JP4132582B2 - Low carbon cold-rolled steel sheet with excellent formability and tensile rigidity and method for producing the same - Google Patents
Low carbon cold-rolled steel sheet with excellent formability and tensile rigidity and method for producing the same Download PDFInfo
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Description
【０００１】
【発明の属する技術分野】
本発明は、主に自動車構造材として、例えば、ルーフ、フード、ドアパネル等のように曲率が大きな部分を有する部品に好適な張り剛性に優れた低炭素冷延鋼板およびその製造方法に関する。
【０００２】
【従来の技術】
近年、地球環境問題に対する関心の高まりと共に自動車の燃費向上のニーズが強くなっている。燃費向上のための有効な方策の一つとして車体重量の軽減があり、その中でも車体を構成す低炭素冷延鋼板の板厚を低減することが重要視されている。板厚を低減する際に最も問題となるのが、成形部品の張り剛性の低下である。張り剛性が低下すると、成形品が外部から力を受けた際に容易にたわみを生じてしまう。一般に張り剛性は式（１）に示すように板厚とヤング率に依存する。
Ｓ∝Ｅ・ｔm ・・・（１）
ここでＳは張り剛性，Ｅはヤング率、ｔは板厚、ｍはパネル形状に依存した乗数で１〜３の値を持つ。
【０００３】
この式からも明らかなように、薄肉化による張り剛性の低下を防ぐためには、鋼板のヤング率を向上させる以外に手段はない。そこで、例えば、特開昭５８−９９３２号公報や特開平３−３７３１号公報に開示されているように、鋼のヤング率の異方性に着目し、成分や圧延方法を限定することで板厚方向に対してヤング率の高い方位の集積度を上げることが行われている。しかし、この方法ではヤング率の向上代は小さく、かつ等方的なヤング率の向上は期待できない。
【０００４】
ところで、ヤング率は物理定数であることから、張り剛性の評価を行う場合も一定値として取り扱われてきた。しかし、パネルのようにプレス成形などによって材料に歪みが与えられた部品に再度力が加わると、一般に弾性域と言われる歪み量（０．１％以下）の範囲においても、歪みの増加に伴い、応力−歪み曲線の刻々の傾きが低下していく。この傾きのことを、以後、瞬間ヤング率と呼ぶ。すなわち、従来完全な弾性範囲内での変形であり、一定値のヤング率で評価できると考えられていた張り剛性は、実際は、歪みの増加に伴う瞬間ヤング率の低下という現象を含めた形で取り扱われるべきである。しかし、これまでにこのような現象に着目して張り剛性向上を検討した例は全くない。
【０００５】
【発明が解決しようとする課題】
そこで、本発明は、上記課題を有利に解決し、張り剛性に優れた低炭素冷延鋼板およびその製造方法を提供することを目的とするものである。
【０００６】
【課題を解決するための手段】
前述のように、本発明者らは瞬間ヤング率の低下という現象と張り剛性との相関に着目し、歪みの増加に伴う瞬間ヤング率の低下を抑制することで張り剛性が著しく向上するという全く新しい知見を得た。すなわち、プレスに相当する２％予歪みを与え、１５０〜１７０℃で２０分以内の熱処理を施し、その後再度引張試験を行った際の瞬間ヤング率をＸ（応力−歪み曲線の傾きに相当）、ヤング率をＹとしたとき、歪み量０．０６％まで式（２）の関係を保つ鋼板で、かつ熱処理前からの降伏応力の上昇代が４０Ｎ／ｍｍ2以上の鋼板は張り剛性が著しく向上することを見いだした。
Ｘ／Ｙ＞０．８ ・・・（２）
【０００７】
瞬間ヤング率の低下には、前述したようにプレス成形などによって材料中に導入された歪みが深く関係している。すなわち、プレス成型時に可動転位が導入されていると、マクロには弾性変形範囲内とされる歪み域においても、徐々に局所的な降伏現象が進行し、それが、瞬間ヤング率の低下の要因になっていると考えられる。そこで本発明者らは鋼中において可動転位の動きを抑制し、瞬間ヤング率の低下を抑制する方法として、成形後に熱処理でＣの様な侵入型固溶元素を可動転位の周囲に偏析させることが極めて効果的であるという事実を新たに見いだした。また、張り剛性はヤング率の他に板厚の影響も著しく受けることから、この瞬間ヤング率の向上による張り剛性改善効果は限定された板厚範囲のみで発揮されることも初めて見いだした。
【０００８】
本発明の要旨は以下の通りである。
（１）質量％で、Ｃ：０．０１超〜０．０５％，Ｓｉ≦１．０％，Ｍｎ≦１．５％，Ｐ≦０．１５％，Ａｌ０．００５〜０．２％，Ｎ≦０．００７％，Ｍｏ：０．０１〜３％を含有し、残部がＦｅ及び不可避的不純物からなり、固溶Ｃ量と固溶Ｎ量の合計が、質量％で、０．０００７〜０．００５％である、板厚０．５〜０．８ｍｍ、降伏応力が１２０〜２５０Ｎ／ｍｍ2の低炭素冷延鋼板であって、相当歪みで２％の予歪みを施し、１５０〜１７０℃で５〜２０分の熱処理を施した後に、再度引張試験を行った際の歪み量０．０６％での応力歪み曲線の傾きＸとヤング率Ｙの比Ａ（＝Ｘ／Ｙ）がＡ＞０．８を満たし、熱処理後の降伏応力の上昇代が４０Ｎ／ｍｍ2以上であることを特徴とする成形性と張り剛性に優れた低炭素冷延鋼板。
【０００９】
（２）板厚０．５〜０．７５ｍｍであることを特徴とする前記（１）に記載の成形性と張り剛性に優れた低炭素冷延鋼板。
【００１０】
（３）前記（１）又は（２）に記載の低炭素冷延鋼板の製造方法であって、前記（１）に記載の成分を有する熱延鋼板を冷間圧延後、７００℃〜Ａｃ3 点で２０秒〜１２０秒の再結晶焼鈍を施し、更に２５０〜４００℃で１〜５分の過時効処理を施すことを特徴とする成形性と張り剛性に優れた低炭素冷延鋼板の製造方法にある。
【００１１】
【発明の実施の形態】
本発明は、プレスに相当する２％の予歪みを与え所定の熱処理を施した後に引張試験を行った際の瞬間ヤング率をＸ（応力−歪み曲線の傾きに相当）、ヤング率をＹとした時、歪み量０．０６％でのＸ／Ｙ＞０．８という関係を有する鋼板に関するものである。以下に、その限定理由を述べる。
板厚：冷延鋼板の板厚は０．５〜０．８ｍｍ、好ましくは０．６〜０．８ｍｍとする。先に述べた様に張り剛性は板厚の１〜３乗に依存するため、板厚の影響を著しく受ける。従って、板厚が０．５ｍｍ未満に薄手化されると板厚の負の効果が大きくなりすぎ、十分な焼付処理を施しても張り剛性向上の効果が得られなくなる。一方、０．８ｍｍ超の厚手材になると板厚の正の効果が大きいために、焼付処理による張り剛性向上効果が見かけ上見えにくくなる。従って、板厚は０．５〜０．８ｍｍ、望ましくは０．６〜０．８ｍｍとする。
【００１２】
降伏応力：プレス前の降伏応力が１２０Ｎ／ｍｍ2未満では、張り剛性向上の効果が顕著に現れないことから本発明に係る冷延鋼板の降伏応力の下限は１２０Ｎ／ｍｍ2とする。また、プレス前の降伏応力が２５０Ｎ／ｍｍ2を超えるとプレス成形が難しくなり、形状凍結性も低下する。従って、プレス前の降伏応力は２５０Ｎ／ｍｍ2以下とする。
瞬間ヤング率と降伏応力の上昇代：まず、張り剛性と瞬間ヤング率の関係は以下の実験によって決定した。表１に示す化学成分の鋼を熱間圧延・冷間圧延後同表中に示した条件で焼鈍し、同表中に示した固溶Ｃ，Ｎ量および機械的性質を有する板厚０．７５ｍｍの冷延鋼板を製造した。
【００１３】
【表１】
これらの鋼板のＬ方向からＪＩＳ５号引張試験片および振動法によるヤング率測定用試験片を切り出し、残部より図１の模式図に示した型のパネルを作製した。各々の試験片およびパネルに表２に示した種々の熱処理を施し、まず、ヤング率測定と引張試験より瞬間ヤング率の測定を行った。図２にＮｏ．２，５，６，１０の歪み量に伴う瞬間ヤング率の変化を示す。歪み量０のところに表示されている値が振動法によって測定されたヤング率である。鋼種によるヤング率の違いはほとんど認められないが、歪み量の増加に伴い、鋼種および熱処理によって瞬間ヤング率の低下の挙動が異なる事がわかる。
【００１５】
【表２】
一方パネルは、周囲を拘束しパネル正面を構成する部分の中央部を押して荷重１００Ｎでのたわみ量を求めた。図３には歪み量０．０６％でのＮｏ．１〜１０のヤング率比Ａ（＝Ｘ／Ｙ）とたわみ量の関係を示す。これより、歪み量０．０６％でのヤング率比が０．８以上、降伏応力の上昇代が４０Ｎ／ｍｍ2以上であれば高い張り剛性が得られることがわかる。ヤング率比は原理的に１．０を越えることはない。また、前記熱処理時の降伏応力の上昇代の上限は特に定めることなく本発明の効果を得ることができる。
熱処理：引張試験の前に施される熱処理条件は実際にそのパネルを製造するラインで塗装焼付等の目的のために行われている条件に準じる。従って、熱処理条件は１５０〜１７０℃で５〜２０分とする。もちろん、張り剛性向上の目的で、更に高温長時間の熱処理を施して特性を評価しても良い。
【００１７】
次に、化学成分の限定理由について説明する。
Ｃ：Ｃを０．０１％未満にすると連続焼鈍における過時効時の炭化物の析出が十分進行せず、固溶Ｃ量を十分低減することが出来なくなる。また、Ｃ量が０．０５％超になると炭化物やパーライトの析出量が増加し、延性や深絞り性などの加工性が劣化する。従って、Ｃ量の範囲は０．０１％超〜０．０５％とする。
固溶Ｃ，Ｎ量：固溶Ｃ，Ｎ量の合計が０．０００７％未満では可動転位を固着する能力が十分ではない。従って、固溶Ｃ，Ｎ量の合計は０．０００７％以上が望ましい。また、固溶Ｃ，Ｎ量の合計が０．００５％を越えると室温で放置している間に時効硬化が進行し、パネルを成形するのが困難になる。従って、固溶Ｃ，Ｎ量の上限は０．００５％とする。
【００１８】
Ｓｉ，Ｍｎ，Ｐ：強度向上のために通常含まれる成分、すなわち、Ｓｉ，Ｍｎ，Ｐの上限をそれぞれＳｉ：１．０％以下、Ｍｎ：１．５％以下、Ｐ：０．１５％以下とする。これはそれ以上の添加は加工性を劣化するためである。また、ＳｉとＭｎは脱酸のため、それぞれ０．０１％以上含まれていることが好ましい。Ｂ：Ｂの添加は二次加工性を向上させるので必要に応じて０．０００２％以上を添加することは効果的であるが、０．００５％超になると加工性の劣化が著しくなるため、上限を０．００５％とする。
【００１９】
Ａｌ：Ａｌは脱酸材として用いる。また、熱延、または焼鈍中にＡｌＮとして析出し固溶Ｎを低減する。そのため少なくとも０．００５％の添加が望ましい。しかし、０．２％超添加すると加工性が劣化することから上限を０．２％とする。 Ｎ：Ｎは不純物であり、０．００７％超含有すると加工性が劣化する。従って、Ｎの上限は０．００７％とする。
Ｃｒ：Ｃｒは強度上昇に有効な元素であり、かつ熱処理後の降伏応力の上昇代を高めることから状況に応じて添加してもよい。しかし、その添加量が０．２％未満では効果が現れないため、その下限を０．２％とする。一方、３％を超えると熱延板の酸洗性が低下したり、製品板の化成処理性が劣化したりするので、上限を３％とする。
【００２０】
Ｍｏ：Ｍｏも強度上昇に有効な元素であり、かつ熱処理後の降伏応力の上昇代を高めることから状況に応じて添加しても良い。しかし、その添加量が０．０１％未満ではその効果が現れないためその下限を０．０１％とする。一方、３％を超えると強度が上昇しすぎて成形性が劣化するだけでなく、降伏応力上昇の効果も飽和することからその上限を３％とする。
上記成分を得るための原料はとくに限定しないが、鉄鉱石を原料として、高炉、転炉により成分を調整する方法以外にスクラップを原料としてもよいし、これを電炉で溶製してもよい。スクラップを原料の全部または一部として使用する際には、Ｃｕ，Ｎｉ，Ｓｎ，Ｓｂ，Ｚｎ，Ｐｂ等の元素を含んでもよい。
【００２１】
以上のように成分調整された鋼を常法にしたがって鋳造、熱延後、冷延鋼板とする。
熱間圧延に供するスラブは特に限定する物ではない。すなわち、連続鋳造スラブや薄スラブキャスターで製造したものなどであればよい。また、鋳造後に直ちに熱間圧延を行う、連続鋳造−直接圧延（ＣＣ−ＤＲ）のようなプロセスにも適合する。熱間圧延の仕上温度はＡｒ3 点より高いことが望ましい。冷間圧延率は５０〜９０％が望ましい。また、加工性向上のため、仕上げ圧延をα域で行うα域連続熱延プロセスによって得られた素材を冷間圧延、焼鈍して冷延鋼板としてもよい。
【００２２】
焼鈍温度：焼鈍温度が７００℃未満では再結晶が完了せず、加工性が劣化したり、再結晶を完了させるのに著しく長い時間を要し、生産性を低下させてしまう。また、Ａｃ3 点超で焼鈍を行うと加工性が劣化する。従って、焼鈍温度の範囲は７００℃〜Ａｃ3 点とする。
焼鈍時間：焼鈍時間が２０秒未満では再結晶が完了せず、加工性が劣化する。また、１２０秒超焼鈍を行っても顕著な効果は得られず、生産性が低下する。従って、焼鈍時間は２０秒〜１２０秒とする。
【００２３】
過時効温度：過時効温度が２５０℃未満では十分炭化物を析出させるため著しく長い時間を要し、生産性を低下させる。また、５００℃超では炭化物の析出が不十分になり、固溶Ｃを低減できなくなる。したがって過時効温度は２５０℃〜５００℃とする。
過時効時間：上記過時効温度範囲で１分未満の過時効を行っても十分な炭化物析出が得られず、また、５分超行ってもその効果は飽和することから、過時効時間は１分〜５分とする。
【００２４】
【実施例】
以下に本発明を実施例をもって詳細に述べる。
（参考例）
表３に示す機械的性質と固溶Ｃ，Ｎ量を有する板厚０．８ｍｍの冷延鋼板を実機にて製造した。冷延後、７８０℃で６０秒の再結晶焼鈍を施し、更に表３中に示した過時効処理を施した。Ｌ方向からＪＩＳ５号引張り試験片およびヤング率測定用試験片を切り出し、残部より図１に示したパネルを作製した。相当歪みで２％の予歪みを施した各々の試験片及びパネルに１７０℃で２０分の熱処理を施した後に、ヤング率測定と瞬間ヤング率測定を行った結果から得られたヤング率比Ａ（＝０．０６％歪みでの瞬間ヤング率／ヤング率）およびパネル周囲を拘束し、パネル正面を構成する部分の中央部を押した時の荷重１００Ｎでのたわみ量を表４に示す。これより、ヤング率比が０．８以上を有し、かつ熱処理による降伏応力の上昇代が４０Ｎ／ｍｍ2の鋼板はいずれも高い張り剛性を示すことがわかる。また、鋼Ｑの鋼板は原板の降伏応力が高すぎるため、適切な曲率を持ったパネルを成形することができなかった。
【００２５】
【表３】
【表４】
（実施例）
表１中の鋼Ｃの冷延率を変えることによってに表５に示した様な種々の板厚にした。冷延後、表１中の鋼Ｃと同じ条件で再結晶焼鈍及び過時効処理を施した。機械的な性質は、表５中に示したようにいずれも大きな違いはない。このような材料の張り剛性を参考例と同様な方法で評価した結果を表５と図４に示す。これより板厚が０．５ｍｍ〜０．８ｍｍの範囲で熱処理を施すによってたわみ量が低下し、優れた張り剛性が得られることがわかる。
【００２８】
【表５】
【発明の効果】
以上のように、歪みの増加に伴う瞬間ヤング率の低下代が低い鋼板の板厚範囲を制限した上でパネルに適用することによって張り剛性が著しく向上させることが出来る。
【図面の簡単な説明】
【図１】本発明で張り剛性を評価するために作製したパネルを示す図
【図２】歪みの増加に伴う瞬間ヤング率の変化を示すグラフ
【図３】ヤング率比と張り剛性との関係を表すグラフ
【図４】張り剛性に及ぼす板厚と熱処理の関係を表すグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention mainly relates to a low carbon cold-rolled steel sheet having excellent tensile rigidity suitable for parts having a large curvature, such as a roof, a hood, a door panel, and the like, and a manufacturing method thereof.
[0002]
[Prior art]
In recent years, with increasing interest in global environmental problems, there is an increasing need for improving the fuel efficiency of automobiles. One of the effective measures for improving the fuel efficiency is to reduce the weight of the vehicle body. Among them, reducing the thickness of the low-carbon cold-rolled steel sheet constituting the vehicle body is regarded as important. When the sheet thickness is reduced, the most serious problem is a decrease in the rigidity of the molded part. When the tension rigidity is lowered, the molded product easily bends when it receives a force from the outside. In general, the stiffness depends on the plate thickness and Young's modulus as shown in Equation (1).
S∝E · t m (1)
Here, S is the tension rigidity, E is the Young's modulus, t is the plate thickness, and m is a multiplier depending on the panel shape and has a value of 1 to 3.
[0003]
As is apparent from this equation, there is no means other than improving the Young's modulus of the steel sheet in order to prevent a decrease in the tension rigidity due to the thinning. Therefore, for example, as disclosed in Japanese Patent Application Laid-Open No. 58-9932 and Japanese Patent Application Laid-Open No. 3-3731, attention is paid to the anisotropy of the Young's modulus of steel, and the plate and the rolling method are limited. Increasing the degree of integration of orientations with a high Young's modulus relative to the thickness direction. However, this method has a small Young's modulus improvement margin and is not expected to improve the isotropic Young's modulus.
[0004]
By the way, since Young's modulus is a physical constant, it has been treated as a constant value when evaluating the stiffness. However, when a force is applied again to a part that has been distorted by press molding or the like, such as a panel, the strain increases with an increase in strain (0.1% or less), which is generally called the elastic range. The slope of the stress-strain curve gradually decreases. This slope is hereinafter referred to as the instantaneous Young's modulus. In other words, the tension stiffness, which has been considered to be a deformation within the complete elastic range and can be evaluated with a constant Young's modulus, is actually in a form that includes the phenomenon that the instantaneous Young's modulus decreases with increasing strain. Should be handled. However, there has been no example of examining the improvement of the tension rigidity by paying attention to such a phenomenon so far.
[0005]
[Problems to be solved by the invention]
Then, this invention aims to solve the said subject advantageously and to provide the low carbon cold-rolled steel plate excellent in tension rigidity, and its manufacturing method.
[0006]
[Means for Solving the Problems]
As described above, the present inventors pay attention to the correlation between the phenomenon of instantaneous Young's modulus reduction and the tension stiffness, and the tension stiffness is remarkably improved by suppressing the decrease in the instantaneous Young's modulus accompanying the increase in strain. I got new knowledge. That is, 2% pre-strain corresponding to the press is given, heat treatment is performed at 150 to 170 ° C. for 20 minutes or less, and then the instantaneous Young's modulus is X (corresponding to the slope of the stress-strain curve) when the tensile test is performed again. When the Young's modulus is Y, a steel plate that maintains the relationship of formula (2) up to a strain amount of 0.06%, and a steel plate with a yield stress increase rate of 40 N / mm 2 or more before heat treatment has a remarkable stiffness. I found it to improve.
X / Y> 0.8 (2)
[0007]
As described above, the strain introduced into the material by press molding or the like is deeply related to the decrease in the instantaneous Young's modulus. In other words, if movable dislocations are introduced during press molding, the local yield phenomenon gradually progresses in the macro even in the strain range that is considered to be within the elastic deformation range, which is the cause of the decrease in instantaneous Young's modulus. It is thought that. Therefore, as a method for suppressing the movement of movable dislocations in steel and suppressing the decrease in instantaneous Young's modulus, the present inventors segregate interstitial solid solution elements such as C around the movable dislocations by heat treatment after forming. Found a new fact that is extremely effective. In addition, since the tensile stiffness is significantly affected by the plate thickness in addition to the Young's modulus, it has been found for the first time that the effect of improving the stiffness stiffness due to the instant increase in the Young's modulus is exerted only in a limited plate thickness range.
[0008]
The gist of the present invention is as follows.
(1) By mass%, C: more than 0.01 to 0.05% , Si ≦ 1.0%, Mn ≦ 1.5%, P ≦ 0.15%, Al 0.005 to 0.2%, N ≦ 0.007%, Mo: 0.01 to 3% , the balance is made of Fe and inevitable impurities, and the total amount of solid solution C and solid solution N is 0.0007 to 0 in mass%. 0.005%, low-carbon cold-rolled steel sheet having a sheet thickness of 0.5 to 0.8 mm and a yield stress of 120 to 250 N / mm 2 , subjected to a pre-strain of 2% at an equivalent strain of 150 to 170 ° C. The ratio A (= X / Y) of the slope X and Young's modulus Y of the stress-strain curve at a strain amount of 0.06% when the tensile test was performed again after 5 to 20 minutes of heat treatment at A> A low carbon cold-rolled steel sheet excellent in formability and tensile rigidity, characterized by satisfying 0.8 and having an increase in yield stress after heat treatment of 40 N / mm 2 or more.
[0009]
(2) The low carbon cold-rolled steel sheet having excellent formability and tensile rigidity as described in (1) above, having a thickness of 0.5 to 0.75 mm.
[0010]
(3) (1) or a method for producing a low carbon cold rolled steel sheet according to (2), wherein after the cold rolling the hot-rolled steel sheet having a component according to (1), 700 ° C. to Ac 3 A low-carbon cold-rolled steel sheet excellent in formability and tensile rigidity, characterized by being subjected to recrystallization annealing for 20 seconds to 120 seconds, and further subjected to an overaging treatment at 250 to 400 ° C. for 1 to 5 minutes Is in the way.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
In the present invention, the instantaneous Young's modulus is X (corresponding to the slope of the stress-strain curve) when the tensile test is performed after giving a pre-strain of 2% corresponding to the press and performing a predetermined heat treatment, and the Young's modulus is Y. The steel sheet having a relationship of X / Y> 0.8 at a strain amount of 0.06%. The reason for limitation will be described below.
Sheet thickness: The thickness of the cold-rolled steel sheet is 0.5 to 0.8 mm, preferably 0.6 to 0.8 mm. As described above, the tension rigidity depends on the 1st to the 3rd power of the plate thickness, and thus is significantly affected by the plate thickness. Therefore, if the plate thickness is reduced to less than 0.5 mm, the negative effect of the plate thickness becomes too large, and the effect of improving the tension rigidity cannot be obtained even if a sufficient baking process is performed. On the other hand, when the thickness of the material exceeds 0.8 mm, the positive effect of the plate thickness is large, so that the effect of improving the stiffness by the baking process is apparently difficult to see. Therefore, the plate thickness is 0.5 to 0.8 mm, preferably 0.6 to 0.8 mm.
[0012]
Yield stress: If the yield stress before pressing is less than 120 N / mm 2 , the effect of improving the tensile stiffness does not appear remarkably, so the lower limit of the yield stress of the cold-rolled steel sheet according to the present invention is 120 N / mm 2 . On the other hand, if the yield stress before pressing exceeds 250 N / mm 2 , press forming becomes difficult, and the shape freezing property also decreases. Therefore, the yield stress before pressing is 250 N / mm 2 or less.
Increase in instantaneous Young's modulus and yield stress: First, the relationship between the stiffness and the instantaneous Young's modulus was determined by the following experiment. The steel having the chemical composition shown in Table 1 was annealed under the conditions shown in the table after hot rolling / cold rolling, and the thickness of the solution having the solid solution C, N amount and mechanical properties shown in the table was 0. A 75 mm cold-rolled steel sheet was produced.
[0013]
[Table 1]
A JIS No. 5 tensile test piece and a test piece for Young's modulus measurement by the vibration method were cut out from the L direction of these steel plates, and a panel of the type shown in the schematic diagram of FIG. 1 was produced from the remainder. Each test piece and panel were subjected to various heat treatments shown in Table 2, and first, the Young's modulus was measured by Young's modulus measurement and a tensile test. In FIG. The change of instantaneous Young's modulus with the amount of distortion of 2, 5, 6, 10 is shown. The value displayed at the strain amount of 0 is the Young's modulus measured by the vibration method. Although there is almost no difference in Young's modulus depending on the steel type, it can be seen that the behavior of the decrease in instantaneous Young's modulus differs depending on the steel type and heat treatment as the strain increases.
[0015]
[Table 2]
On the other hand, the panel restrained the periphery and pushed the center part of the part which comprises the panel front surface, and calculated | required the deflection amount with a load of 100N. In FIG. 3, No. at a strain of 0.06% is shown. A relationship between a Young's modulus ratio A (= X / Y) of 1 to 10 and a deflection amount is shown. From this, it can be seen that when the Young's modulus ratio at a strain amount of 0.06% is 0.8 or more and the yield increase of yield stress is 40 N / mm 2 or more, high tensile rigidity can be obtained. The Young's modulus ratio does not exceed 1.0 in principle. Further, the effect of the present invention can be obtained without particularly determining the upper limit of the increase in yield stress during the heat treatment.
Heat treatment: The heat treatment conditions applied before the tensile test are in accordance with the conditions currently used for the purpose of paint baking in the line for manufacturing the panel. Accordingly, the heat treatment conditions are 150 to 170 ° C. and 5 to 20 minutes. Of course, for the purpose of improving the rigidity of the tension, the characteristics may be evaluated by performing a heat treatment for a long time at a higher temperature.
[0017]
Next, the reason for limiting the chemical components will be described.
C: If C is less than 0.01%, precipitation of carbide during overaging in continuous annealing does not proceed sufficiently, and the amount of C dissolved cannot be sufficiently reduced. On the other hand, if the amount of C exceeds 0.05%, the amount of carbide and pearlite deposited increases, and the workability such as ductility and deep drawability deteriorates. Therefore, the range of C content is more than 0.01% to 0.05%.
Solid solution C, N amount: If the total of the solid solution C, N amount is less than 0.0007%, the ability to fix movable dislocations is not sufficient. Accordingly, the total amount of solid solution C and N is preferably 0.0007% or more. On the other hand, if the total amount of C and N in the solid solution exceeds 0.005%, the age hardening proceeds while being left at room temperature, and it becomes difficult to mold the panel. Therefore, the upper limit of the solid solution C and N amount is 0.005%.
[0018]
Si, Mn, P: components usually included for strength improvement, that is, the upper limit of Si, Mn, P is Si: 1.0% or less, Mn: 1.5% or less, P: 0.15% or less And This is because further addition deteriorates workability. Si and Mn are each preferably contained in an amount of 0.01% or more for deoxidation. B: Since the addition of B improves the secondary workability, it is effective to add 0.0002% or more as necessary. However, if it exceeds 0.005%, the workability deteriorates significantly. The upper limit is made 0.005%.
[0019]
Al: Al is used as a deoxidizer. Moreover, it precipitates as AlN during hot rolling or annealing, and reduces solid solution N. Therefore, addition of at least 0.005% is desirable. However, if over 0.2% is added, the workability deteriorates, so the upper limit is made 0.2%. N: N is an impurity, and if it exceeds 0.007%, workability deteriorates. Therefore, the upper limit of N is set to 0.007%.
Cr: Cr is an element effective for increasing the strength and increases the yield of the yield stress after the heat treatment, so that it may be added depending on the situation. However, if the addition amount is less than 0.2%, no effect appears, so the lower limit is made 0.2%. On the other hand, if it exceeds 3%, the pickling property of the hot-rolled sheet is lowered or the chemical conversion property of the product plate is deteriorated, so the upper limit is made 3%.
[0020]
Mo: Mo is also an element effective for increasing the strength, and may increase depending on the situation because it increases the yield of the yield stress after the heat treatment. However, if the addition amount is less than 0.01%, the effect does not appear, so the lower limit is made 0.01%. On the other hand, if it exceeds 3%, not only does the strength increase excessively and the formability deteriorates, but the effect of increasing the yield stress is saturated, so the upper limit is made 3%.
Although the raw material for obtaining the said component is not specifically limited, Scrap may be used as a raw material other than the method of adjusting a component with an iron ore as a raw material by a blast furnace and a converter, and you may melt this with an electric furnace. When scrap is used as all or part of the raw material, elements such as Cu, Ni, Sn, Sb, Zn, and Pb may be included.
[0021]
The steel whose components have been adjusted as described above is cast and hot-rolled according to a conventional method, and then a cold-rolled steel plate is obtained.
The slab used for hot rolling is not particularly limited. That is, what is necessary is just what was manufactured with the continuous casting slab or the thin slab caster. It is also compatible with processes such as continuous casting-direct rolling (CC-DR) in which hot rolling is performed immediately after casting. The finishing temperature of hot rolling is preferably higher than the Ar 3 point. The cold rolling rate is preferably 50 to 90%. In addition, in order to improve workability, a material obtained by an α region continuous hot rolling process in which finish rolling is performed in the α region may be cold rolled and annealed to form a cold rolled steel sheet.
[0022]
Annealing temperature: When the annealing temperature is less than 700 ° C., recrystallization is not completed, workability is deteriorated, and a remarkably long time is required to complete recrystallization, and productivity is lowered. Moreover, if annealing is performed at a point exceeding Ac 3 , the workability deteriorates. Accordingly, the annealing temperature range is 700 ° C. to Ac 3 points.
Annealing time: When the annealing time is less than 20 seconds, recrystallization is not completed and workability deteriorates. Moreover, even if it anneals for 120 seconds or more, a remarkable effect is not acquired and productivity falls. Accordingly, the annealing time is 20 seconds to 120 seconds.
[0023]
Overaging temperature: If the overaging temperature is less than 250 ° C., a sufficiently long time is required to sufficiently precipitate the carbide, and the productivity is lowered. On the other hand, if it exceeds 500 ° C., the precipitation of carbides becomes insufficient, and the solid solution C cannot be reduced. Therefore, the overaging temperature is 250 ° C to 500 ° C.
Overaging time: Even if overaging is performed for less than 1 minute in the above-mentioned overaging temperature range, sufficient carbide precipitation is not obtained, and the effect is saturated even after over 5 minutes, so the overaging time is 1 Minutes to 5 minutes.
[0024]
【Example】
The present invention will be described in detail below with reference to examples.
( Reference example )
Cold-rolled steel sheets with a thickness of 0.8 mm having the mechanical properties and solid solution C and N contents shown in Table 3 were produced using actual machines. After cold rolling, recrystallization annealing was performed at 780 ° C. for 60 seconds, and the overaging treatment shown in Table 3 was further performed. A JIS No. 5 tensile test piece and a Young's modulus measurement test piece were cut out from the L direction, and the panel shown in FIG. Young's modulus ratio A obtained from the results of Young's modulus measurement and instantaneous Young's modulus measurement after heat treatment at 170 ° C. for 20 minutes on each test piece and panel subjected to pre-strain of 2% at equivalent strain Table 4 shows the amount of deflection at a load of 100 N when the central portion of the portion constituting the front of the panel is pressed while restraining the periphery of the panel (= the instantaneous Young's modulus at 0.06% strain). From this, it can be seen that all steel sheets having a Young's modulus ratio of 0.8 or more and a yield stress increase due to heat treatment of 40 N / mm 2 exhibit high tensile rigidity. Moreover, since the steel plate of steel Q has a yield stress of the original plate that is too high, a panel having an appropriate curvature could not be formed.
[0025]
[Table 3]
[Table 4]
( Example)
Various plate thicknesses as shown in Table 5 were obtained by changing the cold rolling rate of Steel C in Table 1. After cold rolling, recrystallization annealing and overaging treatment were performed under the same conditions as steel C in Table 1. The mechanical properties are not significantly different as shown in Table 5. Table 5 and FIG. 4 show the results of evaluating the tension stiffness of such a material in the same manner as in the reference example . From this, it can be seen that the amount of deflection is reduced by heat treatment in the range of 0.5 mm to 0.8 mm, and excellent tension rigidity is obtained.
[0028]
[Table 5]
【The invention's effect】
As described above, the tension rigidity can be remarkably improved by applying to the panel after limiting the thickness range of the steel sheet in which the margin of decrease in the instantaneous Young's modulus with increasing strain is low.
[Brief description of the drawings]
FIG. 1 is a diagram showing a panel prepared for evaluating the tension stiffness in the present invention. FIG. 2 is a graph showing a change in instantaneous Young's modulus with increasing strain. FIG. 3 is a relationship between Young's modulus ratio and tension stiffness. FIG. 4 is a graph showing the relationship between the plate thickness and heat treatment affecting the stiffness.
Claims (3)
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