JP2004068095A - High strength steel sheet for working excellent in balance of strength-hole expansion ratio and shape fixability and method for producing the same - Google Patents
High strength steel sheet for working excellent in balance of strength-hole expansion ratio and shape fixability 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】
本発明は乗用車、トラック、バス等の自動車、自動二輪車や産業用機械等に使用することを企図した強度−穴広げ率バランスと形状凍結性に優れた加工用高強度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、自動車車体の軽量化と衝突時の乗員安全確保を主な背景として、加工用高強度鋼板の需要が増大してきた。特に引張強さＴＳ５９０ＭＰａ級（６０ｋｇｆ／ｍｍ２級）或いは、それ以上のＴＳ７８０〜９８０ＭＰａ級（８０〜１００ｋｇｆ／ｍｍ２級）等の適用が急速に拡大しつつある。
【０００３】
かかる用途に供される鋼板として、残留オ−ステナイトやマルテンサイトを有する複合組織鋼板が広く知られている。例えば、特開平９−１０４９４７号公報に記載されているように残留オ−ステナイトを適量含有させることにより、優れた強度−伸びバランス（引張強さ６０〜６９ｋｇｆ／ｍｍ２では全伸び３３．８〜４０．５％、引張強さ１１０ｋｇｆ／ｍｍ２では全伸び２２．０％）を有するものが得られている。しかしながら、強度−穴広げバランスに対する技術は不十分であり、強度−穴広げバランスを改善するための化学成分、ミクロ組織や介在物の最大長制御に対する技術要件は全く考慮されていないため、その特性レベルも低く（引張強さ６０〜６９ｋｇｆ／ｍｍ２では穴広げ比ｄ／ｄ０で１．４６〜１．６８、穴広げ率に換算して４６〜６８％、引張強さ１１０ｋｇｆ／ｍｍ２では穴広げ比ｄ／ｄ０で１．２、穴広げ率に換算して２０％）、伸びフランジ割れが発生し易いことから適用用途が限定されていた。
【０００４】
一方、強度−穴広げバランスに優れた鋼板として特開平３−１８０４２６号公報に記載されているようなベイナイト鋼板（引張強さ６０〜６７ｋｇｆ／ｍｍ２では穴広げ比ｄ／ｄ０で１．７２〜２．０２、穴広げ率に換算して７２〜１０２％、引張強さ７７ｋｇｆ／ｍｍ２では穴広げ比ｄ／ｄ０で１．７５、穴広げ率に換算して７５％）があるが、穴広げ率向上のためベイナイトの単一組織化を指向しているため、逆に強度−伸びバランスはかならずしも高くなく（引張強さ６０〜６７ｋｇｆ／ｍｍ２では全伸び２７〜３０％、引張強さ７７ｋｇｆ／ｍｍ２では全伸び２３％）、さらには引張強さの観点でも引張強さ７７ｋｇｆ／ｍｍ２に留まっており、それ以上の強度では特性劣化を生じ、ハイテン化要望に十分には応えられず、適用用途が限定されているのが実情である。
【０００５】
即ち、自動車部品のプレス成形においては強度−伸びバランスに代表される張り出し成形と強度−穴広げバランスに代表される伸びフランジ成形が二大成形要素であるが、相反する性質であり、両立は困難であってその両者に秀でることが適用用途拡大の鍵であった。
【０００６】
近年、地球環境問題から６０ｋｇｆ／ｍｍ２以上の強度を有する、例えば、８０ｋｇｆ／ｍｍ２、１００ｋｇｆ／ｍｍ２等の加工用高強度鋼板（超ハイテン）への置換が加速度的に進む中、成形難度の高い部品への適用が検討されるに及び強度−伸びバランスと強度−穴広げバランスの両者に優れた鋼板が要求されている。特に、８０ｋｇｆ／ｍｍ２を超える鋼板（超ハイテン）では強度が大きくなるので、上記両バランスの両立が一層困難となると共に材質特性のバラツキも大きくなり、それによるプレス成形品でのスプリングバック増大等の寸法精度の劣化（いわゆる形状凍結性の劣化）が顕著となり、実用上、大きな問題となっており、強度−穴広げ率バランスと形状凍結性に優れた加工用高強度鋼板が渇望されていたのである。
【０００７】
【発明が解決しようとする課題】
本発明は上記した従来の鋼板が持つ問題点を解消し、優れた強度−穴広げバランス（引張強さ×穴広げ率で４００００ＭＰａ・％以上、好ましくは５００００ＭＰａ・％以上）と優れた強度−伸びバランス（引張強さ×全伸びで１００００ＭＰａ・％以上、好ましくは１５０００ＭＰａ・％以上、より好ましくは１７０００ＭＰａ・％以上）と形状凍結性とを兼備した鋼板、即ち強度−穴広げ率バランスと形状凍結性に優れた加工用高強度鋼板及びその製造方法を提供することを課題としている。
【０００８】
【課題を解決するための手段】
本発明者らは、製鋼〜熱延の一貫製造の視点から、鋭意検討を加え、強度−穴広げ率バランスと形状凍結性に優れた加工用高強度鋼板とその製造方法を発明するに到った。
【０００９】
一般に加工用鋼板においては、ＴＳを増大させれば、成形性（穴広げ率や伸び）、形状凍結性は劣化することが知られている。
【００１０】
しかしながら、本発明者が、加工用高強度鋼板について鋭意研究したところ、鋼の化学成分、特にＰ、Ｓ、Ｃ、Ｓｉ、Ａｌ等の含有量を制御し、かつ、組織、介在物を仕上温度等によって制御し、そしてこれらを組み合わせることによって、強度の増大と穴広げ性の向上を両立させ、さらには伸びの劣化の抑制が可能であることを知見し、穴広げ性と伸びとが高く形状凍結性に優れた加工用高強度鋼板が得られることを見出し、本発明を完成した。
【００１１】
（１） 化学成分として、質量％で
Ｃ：０．０２〜０．１６％、
Ｐ≦０．０１０％、
Ｓ≦０．００３％、
ＳｉとＡｌの内の１種又は２種を合計量で０．２〜４％、
Ｍｎ、Ｎｉ、Ｃｒ、Ｍｏ、Ｃｕの内の１種又は２種以上を合計量で０．５〜４％を含み、
Ｃ／（Ｓｉ＋Ａｌ＋Ｐ）が０．１以下で、
残部Ｆｅ及び不可避的不純物よりなる鋼板であって、該鋼板断面のミクロ組織として、マルテンサイトと残留オーステナイトの内の１種又は２種を合計面積率で３％未満、フェライトとベイナイトの内の１種又は２種を合計面積率で８０％以上、残部がパーライトよりなると共に、パーライト、マルテンサイト、残留オーステナイトの最大長が１０ミクロン以下であり、さらに、鋼板断面内に２０ミクロン以上の介在物が１平方ｍｍ当たり０．３ケ以下であることを特徴とする強度−穴広げ率バランスと形状凍結性に優れた加工用高強度鋼板。
【００１２】
（２） ベイナイト硬さ／フェライト硬さが１．０〜１．３であることを特徴とする上記（１）記載の強度−穴広げ率バランスと形状凍結性に優れた加工用高強度鋼板。
【００１３】
（３） パ−ライト面積率が７％未満、フェライトとベイナイトの内の１種又は２種の合計面積率が９０％以上であることを特徴とする上記（１）又は（２）記載の強度−穴広げ率バランスと形状凍結性に優れた加工用高強度鋼板。
【００１４】
（４） 化学成分として、質量％でさらにＮｂ、Ｖ、Ｔｉの内の１種又は２種以上を合計量で０．３％以下含むことを特徴とする上記（１）〜（３）に記載の強度−穴広げ率バランスと形状凍結性に優れた加工用高強度鋼板。
【００１５】
（５） 化学成分として、質量％で
さらにＢを０．０１％以下含むことを特徴とする上記（１）〜（４）の内のいずれかに記載の強度−穴広げ率バランスと形状凍結性に優れた加工用高強度鋼板。
【００１６】
（６） 化学成分として、質量％で
さらにＣａ、ＲＥＭの内の１種又は２種を、
Ｃａにおいては０．０１％以下、
ＲＥＭにおいては０．０５％以下、
含むことを特徴とする上記（１）〜（５）の内のいずれかに記載の強度−穴広げ率バランスと形状凍結性に優れた加工用高強度鋼板。
【００１７】
（７） 化学成分として、質量％で、さらに
Ｎ：０．０２％以下を含むことを特徴とする上記（１）〜（６）の内のいずれかに記載の強度−穴広げ率バランスと形状凍結性に優れた加工用高強度鋼板。
【００１８】
（８） 鋼板の板幅方向の引張強さＴＳの偏差ΔＴＳが１００ＭＰａ以下であることを特徴とする上記（１）〜（７）の内のいずれかに記載の強度−穴広げ率バランスと形状凍結性に優れた加工用高強度鋼板。
【００１９】
（９） 質量％で、
Ｃ：０．０２〜０．１６％、
Ｐ≦０．０１０％、
Ｓ≦０．００３％を含み、
ＳｉとＡｌの内の１種又は２種を合計量で０．２〜４％含み、
Ｍｎ、Ｎｉ、Ｃｒ、Ｍｏ、Ｃｕの内の１種又は２種以上を合計量で０．５〜４％含み、
Ｃ／（Ｓｉ＋Ａｌ＋Ｐ）が０．１以下で、
残部Ｆｅ及び不可避的不純物よりなる鋼板の製造方法であって、溶鋼を溶製するに際し、溶鋼脱硫時の脱硫用フラックス添加後に１．５回以上、溶鋼を環流させ、さらに該溶鋼の鋳造後に得られた鋼片を熱間圧延して鋼板を製造するに際し、仕上圧延を仕上入側温度≧１０００℃、かつ仕上出側温度＞９２０℃で実施し、６００℃以下で得られた鋼板を巻き取ることを特徴とする強度−穴広げ率バランスと形状凍結性に優れた加工用高強度鋼板の製造方法。
【００２０】
（１０） 化学成分として、質量％で
さらにＮｂ、Ｖ、Ｔｉの内の１種又は２種以上を合計量で０．３％以下含むことを特徴とする上記（９）に記載の強度−穴広げ率バランスと形状凍結性に優れた加工用高強度鋼板の製造方法。
【００２１】
（１１） 化学成分として、質量％で
さらにＢを０．０１％以下含むことを特徴とする上記（９）又は（１０）に記載の強度−穴広げ率バランスと形状凍結性に優れた加工用高強度鋼板の製造方法。
【００２２】
（１２） 化学成分として、質量％で
さらにＣａ、ＲＥＭの１種又は２種を、
Ｃａにおいては０．０１％以下、
ＲＥＭにおいては０．０５％以下、
含むことを特徴とする上記（９）〜（１１）の内のいずれかに記載の強度−穴広げ率バランスと形状凍結性に優れた加工用高強度鋼板の製造方法。
【００２３】
（１３） 仕上圧延出側における板幅方向の温度偏差を２０℃以下にすることを特徴とする上記（９）〜（１２）の内のいずれかに記載の強度−穴広げ率バランスと形状凍結性に優れた加工用高強度鋼板の製造方法。
【００２４】
（１４） 仕上圧延出側における板幅方向の温度偏差を２０℃以下となるように仕上圧延前又は仕上圧延中に鋼板を加熱することを特徴とする上記（９）〜（１２）の内のいずれかに記載の強度−穴広げ率バランスと形状凍結性に優れた加工用高強度鋼板の製造方法。
【００２５】
【発明の実施の形態】
以下に本発明を詳細に説明する。
【００２６】
まず、鋼板の化学成分について述べる。なお、以下に記載する化学成分の単位の「％」は全て「質量％」を意味する。
【００２７】
Ｃは強度を確保するために必要な元素であり、０．０２％以上とする。好ましくは０．０５％以上とする。但し、その上限は溶接性の劣化を避け、穴広げ率への悪影響を避けるため、０．１６％以下とする。好ましくは０．１２％以下とする。
【００２８】
【表１】
Ｐは本発明の添加元素において、非常にポイントとなる元素である。図１にその効果を示す。図１は表１の鋼番１の成分の鋼板を用いて、Ｐ濃度と鋼板の穴広げ率の関係を調査した結果を示す。穴広げ率は日本鉄鋼連盟規格ＪＦＳ Ｔ１００１−１９９６より求めた。図１よりＰを０．０１０％以下とすることにより穴広げ率は指数関数的に顕著に向上し、従来の延長上では想定しえない穴広げ率への効果が認められる。それによりプレス割れの回避が可能となるのである。その理由は未だ明らかでない面はあるが、粒界に存在するＰが破壊に際し悪影響を生じる原因となると考えられ、Ｐの低減によって打ち抜き穴端面性状が改善され（破断面の破面サイズ極小化や粗さ低減やミクロクラックの低減等、剪断面のミクロ組織の加工劣化抑制等）、穴広げ率の向上につながったものと考えられる。
【００３０】
また、穴広げ率を向上させるためには、プレス割れの発生や亀裂の伝播を抑制することが必要となる。そのためには炭化物の生成抑制が有効であり、炭化物生成の抑制指標としてのＣ／（Ｓｉ＋Ａｌ＋Ｐ）の値を０．１以下にしなければならない。Ｃ／（Ｓｉ＋Ａｌ＋Ｐ）の値が０．１を超えると、本発明が目的とする優れた引張強さ×穴広げ率と優れた引張強さ×全伸びを兼備した鋼板が得られない。
【００３１】
Ｓは硫化物系介在物による穴広げ率と溶接性の劣化防止の観点から、その含有量は０．００３％以下（好ましくは≦０．００１％）とする。
【００３２】
Ｓｉ、Ａｌは炭化物の生成を抑制しフェライトを強化することにより、フェライトとベイナイトの硬度差を減じ組織の一様性を高めることに寄与する。また、脱酸元素としても作用する。上記観点から、ＳｉとＡｌの内の１種もしくは２種の合計添加下限量は０．２％以上とする必要がある。コストと効果の兼ね合いから、その合計添加上限量は４％以下とする。
【００３３】
ＳｉとＡｌの個別添加量については、下記を加味してもよい。
【００３４】
特に優れた表面性状が要求される場合は、Ｓｉ≦０．１％（好ましくはＳｉ≦０．０１％）とすることによりＳｉスケ−ルを回避するか、逆にＳｉ＞１．０％（好ましくはＳｉ＞１．２％）とすることによりＳｉスケ−ルを無害化（全面に発生させ目立たなくする）してもよい。
【００３５】
ＳｉとＡｌのフェライト強化作用の差を利用して引張強さを低くおさえたい場合等の材質上の観点から、Ａｌ添加量を増しＳｉ添加量を減ずることも可能である。
【００３６】
耐火物溶損やノズル閉塞等の製鋼上デメリットや材質との関連で、Ａｌ≦０．２％（好ましくはＡｌ≦０．１％）としてもよい。
【００３７】
Ｍｎ、Ｎｉ、Ｃｒ、Ｍｏ、Ｃｕは強化元素である。上記観点から、それらの内の１種もしくは２種以上の合計添加下限量は０．５％以上とする必要がある。但し、コストと効果の兼ね合いから、その合計添加上限量は４％以下とする。
【００３８】
さらに、選択元素として、Ｎｂ、Ｖ、Ｔｉ、Ｂ、Ｃａ、ＲＥＭの１種又は２種以上を添加してもよい。
【００３９】
Ｎｂ、Ｖ、Ｔｉは高強度化に有効な元素であるが、効果とコストの兼ね合いから、それら添加量は１種又は２種以上を合計量で０．３％以下とする。
【００４０】
Ｂは強化元素としての作用があり、０．０１％以下添加してもよい。また、Ｐの悪影響を軽減する作用も有する。
【００４１】
Ｃａは硫化物系介在物の形態制御（球状化）により、穴広げ率をより向上させるために０．０１％以下添加してもよい。
【００４２】
また、ＲＥＭも同様の理由から０．０５％以下添加してもよい。
【００４３】
なお、オ−ステナイトの安定化や高強度化等を狙って、必要に応じて、Ｎを０．０２％以下、含有してもよい。
【００４４】
次にミクロ組織について述べる。
【００４５】
優れた穴広げ率を得るためには、極低Ｐ化により改善された打ち抜き穴端面性状を損なわないという観点から、ミクロ組織の均一性とサイズ及び量、介在物の量及びサイズの制御が特に重要なポイントであり、まず、これについて述べる。
【００４６】
ミクロ組織の均一性は打ち抜き穴端面の性状改善（破断面の破面サイズ極小化や粗さ低減やミクロクラックの低減等、剪断面のミクロ組織の加工劣化抑制等）に影響を及ぼすため、穴広げ率に大きく影響を及ぼす。
【００４７】
図２は表１の鋼番２の成分の鋼板を用いて、鋼板内のパーライト、マルテンサイト及び残留オーステナイトのミクロ組織の最大長と鋼板の穴広げ率の関係を調査した結果を示す。マルテンサイトはフェライトとベイナイトに比べ、非常に硬質であり、組織の均一性を害するため、穴広げ率に著しい悪影響を及ぼす。また、残留オ−ステナイトもマルテンサイトに変態するため、組織の均一性に対しては好ましくない。その観点からマルテンサイトと残留オーステナイトの内の１種又は２種を合計面積率で３％未満とすることが必要である。また、パ−ライトはマルテンサイトより軟質ではあるものの穴広げ率に対しては好ましくないため、その面積率を７％未満とすることが好ましい。パ−ライト、マルテンサイト及び残留オーステナイトは組織の均一性の観点からは一切含まれないことが好ましいが、工業生産の見地から多少の混入を許容するため、そのサイズ制御により、その悪影響を抑制することができる。図２に示すようにパーライト、マルテンサイト及び残留オーステナイトの最大長が１０ミクロン以下の場合に穴広げ率は指数関数的に顕著に向上し、従来の延長上では想定しえない穴広げ率への効果が認められる。なお、パーライト、マルテンサイト、残留オーステナイトの１個１個の結晶粒が微細であっても、複数個が連鎖状に存在している場合、その連鎖形状の最大長が穴広げ率に悪影響を及ぼすことを見出し、それを制御することにより従来の延長上では想定しえない穴広げ率の顕著な改善を果たしたのである。好ましくは、最大長を２ミクロン以下とすることにより、その効果は一層高まる。
【００４８】
さらにフェライトとベイナイトの１種又は２種の合計面積率を８０％以上（好ましくは９０％以上）とすることにより、組織の均一性が高まると共に、フェライトとベイナイト以外の硬質組織が連鎖状もしくはネットワーク状に存在することに起因するプレス成形性劣化を抑制して優れた強度−伸びバランスと優れた強度−穴広げバランスが得られる。
【００４９】
フェライトとベイナイトの内の１種又は２種の合計面積率が８０％未満では組織の均一性等が劣化し、成形性の改善効果を安定して得ることができなくなるため８０％を下限とし、好ましくは９０％を下限とする。
【００５０】
さらに、より均一な組織とし、より穴広げ率を向上させるためには、ベイナイト硬さ（Ｈｖ）／フェライト硬さ（Ｈｖ）の比を１．０〜１．３とすることが好ましい。
【００５１】
なお、伸びを高くしたい場合には、残留オーステナイトは含有させた方が有利である。但し、前記した理由により、その上限は３％未満とする。
【００５２】
なお、形状凍結性等の観点から、低い降伏比（降伏比ＹＲ＝降伏応力／引張強さ×１００で７５％未満）が望まれる場合にはマルテンサイトを含有させた方が有利である。但し、前記した理由により、その上限は３％未満とする。
【００５３】
なお、パーライト、マルテンサイト及び残留オーステナイトの最大長は特開昭５９−２１９４７３号公報に開示された試薬及び特開平５−１６３５９０号公報で開示された試薬により鋼板圧延方向断面を腐食した倍率１０００倍の光学顕微鏡写真から板厚方向の全断面を加味して、算出した。
【００５４】
また、介在物制御においては粗大介在物の個数を低減することにより穴広げ率を改善できる。介在物は研磨仕上げした鋼板圧延方向断面を顕微鏡観察（倍率４００倍）し、最大長が２０ミクロン以上の粗大介在物の数を積算した。図３は表１の鋼番２の成分の鋼板を用いて鋼板内の粗大介在物（最大長２０ミクロン以上）の個数と穴広げ率の関係を調査した結果を示す。粗大介在物（最大長２０ミクロン以上）が一定個数以下（１平方ｍｍ当たり０．３ケ以下）の場合に穴広げ率が大幅に向上することが判る。
【００５５】
以上述べた効果により、優れた強度−穴広げバランス（引張強さ×穴広げ率で４００００ＭＰａ・％以上、好ましくは５００００ＭＰａ・％以上）と優れた強度−伸びバランス（引張強さ×全伸びで１００００ＭＰａ・％以上、好ましくは１５０００ＭＰａ・％以上、より好ましくは１７０００ＭＰａ・％以上）の両立が可能となり、プレス成形性が大幅に向上する。
【００５６】
次に、その製造方法について述べる。
【００５７】
まず製鋼工程においては、溶鋼を溶製するに際し、ＲＨ等の２次精練装置を用いた溶鋼脱硫時の脱硫用フラックス添加後に１．５回以上溶鋼を環流させることがポイントである。ここでの溶鋼の還流とは、単位時間当たりＲＨ等の２次精練装置内を循環させる溶鋼量を示すものであり、種々の算出式があるが、例えば「大量生産規模における不純物元素の精練限界」（（株）日本鉄鋼協会 高温精練プロセス部会精練フォーラム 日本学術振興会 製鋼第１９委員会反応プロセス研究会，平成８年３月，１８４頁〜１８７頁）に開示されているように、下記（１）で表される溶鋼還流量Ｑを１回と定義したものである。
還流量Ｑ＝１１．４×Ｖ１／３×Ｄ４／３×｛ｌｎ（Ｐ１／Ｐ０）｝１／３×ｋ（式１）を１回とした。
Ｑ：溶鋼環流量（ｔ／ｍｉｎ）、Ｖ：環流ガス流量（Ｎｌ／ｍｉｎ）
Ｄ：浸漬管内径（ｍ） 、Ｐ０：真空槽内圧力（Ｐａ）
Ｐ１：環流ガス吹込位置圧力（Ｐａ）、
ｋ：定数（２次精練装置による定数。今回は４とする）
【００５８】
ここで、ＲＨを用いた場合の溶鋼溶製の模式図を図４に示すが、溶鋼鍋１中に脱ガス槽２の浸漬管３の２本浸漬をさせ、その一方の下方からガスを吹き込み（ここでは浸漬管の下方からインジェクションランス４からＡｒを吹き込む）、溶鋼鍋１内の溶鋼が上昇して脱ガス槽２に入り、脱ガス処理後に他方の浸漬管３から溶鋼鍋に下降して戻るものである。なお、ここではＲＨによる２次精練装置を用いた例を示したが、他の２次精練装置（例えばＤＨ）を用いても構わないことはいうまでもない。
【００５９】
図５は表１の鋼番２の成分の溶鋼を溶製した際の脱硫フラックス添加後の溶鋼環流回数と、得られた溶鋼の鋳造後の鋳片から熱間圧延した後に鋼板断面１平方ｍｍ当たりの２０ミクロン以上の介在物個数との関係を調査した結果を示す。図５に示すように環流回数１．５回以上で脱硫用フラックス系介在物の浮上が顕著に促進され、粗大介在物（２０ミクロン以上）を一定個数以下（１平方ｍｍ当たり０．３ケ以下）とすることが可能となり、穴広げ率を向上させることができるのである。
【００６０】
次に本発明鋼を熱延鋼板にて得る場合の、熱間圧延工程における仕上圧延の温度条件について検討した。図６（ａ）〜（ｅ）は、仕上出側温度（℃）と引張強さ（ＴＳ）、穴広げ率（％）、伸び（％）、引張強さ×穴広げ率（ＭＰａ）及び引張強さ×伸び（ＭＰａ）とのそれぞれの関係を示す図である。仕上出側温度が９２０℃以下の領域では、仕上出側温度が高くなるに従って、ＴＳは増大し、穴広げ率と伸びは劣化するため、高強度（ＴＳ）と成形性（穴広げ率、伸び）の両立を果たすことができない。しかし、仕上出側温度が９２０℃超の領域になると、仕上出側温度が高くなるに従って、引張強さの増大傾向は顕著となり、さらに穴広げ率は向上傾向に転ずる。従って、仕上出側温度が９２０℃超では、従来不可能であった高強度と優れた穴広げ率の両立が可能となった。一方、伸びは、９２０℃超の領域でも劣化傾向にはあるが、強度−伸びバランス（ＴＳ×伸び）としては劣化傾向が緩和され、好ましい傾向に転ずる。即ち、９２０℃を境にして、ミクロ組織の均一性等が顕著に良好となり成形性が著しく改善されるのである。
【００６１】
これらの結果からして、仕上出側温度が９２０℃超では、優れた強度−穴広げバランス、及び優れた強度−伸びバランスを得ることができる。
【００６２】
従って、本発明では仕上出側温度を９２０℃超とした。また、仕上入側温度は組織の均一性等の視点から１０００℃以上とすることが必要である。
【００６３】
また、鋼板の板幅方向の引張強さＴＳの偏差ΔＴＳを１００（好ましくは５０）ＭＰａ以下とするために、仕上圧延出側における板幅方向の温度偏差を２０（好ましくは１０）℃以下にすることが必要である。そのために仕上圧延前又は仕上圧延中に鋼板を加熱装置によって加熱してもよい。この際の加熱装置としては、ガス加熱、直接通電加熱等が考えられるが、加熱制御性に優れた誘導加熱装置が好ましい。さらに、特に板幅中央部や板幅端部の温度低下がある場合には、板に対して上／下にコイル鉄心が独立し、板幅方向で部分的に昇温量を可変でき、板幅方向の温度均一制御性に優れたトランスバース型誘導加熱装置を用いることが特に好ましい。
【００６４】
それによりハイテン適用時の成形上の大きな課題であった形状凍結性に関しても顕著な改善が図れる。即ち、ΔＴＳの低減によりプレス成形品でのスプリングバック増大等の寸法精度の劣化（いわゆる形状凍結性の劣化）の改善が可能となるのである。
【００６５】
なお、パーライト、マルテンサイト、残留オーステナイトの最大長を低減させるために、仕上圧延の圧下率を９０％以上とすることが望ましい。
【００６６】
仕上圧延後の冷却テ−ブルにおける条件は特に規定しないが、組織の均一性向上、ミクロ組織の微細化促進のため、仕上圧延出側での直後冷却や強冷却等の冷却速度の増大手段を実施してもよい。また、ミクロ組織面積率の制御を狙って、一般的に知られている冷却速度の多段制御（急冷、緩冷、等温保持の組み合わせ）や巻取温度制御を実施してもよい。
【００６７】
巻取温度は穴広げ率向上に有害な炭化物の生成及びミクロ組織粗大化を抑制するために、その上限を６００℃とする。巻取温度が６００℃を超えると、優れた強度−穴広げバランス（引張強さ×穴広げ率）が得られなくなる。強度−穴広げバランスは巻取温度が低い方が改善する傾向にあり、強度−穴広げバランスの観点からは４５０℃以下とすることが望ましい。また、強度−伸びバランスの観点からは３５０℃以下で劣化傾向となるので、穴広げと伸びの両立という視点からは３５０〜４５０℃が好ましい。
【００６８】
さらに、巻取後の鋼板の冷却は放冷をおこなってもよいし、強制冷却でもよい。
【００６９】
なお、圧延に供する鋼片はいわゆる冷片再加熱、ＨＣＲ、ＨＤＲのいずれであっても構わない。また、いわゆる薄肉連続鋳造による鋼片であっても構わない。
【００７０】
また、本発明による鋼板にＺｎ等のめっきを施し耐食性の向上を図ったり、潤滑剤等を塗布しプレス成形性の一層の向上を図ってもよい。
【００７１】
【実施例】
供試鋼のＦｅ以外の化学成分を表２に示す。鋼番１〜１０が本発明の鋼成分を満たす例で鋼番１１〜１２が比較例である。
【００７２】
供試鋼の製鋼及び熱間圧延における製造条件を表３に示す。鋼番１の１、１の６及び２〜１０が本発明例で他は比較例である。なお、鋼番１の１〜１の７は、鋼番１の鋼を用いて製造条件を異ならせた例である。得られた熱延鋼板のミクロ組織を表４に、そして鋼の機械的性質を表５に示す。
【００７３】
なお、特性評価やミクロ組織評価は以下の方法で実施した。
【００７４】
引張試験はＪＩＳ５号にて実施し、引張強度（ＴＳ）、降伏強度（ＹＳ）、降伏比（ＹＲ＝ＹＳ／ＴＳ×１００）、全伸び（Ｔ．ＥＬ）、強度−伸びバランス（ＴＳ×Ｔ．ＥＬ）を求めた。
【００７５】
穴広げ率は日本鉄鋼連盟規格ＪＦＳ Ｔ１００１−１９９６により求めた。
【００７６】
ミクロ組織の構成同定と面積率の測定、パーライト、残留オ−ステナイト、マルテンサイトの最大長の測定はナイタ−ル試薬、特開昭５９−２１９４７３号公報に開示された試薬及び特開平５−１６３５９０号公報で開示された試薬により鋼板圧延方向断面を腐食した倍率１０００倍の光学顕微鏡写真とＸ線解析により行った。
【００７７】
鋼板内の介在物は研磨仕上げした鋼板圧延方向断面を顕微鏡観察（倍率４００倍）し、最大長が２０ミクロン以上の粗大介在物の数を積算した。
【００７８】
Ｘ線解析により残留オーステナイト面積率（Ｆγ：単位は％）を算出する場合はＭｏ−Ｋα線により次式に従い、算出した。
Ｆγ（％）＝（２／３）｛１００／（０．７×α（２１１）／γ（２２０）＋１）｝＋（１／３）｛１００／（０．７８×α（２１１）／γ（３１１）＋１）｝
但し、α（２１１）、γ（２２０）、α（２１１）、γ（３１１）は面強度を示す。
【００７９】
【表２】
【表３】
【表４】
【表５】
本発明例（鋼番１の１、１の６及び２〜１０）では、優れた強度−穴広げバランス（ＴＳ×λ）と優れた強度−伸びバランス（ＴＳ×Ｔ・ＥＬ）を兼備したプレス成形性の優れた熱延高強度鋼板が得られている。
【００８４】
一方、比較例（鋼番１の２〜１の５、１の７及び１１〜１２）はそれぞれ表２〜表５に記載のように本発明範囲外であるため、機械的特性が低いものしか得られなかった。
【００８５】
【発明の効果】
本発明により強度−穴広げ率バランスと形状凍結性に優れた加工用高強度鋼板とその製造方法を低コストかつ安定的に提供することが可能となったため、使用用途・使用条件が格段に広がり、工業上、経済上の効果は非常に大きい。
【図面の簡単な説明】
【図１】穴広げ率に及ぼす化学成分Ｐの影響を示す図である。
【図２】穴広げ率に及ぼすミクロ組織（パーライト、マルテンサイト、残留オーステナイト）の最大長の影響を示す図である。
【図３】穴広げ率に及ぼす介在物個数の影響を示す図である。
【図４】ＲＨを用いた場合の溶鋼溶製の模式図である。
【図５】介在物個数に及ぼす脱硫用フラックス添加後の溶鋼環流回数の影響を示す図である。
【図６】鋼板の性質と仕上出側温度の関係を示す図である。
【符号の説明】
１ 溶鋼鍋
２ 脱ガス槽
３ 浸漬管
４ インジェクションランス[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a high-strength steel sheet for processing excellent in strength-hole expansion ratio balance and shape freezing property intended for use in automobiles such as passenger cars, trucks, buses, motorcycles, industrial machines, and the like, and a method for producing the same. It is.
[0002]
[Prior art]
BACKGROUND ART In recent years, demand for high-strength steel sheets for processing has been increasing, mainly on the basis of weight reduction of automobile bodies and occupant safety during collisions. In particular, the TS590 MPa class tensile strength (60 kgf / mm 2 Grade) or higher TS780-980MPa grade (80-100kgf / mm 2 Applications) are rapidly expanding.
[0003]
As a steel sheet provided for such use, a composite structure steel sheet having retained austenite and martensite is widely known. For example, as described in JP-A-9-104947, by adding an appropriate amount of residual austenite, an excellent strength-elongation balance (tensile strength of 60 to 69 kgf / mm) is obtained. 2 Has a total elongation of 33.8-40.5% and a tensile strength of 110 kgf / mm. 2 Has a total elongation of 22.0%). However, the technology for strength-expansion balance is inadequate, and no technical components for improving the strength-expansion balance, and the technical requirements for the maximum length control of microstructures and inclusions are not considered at all. Low level (tensile strength 60-69kgf / mm 2 Is 1.46 to 1.68 in the hole expansion ratio d / d0, 46 to 68% in terms of the hole expansion ratio, and the tensile strength is 110 kgf / mm. 2 , The hole expansion ratio d / d0 is 1.2, and the hole expansion ratio is 20% in terms of the hole expansion ratio).
[0004]
On the other hand, a bainite steel sheet (tensile strength of 60 to 67 kgf / mm) as described in JP-A-3-180426 as a steel sheet excellent in strength-hole expansion balance is used. 2 Is 1.72 to 2.02 in the hole expansion ratio d / d0, 72 to 102% in terms of the hole expansion ratio, and the tensile strength is 77 kgf / mm. 2 Has a hole expansion ratio d / d0 of 1.75, which is equivalent to 75% in terms of hole expansion ratio). However, in order to improve the hole expansion ratio, bainite has a single structure, and consequently strength-elongation. The balance is not always high (tensile strength 60-67kgf / mm 2 Has a total elongation of 27 to 30% and a tensile strength of 77 kgf / mm. 2 In this case, the total elongation is 23%), and also from the viewpoint of tensile strength, the tensile strength is 77 kgf / mm. 2 However, if the strength is higher than that, the characteristics are degraded, the demand for high tensile strength cannot be sufficiently satisfied, and the actual application is limited.
[0005]
That is, in press forming of automobile parts, stretch forming represented by strength-elongation balance and stretch flange forming represented by strength-hole expansion balance are two major forming elements, but they are mutually contradictory properties and it is difficult to achieve both. Therefore, excelling in both is the key to expanding the applications.
[0006]
In recent years, 60kgf / mm 2 Having the above strength, for example, 80 kgf / mm 2 , 100kgf / mm 2 As the replacement with high-strength steel sheets (ultra-high tensile) for processing progresses at an accelerating rate, application to parts with high forming difficulty is considered and both the strength-elongation balance and the strength-hole expansion balance are excellent. Steel sheets are required. In particular, 80 kgf / mm 2 In the case of a steel plate (ultra high tensile steel) that exceeds the maximum strength, it is more difficult to achieve the balance between the two, and the variation in material properties also increases, thereby deteriorating dimensional accuracy such as increased springback in press-formed products ( The so-called “deterioration of shape freezing property” has become remarkable, and has become a serious problem in practical use. Therefore, a high-strength steel sheet for processing excellent in strength-hole expansion ratio balance and shape freezing property has been desired.
[0007]
[Problems to be solved by the invention]
The present invention solves the above-described problems of the conventional steel sheet, and has excellent strength-hole expansion balance (tensile strength × hole expansion ratio of 40,000 MPa ·% or more, preferably 50,000 MPa ·% or more) and excellent strength-elongation. A steel plate having both balance (tensile strength x total elongation of 10,000 MPa ·% or more, preferably 15000 MPa ·% or more, more preferably 17000 MPa ·% or more) and shape freezing property, that is, strength-hole expansion ratio balance and shape freezing property An object of the present invention is to provide a high-strength steel sheet for processing excellent in quality and a method for producing the same.
[0008]
[Means for Solving the Problems]
The present inventors have made intensive studies from the viewpoint of integrated production of steelmaking to hot rolling and have come to invent a high-strength steel sheet for processing excellent in strength-hole expansion ratio balance and shape freezing property, and a method for producing the same. Was.
[0009]
In general, it is known that, when the TS is increased, the formability (hole expansion rate and elongation) and the shape freezing property of a steel sheet for processing are deteriorated.
[0010]
However, the present inventor has conducted intensive studies on high-strength steel sheets for processing, and found that the chemical composition of the steel, in particular, the content of P, S, C, Si, Al, and the like was controlled, and the structure and inclusions were subjected to a finishing temperature. By controlling these factors, etc., and combining them, we found that it was possible to achieve both an increase in strength and an improvement in hole-expandability, and furthermore, it was possible to suppress the deterioration of elongation. The present inventors have found that a high-strength steel sheet for processing excellent in freezing properties can be obtained, and completed the present invention.
[0011]
(1) As a chemical component, in mass%
C: 0.02 to 0.16%,
P ≦ 0.010%,
S ≦ 0.003%,
0.2 to 4% in total of one or two of Si and Al,
One or more of Mn, Ni, Cr, Mo, and Cu containing 0.5 to 4% in total amount;
C / (Si + Al + P) is 0.1 or less,
A steel sheet comprising a balance of Fe and unavoidable impurities, wherein as a microstructure of a cross section of the steel sheet, one or two of martensite and retained austenite are less than 3% in total area ratio and one of ferrite and bainite. 80% or more of the total area ratio of the seeds or two kinds, the balance being pearlite, the maximum length of pearlite, martensite, and retained austenite is 10 microns or less, and further, inclusions of 20 microns or more are present in the cross section of the steel sheet. A high-strength steel sheet for processing excellent in strength-hole expansion ratio balance and shape freezing property, wherein the number is 0.3 or less per square mm.
[0012]
(2) The high-strength steel sheet for processing excellent in strength-hole expansion ratio balance and shape freezing property as described in (1) above, wherein bainite hardness / ferrite hardness is 1.0 to 1.3.
[0013]
(3) The strength according to (1) or (2), wherein the pearlite area ratio is less than 7%, and the total area ratio of one or two of ferrite and bainite is 90% or more. -High-strength steel sheet for processing with excellent hole expansion ratio balance and shape freezing properties.
[0014]
(4) As described in (1) to (3) above, one or more of Nb, V, and Ti are further contained as a chemical component in an amount of 0.3% or less in total. High-strength steel plate for processing with excellent strength-hole expansion ratio balance and shape freezing properties.
[0015]
(5) As a chemical component, in mass%
The high-strength steel sheet for processing according to any one of the above (1) to (4), further comprising 0.01% or less of B, wherein the steel sheet has excellent strength-hole expansion ratio balance and shape freezing property.
[0016]
(6) As a chemical component, in mass%
Further, one or two of Ca, REM,
0.01% or less in Ca,
0.05% or less in REM,
The high-strength steel sheet for processing according to any one of (1) to (5) above, which is excellent in strength-hole expansion ratio balance and shape freezing property.
[0017]
(7) As a chemical component, in mass%,
N: The high-strength steel sheet for processing according to any one of the above (1) to (6), which contains 0.02% or less, and is excellent in balance between strength-hole expansion ratio and shape freezing property.
[0018]
(8) The strength-hole expansion ratio balance and shape according to any one of the above (1) to (7), wherein the deviation ΔTS of the tensile strength TS in the width direction of the steel sheet is 100 MPa or less. High strength steel plate for processing with excellent freezing properties.
[0019]
(9) In mass%,
C: 0.02 to 0.16%,
P ≦ 0.010%,
Including S ≦ 0.003%,
Containing one or two of Si and Al in a total amount of 0.2 to 4%,
One or more of Mn, Ni, Cr, Mo, and Cu are contained in a total amount of 0.5 to 4%;
C / (Si + Al + P) is 0.1 or less,
A method for producing a steel sheet comprising a balance of Fe and unavoidable impurities, wherein when melting molten steel, the molten steel is refluxed 1.5 times or more after the addition of a desulfurization flux at the time of desulfurization of the molten steel, and further obtained after casting the molten steel. When hot rolling the obtained slab to produce a steel sheet, finish rolling is performed at a finishing inlet temperature ≧ 1000 ° C. and a finishing outlet temperature> 920 ° C., and the steel sheet obtained at 600 ° C. or lower is wound. A method for producing a high-strength steel sheet for processing excellent in strength-hole expansion ratio balance and shape freezing property.
[0020]
(10) As a chemical component, in mass%
Further, it is excellent in strength-hole expansion ratio balance and shape freezing property as described in (9) above, wherein one or more of Nb, V, and Ti are contained in a total amount of 0.3% or less. Manufacturing method of high strength steel sheet for processing.
[0021]
(11) As a chemical component, in mass%
The method for producing a high-strength steel sheet for processing according to the above (9) or (10), further comprising B in an amount of 0.01% or less, which is excellent in strength-hole expansion ratio balance and shape freezing property.
[0022]
(12) As a chemical component, in mass%
Furthermore, one or two kinds of Ca and REM are
0.01% or less in Ca,
0.05% or less in REM,
The method for producing a high-strength steel sheet for processing according to any one of the above (9) to (11), which is excellent in strength-hole expansion ratio balance and shape freezing property.
[0023]
(13) The strength-hole expansion ratio balance and shape freezing according to any one of the above (9) to (12), wherein the temperature deviation in the sheet width direction on the finish rolling output side is set to 20 ° C. or less. For manufacturing high-strength steel sheets for processing with excellent heat resistance.
[0024]
(14) Among the above (9) to (12), the steel sheet is heated before or during finish rolling so that the temperature deviation in the sheet width direction at the finish rolling output side is 20 ° C. or less. A method for producing a high-strength steel sheet for processing according to any of the above, which is excellent in strength-hole expansion ratio balance and shape freezing property.
[0025]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in detail.
[0026]
First, the chemical composition of the steel sheet will be described. In addition, “%” of the unit of the chemical component described below means “% by mass”.
[0027]
C is an element necessary for securing strength, and is set to 0.02% or more. Preferably, it is 0.05% or more. However, the upper limit is set to 0.16% or less in order to avoid deterioration of weldability and to avoid an adverse effect on the hole expansion rate. Preferably it is 0.12% or less.
[0028]
[Table 1]
P is a very important element in the additive element of the present invention. FIG. 1 shows the effect. FIG. 1 shows the results of an investigation on the relationship between the P concentration and the hole expansion ratio of a steel sheet using the steel sheet of the steel No. 1 in Table 1. The hole expansion ratio was determined from the Japan Iron and Steel Federation Standard JFS T1001-1996. As shown in FIG. 1, when P is set to 0.010% or less, the hole expanding ratio is remarkably improved exponentially, and an effect on the hole expanding ratio which cannot be expected in the conventional extension is recognized. This makes it possible to avoid press cracking. Although the reason is still not clear, it is considered that P present in the grain boundary causes an adverse effect upon fracture. By reducing P, the end face properties of the punched hole are improved (minimizing the fracture surface size of the fracture surface, It is considered that the reduction in roughness and the reduction in microcracks, the suppression of processing deterioration of the microstructure of the sheared surface, and the like, and the improvement in the hole expansion rate were achieved.
[0030]
Further, in order to improve the hole expansion rate, it is necessary to suppress the occurrence of press cracks and the propagation of cracks. For this purpose, suppression of carbide formation is effective, and the value of C / (Si + Al + P) as an index for suppressing carbide formation must be 0.1 or less. If the value of C / (Si + Al + P) exceeds 0.1, a steel sheet having both excellent tensile strength × hole expansion ratio and excellent tensile strength × total elongation, which is the object of the present invention, cannot be obtained.
[0031]
The content of S is set to 0.003% or less (preferably ≦ 0.001%) from the viewpoint of the hole expansion rate and the prevention of deterioration of weldability due to the sulfide-based inclusions.
[0032]
Si and Al suppress generation of carbides and strengthen ferrite, thereby contributing to reducing the difference in hardness between ferrite and bainite and improving the uniformity of the structure. It also acts as a deoxidizing element. From the above viewpoint, the total lower limit of the addition of one or two of Si and Al needs to be 0.2% or more. In consideration of cost and effect, the total upper limit of the addition is set to 4% or less.
[0033]
Regarding the individual addition amounts of Si and Al, the following may be added.
[0034]
If particularly excellent surface properties are required, Si scale is avoided by setting Si ≦ 0.1% (preferably Si ≦ 0.01%), or conversely, Si> 1.0% ( The Si scale may be rendered harmless (preferably, Si> 1.2%) (to make it less noticeable).
[0035]
It is also possible to increase the amount of Al added and decrease the amount of Si added from the viewpoint of the material when it is desired to reduce the tensile strength by utilizing the difference in the ferrite strengthening action between Si and Al.
[0036]
Al ≦ 0.2% (preferably Al ≦ 0.1%) may be set in relation to steelmaking disadvantages such as refractory erosion and nozzle clogging and materials.
[0037]
Mn, Ni, Cr, Mo, and Cu are strengthening elements. From the above viewpoint, the total lower limit of the addition of one or more of them must be 0.5% or more. However, in view of the balance between cost and effect, the total upper limit is 4% or less.
[0038]
Further, one or more of Nb, V, Ti, B, Ca, and REM may be added as a selection element.
[0039]
Nb, V, and Ti are effective elements for increasing the strength. However, in view of the balance between the effect and the cost, one or more of them are added in a total amount of 0.3% or less.
[0040]
B has a function as a strengthening element, and may be added in an amount of 0.01% or less. It also has the effect of reducing the adverse effects of P.
[0041]
Ca may be added in an amount of 0.01% or less in order to further improve the hole expansion rate by controlling the form (spheroidization) of the sulfide-based inclusions.
[0042]
Also, REM may be added at 0.05% or less for the same reason.
[0043]
In addition, N may be contained in an amount of 0.02% or less as required for the purpose of stabilizing austenite, increasing strength, and the like.
[0044]
Next, the microstructure will be described.
[0045]
In order to obtain an excellent hole expansion ratio, the uniformity and size and amount of the microstructure, and the control of the amount and size of inclusions are particularly important from the viewpoint of not impairing the end surface properties of the punched hole improved by extremely low P. This is an important point. First, this is described.
[0046]
The uniformity of the microstructure affects the properties of the end face of the punched hole (such as minimizing the fracture surface size of the fracture surface, reducing roughness, reducing microcracks, etc., and suppressing processing deterioration of the microstructure of the shear surface). It greatly affects the spread rate.
[0047]
FIG. 2 shows the results of investigating the relationship between the maximum length of the microstructure of pearlite, martensite, and retained austenite in the steel sheet and the hole expansion ratio of the steel sheet, using the steel sheet having the
[0048]
Further, by setting the total area ratio of one or two types of ferrite and bainite to 80% or more (preferably 90% or more), the homogeneity of the structure is enhanced, and the hard structure other than the ferrite and bainite is linked or formed into a network. By suppressing the deterioration of press formability due to the presence in the shape, excellent strength-elongation balance and excellent strength-hole expansion balance can be obtained.
[0049]
If the total area ratio of one or two of ferrite and bainite is less than 80%, the uniformity of the structure is deteriorated, and the effect of improving the formability cannot be stably obtained. Preferably, the lower limit is 90%.
[0050]
Furthermore, in order to form a more uniform structure and to further improve the hole expansion rate, the ratio of bainite hardness (Hv) / ferrite hardness (Hv) is preferably set to 1.0 to 1.3.
[0051]
In order to increase elongation, it is more advantageous to include retained austenite. However, for the reasons described above, the upper limit is set to less than 3%.
[0052]
When a low yield ratio (yield ratio YR = yield stress / tensile strength × 100 × less than 75%) is desired from the viewpoint of shape freezing property, etc., it is more advantageous to include martensite. However, for the reasons described above, the upper limit is set to less than 3%.
[0053]
The maximum length of pearlite, martensite and retained austenite was 1000 times as large as that obtained by corroding the cross section in the rolling direction of the steel sheet with the reagent disclosed in JP-A-59-219473 and the reagent disclosed in JP-A-5-163590. Was calculated by taking into account the entire cross section in the plate thickness direction from the optical micrograph of the above.
[0054]
In addition, in the inclusion control, the hole expansion rate can be improved by reducing the number of coarse inclusions. For the inclusions, the cross section in the rolling direction of the polished steel sheet was observed with a microscope (400 times magnification), and the number of coarse inclusions having a maximum length of 20 microns or more was integrated. FIG. 3 shows the results of investigation on the relationship between the number of coarse inclusions (maximum length of 20 μm or more) in the steel sheet and the hole expansion ratio using the steel sheet having the composition of steel No. 2 in Table 1. It can be seen that when the number of coarse inclusions (
[0055]
Due to the effects described above, excellent strength-hole expansion balance (tensile strength × hole expansion ratio of 40,000 MPa ·% or more, preferably 50,000 MPa ·% or more) and excellent strength-elongation balance (tensile strength × total elongation of 10,000 MPa) % Or more, preferably 15000 MPa ·% or more, more preferably 17000 MPa ·% or more), and the press formability is greatly improved.
[0056]
Next, the manufacturing method will be described.
[0057]
First, in the steel making process, the point is that when the molten steel is smelted, the molten steel is circulated 1.5 times or more after the desulfurization flux is added during the desulfurization of the molten steel using a secondary scouring device such as RH. The term “reflux of molten steel” as used herein refers to the amount of molten steel circulated in a secondary scouring device such as RH per unit time, and there are various calculation formulas. (Refer to the Japan Iron and Steel Association, High-Temperature Refining Process Subcommittee Refining Forum, Japan Society for the Promotion of Science, Steelmaking 19th Committee, Reaction Process Study Group, March 1996, pp. 184-187). The molten steel reflux amount Q represented by 1) is defined as one time.
Reflux amount Q = 11.4 × V 1/3 × D 4/3 × ｛ln (P 1 / P 0 )｝ 1/3 × k (Equation 1) was set to once.
Q: molten steel ring flow rate (t / min), V: reflux gas flow rate (Nl / min)
D: Immersion tube inner diameter (m), P 0 : Pressure in vacuum chamber (Pa)
P 1 ： Reflux gas injection position pressure (Pa),
k: constant (constant by secondary scouring device; this time is 4)
[0058]
Here, FIG. 4 shows a schematic diagram of the molten steel smelting using RH. Two immersion pipes 3 of the
[0059]
FIG. 5 shows the number of times of recirculation of molten steel after the addition of desulfurization flux when the molten steel of the component of steel No. 2 in Table 1 was melted, and the cross section of the steel plate after hot rolling from the cast slab after casting the molten steel. The result of investigating the relationship with the number of inclusions of 20 microns or more per unit is shown. As shown in FIG. 5, when the number of reflux times is 1.5 or more, the floating of the flux-based inclusions for desulfurization is remarkably promoted, and the number of coarse inclusions (20 μm or more) is less than a certain number (0.3 or less per square mm). ), And the hole expansion rate can be improved.
[0060]
Next, when the steel of the present invention was obtained from a hot-rolled steel sheet, the temperature conditions of finish rolling in the hot rolling step were examined. FIGS. 6 (a) to 6 (e) show the finish side temperature (° C.), tensile strength (TS), hole expansion ratio (%), elongation (%), tensile strength × hole expansion ratio (MPa), and tensile strength. It is a figure which shows each relationship with strength x elongation (MPa). In the region where the finishing temperature is 920 ° C. or lower, TS increases as the finishing temperature increases, and the hole expansion ratio and elongation deteriorate. Therefore, the high strength (TS) and moldability (hole expansion ratio, elongation) ) Cannot be achieved. However, when the temperature on the finishing side exceeds 920 ° C., as the temperature on the finishing side becomes higher, the tendency of the tensile strength to increase becomes remarkable, and the hole expansion ratio turns to the tendency to improve. Therefore, when the finishing temperature is higher than 920 ° C., it is possible to achieve both high strength and an excellent hole expanding ratio, which were impossible in the past. On the other hand, the elongation tends to deteriorate even in a region exceeding 920 ° C., but the strength-elongation balance (TS × elongation) eases the deterioration tendency and turns to a favorable tendency. That is, the uniformity of the microstructure and the like are remarkably improved from 920 ° C., and the moldability is remarkably improved.
[0061]
From these results, when the finishing temperature is higher than 920 ° C., an excellent strength-hole expanding balance and an excellent strength-elongation balance can be obtained.
[0062]
Therefore, in the present invention, the finishing side temperature is set to more than 920 ° C. In addition, the temperature on the finishing side needs to be 1000 ° C. or higher from the viewpoint of the uniformity of the structure and the like.
[0063]
Further, in order to make the deviation ΔTS of the tensile strength TS in the sheet width direction of the
[0064]
As a result, a remarkable improvement can be achieved also in the shape freezing property, which has been a major problem in molding when applying high tensile strength. That is, the reduction of ΔTS makes it possible to improve the dimensional accuracy deterioration (so-called shape freezing deterioration) such as an increase in springback in the press-formed product.
[0065]
In order to reduce the maximum length of pearlite, martensite, and retained austenite, the rolling reduction in finish rolling is desirably 90% or more.
[0066]
The conditions in the cooling table after finish rolling are not particularly defined, but in order to improve the uniformity of the structure and promote the miniaturization of the microstructure, means for increasing the cooling rate such as immediately cooling or strong cooling at the finish rolling exit side is used. May be implemented. Further, in order to control the microstructure area ratio, generally known multi-stage control of the cooling rate (combination of rapid cooling, slow cooling, and isothermal holding) or winding temperature control may be performed.
[0067]
The upper limit of the winding temperature is set to 600 ° C. in order to suppress generation of carbides and microstructure coarsening which are harmful to the improvement of the hole expansion ratio. If the winding temperature exceeds 600 ° C., an excellent strength-hole expansion balance (tensile strength × hole expansion ratio) cannot be obtained. The strength-hole expanding balance tends to be improved when the winding temperature is lower, and is preferably 450 ° C. or less from the viewpoint of the strength-hole expanding balance. In addition, from the viewpoint of strength-elongation balance, the temperature tends to deteriorate at 350 ° C. or lower. Therefore, 350-450 ° C. is preferable from the viewpoint of achieving both hole expansion and elongation.
[0068]
Further, the cooling of the steel sheet after winding may be performed by allowing it to be cooled or by forced cooling.
[0069]
The steel slab to be rolled may be any of so-called cold slab reheating, HCR, and HDR. Also, a steel slab by so-called thin continuous casting may be used.
[0070]
Further, the steel sheet according to the present invention may be plated with Zn or the like to improve corrosion resistance, or a lubricant or the like may be applied to further improve press formability.
[0071]
【Example】
Table 2 shows the chemical components of the test steel other than Fe.
[0072]
Table 3 shows the manufacturing conditions in steel making and hot rolling of the test steel.
[0073]
In addition, characteristic evaluation and microstructure evaluation were implemented by the following methods.
[0074]
The tensile test was conducted according to JIS No. 5, and the tensile strength (TS), the yield strength (YS), the yield ratio (YR = YS / TS × 100), the total elongation (T.EL), and the strength-elongation balance (TS × T) .EL).
[0075]
The hole expansion ratio was determined according to the Japan Iron and Steel Federation Standard JFS T1001-1996.
[0076]
The identification of the structure of the microstructure and the measurement of the area ratio, and the measurement of the maximum length of pearlite, retained austenite and martensite were carried out using a nitral reagent, a reagent disclosed in JP-A-59-219473 and JP-A-5-163590. The cross section in the rolling direction of the steel sheet was corroded by the reagent disclosed in Japanese Patent Laid-Open Publication No. H07-163, and an optical microscope photograph at a magnification of 1000 times and X-ray analysis were used.
[0077]
For inclusions in the steel sheet, the cross section in the rolling direction of the polished steel sheet was observed with a microscope (400 times magnification), and the number of coarse inclusions having a maximum length of 20 microns or more was integrated.
[0078]
When calculating the retained austenite area ratio (Fγ: unit is%) by X-ray analysis, it was calculated from Mo-Kα radiation according to the following equation.
Fγ (%) = (2/3) ｛100 / (0.7 × α (211) / γ (220) +1)｝ + (1/3) ｛100 / (0.78 × α (211) / γ) (311) +1)｝
Here, α (211), γ (220), α (211), γ (311) indicate surface strength.
[0079]
[Table 2]
[Table 3]
[Table 4]
[Table 5]
In the present invention examples (Steel No. 1, 1, 6 and 2 to 10), a press having both excellent strength-hole expansion balance (TS × λ) and excellent strength-elongation balance (TS × T · EL) is used. A hot-rolled high-strength steel sheet excellent in formability has been obtained.
[0084]
On the other hand, the comparative examples (Steel No. 1 of 2-1 to 5, 1 of 7 and 11 to 12) are out of the scope of the present invention as described in Tables 2 to 5, respectively, and therefore, only those having low mechanical properties are used. Could not be obtained.
[0085]
【The invention's effect】
The present invention makes it possible to provide a high-strength steel sheet for processing excellent in strength-hole expansion ratio balance and shape freezing property and a method for producing the same at low cost and in a stable manner, so that the uses and conditions of use are greatly expanded. The industrial and economic effects are very large.
[Brief description of the drawings]
FIG. 1 is a diagram showing the effect of a chemical component P on the hole expansion ratio.
FIG. 2 is a view showing the effect of the maximum length of a microstructure (pearlite, martensite, retained austenite) on the hole expansion ratio.
FIG. 3 is a diagram showing the effect of the number of inclusions on the hole expansion ratio.
FIG. 4 is a schematic diagram of molten steel smelting when RH is used.
FIG. 5 is a diagram showing the influence of the number of times of molten steel reflux after addition of a desulfurization flux on the number of inclusions.
FIG. 6 is a diagram showing the relationship between the properties of a steel sheet and the finishing side temperature.
[Explanation of symbols]
1 molten steel pot
2 Degassing tank
3 Immersion tube
4 Injection lance
Claims (14)
Ｃ：０．０２〜０．１６％、
Ｐ≦０．０１０％、
Ｓ≦０．００３％、
ＳｉとＡｌの内の１種又は２種を合計量で０．２〜４％、
Ｍｎ、Ｎｉ、Ｃｒ、Ｍｏ、Ｃｕの内の１種又は２種以上を合計量で０．５〜４％を含み、
Ｃ／（Ｓｉ＋Ａｌ＋Ｐ）が０．１以下で、
残部Ｆｅ及び不可避的不純物よりなる鋼板であって、該鋼板断面のミクロ組織として、マルテンサイトと残留オーステナイトの内の１種又は２種を合計面積率で３％未満、フェライトとベイナイトの内の１種又は２種を合計面積率で８０％以上、残部がパーライトよりなると共に、パーライト、マルテンサイト、残留オーステナイトの最大長が１０ミクロン以下であり、さらに、鋼板断面内に２０ミクロン以上の介在物が１平方ｍｍ当たり０．３ケ以下であることを特徴とする強度−穴広げ率バランスと形状凍結性に優れた加工用高強度鋼板。As a chemical component, C: 0.02 to 0.16% by mass%,
P ≦ 0.010%,
S ≦ 0.003%,
0.2 to 4% in total of one or two of Si and Al,
One or more of Mn, Ni, Cr, Mo, and Cu containing 0.5 to 4% in total amount;
C / (Si + Al + P) is 0.1 or less,
A steel sheet comprising a balance of Fe and unavoidable impurities, wherein as a microstructure of a cross section of the steel sheet, one or two of martensite and retained austenite are less than 3% in total area ratio and one of ferrite and bainite. 80% or more of the total area ratio of the seeds or two kinds, the balance being pearlite, the maximum length of pearlite, martensite, and retained austenite is 10 microns or less, and further, inclusions of 20 microns or more are present in the cross section of the steel sheet. A high-strength steel sheet for processing excellent in strength-hole expansion ratio balance and shape freezing property, wherein the number is 0.3 or less per square mm.
さらにＮｂ、Ｖ、Ｔｉの内の１種又は２種以上を合計量で０．３％以下含むことを特徴とする請求項１〜３に記載の強度−穴広げ率バランスと形状凍結性に優れた加工用高強度鋼板。The strength-hole expanding ratio according to any one of claims 1 to 3, wherein one or more of Nb, V, and Ti are further contained as a chemical component in an amount of 0.3% or less in total. High-strength steel sheet for processing with excellent balance and shape freezing properties.
さらにＢを０．０１％以下含むことを特徴とする請求項１〜４の内のいずれかに記載の強度−穴広げ率バランスと形状凍結性に優れた加工用高強度鋼板。The processing height according to any one of claims 1 to 4, further comprising 0.01% or less of B by mass% as a chemical component. Strength steel plate.
さらにＣａ、ＲＥＭの内の１種又は２種を、
Ｃａにおいては０．０１％以下、
ＲＥＭにおいては０．０５％以下、
含むことを特徴とする請求項１〜５の内のいずれかに記載の強度−穴広げ率バランスと形状凍結性に優れた加工用高強度鋼板。As a chemical component, one or two of Ca and REM are further added in mass%.
0.01% or less in Ca,
0.05% or less in REM,
The high-strength steel sheet for processing according to any one of claims 1 to 5, wherein the high-strength steel sheet has an excellent balance between a strength and a hole expansion ratio and excellent shape freezing properties.
Ｎ：０．０２％以下を含むことを特徴とする請求項１〜６の内のいずれかに記載の強度−穴広げ率バランスと形状凍結性に優れた加工用高強度鋼板。7. The process according to claim 1, further comprising N: 0.02% or less by mass as a chemical component. For high strength steel sheet.
Ｃ：０．０２〜０．１６％、
Ｐ≦０．０１０％、
Ｓ≦０．００３％を含み、
ＳｉとＡｌの内の１種又は２種を合計量で０．２〜４％含み、
Ｍｎ、Ｎｉ、Ｃｒ、Ｍｏ、Ｃｕの内の１種又は２種以上を合計量で０．５〜４％含み、
Ｃ／（Ｓｉ＋Ａｌ＋Ｐ）が０．１以下で、
残部Ｆｅ及び不可避的不純物よりなる鋼板の製造方法であって、溶鋼を溶製するに際し、溶鋼脱硫時の脱硫用フラックス添加後に１．５回以上、溶鋼を環流させ、さらに該溶鋼の鋳造後に得られた鋼片を熱間圧延して鋼板を製造するに際し、仕上圧延を仕上入側温度≧１０００℃、かつ仕上出側温度＞９２０℃で実施し、６００℃以下で得られた鋼板を巻き取ることを特徴とする強度−穴広げ率バランスと形状凍結性に優れた加工用高強度鋼板の製造方法。In mass%,
C: 0.02 to 0.16%,
P ≦ 0.010%,
Including S ≦ 0.003%,
Containing one or two of Si and Al in a total amount of 0.2 to 4%,
One or more of Mn, Ni, Cr, Mo, and Cu are contained in a total amount of 0.5 to 4%;
C / (Si + Al + P) is 0.1 or less,
A method for producing a steel sheet comprising a balance of Fe and unavoidable impurities, wherein when melting molten steel, the molten steel is refluxed 1.5 times or more after the addition of a desulfurization flux at the time of desulfurization of the molten steel, and further obtained after casting the molten steel. When hot rolling the obtained slab to produce a steel sheet, finish rolling is performed at a finishing inlet temperature ≧ 1000 ° C. and a finishing outlet temperature> 920 ° C., and the steel sheet obtained at 600 ° C. or lower is wound. A method for producing a high-strength steel sheet for processing excellent in strength-hole expansion ratio balance and shape freezing property.
さらにＮｂ、Ｖ、Ｔｉの内の１種又は２種以上を合計量で０．３％以下含むことを特徴とする請求項９に記載の強度−穴広げ率バランスと形状凍結性に優れた加工用高強度鋼板の製造方法。The strength-hole expansion ratio balance according to claim 9, wherein one or more of Nb, V, and Ti are further included as a chemical component in an amount of 0.3% or less in total. A method for manufacturing high-strength steel sheets for processing with excellent shape freezing properties.
さらにＢを０．０１％以下含むことを特徴とする請求項９又は１０に記載の強度−穴広げ率バランスと形状凍結性に優れた加工用高強度鋼板の製造方法。The method for producing a high-strength steel sheet for processing according to claim 9 or 10, further comprising 0.01% or less by mass of B as a chemical component. .
さらにＣａ、ＲＥＭの１種又は２種を、
Ｃａにおいては０．０１％以下、
ＲＥＭにおいては０．０５％以下、
含むことを特徴とする請求項９〜１１の内のいずれかに記載の強度−穴広げ率バランスと形状凍結性に優れた加工用高強度鋼板の製造方法。As a chemical component, one or two of Ca and REM are further added in mass%.
0.01% or less in Ca,
0.05% or less in REM,
The method for producing a high-strength steel sheet for processing according to any one of claims 9 to 11, wherein the high-strength steel sheet has excellent strength-hole expansion ratio balance and shape freezing properties.
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