JP4537599B2

JP4537599B2 - High corrosion resistance Al-based plated steel sheet with excellent appearance

Info

Publication number: JP4537599B2
Application number: JP2001060166A
Authority: JP
Inventors: 雅裕布田; 伸一山口; 純真木; 輝明伊崎; 俊治坂本
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2000-03-10
Filing date: 2001-03-05
Publication date: 2010-09-01
Anticipated expiration: 2021-03-05
Also published as: JP2001323357A

Description

【０００１】
【発明の属する技術分野】
本発明は、屋根壁等の金属建材、自動車の排気系部材、ガソリンタンク材、トースター、ストーブ等の家庭用熱器具に使用される外観に優れた高耐食性Ａｌ系めっき鋼板に関する。
【０００２】
【従来の技術】
Ａｌ系めっき鋼板は、高い耐食性と耐熱性、美しい外観等から、自動車部品、建材、家電部品等に使用されている。近年の自動車排気系部材の耐食性向上要求に対応するため、めっき原板にＣｒを含有する鋼板、あるいはステンレスにアルミめっきを施し、高い耐食性を持たせたものが多数開発されている（特開昭６１−２３１１５２号公報、特開平３−２７７７６１号公報等）。原板としてＣｒ含有鋼ないしステンレス鋼を使用すると、当然耐食性は向上するが、製造コストの増大となり、また、加工性は劣化する傾向にある。そこで、めっき浴に耐食性向上元素を添加する検討も種々なされ、特開平２−８８７５４号公報、特開平７−２００９１号公報等において、Ｃｒ，Ｍｎ添加等が開示されている。しかし、これらにおいては厳しい曲げ加工部、あるいは端面部で赤錆の発生を完全に抑制するまでに至っていない。
【０００３】
一方、最近では、自動車燃料タンクのＰｂフリー化が検討されつつあり、この用途へのアルミめっき鋼板の適用が進みつつある。この際の課題は、耐食性と加工性、溶接性の高度なバランスである。一般に表面処理鋼板において、めっきの付着量が増大するほど、耐食性は当然向上するが、加工性、溶接性は低下する傾向にある。この場合の溶接性は連続作業性を意味する。Ａｌは電極材質のＣｕと容易に反応するため、付着量を増すと、電極との反応量が増加し電極寿命の低下を招く。また、プレス加工においても、付着量の増加はめっき層の損傷、剥離などを生じ易くなる。
【０００４】
そこで、これらの特性を両立させるべく、やはり多数の発明がなされている。（特開平１０−４６３５８号公報等）が、耐食性と溶接性、加工性を完全に両立できるとは言い難い状況である。また、アルミめっきは特に乾湿繰り返し環境では非常に耐食性に優れるが、常時濡れた環境では溶解が進行しやすい傾向にある。特に、加工の厳しい側壁部や、塗装ののりにくいフランジ端面部では、めっきが鋼板を犠牲防食して優先的に溶解し、短期間で赤錆発生に至る可能性がある。
【０００５】
また、従来建材用表面処理鋼板としては、主としてＺｎめっき鋼板が使用されてきたが、最近では更なる耐食性の向上、意匠性への要請から、Ａｌ−Ｚｎ系めっき鋼板、あるいはＡｌ−Ｓｉ系めっき鋼板等の使用量が増加傾向にある。Ａｌ−Ｚｎ系合金めっき鋼板においては、Ａｌの添加量が多くなるほど耐食性は優れるが犠牲防食作用が弱まる傾向にあり、使用から数ケ月経つと端面あるいは厳しい折り曲げ加工部等から赤錆の発生が見られる。更にめっき層の耐食性が優れるアルミめっき鋼板においては、通常の大気環境下では地鉄の犠牲防食作用はほとんど有せず、端面は数日で赤錆が発生するため施工後の改修塗装が必要であり、作業が煩雑となる。あるいは補修塗装が不十分であると端面から赤錆が発生して外観を損ねるといった課題があった。
【０００６】
アルミめっき鋼板のこの課題に対応すべく、本発明者らは特開平６−３３０２７４号公報において加工部からの赤錆発生を抑制する技術の開示を行っている。しかし、これにより加工部からの赤錆発生は抑制できるものの、端面からの赤錆発生が抑制できないという欠点があった。また、同様の目的で特開平１１−２７９７３４号公報、特開平１１−２７９７３５号公報ではＡｌ−Ｓｉ−Ｍｇ系もしくはＡｌ−Ｓｉ−Ｍｇ−Ｚｎ系めっき技術が開示されており、この系では表面の外観を確保するためＢｅ、Ｓｒ添加を必須とする技術であるが、Ｂｅは有毒性で環境上問題があり、また、Ｓｒはしわ発生の抑制効果が不十分であるという欠点があった。
【０００７】
【発明が解決しようとする課題】
そこで、本発明において、アルミめっき層に耐食性に寄与するＭｇ，Ｓｉ，Ｓｎ，Ｚｎ，Ｃａを添加することで、めっき層及び端面の耐食性を従来より飛躍的に優れさせ、かつ優れた表面外観と良好な加工性、加工後耐食性を確保し、これにより各種素材として適用可能な外観に優れた高耐食性Ａｌ系めっき鋼板を提供するものである。
【０００８】
【課題を解決するための手段】
本発明者らは、Ａｌ系めっき層の耐食性と加工部及び端面耐食性に及ぼすめっき浴添加元素の効果を詳細に検討した結果、めっき層にＳｉ，Ｓｎ，Ｍｇ，Ｚｎ及びＣａを適正量添加することで、表面外観に優れ、かつめっき表面の耐食性を確保し、さらに加工部及び端面の赤錆発生を抑制することが出来ることを見出し、本発明を完成させた。
【０００９】
以下、本発明を詳細に説明する。
特開昭５６−１２７７６２号公報において、本出願人らは既にＳｉ，Ｍｇを含有するアルミめっき鋼板の製造法を開示している。本発明者らは、Ｓｉ，Ｍｇ，Ｚｎ及びＣａを添加したときのめっき組織、その時の耐食性等を更に詳細に検討し、以下の知見を得た。即ち、Ａｌめっき浴にＳｉ，Ｓｎ，Ｍｇ，Ｚｎ，Ｃａを複合添加することにより、Ｍｇ及びＺｎが大気環境、濡れ環境もしくは塩害環境において溶解し、地鉄の露出面あるいはめっき表面に緻密なＭｇ系もしくはＭｇ−Ｚｎ系皮膜を形成して防食するとともに、Ｚｎの犠牲防食作用が加味され、さらにＣａが溶解することで切断面のＦｅ表面のｐＨ値を上昇させ赤錆発生を抑制するという効果が得られる。これによって、飛躍的に加工部及び端面の耐食性が向上し、かつめっき層表面の耐食性も優れるという知見を得たものである。また、めっき層中にＭｇ₂Ｓｉ，Ｍｇ₂Ｓｎ，ＭｇＺｎ₂の金属間化合物を生成させることでさらに耐食性を向上させることができる。
【００１０】
この金属間化合物は塊状となり、腐食環境に曝されたときに優先的に溶解するためＭｇの供給力が優れている。ただし、ある大きさを超えると、金属間化合物は硬質であるため加工性が低下し、加工部からの腐食を速めてしまう可能性があるため注意が必要である。このＭｇ₂Ｓｉ，Ｍｇ₂Ｓｎ，ＭｇＺｎ₂を望ましい大きさに望ましい量だけ晶出させるには、冷却速度を制御することが有効である。例えばめっき後急冷することが好ましい。本発明に従えば、Ｓｉ，Ｓｎ，Ｍｇ，Ｚｎ，Ｃａを添加し、かつそれらの添加量を適正に制御することで飛躍的に耐食性が向上したＡｌ系めっき鋼板を得ることが可能となり、その優れた耐食性からめっき付着量の低減、あるいは用途によっては必要となる後処理皮膜、潤滑皮膜の簡易化が可能となり、溶接性、あるいは加工性への向上効果も増大する。
【００１１】
本発明の要旨とするところは以下の通りである。
（１）鋼板の表面に、質量％で、Ｍｇ：１〜１５％、Ｓｉ：２〜１５％、Ｚｎ：１〜１０％、Ｃａ：０．０２〜５％、Ｓｎ：１〜１５％を含有し、残部がＡｌ及び不可避的不純物からなるめっき層を有し、めっき層中に存在するＭｇ₂ Ｓｉ相、ＭｇＺｎ₂ 相、Ｍｇ ₂ Ｓｎ相の金属間化合物の大きさが長径１０μｍ未満であることを特徴とする外観に優れた高耐食性Ａｌ系めっき鋼板。
【００１３】
（２）めっき相と鋼板との界面に厚み５μｍ以下のＡｌ−Ｆｅ−Ｓｉ系合金層もしくはＡｌ−Ｆｅ−Ｓｉ−Ｍｇ系合金層を有することを特徴とする前記（１）に記載の外観に優れた高耐食性Ａｌ系めっき鋼板。
（３）めっき層の表面に後処理皮膜を有することを特徴とする前記（１）または（２）に記載の外観に優れた高耐食性Ａｌ系めっき鋼板。
【００１４】
（４）後処理皮膜上に脱膜型もしくは非脱膜型の潤滑皮膜を有することを特徴とする前記（１）〜（３）のいずれか１に記載の外観に優れた高耐食性Ａｌ系めっき鋼板。
（５）Ａｌ系めっき層の付着量が片面あたり２０〜２００ｇ／ｍ² であることを特徴とする前記（１）〜（４）のいずれか１に記載の外観に優れた高耐食性Ａｌ系めっき鋼板である。
【００１５】
次に、本発明の限定理由について説明する。まずＡｌ系被覆層（以降めっき層と略称）の限定理由を説明する。
めっき層はＳｉ：２〜１５％、Ｍｇ：１〜１５％、Ｚｎ：１〜１０％、Ｃａ：０．０２〜５％を含有し、さらに、Ｓｎ：１〜１５％を含み、残部Ａｌ及び不可避的不純物からなるものとする。Ｍｇ，Ｚｎ，Ｓｉ，Ｓｎを複合添加することでめっき層中にＭｇ₂ Ｓｉ、Ｍｇ₂ Ｓｎ、ＭｇＺｎ₂ などの金属間化合物が生成し耐食性を大きく向上させることができる。従って、Ｓｉ，Ｍｇ，Ｚｎ，Ｓｎが複合添加されることが望ましい。
【００１６】
前述したように、これら金属間化合物が使用環境下で溶解することによりＭｇもしくはＭｇ−Ｚｎ系の保護皮膜を形成し、めっき層自体と地鉄の露出した部分を防食する。この量が多いほど耐食性向上効果がある。ただし、この大きさが大きすぎると加工性に悪影響を及ぼし結果として加工部の耐食性を低下させるため、そのめっき層中での状態は分散した状態であっても塊状であってもよいが、その大きさを断面から観察したときの長径を１０μｍ未満とした。この大きさが小さければ小さいほど加工性は良好となる。金属間化合物の大きさは５度の断面傾斜研磨で組織を観察するものとする。
【００１７】
ＳｉはＡｌ系めっき鋼板においては通常合金層成長抑制の目的で添加されるが、本発明ではＭｇ₂Ｓｉ相の晶出に使用される。Ｓｉ量が少なすぎるとその晶出効果がなくなり耐食性の向上効果が発現されず、一方多すぎると粗大なＳｉ初晶が生成して耐食性、加工性を阻害する。従ってＳｉ添加量は２〜１５％に限定する。
【００１８】
Ｍｇの添加は１％以上で耐食性向上効果が発現し、５〜６％で最大の効果を有する。Ｍｇは極めて酸化しやすい元素であるが、溶融めっきの場合、アルミめっき浴中にこの程度の量を添加しても、特にドロスの発生が多くなることはない。但し、Ｍｇ添加量を増大していくと、徐々に浴の粘度が上昇していき、操業性が劣化するため、またその耐食性への効果も飽和する傾向があるため、Ｍｇの上限値を１５％とする。
【００１９】
Ｚｎの添加は１％以上で耐食性向上効果が発現するが、１０％超の添加はＺｎの白錆の発生量を増加させるとともに、製造条件によってはＭｇ₂Ｓｉ、ＭｇＺｎ₃などの金属間化合物が大きく成長しやすくなり加工性の低下が懸念される。したがって、Ｚｎ添加量は１〜１０％とする。
Ｃａは０．０２％以上の添加で特に溶融めっき法における外観のしわ発生抑制に効果があり、また耐食性にも効果を発揮するが、一方添加量が多すぎると浴温が上昇し、さらにめっき層の加工性が低下するため、上限を５％とする。これらの元素はめっき層中においてＳｉ，Ａｌとの化合物を造り得るが、その存在形態は特に限定しない。
【００２０】
Ｓｎは、Ｓｉと異なり合金層成長抑制効果を有さないため、Ｓｉの代替とはなり得ないが、Ｓｉと同様にＭｇと金属間化合物Ｍｇ₂ Ｓｎを形成して、耐食性を改善する効果を有するため、Ｓｉと共にめっき層に含有させる。特に、前記の粗大Ｓｉ晶の抑制のためＳｉ量を制御する場合において、耐食性に必要な金属間化合物を保護するのに有効となる。Ｓｎの含有量は１〜１５％が望ましく、下限は耐食性への効果、上限は浴温や粘性などの操業性への弊害を考慮して限定した。
【００２１】
めっき層には、これら元素のほかに、不可避的不純物としてＦｅを含有しうるが、この量は通常１％以下であり、めっき組織への影響も比較的少ない。更にめっき層中に、Ｃｒ，Ｍｎ，Ｔｉ等の元素を微量添加することも可能である。
本発明においてＡｌ系めっき鋼板の製造法については特に限定するものではなく、溶融めっき法、非水溶媒からの電気めっき法、蒸着法、クラッド法等が適用可能である。
【００２２】
現在最も工業的に普及しているのは溶融Ａｌ系めっき鋼板である。このときにはめっき層と地鉄の界面に金属間化合物からなる合金層が生成する。溶融法でＡｌ−Ｓｉ−Ｍｇ−Ｚｎ系めっき鋼板を製造すると合金層も当然生成するが、この時の合金層の組成はＡｌ−Ｓｉ−Ｆｅ系である。ただしＭｇ量が増大するとＡｌ−Ｆｅ−Ｓｉ−Ｍｇ系の金属間化合物の生成も観察される。合金層の厚みは５μｍ以下であることが望ましい。合金層は硬質で脆性であるため、厚いと鋼板の加工性を大きく阻害するためである。めっき浴にＭｇを添加することで合金層厚みの低減効果も得られ、２μｍ以下の合金層が可能となる。
【００２３】
次に母材の鋼成分について説明する。鋼成分の限定は特に行わず、どのような鋼種に対しても耐食性向上効果を有する。鋼種としては、Ｔｉ，Ｎｂ，Ｂ等を添加したＩＦ鋼、Ａｌ−ｋ鋼、Ｃｒ含有鋼、ステンレス鋼、ハイテンに加え、耐熱性を狙うＴｉ添加鋼、合金化抑制効果を狙うｆｒｅｅ−Ｎ添加鋼等を使用することも可能である。建材用途には、Ａｌ−ｋ系、あるいはステンレス系が、排気系用途には、Ｔｉ−ＩＦ、Ｔｉ添加鋼が、家電用途にはＡｌ−ｋ系、ｆｒｅｅ−Ｎ添加鋼系が、燃料タンク用途にはＢ添加ＩＦ鋼の適用がそれぞれ望ましい。
【００２４】
本発明においてめっきの後処理も特に限定しないが、クロメート、リン酸塩処理等の化成処理を施すことが可能で、樹脂を含有する後処理であっても良い。化成処理としてはリン酸、シリカ等を含有することが可能で、Ｍｇ系の化合物を添加しても良い。樹脂種としては、例えばアクリル系、ポリエチレン系，ポリエステル系，メラミン系，エポキシ系，ウレタン系，フッ素系等、汎用の樹脂を含む処理は全て可能である。最近ではＣｒを使用しない後処理が種々開発されつつあるが、これらを適用することも当然可能である。
【００２５】
用途によって、この処理のさらに表層に有機樹脂で被覆しても良い。その有機樹脂としては、アルミの外観を活かしたクリア処理、あるいは顔料を含有するカラー処理、あるいは溶接性を向上させるための処理等非脱膜型の樹脂皮膜、もしくは脱膜型の樹脂皮膜があり、樹脂系もアクリル系，ポリエチレン系，ポリエステル系，メラミン系，エポキシ系，ウレタン系，フッ素系等、汎用の処理は全て可能である。膜厚も特に限定するものではなく、通常の０．５〜２０μｍ程度の処理が可能である。めっき後の後処理として、これ以外に、ゼロスパングル処理、焼鈍、調質圧延等が付与されることがあるが、これらについても特に限定せず、適用も可能である。
【００２６】
最後にめっきの付着量の限定理由を説明する。本発明ではＡｌ系被覆層と金属間化合物層の合計被覆量（以降めっき付着量と称する）を、片面当たり２０〜２００ｇ／ｍ² とすることが望ましい。建材用途では通常めっき層の寿命も端面加工部からの赤錆発生も付着量増大により抑制する傾向がある。本発明は従来に増す耐食性、端面、加工部耐食性を実現するもので従来より低い目付けでも十分な性能を発揮するが当然付着量が多いほど耐食性向上効果が得られる。片面２０ｇ／ｍ² 未満では長期の耐久性という意味でやや不安があり、また付着量が多すぎると加工性を損なう懸念があるため、好ましい付着量として片面あたり２０〜２００ｇ／ｍ² とする。
【００２７】
次に実施例で本発明をより詳細に説明する。
【実施例】
表１に示す成分の鋼を通常の転炉−真空脱ガス処理により溶製し、鋼片とした後、通常の条件で熱間圧延、冷延工程を行い、冷延鋼板（板厚０．８ｍｍ）を得た。これを材料として溶融Ａｌ系めっき鋼板を得た。溶融Ａｌめっきは無酸化炉−還元炉タイプのラインを使用し、焼鈍もこのライン内で行った。焼鈍温度は８００〜８５０℃とした。めっき浴組成としては、Ａｌ−Ｓｉ−Ｓｎ−Ｍｇ−Ｚｎ−Ｃａ系についても検討した。
【００２８】
この組成を様々に変化させ、また侵入板温、めっき後の冷却速度を制御して、合金層の厚みは低めを狙って製造した。浴温は融点＋６０℃とした。めっき後ガスワイピング法でめっき付着量を調節した。めっき外観は不めっき等なく良好であった。また後処理としてシランカップリング剤系のノンクロメート皮膜をＳｉＯ₂換算で片面あたり１００ｍｇ／ｍ² 処理し、さらに０．５％で調質圧延した。このようにして製造した溶融Ａｌ系めっき鋼板の性能を以下に示す方法で評価した。
【００２９】
【表１】

【００３０】
（１）めっき層組成，合金層厚み分析方法
▲１▼めっき層組成：３％ＮａＯＨ＋１％ＡｌＣｌ₃・６Ｈ₂Ｏ中で定電流電解剥離によりめっき層のみを剥離した。Ｍｇはアルカリ溶液に不溶であるため、電解剥離後、更に２０％硝酸で処理して、電解剥離液と混合してめっき層組成分析液とした。各元素の分析はＩＣＰで行った。なお、クロメート処理した後分析する場合には、表面を軽研磨してクロメート中のＣｒの影響を少なくする必要がある。
▲２▼めっき層組織：めっき層断面の５度傾斜研磨を行い、光学顕微鏡によるめっき組織観察（２００〜５００倍）を行った。任意の５箇所について、めっき１ｍｍ幅視野中でのめっき層中に存在する金属間化合物の最大の長径を測定した。
▲３▼合金層厚み：４００倍の断面顕微鏡写真より合金層厚みを測定した。
【００３１】
（２）外観
目視でしわの発生を評価した。
〔評価基準〕
○：しわ発生無し
×：しわ発生有り
【００３２】
（３）耐食性評価
▲１▼塩害耐食性
寸法７０×１５０ｍｍの試料に対してＪＩＳＺ２３７１に準拠した塩水噴霧試験を３０日行い、腐食生成物を剥離して腐食減量を測定した。この腐食減量の表示はめっき片面に対しての値である。
〔評価基準〕
◎：腐食減量５ｇ／ｍ² 以下
○：腐食減量１０ｇ／ｍ² 未満
△：腐食減量１０〜２５ｇ／ｍ²
×：腐食減量２５ｇ／ｍ² 超
【００３３】
▲２▼加工後耐食性
ロールフォーマーにより、３ｔ（ｔ：板厚）相当の曲げ加工を行った後、ＪＩＳＺ２３７１に準拠した塩水噴霧試験を３０日間行い、加工部付近の腐食状況を観察した。
〔評価基準〕
◎：赤錆発生無し
○：赤錆発生率５％以下
△：赤錆発生率５〜３０％
×：赤錆発生率３０％超
【００３４】
▲３▼屋外暴露試験
寸法５０×２００ｍｍの試料を南面３０度にて屋外暴露試験を行った。３ヶ月経過後の端面からの赤錆発生率、表面の変色状況を観察した。
〔評価基準〕
○：端面からの赤錆発生率３０％未満
△：端面からの赤錆発生率３０〜８０％
×：端面からの赤錆発生率８０％超
【００３５】
（４）加工性
油圧成形試験機により、直径５０ｍｍの円筒ポンチを用いて、絞り比２．２５でカップ成形を行った。試験は塗油して行い、しわ押え圧は５００ｋｇとした。加工性の評価は次の指標によった。
〔評価基準〕
○：異常無し
△：めっきに亀裂有り
×：めっき剥離有り
【００３６】
【表２】

【００３７】
【表３】

【００３８】
表２および表３に示すように、比較例であるＮｏ．４のように、Ｍｇを含有しない場合には、厳しい環境下では耐食性に劣る傾向にある。また、比較例であるＮｏ．４，５，７のようにＣａを含まない場合は、しわが発生し外観が損なわれる傾向であるし、端面からの赤錆も発生しやすい。また、比較例であるＮｏ．５のようにＳｉ量が少ないと合金層が成長して加工性に劣る。また、比較例であるＮｏ．６のように、Ｚｎを含有しない場合には端面からの赤錆発生が生じるし、Ｃａ添加量が多く加工性が低下している。比較例であるＮｏ．７ではＺｎの量が多すぎてもＺｎの溶解から塗膜膨れを助長し耐食性が低下する傾向である。
【００３９】
また、本発明例Ｎｏ．１〜３は、特にめっき層組成にＳｎの含有する場合である。また、比較例であるＮｏ．６のように金属間化合物の大きさが長径１０μｍ以上となる場合には、加工性及び加工後耐食性が低下する。Ｍｇ，Ｓｉ，Ｚｎ，Ｃａ量が適正域にあると、極めて優れた耐食性、加工性を示す。
【００４３】
【発明の効果】
本発明は、従来に比べて飛躍的にめっき層の耐食性及び端面の耐食性に優れる溶融アルミめっき鋼板を提供するものである。アルミめっき鋼板の用途は従来の自動車排気系部材、熱器具、屋根壁等から、最近では自動車燃料タンクへの適用も始まっており、これらの用途における耐食性を向上させることでより材料の長寿命化、メンテナンスフリー化が期待され、産業上の寄与は大きい。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a highly corrosion-resistant Al-based plated steel sheet excellent in appearance that is used in metal building materials such as roof walls, automobile exhaust system members, gasoline tank materials, toasters, stoves, and other household heat appliances.
[0002]
[Prior art]
Al-based plated steel sheets are used for automobile parts, building materials, home appliance parts, etc. because of their high corrosion resistance and heat resistance, beautiful appearance, and the like. In order to meet the recent demands for improving the corrosion resistance of automobile exhaust system members, many steel plates containing Cr are applied to the plating base plate, or aluminum is plated on stainless steel to provide high corrosion resistance (Japanese Patent Laid-Open No. 61). No. -231152, JP-A-3-277761, etc.). When Cr-containing steel or stainless steel is used as the original plate, the corrosion resistance is naturally improved, but the manufacturing cost is increased, and the workability tends to deteriorate. Accordingly, various studies have been made to add an element for improving the corrosion resistance to the plating bath, and JP-A-2-88754, JP-A-7-20091, etc. disclose the addition of Cr and Mn. However, in these, it does not come to completely suppressing generation | occurrence | production of red rust in a severe bending process part or an end surface part.
[0003]
On the other hand, recently, Pb-free automobile fuel tanks are being studied, and the application of aluminum-plated steel sheets for this application is being advanced. The problem at this time is a high balance between corrosion resistance, workability, and weldability. In general, in a surface-treated steel sheet, the corrosion resistance naturally improves as the amount of plating increases, but the workability and weldability tend to decrease. The weldability in this case means continuous workability. Since Al easily reacts with Cu as the electrode material, increasing the amount of adhesion increases the amount of reaction with the electrode, leading to a decrease in electrode life. Also in press working, an increase in the amount of adhesion tends to cause damage or peeling of the plating layer.
[0004]
Therefore, many inventions have been made to achieve both of these characteristics. However, it is difficult to say that the corrosion resistance, weldability, and workability are completely compatible. In addition, aluminum plating is extremely excellent in corrosion resistance especially in a dry and wet repeated environment, but dissolution tends to proceed in an always wet environment. In particular, in the side wall portion that is severely processed and the flange end surface portion that is hard to be painted, the plating may be preferentially dissolved by sacrificing the steel plate, and red rust may be generated in a short period of time.
[0005]
In addition, as a surface-treated steel sheet for building materials, a Zn-plated steel sheet has been mainly used, but recently, due to further demands for improved corrosion resistance and design, Al-Zn-based plated steel sheet, or Al-Si-based plating. The amount of steel sheets used is increasing. In the Al-Zn alloy-plated steel sheet, the corrosion resistance becomes better as the Al content increases, but the sacrificial anti-corrosion action tends to be weakened. After several months from use, red rust is generated from the end face or severely bent parts. . Furthermore, the aluminum-plated steel sheet with excellent corrosion resistance of the plating layer has almost no sacrificial anticorrosive action of the base iron in a normal atmospheric environment, and red rust is generated on the end face in a few days, so renovation coating after construction is necessary The work becomes complicated. Alternatively, if the repair coating is insufficient, there is a problem that red rust is generated from the end face and the appearance is impaired.
[0006]
In order to cope with this problem of the aluminum-plated steel sheet, the present inventors have disclosed a technique for suppressing the occurrence of red rust from the processed part in Japanese Patent Laid-Open No. 6-330274. However, although this can suppress the occurrence of red rust from the processed part, there is a drawback that the occurrence of red rust from the end face cannot be suppressed. For the same purpose, JP-A-11-279734 and JP-A-11-279735 disclose Al-Si-Mg or Al-Si-Mg-Zn-based plating technology. Although it is a technique that requires the addition of Be and Sr in order to ensure the appearance, Be is toxic and has environmental problems, and Sr has a drawback that the effect of suppressing the generation of wrinkles is insufficient.
[0007]
[Problems to be solved by the invention]
Therefore, in the present invention, by adding Mg, Si, Sn, Zn, Ca that contributes to corrosion resistance to the aluminum plating layer, the corrosion resistance of the plating layer and the end face is remarkably improved compared with the conventional one, and the surface appearance is excellent. The present invention provides a highly corrosion-resistant Al-based plated steel sheet that has excellent workability and post-processing corrosion resistance, and thus has an excellent appearance that can be applied as various materials.
[0008]
[Means for Solving the Problems]
As a result of examining the effect of the plating bath additive element on the corrosion resistance of the Al-based plating layer and the processed part and end face corrosion resistance in detail, the present inventors added appropriate amounts of Si, Sn, Mg, Zn and Ca to the plating layer. Thus, the present inventors have found that the surface appearance is excellent, the corrosion resistance of the plating surface is ensured, and the occurrence of red rust on the processed part and the end face can be suppressed, and the present invention has been completed.
[0009]
Hereinafter, the present invention will be described in detail.
In Japanese Patent Application Laid-Open No. 56-127762, the present applicants have already disclosed a method for producing an aluminized steel sheet containing Si and Mg. The inventors of the present invention examined in further detail the plating structure when Si, Mg, Zn and Ca were added, the corrosion resistance at that time, and obtained the following knowledge. That is, by adding Si, Sn, Mg, Zn, and Ca to the Al plating bath, Mg and Zn are dissolved in an air environment, a wet environment, or a salt damage environment, and a dense Mg is formed on the exposed surface of the base metal or the plating surface. In addition to forming a Zn-based or Mg—Zn-based film, the sacrificial anticorrosive action of Zn is taken into account, and further, Ca dissolves to increase the pH value of the Fe surface of the cut surface, thereby suppressing the occurrence of red rust. can get. As a result, the inventors have obtained knowledge that the corrosion resistance of the processed part and the end face is dramatically improved and the corrosion resistance of the plating layer surface is also excellent. Further, the corrosion resistance can be further improved by generating an intermetallic compound of Mg ₂ Si, Mg ₂ Sn, MgZn _{2 in} the plating layer.
[0010]
This intermetallic compound is agglomerated and preferentially dissolves when exposed to a corrosive environment, so that the supply power of Mg is excellent. However, if the size exceeds a certain size, the intermetallic compound is hard, so the workability is lowered and the corrosion from the processed part may be accelerated, so care must be taken. In order to crystallize the Mg ₂ Si, Mg ₂ Sn, MgZn ₂ to a desired size and a desired amount, it is effective to control the cooling rate. For example, it is preferable to rapidly cool after plating. According to the present invention, it is possible to obtain an Al-based plated steel sheet having drastically improved corrosion resistance by adding Si, Sn, Mg, Zn, and Ca, and appropriately controlling the amount of addition. Due to its excellent corrosion resistance, it is possible to reduce the amount of plating deposited, or to simplify the post-treatment film and lubrication film that are required depending on the application, and the effect of improving weldability or workability is also increased.
[0011]
The gist of the present invention is as follows.
(1) On the surface of the steel sheet, by mass, Mg: 1 to 15%, Si: 2 to 15%, Zn: 1 to 10%, Ca: 0.02 to 5%, Sn: 1 to 15% And the balance has a plating layer made of Al and inevitable impurities, and the size of the intermetallic compound of Mg ₂ Si phase, MgZn ₂ phase , Mg ₂ Sn phase existing in the plating layer is less than 10 μm in major axis High corrosion-resistant Al-based plated steel sheet with excellent appearance characterized by
[0013]
(2) the appearance according to (1), characterized by having a plating phase and the steel sheet and the interface to the following Al-Fe-Si alloy layer thickness 5μm or Al-Fe-Si-Mg-based alloy layer Excellent high corrosion resistance Al plated steel sheet.
( 3 ) The highly corrosion-resistant Al-based plated steel sheet having excellent appearance according to the above (1) or (2) , which has a post-treatment film on the surface of the plating layer.
[0014]
( 4 ) A highly corrosion-resistant Al-based plating excellent in appearance according to any one of (1) to (3) above, wherein the post-treatment film has a film removal type or non-film removal type lubricating film. steel sheet.
( 5 ) The high corrosion resistance Al-based plating excellent in appearance according to any one of (1) to (4), wherein the adhesion amount of the Al-based plating layer is ²⁰ to 200 g / m ² per side. It is a steel plate.
[0015]
Next, the reason for limitation of the present invention will be described. First, the reason for limitation of the Al-based coating layer (hereinafter abbreviated as plating layer) will be described.
Plating layer Si: 2~15%, Mg: 1~15 %, Zn: 1~10%, Ca: contains 0.02 to 5%, in further, Sn: comprises 1% to 15%, the balance Al And inevitable impurities. By compositely adding Mg, Zn, Si, and Sn, intermetallic compounds such as Mg ₂ Si, Mg ₂ Sn, and MgZn ₂ are generated in the plating layer, and the corrosion resistance can be greatly improved. Therefore, it is desirable to add Si, Mg, Zn, and Sn in combination.
[0016]
As described above, when these intermetallic compounds are dissolved in the use environment, an Mg or Mg—Zn-based protective film is formed, and the plating layer itself and the exposed portion of the ground iron are anticorrosive. The larger the amount, the more effective the corrosion resistance. However, if this size is too large, the workability is adversely affected and as a result the corrosion resistance of the processed part is reduced, so the state in the plating layer may be in a dispersed state or a lump, The major axis when the size was observed from the cross section was set to less than 10 μm. The smaller this size, the better the workability. As for the size of the intermetallic compound, it is assumed that the structure is observed by 5 ° cross-sectional inclined polishing.
[0017]
Si is usually added in an Al-based plated steel sheet for the purpose of suppressing the growth of the alloy layer. In the present invention, Si is used for crystallization of the Mg ₂ Si phase. If the amount of Si is too small, the crystallization effect is lost and the effect of improving the corrosion resistance is not manifested. On the other hand, if the amount is too large, coarse Si primary crystals are formed and the corrosion resistance and workability are impaired. Therefore, the Si addition amount is limited to 2 to 15%.
[0018]
Addition of Mg exhibits a corrosion resistance improving effect at 1% or more, and has a maximum effect at 5 to 6%. Mg is an element that is very easily oxidized. However, in the case of hot dipping, even when such an amount is added to the aluminum plating bath, dross generation is not particularly increased. However, as the amount of Mg added is increased, the viscosity of the bath gradually increases, the operability deteriorates, and the effect on the corrosion resistance tends to be saturated. %.
[0019]
Addition of Zn exhibits an effect of improving corrosion resistance at 1% or more, but addition of more than 10% increases the amount of Zn white rust generated, and depending on the production conditions, intermetallic compounds such as Mg ₂ Si and MgZn ₃ may be added. It is easy to grow large, and there is a concern that workability will be reduced. Therefore, the Zn addition amount is 1 to 10%.
When Ca is added in an amount of 0.02% or more, it is effective in suppressing the appearance of wrinkles particularly in the hot dipping process, and is effective in corrosion resistance. On the other hand, if the added amount is too large, the bath temperature increases, and further plating is performed. Since the workability of the layer is lowered, the upper limit is made 5%. Although these elements can form a compound with Si and Al in the plating layer, the existence form is not particularly limited.
[0020]
Sn, unlike Si, does not have an effect of suppressing the growth of the alloy layer, so it cannot be substituted for Si. However, as with Si, Mg and the intermetallic compound Mg ₂ Sn are formed to improve the corrosion resistance. order to have, Ru is contained in the plating layer with Si. In particular, when the amount of Si is controlled to suppress the coarse Si crystal, it is effective to protect intermetallic compounds necessary for corrosion resistance. The Sn content is desirably 1 to 15%, the lower limit is limited in consideration of the effect on corrosion resistance, and the upper limit is limited in consideration of adverse effects on operability such as bath temperature and viscosity.
[0021]
In addition to these elements, the plating layer may contain Fe as an inevitable impurity, but this amount is usually 1% or less, and the influence on the plating structure is relatively small. Furthermore, a trace amount of elements such as Cr, Mn and Ti can be added to the plating layer.
In the present invention, the production method of the Al-based plated steel sheet is not particularly limited, and a hot dipping method, an electroplating method from a non-aqueous solvent, a vapor deposition method, a cladding method, and the like are applicable.
[0022]
At present, the most industrially widespread are hot-dip Al-based plated steel sheets. At this time, an alloy layer made of an intermetallic compound is formed at the interface between the plating layer and the ground iron. When an Al—Si—Mg—Zn-based plated steel sheet is produced by the melting method, an alloy layer is naturally formed. At this time, the composition of the alloy layer is Al—Si—Fe. However, when the amount of Mg increases, the formation of Al—Fe—Si—Mg intermetallic compounds is also observed. The thickness of the alloy layer is desirably 5 μm or less. This is because the alloy layer is hard and brittle, and if it is thick, the workability of the steel sheet is greatly hindered. By adding Mg to the plating bath, an effect of reducing the thickness of the alloy layer is also obtained, and an alloy layer of 2 μm or less becomes possible.
[0023]
Next, the steel component of the base material will be described. The steel component is not particularly limited, and any steel type has an effect of improving corrosion resistance. Steel types include IF steel added with Ti, Nb, B, etc., Al-k steel, Cr-containing steel, stainless steel, high tensile steel, Ti-added steel for heat resistance, and free-N addition for alloying suppression effect It is also possible to use steel or the like. For building materials, Al-k or stainless steel is used. For exhaust systems, Ti-IF and Ti-added steel are used. For household appliances, Al-k and free-N-added steel are used for fuel tanks. For each, application of B-added IF steel is desirable.
[0024]
In the present invention, post-treatment of plating is not particularly limited, but chemical conversion treatment such as chromate treatment and phosphate treatment can be performed, and post-treatment containing resin may be used. As the chemical conversion treatment, phosphoric acid, silica and the like can be contained, and an Mg-based compound may be added. As the resin type, for example, all treatments including general-purpose resins such as acrylic, polyethylene, polyester, melamine, epoxy, urethane, and fluorine are possible. Recently, various post-treatments not using Cr have been developed, but it is naturally possible to apply them.
[0025]
Depending on the application, the surface layer of this treatment may be coated with an organic resin. As the organic resin, there is a non-delaminating type resin film such as a clear process utilizing the appearance of aluminum, a color process containing a pigment, or a process for improving weldability, or a delaminating type resin film. As for the resin system, general-purpose treatments such as acrylic system, polyethylene system, polyester system, melamine system, epoxy system, urethane system, and fluorine system are all possible. The film thickness is not particularly limited, and a normal treatment of about 0.5 to 20 μm is possible. Other post-plating treatments such as zero spangle treatment, annealing, temper rolling, etc. may be applied, but these are not particularly limited and can be applied.
[0026]
Finally, the reason for limiting the amount of plating applied will be described. In the present invention, the total coating amount of the Al-based coating layer and the intermetallic compound layer (hereinafter referred to as plating adhesion amount) is preferably ²⁰ to 200 g / m ² per side. In building materials, there is a tendency to suppress both the life of the plating layer and the occurrence of red rust from the end surface processed part by increasing the amount of adhesion. The present invention realizes the corrosion resistance, end face, and processed portion corrosion resistance which are increased conventionally, and exhibits sufficient performance even with a lower basis weight than the conventional one. Naturally, as the amount of adhesion increases, the corrosion resistance improvement effect is obtained. If it is less than 20 g / m ^{2 on} one side, there is a little anxiety in the sense of long-term durability, and if there is too much adhesion, there is a concern that workability will be impaired, so the preferred adhesion amount is ²⁰ to 200 g / m ² per side.
[0027]
Next, the present invention will be described in more detail with reference to examples.
【Example】
Steels having the components shown in Table 1 were melted by a normal converter-vacuum degassing treatment to form a steel piece, and then subjected to hot rolling and a cold rolling process under normal conditions to obtain a cold rolled steel sheet (plate thickness of 0. 8 mm). Using this as a material, a molten Al-based plated steel sheet was obtained. For the hot-dip Al plating, a non-oxidation furnace-reduction furnace type line was used, and annealing was also performed in this line. The annealing temperature was 800 to 850 ° C. As the plating bath composition , Al-Si-Sn-Mg-Zn-Ca system was also examined.
[0028]
The composition was changed in various ways, and the intrusion plate temperature and the cooling rate after plating were controlled to produce a thin alloy layer. The bath temperature was a melting point + 60 ° C. The plating adhesion amount was adjusted by gas wiping after plating. The plating appearance was good with no plating. Further, as a post-treatment, a silane coupling agent-based non-chromate film was treated with 100 mg / m ² per side in terms of SiO ₂ and further temper-rolled at 0.5%. The performance of the hot-dip Al-based plated steel sheet produced in this way was evaluated by the following method.
[0029]
[Table 1]

[0030]
(1) Plating layer composition, alloy layer thickness analysis method (1) Plating layer composition: Only the plating layer was peeled off by constant current electrolytic peeling in 3% NaOH + 1% AlCl ₃ .6H ₂ O. Since Mg is insoluble in an alkaline solution, after electrolytic stripping, it was further treated with 20% nitric acid and mixed with an electrolytic stripper solution to obtain a plating layer composition analysis solution. Each element was analyzed by ICP. When analyzing after chromate treatment, it is necessary to lightly polish the surface to reduce the influence of Cr in chromate.
{Circle around (2)} Plating layer structure: The plating layer cross section was subjected to 5 ° inclined polishing, and the plating structure was observed with an optical microscope (200 to 500 times). The maximum major axis of the intermetallic compound existing in the plating layer in the 1 mm wide visual field was measured at any five locations.
(3) Alloy layer thickness: The alloy layer thickness was measured from a cross-sectional micrograph of 400 times.
[0031]
(2) Appearance Visual evaluation of the occurrence of wrinkles.
〔Evaluation criteria〕
○: No wrinkle occurrence ×: Wrinkle occurrence [0032]
(3) Corrosion resistance evaluation {circle around (1)} Salt damage corrosion resistance A 70 × 150 mm sample was subjected to a salt spray test in accordance with JIS Z 2371 for 30 days, and the corrosion products were peeled to measure the corrosion weight loss. This indication of corrosion weight loss is a value for the plated surface.
〔Evaluation criteria〕
◎: Corrosion weight loss 5 g / m ² or less ○: Corrosion weight loss less than 10 g / m ² Δ: Corrosion weight loss 10-25 g / m ²
×: Corrosion loss more than 25 g / m ²
(2) After processing After bending processing corresponding to 3t (t: plate thickness) with a corrosion-resistant roll former, a salt spray test in accordance with JIS Z 2371 was performed for 30 days, and the corrosion state near the processed portion was observed. .
〔Evaluation criteria〕
◎: No red rust occurrence ○: Red rust occurrence rate 5% or less △: Red rust occurrence rate 5-30%
×: Red rust occurrence rate is over 30%.
(3) Outdoor exposure test A sample having a size of 50 × 200 mm was subjected to an outdoor exposure test at 30 degrees south. The incidence of red rust from the end face after 3 months and the discoloration of the surface were observed.
〔Evaluation criteria〕
○: Red rust occurrence rate from end face is less than 30% △: Red rust occurrence rate from end face is 30-80%
×: Red rust generation rate from the end face is over 80%.
(4) Workability Cup molding was performed with a drawing ratio of 2.25 using a cylindrical punch with a diameter of 50 mm by a hydraulic molding tester. The test was performed by applying oil, and the wrinkle presser pressure was 500 kg. The evaluation of workability was based on the following index.
〔Evaluation criteria〕
○: No abnormality △: There is a crack in the plating ×: There is plating peeling [0036]
[Table 2]

[0037]
[Table 3]

[0038]
As shown in Tables 2 and 3, No. When Mg is not contained as in No. 4 , the corrosion resistance tends to be inferior in a severe environment. Moreover, No. which is a comparative example. When Ca is not included as in 4 , 5 , and 7 , wrinkles are generated and the appearance is liable to be deteriorated, and red rust is easily generated from the end face. Moreover, No. which is a comparative example. When the amount of Si is small as in 5, the alloy layer grows and the workability is inferior. Moreover, No. which is a comparative example. As shown in FIG. 6 , when Zn is not contained, red rust is generated from the end face, and the Ca addition amount is large and the workability is lowered. No. which is a comparative example. In No. 7 , even if the amount of Zn is too large, the coating film bulges from dissolution of Zn, and the corrosion resistance tends to decrease.
[0039]
In addition, Invention Example No. 1 to 3 are particularly cases where Sn is contained in the plating layer composition. Moreover, No. which is a comparative example. When the size of the intermetallic compound is 6 μm or more as in 6 , workability and post-processing corrosion resistance are reduced. When the amount of Mg, Si, Zn, and Ca is in an appropriate range, extremely excellent corrosion resistance and workability are exhibited.
[0043]
【The invention's effect】
The present invention provides a hot dip galvanized steel sheet that is remarkably superior in corrosion resistance of a plating layer and corrosion resistance of an end face as compared with the prior art. The use of aluminized steel sheets has been applied to automobile fuel tanks from the conventional automobile exhaust system members, heat appliances, roof walls, etc., and the life of the materials has been extended by improving the corrosion resistance in these applications. Therefore, maintenance-free operation is expected, and the industrial contribution is significant.

Claims

On the surface of the steel sheet,
Mg: 1 to 15%
Si: 2 to 15%,
Zn: 1 to 10%,
Ca: 0.02 to 5%,
Sn: 1 to 15%
The balance is Al and an inevitable impurity plating layer, and the size of the Mg ₂ Si phase, MgZn ₂ phase , Mg ₂ Sn phase intermetallic compound existing in the plating layer is less than 10 μm in major axis A highly corrosion-resistant Al-based plated steel sheet with an excellent appearance characterized by being.

The high corrosion resistance excellent in appearance according to claim 1, comprising an Al—Fe—Si based alloy layer or an Al—Fe—Si—Mg based alloy layer having a thickness of 5 μm or less at the interface between the plating phase and the steel plate. Al-based plated steel sheet.

The high corrosion-resistant Al-based plated steel sheet having excellent appearance according to claim 1 or 2 , wherein the surface of the plating layer has a post-treatment film.

The highly corrosion-resistant Al-based plated steel sheet having excellent appearance according to any one of claims 1 to 3, which has a film removal type or non-film removal type lubricating film on the post-treatment film.

The highly corrosion-resistant Al-based plated steel sheet having excellent appearance according to any one of claims 1 to 4, wherein an adhesion amount of the Al-based plated layer is ²⁰ to 200 g / m ² per side.