JP3583306B2 - Method for producing high-strength and high-ductility cold-rolled steel sheet with improved variation in elongation in the sheet width direction - Google Patents
Method for producing high-strength and high-ductility cold-rolled steel sheet with improved variation in elongation in the sheet width direction Download PDFInfo
- Publication number
- JP3583306B2 JP3583306B2 JP01246999A JP1246999A JP3583306B2 JP 3583306 B2 JP3583306 B2 JP 3583306B2 JP 01246999 A JP01246999 A JP 01246999A JP 1246999 A JP1246999 A JP 1246999A JP 3583306 B2 JP3583306 B2 JP 3583306B2
- Authority
- JP
- Japan
- Prior art keywords
- steel sheet
- cold
- elongation
- rolled steel
- cooling
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Fee Related
Links
Description
【０００１】
【発明の属する技術分野】
本発明は、板幅方向における伸びのバラツキが改善された均質な高強度高延性冷延鋼板、および高強度高延性冷延鋼板の製造方法に関する。本発明の高強度高延性冷延鋼板は、自動車部品等の如くプレス成形時に加工性が問題となる工業分野に広く用いられる。
【０００２】
【従来の技術】
近年、特に自動車用鋼板では、軽量化を目的としてプレス成形性、とりわけ延性に優れた高強度冷延鋼板が汎用されており、残留オーステナイトの変態誘起塑性を利用して延性を高める方法が種々提案されている。
【０００３】
例えば、▲１▼特開平６２−２１７５２９号には、熱処理の均熱温度をＡｃ３ 温度以上に加熱すると共に、その後の冷却工程を適切に制御する方法が開示されており、これにより、１０％以上の残留オーステナイト相に基づく変態誘起塑性による均一伸びの向上、細粒化したフェライト相による局部伸びの上昇または衝撃特性向上、および残留ベイナイト相またはマルテンサイト相による強度確保を図っている。しかしながら、上記方法の如くＡｃ３ 温度以上に均熱温度を高めるとフェライト析出に時間がかかる為、その後の冷却工程でオーステナイト中へのＣ，Ｍｎ等の濃化が迅速に進まず、目標とする残留オーステナイト量が得られない恐れがある。
【０００４】
また、▲２▼特開平２−２１７４２５号には、冷間圧延後、Ａｃ１ 変態点以上の７００〜８００℃に加熱し、所定の冷却速度で冷却する方法が開示されており、これは、均熱時からオーステナイト中にＣ，Ｍｎ等を濃化させ、フェライトマトリックスの間に残留オーステナイト及びベイナイトを微細分散化させようとするものである。しかしながら、この様な熱サイクルでは均熱時のオーステナイト化面積率が低い為、オーステナイト化されないフェライト部分では熱延時の組織や冷延の影響を受け易く、焼鈍後に板幅方向における伸びのバラツキが大きくなり、均質な組織を得ることは困難である。
【０００５】
また、▲３▼特開平４−３３３５２４号には、Ａｃ１ 〜Ａｃ３ 変態温度の二相域に均熱した後、一段冷却で１〜１０℃／秒の緩冷却を行ってフェライトを析出させてから、続く二段冷却で１０〜２００℃／秒の急冷却を４５０℃以下まで行う方法が開示されている。しかしながら、この様な緩冷却→急冷却を行うと冷却ムラにより均質な鋼板が得られない恐れがある。
【０００６】
更に▲４▼特開平５−５９４２９号には、熱延鋼板の組織に応じて焼鈍時の均熱温度を変化させ、その後、概ね上記▲３▼に記載の方法で緩冷却→急冷却を行う方法が開示されている。上記公報によれば、熱延鋼板の組織をフェライト＋ベイナイトとした場合、焼鈍時の均熱温度はＡｃｌ 〜Ａｃ３ の間の比較的低温で焼鈍することになるが、この様な低温域で焼鈍すると、オーステナイト化されないフェライト部は熱延前の組織や冷延による影響を受け易く、焼鈍後に均質な組織を得ることが困難であることは前述した通りである。一方、熱延時にフェライト＋パーライト組織となる高温で巻取った場合、高Ｓｉ鋼では、熱延時の鋼板表面における粒界酸化が発生し易く、鋼板表面にスケール状の欠陥が発生して美麗な表面を得ることは困難である。
【０００７】
その他、▲５▼特開平５−１２５４４８号には、６００℃以上で熱延巻取した後、冷延、連続焼鈍するに当たり、６００〜４８０℃の温度範囲を２０℃／秒以上の冷却速度で冷却する方法が開示されている。しかしながら、上記方法では高Ｓｉ鋼を用いており、巻取温度が６００℃以上と高くなると、粒界酸化により美麗な表面が得られ難い。
【０００８】
【発明が解決しようとする課題】
本発明は上記事情に鑑みてなされたものであり、その目的は、板幅方向における伸びのバラツキが改善された高強度高延性冷延鋼板、および高強度高延性冷延鋼板を効率よく製造することのできる方法を提供することにある。
【０００９】
【課題を解決するための手段】
上記課題を解決することのできた本発明に係る高強度高延性冷延鋼板とは、フェライト、ベイナイト及び３％以上の残留オーステナイトを有する高強度高延性冷延鋼板であって、更に、下記方法で測定される４個のΔＥｌのうち、最大値が２．０％以下に抑制されることにより板幅方向における伸びのバラツキが改善された高強度高延性冷延鋼板であるところに要旨を有する。
【００１０】
冷延鋼板の板幅をｗ（ｍｍ）としたとき、ＪＩＳ５号引張試験片の圧延方向中心線が上記冷延鋼板の両端部から夫々４０ｍｍずつの位置，及び一方の端部からｗ／４，ｗ／２，３ｗ／４となる各位置から合計５点採取して夫々引張試験を行ったとき、ｗ／２の位置における引張試験片の伸びと、それ以外の各位置における引張試験片の伸びの差をΔＥｌとする。
【００１１】
ここで、Ｃ：０．０５〜０．１５％（ｍａｓｓ％，以下同じ），Ｓｉ：０．５〜２．０％，Ｍｎ：１．０〜２．０％，Ａｌ：０．０１〜２．０％を含有するもの；更に、Ｎｉ≦１．０％（０％を含まない），Ｃｒ≦１．０％（０％を含まない），及びＭｏ≦０．５％（０％を含まない）よりなる群から選択される少なくとも１種を含有するものは本発明の好ましい態様である。
【００１２】
また、上記課題を解決し得た本発明の高強度高延性冷延鋼板を製造する方法は、熱延鋼板を冷間圧延した後、焼鈍して冷延鋼板を製造する方法において、
８００℃超Ａｃ３ 点未満で３０秒〜５分間均熱する工程、
４５０〜５５０℃の温度範囲まで一次冷却する工程、
４５０〜４００℃までの温度範囲を、一次冷却速度に比べて小さい冷却速度で二次冷却する工程、及び
４５０〜４００℃で１分間以上保持する工程
を含むところに要旨を有する。
【００１３】
具体的には、上記一次冷却工程は１０℃／秒を超える冷却速度で冷却し、二次冷却工程は１０℃／秒未満の冷却速度で冷却することが好ましい。
【００１４】
また、二次冷却工程において、歪みを付与し、曲げ戻し応力を１回以上付与するもの；熱間圧延後５５０℃以下で巻取ってから冷間圧延するもの；Ｃ：０．０５〜０．１５％，Ｓｉ：０．５〜２．０％，Ｍｎ：１．０〜２．０％，Ａｌ：０．０１〜２．０％を含有するもの；更に、Ｎｉ≦１．０％（０％を含まない），Ｃｒ≦１．０％（０％を含まない），及びＭｏ≦０．５％（０％を含まない）よりなる群から選択される少なくとも１種を含有するものは本発明の好ましい態様である。
【００１５】
【発明の実施の形態】
前述した通り、残留オーステナイトの変態誘起塑性を利用することにより加工性を向上させる方法は広く知られている。しかしながら、残留オーステナイト量はたとえ僅かな変化であっても伸びに著しく影響する為、所定の残留オーステナイトを効率よく得る為に保持温度を厳密に管理する必要があるのに対し、従来の方法ではその保持温度域まで冷却するに当たり、冷却速度の範囲が広過ぎる為、鋼板の板幅方向（長さ方向）における材質のバラツキが生じ易いことが分かった。更に、未変態オーステナイト中に炭素を効果的に濃化させ、オーステナイトの安定化を図る為に高Ｓｉ鋼が用いられているが、Ｓｉの粒界酸化により表面性状が劣化し、美麗な表面が得られ難いことも懸念される。
【００１６】
本発明者らは上記事情に鑑み、高強度高延性冷延鋼板であって、更に鋼板の板幅方向における伸びのバラツキも改善された均質な鋼板を提供すべく鋭意検討してきた。その結果、冷延鋼板を均熱した後、４５０〜５５０℃の温度範囲まで一次冷却し、４５０〜４００℃の温度範囲を、該一次冷却速度に比べて小さい冷却速度で二次冷却するという「急冷→徐冷」の二段冷却方法を採用すれば所期の目的を達成し得ることを見出し、本発明を完成した。この方法によれば、所望の高強度高延性冷延鋼板が効率よく得られるのみならず、板幅方向における伸びのバラツキも少ない均質な鋼板が得られ、且つ、たとえ高Ｓｉ鋼であっても表面性状の美麗な鋼板が得られる点で極めて有用である。
【００１７】
この様に本発明は、鋼板組織中に残留オーステナイトを導入することにより延性を改善すると共に、特に、連続焼鈍時の冷却工程を「急冷→徐冷」という二段冷却することによりオーステナイトからベイナイトへの変態を速やかに行って残留オーステナイト量を一層増加させ、延性を著しく高めて板幅方向の材質を均質化させたところに最重要ポイントが存在する。焼鈍時の冷却を「急冷→徐冷」することにより残留オーステナイト量を一層高めるという本発明の技術的思想は従来より知られておらず、新規である。
【００１８】
例えば前記従来技術に掲げた公報のうち、▲１▼〜▲４▼の方法には焼鈍時の冷却を二段冷却する方法が開示されている。しかしながら、これらの方法はいずれも、一次冷却に比べ、二次冷却時の冷却速度を大きくする「徐冷→急冷」方法を採用しており、本発明における「急冷→徐冷」方法とは相違する。上記▲１▼〜▲４▼の如く「徐冷→急冷」する方法では、冷却ムラが起こり、板幅方向における伸びのバラツキが大きくなってしまう（後記する実施例を参照）。また、上記▲５▼の方法には、連続焼鈍するに当たり、６００〜４８０℃の温度範囲を２０℃／秒以上の冷却速度で冷却する旨記載されているのみであり、二段冷却すること自体全く記載されていない。このことは、そもそも従来の方法は、「強度及び延性」を高めることのみを目的としており、板幅方向における伸びのバラツキを改善しようという課題すら提起されていなかったことを裏付けるものである。高強度高延性に加えて、更に、板幅方向における伸びのバラツキ（均質化）を改善し得る冷延鋼板を提供しようという課題のもとに検討されたものは従来知られておらず、この様な新規な課題のもと、該課題を解決する為に上記要件を特定したところに本発明の技術的意義が存在するのである。
以下、本発明の冷延鋼板について詳述する。
【００１９】
前述した通り、本発明鋼板は、フェライト、ベイナイト及び３％以上の残留オーステナイトを有する高強度高延性冷延鋼板であって、更に、上記方法で測定される４個のΔＥｌのうち、最大値が２．０％以下に抑制されることにより板幅方向における伸びのバラツキが改善されたものである。本発明鋼板は強度および延性の高められた冷延鋼板であることが前提であり、具体的には、引張強度（ＴＳ）が５５０ＭＰａ以上であり、且つ３％以上の残留オーステナイトを生成させることにより高延性を付与するものである。しかしながら、この点に本発明の技術的特徴があるのではなく、上記ΔＥｌの最大値が２．０％以下に抑制されたものであるところに最重要ポイントが存在するのであり、これにより、従来の方法では達成されなかった、板幅方向における伸びのバラツキを著しく改善し得るという極めて顕著な効果が得られるのである。
【００２０】
この様に本発明は、板幅方向における伸びのバラツキを改善させたところに最大の特徴を有しており、鋼板の成分組成によって本発明を限定する趣旨では決してないが、以下、本発明に適用される好ましい成分組成について説明する。
【００２１】
Ｃ：０．０５〜０．１５％
Ｃは、引張強度（ＴＳ）≧５５０ＭＰａを確保し、３％以上の残留オーステナイトを生成させる為に重要な元素であり、その為には０．０５％以上添加することが好ましい。より好ましくは０．０７％以上である。しかしながら、過剰に添加するとスポット溶接性が著しく阻害され、実用的でない。より好ましくは０．１４％以下である。
【００２２】
Ｓｉ：０．５〜２．０％
Ｓｉは、炭化物の生成を抑制して安定な残留オーステナイトを生成させる為に重要であり、その為には、０．５％以上の添加が好ましい。より好ましくは０．８％以上である。しかしながら、２．０％を超えて過剰に添加すると、鋼の製造費用が上昇するのみならず、スラブ割れや表面拉界酸化に対する感受性が高まるので好ましくない。より好ましくは１．７％以下である。
【００２３】
Ｍｎ：１．０〜２．０％
Ｍｎは、高強度を安定して確保すると共に焼入れ性向上元素として有用であり、この様な効果を有効に得るためには１．０％以上添加することが好ましい。より好ましくは１．２％以上である。しかしながら、２．０％を超えて過剰に添加すると、延性に有害なバンド状組織となり易く、伸びの低下を招く。より好ましくは１．８％以下である。
【００２４】
Ａｌ：０．０１〜２．０％
Ａｌは脱酸剤として有用であるのみならず、Ｓｉと同様、炭化物の生成を抑制し、安定な残留オーステナイトを生成させるのに有用である。この様な効果を有効に発揮させる為には０．０１％以上添加することが好ましい。より好ましくは０．０３％以上である。しかしながら、２．０％を超えて過剰に添加すると、必要以上にオーステナイトを安定化させ、いわゆる変態誘起組成（ＴＲＩＰ効果）が起こらない為、伸びが低下してしまう。より好ましくは１．８％以下である。
【００２５】
本発明鋼板は、基本的に上記成分を含有し、残部：Ｆｅ及び不可避的不純物であるものが好ましいが、残留オーステナイト量の更なる上昇を目指して、更に、下記成分を積極的に含有することが推奨される。
【００２６】
Ｎｉ≦１．０％（０％を含まない），Ｃｒ≦１．０％（０％を含まない），及びＭｏ≦０．５％（０％を含まない）よりなる群から選択される少なくとも１種Ｎｉ，Ｃｒ及びＭｏは、いずれも焼入れ性向上元素として知られており、生成したオーステナイト量を低温域でも確保するのに有効である。この様な効果を有効に発揮させる為には、Ｎｉ：０．１％以上、Ｃｒ：０．１％以上、Ｍｏ：０．１％以上添加することが推奨される。しかしながら、Ｎｉ＞１．０％，Ｃｒ＞１．０％，Ｍｏ＞０．５％になると、その効果が飽和すると共に、鋼板の製造費用が上昇する為、好ましくない。より好ましくは、Ｎｉ≦０．２％，Ｃｒ≦０．２％，Ｍｏ≦０．２％である。尚、これらの元素は一種のみ添加しても良いし、或いは、二種以上を添加しても構わない。
次に、本発明の製造方法について説明する。
【００２７】
上述した通り、本発明の方法は、熱延鋼板を冷間圧延した後、焼鈍して冷延鋼板を製造する方法において、
（１）８００℃超Ａｃ３ 点未満で３０秒〜５分間均熱する工程、
（２）４５０〜５５０℃の温度範囲まで一次冷却する工程、
（３）４５０〜４００℃までの温度範囲を、一次冷却速度に比べて小さい冷却速度で二次冷却する工程、及び
（４）４５０〜４００℃で１分間以上保持する工程
を含むところに特徴を有するものであり、この様にして得られた鋼板は、強度及び延性の非常に高められたものとなる。更に上記方法を採用すれば、強度及び延性の上昇に加え、板幅方向における伸びのバラツキも改善することができ、そのうえ、美麗な表面性状が得られる等、非常に優れた冷延鋼板を得ることができる点で極めて有用である。
以下、上記（１）〜（４）の各工程について詳述する。
【００２８】
（１）８００℃超Ａｃ ３ 点未満で３０秒〜５分間均熱する工程
均熱は８００℃超Ａｃ３ 点未満の二相域で行うことが必要である。８００℃以下では、均熱時のオーステナイト化率が小さい為、オーステナイト化しないフェライト中に熱延時の組織が残存したり、冷延による影響を払拭することができず、所望の均質な鋼板が得られない。一方、Ａｃ３ 点以上で均熱すると、均熱後の冷却過程においてオーステナイトが十分に濃化せず、残留オーステナイト量が低下し、所望の伸びを得るのに悪影響を及ぼす。好ましくは８３０℃以上、８７０℃以下である。
【００２９】
また、均熱時間は３０秒以上行うことが必要である。３０秒未満では、炭化物が十分に溶解せずオーステナイト中のＣ濃度が低下する為、残留オーステナイト量が低下してしまう。一方、５分を超えて均熱しても均熱効果は飽和してしまう他、製造ラインを長くする必要がある等、不経済である。好ましくは６０秒以上である。
【００３０】
（２）４５０〜５５０℃の温度範囲まで一次冷却する工程、及び（３）４５０〜４００℃までの温度範囲を、一次冷却速度に比べて小さい冷却速度で二次冷却する工程
上記の均熱に続く冷却工程が本発明において最も重要であり、詳細には、（２）の一次冷却工程と、（３）の二次冷却工程に分けられる。
【００３１】
まず、上記（１）の如く高温で均熱した後、（２）の一次冷却を行う。本発明では、一次冷却工程の冷却速度を、二次冷却速度の冷却速度よりも大きい速度で冷却する、即ち「急冷→徐冷」という二段冷却するところに最重要ポイントが存在し、これにより、所望の均質化特性を付与することができる。
【００３２】
具体的には、一次冷却は、均熱温度から、４５０〜５５０℃の温度範囲までを、１０℃／秒を超える速度で冷却することが推奨される。この様に一次冷却工程を急速冷却することにより、ベイナイト変態へのドライビングフォースが高まり、オーステナイトからベイナイトへの変態をより速やかに行うことが可能になる。１０℃／秒以下の冷却速度では、冷却途中でパーライトが析出し易くなり、残留オーステナイト量が減少して伸びが低下してしまう。
【００３３】
次に行う（３）の二次冷却工程は、４５０〜４００℃の温度範囲を、一次冷却速度に比べて小さい冷却速度で冷却することが必要であり、具体的には、１０℃／秒未満の冷却速度で冷却することが推奨される。残留オーステナイト組織を効率よく得る為には、（４）で後述する通り、ベイナイトへ効率よく変態させる為に、４００〜４５０℃という非常に狭い温度範囲で等温保持することが必要であるが、二次冷却工程における冷却速度が１０℃／秒以上では、所望の保持温度に調整するのが極めて困難であり、また、板幅方向での冷却ムラが生じ易くなるからである。冷却ムラは鋼板温度の不均一化を招き、鋼板の残留オーステナイト量に悪影響を及ぼす為、本発明で目的とするところの所望の均質特性を発揮でき難くなる。冷却速度は小さくなればなる程好ましく、８℃／秒以下、より好ましくは６℃／秒以下に制御することが推奨される。
【００３４】
尚、二次冷却工程において、歪みを付与し、曲げ戻し応力を１回以上（より好ましくは４回以上）付与することは、ベイナイト変態を一層促進し、結果的に残留オーステナイト量の更なる増加、ひいては伸びの向上が得られる点で極めて有効である。その理由は詳細には不明であるが、ベイナイト変態温度近傍でフェライト中に歪みを加えることにより、Ｃの拡散が一層容易になってオーステナイト中へＣの濃化が進み、残留オーステナイト量が増加するのではないかと考えられる。一層優れた特性を得るためには、歪み量が１．０％以下になる様曲げることが推奨される。
【００３５】
（４）４５０〜４００℃で１分間以上保持する工程
この工程はベイナイト変態を極めて効率的に生成させるのに重要であり、本発に用いられる好ましい鋼成分組成等を考慮すれば、４００〜４５０℃という非常に狭い温度範囲で等温保持することが必要である。保持時間が１分間未満では、所望の効果が得られない。好ましくは２分間以上、より好ましくは２．５分間以上である。一方、保持時間が長過ぎると、ライン長が必要以上に長くなる為、不経済であることから、より好ましくは１０分以下に制御することが推奨される。
【００３６】
本発明の方法は上記（１）〜（４）の工程を必須工程として含むものであり、その他の条件は特に限定されないが、更に一層優れた特性の付与を目指して、熱間圧延後５５０℃以下で巻取ってから冷間圧延することが推奨される。巻取温度が５５０℃を超えると熱延鋼板表面において粒界酸化が起こり易くなり、美麗な表面が得られないからである。より好ましくは５２０℃以下である。尚、その下限は特に限定されないが、巻取温度の安定性、冷却帯が余計に長くなる等の経済性等を考慮すれば、３５０℃以上に制御することが推奨される。
【００３７】
尚、熱間圧延工程は特に限定されず、鋼板を常法に従ってＡｒ３ 点以上の温度で熱間圧延を終了した後、上記温度で巻取れば良い。
【００３８】
以下、実施例を挙げて本発明をより詳細に説明するが、本発明は、もとより下記実施例によって制限されるものでは決してなく、前・後記の主旨に適合し得る範囲で適切に変更して実施することも勿論可能であり、いずれも本発明の技術的範囲内に包含される。
【００３９】
【実施例】
表１に示す成分組成からなるＡ〜Ｎの各鋼を溶製してスラブとした後、１２００℃まで加熱して８８０℃で熱延を終了し、板厚２．０ｍｍ×板幅１２００ｍｍの鋼板を得た。次に、表２に示す温度で巻取った後、酸洗して冷間圧延を行い、板厚０．８ｍｍの鋼板を得た。更に、表２に併記するａ〜ｍの条件で連続焼鈍し、０．３％の調質圧延を行った後、ＪＩＳ５号引張試験片の圧延方向中心線が冷延鋼板の端部から４０ｍｍ、３００ｍｍ（ｗ／４，ｗ＝１６００ｍｍ），６００ｍｍ（ｗ／２），３ｗ／４（９００ｍｍ）、及び１１６０ｍｍとなる各位置から合計５点採取して夫々引張試験を行ったとき、６００ｍｍ（ｗ／２）の位置における引張試験片の伸びと、それ以外の各位置における引張試験片の伸びの差をΔＥｌとして算出すると共に、残留オーステナイト量をＸ線にて測定した。更に、引張試験を行って降伏点（ＹＰ）、引張強度（ＴＳ）、及び伸び（Ｅｌ）を夫々測定すると共に、［ＴＳ×Ｅｌ］も算出した。また、得られた各鋼板の表面性状を肉眼観察し、下記三段階にて評価した。
◎：表面性状に優れる
○：粒界酸化による肌荒れの程度が軽微
×：粒界酸化による肌荒れの程度が大きい
これらの結果を表３及び表４に示す。
【００４０】
【表１】
【表２】
【表３】
【表４】
表３及び表４より以下の様に考察することができる。
まず、Ｎｏ．１〜４、８、１３、１５、１７〜１９、２５は本発明の要件を満足する実施例であり、いずれも残留オーステナイト量を３％以上含有し、強度及び延性が著しく高められると共に、ΔＥｌも２．０％以下に抑制されている為、板幅方向における伸びのバラツキも抑制された均質な冷延鋼板が得られた。尚、二次冷却工程で歪みを加えたＮｏ．２，４及び１３では、残留オーステナイト量が更に上昇し、伸びが一層向上していることが分かる。
【００４５】
これに対し、本発明の要件を満足しないその他の鋼板は、夫々以下の様な不具合を有している。
【００４６】
まず、Ｎｏ．５は一次冷却工程における冷却速度（第一次冷却速度）と二次冷却工程における冷却速度（第二次冷却速度）が等しい比較例であり、所望の残留オーステナイト量が得られず、伸びが低下した。
【００４７】
また、Ｎｏ．６は第一次冷却速度が第二次冷却速度に比べて小さい比較例（徐冷→急冷）であり、ΔＥｌが２．０％超える為、均質な冷延鋼板が得られなかった。
【００４８】
また、Ｎｏ．７，１０〜１２，１６は鋼中成分が本発明の好ましい要件を満足しない比較例であり、Ｎｏ．７はＣ量が少ない為、Ｎｏ．１０はＳｉ量が少ない為、Ｎｏ．１１はＭｎ量が少ない為、いずれも３％以上の残留オーステナイト量が得られず；Ｎｏ．１２はＭｎ量が多い為、バンド状組織が生じ、伸びが低下した。
【００４９】
Ｎｏ．１４／Ｎｏ．２１は、均熱温度が低い／均熱時間が短い比較例であり、均熱時のオーステナイト化率が不足して所望のオーステナイト量が得られなった。
【００５０】
Ｎｏ．１６は、Ａｌ量が多い為オーステナイトが安定化しており、歪みを付与しても変態誘起塑性が起こらず、伸びが低下した。
【００５１】
Ｎｏ．２０はＡｃ３ 点以上で均熱した比較例であり、オーステナイトの濃化が腐食し、残留オーステナイト量が低下した。
【００５２】
Ｎｏ．２２は保持温度が低い為、オーステナイトが効率的に濃化されず、残留オーステナイト量が減少し、結果的にマルテンサイト量が増加して伸びが低下した。
【００５３】
Ｎｏ．２３は保持時間が短い為、残留オーステナイト量が少なく、所望の高い延性が得られなかった。
【００５４】
Ｎｏ．２４は保持温度が高い為、オーステナイトがパーライトまたはベイナイトに変態してしまい、強度の低下および残留オーステナイト量の減少による伸びの低下を招いた。
【００５５】
【発明の効果】
本発明は以上の様に構成されており、表面が美麗であり、且つ板幅方向における伸びのバラツキが改善された高強度高延性冷延鋼板を効率よく得ることができた。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a homogeneous high-strength and high-ductility cold-rolled steel sheet with improved variation in elongation in the sheet width direction, and a method for producing a high-strength and high-ductility cold-rolled steel sheet. The high-strength and high-ductility cold-rolled steel sheet of the present invention is widely used in the industrial field where workability is a problem during press forming, such as automotive parts.
[0002]
[Prior art]
In recent years, especially for automotive steel sheets, high-strength cold-rolled steel sheets with excellent press formability, especially ductility, have been widely used for the purpose of weight reduction, and various methods have been proposed to enhance the ductility by utilizing transformation-induced plasticity of retained austenite. Have been.
[0003]
For example, ▲ 1 ▼ to JP-62-217529, as well as heating the soaking temperature of the heat treatment above Ac 3 temperatures are how to properly control the subsequent cooling step disclosed, thereby, 10% Improvement of uniform elongation by transformation-induced plasticity based on the above-described retained austenite phase, enhancement of local elongation or improvement of impact properties by a fine-grained ferrite phase, and securing of strength by a residual bainite phase or a martensite phase are aimed at. However, when the soaking temperature is increased to the temperature of Ac 3 or more as in the above method, it takes a long time to precipitate ferrite. Therefore, the concentration of C, Mn, etc. in austenite does not progress rapidly in the subsequent cooling step, and the target is set. There is a possibility that the amount of retained austenite cannot be obtained.
[0004]
Also, (2) JP-A-2-217425 discloses a method in which after cold rolling, heating is performed at a temperature of 700 to 800 ° C. which is equal to or higher than the Ac 1 transformation point, and cooling is performed at a predetermined cooling rate. C, Mn, etc. are concentrated in the austenite from the time of soaking, and the retained austenite and bainite are finely dispersed between the ferrite matrices. However, since the austenitized area ratio during soaking is low in such a thermal cycle, the ferrite portion that is not austenitized is easily affected by the structure and cold rolling during hot rolling, and the variation in elongation in the sheet width direction after annealing is large. And it is difficult to obtain a homogeneous tissue.
[0005]
(3) Japanese Patent Application Laid-Open No. Hei 4-333524 discloses that after soaking in a two-phase region of Ac 1 to Ac 3 transformation temperature, slow cooling is performed at 1 to 10 ° C./sec by single-stage cooling to precipitate ferrite. After that, a method of performing rapid cooling at 10 to 200 ° C./sec to 450 ° C. or less in a subsequent two-stage cooling is disclosed. However, if such slow cooling → rapid cooling is performed, there is a possibility that a uniform steel sheet cannot be obtained due to uneven cooling.
[0006]
Further, (4) JP-A-5-59429 discloses that the soaking temperature during annealing is changed in accordance with the structure of the hot-rolled steel sheet, and thereafter, the slow cooling → rapid cooling is generally performed by the method described in (3) above. A method is disclosed. According to the above publication, when the structure of the hot-rolled steel sheet is ferrite + bainite, the soaking temperature during annealing is annealing at a relatively low temperature between Ac 1 and Ac 3. As described above, the ferrite portion that is not austenitized when subjected to annealing is easily affected by the structure before hot rolling or the cold rolling, and it is difficult to obtain a uniform structure after annealing. On the other hand, when the steel sheet is wound at a high temperature, which has a ferrite + pearlite structure during hot rolling, in a high-Si steel, grain boundary oxidation is likely to occur on the steel sheet surface during hot rolling, and scale-like defects are generated on the steel sheet surface, resulting in beautiful appearance. It is difficult to get a surface.
[0007]
In addition, (5) JP-A-5-125448 discloses that, after hot rolling at 600 ° C. or higher, cold rolling and continuous annealing, the temperature range of 600 to 480 ° C. is set at a cooling rate of 20 ° C./sec or higher. A method of cooling is disclosed. However, in the above method, a high Si steel is used, and when the winding temperature is as high as 600 ° C. or more, it is difficult to obtain a beautiful surface due to grain boundary oxidation.
[0008]
[Problems to be solved by the invention]
The present invention has been made in view of the above circumstances, and an object of the present invention is to efficiently produce a high-strength high-ductility cold-rolled steel sheet with improved dispersion in elongation in the sheet width direction, and a high-strength high-ductility cold-rolled steel sheet. It is to provide a method that can do it.
[0009]
[Means for Solving the Problems]
The high-strength high-ductility cold-rolled steel sheet according to the present invention that can solve the above-mentioned problems is a high-strength high-ductility cold-rolled steel sheet having ferrite, bainite, and 3% or more retained austenite, and is further obtained by the following method. The gist lies in that it is a high-strength and high-ductility cold-rolled steel sheet whose variation in elongation in the sheet width direction is improved by suppressing the maximum value among the four measured ΔEls to 2.0% or less.
[0010]
Assuming that the width of the cold-rolled steel sheet is w (mm), the center line in the rolling direction of the JIS No. 5 tensile test piece is 40 mm each from both ends of the cold-rolled steel sheet, and w / 4 from one end. When a total of 5 points were sampled from each of the positions w / 2 and 3w / 4, and the tensile test was performed, the elongation of the tensile test specimen at the position of w / 2 and the elongation of the tensile test specimen at each other position Is defined as ΔEl.
[0011]
Here, C: 0.05 to 0.15% (mass%, the same applies hereinafter), Si: 0.5 to 2.0%, Mn: 1.0 to 2.0%, Al: 0.01 to 2 Further, Ni ≦ 1.0% (not including 0%), Cr ≦ 1.0% (not including 0%), and Mo ≦ 0.5% (including 0%) No.) is a preferred embodiment of the present invention containing at least one selected from the group consisting of:
[0012]
Further, the method for producing a high-strength high-ductility cold-rolled steel sheet of the present invention that has solved the above-mentioned problems includes, after cold-rolling a hot-rolled steel sheet, a method for producing a cold-rolled steel sheet by annealing.
A step of soaking at a temperature of more than 800 ° C. Ac and less than 3 points for 30 seconds to 5 minutes;
Primary cooling to a temperature range of 450 to 550 ° C,
The gist is that the method includes a step of performing secondary cooling at a temperature range of 450 to 400 ° C. at a cooling rate smaller than the primary cooling rate, and a step of maintaining the temperature at 450 to 400 ° C. for 1 minute or more.
[0013]
Specifically, the primary cooling step is preferably performed at a cooling rate exceeding 10 ° C./sec, and the secondary cooling step is preferably performed at a cooling rate of less than 10 ° C./sec.
[0014]
In the secondary cooling step, a strain is imparted and a bending-back stress is imparted one or more times; after hot rolling, it is wound at 550 ° C. or less and then cold-rolled; C: 0.05 to 0. 15%, Si: 0.5 to 2.0%, Mn: 1.0 to 2.0%, Al: 0.01 to 2.0%; Ni ≦ 1.0% (0 %), Cr ≦ 1.0% (not including 0%), and Mo ≦ 0.5% (not including 0%). This is a preferred embodiment of the present invention.
[0015]
BEST MODE FOR CARRYING OUT THE INVENTION
As described above, a method of improving workability by utilizing transformation-induced plasticity of retained austenite is widely known. However, the amount of retained austenite, even a slight change, significantly affects the elongation. Therefore, it is necessary to strictly control the holding temperature in order to efficiently obtain a predetermined retained austenite. It was found that when cooling to the holding temperature range, the range of the cooling rate was too wide, so that the material of the steel sheet tended to vary in the width direction (length direction). Furthermore, high Si steel is used to effectively enrich carbon in untransformed austenite and stabilize austenite, but the surface properties are degraded by the grain boundary oxidation of Si, resulting in a beautiful surface. There is also concern that it is difficult to obtain.
[0016]
In view of the above circumstances, the present inventors have intensively studied to provide a high-strength, high-ductility cold-rolled steel sheet and a homogeneous steel sheet in which the variation in elongation in the sheet width direction is further improved. As a result, after soaking the cold-rolled steel sheet, the primary cooling is performed to a temperature range of 450 to 550 ° C., and the secondary cooling is performed in a temperature range of 450 to 400 ° C. at a cooling rate smaller than the primary cooling rate. The inventor has found that the intended purpose can be achieved by employing a two-stage cooling method of “quenching → gradual cooling”, and completed the present invention. According to this method, not only a desired high-strength high-ductility cold-rolled steel sheet can be efficiently obtained, but also a homogeneous steel sheet having less variation in elongation in the sheet width direction is obtained, and even if a high Si steel is used. This is extremely useful in that a beautiful steel sheet having a good surface texture can be obtained.
[0017]
As described above, the present invention improves ductility by introducing residual austenite into the steel sheet structure, and in particular, converts the austenite to bainite by performing a two-stage cooling process of “quenching → gradual cooling” during the continuous annealing. The most important point exists when the transformation is rapidly performed to further increase the amount of retained austenite, to significantly increase the ductility, and to homogenize the material in the width direction of the sheet. The technical idea of the present invention to further increase the amount of retained austenite by "quenching → slow cooling" during annealing has not been known conventionally and is novel.
[0018]
For example, in the above-mentioned publications, methods (1) to (4) disclose a method of performing two-stage cooling during annealing. However, each of these methods employs a “slow cooling → rapid cooling” method in which the cooling rate during secondary cooling is increased as compared with the primary cooling, which is different from the “rapid cooling → slow cooling” method in the present invention. I do. In the method of “gradual cooling → rapid cooling” as described in the above (1) to (4), cooling unevenness occurs and variation in elongation in the plate width direction becomes large (see examples described later). Further, the method of (5) only describes that in the continuous annealing, the temperature range of 600 to 480 ° C. is cooled at a cooling rate of 20 ° C./sec or more. Not at all described. This confirms that the conventional method is intended only to enhance the "strength and ductility" in the first place, and has not raised the problem of improving the variation in elongation in the plate width direction. In addition to high strength and high ductility, what has been studied under the problem of providing a cold-rolled steel sheet that can further improve the variation (homogenization) of elongation in the sheet width direction has not been known so far. Under such new problems, the technical significance of the present invention exists when the above requirements are specified in order to solve the problems.
Hereinafter, the cold rolled steel sheet of the present invention will be described in detail.
[0019]
As described above, the steel sheet of the present invention is a high-strength high-ductility cold-rolled steel sheet having ferrite, bainite, and 3% or more retained austenite, and further, among the four ΔEl measured by the above method, the maximum value is four. When the content is suppressed to 2.0% or less, the variation in elongation in the plate width direction is improved. It is premised that the steel sheet of the present invention is a cold-rolled steel sheet having enhanced strength and ductility. Specifically, the steel sheet has a tensile strength (TS) of 550 MPa or more and generates 3% or more of retained austenite. It imparts high ductility. However, this point does not have the technical feature of the present invention, and the most important point exists where the maximum value of ΔEl is suppressed to 2.0% or less. Thus, an extremely remarkable effect that variation in elongation in the plate width direction, which has not been achieved by the method described above, can be remarkably improved can be obtained.
[0020]
As described above, the present invention has the greatest feature in that the variation in elongation in the sheet width direction is improved, and is not intended to limit the present invention by the component composition of the steel sheet. The preferred component composition applied will be described.
[0021]
C: 0.05-0.15%
C is an important element for ensuring a tensile strength (TS) ≧ 550 MPa and generating 3% or more of retained austenite. For this reason, it is preferable to add 0.05% or more. More preferably, it is 0.07% or more. However, excessive addition significantly impairs spot weldability and is not practical. More preferably, it is 0.14% or less.
[0022]
Si: 0.5 to 2.0%
Si is important for suppressing the generation of carbides and for generating stable retained austenite. For this purpose, 0.5% or more is preferably added. It is more preferably at least 0.8%. However, an excessive addition exceeding 2.0% is not preferred because not only increases the cost of producing steel, but also increases the susceptibility to slab cracking and surface oxidation. It is more preferably at most 1.7%.
[0023]
Mn: 1.0-2.0%
Mn is useful as an element for improving hardenability while ensuring high strength stably, and it is preferable to add 1.0% or more to effectively obtain such an effect. It is more preferably at least 1.2%. However, if it is added in excess of 2.0%, a band-like structure that is harmful to ductility is likely to occur, resulting in a decrease in elongation. It is more preferably at most 1.8%.
[0024]
Al: 0.01 to 2.0%
Al is not only useful as a deoxidizing agent, but also similar to Si, suppresses the formation of carbides and is useful for forming stable retained austenite. In order to exhibit such effects effectively, it is preferable to add 0.01% or more. More preferably, it is at least 0.03%. However, if added in excess of 2.0%, the austenite is stabilized more than necessary, and the so-called transformation-inducing composition (TRIP effect) does not occur, so that the elongation decreases. It is more preferably at most 1.8%.
[0025]
The steel sheet of the present invention basically contains the above-mentioned components, and the balance: Fe and inevitable impurities are preferable. However, in order to further increase the amount of retained austenite, the steel plate further contains the following components positively. Is recommended.
[0026]
At least one selected from the group consisting of Ni ≦ 1.0% (not including 0%), Cr ≦ 1.0% (not including 0%), and Mo ≦ 0.5% (not including 0%) One of Ni, Cr and Mo is known as a hardenability improving element, and is effective in securing the generated austenite amount even in a low temperature range. In order to effectively exhibit such effects, it is recommended to add Ni: 0.1% or more, Cr: 0.1% or more, and Mo: 0.1% or more. However, when Ni> 1.0%, Cr> 1.0% and Mo> 0.5%, the effects are saturated and the production cost of the steel sheet increases, which is not preferable. More preferably, Ni ≦ 0.2%, Cr ≦ 0.2%, Mo ≦ 0.2%. Incidentally, these elements may be added alone or in combination of two or more.
Next, the manufacturing method of the present invention will be described.
[0027]
As described above, the method of the present invention is a method for producing a cold-rolled steel sheet by cold-rolling a hot-rolled steel sheet, followed by annealing.
(1) a step of soaking at a temperature higher than 800 ° C. and less than 3 points Ac for 30 seconds to 5 minutes;
(2) a step of primary cooling to a temperature range of 450 to 550 ° C,
(3) a step of secondary cooling the temperature range from 450 to 400 ° C. at a lower cooling rate than the primary cooling rate; and (4) a step of holding the temperature at 450 to 400 ° C. for 1 minute or more. The steel sheet obtained in this way has extremely enhanced strength and ductility. Further, if the above method is adopted, in addition to the increase in strength and ductility, the variation in elongation in the sheet width direction can also be improved, and furthermore, a very excellent cold-rolled steel sheet can be obtained, such as obtaining a beautiful surface property. It is extremely useful in that it can be used.
Hereinafter, the respective steps (1) to (4) will be described in detail.
[0028]
(1) Ac 3 exceeding 800 ° C Step of soaking for 30 seconds to 5 minutes below the point It is necessary to perform soaking in a two-phase region of more than 800 ° C. and less than 3 points of Ac. At 800 ° C. or lower, the austenitization rate during soaking is small, so that the structure during hot rolling remains in the ferrite that does not austenite, and the effect of cold rolling cannot be wiped out, and a desired homogeneous steel sheet can be obtained. I can't. On the other hand, if the heat is soaked at three or more points of Ac, austenite is not sufficiently concentrated in the cooling process after the soaking, and the amount of retained austenite is reduced, which adversely affects the desired elongation. Preferably it is 830 ° C or higher and 870 ° C or lower.
[0029]
The soaking time must be 30 seconds or more. If the time is less than 30 seconds, the carbide is not sufficiently dissolved and the C concentration in the austenite decreases, so that the amount of retained austenite decreases. On the other hand, even if the temperature is soaked for more than 5 minutes, the soaking effect is saturated and the production line needs to be lengthened, which is uneconomical. Preferably, it is 60 seconds or more.
[0030]
(2) a step of performing primary cooling to a temperature range of 450 to 550 ° C, and (3) a step of performing secondary cooling in a temperature range of 450 to 400 ° C at a cooling rate smaller than the primary cooling rate. Is most important in the present invention, and is divided into (2) primary cooling step and (3) secondary cooling step.
[0031]
First, after soaking at a high temperature as in the above (1), the primary cooling (2) is performed. In the present invention, the cooling rate of the primary cooling step, cooling at a rate greater than the cooling rate of the secondary cooling rate, that is, the most important point exists in the two-stage cooling of `` rapid cooling → slow cooling '', , The desired homogenizing properties.
[0032]
Specifically, it is recommended that the primary cooling be performed at a rate exceeding 10 ° C./sec from the soaking temperature to a temperature range of 450 to 550 ° C. By rapidly cooling the primary cooling step in this manner, the driving force to bainite transformation is increased, and the transformation from austenite to bainite can be performed more quickly. At a cooling rate of 10 ° C./second or less, pearlite tends to precipitate during cooling, the amount of retained austenite decreases, and elongation decreases.
[0033]
In the secondary cooling step (3) to be performed next, it is necessary to cool the temperature range of 450 to 400 ° C. at a cooling rate smaller than the primary cooling rate, and specifically, less than 10 ° C./sec. It is recommended to cool at a cooling rate of In order to efficiently obtain a retained austenite structure, it is necessary to maintain isothermal temperature in a very narrow temperature range of 400 to 450 ° C. in order to efficiently transform into bainite as described later in (4). If the cooling rate in the next cooling step is 10 ° C./sec or more, it is extremely difficult to adjust the temperature to a desired holding temperature, and uneven cooling in the width direction of the sheet is likely to occur. The cooling unevenness causes the temperature of the steel sheet to become non-uniform and adversely affects the amount of retained austenite of the steel sheet, so that it is difficult to exhibit the desired homogenous properties intended in the present invention. The cooling rate is preferably as low as possible, and it is recommended to control the cooling rate to 8 ° C./sec or less, more preferably 6 ° C./sec or less.
[0034]
In the secondary cooling step, applying the strain and applying the bending-back stress at least once (more preferably at least four times) further promotes the bainite transformation and consequently further increases the amount of retained austenite. This is extremely effective in improving the elongation. The reason is not clear in detail, but by adding strain in the ferrite near the bainite transformation temperature, the diffusion of C is further facilitated, the concentration of C in austenite proceeds, and the amount of retained austenite increases. It is thought that it may be. In order to obtain more excellent characteristics, it is recommended to bend so that the amount of distortion is 1.0% or less.
[0035]
(4) Step of holding at 450 to 400 ° C. for 1 minute or more This step is important for generating bainite transformation extremely efficiently, and considering the preferable steel composition used for the present invention, etc. , 400-450 ° C. in a very narrow temperature range. If the holding time is less than 1 minute, the desired effect cannot be obtained. It is preferably at least 2 minutes, more preferably at least 2.5 minutes. On the other hand, if the holding time is too long, the line length becomes longer than necessary, which is uneconomical. Therefore, it is more preferable to control the line length to 10 minutes or less.
[0036]
The method of the present invention includes the above-described steps (1) to (4) as essential steps, and other conditions are not particularly limited. It is recommended to wind and cold roll below. If the winding temperature exceeds 550 ° C., grain boundary oxidation is likely to occur on the surface of the hot-rolled steel sheet, and a beautiful surface cannot be obtained. More preferably, it is 520 ° C or lower. The lower limit is not particularly limited, but it is recommended to control the temperature to 350 ° C. or higher in consideration of the stability of the winding temperature, the economics such as an extra long cooling zone, and the like.
[0037]
The hot rolling step is not particularly limited, and the steel sheet may be wound at the above temperature after the hot rolling is completed at a temperature of three or more points of Ar according to a conventional method.
[0038]
Hereinafter, the present invention will be described in more detail with reference to examples.However, the present invention is not necessarily limited to the following examples, and may be appropriately modified within a range that can conform to the gist of the preceding and the following. It is of course possible to implement, and all of them are included in the technical scope of the present invention.
[0039]
【Example】
After smelting each of the steels A to N having the component compositions shown in Table 1 to form a slab, heating to 1200 ° C., finishing hot rolling at 880 ° C., a steel sheet having a thickness of 2.0 mm × a width of 1200 mm Got. Next, after winding at the temperature shown in Table 2, pickling and cold rolling were performed to obtain a steel sheet having a thickness of 0.8 mm. Furthermore, after continuous annealing under the conditions of a to m also described in Table 2 and performing temper rolling of 0.3%, the center line in the rolling direction of the JIS No. 5 tensile test piece was 40 mm from the end of the cold-rolled steel sheet, When a total of five points were sampled from each position of 300 mm (w / 4, w = 1600 mm), 600 mm (w / 2), 3w / 4 (900 mm), and 1160 mm and subjected to a tensile test, respectively, 600 mm (w / The difference between the elongation of the tensile test piece at the position 2) and the elongation of the tensile test piece at other positions was calculated as ΔEl, and the amount of retained austenite was measured by X-ray. Further, a tensile test was performed to measure the yield point (YP), the tensile strength (TS), and the elongation (El), respectively, and [TS × El] was calculated. Further, the surface properties of each of the obtained steel sheets were visually observed and evaluated according to the following three grades.
：: excellent surface properties ：: slight degree of skin roughness due to grain boundary oxidation ×: large degree of skin roughness due to grain boundary oxidation These results are shown in Tables 3 and 4.
[0040]
[Table 1]
[Table 2]
[Table 3]
[Table 4]
From Tables 3 and 4, it can be considered as follows.
First, no. Examples 1 to 4, 8, 13, 15, 17 to 19, and 25 satisfy the requirements of the present invention. Each of the examples contains 3% or more of retained austenite, significantly increases strength and ductility, and has a ΔEl Was also suppressed to 2.0% or less, so that a homogeneous cold-rolled steel sheet in which variation in elongation in the sheet width direction was suppressed was obtained. It should be noted that, in the case of No. 2, 4, and 13, the amount of retained austenite is further increased, and the elongation is further improved.
[0045]
On the other hand, other steel plates that do not satisfy the requirements of the present invention have the following disadvantages.
[0046]
First, no. Comparative Example No. 5 is a comparative example in which the cooling rate (primary cooling rate) in the primary cooling step is equal to the cooling rate (secondary cooling rate) in the secondary cooling step, and a desired amount of retained austenite is not obtained, and the elongation is reduced. did.
[0047]
No. No. 6 is a comparative example (slow cooling → rapid cooling) in which the primary cooling rate was lower than the secondary cooling rate. Since ΔEl exceeded 2.0%, a homogeneous cold-rolled steel sheet could not be obtained.
[0048]
No. Nos. 7, 10 to 12, and 16 are comparative examples in which components in steel do not satisfy the preferred requirements of the present invention. No. 7 has a small amount of C, and No. 10 has a small amount of Si. No. 11 did not provide a residual austenite amount of 3% or more because of a small amount of Mn; Since No. 12 had a large Mn content, a band-like structure was formed and elongation was reduced.
[0049]
No. 14 / No. Comparative Example No. 21 is a comparative example having a low soaking temperature / short soaking time, in which the austenite conversion rate at the time of soaking was insufficient and a desired amount of austenite was not obtained.
[0050]
No. In No. 16, austenite was stabilized due to a large amount of Al, and even when strain was applied, transformation-induced plasticity did not occur and elongation was reduced.
[0051]
No. 20 is a comparative example in which the temperature was soaked at three or more points of Ac, where the austenite concentration corroded and the amount of retained austenite decreased.
[0052]
No. In No. 22, since the holding temperature was low, austenite was not efficiently concentrated, the amount of retained austenite decreased, and as a result, the amount of martensite increased and elongation decreased.
[0053]
No. In No. 23, the retention time was short, the amount of retained austenite was small, and desired high ductility was not obtained.
[0054]
No. In No. 24, since the holding temperature was high, austenite was transformed into pearlite or bainite, resulting in a decrease in strength and a decrease in elongation due to a decrease in the amount of retained austenite.
[0055]
【The invention's effect】
The present invention is configured as described above, and a high-strength and high-ductility cold-rolled steel sheet having a beautiful surface and improved variation in elongation in the sheet width direction can be efficiently obtained.
Claims (6)
鋼中成分が、Ｃ：０．０５〜０．１５％（ｍａｓｓ％，以下同じ），Ｓｉ：０．５〜２．０％，Ｍｎ：１．０〜２．０％，Ａｌ：０．０１〜２．０％を含有し、残部：Ｆｅ及び不可避的不純物である鋼板を用い、かつ前記焼鈍工程が、
８００℃超Ａｃ3点未満で３０秒〜５分間均熱する工程、
４５０〜５５０℃の温度範囲まで一次冷却する工程、
４５０〜４００℃までの温度範囲を、一次冷却速度に比べて小さい冷却速度で二次冷却する工程、及び
４５０〜４００℃で１分間以上保持する工程
を含むことを特徴とする板幅方向における伸びのバラツキが改善された高強度高延性冷延鋼板の製造方法。After cold-rolling a hot-rolled steel sheet, in a method of producing a cold-rolled steel sheet by annealing,
The components in steel are C: 0.05 to 0.15% (mass%, the same applies hereinafter), Si: 0.5 to 2.0%, Mn: 1.0 to 2.0%, Al: 0.01 containing 2.0%, the balance with Fe and unavoidable impurities der Ru steel, and the annealing step,
A step of soaking at a temperature higher than 800 ° C. and less than 3 points Ac for 30 seconds to 5 minutes;
Primary cooling to a temperature range of 450 to 550 ° C,
Elongation in the width direction of the sheet , comprising a step of performing secondary cooling at a temperature range from 450 to 400 ° C. at a cooling rate smaller than the primary cooling rate, and a step of holding at 450 to 400 ° C. for 1 minute or more. A method for producing a high-strength, high-ductility cold-rolled steel sheet with improved dispersion of the steel sheet.
冷延鋼板の板幅をｗ（ｍｍ）としたとき、ＪＩＳ５号引張試験片の圧延方向中心線が上記冷延鋼板の両端部から夫々４０ｍｍずつの位置，及び一方の端部からｗ／４，ｗ／２，３ｗ／４となる各位置から合計５点採取して夫々引張試験を行ったとき、ｗ／２の位置における引張試験片の伸びと、それ以外の各位置における引張試験片の伸びの差をΔＥｌとする。Assuming that the width of the cold-rolled steel sheet is w (mm), the center line in the rolling direction of the JIS No. 5 tensile test piece is 40 mm each from both ends of the cold-rolled steel sheet, and w / 4, from one end. When a total of 5 points were sampled from each of the positions w / 2 and 3w / 4 and a tensile test was performed, the elongation of the tensile test specimen at the position of w / 2 and the elongation of the tensile test specimen at each other position Is defined as ΔEl.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP01246999A JP3583306B2 (en) | 1999-01-20 | 1999-01-20 | Method for producing high-strength and high-ductility cold-rolled steel sheet with improved variation in elongation in the sheet width direction |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP01246999A JP3583306B2 (en) | 1999-01-20 | 1999-01-20 | Method for producing high-strength and high-ductility cold-rolled steel sheet with improved variation in elongation in the sheet width direction |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| JP2000212684A JP2000212684A (en) | 2000-08-02 |
| JP3583306B2 true JP3583306B2 (en) | 2004-11-04 |
Family
ID=11806236
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| JP01246999A Expired - Fee Related JP3583306B2 (en) | 1999-01-20 | 1999-01-20 | Method for producing high-strength and high-ductility cold-rolled steel sheet with improved variation in elongation in the sheet width direction |
Country Status (1)
| Country | Link |
|---|---|
| JP (1) | JP3583306B2 (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| KR101297042B1 (en) | 2011-06-16 | 2013-08-14 | 현대하이스코 주식회사 | High strength cold-rolled steel sheet for automobile with excellent formability and method of manufacturing the steel sheet |
| KR101518580B1 (en) | 2013-09-12 | 2015-05-07 | 주식회사 포스코 | Steel sheet having excellent strength and ductility and method for manufacturing the same |
Families Citing this family (9)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP5644095B2 (en) * | 2009-11-30 | 2014-12-24 | 新日鐵住金株式会社 | High strength steel sheet having good tensile maximum strength of 900 MPa or more with good ductility and delayed fracture resistance, manufacturing method of high strength cold rolled steel sheet, manufacturing method of high strength galvanized steel sheet |
| JP5793971B2 (en) | 2011-06-01 | 2015-10-14 | Ｊｆｅスチール株式会社 | Manufacturing method of high-strength hot-dip galvanized steel sheet with excellent material stability, workability, and plating appearance |
| JP5397437B2 (en) | 2011-08-31 | 2014-01-22 | Ｊｆｅスチール株式会社 | Hot-rolled steel sheet for cold-rolled steel sheet, hot-rolled steel sheet for hot-dip galvanized steel sheet, and manufacturing method thereof excellent in workability and material stability |
| EP2792760B1 (en) | 2011-12-15 | 2018-05-30 | Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.) | High-strength cold-rolled steel sheet having small variations in strength and ductility and manufacturing method for the same |
| JP5860343B2 (en) | 2012-05-29 | 2016-02-16 | 株式会社神戸製鋼所 | High strength cold-rolled steel sheet with small variations in strength and ductility and method for producing the same |
| WO2013180180A1 (en) | 2012-05-31 | 2013-12-05 | 株式会社神戸製鋼所 | High strength cold-rolled steel plate and manufacturing method therefor |
| US11008632B2 (en) | 2016-03-31 | 2021-05-18 | Jfe Steel Corporation | Steel sheet, coated steel sheet, method for producing hot-rolled steel sheet, method for producing cold-rolled full hard steel sheet, method for producing heat-treated sheet, method for producing steel sheet, and method for producing coated steel sheet |
| KR102285523B1 (en) * | 2019-11-20 | 2021-08-03 | 현대제철 주식회사 | Steel sheet having high strength and high formability and method for manufacturing the same |
| WO2024070889A1 (en) * | 2022-09-30 | 2024-04-04 | Ｊｆｅスチール株式会社 | Steel sheet, member, and production methods therefor |
Family Cites Families (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP3569307B2 (en) * | 1994-01-12 | 2004-09-22 | 新日本製鐵株式会社 | High strength composite structure cold rolled steel sheet having excellent workability and a tensile strength of 45 to 65 kgf / mm2, and a method for producing the same |
-
1999
- 1999-01-20 JP JP01246999A patent/JP3583306B2/en not_active Expired - Fee Related
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| KR101297042B1 (en) | 2011-06-16 | 2013-08-14 | 현대하이스코 주식회사 | High strength cold-rolled steel sheet for automobile with excellent formability and method of manufacturing the steel sheet |
| KR101518580B1 (en) | 2013-09-12 | 2015-05-07 | 주식회사 포스코 | Steel sheet having excellent strength and ductility and method for manufacturing the same |
Also Published As
| Publication number | Publication date |
|---|---|
| JP2000212684A (en) | 2000-08-02 |
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| JP3857939B2 (en) | High strength and high ductility steel and steel plate excellent in local ductility and method for producing the steel plate | |
| EP2604715A1 (en) | High-strength cold-rolled steel sheet having excellent workability and impact resistance, and method for manufacturing same | |
| JP2004068050A (en) | High tensile strength cold rolled steel sheet and its manufacturing method | |
| JP2020509208A (en) | Tempered martensitic steel with low yield ratio and excellent uniform elongation and method for producing the same | |
| JP4325998B2 (en) | High-strength hot-dip galvanized steel sheet with excellent spot weldability and material stability | |
| JP4910898B2 (en) | High strength steel plate and manufacturing method thereof | |
| JP3583306B2 (en) | Method for producing high-strength and high-ductility cold-rolled steel sheet with improved variation in elongation in the sheet width direction | |
| EP3561121B1 (en) | Cold-rolled steel sheet having excellent bendability and hole expandability and method for manufacturing same | |
| JP4156889B2 (en) | Composite steel sheet with excellent stretch flangeability and method for producing the same | |
| JP3899680B2 (en) | Paint bake-hardening type high-tensile steel sheet and manufacturing method thereof | |
| JP4320198B2 (en) | Manufacturing method of high-strength cold-rolled steel sheets with excellent impact properties and shape freezing properties | |
| JP2004256836A (en) | High tensile strength hot-dip galvanized steel sheet having excellent strength-elongation balance and fatigue characteristic, and its manufacturing method | |
| JPH11193418A (en) | Manufacture of high strength cold rolled steel sheet excellent in flatness characteristic | |
| JP4299774B2 (en) | High strength low specific gravity steel sheet with excellent ductility and fatigue characteristics and method for producing the same | |
| JP3915460B2 (en) | High strength hot rolled steel sheet and method for producing the same | |
| JP5280795B2 (en) | Method for producing high-strength cold-rolled steel sheet with excellent mechanical property stability | |
| US6797078B2 (en) | Strip of hot rolled steel of very high strength, usable for shaping and particularly for stamping | |
| JP3337246B2 (en) | Method for producing thick H-section steel having a thickness of 40 mm or more with small difference in mechanical properties in the thickness direction | |
| JP2621744B2 (en) | Ultra-high tensile cold rolled steel sheet and method for producing the same | |
| JP2006009057A (en) | Method for producing high strength cold-rolled steel sheet excellent in bending and fatigue resistant characteristics | |
| JP5672736B2 (en) | High-strength thin steel sheet with excellent material stability and manufacturing method thereof | |
| JP3945373B2 (en) | Method for producing cold-rolled steel sheet with fine grain structure and excellent fatigue characteristics | |
| JP2004043884A (en) | Thin steel sheet for working having excellent low temperature seizure hardenability and aging resistance | |
| JP3412157B2 (en) | High-ductility hot-rolled high-strength steel sheet and its manufacturing method | |
| KR20190077193A (en) | High strength steel sheet having high yield ratio and method for manufacturing the same |
Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| A977 | Report on retrieval |
Free format text: JAPANESE INTERMEDIATE CODE: A971007 Effective date: 20040113 |
|
| A131 | Notification of reasons for refusal |
Free format text: JAPANESE INTERMEDIATE CODE: A131 Effective date: 20040120 |
|
| A521 | Written amendment |
Free format text: JAPANESE INTERMEDIATE CODE: A523 Effective date: 20040319 |
|
| A131 | Notification of reasons for refusal |
Free format text: JAPANESE INTERMEDIATE CODE: A131 Effective date: 20040413 |
|
| A521 | Written amendment |
Free format text: JAPANESE INTERMEDIATE CODE: A523 Effective date: 20040614 |
|
| TRDD | Decision of grant or rejection written | ||
| A01 | Written decision to grant a patent or to grant a registration (utility model) |
Free format text: JAPANESE INTERMEDIATE CODE: A01 Effective date: 20040721 |
|
| A61 | First payment of annual fees (during grant procedure) |
Free format text: JAPANESE INTERMEDIATE CODE: A61 Effective date: 20040728 |
|
| R150 | Certificate of patent or registration of utility model |
Free format text: JAPANESE INTERMEDIATE CODE: R150 |
|
| FPAY | Renewal fee payment (event date is renewal date of database) |
Free format text: PAYMENT UNTIL: 20070806 Year of fee payment: 3 |
|
| FPAY | Renewal fee payment (event date is renewal date of database) |
Free format text: PAYMENT UNTIL: 20080806 Year of fee payment: 4 |
|
| FPAY | Renewal fee payment (event date is renewal date of database) |
Free format text: PAYMENT UNTIL: 20080806 Year of fee payment: 4 |
|
| FPAY | Renewal fee payment (event date is renewal date of database) |
Free format text: PAYMENT UNTIL: 20090806 Year of fee payment: 5 |
|
| FPAY | Renewal fee payment (event date is renewal date of database) |
Free format text: PAYMENT UNTIL: 20090806 Year of fee payment: 5 |
|
| FPAY | Renewal fee payment (event date is renewal date of database) |
Free format text: PAYMENT UNTIL: 20100806 Year of fee payment: 6 |
|
| FPAY | Renewal fee payment (event date is renewal date of database) |
Free format text: PAYMENT UNTIL: 20110806 Year of fee payment: 7 |
|
| FPAY | Renewal fee payment (event date is renewal date of database) |
Free format text: PAYMENT UNTIL: 20110806 Year of fee payment: 7 |
|
| FPAY | Renewal fee payment (event date is renewal date of database) |
Free format text: PAYMENT UNTIL: 20120806 Year of fee payment: 8 |
|
| FPAY | Renewal fee payment (event date is renewal date of database) |
Free format text: PAYMENT UNTIL: 20120806 Year of fee payment: 8 |
|
| FPAY | Renewal fee payment (event date is renewal date of database) |
Free format text: PAYMENT UNTIL: 20130806 Year of fee payment: 9 |
|
| LAPS | Cancellation because of no payment of annual fees |