JP4102176B2 - Painted steel sheet with excellent corrosion resistance - Google Patents
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Description
【０００１】
【産業上の利用分野】
本発明は、加工によってＡｌ−Ｚｎ合金めっき層に発生しがちな亀裂を塗膜で抑制し、Ａｌ−Ｚｎ合金めっき鋼板の優れた耐食性を活用できる塗装鋼板に関する。
【０００２】
【従来技術及び問題点】
Ａｌ−Ｚｎ合金めっき鋼板は、Ａｌに由来する耐熱性，耐食性及びＺｎに由来する犠牲防食作用を兼ね備え、従来の溶融Ｚｎめっき鋼板に比較して無塗装で数倍〜十数倍の耐食性を呈する。優れた耐食性を活用し、無塗装のままで或いは美観付与の塗装を施した後で屋根材，壁材等の建材として一部で使用されている。
Ａｌ−Ｚｎ合金めっき層は、優れた耐食性を呈するものの、軟質のＺｎリッチ相と硬質のＡｌリッチ相が混在した不均質組織になっているため、Ｚｎリッチ相／Ａｌリッチ相の界面に応力が集中しやすい。そのため、Ａｌ−Ｚｎ合金めっき鋼板を加工すると、Ｚｎリッチ相／Ａｌリッチ相の界面を起点とする亀裂が発生し、亀裂を介して露出した下地鋼が腐食しやすくなる。
めっき層を均質化する熱処理（特開平９−３１６６１８号公報）で亀裂の発生をある程度防止できるが、均質化しためっき層ではＺｎリッチ相の犠牲防食作用が損なわれ、耐食性が劣化する場合がある。
【０００３】
【課題を解決するための手段】
本発明は、優れた耐食性に有効な不均質組織を損なうことなく、鋼板表面に設ける表層塗膜の改善によって加工時に発生しがちなめっき層の亀裂を防止し、加工部耐食性を改善した塗装鋼板を提供することを目的とする。
【０００４】
本発明の塗装鋼板は、その目的を達成するため、Ａｌ：５０〜６０質量％を含むＡｌ−Ｚｎ合金めっき鋼板を基材とし、基材表面に設けた塗膜の少なくとも表層塗膜が伸び率１００％以上，１００％伸ばしたときの引張荷重が塗膜幅１０ｍｍ当り１Ｎ以上の熱硬化性樹脂塗膜であることを特徴とする。
表層塗膜には、高分子ポリエステル等も使用可能であるが、ガラス転移温度Ｔgの高いポリエステルの配合により塗膜強度を高めた膜厚２５μｍ以上のウレタン塗膜が好ましく、外装建材等で要求される耐光性を付与できる。
【０００５】
【作用】
硬質Ａｌリッチ相と軟質Ｚｎリッチ相との混合組織からなるＡｌ−Ｚｎ合金めっき層を形成しためっき鋼板に曲げ，深絞り等の加工を施すと、Ｚｎリッチ相／Ａｌリッチ相の界面に応力が集中し、亀裂が発生する。曲げ曲率や絞り比が大きくなるほど亀裂幅が大きくなり、亀裂を介した下地鋼の露出面積が広がる。本発明者等は、加工を受けたＡｌ−Ｚｎ合金めっき層の亀裂発生メカニズムを考慮し、応力分散，応力緩和を図るため表層塗膜の伸び，強度を利用することに想到した。伸び，強度のある表層塗膜によりＡｌ−Ｚｎ合金めっき層の亀裂が抑えられることは次のように推測される。
【０００６】
たとえば、Ａｌ−Ｚｎ合金めっき鋼板１０を曲げ加工すると、曲げ部では下地鋼１１の塑性変形に伴って塗膜１２も伸びる（図１）。伸び率の小さな塗膜１２では下地鋼１１の塑性変形に追従できずに破断することがあるが、伸び率の大きな塗膜１２は下地鋼１１の塑性変形に十分追従し、曲げ加工後にも連続塗膜として下地鋼１１を覆う。他方、下地鋼１１と塗膜１２との間にあるＡｌ−Ｚｎ合金めっき層１３では、軟質Ｚｎリッチ相１３aと硬質Ａｌリッチ相１３bとの界面に応力が集中して界面に亀裂１３cが発生する（図２）。亀裂１３cの幅は、加工度の大きな曲げ部外側ほど大きくなる。
【０００７】
Ａｌ−Ｚｎ合金めっき層１３の上に形成した伸び率，塗膜強度の大きな表層塗膜１４は、曲げ加工時に下地鋼１１の塑性変形に追従して連続塗膜のまま伸びるが、塗膜伸びに伴ってＡｌ−Ｚｎ合金めっき層１３を下地鋼１１に押し付ける方向の圧縮力Ｆが働く（図３）。圧縮力Ｆが加えられる条件下でＡｌ−Ｚｎ合金めっき層１３が曲げ応力を受けるため、曲げ応力がめっき層１３全体で受け止められ、Ｚｎリッチ相／Ａｌリッチ相の界面への応力集中が緩和される。その結果、めっき層１３の亀裂発生が抑制される。仮に、亀裂が発生しても下地鋼１１の表面に達する亀裂１３c（図２）にまで成長しない。したがって、曲げ加工後にも亀裂を介した下地鋼１１の露出がなく、Ａｌ−Ｚｎ合金めっき層１３本来の優れた防食機能が維持される。
【０００８】
表層塗膜１４の伸び率，塗膜強度がＡｌ−Ｚｎ合金めっき層の亀裂抑制に有効であるとの推論で、塗膜伸び率，塗膜強度と亀裂抑制との関係を調査・検討した結果、塗膜伸び率を１００％以上に調整すると１８０度曲げによっても曲げ部外側で表層塗膜１４が破断せず、塗膜を１００％伸ばした状態で塗膜幅１０ｍｍ当り引張荷重が１Ｎ以上の塗膜強度に調整するときめっき層１３の亀裂が効果的に抑制されることが判った。
【０００９】
塗膜伸び率は一般的に樹脂のガラス転移温度Ｔgに応じて異なり、ガラス転移温度Ｔgが低いほど高い伸び率を示す。ガラス転移温度Ｔgの低いポリエステルをベース樹脂に使用しているウレタン塗膜では、３００％に達する伸び率を呈するものもある。しかし、ガラス転移温度Ｔgが低い表層塗膜であっても、塗膜強度が低いと成形時にＡｌ−Ｚｎ合金めっき層を拘束する圧縮力Ｆが不足する。そこで、ウレタン塗料ではガラス転移温度Ｔgの低いポリエステルとガラス転移温度Ｔgの高いポリエステルとの配合によってガラス転移温度Ｔgを好ましくは５〜１５℃の範囲に調整し、塗膜伸び率，塗膜強度をバランスさせる。
【００１０】
ガラス転移温度Ｔgの高いポリエステルの配合は、塗膜伸び率を低下させるものの、１００％以上の塗膜伸び率を確保することは容易であり、亀裂抑制に有効な圧縮力Ｆが加工時に得られる。硬質ポリエステルは、表層塗膜を硬質化し耐疵付き性，耐候性の向上にも有効である。
具体的には、ガラス転移温度Ｔgの低いポリエステル樹脂とイソシアネート硬化剤からなるウレタン塗料から成膜される塗膜は、ガラス転移温度Ｔgの高いポリエステル樹脂の配合量で塗膜性状を調整でき（表１）、膜厚によっても塗膜伸び率，塗膜強度を調整できる（表２）。また、ガラス転移温度Ｔgの高いポリエステル樹脂を２５質量％配合したウレタン塗膜では、塗膜の伸びと共に引張荷重が増し二次曲線関係になる。塗膜の伸びと共に引張荷重が高くなる傾向は熱硬化型樹脂の特徴であり、高分子ポリエステル等、他の熱硬化型樹脂でも同様である。引張荷重と塗膜伸びの二次関数的な関係は、塗膜伸びに伴って分子密度が上昇して塗膜が硬質化することに起因するものと推察される。
【００１１】
【実施の形態】
本発明で塗装原板に使用するＡｌ−Ｚｎ合金めっき鋼板は、Ａｌ含有量：５０〜６０質量％のＡｌ−Ｚｎ合金めっき層が鋼板表面に形成されているめっき鋼板である。Ａｌ−Ｚｎ合金めっき層は、優れた耐食性を付与するものの、硬質粒子，軟質粒子が混在した金属組織のため加工等の際に亀裂が発生しやすい。加工性に劣る欠点は、Ａｌ−Ｚｎ合金めっき層上に塗膜伸び率の高いウレタン，高分子ポリエステル等の表層塗膜を設けることにより改善できる。
【００１４】
塗膜伸び率の高い熱硬化型樹脂としては、ウレタン，高分子ポリエステルが挙げられる。
ウレタンは、ポリエステルをイソシアネートで架橋したイソシアネート硬化型ポリエステル樹脂である。主樹脂となるポリエステルは、分子量が高すぎると耐候性が劣るので、数平均分子量が３０００〜１０００の範囲にある低ポリエステル，中ポリエステルが好ましい。
高分子ポリエステルは、ポリエステルをメラミンで架橋した樹脂であり、数平均分子量１０００〜２００００の範囲にあるものが好ましい。高分子ポリエステルの伸び率，塗膜強度は、主樹脂単体の構造をコントロールし、或いは伸び率が異なる樹脂を配合することによって調整される。ただし、分子構造や硬化剤によって若干の変化があるものの分子量の高いポリエステル樹脂ほど耐候性が劣るので、耐食性，耐候性が要求される外装建材としての用途を考慮するとウレタン樹脂の方が好ましい。
【００１５】
表層塗膜の伸び率，強度は、塗料樹脂組成物のガラス転移温度Ｔg，膜厚によって調整できる。ガラス転移温度Ｔgが異なる複数のポリエステル樹脂から塗膜の主樹脂を調製する際に各ポリエステル樹脂の配合量にガラス転移温度Ｔgを調整でき、それに応じて塗膜伸び率，塗膜強度も調整できる。なお、塗膜の伸び率は、破断するまで塗膜を引っ張ったときの伸びを当初の塗膜長さに対する割合で示す。
たとえば、ガラス転移温度Ｔgの低いポリエステル樹脂の配合量が多くなるほど伸び率が向上し，ガラス転移温度Ｔgの高いポリエステル樹脂の配合量が多くなるほど塗膜強度が向上する。具体的には、ウレタン塗料に硬質ポリエステルを配合した系では、ガラス転移温度Ｔgの高いポリエステルの増量に応じて軟化開始温度が高くなるが、５０質量％の高ガラス転移温度Ｔgのポリエステルを配合した場合でも塗膜伸び率１００％以上で、塗膜を１００％伸ばしたときの引張荷重１Ｎ以上を確保できる（表１）。また、表層塗膜を厚膜化することによっても、塗膜伸び率１００％以上，引張荷重１Ｎ以上が達成される（表２）。
【００１６】
表層塗膜の伸び率を１００％以上，引張荷重を１Ｎ以上に調整するとき、塗装鋼板を高加工しても表層塗膜が破断することなく、しかも表層塗膜によってＡｌ−Ｚｎ合金めっき層に圧縮力Ｆが加えられるためＺｎリッチ相／Ａｌリッチ相の界面に集中する応力が緩和される。その結果、後述の実施例でも明らかなように、Ａｌ−Ｚｎ合金めっき層に亀裂が発生しがたく、仮に亀裂が発生した場合でもめっき層の表層に留まり下地鋼に達する亀裂とならない。他方、伸び率が１００％に達しない塗膜では、曲げ部外側等、高度の加工を受けた部位で塗膜破断が生じやすく、加工後の塗膜性状が劣化する。塗膜強度が引張荷重１Ｎ未満では、加工時にめっき層が受ける圧縮力Ｆが不足し、Ｚｎリッチ相／Ａｌリッチ相の界面を起点とする亀裂が下地鋼に達するまで成長しやすくなる。
【００１７】
なお、表層塗膜の形成に先立って、必要に応じて塗装原板に塗装前処理を施し、下塗り塗膜，中塗り塗膜等を形成することもできる。塗装前処理としては、たとえばアルカリ脱脂→湯洗→水洗→乾燥→塗布型クロメート処理→乾燥の工程が採られる。腐食雰囲気に曝される外装建材としての用途では、エポキシ，ポリエステル等の下塗り塗膜を設けることが好ましく、長期耐久性の上ではエポキシ樹脂が下塗り塗膜のベース樹脂に使用される。更に長期耐久性，耐疵付き性等を改善するため、中塗り塗膜を設ける場合もある。中塗りには、ウレタン、高分子ポリエステル等をベースとする塗料が使用される。
【００１８】
【実施例１】
片面当りめっき付着量７５ｇ／ｍ2でＡｌ：５５質量％を含むＡｌ−Ｚｎ合金めっき層が形成された板厚０.５ｍｍの溶融Ａｌ−Ｚｎ合金めっき鋼板をゼンジミア方式の連続溶融めっきラインで製造した。
Ａｌ−Ｚｎ合金めっき鋼板をアルカリ脱脂した後、湯洗，水洗により洗浄し、乾燥した。次いで、塗布型クロメート処理液（サーフコートNRC300NS：日本ペイント株式会社製）をロールコーターで塗布し、１００℃で乾燥させ、全Ｃｒ付着量：４０ｍｇ／ｍ2のクロメート皮膜を形成した。
【００１９】
クロメート皮膜の上に、クロム酸ストロンチウムを不揮発分にして２５質量％配合したエポキシ又はポリエステル系樹脂の下塗り塗料を塗装し、２１０℃で乾燥・焼付けすることにより、プライマ塗膜を形成した。
更に、プライマ塗膜の上にウレタン系の熱硬化型樹脂塗料を塗布し、１００％以上の塗膜伸び率，引張荷重（オートグラフを用い遊離塗膜を１００％伸ばしたときの荷重を塗膜幅１０ｍｍ当りで示した値）１Ｎ以上が得られる乾燥膜厚の表層塗膜を形成した（本発明例１〜８）。熱硬化性樹脂塗料にガラス転移温度Ｔgの高いポリエステルを配合することにより、表層塗膜の強度を調整した。
比較のため、同様なクロメート皮膜，エポキシ又はポリエステル樹脂系下塗り塗膜を形成した後、伸び率が１００％に達しない乾燥膜厚の表層塗膜（比較例１〜４）及び引張荷重１Ｎ未満の表層塗膜（比較例５〜６）を設けた塗装鋼板を用意した。
【００２０】
各塗装鋼板から試験片を切り出し、加工性試験及び加工部耐食性試験に供した。
〔加工性試験〕
サイズ５０ｍｍ×５０ｍｍの試験片を使用し、試験片の圧延方向と直角に試験面を外にして直径２ｍｍの棒の回りに約１秒かけて１８０度折り曲げた。折曲げに際し、曲げ部内側に試験片と同じ厚さの板を所定枚数挟み込み、万力を用いて急速に締め付けた。曲げ部内側に何も挟み込まずに締め付けたものを０Ｔ曲げ（密着曲げ）、同じ厚さの板をｎ枚挟み込んで締め付けたものをｎＴ曲げとした。曲げ部外側にある塗膜を倍率２０倍の視野で観察し、割れが検出できなかった塗膜を◎，軽微な割れが発生した塗膜を○，中程度の割れが発生した塗膜を△，著しい亀裂が発生した塗膜を×として加工性を評価した。
【００２２】
〔加工部耐食性試験〕
圧延方向と直交方向に５０ｍｍ，平行方向に２０ｍｍの長さで塗装めっき鋼板から切り出した試験片を使用し、加工性試験と同様な方法で０Ｔ曲げ及び２Ｔ曲げ加工した後、切断端面及び裏面を塗料で補修した。幅６５ｍｍ，高さ１５０ｍｍの樹脂板の面に対して垂直，幅方向に対して平行に試験片をシリコーン系接着剤で貼り付け、試験サンプルを用意した。
試験サンプルを１０００時間の塩水噴霧試験（JIS K−5600−7−1）に供し、錆及び膨れの発生を評価した。白錆発生が検出されなかった加工部を◎、加工部全長さに対して白錆の発生率が１０％以下を○，１０〜３０％を△，３０％を超える白錆が発生した加工部を×として加工部の耐白錆性を評価した。塗膜膨れが発生していない加工部を◎，加工部全長さに対して膨れの発生率が１０％以下を○，１０〜３０％を△，３０％を超えるフクレが発生した加工部を×として加工部の耐膨れ性を評価した。
【００２３】
表４の調査結果にみられるように、本発明例１〜８の塗装めっき鋼板は、加工性及び加工部耐食性共に優れ、加工試験後の塗膜割れや加工部耐食性試験後の白錆，塗膜膨れ等が検出されなかった。この結果から、溶融Ａｌ−Ｚｎ合金めっき鋼板を基材とし、伸び率１００％以上，引張荷重１Ｎ以上の表層塗膜を形成することにより、高加工を受けた部位でも塗膜破断やめっき層の亀裂がなく、優れた加工部耐食性を呈した。
【００２４】
他方、塗膜伸び率１００％未満の表層塗膜を形成した比較例１〜４の塗装めっき鋼板では、塗膜伸び率に応じて若干の差があるものの、０Ｔ曲げ加工部，２Ｔ曲げ加工部何れも塗膜が破断していた。塗膜強度が引張荷重１Ｎ未満の表層塗膜を形成した比較例５〜６の塗装めっき鋼板では、下地鋼に達する亀裂がめっき層に生じており、亀裂を起点とする腐食が検出された。
この対比結果は、伸び率が１００％以上，塗膜強度が引張荷重１Ｎ以上の表層塗膜を形成することにより加工部の塗膜破断や腐食発生が抑制されることを意味する。
【００２５】
【実施例２】
実施例１と同じＡｌ−Ｚｎ合金めっき鋼板を塗装原板に使用し、エポキシ系プライマ塗膜を介しウレタン塗料の塗布・焼付けで表層塗膜を形成した。ガラス転位温度Ｔgの高いポリエステルを２５質量％配合したウレタン塗料を用い、塗布量の調整により表層塗膜の乾燥膜厚を５〜３５μｍの間で変化させた。
得られた塗装鋼板を実施例１と同様に０Ｔ曲げ加工した後、曲げ部外側の断面を観察した。図５〜９の観察結果にみられるように、表層塗膜の厚膜化に応じてＡｌ−Ｚｎ合金めっき層の亀裂が少なくなっており、下地鋼に達する亀裂が膜厚２５μｍで大幅に低減し、膜厚３５μｍでほぼ皆無となった。膜厚２５μｍ以上は、表２から伸び率１００％以上，引張荷重１Ｎ以上の表層塗膜であることが判る。
【００２７】
【発明の効果】
以上に説明したように、塗膜伸びが率１００％以上，塗膜強度が引張荷重１Ｎ以上の表層塗膜をＡｌ−Ｚｎ合金めっき鋼板の表面に設けた塗装鋼板では、高加工しても塗膜破断が生じることなく加工後にも良好な表面状態の塗膜が維持される。しかも、加工時に基材の塑性変形に追従する表層塗膜によって圧縮力がＡｌ−Ｚｎ合金めっき層に加えられるため、Ｚｎリッチ相／Ａｌリッチ相の界面に集中しがちな応力が緩和され、めっき層の亀裂も抑制される。その結果、加工後においても美麗な表面が維持され、Ａｌ−Ｚｎ合金めっき層本来の優れた耐食性を呈する製品として広範な分野で使用される。
【図面の簡単な説明】
【図１】 塗装Ａｌ−Ｚｎ合金めっき鋼板を曲げ加工した曲げ部断面の模式図
【図２】 めっき層に亀裂が発生した塗装Ａｌ−Ｚｎ合金めっき鋼板の曲げ部断面の模式図
【図３】 亀裂発生が表層塗膜の圧縮力で抑えられることを説明する模式図
【図４】 ウレタン塗膜の伸び率と引張荷重との関係を示すグラフ
【図５】 エポキシプライマ塗膜の上に膜厚５μｍのウレタン塗膜を設けた塗装Ａｌ−Ｚｎ合金めっき鋼板の０Ｔ曲げ部断面を示す顕微鏡写真
【図６】 エポキシプライマ塗膜の上に膜厚１０μｍのウレタン塗膜を設けた塗装Ａｌ−Ｚｎ合金めっき鋼板の０Ｔ曲げ部断面を示す顕微鏡写真
【図７】 エポキシプライマ塗膜の上に膜厚１５μｍのウレタン塗膜を設けた塗装Ａｌ−Ｚｎ合金めっき鋼板の０Ｔ曲げ部断面を示す顕微鏡写真
【図８】 エポキシプライマ塗膜の上に膜厚２５μｍのウレタン塗膜を設けた塗装Ａｌ−Ｚｎ合金めっき鋼板の０Ｔ曲げ部断面を示す顕微鏡写真
【図９】 エポキシプライマ塗膜の上に膜厚３０μｍのウレタン塗膜を設けた塗装Ａｌ−Ｚｎ合金めっき鋼板の０Ｔ曲げ部断面を示す顕微鏡写真
【符号の説明】
１０：Ａｌ−Ｚｎ合金めっき鋼板 １１：下地鋼 １２：塗膜 １３：Ａｌ−Ｚｎ合金めっき層 １３a：軟質Ｚｎリッチ相 １３b：硬質Ａｌリッチ相 １３c：めっき層の亀裂 １４：表層塗膜[0001]
[Industrial application fields]
The present invention relates to a coated steel sheet that can suppress cracks that tend to occur in an Al—Zn alloy plated layer by processing with a coating film and that can utilize the excellent corrosion resistance of the Al—Zn alloy plated steel sheet.
[0002]
[Prior art and problems]
The Al-Zn alloy-plated steel sheet has heat resistance, corrosion resistance derived from Al, and sacrificial anticorrosive action derived from Zn, and exhibits corrosion resistance several times to several tens of times without coating compared to conventional hot-dip Zn-plated steel sheets. . Utilizing excellent corrosion resistance, some are used as building materials, such as roofing materials and wall materials, without being painted or after being painted with beauty.
Although the Al-Zn alloy plating layer exhibits excellent corrosion resistance, it has a heterogeneous structure in which a soft Zn-rich phase and a hard Al-rich phase are mixed, so stress is applied to the interface between the Zn-rich phase and the Al-rich phase. Easy to concentrate. For this reason, when an Al—Zn alloy-plated steel sheet is processed, cracks starting from the interface of the Zn-rich phase / Al-rich phase are generated, and the underlying steel exposed through the cracks is easily corroded.
Although heat treatment for homogenizing the plating layer (Japanese Patent Laid-Open No. 9-316618) can prevent cracking to some extent, the sacrificial anticorrosive action of the Zn-rich phase is impaired in the homogenized plating layer, and the corrosion resistance may deteriorate. .
[0003]
[Means for Solving the Problems]
The present invention is a coated steel sheet that prevents cracking of the plating layer that tends to occur during processing by improving the surface coating film provided on the surface of the steel sheet without impairing the heterogeneous structure effective for excellent corrosion resistance, and improves the corrosion resistance of the processed part The purpose is to provide.
[0004]
In order to achieve the purpose of the coated steel sheet of the present invention, Al: Zn alloy-plated steel sheet containing Al: 50 to 60% by mass is used as a base material, and at least the surface coating film of the coating film provided on the base material surface has an elongation percentage. The thermosetting resin coating film is characterized in that the tensile load when stretched by 100% or more or 100% is 1 N or more per 10 mm of the coating film width.
The surface coating, but may also be used polymers such as polyester, film strength of more than the film thickness 25μm with enhanced urethane coating film formulation of high glass transition temperature Tg polyesters are preferable, requests exterior building materials Light resistance can be imparted.
[0005]
[Action]
When a plated steel sheet with an Al-Zn alloy plating layer composed of a mixed structure of a hard Al-rich phase and a soft Zn-rich phase is subjected to bending or deep drawing, stress is applied to the Zn-rich phase / Al-rich phase interface. Concentrate and crack. As the bending curvature and the drawing ratio increase, the crack width increases, and the exposed area of the base steel through the crack increases. The present inventors considered the crack generation mechanism of the processed Al—Zn alloy plating layer, and came up with the idea of using the elongation and strength of the surface coating film in order to achieve stress dispersion and stress relaxation. It is presumed that the crack of the Al—Zn alloy plating layer is suppressed by the stretched and strong surface coating as follows.
[0006]
For example, when the Al—Zn alloy-plated
[0007]
The surface coating film 14 having a high elongation rate and coating strength formed on the Al—Zn
[0008]
The result of investigating and examining the relationship between the elongation rate of coating film, coating strength, and crack suppression, inferring that the elongation rate and coating strength of surface coating film 14 are effective for crack suppression of Al-Zn alloy plating layer When the elongation percentage of the coating film is adjusted to 100% or more, the surface coating film 14 does not break outside the bent portion even by 180-degree bending, and the tensile load is 1 N or more per 10 mm coating film width in a state where the coating film is stretched 100%. It has been found that cracks in the plating
[0009]
Coating growth rate generally depends on the glass transition temperature Tg of the resin, showing a high elongation lower the glass transition temperature Tg. The urethane coating having a low glass transition temperature Tg polyesters are used in the base resin, some exhibit elongation reaches 300%. However, the glass transition temperature Tg is a lower surface layer coating, the compressive force F is insufficient coating film strength to restrain the lower the Al-Zn alloy plating layer at the time of molding. Therefore, preferably the glass transition temperature Tg by blending with high low polyester and glass transition temperature Tg of the glass transition temperature Tg polyester urethane paint was adjusted in the range of 5 to 15 ° C., the coating film elongation coating Balance film strength.
[0010]
Blending the high glass transition temperature Tg polyester, although lowering the coating film elongation, it is easy to secure a coating film elongation above 100%, resulting effective compressive force to crack suppression F is in processing It is done. Hard polyester hardens the surface coating film and is effective in improving wrinkle resistance and weather resistance.
Specifically, the coating film is formed from a urethane coating consisting of lower polyester resins and isocyanate curing agent glass transition temperature Tg can adjust the coating properties by the amount of high polyester resin glass transition temperature Tg (Table 1) The coating film elongation rate and the coating film strength can also be adjusted by the film thickness (Table 2). Further, the urethane coating formulated with higher polyester resin glass transition temperature Tg 25 wt%, a tensile load along with elongation of the coating film is increased becomes quadratic curve relation. The tendency for the tensile load to increase with the elongation of the coating film is a characteristic of thermosetting resins, and the same applies to other thermosetting resins such as polymer polyesters. It is inferred that the quadratic function relationship between the tensile load and the coating film elongation is caused by the molecular density increasing with the coating film elongation and the coating film becoming hard.
[0011]
Embodiment
The Al—Zn alloy-plated steel sheet used for the coating original sheet in the present invention is a plated steel sheet in which an Al—Zn alloy plating layer having an Al content of 50 to 60% by mass is formed on the steel sheet surface. Although the Al—Zn alloy plating layer imparts excellent corrosion resistance, cracks are likely to occur during processing or the like because of the metal structure in which hard particles and soft particles are mixed. The disadvantage of being inferior in workability can be improved by providing a surface coating film such as urethane or polymer polyester having a high coating film elongation on the Al—Zn alloy plating layer.
[0014]
Examples of the thermosetting resin having a high coating film elongation include urethane and polymer polyester.
Urethane is an isocyanate curable polyester resin obtained by crosslinking polyester with isocyanate. Since the polyester used as the main resin has poor weather resistance when the molecular weight is too high, low polyesters and medium polyesters having a number average molecular weight in the range of 3000 to 1000 are preferred.
The high molecular polyester is a resin obtained by crosslinking polyester with melamine, and preferably has a number average molecular weight of 1000 to 20000. The elongation rate and coating film strength of the polymer polyester are adjusted by controlling the structure of the main resin alone or by blending resins having different elongation rates. However, although the weather resistance of a polyester resin having a higher molecular weight is inferior in spite of slight changes depending on the molecular structure and curing agent, the urethane resin is more preferable in consideration of the use as an exterior building material that requires corrosion resistance and weather resistance.
[0015]
Elongation of the surface layer coating film strength can be adjusted glass transition temperature Tg of the coating resin composition, the film thickness. The amount of the polyester resin in the glass transition temperature Tg to prepare a main resin of the coating from different polyester resins can adjust the glass transition temperature Tg, the coating film elongation accordingly, the coating film strength Can be adjusted. In addition, the elongation rate of a coating film shows elongation when a coating film is pulled until it fractures, as a ratio with respect to the initial coating film length.
For example, it improved as the elongation amount of low polyester resin glass transition temperature Tg is increased, thereby improving the film strength as the amount of high polyester resin glass transition temperature Tg is increased. Specifically, in a system obtained by blending hard polyester urethane coating, although initial softening temperature becomes higher in accordance with increase of the high glass transition temperature Tg polyester, 50% by weight of polyester high glass transition temperature Tg Even if it mix | blends, with the coating-film elongation rate of 100% or more, the tensile load of 1 N or more when a coating film is extended 100% is securable (Table 1). Further, by increasing the thickness of the surface coating film, a coating film elongation of 100% or more and a tensile load of 1 N or more can be achieved (Table 2).
[0016]
When adjusting the elongation rate of the surface coating film to 100% or more and the tensile load to 1N or more, the surface coating film does not break even when the coated steel sheet is processed to a high degree, and the surface coating film forms an Al-Zn alloy plating layer. Since the compressive force F is applied, the stress concentrated on the interface between the Zn rich phase / Al rich phase is relieved. As a result, as will be apparent from the examples described later, cracks hardly occur in the Al—Zn alloy plating layer, and even if cracks occur, they remain on the surface of the plating layer and do not reach the base steel. On the other hand, in a coating film whose elongation does not reach 100%, coating film breakage is likely to occur at sites that have undergone a high degree of processing, such as the outside of the bent portion, and the coating film properties after processing deteriorate. When the coating film strength is less than 1 N tensile load, the compressive force F received by the plating layer during processing is insufficient, and cracks starting from the Zn-rich phase / Al-rich phase interface tend to grow until reaching the base steel.
[0017]
Prior to the formation of the surface coating film, a coating pre-treatment can be applied to the coating original plate as necessary to form an undercoating film, an intermediate coating film, or the like. As the pretreatment for coating, for example, a process of alkaline degreasing → washing with water → washing with water → drying → coating chromate treatment → drying is employed. For use as an exterior building material exposed to a corrosive atmosphere, it is preferable to provide an undercoat film such as epoxy or polyester, and epoxy resin is used as the base resin of the undercoat film for long-term durability. Furthermore, in order to improve long-term durability, scratch resistance, etc., an intermediate coating film may be provided. For the intermediate coating, a paint based on urethane, polymer polyester or the like is used.
[0018]
[Example 1]
Manufactured a 0.5 mm thick hot-dip Al-Zn alloy-plated steel sheet with an Al-Zn alloy plating layer containing Al: 55 mass% at a coating weight of 75 g / m 2 per side on a Sendzimir type continuous hot-dip plating line did.
The Al-Zn alloy-plated steel sheet was degreased with alkali, washed with hot water and water, and dried. Next, a coating type chromate treatment solution (Surfcoat NRC300NS: manufactured by Nippon Paint Co., Ltd.) was applied with a roll coater and dried at 100 ° C. to form a chromate film with a total Cr adhesion amount of 40 mg / m 2 .
[0019]
On the chromate film, an undercoat paint of epoxy or polyester resin containing 25% by mass of strontium chromate in a nonvolatile content was applied, dried and baked at 210 ° C. to form a primer film.
Furthermore, a urethane-based thermosetting resin coating is applied on the primer coating, and the coating elongation is 100% or more and the tensile load (the load when the free coating is extended 100% using an autograph is applied to the coating. A value shown per width of 10 mm) A surface coating film having a dry film thickness of 1 N or more was formed (Invention Examples 1 to 8). By blending high glass transition temperature Tg polyester thermosetting resin coating material was adjust the intensity of the surface coating.
For comparison, after forming a similar chromate film, epoxy or polyester resin-based undercoat film, the surface layer film (Comparative Examples 1 to 4 ) having a dry film thickness that does not reach 100% and a tensile load of less than 1 N The coated steel plate which provided the surface layer coating film (Comparative Examples 5-6 ) was prepared.
[0020]
A test piece was cut out from each coated steel sheet and subjected to a workability test and a processed part corrosion resistance test.
[Workability test]
A test piece having a size of 50 mm × 50 mm was used, and the test surface was perpendicular to the rolling direction of the test piece, and the test piece was bent 180 degrees around a 2 mm diameter rod for about 1 second. In bending, a predetermined number of plates having the same thickness as the test piece were sandwiched inside the bent portion and fastened using a vise. What was clamped without pinching anything inside the bent part was 0T bend (contact bend), and what was clamped by sandwiching n sheets of the same thickness was called nT bend. Observe the coating film on the outside of the bending part with a field of view of 20 times magnification, ◎ for coatings where cracks could not be detected, ○ for coatings with minor cracks, and △ for coatings with moderate cracks The workability was evaluated with x indicating a coating film in which significant cracks occurred.
[0022]
[Processed part corrosion resistance test]
Using a test piece cut from a coated steel sheet with a length of 50 mm in the direction perpendicular to the rolling direction and 20 mm in the parallel direction, and after 0T bending and 2T bending by the same method as the workability test, Repaired with paint. A test sample was prepared by attaching a test piece with a silicone adhesive in a direction perpendicular to the surface of a resin plate having a width of 65 mm and a height of 150 mm and parallel to the width direction.
The test sample was subjected to a 1000 hour salt spray test (JIS K-5600-7-1) to evaluate the occurrence of rust and blistering. Machining parts where no white rust was detected were marked with ◎, white rust occurrence rate was 10% or less, and 10-30% was marked with respect to the total length of the machining area, and machining areas with white rust exceeding 30%. The white rust resistance of the processed part was evaluated as x. ◎ for processed parts with no coating bulging, ○ for bulging occurrence rate of 10% or less, △ for 10-30%, and processed part with over 30% bulge The blistering resistance of the processed part was evaluated.
[0023]
As can be seen from the investigation results in Table 4, the coated plated steel sheets of Examples 1 to 8 of the present invention are excellent in both workability and processed part corrosion resistance, and are coated with cracks after processing test and white rust and coated after processed part corrosion resistance test. Film swelling or the like was not detected. From this result, by using a molten Al-Zn alloy-plated steel sheet as a base material and forming a surface coating film having an elongation of 100% or more and a tensile load of 1 N or more, the coating film breakage or the plating layer can be obtained even at sites subjected to high processing. There was no crack, and excellent corrosion resistance of the processed part was exhibited.
[0024]
On the other hand, in the coated plated steel sheets of Comparative Examples 1 to 4 in which a surface coating film having a coating film elongation rate of less than 100% was formed, although there was a slight difference depending on the coating film elongation rate, the 0T bending part and the 2T bending part In all cases, the coating film was broken. In the coated plated steel sheets of Comparative Examples 5 to 6 in which the coating film strength formed a surface coating film having a tensile load of less than 1 N, cracks reaching the base steel occurred in the plated layer, and corrosion starting from the cracks was detected.
This comparison result means that the formation of a surface coating film having an elongation of 100% or more and a coating film strength of 1 N or more of tensile load suppresses coating film breakage and corrosion occurrence in the processed part.
[0025]
[Example 2]
The same Al—Zn alloy-plated steel plate as in Example 1 was used as a coating original plate, and a surface coating film was formed by applying and baking a urethane coating through an epoxy primer coating. A urethane coating containing 25% by mass of polyester having a high glass transition temperature Tg was used, and the dry film thickness of the surface coating film was changed between 5 and 35 μm by adjusting the coating amount.
The obtained coated steel sheet was subjected to 0T bending in the same manner as in Example 1, and then the cross section outside the bent portion was observed. As can be seen from the observation results in FIGS. 5 to 9, cracks in the Al—Zn alloy plating layer are reduced according to the increase in the thickness of the surface coating film, and the crack reaching the base steel is greatly reduced at a film thickness of 25 μm. However, almost no film was formed at a film thickness of 35 μm. It can be seen from Table 2 that a film thickness of 25 μm or more is a surface coating film having an elongation of 100% or more and a tensile load of 1 N or more.
[0027]
【The invention's effect】
As explained above, the coated steel sheet provided with a surface coating film on the surface of the Al-Zn alloy-plated steel sheet having a coating film elongation rate of 100% or more and a coating strength of 1N or more is applied even if it is highly processed. A film having a good surface state is maintained even after processing without film breakage. Moreover, since a compressive force is applied to the Al-Zn alloy plating layer by the surface layer coating that follows the plastic deformation of the base material during processing, the stress that tends to concentrate at the interface between the Zn-rich phase / Al-rich phase is alleviated, and plating Layer cracking is also suppressed. As a result, a beautiful surface is maintained even after processing, and the product is used in a wide range of fields as a product exhibiting excellent corrosion resistance inherent to the Al—Zn alloy plating layer.
[Brief description of the drawings]
FIG. 1 is a schematic diagram of a cross section of a bent portion obtained by bending a coated Al—Zn alloy plated steel sheet. FIG. 2 is a schematic diagram of a cross section of a bent portion of a coated Al—Zn alloy plated steel sheet in which a crack is generated in a plating layer. Schematic diagram explaining that crack generation is suppressed by the compressive force of the surface coating film. [FIG. 4] A graph showing the relationship between the elongation percentage of the urethane coating film and the tensile load. [FIG. 5] Film thickness on the epoxy primer coating film. Micrograph showing cross section of 0T bend of coated Al-Zn alloy plated steel sheet provided with 5 μm urethane coating. [FIG. 6] Painted Al—Zn alloy provided with 10 μm thick urethane coating on epoxy primer coating. Micrograph showing a 0T bend section of a plated steel sheet. [Fig. 7] Micrograph showing a 0T bend section of a coated Al-Zn alloy-plated steel sheet having a 15 μm thick urethane coating on an epoxy primer coating. 8] Micrograph showing cross section of 0T bending part of coated Al-Zn alloy plated steel sheet with 25μm thick urethane coating on Poxy Primer coating. [Fig.9] 30μm thick urethane coating on epoxy primer coating. Micrograph showing cross section of 0T bending part of coated Al-Zn alloy plated steel sheet
DESCRIPTION OF SYMBOLS 10: Al-Zn alloy plating steel plate 11: Base steel 12: Coating film 13: Al-Zn
Claims (1)
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP2002353481A JP4102176B2 (en) | 2002-12-05 | 2002-12-05 | Painted steel sheet with excellent corrosion resistance |
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| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP2002353481A JP4102176B2 (en) | 2002-12-05 | 2002-12-05 | Painted steel sheet with excellent corrosion resistance |
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| JP2004181860A JP2004181860A (en) | 2004-07-02 |
| JP4102176B2 true JP4102176B2 (en) | 2008-06-18 |
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| JP4931673B2 (en) * | 2007-04-05 | 2012-05-16 | 日新製鋼株式会社 | Painted steel sheet with excellent corrosion resistance |
| JP5398161B2 (en) * | 2008-03-31 | 2014-01-29 | 日新製鋼株式会社 | Clear painted steel plate with excellent corrosion resistance |
| CN114250408B (en) * | 2020-09-21 | 2023-02-28 | 上海梅山钢铁股份有限公司 | Hot-dip aluminum-zinc embossed steel plate with yield strength of 280MPa and manufacturing method thereof |
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