JP4936300B2 - High-strength hot-dip galvanized steel sheet excellent in press workability and manufacturing method thereof - Google Patents
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Description
【０００１】
【発明の属する技術分野】
本発明は、プレス成形性に優れた高強度溶融亜鉛めっき鋼板およびその製造方法に関する。
【０００２】
【従来の技術】
近年、地球環境保護の観点より、自動車の燃費向上の動きが盛んになっている。中でも、燃費向上に最も影響を与える軽量化のニーズが強くなっており、主要な車体構成材料である鋼板に対しては、成形性を損なわずに、一層の強度増加を図ることが求められている。また、直近では自動車事故を想定した衝突安全性に関する法規制が急速に拡大・強化されつつあり、高強度鋼板への期待がますます高まっている。
【０００３】
しかし、鋼板を高強度化した場合には延性が低下して、プレス成形が困難となる。そこで、注目されている鋼板としては、フェライトを主体としてマルテンサイト、ベイナイト、残留オーステナイトのような低温生成相からなる複合組織鋼板がある。この複合組織による強化は、析出強化などの他の強化方法よりも、高強度化の際に延性の低下が少なく、強度・延性バランスに優れるという特徴がある。
【０００４】
とくに残留オーステナイト鋼は特開平１−２３０７１５号公報に開示されているように、強度・延性バランスに優れるが、基本的に高いＳｉを含有する成分系なので、溶融亜鉛めっき性が悪い。そのため、溶融亜鉛めっきが必要な部位には、特開昭５７−１５５３２９号公報に開示されているような、フェライトを主体として、低温生成相としてマルテンサイトとを主に含むＤｕａｌ Ｐｈａｓｅ鋼（以後ＤＰ鋼）が用いられている。
【０００５】
【発明が解決しようとしている課題】
近年、この溶融亜鉛めっきを施したＤＰ鋼の実部品への適用が進められているが、この場合、従来は低強度材を使用してきた部材に適用されるために、高い強度レベルにもかかわらず、厳しい加工が要求されることになり、プレス加工が困難となる場合がある。さらに、この場合の課題として材料特性もさることながら、めっき密着性があげられる。前述のようにＤＰ鋼はめっき密着性も考慮してＳｉ添加量が低減されている。
【０００６】
特開平５−１６３５３１号公報に開示された技術のようにＳｉの添加量を従来のＤＰ鋼よりも低減することによりめっき性は向上すると考えられるが、近年要求されている加工レベルに対応するためにはＳｉ低減という手法だけではめっき密着性が不足する。また、Ｓｉをさらに低減するとめっき密着性は良好となるが、延性が低下してプレス時に割れが生じてしまう。
【０００７】
本発明は、以上のような従来技術の問題点を解決し、近年の厳しい加工レベルに適用できるめっき密着性と材料特性との双方を満足するプレス加工性に優れた高強度溶融亜鉛めっき鋼板およびその製造方法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】
本発明は、めっき密着性に対する鋼成分の影響を検討した結果、ＮｉとＣｕを添加することにより、めっき密着性を改善されることを見出した。このメカニズムは明らかではないが、以下のように推察される。ＮｉやＣｕは、熱延中および再結晶焼鈍中に表層に濃化し、めっき密着性を悪化させるＳｉ等の元素の表層への濃化を抑制することによって、めっき密着性が改善されると考えられる。
【０００９】
また、延性を向上させる方法として、残留オーステナイトの量を増加させる方法がある。そこで、本発明では、ＤＰ鋼の低温変態相の主相はマルテンサイトであるが、その中に残留オーステナイトを活用することを考えた。残留オーステナイト鋼においてＳｉは、セメンタイトの析出を抑制して残留オーステナイトを残存させる効果がある。Ｃｕ、Ｎｉは同様の効果を持つ元素と考えられ、これにより残留オーステナイトの残存を図り、延性を向上させることを考えた。このように、Ｃｕ、Ｎｉを添加することにより、めっき密着性と延性を両立したプレス加工性に優れた高強度溶融亜鉛めっき鋼板を得ることができる。
【００１０】
また、上記の思想により設計された成分系の鋼板を、連続焼鈍または連続溶融亜鉛めっきラインにて、フェライト・オーステナイト２相域にて再結晶焼鈍を行った後に、適当な冷却速度にて冷却することにより、フェライトと、低温変態相として残留オーステナイトを含むマルテンサイトとを主相とした金属組織を得ることができる。その際、焼鈍での温度パターンによってはベイナイトも生成するが、ベイナイトが生成しても材質は良好のままである。
【００１１】
本発明の要旨とするところは、特許請求の範囲に記載したように、以下の通りである。
（１）質量％で、C：０．０１〜０．１０％、Ｓｉ：０．３％以下、Ｍｎ：１．０〜３．０％、Ｐ：０．０９％以下、Ｂ：０．００２０〜０．０１００％、Ｃｕ、Ｎｉのうち１種または２種の合計が０．００２０％以上、１．０％以下（ただし、Ｎｉは０．０２７％以下）を含有し、残部Ｆｅおよび不可避的不純物からなり、金属組織がフェライトと、残留オーステナイトを０．２〜３．０体積％含むマルテンサイトからなる低温変態組織とを主相とする鋼板に溶融亜鉛めっきまたは合金化溶融亜鉛めっきが施されたことを特徴とするプレス加工性に優れた高強度溶融亜鉛めっき鋼板。
【００１２】
（２）質量％で、C：０．０１〜０．１０％、Ｓｉ：０．３％以下、Ｍｎ：１．０〜３．０％、Ｐ：０．０９％以下、Ｂ：０．００２０〜０．０１００％、Ｃｕ、Ｎｉのうち１種または２種の合計が、０．００２０％以上、１．０％以下（ただし、Ｎｉは０．０２７％以下）、Ｃｒ：０．５％以下、Ｍｏ：０．５％以下、Ｖ：０．１％以下のうち１種または２種以上を含有し、残部Ｆｅおよび不可避的不純物からなり、金属組織がフェライトと、残留オーステナイトを０．２〜３．０体積％含むマルテンサイトからなる低温変態組織とを主相とする鋼板に溶融亜鉛めっきまたは合金化溶融亜鉛めっきが施されたことを特徴とするプレス加工性に優れた高強度溶融亜鉛めっき鋼板。
【００１３】
（３）質量％で、C：０．０１〜０．１０％、Ｓｉ：０．３％以下、Ｍｎ：１．０〜３．０％、Ｐ：０．０９％以下、Ｂ：０．００２０〜０．０１００％、Ｃｕ、Ｎｉのうち１種または２種の合計が、０．００２０％以上、１．０％以下（ただし、Ｎｉは０．０２７％以下）、Ｃａ：０．０００５〜０．０１％、ＲＥＭ：０．００５〜０．０５％のうち１種または２種を含有し、残部Ｆｅおよび不可避的不純物からなり、金属組織がフェライトと、残留オーステナイトを０．２〜３．０体積％含むマルテンサイトからなる低温変態組織とを主相とする鋼板に溶融亜鉛めっきまたは合金化溶融亜鉛めっきが施されたことを特徴とするプレス加工性に優れた高強度溶融亜鉛めっき鋼板。
【００１４】
（４）質量％で、C：０．０１〜０．１０％、Ｓｉ：０．３％以下、Ｍｎ：１．０〜３．０％、Ｐ：０．０９％以下、Ｂ：０．００２０〜０．０１００％、Ｃｕ、Ｎｉのうち１種または２種の合計が、０．００２０％以上、１．０％以下（ただし、Ｎｉは０．０２７％以下）、Ｃｒ：０．５％以下、Ｍｏ：０．５％以下、Ｖ：０．１％以下のうち１種または２種以上、Ｃａ：０．０００５〜０．０１％、ＲＥＭ：０．００５〜０．０５％のうち１種または２種を含有し、残部Ｆｅおよび不可避的不純物からなり、金属組織がフェライトと、残留オーステナイトを０．２〜３．０体積％含むマルテンサイトからなる低温変態組織とを主相とする鋼板に溶融亜鉛めっきまたは合金化溶融亜鉛めっきが施されたことを特徴とするプレス加工性に優れた高強度溶融亜鉛めっき鋼板。
【００１５】
（５）（１）乃至（４）に記載した成分の鋼塊を熱間圧延して６００〜７００℃の温度で捲取った鋼板を酸洗し、続いて７２５〜８５０℃の温度に加熱した後、平均冷却速度５〜３０℃／ｓでめっき浴温度まで冷却して溶融亜鉛めっきを施し、鋼板の金属組織をフェライトと、残留オーステナイトを０．２〜３．０体積％含むマルテンサイトからなる低温変態組織とを主相とすることを特徴とする、プレス加工性に優れた高強度溶融亜鉛めっき鋼板の製造方法。
【００１６】
（６）（１）乃至（４）に記載した成分の鋼塊を熱間圧延して６００〜７００℃の温度で捲取った鋼板を酸洗し、冷間圧延し、続いて７２５〜８５０℃の温度に加熱した後、平均冷却速度５〜３０℃／ｓでめっき浴温度まで冷却して溶融亜鉛めっきを施し、鋼板の金属組織をフェライトと、残留オーステナイトを０．２〜３．０体積％含むマルテンサイトからなる低温変態組織とを主相とすることを特徴とする、プレス加工性に優れた高強度溶融亜鉛めっき鋼板の製造方法。
【００１７】
（７）（１）乃至（４）に記載した成分の鋼塊を熱間圧延して６００〜７００℃の温度で捲取った鋼板を酸洗し、続いて７２５〜８５０℃の温度に加熱した後、平均冷却速度５〜３０℃／ｓでめっき浴温度まで冷却して溶融亜鉛めっきを施し、しかる後に４５０〜６００℃の温度で合金化熱処理を行い、鋼板の金属組織をフェライトと、残留オーステナイトを０．２〜３．０体積％含むマルテンサイトからなる低温変態組織とを主相とすることを特徴とする、プレス加工性に優れた高強度溶融亜鉛めっき鋼板の製造方法。
【００１８】
（８）（１）乃至（４）に記載した成分の鋼塊を熱間圧延して６００〜７００℃の温度で捲取った鋼板を酸洗し、冷間圧延し、続いて７２５〜８５０℃の温度に加熱した後、平均冷却速度５〜３０℃／ｓでめっき浴温度まで冷却して溶融亜鉛めっきを施し、しかる後に４５０〜６００℃の温度で合金化熱処理を行い、鋼板の金属組織をフェライトと、残留オーステナイトを０．２〜３．０体積％含むマルテンサイトからなる低温変態組織とを主相とすることを特徴とする、プレス加工性に優れた高強度溶融亜鉛めっき鋼板の製造方法。
【００１９】
本発明の成分および金属組織の限定理由は次の通りである。
Ｃの０．０１％以上の含有は、めっきされた鋼板のフェライト地中に微細な低温変態相を分散させた複合組織を生成させて、鋼の高強度化を図るのに有効な成分である。しかし、０．１０％を超える過剰な含有は、加工性、さらには、溶接性を劣化させるため、Cの範囲を０．０１〜０．１０％とした。
【００２０】
Ｓｉは、本発明の目的の一つである強度向上に有効な成分であるが、０．３％を超える過剰な含有は、連続溶融亜鉛めっきラインの焼鈍炉内で還元しがたい酸化膜を形成し、めっき濡れ性を低下させ、まためっき密着性も低下させる。従って、Ｓｉ含有量の上限値を０．３％とした。
【００２１】
Ｍｎは、１．０％以上で鋼の変態点を低減して低冷却速度でも複合組織を生成させる成分である。しかし、３．０％を超えると、圧延加工性、さらには、成形加工性も低下する。従って、Ｍｎの含有量を１．０〜３．０％に規定した。
【００２２】
Ｐは、鋼の強度を高めるのに有効な成分であるが、過度に含有すると、溶融めっき鋼板の合金化処理において、合金化速度を低減するので、０．０９％以下とした。
【００２３】
Ｂは、０．００２０％以上の添加で複合組織を生成し、Ｂを添加することにより、めっき性を低減させる元素であるＭｎを低減できる。さらに、複合組織においては、Ｂは焼き入れ性を向上させる元素であるため、低温変態相の生成を促進することから、強度上昇にも寄与すると考えられる。しかし、Ｂの含有が０．０１００％超となると延性が低下するため、Ｂの含有範囲を０．００２０〜０．０１００％とした。
【００２４】
また、Ｓは本発明のようにＭｎを多く含有する鋼ではＭｎＳとなって介在物となり、プレス加工時に加工割れを誘発するので０．０１％以下、それも極力少ない方が好ましい。また、Ｎは多量に添加した場合には、加工性を劣化させるため０．０００８％以下が好ましい。
【００２５】
Ａｌは、Ａｌ2Ｏ5系介在物を形成して鋼板加工性を阻害するため、０．１％以下が好ましい。下限は、脱酸の観点から０．００５％以上の添加が好ましい。Nは多量に添加した場合は、加工性を劣化させるため、０．００８％以下が好ましい。
【００２６】
ＣｕとＮｉは前述のように、鋼板表面に濃化することにより、Ｓｉ等のめっき性を低下させる元素の表面の濃化を抑制する元素であり、その効果はＣｕとＮｉの含有量の和が０．００２０％以上のときに発揮される。また、これ以上の含有量であると低温変態相中の残留オーステナイトの残存を促進する効果がある。しかし、ＣｕとＮｉの含有量の和が１．０％以上となると延性が低下して加工性が低下するため、ＣｕとＮｉの含有量の和を０．００２〜１．０％とした。
【００２７】
さらに、焼き入れ性を向上する元素としてＣｒ：０．５％以下、Ｍｏ：０．５％以下、Ｖ：０．１％以下のうち１種または２種以上を添加してもよい。また、Ｃａ，ＲＥＭは硫化物系介在物が球状化して穴拡げ性を向上させるので、Ｃａ：０．０００５％〜０．０１％、ＲＥＭ：０．００５〜０．０５％の１種または２種を添加してもよい。
【００２８】
金属組織を、フェライトと残留オーステナイトを含むマルテンサイトとを主相としたのは、このような組織の場合は、強度・延性バランスに優れ、降伏点伸びが発生せずに、プレス時にストレッチャーストレインが発生しない鋼板となるためである。また、熱処理条件によっては、低温変態組織中にベイナイトが生成することがあるが、この場合も強度・延性バランスに優れ、降伏点伸びが発生せずにプレス時にストレッチャーストレインが発生しない鋼板となるため、何ら本発明の趣旨を損なうものではない。尚、主相とは、下記のＸ線回折法により測定した体積分率が５０％以上の相を指し、本発明においては、フェライトと残留オーステナイトを含むマルテンサイトとの合計の体積分率が５０％以上であればよい。
【００２９】
また、残留オーステナイトの体積分率は０．２％以上で延性向上効果が発揮されるが、通常の連続溶融亜鉛めっき工程の熱処理条件では３％以上の体積分率を実現することは困難であって、実現のためには、ライン改造などのコストがかかる。以上の理由により、残留オーステナイトの体積分率は０．２〜３．０％が好ましい。
【００３０】
次に、製造方法の限定理由を以下に示す。捲取温度を６００〜７００℃としたのは、複合組織の脱スケール性から規定したものである。６００℃以上の高い捲取温度は捏延後の冷却過程において複合組織の生成を抑制するのに必要な温度であり、７００℃を超える温度はその後の酸洗工程で脱スケール性を低下させる。そのため、捲取温度を、６００〜７００℃と規定した。 熱延後、または、酸洗、冷延後に連続溶融亜鉛めっきラインにて焼鈍されるが、その焼鈍温度を７２５〜８５０℃としたのは焼鈍にてフェライト・オーステナイトの２相を出現させるためである。また、その後の冷却速度を５〜３０℃／ｓとしたのはパーライト変態を抑制するためである。
【００３１】
また、めっき鋼板を亜鉛浴から引き上げ、ガスワイピング法などでめっき厚みを調整した後に４５０〜６００℃の温度で加熱する合金加熱処理は、めっき層を亜鉛・鉄の合金としてプレス成形での摺動性、化成処理性、スポット溶接性を向上させるために行ってもよい。なお、めっき後に耐食性、溶接性、潤滑性、プレス成形性を向上させる目的で種々の後処理や皮膜を付与してもよいが、これらの処理が施されても何ら本発明の趣旨を損なうものではない。上記の条件を満たすことで、プレス加工性に優れた高強度溶融亜鉛めっき鋼板を実現することができる。
【００３２】
【発明の実施の形態】
以下、本発明の実施の形態を実施例を用いて説明する。
基本的な製造工程は鋼を、転炉にて溶製し、仕上圧延温度８８０℃、捲取温度６５０℃にて熱間圧延を行い、板厚４．０ｍｍの熱延板を得た。これを酸洗後、いくつかの試料を圧下率７０％で冷間圧延を行った。その後、通常の連続溶融亜鉛めっきラインにて所定の温度で焼鈍した後、めっき浴温度（４５０℃）まで所定の冷却速度で冷却し、亜鉛めっきし、その後直ちにガスワイピングにて亜鉛皮膜の付着量を片面４５ｇ／ｍ2にコントロールし、いくつかの試料については引き続いて合金化炉にて４５０〜５５０℃に加熱して合金化処理を施した。表１に実験に用いた材料の成分値を示す。
【００３３】
【表１】
評価は、引張試験、めっき密着性試験、残留γ（オーステナイト）量測定にて行った。引張試験はＪＩＳ５号試験片によるＣ方向引張により求め、強度（ＴＳ）と伸び（Ｅｌ）の積が１７５００以上を良好とした。めっき密着性試験は、密着曲げ試験による剥離性を調査した。残留γ量測定は、Ｘ線回折法を用いた。これは板厚１／４の断面を化学研磨したのち、Ｘ線回折測定を行いフェライトの（２００）および（２１１）とオーステナイトの（２２０）および（３１１）面の回折線積分強度を利用した。また、光学顕微鏡により金属組織も観察した。評価結果を表２および表３に示す。
【００３５】
【表２】
【表３】
実験番号１〜２７にて鋼種の影響について検討した。表１に示す化学成分の鋼を用いて溶融亜鉛めっき鋼板を製造した。そのときの焼鈍温度は８００℃、焼鈍後の冷却速度は１０℃／ｓとした。実験番号１はＣ含有量が制限以下であったために、引張特性が低下した。実験番号５は、Si含有量は本発明の範囲を超えているため、めっき密着性が低下した。実験番号６,９は、Ｂ含有量が本発明の範囲外であったために、めっき密着性が低下した。その他の実験については、本発明の範囲内であるため、引張特性、めっき密着性も良好であり、プレス加工性に優れた高強度溶融亜鉛めっき鋼板を製造できた。
【００３８】
実験番号２８〜３８は製造条件の影響を検討したものである。ここでは、表１に示す、Ｃ，Ｘ，ＡＡを用いた。実験番号２８，３３，３６は焼鈍後の冷却速度が制限以下であったため、引張特性が低下した。実験番号３１，３２，３４，３５，３７，３８は焼鈍温度が本発明の範囲外であるため、引張特性が低下した。その他の実験については、本発明の範囲内であるため、引張特性、めっき密着性も良好であり、プレス加工性に優れた高強度溶融亜鉛めっき鋼板を製造できた。なお、光学顕微鏡による組織観察により、全ての試料で、フェライトと低温変態相の複合組織が観察された。
【００３９】
【発明の効果】
以上述べたように、本発明により、めっき密着性と材料特性の双方を満足するプレス加工性に優れた高強度溶融亜鉛めっき鋼板およびその製造方法を提供することができる。本発明によって、自動車などへの高強度鋼板の適用拡大がさらに進み、産業上多大な効果が期待できる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high-strength hot-dip galvanized steel sheet excellent in press formability and a method for producing the same.
[0002]
[Prior art]
In recent years, from the viewpoint of protecting the global environment, movements for improving the fuel efficiency of automobiles have become active. In particular, there is a growing need for weight reduction that has the most impact on improving fuel efficiency, and steel sheets, which are the main body components, are required to increase strength without sacrificing formability. Yes. In addition, recently, laws and regulations related to collision safety assuming automobile accidents are rapidly expanding and strengthening, and expectations for high-strength steel sheets are increasing.
[0003]
However, when the strength of the steel plate is increased, the ductility is lowered and press forming becomes difficult. Therefore, as a steel sheet that has been attracting attention, there is a composite structure steel sheet mainly composed of ferrite and composed of a low-temperature generation phase such as martensite, bainite, and retained austenite. The strengthening by this composite structure is characterized in that the ductility is less lowered and the balance between strength and ductility is excellent when the strength is increased than other strengthening methods such as precipitation strengthening.
[0004]
Particularly, as disclosed in JP-A-1-230715, the retained austenitic steel is excellent in strength and ductility balance, but is basically a component system containing high Si, so that the hot dip galvanizing property is poor. Therefore, a dual phase steel (hereinafter referred to as DP) containing mainly ferrite and martensite as a low-temperature generation phase as disclosed in Japanese Patent Application Laid-Open No. 57-155329 is provided at a site requiring hot dip galvanization. Steel) is used.
[0005]
[Problems to be solved by the invention]
In recent years, application of this hot-dip galvanized DP steel to actual parts has been promoted, but in this case, since it is applied to members that have conventionally used low-strength materials, it is in spite of high strength levels. However, severe processing is required, and press processing may be difficult. Furthermore, as a problem in this case, plating adhesion is raised as well as material characteristics. As described above, the amount of Si added to DP steel is reduced in consideration of plating adhesion.
[0006]
Although it is considered that the plating property is improved by reducing the amount of Si added as compared with the conventional DP steel as in the technique disclosed in Japanese Patent Laid-Open No. 5-163531, in order to meet the recently required processing level. However, the plating adhesion is insufficient only by the technique of reducing Si. Further, when Si is further reduced, the plating adhesion is improved, but the ductility is lowered and cracking occurs during pressing.
[0007]
The present invention solves the problems of the prior art as described above, and is a high-strength hot-dip galvanized steel sheet excellent in press workability that satisfies both plating adhesion and material properties that can be applied to severe working levels in recent years. It aims at providing the manufacturing method.
[0008]
[Means for Solving the Problems]
As a result of examining the influence of the steel component on the plating adhesion, the present invention has found that the plating adhesion can be improved by adding Ni and Cu. Although this mechanism is not clear, it is guessed as follows. Ni and Cu concentrate on the surface layer during hot rolling and recrystallization annealing, and it is thought that plating adhesion is improved by suppressing the concentration of elements such as Si that deteriorate plating adhesion on the surface layer. It is done.
[0009]
As a method for improving ductility, there is a method for increasing the amount of retained austenite. Therefore, in the present invention, the main phase of the low temperature transformation phase of DP steel is martensite, but it has been considered to utilize retained austenite therein. In the retained austenitic steel, Si has an effect of suppressing the precipitation of cementite and remaining retained austenite. Cu and Ni are considered to be elements having the same effect. Thus, it was considered that residual austenite was retained and ductility was improved. Thus, by adding Cu and Ni, it is possible to obtain a high-strength hot-dip galvanized steel sheet excellent in press workability that achieves both plating adhesion and ductility.
[0010]
In addition, the steel sheet of the component system designed based on the above idea is cooled at an appropriate cooling rate after recrystallization annealing in the ferrite / austenite two-phase region in a continuous annealing or continuous hot dip galvanizing line. Thus, it is possible to obtain a metal structure mainly composed of ferrite and martensite containing retained austenite as a low temperature transformation phase. At that time, bainite is also generated depending on the temperature pattern in annealing, but the material remains good even if bainite is generated.
[0011]
The gist of the present invention is as follows, as described in the claims.
(1) By mass%, C: 0.01 to 0.10%, Si: 0.3% or less, Mn: 1.0 to 3.0%, P: 0.09% or less, B: 0.0020 -0.0100%, the total of one or two of Cu and Ni is 0.0020% or more and 1.0% or less (however, Ni is 0.027% or less), the remainder Fe and unavoidable Hot-dip galvanizing or alloying hot-dip galvanizing is applied to a steel sheet composed of impurities , the main phase of which is the main phase of ferrite and a low-temperature transformation structure consisting of martensite containing 0.2 to 3.0% by volume of retained austenite. A high-strength hot-dip galvanized steel sheet with excellent press workability, characterized by
[0012]
(2) By mass%, C: 0.01 to 0.10%, Si: 0.3% or less, Mn: 1.0 to 3.0%, P: 0.09% or less, B: 0.0020 -0.0100%, the total of one or two of Cu and Ni is 0.0020% or more and 1.0% or less (however, Ni is 0.027% or less), Cr: 0.5% or less , Mo: 0.5% or less, V: containing one or more of 0.1% or less, the balance being Fe and inevitable impurities, the metal structure is ferrite, and the residual austenite is 0.2 to High-strength hot-dip galvanizing with excellent press workability, characterized in that hot-dip galvanizing or alloying hot-dip galvanizing is applied to steel sheets whose main phase is a low-temperature transformation structure consisting of martensite containing 3.0% by volume steel sheet.
[0013]
(3) By mass%, C: 0.01 to 0.10%, Si: 0.3% or less, Mn: 1.0 to 3.0%, P: 0.09% or less, B: 0.0020 -0.0100%, the total of one or two of Cu and Ni is 0.0020% or more and 1.0% or less (however, Ni is 0.027% or less), Ca: 0.0005-0 .01%, REM: One or two of 0.005 to 0.05%, the balance being Fe and inevitable impurities, the metal structure is ferrite, and the residual austenite is 0.2 to 3.0 A high-strength hot-dip galvanized steel sheet excellent in press workability, characterized in that hot-dip galvanizing or galvannealed hot-dip galvanizing is applied to a steel sheet whose main phase is a low-temperature transformation structure composed of martensite containing volume% .
[0014]
(4) By mass%, C: 0.01 to 0.10%, Si: 0.3% or less, Mn: 1.0 to 3.0%, P: 0.09% or less, B: 0.0020 -0.0100%, the total of one or two of Cu and Ni is 0.0020% or more and 1.0% or less (however, Ni is 0.027% or less), Cr: 0.5% or less , Mo: 0.5% or less, V: One or more of 0.1% or less, Ca: 0.0005 to 0.01%, REM: One of 0.005 to 0.05% Or a steel sheet containing two types, the balance being Fe and unavoidable impurities , the main phase being ferrite and the low-temperature transformation structure consisting of martensite containing 0.2 to 3.0% by volume of retained austenite. Excellent press workability, characterized by hot dip galvanization or galvannealing High strength hot dip galvanized steel sheet.
[0015]
(5) A steel ingot having the components described in (1) to (4) is hot-rolled and pickled at a temperature of 600 to 700 ° C, and then pickled, and subsequently heated to a temperature of 725 to 850 ° C. Thereafter, it is cooled to the plating bath temperature at an average cooling rate of 5 to 30 ° C./s and is subjected to hot dip galvanizing, and the steel structure is composed of ferrite and martensite containing 0.2 to 3.0% by volume of retained austenite. A method for producing a high-strength hot-dip galvanized steel sheet excellent in press workability, characterized by comprising a low-temperature transformation structure as a main phase .
[0016]
(6) A steel ingot having the components described in (1) to (4) is hot-rolled and pickled at a temperature of 600 to 700 ° C., pickled, cold-rolled, and subsequently 725 to 850 ° C. Then, it is cooled to a plating bath temperature at an average cooling rate of 5 to 30 ° C./s and subjected to hot dip galvanization, and the metal structure of the steel sheet is ferrite and the residual austenite is 0.2 to 3.0% by volume. A method for producing a high-strength hot-dip galvanized steel sheet having excellent press workability, characterized by comprising a low-temperature transformation structure comprising martensite as a main phase .
[0017]
(7) A steel ingot having the components described in (1) to (4) is hot-rolled and pickled at a temperature of 600 to 700 ° C, and then pickled, and subsequently heated to a temperature of 725 to 850 ° C. Thereafter, it is cooled to a plating bath temperature at an average cooling rate of 5 to 30 ° C./s and subjected to hot dip galvanization, and then alloyed and heat treated at a temperature of 450 to 600 ° C. to change the metal structure of the steel sheet to ferrite and residual austenite. A method for producing a high-strength hot-dip galvanized steel sheet excellent in press workability, characterized in that the main phase is a low-temperature transformation structure comprising martensite containing 0.2 to 3.0% by volume .
[0018]
(8) A steel ingot having the components described in (1) to (4) is hot-rolled and pickled at a temperature of 600 to 700 ° C., pickled, cold-rolled, and subsequently 725 to 850 ° C. Then, it is cooled to a plating bath temperature at an average cooling rate of 5 to 30 ° C./s and subjected to hot dip galvanization, and then subjected to alloying heat treatment at a temperature of 450 to 600 ° C. A method for producing a high-strength hot-dip galvanized steel sheet excellent in press workability, characterized by comprising ferrite and a low-temperature transformation structure composed of martensite containing 0.2 to 3.0% by volume of retained austenite. .
[0019]
The reasons for limiting the components and metal structure of the present invention are as follows.
Inclusion of 0.01% or more of C is an effective component for increasing the strength of steel by generating a composite structure in which fine low-temperature transformation phases are dispersed in the ferrite ground of a plated steel sheet. . However, excessive content exceeding 0.10% deteriorates workability and further weldability, so the C range was made 0.01 to 0.10%.
[0020]
Si is an effective component for improving the strength, which is one of the objects of the present invention. However, if it exceeds 0.3%, it is difficult to reduce the oxide film in an annealing furnace of a continuous hot dip galvanizing line. Forming, lowering the plating wettability, and lowering the plating adhesion. Therefore, the upper limit of the Si content is set to 0.3%.
[0021]
Mn is a component that reduces the transformation point of steel at 1.0% or more and generates a composite structure even at a low cooling rate. However, if it exceeds 3.0%, the rolling processability and the molding processability are also deteriorated. Therefore, the Mn content is specified to be 1.0 to 3.0%.
[0022]
P is an effective component for increasing the strength of the steel, but if contained excessively, the alloying rate of the hot-dip galvanized steel sheet is reduced, so the content was made 0.09% or less.
[0023]
B forms a composite structure when 0.0020% or more is added, and by adding B, Mn, which is an element that reduces the plating property, can be reduced. Furthermore, in the composite structure, since B is an element that improves the hardenability, it promotes the formation of a low-temperature transformation phase, which is considered to contribute to an increase in strength. However, when the B content exceeds 0.0100%, the ductility decreases, so the B content range is set to 0.0020 to 0.0100%.
[0024]
In the steel containing a large amount of Mn as in the present invention, S becomes MnS and becomes an inclusion, and induces a work crack during press working, so 0.01% or less is preferable. Further, when N is added in a large amount, it is preferably 0.0008% or less in order to deteriorate the workability.
[0025]
Since Al forms Al 2 O 5 inclusions and hinders the workability of the steel sheet, 0.1% or less is preferable. The lower limit is preferably 0.005% or more from the viewpoint of deoxidation. When N is added in a large amount, the workability is deteriorated, so 0.008% or less is preferable.
[0026]
As described above, Cu and Ni are elements that suppress the concentration of the surface of an element that lowers the plating properties such as Si by concentrating on the surface of the steel sheet, and the effect is the sum of the contents of Cu and Ni. Is exhibited when the content is 0.0020% or more. Further, when the content is higher than this, there is an effect of promoting the residual austenite in the low temperature transformation phase. However, when the sum of the Cu and Ni contents is 1.0% or more, the ductility is lowered and the workability is lowered. Therefore, the sum of the Cu and Ni contents is set to 0.002 to 1.0%.
[0027]
Furthermore, you may add 1 type (s) or 2 or more types among Cr: 0.5% or less, Mo: 0.5% or less, and V: 0.1% or less as an element which improves hardenability. In addition, since Ca and REM improve the hole expansibility by spheroidizing sulfide inclusions, one or two of Ca: 0.0005% to 0.01% and REM: 0.005 to 0.05%. Seeds may be added.
[0028]
The main structure of the metal structure is ferrite and martensite containing retained austenite. In such a structure, the structure has an excellent balance between strength and ductility, does not cause yield point elongation, and stretcher strain during pressing. This is because the steel sheet is free from the occurrence of defects. Depending on the heat treatment conditions, bainite may be generated in the low-temperature transformation structure, but in this case as well, the strength / ductility balance is excellent, yielding point elongation does not occur and stretcher strain does not occur during pressing. Therefore, the gist of the present invention is not impaired. The main phase refers to a phase having a volume fraction measured by the following X-ray diffraction method of 50% or more. In the present invention, the total volume fraction of ferrite and martensite containing residual austenite is 50. % Or more.
[0029]
In addition, although the volume fraction of retained austenite is 0.2% or more, the effect of improving ductility is exhibited. However, it is difficult to realize a volume fraction of 3% or more under the heat treatment conditions of a normal continuous hot dip galvanizing process. In order to achieve this, costs such as line modification are required. For these reasons, the volume fraction of retained austenite is preferably 0.2 to 3.0%.
[0030]
Next, the reasons for limiting the manufacturing method are shown below. The setting temperature of 600 to 700 ° C. is defined by the descalability of the composite structure. A high cutting temperature of 600 ° C. or higher is a temperature necessary for suppressing the formation of a composite structure in the cooling process after rolling, and a temperature exceeding 700 ° C. reduces descalability in the subsequent pickling step. Therefore, the scraping temperature was defined as 600 to 700 ° C. After hot rolling, pickling and cold rolling, annealing is performed in a continuous hot dip galvanizing line, but the annealing temperature is set to 725-850 ° C. in order to cause two phases of ferrite and austenite to appear by annealing. is there. The subsequent cooling rate was set to 5 to 30 ° C./s in order to suppress pearlite transformation.
[0031]
In addition, the alloy heat treatment, in which the plated steel sheet is lifted from the zinc bath and the plating thickness is adjusted by a gas wiping method and then heated at a temperature of 450 to 600 ° C., slides in press forming with the plated layer as a zinc / iron alloy. May be performed to improve the property, chemical conversion property, and spot weldability. In addition, various post-treatments and coatings may be applied for the purpose of improving corrosion resistance, weldability, lubricity, and press formability after plating, but even if these treatments are performed, the purpose of the present invention is impaired. is not. By satisfy | filling said conditions, the high intensity | strength hot-dip galvanized steel plate excellent in press workability is realizable.
[0032]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described using examples.
In the basic manufacturing process, steel was melted in a converter and hot-rolled at a finishing rolling temperature of 880 ° C. and a cutting temperature of 650 ° C. to obtain a hot rolled sheet having a thickness of 4.0 mm. After pickling this, some samples were cold-rolled at a rolling reduction of 70%. Then, after annealing at a predetermined temperature in a normal continuous hot dip galvanizing line, it is cooled to a plating bath temperature (450 ° C) at a predetermined cooling rate, galvanized, and immediately afterwards, the amount of zinc coating deposited by gas wiping Was controlled to 45 g / m 2 on one side, and several samples were subsequently heated to 450 to 550 ° C. in an alloying furnace to be alloyed. Table 1 shows the component values of the materials used in the experiment.
[0033]
[Table 1]
The evaluation was performed by a tensile test, a plating adhesion test, and a residual γ (austenite) amount measurement. The tensile test was determined by tensile in the C direction using a JIS No. 5 test piece, and the product of strength (TS) and elongation (El) was 17500 or more. In the plating adhesion test, the peelability by the adhesion bending test was investigated. The residual γ amount was measured using an X-ray diffraction method. This was obtained by chemically polishing a section having a thickness of 1/4 and then performing X-ray diffraction measurement to utilize the diffraction line integral intensities of (200) and (211) ferrite and (220) and (311) faces of austenite. The metal structure was also observed with an optical microscope. The evaluation results are shown in Table 2 and Table 3.
[0035]
[Table 2]
[Table 3]
In Experiment Nos. 1-27, the influence of the steel type was examined. Hot dip galvanized steel sheets were manufactured using steels having chemical components shown in Table 1. The annealing temperature at that time was 800 ° C., and the cooling rate after annealing was 10 ° C./s. In Experiment No. 1, since the C content was less than the limit, the tensile properties were deteriorated. In Experiment No. 5, since the Si content exceeded the range of the present invention, the plating adhesion decreased. In Experiment Nos. 6 and 9, since the B content was outside the range of the present invention, the plating adhesion decreased. Since other experiments were within the scope of the present invention, tensile strength and plating adhesion were good, and a high-strength hot-dip galvanized steel sheet excellent in press workability could be produced.
[0038]
Experiment numbers 28 to 38 are for examining the influence of manufacturing conditions. Here, C, X, and AA shown in Table 1 were used. In Experiment Nos. 28, 33, and 36, the cooling rate after annealing was less than the limit, so the tensile properties were degraded. In Experiment Nos. 31, 32, 34, 35, 37, and 38, the annealing temperature was outside the range of the present invention, so the tensile properties were lowered. Since other experiments were within the scope of the present invention, tensile strength and plating adhesion were good, and a high-strength hot-dip galvanized steel sheet excellent in press workability could be produced. It should be noted that a composite structure of ferrite and a low-temperature transformation phase was observed in all samples by structure observation with an optical microscope.
[0039]
【Effect of the invention】
As described above, according to the present invention, it is possible to provide a high-strength hot-dip galvanized steel sheet excellent in press workability that satisfies both plating adhesion and material characteristics, and a method for producing the same. According to the present invention, the application of high-strength steel sheets to automobiles and the like is further expanded, and a great industrial effect can be expected.
Claims (8)
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| JP5194878B2 (en) | 2007-04-13 | 2013-05-08 | Ｊｆｅスチール株式会社 | High-strength hot-dip galvanized steel sheet excellent in workability and weldability and method for producing the same |
| JP4623233B2 (en) | 2009-02-02 | 2011-02-02 | Ｊｆｅスチール株式会社 | High-strength hot-dip galvanized steel sheet and manufacturing method thereof |
| JP4924730B2 (en) | 2009-04-28 | 2012-04-25 | Ｊｆｅスチール株式会社 | High-strength hot-dip galvanized steel sheet excellent in workability, weldability and fatigue characteristics and method for producing the same |
| JP5434960B2 (en) | 2010-05-31 | 2014-03-05 | Ｊｆｅスチール株式会社 | High-strength hot-dip galvanized steel sheet excellent in bendability and weldability and method for producing the same |
| EP2924141B1 (en) | 2014-03-25 | 2017-11-15 | ThyssenKrupp Steel Europe AG | Cold rolled steel flat product and method for its production |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPH05163531A (en) * | 1991-12-13 | 1993-06-29 | Nippon Steel Corp | Manufacture of alloyed galvanized steel sheet with high shape freezing character |
| US6517955B1 (en) * | 1999-02-22 | 2003-02-11 | Nippon Steel Corporation | High strength galvanized steel plate excellent in adhesion of plated metal and formability in press working and high strength alloy galvanized steel plate and method for production thereof |
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