応力とした。図中プロット内の数字がＴＳ／（ρ／ｄ）の値である。図１から、６０％以上の面分率を占めるフェライト組織中に粒径１０ｎｍ以下の極微細な（Ｔｉ，Ｍｏ）複合炭化物を析出させることで、ＴＳ／（ρ／ｄ）が４００ＭＰａ以上の優れた曲げ加工特性が得られることがわかる。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、析出物の組成は、透過型電子顕微鏡に装備されたエネルギー分散型分光装置により分析した。
【００１８】
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。
【００１９】
図２に鋼管のフェライト組織中の析出物の大きさ、析出物のＭｏ／（Ｔｉ＋Ｍｏ）（原子比）の値と鋼管の疲労特性との関係を示す。鋼管の疲労特性は、直管の４点曲げ疲労試験と、曲げ管の一端を固定し、もう一端を曲げ平面に垂直方向に変位させる曲げ捩り疲労試験により評価した。直管の４点曲げ疲労試験は片振り、周波数５Ｈｚの条件での１０６繰り返し疲れ限度σＡ（最大主応力振幅の２倍）と鋼管強度ＴＳとの比（σＡ／ＴＳ）で評価し、曲げ管の曲げ捩り疲労試験は、曲げ半径ρ／ｄ＝２．０、曲げ角度９０°で回転引き曲げした後、両振り、周波数１Ｈｚの条件での５×１０５繰り返し疲れ限度σＢ（最大主応力の応力振幅）と鋼管強度ＴＳとの比（σＢ／ＴＳ）でそれぞれ評価した。素管寸法はφ７０×２．０ｔ（ｍｍ）とした。図２の下段の丸プロット内の数字がσＡ／ＴＳの値であり、図２の上段の四角プロット内の数字がσＢ／ＴＳの値である。図２から、フェライト組織中に粒径が１０ｎｍ以下、原子比でＭｏ／（Ｔｉ＋Ｍｏ）＝０．３３〜０．７７である（Ｔｉ，Ｍｏ）複合炭化物を析出させることで、（σＡ／ＴＳ）≧０．６、（σＢ／ＴＳ）≧０．４５の優れた疲労特性が得られることがわかる。
【００２０】
次に、成分組成について説明する。
本発明の溶接鋼管の成分組成は、重量％で、Ｃ：０．０３５〜０．１８５％、Ｍｎ：０．７５〜１．９５％、Ｍｏ：０．０１〜０．４９％、Ｔｉ：０．０１０〜０．１４５％、Ａｌ：０．０１１〜０．１０％、Ｐ：０．０３％以下、Ｓ：０．００４％以下、Ｎ：０．００６％以下、Ｏ：０．００４％以下、残部がＦｅおよび不可避不純物からなる。さらに、Ｓｉ：０．００５〜１．５０％、Ｃｒ：０．０１〜０．２４％、Ｎｂ：０．００１〜０．０６０％、Ｖ：０．００１〜０．０５０％、Ｗ：０．００１〜０．５０％、Ｎｉ：０．０１〜０．５０％、Ｃｕ：０．０１〜０．２４％、Ｂ：０．０００１〜０．０００６％、Ｃａ：０．０００１〜０．００４０％、ＲＥＭ：０．０００１〜０．００４０％のうちの１種以上を含有することができる。また、０．１５≦｛Ｔｉ−（４８／１４）Ｎ｝／Ｍｏ≦１を満たすことが好ましい。
【００２１】
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、疲労特性を得るために必要なフェライト組織中の（Ｔｉ，Ｍｏ）複合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量の（Ｔｉ，Ｍｏ）複合炭化物が得られず強度不足となり、一方、０．１８５％を超えると炭化物の析出挙動が変化し、加工性と疲労特性が劣化する。したがって、Ｃ含有量を０．０３５〜０．１８５％とする。
【００２２】
Ｍｎ： Ｍｎは（Ｔｉ，Ｍｏ）複合炭化物の成長速度を抑制し、粒径が１０ｎｍ以下の（Ｔｉ，Ｍｏ）複合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％未満では粒径が１０ｎｍ未満の（Ｔｉ，Ｍｏ）複合炭化物が十分に形成されないため所望の強度、加工性、疲労特性が得られず、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。
【００２３】
Ｍｏ： Ｍｏは所望の強度、加工性、疲労特性を得るために必要なフェライト組織中の（Ｔｉ，Ｍｏ）複合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（Ｔｉ，Ｍｏ）複合炭化物が得られず、強度、加工性不足となり、一方、０．４９％を超えると疲労特性が低下する。したがって、Ｍｏ含有量を０．０１〜０．４９％とする。
【００２４】
Ｔｉ： ＴｉはＭｏと同様、所望の強度、加工性、疲労特性を得るために必要なフェライト組織中の（Ｔｉ，Ｍｏ）複合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（Ｔｉ，Ｍｏ）複合炭化物が得られず、強度、加工性不足となり、一方、０．１４５％を超えると疲労特性が低下する。したがって、Ｔｉ含有量を０．０１０〜０．１４５％とする。
【００２５】
Ａｌ： Ａｌは製鋼時の脱酸元素であるとともに、熱延工程でのオーステナイト粒成長を抑制することで（Ｔｉ，Ｍｏ）複合炭化物を微細析出させるための必須元素である。しかし、その量が０．０１１％未満ではその効果に乏しく、一方、０．１０％を超えると疲労特性が劣化する。したがって、Ａｌ含有量を０．０１１〜０．１０％とする。
【００２６】
Ｐ，Ｓ，Ｎ，Ｏ： これらはいずれも加工性、疲労特性を低下させる不純物元素であり、Ｐは０．０３％、Ｓは０．００４％、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。
【００２７】
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、１．５０％を超えると疲労特性が低下するため、Ｓｉを添加する場合には、その含有量を０．００５〜１．５０％とする。
【００２８】
Ｃｒ： ＣｒはＭｎの（Ｔｉ，Ｍｏ）複合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、０．２４％を超えると疲労特性が劣化するため、Ｃｒを添加する場合には、その含有量を０．０１〜０．２４％とする。
【００２９】
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０．０６０％、Ｖは０．０５０％、Ｗは０．５０％を超えると加工性と疲労特性が低下する。したがって、これらを添加する場合には、Ｎｂ：０．００１〜０．０６０％、Ｖ：０．００１〜０．０５０％、Ｗ：０．００１〜０．５０％とする。なお、Ｔｉが０．０４１％未満の場合には、Ｎｂの強度を補完する効果が０．０１７％で飽和するので、Ｎｂを０．０１８％以上添加する場合はＴｉが０．０４１％以上であることが望ましい。
【００３０】
Ｎｉ，Ｃｕ： これらはＭｎの（Ｔｉ，Ｍｏ）複合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、Ｎｉは０．５０％、Ｃｕは０．２４％を超えると加工性と疲労特性が劣化するため、Ｎｉ，Ｃｕを添加する場合には、Ｎｉ：０．０１〜０．５０％、Ｃｕ：０．０１〜０．２４％とする。
【００３１】
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、０．０００６％を超えると加工性と疲労特性が低下するため、Ｂを添加する場合には、その含有量を０．０００１〜０．０００６％とする。
【００３２】
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。
【００３３】
｛Ｔｉ−（４８／１４）Ｎ｝／Ｍｏ： フェライト組織中の（Ｔｉ，Ｍｏ）複合炭化物のサイズを１０ｎｍ以下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好ましい。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て加工性と疲労強度とが低下するおそれがある。
【００３４】
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５秒間以内に７００℃以下まで冷却し、６００℃超〜６７５℃で巻取って熱延鋼帯とし、酸洗、スリット後造管する。
【００３５】
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、（Ｔｉ，Ｍｏ）複合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。
【００３６】
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。
【００３７】
熱延ランナウト冷却条件： （Ｔｉ，Ｍｏ）複合炭化物の析出状態を制御し、粒径１０ｎｍ以下に微細析出させ、原子比で表したＭｏ／（Ｔｉ＋Ｍｏ）の値を０．３３〜０．７７の範囲とす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。（Ｔｉ，Ｍｏ）複合炭化物の成長を抑制し所望の原子比を有する（Ｔｉ，Ｍｏ）複合炭化物を得るには熱延仕上げ圧延終了後、５秒間以内に７００℃まで冷却する必要がある。
【００３８】
巻取温度： 粒径が１０ｎｍ以下の（Ｔｉ，Ｍｏ）複合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〜６７５℃とする必要がある。６７５℃を超えると（Ｔｉ，Ｍｏ）複合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。
【００３９】
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。
幅絞り＝［（素材鋼帯の幅）−π｛（製品外径）−（製品肉厚）｝］／π｛（製品外径）−（製品肉厚）｝×（１００％）
【００４０】
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。
【００４１】
【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１２８０℃に再加熱後、仕上圧延温度約９１５℃、ランナウトでの７００℃までの冷却時間約３秒、巻取温度約６３０℃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。
【００４２】
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、析出物の平均粒径と組成は、薄膜の透過型電子顕微鏡観察とエネルギー分散型分光分析によりそれぞれ評価した。
【００４３】
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、疲労特性を求めた。その結果を表３に示す。
【００４４】
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。
【００４５】
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。
【００４６】
鋼管の疲労特性は、直管の４点曲げ疲労試験と、曲げ管の一端を固定し、もう一端を曲げ平面に垂直方向に変位させる曲げ捩り疲労試験により評価した。直管の４点曲げ疲労試験は片振り、周波数５Ｈｚの条件での１０６繰り返し疲れ限度σＡ（最大主応力振幅の２倍）と鋼管強度ＴＳとの比（σＡ／ＴＳ）で評価し、曲げ管の曲げ捩り疲労試験は、曲げ半径ρ／ｄ＝２．０、曲げ角度９０°で回転引き曲げした後、両振り、周波数１Ｈｚの条件での５×１０５繰り返し疲れ限度σＢ（最大主応力の応力振幅）と鋼管強度ＴＳとの比（σＢ／ＴＳ）でそれぞれ評価した。
【００４７】
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、粒径１０ｎｍ以下、原子比で表したＭｏ／（Ｔｉ＋Ｍｏ）の値が０．３３〜０．７７である（Ｔｉ，Ｍｏ）複合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５９０ＭＰａ以上で、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、（σＡ／ＴＳ）≧０．６、（σＢ／ＴＳ）≧０．４５の優れた疲労特性を示した。
【００４８】
一方、本発明範囲からＣおよびＡｌが低く外れた鋼Ｌ、Ｍｎが低く外れた鋼ＮのＮｏ．１２，１４は、微細な析出物の量が不十分であり、Ｔｉ、Ｍｏが低く外れた鋼Ｑ、ＳのＮｏ．１７，１９は、炭化物の粒径が１０ｎｍ以上であり、また、いずれも析出物の組成が本発明の範囲をはずれており、いずれも引張強度が５９０ＭＰａ未満でかつＴＳ／（ρ／ｄ）が４００ＭＰａ未満と曲げ加工特性が低く、（σＡ／ＴＳ）が０．６未満、（σＢ／ＴＳ）が０．４５未満と疲労特性が低かった。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、フェライト組織面分率、析出物の平均粒径、析出物組成のうち１項目以上が本発明の範囲外となり、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、（σＡ／ＴＳ）が０．６未満、（σＢ／ＴＳ）が０．４５未満と疲労特性も低かった。
【００４９】
【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、疲労特性を表６にそれぞれ示す。
【００５３】
熱延条件が本発明範囲内にあるＮｏ．２１，２６は、粒径１０ｎｍ以下、原子比で表したＭｏ／（Ｔｉ＋Ｍｏ）の値が０．３３〜０．７７である（Ｔｉ，Ｍｏ）複合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５９０ＭＰａ以上で、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、（σＡ／ＴＳ）≧０．６、（σＢ／ＴＳ）≧０．４５の優れた疲労特性を示した。
【００５４】
一方、スラブ加熱温度、仕上圧延温度、仕上圧延後７００℃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．２２〜２５，２７〜３１では、いずれもフェライト中の析出物の粒径が２０ｎｍ以上と大きく、原子比で表したＭｏ／（Ｔｉ＋Ｍｏ）の値が０．３３未満あるいは０．７７を超えるため、ＴＳ／（ρ／ｄ）が４００ＭＰａ未満と曲げ加工特性が低く、強度に比して液圧加工特性が低く、（σＡ／ＴＳ）が０．６未満、（σＢ／ＴＳ）が０．４５未満と疲労特性も低かった。
【００５５】
【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し、複合した成形後の疲労特性にも優れる。
【００５９】
【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５９０ＭＰａ以上の強度を有し、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、優れた疲労特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、疲労特性を満たしており、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曲げ加工特性の関係を示すグラフ。
【図２】フェライト組織中の析出物の大きさ、析出物の原子比で表したＭｏ／（Ｔｉ＋Ｍｏ）の値と鋼管の疲労特性の関係を示すグラフ。[0001]
BACKGROUND OF THE INVENTION
The present invention has a tensile strength of 590 MPa or more, and combines the workability necessary for bending, hydraulic pressure, pipe expansion, contraction, and molding combined with these, and excellent fatigue characteristics. The present invention relates to a high-tensile welded steel pipe suitable for a structural member such as a motorcycle, a manufacturing method thereof, and a steel strip for the material.
[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, although relatively good elongation (El) is obtained when the tensile strength (TS) is in the range of 550 to 780 MPa, there is a problem in fatigue characteristics.
[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. However, this technique has a problem that fatigue characteristics, particularly fatigue characteristics after forming, cannot be obtained sufficiently.
[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, relatively good ERW weld toughness can be obtained when TS is in the range of 680 to 940 MPa, but there are problems in workability and fatigue characteristics, particularly fatigue characteristics after processing.
[0006]
[Problems to be solved by the invention]
As described above, according to the conventional technology, a high-tensile welded steel pipe having both workability and fatigue characteristics required for a structural member such as an automobile, particularly fatigue characteristics after forming a member has not been obtained.
[0007]
The present invention has been made in view of such circumstances, has a tensile strength of 590 MPa or more, workability required for bending, hydraulic pressure, pipe expansion, contraction, and the combination of these, and the like, An object of the present invention is to provide a high-strength welded steel pipe suitable for structural members such as automobiles and motorcycles, a manufacturing method thereof, and a steel strip for a welded steel pipe material, which have excellent fatigue characteristics.
[0008]
[Means for Solving the Problems]
In order to obtain a welded steel pipe that simultaneously satisfies conflicting properties such as strength, workability, and fatigue properties, the present inventors have made various systematic experimental studies by changing the chemical composition, microstructure, and precipitate presence state of the welded steel pipe in various ways. Went. As a result, in the ferrite structure occupying a surface fraction of 60% or more, the (Ti, Mo) composite carbide having a particle size of 10 nm or less and an atomic ratio of Mo / (Ti + Mo) = 0.33 to 0.77 is finely formed. It was found that a welded steel pipe satisfying desired strength, workability, and fatigue characteristics at the same time can be obtained.
[0009]
The present invention has been completed based on such knowledge, and provides the following (1) to (6).
[0010]
(1) By weight, C: 0.035 to 0.185%, Mn: 0.75 to 1.95%, Mo: 0.01 to 0.49%, Ti: 0.010 to 0.145% Al: 0.011-0.10%, P: 0.03% or less, S: 0.004% or less, N: 0.006% or less, O: 0.004% or less, the balance being Fe and inevitable impurities The ferrite structure in which the (Ti, Mo) composite carbide having a particle size of 10 nm or less and an atomic ratio of Mo / (Ti + Mo) = 0.33 to 0.77 is precipitated is 60 to 100% in terms of the structure area fraction. High-tensile welded steel pipe with excellent workability and fatigue characteristics.
[0011]
(2) In the above (1), further, by weight, Si: 0.005 to 1.50%, Cr: 0.01 to 0.24%, Nb: 0.001 to 0.060%, V: 0.001-0.050%, W: 0.001-0.50%, Ni: 0.01-0.50%, Cu: 0.01-0.24%, B: 0.0001-0. High-tensile welded steel pipe excellent in workability and fatigue characteristics characterized by containing one or more of 0006%, Ca: 0.0001-0.0040%, REM: 0.0001-0.0040% .
[0012]
(3) In the above (1) or (2), the following high-strength welded steel pipe excellent in workability and fatigue characteristics characterized by satisfying the following formula (1) represented by weight% of Ti, Mo, N: .
0.15 ≦ {Ti− (48/14) N} / Mo ≦ 1 (1)
[0013]
(4) In manufacturing the welded steel pipe according to any one of (1) to (3) above, the steel slab having the above composition is heated to 1150 ° C. or higher, and then hot rolled to a finish rolling temperature of 850 ° C. or higher. Workability and fatigue characterized by cooling to 700 ° C. or less within 5 seconds after finish rolling, winding at over 600 ° C. to 675 ° C. to form a hot-rolled steel strip, pickling, and pipe forming after slitting Manufacturing method of high-tensile welded steel pipe with excellent characteristics.
[0014]
(5) (Ti, Mo) composite carbide having the component composition of (1) or (2), having a particle size of 10 nm or less, and Mo / (Ti + Mo) = 0.33 to 0.77 in atomic ratio A steel strip for high-strength welded steel pipe material excellent in workability and fatigue characteristics, characterized in that the ferrite structure in which the precipitates are 60% to 100% in terms of the structural area fraction.
[0015]
(6) In the above (5), a steel strip for a high-strength welded steel pipe material excellent in workability and fatigue characteristics characterized by satisfying the following formula (1) represented by weight% of Ti, Mo, N: .
0.15 ≦ {Ti− (48/14) N} / Mo ≦ 1 (1)
[0016]
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 a ferrite structure in which a fine (Ti, Mo) composite carbide having a particle size of 10 nm or less and an atomic ratio of Mo / (Ti + Mo) = 0.33 to 0.77 is precipitated. Is the point which is 60 to 100% in a tissue surface fraction. As a result, a welded steel pipe having desired strength, workability, and fatigue characteristics can be obtained. In this way, high strength can be achieved without degrading fatigue characteristics and workability. It is necessary to obtain the distance between the nearest particles of precipitates necessary for strength increase by fine precipitation of carbides. One of the factors is considered to be a reduction in the amount of elements.
[0017]
FIG. 1 shows the relationship between the ferrite fraction of the steel pipe, the size of precipitates in the ferrite structure, and the bending characteristics. The bending workability is the strength of the steel pipe by the ratio ρ / d between the limit bending radius (bending radius of the pipe center axis) ρ (mm) and the pipe outer shape d (mm) by rotary pulling using both a pressure die and a mandrel. Evaluation was performed by a value TS / (ρ / d) (MPa) obtained by dividing TS (MPa). The larger the value, the better the bending workability. The pressure die pressing force at this time was the maximum stress that does not cause buckling or wrinkling. The numbers in the plot in the figure are the values of TS / (ρ / d). From FIG. 1, by depositing ultrafine (Ti, Mo) composite carbide having a particle size of 10 nm or less in a ferrite structure occupying an area fraction of 60% or more, TS / (ρ / d) is excellent at 400 MPa or more. It can be seen that bending characteristics can be obtained. The size of the precipitate in the ferrite structure is measured from a transmission electron micrograph of a thin film cut out from a steel pipe and polished to obtain the average particle size. The analysis was carried out by an energy dispersive spectroscope equipped in a transmission electron microscope.
[0018]
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.
[0019]
FIG. 2 shows the relationship between the size of the precipitate in the ferrite structure of the steel pipe, the value of Mo / (Ti + Mo) (atomic ratio) of the precipitate, and the fatigue characteristics of the steel pipe. The fatigue characteristics of the steel pipe were evaluated by a four-point bending fatigue test of a straight pipe and a bending torsion fatigue test in which one end of the bending pipe is fixed and the other end is displaced in a direction perpendicular to the bending plane. The straight pipe four-point bending fatigue test is evaluated by the ratio (σ A / TS) of the steel pipe strength TS to 10 6 repeated fatigue limit σ A (twice the maximum principal stress amplitude) under the condition of a single swing and a frequency of 5 Hz. The bending torsional fatigue test of the bent pipe was performed by rotating and bending at a bending radius of ρ / d = 2.0 and a bending angle of 90 °, and then 5 × 10 5 repeated fatigue limit σ B (both swings under a frequency of 1 Hz). The ratio (σ B / TS) between the stress amplitude of the maximum principal stress and the steel pipe strength TS was evaluated. The raw tube dimensions were set to φ70 × 2.0 t (mm). The numbers in the lower circle plot of FIG. 2 are the values of σ A / TS, and the numbers in the upper square plot of FIG. 2 are the values of σ B / TS. From FIG. 2, (σ A / TS) is obtained by precipitating (Ti, Mo) composite carbide having a particle size of 10 nm or less and an atomic ratio of Mo / (Ti + Mo) = 0.33 to 0.77 in the ferrite structure. ) ≧ 0.6 and (σ B /TS)≧0.45 excellent fatigue characteristics can be obtained.
[0020]
Next, the component composition will be described.
The component composition of the welded steel pipe of the present invention is, by weight, C: 0.035 to 0.185%, Mn: 0.75 to 1.95%, Mo: 0.01 to 0.49%, Ti: 0 0.010 to 0.145%, Al: 0.011 to 0.10%, P: 0.03% or less, S: 0.004% or less, N: 0.006% or less, O: 0.004% or less The balance consists of Fe and inevitable impurities. Further, Si: 0.005 to 1.50%, Cr: 0.01 to 0.24%, Nb: 0.001 to 0.060%, V: 0.001 to 0.050%, W: 0.00. 001 to 0.50%, Ni: 0.01 to 0.50%, Cu: 0.01 to 0.24%, B: 0.0001 to 0.0006%, Ca: 0.0001 to 0.0040% , REM: One or more of 0.0001 to 0.0040% can be contained. Moreover, it is preferable to satisfy 0.15 ≦ {Ti− (48/14) N} / Mo ≦ 1.
[0021]
The reasons for these limitations are as follows.
C: C is an essential element constituting the (Ti, Mo) composite carbide in the ferrite structure necessary for obtaining desired strength, workability, and fatigue characteristics. However, if the amount is less than 0.035%, the amount of (Ti, Mo) composite carbide necessary for securing the strength cannot be obtained and the strength is insufficient. On the other hand, if the amount exceeds 0.185%, the precipitation behavior of the carbide changes. However, workability and fatigue characteristics deteriorate. Therefore, the C content is 0.035 to 0.185%.
[0022]
Mn: Mn is an essential element for suppressing the growth rate of (Ti, Mo) composite carbide and forming (Ti, Mo) composite carbide having a particle size of 10 nm or less. However, if the amount is less than 0.75%, a (Ti, Mo) composite carbide having a particle size of less than 10 nm is not sufficiently formed, so that desired strength, workability, and fatigue characteristics 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%.
[0023]
Mo: Mo is an essential element for generating (Ti, Mo) composite carbide in a ferrite structure necessary for obtaining desired strength, workability, and fatigue characteristics. However, if the amount is less than 0.01%, the amount of (Ti, Mo) composite carbide necessary for securing the strength and workability cannot be obtained, and the strength and workability are insufficient. If it exceeds, the fatigue characteristics will deteriorate. Therefore, the Mo content is set to 0.01 to 0.49%.
[0024]
Ti: Like Mo, Ti is an essential element for generating (Ti, Mo) composite carbide in a ferrite structure necessary for obtaining desired strength, workability, and fatigue characteristics. However, if the amount is less than 0.010%, the amount of (Ti, Mo) composite carbide necessary for securing the strength and workability cannot be obtained, and the strength and workability are insufficient. If it exceeds, the fatigue characteristics will deteriorate. Therefore, the Ti content is set to 0.010 to 0.145%.
[0025]
Al: Al is a deoxidizing element during steelmaking, and is an essential element for finely depositing (Ti, Mo) composite carbide by suppressing austenite grain growth in the hot rolling process. However, if the amount is less than 0.011%, the effect is poor, while if it exceeds 0.10%, the fatigue characteristics deteriorate. Therefore, the Al content is 0.011 to 0.10%.
[0026]
P, S, N, O: These are all impurity elements that reduce workability and fatigue characteristics. P is 0.03%, S is 0.004%, N is 0.006%, and O is 0.0. If it exceeds 004%, the adverse effect becomes obvious, so that value is set as the upper limit of each.
[0027]
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 1.50%, the fatigue characteristics are lowered. Therefore, when Si is added, the content is 0.005 to 1.50%. And
[0028]
Cr: Cr can be added because it has a function to supplement the growth rate suppressing action of the (Ti, Mo) composite carbide of Mn. If the amount is less than 0.01%, the effect is poor. On the other hand, if it exceeds 0.24%, the fatigue characteristics deteriorate. Therefore, when Cr is added, the content is 0.01 to 0.24%. And
[0029]
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 0.060%, V is 0.050%, and W exceeds 0.50%, workability and fatigue characteristics deteriorate. Therefore, when adding these, it is set as Nb: 0.001-0.060%, V: 0.001-0.050%, W: 0.001-0.50%. When Ti is less than 0.041%, the effect of complementing the strength of Nb is saturated at 0.017%. Therefore, when Nb is added by 0.018% or more, Ti is 0.041% or more. It is desirable to be.
[0030]
Ni, Cu: Since these have a function to supplement the growth rate suppressing action of the (Ti, Mo) composite carbide of Mn, they can be added. When the amount is less than 0.01%, the effect is poor. On the other hand, when Ni exceeds 0.50% and Cu exceeds 0.24%, workability and fatigue characteristics deteriorate. Are Ni: 0.01 to 0.50% and Cu: 0.01 to 0.24%.
[0031]
B: B can be added to impart hardenability to the steel pipe. If the amount is less than 0.0001%, the effect is poor, and if it exceeds 0.0006%, the workability and fatigue characteristics deteriorate. Therefore, when B is added, the content is 0.0001 to 0.0006. %.
[0032]
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.
[0033]
{Ti- (48/14) N} / Mo: In order to make the size of the (Ti, Mo) composite carbide in the ferrite structure as fine as 10 nm or less, it is expressed in terms of wt% of Ti, Mo, and N. The value of {Ti- (48/14) N} / Mo is preferably 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. There is a possibility that the workability and fatigue strength are lowered due to the behavior change.
[0034]
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 700 ° C. or lower within 5 seconds after finish rolling, and exceeds 600 ° C. It is wound at ˜675 ° C. to form a hot-rolled steel strip, and is piped after pickling and slitting.
[0035]
Hereinafter, these reasons for limitation will be described.
Slab reheating temperature: When rolling a cooled steel slab after reheating, in order to re-dissolve most of the precipitates in the steel and precipitate (Ti, Mo) composite carbide in the ferrite structure, The reheating temperature of the steel slab needs to be 1150 ° C or higher.
[0036]
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.
[0037]
Hot-roll runout cooling conditions: (Ti, Mo) The precipitation state of the composite carbide is controlled, finely precipitated to a particle size of 10 nm or less, and the value of Mo / (Ti + Mo) expressed by atomic ratio is 0.33 to 0.77. In order to achieve this range, it is important to control the hot-roll runout cooling conditions. In order to obtain a (Ti, Mo) composite carbide having a desired atomic ratio by suppressing the growth of the (Ti, Mo) composite carbide, it is necessary to cool to 700 ° C. within 5 seconds after the hot rolling finish rolling.
[0038]
Winding temperature: In order to make the ferrite structure in which the (Ti, Mo) composite carbide having a particle size of 10 nm or less is precipitated in a structure area fraction of 60 to 100%, the hot rolling coiling temperature is over 600 ° C to 675 ° C. There is a need to. When it exceeds 675 ° C., (Ti, Mo) composite carbide grows, so that the strength is lowered. On the other hand, when it is 600 ° C. or less, the ferrite structure fraction is lowered.
[0039]
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%.
Width reduction = [(material steel strip width) −π {(product outer diameter) − (product thickness)}] / π {(product outer diameter) − (product thickness)} × (100%)
[0040]
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.
[0041]
【Example】
(Example 1)
20 types of steel slabs A to T shown in Table 1 were reheated to about 1280 ° C, then the finish rolling temperature was about 915 ° C, the runout cooling time to 700 ° C was about 3 seconds, and the coiling temperature was about 630 ° 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%.
[0042]
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 nital etching, and the average particle size and composition of the precipitates were evaluated by transmission electron microscope observation and energy dispersive spectroscopic analysis of the thin film, respectively.
[0043]
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, and the bending process characteristics, hydraulic process characteristics, and fatigue characteristics of these steel pipes were determined. The results are shown in Table 3.
[0044]
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 processing characteristics, the limit bending radius ρ ′ (mm) and the pipe outer diameter d ′ (mm) at this time are calculated from the limit bending radius ρ after the diameter reduction processing with an outer diameter reduction ratio of 10%. The ratio ρ ′ / d ′ was also determined.
[0045]
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.
[0046]
The fatigue characteristics of the steel pipe were evaluated by a four-point bending fatigue test of a straight pipe and a bending torsion fatigue test in which one end of the bending pipe is fixed and the other end is displaced in a direction perpendicular to the bending plane. The straight pipe four-point bending fatigue test is evaluated by the ratio (σ A / TS) of the steel pipe strength TS to 10 6 repeated fatigue limit σ A (twice the maximum principal stress amplitude) under the condition of a single swing and a frequency of 5 Hz. The bending torsional fatigue test of the bending tube was performed by rotating and bending at a bending radius of ρ / d = 2.0 and a bending angle of 90 °, and then 5 × 10 5 repeated fatigue limit σ B (both swings at a frequency of 1 Hz). The ratio (σ B / TS) between the stress amplitude of the maximum principal stress and the steel pipe strength TS was evaluated.
[0047]
No. in Table 1 Nos. 1 to 11 are composites in which the component composition is within the range of the present invention, the particle size is 10 nm or less, and the value of Mo / (Ti + Mo) expressed in atomic ratio is 0.33 to 0.77. The ferrite structure in which carbides are precipitated is an example of the present invention having a structure area fraction of 60 to 100%, the tensile strength TS is 590 MPa or more, TS / (ρ / d) is 400 MPa or more, and the critical bending radius after shrinkage Excellent bending characteristics with ρ '/ d' of 2.8 or less, circumference increasing rate of 9% or more without axial compression, and excellent hydraulic working characteristics with circumference increasing rate of 17% or more without axial compression Excellent fatigue properties of (σ A /TS)≧0.6 and (σ B /TS)≧0.45 were exhibited.
[0048]
On the other hand, steel L from which the C and Al deviated from the scope of the present invention, and steel N from which the Mn deviated from low were no. Nos. 12 and 14 are steels Q and S No. of steel Q and S in which the amount of fine precipitates is insufficient and Ti and Mo are low. Nos. 17 and 19 have a carbide particle size of 10 nm or more, and the composition of the precipitates is out of the range of the present invention, and the tensile strength is less than 590 MPa and TS / (ρ / d) is all Bending characteristics were low at less than 400 MPa, (σ A / TS) was less than 0.6, and (σ B / TS) was less than 0.45, and fatigue characteristics were 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. As for 13, 15, 16, 18, and 20, one or more items out of the range of the present invention are out of the scope of the present invention, and TS / (ρ / d) is less than 400 MPa. Bending characteristics are low, circumference increase rate is 8% or less without axial compression, circumference increase rate is 14% or less with axial compression, hydraulic processing characteristics are low, and (σ A / TS) is less than 0.6 (Σ B / TS) was less than 0.45, and the fatigue characteristics were also low.
[0049]
[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 characteristics, hydraulic processing characteristics, and fatigue characteristics.
[0053]
No. whose hot rolling conditions are within the scope of the present invention. Nos. 21 and 26 have a grain size of 10 nm or less, and the ferrite structure in which (Ti, Mo) composite carbides having a Mo / (Ti + Mo) value expressed by an atomic ratio of 0.33 to 0.77 is a structure area fraction. Excellent bending properties with a tensile strength TS of 590 MPa or more, TS / (ρ / d) of 400 MPa or more, a peripheral length increase rate of 9% or more without axial compression, and a circumference with axial compression. Excellent hydraulic working characteristics with a long increase rate of 17% or more were exhibited, and excellent fatigue characteristics of (σ A /TS)≧0.6 and (σ B /TS)≧0.45 were exhibited.
[0054]
On the other hand, any of slab heating temperature, finish rolling temperature, cooling time to 700 ° C. after finish rolling, and coiling temperature deviated from the scope of the present invention. In 22-25 and 27-31, since the particle size of the precipitate in ferrite is as large as 20 nm or more, and the value of Mo / (Ti + Mo) expressed in atomic ratio is less than 0.33 or more than 0.77, When TS / (ρ / d) is less than 400 MPa, the bending property is low, the hydraulic processing property is lower than the strength, (σ A / TS) is less than 0.6, and (σ B / TS) is 0. Less than 45 and fatigue properties were also low.
[0055]
[Table 4]
[Table 5]
[Table 6]
The welded steel pipe within the scope of the present invention is excellent in the hydraulic processing characteristics in the mold, exhibits excellent workability even in the combination of bending, hydraulic pressure, pipe expansion, contraction, etc., and fatigue after the composite molding Excellent characteristics.
[0059]
【The invention's effect】
As described above, according to the present invention, the tensile strength is not less than 590 MPa, the workability required for bending, hydraulic pressure, tube expansion, tube contraction, and molding that combines these, and excellent fatigue. It is possible to obtain a high-tensile welded steel pipe that has both properties. The high-tensile welded steel pipe of the present invention satisfies the strength, workability, and fatigue characteristics required for closed-section automotive structural materials such as suspension arms, suspension members, axle beams, stabilizers, frames, and shafts. It is extremely effective.
[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 bending characteristics.
FIG. 2 is a graph showing the relationship between the size of precipitates in a ferrite structure, the value of Mo / (Ti + Mo) expressed by the atomic ratio of precipitates, and the fatigue characteristics of a steel pipe.
Claims (6)
Ｃ：０．０３５〜０．１８５％、
Ｍｎ：０．７５〜１．９５％、
Ｍｏ：０．０１〜０．４９％、
Ｔｉ：０．０１０〜０．１４５％、
Ａｌ：０．０１１〜０．１０％、
Ｐ：０．０３％以下、
Ｓ：０．００４％以下、
Ｎ：０．００６％以下、
Ｏ：０．００４％以下、
残部がＦｅおよび不可避不純物からなり、
粒径が１０ｎｍ以下で、原子比でＭｏ／（Ｔｉ＋Ｍｏ）＝０．３３〜０．７７である（Ｔｉ，Ｍｏ）複合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疲労特性に優れた高張力溶接鋼管。% By weight
C: 0.035 to 0.185%,
Mn: 0.75 to 1.95%,
Mo: 0.01-0.49%,
Ti: 0.010 to 0.145%,
Al: 0.011 to 0.10%,
P: 0.03% or less,
S: 0.004% or less,
N: 0.006% or less,
O: 0.004% or less,
The balance consists of Fe and inevitable impurities ,
The ferrite structure in which (Ti, Mo) composite carbides having a particle size of 10 nm or less and an atomic ratio of Mo / (Ti + Mo) = 0.33 to 0.77 is 60 to 100% in terms of the texture area. High-tensile welded steel pipe with excellent workability and fatigue characteristics.
Ｓｉ：０．００５〜１．５０％、
Ｃｒ：０．０１〜０．２４％、
Ｎｂ：０．００１〜０．０６０％、
Ｖ：０．００１〜０．０５０％、
Ｗ：０．００１〜０．５０％、
Ｎｉ：０．０１〜０．５０％、
Ｃｕ：０．０１〜０．２４％、
Ｂ：０．０００１〜０．０００６％、
Ｃａ：０．０００１〜０．００４０％、
ＲＥＭ：０．０００１〜０．００４０％
のうちの１種以上を含有することを特徴とする、請求項１に記載の加工性と疲労特性に優れた高張力溶接鋼管。In addition, by weight
Si: 0.005 to 1.50%,
Cr: 0.01 to 0.24%,
Nb: 0.001 to 0.060%,
V: 0.001 to 0.050%,
W: 0.001 to 0.50%,
Ni: 0.01 to 0.50%,
Cu: 0.01 to 0.24%,
B: 0.0001 to 0.0006%,
Ca: 0.0001 to 0.0040%,
REM: 0.0001 to 0.0040%
The high-strength welded steel pipe excellent in workability and fatigue characteristics according to claim 1, comprising at least one of the above.
０．１５≦｛Ｔｉ−（４８／１４）Ｎ｝／Ｍｏ≦１ ‥‥（１）The high-tensile welded steel pipe excellent in workability and fatigue characteristics according to claim 1 or 2, characterized by satisfying the following formula (1) expressed by weight% of Ti, Mo, N:
0.15 ≦ {Ti− (48/14) N} / Mo ≦ 1 (1)
０．１５≦｛Ｔｉ−（４８／１４）Ｎ｝／Ｍｏ≦１ ‥‥（１）The steel strip for a high-tensile welded steel pipe material excellent in workability and fatigue characteristics according to claim 5, wherein the following formula (1) expressed by weight percent of Ti, Mo, and N is satisfied.
0.15 ≦ {Ti− (48/14) N} / Mo ≦ 1 (1)
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| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP2002126690A JP4007050B2 (en) | 2002-04-26 | 2002-04-26 | High-tensile welded steel pipe excellent in workability and fatigue characteristics, its manufacturing method, and steel strip for welded steel pipe material |
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| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP2002126690A JP4007050B2 (en) | 2002-04-26 | 2002-04-26 | High-tensile welded steel pipe excellent in workability and fatigue characteristics, its manufacturing method, and steel strip for welded steel pipe material |
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| Publication Number | Publication Date |
|---|---|
| JP2003321748A JP2003321748A (en) | 2003-11-14 |
| JP4007050B2 true JP4007050B2 (en) | 2007-11-14 |
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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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| JP4443910B2 (en) | 2003-12-12 | 2010-03-31 | Ｊｆｅスチール株式会社 | Steel materials for automobile structural members and manufacturing method thereof |
| WO2005087966A1 (en) * | 2004-03-11 | 2005-09-22 | Nippon Steel Corporation | Steel plate excellent in machinability, toughness and weldability, and method for production thereof |
| JP4735315B2 (en) * | 2006-02-15 | 2011-07-27 | Ｊｆｅスチール株式会社 | High tensile welded steel pipe for automobile structural members and method for manufacturing 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 |
| JP4955497B2 (en) * | 2007-09-28 | 2012-06-20 | 株式会社神戸製鋼所 | Hot-rolled steel sheet with excellent fatigue characteristics and stretch flangeability balance |
| JP5499560B2 (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 |
| JP5754279B2 (en) * | 2011-07-20 | 2015-07-29 | Ｊｆｅスチール株式会社 | High strength steel sheet for warm forming and manufacturing method thereof |
| JP7338590B2 (en) * | 2020-08-21 | 2023-09-05 | Ｊｆｅスチール株式会社 | Use of electric resistance welded steel pipes as raw materials for manufacturing modified cross-section steel pipes |
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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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