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JP2004211157A - High-strength high-ductility hot dip galvanized steel sheet and its production method - Google Patents

High-strength high-ductility hot dip galvanized steel sheet and its production method Download PDF

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Publication number
JP2004211157A
JP2004211157A JP2002382250A JP2002382250A JP2004211157A JP 2004211157 A JP2004211157 A JP 2004211157A JP 2002382250 A JP2002382250 A JP 2002382250A JP 2002382250 A JP2002382250 A JP 2002382250A JP 2004211157 A JP2004211157 A JP 2004211157A
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Japan
Prior art keywords
steel sheet
strength
dip galvanized
mass
hot
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JP2002382250A
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JP3887308B2 (en
Inventor
Masashi Azuma
昌史 東
Nobuhiro Fujita
展弘 藤田
Naoki Yoshinaga
直樹 吉永
Manabu Takahashi
学 高橋
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Nippon Steel Corp
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Nippon Steel Corp
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Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-strength high-ductility hot-dip galvanized steel sheet having a strength of >980 MPa, and its production method. <P>SOLUTION: The high-strength high-ductility hot-dip galvanized steel sheet has a tensile strength of >980 MPa and has a hot dip galvanized layer comprising, by mass, 0.001-0.5% Al and 5-20% Fe and the balance being Zn and unavoidable impurities on the surface of a steel sheet. The steel sheet comprises, by mass, 0.0001-0.3% C, 0.1-0.1% Si, 0.001-3% Mn, 0.1-4% Al, 0.0001-0.3% P, ≤0.01% S, 0.01-4% total of Ni and/or Cu, 0.001-1% total of Ti and/or Nb and the balance being iron and unavoidable impurities and has a microstructure containing 50 vol.% ferrite in total as a main phase and ≥3% and <50% austenite and the remaining structure comprising bainite or martensite and bainite. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

【0001】
【発明の属する技術分野】
本発明は、自動車、建材、家電製品などに適する加工性に優れた高強度高延性溶融亜鉛めっき鋼板及びその製造方法に関する。
【0002】
【従来の技術】
自動車のクロスメンバーやサイドメンバー等の部材は、近年の燃費軽量化の動向に対応すべく軽量化が検討されており、材料面では、薄肉化しても強度及び衝突安全性が確保されるという観点から、鋼板の高強度化が進められている。
【0003】
しかしながら、材料の成形性は、強度が上昇するのに伴って劣化するので、上記部材の軽量化を実現するには、成形性と高強度の両方を満足する鋼板を製造する必要がある。
【0004】
成形性を表す指標としては、引張り試験の際の伸び(El.)が用いられ、この値が高いことが、成形性の指標の一つとして用いられている。また、強度としては、引張り試験の際の最大応力である引張強さ(TS)が、その値として用いられており、これらの積(TS×El.)が21000(MPa・%)を上回る鋼板が、高強度かつ高延性鋼板として知られている。
【0005】
これらの特性を有する鋼板としては、特許文献1や特許文献2に開示されているような、残留オーステナイトを含む鋼板がある。
【0006】
これら残留オーステナイトを含む鋼板は、その成分元素として、CとSi及びMnのみを基本的な合金元素とし、冷延鋼板の場合、二相域で焼鈍後、300〜450℃程度の温度範囲でベイナイト変態を行うことを特徴とする熱処理により、残留オーステナイトを金属中に含む鋼板であり、これら鋼板は、残留オーステナイトを含む組織を作るために、特許文献3や特許文献4で開示しているように、Si又はAlを含有させる必要がある。
【0007】
これらの鋼板は、金属組織に含まれている残留オーステナイトを、プレス成形時に、応力誘起変態によりマルテンサイトに変態させることで、優れた延性を得ているものであるから、特許文献5や特許文献6に開示されているように、残留オーステナイトの安定性の向上と体積率増加に着目した研究が、これまで数多く行われてきた。
【0008】
【特許文献1】
特開平1−230715号公報
【特許文献2】
特開平2−217425号公報
【特許文献3】
特開平3−265337号公報
【特許文献4】
特開平5−195143号公報
【特許文献5】
特開平10−130776号公報
【特許文献6】
特開平4−98859号公報
【特許文献7】
特開平3−28359号公報
【特許文献8】
特開平3−64437号公報
【0009】
【発明が解決しようとする課題】
また、自動車用鋼板の高性能化を反映して、耐食性及び外観向上を目的に、自動車部材のめっき化が進んでいるが、現在は、車内に装着される特定の部材を除いて、多くの部材に、溶融亜鉛めっき鋼板が使用されている。
【0010】
しかしながら、これら鋼板は、Siを多量に含むため、鋼板表面が酸化し易く、溶融亜鉛めっきの際に、微少不めっきを生じ、合金化後の加工部において、めっき密着性が劣る等の問題がある。
【0011】
これらの問題を解決するための手段としては、例えば、特許文献7や特許文献8には、鋼板表面に、0.002〜2.0g/mm程度のNi、Cu、Co、Feの単独又は複合めっきを行うことで、めっき性を改善しているが、この方法では、溶融亜鉛めっきライン前段に新たにめっき設備を設けるか、もしくは、あらかじめ電気めっきラインにおいてめっき処理を行わねばならず、大幅なコストアップを招くという問題を有していた。
【0012】
本発明は、上記課題を解決し、めっき密着性に優れた高強度合金化溶融亜鉛めっき鋼板、ならびに、めっき密着性に優れた高強度溶融亜鉛めっき鋼板、及び、その製造法を提供することを目的とする。
【0013】
【課題を解決するための手段】
本発明者等は、種々検討を行った結果、めっき性に関しては、鋼中に、Ni又はCuを単独あるいは複合添加することで、高強度鋼板の溶融亜鉛めっき濡れ性を確保できること、及び、合金化めっきにおける合金化が促進されることを見出した。この効果は、鋼中のこれら元素の含有量を0.001%以上とすることで発揮される。
【0014】
また、鋼中に、主相としてフェライトを体積分率で50%以上含有し、残留オーステナイトを3〜50%未満含有し、残部組織を、マルテンサイト及びベイナイトとすることで、強度−延性バランスに優れた高強度高延性鋼板が製造可能であることを見出した。
【0015】
さらには、鋼中に、Ti、Nbの1種又は2種を添加すると共に、熱延後の焼鈍工程での最高加熱温度を、Ac1+30(℃)以上、1(℃/秒)以上の冷却速度で650〜800(℃)の温度域に冷却し、引き続いて、1〜100(℃/秒)の冷却速度で、Znめっき浴温度〜Znめっき浴温度+100(℃)まで冷却した後、350〜Znめっき浴温度+100(℃)の温度域で、めっき浴の浸漬時間を含めて1〜3000秒保持した後、Znめっき浴に浸漬して、その後、室温まで冷却するか、あるいは、めっき浴温度〜100℃の温度範囲で保持を行うことで、主相であるフェライトの平均粒径を5μm以下とし、さらに、強度−延性バランスに優れた高強度高延性鋼板が製造可能であることを見出した。
【0016】
本発明は、上記知見に基づいて完成されたもので、その要旨とするところは、以下の通りである。
【0017】
[1] 質量%で、
C:0.0001〜0.3%、
Si:0.1〜4%、
Al:0.001〜4%、
Mn:0.001〜3%、
P:0.0001〜0.3%、
S:0.01%以下を含有し、Ni、Cuの1種又は2種を合計で0.001〜4%含有し、さらに、Ti、Nbの1種又は2種を合計で0.001〜1%含有し、残部鉄及び不可避的不純物からなり、ミクロ組織が、体積分率で、主相としてフェライトを50%以上含有し、オーステナイトを3〜50%未満含有し、残部組織が、ベイナイト、又は、マルテンサイト及びベイナイトよりなる鋼板の表面に、質量%で、
Al:0.001〜0.5%、
Fe:5〜20%を含有し、残部がZn及び不可避不純物からなる溶融亜鉛めっき層を有し、引張強さが980MPa超であることを特徴とする高強度高延性溶融亜鉛めっき鋼板。
【0018】
[2] 質量%で、
C:0.0001〜0.3%、
Si:0.1〜4%、
Al:0.001〜4%、
Mn:0.001〜3%、
P:0.0001〜0.3%、
S:0.01%以下を含有し、Ni、Cuの1種又は2種を合計で0.001〜4%含有し、さらに、Ti、Nbの1種又は2種を合計で0.001〜1%含有し、残部鉄及び不可避的不純物からなり、ミクロ組織が、体積分率で、主相としてフェライトを50%以上含有し、オーステナイトを3〜50%未満含有し、残部組織が、ベイナイト、又は、マルテンサイト及びベイナイトよりなる鋼板の表面に、質量%で、
Al:0.001〜0.5%、
Fe:5%未満を含有し、残部がZn及び不可避不純物からなる溶融亜鉛めっき層を有し、引張強さが980MPa超であることを特徴とする高強度高延性溶融亜鉛めっき鋼板。
【0019】
[3] さらに、鋼中に、質量%で、
Mo:0.001〜4%、
Cr:0.001〜4%、
Co:0.001〜4%
の1種又は2種以上を含有することを特徴とする[1]又は[2]に記載の高強度高延性溶融亜鉛めっき鋼板。
【0020】
[4] 前記めっき層中のCu含有率(質量%)と鋼中のCu含有率(質量%)の比をa、めっき層中のNi含有率(質量%)と鋼中のNi含有率(質量%)の比をbとすると
0.0001≦a+b≦0.2
を満たすことを特徴とする[1]〜[3]のいずれかに記載の高強度高延性溶融亜鉛めっき鋼板。
【0021】
[5] さらに、鋼中に、質量%で、
V:0.001〜1%を含有することを特徴とする[1]〜[4]のいずれかに記載の高強度高延性溶融亜鉛めっき鋼板。
【0022】
[6] さらに、鋼中に、質量%で、
B:0.0001〜0.1%を含有することを特徴とする[1]〜[5]のいずれかに記載の高強度高延性溶融亜鉛めっき鋼板。
【0023】
[7] さらに、鋼中に、質量%で、Mg、Zr、Ca、Y、Hf、La−Ceの元素群中から1種又は2種以上を、合計で0.001〜1%含有することを特徴とする[1]〜[6]のいずれかに記載の高強度高延性溶融亜鉛めっき鋼板。
【0024】
[8] 前記めっき鋼板の垂直断面において、めっき相/鋼板界面から、MgO、CaO、ZrO、CeO、La、HfO、TiO、Y、SiO、MnO、及び、Alの1種又は2種以上の内部酸化物が存在する最大深さまでの範囲における鋼板中に、MgO、CaO、ZrO、CeO、La、HfO、TiO、Y、SiO、MnO、及び、Alの1種又は2種以上が、面積率で1%以上含まれることを特徴とする[1]〜[7]のいずれかに記載の高強度溶融亜鉛めっき鋼板。
【0025】
[9] 前記めっき鋼板の垂直断面において、めっき相/鋼板界面から10μmまでの鋼板中に、MgO、CaO、ZrO、CeO、La、HfO、TiO、Y、SiO、MnO、及び、Alの1種又は2種以上の合計を、面積率で0.1%以上含むことを特徴とする[1]〜[8]のいずれかに記載の高強度高延性溶融亜鉛めっき鋼板。
【0026】
[10] 前記フェライトの体積率80%以上の平均粒径が、5μm以下であることを特徴とする[1]〜[9]のいずれかに記載の高強度高延性溶融亜鉛めっき鋼板。
【0027】
[11] [1]〜[7]のいずれかに記載の成分組成からなる鋳造スラブを、鋳造まま、又は、一旦冷却した後に再度加熱し、熱延後巻き取った熱延鋼板を酸洗後冷延した後、めっきラインにおける酸化帯において、燃焼空気比0.9〜1.5の雰囲気内にて酸化せしめ、その後、還元帯にて、水分圧と水素分圧の常用対数log(PHO/PH)が下記(1)式を満たす還元雰囲気にて、Ac1+30(℃)以上、Ac3−10(℃)以下で焼鈍した後に、0.1℃/秒以上の冷却速度で650〜800℃の温度域に冷却し、引き続いて、1〜100℃/秒の冷却速度で、Znめっき浴温度〜Znめっき浴温度+100(℃)まで冷却した後、350〜Znめっき浴温度+100(℃)の温度域で、後続のめっき浴の浸漬時間を含めて1〜3000秒間保持した後、Znめっき浴に浸漬して、その後、室温まで冷却し、980MPa超の引張強さを有する高強度高延性溶融亜鉛めっき鋼板を製造することを特徴とする高強度高延性溶融亜鉛めっき鋼板の製造方法。
−0.8≧log(PHO/PH) ・・・・・(1)
【0028】
[12] 前記Znめっき浴に浸漬した後、浴温〜Znめっき浴温度+100(℃)の温度範囲で1〜300秒間の保持を行い、室温まで冷却することを特徴とする[11]に記載の高強度高延性溶融亜鉛めっき鋼板の製造方法。
【0029】
【発明の実施の形態】
以下に本発明の詳細を説明する。
【0030】
本発明の鋼板は、鋼板の成分組成と製造条件をある範囲内に制御することにより製造された鋼板であって、体積分率で、主相であるフェライトを50%以上含有し、オーステナイト3〜50%未満含有し、残部組織が、ベイナイト、又は、マルテンサイト及びベイナイトよりなる鋼板に溶融亜鉛めっきを行った引張強さ980MPa超の鋼板であり、強度と優れた延性を具備し、かつ、耐食性を兼ね備えることを特徴とするものである。
【0031】
本発明に係る高強度溶融亜鉛めっき鋼板の延性は、製品に含まれる残留オーステナイトの体積分率に左右される。金属組織に含まれている残留オーステナイトは、応力を受けていない状態では、安定に存在するものの、変形が加えられるとマルテンサイトに変態し、変態誘起塑性により優れた延性が得られる。
【0032】
しかしながら、その体積分率が3%未満では、その効果がほとんど表れないことから、下限値を3%以上とした。一方、残留オーステナイトの体積分率が50%以上になると、極端に著しい成形を加えた場合、成形後多量のマルテンサイトが存在することとなり、二次加工性や衝撃特性に問題を生じることがあるので、本発明では、その上限値を50%未満とした。
【0033】
また、主相であるフェライト、第2相の残留オーステナイト、ベイナイト及びマルテンサイト以外の相として、炭化物、合金炭化物、窒化物及び酸化物を含有しても、本発明を逸脱するものではない。
【0034】
さらに、主相であるフェライトの体積率80%以上の平均粒径を5μm以下とすることで、更なる強度−延性バランスの向上を引き起こす。ただし、この効果が顕著になるのは、平均粒径が3μm以下であることから、平均粒径は3μm以下が望ましい。一方、平均粒径の下限は、特に定めることなく本発明の効果を得ることができる。さらには、靭性及び穴拡げ性の向上へも有効である。
【0035】
なお、上記ミクロ組織の、フェライト、ベイナイト、オーステナイト、マルテンサイト、酸化物相及び残部組織の同定、存在位置の観察及び平均粒径(平均円相当径)と占積率の測定は、ナイタール試薬及び特開昭59−219473号公報に開示された試薬により、鋼板圧延方向断面又は圧延直角方向断面を腐食して、500倍〜20000倍の光学顕微鏡、走査型電子顕微鏡及び透過型電子顕微鏡観察により定量化が可能である。
【0036】
また、めっき相/鋼板界面近傍の酸化物の形態及び同定は、走査型顕微鏡、透過電子顕微鏡及びEPMAを用いて行った。
【0037】
本発明者等は、質量%で、C:0.0001〜0.3%、Si:0.1〜4%、Al:0.001〜4%、Mn:0.001〜3%、P:0.0001〜0.3%、S:0.01%以下、Ni:0.01〜4%、Cu:0.01〜4%、Ti:0.001〜1%、Nb:0.001〜1%を含有し、残部Fe及び不可避不純物よりなる鋳造スラブを、鋳造まま、あるいは、一旦冷却した後に再度加熱し、熱延後巻き取った熱延鋼板を、酸洗後冷延し、その後、酸化帯及び還元帯を有する焼鈍炉を用いて、酸化帯にて、燃焼空気比0.9〜1.2の雰囲気内にて酸化せしめ、その後、還元帯にて、水分圧と水素分圧の常用対数log(PHO/PH)が−0.8以下の範囲にて、Ac1+100 (℃)以上1000℃以下で焼鈍し、0.1〜50℃/秒の冷却速度で650〜800℃の温度域に冷却し、引き続いて、1〜100℃/秒の冷却速度で、Znめっき浴温度〜Znめっき浴温度+100(℃)まで冷却した後、350〜Znめっき浴温度+100(℃)の温度域で、後続のめっき浴の浸漬時間を含めて1〜3000秒間保持した後、Znめっき浴に浸漬して、その後、室温まで冷却した。
【0038】
さらに、一部の試料については、Znめっき浴に同条件で浸漬後、Znめっき浴温度〜550℃で5〜60秒間保持することで合金化処理を行った後、350℃までの温度範囲を0.5〜100℃/秒の冷却速度で冷却し、その後室温まで冷却した。
【0039】
上記の試験を行い、めっき表面の欠陥発生率に基づき、外観を5段階評価した。その結果、鋼板成分を上記範囲とすることで、外観欠陥のほとんど生じない評点5〜4を得ることが解った。
【0040】
評点1〜5は、めっきの外観について不めっきの発生状況を目視にて判断して決めた。評価指標は以下の通りである。
【0041】
評点5:不めっきはほとんどなし。(面積率で0.1%以下)
評点4:不めっきは微少。(面積率で0.1%超3%以下)
評点3:不めっきは少。(面積率で3%超5%以下)
評点2:不めっきは多数。(面積率で5%超50%以下)
評点1:めっき塗れず。(面積率で50%超)
めっき付着量については、特に制約は設けないが、耐食性の観点から、片面の付着量が5g/mm以上であることが望ましい。本発明の溶融亜鉛めっき鋼板上に塗装性、耐食性、溶接性を改善する目的で上層めっきを施すことや、各種処理、例えば、クロメート処理、りん酸塩処理、潤滑性向上処理、溶接性向上処理を施しても、本発明を逸脱するものではない。
【0042】
めっき層中Al量を0.001〜0.5%の範囲内としたのは、0.001%未満では、ドロス発生が顕著で良好な外観が得られないこと、0.5%を超えてAlを添加すると、合金化反応を著しく抑制してしまい、合金化溶融亜鉛めっき層を形成することが困難となるためである。
【0043】
さらに、合金化処理を施すことによってよって、めっき層中にFeが取り込まれ、塗装性やスポット溶接性に優れた高強度高延性溶融亜鉛めっき鋼板を得ることができる。めっき層中のFe量が20質量%を超えるとめっき自体の密着性を損ない、加工の際めっき層が破壊、脱落し金型に付着することで、成形時の疵の原因となる。
【0044】
一方、スポット溶接性を良好にするためにはFe量を5%以上にすることが望ましい。従って、合金化処理を行う場合、めっき層中のFe量の範囲は5〜20質量%とする。
【0045】
また、合金化処理を行わない場合めっき層中のFe量が5質量%未満でも、合金化により得られるスポット溶接を除く効果である耐食性と延性や加工性は良好である。
【0046】
次に、本発明における鋼板の成分組成の限定理由について述べる。
【0047】
C:オーステナイト安定化元素であり、二相域加熱時及びベイナイト変態温度域で、フェライトからオーステナイト中へと移動し、オーステナイト中に濃化することでオーステナイトを安定化させる。その結果、室温においても、オーステナイトが安定化することとなり、変態誘起塑性により優れた延性が確保される。
【0048】
Cが0.0001質量%未満だと、体積分率で3%以上のオーステナイトを確保するのが困難であることから、その下限値を0.0001質量%とした。
【0049】
一方では、Cが0.3質量%を超えると溶接が困難となることから、その上限を0.3質量%とした。
【0050】
Si:Siは強化元素であるのに加え、セメンタイトに固溶しないことから、セメンタイト析出を遅らせ、オーステナイトがフェライトとセメンタイトへ分解するのを遅らせる。この間に、オーステナイト中へとCを濃化させることが可能となり、室温でもオーステナイトが存在し得る。
【0051】
ただし、980MPa超の強度を有する強度−延性バランスに優れた鋼板を得るためには、0.1質量%以上の添加が必要となる。一方、4%を超えると溶接性が悪化することから、その上限を4質量%とした。
【0052】
Mn:Mnは強度確保に必要な元素であり、0.001質量%未満では、強化効果が発現しないことから、下限を0.001質量%とした。一方、3質量%を超えると、延性に悪影響を及ぼすことから、3質量%を上限値とした。
【0053】
Al:Alは、脱酸材として用いられるのに加え、セメンタイトに固溶しないことから、セメンタイト析出を遅らせ、オーステナイトがフェライトとセメンタイトへ分解するのを遅らせる。この間に、オーステナイト中へとCを濃化させることが可能となり、室温でもオーステナイトが存在し得る。
【0054】
ただし、0.001質量%未満だとその効果は発揮しない。一方、4質量%を超えると溶接性が悪化することから、その上限を4質量%とした。
【0055】
P:Pを0.0001質量%未満とすることは、経済的に不利であることから、この値を下限値とした。一方、0.3質量%を超える量の添加では、溶接性、ならびに、製造時及び熱延時の製造性に悪影響を及ぼす。このことから、上限値を、0.3質量%とした。
【0056】
S:Sは、溶接性、ならびに、製造時及び熱延時の製造性に悪影響を及ぼす。このことから、その上限値を0.01質量%とした。
【0057】
Ni及びCuは、強化元素であるのに加え、Al及びSiの酸化物形態を変化させ、濡れ性の向上と合金化促進を引き起こす。この効果は、この元素群中から1種又は2種を、合計で0.001質量%未満の添加では効果が少ないことから、その下限を0.001質量%とした。一方、4%を超えると、延性に悪影響を及ぼすことから、上限値を4質量%とした。
【0058】
Ti及びNbは、微細な炭化物、窒化物又は炭窒化物を形成することから、鋼板の強化には極めて有効であるのに加え、主相であるフェライト粒を微細化し、靭性向上へ多大なる寄与をもたらす。この効果は、1種又は2種類を、合計で0.001質量%未満の添加ではその効果が小さいことから、その下限値を0.001質量%とした。
【0059】
一方で、多量の添加は、延性の劣化や残留オーステナイト中へのCの濃化を妨げることから、その添加の上限を、1種又は2種類の合計で1質量%とした。さらには、鋼中へのこれら元素の添加は、スポット溶接、アーク溶接、レーザー溶接等の溶接を行う際の溶接部の軟化を抑制することから、溶接性にも優れている。
【0060】
さらに、本発明が対象とする鋼板は、強度のさらなる向上を目的に、Mo、Cr、Coの1種又は2種以上を含有できる。
【0061】
Mo:Moは、セメンタイト析出およびパーライト変態を遅らせることから、残留オーステナイトの確保に重要である。ただし、0.001質量%未満だと、その効果を発揮しない。一方、4質量%を超えると、延性に悪影響を及ぼすことから、上限値を4質量%とした。
【0062】
Cr:Crは、強化元素である。ただし、0.001質量%未満であると、その効果が発揮されないことから、その下限値を0.001質量%とした。一方、4質量%を超える添加は、延性に悪影響を及ぼすことから、上限値を4質量%とした。
【0063】
Co:Coは、強化元素である。ただし、0.001質量%未満であると、その効果が発揮されないことから、その下限値を0.001質量%とした。一方、4質量%を超える添加は、延性に悪影響を及ぼすとともに、コスト高を招き経済的に不利となることから、上限値を4質量%とした。
【0064】
さらに、めっき層中Cu含有率(質量%)と鋼中Cu含有率(質量%)の比をa(=めっき層中Cu含有率/鋼中Cu含有率)、めっき層中Ni含有率(質量%)と鋼中Ni含有率(質量%)の比をb(=めっき層中Ni含有率/鋼中Ni含有率)とすると、これらが、0.0001≦a+b≦0.2を満たすことにより、さらに優れためっき外観が得られる。
【0065】
a+bが0.0001より小さいと、本発明の効果が発揮されず、0.2を超えるような合金中への過度の添加は、経済的に好ましくないため、前記の範囲を満たすことが好ましい。
【0066】
なお、上記のaとbの関係は、本発明者らが実験を行い、新たに見出した式である。
【0067】
さらに、本発明が対象とする鋼板は、強度のさらなる向上を目的に、強炭化物形成元素であるVを含有できる。Vは、微細な炭化物、窒化物又は炭窒化物を形成することから、鋼板の強化には極めて有効であり、必要に応じて、0.001質量%以上添加できるものとした。一方で、多量の添加は、延性の劣化や残留オーステナイト中へのCの濃化を妨げることから、その添加の上限を、1質量%とした。
【0068】
Bもまた必要に応じて添加できる。Bは、0.0001質量%以上の添加で、粒界の強化や鋼材の強度化に有効であるが、その添加量が0.1質量%を超えると、その効果が飽和するばかりでなく、必要以上に鋼板強度を増加させ、加工性を低下させることから、その上限を0.1質量%とした。
【0069】
Mg、Zr、Ca、Y、Hf、La−Ceの添加は、Al及びSiの酸化物形態を変化させ、濡れ性の向上と合金化促進を引き起こす。この効果は、この元素群中から1種又は2種以上を、合計で0.001質量%以上含有することで発揮される。
【0070】
0.001質量%未満の添加では、その効果が少ないことから、その下限を0.001質量%とした。過度の添加は、コストの増大と鋳造性の劣化を招くことから、その上限を1質量%とした。
【0071】
さらに、めっき鋼板の垂直断面において、めっき相/鋼板界面から10μmまでの範囲における鋼板中に、MgO、CaO、ZrO、CeO、La、HfO、TiO、Y、SiO、MnO及びAlの1種又は2種以上の合計が、面積率で0.1%以上存在すると、MnO、Al、SiOの形態が外部酸化から内部酸化へと変化することにより、さらなる濡れ性の向上と合金化の促進が得られる。
【0072】
また、MgO、CaO、ZrO、CeO、La、HfO、TiO及びYは、フェライト、オーステナイト及びベイナイトの各相の粒界のみならず、これら相の粒内へも形成され、内部酸化物は均一に分散されることから、靭性、延性及び穴拡げ性の向上にも寄与する。
【0073】
一方、合金化促進のためには、これらの酸化物の合計が、面積率で50%以下であることが好ましいが、合金化処理を行わない場合には、これらの酸化物の1種又は2種以上の合計の面積率が50%以上であったとしても、合金化により得られるスポット溶接を除く効果である耐食性と延性や加工性は良好である。
【0074】
また、MgO、CaO、ZrO、CeO、La、HfO、TiO及びYの存在により、内部酸化物は均一に分散されることから、穴拡げ性の向上にも寄与する。
【0075】
また、めっき鋼板の垂直断面において、めっき相/鋼板界面から、MgO、CaO、ZrO、CeO、La、HfO、TiO、Y、SiO、MnO又はAlが存在する最大深さまでの範囲における鋼板組織中に、これらの酸化物の1種又は2種以上の合計が、面積率で1%以上含まれると、更なる濡れ性の向上と合金化の促進が得られる。
【0076】
この効果は、面積率で1%以上となると顕在化することから、この範囲が好ましい。面積率で5%以上とすることが、さらに好ましい。
【0077】
一方、これらの酸化物の面積率が50%超になるとめっき層と鋼板の間の元素拡散を阻害してしまい、合金化が遅れる。このことから合金化促進のためには、内部酸化物の面積率は50%以下であることが好ましい。
【0078】
しかしながら、合金化処理を行わない場合には、内部酸化物の面積率が50%以上であったとしても、合金化により得られるスポット溶接を除く効果である耐食性と延性や加工性は良好である。
【0079】
また、MgO、CaO、ZrO、CeO、La、HfO、TiO及びYにより、内部酸化物は均一に分散されることから、穴拡げ性の向上にも寄与する。
【0080】
なお、上述したような鋼板中に存在する酸化物の同定・観察や面積率測定は、EPMAやFE−SEMなどを用いて行うことができる。本発明に当たっては、2000〜20000倍で50視野以上を測定し、画像解析により面積率を求めた。
【0081】
また、酸化物の同定には、抽出レプリカ試料を作成して、TEM、EPMA及びEBSPを用いた。
【0082】
また、ここでいう、MnO、Al、SiOは、他の原子を含む複合酸化物であったり、欠陥を多く含む場合があるが、元素分析及び構造同定からもっとも近いものを見つけて判別した。
【0083】
面積率測定は、EPMAやFE−SEMなどを用い各成分の面分析を行うことで求めることができる。この場合には、個々の正確な構造の同定は難しいものの、上述した構造解析の結果と併せて形態やその組成から判断し得る。
【0084】
以上のような組織と特性を有する鋼板の製造方法について述べる。上記成分組成の要件を満足する成分組成の鋳造スラブを、鋳造まま、あるいは、一旦冷却した後に再度加熱し、熱延後巻き取った熱延鋼板を酸洗後冷延し、その後、溶融亜鉛めっきラインにおいてめっきを行う。
【0085】
本発明法により、高強度鋼板に溶融亜鉛めっきを行うには、まず、めっきラインにおける酸化帯で、鉄酸化膜を生成させる。鉄酸化膜中は、SiやAlの拡散が難しいため、SiやAlの酸化物形成は抑制される。
【0086】
この際、鉄酸化膜の厚さが数十Å未満であると、SiやAlの拡散抑制効果に乏しく、SiやAlの酸化物形成の抑制は難しい。このことから、鉄酸化膜厚さは数十Å以上が望ましい。
【0087】
ただし、酸化膜を形成させる際の酸化帯の燃焼空気比は、SiやAlの酸化物形成を抑制するのに十分な鉄酸化膜を生成するため、0.9以上必要であり、0.9未満の場合は、十分な鉄酸化膜を形成することができない。
【0088】
また、燃焼空気比が1.5超では、形成される酸化膜が厚くなり、引き続き還元帯で行われる鉄酸化膜の還元に、極端な長時間を要することとなり不経済である。また、めっき層と鋼板の間に酸化物が残るとめっき密着性を阻害してしまう。よって、酸化帯での燃焼空気比は0.9〜1.5とし、さらに、0.9〜1.2の範囲が望ましい。
【0089】
次に、還元帯における水分圧(PHO)と水素分圧(PH)の比を下記(2)式の範囲にする必要がある。
−0.8≧log(PHO/PH) ・・・・・(1)
【0090】
ここで、logは常用対数であり、log(PHO/PH)を−0.8以下としたのは、−0.8を超えると、酸化帯で生成した鉄の酸化膜を還元できず、めっき層と鋼板の間に酸化物が残存し、めっき密着性を阻害してしまうからである。
【0091】
冷延鋼板の連続焼鈍では、主相であるフェライトの体積率を50%以上とするため、Ac1+30(℃)以上に加熱する。この時の加熱温度が、Ac1+30(℃)より低いと、フェライト体積率を50%以上とすることができず、延性に劣る。
【0092】
同時に、加熱温度をAc1+30(℃)以上、Ac3−10(℃)以下にすることで、フェライト粒径を小さくすることができ、さらなる、高強度化、延性及び穴拡げ性の向上を引き起こす。
【0093】
この効果は、加熱温度をAc1+30(℃)以上、Ac3−10(℃)以下にすることで発揮される。ただし、この効果が顕著になるのは、加熱温度がAc3−50(℃)以下であることから、Ac3−50(℃)以下での加熱が望ましい。
【0094】
焼鈍の保持時間が短すぎると、未溶解炭化物が残存する可能性が高く、オーステナイト体積率が少なくなるため、焼鈍の保持時間は、10秒以上とすることが望ましい。一方、保持時間が長すぎると、結晶粒が粗大化する可能性が高くなり強度、延性及び穴拡げ性が低下するため、その上限は1000秒とすることが好ましい。
【0095】
その後、1次冷却として、0.1℃/秒以上の冷却速度で650〜800℃の温度域に冷却し、引き続いて、2次冷却として、1〜100℃/秒の冷却速度で、めっき浴温度〜めっき浴温度+100(℃)まで冷却した後、350〜めっき浴温度+100(℃)の温度域で、めっき浴の侵漬時間を含めて1〜3000秒間保持する。
【0096】
これは、二相域に加熱して生成させたオーステナイトをパーライトに変態させることなく、ベイナイト変態まで維持し、350〜Znめっき浴温度+100(℃)の温度域を1〜3000秒間かけて通過させることで、その組織を、フェライト(+ベイナイト)+残留オーステナイトとして所定の特性を得るためである。
【0097】
焼鈍後、650〜800℃までの1次冷却速度が0.1℃/秒未満では、フェライト粒が粗大化してしまい、好ましくない。そこで、その下限値を0.1℃/秒とした。その上限は、特に定めないが、過度に冷却速度を大きくすることは、過剰な設備投資を招く。このことから、上限は100℃/秒以下が望ましい。しかしながら、この値を越えたとしても、本発明で示す効果は発揮される。
【0098】
また、その後、めっき浴温度〜めっき浴温度+100(℃)までの2次冷却速度が1℃/秒未満では、冷却途中でオーステナイトがパーライトへと変態してしまうため、残留オーステナイトが残らず望ましくない。また、2次冷却速度が100℃/秒より速いと、板幅方向での冷却終点温度がばらつくこととなり、均一な鋼板の製造ができなくなるので好ましくない。
【0099】
めっき浴侵漬前の保持温度を350〜Znめっき浴温度+100(℃)としたのは、この温度域にてベイナイト変態が起こり、残留オーステナイトを室温まで残すためである。350℃未満では、二相域に加熱して生成させたオーステナイトがマルテンサイト変態を起こしてしまい残留オーステナイトが残らないため、下限温度を350℃とした。
【0100】
一方、保持温度がZnめっき浴温度+100(℃)より高いと、二相域に加熱して生成させたオーステナイトがパーライトへと変態するか、あるいは、オーステナイト中からセメンタイトが析出するため、オーステナイトがベイナイトへと分解してしまうため、その上限温度をZnめっき浴温度+100(℃)とした。
【0101】
Znめっき浴浸漬時間を含めた前記の保持時間が1秒未満では、鋼板をめっき浴へ侵漬する時間が十分でないので好ましくなく、3000秒を超えると、設備が巨大になりすぎ、不経済となるため好ましくないので、前記の保持時間は1〜3000秒間とした。
【0102】
めっき浴侵漬後の選択的に行う合金化処理において、保持温度を浴温〜Znめっき浴温度+100(℃)としたのは、この温度より高いと、オーステナイトがパーライトへと変態するか、あるいは、オーステナイト中からセメンタイトが析出するため、オーステナイトがベイナイトへと分解してしまうことから、その上限温度を、Znめっき浴温度+100(℃)とした。保持温度が浴温より低いと合金化に長時間の時間を要するので、下限温度を浴温とした。
【0103】
浴侵漬後、浴温〜めっき浴温度+100(℃)の温度範囲での保持を行わなくても、本発明に記載の高強度高延性溶融亜鉛めっき鋼板を得ることができる。
【0104】
Znめっき浴温度から、又は、Znめっき浸漬後の保持温度から室温までの冷却速度は、特に規定するものではないが、この温度範囲での冷却速度が100℃/秒を超えて、冷却したとしても、材質上はなんら問題を生じないが、過度に冷却速度を上げることは、製造コスト高を招くこととなるので、上限を100℃/秒とすることが好ましい。
【0105】
一方、下限は、特に規定することなく本発明の効果を得ることができるが、0.5℃/秒未満だと設備が大きくなりすぎ不経済であることから、冷却速度は0.5℃/秒以上とすることが望ましい。
【0106】
酸化物を制御し、濡れ性と合金化の向上を図るためには、熱延段階から温度と加工履歴を制御することが望ましい。まず、鋳片の加熱温度を1100〜1250℃として、仕上げ温度を850℃以上として、仕上げ圧延後のデスケには高圧デスケや強酸洗を行い、熱延で形成した酸化相をできるだけ除去することが望ましい。
【0107】
また、本発明の高強度高延性溶融亜鉛めっき鋼板の素材は、通常の製鉄工程である精錬、製鋼、鋳造、熱延、冷延工程を経て製造されることを原則とするが、その一部あるいは全部を省略して製造されるものでも、本発明に係わる条件を満足する限り、本発明の効果を得ることができる。
【0108】
また、めっき密着性をさらに向上させるために、焼鈍前に鋼板に、Ni、Cu、Co、Feの単独あるいは複数よりなるめっきを施しても、本発明を逸脱するものではない。
【0109】
さらには、焼鈍後、切削を行い鋼板表面の酸化物を除去した後、めっき浴に侵漬させ、めっきを施したとしても本発明の効果を得ることができる。
【0110】
【実施例】
以下実施例によって本発明をさらに詳細に説明する。
【0111】
(実施例1)
表1に成分組成を記載したスラブを熱延、酸洗、冷延し、1mm厚とした。その後、各鋼の成分(質量%)よって、下記式に従って、Ac1とAc3変態温度を計算により求めた。
【0112】
Ac1=723−10.7×Mn(%)+29.1×Si(%)、
Ac3=910−203×C(%)1/2+44.7×Si(%)+31.5×Mo(%)−30×Mn(%)−11×Cr(%)+400×Al(%)
【0113】
【表1】

Figure 2004211157
【0114】
これらのAc1とAc3変態温度から計算される焼鈍温度に10%H−N雰囲気で昇温・保定した後、0.1〜10℃/秒の冷却速度で600〜710℃まで冷却し、その後、1〜10℃/秒の冷速でめっき浴温度まで冷却し、浴組成を種々に変化させた460℃の溶融亜鉛めっき浴に3秒間侵漬することで、めっきを行った。この際のめっき付着量は、片面40g/mとした。
【0115】
この後、一部の鋼板については、めっき浴温度〜600℃の温度範囲で保持を行った後、350℃までの温度域を0.1〜100℃/秒の冷却速度で冷却後、室温まで冷却した。製造条件の詳細を表2〜5に示す。
【0116】
【表2】
Figure 2004211157
【0117】
【表3】
Figure 2004211157
【0118】
【表4】
Figure 2004211157
【0119】
【表5】
Figure 2004211157
【0120】
めっき表面外観におけるドロス巻き込みの状況の目視観察及び不めっき部面積の測定により、めっき性を評価した。作製しためっき層中濃度測定は、アミン系インヒビターを入れた5%塩酸でめっき層を溶かした後、ICP発光分析法を用いて行った。
【0121】
めっき密着性は、パウダリングを調査し、その剥離幅が3mmを超えた場合を不合格とした。
【0122】
これらめっきを施した鋼板から、JIS5号試験片を採取し、ゲージ長さ50mm、引張り試験速度10mm/分で、常温引張り試験を行った。
【0123】
穴拡げ性は、直径10mmの円形穴を、クリアランスが12%となる条件にて打ち抜き、かえりがダイ側となるようにし、60°円錐ポンチにて成形し、穴拡げ率λ(%)により評価した。
【0124】
残留オーステナイト体積率Vγの測定は、めっき層/鋼板界面より板厚の7/16内層を化学研磨後、Mo管球を用いたX線回折で、フェライトの(200)の回折強度Iα(200)、フェライトの(211)の回折強度Iα(211)とオーステナイトの(220)の回折強度Iγ(220)及び(311)の回折強度Iγ(311)の強度比より求めた。
Vγ(体積%)=0.25×{Iγ(220)/(1.35×Iα(200)+Iγ(220))+Iγ(220)/(0.69×Iα(211)+Iγ(220))+Iγ(311)/(1.5×Iα(200)+Iγ(311))+Iγ(311)/(0.69×Iα(211)+Iγ(311))}
【0125】
酸化物の体積率と分布については、研磨を行った後、SEM及びEPMAを用いて観察することで、測定を行った。
【0126】
表6〜9に機械特性、めっき特性等を示す。表6〜9に示すように、鋼板成分が所定の範囲を満たすものは、不めっきもなく良好なめっき性が得られている。
【0127】
【表6】
Figure 2004211157
【0128】
【表7】
Figure 2004211157
【0129】
【表8】
Figure 2004211157
【0130】
【表9】
Figure 2004211157
【0131】
鋼板の成分組成が所定の範囲であったとしても、製造条件及び鋼板組織が所定の要件を満たさないものは、高強度、高延性及びめっき密着性に劣り、さらには、残留オーステナイト分率が低くなり、強度−延性バランス(TS×El.)も21000(MPa・%)未満となり、強度−延性バランスに劣る。
【0132】
製造条件が本発明の要件を満たすものであっても、成分組成の範囲が所定の要件を満たさないものは、本発明の効果を得ることができない。
【0133】
【発明の効果】
本発明により、良好なめっき性が得られ、強度−延性バランス(TS×El.)も21000(MPa・%)を超える強度980MPa超の高強度高延性溶融亜鉛めっき鋼板が得られた。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high-strength, high-ductility hot-dip galvanized steel sheet excellent in workability suitable for automobiles, building materials, home appliances, and the like, and a method for producing the same.
[0002]
[Prior art]
Lightweight parts of automobile cross members and side members are being studied in order to respond to recent trends in lighter fuel consumption, and in terms of materials, the strength and collision safety can be ensured even if the thickness is reduced. Therefore, increasing the strength of steel sheets is being promoted.
[0003]
However, since the formability of the material deteriorates as the strength increases, it is necessary to manufacture a steel sheet that satisfies both the formability and the high strength in order to reduce the weight of the member.
[0004]
As an index representing formability, elongation (El.) During a tensile test is used, and a high value is used as one of the moldability indices. Further, as the strength, the tensile strength (TS) which is the maximum stress in the tensile test is used as the value, and the product (TS × El.) Of these steel plates exceeds 21000 (MPa ·%). Is known as a high strength and high ductility steel sheet.
[0005]
As steel plates having these characteristics, there are steel plates containing retained austenite as disclosed in Patent Document 1 and Patent Document 2.
[0006]
These steel sheets containing retained austenite have only C, Si and Mn as their constituent elements, and in the case of cold-rolled steel sheets, after annealing in a two-phase region, bainite in a temperature range of about 300 to 450 ° C. Steel sheets containing residual austenite in the metal by heat treatment characterized by transformation, and these steel sheets are disclosed in Patent Document 3 and Patent Document 4 in order to form a structure containing residual austenite. , Si or Al must be contained.
[0007]
Since these steel sheets have obtained excellent ductility by transforming retained austenite contained in the metal structure into martensite by stress-induced transformation at the time of press forming, Patent Document 5 and Patent Document As disclosed in Japanese Patent No. 6, many studies have been conducted so far focusing on improving the stability and volume ratio of retained austenite.
[0008]
[Patent Document 1]
Japanese Patent Laid-Open No. 1-2230715
[Patent Document 2]
JP-A-2-217425
[Patent Document 3]
JP-A-3-265337
[Patent Document 4]
JP-A-5-195143
[Patent Document 5]
JP-A-10-130776
[Patent Document 6]
Japanese Patent Laid-Open No. 4-9859
[Patent Document 7]
JP-A-3-28359
[Patent Document 8]
Japanese Patent Laid-Open No. 3-64437
[0009]
[Problems to be solved by the invention]
In addition, reflecting the high performance of automobile steel plates, the plating of automobile parts is progressing for the purpose of improving corrosion resistance and appearance, but at present, except for specific parts installed in the car, there are many A hot-dip galvanized steel sheet is used for the member.
[0010]
However, since these steel plates contain a large amount of Si, the surface of the steel plate is likely to be oxidized, causing slight unplating during hot dip galvanization, and problems such as poor plating adhesion in the processed parts after alloying. is there.
[0011]
As means for solving these problems, for example, in Patent Document 7 and Patent Document 8, 0.002 to 2.0 g / mm on the steel plate surface. 2 Plating properties are improved by performing single or composite plating of Ni, Cu, Co, and Fe. However, in this method, a new plating facility is provided in front of the hot dip galvanizing line, or electric There was a problem that a plating process had to be performed in the plating line, resulting in a significant cost increase.
[0012]
The present invention solves the above-mentioned problems, and provides a high-strength galvannealed steel sheet excellent in plating adhesion, a high-strength galvanized steel sheet excellent in plating adhesion, and a method for producing the same. Objective.
[0013]
[Means for Solving the Problems]
As a result of various investigations, the present inventors have confirmed that, with respect to plating properties, by adding Ni or Cu alone or in combination to steel, the hot dip galvanizing wettability of high-strength steel plates can be secured, and alloys It has been found that alloying in chemical plating is promoted. This effect is exhibited when the content of these elements in the steel is 0.001% or more.
[0014]
Further, in the steel, ferrite is contained as a main phase in a volume fraction of 50% or more, residual austenite is contained in an amount of 3 to less than 50%, and the remaining structure is martensite and bainite. It has been found that excellent high strength and high ductility steel sheets can be produced.
[0015]
Furthermore, while adding 1 type or 2 types of Ti and Nb in steel, the maximum heating temperature in the annealing process after hot rolling is set to a cooling rate of Ac1 + 30 (° C.) or higher and 1 (° C./sec) or higher. After cooling to a temperature range of 650 to 800 (° C.), and subsequently cooling at a cooling rate of 1 to 100 (° C./second) from Zn plating bath temperature to Zn plating bath temperature +100 (° C.), 350 to Hold in the temperature range of Zn plating bath temperature +100 (° C.) for 1 to 3000 seconds including the immersion time of the plating bath, then immerse in the Zn plating bath and then cool to room temperature, or the plating bath temperature It has been found that by holding in a temperature range of ˜100 ° C., the average particle diameter of ferrite as the main phase is 5 μm or less, and furthermore, a high-strength, high-ductility steel sheet excellent in strength-ductility balance can be produced. .
[0016]
The present invention has been completed based on the above findings, and the gist thereof is as follows.
[0017]
[1] By mass%
C: 0.0001 to 0.3%,
Si: 0.1 to 4%,
Al: 0.001 to 4%,
Mn: 0.001 to 3%,
P: 0.0001 to 0.3%,
S: 0.01% or less, 0.001 to 4% in total of one or two of Ni and Cu, and further 0.001 to 1 in total of one or two of Ti and Nb 1% content, consisting of the balance iron and inevitable impurities, the microstructure is volume fraction, containing 50% or more of ferrite as the main phase, containing less than 3-50% austenite, the balance structure being bainite, Or, on the surface of the steel plate made of martensite and bainite, in mass%,
Al: 0.001 to 0.5%,
A high-strength, high-ductility hot-dip galvanized steel sheet comprising Fe: 5 to 20%, the balance having a hot-dip galvanized layer composed of Zn and inevitable impurities, and having a tensile strength of over 980 MPa.
[0018]
[2] By mass%
C: 0.0001 to 0.3%,
Si: 0.1 to 4%,
Al: 0.001 to 4%,
Mn: 0.001 to 3%,
P: 0.0001 to 0.3%,
S: 0.01% or less, 0.001 to 4% in total of one or two of Ni and Cu, and further 0.001 to 1 in total of one or two of Ti and Nb 1% content, consisting of the balance iron and inevitable impurities, the microstructure is volume fraction, containing 50% or more of ferrite as the main phase, containing less than 3-50% austenite, the balance structure being bainite, Or, on the surface of the steel plate made of martensite and bainite, in mass%,
Al: 0.001 to 0.5%,
Fe: A high-strength, high-ductility hot-dip galvanized steel sheet having a hot-dip galvanized layer containing less than 5%, the balance being Zn and inevitable impurities, and having a tensile strength of over 980 MPa.
[0019]
[3] Furthermore, in steel, in mass%,
Mo: 0.001 to 4%,
Cr: 0.001 to 4%,
Co: 0.001 to 4%
The high strength and high ductility hot dip galvanized steel sheet according to [1] or [2], characterized by containing one or more of the following.
[0020]
[4] The ratio of the Cu content (mass%) in the plating layer to the Cu content (mass%) in steel is a, the Ni content (mass%) in the plating layer and the Ni content ( Mass ratio) is b
0.0001 ≦ a + b ≦ 0.2
The high strength and high ductility hot dip galvanized steel sheet according to any one of [1] to [3], wherein:
[0021]
[5] Furthermore, in steel, in mass%,
V: 0.001 to 1% is contained. The high strength and high ductility hot dip galvanized steel sheet according to any one of [1] to [4].
[0022]
[6] Furthermore, in steel, in mass%,
B: The high strength and high ductility hot dip galvanized steel sheet according to any one of [1] to [5], which contains 0.0001 to 0.1%.
[0023]
[7] Further, the steel contains 0.001 to 1% in total of one or more of Mg, Zr, Ca, Y, Hf, and La—Ce in elemental mass. The high strength and high ductility hot dip galvanized steel sheet according to any one of [1] to [6].
[0024]
[8] In the vertical cross section of the plated steel sheet, MgO, CaO, ZrO from the plating phase / steel sheet interface. 2 , CeO 2 , La 2 O 3 , HfO 2 TiO 2 , Y 2 O 3 , SiO 2 , MnO and Al 2 O 3 In the steel sheet in the range up to the maximum depth where one or more of the internal oxides exist, MgO, CaO, ZrO 2 , CeO 2 , La 2 O 3 , HfO 2 TiO 2 , Y 2 O 3 , SiO 2 , MnO and Al 2 O 3 The high strength hot-dip galvanized steel sheet according to any one of [1] to [7], wherein 1 type or 2 types or more are included in an area ratio of 1% or more.
[0025]
[9] In the vertical cross section of the plated steel sheet, MgO, CaO, ZrO are included in the steel sheet from the plating phase / steel sheet interface to 10 μm. 2 , CeO 2 , La 2 O 3 , HfO 2 TiO 2 , Y 2 O 3 , SiO 2 , MnO and Al 2 O 3 The high-strength and high-ductility hot-dip galvanized steel sheet according to any one of [1] to [8], wherein the total of one or more of the above is contained in an area ratio of 0.1% or more.
[0026]
[10] The high-strength and high-ductility hot-dip galvanized steel sheet according to any one of [1] to [9], wherein an average particle diameter of the ferrite having a volume ratio of 80% or more is 5 μm or less.
[0027]
[11] After the cast slab having the component composition according to any one of [1] to [7] is cast or heated again after being cooled, the hot-rolled steel sheet wound after hot rolling is pickled. After cold rolling, oxidation is performed in an oxidation zone in a plating line in an atmosphere having a combustion air ratio of 0.9 to 1.5, and thereafter, a common logarithm log (PH of water pressure and hydrogen partial pressure) is used in a reduction zone. 2 O / PH 2 ) In a reducing atmosphere satisfying the following formula (1), after annealing at Ac1 + 30 (° C.) or more and Ac 3-10 (° C.) or less, a temperature range of 650 to 800 ° C. at a cooling rate of 0.1 ° C./second or more. After cooling to Zn plating bath temperature to Zn plating bath temperature +100 (° C.) at a cooling rate of 1 to 100 ° C./second, 350 to Zn plating bath temperature +100 (° C.) The high strength high ductility hot dip galvanized steel sheet having a tensile strength of over 980 MPa after being held for 1-3000 seconds including the immersion time of the subsequent plating bath and then immersed in a Zn plating bath and then cooled to room temperature. A method for producing a high-strength, high-ductility, hot-dip galvanized steel sheet, characterized in that
−0.8 ≧ log (PH 2 O / PH 2 (1)
[0028]
[12] After dipping in the Zn plating bath, holding is performed for 1 to 300 seconds in a temperature range of bath temperature to Zn plating bath temperature + 100 (° C.), and cooling to room temperature is described in [11] Manufacturing method of high strength and high ductility hot dip galvanized steel sheet.
[0029]
DETAILED DESCRIPTION OF THE INVENTION
Details of the present invention will be described below.
[0030]
The steel sheet of the present invention is a steel sheet produced by controlling the component composition and production conditions of the steel sheet within a certain range, and contains 50% or more of ferrite as a main phase in volume fraction, It is a steel sheet containing less than 50% and the balance structure is galvanized or hot-dip galvanized on a steel sheet made of martensite and bainite, and has a tensile strength of over 980 MPa, has strength and excellent ductility, and has corrosion resistance. It is characterized by having.
[0031]
The ductility of the high-strength hot-dip galvanized steel sheet according to the present invention depends on the volume fraction of retained austenite contained in the product. Residual austenite contained in the metal structure stably exists in a state where it is not subjected to stress, but when deformed, it transforms into martensite, and excellent ductility is obtained by transformation-induced plasticity.
[0032]
However, when the volume fraction is less than 3%, the effect hardly appears, so the lower limit is set to 3% or more. On the other hand, when the volume fraction of retained austenite is 50% or more, if extremely remarkable molding is performed, a large amount of martensite is present after molding, which may cause problems in secondary workability and impact characteristics. Therefore, in the present invention, the upper limit value is set to less than 50%.
[0033]
Moreover, even if carbides, alloy carbides, nitrides and oxides are contained as phases other than the main phase ferrite, second phase retained austenite, bainite and martensite, it does not depart from the present invention.
[0034]
Furthermore, when the average particle diameter of the volume ratio of 80% or more of ferrite as the main phase is set to 5 μm or less, the strength-ductility balance is further improved. However, this effect becomes remarkable because the average particle size is 3 μm or less, and the average particle size is preferably 3 μm or less. On the other hand, the lower limit of the average particle diameter is not particularly defined, and the effects of the present invention can be obtained. Furthermore, it is effective for improving toughness and hole expansibility.
[0035]
In addition, the ferrite, bainite, austenite, martensite, oxide phase and the remaining structure of the microstructure, the observation of the existing position, and the measurement of the average particle size (average equivalent circle diameter) and the space factor are as follows: With the reagent disclosed in Japanese Patent Laid-Open No. 59-219473, the cross section in the rolling direction of the steel sheet or the cross section in the direction perpendicular to the rolling direction is corroded, and quantified by observation with an optical microscope, scanning electron microscope, and transmission electron microscope at 500 to 20000 times. Is possible.
[0036]
The form and identification of the oxide in the vicinity of the coating phase / steel plate interface were performed using a scanning microscope, a transmission electron microscope, and EPMA.
[0037]
The present inventors, in mass%, C: 0.0001 to 0.3%, Si: 0.1 to 4%, Al: 0.001 to 4%, Mn: 0.001 to 3%, P: 0.0001-0.3%, S: 0.01% or less, Ni: 0.01-4%, Cu: 0.01-4%, Ti: 0.001-1%, Nb: 0.001- The cast slab containing 1% and the balance Fe and inevitable impurities, as cast, or once again cooled and then heated again, the hot-rolled steel sheet wound up after hot rolling, cold-rolled after pickling, Using an annealing furnace having an oxidation zone and a reduction zone, the oxidation zone is oxidized in an atmosphere having a combustion air ratio of 0.9 to 1.2, and then the moisture pressure and hydrogen partial pressure are reduced in the reduction zone. Common logarithm log (PH 2 O / PH 2 ) In the range of −0.8 or less, annealing at Ac1 + 100 (° C.) or more and 1000 ° C. or less, cooling to a temperature range of 650 to 800 ° C. at a cooling rate of 0.1 to 50 ° C./second, and subsequently, After cooling from Zn plating bath temperature to Zn plating bath temperature +100 (° C.) at a cooling rate of 1 to 100 ° C./second, the subsequent plating bath is immersed in a temperature range of 350 to Zn plating bath temperature +100 (° C.). After holding for 1 to 3000 seconds including time, it was immersed in a Zn plating bath and then cooled to room temperature.
[0038]
Furthermore, about some samples, after immersing in Zn plating bath on the same conditions, after alloying processing by hold | maintaining for 5 to 60 seconds at Zn plating bath temperature -550 degreeC, the temperature range to 350 degreeC is carried out. The solution was cooled at a cooling rate of 0.5 to 100 ° C./second, and then cooled to room temperature.
[0039]
Said test was done and the external appearance was evaluated in five steps based on the defect occurrence rate of the plating surface. As a result, it was found that by setting the steel plate component in the above range, scores 5 to 4 with almost no appearance defects were obtained.
[0040]
The grades 1 to 5 were determined by visually judging the occurrence of non-plating on the appearance of plating. The evaluation index is as follows.
[0041]
Score 5: Almost no plating. (Area ratio is 0.1% or less)
Score 4: Unplated is negligible. (Area ratio is over 0.1% and 3% or less)
Score 3: Less plating. (Area ratio is more than 3% and less than 5%)
Score 2: Many unplated. (Area ratio is more than 5% and less than 50%)
Score 1: Not plated. (Over 50% in area ratio)
There are no particular restrictions on the plating adhesion amount, but from the viewpoint of corrosion resistance, the adhesion amount on one side is 5 g / mm. 2 The above is desirable. For the purpose of improving paintability, corrosion resistance and weldability on the hot dip galvanized steel sheet of the present invention, various treatments such as chromate treatment, phosphate treatment, lubricity improvement treatment, weldability improvement treatment However, the present invention does not depart from the present invention.
[0042]
The reason why the Al content in the plating layer is within the range of 0.001 to 0.5% is that if it is less than 0.001%, dross generation is remarkable and a good appearance cannot be obtained, exceeding 0.5% When Al is added, the alloying reaction is remarkably suppressed, and it becomes difficult to form an alloyed hot-dip galvanized layer.
[0043]
Furthermore, by performing the alloying treatment, Fe is taken into the plating layer, and a high-strength and high-ductility hot-dip galvanized steel sheet excellent in paintability and spot weldability can be obtained. If the amount of Fe in the plating layer exceeds 20% by mass, the adhesion of the plating itself is impaired, and the plating layer breaks and falls off during processing and adheres to the mold, thereby causing defects during molding.
[0044]
On the other hand, in order to improve spot weldability, it is desirable that the Fe content be 5% or more. Therefore, when alloying is performed, the range of the amount of Fe in the plating layer is 5 to 20% by mass.
[0045]
Further, when the alloying treatment is not performed, even if the amount of Fe in the plating layer is less than 5% by mass, the corrosion resistance, ductility and workability, which are the effects excluding spot welding obtained by alloying, are good.
[0046]
Next, the reason for limiting the component composition of the steel sheet in the present invention will be described.
[0047]
C: An austenite stabilizing element, which moves from ferrite to austenite during two-phase heating and bainite transformation temperature range, and stabilizes austenite by concentrating in austenite. As a result, austenite is stabilized even at room temperature, and excellent ductility is ensured by transformation-induced plasticity.
[0048]
If C is less than 0.0001% by mass, it is difficult to secure austenite of 3% or more in terms of volume fraction, so the lower limit was made 0.0001% by mass.
[0049]
On the other hand, if C exceeds 0.3 mass%, welding becomes difficult, so the upper limit was made 0.3 mass%.
[0050]
Si: In addition to Si being a strengthening element, Si does not dissolve in cementite, so it delays cementite precipitation and delays the decomposition of austenite into ferrite and cementite. During this time, it becomes possible to concentrate C into austenite, and austenite can exist even at room temperature.
[0051]
However, in order to obtain a steel sheet having a strength-ductility balance having a strength of over 980 MPa, addition of 0.1% by mass or more is necessary. On the other hand, if it exceeds 4%, the weldability deteriorates, so the upper limit was made 4% by mass.
[0052]
Mn: Mn is an element necessary for securing the strength, and if the amount is less than 0.001% by mass, the reinforcing effect is not exhibited. On the other hand, if it exceeds 3% by mass, the ductility is adversely affected, so 3% by mass was made the upper limit.
[0053]
Al: In addition to being used as a deoxidizer, Al does not dissolve in cementite, so it delays cementite precipitation and delays the decomposition of austenite into ferrite and cementite. During this time, it becomes possible to concentrate C into austenite, and austenite can exist even at room temperature.
[0054]
However, if it is less than 0.001% by mass, the effect is not exhibited. On the other hand, if it exceeds 4% by mass, weldability deteriorates, so the upper limit was made 4% by mass.
[0055]
Since it is economically disadvantageous to make P: P less than 0.0001% by mass, this value is set as the lower limit. On the other hand, addition of more than 0.3% by mass adversely affects weldability and manufacturability during production and hot rolling. Therefore, the upper limit value was set to 0.3% by mass.
[0056]
S: S adversely affects weldability and manufacturability during production and hot rolling. For this reason, the upper limit was set to 0.01% by mass.
[0057]
In addition to being strengthening elements, Ni and Cu change the oxide form of Al and Si, causing improvement in wettability and promotion of alloying. Since this effect is less effective when one or two elements in the element group are added in a total amount of less than 0.001% by mass, the lower limit is set to 0.001% by mass. On the other hand, if it exceeds 4%, the ductility is adversely affected, so the upper limit was made 4% by mass.
[0058]
Since Ti and Nb form fine carbides, nitrides, or carbonitrides, in addition to being extremely effective for strengthening steel sheets, the ferrite grains that are the main phase are refined, contributing greatly to improved toughness. Bring. This effect has a small effect when one or two types are added in a total amount of less than 0.001% by mass, so the lower limit was set to 0.001% by mass.
[0059]
On the other hand, addition of a large amount prevents deterioration of ductility and concentration of C in retained austenite, so the upper limit of addition was set to 1% by mass in one or two types in total. Furthermore, the addition of these elements into steel suppresses softening of the welded portion when performing welding such as spot welding, arc welding, laser welding, etc., and is therefore excellent in weldability.
[0060]
Furthermore, the steel plate which this invention makes object can contain the 1 type (s) or 2 or more types of Mo, Cr, Co for the purpose of the further improvement of intensity | strength.
[0061]
Mo: Mo is important for securing retained austenite because it delays cementite precipitation and pearlite transformation. However, if it is less than 0.001% by mass, the effect is not exhibited. On the other hand, if it exceeds 4% by mass, the ductility is adversely affected, so the upper limit was made 4% by mass.
[0062]
Cr: Cr is a strengthening element. However, since the effect is not exhibited as it is less than 0.001 mass%, the lower limit was set to 0.001 mass%. On the other hand, addition exceeding 4% by mass has an adverse effect on ductility, so the upper limit was made 4% by mass.
[0063]
Co: Co is a strengthening element. However, since the effect is not exhibited as it is less than 0.001 mass%, the lower limit was set to 0.001 mass%. On the other hand, the addition exceeding 4% by mass adversely affects the ductility and increases the cost, which is economically disadvantageous. Therefore, the upper limit is set to 4% by mass.
[0064]
Further, the ratio of the Cu content (mass%) in the plating layer and the Cu content (mass%) in the steel is a (= Cu content in the plating layer / Cu content in the steel), Ni content (mass in the plating layer). %) And the Ni content (mass%) in the steel is b (= Ni content in the plating layer / Ni content in the steel), these satisfy 0.0001 ≦ a + b ≦ 0.2. Further, an excellent plating appearance can be obtained.
[0065]
When a + b is smaller than 0.0001, the effect of the present invention is not exhibited, and excessive addition to the alloy exceeding 0.2 is not economically preferable, and therefore, the above range is preferably satisfied.
[0066]
The above relationship between a and b is an expression newly found by the inventors through experiments.
[0067]
Furthermore, the steel plate targeted by the present invention can contain V, which is a strong carbide forming element, for the purpose of further improving the strength. V forms extremely fine carbides, nitrides, or carbonitrides, so is extremely effective for strengthening steel sheets, and can be added in an amount of 0.001% by mass or more as necessary. On the other hand, addition of a large amount hinders deterioration of ductility and concentration of C in retained austenite, so the upper limit of addition was set to 1% by mass.
[0068]
B can also be added as needed. B is effective for strengthening grain boundaries and strengthening steel by addition of 0.0001% by mass or more, but when the addition amount exceeds 0.1% by mass, not only the effect is saturated, Since the steel sheet strength is increased more than necessary and the workability is lowered, the upper limit was made 0.1 mass%.
[0069]
Addition of Mg, Zr, Ca, Y, Hf, and La—Ce changes the oxide form of Al and Si, and improves wettability and promotes alloying. This effect is exhibited by containing one or more elements in the element group in a total amount of 0.001% by mass or more.
[0070]
The addition of less than 0.001% by mass has little effect, so the lower limit was made 0.001% by mass. Excessive addition causes an increase in cost and deterioration of castability, so the upper limit was made 1 mass%.
[0071]
Further, in the vertical cross section of the plated steel sheet, MgO, CaO, ZrO in the steel sheet in the range from the plating phase / steel sheet interface to 10 μm. 2 , CeO 2 , La 2 O 3 , HfO 2 TiO 2 , Y 2 O 3 , SiO 2 , MnO and Al 2 O 3 When the total of one kind or two kinds or more is 0.1% or more by area ratio, MnO, Al 2 O 3 , SiO 2 By changing the form from external oxidation to internal oxidation, further improvement in wettability and promotion of alloying can be obtained.
[0072]
MgO, CaO, ZrO 2 , CeO 2 , La 2 O 3 , HfO 2 TiO 2 And Y 2 O 3 Is formed not only in the grain boundaries of each phase of ferrite, austenite and bainite, but also in the grains of these phases, and the internal oxide is uniformly dispersed. Contribute.
[0073]
On the other hand, in order to promote alloying, the total of these oxides is preferably 50% or less in terms of area ratio. However, when alloying treatment is not performed, one or two of these oxides are used. Even if the total area ratio of the seeds or more is 50% or more, the corrosion resistance, ductility and workability, which are the effects excluding spot welding obtained by alloying, are good.
[0074]
MgO, CaO, ZrO 2 , CeO 2 , La 2 O 3 , HfO 2 TiO 2 And Y 2 O 3 Due to the presence of the internal oxide, the internal oxide is uniformly dispersed, which contributes to an improvement in hole expansibility.
[0075]
Further, in the vertical cross section of the plated steel plate, from the plating phase / steel plate interface, MgO, CaO, ZrO 2 , CeO 2 , La 2 O 3 , HfO 2 TiO 2 , Y 2 O 3 , SiO 2 , MnO or Al 2 O 3 If the total of one or more of these oxides is contained in an area ratio of 1% or more in the steel sheet structure in the range up to the maximum depth in which there is a presence, further improvement in wettability and promotion of alloying Is obtained.
[0076]
Since this effect becomes apparent when the area ratio is 1% or more, this range is preferable. More preferably, the area ratio is 5% or more.
[0077]
On the other hand, when the area ratio of these oxides exceeds 50%, element diffusion between the plating layer and the steel sheet is inhibited, and alloying is delayed. Therefore, in order to promote alloying, the area ratio of the internal oxide is preferably 50% or less.
[0078]
However, when the alloying treatment is not performed, even if the area ratio of the internal oxide is 50% or more, the corrosion resistance, ductility and workability, which are the effects excluding spot welding obtained by alloying, are good. .
[0079]
MgO, CaO, ZrO 2 , CeO 2 , La 2 O 3 , HfO 2 TiO 2 And Y 2 O 3 As a result, the internal oxide is uniformly dispersed, which contributes to improvement of hole expansibility.
[0080]
Note that identification / observation and area ratio measurement of the oxides present in the steel sheet as described above can be performed using EPMA, FE-SEM, or the like. In the present invention, 50 fields or more were measured at 2000 to 20000 times, and the area ratio was determined by image analysis.
[0081]
In addition, for identification of oxides, extraction replica samples were prepared and TEM, EPMA, and EBSP were used.
[0082]
Also, here, MnO, Al 2 O 3 , SiO 2 May be a complex oxide containing other atoms or a lot of defects, but it was determined by finding the closest one from elemental analysis and structural identification.
[0083]
The area ratio measurement can be obtained by performing surface analysis of each component using EPMA, FE-SEM, or the like. In this case, although it is difficult to identify an individual accurate structure, it can be determined from the form and the composition thereof together with the result of the structural analysis described above.
[0084]
A method for producing a steel sheet having the above structure and characteristics will be described. A cast slab having a component composition that satisfies the above-described component composition is cast as it is, or once heated after being cooled, the hot-rolled steel sheet wound after hot rolling is pickled and cold-rolled, and then hot-dip galvanized. Plating is performed on the line.
[0085]
In order to perform hot dip galvanizing on a high-strength steel sheet according to the method of the present invention, first, an iron oxide film is formed in an oxidation zone in a plating line. In the iron oxide film, since diffusion of Si and Al is difficult, formation of oxides of Si and Al is suppressed.
[0086]
At this time, if the thickness of the iron oxide film is less than several tens of mm, the effect of suppressing the diffusion of Si and Al is poor, and the formation of oxides of Si and Al is difficult to suppress. For this reason, the iron oxide film thickness is preferably several tens of mm or more.
[0087]
However, the combustion air ratio in the oxidation zone when forming the oxide film needs to be 0.9 or more in order to generate an iron oxide film sufficient to suppress the formation of oxides of Si and Al. If it is less, a sufficient iron oxide film cannot be formed.
[0088]
On the other hand, if the combustion air ratio is more than 1.5, the oxide film formed becomes thick, and it takes an extremely long time to reduce the iron oxide film continuously in the reduction zone, which is uneconomical. Moreover, when an oxide remains between a plating layer and a steel plate, plating adhesion will be inhibited. Therefore, the combustion air ratio in the oxidation zone is set to 0.9 to 1.5, and more preferably 0.9 to 1.2.
[0089]
Next, the water pressure (PH 2 O) and hydrogen partial pressure (PH 2 ) Ratio within the range of the following formula (2).
−0.8 ≧ log (PH 2 O / PH 2 (1)
[0090]
Where log is a common logarithm and log (PH 2 O / PH 2 ) Is -0.8 or less, if it exceeds -0.8, the iron oxide film formed in the oxidation zone cannot be reduced, and an oxide remains between the plating layer and the steel sheet, resulting in plating adhesion. It is because it will inhibit.
[0091]
In the continuous annealing of the cold-rolled steel sheet, it is heated to Ac1 + 30 (° C.) or more in order to make the volume ratio of ferrite as the main phase 50% or more. If the heating temperature at this time is lower than Ac1 + 30 (° C.), the ferrite volume fraction cannot be made 50% or more, and the ductility is inferior.
[0092]
At the same time, when the heating temperature is set to Ac1 + 30 (° C.) or more and Ac 3-10 (° C.) or less, the ferrite particle size can be reduced, and further increase in strength, ductility and hole expandability are caused.
[0093]
This effect is exhibited when the heating temperature is set to Ac1 + 30 (° C.) or higher and Ac 3-10 (° C.) or lower. However, this effect becomes remarkable because the heating temperature is Ac3-50 (° C.) or lower, and heating at Ac3-50 (° C.) or lower is desirable.
[0094]
If the annealing holding time is too short, there is a high possibility that undissolved carbides remain, and the austenite volume fraction decreases. Therefore, the annealing holding time is desirably 10 seconds or more. On the other hand, if the holding time is too long, there is a high possibility that the crystal grains are coarsened, and the strength, ductility and hole expansibility are lowered. Therefore, the upper limit is preferably set to 1000 seconds.
[0095]
Thereafter, as the primary cooling, the cooling is performed at a cooling rate of 0.1 ° C./second or more to a temperature range of 650 to 800 ° C., and subsequently, as the secondary cooling, the plating bath is performed at a cooling rate of 1 to 100 ° C./second. After cooling from temperature to plating bath temperature + 100 (° C.), the temperature is maintained within the range of 350 to plating bath temperature + 100 (° C.) for 1-3000 seconds including the immersion time of the plating bath.
[0096]
This maintains the bainite transformation without transforming the austenite generated by heating in the two-phase region to pearlite, and passes the temperature region of 350 to Zn plating bath temperature +100 (° C.) over 1 to 3000 seconds. This is because the structure is obtained as ferrite (+ bainite) + residual austenite to obtain predetermined characteristics.
[0097]
If the primary cooling rate from 650 to 800 ° C. after annealing is less than 0.1 ° C./second, the ferrite grains become coarse, which is not preferable. Therefore, the lower limit was set to 0.1 ° C./second. The upper limit is not particularly defined, but excessively increasing the cooling rate causes excessive capital investment. Therefore, the upper limit is desirably 100 ° C./second or less. However, even if this value is exceeded, the effects shown in the present invention are exhibited.
[0098]
In addition, when the secondary cooling rate from the plating bath temperature to the plating bath temperature +100 (° C.) is less than 1 ° C./second, austenite is transformed into pearlite during cooling, so that residual austenite does not remain and is not desirable. . On the other hand, if the secondary cooling rate is faster than 100 ° C./sec, the cooling end point temperature in the plate width direction varies, and it is not preferable because a uniform steel plate cannot be produced.
[0099]
The reason why the holding temperature before plating bath immersion is set to 350 to Zn plating bath temperature +100 (° C.) is that bainite transformation occurs in this temperature range, and residual austenite remains at room temperature. When the temperature is lower than 350 ° C., the austenite generated by heating in the two-phase region causes martensitic transformation and no residual austenite remains, so the lower limit temperature was set to 350 ° C.
[0100]
On the other hand, if the holding temperature is higher than the Zn plating bath temperature +100 (° C.), the austenite generated by heating in the two-phase region is transformed into pearlite, or cementite is precipitated from the austenite, so the austenite is bainite. Therefore, the upper limit temperature was set to Zn plating bath temperature +100 (° C.).
[0101]
If the holding time including the Zn plating bath immersion time is less than 1 second, it is not preferable because the time for immersing the steel sheet in the plating bath is not sufficient, and if it exceeds 3000 seconds, the equipment becomes too large and uneconomical. Therefore, the holding time is set to 1 to 3000 seconds.
[0102]
In the alloying treatment that is selectively performed after immersion in the plating bath, the holding temperature is set to bath temperature to Zn plating bath temperature +100 (° C.). If the temperature is higher than this temperature, the austenite is transformed into pearlite, or Since cementite is precipitated from austenite, austenite is decomposed into bainite, so the upper limit temperature was set to Zn plating bath temperature +100 (° C.). If the holding temperature is lower than the bath temperature, it takes a long time for alloying, so the lower limit temperature was taken as the bath temperature.
[0103]
The high strength and high ductility hot dip galvanized steel sheet described in the present invention can be obtained without holding in the temperature range of bath temperature to plating bath temperature +100 (° C.) after bath immersion.
[0104]
The cooling rate from the temperature of the Zn plating bath or from the holding temperature after immersion of the Zn plating to room temperature is not particularly specified, but the cooling rate in this temperature range exceeds 100 ° C./sec. However, there is no problem with the material, but excessively increasing the cooling rate leads to high manufacturing costs, so the upper limit is preferably set to 100 ° C./second.
[0105]
On the other hand, the lower limit can provide the effects of the present invention without any particular limitation, but if it is less than 0.5 ° C./second, the equipment becomes too large and uneconomical, so the cooling rate is 0.5 ° C. / It is desirable to set it to 2 seconds or more.
[0106]
In order to control oxides and improve wettability and alloying, it is desirable to control temperature and processing history from the hot rolling stage. First, the heating temperature of the slab is set to 1100 to 1250 ° C., the finishing temperature is set to 850 ° C. or higher, and the deske after finish rolling is subjected to high pressure deske and strong pickling to remove as much as possible the oxidized phase formed by hot rolling desirable.
[0107]
In addition, the material of the high-strength and high-ductility hot-dip galvanized steel sheet of the present invention is generally manufactured through refining, steelmaking, casting, hot rolling, and cold rolling processes, which are ordinary iron making processes, but part of them. Or even if it manufactures omitting all, as long as the conditions concerning this invention are satisfied, the effect of this invention can be acquired.
[0108]
Further, in order to further improve the plating adhesion, the present invention does not depart from the present invention even if the steel sheet is plated with Ni, Cu, Co, or Fe alone or plurally before annealing.
[0109]
Furthermore, the effect of the present invention can be obtained even when annealing is performed to remove oxides on the surface of the steel sheet, and then immersed in a plating bath to perform plating.
[0110]
【Example】
Hereinafter, the present invention will be described in more detail by way of examples.
[0111]
Example 1
The slab whose component composition is listed in Table 1 was hot rolled, pickled, and cold rolled to a thickness of 1 mm. Then, Ac1 and Ac3 transformation temperature was calculated | required by calculation according to the following formula with the component (mass%) of each steel.
[0112]
Ac1 = 723-10.7 × Mn (%) + 29.1 × Si (%),
Ac3 = 910-203 * C (%) 1/2 + 44.7 * Si (%) + 31.5 * Mo (%)-30 * Mn (%)-11 * Cr (%) + 400 * Al (%)
[0113]
[Table 1]
Figure 2004211157
[0114]
The annealing temperature calculated from these Ac1 and Ac3 transformation temperatures is 10% H 2 -N 2 After the temperature is raised and maintained in the atmosphere, it is cooled to 600 to 710 ° C. at a cooling rate of 0.1 to 10 ° C./second, and then cooled to the plating bath temperature at a cooling rate of 1 to 10 ° C./second. Plating was carried out by immersing in a hot-dip galvanizing bath at 460 ° C. with various changes in the temperature for 3 seconds. In this case, the plating adhesion amount is 40 g / m on one side. 2 It was.
[0115]
Then, about some steel plates, after hold | maintaining in the temperature range of plating bath temperature -600 degreeC, the temperature range to 350 degreeC is cooled by the cooling rate of 0.1-100 degrees C / sec. Cooled down. Details of the production conditions are shown in Tables 2-5.
[0116]
[Table 2]
Figure 2004211157
[0117]
[Table 3]
Figure 2004211157
[0118]
[Table 4]
Figure 2004211157
[0119]
[Table 5]
Figure 2004211157
[0120]
The plating property was evaluated by visual observation of the state of dross entrainment on the plating surface appearance and measurement of the area of the non-plated part. The concentration in the produced plating layer was measured using ICP emission analysis after dissolving the plating layer with 5% hydrochloric acid containing an amine-based inhibitor.
[0121]
For plating adhesion, powdering was investigated and the case where the peel width exceeded 3 mm was rejected.
[0122]
JIS No. 5 specimens were collected from these plated steel plates and subjected to a room temperature tensile test at a gauge length of 50 mm and a tensile test speed of 10 mm / min.
[0123]
Hole expandability is evaluated by punching a circular hole with a diameter of 10 mm under the condition that the clearance is 12%, forming the burr on the die side, forming with a 60 ° conical punch, and evaluating the hole expansion ratio λ (%). did.
[0124]
The residual austenite volume fraction Vγ is measured by X-ray diffraction using a Mo tube after chemically polishing the 7/16 inner layer of the plate thickness from the plating layer / steel plate interface, and the diffraction intensity Iα (200) of ferrite (200). The diffraction intensity Iα (211) of ferrite (211) and the diffraction intensity Iγ (220) of austenite (220) and the diffraction intensity Iγ (311) of (311) were obtained from the intensity ratio.
Vγ (volume%) = 0.25 × {Iγ (220) / (1.35 × Iα (200) + Iγ (220)) + Iγ (220) / (0.69 × Iα (211) + Iγ (220)) + Iγ (311) / (1.5 × Iα (200) + Iγ (311)) + Iγ (311) / (0.69 × Iα (211) + Iγ (311))}
[0125]
The volume ratio and distribution of the oxide were measured by observing with SEM and EPMA after polishing.
[0126]
Tables 6 to 9 show mechanical characteristics, plating characteristics, and the like. As shown in Tables 6 to 9, when the steel plate component satisfies the predetermined range, good plating properties are obtained without any non-plating.
[0127]
[Table 6]
Figure 2004211157
[0128]
[Table 7]
Figure 2004211157
[0129]
[Table 8]
Figure 2004211157
[0130]
[Table 9]
Figure 2004211157
[0131]
Even if the component composition of the steel sheet is within the predetermined range, the manufacturing conditions and the steel sheet structure that do not satisfy the predetermined requirements are inferior in high strength, high ductility and plating adhesion, and further, the retained austenite fraction is low. Therefore, the strength-ductility balance (TS × El.) Is also less than 21000 (MPa ·%), which is inferior to the strength-ductility balance.
[0132]
Even if the manufacturing conditions satisfy the requirements of the present invention, the effects of the present invention cannot be obtained if the component composition range does not satisfy the predetermined requirements.
[0133]
【The invention's effect】
According to the present invention, a good platability was obtained, and a high-strength, high-ductility hot-dip galvanized steel sheet having a strength exceeding 980 MPa and a strength-ductility balance (TS × El.) Exceeding 21000 (MPa ·%) was obtained.

Claims (12)

質量%で、
C:0.0001〜0.3%、
Si:0.1〜4%、
Al:0.001〜4%、
Mn:0.001〜3%、
P:0.0001〜0.3%、
S:0.01%以下を含有し、Ni、Cuの1種又は2種を合計で0.001〜4%含有し、さらに、Ti、Nbの1種又は2種を合計で0.001〜1%含有し、残部鉄及び不可避的不純物からなり、ミクロ組織が、体積分率で、主相としてフェライトを50%以上含有し、オーステナイトを3〜50%未満含有し、残部組織が、ベイナイト、又は、マルテンサイト及びベイナイトよりなる鋼板の表面に、質量%で、
Al:0.001〜0.5%、
Fe:5〜20%を含有し、残部がZn及び不可避不純物からなる溶融亜鉛めっき層を有し、引張強さが980MPa超であることを特徴とする高強度高延性溶融亜鉛めっき鋼板。
% By mass
C: 0.0001 to 0.3%,
Si: 0.1 to 4%,
Al: 0.001 to 4%,
Mn: 0.001 to 3%,
P: 0.0001 to 0.3%,
S: 0.01% or less, 0.001 to 4% in total of one or two of Ni and Cu, and further 0.001 to 1 in total of one or two of Ti and Nb 1% content, consisting of the balance iron and inevitable impurities, the microstructure is volume fraction, containing 50% or more of ferrite as the main phase, containing less than 3-50% austenite, the balance structure being bainite, Or, on the surface of the steel plate made of martensite and bainite, in mass%,
Al: 0.001 to 0.5%,
A high-strength, high-ductility hot-dip galvanized steel sheet comprising Fe: 5 to 20%, the balance having a hot-dip galvanized layer composed of Zn and inevitable impurities, and having a tensile strength of over 980 MPa.
質量%で、
C:0.0001〜0.3%、
Si:0.1〜4%、
Al:0.001〜4%、
Mn:0.001〜3%、
P:0.0001〜0.3%、
S:0.01%以下を含有し、Ni、Cuの1種又は2種を合計で0.001〜4%含有し、さらに、Ti、Nbの1種又は2種を合計で0.001〜1%含有し、残部鉄及び不可避的不純物からなり、ミクロ組織が、体積分率で、主相としてフェライトを50%以上含有し、オーステナイトを3〜50%未満含有し、残部組織が、ベイナイト、又は、マルテンサイト及びベイナイトよりなる鋼板の表面に、質量%で、
Al:0.001〜0.5%、
Fe:5%未満を含有し、残部がZn及び不可避不純物からなる溶融亜鉛めっき層を有し、引張強さが980MPa超であることを特徴とする高強度高延性溶融亜鉛めっき鋼板。
% By mass
C: 0.0001 to 0.3%,
Si: 0.1 to 4%,
Al: 0.001 to 4%,
Mn: 0.001 to 3%,
P: 0.0001 to 0.3%,
S: 0.01% or less, 0.001 to 4% in total of one or two of Ni and Cu, and further 0.001 to 1 in total of one or two of Ti and Nb 1% content, consisting of the balance iron and inevitable impurities, the microstructure is volume fraction, containing 50% or more of ferrite as the main phase, containing less than 3-50% austenite, the balance structure being bainite, Or, on the surface of the steel plate made of martensite and bainite, in mass%,
Al: 0.001 to 0.5%,
Fe: A high-strength, high-ductility hot-dip galvanized steel sheet having a hot-dip galvanized layer containing less than 5%, the balance being Zn and inevitable impurities, and having a tensile strength of over 980 MPa.
さらに、鋼中に、質量%で、
Mo:0.001〜4%、
Cr:0.001〜4%、
Co:0.001〜4%
の1種又は2種以上を含有することを特徴とする請求項1又は2に記載の高強度高延性溶融亜鉛めっき鋼板。
Furthermore, in steel,
Mo: 0.001 to 4%,
Cr: 0.001 to 4%,
Co: 0.001 to 4%
The high-strength, high-ductility hot-dip galvanized steel sheet according to claim 1, comprising one or more of the following.
前記めっき層中のCu含有率(質量%)と鋼中のCu含有率(質量%)の比をa、めっき層中のNi含有率(質量%)と鋼中のNi含有率(質量%)の比をbとすると、
0.0001≦a+b≦0.2
を満たすことを特徴とする請求項1〜3のいずれか1項に記載の高強度高延性溶融亜鉛めっき鋼板。
The ratio of the Cu content (mass%) in the plating layer to the Cu content (mass%) in steel is a, the Ni content (mass%) in the plating layer and the Ni content (mass%) in the steel. If the ratio of is b,
0.0001 ≦ a + b ≦ 0.2
The high strength and high ductility hot dip galvanized steel sheet according to any one of claims 1 to 3, wherein:
さらに、鋼中に、質量%で、
V:0.001〜1%を含有することを特徴とする請求項1〜4のいずれか1項に記載の高強度高延性溶融亜鉛めっき鋼板。
Furthermore, in steel,
V: 0.001-1% is contained, The high intensity | strength high ductility hot dip galvanized steel plate of any one of Claims 1-4 characterized by the above-mentioned.
さらに、鋼中に、質量%で、
B:0.0001〜0.1%を含有することを特徴とする請求項1〜5のいずれか1項に記載の高強度高延性溶融亜鉛めっき鋼板。
Furthermore, in steel,
B: 0.0001-0.1% is contained, The high intensity | strength highly ductile hot-dip galvanized steel plate of any one of Claims 1-5 characterized by the above-mentioned.
さらに、鋼中に、質量%で、Mg、Zr、Ca、Y、Hf、La−Ceの元素群中から1種又は2種以上を、合計で0.001〜1%含有することを特徴とする請求項 1〜6のいずれか1項に記載の高強度高延性溶融亜鉛めっき鋼板。Furthermore, the steel contains 0.001 to 1% in total of one or more of Mg, Zr, Ca, Y, Hf, and La—Ce in an element group by mass%. The high strength and high ductility hot dip galvanized steel sheet according to any one of claims 1 to 6. 前記めっき鋼板の垂直断面において、めっき相/鋼板界面から、MgO、CaO、ZrO、CeO、La、HfO、TiO、Y、SiO、MnO、及び、Alの1種又は2種以上の内部酸化物が存在する最大深さまでの範囲における鋼板中に、MgO、CaO、ZrO、CeO、La、HfO、TiO、Y、SiO、MnO、及び、Alの1種又は2種以上が、面積率で1%以上含まれることを特徴とする請求項1〜7のいずれか1項に記載の高強度溶融亜鉛めっき鋼板。In the vertical cross section of the plated steel sheet, MgO, CaO, ZrO 2 , CeO 2 , La 2 O 3 , HfO 2 , TiO 2 , Y 2 O 3 , SiO 2 , MnO, and Al 2 from the plating phase / steel sheet interface. In the steel sheet in the range up to the maximum depth where one or more internal oxides of O 3 are present, MgO, CaO, ZrO 2 , CeO 2 , La 2 O 3 , HfO 2 , TiO 2 , Y 2 O 3, SiO 2, MnO, and one or two or more of Al 2 O 3 is a high strength according to claim 1, characterized in that contained 1% or more in area ratio Hot dip galvanized steel sheet. 前記めっき鋼板の垂直断面において、めっき相/鋼板界面から10μmまでの鋼板中に、MgO、CaO、ZrO、CeO、La、HfO、TiO、Y、SiO、MnO、及び、Alの1種又は2種以上の合計を、面積率で0.1%以上含むことを特徴とする請求項 1〜8のいずれか1項に記載の高強度高延性溶融亜鉛めっき鋼板。In the vertical section of the plated steel sheet, in the steel sheet from the plating phase / steel sheet interface to 10 μm, MgO, CaO, ZrO 2 , CeO 2 , La 2 O 3 , HfO 2 , TiO 2 , Y 2 O 3 , SiO 2 , MnO, and high strength and high ductility according to any one of claims 1-8, characterized in that it comprises one or sum of two or more of Al 2 O 3, an area ratio of 0.1% or more Hot dip galvanized steel sheet. 前記フェライトの体積率80%以上の平均粒径が、5μm以下であることを特徴とする請求項1〜9のいずれか1項に記載の高強度高延性溶融亜鉛めっき鋼板。10. The high-strength, high-ductility hot-dip galvanized steel sheet according to claim 1, wherein an average particle diameter of the ferrite having a volume ratio of 80% or more is 5 μm or less. 請求項1〜7のいずれか1項に記載の成分組成からなる鋳造スラブを、鋳造まま、又は、一旦冷却した後に再度加熱し、熱延後巻き取った熱延鋼板を酸洗後冷延した後、めっきラインにおける酸化帯において、燃焼空気比0.9〜1.5の雰囲気内にて酸化せしめ、その後、還元帯にて、水分圧と水素分圧の常用対数log(PHO/PH)が下記(1)式を満たす還元雰囲気にて、Ac1+30(℃)以上、Ac3−10(℃)以下で焼鈍した後に、0.1℃/秒以上の冷却速度で650〜800℃の温度域に冷却し、引き続いて、1〜100℃/秒の冷却速度で、Znめっき浴温度〜Znめっき浴温度+100(℃)まで冷却した後、350〜Znめっき浴温度+100(℃)の温度域で、後続のめっき浴の浸漬時間を含めて1〜3000秒間保持した後、Znめっき浴に浸漬して、その後、室温まで冷却し、980MPa超の引張強さを有する高強度高延性溶融亜鉛めっき鋼板を製造することを特徴とする高強度高延性溶融亜鉛めっき鋼板の製造方法。
−0.8≧log(PHO/PH) ・・・・・(1)
The cast slab having the component composition according to any one of claims 1 to 7 is cast as it is or after being cooled again, and then heated again, and the hot-rolled steel sheet wound up after hot rolling is pickled and then cold-rolled. Then, in the oxidation zone in the plating line, it was oxidized in an atmosphere with a combustion air ratio of 0.9 to 1.5, and then in the reduction zone, the logarithm logarithm of the water pressure and hydrogen partial pressure (PH 2 O / PH 2 ) In a reducing atmosphere satisfying the following formula (1), after annealing at Ac1 + 30 (° C.) or more and Ac 3-10 (° C.) or less, a temperature of 650 to 800 ° C. at a cooling rate of 0.1 ° C./second or more. Then, after cooling to a Zn plating bath temperature to a Zn plating bath temperature +100 (° C.) at a cooling rate of 1 to 100 ° C./second, a temperature range of 350 to a Zn plating bath temperature +100 (° C.) Including the immersion time of the subsequent plating bath, After holding for 3000 seconds, immersed in a Zn plating bath, and then cooled to room temperature, producing a high strength and high ductility hot dip galvanized steel sheet having a tensile strength of over 980 MPa. Manufacturing method of galvanized steel sheet.
−0.8 ≧ log (PH 2 O / PH 2 ) (1)
前記Znめっき浴に浸漬した後、浴温〜Znめっき浴温度+100(℃)の温度範囲で1〜300秒間の保持を行い、室温まで冷却することを特徴とする請求項11に記載の高強度高延性溶融亜鉛めっき鋼板の製造方法。The high strength according to claim 11, wherein after being immersed in the Zn plating bath, holding is performed in a temperature range of bath temperature to Zn plating bath temperature + 100 (° C.) for 1 to 300 seconds and cooling to room temperature. A method for producing a high ductility hot-dip galvanized steel sheet.
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