JP4055486B2 - High-tensile hot-rolled steel sheet excellent in high-speed deformation characteristics and elongation characteristics and method for producing the same - Google Patents
High-tensile hot-rolled steel sheet excellent in high-speed deformation characteristics and elongation characteristics and method for producing the same Download PDFInfo
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Description
【０００１】
【発明の属する技術分野】
本発明は、高速変形特性および伸び特性に優れる高張力熱延鋼板およびその製造方法に関する。本発明に係る熱延鋼板は、例えば自動車用のバンパーやドア、キャビンの補強材等の、衝突時に許容される変形量が小さい部材で、高速変形特性、具体的には真ひずみ量0.1 までの吸収エネルギーが高く、加えて成形性を要する部品への適用を意図したものである。
【０００２】
本発明において、炭化物は、その大部分（95面積％以上の部分）がセメンタイトもしくはセメンタイト中のFeの一部がCrやMn等で置換されたものからなる炭化物である。
【０００３】
【従来の技術】
上述のような用途に用いられる熱延鋼板においては、従来は、通常の引張試験における強度が高いことのみが要求されている。しかし、自動車の衝突時の特性を評価するためには、衝突時のひずみ速度での特性（高速変形特性）を把握することが重要である。
【０００４】
一方で、昨今の地球環境問題からの排出ガス規制に関連し、車体重量の軽減は極めて重要な問題である。それとともに乗員の安全性確保も同様に重要である。この両者を両立させるために、使用する鋼板の強度を高めることは勿論のこと、部品形状を工夫し、剛性向上および耐衝突特性向上を試みている。そのため、素材となる鋼板には従来鋼板を凌ぐ成形性および高速変形特性を兼備したものが要求される。
【０００５】
この要求に応じるべく、例えば特開平10-95588号公報では、真ひずみ0.3 ％までの吸収エネルギーの高い鋼板を製造するための技術が提案されている。しかし、前述した部品、すなわち自動車用のバンパーやドア、キャビンの補強材等では、乗員の安全性確保の面から、衝突時に大きく変形して衝突のエネルギーを吸収することは好ましくなく、小さな変形量で大きなエネルギー吸収能を発揮できる高速変形特性が重要であることに照らすと、前記特開平10-95588号公報所載の技術では、小さな変形量での高速変形特性についての特性向上には十分でない場合があった。
【０００６】
一方、成形性向上に関して特開平７−62485 号公報所載の技術が知られているが、そこでは高速変形特性についての検討がされていない。また、特開平11−43740 号公報には高速変形特性および成形性の向上に関する技術が開示されているが、そこでの高速変形特性の向上とは高速変形時のｎ値を向上させることであり、真ひずみ0.1 までといった小さな変形時の吸収エネルギーを向上させることに対しては何の指針も与えない。
【０００７】
【発明が解決しようとする課題】
本発明は、上記問題に鑑みて、高速変形特性および伸び特性に優れる高張力熱延鋼板、すなわち真ひずみ0.1 の変形量での高速変形時の吸収エネルギーが大きくかつ伸び特性に優れる高張力熱延鋼板、およびその製造方法を提供することを目的とする。
【０００８】
【課題を解決するための手段】
本発明者らは、上記目的を達成するために鋭意検討を重ねた結果、鋼板組織中の微細炭化物、残留オーステナイト、マルテンサイト、およびフェライトの分率を適正範囲に制御することにより、真ひずみ0.1 の変形量での高速変形時の吸収エネルギーが上昇し、かつ伸び特性も向上するという知見を得るに至った。本発明はかかる知見に基づいてなされたものであり、その要旨とするところは以下のとおりである。
【０００９】
（１）質量％で、Ｃ：０．０５％以上０．２％以下、Ｓｉ：０．８％以上２．５％以下、Ｍｎ：０．５％以上２．５％以下、Ｐ：０．０１５％以下、Ｓ：０．０１％以下、Ａｌ：０．０５％以下を含み、残部がＦｅおよび不可避的不純物からなる組成、および、面積％で、円相当径０．５μｍ以下の炭化物：０．０５％以上０．３％以下、残留オーステナイト：３％以上１５％以下、フェライト：６０％以上を含み、マルテンサイト：５％以下である（ただし、マルテンサイトと残留オーステナイトの合計が６％未満である範囲を除く）組織を有することを特徴とする高速変形特性および伸び特性に優れる高張力熱延鋼板。
【００１０】
（２）前記（１）において鋼板がさらに質量％でCr：0.5 ％以下、Mo：0.3 ％以下の１種または２種を含む組成になることを特徴とする高速変形特性および伸び特性に優れる高張力熱延鋼板。
（３）前記（１）または（２）において鋼板がさらに質量％でTi：0.05％以下を含む組成になることを特徴とする高速変形特性および伸び特性に優れる高張力熱延鋼板。
【００１１】
（４）質量％で、Ｃ：0.05％以上0.2 ％以下、Si：0.8 ％以上2.5 ％以下、Mn：0.5 ％以上2.5 ％以下、Ｐ：0.015 ％以下、Ｓ：0.01％以下、Al：0.05％以下を含み、残部がFeおよび不可避的不純物からなる組成を有する鋼スラブを、1000℃以上1300℃以下に加熱後圧延し、該圧延をＡr3点以上の温度で仕上げた後、650 ℃以上800 ℃未満の温度まで平均冷却速度30℃/s以上で冷却し、次いで640 ℃以上780 ℃以下の温度まで３〜10秒間で空冷し、次いで下記(1) 式の値Ψ以上の平均冷却速度で10×Ψ（℃）以上10×Ψ＋50（℃）以下の温度まで冷却し、該温度で巻き取ることを特徴とする高速変形特性および伸び特性に優れる高張力熱延鋼板の製造方法。
【００１２】
記
Ψ（℃/s）＝30000 ／｛SRT ×（〔％Ｃ〕＋〔％Mn〕／６＋〔％Si〕／24＋〔％Mo〕／４）｝ ‥‥(1)
SRT ：スラブ加熱温度（℃）
〔％Ｃ〕、〔％Mn〕、〔％Si〕、〔％Mo〕：各元素の含有量（質量％）
（５）前記（４）において、鋼スラブがさらに質量％で、Cr：0.5 ％以下、Mo：0.3 ％以下の１種または２種を含む組成になることを特徴とする高速変形特性および伸び特性に優れる高張力熱延鋼板の製造方法。
【００１３】
（６）前記（４）または（５）において、鋼スラブがさらに質量％で、Ti：0.05％以下を含む組成になることを特徴とする高速変形特性および伸び特性に優れる高張力熱延鋼板の製造方法。
【００１４】
【発明の実施の形態】
まず、本発明に係る高速変形特性および伸び特性に優れる高張力熱延鋼板（本発明鋼板）の組成の限定理由について述べる。
Ｃ：0.05％以上0.2 ％以下
Ｃは、所定の強度を確保しかつ残留オーステナイトを生成させるために0.05％以上の含有を必要とするが、0.2 ％を超えて含有すると溶接性が著しく劣化し実使用に耐えなくなるから0.2 ％以下とする。好ましくは0.08％以上0.16％以下である。
【００１５】
Si：0.8 ％以上2.5 ％以下
Siは、円相当径0.5 μｍ以下の微細な炭化物の生成を促進するために重要な元素であり、残留オーステナイトの生成を促進するためにも必要である。とくに鋼組織の円相当径0.5 μｍ以下の微細な炭化物分率を0.05％以上0.3 ％以下に制御し、かつ残留オーステナイト分率を３％以上15％以下に制御するためには0.8 ％以上のSi含有を必要とする。しかし、多量に添加すると冷間圧延性の低下をもたらすので、Siの上限含有量を2.5 ％とする。好ましくは1.2 ％以上2.0 ％以下である。
【００１６】
Mn：0.5 ％以上2.5 ％以下
Mnは、強度上昇のために0.5 ％以上の含有を必要とするが、過剰な添加は鋼板の溶接性を著しく低下させるので、上限含有量を2.5 ％とする。好ましくは0.8 ％以上1.8 ％以下である。
Ｐ：0.015 ％以下
Ｐは、旧オーステナイト粒界に偏析して低温靭性を劣化させ、また強い鋼中偏析傾向により鋼板の異方性を増大させ加工性を低下させるので、極力低減することが望ましいが、0.015 ％までは許容される。
【００１７】
Ｓ：0.01％以下
Ｓは、旧オーステナイト粒界に偏析し、あるいはMnS を多量に生成した場合、低温靭性を劣化させ、寒冷地での鋼板使用を困難にするので、極力低減することが望ましいが、0.01％までは許容される。
Al：0.05％以下
Alは、鋼の脱酸剤として添加され、鋼の清浄度の向上に寄与し、また鋼板組織の微細化のためにも有用な元素である。この効果を得るためには0.001 ％以上含有させることが好ましい。しかし、0.05％を超えて含有すると介在物が多量に発生し、冷延鋼板の疵の原因となるので、上限を0.05％とした。
【００１８】
Cr：0.5 ％以下
Crは、鋼板中の円相当径0.5 μｍ以下の微細炭化物および残留オーステナイトの分率の制御因子として適宜使用しうる。この制御効果を得るためには0.1 ％以上含有させることが好ましいが、多量に添加すると部品に成形後の鋼板の電着塗装性を低下させるので、上限含有量を0.5 ％とした。好ましくは0.3 ％以下である。
【００１９】
Mo：0.3 ％以下
Moは、Crと同様に鋼板中の微細炭化物および残留オーステナイトの分率の制御因子として適宜使用しうる。この制御効果を得るためには0.05％以上含有させることが好ましいが、多量に添加すると冷間圧延を困難ならしめるので、上限含有量を0.3 ％とした。好ましくは0.2 ％以下である。
【００２０】
Ti：0.05％以下
Tiは、鋼板中の円相当径0.5 μｍ以下の微細炭化物の分率の制御因子として適宜使用しうる。この制御効果を得るためには0.001 ％以上含有させることが好ましいが、多量に添加してもその効果が飽和するので、上限含有量を0.05％とした。好ましくは0.005 ％以上0.03％以下である。
【００２１】
上記した成分以外の残部はFeおよび不可避的不純物である。
次に、本発明鋼板の組織の限定理由について述べる。
円相当径0.5 μｍ以下の炭化物：0.05％以上0.3 ％以下
炭化物の円相当径が小さいほど真ひずみ0.1 までの高速変形時の吸収エネルギーが増加するが、その効果は円相当径0.5 μｍ以下から現れるため、組織は円相当径0.5 μｍ以下の炭化物を含むものとした。さらに組織全部に対する円相当径0.5 μｍ以下の炭化物の分率（面積％で表される）について、それが大きいほど真ひずみ0.1 までの高速変形時の吸収エネルギーが増加するが、その効果は0.05％以上から現れるため、下限を0.05％とした。しかし、0.3 ％を超えると残留オーステナイト中に濃化する炭素量が減少しすぎて、伸び特性が劣化するので、上限を0.3 ％とした。好ましくは0.05％以上0.2 ％以下である。
【００２２】
残留オーステナイト：３％以上15％以下
残留オーステナイトは、伸び特性を良好として引張強さ（ＴＳ）- 伸び（Ｅｌ）バランスを良好とする、すなわちTS×Elを大きくするために必要である。本発明において、伸び特性に優れるとは、TS×El≧24000 ＭPa・％の特性を有することを意味する。TS×Elを24000 ＭPa・％以上とするためには残留オーステナイトの分率を面積率で３％以上15％以下とする必要がある。残留オーステナイトが３％未満では伸びが低下し、TS×El≧24000 ＭPa・％を達成することが困難となる。また、残留オーステナイトが15％を超えると、残留オーステナイト中への炭素の濃化が十分でないためと考えられるが、やはり伸びが低下し、TS×El≧24000 ＭPa・％を達成することが困難となる。なお、好ましくは５％以上10％以下である。
【００２３】
フェライト：60％以上
本発明鋼板において、フェライトは、伸び特性を良好としてTS-El バランスを良好とし、TS×Elを24000 ＭPa・％以上とするために必要であり、フェライトの分率を面積率で60％以上とする必要がある。フェライトの分率が60％未満では、伸びが低下し、TS×El≧24000 ＭPa・％を達成することが困難となる。なお、このフェライトとしては、ポリゴナルフェライト（粒状フェライト）の他、ベイニティックフェライト（ラス状フェライト）が存在する。このベイニティックフェライトは、後述するように、本発明の特徴である円相当径0.5 μｍ以下の微細炭化物の生成と同期して生成し、該炭化物の分率0.05〜0.3 ％での生成に対応して、10〜40％程度の分率で生成する。
【００２４】
マルテンサイト：５％以下
マルテンサイトが存在すると伸びが低下するので、本発明では極力形成させないようにすることが好ましいが、本発明の目的の特性を得るために、面積率で５％までは許容される。
なお、本発明では他に、円相当径0.5 μｍを超える炭化物あるいはさらに前記以外の低温変態相が存在し得るが、円相当径0.5 μｍ以下の炭化物、残留オーステナイトあるいはさらにマルテンサイトの他の残部の組織は、主に前記フェライトとすることが好ましく、円相当径0.5 μｍ以下の炭化物、残留オーステナイトあるいはさらにマルテンサイト以外の残部のうち95％以上をフェライトとすることが好ましい。
【００２５】
次に、本発明鋼板の製造方法の限定理由について述べる。
本発明の製造方法に用いられる鋼スラブの組成は、上述した本発明鋼板の組成の範囲内とすればよい。
スラブ加熱温度：1000℃以上1300℃以下
スラブ加熱温度は、後述の圧延仕上げ温度をＡr3点以上とするために最低でも1000℃とする必要があり、オーステナイト粒径の粗大化を抑制して本発明の目的の特性を確保するために1300℃以下とする必要がある。
【００２６】
圧延仕上げ温度（仕上圧延終了温度）：Ａr3点以上
圧延をＡr3点未満の温度で仕上げると、高速変形時の吸収エネルギーが低下するので、圧延仕上げ温度はＡr3点以上とした。
圧延仕上げ後、650 ℃以上800 ℃未満の温度まで平均冷却速度30℃/s以上で冷却（：第１冷却）
仕上圧延終了後、直ちに650 ℃以上800 ℃未満の温度まで急冷することにより、パーライト変態を抑制し、後述の第２〜第３冷却において目的とする組織を得やすくする。該急冷の平均冷却速度が30℃/s未満ではパーライト変態が起こり、高速変形時の吸収エネルギーが低下するので、平均冷却速度は30℃/s以上とした。
【００２７】
次いで640 ℃以上780 ℃以下の温度まで３〜10秒間で空冷（：第２冷却）
前記急冷（強制冷却）の停止に引き続き640 ℃以上780 ℃以下の温度まで空冷（放冷）を行うことにより、フェライト変態を促進し、未変態のオーステナイトへ炭素を濃化させる。２秒以下の空冷ではフェライトが十分に析出せず目的の組織が得られず、一方、11秒以上空冷しても組織の変化は小さく製造効率が悪いので、空冷時間は３〜10秒間とした。なお、ここで析出するフェライトはポリゴナルフェライトである。
【００２８】
次いで前記(1) 式の値Ψ以上の平均冷却速度で10×Ψ（℃）以上10×Ψ＋50（℃）以下の温度まで冷却（：第３冷却）、該温度での巻取り
この条件が、炭化物制御に最も重要である。前記（１）〜（３）のいずれかに記載の組成条件とこの第３冷却条件（および巻き取り条件）との両方が満たされた場合にのみ、鋼板中に円相当径0.5 μｍ以下の炭化物が0.05％以上0.3 ％以下の分率で存在するようになり、高速変形時の吸収エネルギーと伸びとを共に増大させることができる。
【００２９】
すなわち、目的とする微細炭化物は、Ψ（℃/s）以上の平均冷却速度で10×Ψ（℃）以上10×Ψ＋50（℃）以下の温度まで冷却し、該温度で巻き取った後に析出する。この巻き取り中にオーステナイトからフェライトへの変態が起こってベイニティックフェライトが生成すると共に、この生成したベイニティックフェライトの粒界を主な析出サイトとして炭化物が析出する。また、オーステナイトからフェライトへの変態に伴い、オーステナイトへの炭素の濃化も進行してオーステナイトを安定化し、分率３〜15％の残留オーステナイトを確保できる。
【００３０】
Ψ（℃/s）よりも遅い平均冷却速度で冷却すると、冷却中に炭化物が析出し始めるため、最終的に得られる炭化物が粗大化し、目的とする炭化物組織を得ることができない。また、巻取り温度を10×Ψ（℃）未満とすると、炭化物を析出させる温度が低くなりすぎるため、充分な炭化物量を得ることができず、また、マルテンサイトが多量に生成するようになり、残留オーステナイトの分率も過剰となる。一方、巻取り温度を10×Ψ＋50（℃）超えとすると、炭化物が多量に析出すると共に粗大に成長するため、目的とする炭化物組織を得ることができず、オーステナイトへの炭素の濃化が不充分となり、充分な残留オーステナイト量を確保できなくなる。
【００３１】
【実施例】
表１に示す組成になる鋼を転炉で溶製し、連続鋳造によりスラブとなし、該スラブを、表２に示す条件で処理（熱間圧延→第１〜第３冷却→巻取り）して、板厚2.0mm の熱延鋼板となした。得られた鋼板について、ミクロ組織、引張特性、真ひずみ0.1 までの高速変形時の吸収エネルギーを、それぞれ次の要領で調査した。
【００３２】
・円相当径：0.5 μｍ以下の炭化物の分率：鋼板被検面（：Ｌ断面（圧延方向に平行な断面）の板厚方向の両端の10％厚み部を除いた部分）について、体積％で過塩素酸7.8 ％、エタノール70％、2-ブトキシエタノール10％を含み残部が蒸留水からなる研磨液で電解研磨を施した後、原子間力顕微鏡（ＡＦＭ）で10000 倍に拡大した組織を観察して炭化物を同定し、個々の炭化物の円相当径を決定するとともに面積率を測定し、該円相当径が0.5 μｍ以下の炭化物の総面積率を求め、これを表３に示した。
【００３３】
なお、ここで円相当径は、組織の炭化物部分を各々画像処理により求めた面積から計算されたのと同じ面積の円の直径である。
・残留オーステナイトの分率：鋼板被検面について、Ｘ線回折により残留オーステナイトを同定し、その面積率を測定し、該面積率を分率とした。
・マルテンサイト、フェライト（ポリゴナルフェライト、ベイニティックフェライト）の分率：鋼板被検面について、ナイタール腐食現出組織の走査型電子顕微鏡（ＳＥＭ）による2000倍拡大像を目視でそれぞれの組織に分別し、それぞれの領域を色分けした後に画像解析により定量化して面積率を測定し、これをこれら組織の分率とした。表３にはベイニティックフェライトおよびフェライト全量の面積率としてベイニティックフェライトとポリゴナルフェライトの合計量を示している。
【００３４】
・引張特性：圧延方向に直交する方向を試験方向として採取したＪＩＳ５号試験片を用い、ＪＩＳ Ｚ 2241に準拠した方法で引張強さ（ＴＳ）、伸び（Ｅｌ）を測定した。
・真ひずみ0.1 までの高速変形時の吸収エネルギー：圧延方向を試験方向として試験片を採取し、ひずみ速度2000/sで文献（鉄と鋼，83(1997)748 ）に示されるようなホプキンソン・プレッシャー・バーという方法で引張試験を行い、得られた応力- ひずみ曲線を真ひずみ0.0 〜0.1 の範囲で積分することにより吸収エネルギーを求めた。
【００３５】
これらの調査結果を表３に示す。また、表３に示した実施例と比較例について、ＴＳと真ひずみ0.1 までの吸収エネルギーの関係をグラフにして図１に示す。表３および図１より明らかなように、実施例では、ひずみ速度2000/sでの真ひずみ0.1 までの吸収エネルギーが同じＴＳレベルの比較例よりも格段に高く、かつＴＳ×Ｅｌが24000 ＭPa・％以上を呈する。また、ＴＳレベルの異なる鋼板同士の高速変形特性を比較するため、ＴＳ１ＭPa当りの真ひずみ0.1 までの吸収エネルギーを計算し、表３に結果を示す。該結果より、実施例ではＴＳ１ＭPa当り0.110 ＭＪ・ｍ-3以上の吸収エネルギーを確保できることがわかる。
【００３６】
【表１】
【表２】
【表３】
【発明の効果】
かくして本発明によれば、炭化物組織を制御し、さらに残留オーステナイト、マルテンサイト、フェライトの分率を調整することにより、ひずみ速度2000/sでの真ひずみ0.1 までの吸収エネルギーが高く、かつＴＳ×Ｅｌが24000 ＭPa・％以上になる高速変形特性および伸び特性に優れる高張力熱延鋼板が得られるから、自動車用部品の板厚低減および自動車の衝突安全性向上を助成し、自動車車体の高性能化に大きく寄与するという優れた効果を奏する。
【図面の簡単な説明】
【図１】実施例と比較例についてＴＳと真ひずみ0.1 までの吸収エネルギーの関係を示すグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high-tensile hot-rolled steel sheet excellent in high-speed deformation characteristics and elongation characteristics and a method for producing the same. The hot-rolled steel sheet according to the present invention is a member having a small deformation amount allowed at the time of collision, such as an automobile bumper, door, cabin reinforcement, etc., and has a high-speed deformation characteristic, specifically, a true strain amount of up to 0.1. It is intended for application to parts that have high energy absorption and require formability.
[0002]
In the present invention, the carbide is a carbide comprising a majority (95 area% or more) of cementite or a part of Fe in the cementite substituted with Cr, Mn or the like.
[0003]
[Prior art]
Conventionally, hot-rolled steel sheets used for the above-described applications are only required to have high strength in a normal tensile test. However, in order to evaluate the characteristics at the time of collision of an automobile, it is important to grasp the characteristics at the strain rate at the time of collision (high-speed deformation characteristics).
[0004]
On the other hand, in connection with recent exhaust gas regulations due to global environmental problems, the reduction of vehicle weight is a very important issue. At the same time, ensuring the safety of passengers is equally important. In order to achieve both of these, not only the strength of the steel sheet to be used is increased, but also the shape of the part is devised to improve the rigidity and the impact resistance. Therefore, the steel plate used as a material is required to have both formability and high-speed deformation characteristics that surpass conventional steel plates.
[0005]
In order to meet this requirement, for example, Japanese Patent Application Laid-Open No. 10-95588 proposes a technique for manufacturing a steel plate having high absorbed energy up to a true strain of 0.3%. However, in the above-mentioned parts, ie, bumpers and doors for automobiles, cabin reinforcements, etc., from the viewpoint of ensuring the safety of passengers, it is not desirable to absorb the energy of the collision by greatly deforming at the time of collision, and the small amount of deformation In light of the importance of high-speed deformation characteristics that can exhibit a large energy absorption capability, the technology described in the above-mentioned Japanese Patent Application Laid-Open No. 10-95588 is not sufficient for improving the characteristics of high-speed deformation characteristics with a small amount of deformation. There was a case.
[0006]
On the other hand, a technique described in Japanese Patent Application Laid-Open No. 7-62485 is known for improving the moldability, but the high-speed deformation characteristics are not studied there. Japanese Patent Application Laid-Open No. 11-43740 discloses a technique related to improvement of high-speed deformation characteristics and formability, and the improvement of high-speed deformation characteristics is to improve the n value during high-speed deformation, No guidance is given for improving the absorbed energy at small deformations, such as true strain up to 0.1.
[0007]
[Problems to be solved by the invention]
In view of the above problems, the present invention is a high-tensile hot-rolled steel sheet that is excellent in high-speed deformation characteristics and elongation characteristics, that is, high-strength hot-rolling that has high absorbed energy during high-speed deformation at a deformation amount of true strain 0.1 and excellent elongation characteristics. It aims at providing a steel plate and its manufacturing method.
[0008]
[Means for Solving the Problems]
As a result of intensive studies to achieve the above object, the present inventors have controlled the fractions of fine carbides, retained austenite, martensite, and ferrite in the steel sheet structure to an appropriate range, thereby achieving a true strain of 0.1. As a result, the inventors have found that the absorbed energy during high-speed deformation with a large amount of deformation increases and the elongation characteristics also improve. This invention is made | formed based on this knowledge, and the place made into the summary is as follows.
[0009]
(1) By mass%, C: 0.05% to 0.2%, Si: 0.8% to 2.5%, Mn: 0.5% to 2.5%, P: 0.0. 015% or less, S: 0.01% or less, Al: 0.05% or less, the composition comprising the balance of Fe and inevitable impurities, and carbide having an equivalent circle diameter of 0.5 μm or less in area%: 0 0.05% to 0.3%, retained austenite: 3% to 15%, ferrite: 60% or more, martensite: 5% or less (however, the total of martensite and retained austenite is less than 6% A high-tensile hot-rolled steel sheet excellent in high-speed deformation characteristics and elongation characteristics characterized by having a structure ( except for a range of
[0010]
(2) In the above (1), the steel sheet further has a composition containing one or two of Cr: 0.5% or less and Mo: 0.3% or less by mass%. Tensile hot-rolled steel sheet.
(3) A high-tensile hot-rolled steel sheet excellent in high-speed deformation characteristics and elongation characteristics, wherein the steel sheet further comprises a composition containing Ti: 0.05% or less by mass% in (1) or (2).
[0011]
(4) By mass%, C: 0.05% to 0.2%, Si: 0.8% to 2.5%, Mn: 0.5% to 2.5%, P: 0.015% or less, S: 0.01% or less, Al: 0.05% A steel slab having a composition including the following, with the balance consisting of Fe and unavoidable impurities, is heated to 1000 ° C. to 1300 ° C. and then rolled, and the rolling is finished at a temperature of Ar 3 points or more, and then 650 ° C. to 800 ° C. Cool to a temperature below 30 ° C at an average cooling rate of 30 ° C / s or higher, then air cool to a temperature of 640 ° C or higher and 780 ° C or lower for 3 to 10 seconds, and then at an average cooling rate equal to or higher than the value ψ A method for producing a high-tensile hot-rolled steel sheet excellent in high-speed deformation characteristics and elongation characteristics, characterized by cooling to a temperature of 10 × Ψ (° C.) or more and 10 × Ψ + 50 (° C.) or less, and winding at the temperature.
[0012]
Ψ (° C / s) = 30000 / {SRT × ([% C] + [% Mn] / 6 + [% Si] / 24 + [% Mo] / 4)} (1)
SRT: Slab heating temperature (℃)
[% C], [% Mn], [% Si], [% Mo]: Content of each element (mass%)
(5) In the above (4), the steel slab further has a composition containing one or two of Cr: 0.5% or less and Mo: 0.3% or less in terms of mass%, and high-speed deformation characteristics and elongation characteristics A method for producing high-tensile hot-rolled steel sheets with excellent resistance.
[0013]
(6) In the above-mentioned (4) or (5), a high-tensile hot-rolled steel sheet excellent in high-speed deformation characteristics and elongation characteristics characterized in that the steel slab further has a composition containing mass% and Ti: 0.05% or less. Production method.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
First, the reasons for limiting the composition of the high-tensile hot-rolled steel sheet (the steel sheet of the present invention) excellent in high-speed deformation characteristics and elongation characteristics according to the present invention will be described.
C: 0.05% or more and 0.2% or less C needs to be contained in an amount of 0.05% or more in order to secure a predetermined strength and generate retained austenite. 0.2% or less because it cannot withstand use. Preferably they are 0.08% or more and 0.16% or less.
[0015]
Si: 0.8% or more and 2.5% or less
Si is an important element for promoting the formation of fine carbides having an equivalent circle diameter of 0.5 μm or less, and is also necessary for promoting the formation of retained austenite. In particular, in order to control the fine carbide fraction of steel structure with an equivalent circle diameter of 0.5 μm or less to 0.05% or more and 0.3% or less and to control the retained austenite fraction to 3% or more and 15% or less, 0.8% or more of Si is required. Containing is required. However, if added in a large amount, the cold rolling property is lowered, so the upper limit of Si content is set to 2.5%. Preferably they are 1.2% or more and 2.0% or less.
[0016]
Mn: 0.5% to 2.5%
Mn needs to be contained in an amount of 0.5% or more in order to increase the strength, but excessive addition significantly reduces the weldability of the steel sheet, so the upper limit content is set to 2.5%. Preferably they are 0.8% or more and 1.8% or less.
P: 0.015% or less P is segregated at the prior austenite grain boundaries to deteriorate the low temperature toughness, and also increases the anisotropy of the steel sheet due to the strong segregation tendency in the steel and lowers the workability. However, up to 0.015% is allowed.
[0017]
S: 0.01% or less S is segregated at the prior austenite grain boundaries, or if a large amount of MnS is produced, it degrades low-temperature toughness and makes it difficult to use steel sheets in cold regions. , Up to 0.01% is allowed.
Al: 0.05% or less
Al is added as a deoxidizer for steel, contributes to improving the cleanliness of steel, and is a useful element for refining the structure of steel sheets. In order to acquire this effect, it is preferable to make it contain 0.001% or more. However, if the content exceeds 0.05%, a large amount of inclusions are generated, which causes wrinkling of the cold-rolled steel sheet, so the upper limit was made 0.05%.
[0018]
Cr: 0.5% or less
Cr can be appropriately used as a control factor for the fraction of fine carbide and retained austenite having an equivalent circle diameter of 0.5 μm or less in the steel sheet. In order to obtain this control effect, it is preferably contained in an amount of 0.1% or more. However, if added in a large amount, the electrodeposition coatability of the formed steel sheet on the part is lowered, so the upper limit content was set to 0.5%. Preferably it is 0.3% or less.
[0019]
Mo: 0.3% or less
Mo can be appropriately used as a control factor for the fraction of fine carbides and retained austenite in the steel sheet, as with Cr. In order to obtain this control effect, it is preferable to contain 0.05% or more, but if added in a large amount, cold rolling becomes difficult, so the upper limit content was made 0.3%. Preferably it is 0.2% or less.
[0020]
Ti: 0.05% or less
Ti can be appropriately used as a control factor for the fraction of fine carbide having an equivalent circle diameter of 0.5 μm or less in the steel sheet. In order to obtain this control effect, it is preferable to contain 0.001% or more, but even if added in a large amount, the effect is saturated, so the upper limit content was made 0.05%. Preferably they are 0.005% or more and 0.03% or less.
[0021]
The balance other than the above components is Fe and inevitable impurities.
Next, the reason for limiting the structure of the steel sheet of the present invention will be described.
Carbide with equivalent circle diameter of 0.5 μm or less: 0.05% or more and 0.3% or less The smaller the equivalent circle diameter of carbide, the higher the absorbed energy during high-speed deformation up to true strain 0.1, but the effect appears from the equivalent circle diameter of 0.5 μm or less. For this reason, the structure contains carbide with an equivalent circle diameter of 0.5 μm or less. Furthermore, with regard to the fraction of carbide with an equivalent circle diameter of 0.5 μm or less (represented in area%) with respect to the entire structure, the larger the amount, the higher the absorbed energy during high-speed deformation up to true strain 0.1, but the effect is 0.05% Since it appears from the above, the lower limit was made 0.05%. However, if it exceeds 0.3%, the amount of carbon concentrated in the retained austenite is excessively reduced and the elongation characteristics deteriorate, so the upper limit was made 0.3%. Preferably they are 0.05% or more and 0.2% or less.
[0022]
Residual austenite: not less than 3% and not more than 15% Residual austenite is necessary to improve the tensile properties (TS) -elongation (El) balance with good elongation characteristics, that is, to increase TS × El. In the present invention, “excellent in elongation characteristics” means having the characteristics of TS × El ≧ 24000 MPa ·%. In order to make TS × El 24000 MPa ·% or more, the fraction of retained austenite needs to be 3% or more and 15% or less in terms of area ratio. If the retained austenite is less than 3%, the elongation decreases and it becomes difficult to achieve TS × El ≧ 24000 MPa ·%. In addition, if the retained austenite exceeds 15%, it is considered that the carbon concentration in the retained austenite is not sufficient, but the elongation is still lowered and it is difficult to achieve TS × El ≧ 24000 MPa ·%. Become. In addition, Preferably they are 5% or more and 10% or less.
[0023]
Ferrite: 60% or more In the steel sheet of the present invention, ferrite is necessary to make the elongation characteristics good, TS-El balance good, and TS x El to be 24000 MPa ·% or more. It is necessary to make it 60% or more. If the ferrite fraction is less than 60%, the elongation decreases and it is difficult to achieve TS × El ≧ 24000 MPa ·%. In addition to the polygonal ferrite (granular ferrite), bainitic ferrite (laser-like ferrite) exists as this ferrite. As will be described later, this bainitic ferrite is generated in synchronism with the generation of fine carbides having an equivalent circle diameter of 0.5 μm or less, which is a feature of the present invention, and corresponds to the generation of the carbides at a fraction of 0.05 to 0.3%. Then, it is produced at a fraction of about 10 to 40%.
[0024]
Martensite: 5% or less Since the elongation decreases when martensite is present, it is preferable not to form as much as possible in the present invention. However, in order to obtain the target characteristics of the present invention, an area ratio of up to 5% is acceptable. Is done.
In the present invention, there may be other carbides having an equivalent circle diameter of more than 0.5 μm or low-temperature transformation phases other than the above, but carbides having an equivalent circle diameter of 0.5 μm or less, residual austenite, or other remaining martensite. It is preferable that the structure is mainly composed of the ferrite, and it is preferable that 95% or more of the carbide, the retained austenite having an equivalent circle diameter of 0.5 μm or less, or the remaining part other than martensite is composed of ferrite.
[0025]
Next, the reasons for limiting the manufacturing method of the steel sheet of the present invention will be described.
What is necessary is just to let the composition of the steel slab used for the manufacturing method of this invention be in the range of the composition of this invention steel plate mentioned above.
Slab heating temperature: 1000 ° C or more and 1300 ° C or less The slab heating temperature must be at least 1000 ° C in order to set the rolling finishing temperature described later to 3 points or more, and this is achieved by suppressing the coarsening of the austenite grain size. In order to ensure the target characteristics of the invention, it is necessary to set the temperature to 1300 ° C. or lower.
[0026]
Rolling finishing temperature (finish rolling temperature): When finish the Ar 3 point or more rolling at a temperature of Ar less than 3 points, since the energy absorption during high-speed deformation decreases, rolling finishing temperature was Ar 3 point or more.
After rolling finish, cool to 650 ℃ or more and less than 800 ℃ with average cooling rate of 30 ℃ / s or more (: 1st cooling)
Immediately after the finish rolling is completed, the pearlite transformation is suppressed by rapidly cooling to a temperature of 650 ° C. or higher and lower than 800 ° C., and the desired structure is easily obtained in the second to third cooling described later. When the average cooling rate of the quenching is less than 30 ° C./s, pearlite transformation occurs and the absorbed energy during high-speed deformation decreases, so the average cooling rate was set to 30 ° C./s or more.
[0027]
Next, air cooling to 640 ° C or higher and 780 ° C or lower in 3 to 10 seconds (: second cooling)
Subsequent to stopping the rapid cooling (forced cooling), air cooling (cooling) is performed to a temperature of 640 ° C. or higher and 780 ° C. or lower, thereby promoting ferrite transformation and concentrating carbon into untransformed austenite. When air cooling is performed for 2 seconds or less, ferrite does not sufficiently precipitate and the desired structure cannot be obtained. On the other hand, even if air cooling is performed for 11 seconds or more, the change in the structure is small and the manufacturing efficiency is poor. . The ferrite precipitated here is polygonal ferrite.
[0028]
Next, cooling to a temperature of 10 × Ψ (° C.) or more and 10 × Ψ + 50 (° C.) or less at an average cooling rate equal to or higher than the value Ψ in the above equation (1), and winding at that temperature Most important for carbide control. Only when both the composition condition described in any one of (1) to (3) and the third cooling condition (and the winding condition) are satisfied, a carbide having an equivalent circle diameter of 0.5 μm or less in the steel sheet. Exists at a fraction of 0.05% or more and 0.3% or less, and both the absorbed energy and the elongation during high-speed deformation can be increased.
[0029]
That is, the target fine carbide is cooled to a temperature of 10 × Ψ (° C.) or more and 10 × Ψ + 50 (° C.) or less at an average cooling rate of Ψ (° C./s) or more, and is precipitated after winding at the temperature. . During this winding, transformation from austenite to ferrite occurs to generate bainitic ferrite, and carbide precipitates with the grain boundaries of the generated bainitic ferrite as main precipitation sites. Further, along with the transformation from austenite to ferrite, the concentration of carbon to austenite also proceeds to stabilize austenite, and retained austenite with a fraction of 3 to 15% can be secured.
[0030]
When cooling at an average cooling rate slower than Ψ (° C./s), carbide begins to precipitate during cooling, and the finally obtained carbide is coarsened, and the desired carbide structure cannot be obtained. If the coiling temperature is less than 10 × Ψ (° C.), the temperature at which the carbide is precipitated becomes too low, so that a sufficient amount of carbide cannot be obtained, and a large amount of martensite is generated. The fraction of retained austenite is also excessive. On the other hand, when the coiling temperature exceeds 10 × Ψ + 50 (° C.), a large amount of carbide precipitates and grows coarsely, so that the target carbide structure cannot be obtained, and the concentration of carbon to austenite is not possible. As a result, the amount of retained austenite cannot be secured.
[0031]
【Example】
Steel having the composition shown in Table 1 is melted in a converter and made into a slab by continuous casting. The slab is processed under the conditions shown in Table 2 (hot rolling → first to third cooling → winding). Thus, a hot rolled steel sheet having a thickness of 2.0 mm was obtained. The obtained steel sheet was examined for the microstructure, tensile properties, and absorbed energy during high-speed deformation up to true strain of 0.1 in the following manner.
[0032]
-Equivalent circle diameter: fraction of carbide of 0.5 μm or less: steel plate test surface (: portion excluding 10% thick part at both ends in the plate thickness direction of L cross section (cross section parallel to rolling direction)) After electropolishing with a polishing solution containing 7.8% perchloric acid, 70% ethanol, 10% 2-butoxyethanol, and the remainder consisting of distilled water, the structure was magnified 10,000 times with an atomic force microscope (AFM). The carbides were identified by observation, the equivalent circle diameter of each carbide was determined and the area ratio was measured, and the total area ratio of carbides having an equivalent circle diameter of 0.5 μm or less was determined. This is shown in Table 3.
[0033]
Here, the equivalent circle diameter is the diameter of a circle having the same area as that calculated from the area obtained by image processing of the carbide portions of the tissue.
-Fraction of retained austenite: About the test surface of the steel sheet, retained austenite was identified by X-ray diffraction, the area ratio was measured, and the area ratio was defined as the fraction.
・ Martensite and ferrite (polygonal ferrite, bainitic ferrite) fraction: 2,000-fold magnified image of Nital corrosion appearing structure by scanning electron microscope (SEM) on each steel sheet test surface. After sorting, each area was color-coded and then quantified by image analysis to measure the area ratio, which was used as the fraction of these tissues. Table 3 shows the total amount of bainitic ferrite and polygonal ferrite as the area ratio of bainitic ferrite and the total amount of ferrite.
[0034]
-Tensile properties: Tensile strength (TS) and elongation (El) were measured by a method based on JIS Z 2241 using a JIS No. 5 test piece taken with the direction orthogonal to the rolling direction as the test direction.
-Absorbed energy during high-speed deformation up to true strain of 0.1: Hopkinson as shown in the literature (iron and steel, 83 (1997) 748) at a strain rate of 2000 / s with the rolling direction taken as the test direction. A tensile test was performed by a method called a pressure bar, and the absorbed energy was determined by integrating the obtained stress-strain curve in the range of true strain of 0.0 to 0.1.
[0035]
Table 3 shows the results of these investigations. Moreover, about the Example and comparative example which are shown in Table 3, the relationship between TS and the absorbed energy to true strain 0.1 is made into a graph, and it shows in FIG. As is apparent from Table 3 and FIG. 1, in the examples, the absorbed energy up to a true strain of 0.1 at a strain rate of 2000 / s is much higher than that of the comparative example of the same TS level, and TS × El is 24000 MPa · % Or more. Moreover, in order to compare the high-speed deformation characteristics of steel plates having different TS levels, the absorbed energy up to a true strain of 0.1 per TS1MPa was calculated, and the results are shown in Table 3. From the results, it can be seen that the absorbed energy of 0.110 MJ · m −3 or more per 1 MPa of TS can be secured in the example.
[0036]
[Table 1]
[Table 2]
[Table 3]
【The invention's effect】
Thus, according to the present invention, by controlling the carbide structure and further adjusting the fraction of retained austenite, martensite, and ferrite, the absorbed energy up to a true strain of 0.1 at a strain rate of 2000 / s is high, and TS × A high-tensile hot-rolled steel sheet with excellent high-speed deformation characteristics and elongation characteristics with an El of 24000 MPa ·% or more can be obtained, which helps to reduce the thickness of automotive parts and improve the collision safety of automobiles. It has an excellent effect of greatly contributing to the process.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between TS and absorbed energy up to a true strain of 0.1 for an example and a comparative example.
Claims (6)
記
Ψ（℃/s）＝30000 ／｛SRT ×（〔％Ｃ〕＋〔％Mn〕／６＋〔％Si〕／24＋〔％Mo〕／４）｝ ‥‥(1)
SRT ：スラブ加熱温度（℃）
〔％Ｃ〕、〔％Mn〕、〔％Si〕、〔％Mo〕：各元素の含有量（質量％）In mass%, C: 0.05% or more and 0.2% or less, Si: 0.8% or more and 2.5% or less, Mn: 0.5% or more and 2.5% or less, P: 0.015% or less, S: 0.01% or less, Al: 0.05% or less A steel slab having a composition composed of Fe and unavoidable impurities is rolled after being heated to 1000 ° C. or higher and 1300 ° C. or lower, and the rolling is finished at a temperature of Ar 3 or higher, and then 650 ° C. or higher and lower than 800 ° C. Cool to temperature at an average cooling rate of 30 ° C / s or higher, then cool to 640 ° C or higher and 780 ° C or lower for 3 to 10 seconds, then 10 × Ψ at an average cooling rate equal to or higher than the value Ψ in the following equation (1) A method for producing a high-tensile hot-rolled steel sheet excellent in high-speed deformation characteristics and elongation characteristics, characterized by cooling to a temperature of (° C.) to 10 × Ψ + 50 (° C.) and winding at the temperature.
Ψ (° C / s) = 30000 / {SRT × ([% C] + [% Mn] / 6 + [% Si] / 24 + [% Mo] / 4)} (1)
SRT: Slab heating temperature (℃)
[% C], [% Mn], [% Si], [% Mo]: Content of each element (mass%)
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