JP3545696B2 - High strength hot rolled steel sheet excellent in hole expandability and ductility and method for producing the same - Google Patents
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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用鋼板を対象とし、１．０〜６．０ｍｍ程度の板厚で、５９０Ｎ／ｍｍ２ 以上の引張強度を有し、穴拡げ性と延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】
近年、自動車の燃費改善対策としての車体軽量化、部品の一体成形によるコストダウンのニーズが強まり、プレス成形性に優れた熱延高強度鋼板の開発が進められてきた。従来、加工用熱延鋼板としてはフェライト・マルテンサイト組織からなるＤｕａｌ Ｐｈａｓｅ鋼板が知られている。Ｄｕａｌ Ｐｈａｓｅ鋼板は、軟質なフェライト相と硬質なマルテンサイト相の複合組織で構成されており、著しく硬度の異なる両相の界面から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。
【０００３】
このため特開平４−８８１２５号公報、特開平３−１８０４２６号公報にベイナイトを主体とした組織から構成される、穴拡げ性の優れた熱延鋼板の製造方法が提案されている。さらに、特開平６−２９３９１０号公報では２段冷却を用いることによってフェライト占積率を制御することで穴拡げ性と延性を両立する製造方法が提案されている。しかしながら、自動車のさらなる軽量化指向、部品の複雑化等を背景に更に高い穴拡げ性が求められ上記技術では対応しきれない高度な加工性、高強度化が要求されている。
【０００４】
【発明が解決しようとする課題】
本発明は５９０Ｎ／ｍｍ２ クラス以上の熱延鋼板に関するもので、優れた穴拡げ性と延性を両立した高強度熱延鋼板を提供しようとするものである。
【０００５】
【課題を解決するための手段】
本発明の課題解決のため種々実験、検討を重ねた結果、穴拡げ性の改善には打抜き穴のクラックの状態が重要であることはよく知られるところであるが、本発明者らが鋭意検討した結果、Ｍｇを添加することで打抜き穴の断面に発生するクラックを微細均一化することが可能であることを見出した。そして、鋼板中に存在する酸化物とこれらを核にした（Ｎｂ、Ｔｉ）Ｎの複合析出物を均一微細に分散析出させることにより、打抜き時に微細ボイドを発生させることでの応力の集中を緩和しうることが考えられ、粗大クラックの発生を抑制し穴拡げ性を向上させていると考えられる。これより、この発明をなすに至ったのである。これまで、Ｍｇ添加による酸化物を利用した提案には、例えば特開平１１−３２３４８８号公報による面内異方性改善に関する提案ではＭｇ酸化物による再結晶時の面方位の優先的な核生成・成長を抑制する事を目的にしており、特開平１１−２３６６４５号公報の溶接部の靭性に関する提案ではＭｇ複合酸化物により超大入熱溶接時のＨＡＺ部のγ粒の成長を抑制することを目的としている．これらはいずれも微細酸化物によるピンニングによる効果を利用したものであり、本発明の打抜き時、介在物により発生する微細ボイドを利用するものとは異なり、これらを目的とする鋼板において穴拡げ性が向上しているかはさだかではない。本発明の要旨は、下記の通りである。
【０００６】
１）重量％にて
Ｃ ：０．０１％以上、０．２０％以下、
Ｓｉ：０．３％以上、１．５％以下、
Ｍｎ：０．５％以上、２．５％以下、
Ｐ ：０．１０％以下、
Ｓ ：０．００９％以下、
Ｎ ：０．０１０％以下、
Ｍｇ：０．０００５％以上、０．０１％以下、
Ａｌ：０．００２％以上、０．０７％以下、
および
Ｔｉ：０．００３％以上、０．２５％以下、
Ｎｂ：０．００３％以上、０．０４％以下
の１種または２種を含有し、残部が鉄および不可避的不純物からなり、
粒子径が０．００５μｍ〜５．０μｍの範囲にあるＭｇＯまたは、ＭｇＯを含むＡｌ ２ Ｏ ３ 、ＭｇＯを含むＳｉＯ ２ 、ＭｇＯを含むＭｎＯ、ＭｇＯを含むＴｉ ２ Ｏ ３ の１種もしくは２種以上の複合酸化物が１平方ｍｍあたり１．０×１０３個以上、１．０×１０７個以下含む、鋼組織をフェライト組織を主とし残ベイナイト組織とすることを特徴とする穴拡げ性と延性に優れた高強度熱延鋼板。
【０００７】
２）重量％にて
Ｃ ：０．０１％以上、０．２０％以下、
Ｓｉ：０．３％以上、１．５％以下、
Ｍｎ：０．５％以上、２．５％以下、
Ｐ ：０．１０％以下、
Ｓ ：０．００９％以下、
Ｎ ：０．０１０％以下、
Ｍｇ：０．０００５％以上、０．０１％以下、
Ａｌ：０．００２％以上、０．０７％以下、
および
Ｔｉ：０．００３％以上、０．２５％以下、
Ｎｂ：０．００３％以上、０．０４％以下
の１種または２種を含有し、残部が鉄および不可避的不純物からなり、
粒子径が０．００５μｍ〜５．０μｍ以下の範囲にあるＭｇＯまたは、ＭｇＯを含むＡｌ ２ Ｏ ３ 、ＭｇＯを含むＳｉＯ ２ 、ＭｇＯを含むＭｎＯ、ＭｇＯを含むＴｉ ２ Ｏ ３ の１種もしくは２種以上の複合酸化物と、これを核にして、その周辺に（Ｎｂ、Ｔｉ）Ｎを有する複合析出物のうち、そのサイズが０．０５μｍ〜５．０μｍの範囲の析出物が１平方ｍｍあたり１．０×１０３個以上、１．０×１０７個以下含む、鋼組織をフェライト組織を主とし残ベイナイト組織とすることを特徴とする穴拡げ性と延性に優れた高強度熱延鋼板。
【０００８】
３）重量％にて
Ｃ ：０．０１％以上、０．２０％以下、
Ｓｉ：０．３％以上、１．５％以下、
Ｍｎ：０．５％以上、２．５％以下、
Ｐ ：０．１０％以下、
Ｓ ：０．００９％以下、
Ｎ ：０．０１０％以下、
Ｍｇ：０．０００５％以上、０．０１％以下、
Ａｌ：０．００２％以上、０．０７％以下、
および
Ｔｉ：０．００３％以上、０．２５％以下、
Ｎｂ：０．００３％以上、０．０４％以下
の１種または２種含有し、残部が鉄および不可避的不純物からなり、さらに、
Ｃａ：０．０００５％以上、０．０１００％以下、
ＲＥＭ元素の合計：０．０００５％以上、０．０１００％以下
の１種または２種含有し、残部が鉄および不可避的不純物からなり、粒子径が０．００５μｍ〜５．０μｍの範囲にあるＭｇＯまたは、ＭｇＯを含むＡｌ ２ Ｏ ３ 、ＭｇＯを含むＳｉＯ ２ 、ＭｇＯを含むＭｎＯ、ＭｇＯを含むＴｉ ２ Ｏ ３ の１種もしくは２種以上の複合酸化物が１平方ｍｍあたり１．０×１０３個以上、１．０×１０７個以下含む、鋼組織をフェライト組織を主とし残ベイナイト組織とすることを特徴とする穴拡げ性と延性に優れた高強度熱延鋼板。
【０００９】
４）重量％にて
Ｃ ：０．０１％以上、０．２０％以下、
Ｓｉ：０．３％以上、１．５％以下、
Ｍｎ：０．５％以上、２．５％以下、
Ｐ ：０．１０％以下、
Ｓ ：０．００９％以下、
Ｎ ：０．０１０％以下、
Ｍｇ：０．０００５％以上、０．０１％以下、
Ａｌ：０．００２％以上、０．０７％以下、
および
Ｔｉ：０．００３％以上、０．２５％以下、
Ｎｂ：０．００３％以上、０．０４％以下
の１種または２種含有し、残部が鉄および不可避的不純物からなり、さらに、
Ｃａ：０．０００５％以上、０．０１００％以下、
ＲＥＭ元素の合計：０．０００５％以上、０．０１００％以下
の１種または２種含有し、残部が鉄および不可避的不純物からなり、粒子径が０．００５μｍ〜５．０μｍのＭｇＯまたは、ＭｇＯを含むＡｌ ２ Ｏ ３ 、ＭｇＯを含むＳｉＯ ２ 、ＭｇＯを含むＭｎＯ、ＭｇＯを含むＴｉ ２ Ｏ ３ の１種もしくは２種以上の複合酸化物と、これを核にして、その周辺に（Ｎｂ、Ｔｉ）Ｎを有する複合析出物のうち、そのサイズが０．０５μｍ〜５．０μｍの範囲の析出物が１平方ｍｍあたり１．０×１０３個以上、１．０×１０７個以下含む、鋼組織をフェライト組織を主とし残ベイナイト組織とすることを特徴とする穴拡げ性と延性に優れた高強度熱延鋼板。
【００１０】
５） １）又は２）又は３）又は４）に記した鋼を、圧延終了温度をＡｒ３ 変態点以上とする圧延をし、引き続き２０℃／ｓｅｃ以上の冷却速度で冷却し、３５０℃〜６００℃で捲取ることを特徴とする、鋼組織をフェライト組織を主とし残ベイナイト組織とすることを特徴とする穴拡げ性と延性に優れた高強度熱延鋼板の製造方法。
【００１１】
６） １）又は２）又は３）又は４）に記した鋼を、圧延終了温度をＡｒ３変態点以上とする圧延をした後、２０℃／ｓｅｃ以上の冷却速度で６５０℃〜７００℃まで冷却し、該温度で15秒以下空冷した後、再度前記冷却速度で冷却して、３５０℃〜６００℃で捲取ることを特徴とする、鋼組織をフェライト組織を主とし残ベイナイト組織とすることを特徴とする穴拡げ性と延性に優れた高強度熱延鋼板の製造方法。
【００１２】
７） ５）又は６）において、１）又は２）又は３）又は４）に記した鋼の溶製工程の成分調整段階において、ＳｉとＭｎを添加した後、Ｔｉを添加、その後にＭｇとＡｌを添加することを特徴とする穴拡げ性と延性に優れた高強度熱延鋼板の製造方法。
【００１３】
８） ７）において、Ｍｇの希釈溶媒金属としてＳｉ、Ｎｉ、Ｃｕ、Ａｌ、ＲＥＭ（希土類元素）の１種あるいは２種以上から成るＭｇ合金を用いることを特徴とする穴拡げ性と延性に優れた高強度熱延鋼板の製造方法。
【００１４】
９） ８）において、Ｍｇ合金中のＭｇ濃度が１％以上１０％未満であることを特徴とする穴拡げ性と延性に優れた高強度熱延鋼板の製造方法。
【００１５】
１０） ７）から９）において、Ｍｇ合金中のＦｅ、Ｍｎ、Ｃｒの濃度の和が１０％未満であることを特徴とする穴拡げ性と延性に優れた高強度熱延鋼板の製造方法。
【００１６】
【発明の実施の形態】
本発明は穴拡げ性の改善のために打抜き穴の粗大クラックを抑制するため、Ｍｇを添加し、酸化物を均一微細析出させ、これにより、打抜き時の粗大クラックの発生を抑制させ穴拡げ性を改善させるものである。以下に本発明の個々の構成要件について詳細に説明する。
【００１７】
まず、本発明の成分の限定理由について述べる。
Ｃは、鋼の加工性に影響を及ぼす元素であり、含有量が多くなると、加工性は劣化する。特に０．２０％を超えると穴拡げ性に有害な炭化物（パーライト、セメンタイト）が生成するので、０．２０％以下、ただし、好ましくは０．１５％以下が望ましい。また、強度確保の面で０．０１％以上は必要である。
【００１８】
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。また、Ｓｉ添加により強度と延性の両立させる作用もある。このような作用を得るためには０．３％以上の添加が必要である。しかし、添加量が増加すると化成処理性が低下するほか、点溶接性も劣化するため１．５％を上限とする。
【００１９】
Ｍｎは、強度確保に必要な元素であり、最低０．５０％の添加が必要である。しかし、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、これらは穴拡げ性を劣化させる。これより２．５０％を上限とする。
【００２０】
Ｐは鋼板の強度を上げる元素であるが、添加量が高いと溶接性、加工性、靱性の劣化を引き起こす元素である。これより、０．１０％以下とし、好ましくは０．０３％以下が望ましい。
【００２１】
ＳはＭｎＳ等の非金属介在物を生成し、延性穴拡げ性を劣化させるので鋼中に存在しない方が好ましい元素であり、添加量は少ない程望ましく、０．００９％以下とする。ただし、０．００５％以下でこの効果は顕著に現れるため０．００５％以下が望ましい。
【００２２】
Ｎは、加工性を確保するためには少ない方が良い。０．０１０％を越えると加工性が劣化してくるので、０．０１０％以下とし、０．００５％以下が望ましい。
【００２３】
Ｍｇは、本発明における最も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時に微細ボイドを形成し、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ると考えられ、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％超の添加は添加量に対する改善代が飽和するばかりでなく、逆に鋼の清浄度を劣化させ、穴拡げ性、延性を劣化させるため上限を０．０１％とする。
【００２４】
Ａｌは本発明における最も重要な添加元素の一つである。ＡｌはＭｇが添加されている時、スピネル構造をもつＭｇＡｌ２Ｏ４複合酸化物を生成しやすい。ＭｇＡｌ２Ｏ４複合酸化物はＭｇＯを含むＳｉＯ ２ 、ＭｇＯを含むＭｎＯ、ＭｇＯを含むＴｉ ２ Ｏ ３ の複合酸化物のうち最も微細な酸化物の存在状態のひとつであり、酸化物の分散状態を均一微細化するのに効果的であると考えられる。このため、打抜き時に微細ボイドを形成し、これが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ると考えられ、穴広げ性の向上に効果があると考えられる。これより０．００２％以上添加する。ただし添加量が増加するとＭｇ添加の効果を阻害するため、０．０７％以下とする。特に複合酸化物のうちＭｇＡｌ複合酸化物の酸化物に占める割合を向上し酸化物の微細化を効率よく達成させるためには添加量は０．０２％〜０．０７％が望ましい。
【００２５】
Ｔｉ、Ｎｂは本発明における最も重要な添加元素の一つである。Ｔｉ、Ｎｂは微細均一に析出している酸化物のうち特に小さいＭｇＯまたはＭｇＡｌ２ Ｏ４ を主とする複合酸化物を核に析出し、これら酸化物上に析出することで析出物サイズを大きくし、ＭｇＯまたはＭｇＡｌ２ Ｏ４ の微細ボイド形成を助成する働きがあると考えられる。また、強度の増加にも有効である。これらの結果を有効に発揮させるためにはＮｂ、Ｔｉともに少なくとも０．００３％の添加が必要であり、０．０１％以上の添加が望ましい。しかし、これらの添加が過度になると析出強化により延性が劣化するため、上限としてＴｉは０．２５％以下、Ｎｂは０．０４％以下とする。これらの元素は単独で添加しても効果があり、複合添加しても効果がある。
【００２６】
Ｃａは硫化物系の介在物の形状制御し、穴拡げ性の向上に有効である。これを有効に発揮させるためには０．０００５％以上の添加が必要である。一方、多量の添加は逆に鋼の清浄度を悪化させるため穴拡げ性、延性を損なう。これより上限を０．０１００％とする。
ＲＥＭ元素はＣａと同様の効果を有する。すなわち、ＲＥＭは硫化物系の介在物の形状制御し、穴拡げ性の向上に有効である。これを有効に発揮させるためにはＲＥＭ元素の合計で０．０００５％以上の添加が必要である。一方、多量の添加は逆に鋼の清浄度を悪化させるため穴拡げ性、延性を損なう。また、製造コストも高いため上限を０．０１００％とする。
【００２７】
酸化物としてはＭｇＯまたは、ＭｇＯを含むＡｌ ２ Ｏ ３ 、ＭｇＯを含むＳｉＯ ２ 、ＭｇＯを含むＭｎＯ、ＭｇＯを含むＴｉ ２ Ｏ ３ の１種もしくは２種以上の複合酸化物がよい。本発明者らが鋭意検討した結果、複合酸化物のうちＭｇを含む複合酸化物（例、ＭｇＯ、ＭｇＡｌ ２ Ｏ ４ ）と、これ以外のＭｇＯを含まない複合酸化物とで異なった存在状態にて微細クラックの形成に効果を発揮しており、これらはともにＭｇ添加によって得られる効果であり、相乗効果によって穴拡げ性を向上させていることがわかった。
【００２８】
ＭｇＯ、ＭｇＡｌ２ Ｏ４ は主に（Ｎｂ、Ｔｉ）Ｎを周辺に析出させることで微細ボイド形成の効果を得ており、ＭｇＯ、ＭｇＡｌ２ Ｏ４ は均一な分散析出の核として寄与していると考えられる。一方で、ＭｇＯ、ＭｇＡｌ２ Ｏ４ 以外の微細な複合酸化物はＭｇＯとの複合酸化物化により微細分散析出し、（Ｎｂ、Ｔｉ）Ｎを周辺に析出させることなく酸化物単独にて微細ボイド形成の効果がある。特に、ＭｇＯ、ＭｇＡｌ２ Ｏ４ 以外の微細な複合酸化物としてはＭｇＯ、Ａｌ２ Ｏ３ 、ＳｉＯ２ 主体の複合酸化物がほとんどであり、この時、全体に占めるＭｇＯ、Ａｌ２ Ｏ３ 、ＳｉＯ２ 酸化物の割合は９０％以上である。
【００２９】
酸化物の粒子径は０．００５μｍ未満ではこれを核にした（Ｎｂ、Ｔｉ）Ｎの析出も少ないこと、一方で、このサイズの酸化物は（Ｎｂ、Ｔｉ）Ｎの複合析出なしでは微細クラックを発生させる核とはなり難く、微細ボイド生成の効果が得られ難くなるため０．００５μｍ以上とする。逆に５．０μｍ超では粒子数の確保が困難であり、また、粗大析出物は延性の劣化を招くため５．０μｍ以下とする。
【００３０】
酸化物と複合析出物のサイズはこれが小さい時、微細ボイドの起点とならないため効果を発揮できない。従って、０．０５μｍ以上とする。一方、５．０μｍ超では粒子数の確保が困難であり、これが粗大クラックの生成を助長し穴拡げ性を低減させるため５．０μｍ以下とする。
【００３１】
析出物密度は個数が少ないと、打抜き時に発生する微細ボイドが不足し、粗大なクラックの発生を抑制する効果が得られないと考えられる。この効果を得るには１平方ｍｍあたり１．０×１０３ 以上必要である。一方で個数が多くなると効果は飽和し、逆に延性を劣化させるため、１．０×１０７ 個以下とする。ただし、この効果の飽和と延性のバランスから１．０×１０６ 個以下が望ましい。
【００３２】
また、穴拡げ性を高める手段として打抜き穴の性状の他、母材の局部延性能を高めることが効果的である。母材の局部延性能を高めるためには組織の均一化が有効であるが、単相鋼では本発明の目的とする強度において延性の劣化が大きく、目的とする特性が得られない。このため、鋼の組織としてはフェライト組織主体の複合組織とする。但し、フェライト組織の占有率が高く単相鋼となると延性または強度の低下を引き起こし、また、この占有率が低い時、伸びの低い第２相の影響を受け、延性が低下する。このため、フェライト組織の占有率は５０％以上、９５％以下が望ましい。また、残りの組織はこれが、マルテンサイト、粗大セメンタイト、パーライト組織であるとき、フェライト組織とこれらの組織の界面でクラックが発生し局部変形能が低下する。一方で、ベイナイト組織はフェライト組織中に微細なセメンタイトの分散した組織であり、母材の局部延性能を低下させないため、鋼の組織としてフェライト組織を主体とし、残ベイナイト組織とする。
【００３３】
本発明で規定した介在物の分散状態は例えば以下の方法により定量的に測定される。母材鋼板の任意の場所から抽出レプリカ試料を作成し、これを前記の透過電子顕微鏡（ＴＥＭ）を用いて倍率は５０００〜２００００倍で少なくとも５０００μｍ２ 以上の面積にわたって観察し、対象となる複合介在物の個数を測定し、単位面積当たりの個数に換算する。この時、酸化物と（Ｎｂ、Ｔｉ）Ｎの同定にはＴＥＭに付属のエネルギー分散型Ｘ線分光法（ＥＤＳ）による組成分析とＴＥＭによる電子線回折像の結晶構造解析によって行われる。このような同定を測定する全ての複合介在物に対して行うことが煩雑な場合、簡易的に次に手順による。まず、対象となるサイズの個数を形状、サイズ別に上記の要領にて測定し、これらのうち、形状、サイズの異なる全てに対し、各々１０個以上に対し上記の要領にて同定を行い、酸化物と（Ｎｂ、Ｔｉ）Ｎの割合を算出する。そして、はじめに測定された介在物の個数にこの割合を掛け合わせる。鋼中の炭化物が以上のＴＥＭ観察を邪魔する場合、熱処理によって炭化物を凝集粗大化、または溶解させ対象とする複合介在物の観察を容易にすることができる。
【００３４】
次に製造方法について説明する。
仕上圧延終了温度はフェライトの生成を妨げ、穴拡げ性を良好にするためＡｒ３ 変態点以上とする必要がある。しかしあまり高温にすると組織の粗大化による強度低減、延性の低下を招くため９５０℃以下とすることが望ましい。冷却速度は穴拡げ性に有害な炭化物形成を抑制し、高い穴拡げ比を得るためには２０℃／ｓ以上が必要である。捲取温度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３５０℃以上とする。一方、上限は６００℃超になると穴拡げ性に有害な、パーライト、セメンタイトが生成するため６００℃以下とする。
【００３５】
連続冷却中空冷はフェライト相の占有率を増加させ、延性を向上させるために有効である。しかし、空冷温度、空冷時間により、パーライトが生成されると逆に延性が低下するばかりでなく、穴拡げ性が著しく低下する。空冷温度が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ため、６５０℃以上とする。一方で７００℃超ではフェライト生成が遅く空冷の効果を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生成が発生しやすくため７００℃以下とする。１５秒間超の空冷はフェライト相の増加が飽和するばかりでなく、その後の冷却速度、捲取温度の制御に負荷をかける。このため、空冷時間は１５秒以下とする。
【００３６】
次に溶製工程における成分調整段階の添加順序は本発明者らが鋭意検討した結果、ＳｉとＭｎを添加した後、Ｔｉを添加、その後にＭｇとＡｌを添加することを行うとき、溶鋼中へのＭｇ歩留が増加することと、さらに酸化物のサイズがより微細化して、本発明で請求している酸化物のサイズの分散状態が安定に得られ易くなることから、より好ましい。
【００３７】
Ｍｇは溶鋼中での揮発性が高く、Ｍｇ純金属で溶鋼中へ投入するとＭｇ歩留が非常に低い。このため、Ｍｇは希釈溶媒金属との合金の形で溶鋼中へ投入する。このとき、本発明者らが鋭意検討した結果、Ｍｇの希釈溶媒金属としてＳｉ、Ｎｉ、Ｃｕ、Ａｌ、ＲＥＭ（希土類元素）の１種あるいは２種以上から成るＭｇ合金を用いたとき、鋼中へ残存するＭｇ量は向上し、これら以外の金属を主体とする合金では、効果が得られなかった。Ｍｇの希釈溶媒金属としてＭｇと原子間引力の相互作用を有するＳｉ、Ｎｉ、Ｃｕ、Ａｌ、ＲＥＭ（希土類元素）を選び、これらのうち、１種あるいは２種以上から成るＭｇ合金を用いて溶鋼中へのＭｇの投入を行うことが好ましい。
ここで希土類元素の範囲は、例えば理化学辞典第５版、３０９頁、岩波書店、１９９８年発行の記載通り、周期律表３族に属するＳｃ、Ｙおよびランタノイド（原子番号５７のＬａから７１のＬｕ）の総称である。
【００３８】
また、本発明者らが鋭意検討した結果、Ｍｇ合金中のＭｇ濃度としては１０％未満ではＭｇ歩留が顕著に増加することと、さらに適正な酸化物サイズと個数が安定に得やすくなり好ましいことを見出した．一方、１％未満であるとＭｇ合金添加時に希釈溶媒金属が鋼中へ過剰に溶解するため、成分調整が困難となる。従って、合金中のＭｇ濃度は１％以上１０％未満とすることが好ましい。
【００３９】
Ｍｇ合金中のＦｅ、Ｍｎ、Ｃｒの濃度の和として１０％未満の時、Ｍｇ歩留が顕著に増加することと、さらに適正な酸化物サイズと個数が安定に得やすくなり好ましいことを見出した。これはＭｇ合金が溶鋼に溶解中に生じるＭｇとこれらの元素との間の原子間反発作用によると解釈される。従って、Ｍｇ合金中のＦｅ、Ｍｎ、Ｃｒの濃度の和は１０％未満とすることが好ましい。
本発明の鋼板は上記のように熱延の後、溶融亜鉛めっきのように焼鈍によりめっきを施しても本発明の効果は損なわれない。また、熱延後、電気めっき、有機複合皮膜を施した場合も効果は損なわれない。
【００４０】
【実施例】
次に本発明を実施例に基づいて説明する．
表１に示す鋼成分の鋼を溶製するために、溶銑２７０ｔを転炉で目標Ｃ 濃度に脱炭したのち取鍋に溶鋼を移し、脱酸と合金調整をＣＡＳ法（日本鉄鋼協会編、梶岡博幸著、取鍋精錬法、１０４頁、地人書館、１９９７年発行に記載）により実施した。溶鋼の脱酸をＳｉとＭｎを添加した後、Ｔｉを添加、その後にＭｇとＡｌを添加する順序で行った例とそれ以外の例を表１に示す。ここではＳｉ、Ｍｎ、Ｔｉ原料としてＦｅＳｉ、ＦｅＭｎ、ＦｅＴｉを用いた。また、Ｍｇ、Ａｌは希釈溶媒金属としてＳｉ、Ｎｉ、Ｃｕ、Ａｌ、ＲＥＭ（希土類元素）の１種あるいは２種以上を用い、Ｍｇ合金中のＭｇ濃度が１％以上１０％未満であり、Ｍｇ合金中のＦｅ、Ｍｎ、Ｃｒの濃度の和が１０％未満のＭｇ合金を用いた例とこれら以外の合金を用いた例も表１に示した。脱酸後、必要元素を目標成分濃度範囲に調整した後、ただちに連続鋳造機により厚さ２５０ｍｍ、幅１３００ｍｍのスラブを製造した。これらの鋼を１２００℃以上にて加熱炉中で加熱し、表２に示す熱延条件にて圧延・冷却し、板厚２．６〜３．２ｍｍの熱延鋼板を得た。
【００４１】
一方、表３にＭｇの添加は希釈溶媒金属としてＳｉ、Ｎｉ、Ｃｕ、Ａｌ、ＲＥＭ（希土類元素）の１種あるいは２種以上を用い、Ｍｇ合金中のＭｇ濃度が１％以上１０％未満であり、Ｍｇ合金中のＦｅ、Ｍｎ、Ｃｒの濃度の和が１０％未満のＭｇ合金を用い、溶製工程の成分調整段階において、ＳｉとＭｎを添加した後、Ｔｉを添加、その後にＭｇとＡｌを添加する脱酸を行ったもので、成分を変化させたものを示す。符号Ｄ〜Ｙが本発明に従った鋼でこれ以外はＣ、Ｓｉ、Ｍｎ、Ｓ、Ａｌ、Ｍｇ、Ｎｂ、Ｔｉの添加量が本発明の範囲外である。これらの鋼を１２００℃以上にて加熱炉中で加熱し、表４に示す熱延条件にて圧延・冷却し、板厚２．６〜３．２ｍｍの熱延鋼板を得た。
【００４２】
また、鋼板母材より抽出レプリカ試料を作成し、前述の方法にて酸化物とこれを核に存在する（Ｔｉ、Ｎｂ）Ｎ複合酸化物の粒径、個数を測定し、単位面積当たりの個数に換算した。これを表１、３に表記する。
【００４３】
このようにして得られた熱延鋼板についてＪＩＳ５号片による引張試験、穴拡げ試験、組織観察を行った。穴拡げ性（λ）は径１２ｍｍ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）と初期穴径（ｄ０：１２ｍｍ）から λ＝（ｄ−ｄ０）／ｄ０×１００ で評価した。
【００４４】
各試験片のＴＳ、Ｅｌ、λを表２、４に示す、図１に強度と伸びの関係を図２に強度と穴拡げ比の関係を示す。本発明鋼は比較鋼１と比べて穴拡げ比が、比較鋼２と比べると穴拡げ比と伸びの両特性が高くなっていることがわかる。このように、本発明の鋼板は穴拡げ比、延性をともに優れていることがわかる。
【００４５】
なお、ここでは合金投入をＣＡＳ 法で行ったがこれは特に限定するものではなく、ＲＨ脱ガス装置の真空槽内合金添加法、溶鋼取鍋内ワイヤー添加法、粉体インジェクション法等の公知の方法も問題なく使用できることを付記する。
【００４６】
【表１】
【表２】
【表３】
【表４】
【発明の効果】
本発明によれば強度レベルが５９０Ｎ／ｍｍ２ クラス以上で、従来にない伸び−延性バランスを有した熱延高強度鋼板を供給できるようになったもので、産業上極めて有用なものである。
【図面の簡単な説明】
【図１】引張強度と伸びの関係を示すグラフである。
【図２】引張強度と穴拡げ比の関係を示すグラフである。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention is mainly intended for a steel sheet for automobiles to be pressed, has a thickness of about 1.0 to 6.0 mm, has a tensile strength of 590 N / mm 2 or more, and has excellent hole expandability and ductility. The present invention relates to a high-strength hot-rolled steel sheet and a method for producing the same.
[0002]
[Prior art]
In recent years, there has been a growing need for weight reduction of the vehicle body as a measure for improving fuel efficiency of automobiles and cost reduction by integral molding of parts, and development of hot-rolled high-strength steel sheets excellent in press formability has been promoted. Conventionally, as a hot rolled steel sheet for processing, a Dual Phase steel sheet having a ferrite-martensite structure has been known. The dual phase steel sheet is composed of a composite structure of a soft ferrite phase and a hard martensite phase, and has a problem of poor hole expandability because voids are generated from the interface between the two phases with remarkably different hardness to cause cracking. However, it is not suitable for applications requiring high hole expandability, such as underbody parts.
[0003]
For this reason, Japanese Patent Application Laid-Open Nos. 4-88125 and 3-180426 propose a method for producing a hot-rolled steel sheet having a structure mainly composed of bainite and having excellent hole expandability. Further, Japanese Patent Application Laid-Open No. Hei 6-293910 proposes a manufacturing method in which hole expansion and ductility are compatible by controlling the ferrite space factor by using two-stage cooling. However, in view of the trend toward lighter automobiles and more complicated parts, higher hole expandability is required, and high workability and high strength that cannot be handled by the above technology are required.
[0004]
[Problems to be solved by the invention]
The present invention relates to 590N / mm 2 or more classes of hot-rolled steel sheet, it is intended to provide a high strength hot rolled steel sheet having both excellent hole expandability and ductility.
[0005]
[Means for Solving the Problems]
As a result of repeated experiments and studies for solving the problems of the present invention, it is well known that the state of cracks in punched holes is important for improving hole expandability, but the present inventors have conducted extensive studies. As a result, it has been found that by adding Mg, it is possible to finely uniform the cracks generated in the cross section of the punched hole. The oxides present in the steel sheet and the composite precipitates of (Nb, Ti) N having these as nuclei are uniformly and finely dispersed and precipitated, thereby alleviating the concentration of stress due to generation of fine voids at the time of punching. It is considered that the generation of coarse cracks is suppressed, and the hole expandability is improved. Thus, the present invention has been accomplished. Until now, proposals using oxides by adding Mg include, for example, Japanese Patent Application Laid-Open No. H11-323488, which proposes improvement of in-plane anisotropy, preferential nucleation of plane orientation during recrystallization by Mg oxide. Japanese Patent Application Laid-Open No. H11-236645 proposes toughness of a welded portion. The purpose of the proposed method is to suppress the growth of γ grains in a HAZ portion during ultra-high heat input welding by using a Mg composite oxide. And These are all based on the effect of pinning by fine oxides.Unlike those utilizing fine voids generated by inclusions at the time of punching of the present invention, the hole expandability in a steel plate intended for these is It's not up to you to improve. The gist of the present invention is as follows.
[0006]
1) C: 0.01% or more and 0.20% or less by weight%
Si: 0.3% or more, 1.5% or less,
Mn: 0.5% or more, 2.5% or less,
P: 0.10% or less,
S: 0.009% or less,
N: 0.010% or less,
Mg: 0.0005% or more, 0.01% or less,
Al: 0.002% or more, 0.07% or less,
And Ti: 0.003% or more, 0.25% or less,
Nb: contains one or more of 0.003% or more and 0.04% or less, and the balance consists of iron and inevitable impurities;
One or more of MgO having a particle diameter in the range of 0.005 μm to 5.0 μm, Al 2 O 3 containing MgO, SiO 2 containing MgO, MnO containing MgO, and Ti 2 O 3 containing MgO. A hole diameter of 1.0 × 10 3 or more and 1.0 × 10 7 or less per square mm, wherein the steel structure is mainly a ferrite structure and a residual bainite structure. High strength hot rolled steel sheet with excellent ductility.
[0007]
2) C in weight%: 0.01% or more, 0.20% or less,
Si: 0.3% or more, 1.5% or less,
Mn: 0.5% or more, 2.5% or less,
P: 0.10% or less,
S: 0.009% or less,
N: 0.010% or less,
Mg: 0.0005% or more, 0.01% or less,
Al: 0.002% or more, 0.07% or less,
And Ti: 0.003% or more, 0.25% or less,
Nb: contains one or more of 0.003% or more and 0.04% or less, and the balance consists of iron and inevitable impurities;
MgO particle diameter is in the range 0.005μm~5.0μm or, MnO containing SiO 2, MgO containing Al 2 O 3, MgO containing MgO, 1 kind or two kinds of Ti 2 O 3 containing MgO Among the above composite oxides and the composite precipitates having (Nb, Ti) N around the cores, precipitates having a size in the range of 0.05 μm to 5.0 μm per square mm A high-strength hot-rolled steel sheet excellent in hole expandability and ductility, characterized in that the steel structure is mainly a ferrite structure and a residual bainite structure, including not less than 1.0 × 10 3 and not more than 1.0 × 10 7 pieces. .
[0008]
3) C in weight%: 0.01% or more, 0.20% or less,
Si: 0.3% or more, 1.5% or less,
Mn: 0.5% or more, 2.5% or less,
P: 0.10% or less,
S: 0.009% or less,
N: 0.010% or less,
Mg: 0.0005% or more, 0.01% or less,
Al: 0.002% or more, 0.07% or less,
And Ti: 0.003% or more, 0.25% or less,
Nb: One or two or more elements of 0.003% or more and 0.04% or less, with the balance being iron and unavoidable impurities.
Ca: 0.0005% or more, 0.0100% or less,
Total of REM elements: MgO containing 0.0005% or more and 0.0100% or less, the balance being iron and unavoidable impurities, and having a particle size in the range of 0.005 μm to 5.0 μm. Alternatively, one or more composite oxides of Al 2 O 3 containing MgO, SiO 2 containing MgO, MnO containing MgO, and Ti 2 O 3 containing MgO are 1.0 × 10 3 per square mm. A high-strength hot-rolled steel sheet excellent in hole expandability and ductility, characterized in that the steel structure is mainly a ferrite structure and has a residual bainite structure, including not less than 1.0 × 10 7 and not more than 1.0 × 10 7 pieces.
[0009]
4) C: 0.01% or more, 0.20% or less in weight%,
Si: 0.3% or more, 1.5% or less,
Mn: 0.5% or more, 2.5% or less,
P: 0.10% or less,
S: 0.009% or less,
N: 0.010% or less,
Mg: 0.0005% or more, 0.01% or less,
Al: 0.002% or more, 0.07% or less,
And Ti: 0.003% or more, 0.25% or less,
Nb: One or two or more elements of 0.003% or more and 0.04% or less, with the balance being iron and unavoidable impurities.
Ca: 0.0005% or more, 0.0100% or less,
Total of REM elements: One or two of 0.0005% or more and 0.0100% or less, the balance being iron and unavoidable impurities, and MgO or MgO having a particle size of 0.005 μm to 5.0 μm. And / or two or more complex oxides of Al 2 O 3 containing Mg, SiO 2 containing MgO, MnO containing MgO, and Ti 2 O 3 containing MgO, with (Nb, Ti) Among the composite precipitates having N, the precipitates having a size in the range of 0.05 μm to 5.0 μm include 1.0 × 10 3 or more and 1.0 × 10 7 or less per 1
[0010]
5) The steel described in 1) or 2) or 3) or 4) is rolled so that the rolling end temperature is equal to or higher than the Ar 3 transformation point, and then cooled at a cooling rate of 20 ° C./sec or higher, and 350 ° C. or higher. A method for producing a high-strength hot-rolled steel sheet excellent in hole expandability and ductility, characterized in that the steel structure is mainly a ferrite structure and the remaining bainite structure is characterized by winding at 600 ° C.
[0011]
6) After rolling the steel described in 1) or 2) or 3) or 4) so that the rolling end temperature is equal to or higher than the Ar 3 transformation point, from 650 ° C to 700 ° C at a cooling rate of 20 ° C / sec or higher. Cooling, after air cooling at this temperature for 15 seconds or less, cooling again at the above-mentioned cooling rate, and winding at 350 ° C to 600 ° C, wherein the steel structure is mainly a ferrite structure and a residual bainite structure. The method for producing a high-strength hot-rolled steel sheet having excellent hole expandability and ductility characterized by the following.
[0012]
7) In 5) or 6), in the component adjustment stage of the steel melting process described in 1) or 2) or 3) or 4), after adding Si and Mn, then adding Ti, and then adding Mg and A method for producing a high-strength hot-rolled steel sheet having excellent hole expandability and ductility, characterized by adding Al.
[0013]
8) In 7), excellent in hole-expanding property and ductility, characterized in that a Mg alloy composed of one or more of Si, Ni, Cu, Al, and REM (rare earth element) is used as a diluting solvent metal of Mg. Manufacturing method of high strength hot rolled steel sheet.
[0014]
9) The method for manufacturing a high-strength hot-rolled steel sheet according to 8), wherein the Mg concentration in the Mg alloy is 1% or more and less than 10%.
[0015]
10) The method for producing a high-strength hot-rolled steel sheet excellent in hole expandability and ductility according to 7) to 9), wherein the sum of the concentrations of Fe, Mn, and Cr in the Mg alloy is less than 10%.
[0016]
BEST MODE FOR CARRYING OUT THE INVENTION
In the present invention, Mg is added to suppress coarse cracks in a punched hole in order to improve hole expandability, and oxides are uniformly and finely precipitated, thereby suppressing the occurrence of coarse cracks during punching and improving hole expandability. Is to improve. Hereinafter, individual components of the present invention will be described in detail.
[0017]
First, the reasons for limiting the components of the present invention will be described.
C is an element that affects the workability of steel, and as the content increases, the workability deteriorates. In particular, if it exceeds 0.20%, carbides (pearlite, cementite) which are harmful to the hole-expanding property are generated, so that the content is preferably 0.20% or less, but preferably 0.15% or less. In addition, 0.01% or more is necessary for securing strength.
[0018]
Si is an important element for suppressing generation of harmful carbides and obtaining a composite structure mainly composed of ferrite structure and residual bainite. The addition of Si also has the effect of balancing strength and ductility. To obtain such an effect, it is necessary to add 0.3% or more. However, when the added amount increases, the chemical conversion property decreases and the spot weldability also deteriorates, so the upper limit is 1.5%.
[0019]
Mn is an element necessary for ensuring strength, and needs to be added at least 0.50%. However, when added in a large amount, micro segregation and macro segregation are likely to occur, and these deteriorate the hole expandability. From this, the upper limit is 2.50%.
[0020]
P is an element that increases the strength of the steel sheet, but is an element that causes deterioration in weldability, workability, and toughness when the addition amount is high. Accordingly, the content is set to 0.10% or less, preferably 0.03% or less.
[0021]
S is an element that is preferably not present in steel because it forms non-metallic inclusions such as MnS and deteriorates ductile hole expandability. The smaller the addition amount, the more desirable, the content is 0.009% or less. However, this effect is remarkably exhibited at 0.005% or less, so 0.005% or less is desirable.
[0022]
N should preferably be small in order to ensure workability. If the content exceeds 0.010%, the workability deteriorates. Therefore, the content is set to 0.010% or less, preferably 0.005% or less.
[0023]
Mg is one of the most important additive elements in the present invention. The Mg this addition, the oxygen combines with to form an oxide, a composite oxide refinement of Al 2 O 3, SiO 2, MnO, Ti 2 O 3 containing MgO or MgO are generated at this time is Mg It has been found that the size of each oxide is smaller than that of a conventional steel containing no, and a uniformly dispersed state is obtained. These oxides, which are finely dispersed in the steel, are not clear, but are thought to have the effect of forming fine voids at the time of punching and suppressing the occurrence of coarse cracks by suppressing stress concentration. It is considered effective for improvement. However, if it is less than 0.0005%, the effect is insufficient. On the other hand, the addition of more than 0.01% not only saturates the improvement margin with respect to the addition amount but also conversely deteriorates the cleanliness of the steel and deteriorates the hole expandability and ductility, so the upper limit is made 0.01%.
[0024]
Al is one of the most important additive elements in the present invention. Al tends to form a MgAl 2 O 4 composite oxide having a spinel structure when Mg is added. MgAl 2 O 4 composite oxide is one of the finest oxides among the composite oxides of SiO 2 containing MgO, MnO containing MgO, and Ti 2 O 3 containing MgO , and the dispersed state of the oxide. Is considered to be effective for uniformly reducing the size. For this reason, fine voids are formed at the time of punching, and this is considered to be effective in suppressing the occurrence of coarse cracks by suppressing stress concentration, which is considered to be effective in improving hole expandability. 0.002% or more is added. However, if the added amount increases, the effect of Mg addition is hindered, so the content is made 0.07% or less. In particular, in order to improve the ratio of the MgAl composite oxide to the oxide among the composite oxides and to efficiently achieve the miniaturization of the oxide, the addition amount is desirably 0.02% to 0.07%.
[0025]
Ti and Nb are one of the most important additive elements in the present invention. Ti, Nb precipitates in the nucleus a particularly small composite oxide mainly composed of MgO or MgAl 2 O 4 among oxides that are finely and uniformly deposited, and deposits on these oxides to increase the precipitate size. However, it is considered that MgO or MgAl 2 O 4 has a function of promoting the formation of fine voids. It is also effective in increasing the strength. In order to exert these results effectively, it is necessary to add at least 0.003% of both Nb and Ti, and it is desirable to add 0.01% or more. However, if these additions are excessive, ductility is deteriorated due to precipitation strengthening. Therefore, the upper limit is set to 0.25% or less for Ti and 0.04% or less for Nb. These elements are effective even if added alone, and are effective even if added in combination.
[0026]
Ca is effective in controlling the shape of the sulfide-based inclusions and improving hole expandability. In order to exhibit this effectively, it is necessary to add 0.0005% or more. On the other hand, the addition of a large amount adversely deteriorates the cleanliness of the steel, thereby impairing the hole expandability and ductility. From this, the upper limit is set to 0.0100%.
The REM element has the same effect as Ca. That is, REM is effective in controlling the shape of sulfide-based inclusions and improving hole expandability. In order to exhibit this effectively, it is necessary to add 0.0005% or more in total of the REM elements. On the other hand, the addition of a large amount adversely deteriorates the cleanliness of the steel, thereby impairing the hole expandability and ductility. Further, since the manufacturing cost is high, the upper limit is set to 0.0100%.
[0027]
The oxide MgO or, MnO containing SiO 2, MgO containing Al 2 O 3, MgO containing MgO, it is one or two or more composite oxides of Ti 2 O 3 containing MgO. As a result of the inventor's intensive studies, the composite oxides containing Mg (eg, MgO, MgAl 2 O 4 ) and other composite oxides containing no MgO among the composite oxides have different existence states. It has been found that they are effective in forming fine cracks, and these are both effects obtained by adding Mg, and that the hole expandability is improved by a synergistic effect.
[0028]
MgO and MgAl 2 O 4 mainly obtain the effect of forming fine voids by precipitating (Nb, Ti) N around, and MgO and MgAl 2 O 4 contribute as nuclei for uniform dispersion precipitation. it is conceivable that. On the other hand, fine composite oxides other than MgO and MgAl 2 O 4 are finely dispersed and precipitated by forming a composite oxide with MgO, and fine voids are formed by the oxide alone without depositing (Nb, Ti) N around. Has the effect of Particularly, most of the fine composite oxides other than MgO and MgAl 2 O 4 are composite oxides mainly composed of MgO, Al 2 O 3 , and SiO 2. At this time, MgO, Al 2 O 3 , SiO 2 ratio of 2 oxide is 90% or more.
[0029]
If the particle size of the oxide is less than 0.005 μm, the precipitation of (Nb, Ti) N with this as the nucleus is small. On the other hand, the oxide of this size has fine cracks without the composite precipitation of (Nb, Ti) N. Since it is difficult to become a nucleus for generating fine particles and it is difficult to obtain the effect of generating fine voids, the thickness is set to 0.005 μm or more. Conversely, if it exceeds 5.0 μm, it is difficult to secure the number of particles, and coarse precipitates cause deterioration of ductility, so the thickness is set to 5.0 μm or less.
[0030]
When the size of the oxide and the composite precipitate is small, the size of the oxide and the composite precipitate does not become a starting point of a fine void, so that the effect cannot be exhibited. Therefore, the thickness is set to 0.05 μm or more. On the other hand, if it is more than 5.0 μm, it is difficult to secure the number of particles.
[0031]
If the number of precipitates is small, it is considered that fine voids generated at the time of punching are insufficient, and the effect of suppressing generation of coarse cracks cannot be obtained. To obtain this effect, 1.0 × 10 3 or more per square mm is required. On the other hand becomes large number in the effect is saturated, in order to degrade the ductility Conversely, a 1.0 × 10 7 or less. However, from the balance between the saturation of this effect and ductility, the number is desirably 1.0 × 10 6 or less.
[0032]
In addition, as means for improving the hole expandability, it is effective to enhance the local rolling performance of the base material in addition to the properties of the punched holes. In order to enhance the local rolling performance of the base material, it is effective to homogenize the structure. However, in the case of a single-phase steel, the ductility is largely deteriorated at the target strength of the present invention, and the desired characteristics cannot be obtained. For this reason, the steel has a composite structure mainly composed of a ferrite structure. However, when the occupation ratio of the ferrite structure is high and a single-phase steel is formed, the ductility or strength is reduced. When the occupation ratio is low, the ductility is reduced due to the influence of the second phase having low elongation. Therefore, the occupation ratio of the ferrite structure is desirably 50% or more and 95% or less. When the remaining structure is martensite, coarse cementite, or pearlite, cracks occur at the interface between the ferrite structure and these structures, and the local deformability decreases. On the other hand, the bainite structure is a structure in which fine cementite is dispersed in the ferrite structure. In order not to reduce the local rolling performance of the base material, the steel structure is mainly composed of a ferrite structure and the remaining bainite structure.
[0033]
The dispersion state of the inclusions specified in the present invention is quantitatively measured, for example, by the following method. An extracted replica sample was prepared from an arbitrary location on the base steel sheet, and observed using a transmission electron microscope (TEM) at a magnification of 5000 to 20000 times over an area of at least 5000 μm 2 or more. The number of objects is measured and converted to the number per unit area. At this time, the oxide and (Nb, Ti) N are identified by a composition analysis by an energy dispersive X-ray spectroscopy (EDS) attached to the TEM and a crystal structure analysis of an electron diffraction image by the TEM. When it is complicated to perform such identification for all the composite inclusions to be measured, the procedure will be simply described next. First, the number of target sizes is measured according to the above-described procedure for each shape and size. Of these, all of the different shapes and sizes are identified in the above-described manner for at least 10 each, and oxidation is performed. The ratio between the substance and (Nb, Ti) N is calculated. Then, the number of inclusions measured first is multiplied by this ratio. When the carbide in the steel hinders the above TEM observation, the carbide can be coagulated and coarsened or dissolved by heat treatment to facilitate observation of the target composite inclusion.
[0034]
Next, a manufacturing method will be described.
The finish rolling end temperature must be equal to or higher than the Ar 3 transformation point in order to prevent the formation of ferrite and improve the hole expandability. However, if the temperature is too high, the strength is reduced due to the coarsening of the structure, and the ductility is reduced. The cooling rate needs to be 20 ° C./s or more in order to suppress the formation of carbides harmful to the hole expanding property and to obtain a high hole expanding ratio. If the winding temperature is lower than 350 ° C., hard martensite which is harmful to the hole expandability is generated. On the other hand, if the upper limit exceeds 600 ° C., pearlite and cementite, which are harmful to hole expandability, are generated, so the upper limit is set to 600 ° C. or less.
[0035]
Continuous cooling and hollow cooling are effective for increasing the occupancy of the ferrite phase and improving ductility. However, when pearlite is produced due to the air cooling temperature and the air cooling time, not only does the ductility decrease, but also the hole expandability decreases significantly. If the air cooling temperature is lower than 650 ° C., pearlite which is harmful to hole expandability is generated from an early stage. On the other hand, if the temperature exceeds 700 ° C., ferrite formation is slow and it is difficult to obtain the effect of air cooling, and further, pearlite is likely to be generated during subsequent cooling, so that the temperature is set to 700 ° C. or lower. Air cooling for more than 15 seconds not only saturates the increase in ferrite phase, but also places a load on the control of the subsequent cooling rate and winding temperature. For this reason, the air cooling time is set to 15 seconds or less.
[0036]
The inventors of the present invention have conducted intensive studies on the order of addition in the component adjustment step in the smelting process. As a result, after adding Si and Mn, adding Ti, and then adding Mg and Al, This is more preferable because the Mg yield to the oxide is increased and the oxide size is further refined, so that the dispersion state of the oxide size claimed in the present invention can be easily obtained in a stable manner.
[0037]
Mg has high volatility in molten steel, and the Mg yield is extremely low when it is introduced into molten steel with pure Mg metal. For this reason, Mg is introduced into molten steel in the form of an alloy with a diluent solvent metal. At this time, as a result of extensive studies by the present inventors, when a Mg alloy composed of one or more of Si, Ni, Cu, Al, and REM (rare earth element) is used as a diluting solvent metal for Mg, it is found that steel containing The amount of Mg remaining in the alloy increased, and no effect was obtained with alloys mainly composed of other metals. As a diluting solvent metal of Mg, Si, Ni, Cu, Al, REM (rare earth element) having interaction of attractive force with Mg are selected, and molten steel is prepared by using one or two or more of these Mg alloys. It is preferable to introduce Mg into the inside.
Here, the range of the rare earth element is, for example, Sc, Y and lanthanoids belonging to Group 3 of the Periodic Table (Lu from atomic number 57 to Lu of 71, as described in the Physical and Chemical Dictionary, 5th Edition, page 309, Iwanami Shoten, 1998). ).
[0038]
Further, as a result of intensive studies by the present inventors, when the Mg concentration in the Mg alloy is less than 10%, the Mg yield is remarkably increased, and more appropriate oxide size and number are easily obtained stably. I found that. On the other hand, if the content is less than 1%, the dilution solvent metal is excessively dissolved in the steel when the Mg alloy is added, so that it becomes difficult to adjust the components. Therefore, it is preferable that the Mg concentration in the alloy be 1% or more and less than 10%.
[0039]
When the sum of the concentrations of Fe, Mn, and Cr in the Mg alloy is less than 10%, it has been found that the Mg yield is remarkably increased, and that a more appropriate oxide size and number are more easily obtained stably, which is preferable. . This is interpreted as being due to the interatomic repulsion between Mg and these elements generated during melting of the Mg alloy in the molten steel. Therefore, the sum of the concentrations of Fe, Mn, and Cr in the Mg alloy is preferably less than 10%.
The effect of the present invention is not impaired even if the steel sheet of the present invention is plated by annealing such as hot dip galvanizing after hot rolling as described above. Also, the effect is not impaired when electroplating or organic composite coating is applied after hot rolling.
[0040]
【Example】
Next, the present invention will be described based on examples.
In order to smelt steel having the steel components shown in Table 1, 270 t of hot metal was decarburized in a converter to a target C concentration, then the molten steel was transferred to a ladle, and deoxidation and alloy adjustment were performed by CAS method (edited by the Iron and Steel Institute of Japan (By Hiroyuki Kajioka, Ladle Refining Method, p. 104, Jinjinshokan, published in 1997). Table 1 shows examples in which molten steel was deoxidized in the order of adding Si and Mn, then adding Ti, and then adding Mg and Al, and other examples. Here, FeSi, FeMn, and FeTi were used as Si, Mn, and Ti raw materials. Mg and Al use one or more of Si, Ni, Cu, Al and REM (rare earth element) as a diluting solvent metal, and the Mg concentration in the Mg alloy is 1% or more and less than 10%. Table 1 also shows an example using an Mg alloy in which the sum of the concentrations of Fe, Mn, and Cr in the alloy is less than 10% and an example using an alloy other than these. After the deoxidation, necessary elements were adjusted to the target component concentration range, and immediately, a slab having a thickness of 250 mm and a width of 1300 mm was produced by a continuous casting machine. These steels were heated in a heating furnace at 1200 ° C. or higher, rolled and cooled under the hot rolling conditions shown in Table 2, and hot rolled steel sheets having a sheet thickness of 2.6 to 3.2 mm were obtained.
[0041]
On the other hand, Table 3 shows that the addition of Mg uses one or more of Si, Ni, Cu, Al, and REM (rare earth element) as diluting solvent metals, and the Mg concentration in the Mg alloy is 1% or more and less than 10%. Yes, using a Mg alloy in which the sum of the concentrations of Fe, Mn, and Cr in the Mg alloy is less than 10%, and adding Si and Mn, then adding Ti, and then adding Mg and The figure shows the result of deoxidation in which Al is added, in which the components are changed. Symbols D to Y are steels according to the present invention, and the other amounts of C, Si, Mn, S, Al, Mg, Nb, and Ti are outside the scope of the present invention. These steels were heated in a heating furnace at 1200 ° C. or higher, rolled and cooled under the hot rolling conditions shown in Table 4, to obtain hot rolled steel sheets having a sheet thickness of 2.6 to 3.2 mm.
[0042]
In addition, an extracted replica sample was prepared from the base material of the steel sheet, and the particle size and the number of the oxide and the (Ti, Nb) N composite oxide existing in the nucleus were measured by the above-described method, and the number per unit area was measured. Was converted to This is shown in Tables 1 and 3.
[0043]
The thus obtained hot-rolled steel sheet was subjected to a tensile test using a JIS No. 5 piece, a hole expanding test, and a structure observation. The hole expandability (λ) is obtained by pushing a punched hole having a diameter of 12 mm with a 60 ° conical punch, and obtaining λ = (from the hole diameter (d) when the crack penetrates the plate thickness and the initial hole diameter (d0: 12 mm). d−d0) / d0 × 100.
[0044]
Tables 2 and 4 show TS, El, and λ of each test piece. FIG. 1 shows the relationship between strength and elongation, and FIG. 2 shows the relationship between strength and hole expansion ratio. It can be seen that the steel of the present invention has a higher hole expansion ratio than Comparative Steel 1 and both higher hole expansion ratio and elongation characteristics than
[0045]
In this case, the alloy was introduced by the CAS method, but this is not particularly limited, and a known method such as an alloy addition method in a vacuum tank of an RH degassing apparatus, a wire addition method in a molten steel ladle, and a powder injection method is used. Note that the method can be used without any problem.
[0046]
[Table 1]
[Table 2]
[Table 3]
[Table 4]
【The invention's effect】
In the accordance if intensity levels present invention is 590N / mm 2 or more classes, unprecedented growth - which was can be supplied hot-rolled high strength steel sheet having ductility balance, but on a very useful industrial.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between tensile strength and elongation.
FIG. 2 is a graph showing the relationship between tensile strength and hole expansion ratio.
Claims (10)
Ｃ ：０．０１％以上、０．２０％以下、
Ｓｉ：０．３％以上、１．５％以下、
Ｍｎ：０．５％以上、２．５％以下、
Ｐ ：０．１０％以下、
Ｓ ：０．００９％以下、
Ｎ ：０．０１０％以下、
Ｍｇ：０．０００５％以上、０．０１％以下、
Ａｌ：０．００２％以上、０．０７％以下、
および
Ｔｉ：０．００３％以上、０．２５％以下、
Ｎｂ：０．００３％以上、０．０４％以下
の１種または２種を含有し、残部が鉄および不可避的不純物からなり、
粒子径が０．００５μｍ〜５．０μｍの範囲にあるＭｇＯまたは、ＭｇＯを含むＡｌ ２ Ｏ ３ 、ＭｇＯを含むＳｉＯ ２ 、ＭｇＯを含むＭｎＯ、ＭｇＯを含むＴｉ ２ Ｏ ３ の１種もしくは２種以上の複合酸化物が１平方ｍｍあたり１．０×１０３個以上、１．０×１０７個以下含む、鋼組織をフェライト組織を主とし残ベイナイト組織とすることを特徴とする穴拡げ性と延性に優れた高強度熱延鋼板。C in weight%: 0.01% or more, 0.20% or less,
Si: 0.3% or more, 1.5% or less,
Mn: 0.5% or more, 2.5% or less,
P: 0.10% or less,
S: 0.009% or less,
N: 0.010% or less,
Mg: 0.0005% or more, 0.01% or less,
Al: 0.002% or more, 0.07% or less,
And Ti: 0.003% or more, 0.25% or less,
Nb: contains one or more of 0.003% or more and 0.04% or less, and the balance consists of iron and inevitable impurities;
One or more of MgO having a particle diameter in the range of 0.005 μm to 5.0 μm, Al 2 O 3 containing MgO, SiO 2 containing MgO, MnO containing MgO, and Ti 2 O 3 containing MgO. A hole diameter of 1.0 × 10 3 or more and 1.0 × 10 7 or less per square mm, wherein the steel structure is mainly a ferrite structure and a residual bainite structure. High strength hot rolled steel sheet with excellent ductility.
Ｃ ：０．０１％以上、０．２０％以下、
Ｓｉ：０．３％以上、１．５％以下、
Ｍｎ：０．５％以上、２．５％以下、
Ｐ ：０．１０％以下、
Ｓ ：０．００９％以下、
Ｎ ：０．０１０％以下、
Ｍｇ：０．０００５％以上、０．０１％以下、
Ａｌ：０．００２％以上、０．０７％以下、
および
Ｔｉ：０．００３％以上、０．２５％以下、
Ｎｂ：０．００３％以上、０．０４％以下
の１種または２種を含有し、残部が鉄および不可避的不純物からなり、
粒子径が０．００５μｍ〜５．０μｍの範囲にあるＭｇＯまたは、ＭｇＯを含むＡｌ ２ Ｏ ３ 、ＭｇＯを含むＳｉＯ ２ 、ＭｇＯを含むＭｎＯ、ＭｇＯを含むＴｉ ２ Ｏ ３ の１種もしくは２種以上の複合酸化物と、これを核にして、その周辺に（Ｎｂ、Ｔｉ）Ｎを有する複合析出物のうち、そのサイズが０．０５μｍ〜５．０μｍの範囲の析出物が１平方ｍｍあたり１．０×１０３個以上、１．０×１０７個以下含む、鋼組織をフェライト組織を主とし残ベイナイト組織とすることを特徴とする穴拡げ性と延性に優れた高強度熱延鋼板。C in weight%: 0.01% or more, 0.20% or less,
Si: 0.3% or more, 1.5% or less,
Mn: 0.5% or more, 2.5% or less,
P: 0.10% or less,
S: 0.009% or less,
N: 0.010% or less,
Mg: 0.0005% or more, 0.01% or less,
Al: 0.002% or more, 0.07% or less,
And Ti: 0.003% or more, 0.25% or less,
Nb: contains one or more of 0.003% or more and 0.04% or less, and the balance consists of iron and inevitable impurities;
One or more of MgO having a particle diameter in the range of 0.005 μm to 5.0 μm, Al 2 O 3 containing MgO, SiO 2 containing MgO, MnO containing MgO, and Ti 2 O 3 containing MgO. Of composite oxides having a core of (Nb, Ti) N and having a size in the range of 0.05 μm to 5.0 μm per square mm, A high-strength hot-rolled steel sheet excellent in hole expandability and ductility, characterized in that the steel structure is mainly a ferrite structure and has a residual bainite structure, containing not less than 0.0 × 10 3 and not more than 1.0 × 10 7 .
Ｃ ：０．０１％以上、０．２０％以下、
Ｓｉ：０．３％以上、１．５％以下、
Ｍｎ：０．５％以上、２．５％以下、
Ｐ ：０．１０％以下、
Ｓ ：０．００９％以下、
Ｎ ：０．０１０％以下、
Ｍｇ：０．０００５％以上、０．０１％以下、
Ａｌ：０．００２％以上、０．０７％以下、
および
Ｔｉ：０．００３％以上、０．２５％以下、
Ｎｂ：０．００３％以上、０．０４％以下
の１種または２種含有し、残部が鉄および不可避的不純物からなり、さらに、
Ｃａ：０．０００５％以上、０．０１００％以下、
ＲＥＭ元素の合計：０．０００５％以上、０．０１００％以下
の１種または２種含有し、残部が鉄および不可避的不純物からなり、粒子径が０．００５μｍ〜５．０μｍの範囲にあるＭｇＯまたは、ＭｇＯを含むＡｌ ２ Ｏ ３ 、ＭｇＯを含むＳｉＯ ２ 、ＭｇＯを含むＭｎＯ、ＭｇＯを含むＴｉ ２ Ｏ ３ の１種もしくは２種以上の複合酸化物が１平方ｍｍあたり１．０×１０３個以上、１．０×１０７個以下含む、鋼組織をフェライト組織を主とし残ベイナイト組織とすることを特徴とする穴拡げ性と延性に優れた高強度熱延鋼板。C in weight%: 0.01% or more, 0.20% or less,
Si: 0.3% or more, 1.5% or less,
Mn: 0.5% or more, 2.5% or less,
P: 0.10% or less,
S: 0.009% or less,
N: 0.010% or less,
Mg: 0.0005% or more, 0.01% or less,
Al: 0.002% or more, 0.07% or less,
And Ti: 0.003% or more, 0.25% or less,
Nb: One or two or more elements of 0.003% or more and 0.04% or less, with the balance being iron and unavoidable impurities.
Ca: 0.0005% or more, 0.0100% or less,
Total of REM elements: MgO containing 0.0005% or more and 0.0100% or less, the balance being iron and unavoidable impurities, and having a particle size in the range of 0.005 μm to 5.0 μm. Alternatively, one or more composite oxides of Al 2 O 3 containing MgO, SiO 2 containing MgO, MnO containing MgO, and Ti 2 O 3 containing MgO are 1.0 × 10 3 per square mm. A high-strength hot-rolled steel sheet excellent in hole expandability and ductility, characterized in that the steel structure is mainly a ferrite structure and has a residual bainite structure, including not less than 1.0 × 10 7 and not more than 1.0 × 10 7 pieces.
Ｃ ：０．０１％以上、０．２０％以下、
Ｓｉ：０．３％以上、１．５％以下、
Ｍｎ：０．５％以上、２．５％以下、
Ｐ ：０．１０％以下、
Ｓ ：０．００９％以下、
Ｎ ：０．０１０％以下、
Ｍｇ：０．０００５％以上、０．０１％以下、
Ａｌ：０．００２％以上、０．０７％以下、
および
Ｔｉ：０．００３％以上、０．２５％以下、
Ｎｂ：０．００３％以上、０．０４％以下
の１種または２種含有し、残部が鉄および不可避的不純物からなり、さらに、
Ｃａ：０．０００５％以上、０．０１００％以下、
ＲＥＭ元素の合計：０．０００５％以上、０．０１００％以下
の１種または２種含有し、残部が鉄および不可避的不純物からなり、粒子径が０．００５μｍ〜５．０μｍのＭｇＯまたは、ＭｇＯを含むＡｌ ２ Ｏ ３ 、ＭｇＯを含むＳｉＯ ２ 、ＭｇＯを含むＭｎＯ、ＭｇＯを含むＴｉ ２ Ｏ ３ の１種もしくは２種以上の複合酸化物と、これを核にして、その周辺に（Ｎｂ、Ｔｉ）Ｎを有する複合析出物のうち、そのサイズが０．０５μｍ〜５．０μｍの範囲の析出物が１平方ｍｍあたり１．０×１０３個以上、１．０×１０７個以下含む、鋼組織をフェライト組織を主とし残ベイナイト組織とすることを特徴とする穴拡げ性と延性に優れた高強度熱延鋼板。C in weight%: 0.01% or more, 0.20% or less,
Si: 0.3% or more, 1.5% or less,
Mn: 0.5% or more, 2.5% or less,
P: 0.10% or less,
S: 0.009% or less,
N: 0.010% or less,
Mg: 0.0005% or more, 0.01% or less,
Al: 0.002% or more, 0.07% or less,
And Ti: 0.003% or more, 0.25% or less,
Nb: One or two or more elements of 0.003% or more and 0.04% or less, with the balance being iron and unavoidable impurities.
Ca: 0.0005% or more, 0.0100% or less,
Total of REM elements: One or two of 0.0005% or more and 0.0100% or less, the balance being iron and unavoidable impurities, and MgO or MgO having a particle size of 0.005 μm to 5.0 μm. And / or two or more complex oxides of Al 2 O 3 containing Mg, SiO 2 containing MgO, MnO containing MgO, and Ti 2 O 3 containing MgO, with (Nb, Ti) Among the composite precipitates having N, the precipitates having a size in the range of 0.05 μm to 5.0 μm include 1.0 × 10 3 or more and 1.0 × 10 7 or less per 1 mm 2, A high-strength hot-rolled steel sheet excellent in hole expandability and ductility, characterized by having a steel structure mainly of a ferrite structure and a residual bainite structure.
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| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP2000357752A JP3545696B2 (en) | 2000-03-30 | 2000-11-24 | High strength hot rolled steel sheet excellent in hole expandability and ductility and method for producing the same |
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|---|---|---|---|
| JP2000093590 | 2000-03-30 | ||
| JP2000-93590 | 2000-03-30 | ||
| JP2000357752A JP3545696B2 (en) | 2000-03-30 | 2000-11-24 | High strength hot rolled steel sheet excellent in hole expandability and ductility and method for producing the same |
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|---|---|
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| JP3545696B2 true JP3545696B2 (en) | 2004-07-21 |
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