JP2004052000A

JP2004052000A - Heat resistant precoated steel sheet having excellent workability and corrosion resistance

Info

Publication number: JP2004052000A
Application number: JP2002207002A
Authority: JP
Inventors: Keiji Izumi; 和泉　圭二; Takeshi Shimizu; 清水　剛
Original assignee: Nisshin Steel Co Ltd
Current assignee: Nippon Steel Nisshin Co Ltd
Priority date: 2002-07-16
Filing date: 2002-07-16
Publication date: 2004-02-19
Anticipated expiration: 2022-07-16
Also published as: JP4055942B2

Abstract

<P>PROBLEM TO BE SOLVED: To provide a heat resistant precoated steel sheet which has excellent heat resistance, workability and corrosion resistance, and is suitable as the stock for a cooker, a heating machine, an air conditioner, an automobile exhaust gas flow passage member or the like. <P>SOLUTION: In the steel sheet, an Al-Si alloy plated steel sheet is used as a base material, and one side or both sides thereof are provided with a coating film formed from a coating material obtained by blending flaky powder into a methyl silicone resin expressed by general formula of (CH<SB>3</SB>)<SB>a</SB>SiO<SB>(4-a-b)/2</SB>(OH)<SB>b</SB>[wherein, a=0.5 to 1.5, and b=0.5 to 1.05] via a chemical conversion-treated film mainly made up of of the oxide and hydroxide of valve metal. The chemical conversion-treated film contains, as major components, the oxide and hydroxide of valve metal such as Ti, Zr, Hf, V, Nb, Ta, Mo and W. As the flaky powder dispersed into the coating film, aluminum flakes, alumina flakes, stainless steel flakes, glass flakes, mica powder, talc powder, planar kaolin, barium sulfate flakes or the like can be cited. <P>COPYRIGHT: (C)2004,JPO

Description

【０００１】
【産業上の利用分野】
本発明は、加熱調理器具，暖房機器，空調機器，自動車排ガス流路部材等に使用され、耐食品汚染性，耐食性，耐熱性に優れた耐熱プレコート鋼板に関する。
【０００２】
【従来の技術】
耐食性の良好な鋼材としてＡｌ−Ｓｉ合金めっき鋼板が多用されているが、湿潤雰囲気，排ガス雰囲気，海塩粒子飛散雰囲気等にＡｌ−Ｓｉ合金めっき鋼板を長期間放置すると、鋼板表面に白錆が発生し外観が劣化する。白錆の発生はＡｌ−Ｓｉ合金めっき鋼板をクロメート処理することにより防止でき、クロメート皮膜は塗膜密着性の向上にも有効である。
【０００３】
クロメート処理では、環境にとって有害な水溶性のＣｒイオンを含むクロメート処理液，リンス液等の廃液処理に多大な負担がかかる。そこで、チタン系，ジルコニウム系，モリブデン系，リン酸塩系等の薬液を使用したＣｒフリーの化成処理方法が検討されており、アルミニウム材料ではＤＩ缶等への適用を主目的として多数の提案がある。
たとえば、チタン系では、チタン化合物，燐酸イオン，フッ化物，促進剤を含む水溶液をアルミニウム含有金属材料に接触させ、水洗・乾燥することにより化成処理皮膜を形成する方法が特開平９−２０９８４号公報で紹介されている。本発明者等も、バルブメタルの酸化物，水酸化物を主成分とするＣｒフリーの化成処理皮膜を形成するとき、Ａｌ−Ｓｉ合金めっき鋼板の耐食性が改善されることを提案した（特願２００２−０１５６６１号）。
【０００４】
ところで、耐熱性が要求される加熱調理器具，暖房機器，空調機器，自動車排ガス流路部材等の素材にＡｌ−Ｓｉ合金めっき鋼板を使用するとき、周囲部材との調和を損なう金属光沢を嫌って鋼板表面に樹脂塗膜を設けた塗装鋼板が好まれている。なかでも、製品形状に加工した後での塗装が不要な耐熱プレコート鋼板の提供が強く望まれている。
耐熱プレコート鋼板の塗膜には、４００℃以上の耐熱温度，２〜４ｔ程度の１８０度折曲げ加工によっても塗膜剥離が生じない加工性が要求される。そこで、アルキル基，アルケニル基，フェニル基を配合したシリコーン樹脂塗料（特開昭６３−１７２６４０号公報，特開平２−２６５７４２号公報）を使用し、シロキサン結合のネットワークによって塗膜を強化している。
【０００５】
シリコーン樹脂は、従来の有機樹脂に比較して耐熱性に優れたシロキサン結合を主骨格にしているが、樹脂に導入される有機基の種類や含有量によって樹脂塗膜の特性が大きく変動する。一般的には、シリコーン樹脂中の有機基の含有量を減らすと耐熱性が向上するものの加工性が低下する。この点、従来のシリコーン樹脂は、樹脂中の有機基比率が比較的高いためプレコート化に耐える加工性をもつが４００℃以上の耐熱性に劣る。しかも、４００℃以上に加熱されると、樹脂中のＳｉ−Ｃ_６Ｈ_５の結合が徐々に切断されて少量ながらもベンゼンガスが放出され、発煙もある。
【０００６】
本出願人は、耐熱性に優れたプレコート鋼板用の塗料としてモノメチルシラノールゾルにエポキシ樹脂及びイソシアネートを配合した塗料を紹介した（特開平８−２４５９２２号公報）。当該塗料は、シリコーン樹脂中の有機基比率が比較的低いため４００℃以上の耐熱性に優れているが、加工性に劣るため有機樹脂の添加によってプレコート化を可能にしている。
【０００７】
当該塗料を用いて製造された塗装鋼板から作られた製品では、使用初期に添加有機樹脂が加熱分解する欠点がある。特にイソシアネートの加熱分解に際しては特有の臭気又は煙が発生する。添加有機樹脂が加熱分解した直後にはモノメチルシラノールから形成されるオルガノポリシロキサン樹脂もシロキサン結合の十分なネットワークを形成しておらず、一時的に不安定な皮膜状態を経る。そのため、かかる問題が懸念される用途、たとえば身近に使用される加熱調理機器，耐食性が要求される自動車排気系部材等では、添加有機樹脂の加熱分解及びオルガノポリシロキサン樹脂のネットワーク形成促進による耐食性向上を狙って、プレス加工後に４００〜５００℃に数分加熱する予備加熱を施した後で本製品に組み込む工程が採用される。
【０００８】
【発明が解決しようとする課題】
予備加熱の必要なく成形加工された耐熱プレコート鋼板をそのまま各種機器に組み込めると、製造工程が簡略化され、製品コストの低減も図られる。臭気や煙を予め除去する予備加熱が省略可能で耐熱性も良好な塗膜を形成する耐熱塗料としては、フェニル基を有するシリコーン樹脂に代えてメチルシリコーン樹脂をベース樹脂に使用することが考えられる。しかし、メチルシリコーン樹脂をベースとする塗膜を形成した耐熱プレコート鋼板は、３００〜３５０℃の加熱及び４００℃の短時間加熱で塗膜性能が著しく低下する。塗膜の性能低下は、次のように推察される。
【０００９】
樹脂に含まれるＳｉ−ＣＨ_３結合は、３００〜３５０℃の加熱で分解反応を開始する。他方、シラノール基（Ｓｉ−ＯＨ）間の脱水縮合反応によりＳｉ−Ｏ−Ｓｉのシロキサン結合を進行させて十分なネットワーク構造を形成する上では４００℃以上の高温長時間加熱が必要とされる。そのため、メチルシリコーン樹脂単独の系が３００〜４００℃の中間温度域に加熱されると、Ｓｉ−ＣＨ_３結合が加熱分解する一方で、ネットワーク構造の形成に必要なＳｉ−Ｏ−Ｓｉのシロキサン結合が十分に進行しない。その結果、メチルシリコーン樹脂から成膜された塗膜は、一時的に不安定な皮膜状態になる。
【００１０】
また、加熱調理器具等の実使用状態を想定すると、食品類や調味料の飛散・付着が予想される。食品類や調味料はＮａＣｌ，ミネラル，有機酸，硫酸イオン等を多く含み、酸性の調味料が多く、ｐＨ３以下の強酸性調味料もある。付着した食品類の腐敗によるｐＨ値の低下も予想される。付着した食品類や調味料が腐食性因子となって、加熱調理器具等の構成材料である鋼板が過酷な腐食性雰囲気に曝される。
耐熱プレコート鋼板を加熱調理器具等の素材に使用する場合、所定形状に成形加工した耐熱プレコート鋼板を空焼き（予備加熱）せず加熱調理器具等に組み込むことがある。この場合、加熱調理器具等の使用中に耐熱プレコート鋼板が加熱されるが、幅広い温度域での後加熱が余儀なくされ、コーナー部，合せ部や加熱源から離れた個所等では後加熱温度が低くなる。
【００１１】
【課題を解決するための手段】
本発明は、Ａｌ−Ｓｉ合金めっき鋼板にＣｒフリーの皮膜を設けた化成処理鋼板が塗膜密着性に優れていることに着目し、耐熱性に優れた基材，化成処理皮膜，樹脂塗膜の特定された組合せにより、予備加熱や後加熱を省略しても使用時に異臭や煙が発生せず、耐食性，加工性，耐食品汚染性に優れた耐熱プレコート鋼板を提供することを目的とする。
【００１２】
本発明の耐熱プレコート鋼板は、その目的を達成するため、Ａｌ−Ｓｉ合金めっき鋼板を基材とし、バルブメタルの酸化物及び水酸化物を主体とする化成処理皮膜を介し、一般式（ＣＨ_３）_ａＳｉＯ_{（４−ａ−ｂ）／２}（ＯＨ）_ｂ〔ただし、ａ＝０．５〜１．５，ｂ＝０．５〜１．０５〕で表されるメチルシリコーン樹脂に鱗片状粉末を配合した塗料から形成された塗膜が片面又は両面に設けられていることを特徴とする。
【００１３】
基材としては、めっき層全体としてＳｉ含有量が５〜１３質量％，表層のＳｉ含有量が７〜８０質量％のＡｌ−Ｓｉ合金めっき層が下地鋼板の表面に形成されているＡｌ−Ｓｉ合金めっき鋼板が好適である。化成処理皮膜は、Ｔｉ，Ｚｒ，Ｈｆ，Ｖ，Ｎｂ，Ｔａ，Ｍｏ，Ｗから選ばれた１種又は２種以上のバルブメタルの酸化物，水酸化物を主成分にしている。塗膜には、アルミフレーク，アルミナフレーク，ステンレス鋼フレーク，ガラスフレーク，アルミナフレーク，マイカ粉，タルク粉，板状カオリン，硫酸バリウムフレークから選ばれた１種又は２種以上の鱗片状粉末が分散している。
【００１４】
【実施の形態】
Ａｌ−Ｓｉ合金めっき鋼板の下地鋼には、低炭素鋼，中炭素鋼，高炭素鋼，合金鋼等が使用される。なかでも、良好なプレス成形性が要求される用途では、低炭素Ｔｉ添加鋼，低炭素Ｎｂ添加鋼等の深絞り用鋼板が好ましい。下地鋼は、常法に従って溶融アルミニウムめっきされるが、Ａｌ−Ｓｉ合金めっき層のＳｉ含有量を５〜１３質量％の範囲に調整することが好ましい。Ｓｉ含有量を５質量％以上とすることにより、めっき層表層にＳｉが濃化しやすくなると共に、下地鋼とめっき層との界面に生じ加工性に有害な合金層の成長が抑制される。しかし、１３質量％を超える過剰量のＳｉが含まれると、溶融めっき後の冷却過程で初晶Ｓｉがめっき層に晶出し、加工性が著しく劣化する。
【００１５】
Ｓｉ含有量を５〜１３質量％に調整したＡｌ−Ｓｉ合金めっき鋼板を溶融めっき浴から引き上げ、冷却速度等を調整することによって予めめっき層の表層にＳｉを濃化させた後、酸洗，アルカリ洗浄等を施すことにより金属Ｓｉ主体の凸部及びＡｌリッチの凹部がめっき層の表層に形成される。酸洗，アルカリ洗浄等で金属Ｓｉ主体の凸部及びＡｌリッチの凹部を形成する場合、水洗，乾燥工程を必要とする。他方、Ａｌに対してエッチング作用のある化成処理液を使用する場合、化成処理液をめっき層に塗布して乾燥させる化成処理皮膜の生成過程で表層のＡｌが選択的にエッチング除去され、Ａｌリッチの凹部が形成される。
【００１６】
金属Ｓｉ主体の凸部及びＡｌリッチの凹部がめっき層の表層に分散している状況は、ＡＥＳ分析法を用いて１０００μｍ四方のエリアを走査・分析し、同様にＡｒスパッタで表層から１００ｎｍの深さまで繰返し分析することにより確認できる。本発明者等による実験結果から、めっき層の表層から１００ｎｍまでの深さにおけるＳｉ濃度を７質量％以上にするとき、目標とする平坦部耐食性及び加工部耐食性が得られることが判った。しかし、表層のＳｉ濃度が８０質量％を超えるまでＡｌがエッチング除去されると、めっき層の表層が脆くなり、プレス加工時等の際に鋼板が変形すると化成処理皮膜が脱落しやすくなる。
【００１７】
Ａｌ−Ｓｉ合金めっき鋼板に化成処理液を塗布し、水洗せずに乾燥することによりバルブメタルの酸化物，水酸化物を主成分にする化成処理皮膜が形成される。化成処理液は、バルブメタルを含む限り塗布型又は反応型の何れであってもよいが、反応型化成処理では処理液の安定性を維持する上からｐＨを若干低く調整する。
バルブメタルは、酸化物が高い絶縁抵抗を示す金属を指し、Ｔｉ，Ｚｒ，Ｈｆ，Ｖ，Ｎｂ，Ｔａ，Ｍｏ，Ｗの１種又は２種以上が使用される。たとえば、バルブメタルにＴｉを使用した化成処理液は、ＴｉＣｌ_４，（ＮＨ_４）_２ＴｉＦ_６，ＴｉＯＳＯ_４，Ｔｉ（ＳＯ_４）_２，Ｔｉ（ＯＨ）_２，Ｋ_２［ＴｉＯ（ＣＯＯ）_２］，ＸＴｉＦ_６（Ｘ：アルカリ金属又はアルカリ土類金属，ｎ：１又は２）等をＴｉソースとして含む。ＴｉＦ_６ ^２−＋４Ｈ_２Ｏ→Ｔｉ（ＯＨ）_４＋６Ｆ⁻等の反応によって皮膜の最終的な主成分がバルブメタルの酸化物，水酸化物となる限り、フッ化物，硫酸塩等もＴｉソースに使用できる。
【００１８】
化成処理液には、Ａｌ−Ｓｉ合金めっき層の表面をエッチングにより活性化するため、フッ化物や各種酸類を複合添加しても良い。キレート作用のある酒石酸，タンニン酸，クエン酸，蓚酸，マロン酸，乳酸，酪酸等の有機酸を添加すると、化成処理液の安定性が向上する。更に、金属リン酸塩又は複合リン酸塩を化成処理液に添加すると、化成処理皮膜の欠陥部に溶出してＡｌと反応し不溶性リン酸塩となって析出するので、化成処理皮膜に自己修復機能が付与される。
【００１９】
バルブメタルの酸化物又は水酸化物からなる皮膜は、電子の移動に対する抵抗体として働き、雰囲気中の水分に含まれている溶存酸素による還元反応（下地鋼との酸化反応）が抑えられる。高い絶縁抵抗を示すバルブメタルの酸化物は、腐食促進因子や水，プロトン，酸素等の腐食作用を呈する物質を効果的に遮蔽する。その結果、下地鋼からの金属成分の溶出（腐食）が防止される。
バルブメタルの酸化物の他に、水酸化物も化成処理皮膜に存在する。酸化物，水酸化物は、Ｓｉ−Ｏ結合の進展によりメチルシリコーン樹脂に十分なネットワーク構造が形成されるまでの３００〜４００℃の中間温度域で、シラノール基（Ｓｉ−ＯＨ）又はシロキサン結合（Ｓｉ−Ｏ）と水素結合する。水素結合の形成により、化成処理皮膜に塗布されたメチルシリコーン樹脂との密着性が向上し、３００〜４００℃の中間温度域における耐食品汚染性，耐食性が飛躍的に向上する。
【００２０】
化成処理皮膜は、ロールコート法，スピンコート法，スプレー法等でＡｌ−Ｓｉ合金めっき鋼板に化成処理液を塗布し、水洗せずに乾燥することにより形成される。常温乾燥も可能であるが、連続操業を考慮すると５０℃以上に保持して乾燥時間を短縮することが好ましい。しかし、乾燥温度が高温になるほど化成処理皮膜に含まれているバルブメタルの水酸化物が減少するので、乾燥温度の上限を３００℃に設定することが好ましい。化成処理液の塗布量は、十分な耐食性を確保するため１ｍｇ／ｍ^２以上のチタン付着量となるように調整することが好ましい。なお、化成処理に先立って、必要に応じＡｌ−Ｓｉ合金めっき鋼板がアルカリ脱脂される。
【００２１】
化成処理皮膜形成後、直ちにメチルシリコーン樹脂に鱗片状粉末を配合した樹脂塗料を塗布し焼付け・乾燥によって樹脂塗膜を形成する。樹脂塗料の塗布にはスプレー法，ロールコート法，バーコート法等が採用され、塗膜の硬度を保証し加工密着性を確保するために好ましくは１５０〜３００℃の範囲で加熱・乾燥される。３００℃を超える高温乾燥では、耐食品汚染性，耐食性に悪影響が現れるだけでなく塗膜の加工性も損なわれ、プレコート鋼板としての特性が得られない。樹脂塗膜の膜厚は、耐食性，加工密着性を勘案して好ましくは２〜１５μｍ（好ましくは、１０μｍ以下）の範囲に調整される。膜厚２μｍ以上で耐食性の改善効果がみられるが、１５μｍを超える厚膜では塗膜の加工密着性が低下する。
【００２２】
メチルシリコーン樹脂は、一般式（ＣＨ_３）_ａＳｉＯ_{（４−ａ−ｂ）／２}（ＯＨ）_ｂをもつ化合物である。指数ａ，ｂは、ブロッキングを起こすことなく塗膜に必要硬度を付与し、長時間焼付けを必要とせずにコイルでの連続塗装を可能にするため、それぞれａ＝０．５〜１．５，ｂ＝０．５〜１．０５の範囲に調整することが好ましい。ａ＜０．５では塗膜の加工密着性が低下し、ａ＞１．５では耐熱性が劣る。ｂ＜０．５では、塗装原板に配向した水酸基との脱水縮合によって結合する起点が少なくなるため加工密着性が低下し、塗膜の硬化性も劣る。逆にｂ＞１．０５では、焼成時に三次元架橋が過度に進行して塗膜の加工密着性が低下する。
【００２３】
メチルシリコーン樹脂に配合する鱗片状粉末には、アルミフレーク，アルミナフレーク，ステンレス鋼フレーク，ガラスフレーク，マイカ粉，タルク粉，板状カオリン，硫酸バリウムフレーク等、５００℃の耐熱性をもつものが好ましく、各種フレークを単独で或いは２種以上組み合わせて使用できる。電子オーブンレンジ等の赤外線放射特性が要求される用途や樹脂塗料の化学的安定性を考慮すると、アルミナフレーク，マイカ粉，タルク粉，板状カオリン，硫酸バリウムフレーク等の難水溶性酸化物のフレークが好適である。
【００２４】
鱗片状粉末は、塗膜に分散した状態で基材・Ａｌ−Ｓｉ合金めっき鋼板の面内方向と平行に配位し、Ｓｉ−ＣＨ_３結合の加熱分解が進行している過程で塗膜密着性，耐水性等の物性を改善する。物性改善効果を発現させる上で、鱗片状粉末の平均粒度を０．５〜５０μｍ，樹脂固形分に対する配合量を５〜１００質量部に設定することが好ましい。５質量部未満の配合量では鱗片状粉末の添加効果が不十分であり、１００質量部を超える配合量では加工密着性が低下する傾向がみられる。鱗片状粉末は各種の焼成顔料，防錆顔料，金属粉等と併用添加でき、併用添加する場合には合計配合量を２００質量部以下に調整することが好ましい。
【００２５】
鱗片状粉末の添加効果は、配合量の他に鱗片状粉末の形状によっても異なり、０．５μｍ以上の平均粒径で顕著になる。しかし、５０μｍを超える平均粒径では、加工密着性が低下する傾向にある。鱗片状粉末の平均粒径は、より好ましくは１〜３０μｍの範囲にある。シロキサン結合のネットワーク構造を補強する上では、厚みが平均粒径の１／５以下の鱗片状粉末を塗膜に分散させ、下地表面と平行な方向に関する鱗片状粉末の配向性を高めることが好ましい。厚みが平均粒径の１／５を超える鱗片状粉末では、下地表面に交叉する方向に配位する鱗片状粉末の分散状態が避けられず、また鱗片状粉末単位重量当りのシリコーン樹脂との接触面積が小さくなることから鱗片状粉末の添加効果が十分に発現しない。
【００２６】
樹脂塗料には、意匠性，耐食性を改善し、触媒機能等の各種機能を付与するために着色顔料，体質顔料，メタリック顔料，触媒，防錆顔料，金属粉等、各種添加剤が必要に応じて配合される。着色顔料には、Ｍｎ，Ｆｅ，Ｃｒ，Ｃｕ，Ｔｉ等の酸化物や複合酸化物，グラファイト，カーボンブラック等がある。防錆顔料には、従来のクロム系の他に、環境を考慮したモリブデン酸カルシウム，リンモリブデン酸カルシウム，リンモリブデン酸アルミニウム等の非クロム系も使用される。触媒にはＴｉＯ_２を始めとする光触媒、金属粉にはＮｉ，Ｃｏ，Ｃｕ等がある。
【００２７】
【実施例】
板厚０．４ｍｍの極低炭素鋼板を溶融めっきし、めっき付着量４０ｇ／ｍ^２でＳｉ：６〜１１質量％のＡｌ−Ｓｉ合金めっき層を形成した。このＡｌ−Ｓｉ合金めっき鋼板を原板に使用し、バルブメタルの化合物を種々の比率で含む化成処理液を塗布した後、水洗せずにオーブンに装入し、最高到達板温１５０℃で乾燥することにより、バルブメタルの酸化物，水酸化物を主成分とする化成処理皮膜を形成した。
化成処理皮膜形成後、直ちに樹脂塗料をバーコート法で塗布し、最高到達板温２３０℃で加熱焼成することにより、乾燥膜厚６μｍの塗膜を形成した。樹脂塗料には、一般式（ＣＨ_３）_０．９５ＳｉＯ_１．０５（ＯＨ）_０．９５のメチルシリコーン樹脂をベースとし、樹脂の合計重量１００質量部に対し黒色顔料（ＭｎＣｕＣｒＯ_ｘ焼成顔料）：１００質量部，各種鱗片状粉末：２０質量部を配合した塗料を使用した。
原板のＡｌ−Ｓｉ合金めっき層，化成処理液の組成，樹脂塗料に配合した鱗片状粉末を表１に示す。
【００２８】

【００２９】
得られた塗装鋼板から試験片を切り出し、加工試験，腐食試験に供した。
加工試験では、試験片を１８０度折曲げ加工（３〜６ｔ）し、加工部に粘着テープを貼り付けた後で引き剥がすテープ剥離試験によって塗膜の剥離状況を調査した。剥離が検出されない塗膜を◎，僅かに剥離が検出された塗膜を○，下地鋼から剥離した塗膜を×として加工密着性を評価した。
【００３０】
腐食試験では、未加熱の試験片の他に３００℃×１０時間，３５０℃×１０時間，４００℃×１時間，５００℃×１００時間の加熱を施した試験片を用意した。ＪＩＳ　Ｚ２３７１に準拠した塩水噴霧を１００時間継続した後、テープ剥離試験で各試験片の塗膜密着性（二次密着性）を調査した。剥離が検出されなかった塗膜を◎，点状の剥離が一部に生じた塗膜を○，点状の剥離が著しく生じた塗膜を△，全面剥離した塗膜を×として二次密着性を評価した。
【００３１】
表２の調査結果にみられるように、Ａｌ−Ｓｉ合金めっき鋼板を塗装原板とし、化成処理皮膜を介して鱗片状粉末分散塗膜を設けた本発明例Ｎｏ．１〜８は、耐熱性，加工密着性，耐食性の何れにも優れていた。加熱された後でも良好な耐食性が維持されており、加熱中に異臭ガスも発生しなかった。
他方、鱗片状粉末無添加の塗膜を形成した塗装鋼板Ｎｏ．１０は、３００〜４００℃の温度域に加熱されたときの耐食性に劣っていた。化成処理皮膜を設けることなく樹脂塗膜を直接形成した塗装鋼板Ｎｏ．９は、耐熱性，加工性の何れにも劣っていた。タンニン酸処理後に樹脂塗膜を形成した塗装鋼板Ｎｏ．１１は、基材に対する塗膜の密着性が不足し、加工後の塗膜に剥離が生じた。
【００３２】

【００３３】
更に、未加熱，３００℃×１０時間加熱，３５０℃×１０時間加熱，４００℃×１時間加熱，５００℃×１００時間加熱後の各試験片を食品汚染試験に供した。食品汚染試験では、各種調味料１ｇを試験片に滴下し、試験片を水平に維持したままで６０℃，９０％ＲＨの恒温恒湿槽に装入した。恒温恒湿槽で８時間保持した後、室温に１６時間放置した。調味料滴下→高温高湿→室温放置を１サイクルとし、３サイクル繰り返した後で試験片をテープ剥離試験にかけ、塗膜の密着性を調査した。剥離しなかった塗膜を◎，点状の剥離が一部に生じた塗膜を○，点状の剥離が著しく進行した塗膜を△，全面剥離した塗膜を×とし、各種調味料に対する耐食品汚染性を評価した。
【００３４】
表３の結果にみられるように、本発明例の塗装鋼板Ｎｏ．１〜８は、何れの調味料に対しても汚染されることなく、美麗な表面及び塗膜性状を維持していた。他方、比較例の塗装鋼板Ｎｏ．９〜１１は調味料による塗膜の劣化が激しく、加熱後の試験片から大半の塗膜が全面剥離していた。この対比から明らかなように、基材，化成処理皮膜，樹脂塗膜を適正に組み合わせることによって、３００〜４００℃の中間温度域においても十分な特性を維持し、加熱調理器具等に好適な素材として使用できることが判る。
【００３５】

【００３６】
【発明の効果】
以上に説明したように、基材としてのＡｌ−Ｓｉ合金めっき鋼板，バルブメタルの酸化物，水酸化物を主成分とする化成処理皮膜，メチルシリコーン樹脂に鱗片状粉末を配合した樹脂塗料から成膜された塗膜を組み合わせることにより、３００〜４００℃の中間温度域においても優れた耐熱性を呈し、加熱後にも十分な耐食性，耐食品汚染性を維持する耐熱プレコート鋼板が得られる。この耐熱プレコート鋼板は、予備加熱や後加熱を必要とすることなく所定形状に加工したまま各種機器に組み込むことができ、使用時に異臭や煙の発生がないため、加熱調理器具，暖房機器，空調機器，自動車排ガス流路部材等の素材として重宝される。[0001]
[Industrial applications]
TECHNICAL FIELD The present invention relates to a heat-resistant pre-coated steel sheet which is used for a heating cooker, a heating device, an air conditioner, a vehicle exhaust gas flow passage member, and the like, and has excellent food contamination resistance, corrosion resistance, and heat resistance.
[0002]
[Prior art]
Al-Si alloy-plated steel sheets are frequently used as steel materials having good corrosion resistance. However, if the Al-Si alloy-plated steel sheets are left for a long time in a humid atmosphere, an exhaust gas atmosphere, a sea salt particle scattering atmosphere, or the like, white rust is formed on the steel sheet surface. It occurs and the appearance deteriorates. The generation of white rust can be prevented by subjecting the Al-Si alloy plated steel sheet to a chromate treatment, and the chromate film is also effective for improving the adhesion of the coating film.
[0003]
In the chromate treatment, a great burden is imposed on the treatment of waste liquids such as a chromate treatment solution and a rinsing solution containing water-soluble Cr ions that are harmful to the environment. Therefore, Cr-free chemical conversion treatment methods using chemicals such as titanium, zirconium, molybdenum, and phosphate have been studied. For aluminum materials, many proposals have been made mainly for application to DI cans and the like. is there.
For example, in the case of titanium, a method of forming a chemical conversion coating by contacting an aqueous solution containing a titanium compound, phosphate ions, fluoride, and an accelerator with an aluminum-containing metal material, and washing and drying with water is disclosed in JP-A-9-20984. Introduced in The present inventors have also proposed that when forming a Cr-free chemical conversion coating mainly containing oxides and hydroxides of valve metals, the corrosion resistance of the Al—Si alloy-plated steel sheet is improved (Japanese Patent Application No. 2002-214,197). 2002-015661).
[0004]
By the way, when using an Al-Si alloy plated steel sheet for a material such as a heating cooker, a heating apparatus, an air conditioner, and an exhaust gas flow path member which requires heat resistance, the metal luster which impairs the harmony with the surrounding members is disliked. Painted steel sheets provided with a resin coating on the steel sheet surface are preferred. Above all, it is strongly desired to provide a heat-resistant pre-coated steel sheet that does not require painting after being processed into a product shape.
The coating film of the heat-resistant pre-coated steel sheet is required to have a heat resistance temperature of 400 ° C. or higher and a workability such that the coating film is not peeled even by a 180 ° bending process of about 2 to 4 t. Therefore, a silicone resin paint containing an alkyl group, an alkenyl group, and a phenyl group (JP-A-63-172640, JP-A-2-265742) is used to reinforce the coating by a siloxane bond network. .
[0005]
The silicone resin has a siloxane bond as a main skeleton having better heat resistance than conventional organic resins, but the characteristics of the resin coating film vary greatly depending on the type and content of the organic group introduced into the resin. Generally, when the content of the organic group in the silicone resin is reduced, heat resistance is improved, but processability is lowered. In this regard, the conventional silicone resin has a relatively high ratio of organic groups in the resin and thus has workability to withstand pre-coating, but is inferior in heat resistance at 400 ° C. or higher. In addition, when heated to 400 ° C. or higher, the bond of Si—C ₆ H _{5 in} the resin is gradually broken, benzene gas is emitted in a small amount, and smoke is generated.
[0006]
The present applicant introduced a paint prepared by blending an epoxy resin and an isocyanate with monomethylsilanol sol as a paint for a precoated steel sheet having excellent heat resistance (Japanese Patent Application Laid-Open No. 8-245922). The paint is excellent in heat resistance at 400 ° C. or higher because the ratio of organic groups in the silicone resin is relatively low, but is inferior in workability, so that it can be precoated by adding an organic resin.
[0007]
A product made from a coated steel sheet manufactured using the paint has a disadvantage that an added organic resin is thermally decomposed in an early stage of use. In particular, when the isocyanate is thermally decomposed, a characteristic odor or smoke is generated. Immediately after the added organic resin is thermally decomposed, the organopolysiloxane resin formed from monomethylsilanol does not form a sufficient network of siloxane bonds, and temporarily goes through an unstable film state. Therefore, in applications in which such a problem is concerned, for example, heating cooking equipment used in close proximity, automobile exhaust system members requiring corrosion resistance, etc., the corrosion resistance is improved by the thermal decomposition of the added organic resin and the promotion of the formation of the organopolysiloxane resin network. In order to achieve this, a process is adopted in which after the press working, a preliminary heating of heating to 400 to 500 ° C. for several minutes is performed and then incorporated into the product.
[0008]
[Problems to be solved by the invention]
If the heat-resistant pre-coated steel sheet formed and processed without the need for preheating can be directly incorporated into various devices, the manufacturing process can be simplified and the product cost can be reduced. As a heat-resistant paint that can omit preheating for removing odors and smoke in advance and forms a coating film having good heat resistance, it is conceivable to use methyl silicone resin as a base resin instead of silicone resin having a phenyl group. . However, the heat-resistant precoated steel sheet on which a coating film based on a methyl silicone resin is formed has a significantly reduced coating film performance when heated at 300 to 350 ° C. and for a short time at 400 ° C. The performance degradation of the coating film is presumed as follows.
[0009]
The Si—CH ₃ bond contained in the resin starts a decomposition reaction by heating at 300 to 350 ° C. On the other hand, heating at 400 ° C. or higher for a long time is required to form a sufficient network structure by promoting a siloxane bond of Si—O—Si by a dehydration condensation reaction between silanol groups (Si—OH). Therefore, when the system of the methyl silicone resin alone is heated to an intermediate temperature range of 300 to 400 ° C., while the Si—CH ₃ bond is thermally decomposed, the siloxane bond of Si—O—Si necessary for forming the network structure is formed. Does not progress sufficiently. As a result, the coating film formed from the methyl silicone resin temporarily becomes an unstable coating state.
[0010]
In addition, assuming the actual use state of the heating cooker and the like, scattering and adhesion of foods and seasonings are expected. Foods and seasonings contain a large amount of NaCl, minerals, organic acids, sulfate ions, etc., and many are acidic seasonings, and some are strongly acidic seasonings having a pH of 3 or less. A decrease in pH value due to putrefaction of the attached foods is also expected. The attached foods and seasonings serve as corrosive factors, and the steel sheet, which is a constituent material of the cooking utensil, is exposed to a severe corrosive atmosphere.
When a heat-resistant pre-coated steel sheet is used as a material for a heating cooker or the like, a heat-resistant pre-coated steel sheet formed into a predetermined shape may be incorporated into a heating cooker or the like without baking (preheating). In this case, the heat-resistant pre-coated steel sheet is heated during use of the cooking utensil, etc., but post-heating is inevitable in a wide temperature range, and the post-heating temperature is low in corners, joints, and places away from a heating source. Become.
[0011]
[Means for Solving the Problems]
The present invention focuses on the fact that a chemical conversion treated steel sheet provided with a Cr-free film on an Al—Si alloy plated steel sheet has excellent coating film adhesion, and has a base material, a chemical conversion coating film, and a resin coating film having excellent heat resistance. It is an object of the present invention to provide a heat-resistant pre-coated steel sheet which does not generate an offensive odor or smoke when used even if pre-heating and post-heating are omitted, and is excellent in corrosion resistance, workability, and food stain resistance. .
[0012]
In order to achieve the object, the heat-resistant pre-coated steel sheet of the present invention uses an Al—Si alloy-plated steel sheet as a base material and a chemical conversion treatment film mainly composed of an oxide and a hydroxide of a valve metal, and has a general formula (CH ₃ _{_{) a SiO (4-a-}} b) / 2 (OH) b [However, a = 0.5~1.5, b = 0.5~1.05] scaly powder methyl silicone resin represented by Characterized in that a coating film formed from a paint containing is provided on one or both surfaces.
[0013]
As the base material, an Al-Si alloy plating layer having an Si content of 5 to 13% by mass and a surface Si content of 7 to 80% by mass is formed on the surface of the base steel sheet as a whole plating layer. Alloy-plated steel sheets are preferred. The chemical conversion coating mainly contains oxides or hydroxides of one or more valve metals selected from Ti, Zr, Hf, V, Nb, Ta, Mo, and W. One or more flaky powders selected from aluminum flakes, alumina flakes, stainless steel flakes, glass flakes, alumina flakes, mica powder, talc powder, plate kaolin, and barium sulfate flakes are dispersed in the coating film. are doing.
[0014]
Embodiment
Low carbon steel, medium carbon steel, high carbon steel, alloy steel, or the like is used as the base steel of the Al—Si alloy plated steel sheet. Among them, for applications requiring good press formability, steel sheets for deep drawing such as low carbon Ti added steel and low carbon Nb added steel are preferable. The base steel is subjected to hot-dip aluminum plating according to a conventional method, but it is preferable to adjust the Si content of the Al—Si alloy plating layer to a range of 5 to 13% by mass. By setting the Si content to 5% by mass or more, Si is easily concentrated on the surface layer of the plating layer, and the growth of an alloy layer which occurs at the interface between the base steel and the plating layer and is harmful to the workability is suppressed. However, when an excessive amount of Si exceeding 13% by mass is contained, primary crystal Si is crystallized in the plating layer in a cooling process after hot-dip plating, and workability is significantly deteriorated.
[0015]
An Al-Si alloy-plated steel sheet whose Si content is adjusted to 5 to 13% by mass is pulled up from the hot-dip plating bath, and the cooling rate or the like is adjusted to concentrate Si in the surface layer of the plating layer in advance. By performing alkali cleaning or the like, a convex portion mainly composed of metal Si and a concave portion rich in Al are formed on the surface layer of the plating layer. When a convex portion mainly composed of metal Si and a concave portion rich in Al are formed by pickling, alkali cleaning, or the like, a water washing and drying process is required. On the other hand, when a chemical conversion treatment solution having an etching effect on Al is used, the surface layer of Al is selectively removed by etching in the process of forming a chemical conversion treatment film in which the chemical conversion treatment solution is applied to the plating layer and dried. Is formed.
[0016]
The situation in which the convex portion mainly composed of metal Si and the concave portion rich in Al are dispersed in the surface layer of the plating layer is obtained by scanning and analyzing an area of 1000 μm square using the AES analysis method, and similarly performing a depth of 100 nm from the surface layer by Ar sputtering. It can be confirmed by repeated analysis. From the experimental results by the present inventors, it was found that when the Si concentration at a depth of 100 nm or more from the surface layer of the plating layer was 7% by mass or more, the target flat portion corrosion resistance and processed portion corrosion resistance were obtained. However, if Al is removed by etching until the Si concentration of the surface layer exceeds 80% by mass, the surface layer of the plating layer becomes brittle, and if the steel sheet is deformed at the time of press working or the like, the chemical conversion treatment film tends to fall off.
[0017]
A chemical conversion treatment liquid is applied to an Al—Si alloy plated steel sheet and dried without washing with water, thereby forming a chemical conversion coating mainly containing oxides and hydroxides of a valve metal. The chemical conversion treatment liquid may be either a coating type or a reaction type as long as it contains a valve metal. In the reaction type chemical conversion treatment, the pH is adjusted to be slightly lower in order to maintain the stability of the treatment solution.
The valve metal refers to a metal in which an oxide exhibits high insulation resistance, and one or more of Ti, Zr, Hf, V, Nb, Ta, Mo, and W are used. For example, chemical conversion treatment solutions using Ti as the valve metal include TiCl ₄ , (NH ₄ ) ₂ TiF ₆ , TiOSO ₄ , Ti (SO ₄ ) ₂ , Ti (OH) ₂ , and K ₂ [TiO (COO) ₂ ]. , XTiF ₆ (X: alkali metal or alkaline earth metal, n: 1 or 2) or the like as a Ti source. _{^{_{TiF 6 2- + 4H 2 O →}}} Ti (OH) 4 + 6F - reaction final main component oxides of valve metal coating by such as long as the hydroxide, fluoride, sulfate, etc. to Ti source Can be used.
[0018]
In order to activate the surface of the Al—Si alloy plating layer by etching, a fluoride or various acids may be added to the chemical conversion treatment solution. Addition of an organic acid having a chelating effect such as tartaric acid, tannic acid, citric acid, oxalic acid, malonic acid, lactic acid, and butyric acid improves the stability of the chemical conversion solution. Furthermore, when a metal phosphate or a composite phosphate is added to the chemical conversion treatment solution, it is eluted into a defect of the chemical conversion treatment film, reacts with Al and precipitates as an insoluble phosphate, so that the chemical conversion treatment film is self-repaired. A function is provided.
[0019]
The coating made of the oxide or hydroxide of the valve metal acts as a resistor against the transfer of electrons, and suppresses the reduction reaction (oxidation reaction with the underlying steel) due to dissolved oxygen contained in the moisture in the atmosphere. The oxide of the valve metal exhibiting high insulation resistance effectively shields corrosion-promoting factors and substances exhibiting corrosive effects such as water, protons, and oxygen. As a result, elution (corrosion) of the metal component from the base steel is prevented.
In addition to the oxide of the valve metal, a hydroxide is also present in the chemical conversion coating. Oxides and hydroxides form a silanol group (Si-OH) or a siloxane bond (Si-OH) in an intermediate temperature range of 300 to 400C until a sufficient network structure is formed in the methyl silicone resin due to the progress of Si-O bonds. Hydrogen bond with Si—O). Due to the formation of hydrogen bonds, the adhesion to the methyl silicone resin applied to the chemical conversion treatment film is improved, and the food contamination resistance and corrosion resistance in the intermediate temperature range of 300 to 400 ° C are dramatically improved.
[0020]
The chemical conversion coating is formed by applying a chemical conversion solution to an Al—Si alloy-plated steel sheet by a roll coating method, a spin coating method, a spray method, or the like, and drying the steel sheet without washing with water. Drying at room temperature is also possible, but considering continuous operation, it is preferable to keep the temperature at 50 ° C. or higher to shorten the drying time. However, since the hydroxide of the valve metal contained in the chemical conversion treatment film decreases as the drying temperature increases, it is preferable to set the upper limit of the drying temperature to 300 ° C. The application amount of the chemical conversion treatment liquid is preferably adjusted so that the titanium adhesion amount is 1 mg / m ² or more in order to secure sufficient corrosion resistance. Prior to the chemical conversion treatment, the Al—Si alloy plated steel sheet is alkali-degreased as necessary.
[0021]
Immediately after the formation of the chemical conversion coating, a resin coating in which flaky powder is mixed with a methyl silicone resin is applied, baked and dried to form a resin coating. Spray method, roll coating method, bar coating method, etc. are adopted for application of the resin paint, and the coating is preferably heated and dried at a temperature in the range of 150 to 300 ° C. in order to guarantee the hardness of the coating film and ensure the processing adhesion. . High-temperature drying exceeding 300 ° C. not only adversely affects food stain resistance and corrosion resistance, but also impairs the workability of the coating film, making it impossible to obtain properties as a precoated steel sheet. The thickness of the resin coating is preferably adjusted to 2 to 15 μm (preferably 10 μm or less) in consideration of corrosion resistance and processing adhesion. The effect of improving the corrosion resistance is seen when the film thickness is 2 μm or more, but when the film thickness is more than 15 μm, the processing adhesion of the coating film decreases.
[0022]
Methyl silicone resin is a general formula _{_{_{(CH 3) a SiO (4}}} -a-b) / 2 (OH) compound with _b. The indices a and b are a = 0.5 to 1.5, respectively, in order to impart the required hardness to the coating film without blocking and to enable continuous coating with the coil without requiring long-time baking. It is preferable to adjust b in the range of 0.5 to 1.05. When a <0.5, the processing adhesion of the coating film decreases, and when a> 1.5, the heat resistance is poor. In the case of b <0.5, the number of starting points for bonding by dehydration and condensation with the hydroxyl groups oriented on the original coating plate is reduced, whereby the processing adhesion is reduced and the curability of the coating film is also poor. On the other hand, when b> 1.05, the three-dimensional cross-linking proceeds excessively during baking, and the processing adhesion of the coating film decreases.
[0023]
As the flaky powder to be mixed with the methyl silicone resin, those having a heat resistance of 500 ° C., such as aluminum flake, alumina flake, stainless steel flake, glass flake, mica powder, talc powder, plate kaolin and barium sulfate flake, are preferable. And various flakes can be used alone or in combination of two or more. Considering applications such as microwave ovens that require infrared radiation characteristics and the chemical stability of resin coatings, flakes of poorly water-soluble oxides such as alumina flake, mica powder, talc powder, plate kaolin, barium sulfate flake, etc. Is preferred.
[0024]
The scaly powder is dispersed in the coating film and is arranged parallel to the in-plane direction of the base material and the Al-Si alloy-plated steel sheet, and adheres to the coating film while the thermal decomposition of the Si-CH ₃ bond is progressing. Improve physical properties such as water resistance and water resistance. In order to exhibit the effect of improving the physical properties, it is preferable to set the average particle size of the flaky powder to 0.5 to 50 μm and the blending amount to the resin solid content to 5 to 100 parts by mass. When the amount is less than 5 parts by mass, the effect of adding the flaky powder is insufficient, and when the amount is more than 100 parts by mass, the processing adhesion tends to decrease. The scaly powder can be added in combination with various types of calcined pigments, rust-preventive pigments, metal powders, and the like, and when added in combination, the total amount is preferably adjusted to 200 parts by mass or less.
[0025]
The effect of adding the scaly powder varies depending on the shape of the scaly powder in addition to the amount of the scaly powder, and becomes remarkable at an average particle size of 0.5 μm or more. However, if the average particle diameter exceeds 50 μm, the processing adhesion tends to decrease. The average particle size of the scaly powder is more preferably in the range of 1 to 30 μm. In order to reinforce the network structure of the siloxane bond, it is preferable to disperse the flaky powder having a thickness of 1/5 or less of the average particle size in the coating film to enhance the orientation of the flaky powder in a direction parallel to the base surface. . In the case of a flaky powder having a thickness exceeding 1/5 of the average particle size, the dispersed state of the flaky powder which is coordinated in the direction crossing the base surface is inevitable, and the contact with the silicone resin per unit weight of the flaky powder is inevitable. Since the area becomes small, the effect of adding the flaky powder is not sufficiently exhibited.
[0026]
Various additives, such as coloring pigments, extender pigments, metallic pigments, catalysts, rust-preventive pigments, metal powders, etc., are added to the resin paint as needed to improve design and corrosion resistance and to provide various functions such as catalytic functions. Mixed. Color pigments include oxides and composite oxides such as Mn, Fe, Cr, Cu, and Ti, graphite, and carbon black. In addition to the conventional chromium-based pigments, non-chromium-based pigments such as calcium molybdate, calcium phosphomolybdate, and aluminum phosphomolybdate are used in addition to conventional chromium-based pigments. The catalyst is a photocatalyst, metal powder including TiO ₂ is Ni, Co, Cu and the like.
[0027]
【Example】
An ultra-low carbon steel sheet having a thickness of 0.4 mm was hot-dip plated to form an Al—Si alloy plating layer of 6 to 11% by mass of Si with a coating weight of 40 g / m ² . Using this Al-Si alloy-plated steel sheet as a base sheet, after applying a chemical conversion treatment solution containing a compound of valve metal in various ratios, it is charged into an oven without washing with water, and dried at a maximum ultimate sheet temperature of 150 ° C. As a result, a chemical conversion coating mainly containing oxides and hydroxides of the valve metal was formed.
Immediately after the formation of the chemical conversion coating, a resin coating was applied by a bar coating method, and heated and baked at a maximum temperature of 230 ° C. to form a coating having a dry film thickness of 6 μm. The resin paint is based on a methyl silicone resin of the general formula (CH ₃ ) _0.95 SiO _1.05 (OH) _0.95 , and a black pigment (MnCuCrO _x baked pigment) based on 100 parts by weight of the total weight of the resin: 100 parts by mass, various scaly powders: paints containing 20 parts by mass were used.
Table 1 shows the Al-Si alloy plating layer of the original plate, the composition of the chemical conversion treatment solution, and the flaky powder mixed in the resin paint.
[0028]

[0029]
A test piece was cut out from the obtained coated steel sheet and subjected to a processing test and a corrosion test.
In the processing test, the test piece was bent 180 degrees (3 to 6 t), and the peeling state of the coating film was investigated by a tape peeling test in which an adhesive tape was attached to the processed portion and then peeled off. The coating adhesion where no peeling was detected was evaluated as ◎, the coating where slight peeling was detected as ○, and the coating peeled from the base steel as ×, and the working adhesion was evaluated.
[0030]
In the corrosion test, a test piece heated at 300 ° C. × 10 hours, 350 ° C. × 10 hours, 400 ° C. × 1 hour, and 500 ° C. × 100 hours was prepared in addition to the unheated test piece. After continuous salt spray according to JIS Z2371 for 100 hours, the coating film adhesion (secondary adhesion) of each test piece was examined by a tape peeling test. Secondary adhesion: coatings with no peeling detected are marked with ◎, coatings with spotted peeling in some areas are marked with ○, coatings with marked spotting are marked with △, and coatings with full peeling are marked as x The sex was evaluated.
[0031]
As can be seen from the investigation results in Table 2, the present invention example No. in which an Al-Si alloy plated steel sheet was used as a coating base plate and a flaky powder-dispersed coating film was provided via a chemical conversion coating film. Nos. 1 to 8 were excellent in all of heat resistance, processing adhesion and corrosion resistance. Good corrosion resistance was maintained even after heating, and no off-flavor gas was generated during heating.
On the other hand, the coated steel sheet No. having formed a coating film without the addition of the flaky powder. No. 10 was inferior in corrosion resistance when heated to a temperature range of 300 to 400 ° C. The coated steel sheet No. in which a resin coating was directly formed without providing a chemical conversion coating. No. 9 was inferior in both heat resistance and workability. The coated steel sheet No. in which a resin coating film was formed after the tannic acid treatment. In No. 11, the adhesion of the coating film to the substrate was insufficient, and the coating film after processing was peeled off.
[0032]

[0033]
Furthermore, each test piece after unheating, heating at 300 ° C. × 10 hours, heating at 350 ° C. × 10 hours, heating at 400 ° C. × 1 hour, and heating at 500 ° C. × 100 hours was subjected to a food contamination test. In the food contamination test, 1 g of various seasonings was dropped on the test piece, and the test piece was placed in a constant temperature and humidity chamber at 60 ° C. and 90% RH while the test piece was kept horizontal. After being kept in a thermo-hygrostat for 8 hours, it was left at room temperature for 16 hours. The test piece was subjected to a tape peeling test after repeating three cycles with one cycle of seasoning dripping → high temperature and high humidity → standing at room temperature, and the adhesion of the coating film was investigated. The coatings that were not peeled off were marked with ，, the coatings with spot-like peeling partially occurred were marked with ○, the coatings with spot-like peeling markedly advanced were marked with Δ, and the coatings that were peeled off entirely were marked with ×. The food stain resistance was evaluated.
[0034]
As can be seen from the results in Table 3, the coated steel sheet No. of the present invention example. Nos. 1 to 8 maintained a beautiful surface and coating properties without being contaminated by any of the seasonings. On the other hand, the coated steel sheet No. of the comparative example. In Nos. 9 to 11, deterioration of the coating film due to the seasoning was severe, and most of the coating films were completely peeled off from the test pieces after heating. As is clear from this comparison, by appropriately combining the base material, the chemical conversion coating film, and the resin coating film, sufficient properties are maintained even in the intermediate temperature range of 300 to 400 ° C. It turns out that it can be used as.
[0035]

[0036]
【The invention's effect】
As described above, an Al-Si alloy-plated steel sheet as a base material, a chemical conversion coating mainly containing oxides and hydroxides of a valve metal, and a resin paint in which flaky powder is mixed with methyl silicone resin. By combining the formed coating films, a heat-resistant precoated steel sheet exhibiting excellent heat resistance even in an intermediate temperature range of 300 to 400 ° C. and maintaining sufficient corrosion resistance and food stain resistance even after heating can be obtained. This heat-resistant pre-coated steel sheet can be assembled into various equipment while processing it into a predetermined shape without the need for pre-heating or post-heating. It is useful as a material for equipment and automobile exhaust gas flow path members.

Claims

Ａｌ−Ｓｉ合金めっき鋼板を基材とし、バルブメタルの酸化物及び水酸化物を主体とする化成処理皮膜を介し、一般式（ＣＨ_３）_ａＳｉＯ_{（４−ａ−ｂ）／２}（ＯＨ）_ｂ〔ただし、ａ＝０．５〜１．５，ｂ＝０．５〜１．０５〕で表されるメチルシリコーン樹脂に鱗片状粉末を配合した塗料から形成された塗膜が片面又は両面に設けられていることを特徴とする加工性，耐食性に優れた耐熱プレコート鋼板。Using an Al-Si alloy-plated steel sheet as a base material and a chemical conversion coating mainly comprising an oxide and a hydroxide of a valve metal, a general formula (CH ₃ ) _a SiO _{(4-ab) / 2} (OH) _b [where a = 0.5 to 1.5, b = 0.5 to 1.05] A coating film formed from a paint obtained by blending flaky powder with methyl silicone resin represented on one or both surfaces Heat-resistant pre-coated steel sheet with excellent workability and corrosion resistance characterized by being provided.

めっき層全体としてＳｉ含有量が５〜１３質量％，表層のＳｉ含有量が７〜８０質量％のＡｌ−Ｓｉ合金めっき層が下地鋼板の表面に形成されている請求項１記載の耐熱プレコート鋼板。The heat-resistant precoated steel sheet according to claim 1, wherein an Al-Si alloy plating layer having a Si content of 5 to 13% by mass and a surface Si content of 7 to 80% by mass is formed on the surface of the base steel sheet. .

バルブメタルがＴｉ，Ｚｒ，Ｈｆ，Ｖ，Ｎｂ，Ｔａ，Ｍｏ，Ｗから選ばれた１種又は２種以上である請求項１記載の耐熱プレコート鋼板。The heat-resistant precoated steel sheet according to claim 1, wherein the valve metal is at least one selected from Ti, Zr, Hf, V, Nb, Ta, Mo, and W.

アルミフレーク，アルミナフレーク，ステンレス鋼フレーク，ガラスフレーク，アルミナフレーク，マイカ粉，タルク粉，板状カオリン，硫酸バリウムフレークから選ばれた１種又は２種以上を鱗片状粉末とする請求項１記載の耐熱プレコート鋼板。2. The scaly powder according to claim 1, wherein one or more selected from aluminum flake, alumina flake, stainless steel flake, glass flake, alumina flake, mica powder, talc powder, plate kaolin, and barium sulfate flake are used. Heat-resistant pre-coated steel sheet.