JP3777033B2

JP3777033B2 - Method for producing cold-rolled steel sheet having anti-aging characteristics with very little variation

Info

Publication number: JP3777033B2
Application number: JP33279897A
Authority: JP
Inventors: 治秋末; 学 ▲高▼橋; 正芳末廣
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 1997-12-03
Filing date: 1997-12-03
Publication date: 2006-05-24
Anticipated expiration: 2017-12-03
Also published as: JPH11158554A

Description

【０００１】
【発明の属する技術分野】
本発明は、優れた加工性と、耐時効特性の、ばらつきが極めて少ない冷延鋼板の連続焼鈍による製造方法に関わり、その主たる用途は、自動車、家庭電化製品、建築材料等である。さらに、本発明による高強度鋼板を自動車に使用すると、板厚を薄くすることができるため自動車の燃費の向上に寄与し、近年、大きな課題となっている地球環境の改善にも役立つことになる。
【０００２】
【従来の技術】
低炭素鋼板を用い、耐時効特性のよい冷延鋼板を連続焼鈍によって製造する方法は、例えば、特公平５−５５５７３号公報に開示されている。すなわち、この発明は、冷延された低炭素鋼板を連続焼鈍するにあたり、通常の方法で熱延された板を冷延し、その冷延鋼板を再結晶・粒成長させた後、急冷・過冷却・再加熱・傾斜過時効処理を行う連続焼鈍法に関するものであり、炭素量が０．０１〜０．０６％の範囲で変化する低炭素鋼板を、７２０〜６００℃の高温域から４５０〜３００℃の低温域までを、５０〜２５０℃／ｓｅｃの冷却速度で一段で一気に冷却するものである。この急冷によって多量の固溶炭素を確保し炭化物の核発生を促進させようとするものである。
【０００３】
【発明が解決しようとする課題】
しかしながら、上記従来技術の特公平５−５５５７３号公報に記載の発明では、炭素量が０．０１〜０．０６％の範囲で変化すると炭化物の核発生の数が不安定になり、その結果、過時効処理を行っても最終的な固溶炭素量にばらつきが発生し易いことになり、冷延鋼板の耐時効特性がばらつくという問題がある。このように、従来の連続焼鈍方法で冷延鋼板を焼鈍すると、一般に、鋼板のＣやＭｎ等の成分量の変動や熱延、冷延等の工程条件の変動の存在によって、連続焼鈍後の冷延鋼板の耐時効特性に大きなばらつきが発生する。このばらつきは、製品価値を低めるばかりでなく、出荷管理上の困難さをも生じさせることになる。
【０００４】
そこで、本発明の第一の狙いは、耐時効特性のばらつきを減少させることのできる冷延鋼板の製造方法を提供することである。
冷延鋼板の時効指数（ＡｇｉｎｇＩｎｄｅｘ、以下単にＡＩともいう）と焼付硬化量（Ｂａｋｅ−ｈａｒｄｅｎａｂｉｌｉｔｙ、以下単にＢＨともいう）との間には、図５に示すような関係がある。鋼板の出荷管理上、時効指数には上限があり、これ以上の時効指数をもつ鋼板は出荷できない。通常、時効指数の上限値としては、３０ＭＰａが使われている。鋼板の出荷を管理する上で時効指数には上限があるが、それでも、焼付硬化量を、より高くすることが一方で要請されている。そこで、本発明の第二のねらいは、耐時効特性のばらつきを抑えることによって、実質的に焼付硬化量の高い冷延鋼板の製造方法を提供することである。
【０００５】
【課題を解決するための手段】
本発明者らは、上記の課題を解決するために、鋼板の成分や熱延、冷延等の工程条件が、ある範囲内で変動しても耐時効特性のばらつきを十分に小さくすることのできる連続焼鈍熱サイクルについて鋭意検討した。その結果、図２に示すように、急冷開始温度（Ｔ１）から急冷終点温度（Ｔ３）までを直線的に一気に冷却する従来の方法に対して、Ｔ１から急冷中間温度のＴ２までを急冷し、その後Ｔ２からＴ３にいたる温度の間では緩冷却をすれば、冷延鋼板の耐時効特性のばらつきが極めて小さくなるという新しい作用効果を見出した。この二段冷却法ともいうべき新連続焼鈍の熱サイクルを使用して鋼板を連続焼鈍すると、鋼板の成分系や熱延、冷延等の工程条件に、ある範囲内での変動があっても、耐時効特性のばらつきを十分に小さくすることができる。
【０００６】
Ｔ１からＴ３にいたる冷却処理の技術的役割は、（１）過冷による多量の固溶炭素の確保と、（２）過時効処理に先立ち炭化物の核発生の誘起の２点である。
まず、（１）の多量の固溶炭素量の確保は、炭化物の核発生が容易に起こるための駆動力を発生させるために不可欠である。そして、（２）は、その後に続く過時効処理で固溶炭素を迅速に減少させるために、固溶炭素の析出サイトの数を十分に確保することと、それを板内で均一に発生させるために必要である。
【０００７】
この技術的目的を達成させるためのＴ１からＴ３までの熱サイクル条件を詳細に検討した。その結果、（１）の固溶炭素量を十分確保するためには、特に炭素の拡散速度の速い７２０℃〜６００℃から中間温度の４５０〜３００℃の温度域において高冷却速度が必要であることを見出した。すなわち、この場合に必要な冷却速度は、５０℃／ｓｅｃ以上の高冷却速度である。一方、上限の冷却速度は２５０℃／ｓｅｃであり、これを超える高冷却速度は固溶炭素量を高める点では不必要である。また、（２）の十分な炭化物の核を板内に均一に発生させるためには、４５０〜３００℃の急冷中間温度（Ｔ２）から３００〜２００℃までの急冷終点温度（Ｔ３）を５〜４０℃／ｓのやや緩い速度で冷却することが効果的であることを見出した。
【０００８】
この本発明の方法を、再結晶・粒成長処理後、急冷・過冷却・再加熱・傾斜過時効を行う連続焼鈍法に適用すると、従来より極めて、ばらつきの少ない耐時効特性を持った冷延鋼板を製造することができる。更に、耐時効特性のばらつきを少なくすることによって、鋼板にはより高い焼付硬化量を実質的に持たせることができるのである。
【０００９】
本発明による、ばらつきの極めて少ない耐時効特性を有する冷延鋼板の製造方法は以下の通りである。
（１）質量％で、Ｃ：０．０１〜０．０６％、Ｓｉ：０．１％以下、Ｍｎ：０．０５〜０．４０％、Ｐ：０．１０％以下、Ｓ：０．００２〜０．０２５％、ｓｏｌ．Ａｌ：０．０１〜０．１０％、Ｎ：０．００１〜０．００６％、残部鉄および不可避的不純物からなる鋼を通常の方法で熱延し、冷延した冷延鋼板を再結晶・粒成長処理後、急冷・過冷却・再加熱・傾斜過時効処理する連続焼鈍を行う際、再結晶・粒成長後に７２０〜６００℃の温度域から４５０〜３００℃の温度域までを５０〜２５０℃／ｓｅｃの冷却速度で急冷し、次いで３００〜２００℃の温度域までを５〜４０℃／ｓｅｃの冷却速度で冷却し、該温度域で０〜１５秒間保持した後、４００〜３２０℃の温度域に少なくとも４０℃以上の昇温を伴う再加熱を行い、該温度域から３８５〜２２０℃の温度域まで冷却しながら過時効処理することを特徴とする、ばらつきの極めて少ない耐時効特性を有する冷延鋼板の製造方法。
（２）前記鋼が、ＢとＮの含有量の比でＢ／Ｎ＝０．５〜２．０の関係を満たして含有することを特徴とする、上記（１）に記載のばらつきの極めて少ない耐時効特性を有する冷延鋼板の製造方法。
【００１０】
【発明の実施の形態】
本発明における冷延鋼板の製造方法は、低炭素鋼板のＣ、Ｓｉ、Ｍｎ、Ｐ、Ｓ、Ａｌ、Ｎ量を限定し、通常の方法で熱延板とし、冷延した冷延鋼板を再結晶・粒成長処理後、急冷・過冷却・再加熱・傾斜過時効処理する連続焼鈍法に関するものである。特に、急冷開始温度（Ｔ１）から急冷終点温度（Ｔ３）までを二段階に冷却する熱サイクルを採用するものである。これらによって、耐時効特性のばらつきが抑えられ、高い焼付硬化量を持った冷延鋼板が製造される。以下に本発明の限定理由を述べる。
【００１１】
先ず化学成分についてである。
Ｃは、その含有量が少なくなるに従って、延性および深絞り性が向上すること、また、含有量が少なくなり過ぎる耐時効特性が劣化するるので、本発明の場合は、Ｃ：０．０１〜０．０６％の範囲で加工性と耐時効特性の優れた冷延鋼板が得られる。
【００１２】
ＭｎおよびＳは、熱間圧延時のＳによる脆性を防止することと、ＭｎＳを炭化物の優先析出サイトとして利用するので、それらの量には限定された範囲が必要である。これらの観点から、Ｍｎを０．０５〜０．４０％、Ｓを０．００２〜０．０２５％に規制した。このことと二段急速冷却の組み合わせによって、一層、耐時効特性の優れた冷延鋼板が製造されることになるのである。
【００１３】
Ｓｉは、フェライトに固溶し、マトリックスを硬質化させるのみならず、大量に添加された場合にはスケール起因の表面キズ等による表面品位の劣化や、化成処理性の低下をもたらすために、０．１％以下の添加量に制限する。しかしながら、Ｓｉ添加量を０．００３％未満に制御することは実製造条件から困難であることから、Ｓｉ添加量の下限を０．００３％とすることが好ましい。Ｐは、耐時効特性に大きく影響しない元素であるが、自動車用鋼板等を製造する場合はその上限を０．１０％としなければならない。Ｐがこれを越えると鋼板の加工性が著しく劣化するからである。
【００１４】
ｓｏｌ．Ａｌは、鋼中の酸素、窒素量を制御するのに必要な元素であるが、これが多すぎると硬質化するので上限を０．１０％とした。一方、これが少な過ぎると窒素に起因する時効性を抑えることができないので、下限を０．０１％とした。
Ｎは、鋼中のｓｏｌ．Ａｌと結びついてＡｌＮ（Ｂが添加されるときはＢＮ）となり材質を硬質化させるので、その上限を０．００６％とした。なお、下限を０．００１％としたのは、現在の製鋼技術ではＮ量をこれより低くすることは困難なためである。
【００１５】
Ｂは、本発明においては、必要に応じて含有させるものである。Ｂは、Ｂ／Ｎで０．５以上含有すると鋼中のＮと結びついてＢＮとなり窒素時効を防止できるが、Ｂ／Ｎが２．０を越えると固溶のＢ量が増えて材質を硬質化させるので、Ｂ／Ｎの下限を０．５、上限を２．０とした。次に、鋳造から熱間圧延にいたるまでの工程であるが、スラブを冷片とした後再加熱する方法でも、連続鋳造−直送熱間圧延（ＣＣ−ＤＲ）法でもいずれを採用してもよい。従って、スラブを加熱する場合の温度は、特に限定しなくてもよいことになる。また、熱延後の巻取温度は、鋼板の耐時効特性にあまり影響を与えず、６００℃程度の低温巻取の場合でも本発明の効果は十分に得られる。７００℃以上の高温巻取をした場合には、焼鈍後の結晶粒が大きくなり加工性が向上する。次に連続焼鈍工程について述べる。
【００１６】
冷間圧延された鋼板を再結晶・粒成長させる工程は、特に限定する必要は無く、通常の方法の再結晶温度以上に加熱し均熱すればよい。均熱後の急冷は、７２０〜６００℃の温度域から４５０〜３００℃の温度域までを５０〜２５０℃／ｓｅｃの冷却速度で冷却し、次いで３５０〜２００℃の温度域までを５〜４０℃／ｓｅｃの冷却速度で冷却する必要がある。この二段階の冷却方法が、ばらつきの少ない耐時効特性を得る上で最も重要な技術的ポイントである。
【００１７】
鋼板中への変態による歪みの導入を避けるために、均熱後は２０℃／ｓｅｃ以下の冷却速度で冷却する。同じ理由で急冷開始温度の上限は７２０℃とする。急冷開始温度が６００℃未満になると、急冷中間温度（Ｔ２）での固溶炭素量が十分でなくなるので、６００℃を急冷開始温度の下限とする。
急冷中間温度（Ｔ２）の上限温度は４５０℃とする。この温度より高くなると固溶炭素量が不足し、炭化物の核発生が十分に起こらなくなるからである。また、急冷中間温度が３００℃未満になると、炭化物の核発生の、ばらつきが大きくなり、結果として耐時効特性の、ばらつきを大きくさせるので、３００℃を急冷中間温度の下限とする。
【００１８】
急冷開始温度（Ｔ１）から急冷中間温度（Ｔ２）までの冷却速度の下限は５０℃／ｓｅｃである。これより遅くなると十分な固溶炭素量が確保されず、高い耐時効特性は得られない。また、冷却速度を２５０℃／ｓｅｃ超にしても、温度の制御性が悪くなり耐時効特性が不安定になるので、２５０℃／ｓｅｃを冷却速度の上限とする。
【００１９】
十分な炭化物の析出密度を確保するために、急冷終点温度を３００℃以下にする。一方、急冷終点温度を２００℃未満にすると、炭化物の核の密度が多くなりすぎて鋼板が硬質化するので、２００℃をその下限とする。
急冷中間温度（Ｔ２）から急冷終点温度（Ｔ３）までは、５〜４０℃／ｓｅｃの冷却速度で冷却する必要がある。４０℃／ｓｅｃを超える冷却速度になると、鋼板の耐時効特性のばらつきが大きくなるので、４０℃／ｓｅｃを上限とする。一方、５℃／ｓｅｃ未満の冷却速度になると連続焼鈍炉が長大になるので、５℃／ｓｅｃを下限とした。
【００２０】
急冷終点温度（Ｔ３）に到達後は、この温度域で０〜１５秒間保持する。この保持時間は、長いほうが耐時効特性が安定するが、焼鈍炉を不必要に長くするので１５秒をその上限とする。なお、保持時間が０秒であっても本発明の目的は達成される。
この後は、４００〜３２０℃（過時効開始温度Ｔ４）の温度域に再加熱し、徐冷させながら３８５〜２２０℃の温度域（過時効終了温度Ｔ５）までで過時効処理する。これらの温度範囲内で過時効処理すると、炭素の効率的な拡散速度が活用でき、迅速に固溶炭素量の減少が図れる。更に、過時効終了温度を３８５〜２２０℃にすることによって、到達固溶炭素量を低く抑えることができる。急冷終点温度（Ｔ３）から過時効開始温度（Ｔ４）までの再加熱は、５℃／ｓｅｃ以上の加熱速度で行えば、最終的な特性を大きく劣化させることはない。好ましくは、過度に連続焼鈍炉のライン長さを長くしないために、１５℃／ｓｅｃ以上とするのが好ましく、また、再加熱のコスト上昇を最小限にとどめるために６０℃／ｓｅｃ以下とするのが好ましい。
【００２１】
【実施例】
以下、実施例により本発明をさらに説明する。
（実施例１）
表１に、熱延鋼板の製造現場から採取したＡからＶまでの熱延鋼板の成分組成を記載する。この熱延鋼板のＢからＪまでを実験室で冷間圧延をして、０．８ｍｍの冷延板とし、それに以下の熱処理を施した。
【００２２】
一つは従来の熱処理の代表的なもので、８００℃−６０秒の均熱焼鈍後、６７５℃から２５０℃までを１００℃／ｓｅｃの冷却速度で急冷し、続いて３５０℃まで再加熱し、３５０℃から２７０℃までの過時効時間を変化させながら徐冷させて過時効処理をした。
一方、本発明の代表的な方法によるもので、８００℃−６０秒の均熱焼鈍後、６７５℃から３５０℃までを８０℃／ｓｅｃの冷却速度で冷却し、その後２５０℃までを２０℃／ｓｅｃの冷却速度で冷却した。その後は、３５０℃まで再加熱し、３５０℃から２７０℃までは過時効時間を変えながら過時効処理を施した。
【００２３】
これらの鋼板の時効指数（ＡＩ）のばらつきを、過時効時間の関数として図３、図４に示す。図３は、炭素量が０．０１０〜０．０２５％のものであり、図４は、炭素量が０．０２５〜０．０５５％の結果である。さらに、図１には、時効指数（ＡＩ）のばらつきの範囲の変化を示す。なお、時効指数は焼鈍した鋼板中に残存する固溶炭素量にほぼ比例する指標である。この時効指数（ＡＩ）は、１０％の引張歪みを与えた後に１００℃−１ｈｒの熱処理を施し、再度引張試験をして、その降伏応力の上昇代で表現する。
【００２４】
図３、図４および図１に示す結果から明らかなように、本発明による二段冷却方法によれば、鋼板の炭素量の範囲にほとんど関係なく時効指数のばらつきを非常に低く抑えることが可能である。時効指数のばらつきを低く抑えることができれば、図５からわかるように、高い焼付硬化量を持った鋼板の製造が可能になるのである。
（実施例２）
表１に示すＡ〜Ｖの熱延鋼板を実験室で冷間圧延によって板圧０．８０ｍｍの鋼板とした。それらを８００℃−６０ｓの均熱処理後、５〜１０℃／ｓｅｃで急冷開始温度Ｔ１まで冷却し、急冷中間温度Ｔ２までおよびその後急冷終点温度Ｔ３までをそれぞれ表２〜３に示す冷却速度で冷却し、Ｔ３温度で所定の等温保持を行い、過時効開始温度まで１０〜３０℃／ｓｅｃの加熱速度で再加熱した。その後過時効処理の開始温度から終了温度へ冷却しながら所定の時間の時効処理を行い、時効処理完了後、室温まで冷却して鋼板の材質を調査した。この様にして得られた鋼板の特性を焼鈍処理条件と共に表２〜３に示す。各条件で、１０回の実験を行い、各材質は１２回の平均値を示した。また、ＡＩのばらつきは、測定されたＡＩの範囲で示した。
【００２５】
Ｎｏ．１は、Ｃ量が本発明の範囲外であるために、ＡＩが３０ＭＰａ以上であるばかりでなく、ＡＩのばらつきも１０ＭＰａ以上の大きな値となっている。
また、Ｎｏ．４２〜５１はすべて、化学成分が本発明の範囲外の鋼である。まず、Ｎｏ．４２は、Ｃ量が過剰であるために、ＹＰが２５０ＭＰａ以上と硬質化しているばかりでなく、ＡＩ及びＡＩのばらつきも大きくなっている。Ｎｏ．４３は、Ｓｉ添加量が本発明の範囲外であったために熱延鋼板にスケール起因のキズが発生しており、冷延鋼板の表面品位が悪い。そのため、他の評価は行わなかった。Ｎｏ．４４および４７は、ＭｎとＳの比が１０以下と小さいために、スラブ段階での割れが生じ、冷延鋼板の表面品位が悪い。そのため、他の評価は行わなかった。Ｎｏ．４５、４６、４９、５０、５１はそれぞれ、Ｍｎ、Ｐ、Ａｌ、Ｎ、Ｂの添加量が本発明範囲より多いために、鋼板が硬質化し、ＹＰが２５０ＭＰａを越えて、加工性が劣化している。Ｎｏ．４８は、Ａｌ添加量が本発明範囲より少ないために、Ｎ起因の時効性が現れ、ＡＩ及びＡＩのばらつき共に大きくなっている。
【００２６】
Ｎｏ．１１と１２は急冷開始温度が、また、Ｎｏ．３、４、１８、３１、３９は急冷中間温度が、また、Ｎｏ．２４、２５は急冷終点温度が本発明の範囲外であるために、ＡＩが３０ＭＰａ以上と大きく、また、ＡＩのばらつきが１０ＭＰａ以上と大きくなっている。また、Ｎｏ．２４は、急冷終点温度から過時効までに再加熱が行われていないことも本発明の範囲外である。
【００２７】
Ｎｏ８はＴ１からＴ２の冷却速度が、Ｎｏ．１０はＴ２からＴ３の冷却速度が本発明の範囲外で遅すぎるために、ＡＩが３０ＭＰａ以上と大きく、また、ＡＩのばらつきが１０ＭＰａ以上と大きくなっている。
Ｎｏ．９、１３、１４、２６、４０、４１は、冷却速度が本発明の範囲外で速すぎるか、もしくは、冷却が１段で行われたために、ＡＩ自身は低くなっているものの、ＡＩのばらつきが１０ＭＰａ以上となって、安定したＡＩの制御ができていない。
【００２８】
Ｎｏ．２２は、Ｔ３での保持時間が１８秒と本発明の範囲よりも長いために、ＡＩのばらつきが１０ＭＰａ以上と大きくなっている。
Ｎｏ．３５、３６、３７、３８は、過時効温度が本発明の範囲外であることから、ＡＩ及びＡＩのばらつきが大きくなっている。
以上の比較例以外の例はすべて本発明例であり、焼鈍の均熱処理以降の冷却条件、過時効条件が、本発明の二段冷却−再加熱−過時効処理による場合には、ＡＩとして３０ＭＰａ以内でかつ、ＡＩのばらつきが１０ＭＰａ以内である様な耐時効性のばらつきが極めて少ない冷延鋼板を製造できることを示している。
【００２９】
【表１】

【００３０】
【表２】

【００３１】
【表３】

【００３２】
【発明の効果】
以上詳述したように、本発明によれば、実質的に加工用冷延鋼板のＡＩのばらつきを低減でき、図５に概念的に示したように、品質管理上、より高いＢＨ性を持った非時効性鋼板を安定して提供することができる。これによって、鋼板の出荷管理が容易になるばかりでなく、鋼板の使用者である自動車メーカー等で、ストレッチャーストレインが発生しない条件下でこれまでよりも高いＢＨ性の鋼板を使用することができるために、製品の高品質化、安全性向上、更には、車体重量の低下等を通じて、省エネルギー、環境負荷の低減等に貢献する事ができる。
【図面の簡単な説明】
【図１】図１は、本発明と従来方法について、時効指数の範囲を示すグラフである。
【図２】図２は、本発明と従来方法の焼鈍サイクルを比較して概念的に示すグラフである。
【図３】図３は、本発明と従来方法について、Ｃ：０．０１０〜０．０２５％の鋼板の時効指数のばらつきを示すグラフである。
【図４】図４は、本発明と従来方法について、Ｃ：０．０２５〜０．０５５％の鋼板の時効指数のばらつきを示すグラフである。
【図５】図５は、焼き付け硬化量（ＢＨ）と時効指数（ＡＩ）の関係により、本発明によるばらつき低減の効果を概念的に説明するグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a manufacturing method by continuous annealing of a cold-rolled steel sheet having excellent workability and aging resistance with very little variation, and its main application is automobiles, home appliances, building materials and the like. Furthermore, when the high-strength steel sheet according to the present invention is used in an automobile, the thickness can be reduced, which contributes to an improvement in automobile fuel consumption, and also helps to improve the global environment, which has become a major issue in recent years. .
[0002]
[Prior art]
A method for producing a cold-rolled steel sheet having good aging resistance by using a low-carbon steel sheet by continuous annealing is disclosed in, for example, Japanese Patent Publication No. 5-55573. That is, according to the present invention, in continuously annealing a cold-rolled low carbon steel sheet, the hot-rolled sheet is cold-rolled by a normal method, and the cold-rolled steel sheet is recrystallized / granulated, and then rapidly cooled / superheated. The present invention relates to a continuous annealing method in which cooling, reheating, and gradient overaging treatment are performed, and a low carbon steel plate whose carbon content changes in a range of 0.01 to 0.06% is changed from a high temperature range of 720 to 600 ° C. to 450 to 450 ° C. A low temperature range of 300 ° C. is cooled in one step at a cooling rate of 50 to 250 ° C./sec. This rapid cooling secures a large amount of solute carbon and promotes the nucleation of carbides.
[0003]
[Problems to be solved by the invention]
However, in the invention described in Japanese Patent Publication No. 5-55573 of the above prior art, when the carbon content changes in the range of 0.01 to 0.06%, the number of carbide nucleation becomes unstable. Even if an overaging treatment is performed, the final amount of dissolved carbon tends to vary, and there is a problem that the aging resistance characteristics of the cold-rolled steel sheet vary. Thus, when a cold-rolled steel sheet is annealed by the conventional continuous annealing method, in general, due to the presence of fluctuations in the amount of components such as C and Mn of the steel sheet and process conditions such as hot rolling and cold rolling, Large variations occur in the aging resistance characteristics of cold-rolled steel sheets. This variation not only lowers the product value, but also creates difficulty in shipping management.
[0004]
Then, the 1st aim of this invention is providing the manufacturing method of the cold-rolled steel plate which can reduce the dispersion | variation in an anti-aging characteristic.
There is a relationship as shown in FIG. 5 between the aging index (Aging Index, hereinafter simply referred to as AI) and the bake-hardenability (hereinafter also simply referred to as BH) of the cold rolled steel sheet. There is an upper limit to the aging index in the shipping management of steel sheets, and steel sheets with an aging index higher than this cannot be shipped. Usually, 30 MPa is used as the upper limit of the aging index. Although there is an upper limit to the aging index in managing the shipment of steel sheets, there is still a demand for higher bake hardening. Then, the 2nd aim of this invention is providing the manufacturing method of a cold-rolled steel plate with a substantially high bake hardening amount by suppressing the dispersion | variation in an anti-aging characteristic.
[0005]
[Means for Solving the Problems]
In order to solve the above-mentioned problems, the present inventors have made it possible to sufficiently reduce the variation in anti-aging characteristics even if the process conditions such as the steel sheet components, hot rolling, and cold rolling fluctuate within a certain range. The continuous annealing heat cycle that can be done was studied earnestly. As a result, as shown in FIG. 2, in contrast to the conventional method of linearly cooling from the rapid cooling start temperature (T1) to the rapid cooling end point temperature (T3), the rapid cooling is performed from T1 to the rapid cooling intermediate temperature T2. Thereafter, a new effect was found that if the gradual cooling is carried out between the temperatures from T2 to T3, the variation in the aging characteristics of the cold-rolled steel sheet becomes extremely small. When the steel sheet is continuously annealed using the thermal cycle of the new continuous annealing, which is also called the two-stage cooling method, even if there are fluctuations within a certain range in the process conditions such as the component system of the steel sheet, hot rolling, and cold rolling. The variation in anti-aging characteristics can be sufficiently reduced.
[0006]
The technical roles of the cooling process from T1 to T3 are two points: (1) securing a large amount of solute carbon by supercooling and (2) inducing nucleation of carbides prior to the overaging process.
First, securing a large amount of solute carbon in (1) is indispensable for generating a driving force for easily generating carbide nuclei. And (2) is to ensure a sufficient number of solute carbon precipitation sites and uniformly generate it in the plate in order to rapidly reduce the solute carbon in the subsequent overaging treatment. Is necessary for.
[0007]
The thermal cycle conditions from T1 to T3 for achieving this technical purpose were examined in detail. As a result, in order to sufficiently secure the amount of dissolved carbon in (1), a high cooling rate is required particularly in the temperature range from 720 ° C. to 600 ° C. where the carbon diffusion rate is high to an intermediate temperature of 450 to 300 ° C. I found out. That is, the cooling rate required in this case is a high cooling rate of 50 ° C./sec or more. On the other hand, the upper limit cooling rate is 250 ° C./sec, and a high cooling rate exceeding this is unnecessary in terms of increasing the amount of dissolved carbon. Further, in order to uniformly generate sufficient carbide nuclei in (2) in the plate, the quenching end point temperature (T3) from 450 to 300 ° C of the quenching intermediate temperature (T2) to 300 to 200 ° C is set to 5 to 5. It has been found that it is effective to cool at a slightly loose rate of 40 ° C./s.
[0008]
When the method of the present invention is applied to a continuous annealing method in which rapid cooling, supercooling, reheating, and gradient overaging are performed after recrystallization and grain growth treatment, cold rolling with extremely low aging resistance characteristics is achieved. Steel sheets can be manufactured. Furthermore, by reducing the variation in the aging resistance characteristics, the steel sheet can be substantially given a higher bake hardening amount.
[0009]
A method for producing a cold-rolled steel sheet having anti-aging characteristics with very little variation according to the present invention is as follows.
(1) By mass%, C: 0.01 to 0.06%, Si: 0.1% or less, Mn: 0.05 to 0.40%, P: 0.10% or less, S: 0.002 -0.025%, sol. Al: 0.01 to 0.10%, N: 0.001 to 0.006%, steel comprising the balance iron and unavoidable impurities is hot-rolled by a normal method, and the cold-rolled cold-rolled steel sheet is recrystallized. When performing continuous annealing with rapid cooling, supercooling, reheating, and gradient overaging after the grain growth treatment, the temperature range from 720 to 600 ° C to 450 to 300 ° C after the recrystallization and grain growth is 50 to 250. ° C. / quenched with sec cooling rate, and then to a temperature range of 300 to 200 DEG ° C. and cooled at a cooling rate of 5 to 40 ° C. / sec, after holding 0-15 seconds at that temperature range, the four hundred to three hundred twenty ° C. Reheating with a temperature increase of at least 40 ° C. in the temperature range, and overaging while cooling from the temperature range to a temperature range of 385 to 220 ° C. A method for producing a cold-rolled steel sheet.
(2) The steel contains a B / N content ratio of B / N = 0.5 to 2.0, and the variation is extremely high as described in (1) above. A method for producing a cold-rolled steel sheet having low aging resistance.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
The manufacturing method of the cold-rolled steel sheet in the present invention is limited to the amount of C, Si, Mn, P, S, Al, and N of the low-carbon steel sheet. The present invention relates to a continuous annealing method in which rapid cooling, supercooling, reheating, and gradient overaging are performed after crystal / grain growth treatment. In particular, a thermal cycle in which the quenching start temperature (T1) to the quenching end point temperature (T3) is cooled in two stages is employed. As a result, variations in the aging resistance are suppressed, and a cold-rolled steel sheet having a high bake hardening amount is manufactured. The reasons for limiting the present invention will be described below.
[0011]
First, about chemical components.
As the content of C decreases, the ductility and deep drawability are improved, and the aging resistance characteristics that the content is too low deteriorates. Therefore, in the case of the present invention, C: 0.01 to A cold-rolled steel sheet having excellent workability and aging resistance can be obtained in the range of 0.06%.
[0012]
Since Mn and S prevent brittleness due to S during hot rolling, and MnS is used as a preferential precipitation site for carbide, a limited range is required for their amounts. From these viewpoints, Mn was regulated to 0.05 to 0.40%, and S was regulated to 0.002 to 0.025%. The combination of this and the two-stage rapid cooling will produce a cold-rolled steel sheet with even better aging resistance.
[0013]
Si not only dissolves in ferrite and hardens the matrix, but when added in a large amount, it causes deterioration of the surface quality due to surface scratches and the like due to scale and lowering of chemical conversion treatment. Limit to 1% or less. However, since it is difficult to control the Si addition amount to less than 0.003% from the actual manufacturing conditions, the lower limit of the Si addition amount is preferably set to 0.003%. P is an element that does not significantly affect the aging resistance, but when manufacturing steel sheets for automobiles, the upper limit must be 0.10%. This is because when P exceeds this range, the workability of the steel sheet is significantly deteriorated.
[0014]
sol. Al is an element necessary for controlling the amounts of oxygen and nitrogen in the steel, but if it is too much, it hardens, so the upper limit was made 0.10%. On the other hand, if the amount is too small, aging caused by nitrogen cannot be suppressed, so the lower limit was made 0.01%.
N is sol. Since it is combined with Al to become AlN (BN when B is added) and the material is hardened, the upper limit was made 0.006%. Note that the lower limit is set to 0.001% because it is difficult for the current steelmaking technology to lower the N content.
[0015]
In the present invention, B is contained as necessary. When B is contained at 0.5 or more in B / N, it is combined with N in the steel to become BN and nitrogen aging can be prevented. However, when B / N exceeds 2.0, the amount of solid solution B increases and the material becomes hard. Therefore, the lower limit of B / N was set to 0.5 and the upper limit was set to 2.0. Next, although it is a process from casting to hot rolling, either a method of reheating the slab as a cold piece or a continuous casting-direct hot rolling (CC-DR) method may be adopted. Good. Therefore, the temperature when heating the slab need not be particularly limited. Moreover, the coiling temperature after hot rolling does not significantly affect the aging resistance of the steel sheet, and the effect of the present invention can be sufficiently obtained even when coiling at a low temperature of about 600 ° C. When high temperature winding at 700 ° C. or higher is performed, the crystal grains after annealing become large and the workability is improved. Next, the continuous annealing process will be described.
[0016]
The step of recrystallization and grain growth of the cold-rolled steel sheet is not particularly limited, and may be performed by heating above the recrystallization temperature of a normal method and soaking. The rapid cooling after soaking is performed by cooling from a temperature range of 720 to 600 ° C. to a temperature range of 450 to 300 ° C. at a cooling rate of 50 to 250 ° C./sec and then to a temperature range of 350 to 200 ° C. It is necessary to cool at a cooling rate of ° C / sec. This two-stage cooling method is the most important technical point in obtaining anti-aging characteristics with little variation.
[0017]
In order to avoid the introduction of strain due to transformation into the steel sheet, cooling is performed at a cooling rate of 20 ° C./sec or less after soaking. For the same reason, the upper limit of the rapid cooling start temperature is 720 ° C. If the rapid cooling start temperature is less than 600 ° C., the amount of solid solution carbon at the rapid cooling intermediate temperature (T2) becomes insufficient, so 600 ° C. is set as the lower limit of the rapid cooling start temperature.
The upper limit temperature of the quenching intermediate temperature (T2) is 450 ° C. This is because if the temperature is higher than this temperature, the amount of dissolved carbon is insufficient, and nucleation of carbides does not occur sufficiently. Moreover, when the quenching intermediate temperature is less than 300 ° C., the variation in the nucleation of carbides increases, and as a result, the variation in anti-aging characteristics increases, so 300 ° C. is set as the lower limit of the quenching intermediate temperature.
[0018]
The lower limit of the cooling rate from the rapid cooling start temperature (T1) to the rapid cooling intermediate temperature (T2) is 50 ° C./sec. If it is slower than this, a sufficient amount of solute carbon is not secured, and high aging resistance characteristics cannot be obtained. Further, even if the cooling rate exceeds 250 ° C./sec, the temperature controllability deteriorates and the aging resistance becomes unstable, so 250 ° C./sec is set as the upper limit of the cooling rate.
[0019]
In order to ensure a sufficient carbide precipitation density, the quenching end point temperature is set to 300 ° C. or lower. On the other hand, when the quenching end point temperature is less than 200 ° C., the density of carbide nuclei increases too much and the steel plate becomes hard, so 200 ° C. is set as the lower limit.
From the quenching intermediate temperature (T2) to the quenching end point temperature (T3), it is necessary to cool at a cooling rate of 5 to 40 ° C./sec. When the cooling rate exceeds 40 ° C./sec, the variation in the aging resistance characteristics of the steel sheet increases, so 40 ° C./sec is set as the upper limit. On the other hand, when the cooling rate is less than 5 ° C./sec, the continuous annealing furnace becomes long, so 5 ° C./sec was set as the lower limit.
[0020]
After reaching the quenching end point temperature (T3), this temperature range is maintained for 0 to 15 seconds. The longer the holding time, the more stable the aging resistance, but the annealing furnace is unnecessarily lengthened, so the upper limit is 15 seconds. Even if the holding time is 0 second, the object of the present invention is achieved.
Thereafter, overheating treatment is performed up to a temperature range of 385 to 220 ° C. (overaging end temperature T5) while reheating to 400 to 320 ° C. (overaging start temperature T4) and gradually cooling. By over-aging within these temperature ranges, the efficient diffusion rate of carbon can be utilized, and the amount of dissolved carbon can be reduced quickly. Furthermore, by setting the overaging end temperature to 385 to 220 ° C., the amount of reached solid solution carbon can be kept low. If reheating from the quenching end point temperature (T3) to the overaging start temperature (T4) is performed at a heating rate of 5 ° C./sec or more, the final characteristics are not greatly deteriorated. Preferably, it is preferably 15 ° C./sec or more so as not to excessively increase the line length of the continuous annealing furnace, and 60 ° C./sec or less in order to minimize the increase in reheating costs. Is preferred.
[0021]
【Example】
Hereinafter, the present invention will be further described by examples.
Example 1
In Table 1, the component composition of the hot rolled steel sheet from A to V collected from the production site of the hot rolled steel sheet is described. This hot-rolled steel sheet from B to J was cold-rolled in a laboratory to obtain a 0.8 mm cold-rolled sheet, which was subjected to the following heat treatment.
[0022]
One is a typical conventional heat treatment. After soaking at 800 ° C. for 60 seconds, quench from 675 ° C. to 250 ° C. at a cooling rate of 100 ° C./sec, and then reheat to 350 ° C. The film was slowly cooled while changing the overaging time from 350 ° C. to 270 ° C. to perform overaging treatment.
On the other hand, according to a typical method of the present invention, after soaking at 800 ° C. for 60 seconds, 675 ° C. to 350 ° C. is cooled at a cooling rate of 80 ° C./sec. Cooled at a cooling rate of sec. After that, it was reheated to 350 ° C., and overaging treatment was performed from 350 ° C. to 270 ° C. while changing the overaging time.
[0023]
Variations in the aging index (AI) of these steel sheets are shown in FIGS. 3 and 4 as a function of overaging time. FIG. 3 shows the results when the carbon content is 0.010 to 0.025%, and FIG. 4 shows the results when the carbon content is 0.025 to 0.055%. Further, FIG. 1 shows changes in the variation range of the aging index (AI). The aging index is an index that is substantially proportional to the amount of dissolved carbon remaining in the annealed steel sheet. This aging index (AI) is expressed in terms of an increase in yield stress after applying a heat treatment of 100 ° C. to 1 hr after giving a tensile strain of 10%, performing a tensile test again.
[0024]
As is clear from the results shown in FIGS. 3, 4 and 1, according to the two-stage cooling method of the present invention, it is possible to keep the variation in the aging index very low regardless of the carbon content range of the steel sheet. It is. If variation in the aging index can be suppressed to a low level, as can be seen from FIG. 5, it is possible to produce a steel sheet having a high bake hardening amount.
(Example 2)
The hot rolled steel sheets A to V shown in Table 1 were made into steel sheets having a plate pressure of 0.80 mm by cold rolling in a laboratory. They are soaked at 800 ° C. for 60 seconds, cooled to 5 to 10 ° C./sec to the quenching start temperature T1, and then cooled to the quenching intermediate temperature T2 and then to the quenching end point temperature T3 at the cooling rates shown in Tables 2 and 3, respectively. Then, predetermined isothermal holding was performed at the T3 temperature, and reheating was performed at a heating rate of 10 to 30 ° C./sec to the overaging start temperature. Thereafter, an aging treatment was performed for a predetermined time while cooling from the start temperature to the end temperature of the overaging treatment, and after completion of the aging treatment, the material was cooled to room temperature and the material of the steel sheet was investigated. The properties of the steel sheet thus obtained are shown in Tables 2-3 together with the annealing treatment conditions. Under each condition, 10 experiments were conducted, and each material showed an average value of 12 times. Further, the variation in AI is shown in the range of the measured AI.
[0025]
No. In the case of No. 1, since the C amount is outside the range of the present invention, not only the AI is 30 MPa or more, but also the variation in AI is a large value of 10 MPa or more.
No. 42 to 51 are all steels whose chemical components are outside the scope of the present invention. First, no. No. 42 has not only hardened YP at 250 MPa or more due to an excessive amount of C, but also has a large variation in AI and AI. No. In No. 43, since the Si addition amount was outside the range of the present invention, scratches due to scale occurred in the hot-rolled steel sheet, and the surface quality of the cold-rolled steel sheet was poor. Therefore, no other evaluation was performed. No. In 44 and 47, since the ratio of Mn and S is as small as 10 or less, cracks occur at the slab stage, and the surface quality of the cold-rolled steel sheet is poor. Therefore, no other evaluation was performed. No. 45, 46, 49, 50, and 51 have Mn, P, Al, N, and B added in an amount greater than the range of the present invention, so that the steel plate becomes hard, YP exceeds 250 MPa, and workability deteriorates. ing. No. In No. 48, since the Al addition amount is less than the range of the present invention, aging due to N appears, and both AI and AI variation are large.
[0026]
No. Nos. 11 and 12 have rapid cooling start temperatures. Nos. 3, 4, 18, 31, and 39 have quenching intermediate temperatures. In 24 and 25, since the quenching end point temperature is outside the range of the present invention, AI is as large as 30 MPa or more, and variation in AI is as large as 10 MPa or more. No. It is also outside the scope of the present invention that 24 is not reheated from the quenching end point temperature to overaging.
[0027]
In No. 8, the cooling rate from T1 to T2 is No. No. 10 has a cooling rate from T2 to T3 that is too slow outside the scope of the present invention, so AI is as large as 30 MPa or more, and variation in AI is as large as 10 MPa or more.
No. 9, 13, 14, 26, 40, and 41, although the cooling rate is too fast outside the scope of the present invention or the cooling is performed in one stage, the AI itself is low, but the variation in AI Is 10 MPa or more, and stable AI control is not achieved.
[0028]
No. No. 22 has a retention time at T3 of 18 seconds, which is longer than the range of the present invention, and thus the variation in AI is as large as 10 MPa or more.
No. In 35, 36, 37, and 38, the overaging temperature is outside the range of the present invention, and therefore, the variation in AI and AI is large.
Examples other than the above comparative examples are all examples of the present invention, and when the cooling conditions and soaking conditions after annealing soaking are based on the two-stage cooling-reheating-overaging process of the present invention, the AI is 30 MPa. It is shown that it is possible to manufacture a cold-rolled steel sheet with extremely little variation in aging resistance such that the variation in AI is within 10 MPa.
[0029]
[Table 1]

[0030]
[Table 2]

[0031]
[Table 3]

[0032]
【The invention's effect】
As described above in detail, according to the present invention, it is possible to substantially reduce the variation in AI of the cold-rolled steel sheet for processing, and as shown conceptually in FIG. In addition, a non-aging steel sheet can be provided stably. This not only facilitates the shipping management of steel sheets, but also makes it possible for automobile manufacturers who are steel sheet users to use steel sheets with higher BH properties than before under conditions where stretcher strain does not occur. For this reason, it is possible to contribute to energy saving, reduction of environmental load, and the like by improving the quality of products, improving safety, and lowering the weight of the vehicle body.
[Brief description of the drawings]
FIG. 1 is a graph showing the range of aging index for the present invention and the conventional method.
FIG. 2 is a graph conceptually showing a comparison between annealing cycles of the present invention and a conventional method.
FIG. 3 is a graph showing variations in the aging index of steel sheets with C: 0.010 to 0.025% for the present invention and the conventional method.
FIG. 4 is a graph showing variations in the aging index of steel sheets with C: 0.025 to 0.055% for the present invention and the conventional method.
FIG. 5 is a graph conceptually illustrating the effect of reducing variation according to the present invention, based on the relationship between the bake hardening amount (BH) and the aging index (AI).

Claims

In mass%, C: 0.01 to 0.06%, Si: 0.1% or less, Mn: 0.05 to 0.40%, P: 0.10% or less, S: 0.002 to 0. 025%, sol. Al: 0.01 to 0.10%, N: 0.001 to 0.006%, the steel made of the remaining iron and unavoidable impurities is hot-rolled by a normal method, and the cold-rolled cold-rolled steel sheet is recrystallized. When continuous annealing is performed by rapid cooling, supercooling, reheating, and gradient overaging after the grain growth treatment, the temperature range from 720 to 600 ° C to 450 to 300 ° C after the recrystallization and grain growth is 50 to 250. ° C. / quenched with sec cooling rate, and then to a temperature range of 300 to 200 DEG ° C. and cooled at a cooling rate of 5 to 40 ° C. / sec, after holding 0-15 seconds at that temperature range, the four hundred to three hundred twenty ° C. Reheating with a temperature increase of at least 40 ° C. in the temperature range, and overaging treatment while cooling from the temperature range to 385 to 220 ° C. A method for producing a cold-rolled steel sheet.

The aging resistance characteristic of the present invention is extremely low in variation according to claim 1, characterized in that the steel contains a B / N content ratio satisfying the relationship of B / N = 0.5 to 2.0. The manufacturing method of the cold-rolled steel plate which has this.