JP3640688B2 - Zn-Mg alloy plated steel sheet and manufacturing method - Google Patents

Description

【０００１】
【産業上の利用分野】
本発明は、耐食性，耐パウダリング性，密着性，スポット溶接性，耐変色性等に優れたＺｎ−Ｍｇ合金めっき鋼板及びその製造方法に関する。
【０００２】
【従来の技術】
鋼板の耐食性を向上させるため、従来から各種の表面処理が採用されている。なかでも、代表的な表面処理方法であるＺｎめっきには、主として電気めっき法，溶融めっき法等が採用されている。
耐食性の向上に対する要求は年々高まる傾向にあり、これに伴って溶融めっき法，電気めっき法等において種々の改良が提案されている。
溶融めっき法でＺｎめっき鋼板の耐食性を向上させようとすると、Ｚｎめっき層の付着量を増加させることが先ず考えられる。しかし、製造面から付着量の上限が制約されるため、付着量の増加によって耐食性の向上を図ることには限界がある。また、付着量の増加、すなわちめっき層の厚膜化は、めっき鋼板をプレス成形するときにカジリ，フレーキング等の欠陥を発生させる原因になり易い。
【０００３】
電気めっき法で同様に高付着量のめっき層を形成しようとすると、ラインスピードを遅くすることが必要になり、生産性が著しく損なわれる。そこで、電気めっき法では、Ｚｎ−Ｎｉ系等のＺｎ合金めっきを施すことによって耐食性の向上を図っている。しかし、Ｚｎ−Ｎｉ合金めっき層は、硬質で脆いため、成形時にめっき層に割れや欠け等の欠陥を発生させ易い。このような欠陥がめっき層に発生すると、欠陥部を介して下地鋼が露出するため、めっき層本来の性能が発揮されず、欠陥部を起点とした腐食が進行する。
以上のような背景から、高耐食性のＺｎ系合金めっき鋼板を蒸着で製造することが試みられている。なかでも、Ｚｎ−Ｍｇ合金めっきは、優れた防食作用を有する。たとえば、特開昭６４−１７８５３号公報では、０．５〜４０重量％のＭｇを含むＺｎ−Ｍｇ合金めっき層を形成することを開示している。また、Ｚｎ−Ｍｇ合金めっき層と下地鋼との間にＺｎ，Ｎｉ，Ｃｕ，Ｍｇ，Ａｌ，Ｆｅ，Ｃｏ，Ｔｉ等の中間層を介在させるとき、めっき層の密着性及び加工性が向上することが特開平２−１４１５８８号公報で紹介されている。
【０００４】
【発明が解決しようとする課題】
高付着量のＺｎ−Ｍｇ合金めっき鋼板では、プレス加工時にパウダリングが発生し易い欠点がある。特に、Ｍｇ濃度の上昇に伴ってめっき層にＺｎ−Ｍｇの金属間化合物が多くなった場合、Ｍｇの平均濃度が低いときでも下地側に金属間化合物が存在する場合等において、この傾向が顕著になる。これは、Ｚｎ−Ｍｇ金属間化合物層が硬くて脆いためであり、高い延性をもつ下地鋼板の変形に追従できず、界面剥離や亀裂を起こすことに原因がある。
パウダリングは、Ｍｇ濃度の低下によってめっき層に含まれる金属間化合物の量を下げ、延性を高くすることによって解消される。しかし、Ｍｇ濃度の低下は、めっき層の防食作用を低下させる。また、表面側のみを高Ｍｇ濃度にしても耐パウダリング性が改善されるが、めっき層の表面がＭｇ濃化によって黒色に変色し、めっき鋼板の商品価値を下げる。しかも、めっき層表面の高Ｍｇ濃度は、溶接電極に対するＭｇの拡散を促進させ、スポット溶接性を低下させる。
本発明は、このような問題を解消すべく案出されたものであり、Ｚｎ−Ｍｇ合金めっき層を特定組成の多層構造とすることにより、高Ｍｇ濃度に起因する欠陥を抑制し、耐食性，耐パウダリング性，密着性，スポット溶接性，耐変色性等に優れたＺｎ−Ｍｇ合金めっき鋼板を得ること目的とする。
【０００５】
【課題を解決するための手段】
本発明のＺｎ−Ｍｇ合金めっき鋼板は、その目的を達成するため、図１に示す基本的な層構成をもっている。下地鋼の表面にはＭｇ濃度０．５重量％以下のＺｎ−Ｍｇ合金層があり、その上にＭｇ濃度７重量％以上のＺｎ−Ｍｇ合金層及びＭｇ濃度０．５重量％以下のＺｎ−Ｍｇ合金層が順次積層されている。
この基本層構成よりも高耐食性が要求される場合、図２に示すように、Ｍｇ濃度０．５重量％以下のＺｎ−Ｍｇ合金層とＭｇ濃度７重量％以上のＺｎ−Ｍｇ合金層との間にＭｇ濃度２〜７重量％のＺｎ−Ｍｇ合金層を設ける。
Ｍｇ濃度７重量％以上のＺｎ−Ｍｇ合金層は、高湿潤環境における耐食性を確保する観点から、Ｍｇ含有量の上限を２０重量％にすることが好ましい。
【０００６】
蒸着時の雰囲気にＯ₂ やＨ₂ Ｏが含まれていると、鋼板表面が酸化され、めっき層の密着性が低下する。また、表面活性化後に直ちに蒸着が行われず、鋼板表面の汚染が予想される場合にあっても、めっき層の密着性が低下する虞れがある。このような場合、鋼板表面に対するＺｎ−Ｍｇ合金めっき層の密着性を図るため、図３又は図４に示すようにＺｎ−Ｆｅ合金層又はＺｎ−Ｆｅ−Ｍｇ合金層を形成する。
Ｚｎ−Ｆｅ合金層又はＺｎ−Ｆｅ−Ｍｇ合金層は、それらによる成形時のパウダリングを防止するため、層厚を０．５μｍ以下にすることが好ましい。また、Ｚｎ−Ｆｅ合金層又はＺｎ−Ｆｅ−Ｍｇ合金層に含まれるＦｅ濃度は、通常６重量％以上にする。
【０００７】
これらのめっき鋼板は、鋼板表面にＺｎ蒸着→Ｍｇ蒸着→Ｚｎ蒸着を順次行った後で加熱することによって形成される。或いは、蒸着割合を変えながら、Ｚｎ及びＭｇを同時蒸着することによっても、必要とする層構成をもつＺｎ−Ｍｇ合金めっき層が形成される。
蒸着後の加熱温度，加熱時間等の条件を制御することにより、必要とする厚みをもったＺｎ−Ｆｅ合金層又はＺｎ−Ｆｅ−Ｍｇ合金層が下地鋼とめっき層との界面に形成される。たとえば、５０秒以内の短時間加熱では加熱温度を２７０〜３７０℃に、１時間以上の長時間加熱では加熱温度を１５０〜２５０℃に設定する。また、加熱処理に代えて、蒸着終了時の鋼板温度が２７０〜３７０℃となるように条件を設定することによっても、所定の層構成をもったＺｎ−Ｍｇ合金めっき層を形成することができる。
蒸着法で形成されたＺｎ−Ｍｇ合金めっき層には、通常、酸化したＭｇ濃化層が表面に存在する。初期の耐食性が重視される用途ではＭｇ濃化層をそのまま残しておくが、スポット溶接性が重視される用途や表面の黒変色を防止する場合には、酸洗等によってＭｇ濃化層を除去する。
【０００８】
【作用】
本発明に従ったＺｎ−Ｍｇ合金めっき層は、中心部にＭｇ濃度の高い部分があり、その上下にある層のＭｇ濃度が低くなった多層構成をもっている。Ｍｇ濃度が０．５重量％以下の低い層は、比較的溶出速度が大きく、犠牲防食作用を呈し、疵付き部等における鋼板露出部の赤錆発生を防止する。特に、初期の赤錆発生防止に有効である。Ｍｇ濃度が７重量％以上と高い層は、耐食性が高く、めっき層自体の腐食寿命を長くする。また、Ｍｇ濃度が高い層から溶け出したＭｇは、防食性に優れたＺｎの腐食生成物であるＺｎＣｌ₂・４Ｚｎ (ＯＨ)₂やＺｎ (ＯＨ)₂の生成を促進させ、耐食性を向上させる。Ｍｇ濃度が２〜７重量％と中間濃度のＺｎ−Ｍｇ合金層は、低Ｍｇ濃度層と高Ｍｇ濃度層との中間の性質を示し、更に耐食性を向上させる。このような各層の作用が相乗的に働き、従来にない優れた耐食性が発揮される。
【０００９】
めっきされた鋼板をプレス加工等で変形させるとき、Ｍｇ濃度０．５重量％以下のＺｎ−Ｍｇ合金層に延性があるため、ほとんど変形しない高Ｍｇ濃度のＺｎ−Ｍｇ合金層と下地鋼との間の加工時における変形量の差を吸収する。その結果、パウダリングの発生が抑制される。
めっき層の表層は、めっき後の鋼板を酸洗し、表面の酸化したＭｇ濃化層を除去することによって、スポット溶接時に溶接電極と接する面のＭｇ濃度が０．５重量％以下になる。低Ｍｇ層は、溶接電極へのＭｇ拡散を少なくし、スポット溶接性を向上させる。また、表面に多量のＭｇが存在すると、Ｚｎの酸化物や水酸化物が不飽和になり、黒変色が生じる。この黒変色も、表層のＭｇ濃度を０．５重量％以下にすることによって防止できる。
【００１０】
【実施例】
本発明に従っためっき鋼板の作製
めっき原板として、Ｃ：０．００２重量％，Ｓｉ：０．０２重量％，Ｍｎ：０．２１重量％，Ｐ：０．００７重量％，Ｓ：０．００１重量％，Ｔｉ：０．０７６重量％，Ａｌ：０．０３１重量％の組成を持ち、板厚０．８ｍｍの鋼板を使用した。めっき原板をＮ₂ −５０％Ｈ₂ ガス雰囲気中で還元加熱することにより表面の酸化膜を除去した後、真空室にセットした。真空室は、ポンプで排気しながら露点−６０℃のＮ₂ ガスを導入し、Ｎ₂ 分圧を５×１０^-2トールに維持した。この真空室内で、次の手順で蒸着した。
Ｚｎ→Ｍｇ→Ｚｎの順番で蒸着し、トータルで片面当り１００ｇ／ｍ² の蒸着量に設定し、最初に蒸着するＺｎ量と最後に蒸着するＺｎ量を同一にした。
【００１１】
下地鋼とめっき層との界面にＺｎ−Ｆｅ合金層又はＺｎ−Ｆｅ−Ｍｇ合金層がある３層構造（図３）又は５層構造（図４）のめっき鋼板を製造する場合、２００℃に保持した鋼板表面にＺｎ→Ｍｇ→Ｚｎの順番で蒸着した後、真空室を７００トールのＮ₂ で満たし、５〜１０秒間加熱した。加熱温度は、３層構造（図３）のめっき層を形成する場合には２７０〜３３０℃、５層構造（図４）のめっき層を形成する場合には３３０〜３７０℃とした。これにより、Ｚｎ−Ｆｅ合金層又はＺｎ−Ｆｅ−Ｍｇ合金層は、約０．２μｍの厚みとなった。これにより、中心付近のＭｇ濃度が最も高い層では約１０重量％のＭｇ濃度となり、上下層のＭｇ濃度は０．５重量％以下になった。また、中間にある層のＭｇ濃度は、約４重量％であった。
また、蒸着割合を変化させながらＺｎ及びＭｇを同時に蒸着することにより、図１及び図２に示す層構成をもった片面当りのＺｎ付着量が１００ｇ／ｍ² のＺｎ−Ｍｇ合金めっき層を形成した。各層の濃度は、先のもの（図３，４）と同じにした。このとき、蒸着時の鋼板を板温１２０℃に保持した。なお、蒸着後に、加熱処理は行わなかった。
【００１２】
以上の各めっき鋼板を０．５％ＨＣｌ水溶液で酸洗し、表面のＭｇ濃化層を除去した。酸洗後のめっき鋼板は、十分に水洗した。得られためっき鋼板を観察したところ、表１に示す層構成をもつめっき層となっていた。
各めっき鋼板の特性を調査した。耐食性は、ＪＩＳ Ｚ２３７１に準拠した塩水噴霧試験を行い、赤錆発生時間で評価した。パウダリング性は、高さ４ｍｍ，Ｒ＝０．５ｍｍのビードを付けた金型に試験片を挟み、金型への加圧力５００ｋｇｆ及び引抜き速度２００ｍ／分で金型から試験片を引き抜くド−ロービード試験を行い、発生したパウダリングの量で評価した。スポット溶接性は、単相交流型の溶接機に先端径４．５ｍｍのＣＦ型Ｃｕ−１％Ｃｒ電極を装着し、連続溶接が可能な打点数によって評価した。黒変色は、温度５０℃及び相対湿度６０％の促進試験機の中に試験片を１０００時間放置し、試験前後の明度差ΔＬ^* によって評価した。
試験結果を示す表１から明らかなように、本発明に従った３層構造又は５層構造をもつＺｎ−Ｍｇ合金めっき鋼板は、耐食性，耐パウダリング性，スポット溶接性，耐黒変色性の何れにおいても優れていた。また、図３及び図４の構造をもつめっき層が形成されたものでは、Ｈ₂ ＯやＯ₂ が数十ｐｐｍ存在する酸化性の雰囲気で蒸着した場合でも、めっき層の密着性が良好であった。
【００１３】
【表１】

【００１４】
比較例１
同じめっき原板を１２０℃に保持し、Ｚｎ及びＭｇの同時蒸着によって、片面当りの付着量が１００ｇ／ｍ^２で均一組成をもつＺｎ−Ｍｇ合金めっき層を形成した。
比較例２
鋼板温度を２００℃に保持し、Ｚｎ→Ｍｇの順に蒸着した後、２７０〜３３０℃に加熱することによって、下層がＭｇ濃度０．５重量％以下のＺｎ−Ｍｇ合金層，上層がＭｇ濃度１０重量％のＺｎ−Ｍｇ合金層からなる２層構造のＺｎ−Ｍｇ合金めっき鋼板をも作製した。このとき、付着量を片面当り１００ｇ／ｍ^２に設定し、界面にＺｎ−Ｆｅ合金層又はＺｎ−Ｆｅ−Ｍｇ合金層が形成されるように温度管理した。
【００１５】
比較例３
片面当り１００ｇ／ｍ² の付着量でＭｇ→Ｚｎの順に蒸着し、２７０〜３３０℃に加熱することにより、Ｍｇ濃度約１０重量％の下層とＭｇ濃度０．５重量％の上層からなる２層構造のＺｎ−Ｍｇ合金めっき鋼板を作製した。
比較例４
鋼板温度を９０℃に保持し、Ｚｎ蒸着→Ｚｎ，Ｍｇ同時蒸着→Ｚｎ蒸着を順次行い、下層からＺｎ層，Ｍｇ濃度約１０重量％のＺｎ−Ｍｇ合金層及びＺｎ層の３層が積層したＺｎ−Ｍｇ合金めっき鋼板を作製した。この場合も、付着量を片面当り１００ｇ／ｍ² に設定した。
めっき層表面にあるＭｇ濃化層を除去するため、各めっき鋼板を０．５％ＨＣｌ水溶液で１０秒間酸洗し、十分に水洗した。得られたＺｎ−Ｍｇ合金めっき鋼板について、同様な特性調査試験を行った結果を表２に示す。
【００１６】
【表２】

【００１７】
表２の結果から明らかなように、比較例のめっき鋼板は、耐食性，耐パウダリング性，スポット溶接性及び黒変性の何れにおいても劣っていた。特に表層までＭｇ濃度が高い比較例１，２では、明度変化 (ΔＬ^*)が大きく、良好な表面状態が維持されなかった。比較例３のめっき鋼板は、表層のＭｇ濃度が低いものの、パウダリングが多量に発生し、加工性に劣っていた。
【００１８】
【発明の効果】
以上に説明したように、本発明のＺｎ−Ｍｇ合金めっき鋼板は、下地鋼と高Ｍｇ濃度のＺｎ−Ｍｇ合金層との間に延性のある低Ｍｇ濃度のＺｎ−Ｍｇ合金層を介在させることにより、高Ｍｇ濃度のＺｎ−Ｍｇ合金層の耐食性を維持し、且つ硬質な高Ｍｇ濃度のＺｎ−Ｍｇ合金層と下地鋼との間で加工時に生じる変形量の差を低Ｍｇ濃度のＺｎ−Ｍｇ合金層で吸収している。これにより、Ｚｎ−Ｍｇ合金めっき層本来の高耐食性が活用され、加工性が良好なめっき鋼板が得られる。しかも、表層のＭｇ濃度が低下しているので、スポット溶接性も向上しためっき鋼板となる。
【図面の簡単な説明】
【図１】 ３層構造をもつＺｎ−Ｍｇ合金めっき層
【図２】 ５層構造をもつＺｎ−Ｍｇ合金めっき層
【図３】 下地鋼とめっき層との界面にＺｎ−Ｆｅ合金層又はＺｎ−Ｆｅ−Ｍｇ合金層を形成させた３層構造のＺｎ−Ｍｇ合金めっき層
【図４】 下地鋼とめっき層との界面にＺｎ−Ｆｅ合金層又はＺｎ−Ｆｅ−Ｍｇ合金層を形成させた５層構造のＺｎ−Ｍｇ合金めっき層[0001]
[Industrial application fields]
The present invention relates to a Zn-Mg alloy-plated steel sheet excellent in corrosion resistance, powdering resistance, adhesion, spot weldability, discoloration resistance, and the like, and a method for producing the same.
[0002]
[Prior art]
Conventionally, various surface treatments have been adopted to improve the corrosion resistance of the steel sheet. In particular, electroplating, hot dipping, etc. are mainly employed for Zn plating, which is a typical surface treatment method.
The demand for improved corrosion resistance tends to increase year by year, and various improvements have been proposed in the hot dipping method, electroplating method, and the like.
In order to improve the corrosion resistance of the Zn-plated steel sheet by the hot dipping method, it is considered firstly to increase the adhesion amount of the Zn plating layer. However, since the upper limit of the adhesion amount is restricted from the manufacturing aspect, there is a limit in improving the corrosion resistance by increasing the adhesion amount. Further, the increase in the amount of adhesion, that is, the thickening of the plating layer tends to cause defects such as galling and flaking when the plated steel sheet is press-formed.
[0003]
Similarly, when an electroplating method is used to form a plating layer having a high adhesion amount, it is necessary to reduce the line speed, and productivity is significantly impaired. Therefore, in the electroplating method, corrosion resistance is improved by applying Zn alloy plating such as Zn—Ni. However, since the Zn—Ni alloy plating layer is hard and brittle, defects such as cracks and chips are easily generated in the plating layer during molding. When such a defect occurs in the plated layer, the base steel is exposed through the defective portion, so that the original performance of the plated layer is not exhibited, and corrosion starts from the defective portion.
From the above background, it has been attempted to produce a highly corrosion-resistant Zn-based alloy-plated steel sheet by vapor deposition. Among these, the Zn—Mg alloy plating has an excellent anticorrosive action. For example, Japanese Patent Application Laid-Open No. 64-17853 discloses forming a Zn—Mg alloy plating layer containing 0.5 to 40% by weight of Mg. Further, when an intermediate layer such as Zn, Ni, Cu, Mg, Al, Fe, Co, Ti or the like is interposed between the Zn—Mg alloy plating layer and the base steel, the adhesion and workability of the plating layer are improved. This is introduced in Japanese Patent Laid-Open No. 2-141588.
[0004]
[Problems to be solved by the invention]
A Zn-Mg alloy-plated steel sheet with a high adhesion amount has a drawback that powdering is likely to occur during press working. In particular, when the Zn-Mg intermetallic compound increases in the plating layer as the Mg concentration increases, this tendency is remarkable when the intermetallic compound is present on the underlayer even when the average Mg concentration is low. become. This is because the Zn—Mg intermetallic compound layer is hard and brittle, and cannot follow the deformation of the underlying steel sheet having high ductility, and causes interfacial peeling and cracking.
Powdering is eliminated by lowering the amount of intermetallic compounds contained in the plating layer by increasing the Mg concentration and increasing ductility. However, a decrease in Mg concentration reduces the anticorrosive action of the plating layer. Further, even if only the surface side has a high Mg concentration, the powdering resistance is improved, but the surface of the plating layer turns black due to the Mg concentration, and the commercial value of the plated steel sheet is lowered. In addition, the high Mg concentration on the surface of the plating layer promotes the diffusion of Mg to the welding electrode and reduces spot weldability.
The present invention has been devised to solve such problems, and by making the Zn-Mg alloy plating layer a multilayer structure of a specific composition, defects due to high Mg concentration are suppressed, corrosion resistance, The object is to obtain a Zn-Mg alloy-plated steel sheet excellent in powdering resistance, adhesion, spot weldability, discoloration resistance and the like.
[0005]
[Means for Solving the Problems]
The Zn—Mg alloy plated steel sheet of the present invention has the basic layer structure shown in FIG. 1 in order to achieve the object. On the surface of the base steel, there is a Zn—Mg alloy layer with an Mg concentration of 0.5% by weight or less, and a Zn—Mg alloy layer with an Mg concentration of 7% by weight or more and a Zn—Mg alloy layer with an Mg concentration of 0.5% by weight or less. Mg alloy layers are sequentially stacked.
When higher corrosion resistance is required than the basic layer structure, as shown in FIG. 2, a Zn—Mg alloy layer having an Mg concentration of 0.5 wt% or less and a Zn—Mg alloy layer having an Mg concentration of 7 wt% or more are used. A Zn—Mg alloy layer having an Mg concentration of 2 to 7% by weight is provided therebetween.
In the Zn-Mg alloy layer having an Mg concentration of 7% by weight or more, the upper limit of the Mg content is preferably 20% by weight from the viewpoint of ensuring corrosion resistance in a highly humid environment.
[0006]
When the atmosphere at the time of vapor deposition contains O ₂ or H ₂ O, the surface of the steel sheet is oxidized and the adhesion of the plating layer is lowered. Further, even if the deposition is not performed immediately after the surface activation and the steel plate surface is expected to be contaminated, the adhesion of the plating layer may be lowered. In such a case, a Zn—Fe alloy layer or a Zn—Fe—Mg alloy layer is formed as shown in FIG. 3 or FIG. 4 in order to achieve adhesion of the Zn—Mg alloy plating layer to the steel sheet surface.
The Zn—Fe alloy layer or the Zn—Fe—Mg alloy layer preferably has a layer thickness of 0.5 μm or less in order to prevent powdering during molding. Moreover, the Fe concentration contained in the Zn—Fe alloy layer or Zn—Fe—Mg alloy layer is usually 6 wt% or more.
[0007]
These plated steel sheets are formed by heating after sequentially performing Zn deposition → Mg deposition → Zn deposition on the steel sheet surface. Alternatively, a Zn—Mg alloy plating layer having a required layer structure can be formed by simultaneously depositing Zn and Mg while changing the deposition rate.
By controlling conditions such as heating temperature and heating time after vapor deposition, a Zn-Fe alloy layer or Zn-Fe-Mg alloy layer having the required thickness is formed at the interface between the base steel and the plating layer. . For example, the heating temperature is set to 270 to 370 ° C. for short time heating within 50 seconds, and the heating temperature is set to 150 to 250 ° C. for long time heating for 1 hour or more. Moreover, it can replace with heat processing and can form the Zn-Mg alloy plating layer with a predetermined | prescribed layer structure also by setting conditions so that the steel plate temperature at the time of completion | finish of vapor deposition may be 270-370 degreeC. .
In the Zn—Mg alloy plating layer formed by the vapor deposition method, an oxidized Mg concentrated layer is usually present on the surface. In applications where the initial corrosion resistance is important, the Mg-concentrated layer is left as it is. However, in cases where spot weldability is important or when the surface is prevented from blackening, the Mg-concentrated layer is removed by pickling or the like. To do.
[0008]
[Action]
The Zn—Mg alloy plating layer according to the present invention has a multilayer structure in which there is a portion with a high Mg concentration at the center, and the Mg concentration of the layers above and below it is low. A layer having a low Mg concentration of 0.5% by weight or less has a relatively high elution rate, exhibits a sacrificial anticorrosive action, and prevents red rust from appearing on the exposed portion of the steel sheet in the brazed portion or the like. In particular, it is effective for preventing the initial occurrence of red rust. A layer having a high Mg concentration of 7% by weight or more has high corrosion resistance and extends the corrosion life of the plating layer itself. In addition, Mg dissolved from a layer having a high Mg concentration promotes the generation of ZnCl ₂ .4Zn (OH) ₂ and Zn (OH) ₂ which are corrosion products of Zn having excellent anticorrosion properties, and improves corrosion resistance. . A Zn—Mg alloy layer having an Mg concentration of 2 to 7 wt% and an intermediate concentration exhibits intermediate properties between the low Mg concentration layer and the high Mg concentration layer, and further improves the corrosion resistance. Such an action of each layer works synergistically, and excellent corrosion resistance unprecedented is exhibited.
[0009]
When a plated steel sheet is deformed by press working or the like, the Zn-Mg alloy layer having an Mg concentration of 0.5 wt% or less is ductile, so there is little deformation between the high Mg concentration Zn-Mg alloy layer and the underlying steel. Absorbs the difference in deformation during machining. As a result, the occurrence of powdering is suppressed.
As for the surface layer of the plating layer, the plated steel sheet is pickled, and the oxidized Mg concentrated layer on the surface is removed, so that the Mg concentration on the surface in contact with the welding electrode during spot welding becomes 0.5% by weight or less. The low Mg layer reduces Mg diffusion to the welding electrode and improves spot weldability. Further, when a large amount of Mg is present on the surface, the oxide or hydroxide of Zn becomes unsaturated and black discoloration occurs. This black discoloration can also be prevented by setting the Mg concentration in the surface layer to 0.5% by weight or less.
[0010]
【Example】
Production of Plated Steel Sheet According to the Present Invention As a plating base plate, C: 0.002 wt%, Si: 0.02 wt%, Mn: 0.21 wt%, P: 0.007 wt%, S: 0.001 A steel plate having a composition of wt%, Ti: 0.076 wt%, Al: 0.031 wt% and a plate thickness of 0.8 mm was used. The plating original plate was reduced and heated in an N ₂ -50% H ₂ gas atmosphere to remove the surface oxide film, and then set in a vacuum chamber. In the vacuum chamber, N ₂ gas having a dew point of −60 ° C. was introduced while evacuating with a pump, and the N ₂ partial pressure was maintained at 5 × 10 ⁻² Torr. In this vacuum chamber, vapor deposition was performed by the following procedure.
Evaporation was performed in the order of Zn → Mg → Zn, and the total deposition amount was set to 100 g / m ² per side, and the Zn amount deposited first and the Zn amount deposited last were the same.
[0011]
When manufacturing a plated steel sheet having a three-layer structure (FIG. 3) or a five-layer structure (FIG. 4) having a Zn—Fe alloy layer or a Zn—Fe—Mg alloy layer at the interface between the base steel and the plated layer, the temperature is set to 200 ° C. After vapor deposition in the order of Zn → Mg → Zn on the retained steel plate surface, the vacuum chamber was filled with N ₂ of 700 Torr and heated for 5 to 10 seconds. The heating temperature was set to 270 to 330 ° C. when forming a plating layer having a three-layer structure (FIG. 3) and 330 to 370 ° C. when forming a plating layer having a five-layer structure (FIG. 4). Thereby, the thickness of the Zn—Fe alloy layer or the Zn—Fe—Mg alloy layer was about 0.2 μm. As a result, the Mg concentration in the vicinity of the center with the highest Mg concentration was about 10 wt%, and the Mg concentration in the upper and lower layers was 0.5 wt% or less. Further, the Mg concentration of the intermediate layer was about 4% by weight.
Further, by simultaneously depositing Zn and Mg while changing the deposition rate, a Zn-Mg alloy plating layer having a layer structure shown in FIGS. 1 and 2 and having a Zn deposition amount of 100 g / m ² per side is formed. did. The concentration of each layer was the same as the previous one (FIGS. 3 and 4). At this time, the steel plate during vapor deposition was kept at a plate temperature of 120 ° C. Note that no heat treatment was performed after vapor deposition.
[0012]
Each of the above plated steel sheets was pickled with a 0.5% HCl aqueous solution to remove the Mg concentrated layer on the surface. The plated steel sheet after pickling was thoroughly washed with water. When the obtained plated steel sheet was observed, it was a plated layer having the layer structure shown in Table 1.
The characteristics of each plated steel sheet were investigated. Corrosion resistance was evaluated by a red rust occurrence time by performing a salt spray test based on JIS Z2371. The powdering property is such that a test piece is sandwiched between a mold having a bead with a height of 4 mm and R = 0.5 mm, and the test piece is pulled out of the mold at a pressure of 500 kgf and a drawing speed of 200 m / min. A low bead test was performed and the amount of generated powdering was evaluated. Spot weldability was evaluated based on the number of striking points on which a CF-type Cu-1% Cr electrode having a tip diameter of 4.5 mm was attached to a single-phase AC type welding machine and continuous welding was possible. The black discoloration was evaluated by a lightness difference ΔL ^* before and after the test, which was left for 1000 hours in an accelerated tester at a temperature of 50 ° C. and a relative humidity of 60%.
As is apparent from Table 1 showing the test results, the Zn-Mg alloy-plated steel sheet having a three-layer structure or a five-layer structure according to the present invention has corrosion resistance, powdering resistance, spot weldability, and black discoloration resistance. Both were excellent. Further, in the case where the plating layer having the structure of FIGS. 3 and 4 is formed, the adhesion of the plating layer is good even when vapor deposition is performed in an oxidizing atmosphere where H ₂ O or O ₂ is present at several tens of ppm. there were.
[0013]
[Table 1]

[0014]
Comparative Example 1
The same plating base plate was kept at 120 ° C., and a Zn—Mg alloy plating layer having a uniform composition with an adhesion amount per side of 100 g / m ² was formed by simultaneous vapor deposition of Zn and Mg.
Comparative Example 2
The steel plate temperature is maintained at 200 ° C., and after vapor deposition in the order of Zn → Mg, the lower layer is heated to 270 to 330 ° C., so that the lower layer is a Zn—Mg alloy layer having an Mg concentration of 0.5 wt% or less, and the upper layer has an Mg concentration of 10 A Zn-Mg alloy-plated steel sheet having a two-layer structure consisting of a wt% Zn-Mg alloy layer was also produced. At this time, the adhesion amount was set to 100 g / m ² per side, and the temperature was controlled so that a Zn—Fe alloy layer or a Zn—Fe—Mg alloy layer was formed at the interface.
[0015]
Comparative Example 3
Two layers consisting of a lower layer with an Mg concentration of about 10% by weight and an upper layer with an Mg concentration of 0.5% by weight are deposited in the order of Mg → Zn in an adhesion amount of 100 g / m ² per side and heated to 270 to 330 ° C. A Zn—Mg alloy-plated steel sheet having a structure was prepared.
Comparative Example 4
The steel plate temperature was maintained at 90 ° C., and Zn vapor deposition → Zn and Mg simultaneous vapor deposition → Zn vapor deposition were sequentially performed, and a Zn layer, a Zn—Mg alloy layer having an Mg concentration of about 10 wt%, and a Zn layer were laminated from the lower layer. A Zn-Mg alloy plated steel sheet was prepared. Also in this case, the adhesion amount was set to 100 g / m ² per side.
In order to remove the Mg concentrated layer on the surface of the plating layer, each plated steel sheet was pickled with a 0.5% HCl aqueous solution for 10 seconds and sufficiently washed with water. Table 2 shows the results of a similar characteristic investigation test performed on the obtained Zn-Mg alloy-plated steel sheet.
[0016]
[Table 2]

[0017]
As is clear from the results in Table 2, the plated steel sheet of the comparative example was inferior in all of corrosion resistance, powdering resistance, spot weldability and blackening. Particularly in Comparative Examples 1 and 2 where the Mg concentration was high up to the surface layer, the brightness change (ΔL ^* ) was large, and a good surface state was not maintained. Although the plated steel sheet of Comparative Example 3 had a low surface layer Mg concentration, a large amount of powdering occurred and the workability was poor.
[0018]
【The invention's effect】
As explained above, the Zn-Mg alloy plated steel sheet of the present invention has a ductile low Mg concentration Zn-Mg alloy layer interposed between the base steel and the high Mg concentration Zn-Mg alloy layer. Thus, the corrosion resistance of the Zn-Mg alloy layer having a high Mg concentration is maintained, and the difference in deformation caused during processing between the hard Zn-Mg alloy layer having a high Mg concentration and the base steel is reduced by Zn- having a low Mg concentration. It is absorbed by the Mg alloy layer. Thereby, the high corrosion resistance inherent in the Zn—Mg alloy plating layer is utilized, and a plated steel sheet having good workability can be obtained. And since the Mg density | concentration of surface layer is falling, it becomes a plated steel plate which also improved the spot weldability.
[Brief description of the drawings]
[Fig. 1] Zn-Mg alloy plating layer having a three-layer structure [Fig. 2] Zn-Mg alloy plating layer having a five-layer structure [Fig. 3] Zn-Fe alloy layer or Zn at the interface between the base steel and the plating layer A Zn-Mg alloy plating layer having a three-layer structure in which a -Fe-Mg alloy layer is formed. Fig. 4 A Zn-Fe alloy layer or a Zn-Fe-Mg alloy layer is formed at the interface between the base steel and the plating layer. 5-layer Zn-Mg alloy plating layer

Claims (4)

Ｍｇ濃度０．５重量％以下のＺｎ−Ｍｇ合金層，Ｍｇ濃度７重量％以上のＺｎ−Ｍｇ合金層及びＭｇ濃度０．５重量％以下のＺｎ−Ｍｇ合金層が順次積層されているＺｎ−Ｍｇ合金めっき鋼板。A Zn—Mg alloy layer having an Mg concentration of 0.5 wt% or less, a Zn—Mg alloy layer having an Mg concentration of 7 wt% or more, and a Zn—Mg alloy layer having an Mg concentration of 0.5 wt% or less are sequentially laminated. Mg alloy plated steel sheet. Ｍｇ濃度０．５重量％以下のＺｎ−Ｍｇ合金層，Ｍｇ濃度２〜７重量％のＺｎ−Ｍｇ合金層，Ｍｇ濃度７重量％以上のＺｎ−Ｍｇ合金層，Ｍｇ濃度７重量％以上のＺｎ−Ｍｇ合金層，Ｍｇ濃度２〜７重量％のＺｎ−Ｍｇ合金層及びＭｇ濃度０．５重量％以下のＺｎ−Ｍｇ合金層が順次積層されているＺｎ−Ｍｇ合金めっき鋼板。Zn-Mg alloy layer with Mg concentration of 0.5 wt% or less, Zn-Mg alloy layer with Mg concentration of 2-7 wt%, Zn-Mg alloy layer with Mg concentration of 7 wt% or more, Zn with Mg concentration of 7 wt% or more A Zn-Mg alloy-plated steel sheet in which a Mg alloy layer, a Zn-Mg alloy layer with a Mg concentration of 2 to 7% by weight, and a Zn-Mg alloy layer with a Mg concentration of 0.5% by weight or less are sequentially laminated. 下地鋼との界面にＺｎ−Ｆｅ合金層又はＺｎ−Ｆｅ−Ｍｇ合金層が形成されている請求項１又は２記載のＺｎ−Ｍｇ合金めっき鋼板。The Zn-Mg alloy plated steel sheet according to claim 1 or 2, wherein a Zn-Fe alloy layer or a Zn-Fe-Mg alloy layer is formed at an interface with the base steel. 鋼板にＺｎ，Ｍｇ及びＺｎを順次蒸着し、次いで加熱することを特徴とする請求項１〜３の何れかに記載のＺｎ−Ｍｇ合金めっき鋼板の製造方法。The method for producing a Zn-Mg alloy-plated steel sheet according to any one of claims 1 to 3, wherein Zn, Mg and Zn are sequentially deposited on the steel sheet and then heated.

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