JP3927384B2 - Thin steel sheet for automobiles with excellent notch fatigue strength and method for producing the same - Google Patents

Description

なお、Ｔ₀ に及ぼす本発明で規定した上記以外の成分の質量％の影響はそれほど大きくないので、ここでは無視できる。
【００７４】
また巻取温度の下限値は、その切り欠き疲労強度を向上させるという目的のためだけに鋼板のミクロ組織を特に限定する必要はないので、特に限定する必要はないが、コイルが長時間水濡れの状態にあると錆による外観不良が懸念されるため、５０℃以上が望ましい。
本発明において、切り欠き疲労強度の向上の他に良好なバーリング加工性を付与する目的で、ミクロ組織の体積分率最大の相をベイナイト，またはフェライトおよびベイナイトの複合組織、とするためには、巻取温度が４５０℃未満では、バーリング性に有害と考えられている残留オーステナイトまたはマルテンサイトが多量に生成する恐れがあり、目的とする体積率最大のミクロ組織であるベイナイト，またはフェライトおよびベイナイトからなる複合組織が得られないため、巻取温度は４５０℃以上と限定する。
さらに、巻取り後の冷却速度は特に限定しないが、Ｃｕを１．２％以上添加した場合、巻取り後にＣｕが析出して加工性が劣化するばかりでなく、疲労特性向上に有効な固溶状態のＣｕが失われる恐れがあるので、巻取り後の冷却速度は２００℃までを３０℃／ｓ以上とすることが望ましい。
【００７５】
また、本発明において切り欠き疲労強度の向上の他に良好な延性を付与する目的で、ミクロ組織を体積分率５％以上２５％以下の残留オーステナイトを含み、残部が主にフェライト、ベイナイトからなる複合組織とするためには、巻取温度が４５０℃以上では、炭化物を含むベイナイトが生成して十分な残留オーステナイトが得られず、目的とする体積分率５％以上２５％以下の残留オーステナイトを含み、残部が主にフェライト、ベイナイトからなるミクロ組織が得られないため、巻取温度は４５０℃未満と限定する。また巻取温度が３５０℃以下では、マルテンサイトが多量に生成して十分な残留オーステナイトが得られず、目的とする体積分率５％以上２５％以下の残留オーステナイトを含み、残部が主にフェライト、ベイナイトからなるミクロ組織が得られないため、巻取温度は３５０℃超と限定する。
さらに、巻取り後の冷却速度は特に限定しないが、Ｃｕを１％以上添加した場合、巻取り後にＣｕが析出して加工性が劣化するばかりでなく、疲労特性向上に有効な固溶状態のＣｕが失われる恐れがあるので、巻取り後の冷却速度は２００℃までを３０℃／ｓ以上とすることが望ましい。
【００７６】
さらに、本発明において切り欠き疲労強度の向上の他に良好な形状凍結性を得るための低降伏比を付与する目的で、ミクロ組織の体積分率最大の相をフェライトとし、第二相を主にマルテンサイトとする複合組織とするためには、巻取温度が３５０℃超では、ベイナイトが生成して十分なマルテンサイトが得られず、目的とするフェライトを体積分率最大の相とし、マルテンサイトを第二相とするミクロ組織が得られないため、巻取温度は３５０℃以下と限定する。また、巻取温度の下限値は特に限定する必要はないが、コイルが長時間水濡れの状態にあると錆による外観不良が懸念されるため、５０℃以上が望ましい。
熱間圧延工程終了後は必要に応じて酸洗し、その後インラインまたはオフラインで、圧下率１０％以下のスキンパスまたは圧下率４０％程度までの冷間圧延を施しても構わない。
【００７７】
次に、冷延鋼板として最終製品にする場合であるが、熱間での仕上げ圧延条件は特に限定しない。ただし、より良好な切り欠き疲労強度を得るためには、Ａｒ3 変態点温度＋１００℃以下の温度域での合計圧下率が２５％以上であることが望ましい。また、仕上げ圧延の最終パス温度（ＦＴ）はＡｒ3 変態点温度未満で終了しても差し支えないが、その場合は、圧延前もしくは圧延中に析出したフェライトに強い加工組織が残留するため、続く巻取処理または加熱処理により回復、再結晶させることが望ましい。
【００７８】
続く酸洗後の冷間圧延の合計圧下率は８０％未満とする。これは、冷間圧延の合計圧下率は８０％以上であると、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｛１１１｝面や｛５５４｝面のＸ線回折積分面強度比が高くなるためである。また、望ましくは７０％以下である。冷間圧延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％以上とすることが望ましい。
【００７９】
この様に冷間圧延された鋼板の熱処理は連続焼鈍工程を前提としている。
まず、Ａｃ3 変態点温度＋１００℃以下の温度域で５〜１５０秒間行う。この熱処理温度の上限がＡｃ3 変態点温度＋１００℃超では、再結晶によって生成したフェライトがオーステナイトへ変態し、オーステナイトの粒成長によっての集合組織がランダム化され、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てしまうので、熱処理の上限温度Ａｃ3 変態点温度＋１００℃以下とする。ここでＡｃ1 変態点温度およびＡｃ3 変態点温度とは、例えば「レスリー鉄鋼材科学」（１９８５年発行、熊井浩・野田龍彦訳、丸善株式会社）２７３頁に記載の計算式により、鋼成分との関係で示される。
一方、この熱処理温度の下限は、その切り欠き疲労強度を向上させるという目的のために鋼板のミクロ組織を特に限定する必要はないので、回復温度以上で構わないが、回復温度未満の場合には加工組織が残留し成形性を著しく劣化させるので、熱処理の下限温度は回復温度以上とする。また、この温度域での保持時間は、５秒未満では、セメンタイトが完全に再固溶するのに不十分であり、一方、１５０秒超の熱処理を行ってもその効果が飽和するばかりでなく生産性を低下させるので、保持時間は５〜１５０秒間とする。
【００８０】
その後の冷却条件については特に限定しないが、ミクロ組織を制御するために、必要に応じて以下の冷却または任意温度での保持および冷却を行ってもよい。本発明において、切り欠き疲労強度の向上の他に良好なバーリング加工性を付与する目的で、ミクロ組織の体積分率最大の相をベイナイト，またはフェライトおよびベイナイトの複合組織、とするためには、その熱処理温度の下限温度をＡｃ1 変態点温度以上とする。この下限温度がＡｃ1 変態点温度未満の場合には、目的とする体積分率最大の相をベイナイト，またはフェライトおよびベイナイトの複合組織、が得られない。ここで、バーリング性をそれほど劣化させずに延性との両立を目指す場合は、フェライトの体積分率を増加させるために、その温度域をＡｃ1 変態点温度以上Ａｃ3 変態点温度以下（フェライトとオーステナイトの二相域）の温度域とする。また、更に良好なバーリング性を得るためには、ベイナイトの体積分率を増加させるために、Ａｃ3 変態点温度以上Ａｃ3 変態点温度＋１００℃以下の温度域が望ましい。
【００８１】
次に、冷却工程については本発明で特に定めないが、前記熱処理温度がＡｃ1 変態点温度以上Ａｃ3 変態点温度以下の場合においては、２０℃／ｓ以上の冷却速度で３５０℃超前記Ｔ₀ 温度以下の温度域まで冷却することが望ましい。これは、冷却速度が２０℃／ｓ未満では、炭化物を多量に含むベイナイトもしくはパーライト変態のノーズにかかる恐れがあるためである。また、冷却終了温度は、３５０℃以下ではバーリング性に有害と考えられているマルテンサイトが多量に生成する恐れがあり、目的とする体積率最大のミクロ組織であるベイナイト，またはフェライトおよびベイナイトからなる複合組織が得られないため、３５０℃超が望ましい。さらに、前工程までに得られた集合組織を遺伝させるためにはＴ₀ 以下が望ましい。
【００８２】
最後に冷却工程の終了温度までの冷却速度は、２０℃／ｓ以上では冷却中にバーリング性に有害と考えられているマルテンサイトが多量に生成する恐れがあり、目的とする体積率最大のミクロ組織であるベイナイト，またはフェライトおよびベイナイトからなる複合組織が得られない恐れがあるので、２０℃／ｓ未満とすることが望ましい。また冷却工程の終了温度は、２００℃超では時効性が劣化する恐れがあるので、２００℃以下とすることが望ましい。また下限は、水冷もしくはミストで冷却する場合コイルが長時間水濡れの状態にあると、錆による外観不良が懸念されるため、５０℃以上が望ましい。
一方、前記熱処理温度がＡｃ3 変態点温度超Ａｃ3 変態点温度＋１００℃以下の場合においては、２０℃／ｓ以上の冷却速度で２００℃以下の温度まで冷却することが望ましい。これは、２０℃／ｓ以上では、炭化物を多量に含むベイナイトもしくはパーライト変態のノーズにかかる恐れがあるためである。また冷却終了温度は、２００℃超では時効性が劣化する恐れがあるので、２００℃以下が望ましい。下限は、水冷もしくはミストで冷却する場合、コイルが長時間水濡れの状態にあると錆による外観不良が懸念されるため、５０℃以上が望ましい。
【００８３】
また、本発明において切り欠き疲労強度の向上の他に良好な延性を付与する目的で、ミクロ組織を体積分率５％以上２５％以下の残留オーステナイトを含み、残部が主にフェライト、ベイナイトからなる複合組織とするためには、前記同様にＡｃ1 変態点温度以上Ａｃ3 変態点温度＋１００℃以下の温度域で５〜１５０秒間行う。このとき、その温度域内でも低温すぎると、熱延板段階でセメンタイトが析出していた場合、セメンタイトが再固溶するのに時間がかかりすぎ、高温すぎるとオーステナイトの体積率が大きくなりすぎて、オーステナイト中のＣ濃度が低下し炭化物を多量に含むベイナイトもしくはパーライト変態のノーズにかかりやすくなるため、７８０℃以上８５０℃以下で加熱するのが好ましい。保持後の冷却速度が２０℃／ｓ未満では、炭化物を多量に含むベイナイトもしくはパーライト変態のノーズにかかる恐れがあるため、２０℃／ｓ以上の冷却速度とする。
【００８４】
次に、ベイナイト変態を促進し必要な量の残留オーステナイトを安定化する工程であるが、冷却終了温度が４５０℃以上では、残留したオーステナイトが炭化物を多量に含むベイナイトまたはパーライトに分解してしまい、目的とする体積分率５％以上２５％以下の残留オーステナイトを含み、残部が主にフェライト、ベイナイトからなるミクロ組織が得られない。また３５０℃未満では、マルテンサイトが多量に生成する可能性があり十分な残留オーステナイトが得られず、目的とする体積分率５％以上２５％以下の残留オーステナイトを含み、残部が主にフェライト、ベイナイトからなるミクロ組織が得られないため、３５０℃超の温度域まで冷却する。
【００８５】
さらに、その温度域での保持時間であるが、５秒未満では残留オーステナイトを安定化するためのベイナイト変態が不十分であり、不安定な残留オーステナイトが続く冷却終了時にマルテンサイト変態する恐れがあり、目的とする体積分率５％以上２５％以下の残留オーステナイトを含み、残部が主にフェライト、ベイナイトからなるミクロ組織が得られない。また６００秒超ではベイナイト変態が促進しすぎて、必要な量の安定した残留オーステナイトを得ることができず、目的とする体積分率５％以上２５％以下の残留オーステナイトを含み、残部が主にフェライト、ベイナイトからなるミクロ組織が得られない。従って、その温度域での保持時間は５秒以上６００秒以下とする。
【００８６】
最後に冷却終了までの冷却速度は、５℃／ｓ未満では冷却中にベイナイト変態が促進しすぎる可能性があり、必要な量の安定した残留オーステナイトを得ることができず、目的とする体積分率５％以上２５％以下の残留オーステナイトを含み、残部が主にフェライト、ベイナイトからなるミクロ組織が得られない恐れがあるので、５℃／ｓ以上とする。
また冷却終了温度は、２００℃超では時効性が劣化する恐れがあるので、２００℃以下とする。冷却終了温度の下限については特に限定しないが、水冷もしくはミストで冷却する場合、コイルが長時間水濡れの状態にあると、錆による外観不良が懸念されるため、５０℃以上が望ましい。
【００８７】
さらに、本発明において切り欠き疲労強度の向上の他に良好な形状凍結性を得るための低降伏比を付与する目的で、ミクロ組織の体積分率最大の相をフェライトとし、第二相を主にマルテンサイトとする複合組織とするために、前記同様にＡｃ1 変態点温度以上Ａｃ3 変態点温度＋１００℃以下の温度域で５〜１５０秒間行う。このとき、その温度範囲内でも低温すぎると、熱延板段階でセメンタイトが析出していた場合、セメンタイトが再固溶するのに時間がかかりすぎ、高温すぎるとオーステナイトの体積率が大きくなりすぎて、オーステナイト中のＣ濃度が低下し、炭化物を多量に含むベイナイトもしくはパーライト変態のノーズにかかりやすくなるため、７８０℃以上８５０℃以下で加熱するのが好ましい。
【００８８】
保持後の冷却速度は、２０℃／ｓ未満では炭化物を多量に含むベイナイトもしくはパーライト変態のノーズにかかる恐れがあるため、２０℃／ｓ以上の冷却速度とする。冷却終了温度が３５０℃超では、目的とするフェライトを体積分率最大の相とし、マルテンサイトを第二相とするミクロ組織が得られないので、３５０℃以下の温度域まで冷却する。冷却工程の終了温度の下限については特に限定しないが、水冷もしくはミストで冷却する場合、コイルが長時間水濡れの状態にあると錆による外観不良が懸念されるため、５０℃以上が望ましい。
さらにその後、必要に応じてスキンパス圧延を施してもよい。
酸洗後の熱延鋼板、または上記の再結晶焼鈍終了後の冷延鋼板に亜鉛めっきを施すためには、亜鉛めっき浴中に浸漬し、必要に応じて合金化処理してもよい。
【００８９】
【実施例】
（実施例１）
以下に、実施例１により本発明をさらに説明する。
表１に示す化学成分を有するＡ〜Ｌの鋼は、転炉にて溶製して、連続鋳造後、再加熱し、粗圧延後に続く仕上げ圧延で１．２〜５．５ｍｍの板厚にした後に巻き取った。ただし、表中の化学組成についての表示は質量％である。
次に製造条件の詳細を表２に示す。ここで、「ＳＲＴ」はスラブ加熱温度、 「ＦＴ」は最終パス仕上げ圧延温度、「圧延率」とはＡｒ3 変態点温度＋１００℃以下の温度域での圧下率の合計を示す。ただし、後に冷延工程にて圧延を行う場合はこのような制限の限りではないので「―」とした。また、「潤滑」はＡｒ3 変態点温度＋１００℃以下の温度域での潤滑の有無を示した。
さらに「巻取」とは、巻取温度（ＣＴ）がＴ₀ 以下ならば「○」、Ｔ₀ 超の場合には「×」とした。ただし、冷延鋼板の場合は製造の条件として特に限定する必要がないので「―」とした。
【００９０】
次に、一部については熱間圧延後、酸洗、冷延、焼鈍を行った。板厚は０．７〜２．３ｍｍである。ここで、「冷延率」とは合計冷間圧延率、「Ｔｉｍｅ」は焼鈍時間、「焼鈍」とは、焼鈍温度が回復温度以上Ａｃ3 変態点温度＋１００℃以下の温度域に含まれていれば「○」、外れていれば「×」とした。なお、鋼Ｌについては粗圧延後に衝突圧２．７ＭＰａ、流量０．００１リットル／ｃｍ² の条件でデスケーリングを施した。一方、上記鋼板のうち鋼Ｇおよび鋼Ｆ−５については、亜鉛めっきを施した。
このようにして得られた熱延板の引張試験は、供試材を、まず、ＪＩＳ Ｚ ２２０１記載の５号試験片に加工し、ＪＩＳ Ｚ ２２４１記載の試験方法に従って行った。表２に降伏強度（σY ）、引張強度（σB ）、破断伸び（Ｅｌ）を併せて示す。
【００９１】
さらに、板幅の１／４Ｗもしくは３／４Ｗ位置より３０ｍｍφに切り取った試片の、最表層より０．０５ｍｍ程度の深さまで三山仕上の研削を行い、次いで化学研磨または電解研磨によって歪みを除去して作製し、「新版カリティＸ線回折要論」（１９８６年発行、松村源太郎訳、株式会社アグネ）２７４〜２９６頁に記載の方法に従ってＸ線回折強度の測定を行った。
ここで｛１００｝＜０１１＞〜｛２２３｝＜１１０＞方位群のＸ線ランダム強度比の平均値とは、この方位群に含まれる主な方位、｛１００｝＜０１１＞、 ｛１１６｝＜１１０＞、｛１１４｝＜１１０＞、｛１１３｝＜１１０＞、｛１１２｝＜１１０＞、｛３３５｝＜１１０＞および｛２２３｝＜１１０＞のＸ線回折強度を、｛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、または｛１１０｝、｛１００｝、｛２１１｝、｛３１０｝極点図のうち複数の極点図（望ましくは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た。
【００９２】
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ればよい。ただし｛１００｝＜０１１＞〜｛２２３｝＜１１０＞方位群のＸ線ランダム強度比の平均値とは、上記の各方位の相加平均である。
上記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＞の各方位の相加平均で代替してもよい。
次に｛５５４｝＜２２５＞、｛１１１｝＜１１２＞および｛１１１｝＜１１０＞の３方位のＸ線ランダム強度比の平均値とは、上記の方法と同様に計算した３次元集合組織から求めればよい。
【００９３】
表２において、Ｘ線ランダム強度比のうち「強度比１」とは、｛１００｝＜０１１＞〜｛２２３｝＜１１０＞方位群のＸ線ランダム強度比の平均値、「強度比２」とは｛５５４｝＜２２５＞、｛１１１｝＜１１２＞および｛１１１｝＜１１０＞の３方位のＸ線ランダム強度比の平均値である。
【００９４】
次に、上記鋼板の切り欠き疲労強度を調査するために、板幅の１／４Ｗもしくは３／４Ｗ位置から圧延方向が長辺になるように、図１（ｂ）に示す形状の疲労試験片を採取し疲労試験に供した。ただし、疲労試験片には最表層より０．０５ｍｍ程度の深さまで三山仕上の研削を施した。疲労試験は電気油圧サーボ型疲労試験機を用い、試験方法はＪＩＳ Ｚ ２２７３−１９７８およびＪＩＳ Ｚ ２２７５−１９７８に準じた。表２に切り欠き疲労限（σWK）、切り欠き疲労限度比（σWK／σB ）を併せて示す。
【００９５】
本発明に沿うものは、鋼Ａ，Ｅ，Ｆ−１，Ｆ−２，Ｆ−５，Ｇ，Ｈ，Ｉ，Ｊ，Ｋ，Ｌの１１鋼であり、所定の量の鋼成分を含有し、最表面から板厚方向に０．５ｍｍまでの任意深さにおける板面の｛１００｝＜０１１＞〜｛２２３｝＜１１０＞方位群のＸ線ランダム強度比の平均値が２以上かつ、｛５５４｝＜２２５＞、｛１１１｝＜１１２＞および｛１１１｝＜１１０＞の３方位のＸ線ランダム強度比の平均値が４以下であり、板厚が０．５ｍｍ以上１２ｍｍ以下であることを特徴とする切り欠き疲労強度に優れる自動車用薄鋼板が得られており、従って、本発明記載の方法によって評価した従来鋼の疲労限度比０．２〜０．３を上回っている。
【００９６】
上記以外の鋼は、以下の理由によって本発明の範囲外である。
すなわち、鋼Ｂは、Ｃの含有量が本願請求項５の範囲外であるので、十分な強度（σB ）が得られていない。鋼Ｃは、Ｐの含有量が本願請求項５の範囲外であるので、十分な切り欠き疲労強度（σWK／σB ）が得られていない。鋼Ｄは、Ｓの含有量が本願請求項５の範囲外であるので、十分な伸び（Ｅｌ）が得られていない。鋼Ｆ−３は、Ａｒ3 変態点温度＋１００℃以下の温度域での合計圧下率が本願請求項１２の範囲外であるので、請求項１記載の目的とする集合組織が得られず、十分な切り欠き疲労強度（σWK／σB ）が得られていない。
【００９７】
鋼Ｆ−４は、仕上圧延終了温度（ＦＴ）が本願請求項１２の範囲外で、かつ巻取温度も本願明細書記載の範囲外でかつ巻取温度も本願発明範囲外であるので、請求項１記載の目的とする集合組織が得られず、十分な切り欠き疲労強度（σWK／σB ）が得られていない。鋼Ｆ−６は、冷延率が本願請求項１８の範囲外であるので、請求項１記載の集合組織が得られず、十分な切り欠き疲労強度（σWK／σB ）が得られていない。鋼Ｆ−７は、焼鈍温度が本願請求項１８の範囲外であるので、請求項１記載の目的とする集合組織が得られず、十分な切り欠き疲労強度（σWK／σB ）が得られていない。鋼Ｆ−８は、焼鈍時間が本願請求項１８の範囲外であるので、請求項１記載の集合組織が得られず、十分な切り欠き疲労強度（σWK／σB ）が得られていない。
【００９８】
（実施例２）
次に、実施例２により本発明をさらに詳しく説明する。
表１に示す化学成分を有するＧ、Ｈの２鋼を表３に示す加熱温度で再加熱し、粗圧延後に続く仕上げ圧延で１．２〜５．５ｍｍの板厚にした後に巻き取った。また、表３に示すようにいくつかについては、粗圧延後に衝突圧２．７ＭＰａ、流量０．００１リットル／ｃｍ² の条件でデスケーリングを施した。
製造条件の詳細を表３に示す。ここで、「ＳＲＴ」はスラブ加熱温度、「ＦＴ」は最終パス仕上げ圧延温度、「圧延率」とはＡｒ3 変態点温度＋１００℃以下の温度域での圧下率の合計を示す。ただし、後に冷延工程にて圧延を行う場合はこのような制限の限りではないので「―」とした。また、「潤滑」はＡｒ3 変態点温度＋１００℃以下の温度域での潤滑の有無を示した。さらに「ＣＴ」とは巻取温度を示している。ただし、冷延鋼板の場合は製造の条件として特に限定する必要がないので「―」とした。次に、一部については熱間圧延後、酸洗、冷延、熱処理を行った。板厚は０．７〜２．３ｍｍである。「冷延率」とは合計冷間圧延率、「ＳＴ」とは、熱処理温度、「Ｔｉｍｅ」は熱処理時間である。なお、上記鋼板のうちいくつかについては、亜鉛めっきを施した。
【００９９】
このようにして得られた熱延板および冷延板の引張試験は、上記同様な方法にて実施した。
表４に降伏強度（σY ）、引張強度（σB ）、破断伸び（Ｅｌ）および降伏比（ＹR ）、強度−延性バランス（σB ×Ｅｌ）を示す。一方、バーリング加工性（穴拡げ性）については、日本鉄鋼連盟規格ＪＦＳ Ｔ １００１−１９９６記載の穴拡げ試験方法に従って評価した。表４に穴拡げ率（λ）を示す。
さらにミクロ組織についても表４に示す。ここで、その他とはパーライト、および／または表４に個別に示すフェライト、ベイナイト、残留オーステナイト、マルテンサイト以外の組織である。鋼板のミクロ組織において、フェライト、ベイナイト、残留オーステナイト、パーライト、マルテンサイトの体積分率とは、鋼板板幅の１／４Ｗもしくは３／４Ｗ位置より切出した試料を圧延方向断面に研磨し、ナイタール試薬および特開平５−１６３５９０号公報で開示されている試薬を用いてエッチングし、光学顕微鏡を用い２００〜５００倍の倍率で観察された板厚の１／４ｔにおけるミクロ組織の面積分率で定義される。
【０１００】
一方、オーステナイトはフェライトと結晶構造が違うため結晶学的に容易に識別できる。従って、残留オーステナイトの体積分率はＸ線回折法によっても実験的に求めることができる。すなわち、ＭｏのＫα線を用いてオーステナイトとフェライトとの反射面強度の違いより次式を用いてその体積分率を簡便に求める方法である。
Ｖγ＝（２／３）｛１００／（０．７×α（２１１）／γ（２２０）＋１）｝＋（１／３）｛１００／（０．７８×α（２１１）／γ（３１１）＋１）｝
ただし、α（２１１）、γ（２２０）およびγ（３１１）は、それぞれフェライト（α）オーステナイト（γ）のＸ線反射面強度である。残留オーステナイトの体積分率は、光学顕微鏡観察およびＸ線回折法のいずれの方法を用いてもほぼ一致した値が得られたので、いずれの測定値を用いても差し支えない。
さらに、前記と同様な方法に従ってＸ線回折強度の測定、疲労試験を行った。
また、疲労試験は前記と同様な方法に従って行った。表４に切り欠き疲労限 （σWK）、切り欠き疲労限度比（σWK／σB ）を示す。
【０１０１】
本発明に沿うものは、鋼ｇ−１、ｇ−２、ｇ−３、ｇ−５、ｇ−６、ｇ−７、ｈ−１、ｈ−２、ｈ−３の９鋼であり、所定の量の鋼成分を含有し、最表面から板厚方向に０．５ｍｍまでの任意深さにおける板面の｛１００｝＜０１１＞〜 ｛２２３｝＜１１０＞方位群のＸ線ランダム強度比の平均値が２以上かつ、｛５５４｝＜２２５＞、｛１１１｝＜１１２＞および｛１１１｝＜１１０＞の３方位のＸ線ランダム強度比の平均値が４以下、かつ板厚が０．５ｍｍ以上１２ｍｍ以下であり、かつ、体積分率最大の相をベイナイト，またはフェライトおよびベイナイトの複合組織、または、体積分率５％以上２５％以下の残留オーステナイトを含み、残部が主にフェライト、ベイナイトからなる複合組織、または、体積分率最大の相をフェライトとし、第二相を主にマルテンサイトとする複合組織であることを特徴とする切り欠き疲労強度に優れる自動車用薄鋼板が得られており、従って、本発明記載の方法によって評価した従来鋼の疲労限度比２０〜３０％に対して有意差が認められる。
【０１０２】
上記以外の鋼は、以下の理由によって本発明の範囲外である。
すなわち、鋼ｇ−４は、仕上圧延終了温度（ＦＴ）およびＡｒ3 変態点温度＋１００℃以下の温度域での合計圧下率が本発明請求項１２の範囲外であるので、請求項１記載の目的とする集合組織が得られず、十分な切り欠き疲労強度（σWKｋ／σB ）が得られていない。鋼ｇ−８は、冷延率が本発明請求項１３の範囲外であるので、請求項１記載の目的とする集合組織が得られず、十分な切り欠き疲労強度（σWK／σB ）が得られていない。鋼ｈ−４は、仕上圧延終了温度（ＦＴ）およびＡｒ3 変態点温度＋１００℃以下の温度域での合計圧下率が本発明請求項１２の範囲外であるので、請求項１記載の目的とする集合組織が得られず、十分な切り欠き疲労強度（σWK／σB ）が得られていない。
【０１０３】
【表１】

【０１０４】
【表２】

【０１０５】
【表３】

【０１０６】
【表４】

【０１０７】
【発明の効果】
以上詳述したように、本発明は、切り欠き疲労強度に優れる自動車用薄鋼板およびその製造方法に関するものであり、これらの薄鋼板を用いることにより、打ち抜き加工部や溶接部等の応力集中部からの疲労き裂の進展が問題となるような、自動車足廻り部品等の耐久性が求められる部材における重要な特性の一つである切り欠き疲労強度の大幅な改善が期待できるため、工業的価値が高い発明である。
【図面の簡単な説明】
【図１】疲労試験片の形状を説明する図であり、（ａ）は平滑疲労試験片、（ｂ）は切り欠き疲労試験片を示す。
【図２】本発明に至る予備実験の結果を、｛１００｝＜０１１＞〜｛２２３｝＜１１０＞方位群のＸ線ランダム強度比の平均値および、｛５５４｝＜２２５＞、｛１１１｝＜１１２＞および｛１１１｝＜１１０＞の３方位のＸ線ランダム強度比の平均値と切り欠き疲労強度（１０⁷ 回での時間強度：疲労限）の関係において示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an automotive thin steel sheet having excellent notch fatigue strength and a method for producing the same, and in particular, an automobile in which the development of a fatigue crack from a stress concentrated portion such as a punched portion or a welded portion becomes a problem. The present invention relates to a thin steel sheet for automobiles excellent in notch fatigue strength, which is suitable as a material for suspension parts and the like, and a method for producing the same.
[0002]
[Prior art]
In recent years, application of light metals such as Al alloys and high-strength steel sheets to automobile members has been promoted for the purpose of reducing the weight in order to improve the fuel efficiency of automobiles. However, although light metals such as Al alloys have the advantage of high specific strength, their application is limited to special applications because they are significantly more expensive than steel. Therefore, in order to promote weight reduction of automobiles in a wider range, application of inexpensive high-strength steel sheets is strongly demanded.
[0003]
In response to such demands for high strength, steel sheets that have both strength and deep drawability in the field of cold rolled steel sheets used for white bodies and panels, which occupy about 1/4 of the weight of the vehicle body, The development of bake-hardening steel sheets has been promoted and contributed to the weight reduction of the car body. However, at present, the object of weight reduction has shifted to structural members and suspension members that account for about 20% of the weight of the vehicle body, and the development of high-strength thin steel plates used for these members is an urgent task.
[0004]
However, increasing strength generally degrades material properties such as formability (workability), so the key to developing high-strength steel sheets is how to increase strength without deteriorating material properties. In particular, not only elongation but also shearing, punching workability, burring workability, fatigue durability, and corrosion resistance are important properties required for steel plates for structural members and suspension members, and how high these strengths are. It is important to balance in dimension.
For example, parts such as suspension arms are press-molded after blanking or punching by shearing or punching, and some parts are further welded into parts. In such parts, cracks often propagate from the end face of the shearing process and the vicinity of the welded portion, resulting in fatigue failure. That is, the sheared end face and the welded portion become a stress concentration portion such as a notch, and a fatigue crack develops therefrom.
[0005]
On the other hand, the fatigue limit of a material generally decreases as the notch becomes sharper. However, when the notch becomes sharp to some extent, a phenomenon occurs in which the fatigue limit does not decrease any more. This is because the fatigue limit transitions from the crack initiation limit to the crack growth limit. When the material is strengthened, the crack initiation limit is improved, but the crack growth limit is not improved, so the point at which the fatigue limit transitions from the crack initiation limit to the crack growth limit moves to the sharp side of the notch. . Therefore, even if the strength of the material is increased, the fatigue limit significantly decreases due to the notch, and the fatigue limit when the notch is sharp cannot enjoy the advantage of high strength. That is, when the strength is increased, the sensitivity to notches increases.
[0006]
Currently, steel plates of 340 to 440 MPa class are used as thin steel plates for automobile undercarriage, but the strength level required for these member steel plates is being further increased to the 590 to 780 MPa class. Therefore, in order to meet these demands, it is indispensable to develop a steel plate that can enjoy the benefits of high strength even when there are sharp notches.
[0007]
There are roughly two methods for improving the fatigue strength when there is a punched or sheared end face. One is to eliminate sharp notches such as burrs generated on the punching and shearing end faces, and the other is to increase resistance to crack propagation even if such sharp notches are present.
[0008]
As an invention belonging to the former, for example, in Japanese Patent Laid-Open No. 5-51695, the amount of Si is reduced, and the occurrence of burrs is suppressed by reducing the elongation at break with precipitates of Ti, Nb, and V. A technique for improving fatigue strength in a shearing process is disclosed. Japanese Patent Application Laid-Open No. 5-179346 discloses a technique for limiting the upper limit of the bainite volume fraction by defining the upper limit of the rolling finishing temperature and improving the fatigue strength in the punching or shearing process. Yes. Japanese Patent Application Laid-Open No. 8-13033 discloses a technique for improving the fatigue strength while punching or shearing by regulating the cooling rate after rolling and suppressing the formation of martensite.
[0009]
In JP-A-8-302446, the hardness of the second phase is specified to be 1.3 times or more that of ferrite in the composite structure steel, so that the strain energy at the time of punching or shearing is reduced, and the punching or shearing process is maintained. A technique for improving the fatigue strength at the same time is disclosed. Japanese Patent Application Laid-Open No. 9-170048 discloses a technique for reducing the burrs during punching or shearing by defining the length of grain boundary cementite and improving the fatigue strength in the punching or shearing process. Furthermore, Japanese Patent Application Laid-Open No. 9-202940 discloses a technique for improving the punching property by defining the parameters arranged by adding amounts of Ti, Nb, and Cr, and improving the fatigue strength while punching.
[0010]
On the other hand, as an invention belonging to the latter, Japanese Patent Application Laid-Open No. 6-88161 discloses that the (100) plane strength of the texture parallel to the rolling surface in the surface layer is defined as 1.5 or more to reduce the fatigue crack propagation rate. Technology is disclosed. In JP-A-8-199286 and JP-A-10-147846, the (200) diffraction intensity ratio in the plate thickness direction measured by X-ray is defined as 2.0 to 15.0, and recovery or recrystallization is performed. A technique for reducing the fatigue crack propagation rate by setting the area ratio of ferrite to 15 to 40% is disclosed.
[0011]
However, as disclosed in the publications such as JP-A-5-51695, JP-A-5-179346, JP-A-8-13033, JP-A-8-302446, JP-A-9-170048 and JP-A-9-202940, The technology to reduce sharp notches such as burrs that occur on the shearing end face is not a technology that can be applied under any conditions because the degree of burrs that are generated varies greatly depending on the clearance during punching or shearing. It must be said that it is insufficient as a steel sheet having excellent notch fatigue strength.
[0012]
  On the other hand, the techniques disclosed in JP-A-6-88161, 8-199286, and 10-147846 that control the texture and increase the resistance to crack propagation are mainly construction machines, ships, This invention is intended for steel for large structures such as bridges, and is not intended for thin steel sheets for automobiles as in the present invention.
  In addition, the above-mentioned technique mainly controls the crack propagation speed in the PARIS region, which is the fracture mechanics of fatigue cracks that propagate from the weld toe. Since it is thin, it is insufficient as a technique when there is almost no crack propagation region in the PARIS region.
  Also used for thin steel platebendingThe invention which evaluated the notch fatigue characteristic by the fatigue test method using the test piece shown in FIG.1 (b) is not found until now.
[0013]
[Problems to be solved by the invention]
Therefore, in the present invention, in an automotive thin steel sheet, a fatigue crack that propagates from a notch such as a punched or sheared end face is controlled by controlling the texture regardless of conditions such as a clearance during punching or shearing. TECHNICAL FIELD OF THE INVENTION That is, an object of the present invention is to provide a thin steel sheet for automobiles excellent in notch fatigue strength, and a manufacturing method capable of stably and inexpensively manufacturing the steel sheet.
[0014]
[Means for Solving the Problems]
The present inventors have conducted intensive research to achieve improvement in notch fatigue strength of thin steel sheets for automobiles, keeping in mind the manufacturing process of thin steel sheets produced on an industrial scale by production equipment that is currently employed normally. Repeated. As a result, the average value of the X-ray random intensity ratio of the {100} <011> to {223} <110> orientation groups of the plate surface at an arbitrary depth from the outermost surface to 0.5 mm in the plate thickness direction is 2 or more and , {554} <225>, {111} <112> and {111} <110> have an average value of the X-ray random intensity ratio in three directions of 4 or less, and a plate thickness of 0.5 mm or more and 12 mm or less. Is newly found to be very effective in improving notch fatigue strength, and the present invention has been made.
[0015]
  That is, the gist of the present invention is as follows.
(1)In mass%,
        C: 0.01 to 0.3%,
        Si: 0.01-2%
        Mn: 0.05-3%,
        P ≦ 0.1%,
        S ≦ 0.01%,
        Al: 0.005 to 1%
And the balance is made of steel consisting of Fe and inevitable impurities,The average value of the X-ray random intensity ratios in the {100} <011> to {223} <110> orientation groups of the plate surface at an arbitrary depth from the outermost surface to 0.5 mm in the plate thickness direction is 2 or more and {554 } <225>, {111} <112>, and {111} <110>, the average value of the three-direction X-ray random intensity ratio is 4 or less, and the plate thickness is 0.5 mm or more and 12 mm or less. A thin steel sheet for automobiles with excellent notch fatigue strength.
(2) The thin structure for automobiles having excellent notch fatigue strength, wherein the microstructure of the steel sheet according to (1) is bainite having a maximum volume fraction or a composite structure of ferrite and bainite. steel sheet.
(3) The microstructure of the steel sheet according to (1) is characterized in that it includes a retained austenite having a volume fraction of 5% or more and 25% or less, and the balance is a composite structure mainly composed of ferrite and bainite. Thin steel sheet for automobiles with excellent notch fatigue strength.
(4) The notch fatigue strength, wherein the microstructure of the steel sheet according to (1) is a composite structure in which the phase with the largest volume fraction is ferrite and the second phase is mainly martensite. Excellent steel sheet for automobiles.
(5) Steel component is further mass%,
        Cu: 0.2-2%When,
        B: 0.0002 to 0.002% and
        Ni: 0.1 to 1% of one or two
Characterized in that it contains(1) to (4)The thin steel plate for automobiles which is excellent in notch fatigue strength of any one of these.
(6) Steel component is further mass%,
        Ca: 0.0005 to 0.002%,
        REM: 0.0005 to 0.02%
Characterized by containing one or two of the above,(1) to (5)The thin steel plate for automobiles which is excellent in notch fatigue strength of any one of these.
(7) Steel component is further mass%,
      Ti: 0.05 to 0.5%,
      Nb: 0.01-0.5%
      Mo: 0.05 to 1%
      V: 0.02-0.2%,
      Cr: 0.01-1%,
      Zr: 0.02 to 0.2%
Characterized by containing one or more of the above,(1) to (6)The thin steel plate for motor vehicles which is excellent in the notch fatigue strength of any one of Claims.
[0016]
(8) (1) and (5) to (7)The component according to any one ofBilletAfter rough rolling, finish rolling with a total reduction of 25% or more of the steel sheet thickness in the temperature range of Ar3 transformation point temperature + 100 ° C or lowerTheFor automobiles with excellent notch fatigue strengthHot rollingManufacturing method of thin steel sheet.
(9) In the hot rolling, lubrication rolling is performed in finish rolling after rough rolling,8) For automobiles with excellent notch fatigue strengthHot rollingManufacturing method of thin steel sheet.
(10) (8) or (9)In the hot rolling described in 1, for automobiles with excellent notch fatigue strength, characterized by performing descaling after completion of rough rollingHot rollingManufacturing method of thin steel sheet.
[0017]
(11) (1) and (5) to (7)The component according to any one ofBilletAfter hot rolling, subsequent pickling, cold rolling with a steel sheet thickness reduction rate of less than 80%, holding at a temperature range from the recovery temperature to the Ac3 transformation point temperature + 100 ° C for 5 to 150 seconds, For automobiles with excellent notch fatigue strength, characterized by recrystallization annealingCold rollingManufacturing method of thin steel sheet
.
(12) (1) and (5) to (7)The component according to any one ofBilletAfter hot rolling, after subsequent pickling and cold rolling with a steel sheet thickness reduction rate of less than 80%, hold in the temperature range of Ac1 transformation point temperature to Ac3 transformation point temperature + 100 ° C for 5 to 150 seconds, and then cool. For automobiles with excellent notch fatigue strength, characterized by heat treatment in the processCold rollingManufacturing method of thin steel sheet.
[0018]
DETAILED DESCRIPTION OF THE INVENTION
First, the basic research results that led to the present invention will be described below.
Generally, fatigue cracks are generated from the surface. This is no exception even when a stress concentration part such as a notch exists. Further, even when there is a punched or sheared end face, it is often observed that a fatigue crack propagates from the steel sheet surface end under a repeated load including a load mode in the out-of-plane bending direction. Therefore, even in such a case, it is clear that an increase in crack propagation resistance up to the outermost surface of the steel sheet or a depth of several crystal grains is effective in improving the notch fatigue strength. Even if the crack propagation resistance is increased at the center of the plate thickness, it is difficult to stop the crack already. Therefore, in the present invention, the range of the texture effective for improving the fatigue strength is limited to 0.5 mm in the plate thickness direction from the outermost surface. Desirably, it is up to 0.1 mm.
[0019]
Average value of the X-ray random intensity ratio of {100} <011> to {223} <110> orientation groups on the plate surface at an arbitrary depth from the outermost surface to 0.5 mm in the plate thickness direction affecting the notch fatigue strength And the influence of the average value of the X-ray random intensity ratio of three directions of {554} <225>, {111} <112> and {111} <110> was investigated. The test material for that purpose was prepared as follows. That is, 0.08% C-0.9% Si-1.2% Mn-0.01% P-0.001% S-0.03% Al slab was prepared by adjusting the composition to the Ar3 transformation. After hot finish rolling was finished so that the plate thickness would be 3.5 mm at any temperature not lower than the spot temperature, it was wound up.
[0020]
The X-ray random intensity ratio of {100} <011> to {223} <110> orientation groups on the plate surface at an arbitrary depth from the outermost surface of the steel plate thus obtained to 0.5 mm in the plate thickness direction. And the average value of the X-ray random intensity ratios in the three directions of {554} <225>, {111} <112> and {111} <110> A specimen cut to 30 mmφ from the / 4 W position was ground to a depth of about 0.05 mm from the outermost layer, and then strain was removed by chemical polishing or electrolytic polishing.
[0021]
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>. X-ray crystal orientation was measured according to the method described in, for example, “New edition of Karity X-ray diffraction theory” (published in 1986, translated by Gentaro Matsumura, Agne Co., Ltd.), pages 274-296.
[0022]
Here, the average value of the X-ray random intensity ratio of the {100} <011> to {223} <110> azimuth group is the main azimuth included in this azimuth group, {100} <011>, {116} <110>, {114} <110>, {113} <110>, {112} <110>, {335} <110> and {223} <110> X-ray diffraction intensities represented by a {110} pole figure Or a series expansion method using a plurality of pole figures (preferably three or more) out of {110}, {100}, {211}, {310} pole figures. Obtained from the calculated three-dimensional texture.
[0023]
For example, the X-ray random intensity ratio of each crystal orientation in the latter method is (001) [1-10], (116) [1-10], (114) in the φ2 = 45 ° cross section of the three-dimensional texture. The intensities of [1-10], (113) [1-10], (112) [1-10], (335) [1-10], (223) [1-10] may be used as they are. However, the average value of the X-ray random intensity ratio of the {100} <011> to {223} <110> azimuth group is an arithmetic average of the above azimuths.
[0024]
When the intensity of all the directions cannot be obtained, {100} <011>, {116} <110>, {114} <110>, {112} <110>, {223} <110> You may substitute with the arithmetic mean of each direction.
Next, the average value of the three-direction X-ray random intensity ratios of {554} <225>, {111} <112> and {111} <110> is obtained from a three-dimensional texture calculated in the same manner as the above method. Find it.
[0025]
Next, in order to investigate the notch fatigue strength of the steel sheet, a fatigue test piece having the shape shown in FIG. 1B is set so that the rolling direction becomes the long side from the 1/4 W or 3/4 W position of the plate width. Were collected and subjected to a fatigue test. Here, the fatigue test piece shown in FIG. 1 (a) is a smooth test piece for obtaining fatigue strength of a general material, whereas the fatigue test piece shown in FIG. 1 (b) is notched. It is a notch test piece produced in order to obtain fatigue strength. However, the fatigue test piece was ground to a depth of about 0.05 mm from the outermost layer. For the fatigue test, an electrohydraulic servo type fatigue tester was used, and the test method was based on JIS Z 2273-1978 and JIS Z 2275-1978.
[0026]
The average value of the X-ray random intensity ratios of the {100} <011> to {223} <110> orientation groups, which affect the notch fatigue strength, and {554} <225>, {111} <112> and {111} < FIG. 2 shows the result of investigating the influence of the average value of the X-ray random intensity ratios in three directions of 110>. Here, the numbers in the circles indicate the fatigue limit (10) obtained from the fatigue test performed using the notch fatigue test piece having the shape shown in FIG.⁷Time strength), and hereinafter referred to as notch fatigue strength.
[0027]
The average value of the X-ray random intensity ratio of the {100} <011> to {223} <110> azimuth group and the three azimuths of {554} <225>, {111} <112> and {111} <110> There was a strong correlation between the average value of the X-ray random intensity ratio and the notch fatigue strength, and it was shown that the notch fatigue strength is significantly improved when the average value is 2 or more and 4 or less.
[0028]
As a result of examining these experimental results in detail, the inventors of the present invention have found that in order to improve the notch fatigue strength, {100 of the plate surface at an arbitrary depth from the outermost surface to 0.5 mm in the plate thickness direction. } <011> to {223} <110> orientation group has an average X-ray random intensity ratio of 2 or more, and three orientations {554} <225>, {111} <112>, and {111} <110> It came to newly discover that it is very important that the average value of the X-ray random intensity ratio is 4 or less.
[0029]
However, in order to improve not only the notch but also smooth fatigue crack resistance, {100} <011> to {100} of the plate surface at an arbitrary depth from the outermost surface to 0.5 mm in the plate thickness direction. 223} <110> orientation group having an average X-ray random intensity ratio of 4 or more and {554} <225>, {111} <112> and {111} <110> in three directions of X-ray random intensity ratio It is desirable that the average value of is 2.5 or less.
[0030]
Although this mechanism is not necessarily clear, it is presumed as follows.
In general, the fatigue limit in the presence of a sharp notch is determined by the crack growth limit, that is, the magnitude of the crack growth resistance for retaining the crack. Fatigue crack growth is a repetitive small-scale plastic deformation at the notch bottom or stress concentration point, but when the crack length is relatively short and the plastic deformation occurs within the size of a crystal grain, It is estimated that the influence of the crystallographic slip surface and the slip direction is large. Therefore, if the ratio of the crystal having a slip surface and a slip direction having a high crack propagation resistance with respect to the crack propagation direction and the crack surface is large, the progress of the fatigue crack is suppressed.
[0031]
Next, the reason for limiting the thickness of the steel sheet in the present invention will be described.
If the plate thickness is less than 0.5 mm, the small-scale yield condition cannot be satisfied regardless of the degree of stress concentration, and there is a risk of monotonic ductile fracture. Further, since sufficient plastic restraint is necessary from the viewpoint of crack retention, it is desirable that the thickness is at least 1.2 mm or more in order to maintain a plane strain state.
On the other hand, if the plate thickness exceeds 12 mm, the fatigue strength is significantly reduced due to the plate thickness effect (size effect). In addition, if the plate thickness exceeds 8 mm, an excessive load may be applied to the equipment in order to achieve hot or cold rolling conditions for obtaining a texture effective in improving notch fatigue strength. Therefore, 8 mm or less is desirable. Therefore, in the present invention, the plate thickness is limited to 0.5 mm or more and 12 mm or less. Desirably, it is 1.2 mm or more and 8 mm or less.
[0032]
Next, the microstructure of the steel sheet in the present invention will be described.
In the present invention, it is not necessary to specifically limit the microstructure of the steel sheet for the purpose of improving the notch fatigue strength, and the texture within the scope of the present invention in the ferrite, bainite, pearlite, and martensite structures exhibited by ordinary steel If the (X-ray random intensity ratio within the range of the present invention) is obtained, the effect of improving the notch fatigue strength of the present invention can be obtained, so it is preferable to define the microstructure according to other necessary characteristics. . However, a specific microstructure, for example, retained austenite having a volume fraction of 5% or more and 25% or less, with the balance being mainly composed of ferrite and bainite, or the phase having the largest volume fraction as ferrite and the second phase as This effect can be further enhanced in a composite structure mainly composed of martensite.
[0033]
In addition, the bainite mentioned here includes bainitic ferrite and ashular ferrite structures. However, when the composite structure of two or more phases includes a crystal structure such as retained austenite that is not bcc, the X-ray random intensity ratio converted by the volume fraction of the other structure is within the range of the present invention. Absent. Further, since pearlite containing coarse carbides becomes a site where fatigue cracks are generated and there is a risk of extremely reducing fatigue strength, the volume fraction of pearlite containing coarse carbides is preferably 15% or less. In order to secure better fatigue characteristics, the volume fraction of pearlite containing coarse carbides is desirably 5% or less.
[0034]
Here, the volume fraction of ferrite, bainite, pearlite, martensite, and retained austenite is a sample cut from the 1/4 W or 3/4 W position of the steel plate width in the rolling direction cross section, and the Nital reagent and / Or etched with the reagent disclosed in JP-A-5-163590, and defined by the area fraction of the microstructure at 1 / 4t of the plate thickness observed at a magnification of 200 to 500 times using an optical microscope Is done. However, since the retained austenite may not be easily determined by etching with the above reagent, the volume fraction may be calculated by the following method.
That is, since austenite has a crystal structure different from that of ferrite, it can be easily distinguished crystallographically. Therefore, the volume fraction of retained austenite can be experimentally determined also by the X-ray diffraction method. That is, it is a method of simply obtaining the volume fraction using the following equation from the difference in the intensity of the reflection surface between austenite and ferrite using the Kα ray of Mo.
Vγ = (2/3) {100 / (0.7 × α (211) / γ (220) +1)} + (1/3) {100 / (0.78 × α (211) / γ (311) +1)}
However, α (211), γ (220) and γ (311) are the X-ray reflection surface strengths of ferrite (α) austenite (γ), respectively.
[0035]
In the present invention, in order to give good burring workability in addition to the improvement of the notch fatigue strength, the microstructure has a maximum volume fraction of bainite or a composite structure of ferrite and bainite. However, inevitable inclusion of martensite, retained austenite and pearlite is permitted. In order to obtain good burring workability (hole expansion value), the total volume fraction of hard retained austenite and martensite is preferably less than 5%. The volume fraction of bainite is preferably 30% or more. Furthermore, in order to obtain good ductility, the volume fraction of bainite is desirably 70% or less.
[0036]
In order to give good ductility in addition to improving notch fatigue strength in the present invention, the microstructure contains residual austenite having a volume fraction of 5% or more and 25% or less, with the balance mainly consisting of ferrite and bainite. A composite organization consisting of However, a total of less than 5% of inevitable martensite and pearlite is allowed.
Furthermore, in order to provide a low yield ratio for obtaining good shape freezing properties in addition to the improvement of notch fatigue strength in the present invention, the microstructure has a maximum volume fraction of ferrite and the second phase. Is a composite organization mainly composed of martensite. However, a total of less than 5% of inevitable bainite, retained austenite and pearlite is allowed. In order to secure a low yield ratio of 70% or less, the volume fraction of ferrite is desirably 50% or more.
[0037]
Then, the reason for limitation of the chemical component of this invention is demonstrated.
C is an element necessary for obtaining a desired microstructure. However, if it exceeds 0.3%, the workability deteriorates, so the content is made 0.3% or less. Moreover, since there exists a tendency for weldability to deteriorate when it contains more than 0.2%, 0.2% or less is desirable. On the other hand, if the content is less than 0.01%, the strength decreases, so the content is made 0.01% or more. Moreover, in order to stably obtain a sufficient amount of retained austenite for obtaining good ductility, 0.05% or more is desirable.
[0038]
Si is effective for increasing the strength as a solid solution strengthening element. In order to obtain a desired strength, it is necessary to contain 0.01% or more. However, if it exceeds 2%, workability deteriorates. Therefore, the Si content is set to 0.01 to 2%.
[0039]
Mn is effective for increasing the strength as a solid solution strengthening element. In order to obtain a desired strength, 0.05% or more is necessary. 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%. Further, Mn is an austenite stabilizing element, and the amount added is preferably 0.1% or more in order to stably obtain a sufficient amount of retained austenite for obtaining good ductility. On the other hand, if over 3% is added, slab cracking occurs, so the content is made 3% or less.
[0040]
P is an impurity and is preferably as low as possible. If contained over 0.1%, the workability and weldability are adversely affected and the fatigue characteristics are also reduced.
[0041]
S is an impurity and is preferably as low as possible. If it is too large, S-based inclusions that deteriorate local ductility and burring workability are generated. Therefore, S should be reduced as much as possible. is there.
[0042]
Al needs to be added in an amount of 0.005% or more for deoxidation of molten steel, but the upper limit is set to 1.0% because of an increase in cost. Further, if added too much, non-metallic inclusions are increased and elongation is deteriorated, so the content is desirably 0.5% or less.
[0043]
Since Cu has an effect of improving fatigue properties in a solid solution state, Cu is added as necessary. However, if the content is less than 0.2%, the effect is small, and even if the content exceeds 2%, the effect is saturated. Therefore, the Cu content is in the range of 0.2 to 2%. However, when the coiling temperature is 450 ° C. or higher, if it exceeds 1.2%, there is a possibility that it will precipitate after coiling and the workability will be remarkably deteriorated.
[0044]
Since B has an effect of increasing the fatigue limit by being added in combination with Cu, it is added as necessary. However, if it is less than 0.0002%, it is insufficient for obtaining the effect, and if added over 0.002%, slab cracking occurs. Therefore, the addition of B is set to 0.0002 to 0.002%.
[0045]
Ni is added as necessary to prevent hot brittleness due to Cu inclusion. However, if the content is less than 0.1%, the effect is small, and even if added over 1%, the effect is saturated.
[0046]
Ca and REM are elements that are detoxified by changing the form of non-metallic inclusions that become the starting point of destruction or deteriorate workability. However, even if less than 0.0005% is added, there is no effect, and if Ca exceeds 0.002% and REM exceeds 0.02%, the effect is saturated, so Ca: 0.0005 -0.002%, REM: It is desirable to add 0.0005-0.02%.
[0047]
Further, in order to impart strength, one or more of precipitation strengthening or solid solution strengthening elements of Ti, Nb, Mo, V, Cr, and Zr may be added. However, if it is less than 0.05%, 0.01%, 0.05%, 0.02%, 0.01%, and 0.02%, the effect cannot be obtained. Moreover, the effect will be saturated even if it adds exceeding 0.5%, 0.5%, 1%, 0.2%, 1%, and 0.2%, respectively.
[0048]
Note that Sn, Co, Zn, W, and Mg may be contained in a total amount of 1% or less in steel containing these as main components. However, since Sn may cause wrinkles during hot rolling, 0.05% or less is desirable.
[0049]
Next, the reasons for limiting the production method of the present invention will be described in detail below.
The present invention, after casting, after cooling after hot rolling, or after cooling, pickling and cooling after hot rolling is annealed, or hot-rolled steel sheet or cold-rolled steel sheet is subjected to heat treatment in a hot dipping line, Can also be obtained by subjecting these steel plates to a separate surface treatment.
[0050]
In the present invention, the production method preceding hot rolling is not particularly limited. In other words, following the smelting by blast furnace or electric furnace, etc., the components are adjusted so that the desired component content is obtained by various secondary smelting, and then, in addition to normal continuous casting, casting by ingot method, thin slab casting, etc. It can be cast by the method. Scrap may be used as a raw material. In the case of a slab obtained by continuous casting, it may be directly sent to a hot rolling mill as it is a high-temperature slab, or may be hot-rolled after being reheated in a heating furnace after being cooled to room temperature.
[0051]
The reheating temperature is not particularly limited, but if it is 1400 ° C. or higher, the scale-off amount becomes large and the yield decreases, so the reheating temperature is preferably less than 1400 ° C. In addition, since heating below 1000 ° C. significantly impairs the operation efficiency on the schedule, the reheating temperature is desirably 1000 ° C. or higher.
In the hot rolling process, finish rolling is performed after finishing rough rolling. When descaling is performed after finishing rough rolling, the collision pressure P (MPa) of high-pressure water on the steel sheet surface × flow rate L (liter / cm).²It is desirable to satisfy the condition of ≧ 0.0025.
[0052]
The collision pressure P of high-pressure water on the steel sheet surface is described as follows (see “Iron and Steel” 1991, vol. 77, No. 9, p1450).
P (MPa) = 5.64 × P₀× V / H²
However,
P₀(MPa): Fluid pressure
V (liter / min): Nozzle flow rate
H (cm): distance between the steel plate surface and the nozzle
[0053]
The flow rate L is described as follows.
L (liters / cm²) = V / (W × v)
However,
V (liter / min): Nozzle flow rate
W (cm): Width of spray liquid per nozzle hitting steel plate surface
v (cm / min): Feeding speed
The upper limit of the collision pressure P × flow rate L is not particularly required to obtain the effect of the present invention. However, increasing the nozzle flow rate causes inconveniences such as severe wear of the nozzle. The following is desirable.
[0054]
Furthermore, it is desirable that the maximum height Ry of the steel sheet after finish rolling is 15 μm (15 μm Ry, l2.5 mm, ln12.5 mm) or less. For example, as described in “Handbook of Fatigue Design for Metallic Materials”, edited by the Japan Society of Materials Science, page 84, the fatigue strength of a hot-rolled or pickled steel sheet correlates with the maximum height Ry of the steel sheet surface. It is clear from this. Further, the subsequent finish rolling is desirably performed within 5 seconds in order to prevent the scale from being generated again after descaling.
Further, after rough rolling or subsequent descaling, the sheet bar may be joined and finish rolled 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.
[0055]
  In finish rolling, when making the final product as a hot-rolled steel sheet,Is Ar Three It is necessary to perform rolling with a total rolling reduction of 25% or more in the temperature range of the transformation point temperature + 100 ° C. or less. Here, the Ar3 transformation point temperature is simply shown in relation to the steel components by the following calculation formula, for example. That is,
    Ar 3 = 910-310 ×% C + 25 ×% Si-80 ×% Mn
[0056]
If the total rolling reduction in the temperature range of Ar3 transformation temperature + 100 ° C. or less is less than 25%, the texture of rolled austenite will not be sufficiently developed. The effect of the invention cannot be obtained. In order to obtain a sharper texture, the total rolling reduction in the temperature range of Ar3 transformation point temperature + 100 ° C. or lower is desirably 35% or more.
[0057]
Further, the lower limit of the temperature range for rolling at a total rolling reduction of 25% or more is not particularly limited, but if it is lower than the Ar3 transformation point temperature, the work structure remains in the ferrite precipitated during rolling, and the ductility is lowered. Since the workability deteriorates, the lower limit of the temperature range at which rolling with a total rolling reduction of 25% or more is desirable is the Ar3 transformation point temperature or more. However, even if this temperature is lower than the Ar3 transformation point temperature, this does not apply if recovery or recrystallization has progressed to some extent by subsequent winding treatment or heat treatment after winding treatment.
In the present invention, the upper limit of the total rolling reduction in the temperature range of Ar3 transformation point temperature + 100 ° C. or lower is not particularly limited, but if the total rolling reduction exceeds 97.5%, the rolling load increases and the rigidity of the rolling mill is increased. Since it is necessary to increase excessively and an economic disadvantage is caused, it is desirably 97.5% or less.
[0058]
Here, when the friction between the hot rolling roll and the steel plate at the time of hot rolling in the temperature range of Ar3 transformation point temperature + 100 ° C. or less is large, the {110} plane is mainly used as the plate surface in the vicinity of the steel plate surface. Since the crystal orientation develops and the notch fatigue strength deteriorates, lubrication is performed as necessary in order to reduce the friction between the hot rolling roll and the steel sheet.
[0059]
In the present invention, the upper limit of the friction coefficient between the hot rolling roll and the steel sheet is not particularly limited, but if it exceeds 0.2, the development of crystal orientation mainly consisting of {110} faces becomes remarkable, and the notch fatigue strength deteriorates. Therefore, it is desirable that the friction coefficient between the hot rolling roll and the steel sheet is 0.2 or less for at least one pass during hot rolling in the temperature range of Ar3 transformation point temperature + 100 ° C. or less. More preferably, the friction coefficient between the hot rolling roll and the steel sheet is set to 0.15 or less for all passes during hot rolling in the temperature range of Ar3 transformation point temperature + 100 ° C. or less.
Here, the friction coefficient between the hot rolling roll and the steel sheet is a value obtained by calculation based on the rolling theory from the values of the advance rate, rolling load, rolling torque, and the like.
[0060]
The final pass temperature (FT) of finish rolling is not particularly limited, but it is preferable that the final pass temperature (FT) of finish rolling is finished at an Ar3 transformation point temperature or higher. This is because if the rolling temperature is lower than the Ar3 transformation point temperature during hot rolling, the work structure remains in the ferrite deposited before or during rolling, resulting in reduced ductility and deteriorated workability. is there. However, even if the final pass temperature (FT) of finish rolling is lower than the Ar3 transformation point temperature, this is not the case when a subsequent winding process or a heat treatment for recovery and recrystallization is performed after the winding process.
[0061]
On the other hand, the upper limit of the finishing temperature is not particularly limited. However, when the Ar3 transformation point temperature is higher than 100 ° C, it is practically impossible to perform rolling at a total reduction rate of 25% or more in the temperature range of Ar3 transformation point temperature + 100 ° C or lower. Therefore, the upper limit of the finishing temperature is preferably Ar3 transformation point temperature + 100 ° C. or less.
[0062]
In the present invention, there is no need to specifically limit the microstructure of the steel plate only for the purpose of improving the notch fatigue strength, so cooling after completion of finish rolling and winding at a predetermined winding temperature. Although the process is not particularly defined, cooling is performed as necessary to wind at a predetermined winding temperature or to control the microstructure. The upper limit of the cooling rate is not particularly limited, but it is desirable that the cooling rate be 300 ° C./s or less because there is a concern about plate warpage due to thermal strain. Furthermore, if this cooling rate is too fast, the cooling end temperature cannot be controlled, and overcooling may cause overcooling to a predetermined coiling temperature or lower, so the cooling rate here is 150 ° C./s. The following is desirable. The lower limit of the cooling rate is not particularly defined, but the air cooling rate when cooling is not performed is 5 ° C./s or more.
[0063]
In the present invention, in order to give good burring workability in addition to improving notch fatigue strength, finish rolling is performed in order to make the maximum volume fraction of the microstructure a bainite or a composite structure of ferrite and bainite. After completion, the process until winding at a predetermined winding temperature is not particularly defined except for the cooling rate during that time, but when aiming to achieve both ductility without significantly degrading burring properties, the Ar3 transformation point May be retained for 1 to 20 seconds in the temperature range (two-phase region of ferrite and austenite) from the transition point to the Ar1 transformation point. The residence here is carried out in order to promote ferrite transformation in the two-phase region, but if it is less than 1 second, ferrite transformation in the two-phase region is insufficient, so that sufficient ductility cannot be obtained. Pearlite is generated, and bainite or a composite structure of ferrite and bainite cannot be obtained as the target microstructure having the maximum volume fraction.
[0064]
Further, the temperature range in which the residence is performed for 1 to 20 seconds is preferably not less than the Ar1 transformation point and not more than 800 ° C. in order to facilitate the ferrite transformation. Furthermore, the residence time of 1 to 20 seconds is desirably 1 to 10 seconds so as not to extremely reduce productivity. Moreover, in order to satisfy these conditions, it is necessary to quickly reach the temperature range at a cooling rate of 20 ° C./s or higher after the finish rolling.
Although the upper limit of the cooling rate is not particularly defined, an appropriate cooling rate is 300 ° C./s or less because of the capacity of the cooling facility. Furthermore, if this cooling rate is too fast, the cooling end temperature cannot be controlled, and overshooting may result in overcooling to below the Ar1 transformation point, and the effect of improving ductility is lost. The speed is desirably 150 ° C./s or less.
[0065]
Next, from the temperature range to the coiling temperature (CT), cooling is performed at a cooling rate of 20 ° C./s or more, but at a cooling rate of less than 20 ° C./s, bainite containing pearlite or carbide is generated, As the target microstructure having the maximum volume fraction, bainite or a composite structure of ferrite and bainite cannot be obtained. The upper limit of the cooling rate up to the coiling temperature is not particularly defined, but the effect of the present invention can be obtained. However, since there is a concern about plate warpage due to thermal strain, it is desirable to set it at 300 ° C./s or less.
[0066]
In the present invention, in addition to improving notch fatigue strength, the microstructure contains residual austenite having a volume fraction of 5% or more and 25% or less, with the balance being mainly composed of ferrite and bainite. In order to obtain a composite structure, the process after finishing rolling is first retained for 1 to 20 seconds in a temperature range (two-phase region of ferrite and austenite) from the Ar3 transformation point temperature to the Ar1 transformation point temperature. The residence here is carried out in order to promote ferrite transformation in the two-phase region, but if it is less than 1 second, ferrite transformation in the two-phase region is insufficient, so that sufficient ductility cannot be obtained. Pearlite is generated, and the target austenite having a volume fraction of 5% or more and 25% or less is contained, and the microstructure mainly composed of ferrite and bainite cannot be obtained.
[0067]
Further, the temperature range in which the residence time is 1 to 20 seconds is preferably from Ar1 transformation point temperature to 800 ° C. in order to facilitate the ferrite transformation. Further, the residence time of 1 to 20 seconds is desirably 1 to 10 seconds so as not to extremely reduce the productivity. Moreover, in order to satisfy these conditions, it is necessary to quickly reach the temperature range at a cooling rate of 20 ° C./s or higher after the finish rolling. Although the upper limit of the cooling rate is not particularly defined, an appropriate cooling rate is 300 ° C./s or less because of the capacity of the cooling facility. Furthermore, if the cooling rate is too fast, the cooling end temperature cannot be controlled, and overshooting may result in overcooling to the Ar1 transformation point temperature or lower, so the cooling rate here is 150 ° C / s or less. Is desirable.
[0068]
Next, from the temperature range to the coiling temperature (CT), cooling is performed at a cooling rate of 20 ° C./s or more, but at a cooling rate of less than 20 ° C./s, bainite containing pearlite or carbide is generated, Sufficient retained austenite cannot be obtained, and a microstructure containing the intended retained austenite with a volume fraction of 5% or more and 25% or less and the balance mainly composed of ferrite and bainite cannot be obtained. The upper limit of the cooling rate up to the coiling temperature is not particularly defined, but the effect of the present invention can be obtained. However, since there is a concern about plate warpage due to thermal strain, it is desirable to set it at 300 ° C./s or less.
[0069]
Further, in the present invention, in addition to improving notch fatigue strength, in order to provide a low yield ratio for obtaining good shape freezing properties, the phase having the largest volume fraction of the microstructure is ferrite, and the second phase is mainly used. In order to obtain a composite structure of martensite, the process after finishing rolling is first to 20 in the temperature range from the Ar3 transformation point temperature to the Ar1 transformation point temperature (two-phase region of ferrite and austenite). Stay for 2 seconds. The residence here is performed to promote ferrite transformation in the two-phase region, but if it is less than 1 second, the ferrite transformation in the two-phase region is insufficient, so that sufficient ductility cannot be obtained, and if it exceeds 20 seconds, Pearlite is generated, and a composite structure in which the target phase with the maximum volume fraction is ferrite and the second phase is mainly martensite cannot be obtained.
[0070]
The temperature range for 1 to 20 seconds of residence is preferably not lower than the Ar1 transformation point temperature and not higher than 800 ° C. in order to facilitate the ferrite transformation. Further, the residence time of 1 to 20 seconds is desirably 1 to 10 seconds so as not to extremely reduce productivity. Moreover, in order to satisfy these conditions, it is necessary to quickly reach the temperature range at a cooling rate of 20 ° C./s or higher after the finish rolling. Although the upper limit of the cooling rate is not particularly defined, an appropriate cooling rate is 300 ° C./s or less because of the capacity of the cooling facility. Furthermore, if the cooling rate is too fast, the cooling end temperature cannot be controlled, and overshooting may result in overcooling to the Ar1 transformation point temperature or lower, so the cooling rate here is 150 ° C / s or less. Is desirable.
[0071]
Next, from the temperature range to the coiling temperature (CT), cooling is performed at a cooling rate of 20 ° C./s or more, but at a cooling rate of less than 20 ° C./s, pearlite or bainite is generated, and sufficient martensite is generated. A site cannot be obtained, and a microstructure with the target ferrite as the phase with the largest volume fraction and the martensite as the second phase cannot be obtained.
The upper limit of the cooling rate up to the coiling temperature is not particularly defined, but the effect of the present invention can be obtained. However, since there is a concern about plate warpage due to thermal strain, it is desirable to set it at 300 ° C./s or less.
[0072]
In the present invention, there is no need to particularly limit the microstructure of the steel sheet only for the purpose of improving the notch fatigue strength, so the upper limit of the coiling temperature is not particularly defined, but the Ar3 transformation temperature + 100 ° C. or less. In order to inherit the texture of austenite obtained by rolling with a total rolling reduction of 25% or more in the temperature range of₀It is desirable to take up the following. T₀Need not be below room temperature. This T₀Is a temperature that is thermodynamically defined as the temperature at which austenite and ferrite of the same component as austenite have the same free energy, and considering the influence of components other than C, it can be simplified using the following formula: Can be calculated.
T₀= −650.4 ×% C + B
[0073]
Here, B is determined as follows.
B = −50.6 × Mneq + 894.3
Here, Mneq is determined from the mass% of the contained elements shown below.

T₀Since the influence of the mass% of the components other than the above specified on the present invention is not so large, it can be ignored here.
[0074]
In addition, the lower limit of the coiling temperature is not particularly limited because the microstructure of the steel sheet is not particularly limited only for the purpose of improving the notch fatigue strength, but the coil is wetted for a long time. Since there is a concern about appearance defects due to rust in this state, 50 ° C. or higher is desirable.
In the present invention, for the purpose of giving good burring workability in addition to the improvement of notch fatigue strength, in order to make the phase with the largest volume fraction of the microstructure bainite, or a composite structure of ferrite and bainite, If the coiling temperature is less than 450 ° C, a large amount of retained austenite or martensite, which is considered to be harmful to burring properties, may be generated. From the target microstructure, bainite, ferrite or bainite, which has the maximum volume fraction. Therefore, the coiling temperature is limited to 450 ° C. or higher.
Furthermore, the cooling rate after winding is not particularly limited, but when Cu is added in an amount of 1.2% or more, not only Cu precipitates after winding but the workability deteriorates, but also a solid solution effective for improving fatigue characteristics. Since Cu in the state may be lost, the cooling rate after winding is preferably 30 ° C./s or higher up to 200 ° C.
[0075]
In the present invention, in addition to improving notch fatigue strength, the microstructure contains residual austenite having a volume fraction of 5% or more and 25% or less, with the balance being mainly composed of ferrite and bainite. In order to obtain a composite structure, when the coiling temperature is 450 ° C. or more, bainite containing carbide is generated and sufficient retained austenite cannot be obtained, and the desired retained austenite having a volume fraction of 5% to 25% is not obtained. In addition, the coiling temperature is limited to less than 450 ° C. because a microstructure mainly comprising ferrite and bainite cannot be obtained. When the coiling temperature is 350 ° C. or less, a large amount of martensite is generated and sufficient retained austenite cannot be obtained, and the target austenite with a volume fraction of 5% or more and 25% or less is contained, with the balance being mainly ferrite. Since the microstructure composed of bainite cannot be obtained, the coiling temperature is limited to over 350 ° C.
Further, the cooling rate after winding is not particularly limited, but when Cu is added in an amount of 1% or more, not only does Cu precipitate after winding but the workability deteriorates, but also a solid solution state effective for improving fatigue characteristics. Since Cu may be lost, the cooling rate after winding is preferably 30 ° C./s or higher up to 200 ° C.
[0076]
Further, in the present invention, in addition to improving notch fatigue strength, in order to provide a low yield ratio for obtaining good shape freezing properties, the phase having the largest volume fraction of the microstructure is ferrite, and the second phase is mainly used. In order to obtain a martensitic composite structure, when the coiling temperature exceeds 350 ° C., bainite is generated and sufficient martensite cannot be obtained, and the target ferrite is the phase with the maximum volume fraction, and martensite is obtained. The coiling temperature is limited to 350 ° C. or less because a microstructure with the site as the second phase cannot be obtained. Further, the lower limit value of the coiling temperature is not particularly limited. However, if the coil is wet for a long time, there is a concern about poor appearance due to rust.
After completion of the hot rolling step, pickling may be performed as necessary, and then in-line or off-line, a skin pass with a reduction rate of 10% or less or cold rolling to a reduction rate of about 40% may be performed.
[0077]
Next, although it is a case where it is set as a final product as a cold-rolled steel plate, the hot finish rolling conditions are not specifically limited. However, in order to obtain better notch fatigue strength, it is desirable that the total rolling reduction in the temperature range of Ar3 transformation point temperature + 100 ° C. or lower is 25% or more. The final pass temperature (FT) of the finish rolling may be finished below the Ar3 transformation point temperature, but in that case, a strong work structure remains in the ferrite precipitated before or during rolling, so that the subsequent winding It is desirable to recover and recrystallize by a collection treatment or a heat treatment.
[0078]
The total rolling reduction of the subsequent cold rolling after pickling is less than 80%. This is because when the total rolling reduction of cold rolling is 80% or more, the X direction of {111} plane or {554} plane parallel to the plate surface, which is a general cold rolling-recrystallization texture This is because the line diffraction integration plane intensity ratio becomes high. Moreover, it is 70% or less desirably. Although the lower limit of the cold rolling rate is not particularly defined, the effect of the present invention can be obtained. However, in order to control the strength of the crystal orientation to an appropriate range, it is desirable to set it to 3% or more.
[0079]
The heat treatment of the steel sheet thus cold-rolled is premised on a continuous annealing process.
First, it is carried out for 5 to 150 seconds in a temperature range of Ac3 transformation point temperature + 100 ° C. or lower. When the upper limit of the heat treatment temperature is higher than the Ac3 transformation point temperature + 100 ° C., the ferrite formed by recrystallization transforms to austenite, the texture due to austenite grain growth is randomized, and the finally obtained ferrite texture is also Since it is randomized, the heat treatment upper limit temperature Ac3 transformation point temperature is set to 100 ° C. or lower. Here, the Ac1 transformation point temperature and the Ac3 transformation point temperature are, for example, calculated by the calculation formula described in “Leslie Steel Science” (published in 1985, Hiroshi Kumai, Tatsuhiko Noda, Maruzen Co., Ltd.), page 273. Shown in relationship.
On the other hand, the lower limit of the heat treatment temperature is not particularly limited to the microstructure of the steel sheet for the purpose of improving the notch fatigue strength, and may be higher than the recovery temperature. Since the processed structure remains and the formability is remarkably deteriorated, the lower limit temperature of the heat treatment is set to the recovery temperature or more. Further, if the holding time in this temperature range is less than 5 seconds, the cementite is insufficient to completely re-dissolve. On the other hand, not only the effect is saturated even if heat treatment for more than 150 seconds is performed. Since the productivity is lowered, the holding time is set to 5 to 150 seconds.
[0080]
Although it does not specifically limit about the subsequent cooling conditions, In order to control a microstructure, you may perform the following cooling or cooling and arbitrary temperature cooling as needed. In the present invention, for the purpose of giving good burring workability in addition to the improvement of notch fatigue strength, in order to make the phase with the largest volume fraction of the microstructure bainite, or a composite structure of ferrite and bainite, The lower limit temperature of the heat treatment temperature is set to the Ac1 transformation point temperature or higher. When this lower limit temperature is lower than the Ac1 transformation point temperature, the target phase with the maximum volume fraction cannot be obtained as bainite or a composite structure of ferrite and bainite. Here, when aiming at coexistence with ductility without significantly degrading the burring property, in order to increase the volume fraction of ferrite, the temperature range is not less than Ac1 transformation point temperature and not more than Ac3 transformation point temperature (of ferrite and austenite). Two-phase region). Further, in order to obtain better burring properties, in order to increase the volume fraction of bainite, a temperature range from the Ac3 transformation point temperature to the Ac3 transformation point temperature + 100 ° C. is desirable.
[0081]
Next, although the cooling step is not particularly defined in the present invention, when the heat treatment temperature is not lower than the Ac1 transformation point temperature and not higher than the Ac3 transformation point temperature, the temperature T exceeds 350 ° C. at a cooling rate of 20 ° C./s or higher.₀It is desirable to cool to a temperature range below the temperature. This is because if the cooling rate is less than 20 ° C./s, there is a risk of bainite containing a large amount of carbide or nose of pearlite transformation. Further, when the cooling end temperature is 350 ° C. or less, there is a possibility that a large amount of martensite, which is considered to be harmful to burring properties, may be formed, and it is composed of bainite or ferrite and bainite, which is the target microstructure with the maximum volume fraction. Since a composite structure cannot be obtained, it is desirable to exceed 350 ° C. Furthermore, in order to inherit the texture obtained by the previous process, T₀The following is desirable.
[0082]
Finally, if the cooling rate to the end temperature of the cooling process is 20 ° C./s or more, a large amount of martensite, which is considered to be harmful to burring properties, may be generated during cooling. Since there is a possibility that a bainite structure or a composite structure composed of ferrite and bainite may not be obtained, it is desirable that the temperature be less than 20 ° C./s. Further, if the end temperature of the cooling step exceeds 200 ° C., the aging property may be deteriorated. Further, the lower limit is preferably 50 ° C. or higher because when cooling with water or mist, if the coil remains wet for a long time, there is a concern about appearance defects due to rust.
On the other hand, when the heat treatment temperature is higher than Ac3 transformation point temperature and Ac3 transformation point temperature + 100 ° C. or lower, it is desirable to cool to 200 ° C. or lower at a cooling rate of 20 ° C./s or higher. This is because at 20 ° C./s or more, there is a risk of bainite containing a large amount of carbide or nose of pearlite transformation. The cooling end temperature is preferably 200 ° C. or lower because aging may be deteriorated if it exceeds 200 ° C. The lower limit is preferably 50 ° C. or higher because when cooling with water or mist, if the coil remains wet for a long time, there is a concern of poor appearance due to rust.
[0083]
In the present invention, in addition to improving notch fatigue strength, the microstructure contains residual austenite having a volume fraction of 5% or more and 25% or less, with the balance being mainly composed of ferrite and bainite. In order to obtain a composite structure, it is performed for 5 to 150 seconds in a temperature range of Ac1 transformation point temperature to Ac3 transformation point temperature + 100 ° C. or less as described above. At this time, if the temperature range is too low, if cementite is precipitated in the hot-rolled sheet stage, it takes too much time for the cementite to re-dissolve, and if it is too high, the volume ratio of austenite becomes too large. It is preferable to heat at 780 ° C. or higher and 850 ° C. or lower because the C concentration in the austenite is lowered and the bainite or pearlite transformation nose containing a large amount of carbide is easily applied. If the cooling rate after holding is less than 20 ° C./s, there is a risk of bainite containing a large amount of carbide or nose of pearlite transformation, so the cooling rate is 20 ° C./s or more.
[0084]
Next, it is a step of promoting the bainite transformation and stabilizing the required amount of retained austenite, but when the cooling end temperature is 450 ° C. or higher, the remaining austenite is decomposed into bainite or pearlite containing a large amount of carbide, A microstructure containing the target austenite with a volume fraction of 5% or more and 25% or less, and the balance mainly composed of ferrite and bainite cannot be obtained. If it is less than 350 ° C., a large amount of martensite may be generated, and sufficient retained austenite cannot be obtained, including the intended retained austenite with a volume fraction of 5% or more and 25% or less, with the balance being mainly ferrite, Since a microstructure composed of bainite cannot be obtained, cooling is performed to a temperature range higher than 350 ° C.
[0085]
Furthermore, although it is the holding time in that temperature range, if it is less than 5 seconds, the bainite transformation for stabilizing the retained austenite is insufficient, and there is a possibility that the martensitic transformation may occur at the end of cooling where unstable retained austenite continues. In addition, a microstructure containing a target austenite with a volume fraction of 5% or more and 25% or less and a balance mainly composed of ferrite and bainite cannot be obtained. Further, if it exceeds 600 seconds, the bainite transformation is promoted too much to obtain a necessary amount of stable retained austenite, including the intended retained austenite with a volume fraction of 5% or more and 25% or less, with the balance mainly. A microstructure composed of ferrite and bainite cannot be obtained. Therefore, the holding time in the temperature range is set to 5 seconds or more and 600 seconds or less.
[0086]
If the cooling rate until the end of cooling is less than 5 ° C./s, bainite transformation may be promoted too much during cooling, and a necessary amount of stable retained austenite cannot be obtained. Since there is a possibility that a microstructure containing residual austenite at a rate of 5% or more and 25% or less and the balance mainly consisting of ferrite and bainite cannot be obtained, the temperature is set to 5 ° C./s or more.
The cooling end temperature is set to 200 ° C. or lower because aging may be deteriorated if it exceeds 200 ° C. The lower limit of the cooling end temperature is not particularly limited. However, when cooling with water or mist, if the coil remains wet for a long time, there is a concern about appearance defects due to rust.
[0087]
Further, in the present invention, in addition to improving notch fatigue strength, in order to provide a low yield ratio for obtaining good shape freezing properties, the phase having the largest volume fraction of the microstructure is ferrite, and the second phase is mainly used. In order to obtain a composite structure of martensite, it is carried out for 5 to 150 seconds in the temperature range from the Ac1 transformation point temperature to the Ac3 transformation point temperature + 100 ° C. as described above. At this time, if the temperature range is too low, if cementite is precipitated in the hot-rolled sheet stage, it takes too much time for the cementite to re-dissolve, and if it is too high, the volume fraction of austenite becomes too large. It is preferable to heat at 780 ° C. or higher and 850 ° C. or lower because the C concentration in austenite is lowered and the bainite or pearlite transformation nose containing a large amount of carbide is easily applied.
[0088]
The cooling rate after holding may be 20 ° C./s or more because there is a risk of bainite containing a large amount of carbide or nose of pearlite transformation if it is less than 20 ° C./s. When the cooling end temperature is higher than 350 ° C., a microstructure with the target ferrite as the phase with the maximum volume fraction and the martensite as the second phase cannot be obtained, so the cooling is performed to a temperature range of 350 ° C. or lower. The lower limit of the end temperature of the cooling step is not particularly limited. However, when cooling with water or mist, if the coil is in a wet state for a long time, there is a concern about poor appearance due to rust.
Further, after that, skin pass rolling may be performed as necessary.
In order to galvanize the hot-rolled steel sheet after pickling or the cold-rolled steel sheet after completion of the recrystallization annealing, it may be immersed in a galvanizing bath and alloyed as necessary.
[0089]
【Example】
Example 1
In the following, the present invention will be further described with reference to Example 1.
The steels A to L having the chemical components shown in Table 1 are melted in a converter, re-heated after continuous casting, and finished to a plate thickness of 1.2 to 5.5 mm by finish rolling after rough rolling. And then wound up. However, the display about the chemical composition in a table | surface is the mass%.
Details of the manufacturing conditions are shown in Table 2. Here, “SRT” is the slab heating temperature, “FT” is the final pass finish rolling temperature, and “rolling ratio” is the total rolling reduction in the temperature range of Ar 3 transformation point temperature + 100 ° C. or less. However, if the rolling is performed later in the cold rolling process, it is not limited to such a restriction, so “-” is given. “Lubrication” indicates the presence or absence of lubrication in the temperature range of Ar3 transformation point temperature + 100 ° C. or lower.
Furthermore, “winding” means that the winding temperature (CT) is T₀“○”, T if below₀In the case of super, “x” was given. However, in the case of a cold-rolled steel sheet, “−” is given because it is not necessary to limit the production conditions.
[0090]
Next, for some, after hot rolling, pickling, cold rolling, and annealing were performed. The plate thickness is 0.7 to 2.3 mm. Here, “cold rolling ratio” is the total cold rolling ratio, “Time” is the annealing time, and “annealing” is included in the temperature range where the annealing temperature is higher than the recovery temperature and not higher than Ac3 transformation point temperature + 100 ° C. “○” if it was not, and “×” if it was not. For steel L, after rough rolling, the impact pressure was 2.7 MPa, and the flow rate was 0.001 liter / cm.²Descaling was performed under the following conditions. On the other hand, among the steel plates, Steel G and Steel F-5 were galvanized.
The tensile test of the hot-rolled sheet thus obtained was performed by first processing the specimen into a No. 5 test piece described in JIS Z 2201, and following the test method described in JIS Z 2241. Table 2 shows the yield strength (σY), tensile strength (σB), and elongation at break (El).
[0091]
Further, the specimen cut to 30 mmφ from the 1/4 W or 3/4 W position of the plate width is ground to a depth of about 0.05 mm from the outermost layer, and then the distortion is removed by chemical polishing or electrolytic polishing. The X-ray diffraction intensity was measured according to the method described in pages 274 to 296 of “New edition of Karity X-ray diffraction theory” (published in 1986, translated by Gentaro Matsumura, Agne Co., Ltd.).
Here, the average value of the X-ray random intensity ratios of the {100} <011> to {223} <110> azimuth group is the main azimuth included in this azimuth group, {100} <011>, {116} < 110>, {114} <110>, {113} <110>, {112} <110>, {335} <110> and {223} <110> X-ray diffraction intensities in a {110} pole figure Based on the three-dimensional texture calculated by the vector method based on the above, or by the series expansion method using a plurality of pole figures (preferably 3 or more) among {110}, {100}, {211}, {310} pole figures Obtained from the three-dimensional texture.
[0092]
For example, the X-ray random intensity ratio of each crystal orientation in the latter method is (001) [1-10], (116) [1-10], (114) in the φ2 = 45 ° cross section of the three-dimensional texture. The strengths of [1-10], (113) [1-10], (112) [1-10], (335) [1-10], (223) [1-10] may be used as they are. However, the average value of the X-ray random intensity ratio of the {100} <011> to {223} <110> azimuth group is an arithmetic average of the above azimuths.
When the intensity of all the directions cannot be obtained, {100} <011>, {116} <110>, {114} <110>, {112} <110>, {223} <110> You may substitute with the arithmetic mean of each direction.
Next, the average value of the three-direction X-ray random intensity ratios of {554} <225>, {111} <112> and {111} <110> is obtained from a three-dimensional texture calculated in the same manner as the above method. Find it.
[0093]
In Table 2, among the X-ray random intensity ratios, “intensity ratio 1” means an average value of X-ray random intensity ratios of {100} <011> to {223} <110> orientation groups, “intensity ratio 2” Is the average value of the three-direction X-ray random intensity ratio of {554} <225>, {111} <112> and {111} <110>.
[0094]
Next, in order to investigate the notch fatigue strength of the steel sheet, a fatigue test piece having the shape shown in FIG. 1B is set so that the rolling direction becomes the long side from the 1/4 W or 3/4 W position of the plate width. Were collected and subjected to a fatigue test. However, the fatigue test piece was ground to a depth of about 0.05 mm from the outermost layer. For the fatigue test, an electrohydraulic servo type fatigue tester was used, and the test method was based on JIS Z 2273-1978 and JIS Z 2275-1978. Table 2 also shows notch fatigue limit (σWK) and notch fatigue limit ratio (σWK / σB).
[0095]
Consistent with the present invention are steels A, E, F-1, F-2, F-5, G, H, I, J, K, and L, which contain a predetermined amount of steel components. The average value of the X-ray random intensity ratios of {100} <011> to {223} <110> orientation groups on the plate surface at an arbitrary depth from the outermost surface to 0.5 mm in the plate thickness direction is { 554} <225>, {111} <112> and {111} <110>, the average value of the three-direction X-ray random intensity ratio is 4 or less, and the plate thickness is 0.5 mm or more and 12 mm or less. The thin steel sheet for automobiles having excellent notch fatigue strength is obtained, and thus exceeds the fatigue limit ratio of 0.2 to 0.3 of the conventional steel evaluated by the method described in the present invention.
[0096]
Steels other than the above are outside the scope of the present invention for the following reasons.
That is, steel B has a C content outside the range of claim 5 of the present application, so that sufficient strength (σB) is not obtained. In Steel C, the P content is outside the range of Claim 5 of the present application, so that a sufficient notch fatigue strength (σWK / σB) is not obtained. Steel D does not have a sufficient elongation (El) because the S content is outside the range of claim 5 of the present application. In Steel F-3, the total reduction ratio in the temperature range of Ar3 transformation point temperature + 100 ° C. or less is outside the range of Claim 12 of the present application. Notch fatigue strength (σWK / σB) has not been obtained.
[0097]
Steel F-4 has a finish rolling finish temperature (FT) outside the scope of claim 12, the coiling temperature is outside the scope of the present specification, and the coiling temperature is outside the scope of the present invention. The target texture described in Item 1 is not obtained, and sufficient notch fatigue strength (σWK / σB) is not obtained. Steel F-6 has a cold rolling rate outside the scope of claim 18 of the present application, so the texture of claim 1 cannot be obtained, and sufficient notch fatigue strength (σWK / σB) cannot be obtained. Since the annealing temperature of steel F-7 is outside the range of claim 18 of the present application, the desired texture of claim 1 cannot be obtained, and sufficient notch fatigue strength (σWK / σB) is obtained. Absent. Since the annealing time of steel F-8 is outside the range of claim 18 of the present application, the texture of claim 1 cannot be obtained, and sufficient notch fatigue strength (σWK / σB) cannot be obtained.
[0098]
(Example 2)
Next, the present invention will be described in more detail with reference to Example 2.
Two steels of G and H having the chemical components shown in Table 1 were reheated at the heating temperature shown in Table 3, and were rolled after finishing to a sheet thickness of 1.2 to 5.5 mm by finish rolling after rough rolling. Moreover, as shown in Table 3, for some, after rough rolling, the impact pressure is 2.7 MPa, the flow rate is 0.001 liter / cm.²Descaling was performed under the following conditions.
Details of the manufacturing conditions are shown in Table 3. Here, “SRT” is the slab heating temperature, “FT” is the final pass finish rolling temperature, and “rolling ratio” is the total rolling reduction in the temperature range of Ar 3 transformation point temperature + 100 ° C. or less. However, if the rolling is performed later in the cold rolling process, it is not limited to such a restriction, so “-” is given. “Lubrication” indicates the presence or absence of lubrication in the temperature range of Ar3 transformation point temperature + 100 ° C. or lower. Further, “CT” indicates a winding temperature. However, in the case of a cold-rolled steel sheet, “−” is given because it is not necessary to limit the production conditions. Next, for some, after hot rolling, pickling, cold rolling, and heat treatment were performed. The plate thickness is 0.7 to 2.3 mm. “Cold rolling ratio” is the total cold rolling ratio, “ST” is the heat treatment temperature, and “Time” is the heat treatment time. Note that some of the steel plates were galvanized.
[0099]
The tensile test of the hot-rolled sheet and the cold-rolled sheet thus obtained was performed by the same method as described above.
Table 4 shows the yield strength (σY), tensile strength (σB), elongation at break (El), yield ratio (YR), and strength-ductility balance (σB x El). On the other hand, burring workability (hole expandability) was evaluated according to the hole expansion test method described in Japan Iron and Steel Federation Standard JFS T 1001-1996. Table 4 shows the hole expansion rate (λ).
Further, the microstructure is also shown in Table 4. Here, the other is pearlite and / or a structure other than ferrite, bainite, retained austenite, and martensite individually shown in Table 4. In the microstructure of the steel sheet, the volume fraction of ferrite, bainite, retained austenite, pearlite, and martensite is determined by polishing a sample cut from a 1/4 W or 3/4 W position of the steel sheet width to the cross section in the rolling direction. And an area fraction of the microstructure at 1/4 t of the plate thickness, which was etched using a reagent disclosed in JP-A-5-163590 and observed at a magnification of 200 to 500 times using an optical microscope. The
[0100]
On the other hand, since austenite has a different crystal structure from ferrite, it can be easily distinguished crystallographically. Therefore, the volume fraction of retained austenite can be experimentally determined also by the X-ray diffraction method. That is, it is a method of simply obtaining the volume fraction using the following equation from the difference in the intensity of the reflection surface between austenite and ferrite using the Kα ray of Mo.
Vγ = (2/3) {100 / (0.7 × α (211) / γ (220) +1)} + (1/3) {100 / (0.78 × α (211) / γ (311) +1)}
However, α (211), γ (220) and γ (311) are the X-ray reflection surface strengths of ferrite (α) austenite (γ), respectively. As the volume fraction of retained austenite, an almost consistent value was obtained using any of the optical microscope observation and the X-ray diffraction method, so any measured value may be used.
Further, according to the same method as described above, measurement of X-ray diffraction intensity and fatigue test were performed.
The fatigue test was conducted according to the same method as described above. Table 4 shows the notch fatigue limit (σWK) and the notch fatigue limit ratio (σWK / σB).
[0101]
In accordance with the present invention are steels g-1, g-2, g-3, g-5, g-6, g-7, h-1, h-2, h-3, which are predetermined steels. Of the X-ray random intensity ratio of the {100} <011> to {223} <110> orientation groups of the plate surface at an arbitrary depth from the outermost surface to 0.5 mm in the plate thickness direction. The average value is 2 or more, the average value of X-ray random intensity ratios in three directions of {554} <225>, {111} <112> and {111} <110> is 4 or less, and the plate thickness is 0.5 mm. More than 12 mm and the phase with the largest volume fraction includes bainite, or a composite structure of ferrite and bainite, or residual austenite with a volume fraction of 5% or more and 25% or less, with the balance mainly from ferrite or bainite. The complex tissue or the phase with the highest volume fraction And a thin steel sheet for automobiles having excellent notch fatigue strength, characterized by being a composite structure mainly composed of martensite in the second phase, and therefore, the conventional steel evaluated by the method described in the present invention. A significant difference is observed with respect to the fatigue limit ratio of 20 to 30%.
[0102]
Steels other than the above are outside the scope of the present invention for the following reasons.
That is, Steel g-4 has a finish rolling finish temperature (FT) and an Ar3 transformation point temperature of + 100 ° C. or less, and the total rolling reduction is outside the scope of Claim 12 of the present invention. And a sufficient notch fatigue strength (σWKk / σB) cannot be obtained. Steel g-8 has a cold rolling rate outside the range of claim 13 of the present invention, so that the target texture of claim 1 cannot be obtained, and sufficient notch fatigue strength (σWK / σB) is obtained. It is not done. Steel h-4 has a finish rolling finish temperature (FT) and an Ar3 transformation point temperature of + 100 ° C. or less, and the total rolling reduction is outside the scope of claim 12 of the present invention. A texture cannot be obtained, and a sufficient notch fatigue strength (σWK / σB) cannot be obtained.
[0103]
[Table 1]

[0104]
[Table 2]

[0105]
[Table 3]

[0106]
[Table 4]

[0107]
【The invention's effect】
As described above in detail, the present invention relates to an automotive thin steel sheet having excellent notch fatigue strength and a method for producing the same, and by using these thin steel sheets, stress concentration parts such as punched parts and welded parts are provided. As a result, it can be expected to greatly improve notch fatigue strength, which is one of the important characteristics of parts that require durability such as automobile undercarriage parts, where the development of fatigue cracks from It is a highly valuable invention.
[Brief description of the drawings]
FIG. 1 is a diagram for explaining the shape of a fatigue test piece, where (a) shows a smooth fatigue test piece and (b) shows a notched fatigue test piece.
FIG. 2 shows the results of preliminary experiments leading to the present invention, the average value of the X-ray random intensity ratio of {100} <011> to {223} <110> orientation groups, and {554} <225>, {111} <112> and {111} <110> three-direction average X-ray random strength ratio and notch fatigue strength (10⁷It is a figure shown in the relationship of time intensity | strength in times: fatigue limit).

Claims (12)

% By mass
C: 0.01 to 0.3%
Si: 0.01-2%
Mn: 0.05-3%,
P ≦ 0.1%,
S ≦ 0.01%,
Al: 0.005 to 1%
{100} <011> to {223} <110> orientation group of the plate surface at an arbitrary depth from the outermost surface to 0.5 mm in the plate thickness direction. The average value of the X-ray random intensity ratio is 2 or more, and the average value of the three-direction X-ray random intensity ratios of {554} <225>, {111} <112> and {111} <110> is 4 or less A thin steel sheet for automobiles having excellent notch fatigue strength, wherein the sheet thickness is 0.5 mm or more and 12 mm or less.   A thin steel sheet for automobiles having excellent notch fatigue strength, wherein the microstructure of the steel sheet according to claim 1 is a bainite or a composite structure of ferrite and bainite having a maximum volume fraction.   The notch fatigue strength, wherein the microstructure of the steel sheet according to claim 1 contains a retained austenite having a volume fraction of 5% or more and 25% or less, and the balance is a composite structure mainly composed of ferrite and bainite. Excellent steel sheet for automobiles.   The microstructure of the steel sheet according to claim 1 is a composite structure in which the phase having the largest volume fraction is ferrite and the second phase is mainly martensite, and is excellent in notch fatigue strength. Thin steel plate. Steel component is further mass%,
Cu: 0.2-2% ,
B: 0.0002 to 0.002% and
The thin steel sheet for automobiles having excellent notch fatigue strength according to any one of claims 1 to 4 , wherein Ni: 0.1 to 1% of one kind or two kinds are contained. Steel component is further mass%,
Ca: 0.0005 to 0.002%,
REM: 0.0005 to 0.02%
The thin steel plate for automobiles having excellent notch fatigue strength according to any one of claims 1 to 5 , wherein one or two of the above are contained. Steel component is further mass%,
Ti: 0.05 to 0.5%,
Nb: 0.01-0.5%
Mo: 0.05 to 1%
V: 0.02-0.2%,
Cr: 0.01-1%,
Zr: 0.02 to 0.2%
The thin steel plate for automobiles having excellent notch fatigue strength according to any one of claims 1 to 6 , wherein the thin steel plate is excellent in notch fatigue strength. A steel slab having the component according to any one of claims 1 and 5 to 7 is subjected to finish rolling with a total reduction ratio of 25% or more of the steel sheet thickness in a temperature range of Ar3 transformation point temperature + 100 ° C or less after rough rolling. A method for producing a hot-rolled thin steel sheet for automobiles having excellent notch fatigue strength. The method for producing a hot-rolled thin steel sheet for automobiles having excellent notch fatigue strength according to claim 8 , wherein lubrication rolling is performed in finish rolling after rough rolling during hot rolling. A method for producing a hot-rolled thin steel sheet for automobiles having excellent notch fatigue strength, wherein descaling is performed after completion of rough rolling in the hot rolling according to claim 8 or 9 . A steel slab having the component according to any one of claims 1 and 5 to 7 is hot-rolled, followed by pickling, cold-rolling with a steel sheet thickness reduction of less than 80%, and a recovery temperature or higher and an Ac3 transformation point temperature. A method for producing a cold-rolled thin steel sheet for automobiles having excellent notch fatigue strength, wherein the process is held at a temperature range of + 100 ° C. or lower for 5 to 150 seconds, and cooling is recovered or recrystallization annealing is performed. A steel slab having the component according to any one of claims 1 and 5 to 7 is hot-rolled, followed by pickling, cold-rolling with a steel sheet thickness reduction of less than 80%, and an Ac3 transformation point temperature or higher. A method for producing a cold-rolled thin steel sheet for automobiles having excellent notch fatigue strength, characterized by performing heat treatment in a step of holding at a point temperature of + 100 ° C or lower for 5 to 150 seconds and then cooling.

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