JP3584911B2

JP3584911B2 - High tensile hot dip galvanized steel sheet and high tensile alloyed hot dip galvanized steel sheet

Info

Publication number: JP3584911B2
Application number: JP2001263156A
Authority: JP
Inventors: 善継鈴木; 一章京野
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2000-12-05
Filing date: 2001-08-31
Publication date: 2004-11-04
Anticipated expiration: 2021-08-31
Also published as: JP2002235160A

Description

【０００１】
【発明の属する技術分野】
本発明は、高張力（合金化）溶融亜鉛めっき鋼板（鋼帯を含む）に係わり、特に複雑な形状にプレス成形する場合にも充分に耐え得る強度延性バランスおよびめっき密着性に優れた高張力（合金化）溶融亜鉛めっき鋼板、加えて、さらに耐食性に優れた高張力（合金化）溶融亜鉛めっき鋼板に関するものである。
【０００２】
【従来の技術】
近年、地球環境の保全という観点から、自動車の燃費改善が要求されている。さらに、衝突時に乗員を保護するため、自動車車体の安全性向上も要求されている。このような状況から、自動車車体の軽量化および自動車車体の強化が積極的に進められている。特に、自動車車体の軽量化のために、熱延鋼板および冷延鋼板等の自動車用鋼板を高強度化し、鋼板板厚を低減することが提案されている。一方、鋼板を素材とする自動車用部品の多くがプレス加工によって成形されるため、自動車用鋼板には優れたプレス成形性が要求される。また、溶融亜鉛めっき鋼板は防錆性（耐食性）に優れ、安価に製造できるため、自動車車体用防錆表面処理鋼板として多用されている。
【０００３】
鋼板を高強度化するには、Ｓｉ、Ｍｎ等の元素を添加し、固溶強化等を図る必要があるが、Ｓｉ、Ｍｎは易酸化性元素であるため焼鈍時にＳｉ、Ｍｎ等が表面に濃化して、その表面に施される溶融亜鉛めっきの濡れ性が悪化し、めっき密着性が劣化する。
上記問題を解決するために、例えば特開平５−１７９３５６号公報や特開平５−５１６４７号公報では、Ｓｉ、Ｍｎの添加量を減らし、熱延巻取り時に焼き入れ急冷し、溶融亜鉛めっきラインにおいて、二相域で焼鈍した後、めっきする方法が提案されている。しかしながら、実際には、Ｓｉが少しでも添加されていると、めっき密着性が劣化して、めっき剥離が生じやすいため、従来は、Ｓｉ、Ｍｎ含有量が多い鋼板に、めっき密着性が良好な溶融亜鉛めっきを施すことは事実上不可能とされていた。
【０００４】
また、良好な延びと強度を両立するために、溶融亜鉛めっき鋼板の最終組織を、焼き戻しマルテンサイト、残留オーステナイトを含み、残部をフェライトと低温変態相からなる複合組織とする必要がある。そのためにはＳｉ、Ｍｎを多量に複合添加することが有効であるが、上記のようにＳｉ、Ｍｎを多量に含むとめっき密着性が劣化する。
【０００５】
【発明が解決しようとする課題】
本発明は、上記問題を解決しようとするもので、下地鋼板がＳｉ、Ｍｎを多量に含んでいても溶融亜鉛めっき密着性に優れ、かつ、プレス成形性等の機械的特性、強度延性バランスに優れた高張力（合金化）溶融亜鉛めっき鋼板を提供することを目的とする。加えて、さらに耐食性に優れた高張力（合金化）溶融亜鉛めっき鋼板を提供することを目的とする。
【０００６】
【課題を解決するための手段】
本発明者は、高張力溶融亜鉛めっき鋼板において、Ｓｉ、Ｍｎを多量に含有し機械的特性を維持したままの鋼板の表層への溶融亜鉛めっき密着性の劣化を阻止するための条件を鋭意調査したところ、鋼中のＭｎ／Ｓｉ質量比を一定範囲内とし、かつ鋼中のＭｎ／Ｓｉ質量比とめっき層中のＡｌ濃度の関係を特定化すれば、上記目的を達成できることを見出した。
【０００７】
すなわち、本発明は、
質量％で、
Ｃ：０．０５〜０．２５％、
Ｓｉ：０．５０％超２．００％未満、
Ｍｎ：３．５％以下および、
Ａｌ：０．０１〜１．０％を含み、
残部がＦｅおよび不可避不純物からなる組成で、鋼中のＭｎ／Ｓｉ質量比が２以上であり、かつ焼き戻しマルテンサイト、残留オーステナイト、フェライトおよび低温変態相からなり、体積分率で、前記焼き戻しマルテンサイトが２０％以上で、前記残留オーステナイトが２％以上の複合組織を有する鋼板上に、めっき層中のＡｌ濃度が下記の式（１）を満たす溶融亜鉛めっき層を有することを特徴とする強度延性バランスおよびめっき密着性に優れた高張力溶融亜鉛めっき鋼板である。
【０００８】
０．６７−１／５０（Ｍｎ／Ｓｉ）≧［めっき層中のＡｌ濃度（質量％）］≧０．３７−１／５０（Ｍｎ／Ｓｉ）・・・（１）
（式（１）のＭｎ／Ｓｉは鋼中のＭｎ／Ｓｉ質量比を表わす。）
【０００９】
好ましい本発明は、前記鋼板成分に加えて、さらに、下記の群から選択された少なくとも１種の成分を含む強度延性バランスおよびめっき密着性に優れた高張力溶融亜鉛めっき鋼板である。
（第１群）質量％で、Ｃｒ：１．０％以下、Ｍｏ：１．０％以下およびＢ：０．００３％以下からなる群から選択された少なくとも１種。
（第２群）質量％で、Ｔｉ：０．１％以下、Ｎｂ：０．１％以下およびＶ：０．１％以下からなる群から選択された少なくとも１種。
（第３群）質量％で、Ｃｕ：１．０％以下およびＮｉ：１．０％以下からなる群から選択された少なくとも１種。
【００１０】
好ましい本発明は、前記鋼板組成において、質量％で、鋼中のＳｉとＭｎの合計が３％以上である強度延性バランスおよびめっき密着性に加えて、耐食性に優れた高張力溶融亜鉛めっき鋼板である。
【００１１】
また、本発明は、
質量％で、
Ｃ：０．０５〜０．２５％、
Ｓｉ：０．５０％超２．００％未満、
Ｍｎ：３．５％以下および、
Ａｌ：０．０１〜１．０％を含み、
残部がＦｅおよび不可避不純物からなる組成で、鋼中のＭｎ／Ｓｉ質量比が２以上であり、かつ焼き戻しマルテンサイト、残留オーステナイト、フェライトおよび低温変態相からなり、体積分率で、前記焼き戻しマルテンサイトが２０％以上で、前記残留オーステナイトが２％以上の複合組織を有する鋼板上に、めっき層中のＡｌ濃度が下記の式（２）を満たす溶融亜鉛めっき層を有することを特徴とする強度延性バランスおよびめっき密着性に優れた高張力溶融亜鉛めっき鋼板である。
【００１２】
０．５−１／５０（Ｍｎ／Ｓｉ） ≧［めっき層中のＡｌ濃度（質量％）］≧０．２−１／５０（Ｍｎ／Ｓｉ）
・・・（２）
（式（２）のＭｎ／Ｓｉは鋼中のＭｎ／Ｓｉ質量比を表わす。）
【００１３】
好ましい本発明は、前記鋼板成分に加えて、さらに、下記の群から選択された少なくとも１種の成分を含む強度延性バランスおよびめっき密着性に優れた高張力溶融亜鉛めっき鋼板である。
（第１群）質量％で、Ｃｒ：１．０％以下、Ｍｏ：１．０％以下および
Ｂ：０．００３％以下からなる群から選択された少なくとも１種。
（第２群）質量％で、Ｔｉ：０．１％以下、Ｎｂ：０．１％以下および
Ｖ：０．１％以下からなる群から選択された少なくとも１種。
（第３群）質量％で、Ｃｕ：１．０％以下およびＮｉ：１．０％以下からなる群から選択された少なくとも１種。
【００１４】
好ましい本発明は、前記鋼板組成において、質量％で、鋼中のＳｉとＭｎの合計が３％以上である強度延性バランスおよびめっき密着性に加えて、耐食性に優れた高張力溶融亜鉛めっき鋼板である。
【００１５】
【発明の実施の形態】
本発明は以下の実験事実に基づいて完成されたものである。以下、鋼組成は質量分率で表す。
Ｃ：０．１０％、Ｓｉ：０．３〜２％、Ｍｎ：１．０〜３．５％、Ｐ：０．０１％、Ａｌ：０．０４％を含有する厚さ３０ｍｍのシートバーを１２５０℃で加熱し、５パスで厚さ２．３ｍｍの熱延鋼板とした後、６２０℃で巻き取った。次いで、酸洗により黒皮を除去し、１．０ｍｍまで冷間圧延し、焼鈍炉において８００〜９００℃で加熱後、６０℃５％塩酸で６秒間酸洗した。その後、溶融亜鉛めっきシミュレーターにより７８０℃で５％Ｈ_２−Ｎ_２還元性雰囲気中で焼鈍し、４６０℃の溶融亜鉛めっき浴（０．０８〜０．２５％Ａｌ−Ｚｎ）で溶融亜鉛めっきし、付着量を片面５０ｇ／ｍ^２になるようにガスワイピングした。合金化は通電炉において５００℃で実施した。得られためっき鋼板のめっき密着性および機械的特性を調査した。
【００１６】
以下、合金化した溶融亜鉛めっき鋼板をＧＡ、合金化していない溶融亜鉛めっき鋼板をＧＩで表し、また冷延鋼板をＣＲ、熱延鋼板をＨｏｔで表して、鋼中のＭｎ／Ｓｉ質量比とめっき層中のＡｌ濃度との関係について説明する。
上記実験結果から、図１（ＧＩの鋼中のＭｎ／Ｓｉ質量比とめっき層中のＡｌ濃度との関係）および図２（ＧＡの鋼中のＭｎ／Ｓｉ質量比とめっき層中のＡｌ濃度との関係）に示すように、鋼中のＭｎ／Ｓｉ質量比が２未満であると、機械的特性やめっき品質が不良であること、およびめっき層中のＡｌ濃度が低過ぎたり、逆に高過ぎる場合には、めっき密着性の劣化や合金化遅延あるいは溶接性不良などが起きることを初めて知見し、前記式（１）または式（２）を誘導した。
【００１７】
そして、鋼中のＭｎ／Ｓｉ質量比とめっき層中のＡｌ濃度を、前記式（１）または式（２）を満足するように調整すれば、驚くべきことに鋼中にＳｉ、Ｍｎが存在しても、その表面への溶融亜鉛めっきの密着性が良好なＧＩおよびＧＡが製造できることを知見し、本発明を完成した。
そして、前記式（１）を満たすＧ１について、鋼中のＳｉ含有量とＭｎ含有量が各種特性に及ぼす影響を図３に示した。鋼中のＳｉ含有量が０．５０％超２．００％未満、Ｍｎ／Ｓｉ質量比が２以下のいずれをも満たす場合には、機械的特性不良、めっき密着性不良が生じず、耐食性も平均レベルを維持できる。
【００１８】
また、前記式（２）を満たすＧＡについて、鋼中のＳｉ含有量とＭｎ含有量が各種特性に及ぼす影響を図４に示した。ＧＩの場合と同様、鋼中のＳｉ含有量が０．５０％超２．００％未満、Ｍｎ／Ｓｉ質量比が２以下のいずれをも満たす場合には、機械的特性不良、めっき密着性不良が生じず、耐食性についても平均レベル維持できる。
さらに、ＧＩ，ＧＡのいずれの場合においても、Ｓｉ含有量とＭｎ含有量の合計量が３％以上となると耐食性の向上に極めて有利になることがわかる。
【００１９】
なお、図３、図４において、機械的特性については、引張強さが５９０ＭＰａ以上でかつ伸び率が３５％以上であるものを良好とし、これ以外を不良とした。まためっき密着性については、後述する表５の基準を用いた密着性試験を行い、ランク３以上のものを不良とした。
耐食性の評価は、ＧＩについては、後述する複合腐食サイクル試験の結果、最大侵食深さが軟鋼（ＳＰＣＣ）の溶融亜鉛めっき鋼板を比較鋼板とし、これと同等の場合を耐食性平均レベルとし、孔食が見られないものを極めて良好と評価した。ＧＡについては、比較鋼板を軟鋼（ＳＰＣＣ）の合金化溶融亜鉛めっき鋼板として、ＧＩの場合と同様に評価した。
【００２０】
本発明において鋼中の構成成分の含有量およびその複合組織を規定した理由は次の通りである。なお、各元素の含有量（％）は質量％を意味する。
Ｃ：０．０５〜０．２５％
Ｃは、必要強度を得るためと残留オーステナイト等の所望の組織を得るために不可欠な成分であり、少なくとも０．０５％が必要であるが、０．２５％を超えると溶接性が悪化するため上記範囲とした。好ましくは０．０７〜０．１８％である。
【００２１】
Ｓｉ：０．５０％超２．００％未満
Ｓｉは、固溶強化と所望の組織を得るために不可欠な成分であり、延性を劣化させずに高強度化を図ることを可能にする成分である。その効果は０．５０％より多くないと発揮されない。一方、２．００％以上になると、めっき密着性が劣化する。そのため上記範囲とした。好ましくは０．６〜１．６％である。
【００２２】
Ｍｎ：３．５％以下
Ｃと同様に必要強度を得るため、また鋼の焼き入れ性を向上させ所望の組織を得るために不可欠な成分であり、１％以上であれば、効果が十分発揮され、好ましい。しかし、３．５％を超えても効果が飽和しコストの上昇を招くため上記範囲とした。
【００２３】
Ａｌ：０．０１〜１．０％
Ａｌは必要強度を得るため、また所望の組織を得るために重要であり、結果としてＳｉ添加量を低減できるため、同等の引張強さを有するＳｉ添加鋼よりめっき密着性の改善に有利である。所望の効果を得るためには最低０．０１％必要である。１．０％を超えると経済性が劣化するため上限を１．０％とした。好ましくは０．０２〜０．５％である。
【００２４】
鋼中のＭｎ／Ｓｉ質量比：２以上
めっき密着性向上効果を得るためには、鋼中のＭｎ／Ｓｉ質量比が高い方が有利である。これはめっき直前の焼鈍時に生成する表面濃化物が、Ｓｉ主体の酸化物から、溶融亜鉛との濡れ性が良好であるＳｉ−Ｍｎ系複合酸化物に変化するためである。さらには、鋼板を連続溶融亜鉛めっき設備（ＣＧＬと表す。）通板前にあらかじめ加熱処理し、次いで、冷却後表面を酸洗処理することにより表面を活性化する場合に、鋼中のＭｎ／Ｓｉ質量比が高い方が酸化皮膜の酸洗性向上効果が得られる。これは、鋼板表面の酸化物が難酸洗性であるＳｉ主体の酸化物から酸洗性良好なＳｉ−Ｍｎ系複合酸化物に変化するためと考えられる。
【００２５】
また、鋼中のＭｎ／Ｓｉ質量比が高い方が合金化を遅延させずにすむため、溶融亜鉛めっき後、合金化処理する場合の生産性も向上する。鋼中のＭｎ／Ｓｉ質量比が２未満であると、めっき密着性が劣化するため、鋼中のＭｎ／Ｓｉ質量比を２以上と規定した。ただし、鋼中のＭｎ／Ｓｉ質量比が７より高くなると、相対的にＳｉ含有量が小さくなるため、所望のミクロ複合組織を得ることが困難になることがあるので、鋼中のＭｎ／Ｓｉ質量比は７以下であるのが好ましい。より好ましいのは２．２〜４．５である。
【００２６】
鋼中の（Ｓｉ＋Ｍｎ）量：３質量％以上
そもそも亜鉛系めっき鋼板は、耐食性は元来良好であるものの、近年の自動車用鋼板における耐孔食性向上の要求など、さらなる耐食性の向上が望まれており、耐食性をさらに向上させるためには、鋼中のＭｎ含有量とＳｉ含有量の合計を３質量％以上とするのが好ましい。各鋼中成分の含有量が前記範囲にあり、かつ（Ｓｉ＋Ｍｎ）含有量が３質量％以上になると、緻密で不活性なＳｉＯ_２皮膜の鋼板表面への生成が抑制され、鋼中への酸素拡散が進みやすくなるため、熱延鋼板巻取り時や一次加工工程における加熱時に内部酸化が促進される。鋼中の（Ｓｉ＋Ｍｎ）含有量が３質量％以上であっても、鋼中のＭｎ／Ｓｉ質量比が低く、２未満の場合には、めっき密着性が悪く、耐食性も充分でない。
【００２７】
耐食性が向上する作用機構は明らかでないが、内部酸化により、溶融亜鉛めっき時にアンカー効果などにより、めっき密着性がさらに向上し、耐食性向上に寄与するものと推定される。
【００２８】
焼き戻しマルテンサイト
焼き戻しマルテンサイトは、焼き戻しにより軟質化し充分な塑性変形能を有するため、伸び特性の向上に有効である。体積分率２０％未満では、延性向上効果が認められないため２０％以上とした。但し８０％を超えると鋼板の高強度化が困難となるため８０％以下が好ましい。
【００２９】
残留オーステナイト
残留オーステナイトは加工時にマルテンサイトに歪誘起変態し、局部的に加えられた加工歪みを広く分散させ、鋼板の延性を向上させる効果を有する。体積分率２％未満では延性の顕著な向上が期待できないため、２％以上であることが必要であり、５％以上であると延性向上効果がより顕著であるため好ましい。
【００３０】
上記複合組織を得る製造法は特に限定されないが、例えば、鋼板をあらかじめ（Ａｃ_３変態点−５０℃）以上に加熱した後、１０℃／秒以上の冷却速度で組織を焼き入れし、次いでＣＧＬにてＡｃ_１〜Ａｃ_３変態点の間で加熱し、５℃／秒以上の冷却速度で冷却して組織を焼き戻すことにより得られる。ただし、これは１例であり、本発明が規定する成分と相を有する高張力鋼板であれば、どのような製造方法で得られたものでもよい。
【００３１】
本発明でいう低温変態相とは、マルテンサイトあるいはベイナイトを指す。
マルテンサイト、ベイナイトとも硬質相であり、組織強化によって鋼板強度を増加させる作用を有する。また、変態時に可動転位の発生を伴うため、鋼板の降伏比を低下させる作用も有する。なお、このような効果を充分に得るためには、低温変態相はマルテンサイトとするのが好適である。本発明において低温変態相の量は特に限定されない。鋼板の強度に応じて適宜配分すればよい。
【００３２】
めっき層中のＡｌ濃度
本発明では、めっき層中のＡｌ濃度は重要である。本発明者は、前述のようにＭｎ／Ｓｉ質量比が２以上を満たして、Ｍｎ、Ｓｉを含有する鋼板では、ＧＩの場合は前記式（１）、ＧＡの場合は前記式（２）の範囲内で所定のめっき層中のＡｌ濃度を確保することにより良好なめっき密着性が確保できることを知見した。ＧＩの場合は、めっき後の復熱などによる合金化反応の開始を抑制する必要があるため、ＧＡの場合よりＡｌ濃度を高めにする。また、Ｓｉ含有量が多い場合には、局所的合金化反応が多発するので、これに基づく合金化反応の開始を抑制するため、Ｍｎ／Ｓｉ質量比が低いほど、Ａｌ含有量を多くする。
【００３３】
Ａｌ濃度が前記式（１）または式（２）の左辺の値より高いと、ＧＩの場合はめっき時の初期に生成するＦｅ−Ａｌ合金層が厚いため溶接性が劣化したり、ＧＡの場合は合金化が著しく遅延する。Ａｌ濃度が前記式（１）または式（２）の右辺の値より低いと、Ｆｅ−Ａｌ合金層の生成が抑えられ、硬くて脆いΓ相がめっき初期に生成しやすくなり、めっき密着性が劣化する。したがって、良好なめっき密着性を確保するためには、ＧＩの場合は前記式（１）、ＧＡの場合は前記式（２）の範囲内に所定のＡｌ濃度を維持することが必要である。
【００３４】
めっき層中のＡｌ濃度を所定量にするための方法は特に限定されないが、例えば、めっき浴中のＡｌ量を高めにしたり、めっき時間を長くすることによりＡｌと地鉄の反応を促進してめっき層中のＡｌ濃度を高くする方法が例示される。また、本発明の鋼中成分を有する鋼板をあらかじめ加熱し、冷却後に表面を軽く酸洗し、表面を活性化した後に、ＣＧＬに通板してもよい。これらの製造方法の例が本発明を限定するものではない。
【００３５】
さらに、本発明の高張力溶融亜鉛めっき鋼板または高張力合金化溶融亜鉛めっき鋼板の鋼板中には、下記の元素を下記の量（質量％）で、１種以上含有していてもよい。その場合はさらに、以下の効果を有する。
Ｃｒ：１．０％以下
Ｃｒは焼き入れ性を向上し、低温変態相の生成を促進する作用を有するため必要に応じて添加する。好ましくは０．０５％以上であるが、１．０％を超えるとめっき密着性が劣化するため上限を１．０％とするのが好ましい。
【００３６】
Ｍｏ：１．０％以下
ＭｏはＣｒと同様に焼き入れ性を向上し、低温変態相の生成を促進する作用を有するため必要に応じて添加する。好ましくは０．０５％以上であるが、１．０％を超えるとコスト上昇を招くため、上限は１．０％とするのが好ましい。
【００３７】
Ｂ：０．００３％以下
Ｂは焼き入れ性を向上させる作用を有するため必要に応じて添加する。ただし、０．００３％を超えるとめっき密着性が劣化するため、０．００３％を上限とするのが好ましい。
【００３８】
Ｔｉ、Ｎｂ、Ｖ：０．１％以下
Ｔｉ、ＮｂおよびＶは炭窒化物を形成し、鋼を析出強化により高強度化する作用を有するため必要に応じて添加する。これらを添加する場合は、それぞれ０．０１％以上添加するのが好ましい。ただし、０．１％を超えても過度に高強度化し、延性がかえって劣化するため、上限は０．１％とするのが好ましい。
【００３９】
Ｃｕ：１．０％以下
Ｃｕはオーステナイト中に偏析し、必要強度を得るためと所望の組織を得るために重要であるだけでなく、めっき密着性を向上させる効果もあるため必要に応じて添加する。めっき密着性が向上する理由は現時点では明らかになっていないが、所望の効果を得るためには０．０１％以上の添加が好ましい。ただし、１．０％を超えると経済性が劣化するため、上限を１．０％とするのが好ましい。
【００４０】
Ｎｉ：１．０％以下
ＮｉはＣｕと同様オーステナイト中に偏析し、必要強度を得るためと所望の組織を得るため重要であるだけでなく、めっき密着性を向上させる効果もあるので必要に応じて添加する。めっき密着性が向上する理由は現時点では明らかになっていないが、所望の効果を得るためには、０．０１％以上添加するのが好ましい。ただし、１．０％を超えると経済性が劣化するため、上限を１．０％とするのが好ましい。
【００４１】
ＣＧＬ（溶融亜鉛めっき）条件
本発明の溶融亜鉛めっき鋼板を製造するための鋼板のＣＧＬ条件は特に限定されず、定法により実施可能である。ただし、ＣＧＬ加熱温度（二次加熱温度）が６５０℃以下であると、鋼板表面の酸化皮膜が還元できず、不めっきが発生しやすくなる。一方、８５０℃以上であると、加熱時にＳｉ、Ｍｎの表面濃化が多いため、同様に不めっきが発生しやすい。よって６５０〜８５０℃が好ましい。
【００４２】
また、溶融亜鉛めっき浴は、めっき層の合金化後の密着性を確保するために、Ａｌ濃度を０．０８％以上とすることが好ましい。ただし、０．２０％を超えると合金化が困難になったり、得られる溶融亜鉛めっき鋼板の溶接性が劣化することがあるため、上限は０．２０％が好ましい。なお、前記したように、めっき層中のＡｌ濃度を本発明の範囲に制御するために、浴中のＡｌ濃度の他に、進入板温、めっき浴浸漬時間、その他の操業条件を調整すればよい。
浴温が４４０℃以下であると、めっき浴の浴温変動により凝固点（４２０℃）を下回る箇所が出てくる可能性があり、操業上安定性に欠ける。また、４８０℃を超えると加熱保持にかかるコストがかさむ。そのため浴温は４４０〜４８０℃が好ましい。
【００４３】
合金化する場合には、合金化温度が４５０℃以下だとζ相が生成しやすくなり、ＧＡの摺動性に欠けるおそれがあるだけでなく、合金化に時間がかかるため生産性が劣化する。また、６００℃を超えるとΓ相が生成しやすくなり、ＧＡのめっき密着性に欠けるおそれがある。そのため合金化温度は４５０〜６００℃が好ましい。
【００４４】
さらには、合金化度はＦｅ拡散量が８〜１３％の範囲に収まることが好ましい。８％未満であるとζ相が残存し、耐フレーキング性が劣化しやすく、１３％以上だとΓ相が生成し、めっき密着性が劣化する場合がある。ただし、これらの製造方法の条件は例示であり、本発明は特定の製造方法に限定されない。
【００４５】
【実施例】
以下、実施例を挙げて本発明をより具体的に説明するが、本発明はこれらの実施例により何ら限定されるものではない。
（実施例１〜１０、比較例１〜４）
表１に示した化学組成（Ｃ、Ｓｉ、Ｍｎ、Ｐ、Ｓ、Ａｌ、Ｃｒ、Ｃｕ、Ｎｉ、Ｍｏ、Ｔｉ、Ｎｂ、ＶおよびＢ）のスラブ（厚さ３００ｍｍ）を１２５０℃で加熱し、熱間圧延により厚さ２．０ｍｍの熱延鋼板（鋼Ａ〜Ｈが実施例；鋼Ｉ〜Ｊが比較例）とした後、６２０℃で巻き取った。次いで、酸洗により黒皮除去した後、必要に応じ冷間圧延を行う場合には５０％の圧下率で圧延し、加熱炉で加熱（一次加熱）した後、ＣＧＬに通板して酸洗、焼鈍、溶融亜鉛めっきおよび合金化処理を行った。冷間圧延工程の有無、一次加熱温度、めっき浴中のＡｌ濃度と合金化の有無を表２に示した。なお、一次加熱後の冷却速度は３０℃／秒、二次加熱温度は７８０℃、二次加熱後の冷却速度は１０℃／秒であり、めっき付着量は片面で５０ｇ／ｍ^２ずつであった。合金化温度は４５０〜６００℃の範囲とした。
【００４６】
【表１】

【００４７】
【表２】

【００４８】
得られた溶融亜鉛めっき鋼板および合金化溶融亜鉛めっき鋼板のめっき層中のＡｌ濃度、合金化度（Ｆｅ拡散量）、めっき後の外観、めっき密着性および耐食性についての調査結果を表３に示した。また、得られためっき鋼板の焼き戻しマルテンサイト分率、残留オーステナイト分率、フェライト分率、低温変態相および機械的特性（延び率、引っ張り強度）についての調査結果を表４に示した。
【００４９】
【表３】

【００５０】
【表４】

【００５１】
めっき層中のＡｌ濃度は、めっき層をインヒビターを添加したＮａＯＨ、ＫＯＨなどのアルカリまたはＨＣｌ、Ｈ_２ＳＯ_４などの酸で溶解し、その液をプラズマ発光分光機（ＩＣＰ）などで分析定量することにより測定した。
合金化度（Ｆｅ拡散量）は、同様にＩＣＰなどでＦｅを分析定量することにより測定した。
めっき後の外観は、めっき直後の鋼板の不めっき部の面積を画像処理により定量し、不めっきの面積率が０．１％未満を良好、０．１％以上を不良と評価した。
【００５２】
めっき密着性は、めっき鋼板にセロファンテープを貼りテープ面を９０°内に曲げ、曲げ戻しをした後、テープを剥したときの単位長さ当りのめっき剥離量を蛍光Ｘ線によりＺｎカウント数として測定し、表５の基準に照らしてランク１、２のものを良好（○、△）、３以上のものを不良として評価した。
【００５３】

【００５４】
耐食性評価は、軟鋼板（ＳＰＣＣ）の溶融亜鉛めっき鋼板または合金化溶融亜鉛めっき鋼板を基準に取り、それぞれの試験片を複合腐食サイクル試験（０．５％塩水を３５℃で６Ｈｒ噴霧後、７０℃で６Ｈｒ乾燥し、その後、温度４０℃、湿度９０％の湿潤状態に１２Ｈｒ維持）を６０サイクル、すなわち、６０日間施した後、最大腐食深さを比較した。腐食試験後の試験片は、腐食深さ（孔食）を極値統計処理により最大腐食深さを求めた。基準の鋼板の最大腐食深さとほぼ同等の深さのものを平均レベル、半分以下のものを良好、孔食が見られないものを極めて良好、１．５倍以上のものを不良として評価した。
【００５５】
鋼板の焼き戻しマルテンサイト相の体積分率は、樹脂に埋め込んだ鋼板断面を研磨した後、１ｍａｓｓ％ピロ亜硫酸ナトリウムのピクラール溶液（４ｇピクリン酸／１００ｃｃエタノール）を用いてエッチングした後、電子顕微鏡によって倍率１０００倍で観察後、画像解析によって１００ｍｍ四方の正方形領域内に存在するマルテンサイト相の占有面積率を求め、マルテンサイト相の体積率とした。
【００５６】
フェライト相は、樹脂に埋め込んだ鋼板断面を研磨し、ナイタール溶液（６９ｍａｓｓ％ＨＮＯ_３溶液３ｖｏｌ％、エタノール９７ｖｏｌ％）で組織をエッチングした後、光学顕微鏡で２５０倍で観察した１００ｍｍ四方の正方形領域内の写真を画像処理することにより、フェライト相の占有面積率を求め、フェライト相の体積率とした。
【００５７】
鋼板の残留オーステナイト相の体積分率は、鋼板より採取した試験片を板厚方向の中心面まで研磨し、板厚中心面でのＸ線強度測定により求めた。すなわち、ＭｏＫα線を使用し、フェライトの（２００）（２１１）各面の回折Ｘ線強度と、オーステナイトの（２００）（２２０）各面の回折Ｘ線強度を求め、フェライト（マルテンサイトを含む）の（２００）（２１１）の積分強度とオーステナイト（２００）（２２０）の積分強度の比を求め、これをオーステナイト相の体積分率とした。
【００５８】
機械的特性は、鋼板から圧延方向と直交する方向を引張方向として採取したＪＩＳ５号引張試験片を用いて、降伏強さ（降伏点）ＹＰ、引張強さＴＳ、伸びＥｌを測定し、引張強さが５９０ＭＰａ以上で、かつ伸び率が３５％以上であるものを良好と評価した。
【００５９】
鋼組成、鋼組織およびめっき層が本発明範囲内のものは、いずれもめっき密着性、溶融亜鉛めっき鋼板の機械的特性が良好であった。特に、実施例３は耐食性が極めて優れていた。一方、本発明の範囲外のものは、めっき密着性、機械的特性が劣っていた。
【００６０】
【発明の効果】
以上のように、本発明によれば、めっき密着性、溶融亜鉛めっき鋼板の機械的特性に優れ、さらには耐食性にも優れた高張力溶融亜鉛めっき鋼板および高張力合金化溶融亜鉛めっき鋼板が得られる。本発明の鋼板を適用することにより、自動車車体の軽量化および低燃費化が可能となり、ひいては地球環境の改善にも大きく貢献する。
【図面の簡単な説明】
【図１】溶融亜鉛めっき鋼板（ＧＩ）の鋼中のＭｎ／Ｓｉ質量比とめっき層中のＡｌ濃度との関係を表す図である。
【図２】合金化溶融亜鉛めっき鋼板（ＧＡ）の鋼中のＭｎ／Ｓｉ質量比とめっき層中のＡｌ濃度との関係を表す図である。
【図３】溶融亜鉛めっき鋼板（ＧＩ）の鋼中のＭｎ、Ｓｉ含有量と耐食性との関係を表す図である。
【図４】合金化溶融亜鉛めっき鋼板（ＧＡ）の鋼中のＭｎ、Ｓｉ含有量と耐食性との関係を表す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to high-tensile (alloyed) hot-dip galvanized steel sheets (including steel strips), particularly high tensile strength with excellent strength-ductility balance and plating adhesion that can withstand even when pressed into complex shapes. The present invention relates to a (alloyed) hot-dip galvanized steel sheet and, in addition, to a high-tensile (alloyed) hot-dip galvanized steel sheet having excellent corrosion resistance.
[0002]
[Prior art]
In recent years, there has been a demand for improvement in fuel efficiency of automobiles from the viewpoint of conservation of the global environment. Furthermore, in order to protect a passenger | crew at the time of a collision, the safety improvement of a motor vehicle body is also requested | required. Under such circumstances, the weight reduction of the automobile body and the reinforcement of the automobile body are being actively promoted. In particular, in order to reduce the weight of an automobile body, it has been proposed to increase the strength of steel sheets for automobiles such as hot-rolled steel sheets and cold-rolled steel sheets and reduce the thickness of the steel sheet. On the other hand, since many automotive parts made of steel plates are formed by press working, the automotive steel plates are required to have excellent press formability. Moreover, since the hot dip galvanized steel sheet is excellent in rust prevention (corrosion resistance) and can be manufactured at low cost, it is frequently used as a rust-proof surface-treated steel sheet for automobile bodies.
[0003]
In order to increase the strength of the steel sheet, it is necessary to add elements such as Si and Mn to enhance solid solution. However, since Si and Mn are oxidizable elements, Si, Mn, etc. are on the surface during annealing. It concentrates, the wettability of the hot dip galvanizing applied to the surface deteriorates, and the plating adhesion deteriorates.
In order to solve the above problem, for example, in JP-A-5-179356 and JP-A-5-51647, the addition amount of Si and Mn is reduced, quenching and quenching at the time of hot rolling, and in a hot dip galvanizing line A method of plating after annealing in a two-phase region has been proposed. However, in practice, if Si is added even a little, plating adhesion deteriorates and plating peeling is likely to occur. Conventionally, plating adhesion is good for steel sheets having a high Si and Mn content. It was virtually impossible to apply hot dip galvanizing.
[0004]
In order to achieve both good elongation and strength, it is necessary that the final structure of the hot dip galvanized steel sheet includes a tempered martensite and residual austenite, and the balance is a composite structure composed of ferrite and a low-temperature transformation phase. For this purpose, it is effective to add a large amount of Si and Mn. However, if a large amount of Si and Mn is contained as described above, the plating adhesion deteriorates.
[0005]
[Problems to be solved by the invention]
The present invention is intended to solve the above problem, and even if the base steel sheet contains a large amount of Si and Mn, it has excellent hot dip galvanizing adhesion, and has a mechanical property such as press formability and a balance of strength ductility. An object is to provide an excellent high-tensile (alloyed) hot-dip galvanized steel sheet. In addition, an object is to provide a high-tensile (alloyed) hot-dip galvanized steel sheet that is further excellent in corrosion resistance.
[0006]
[Means for Solving the Problems]
The present inventor has intensively investigated conditions for preventing deterioration of adhesion of hot dip galvanizing to the surface layer of a steel sheet containing a large amount of Si and Mn and maintaining mechanical properties in a high-tensile hot dip galvanized steel sheet. As a result, it has been found that the above object can be achieved if the Mn / Si mass ratio in the steel is within a certain range and the relationship between the Mn / Si mass ratio in the steel and the Al concentration in the plating layer is specified.
[0007]
That is, the present invention
% By mass
C: 0.05 to 0.25%,
Si: more than 0.50% and less than 2.00%,
Mn: 3.5% or less and
Al: 0.01-1.0% included,
The balance is composed of Fe and inevitable impurities, the Mn / Si mass ratio in the steel is 2 or more, and consists of tempered martensite, residual austenite, ferrite and a low-temperature transformation phase, and is tempered in volume fraction. A hot-dip galvanized layer satisfying the following formula (1) is provided on a steel sheet having a composite structure in which martensite is 20% or more and the retained austenite is 2% or more. It is a high-tensile hot-dip galvanized steel sheet with excellent strength ductility balance and plating adhesion.
[0008]
0.67-1 / 50 (Mn / Si) ≧ [Al concentration in plating layer (mass%)] ≧ 0.37-1 / 50 (Mn / Si) (1)
(Mn / Si in formula (1) represents the Mn / Si mass ratio in the steel.)
[0009]
A preferred present invention is a high-tensile hot-dip galvanized steel sheet excellent in strength ductility balance and plating adhesion, which contains at least one component selected from the following group in addition to the steel sheet components.
(First group) At least one selected from the group consisting of Cr: 1.0% or less, Mo: 1.0% or less, and B: 0.003% or less in mass%.
(Second group) At least one selected from the group consisting of Ti: 0.1% or less, Nb: 0.1% or less, and V: 0.1% or less in terms of mass%.
(Third Group) At least one selected from the group consisting of Cu: 1.0% or less and Ni: 1.0% or less by mass%.
[0010]
The present invention is preferably a high-tensile hot-dip galvanized steel sheet having excellent corrosion resistance, in addition to the strength ductility balance and plating adhesion, in which the steel sheet composition has a mass% and the sum of Si and Mn in the steel is 3% or more. is there.
[0011]
The present invention also provides:
% By mass
C: 0.05 to 0.25%
Si: more than 0.50% and less than 2.00%,
Mn: 3.5% or less and
Al: 0.01-1.0% included,
The balance is composed of Fe and inevitable impurities, the Mn / Si mass ratio in the steel is 2 or more, and consists of tempered martensite, residual austenite, ferrite and a low-temperature transformation phase, and is tempered in volume fraction. A hot-dip galvanized layer satisfying the following expression (2) is provided on a steel sheet having a composite structure in which martensite is 20% or more and the retained austenite is 2% or more. It is a high-tensile hot-dip galvanized steel sheet with excellent strength ductility balance and plating adhesion.
[0012]
0.5-1 / 50 (Mn / Si) ≧ [Al concentration in plating layer (mass%)] ≧ 0.2-1 / 50 (Mn / Si)
(2)
(Mn / Si in formula (2) represents the Mn / Si mass ratio in the steel.)
[0013]
A preferred present invention is a high-tensile hot-dip galvanized steel sheet excellent in strength ductility balance and plating adhesion, which contains at least one component selected from the following group in addition to the steel sheet components.
(Group 1)% by mass, Cr: 1.0% or less, Mo: 1.0% or less and
B: At least one selected from the group consisting of 0.003% or less.
(Group 2) By mass%, Ti: 0.1% or less, Nb: 0.1% or less and
V: At least one selected from the group consisting of 0.1% or less.
(Third Group) At least one selected from the group consisting of Cu: 1.0% or less and Ni: 1.0% or less by mass%.
[0014]
The present invention is preferably a high-tensile hot-dip galvanized steel sheet having excellent corrosion resistance, in addition to the strength ductility balance and plating adhesion, in which the steel sheet composition has a mass% and the sum of Si and Mn in the steel is 3% or more. is there.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
The present invention has been completed based on the following experimental facts. Hereinafter, the steel composition is expressed by mass fraction.
A sheet bar having a thickness of 30 mm containing C: 0.10%, Si: 0.3-2%, Mn: 1.0-3.5%, P: 0.01%, Al: 0.04% After heating at 1250 ° C. to obtain a hot-rolled steel sheet having a thickness of 2.3 mm by 5 passes, the film was wound at 620 ° C. Next, the black skin was removed by pickling, cold rolled to 1.0 mm, heated at 800 to 900 ° C. in an annealing furnace, and then pickled with 60% 5% hydrochloric acid for 6 seconds. Then, 5% H at 780 ° C by hot dip galvanizing simulator ₂ -N ₂ Annealed in a reducing atmosphere, hot dip galvanized in a hot dip galvanizing bath (0.08 to 0.25% Al—Zn) at 460 ° C., the adhesion amount was 50 g / m on one side ² Gas wiping was performed. Alloying was performed at 500 ° C. in an electric furnace. The plating adhesion and mechanical properties of the obtained plated steel sheet were investigated.
[0016]
Hereinafter, the alloyed hot-dip galvanized steel sheet is represented by GA, the non-alloyed hot-dip galvanized steel sheet is represented by GI, the cold-rolled steel sheet is represented by CR, the hot-rolled steel sheet is represented by Hot, and the Mn / Si mass ratio in the steel The relationship with the Al concentration in the plating layer will be described.
From the above experimental results, FIG. 1 (relation between the Mn / Si mass ratio in the steel of GI and the Al concentration in the plating layer) and FIG. 2 (Mn / Si mass ratio in the steel of GA and the Al concentration in the plating layer). As shown in Fig. 3), if the Mn / Si mass ratio in the steel is less than 2, the mechanical properties and plating quality are poor, and the Al concentration in the plating layer is too low. When it was too high, it was discovered for the first time that plating adhesion deterioration, alloying delay or poor weldability occurred, and the formula (1) or formula (2) was derived.
[0017]
If the Mn / Si mass ratio in the steel and the Al concentration in the plating layer are adjusted so as to satisfy the formula (1) or the formula (2), surprisingly, Si and Mn exist in the steel. Even then, it was found that GI and GA having good adhesion of hot dip galvanizing to the surface could be produced, and the present invention was completed.
And about G1 which satisfy | fills said Formula (1), the influence which Si content and Mn content in steel exert on various characteristics was shown in FIG. When the Si content in the steel is more than 0.50% and less than 2.00% and the Mn / Si mass ratio is 2 or less, there is no mechanical property failure, poor plating adhesion, and corrosion resistance. The average level can be maintained.
[0018]
Moreover, about GA which satisfy | fills said Formula (2), the influence which Si content and Mn content in steel exert on various characteristics was shown in FIG. As in the case of GI, if the Si content in the steel is more than 0.50% and less than 2.00% and the Mn / Si mass ratio is 2 or less, poor mechanical properties and poor plating adhesion The average level of corrosion resistance can be maintained.
Furthermore, in both cases of GI and GA, it is found that when the total amount of Si content and Mn content is 3% or more, it is extremely advantageous for improving the corrosion resistance.
[0019]
3 and 4, regarding the mechanical properties, those having a tensile strength of 590 MPa or more and an elongation of 35% or more were considered good, and the others were regarded as poor. As for the plating adhesion, an adhesion test using the criteria of Table 5 described later was conducted, and those with rank 3 or higher were regarded as defective.
As for the evaluation of corrosion resistance, as for GI, as a result of the combined corrosion cycle test described later, a hot dip galvanized steel sheet having a maximum erosion depth of mild steel (SPCC) is used as a comparative steel sheet. The case where no was observed was evaluated as extremely good. About GA, it evaluated similarly to the case of GI by making a comparative steel plate into an galvannealed steel plate of mild steel (SPCC).
[0020]
The reason why the content of the constituent components in the steel and the composite structure thereof are defined in the present invention is as follows. In addition, content (%) of each element means the mass%.
C: 0.05-0.25%
C is an indispensable component for obtaining the required strength and for obtaining a desired structure such as retained austenite. At least 0.05% is necessary, but if it exceeds 0.25%, the weldability deteriorates. It was set as the said range. Preferably it is 0.07 to 0.18%.
[0021]
Si: more than 0.50% and less than 2.00%
Si is an indispensable component for obtaining solid solution strengthening and a desired structure, and is a component that makes it possible to increase the strength without deteriorating ductility. The effect is not exhibited unless it exceeds 0.50%. On the other hand, when it is 2.00% or more, the plating adhesion deteriorates. Therefore, the above range was adopted. Preferably it is 0.6 to 1.6%.
[0022]
Mn: 3.5% or less
Similar to C, it is an essential component for obtaining the required strength, and improving the hardenability of the steel to obtain the desired structure. If it is 1% or more, the effect is sufficiently exhibited, which is preferable. However, even if it exceeds 3.5%, the effect is saturated and the cost is increased.
[0023]
Al: 0.01 to 1.0%
Al is important for obtaining the required strength and for obtaining a desired structure. As a result, the amount of Si added can be reduced, so that it is more advantageous for improving the plating adhesion than Si-added steel having the same tensile strength. . A minimum of 0.01% is necessary to obtain the desired effect. If it exceeds 1.0%, the economy deteriorates, so the upper limit was made 1.0%. Preferably it is 0.02 to 0.5%.
[0024]
Mn / Si mass ratio in steel: 2 or more
In order to obtain the plating adhesion improving effect, it is advantageous that the Mn / Si mass ratio in the steel is higher. This is because the surface concentrate produced during annealing immediately before plating changes from an Si-based oxide to an Si—Mn based complex oxide having good wettability with molten zinc. Further, when the surface is activated by subjecting the steel sheet to heat treatment before passing through a continuous hot dip galvanizing facility (CGL) and then pickling the surface after cooling, Mn / Si in the steel is used. The higher the mass ratio, the better the pickling performance of the oxide film. This is presumably because the oxide on the surface of the steel sheet changes from a Si-based oxide having poor pickling properties to a Si—Mn-based composite oxide having good pickling properties.
[0025]
Moreover, since the one where the Mn / Si mass ratio in steel is higher does not delay the alloying, the productivity in the case of alloying after hot dip galvanizing is also improved. When the Mn / Si mass ratio in the steel is less than 2, the plating adhesion deteriorates, so the Mn / Si mass ratio in the steel is defined as 2 or more. However, when the Mn / Si mass ratio in the steel is higher than 7, since the Si content is relatively small, it may be difficult to obtain a desired microcomposite structure. The mass ratio is preferably 7 or less. More preferred is 2.2 to 4.5.
[0026]
(Si + Mn) content in steel: 3% by mass or more
In the first place, although zinc-based plated steel sheets are originally good in corrosion resistance, further improvements in corrosion resistance are desired, such as the recent demand for improved pitting corrosion resistance in automotive steel sheets. To further improve corrosion resistance, The total of the Mn content and the Si content is preferably 3% by mass or more. When the content of each steel component is in the above range and the (Si + Mn) content is 3% by mass or more, dense and inert SiO. ₂ Since generation of the coating on the steel sheet surface is suppressed and oxygen diffusion into the steel is facilitated, internal oxidation is promoted during winding of the hot-rolled steel sheet or during heating in the primary processing step. Even if the (Si + Mn) content in the steel is 3% by mass or more, if the Mn / Si mass ratio in the steel is low and less than 2, the plating adhesion is poor and the corrosion resistance is not sufficient.
[0027]
The mechanism of action for improving the corrosion resistance is not clear, but it is presumed that due to internal oxidation, the plating adhesion is further improved by the anchor effect during hot dip galvanization and contributes to the improvement of the corrosion resistance.
[0028]
Tempered martensite
Tempered martensite is softened by tempering and has sufficient plastic deformability, and is therefore effective in improving elongation characteristics. If the volume fraction is less than 20%, the effect of improving ductility is not recognized, so the content was set to 20% or more. However, if it exceeds 80%, it is difficult to increase the strength of the steel sheet, so 80% or less is preferable.
[0029]
Retained austenite
Residual austenite has an effect of strain-induced transformation into martensite during processing, widely dispersing locally applied processing strain, and improving the ductility of the steel sheet. If the volume fraction is less than 2%, a significant improvement in ductility cannot be expected. Therefore, it is necessary that the volume fraction be 2% or more, and if it is 5% or more, the effect of improving the ductility is more remarkable.
[0030]
The production method for obtaining the composite structure is not particularly limited. ₃ Transformation point−50 ° C.) or higher, and the structure was quenched at a cooling rate of 10 ° C./second or higher. ₁ ~ Ac ₃ It is obtained by heating between transformation points, cooling at a cooling rate of 5 ° C./second or more, and tempering the structure. However, this is only an example, and any high-strength steel sheet having a component and phase defined by the present invention may be used.
[0031]
The low temperature transformation phase referred to in the present invention refers to martensite or bainite.
Both martensite and bainite are hard phases and have the effect of increasing steel sheet strength by strengthening the structure. Moreover, since it involves the generation of movable dislocations during transformation, it also has the effect of reducing the yield ratio of the steel sheet. In order to sufficiently obtain such an effect, it is preferable that the low temperature transformation phase is martensite. In the present invention, the amount of the low temperature transformation phase is not particularly limited. What is necessary is just to distribute suitably according to the intensity | strength of a steel plate.
[0032]
Al concentration in the plating layer
In the present invention, the Al concentration in the plating layer is important. As described above, the inventor of the present invention has a Mn / Si mass ratio of 2 or more, and in a steel sheet containing Mn and Si, the formula (1) in the case of GI and the formula (2) in the case of GA. It has been found that good plating adhesion can be secured by securing the Al concentration in the predetermined plating layer within the range. In the case of GI, since it is necessary to suppress the start of the alloying reaction due to recuperation after plating, the Al concentration is made higher than in the case of GA. Further, when the Si content is high, local alloying reactions occur frequently. Therefore, in order to suppress the initiation of the alloying reaction based on this, the Al content is increased as the Mn / Si mass ratio is lower.
[0033]
When the Al concentration is higher than the value on the left side of the formula (1) or formula (2), in the case of GI, the Fe-Al alloy layer generated at the initial stage of plating is thick, so that the weldability deteriorates or the case of GA Significantly delays alloying. When the Al concentration is lower than the value on the right side of the formula (1) or formula (2), the formation of the Fe—Al alloy layer is suppressed, and a hard and brittle Γ phase is easily generated at the initial stage of plating, and the plating adhesion is improved. to degrade. Therefore, in order to ensure good plating adhesion, it is necessary to maintain a predetermined Al concentration within the range of the formula (1) in the case of GI and the range of the formula (2) in the case of GA.
[0034]
The method for setting the Al concentration in the plating layer to a predetermined amount is not particularly limited. For example, by increasing the amount of Al in the plating bath or by increasing the plating time, the reaction between Al and the ground iron is promoted. A method for increasing the Al concentration in the plating layer is exemplified. Moreover, after heating the steel plate which has the component in steel of this invention in advance, pickling the surface lightly after cooling, and activating the surface, you may pass to CGL. These examples of the manufacturing method do not limit the present invention.
[0035]
Furthermore, in the steel plate of the high-tensile hot-dip galvanized steel plate or high-tensile alloyed hot-dip galvanized steel plate of the present invention, one or more of the following elements may be contained in the following amounts (mass%). In that case, the following effects are further obtained.
Cr: 1.0% or less
Cr improves the hardenability and has the effect of promoting the formation of a low temperature transformation phase, so it is added as necessary. Preferably, it is 0.05% or more, but if it exceeds 1.0%, the plating adhesion deteriorates, so the upper limit is preferably made 1.0%.
[0036]
Mo: 1.0% or less
Mo, like Cr, improves the hardenability and has the effect of promoting the formation of a low-temperature transformation phase, so it is added as necessary. The upper limit is preferably 0.05% or more, but if it exceeds 1.0%, the cost is increased, so the upper limit is preferably set to 1.0%.
[0037]
B: 0.003% or less
B has the effect of improving the hardenability, so is added as necessary. However, if it exceeds 0.003%, the plating adhesion deteriorates, so 0.003% is preferably the upper limit.
[0038]
Ti, Nb, V: 0.1% or less
Ti, Nb, and V form carbonitrides and have the effect of increasing the strength of steel by precipitation strengthening, so are added as necessary. When adding these, it is preferable to add 0.01% or more respectively. However, even if it exceeds 0.1%, the strength is excessively increased and ductility is deteriorated. Therefore, the upper limit is preferably set to 0.1%.
[0039]
Cu: 1.0% or less
Cu is segregated in austenite and is important not only for obtaining the required strength and for obtaining the desired structure, but also for improving the plating adhesion, and is added as necessary. The reason why the plating adhesion is improved is not clarified at the present time, but 0.01% or more is preferable to obtain a desired effect. However, if it exceeds 1.0%, the economic efficiency deteriorates, so the upper limit is preferably made 1.0%.
[0040]
Ni: 1.0% or less
Ni is segregated in austenite, like Cu, and is important not only for obtaining the required strength and for obtaining the desired structure, but also for improving the plating adhesion, and is added as necessary. The reason why the plating adhesion is improved is not clear at present, but in order to obtain a desired effect, it is preferable to add 0.01% or more. However, if it exceeds 1.0%, the economic efficiency deteriorates, so the upper limit is preferably made 1.0%.
[0041]
CGL (hot dip galvanizing) conditions
The CGL conditions of the steel sheet for producing the hot dip galvanized steel sheet of the present invention are not particularly limited and can be carried out by a usual method. However, when the CGL heating temperature (secondary heating temperature) is 650 ° C. or less, the oxide film on the steel sheet surface cannot be reduced, and non-plating is likely to occur. On the other hand, when the temperature is 850 ° C. or higher, non-plating is likely to occur in the same manner because the surface concentration of Si and Mn is large during heating. Therefore, 650-850 degreeC is preferable.
[0042]
The hot dip galvanizing bath preferably has an Al concentration of 0.08% or more in order to ensure adhesion after alloying of the plating layer. However, if it exceeds 0.20%, alloying becomes difficult, and the weldability of the resulting hot-dip galvanized steel sheet may be deteriorated, so the upper limit is preferably 0.20%. In addition, as described above, in order to control the Al concentration in the plating layer within the range of the present invention, in addition to the Al concentration in the bath, the ingress plate temperature, the plating bath immersion time, and other operating conditions may be adjusted. Good.
When the bath temperature is 440 ° C. or lower, there is a possibility that a portion below the freezing point (420 ° C.) may appear due to the bath temperature fluctuation of the plating bath, resulting in lack of operational stability. Moreover, when it exceeds 480 degreeC, the cost concerning heating holding will increase. Therefore, the bath temperature is preferably 440 to 480 ° C.
[0043]
In the case of alloying, if the alloying temperature is 450 ° C. or lower, the ζ phase is likely to be generated, and there is a risk that the slidability of GA may be lacking, and the productivity is deteriorated due to the time required for alloying. . Moreover, when it exceeds 600 degreeC, it will become easy to produce | generate a (GAMMA) phase and there exists a possibility that the plating adhesiveness of GA may be missing. Therefore, the alloying temperature is preferably 450 to 600 ° C.
[0044]
Furthermore, the degree of alloying is preferably within the range of 8 to 13% of the amount of Fe diffusion. If it is less than 8%, the ζ phase remains and the flaking resistance tends to deteriorate, and if it is 13% or more, the Γ phase is generated and the plating adhesion may deteriorate. However, the conditions of these manufacturing methods are examples, and the present invention is not limited to a specific manufacturing method.
[0045]
【Example】
EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated more concretely, this invention is not limited at all by these Examples.
(Examples 1-10, Comparative Examples 1-4)
A slab (thickness 300 mm) having a chemical composition (C, Si, Mn, P, S, Al, Cr, Cu, Ni, Mo, Ti, Nb, V, and B) shown in Table 1 was heated at 1250 ° C. A hot-rolled steel sheet having a thickness of 2.0 mm (steel A to H is an example; steels I to J are comparative examples) was hot-rolled and then wound at 620 ° C. Next, after removing the black skin by pickling, if cold rolling is performed as necessary, it is rolled at a reduction rate of 50%, heated in a heating furnace (primary heating), passed through CGL and pickled. Annealing, hot dip galvanizing and alloying treatment were performed. Table 2 shows the presence or absence of the cold rolling process, the primary heating temperature, the Al concentration in the plating bath, and the presence or absence of alloying. The cooling rate after primary heating is 30 ° C./second, the secondary heating temperature is 780 ° C., the cooling rate after secondary heating is 10 ° C./second, and the amount of plating adhesion is 50 g / m on one side. ² It was one by one. The alloying temperature was in the range of 450 to 600 ° C.
[0046]
[Table 1]

[0047]
[Table 2]

[0048]
Table 3 shows the results of investigations on the Al concentration, degree of alloying (Fe diffusion amount), appearance after plating, plating adhesion and corrosion resistance of the obtained hot-dip galvanized steel sheet and galvannealed steel sheet. It was. Table 4 shows the results of investigations on the tempered martensite fraction, retained austenite fraction, ferrite fraction, low-temperature transformation phase, and mechanical properties (elongation rate and tensile strength) of the obtained plated steel sheet.
[0049]
[Table 3]

[0050]
[Table 4]

[0051]
The Al concentration in the plating layer is determined by adding alkali to the plating layer to which an inhibitor is added, such as NaOH or KOH, or HCl, H ₂ SO ₄ It measured by melt | dissolving with acids, such as, and analyzing and quantifying the liquid with a plasma emission spectrometer (ICP) etc.
Similarly, the degree of alloying (Fe diffusion amount) was measured by analyzing and quantifying Fe with ICP or the like.
As for the appearance after plating, the area of the non-plated portion of the steel sheet immediately after plating was quantified by image processing, and the area ratio of non-plating was evaluated as good when the area ratio was less than 0.1% and poor when 0.1% or more.
[0052]
For plating adhesion, cellophane tape is applied to the plated steel sheet, the tape surface is bent within 90 °, bent back, and the amount of plating peeled off per unit length when the tape is peeled is expressed as a Zn count by fluorescent X-rays. In the light of the criteria in Table 5, those with

ranks

1 and 2 were evaluated as good (◯, Δ), and those with 3 or more were evaluated as bad.
[0053]

[0054]
Corrosion resistance evaluation is based on a hot dip galvanized steel plate or an alloyed hot dip galvanized steel plate of mild steel plate (SPCC), and each test piece was subjected to a combined corrosion cycle test (after spraying 0.5% salt water at 35 ° C. for 6 hours, 70% The film was dried at 6 ° C. for 6 hours and then maintained at a temperature of 40 ° C. and a humidity of 90% for 12 hours for 60 cycles, that is, after 60 days, the maximum corrosion depth was compared. For the test piece after the corrosion test, the maximum corrosion depth was determined by extreme statistical processing of the corrosion depth (pitting corrosion). A steel sheet having a depth substantially equal to the maximum corrosion depth of the reference steel sheet was evaluated as an average level, a steel sheet having a half or less was evaluated as good, a steel sheet having no pitting corrosion was evaluated as extremely good, and a steel sheet having a depth of 1.5 times or more was evaluated as defective.
[0055]
The volume fraction of the tempered martensite phase of the steel sheet was determined by polishing the cross-section of the steel sheet embedded in the resin, etching using a picral solution of 1 mass% sodium pyrosulfite (4 g picric acid / 100 cc ethanol), and then using an electron microscope. After observing at a magnification of 1000 times, the area ratio of the martensite phase present in a square area of 100 mm square was determined by image analysis, and the volume ratio of the martensite phase was obtained.
[0056]
The ferrite phase is obtained by polishing the cross section of the steel plate embedded in the resin to obtain a nital solution (69 mass% HNO ₃ After etching the structure with 3 vol% solution and 97 vol% ethanol), the area occupied by the ferrite phase was obtained by image processing of a photograph in a 100 mm square area observed at 250 times with an optical microscope. The volume ratio was used.
[0057]
The volume fraction of the retained austenite phase of the steel plate was obtained by polishing a test piece collected from the steel plate to the center plane in the plate thickness direction and measuring the X-ray intensity at the plate thickness center plane. That is, using MoKα rays, the diffraction X-ray intensity of each surface of ferrite (200) (211) and the diffraction X-ray intensity of each surface of (200) (220) of austenite are obtained, and ferrite (including martensite) is obtained. The ratio of the integrated intensity of (200) (211) and the integrated intensity of austenite (200) (220) was obtained, and this was used as the volume fraction of the austenite phase.
[0058]
Mechanical properties were measured by measuring the yield strength (yield point) YP, tensile strength TS, and elongation El using a JIS No. 5 tensile test specimen taken from a steel sheet with the direction orthogonal to the rolling direction as the tensile direction. Those having a thickness of 590 MPa or more and an elongation of 35% or more were evaluated as good.
[0059]
The steel composition, steel structure and plating layer within the scope of the present invention all had good plating adhesion and mechanical properties of the hot dip galvanized steel sheet. In particular, Example 3 was extremely excellent in corrosion resistance. On the other hand, those outside the scope of the present invention were inferior in plating adhesion and mechanical properties.
[0060]
【The invention's effect】
As described above, according to the present invention, a high-tensile hot-dip galvanized steel sheet and a high-tensile alloyed hot-dip galvanized steel sheet that are excellent in plating adhesion, mechanical properties of hot-dip galvanized steel sheet, and excellent in corrosion resistance are obtained. It is done. By applying the steel plate of the present invention, it is possible to reduce the weight and fuel consumption of an automobile body, and thus greatly contribute to the improvement of the global environment.
[Brief description of the drawings]
FIG. 1 is a diagram showing the relationship between the Mn / Si mass ratio in hot dip galvanized steel sheet (GI) and the Al concentration in a plated layer.
FIG. 2 is a diagram showing the relationship between the Mn / Si mass ratio in steel of a galvannealed steel sheet (GA) and the Al concentration in the plating layer.
FIG. 3 is a diagram showing the relationship between the Mn and Si contents in the steel of a hot dip galvanized steel sheet (GI) and the corrosion resistance.
FIG. 4 is a diagram showing the relationship between the Mn and Si contents in the steel of an alloyed hot-dip galvanized steel sheet (GA) and the corrosion resistance.

Claims

% By mass
C: 0.05 to 0.25%,
Si: more than 0.50% and less than 2.00%,
Mn: 3.5% or less and
Al: 0.01-1.0% included,
The balance is composed of Fe and inevitable impurities, the Mn / Si mass ratio in the steel is 2 or more, and consists of tempered martensite, residual austenite, ferrite and a low-temperature transformation phase, and is tempered in volume fraction. A hot-dip galvanized layer satisfying the following formula (1) is provided on a steel sheet having a composite structure in which martensite is 20% or more and the retained austenite is 2% or more. High-tensile hot-dip galvanized steel sheet with excellent strength ductility balance and plating adhesion.
0.67-1 / 50 (Mn / Si) ≧ [Al concentration in plating layer (mass%)] ≧ 0.37-1 / 50 (Mn / Si) (1)
(Mn / Si in formula (1) represents the Mn / Si mass ratio in the steel.)

The high-tensile hot-dip galvanized steel sheet excellent in strength ductility balance and plating adhesion according to claim 1, further comprising at least one component selected from the following group in addition to the steel sheet component.
(First group) At least one selected from the group consisting of Cr: 1.0% or less, Mo: 1.0% or less, and B: 0.003% or less in mass%.
(Second group) At least one selected from the group consisting of Ti: 0.1% or less, Nb: 0.1% or less, and V: 0.1% or less in terms of mass%.
(Third Group) At least one selected from the group consisting of Cu: 1.0% or less and Ni: 1.0% or less by mass%.

In the steel plate composition, the total of Si and Mn in the steel is 3% or more by mass%, and excellent in strength ductility balance, plating adhesion and corrosion resistance according to claim 1 or 2. High tensile hot dip galvanized steel sheet.

% By mass
C: 0.05 to 0.25%
Si: more than 0.50% and less than 2.00%,
Mn: 3.5% or less and
Al: 0.01-1.0% included,
The balance is composed of Fe and inevitable impurities, the Mn / Si mass ratio in the steel is 2 or more, and consists of tempered martensite, residual austenite, ferrite and a low-temperature transformation phase, and is tempered in volume fraction. The alloyed hot-dip galvanized layer having an Al concentration in the plated layer satisfying the following formula (2) is provided on a steel sheet having a composite structure in which martensite is 20% or more and the retained austenite is 2% or more. A high-tensile alloyed hot-dip galvanized steel sheet with excellent strength ductility balance and plating adhesion.
0.5-1 / 50 (Mn / Si) ≧ [Al concentration in plating layer (mass%)] ≧ 0.2-1 / 50 (Mn / Si) (2)
(Mn / Si in formula (2) represents the Mn / Si mass ratio in the steel.)

The high-tensile alloyed hot-dip galvanized steel sheet excellent in strength ductility balance and plating adhesion according to claim 4, further comprising at least one component selected from the following group in addition to the steel sheet components.
(First group) At least one selected from the group consisting of Cr: 1.0% or less, Mo: 1.0% or less, and B: 0.003% or less in mass%.
(Second group) At least one selected from the group consisting of Ti: 0.1% or less, Nb: 0.1% or less, and V: 0.1% or less in terms of mass%.
(Third Group) At least one selected from the group consisting of Cu: 1.0% or less and Ni: 1.0% or less by mass%.

In the said steel plate composition, the sum total of Si and Mn in steel is 3% or more by the mass%, It was excellent in the strength ductility balance of Claim 4, or the plating adhesiveness, and corrosion resistance High tensile alloyed hot dip galvanized steel sheet.