JP4482213B2

JP4482213B2 - Fuel tank and manufacturing method thereof

Info

Publication number: JP4482213B2
Application number: JP2000296445A
Authority: JP
Inventors: 雅也山本; 光夫朝吹; 幸弘守田; 博文武津
Original assignee: Nisshin Steel Co Ltd
Current assignee: Nippon Steel Nisshin Co Ltd
Priority date: 2000-09-28
Filing date: 2000-09-28
Publication date: 2010-06-16
Anticipated expiration: 2020-09-28
Also published as: JP2002105660A

Description

【０００１】
【産業上の利用分野】
本発明は、内面耐食性及び加工性に優れた燃料タンク用アルミニウム系めっき鋼板を加工成形した燃料タンクおよびその製造方法に関する。
【０００２】
【従来の技術】
アルミニウム系めっき鋼板は、めっきしたままでは白錆が発生して外観が劣化しやすいため、クロメート処理によって耐白錆性を改善している。また、６価クロムイオンの溶出を抑制した皮膜として、燐酸−クロム−アルミニウム複合皮膜も知られている（特願平１０−３６７４７８号）。
クロメート処理されたアルミニウム系めっき鋼板をプレス成形して自動車用燃料タンクを製造する場合、カジリ，クラック等の欠陥がクロメート皮膜やめっき層に生じやすい。すなわち、自動車用燃料タンクの製造では、プレス成形したアッパータンク１ｕ及びロアータンク１ｄをシーム溶接することにより燃料タンク本体１とし、インレットパイプ２，フュエルパイプ３，フュエルリターンパイプ４，サブタンク５，ドレーンプラグ６等の各種部材を取り付ける（図１）。
【０００３】
アルミニウム系めっき鋼板をアッパータンク１ｕ，ロアータンク１ｄに成形するプレス加工は、伸び，圧縮等が複合された複雑な塑性変形を伴う加工である。そのため、従来のクロメート皮膜では潤滑性が不足し、プレス油を塗布した場合にあっても加工時の塑性変形にアルミニウム系めっき層やクロメート皮膜が追従できず、カジリ，クラック等の欠陥が発生しやすい。
【０００４】
【発明が解決しようとする課題】
欠陥発生個所では、アルミニウム系めっき層及びクロメート皮膜皮膜本来の長所が発現されず、燃料タンクが腐食環境に曝されたとき腐食発生の起点となり、穴開きに至る腐食が生じることもある。また、アルコール系燃料が使用される燃料タンクでは、アルミニウム系めっき層の犠牲防食作用が期待できず、プレス成形時に生じた欠陥発生個所で露出している鋼素地の腐食が進行し、耐久性が低下する。
【０００５】
クロメート皮膜の上に高分子樹脂粉末を含む有機樹脂層を形成してアルミニウム系めっき鋼板表面の潤滑性を改善することにより、カジリ，クラック等の欠陥発生が抑制される（特開平８−４１６５１号公報，特開平８−３１９５５０号公報）。しかし、形成された有機樹脂層は、燃料タンク製造時のシーム溶接やスポット溶接工程で熱分解し、煙や臭気を発生させて作業環境を悪化させる原因となる。しかも、樹脂皮膜の劣化や下地めっき層の腐食等に起因してアルミニウム系めっき鋼板に対する樹脂皮膜の密着性が低下する。その結果、脱落した樹脂皮膜片が燃料中に混入し、フィルタの目詰り等のトラブルを発生させることにもなりかねない。この点、脱膜型の樹脂皮膜を採用し、プレス成形後に樹脂皮膜を除去することも提案されているが、脱膜工程を必要とすることから作業性及び製造性に問題がある。
【０００６】
【課題を解決するための手段】
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、フッ化アルミニウムを主成分とする粒状物をクロメート皮膜又は燐酸−クロム−アルミニウム複合皮膜に分散させることにより、皮膜自体の潤滑性を向上させ、自動車用燃料タンクに成形した場合でも欠陥発生が抑制されるアルミニウム系めっき鋼板を提供することを目的とする。
【０００７】
本発明の燃料タンク用アルミニウム系めっき鋼板は、その目的を達成するため、下地鋼の表面に形成されたＡｌ−Ｓｉ合金めっき層と、該Ａｌ−Ｓｉ合金めっき層の上に形成されたクロメート皮膜又は燐酸−クロム−アルミニウム複合皮膜とを備え、フッ化アルミニウムを主成分とする粒状物がクロメート皮膜又は燐酸−クロム−アルミニウム複合皮膜に分散していることを特徴とする。粒状物は、平均粒径が０．０１〜１０μｍ、めっき層表面に占める面積比率が５％以上であることが好ましい。
【０００８】
【作用】
本発明者等は、クロメート皮膜又は燐酸−クロム−アルミニウム複合皮膜（以下、「化成処理皮膜」で総称する）が形成されたアルミニウム系めっき鋼板をプレス加工して燃料タンクを製造する際、プレス加工されたアルミニウム系めっき鋼板を表面観察した。その結果、プレス加工されたアルミニウム系めっき鋼板表面におけるカジリ，クラック等の欠陥発生状況が皮膜によって大きく異なることを見出した。欠陥発生のない又は欠陥発生が少ないアルミニウム系めっき鋼板について、化成処理皮膜をミクロ観察したところ、微細な粒状物が多数分散していた（図２）。
【０００９】
化成処理皮膜に粒状物が分散析出する原因は、次のように推察される。
アルミニウム系めっき鋼板に化成処理液が接触すると、アルミニウム系めっき層は、化成処理液のフッ素によるエッチング作用で表面が活性化すると共に、Ａｌの一部がイオン化して化成処理液に溶出する。溶出したＡｌイオンは、化成処理液中のフッ素と反応してフッ化アルミニウムＡｌＦ₃となって、アルミニウム系めっき層表面にある多数の析出サイトに分散析出する。析出したフッ化アルミニウムＡｌＦ₃を核として、後続する反応で生成したフッ化アルミニウムＡｌＦ₃が順次沈積し、粒状物が成長する。生成・成長した粒状物は、フッ化アルミニウムＡｌＦ₃を主成分とし、燐酸アルミニウムＡｌＰＯ₄，フッ化カリウムＫＦ等を含んでいる。
【００１０】
他方、析出サイト以外のアルミニウム系めっき層表面では、表面近傍のＡｌイオン濃度の増加に伴ってＡｌの溶出速度が低下し、ＣｒＰＯ₄，ＡｌＰＯ₄等の難溶性反応生成物が沈積して皮膜となる。皮膜が形成されると、アルミニウム系めっき層表面からＡｌが溶出しなくなり、粒状物成長に必要なＡｌ供給源がなくなる。そのため、粒状物は過度に成長することがない。
【００１１】
化成処理皮膜に分散析出した粒状物は、プレス加工等の成形時に化成処理皮膜が金型に直接接触することを防ぐと共に、固形潤滑剤として働き、アルミニウム系めっき鋼板の成形性を向上させる。その結果、成形されたアルミニウム系めっき鋼板を観察してもカジリ，クラック等の欠陥が化成処理皮膜及びアルミニウム系めっき層に検出されず、化成処理皮膜及びアルミニウム系めっき層本体の特性が発現され、耐白錆性に優れたアルミニウム系めっき鋼板製の製品が得られる。
【００１２】
このように化成処理皮膜に粒状物を分散させることにより潤滑性が高められるため、プレス加工時にカジリ，クラック等の発生が抑制され、Ａｌ−Ｓｉ合金めっき層及び化成処理皮膜が欠陥のない又は欠陥の少ない状態に維持される。したがって、Ａｌ−Ｓｉ合金めっき層及び化成処理皮膜本来の作用により、劣化ガソリンを含む雰囲気でも優れた内面耐食性が発現する。また、アルミニウムの犠牲防食作用が期待できないアルコール燃料が使用される燃料タンクでも、健全な化成処理皮膜及びＡｌ−Ｓｉ合金めっき層で下地鋼が覆われているため、耐久性に優れた燃料タンクとなる。
【００１３】
【実施の形態】
下地鋼としては、低炭素鋼，中炭素鋼，高炭素鋼，合金鋼等が使用される。なかでも、良好なプレス成形性が要求されることから低炭素Ｔｉ添加鋼，低炭素Ｎｂ添加鋼等の深絞り用鋼板が好ましい。
下地鋼は常法に従って溶融アルミニウムめっきされるが、加工性を劣化させる合金層が下地鋼とめっき層との界面に過度に成長しないようにＳｉ含有量５〜１３質量％のＡｌ−Ｓｉ合金めっき層を形成することが好ましい。Ｓｉ添加により合金層の生成が抑制され、プレス加工時にめっき層の剥離やカジリが抑制される。このような効果は、５質量％以上のＳｉ含有量で顕著になる。しかし、１３質量％を超える過剰量のＳｉが含まれると、溶融めっき後の冷却過程で初晶Ｓｉがめっき層に晶出し、加工性が著しく低下する。
【００１４】
溶融めっきラインから送り出されたアルミニウム系めっき鋼板は、次いでクロメート処理又は燐酸クロメート処理が施される。短時間処理で十分な化成処理皮膜を形成するために、全Ｃｒ量が０．１ｇ／ｌ以上の処理液が好ましい。しかし、２０ｇ／ｌを超える過剰量のＣｒを含む処理液を使用すると、クロメート付着量が多くなりすぎ抵抗溶接性が低下する。
アルミニウム系めっき鋼板の表面をエッチングにより活性化するためフッ素イオンが処理液に通常含まれるが、本発明では、生成した化成処理皮膜にフッ化アルミニウム，燐酸アルミニウム，フッ化カリウム等の粒状物を分散析出させることからフッ素イオン濃度を０．１ｇ／ｌ以上にすることが好ましい。しかし、３０ｇ／ｌを超える高濃度では、処理液のエッチング作用が強くなりすぎ、析出した粒状物が除去されてしまう。フッ素イオンの供給源としては、ＫＦ，ＮａＦ，ＮＨ₄Ｆ等、フッ素イオンを解離しやすいフッ化物が好ましい。フッ素イオンの解離が小さなケイフッ化物等の化合物であっても、フッ素イオンを解離しやすいフッ化物と併用することによりフッ素イオン供給源となる。
【００１５】
粒状物の生成には、化成処理液に含まれるＡｌイオン及びＦイオンの濃度及び比率が大きく影響する。Ｆイオン濃度は、めっき層を十分にエッチングでき、且つ粒状物が可溶化するほど過剰とならないように調整することが重要である。粒状物の粒径及び面積率は、使用するフッ化物の種類，クロム酸や燐酸濃度に対するフッ化物イオンの濃度比率，化成処理液のｐＨや温度等によって制御できる。
【００１６】
粒状物の分散析出により化成処理皮膜の潤滑性及び耐カジリ性を改善させるため、粒状物の平均粒径を０．０１〜１０μｍ，面積率を５％以上に調整することが好ましい。平均粒径が０．０１μｍより小さいと潤滑性向上効果が不充分で、逆に１０μｍより大きな平均粒径や５％未満の面積率ではアルミニウム系めっき鋼板のプレス加工時に粒状物が脱落しやすくなり耐カジリ性が低下する。
クロメート処理液に燐酸イオンを添加すると、難溶性のＣｒＰＯ₄及びＡｌＰＯ₄が生成してめっき層表面に沈殿し、一層優れた耐白錆性を呈する燐酸−クロム−アルミニウム複合皮膜が形成される。耐白錆性の改善は、Ｐ／全Ｃｒの質量比を０．１以上にすると顕著になる。しかし、過剰量のＡｌＰＯ₄が含まれる皮膜構成にすると、未反応の吸湿性燐酸が皮膜中に残存しやすくなり、却って耐白錆性が低下するので、Ｐ／全Ｃｒの上限を５．０に設定することが好ましい。
【００１７】
【実施例】
板厚０．８ｍｍの普通鋼鋼板をめっき原板に使用し、還元焼鈍後にライン速度１００ｍ／分で浴温６４０℃，Ｓｉ：９．５質量％の溶融アルミニウムめっき浴に導入し、溶融アルミニウムめっき浴から引き上げられた鋼板をガスワイピングしてめっき付着量を片面当り３５ｇ／ｍ²に調整した。得られたアルミニウム系めっき鋼板には、Ｓｉ：９．０質量％を含む平均層厚１３μｍのＡｌ−Ｓｉ合金めっき層が形成されていた。
【００１８】
アルミニウム系めっき鋼板から試験片を切り出し、表１に示す各種組成の反応型クロメート処理液をスプレーし、ゴムロールで過剰の処理液を除去した後、乾燥した。クロメート処理された試験片の表面を観察すると、試験番号４の試験片表面を観察した図２にみられるように、粒状物が分散析出したクロメート皮膜が形成されていた。粒状物の平均粒径及びめっき層表面に占める面積率は、処理液の組成によって異なっていた。
【００１９】

【００２０】
各種化成処理皮膜が形成された試験片を裸腐食試験，内面腐食試験，加工試験，抵抗溶接試験に供した。
裸腐食試験：ＪＩＳＺ２３７１に準拠して５％塩水噴霧を１００時間継続した後、試験片表面を観察し白錆発生状況を調査した。試験片全体に占める白錆の面積率が３％未満を◎，３〜１０％を○，１０〜３０％を△，３０％以上を×として裸耐食性を評価した。
【００２１】
内面腐食試験：劣化ガソリンからなる腐食性雰囲気に燃料タンクの内面が曝されることを想定し、自動車燃料タンク形状にプレス成形したアルミニウム系めっき鋼板から切り出した試験片を用い、５０℃に加温した試験液Ａ，Ｂに試験片を浸漬し、１週間ごとに試験液を取り替えながら浸漬しつづけた。９週間後に試験液から試験片を引き上げ、試験片に生じた侵食の最大深さを測定した。最大侵食深さが１００μｍ以下を◎，１００〜２００μｍを○，２００〜３００μｍを△，３００μｍ以上を×として内面耐食性を評価した。
試験液Ａ：蟻酸３５０ｐｐｍを含む水＋等量のガソリン
試験液Ｂ：メタノール８５質量％＋ガソリン１５質量％＋蟻酸３５０ｐｐｍ
【００２２】
加工試験：
直径８０ｍｍの円板状試験片に防錆油を１ｇ／ｍ²の割合で塗布し、絞り比２．０で筒状に成形した。加工前の試験片の直径をＬ₁，加工後の試験片の平均直径をＬ₂とし、比Ｌ₂／Ｌ₁を算出した。比Ｌ₂／Ｌ₁が０．８０未満を◎，０．８０〜０．８６を○，０．８６〜０．９０を△，０．９０以上を×として絞り加工性を評価した。
また、３０ｍｍ×２０ｍｍの試験片に防錆油を１ｇ／ｍ²の割合で塗布し、ビード高さ：５ｍｍ，加圧力：９．０ｋＮ，引抜き速度：５００ｍｍ／分の条件でドロービード試験した。そのときの引き抜き力が４．５ｋＮ以下を◎，４．５〜５．０ｋＮを○，５．０〜５．５ｋＮを△，５．５ｋＮ以上を×として滑り込み性を評価した。
【００２３】
抵抗溶接試験：Ｃｒ−Ｃｕ合金電極を用い、重ね合わせた２枚の試験片をスポット溶接した。溶接条件は、各試験片ごとに予め適正電流及び適正荷重を求めておき、一定打点ごとに一定比率で溶接電流を増加させる方法を採用した。溶接打点数の増加に伴ってＣｒ−Ｃｕ合金電極にめっき層中のＡｌが付着し、電極先端の損傷に起因した電流密度の低下等によって溶接できなくなるので、溶接打点数から試験片の抵抗溶接性を判定できる。溶接打点数が５００〜１０００打点を○，５００打点以下を×として抵抗溶接性を評価した。
【００２４】
表２の調査結果にみられるように、粒状物が析出していない比較例１〜３では、絞り加工性，滑り込み性，内面耐食性の何れも不充分であった。これに対し、粒状物が析出した化成処理皮膜を形成した本発明例１〜１０では、絞り加工性，滑り込み性，内面耐食性の何れも良好な水準にあった。また，燐酸イオンが比較的多い比較例１では裸耐食性が劣り、高Ｃｒ付着量で化成処理皮膜を形成した比較例３では抵抗溶接性が不充分であった。
【００２５】

【００２６】
【発明の効果】
以上に説明したように、本発明の燃料タンク用アルミニウム系めっき鋼板は、Ａｌ−Ｓｉ合金めっき層の上にフッ化アルミニウムを主成分とする粒状物を分散析出させた化成処理皮膜を形成している。粒状物によって化成処理皮膜の潤滑性が改善されているため、過酷な条件下で燃料タンクに成形されるプレス工程の際に素材が金型にスムーズに流れ込み、カジリ，クラック等の欠陥発生が抑制され、アルミニウム系めっき鋼板を所定形状に成形できる。そのため、プレス成形後にもＡｌ−Ｓｉ合金めっき層及び化成処理皮膜が健全な状態に維持され、耐食性，耐久性に優れた燃料タンクが得られる。
【図面の簡単な説明】
【図１】燃料タンクの概略図
【図２】化成処理皮膜に粒状物が分散析出することを説明する図
【図３】粒状物が分散析出している化成処理皮膜の写真[0001]
[Industrial application fields]
The present invention relates to a fuel tank obtained by processing and forming an aluminum-based plated steel sheet for a fuel tank excellent in inner surface corrosion resistance and workability, and a manufacturing method thereof .
[0002]
[Prior art]
An aluminum-plated steel sheet has white rust resistance and is improved by chromate treatment because white rust is generated and the appearance is likely to deteriorate if the steel is plated. A phosphate-chromium-aluminum composite coating is also known as a coating that suppresses the elution of hexavalent chromium ions (Japanese Patent Application No. 10-367478).
When an automobile fuel tank is manufactured by press-molding a chromate-treated aluminum-based plated steel sheet, defects such as galling and cracking are likely to occur in the chromate film and plating layer. That is, in the manufacture of a fuel tank for automobiles, a press-molded upper tank 1u and lower tank 1d are seam welded to form a fuel tank body 1, and an inlet pipe 2, a fuel pipe 3, a fuel return pipe 4, a sub tank 5, a drain plug 6 Various members such as are attached (FIG. 1).
[0003]
The press work for forming the aluminum-based plated steel sheet into the upper tank 1u and the lower tank 1d is a process involving complicated plastic deformation in which elongation, compression and the like are combined. Therefore, the conventional chromate film lacks lubricity, and even when press oil is applied, the aluminum plating layer and chromate film cannot follow the plastic deformation during processing, resulting in defects such as galling and cracking. Cheap.
[0004]
[Problems to be solved by the invention]
At the defect occurrence point, the original advantages of the aluminum-based plating layer and the chromate film are not manifested, and when the fuel tank is exposed to a corrosive environment, corrosion starts, and corrosion leading to opening may occur. Also, in fuel tanks that use alcohol-based fuels, the sacrificial anticorrosive action of the aluminum-based plating layer cannot be expected, and the corrosion of the steel substrate exposed at the locations where defects occurred during press forming progresses, resulting in durability. descend.
[0005]
By forming an organic resin layer containing polymer resin powder on the chromate film to improve the lubricity of the surface of the aluminum-based plated steel sheet, the occurrence of defects such as galling and cracking is suppressed (Japanese Patent Laid-Open No. 8-41651). Gazette, JP-A-8-319550). However, the formed organic resin layer is thermally decomposed in the seam welding or spot welding process at the time of manufacturing the fuel tank, and causes smoke and odor to deteriorate the working environment. In addition, the adhesion of the resin film to the aluminum-based plated steel sheet decreases due to deterioration of the resin film, corrosion of the underlying plating layer, and the like. As a result, the dropped resin film piece may be mixed into the fuel and cause problems such as clogging of the filter. Although it has been proposed to remove the resin film after press molding by adopting a film removal type resin film in this respect, there is a problem in workability and manufacturability because a film removal process is required.
[0006]
[Means for Solving the Problems]
The present invention has been devised to solve such a problem. By dispersing a particulate material mainly composed of aluminum fluoride in a chromate film or a phosphoric acid-chromium-aluminum composite film, the film itself can be obtained. An object of the present invention is to provide an aluminum-plated steel sheet that improves lubricity and suppresses generation of defects even when molded into a fuel tank for automobiles.
[0007]
In order to achieve the object, an aluminum-based plated steel sheet for a fuel tank according to the present invention has an Al—Si alloy plating layer formed on the surface of a base steel, and a chromate film formed on the Al—Si alloy plating layer. Or a phosphoric acid-chromium-aluminum composite film, wherein a particulate material containing aluminum fluoride as a main component is dispersed in the chromate film or the phosphoric acid-chromium-aluminum composite film. The granular material preferably has an average particle diameter of 0.01 to 10 μm and an area ratio of 5% or more on the surface of the plating layer.
[0008]
[Action]
When the present inventors press the aluminum-plated steel sheet on which a chromate film or a phosphate-chromium-aluminum composite film (hereinafter collectively referred to as “chemical conversion film”) is formed to produce a fuel tank, the press work The surface of the aluminum-based plated steel sheet was observed. As a result, it was found that the occurrence of defects such as galling and cracks on the surface of the pressed aluminum-plated steel sheet varies greatly depending on the film. When the chemical conversion treatment film was micro-observed with respect to the aluminum-based plated steel sheet having no defects or few defects, many fine particles were dispersed (FIG. 2).
[0009]
The reason why the granular material is dispersed and deposited on the chemical conversion coating is presumed as follows.
When the chemical conversion treatment liquid comes into contact with the aluminum-based plated steel sheet, the surface of the aluminum-based plating layer is activated by the etching action of fluorine of the chemical conversion treatment solution, and a part of Al is ionized and eluted into the chemical conversion treatment solution. The eluted Al ions react with fluorine in the chemical conversion solution to form aluminum fluoride AlF _3, and are dispersed and precipitated at a number of precipitation sites on the surface of the aluminum-based plating layer. Precipitated aluminum fluoride AlF ₃ as a nucleus, sequentially deposited aluminum fluoride AlF ₃ produced in subsequent reaction, granules grow. The produced and grown granular material contains aluminum fluoride AlF ₃ as a main component and contains aluminum phosphate AlPO ₄ , potassium fluoride KF, and the like.
[0010]
On the other hand, on the surface of the aluminum-based plating layer other than the precipitation sites, the elution rate of Al decreases as the Al ion concentration in the vicinity of the surface increases, and hardly soluble reaction products such as CrPO ₄ and AlPO ₄ are deposited. Become. When the film is formed, Al is not eluted from the surface of the aluminum-based plating layer, and there is no Al supply source necessary for the growth of the granular material. Therefore, the granular material does not grow excessively.
[0011]
The granular material dispersed and deposited on the chemical conversion coating film prevents the chemical conversion coating film from coming into direct contact with the mold during molding such as pressing, and acts as a solid lubricant to improve the formability of the aluminum-based plated steel sheet. As a result, even when the formed aluminum-plated steel sheet is observed, defects such as galling and cracks are not detected in the chemical conversion coating and the aluminum plating layer, and the characteristics of the chemical conversion coating and the aluminum plating layer main body are expressed. A product made of an aluminum-based plated steel sheet having excellent white rust resistance can be obtained.
[0012]
Since the lubricity is enhanced by dispersing the particulates in the chemical conversion treatment film in this way, the occurrence of galling, cracks, etc. during pressing is suppressed, and the Al-Si alloy plating layer and the chemical conversion treatment coating are free of defects or defective. It is maintained in a state with little. Therefore, excellent inner surface corrosion resistance is exhibited even in an atmosphere containing deteriorated gasoline due to the original action of the Al—Si alloy plating layer and the chemical conversion film. Even in fuel tanks that use alcohol fuel that cannot be expected to have sacrificial anticorrosive action, aluminum is covered with a sound chemical conversion coating and an Al-Si alloy plating layer. Become.
[0013]
Embodiment
As the base steel, low carbon steel, medium carbon steel, high carbon steel, alloy steel or the like is used. Of these, steels for deep drawing such as low carbon Ti-added steel and low carbon Nb-added steel are preferred because good press formability is required.
The base steel is hot-dip aluminized according to a conventional method, but Al—Si alloy plating with a Si content of 5 to 13% by mass so that an alloy layer that deteriorates workability does not grow excessively at the interface between the base steel and the plating layer. It is preferable to form a layer. The addition of Si suppresses generation of an alloy layer, and suppresses peeling and galling of the plating layer during press working. Such an effect becomes remarkable when the Si content is 5% by mass or more. However, if an excessive amount of Si exceeding 13% by mass is contained, primary Si crystallizes in the plating layer in the cooling process after hot dipping, and the workability is remarkably reduced.
[0014]
The aluminum-based plated steel sheet fed from the hot dipping line is then subjected to chromate treatment or phosphoric acid chromate treatment. In order to form a sufficient chemical conversion film in a short time treatment, a treatment liquid having a total Cr amount of 0.1 g / l or more is preferable. However, when a treatment liquid containing an excessive amount of Cr exceeding 20 g / l is used, the amount of chromate attached becomes too large, and resistance weldability deteriorates.
Fluorine ions are usually included in the treatment liquid to activate the surface of the aluminum-plated steel sheet by etching. In the present invention, however, particulates such as aluminum fluoride, aluminum phosphate, and potassium fluoride are dispersed in the produced chemical conversion coating. It is preferable to make the fluorine ion concentration 0.1 g / l or more because of precipitation. However, at a high concentration exceeding 30 g / l, the etching action of the treatment liquid becomes too strong, and the precipitated particulate matter is removed. As a supply source of fluorine ions, fluorides that easily dissociate fluorine ions, such as KF, NaF, and NH ₄ F, are preferable. Even a compound such as silicofluoride having a small dissociation of fluorine ions can be used as a fluorine ion supply source by using it together with a fluoride that easily dissociates fluorine ions.
[0015]
The concentration and ratio of Al ions and F ions contained in the chemical conversion solution are greatly affected in the generation of the granular material. It is important to adjust the F ion concentration so that the plating layer can be sufficiently etched and the particulate matter is not so excessive that it is solubilized. The particle size and area ratio of the granular material can be controlled by the type of fluoride used, the concentration ratio of fluoride ion to the chromic acid or phosphoric acid concentration, the pH and temperature of the chemical conversion treatment liquid, and the like.
[0016]
In order to improve the lubricity and galling resistance of the chemical conversion film by dispersing and dispersing the granular material, it is preferable to adjust the average particle size of the granular material to 0.01 to 10 μm and the area ratio to 5% or more. If the average particle size is smaller than 0.01 μm, the effect of improving the lubricity is insufficient. Conversely, if the average particle size is larger than 10 μm or the area ratio is less than 5%, the granular material is likely to fall off when the aluminum-based plated steel sheet is pressed. The galling resistance is reduced.
When phosphate ions are added to the chromate treatment solution, poorly soluble CrPO ₄ and AlPO ₄ are generated and precipitated on the surface of the plating layer, and a phosphate-chromium-aluminum composite film exhibiting more excellent white rust resistance is formed. The improvement in white rust resistance becomes significant when the mass ratio of P / total Cr is 0.1 or more. However, when the coating composition contains an excessive amount of AlPO ₄ , unreacted hygroscopic phosphoric acid tends to remain in the coating film, and the white rust resistance is lowered, so the upper limit of P / total Cr is 5.0. It is preferable to set to.
[0017]
【Example】
A normal steel plate with a thickness of 0.8 mm is used as the plating plate, and after reduction annealing, it is introduced into a molten aluminum plating bath with a line temperature of 100 m / min and a bath temperature of 640 ° C., Si: 9.5 mass%, and a molten aluminum plating bath The steel plate pulled up from was gas-wiped to adjust the coating amount to 35 g / m ² per side. On the obtained aluminum-based plated steel sheet, an Al—Si alloy plating layer having an average layer thickness of 13 μm containing Si: 9.0% by mass was formed.
[0018]
A test piece was cut out from the aluminum-based plated steel sheet, sprayed with a reactive chromate treatment liquid having various compositions shown in Table 1, and after removing the excess treatment liquid with a rubber roll, it was dried. When the surface of the chromate-treated test piece was observed, a chromate film in which particulate matter was dispersed and formed was formed as shown in FIG. The average particle diameter of the granular material and the area ratio in the plating layer surface differed depending on the composition of the treatment liquid.
[0019]

[0020]
The specimens with various chemical conversion coatings were subjected to bare corrosion test, internal corrosion test, processing test, and resistance welding test.
Bare corrosion test: 5% salt spray was continued for 100 hours in accordance with JIS Z2371, and then the surface of the test specimen was observed to investigate the occurrence of white rust. Bare corrosion resistance was evaluated with the area ratio of white rust in the entire test piece being less than 3% as ◎, 3-10% as ○, 10-30% as Δ, and 30% or more as ×.
[0021]
Inner surface corrosion test: Assuming that the inner surface of the fuel tank is exposed to a corrosive atmosphere made of deteriorated gasoline, use a test piece cut out from an aluminum-plated steel sheet press-formed into an automobile fuel tank shape, and heat to 50 ° C. The test piece was immersed in the test liquids A and B, and the test pieces were continuously immersed while replacing the test liquid every week. After 9 weeks, the test piece was pulled up from the test solution, and the maximum depth of erosion generated on the test piece was measured. The inner surface corrosion resistance was evaluated with a maximum erosion depth of 100 μm or less as ◎, 100 to 200 μm as ◯, 200 to 300 μm as Δ, and 300 μm or more as x.
Test solution A: Water containing 350 ppm formic acid + equal gasoline test solution B: 85% methanol + 15% gasoline + 350 ppm formic acid
[0022]
Processing test:
A rust preventive oil was applied to a disk-shaped test piece having a diameter of 80 mm at a rate of 1 g / m ² and molded into a cylinder with a drawing ratio of 2.0. The ratio L ₂ / L ₁ was calculated assuming that the diameter of the test piece before processing was L ₁ and the average diameter of the test piece after processing was L ₂ . The drawability was evaluated with the ratio L ₂ / L ₁ being less than 0.80, ０．８, 0.80 to 0.86 being ◯, 0.86 to 0.90 being Δ, and 0.90 or more being x.
Further, a rust preventive oil was applied to a 30 mm × 20 mm test piece at a rate of 1 g / m ² , and a draw bead test was performed under the conditions of a bead height: 5 mm, a pressing force: 9.0 kN, and a drawing speed: 500 mm / min. The pulling force at that time was 4.5 kN or less, ◎, 4.5 to 5.0 kN was evaluated as ◯, 5.0 to 5.5 kN as Δ, and 5.5 kN or more as ×.
[0023]
Resistance welding test: Using a Cr—Cu alloy electrode, two superposed test pieces were spot welded. As the welding conditions, a method was adopted in which an appropriate current and an appropriate load were obtained in advance for each test piece, and the welding current was increased at a constant ratio for each fixed spot. As the number of welding points increases, Al in the plating layer adheres to the Cr-Cu alloy electrode, making it impossible to weld due to a decrease in current density due to damage at the electrode tip, etc. Therefore, resistance welding of the test piece from the number of welding points Can determine gender. The resistance weldability was evaluated by setting the number of welding points to 500 to 1000, and setting the number to 500 or less to x.
[0024]
As can be seen from the results of the investigation in Table 2, Comparative Examples 1 to 3 in which no particulate matter was precipitated had insufficient drawability, sliding property, and inner surface corrosion resistance. On the other hand, in Examples 1 to 10 of the present invention in which the chemical conversion film on which the particulate matter was deposited was formed, all of the drawing workability, slipping property, and inner surface corrosion resistance were at good levels. Further, Comparative Example 1 having a relatively large amount of phosphate ions was inferior in bare corrosion resistance, and Comparative Example 3 in which a chemical conversion treatment film was formed with a high Cr adhesion amount was insufficient in resistance weldability.
[0025]

[0026]
【The invention's effect】
As described above, the aluminum-based plated steel sheet for a fuel tank according to the present invention is formed by forming a chemical conversion treatment film on which an aluminum fluoride-based granular material is dispersed and deposited on an Al-Si alloy plating layer. Yes. Since the lubricity of the chemical conversion coating is improved by the granular material, the material smoothly flows into the mold during the pressing process that is molded into the fuel tank under severe conditions, and the occurrence of defects such as galling and cracking is suppressed. Thus, the aluminum-based plated steel sheet can be formed into a predetermined shape. Therefore, even after press molding, the Al—Si alloy plating layer and the chemical conversion film are maintained in a healthy state, and a fuel tank excellent in corrosion resistance and durability can be obtained.
[Brief description of the drawings]
1 is a schematic diagram of a fuel tank. FIG. 2 is a diagram for explaining that particles are dispersed and deposited on the chemical conversion coating. FIG. 3 is a photograph of the chemical conversion coating on which particulates are dispersed and deposited.

Claims

A fuel tank that is formed by processing an aluminum-plated steel sheet,
The aluminum-based plated steel sheet is
An Al-Si alloy plating layer formed on the surface of the base steel, and a chromate film or a phosphate-chromium-aluminum composite film formed on the Al-Si alloy plating layer,
Granules mainly composed of aluminum fluoride are dispersed in the chromate film or the phosphate-chromium-aluminum composite film,
The average particle diameter of the granular material is 0.01 to 10 μm, and the area ratio of the granular material occupying the surface of the plating layer is 5% or more.
Fuel tank.

The aluminum-plated steel sheet, the Cr deposition amount of the Al-Si alloy plating layer is 50 mg / ^{m 2} or less, the fuel tank according to claim 1.

A method for manufacturing a fuel tank, comprising a step of pressing an aluminum-plated steel sheet,
The aluminum-based plated steel sheet is
An Al-Si alloy plating layer formed on the surface of the base steel, and a chromate film or a phosphate-chromium-aluminum composite film formed on the Al-Si alloy plating layer,
Granules mainly composed of aluminum fluoride are dispersed in the chromate film or the phosphate-chromium-aluminum composite film,
The average particle diameter of the granular material is 0.01 to 10 μm, and the area ratio of the granular material occupying the surface of the plating layer is 5% or more.
Fuel tank manufacturing method.

The aluminum-based plated steel sheet has a Cr adhesion amount of 50 mg / m on the Al-Si alloy plating layer. ^２2 The manufacturing method of the fuel tank of Claim 3 which is the following.