JP2004068040A - High-strength steel pipe superior in workability, and manufacturing method therefor - Google Patents
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Description
【０００１】
【発明の属する技術分野】
本発明は、伸管、曲げ、ハイドロフォーム等によって成形する、構造用部品、配管等に好適な、加工性に優れた鋼管とその製造方法に関する。
【０００２】
【従来の技術】
最近、鋼板のプレス成形及び接合によって製造していた部材を、鋼管の一体成形によって製造する方法が提案されている。例えば、工程省略及び部品点数減少による自動車の軽量化及び製造コスト低減を目的として、複雑形状の部品を鋼管のハイドロフォーム成形により製造する技術が、特開平１０−１７５０２６号公報に開示されている。
【０００３】
ハイドロフォーム成形性を向上させる材料因子は、加工硬化指数ｎ値と塑性異方性の指標である管軸方向のｒ値（以下、ｒＬ）であることが、塑性と加工、第４１巻、第４７８号（２０００）第１０７５〜１０８１頁に報告されている。
【０００４】
このうちｒＬが高く、ハイドロフォーム加工性に優れた鋼管及び縮径圧延による製造方法が、特開２００１−２１４２１８号公報、特開２００１−３４８６４３号公報、特開２００１−３４８６４７号公報、特開２００１−３４８６４８号公報、特開２００１−３５５０３４号公報、特開２００１−３５５０４７号公報、特開２００２−２０８４１号公報、特開２００２−９７５４９号公報、特開２００２−１１５７８０号公報、特開２００２−１１５０２９号公報、特開２００２−１１５０１２号公報に、開示されている。
【０００５】
しかし、縮経率を高くした縮径圧延では、２．０以上という極めて高いｒＬが得られるものの、ｎ値を高めることは困難であった。
【０００６】
また、圧延方向及び圧延方向に直交する幅方向のｒ値が高い冷延鋼板を造管し、鋼管を熱処理する方法が特開２００２−１１５７８０号公報に開示されている。しかし、この方法では、ｒ値の高い冷延鋼板を造管する際に、冷間加工歪みが導入されてｎ値が低下し、高いｒ値を維持する条件で鋼管を熱処理するとｎ値が十分に回復せず、ｒ値及びｎ値を同時に高めることは困難であった。
【０００７】
【発明が解決しようとする課題】
本発明は、ハイドロフォーム成形によって製造する構造用部品、配管等に好適な、ｒ値及びｎ値をともに従来にないレベルまで向上させた、加工性に優れた高強度鋼管及びその製造方法を提供するものである。
【０００８】
【課題を解決するための手段】
本発明は上記のような課題を解決すべく検討を鋭意進めたところ、ハイドロフォーム成形性を向上させる材料因子は、ｎ値及びｒＬだけでなく、管周方向のｒ値（以下、ｒＣ）の向上も効果的であることを見出した。
【０００９】
また、冷間加工歪みが導入された鋼板又は造管後の鋼管を再結晶させずにＡｌＮを析出させ、その後、再結晶させることにより、ｒＬ、ｒＣ、及びｎ値を同時に向上させた高強度鋼管を発明するに至った。
【００１０】
すなわち、本発明の要旨とするところは、以下のとおりである。
【００１１】
（１） 質量％で、Ｃ：０．０３〜０．５％、Ｓｉ：０．００１〜３．０％、Ｍｎ：０．０１〜３．０％、Ｐ：０．００１〜０．１５％、Ｓ：０．０５％以下、Ａｌ：０．００８〜０．３％、Ｎ：０．００１〜０．０３％を含有し、残部がＦｅ及び不可避的不純物からなり、引張強度が３５０ＭＰａ以上であり、管軸方向及び円周方向のｒ値がともに１．３以上であって、管軸方向のｎ値「ｎ」と引張強度「ＴＳ［ＭＰａ］」が、ＴＳ＋３２８５×ｎ＞１０８２の関係を満たすことを特徴とする、加工性に優れた高強度鋼管。
【００１２】
（２） 前記管軸方向及び円周方向の降伏比が０．８以下であることを特徴とする、前記（１）に記載の加工性に優れた高強度鋼管。
【００１３】
（３） 質量％で、Ｚｒ及びＭｇの１種又は２種を合計で０．０００１〜０．５％含有することを特徴とする、前記（１）又は（２）に記載の加工性に優れた高強度鋼管。
【００１４】
（４） 質量％で、Ｔｉ、Ｎｂ、Ｖの１種又は２種以上を合計で０．００１〜０．２％含有することを特徴とする、前記（１）〜（３）のいずれか１項に記載の加工性に優れた高強度鋼管。
【００１５】
（５） 質量％で、Ｓｎ、Ｃｒ、Ｃｕ、Ｎｉ、Ｃｏ、Ｗ、Ｍｏの１種又は２種以上を合計で０．００１〜２．５％含有することを特徴とする、前記（１）〜（４）のいずれか１項に記載の加工性に優れた高強度鋼管。
【００１６】
（６） 質量％で、Ｃａを０．０００１〜０．０１％含有することを特徴とする、前記（１）〜（５）のいずれか１項に記載の加工性に優れた高強度鋼管。
【００１７】
（７） 質量％で、Ｂを０．０００１〜０．０１％含有することを特徴とする、前記（１）〜（６）のいずれか１項に記載の加工性に優れた高強度鋼管。
【００１８】
（８） 鋼片を加熱して熱間圧延を８３０℃以上で終了し、冷却して６００℃以下で巻き取り、冷延率３０％以上、７５％未満の冷間圧延後、４５０℃〜６００℃の範囲の保持時間を１００ｓ以上として、４５０℃以上、再結晶温度未満に加熱して冷却し、造管した後、再結晶温度以上、Ａｃ１＋５０℃以下に加熱することを特徴とする、前記（１）〜（７）のいずれか１項に記載の加工性に優れた鋼管の製造方法。
【００１９】
（９） 鋼片を加熱して熱間圧延を８３０℃以上で終了し、冷却して６００℃以下で巻き取り、３０％〜７５％未満の冷間圧延後、造管し、４５０℃〜６００℃の範囲の保持時間を１００ｓ以上として、再結晶温度以上、Ａｃ１＋５０℃以下に加熱することを特徴とする、前記（１）〜（７）のいずれか１項に記載の加工性に優れた鋼管の製造方法。
【００２０】
（１０） 鋼片を加熱して熱間圧延を８３０℃以上で終了し、冷却して６００℃以下で巻き取り、３０％〜７５％未満の冷間圧延後、造管し、室温から４５０℃以上再結晶温度未満まで１．５℃／ｓ以下で加熱し、さらに再結晶温度以上、Ａｃ１＋５０℃以下に１．５℃／ｓ超で加熱することを特徴とする、前記（１）〜（７）のいずれか１項に記載の加工性に優れた鋼管の製造方法。
【００２１】
（１１） 鋼片を加熱して熱間圧延を８３０℃以上で終了し、冷却して６００℃以下で巻き取り、３０％〜７５％未満の冷間圧延後、造管し、４５０℃〜６００℃の範囲の保持時間を１００ｓ以上として、４５０℃以上、再結晶温度未満に加熱して冷却し、再結晶温度以上、Ａｃ１＋５０℃以下に加熱することを特徴とする、前記（１）〜（７）のいずれか１項に記載の加工性に優れた鋼管の製造方法。
【００２２】
（１２） ４５０℃以上、再結晶温度未満に加熱する際に、１．５℃／ｓ以下で加熱することを特徴とする、前記（８）又は（１１）に記載の加工性に優れた鋼管の製造方法。
【００２３】
【発明の実施の形態】
本発明者は、ハイドロフォーム成形性を向上させる材料因子について有限要素法解析によって詳細な検討を行った。その結果、ｎ値及びｒＬだけでなくｒＣの向上も、ハイドロフォーム成形性の改善には効果的であることを見出し、ハイドロフォーム成形性に優れた鋼管を開発すべく、鋼管のｎ値及びｒ値に及ぼす成分及び製造方法の影響を詳細に調査した。
【００２４】
まず、優れたｒ値及びｎ値を有する冷延鋼板を造管し、加熱する方法を試みた。冷間圧延後、ｒ値及びｎ値を向上させるため、５〜５０℃／ｈで再結晶温度以上に加熱し、再結晶させた冷延鋼板を造管した。再結晶温度は、成分によって変化するが、７００〜７５０℃の範囲内であった。
【００２５】
鋼板の圧延方向及び板幅方向を長手として、ＪＩＳ Ｚ ２２０１の１３Ｂ号試験片を採取して板厚及び板幅をマイクロメータにより測定し、ＪＩＳ Ｚ ２２４１に準拠して引張試験を行った。
【００２６】
圧延方向及び板幅方向のｒ値は、１０％の引張歪みを導入して、引張歪み導入前後の試験片の幅及び引張歪み導入前後の標点距離により、ｒ値の定義に従って算出した。また、圧延方向のｎ値は、引張歪み５〜１５％の範囲において、引張応力と歪みから加工硬化指数として計算した。
【００２７】
その結果、圧延方向及び板幅方向のｒ値は１．３以上であり、圧延方向のｎ値は０．２以上であることを確認した。
【００２８】
これらの鋼管を１〜５℃／ｓで再結晶温度未満に加熱し、鋼管からＪＩＳ Ｚ２２０１に準拠し、管軸方向を長手として１２号円弧状試験片を採取し、試験片平行部に標点をマーキングし、標点距離を測定した。
【００２９】
標点の中央に幅方向に歪みゲージを貼った後、伸び計を取付けて引張試験機にて１０％の引張歪みを与え、標点距離の変化と歪みゲージにより測定した幅方向の歪み変化からｒＬを算出した。
【００３０】
また、鋼管を切断してプレス等で平板上の板とし、円周方向を長手としてＪＩＳ Ｚ ２２０１の１３Ｂ号試験片を採取し、試験片平行部に標点をマーキングして標点距離並びに試験片平行部の板厚及び板幅を測定した。
【００３１】
試験片に伸び計を取付けて、引張試験機にて１０％の引張歪みを与え、引張歪み導入前後の試験片の板幅及び標点距離により、ｒ値の定義に従ってｒＣを算出した。
【００３２】
また、ｎ値は、ＪＩＳ Ｚ ２２０１に準拠し、管軸方向を長手として１１号試験片を採取して、ＪＩＳ Ｚ ２２４１に準拠して引張引張試験を行い、引張歪みが５〜１５％の範囲において、応力と歪みから加工硬化指数として計算した。
【００３３】
その結果、造管後１〜５℃／ｓで再結晶温度未満に加熱した鋼管のｒ値は、造管前の冷延鋼板と同等であるが、ｎ値は低下していることがわかった。
【００３４】
一方、造管後の鋼管を１〜５℃／ｓで再結晶温度以上に加熱し、ｒ値及びｎ値を測定し、造管前の冷延鋼板と比較すると、ｎ値は同等であるが、ｒ値が低下することがわかった。
【００３５】
すなわち、再結晶させた冷延鋼板を造管した鋼管は、再結晶させずに冷間加工歪を回復させるとｎ値が低下し、再結晶させるとｒ値が低下するため、ｒ値及びｎ値を共に向上させることは極めて困難であった。
【００３６】
そこで、本発明者は、冷延鋼板を再結晶させずに造管した後、再結晶温度以上に加熱して、ミクロ組織を再結晶フェライトとすることにより、ｒ値とｎ値を同時に向上させる製造方法を指向した。
【００３７】
まず、冷延圧延後、鋼板を造管し、１〜５℃／ｓで再結晶温度以上に加熱した結果、ｎ値は向上したが、ｒ値が低下した。この原因を明らかにするために、冷延鋼板及び鋼管の板厚中心部より小片を採取して、集合組織をＸ線回折法により調査した。
【００３８】
その結果、鋼管の集合組織は、冷延鋼板に比べて、（１１１）［１−１０］方位が集まった再結晶集合組織（以下、γファイバー）の集積が弱いことがわかった。
【００３９】
これは、加熱時にＡｌＮが微細析出していないことに起因するものであると考え、鋼管を再結晶させる前に、ＡｌＮを微細析出させる製造方法を検討した。
【００４０】
すなわち、冷間圧延後、再結晶させることなくＡｌＮを微細析出させて造管し、再結晶させるか、又は、冷間圧延、造管後、再結晶させることなくＡｌＮを微細析出させて、再結晶させる製造方法である。
【００４１】
本発明者は、再結晶させずにＡｌＮを微細析出させる加熱条件を検討した結果、４５０〜６００℃の温度域に１００ｓ以上保持することが必要であることを見出した。
【００４２】
本発明者は、さらに、冷間圧延の冷延率によるｒ値の変化について詳細な検討を行った。なお、冷延率は冷間圧延後の板厚と冷間圧延前の板厚の差を冷間圧延前の板厚で除した百分率である。
【００４３】
従来、Ｃ等の添加量が低い鋼では、ｒ値を向上させるには、冷延率を７０％以上とすることが有効であったが、本発明の成分の鋼では、冷延率を低くすることでｒ値が向上することを見出した。
【００４４】
これは、第二相が多い鋼は、冷延率を高くすると第二相の周囲に歪みが蓄積し、その後、再結晶温度以上に加熱するとγファイバーが形成されずに、ｒ値が向上しないためである。
【００４５】
本発明者は、このような知見に基づいて、さらに鋼板及び鋼管のｒ値及びｎ値に及ぼす製造条件の影響を詳細に調査し、ｒ値及びｎ値がともに高い、加工性に優れた鋼管及びその製造方法を発明するに至った。
【００４６】
以下、本発明を詳細に説明する。
【００４７】
Ｃは、高強度化に最も有効な元素である。その効果を発現するためには、０．０３％以上の添加を必要とするが、０．５％を超えて添加するとｒ値及び溶接性が低下する。従って、Ｃ量を０．０３〜０．５％の範囲とする。なお、強度、ｒ値、及び、溶接性のバランスを考慮すると、０．０５〜０．２％が好ましい範囲で、０．０８〜０．１５％が最適な範囲である。
【００４８】
Ｓｉは、高強度化に有効な元素であり、また、鋼中の炭化物を低減、又は、微細化し、ｒ値の向上にも有効である。この効果は、Ｓｉが０．００１％未満では不十分であり、３．０％を超えて添加するとめっき濡れ性や加工性が劣化する。従ってＳｉ量を０．００１〜３．０％の範囲とした。
【００４９】
Ｍｎは、高強度化に有効な元素であり、その効果を発現するためには、０．０１％以上の添加が必要である。しかし、３．０％を超える過度の添加はｒ値を劣化させるので、上限を３．０％とする。
【００５０】
Ｐは、高強度化に有効な元素であり、この効果を発揮するには、０．００１％以上の添加がで必要である。ただし、０．１５％超添加すると溶接性や耐二次加工脆性が劣化するので、上限を０．１５％以下とする。好ましくは、０．０３％以下である。
【００５１】
Ｓは、不純物元素であり、熱間割れを防止するために、０．０５％を上限とするが、０．０１％以下にすることが好ましい。Ｓ量は低いほど好ましいが、現状の技術では０．０００１％未満に低減することは困難である。
【００５２】
Ａｌは、脱酸元素であるとともに、冷延鋼板又は鋼管を再結晶温度未満に加熱する際に微細なＡｌＮ及び／又はＮとのクラスタを生成する、極めて重要な元素である。この効果を発現するには、０．００８％以上の添加が必要である。一方、０．３％を超えて過度に添加すると鋼板の表面欠陥あるいは溶接性の低下が発生するため、上限を０．３％とする。
【００５３】
鋼管を再結晶温度以上に加熱して集合組織をγファイバーとしてｒ値を高める効果を十分に発現するには、Ａｌ量を０．０１％以上とすることが好ましく、溶接性を極めて良好とするためには、０．１％以下とすることが好ましい。
【００５４】
Ｎも、Ａｌと同様に本発明において極めて重要な元素であり、０．００１％以上の添加によりｒ値が向上する。一方、０．０３％を超えて添加すると時効性が劣化して強度が低下する。従って、Ｎ量を０．００１〜０．０３％の範囲とする。ｒ値及び強度がともに極めて良好であるＮ量の好ましい範囲は、０．００２〜０．００７％である。
【００５５】
さらに、必要に応じて、Ｚｒ、Ｍｇ、Ｔｉ、Ｎｂ、Ｖ、Ｓｎ、Ｃｒ、Ｃｕ、Ｎｉ、Ｃｏ、Ｗ、Ｍｏ、Ｃａ、Ｂの１種又は２種以上を含有しても良い。
【００５６】
ＺｒとＭｇは、脱酸元素であり、１種又は２種を合計で０．０００１％以上添加すると効果的であるが、０．５％を超える過剰な添加は、酸化物が粗大に析出し延性を劣化させる。従って、Ｚｒ、Ｍｇの１種又は２種を合計で０．０００１〜０．５％添加することが好ましい。
【００５７】
Ｔｉ、Ｎｂ、Ｖは、炭化物、窒化物及び炭窒化物を生成し、高強度化及び加工性向上に寄与するため、１種また２種以上で合計０．００１％以上添加することが好ましい。一方、０．２％を超えて過剰に添加すると、フェライト粒界に粗大かつ多量の炭化物、窒化物あるいは炭窒化物が生成し延性を低下させる。従って、Ｔｉ、Ｎｂ、Ｖを１種また２種以上で合計０．００１〜０．２％とする。
【００５８】
Ｓｎ、Ｃｒ、Ｃｕ、Ｎｉ、Ｃｏ、Ｗ、Ｍｏは強化元素として有効で、１種又は２種以上を合計で０．００１％以上添加することが好ましい。しかし、２．５％を超えて過剰添加するとコストアップあるいは延性低下という問題を生じるため、上限を２．５％以下とすることが好ましい。
【００５９】
Ｃａは、硫化物等の介在物制御あるいは脱酸に有効な元素であるが、過剰添加すると熱間脆化を引き起こすので、範囲を０．０００１〜０．０１％とすることが好ましい。
【００６０】
Ｂは、ｒ値向上や、耐２次加工性脆性の改善に有効な元素であるが、０．０００１％未満ではその効果が小さく、また、０．０１％超添加すると、ｒ値を大きく低下させる。従って、０．０００１〜０．０１％添加することが好ましい。
【００６１】
本発明によって得られる鋼管の管軸方向及び円周方向のｒ値、すなわち、ｒＬ及びｒｃは、ともに１．３以上であり、高ｎ値の組み合わせによって、優れたハイドロフォーム成形性を発現する。
【００６２】
ｒ値は高いほど好ましいため、上限を規定しないが、現状の技術では３を超えることは困難である。
【００６３】
ｒＬは、以下の方法によって測定することができる。鋼管からＪＩＳ Ｚ ２２０１に準拠し、管軸方向を長手として１２号円弧状試験片を採取し、試験片平行部に標点をマーキングし、標点距離を測定する。
【００６４】
標点の中央に幅方向に歪みゲージを貼った後、伸び計を取付けて引張試験機にて１０％の引張歪みを与え、標点距離の変化により測定した試験片長手方向の歪みと歪みゲージにより測定した幅方向の歪み変化から、ｒ値の定義に従ってｒＬを算出する。
【００６５】
また、ｒＣは、以下の方法によって測定することができる。鋼管を切断してプレス等で平板上の板とし、円周方向を長手としてＪＩＳ Ｚ ２２０１の１３Ｂ号試験片を採取し、試験片平行部に標点をマーキングして標点距離並びに試験片平行部の板厚及び板幅を測定する。
【００６６】
試験片に伸び計を取付けて、引張試験機にて１０％の引張歪みを与え、標点距離の変化により試験片長手方向の歪みを測定し、試験後の板幅を測定して、試験前後の板幅の変化により測定した幅方向の歪み変化から、ｒ値の定義に従って、ｒＣを算出する。
【００６７】
また、引張強度「ＴＳ［ＭＰａ］」及び管軸方向のｎ値「ｎ」は、ＴＳ＋３２８５×ｎ＞１０８２の関係を満たすことが必要である。このＴＳ＋３２８５×ｎはハイドロフォーム加工の指標であり、これが１０８２より大きいと、ハイドロフォーム加工性が、極めて良好である。
【００６８】
これは、造管後に再結晶温度以上に加熱し、転位密度が少ない再結晶フェライト組織とすることにより、得られる特性である。
【００６９】
なお、管周方向のｎ値も高い方が好ましいが、鋼管の管周方向のｎ値の測定は、試験片を作製する際の加工歪みの導入が避けられないため困難である。
【００７０】
ＴＳ＋３２８５×ｎ＞１０８２を満たすには、ｎ値を向上させることが重要である。これは、未再結晶でＡｌＮが微細析出した鋼管を再結晶させること、すなわち、４５０〜６００℃の保持時間を１００ｓ以上として再結晶温度未満に加熱し、さらに再結晶温度以上に加熱する製造方法によって達成できる。
【００７１】
なお、引張強度は、鋼管からＪＩＳ Ｚ ２２０１に準拠し、管軸方向を長手として１２号円弧状試験片を採取し、ＪＩＳ Ｚ ２２４１に準拠して測定する。
【００７２】
ｎ値は、ＪＩＳ Ｚ ２２０１に準拠し、管軸方向を長手として１１号試験片を採取して、ＪＩＳ Ｚ ２２４１に準拠して引張引張試験を行い、引張歪み５〜１５％の範囲において応力及び歪みから加工硬化指数を算出する。
【００７３】
降伏比は、降伏強度と引張強度の比であり、ｎ値及びｒ値と同様、成形性の重要な指標である。なお、降伏強度は、応力−歪み曲線において降伏点を示す場合は下降伏点強度とし、降伏点を示さない場合は０．２％耐力とする。
【００７４】
降伏比は、小さいほど成形しやすく、０．８を超えるとハイドロフォーム成形が低下するため、０．８以下を上限とすることが好ましい。降伏比が低いほど、ハイドロフォーム成形性が良いが、通常の鋼成分では降伏比の下限は０．５程度である。
【００７５】
造管後、鋼管を再結晶温度未満に加熱すると、降伏比を０．８以下とすることは困難であったが、鋼管を再結晶温度以上に加熱してミクロ組織を再結晶フェライトとし、降伏比を０．８以下にすることができる。
【００７６】
次に、製造方法について説明する。
【００７７】
製造にあたって、高炉、転炉、電炉等による溶製に引き続き各種の２次精錬を行い、インゴット鋳造あるいは連続鋳造して所定の成分の鋼片を製造する。これらの鋳造インゴット又は鋳造スラブを再加熱して熱間圧延するが、連続鋳造の場合、室温まで冷却することなく熱間圧延するＣＣ−ＤＲなどの製法を組み合わせて製造しても構わない。
【００７８】
熱間圧延時の再加熱温度は特に限定しないが、Ａｌ及びＮを固溶させるために、１１００℃以上とすることが好ましい。再加熱後、熱間圧延し、冷却して巻取る。熱間圧延の終了温度は、ＡｌＮの析出を抑制するため、８３０℃以上とする必要がある。
【００７９】
これは、熱延中に析出するＡｌＮが粗大であり、Ａｌの固溶量が減少してＡｌＮ析出処理時に微細なＡｌＮが析出せず、結晶熱処理後のｒ値が低下するためである。熱間圧延の終了温度は、上限を特に規定しないが、設備上の制限から、通常、１１００℃を超えることはない。
【００８０】
熱延後、冷却して巻取るが、冷却は水冷であることが好ましい。また、巻取り温度が６００℃超では、巻取り中に粗大なＡｌＮが析出し、Ａｌの固溶量が減少するため、冷延鋼板又は鋼管を再結晶温度未満に加熱する際に、微細なＡｌＮが析出しないため、鋼管を再結晶温度以上に加熱した後、ｒ値が１．３よりも低下する。
【００８１】
粗大な析出物の生成を抑制するためには、巻取り温度を５００℃以下とすることが好ましい。巻き取り温度の下限は特に定めるところではないが、固溶Ｃの低減あるいは設備負荷の軽減の観点から３５０℃以上とすることが好ましい。
【００８２】
熱間圧延後は酸洗することが好ましい。さらに冷間圧延を行う。
【００８３】
冷延率は高ｒ値化達成に重要な要素である。冷延率が３０％未満あるいは７５％より大きいとｒ値が低くなるので、３０〜７５％に限定する。好ましくは４５〜６５％である。
【００８４】
冷間圧延後、再結晶させずに、ＡｌＮを微細析出させるため４５０℃以上、再結晶温度未満に加熱する。これには、冷延鋼板を再結晶温度未満に加熱する際に、４５０〜６００℃に１００ｓ以上保持することが必要である。
【００８５】
保持温度が４５０℃未満では、ＡｌＮが析出せず、６００℃超では、ＡｌＮが粗大化するため、再結晶処理後のγファイバーの集積が不十分なり、高ｒ値化は達成できない。
【００８６】
さらに、４５０〜６００℃に保持する時間が１００ｓ未満では、ＡｌＮがほとんど析出しないため、下限を１００ｓとする。上限は規定しないが、３００ｓを超えて保持することは、設備上の制限により難しい。
【００８７】
なお、４５０〜６００℃で保持する際には、温度は一定でも良いが、昇温しても良く、降温しても良い。
【００８８】
また、加熱時に再結晶させるとｒ値及びｎ値を共に向上させることが困難であるため、加熱温度の上限を再結晶温度未満とする。再結晶温度は成分によって変化するが、７００〜７５０℃の範囲である。加熱温度の下限は、ＡｌＮが析出する温度であれば良いため、４５０℃以上とする。
【００８９】
ＡｌＮ析出に重要な４５０〜６００℃の温度域以外での保持時間及び加熱速度は限定しない。また、４５０〜６００℃では一定温度に保持しても良く、加熱しても冷却しても良い。熱処理は、高周波、炉加熱を単独あるいは併用、いずれの場合でも構わない。
【００９０】
ＡｌＮを析出させるために再結晶温度未満に加熱する際には、室温から一定の加熱速度で加熱しても良い。この場合には、４５０℃から６００℃までの昇温時間が１００ｓ以上となるよう、加熱速度を１．５℃／ｓ以下とすることが必要である。下限は、規定しないが生産性を考慮すると０．００１℃／ｓ以上とすることが好ましい。
【００９１】
造管は、鋼板長手方向を管軸方向と一致させて冷間あるいはＡｃ１点以下の温間域で中空状に成形し、電縫、鍛接、レーザ、ＴＩＧ等で溶接して造管する。
【００９２】
造管後の加熱温度は、再結晶温度未満では、ミクロ組織が再結晶フェライトにならず、ｎ値が不十分であるため、再結晶温度以上にすることが必要である。
【００９３】
加熱温度がＡｃ１＋５０℃を超えると、再結晶フェライトから変態したγ量が急激に増加し、γファイバーの集積が弱まってｒ値が低下することから、上限をＡｃ１＋５０℃以下にする必要がある。加熱方法は、高周波、炉加熱どちらでも良く、併用しても構わない。
【００９４】
なお、生産性を向上させるために、冷延鋼板を造管して、ＡｌＮを微細析出させ、再結晶温度以上に加熱しても良い。この際にも、冷延鋼板の加熱と同様に、４５０〜６００℃の保持時間を１００ｓ以上とすることが必要である。また、加熱温度は、造間後の鋼管の加熱と同様に、再結晶温度以上Ａｃ１＋５０℃以下とすることが必要である。
【００９５】
また、室温から４５０℃以上、再結晶温度未満までを１．５℃／ｓ以下で加熱することにより、ＡｌＮを微細に析出させることが可能である。加熱速度の下限は、規定しないが生産性を考慮すると０．００１℃／ｓ以上とすることが好ましい。
【００９６】
さらに、再結晶温度以上Ａｃ１＋５０℃以下まで、１．５℃／ｓ超で加熱することにより、従来方法と比較して短時間で高ｒ値化が達成できる。加熱速度の上限は、規定しないが、通常は１００℃／ｓ以下である。加熱方法は、高周波、炉加熱どちらでも良く、併用しても良い。
【００９７】
また、冷延鋼板を造管して、再結晶温度未満に加熱して冷却し、再結晶温度以上に加熱しても良い。この際にも、再結晶温度未満に加熱する際には、ＡｌＮを微細に析出させるため、４５０〜６００℃の保持時間を１００ｓ以上とすることが必要である。
【００９８】
さらに、冷却した後、再結晶温度以上Ａｃ１＋５０℃以下に加熱することにより、γファイバーが集積した再結晶フェライトが生成し、ｒ値とｎ値が同時に向上する。
【００９９】
ＡｌＮを析出させるために再結晶温度未満に加熱する際には、室温から一定の加熱速度で加熱しても良い。この場合には、４５０℃から６００℃までの昇温時間が１００ｓ以上となるよう、加熱速度を１．５℃／ｓ以下とすることが必要である。下限は、規定しないが生産性を考慮すると０．００１℃／ｓ以上とすることが好ましい。
【０１００】
【実施例】
（実施例）
表１に示す成分の各鋼を溶製して１２５０℃に加熱後、表１に示す仕上げ温度で熱間圧延して巻取り、熱延鋼板とした。熱延仕上げ温度及び巻取り温度は放射温度計によって測定した。
【０１０１】
この熱延板を、酸洗後、表２及び表３に示す冷延率で冷間圧延し、板厚２．３ｍｍの鋼板とした。これらの冷延鋼板より小片を採取し、６００〜７５０℃に加熱して冷却し、ミクロ組織の変化により再結晶温度を求めた。
【０１０２】
また、Ａｃ１はＡｃ１＝７２３−１０．７Ｍｎ−１６．９Ｎｉ＋２９．１Ｓｉ＋１６．９Ｃｒによって計算した。再結晶温度及びＡｃ１を表１に示す。
【０１０３】
さらに、表２に、ＡｌＮ析出処理と示した条件で炉加熱した後、電縫溶接によって外径６３．５ｍｍ、板厚２．３ｍｍの管に造管した。これらの鋼管を、光輝炉によって表２に再結晶熱処理と示した条件で加熱した。
【０１０４】
また、冷延ままの鋼板を電縫溶接によって外径６３．５ｍｍ、板厚２．３ｍｍの管に造管し、表３に、ＡｌＮ析出処理と示した条件で、光輝炉によって加熱した後、表３に再結晶熱処理と示した条件で、高周波加熱により加熱した。温度測定は鋼管の表面に取付けた熱電対によって行った。
【０１０５】
【表１】
鋼管からＪＩＳ Ｚ ２２０１に準拠し、管軸方向を長手として１２号円弧状試験片を採取し、試験片平行部に標点をマーキングし、標点距離を測定した。ＪＩＳ Ｚ ２２４１に準拠して引張引張試験を行い、得られた降伏強度及び引張強度から降伏比を算出した。
【０１０７】
なお、降伏強度は、応力−歪み曲線において降伏点を示す場合は下降伏点強度とし、降伏点を示さない場合は０．２％耐力とした。
【０１０８】
また、標点の中央に幅方向に歪みゲージを貼った後、伸び計を取付けて引張試験機にて１０％の引張歪みを与え、引張歪みを与える前後の標点距離及び歪みゲージにより測定した幅方向の歪みから、ｒ値の定義に従ってｒＬを算出した。
【０１０９】
また、鋼管を切断してプレス等で平板上の板とし、円周方向を長手としてＪＩＳ Ｚ ２２０１の１３Ｂ号試験片を採取し、試験片平行部に標点をマーキングして標点距離並びに試験片平行部の板厚及び板幅を測定した。
【０１１０】
試験片に伸び計を取付けて、引張試験機にて１０％の引張歪みを与え、引張歪みを与える前後の標点距離及び板幅の変化により、ｒ値の定義に従ってｒＣを算出した。
【０１１１】
ｎ値は、ＪＩＳ Ｚ ２２０１に準拠し、管軸方向を長手として１１号試験片を採取して、ＪＩＳ Ｚ ２２４１に準拠して引張引張試験を行い、引張歪みが５〜１５％の範囲において、応力と歪みから加工硬化指数として計算した。
【０１１２】
得られた鋼管を軸押しと内圧を制御して張り出し成形し、鋼管の破裂であるバースト及び座屈の有無を確認した。なお、ハイドロフォーム成形試験は、鋼管に１０ｍｍφのスクライブドサークルを転写し、金型に鋼管を設置して軸方向に押し込み、かつ鋼管内に静水圧を負荷して行った。
【０１１３】
内圧と軸押し量を制御して、バースト及び座屈の無い試験片の成形後の最大の外径を測定し、その値を成形前の鋼管の外径で除した値を最大拡管率とした。
【０１１４】
さらに、外径が最大である部位において、軸方向の歪み及び円周方向の歪みをスクライブドサークルによって測定し、絶対値が最大である軸方向の歪みεΦ及び円周方向の歪みεθを求めた。
【０１１５】
この２つの歪の比ρ＝εΦ／εθが、ρ＝−０．５となった鋼管の最大拡管率Ｒｅをハイドロフォームの成形性指標とし、表２及び表３に示した。
【０１１６】
【表２】
【表３】
表２及び表３より明らかなとおり、製造Ｎｏ．１〜１７及びＮｏ．３４〜５０の本発明例では、いずれも良好なｒ値を有して、ＴＳ＋３２８５×ｎが高く、最大拡管率が良好である。
【０１１９】
一方、製造Ｎｏ．１８及び５１は冷延率が低く、製造Ｎｏ．２１及び６１はＡｌＮ析出処理の加熱温度が低く、製造Ｎｏ．２２及び６２は再結晶処理の加熱温度が高いため、ｒＬ及びｒＣが低下している。
【０１２０】
また、製造Ｎｏ．２３及び５３は冷延率が低く、ＡｌＮ析出処理の保持時間が短いため、製造Ｎｏ．２７及び６３はＡｌＮ析出処理の保持時間が短く、加熱温度が低く、製造Ｎｏ．２６及び６４はＡｌＮ析出処理の保持時間が短く、再結晶処理の加熱温度が高いため、ｒＬ及びｒＣが低下している。
【０１２１】
さらに、製造Ｎｏ．３１及び５６は、冷延率が低く、ＡｌＮ析出処理の保持時間が短く、再結晶処理の加熱温度が高いため、ｒＬ及びｒＣが低下している。
【０１２２】
また、製造Ｎｏ．５４は冷延率が高く、ＡｌＮ析出処理の加熱温度が低いため、製造Ｎｏ．６５は、ＡｌＮ析出処理の加熱温度が低く、再結晶処理の加熱温度が高いため、製造Ｎｏ．２０及び６０は、ＡｌＮ析出処理の保持時間が短いため、ｒＬ及びｒＣが低下している。
【０１２３】
製造Ｎｏ．３０は、冷延率が低く、ＡｌＮ析出処理の保持時間が短く、加熱温度が高いため、製造Ｎｏ．５７は冷延率が高く、ＡｌＮ析出処理の保持時間が短く、加熱温度が低いため、製造Ｎｏ．３３は、冷延率が高く、ＡｌＮ析出処理の保持時間が短く、加熱温度が低く、再結晶処理の加熱温度が高いため、ｒＬ及びｒＣが低下している。
【０１２４】
製造Ｎｏ．１９及び５２は、冷延率が高いため、製造Ｎｏ．３２は、冷延率が高く、ＡｌＮ析出処理の加熱温度が低く、再結晶処理の加熱温度が高いため、ｒＬが低下している。
【０１２５】
製造Ｎｏ．２４は冷延率が低く、ＡｌＮ析出処理の加熱温度が高いため、ｒＣが低下している。製造Ｎｏ．２８は、ＡｌＮ析出処理の加熱温度が高く、再結晶処理の加熱温度が低いため、ＴＳ＋３２８５×ｎが低下している。
【０１２６】
製造Ｎｏ．２９及び６６は、ＡｌＮ析出処理の保持時間が短く、加熱温度が低く、再結晶処理の加熱温度が低いため、製造Ｎｏ．２５は、冷延率が高く、再結晶処理の加熱温度が低いため、製造Ｎｏ．５５は、冷延率が低く、再結晶処理の加熱温度が低いため、製造Ｎｏ．５８は、冷延率が高く、ＡｌＮ析出処理の加熱温度が低く、再結晶処理の加熱温度が低いため、製造Ｎｏ．５９は、冷延率が低く、ＡｌＮ析出処理の保持時間が短く、加熱温度が低く、再結晶処理の加熱温度が低いため、ｒＬ、ｒＣ及びＴＳ＋３２８５×ｎが低下している。
【０１２７】
【発明の効果】
本発明によれば、ハイドロフォーム成形によって製造する構造用部品、配管等に好適な、ｒ値及びｎ値をともに従来にないレベルまで向上させた、加工性に優れた高強度鋼管を提供することが可能になり、産業上の貢献が極めて大きい。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a steel pipe excellent in workability, suitable for structural parts, piping, and the like, formed by drawing, bending, hydroforming, and the like, and a method for producing the same.
[0002]
[Prior art]
Recently, there has been proposed a method of manufacturing a member that has been manufactured by press forming and joining a steel plate by integrally forming a steel pipe. For example, Japanese Patent Application Laid-Open No. H10-175026 discloses a technique for manufacturing a part having a complicated shape by hydroforming a steel pipe for the purpose of reducing the weight and manufacturing cost of an automobile by omitting steps and reducing the number of parts.
[0003]
The material factors for improving the hydroform moldability include a work hardening index n value and an r value in the pipe axis direction (hereinafter, r) which is an index of plastic anisotropy. L ) Is reported in Plasticity and Processing, Vol. 41, No. 478 (2000), pp. 1075-1081.
[0004]
R L And a production method by reducing diameter rolling, which is excellent in hydroform workability, is disclosed in JP-A-2001-214218, JP-A-2001-348463, JP-A-2001-348647, and JP-A-2001-348648. JP, JP-A-2001-355034, JP-A-2001-355047, JP-A-2002-20841, JP-A-2002-97549, JP-A-2002-115780, JP-A-2002-115029, It is disclosed in JP-A-2002-115012.
[0005]
However, in diameter reduction rolling with a high degree of shrinkage, an extremely high r of 2.0 or more. L However, it was difficult to increase the n value.
[0006]
JP-A-2002-115780 discloses a method of forming a cold-rolled steel sheet having a high r-value in a rolling direction and a width direction orthogonal to the rolling direction and heat-treating the steel pipe. However, in this method, when a cold-rolled steel sheet having a high r value is formed, cold working strain is introduced to reduce the n value, and when the steel pipe is heat-treated under the condition of maintaining a high r value, the n value is sufficiently increased. And it was difficult to simultaneously increase the r value and the n value.
[0007]
[Problems to be solved by the invention]
The present invention provides a high-strength steel pipe excellent in workability, in which both the r value and the n value are improved to unprecedented levels, which are suitable for structural parts, pipes and the like manufactured by hydroform molding, and a method for manufacturing the same. Is what you do.
[0008]
[Means for Solving the Problems]
The present invention has intensively studied to solve the above-mentioned problems, and the material factors for improving the hydroform moldability are n value and r value. L Not only the r value in the pipe circumferential direction (hereinafter, r C ) Is also effective.
[0009]
Further, AlN is precipitated without recrystallizing the steel sheet into which the cold working strain has been introduced or the steel pipe after pipe forming, and then recrystallized, whereby r L , R C And a high-strength steel pipe in which the n value is improved at the same time.
[0010]
That is, the gist of the present invention is as follows.
[0011]
(1) In mass%, C: 0.03 to 0.5%, Si: 0.001 to 3.0%, Mn: 0.01 to 3.0%, P: 0.001 to 0.15% , S: 0.05% or less, Al: 0.008 to 0.3%, N: 0.001 to 0.03%, the balance being Fe and unavoidable impurities, and a tensile strength of 350 MPa or more. The r value in both the tube axis direction and the circumferential direction is 1.3 or more, and the n value “n” in the tube axis direction and the tensile strength “TS [MPa]” are in the relationship of TS + 3285 × n> 1082. High-strength steel pipe with excellent workability, characterized by satisfying.
[0012]
(2) The high-strength steel pipe excellent in workability according to (1), wherein the yield ratio in the pipe axis direction and the circumferential direction is 0.8 or less.
[0013]
(3) Excellent workability according to (1) or (2), wherein one or two of Zr and Mg are contained in a total of 0.0001 to 0.5% by mass%. High strength steel pipe.
[0014]
(4) Any one of the above (1) to (3), wherein one or more of Ti, Nb and V are contained in a total of 0.001 to 0.2% by mass%. High-strength steel pipe with excellent workability as described in the item.
[0015]
(5) The above-mentioned (1), wherein one or more of Sn, Cr, Cu, Ni, Co, W and Mo are contained in a mass% of 0.001 to 2.5% in total. A high-strength steel pipe excellent in workability according to any one of (1) to (4).
[0016]
(6) The high-strength steel pipe excellent in workability according to any one of the above (1) to (5), characterized by containing 0.0001 to 0.01% by mass of Ca.
[0017]
(7) The high-strength steel pipe excellent in workability according to any one of (1) to (6), wherein B is contained in an amount of 0.0001 to 0.01% by mass%.
[0018]
(8) The slab is heated to complete hot rolling at 830 ° C. or higher, cooled and wound at 600 ° C. or lower, and after cold rolling at a cold rolling rate of 30% or more and less than 75%, 450 ° C. to 600 ° C. The temperature is maintained at 450 ° C. or higher and lower than the recrystallization temperature by setting the holding time in the range of 100 ° C. or higher to 100 s or higher, and then cooled to form a tube. c1 The method for producing a steel pipe excellent in workability according to any one of (1) to (7), wherein the steel pipe is heated to + 50 ° C. or lower.
[0019]
(9) The slab is heated to complete hot rolling at 830 ° C. or higher, cooled and wound up at 600 ° C. or lower, cold rolled to 30% to less than 75%, pipe-formed, and 450 ° C. to 600 ° C. The retention time in the range of 100 ° C. is 100 s or more, and c1 The method for producing a steel pipe excellent in workability according to any one of (1) to (7), wherein the steel pipe is heated to + 50 ° C. or lower.
[0020]
(10) Heating the slab to complete hot rolling at 830 ° C. or higher, cooling and winding at 600 ° C. or lower, cold rolling of 30% to less than 75%, pipe forming, and room temperature to 450 ° C. Heat at 1.5 ° C./s or less to a temperature lower than the recrystallization temperature. c1 The method for producing a steel pipe excellent in workability according to any one of the above (1) to (7), wherein the steel pipe is heated to + 50 ° C or less at a rate of more than 1.5 ° C / s.
[0021]
(11) The slab is heated to complete hot rolling at 830 ° C. or higher, cooled and wound up at 600 ° C. or lower, cold rolled to 30% to less than 75%, piped, and 450 to 600 ° C. When the holding time in the range of 100 ° C. is 100 s or more, the mixture is heated to 450 ° C. or more and lower than the recrystallization temperature and cooled. c1 The method for producing a steel pipe excellent in workability according to any one of (1) to (7), wherein the steel pipe is heated to + 50 ° C. or lower.
[0022]
(12) The steel pipe having excellent workability according to the above (8) or (11), wherein the heating is performed at a temperature of 450 ° C. or higher and lower than the recrystallization temperature, at a rate of 1.5 ° C./s or lower. Manufacturing method.
[0023]
BEST MODE FOR CARRYING OUT THE INVENTION
The present inventor has conducted detailed studies on material factors that improve hydroform moldability by finite element analysis. As a result, the n value and r L Not only r C Is also effective in improving the hydroform formability, and in order to develop a steel pipe with excellent hydroform formability, the effects of the components and the manufacturing method on the n-value and r-value of the steel pipe are described in detail. Investigated.
[0024]
First, a method of forming and heating a cold-rolled steel sheet having excellent r-value and n-value was attempted. After cold rolling, in order to improve the r-value and the n-value, the re-crystallized cold-rolled steel sheet was heated at 5 to 50 ° C / h at a recrystallization temperature or higher to produce a tube. The recrystallization temperature varied depending on the components, but was in the range of 700 to 750 ° C.
[0025]
With the rolling direction and the sheet width direction of the steel sheet taken as the length, a JIS Z 2201 No. 13B test piece was sampled, the sheet thickness and the sheet width were measured with a micrometer, and a tensile test was performed in accordance with JIS Z 2241.
[0026]
The r-values in the rolling direction and the plate width direction were calculated according to the definition of the r-value by introducing a 10% tensile strain and using the width of the test piece before and after the tensile strain was introduced and the gauge length before and after the tensile strain was introduced. Further, the n value in the rolling direction was calculated as a work hardening index from tensile stress and strain in the range of tensile strain of 5 to 15%.
[0027]
As a result, it was confirmed that the r value in the rolling direction and the sheet width direction was 1.3 or more, and the n value in the rolling direction was 0.2 or more.
[0028]
These steel pipes were heated at a temperature of less than the recrystallization temperature at 1 to 5 ° C./s, and a No. 12 arc-shaped test piece was taken from the steel pipe in accordance with JIS Z2201, with the pipe axis direction as a longitudinal direction, and a mark was placed on the parallel part of the test piece. And the gauge length was measured.
[0029]
After attaching a strain gauge in the width direction at the center of the gauge, attach an extensometer and give a 10% tensile strain with a tensile tester. From the change in gauge length and the change in strain in the width direction measured by the strain gauge, r L Was calculated.
[0030]
In addition, a steel pipe is cut into a plate on a flat plate with a press or the like, a test piece of JIS Z 2201 No. 13B is taken with the circumferential direction as a longitudinal direction, and a mark is marked on a parallel portion of the test piece, and a mark length and a test are performed. The plate thickness and plate width of the partially parallel portion were measured.
[0031]
An extensometer was attached to the test piece, a 10% tensile strain was given by a tensile tester, and r was defined according to the definition of the r value according to the plate width and gauge length of the test piece before and after the tensile strain was introduced. C Was calculated.
[0032]
In addition, the n value is based on JIS Z 2201, and a No. 11 test piece is taken with the tube axis direction as a longitudinal direction, and a tensile test is performed in accordance with JIS Z 2241. Was calculated as a work hardening index from stress and strain.
[0033]
As a result, it was found that the r-value of the steel pipe heated to less than the recrystallization temperature at 1 to 5 ° C./s after the pipe making was equivalent to that of the cold-rolled steel sheet before the pipe making, but the n-value was reduced. .
[0034]
On the other hand, when the steel pipe after pipe forming is heated at a recrystallization temperature of 1 to 5 ° C./s or higher, the r value and the n value are measured, and when compared with the cold-rolled steel sheet before pipe forming, the n values are equivalent. , R values were found to decrease.
[0035]
That is, in a steel pipe formed from a recrystallized cold-rolled steel sheet, the n value decreases when the cold working strain is recovered without recrystallization, and the r value decreases when recrystallized. It was extremely difficult to improve both values.
[0036]
Therefore, the present inventor, after forming a tube without recrystallizing a cold-rolled steel sheet, by heating the recrystallization temperature or higher to make the microstructure a recrystallized ferrite, thereby simultaneously improving the r value and the n value. Oriented manufacturing method.
[0037]
First, after cold rolling, a steel sheet was formed into a tube and heated at a temperature of 1 to 5 ° C./s or more at a recrystallization temperature or higher. As a result, the n value was improved, but the r value was lowered. In order to clarify the cause, small pieces were collected from the center of the thickness of the cold-rolled steel sheet and the steel pipe, and the texture was examined by an X-ray diffraction method.
[0038]
As a result, the texture of the steel pipe was found to be weaker in the accumulation of the recrystallized texture (hereinafter referred to as γ-fiber) in which the (111) [1-10] orientation was gathered than in the cold-rolled steel sheet.
[0039]
This was attributed to the fact that AlN was not finely precipitated at the time of heating, and a production method of finely depositing AlN before recrystallizing the steel pipe was examined.
[0040]
That is, after cold rolling, AlN is finely precipitated without recrystallization to form a tube and recrystallized, or after cold rolling and tube forming, AlN is finely precipitated without recrystallization and recrystallized, This is a manufacturing method for crystallizing.
[0041]
As a result of studying heating conditions for finely depositing AlN without recrystallization, the present inventors have found that it is necessary to maintain the temperature in a temperature range of 450 to 600 ° C for 100 s or more.
[0042]
The present inventor further studied in detail the change of the r value depending on the cold rolling rate of the cold rolling. The cold rolling ratio is a percentage obtained by dividing the difference between the sheet thickness after cold rolling and the sheet thickness before cold rolling by the sheet thickness before cold rolling.
[0043]
Conventionally, it has been effective to increase the cold-rolling rate to 70% or more to improve the r-value in steels with a low added amount of C and the like. However, in the steel of the present invention, the cold-rolling rate is low. It has been found that the r value is improved by doing so.
[0044]
This is because steel with a large amount of the second phase, when the cold rolling reduction is increased, strain accumulates around the second phase, and thereafter, when heated above the recrystallization temperature, γ fibers are not formed and the r value does not improve. That's why.
[0045]
The present inventor has further investigated in detail the effects of the manufacturing conditions on the r value and the n value of the steel plate and the steel tube based on such knowledge, and found that both the r value and the n value are high, and that the steel tube has excellent workability. And a method for producing the same.
[0046]
Hereinafter, the present invention will be described in detail.
[0047]
C is the most effective element for increasing the strength. In order to exhibit the effect, addition of 0.03% or more is required. However, if it exceeds 0.5%, the r value and the weldability decrease. Therefore, the C content is set in the range of 0.03 to 0.5%. In consideration of the balance between the strength, the r value, and the weldability, 0.05 to 0.2% is a preferable range, and 0.08 to 0.15% is an optimum range.
[0048]
Si is an element effective for increasing the strength, and is also effective for reducing or miniaturizing carbides in steel and improving the r value. This effect is insufficient when Si is less than 0.001%, and when added over 3.0%, plating wettability and workability are deteriorated. Therefore, the Si content is set in the range of 0.001 to 3.0%.
[0049]
Mn is an element effective for increasing the strength, and 0.01% or more of Mn is required to exhibit its effect. However, an excessive addition exceeding 3.0% deteriorates the r value, so the upper limit is made 3.0%.
[0050]
P is an element effective for increasing the strength, and in order to exhibit this effect, it is necessary to add 0.001% or more. However, if added over 0.15%, the weldability and the resistance to secondary working brittleness deteriorate, so the upper limit is made 0.15% or less. Preferably, it is at most 0.03%.
[0051]
S is an impurity element, and has an upper limit of 0.05% in order to prevent hot cracking, but preferably 0.01% or less. The lower the S content, the better, but it is difficult to reduce the S content to less than 0.0001% with the current technology.
[0052]
Al is a deoxidizing element and an extremely important element that forms fine AlN and / or clusters with N when a cold-rolled steel sheet or a steel pipe is heated below a recrystallization temperature. In order to exhibit this effect, 0.008% or more must be added. On the other hand, an excessive addition exceeding 0.3% causes a surface defect of the steel sheet or a decrease in weldability, so the upper limit is made 0.3%.
[0053]
In order to sufficiently exhibit the effect of heating the steel pipe to a temperature higher than the recrystallization temperature and increasing the r-value with the texture as a γ-fiber, the Al content is preferably 0.01% or more, and the weldability is extremely good. For this purpose, the content is preferably set to 0.1% or less.
[0054]
N is also an extremely important element in the present invention like Al, and the r value is improved by adding 0.001% or more. On the other hand, if it exceeds 0.03%, the aging property is deteriorated and the strength is reduced. Therefore, the N content is set in the range of 0.001 to 0.03%. A preferable range of the N amount at which both the r value and the strength are extremely good is 0.002 to 0.007%.
[0055]
Further, if necessary, one or more of Zr, Mg, Ti, Nb, V, Sn, Cr, Cu, Ni, Co, W, Mo, Ca, and B may be contained.
[0056]
Zr and Mg are deoxidizing elements, and it is effective to add one or two of them in a total amount of 0.0001% or more. However, excessive addition of more than 0.5% results in coarse precipitation of oxides. Deterioration of ductility. Therefore, it is preferable to add one or two of Zr and Mg in a total amount of 0.0001 to 0.5%.
[0057]
Since Ti, Nb, and V generate carbides, nitrides, and carbonitrides and contribute to higher strength and improved workability, it is preferable to add one or more of them in a total amount of 0.001% or more. On the other hand, if it is added in excess of 0.2%, coarse and large amounts of carbides, nitrides or carbonitrides are formed at the ferrite grain boundaries, and the ductility is reduced. Therefore, the total content of Ti, Nb, and V is set to 0.001 to 0.2% by one kind or two or more kinds.
[0058]
Sn, Cr, Cu, Ni, Co, W, and Mo are effective as reinforcing elements, and it is preferable to add one or more of them in a total amount of 0.001% or more. However, an excessive addition exceeding 2.5% causes a problem of cost increase or ductility reduction. Therefore, it is preferable to set the upper limit to 2.5% or less.
[0059]
Ca is an element effective for controlling inclusions such as sulfides or deoxidizing, but if added excessively, causes hot embrittlement, so the range is preferably 0.0001 to 0.01%.
[0060]
B is an element effective for improving the r value and improving the brittleness resistance to secondary workability, but its effect is small when it is less than 0.0001%, and when it exceeds 0.01%, the r value is greatly reduced. Let it. Therefore, it is preferable to add 0.0001 to 0.01%.
[0061]
R value of the steel pipe obtained by the present invention in the pipe axis direction and the circumferential direction, that is, r L And r c Are 1.3 or more, and exhibit excellent hydroform moldability by a combination of high n values.
[0062]
The higher the r value, the better, so no upper limit is specified, but it is difficult for the current technology to exceed 3.
[0063]
r L Can be measured by the following method. In accordance with JIS Z 2201, a No. 12 arc-shaped test piece is sampled from a steel pipe with the pipe axis direction as a longitudinal direction, a mark is marked on a parallel part of the test piece, and a gauge distance is measured.
[0064]
After attaching a strain gauge in the width direction at the center of the gauge, attach an extensometer to give 10% tensile strain with a tensile tester, and measure the strain in the longitudinal direction of the test piece and the strain gauge measured by the change in gauge length. From the change in strain in the width direction measured by L Is calculated.
[0065]
Also, r C Can be measured by the following method. A steel pipe is cut into a plate on a flat plate with a press or the like. A 13B test piece of JIS Z 2201 is sampled with the circumferential direction as a longitudinal direction, and a mark is marked on a parallel portion of the test piece to obtain a gauge length and a test piece parallel. Measure the thickness and width of the part.
[0066]
Attach an extensometer to the test piece, give 10% tensile strain with a tensile tester, measure the strain in the longitudinal direction of the test piece by changing the gauge length, measure the plate width after the test, and before and after the test From the change in strain in the width direction measured by the change in the plate width of C Is calculated.
[0067]
Further, the tensile strength “TS [MPa]” and the n value “n” in the tube axis direction need to satisfy the relationship of TS + 3285 × n> 1082. This TS + 3285 × n is an index for hydroforming, and when it is larger than 1082, hydroforming properties are extremely good.
[0068]
This is a characteristic that can be obtained by heating the tube to a temperature higher than the recrystallization temperature after forming the tube to form a recrystallized ferrite structure having a low dislocation density.
[0069]
It is preferable that the n value in the circumferential direction of the steel pipe is also high, but it is difficult to measure the n value in the circumferential direction of the steel pipe because it is unavoidable to introduce processing strain when producing a test piece.
[0070]
In order to satisfy TS + 3285 × n> 1082, it is important to improve the n value. This is to recrystallize a steel pipe in which AlN is finely precipitated in a non-recrystallized state, that is, a manufacturing method in which the holding time at 450 to 600 ° C. is set to 100 s or more, heating to a temperature lower than the recrystallization temperature, and further heating to a temperature higher than the recrystallization temperature. Can be achieved by:
[0071]
The tensile strength is measured in accordance with JIS Z 2241 by extracting a No. 12 arc-shaped test piece from a steel pipe in accordance with JIS Z 2201 with the pipe axis direction as a longitudinal direction.
[0072]
The n value is based on JIS Z 2201 and a No. 11 test piece is taken with the pipe axis direction as a longitudinal direction, and a tensile test is performed in accordance with JIS Z 2241. The work hardening index is calculated from the strain.
[0073]
The yield ratio is a ratio between the yield strength and the tensile strength, and is an important index of the formability, like the n value and the r value. The yield strength is defined as the falling yield point strength when the yield point is indicated in the stress-strain curve, and when the yield point is not indicated, the yield strength is 0.2%.
[0074]
If the yield ratio is smaller, the molding is easier, and if it exceeds 0.8, the hydroform molding is reduced. Therefore, the upper limit is preferably 0.8 or less. The lower the yield ratio is, the better the hydroform formability is, but the lower limit of the yield ratio is about 0.5 for ordinary steel components.
[0075]
After pipe forming, if the steel pipe was heated below the recrystallization temperature, it was difficult to reduce the yield ratio to 0.8 or less, but the steel pipe was heated above the recrystallization temperature to change the microstructure to recrystallized ferrite and yield. The ratio can be less than 0.8.
[0076]
Next, a manufacturing method will be described.
[0077]
In the production, various secondary refining is performed following smelting using a blast furnace, a converter, an electric furnace, or the like, and ingot casting or continuous casting is performed to produce a steel slab having a predetermined component. These cast ingots or cast slabs are reheated and hot-rolled. In the case of continuous casting, the cast ingots or cast slabs may be manufactured by combining manufacturing methods such as CC-DR in which hot rolling is performed without cooling to room temperature.
[0078]
The reheating temperature during the hot rolling is not particularly limited, but is preferably 1100 ° C. or higher in order to form a solid solution of Al and N. After reheating, hot rolling, cooling and winding. The end temperature of the hot rolling needs to be 830 ° C. or higher in order to suppress the precipitation of AlN.
[0079]
This is because AlN precipitated during hot rolling is coarse, the solid solution amount of Al decreases, and fine AlN does not precipitate during the AlN precipitation treatment, and the r value after the crystal heat treatment decreases. Although the upper limit temperature of the hot rolling is not particularly defined, it does not usually exceed 1100 ° C. due to facility restrictions.
[0080]
After hot rolling, it is cooled and wound up, but preferably cooled by water. Further, if the winding temperature is higher than 600 ° C., coarse AlN precipitates during winding and the amount of solid solution of Al decreases, so when the cold-rolled steel sheet or the steel pipe is heated to a temperature lower than the recrystallization temperature, fine Since AlN does not precipitate, the r-value drops below 1.3 after the steel pipe is heated above the recrystallization temperature.
[0081]
In order to suppress the formation of coarse precipitates, the winding temperature is preferably set to 500 ° C. or lower. Although the lower limit of the winding temperature is not particularly specified, it is preferably 350 ° C. or higher from the viewpoint of reducing solid solution C or reducing the facility load.
[0082]
After hot rolling, it is preferable to perform pickling. Further, cold rolling is performed.
[0083]
The cold rolling reduction is an important factor for achieving a high r-value. If the cold rolling reduction is less than 30% or more than 75%, the r-value will be low, so it is limited to 30 to 75%. Preferably it is 45 to 65%.
[0084]
After cold rolling, heating is performed at 450 ° C. or higher and lower than the recrystallization temperature in order to finely precipitate AlN without recrystallization. For this purpose, when the cold-rolled steel sheet is heated to a temperature lower than the recrystallization temperature, it is necessary to maintain the temperature at 450 to 600 ° C. for 100 seconds or more.
[0085]
If the holding temperature is lower than 450 ° C., AlN does not precipitate, and if it is higher than 600 ° C., AlN coarsens, so that the accumulation of γ-fibers after the recrystallization treatment becomes insufficient and a high r-value cannot be achieved.
[0086]
Further, if the time of holding at 450 to 600 ° C. is less than 100 s, AlN hardly precipitates, so the lower limit is set to 100 s. The upper limit is not specified, but it is difficult to keep it longer than 300 s due to facility restrictions.
[0087]
When the temperature is maintained at 450 to 600 ° C., the temperature may be constant, but the temperature may be raised or lowered.
[0088]
In addition, when recrystallization is performed during heating, it is difficult to improve both the r value and the n value. Therefore, the upper limit of the heating temperature is set to be lower than the recrystallization temperature. The recrystallization temperature varies depending on the components, but is in the range of 700 to 750 ° C. The lower limit of the heating temperature is set to 450 ° C. or higher because it is sufficient that AlN is deposited.
[0089]
The holding time and heating rate in a temperature range other than the temperature range of 450 to 600 ° C. important for AlN precipitation are not limited. Further, at a temperature of 450 to 600 ° C., the temperature may be maintained at a constant value, and the material may be heated or cooled. In the heat treatment, high frequency heating and furnace heating may be used alone or in combination.
[0090]
When heating below the recrystallization temperature to precipitate AlN, heating may be performed at a constant heating rate from room temperature. In this case, it is necessary to set the heating rate to 1.5 ° C./s or less so that the temperature rise time from 450 ° C. to 600 ° C. becomes 100 s or more. The lower limit is not specified, but is preferably 0.001 ° C./s or more in consideration of productivity.
[0091]
Pipe forming is performed cold or A by setting the longitudinal direction of the steel sheet to the pipe axis direction. c1 The pipe is formed into a hollow shape in the warm zone below the point, and welded by electric resistance welding, forge welding, laser, TIG or the like to form a pipe.
[0092]
If the heating temperature after pipe formation is lower than the recrystallization temperature, the microstructure does not become recrystallized ferrite, and the n value is insufficient.
[0093]
Heating temperature is A c1 When the temperature exceeds + 50 ° C., the amount of γ transformed from the recrystallized ferrite rapidly increases, the accumulation of γ fibers weakens, and the r value decreases. c1 It is necessary to be lower than + 50 ° C. The heating method may be either high frequency heating or furnace heating, and may be used in combination.
[0094]
In addition, in order to improve productivity, a cold rolled steel sheet may be formed into a tube, AlN may be finely precipitated, and heated to a recrystallization temperature or higher. At this time, similarly to the heating of the cold-rolled steel sheet, the holding time at 450 to 600 ° C. needs to be 100 s or more. Further, the heating temperature is equal to or higher than the recrystallization temperature in the same manner as the heating of the steel pipe after the sintering. c1 It is necessary to keep the temperature at + 50 ° C. or lower.
[0095]
By heating from room temperature to 450 ° C. or higher and lower than the recrystallization temperature at 1.5 ° C./s or lower, AlN can be finely precipitated. The lower limit of the heating rate is not specified, but is preferably 0.001 ° C./s or more in consideration of productivity.
[0096]
In addition, the recrystallization temperature c1 By heating to + 50 ° C. or lower at a rate exceeding 1.5 ° C./s, it is possible to achieve a high r-value in a short time as compared with the conventional method. The upper limit of the heating rate is not specified, but is usually 100 ° C./s or less. The heating method may be either high frequency heating or furnace heating, or may be used in combination.
[0097]
Alternatively, a cold-rolled steel sheet may be formed into a tube, heated to a temperature lower than the recrystallization temperature, cooled, and heated to a temperature higher than the recrystallization temperature. Also in this case, when heating to a temperature lower than the recrystallization temperature, the retention time at 450 to 600 ° C. needs to be 100 s or more to precipitate AlN finely.
[0098]
Further, after cooling, A c1 By heating to + 50 ° C. or lower, recrystallized ferrite in which γ fibers are accumulated is generated, and the r value and the n value are simultaneously improved.
[0099]
When heating below the recrystallization temperature to precipitate AlN, heating may be performed at a constant heating rate from room temperature. In this case, it is necessary to set the heating rate to 1.5 ° C./s or less so that the temperature rise time from 450 ° C. to 600 ° C. becomes 100 s or more. The lower limit is not specified, but is preferably 0.001 ° C./s or more in consideration of productivity.
[0100]
【Example】
(Example)
Each steel having the components shown in Table 1 was melted and heated to 1250 ° C., and then hot-rolled and wound at the finishing temperature shown in Table 1 to obtain a hot-rolled steel sheet. The hot rolling finishing temperature and the winding temperature were measured by a radiation thermometer.
[0101]
After pickling, the hot-rolled sheet was cold-rolled at a cold-rolling rate shown in Tables 2 and 3 to obtain a steel sheet having a thickness of 2.3 mm. Small pieces were collected from these cold rolled steel sheets, heated to 600 to 750 ° C. and cooled, and the recrystallization temperature was determined from the change in microstructure.
[0102]
Also, A c1 Is A c1 = 723-10.7 Mn-16.9 Ni + 29.1 Si + 16.9 Cr. Recrystallization temperature and A c1 Are shown in Table 1.
[0103]
Further, after heating the furnace under the conditions indicated as AlN precipitation treatment in Table 2, pipes having an outer diameter of 63.5 mm and a plate thickness of 2.3 mm were formed by electric resistance welding. These steel tubes were heated in a bright furnace under the conditions shown in Table 2 as recrystallization heat treatment.
[0104]
Further, a cold-rolled steel plate was formed into a tube having an outer diameter of 63.5 mm and a plate thickness of 2.3 mm by electric resistance welding, and heated in a bright furnace under the conditions indicated in Table 3 as AlN precipitation treatment. Heating was performed by high frequency heating under the conditions shown in Table 3 as recrystallization heat treatment. The temperature was measured with a thermocouple attached to the surface of the steel pipe.
[0105]
[Table 1]
In accordance with JIS Z 2201, a No. 12 arc-shaped test piece was taken from a steel pipe with the pipe axis direction as a longitudinal direction, a mark was marked on a parallel part of the test piece, and a gauge length was measured. A tensile test was performed in accordance with JIS Z 2241, and the yield ratio was calculated from the obtained yield strength and tensile strength.
[0107]
The yield strength was defined as the falling yield point strength when the yield point was shown in the stress-strain curve, and was 0.2% proof stress when not showing the yield point.
[0108]
Also, after a strain gauge was attached in the width direction at the center of the gauge, a 10% tensile strain was applied by a tensile tester by attaching an extensometer, and the gauge distance before and after the tensile strain was applied and the strain gauge was measured. From the distortion in the width direction, r L Was calculated.
[0109]
In addition, a steel pipe is cut into a plate on a flat plate with a press or the like, a test piece of JIS Z 2201 No. 13B is taken with the circumferential direction as a longitudinal direction, and a mark is marked on a parallel portion of the test piece, and a mark length and a test are performed. The plate thickness and plate width of the partially parallel portion were measured.
[0110]
An extensometer is attached to the test piece, 10% tensile strain is given by a tensile tester, and a change in gauge length and plate width before and after the tensile strain is applied, and r is defined according to the definition of r value. C Was calculated.
[0111]
The n value is based on JIS Z 2201 and a No. 11 test piece is taken with the pipe axis direction as a longitudinal direction, and a tensile tensile test is performed according to JIS Z 2241. When the tensile strain is in the range of 5 to 15%, The work hardening index was calculated from the stress and strain.
[0112]
The obtained steel pipe was stretched and formed by controlling the axial pressing and the internal pressure, and the presence or absence of burst and buckling, which are bursts of the steel pipe, was confirmed. In the hydroform molding test, a scribed circle having a diameter of 10 mm was transferred to a steel pipe, the steel pipe was placed in a metal mold and pushed in the axial direction, and a hydrostatic pressure was applied in the steel pipe.
[0113]
By controlling the internal pressure and the amount of axial pressing, the maximum outer diameter of the test piece without burst and buckling was measured, and the value obtained by dividing the value by the outer diameter of the steel pipe before forming was taken as the maximum expansion ratio. .
[0114]
Further, at the portion where the outer diameter is the largest, the axial strain and the circumferential strain are measured by a scribed circle, and the axial strain ε having the largest absolute value is obtained. Φ And circumferential strain ε θ I asked.
[0115]
Ratio of these two strains ρ = ε Φ / Ε θ Is the maximum expansion ratio R of the steel pipe with ρ = -0.5 e Is the index of the moldability of the hydroform, and is shown in Tables 2 and 3.
[0116]
[Table 2]
[Table 3]
As is clear from Tables 2 and 3, Production No. Nos. 1 to 17 and Nos. In the examples of the present invention of 34 to 50, all have a good r value, TS + 3285 × n is high, and the maximum expansion ratio is good.
[0119]
On the other hand, the production No. Nos. 18 and 51 have low cold-rolling ratios. Nos. 21 and 61 have a low heating temperature for the AlN precipitation treatment. In Nos. 22 and 62, since the heating temperature of the recrystallization treatment is high, r L And r C Is declining.
[0120]
In addition, the production No. Production Nos. 23 and 53 have a low cold rolling rate and a short retention time of the AlN precipitation treatment. In Nos. 27 and 63, the holding time of the AlN precipitation treatment was short, the heating temperature was low, and In Nos. 26 and 64, the holding time of the AlN precipitation treatment was short, and the heating temperature of the recrystallization treatment was high. L And r C Is declining.
[0121]
Furthermore, the production No. 31 and 56 have a low cold rolling rate, a short holding time for the AlN precipitation treatment, and a high heating temperature for the recrystallization treatment. L And r C Is declining.
[0122]
In addition, the production No. Production No. 54 has a high cold rolling ratio and a low heating temperature for the AlN precipitation treatment. Production No. 65 has a low heating temperature for the AlN precipitation treatment and a high heating temperature for the recrystallization treatment. In Nos. 20 and 60, since the holding time of the AlN precipitation treatment was short, r L And r C Is declining.
[0123]
Production No. Production No. 30 has a low cold rolling rate, a short holding time for the AlN precipitation treatment, and a high heating temperature. Production No. 57 has a high cold rolling rate, a short holding time for the AlN precipitation treatment, and a low heating temperature. No. 33 has a high cold-rolling rate, a short holding time for the AlN precipitation treatment, a low heating temperature, and a high heating temperature for the recrystallization treatment. L And r C Is declining.
[0124]
Production No. Production Nos. 19 and 52 have high cold rolling rates, and No. 32 has a high cold rolling rate, a low heating temperature for the AlN precipitation treatment, and a high heating temperature for the recrystallization treatment. L Is declining.
[0125]
Production No. 24 has a low cold rolling rate and a high heating temperature for the AlN precipitation treatment, C Is declining. Production No. Sample No. 28 has a lower TS + 3285 × n because the heating temperature of the AlN precipitation treatment is high and the heating temperature of the recrystallization treatment is low.
[0126]
Production No. Production Nos. 29 and 66 have a short holding time in the AlN precipitation treatment, a low heating temperature, and a low heating temperature in the recrystallization treatment. Production No. 25 has a high cold rolling rate and a low heating temperature for the recrystallization treatment. Production No. 55 has a low cold rolling rate and a low heating temperature for the recrystallization treatment. Production No. 58 has a high cold rolling rate, a low heating temperature for the AlN precipitation treatment, and a low heating temperature for the recrystallization treatment. No. 59 has a low cold rolling rate, a short holding time for the AlN precipitation treatment, a low heating temperature, and a low heating temperature for the recrystallization treatment. L , R C And TS + 3285 × n.
[0127]
【The invention's effect】
According to the present invention, there is provided a high-strength steel pipe excellent in workability, in which both r value and n value are improved to unprecedented levels, which are suitable for structural parts, pipes and the like manufactured by hydroforming. And the industrial contribution is extremely large.
Claims (12)
Ｃ：０．０３〜０．５％、
Ｓｉ：０．００１〜３．０％、
Ｍｎ：０．０１〜３．０％、
Ｐ：０．００１〜０．１５％、
Ｓ：０．０５％以下、
Ａｌ：０．００８〜０．３％、
Ｎ：０．００１〜０．０３％
を含有し、残部がＦｅ及び不可避的不純物からなり、引張強度が３５０ＭＰａ以上であり、管軸方向及び円周方向のｒ値がともに１．３以上であって、管軸方向のｎ値「ｎ」と引張強度「ＴＳ［ＭＰａ］」が、
ＴＳ＋３２８５×ｎ＞１０８２
の関係を満たすことを特徴とする、加工性に優れた高強度鋼管。In mass%,
C: 0.03 to 0.5%,
Si: 0.001 to 3.0%,
Mn: 0.01 to 3.0%,
P: 0.001 to 0.15%,
S: 0.05% or less,
Al: 0.008 to 0.3%,
N: 0.001 to 0.03%
, The balance consisting of Fe and unavoidable impurities, tensile strength of 350 MPa or more, r values in the tube axis direction and circumferential direction of both 1.3 or more, and n value "n" in the tube axis direction ”And tensile strength“ TS [MPa] ”
TS + 3285 × n> 1082
A high-strength steel pipe with excellent workability, characterized by satisfying the following relationship:
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Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP2008174834A (en) * | 2006-12-18 | 2008-07-31 | Nippon Steel Corp | Steel tube excellent in workability and manufacturing method therefor |
| JP2008255395A (en) * | 2007-04-03 | 2008-10-23 | Nippon Steel Corp | High-strength steel pipe having excellent workability, and method for producing the same |
| JP2008255394A (en) * | 2007-04-03 | 2008-10-23 | Nippon Steel Corp | Steel pipe having excellent workability, and method for producing the same |
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Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP2008174834A (en) * | 2006-12-18 | 2008-07-31 | Nippon Steel Corp | Steel tube excellent in workability and manufacturing method therefor |
| JP2008255395A (en) * | 2007-04-03 | 2008-10-23 | Nippon Steel Corp | High-strength steel pipe having excellent workability, and method for producing the same |
| JP2008255394A (en) * | 2007-04-03 | 2008-10-23 | Nippon Steel Corp | Steel pipe having excellent workability, and method for producing the same |
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