JP4306202B2 - High tensile cold-rolled steel sheet and method for producing the same - Google Patents
High tensile cold-rolled steel sheet and method for producing the same Download PDFInfo
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Description
【０００１】
【発明の属する技術分野】
この発明は、ドアインパクトビ−ム等のような“自動車の衝突安全確保のための部品”やシ−トレ−ル等の“乗員の安全に関わる部品”の素材として好適な、曲げ加工性に優れた引張強度が７８０ＭPa以上の高張力冷延鋼板及びその製造方法に関するものである。
【０００２】
【従来の技術】
近年、自動車の安全及び軽量化対策としての“引張強度が７８０ＭPa以上の高張力鋼板”に対する要望が高まっている。
しかしながら、鋼板を高強度化していくと、成形性、特に曲げ性の劣化が問題になってくる。特に、引張強度が７８０ＭPa以上の超高張力鋼板では曲げ性の確保は一層深刻な問題となる。
【０００３】
高強度冷延鋼板の製造方法としては、例えば特開平７−１８８７６７号公報にベイナイト主体の金属組織にすることで伸びフランジ性を改善する製造方法が開示されている。
しかしながら、ベイナイトは延性が低く、ベイナイト主体の組織にするだけでは十分な曲げ性の確保は困難であった。
更に、上記特開平７−１８８７６７号公報に記載の高強度冷延鋼板の製造方法では伸びフランジ性を改善するために鋼板を低合金としており、そのため高強度を得るためには焼鈍後に１００℃/c以上の冷却速度で冷却することが必要で、冷却中に生じる歪によって鋼板の平坦度を確保することが困難であるとの問題もあった。
【０００４】
また、特開平９−２６３８３８号公報には、鋼板の金属組織をフェライトとベイナイトの混合組織にして穴拡げ性を改善する方法が開示されている。
しかし、引張強度で７８０ＭPa以上の高強度冷延鋼板においては、上述のような複合組織化を行うだけでは十分な曲げ性を得ることができなかった。
【０００５】
【発明が解決しようとする課題】
このようなことから、本発明が目的としたのは、前述したような従来技術の問題点を解決し、引張強度で７８０ＭPa以上の高強度と良好な曲げ加工性を有する高強度冷延鋼板及びその製造方法を提供することであった。
【０００６】
【課題を解決するための手段】
本発明者らは上記目的を達成すべく鋭意研究を行った結果、引張強度で７８０ＭPa以上の高強度と良好な曲げ加工性が両立した鋼板を実現するためには、その金属組織を、フェライトを体積率にて３％以上と、“炭化物を含むベイナイト”及び“炭化物を含むマルテンサイト”を合計の体積率にて４０％以上含み、かつ上記フェライトと上記ベイナイト及びマルテンサイトとの合計量が体積率にて６０％以上であって、更に粒内にセメンタイト又はマルテンサイト又は残留オ−ステナイトを有しているフェライト粒の数が総フェライトの数の３０％以上である組織とすることが重要であるとの知見を得ることができた。
【０００７】
即ち、本発明者らは、まず、曲げ加工における割れの起点はフェライト粒界に存在する非常に硬質な“炭化物を含まないマルテンサイト”や“粗大化して脆い炭化物を含むパ−ライト”であり、フェライト粒界に存在する炭化物を含む適当な硬さのベイナイトや、焼き戻されて軟質化した炭化物を含むマルテンサイトでは割れが発生しにくいことを見出した。更に、高張力を得るために必要な炭化物を含まないマルテンサイトやセメンタイトや残留オ−ステナイトは、フェライト粒内にあれば割れの起点になりにくいことも明らかとした。
そして、上記解明事項と、引張強度で７８０ＭPa以上の強度が得られる金属組織を検討し、引張強度で７８０ＭPa以上の高強度と良好な曲げ加工性が両立した鋼板を実現する上での前記金属組織の有効性を確認するに至ったわけである。
【０００８】
本発明は、上記知見事項等を基に完成されたものであり、次の 1）〜 5）項に示す曲げ加工性に優れた引張強度が７８０ＭPa以上の高張力冷延鋼板並びにその製造方法を提供するものである。
1） Ｃ：0.05〜0.10％（以降、成分割合を表す％は重量％とする），Si：0.1 〜2.0 ％，Mn：1.3 〜3.0 ％，Ｐ：0.10％以下，Ｓ：0.010 ％以下，Al：0.001 〜0.20％，Ｎ：0.020 ％以下を含むと共に残部がFe及び不可避的不純物から成り、また金属組織として、フェライトを体積率にて３％以上と、更に“炭化物を含むベイナイト”及び“炭化物を含むマルテンサイト”の１種又は２種を合計の体積率にて４０％以上含み、かつ上記フェライトと上記ベイナイト及びマルテンサイトとの合計量が体積率にて６０％以上であって、更に粒内にセメンタイト又はマルテンサイト又は残留オ−ステナイトを有しているフェライト粒の数が総フェライトの数の３０％以上である組織を持つことを特徴とする、引張強度７８０ＭPa以上を示す高張力冷延鋼板。
2） Ｃ：0.05〜0.10％，Si：0.1 〜2.0 ％，Mn：1.3 〜3.0 ％，Ｐ：0.10％以下，Ｓ：0.010 ％以下，Al：0.001 〜0.20％，Ｎ：0.020 ％以下を含み、更にTi：0.20％以下，Nb：0.20％以下，Ｖ：0.10％以下，Ｂ：0.01％以下，Cr：1.0 ％以下，Mo：1.0 ％以下，Cu：1.0 ％以下，Ni：1.0 ％以下，Ca：0.01％以下の１種以上をも含むと共に残部がFe及び不可避的不純物から成り、また金属組織として、フェライトを体積率にて３％以上と、更に“炭化物を含むベイナイト”及び“炭化物を含むマルテンサイト”の１種又は２種を合計の体積率にて４０％以上含み、かつ上記フェライトと上記ベイナイト及びマルテンサイトとの合計量が体積率にて６０％以上であって、更に粒内にセメンタイト又はマルテンサイト又は残留オ−ステナイトを有しているフェライト粒の数が総フェライトの数の３０％以上である組織を持つことを特徴とする、引張強度７８０ＭPa以上を示す高張力冷延鋼板。
3） 前記 1）項又は 2）項に示す成分組成の高張力冷延鋼板であって、表層から２０μｍまでの範囲の金属組織として、フェライトを体積率にて３％以上と、更に“炭化物を含むベイナイト”及び“炭化物を含むマルテンサイト”の１種又は２種を合計の体積率にて４０％以上含み、かつ上記フェライトと上記ベイナイト及びマルテンサイトとの合計量が体積率にて６０％以上であって、更に粒内にセメンタイト又はマルテンサイト又は残留オ−ステナイトを有しているフェライト粒の数が総フェライトの数の３０％以上である組織を持つことを特徴とする、引張強度７８０ＭPa以上を示す高張力冷延鋼板。
4） 表層に亜鉛めっき層を有する前記 1）項乃至 3）項の何れかに記載の冷延鋼板。
5） 前記 1）項乃至 3）項の何れかに記載の成分組成を有した鋼片を、１０５０℃以上に加熱した後に粗圧延を開始し、粗圧延終了後、そのまま或いは粗バ−の加熱又は温度保定を実施してから仕上圧延を開始し、仕上温度７８０〜１０３０℃で圧延を終了した後、平均冷却速度５℃／s 以上で冷却して７００℃以下で巻き取り、更に巻き戻してからそのまま酸洗、或いはスキンパス圧延と酸洗を施した後に冷間圧延を施し、次いで７２０〜９００℃の温度範囲で５秒以上保持する焼鈍を行った後、２〜２０℃／s の平均冷却速度で７００〜７６０℃まで冷却し、更に１０超〜１００℃／s の平均冷却速度で２００〜４２０℃まで冷却して、２００〜４２０℃の温度範囲に６０〜３００秒間保持してから、３００秒以内に１００℃以下にまで冷却することを特徴とする、引張強度７８０ＭPa以上を示す高張力冷延鋼板の製造方法。
【０００９】
【発明の実施の形態】
次に、本発明において鋼板の金属組織，鋼片乃至は鋼板の成分組成、並びに鋼板の製造処理条件を前記の如くに限定した理由を説明する。
(A) 金属組織
金属組織は本発明鋼板の重要な要素であって、フェライトを体積率で３％以上と、更に“炭化物を含むベイナイト”及び“炭化物を含むマルテンサイト”の１種又は２種を合計の体積率で４０％以上含み、更にフェライトと上記ベイナイト及びマルテンサイトとの合計が６０％以上で、かつ粒内にセメンタイト又はマルテンサイト又は残留オ−ステナイトを有しているフェライト粒の数が総フェライトの数の３０％以上となる金属組織とすることにより、引張強度が７８０ＭPa以上で優れた曲げ加工性を有する高張力冷延鋼板が得られる。
【００１０】
なお、フェライトはそれ自体優れた延性を有しているので、鋼板に優れた曲げ加工性を確保するためには体積率で３％以上必要である。フェライトが体積率で３％未満であって残部が延性の不十分なベイナイト等のフェライト以外の相である場合には、優れた曲げ性を確保することができない。しかしながら、フェライトのみでは鋼板に引張強度７８０ＭPa以上を確保することが困難であり、従って“炭化物を含むベイナイト”と“炭化物を含むマルテンサイト”とを合計の体積率で４０％以上含む必要がある。
即ち、本発明者らは、曲げ加工における割れの起点はフェライト粒界に存在する非常に硬質な炭化物を含まないマルテンサイトや粗大で脆い炭化物であるセメンタイト又は粗大な炭化物を含むパ−ライトであり、フェライト粒界に存在する炭化物を含む適当な硬さのベイナイトや焼き戻されて軟質化した炭化物を含むマルテンサイトでは割れが発生しにくいことを見出した。そして、優れた曲げ加工性を有すると共に引張強度７８０ＭPa以上の鋼板を実現するためには、炭化物を含むベイナイトと炭化物を含むマルテンサイトの合計を体積率で４０％以上とし、更にフェライトと上記ベイナイト及びマルテンサイトの合計が体積率で６０％以上とする必要のあることを確認した。
好ましくは、フェライトは１５％以上、また炭化物を含むベイナイト又は炭化物を含むマルテンサイトの合計を体積率で６０％以上、そしてフェライトと炭化物を含むベイナイト又は炭化物を含むマルテンサイトとの合計量を体積率で８０％以上とするのが良い。
【００１１】
なお、上記炭化物は、ベイナイト又はマルテンサイト中において体積率で３０％以上占有しているのが好ましい。
そして、上記理由によりフェライトと粒界で接する金属組織は“フェライト”又は“炭化物を含むベイナイト”又は“炭化物を含むマルテンサイト”であることが好ましく、それが粒界の５０％以上であるのが望ましい。
【００１２】
また、先に述べた理由により高張力を得るために必要なマルテンサイトやセメンタイトや残留オ−ステナイトはフェライト粒界ではなくてフェライト粒内にあるのが好ましく、所望の高張力を確保するには粒内にセメンタイト又はマルテンサイト又は残留オ−ステナイトを有しているフェライト粒の数が総フェライトの数の３０％以上である必要がある。
【００１３】
ところで、鋼板の曲げ加工では表層よりクラックが生じるため、鋼板表層から２０μｍまでの範囲を上記金属組織とするだけであっても優れた曲げ加工性の確保が可能であることも本発明者らは確認済である。
【００１４】
（Ｂ）鋼片乃至は鋼板の化学組成
Ｃ：Ｃは鋼板に高張力（引張強度）を得るのに重要な成分である。Ｃの含有量が0.05％未満では必要な高張力が得られず、また上限を超えてＣを含有させると靱性や溶接性が低下すると共にフェライト生成量が不足して所望の延性を確保することができない。従って、Ｃ含有量は0.05〜0.10％に調整するのが良い。
【００１５】
Si： Siも鋼板を高強度化するのに有効な成分であり、必要な強度を確保するために 0.1％以上を含有させる。しかし、 2.0％を超えてSiを含有させると化成処理性が劣化すると共に、フェライト粒内ではなくて粒界に生成するマルテンサイト量が増加するために曲げ加工性が劣化する。従って、Si含有量は 0.1〜 2.0％と定めたが、好ましくは 0.7〜 1.6％に、より好ましくは 1.0〜 1.6％に調整するのが良い。
【００１６】
Mn： Mnにはオ−ステナイトを安定化することでベイナイトを生成させる働きがある。Mnの含有量が 1.3％未満ではベイナイト生成が不十分で良好な曲げ性と高張力の両立が得られず、また 3.0％を超えてMnを含有させるとフェライトが生成しにくくなると共にバンド組織が発達して曲げ性が低下する。従って、Mn含有量は 1.3〜 3.0％と定めたが、好ましくは 2.0〜 3.0％に、より好ましくは 2.2〜 3.0％に調整するのが良い。
【００１７】
Ｐ： Ｐは靱性を劣化させる好ましくない元素である。従って、その許容量を確認し、Ｐ含有量は0.10％以下と定めた。
Ｓ： ＳはMnＳを形成して鋼板の曲げ加工性を劣化させる好ましくない元素である。従って、その許容量を確認し、Ｓ含有量を 0.010％以下と定めたが、好ましくは0.0040％以下に、より好ましくは0.0015％以下とするのが良い。
【００１８】
Al： Alは脱酸のために添加される元素であるが、その効果は 0.001％未満では不十分であり、また0.20％を超えて含有させても効果が飽和し経済的に不利となる。従って、Al含有量は 0.001〜0.20％と定めた。
Ｎ： Ｎは、連続鋳造中に窒化物を形成してスラブのひび割れの原因となるので、その含有量は低い方が好ましい。従って、その許容量を確認し、Ｎ含有量は0.020 ％以下と定めた。
【００１９】
Ti，Nb，Ｖ，Ｂ： Ti，Nb，Ｖ及びＢは何れも再結晶を遅らせて結晶粒を微細化させる効果を有しているので、必要に応じて１種以上が含有せしめられる。しかしながら、その効果は、Ti含有量が0.20％を超え、Nb含有量が0.20％を超え、Ｖ含有量が0.10％を超え、そしてＢ含有量が 0.010％を超えると飽和してしまいコスト的に不利となる。そのため、Ti含有量は0.20％以下、Nb含有量は0.20％以下、Ｖ含有量は0.10％以下、Ｂ含有量は 0.010％以下とそれぞれ定めた。
【００２０】
Cr，Mo： Cr及びMoには何れもMnと同様にオ−ステナイトを安定化することでベイナイトを生成させる働きがあるので、必要に応じて１種以上が含有せしめられる。しかし、Cr含有量が 1.0％を超え、そしてMo含有量が 1.0％を超えると鋼板の化成処理性に問題が出てくる。従って、Cr含有量は 1.0％以下、Mo含有量は1.0 以下とそれぞれ定めた。
【００２１】
Cu，Ni： Cu及びNiには何れも腐食抑制効果があり、鋼板表面に濃化して水素の侵入を抑え遅れ破壊を抑制する働きがあるので、必要に応じて１種以上が含有せしめられる。しかしながら、何れもその含有量が 1.0％を超えると前記効果は飽和しコスト的に不利となる。従って、Cu含有量もNi含有量も 1.0％以下とそれぞれ定めたが、何れも好ましくは0.01〜 1.0％に調整するのが良い。
【００２２】
Ca： CaはＳと結合し、硫化物を球状化させて曲げ加工性や耐遅れ破壊性の改善に効果があるので、必要に応じて添加される。しかし、0.01％を超えて含有させてもその効果は飽和しコスト的に不利となることから、Ca含有量は0.01％以下と定めた。
【００２３】
なお、上記以外の成分はFe及び不可避的不純物である。
そして、上記組成の鋼は、例えば転炉，電気炉又は平炉等により溶製される。鋼種もリムド鋼，キャップド鋼，セミキルド鋼又はキルド鋼の何れでも良い。更に、鋼片の製造は“造塊−分塊圧延”あるいは“連続鋳造”の何れの手段によっても構わない。
【００２４】
(C) 製造条件
本発明に係る“曲げ加工性に優れた引張強度７８０ＭPa以上を示す高張力冷延鋼板”を製造するには、まず本発明が規定する範囲の化学組成の鋼片を１０５０℃以上に加熱後粗圧延を開始し、粗圧延終了後、そのまま或いは必要に応じて粗バ−に加熱又は温度保定を施してから仕上圧延を開始し、仕上温度７８０〜１０３０℃で圧延を終了後、平均冷却速度５℃/s以上で冷却し、７００℃以下で巻き取る熱間圧延を施す。
【００２５】
１０５０℃以上への鋼片の加熱は、仕上温度を確保するのに必要である。そして、仕上温度７８０℃以上は変態点以下の圧延に伴って表層に形成されるフェライトの粗大組織を抑制するのに必要な条件であり、また１０３０℃以下の仕上温度は組織を微細化して冷延板焼鈍後に十分なフェライトを生成させるのに必要な条件である。
なお、仕上温度を確保するために仕上圧延前に粗バ−を加熱あるいは温度保定することは有効である。また、粗バ−を接合して連続圧延を施しても何ら問題はない。
熱間圧延に際して加熱炉に挿入する鋼片は“鋳造後の高温ままでのスラブ”でも“室温で放置されたスラブ”でも構わない。
【００２６】
仕上圧延を終了した後は、バンド状組織軽減のために平均冷却速度５℃/s以上で冷却し、７００℃以下で巻き取る。巻取温度が７００℃を上回ると製品においても曲げ性を低下させるバンド状組織が発達するので好ましくない。巻取温度は望ましくは５９０℃以下とするのが良い。
【００２７】
熱間圧延後は、必要に応じて平坦矯正のためのスキンパス圧延やスケ−ル除去のための酸洗を施し、好ましくは圧下率３０％以上の冷間圧延を施して焼鈍（連続焼鈍）を施す。焼鈍では７２０〜９００℃の温度範囲で５秒以上保持する処理を行い、引き続いて２〜２０℃／s の平均冷却速度で７００〜７６０℃まで冷却した後、更に１０超〜１００℃／s の平均冷却速度で２００〜４２０℃まで冷却し、この２００〜４２０℃の温度範囲で６０〜３００秒保持してから、３００秒以内に１００℃以下にまで冷却する。
【００２８】
なお、７２０℃未満の焼鈍温度ではオ−ステナイト化が不十分で、得られる炭化物を含むベイナイト又は炭化物を含むマルテンサイトの量が少ない。一方、焼鈍温度が９００℃を超えると粒の粗大化が生じ、フェライトが得られなくなる。
また、加熱時間は５秒以上必要で、５秒未満ではオ−ステナイト化が不十分であって得られる炭化物を含むベイナイト又は炭化物を含むマルテンサイト量が少ない上に、組織が安定せず、コイル内の強度バラツキの原因となる。
【００２９】
焼鈍後の冷却過程で、７００〜７６０℃までは２〜２０℃／s の平均冷却速度で徐冷され、更に１０超〜１００℃／s の冷却速度で２００〜４２０℃まで冷却される。
７００〜７６０℃までの徐冷は、３％以上のフェライトを分散して生成させるために必要である。その後１０超〜１００℃／s の平均冷却速度で急冷するのは、パ−ライトやセメンタイトの生成を抑制して４０％以上のベイナイト又はマルテンサイトを生成させるためである。この第２段目の冷却において、１００℃／s を超える平均冷却速度では平坦不良の原因となると共に、フェライトの成長不足に伴いセメンタイト又はマルテンサイト又は残留オ−ステナイトがフェライト粒内ではなくて粒界に生成し曲げ加工性劣化の原因となる。なお、第２段目の冷却における冷却速度は、好ましくは２０超〜８０℃／s とするのが良い。
【００３０】
急冷停止及び保持温度範囲は２００〜４２０℃とし、保持時間は６０〜３００秒にする必要があるが、急冷停止及び保持温度が２００℃未満であったり、保持時間が６０秒を下回ったりすると炭化物を含まないマルテンサイト量が増えすぎる。また、急冷停止及び保持温度が４２０℃を上回っていたりあるいは保持時間３００秒を超えたりすると、粒界に出てくるセメンタイトの析出量が増加しすぎる。なお、急冷停止及び保持温度の好ましい範囲は２００〜３５０℃、より好ましくは２００〜３００℃である。
【００３１】
更に、２００〜４２０℃の温度範囲に保持した後で３００秒以内に１００℃以下にまで冷却することは重要である。これによって、フェライト粒界に粗大なセメンタイトが析出するのが抑制できる。なお、２００℃から１００℃以下までの冷却は６０秒以内とするのが好ましい。
【００３２】
連続焼鈍後、鋼板へは必要に応じて更に平坦矯正のため伸び率４％以下のスキンパスを付与しても何ら問題がない。
また、本発明に係る成分組成並びに金属組織を有する鋼板の表面に亜鉛めっき等の表面処理を施しても何ら問題はない。
【００３３】
以下、本発明を実施例によって更に具体的に説明する。
【実施例】
表１に示す化学組成の鋼を転炉にて溶製した後、連続鋳造にてスラブとした。
そして、そのスラブを表２に示す条件で熱間圧延し、 2.6mm厚の熱延鋼板を製造した。
次に、得られた熱延鋼板を酸洗してから 1.2mm厚まで冷間圧延し、その後、表３に示す条件の連続焼鈍で冷延鋼板を製造した。
【００３４】
【表１】
【表２】
【表３】
このようにして得られた冷延鋼板につき、ナイタル腐食後に光学顕微鏡及びＳＥＭ観察、更に電子顕微鏡での観察にて金属組織の観察及び特定を行った。
また、圧延直角方向にＪＩＳ５号試験片と曲げ試験片を採取し、引張試験及び曲げ試験を実施した。曲げ試験はＪＩＳ法に従い実施し、「亀裂が発生する限界曲げ半径×板厚」で評価した。
【００３８】
また、粒内にセメンタイト又はマルテンサイトを含むフェライトの数は、鋼板表層より２０μｍまでの範囲のフェライト１００個を観察して調査し、相当する粒数の割合（％）を把握した。
表層より２０μｍの範囲で得られた金属組織及び材料特性の調査結果を、表４及び表５に示す。
【００３９】
【表４】
【表５】
表４及び表５に示す結果からも明らかなように、本発明に係る冷延鋼板は１４％以上の伸びと 0.5ｔ以下の良好な曲げ性を示した。
これに対して、試験番号８〜25及び試験番号40〜42に係る冷延鋼板では本発明が規定する金属組織が得られずに曲げ性に劣っており、フェライト量が不足したものは伸びも低かった。
また、Ｃ含有量が高い試験番号40に係る冷延鋼板はスポット溶接性に劣っており、Si含有量の高い試験番号41に係る冷延鋼板は化成処理性にも問題のあることが確認された。
【００４２】
【発明の効果】
以上に説明した如く、この発明によれば、優れた曲げ加工性を有し、バンパ−レインフォ−ス等といった自動車の補強部品やシ−トレ−ル等の自動車部品等に好適な高張力冷延鋼板を安定して得ることができるなど、産業上有用な効果が得られる。[0001]
BACKGROUND OF THE INVENTION
The present invention is suitable for bending workability as a material for “parts for ensuring collision safety of automobiles” such as door impact beams and “parts related to passenger safety” such as seat rails. The present invention relates to a high-tensile cold-rolled steel sheet having an excellent tensile strength of 780 MPa or more and a method for producing the same.
[0002]
[Prior art]
In recent years, there has been an increasing demand for “high-tensile steel sheets having a tensile strength of 780 MPa or more” as measures for safety and weight reduction of automobiles.
However, as the strength of the steel sheet increases, deterioration of formability, particularly bendability, becomes a problem. In particular, in an ultra-high-strength steel sheet having a tensile strength of 780 MPa or more, securing bendability becomes a more serious problem.
[0003]
As a method for producing a high-strength cold-rolled steel sheet, for example, Japanese Patent Application Laid-Open No. 7-188767 discloses a production method for improving stretch flangeability by forming a bainite-based metal structure.
However, bainite has low ductility, and it has been difficult to ensure sufficient bendability simply by using a bainite-based structure.
Furthermore, in the manufacturing method of the high strength cold-rolled steel sheet described in JP-A-7-188767, the steel sheet is made of a low alloy in order to improve stretch flangeability. Therefore, in order to obtain high strength, 100 ° C. / There is also a problem that it is necessary to cool at a cooling rate of c or more, and it is difficult to ensure the flatness of the steel sheet due to strain generated during cooling.
[0004]
Japanese Patent Application Laid-Open No. 9-263838 discloses a method for improving hole expansibility by making the metal structure of a steel plate a mixed structure of ferrite and bainite.
However, in a high-strength cold-rolled steel sheet having a tensile strength of 780 MPa or more, sufficient bendability cannot be obtained only by performing the above-described composite structure.
[0005]
[Problems to be solved by the invention]
For this reason, the present invention aims to solve the problems of the prior art as described above, a high-strength cold-rolled steel sheet having high strength of 780 MPa or more in tensile strength and good bending workability, and It was to provide a manufacturing method thereof.
[0006]
[Means for Solving the Problems]
As a result of intensive studies to achieve the above object, the present inventors have found that the metal structure is made of ferrite in order to realize a steel sheet that has both a high strength of 780 MPa or more in tensile strength and good bending workability. 3% or more in volume ratio, 40% or more in total volume ratio of “bainite containing carbide” and “martensite containing carbide”, and the total amount of ferrite, bainite and martensite is volume It is important to have a structure in which the number of ferrite grains having cementite, martensite, or retained austenite in the grains is 30% or more of the total number of ferrites. We were able to obtain the knowledge that there was.
[0007]
That is, the inventors of the present invention, first, the starting point of the crack in bending is very hard “martensite containing no carbide” or “pearlite containing coarse and brittle carbide” present in the ferrite grain boundary. It has been found that cracks are unlikely to occur in bainite having an appropriate hardness including carbides present in ferrite grain boundaries and martensite including carbides that have been tempered and softened. Furthermore, it has also been clarified that martensite, cementite, and retained austenite that do not contain carbides necessary for obtaining high tension are less likely to start cracks within the ferrite grains.
Then, the above-mentioned elucidated matters and a metal structure capable of obtaining a strength of 780 MPa or more in tensile strength are studied, and the metal structure for realizing a steel sheet having both a high strength of 780 MPa or more in tensile strength and good bending workability. It came to confirm the effectiveness of.
[0008]
The present invention has been completed on the basis of the above-mentioned findings and the like, and includes a high-tensile cold-rolled steel sheet having excellent bending workability and having a bending strength of 780 MPa or more as shown in the following items 1) to 5) and a method for producing the same. It is to provide.
1) C: 0.05 to 0.10 % (Hereinafter, “%” represents percentage by weight), Si: 0.1 to 2.0%, Mn: 1.3 to 3.0%, P: 0.10% or less, S: 0.010% or less, Al : 0.001 to 0.20%, N: 0.020% or less and the balance is composed of Fe and unavoidable impurities, and the metal structure is 3% or more by volume of ferrite, and “carbite containing carbide” and “carbide” Including one or two of "martensite containing" in a total volume ratio of 40% or more, and the total amount of the ferrite, the bainite and martensite is 60% or more in volume ratio, A high-tensile cold-rolling exhibiting a tensile strength of 780 MPa or more, characterized by having a structure in which the number of ferrite grains having cementite, martensite, or retained austenite is 30% or more of the total number of ferrites steel sheet.
2) C: 0.05 to 0.10 %, Si: 0.1 to 2.0%, Mn: 1.3 to 3.0%, P: 0.10% or less, S: 0.010% or less, Al: 0.001 to 0.20%, N: 0.020% or less, Furthermore, Ti: 0.20% or less, Nb: 0.20% or less, V: 0.10% or less, B: 0.01% or less, Cr: 1.0% or less, Mo: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, Ca : Containing not less than 0.01% of one or more, the balance being composed of Fe and inevitable impurities, and having a metal structure of not less than 3% by volume of ferrite, and further including “bainite containing carbide” and “carbide” 1 type or 2 types of “martensite” is contained in a total volume ratio of 40% or more, and the total amount of the ferrite, the bainite and martensite is 60% or more in volume ratio, The number of ferrite grains with cementite or martensite or residual austenite A high-tensile cold-rolled steel sheet having a tensile strength of 780 MPa or more, characterized in that it has a structure that is 30% or more of the number of steel sheets.
3) A high-tensile cold-rolled steel sheet having the composition shown in 1) or 2) above, and having a metal structure in the range from the surface layer to 20 μm, ferrite is 3% or more by volume ratio, Containing one or two kinds of “bearing bainite” and “carbide containing martensite” in a total volume ratio of 40% or more, and the total amount of the ferrite and the bainite and martensite in a volume ratio of 60% or more. And having a structure in which the number of ferrite grains having cementite, martensite or residual austenite in the grains is 30% or more of the total number of ferrites, and having a tensile strength of 780 MPa or more High tensile cold-rolled steel sheet.
4) The cold-rolled steel sheet according to any one of 1) to 3) above, wherein the surface layer has a galvanized layer.
5) The steel slab having the composition described in any one of 1) to 3) above is heated to 1050 ° C. or higher, and then rough rolling is started. Alternatively, finish rolling is started after the temperature is maintained, and after finishing rolling at a finishing temperature of 780 to 1030 ° C., the steel is cooled at an average cooling rate of 5 ° C./s or more, wound up at 700 ° C. or less, and further rewound. Then, after pickling as it is, or after performing skin pass rolling and pickling, cold rolling is performed, and then annealing is performed at a temperature range of 720 to 900 ° C for 5 seconds or more, and then an average cooling of 2 to 20 ° C / s Cooling to 700 to 760 ° C. at a rate, further cooling to 200 to 420 ° C. at an average cooling rate of more than 10 to 100 ° C./s, and holding in a temperature range of 200 to 420 ° C. for 60 to 300 seconds, then 300 Cool to below 100 ° C within seconds Characterized Rukoto, method for producing a high strength cold-rolled steel sheet showing a more tensile strength 780 MPa.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
Next, the reason why the metal structure of the steel sheet, the steel slab or the component composition of the steel sheet, and the manufacturing conditions of the steel sheet are limited as described above in the present invention will be described.
(A) Metal structure The metal structure is an important element of the steel sheet of the present invention, and ferrite is 3% or more by volume, and one or two kinds of “bainite containing carbide” and “martensite containing carbide”. The number of ferrite grains containing 40% or more in total volume ratio, the sum of ferrite, bainite and martensite being 60% or more, and having cementite, martensite or residual austenite in the grains By making the metal structure 30% or more of the total number of ferrite, a high-tensile cold-rolled steel sheet having excellent bending workability with a tensile strength of 780 MPa or more can be obtained.
[0010]
In addition, since ferrite has excellent ductility itself, 3% or more is required by volume ratio in order to ensure excellent bending workability for the steel sheet. When the ferrite is less than 3% by volume and the balance is a phase other than ferrite such as bainite having insufficient ductility, excellent bendability cannot be ensured. However, with ferrite alone, it is difficult to secure a tensile strength of 780 MPa or more in the steel sheet. Therefore, it is necessary to include “bainite containing carbide” and “martensite containing carbide” in a total volume ratio of 40% or more.
That is, the inventors of the present invention are the starting point of cracking in bending work is martensite that does not contain very hard carbides present in ferrite grain boundaries, or cementite that is coarse and brittle carbide, or pearlite that contains coarse carbides. It has been found that cracks are unlikely to occur in bainite having an appropriate hardness including carbides present in ferrite grain boundaries and martensite including carbides that have been tempered and softened. And in order to implement | achieve the steel plate which has the outstanding bending workability and tensile strength of 780 Mpa or more, the sum total of the volume ratio of the bainite containing a carbide | carbonized_material and the martensite containing a carbide | carbonized_material is set to 40% or more, and also ferrite, the said bainite, It was confirmed that the total martensite needs to be 60% or more by volume ratio.
Preferably, the ferrite is 15% or more, and the total amount of bainite containing carbide or martensite containing carbide is 60% or more by volume, and the total amount of ferrite and bainite containing carbide or carbide containing martensite is volume fraction. 80% or more is preferable.
[0011]
In addition, it is preferable that the said carbide | carbonized_material occupies 30% or more by volume ratio in bainite or a martensite.
For the above reason, the metal structure in contact with ferrite at the grain boundary is preferably “ferrite” or “bainite containing carbide” or “martensite containing carbide”, which is 50% or more of the grain boundary. desirable.
[0012]
In addition, martensite, cementite and residual austenite necessary for obtaining high tension for the reasons described above are preferably not in ferrite grain boundaries but in ferrite grains, and in order to secure a desired high tension. The number of ferrite grains having cementite, martensite, or retained austenite in the grains needs to be 30% or more of the total number of ferrites.
[0013]
By the way, since cracks are generated from the surface layer in the bending process of the steel sheet, the present inventors can also ensure excellent bending workability even if only the range from the steel sheet surface layer to 20 μm is the above metal structure. Confirmed.
[0014]
(B) Chemical composition of steel slab or steel plate C: C is an important component for obtaining high tension (tensile strength) in the steel plate. If the C content is less than 0.05%, the necessary high tension cannot be obtained. If the C content exceeds the upper limit , the toughness and weldability are deteriorated and the ferrite generation amount is insufficient to ensure the desired ductility. I can't. Therefore, the C content is preferably adjusted to 0.05 to 0.10%.
[0015]
Si: Si is also an effective component for increasing the strength of steel sheets, and is contained in an amount of 0.1% or more to ensure the required strength. However, if Si is contained in excess of 2.0%, the chemical conversion processability deteriorates and the bending workability deteriorates because the amount of martensite generated in the grain boundaries rather than in the ferrite grains increases. Accordingly, the Si content is determined to be 0.1 to 2.0%, but is preferably adjusted to 0.7 to 1.6%, more preferably 1.0 to 1.6%.
[0016]
Mn: Mn has the function of generating bainite by stabilizing austenite. If the Mn content is less than 1.3%, the bainite formation is insufficient and good bendability and high tension cannot be achieved at the same time. If the Mn content exceeds 3.0%, ferrite is difficult to form and the band structure is reduced. It develops and the bendability decreases. Therefore, the Mn content is determined to be 1.3 to 3.0%, but is preferably adjusted to 2.0 to 3.0%, more preferably 2.2 to 3.0%.
[0017]
P: P is an undesirable element that deteriorates toughness. Therefore, the allowable amount was confirmed, and the P content was determined to be 0.10% or less.
S: S is an undesirable element that forms MnS and degrades the bending workability of the steel sheet. Therefore, the allowable amount was confirmed, and the S content was determined to be 0.010% or less, but is preferably 0.0040% or less, more preferably 0.0015% or less.
[0018]
Al: Al is an element added for deoxidation, but its effect is insufficient if it is less than 0.001%, and if it exceeds 0.20%, the effect is saturated and economically disadvantageous. Therefore, the Al content is determined to be 0.001 to 0.20%.
N: Since N forms a nitride during continuous casting and causes cracks in the slab, its content is preferably low. Therefore, the allowable amount was confirmed, and the N content was determined to be 0.020% or less.
[0019]
Ti, Nb, V, B: Ti, Nb, V, and B all have the effect of delaying recrystallization and refining crystal grains, so that one or more of them can be contained as required. However, the effect is saturated when the Ti content exceeds 0.20%, the Nb content exceeds 0.20%, the V content exceeds 0.10%, and the B content exceeds 0.010%. Disadvantageous. Therefore, the Ti content is 0.20% or less, the Nb content is 0.20% or less, the V content is 0.10% or less, and the B content is 0.010% or less.
[0020]
Cr, Mo: Since Cr and Mo all have a function of generating bainite by stabilizing austenite in the same manner as Mn, one or more kinds may be contained as required. However, if the Cr content exceeds 1.0% and the Mo content exceeds 1.0%, a problem arises in the chemical conversion properties of the steel sheet. Therefore, the Cr content is set to 1.0% or less, and the Mo content is set to 1.0 or less.
[0021]
Cu, Ni: Cu and Ni both have a corrosion-inhibiting effect and have a function of concentrating on the steel sheet surface to suppress hydrogen penetration and delayed fracture, so that one or more of them can be contained as required. However, in any case, if the content exceeds 1.0%, the effect is saturated and disadvantageous in cost. Accordingly, the Cu content and the Ni content are respectively determined to be 1.0% or less, but both are preferably adjusted to 0.01 to 1.0%.
[0022]
Ca: Ca binds to S and spheroidizes a sulfide to improve bending workability and delayed fracture resistance. Therefore, Ca is added as necessary. However, even if the content exceeds 0.01%, the effect is saturated and disadvantageous in terms of cost, so the Ca content is determined to be 0.01% or less.
[0023]
Components other than the above are Fe and inevitable impurities.
And the steel of the said composition is smelted by a converter, an electric furnace, a flat furnace, etc., for example. The steel type may be any of rimmed steel, capped steel, semi-killed steel or killed steel. Further, the steel slab may be produced by any means of “ingot-bundling rolling” or “continuous casting”.
[0024]
(C) Production conditions In order to produce a “high-tensile cold-rolled steel sheet having a tensile strength of 780 MPa or more excellent in bending workability” according to the present invention, first, a steel slab having a chemical composition in a range specified by the present invention is 1050 ° C. After starting the rough rolling after heating, after finishing the rough rolling, heat rolling or temperature holding is applied to the rough bar as needed or after finishing rolling, and finishing at a finishing temperature of 780 to 1030 ° C. Then, it is cooled at an average cooling rate of 5 ° C./s or more, and is hot-rolled at 700 ° C. or less.
[0025]
Heating the steel slab to 1050 ° C. or higher is necessary to ensure the finishing temperature. A finishing temperature of 780 ° C. or higher is a necessary condition for suppressing the coarse structure of ferrite formed on the surface layer with rolling below the transformation point, and a finishing temperature of 1030 ° C. or lower is refined to cool the structure. This is a necessary condition for generating sufficient ferrite after annealing.
In order to secure the finishing temperature, it is effective to heat or hold the temperature of the rough bar before finishing rolling. Further, there is no problem even if the rough bar is joined and subjected to continuous rolling.
The steel slab inserted into the heating furnace during the hot rolling may be “slab as it is after casting” or “slab left at room temperature”.
[0026]
After finishing rolling, the steel sheet is cooled at an average cooling rate of 5 ° C./s or more and wound at 700 ° C. or less in order to reduce the band-like structure. When the coiling temperature exceeds 700 ° C., a band-like structure that lowers the bendability develops even in the product, which is not preferable. The winding temperature is desirably 590 ° C. or lower.
[0027]
After hot rolling, subject to skin pass rolling for flattening and pickling to remove scale, if necessary, preferably cold rolling with a reduction rate of 30% or more for annealing (continuous annealing). Apply. In the annealing, a treatment is performed for 5 seconds or more in a temperature range of 720 to 900 ° C., followed by cooling to 700 to 760 ° C. at an average cooling rate of 2 to 20 ° C./s, and further over 10 to 100 ° C./s. It cools to 200-420 degreeC with an average cooling rate, It hold | maintains for 60-300 seconds in this 200-420 degreeC temperature range, Then, it cools to 100 degrees C or less within 300 seconds.
[0028]
Note that at an annealing temperature of less than 720 ° C., austenitization is insufficient, and the amount of bainite containing carbide or martensite containing carbide is small. On the other hand, if the annealing temperature exceeds 900 ° C., grain coarsening occurs and ferrite cannot be obtained.
Further, the heating time is required to be 5 seconds or more. If it is less than 5 seconds, the austenitization is insufficient, and the resulting bainite containing carbide or the amount of martensite containing carbide is small, and the structure is not stable, and the coil It causes the intensity variation in the inside.
[0029]
In the cooling process after annealing, it is gradually cooled to 700 to 760 ° C. at an average cooling rate of 2 to 20 ° C./s, and further cooled to 200 to 420 ° C. at a cooling rate of more than 10 to 100 ° C./s.
Slow cooling to 700 to 760 ° C. is necessary for dispersing and producing 3% or more of ferrite. Then, the reason for quenching at an average cooling rate of more than 10 to 100 ° C./s is to suppress the formation of pearlite and cementite and to produce 40% or more of bainite or martensite. In this second stage cooling, an average cooling rate exceeding 100 ° C./s causes a failure in flatness, and due to insufficient growth of ferrite, cementite, martensite, or residual austenite is not in the ferrite grains but in the grains. It generates in the boundary and causes bending workability deterioration. The cooling rate in the second stage cooling is preferably more than 20 to 80 ° C./s.
[0030]
The quenching stop and holding temperature range is 200 to 420 ° C. and the holding time needs to be 60 to 300 seconds. If the quenching stop and holding temperature is less than 200 ° C. or the holding time falls below 60 seconds, the carbide The amount of martensite that does not contain is increased too much. In addition, when the rapid cooling stop and holding temperature exceed 420 ° C. or when the holding time exceeds 300 seconds, the amount of cementite precipitated at the grain boundaries increases excessively. In addition, the preferable range of rapid cooling stop and holding temperature is 200-350 degreeC, More preferably, it is 200-300 degreeC.
[0031]
Furthermore, it is important to cool to 100 ° C. or less within 300 seconds after being kept in the temperature range of 200 to 420 ° C. This can suppress the precipitation of coarse cementite at the ferrite grain boundaries. The cooling from 200 ° C. to 100 ° C. or less is preferably within 60 seconds.
[0032]
After continuous annealing, there is no problem even if a skin pass having an elongation of 4% or less is applied to the steel sheet as necessary for further flattening.
Moreover, there is no problem even if surface treatment such as galvanization is applied to the surface of the steel sheet having the component composition and metal structure according to the present invention.
[0033]
Hereinafter, the present invention will be described more specifically with reference to examples.
【Example】
The steel having the chemical composition shown in Table 1 was melted in a converter and then made into a slab by continuous casting.
The slab was hot rolled under the conditions shown in Table 2 to produce a 2.6 mm thick hot rolled steel sheet.
Next, the obtained hot-rolled steel sheet was pickled and cold-rolled to a thickness of 1.2 mm, and then a cold-rolled steel sheet was manufactured by continuous annealing under the conditions shown in Table 3.
[0034]
[Table 1]
[Table 2]
[Table 3]
The cold-rolled steel sheet thus obtained was observed and specified for the metal structure by observation with an optical microscope and SEM, and further observation with an electron microscope after nitral corrosion.
Moreover, a JIS No. 5 test piece and a bending test piece were sampled in the direction perpendicular to the rolling direction, and a tensile test and a bending test were performed. The bending test was carried out according to the JIS method and evaluated by “limit bending radius at which cracks occur × sheet thickness”.
[0038]
Further, the number of ferrites containing cementite or martensite in the grains was investigated by observing 100 ferrites in the range of 20 μm from the steel sheet surface layer, and the proportion (%) of the corresponding number of grains was grasped.
Tables 4 and 5 show the investigation results of the metal structure and material characteristics obtained in the range of 20 μm from the surface layer.
[0039]
[Table 4]
[Table 5]
As is clear from the results shown in Tables 4 and 5, the cold-rolled steel sheet according to the present invention exhibited an elongation of 14% or more and a good bendability of 0.5 t or less.
On the other hand, in the cold-rolled steel sheets according to test numbers 8 to 25 and test numbers 40 to 42, the metal structure defined by the present invention is not obtained and the bendability is inferior. It was low.
Further, it was confirmed that the cold-rolled steel sheet according to test number 40 having a high C content is inferior in spot weldability, and the cold-rolled steel sheet according to test number 41 having a high Si content has a problem in chemical conversion treatment. It was.
[0042]
【The invention's effect】
As described above, according to the present invention, high-tensile cold rolling has excellent bending workability and is suitable for automobile reinforcement parts such as bumper reinforcements and automobile parts such as seat rails. Industrially useful effects can be obtained, for example, a steel plate can be stably obtained.
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