JP4516653B2

JP4516653B2 - Al-based plated steel sheet for automobiles

Info

Publication number: JP4516653B2
Application number: JP2000023734A
Authority: JP
Inventors: 將夫黒崎; 亮介和気; 達也崎山; 英俊新頭; 伸一山口
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2000-02-01
Filing date: 2000-02-01
Publication date: 2010-08-04
Anticipated expiration: 2020-02-01
Also published as: JP2001220690A

Description

【０００１】
【発明の属する技術分野】
本発明は、自動車用Ａｌ系めっき鋼板に関するものである。
【０００２】
【従来の技術】
耐食性に優れた自動車用防錆鋼板として、Ｚｎ系めっき鋼板は幅広く用いられており、その種類も電気Ｚｎめっき、電気Ｚｎ−Ｆｅめっき、電気Ｚｎ−Ｎｉめっき、溶融Ｚｎめっき、溶融Ｚｎ−Ｆｅめっきと多種に及ぶ、また、その防錆能はめっき付着量に依存するところが大きく、近年の防錆能向上ニーズに対応するためにより厚目付の材料を用いる傾向が強まっている。
一方で、鋼板のリサイクルを考慮するとＺｎ系めっき鋼板は、転炉スクラップとして用いた場合にはＺｎヒュームとして蒸発し、転炉発生ダスト内に蓄積される。Ｚｎヒューム発生量が少量であれば問題無いが、目付量の多いＺｎ系めっき鋼板のリサイクル量が増加すると、転炉ダストのべたつきが増加しダスト利用時の棚欠け等の操業問題を誘因する。
【０００３】
これに対処するためＺｎ系めっき鋼板を転炉にリサイクルする前に真空雰囲気で加熱してＺｎを除去する方法や、酸に溶解する方法が検討されているが、いずれも処理コストの増加を引き起こし経済的観点から望ましいものでは無い。
一方、従来用いられているＡｌ系のめっき鋼板の主成分はＡｌ、Ｓｉであり、いずれの成分も鉱石の脈石分、あるいは溶銑成分として炉内に持ち込まれる。これらの成分は吹錬後は殆ど全量がスラグ中に酸化物として移行し、発生する転炉ダストへは影響を及ぼさない。また、ホタル石に含まれるフッ素の様な環境への悪影響が無いため、埋め立て用としてのスラグの再利用にも全く環境への影響は及ぼさない。また、Ｍｇに関しても炉の煉瓦がＭｇＯを主成分としており、転炉スラグ中には含まれるがダストには含まれない成分である。
【０００４】
ところで、転炉でのリサイクルを前提として、転炉ダストにＺｎを含まないＡｌめっき鋼板の使用を考えた場合、従来のＡｌめっき鋼板は耐食性において以下の様な難点がある。すなわち、Ａｌめっき鋼板は乾燥が入る腐食環境では腐食生成物が安定化して優れた耐食性を示すのに対し、融雪塩を散布する地域など、比較的湿った環境でのめっき溶出速度は極めて速く、容易に鋼板腐食に至ってしまうために、充分な耐食性を発揮しないといった欠点がある。
また、自動車外板のように塗装して用いる場合には、塗膜下がアルカリ雰囲気になるためＡｌの腐食速度が極端に増加し容易にブリスターを引き起こしてしまうという欠点もあった。
【０００５】
さらに、Ａｌめっき鋼板は、めっき層と鋼板との界面にＡｌ−Ｆｅ−Ｓｉを主成分とする脆い合金層を有しており、加工時に地鉄まで貫通しためっき割れが生じやすく、また、めっき金属であるであるＡｌが比較的柔らかいため、プレス成形時に金型との接触による線状の傷、いわゆるかじりが発生しやすいといった問題があった。これらめっき割れ、かじりといっためっき鋼板の欠陥は、腐食環境下での耐食性を劣化させる。特に地鉄まで貫通した欠陥が存在すると腐食起点として作用し腐食を誘発する。また、めっき層の割れが存在すると、かじり部、端面部で発生した腐食が伝播しやすいため、耐食性が著しく劣化する。
【０００６】
Ａｌめっきの耐食性を改善する方策としてのＭｇ添加も検討されており、例えば、特公平１−２０２２４号公報に記載されているようにＳｉ：３．０〜１３％、Ｍｇ：０．５〜３．０％、残部Ａｌおよび不可避的不純物を含んだめっき鋼板の製造例が開示されている。この開示例での最適なＭｇの添加量は０．５〜３％と記されているが、発明者らの詳細な研究ではＭｇ３％未満では耐食性向上効果は不十分であることが明らかになった。
【０００７】
また、金属表面技術ｖｏｌ．１１，Ｎｏ．２，１９６０の４１〜４４ページにはＡｌにＭｇを０．５％、５％、１０％添加したＡｌ−Ｍｇ合金めっきの耐食性に関して触れられており、無加工の平板状態での暴露試験での裸耐食性はＭｇが０．５％では不十分であるがＭｇを５％、１０％とすることで向上することが述べられれいる。しかし一般的には無加工状態でめっき鋼板を用いることは非常に少なく、加工後にも安定した耐食性を発揮するためのＳｉ並びにＭｇの濃度の適正範囲、さらにはめっき組織の構造に関しては一切触れられていない。また、Ｍｇを含んだＡｌめっき鋼板の塗装後耐食性に関しては、殆ど検討されていないのが現状である。
【０００８】
また、スポット溶接を考えると、Ａｌめっき鋼板は電極であるＣｕとの合金化が促進されやすいため連続溶接性は非常に悪く、連続打点はせいぜい３００点程度であり、通常の合金化溶融Ｚｎめっき鋼板の２５００点に比べると極端に劣るため、自動車用外板としては適さないめっき鋼板であると考えられて来た。
本発明は、Ｚｎをほとんど含まないために転炉でのリサイクル性が優れたＡｌ系めっき鋼板において、自動車用途に必要な、耐食性、加工性、溶接性の諸特性を向上させ、耐食性、加工性、溶接性、リサイクル性のいずれにも優れた自動車用Ａｌ系めっき鋼板を提供せんとするものである。
【０００９】
【課題を解決するための手段】
本発明者らは、これらの諸問題を解決すべく鋭意検討を重ねた結果、ＡｌにＭｇ並びにＳｉを適正範囲で添加しその組織形態を制御し、さらに、めっきの表面状態を制御した後に、りん酸亜鉛皮膜層あるいは有機樹脂皮膜層を適正量付与することで優れた防錆能を発揮し、しかもプレス成形性、スポット溶接性に優れ、さらには転炉製鋼法を前提としたリサイクル性にも優れた自動車用Ａｌ系めっき鋼板の提供が可能であることを見い出した。本発明は、かかる知見に基づいて完成されたもので、その要旨とするところは、
（１）鋼板表面に、質量％で、Ｍｇ：３〜９％、Ｓｉ：６〜１０％を含有し、残部Ａｌ及び不可避的不純物からなるＡｌ系めっき層を片面当たり１０〜１００ｇ／ｍ²有し、該Ａｌ系めっき層の表面のＭｇを主体とする酸化皮膜層を２０Å以下とし、その上層に、亜鉛とりんとを重量比（亜鉛／りん）２．３〜３．２で含有するりん酸亜鉛皮膜層を０．３〜２．５ｇ／ｍ²有するＡｌ系めっき鋼板であって、前記Ａｌ系めっき層を５°の断面傾斜研磨で観察したとき、めっき層中に、最も長い辺が５〜４０μｍである塊状のＭｇ₂Ｓｉ相を、面積分率で０．５〜３０％含有することを特徴とする自動車用Ａｌ系めっき鋼板。
【００１０】
（２）鋼板表面に、質量％で、Ｍｇ：３〜９％、Ｓｉ：６〜１０％を含有し、残部Ａｌ及び不可避的不純物からなるＡｌ系めっき層を片面当たり１０〜１００ｇ／ｍ²有し、該Ａｌ系めっき層の表面のＭｇを主体とする酸化皮膜層を２０Å以下とし、その上層に、有機樹脂と高分子固体潤滑剤の合計量に対して１〜２０質量％の高分子固体潤滑剤を含有する有機樹脂皮膜層を０．３〜３μｍ有するＡｌ系めっき鋼板であって、前記Ａｌ系めっき層を５°の断面傾斜研磨で観察したとき、めっき層中に、最も長い辺が５〜４０μｍである塊状のＭｇ₂Ｓｉ相を、面積分率で０．５〜３０％含有することを特徴とする自動車用Ａｌ系めっき鋼板である。
【００１１】
【発明の実施形態】
まず、めっき組成の決定理由に関して説明する。
めっき層中のＭｇは、当該めっき鋼板の耐食性を向上させる作用を供する。Ｍｇの添加は０．５％以上で塩水環境での耐食性向上効果を発揮するが、さらに、電着塗装後の耐食性を向上させるためには３％以上の添加が必要である。また、Ｍｇの添加量が３％以上となる段階から、塊状のＭｇ₂Ｓｉ相の析出が開始し、Ｍｇ添加量とともにその量は増加する。
【００１２】
他方、Ｍｇの添加量を増大して行くと徐々に浴の粘度が上昇し操業性を劣化させると同時に、９％を超えると耐食性が劣化する。これらのことを考慮すると好ましいＭｇ含有量は３〜９％である。
めっき層のＳｉは、４％以上添加すれば、加工性に劣るＦｅ−Ａｌ系合金層が生成せず、十分な加工性が得られるが、Ｓｉが６％未満であると、塊状のＭｇ₂Ｓｉ相の析出が起こらないので６％以上の添加が必要である。
一方で、Ｓｉ含有率が１０％を超えると耐食性が極端に劣化する。これらの理由からめっき層中Ｓｉ含有率は６〜１０％とした。
【００１３】
さらに、発明者らは、めっきの耐食性に与える影響を鋭意検討した結果、めっき組織、特にめっき層中に存在するＭｇ₂Ｓｉの形態並びに量の影響が大きいことを突き止めた。従来から、Ａｌ−Ｍｇ−Ｓｉ系合金である５０００系Ａｌ合金では、Ｍｇを４〜６％、Ｓｉは０．２％程度添加しているため、微細なＭｇ₂Ｓｉが析出している。Ａｌ系合金を母材として使用する場合には、このＭｇ₂Ｓｉが加工性を劣化させてしまうため析出量を最小限度に抑える必要があり、耐食性の向上効果にもおのずと限界があった。ところが、めっき層として用いる場合にはめっき層中のＭｇ並びにＳｉの含有量をさらに増加し、Ｍｇ₂Ｓｉの析出量を増加させても、りん酸亜鉛皮膜または有機樹脂皮膜を適正量付与すれば加工性の劣化は顕著でなく、むしろ塊状のＭｇ₂Ｓｉを積極的に析出させることで耐食性が大きく向上することが明らかになった。
【００１４】
従来のＡｌ系合金における組成範囲でのＭｇ、Ｓｉ添加（Ｍｇ：４〜６％、Ｓｉ：１％以下程度）では隣片状のＭｇ₂Ｓｉしか析出せず、断面光学顕微鏡観察で確認出来る大きさは５μｍ以下である。本発明に従うＭｇ₂Ｓｉはめっきの凝固過程で初晶として析出するため塊状の形態を示し、断面光学顕微鏡観察では多角形の析出物として確認できる。
この塊状のＭｇ₂Ｓｉ相は５°の断面傾斜研磨で観察したときに、面積分率で０．５％以上含有しないと耐食性の向上が期待出来ない。一方で、面積分率で３０％を超えると、めっき層加工性の劣化が著しいため上限は３０％とした。
めっき付着量は片面当たり１０ｇ／ｍ²未満であると自動車材料として十分な耐食性を発揮しない。また、１００ｇ／ｍ²を超えると加工性並びに溶接性が極端に劣ってしまう。これらの理由から、めっき付着量は１０〜１００ｇ／ｍ²とした。
【００１５】
また、自動車材料として用いるためには上述した耐食性はもとより、優れた加工性とスポット溶接性を有することが必須条件となる。この意味でりん酸亜鉛皮膜あるいは高分子固体潤滑剤を含有した有機樹脂皮膜を上層被膜層として付与することが必須条件となる。
まず、りん酸亜鉛皮膜に関してであるが、亜鉛とりんとの重量比（亜鉛／りん）が３．２を超える場合には、皮膜の硬度が不十分で、めっき鋼板の加工性向上に寄与しない。一方、亜鉛とりんとの重量比が２．３未満の場合には、めっき鋼板の加工性は十分であるが、その向上効果は飽和し、製造コストの増加を引き起こす。このため、亜鉛とりんとの重量比（亜鉛／りん）は２．３〜３．２の範囲とする。
【００１６】
このりん酸亜鉛皮膜は、亜鉛とりん以外に、ニッケル、マグネシウム、鉄、コバルト、カルシウム、マンガンの１種以上を合計で０．１〜１０重量％含有しても、上記の亜鉛とりんとの重量比さえ満たしていれば、他の特性に影響を与えることなく加工性向上に有効に作用する。
りん酸亜鉛皮膜の付着量は、０．３〜２．５ｇ／ｍ²の範囲で付与する。付着量が０．３ｇ／ｍ²未満では皮膜の効果が発揮されず加工性が不十分であり、２．５ｇ／ｍ²を超えると皮膜付与に要する処理時間が長くなるうえ、プレス成形時に複合皮膜が剥離しプレス金型に付着し鋼板に押し込まれるため、かえってプレス傷を誘発してしまうからである。また、皮膜の電気抵抗が増加し、スポット溶接が不可能になってしまう。好ましくは、１〜２ｇ／ｍ²の範囲がよく、この範囲でプレス成形時の加工性ならびにスポット溶接性が最も良好となる。
【００１７】
また、加工性、溶接性を向上させる手段として上記りん酸亜鉛皮膜の代わりに高分子固体潤滑剤を含有した有機樹脂皮膜を付与することも可能である。
この場合、有機樹脂と高分子固体潤滑剤の合計量に対して、高分子固体潤滑剤の添加比率が１質量％以下であると十分な潤滑性が発揮されないため加工性が改善されず、また、２０質量％を超えると効果が飽和する上、その後、行なわれる電着塗装工程でブツと呼ばれる微小なピンホールが生じるために外観劣化を引き起こす。これらの理由から高分子固体潤滑剤の添加比率は、有機樹脂と高分子固体潤滑剤の合計量に対して、１〜２０質量％とした。
【００１８】
有機樹脂被膜は、その厚みが０．３μｍ以下であると加工性改善に寄与せず、３μｍを超えると皮膜による抵抗が高くなりすぎ溶接が不可能となってしまう。また、有機皮膜の下に施す化成処理に関しては特に限定するものでは無く、通常用いられているクロメート処理、Ｃｒを含有しないノンクロメート処理等の適用が可能である。また密着性の良好な樹脂を用いる場合には化成処理を省略することも可能である。
また、使用する有機樹脂は、特に限定するものではなく、エポキシ系、フェノキシ系、フェノール系、ポリエステル系、ポリウレタン系、フタル酸系、アクリル系、フッ素系、シリコーン系などの有機樹脂を利用できる、またこれらの２種類以上の混合物でも良い。
【００１９】
次に、めっき層とりん酸亜鉛皮膜層あるいは有機樹脂皮膜層との界面に関しては、Ｍｇを主体とする酸化皮膜層の厚みを２０Å以下に制御する必要がある。これは酸化皮膜層の厚みが２０Åを超えると亜鉛とりんとを含有する皮膜あるいは有機皮膜の２次密着性が劣化してしまうからである。この酸化皮膜の厚みはめっき製造時のポット回りを窒素雰囲気、あるいは水素を適量加え還元雰囲気にすることで制御できる。もし、これらの雰囲気制御が困難で酸化膜が２０Åを超えて生成する条件で製造せざるをえない場合には、めっき製造後に酸洗することで容易に２０Å以下に制御することが可能である。
【００２０】
酸化皮膜の厚みはりん酸亜鉛皮膜層あるいは有機樹脂皮膜層を施した試料の表面からＧＤＳでめっき厚み方向にＭｇ、並びに酸素の分析を行ないその濃度分布を調査することで測定出来る。また、ＸＰＳでめっき厚み方向にスパッタリングしながらＭｇ、酸素のピークを分析することでも測定できる。
また、リサイクル性に関しては、スクラップとして転炉、電気炉に用いた場合の発生ダスト中のＺｎ濃度が重要である。発明者らが温度１４００℃、Ｓｉ＝０．１５％，Ｃ＝４．１５％の溶銑を全体の９０％、残りの１０％にスクラップとして各種表面処理鋼板を用い、吹止Ｃ＝０．０５％、温度１６４０℃で転炉吹錬したところ、溶鋼１ｔｏｎあたり約１３〜１７ｋｇのダストが発生した。
【００２１】
表面処理鋼板としては亜鉛めっき鋼板、Ｚｎ−Ａｌ合金めっき、Ａｌ系めっき鋼板で付着量が３０〜１００ｇ／ｍ²のもので、自動車用のりん酸亜鉛化成処理（皮膜付着量３〜５ｇ／ｍ²）施した後用いた。めっき種・付着量を変化させてもダスト発生量に大差は無くダスト中のＺｎ濃度が大きく変化した。Ｚｎ付着量の多いめっき鋼板を用いた場合にダスト中Ｚｎ濃度が上昇し、場合によっては２５％まで上昇した。
【００２２】
この方法で作製したＺｎ濃度の異なるダストを収集し、高炉原料であるペレット製造ポッパーを模擬したホッパー（払出部の傾斜角度６０°）を用いて払出試験を行なったところ、ダスト中Ｚｎ濃度が２％を超えた所でダストの流動性が低下し払出し不可（いわゆる棚つり）を生じてしまった。ダスト中Ｚｎ濃度が２％以下であれば問題なく払出し出来き、良好なペレットの製造が可能であった。これらの結果から表面処理鋼板のリサイクル性、すなわち発生ダストの流動性維持という観点からは発生ダスト中のＺｎ濃度を２％以下にする必要がある。
本発明のＡｌ系めっき鋼板は、従来のＺｎめっき鋼板以上の耐食性を有するにもかかわらず、めっき層はもちろん、上層の皮膜層にもＺｎをほとんど含まないため、発生ダスト中のＺｎ濃度は２％を超えることはない。
【００２３】
【実施例】
（実施例１）
冷延鋼板を酸化方式により脱脂し、７５％Ｈ₂−２５％Ｎ₂からなる還元ガス中で７５０℃で３０秒間還元、焼鈍し、７００℃に冷却後６３０℃に保持されためっき浴に浸漬し溶融めっきを行なった。
上記浴中に３．５秒浸漬後Ｎ₂ガスワイピングでめっき付着量を片面当たり５〜２００ｇ／ｍ²に調整し、めっき浴組成とほぼ同一組成のＡｌ−Ｓｉ−Ｍｇ合金めっき鋼板を作成した。めっき浴組成は、Ｓｉ：２０質量％以下、Ｍｇ：２０質量％以下の範囲で変化させた。得られためっき鋼板は化学分析でめっき付着量と組成を分析すると同時に、めっき層の表層を５°の傾斜をつけて研磨した後、光学顕微鏡で５００倍にて組織の観察を行った。
【００２４】
その後、一部の試料においては、５％硫酸溶液に５秒浸漬して酸洗を行ない、めっき表層のＭｇ系酸化皮膜層を除去し、市販のチタンコロイドをスプレー塗布して活性化処理を施した。酸洗処理を行わなかったものは、りん酸ナトリウム系のアルカリ脱脂剤で脱脂した後、市販のチタンコロイドをスプレー塗布して活性化処理を施した。その後、りん酸をＰＯ₄として１０〜３０ｇ／リットル、硝酸をＮＯ₃として２〜６ｇ／リットル、フッ化水素酸をフッ素として２ｇ／リットル、亜鉛イオンを１〜３ｇ／リットル含有した化成処理液を基本浴として、さらに、ニッケルイオンを１ｇ／リットル含有した化成処理液を調合した。
上記活性化処理を施した鋼板を４５℃に加熱した化成処理液に０．５〜２秒浸漬したのち、水洗、乾燥を施した。その後、試験片の一部を切り出し、蛍光Ｘ線にてりん酸亜鉛皮膜層の付着量ならびに組成を分析した。この方法で得られた試料に関し、以下の方法で各種特性を評価した。
【００２５】
１）酸化層厚み測定
りん酸亜鉛を施したサンプルを表層から地鉄までＧＤＳで連続的に分析した。予め測定したＧＤＳエッチング速度からめっき層とりん酸亜鉛皮膜との界面に存在するＭｇ系酸化厚みを求めた。
２）加工性
ブランク径１６０ｍｍφの試験片に鉱油を主成分とする防錆油を１．５ｇ／ｍ²塗布したのち、ポンチ径７８ｍｍφで押し付け荷重５ｋｇｆ／ｃｍ²で４５ｍｍの深絞り成形を行なった。
このとき、絞り成形されたサンプルのプレスかじりを、目視で確認すると同時に、側壁部から１０×１０ｍｍの試片を切り出し、傾斜研磨後、光学顕微鏡でめっき割れの状況を観察した。
【００２６】
なお、それぞれの評価基準は以下の通りである。
（プレスかじり評価基準）
○：かじりなし
△：微小なかじり有り
×：激しいかじり存在
（めっき割れ評価基準）
○：めっき割れなし
△：めっき層のみに割れ存在
×：地鉄まで貫通する割れ存在
【００２７】
３）塗装後耐食性
７０×１５０ｍｍの試料をトリクレン脱脂した後、市販の自動車用のりん酸亜鉛化成処理液を用い４３℃で３．５分の浸漬処理を行ない、化成皮膜を２〜３ｇ／ｍ²付与した後、市販のカチオン電着塗装剤を用いて２８℃で到達電圧２００Ｖの状態で３分通電し、電着塗装を２０μｍ施した。その後市販の塗料を用い中塗、上塗塗装を各４０μｍ実施した後、試片中心部にクロスカットを入れ腐食試験に供した。
腐食試験は塩水噴霧４時間→６０℃乾燥２時間→５０℃、湿度９５％雰囲気２時間のサイクルで１２０サイクル行ない、クロスカット部の最大膨れ幅を測定した。塗装後耐食性の評価基準は以下の通りである。
【００２８】
（塗装後耐食性評価基準）
○：膨れ幅３ｍｍ以下
△：膨れ幅３〜５ｍｍ
×：膨れ幅５ｍｍ超
４）孔あき耐食性
７０×１５０ｍｍ、厚さ０．８ｍｍの試料をトリクレン脱脂した後、試料中心部５０×５０ｍｍに絶縁テープを張り、前述の塗装後耐食性試験片と同様の方法で電着塗装まで施した。その後、絶縁テープを剥がし、５０×５０ｍｍの部位のみ塗装が施されていない試料を作製し、この未塗装部が対向するような状態で間隔０．５ｍｍあけて２枚の試片を接合した。
その後、前述の塗装後耐食性試験と同様なサイクルで１６０サイクルの試験を行ない、完了後２枚の板を開き腐食生成物を除去した後、未塗装部における最大浸食深さを測定した。孔あき耐食性の評価基準は以下の通りである。
【００２９】
（孔あき耐食性評価基準）
○：浸食深さ０．４ｍｍ以下
△：浸食深さ０．４〜０．８ｍｍ
×：孔あき発生
５）溶接性
下に示す溶接条件で通常のＣｕ−Ｃｒ合金電極を用いたスポット溶接を行い，ナゲット径が４√ｔ（ｔ：板厚）を切った時点までの連続打点数を評価した。溶接条件と溶接性評価基準は以下の通りである。
【００３０】
（溶接条件）
溶接電流：１０ｋＡ
加圧力：２２０ｋｇ
溶接時間：１２サイクル
電極径：６ｍｍ
電極形状：ドーム型、先端６φ−４０Ｒ
【００３１】
（溶接性評価基準）
○：連続打点１５００点超
△：連続打点８００〜１５００点
×：連続打点８００点未満
６）ダスト中Ｚｎ濃度
温度１４００℃、Ｓｉ＝０．１５％，Ｃ＝４．１５％の溶銑を９００ｋｇ、めっき鋼板を１００ｋｇを１ｔｏｎ小型転炉に装入し、送酸速度０．０５Ｎｍ³／ｓで吹止Ｃ＝０．０５％のなるように吹錬した。発生したダストを回収し、ダスト中のＺｎ濃度を分析した。ダスト中Ｚｎ濃度の評価基準は以下の通りである。
（評価基準）
○：ダスト中Ｚｎ濃度２％以下、
×：ダスト中Ｚｎ濃度２％超
以上の試験結果をを表１に示す。
【００３２】
【表１】

【００３３】
めっき組成・組織・付着量、Ｍｇ系酸化皮膜厚み、りん酸亜鉛皮膜層の組成・皮膜量を適正範囲に制御した場合には、本発明例１〜６に見るように良好な加工性、耐食性、溶接性、リサイクル性を示す。一方比較例の７に示すようにめっき層中Ｍｇ濃度が低い場合には析出するＭｇ₂Ｓｉのサイズ、容量とも不足して充分な耐食性が発揮されなくなる。
逆に、比較例８のようにめっき層中Ｍｇが適正範囲を超えて高い場合には析出するＭｇ₂Ｓｉの容量が過大となり加工性を阻害する。同様のことはめっき層中Ｓｉが適正範囲からはずれた９、１０にも当てはまり、低い場合には加工性が、また高い場合には耐食性が劣化する。
【００３４】
また、比較例１１に示すように、めっき層がＺｎ単独の場合には耐食性が劣る上ダスト中のＺｎ濃度高くなってしまう。これに、Ｍｇを添加した比較例１２では耐食性は良好なものの、やはりダスト中に適正量以上のＺｎを含有するためリサイクル性に支障をきたす。
めっき組成は適正範囲ひ制御されても付着量が少ない場合（比較例１３）には耐食性が、付着量が多い場合（比較例１４）には溶接性と加工性に問題が生じる。まためっき層とりん酸亜鉛皮膜層との界面に存在するＭｇ系酸化皮膜厚みが厚いと（比較例１５）、りん酸亜鉛皮膜層の密着性が劣り結果として塗装後耐食性が不十分となる。また比較例１６〜１８の様にりん酸亜鉛皮膜層の組成、付着量が適正範囲からはずれると加工性に大きな障害をきたす。
【００３５】
（実施例２）
実施例１と同様の方法でＡｌ−Ｍｇ−Ｓｉ系、並びにＺｎ系のめっきを作製した。その後、一部のものを除いて５％硫酸溶液に５ｓ間浸漬し酸洗処理を行なった後、水洗処理しＣｒ付着量が３０ｍｇ／ｍ²になるようにクロメート処理を行なった。酸洗処理を行なわなかったものに関しては、りん酸ナトリウム系のアルカリ脱脂剤で脱脂した後同様のクロメート処理を行なった。
次に各種有機樹脂に高分子固体潤滑剤をしてポリエチレンワックスをを０〜３０％の範囲で変化させ添加し、膜厚を０．１〜５μｍの範囲で制御し塗布した後１００℃で焼き付けた。
この方法で作製した有機樹脂被覆めっき鋼板に関し実施例１と同様の方法で酸化層厚み測定、加工性、塗装後耐食性、孔あき耐食性、溶接性、ダスト中Ｚｎ濃度を評価した。結果を表２に示す。
【００３６】
【表２】

【００３７】
めっき組成・組織・付着量、Ｍｇ系酸化皮膜厚み、有機樹脂皮膜の高分子固体潤滑剤配合比・皮膜厚を適正範囲に制御した場合には、本発明例１〜６に見るように良好な加工性、耐食性、溶接性、リサイクル性を示す。一方比較例７〜１１の様にＡｌ系めっきでめっき組成が適正範囲を外れた場合には加工性、耐食性で問題が生じ、比較例１２、１３の様にＺｎ系めっきを用いた場合にはダスト中に適正量以上のＺｎを含有するためリサイクル性に支障をきたす。
めっき組成は適正範囲に制御されても付着量が少ない場合（比較例１３）には耐食性が、付着量が多い場合（比較例１４）には溶接性と加工性に問題が生じる。まためっき層ととの界面に存在するＭｇ系酸化皮膜厚みが厚いと（比較例１５）、有機樹脂皮膜の密着性が劣り、結果として塗装後耐食性が不十分となる。また比較例１６〜１８の様にりん有機樹脂皮膜の高分子固体潤滑剤配合比・皮膜厚が適正範囲からはずれると加工性に大きな障害をきたす。
【００３８】
【発明の効果】
本発明は、めっき組成・組織制御で耐食性を解決し、りん酸亜鉛皮膜層、有機樹脂皮膜層を適用することで加工性、溶接性を向上させることにより、Ａｌ系めっき鋼板を自動車内外板に適用する際の課題を解決し、転炉ダストにおけるＺｎ含有量が極めて少なく、リサイクル性に優れた自動車用Ａｌ系めっき鋼板を提供するものであり、その産業上の価値は極めて高いといえる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an Al-based plated steel sheet for automobiles.
[0002]
[Prior art]
Zn-plated steel sheets are widely used as anticorrosive steel sheets for automobiles with excellent corrosion resistance, and the types thereof are electric Zn plating, electric Zn-Fe plating, electric Zn-Ni plating, hot Zn plating, hot Zn-Fe plating. In addition, the rust prevention ability largely depends on the amount of plating, and the tendency to use thicker materials is increasing in order to meet the recent needs for improvement in rust prevention ability.
On the other hand, in consideration of recycling of the steel sheet, the Zn-based plated steel sheet evaporates as Zn fume when accumulated as converter scrap and accumulates in converter generated dust. If the amount of generated Zn fume is small, there is no problem. However, when the amount of Zn-based plated steel sheet with a large basis weight increases, the stickiness of converter dust increases, which causes operational problems such as lack of shelves when using dust.
[0003]
In order to cope with this, a method of removing Zn by heating in a vacuum atmosphere before recycling a Zn-based plated steel sheet to a converter and a method of dissolving in an acid have been studied, both of which cause an increase in processing costs. This is not desirable from an economic point of view.
On the other hand, the main components of conventionally used Al-based plated steel sheets are Al and Si, and both components are brought into the furnace as ore gangue or hot metal components. Most of these components are transferred as oxides in the slag after blowing and do not affect the generated converter dust. In addition, since there is no adverse environmental impact such as fluorine contained in fluorite, there is no environmental impact at all on the reuse of slag for landfill. Further, regarding the Mg, the brick of the furnace is mainly composed of MgO, and is included in the converter slag but not in the dust.
[0004]
By the way, on the premise of recycling in a converter, when considering the use of an Al-plated steel sheet that does not contain Zn in the converter dust, the conventional Al-plated steel sheet has the following difficulties in corrosion resistance. In other words, in the corrosive environment where dryness enters the Al plated steel sheet, the corrosion products stabilize and show excellent corrosion resistance, whereas the plating elution rate in a relatively moist environment such as an area where snow melting salt is sprayed is extremely fast, Since it easily leads to corrosion of the steel sheet, there is a drawback that sufficient corrosion resistance is not exhibited.
In addition, when used as an automobile outer plate, there is also a drawback that the corrosion rate of Al is extremely increased and blisters are easily caused because the undercoat is in an alkaline atmosphere.
[0005]
Furthermore, the Al-plated steel sheet has a brittle alloy layer mainly composed of Al-Fe-Si at the interface between the plating layer and the steel sheet, and is likely to cause plating cracks penetrating to the ground metal during processing. Since Al, which is a metal, is relatively soft, there has been a problem that linear scratches due to contact with the mold, that is, so-called galling, are likely to occur during press molding. Defects in the plated steel sheet such as plating cracking and galling deteriorate the corrosion resistance in a corrosive environment. In particular, if there is a defect penetrating to the ground iron, it acts as a starting point for corrosion and induces corrosion. Further, if there is a crack in the plating layer, corrosion generated at the galling part and the end face part easily propagates, so that the corrosion resistance is remarkably deteriorated.
[0006]
Addition of Mg as a measure for improving the corrosion resistance of Al plating has been studied. For example, as described in JP-B-1-20224, Si: 3.0 to 13%, Mg: 0.5 to 3 An example of manufacturing a plated steel sheet containing 0.0%, the balance Al and inevitable impurities is disclosed. Although the optimum amount of Mg added in this disclosed example is 0.5 to 3%, the inventors' detailed research revealed that the effect of improving corrosion resistance is insufficient when the amount is less than 3%. It was.
[0007]
In addition, metal surface technology vol. 11, no. 2, 1960, pages 41-44, touches on the corrosion resistance of Al-Mg alloy plating with 0.5%, 5%, and 10% Mg added to Al. It is stated that the bare corrosion resistance is insufficient when Mg is 0.5%, but is improved when Mg is 5% or 10%. However, in general, there is very little use of plated steel sheets in the unprocessed state, and there is no mention of the appropriate range of Si and Mg concentrations for exhibiting stable corrosion resistance after processing, as well as the structure of the plated structure. Not. In addition, regarding the corrosion resistance after coating of the Al-plated steel sheet containing Mg, there is almost no investigation.
[0008]
In addition, when considering spot welding, the Al-plated steel sheet is easily promoted to be alloyed with Cu, which is an electrode, so that the continuous weldability is very poor, and the continuous striking point is at most about 300 points. Since it is extremely inferior to the 2500 points of the steel plate, it has been considered to be a plated steel plate that is not suitable as an automobile outer plate.
The present invention improves the corrosion resistance, workability, and weldability characteristics required for automotive applications in Al-plated steel sheets with excellent recyclability in converters because they contain almost no Zn. Corrosion resistance, workability It is intended to provide an Al-plated steel sheet for automobiles that is excellent in both weldability and recyclability.
[0009]
[Means for Solving the Problems]
As a result of intensive studies to solve these problems, the present inventors added Mg and Si to Al in an appropriate range to control the structure morphology, and further, after controlling the surface state of plating, By applying an appropriate amount of zinc phosphate coating layer or organic resin coating layer, it exhibits excellent rust-preventing ability, and is excellent in press formability and spot weldability, and also on recyclability based on the converter steelmaking process. Has also found that it is possible to provide excellent automotive Al-based plated steel sheets. The present invention has been completed based on such findings, and the gist thereof is as follows:
(1) On the steel sheet surface, by mass%, Mg: 3 to 9%, Si: 6 to 10%, Al-based plating layer consisting of the balance Al and inevitable impurities, 10 to 100 g / m ² per side And an oxide film layer mainly composed of Mg on the surface of the Al-based plating layer having a thickness of 20 mm or less, and an upper layer containing phosphoric acid containing zinc and phosphorus in a weight ratio (zinc / phosphorus) of 2.3 to 3.2. An Al-based plated steel sheet having a zinc coating layer of 0.3 to 2.5 g / m ² , and when the Al-based plated layer is observed with a 5 ° cross-sectional inclined polishing, the longest side is 5 in the plated layer. An Al-based plated steel sheet for automobiles, containing 0.5 to 30% of an agglomerated Mg ₂ Si phase having an area fraction of 40 μm.
[0010]
(2) On the surface of the steel sheet, by mass%, Mg: 3 to 9%, Si: 6 to 10%, Al-based plating layer consisting of the balance Al and inevitable impurities is 10 to 100 g / m ² per side. And the oxide film layer mainly composed of Mg on the surface of the Al-based plating layer is 20 mm or less, and on the upper layer, 1 to 20% by mass of the polymer solid with respect to the total amount of the organic resin and the polymer solid lubricant An Al-based plated steel sheet having an organic resin film layer containing a lubricant of 0.3 to 3 μm, and when the Al-based plated layer is observed with a 5 ° cross-sectional inclined polishing, the longest side is present in the plated layer. It is an Al-based plated steel sheet for automobiles characterized by containing a bulk Mg ₂ Si phase of 5 to 40 μm in an area fraction of 0.5 to 30%.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
First, the reason for determining the plating composition will be described.
Mg in the plating layer serves to improve the corrosion resistance of the plated steel sheet. Addition of Mg exhibits an effect of improving the corrosion resistance in a salt water environment at 0.5% or more, and further, addition of 3% or more is necessary to improve the corrosion resistance after electrodeposition coating. In addition, the precipitation of the massive Mg ₂ Si phase starts from the stage where the added amount of Mg becomes 3% or more, and the amount increases with the added amount of Mg.
[0012]
On the other hand, when the amount of Mg added is increased, the viscosity of the bath gradually increases and the operability is deteriorated. At the same time, when it exceeds 9%, the corrosion resistance is deteriorated. Considering these matters, the preferable Mg content is 3 to 9%.
If Si of the plating layer is added in an amount of 4% or more, an Fe—Al alloy layer inferior in workability is not formed and sufficient workability is obtained. However, if Si is less than 6%, massive Mg ₂ Since precipitation of Si phase does not occur, addition of 6% or more is necessary.
On the other hand, if the Si content exceeds 10%, the corrosion resistance is extremely deteriorated. For these reasons, the Si content in the plating layer is set to 6 to 10%.
[0013]
Furthermore, as a result of intensive studies on the influence of the plating on the corrosion resistance, the inventors have found that the influence of the plating structure, particularly the form and amount of Mg ₂ Si present in the plating layer, is large. Conventionally, in a 5000 series Al alloy, which is an Al—Mg—Si based alloy, 4 to 6% Mg and about 0.2% Si have been added, so fine Mg ₂ Si is precipitated. When using an Al-based alloy as a base material, this Mg ₂ Si deteriorates workability, so the amount of precipitation must be minimized, and the effect of improving corrosion resistance is naturally limited. However, when used as a plating layer, even if the contents of Mg and Si in the plating layer are further increased and the precipitation amount of Mg ₂ Si is increased, an appropriate amount of zinc phosphate coating or organic resin coating is applied. It has been found that the deterioration of workability is not remarkable, but rather the corrosion resistance is greatly improved by aggressively precipitating massive Mg ₂ Si.
[0014]
With Mg and Si addition (Mg: 4-6%, Si: about 1% or less) in the composition range of conventional Al-based alloys, only adjacent piece-like Mg ₂ Si precipitates, and can be confirmed by observation with a cross-sectional optical microscope. The thickness is 5 μm or less. Since Mg ₂ Si according to the present invention precipitates as a primary crystal during the solidification process of plating, it shows a massive form and can be confirmed as a polygonal precipitate by observation with a cross-sectional optical microscope.
When this massive Mg ₂ Si phase was observed by 5 ° cross-sectional slant polishing, the area fraction was 0.00. If 5 % or more is not contained, improvement in corrosion resistance cannot be expected. On the other hand, if the area fraction exceeds 30%, the plating layer workability is significantly deteriorated, so the upper limit was made 30%.
If the plating adhesion amount is less than 10 g / m ² per side, sufficient corrosion resistance as an automobile material cannot be exhibited. Moreover, when it exceeds 100 g / m < ² >, workability and weldability will be extremely inferior. For these reasons, the plating adhesion amount was set to 10 to 100 g / m ² .
[0015]
Moreover, in order to use as an automobile material, it is essential to have excellent workability and spot weldability as well as the above-described corrosion resistance. In this sense, it is an essential condition that an organic resin film containing a zinc phosphate film or a polymer solid lubricant is applied as an upper film layer.
First, regarding the zinc phosphate coating, when the weight ratio of zinc to phosphorus (zinc / phosphorus) exceeds 3.2, the coating has insufficient hardness and does not contribute to the improvement of the workability of the plated steel sheet. On the other hand, when the weight ratio of zinc and phosphorus is less than 2.3, the workability of the plated steel sheet is sufficient, but the improvement effect is saturated and the manufacturing cost is increased. For this reason, the weight ratio of zinc to phosphorus (zinc / phosphorus) is in the range of 2.3 to 3.2.
[0016]
This zinc phosphate coating is not limited to zinc and phosphorus, but contains one or more of nickel, magnesium, iron, cobalt, calcium and manganese in a total amount of 0.1 to 10% by weight. If the ratio is satisfied, it will work effectively to improve workability without affecting other properties.
The amount of zinc phosphate coating applied is in the range of 0.3 to 2.5 g / m ² . If the adhesion amount is less than 0.3 g / m ² , the effect of the film is not exhibited and the workability is insufficient, and if it exceeds 2.5 g / m ² , the treatment time required for applying the film becomes long and the compound is combined during press molding. This is because the film peels off and adheres to the press mold and is pushed into the steel plate, which in turn induces press flaws. In addition, the electrical resistance of the film increases and spot welding becomes impossible. Preferably, the range of 1 to ² g / m ² is good, and the workability at the time of press molding and the spot weldability are the best in this range.
[0017]
Further, as a means for improving workability and weldability, an organic resin film containing a polymer solid lubricant can be applied instead of the zinc phosphate film.
In this case, if the addition ratio of the solid polymer lubricant is 1% by mass or less with respect to the total amount of the organic resin and the solid polymer lubricant, sufficient lubricity is not exhibited, and the workability is not improved. When the content exceeds 20% by mass, the effect is saturated, and thereafter, a minute pinhole called “butsu” is generated in the electrodeposition coating process, which causes deterioration in appearance. For these reasons, the addition ratio of the polymer solid lubricant is 1 to 20% by mass with respect to the total amount of the organic resin and the polymer solid lubricant.
[0018]
If the thickness of the organic resin film is 0.3 μm or less, it does not contribute to the improvement of workability, and if it exceeds 3 μm, the resistance due to the film becomes too high and welding becomes impossible. Moreover, it does not specifically limit about the chemical conversion treatment performed under an organic film, The application of the chromate process normally used, the non-chromate process which does not contain Cr, etc. is possible. Further, when a resin having good adhesion is used, the chemical conversion treatment can be omitted.
In addition, the organic resin to be used is not particularly limited, and organic resins such as epoxy, phenoxy, phenol, polyester, polyurethane, phthalic acid, acrylic, fluorine, and silicone can be used. Also, a mixture of two or more of these may be used.
[0019]
Next, regarding the interface between the plating layer and the zinc phosphate coating layer or the organic resin coating layer, it is necessary to control the thickness of the oxide coating layer mainly composed of Mg to 20 mm or less. This is because when the thickness of the oxide film layer exceeds 20 mm, the secondary adhesion of the film containing zinc and phosphorus or the organic film deteriorates. The thickness of the oxide film can be controlled by making a pot atmosphere during plating production a nitrogen atmosphere or a reducing atmosphere by adding an appropriate amount of hydrogen. If it is difficult to control these atmospheres and the oxide film must be produced under the condition that it exceeds 20 mm , it can be easily controlled to 20 mm or less by pickling after plating. is there.
[0020]
The thickness of the oxide film can be measured by analyzing the concentration distribution of Mg and oxygen in the plating thickness direction by GDS from the surface of the sample coated with the zinc phosphate film layer or the organic resin film layer. It can also be measured by analyzing Mg and oxygen peaks while sputtering in the plating thickness direction with XPS.
Regarding recyclability, the Zn concentration in the generated dust when used as scrap in a converter or electric furnace is important. The inventors used various surface-treated steel sheets as scrap for 90% of the hot metal with a temperature of 1400 ° C., Si = 0.15%, and C = 4.15%, and the remaining 10%. % And a temperature of 1640 ° C. when the converter was blown, about 13 to 17 kg of dust was generated per ton of molten steel.
[0021]
Surface-treated steel sheets are galvanized steel sheets, Zn-Al alloy plated, Al-based steel sheets with an adhesion amount of 30 to 100 g / m ² , and zinc phosphate conversion treatment for automobiles (film adhesion amount 3 to 5 g / m ² ) Used after application. There was no big difference in the amount of dust generated even when the plating type and the amount of adhesion were changed, and the Zn concentration in the dust changed greatly. When a plated steel sheet with a large amount of Zn adhered was used, the Zn concentration in the dust increased, and in some cases increased to 25%.
[0022]
Dust with different Zn concentrations produced by this method was collected, and when a discharge test was performed using a hopper (slope angle of discharge portion 60 °) simulating a pellet production popper as a blast furnace raw material, the Zn concentration in the dust was 2 When the amount exceeds 50%, the fluidity of the dust is reduced, and it is impossible to dispense (so-called shelf hanging). If the Zn concentration in the dust was 2% or less, it could be dispensed without problems, and good pellets could be produced. From these results, it is necessary to reduce the Zn concentration in the generated dust to 2% or less from the viewpoint of recyclability of the surface-treated steel sheet, that is, maintaining the fluidity of the generated dust.
Although the Al-based plated steel sheet of the present invention has a corrosion resistance higher than that of the conventional Zn-plated steel sheet, it contains almost no Zn in the upper coating layer as well as the plated layer, so the Zn concentration in the generated dust is 2 % Will not exceed.
[0023]
【Example】
Example 1
Cold rolled steel sheet is degreased by an oxidation method, reduced and annealed at 750 ° C. for 30 seconds in a reducing gas consisting of 75% H ₂ -25% N ₂ , immersed in a plating bath maintained at 630 ° C. after cooling to 700 ° C. Then, hot dip plating was performed.
After being immersed in the bath for 3.5 seconds, the amount of plating adhesion was adjusted to ⁵ to 200 g / m ² per side by N ₂ gas wiping, and an Al—Si—Mg alloy plated steel sheet having almost the same composition as the plating bath composition was prepared. . The plating bath composition was changed in a range of Si: 20% by mass or less and Mg: 20% by mass or less. The obtained plated steel sheet was analyzed for the amount and composition of plating by chemical analysis, and at the same time, the surface layer of the plating layer was polished with an inclination of 5 °, and then the structure was observed 500 times with an optical microscope.
[0024]
After that, some samples were immersed in a 5% sulfuric acid solution for 5 seconds to pickle, remove the Mg-based oxide layer on the plating surface layer, and spray-coated with commercially available titanium colloid for activation treatment. did. Those not pickled were degreased with a sodium phosphate alkaline degreasing agent and then spray-coated with a commercially available titanium colloid for activation treatment. Thereafter, a chemical conversion treatment solution containing 10 to ₃₀ g / liter of phosphoric acid as PO ₄ , 2 to 6 g / liter of nitric acid as NO ₃ , 2 g / liter of hydrofluoric acid as fluorine, and 1 to 3 g / liter of zinc ions. As a basic bath, a chemical conversion treatment solution containing 1 g / liter of nickel ions was further prepared.
The steel plate subjected to the activation treatment was immersed in a chemical conversion treatment liquid heated to 45 ° C. for 0.5 to 2 seconds, and then washed with water and dried. Thereafter, a part of the test piece was cut out, and the amount and composition of the zinc phosphate coating layer were analyzed by fluorescent X-ray. Various characteristics of the sample obtained by this method were evaluated by the following methods.
[0025]
1) Measurement of oxide layer thickness Samples subjected to zinc phosphate were continuously analyzed from the surface layer to the iron base by GDS. The Mg-based oxide thickness present at the interface between the plating layer and the zinc phosphate film was determined from the previously measured GDS etching rate.
2) Processability After applying 1.5 g / m ^{2 of} a rust-preventive oil mainly composed of mineral oil to a test piece having a diameter of 160 mmφ, a deep drawing was performed at a punch diameter of 78 mmφ and a pressing load of 5 kgf / cm ² to 45 mm. .
At this time, the press galling of the drawn sample was confirmed visually, and at the same time, a 10 × 10 mm specimen was cut out from the side wall, and after the slant polishing, the state of plating cracking was observed with an optical microscope.
[0026]
In addition, each evaluation standard is as follows.
(Press galling evaluation criteria)
○: No galling △: Small galling ×: Existence of galling (plating crack evaluation criteria)
○: No plating cracking △: Cracking only in the plating layer ×: Cracking penetrating to the ground iron [0027]
3) Corrosion resistance after coating After degreasing a 70 x 150 mm sample, it was immersed in a commercially available zinc phosphate chemical treatment solution for 3.5 minutes at 43 ° C to give a chemical coating of 2 to 3 g / m. ² After application, a commercially available cationic electrodeposition coating agent was applied at 28 ° C. for 3 minutes at an ultimate voltage of 200 V, and 20 μm of electrodeposition coating was applied. Then, after carrying out an intermediate coating and a top coating using commercially available paints for 40 μm, a cross cut was made at the center of the specimen and subjected to a corrosion test.
The corrosion test was carried out for 120 cycles with a cycle of salt spray 4 hours → 60 ° C. drying 2 hours → 50 ° C., humidity 95% atmosphere 2 hours, and the maximum swollen width of the cross-cut portion was measured. Evaluation criteria for post-coating corrosion resistance are as follows.
[0028]
(Evaluation criteria for corrosion resistance after painting)
○: Swelling width 3 mm or less △: Swelling width 3 to 5 mm
×: Swelling width greater than 5 mm 4) After perforating a sample having a perforated corrosion resistance of 70 × 150 mm and a thickness of 0.8 mm, an insulating tape is applied to the sample center 50 × 50 mm, and the same as the above-mentioned corrosion resistance test piece after coating The method was applied up to electrodeposition coating. Thereafter, the insulating tape was peeled off to prepare a sample in which only a 50 × 50 mm portion was not coated, and two specimens were joined with a gap of 0.5 mm in a state where the unpainted portions face each other.
Thereafter, 160 cycles of the test were performed in the same cycle as the above-described post-coating corrosion resistance test. After completion, the two plates were opened to remove the corrosion products, and then the maximum erosion depth in the unpainted portion was measured. The evaluation criteria for perforated corrosion resistance are as follows.
[0029]
(Perforated corrosion resistance evaluation standard)
○: Erosion depth of 0.4 mm or less Δ: Erosion depth of 0.4 to 0.8 mm
×: Perforated generation 5) Spot welding using a normal Cu—Cr alloy electrode under the welding conditions shown below, and continuous punching until the nugget diameter cuts 4√t (t: plate thickness) Score was evaluated. Welding conditions and weldability evaluation criteria are as follows.
[0030]
(Welding conditions)
Welding current: 10 kA
Applied pressure: 220kg
Welding time: 12 cycles Electrode diameter: 6mm
Electrode shape: dome shape, tip 6φ-40R
[0031]
(Weldability evaluation criteria)
○: Continuous hitting point over 1500 points Δ: Continuous hitting point 800 to 1500 points ×: Continuous hitting point less than 800 points 6) 900 kg of molten iron with a Zn concentration temperature of 1400 ° C., Si = 0.15%, C = 4.15%, 100 kg of the plated steel sheet was charged into a 1 ton small converter and blown so that the blow-off C = 0.05% at an acid feed rate of 0.05 Nm ³ / s. The generated dust was collected, and the Zn concentration in the dust was analyzed. The evaluation criteria for the Zn concentration in the dust are as follows.
(Evaluation criteria)
○: Zn concentration in dust is 2% or less,
X: Table 1 shows test results of Zn concentration in dust exceeding 2%.
[0032]
[Table 1]

[0033]
When the plating composition / structure / adhesion amount, Mg-based oxide film thickness, zinc phosphate film layer composition / film amount are controlled within the appropriate ranges, good workability and corrosion resistance can be seen in Examples 1-6 of the present invention. Shows weldability and recyclability. On the other hand, as shown in Comparative Example 7, when the Mg concentration in the plating layer is low, the size and capacity of the deposited Mg ₂ Si are insufficient and sufficient corrosion resistance cannot be exhibited.
On the other hand, when Mg in the plating layer is higher than the appropriate range as in Comparative Example 8, the capacity of the deposited Mg ₂ Si is excessive and the workability is hindered. The same applies to 9, 10 where Si in the plating layer deviates from the appropriate range. When it is low, workability is deteriorated, and when it is high, corrosion resistance is deteriorated.
[0034]
Further, as shown in Comparative Example 11, when the plating layer is made of Zn alone, the corrosion resistance is inferior and the Zn concentration in the dust becomes high. In Comparative Example 12, to which Mg is added, the corrosion resistance is good, but the dust is also contained in the dust in an appropriate amount or more, so that the recyclability is hindered.
Even if the plating composition is controlled within an appropriate range, there is a problem in corrosion resistance when the adhesion amount is small (Comparative Example 13), and there is a problem in weldability and workability when the adhesion amount is large (Comparative Example 14). Further, when the Mg-based oxide film thickness present at the interface between the plating layer and the zinc phosphate coating layer is thick (Comparative Example 15), the adhesion of the zinc phosphate coating layer is inferior, resulting in insufficient post-coating corrosion resistance. Moreover, if the composition and the amount of adhesion of the zinc phosphate coating layer deviate from the appropriate ranges as in Comparative Examples 16 to 18, the workability is greatly impaired.
[0035]
(Example 2)
Al-Mg-Si-based and Zn-based platings were produced in the same manner as in Example 1. Then, except for some, after being immersed in a 5% sulfuric acid solution for 5 s and pickling treatment, it was washed with water and chromated so that the amount of Cr deposited was 30 mg / m ² . For those not pickled, the same chromate treatment was performed after degreasing with a sodium phosphate alkaline degreasing agent.
Next, a solid polymer lubricant is added to various organic resins, polyethylene wax is added in a range of 0 to 30%, and the film thickness is controlled in a range of 0.1 to 5 μm and then baked at 100 ° C. It was.
The organic resin-coated plated steel sheet produced by this method was evaluated for oxide layer thickness measurement, workability, post-coating corrosion resistance, perforated corrosion resistance, weldability, and Zn concentration in dust in the same manner as in Example 1. The results are shown in Table 2.
[0036]
[Table 2]

[0037]
When the plating composition / structure / adhesion amount, Mg-based oxide film thickness, organic resin film polymer solid lubricant compounding ratio / film thickness are controlled within an appropriate range, as shown in Examples 1 to 6 of the present invention, it is good. Shows workability, corrosion resistance, weldability, and recyclability. On the other hand, when the plating composition is out of the proper range with Al-based plating as in Comparative Examples 7 to 11, problems occur in workability and corrosion resistance, and when Zn-based plating is used as in Comparative Examples 12 and 13, Recyclability is hindered because the dust contains more than the proper amount of Zn.
Even if the plating composition is controlled within an appropriate range, when the adhesion amount is small (Comparative Example 13), there is a problem in corrosion resistance, and when the adhesion amount is large (Comparative Example 14), there is a problem in weldability and workability. Moreover, when the Mg-type oxide film thickness which exists in an interface with a plating layer is thick (Comparative Example 15), the adhesiveness of an organic resin film is inferior, and as a result, the corrosion resistance after coating becomes insufficient. Further, as in Comparative Examples 16 to 18, if the polymer solid lubricant compounding ratio and the film thickness of the phosphorus organic resin film deviate from the proper ranges, the workability is greatly hindered.
[0038]
【The invention's effect】
The present invention solves corrosion resistance by controlling the plating composition and structure, and improves the workability and weldability by applying a zinc phosphate coating layer and an organic resin coating layer. It solves the problems in application, provides an Al-plated steel sheet for automobiles that has an extremely low Zn content in converter dust and is excellent in recyclability, and it can be said that its industrial value is extremely high.

Claims

On the steel sheet surface, by mass%, Mg: 3 to 9%, Si: 6 to 10%, Al-based plating layer consisting of the balance Al and inevitable impurities has 10 to 100 g / m ² per side, A zinc phosphate coating layer containing an oxide coating layer mainly composed of Mg on the surface of the Al-based plating layer of 20 mm or less and containing zinc and phosphorus in a weight ratio (zinc / phosphorous) of 2.3 to 3.2. the a Al-based plated steel sheet having 0.3~2.5g / m ^2, when observing the Al-based plating layer in cross-section inclined polishing 5 °, the plating layer, the longest side is. 5 to 40 [mu] m An Al-based plated steel sheet for automobiles containing 0.5 to 30% of the massive Mg ₂ Si phase as an area fraction.

On the steel sheet surface, by mass%, Mg: 3 to 9%, Si: 6 to 10%, Al-based plating layer consisting of the balance Al and inevitable impurities has 10 to 100 g / m ² per side, The oxide film layer mainly composed of Mg on the surface of the Al-based plating layer has a thickness of 20 mm or less, and a polymer solid lubricant of 1 to 20% by mass with respect to the total amount of the organic resin and the polymer solid lubricant is formed thereon. An Al-based plated steel sheet having an organic resin coating layer content of 0.3 to 3 μm, and when the Al-based plated layer is observed by 5 ° cross-sectional inclined polishing, the longest side is 5 to 40 in the plated layer. An Al-based plated steel sheet for automobiles containing 0.5 to 30% of an agglomerated Mg ₂ Si phase having an area fraction of μm.