JP3729233B2 - Hot-dip galvanized steel sheet with blackening resistance - Google Patents
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【０００１】
【発明の属する技術分野】
本発明は黒変抵抗をもつ溶融亜鉛基めっき鋼板に関する。
【０００２】
【従来の技術】
溶融亜鉛めっき鋼板は黒変することがある。特に，Ａｌを含有したＺｎ−Ａｌ系合金めっき鋼板では黒変する傾向が強く，クロメート処理するとその傾向は一層顕著になることがある。
【０００３】
ここで“黒変”とは，該溶融亜鉛めっき鋼板を主にコイル状で保管したときや，該鋼板が高温多湿な環境にさらされたときに，めっき表面が灰黒色に変色する現象を言う。この黒変現象が発生しても，めっき本来の目的である耐食性には直接悪い影響を与えないが，見栄えが悪くなって表面外観が損なわれる。
【０００４】
このような黒変現象は，亜鉛めっき浴にＡｌを添加したＺｎ−Ａｌ系合金めっき鋼板で起きやすいが，これは，例えば「鉄と鋼」第72年(1986)，第８号，P.1013〜1020によれば，めっき表層部に生成した亜鉛酸化物（ＺｎＯ）からその酸素の一部がＡｌによって奪われ, ＺｎＯ1-X 系の酸化物が最表層部に形成するからであるとされている。また，クロメート処理した場合には，活性化されためっき表面に形成されたクロメート皮膜によって，該めっき表面は酸素の供給が抑制された状態となり，いっそうＺｎＯが酸素欠乏型のＺｎＯ1-X に変化しやすく，すなわち，黒変しやすくなる。
【０００５】
その発生原因や挙動がどのようであるにしても，Ａｌを含有するＺｎ−Ａｌ系溶融めっき鋼板において，さらには，これにクロメート処理した鋼板において，黒変現象が起き易いことは事実であり，その根本的な解決策は見い出されていないというのが実情である。
【０００６】
【発明が解決しようとする課題】
したがって，本発明の課題は，Ａｌを含有するＺｎ−Ａｌ系溶融めっき鋼板，さらにはこれにクロメート処理した鋼板において，黒変現象の発生を防止することにある。
【０００７】
【課題を解決するための手段】
前記の課題は，Ａｌ含有溶融亜鉛めっき浴に１.０重量％以上のＭｇを添加しためっき浴組成とすることによって解決できることがわかった。より具体的には，４.０重量％以上のＡｌを含有する溶融亜鉛めっき浴に１.０重量％以上のＭｇを添加し，このＡｌ・Ｍｇ含有溶融亜鉛基めっき浴に鋼板を浸漬してＡｌ・Ｍｇ含有溶融亜鉛基めっき鋼板を製造し，このめっき鋼板にクロメート処理を施した場合には，前述のような黒変の発生が抑制されることがわかった。
【０００８】
すなわち本発明によれば，４.０重量％以上のＡｌおよび１.０重量％以上のＭｇを含有する溶融亜鉛基めっきを施した溶融亜鉛基めっき鋼板の該めっき層表面にクロメート処理を施してなる黒変抵抗をもつ溶融亜鉛基めっき鋼板を提供する。めっき層の好ましいＡｌ含有量は４.０〜１０重量％，めっき層の好ましいＭｇ含有量は１.０〜４.０重量％である。
【０００９】
また，該溶融亜鉛基めっき層が，めっき層表面における〔Ａｌ／Ｚｎ／Ｚｎ2Ｍｇの三元共晶組織〕の面積率が７０％以上である金属組織を有する場合には，これにクロメート処理を施しても，黒変発生を顕著に抑制できる。当該めっき浴中に適量のＴｉとＢを含有させておくと，より具体的には０.００２〜０.１重量％のＴｉおよび０.００１〜０.０４５重量％のＢを含有させておくと，前記のようなＺｎ2Ｍｇ系の三元共晶組織を得やすくなる。
【００１０】
【発明の実施の形態】
Ａｌを含有するＺｎ−Ａｌ系合金めっき鋼板ではＡｌの含有量の多少に拘わらず黒変が発生しやすく，この場合，めっき層にさらにＭｇを含有すると，Ｍｇ量が０.５重量％程度では，Ｍｇを含有しない場合よりも黒変が発生しやすくなる。例えば前掲の「鉄と鋼」第72年(1986)，第８号，P.1013〜1020には，０.１８〜１３％Ａｌ−Ｚｎ合金めっき鋼板において，０.１％および０.５％のＭｇを添加した場合の黒変発生状況が報告されており，めっき層へのＭｇの添加は無添加の場合に比べて，Ａｌの含有量に拘わらず，黒変を発生しやすくなることが示されている。
【００１１】
また，このようなめっき鋼板にクロメート処理を施すと，一般に，黒変が促進される。例えば前掲文献においても，めっき層中のＡｌおよびＭｇの含有率の増加と共に，これにクロメート処理した場合には黒変化が促進することが実験的に示されている。ただし，この文献でのめっき層中のＭｇ含有量は最大で０.５重量％である。
【００１２】
ところが，４.０重量％以上のＡｌと１.０重量％以上のＭｇを含有する溶融亜鉛基めっき鋼板の場合には，クロメート処理しても，黒変が発生し難くなることを本発明者らは見い出した。とくに，４.０重量％以上のＡｌと１.０重量％以上のＭｇを含有させ且つ〔Ａｌ／Ｚｎ／Ｚｎ2Ｍｇの三元共晶組織〕を晶出させた金属組織としためっき層の場合には，この三元共晶組織の量が多いほど，黒変が発生し難くなることがわかった。本発明のめっき鋼板では，Ｍｇはむしろ黒変を抑制する作用を果たすのである。
【００１３】
その理由は必ずしも明確ではないが，本発明のめっき鋼板ではめっき最表層部に薄く且つ安定なＭｇの酸化皮膜が均一に形成するので，この状態でクロメート処理を施すと，このＭｇ酸化皮膜が亜鉛の酸化そのものを抑制し，結果として酸素欠乏型のＺｎＯ1-Xの生成を抑制するからであろうと考えられる。そのさい，前記の三元共晶組織が充分に晶出していると，Ｍｇの分布が緻密且つ均一となり，一層，均一で且つ安定なＭｇ酸化皮膜の生成を促すものと考えられる。この三元共晶組織はめっき層中のＭｇが１重量％以上になると充分な量晶出する。その際Ａｌ量が４重量％未満であると，すなわち初晶にＺｎが晶出すると，その部分でＺｎＯ1-Xが生成し，充分な黒変抵抗が得られない。
【００１４】
以下に本発明の溶融亜鉛基めっき鋼板の好ましい形態を述べる。
【００１５】
本発明の溶融亜鉛基めっき鋼板は，Ｚｎ中に４.０重量％以上のＡｌと１.０重量％以上のＭｇを含有させた溶融めっき浴に鋼板を浸漬することによって製造されるものであるが，このようなＡｌとＭｇを含有するＡｌ−Ｍｇ系の溶融亜鉛基めっき鋼板は良好な耐食性を示す。
【００１６】
すなわち，めっき層中のＭｇはめっき層表面に均一な腐食生成物を生成させ，これが当該めっき鋼板の耐食性を著しく高める作用を供する。Ｍｇ含有量が１.０重量％未満ではかような腐食生成物を均一に生成させる作用が十分ではなく，他方，Ｍｇ含有量が４.０重量％を越えてもＭｇによる耐食性向上効果は飽和し，かえってめっき浴中にＭｇ酸化物系のドロスが発生しやすくなるので，Ｍｇ含有量は１.０〜４.０重量％とするのがよい。好ましいＭｇ含有量は１.５〜４.０重量％，さらに好ましいＭｇ含有量は２.０〜３.５重量％，一層好ましいＭｇ含有量は２.５〜３.５重量％である。
【００１７】
また，めっき層中のＡｌは，当該めっき鋼板の耐食性を向上させる作用を供するとともにめっき浴中のＡｌはめっき浴表面にＭｇ酸化物系のドロスが発生するのを抑制する作用を供する。Ａｌ含有量が４.０重量％未満では該鋼板の耐食性向上効果が十分ではなく，Ｍｇ酸化物系のドロス発生を抑制する効果も低い。他方，Ａｌ含有量が１０重量％を越えると，めっき層と母材鋼板との界面でＦｅ−Ａｌ合金層の成長が著しくなり，めっき密着性が悪くなる。好ましいＡｌ含有量は４.０〜９.０重量％，更に好ましいＡｌ含有量は５.０〜８.５重量％，一層好ましいＡｌ含有量は５.０〜７.０重量％である。
【００１８】
このようなＡｌ量とＭｇ量をＺｎ中に含むＺｎ−Ａｌ−Ｍｇの三元組成において，めっき層表面における〔Ａｌ／Ｚｎ／Ｚｎ2Ｍｇの三元共晶組織〕の面積率が大きいほどクロメート処理後の黒変を抑制する作用が増す。
【００１９】
ここで〔Ａｌ／Ｚｎ／Ｚｎ2Ｍｇの三元共晶組織〕は，〔Ａｌ／Ｚｎ／Ｚｎ11Ｍｇ2の三元共晶組織〕とは区別される。Ｚｎ−Ａｌ−Ｍｇの三元平衡状態図上では，Ａｌが約４重量％付近でＭｇが約３重量％近傍において，融点が最も低くなる三元共晶点（融点＝３４３℃）が見られる。したがって，この三元共晶点近傍の浴組成を採用した場合に，めっき層の金属組織中に，後者の〔Ａｌ／Ｚｎ／Ｚｎ11Ｍｇ2の三元共晶組織の素地自体或いはこの素地中に〔初晶Ａｌ相〕または〔初晶Ａｌ相〕と〔Ｚｎ単相〕が混在したもの或いは〔初晶Ａｌ相〕と〔Ａｌ／Ｚｎ11Ｍｇ2共晶〕が混在したもの（Ｚｎ11Ｍｇ2系の相という）が局部的に晶出する現象が起きやすいが，この局部的に晶出したＺｎ11Ｍｇ2系の相は，他の相（Ｚｎ2Ｍｇ系の相）よりも変色しやすく，放置しておくと，この部分が非常に目立った色調となり，表面外観を著しく悪くするので好ましくない。したがってＺｎ11Ｍｇ2系の相が局部的に晶出するのを防止し，Ｚｎ2Ｍｇ系の相を多くすることによって表面外観に優れた且つ耐食性にも優れたものとすることができるが，このような組織制御は，めっき浴から引き上げられるめっき鋼板の冷却速度を調整することによって行ない得る。
【００２０】
例えば，該めっき浴の浴温を融点以上４７０℃未満とし且つ溶融めっき層の凝固完了までの冷却速度を１０℃／秒以上に制御するか，或いは該めっき浴の浴温を４７０℃以上とし且つ溶融めっき層の凝固完了までの冷却速度を０.５℃／秒以上に制御すれば，Ｚｎ11Ｍｇ2系の相の局部的な晶出を防止し，Ｚｎ2Ｍｇ系の相を多くすることができる。
【００２１】
この組織制御はめっき浴に適量のＴｉとＢを添加しておくと一層容易となる。例えば０.００２〜０.１重量％のＴｉおよび０.００１〜０.０４５重量％のＢを含有させておくと，該めっき浴の浴温を融点以上４１０℃未満とし且つめっき後の冷却速度を７℃／秒以上に制御するか，或いはめっき浴の浴温を４１０℃以上とし且つめっき後の冷却速度を０.５℃／秒以上に制御すればＺｎ11Ｍｇ2系の相の局部的な晶出を防止しながらＺｎ2Ｍｇ系の三元共晶組織を得ることができる。
【００２２】
本発明者らは，このようなＺｎ2Ｍｇ系の相，すなわち〔Ａｌ／Ｚｎ／Ｚｎ2Ｍｇの三元共晶組織〕の素地中に〔初晶Ａｌ相〕，または〔初晶Ａｌ相〕と〔Ｚｎ単相〕，或いは〔初晶Ａｌ相〕と〔Ａｌ／Ｚｎ2Ｍｇ共晶組織〕が混在した金属組織を有するめっき鋼板にクロメート処理を施した場合，めっき層表面における〔Ａｌ／Ｚｎ／Ｚｎ2Ｍｇの三元共晶組織〕の含有割合と黒変との間に所定の関係があることを見い出した。
【００２３】
すなわち，〔Ａｌ／Ｚｎ／Ｚｎ2Ｍｇの三元共晶組織〕の晶出量が多いほど，クロメート処理後の黒変発生頻度が低くなり，具体的には該三元共晶組織がめっき層の表面の７０％以上であれば黒変発生をほぼ抑制できることがわかった。
【００２４】
一般に，亜鉛めっき鋼板に施すクロメート処理には，大きく分けて反応型のものと塗布型のものがある。反応型のものはエッチング機能を有するもので，液中にＦ-, Ｃｌ-, ＳＯ4 2+, ＮＯ3 -, ＰＯ3 -, ＣＨ3ＣＯＯ-, しゅう酸イオンなどを含むのが通常であるのに対し，塗布型のものでは，このようなイオンを通常は含有しない。本発明で適用するクロメート処理としては反応型クロメート処理を採用することもできるが，塗布型のクロメート処理を採用した場合には一層顕著に黒変抵抗を有するものとなる。
【００２５】
【実施例】
〔実施例１〕
ゼンジマータイプの連続溶融めっきラインを想定した条件で，中炭素鋼の熱延鋼板（厚み：２.３ｍｍ）を，最高到達板温が６００℃，炉内雰囲気の露点が−４０℃のもとで熱処理したあと，各種組成と浴温（表１に示す）の溶融亜鉛基めっき浴に３秒間浸漬し，該浴から引き上げたあとの冷却速度（浴温からめっき層凝固温度までの平均値）を表１のように種々変化させて溶融めっきを施した。
【００２６】
得られためっき鋼板から電顕用サンプルを採取し，電子顕微鏡像でめっき層表面の金属組織を調査し，〔Ａｌ／Ｚｎ／Ｚｎ2Ｍｇの三元共晶組織〕の占める割合（面積率％）を評価した。その結果を表１に示した。
【００２７】
ついで，各めっき鋼板にクロメート処理を施した。処理液は日本パーカライジング株式会社製の商品名ジンクロム３３８７Ｎのもの（塗布型クロメート液）を用いた。処理条件は次のとおりである。
【００２８】
〔クロメート処理条件〕
塗布方法：ロールコーター方式
液中トータルクロム濃度：１３ｇ／Ｌ
クロム付着量：４０ｍｇ／ｍ2
【００２９】
得られたクロメート処理鋼板を，湿度ＲＨ６０％，温度５０℃の炉内に２５日間保持し，黒変の発生状況を次のようにして調べた。すなわち，試験後の黒変の程度を分光光度計を用いてＬａｂ法のＬ値で測定し，試験前のＬ値との差ΔＬを指標とした。ΔＬ値が５以下では目視で変色が殆んど分からないことからΔＬ≦５を合格とする。その結果を表１に示した。
【００３０】
【表１】
〔実施例２〕
クロメート処理を次の処理に代えた以外は，実施例１を繰り返した。
〔適用したクロメート処理〕
処理液：ジンクロム３３８７Ｎにフッ化ジルコニウムを１ｇ／Ｌ添加した液
処理方法：実施例１に同じ
クロム付着量：４０ｍｇ／ｍ2
【００３２】
得られた各クロメート処理鋼板について実施例１と同様の黒変評価を行ない，表２に示す結果を得た。
【００３３】
【表２】
〔実施例３〕
クロメート処理を次の処理に代えた以外は，実施例１を繰り返した。
〔適用したクロメート処理〕
処理液：ＣｒＯ3 水溶液
処理条件：スプレーリンガーロール方式
液中トータルＣｒ濃度：１８ｇ／Ｌ
クロム付着量：１８ｍｇ／ｍ2
【００３５】
得られた各クロメート処理鋼板について実施例１と同様の黒変評価を行ない，表３に示す結果を得た。
【００３６】
【表３】
表１〜３の結果に見られるように，４.０重量％以上のＡｌと１.０重量％以上のＭｇを含有する溶融亜鉛基めっき鋼板にクロメート処理を施したものは優れた黒変抵抗を有することがわかる。また，めっき層表面における〔Ａｌ／Ｚｎ／Ｚｎ2Ｍｇの三元共晶組織〕の面積率が７０％以上において優れた黒変抵抗を有することがわかる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a hot dip galvanized steel sheet having blackening resistance.
[0002]
[Prior art]
Hot dip galvanized steel sheets may turn black. In particular, a Zn-Al alloy-plated steel sheet containing Al has a strong tendency to turn black, and this tendency may become more pronounced when chromate treatment is performed.
[0003]
Here, “black discoloration” refers to a phenomenon in which the plated surface turns grayish gray when the hot dip galvanized steel sheet is stored mainly in a coil shape or when the steel sheet is exposed to a high temperature and humidity environment. . Even if this blackening phenomenon occurs, the corrosion resistance, which is the original purpose of plating, is not directly adversely affected, but the appearance is deteriorated and the surface appearance is impaired.
[0004]
Such a blackening phenomenon is likely to occur in a Zn-Al alloy-plated steel sheet in which Al is added to a galvanizing bath. For example, “Iron and Steel” 72nd year (1986), No. 8, p. According to 1013 to 1020, part of the oxygen is removed from the zinc oxide (ZnO) formed on the plating surface layer part by Al, and the ZnO 1-X oxide is formed on the outermost layer part. Has been. In addition, when chromate treatment is performed, the chromate film formed on the activated plating surface causes the supply of oxygen to the plating surface to be suppressed, and ZnO changes to oxygen-deficient ZnO 1-X . It is easy to do, that is, it becomes easy to turn black.
[0005]
Whatever the cause and behavior, blackening is likely to occur in Zn-Al hot-dip galvanized steel sheets containing Al, and even in chromated steel sheets. The fact is that no fundamental solution has been found.
[0006]
[Problems to be solved by the invention]
Accordingly, an object of the present invention is to prevent the occurrence of the blackening phenomenon in a Zn-Al hot dip plated steel sheet containing Al and further a chromate-treated steel sheet.
[0007]
[Means for Solving the Problems]
It has been found that the above problem can be solved by using a plating bath composition in which 1.0 wt% or more of Mg is added to an Al-containing hot dip galvanizing bath. More specifically, 1.0 wt% or more of Mg is added to a hot dip galvanizing bath containing 4.0 wt% or more of Al, and the steel sheet is immersed in this Al / Mg containing hot dip galvanizing bath. It was found that when an Al / Mg-containing hot dip galvanized steel sheet was produced and the plated steel sheet was subjected to chromate treatment, the occurrence of blackening as described above was suppressed.
[0008]
That is, according to the present invention, the surface of the plated layer of the hot dip galvanized steel sheet subjected to hot dip galvanized steel containing 4.0% by weight or more of Al and 1.0% by weight or more of Mg is subjected to chromate treatment. A hot-dip galvanized steel sheet having blackening resistance is provided. Preferred Al content of the plating layer is 4.0 to 10 wt%, preferably Mg content of the plating layer is 1.0 to 4.0 wt%.
[0009]
In addition, when the hot dip zinc-based plating layer has a metal structure having an area ratio of [Al / Zn / Zn 2 Mg ternary eutectic structure] of 70% or more on the surface of the plating layer, it is chromated. Even if applied, blackening can be remarkably suppressed. When an appropriate amount of Ti and B is contained in the plating bath, more specifically, 0.002 to 0.1% by weight of Ti and 0.001 to 0.045% by weight of B are contained. Thus, it becomes easier to obtain the Zn 2 Mg-based ternary eutectic structure as described above.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
In Zn-Al alloy-plated steel sheet containing Al, blackening is likely to occur regardless of the Al content. In this case, if Mg is further contained in the plating layer, the Mg content is about 0.5% by weight. , Blackening is more likely to occur than when Mg is not contained. For example, in “Iron and Steel” 72nd year (1986), No. 8, pages 1013 to 1020, 0.1% and 0.5% in 0.18 to 13% Al—Zn alloy plated steel sheet It has been reported that blackening occurs when Mg is added. Compared to the case where Mg is not added to the plating layer, blackening is likely to occur regardless of the Al content. It is shown.
[0011]
In addition, when such a plated steel sheet is chromated, blackening is generally promoted. For example, also in the above-mentioned literature, it has been experimentally shown that when the content of Al and Mg in the plating layer is increased, black change is promoted when the chromate treatment is applied thereto. However, the maximum Mg content in the plating layer in this document is 0.5% by weight.
[0012]
However, in the case of a hot dip galvanized steel sheet containing 4.0% by weight or more of Al and 1.0% by weight or more of Mg, it is difficult to cause blackening even when chromated. Found. In particular, a plated layer having a metal structure containing 4.0 wt% or more Al and 1.0 wt% or more Mg and crystallizing [Al / Zn / Zn 2 Mg ternary eutectic structure]. In some cases, it was found that the larger the amount of this ternary eutectic structure, the less likely blackening occurred. In the plated steel sheet of the present invention, Mg rather acts to suppress blackening.
[0013]
The reason for this is not necessarily clear, but in the plated steel sheet of the present invention, a thin and stable Mg oxide film is uniformly formed on the outermost layer portion of the plating. This is thought to be because the oxidation itself of the material is suppressed, and as a result, the generation of oxygen-deficient ZnO 1-X is suppressed. At that time, if the ternary eutectic structure is sufficiently crystallized, it is considered that the distribution of Mg becomes dense and uniform, and promotes the formation of a more uniform and stable Mg oxide film. The ternary eutectic structure crystallizes in a sufficient amount when Mg in the plating layer is 1 wt% or more . At that time, if the Al amount is less than 4% by weight, that is, if Zn crystallizes in the primary crystal, ZnO 1-X is generated in that portion, and sufficient blackening resistance cannot be obtained.
[0014]
Below, the preferable form of the hot dip galvanized steel plate of this invention is described.
[0015]
The hot dip galvanized steel sheet of the present invention is manufactured by immersing the steel sheet in a hot dipping bath containing 4.0 wt% or more of Al and 1.0 wt% or more of Mg in Zn. However, such an Al—Mg hot-dip galvanized steel sheet containing Al and Mg exhibits good corrosion resistance.
[0016]
That is, Mg in the plating layer generates a uniform corrosion product on the surface of the plating layer, and this serves to remarkably increase the corrosion resistance of the plated steel sheet. If the Mg content is less than 1.0% by weight, the effect of uniformly generating such corrosion products is not sufficient. On the other hand, even if the Mg content exceeds 4.0% by weight, the effect of improving the corrosion resistance by Mg is saturated. However, since Mg oxide-based dross tends to occur in the plating bath, the Mg content is preferably set to 1.0 to 4.0% by weight. A preferable Mg content is 1.5 to 4.0% by weight, a more preferable Mg content is 2.0 to 3.5% by weight, and a more preferable Mg content is 2.5 to 3.5% by weight.
[0017]
Further, Al in the plating layer serves to improve the corrosion resistance of the plated steel sheet, and Al in the plating bath serves to suppress the occurrence of Mg oxide-based dross on the plating bath surface. If the Al content is less than 4.0% by weight, the effect of improving the corrosion resistance of the steel sheet is not sufficient, and the effect of suppressing the generation of Mg oxide-based dross is low. On the other hand, if the Al content exceeds 10% by weight, the growth of the Fe—Al alloy layer becomes remarkable at the interface between the plating layer and the base steel plate, resulting in poor plating adhesion. The preferable Al content is 4.0 to 9.0% by weight, the more preferable Al content is 5.0 to 8.5% by weight, and the more preferable Al content is 5.0 to 7.0% by weight.
[0018]
In such a Zn-Al-Mg ternary composition containing Al and Mg in Zn, the larger the area ratio of [Al / Zn / Zn 2 Mg ternary eutectic structure] on the plating layer surface, the chromate The effect of suppressing blackening after the treatment is increased.
[0019]
Here, [Al / Zn / Zn 2 Mg ternary eutectic structure] is distinguished from [Al / Zn / Zn 11 Mg 2 ternary eutectic structure]. On the Zn-Al-Mg ternary equilibrium diagram, there is a ternary eutectic point (melting point = 343 ° C) where the melting point is the lowest when Al is about 4 wt% and Mg is about 3 wt%. . Therefore, when a bath composition in the vicinity of this ternary eutectic point is adopted, the latter [Al / Zn / Zn 11 Mg 2 ternary eutectic structure substrate itself or the substrate [Primary Al phase] or [Primary Al phase] and [Zn single phase] mixed or [Primary Al phase] and [Al / Zn 11 Mg 2 eutectic] mixed (Zn 11 Mg The local crystallization phenomenon is likely to occur locally, but this locally crystallized Zn 11 Mg 2 phase is more discolored than the other phases (Zn 2 Mg phase). If left unattended, this part is not preferable because the color tone becomes very conspicuous and the surface appearance is remarkably deteriorated. Accordingly, it is possible to prevent the Zn 11 Mg 2 phase from locally crystallizing, and by increasing the number of Zn 2 Mg phases, the surface appearance and the corrosion resistance can be improved. Such a structure control can be performed by adjusting the cooling rate of the plated steel sheet pulled up from the plating bath.
[0020]
For example, the bath temperature of the plating bath is set to the melting point or more and less than 470 ° C., and the cooling rate until the completion of solidification of the hot-dip plating layer is controlled to 10 ° C./second or more, or the bath temperature of the plating bath is set to 470 ° C. or more and If the cooling rate until the completion of solidification of the hot-dip plating layer is controlled to 0.5 ° C / second or more, local crystallization of the Zn 11 Mg 2 phase can be prevented and the number of Zn 2 Mg phases can be increased. Can do.
[0021]
This structure control is further facilitated by adding appropriate amounts of Ti and B to the plating bath. For example, if 0.002 to 0.1% by weight of Ti and 0.001 to 0.045% by weight of B are contained, the bath temperature of the plating bath is set to the melting point or higher and lower than 410 ° C., and the cooling rate after plating Is controlled to 7 ° C./second or more, or the plating bath temperature is set to 410 ° C. or more and the cooling rate after plating is controlled to 0.5 ° C./second or more, the local phase of the Zn 11 Mg 2 system Zn 2 Mg-based ternary eutectic structure can be obtained while preventing crystallization.
[0022]
The inventors of the present invention made [Primary Al phase] or [Primary Al phase] in such a Zn 2 Mg phase, ie, [Al / Zn / Zn 2 Mg ternary eutectic structure]. And [Zn single phase], or [Al / Zn 2 Mg eutectic structure] and [Al / Zn 2 Mg eutectic structure] on the plated layer surface, / Zn 2 Mg ternary eutectic structure] was found to have a predetermined relationship with the blackening.
[0023]
That is, the greater the amount of crystallization of [Al / Zn / Zn 2 Mg ternary eutectic structure], the lower the occurrence of blackening after chromate treatment. It was found that the occurrence of blackening can be substantially suppressed when the surface is 70% or more of the surface.
[0024]
In general, chromate treatments applied to galvanized steel sheets are broadly divided into reactive and coated types. The reactive type has an etching function, and usually contains F − , Cl − , SO 4 2+ , NO 3 − , PO 3 − , CH 3 COO − , oxalate ions, etc. in the liquid. On the other hand, the coating type does not normally contain such ions. As the chromate treatment applied in the present invention, a reactive chromate treatment can be adopted. However, when a coating-type chromate treatment is adopted, the resistance to blackening is more remarkable.
[0025]
【Example】
[Example 1]
A medium-carbon steel hot-rolled steel sheet (thickness: 2.3 mm) with a maximum temperature of 600 ° C and a dew point in the furnace atmosphere of -40 ° C, assuming a Sendzimer type continuous hot dipping line. After heat treatment in, immersing in a hot dip zinc-based plating bath of various compositions and bath temperatures (shown in Table 1) for 3 seconds and cooling rate after pulling up from the bath (average value from bath temperature to plating layer solidification temperature) As shown in Table 1, hot dip plating was performed.
[0026]
An electron microscope sample was taken from the obtained plated steel sheet, and the metallographic structure of the plating layer surface was examined with an electron microscope image, and the proportion of [Al / Zn / Zn 2 Mg ternary eutectic structure] (area ratio% ) Was evaluated. The results are shown in Table 1.
[0027]
Next, chromate treatment was applied to each plated steel sheet. The treatment liquid used was a product name Zincrom 3387N manufactured by Nippon Parkerizing Co., Ltd. (coating chromate liquid). The processing conditions are as follows.
[0028]
[Chromate treatment conditions]
Application method: Roll coater method Total chromium concentration in liquid: 13 g / L
Chromium adhesion amount: 40 mg / m 2
[0029]
The obtained chromate-treated steel sheet was kept in a furnace with a humidity of RH 60% and a temperature of 50 ° C. for 25 days, and the occurrence of blackening was examined as follows. That is, the degree of blackening after the test was measured with the L value of the Lab method using a spectrophotometer, and the difference ΔL from the L value before the test was used as an index. If the ΔL value is 5 or less, almost no discoloration is visually recognized, so ΔL ≦ 5 is accepted. The results are shown in Table 1.
[0030]
[Table 1]
[Example 2]
Example 1 was repeated except that the chromate treatment was replaced with the next treatment.
[Applied chromate treatment]
Treatment liquid: Zinc chromium 3387N added with 1 g / L of zirconium fluoride Treatment method: Same chromium adhesion amount as Example 1 40 mg / m 2
[0032]
Each of the obtained chromate-treated steel sheets was evaluated for blackening as in Example 1, and the results shown in Table 2 were obtained.
[0033]
[Table 2]
Example 3
Example 1 was repeated except that the chromate treatment was replaced with the next treatment.
[Applied chromate treatment]
Treatment liquid: CrO 3 aqueous solution Treatment conditions: Total Cr concentration in spray ringer roll method liquid: 18 g / L
Chromium adhesion amount: 18 mg / m 2
[0035]
Each of the obtained chromate-treated steel sheets was subjected to the same blackening evaluation as in Example 1, and the results shown in Table 3 were obtained.
[0036]
[Table 3]
As can be seen from the results in Tables 1 to 3, a steel sheet with a chromate treatment on a hot-dip galvanized steel sheet containing 4.0 wt% or more of Al and 1.0 wt% or more of Mg has excellent blackening resistance. It can be seen that Further, it can be seen that when the area ratio of [Al / Zn / Zn 2 Mg ternary eutectic structure] on the surface of the plating layer is 70% or more, it has excellent blackening resistance.
Claims (4)
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| JP10175898A JP3729233B2 (en) | 1998-03-31 | 1998-03-31 | Hot-dip galvanized steel sheet with blackening resistance |
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| JP10175898A JP3729233B2 (en) | 1998-03-31 | 1998-03-31 | Hot-dip galvanized steel sheet with blackening resistance |
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| JP3729233B2 true JP3729233B2 (en) | 2005-12-21 |
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| JP3954766B2 (en) * | 1999-12-22 | 2007-08-08 | 新日本製鐵株式会社 | Organic composite coated metal plate with excellent corrosion resistance and press formability that can be resistance welded |
| JP4555491B2 (en) * | 2000-03-16 | 2010-09-29 | 新日本製鐵株式会社 | Hot-dip zinc-aluminum alloy-plated steel sheet with excellent chemical conversion and its manufacturing method |
| JP4528187B2 (en) * | 2005-04-01 | 2010-08-18 | 新日本製鐵株式会社 | Hot-dip steel sheet with good appearance |
| JP4940813B2 (en) * | 2006-03-06 | 2012-05-30 | Ｊｆｅスチール株式会社 | Method for producing hot-dip galvanized steel sheet having a value of TS × El of 21000 MPa ·% or more |
| JP5356616B1 (en) | 2012-11-27 | 2013-12-04 | 日新製鋼株式会社 | Method for producing hot-dip Zn alloy-plated steel sheet |
| KR101767788B1 (en) | 2015-12-24 | 2017-08-14 | 주식회사 포스코 | Plating steel material having excellent friction resistance and white rust resistance and method for manufacturing same |
| KR102453011B1 (en) * | 2020-12-21 | 2022-10-12 | 주식회사 포스코 | Plated steel sheet having excellent sealer adhesion and method for manufacturing the same |
| CN115287656B (en) * | 2022-07-13 | 2023-10-13 | 首钢集团有限公司 | Zinc-aluminum-magnesium composite coating steel plate and preparation method thereof |
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