JP4287976B2 - High-strength hot-rolled steel sheet having high dynamic deformation resistance and good formability and manufacturing method thereof - Google Patents

Description

【００１８】
Ａｌ、Ｓｉ：ＡｌとＳｉは共にフェライトの安定化元素であり、フェライト体積率を増加させることによって鋼材の加工性を向上させる働きがある。また、Ａｌ、Ｓｉ共にセメンタイトの生成を抑制することから、効果的にオーステナイト中へのＣを濃化させることを可能とすることから、室温で適当な体積分率のオーステナイトを残留させるためには不可避的な添加元素である。この様な機能を持つ添加元素としては、Ａｌ、Ｓｉ以外に、ＰやＣｕ、Ｃｒ、Ｍｏ等があげられ、この様な元素を適当に添加することも同様な効果が期待される。
【００１９】
しかしながら、ＡｌとＳｉの一種もしくは双方の合計が０．５質量％未満の場合には、セメンタイト生成抑制の効果が十分でなく、オーステナイトの安定化に最も効果的な添加されたＣの多くが炭化物の形で浪費され、本発明に必要な残留オーステナイト体積率を確保することが出来ないかもしくは残留オーステナイトの確保に必要な製造条件が大量生産工程の条件に適しない。従って下限を０．５質量％とした。また、ＡｌとＳｉの一種もしくは双方の合計が３．０％を越える場合には、母相であるフェライトもしくはベイナイトの硬質化や脆化を招き、歪み速度上昇による変形抵抗の増加を阻害するばかりでなく、鋼材の加工性の低下、靱性の低下、さらには鋼材コストの上昇を招き、また化成処理性等の表面処理特性が著しく劣化するために、３．０質量％を上限値とした。
【００２０】
Ｍｎ、Ｎｉ、Ｃｒ、Ｃｕ、Ｍｏ：Ｍｎ、Ｎｉ、Ｃｒ、Ｃｕ、Ｍｏは全てオーステナイト安定化元素であり、室温でオーステナイトを安定化させるためには有効な元素である。特に、溶接性の観点からＣの添加量が制限される場合には、この様なオーステナイト安定化元素を適量添加することによって効果的にオーステナイトを残留させることが可能となる。また、これらの元素はＡｌやＳｉ程ではないがセメンタイトの生成を抑制する効果があり、オーステナイトへのＣの濃化を助ける働きもする。更に、これらの元素はＡｌ、Ｓｉと共にマトリックスであるフェライトやベイナイトを固溶強化させることによって、高速での動的変形抵抗を高める働きも持つ。
【００２１】
しかしながら、これらの元素の１種もしくは２種以上の添加の合計が０．５質量％未満の場合には、必要な残留オーステナイトの確保が出来なくなるとともに、鋼材の強度が低くなり、有効な車体軽量化が達成できなくなることから、下限を０．５質量％とした。一方、これらの合計が３．５質量％を越える場合には、母相であるフェライトもしくはベイナイトの硬質化を招き、歪み速度上昇による変形抵抗の増加を阻害するばかりでなく、鋼材の加工性の低下、靱性の低下、さらには鋼材コストの上昇を招くために、上限を３．５質量％とした。
【００２２】
Ｎｂ、Ｔｉ、Ｖ：また、必要に応じて添加するＮｂ、Ｔｉ、Ｖは、炭化物、窒化物もしくは炭窒化物を形成することによって鋼材を高強度化する事が出来るが、その合計が０．３％を越えた場合には母相であるフェライトやベイナイト粒内もしくは粒界に多量の炭化物、窒化物もしくは炭窒化物として析出し、高速変形時の可動転位発生源となって、高い動的変形抵抗を得ることが出来なくなる。また、炭化物の生成は、本発明にとって最も重要な残留オーステナイト中へのＣの濃化を阻害し、Ｃを浪費することから上限を０．３質量％とした。但し、これらの元素の添加によって高強度化するためには、Ｎｂ、Ｔｉ、Ｖの合計で０．００５質量％以上添加することが好ましい。
【００２３】
Ｐ：更に、必要に応じて添加するＰは、鋼材の高強度化や前述のように残留オーステナイトの確保に有効ではあるが、０．２質量％を越えて添加された場合には鋼材のコストの上昇を招くばかりでなく、体積分率最大の相であるフェライトやベイナイトの変形抵抗を必要以上に高め、かつ高速変形時の変形抵抗の上昇を阻害する。更に、耐置き割れ性の劣化や疲労特性、靱性の劣化を招くことから、０．２質量％をその上限とした。但し、Ｐの添加の効果を得るためには、０．００５質量％以上含有することが好ましい。
【００２４】
Ｂ：また、必要に応じて添加するＢは、粒界の強化や鋼材の高強度化に有効ではあるが、その添加量が０．０１質量％を越えるとその効果が飽和するばかりでなく、必要以上に鋼板強度を上昇させ、高速変形時の変形抵抗の上昇を阻害すると共に、部品への加工性も低下させることから、上限を０．０１質量％とした。但し、Ｂの添加効果を得るためには、０．０００５質量％以上含有することが好ましい。
【００２５】
Ｃａ、Ｒｅｍ：選択的に添加するＣａ、Ｒｅｍはいずれも、硫化物の形態を制御することで、靱性、溶接性等を向上させ、更には特に伸びフランジ成形性に代表されるプレス成形性を向上させる元素である。しかしながら、Ｃａが０．０００５質量％未満、Ｒｅｍが０．００１質量％未満の場合にはその効果が発揮されず、また、Ｃａが０．００５質量％超、Ｒｅｍが０．０２質量％超ではこれらの効果が飽和するばかりでなく逆に酸化物起因で靱性を劣化させるために、これらを各々の添加量の上限、下限とした。
【００２６】
次に、フロントサイドメンバー等の衝撃吸収用部材は、特徴的にハット型の断面形状をしており、この様な部材の高速での衝突圧潰時の変形を本発明者らが解析した結果、最大では４０％以上の高い歪みまで変形が進んでいるものの、吸収エネルギー全体の約７０％以上が、高速の応力−歪み線図の１０％以下の歪み範囲で吸収されていることを見いだした。従って、高速での衝突エネルギーの吸収能の指標として、１０％以下での高速変形時の動的変形抵抗を採用した。特に、歪み量として３％〜１０％の範囲が最も重要であることから、高速引張り変形時の相当歪みで３％〜１０％の範囲の平均応力σｄｙｎをもって衝撃エネルギー吸収能の指標とした。
【００２７】
この高速変形時の３％〜１０％の平均応力σｄｙｎは、鋼材の静的な引張り強度｛５×１０^-4〜５×１０^-3（１／ｓ）の歪み速度範囲で測定された静的な引張り試験における最大応力ＴＳ｝の上昇に伴って大きくなることが一般的である。従って、鋼材の静的な引張り強度を増加させることは部材の衝撃エネルギー吸収能の向上に直接寄与する。しかしながら、鋼材の強度が上昇すると部材への成形性が劣化し、必要な部材形状を得ることが困難となる。従って、同一のＴＳで高いσｄｙｎを持つ鋼材が好ましい。特に部材への加工時の歪みレベルが主に１０％以下であることから、部材への成型時に考慮すべき形状凍結性等の成形性の指標となる低歪み領域での応力が低いことが成形性向上のためには重要である。
【００２８】
従って、σｄｙｎと５×１０^-4〜５×１０^-3（１／ｓ）の歪み速度範囲で変形した時の３〜１０％の相当歪み範囲における変形応力の平均値σｓｔの差が大きいほど静的には成形性に優れ、動的には高い衝撃エネルギーの吸収能を持つと言える。この関係で、特に（σｄｙｎ−σｓｔ）×ＴＳ／１０００≧４０の関係を満足する鋼材は、実部材への成形性に優れると同時に衝撃エネルギー吸収能が他の鋼材に比べて高く、部材の総質量を増加させることなく衝撃エネルギー吸収能を向上させることができる。本発明者らの実験検討の結果、同一レベルのＴＳに対して、（σｄｙｎ−σｓｔ）は部材への加工が行われる以前の鋼板中に含まれる残留オーステナイト中の固溶炭素量［Ｃ］鋼材の平均Ｍｎ等量質量％（Ｍｎｅｑ＝Ｍｎ＋（Ｎｉ＋Ｃｒ＋Ｃｕ＋Ｍｏ）／２）によって変化することが見いだされた。
【００２９】
残留オーステナイト中の炭素濃度は、Ｘ線解析やメスバウアー分光により実験的に求めることが出来、例えば、板状の資料に対してＣｏ、Ｃｕ、ＦｅのＫα線を用いたＸ線解析により、オーステナイトの（００２）、（０２２）、（１１３）、（２２２）面の反射角度を測定し、「Ｘ線回折要論」、Ｂ．Ｄ．Ｃｕｌｌｉｔｙ著（松村源太郎訳）、株式会社アグネの第１１章に記述されているように、反射角度から格子常数を計算し、ｃｏｓ² Θ＝０（但しΘは反射角度）に外挿する事で得られる格子常数の値から、オーステナイトの格子常数とオーステナイト中の固溶Ｃ濃度との関係｛例えばＲ．Ｃ．Ｒｕｈｌ ａｎｄ Ｍ．Ｃｏｈｅｎ，Ｔｒａｎｓａｃｔｉｏｎ ｏｆ Ｔｈｅ Ｍｅｔａｌｌｕｒｇｉｃａｌ Ｓｏｃｉｅｔｙ ｏｆ ＡＩＭＥ，ｖｏｌ ２４５（１９６９）ｐｐ２４１−２５１に記述されている式［１］、即ち、格子常数＝３．５７２＋０．０３３（質量％Ｃ）の関係｝を用いてオーステナイト中のＣ濃度に換算する事によってなされる。また、オーステナイトの格子常数に及ぼすその他の元素の効果はそれほど大きく無いことから、無視しても差し支えないことがわかっている。
【００３０】
本発明者らが行った実験結果から、この様にして得られた残留オーステナイト中の固溶Ｃ（［Ｃ］）と鋼材に添加されている置換型合金元素から求められるＭｎｅｑを用いて計算される値（Ｍ＝６７８−４２８×［Ｃ］−３３×Ｍｎｅｑ）が−１４０以上１８０以下の場合に、同一の静的な引張り強度ＴＳに対して大きな（σｄｙｎ−σｓｔ）を示すことが見いだされた。このときＭが１８０超では、残留オーステナイトが低歪み領域で硬質のマルテンサイトに変態することから、成形性を支配する低歪み領域での静的な応力を上昇させてしまい、形状凍結性等の成形性を劣化させるのみならず、（σｄｙｎ−σｓｔ）の値を小さくすることから、良好な成形性と高い衝撃エネルギー吸収能の両立が得られないためにＭを１８０以下とした。また、Ｍが−１４０未満の場合には、残留オーステナイトの変態が高い歪み領域に限定されるために、良好な成形性は得られるものの（σｄｙｎ−σｓｔ）を増大させる効果がなくなることからＭの下限を−１４０とした。
【００３１】
残留オーステナイトの量は例えばＭｏのＫα線を用いたＸ線解析によりフェライトの（２００）面、（２１１）面及びオーステナイトの（２００）面、（２２０）面、（３１１）面の積分反射強度をもちいて、Ｊｏｕｒｎａｌ ｏｆ Ｔｈｅ Ｉｒｏｎ ａｎｄ Ｓｔｅｅｌ Ｉｎｓｔｉｔｕｔｅ，２０６（１９６８）ｐ６０に示された方法にて算出できる。また、体積分率最大の相であるフェライト又はベイナイトはナイタール腐食写真を元に画像処理もしくはポイントカウント法などを用いて測定することができる。
【００３２】
相当歪みで０％超１０％以下の予変形を与えた後の残留オーステナイト体積分率の測定も上記の方法によって行うことができる。この時、予変形後の残留オーステナイト体積分率が２．５％未満になると、衝撃エネルギー吸収能が著しく劣化するために、これを相当歪みで０％超１０％以下の予変形を与えた後の残留オーステナイト体積分率の下限値とした。予変形後の残留オーステナイト体積分率の上限は特に定めることなく本発明の効果を得ることができるが、その量（％）が鋼板のＣ濃度（質量％）の１２０倍を越える場合にはオーステナイトの安定性が十分でなく、結果として成形性や衝撃エネルギー吸収能を低下させるために１２０×Ｃ（％）以下とする事が好ましい。
【００３３】
ここで、予変形の様式は、単軸引張り、曲げ、プレス成形、鍛造、圧延、造管、拡管等のどの様な変形様式でもかまわない。また、この予変形前後での残留オーステナイト体積分率の比が０．４未満である場合には、衝撃エネルギー吸収能に及ぼす残留オーステナイトの効果が現れないためにこれを下限値とした。また、この比の上限は特に定めることなく本発明の効果を得ることができるが、今想定している最大の予変形量である相当歪みで１０％の予変形を与えた際に、この比が０．９を越えるような場合には、残留オーステナイトが必要以上に安定となり、効果が小さくなるため、相当歪みで１０％の予変形を与えた際の予変形前後での残留オーステナイト体積分率の比は０．９以下とすることが好ましい。
【００３４】
体積分率最大の相であるフェライトやベイナイトの粒径に比べ、残留オーステナイトの平均粒径が大きくなると、残留オーステナイトの安定性そのものが低下し、成形性も衝撃エネルギー吸収能も低下させるために、残留オーステナイト粒はできるだけ細粒にすることが好ましい。特に体積分率最大の相であるフェライトやベイナイトの粒径に対する残留オーステナイトの平均粒径の比が０．６超となった場合にはこの傾向が顕著であるために、これを粒径比の上限とした。この比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、残留オーステナイト粒を極度に細粒化することは必要以上にオーステナイトを安定化することによって残留オーステナイトの効果を小さくするため、体積分率最大の相であるフェライトやベイナイトの粒径に対する残留オーステナイトの平均粒径の比は０．０５以上であることが好ましい。
【００３５】
本発明鋼は成形性の中でも延性のみならず、穴広げ加工性や曲げ性にも優れていることが確認されている。また、成形性と衝突エネルギー吸収能以外に、疲労特性や衝撃特性も良好である。
本発明鋼に通常のスキンパス圧延や熱延板熱処理、表面処理等を施しても本発明の特性を阻害するものではない。
【００３６】
熱延条件：熱延ままで本発明の鋼板を製造する場合には、所定の成分に調整されたスラブを鋳造ままもしくは一旦冷却した後に１０００℃〜１３００℃の範囲に再度加熱し、熱間圧延を行う。再加熱温度を１０００℃未満とする場合には、スラブの均一加熱が困難となり、表面キズ発生等の問題を生じるので、再加熱温度の下限を１０００℃とした。また、再加熱温度が１３００℃以上では、スラブの変形が激しくなると同時にコスト高となることから、これを上限とした。また、熱延完了温度ＦＴが鋼材の化学成分で決まるＡｒ₃ 変態温度−１０℃未満である場合には（σｄｙｎ−σｓｔ）が低くなる。従ってこれを熱延完了温度の下限値とする。
【００３７】
また熱延完了温度がＡｒ₃ ＋１２０℃以上の場合には必要以上に鋼板の強度が上昇するのみならず、組織の粗大化が起こり、鋼板動的変形抵抗の上昇を阻害する。またこの様な高温で熱延が完了された場合には鋼板の表面粗度が大きくなり、表面品位を落とす。従ってこれを熱延完了温度の上限値とする。熱延完了後に冷却されるが、このときの冷却速度を５℃／秒未満もしくは１００℃／秒超とすることは、大量生産の工程条件上困難であることから、これを下限、上限とした。また冷却の方法は一定の冷却速度で行っても、途中で低冷却速度の領域を含むような複数種類の冷却速度の組み合わせであってもよい。
【００３８】
冷却後鋼板は巻き取り処理が行われるが、巻き取り温度が５００℃以上では所定の量の残留オーステナイトを確保することができないためにこれを上限とした。また、熱延完了温度ＦＴがＡｒ₃ ＋５０℃以上の場合には３００℃未満で巻取ると、必要以上の強度上昇を招くと共に、（σｄｙｎ−σｓｔ）の値が小さくなることからＦＴがＡｒ₃ ＋５０℃以上の場合の巻取温度の下限を３００℃とした。また、ＦＴがＡｒ₃ ＋５０℃未満の場合には、３５０℃未満で巻き取った場合に（σｄｙｎ−σｓｔ）の値が小さくなることから、ＦＴがＡｒ₃ ＋５０℃未満の場合の巻取温度の下限を３５０℃とした。最終的に得られた鋼板の（σｄｙｎ−σｓｔ）を高めるためには巻き取り温度を４６０℃以下とすることが好ましい。
【００３９】
【実施例】
（実施例１）
表２に示す２５種類の鋼材を１２００℃に加熱し、各鋼の成分からＡｒ₃ ＝９０１−３２５×％Ｃ＋３３×％Ｓｉ−９２×％Ｍｎｅｑの式（％Ｍｎｅｑ＝％Ｍｎ＋％Ｎｉ／２＋％Ｃｒ／２＋％Ｃｕ／２＋％Ｍｏ／２）で計算されるＡｒ₃ 変態温度＋５０℃〜Ａｒ₃ 変態温度＋１００℃の範囲内で熱延を完了し、４５℃／秒の冷却速度で冷却し、４００℃〜４５０℃の範囲で巻き取った。この様にして得られた熱延鋼板の動的な特性を調査し、静的な特性と比較した結果を表３に示した。鋼の成分が本発明の範囲内のものについては表中の＊１の欄に示した値、すなわち、（σｄｙｎ−σｓｔ）×ＴＳ／１０００が４０以上であることがわかる。
【００４０】
【表２】

【００４１】
【表３】

【００４２】
（実施例２）
表２に示した本発明の成分範囲内である鋼Ｐ２を用いて、熱延条件を変化させた場合の特性を調査した結果を表４に示す。Ｐ２鋼のＡｒ₃ 変態温度は上記の式から７６４℃と計算された。加熱温度は１２００℃一定とした。熱延完了温度ＦＴがＡｒ₃ ＋５０℃以上の８３０℃の場合には、Ｎｏ．１、４では巻き取り温度ＣＴが本発明の範囲外であるために所定の動的変形抵抗の上昇（σｄｙｎ−σｓｔ）が得られていない。また、Ｎｏ．５では、ＦＴが本発明の範囲外であるために結果的に残留オーステナイト粒径とフェライト粒径の比が０．６よりも大きくなり、所定の動的変形抵抗σｄｙｎが得られていない。他の例はすべて本発明の例であり、熱延完了温度、ＣＴが本発明の範囲内であれば所定の動的変形抵抗の上昇（σｄｙｎ−σｓｔ）が得られることがわかる（表中の＊１の欄の値が４０以上）。
【００４３】
【表４】

【００４４】
【発明の効果】
本発明により、自動車の軽量化と衝突安全性の確保の要求に応えることのできる高い動的変形抵抗を有する良加工性高強度熱延鋼板を確実に提供することができる。
【図面の簡単な説明】
【図１】本発明における、加工性と衝撃エネルギー吸収能のバランスを表す｛（σｄｙｎ−σｓｔ）×ＴＳ／１０００｝とＣｏ添加量の関係を示す図、
【図２】本発明における、衝突時の衝撃エネルギー吸収能の指標である、５×１０² 〜５×１０³ （１／ｓ）の歪み速度範囲で変形した時の３〜１０％の相当歪み範囲における変形応力の平均値σｄｙｎと５×１０^-4〜５×１０^-3（１／ｓ）の歪み速度範囲で変形した時の３〜１０％の相当歪み範囲における変形応力の平均値σstの差（σｄｙｎ−σｓｔ）と静的な素材強度との関係を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention is used for automobile members and the like, and a high-strength hot-rolled steel sheet exhibiting high dynamic deformation resistance that can contribute to ensuring the safety of passengers by efficiently absorbing impact energy at the time of collision, and a method for producing the same It is about.
[0002]
[Prior art]
In recent years, occupant protection in the event of an automobile collision has been recognized as the most important performance of an automobile, and there is an increasing expectation for a material that exhibits high high-speed deformation resistance to cope with it. For example, in the case of a frontal collision of a passenger car, if such a material is applied to a member called a front side member, the member is crushed so that the energy of the impact is absorbed and the impact on the occupant can be reduced.
Since the strain rate of deformation that each part undergoes at the time of a car collision reaches about 10 ³ (1 / s), when considering the shock absorption performance of the material, the dynamic deformation characteristics in such a high strain rate region Elucidation is necessary. At the same time, it is considered essential to simultaneously reduce the weight of an automobile body with the aim of saving energy and reducing CO ₂ emissions, and the need for an effective high-strength steel sheet is increasing.
[0003]
For example, the present inventors have used CAMP-ISIJ Vol. 9 (1996) P.I. 1112 to 1115 report on the high-speed deformation characteristics and impact energy absorption ability of the high-strength thin steel sheet, and the dynamic strength in the high strain rate region of about 10 ³ (1 / s) is 10 ⁻³ ( 1 / s), which is greatly increased compared to the static strength at a low strain rate, and the strain rate dependency of deformation resistance is changed by the strengthening mechanism of the material. Among them, TRIP (transformation induced plasticity) type steel And DP (ferritic martensite two-phase) type steel have been reported to have excellent formability and shock absorption capacity compared to other high-strength steel sheets. Japanese Patent Laid-Open No. 7-18372 discloses a high-strength steel sheet excellent in impact resistance including residual austenite and a method for producing the same. However, the impact absorption capacity of the yield stress accompanying the increase in deformation speed is disclosed. Since only the increase is expressed, it is not clarified how to control the properties of retained austenite in addition to the amount of retained austenite in order to improve the shock absorbing ability.
[0004]
[Problems to be solved by the invention]
As described above, the dynamic deformation characteristics of component materials that affect the absorption of impact energy in the event of an automobile collision are being elucidated little by little. However, it has not been fully clarified what criteria should be used for material selection. In addition, automobile parts are formed into a required part shape by pressing a steel material, and then generally painted and baked, and then incorporated into an automobile to face an actual collision phenomenon. Therefore, it is considered that both press workability and impact absorption energy capability at the time of collision are important. The present invention is a steel material that is molded and used in a part that bears impact energy absorption at the time of a collision such as a front side member, a high-strength hot-rolled steel sheet with good workability that exhibits high impact energy absorption capability, and a method for manufacturing the same. It is intended to provide.
[0005]
[Means for Solving the Problems]
In order to achieve the above object, the present invention provides:
(1) By mass%, C is 0.04% or more and 0.3% or less, and one or two or more of Mn, Ni, Cr, Cu, and Mo are added in a total of 0.5% to 3.5%. In the following, one or both of Si and Al in total includes 0.5% or more and 3% or less, Co includes 0.01% or more and 3% or less, and the balance is Fe and inevitable impurities, and finally obtained microstructure High strength hot rolling with high dynamic deformation resistance and good formability, which is a composite structure with ferrite or bainite having the largest volume fraction and a second phase containing 3% or more residual austenite by volume fraction steel sheet.
[0006]
(2) Value determined by solid solution C mass% [C] in retained austenite and average Mn equivalent mass% of steel (Mneq = Mn + (Ni + Cr + Cu + Mo) / 2) (M = 678−428 × [C] −33 × Mneq) is a high-strength hot-rolled steel sheet having high dynamic deformation resistance and good formability as described in (1) above, which is from -140 to 180.
(3) Deformation stress average value σst and maximum stress TS in an equivalent strain range of 3 to 10% when deformed in a strain rate range of 5 × 10 ^{−4 to} 5 × 10 ⁻³ (1 / s), and 5 The average value σdyn of deformation stress in the equivalent strain range of 3 to 10% when deformed in the strain rate range of × 10 ^{2 to} 5 × 10 ³ (1 / s) is the formula (σdyn−σst) × TS / 1000. The high-strength hot-rolled steel sheet having high dynamic deformation resistance and good formability as described in (1) or (2), wherein ≧ 40 is satisfied.
[0007]
(4) The high dynamic deformation resistance according to any one of (1) to (3) , wherein one or more of Nb, Ti, and V are included in a total of 0.3% by mass or less. High strength hot rolled steel sheet with good formability.
(5) The high-strength hot-rolled steel sheet having high dynamic deformation resistance and good formability according to any one of (1) to (4) , wherein P is contained in an amount of 0.2% by mass or less.
(6) The high-strength hot-rolled steel sheet having high dynamic deformation resistance and good formability according to any one of (1) to (5) , wherein B is contained in an amount of 0.01% by mass or less.
[0008]
(7) Any one of the above (1) to (6) , which contains one or two of Ca: 0.0005 to 0.005% and Rem: 0.001 to 0.02% by mass% high strength hot rolled steel sheet having a high dynamic deformation resistance and good moldability according to one.
(8) The residual austenite volume fraction of the steel material after pre-deformation of more than 0% and not more than 10% exceeds 2.5%, and the residual austenite volume fraction before pre-deformation and the residual after pre-deformation The high-strength hot-rolled steel sheet having high dynamic deformation resistance and good formability according to any one of (1) to ( 7 ) , wherein the austenite volume fraction ratio is 0.4 or more .
(9) The ratio of the average particle size of retained austenite in the finally obtained microstructure to the average particle size of ferrite or bainite that is the phase with the largest volume fraction is 0.6 or less. A high strength rolled steel sheet having high dynamic deformation resistance and good formability according to any one of (1) to ( 8 ).
[0009]
(10) The cast slab having the component according to any one of (1) and (4) to (7) is heated again in a range of 1000 ° C. to 1300 ° C. after being cast or once cooled, and hot rolled Is completed at a hot rolling completion temperature FT of Ar ₃ transformation temperature −10 ° C. or more and Ar ₃ transformation temperature + 120 ° C. or less determined by the components of the steel material, and then cooled and wound at a cooling rate of 5 ° C./second or more and 100 ° C./second or less. When FT is Ar ₃ transformation temperature + 50 ° C. or higher, it is wound at a temperature of 300 ° C. or higher and lower than 500 ° C. When FT is Ar ₃ transformation temperature + 50 ° C. or lower, it is 350 ° C. or higher and lower than 500 ° C. The final microstructure obtained by winding is ferrite or bainite with the largest volume fraction, and is a composite structure with the second phase containing 3% or more of retained austenite. The value (M = 678−428 × [C] −33 × Mneq) determined by the solid solution C mass% [C] and the average Mn equivalent mass% (Mneq = Mn + (Ni + Cr + Cu + Mo) / 2) of the steel material is −140. The average value σst and the maximum stress TS of the deformation stress in the equivalent strain range of 3 to 10% when deformed in the strain rate range of 5 × 10 −4 to 5 × 10 −3 (1 / s) at 180 or less. The average value σdyn of the deformation stress in the equivalent strain range of 3 to 10% when deformed in the strain rate range of 5 × 102 to 5 × 103 (1 / s) is the formula (σdyn−σst) × TS / 1000. A method for producing a high-strength hot-rolled steel sheet having high dynamic deformation resistance and good formability, characterized by satisfying ≧ 40.
[0010]
(11) The retained austenite volume fraction of the steel material after pre-deformation of more than 0% and not more than 10% is more than 2.5%, and the retained austenite volume fraction before pre-deformation and the residual after pre-deformation The method for producing a high-strength hot-rolled steel sheet having high dynamic deformation resistance and good formability as described in (10) above, wherein the ratio of austenite volume fraction is 0.4 or more.
(12) The ratio of the average particle diameter of the retained austenite in the finally obtained microstructure and the average particle diameter of ferrite or bainite that is the phase with the largest volume fraction is 0.6 or less. It exists in the manufacturing method of the high strength hot-rolled steel plate which has the dynamic deformation resistance of said (10) or (11), and favorable formability.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
An impact absorbing member such as a front side member of an automobile is manufactured by press forming of a steel plate or the like, and is required to efficiently absorb impact energy at the time of automobile collision. Therefore, it is necessary to satisfy both good press formability and high energy absorption ability at the time of impact. As a result of the study by the present inventors, it has been found that it is suitable for the steel sheet to contain an appropriate amount of retained austenite as a high-strength steel sheet capable of achieving both such formability and excellent shock absorption characteristics. That is, the optimal microstructure is the volume fraction of retained austenite that transforms into hard martensite during deformation, with ferrite or bainite easily solid solution strengthened by various substitutional elements as the phase with the largest volume fraction. When 3% or more is included, it has been found that both of the above characteristics can be achieved.
[0012]
Although the upper limit of the volume fraction of retained austenite is not particularly defined, the effects of the present invention can be obtained, but when the amount (%) exceeds 120 times the C concentration (mass%) of the steel sheet, the stability of austenite Is not sufficient, and as a result, in order to reduce the moldability and impact energy absorption ability, it is preferable to be 120 × C (%) or less. Further, even when martensite particles are included in the initial microstructure, it is within the scope of the present invention as long as other conditions are satisfied.
[0013]
The reasons for limiting each component at this time are as follows.
C: C is the most important element in the present invention because it is the cheapest element that contributes to the stabilization of austenite necessary for stabilizing and retaining austenite at room temperature. The average C content of the steel material not only affects the retained austenite volume fraction that can be secured at room temperature, but also improves the stability of residual austenite to processing by concentrating in the untransformed austenite during the manufacturing heat treatment. It can be made. However, when the amount added is less than 0.04% by mass, the final obtained austenite volume fraction cannot be ensured to be 3% or more, so 0.04% was made the lower limit.
[0014]
On the other hand, the retained austenite volume fraction that can be secured increases as the average C content of the steel material increases, and the stability of retained austenite can be secured while securing the retained austenite volume fraction. However, when the amount of C added to the steel material is excessive, the strength of the steel material is increased more than necessary, and not only the formability such as press working is hindered, but also the dynamic stress increase compared to the static strength increase. In addition to being hindered, the use of steel as a part is limited by reducing weldability. Therefore, the upper limit of C mass% of the steel material is set to 0.3%.
[0015]
Co: Co is one of the most important additive elements in the present invention. Improving the stability of retained austenite not only improves the formability of the steel sheet as described later, but also improves the dynamic deformation resistance, thereby improving the energy absorption capability at the time of collision. At this time, it is a chemical component in retained austenite that determines the stability of austenite. As a result of investigating austenite having various chemical compositions, the inventors have found that Co can efficiently stabilize the retained austenite by increasing the carbon concentration in the retained austenite.
[0016]
FIG. 1 shows the average stress σdyn during high-speed deformation of a steel sheet of 3 to 10% strain re-heated and hot-rolled in Table 1 with varying amounts of Co and rolled up at 380 to 400 ° C. And the value {(σdyn−σst) × TS / 1000} determined by the average stress σst during the static tensile test and the maximum stress TS during the static tensile test were plotted against the amount of Co added. The larger the vertical axis, the higher the dynamic deformation resistance when compared at the same strength level. As is apparent from the figure, the addition of Co improves the dynamic deformation resistance. At this time, when the amount of Co added is less than 0.01% by mass, the value of the above formula is less than 40, and the demand from the actual member cannot be met as will be described later. In addition, adding more than 3% by mass of Co causes a large economic disadvantage, so this was made the upper limit of Co addition.
[0017]
[Table 1]

[0018]
Al, Si: Both Al and Si are stabilizing elements of ferrite and have a function of improving the workability of the steel material by increasing the ferrite volume fraction. In addition, since it suppresses the formation of cementite for both Al and Si, it is possible to effectively concentrate C in austenite, so that austenite having an appropriate volume fraction remains at room temperature. Inevitable additive element. Examples of the additive element having such a function include P, Cu, Cr, Mo and the like in addition to Al and Si, and the same effect can be expected by appropriately adding such an element.
[0019]
However, when the total of one or both of Al and Si is less than 0.5% by mass, the effect of suppressing cementite formation is not sufficient, and most of the added C that is most effective for stabilizing austenite is carbide. The volume of retained austenite required for the present invention cannot be ensured, or the manufacturing conditions necessary for securing retained austenite are not suitable for the conditions of mass production processes. Therefore, the lower limit was set to 0.5% by mass. In addition, when the total of one or both of Al and Si exceeds 3.0%, the parent phase ferrite or bainite is hardened or embrittled, and only increases the deformation resistance due to an increase in strain rate. In addition, the workability of the steel material, the toughness, and the steel material cost are increased, and the surface treatment characteristics such as the chemical conversion property are remarkably deteriorated.
[0020]
Mn, Ni, Cr, Cu, Mo: Mn, Ni, Cr, Cu, and Mo are all austenite stabilizing elements, and are effective elements for stabilizing austenite at room temperature. In particular, when the amount of addition of C is limited from the viewpoint of weldability, austenite can effectively remain by adding an appropriate amount of such an austenite stabilizing element. In addition, these elements have an effect of suppressing the formation of cementite although not as much as Al and Si, and also serve to assist the concentration of C in austenite. Furthermore, these elements have the function of increasing the dynamic deformation resistance at high speed by strengthening the solution of ferrite and bainite as a matrix together with Al and Si.
[0021]
However, if the total amount of one or more of these elements is less than 0.5% by mass, the necessary retained austenite cannot be secured, and the strength of the steel material is reduced, resulting in effective vehicle weight reduction. Therefore, the lower limit was set to 0.5% by mass. On the other hand, when the total of these exceeds 3.5 mass%, the hard phase ferrite or bainite is hardened, which not only inhibits an increase in deformation resistance due to an increase in strain rate but also improves the workability of the steel material. In order to cause a decrease, a decrease in toughness, and an increase in steel material cost, the upper limit is set to 3.5% by mass.
[0022]
Nb, Ti, V: Nb, Ti, V added as necessary can increase the strength of the steel material by forming carbides, nitrides, or carbonitrides. If it exceeds 3%, it will precipitate as a large amount of carbide, nitride, or carbonitride in ferrite or bainite grains as the parent phase or at grain boundaries, and it will become a source of movable dislocations during high-speed deformation, resulting in high dynamics. Unable to obtain deformation resistance. Further, the formation of carbides inhibits the concentration of C in the retained austenite, which is the most important for the present invention, and wastes C, so the upper limit was made 0.3 mass%. However, in order to increase the strength by adding these elements, it is preferable to add 0.005% by mass or more in total of Nb, Ti, and V.
[0023]
P: Furthermore, P added if necessary is effective in increasing the strength of the steel material and securing retained austenite as described above, but if added over 0.2% by mass, the cost of the steel material is increased. Not only increases the deformation resistance of ferrite or bainite, which is the phase with the largest volume fraction, but also inhibits the increase in deformation resistance during high-speed deformation. Furthermore, since the crack resistance, fatigue characteristics, and toughness are deteriorated, the upper limit is set to 0.2% by mass. However, in order to acquire the effect of addition of P, it is preferable to contain 0.005 mass% or more.
[0024]
B: In addition, B added as necessary is effective for strengthening grain boundaries and increasing the strength of steel materials, but when the added amount exceeds 0.01% by mass, not only the effect is saturated, Since the steel sheet strength is increased more than necessary, the increase in deformation resistance at the time of high-speed deformation is inhibited, and the workability to parts is also reduced, so the upper limit was made 0.01% by mass. However, in order to acquire the addition effect of B, it is preferable to contain 0.0005 mass% or more.
[0025]
Ca, Rem: Both Ca and Rem to be selectively added improve the toughness, weldability, etc. by controlling the form of the sulfide, and more particularly press formability represented by stretch flange formability. It is an element to improve. However, when Ca is less than 0.0005 mass% and Rem is less than 0.001 mass%, the effect is not exhibited, and when Ca exceeds 0.005 mass% and Rem exceeds 0.02 mass%, In order not only to saturate these effects but also to deteriorate the toughness due to oxides, these were made the upper limit and the lower limit of each addition amount.
[0026]
Next, the impact absorbing member such as the front side member has a hat-shaped cross-sectional shape, and as a result of analysis by the inventors of the deformation at the time of collision crushing of such a member at high speed, Although the deformation progressed to a high strain of 40% or more at the maximum, it was found that about 70% or more of the total absorbed energy was absorbed in a strain range of 10% or less of the high-speed stress-strain diagram. Therefore, dynamic deformation resistance during high-speed deformation at 10% or less was adopted as an index of the collision energy absorption capability at high speed. In particular, since the range of 3% to 10% is the most important as the amount of strain, the average stress σdyn in the range of 3% to 10% as the equivalent strain during high-speed tensile deformation was used as an index of the impact energy absorption ability.
[0027]
The average stress σdyn of 3% to 10% during this high-speed deformation is a static tensile strength of the steel material {static force measured in the strain rate range of 5 × 10 ^{−4 to} 5 × 10 ⁻³ (1 / s). In general, the maximum stress TS} in a simple tensile test increases with an increase. Therefore, increasing the static tensile strength of the steel material directly contributes to improving the impact energy absorption capacity of the member. However, when the strength of the steel material increases, the formability of the member deteriorates, and it becomes difficult to obtain a necessary member shape. Therefore, steel materials having high σdyn with the same TS are preferable. In particular, since the strain level during processing of the member is mainly 10% or less, the stress is low in the low strain region, which is an index of formability such as shape freezing property to be considered when forming the member. It is important to improve performance.
[0028]
Accordingly, the larger the difference between the σdyn and the average value σst of the deformation stress in the equivalent strain range of 3 to 10% when the deformation is performed in the strain rate range of 5 × 10 ^{−4 to} 5 × 10 ⁻³ (1 / s), the static is increased. Therefore, it can be said that it has excellent moldability and dynamically absorbs high impact energy. In this relationship, steel materials satisfying the relationship of (σdyn−σst) × TS / 1000 ≧ 40 are particularly excellent in formability into actual members and at the same time have higher impact energy absorption capacity than other steel materials, and the total number of members The impact energy absorption ability can be improved without increasing the mass. As a result of the experimental study by the present inventors, for the same level of TS, (σdyn−σst) is the amount of solute carbon [C] in the retained austenite contained in the steel plate before being processed into the member It was found that it varies depending on the average Mn equivalent mass% (Mneq = Mn + (Ni + Cr + Cu + Mo) / 2).
[0029]
The carbon concentration in the retained austenite can be experimentally determined by X-ray analysis or Mossbauer spectroscopy. For example, the austenite is obtained by X-ray analysis using Co, Cu, Fe Kα rays on a plate-like material. Of (002), (022), (113), and (222) planes are measured. D. As described by Cullity (translated by Gentaro Matsumura) and Chapter 11 of Agne, Inc., the lattice constant is calculated from the reflection angle and extrapolated to cos ² Θ = 0 (where Θ is the reflection angle). From the value of the obtained lattice constant, the relationship between the lattice constant of austenite and the solute C concentration in austenite {for example, R.I. C. Ruhl and M.M. Cohen, Transaction of The Metallurgical Society of AIME, vol 245 (1969) pp 241-251, ie, austenite using the relation of lattice constant = 3.572 + 0.033 (mass% C)} It is done by converting to the C concentration in the medium. Moreover, since the effect of other elements on the lattice constant of austenite is not so great, it is known that it can be ignored.
[0030]
From the results of experiments conducted by the present inventors, the calculation was performed using Mneq obtained from the solid solution C ([C]) in the retained austenite thus obtained and the substitutional alloy element added to the steel material. When the value (M = 678−428 × [C] −33 × Mneq) is −140 or more and 180 or less, it is found that the same static tensile strength TS shows a large (σdyn−σst). It was. At this time, if M is more than 180, the retained austenite is transformed into hard martensite in the low strain region, so that the static stress in the low strain region governing the formability is increased, and the shape freezing property and the like are increased. Since not only the moldability is deteriorated but also the value of (σdyn−σst) is reduced, M is set to 180 or less because good moldability and high impact energy absorption ability cannot be achieved at the same time. Further, when M is less than −140, the transformation of retained austenite is limited to a high strain region, so that although good formability is obtained, the effect of increasing (σdyn−σst) is lost. The lower limit was −140.
[0031]
The amount of retained austenite is determined by, for example, the integrated reflection intensity of ferrite (200) plane, (211) plane and austenite (200) plane, (220) plane, (311) plane by X-ray analysis using Mo Kα ray. It can be calculated by the method shown in Journal of The Iron and Steel Institute, 206 (1968) p60. Further, ferrite or bainite, which is the phase with the maximum volume fraction, can be measured by image processing or a point counting method based on a nital corrosion photograph.
[0032]
Measurement of the retained austenite volume fraction after pre-deformation of more than 0% and not more than 10% with an equivalent strain can also be performed by the above method. At this time, if the retained austenite volume fraction after the pre-deformation is less than 2.5%, the impact energy absorption ability is remarkably deteriorated. The lower limit value of the retained austenite volume fraction. The upper limit of the retained austenite volume fraction after pre-deformation is not particularly defined, and the effects of the present invention can be obtained. However, when the amount (%) exceeds 120 times the C concentration (mass%) of the steel sheet, austenite Is not sufficient, and as a result, in order to reduce moldability and impact energy absorption capacity, it is preferable to be 120 × C (%) or less.
[0033]
Here, the pre-deformation mode may be any deformation mode such as uniaxial tension, bending, press molding, forging, rolling, pipe making, and pipe expansion. In addition, when the ratio of the retained austenite volume fraction before and after this pre-deformation is less than 0.4, the effect of retained austenite on the impact energy absorption ability does not appear, so this was made the lower limit value. Further, the upper limit of this ratio is not particularly defined, and the effect of the present invention can be obtained. However, when 10% pre-deformation is applied at the equivalent strain that is the maximum pre-deformation amount currently assumed, this ratio Is more than necessary, the effect becomes less effective, so the retained austenite volume fraction before and after pre-deformation when pre-deformation of 10% is applied with considerable strain. The ratio is preferably 0.9 or less.
[0034]
When the average particle size of retained austenite is larger than the particle size of ferrite or bainite, which is the phase with the largest volume fraction, the stability of retained austenite itself decreases, and the formability and impact energy absorption capacity also decrease. The retained austenite grains are preferably made as fine as possible. In particular, when the ratio of the average particle size of retained austenite to the particle size of ferrite or bainite, which is the phase with the largest volume fraction, exceeds 0.6, this tendency is significant. The upper limit. Although the lower limit of this ratio is not particularly defined, the effect of the present invention can be obtained. However, extremely reducing the retained austenite grains to reduce the effect of retained austenite by stabilizing the austenite more than necessary. The ratio of the average grain size of retained austenite to the grain size of ferrite or bainite, which is the phase with the largest volume fraction, is preferably 0.05 or more.
[0035]
It has been confirmed that the steel of the present invention is excellent not only in ductility but also in hole expanding workability and bendability. In addition to formability and impact energy absorption, fatigue properties and impact properties are also good.
Even if the steel of the present invention is subjected to ordinary skin pass rolling, hot-rolled sheet heat treatment, surface treatment or the like, the characteristics of the present invention are not impaired.
[0036]
Hot rolling conditions: When the steel sheet of the present invention is produced as it is, it is heated again in the range of 1000 ° C to 1300 ° C after being cast or once cooled, and then hot rolled. I do. When the reheating temperature is less than 1000 ° C., uniform heating of the slab becomes difficult and problems such as surface scratches occur, so the lower limit of the reheating temperature is set to 1000 ° C. In addition, when the reheating temperature is 1300 ° C. or higher, the deformation of the slab becomes severe and the cost increases at the same time. Further, when the hot rolling completion temperature FT is less than Ar ₃ transformation temperature −10 ° C. determined by the chemical composition of the steel material, (σdyn−σst) becomes low. Therefore, this is the lower limit of the hot rolling completion temperature.
[0037]
Further, when the hot rolling completion temperature is Ar ₃ + 120 ° C. or higher, not only the strength of the steel sheet is increased more than necessary, but also the structure is coarsened and the increase in the steel sheet dynamic deformation resistance is hindered. In addition, when hot rolling is completed at such a high temperature, the surface roughness of the steel sheet increases and the surface quality is degraded. Therefore, this is the upper limit of the hot rolling completion temperature. Although it is cooled after completion of hot rolling, it is difficult to set the cooling rate at less than 5 ° C./second or more than 100 ° C./second in terms of process conditions for mass production. . The cooling method may be performed at a constant cooling rate, or may be a combination of a plurality of types of cooling rates including a low cooling rate region on the way.
[0038]
After cooling, the steel sheet is subjected to a winding process, but when the winding temperature is 500 ° C. or higher, a predetermined amount of retained austenite cannot be secured, so this was made the upper limit. Further, when the hot rolling completion temperature FT is Ar ₃ + 50 ° C. or higher, winding at less than 300 ° C. causes an increase in strength more than necessary and decreases the value of (σdyn−σst), so that the FT becomes Ar _3. The lower limit of the coiling temperature in the case of + 50 ° C. or higher was set to 300 ° C. In addition, when FT is less than Ar ₃ + 50 ° C., the value of (σdyn−σst) decreases when the coil is wound at less than 350 ° C. Therefore, the winding temperature when FT is less than Ar ₃ + 50 ° C. The lower limit was 350 ° C. In order to increase the (σdyn−σst) of the finally obtained steel plate, it is preferable to set the coiling temperature to 460 ° C. or less.
[0039]
【Example】
Example 1
25 kinds of steel materials shown in Table 2 were heated to 1200 ° C., and Ar ₃ = 901-325 ×% C + 33 ×% Si-92 ×% Mneq formula (% Mneq =% Mn +% Ni / 2 +%) from each steel component (Cr / 2 +% Cu / 2 +% Mo / 2) calculated in the range of Ar ₃ transformation temperature + 50 ° C. to Ar ₃ transformation temperature + 100 ° C. calculated by (Cr / 2 +% Cu / 2 +% Mo / 2) and cooled at a cooling rate of 45 ° C./second It wound up in the range of 400 to 450 degreeC. The dynamic characteristics of the hot-rolled steel sheet thus obtained were investigated, and the results compared with the static characteristics are shown in Table 3. It can be seen that the steel component within the scope of the present invention has a value shown in the column of * 1 in the table, that is, (σdyn−σst) × TS / 1000 is 40 or more.
[0040]
[Table 2]

[0041]
[Table 3]

[0042]
(Example 2)
Table 4 shows the results of investigating the characteristics when the hot rolling conditions were changed using steel P2 within the component range of the present invention shown in Table 2. The Ar ₃ transformation temperature of P2 steel was calculated as 764 ° C. from the above formula. The heating temperature was kept constant at 1200 ° C. In the case where the hot rolling completion temperature FT is 830 ° C. of Ar ₃ + 50 ° C. or higher, no. In 1 and 4, since the coiling temperature CT is outside the range of the present invention, a predetermined increase in dynamic deformation resistance (σdyn−σst) is not obtained. No. In No. 5, since the FT is outside the range of the present invention, the ratio of the retained austenite grain size to the ferrite grain size is larger than 0.6, and the predetermined dynamic deformation resistance σdyn is not obtained. All other examples are examples of the present invention, and it can be seen that if the hot rolling completion temperature and CT are within the range of the present invention, a predetermined increase in dynamic deformation resistance (σdyn−σst) can be obtained (in the table) * Value in column 1 is 40 or more).
[0043]
[Table 4]

[0044]
【The invention's effect】
According to the present invention, it is possible to reliably provide a high workability high-strength hot-rolled steel sheet having high dynamic deformation resistance that can meet demands for reducing the weight of automobiles and ensuring collision safety.
[Brief description of the drawings]
FIG. 1 is a diagram showing the relationship between {(σdyn−σst) × TS / 1000} representing the balance between workability and impact energy absorption capacity and the amount of Co addition in the present invention;
FIG. 2 is an equivalent strain of 3 to 10% when deformed in a strain rate range of 5 × 10 ^{2 to} 5 × 10 ³ (1 / s), which is an index of impact energy absorption capability in the present invention. The average value σdyn of the deformation stress in the range and the average value σst of the deformation stress in the equivalent strain range of 3 to 10% when deformed in the strain rate range of 5 × 10 ^{−4 to} 5 × 10 ⁻³ (1 / s). It is a figure which shows the relationship between a difference ((sigma) dyn- (sigma) st) and static raw material intensity | strength.

Claims (12)

  In mass%, C is 0.04% or more and 0.3% or less, and one or more of Mn, Ni, Cr, Cu, and Mo are combined in a total of 0.5% to 3.5%, Si One or both of Al and Al in total 0.5% to 3%, Co 0.01% to 3%, the balance consisting of Fe and inevitable impurities, and the final microstructure is ferrite or A high-strength hot-rolled steel sheet having high dynamic deformation resistance and good formability, which is a composite structure with a second phase containing bainite as a phase with the largest volume fraction and containing 3% or more of retained austenite.   The value (M = 678−428 × [C] −33 × Mneq) determined by the solid solution C mass% [C] in the retained austenite and the average Mn equivalent mass% (Mneq = Mn + (Ni + Cr + Cu + Mo) / 2) of the steel material. The high-strength hot-rolled steel sheet having high dynamic deformation resistance and good formability according to claim 1, which is from -140 to 180. An average value σst and maximum stress TS of deformation stress in an equivalent strain range of 3 to 10% when deformed in a strain rate range of 5 × 10 ^{−4 to} 5 × 10 ⁻³ (1 / s), and 5 × 10 ^2. The average value σdyn of the deformation stress in the equivalent strain range of 3 to 10% when deformed in the strain rate range of ˜5 × 10 ³ (1 / s) is expressed by the formula (σdyn−σst) × TS / 1000 ≧ 40 The high-strength hot-rolled steel sheet having high dynamic deformation resistance and good formability according to claim 1 or 2, characterized by being satisfied. The high dynamic deformation resistance and good moldability according to any one of claims 1 to 3 , characterized by containing one or more of Nb, Ti and V in a total amount of 0.3 mass% or less. High strength hot-rolled steel sheet. The high-strength hot-rolled steel sheet having high dynamic deformation resistance and good formability according to any one of claims 1 to 4 , wherein P is contained in an amount of 0.2% by mass or less. The high-strength hot-rolled steel sheet having high dynamic deformation resistance and good formability according to any one of claims 1 to 5 , wherein B is contained in an amount of 0.01% by mass or less. The composition according to any one of claims 1 to 6 , comprising one or two of Ca: 0.0005 to 0.005% and Rem: 0.001 to 0.02% in mass%. High strength hot rolled steel sheet with high dynamic deformation resistance and good formability. The retained austenite volume fraction of the steel material after pre-deformation of more than 0% and less than 10% is more than 2.5%, and the residual austenite volume fraction before pre-deformation and the residual austenite volume fraction after pre-deformation The high-strength hot-rolled steel sheet having high dynamic deformation resistance and good formability according to any one of claims 1 to 7, wherein the ratio of the ratios is 0.4 or more. The ratio of the average particle size of the retained austenite in the finally obtained microstructure to the average particle size of ferrite or bainite, which is the phase with the largest volume fraction, is 0.6 or less. A high-strength rolled steel sheet having high dynamic deformation resistance and good formability according to any one of -8. The cast slab having the component according to any one of claims 1 and 4 to 7 is heated again in a range of 1000 ° C to 1300 ° C after being cast or once cooled, and hot rolling is determined by a component of the steel material. ₃ transformation temperature −10 ° C. or higher Ar ₃ transformation temperature + 120 ° C. Completion at a hot rolling completion temperature FT of less than 120 ° C. Then, when cooling and winding at a cooling rate of 5 ° C./second or more and 100 ° C./second or less, the FT becomes Ar ₃ When the transformation temperature is 50 ° C. or higher, winding is performed at a temperature of 300 ° C. or higher and lower than 500 ° C., and when the FT is Ar ₃ transformation temperature + 50 ° C. or lower, winding is performed at a temperature of 350 ° C. or higher and lower than 500 ° C. The microstructure obtained in the above is a composite structure with the second phase containing ferrite or bainite having the largest volume fraction and containing 3% or more of retained austenite, and the solid solution C mass in the retained austenite. The value (M = 678−428 × [C] −33 × Mneq) determined by [C] and the average Mn equivalent mass% (Mneq = Mn + (Ni + Cr + Cu + Mo) / 2) of the steel material is −140 to 180 and 5 × The average value σst and the maximum stress TS of the deformation stress in the equivalent strain range of 3 to 10% when deformed in the strain rate range of 10 −4 to 5 × 10 −3 (1 / s), and 5 × 10 2 to 5 ×. The deformation stress average value σdyn in the equivalent strain range of 3 to 10% when deformed in the strain rate range of 103 (1 / s) satisfies the formula (σdyn−σst) × TS / 1000 ≧ 40. A method for producing a high-strength hot-rolled steel sheet having high dynamic deformation resistance and good formability.   The retained austenite volume fraction of the steel material after pre-deformation of more than 0% and less than 10% is more than 2.5%, and the residual austenite volume fraction before pre-deformation and the residual austenite volume fraction after pre-deformation The method for producing a high-strength hot-rolled steel sheet having high dynamic deformation resistance and good formability according to claim 10, wherein the rate ratio is 0.4 or more.   The ratio of the average particle diameter of the retained austenite in the finally obtained microstructure to the average particle diameter of ferrite or bainite, which is the phase with the largest volume fraction, is 0.6 or less. Or the manufacturing method of the high intensity | strength hot-rolled steel plate which has the dynamic deformation resistance of 11 and favorable formability.

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