JP2004204326A - High-strength hot-rolled steel sheet excellent in hole expandability, ductility, and chemical conversion treatment amenability and method for producing the same - Google Patents
High-strength hot-rolled steel sheet excellent in hole expandability, ductility, and chemical conversion treatment amenability and method for producing the same Download PDFInfo
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Abstract
Description
【０００１】
【発明の属する技術分野】
本発明は、主としてプレス加工される自動車足廻り部品等を対象とし、0.6 〜6.0mm 程度の板厚で、590N/mm2以上の強度を有する穴拡げ性、延性及び化成処理性に優れた高強度熱延鋼板及びその製造方法に関するものである。
【０００２】
【従来の技術】
【特許文献１】特開平４−８８１２５号公報
【特許文献２】特開平３−１８０４２６号公報
【特許文献３】特開平６−１７２９２４号公報
【特許文献４】特開平７−１１３８２号公報
【特許文献５】特開平６−２００３５１号公報
【特許文献６】特開平６−２９３９１０号公報
【特許文献７】特開２００２−１８０１９０号公報
【０００３】
近年、自動車の環境問題を契機に燃費改善対策としての車体軽量化、部品の一体成形化、加工工程の合理化によるコストダウンのニーズが強まり、プレス加工性に優れた高強度熱延鋼板の開発が進められてきた。従来、かかる高い加工性を有する高強度熱延鋼板としては、フェライト・マルテンサイト組織、フェライト・ベイナイト組織からなる混合組織のもの、或いはベイナイト、フェライト主体のほぼ単相組織のものが広く知られている。
【０００４】
しかし、フェライト・マルテンサイト組織においては、変形の初期からマルテンサイトの周囲にミクロボイドが発生して割れを生じるため、穴拡げ性に劣る問題があり、足廻り部品等の高い穴拡げ性が要求される用途には不向きであった。
【０００５】
また、特開平４−８８１２５号公報、特開平３−１８０４２６号公報には、ベイナイトを主体とした組織を有する鋼板が開示されているが、ベイナイトを主体とした組織であるため穴拡げ性は優れるものの、軟質なフェライト相が少ないので延性に劣る。さらに、特開平６−１７２９２４号公報、特開平７−１１３８２号公報ではフェライトを主体とした組織を有する鋼板が開示されているが、同様に穴拡げ性は優れているものの、強度を確保するために硬質な炭化物を析出させているので延性に劣る。
【０００６】
また、特開平６−２００３５１号公報にはフェライト・ベイナイト組織を有する穴拡げ性、延性に優れた鋼板が開示されており、特開平６−２９３９１０号公報には２段冷却を用いることによってフェライト占有率を制御することで穴拡げ性、延性が両立する鋼板の製造方法が開示されている。しかしながら、自動車のさらなる軽量化、部品の複雑化等を背景にさらに高い穴拡げ性、延性が求められ、最近の高強度熱延鋼板には上記した技術では対応しきれない高度な加工性が求められている。
【０００７】
更に、特開２００２−１８０１９０号公報には、穴拡げ性及び延性に優れた高強度熱延鋼板に関する発明が開示してある。穴拡げ性及び延性の相反する特性には優れた高強度熱延鋼板が得られたが、熱延工程で、Ｓｉスケールと呼ばれる表面の凹凸疵が発生する場合があり、製品での外観が損なわれる場合が生じた。また、足回り部品等の高強度熱延鋼板は、通常、プレス成形した後に化成処理と塗装が施される。しかし、化成皮膜の生成が良くない（化成処理性が悪い）ケースや、塗装後の塗膜の密着が悪いケースなどの問題が生じる場合があった。これらの問題は、鋼中の多量のＳｉ含有が原因と考えられている。このように、高強度熱延鋼板には、Ｓｉがよく使用されるが、各種のトラブルが生じている。
【０００８】
【発明が解決しようとする課題】
本発明は上記した従来の問題点を解決するためになされたものであって、590N/mm2以上の高強度化に伴う穴拡げ性と延性の低下を防ぎ、かつＳｉスケールの発生を防いだ、穴拡げ性、延性に優れた高強度熱延鋼板において、化成処理性を格段に向上するものである。即ち、本発明は穴拡げ性、延性及び化成処理性に優れた高強度熱延鋼板およびその鋼板の製造方法を提供することを目的とする。
【０００９】
【課題を解決するための手段】
上記の課題を解決するためになされた本発明の穴拡げ性、延性及び化成処理性に優れた高強度熱延鋼板は、
（１）質量％で、C ：0.02〜0.08％、Si：0.50％以下、Mn：0.50〜3.50％、P ：0.03％以下、S ：0.01% 以下、Al：0.4 〜2.0 ％、かつ Mn＋0.5 ×Al＜４ ・・・式（Ａ）、及びTi：0.003 〜0.20％、Nb：0.003 〜0.04％、Ｖ：0.003 〜0.20％の１種または２種以上を含有し、残部鉄及び不可避的不純物からなる鋼組成の高強度熱延鋼板であって、該鋼板の金属組織が粒径 2μｍ以上のフェライトの割合が40% 以上であるフェライト・ベイナイトの二相組織で、強度が590N/mm2以上であることを特徴とする穴拡げ性、延性及び化成処理性に優れた高強度熱延鋼板。
（２）質量％で、更に、Ca：0.0005〜0.01％、Zr：0.0005〜0.01％、REM ：0.0005〜0.05％、の１種または２種以上を含有する前記（１）記載の穴拡げ性、延性及び化成処理性に優れた高強度熱延鋼板。
（３）質量％で、更に、Mg：0.0005〜0.01％含有する前記（１）または（２）記載の穴拡げ性、延性及び化成処理性に優れた高強度熱延鋼板。
（４）質量％で、C ：0.02〜0.08％、Si：0.50％以下、Mn：0.50〜3.50％、P ：0.03％以下、S ：0.01％以下、Al：0.4 〜2.0 ％、かつ Mn＋0.5 ×Al＜４ ・・・式（Ａ）、及びTi：0.003 〜0.20％、Nb：0.003 〜0.04％、V ：0.003 〜0.20％の１種または２種以上を含有し、残部鉄及び不可避的不純物からなる鋼組成の鋳片を、圧延終了温度をAr3 点以上として熱間圧延を終了したのち20℃/sec以上の冷却速度にて650 〜800 ℃にまで冷却し、次いで2 〜15秒空冷したのち、さらに20℃/sec以上の冷却速度にて350 〜600 ℃に冷却して巻き取ることを特徴とする穴拡げ性、延性及び化成処理性に優れた高強度熱延鋼板の製造方法。
（５）鋳片に、質量％で、更に、Ca：0.0005〜0.01％、Zr：0.0005〜0.01％、REM ：0.0005〜0.05％、の１種または２種以上を含有する前記（４）に記載の穴拡げ性、延性及び化成処理性に優れた高強度熱延鋼板の製造方法。
（６）鋳片に、質量％で、更に、Mg：0.0005〜0.01％含有する前記（４）または（５）に記載の穴拡げ性、延性及び化成処理性に優れた高強度熱延鋼板の製造方法。
【００１０】
【発明の実施の形態】
高強度熱延鋼板において、穴拡げ性と延性とは相反する傾向を示すことは良く知られている。本発明者らは上記課題を解決するために鋭意研究した結果、フェライト・ベイナイト鋼においてフェライト結晶粒をできる限り一定値以上の粒径とすることによって穴拡げ性を劣化させることなく延性が改善できることを知見し、本発明を完成するに至った。即ち、フェライト・ベイナイトの実質的な二相組織鋼において延性を高めるフェライトと強度を確保するTiC 、NbC 、VCからなる析出物に着目し、フェライト粒を十分成長させることにより穴拡げ性を低下させずに延性を改善し、その後に析出物を生成させて強度を確保することによって上記課題を解決したものである。即ち、低C −低Si−高Al−(Ti 、Ｎb 、V)成分系で、MnとAlが特定の関係のもとで本発明鋼板の特定の金属組織を得ることによって、穴拡げ性、延性及び化成処理性の三つの特性を同時に満足する高強度熱延鋼板が得られることを本発明者らは新たに見出したものである。更にはその工業的に有利な製造方法を見出したものである。尚、(Ti 、Ｎb 、V)とは、Ti、Nb、V の１種又は２種以上の特定量の含有を意味する。
【００１１】
以下、鋼組成の各元素含有量の％は質量％を示す。本発明において高強度熱延鋼板中のＣは 0.02 〜0.08％とする。C は炭化物を析出して強度を確保するに必要な元素であって0.02％未満では、たとえ本発明の高Al−(Ti 、Ｎb 、V)成分系でも所望の強度を確保することが困難になる。一方、0.08％を超えると延性の低下が大きくなるからである。
【００１２】
Siは、有害な炭化物の生成を抑えフェライト組織主体＋残ベイナイトの複合組織を得るために重要な元素であるが、化成処理性を悪化させ、また、Ｓｉスケールも発生するため、0.5 ％を上限とする。0.25％超では、熱延鋼板の製造時に前記の金属組織を得るための温度管理が厳しい場合があるので、Si含有量は、0.25％以下が更に好ましい。
【００１３】
Mnは、強度の確保に必要な元素であり、このためには0.50％以上の添加を必要とする。しかし、3.5 ％を超えて多量に添加するとミクロ偏析、マクロ偏析が起こりやすくなり、穴拡げ性を劣化させる。なお、穴拡げ性と延性を効果的に両立させるにはMnの範囲を1.00〜3.50％とするのが望ましい。
【００１４】
P はフェライトに固溶してその延性を低下させるので、その含有量は0.03％以下とする。また、S はMnS を形成して破壊の起点として作用し著しく穴拡げ性、延性を低下させるので0.01% 以下とする。
【００１５】
Alは、本発明において重要な元素の一つで、延性と化成処理性の両立に必要な元素であり、このため0.4 ％以上の添加を必要とする。Alは、従来より熱延鋼板において脱酸に必要な元素であり、通常0.01〜0.07％程度添加してきた。本発明者らは、低C −低Si系でAlを著しく多量に含有させた鋼組成をベースに金属組織の異なる高強度熱延鋼板で各種実験を行い、本発明に至ったものである。すなわち、Alが0.4 ％以上で、前記の金属組織を形成することにより化成処理性を損なうことなく、延性を大幅に向上できることを見出した。Alは、2.0 ％で延性向上効果が飽和してしまうばかりか、2.0 ％超の添加では穴拡げ性、延性と化成処理性の両立が逆に困難になってしまうので、2.0 ％超の添加は、経済的に不利である。
【００１６】
また、延性と化成処理性の両立には、ＭｎとＡｌの関係の規定も重要である。図１にＭｎとＡｌの含有量と化成処理性の関係を示す。図中の太実線は線上を含むが、破線は線上は除く。また、図１中の○、×は、後述の注４）の化成処理性評価の○、×と同様に評価した。図１に示すように、Mn＋0.5 ×Al＜４ ・・・式（Ａ）の条件の場合、化成処理性が損なわれない事を本発明者らは新たに見出した。
理由は不明であるが、高Al系で（Ａ）の成分を満足した前記の本発明金属組織の場合には、フェライト・ベイナイトの実質的な二相組織鋼で粒径 2μｍ以上のフェライトの割合が40% 以上であるフェライト粒にAlが多量に固溶し、下記のTi、Nb、V の炭化物析出強化が、従来の低Al系の異なる金属組織（例えばフェライト・マルテンサイト二相組織、実質的にベイナイト単相からなる組織）の場合よりも有効に発揮されるので、高強度熱延鋼板において化成処理性が良好になると発明者らは推定している。
【００１７】
Ti、Nb、V も本発明において最も重要な元素の一つであり、TiC 、NbC 、VCなどの微細な炭化物を析出させて本発明では高強度を可能にする。この目的のためにはTi 0.003 〜0.20％、Nb 0.003〜0.04％、Ｖ 0.003〜0.20％の１種または２種以上を添加することが必要である、Tiが0.003 ％未満、Nbが0.003 ％未満、Ｖ0.003％未満では、本発明の高Ａｌ系でも高強度を得ることが困難であり、Tiが0.20％、Nbが0.04％、Ｖが0.20％を超えると析出物が多量生成しすぎて延性が劣化するからである。尚、Ti、Nb、Ｖは、析出物を更に有効に活用するには、Ti 0.010％以上、Nb 0.010％以上、V 0.030 ％以上の含有が好ましい。
【００１８】
Ca、Zr、REM は硫化物系介在物の形態を制御し穴拡げ性の向上に有効な元素である。この形態制御効果を有効ならしめるためにはCa、Zr、REM の１種または２種以上を0.0005％以上添加するのが望ましい。一方、多量の添加は硫化物系介在物の粗大化を招き、清浄度を悪化させて本発明の低C −低Si−高Al−(Ti 、Ｎb 、V)成分系であっても延性を低下させるのみならず、コストの上昇を招くので、CaとZrの上限を0.01％とし、REM の上限を0.05％とする。尚、REM とは、例えば、元素番号21,39,57〜71の元素である。
【００１９】
Mgは、本発明における重要な添加元素の一つである。Mgはこの添加により、酸素と結合して酸化物を形成するが、このとき生成されるＭｇＯ、またはＭｇＯを含むＡｌ2 Ｏ3 、ＳｉＯ2 、ＭｎＯ、Ｔｉ2 Ｏ3 の複合酸化物微細化は、Ｍｇを添加しない従来の低Al鋼に比べ、個々の酸化物のサイズが小さく、均一に分散した分布状態となることを本発明者らは新たに見出した。鋼中に微細に分散したこれらの酸化物は、明確ではないが打抜き加工時に微細ボイドを形成し、応力の分散に寄与し応力集中を抑制することで粗大クラックの発生を抑制する効果があり、穴広げ性の向上に効果があると考えられる。ただし、0.0005％未満ではその効果が不十分である。一方で0.01％超を含有せしめても改善効果が飽和するばかりでなく、逆に凝集粗大化した酸化物が生成し易く鋼の清浄度を劣化させ、本発明の低C −低Si−高Al−(Ti 、Ｎb 、V)成分系であっても穴拡げ性、延性を劣化させるため上限を0.01％とする。
不可避不純物としては、例えば、Ｎ≦0.01％、Ｃｕ≦0.3 ％、Ｎｉ≦0.3 ％、Ｃｒ≦0.3 ％、Ｍｏ≦0.3 ％、Ｃｏ≦0.05％、Ｚｎ≦0.05％、Ｓｎ≦0.05％、Ｎａ≦0.02％、Ｋ≦0.02％、Ｂ≦0.0005％で含有していても本発明を逸脱するものではない。
【００２０】
フェライト粒径の大きさは、本発明において最も重要な指標の一つである。本発明者らは鋭意研究した結果、粒径が２μｍ以上のフェライトの占める面積率が４0 ％以上となると穴拡げ性と延性（例えば伸び）が共に優れた性能になることも見出した。図２は、引張強さ780 〜820N/mm2、λ値100 〜115 の高強度熱延鋼板において、粒径が２μｍ以上のフェライトの占める面積率が４0 ％以上（実施例）の引張強度と伸びの関係と，粒径が２μｍ以上のフェライトの占める面積率が４0 ％以下（比較例）での引張強度と伸びの関係を示した図である。比較例もMn＋0.5 ×Al＜４ ・・・式（Ａ）を満足する成分系であるが、製造方法が異なり、粒径が２μｍ以上のフェライトの占める面積率が４0 ％以下であった。図２に示すように粒径が２μｍ以上のフェライトの占める面積率が４0 ％以上の場合、４０％以下に比較して同一強度にて伸びが３〜５％程度高い値を示している。穴拡げ性、延性を良好にして両立させるには、粒径が２μｍ以上のフェライト粒の割合を40％以上とする必要がある。なお、より顕著な効果を得るには粒径が３μｍ以上のフェライト粒の割合を40％以上とするのが望ましい。尚、粒径は各粒の面積を円相当径に換算して求めることができる。この換算には、画像解析装置を用いることが有効である。
【００２１】
高強度熱延鋼板における金属組織は実質的にフェライトとベイナイトの二相組織よりなるものとする。ここで、金属組織には粒径２μｍ以上のフェライトが40％以上含まれるので、金属組織はフェライト40％以上のフェライト・ベイナイトの実質的に二相組織となる。例えば、本発明の金属組織としては、２μｍ以上の粒径のフェライトが40％以上で、残部が２μｍ未満の粒径のフェライトとベイナイトのもの、又は、２μｍ以上の粒径のフェライトが40％以上で残部がベイナイトのみのものとすることができる。このようにベイナイトを60％以下とするのは、ベイナイトの量がこれより多くなると延性の低下が著しく大きくなリ、高Al系であっても良好な引張強度と伸びの関係を得にくくなるからである。但し、残留オーステナイトが通常のＸ線回折強度で測定した場合に３％以下含有していても、本発明のフェライト・ベイナイトの実質的な二相組織を逸脱するものではない。また、熱延鋼板の表面近傍に、極薄（例えば厚みで0.05〜0.3 ｍｍ程度）の炭素等の鋼組成が若干低下した領域が一部存在し、金属組織が若干異なるとしても、熱延鋼板の板厚方向の大部 分が上記のフェライト・ベイナイトの実質的な二相組織で粒径２μｍ以上のフェライトが40％以上含まれる金属組織であれば本発明の作用効果を有するものである。
【００２２】
本発明は上記の鋼組成と金属組織を有する高強度熱延鋼板と、更にその鋼板を工業的に有利に製造するための高強度熱延鋼板の製造方法である。
高強度熱延鋼板を熱間圧延により製造するに際して、本発明の低C −低Si−高Al成分系では、熱間圧延終了温度、即ち熱間圧延の仕上圧延終了温度は、フェライトの生成を抑え穴拡げ性を良好にし、良好な化成処理性を得るにはAr3 点以上とすることが好ましい。Ar3 点は、熱間圧延にてフェライトが生成し始める温度であるが、公知の計算式または経験的に求めた計算式等を用いて決めてかまわない。しかし、あまり高温にすると金属組織の粗大化による強度及び延性の低下を招く場合があるので仕上げ圧延終了温度は1050℃以下が望ましい。鋳片を加熱するか否かは、鋼板の圧延条件により適宜決めればよいし、熱延鋼板を熱間圧延中に次の熱延鋼板又は鋳片を接合して連続圧延するかは、本発明の金属組織が得らるなら適宜選択することができる。尚、鋼溶製は、転炉方式でも電炉方式でも、溶解して鋼組成が得られれば良い。また、不純物などの制御のための、溶銑予備処理、精錬、脱ガス処理などは適宜選択すれば良い。
【００２３】
熱間圧延の仕上圧延終了直後に鋼板を急速冷却（一次急冷）することは、本発明の低C −低Si−高Al成分系において高い穴拡げ性を得るために有効であって、その冷却速度は20℃/sec以上が好ましい。20℃/sec未満では穴拡げ性に有害な炭化物形成を抑制するのが困難な場合がある。尚、250 ℃/secで炭化物析出の粗大化抑制効果は飽和するが、250 ℃/sec以上でもフェライト結晶粒が成長してフェライト結晶粒径が２μｍ以上を金属組織の４０％以上確保するには有効である。600 ℃/sec超では、フェライト結晶粒の成長効果も飽和し、逆に、熱延鋼板の形状の維持が現状容易ではないので600 ℃/sec以下が望ましい。
【００２４】
鋼板の急速冷却（一次急冷）を一旦停止して空冷を施すことは本発明の低C −低Si−高Al成分系において、フェライトを析出してその占有率を増加させ、延性を向上させるために重要である。しかしながら、空冷開始温度が 650℃未満では穴拡げ性に有害なパーライトが早期より発生する。一方、空冷開始温度が 800℃を超える場合にはフェライトの生成が遅く空冷の効果が得にくいばかりでなく、その後の冷却中におけるパーライトの生成が起こりやすいので望ましくない。従って、空冷開始温度は 650〜800 ℃とするのが好ましい。また、空冷時間が15秒を超えてもフェライトの増加は飽和するばかりでなく、その後の冷却速度、巻取温度の制御に負荷がかかるので工業的に好ましくない。従って、空冷時間は15秒以下とする。なお、空冷時間が２秒未満ではフェライトを十分析出させることはできないので好ましくない。また、本発明の空冷には、その後の金属組織の生成に影響を及ぼさない程度に、熱延鋼板表面付近のスケール改質の目的で霧状の冷媒を少量吹き付けることも含まれる。
【００２５】
空冷後は再度該熱延鋼板を急速に冷却（二次急冷）するが、その冷却速度はやはり20℃/sec以上が好ましい。20℃/sec未満では有害なパーライトが生成し易くなるから好ましくない。200 ℃/se でベイナイトの生成はほぼ飽和するが、200 ℃/sec以上でもTiC 、NbC 、VCなどの微細な炭化物の粗大化抑制には有効である。尚、600 ℃超では、鋼板が部分的に過冷される場合があり、局部的に硬質なマルテンサイトが生成し易いので好ましくはない。
【００２６】
そして、この急冷（二次急冷）の停止温度、即ち巻取温度は350 〜600 ℃とする。巻取温度が350 ℃未満では穴拡げ性に有害な硬質のマルテンサイトが発生するためであり、一方、600 ℃を超えると穴拡げ性に有害なパーライトが生成し易くなるからである。
【００２７】
以上のように本鋼組成と熱延条件の組み合わせにより、鋼板の金属組織が粒径2μｍ以上のフェライトの割合が40% 以上であるフェライト・ベイナイトの実質的な二相組織であって、引張強度590N/mm2以上である穴拡げ性、延性及び化成処理性に優れた高強度熱延鋼板を製造することができる。更に、本発明鋼板の表面に表面処理（例えば亜鉛メッキ、潤滑処理等）が施されていても本発明の効果を有し、本発明を逸脱するものではない。
【００２８】
【実施例】
表１に示す化学成分組成（含有量は質量％、空欄は無添加を示す）を有する鋼を転炉溶製して、連続鋳造により鋳片とし、表２に示す熱延条件にて圧延・冷却し、板厚2.6 （実施例１〜１６、比較例１〜３）、3.2mm （実施例１７〜３２、比較例４〜６）の熱延鋼板を製造した。なお、急速冷却の速度を40℃/sec（実施例１〜１５、比較例１〜４）、120 ℃/sec（実施例１６〜３０、比較例５）、300 ℃/sec（実施例３１、３２、比較例６）、空冷時間は10秒（実施例１〜３２、比較例１〜６）とした。 但し、熱間圧延の仕上圧延終了温度は、９００℃（実施例１〜３２、比較例４〜９）、９３０℃（比較例１〜３）であった。
【００２９】
【表１】
【表２】
このようにして得られた熱延鋼板について、引張試験、穴拡げ試験、金属組織観察、化成処理性評価を行なった。その結果を表２に示す。
注１） 引張強度、延性
ＪＩＳ Ｚ ２２０１に準拠して、試験片はＪＩＳ５号を用いて引張試験を行った。
注２）穴拡げ性
穴拡げ試験は初期穴径（ｄ0:10mm） の打抜き穴を60°円錐ポンチにて押し拡げ、クラックが板厚を貫通した時点での穴径（d)から穴拡げ値（λ値）＝(d-d0)/d0 ×100 を求めて穴拡げ性を評価した。これらの結果を表２に示す。
注３）鋼板の金属組織
金属組織観察においては、ナイタールで腐食後、走査電子顕微鏡にてフェライト、ベイナイトを同定し、粒径２μｍ以上のフェライトの面積率を画像解析により測定した。
注４）化成処理性
熱延鋼板の化成処理性は、表面スケールを除去後に、化成処理液SD5000（日本ペイント社製）を用い、処方どおり脱脂、表面調整を行った後化成処理を行った。化成処理皮膜の判定は、SEM （２次電子線像）により、均一に皮膜が形成されているものは○、皮膜が一部形成されていないものは×と判定した。
【００３２】
実施例１〜３２は、化学成分、仕上圧延終了温度、空冷開始温度、巻取温度の何れも本発明の範囲内であって、金属組織がフェライト・ベイナイト二相よりなり、且つ、粒径２μｍ以上のフェライトの割合が40％以上である本発明例であり、高いλ値と伸びを有する穴 拡げ性、延性及び化成処理性に優れた高強度熱延鋼板である。一方、比較例１〜９の本発明の条件を外れた比較例のものは強度、穴拡げ性、延性のバランス、化成処理性に劣るものである。
【００３３】
また、表１、表２には示していないが、実施例１に示す鋼成分の鋳片を用いて熱間圧延終了温度 920℃、その後625 ℃まで一次急冷（冷却速度40℃／sec ）し、空冷開始温度 625℃で10秒空冷し、更に二次急冷（冷却速度20℃／sec ）し、巻取温度 460℃として熱間圧延した場合には空冷開始温度が本発明の範囲より低過ぎたために金属組織にパーライトが数％生成し、粒径２μｍ以上のフェライトの面積率も36％が低く本発明の範囲外であった。従って伸び19％、λ値95％となり、穴拡げ性、延性バランスの劣るものであった。また、同様に実施例１に示す鋼成分の鋳片を用いて熱間圧延終了温度 910℃、その後675 ℃まで一次急冷（冷却速度100 ℃／sec)し、空冷開始温度 680℃で10秒空冷し、更に二次急冷（冷却速度20℃／sec)し、巻取温度 320℃として熱間圧延した場合には巻取温度が本発明の範囲より低過ぎたために金属組織にマルテンサイトが１０％程度生成し、粒径２μｍ以上のフェライトの面積率が３３％と低いものであって、このため伸び20％、λ値63％となり、やはり穴拡げ性、延性バランスの劣るものとなってしまった。
【００３４】
【発明の効果】
以上に詳述したように、本発明によれば引張強度が590N/mm 2 以上の高強度であって穴拡げ性、延性および化成処理性に優れた高強度熱延鋼板を経済的に提供することができるで本発明は高い加工性を有する高強度熱延鋼板として好適である。また、本発明の高強度熱延鋼板は車体の軽量化、部品の一体成形化、加工工程の合理化が可能であって、燃費の向上、製造コストの低減を図ることができるものとして工業的価値大なものである。
【図面の簡単な説明】
【図１】穴拡げ性、延性及び化成処理性に優れたMnとAlの関係を示す説明図である。
【図２】フェライト・ベイナイトの実質的な二相組織の高強度熱延鋼板において、フェライト分布と引張強度−伸びバランスとの関係を示す説明図である。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention is primarily directed to automotive underbody parts or the like which is press working a plate thickness of about 0.6 ～6.0Mm, hole expandability having a 590N / mm 2 or more strength, high excellent ductility and chemical conversion treatability The present invention relates to a high-strength hot-rolled steel sheet and a method for producing the same.
[0002]
[Prior art]
[Patent Document 1] JP-A-4-88125 [Patent Document 2] JP-A-3-180426 [Patent Document 3] JP-A-6-172924 [Patent Document 4] JP-A 7-11382 [Patent]
In recent years, due to the environmental problems of automobiles, there has been a growing need for cost reduction by reducing body weight, integrating parts, and streamlining the processing process as measures to improve fuel efficiency, and the development of high-strength hot-rolled steel sheets with excellent press workability has been developed. It has been advanced. Conventionally, as such a high-strength hot-rolled steel sheet having such high workability, a ferrite-martensite structure, a mixed structure composed of a ferrite-bainite structure, or a bainite, a substantially single-phase structure mainly composed of ferrite are widely known. I have.
[0004]
However, in the ferrite-martensite structure, microvoids are generated around the martensite from the initial stage of deformation and cracks are generated.Therefore, there is a problem of poor hole expandability, and high hole expandability of undercarriage parts and the like is required. It was not suitable for the use which was.
[0005]
JP-A-4-88125 and JP-A-3-180426 disclose steel sheets having a structure mainly composed of bainite. However, since the steel sheet is mainly composed of bainite, hole expandability is excellent. However, ductility is inferior because there is little soft ferrite phase. Further, JP-A-6-172924 and JP-A-7-11382 disclose a steel sheet having a structure mainly composed of ferrite. Similarly, although the steel sheet has excellent hole expandability, it is required to secure strength. Hard carbide is precipitated in the steel, so that the ductility is poor.
[0006]
JP-A-6-200351 discloses a steel sheet having a ferrite-bainite structure and having excellent hole-expanding properties and ductility, and JP-A-6-293910 discloses the use of two-stage cooling to occupy ferrite. A method for manufacturing a steel sheet that achieves both hole expandability and ductility by controlling the rate is disclosed. However, with the background of lighter automobiles and more complex parts, higher hole expandability and ductility are required, and recent high-strength hot-rolled steel sheets require advanced workability that cannot be handled by the above-mentioned technologies. Have been.
[0007]
Further, Japanese Patent Application Laid-Open No. 2002-180190 discloses an invention relating to a high-strength hot-rolled steel sheet excellent in hole expandability and ductility. Although a high strength hot rolled steel sheet excellent in the contradictory properties of hole expandability and ductility was obtained, in the hot rolling process, irregularities on the surface called Si scale may occur, which impairs the appearance of the product. Occurred. Further, a high-strength hot-rolled steel sheet such as underbody parts is usually subjected to a chemical conversion treatment and coating after press forming. However, there were cases where problems such as a case where the formation of a chemical conversion film was not good (poor chemical conversion treatment) and a case where the adhesion of a coating film after coating was poor. These problems are believed to be due to the large amount of Si contained in the steel. As described above, Si is often used for a high-strength hot-rolled steel sheet, but various troubles have occurred.
[0008]
[Problems to be solved by the invention]
The present invention has been made in order to solve the above-described conventional problems, and has prevented a decrease in hole expandability and ductility due to an increase in strength of 590 N / mm 2 or more, and has prevented the occurrence of Si scale. In a high-strength hot-rolled steel sheet having excellent hole expandability and ductility, the chemical conversion property is remarkably improved. That is, an object of the present invention is to provide a high-strength hot-rolled steel sheet excellent in hole expandability, ductility, and chemical conversion treatment properties, and a method for manufacturing the steel sheet.
[0009]
[Means for Solving the Problems]
High-strength hot-rolled steel sheet excellent in hole expandability, ductility and chemical conversion treatment properties of the present invention made in order to solve the above problems,
(1) In mass%, C: 0.02 to 0.08%, Si: 0.50% or less, Mn: 0.50 to 3.50%, P: 0.03% or less, S: 0.01% or less, Al: 0.4 to 2.0%, and Mn + 0.5 × Al <4: Formula (A), containing one or more of Ti: 0.003 to 0.20%, Nb: 0.003 to 0.04%, V: 0.003 to 0.20%, with the balance being iron and unavoidable impurities A high-strength hot-rolled steel sheet having a steel composition consisting of a two-phase structure of ferrite bainite in which the metal structure of the steel plate has a grain size of 2 μm or more and a ratio of ferrite of 40% or more, and a strength of 590 N / mm 2 or more. A high-strength hot-rolled steel sheet having excellent hole expandability, ductility, and chemical conversion treatment characteristics.
(2) The hole-expanding ability according to (1), further containing one or more of, by mass%, Ca: 0.0005 to 0.01%, Zr: 0.0005 to 0.01%, and REM: 0.0005 to 0.05%. High strength hot rolled steel sheet with excellent ductility and chemical conversion treatment.
(3) The high-strength hot-rolled steel sheet according to the above (1) or (2), further containing 0.0005 to 0.01% by mass of Mg and having excellent hole expandability, ductility, and chemical conversion treatment properties.
(4) In mass%, C: 0.02 to 0.08%, Si: 0.50% or less, Mn: 0.50 to 3.50%, P: 0.03% or less, S: 0.01% or less, Al: 0.4 to 2.0%, and Mn + 0.5 × Al <4: Formula (A), containing one or more of Ti: 0.003 to 0.20%, Nb: 0.003 to 0.04%, and V: 0.003 to 0.20%, with the balance being iron and unavoidable impurities After the hot rolling is completed at a rolling end temperature of Ar 3 or higher, the steel slab is cooled to 650 to 800 ° C. at a cooling rate of 20 ° C./sec or higher, and then air-cooled for 2 to 15 seconds. Thereafter, it is further cooled to a temperature of 350 to 600 ° C. at a cooling rate of 20 ° C./sec or more and wound up, thereby producing a high-strength hot-rolled steel sheet excellent in hole expandability, ductility and chemical conversion treatment.
(5) The slab according to (4), wherein the slab further contains one or more of Ca: 0.0005 to 0.01%, Zr: 0.0005 to 0.01%, and REM: 0.0005 to 0.05% by mass%. For producing a high-strength hot-rolled steel sheet having excellent hole expandability, ductility and chemical conversion treatment properties.
(6) A high-strength hot-rolled steel sheet excellent in hole expandability, ductility, and chemical conversion treatment according to (4) or (5), wherein the cast slab further contains, by mass%, Mg: 0.0005 to 0.01%. Production method.
[0010]
BEST MODE FOR CARRYING OUT THE INVENTION
It is well known that, in a high-strength hot-rolled steel sheet, hole expandability and ductility tend to contradict each other. The present inventors have conducted intensive studies to solve the above-mentioned problems, and as a result, it has been found that ductility can be improved without deteriorating hole expandability by making ferrite grains in a ferrite bainite steel a grain size of a certain value or more as much as possible. And completed the present invention. In other words, in the ferritic bainite substantial dual phase steel, attention is paid to the ferrite that increases ductility and the precipitates of TiC, NbC, and VC that secure the strength, and the hole expandability is reduced by sufficiently growing the ferrite grains. The above-mentioned problem has been solved by improving the ductility of the steel without forming the precipitate and securing the strength afterwards. That is, in a low C-low Si-high Al- (Ti, Nb, V) component system, Mn and Al obtain a specific metallographic structure of the steel sheet of the present invention under a specific relationship, so that the hole expandability, The present inventors have newly found that a high-strength hot-rolled steel sheet that simultaneously satisfies the three properties of ductility and chemical conversion treatment can be obtained. Furthermore, they have found an industrially advantageous production method. In addition, (Ti, Nb, V) means the inclusion of one or more specific amounts of Ti, Nb, V.
[0011]
Hereinafter,% of each element content in the steel composition indicates mass%. In the present invention, C in the high-strength hot-rolled steel sheet is set to 0.02 to 0.08%. C is an element necessary to secure the strength by precipitating carbides, and if it is less than 0.02%, it becomes difficult to secure the desired strength even in the high Al- (Ti, Nb, V) component system of the present invention. Become. On the other hand, if it exceeds 0.08%, the ductility is greatly reduced.
[0012]
Si is an important element for suppressing the formation of harmful carbides and obtaining a composite structure consisting mainly of ferrite structure and residual bainite, but deteriorates chemical conversion treatment and generates Si scale, so the upper limit is 0.5%. And If it exceeds 0.25%, the temperature control for obtaining the above-mentioned metal structure may be strict during the production of a hot-rolled steel sheet, so the Si content is more preferably 0.25% or less.
[0013]
Mn is an element necessary for securing the strength, and for this purpose, addition of 0.50% or more is required. However, when added in a large amount exceeding 3.5%, micro-segregation and macro-segregation are apt to occur, and the hole expandability is deteriorated. In order to effectively achieve both hole expandability and ductility, the range of Mn is preferably set to 1.00 to 3.50%.
[0014]
Since P forms a solid solution in ferrite and lowers its ductility, its content should be 0.03% or less. Further, S forms MnS and acts as a starting point of fracture, which significantly reduces hole expandability and ductility.
[0015]
Al is one of the important elements in the present invention and is an element necessary for achieving both ductility and chemical conversion treatment. Therefore, it is necessary to add 0.4% or more of Al. Al has conventionally been an element necessary for deoxidation in a hot-rolled steel sheet, and is usually added in an amount of about 0.01 to 0.07%. The present inventors have conducted various experiments on high-strength hot-rolled steel sheets having different metal structures based on a steel composition containing a very low amount of Al in a low C-low Si system, and have reached the present invention. That is, it has been found that when Al is 0.4% or more, ductility can be greatly improved by forming the above-mentioned metal structure without impairing the chemical conversion property. Al not only saturates the ductility-improving effect at 2.0%, but the addition of more than 2.0% makes it difficult to balance hole-expandability, ductility and chemical conversion treatment. , Economically disadvantageous.
[0016]
In order to achieve both ductility and chemical conversion treatment, it is important to define the relationship between Mn and Al. FIG. 1 shows the relationship between the contents of Mn and Al and the chemical conversion property. Thick solid lines in the figure include lines, but broken lines exclude lines. In addition, 、 and × in FIG. 1 were evaluated in the same manner as ○ and × in the chemical conversion treatment evaluation of Note 4) described later. As shown in FIG. 1, the present inventors have newly found that the chemical conversion property is not impaired under the condition of Mn + 0.5 × Al <4...
Although the reason is unknown, in the case of the above-mentioned metal structure of the present invention satisfying the component (A) with a high Al content, the proportion of ferrite having a grain size of 2 μm or more in a substantially two-phase structure steel of ferrite bainite Is a solid solution in ferrite grains with more than 40%, and the following precipitation strengthening of carbides of Ti, Nb and V is enhanced by the different low Al-based metal structures (for example, ferrite-martensitic two-phase structure, The present inventors presume that the chemical conversion processability of a high-strength hot-rolled steel sheet is improved, since it is more effectively exhibited than in the case of a structure composed of bainite single phase.
[0017]
Ti, Nb, and V are also one of the most important elements in the present invention, and precipitate fine carbides such as TiC, NbC, and VC to enable high strength in the present invention. For this purpose, it is necessary to add one or more of 0.003 to 0.20% of Ti, 0.003 to 0.04% of Nb, and 0.003 to 0.20% of V. Ti is less than 0.003% and Nb is less than 0.003% , V less than 0.003%, it is difficult to obtain high strength even with the high Al type of the present invention, and if Ti exceeds 0.20%, Nb exceeds 0.04%, and V exceeds 0.20%, a large amount of precipitates is formed. This is because ductility deteriorates. In order to make more effective use of the precipitates, Ti, Nb and V are preferably contained in an amount of 0.010% or more of Ti, 0.010% or more of Nb, and 0.030% or more of V.
[0018]
Ca, Zr, and REM are effective elements for controlling the morphology of sulfide inclusions and improving hole expansion properties. In order to make this morphological control effect effective, it is desirable to add one or more of Ca, Zr and REM in an amount of 0.0005% or more. On the other hand, a large amount of addition causes coarsening of the sulfide-based inclusions, deteriorating the cleanliness, and reducing the ductility even in the low C-low Si-high Al- (Ti, Nb, V) component system of the present invention. Since not only lowering, but also increasing the cost, the upper limits of Ca and Zr are set to 0.01%, and the upper limit of REM is set to 0.05%. REM is, for example, an element having an element number of 21, 39, 57 to 71.
[0019]
Mg is one of the important additional elements in the present invention. By this addition, Mg combines with oxygen to form an oxide. At this time, MgO or Al 2 O 3 containing MgO, SiO 2 , MnO, and Ti 2 O 3 are finely divided. The present inventors have newly found that each oxide has a smaller size and a more uniformly dispersed state as compared with the conventional low Al steel to which Mg is not added. These oxides, which are finely dispersed in the steel, are not clear, but form minute voids during punching, have the effect of contributing to the dispersion of stress and suppressing the occurrence of coarse cracks by suppressing stress concentration, It is considered to be effective in improving the hole spreading property. However, if less than 0.0005%, the effect is insufficient. On the other hand, even if the content exceeds 0.01%, not only the improvement effect is saturated, but also, on the contrary, the oxide which is agglomerated and coarse tends to be generated and the cleanliness of the steel is deteriorated, and the low C-low Si-high Al -Even in the case of the (Ti, Nb, V) component system, the upper limit is made 0.01% because the hole expandability and ductility are deteriorated.
Examples of the inevitable impurities include N ≦ 0.01%, Cu ≦ 0.3%, Ni ≦ 0.3%, Cr ≦ 0.3%, Mo ≦ 0.3%, Co ≦ 0.05%, Zn ≦ 0.05%, Sn ≦ 0.05%, Na ≦ 0.02. %, K ≦ 0.02% and B ≦ 0.0005% do not depart from the present invention.
[0020]
The size of the ferrite grain size is one of the most important indicators in the present invention. As a result of intensive studies, the present inventors have also found that when the area ratio of ferrite having a grain size of 2 μm or more is 40% or more, both the hole expandability and the ductility (for example, elongation) are excellent. FIG. 2 shows that, in a high-strength hot-rolled steel sheet having a tensile strength of 780 to 820 N / mm 2 and a λ value of 100 to 115, the area ratio of ferrite having a grain size of 2 μm or more is 40% or more (Example). FIG. 4 is a diagram showing the relationship between elongation and the relationship between tensile strength and elongation when the area ratio of ferrite having a grain size of 2 μm or more is 40% or less (comparative example). Mn + 0.5 × Al <4 The comparative example is also a component system that satisfies the formula (A), but the production method is different, and the area ratio of the ferrite having a particle size of 2 μm or more is 40% or less. As shown in FIG. 2, when the area ratio of the ferrite having a grain size of 2 μm or more is 40% or more, the elongation is higher by about 3 to 5% at the same strength than that of 40% or less. In order to achieve both good hole expandability and ductility, it is necessary that the ratio of ferrite grains having a grain size of 2 μm or more be 40% or more. In order to obtain a more remarkable effect, the ratio of ferrite grains having a grain size of 3 μm or more is desirably 40% or more. The particle size can be determined by converting the area of each particle into a circle equivalent diameter. It is effective to use an image analysis device for this conversion.
[0021]
The metal structure of the high-strength hot-rolled steel sheet substantially consists of a two-phase structure of ferrite and bainite. Here, since the metal structure contains 40% or more of ferrite having a grain size of 2 μm or more, the metal structure is substantially a two-phase structure of ferrite bainite having 40% or more of ferrite. For example, as the metallographic structure of the present invention, ferrite having a grain size of 2 μm or more is 40% or more and the balance is less than 2 μm ferrite and bainite, or ferrite having a grain size of 2 μm or more is 40% or more. The rest can be made only bainite. The reason why the content of bainite is set to 60% or less is that when the amount of bainite is larger than this, ductility is significantly reduced, and it becomes difficult to obtain a good relationship between tensile strength and elongation even in a high Al-based material. It is. However, even if the content of retained austenite is 3% or less as measured by ordinary X-ray diffraction intensity, it does not deviate from the substantial two-phase structure of the ferrite bainite of the present invention. In addition, in the vicinity of the surface of the hot-rolled steel sheet, there is a region where the composition of steel such as ultra-thin (for example, about 0.05 to 0.3 mm in thickness) carbon is slightly reduced. Most of the above in the sheet thickness direction has a substantial two-phase structure of the above ferrite bainite and a metal structure containing 40% or more of ferrite having a grain size of 2 μm or more has the function and effect of the present invention.
[0022]
The present invention is a high-strength hot-rolled steel sheet having the above-described steel composition and metal structure, and a method of manufacturing a high-strength hot-rolled steel sheet for producing the steel sheet industrially advantageously.
In producing a high-strength hot-rolled steel sheet by hot rolling, in the low C-low Si-high Al component system of the present invention, the hot rolling end temperature, that is, the finish rolling end temperature of hot rolling, ferrite generation. In order to improve the suppression hole expanding property and obtain good chemical conversion treatment property, it is preferable that the number of Ar is 3 or more. Ar 3 point is a temperature at which ferrite starts to be formed in hot rolling, but may be determined using a known calculation formula or a calculation formula obtained empirically. However, if the temperature is too high, the strength and ductility may be reduced due to the coarsening of the metal structure, so that the finish rolling end temperature is desirably 1050 ° C. or lower. Whether the slab is heated or not may be appropriately determined according to the rolling conditions of the steel sheet, and whether the next hot-rolled steel sheet or slab is continuously rolled by joining the next hot-rolled steel sheet or slab during hot rolling is determined according to the present invention. If a metal structure of the following can be obtained, it can be appropriately selected. It should be noted that the steel smelting may be performed by a converter system or an electric furnace system as long as the steel composition can be obtained by melting. Further, the hot metal pretreatment, refining, degassing, and the like for controlling impurities and the like may be appropriately selected.
[0023]
Rapid cooling (primary quenching) of the steel sheet immediately after the finish rolling of hot rolling is effective for obtaining high hole expandability in the low C-low Si-high Al component system of the present invention. The speed is preferably 20 ° C./sec or more. If the temperature is less than 20 ° C./sec, it may be difficult to suppress the formation of carbides harmful to hole expandability. The effect of suppressing coarsening of carbide precipitation is saturated at 250 ° C./sec. However, even at 250 ° C./sec or more, ferrite crystal grains grow to secure a ferrite crystal grain size of 2 μm or more to 40% or more of the metal structure. It is valid. When the temperature exceeds 600 ° C / sec, the growth effect of ferrite crystal grains is saturated, and conversely, it is not easy to maintain the shape of the hot-rolled steel sheet.
[0024]
Temporarily stopping the rapid cooling (primary quenching) of the steel sheet and performing air cooling is intended to increase the occupancy rate of ferrite in the low C-low Si-high Al component system of the present invention and improve ductility. Is important. However, if the air-cooling start temperature is lower than 650 ° C, pearlite which is harmful to the hole expandability will be generated from an early stage. On the other hand, if the air-cooling start temperature exceeds 800 ° C., it is not desirable because the formation of ferrite is slow and it is difficult to obtain the effect of air-cooling, and pearlite is easily formed during the subsequent cooling. Therefore, the air-cooling start temperature is preferably set to 650 to 800 ° C. Further, even if the air cooling time exceeds 15 seconds, the increase in ferrite is not only saturated, but also a load is imposed on control of the subsequent cooling rate and winding temperature, which is not industrially preferable. Therefore, the air cooling time is set to 15 seconds or less. If the air cooling time is less than 2 seconds, ferrite cannot be sufficiently precipitated, which is not preferable. Further, the air cooling of the present invention includes spraying a small amount of a mist-like refrigerant for the purpose of modifying the scale near the surface of the hot-rolled steel sheet so as not to affect the subsequent formation of the metal structure.
[0025]
After air cooling, the hot-rolled steel sheet is rapidly cooled again (secondary quenching), and the cooling rate is also preferably 20 ° C./sec or more. When the temperature is lower than 20 ° C./sec, harmful pearlite is easily generated, which is not preferable. At 200 ° C./se, the formation of bainite is almost saturated, but even at 200 ° C./sec or more, it is effective for suppressing coarsening of fine carbides such as TiC, NbC, and VC. If the temperature exceeds 600 ° C., the steel sheet may be partially supercooled, and hard martensite is easily formed locally, which is not preferable.
[0026]
The quenching (secondary quenching) stop temperature, ie, the winding temperature, is set to 350 to 600 ° C. If the winding temperature is lower than 350 ° C., hard martensite which is harmful to the hole expandability is generated, while if it exceeds 600 ° C., pearlite which is harmful to the hole expandability tends to be generated.
[0027]
As described above, due to the combination of the steel composition and the hot rolling conditions, the metal structure of the steel sheet is a substantial two-phase structure of ferrite bainite in which the ratio of ferrite with a grain size of 2 μm or more is 40% or more, and the tensile strength A high-strength hot-rolled steel sheet excellent in hole expandability, ductility, and chemical conversion treatment property of 590 N / mm 2 or more can be manufactured. Furthermore, even if a surface treatment (for example, galvanizing, lubrication treatment, etc.) is performed on the surface of the steel sheet of the present invention, it has the effects of the present invention and does not deviate from the present invention.
[0028]
【Example】
A steel having a chemical composition shown in Table 1 (content is mass%, blank column indicates no addition) is melted in a converter, cast into slabs by continuous casting, and rolled under hot rolling conditions shown in Table 2. After cooling, hot-rolled steel sheets having a sheet thickness of 2.6 (Examples 1 to 16, Comparative Examples 1 to 3) and 3.2 mm (Examples 17 to 32, Comparative Examples 4 to 6) were produced. The rapid cooling rate was set at 40 ° C./sec (Examples 1 to 15 and Comparative Examples 1 to 4), 120 ° C./sec (Examples 16 to 30 and Comparative Example 5), and 300 ° C./sec (Example 31 and 32, Comparative Example 6), and the air cooling time was 10 seconds (Examples 1 to 32, Comparative Examples 1 to 6). However, the finish rolling end temperatures of the hot rolling were 900 ° C (Examples 1 to 32, Comparative Examples 4 to 9) and 930 ° C (Comparative Examples 1 to 3).
[0029]
[Table 1]
[Table 2]
The hot-rolled steel sheet thus obtained was subjected to a tensile test, a hole expansion test, observation of a metal structure, and evaluation of chemical conversion treatment. Table 2 shows the results.
Note 1) Tensile strength and ductility In accordance with JIS Z 2201, a test specimen was subjected to a tensile test using JIS No. 5.
Note 2) Hole expansion test In the hole expansion test, a punched hole with an initial hole diameter (d0: 10 mm) is pushed and expanded with a 60 ° conical punch, and the hole expansion value is calculated from the hole diameter (d) when the crack penetrates the plate thickness. (Λ value) = (d−d0) / d0 × 100 was determined and the hole expandability was evaluated. Table 2 shows the results.
Note 3) Metallographic structure of steel sheet In the metallographic observation, ferrite and bainite were identified by scanning electron microscope after corrosion with nital, and the area ratio of ferrite having a grain size of 2 μm or more was measured by image analysis.
Note 4) Chemical conversion property The chemical conversion property of the hot-rolled steel sheet was determined by removing the surface scale, using a chemical conversion solution SD5000 (manufactured by Nippon Paint Co., Ltd.), performing degreasing and surface conditioning as prescribed, and then performing a chemical conversion treatment. The chemical conversion coating was evaluated by SEM (secondary electron beam image) as ○ when the coating was uniformly formed and x when the coating was not partially formed.
[0032]
In Examples 1 to 32, the chemical composition, the finish rolling end temperature, the air cooling start temperature, and the winding temperature were all within the scope of the present invention, the metal structure was composed of two phases of ferrite and bainite, and the particle size was 2 μm. This is a high-strength hot-rolled steel sheet which is an example of the present invention in which the ratio of the above ferrite is 40% or more and has a high λ value and elongation, and is excellent in hole expandability, ductility and chemical conversion treatment. On the other hand, those of Comparative Examples 1 to 9 which deviate from the conditions of the present invention are inferior in strength, hole expanding property, ductility balance and chemical conversion treatment property.
[0033]
Although not shown in Tables 1 and 2, the hot rolling end temperature was 920 ° C using the steel slab shown in Example 1 and then primary quenched to 625 ° C (cooling rate 40 ° C / sec). When the air-cooling start temperature is 625 ° C and air-cooled for 10 seconds, and the secondary quenching (cooling
[0034]
【The invention's effect】
As described in detail above, the present invention economically provides a high-strength hot-rolled steel sheet having a high tensile strength of 590 N / mm 2 or more and excellent in hole expandability, ductility, and chemical conversion treatment properties. Therefore, the present invention is suitable as a high-strength hot-rolled steel sheet having high workability. In addition, the high-strength hot-rolled steel sheet of the present invention is industrially valuable as it can reduce the weight of a vehicle body, integrate parts, and streamline processing steps, thereby improving fuel efficiency and reducing manufacturing costs. It is a big thing.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram showing the relationship between Mn and Al, which is excellent in hole expanding properties, ductility, and chemical conversion treatment properties.
FIG. 2 is an explanatory diagram showing the relationship between ferrite distribution and tensile strength-elongation balance in a high-strength hot-rolled steel sheet having a substantially two-phase structure of ferrite bainite.
Claims (6)
Priority Applications (10)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP2002377097A JP4180909B2 (en) | 2002-12-26 | 2002-12-26 | High-strength hot-rolled steel sheet excellent in hole expansibility, ductility and chemical conversion treatment, and method for producing the same |
| CNB2003801078060A CN100345993C (en) | 2002-12-26 | 2003-12-24 | High strength thin steel sheet excellent in hole expansibility, ductility and chemical treatment characteristics, and method for production thereof |
| EP03786277A EP1595965B1 (en) | 2002-12-26 | 2003-12-24 | High strength thin steel sheet excellent in hole expansibility, ductility and chemical treatment characteristics, and method for production thereof |
| DE60324333T DE60324333D1 (en) | 2002-12-26 | 2003-12-24 | THIN STAINLESS STEEL WITH HIGH STRENGTH AND EXCELLENT LOCHENESSABILITY, BENDING TOLERANCE AND EXCELLENT CHEMICAL TREATMENT PROPERTIES AND METHOD OF MANUFACTURING THEREOF |
| AU2003296089A AU2003296089A1 (en) | 2002-12-26 | 2003-12-24 | High strength thin steel sheet excellent in hole expansibility, ductility and chemical treatment characteristics, and method for production thereof |
| KR1020077009825A KR20070050108A (en) | 2002-12-26 | 2003-12-24 | High strength thin steel sheet excellent in hole expansibility, ductility and chemical treatment characteristics, and method for production thereof |
| KR1020057011928A KR100756114B1 (en) | 2002-12-26 | 2003-12-24 | Method for production of high strength thin steel sheet excellent in hole expansibility, ductility and chemical treatment characteristics |
| US10/540,418 US7780797B2 (en) | 2002-12-26 | 2003-12-24 | High strength thin steel excellent in hole expansibility, ductility and chemical treatment characteristics |
| PCT/JP2003/016614 WO2004059024A1 (en) | 2002-12-26 | 2003-12-24 | High strength thin steel sheet excellent in hole expansibility, ductility and chemical treatment characteristics, and method for production thereof |
| CA2511666A CA2511666C (en) | 2002-12-26 | 2003-12-24 | High strength thin steel sheet excellent in burring, elongation, and ability of phosphate coating and a method of production of the same |
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| JP2002377097A JP4180909B2 (en) | 2002-12-26 | 2002-12-26 | High-strength hot-rolled steel sheet excellent in hole expansibility, ductility and chemical conversion treatment, and method for producing the same |
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| JP2004204326A true JP2004204326A (en) | 2004-07-22 |
| JP4180909B2 JP4180909B2 (en) | 2008-11-12 |
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| CN1732281A (en) | 2006-02-08 |
| CN100345993C (en) | 2007-10-31 |
| JP4180909B2 (en) | 2008-11-12 |
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