JP5884146B2 - Hot-dip Zn-Al alloy-plated steel sheet - Google Patents
Hot-dip Zn-Al alloy-plated steel sheet Download PDFInfo
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- JP5884146B2 JP5884146B2 JP2010229211A JP2010229211A JP5884146B2 JP 5884146 B2 JP5884146 B2 JP 5884146B2 JP 2010229211 A JP2010229211 A JP 2010229211A JP 2010229211 A JP2010229211 A JP 2010229211A JP 5884146 B2 JP5884146 B2 JP 5884146B2
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- 229910007570 Zn-Al Inorganic materials 0.000 title claims description 42
- 229910000831 Steel Inorganic materials 0.000 title claims description 32
- 239000010959 steel Substances 0.000 title claims description 32
- 238000007747 plating Methods 0.000 claims description 60
- 229910045601 alloy Inorganic materials 0.000 claims description 17
- 239000000956 alloy Substances 0.000 claims description 17
- 230000005496 eutectics Effects 0.000 claims description 15
- 239000012535 impurity Substances 0.000 claims description 7
- 239000011701 zinc Substances 0.000 description 16
- XLOMVQKBTHCTTD-UHFFFAOYSA-N Zinc monoxide Chemical compound [Zn]=O XLOMVQKBTHCTTD-UHFFFAOYSA-N 0.000 description 10
- 238000006243 chemical reaction Methods 0.000 description 8
- 239000000126 substance Substances 0.000 description 8
- ZCDOYSPFYFSLEW-UHFFFAOYSA-N chromate(2-) Chemical compound [O-][Cr]([O-])(=O)=O ZCDOYSPFYFSLEW-UHFFFAOYSA-N 0.000 description 5
- 230000007797 corrosion Effects 0.000 description 5
- 238000005260 corrosion Methods 0.000 description 5
- 230000006866 deterioration Effects 0.000 description 5
- 238000000034 method Methods 0.000 description 5
- 239000011787 zinc oxide Substances 0.000 description 5
- 229910018134 Al-Mg Inorganic materials 0.000 description 4
- 229910018467 Al—Mg Inorganic materials 0.000 description 4
- 229910052782 aluminium Inorganic materials 0.000 description 4
- 230000007547 defect Effects 0.000 description 4
- 229910052749 magnesium Inorganic materials 0.000 description 4
- 239000011248 coating agent Substances 0.000 description 3
- 238000000576 coating method Methods 0.000 description 3
- 239000000463 material Substances 0.000 description 3
- 238000010422 painting Methods 0.000 description 3
- 238000000137 annealing Methods 0.000 description 2
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 2
- 230000002950 deficient Effects 0.000 description 2
- 238000007598 dipping method Methods 0.000 description 2
- 230000001771 impaired effect Effects 0.000 description 2
- 239000000203 mixture Substances 0.000 description 2
- 229910052759 nickel Inorganic materials 0.000 description 2
- 229910052760 oxygen Inorganic materials 0.000 description 2
- 239000001301 oxygen Substances 0.000 description 2
- GYHNNYVSQQEPJS-UHFFFAOYSA-N Gallium Chemical compound [Ga] GYHNNYVSQQEPJS-UHFFFAOYSA-N 0.000 description 1
- 229910001335 Galvanized steel Inorganic materials 0.000 description 1
- 206010066901 Treatment failure Diseases 0.000 description 1
- 238000002441 X-ray diffraction Methods 0.000 description 1
- 229910001297 Zn alloy Inorganic materials 0.000 description 1
- 230000001133 acceleration Effects 0.000 description 1
- 229910052799 carbon Inorganic materials 0.000 description 1
- 239000010960 cold rolled steel Substances 0.000 description 1
- 238000010276 construction Methods 0.000 description 1
- 238000001816 cooling Methods 0.000 description 1
- 239000013078 crystal Substances 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 238000011156 evaluation Methods 0.000 description 1
- 229910052733 gallium Inorganic materials 0.000 description 1
- 239000008397 galvanized steel Substances 0.000 description 1
- 238000009499 grossing Methods 0.000 description 1
- 229910000765 intermetallic Inorganic materials 0.000 description 1
- 238000011835 investigation Methods 0.000 description 1
- 239000007788 liquid Substances 0.000 description 1
- 239000002932 luster Substances 0.000 description 1
- 238000004519 manufacturing process Methods 0.000 description 1
- 239000011159 matrix material Substances 0.000 description 1
- 229910052751 metal Inorganic materials 0.000 description 1
- 239000002184 metal Substances 0.000 description 1
- 150000002739 metals Chemical class 0.000 description 1
- 239000003595 mist Substances 0.000 description 1
- 230000006911 nucleation Effects 0.000 description 1
- 238000010899 nucleation Methods 0.000 description 1
- 239000000843 powder Substances 0.000 description 1
- 238000001878 scanning electron micrograph Methods 0.000 description 1
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Description
本発明は、自動車、建築、土木、家電等の分野で利用されるめっき外観および耐黒変性に優れた溶融Zn-Al系合金めっき鋼板、特に、めっき層中のAl含有量が5質量%前後のガルファンと呼ばれる溶融Zn-Al系合金めっき鋼板に関する。 The present invention is a hot-dip Zn-Al alloy-plated steel sheet excellent in plating appearance and blackening resistance used in the fields of automobiles, architecture, civil engineering, home appliances, etc., in particular, the Al content in the plating layer is around 5% by mass. The present invention relates to a hot-dip Zn-Al alloy-plated steel sheet called Galfan.
従来、溶融Zn-Al系合金めっき鋼板は、その表面に塗装を施したいわゆるプレコート鋼板として、自動車、建築、土木、家電等の分野で広く利用されている。この溶融Zn-Al系合金めっき鋼板としては、主に、めっき層中のAl含有量が0.2質量%以下の溶融Znめっき鋼板(以下、GIという)、同Al含有量が約5質量%のガルファン(以下、GFという)、同Al含有量が約55質量%のガルバリュウム鋼板(以下、GLという)が使用されているが、特に建築や土木の分野では、GLより低コストであること、GIより耐食性が優れていること等の理由から、GFが使用されることが多い。しかし、GFには、一般に以下のような問題がある。 Conventionally, a hot-dip Zn—Al-based alloy-plated steel sheet has been widely used in the fields of automobiles, architecture, civil engineering, home appliances and the like as so-called pre-coated steel sheets whose surfaces are coated. The hot-dip Zn-Al alloy-plated steel sheet is mainly a hot-dip Zn-plated steel sheet (hereinafter referred to as GI) with an Al content of 0.2% by mass or less in the plating layer, and a galfan with the Al content of about 5% by mass. (Hereinafter referred to as GF), gallium steel sheet (hereinafter referred to as GL) with the same Al content of about 55% by mass is used, but in the field of construction and civil engineering, it is lower in cost than GL. GF is often used for reasons such as excellent corrosion resistance. However, GF generally has the following problems.
(i) 外観不良
めっき層には亀甲模様のスパングルが形成されるが、このスパングルは、めっき条件(例えば、めっき前の焼鈍条件、めっき浴成分、めっき後の冷却条件)によって形態が異なるため、無塗装でそのまま使用する場合に外観不良を招くことがある。また、塗装を施してカラー鋼板として使用する場合にはスパングルが塗装面に浮き上がり、外観不良を招くこともある。
(i) Appearance failure Although a tortoise-shaped spangle is formed on the plating layer, the shape of this spangle varies depending on the plating conditions (e.g., annealing conditions before plating, plating bath components, cooling conditions after plating). Appearance defects may occur when used as is without painting. Further, when used as a color steel plate after painting, spangles may float on the painted surface, leading to poor appearance.
このため、近年では、スパングルのない金属光沢の美麗なめっき層を有するGFに対する要求が増加している。 For this reason, in recent years, there has been an increasing demand for GF having a beautiful plated layer with a metallic luster without spangles.
(ii) めっき層表面の黒変
腐食環境によっては、めっき層表面が局所的に黒灰色に変色する、いわゆる黒変が発生して商品価値を著しく損なうことがある。この黒変は、めっき後の鋼板が高温多湿の環境に置かれた場合、めっき層表面の酸化亜鉛が酸素欠乏型酸化亜鉛に変化することによって生じるといわれている。めっき後の鋼板を直ちに化成処理して塗装を行う場合は比較的黒変の問題は少ないが、実際にはめっき後の鋼板をコイル状態で梱包し、ある期間放置してから化成処理および塗装することが多く、その放置期間に黒変が発生する。黒変が発生すると、化成処理不良が発生し、結果として塗装後の塗膜の密着性、加工性、耐食性等が低下し、商品価値を著しく損なう。
(ii) Blackening of the plating layer surface Depending on the corrosive environment, the plating layer surface may locally turn black-gray, so-called blackening may occur and the commercial value may be significantly impaired. This blackening is said to occur when zinc oxide on the surface of the plating layer changes to oxygen-deficient zinc oxide when the plated steel sheet is placed in a high-temperature and high-humidity environment. When the coated steel sheet is immediately subjected to chemical conversion treatment and coating, there is relatively little problem of blackening, but actually, the plated steel sheet is packed in a coil state and left for a certain period of time before chemical conversion treatment and painting. In many cases, blackening occurs during that period. When blackening occurs, a chemical conversion treatment failure occurs, and as a result, the adhesion, workability, corrosion resistance, etc. of the coated film after coating are lowered, and the commercial value is significantly impaired.
このため、GFの黒変改善に対して種々の提案がなされている。例えば、特許文献1には、質量%で、Al:0.5〜20%、Mg:2%超〜10%、残部Znおよび不可避的不純物とからなるめっき層を有し、めっき表面のZn-Al-Mg共晶+Zn単相の表面長さ率を50%以上とした耐食性、耐黒変性が良好で、化成処理性に優れた溶融Zn-Al系合金めっき鋼板が開示されている。特許文献2には、表面に、溶融Zn系めっき層、次いでクロメート層を有する鋼板であって、前記溶融Zn系めっき層は、めっき層平均で2〜15質量%のAlを含有し、かつ該めっき層の最表面部には、濃化されたNiおよび／またはTiが存在する、耐黒変性および耐食性に優れたクロメート処理Zn系めっき鋼板が開示されている。特許文献3には、鋼板に対して、Al:4.0〜7.0質量%、Pb:0.01質量%以下、Sn:0.005質量%以下を含有し、残部Znおよび不可避不純物からなる溶融めっき浴によりめっきを行った後、伸び率0.3〜2.0%のスキンパスを行い、その後、クロメート処理を行う耐黒変性に優れた溶融Zn-Al合金めっき鋼板の製造方法が開示されている。特許文献4には、鋼板の少なくとも一方の表面に、Al:1.0〜10質量%、Mg:0.2〜1.0質量%、Ni:0.005〜0.1質量%を含有し、残部がZnおよび不可避的不純物からなる溶融Zn-Al系合金めっき層を有する溶融Zn-Al系合金めっき鋼板が開示されている。また、めっき層表面の黒変改善に関するものではないが、特許文献5には、質量%で、Al:0.1〜40%、Mg:0.1〜10%含有し、残部がZnおよび不可避不純物からなり、かつ、Zn-Al-Mg系の母相に、長径が1μm以上のMg系金属間化合物相が、含有率:0.1〜50容積%にて分散している組織を有するめっき層を表面に有する加工性に優れる溶融めっき鋼材が開示されている。 For this reason, various proposals have been made for improving the blackening of GF. For example, Patent Document 1 has a plating layer consisting of Al: 0.5 to 20%, Mg: more than 2% to 10%, the balance Zn and unavoidable impurities in mass%, and Zn-Al- A hot-dip Zn-Al alloy-plated steel sheet having excellent corrosion resistance and blackening resistance and excellent chemical conversion treatment with a surface length ratio of Mg eutectic + Zn single phase of 50% or more is disclosed. Patent Document 2 discloses a steel sheet having a molten Zn-based plating layer and then a chromate layer on the surface, and the molten Zn-based plating layer contains 2 to 15% by mass of Al on an average plating layer, and There is disclosed a chromate-treated Zn-plated steel sheet excellent in blackening resistance and corrosion resistance, in which concentrated Ni and / or Ti are present on the outermost surface portion of the plating layer. In Patent Document 3, Al: 4.0 to 7.0% by mass, Pb: 0.01% by mass or less, Sn: 0.005% by mass or less is plated on a steel sheet by a hot dipping bath composed of the balance Zn and inevitable impurities. After that, a method for producing a hot-dip Zn—Al alloy-plated steel sheet excellent in blackening resistance is disclosed, in which a skin pass having an elongation of 0.3 to 2.0% is performed, followed by chromate treatment. In Patent Document 4, Al: 1.0 to 10% by mass, Mg: 0.2 to 1.0% by mass, Ni: 0.005 to 0.1% by mass is contained on at least one surface of a steel plate, and the balance is composed of Zn and inevitable impurities. A hot-dip Zn-Al alloy-plated steel sheet having a hot-dip Zn-Al alloy plating layer is disclosed. Although not related to the blackening improvement of the plating layer surface, Patent Document 5 contains, by mass%, Al: 0.1 to 40%, Mg: 0.1 to 10%, and the balance is composed of Zn and inevitable impurities. In addition, a Zn-Al-Mg matrix with a plating layer on the surface having a structure in which a Mg-based intermetallic compound phase having a major axis of 1 μm or more is dispersed at a content rate of 0.1 to 50% by volume. A hot-dip galvanized steel material having excellent properties is disclosed.
しかしながら、こうした特許文献に記載のGFには、以下のような問題がある。 However, the GF described in these patent documents has the following problems.
特許文献1：色調の低下やドロスの付着に起因する外観不良が生じやすく、また、めっき層に亀裂が生じやすいため加工性の劣化を招く。さらに、Mg量が多くなると耐黒変性が劣化する。 Patent Document 1: Appearance defects due to a decrease in color tone and dross adhesion are likely to occur, and cracks are likely to occur in the plating layer, resulting in deterioration of workability. Furthermore, the blackening resistance deteriorates when the amount of Mg increases.
特許文献2、3：通常のクロメート処理液を用いて化成処理を行うと、耐黒変性の改善効果が十分でなく、特殊なクロメート処理液が必要である。また、スパングルに起因する外観不良が生じやすい。 Patent Documents 2 and 3: When a chemical conversion treatment is performed using a normal chromate treatment solution, the effect of improving blackening resistance is not sufficient, and a special chromate treatment solution is required. In addition, poor appearance due to spangles tends to occur.
特許文献4：クロメートフリーの化成処理を行うと耐黒変性が劣化する。 Patent Document 4: Blackening resistance deteriorates when a chromate-free chemical conversion treatment is performed.
特許文献5：スパングル、色調の低下、ドロスの付着に起因する外観不良が生じるとともに、耐黒変性が劣化する。 Patent Document 5: Appearance defects due to spangles, color tone deterioration, and dross adhesion occur, and blackening resistance deteriorates.
本発明は、外観不良や加工性の劣化を招くことなく、クロメートフリーの化成処理を行っても優れた耐黒変性を有するGFを提供することを目的とする。 An object of the present invention is to provide a GF having excellent blackening resistance even when a chromate-free chemical conversion treatment is performed without causing poor appearance or deterioration of workability.
本発明者らは、上記課題を解決するためにGFのめっき層について鋭意検討したところ、めっき層に0.2〜1質量%のMgを含有させ、かつめっき層表面に占めるZn-Al共晶の比率を低下させることが効果的であることを見出した。 In order to solve the above-mentioned problems, the present inventors diligently studied about the GF plating layer, and contained 0.2 to 1% by mass of Mg in the plating layer, and the ratio of the Zn—Al eutectic in the plating layer surface. Has been found to be effective.
本発明は、このような知見に基づきなされたもので、鋼板の少なくとも一方の表面に、Al:1.0〜10質量%およびMg:0.2〜1質量%を含有し、残部がZnおよび不可避的不純物からなる溶融Zn-Al系合金めっき層を有し、前記溶融Zn-Al系合金めっき層表面に占めるZn-Al共晶の面積率が10%以下であることを特徴とする溶融Zn-Al系合金めっき鋼板(GF)を提供する。 The present invention has been made on the basis of such findings, and on at least one surface of the steel sheet, Al: 1.0 to 10% by mass and Mg: 0.2 to 1% by mass, with the balance being Zn and inevitable impurities. A molten Zn-Al-based alloy, characterized in that the area ratio of Zn-Al eutectic in the surface of the molten Zn-Al-based alloy plated layer is 10% or less Provides plated steel sheet (GF).
本発明のGFでは、溶融Zn-Al系合金めっき層に、さらにNi:0.005〜0.1質量%が含有されることが好ましい。 In the GF of the present invention, it is preferable that Ni: 0.005 to 0.1% by mass is further contained in the molten Zn—Al alloy plating layer.
本発明により、外観不良や加工性の劣化を招くことなく、クロメートフリーの化成処理を行っても優れた耐黒変性を有するGFを製造できるようになった。 According to the present invention, it has become possible to produce a GF having excellent blackening resistance even when a chromate-free chemical conversion treatment is performed without causing poor appearance or deterioration of workability.
以下に、本発明の詳細を説明する。 Details of the present invention will be described below.
1) めっき層の組成について
溶融Zn-Al系合金めっき層のAl含有量が1.0質量%未満では、めっき層と下地鋼板との界面に厚いFe-Zn系合金層が形成されるため、加工性が劣化する。一方、Al含有量が10質量%を超えると、Alリッチ層の増加により犠牲防食作用が低下するため、端面部の耐食性が劣化する。また、Al含有量が10質量%を超えるめっき層を形成しようとすると、めっき浴中にAlを主体としたトップドロスが発生しやすくなるため、ドロスの付着に起因する外観不良を招きやすい。したがって、溶融Zn-Al系合金めっき層のAl含有量は1.0〜10質量%、好ましくは3〜7質量%とする。
1) Plating layer composition When the Al content of the molten Zn-Al alloy plating layer is less than 1.0% by mass, a thick Fe-Zn alloy layer is formed at the interface between the plating layer and the underlying steel plate. Deteriorates. On the other hand, when the Al content exceeds 10% by mass, the sacrificial anticorrosive action is reduced due to the increase in the Al-rich layer, so that the corrosion resistance of the end face portion is deteriorated. Further, when a plating layer having an Al content exceeding 10% by mass is formed, a top dross mainly composed of Al is likely to be generated in the plating bath, and thus appearance defects due to the adhesion of dross are likely to be caused. Therefore, the Al content of the molten Zn—Al-based alloy plating layer is 1.0 to 10% by mass, preferably 3 to 7% by mass.
溶融Zn-Al系合金めっき層のMg含有量が0.2質量%未満では、スパングルや色調の低下に起因する外観不良を招きやすくなるとともに、耐黒変性が劣化する。一方、Mg含有量が1質量%を超えると、色調の低下やドロスの付着に起因する外観不良を招くとともに、加工性や耐黒変性が劣化する。したがって、溶融Zn-Al系合金めっき層のMg含有量は0.2〜1質量%とする。 If the Mg content of the molten Zn—Al-based alloy plating layer is less than 0.2% by mass, poor appearance due to spangles and a decrease in color tone tends to occur, and blackening resistance deteriorates. On the other hand, if the Mg content exceeds 1% by mass, appearance deterioration due to a decrease in color tone or adhesion of dross is caused, and workability and blackening resistance are deteriorated. Therefore, the Mg content of the molten Zn—Al-based alloy plating layer is set to 0.2 to 1 mass%.
溶融Zn-Al系合金めっき層の残部はZnおよび不可避的不純物であるが、以下の理由により、Ni:0.005〜0.1質量%を含有させることが好ましい。すなわち、Ni含有量を0.005質量%以上にすることにより、より優れた耐黒変性が得られるが、0.1質量%を超えると、めっき浴中にAl-Mg系のドロスが発生し、ドロスの付着に起因する外観不良を招く。 The balance of the molten Zn—Al-based alloy plating layer is Zn and inevitable impurities, but it is preferable to contain Ni: 0.005 to 0.1 mass% for the following reasons. In other words, when the Ni content is 0.005% by mass or more, better blackening resistance can be obtained, but if it exceeds 0.1% by mass, Al-Mg-based dross is generated in the plating bath, and the dross adheres. Inferior appearance due to.
2) めっき層の組織について
上述したように、一般的に、Zn系めっき鋼板の黒変は、酸素欠乏型酸化亜鉛の生成と関係するといわれている。本発明者らが調査した結果、溶融Zn-Al系合金めっき鋼板においても、黒変の程度はX線回折により測定されるZnOのピーク強度と良い相関があり、黒変にともない、めっき層表面に露出するZn-Al共晶が優先的に腐食されることが明らかになった。以上のことから、溶融Zn-Al系合金めっき鋼板では、Zn-Al共晶が優先腐食することでZnOの生成量が増加し、黒変の程度を悪化させていると考えられるので、めっき層表面に露出するZn-Al共晶の面積率を低下させたところ、黒変の改善が認められた。さらに、耐黒変性の観点からは、めっき層表面に占めるZn-Al共晶の面積率を10%以下にすることが効果的であることを見出した。
2) Regarding the structure of the plating layer As described above, it is generally said that the blackening of the Zn-based plated steel sheet is related to the generation of oxygen-deficient zinc oxide. As a result of the investigation by the present inventors, the degree of blackening also has a good correlation with the peak intensity of ZnO measured by X-ray diffraction in the hot-dip Zn-Al alloy-plated steel sheet. It was revealed that the Zn-Al eutectic exposed to the preferentially corrodes. From the above, in the hot-dip Zn-Al alloy-plated steel sheet, the Zn-Al eutectic is preferentially corroded, so the amount of ZnO produced is thought to increase and the degree of blackening is worsened. When the area ratio of the Zn—Al eutectic exposed on the surface was lowered, improvement in blackening was observed. Furthermore, from the viewpoint of blackening resistance, it has been found that it is effective to reduce the area ratio of Zn—Al eutectic to the plating layer surface to 10% or less.
ここで、本発明のZn-Al-Mg系めっきを例に、Zn-Al共晶の面積率の算出方法を説明する。すなわち、走査電子顕微鏡を用いて、加速電圧10kVにおいて、反射電子像を2000倍以上の倍率で撮影する。図1(a)に、このようにして撮影した、Zn-Al-Mgめっきの典型的なめっき表面の反射電子像を示す。図1(b)に示すように、画像処理ソフトを用いてZn-Al共晶を黒く塗りつぶし、これを二値化することでZn-Al共晶の面積率を算出する。1枚の画像での評価領域を50μm×30μm以上の視野とし、計10枚の画像についての平均値をZn-Al共晶の面積率とする。 Here, a method for calculating the area ratio of the Zn—Al eutectic will be described using the Zn—Al—Mg based plating of the present invention as an example. That is, using a scanning electron microscope, a reflected electron image is taken at a magnification of 2000 times or more at an acceleration voltage of 10 kV. FIG. 1 (a) shows a backscattered electron image of a typical plated surface of Zn—Al—Mg plating taken in this manner. As shown in FIG. 1 (b), the Zn—Al eutectic is blacked out using image processing software, and the area ratio of the Zn—Al eutectic is calculated by binarizing this. The evaluation area of one image is a field of view of 50 μm × 30 μm or more, and the average value for a total of 10 images is the area ratio of the Zn—Al eutectic.
本発明であるGFは、従来と同様、所望の含有量のAl、MgあるいはさらにNiを含むZnめっき浴に浸漬後、めっき浴から引き上げ、(1) ミストやZn微粉を吹き付けてめっき表面にZn初晶の核生成サイトを付与したり、(2) 溶融めっき処理後に電気Znめっき処理を施して、めっき層表面にあるZn-Al共晶を電気Znめっきで被覆することによって製造できる。 The GF according to the present invention is immersed in a Zn plating bath containing a desired content of Al, Mg or Ni, and then lifted from the plating bath, and (1) sprayed mist or fine Zn powder on the plating surface. It can be produced by providing an nucleation site of primary crystals or (2) applying an electro-Zn plating process after the hot dipping process and coating the Zn—Al eutectic on the surface of the plating layer with the electro-Zn plating.
このとき、めっき浴の流動性を増して、微細なピンホールの発生を防止したり、めっき層表面の平滑化を促進する上で、めっき浴にはCeおよび／またはLaのミッシュメタルを合計で0.005〜0.05質量%含有させることが好ましい。また、めっき浴の流動性を増すとともに、ドロスの発生を防止する上で、めっき浴の温度は300〜500℃にすることが好ましい。 At this time, in order to increase the fluidity of the plating bath to prevent the generation of fine pinholes and to promote the smoothing of the plating layer surface, the plating bath contains a total of Ce and / or La misch metals. It is preferable to contain 0.005-0.05 mass%. Further, in order to increase the fluidity of the plating bath and prevent the occurrence of dross, the temperature of the plating bath is preferably 300 to 500 ° C.
連続式溶融Zn-Al系合金めっき設備を用いて、板厚0.5mm、板幅1500mmの焼鈍前の低炭素Alキルド冷延鋼板にZn-5質量%Al、Zn-5質量%Al-0.4質量%Mg、Zn-5質量%Al-0.4質量%Mg-0.1質量%Niの組成を有する溶融Zn-Al系合金めっき層を形成して、溶融Zn-Al系合金めっき鋼板GF1、GF2、GF3を製造した。このとき、めっき浴の温度は480℃とし、めっき層の厚みはワイピングにより12〜16μmに調整した。次に、シャドウマスク材を鋼板表面に設置し、その上からZnを真空蒸着してめっき層表面をZnで被覆した。このとき、シャドウマスク材の開口率や鋼板表面とシャドウマスク間の距離を変えて、めっき層表面に占めるZn-Al共晶の面積率を変化させた。最後に、特許文献6の発明例59に記載の処理液、および実施例に記載された発明例59の処理方法を用いて、クロメートフリー鋼板の試料No.1〜10を作製し、以下の方法で耐黒変性を評価した。
耐黒変性：温度80℃、相対湿度95%の雰囲気中に24時間放置して黒変化し、JIS-Z-8722に準拠して黒変化前後の明度(L値)を測定し、ΔL(=黒変化前のL値-黒変化後のL値)を求め、以下のように評価した。
○：-6≦ΔL
△：-10≦ΔL<-6
×：ΔL<-10
結果を表1に示す。本発明例である溶融Zn-Al系合金めっき層にMgを含み、めっき層表面に占めるZn-Al共晶の面積率が10%以下である試料は耐黒変性に優れていることがわかる。なお、本発明例の試料は、良好な外観を示し、加工性にも問題がなかった。
Using a continuous hot-dip Zn-Al alloy plating facility, Zn-5 mass% Al, Zn-5 mass% Al-0.4 mass on a low-carbon Al-killed cold-rolled steel sheet with a thickness of 0.5 mm and a width of 1500 mm before annealing. A molten Zn-Al alloy plating layer having a composition of% Mg, Zn-5 mass% Al-0.4 mass% Mg-0.1 mass% Ni is formed, and the molten Zn-Al alloy plated steel sheets GF1, GF2, and GF3 are formed. Manufactured. At this time, the temperature of the plating bath was 480 ° C., and the thickness of the plating layer was adjusted to 12 to 16 μm by wiping. Next, a shadow mask material was placed on the surface of the steel plate, and Zn was vacuum-deposited thereon to coat the surface of the plating layer with Zn. At this time, the area ratio of the Zn—Al eutectic in the plating layer surface was changed by changing the aperture ratio of the shadow mask material and the distance between the steel plate surface and the shadow mask. Finally, using the treatment liquid described in Invention Example 59 of Patent Document 6 and the treatment method of Invention Example 59 described in the Examples, sample Nos. 1 to 10 for chromate-free steel plates were prepared, and the following method was used. Was evaluated for resistance to blackening.
Blackening resistance: Stands in an atmosphere of 80 ° C and 95% relative humidity for 24 hours to change black, measure the brightness (L value) before and after black change according to JIS-Z-8722, and ΔL (= L value before black change-L value after black change) was determined and evaluated as follows.
○: -6 ≤ ΔL
Δ: -10 ≦ ΔL <-6
×: ΔL <-10
The results are shown in Table 1. It can be seen that the sample containing Mg in the molten Zn—Al alloy plating layer as an example of the present invention and having an area ratio of Zn—Al eutectic in the plating layer surface of 10% or less is excellent in blackening resistance. In addition, the sample of the example of the present invention showed a good appearance and had no problem in workability.
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