JP3908954B2 - Ferritic thin steel sheet with excellent shape freezing property and manufacturing method thereof - Google Patents

Description

ここで、ｎは仕上げ熱延の圧延スタンド数、ε _i はｉ番目のスタンドで加えられたひずみ、ｔ _i はｉ〜ｉ＋１番目のスタンド間の走行時間（秒）、τ _i は気体常数Ｒ（＝１．９８７）とｉ番目のスタンドの圧延温度Ｔｉ（Ｋ）によって下式で計算できる。
τ _i ＝８．４６×１０ ^-9 ・ｅｘｐ｛（４３８００／Ｒ）／Ｔｉ｝
【００２０】
（７）熱間圧延を終了し、前記（１）式に示す鋼の化学成分（質量％）で決まる臨界温度Ｔｏ（℃）以下まで冷却後、３００℃〜Ａｃ₁変態温度（℃）に加熱することを特徴とする前記（６）記載の形状凍結性に優れたフェライト系薄鋼板の製造方法。
【００２２】
（８）前記（Ａｒ₃−１００）〜（Ａｒ₃＋１５０）℃において、少なくとも１パス以上を摩擦係数が０．２以下となるように圧延することを特徴とする前記（６）または（７）に記載の形状凍結性に優れたフェライト系薄鋼板の製造方法。
【００２３】
（９）前記（６）〜（８）のいずれかに記載のフェライト系薄鋼板を酸洗し、圧下率が８０％未満の冷間圧延を施した後、６００〜（Ａｃ₃＋１００）℃に加熱し、冷却することを特徴とする形状凍結性に優れたフェライト系薄鋼板の製造方法。
【００２４】
【発明の実施の形態】
以下に本発明の内容を詳細に説明する。まず、Ｘ線ランダム強度比とｒ値について説明する。
（ａ）１／２板厚における板面の｛１００｝＜０１１＞〜｛２２３｝＜１１０＞方位群のＸ線ランダム強度比の平均値、｛１００｝＜０１１＞方位のＸ線ランダム強度比並びに｛５５４｝＜２２５＞、｛１１１｝＜１１２＞及び｛１１１｝＜１１０＞の３つの結晶方位のＸ線ランダム強度比の平均値：
この平均値は本発明で、特に重要な特性値である。板厚中心位置での板面のＸ線回折を行い、ランダム試料に対する各方位の強度比を求めたときの、｛１００｝＜０１１＞〜｛２２３｝＜１１０＞方位群における平均値が３．０以上でなくてはならない。これが３．０未満では形状凍結性が劣悪となる。
【００２５】
この方位群に含まれる主な方位は、｛１００｝＜０１１＞、｛１１６｝＜１１０＞、｛１１４｝＜１１０＞、｛１１３｝＜１１０＞、｛１１２｝＜１１０＞、｛３３５｝＜１１０＞および｛２２３｝＜１１０＞である。
【００２６】
これら各方位のＸ線ランダム強度比は、｛１１０｝極点図に基づきベクトル法により計算した３次元集合組織や、｛１１０｝、｛１００｝、｛２１１｝、｛３１０｝極点図のうち複数の極点図（好ましくは３つ以上）を用いて級数展開法で計算した３次元集合組織から求めればよい。
【００２７】
例えば、後者の方法における上記各結晶方位のＸ線ランダム強度比には、３次元集合組織のφ２＝４５゜断面における（００１）［１−１０］、（１１６）［１−１０］、（１１４）［１−１０］、（１１３）［１−１０］、（１１２）［１−１０］、（３３５）［１−１０］、（２２３）［１−１０］の強度をそのまま用いればよい。
【００２８】
｛１００｝＜０１１＞〜｛２２３｝＜１１０＞方位群の平均値とは、上記の各方位の強度の相加平均である。上記の全ての方位の強度を得ることができない場合には、｛１００｝＜０１１＞、｛１１６｝＜１１０＞、｛１１４｝＜１１０＞、｛１１２｝＜１１０＞、｛２２３｝＜１１０＞の各方位の強度の相加平均で代替してもよい。｛１００｝＜０１１＞方位は、スプリングバックと壁そりの低減に特に効果のある方位である。
【００２９】
したがって、｛１００｝＜０１１＞方位のＸ線ランダム強度は｛１００｝＜０１１＞〜｛２２３｝＜１１０＞方位群の中で、最大かつ４．０以上でなければならない。これが４．０未満であると、スプリングバックや壁そりの低減量が十分得られず、極めて良好な形状凍結性を確保することが困難になる。
【００３０】
なお、ここで述べる｛１００｝＜０１１＞方位は、同様の効果を有する方位の範囲として、圧延方向を回転軸として、１２°の範囲の傾きを許容する。この角度は、更に望ましくは、６°とする。
【００３１】
更に、１／２板厚における板面の｛５５４｝＜２２５＞、｛１１１｝＜１１２＞および｛１１１｝＜１１０＞の３つの結晶方位のＸ線ランダム強度比の平均値は３．５以下でなくてはならない。これが３．５超であると、｛１００｝＜０１１＞〜｛２２３｝＜１１０＞方位群の強度が適正であっても良好な形状凍結性を得ることが困難となる。
【００３２】
｛５５４｝＜２２５＞、｛１１１｝＜１１２＞および｛１１１｝＜１１０＞のＸ線ランダム強度比も上記の方法に従って計算した３次元集合組織から求めればよい。
【００３３】
より望ましくは、｛１００｝＜０１１＞〜｛２２３｝＜１１０＞方位群のＸ線ランダム強度比の平均値が４．０以上、｛１００｝＜０１１＞のＸ線ランダム強度比が５．０以上、｛５５４｝＜２２５＞、｛１１１｝＜１１２＞および｛１１１｝＜１１０＞のＸ線ランダム強度比の相加平均値が２．５未満である。
【００３４】
以上述べた結晶方位のＸ線強度が曲げ加工時の形状凍結性に対して重要であることの理由は、必ずしも明らかではないが、曲げ変形時の結晶のすべり挙動と関係があるものと推測される。
【００３５】
Ｘ線回折に供する試料は、機械研磨などによって鋼板を所定の板厚まで減厚し、次いで、化学研磨や電解研磨などによって歪みを除去すると同時に、板厚１／２面が測定面となるように作製する。鋼板の板厚中心層に偏析帯や欠陥などが存在し、測定上不都合が生ずる場合には、板厚の３／８〜５／８の範囲で適当な面が測定面となるように、上述の方法に従って試料を調整して測定すればよい。
【００３６】
当然のことであるが、上述のＸ線強度の限定が板厚１／２近傍だけでなく、なるべく多くの厚みについて満たされることで、より一層、形状凍結性が良好になる。なお、｛ｈｋｌ｝＜ｕｖｗ＞で表される結晶方位は、板面の法線方向が＜ｈｋｌ＞に平行で、圧延方向が＜ｕｖｗ＞と平行であることを示している。
【００３７】
（ｂ）圧延方向のｒ値（ｒＬ）および圧延方向と直角方向のｒ値（ｒＣ）：
これらのｒ値は、本発明において重要である。すなわち、本発明者等が鋭意検討の結果、上述した種々の結晶方位のＸ線強度が適正であっても、必ずしも良好な形状凍結性が得られないことが判明した。上記のＸ線強度と同時に、ｒＬおよびｒＣのうち少なくとも１つが０．７以下であることが必須である。より好ましくは０．５５以下である。
【００３８】
ｒＬおよびｒＣの下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が、ｒ値はＪＩＳ５号引張試験片を用いた引張試験により評価する。引張歪みは通常１５％であるが、均一伸びが１５％を下回る場合には、均一伸びの範囲で、できるだけ１５％に近い歪みで評価すればよい。
【００３９】
なお、曲げ加工を施す方向は加工部品によって異なるので特に限定するものではないが、ｒ値が小さい方向に対して垂直もしくは垂直に近い方向に折り曲げる加工を主とすることが好ましい。
【００４０】
ところで、一般に集合組織とｒ値とは相関があることが知られているが、本発明においては、既述の結晶方位のＸ線強度比に関する規定とｒ値に関する規定とは互いに同義ではなく、両方の規定が同時に満たされなくては良好な形状凍結性を得ることはできない。
【００４１】
本発明は、引張強度レベルの低い軟鋼板から高強度鋼板にいたる全ての薄鋼板に適用できるものであり、上記規定が満たされれば、薄鋼板の曲げ加工性は飛躍的に向上する。換言すれば、上記規定は、薄鋼板の機械的強度レベルの制約を越えた、曲げ加工変形に関する基本的材料指標であるということである。
【００４２】
薄鋼板であれば上記の規定は普遍的に適用できるので、特に薄鋼板の種類を限定することは基本的に必要のないことである。そして、勿論のこととして、熱延鋼板や冷延鋼板の区別は何ら問うものではない。
【００４３】
次に化学成分の限定理由について説明する。
【００４４】
Ｃの下限を０．０００１％としたのは、実用鋼で得られる下限値を用いることにしたためである。Ｃが０．２５％超になると加工性や溶接性が悪くなるので、上限は０．２５％に設定する。
【００４５】
Ｓｉは鋼板の機械的強度を高めるのに有効な元素であるが、２．５％超となると加工性が劣化したり、表面疵が発生したりするので、２．５％を上限とする。一方、実用鋼でＳｉを０．００１％未満とするのは困難であるので、０．００１％を下限とする。
【００４６】
Ｍｎも鋼板の機械的強度を高めるのに有効な元素であるが、２．５％超となると加工性が劣化するので、２．５％を上限とする。一方、実用鋼でＭｎを０．０１％未満とするのは困難であるので、０．０１％を下限とする。また、Ｍｎ以外に、Ｓによる熱間割れの発生を抑制するＴｉなどの元素が十分に添加されない場合には、質量％で、Ｍｎ／Ｓ≧２０となるＭｎ量を添加することが望ましい。
【００４７】
ＰとＳは、それぞれ、０．２％以下、０．０３％以下とする。これは、加工性の劣化や、熱間圧延または冷間圧延時の割れを防ぐためである。
【００４８】
Ａｌは脱酸のために０．０１％以上添加する。また、Ａｌはγ→α変態点を顕著に上昇させるので、特に、Ａｒ₃点以下での熱延を指向する場合には有効な元素である。しかし、多すぎると加工性が低下したり、表面性状が劣悪となるため、上限を２．０％とする。
【００４９】
ＮとＯは不純物であり、加工性を悪くさせないように、それぞれ、０．０１％以下、０．０１％以下とする。
【００５０】
Ｔｉ、Ｎｂ、Ｖ、Ｃｒ、Ｂは、炭素、窒素の固定、析出強化、組織制御、細粒強化などの機構を通じて材質を改善するので、必要に応じて、それぞれ、０．００５％以上、０．００１％以上、０．００１％以上、０．０１％以上、０．０００１％以上添加することが望ましい。しかし、過度に添加しても格段の効果はなく、むしろ加工性や表面性状を劣化させるので、それぞれに上限を設定した。その上限は、Ｔｉ：０．２％、Ｎｂ：０．２％、Ｖ：０．２％、Ｃｒ：１．５％、Ｂ：０．００７％である。
【００５１】
Ｍｏ、Ｃｕ、Ｎｉ、Ｓｎは機械的強度を高めたり材質を改善する効果があるので、必要に応じて、各成分とも０．００１％以上を添加することが望ましい。しかし、過度の添加は逆に加工性を劣化させるので、上限を、Ｍｏ：１％、Ｃｕ：２％、Ｎｉ：１％、Ｓｎ：０．２％とする。
【００５２】
なお、本発明では、脱酸の目的や硫化物の形態制御の目的でＣａやＭｇを０．０１％以下添加しても構わない。
【００５３】
メッキの種類は特に限定されるものではなく、電気めっき、溶融めっき、蒸着めっき等の何れでも本発明の効果が得られる。
【００５４】
次に、本発明薄鋼板の製造方法について説明する。
【００５５】
熱間圧延に先行する製造方法は、特に限定されるものではない。すなわち、高炉や電炉等による溶製に引き続き各種の２次製錬を行い、次いで、通常の連続鋳造、インゴット法による鋳造の他、薄スラブ鋳造などの方法で鋳造すればよい。連続鋳造の場合には、一度低温まで冷却したのち、再度加熱してから熱間圧延してもよいし、鋳造スラブを連続的に熱延してもよい。原料にはスクラップを使用しても構わない。
【００５６】
本発明の形状凍結性に優れたフェライト系薄鋼板は、上記化学成分の鋼を鋳造した後、熱間圧延後冷却まま、熱間圧延後冷却ままもしくは酸洗後に熱処理を施したまま、熱間圧延後冷却・酸洗し冷延した後に焼鈍、あるいは、熱延鋼板もしくは冷延鋼板を溶融めっきラインにて熱処理を施したまま、更には、これらの鋼板に別途表面処理を施すことによっても得られる。
【００５７】
熱間圧延の後半に、（Ａｒ₃＋５０）℃以上（Ａｒ₃＋１５０）℃以下で合計２５％以上の圧延が行われない場合には、オーステナイトの加工が不十分で集合組織が十分に発達しないために、どのような冷却を施しても、最終的に得られる熱延鋼板の板面に、前記（１）の発明で規定する所定のＸ線強度レベルの各結晶方位が得られないので、（Ａｒ₃＋５０）〜（Ａｒ₃＋１５０）℃での圧下率の合計の下限値を２５％とした。
【００５８】
（Ａｒ₃＋５０）〜（Ａｒ₃＋１５０）℃での合計圧下率が高いほど、よりシャープな集合組織形成が期待されるので、この合計圧下率は、３５％以上とすることが好ましいが、９７．５％を越えると、圧延機の剛性を過剰に高める必要があり、経済上のデメリットを生じるので、望ましくは９７．５％以下とする。
【００５９】
｛１００｝＜０１１＞方位への集合組織の集積を著しく高めるためには、更に、（Ａｒ₃−１００）〜（Ａｒ₃＋５０）℃で５〜３５％の圧下を加えることが極めて重要である。
【００６０】
なぜならば、高温域で十分に加工されたオーステナイトが、少なくとも部分的に再結晶した段階で、更に、適量の圧下を加え、その直後に、フェライト変態させることが、｛１００｝＜０１１＞方位の発達に極めて重要だからである。したがって、（Ａｒ₃−１００）℃未満で圧下しても、すでにフェライト変態が完了した領域が大きすぎるために、｛１００｝＜０１１＞が発達しない。
【００６１】
（Ａｒ₃＋５０）℃超で圧下を加えると、フェライト変態までに導入した歪みが回復してしまうために、｛１００｝＜０１１＞が発達しない。また、圧下率が５％未満では、｛１００｝＜０１１＞〜｛２２３｝＜１１０＞を含む集合組織全体がランダム化してしまい、一方、圧下率が３５％を超えると、｛１００｝＜０１１＞方位への集積が低くなるので、（Ａｒ₃−１００）〜（Ａｒ₃＋５０）℃の温度範囲での圧下率は５〜３５％とする。上述の観点から、圧下率は、望ましくは１０〜２５％とする。
【００６２】
熱間圧延は（Ａｒ₃−１００）〜（Ａｒ₃＋５０）℃の温度範囲で終了する。熱延終了温度が（Ａｒ₃−１００）未満になると加工性が著しく劣化し、（Ａｒ₃＋５０）℃超になると集合組織の集積が不十分なため形状凍結性が劣化する。
【００６３】
また、熱延工程では、多段の圧延スタンドで加えられるひずみの累積的な効果が重要である。しかしながら、このひずみの累積的な効果は、加工温度が高温ほど、また、スタンド間の走行時間が長いほど低下する。仕上げ熱延がｎスタンドで行われる際に、ｉ番目のスタンドでの圧延温度をＴｉ（Ｋ）、加工ひずみをε_i（真ひずみでｉ番目の圧下率ｒ_iとは、ε_i＝ｌｎ｛１／（１−ｒ_i）｝の関係を持つ）、ｉ番目とｉ＋１番目のスタンド間の走行時間（パス間時間：秒）をｔ_iとすると、累積効果を考慮したひずみ（有効ひずみε^*）は実験により求めた（２）式で表現できる。
【００６４】
【数３】

ここで、τ_iは気体常数Ｒ（Ｒ＝１．９８７）と圧延温度Ｔｉによって下式で計算できる。
τ_i＝８．４６×１０^-9・ｅｘｐ｛（４３８００／Ｒ）／Ｔｉ｝
【００６５】
この有効ひずみε^*が０．４未満の場合には、集合組織を著しく高く集積させることができない。したがって、有効ひずみε^*は０．４以上とする。
【００６６】
実際の熱延工程で、前記（２）式の計算を行う場合には、Ｔｉは、仕上げ熱延入側温度ＦＴ₀と、仕上げ熱延出側温度ＦＴｎを用い、式Ｔｉ＝ＦＴ₀−（ＦＴ₀−ＦＴｎ）／（ｎ＋１）×（ｉ＋１）で計算した値を用いるとよい。有効ひずみが高いほど集合組織が発達することから、有効ひずみは０．４５以上であればより好ましい。また、有効ひずみが０．９以上であれば、更に好ましい。
【００６７】
ここで、（Ａｒ₃−１００）〜（Ａｒ₃＋１５０）℃の温度範囲において、熱間圧延ロールと鋼板との摩擦係数が０．２を越えている場合には、鋼板表面近傍における板面に、｛１１０｝面を主とする結晶方位が発達し、形状凍結性が劣化するので、より良好な形状凍結性を指向する場合には、熱間圧延時における少なくとも１パスについて、熱間圧延ロールと鋼板との摩擦係数を０．２以下とすることが望ましい。
【００６８】
この摩擦係数は低ければ低いほど好ましく、下限は定めないが、さらに良好な形状凍結性が要求される場合には、摩擦係数を０．１５以下とすることが望ましい。摩擦係数は従来から知られているように、圧延時の先進率と圧延荷重から求めるものとする。
【００６９】
このようにして形成されたオーステナイトの集合組織を最終的な熱延鋼板に受け継がせるためには、Ｔｏ（℃）以下まで冷却または巻き取る必要がある。
【００７０】
このＴｏ（℃）は、オーステナイトと、オーステナイトと同一成分のフェライトが同一の自由エネルギーを持つ温度として熱力学的に定義され、Ｃ以外の成分の影響も考慮して、前記（１）式を用いて、鋼板の化学成分（質量％）で簡易的に計算することができる。本発明において規定した成分以外の成分のＴｏ（℃）に及ぼす影響はそれほど大きくないので、ここでは無視した。
【００７１】

【００７２】
また、巻き取温度または冷却停止温度の下限は特に限定しないが、２５０℃より低くしても加工性が劣化するばかりで格段の効果は得られないことから、２５０℃以上で巻き取るか，冷却を停止することが望ましい。
冷却をする場合、冷却速度が大きいほど集合組織が先鋭化するので、冷却速度は１０℃／ｓ以上とすることが望ましい。
【００７３】
冷却後、加工ままのフェライトが残存していると、機械的性質が劣化する。したがって、回復・再結晶を目的とした付加的な熱処理を行うことが好ましいが、その温度範囲は３００℃〜Ａｃ₁変態温度（℃）とする。熱処理温度が３００℃未満であると、回復・再結晶が進行せず機械的性質が劣化する。一方、Ａｃ₁変態温度超になると、熱間圧延中に形成された集合組織が壊れ、形状凍結性が劣化する。
【００７４】
熱間圧延においては粗圧延後にシートバーを接合し、連続的に仕上げ圧延をしてもよい。その際に、粗バーを一旦コイル状に巻き、必要に応じて保温機能を有するカバーに格納し、再度巻き戻してから接合を行ってもよい。熱延鋼板には、必要に応じてスキンパス圧延を施してもよい。スキンパス圧延には、加工成形時に発生するストレッチャーストレインの防止や形状矯正の効果があることはいうまでもない。
【００７５】
このようにして得られた熱延鋼板（もしくは熱処理された熱延鋼板）を冷間圧延し、焼鈍して最終的な薄鋼板とする際において、冷間圧延の全圧下率が８０％以上となる場合には、一般的な冷間圧延−再結晶集合組織である板面に平行な結晶面のＸ線回折積分面強度比の｛１１１｝面や｛５５４｝面成分が高くなり、本発明の特徴である前記（１）の発明における結晶方位の規定を満たなくなるので、冷間圧延の圧下率の上限を８０％未満とした。
【００７６】
形状凍結性を高めるためには、冷間圧下率を７０％以下に制限することが望ましい。冷間圧下率の下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が、結晶方位の強度を適当な範囲に制御するためには、３％以上とすることが好ましい。
【００７７】
このような範囲で冷間加工された冷延鋼板を焼鈍する際に、焼鈍温度が６００℃未満の場合には、加工組織が残留し成形性が著しく劣化するので、焼鈍温度の下限を６００℃とする。一方、焼鈍温度が過度に高い場合には、再結晶によって生成したフェライトの集合組織が、オーステナイトへ変態後、オーステナイトの粒成長によってランダム化され、最終的に得られるフェライトの集合組織もランダム化される。
【００７８】
特に、焼鈍温度が（Ａｃ₃＋１００）℃を越える場合には、そのような傾向が顕著となる。従って、焼鈍温度は（Ａｃ₃＋１００）℃以下とする。冷延鋼板には必要に応じてスキンパス圧延を施してもよい。
【００７９】
本発明で得られる組織は、フェライトを主体とするものであるが、フェライト以外の金属組織として、パーライト、ベイナイト、マルテンサイト、オーステナイトおよび／または炭窒化物等の化合物を含有しても構わない。特に、マルテンサイトやベイナイトの結晶構造は、フェライトのそれと同等もしくは類似しているので、フェライトの代わりにこれらの組織が主体であっても差し支えない。
【００８０】
なお、本発明に係る鋼板は、曲げ加工だけでなく、曲げ、張り出し、絞り等、曲げ加工を主体とする複合成形にも適用できる。
【００８１】
【実施例】
（実施例）
本発明の実施例を挙げながら、本発明の技術的内容について説明する。
【００８２】
実施例として、表１に示す成分組成を有する鋼種ＡからＬまでの鋼を用いて検討した結果について説明する。これらの鋼は、鋳造後、そのままもしくは一旦室温まで冷却された後に再加熱し、９００〜１３００℃に加熱され、その後熱間圧延が施され、最終的には１．４ｍｍ、３．０ｍｍもしくは８．０ｍｍ厚の熱延鋼板にされた。
【００８３】
３．０ｍｍおよび８．０ｍｍ厚の熱延鋼板は、冷間圧延することによって１．４ｍｍ厚とし、その後連続焼鈍工程にて焼鈍を行った。
【００８４】
これら１．４ｍｍ厚の鋼板から５０ｍｍ幅、２７０ｍｍ長さの試験片を作成し、ポンチ幅７８ｍｍ、ポンチ肩Ｒ５、ダイ肩Ｒ５の金型を用いてハット曲げ試験を行った。
【００８５】
曲げ試験を行った試験片については、三次元形状測定装置にて板幅中心部の形状を測定し、図１に示したように、点（１）と点（２）の接線と点（３）と点（４）の接線の交点の角度から９０°を引いた値の左右での平均値をスプリング・バック量、点（３）と点（５）間の曲率の逆数を左右で平均化した値を１０００倍したものを壁そり量、左右の点（５）間の長さからポンチ幅を引いた値を寸法精度として形状凍結性を評価した。なお、曲げは、ｒ値の低い方向と垂直に折れ線が入るように行った。
【００８６】
ところで、図２および図３に示したように、スプリングバック量や壁そり量はＢＨＦ（しわ押さえ力）によっても変化する。本発明の効果は、いずれのＢＨＦで評価を行ってもその傾向は変わらないが、実機で実部品をプレスする際には、設備上の制約からあまり高いＢＨＦはかけられないため、今回は、ＢＨＦ２９ｋＮで各鋼種のハット曲げ試験を行った。
【００８７】
【表１】

【００８８】
【表２】

【００８９】
表２には、各鋼板の製造条件が本発明の範囲内にあるか否かを示している。「熱延条件１」の「圧下率」の欄には、（Ａｒ₃＋５０）〜（Ａｒ₃＋１５０）℃の温度範囲における圧下率の合計が２５％以上の場合に「○」、圧下率の合計が２５％未満の場合に「×」を記し、「熱延条件１」の「有効ひずみε^*」の欄には、（Ａｒ₃＋５０）〜（Ａｒ₃＋１５０）℃の温度範囲における有効ひずみ量ε^*を記した。また、「熱延条件２」の欄には、（Ａｒ₃−１００）〜（Ａｒ₃＋３０）℃の温度範囲で圧下率の合計が５〜３５％である場合について「○」、この条件を満たさない場合を「×」を記した。
【００９０】
以上のいずれの場合にも、それぞれの温度範囲で少なくとも１パス以上についての摩擦係数が０．２以下の場合には、「潤滑」の欄に「○」、全パスにおける摩擦係数が０．２超の場合には「△」を記した。熱延後鋼種、「Ｃ−３」は、室温まで５０℃／ｓで冷却後、６５０℃で回復熱処理を施した。それ以外は全て２５０℃以上、前記（１）式で求まるＴｏ温度以下で巻き取った。
【００９１】
このような熱延鋼板を１．４ｍｍ厚に冷延する場合、冷延圧下率が８０％以上の場合には「冷延圧下率」を「×」とし、「８０％未満」の場合に「○」とした。また、焼鈍温度が６００℃以上（Ａｃ３＋１００）℃以下の場合は「焼鈍温度」を「○」とし、それ以外の場合を「×」とした。製造の条件として関係のない項目は「―」とした。熱延鋼板および冷延鋼板のいずれに対してもスキンパス圧延を０．５〜１．５％の範囲で施した。
【００９２】
Ｘ線の測定は、鋼板の代表値として板厚の７／１６厚の位置で板面に平行なサンプルを調整して、実施した。
【００９３】
表３に、前記の方法によって製造した１．４ｍｍ厚の熱延鋼板と冷延鋼板の機械的特性値を、表４に、Ｘ線で測定したランダム強度比、寸法精度、スプリング・バック量、壁そり量を示した。表３および表４中の鋼種Ｌを除いた全鋼種において、各鋼種の「−２」および「−３」の番号が本発明の実施例である。これらは、発明外の「−１」と「−４」の番号のものに比べて、スプリング・バック量と壁そり量が小さく、寸法精度が向上していることがわかる。
【００９４】
また、図４に、表３および表４に示した引張強度と寸法精度の関係をグラフにして示す。この図からも明らかなように、いずれの強度レベルにおいても、本発明で規定される各結晶方位のＸ線ランダム強度比とｒ値を満たして、初めて良好な薄鋼板の形状凍結性を達成することができるのである。
【００９５】
各結晶方位のＸ線ランダム強度比やｒ値が形状凍結性に重要であることの機構については、現在のところ必ずしも明らかとはなっていない。おそらく、曲げ変形時にすべり変形の進行を容易にすることで、結果的に曲げ変形時のスプリング・バック量や壁そり量が小さくなり、寸法精度が向上するものと理解される。
【００９６】
【表３】

【００９７】
【表４】

【００９８】
【発明の効果】
薄鋼板の集合組織とｒ値を制御すると、その曲げ加工性は著しく向上することを以上に詳述した。本発明によって、ハット型成形のような曲げ加工を主体とする加工を行った際に、スプリング・バック量および側壁部の壁そり量が少なく、形状凍結性に優れた薄鋼板を提供することができる。特に、従来は形状不良の問題から高強度鋼板の適用が難しかった部品にも高強度鋼板を使用できるようになる。自動車の軽量化を推進するためには、高強度鋼板の使用は是非とも必要である。スプリング・バック量や壁そり量が少なく、形状凍結性に優れた高強度鋼板を適用できるようになると、自動車車体の軽量化をより一層推進することができる。従って、本発明は、工業的に極めて高い価値のある発明である。
【図面の簡単な説明】
【図１】ハット曲げ試験に用いた試験片の断面図である。
【図２】スプリングバック量とＢＨＦ（しわ押さえ力）の関係を示す図である。
【図３】壁そり量とＢＨＦ（しわ押さえ力）の関係を示す図である。
【図４】引張強度と寸法精度の関係を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a ferritic thin steel sheet (hereinafter, also simply referred to as a steel sheet or a thin steel sheet) having excellent shape freezing properties, mainly bending, and is mainly used for automobile parts.
[0002]
[Prior art]
In order to reduce carbon dioxide emissions from automobiles, the weight of automobile bodies is being reduced using high-strength steel sheets. In addition, in order to ensure the safety of passengers, high strength steel plates are often used in automobile bodies in addition to mild steel plates. Furthermore, in order to further reduce the weight of automobile bodies in the future, new demands for increasing the strength level of use of high-strength steel sheets are increasing.
[0003]
However, when bending deformation is applied to a high-strength steel plate, the shape after processing is high-strength, so the spring-back phenomenon that makes it easy to return to the direction of the shape before processing away from the shape of the processing jig, The wall warpage phenomenon that the flat surface of the side wall becomes a curved surface due to the elastic recovery from the bending back occurs, resulting in a dimensional accuracy defect in which the shape of the target processed part cannot be obtained.
[0004]
Therefore, the conventional automobile body has been mainly used only for high-strength steel sheets of 440 MPa or less. For automobile bodies, there is no high-strength steel sheet with less spring back and wall warpage and good shape freezing, although it is necessary to reduce the weight of the body by using a high-strength steel sheet of 490 MPa or higher. This is the actual situation.
[0005]
Needless to add, it is needless to say that increasing the shape freezing property after processing of a high-strength steel plate or mild steel plate of 440 MPa or less is extremely important for improving the shape accuracy of products such as automobiles and home appliances. .
[0006]
Japanese Patent Application Laid-Open No. 10-72644 discloses a spring back amount (dimensional accuracy in the present invention) characterized in that the accumulation degree of {200} texture in a plane parallel to the rolling surface is 1.5 or more. An austenitic stainless cold-rolled steel sheet having a small size is disclosed. However, there is no description about a technique for reducing the amount of spring back of the ferritic steel sheet.
[0007]
As a technique for reducing the amount of springback of ferritic stainless steel, Japanese Patent Laid-Open No. 2001-32050 discloses a reflection X-ray intensity ratio of a {100} plane parallel to the plate surface in the texture at the center of the plate thickness. Two or more inventions are disclosed. However, in the present invention, there is no description regarding the reduction of wall warpage, and there is no description regarding the average X-ray random intensity ratio as {100} <011> to {223} <110> orientation groups. Also, the range of the components is completely different from the present invention.
[0008]
Further, some of the present inventors disclosed a ferritic thin steel sheet in which the ratio of the {100} plane to the {111} plane is 1 or more for the purpose of improving the shape freezing property in WO 00/066791. However, there is no disclosure regarding the reduction of wall warp, and therefore, the {100} <011> to {223} <110> orientation groups and the {100} <110> X-ray random intensity ratio are also not disclosed.
[0009]
Also, some of the present inventors disclosed in Japanese Patent Laid-Open No. 2001-64750 as a technique for reducing the amount of springback, a cold X-ray intensity ratio of the {100} plane parallel to the plate surface is 3 or more. Although the rolled steel sheet has been disclosed, the present invention is characterized by the definition of the X-ray intensity ratio at the outermost surface of the sheet thickness, and is completely different from the present invention.
[0010]
Japanese Patent Application Laid-Open No. 2000-297349 discloses a hot-rolled steel sheet having an r-value in-plane anisotropy Δr of 0.2 or less as a steel sheet having a good shape freezing property. However, the present invention is characterized in that the shape freezing property is improved by lowering the yield ratio, and in the above publication, the texture control for the purpose of improving the shape freezing property based on the technical idea of the present invention is described. Not listed.
[0011]
[Problems to be solved by the invention]
When bending a mild steel plate or a high-strength steel plate, a large spring back or wall warp occurs depending on the strength of the steel plate, and the shape freezing property of the formed part is poor. The present invention fundamentally solves this problem and provides a ferritic thin steel sheet having excellent shape freezing properties.
[0012]
[Means for Solving the Problems]
According to the conventional knowledge, it was considered to be important for the time being to lower the deformation stress of the steel sheet as a measure for suppressing the spring back and the wall warp. And in order to make a deformation stress low, the steel plate with low yield strength and tensile strength had to be used. However, this alone does not provide a fundamental solution for improving the bending workability of the steel sheet and keeping the amount of spring back and wall warpage low.
[0013]
Therefore, in order to improve the bending workability and fundamentally solve the occurrence of springback and wall warpage, the present inventors have newly focused on the influence of the texture of the steel sheet on the bending workability. The effect was investigated and studied in detail. And the steel plate excellent in bending workability was found.
[0014]
As a result, the {100} <011> to {223} <110> orientation groups and the {100} <011>, {554} <225>, {111} <112>, {111} <110> orientations. It is clearly shown that bending workability is drastically improved by controlling the strength, and by setting at least one of the r value in the rolling direction and the r value in the direction perpendicular to the rolling direction as low as possible. did.
[0015]
This invention is comprised based on the said knowledge, The place made into the main point is as follows.
(1) In mass%,
C: 0.0001% or more, 0.25% or less,
Si: 0.001% or more, 2.5% or less,
Mn: 0.01% or more, 2.5% or less,
P: 0.2% or less,
S: 0.03% or less,
Al: 0.01% or more, 2.0% or less,
N: 0.01% or less,
O: 0.01% or less
Contain, with the remainder being iron and unavoidable impurities, the average value of the {100} <011> - {223} <110> orientation component group X-ray random intensity ratio of plate surface at 1/2 sheet thickness is 3.0 As described above, among these orientation groups, the X-ray random intensity ratio of {100} <011> orientation is maximum and satisfies 4.0 or more, and {554} <225>, {111} <112 > And {111} <110>, the average value of the X-ray random intensity ratio of the three crystal orientations is 3.5 or less, and in addition, at least of the r value in the rolling direction and the r value in the direction perpendicular to the rolling direction A ferritic thin steel sheet excellent in shape freezing property, wherein one is 0.7 or less.
[0016]
( 2 ) Furthermore, in mass%,
Ti: 0.2% or less,
Nb: 0.2% or less,
V: 0.2% or less,
Cr: 1.5% or less,
B: Ferritic thin steel sheet having excellent shape freezing property as described in ( 1 ) above, containing one or more of 0.007% or less.
[0017]
( 3 ) Furthermore, in mass%,
Mo: 1% or less,
Cu: 2% or less,
Ni: 1% or less,
Sn: A ferritic thin steel sheet having excellent shape freezing property as described in ( 1 ) or ( 2 ) above, containing one or more of 0.2% or less.
(4) Furthermore, in mass%,
Ca: 0.01% or less
The ferritic thin steel sheet having excellent shape freezing property according to any one of the above (1) to (3).
[0018]
(5) A ferritic thin steel sheet having excellent shape freezing property, wherein the steel sheet according to any one of (1) to (4) is plated.
[0019]
(6) When hot-rolling the steel slab comprising the chemical component according to any one of (1) to (4), the total rolling reduction at (Ar ₃ +50) to (Ar ₃ +150) ° C. is 25%. As described above , the effective strain amount ε * calculated by the equation (2) is 0.4 or more , and the total rolling reduction at (Ar ₃ −100) to (Ar ₃ +50) ° C. is subsequently 5 to 35%. To (Ar ₃ -100) to (Ar ₃ +50) ° C. and finished at the critical temperature To (° C.) determined by the chemical composition (mass%) of the steel shown in formula (1). A method for producing a ferritic thin steel sheet having excellent shape freezing property, characterized by being wound up by a wire.
To = −650.4 × C% + B (1)
here,
B = −50.6 × Mneq + 894.3
Mneq = Mn% + 0.24 × Ni% + 0.13 × Si%
+ 0.38 × Mo% + 0.55 × Cr% + 0.16 × Cu%
-0.50 x Al% -0.45 x Co% + 0.90 x V%
[Expression 2]

Here, n is the number of rolling stands for finish hot rolling, ε _i is the strain applied at the i-th stand, t _i is the travel time (seconds) between the _{i to i} + 1th stands, and τ _i is the gas constant R ( = 1.987) and the rolling temperature Ti (K) of the i-th stand can be calculated by the following equation.
τ _i = 8.46 × 10 ⁻⁹ · exp {(43800 / R) / Ti}
[0020]
(7) Finish hot rolling, cool to below critical temperature To (° C.) determined by the chemical composition (mass%) of steel shown in formula (1), and then heat to 300 ° C. to Ac ₁ transformation temperature (° C.) The method for producing a ferritic thin steel sheet having excellent shape freezing properties as described in (6) above.
[0022]
( 8 ) The above (6) or (7), wherein at least one pass or more is rolled at a friction coefficient of 0.2 or less at (Ar ₃ −100) to (Ar ₃ +150) ° C. A method for producing a ferritic thin steel sheet having excellent shape freezing property as described in 1.
[0023]
( 9 ) After pickling the ferritic steel sheet according to any one of (6) to ( 8 ) and performing cold rolling with a rolling reduction of less than 80%, the temperature is reduced to 600 to (Ac ₃ +100) ° C. A method for producing a ferritic thin steel sheet having excellent shape freezing property, characterized by heating and cooling.
[0024]
DETAILED DESCRIPTION OF THE INVENTION
The contents of the present invention will be described in detail below. First, the X-ray random intensity ratio and the r value will be described.
(A) X-ray random intensity ratio of {100} <011> to {223} <110> orientation group on the plate surface at 1/2 plate thickness, X-ray random intensity ratio of {100} <011> orientation And the average value of the X-ray random intensity ratio of the three crystal orientations of {554} <225>, {111} <112> and {111} <110>:
This average value is a particularly important characteristic value in the present invention. The average value in the {100} <011> to {223} <110> azimuth group when the X-ray diffraction of the plate surface at the plate thickness center position and the intensity ratio of each azimuth relative to the random sample is obtained is 3. Must be greater than or equal to zero. If this is less than 3.0, shape freezing property will be inferior.
[0025]
The main orientations included in this orientation group are {100} <011>, {116} <110>, {114} <110>, {113} <110>, {112} <110>, {335} <110> and {223} <110>.
[0026]
The X-ray random intensity ratio of each of these orientations can be calculated by using a three-dimensional texture calculated by the vector method based on the {110} pole figure, and a plurality of {110}, {100}, {211}, {310} pole figures. What is necessary is just to obtain | require from the three-dimensional texture calculated by the series expansion method using the pole figure (preferably three or more).
[0027]
For example, the X-ray random intensity ratio of each crystal orientation in the latter method includes (001) [1-10], (116) [1-10], (114 in the φ2 = 45 ° cross section of the three-dimensional texture. ) [1-10], (113) [1-10], (112) [1-10], (335) [1-10], (223) [1-10] may be used as they are.
[0028]
The average value of {100} <011> to {223} <110> azimuth group is an arithmetic average of the intensity of each azimuth described above. When the strengths of all the above directions cannot be obtained, {100} <011>, {116} <110>, {114} <110>, {112} <110>, {223} <110> Alternatively, an arithmetic average of the intensity in each direction may be substituted. The {100} <011> orientation is a particularly effective orientation for reducing springback and wall warpage.
[0029]
Therefore, the X-ray random intensity in the {100} <011> orientation must be at most 4.0 or more in the {100} <011> to {223} <110> orientation groups. If this is less than 4.0, a sufficient amount of reduction in springback or wall warp cannot be obtained, and it becomes difficult to ensure extremely good shape freezing properties.
[0030]
Here, described {100} <011> orientation, as the range of orientation having similar effects, the rolling Direction as a rotation axis, allowing the inclination of the range of 12 °. This angle, more desirably, to 6 °.
[0031]
Furthermore, the average value of the X-ray random intensity ratio of the three crystal orientations of {554} <225>, {111} <112> and {111} <110> on the plate surface at 1/2 plate thickness is 3.5 or less. It must be. If this exceeds 3.5, it will be difficult to obtain good shape freezing properties even if the strength of the {100} <011> to {223} <110> orientation groups is appropriate.
[0032]
The X-ray random intensity ratios of {554} <225>, {111} <112> and {111} <110> may be obtained from the three-dimensional texture calculated according to the above method.
[0033]
More preferably, the average X-ray random intensity ratio of the {100} <011> to {223} <110> orientation groups is 4.0 or more, and the X-ray random intensity ratio of {100} <011> is 5.0. As mentioned above, the arithmetic mean value of the X-ray random intensity ratio of {554} <225>, {111} <112> and {111} <110> is less than 2.5.
[0034]
The reason why the X-ray intensity of the crystal orientation described above is important for the shape freezing property during bending is not necessarily clear, but it is presumed to be related to the sliding behavior of the crystal during bending deformation. The
[0035]
The sample to be subjected to X-ray diffraction is thinned to a predetermined plate thickness by mechanical polishing or the like, and then the distortion is removed by chemical polishing or electrolytic polishing, and at the same time, the 1/2 plate thickness becomes the measurement surface. To make. When there is a segregation zone or a defect in the thickness center layer of the steel sheet, which causes inconvenience in measurement, the above-described surface is set so that an appropriate surface becomes the measurement surface in the range of 3/8 to 5/8 of the plate thickness. The sample may be adjusted according to the above method.
[0036]
As a matter of course, the above-mentioned limitation of the X-ray intensity is satisfied not only in the vicinity of the plate thickness ½ but also as much as possible, so that the shape freezing property is further improved. The crystal orientation represented by {hkl} <uvw> indicates that the normal direction of the plate surface is parallel to <hkl> and the rolling direction is parallel to <uvw>.
[0037]
(B) r value (rL) in the rolling direction and r value (rC) in the direction perpendicular to the rolling direction:
These r values are important in the present invention. That is, as a result of intensive studies by the present inventors, it has been found that even if the X-ray intensities of the various crystal orientations described above are appropriate, good shape freezing properties cannot always be obtained. At the same time as the above X-ray intensity, it is essential that at least one of rL and rC is 0.7 or less. More preferably, it is 0.55 or less.
[0038]
The lower limit of rL and rC is not particularly defined, and the effect of the present invention can be obtained. The r value is evaluated by a tensile test using a JIS No. 5 tensile test piece. The tensile strain is usually 15%. However, when the uniform elongation is less than 15%, the strain may be evaluated as close to 15% as possible within the range of uniform elongation.
[0039]
The direction in which the bending process is performed is not particularly limited because it varies depending on the processed part. However, it is preferable that the bending process is mainly performed in a direction that is perpendicular or nearly perpendicular to the direction in which the r value is small.
[0040]
Incidentally, it is generally known that there is a correlation between the texture and the r value. However, in the present invention, the above-mentioned definition regarding the X-ray intensity ratio of the crystal orientation and the definition regarding the r value are not synonymous with each other, If both regulations are satisfied at the same time, good shape freezing property cannot be obtained.
[0041]
The present invention can be applied to all thin steel sheets ranging from a mild steel sheet having a low tensile strength level to a high-strength steel sheet, and the bending workability of the thin steel sheet is remarkably improved if the above rules are satisfied. In other words, the above definition is a basic material index related to bending deformation that exceeds the constraints of the mechanical strength level of the thin steel plate.
[0042]
In the case of a thin steel plate, the above-mentioned provision can be applied universally, and thus it is basically unnecessary to limit the type of the thin steel plate. And of course, the distinction between a hot-rolled steel sheet and a cold-rolled steel sheet is not questioned at all.
[0043]
Next, the reasons for limiting chemical components will be described.
[0044]
The reason why the lower limit of C is set to 0.0001% is that the lower limit value obtained from practical steel is used. If C exceeds 0.25%, workability and weldability deteriorate, so the upper limit is set to 0.25%.
[0045]
Si is an effective element for increasing the mechanical strength of the steel sheet, but if it exceeds 2.5%, workability deteriorates or surface flaws occur, so 2.5% is made the upper limit. On the other hand, since it is difficult to make Si less than 0.001% in practical steel, 0.001% is made the lower limit.
[0046]
Mn is also an effective element for increasing the mechanical strength of the steel sheet, but if it exceeds 2.5%, the workability deteriorates, so 2.5% is made the upper limit. On the other hand, since it is difficult to make Mn less than 0.01% in practical steel, 0.01% is made the lower limit. In addition to Mn, when an element such as Ti that suppresses the occurrence of hot cracking due to S is not sufficiently added, it is desirable to add an amount of Mn that satisfies Mn / S ≧ 20 by mass%.
[0047]
P and S are 0.2% or less and 0.03% or less, respectively. This is for preventing deterioration of workability and cracking during hot rolling or cold rolling.
[0048]
Al is added in an amount of 0.01% or more for deoxidation. Moreover, since Al significantly raises the γ → α transformation point, it is an effective element particularly when directing hot rolling below the Ar ₃ point. However, if the amount is too large, the workability deteriorates or the surface properties become poor, so the upper limit is made 2.0%.
[0049]
N and O are impurities, and are 0.01% or less and 0.01% or less, respectively, so as not to deteriorate the workability.
[0050]
Ti, Nb, V, Cr, and B improve the material through mechanisms such as carbon and nitrogen fixation, precipitation strengthening, structure control, and fine grain strengthening, so that 0.005% or more, 0, respectively, as necessary. It is desirable to add 0.001% or more, 0.001% or more, 0.01% or more, or 0.0001% or more. However, even if added excessively, there is no remarkable effect, but rather the workability and surface properties are deteriorated, so an upper limit was set for each. The upper limit is Ti: 0.2%, Nb: 0.2%, V: 0.2%, Cr: 1.5%, B: 0.007%.
[0051]
Since Mo, Cu, Ni, and Sn have the effect of increasing mechanical strength and improving the material, it is desirable to add 0.001% or more for each component as necessary. However, since excessive addition conversely degrades workability, the upper limit is set to Mo: 1%, Cu: 2%, Ni: 1%, Sn: 0.2%.
[0052]
In the present invention, may be added more than 0.01% of interest in Ca and Mg in the form control objects and sulfides deoxidation.
[0053]
The type of plating is not particularly limited, and the effect of the present invention can be obtained by any of electroplating, hot dipping, vapor deposition plating and the like.
[0054]
Next, the manufacturing method of this invention thin steel plate is demonstrated.
[0055]
The manufacturing method preceding hot rolling is not particularly limited. That is, various secondary smelting may be performed following smelting in a blast furnace, electric furnace, etc., and then cast by a method such as thin continuous slab casting in addition to normal continuous casting and casting by an ingot method. In the case of continuous casting, after cooling to a low temperature once, it may be heated again and then hot rolled, or the cast slab may be continuously hot rolled. Scrap may be used as a raw material.
[0056]
The ferritic thin steel sheet having excellent shape freezing property of the present invention is obtained by casting a steel having the above chemical components, after cooling after hot rolling, after cooling after hot rolling, or after heat treatment after pickling. It can also be obtained by annealing after cooling, pickling and cold rolling after rolling, or by subjecting these steel sheets to surface treatment with heat treatment in hot-dip or cold-rolled steel sheets, and further subjecting these steel sheets to surface treatment. It is done.
[0057]
In the latter half of the hot rolling, if rolling of 25% or more in total is not performed at (Ar ₃ +50) ° C. or more and (Ar ₃ +150) ° C. or less, the austenite processing is insufficient and the texture is not sufficiently developed. Therefore, no matter what cooling is performed, each crystal orientation of the predetermined X-ray intensity level defined in the invention of (1) above cannot be obtained on the plate surface of the finally obtained hot-rolled steel sheet. The lower limit of the total rolling reduction at (Ar ₃ +50) to (Ar ₃ +150) ° C. was 25%.
[0058]
The higher the total rolling reduction at (Ar ₃ +50) to (Ar ₃ +150) ° C., the sharper the texture formation is expected. Therefore, the total rolling reduction is preferably 35% or more. If it exceeds 0.5%, it is necessary to excessively increase the rigidity of the rolling mill, resulting in economic disadvantages.
[0059]
In order to remarkably increase the texture accumulation in the {100} <011> orientation, it is extremely important to further apply a reduction of 5 to 35% at (Ar ₃ −100) to (Ar ₃ +50) ° C. .
[0060]
This is because, when austenite sufficiently processed in a high temperature region is at least partially recrystallized, an appropriate amount of reduction is further applied and immediately after that, ferrite transformation is performed in the {100} <011> orientation. It is very important for development. Therefore, even if the rolling is performed at a temperature lower than (Ar ₃ −100) ° C., {100} <011> does not develop because the region where the ferrite transformation has been completed is too large.
[0061]
When the reduction is applied above (Ar ₃ +50) ° C., the strain introduced before the ferrite transformation is recovered, and {100} <011> does not develop. When the rolling reduction is less than 5%, the entire texture including {100} <011> to {223} <110> is randomized. On the other hand, when the rolling reduction exceeds 35%, {100} <011 > Because accumulation in the orientation becomes low, the rolling reduction in the temperature range of (Ar ₃ -100) to (Ar ₃ +50) ° C. is set to 5 to 35%. From the above viewpoint, the rolling reduction is desirably 10 to 25%.
[0062]
Hot rolling ends in a temperature range of (Ar ₃ -100) to (Ar ₃ +50) ° C. When the hot rolling end temperature is less than (Ar ₃ -100), the workability is remarkably deteriorated, and when it exceeds (Ar ₃ +50) ° C., the texture freezing property is deteriorated due to insufficient accumulation of the texture.
[0063]
In the hot rolling process, the cumulative effect of strain applied by a multi-stage rolling stand is important. However, the cumulative effect of this strain decreases as the processing temperature increases and the traveling time between stands increases. When finishing hot rolling is performed in n stands, the rolling temperature in the i-th stand is Ti (K), the working strain is ε _i (the true strain and the i-th rolling reduction r _i is ε _i = ln { 1 / (1-r _i )}), and the traveling time (inter-pass time: second) between the i-th and i + 1-th stands is t _i , the strain considering the cumulative effect (effective strain ε ^* ) Can be expressed by equation (2) obtained through experiments.
[0064]
[Equation 3]

Here, τ _i can be calculated by the following equation using the gas constant R (R = 1.987) and the rolling temperature Ti.
τ _i = 8.46 × 10 ⁻⁹ · exp { ( 43800 / R ) / Ti}
[0065]
When this effective strain ε ^* is less than 0.4, the texture cannot be remarkably accumulated. Therefore, the effective strain ε ^* is set to 0.4 or more.
[0066]
In the actual hot rolling process, when the calculation of the formula (2) is performed, Ti uses the finish heat extension side temperature FT ₀ and the finish heat extension side temperature FTn, and the formula Ti = FT ₀ − ( A value calculated by FT ₀ −FTn) / (n + 1) × (i + 1) may be used. Since the texture develops as the effective strain increases, the effective strain is more preferably 0.45 or more. Further, it is more preferable that the effective strain is 0.9 or more.
[0067]
Here, in the temperature range of (Ar ₃ −100) to (Ar ₃ +150) ° C., when the friction coefficient between the hot rolling roll and the steel sheet exceeds 0.2, , The crystal orientation mainly of the {110} plane develops, and the shape freezing property deteriorates. Therefore, when aiming at better shape freezing property, at least one pass during hot rolling is a hot rolling roll. It is desirable that the coefficient of friction between the steel plate and the steel plate be 0.2 or less.
[0068]
The lower the coefficient of friction, the better. The lower limit is not set, but when a better shape freezing property is required, the coefficient of friction is desirably 0.15 or less. As is conventionally known, the friction coefficient is obtained from the advanced rate during rolling and the rolling load.
[0069]
In order to transfer the austenite texture formed in this way to the final hot-rolled steel sheet, it is necessary to cool or wind it to To (° C.) or lower.
[0070]
This To (° C.) is thermodynamically defined as the temperature at which austenite and ferrite of the same component as austenite have the same free energy, and the above formula (1) is used in consideration of the influence of components other than C. Thus, the chemical composition (mass%) of the steel sheet can be simply calculated. Since the influence of the components other than those specified in the present invention on To (° C.) is not so great, they are ignored here.
[0071]

[0072]
In addition, the lower limit of the coiling temperature or the cooling stop temperature is not particularly limited. However, even if the temperature is lower than 250 ° C., the workability is deteriorated and a remarkable effect cannot be obtained. It is desirable to stop.
In the case of cooling, the texture becomes sharper as the cooling rate increases, and therefore the cooling rate is preferably 10 ° C./s or more.
[0073]
If the as-processed ferrite remains after cooling, the mechanical properties deteriorate. Therefore, although it is preferable to perform an additional heat treatment for the purpose of recovery / recrystallization, the temperature range is 300 ° C. to Ac ₁ transformation temperature (° C.). When the heat treatment temperature is less than 300 ° C., recovery / recrystallization does not proceed and mechanical properties deteriorate. On the other hand, when the Ac ₁ transformation temperature is exceeded, the texture formed during hot rolling is broken and the shape freezing property is deteriorated.
[0074]
In hot rolling, sheet bars may be joined after rough rolling, and finish rolling may be performed continuously. At that time, the coarse bar may be wound once in a coil shape, stored in a cover having a heat retaining function as necessary, and rewound again before joining. The hot-rolled steel sheet may be subjected to skin pass rolling as necessary. Needless to say, the skin pass rolling has the effect of preventing stretcher strain generated during processing and shape correction.
[0075]
When the hot-rolled steel sheet (or heat-treated hot-rolled steel sheet) thus obtained is cold-rolled and annealed to obtain a final thin steel sheet, the total rolling reduction of cold-rolling is 80% or more. In this case, the {111} plane and {554} plane components of the X-ray diffraction integral plane intensity ratio of the crystal plane parallel to the plate surface, which is a general cold rolling-recrystallization texture, increase, and the present invention Therefore, the upper limit of the cold rolling reduction is set to less than 80%.
[0076]
In order to increase the shape freezing property, it is desirable to limit the cold rolling reduction to 70% or less. Although the lower limit of the cold rolling reduction is not particularly defined, the effects of the present invention can be obtained. However, in order to control the crystal orientation strength within an appropriate range, it is preferably 3% or more.
[0077]
When annealing a cold-rolled steel sheet cold worked in such a range, if the annealing temperature is less than 600 ° C., the processed structure remains and the formability deteriorates significantly, so the lower limit of the annealing temperature is 600 ° C. And On the other hand, when the annealing temperature is excessively high, the ferrite texture formed by recrystallization is randomized by austenite grain growth after transformation to austenite, and the finally obtained ferrite texture is also randomized. The
[0078]
In particular, when the annealing temperature exceeds (Ac ₃ +100) ° C., such a tendency becomes remarkable. Accordingly, the annealing temperature is set to (Ac ₃ +100) ° C. or lower. The cold-rolled steel sheet may be subjected to skin pass rolling as necessary.
[0079]
The structure obtained by the present invention is mainly composed of ferrite, but as a metal structure other than ferrite, a compound such as pearlite, bainite, martensite, austenite and / or carbonitride may be contained. In particular, since the crystal structure of martensite and bainite is equivalent to or similar to that of ferrite, these structures may be mainly used instead of ferrite.
[0080]
Note that the steel sheet according to the present invention can be applied not only to bending, but also to composite forming mainly composed of bending, such as bending, overhanging and drawing.
[0081]
【Example】
(Example)
The technical contents of the present invention will be described with reference to examples of the present invention.
[0082]
As an Example, the result examined using the steel types A to L having the composition shown in Table 1 will be described. After casting, these steels are reheated as they are or once cooled to room temperature, heated to 900-1300 ° C., and then hot-rolled, and finally 1.4 mm, 3.0 mm or 8 A hot-rolled steel sheet having a thickness of 0.0 mm was obtained.
[0083]
The 3.0 mm and 8.0 mm thick hot rolled steel sheets were cold rolled to a thickness of 1.4 mm and then annealed in a continuous annealing process.
[0084]
Test pieces having a width of 50 mm and a length of 270 mm were prepared from these 1.4 mm thick steel plates, and a hat bending test was performed using a die having a punch width of 78 mm, a punch shoulder R5, and a die shoulder R5.
[0085]
About the test piece which performed the bending test, as shown in FIG. 1, the shape of the center part of plate | board width was measured with the three-dimensional shape measuring apparatus, and the tangent of the point (1) and the point (2), and the point (3 ) And the point (4) tangent intersection angle minus 90 degrees, the average value on the left and right is the spring back amount, and the inverse of the curvature between points (3) and (5) is averaged on the left and right The shape freezing property was evaluated by taking the value obtained by multiplying the value 1000 times as the amount of wall warpage and subtracting the punch width from the length between the left and right points (5) as the dimensional accuracy. The bending was performed so that a polygonal line was inserted perpendicular to the direction of low r value.
[0086]
By the way, as shown in FIG. 2 and FIG. 3, the amount of springback and the amount of wall warp also change depending on BHF (wrinkle pressing force). The effect of the present invention does not change its tendency even if it is evaluated with any BHF, but when pressing an actual part with an actual machine, a very high BHF cannot be applied due to equipment limitations. A hat bending test of each steel type was performed at BHF 29 kN.
[0087]
[Table 1]

[0088]
[Table 2]

[0089]
Table 2 shows whether the manufacturing conditions of each steel sheet are within the scope of the present invention. In the “rolling ratio” column of “hot rolling condition 1”, when the total rolling reduction in the temperature range of (Ar ₃ +50) to (Ar ₃ +150) ° C. is 25% or more, “◯” When the total is less than 25%, “x” is marked, and the “effective strain ε ^* ” column of “hot rolling condition 1” indicates the effective strain in the temperature range of (Ar ₃ +50) to (Ar ₃ +150) ° C. The quantity ε ^* is noted. In the column of “Hot Rolling Condition 2”, “○” is indicated for the case where the total rolling reduction is 5 to 35% in the temperature range of (Ar ₃ -100) to (Ar ₃ +30) ° C. The case where it was not satisfied was marked with “x”.
[0090]
In any of the above cases, when the friction coefficient for at least one pass or more in each temperature range is 0.2 or less, “◯” is displayed in the “Lubrication” column, and the friction coefficient in all passes is 0.2. In the case of super, “△” is marked. The steel type “C-3” after hot rolling was subjected to recovery heat treatment at 650 ° C. after cooling to room temperature at 50 ° C./s. In all other cases, winding was performed at 250 ° C. or more and below the To temperature determined by the above formula (1).
[0091]
When such a hot-rolled steel sheet is cold-rolled to a thickness of 1.4 mm, when the cold-rolling reduction ratio is 80% or more, the “cold-rolling reduction ratio” is “x”, and when it is “less than 80%” ○ ”. In addition, when the annealing temperature was 600 ° C. or higher (Ac3 + 100) ° C. or lower, the “annealing temperature” was “◯”, and the other cases were “X”. Items that have no relation to manufacturing conditions were marked with “-”. Skin pass rolling was performed in a range of 0.5 to 1.5% for both hot-rolled steel sheets and cold-rolled steel sheets.
[0092]
X-ray measurement was performed by adjusting a sample parallel to the plate surface at a position of 7/16 thickness as a representative value of the steel plate.
[0093]
Table 3 shows the mechanical property values of a 1.4 mm thick hot-rolled steel sheet and cold-rolled steel sheet manufactured by the above method, and Table 4 shows the random strength ratio, dimensional accuracy, spring back amount measured by X-ray, The amount of wall warping was shown. In all steel types except for steel type L in Table 3 and Table 4, the numbers “−2” and “−3” of each steel type are examples of the present invention. It can be seen that the spring back amount and the wall warp amount are small and the dimensional accuracy is improved as compared with those of the numbers “−1” and “−4” outside the invention.
[0094]
FIG. 4 is a graph showing the relationship between tensile strength and dimensional accuracy shown in Tables 3 and 4. As is clear from this figure, at any strength level, the satisfactory shape freezing property of the thin steel sheet is achieved for the first time by satisfying the X-ray random intensity ratio and r value of each crystal orientation defined in the present invention. It can be done.
[0095]
The mechanism that the X-ray random intensity ratio and the r value of each crystal orientation are important for the shape freezing property is not always clear at present. Perhaps, it is understood that by facilitating the progress of slip deformation during bending deformation, the amount of spring back and wall warpage during bending deformation is reduced, resulting in improved dimensional accuracy.
[0096]
[Table 3]

[0097]
[Table 4]

[0098]
【The invention's effect】
It has been described in detail above that when the texture and r value of the thin steel plate are controlled, the bending workability is remarkably improved. According to the present invention, it is possible to provide a thin steel plate that has a small amount of spring back and a small amount of wall warpage of a side wall portion and has excellent shape freezing property when performing processing such as hat-shaped forming as a main component. it can. In particular, high strength steel sheets can be used for parts that have conventionally been difficult to apply high strength steel sheets due to the problem of shape defects. In order to promote the weight reduction of automobiles, the use of high-strength steel sheets is absolutely necessary. If a high-strength steel plate with a small amount of spring back and wall warpage and excellent shape freezing properties can be applied, the weight reduction of the automobile body can be further promoted. Therefore, the present invention is industrially extremely valuable.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view of a test piece used in a hat bending test.
FIG. 2 is a diagram showing a relationship between a springback amount and BHF (wrinkle pressing force).
FIG. 3 is a diagram showing the relationship between the amount of wall warpage and BHF (wrinkle holding force).
FIG. 4 is a diagram showing the relationship between tensile strength and dimensional accuracy.

Claims (9)

% By mass
C: 0.0001% or more, 0.25% or less,
Si: 0.001% or more, 2.5% or less,
Mn: 0.01% or more, 2.5% or less,
P: 0.2% or less,
S: 0.03% or less,
Al: 0.01% or more, 2.0% or less,
N: 0.01% or less,
O: 0.01% or less
Contain, with the remainder being iron and unavoidable impurities, the average value of the {100} <011> - {223} <110> orientation component group X-ray random intensity ratio of plate surface at 1/2 sheet thickness is 3.0 As described above, among these orientation groups, the X-ray random intensity ratio of {100} <011> orientation is maximum and satisfies 4.0 or more, and {554} <225>, {111} <112 > And {111} <110>, the average value of the X-ray random intensity ratio of the three crystal orientations is 3.5 or less, and in addition, at least of the r value in the rolling direction and the r value in the direction perpendicular to the rolling direction A ferritic thin steel sheet excellent in shape freezing property, wherein one is 0.7 or less. Furthermore, in mass%,
Ti: 0.2% or less,
Nb: 0.2% or less,
V: 0.2% or less,
Cr: 1.5% or less,
B: Excellent ferritic steel sheet in shape fixability according to claim 1, characterized in that it contains one or two or more 0.007% or less. Furthermore, in mass%,
Mo: 1% or less,
Cu: 2% or less,
Ni: 1% or less,
The ferritic thin steel sheet having excellent shape freezing property according to claim 1 or 2 , wherein Sn: 0.2% or less is contained. Furthermore, in mass%,
Ca: 0.01% or less
The ferritic thin steel sheet excellent in shape freezing property according to any one of claims 1 to 3.   A ferritic thin steel sheet having excellent shape freezing property, wherein the steel sheet according to any one of claims 1 to 4 is plated. When hot-rolling a steel slab comprising the chemical component according to any one of claims 1 to 4, the total rolling reduction at (Ar ₃ +50) to (Ar ₃ +150) ° C is 25% or more , (2 ) The effective strain amount ε * calculated by the formula is 0.4 or more , and subsequently, hot rolling is performed so that the total rolling reduction at (Ar ₃ −100) to (Ar ₃ +50) ° C. is 5 to 35%. Then, the hot rolling is finished at (Ar ₃ −100) to (Ar ₃ +50) ° C., and winding is performed at a critical temperature To (° C.) or less determined by the chemical composition (mass%) of the steel shown in the formula (1). A method for producing a ferritic thin steel sheet having excellent shape freezing properties, characterized by
To = −650.4 × C% + B (1)
here,
B = −50.6 × Mneq + 894.3
Mneq = Mn% + 0.24 × Ni% + 0.13 × Si%
+ 0.38 × Mo% + 0.55 × Cr% + 0.16 × Cu%
-0.50 x Al% -0.45 x Co% + 0.90 x V%

Here, n is the number of rolling stands for finish hot rolling, ε _i is the strain applied at the i-th stand, t _i is the travel time (seconds) between _{i to i} + 1th stands, and τ _i is the gas constant R ( = 1.987) and the rolling temperature Ti (K) of the i-th stand can be calculated by the following equation.
τ _i = 8.46 × 10 ⁻⁹ · exp {(43800 / R) / Ti} Finishing hot rolling, cooling to a critical temperature To (° C.) or less determined by the chemical composition (mass%) of the steel shown in the formula (1), and then heating to 300 ° C. to Ac ₁ transformation temperature (° C.). The method for producing a ferritic thin steel sheet having excellent shape freezing property according to claim 6. The shape freezing property according to claim 6 or 7 , wherein at least one pass or more is rolled at the (Ar ₃ -100) to (Ar ₃ +150) ° C so that the friction coefficient is 0.2 or less. Of excellent ferritic steel sheet. The ferritic thin steel sheet according to any one of claims 6 to 8 , which is pickled, cold-rolled with a rolling reduction of less than 80%, heated to 600 to (Ac ₃ +100) ° C, and cooled. A method for producing a ferritic thin steel sheet having excellent shape freezing properties.

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