JP2004232029A

JP2004232029A - COATED HOT DIP Al-Zn BASED ALLOY PLATED STEEL SHEET HAVING EXCELLENT SURFACE APPEARANCE, AND PRODUCTION METHOD THEREFOR

Info

Publication number: JP2004232029A
Application number: JP2003022389A
Authority: JP
Inventors: Chizuko Gotou; 千寿子後藤; Yoichi Tobiyama; 洋一飛山; Hideo Sasaoka; 英男笹岡; Hideo Takamura; 日出夫高村
Original assignee: JFE Steel Corp; JFE Galvanizing and Coating Co Ltd
Current assignee: JFE Steel Corp; JFE Galvanizing and Coating Co Ltd
Priority date: 2003-01-30
Filing date: 2003-01-30
Publication date: 2004-08-19
Anticipated expiration: 2023-01-30
Also published as: JP4054689B2

Abstract

<P>PROBLEM TO BE SOLVED: To provide a coated hot dip Al-Zn based alloy plated steel sheet having excellent surface appearance in which surface defects are not generated in a coated film particularly in a region subjected to forced bending as for a coated steel plate using a high Al-Zn based alloy plated steel sheet as an original sheet. <P>SOLUTION: The coated hot dip Al-Zn based alloy plated steel sheet is obtained by forming a hot dip Al-Zn based alloy plating layer comprising 40 to 70 mass% Al on the surface of a steel sheet, and further forming a film on the plating layer. The hot dip Al-Zn based alloy plating layer has an alloy layer on the boundary with the steel sheet surface, and the thickness of the alloy layer is 7 to 30% of the thickness of the whole plating layer including the alloy layer. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

【０００１】
【発明の属する技術分野】
本発明は、建材、家電などの分野で広く利用されている溶融Ａｌ−Ｚｎ系合金めっき鋼板に塗装を施した塗装鋼板に係わり、とくに曲げ加工を強制された部位においても塗装被膜の表面外観が損なわれることのない表面外観性に優れた塗装溶融Ａｌ−Ｚｎ系合金めっき鋼板に関する。
【０００２】
【従来の技術】
主に、屋根や外壁用のサイディング等の建築材料に用いられる塗装鋼板は、その原板に、各種鋼板の他、表面処理鋼板が用いられている。表面処理鋼板としては、例えば合金化溶融亜鉛めっき鋼板が用いられ、そのめっき層上に、化成処理被膜や有機樹脂塗装被膜を形成したものが上記使途に供されている。
【０００３】
塗装鋼板の原板に適用される合金化溶融亜鉛めっき鋼板の中でも、Ａｌを３０〜７０ｍａｓｓ％含むＡｌ−Ｚｎ系合金めっき層を有する高Ａｌ−Ｚｎ系合金めっき鋼板は、溶融Ｚｎめっき鋼板やめっき層のＡｌ含有量が３０ｍａｓｓ％以下の低Ａｌ−Ｚｎ系合金めっき鋼板に比べて、耐食性、耐熱牲に優れているので、屋根、サイディング等の建築材料を中心に需要が急激に伸びている。
【０００４】
この高Ａｌ−Ｚｎ系合金めっき鋼板は、Ａｌ濃度が高くなるほど加工性が劣化するという問題点がある。これはＡｌ濃度とともにめっき層が硬くなり、鋼板の変形にめっき層が追随できなくなるためである。例えば、建物の施工時にしばしば行われる、曲げ並びに曲げ戻し加工や高エンボス加工のコーナー部などの厳しい加工箇所では、めっき層にクラックが不可避に発生するのである。
【０００５】
かようにめっき層にクラックが発生すると、その上に施した塗装被膜にも影響が及ぶ結果、この塗装被膜にひび割れや、極端な場合は亀裂が発生し、その表面外観が著しく損なわれ、商品価値の低下をまねくことになる。
【０００６】
ここで、特許文献１には、高Ａｌ−Ｚｎ系合金めっき塗装鋼板の塗膜クラックを課題の１つに掲げ、この課題をおもに塗装中の樹脂成分の配合比を変えて、塗装の延性とめっき層との密着性を両立させることによって、解決することが記載されている。しかし、この技術は、塗料のコストアップとなるばかりでなく、めっき層の延性が塗膜より劣る場合は、密着性が良好なためにかえってめっき層にクラックが発生するという問題がある。
【０００７】
【特許文献１】
特開２００２−２２６９６０号公報
【０００８】
【発明が解決しようとする課題】
そこで、本発明の目的は、高Ａｌ−Ｚｎ系合金めっき鋼板を原板とする塗装鋼板において、とくに曲げ加工を強制された部位の塗装被膜に表面欠陥の発生がない、表面外観性に優れた塗装溶融Ａｌ−Ｚｎ系合金めっき鋼板を、その製造方法に併せて提供することにある。
【０００９】
【課題を解決するための手段】
本発明の要旨構成は、次のとおりである。
（１）鋼板の表面に、Ａｌ：４０〜７０ｍａｓｓ％を含有する溶融Ａｌ−Ｚｎ系合金めっき層を形成し、さらにこのめっき層上に被膜を形成した塗装溶融Ａｌ−Ｚｎ系合金めっき鋼板であって、溶融Ａｌ−Ｚｎ系合金めっき層は鋼板表面との界面に合金層を有し、この合金層の厚みが該合金層を含む全めっき層の厚みの７％以上３０％以下であることを特徴とする表面外観性に優れた塗装溶融Ａｌ−Ｚｎ系合金めっき鋼板。
【００１０】
（２）前記合金層の厚みが該合金層を含む全めっき層の厚みの１５％以上２４％以下であることを特徴とする上記（１）に記載の表面外観性に優れた塗装溶融Ａｌ−Ｚｎ系合金めっき鋼板。
【００１１】
（３）前記被膜は、めっき層側から順に化成処理層、プライマー層および有機塗膜層の積層に成ることを特徴とする上記（１）または（２）に記載の表面外観性に優れた塗装溶融Ａｌ−Ｚｎ系合金めっき鋼板。
【００１２】
（４）Ａｌ：４０〜７０ｍａｓｓ％およびＳｉ：０．５ｍａｓｓ％以上２．０ｍａｓｓ％未満を含み、かつＳｒをＳｉ含有量の０．５〜２ｍａｓｓ％含有する組成のめっき浴に、鋼板を浸漬してめっきを施す際に、そのめっき浴温および鋼板のめっき浴浸入時温度を（液相線＋２５℃）以下の範囲に制御し、このめっき浴から鋼板を引き上げてから５６０℃までの間を３℃／ｓ以上で冷却し、引き続き５６０℃〜固相線までの温度域を、１０℃／ｓ以下で冷却するか、または同温度域に６秒以上保持し、その後被膜を形成することを特徴とする表面外観性に優れた塗装溶融Ａｌ−Ｚｎ系合金めっき鋼板の製造方法。
【００１３】
（５）Ａｌ：４０〜７０ｍａｓｓ％およびＳｉ：２．０〜４．５ｍａｓｓ％を含み、かつＳｒをＳｉ含有量の０．５〜２ｍａｓｓ％含有する組成のめっき浴に、鋼板を浸漬してめっきを施し、このめっき浴から鋼板を引き上げてから５６０℃までの間を１５℃／ｓ以下で冷却し、その後被膜を形成することを特徴とする表面外観性に優れた塗装溶融Ａｌ−Ｚｎ系合金めっき鋼板の製造方法。
【００１４】
【発明の実施の形態】
以下、本発明を導くに到った実験結果について、詳しく説明する。
すなわち、Ｓｉ添加量、めっき浴温とその浸入板温、引き上げ後のヒートパターンなどを変化させることにより、めっき合金層厚やめっき層の構造を様々に変化させた、５５ｍａｓｓ％Ａｌ−Ｚｎ系合金めっき鋼板を作製し、この試料にＣｒ付着量３５ｍｇ／ｍ^２のクロメート処理、付着量１０ｇ／ｍ^２のプライマー塗装を介して、付着量３４ｇ／ｍ^２の高分子ポリエステル系の一般的な有機樹脂塗装を施し塗装鋼板とした。
【００１５】
これらの試料にＪＩＳＺ２２４８の規定に準拠して２ｔ曲げ試験を行い、その曲げ部の断面を走査型電子顕微鏡（ＳＥＭ）で５００倍および２０００倍で撮影し、クラック発生部位におけるめっき層および塗装被膜の開口幅と、めっき合金層の厚みおよび同合金層を含むめっき層の全厚とを測定した。それらの測定結果を、めっき層および塗装被膜の開口幅と、めっき層の全厚に占める合金層厚の比率（以下、合金層厚比と示す）とに整理し、さらにめっき層のＳｉ含有量に応じて図１（ａ）および（ｂ）に分けて示す。この実験に用いた一般的な塗料の場合、めっき層にクラックが発生しても、その開口幅が約２０μｍ以下であれば、塗装被膜側にはクラックが発生しないことがわかる。
【００１６】
このようなめっき層における開口幅の小さいクラックは、合金層厚比が７％未満の試料では少なく、合金層厚比が７％以上の試料、とくに１５〜２４％の試料に多数存在することがわかる。
以上から、塗装鋼板の表面外観性を改善するための要件は、
（ｉ）合金層をある程度発達させ、上層のめっき層への応力集中を緩和させること、
（ｉｉ）めっき層に垂直方向にＡｌ−Ｚｎ共晶組織やＳｉ析出物を発達させることにより、クラックの伝播経路を分散させること、
が重要であることを見出し、本発明を完成したのである。
【００１７】
次に、本発明の塗装溶融Ａｌ−Ｚｎ系合金めっき鋼板について、その原板である溶融Ａｌ−Ｚｎ系合金めっき鋼板のめっき層の組成から順に説明する。
（めっき層の組成）
まず、溶融Ａｌ−Ｚｎ系合金めっき層は、４０〜７０ｍａｓｓ％のＡｌを含有することが肝要である。すなわち、Ａｌはめっき層の主要元素であり、４０ｍａｓｓ％未満ではめっき層の耐食性が不十分となり、一方７０ｍａｓｓ％を超えるとめっき層が硬くなり曲げ加工性が劣化し、また切断部端面の耐食牲も劣化する。より好ましくは、４５〜６０ｍａｓｓ％の範囲である。
【００１８】
さらに、Ｓｉを０．５〜４．５ｍａｓｓ％の範囲で含むことが好ましい。Ｓｉは、従来、界面合金層の厚みを抑制するためにめっき浴に添加する元素で、５５ｍａｓｓ％Ａｌ−Ｚｎ系合金めっきの場合、約１．６ｍａｓｓ％の含有量が一般的である。すなわち、０．５ｍａｓｓ％より少ないと、浴温、浸入板温を下げても界面合金層が３０％より厚く生成してしまい、表面外観性は改善しない。一方４．５ｍａｓｓ％より多いと、浴中に多量のドロスが発生し、浴管理がきわめて煩雑になる。また、Ｓｉ含有量が１．６〜１．９ｍａｓｓ％の範囲で合金層の厚みを７〜３０％に制御しようとすると、浴温を高くする必要があり、スパングルが粗大化するので、加工性改善効果はそれほど大きくない。スパングルを粗大化させることなく界面合金層を全めっき厚みの７〜３０％に規制するためには、Ｓｉを０．５ｍａｓｓ％以上２．０ｍａｓｓ％未満の範囲とすることが好ましい。また、めっき層中にＳｉ析出物を多数存在させて、めっき層に開口幅の小さいクラックの生成を図る場合は、Ｓｉを２．０〜４．５ｍａｓｓ％の範囲とすることが好ましい。
【００１９】
さらにまた、ＳｒをＳｉ含有量の０．５〜２ｍａｓｓ％で含むことが好ましい。この元素は、合金層生成に消費されずにめっき層中に析出するＳｉの表面に偏析し、粗大化を抑制するとともに、めっき表面に濃化し表面性状を改善する効果を持つ。Ｓｒ量がＳｉ含有量の０．５ｍａｓｓ％より少ないと、これらの効果が発揮されない。一方、同２ｍａｓｓ％を超えて過剰に添加されると、浴中ドロスが増えるばかりでなくＳｒ主体の析出物がめっき層中に存在するようになり、加工牲が劣化する。
【００２０】
（合金層の厚み）
本発明では、合金層の厚みを合金層を含む全めっき層厚の７〜３０％とすることが肝要である。合金層がこの範囲のとき、めっき層の応力集中が緩和され、しかも鋼板と合金層および上層めっきとの密着性も良好になる。これより薄いと、曲げ変形に対する合金層の応力緩和効果が起こらず、めっき層に開口部の広いクラックが発生し、その影響が塗装被膜にまで及ぶことになる。これより厚くなると、合金層が粉状に脆性破壊し、いわゆるパウダリングが発生し易くなり、やはりその影響が塗装被膜にまで及ぶことになる。より好ましい範囲は１５〜２４％である。
【００２１】
（めっき付着量）
鋼板片面当たり４０ｇ／ｍ^２以上とすることが好ましい。これより薄いと、十分な耐食性を発揮できなくなる。
【００２２】
（被膜）
めっき層上の被膜は、めっき層側から順に化成処理層、プライマー層および有機塗膜層から構成される積層に成ることが好ましい。通常は、化成処理層、プライマー層そして有機塗膜層の形成は、通常の塗装鋼板、例えばＰＣＭ（プレコート鋼板）を製造するのに採用されているものを用いればよい。詳しくは、次の通りである。
【００２３】
・化成処理層：通常のクロメート処理、リン酸処理などを用いることができる。
・プライマー層：エポキシ、変性エポキシ、ポリエステル、変性ポリエステル樹脂等に必要に応じて防錆顔料（たとえばジンククロメート、クロム酸ストロンチウム、クロム酸バリウム等）、硬化剤（メラニン、イソシアネート樹脂等）を混ぜたものを用いることができる。
【００２４】
・有機塗膜層：一般に知られているポリエステル系塗料、フッ素樹脂系塗料、アクリル樹脂系塗料、塩ビニル系塗料、シリコーン系塗料等の上塗り塗料を適当量塗布・焼付けすることによって得ることができる。なお、プライマーと上塗り塗料に体質顔料を添加して高延性を付与することは好ましい。また、これらの塗膜に種々の着色顔料や無機質骨材を適宜添加することも可能である。
【００２５】
次に、上記の塗装溶融Ａｌ−Ｚｎ系合金めっき鋼板の製造方法について説明する。
まず、本発明で使用する鋼板は、通常の方法で製造した鋼板、例えば低炭素アルミキルド鋼板や極低炭素鋼板がいずれも好適に使用できる。これらの鋼板は冷間圧延後、電解脱脂と酸洗により洗浄され、連続焼鈍により再結晶を兼ねた熱処理が施され、さらにめっき浴に浸漬され、めっき層が形成される。めっき浴の組成は、めっき層の組成と実質同一でよい。
【００２６】
めっき浴温と鋼板の浸入板温は、合金層厚の制御のために浴中のＳｉ含有量に応じて以下のように調節する必要がある。
まず、Ｓｉ量が０．５ｍａｓｓ％以上２．０ｍａｓｓ％未満の浴を使用する場合は、鋼板のめっき浴浸入温度を（液相線＋２５℃）以下とする。これより高温では合金層を全めっき厚みの３０％以下にできないばかりか、合金層の形態も変化してしまうので、加工性の改善は得られない。より好適な範囲は、（液相線＋１５℃）以下の温度である。
一方、Ｓｉ量が２．０〜４．５ｍａｓｓ％の浴を使用する場合は、とくに制限はなく、概ね液相線より５０℃高い温度まで可能である。
【００２７】
めっき付着量の調整は、通常用いられているガスワイピングを用いることができる。合金層厚の制御のために、ワイピング後の冷却パターンも、浴中のＳｉ含有量に応じて以下のように変えなければならない。
まず、Ｓｉ量が０．５ｍａｓｓ％以上２．０ｍａｓｓ％未満の浴を使用する場合は、めっき浴引き上げ後５６０℃までの間を３℃／ｓ以上の速度で冷却し、５６０℃〜固相線の温度までの温度域を１０℃／以下で冷却するか、または同温度域での６秒以上の保持を行う。すなわち、前者の冷却速度が３℃／ｓより遅いと、合金層を全めっき厚の３０％以下にできず、表面外観性の改善効果が得られない。
【００２８】
後者の５６０℃〜固相線までの徐冷または保持は、めっき層に垂直方向にＡｌ−Ｚｎ共晶組織を発達させることを主目的としているため、これより冷却速度が速いか、５秒以下の保持では、その効果が得られない。また、この処理により若干であるが、デンドライトの内部歪みも緩和されることから、５℃以下の徐冷または１０秒以上の処理温度の範囲、冷却速度、そして保持時間が上記の範囲を外れると、効果が得られない。
【００２９】
一方、Ｓｉ量が２．０〜４．５ｍａｓｓ％の浴を使用する場合は、５６０℃までの間を１５℃／ｓ以下で冷却しなければならない。これより速いと、合金層厚を全めっき層厚の７％以上にすることができない。浴中のＳｉが高い場合は、インターデンドライトヘのＳｉ析出が多いため、Ａｌ−Ｚｎ共晶組織を発達させるための徐冷または一時保持、すなわち５６０℃〜固相線までの間を、１０℃／ｓ以下で冷却または６秒以上で保持することは、とくに必要ではないが、デンドライトの内部歪みが緩和するので、これらの処理を行うほうが好ましい。
【００３０】
なお、固相線以下の温度域でのヒートパターンはとくに限定する必要はないが、デンドライト相の内部歪みを緩和させる目的で、２５０〜１７５℃の間を３℃／ｓ以下の徐冷または１５秒以上の保持を行うことは可能である。
【００３１】
このようにして製造された溶融Ａｌ−Ｚｎ系合金めっき鋼板に、化成処理を行い、さらにプライマー層を形成した後有機塗膜層を形成する。化成処理は、通常用いられているクロメート処理液、リン酸処理液をスプレー、浸漬、ロールコーター等によりめっき面に塗布し、乾燥することにより得られる。プライマー層と有機塗膜層の形成も、塗料の塗布厚み、塗布方法（スプレー、ロールコーティング、刷毛塗り等）のいずれも通常のＰＣＭで採用されている程度で十分である。
【００３２】
なお、上記した化成処理層、プライマー層、有機塗膜層の形成に際し、塗布後の乾燥・焼き付け処理の最高到達温度を１６０〜２５０℃とし、この間を合計で１５秒以上、より好ましくは３０秒以上保持することが好ましい。
【００３３】
【実施例】
実施例１
Ｃ：０．０４３ｍａｓｓ％、Ｓｉ：０．０１ｍａｓｓ％、Ｓ：０．００５ｍａｓｓ％およびＡｌ：０．００１３ｍａｓｓ％を含み、残部Ｆｅおよび不可避的不純物からなる低炭素冷延鋼板に、連続式溶融めっき設備によって、溶融Ａｌ−Ｚｎ合金めっきを施し、溶融Ａｌ−Ｚｎ合金めっき合金鋼板とした。めっき浴の母合金には、９９．９ｍａｓｓ％Ｚｎインゴット、９９．９９ｍａｓｓ％Ａｌインゴットを用い、これに１３ｍａｓｓ％Ｓｉ−Ａｌ合金、１０ｍａｓｓ％Ｓｒ−Ａｌ合金インゴットを用いて、表１の組成になるように、めっき浴の成分調整を行った。
【００３４】
【表１】

【００３５】
次いで、成分調整されためっき浴に、表２に示す浴温と浸入板温で鋼板を２秒間浸漬させた後引き上げ、付着量（片面）が４５ｇ／ｍ^２となるようにガスワイピングを行い、その後表２に示すヒートパターンで冷却し、伸び率０．７％のテンションレベラーで平滑化し、コイルに巻き取った。得られた溶融Ａｌ−Ｚｎ合金めっき鋼板を、幅１００ｍｍ、長さ２５０ｍｍの試験片に切断し、伸び率０．５％のスキンパス圧延を行った後、クロメート処理、プライマー塗装および有機樹脂塗装を、それぞれの付着量がＣｒ：３５ｍｇ／ｍ^２、９ｇ／ｍ^２および３０ｇ／ｍ^２となるように行い、塗装溶融Ａｌ−Ｚｎ合金めっき鋼板とした。かくして得られた塗装鋼板の、めっき層および合金層の厚みを測定するとともに、塗装後の表面外観性を曲げ加工部位について評価した。その結果を、表２に示す。なお、測定および評価方法は、次のとおりである。
【００３６】
めっき層および合金層の厚み測定：コイルの幅方向の任意の位置から５箇所、幅１５ｍｍ、長さ１０ｍｍの試験片を切断し、断面が出るようにカーボン樹脂に埋め込み、ダイヤモンドペーストでバフ研磨後、直ちにＳＥＭ観察した。各試料から２０００倍で３視野撮影し、写真からそれぞれの厚みを測定し、合金層厚比（合金層の厚み／合金増を含むめっき層の厚み×１００％）を計算し、その平均値を求めた。
【００３７】
・塗装後の表面外観性を曲げ加工部位についての評価：２ｔ曲げ試験およびエリクセン５ｍｍ押し出し試験を実施した。２ｔ曲げ試験は、試料を幅５０ｍｍ、長さ５０ｍｍに切断して試験片とし、ＪＩＳＺ２２４８に準拠して曲げ半径１ｔで行った。２ｔ曲げの亀裂発生状況は、１００倍のマイクロスコープで幅方向に１０ｍｍ間隔で４箇所写真撮影し、各写真の亀裂の面積占有率を５段階で評価し、平均値を算出した。
【００３８】
エリクセン５ｍｍ押し出し試験は、試料を幅１００ｍｍ、長さ６０ｍｍに切断し、幅方向に、３箇所押し出し試験を行なった。なお、エリクセン５ｍｍ押し出し試験試料の亀裂発生状況は、以下に述べる複合サイクル腐食試験を行い、３箇所の白錆発生の面積率を測定し、平均値を算出した。
・複合サイクル腐食試験
１サイクル：５％ＮａＣｌ水溶液噴射（３５℃）を４時間→乾燥２時間→恒温湿潤雰囲気（４９℃、相対根付湿潤率９５％）に４時間保管
以上を２００サイクル実施
【００３９】
【表２】

【００４０】
実施例２
Ｃ：０．００１４ｍａｓｓ％、Ｓｉ：＜０．００４ｍａｓｓ％、Ｍｎ：０．０１２ｍａｓｓ％，Ｐ：０．００８ｍａｓｓ％，Ａｌ：０．０３１ｍａｓｓ％を含み、残部Ｆｅおよび不可避不純物からなる極低炭素鋼の冷延板に、連続式溶融めっき設備によって溶融合金めっきを施し、溶融Ａｌ−Ｚｎ合金めっき鋼板とした。めっき浴の母合金には９９．９ｍａｓｓ％Ｚｎインゴット、９９．９９ｍａｓｓ％Ａｌインゴットを用い、これに１３ｍａｓｓ％Ｓｉ−Ａｌ合金、１０ｍａｓｓ％Ｓｒ−Ｓｉ−Ａｌ合金インゴットを用いて、表３の組成になるようにめっき浴の成分調整を行なった。
【００４１】
【表３】

【００４２】
成分調整されためっき浴に、表４に示す浴温と浸入板温で鋼板を２秒間浸漬させた後引き上げ、付着量（片面）が５０ｇ／ｍ^２となるようにガスワイピングを行い、次いで表４に示すヒートパターンで冷却し、伸び率０．７％のテンションレベラーで平滑化し、コイルに巻き取った。得られた溶融Ａｌ−Ｚｎ合金めっき鋼板を幅１００ｍｍ、長さ２５０ｍｍの試験片に切断し、伸び率０．５％のスキンパス圧延を行った後、クロメート処理、プライマー塗装、有機樹脂塗装を、それぞれの付着量（片面）がＣｒ：３３ｍｇ／ｍ^２，１０ｇ／ｍ^２，２８ｇ／ｍ^２となるように行い、塗装溶融Ａｌ−Ｚｎ合金めっき鋼板とした。これらの試料に２ｔ曲げ試験およびエリクセン５ｍｍ押し出し試験を実施し、塗装鋼板の表面外観性を評価した。その結果を表４に示す。
【００４３】
【表４】

【００４４】
【発明の効果】
本発明によれば、高Ａｌ−Ｚｎ系合金めっき鋼板を原板とする塗装鋼板において、とくに曲げ加工を強制された部位の塗装被膜に表面欠陥の発生が有利に抑制されるため、表面外観性に優れた商品価値の高い塗装溶融Ａｌ−Ｚｎ系合金めっき鋼板を提供することができる。
【図面の簡単な説明】
【図１】めっき層および塗装被膜の開口幅と合金層厚比との関係を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a coated steel sheet coated with a molten Al-Zn alloy-plated steel sheet that is widely used in the fields of building materials, home appliances, etc., and the surface appearance of the coating film is particularly effective even in a region where bending is forced. The present invention relates to a coated molten Al—Zn-based alloy-plated steel sheet having an excellent surface appearance that is not impaired.
[0002]
[Prior art]
Mainly, as for the coated steel plate used for building materials such as siding for roofs and outer walls, a surface-treated steel plate is used in addition to various steel plates. As the surface-treated steel sheet, for example, an alloyed hot-dip galvanized steel sheet is used, and a chemical conversion coating film or an organic resin coating film formed on the plating layer is used for the above purpose.
[0003]
Among the alloyed hot-dip galvanized steel sheets applied to the coated steel sheet, high Al-Zn alloy-plated steel sheets having an Al-Zn-based alloy plated layer containing 30 to 70 mass% Al are hot-dip Zn-plated steel sheets and plated layers. Compared with a low Al—Zn alloy-plated steel sheet having an Al content of 30 mass% or less, the corrosion resistance and heat resistance are excellent, and therefore, demand is growing rapidly mainly for building materials such as roofs and siding.
[0004]
This high Al—Zn alloy-plated steel sheet has a problem that workability deteriorates as the Al concentration increases. This is because the plating layer becomes hard with the Al concentration, and the plating layer cannot follow the deformation of the steel sheet. For example, cracks inevitably occur in the plating layer at severely processed points such as corner portions for bending and unbending and high embossing, which are often performed during construction of buildings.
[0005]
If cracks occur in the plating layer, the coating film applied to the plating layer is also affected. As a result, cracks occur in this coating film, and in extreme cases, cracks occur. This will lead to a decline in value.
[0006]
Here, in Patent Document 1, the coating film crack of the high Al—Zn alloy-plated coated steel sheet is listed as one of the problems, and the blending ratio of the resin component in the coating is changed mainly by this problem, It is described that the problem is solved by making the adhesion with the plating layer compatible. However, this technique not only increases the cost of the paint, but also has a problem that when the ductility of the plating layer is inferior to that of the coating film, cracks occur in the plating layer because of good adhesion.
[0007]
[Patent Document 1]
Japanese Patent Laid-Open No. 2002-226960
[Problems to be solved by the invention]
Accordingly, an object of the present invention is a coated steel sheet using a high Al-Zn alloy-plated steel sheet as a base sheet, and in particular, a coating film having a surface appearance that is free from surface defects in a coating film at a portion where bending is forced. The object is to provide a molten Al—Zn-based alloy-plated steel sheet together with its production method.
[0009]
[Means for Solving the Problems]
The gist configuration of the present invention is as follows.
(1) A coated molten Al—Zn alloy-plated steel sheet in which a molten Al—Zn alloy plating layer containing Al: 40 to 70 mass% is formed on the surface of the steel sheet, and a film is further formed on this plating layer. The molten Al—Zn alloy plating layer has an alloy layer at the interface with the steel plate surface, and the thickness of the alloy layer is 7% or more and 30% or less of the total plating layer including the alloy layer. Painted molten Al-Zn alloy-plated steel sheet with excellent surface appearance.
[0010]
(2) The thickness of the alloy layer is 15% or more and 24% or less of the thickness of the entire plating layer including the alloy layer, and the coated molten Al- having excellent surface appearance according to the above (1) Zn alloy-plated steel sheet.
[0011]
(3) The coating having excellent surface appearance according to the above (1) or (2), wherein the coating is formed by laminating a chemical conversion treatment layer, a primer layer, and an organic coating layer in order from the plating layer side. Fused Al-Zn alloy plated steel sheet.
[0012]
(4) A steel plate is immersed in a plating bath having a composition containing Al: 40 to 70 mass% and Si: 0.5 mass% or more and less than 2.0 mass% and containing Sr in an amount of 0.5 to 2 mass% of the Si content. When the plating is performed, the temperature of the plating bath and the temperature at which the steel sheet enters the plating bath is controlled to the range of (liquidus + 25 ° C.) or less, and the time from when the steel plate is pulled up from this plating bath to 560 ° C. is 3 Cooling at ℃ / s or higher, and subsequently cooling the temperature range from 560 ℃ to the solidus at 10 ℃ / s or lower, or holding the same temperature range for 6 seconds or longer, and then forming a film The manufacturing method of the coating fusion | melting Al-Zn type alloy plating steel plate excellent in the surface appearance property to make.
[0013]
(5) Plating by dipping a steel plate in a plating bath having a composition containing Al: 40 to 70 mass% and Si: 2.0 to 4.5 mass% and containing Sr in an amount of 0.5 to 2 mass% of the Si content. The coated molten Al—Zn alloy with excellent surface appearance is characterized in that the steel sheet is pulled up from the plating bath and cooled to 560 ° C. at 15 ° C./s or less, and then a film is formed. Manufacturing method of plated steel sheet.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the experimental results that led to the present invention will be described in detail.
That is, 55 mass% Al—Zn alloy with various changes in plating alloy layer thickness and plating layer structure by changing Si addition amount, plating bath temperature and its infiltration plate temperature, heat pattern after pulling up, etc. the plated steel sheet was produced, chromate treatment of Cr deposition amount 35 mg / ^{m 2} in this sample, the adhesion amount over the primer coating of 10 g / ^{m 2,} typical organic resin polymer polyester-based coating weight 34g / ^{m 2} The coated steel sheet was applied.
[0015]
These samples were subjected to a 2t bending test in accordance with the provisions of JIS Z2248, and the cross section of the bent portion was photographed at 500 times and 2000 times with a scanning electron microscope (SEM), and the plating layer and the paint film at the crack occurrence site , The thickness of the plating alloy layer, and the total thickness of the plating layer including the alloy layer were measured. The measurement results are organized into the opening width of the plating layer and coating film and the ratio of the alloy layer thickness to the total thickness of the plating layer (hereinafter referred to as the alloy layer thickness ratio), and the Si content of the plating layer FIG. 1 (a) and FIG. In the case of the general paint used in this experiment, it can be seen that even if cracks occur in the plating layer, cracks do not occur on the coating film side if the opening width is about 20 μm or less.
[0016]
Such cracks with a small opening width in the plating layer are rare in samples having an alloy layer thickness ratio of less than 7%, and many cracks may exist in samples having an alloy layer thickness ratio of 7% or more, particularly 15-24%. Understand.
From the above, the requirements for improving the surface appearance of the coated steel sheet are:
(I) To develop an alloy layer to some extent and relieve stress concentration on the upper plating layer;
(Ii) dispersing a crack propagation path by developing an Al—Zn eutectic structure or Si precipitate in a direction perpendicular to the plating layer;
Was found to be important, and the present invention was completed.
[0017]
Next, the coated molten Al—Zn alloy-plated steel sheet of the present invention will be described in order from the composition of the plating layer of the molten Al—Zn-based alloy plated steel sheet as the original plate.
(Composition of plating layer)
First, it is important that the molten Al—Zn alloy plating layer contains 40 to 70 mass% of Al. That is, Al is a main element of the plating layer, and if it is less than 40 mass%, the corrosion resistance of the plating layer becomes insufficient. On the other hand, if it exceeds 70 mass%, the plating layer becomes hard and bending workability deteriorates, and the corrosion resistance of the end face of the cut portion Deteriorate. More preferably, it is the range of 45-60 mass%.
[0018]
Furthermore, it is preferable to contain Si in the range of 0.5 to 4.5 mass%. Conventionally, Si is an element added to the plating bath in order to suppress the thickness of the interface alloy layer. In the case of 55 mass% Al—Zn alloy plating, a content of about 1.6 mass% is generally used. That is, if it is less than 0.5 mass%, the interface alloy layer is formed thicker than 30% even if the bath temperature and the infiltration plate temperature are lowered, and the surface appearance is not improved. On the other hand, if it exceeds 4.5 mass%, a large amount of dross is generated in the bath, and bath management becomes extremely complicated. In addition, if the Si content is in the range of 1.6 to 1.9 mass% and the thickness of the alloy layer is to be controlled to 7 to 30%, it is necessary to increase the bath temperature, and the spangles become coarser. The improvement effect is not so great. In order to regulate the interface alloy layer to 7 to 30% of the total plating thickness without coarsening spangles, it is preferable to set Si in the range of 0.5 mass% or more and less than 2.0 mass%. Moreover, when many Si deposits are made to exist in a plating layer and the production | generation of a crack with a small opening width is aimed at in a plating layer, it is preferable to make Si into the range of 2.0-4.5 mass%.
[0019]
Furthermore, it is preferable that Sr is contained at 0.5 to 2 mass% of the Si content. This element segregates on the surface of Si deposited in the plating layer without being consumed in the formation of the alloy layer, and suppresses the coarsening and has the effect of concentrating on the plating surface and improving the surface properties. If the Sr content is less than 0.5 mass% of the Si content, these effects cannot be exhibited. On the other hand, when it is added excessively exceeding 2 mass%, not only dross in the bath increases, but also Sr-based precipitates are present in the plating layer, which deteriorates workability.
[0020]
(Alloy layer thickness)
In the present invention, it is important that the thickness of the alloy layer is 7 to 30% of the total plating layer thickness including the alloy layer. When the alloy layer is in this range, the stress concentration in the plating layer is relaxed, and the adhesion between the steel sheet and the alloy layer and the upper layer plating is also improved. If it is thinner than this, the stress relaxation effect of the alloy layer against bending deformation does not occur, and a crack with a wide opening is generated in the plating layer, and the influence reaches the coating film. If it becomes thicker than this, the alloy layer is brittlely broken in a powdery state, so that so-called powdering is likely to occur, and the effect also reaches the coating film. A more preferable range is 15 to 24%.
[0021]
(Amount of plating adhesion)
It is preferable to set it as 40 g / m < ² > or more per one surface of a steel plate. If it is thinner than this, sufficient corrosion resistance cannot be exhibited.
[0022]
(Coating)
The coating on the plating layer is preferably a laminate composed of a chemical conversion treatment layer, a primer layer, and an organic coating layer in order from the plating layer side. In general, the chemical conversion treatment layer, the primer layer, and the organic coating layer may be formed by using a conventional coated steel sheet, for example, a PCM (pre-coated steel sheet). Details are as follows.
[0023]
-Chemical conversion treatment layer: Normal chromate treatment, phosphoric acid treatment, etc. can be used.
・ Primer layer: Epoxy, modified epoxy, polyester, modified polyester resin, etc. are mixed with anti-rust pigments (eg zinc chromate, strontium chromate, barium chromate) and curing agents (melanin, isocyanate resin, etc.) as necessary. Things can be used.
[0024]
・ Organic coating layer: It can be obtained by applying and baking an appropriate amount of top coating such as commonly known polyester paint, fluororesin paint, acrylic resin paint, vinyl chloride paint, silicone paint, etc. . It is preferable to add extender pigments to the primer and the top coat to impart high ductility. In addition, various color pigments and inorganic aggregates can be appropriately added to these coating films.
[0025]
Next, the manufacturing method of said coating fusion | melting Al-Zn type alloy plating steel plate is demonstrated.
First, the steel plate used by this invention can use suitably the steel plate manufactured by the normal method, for example, all, such as a low carbon aluminum killed steel plate and a very low carbon steel plate. After cold rolling, these steel plates are washed by electrolytic degreasing and pickling, subjected to heat treatment that also serves as recrystallization by continuous annealing, and further immersed in a plating bath to form a plating layer. The composition of the plating bath may be substantially the same as the composition of the plating layer.
[0026]
The plating bath temperature and the intrusion plate temperature of the steel plate need to be adjusted as follows according to the Si content in the bath for controlling the alloy layer thickness.
First, when using a bath having an Si amount of 0.5 mass% or more and less than 2.0 mass%, the infiltration temperature of the steel sheet is set to (liquidus + 25 ° C.) or less. If the temperature is higher than this, the alloy layer cannot be reduced to 30% or less of the total plating thickness, and the form of the alloy layer is also changed, so that the workability cannot be improved. A more preferable range is a temperature of (liquidus + 15 ° C.) or lower.
On the other hand, when using a bath having an Si amount of 2.0 to 4.5 mass%, there is no particular limitation, and it is possible to reach a temperature approximately 50 ° C. higher than the liquidus.
[0027]
The gas wiping which is usually used can be used for adjusting the plating adhesion amount. In order to control the alloy layer thickness, the cooling pattern after wiping must be changed as follows according to the Si content in the bath.
First, in the case of using a bath having an Si amount of 0.5 mass% or more and less than 2.0 mass%, it is cooled at a rate of 3 ° C / s or more until 560 ° C after pulling up the plating bath. The temperature range up to the temperature is cooled at 10 ° C./less or held for 6 seconds or more in the same temperature range. That is, if the former cooling rate is slower than 3 ° C./s, the alloy layer cannot be made 30% or less of the total plating thickness, and the effect of improving the surface appearance cannot be obtained.
[0028]
The latter slow cooling or holding from 560 ° C. to the solidus is mainly intended to develop an Al—Zn eutectic structure in the direction perpendicular to the plating layer, so the cooling rate is faster than this, or 5 seconds or less. The effect cannot be obtained by holding. In addition, this treatment slightly reduces the internal strain of the dendrite. Therefore, when the cooling temperature is 5 ° C. or lower or the processing temperature is 10 seconds or longer, the cooling rate, and the holding time are out of the above ranges. The effect is not obtained.
[0029]
On the other hand, when using a bath having an Si amount of 2.0 to 4.5 mass%, the temperature up to 560 ° C. must be cooled at 15 ° C./s or less. If it is faster, the alloy layer thickness cannot be 7% or more of the total plating layer thickness. When Si in the bath is high, there is much Si precipitation on the interdendrite, so slow cooling or temporary holding for developing the Al—Zn eutectic structure, that is, between 560 ° C. and the solidus is 10 ° C. It is not particularly necessary to cool at 6 s or less or hold for 6 seconds or more, but it is preferable to perform these treatments because the internal strain of the dendrite is reduced.
[0030]
The heat pattern in the temperature range below the solidus need not be particularly limited, but for the purpose of alleviating the internal strain of the dendrite phase, it is gradually cooled between 250 and 175 ° C. at 3 ° C./s or less. It is possible to hold for more than a second.
[0031]
The molten Al—Zn alloy-plated steel sheet thus manufactured is subjected to chemical conversion treatment, and after forming a primer layer, an organic coating layer is formed. The chemical conversion treatment is obtained by applying a commonly used chromate treatment solution or phosphoric acid treatment solution to the plated surface by spraying, dipping, roll coater or the like, and drying. For the formation of the primer layer and the organic coating layer, it is sufficient that both the coating thickness and the coating method (spray, roll coating, brush coating, etc.) are employed in ordinary PCM.
[0032]
In addition, when forming the above-mentioned chemical conversion treatment layer, primer layer, and organic coating layer, the maximum reached temperature of the drying / baking treatment after coating is set to 160 to 250 ° C., and the total time is 15 seconds or more, more preferably 30 seconds. It is preferable to hold the above.
[0033]
【Example】
Example 1
C: 0.043 mass%, Si: 0.01 mass%, S: 0.005 mass%, and Al: 0.0013 mass%, and a low-temperature cold-rolled steel sheet comprising the balance Fe and inevitable impurities, continuous hot-dip plating equipment Thus, molten Al—Zn alloy plating was performed to obtain a molten Al—Zn alloy plated alloy steel sheet. A 99.9 mass% Zn ingot and a 99.99 mass% Al ingot were used for the mother alloy of the plating bath, and a 13 mass% Si-Al alloy and a 10 mass% Sr-Al alloy ingot were used for the composition shown in Table 1. Thus, the component adjustment of the plating bath was performed.
[0034]
[Table 1]

[0035]
Next, the steel sheet was immersed in the component-adjusted plating bath at the bath temperature and the infiltration plate temperature shown in Table 2 for 2 seconds and then pulled up, and gas wiping was performed so that the adhesion amount (one side) was 45 g / m ² . Thereafter, it was cooled with a heat pattern shown in Table 2, smoothed with a tension leveler having an elongation of 0.7%, and wound around a coil. The obtained molten Al—Zn alloy-plated steel sheet was cut into a test piece having a width of 100 mm and a length of 250 mm and subjected to skin pass rolling with an elongation of 0.5%, followed by chromate treatment, primer coating, and organic resin coating. Each adhesion amount was Cr: 35 mg / m ² , 9 g / m ^2, and 30 g / m ² , and a coated molten Al—Zn alloy plated steel sheet was obtained. While measuring the thickness of the plating layer and the alloy layer of the coated steel sheet thus obtained, the surface appearance after coating was evaluated for the bent portion. The results are shown in Table 2. The measurement and evaluation methods are as follows.
[0036]
Measurement of thickness of plating layer and alloy layer: After cutting test pieces with a width of 15 mm and a length of 10 mm from arbitrary positions in the width direction of the coil, they are embedded in carbon resin so that a cross section appears, and then buffed with diamond paste The SEM was immediately observed. Three fields of view were taken from each sample at a magnification of 2000, the thicknesses were measured from the photographs, and the alloy layer thickness ratio (alloy layer thickness / plated layer thickness including alloy increase × 100%) was calculated. Asked.
[0037]
-Evaluation of the surface appearance after coating with respect to the bending part: a 2t bending test and an Erichsen 5 mm extrusion test were carried out. In the 2t bending test, the sample was cut into a width of 50 mm and a length of 50 mm to obtain a test piece, and the bending radius was 1 t according to JIS Z2248. Regarding the occurrence of 2t-bending cracks, four photographs were taken at 10 mm intervals in the width direction with a 100 × microscope, and the area occupancy ratio of cracks in each photograph was evaluated in five stages, and an average value was calculated.
[0038]
In the Eriksen 5 mm extrusion test, a sample was cut into a width of 100 mm and a length of 60 mm, and a three-point extrusion test was performed in the width direction. In addition, the crack generation condition of the Erichsen 5 mm extrusion test sample was subjected to the combined cycle corrosion test described below, the area ratio of white rust generation at three locations was measured, and the average value was calculated.
Combined cycle corrosion test 1 cycle: 5% NaCl aqueous solution injection (35 ° C.) 4 hours → Dry 2 hours → Constant temperature and humidity atmosphere (49 ° C., relative netting wet rate 95%) 4 hours storage or more 200 cycles ]
[Table 2]

[0040]
Example 2
C: 0.0014 mass%, Si: <0.004 mass%, Mn: 0.012 mass%, P: 0.008 mass%, Al: 0.031 mass%, and a very low carbon steel made of the balance Fe and inevitable impurities The cold-rolled sheet was subjected to hot-dip alloy plating by a continuous hot-dip plating equipment to obtain a hot-dip Al-Zn alloy-plated steel sheet. A 99.9 mass% Zn ingot and a 99.99 mass% Al ingot were used as the master alloy for the plating bath, and a 13 mass% Si-Al alloy and a 10 mass% Sr-Si-Al alloy ingot were used. The components of the plating bath were adjusted so as to have a composition.
[0041]
[Table 3]

[0042]
The steel plate was immersed in the component-adjusted plating bath at the bath temperature and the infiltration plate temperature shown in Table 2 for 2 seconds and then pulled up, and gas wiping was performed so that the adhesion amount (one side) was 50 g / m ^2. Cooled with the heat pattern shown in FIG. 4, smoothed with a tension leveler having an elongation of 0.7%, and wound around a coil. The obtained molten Al—Zn alloy-plated steel sheet was cut into a test piece having a width of 100 mm and a length of 250 mm, subjected to skin pass rolling with an elongation of 0.5%, followed by chromate treatment, primer coating, and organic resin coating. The adhesion amount (one side) of Cr was 33 mg / m ² , 10 g / m ² , and 28 g / m ² , and a coated molten Al—Zn alloy plated steel sheet was obtained. These samples were subjected to a 2t bending test and an Erichsen 5 mm extrusion test to evaluate the surface appearance of the coated steel sheet. The results are shown in Table 4.
[0043]
[Table 4]

[0044]
【The invention's effect】
According to the present invention, in the coated steel sheet using the high Al—Zn alloy-plated steel sheet as the original sheet, the occurrence of surface defects is advantageously suppressed particularly in the coating film at the site where the bending process is forced, so that the surface appearance is improved. It is possible to provide a coated hot-melt Al—Zn alloy-plated steel sheet having excellent commercial value.
[Brief description of the drawings]
FIG. 1 is a diagram showing a relationship between an opening width of a plating layer and a paint film and an alloy layer thickness ratio.

Claims

A coated molten Al—Zn alloy plated steel sheet in which a molten Al—Zn alloy plating layer containing Al: 40 to 70 mass% is formed on the surface of the steel sheet, and a coating film is further formed on the plated layer. The Al—Zn alloy plating layer has an alloy layer at the interface with the steel sheet surface, and the thickness of the alloy layer is 7% or more and 30% or less of the thickness of the entire plating layer including the alloy layer. Painted molten Al-Zn alloy-plated steel sheet with excellent surface appearance.

The thickness of the said alloy layer is 15 to 24% of the thickness of all the plating layers containing this alloy layer, The coating hot-dip Al-Zn type alloy plating excellent in the surface appearance property of Claim 1 characterized by the above-mentioned steel sheet.

3. The coated molten Al—Zn alloy having excellent surface appearance according to claim 1, wherein the coating is formed by laminating a chemical conversion treatment layer, a primer layer, and an organic coating layer in order from the plating layer side. Plated steel sheet.

Plating is performed by immersing the steel sheet in a plating bath containing Al: 40 to 70 mass% and Si: 0.5 mass% or more and less than 2.0 mass% and containing Sr in an amount of 0.5 to 2 mass% of the Si content. At the time of application, the temperature of the plating bath and the temperature when the steel sheet enters the plating bath is controlled to the range of (liquidus + 25 ° C.) or less, and the temperature up to 560 ° C. after the steel plate is pulled up from this plating bath is 3 ° C./s. Surface appearance characterized by cooling in the above manner and subsequently cooling the temperature range from 560 ° C. to the solidus at 10 ° C./s or less, or maintaining in the same temperature range for 6 seconds or more, and then forming a film For producing a coated molten Al-Zn alloy-plated steel sheet having excellent properties.

A steel plate is immersed in a plating bath having a composition containing Al: 40 to 70 mass% and Si: 2.0 to 4.5 mass% and containing 0.5 to 2 mass% of Sr. A paint-melted Al—Zn alloy-plated steel sheet excellent in surface appearance, characterized in that the steel sheet is pulled up from this plating bath and then cooled to 560 ° C. at 15 ° C./s or less, and then a film is formed. Production method.