JP4555499B2

JP4555499B2 - Hot-dip Zn-Al-Mg-Si plated steel with excellent surface properties and method for producing the same

Info

Publication number: JP4555499B2
Application number: JP2001111342A
Authority: JP
Inventors: 曉田中; 義広末宗; 高橋　　彰; 久芳小松; 豪三宅; 元小野澤; 卓穂池田; 康秀森本; 和彦本田
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2000-04-11
Filing date: 2001-04-10
Publication date: 2010-09-29
Anticipated expiration: 2021-04-10
Also published as: JP2001355053A

Description

【０００１】
【発明の属する技術分野】
本発明は、表面性状に優れた溶融Ｚｎ−Ａｌ−Ｍｇ−Ｓｉめっき鋼材とその製造方法に関するものである。
【０００２】
【従来の技術】
近年、自動車、家庭電気製品、建材等の耐用年数の長期化に対応するため、表面処理鋼材の使用が拡大している。特にＺｎ−５質量％Ａｌ溶融めっき鋼材は、今までの溶融亜鉛めっきに比較して耐食性が優れていることから、建材などを中心に使用されている。またＺｎにＡｌやＭｇを添加する溶融めっきについては、米国特許第３５０５０４３号公報で、Ａｌ：３〜１７％、Ｍｇ：１〜５％、残部Ｚｎからなる溶融めっき浴を用いた高耐食性溶融Ｚｎ−Ａｌ−Ｍｇめっき鋼板が提案されて以来、様々な技術が提案されてきた。たとえば、特開平８−６０３２４号公報では、Ａｌが最大０．２５％、Ｍｇが最大３％である。また、特開平９−１４３６５９号公報では、Ｍｇ：０．０５〜３％、Ａｌ：０．１〜１％、Ｎｉ：０．０１〜０．２％を含有している。これらは、確かに耐食性向上効果はあるものの、Ｍｇ含有量が３％近くになると、浴上に酸化物（ドロス）が厚く堆積し、工業生産に不向きである。最近では、特開平１０−２２６８６５号公報のように、Ａｌ：４〜１０％、Ｍｇ:１〜４％添加しためっき鋼板や、特願平１１−１７９９１３号公報のようにＺｎ−Ａｌ−Ｍｇ系めっき鋼板に更にＳｉを添加させた溶融めっき鋼板が提案されている。
【０００３】
ところが、Ｚｎ−Ａｌ−Ｍｇ系めっきでは数ｍｍ〜１０ｍｍ程度の点状の外観模様が発生することがあり、商品上問題になる場合がある。この解決方案としては、たとえば前述した特開平１０−２２６８６５号公報では、Ｍｇの存在形態が重要であるとし、製造時めっき浴温を４５０℃以下ないし、４７０℃未満とし、冷却速度を１０℃／ｓ以上と速くするか或いは浴温を４７０℃以上とし冷却速度を０．５℃／ｓ以上に制御することにより、これを回避できるとしている。更に、特開平１０−３０６３５７号公報では、めっきにＴｉ、Ｂを添加することにより、製造条件の制約が緩和されるとしている。ところが、われわれの調査によれば、これらの技術は、▲１▼Ｚｎ−Ａｌ−Ｍｇ−Ｓｉめっきの点状の表面不良を回避することが出来ないこと、▲２▼浴温はめっき釜、めっき機器の溶食の問題があり、４７０℃以上に、上げることが困難であること、▲３▼ＴｉやＢの添加はドロス生成を招き、表面性状が悪化する等、数々の致命的な欠点があることが明らかとなった。
【０００４】
【発明が解決しようとする課題】
このような状況に鑑み、表面性状を改善した溶融Ｚｎ−Ａｌ−Ｍｇ−Ｓｉめっき鋼材を提供することが本発明の目的である。
【０００５】
【課題を解決するための手段】
本発明者等は、先の検討から、Ｚｎ−Ａｌ−ＭｇめっきとＺｎ−Ａｌ−Ｍｇ−Ｓｉめっきの点状欠陥の生成メカニズムが異なることが推察されることから、Ｚｎ−Ａｌ−Ｍｇ−Ｓｉめっきの表面性状について鋭意検討した。そして、斑点状欠陥が、従来知見と異なり、冷却速度を上げると顕著に発生することを突き止めた。この斑点部分は、大きさは約１．５ｍｍ程度、周囲に比して、黒っぽい平滑な外観を呈している。この斑点部の表面分析を光顕、電子線プローブマイクロアナリシス装置（ＣＭＡ）などで実施したところ、斑点部ではＡｌ相の樹枝状構造が表面に突起状に突き出しておらず、一方、点状欠陥のない正常部では、Ａｌ相の樹枝状構造が表面に突起状に突き出しており、点状欠陥は、Ａｌ相の析出形態の違いであることを見いだし本発明を完成するに至った。ここで、Ａｌ相とは、めっき浴中から最初に凝固析出を開始するＡｌリッチ相と定義される。
【０００６】
そして、このＡｌ相の析出を制御することを指向し、めっき浴温度や冷却速度と、点状欠陥との関係を調査した。その結果、▲１▼従来知見と異なりめっき浴温は点状欠陥生成に影響しないこと、▲２▼めっき後の冷却速度が非常に重要であることが判った。特に後者については、凝固終了温度近傍の温度範囲の冷却速度が斑点の生成に非常に重要であり、この範囲の冷却速度を小さくし、Ａｌ相の析出を制御することで、点状欠陥生成を回避できることを知見した。このメカニズムは未だ明らかではないが、冷却速度が大きいと局所的に過冷却状態が生じ、Ａｌの析出挙動が変化し、過冷部ではＡｌ相の成長が遅れ、表面へのＡｌ相の突起が少なくなり、斑点が生じるものと推察される。ここで、Ａｌ相とは、めっき浴中から最初に凝固析出を開始するＡｌリッチ相と定義される。本発明は、このような種々の新規知見に基づき完成されたものであり、要旨とするところは、以下に示す通りである。
【０００７】
（１）鋼材の表面に、Ａｌ：５〜１８質量％、Ｍｇ：１〜１０質量％、Ｓｉ：０．０１〜２質量％、残部Ｚｎ及び不可避的不純物とからなるめっき層を有するめっき鋼材表面に、Ａｌ相が１ｍｍ²当たり２００個以上存在することを特徴とする表面性状に優れた溶融Ｚｎ−Ａｌ−Ｍｇ−Ｓｉめっき鋼材。
【０００８】
（２）上記（１）に記載のめっき鋼材のめっき層中に、更にＦｅ：１質量％以下含有することを特徴とする表面性状に優れた溶融Ｚｎ−Ａｌ−Ｍｇ−Ｓｉめっき鋼材。
【０００９】
（３）上記（１）または（２）に記載のめっき鋼材のめっき層中に、更にＳｎ：０．１〜２質量％含有することを特徴とする表面性状に優れた溶融Ｚｎ−Ａｌ−Ｍｇ−Ｓｉめっき鋼材。
【００１０】
（４）上記（１）乃至（３）のいずれかに記載のめっき鋼材のめっき層上に更に、無機酸化物皮膜を７０ｍｇ／ｍ²〜２ｇ／ｍ²有することを特徴とする表面性状に優れた溶融Ｚｎ−Ａｌ−Ｍｇ−Ｓｉめっき鋼材。
【００１１】
（５）上記（１）乃至（３）のいずれかに記載のめっき鋼材のめっき層上に更に、有機樹脂皮膜を１００ｍｇ／ｍ²〜２．０ｇ／ｍ²有することを特徴とする表面性状に優れた溶融Ｚｎ−Ａｌ−Ｍｇ−Ｓｉめっき鋼材。
【００１２】
（６）めっき後の冷却速度を１０℃／ｓ未満にすることを特徴とする上記（１）乃至（５）のいずれかに記載の表面性状に優れた溶融Ｚｎ−Ａｌ−Ｍｇ−Ｓｉめっき鋼材の製造方法。
【００１３】
（７）めっき後、凝固終了温度±１０℃の温度範囲の冷却速度を１０℃／ｓ未満にすることを特徴とする上記（１）乃至（５）のいずれかに記載の表面性状に優れた溶融Ｚｎ−Ａｌ−Ｍｇ−Ｓｉめっき鋼材の製造方法。
【００１４】
（８）めっき後、凝固終了温度＋１０℃までの冷却速度を１０℃／ｓ以上とし、更に続いて凝固終了温度±１０℃の温度範囲の冷却速度を１０℃／ｓ未満にすることを特徴とする上記（１）乃至（５）のいずれかに記載の表面性状に優れた溶融Ｚｎ−Ａｌ−Ｍｇ−Ｓｉ鋼材の製造方法。
【００１５】
【発明の実施の形態】
以下、本発明を詳細に説明する。まずめっき層中に含有させる元素について説明する。
【００１６】
めっき層中のＡｌは耐食性の向上と、めっき表面の突起状Ａｌ相樹枝状構造を生成させる目的で添加する。５％（ｍａｓｓ％、以下同じ）未満では表面の突起状Ａｌ相の生成が不十分であり、耐食性も劣る。一方、１８％を超えるとひけ状の凹凸が大きくなり表面外観が劣化すると共に耐食性向上効果が飽和するため、範囲を５〜１８％とした。
【００１７】
Ｍｇは一般には耐食性向上効果があると言われているが、前述しためっき表面のＡｌ相の樹枝状突起物の生成に影響を与え、１％未満ではこの生成が実質的にない。一方、１０％を超えると、めっき浴が大気接触により酸化が進行し黒色酸化物（ドロス）を生成し、めっき製造が困難になるため、範囲を１〜１０％とした。
【００１８】
Ｓｉは、一般的には耐食性向上、めっき密着性向上のために添加される。０．０１％未満では、これらの効果が小さく、２％以上では、ドロス生成が増加し、またＳｉ酸化物が斑点状模様が増加するので、０．０１〜２％とした。
【００１９】
Ｆｅは、Ｆｅ−Ｚｎ−Ａｌ合金、Ｆｅ−Ｚｎ、Ｆｅ−Ａｌ金属間化合物等を形成し、初晶Ａｌ樹枝状突起物の形成を妨げ、斑点模様の発生を誘発することがあるので１％以下とした。
【００２０】
Ｓｎは、耐食性向上のために必要に応じて添加される元素であり、耐食性向上効果のためには０．１％以上の添加が必要である。２％を超えるとめっきひけ状の凹凸が出やすくなり外観が悪化するので、０．１〜２％とした。
【００２１】
次に製造方法について説明する。使用するめっき原材の材質には特に限定はなく、Ａｌキルド鋼、極低炭素鋼、高Ｃ鋼、各種高張力鋼、Ｎｉ、Ｃｒ含有鋼等が使用可能である。また、鋼板、鋼線等のいずれについても適用可能であり、製鋼方法や鋼の強度、熱間圧延方法、酸洗方法、冷延方法等の鋼材の前処理加工についても特に制限がない。めっきの製造方法に関しては、ゼンジミアタイプ、フラックスタイプ、または、プレめっきタイプ等、２浴法等の製造方法によらず、本技術は適用可能である。
【００２２】
めっき時の浴温については、特に制限はないが、めっき機器溶損の点から４７０℃未満で行うことが望ましい。めっき後のワイピング方法には、制限がなく、空気、及び窒素ワイピングの使用が可能である。また、冷却方法にも特に制限がないが、局所的過冷却を避けるために気体冷却をすることが望ましい。そして、このＡｌ相の析出を制御することを指向し、めっき浴温度や冷却速度と、点状欠陥との関係を調査した。その結果、▲１▼従来知見と異なりめっき浴温は点状欠陥生成に影響しないこと、▲２▼めっき後の冷却速度が非常に重要であることが判った。
特に後者については、凝固終了温度近傍の温度範囲の冷却速度が斑点の生成に非常に重要であり、この範囲の冷却速度を小さくし、Ａｌ相の析出を制御することで、点状欠陥生成を回避できることを知見した。このメカニズムは未だ明らかではないが、冷却速度が大きいと局所的に過冷却状態が生じ、Ａｌの析出挙動が変化し、過冷部ではＡｌ相の成長が遅れ、表面へのＡｌ相の突起が少なくなり、斑点が生じるものと推察される。めっき後の冷却速度は本発明にとり非常に重要であり、斑点生成を回避するにはめっき後から凝固終了温度までの冷却速度が１０℃／ｓ未満とする必要がある。更に言えば、凝固点近傍、すなわち凝固終了温度±１０℃の範囲における冷却速度を１０℃／ｓ未満とすることで、斑点生成を回避できる。この温度範囲を除けば、冷却速度を１０℃／ｓ以上としてもかまわない。
【００２３】
めっき後に後処理であるＣｏフラッシュ、Ｃｏ−Ｎｉフラッシュなどの水系後処理をしても本発明の効果を損なうことはない。
【００２４】
めっき後のめっき層上に、更に、Ｍｇ、Ｚｒ、Ｍｏ、Ｃｅ、Ｃａの酸化物から選ばれる少なくとも１種以上の無機酸化物を被覆させることにより、耐食性を更に向上させることが出来る。この場合、たとえば硫酸塩、硝酸塩、燐酸塩等の複合酸化物とすることも何ら問題ない。これらの合計が、７０ｍｇ／ｍ²未満であると、耐食性向上効果が小さい。これらの合計が２．０ｇ／ｍ²を超えると、耐食性向上効果が飽和するので７０ｍｇ／ｍ²〜２．０ｇ／ｍ²の範囲とする。
【００２５】
また、この代わりに有機樹脂皮膜を被覆させることによっても耐食性向上を図ることが出来る。この皮膜の付着量が１００ｍｇ／ｍ²以下では、この効果が少なく、２．０ｇ／ｍ²を超えると耐食性向上効果が飽和するので１００ｍｇ／ｍ²〜２．０ｇ／ｍ²の範囲とする。有機樹脂としては、水系樹脂、溶剤系樹脂、粉体系樹脂、無溶剤系樹脂のどのような形態でもよい。ここで言う水系樹脂とは水溶性樹脂のほか、本来水不溶性でありながらエマルジョンやサスペンジョンのように水不溶性樹脂が水中に微分散された状態になりうるもの（水分散性樹脂）を含めて言う。有機樹脂として使用できる樹脂としては、特に制限はないが、ポリオレフィン系樹脂、アクリルオレフィン系樹脂、ポリウレタン系樹脂、アクリル系樹脂、ポリカーボネート系樹脂、エポキシ系樹脂、ポリエステル系樹脂、アルキド系樹脂、フェノール系樹脂、その他の加熱硬化型の樹脂などを例示でき、架橋可能であることがより好ましい。有機樹脂は２種類以上を混合してあるいは共重合して使用してもよい。また、必要により各種メラミン樹脂、アミノ樹脂等の架橋剤を添加してもよい。有機樹脂に加えて微粒シリカや潤滑剤の若干の添加も問題ない。もちろん各種クロメート処理を行うことも問題ない。
【００２６】
これら、無機酸化物或いは有機皮膜を形成させるための塗布方法としては、スプレー、カーテン、フローコーター、ロールコーター、バーコーター、刷毛塗り、浸漬及びエアナイフ絞り等のいずれの方法を用いてもよい。また、到達焼き付け温度は８０〜２５０℃とするのが望ましい。８０℃未満では、塗料中の水が完全に揮発しづらいため耐食性が低下し、２５０℃を超えると有機物である樹脂のアルキル部分が熱分解等の変性を起こしたり、皮膜の硬化が進みすぎて耐食性や加工性が低下したりするため好ましくない。８０〜１６０℃がより好ましい。また、乾燥設備については特に規制するものではないが、熱風吹き付けによる方法や、ヒーターによる間接加熱方法、赤外線による方法、誘導加熱による方法、並びにこれらを併用する方法が採用できる。また、使用する有機樹脂の種類によっては、紫外線や電子線などのエネルギー線によって硬化させることも出来る。
【００２７】
また、調質圧延を行ってもかまわない。
【００２８】
めっき表面に現れる樹枝状Ａｌ相は、直径数ミクロンから数十ミクロンの大きさのＡｌ相であり、これらが線状に並び、樹枝状模様を形成している。
【００２９】
光学顕微鏡とＣＭＡを併用することで、容易に調査でき、１ｍｍ²の範囲に、２００個以上あると点状欠陥がなくなり、外観が良好となる。
【００３０】
【実施例】
（実施例１）
鋼スラブを溶製して通常の方法で製造した板厚０．８ｍｍのＳＰＣＣ板をめっき原板とした。めっきは無酸化炉タイプの連続溶融亜鉛めっきラインにて加熱、焼鈍、めっきを行った。焼鈍雰囲気は、１０％水素、残９０％窒素ガス雰囲気であり、露点を−３０度とした。焼鈍温度は７３０℃、焼鈍時間は３分である。めっき浴組成はＡｌ：３．９〜１９％、Ｍｇ：０．３〜１１．３％、Ｓｉ：２．２％以下、Ｆｅ：０．０１〜１．２％、Ｓｎ：０．０５〜２．５％、残Ｚｎ及び不可避的不純物とからなり、めっき浴温は４３０℃である。めっき付着量は通常の窒素ガスワイピング法によりめっき付着量を片面当たり９０ｇ／ｍ²とした。めっき後の冷却は空冷にて、８℃／ｓで３２０℃まで行ったのち、気水冷却を実施した。なお、用いた浴の凝固終了温度は３４０℃である。その後調質圧延を１％行った。
【００３１】
その後必要に応じて、後処理を行った。後処理は▲１▼無機酸化物被覆または▲２▼有機樹脂被覆、▲３▼クロメート処理を行った。めっき製造時には、めっき浴表面酸化物（ドロス）生成量を目視で確認し、ドロスの多いものを×、やや多いものを△、少ないものを○とした。また、耐食性については、ＪＩＳ−Ｚ−２３７１に記載されている塩水噴霧試験（ＳＳＴ）を１０００時間行った後の腐食減量で評価した。５ｇ／ｍ²未満を◎◎◎、５ｇ／ｍ²以上１０ｇ／ｍ²未満を◎◎、１０ｇ／ｍ²以上３０ｇ／ｍ²未満を◎、３０ｇ／ｍ²以上４０ｇ／ｍ²未満を○、４０ｇ／ｍ²以上６０ｇ／ｍ²未満を△、６０ｇ／ｍ²以上を×とし、○以上を合格とした。めっき外観は、斑点状模様の有無、ヒケの生成、ドロス付着などが現れているものは、×とし、良好なものを○とした。めっき表面に現れる初晶Ａｌは、光学顕微鏡写真と一般にＣＭＡと呼ばれる電子線プローブマイクロアナリシス装置（島津製作所）で、１ｍｍ×１ｍｍの範囲でめっき表面の測定により、突起状のＡｌ相の数を数えた。めっき密着性は、１８０度の曲げを実施後、曲げ部を粘着テープで剥離試験した。剥離無しを○、剥離有りを×とした。これらの結果を表１及び表２に示した。Ａｌが５％以上で、かつＭｇが１％以上であると、耐食性は良好である。
【００３２】
Ｎｏ．１からＮｏ．７３は本発明例であり、耐食性と表面性状に優れている。
Ｎｏ．７４からＮｏ．７６はＡｌが少なすぎるため、耐食性が悪い。Ｎｏ．７７からＮｏ．７９のようにＡｌ量が多すぎると、冷却時にヒケが発生し、めっき外観が悪い。
【００３３】
Ｎｏ．８０からＮｏ．８２はＭｇ量が低すぎるため耐食性が悪い。Ｎｏ．８３からＮｏ．８５はＭｇが高すぎるため浴ドロスが多くなり、めっき外観も悪化する。Ｎｏ．８６からＮｏ．８８は、Ｓｉ量が少なく、めっき密着性が悪い。Ｎｏ．８９はＳｎが多すぎてめっき外観が悪い。Ｎｏ．９０からＮｏ．９２はＳｉが多すぎてドロスが多く、外観が悪い。Ｎｏ．９３からＮｏ．９５はＦｅ量が大きすぎて、点状欠陥が出やすくなり、浴ドロス、外観、めっき密着性のいずれも悪い。Ｎｏ．９６からＮｏ．１００は、無機酸化物皮膜や、有機物皮膜の量が小さく、耐食性向上効果が小さい。
【００３４】
【表１】

【００３５】
【表２】

【００３６】
（実施例２）
鋼スラブを溶製して通常の方法で製造した板厚２．３ｍｍのＳＰＣＣ板をめっき原板とした。めっきは無酸化炉タイプの連続溶融亜鉛めっきラインにて加熱、焼鈍、めっきを行った。焼鈍雰囲気は、１０％水素、残９０％窒素ガス雰囲気であり、露点を−３０度とした。焼鈍温度は７３０℃、焼鈍時間は３分である。めっき浴温は４４０℃である。めっき付着量は通常の窒素ガスワイピング法によりめっき付着量を片面当たり１００ｇ／ｍ²とした。めっき後の冷却は空冷にて、５〜２２℃／ｓで３２０℃まで連続的に行ったのち、気水冷却を実施した。なお、用いた浴の凝固終了温度は３４０℃である。その後調質圧延を１％行った。斑点状模様の発生の有無とめっき表面の突起状のＡｌ相の数を調査した。結果を表３に示す。冷却速度が１０℃／ｓ未満であると、Ａｌ相の個数が２００個／ｍｍ²以上となり点状欠陥が発生しないことが判る。
【００３７】
【表３】

【００３８】
（実施例３）
鋼スラブを溶製して通常の方法で製造した板厚０．６ｍｍのＳＰＣＣ板をめっき原板とした。めっきは無酸化炉タイプの連続溶融亜鉛めっきラインにて加熱、焼鈍、めっきを行った。焼鈍雰囲気は、１０％水素、残９０％窒素ガス雰囲気であり、露点を−３０度とした。焼鈍温度は７３０℃、焼鈍時間は３分である。めっき浴温は４２０℃である。めっき付着量は通常の窒素ガスワイピング法によりめっき付着量を片面当たり１００ｇ／ｍ²としためっき後の冷却は、１次から３次までの段階に分けて実施した。なお、用いた浴の凝固終了温度は３４０℃である。この結果を表４に示す。Ｎｏ．１３３からＮｏ．１４７について、例にとり説明する。まず、１次冷却速度は、１４℃／ｓ〜３０℃／ｓとしており、冷却方法も空冷、気水冷却、水スプレー冷却としているが、これは、Ｎｏ．１３３〜Ｎｏ．１３６、Ｎｏ．１３８、Ｎｏ．１４０〜Ｎｏ．１４３、Ｎｏ．１４５に示すように１次冷却終了温度を凝固温度＋１０℃以上とすれば、点状欠陥はなくなり良好となる。一方、Ｎｏ．１４７のように１次冷却終了温度が凝固温度＋１０℃未満であると、点状欠陥が発生する。
【００３９】
また、Ｎｏ．１３９、Ｎｏ．１４６のように２次冷却速度が１０℃／ｓ以上となり、すなわち凝固温度近傍の冷却速度が速いと、点状欠陥が生成する。また、Ｎｏ．１３７、Ｎｏ．１４４のように、２次冷却終了温度が凝固温度−１０℃以上となり、凝固点±１０℃の範囲の冷却速度が１０℃以上となると点状欠陥が現れる。以下、Ｎｏ．１４８〜Ｎｏ．１７５に示すようにＡｌやＭｇがより高い場合でも、同様な結果となった。
【００４０】
【表４】

【００４１】
【発明の効果】
以上に述べたように本発明によれば、表面性状に優れた高耐食性Ｚｎ−Ａｌ−Ｍｇ−Ｓｉめっき鋼材を製造でき、自動車、建材等の産業上きわめて影響が大きい。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a molten Zn—Al—Mg—Si plated steel material having excellent surface properties and a method for producing the same.
[0002]
[Prior art]
In recent years, the use of surface-treated steel materials has been expanded in order to cope with the prolonged service life of automobiles, household electrical products, building materials, and the like. In particular, Zn-5 mass% Al hot-dip galvanized steel is used mainly for building materials because it has superior corrosion resistance compared to conventional hot-dip galvanized steel. As for hot dip plating in which Al or Mg is added to Zn, U.S. Pat. No. 3,504,043 discloses a highly corrosion-resistant hot dip Zn using a hot dip plating bath comprising Al: 3 to 17%, Mg: 1 to 5%, and the balance Zn. -Various techniques have been proposed since the Al-Mg plated steel sheet was proposed. For example, in JP-A-8-60324, Al is 0.25% at the maximum and Mg is 3% at the maximum. JP-A-9-143659 includes Mg: 0.05 to 3%, Al: 0.1 to 1%, and Ni: 0.01 to 0.2%. Although these have an effect of improving the corrosion resistance, when the Mg content is close to 3%, a thick oxide (dross) is deposited on the bath, which is not suitable for industrial production. Recently, as disclosed in JP-A-10-226865, a plated steel sheet containing Al: 4 to 10% and Mg: 1 to 4%, or a Zn-Al-Mg system as disclosed in Japanese Patent Application No. 11-179913. A hot-dip plated steel sheet in which Si is further added to a plated steel sheet has been proposed.
[0003]
However, in the case of Zn—Al—Mg-based plating, a dot-like appearance pattern of about several mm to 10 mm may occur, which may cause a problem in terms of products. As a solution for this, for example, in Japanese Patent Laid-Open No. 10-226865 described above, the existence form of Mg is important, the plating bath temperature during production is set to 450 ° C. or lower and lower than 470 ° C., and the cooling rate is set to 10 ° C. / This can be avoided by increasing the temperature to s or more or by setting the bath temperature to 470 ° C. or more and controlling the cooling rate to 0.5 ° C./s or more. Furthermore, Japanese Patent Laid-Open No. 10-306357 discloses that the restrictions on the manufacturing conditions are eased by adding Ti and B to the plating. However, according to our research, these technologies cannot prevent (1) the point-like surface defects of Zn-Al-Mg-Si plating, and (2) the bath temperature is a plating pot, plating. There are problems of equipment corrosion, and it is difficult to raise the temperature to 470 ° C or higher. (3) Addition of Ti and B causes dross generation and surface properties deteriorate. It became clear that there was.
[0004]
[Problems to be solved by the invention]
In view of such a situation, it is an object of the present invention to provide a molten Zn—Al—Mg—Si plated steel material with improved surface properties.
[0005]
[Means for Solving the Problems]
From the above examination, the present inventors speculate that the generation mechanism of the point-like defects in the Zn—Al—Mg plating and the Zn—Al—Mg—Si plating is different. The surface properties of the plating were studied earnestly. Then, it was found that the spot-like defects are conspicuously generated when the cooling rate is increased, unlike the conventional knowledge. This spot portion is about 1.5 mm in size and has a dark and smooth appearance compared to the surroundings. When the surface analysis of the spot portion was carried out with a light microscope, an electron beam probe microanalysis apparatus (CMA), etc., the dendritic structure of the Al phase did not protrude into the surface at the spot portion. In the normal part, the dendritic structure of the Al phase protrudes in a protruding shape on the surface, and it was found that the point defects are the difference in the precipitation form of the Al phase, and the present invention has been completed. Here, the Al phase is defined as an Al-rich phase that starts solidification precipitation first from the plating bath.
[0006]
And it aimed at controlling precipitation of this Al phase, and investigated the relationship between a plating bath temperature and a cooling rate, and a point defect. As a result, it was found that (1) unlike the conventional knowledge, the plating bath temperature does not affect the generation of point defects, and (2) the cooling rate after plating is very important. Especially for the latter, the cooling rate in the temperature range near the solidification end temperature is very important for the generation of spots. By reducing the cooling rate in this range and controlling the precipitation of the Al phase, the generation of point defects is achieved. I found that it can be avoided. This mechanism is not yet clear, but when the cooling rate is high, a supercooled state occurs locally, the Al precipitation behavior changes, the growth of the Al phase is delayed in the supercooled part, and Al phase protrusions on the surface It is presumed that there will be less spots and spots. Here, the Al phase is defined as an Al-rich phase that starts solidification precipitation first from the plating bath. The present invention has been completed based on such various new findings, and the gist of the present invention is as follows.
[0007]
(1) The surface of the steel material having a plating layer composed of Al: 5 to 18% by mass, Mg: 1 to 10% by mass, Si: 0.01 to 2% by mass, the balance Zn and unavoidable impurities. Further, 200 or more Al phases are present per 1 mm ^2, and a hot-dip Zn—Al—Mg—Si plated steel material having excellent surface properties.
[0008]
(2) A molten Zn—Al—Mg—Si plated steel material having excellent surface properties, wherein the plated layer of the plated steel material according to (1) further contains Fe: 1 mass% or less.
[0009]
(3) Molten Zn—Al—Mg excellent in surface properties characterized by further containing Sn: 0.1 to 2% by mass in the plated layer of the plated steel material according to (1) or (2) above -Si plated steel.
[0010]
(4) Excellent surface properties characterized by further having an inorganic oxide film of 70 mg / m ^{2 to 2} g / m ² on the plated layer of the plated steel material according to any one of (1) to (3) above. Hot-dip Zn—Al—Mg—Si plated steel.
[0011]
(5) Surface properties characterized by further having an organic resin film of 100 mg / m ^{2 to} 2.0 g / m ² on the plating layer of the plated steel material according to any one of (1) to (3) above. Excellent hot dip Zn-Al-Mg-Si plated steel.
[0012]
(6) The molten Zn—Al—Mg—Si plated steel material having excellent surface properties according to any one of (1) to (5), wherein a cooling rate after plating is less than 10 ° C./s. Manufacturing method.
[0013]
(7) The surface property according to any one of (1) to (5) above, wherein the cooling rate in the temperature range of the solidification end temperature ± 10 ° C. is less than 10 ° C./s after plating. A method for producing a molten Zn—Al—Mg—Si plated steel material.
[0014]
(8) After the plating, the cooling rate to the solidification end temperature + 10 ° C. is set to 10 ° C./s or more, and the cooling rate in the temperature range of the solidification end temperature ± 10 ° C. is set to less than 10 ° C./s. The manufacturing method of the molten Zn-Al-Mg-Si steel material which was excellent in the surface property in any one of said (1) thru | or (5).
[0015]
DETAILED DESCRIPTION OF THE INVENTION
The present invention will be described in detail below. First, elements contained in the plating layer will be described.
[0016]
Al in the plating layer is added for the purpose of improving corrosion resistance and generating a protruding Al-phase dendritic structure on the plating surface. If it is less than 5% (mass%, the same shall apply hereinafter), the formation of the protruding Al phase on the surface is insufficient, and the corrosion resistance is also poor. On the other hand, if it exceeds 18%, sink-like irregularities become large, the surface appearance is deteriorated, and the effect of improving corrosion resistance is saturated, so the range is made 5 to 18%.
[0017]
Mg is generally said to have an effect of improving corrosion resistance, but affects the generation of the Al phase dendrite on the plating surface described above, and if it is less than 1%, this generation is substantially absent. On the other hand, if it exceeds 10%, the plating bath is oxidized by contact with the atmosphere to produce black oxide (dross), making plating production difficult, so the range is set to 1 to 10%.
[0018]
Si is generally added to improve corrosion resistance and plating adhesion. If it is less than 0.01%, these effects are small, and if it is 2% or more, dross generation increases and the spot pattern of Si oxide increases.
[0019]
Fe forms an Fe-Zn-Al alloy, Fe-Zn, Fe-Al intermetallic compound, etc., prevents the formation of primary Al dendrites, and may induce the occurrence of spotted patterns. It was as follows.
[0020]
Sn is an element added as necessary to improve the corrosion resistance, and 0.1% or more is necessary for the effect of improving the corrosion resistance. If it exceeds 2%, plating sink-like irregularities tend to appear and the appearance deteriorates, so the content was made 0.1 to 2%.
[0021]
Next, a manufacturing method will be described. The material of the plating raw material to be used is not particularly limited, and Al killed steel, extremely low carbon steel, high C steel, various high tensile steels, Ni, Cr-containing steel, and the like can be used. Moreover, it can apply also to any of a steel plate, a steel wire, etc. There is no restriction | limiting in particular about pre-processing processing of steel materials, such as a steel manufacturing method, the intensity | strength of steel, a hot rolling method, a pickling method, and a cold rolling method. With respect to the plating manufacturing method, the present technology can be applied regardless of the two-bath method such as Sendzimir type, flux type, or pre-plating type.
[0022]
Although there is no restriction | limiting in particular about the bath temperature at the time of metal plating, it is desirable to carry out at less than 470 degreeC from the point of metal-plating equipment melting. The wiping method after plating is not limited, and air and nitrogen wiping can be used. Moreover, although there is no restriction | limiting in particular also in the cooling method, in order to avoid local supercooling, it is desirable to perform gas cooling. And it aimed at controlling precipitation of this Al phase, and investigated the relationship between a plating bath temperature and a cooling rate, and a point defect. As a result, it was found that (1) unlike the conventional knowledge, the plating bath temperature does not affect the generation of point defects, and (2) the cooling rate after plating is very important.
Especially for the latter, the cooling rate in the temperature range near the solidification end temperature is very important for the generation of spots. By reducing the cooling rate in this range and controlling the precipitation of the Al phase, the generation of point defects is achieved. I found that it can be avoided. This mechanism is not yet clear, but when the cooling rate is high, a supercooled state occurs locally, the Al precipitation behavior changes, the growth of the Al phase is delayed in the supercooled part, and Al phase protrusions on the surface It is presumed that there will be less spots and spots. The cooling rate after plating is very important for the present invention, and in order to avoid the generation of spots, the cooling rate from after plating to the solidification end temperature needs to be less than 10 ° C./s. Furthermore, speckle generation can be avoided by setting the cooling rate in the vicinity of the freezing point, that is, in the range of the solidification end temperature ± 10 ° C. to less than 10 ° C./s. Except for this temperature range, the cooling rate may be 10 ° C./s or more.
[0023]
Even if the aqueous post-treatment such as Co flash or Co-Ni flash, which is a post-treatment after plating, does not impair the effects of the present invention.
[0024]
Corrosion resistance can be further improved by coating at least one inorganic oxide selected from oxides of Mg, Zr, Mo, Ce, and Ca on the plated layer after plating. In this case, for example, there is no problem in forming a composite oxide such as sulfate, nitrate, and phosphate. If the total of these is less than 70 mg / m ² , the effect of improving corrosion resistance is small. When these total exceeds 2.0 g / m ^2, the corrosion resistance improvement effect is in the range of 70mg / m ^² ~2.0g / m ² so saturated.
[0025]
Alternatively, the corrosion resistance can be improved by coating an organic resin film instead. Adhesion amount of the coating in 100 mg / m ² or less, this effect is small, the corrosion resistance improving effect exceeds 2.0 g / m ² is in the range of 100mg / m ^² ~2.0g / m ² so saturated. The organic resin may be in any form of water-based resin, solvent-based resin, powder-based resin, and solvent-free resin. The water-based resin mentioned here includes, in addition to water-soluble resins, water-insoluble resins that are water-insoluble but can be in a state of being finely dispersed in water, such as emulsions and suspensions (water-dispersible resins). . The resin that can be used as the organic resin is not particularly limited, but polyolefin resin, acrylic olefin resin, polyurethane resin, acrylic resin, polycarbonate resin, epoxy resin, polyester resin, alkyd resin, phenolic resin Resins, other thermosetting resins, and the like can be exemplified, and crosslinkable is more preferable. Two or more kinds of organic resins may be mixed or copolymerized. Moreover, you may add crosslinking agents, such as various melamine resin and an amino resin, as needed. In addition to the organic resin, there is no problem with the addition of fine silica or a lubricant. Of course, there is no problem with various chromate treatments.
[0026]
As a coating method for forming these inorganic oxides or organic films, any method such as spray, curtain, flow coater, roll coater, bar coater, brush coating, dipping and air knife squeezing may be used. The ultimate baking temperature is desirably 80 to 250 ° C. If it is less than 80 ° C, the water in the paint is difficult to completely volatilize and the corrosion resistance is lowered, and if it exceeds 250 ° C, the alkyl part of the resin, which is an organic substance, undergoes modification such as thermal decomposition, or the film is cured too much. Since corrosion resistance and workability will fall, it is not preferable. 80-160 degreeC is more preferable. Moreover, although it does not regulate in particular about drying equipment, the method by hot air blowing, the indirect heating method by a heater, the method by infrared rays, the method by induction heating, and the method of using these together can be employ | adopted. Further, depending on the type of organic resin to be used, it can be cured by energy rays such as ultraviolet rays and electron beams.
[0027]
Further, temper rolling may be performed.
[0028]
The dendritic Al phase appearing on the plating surface is an Al phase having a diameter of several microns to several tens of microns, and these are arranged in a line to form a dendritic pattern.
[0029]
By using an optical microscope and CMA in combination, it is easy to investigate, and when there are 200 or more in the range of 1 mm ² , point defects are eliminated and the appearance is improved.
[0030]
【Example】
Example 1
An SPCC plate having a plate thickness of 0.8 mm produced by melting a steel slab by a normal method was used as a plating base plate. Plating was performed by heating, annealing, and plating in a non-oxidizing furnace type continuous hot dip galvanizing line. The annealing atmosphere was an atmosphere of 10% hydrogen and the remaining 90% nitrogen gas, and the dew point was −30 degrees. The annealing temperature is 730 ° C., and the annealing time is 3 minutes. The plating bath composition is Al: 3.9-19%, Mg: 0.3-11.3%, Si: 2.2% or less, Fe: 0.01-1.2%, Sn: 0.05-2 .5%, remaining Zn and inevitable impurities, and the plating bath temperature is 430.degree. The plating adhesion amount was 90 g / m ² per side by a normal nitrogen gas wiping method. Cooling after plating was performed by air cooling at 8 ° C./s up to 320 ° C., followed by air-water cooling. The solidification end temperature of the bath used is 340 ° C. Thereafter, temper rolling was performed at 1%.
[0031]
Thereafter, post-treatment was performed as necessary. The post-treatment was (1) inorganic oxide coating or (2) organic resin coating, and (3) chromate treatment. At the time of plating production, the amount of plating bath surface oxide (dross) produced was visually confirmed, with x being a lot of dross, Δ being a little more, and ○ being a little. Moreover, about corrosion resistance, it evaluated by the corrosion weight loss after performing the salt spray test (SST) described in JIS-Z-2371 for 1000 hours. 5 g / m ² less than the ◎◎◎, 5g / m ² or more 10 g / m ² less than the ◎◎, 10g / m ² or more 30 g / m less than ² ◎, less than 30 g / m ² or more 40g / m ² ○, A value of 40 g / m ² or more and less than 60 g / m ² was evaluated as Δ, a value of 60 g / m ² or more as x, and a value of ○ or more as a pass. As for the plating appearance, the case where the presence or absence of a spot-like pattern, the generation of sink marks, the adhesion of dross, etc. appeared was indicated as x, and the good one was indicated as ◯. Primary crystal Al appearing on the plating surface is counted by measuring the surface of the plating in the range of 1 mm x 1 mm with an optical micrograph and an electron probe microanalysis device (Shimadzu Corporation) generally called CMA. It was. The plating adhesion was subjected to a peel test with an adhesive tape after bending at 180 degrees. No peeling was marked with ◯, and peeling was marked with x. These results are shown in Tables 1 and 2. When Al is 5% or more and Mg is 1% or more, the corrosion resistance is good.
[0032]
No. 1 to No. 73 is an example of the present invention, and is excellent in corrosion resistance and surface properties.
No. 74 to No. Since 76 has too little Al, corrosion resistance is bad. No. 77 to No. When the amount of Al is too large as in 79, sink marks occur during cooling and the plating appearance is poor.
[0033]
No. 80 to No. No. 82 has poor corrosion resistance because the amount of Mg is too low. No. 83 to No. In 85, Mg is too high, so the bath dross increases and the plating appearance also deteriorates. No. 86 to No. No. 88 has a small amount of Si and poor plating adhesion. No. No. 89 has too much Sn and the plating appearance is poor. No. 90 to No. 92 has too much Si and a lot of dross, and the appearance is bad. No. 93 to No. No. 95 has an excessively large amount of Fe, so that point-like defects are likely to occur, and bath dross, appearance, and plating adhesion are all poor. No. 96 to No. No. 100 has a small amount of inorganic oxide film or organic film and a small effect of improving corrosion resistance.
[0034]
[Table 1]

[0035]
[Table 2]

[0036]
(Example 2)
An SPCC plate having a plate thickness of 2.3 mm produced by melting a steel slab by a normal method was used as a plating base plate. Plating was performed by heating, annealing, and plating in a non-oxidizing furnace type continuous hot dip galvanizing line. The annealing atmosphere was an atmosphere of 10% hydrogen and the remaining 90% nitrogen gas, and the dew point was −30 degrees. The annealing temperature is 730 ° C., and the annealing time is 3 minutes. The plating bath temperature is 440 ° C. The plating adhesion amount was set to 100 g / m ² per side by a normal nitrogen gas wiping method. Cooling after plating was performed by air cooling and continuously at 5 to 22 ° C./s up to 320 ° C., followed by air-water cooling. The solidification end temperature of the bath used is 340 ° C. Thereafter, temper rolling was performed at 1%. The presence or absence of spotted patterns and the number of protruding Al phases on the plating surface were investigated. The results are shown in Table 3. It can be seen that when the cooling rate is less than 10 ° C./s, the number of Al phases is 200 / mm ² or more, and no point defects are generated.
[0037]
[Table 3]

[0038]
(Example 3)
An SPCC plate having a thickness of 0.6 mm produced by melting a steel slab by a normal method was used as a plating original plate. Plating was performed by heating, annealing, and plating in a non-oxidizing furnace type continuous hot dip galvanizing line. The annealing atmosphere was an atmosphere of 10% hydrogen and the remaining 90% nitrogen gas, and the dew point was −30 degrees. The annealing temperature is 730 ° C., and the annealing time is 3 minutes. The plating bath temperature is 420 ° C. The amount of plating adhered was cooled by a normal nitrogen gas wiping method in which the amount of plating deposited was 100 g / m ² per side and the cooling after plating was performed in stages from primary to tertiary. The solidification end temperature of the bath used is 340 ° C. The results are shown in Table 4. No. 133 to No. 147 will be described as an example. First, the primary cooling rate is 14 ° C./s to 30 ° C./s, and the cooling method is air cooling, air-water cooling, and water spray cooling. 133-No. 136, no. 138, no. 140-No. 143, no. As shown in 145, when the primary cooling end temperature is set to the solidification temperature + 10 ° C. or higher, the point defects are eliminated and the temperature is improved. On the other hand, no. If the primary cooling end temperature is less than the solidification temperature + 10 ° C. as in 147, point defects are generated.
[0039]
No. 139, no. When the secondary cooling rate is 10 ° C./s or more as in 146, that is, when the cooling rate in the vicinity of the solidification temperature is high, point defects are generated. No. 137, no. As in 144, when the secondary cooling end temperature is equal to or higher than the solidification temperature −10 ° C. and the cooling rate in the range of the freezing point ± 10 ° C. is equal to or higher than 10 ° C., point defects appear. Hereinafter, no. 148-No. As shown by 175, similar results were obtained even when Al and Mg were higher.
[0040]
[Table 4]

[0041]
【The invention's effect】
As described above, according to the present invention, a highly corrosion-resistant Zn—Al—Mg—Si plated steel material having excellent surface properties can be produced, which has a great influence on industries such as automobiles and building materials.

Claims

On the surface of the steel material, Al: 5 to 18% by mass, Mg: 1 to 10% by mass, Si: 0.01 to 2% by mass, the remaining Zn and a plating layer made of unavoidable impurities, A molten Zn—Al—Mg—Si plated steel material having excellent surface properties, wherein 200 or more phases are present per 1 mm ² .

2. A molten Zn—Al—Mg—Si plated steel material having excellent surface properties, wherein the plated layer of the plated steel material according to claim 1 further contains Fe: 1% by mass or less.

3. A molten Zn—Al—Mg—Si plated steel material having excellent surface properties, wherein the plated layer of the plated steel material according to claim 1 or 2 further contains Sn: 0.1 to 2 mass%.

The molten Zn- having excellent surface properties, further comprising an inorganic oxide film of 70 mg / m ^{2 to 2} g / m ² on the plating layer of the plated steel material according to claim 1. Al-Mg-Si plated steel.

A molten Zn having excellent surface properties, further comprising an organic resin film of 100 mg / m ^{2 to} 2.0 g / m ² on the plated layer of the plated steel material according to any one of claims 1 to 3. -Al-Mg-Si plated steel.

The method for producing a molten Zn-Al-Mg-Si plated steel material having excellent surface properties according to any one of claims 1 to 5, wherein a cooling rate after plating is less than 10 ° C / s.

6. The molten Zn—Al excellent in surface properties according to claim 1, wherein a cooling rate in a temperature range of solidification end temperature ± 10 ° C. is less than 10 ° C./s after plating. -Manufacturing method of Mg-Si plating steel materials.

After the plating, the cooling rate to the solidification end temperature + 10 ° C is set to 10 ° C / s or more, and subsequently the cooling rate in the temperature range of the solidification end temperature ± 10 ° C is set to less than 10 ° C / s. The manufacturing method of the molten Zn-Al-Mg-Si steel material excellent in the surface property in any one of Claim 1 thru | or 5.