JP3613021B2 - Method for producing cold-rolled steel sheet with excellent press formability and little variation in press formability in the coil - Google Patents
Method for producing cold-rolled steel sheet with excellent press formability and little variation in press formability in the coil Download PDFInfo
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【０００１】
【発明が属する技術分野】
本発明は、自動車の外板等の用途に好適な深絞り性に優れた冷延鋼板の製造方法に関するものである。
【０００２】
【従来の技術】
冷延鋼板は自動車用途等に広く使用されている。自動車用途ではプレス成形される部材が多いため、その部材の形状に応じて様々な加工性が要求され、特に、自動車の外板等の用途では深絞り性に優れたプレス加工用冷延鋼板が求められる。また、最近では自動車メーカーからの合理化の要求が厳しく、特に素材の低廉化及び製品製造時における歩留まり向上に関する要求が強まりつつある。このため冷延鋼板の材質面では、特に材質の均一性の高いことが重要になっている。
【０００３】
このような背景の下、例えば特公昭６０−４５６９２号公報には、連続鋳造−直送圧延プロセスにおいて、０．０１５ｗｔ％以下の極低炭鋼スラブの幅中央での表面温度が９００℃未満６００℃以上の温度範囲で熱間圧延を開始し、さらに熱間圧延工程の途中段階で３０分以内の保持処理を施すことにより、鋼板の表面性状及び深絞り性を向上させようとする技術が開示されている。
【０００４】
【発明が解決しようとする課題】
しかし、この技術では鋼板の表面性状と深絞り性を良好なレベルまで改善はしているものの、コイル内の機械的性質の均一性に大きな問題がある。すなわち、この技術では熱間圧延での加熱温度を低温のフェライト域としているため、圧延中の材料幅方向での温度分布（エッジ及びその近傍での著しい温度低下による温度分布）により熱間圧延後のコイル幅方向での集合組織に差を生じ、この結果、冷間圧延・最終焼鈍後のコイル幅方向の機械的性質にバラツキを生じるという問題がある。
【０００５】
このようにコイルの幅方向で機械的性質にバラツキを生じると、材料（コイル）内での加工性が均一でなくなり、特に自動車外板等の用途のように優れた深絞り性が求められる場合には、プレス成形後の品質に大きな問題（例えば、割れやしわ等の発生）が生じる。この結果、自動車メーカーではコイル内での板取りを歩留まりが低い条件（例えば、板取方向を４５度等の不合理な方向としたり、コイルエッジ近傍からは板取りしない等の条件）で行なわざるを得なくなる。
【０００６】
したがって本発明の目的は、このような従来技術の課題を解決し、自動車の外板等の用途に好適な優れたプレス成形性（深絞り性等）を有し、しかもコイル内でのプレス成形性の変動も少ない冷延鋼板の製造方法を提供することにある。
【０００７】
【課題を解決するための手段】
本発明者等は上述した課題を解決すべく鋭意研究を重ね、その結果、自動車の外板等の用途に好適な優れたプレス成形性を有し、且つコイル内でのプレス成形性の変動も少ない冷延鋼板を得るためには、まず、素材鋼の成分組成を適正化した上で、熱間圧延後の平均結晶粒径［Ｄ］及びコイル幅方向での平均結晶粒径の変動度［Ｄｍａｘ／Ｄｍｉｎ］を特定範囲内にすることが必要であることが判明した。
【０００８】
そして、そのような冷延鋼板は、素材鋼成分を特定の条件に適正化するとともに、(1) 熱間圧延工程での粗圧延バーの仕上げ圧延において、特定の加熱手段で粗圧延バー全体をその全長に亘って加熱して温度調整することにより、熱延コイルの先端部から後端部に至るまでの仕上げ温度を特定の温度範囲に制御する、(2) 熱間圧延後のランナウトでの冷却開始時間と特定の温度域までの平均冷却速度を適正化する、(3) 熱延巻取温度を適正化する、という限定された条件で熱間圧延を行い、このようにして得られた熱延鋼帯を順次酸洗、冷間圧延、最終焼鈍及び調質圧延することにより製造できることが判った。
さらに、上記の冷延鋼板をより合理的に得るには、仕上げ圧延において粗圧延バーを所定の条件で加速圧延することが有効であることも判った。
【０００９】
本発明はこのような知見に基づきなされたもので、その特徴は以下の通りである。
[1] Ｃ：０．０２ｗｔ％以下、Ｓｉ：０．６ｗｔ％以下、Ｍｎ：２．５ｗｔ％以下、Ｐ：０．１０ｗｔ％以下、Ｓ：０．０５ｗｔ％以下、Ｏ：０．００４ｗｔ％以下、Ｓｏｌ．Ａｌ：０．０１〜０．１０ｗｔ％を含有し、さらに、Ｔｉ、Ｎｂ、Ｖ、Ｚｒの中から選ばれる１種又は２種以上を合計で０．０１〜０．４０ｗｔ％含有する鋼からなる鋳片を、熱間圧延工程において粗圧延機により粗圧延し、この粗圧延バーを引き続き連続熱間仕上げ圧延機により仕上げ圧延するに際し、仕上げ圧延機の入側及び／又は仕上げ圧延機のスタンド間に設けられた誘導加熱装置で粗圧延バー全体をその全長に亘って加熱することにより、仕上げ温度が熱延コイルの先端部から後端部に至るまでＡｒ３点〜Ａｒ３点＋３０℃の範囲になるように圧延し、続くランナウトでの冷却を仕上げ圧延終了後１．５秒以内に開始するとともに、仕上げ温度から７５０℃までを２０℃／ｓｅｃ以上の平均冷却速度で冷却した後、巻取温度７００℃以下で巻き取り、得られた熱延鋼帯に酸洗、冷間圧延、最終焼鈍、調質圧延を順次施すことを特徴とするプレス成形性に優れ且つコイル内でのプレス成形性の変動が少ない冷延鋼板の製造方法。
【００１０】
［２］ 上記［１］の製造方法において、スラブがＢ：０．０００１〜０．００５ｗｔ％をさらに含有することを特徴とするプレス成形性に優れ且つコイル内でのプレス成形性の変動が少ない冷延鋼板の製造方法。
［３］ 上記［１］または［２］の製造方法において、仕上げ圧延機の入側及び／又は仕上げ圧延機のスタンド間において、粗圧延バーの幅方向エッジ部を誘導加熱装置により加熱することを特徴とするプレス成形性に優れ且つコイル内でのプレス成形性の変動が少ない冷延鋼板の製造方法。
【００１１】
［４］ 上記［１］〜［３］のいずれかの製造方法において、仕上げ圧延される粗圧延バーの圧延速度を、下記（Ａ）〜（Ｃ）のいずれかに制御することを特徴とするプレス成形性に優れ且つコイル内でのプレス成形性の変動が少ない冷延鋼板の製造方法。
（Ａ） 粗圧延バーの先端部が仕上げ圧延機に入ってから圧延速度を加速し、そののち一定速度で圧延する。
（Ｂ） 粗圧延バーの先端部が仕上げ圧延機に入ってから圧延速度を加速し、そののち一定速度で圧延した後、さらに圧延速度を加速する。
（Ｃ） 粗圧延バーの先端部が仕上げ圧延機に入ってから圧延速度を加速し、そののち圧延速度をさらに加速する。
【００１２】
【発明の実施の形態】
以下、本発明の詳細とその限定理由について説明する。
上述したように自動車の外板等の用途に好適なプレス成形性（深絞り性等）に優れ、且つコイル内でのプレス成形性の変動（バラツキ）が少ない冷延鋼板を製造するためには、素材鋼の成分を最適化した上で、熱間圧延後の平均結晶粒径［Ｄ］とコイル幅方向での平均結晶粒径の変動度［Ｄｍａｘ／Ｄｍｉｎ］、すなわちコイル幅方向での平均結晶粒径の最大値［Ｄｍａｘ］と最小値［Ｄｍｉｎ］の比［Ｄｍａｘ／Ｄｍｉｎ］を特定範囲内にすることが必要である。
【００１３】
そこでまず、本発明における素材鋼の成分組成の限定理由について説明する。Ｃは鋼板の深絞り性に悪影響を及ぼす元素であるため、その含有量は少ない方が好ましい。Ｃ量が０．０２ｗｔ％を超えると本発明が狙いとする深絞り性が得られないため、その含有量は０．０２ｗｔ％以下（但し、無添加の場合を含む）とする。また、深絞り性をさらに向上させるためはＣ量を０．００２０ｗｔ％以下とすることが、また、加工性をさらに高いレベルとするためにはＣ量を０．００１５ｗｔ％以下とすることが好ましい。
【００１４】
Ｓｉは鋼板を固溶強化する作用を有するが、深絞り性に悪影響を及ぼす元素であるため、その含有量は少ないほうが好ましい。Ｓｉ量が０．６ｗｔ％を超えるとめっき性および深絞り性が劣化するため、その含有量は０．６ｗｔ％以下（但し、無添加の場合を含む）とする。また、めっき性をさらに向上させるためにはＳｉ量を０．１ｗｔ％以下とすることが、また、加工性をさらに高いレベルとするためにはＳｉ量を０．０３ｗｔ％以下とすることが好ましい。
【００１５】
Ｍｎは靭性を改善して鋼板を固溶強化する作用を有するが、一方において加工性に悪影響を及ぼす元素でもある。Ｍｎ量が２．５ｗｔ％を超えると強度が上昇し、深絞り性の劣化が著しくなることから、その含有量は２．５ｗｔ％以下（但し、無添加の場合を含む）とする。また、深絞り性の向上のためにはＭｎ量を２．０ｗｔ％以下とすることが、また、加工性をさらに高いレベルとするためにはＭｎ量を０．５ｗｔ％以下とすることが好ましい。
【００１６】
Ｐは鋼板を固溶強化する作用を有するが、Ｐ量が０．１０ｗｔ％を超えると粒界偏析による粒界脆化が生じやすくなり、延性も劣化する。このためＰの含有量は０．１０ｗｔ％以下（但し、無添加の場合を含む）する。また、延性をより向上させるためにはＰ量を０．０５ｗｔ％以下とすることが、また、延性をさらに高いレベルとするためにはＰ量を０．０２ｗｔ％以下とすることが好ましい。
【００１７】
Ｓの含有量が０．０５ｗｔ％を超えると硫化物の析出量が多くなり、深絞り性及び延性が劣化する。このためＳの含有量は０．０５ｗｔ％以下（但し、無添加の場合を含む）とする。また、加工性をより向上させるためにはＳ量を０．０２ｗｔ％以下とすることが、また、加工性をさらに高いレベルとするためにはＳ量を０．０１ｗｔ％以下とすることが好ましい。
【００１８】
Ｓｏｌ．Ａｌは鋼の脱酸材として使用され、さらには後述するＴｉ，Ｎｂ，Ｚｒ，Ｖの添加歩留まりを向上させるために必須の添加元素である。Ｓｏｌ．Ａｌが０．０１ｗｔ％未満では上記の効果が得られず、一方、０．１０ｗｔ％を超えて添加しても効果が飽和するため却って不経済となる。このためＳｏｌ．Ａｌの含有量は０．０１〜０．１０ｗｔ％とする。
【００１９】
Ｏは、その含有量が少ないほど加工性に対しては好ましい。Ｏ量が０．００４ｗｔ％を超えると鋼板の加工性の低下が避けられない。このためＯの含有量は０．００４ｗｔ％以下（但し、無添加の場合を含む）とする。なお、このようなＯ含有量は上記したＳｏｌ．Ａｌ量の調整により達成される。
【００２０】
素材鋼は上記した各成分に加えて、さらにＴｉ，Ｎｂ，Ｖ，Ｚｒの中から選ばれる１種又は２種以上を合計で０．０１〜０．４０ｗｔ％含有する。これらの成分は炭窒化物や硫化物を形成して鋼中のＣ，Ｎ，Ｓを減少させ、加工性を改善するのに役立つ。しかし、これらの合計含有量が０．０１ｗｔ％未満では所望の効果が得られず、一方、０．４０ｗｔ％を超えると強度が上昇し過ぎて加工性が劣化するため、その添加量は０．０１〜０．４０ｗｔ％とする。
【００２１】
さらに、本発明の素材鋼には、耐縦割れ性の向上を目的としてＢを０．０００１〜０．００５ｗｔ％の範囲で添加してもよい。Ｂ量が０．０００１ｗｔ％未満では耐縦割れ性の改善効果が十分に得られず、一方、０．００５ｗｔ％を超えると効果が飽和するため却って経済性を損なう。このためＢの添加量は０．０００１〜０．００５ｗｔ％とする。
【００２２】
次に、本発明の製造条件について説明する。
自動車メーカー等での冷延鋼板の使用条件からして、冷延コイルからの製品の板取りを高歩留まりで行なうためには、上記した素材鋼成分の適正化に加えて熱延コイルの平均結晶粒径［Ｄ］及びコイル幅方向での平均結晶粒径の変動度［Ｄｍａｘ／Ｄｍｉｎ］を適正化することが必要であり、具体的には、熱間圧延後の平均結晶粒径［Ｄ］を５〜５０μｍ、コイル幅方向での平均結晶粒径の変動度［Ｄｍａｘ／Ｄｍｉｎ］を２．０以下とする必要がある。
【００２３】
熱間圧延後の平均結晶粒径［Ｄ］が５μｍ未満では本発明で意図する優れた深絞り性が得られず、一方、平均結晶粒径［Ｄ］が５０μｍを超えると加工時に肌荒れが問題となる。これに対して平均結晶粒径［Ｄ］が５〜５０μｍでは優れた深絞り性が得られ、且つ肌荒れも発生しない。また、より優れた深絞り性を得るためには［Ｄ］を１５〜３０μｍとすることが、また、優れた深絞り性を確保しつつ、強度を高めるためには［Ｄ］を５〜１５μｍとすることが好ましい。
【００２４】
また、熱延コイル幅方向での平均結晶粒径の変動度［Ｄｍａｘ／Ｄｍｉｎ］が２．０を超えると、冷間圧延・最終焼鈍後のコイル幅方向での機械的性質（ｍｅａｎ−ｒ値、破断伸び）の均一性が悪くなり、［Ｄｍａｘ／Ｄｍｉｎ］を２．０以下とすることによりコイル幅方向での機械的性質を均一に保つことができる。
【００２５】
図１は、後述する実施例（表２及び表３）に示された本発明材（材料Ｎｏ．１，［Ｄｍａｘ／Ｄｍｉｎ］：１．５０）と比較材（材料Ｎｏ．２，［Ｄｍａｘ／Ｄｍｉｎ］：２．３５）について、冷間圧延・最終焼鈍後のコイル幅方向でのｍｅａｎ−ｒ値と破断伸び（Ｌ方向）の分布を示しており、［Ｄｍａｘ／Ｄｍｉｎ］が２．０以下である本発明材はコイル幅方向での機械的性質の変動（バラツキ）が小さいのに対し、［Ｄｍａｘ／Ｄｍｉｎ］が２．０を超えた比較材はコイル幅方向での機械的性質が大きく変動している。
【００２６】
本発明の製造方法では、上記のような適正化された熱延コイルの平均結晶粒径［Ｄ］と熱延コイル幅方向での平均結晶粒径の変動度［Ｄｍａｘ／Ｄｍｉｎ］を確保するために、以下のような製造条件を採用する。
まず、上記の成分組成を有する鋼からなる鋳片を、熱間圧延工程において粗圧延機によって粗圧延し、この粗圧延バーを引き続き連続熱間仕上げ圧延機によって仕上げ圧延するが、この際に、仕上げ圧延機の入側及び／又は仕上げ圧延機のスタンド間に設けられた誘導加熱装置で粗圧延バー全体をその全長に亘って加熱することにより、仕上げ温度（仕上げ圧延機の最終スタンドにおける温度）が熱延コイルの先端部から後端部に至るまでＡｒ３点〜Ａｒ３点＋３０℃の範囲となるように圧延する。
【００２７】
このような条件で熱間圧延を行うことによってのみ、コイル幅方向及び長手方向を含めたコイル内のmean-ｒ値及び延性（破断伸び）を本発明が意図するレベルとすることが可能となる。
【００２８】
ここで、仕上げ温度がＡｒ３点未満ではフェライト変態後の熱延となってしまうため［Ｄ］及び［Ｄｍａｘ／Ｄｍｉｎ］が本発明の意図する範囲を超えてしまう。また、仕上げ温度がＡｒ３点＋３０℃を超えると、仕上げ圧延後でもオーステナイト結晶粒の粒成長が著しく、しかもコイル幅方向の温度差もあるため、［Ｄ］及び［Ｄｍａｘ／Ｄｍｉｎ］が本発明の意図する範囲を超えてしまう。
【００２９】
誘導加熱装置による粗圧延バーの加熱は、仕上げ圧延機の入側、仕上げ圧延機のスタンド間の任意の位置で行うことができ、これらのうちの複数の位置（例えば、仕上げ圧延機の入側と仕上げ圧延機のスタンド間の１ヶ所以上）で行ってもよい。
なお、粗圧延バーの温度調整用の加熱装置として誘導加熱装置を用いるのは、この誘導加熱装置を用いることにより、粗圧延バー長手方向の温度分布（変動）に対して、良好な制御性（追従性）で温度制御を行うことができるためである。
【００３０】
図２に、先に述べた本発明材（材料Ｎｏ．１，［Ｄ］：１５μｍ，［Ｄｍａｘ／Ｄｍｉｎ］：１．５０）と比較材（材料Ｎｏ．２，［Ｄ］：１６μｍ，［Ｄｍａｘ／Ｄｍｉｎ］：２．３５）について、コイル先端部から後端部に至るまでの熱延仕上げ温度と冷間圧延・最終焼鈍後のコイル内（コイル幅方向及び長手方向）での機械的性質の変動との関係を示す。これによれば、コイルの先端部から後端部に至るまでの熱延仕上げ温度がＡｒ３点−１０℃〜Ａｒ３点＋３５℃である比較材では［Ｄｍａｘ／Ｄｍｉｎ］が２．０を超え、このためコイル内（コイル幅方向及び長手方向）での冷間圧延・最終焼鈍後のｍｅａｎ−ｒ値及び破断伸びが大きく変動している。これに対して、コイルの先端部から後端部に至るまでの熱延仕上げ温度がＡｒ３点〜Ａｒ３点＋２２℃である本発明材では［Ｄｍａｘ／Ｄｍｉｎ］が２．０以下に抑えられ、このためコイル内（コイル幅方向及び長手方向）での冷間圧延・最終焼鈍後のｍｅａｎ−ｒ値及び破断伸びの変動が適切に抑えられている。
【００３１】
また、本発明の製造方法では、上記の仕上げ温度（仕上げ圧延機の最終スタンドにおける圧延温度）の制御に加えて、仕上げ圧延機の最終スタンドより上流側の各スタンドにおける圧延温度についても、粗圧延バーの先端部から後端部に至るまでの温度をＡｒ３点〜Ａｒ３点＋３０℃となるように圧延することにより、より優れた深絞り性を有し、且つコイル内（コイル長手方向及び幅手方向）での機械的性質の変動のより少ない鋼板を製造することができる。このような粗圧延バーの温度調整も、前記誘導加熱装置により粗圧延バー長手方向の一部又は全部を加熱することによって行う。
【００３２】
本発明の製造方法では、続くランナウトでの冷却を仕上げ圧延終了後１．５秒以内に開始するとともに、仕上げ温度から７５０℃までを２０℃／ｓｅｃ以上の平均冷却速度で冷却した後、巻取温度７００℃以下で巻き取る。
ランナウトでの冷却を仕上げ圧延終了後１．５秒以内に開始することにより、仕上げ圧延後の変態前のオーステナイト結晶粒の粒成長を抑制することができ、本発明が意図する［Ｄ］および［Ｄｍａｘ／Ｄｍｉｎ］を得ることが可能となる。仕上げ圧延終了後冷却開始までの時間が１．５秒を超えると、上記した結晶粒の粒成長を抑制できず、本発明で意図する［Ｄ］および［Ｄｍａｘ／Ｄｍｉｎ］を得ることができない。
【００３３】
また、仕上げ温度から７５０℃までの平均冷却速度を２０℃／ｓｅｃ以上とすることにより、オーステナイト−フェライト変態時のフェライトの核生成頻度が増加し、フェライト粒径を本発明で意図する［Ｄ］および［Ｄｍａｘ／Ｄｍｉｎ］とすることが可能となる。平均冷却速度が２０℃／ｓｅｃ未満では、上記したフェライトの核生成頻度が低く、本発明で意図する［Ｄ］および［Ｄｍａｘ／Ｄｍｉｎ］を得ることができない。
【００３４】
また、熱間圧延後の巻取温度を７００℃以下とすることにより、［Ｄ］および［Ｄｍａｘ／Ｄｍｉｎ］が本発明の意図する範囲に制御された鋼帯において、フェライトの粒成長による粗粒化を抑制することができる。巻取温度が７００℃を超えるとフェライトの粒成長による粗粒化により［Ｄ］および［Ｄｍａｘ／Ｄｍｉｎ］が本発明が意図する範囲を超えてしまう。
【００３５】
また、本発明の製造方法において目的とする鋼板をより合理的に得るためには、仕上げ圧延される粗圧延バーの圧延速度を、粗圧延バーの先端部が仕上げ圧延機に入ってから加速すること、具体的には圧延速度を下記（Ａ）〜（Ｃ）のいずれかに制御することが好ましい。
（Ａ） 粗圧延バーの先端部が仕上げ圧延機に入ってから圧延速度を加速し、そののち一定速度で圧延する。
（Ｂ） 粗圧延バーの先端部が仕上げ圧延機に入ってから圧延速度を加速し、そののち一定速度で圧延した後、圧延速度を加速する。
（Ｃ） 粗圧延バーの先端部が仕上げ圧延機に入ってから圧延速度を加速し、そののち圧延速度をさらに加速する。
【００３６】
このような加速圧延を伴う仕上げ圧延を行うことにより材料の温度低下を極力抑えることができ、このため仕上げ圧延機の入側及び／又は仕上げ圧延機のスタンド間に設けられた誘導加熱装置による粗圧延バーの加熱量を必要最小限に抑え、加熱のためエネルギーを節減することができる。
また、上記（Ａ）〜（Ｃ）の圧延パターンのなかでは、仕上げ圧延機内での加速圧延の度合いは（Ｃ）＞（Ｂ）＞（Ａ）となるため、（Ａ）よりも（Ｂ）、（Ｂ）よりも（Ｃ）の方がより高い効果が得られる。
【００３７】
また、本発明の製造方法においては、仕上げ圧延機の入側及び／又は仕上圧延機のスタンド間において粗圧延バーの幅方向エッジ部を誘導加熱装置により加熱する工程を付加することが好ましい。このようなエッジ部の加熱により粗圧延バー幅方向での温度のバラツキが小さくなり、［Ｄｍａｘ／Ｄｍｉｎ］をより小さい値とすることが可能となる。
また、この誘導加熱装置による粗圧延バーエッジ部の加熱についても、仕上げ圧延機の入側、仕上げ圧延機のスタンド間の任意の位置で行うことができ、これらのうちの複数の位置（例えば、仕上げ圧延機の入側と仕上げ圧延機のスタンド間の１ヶ所以上）で行ってもよい。
【００３８】
本発明の製造方法では、上記のような熱間圧延で得られた熱延鋼帯に対して酸洗、冷間圧延、最終焼鈍及び調質圧延を順次施すことによりプレス成形性に優れ、且つコイル内でのプレス成形性の変動が少ない冷延鋼板を得ることができる。冷間圧延は熱延鋼帯を所定の板厚にするとともに、圧延集合組織を発達させ、続く最終焼鈍（再結晶焼鈍）工程において加工性向上のために好ましい集合組織を発達させるために実施される。そのためには、冷間圧延において５０％以上、好ましくは７６％以上の圧下率で最終板厚に加工することが好ましい。
【００３９】
また、最終焼鈍（再結晶焼鈍）は５５０〜９００℃の温度範囲で行なうことが好ましく、この最終焼鈍においてフェライトを再結晶させる。焼鈍温度が５５０℃未満では、長時間の箱焼鈍でも再結晶が十分に生じない。一方、９００℃を超える焼鈍温度では、連続焼鈍においてもオーステナイト化が進行し、加工性が劣化する。
最終焼鈍を行なう方法としては、連続焼鈍、箱焼鈍、または溶融亜鉛めっき処理に先行する連続熱処理のいずれでもよい。
調質圧延は、主に冷延鋼帯の形状性を良好なものにするために行われる。
【００４０】
本発明の製造方法で用いる素材鋼は、例えば転炉、電気炉等により溶製される。鋳片 の製造は造塊−分塊圧延法、連続鋳造法、薄鋳片鋳造法、ストリップ鋳造法のいずれでもよい。連続鋳造で得られた鋳片については、連続鋳造ままで直ちに熱間圧延する方法（所謂直送圧延法）、連続鋳造された鋳片を常温まで冷却することなく保熱または加熱した後、熱間圧延する方法、連続鋳造された鋳片を常温まで冷却した後、再加熱して熱間圧延する方法、のいずれを採用してもよい。
【００４１】
なお、本発明の製造方法において、連続鋳造または造塊−分塊圧延により得られた鋳片（スラブ）を加熱して熱間圧延する場合には、連続鋳造または造塊−分塊圧延された鋳片を室温以上の任意の温度まで冷却した後、熱延加熱炉に装入して鋳片を所定温度まで加熱するが、その場合、熱延加熱炉への鋳片装入温度はＡｒ３点以下であることが熱延組織を制御する上で好ましい。
【００４２】
なお、本発明の製造方法では、誘導加熱装置で粗圧延バーを加熱する工程の前工程または後工程としてレベラー等の矯正装置による形状矯正を行ってもよい。この形状矯正を、粗圧延バーを誘導加熱装置で加熱する工程の前工程として行なった場合、粗圧延バーの形状が良好になるため粗圧延バーを誘導加熱装置により均一に加熱することができ、この結果、粗圧延バー内の組織の均一性が高くなる。また、仕上げ圧延機に挿入される粗圧延バーの形状も良好になるため、仕上げ圧延による塑性変形時の均一性が高くなり、この結果、得られる鋼板の組織も均一になる。また、上記の形状矯正を、粗圧延バーを誘導加熱装置で加熱する工程の後工程として行なった場合には、仕上げ圧延機に挿入される粗圧延バーの形状が良好になるため、仕上げ圧延による塑性変形時の均一性が高くなり、この結果、鋼板の組織が均一となる。
【００４３】
本発明の製造方法によって得られた冷延鋼板は、必要に応じて表面処理（溶融亜鉛めっき、合金化溶融亜鉛めっき、電気めっき、有機被覆コーテング等）やプレス加工を施された後、例えば、自動車、家電製品（テレビ用のフレーム材、各種容器材等）、産業機器、鋼構造物、ほうろう製品等をはじめとする各種の用途に使用されるが、これらの用途において要求される高加工性と強度を兼ね備えている。
【００４４】
【実施例】
表１に示す化学組成を有する鋼（材料Ｎｏ．１〜１１）を溶製し、これを連続鋳造により鋳片とし、この鋳片を表２に示す条件で熱間圧延して熱延コイルとし、この熱延コイルの平均結晶粒径［Ｄ］とコイル幅方向での平均結晶粒径の変動度［Ｄｍａｘ／Ｄｍｉｎ］を測定した。
上記の熱延コイルを酸洗した後、圧下率７５％で冷間圧延し、次いで、再結晶焼鈍（８００℃×４０秒）と調質圧延を順次行なって製品鋼帯とし、得られた鋼帯コイルの機械的性質を調べた（サンプルの採取条件は、表３の脚注を参照）。各材料の熱延条件と測定された［Ｄ］及び［Ｄｍａｘ／Ｄｍｉｎ］の値を表２に、測定された各製品鋼帯の機械的性質を表３に示す。
【００４５】
表３によれば、本発明材ではｍｅａｎ−ｒ値及び破断伸びのレベルが高く、且つコイル内での機械的性質の変動も小さく、したがって、プレス成形性が優れ且つコイル内でのプレス成形性も均一であることが判る。また、仕上げ圧延で加速圧延を実施することにより、粗圧延バー全体の加熱温度をあまり高くすることなく仕上げ温度を本発明範囲内に制御できることが判る。また、加速圧延による上記の効果は、上述したパターン（Ａ）〜（Ｃ）のなかでは（Ｃ）＞（Ｂ）＞（Ａ）であることが判る。さらに、粗圧延バーのエッジ部の加熱を行った材料Ｎｏ．１、Ｎｏ．３、Ｎｏ．５、Ｎｏ．８は、本発明例のなかでも［Ｄｍａｘ／Ｄｍｉｎ］の値が特に小さく、このためコイル幅方向の特性値（ｍｅａｎ−ｒ値、破断伸び）の変動が特に小さく、優れた材料であることが判る。また、比較例である材料Ｎｏ．２のように粗圧延バー全体の加熱を施さない場合には、加速圧延を行なっても仕上げ圧延を本発明範囲に制御することができないことも判る。
【００４６】
【表１】
【表２】
【表３】
【発明の効果】
以上述べたように本発明の製造方法によれば、自動車の外板等の用途に好適な優れたプレス成形性を有し、しかもコイル内でのプレス成形性の変動が少ない冷延鋼板を安定して製造することができる。
【図面の簡単な説明】
【図１】実施例に示された本発明材（［Ｄｍａｘ／Ｄｍｉｎ］：１．５０）と比較材（［Ｄｍａｘ／Ｄｍｉｎ］：２．３５）について、冷間圧延・最終焼鈍後のコイル幅方向でのｍｅａｎ−ｒ値、破断伸びを示すグラフ
【図２】実施例に示された本発明材（［Ｄ］：１５μｍ，［Ｄｍａｘ／Ｄｍｉｎ］：１．５０）と比較材（［Ｄ］：１６μｍ，［Ｄｍａｘ／Ｄｍｉｎ］：２．３５）について、熱延コイル先端部から後端部に至るまでの仕上げ温度と冷間圧延・最終焼鈍後のコイル内の機械的性質との関係を示すグラフ[0001]
[Technical field to which the invention belongs]
The present invention relates to a method for producing a cold-rolled steel sheet excellent in deep drawability suitable for uses such as an outer plate of an automobile.
[0002]
[Prior art]
Cold-rolled steel sheets are widely used for automobile applications. Since there are many members that are press-molded in automobile applications, various workability is required depending on the shape of the members.In particular, cold-rolled steel sheets for press working that have excellent deep drawability are used for automotive outer panels. Desired. In recent years, demands for rationalization from automobile manufacturers have become strict, and in particular, there are increasing demands for cost reduction of materials and improvement of yields during product manufacturing. For this reason, it is important that the material of the cold-rolled steel sheet has high uniformity of material.
[0003]
Under such a background, for example, Japanese Patent Publication No. 60-45692 discloses that the surface temperature at the center of the width of 0.015 wt% or less of ultra-low carbon steel slab is less than 900 ° C. and 600 ° C. in the continuous casting-direct rolling process. Disclosed is a technique for improving the surface properties and deep drawability of a steel sheet by starting hot rolling in the above temperature range and further performing a holding treatment within 30 minutes in the middle stage of the hot rolling process. ing.
[0004]
[Problems to be solved by the invention]
However, although this technique improves the surface properties and deep drawability of the steel sheet to a good level, there is a big problem in the uniformity of the mechanical properties in the coil. That is, in this technique, the heating temperature in hot rolling is set to a low temperature ferrite region, and therefore, after hot rolling due to the temperature distribution in the material width direction during rolling (temperature distribution due to a significant temperature drop at the edge and its vicinity). There is a difference in the texture in the coil width direction, resulting in variations in the mechanical properties in the coil width direction after cold rolling and final annealing.
[0005]
When there is a variation in the mechanical properties in the width direction of the coil in this way, the workability in the material (coil) is not uniform, and particularly when deep drawability is required, such as in applications such as automobile outer plates In this case, a big problem (for example, generation of cracks and wrinkles) occurs in the quality after press molding. As a result, automobile manufacturers must perform plate cutting in the coil under conditions with a low yield (for example, the plate cutting direction is an unreasonable direction such as 45 degrees or the plate edge is not cut from the vicinity of the coil edge). You won't get.
[0006]
Accordingly, the object of the present invention is to solve such problems of the prior art, have excellent press formability (such as deep drawability) suitable for applications such as automobile outer plates, and press forming in a coil. An object of the present invention is to provide a method for producing a cold-rolled steel sheet with little variation in properties.
[0007]
[Means for Solving the Problems]
The inventors of the present invention have made extensive studies to solve the above-mentioned problems. As a result, the present inventors have excellent press formability suitable for applications such as automobile outer plates, and fluctuations in press formability within the coil. In order to obtain a small number of cold-rolled steel sheets, first, after optimizing the component composition of the raw steel, the average crystal grain size [D] after hot rolling and the variation degree of the average crystal grain size in the coil width direction [ Dmax / Dmin] was found to be in a specific range.
[0008]
Such a cold-rolled steel sheet optimizes the raw steel components to specific conditions, and (1) in the finish rolling of the rough rolled bar in the hot rolling process, the entire rough rolled bar is covered with a specific heating means. By controlling the temperature by heating over its entire length, the finishing temperature from the front end to the rear end of the hot-rolled coil is controlled within a specific temperature range. (2) In the runout after hot rolling It was obtained in this way by performing hot rolling under limited conditions of optimizing the cooling start time and the average cooling rate to a specific temperature range, and (3) optimizing the hot rolling coiling temperature. It was found that the hot-rolled steel strip can be manufactured by sequentially pickling, cold rolling, final annealing and temper rolling.
Furthermore, in order to obtain the above-mentioned cold-rolled steel sheet more reasonably, it has been found that it is effective to accelerate the rough rolling bar under predetermined conditions in finish rolling.
[0009]
The present invention has been made based on such findings, and the features thereof are as follows.
[1] C: 0.02 wt% or less, Si: 0.6 wt% or less, Mn: 2.5 wt% or less, P: 0.10 wt% or less, S: 0.05 wt% or less, O: 0.004 wt% or less Sol. Al: It contains 0.01 to 0.10 wt%, and further comprises a steel containing 0.01 to 0.40 wt% in total of one or more selected from Ti, Nb, V, and Zr. When the slab is rough-rolled by a roughing mill in a hot rolling process and this rough-rolled bar is subsequently finish-rolled by a continuous hot-finishing rolling mill, between the entrance side of the finishing mill and / or the stand of the finishing mill In the range of Ar 3 point to Ar 3 point + 30 ° C. until the finishing temperature reaches from the front end part to the rear end part of the hot-rolled coil by heating the entire rough rolling bar over its entire length with the induction heating device provided in Then, the subsequent run-out cooling is started within 1.5 seconds after the finish rolling is completed, and after cooling from the finish temperature to 750 ° C. at an average cooling rate of 20 ° C./sec or more, winding is performed. Temperature 70 The hot-rolled steel strip is wound at a temperature of ℃ or less, and the resulting hot-rolled steel strip is subjected to pickling, cold rolling, final annealing, and temper rolling in order. Method for producing cold-rolled steel sheets with a small amount.
[0010]
[2] In the production method of [1], the slab further contains B: 0.0001 to 0.005 wt%, and is excellent in press formability and has little variation in press formability in the coil. A method for producing a cold-rolled steel sheet.
[3] In the manufacturing method according to [1] or [2], the widthwise edge portion of the rough rolling bar is heated by an induction heating device between the entry side of the finishing mill and / or between the stands of the finishing mill. A method for producing a cold-rolled steel sheet, which is excellent in press formability and has little fluctuation in press formability in a coil.
[0011]
[4] In the manufacturing method according to any one of [1] to [3], the rolling speed of the rough-rolled bar to be finish-rolled is controlled to any of the following (A) to (C). A method for producing a cold-rolled steel sheet having excellent press formability and little variation in press formability in a coil.
(A) The rolling speed is accelerated after the end of the rough rolling bar enters the finish rolling mill, and then rolling is performed at a constant speed.
(B) After the tip of the rough rolling bar enters the finish rolling mill, the rolling speed is accelerated, and after rolling at a constant speed, the rolling speed is further accelerated.
(C) The rolling speed is accelerated after the end of the rough rolling bar enters the finish rolling mill, and then the rolling speed is further accelerated.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
The details of the present invention and the reasons for limitation will be described below.
As described above, in order to produce a cold-rolled steel sheet that is excellent in press formability (deep drawability, etc.) suitable for applications such as an outer plate of an automobile and has little fluctuation (variation) in press formability in a coil. In addition, after optimizing the components of the raw steel, the average crystal grain size [D] after hot rolling and the degree of variation of the average crystal grain size in the coil width direction [Dmax / Dmin], that is, the average in the coil width direction It is necessary that the ratio [Dmax / Dmin] of the maximum value [Dmax] and the minimum value [Dmin] of the crystal grain size be within a specific range.
[0013]
Therefore, first, the reason for limiting the component composition of the material steel in the present invention will be described. Since C is an element that adversely affects the deep drawability of the steel sheet, it is preferable that its content is small. If the amount of C exceeds 0.02 wt%, the deep drawability targeted by the present invention cannot be obtained, so the content is 0.02 wt% or less (including the case of no addition). In order to further improve the deep drawability, the C amount is preferably 0.0020 wt% or less, and in order to further improve the workability, the C amount is preferably 0.0015 wt% or less. .
[0014]
Si has an effect of strengthening the steel sheet by solid solution strengthening, but since it is an element that adversely affects the deep drawability, the content is preferably small. If the Si content exceeds 0.6 wt%, the plating properties and deep drawability deteriorate, so the content is 0.6 wt% or less (including the case where no additive is added). In order to further improve the plating property, the Si amount is preferably 0.1 wt% or less, and in order to further improve the workability, the Si amount is preferably 0.03 wt% or less. .
[0015]
Mn has the effect of improving the toughness and strengthening the steel sheet by solid solution, but it is also an element that adversely affects the workability. If the amount of Mn exceeds 2.5 wt%, the strength increases and the deep drawability deteriorates remarkably, so the content is made 2.5 wt% or less (including the case of no addition). In order to improve deep drawability, the Mn content is preferably 2.0 wt% or less, and in order to further improve the workability, the Mn content is preferably 0.5 wt% or less. .
[0016]
P has the effect of strengthening the steel sheet by solid solution strengthening, but if the amount of P exceeds 0.10 wt%, grain boundary embrittlement due to grain boundary segregation tends to occur, and ductility also deteriorates. For this reason, the P content is 0.10 wt% or less (including the case of no addition). In order to further improve the ductility, the P amount is preferably 0.05 wt% or less, and in order to further increase the ductility, the P amount is preferably 0.02 wt% or less.
[0017]
If the S content exceeds 0.05 wt%, the amount of sulfide deposited increases and the deep drawability and ductility deteriorate. For this reason, the S content is 0.05 wt% or less (including the case where no additive is added). In order to further improve the workability, the S amount is preferably 0.02 wt% or less, and in order to further improve the workability, the S amount is preferably 0.01 wt% or less. .
[0018]
Sol. Al is used as a deoxidizing material for steel, and is an additional element essential for improving the yield of addition of Ti, Nb, Zr, and V, which will be described later. Sol. If Al is less than 0.01 wt%, the above effect cannot be obtained. On the other hand, adding more than 0.10 wt% saturates the effect and is uneconomical. For this reason, Sol. The Al content is 0.01 to 0.10 wt%.
[0019]
The smaller the content of O, the better for processability. When the amount of O exceeds 0.004 wt%, the workability of the steel sheet is inevitably lowered. For this reason, the content of O is 0.004 wt% or less (however, including the case of no addition). In addition, such O content is the above-mentioned Sol. This is achieved by adjusting the amount of Al.
[0020]
In addition to the above-described components, the raw steel further contains one or more selected from Ti, Nb, V, and Zr in a total amount of 0.01 to 0.40 wt%. These components form carbonitrides and sulfides, reduce C, N, and S in the steel and are useful for improving workability. However, if the total content is less than 0.01 wt%, the desired effect cannot be obtained. On the other hand, if the total content exceeds 0.40 wt%, the strength increases so much that the workability deteriorates. 01 to 0.40 wt%.
[0021]
Furthermore, B may be added to the material steel of the present invention in the range of 0.0001 to 0.005 wt% for the purpose of improving the resistance to vertical cracking. If the amount of B is less than 0.0001 wt%, the effect of improving the longitudinal crack resistance cannot be sufficiently obtained. On the other hand, if the amount exceeds 0.005 wt%, the effect is saturated and the economy is impaired. For this reason, the addition amount of B shall be 0.0001-0.005 wt%.
[0022]
Next, the manufacturing conditions of the present invention will be described.
In view of the usage conditions of cold-rolled steel sheets at automakers, etc., in order to plate products from cold-rolled coils at a high yield, in addition to the optimization of the above steel components, the average crystal of hot-rolled coils It is necessary to optimize the grain size [D] and the variation degree [Dmax / Dmin] of the average crystal grain size in the coil width direction, specifically, the average crystal grain size [D] after hot rolling. 5 to 50 μm, and the variation degree [Dmax / Dmin] of the average crystal grain size in the coil width direction needs to be 2.0 or less.
[0023]
If the average crystal grain size [D] after hot rolling is less than 5 μm, the excellent deep drawability intended in the present invention cannot be obtained. On the other hand, if the average crystal grain size [D] exceeds 50 μm, rough skin is a problem during processing. It becomes. On the other hand, when the average crystal grain size [D] is 5 to 50 μm, excellent deep drawability is obtained and rough skin does not occur. Further, in order to obtain a better deep drawability, [D] should be 15-30 μm, and in order to increase the strength while ensuring excellent deep drawability, [D] should be 5-15 μm. It is preferable that
[0024]
Further, if the degree of variability [Dmax / Dmin] of the average crystal grain size in the hot rolled coil width direction exceeds 2.0, the mechanical properties (mean-r value) in the coil width direction after cold rolling and final annealing. ), The uniformity of mechanical properties in the coil width direction can be kept uniform by setting [Dmax / Dmin] to 2.0 or less.
[0025]
1 shows the material of the present invention (material No. 1, [Dmax / Dmin]: 1.50) and the comparative material (material No. 2, [Dmax / Dmax /) shown in Examples (Tables 2 and 3) described later. Dmin]: 2.35) shows the distribution of mean-r value and breaking elongation (L direction) in the coil width direction after cold rolling and final annealing, and [Dmax / Dmin] is 2.0 or less. The material according to the present invention has a small fluctuation (variation) in mechanical properties in the coil width direction, whereas the comparative material having [Dmax / Dmin] exceeding 2.0 has large mechanical properties in the coil width direction. It has fluctuated.
[0026]
In the production method of the present invention, in order to ensure the above-described optimized average crystal grain size [D] of the hot rolled coil and the degree of variation [Dmax / Dmin] of the average crystal grain size in the width direction of the hot rolled coil. In addition, the following manufacturing conditions are adopted.
First, a slab made of steel having the above component composition is roughly rolled by a roughing mill in a hot rolling process, and this rough rolling bar is subsequently finish-rolled by a continuous hot finish rolling mill. Finishing temperature (temperature at the final stand of the finishing mill) by heating the entire rough rolling bar over its entire length with an induction heating device provided on the entrance side of the finishing mill and / or between the stands of the finishing mill Is rolled so as to be in the range of Ar 3 point to Ar 3 point + 30 ° C. from the front end to the rear end of the hot rolled coil.
[0027]
Only by performing the hot rolling under such conditions, it is possible to mean-r value and ductility (elongation at break) the level at which the present invention is intended in the coil, including the coil width direction and the longitudinal direction .
[0028]
Here, when the finishing temperature is less than the Ar 3 point, hot rolling after ferrite transformation occurs, so [D] and [Dmax / Dmin] exceed the range intended by the present invention. Further, if the finishing temperature exceeds Ar 3 point + 30 ° C., the grain growth of austenite crystal grains is remarkable even after finish rolling, and there is also a temperature difference in the coil width direction, so that [D] and [Dmax / Dmin] are the present invention. Exceeds the intended range.
[0029]
The heating of the rough rolling bar by the induction heating device can be performed at any position between the entrance of the finish rolling mill and the stand of the finish rolling mill, and a plurality of these positions (for example, the entrance of the finish rolling mill) And at least one place between the stands of the finishing mill.
The induction heating device is used as a heating device for adjusting the temperature of the rough rolling bar. By using this induction heating device, good controllability (with respect to temperature distribution (variation) in the longitudinal direction of the rough rolling bar ( This is because the temperature can be controlled with the following capability.
[0030]
FIG. 2 shows the above-described material of the present invention (material No. 1, [D]: 15 μm, [Dmax / Dmin]: 1.50) and a comparative material (material No. 2, [D]: 16 μm, [Dmax]. / Dmin]: 2.35), the hot rolling finish temperature from the coil front end to the rear end and the mechanical properties in the coil (coil width direction and longitudinal direction) after cold rolling and final annealing. The relationship with fluctuation is shown. According to this, [Dmax / Dmin] exceeds 2.0 in the comparative material in which the hot rolling finishing temperature from the leading end portion to the trailing end portion of the coil is Ar 3 point−10 ° C. to Ar 3 point + 35 ° C. For this reason, the mean-r value and the elongation at break after cold rolling and final annealing in the coil (coil width direction and longitudinal direction) vary greatly. On the other hand, in the present invention material in which the hot rolling finishing temperature from the front end portion to the rear end portion of the coil is Ar 3 point to Ar 3 point + 22 ° C., [Dmax / Dmin] is suppressed to 2.0 or less. For this reason, fluctuations in the mean-r value and elongation at break after cold rolling and final annealing in the coil (coil width direction and longitudinal direction) are appropriately suppressed.
[0031]
In the production method of the present invention, in addition to the control of the finishing temperature (rolling temperature at the final stand of the finishing mill), the rolling temperature at each stand upstream from the final stand of the finishing mill is also roughly rolled. by rolling the temperature of the bar of the tip up to the rear end portion so that the Ar 3 point to Ar 3 point + 30 ° C., has a more excellent deep drawability, and a coil (coil longitudinal direction and It is possible to produce a steel plate with less variation in mechanical properties in the width direction). Such temperature adjustment of the rough rolling bar is also performed by heating a part or all of the longitudinal direction of the rough rolling bar with the induction heating device.
[0032]
In the production method of the present invention, the subsequent run-out cooling is started within 1.5 seconds after the finish rolling is finished, and after cooling from the finish temperature to 750 ° C. at an average cooling rate of 20 ° C./sec or more, winding is performed. Winding is performed at a temperature of 700 ° C. or lower.
By starting the cooling in the run-out within 1.5 seconds after the finish rolling, the grain growth of the austenite crystal grains before the transformation after the finish rolling can be suppressed, and the present invention intends [D] and [D] Dmax / Dmin] can be obtained. When the time from the end of finish rolling to the start of cooling exceeds 1.5 seconds, the above-described crystal grain growth cannot be suppressed, and [D] and [Dmax / Dmin] intended in the present invention cannot be obtained.
[0033]
Further, by setting the average cooling rate from the finishing temperature to 750 ° C. to 20 ° C./sec or more, the nucleation frequency of ferrite during the austenite-ferrite transformation is increased, and the ferrite particle size is intended in the present invention [D]. And [Dmax / Dmin]. When the average cooling rate is less than 20 ° C./sec, the above-described ferrite nucleation frequency is low, and [D] and [Dmax / Dmin] intended in the present invention cannot be obtained.
[0034]
Moreover, in the steel strip in which [D] and [Dmax / Dmin] are controlled within the range intended by the present invention by setting the coiling temperature after hot rolling to 700 ° C. or less, coarse grains due to ferrite grain growth are obtained. Can be suppressed. When the coiling temperature exceeds 700 ° C., [D] and [Dmax / Dmin] exceed the range intended by the present invention due to the coarsening by the grain growth of ferrite.
[0035]
Further, in order to more reasonably obtain the target steel sheet in the production method of the present invention, the rolling speed of the rough rolling bar to be finish-rolled is accelerated after the tip of the rough rolling bar enters the finish rolling mill. Specifically, it is preferable to control the rolling speed to any of the following (A) to (C).
(A) The rolling speed is accelerated after the end of the rough rolling bar enters the finish rolling mill, and then rolling is performed at a constant speed.
(B) After the tip of the rough rolling bar enters the finish rolling mill, the rolling speed is accelerated, and after rolling at a constant speed, the rolling speed is accelerated.
(C) The rolling speed is accelerated after the end of the rough rolling bar enters the finish rolling mill, and then the rolling speed is further accelerated.
[0036]
By performing such finish rolling with accelerated rolling, it is possible to suppress the temperature drop of the material as much as possible. For this reason, roughing by an induction heating device provided on the entrance side of the finishing mill and / or between the stands of the finishing mill is performed. The heating amount of the rolling bar can be minimized and energy can be saved due to heating.
In the rolling patterns (A) to (C) above, the degree of accelerated rolling in the finish rolling mill is (C)>(B)> (A), so that (B) rather than (A). , (C) is more effective than (B).
[0037]
Moreover, in the manufacturing method of this invention, it is preferable to add the process of heating the width direction edge part of a rough-rolling bar with the induction heating apparatus between the entrance side of a finishing mill and / or the stand of a finishing mill. By such heating of the edge portion, the temperature variation in the rough rolling bar width direction is reduced, and [Dmax / Dmin] can be set to a smaller value.
Further, the heating of the rough rolled bar edge portion by this induction heating device can be performed at any position between the entrance of the finish rolling mill and the stand of the finish rolling mill, and a plurality of these positions (for example, finishing) It may be performed at one or more places between the entrance side of the rolling mill and the stand of the finishing mill.
[0038]
In the production method of the present invention, it is excellent in press formability by sequentially performing pickling, cold rolling, final annealing and temper rolling on the hot-rolled steel strip obtained by hot rolling as described above, and A cold-rolled steel sheet with little variation in press formability in the coil can be obtained. Cold rolling is carried out in order to make the hot-rolled steel strip a predetermined thickness, develop a rolled texture, and develop a favorable texture for improving workability in the subsequent final annealing (recrystallization annealing) process. The For that purpose, it is preferable to process to the final plate thickness at a rolling reduction of 50% or more, preferably 76% or more in cold rolling.
[0039]
The final annealing (recrystallization annealing) is preferably performed in a temperature range of 550 to 900 ° C., and ferrite is recrystallized in this final annealing. When the annealing temperature is less than 550 ° C., recrystallization does not occur sufficiently even when box annealing is performed for a long time. On the other hand, at an annealing temperature exceeding 900 ° C., austenitization proceeds even in continuous annealing, and workability deteriorates.
As a method of performing final annealing, any of continuous annealing, box annealing, or continuous heat treatment preceding hot dip galvanizing treatment may be used.
The temper rolling is performed mainly to improve the shape of the cold-rolled steel strip.
[0040]
The material steel used in the production method of the present invention is melted by, for example, a converter, an electric furnace or the like. The slab may be manufactured by any of the ingot-bundling rolling method, continuous casting method, thin slab casting method, and strip casting method. For slabs obtained by continuous casting, a method in which hot casting is performed immediately in a continuous casting (so-called direct feed rolling method), and after continuously maintaining or heating the cast slab continuously cooled to room temperature, Either a rolling method or a method of reheating and hot rolling after cooling a continuously cast slab to room temperature may be employed.
[0041]
In the production method of the present invention, when a slab (slab) obtained by continuous casting or ingot-bundling is heated and hot-rolled, it is continuously cast or ingot-bundled. After cooling the slab to any temperature above room temperature, the slab is charged into a hot rolling furnace and heated to a predetermined temperature. In this case, the slab charging temperature into the hot rolling furnace is Ar 3. It is preferable to control the hot-rolled structure to be below the point.
[0042]
In addition, in the manufacturing method of this invention, you may perform shape correction | amendment by correction apparatuses, such as a leveler, as a pre-process or a post process of the process of heating a rough rolling bar with an induction heating apparatus. When this shape correction is performed as a pre-process of the step of heating the rough rolling bar with the induction heating device, the shape of the rough rolling bar becomes good, so the rough rolling bar can be heated uniformly with the induction heating device, As a result, the uniformity of the structure in the rough rolling bar is increased. Moreover, since the shape of the rough rolling bar inserted into the finish rolling mill is also improved, the uniformity during plastic deformation by finish rolling is increased, and as a result, the structure of the obtained steel sheet is also uniform. In addition, when the above-mentioned shape correction is performed as a post-process of the step of heating the rough rolling bar with an induction heating device, the shape of the rough rolling bar inserted into the finish rolling mill becomes good. Uniformity at the time of plastic deformation becomes high, and as a result, the structure of the steel sheet becomes uniform.
[0043]
After the cold-rolled steel sheet obtained by the production method of the present invention is subjected to surface treatment (hot dip galvanization, alloyed hot dip galvanization, electroplating, organic coating, etc.) and press work as necessary, for example, Used for various applications including automobiles, home appliances (frame materials for TV, various container materials, etc.), industrial equipment, steel structures, enamel products, etc., but the high workability required in these applications And strength.
[0044]
【Example】
Steel having the chemical composition shown in Table 1 (Material Nos. 1 to 11) is melted and made into a slab by continuous casting, and this slab is hot rolled under the conditions shown in Table 2 to form a hot rolled coil. The average crystal grain size [D] of this hot rolled coil and the variation degree [Dmax / Dmin] of the average crystal grain size in the coil width direction were measured.
After pickling the hot-rolled coil, it is cold-rolled at a reduction ratio of 75%, and then recrystallized annealing (800 ° C. × 40 seconds) and temper rolling are sequentially performed to obtain a product steel strip, and the obtained steel The mechanical properties of the strip coil were examined (see footnotes in Table 3 for sample collection conditions). Table 2 shows the hot rolling conditions of each material and the measured [D] and [Dmax / Dmin] values, and Table 3 shows the measured mechanical properties of each product steel strip.
[0045]
According to Table 3, the material of the present invention has a high mean-r value and a high level of elongation at break, and a small fluctuation in mechanical properties in the coil. Therefore, the press formability is excellent and the press formability in the coil is high. Is also uniform. It can also be seen that by performing accelerated rolling by finish rolling, the finishing temperature can be controlled within the range of the present invention without excessively increasing the heating temperature of the entire rough rolling bar. Moreover, it turns out that said effect by accelerated rolling is (C)>(B)> (A) in pattern (A)-(C) mentioned above. Furthermore, the material No. which heated the edge part of the rough-rolling bar was used. 1, no. 3, no. 5, no. 8 has a particularly small value of [Dmax / Dmin] among the examples of the present invention. Therefore, the variation in the characteristic value (mean-r value, elongation at break) in the coil width direction is particularly small, and is an excellent material. I understand. Moreover, material No. which is a comparative example. It can also be seen that when the entire rough rolling bar is not heated as in 2, finish rolling cannot be controlled within the scope of the present invention even if accelerated rolling is performed.
[0046]
[Table 1]
[Table 2]
[Table 3]
【The invention's effect】
As described above, according to the manufacturing method of the present invention, a cold-rolled steel sheet having excellent press formability suitable for uses such as an outer panel of an automobile and having little fluctuation in press formability in a coil can be stably obtained. Can be manufactured.
[Brief description of the drawings]
FIG. 1 shows the coil width after cold rolling and final annealing of the inventive material ([Dmax / Dmin]: 1.50) and the comparative material ([Dmax / Dmin]: 2.35) shown in the examples. FIG. 2 is a graph showing the mean-r value in the direction and elongation at break. FIG. 2 shows the material of the present invention ([D]: 15 μm, [Dmax / Dmin]: 1.50) shown in the examples and the comparative material ([D]). : 16 μm, [Dmax / Dmin]: 2.35) shows the relationship between the finishing temperature from the front end of the hot rolled coil to the rear end and the mechanical properties in the coil after cold rolling and final annealing. Graph
Claims (4)
(Ａ) 粗圧延バーの先端部が仕上げ圧延機に入ってから圧延速度を加速し、そののち一定速度で圧延する。
(Ｂ) 粗圧延バーの先端部が仕上げ圧延機に入ってから圧延速度を加速し、そののち一定速度で圧延した後、さらに圧延速度を加速する。
(Ｃ) 粗圧延バーの先端部が仕上げ圧延機に入ってから圧延速度を加速し、そののち圧延速度をさらに加速する。The rolling speed of the rough-rolled bar to be finish-rolled is controlled to any one of the following (A) to (C): A method for producing a cold-rolled steel sheet with less variation in press formability.
(A) The rolling speed is accelerated after the end of the rough rolling bar enters the finish rolling mill, and then rolling is performed at a constant speed.
(B) After the tip of the rough rolling bar enters the finish rolling mill, the rolling speed is accelerated, and after rolling at a constant speed, the rolling speed is further accelerated.
(C) The rolling speed is accelerated after the end of the rough rolling bar enters the finish rolling mill, and then the rolling speed is further accelerated.
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| JP23011898A JP3613021B2 (en) | 1998-07-31 | 1998-07-31 | Method for producing cold-rolled steel sheet with excellent press formability and little variation in press formability in the coil |
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| Application Number | Priority Date | Filing Date | Title |
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| JP23011898A JP3613021B2 (en) | 1998-07-31 | 1998-07-31 | Method for producing cold-rolled steel sheet with excellent press formability and little variation in press formability in the coil |
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| JP3613021B2 true JP3613021B2 (en) | 2005-01-26 |
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| JP2016164291A (en) * | 2015-03-06 | 2016-09-08 | 株式会社神戸製鋼所 | Steel for cold working |
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