JP4358418B2 - Low yield ratio high strength cold-rolled steel sheet and plated steel sheet excellent in hole expansibility and method for producing the same - Google Patents
Low yield ratio high strength cold-rolled steel sheet and plated steel sheet excellent in hole expansibility and method for producing the same Download PDFInfo
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【０００１】
【発明の属する技術分野】
本発明は穴拡げ性に優れた低降伏比高強度鋼板およびめっき鋼板とその製造方法に関わるものである。本発明が係わる高強度鋼板とは、自動車部品、家庭電気製品、建築などの用途にプレス加工をして使用されるものであり、防錆の改善のために溶融亜鉛めっきや電気亜鉛めっきを施したり、さらにそのいっそうの改善を図るために金属酸化物皮膜、有機皮膜を表面処理した鋼板やプレス成形性の改善のために上層に鉄めっきを施した鋼板を含む。
【０００２】
【従来の技術】
自動車車体には衝突時に乗員を保護するような機能の確保がＣＯ2をはじめとした排出ガスの低減を燃費向上により実現するような軽量化とともに要求されている。これに呼応し、プレス加工性を悪化させずに鋼板を高強度化する強化機構として一般に考えられている固溶強化やマルテンサイトやベイナイトをフェライトマトリクス中に微細分散させた複合組織強化を用いた高強度鋼板が開発されてきた。さらに近年では登録特許第２０１７３２０号公報や登録特許第２５４５３１６号公報にあるように残留オーステナイトの変態誘起塑性を利用し、引張試験における伸びの改善を試みた鋼板が開発されているが、プレス加工性の良否は引張試験の伸びにだけ依存するものではなく、伸びフランジ加工の厳しい部材では穴拡げ性と言われる剪断端面の延性の良否にも大きく依存するため、穴拡げ性が必ずしも良好ではないかかる高強度鋼板の使用は限定された範囲にとどまっている。
【０００３】
鋼板の穴拡げ性を改善する手法としては、ＭｎＳのように展伸した介在物の増加につながるＳを低減することや、変形応力が大きく異なる相境界に形成される鋭いボイドを裂けるためマルテンサイトの混在を避けることが提案されている。しかしながら鋼中のＳを極端に低減することはコストの著しい増加を招くばかりか、産業上容易に実現できるものではないうえ、Ｓを含む介在物が見られなくなれば別の介在物の悪影響が顕在化する可能性もあり、現実的なものではない。またマルテンサイトの存在を避けて高強度化しようとしても引張強さ５９０ＭＰａ以上の高強度を得ようとすれば、ベイナイトを主体とする金属組織としたり、Ｔｉ、Ｎｂ、Ｍｏ、Ｖといった元素を添加して析出強化を図らなければならないが、一般にこれらの方法では降伏強さの引張強さに対する比、すなわち降伏比が高くなるため、プレス加工性からは好ましくなく、さらにＴｉ、Ｎｂ、Ｍｏ、Ｖといった元素は再結晶温度を上げるため、冷延鋼板では再結晶状態でその析出強化を十分に活用することが難しい。
【０００４】
【発明が解決しようとする課題】
そこで、本発明は、マルテンサイト、残留オーステナイトおよびベイナイトをフェライトマトリクス中に微細分散させ、プレス加工性を改善しようとした複合組織高強度鋼板において、低降伏比を確保しながら、伸びフランジ加工の厳しい部材にもプレス加工できるように、引張試験における伸びと穴拡げ性の両立を図ることを課題とする。
【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べく、剪断端面においてフェライトとマルテンサイトの相境界に鋭いボイドが形成されないような金属組織はマルテンサイト近傍のフェライト自体を微細なＴｉ、Ｎｂ、Ｍｏといった元素を含む微細析出物で強化し、隣接組織間の変形応力差を低減することによって実現できるという見地から、Ｃ、Ｓｉ、Ｍｎ、Ｓの含有量を調節したうえで、Ｔｉ、Ｎｂ、Ｍｏといった析出強化元素を添加した鋼を用いて、熱間圧延温度と冷間圧延後の焼鈍温度条件について鋭意検討を加えた結果、％Ｓ、％ＴｉをそれぞれＳ、Ｔｉ含有量とした時に（％Ｔｉ）／（％Ｓ）≧５となるように成分調整された鋼を、％ＳｉをＳｉ含有量とした時に仕上出側温度を（９００＋５０×％Ｓｉ）℃以下とする熱間圧延を行ったうえで、７００〜９００℃のフェライト、オーステナイトの二相共存温度域で焼鈍し、マルテンサイトおよび残留オーステナイトが合計で体積率６％以上で、かつマルテンサイト、残留オーステナイトおよびベイナイトの硬質相組織がＴｉ、Ｎｂ、Ｍｏといった析出強化元素の添加量に対応した一定割合で含まれる金属組織を形成すれば、必要な強度が確保されたうえで低降伏比で、引張試験における伸びと穴拡げ性を両立できることを見出した。
【０００６】
本発明はこのような思想と新知見に基づいて構成された従来にはない全く新しい伸びおよび穴拡げ性に優れた低降伏比高強度鋼板およびそのめっき鋼板並びにそれらの製造方法であり、その要旨とするところは以下のとおりである。
【０００７】
（１） 質量％で、
Ｃ：０．０４〜０．１４％、
Ｓｉ：０．４〜２．２％、
Ｍｎ：１．５２〜２．４％、
Ｐ：０．０２％以下、
Ｓ：０．０１％以下、
Ａｌ：０．００２〜０．５％、
Ｔｉ：０．００５〜０．１％、
Ｎ：０．００６％以下
を含有し、さらに％Ｓ、％ＴｉをそれぞれＳ、Ｔｉ含有量とした時に（％Ｔｉ）／（％Ｓ）≧５を満足し、残部Ｆｅおよび不可避的不純物からなり、金属組織はフェライトマトリックス中にマルテンサイト、残留オーステナイトおよびベイナイトが微細分散しており、その金属組織におけるマルテンサイトおよび残留オーステナイトの体積率が合計で６％以上で、かつマルテンサイト、残留オーステナイトおよびベイナイトの硬質相組織の体積率をα％、Ｔｉの添加量を％Ｔｉとした時に、α≦５００００×（（％Ｔｉ）／４８）であることを特徴とする、伸びおよび穴拡げ性に優れた低降伏比高強度冷延鋼板。
【０００８】
（２） 質量％で、
Ｃ：０．０４〜０．１４％、
Ｓｉ：０．４〜２．２％、
Ｍｎ：１．５２〜２．４％、
Ｐ：０．０２％以下、
Ｓ：０．０１％以下、
Ａｌ：０．００２〜０．５％、
Ｔｉ：０．００５〜０．１％、
Ｎ：０．００６％以下
とさらにＮｂ、Ｍｏの１種以上を合計で０．００５〜０．１％含有し、さらに％Ｓ、％ＴｉをそれぞれＳ、Ｔｉ含有量とした時に（％Ｔｉ）／（％Ｓ）≧５を満足し、残部Ｆｅおよび不可避的不純物からなり、金属組織はフェライトマトリックス中にマルテンサイト、残留オーステナイトおよびベイナイトが微細分散しており、その金属組織におけるマルテンサイトおよび残留オーステナイトの体積率が合計で６％以上で、かつマルテンサイト、残留オーステナイトおよびベイナイトの硬質相組織の体積率をα％、Ｔｉ、Ｎｂ、Ｍｏの添加量をそれぞれ％Ｔｉ、％Ｎｂ、％Ｍｏとした時に、α≦５００００×（（％Ｔｉ）／４８＋（％Ｎｂ）／９３＋（％Ｍｏ）／９６）であることを特徴とする、伸びおよび穴拡げ性に優れた低降伏比高強度冷延鋼板。
【０００９】
（３） さらに、質量％で、
Ｂ：０．００１０〜０．００３％
を含有することを特徴とする上記（１）または（２）に記載の、伸びおよび穴拡げ性に優れた低降伏比高強度冷延鋼板。
【００１０】
（４） 上記（１）〜（３）のいずれかに記載の鋼板にめっきを施したことを特徴とする、伸びおよび穴拡げ性に優れた低降伏比高強度めっき鋼板。
【００１１】
（５） 上記（１）〜（３）のいずれかに記載の化学成分からなる組成のスラブに％ＳｉをＳｉ含有量とした時、仕上出側温度を（９００＋５０×％Ｓｉ）℃以下とする熱間圧延を行い、５０〜８５％の冷間圧延を施した冷延板を７００〜９００℃のフェライト、オーステナイトの二相共存温度域で１０秒〜５分焼鈍し、７００℃から５００℃までの間の平均冷却速度を１〜１２０℃／秒として２５０〜５００℃に冷却し、その冷却温度域に３０秒〜１０分保持してから常温まで冷却して、フェライトマトリックス中にマルテンサイト、残留オーステナイトおよびベイナイトを微細分散させた金属組織とし、その金属組織におけるマルテンサイトおよび残留オーステナイトの体積率が合計で６％以上で、かつマルテンサイト、残留オーステナイトおよびベイナイトの硬質相組織の体積率をα％、Ｔｉ、Ｎｂ、Ｍｏの添加量をそれぞれ％Ｔｉ、％Ｎｂ、％Ｍｏとした時に、α≦５００００×（（％Ｔｉ）／４８＋（％Ｎｂ）／９３＋（％Ｍｏ）／９６）とすることを特徴とする、伸びおよび穴拡げ性に優れた低降伏比高強度冷延鋼板の製造方法。
【００１２】
（６） 上記（１）〜（３）のいずれかに記載の化学成分からなる組成のスラブに％ＳｉをＳｉ含有量とした時、仕上出側温度を（９００＋５０×％Ｓｉ）℃以下とする熱間圧延を行い、５０〜８５％の冷間圧延を施した冷延板を７００〜９００℃のフェライト、オーステナイトの二相共存温度域で１０秒〜５分焼鈍し、７００℃から５００℃までの間の平均冷却速度を１〜１２０℃／秒として２５０〜５００℃に冷却し、再加熱した後２５０〜６００℃の範囲の温度域に３０秒〜１０分保持してから常温まで冷却して、フェライトマトリックス中にマルテンサイト、残留オーステナイトおよびベイナイトを微細分散させた金属組織とし、その金属組織におけるマルテンサイトおよび残留オーステナイトの体積率が合計で６％以上で、かつマルテンサイト、残留オーステナイトおよびベイナイトの硬質相組織の体積率をα％、Ｔｉ、Ｎｂ、Ｍｏの添加量をそれぞれ％Ｔｉ、％Ｎｂ、％Ｍｏとした時に、α≦５００００×（（％Ｔｉ）／４８＋（％Ｎｂ）／９３＋（％Ｍｏ）／９６）とすることを特徴とする、伸びおよび穴拡げ性に優れた低降伏比高強度冷延鋼板の製造方法。
【００１３】
（７）上記（５）または上記（６）に引き続き、さらに、めっきを行なうことを特徴とする、伸びおよび穴拡げ性に優れた低降伏比高強度めっき鋼板の製造方法。
【００１４】
【発明の実施の形態】
以下、本発明を詳細に説明する。
【００１５】
まず、Ｃ、Ｓｉ、Ｍｎ、Ｐ、Ｓ、Ａｌ、Ｔｉ、Ｎｂ、Ｍｏ、Ｎの数値限定理由について述べる。
【００１６】
Ｃはマルテンサイトや残留オーステナイトによる組織強化を低コストで実現する場合に必須の元素であり、Ｃが０．０４％未満ではマルテンサイトや残留オーステナイトの体積率が低下するため、必要とする引張強さの確保が困難である。一方Ｃが０．１４％を超えると、マルテンサイトや残留オーステナイトの体積率が増加し、引張強さを高めることは容易であるものの、剪断端面において鋭いボイドが形成され穴拡げ性の劣化が著しい。
【００１７】
Ｓｉの添加により鋼板の伸びが大きく損なわれずに強度が増すことはよく知られており、またパーライトおよびベイナイト変態の進行を著しく遅滞させて粗大炭化物の形成を阻害することにより穴拡げ性を改善すると同時に、室温まで冷却後の金属組織にマルテンサイトおよび残留オーステナイトの存在が容易として引張試験における伸びを大きくするうえ、特に本発明では一定の条件で熱間圧延することにより焼鈍後の金属組織においてマルテンサイト近傍のフェライトを主体にＴｉ、Ｎｂ、Ｍｏといった元素を含む微細析出物で強化し、隣接組織間の変形応力差を低減するうえで有用であり、本発明の特徴とする低降伏比で、引張試験における伸びと穴拡げ性の両立を実現するために重要である。パーライトおよびベイナイト変態の進行を著しく遅滞させて粗大炭化物の形成を阻害することにより穴拡げ性を改善し、また室温まで冷却後の金属組織にマルテンサイトおよび残留オーステナイトの存在を容易とし、引張試験における伸びを大きくするためには０．４％を超す添加が必要である。しかしその添加量が２．２％を超えると、焼鈍後の金属組織においてマルテンサイト近傍のフェライトを主体にＴｉ、Ｎｂ、Ｍｏといった元素を含む微細析出物で強化するように熱間圧延を行うことが困難となり、低降伏比と穴拡げ性の両立が困難となる。
【００１８】
ＭｎはＣとともにオーステナイトの自由エネルギーを下げ、鋼の焼入れ性を増す元素として知られており、必要とする引張強さで伸びを改善するためにマルテンサイトおよび残留オーステナイトを合計で体積率６％以上存在する金属組織とすることを目的に１．５２％以上添加する。しかし添加量が過大になると低降伏比のままで穴拡げ性の劣化しないようなＴｉの添加量が見出せないため、２．２％を上限とする。
【００１９】
Ｐは一般に不可避的不純物として鋼に含まれるが、その量が０．０２％を超えると、本発明におけるような引張強さが５９０ＭＰａを超すような高強度鋼板では靭性とともに冷間圧延性が著しく劣化し、工業的に多量生産することが困難となる。
【００２０】
Ｓも一般に不可避的不純物として鋼に含まれるが、その量が０．０１％を超えると鋼板のスポット溶接性におよぼす悪影響が顕著となる。
【００２１】
Ａｌは鋼の脱酸元素として、またＡｌＮによる熱延素材の細粒化、および一連の熱処理工程における結晶粒の粗大化を抑制し材質を改善するために０．００２％以上添加する必要があるが、０．５％を超えることはコスト高となるばかりか、表面性状を劣化させる。
【００２２】
Ｔｉはマルテンサイト近傍のフェライトを主体に微細析出し、隣接組織間の変形応力差を低減するとともに、ＳがＭｎＳとして展伸介在物となり穴拡げ性を劣化するのを防ぐ目的で添加する。一般にはＴｉが微細析出すると引張強さが高くなると同時に再結晶温度も上昇し、フェライト、オーステナイトの二相共存温度域でもフェライトが未再結晶状態となることがあるが、本発明では鋼のＳｉを添加のうえで熱間圧延条件を調整することにより焼鈍温度にかかわらず、十分に再結晶したフェライトを主体にマルテンサイトおよび残留オーステナイトが合計で体積率６％以上含まれる金属組織を形成できる。Ｔｉの添加量が０．００５％未満ではマルテンサイト近傍のフェライトの析出強化に効果がなく、また％Ｓ、％ＴｉをそれぞれＳ、Ｔｉ含有量とした時に（％Ｔｉ）／（％Ｓ）が５未満では展伸したＭｎＳの生成を抑制することが困難であり、穴拡げ性は改善できない。しかしＴｉの量が０．１％を超えるとフェライトの析出強化が著しいために降伏比が増大するばかりか、本発明で規定するように鋼のＳｉ含有量とともに熱間圧延条件を調整したとしても短時間の焼鈍でフェライトを再結晶させることが難しいため、引張試験における伸びも小さくなる。
【００２３】
Ｎは不可避的不純物として鋼に含まれるとともに、Ｔｉ、Ｎｂ、Ｍｏと窒化物を析出し、鋼を高強度化するが、その量が０．００６％を超えると、引張試験における伸びが劣化するため、これを上限とする。
【００２４】
Ｎｂ、ＭｏもまたＴｉと同じように析出強化元素であり、マルテンサイト近傍のフェライトを主体に微細析出し、隣接組織間の変形応力差を低減することで穴拡げ性を改善し、合わせて必要とする引張強さを確保する目的で合計で０．００５〜０．１％を添加してもよい。しかしその添加量が０．１％を超すと、Ｔｉとともにフェライトの再結晶を著しく阻害し、加工性が劣化する。
【００２５】
これらを主成分とする鋼にＣｕ、Ｓｎ、Ｚｎ、Ｚｒ、Ｗ、Ｃｒ、Ｎｉ、Ｂを合計で１％以下含有しても本発明の効果を損なわず、その量によっては耐食性が改善される等好ましい場合もある。
【００２６】
次に製造条件の限定理由について述べる。その目的はマルテンサイト、残留オーステナイトおよびベイナイトをフェライトマトリクス中に微細分散させた金属組織を有し、引張試験における伸びが改善された複合組織高強度鋼板において、低降伏比を確保しながら穴拡げ性を改善するため、剪断端面においてフェライトとマルテンサイトの相境界に鋭いボイドが形成されないようにマルテンサイト近傍を主体にフェライト自体を微細なＴｉ、Ｎｂ、Ｍｏといった元素を含む微細析出物で強化することにある。高強度にして低降伏比で、引張試験における伸びが大きいという複合組織強化の特徴が認められるのは、マルテンサイトおよび残留オーステナイトの体積率が６％以上の場合であり、特に残留オーステナイトの変態誘起塑性による効果をはっきりと認めるには、残留オーステナイトの体積率がマルテンサイトの体積率を上回ることが好ましい。またベイナイトが共存することは高強度にして低降伏比で、引張試験における伸びが大きいという複合組織強化の特徴を発現するうえでは好ましくはないものの、その体積率がマルテンサイトおよび残留オーステナイトの体積率の３倍を超えない場合には降伏比を著しく増加させることなく、より少ないＣ含有量で鋼板の引張強さを高めることができ、本発明において有用である。またマルテンサイト、残留オーステナイトおよびベイナイトの硬質相組織の体積率をα％、Ｔｉ、Ｎｂ、Ｍｏの添加量をそれぞれ％Ｔｉ、％Ｎｂ、％Ｍｏとした時に、α≦５００００×（（％Ｔｉ）／４８＋（％Ｎｂ）／９３＋（％Ｍｏ）／９６）であると、マルテンサイト近傍を主体にフェライト自体を微細なＴｉ、Ｎｂ、Ｍｏといった元素を含む微細析出物で強化し、剪断端面においてフェライトとマルテンサイトの相境界に鋭いボイドが形成されるのを抑制し、穴拡げ性が改善される。α／（（％Ｔｉ）／４８＋（％Ｎｂ）／９３＋（％Ｍｏ）／９６）が５００００を超えるような場合にはフェライトの析出強化が不十分となるため、剪断端面においてフェライトとマルテンサイトの相境界に鋭いボイドが形成されやすく、穴拡げ性が劣化する。
【００２７】
熱間圧延に供するスラブは特に限定するものではない。すなわち、連続鋳造スラブや薄スラブキャスター等で製造したものであればよい。また鋳造後直ちに熱間圧延を行う連続鋳造−直送圧延（ＣＣ−ＤＲ）のようなプロセスにも適合する。
【００２８】
熱間圧延では％ＳｉをＳｉ含有量とした時、仕上出側温度を（９００＋５０×％Ｓｉ）℃以下とする必要がある。これはＴｉ、Ｎｂ、Ｍｏの炭窒化物を引き続く冷間圧延後の再結晶温度を異常に上昇させないように析出させるとともに、フェライト、オーステナイトの二相共存温度域で焼鈍した場合には焼鈍温度にかかわらず、十分に再結晶したフェライトを主体にマルテンサイトおよび残留オーステナイトを合計で体積率６％以上含み、かつマルテンサイト、残留オーステナイトおよびベイナイトの硬質相組織の体積率をα％、Ｔｉ、Ｎｂ、Ｍｏの添加量をそれぞれ％Ｔｉ、％Ｎｂ、％Ｍｏとした時にはα≦５００００×（（％Ｔｉ）／４８＋（％Ｎｂ）／９３＋（％Ｍｏ）／９６）となるようにマルテンサイト近傍のフェライトを主体にＴｉ、Ｎｂ、Ｍｏといった元素を含む微細析出物で強化し、隣接組織間の変形応力差が低減された金属組織を形成させることを目的とする。仕上出側温度が（９００＋５０×％Ｓｉ）℃を超えると、炭窒化物が微細分散するようになり、引張強さが高くはなるものの、フェライト、オーステナイトの二相共存温度域で焼鈍した場合にも未再結晶のフェライトが残存するため、引張試験における伸びが劣化する。熱延後の冷却条件や巻取温度は特に限定しないが、巻取温度はコイル両端部での材質ばらつきが大ききなることを避け、またスケール厚の増加による酸洗性の劣化を避けるためには７５０℃以下とし、またＴｉ、Ｎｂ、Ｍｏの析出が不十分となり、材質ばらつきが生じるのを避けるために５００℃以上とすることが望ましい。
【００２９】
冷間圧延は通常の条件でよく、一連の熱処理が終了後の金属組織を微細化し、引張試験における伸びの向上を最大限に得る目的からその圧延率は５０％以上とする。一方、８５％を超す圧延率で冷間圧延を行うことは多大の冷延負荷が必要となるため現実的ではない。
【００３０】
冷間圧延した鋼板はまず７００〜９００℃のフェライト、オーステナイトの二相共存温度域で１０秒〜５分焼鈍される。この焼鈍はフェライトとオーステナイトの共存する微細な再結晶組織を形成し、同時にＣやＭｎ等のオーステナイト安定化元素をある程度オーステナイト中に濃化し、引き続く一連の熱処理に伴う組織変化に際してオーステナイトを安定化することを目的とする。この焼鈍温度が７００℃未満では再結晶が不十分であり、引張試験における伸びが劣化する。一方９００℃を超すような温度で焼鈍するとＴｉ、Ｎｂ、Ｍｏの炭窒化物が一部溶解し、またオーステナイトの体積率が必要以上に大きくなるとともに、オーステナイトとフェライトの間でＣの分配比が小さくオーステナイトの化学的安定性が悪くなるために以降の工程が厳しく制約されるため金属組織を意図したものとし、必要な引張強さが確保され、低降伏比で、引張試験における伸びと穴拡げ性の両立が容易ではなくなる。焼鈍時間が１０秒未満では炭化物が十分に固溶せず、焼鈍温度が高くともオーステナイトが僅かしか形成されない。５分を超える焼鈍はエネルギーの無駄となるばかりか連続ラインでの生産性低下を引き起こす。
【００３１】
焼鈍後の鋼板は引き続いて２５０〜５００℃に冷却されるが、その際、二相共存温度域で形成されたオーステナイトがパーライトに変態するのを避けるため７００℃から５００℃までの冷却速度を１〜１２０℃／秒とする。５００℃を超える温度で冷却を停止するとパーライト変態が急激に始まり、オーステナイトを残存できないため、低降伏比で穴拡げ性が優れるといった特徴が得られない。一方、冷却終了温度が２５０℃未満になるとオーステナイトの過半がマルテンサイトに変態するため、その後の保持や再加熱によりマルテンサイトが焼き戻されてセメンタイトが析出し、高強度ではあっても降伏比は高く、引張試験における伸びも劣化し、穴拡げ性も良くない。また７００℃から５００℃までの間の冷却速度が１℃／秒未満ではパーライトが生成し、降伏比も高く、引張試験における伸び、穴拡げ性とも劣る。８０℃／秒を超すような冷却速度で冷却すると、冷却途上でオーステナイトからフェライトへの変態が起こりにくく、特に焼鈍温度が高い場合には引き続く２５０〜６００℃での保持で生成するベイナイトの体積率が大きくなり、高強度ではあっても降伏比が高く、引張試験における伸び、穴拡げ性とも劣る。
【００３２】
この後引き続き、本発明では、冷却温度域に３０秒〜１０分保持してから常温まで冷却する。そして、冷却後の保持温度としては、２５０〜６００℃の範囲の温度域に３０秒〜１０分保持してから常温まで冷却することが適していて、冷却後の温度が低い場合（過冷却）には再加熱して、この温度域とすることが好ましい。その目的は７００℃以上から冷却した際にフェライト中に過飽和な状態で存在するＣをフェライトの粒界および粒内に析出させ、フェライトの延性を高めることにより、マルテンサイトおよび残留オーステナイトとの複合組織強化に由来する引張試験における伸びの向上と降伏比の低下を最大限に引き出すことにある。保持する温度が６００℃を超えるとＣのフェライトへの固溶限が高く、何ら効果がないばかりかオーステナイトからはパーライトやベイナイトが生成しやすく、降伏比が高く、引張試験における伸び、穴拡げ性とも良くない。一方２５０℃未満ではフェライト中に微細な炭化物が析出し、フェライトの延性が低下するため、マルテンサイトおよびオーステナイトが合計で体積率６％以上存在し、高強度で低降伏比ではあっても引張試験における伸び、穴拡げ性が良くない。また保持時間が３０秒未満ではＣの析出が極めて不十分で、フェライトの延性が高くないため、加工性が不足する。一方１０分を超えて保持した場合にはオーステナイトの過半がベイナイトに変態し、複合組織強化の効果が発揮されず、加工性も良くない。
【００３３】
この一連の熱処理においては規定した温度域内であれば保持温度は一定である必要はなく、また冷却速度が冷却途中に規定した範囲内で変化することも本発明の趣旨を損なわない。特に２５０〜６００℃の範囲の温度域での３０秒〜１０分保持はこの温度の範囲内で過冷却後再加熱されるものであってもよく、図１に例示するいずれも可能である。また熱履歴さえ満足されれば、鋼板は連続焼鈍設備やライン内焼鈍方式の連続溶融亜鉛めっき設備をはじめとしたいかなる設備で熱処理されてもかまわない。熱処理後形状矯正のために調質圧延を行ったり、また防錆の改善のために電気亜鉛めっきを施したり、またそのいっそうの改善を図るために金属酸化物皮膜、有機皮膜などの表面処理を施しても、さらに上層に鉄めっきを施してプレス成形性の改善を図っても、伸びフランジ加工を含めたプレス加工性に優れた高強度鋼板という本発明の特徴は阻害されず、プレス加工性や防錆の一層の改善につながるため本発明の目的を達成する上で好ましい。
【００３４】
【実施例】
次に本発明例を実施例にて説明する。
【００３５】
（実施例１）
表１に示す組成からなる組成の鋼を１１９０℃に加熱し、表２に示す仕上温度で３．０〜７．５ｍｍの熱間圧延鋼帯とし、５４０〜６７０℃で巻き取った。酸洗後、５５〜７５％の圧下率の冷間圧延を施して表２に示す板厚の冷間圧延鋼帯とした後、連続焼鈍設備を用いて表２に示すような条件の熱処理と伸び率０．３〜１．５％の調質圧延を行い、冷延鋼板を製造した。この鋼帯から切り出した試料の金属組織を光学顕微鏡で観察し、マルテンサイトおよび残留オーステナイトを合わせた体積率を求めるとともに、ＪＩＳ５号試験片を加工して常温での引張試験を行うことにより、降伏強さ（ＹＰ）、引張強さ（ＴＳ）、伸び（Ｅｌ）を求めた。また１５０ｍｍ×１５０ｍｍの長方形に鋼板を切り出し、その中央に直径ｄ0＝１０ｍｍの穴を約１２％のクリアランスで打ち抜き加工後、そのかえりをダイ側になるように試験機にセットし、５ｔｏｎのしわ押え力で押し拡げ部へ材料が流入しないように拘束のうえ、６０°円錐ポンチで穴を押し拡げ、穴縁端面にクラックが板厚貫通した時の穴径ｄを測定し、穴径の増加率、すなわちλ＝（ｄ−ｄ0）／ｄ0を穴拡げ率として評価した結果を表２に示す。
【００３６】
この表から明らかなように、本発明試料である試料Ｎｏ．２、５、７、１０、１２、１６、２２、３４は５９０ＭＰａ以上の引張強さを有しながら、その金属組織にマルテンサイトおよび残留オーステナイトを合わせて６％以上含み、降伏比が０．７以下と小さく、引張試験における伸びが大きいことに加えて、マルテンサイト、残留オーステナイトおよびベイナイトの硬質相組織の体積率をα％、Ｔｉ、Ｎｂ、Ｍｏの添加量をそれぞれ％Ｔｉ、％Ｎｂ、％Ｍｏとした時にはα≦５００００×（（％Ｔｉ)／４８＋（％Ｎｂ）／９３＋（％Ｍｏ）／９６）となり、マルテンサイト近傍を主体にフェライト自体がＴｉ、Ｎｂ、Ｍｏといった元素を含む微細析出物で強化され、隣接組織間の変形応力差が低減されているため、穴拡げ性も優れる。これに対し本発明成分鋼であっても、熱間圧延や冷間圧延後に行う熱処理の条件が不適切な場合には、試料Ｎｏ．４、８、９、１１、１３〜１５、１７〜２１、２８〜３１のようにマルテンサイトおよび残留オーステナイトを合わせた体積率が６％未満であったり、マルテンサイト、残留オーステナイトおよびベイナイトの硬質相組織の体積率をα％、Ｔｉ、Ｎｂ、Ｍｏの添加量をそれぞれ％Ｔｉ、％Ｎｂ、％Ｍｏとした時にはαが５００００×（（％Ｔｉ）／４８＋（％Ｎｂ）／９３＋（％Ｍｏ）／９６）を超え、高強度ではあっても降伏比が低いこと、引張試験における伸びが大きいこと、穴拡げ性が優れることのいずれかが満足されないためにプレス加工性が改善されず、あるいは低降伏比で引張試験における伸びが大きく、穴拡げ性が優れていて、プレス加工性が良くても強度が低い。一方、本発明成分以外の鋼では試料Ｎｏ．１、３、６、２６のように、マルテンサイトおよび残留オーステナイトを合わせた体積率を６％以上としたり、あるいはマルテンサイト、残留オーステナイトおよびベイナイトの硬質相組織の体積率をα％、Ｔｉ、Ｎｂ、Ｍｏの添加量をそれぞれ％Ｔｉ、％Ｎｂ、％Ｍｏとした時に、α≦５００００×（（％Ｔｉ）／４８＋（％Ｎｂ）／９３＋（％Ｍｏ）／９６）とすることが難しく、また試料Ｎｏ．２６、３７、３８のようにマルテンサイトおよび残留オーステナイトを合わせて６％以上含み、マルテンサイト、残留オーステナイトおよびベイナイトの硬質相組織の体積率をα％、Ｔｉ、Ｎｂ、Ｍｏの添加量をそれぞれ％Ｔｉ、％Ｎｂ、％Ｍｏとした時にα≦５００００×（（％Ｔｉ）／４８＋（％Ｎｂ）／９３＋（％Ｍｏ）／９６）となったとしても、高強度ではあっても降伏比が低いこと、引張試験における伸びが大きいこと、穴拡げ性が優れることのいずれかが満足されないためにプレス加工性が改善されず、あるいは低降伏比で引張試験における伸びが大きく、穴拡げ性が優れていて、プレス加工性が良くても強度が低い。
【００３７】
【表１】
【表２】
（実施例２）
表１に示す組成からなる組成の鋼を１２２０℃に加熱し、表３に示す仕上温度で３．０〜６．０ｍｍの熱間圧延鋼帯とし、５８０〜６８０℃で巻き取った。酸洗後、５５〜７３％の圧下率の冷間圧延を施して表３に示す板厚の冷間圧延鋼帯とした後、ライン内焼鈍方式の連続溶融亜鉛めっき設備を用いて表３に示すような条件の熱処理と伸び率０．８〜１．２％の調質圧延を行い、溶融亜鉛めっき鋼板（ＧＩ）および合金化溶融亜鉛めっき鋼板（ＧＡ）を製造した。この鋼帯から切り出した試料の金属組織を光学顕微鏡で観察し、マルテンサイトおよび残留オーステナイトを合わせた体積率を求めるとともに、ＪＩＳ５号試験片を加工して常温での引張試験を行うことにより、降伏強さ（ＹＰ）、引張強さ（ＴＳ）、伸び（Ｅｌ）を求めた。また１５０ｍｍ×１５０ｍｍの長方形に鋼板を切り出し、その中央に直径ｄ0＝１０ｍｍの穴を約１２％のクリアランスで打ち抜き加工後、そのかえりをダイ側になるように試験機にセットし、５ｔｏｎのしわ押え力で押し拡げ部へ材料が流入しないように拘束のうえ、６０°円錐ポンチで穴を押し拡げ、穴縁端面にクラックが板厚貫通した時の穴径ｄを測定し、穴径の増加率、すなわちλ＝（ｄ−ｄ0）／ｄ0を穴拡げ率として評価した結果を表３に示す。
【００４０】
この表から明らかなように、本発明試料である試料Ｎｏ．３９、４０、４３〜４５は５９０ＭＰａ以上の引張強さを有しながら、その金属組織にマルテンサイトおよび残留オーステナイトを合わせて６％以上含み、降伏比が０．７以下と小さく、引張試験における伸びが大きいことに加えて、マルテンサイト、残留オーステナイトおよびベイナイトの硬質相組織の体積率をα％、Ｔｉ、Ｎｂ、Ｍｏの添加量をそれぞれ％Ｔｉ、％Ｎｂ、％Ｍｏとした時にはα≦５００００×（（％Ｔｉ）／４８＋（％Ｎｂ）／９３＋（％Ｍｏ）／９６）となり、マルテンサイト近傍を主体にフェライト自体がＴｉ、Ｎｂ、Ｍｏといった元素を含む微細析出物で強化され、隣接組織間の変形応力差が低減されているため、穴拡げ性も優れる。これに対し本発明成分鋼であっても、熱間圧延や冷間圧延後に行う熱処理の条件が不適切な場合には、試料Ｎｏ．４６のようにマルテンサイトおよび残留オーステナイトを合わせた体積率が６％未満であったり、マルテンサイト、残留オーステナイトおよびベイナイトの硬質相組織の体積率をα％、Ｔｉ、Ｎｂ、Ｍｏの添加量をそれぞれ％Ｔｉ、％Ｎｂ、％Ｍｏとした時にはαが５００００×（（％Ｔｉ）／４８＋（％Ｎｂ）／９３＋（％Ｍｏ）／９６）を超え、高強度ではあっても降伏比が低いこと、引張試験における伸びが大きいこと、穴拡げ性が優れることのいずれかが満足されないためにプレス加工性が改善されず、あるいは低降伏比で引張試験における伸びが大きく、穴拡げ性が優れていて、プレス加工性が良くても強度が低い。一方、本発明成分以外の鋼では試料Ｎｏ．４１、４２のように、マルテンサイトおよび残留オーステナイトを合わせた体積率を６％以上としたり、あるいはマルテンサイト、残留オーステナイトおよびベイナイトの硬質相組織の体積率をα％、Ｔｉ、Ｎｂ、Ｍｏの添加量をそれぞれ％Ｔｉ、％Ｎｂ、％Ｍｏとした時に、α≦５００００×（（％Ｔｉ）／４８＋（％Ｎｂ）／９３＋（％Ｍｏ）／９６）とすることが難しく、また試料Ｎｏ．４７、４８のようにマルテンサイトおよび残留オーステナイトを合わせて６％以上含み、マルテンサイト、残留オーステナイトおよびベイナイトの硬質相組織の体積率をα％、Ｔｉ、Ｎｂ、Ｍｏの添加量をそれぞれ％Ｔｉ、％Ｎｂ、％Ｍｏとした時にα≦５００００×（（％Ｔｉ）／４８＋（％Ｎｂ）／９３＋（％Ｍｏ）／９６）となったとしても、高強度ではあっても降伏比が低いこと、引張試験における伸びが大きいこと、穴拡げ性が優れることのいずれかが満足されないためにプレス加工性が改善されず、あるいは低降伏比で引張試験における伸びが大きく、穴拡げ性が優れていて、プレス加工性が良くても強度が低い。
【００４１】
【表３】
【発明の効果】
以上詳述したように、本発明によればマルテンサイト、残留オーステナイトおよびベイナイトがフェライトマトリクス中に微細分散した低降伏比高強度鋼板において、引張試験における伸びだけではなく、穴拡げ性も改善される。すなわち引張強さ５９０〜８８０ＭＰａの高強度鋼板を伸びフランジ加工性の厳しい部材にもプレス加工により適用することを可能とし、自動車、家庭電気製品、建築等の分野でそれぞれが持つべき機能を向上させながら軽量化を図ることができるため産業上極めて大きな効果を有する。
【図面の簡単な説明】
【図１】 本発明における熱処理条件のいくつかを例示した図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a low-yield ratio high-strength steel plate and plated steel plate excellent in hole expansibility and a method for producing the same. The high-strength steel sheet according to the present invention is used by pressing in applications such as automobile parts, household electrical appliances, and architecture, and is hot dip galvanized or electrogalvanized to improve rust prevention. In order to further improve it, a steel plate having a metal oxide film and an organic film surface-treated, and a steel plate in which an upper layer is iron-plated to improve press formability are included.
[0002]
[Prior art]
The car body must have a function to protect passengers in the event of a collision.2As well as lighter weight, it is required to reduce exhaust emissions by improving fuel efficiency. In response to this, solid solution strengthening, which is generally considered as a strengthening mechanism to increase the strength of a steel sheet without deteriorating press workability, and composite structure strengthening in which martensite and bainite are finely dispersed in a ferrite matrix were used. High strength steel sheets have been developed. In recent years, steel sheets have been developed that attempt to improve elongation in tensile tests using transformation-induced plasticity of retained austenite, as described in Japanese Patent No. 2017320 and Japanese Patent No. 2545316. The hole expandability is not necessarily good because it depends not only on the elongation of the tensile test but also greatly depends on the ductility of the shear end face, which is said to be the hole expandability, in the case of a member with severe stretch flange processing. The use of high-strength steel sheets remains limited.
[0003]
As a technique for improving the hole expandability of a steel sheet, martensite is used to reduce S that leads to an increase in the number of inclusions expanded like MnS, or to break sharp voids formed at phase boundaries with greatly different deformation stresses. It has been proposed to avoid mixing. However, extreme reduction of S in steel not only leads to a significant increase in cost, but also cannot be easily realized industrially, and if inclusions containing S are not found, the adverse effect of other inclusions becomes obvious. This is not realistic. Even if it is attempted to increase the strength by avoiding the presence of martensite, if a high strength of 590 MPa or more is to be obtained, a metal structure mainly composed of bainite or elements such as Ti, Nb, Mo, V are added. However, in general, these methods are not preferable in terms of press workability because the ratio of the yield strength to the tensile strength, that is, the yield ratio is high. Further, Ti, Nb, Mo, V, Since such elements raise the recrystallization temperature, it is difficult to fully utilize the precipitation strengthening in a recrystallized state in a cold rolled steel sheet.
[0004]
[Problems to be solved by the invention]
Therefore, the present invention is a composite high strength steel sheet in which martensite, retained austenite and bainite are finely dispersed in a ferrite matrix to improve the press workability, while ensuring a low yield ratio and severe stretch flange processing. An object is to achieve both elongation and hole expansibility in a tensile test so that the member can also be pressed.
[0005]
[Means for Solving the Problems]
In order to solve the above problems, the inventors of the present invention have a metal structure in which sharp voids are not formed at the phase boundary between ferrite and martensite on the shear end face, and the ferrite itself in the vicinity of martensite is fine Ti, Nb,With MoFrom the standpoint that it can be realized by strengthening with fine precipitates containing such elements and reducing the deformation stress difference between adjacent structures, after adjusting the content of C, Si, Mn, S, Ti, Nb,With MoAs a result of intensive studies on the hot rolling temperature and the annealing temperature condition after the cold rolling using the steel to which the precipitation strengthening element is added, when% S and% Ti are respectively set to S and Ti contents ( % Ti) / (% S) ≧ 5, and hot-rolling the steel with the component adjusted so that the Si content is% Si and the finish side temperature is (900 + 50 ×% Si) ° C. or less. In addition, annealing is performed in the two-phase coexisting temperature range of 700 to 900 ° C. of ferrite and austenite, and the martensite and residual austenite have a total volume ratio of 6% or more, and the hard phase structure of martensite, residual austenite and bainite Ti, Nb,With MoIt has been found that if a metal structure contained at a certain ratio corresponding to the amount of precipitation strengthening element is formed, it is possible to achieve both the elongation and hole expansibility in a tensile test with a low yield ratio while ensuring the necessary strength. It was.
[0006]
The present invention is a low yield ratio high strength steel plate excellent in completely unprecedented elongation and hole expansibility constructed based on such idea and new knowledge, a plated steel plate thereof, and a production method thereof, and its summary Is as follows.
[0007]
(1) In mass%,
C: 0.04 to 0.14%,
Si: 0.4-2.2%
Mn: 1.52 to 2.4%,
P: 0.02% or less,
S: 0.01% or less,
Al: 0.002 to 0.5%,
Ti: 0.005 to 0.1%,
N: 0.006% or less
In addition, when% S and% Ti are respectively S and Ti content, (% Ti) / (% S) ≧ 5 is satisfied, and the balance is composed of Fe and inevitable impurities, and the metal structure is in the ferrite matrix. Martensite, retained austenite, and bainite are finely dispersed in the metal structure. The volume ratio of martensite and retained austenite in the metal structure is 6% or more in total, and the volume ratio of the hard phase structure of martensite, retained austenite, and bainite. Α% and Ti addition amount% Ti, α ≦ 50000 × ((% Ti) / 48), low yield ratio and high strength cold rolling excellent in elongation and hole expansibility steel sheet.
[0008]
(2) By mass%
C: 0.04 to 0.14%,
Si: 0.4-2.2%
Mn: 1.52 to 2.4%,
P: 0.02% or less,
S: 0.01% or less,
Al: 0.002 to 0.5%,
Ti: 0.005 to 0.1%,
N: 0.006% or less
And Nb,Mo'sWhen one or more types are contained in a total amount of 0.005 to 0.1%, and when S and Ti content are S and Ti, respectively, (% Ti) / (% S) ≧ 5 is satisfied, and the balance Fe In the metal structure, martensite, retained austenite and bainite are finely dispersed in the ferrite matrix, and the total volume ratio of martensite and retained austenite in the metal structure is 6% or more. The volume fraction of the hard phase structure of the site, retained austenite and bainite is α%, Ti, Nb,Mo'sAddition amounts are% Ti,% Nb,%With MoΑ ≦ 50000 × ((% Ti) / 48 + (% Nb) / 93 + (% Mo) / 96)A low-yield-ratio, high-strength cold-rolled steel sheet excellent in elongation and hole expansibility.
[0009]
(3) Furthermore, in mass%,
B: 0.0010 to 0.003%
The low yield ratio high strength cold-rolled steel sheet having excellent elongation and hole expansibility as described in (1) or (2) above.
[0010]
(4) A low-yield ratio high-strength plated steel sheet excellent in elongation and hole expansibility, wherein the steel sheet according to any one of (1) to (3) is plated.
[0011]
(5) When% Si is made into Si content to the slab of the composition which consists of a chemical component in any one of said (1)-(3), a finishing side temperature shall be (900 + 50 *% Si) degrees C or less. Cold-rolled sheet that has been hot-rolled and cold-rolled by 50 to 85% is annealed for 10 seconds to 5 minutes in a two-phase temperature range of 700 to 900 ° C. of ferrite and austenite, and 700 to 500 ° C. The average cooling rate during the period is 1 to 120 ° C./second and is cooled to 250 to 500 ° C.,In its cooling temperature rangeHold for 30 seconds to 10 minutes and then cool to room temperature to finely disperse martensite, retained austenite and bainite in the ferrite matrixMetal structureThe volume ratio of martensite and retained austenite in the metal structure is 6% or more in total, and the volume ratio of the hard phase structure of martensite, retained austenite and bainite is α%, Ti, Nb,Mo'sAddition amounts are% Ti,% Nb,%With MoΑ ≦ 50000 × ((% Ti) / 48 + (% Nb) / 93 + (% Mo) / 96)A method for producing a cold rolled steel sheet having a low yield ratio and a high strength excellent in elongation and hole expansibility.
[0012]
(6) When% Si is made into Si content to the slab of the composition which consists of a chemical component in any one of said (1)-(3), a finishing side temperature shall be (900 + 50 *% Si) degrees C or less. Cold-rolled sheet that has been hot-rolled and cold-rolled by 50 to 85% is annealed for 10 seconds to 5 minutes in a two-phase temperature range of 700 to 900 ° C. of ferrite and austenite, and 700 to 500 ° C. Cooling to 250 to 500 ° C. with an average cooling rate between 1 and 120 ° C./secAnd reAfter heating, hold in a temperature range of 250 to 600 ° C. for 30 seconds to 10 minutes and then cool to room temperature to finely disperse martensite, residual austenite and bainite in the ferrite matrix.Metal structureThe volume ratio of martensite and retained austenite in the metal structure is 6% or more in total, and the volume ratio of the hard phase structure of martensite, retained austenite and bainite is α%, Ti, Nb,Mo'sAddition amounts are% Ti,% Nb,%With MoΑ ≦ 50000 × ((% Ti) / 48 + (% Nb) / 93 + (% Mo) / 96) A method for producing a low yield ratio high strength cold-rolled steel sheet excellent in elongation and hole expansibility.
[0013]
(7) A method for producing a low-yield ratio, high-strength plated steel sheet excellent in elongation and hole expansibility, characterized by further plating following (5) or (6) above.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail.
[0015]
First, C, Si, Mn, P, S, Al, Ti, Nb,Mo,The reason for limiting the numerical value of N will be described.
[0016]
C is an essential element for realizing structure strengthening by martensite and retained austenite at low cost. If C is less than 0.04%, the volume fraction of martensite and retained austenite decreases, so the required tensile strength is required. It is difficult to ensure. On the other hand, if C exceeds 0.14%, the volume fraction of martensite and retained austenite increases and it is easy to increase the tensile strength, but sharp voids are formed at the shear end face, and the hole expandability is significantly deteriorated. .
[0017]
It is well known that the addition of Si increases the strength of the steel sheet without significantly damaging the elongation of the steel sheet, and improves the hole expandability by significantly retarding the progress of pearlite and bainite transformation and inhibiting the formation of coarse carbides. At the same time, the presence of martensite and retained austenite in the metal structure after cooling to room temperature is facilitated, and the elongation in the tensile test is increased. In the present invention, the martensite is annealed by hot rolling under certain conditions. Ti, Nb, mainly ferrite near the siteWith MoIt is useful for reducing the difference in deformation stress between adjacent structures by strengthening with fine precipitates containing such elements, and with the low yield ratio characteristic of the present invention, it is possible to achieve both elongation and hole expansibility in tensile tests. It is important to realize. In the tensile test, the progress of the pearlite and bainite transformation is significantly retarded to inhibit the formation of coarse carbides, improving the hole expandability, and facilitating the presence of martensite and residual austenite in the metal structure after cooling to room temperature. In order to increase the elongation, it is necessary to add more than 0.4%. However, if the amount of addition exceeds 2.2%, Ti, Nb, mainly ferrite in the vicinity of martensite in the annealed metal structure.With MoIt becomes difficult to perform hot rolling so as to strengthen with fine precipitates containing such elements, and it becomes difficult to achieve both a low yield ratio and hole expandability.
[0018]
Mn is known as an element that lowers the free energy of austenite together with C and increases the hardenability of steel. In order to improve elongation with the required tensile strength, the total volume ratio of martensite and residual austenite is 6% or more. Add 1.52% or more for the purpose of forming an existing metal structure. However, if the addition amount is excessive, an addition amount of Ti that does not deteriorate the hole expandability with a low yield ratio cannot be found, so the upper limit is set to 2.2%.
[0019]
P is generally contained in steel as an unavoidable impurity. However, if the amount exceeds 0.02%, a high strength steel sheet having a tensile strength exceeding 590 MPa as in the present invention has remarkable cold rolling properties as well as toughness. It deteriorates and it becomes difficult to mass-produce industrially.
[0020]
S is also generally contained in steel as an unavoidable impurity, but if its amount exceeds 0.01%, the adverse effect on the spot weldability of the steel sheet becomes significant.
[0021]
Al is a deoxidizing element of steel, and it is necessary to add 0.002% or more in order to improve the material by suppressing the grain refinement of the hot rolled material by AlN and the coarsening of crystal grains in a series of heat treatment steps. However, exceeding 0.5% not only increases the cost, but also deteriorates the surface properties.
[0022]
Ti is finely precipitated mainly from ferrite in the vicinity of martensite, and is added for the purpose of reducing the difference in deformation stress between adjacent structures and preventing S from becoming expanded inclusions and deteriorating hole expandability. Generally, when Ti precipitates finely, the tensile strength increases and the recrystallization temperature also rises, and in some cases, the ferrite may be in an unrecrystallized state even in the two-phase coexisting temperature range of ferrite and austenite. Regardless of the annealing temperature, a metal structure containing martensite and residual austenite in a total volume ratio of 6% or more can be formed mainly by sufficiently recrystallized ferrite regardless of the annealing temperature. If the addition amount of Ti is less than 0.005%, there is no effect on precipitation strengthening of ferrite in the vicinity of martensite, and when% S and% Ti are respectively S and Ti contents, (% Ti) / (% S) is If it is less than 5, it is difficult to suppress the generation of expanded MnS, and the hole expandability cannot be improved. However, if the amount of Ti exceeds 0.1%, not only the yield ratio increases due to the remarkable precipitation strengthening of ferrite, but even if the hot rolling conditions are adjusted together with the Si content of steel as specified in the present invention. Since it is difficult to recrystallize ferrite by short-time annealing, the elongation in the tensile test is also reduced.
[0023]
N is contained in steel as an unavoidable impurity, and Ti, Nb,With MoNitride is precipitated to increase the strength of the steel, but if the amount exceeds 0.006%, the elongation in the tensile test deteriorates, so this is the upper limit.
[0024]
Nb,Mo tooMoreover, it is a precipitation strengthening element like Ti, and finely precipitates mainly from ferrite in the vicinity of martensite, improving the hole expansibility by reducing the difference in deformation stress between adjacent structures, and the required tensile strength. In order to ensure the thickness, 0.005 to 0.1% in total may be added. However, if the added amount exceeds 0.1%, recrystallization of ferrite together with Ti is remarkably inhibited, and workability deteriorates.
[0025]
Even if Cu, Sn, Zn, Zr, W, Cr, Ni, and B are contained in a total amount of 1% or less in the steel containing these as main components, the effect of the present invention is not impaired, and the corrosion resistance is improved depending on the amount. In some cases, it is preferable.
[0026]
Next, the reasons for limiting the manufacturing conditions will be described. Its purpose is to have a metal structure in which martensite, retained austenite and bainite are finely dispersed in a ferrite matrix, and in a high-strength steel sheet with improved microstructure in tensile testing, hole expandability while ensuring a low yield ratio. Therefore, the ferrite itself is mainly made of fine Ti, Nb, mainly in the vicinity of martensite so that sharp voids are not formed at the phase boundary between ferrite and martensite at the shear end face.With MoIt is to strengthen with fine precipitates containing such elements. The characteristics of the strengthening of the composite structure such as high strength, low yield ratio, and large elongation in the tensile test are recognized when the volume ratio of martensite and retained austenite is 6% or more. In order to clearly recognize the effect of plasticity, it is preferable that the volume ratio of retained austenite exceeds the volume ratio of martensite. The coexistence of bainite is not preferable for exhibiting the characteristics of strengthening of the composite structure, ie, high strength, low yield ratio, and large elongation in the tensile test, but the volume ratio is the volume ratio of martensite and retained austenite. If it does not exceed 3 times, the tensile strength of the steel sheet can be increased with a lower C content without significantly increasing the yield ratio, which is useful in the present invention. Also, the volume fraction of the hard phase structure of martensite, retained austenite and bainite is α%, Ti, Nb,Mo'sAddition amounts are% Ti,% Nb,%With MoΑ ≦ 50000 × ((% Ti) / 48 + (% Nb) / 93 + (% Mo) / 96)The ferrite itself, mainly in the vicinity of martensite, is fine Ti, Nb,With MoStrengthening with fine precipitates containing such elements suppresses the formation of sharp voids at the phase boundary between ferrite and martensite at the shear end face, thereby improving hole expansibility. α / ((% Ti) / 48 + (% Nb) / 93 + (% Mo) / 96)When the value exceeds 50,000, the precipitation strengthening of ferrite becomes insufficient, so that sharp voids are easily formed at the phase boundary between ferrite and martensite on the shear end face, and the hole expansibility is deteriorated.
[0027]
The slab used for hot rolling is not particularly limited. That is, what was manufactured with the continuous casting slab, the thin slab caster, etc. should just be used. It is also compatible with processes such as continuous casting-direct rolling (CC-DR) in which hot rolling is performed immediately after casting.
[0028]
In hot rolling, when% Si is defined as the Si content, the finish side temperature needs to be (900 + 50 ×% Si) ° C. or less. This is Ti, Nb,Mo'sCarbonitride is precipitated so that the recrystallization temperature after subsequent cold rolling does not rise abnormally, and when it is annealed in the two-phase coexisting temperature range of ferrite and austenite, it is sufficiently recrystallized regardless of the annealing temperature. The ferrite mainly contains martensite and residual austenite in a volume ratio of 6% or more, and the volume ratio of the hard phase structure of martensite, residual austenite and bainite is α%, Ti, Nb,Mo'sAddition amounts are% Ti,% Nb,%With MoΑ ≦ 50000 × ((% Ti) / 48 + (% Nb) / 93 + (% Mo) / 96)Ti, Nb, mainly ferrite in the vicinity of martensite so thatWith MoIt aims at strengthening with the fine precipitate containing such an element and forming the metal structure with which the deformation stress difference between adjacent structures was reduced. When the finish side temperature exceeds (900 + 50 ×% Si) ° C., carbonitrides are finely dispersed and the tensile strength is increased, but when annealing is performed in the two-phase coexisting temperature range of ferrite and austenite. However, since unrecrystallized ferrite remains, the elongation in the tensile test deteriorates. The cooling conditions and coiling temperature after hot rolling are not particularly limited, but the coiling temperature is to avoid large material variations at both ends of the coil and to avoid pickling deterioration due to increased scale thickness. Is 750 ° C. or lower, and Ti, Nb,Mo'sIn order to avoid insufficient precipitation and material variations, it is desirable that the temperature be 500 ° C. or higher.
[0029]
Cold rolling may be performed under normal conditions, and the rolling rate is set to 50% or more for the purpose of miniaturizing the metal structure after a series of heat treatments and maximizing the improvement of elongation in a tensile test. On the other hand, it is not realistic to perform cold rolling at a rolling rate exceeding 85% because a large cold rolling load is required.
[0030]
The cold-rolled steel sheet is first annealed for 10 seconds to 5 minutes in a two-phase coexisting temperature range of 700 to 900 ° C. of ferrite and austenite. This annealing forms a fine recrystallized structure in which ferrite and austenite coexist, and at the same time, austenite stabilizing elements such as C and Mn are concentrated to some extent in austenite, and the austenite is stabilized during the structural change accompanying the subsequent heat treatment. For the purpose. When the annealing temperature is less than 700 ° C., recrystallization is insufficient and elongation in a tensile test is deteriorated. On the other hand, when annealing at a temperature exceeding 900 ° C., Ti, Nb,Mo'sSince the carbonitride partially dissolves and the volume fraction of austenite becomes larger than necessary, the distribution ratio of C between austenite and ferrite is small and the chemical stability of austenite is deteriorated, so the subsequent steps are severe. It is assumed that the metal structure is intended because it is restricted, the necessary tensile strength is ensured, and it is not easy to achieve both elongation and hole expansibility in a tensile test at a low yield ratio. When the annealing time is less than 10 seconds, the carbide is not sufficiently dissolved, and only austenite is formed even if the annealing temperature is high. Annealing for more than 5 minutes not only wastes energy but also reduces productivity in the continuous line.
[0031]
The annealed steel sheet is subsequently cooled to 250 to 500 ° C., but at this time, the cooling rate from 700 ° C. to 500 ° C. is set to 1 in order to avoid the transformation of austenite formed in the two-phase coexisting temperature range to pearlite. ˜120 ° C./second. When the cooling is stopped at a temperature exceeding 500 ° C., the pearlite transformation starts abruptly and austenite cannot remain, so that the characteristics such as excellent hole expandability at a low yield ratio cannot be obtained. On the other hand, since the majority of austenite is transformed into martensite when the cooling end temperature is less than 250 ° C., martensite is tempered by subsequent holding and reheating, and cementite is precipitated, and the yield ratio is high even though the strength is high. It is high, the elongation in the tensile test is deteriorated, and the hole expansibility is not good. Further, when the cooling rate between 700 ° C. and 500 ° C. is less than 1 ° C./second, pearlite is generated, the yield ratio is high, and the elongation and hole expansibility in the tensile test are inferior. When cooling at a cooling rate exceeding 80 ° C./second, transformation from austenite to ferrite is unlikely to occur during cooling, and particularly when the annealing temperature is high, the volume fraction of bainite produced by subsequent holding at 250 to 600 ° C. The yield ratio is high even with high strength, and the elongation and hole expansibility in the tensile test are also inferior.
[0032]
Subsequently, in the present invention,Hold in the cooling temperature range for 30 seconds to 10 minutes and then cool to room temperature. And as the holding temperature after cooling,Hold in a temperature range of 250 to 600 ° C. for 30 seconds to 10 minutes and then cool to room temperatureWhen the temperature after cooling is low (supercooling), it is preferable to reheat to this temperature range.The purpose is to precipitate C in the ferrite in a supersaturated state when cooled from 700 ° C. or more, and to precipitate in the grain boundaries and grains of the ferrite, thereby increasing the ductility of the ferrite, thereby forming a composite structure of martensite and retained austenite. The purpose is to maximize the improvement in elongation and the decrease in yield ratio in the tensile test derived from strengthening. When the holding temperature exceeds 600 ° C, the solid solubility limit of C in ferrite is high, not only having no effect, but also pearlite and bainite are easily formed from austenite, the yield ratio is high, elongation in tensile tests, and hole expansibility. Not good. On the other hand, when the temperature is less than 250 ° C., fine carbides precipitate in the ferrite and the ductility of the ferrite decreases, so that a total tensile strength of martensite and austenite is 6% or more, even if the strength is low and the yield ratio is low. Elongation and hole expansibility are not good. Further, if the holding time is less than 30 seconds, the precipitation of C is extremely insufficient and the ductility of ferrite is not high, so that the workability is insufficient. On the other hand, when holding for more than 10 minutes, the majority of austenite is transformed into bainite, the effect of strengthening the composite structure is not exhibited, and the workability is not good.
[0033]
In this series of heat treatments, the holding temperature does not need to be constant as long as it is within the specified temperature range, and the spirit of the present invention is not impaired by the cooling rate changing within the range specified during cooling. In particular, holding for 30 seconds to 10 minutes in a temperature range of 250 to 600 ° C. may be reheated after supercooling within this temperature range, and any of those illustrated in FIG. 1 is possible. Further, as long as the thermal history is satisfied, the steel sheet may be heat-treated in any equipment including a continuous annealing equipment and a continuous hot dip galvanizing equipment using an in-line annealing method. After heat treatment, temper rolling is performed to correct the shape, electrogalvanization is applied to improve rust prevention, and surface treatments such as metal oxide film and organic film are applied for further improvement. Even if the upper layer is further iron-plated to improve the press formability, the features of the present invention of a high-strength steel sheet excellent in press workability including stretch flange processing are not hindered, and press workability It leads to further improvement of rust prevention and is preferable for achieving the object of the present invention.
[0034]
【Example】
Next, examples of the present invention will be described in Examples.
[0035]
Example 1
The steel having the composition shown in Table 1 was heated to 1190 ° C. to form a hot-rolled steel strip of 3.0 to 7.5 mm at the finishing temperature shown in Table 2, and wound at 540 to 670 ° C. After pickling and cold rolling at a rolling reduction of 55 to 75% to obtain a cold rolled steel strip having a thickness shown in Table 2, heat treatment under conditions as shown in Table 2 using a continuous annealing facility Temper rolling was performed at an elongation of 0.3 to 1.5% to produce a cold-rolled steel sheet. Yield by observing the metallographic structure of the sample cut out from this steel strip with an optical microscope, obtaining the volume ratio of martensite and retained austenite, and processing a JIS No. 5 test piece and conducting a tensile test at room temperature. Strength (YP), tensile strength (TS), and elongation (El) were determined. In addition, a steel plate is cut out into a 150 mm × 150 mm rectangle, and a diameter d is formed at the center.0= After punching a 10mm hole with a clearance of about 12%, set the burr on the die so that it is on the die side, and constrain the material so that it does not flow into the expanded portion with a wrinkle pressing force of 5 tons. The hole is expanded with a 60 ° conical punch, and the hole diameter d is measured when the crack penetrates the thickness of the hole edge, and the increase rate of the hole diameter, that is, λ = (dd−d0) / D0Table 2 shows the results of the evaluation of the hole expansion rate.
[0036]
As is apparent from this table, sample No. 2, 5, 7, 10, 12, 16, 22, and 34 have a tensile strength of 590 MPa or more, and contain 6% or more of martensite and residual austenite in the metal structure, and the yield ratio is 0.7. In addition to the following small and large elongation in the tensile test, the volume fraction of the hard phase structure of martensite, retained austenite and bainite is α%, Ti, Nb,Mo'sAddition amounts are% Ti,% Nb,%With MoΑ ≦ 50000 × ((% Ti) / 48 + (% Nb) / 93 + (% Mo) / 96)And the ferrite itself mainly in the vicinity of martensite is Ti, Nb,With MoSince it is strengthened by fine precipitates containing such elements and the difference in deformation stress between adjacent structures is reduced, the hole expandability is also excellent. On the other hand, even in the case of the component steel of the present invention, when the conditions of the heat treatment performed after hot rolling or cold rolling are inappropriate, sample No. 4, 8, 9, 11, 13-15, 17-21, 28-31, the combined volume fraction of martensite and residual austenite is less than 6%, or the hard phase of martensite, residual austenite and bainite The volume fraction of the tissue is α%, Ti, Nb,Mo'sAddition amounts are% Ti,% Nb,%With MoΑ is 50000 × ((% Ti) / 48 + (% Nb) / 93 + (% Mo) / 96)However, even if it is high strength, the yield ratio is low, the elongation in the tensile test is large, or the hole expansibility is not satisfactory, so the press workability is not improved, or the yield ratio is low The elongation in the tensile test is large, the hole expandability is excellent, and the strength is low even if the press workability is good. On the other hand, in steels other than the components of the present invention, sample No. 1, 3, 626, the combined volume ratio of martensite and retained austenite is 6% or more, or the volume ratio of the hard phase structure of martensite, retained austenite and bainite is α%, Ti, Nb,Mo'sAddition amounts are% Ti,% Nb,%With MoΑ ≦ 50000 × ((% Ti) / 48 + (% Nb) / 93 + (% Mo) / 96)Sample No. 26, 37, 38, including martensite and retained austenite in total of 6% or more, and the volume fraction of the hard phase structure of martensite, retained austenite and bainite is α%, Ti, Nb,Mo'sAddition amounts are% Ti,% Nb,%With MoΑ ≦ 50000 × ((% Ti) / 48 + (% Nb) / 93 + (% Mo) / 96)Even if it is high strength, the press workability is not improved because the yield ratio is low, the elongation in the tensile test is large, or the hole expandability is not satisfactory. The yield ratio is large in the tensile test, the hole expandability is excellent, and the strength is low even if the press workability is good.
[0037]
[Table 1]
[Table 2]
(Example 2)
The steel having the composition shown in Table 1 was heated to 1220 ° C. to form a hot-rolled steel strip of 3.0 to 6.0 mm at the finishing temperature shown in Table 3, and wound at 580 to 680 ° C. After pickling, cold rolling at a rolling reduction of 55 to 73% is performed to form a cold-rolled steel strip having the thickness shown in Table 3, and then in Table 3 using a continuous hot-dip galvanizing facility of an in-line annealing method. Heat treatment under the conditions as shown and temper rolling with an elongation of 0.8 to 1.2% were performed to produce a hot dip galvanized steel sheet (GI) and an alloyed hot dip galvanized steel sheet (GA). Yield by observing the metallographic structure of the sample cut out from this steel strip with an optical microscope, obtaining the volume ratio of martensite and retained austenite, and processing a JIS No. 5 test piece and conducting a tensile test at room temperature. Strength (YP), tensile strength (TS), and elongation (El) were determined. In addition, a steel plate is cut out into a 150 mm × 150 mm rectangle, and a diameter d is formed at the center.0= After punching a 10mm hole with a clearance of about 12%, set the burr on the die so that it is on the die side, and constrain the material so that it does not flow into the expanded portion with a wrinkle pressing force of 5 tons. The hole is expanded with a 60 ° conical punch, and the hole diameter d is measured when the crack penetrates the thickness of the hole edge, and the increase rate of the hole diameter, that is, λ = (dd−d0) / D0Table 3 shows the results of the evaluation of the hole expansion rate.
[0040]
As is apparent from this table, sample No. 39, 40, 43 to 45 have a tensile strength of 590 MPa or more, but contain 6% or more of martensite and residual austenite in the metal structure, and the yield ratio is as small as 0.7 or less. In addition to a large volume ratio of the hard phase structure of martensite, retained austenite and bainite by α%, Ti, Nb,Mo'sAddition amounts are% Ti,% Nb,%With MoΑ ≦ 50000 × ((% Ti) / 48 + (% Nb) / 93 + (% Mo) / 96)And the ferrite itself mainly in the vicinity of martensite is Ti, Nb,With MoSince it is strengthened by fine precipitates containing such elements and the difference in deformation stress between adjacent structures is reduced, the hole expandability is also excellent. On the other hand, even in the case of the component steel of the present invention, when the conditions of the heat treatment performed after hot rolling or cold rolling are inappropriate, sample No. 46, the combined volume fraction of martensite and residual austenite is less than 6%, or the volume fraction of the hard phase structure of martensite, residual austenite and bainite is α%, Ti, Nb,Mo'sAddition amounts are% Ti,% Nb,%With MoΑ is 50000 × ((% Ti) / 48 + (% Nb) / 93 + (% Mo) / 96)However, even if it is high strength, the yield ratio is low, the elongation in the tensile test is large, or the hole expansibility is not satisfactory, so the press workability is not improved, or the yield ratio is low The elongation in the tensile test is large, the hole expandability is excellent, and the strength is low even if the press workability is good. On the other hand, in steels other than the components of the present invention, sample No. 41, 42, the combined volume ratio of martensite and retained austenite is 6% or more, or the volume ratio of the hard phase structure of martensite, retained austenite and bainite is α%, Ti, Nb,Mo'sAddition amounts are% Ti,% Nb,%With MoΑ ≦ 50000 × ((% Ti) / 48 + (% Nb) / 93 + (% Mo) / 96)It is difficult to obtain a sample No. 47, 48, including martensite and retained austenite in total of 6% or more, the volume ratio of the hard phase structure of martensite, retained austenite and bainite is α%, Ti, Nb,Mo'sAddition amounts are% Ti,% Nb,%With MoΑ ≦ 50000 × ((% Ti) / 48 + (% Nb) / 93 + (% Mo) / 96)Even if it is high strength, the press workability is not improved because the yield ratio is low, the elongation in the tensile test is large, or the hole expandability is not satisfactory. The yield ratio is large in the tensile test, the hole expandability is excellent, and the strength is low even if the press workability is good.
[0041]
[Table 3]
【The invention's effect】
As described above in detail, according to the present invention, in the low yield ratio high strength steel sheet in which martensite, retained austenite and bainite are finely dispersed in the ferrite matrix, not only the elongation in the tensile test but also the hole expandability is improved. . In other words, it is possible to apply high-strength steel sheets with a tensile strength of 590-880 MPa to members with severe stretch flangeability by pressing, and improve the functions that each should have in the fields of automobiles, home appliances, architecture, etc. However, since the weight can be reduced, it has an extremely large industrial effect.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating some of heat treatment conditions in the present invention.
Claims (7)
Ｃ：０．０４〜０．１４％、
Ｓｉ：０．４〜２．２％、
Ｍｎ：１．５２〜２．４％、
Ｐ：０．０２％以下、
Ｓ：０．０１％以下、
Ａｌ：０．００２〜０．５％、
Ｔｉ：０．００５〜０．１％、
Ｎ：０．００６％以下
を含有し、さらに％Ｓ、％ＴｉをそれぞれＳ、Ｔｉ含有量とした時に（％Ｔｉ）／（％Ｓ）≧５を満足し、残部Ｆｅおよび不可避的不純物からなり、金属組織はフェライトマトリックス中にマルテンサイト、残留オーステナイトおよびベイナイトが微細分散しており、その金属組織におけるマルテンサイトおよび残留オーステナイトの体積率が合計で６％以上で、かつマルテンサイト、残留オーステナイトおよびベイナイトの硬質相組織の体積率をα％、Ｔｉの添加量を％Ｔｉとした時に、α≦５００００×（（％Ｔｉ）／４８）であることを特徴とする、伸びおよび穴拡げ性に優れた低降伏比高強度冷延鋼板。% By mass
C: 0.04 to 0.14%,
Si: 0.4-2.2%
Mn: 1.52 to 2.4%,
P: 0.02% or less,
S: 0.01% or less,
Al: 0.002 to 0.5%,
Ti: 0.005 to 0.1%,
N: not more than 0.006%, further satisfying (% Ti) / (% S) ≧ 5 when the contents of S and Ti are S and Ti respectively, and consists of the balance Fe and inevitable impurities In the metal structure, martensite, retained austenite and bainite are finely dispersed in the ferrite matrix, and the volume ratio of martensite and retained austenite in the metal structure is 6% or more in total, and martensite, retained austenite and bainite. When the volume fraction of the hard phase structure of α is α% and the addition amount of Ti is% Ti, α ≦ 50000 × ((% Ti) / 48), which is excellent in elongation and hole expansibility Low yield ratio high strength cold-rolled steel sheet.
Ｃ：０．０４〜０．１４％、
Ｓｉ：０．４〜２．２％、
Ｍｎ：１．５２〜２．４％、
Ｐ：０．０２％以下、
Ｓ：０．０１％以下、
Ａｌ：０．００２〜０．５％、
Ｔｉ：０．００５〜０．１％、
Ｎ：０．００６％以下
とさらにＮｂ、Ｍｏの１種以上を合計で０．００５〜０．１％含有し、さらに％Ｓ、％ＴｉをそれぞれＳ、Ｔｉ含有量とした時に（％Ｔｉ）／（％Ｓ）≧５を満足し、残部Ｆｅおよび不可避的不純物からなり、金属組織はフェライトマトリックス中にマルテンサイト、残留オーステナイトおよびベイナイトが微細分散しており、その金属組織におけるマルテンサイトおよび残留オーステナイトの体積率が合計で６％以上で、かつマルテンサイト、残留オーステナイトおよびベイナイトの硬質相組織の体積率をα％、Ｔｉ、Ｎｂ、Ｍｏの添加量をそれぞれ％Ｔｉ、％Ｎｂ、％Ｍｏとした時に、α≦５００００×（（％Ｔｉ）／４８＋（％Ｎｂ）／９３＋（％Ｍｏ）／９６）であることを特徴とする、伸びおよび穴拡げ性に優れた低降伏比高強度冷延鋼板。% By mass
C: 0.04 to 0.14%,
Si: 0.4-2.2%
Mn: 1.52 to 2.4%,
P: 0.02% or less,
S: 0.01% or less,
Al: 0.002 to 0.5%,
Ti: 0.005 to 0.1%,
N: 0.006% or less and further containing one or more of Nb and Mo in a total amount of 0.005 to 0.1%, and when% S and% Ti are respectively S and Ti contents (% Ti) / (% S) ≧ 5, consisting of the balance Fe and inevitable impurities, and the metal structure has finely dispersed martensite, residual austenite and bainite in the ferrite matrix, and the martensite and residual austenite in the metal structure The volume ratio of the hard phase structure of martensite, retained austenite, and bainite is α%, and the addition amounts of Ti, Nb, and Mo are% Ti,% Nb, and% Mo , respectively. Sometimes, the elongation and hole expansibility characterized by α ≦ 50000 × ((% Ti) / 48 + (% Nb) / 93 + (% Mo) / 96) Excellent low yield ratio high strength cold rolled steel sheet.
Ｂ：０．００１０〜０．００３％
を含有することを特徴とする請求項１または請求項２に記載の、伸びおよび穴拡げ性に優れた低降伏比高強度冷延鋼板。Furthermore, in mass%,
B: 0.0010 to 0.003%
The low yield ratio high-strength cold-rolled steel sheet excellent in elongation and hole expansibility according to claim 1 or 2, characterized by comprising:
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| IN2015DN00521A (en) | 2012-08-06 | 2015-06-26 | Nippon Steel & Sumitomo Metal Corp | |
| BR112018003267A2 (en) * | 2015-08-21 | 2018-09-25 | Nippon Steel & Sumitomo Metal Corporation | steel sheet |
| CN113584393A (en) * | 2021-08-05 | 2021-11-02 | 马钢(合肥)板材有限责任公司 | Tensile strength 780MPa grade dual-phase steel and production method thereof |
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