JP2004211197A - Steel sheet having excellent hardenability after hot forming and impact property, and its using method - Google Patents
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【０００１】
【発明の属する技術分野】
本発明は、自動車部品の構造部材に使用されるような強度が必要とされる部材に関し、特に熱間成形後の硬化能および衝撃特性に優れた鋼板およびその使用方法に関するものである。
【０００２】
【従来の技術】
地球環境問題に端を発する自動車の燃費向上対策の一つとして車体の軽量化が進められており、自動車に使用される鋼板をできるだけ高強度化することが必要となる。しかし、自動車の軽量化のために一般に鋼板を高強度化していくと伸びやｒ値が低下し、成形性および形状凍結性が劣化していく。
このような課題を解決するために、温間で成形し、その際の熱を利用して強度上昇を図る技術が、特開２０００−２３４１５３号公報（特許文献１）に開示されている。この技術では、鋼中成分を適切に制御し、２００〜８５０℃の温度域で保持・成形加工し、この温度域での析出強化を利用して強度を上昇させることを狙っている。
【０００３】
また、特開２０００−８７１８３号公報（特許文献２）では、プレス成形精度を向上させる目的で温間プレス時での降伏強度を低く、常温での降伏強度を高くする高強度鋼板が提案されている。しかしながら、これらの技術では得られる強度に限度がある可能性がある。
一方、より高強度を得る目的で、成形後に高温のオーステナイト単相域に加熱し、その後の冷却過程で硬質の相に変態させる技術が特開２０００−３８６４０号公報（特許文献３）に提案されている。さらに、成形性および焼入れ性に優れた薄鋼板の製造方法が特開２０００−３３９０２５号公報（特許文献４）に提案されている。
【０００４】
【引用文献】
（１）特許文献１（特開２０００−２３４１５３号公報）
（２）特許文献２（特開２０００−８７１８３号公報）
（３）特許文献３（特開２０００−３８６４０号公報）
（４）特許文献４（特開２０００−３３９０２５号公報）
【０００５】
【発明が解決しようとする課題】
このように、これまでに開示されている技術を用い、熱間成形直後に高強度となる熱間プレスに適した鋼板を製造することは困難である。本発明は、上記課題を解決するためになされたものであり、熱間成形後にＨｖ４２０以上の高い硬度を得ることができる熱間成形後硬化能および衝撃特性に優れた鋼板を提供することを目的とする。
【０００６】
【課題を解決するための手段】
本発明は、
（１）質量％で、Ｃ：０．２０〜０．３５％、Ｓｉ：０．０１〜１．０％、Ｍｎ：０．３〜２．０％、Ａｌ：０．０１〜０．１０％、Ｎｉ：１％超〜５％、Ｔｉ：０．００５〜０．１％、Ｂ：０．０００５〜０．００５％、Ｎ：０．００１〜０．０１０％、Ｐ：０．０２％以下、Ｓ：０．０１％以下、Ｏ：０．０１％以下、残部がＦｅおよび不可避的不純物および／又は付随的成分よりなり、（１）式、（２）式、及び（３）式を満足することを特徴とする熱間成形加工後の硬化能および衝撃特性に優れた鋼板。
０≦Ｔｉ／４７．８８−Ｎ／１４．０１≦０．００１ ・・・（１）式
０≦Ａｌ／２６．９８−Ｎ／１４．０１≦０．００１５・・・（２）式
−２８．８×％Ｃ２ ＋３８．０×％Ｃ＋１．７×％Ｓｉ＋７．５×％Ｍｎ＋１．４×％Ｎｉ＋４３．２×％Ｃｒ＋８．７×％Ｃｕ−５３．５×％Ｓ−５７．１×％Ｐ＋１７．９≧３５・・・（３）式
【０００７】
（２）付随的成分として質量％で、Ｃｒ：０．０１〜１％、Ｍｏ：０．００５〜１％の１種あるいは２種を含有することを特徴とする前記（１）に記載の熱間成形加工後の硬化能および衝撃特性に優れた鋼板。
（３）付随的成分として質量％で、Ｎｂ：０．００５〜０．５％、Ｖ：０．０１〜０．５％の１種あるいは２種を含有することを特徴とする前記（１）または前記（２）に記載の熱間成形加工後の硬化能および衝撃特性に優れた鋼板。
（４）付随的成分として質量％で、Ｃｕ：０．０１〜１％を含有することを特徴とする前記（１）から前記（３）に記載の熱間成形加工後の硬化能および衝撃特性に優れた鋼板。
【０００８】
（５）質量％で、Ｓｉ：０．１５％未満に制限することを特徴とする前記（１）から前記（４）に記載の熱間成形加工後の硬化能および衝撃特性に優れた鋼板。
（６）前記（１）から前記（５）に記載の鋼板をＡｃ３ 変態点以上のオーステナイト領域に加熱後、Ａｒ３ 変態点以上の温度で成形加工を開始し、加工と同時に金型で抜熱することにより急速冷却し、マルテンサイト変態させて硬化させることを特徴とする熱間成形加工後の硬化能および衝撃特性に優れた鋼板の使用方法である。
【０００９】
【発明の実施の形態】
本発明では、特定の化学組成を有する熱延素材あるいは冷延素材を用いる。冷延素材にはめっき処理が施されていてもよい。また、熱間成形の方法としては、Ａｃ３ 変態点以上のオーステナイト領域に加熱後、Ａｒ３ 変態点以上の温度で成形加工（例えばプレス加工）を開始し、加工と同時に金型で抜熱することにより急速冷却し、マルテンサイト変態させて硬化させる方法である。
【００１０】
次に、鋼板の化学成分について説明する。
Ｃは、基地中に固溶あるいは炭化物として析出し、鋼の強度を増加させる元素であり、また、セメンタイト、パーライト、ベイナイト、マルテンサイト等の硬質な第２相として析出し、高強度化と一様伸びの向上に寄与する。強度向上のために０．２０％以上のＣが必要であるが、Ｃ含有量が０．３５％を超えると、焼入れまま状態での衝撃特性が低下するため、Ｃは０．２０〜０．３５％の範囲に規定した。尚、強度と衝撃特性のバランスを考慮すると、好ましい範囲は０．２０〜０．３０％である。
【００１１】
Ｓｉは、固溶強化型の合金元素であり、強度を確保するために０．０１％のＳｉが必要であるが、１．０％を超えると、表面スケールの問題が生じる。このため、Ｓｉは０．０１〜１．０％の範囲に規定した。また、鋼板表面にメッキ処理を行う場合は、Ｓｉの添加量が多いとメッキ性が劣化するため、上限を０．５％とすることが好ましい。また、Ｓｉの含有量が多いと、衝撃特性や延性が低下するため、Ｓｉの添加量は０．５％以下とすることが望ましい。なお、Ｓｉ含有量を０．１５％未満に低減するとシャルピー吸収エネルギーが向上し、同時に延性脆性遷移温度も低温化させることが期待できるため、衝撃特性は向上する。このため、衝撃特性を重要視する場合のＳｉ含有量のより好ましい範囲は０．０１〜０．１５％未満である。
【００１２】
Ｍｎは、強度および焼入れ性を向上させる元素であり、０．３％未満では焼入れ時の強度を十分に得られず、また、２．０％を超えて添加しても効果が飽和するとともに衝撃特性が劣化するため、Ｍｎは０．３〜２．０％の範囲に規定した。なお、強度と衝撃特性のバランスを考慮すると、好ましい範囲は０．５〜１．５％である。
【００１３】
Ａｌは、溶鋼の脱酸材として使われる必要な元素であり、またＮを固定する元素でもあり、その量は結晶粒径や機械的性質に大きな影響を及ぼす。このような効果を有するためには０．０１％以上の含有量が必要であるが、０．１％を超えると非金属介在物が多くなり製品に表面疵が発生しやすくなる。このため、Ａｌは０．０１〜０．１％の範囲に規定した。
Ｂ添加の効果を阻止するＮによる阻害を抑制するためには、式０≦Ａｌ／２６．９８−Ｎ／１４．０１を満足するＡｌを含有させる必要がある。しかし、式Ａｌ／２６．９８−Ｎ／１４．０１が０．００１５を超えるＡｌを含有すると粗大なＡｌＮが生成しやすくなるため、ＡｌとＮの関係を式０≦Ａｌ／２６．９８−Ｎ／１４．０１≦０．００１５の範囲に規定した。
【００１４】
Ｎｉは、強度および衝撃特性を向上させる元素であり、特に遷移温度を低下させる効果を有するが、５％を超えて添加すると、降伏強度の上昇が過度に大きくなるとともにコストも上昇する。１％未満では衝撃特性の大きな向上効果が発揮されないため、Ｎｉは１％超〜５％の範囲に規定した。
Ｔｉは、Ｂ添加による焼入れ性を安定かつ効果的に向上させるために作用するが、０．００５％未満およびＴｉ／４７．８８−Ｎ／１４．０１≧０式を満足しない範囲では効果が期待できず、０．１％超およびＴｉ／４７．８８−Ｎ／１４．０１≦０．００１ではＴｉの窒化物が多く生成して、衝撃特性が劣化する傾向があるため、Ｔｉは０．００５〜０．１％の範囲および式０≦Ｔｉ／４７．８８−Ｎ／１４．０１≦０．００１の範囲に規定した。
【００１５】
Ｂは、微量添加で鋼材の焼入れ性を大幅に向上させる元素であり、また、粒界強化およびＭ２３（Ｃ，Ｂ）６ などとして析出強化の効果もある。添加量が０．０００５％未満では焼入れ性に効果が期待できず、また、０．００５％を超えると粗大なＢ含有相を生成する傾向があり、また、脆化が起こりやすくなる。このため、Ｂは０．０００５〜０．００５％の範囲に規定した。
Ｎは、窒化物または炭窒化物を析出させ、強度を高める重要な元素の一つである。０．００１％以上の添加により効果を発揮するが、０．０１％を超えると窒化物の粗大化および固溶Ｎによる時効硬化により、衝撃特性が劣化する傾向がみられる。このため、Ｎは０．００１〜０．０１％の範囲に規定した。
【００１６】
Ｐは、溶接割れ性および衝撃特性に悪影響を及ぼす元素であるため、Ｐは０．０２％以下に規制した。なお、好ましくは、０．０１５％以下である。
Ｓは、鋼中の非金属介在物に影響し、加工性を劣化させるとともに、衝撃特性劣化、異方性および再熱割れ感受性の増大の原因となる。このため、Ｓは０．０１％以下に規定した。なお、好ましくは、０．００５％以下である。
Ｏは、衝撃特性に悪影響を及ぼす酸化物の生成の原因となるとともに、疲労破壊の起点となる酸化物を生成するため、上限を０．０１％に規定した。
【００１７】
Ｃｒは、焼入れ性を向上させる元素であり、またマトリックス中へＭ２３Ｃ６ 型炭化物を析出させる効果を有し、強度を高めるとともに、炭化物を微細化する作用を有する。０．０１％未満ではこれらの効果が十分期待できず、また、１％を超えると降伏強度が過度に上昇するため、Ｃｒは０．０１〜１％の範囲に規定した。
Ｍｏは、焼入れ性を向上させる元素であり、また固溶強化をもたらす元素であるとともに、マトリックス中のＭ２３Ｃ６ 型炭化物を安定化させる元素である。０．００５％未満ではこの効果が十分期待できず、１％を超えると降伏強度が過度に上昇し、また衝撃特性を劣化させるため、Ｍｏは０．００５〜１％の範囲に規定した。
【００１８】
Ｎｂは、炭窒化物を形成し、強度を向上させる元素であるが、０．５％を超えて添加すると降伏強度の上昇が過度に大きくなる。０．００５％未満では強度向上の効果が発揮されないため、Ｎｂは０．００５〜０．５％の範囲に規定した。
Ｖは、炭窒化物を形成し、強度を向上させる元素であるが、０．５％を超えて添加すると、降伏強度の上昇が過度に大きくなる。０．０１％未満では強度向上の効果が発揮されないため、Ｖは０．０１〜０．５％の範囲に規定した。
【００１９】
Ｃｕは、強度を向上させる元素であるが、１％を超えて添加すると、降伏強度の上昇が過度に大きくなる。０．０１％未満では強度向上の効果が発揮されないため、Ｃｕは０．０１〜１％の範囲に規定した。
下式にしたがう値は、熱間成形後の硬さに影響し、その値が３５未満では必要硬さが得られないため、その下限を３５に規定した。
−２８．８×％Ｃ２ ＋３８．０×％Ｃ＋１．７×％Ｓｉ＋７．５×％Ｍｎ＋ １．４×％Ｎｉ＋４３．２×％Ｃｒ＋８．７×％Ｃｕ−５３．５×％Ｓ−５ ７．１×％Ｐ＋１７．９≧３５
【００２０】
次に、熱間成形方法について説明する。
マルテンサイト変態させるためには、加熱時にオーステナイト組織になっている必要があるため、熱間成形前の加熱温度をオーステナイト領域となるＡｃ３ 変態点以上とした。熱間成形前の加熱方法は、炉加熱、誘導加熱、通電加熱等のいずれの加熱方法でもよいが、成形対象部分がほぼ均一な温度となっていることが望ましい。また、熱間成形開始温度がＡｒ３ 変態点よりも低くなると、フェライト相が出現するため、マルテンサイト変態後の強度が低下する。このため、熱間成形開始温度をＡｒ３ 変態点以上に規定した。成形方法は通常のプレス方法と同等で良いが、マルテンサイト変態させるために、加熱した材料を金型で抜熱し急速冷却するために、水冷式金型を用いてもよい。なお、金型の温度は１００℃以下が望ましいが、２００℃程度でも急速冷却は可能である。
【００２１】
【実施例】
表１の組成をもつ各種鋼スラブに鋳造した。これらのスラブを１２００℃に加熱し、熱間圧延にて仕上温度８５０℃、巻取温度６００℃で板厚４ｍｍの熱延鋼板とした。また、一部の熱延鋼板を冷間圧延により板厚１．２ｍｍの冷延鋼板とした。炉加熱によりＡｃ３ 点以上である９５０℃のオーステナイト領域に加熱した後、Ａｃ３ 点以上である９００℃から水冷式金型を有するプレス機にてフォーム成形加工を行った。成形時間を約１秒とし、成形完了１０秒間はプレス金型をそのままの状態にして金型による冷却を行った。また、１０秒後の鋼板温度を測定した。成形された鋼板について、冷延鋼板の圧延方向に垂直な断面をビッカース硬度計にて硬度測定を実施し、更に光学顕微鏡にて金属組織を観察し、マルテンサイト率を測定した。また更に、板厚４ｍｍの熱延鋼板を炉加熱によりＡｃ３ 点以上である９５０℃のオーステナイト領域に加熱した後、９００℃から水冷した素材を用いて衝撃試験を実施した。その条件および結果を表２に示す。
【００２２】
【表１】
【表２】
表２に示した本発明例（Ｎｏ．１〜Ｎｏ．１３）は、マルテンサイト率を９０％以上とすることで熱間成形後の硬さがＨｖ４２０以上、−６０℃の脆性破面率が５０％以下であり、形状凍結性も良く、自動車の構造部材として必要な特性を満足している。それに比較し、本発明の範囲を外れた比較例では、焼入れ硬さ、脆性破面率および形状凍結性が劣化している。
比較例（Ｎｏ．１４、Ｎｏ．２１、Ｎｏ．２６、Ｎｏ．２７、Ｎｏ．２８、Ｎｏ．３１）は、式０≦Ｔｉ／４７．８８−Ｎ／１４．０１≦０．００１を満足していないため、焼入れ性が不足し、焼入れ硬さを満足していない例。
【００２５】
比較例（Ｎｏ．２２）は、式０≦Ａｌ／２６．９８−Ｎ／１４．０１を満足していないため、焼入れ性が不足し、焼入れ硬さを満足していない例。比較例（Ｎｏ．２３、Ｎｏ．３２）は、式Ａｌ／２６．９８−Ｎ／１４．０１≦０．００１５を満足していないため、粗大なＡｌＮが生成し、衝撃特性が劣化した例。
比較例（Ｎｏ．１４、Ｎｏ．１６、Ｎｏ．１８、Ｎｏ．２０、Ｎｏ．２１、Ｎｏ．２２、Ｎｏ．２４、Ｎｏ．２６、Ｎｏ．２７、Ｎｏ．２８、Ｎｏ．３１、Ｎｏ．３２）は、式−２８．８×％Ｃ２ ＋３８．０×％Ｃ＋１．７×％Ｓｉ＋７．５×％Ｍｎ＋１．４×％Ｎｉ＋４３．２×％Ｃｒ＋８．７×％Ｃｕ−５３．５×％Ｓ−５７．１×％Ｐ＋１７．９≧３５を満足していないため、焼入れ性が不足し、焼入れ硬さを満足していない例。
【００２６】
比較例Ｎｏ．１４は、Ｃ量が規定値に達していないために焼入れ硬さが不足した例。比較例Ｎｏ．１５は、Ｃ量が規定値を超えているために衝撃特性が低下した例。比較例Ｎｏ．１６は、Ｓｉ量が規定値に達していないために焼入れ硬さが不足した例。比較例Ｎｏ．１７はＳｉ量が規定値を超えているために、衝撃値が低下した例。比較例Ｎｏ．１８は、Ｍｎ量が規定値に達していないために焼入れ性が低下し焼入れ硬さが不足した例。比較例Ｎｏ．１９はＭｎ量が規定値を超えているために、衝撃値が低下した例。
【００２７】
比較例Ｎｏ．２０はＰ量が、比較例Ｎｏ．２１はＳ量が、それぞれ規定値を超えているために、衝撃値が劣化した例。
比較例Ｎｏ．２２は、Ａｌ量が規定値に達していないために焼入れ硬さが不足した例。比較例Ｎｏ．２３は、Ａｌ量が規定値を超えているために多くの非金属介在物が生成し、また粗大なＡｌＮが生成したために衝撃特性が低下した例。
【００２８】
比較例Ｎｏ．２４は、Ｎｉ量が規定値に達していないために焼入れ硬さが不足した例。比較例Ｎｏ．２５は、Ｎｉ量が規定値を超えているために降伏強度が高くなり形状凍結性が劣化した例。
比較例Ｎｏ．２６は、Ｔｉ量が規定値に達していないために焼入れ硬さが不足した例。比較例Ｎｏ．２７は、Ｔｉ量が規定値を超えているためにＴｉの窒化物が多く生成して、衝撃特性が劣化した例。
【００２９】
比較例Ｎｏ．２８は、Ｂ量が規定値に達していないために焼入れ硬さが不足した例。比較例Ｎｏ．２９は、Ｂ量が規定値を超えているために粗大なＢ含有相を生成し、衝撃特性が劣化した例。
比較例Ｎｏ．３０は、Ｎ量が規定値に達していないために焼入れ硬さが不足した例。比較例Ｎｏ．３１は、Ｎ量が規定値を超えているために粗大な窒化物の生成および固溶Ｎによる時効硬化により、衝撃特性が劣化した例。
【００３０】
比較例Ｎｏ．３２は、Ｏ量が規定値を超えているために酸化物が多く生成し、衝撃特性が劣化した例。
比較例Ｎｏ．３３は、加熱温度がＡｃ３ 変態点を超えていないために、十分な焼入れが行えず焼入れ硬さが不足し、衝撃特性も低下した例。
比較例Ｎｏ．３４は、成形開始温度はＡｒ３ 変態点を下回ったために、フェライト変態が生じ十分な焼入れが行えず焼入れ硬さが不足し、衝撃特性も低下した例である。
【００３１】
【発明の効果】
以上述べたように、本発明鋼は、自動車部品の構造部材に使用され、熱間成形後の硬化能が高く高強度となる鋼板であり、また衝撃特性および形状凍結性にも優れており、加工工程の省略化に貢献することが可能である。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a member requiring strength as used for a structural member of an automobile part, and more particularly to a steel sheet having excellent hardening ability and impact characteristics after hot forming and a method of using the same.
[0002]
[Prior art]
2. Description of the Related Art As one of measures for improving fuel efficiency of automobiles originating from global environmental problems, weight reduction of vehicle bodies is being promoted, and it is necessary to increase the strength of steel sheets used in automobiles as much as possible. However, in general, when the strength of a steel sheet is increased to reduce the weight of an automobile, elongation and r-value decrease, and formability and shape freezing property deteriorate.
To solve such a problem, Japanese Patent Application Laid-Open No. 2000-234153 (Patent Literature 1) discloses a technique in which molding is performed in a warm state and the strength is increased by using heat at that time. This technique aims at appropriately controlling the components in steel, holding and forming in a temperature range of 200 to 850 ° C., and increasing the strength by utilizing precipitation strengthening in this temperature range.
[0003]
Also, Japanese Patent Application Laid-Open No. 2000-87183 (Patent Document 2) proposes a high-strength steel sheet that lowers the yield strength at the time of warm pressing and increases the yield strength at normal temperature for the purpose of improving the press forming accuracy. I have. However, these techniques may have limitations on the strength obtained.
On the other hand, for the purpose of obtaining higher strength, a technique is proposed in Japanese Patent Application Laid-Open No. 2000-38640 (Patent Document 3) in which after forming, a high-temperature austenitic single-phase region is heated and then transformed into a hard phase in the subsequent cooling process. ing. Furthermore, a method for producing a thin steel sheet having excellent formability and hardenability has been proposed in Japanese Patent Application Laid-Open No. 2000-339025 (Patent Document 4).
[0004]
[References]
(1) Patent Document 1 (JP-A-2000-234153)
(2) Patent Document 2 (JP-A-2000-87183)
(3) Patent Document 3 (JP-A-2000-38640)
(4) Patent Document 4 (Japanese Patent Application Laid-Open No. 2000-339025)
[0005]
[Problems to be solved by the invention]
As described above, it is difficult to manufacture a steel sheet suitable for hot pressing, which has high strength immediately after hot forming, using the techniques disclosed so far. The present invention has been made in order to solve the above problems, and an object of the present invention is to provide a steel sheet having excellent post-hot-forming hardening ability and impact characteristics capable of obtaining a high hardness of Hv 420 or more after hot-forming. And
[0006]
[Means for Solving the Problems]
The present invention
(1) In mass%, C: 0.20 to 0.35%, Si: 0.01 to 1.0%, Mn: 0.3 to 2.0%, Al: 0.01 to 0.10% , Ni: more than 1% to 5%, Ti: 0.005 to 0.1%, B: 0.0005 to 0.005%, N: 0.001 to 0.010%, P: 0.02% or less , S: 0.01% or less, O: 0.01% or less, with the balance being Fe and inevitable impurities and / or incidental components, satisfying the equations (1), (2) and (3) A steel sheet having excellent hardening ability and impact properties after hot forming.
0 ≦ Ti / 47.88-N / 14.01 ≦ 0.001 Expression (1) 0 ≦ Al / 26.98-N / 14.01 ≦ 0.0015 Expression (2) 0.8 ×% C 2 + 38.0 ×% C + 1.7 ×% Si + 7.5 ×% Mn + 1.4 ×% Ni + 43.2 ×% Cr + 8.7 ×% Cu-53.5 ×% S-57.1 ×% P + 17.9 ≧ 35 Expression (3)
(2) The heat as described in (1) above, wherein one or two types of Cr: 0.01 to 1% and Mo: 0.005 to 1% are contained as mass components in mass%. Steel sheet with excellent hardening ability and impact properties after hot working.
(3) The above-mentioned (1), wherein one or two of Nb: 0.005 to 0.5% and V: 0.01 to 0.5% by mass% are contained as accessory components. Alternatively, a steel sheet having excellent hardening ability and impact properties after the hot forming described in (2).
(4) Curing ability and impact properties after hot forming according to (1) to (3), wherein Cu: 0.01 to 1% by mass as an ancillary component is contained. Excellent steel plate.
[0008]
(5) The steel sheet having excellent hardening ability and impact properties after hot forming according to (1) to (4), wherein the steel is limited to less than 0.15% by mass of Si in mass%.
(6) After the steel sheet according to (1) to (5) is heated to an austenite region at or above the Ac 3 transformation point, the forming process is started at a temperature at or above the Ar 3 transformation point, and the mold is simultaneously extracted with a die. This is a method for using a steel sheet having excellent hardening ability and impact properties after hot forming, characterized in that it is rapidly cooled by heating, transformed into martensite and hardened.
[0009]
BEST MODE FOR CARRYING OUT THE INVENTION
In the present invention, a hot rolled material or a cold rolled material having a specific chemical composition is used. The cold-rolled material may be plated. Further, as a method of hot forming, after heating to an austenite region at or above the Ac 3 transformation point, forming (for example, pressing) is started at a temperature at or above the Ar 3 transformation point, and heat is removed by a mold at the same time as the working. This is a method of rapidly cooling, transforming into martensite and curing.
[0010]
Next, the chemical composition of the steel sheet will be described.
C is an element that precipitates as a solid solution or carbide in the matrix and increases the strength of steel, and also precipitates as a hard second phase such as cementite, pearlite, bainite, martensite, etc. It contributes to the improvement of elongation. To improve the strength, 0.20% or more of C is required. However, if the C content exceeds 0.35%, the impact characteristics in the as-quenched state deteriorate, so that C is 0.20 to 0.1%. It was specified in the range of 35%. In consideration of a balance between strength and impact characteristics, a preferable range is 0.20 to 0.30%.
[0011]
Si is a solid solution hardening type alloy element, and 0.01% of Si is required to secure strength. However, if it exceeds 1.0%, a problem of surface scale occurs. For this reason, Si was specified in the range of 0.01 to 1.0%. Further, when plating is performed on the surface of the steel sheet, if the amount of Si added is large, the plating property is deteriorated. Therefore, the upper limit is preferably set to 0.5%. Further, when the content of Si is large, impact characteristics and ductility are reduced. Therefore, the addition amount of Si is desirably 0.5% or less. When the Si content is reduced to less than 0.15%, the Charpy absorbed energy is improved, and at the same time, the ductile brittle transition temperature can be expected to be lowered, so that the impact characteristics are improved. For this reason, a more preferable range of the Si content when the impact characteristics are regarded as important is 0.01 to less than 0.15%.
[0012]
Mn is an element that improves strength and hardenability. If it is less than 0.3%, sufficient strength at the time of hardening cannot be obtained. Since the characteristics deteriorate, Mn is specified in the range of 0.3 to 2.0%. In consideration of the balance between strength and impact characteristics, a preferable range is 0.5 to 1.5%.
[0013]
Al is a necessary element used as a deoxidizing material for molten steel, and is also an element fixing N, and its amount has a great effect on the crystal grain size and mechanical properties. To have such an effect, the content is required to be 0.01% or more. However, if the content exceeds 0.1%, nonmetallic inclusions increase and a surface flaw is easily generated in a product. For this reason, Al was specified in the range of 0.01 to 0.1%.
In order to suppress the inhibition by N which inhibits the effect of the addition of B, it is necessary to contain Al satisfying the expression 0 ≦ Al / 26.98-N / 14.01. However, if the formula Al / 26.98-N / 14.01 contains more than 0.0015 of Al, coarse AlN is likely to be generated, so the relationship between Al and N is determined by the formula 0 ≦ Al / 26.98-N. /14.01≦0.0015.
[0014]
Ni is an element that improves the strength and impact characteristics, and particularly has an effect of lowering the transition temperature. However, if it exceeds 5%, the yield strength increases excessively and the cost also increases. If it is less than 1%, the effect of greatly improving the impact characteristics is not exhibited, so Ni is specified in a range of more than 1% to 5%.
Ti acts to stably and effectively improve the hardenability due to the addition of B, but an effect is expected in a range of less than 0.005% and a range not satisfying the expression Ti / 47.88-N / 14.01 ≧ 0. If it exceeds 0.1% and Ti / 47.88-N / 14.01 ≦ 0.001, a large amount of Ti nitride is formed, and the impact characteristics tend to deteriorate. -0.1% and the formula 0 ≦ Ti / 47.88-N / 14.01 ≦ 0.001.
[0015]
B is an element that greatly improves the hardenability of steel material when added in a small amount, and has the effect of strengthening grain boundaries and strengthening precipitation as M 23 (C, B) 6 . If the addition amount is less than 0.0005%, no effect on hardenability can be expected, and if it exceeds 0.005%, a coarse B-containing phase tends to be formed, and embrittlement tends to occur. For this reason, B is specified in the range of 0.0005 to 0.005%.
N is one of the important elements that precipitate nitride or carbonitride and increase the strength. The effect is exhibited by the addition of 0.001% or more, but if it exceeds 0.01%, impact characteristics tend to be deteriorated due to coarsening of the nitride and age hardening due to solid solution N. For this reason, N was specified in the range of 0.001 to 0.01%.
[0016]
Since P is an element that has an adverse effect on weld cracking and impact properties, P is regulated to 0.02% or less. In addition, preferably, it is 0.015% or less.
S affects nonmetallic inclusions in steel, deteriorating workability, and also causes deterioration of impact characteristics, anisotropy and increase in susceptibility to reheat cracking. For this reason, S was specified to be 0.01% or less. Note that the content is preferably 0.005% or less.
O has an upper limit of 0.01% because it causes the generation of oxides that adversely affect the impact characteristics and generates oxides that serve as starting points for fatigue fracture.
[0017]
Cr is an element that improves hardenability, has the effect of precipitating M 23 C 6 type carbide in the matrix, has the effect of increasing the strength, and has the effect of making the carbide finer. If it is less than 0.01%, these effects cannot be expected sufficiently, and if it exceeds 1%, the yield strength is excessively increased. Therefore, Cr is specified in the range of 0.01 to 1%.
Mo is an element that improves quenching properties, is an element that enhances solid solution strengthening, and is an element that stabilizes the M 23 C 6 type carbide in the matrix. If it is less than 0.005%, this effect cannot be sufficiently expected, and if it exceeds 1%, the yield strength is excessively increased and the impact characteristics are deteriorated. Therefore, Mo is specified in the range of 0.005 to 1%.
[0018]
Nb is an element that forms carbonitrides and improves the strength, but when added in excess of 0.5%, the increase in yield strength becomes excessively large. If less than 0.005%, the effect of improving strength is not exhibited, so Nb is specified in the range of 0.005 to 0.5%.
V is an element that forms carbonitride and improves the strength, but when added in excess of 0.5%, the increase in yield strength becomes excessively large. If less than 0.01%, the effect of improving the strength is not exhibited, so V is specified in the range of 0.01 to 0.5%.
[0019]
Cu is an element that improves the strength, but if added in excess of 1%, the increase in yield strength becomes excessively large. If less than 0.01%, the effect of improving the strength is not exhibited, so Cu is specified in the range of 0.01 to 1%.
The value according to the following equation affects the hardness after hot forming, and if the value is less than 35, the required hardness cannot be obtained.
−28.8 ×% C 2 + 38.0 ×% C + 1.7 ×% Si + 7.5 ×% Mn + 1.4 ×% Ni + 43.2 ×% Cr + 8.7 ×% Cu-53.5 ×% S−57 .1 ×% P + 17.9 ≧ 35
[0020]
Next, the hot forming method will be described.
Since the martensitic transformation requires an austenite structure at the time of heating, the heating temperature before hot forming was set to an Ac 3 transformation point or more in the austenite region. The heating method before hot forming may be any of heating methods such as furnace heating, induction heating, and electric heating, but it is desirable that the forming target portion has a substantially uniform temperature. When the hot forming start temperature is lower than the Ar 3 transformation point, a ferrite phase appears, so that the strength after martensitic transformation is reduced. For this reason, the hot forming start temperature is specified to be equal to or higher than the Ar 3 transformation point. The molding method may be the same as a normal pressing method, but a water-cooled mold may be used to remove heat from a heated material and rapidly cool the material in order to transform into martensite. The temperature of the mold is desirably 100 ° C. or lower, but rapid cooling is possible even at about 200 ° C.
[0021]
【Example】
Various steel slabs having the compositions shown in Table 1 were cast. These slabs were heated to 1200 ° C., and hot-rolled to obtain a hot-rolled steel sheet having a finishing temperature of 850 ° C. and a winding temperature of 600 ° C. and a thickness of 4 mm. Some of the hot-rolled steel sheets were cold-rolled into cold-rolled steel sheets having a thickness of 1.2 mm. After heating to an austenitic region of 950 ° C., which is three or more Ac, by heating in a furnace, foam molding was performed from 900 ° C., which is three or more Ac, using a press having a water-cooled mold. The molding time was set to about 1 second, and the mold was cooled by keeping the press mold for 10 seconds after the completion of the molding. Further, the steel sheet temperature after 10 seconds was measured. With respect to the formed steel sheet, a cross section perpendicular to the rolling direction of the cold-rolled steel sheet was subjected to hardness measurement with a Vickers hardness meter, and further, the metal structure was observed with an optical microscope, and the martensite ratio was measured. Further, after a hot-rolled steel sheet having a thickness of 4 mm was heated in a furnace to an austenitic region of 950 ° C., which is three or more Ac points, an impact test was performed using a material that was water-cooled from 900 ° C. Table 2 shows the conditions and results.
[0022]
[Table 1]
[Table 2]
Examples of the present invention (No. 1 to No. 13) shown in Table 2 have a hardness after hot forming of Hv 420 or more and a brittle fracture rate at −60 ° C. by setting the martensite ratio to 90% or more. It is 50% or less, has a good shape freezing property, and satisfies the characteristics required as a structural member of an automobile. On the other hand, in the comparative examples out of the range of the present invention, the quenching hardness, the brittle fracture ratio and the shape freezing property are deteriorated.
Comparative examples (No. 14, No. 21, No. 26, No. 27, No. 28, No. 31) satisfy the expression 0 ≦ Ti / 47.88-N / 14.01 ≦ 0.001. The case where the hardenability is insufficient and the hardenability is not satisfied.
[0025]
Comparative Example (No. 22) is an example in which the quenchability is insufficient and the quench hardness is not satisfied because the formula 0 ≦ Al / 26.98-N / 14.01 is not satisfied. Comparative Examples (No. 23 and No. 32) do not satisfy the formula: Al / 26.98-N / 14.01 ≦ 0.0015, so that coarse AlN was generated and the impact characteristics were deteriorated.
Comparative Examples (No. 14, No. 16, No. 18, No. 20, No. 21, No. 22, No. 24, No. 26, No. 27, No. 28, No. 31, No. 32) ) Is the formula −28.8 ×% C 2 + 38.0 ×% C + 1.7 ×% Si + 7.5 ×% Mn + 1.4 ×% Ni + 43.2 ×% Cr + 8.7 ×% Cu-53.5 ×% S An example in which quenching properties are insufficient and quenching hardness is not satisfied because -57.1 ×% P + 17.9 ≧ 35 is not satisfied.
[0026]
Comparative Example No. 14 is an example in which the quenching hardness was insufficient because the C amount did not reach the specified value. Comparative Example No. No. 15 is an example in which the impact characteristics deteriorated because the C amount exceeded the specified value. Comparative Example No. 16 is an example in which the quenching hardness was insufficient because the Si amount did not reach the specified value. Comparative Example No. 17 is an example in which the impact value was reduced because the amount of Si exceeded the specified value. Comparative Example No. No. 18 is an example in which the quenchability was reduced and the quench hardness was insufficient because the Mn amount did not reach the specified value. Comparative Example No. 19 is an example in which the impact value was reduced because the Mn amount exceeded the specified value.
[0027]
Comparative Example No. No. 20 has a comparative example No. 21 is an example in which the impact value deteriorated because the S amount exceeded the specified value.
Comparative Example No. 22 is an example in which the quenching hardness was insufficient because the Al amount did not reach the specified value. Comparative Example No. No. 23 is an example in which many nonmetallic inclusions were generated because the Al amount exceeded the specified value, and impact characteristics were deteriorated because coarse AlN was generated.
[0028]
Comparative Example No. 24 is an example in which the quenching hardness was insufficient because the Ni amount did not reach the specified value. Comparative Example No. No. 25 is an example in which the yield strength was increased and the shape freezing property was deteriorated because the Ni amount exceeded the specified value.
Comparative Example No. 26 is an example in which the quenching hardness was insufficient because the Ti amount did not reach the specified value. Comparative Example No. 27 is an example in which a large amount of Ti nitride was generated because the amount of Ti exceeded the specified value, and the impact characteristics were deteriorated.
[0029]
Comparative Example No. 28 is an example in which the quenching hardness was insufficient because the B amount did not reach the specified value. Comparative Example No. 29 is an example in which a coarse B-containing phase was generated because the B amount exceeded a specified value, and the impact characteristics were deteriorated.
Comparative Example No. 30 is an example in which the quenching hardness was insufficient because the N amount did not reach the specified value. Comparative Example No. 31 is an example in which the impact characteristics deteriorated due to the formation of coarse nitrides and age hardening due to solid solution N because the N amount exceeded the specified value.
[0030]
Comparative Example No. 32 shows an example in which a large amount of oxide was generated because the O amount exceeded the specified value, and the impact characteristics were deteriorated.
Comparative Example No. No. 33 is an example in which the heating temperature did not exceed the Ac 3 transformation point, so that sufficient quenching could not be performed, the quenching hardness was insufficient, and the impact characteristics were reduced.
Comparative Example No. No. 34 is an example in which, since the molding start temperature was lower than the Ar 3 transformation point, ferrite transformation occurred, sufficient quenching could not be performed, quenching hardness was insufficient, and impact characteristics were also reduced.
[0031]
【The invention's effect】
As described above, the steel of the present invention is used for structural members of automobile parts, is a steel sheet having a high hardening ability after hot forming and a high strength, and is also excellent in impact properties and shape freezing properties, It is possible to contribute to elimination of a processing step.
Claims (6)
Ｃ ：０．２０〜０．３５％、
Ｓｉ：０．０１〜１．０％、
Ｍｎ：０．３〜２．０％、
Ａｌ：０．０１〜０．１０％、
Ｎｉ：１％超〜５％、
Ｔｉ：０．００５〜０．１％、
Ｂ ：０．０００５〜０．００５％、
Ｎ ：０．００１〜０．０１％、
Ｐ ：０．０２％以下、
Ｓ ：０．０１％以下、
Ｏ ：０．０１％以下、
残部がＦｅおよび不可避的不純物および／又は付随的成分よりなり、（１）式、（２）式、及び（３）式を満足することを特徴とする熱間成形加工後の硬化能および衝撃特性に優れた鋼板。
０≦Ｔｉ／４７．８８−Ｎ／１４．０１≦０．００１ ・・・（１）式
０≦Ａｌ／２６．９８−Ｎ／１４．０１≦０．００１５・・・（２）式
−２８．８×％Ｃ２ ＋３８．０×％Ｃ＋１．７×％Ｓｉ＋７．５×％Ｍｎ＋１．４×％Ｎｉ＋４３．２×％Ｃｒ＋８．７×％Ｃｕ−５３．５×％Ｓ−５７．１×％Ｐ＋１７．９≧３５・・・（３）式In mass%,
C: 0.20 to 0.35%,
Si: 0.01 to 1.0%,
Mn: 0.3 to 2.0%,
Al: 0.01 to 0.10%,
Ni: more than 1% to 5%,
Ti: 0.005 to 0.1%,
B: 0.0005 to 0.005%,
N: 0.001 to 0.01%,
P: 0.02% or less,
S: 0.01% or less,
O: 0.01% or less,
The balance consists of Fe and unavoidable impurities and / or ancillary components, and satisfies the formulas (1), (2) and (3). Excellent steel plate.
0 ≦ Ti / 47.88-N / 14.01 ≦ 0.001 Expression (1) 0 ≦ Al / 26.98-N / 14.01 ≦ 0.0015 Expression (2) 0.8 ×% C 2 + 38.0 ×% C + 1.7 ×% Si + 7.5 ×% Mn + 1.4 ×% Ni + 43.2 ×% Cr + 8.7 ×% Cu-53.5 ×% S-57.1 ×% P + 17.9 ≧ 35 Expression (3)
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| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP2003056290A JP3993831B2 (en) | 2002-11-14 | 2003-03-03 | Steel sheet with excellent curability and impact properties after hot forming and method of using the same |
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| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP2002330369 | 2002-11-14 | ||
| JP2003056290A JP3993831B2 (en) | 2002-11-14 | 2003-03-03 | Steel sheet with excellent curability and impact properties after hot forming and method of using the same |
Publications (2)
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