JP2004360056A

JP2004360056A - BLACKENED HOT DIP Zn-Al-Mg BASED ALLOY PLATED STEEL SHEET, AND ITS PRODUCTION METHOD

Info

Publication number: JP2004360056A
Application number: JP2003163450A
Authority: JP
Inventors: Koichi Watanabe; 幸一渡辺; Takeshi Shimizu; 剛清水; Atsushi Ando; 敦司安藤
Original assignee: Nisshin Steel Co Ltd
Current assignee: Nippon Steel Nisshin Co Ltd
Priority date: 2003-06-09
Filing date: 2003-06-09
Publication date: 2004-12-24

Abstract

<P>PROBLEM TO BE SOLVED: To provide a blackened hot dip plated steel sheet in which a blackened surface having a color tone with a more profound feeling can be produced in a short time by surface treatment with a phosphate treatment liquid. <P>SOLUTION: A hot dip Zn-Al-Mg based alloy plated steel sheet in which the ratio of the phase of (the three-component eutectic structure of Al/Zn/Zn<SB>2</SB>Mg) occupied in the outermost surface of a plated layer is ≥60% by area rate is brought into contact with a phosphate treatment liquid comprising 5 to 10 g/L Zn ions, 3 to 15 g/L nitric acid ions, and 3 to 15 g/L Ni ions and/or Co ions, so that a film consisting of an Ni and/or Co-precipitated layer with the Ni and/or Co coating weight of ≥20 mg/m<SP>2</SP>and zinc phosphate crystals with the zinc phosphate coating weight of ≥2 g/m<SP>2</SP>is formed on the plated layer. When the surface of the film is further stacked with a chemical conversion treatment film and a clear coating film, its corrosion resistance and durability increase. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【０００１】
【産業上の利用分野】
本発明は、溶融Ｚｎ−Ａｌ−Ｍｇ系合金めっき鋼板にリン酸塩処理を施して黒色の色調を発現させた黒色化溶融Ｚｎ−Ａｌ−Ｍｇ系合金めっき鋼板に関する。
【０００２】
【従来の技術】
溶融Ｚｎ系めっき鋼板にリン酸塩処理を施して黒色化すると、塗装鋼板では出せない独特な色調を発現するため、例えば、特開平３−２２６５８３号公報、特開平５−２３７４５０号公報にみられるように、建築物の屋根材や外装材として広く使用されている。家電製品部材にも使用されようとしている。
また、その量産方法として、連続ラインでＺｎ系めっき鋼板を短時間でリン酸塩処理する技術が特開平８−２１８１８１号公報や特開２０００−３０９８８１号公報で紹介されている。連続ラインでの操業では、リン酸塩処理に要する時間は短いほど作業の効率は上がり、有利になる。特開平８−２１８１８１号公報では、処理時間が３秒ほどでＬ値４５〜６０の黒色化外観が得られると記載されている。
【０００３】
また、Ｚｎ系めっき材料の黒色化に関して、特開平７−６２５５４号公報や特開平８−２１８１５８号公報には、Ｚｎめっき表面でＮｉあるいはＣｏが微細粒状に置換析出して光の散乱効果を生み出し、黒色の外観（Ｌ^＊値で１０〜２０（Ｌ値１８〜３０に相当）、６０°光沢度９〜１５）を呈するようになると記載されている。
色調に関して、目視による評価と機器分析による明度や光沢度との関係を調査すると、明度指数Ｌ値が低く（すなわち黒色度が高く）、かつ６０°光沢度（６０°の入射角の光の正反射成分）が低いほど、重厚感が増すことがわかった。
【０００４】
黒色化したＺｎ系めっき鋼板についてＬ値と６０°光沢度を支配する要因を調べたところ、Ｌ値はリン酸塩処理液との接触時にめっき表面に置換析出するＮｉあるいはＣｏの析出量に、６０°光沢度はリン酸塩結晶の析出量にそれぞれ関係することがわかった。Ｎｉ，Ｃｏの置換析出量が多いほどＬ値は低く、リン酸塩結晶の析出量が多いほど低光沢度を示すことになる。
しかしながら、溶融Ｚｎ系めっき鋼板をリン酸塩処理液を用いて黒色化処理を施そうとするとき、黒色化を強くするためにＮｉイオンあるいはＣｏイオン濃度の高い処理液を使用すると、ＮｉあるいはＣｏの置換析出量は多くなるがリン酸塩結晶の析出量が少なくなって６０°光沢度が高くなる。したがって、Ｌ値４０以下及び６０°光沢度５以下になるような黒色化溶融Ｚｎ系めっき鋼は非常に得難い。
【０００５】
【発明が解決しようとする課題】
本発明は、このような問題を解消すべく案出されたものであり、リン酸塩処理液による表面処理で、より重厚感のある色調、具体的にはＬ値が４０以下で、かつ６０°光沢度が５以下を満足し、しかもそのような黒色化表面が短時間の処理で生成可能な溶融めっき鋼板及びその鋼板を用いて黒色化溶融めっき鋼板を提供するものである。
【０００６】
【課題を解決するための手段】
本発明の黒色化処理用溶融Ｚｎ−Ａｌ−Ｍｇ系合金めっき鋼板は、その目的を達成するため、リン酸塩処理により黒色化される溶融Ｚｎ−Ａｌ−Ｍｇ系合金めっき鋼板であって、めっき層最表面で〔Ａｌ／Ｚｎ／Ｚｎ_２Ｍｇの三元共晶組織〕の相が占める割合が面積率で６０％以上であることを特徴とする。
また、本発明の黒色化溶融Ｚｎ−Ａｌ−Ｍｇ系合金めっき鋼板は、めっき層最表面で〔Ａｌ／Ｚｎ／Ｚｎ_２Ｍｇの三元共晶組織〕の相が占める割合が面積率で６０％以上になるように調整された溶融Ｚｎ−Ａｌ−Ｍｇ系合金めっき鋼板の合金めっき層上に、Ｎｉ及び／又はＣｏ付着量２０ｍｇ／ｍ^２以上のＮｉ及び／又はＣｏ析出層とリン酸亜鉛付着量２ｇ／ｍ^２以上のリン酸亜鉛結晶からなる皮膜が形成されていることを特徴とする。
Ｎｉ及び／又はＣｏ析出層とリン酸亜鉛結晶からなる皮膜の上に、更にクロメート処理皮膜或いはノンクロム系処理皮膜と５〜２０μｍのクリア塗膜が積層されていることが好ましい。
このような黒色化溶融Ｚｎ−Ａｌ−Ｍｇ系合金めっき鋼板は、めっき層最表面で〔Ａｌ／Ｚｎ／Ｚｎ_２Ｍｇの三元共晶組織〕の相が占める割合を面積率で６０％以上になるように調整した溶融Ｚｎ−Ａｌ−Ｍｇ系合金めっき鋼板を、Ｚｎイオンを５〜１０ｇ／Ｌ，硝酸イオンを３〜１５ｇ／Ｌ，Ｎｉイオン及び／又はＣｏイオンを３〜１５ｇ／Ｌを含むリン酸塩処理液と接触させることにより製造される。
【０００７】
【作用】
本発明者等は、まず、溶融Ｚｎ系めっき鋼板をリン酸塩処理液を用いて黒色化処理を施したとき、低明度化と低光沢度化が両立し難い原因について検討した。
その一環として、溶融Ｚｎめっき鋼板をベースに、リン酸塩溶液中のＮｉイオン濃度を変えてより重厚感のある色調を出せるリン酸塩処理条件を調べた。
なお、ＮｉとＣｏはほとんど同じ挙動を示すので、以下の説明ではＮｉを例示して説明している。したがって「Ｎｉ」の記載は、「Ｎｉ及び／又はＣｏ」を意味するものである。
図１に、溶融Ｚｎめっき鋼板をベースに処理液中のＺｎイオン濃度７ｇ／Ｌ，処理時間を５秒，処理液温度を６５℃としてスプレー処理によりリン酸塩処理を行ったときの、Ｌ値と６０°光沢度の変化を示す。また図２には、処理液中のＮｉイオン濃度とＺｎめっき表面のＮｉ置換析出量並びにリン酸塩結晶の析出量の関係を示す。さらに図３には、Ｎｉイオン濃度が２．５，５．２，１０．０ｇ／Ｌであるときのリン酸塩処理後の外観を示す。ここで、Ｎｉ置換析出量とリン酸塩結晶の析出量は次の手順で求めた。
【０００８】
〔リン酸塩結晶の析出量〕
リン酸塩処理後の鋼板を１ｄｍ^２の大きさに切り出して重量を測定した後、二クロム酸をアンモニウム水に（アンモニア：蒸留水＝１：１）に溶解した水溶液（常温）に１５分間浸漬し、その浸漬の前後の重量差からリン酸塩結晶の析出量を算出した。
〔Ｎｉ置換析出量〕
上記リン酸塩結晶溶解後の試験片を用い、常温，１０％ＨＣｌ中でめっき層を溶解し、その溶解液をＩＣＰで分析することによりＮｉ置換析出量を算出した。
なお、本明細書中における明度指数Ｌ値は、スガ試験機（株）製のＳＭカラーコンピューターＳＭ−７で測定した値で、ＪＩＳＺ８７３０に規定されたハンターの色差式に基づく数値であり、同じく光沢度は、ＪＩＳＺ８７４１の「鏡面光沢度測定方法」の規定に準拠して測定された数値である。
【０００９】
図１，２からわかるように、処理液中のＮｉイオン濃度を上げるとＮｉ置換析出量は増大し、Ｌ値は低下する。しかし、リン酸塩結晶の析出量は処理液中のＮｉイオン濃度が５ｇ／Ｌを超えると急激に低下し、それにともない光沢度の増大が認められる。このことから、処理液中のＮｉイオン濃度を増やすことは、Ｎｉ置換析出量を上げてＬ値を低下させる反面、ある濃度以上になるとリン酸塩結晶の析出を阻害していることになる。図３にも見られるように、処理液中のＮｉイオン濃度が２．５，５．２ｇ／Ｌのときはめっき表面にリン酸塩結晶が緻密に発生しているのに対して、Ｎｉイオン濃度が１０．０ｇ／Ｌになるとリン酸塩結晶はまばらにしか析出していない。
【００１０】
このような結果から、処理液中のＮｉイオン濃度の増加がリン酸塩結晶の析出量の低下につながる理由として次のようなことが考えられる。
すなわち、Ｚｎめっき表面ではリン酸塩処理液と接触すると始めにＮｉの置換析出が起こる。このとき処理液中のＮｉイオン濃度の増加に応じてＺｎめっき表面でＮｉ置換析出層の占める面積は高まる。一端、Ｎｉ置換析出層が形成された部位ではＺｎめっき層の溶解が起こり難く、Ｎｉ置換析出層が占める面積が大きくなるにしたがってその後のリン酸塩結晶の析出反応が妨げられるものと考えられる。
【００１１】
そこで、本発明者等は、リン酸塩処理液と接触させてＮｉの置換析出を起こさせる際に、Ｎｉ置換析出層が占める面積率を小さくする手段について探索した。その結果、溶融めっき合金として、Ｚｎ−Ａｌ−Ｍｇ系のめっき合金を採用し、めっき層最表層の組織を、〔Ａｌ／Ｚｎ／Ｚｎ_２Ｍｇの三元共晶〕が占める割合を多くすることで、上記目的が達成できることに到達した。
すなわち、鋼板表面にＺｎ−Ａｌ−Ｍｇ系の合金を溶融めっきすると、本発明者等が特開２００２−２２６９５８号公報で報告しているように、めっき鋼板のめっき層は〔Ａｌ／Ｚｎ／Ｚｎ_２Ｍｇの三元共晶組織〕の素地中に〔初晶Ａｌ相〕，更には〔Ｚｎ単相〕が混在した金属組織が形成される。
【００１２】
例えば図４は、溶融Ｚｎめっき鋼板と溶融Ｚｎ−Ａｌ−Ｍｇ合金めっき鋼板をＮｉ含有リン酸塩処理液でスプレー処理したときの反応表面を、模式的に比較して示したものである。めっき層がほぼ単一組織である溶融Ｚｎめっき鋼板に比べて、溶融Ｚｎ−Ａｌ−Ｍｇ合金めっき鋼板では、めっき層の大部分は〔Ａｌ／Ｚｎ／Ｚｎ_２Ｍｇの三元共晶〕の微細組織からなっている。
めっき層最表面の組織が〔Ａｌ／Ｚｎ／Ｚｎ_２Ｍｇの三元共晶組織〕になっているため、Ｎｉ含有リン酸塩処理液と接触しても、三元共晶組織の中で表面電位が最も低いとみられるＺｎ_２Ｍｇ相で優先的にＮｉの置換析出反応が起こり、溶融Ｚｎめっきのようにほぼ全面がＮｉ置換析出層で覆われることはないとみられる。その後、溶融Ｚｎめっき鋼板においてはＮｉ置換層の未析出部近傍でリン酸塩結晶が析出するだけであるのに対して、溶融Ｚｎ−Ａｌ−Ｍｇ合金めっき鋼板では、図４に示すように、全面にリン酸塩結晶が析出すると考えられる。
【００１３】
上記のような反応機構により、リン酸塩処理の反応性を高めて処理時間の短縮化が可能になるとともに、Ｎｉイオン含有量の多いリン酸塩処理液を接触させた場合にあっても、Ｌ値が低くかつ低光沢感の表面を安定して製造できることを可能にしていると考えられる。
したがって、リン酸塩処理液と接触させる溶融Ｚｎ−Ａｌ−Ｍｇ合金めっき鋼板のめっき層最表面には、〔Ａｌ／Ｚｎ／Ｚｎ_２Ｍｇの三元共晶組織〕の相がより多く存在することが重要となる。
【００１４】
本発明では、後述するように、各種試験を繰り返すことにより、めっき層最表面で〔Ａｌ／Ｚｎ／Ｚｎ_２Ｍｇの三元共晶組織〕の相が占める割合が面積率で６０％以上であると、リン酸塩処理の反応性を高めて処理時間の短縮化が可能になることを確認した。面積率が６０％に満たないと、リン酸塩処理の反応性が低下し、処理時間の短縮化が図れない。またＮｉイオン濃度の高いリン酸塩処理液を用いたとき、リン酸塩結晶の析出量が少なくなって低光沢感の表面が得られなくなる。より好ましくは８０％以上である。
【００１５】
めっき層最表面で〔Ａｌ／Ｚｎ／Ｚｎ_２Ｍｇの三元共晶組織〕の相の面積率が８０％以上である溶融Ｚｎ−Ａｌ−Ｍｇ合金めっき鋼板をベースに、Ｚｎ金属イオンが８ｇ／Ｌ，硝酸イオンが６ｇ／Ｌで、処理温度を６５℃にしてスプレー処理を３秒間実施したときの処理液中のＮｉイオン濃度とめっき表面のＮｉ置換析出量並びにリン酸塩結晶の析出量の関係を図５に示す。また、そのときのＬ値と６０°光沢度を図６に示す。
溶融Ｚｎめっき鋼板をベースとしたときとは異なり、３秒という短時間処理にもかかわらず、処理液中のＮｉイオン濃度を１４．８ｇ／Ｌまで上げてもリン酸塩結晶の析出量の大幅な低下はみられず、Ｌ値が３５以下の範囲でも安定して低光沢感が維持されている。上記考え方が裏付けられている。
【００１６】
なお、本明細書中では、めっき層最表面での〔Ａｌ／Ｚｎ／Ｚｎ_２Ｍｇ三元共晶組織〕の面積率は次のようにして求めている。
図７に、溶融Ｚｎ−Ａｌ−Ｍｇ合金めっき鋼板のめっき層断面の顕微鏡写真の一例を示す。溶融Ｚｎ−Ａｌ−Ｍｇ合金めっき鋼板のめっき層は、〔Ａｌ／Ｚｎ／Ｚｎ_２Ｍｇ三元共晶組織〕の素地中に〔初晶Ａｌ相〕又は〔初晶Ａｌ相〕と〔Ｚｎ単相〕、あるいは〔初晶Ａｌ相〕と〔Ｚｎ単相〕及び一部の〔Ｚｎ_２Ｍｇ相〕が混在した金属組織を有している。リン酸塩処理に供する溶融Ｚｎ−Ａｌ−Ｍｇ合金めっき鋼板の任意の場所から５つのサンプルを切り出し、１つのサンプルにつき断面から２０００倍の視野でめっき層の観察を３ヶ所行い、めっき層最表面での〔Ａｌ／Ｚｎ／Ｚｎ_２Ｍｇ三元共晶組織〕が占める割合を求め、その平均値を取ったものが、本発明におけるめっき層最表面での〔Ａｌ／Ｚｎ／Ｚｎ_２Ｍｇ三元共晶組織〕の面積率である。めっき層最表面での〔Ａｌ／Ｚｎ／Ｚｎ_２Ｍｇ三元共晶組織〕が占める割合とは、図７中、Ａ，Ｂで示した長さを次式で計算した値である。
ちなみに、図７のめっき層最表面での〔Ａｌ／Ｚｎ／Ｚｎ_２Ｍｇ三元共晶組織〕が占める割合は６６％である。
（Ｂ_１＋Ｂ_２＋Ｂ_３）×１００／Ａ
ただし、Ａ；観察した部分の長さ
Ｂ_１，Ｂ_２，Ｂ_３；めっき層最表面に露出している〔Ａｌ／Ｚｎ／Ｚｎ_２Ｍｇ三元共晶組織〕の長さ
【００１７】
【実施の形態】
次に本発明の好ましい実施形態について説明する。
ベースとなる溶融Ｚｎ−Ａｌ−Ｍｇ合金めっき鋼板は、本発明者等が先に提案した特開２００２−２２６９５８号公報に記載の方法で調製できる。
めっきする鋼板の成分組成は、特に限定されない。普通鋼冷延鋼板やステンレス鋼冷延鋼板が使用できる。ただし、めっき原板にステンレス鋼板を使用する際には、溶融Ｚｎ−Ａｌ−Ｍｇ合金めっき層との密着性を高めるために、めっき前にＦｅ−Ｂ系のプレめっきを施しておくことが好ましい。
【００１８】
溶融めっきは、Ｚｎ浴中にＡｌ：２．５〜１５質量％，Ｍｇ：２．０〜４．０質量％を含有し、さらに必要に応じてＴｉ：０．００１〜０．１質量％，Ｂ：０．００１〜０．０４５質量％，Ｓｉ：０．００５〜０．５質量％を含有させた溶融Ｚｎ−Ａｌ−Ｍｇめっき浴にめっき原板鋼帯を連続的に浸漬して引き上げたあと，めっき層表面をエアーにて冷却して溶融Ｚｎ−Ａｌ−Ｍｇ合金めっき層を形成すればよい。
【００１９】
この際、めっき浴中のＡｌ濃度は、２．５〜１５質量％の範囲とする。Ａｌ含有量が１５質量％を超えると、めっき後のめっき層最表面での〔初晶Ａｌ相〕の占める割合が増えて〔Ａｌ／Ｚｎ／Ｚｎ_２Ｍｇ三元共晶組織〕の占める面積率が６０％未満になる。一方、Ａｌが２．５質量％に満たないとめっき層中の〔Ｚｎ単相〕が増えて、同様に表面での〔Ａｌ／Ｚｎ／Ｚｎ_２Ｍｇ三元共晶組織〕の占める面積率が６０％未満になる。
〔Ａｌ／Ｚｎ／Ｚｎ_２Ｍｇ三元共晶組織〕の占める面積率を８０％以上にするためには、Ａｌ含有量は４〜８質量％の範囲にすることが好ましい。
【００２０】
めっき浴中のＭｇ濃度は、２．０〜４．０質量％の範囲とする。Ｍｇ含有量が４．０質量％を超えると、めっき層内に〔Ｚｎ_２Ｍｇ単相〕の出現が目立ち始め、めっき層表面での〔Ａｌ／Ｚｎ／Ｚｎ_２Ｍｇ三元共晶組織〕の占める面積率が６０％未満になる。一方、Ｍｇが２．０質量％に満たないと〔初晶Ａｌ相〕と〔Ｚｎ単相〕が増えて、同様にめっき表面での〔Ａｌ／Ｚｎ／Ｚｎ_２Ｍｇ三元共晶組織〕の占める面積率が６０％未満になる。
〔Ａｌ／Ｚｎ／Ｚｎ_２Ｍｇ三元共晶組織〕の占める面積率を８０％以上にするためには、Ｍｇ含有量は２．５〜３．５質量％の範囲にすることが好ましい。
【００２１】
ＴｉとＢについては，めっき層の金属組織を〔Ａｌ／Ｚｎ／Ｚｎ_２Ｍｇ三元共晶組織〕の金属組織とする場合に、外観および耐食性に悪い影響を与えるＺｎ_１１Ｍｇ_２相の生成・成長を抑制する作用を供するのでその添加が有益である。この効果を得るために，Ｔｉ，ＢまたはＴｉ−Ｂ合金もしくは化合物をめっき浴に添加する場合、浴中の含有量がＴｉ：０．００１〜０．１質量％，Ｂ：０．００１〜０．０４５質量％となるように添加することが好ましい。これより多く含有させると、めっき層中に析出物が成長し、めっき層に凹凸が生じて外観を損ねることがある。
Ｓｉについては、めっき層と素地鋼との界面でのＡｌ−Ｆｅ系合金層の成長を抑制するために、０．００５〜０．５質量％の範囲で添加しておくことが好ましい。
【００２２】
めっき後の冷却速度もめっき層に出現する相組織に影響を及ぼすので、調整することが好ましい。耐食性を低下させるＺｎ_１１Ｍｇ_２相を出現させないように、７℃／秒以上の速度で冷却することが好ましい。エアー量，エアー圧の変更により調整すればよい。
溶融Ｚｎ−Ａｌ−Ｍｇ合金めっきされた鋼板は、リン酸塩処理液をスプレーする方法，あるいはリン酸塩処理液に浸漬する方法等により、黒色化処理される。
この際の処理液組成も黒色化に大きな影響を及ぼす。
【００２３】
黒色化用リン酸塩処理液は、基本的にはリン酸溶液に酸化亜鉛及びニッケルを溶解したリン酸酸性溶液であり、亜鉛はリン酸水素亜鉛［Ｚｎ（Ｈ_２ＰＯ_４）_２］の状態で溶解し、一定の平衡状態を保っている。
このリン酸亜鉛処理液を、母材の溶融Ｚｎ−Ａｌ−Ｍｇ合金めっき鋼板のめっき面に接触させると、液中のＮｉイオンはめっき層の亜鉛（遊離リン酸により液中に溶出）と置換し、めっき層表面にＮｉ置換析出層が形成される。また亜鉛の溶解に伴う処理液のｐＨ上昇によりリン酸亜鉛［Ｚｎ_３（ＰＯ_４）_２］が生成し、水化物［Ｚｎ_３（ＰＯ_４）_２・４Ｈ_２Ｏ］として析出することによりリン酸亜鉛結晶層が形成される。
【００２４】
リン酸塩処理液中のＺｎイオン濃度は５〜１０ｇ／Ｌの範囲にする。Ｚｎイオン濃度が５ｇ／Ｌに満たないと、短時間処理でリン酸塩結晶が十分に析出せず、低光沢感が得られない。１０ｇ／Ｌを超えるとリン酸塩処理液の安定性が劣り、スラッジが発生しやすくなる。
また、リン酸塩処理液中のリン酸イオン濃度は２５〜４５ｇ／Ｌの範囲にすることが好ましい。リン酸イオン濃度が２５ｇ／Ｌに満たないと、短時間処理でリン酸塩結晶が十分に析出せず、低光沢感が得られない。４５ｇ／Ｌを超えるとリン酸塩処理液の安定性が劣り、スラッジが発生しやすくなる。
【００２５】
リン酸塩処理液中のＮｉイオン，Ｃｏイオン濃度は合わせて３〜１５ｇ／Ｌの範囲にする。ＮｉあるいはＣｏ又はＮｉとＣｏを併せたイオンの濃度が３ｇ／Ｌに満たないと、Ｌ値４０以下の色調が得られない。一方、１５ｇ／Ｌを超えて増加させても黒色度を増す効果は飽和する。
さらに、リン酸塩処理液中の硝酸イオンも３〜１５ｇ／Ｌの範囲にする。硝酸イオンは反応促進剤として含有させる必要があり、３ｇ／Ｌに満たないと反応促進効果が不十分である。逆に１５ｇ／Ｌを超えると硝酸イオンの酸化作用によりめき表面が不活性になり、ＮｉあるいはＣｏの置換析出反応が低下し、十分な黒色度が得られなくなる。
【００２６】
リン酸塩処理は液温５０〜８０℃の範囲で行うことが好ましい。５０℃に満たないと、反応性が低下して短時間処理が困難になる。８０℃を超える温度にすると処理液の安定性が低下し、スラッジの発生が多くなるとともに、水分の蒸発も多くなって連続操業での濃度管理が難しくなる。
リン酸塩処理は、処理液の温度が５０〜８０℃の範囲では、スプレー処理であれば２〜５秒程度、浸漬処理であれば３〜８秒程度必要であり、それ以上しても問題はないが色調の変化はほとんどみられず効果は飽和する。
【００２７】
ところで、当該リン酸塩処理を、溶融めっき工程に連続したライン等で連続的に行おうとする場合、めっき層より溶出したＡｌがリン酸塩反応を阻害する。そこで、処理液中のフリーフッ素イオン濃度が常に３０ｐｐｍ以上になるように保ち、溶出してきたＡｌをリン酸塩処理反応に対して無害化するようにすることが好ましい。具体的には、一定量のフッ化物を連続的に添加することにより、連続的な操業が可能となる。使用するフッ化物としては、フッ化ナトリウム，フッ化カリウム，フッ化水素ナトリウム等が挙げられる。
【００２８】
リン酸塩処理による黒色化後、耐食性を向上させるためにクロム酸シーリング処理あるいは酸化ジルコニウムと酸化バナジウムを含むクロムフリーのシーリング処理を施してもよい。
溶融めっき工程に連続したラインで連続的に本発明リン酸塩処理を実施する場合には、リン酸塩処理の後工程に上記シーリング処理を行うためのスプレー装置あるいはロールコータ装置を附設すればよい。
さらに、リン酸塩処理による黒色化後、加工時のリン酸塩処理面の保護及び耐食性を向上させるために、必要に応じてクリア塗装を施してもよい。クリア塗装は、公知の塗料、例えばポリエステル系，エポキシ系，塩化ビニル系，アクリル系，フッ素系等を使用し、常法にしたがって行えばよい。塗膜厚は乾燥後５〜２０μｍもあれば十分である。
【００２９】
【実施例】
実施例１：
下記に示す条件１にてめっき浴成分を種々変えて溶融Ｚｎ−Ａｌ−Ｍｇ合金めっき鋼板を作製し、脱脂→水洗→表面調整を行った後、下記条件２にしたがいラボ処理したリン酸塩処理板の各種特性値を測定した。
また、比較として板厚が０．５ｍｍの溶融亜鉛めっき鋼板，溶融Ｚｎ−５％Ａｌ合金めっき鋼板も評価した。
なお、明度指数Ｌ値及び光沢度は、前記したように、ＪＩＳＺ８７３０及びＪＩＳＺ８７４１の規定に準拠して測定したものである。
めっき層最表層の三元共晶組織の面積率と各種特性値の関係を整理した結果を併せて表１に示す。
【００３０】
（条件１）
処理設備：ラボバッチ式溶融めっき装置
処理めっき原板：板厚０．５ｍｍの低炭素冷延鋼板
還元炉最高到達温度：７２０℃，露点：−３０℃
めっき浴組成：Ａ１＝３〜１２質量％，Ｍｇ＝１〜５質量％，残部＝Ｚｎ
めつき浴温：４５０〜５１０℃
浸漬時間：３秒
めっき後の冷却速度：強制空冷方式で２０℃／秒
（冷却速度はめっき浴温からめっき層凝固温度までの平均値）
めっき付着量：約２００ｇ／ｍ^２（片面）
【００３１】

【００３２】

【００３３】
上記結果からもわかるように、めっき層最表面での［Ａｌ／Ｚｎ／Ｚｎ_２Ｍｇ三元共晶組織］の面積率が６０％以上の本発明例では、Ｌ値及び６０°光沢度のいずれも満足できる値となっている。
しかしながら、めっき層最表面での［Ａｌ／Ｚｎ／Ｚｎ_２Ｍｇ三元共晶組織］の面積率が６０％に満たない比較例では、Ｌ値で満足できるものはあっても、６０°光沢度の点では不十分であった。三元共晶組織の面積率が低かったために、リン酸塩結晶が十分に析出できなかったことに原因があると思われる。
【００３４】
実施例２：
下記に示す条件３にて作製した溶融Ｚｎ−Ａｌ−Ｍｇ合金めっき鋼板を、脱脂→水洗→表面調整を行った後、下記条件４にしたがいラボ処理したリン酸塩処理板の各種特性値を測定した。その結果を表２に示す。
また、比較として板厚が０．５ｍｍの溶融亜鉛めっき鋼板，溶融Ｚｎ−５％Ａｌ合金めっき鋼板を評価した結果を表３，４にそれぞれ示す。
なお、リン酸塩結晶析出量，Ｎｉ及び／又はＣｏ置換析出量は前記の方法で測定したものであり、Ｌ値及び６０°光沢度は実施例１と同様ＪＩＳに準拠して測定したものである。
【００３５】
（条件３）
処理設備：連続式溶融めっきライン
処理めっき原板：板厚０．５ｍｍの低炭素冷延鋼板
還元炉最高到達温度：７２０℃，露点：−３０℃
めっき浴組成：Ａ１＝６．１質量％，Ｍｇ＝３．１質量％，残部＝Ｚｎ
めつき浴温：４５０℃
浸漬時間：３秒
めっき後の冷却速度：強制空冷方式で２５℃／秒
（冷却速度はめっき浴温からめっき層凝固温度までの平均値）
めっき付着量：約１５０ｇ／ｍ^２（片面）
めっき最表面での〔Ａｌ／Ｚｎ／Ｚｎ_２Ｍｇ三元共晶組織〕の面積率：８７％
【００３６】

【００３７】

【００３８】

【００３９】

【００４０】
表２，３，４に示す結果からもわかるように、Ｎｉ及び／又はＣｏの置換析出量が２０ｍｇ／ｍ^２以上で，リン酸塩結晶の析出量が２ｇ／ｍ^２以上の本発明例は、Ｌ値及び６０°光沢度のいずれも満足できる値となっている。
これに対して、Ｎｉ及び／又はＣｏの置換析出量が２０ｍｇ／ｍ^２に満たない比較例では、Ｌ値が目標の数値に達していない。また、リン酸塩結晶の析出量が２ｇ／ｍ^２に満たない比較例では、６０°光沢度が目標の数値に達していない。
【００４１】
実施例３：
実施例２と同じ条件で作製した幅１０００ｍｍの溶融Ｚｎ―Ａｌ−Ｍｇ合金めっき鋼帯をカラー塗装ラインにて、脱脂→水洗→表面調整実施後、下記条件５にてリン酸塩処理を施した。さらに連続して下記条件６にて後処理ならびにクリア塗装を施した。
ただし、後処理は２種類の処理液を用いて後処理の異なる鋼帯を製造した。
得られた鋼帯のＬ値は幅方向、長手方向で３０±１の範囲で変動し、６０°光沢度も３以下と安定しており、均一処理性が良好であった。鋼帯より切出した端板を用いて、ロールフォーミングによる波板成形を実施したが、クリア塗膜，リン酸塩処理皮膜とも剥離や脱落は見られず、良好な加工性を有していた。
【００４２】

【００４３】
（条件６）
・クロムシーリング後処理
処理法：スプレー＋リンガーロール
付着量：Ｃｒ換算で約８ｍｇ／ｍ^２（片面）
・クロムフリーシーリング後処理（酸化ジルコニウム，酸化バナジウム含有処理）
処理法：スプレー＋リンガーロール
付着量：Ｚｒ＋Ｖ換算で約１０ｍｇ／ｍ^２（片面）
・クリア塗装
塗装法：ロールコーター
塗布膜厚：８μｍ
塗料種類：シリカ含有ポリエステル系樹脂
【００４４】
【発明の効果】
以上に説明したように、本発明によれば、溶融Ｚｎ−Ａｌ−Ｍｇ系合金めっき鋼板にリン酸塩処理液による表面処理を施すことにより、明度及び光沢が制御され、より重厚感のある色調を付与することができる。
光沢が抑制され、深い黒灰色をもつ外観を得ることも容易であり、建材や家電製品、その他の各種分野における個々の具体的用途・使用態様に応じて要望される種々の色調を有し、意匠性の優れた黒色溶融Ｚｎ−Ａｌ−Ｍｇ系合金めっき鋼板を得ることができる。
黒色化を施すめっき鋼板が、耐食性に極めて優れた溶融Ｚｎ−Ａｌ−Ｍｇ系合金めっきが施されたものであるために、黒色化されためっき鋼板は耐食性にも優れており、上記建材や家電製品、その他の各種分野に幅広く使用されることが期待される。
【図面の簡単な説明】
【図１】溶融Ｚｎめっき鋼板にリン酸塩処理を施したときの、処理液中のＮｉイオン濃度とＺｎめっき表面のＬ値と６０°光沢度の変化を示す図
【図２】溶融Ｚｎめっき鋼板にリン酸塩処理を施したときの、処理液中のＮｉイオン濃度とＺｎめっき表面のＮｉ置換析出量並びにリン酸塩結晶の析出量の関係を示す図
【図３】溶融Ｚｎめっき鋼板にリン酸塩処理を施したときの、Ｎｉイオン濃度に応じた表面外観の違いも模式的に示す図
【図４】溶融Ｚｎめっき鋼板と溶融Ｚｎ−Ａｌ−Ｍｇ合金めっき鋼板をＮｉ含有リン酸塩処理液でスプレー処理したときの反応表面を、模式的に比較して示した図
【図５】めっき層最表面で〔Ａｌ／Ｚｎ／Ｚｎ_２Ｍｇの三元共晶組織〕の相の面積率が９０％以上である溶融Ｚｎ−Ａｌ−Ｍｇ合金めっき鋼板に、Ｚｎ金属イオンが８ｇ／Ｌ，硝酸イオンが６ｇ／Ｌで、処理温度を６５℃にしてスプレー処理を３秒間実施したときの処理液中のＮｉイオン濃度とめっき表面のＮｉ置換析出量並びにリン酸塩結晶の析出量の関係を示す図
【図６】めっき層最表面で〔Ａｌ／Ｚｎ／Ｚｎ_２Ｍｇの三元共晶組織〕の相の面積率が９０％以上である溶融Ｚｎ−Ａｌ−Ｍｇ合金めっき鋼板に、Ｚｎ金属イオンが８ｇ／Ｌ，硝酸イオンが６ｇ／Ｌで、処理温度を６５℃にしてスプレー処理を３秒間実施したときの処理液中のＮｉイオン濃度とめっき表面のＬ値並びに６０°光沢度の関係を示す図
【図７】溶融Ｚｎ−Ａｌ−Ｍｇ合金めっき鋼板のめっき層断面を顕微鏡観察したときの観察表面を模式的に表示した図及びそれを簡略的に表示した図[0001]
[Industrial applications]
The present invention relates to a blackened hot-dip Zn-Al-Mg-based alloy-plated steel sheet obtained by subjecting a hot-dip Zn-Al-Mg-based alloy-plated steel sheet to a phosphate treatment to develop a black color tone.
[0002]
[Prior art]
When a hot-dip Zn-coated steel sheet is subjected to a phosphate treatment to be blackened, a unique color tone that cannot be obtained with a coated steel sheet is developed. For example, it is found in JP-A-3-226584 and JP-A-5-237450. Thus, it is widely used as a roofing material and exterior material for buildings. It is about to be used for home appliance parts.
As a method for mass production, a technique of treating a Zn-based plated steel sheet with phosphate in a short time in a continuous line is introduced in Japanese Patent Application Laid-Open Nos. 8-218181 and 2000-309881. In a continuous line operation, the shorter the time required for phosphating, the higher the work efficiency and the more advantageous. JP-A-8-218181 describes that a blackened appearance having an L value of 45 to 60 can be obtained in a processing time of about 3 seconds.
[0003]
Regarding the blackening of Zn-based plating materials, JP-A-7-62554 and JP-A-8-218158 disclose that Ni or Co is substituted and precipitated into fine particles on the Zn plating surface to produce a light scattering effect. , Black appearance (L^*It is described that it will exhibit a value of 10 to 20 (corresponding to an L value of 18 to 30) and a 60 ° glossiness of 9 to 15).
Investigating the relationship between color evaluation and visual evaluation and lightness and glossiness by instrumental analysis reveals that the lightness index L value is low (that is, blackness is high) and 60 ° glossiness (positive light at an incident angle of 60 °). It was found that the lower the (reflection component), the greater the profound feeling.
[0004]
The L value and the factor governing the 60 ° glossiness of the blackened Zn-based plated steel sheet were examined, and the L value was determined by the amount of Ni or Co deposited and substituted on the plating surface at the time of contact with the phosphating solution. It was found that the 60 ° glossiness was related to the precipitation amount of phosphate crystals. The larger the substitution amount of Ni and Co, the lower the L value, and the larger the precipitation amount of phosphate crystals, the lower the glossiness.
However, when a blackening treatment is to be performed on a hot-dip Zn-based plated steel sheet using a phosphate treatment solution, if a treatment solution having a high concentration of Ni ions or Co ions is used in order to strengthen the blackening, Ni or Co Is increased, but the amount of precipitated phosphate crystals is reduced, and the 60 ° glossiness is increased. Therefore, it is very difficult to obtain a blackened hot-dip Zn-based plated steel having an L value of 40 or less and a 60 ° glossiness of 5 or less.
[0005]
[Problems to be solved by the invention]
The present invention has been devised to solve such a problem, and a surface treatment with a phosphating solution provides a more profound color tone, specifically, an L value of 40 or less and 60 The present invention is to provide a hot-dip coated steel sheet which satisfies a glossiness of 5 or less and which can produce such a blackened surface in a short time, and a blackened hot-dip coated steel sheet using the steel sheet.
[0006]
[Means for Solving the Problems]
The hot-dip Zn-Al-Mg-based alloy-plated steel sheet for blackening treatment of the present invention is a hot-dip Zn-Al-Mg-based alloy-plated steel sheet that is blackened by a phosphate treatment in order to achieve its object. [Al / Zn / Zn₂Ternary eutectic structure of Mg] is 60% or more in area ratio.
In addition, the blackened hot-dip Zn-Al-Mg-based alloy-plated steel sheet of the present invention has [Al / Zn / Zn₂Mg ternary eutectic structure] Ni and / or Co adhered on the alloy plating layer of the hot-dip Zn-Al-Mg-based alloy-plated steel sheet adjusted so that the area occupied by the phase is 60% or more in area ratio. 20mg / m²The Ni and / or Co deposited layer and zinc phosphate adhesion amount of 2 g / m²It is characterized in that a film composed of the above zinc phosphate crystals is formed.
It is preferable that a chromate coating or a non-chromium coating and a clear coating of 5 to 20 μm are further laminated on the coating composed of the Ni and / or Co deposition layer and the zinc phosphate crystal.
Such a blackened hot-dip Zn-Al-Mg-based alloy-plated steel sheet has [Al / Zn / Zn₂[Ternary eutectic structure of Mg]], a molten Zn-Al-Mg-based alloy-plated steel sheet in which the proportion of the phase occupied by the alloy is 60% or more in terms of area ratio, Zn ions of 5 to 10 g / L, nitrate ions of It is manufactured by contacting a phosphating solution containing 3 to 15 g / L, Ni ions and / or Co ions with 3 to 15 g / L.
[0007]
[Action]
The present inventors first studied the cause of the difficulty in achieving low brightness and low gloss when a hot-dip Zn-coated steel sheet was subjected to a blackening treatment using a phosphate treatment solution.
As part of this, based on the hot-dip galvanized steel sheet, the phosphate treatment conditions were examined in which the Ni ion concentration in the phosphate solution was changed to produce a more profound color tone.
Since Ni and Co exhibit almost the same behavior, in the following description, Ni is exemplified and described. Therefore, description of “Ni” means “Ni and / or Co”.
FIG. 1 shows the L value when the phosphate treatment was carried out by spraying at a Zn ion concentration of 7 g / L in the treatment solution, a treatment time of 5 seconds and a treatment solution temperature of 65 ° C. based on the hot-dip Zn-coated steel sheet. And the change in 60 ° gloss. FIG. 2 shows the relationship between the concentration of Ni ions in the treatment solution and the amount of Ni-substituted precipitates on the Zn plating surface and the amount of phosphate crystal precipitated. FIG. 3 shows the appearance after the phosphate treatment when the Ni ion concentration is 2.5, 5.2, 10.0 g / L. Here, the Ni-substituted precipitation amount and the phosphate crystal precipitation amount were determined by the following procedure.
[0008]
(Precipitation amount of phosphate crystals)
1dm steel plate after phosphate treatment²After slicing into a size and weighing it, it was immersed in an aqueous solution (normal temperature) in which dichromic acid was dissolved in ammonium water (ammonia: distilled water = 1: 1) for 15 minutes. From the weight difference before and after the immersion, The amount of precipitated phosphate crystals was calculated.
[Ni substitution precipitation amount]
Using the test piece after dissolving the phosphate crystals, the plating layer was dissolved in 10% HCl at room temperature, and the solution was analyzed by ICP to calculate the Ni-substituted precipitation amount.
The lightness index L value in the present specification is a value measured by an SM color computer SM-7 manufactured by Suga Test Instruments Co., Ltd., and is a numerical value based on a hunter color difference formula defined in JIS Z8730. The gloss is a numerical value measured in accordance with JIS Z8741 “Method for measuring specular gloss”.
[0009]
As can be seen from FIGS. 1 and 2, when the concentration of Ni ions in the treatment liquid is increased, the amount of Ni-substituted precipitates increases, and the L value decreases. However, the amount of precipitated phosphate crystals sharply decreases when the Ni ion concentration in the treatment liquid exceeds 5 g / L, and an increase in glossiness is observed. This indicates that increasing the Ni ion concentration in the processing solution increases the amount of Ni-substituted precipitates and lowers the L value, but inhibits the precipitation of phosphate crystals at a certain concentration or more. As can be seen from FIG. 3, when the concentration of Ni ions in the processing solution is 2.5, 5.2 g / L, phosphate crystals are densely generated on the plating surface, whereas At a concentration of 10.0 g / L, phosphate crystals are sparsely precipitated.
[0010]
From these results, the following can be considered as a reason why an increase in the concentration of Ni ions in the treatment liquid leads to a decrease in the amount of precipitated phosphate crystals.
That is, when the surface of the Zn plating is brought into contact with the phosphating solution, substitutional precipitation of Ni occurs first. At this time, the area occupied by the Ni-substituted precipitation layer on the Zn plating surface increases as the Ni ion concentration in the processing solution increases. At one end, the dissolution of the Zn plating layer hardly occurs at the portion where the Ni-substituted precipitation layer is formed, and it is considered that as the area occupied by the Ni-substitution precipitation layer increases, the subsequent precipitation reaction of phosphate crystals is hindered.
[0011]
Therefore, the present inventors searched for means for reducing the area ratio occupied by the Ni-substituted precipitation layer when causing the Ni-substituted precipitation by contact with the phosphating solution. As a result, a Zn-Al-Mg-based plating alloy was adopted as the hot-dip plating alloy, and the structure of the outermost layer of the plating layer was changed to [Al / Zn / Zn.₂The above object can be achieved by increasing the proportion of Mg ternary eutectic].
That is, when a Zn—Al—Mg-based alloy is hot-dip coated on the surface of a steel sheet, as reported by the present inventors in JP-A-2002-226958, the plating layer of the plated steel sheet is [Al / Zn / Zn].₂A metal structure in which [primary crystal Al phase] and further [Zn single phase] are mixed in the matrix having the ternary eutectic structure of Mg is formed.
[0012]
For example, FIG. 4 schematically shows reaction surfaces when a hot-dip galvanized steel sheet and a hot-dip Zn-Al-Mg alloy-plated steel sheet are spray-treated with a Ni-containing phosphating solution. Compared to a hot-dip Zn-plated steel sheet in which the plating layer has a substantially single structure, in the hot-dip Zn-Al-Mg alloy-plated steel sheet, most of the plated layer is [Al / Zn / Zn₂Mg ternary eutectic].
The structure of the outermost surface of the plating layer is [Al / Zn / Zn₂Mg ternary eutectic structure], so even if it comes into contact with a Ni-containing phosphating solution, Zn, which is considered to have the lowest surface potential in the ternary eutectic structure₂It is presumed that Ni substitution precipitation reaction occurs preferentially in the Mg phase, and almost the entire surface is not covered with the Ni substitution precipitation layer unlike hot-dip Zn plating. Thereafter, in the hot-dip Zn-plated steel sheet, phosphate crystals only precipitate near the undeposited portion of the Ni-substituted layer, whereas in the hot-dip Zn-Al-Mg alloy-plated steel sheet, as shown in FIG. It is considered that phosphate crystals precipitate on the entire surface.
[0013]
By the reaction mechanism as described above, it is possible to increase the reactivity of the phosphating treatment to shorten the treatment time, and even when the phosphating treatment solution having a high Ni ion content is brought into contact with the phosphating treatment solution, It is considered that it is possible to stably produce a surface having a low L value and low glossiness.
Therefore, [Al / Zn / Zn / Zn is placed on the outermost surface of the plating layer of the hot-dip Zn-Al-Mg alloy-plated steel sheet brought into contact with the phosphating solution.₂It is important that there be more phases of [ternary eutectic structure of Mg].
[0014]
In the present invention, as will be described later, by repeating various tests, [Al / Zn / Zn₂It has been confirmed that when the ratio of the phase of the [ternary eutectic structure of Mg] is 60% or more in terms of the area ratio, the reactivity of the phosphate treatment can be increased and the treatment time can be shortened. If the area ratio is less than 60%, the reactivity of the phosphate treatment decreases, and the treatment time cannot be reduced. Further, when a phosphating solution having a high Ni ion concentration is used, the amount of precipitated phosphate crystals is reduced, and a low gloss surface cannot be obtained. It is more preferably at least 80%.
[0015]
[Al / Zn / Zn]₂Mg ternary eutectic structure] is based on a hot-dip Zn-Al-Mg alloy-plated steel sheet having a phase area ratio of 80% or more, with Zn metal ions at 8 g / L and nitrate ions at 6 g / L at a processing temperature of FIG. 5 shows the relationship between the Ni ion concentration in the processing solution, the amount of Ni-substituted precipitates on the plating surface, and the amount of phosphate crystals precipitated when spray treatment was performed for 3 seconds at a temperature of 65 ° C. FIG. 6 shows the L value and the 60 ° gloss at that time.
Unlike when using hot-dip galvanized steel sheet as a base, despite the short treatment time of 3 seconds, even when the Ni ion concentration in the processing solution is increased to 14.8 g / L, the precipitation amount of phosphate crystals is large. No significant decrease was observed, and the low glossiness was stably maintained even when the L value was in the range of 35 or less. The above idea is supported.
[0016]
In this specification, [Al / Zn / Zn]₂Mg ternary eutectic structure] is determined as follows.
FIG. 7 shows an example of a micrograph of a section of a plating layer of a hot-dip Zn-Al-Mg alloy-plated steel sheet. The plating layer of the hot-dip Zn-Al-Mg alloy plated steel sheet is [Al / Zn / Zn₂[Primary Al phase] or [Primary Al phase] and [Zn single phase], or [Primary Al phase] and [Zn single phase] and a part of [Zn₂Mg phase]. Five samples are cut out from an arbitrary place of the hot-dip Zn-Al-Mg alloy plated steel sheet to be subjected to the phosphate treatment, and the plating layer is observed at three places in a cross-section of 2000 times from each sample, and the outermost surface of the plating layer is obtained. [Al / Zn / Zn₂Mg ternary eutectic structure] was determined, and the average value thereof was taken to be [Al / Zn / Zn] at the outermost surface of the plating layer in the present invention.₂Mg ternary eutectic structure]. [Al / Zn / Zn at the outermost surface of the plating layer₂The ratio occupied by the Mg ternary eutectic structure] is a value obtained by calculating the length indicated by A and B in FIG.
Incidentally, [Al / Zn / Zn] on the outermost surface of the plating layer in FIG.₂Mg ternary eutectic structure] accounts for 66%.
(B₁+ B₂+ B₃) × 100 / A
Where A is the length of the observed part
B₁, B₂, B₃; Exposed on the outermost surface of the plating layer [Al / Zn / Zn₂Mg ternary eutectic structure]
[0017]
Embodiment
Next, a preferred embodiment of the present invention will be described.
A hot-dip Zn-Al-Mg alloy-plated steel sheet serving as a base can be prepared by the method described in JP-A-2002-226958 previously proposed by the present inventors.
The component composition of the steel sheet to be plated is not particularly limited. Normal steel cold-rolled steel sheets and stainless steel cold-rolled steel sheets can be used. However, when a stainless steel plate is used as the plating base plate, it is preferable to perform Fe-B pre-plating before plating in order to increase the adhesion with the molten Zn-Al-Mg alloy plating layer.
[0018]
The hot-dip plating contains 2.5 to 15% by mass of Al and 2.0 to 4.0% by mass of Mg in a Zn bath, and further contains 0.001 to 0.1% by mass of Ti as necessary. B: After immersing the original steel strip continuously in a hot-dip Zn-Al-Mg plating bath containing 0.001 to 0.045 mass% and Si: 0.005 to 0.5 mass%, and pulling it up. Then, the surface of the plating layer may be cooled by air to form a molten Zn—Al—Mg alloy plating layer.
[0019]
At this time, the Al concentration in the plating bath is in the range of 2.5 to 15% by mass. If the Al content exceeds 15% by mass, the proportion of the [primary Al phase] on the outermost surface of the plated layer after plating increases, and [Al / Zn / Zn]₂Mg ternary eutectic structure] is less than 60%. On the other hand, if the Al content is less than 2.5% by mass, the [Zn single phase] in the plating layer increases, and the [Al / Zn / Zn]₂Mg ternary eutectic structure] is less than 60%.
[Al / Zn / Zn₂In order to make the area ratio occupied by the Mg ternary eutectic structure] 80% or more, the Al content is preferably in the range of 4 to 8% by mass.
[0020]
The Mg concentration in the plating bath is in the range of 2.0 to 4.0% by mass. If the Mg content exceeds 4.0% by mass, [Zn]₂Mg single phase] began to be noticeable, and [Al / Zn / Zn]₂Mg ternary eutectic structure] is less than 60%. On the other hand, if the Mg content is less than 2.0% by mass, the [primary Al phase] and the [Zn single phase] increase, and the [Al / Zn / Zn]₂Mg ternary eutectic structure] is less than 60%.
[Al / Zn / Zn₂In order to make the area ratio occupied by the Mg ternary eutectic structure] 80% or more, the Mg content is preferably in the range of 2.5 to 3.5% by mass.
[0021]
For Ti and B, the metallographic structure of the plating layer was changed to [Al / Zn / Zn₂Mg ternary eutectic structure] has a bad influence on the appearance and corrosion resistance.₁₁Mg₂The addition is beneficial because it provides a function of suppressing the formation and growth of the phase. When Ti, B or Ti-B alloy or compound is added to the plating bath to obtain this effect, the content of the bath is 0.001 to 0.1% by mass of Ti, and 0.001 to 0% of B. It is preferable to add so that it may become 0.045 mass%. When the content is more than this, a precipitate grows in the plating layer, and the plating layer may have irregularities to impair the appearance.
Si is preferably added in the range of 0.005 to 0.5% by mass in order to suppress the growth of the Al—Fe alloy layer at the interface between the plating layer and the base steel.
[0022]
Since the cooling rate after plating also affects the phase structure appearing in the plating layer, it is preferable to adjust the cooling rate. Zn that reduces corrosion resistance₁₁Mg₂It is preferable to cool at a rate of 7 ° C./second or more so that no phase appears. It may be adjusted by changing the air amount and air pressure.
The steel sheet plated with the molten Zn—Al—Mg alloy is blackened by a method of spraying a phosphating solution or a method of dipping in a phosphating solution.
The composition of the processing solution at this time also has a large effect on blackening.
[0023]
The phosphating solution for blackening is basically a phosphoric acid solution in which zinc oxide and nickel are dissolved in a phosphoric acid solution, and zinc is zinc hydrogen phosphate [Zn (H₂PO₄)₂] To maintain a certain equilibrium state.
When this zinc phosphate treatment liquid is brought into contact with the plating surface of a base material of a molten Zn—Al—Mg alloy plated steel sheet, Ni ions in the liquid are replaced with zinc (eluted into the liquid by free phosphoric acid) in the plating layer. Thus, a Ni-substituted precipitation layer is formed on the plating layer surface. In addition, zinc phosphate [Zn]₃(PO₄)₂Is produced and the hydrate [Zn₃(PO₄)₂・ 4H₂O], a zinc phosphate crystal layer is formed.
[0024]
The Zn ion concentration in the phosphating solution is in the range of 5 to 10 g / L. If the Zn ion concentration is less than 5 g / L, phosphate crystals are not sufficiently precipitated in a short time treatment, and a low gloss feeling cannot be obtained. If it exceeds 10 g / L, the stability of the phosphating solution will be poor, and sludge will easily be generated.
The phosphate ion concentration in the phosphating solution is preferably in the range of 25 to 45 g / L. When the phosphate ion concentration is less than 25 g / L, phosphate crystals are not sufficiently precipitated in a short time treatment, and a low gloss feeling cannot be obtained. If it exceeds 45 g / L, the stability of the phosphating solution will be poor, and sludge will easily be generated.
[0025]
The total concentration of Ni ions and Co ions in the phosphating solution is in the range of 3 to 15 g / L. If the concentration of Ni or Co or the combined ion of Ni and Co is less than 3 g / L, a color tone having an L value of 40 or less cannot be obtained. On the other hand, the effect of increasing blackness saturates even if it is increased beyond 15 g / L.
Further, the nitrate ion in the phosphating solution is also in the range of 3 to 15 g / L. Nitrate ions must be contained as a reaction accelerator, and if less than 3 g / L, the effect of promoting the reaction is insufficient. Conversely, if it exceeds 15 g / L, the surface of the metal becomes inactive due to the oxidizing effect of nitrate ions, the substitution precipitation reaction of Ni or Co is reduced, and sufficient blackness cannot be obtained.
[0026]
The phosphating treatment is preferably performed at a liquid temperature of 50 to 80 ° C. If the temperature is lower than 50 ° C., the reactivity is lowered, and the treatment in a short time becomes difficult. When the temperature is higher than 80 ° C., the stability of the processing liquid is lowered, sludge is generated more, and the evaporation of water is increased, making it difficult to control the concentration in continuous operation.
When the temperature of the treatment liquid is in the range of 50 to 80 ° C., the phosphate treatment requires about 2 to 5 seconds for the spray treatment and about 3 to 8 seconds for the immersion treatment. There is no color change, but the effect is saturated.
[0027]
By the way, in the case where the phosphate treatment is to be continuously performed in a line or the like continuous to the hot-dip plating step, Al eluted from the plating layer inhibits the phosphate reaction. Therefore, it is preferable that the concentration of free fluorine ions in the treatment liquid is always maintained at 30 ppm or more so that the eluted Al is rendered harmless to the phosphate treatment reaction. Specifically, continuous operation becomes possible by continuously adding a certain amount of fluoride. Examples of the fluoride to be used include sodium fluoride, potassium fluoride, sodium hydrogen fluoride and the like.
[0028]
After the blackening by the phosphate treatment, a chromate sealing treatment or a chromium-free sealing treatment containing zirconium oxide and vanadium oxide may be performed to improve corrosion resistance.
In the case where the phosphating treatment of the present invention is continuously performed in a line continuous to the hot-dip plating process, a spray device or a roll coater device for performing the sealing treatment may be attached to the post-treatment of the phosphating treatment. .
Further, after blackening by the phosphate treatment, a clear coating may be applied as necessary to protect the phosphate treated surface during processing and to improve corrosion resistance. The clear coating may be performed according to a conventional method using a known coating material, for example, a polyester type, an epoxy type, a vinyl chloride type, an acrylic type, a fluorine type or the like. It is sufficient that the thickness of the coating is 5 to 20 μm after drying.
[0029]
【Example】
Example 1
Prepare a hot-dip Zn-Al-Mg alloy plated steel sheet with various plating bath components under the conditions 1 shown below, and perform degreasing → water washing → surface conditioning, then phosphate treatment according to condition 2 below Various characteristic values of the plate were measured.
For comparison, a hot-dip galvanized steel sheet and a hot-dip Zn-5% Al alloy-plated steel sheet having a thickness of 0.5 mm were also evaluated.
In addition, the lightness index L value and the glossiness are measured according to JIS Z8730 and JIS Z8741 as described above.
Table 1 also shows the results of arranging the relationship between the area ratio of the ternary eutectic structure of the outermost layer of the plating layer and various characteristic values.
[0030]
(Condition 1)
Processing equipment: Lab batch type hot-dip plating equipment
Raw plating: Low-carbon cold-rolled steel sheet with a thickness of 0.5 mm
Maximum reduction furnace temperature: 720 ° C, dew point: -30 ° C
Plating bath composition: A1 = 3-12% by mass, Mg = 1-5% by mass, balance = Zn
Megumi bath temperature: 450-510 ° C
Immersion time: 3 seconds
Cooling rate after plating: 20 ° C / sec by forced air cooling
(The cooling rate is the average value from the plating bath temperature to the plating layer solidification temperature.)
Plating weight: about 200 g / m²(One side)
[0031]

[0032]

[0033]
As can be seen from the above results, [Al / Zn / Zn]₂Mg ternary eutectic structure] of the present invention in which the area ratio is 60% or more, both the L value and the 60 ° gloss are satisfactory values.
However, [Al / Zn / Zn on the outermost surface of the plating layer₂Mg ternary eutectic structure] in Comparative Examples having an area ratio of less than 60%, the L value was satisfactory, but the gloss at 60 ° was insufficient. This is probably because phosphate crystals could not be sufficiently precipitated because the area ratio of the ternary eutectic structure was low.
[0034]
Example 2:
After performing degreasing → water washing → surface conditioning on the hot-dip Zn-Al-Mg alloy-plated steel sheet prepared under the following condition 3, various characteristic values of the phosphate-treated plate subjected to the lab treatment according to the following condition 4 were measured. did. Table 2 shows the results.
Tables 3 and 4 show the results of evaluation of a hot-dip galvanized steel sheet and a hot-dip Zn-5% Al alloy-plated steel sheet having a thickness of 0.5 mm for comparison.
Incidentally, the phosphate crystal precipitation amount and the Ni and / or Co substitution precipitation amount were measured by the above-mentioned method, and the L value and the 60 ° glossiness were measured in accordance with JIS as in Example 1. is there.
[0035]
(Condition 3)
Processing equipment: continuous hot-dip plating line
Raw plating: Low-carbon cold-rolled steel sheet with a thickness of 0.5 mm
Maximum reduction furnace temperature: 720 ° C, dew point: -30 ° C
Plating bath composition: A1 = 6.1% by mass, Mg = 3.1% by mass, balance = Zn
Melting bath temperature: 450 ° C
Immersion time: 3 seconds
Cooling rate after plating: 25 ° C / sec by forced air cooling
(The cooling rate is the average value from the plating bath temperature to the plating layer solidification temperature.)
Plating weight: about 150 g / m²(One side)
[Al / Zn / Zn on the outermost surface of plating₂Mg ternary eutectic structure]: 87%
[0036]

[0037]

[0038]

[0039]

[0040]
As can be seen from the results shown in Tables 2, 3 and 4, the substitution precipitation amount of Ni and / or Co was 20 mg / m 2.²As described above, the precipitation amount of phosphate crystals was 2 g / m²In the above examples of the present invention, both the L value and the 60 ° gloss are satisfactory values.
On the other hand, the substitution precipitation amount of Ni and / or Co is 20 mg / m²In the comparative examples less than the above, the L value has not reached the target numerical value. Further, the precipitation amount of phosphate crystals is 2 g / m 2.²In Comparative Examples less than, the 60 ° glossiness did not reach the target value.
[0041]
Example 3
A hot-dip Zn—Al—Mg alloy-plated steel strip having a width of 1000 mm produced under the same conditions as in Example 2 was degreased, washed with water, and surface-adjusted in a color coating line, and then subjected to phosphate treatment under the following condition 5. . Further, a post-treatment and a clear coating were continuously performed under the following condition 6.
However, in the post-treatment, steel strips having different post-treatments were produced using two types of treatment liquids.
The L value of the obtained steel strip fluctuated in the width direction and the longitudinal direction within a range of 30 ± 1, the glossiness at 60 ° was stable at 3 or less, and the uniform processability was good. Using the end plate cut out from the steel strip, the corrugated sheet was formed by roll forming. However, neither the clear coating film nor the phosphate treatment film was peeled off or dropped off, and had good workability.
[0042]

[0043]
(Condition 6)
・ Chrome sealing post-treatment
Treatment method: spray + ringer roll
Attached amount: about 8 mg / m in terms of Cr²(One side)
・ Chromium-free sealing post-treatment (treatment containing zirconium oxide and vanadium oxide)
Treatment method: spray + ringer roll
Attached amount: about 10 mg / m in terms of Zr + V²(One side)
・ Clear painting
Painting method: roll coater
Coating thickness: 8 μm
Paint type: Silica-containing polyester resin
[0044]
【The invention's effect】
As described above, according to the present invention, by subjecting a hot-dip Zn-Al-Mg-based alloy-plated steel sheet to a surface treatment with a phosphating solution, the brightness and gloss are controlled, and a more profound color tone is obtained. Can be given.
The gloss is suppressed, it is easy to obtain an appearance with a deep black gray, and it has various color tones depending on individual specific applications and usage modes in building materials, home appliances, and various other fields, A black hot-dip Zn-Al-Mg-based alloy plated steel sheet having excellent design properties can be obtained.
Since the plated steel sheet to be blackened is coated with a hot-dip Zn-Al-Mg-based alloy that is extremely excellent in corrosion resistance, the blackened plated steel sheet is also excellent in corrosion resistance. It is expected to be widely used in products and other various fields.
[Brief description of the drawings]
FIG. 1 is a graph showing changes in the Ni ion concentration in the treatment solution, the L value of the Zn plating surface, and the 60 ° gloss when a phosphate treatment is applied to a hot-dip Zn-coated steel sheet.
FIG. 2 is a graph showing the relationship between the concentration of Ni ions in a treatment solution, the amount of Ni-substituted precipitates on the Zn-plated surface, and the amount of phosphate crystals precipitated when a hot-dip Zn-coated steel sheet is subjected to phosphate treatment.
FIG. 3 is a diagram schematically showing a difference in surface appearance according to a Ni ion concentration when a phosphate treatment is performed on a hot-dip galvanized steel sheet.
FIG. 4 is a diagram schematically showing reaction surfaces when a hot-dip Zn-plated steel sheet and a hot-dip Zn-Al-Mg alloy-plated steel sheet are spray-treated with a Ni-containing phosphating solution;
FIG. 5: [Al / Zn / Zn]₂Mg ternary eutectic structure] was applied to a hot-dip Zn-Al-Mg alloy-plated steel sheet having a phase area ratio of 90% or more, with Zn metal ions of 8 g / L and nitrate ions of 6 g / L, and a treatment temperature of 65 g / L. FIG. 6 is a graph showing the relationship between the Ni ion concentration in the processing solution, the amount of Ni-substituted precipitates on the plating surface, and the amount of phosphate crystals deposited when spraying was performed at 3 ° C. for 3 seconds.
FIG. 6: [Al / Zn / Zn]₂Mg ternary eutectic structure] was applied to a hot-dip Zn-Al-Mg alloy-plated steel sheet having a phase area ratio of 90% or more, with Zn metal ions of 8 g / L and nitrate ions of 6 g / L, and a treatment temperature of 65 g / L. FIG. 6 is a diagram showing the relationship between the Ni ion concentration in the treatment solution, the L value of the plating surface, and the 60 ° gloss when the spray treatment was performed at 3 ° C. for 3 seconds.
FIG. 7 is a diagram schematically showing an observation surface when a section of a plating layer of a hot-dip Zn—Al—Mg alloy plated steel plate is observed by a microscope, and a diagram schematically showing the observation surface.

Claims

This is a hot-dip Zn-Al-Mg-based alloy-plated steel sheet that is blackened by phosphate treatment, and the ratio of the [Al / Zn / Zn ₂ Mg ternary eutectic structure] phase to the outermost surface of the plating layer is the area. A hot-dip Zn-Al-Mg-based alloy-plated steel sheet for blackening treatment, wherein the rate is 60% or more.

The ratio of the [Al / Zn / Zn ₂ Mg ternary eutectic structure] phase at the outermost surface of the plating layer of the hot-dip Zn-Al-Mg-based alloy-plated steel sheet was adjusted so that the area ratio was 60% or more. A Ni and / or Co deposition layer having a Ni and / or Co adhesion of 20 mg / m ² or more and a phosphorus having a zinc phosphate adhesion of 2 g / m ² or more on an alloy plating layer of a hot-dip Zn—Al—Mg alloy plated steel sheet. A blackened hot-dip Zn-Al-Mg-based alloy-plated steel sheet having a coating made of zinc oxide crystals formed thereon.

A chromate-treated coating or a non-chromium-based coating on a coating consisting of a Ni and / or Co deposition layer with a Ni and / or Co adhesion amount of 20 mg / m ² or more and a zinc phosphate crystal with a zinc phosphate adhesion amount of 2 g / m ² or more The blackened hot-dip Zn-Al-Mg-based alloy-plated steel sheet according to claim 2, wherein the treated film and a clear coating of 5 to 20 μm are laminated.

A hot-dip Zn-Al-Mg-based alloy-plated steel sheet adjusted so that the ratio of the phase of [Al / Zn / Zn ₂ Mg ternary eutectic] on the outermost surface of the plating layer is 60% or more in area ratio. A blackening melt characterized by contacting with a phosphating solution containing 5 to 10 g / L of Zn ions, 3 to 15 g / L of nitrate ions, and 3 to 15 g / L of Ni ions and / or Co ions. A method for producing a Zn-Al-Mg alloy plated steel sheet.