JP3826526B2

JP3826526B2 - Hot-rolled steel sheet with excellent paintability and corrosion resistance

Info

Publication number: JP3826526B2
Application number: JP35041797A
Authority: JP
Inventors: 重男黒川
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 1997-12-19
Filing date: 1997-12-19
Publication date: 2006-09-27
Anticipated expiration: 2017-12-19
Also published as: JPH11181579A

Description

【０００１】
【発明の属する技術分野】
本発明は、自動車、家電、建材等の用途に適する高耐食性熱延鋼板に係り、とくに塗装性に優れた熱延鋼板に関する。
【０００２】
【従来の技術】
熱間圧延された鋼板の表面には、通常、薄いスケール（黒皮）が形成されている。このスケールは、一般に、地鉄側からウスタイト（FeO ）、マグネタイト（Fe₃O₄）、ヘマタイト（Fe₂O₃）からなる３層構造を有することが知られている。
【０００３】
この熱延鋼板に、脱スケール処理を施さず、化成処理を行うと、スケールのため化成処理による被膜が形成されにくく、その後、塗装処理を行っても、塗膜密着性が劣化し、塗装後の耐食性が劣化する。
例えば、黒皮付熱延鋼板に、通常の自動車塗装工程と同じ条件の化成処理、すなわち、液温43℃のりん酸亜鉛浴に120sec浸漬する処理を施した場合、鋼板表面に付着するりん酸亜鉛被膜の量は1.0g/m²未満と非常に少ない。この鋼板に、引続き電着塗装、スプレー塗装などの処理を行っても、電着塗装の欠陥、スプレー塗装後の外観不良等が発生する。
【０００４】
このようなことから、熱延鋼板では、酸洗等により完全に脱スケールを行ってから、加工や、スポット溶接等の接合を行い、化成処理を施し、塗装するという工程が採用されていた。しかしながら、脱スケールされた熱延鋼板は、酸洗後の腐食生成物のため化成結晶の核生成および成長が阻害され、冷延鋼板に比べ化成処理性、塗膜密着性が劣るという問題があった。酸洗後の腐食生成物の発生原因としては、酸洗残渣の表面吸着、水洗後の水膜などが考えられる。
【０００５】
上記した問題に対し、特開昭59-64783号公報では、脱スケールされた熱延鋼板を陽極として電解し、熱延鋼板表面上の腐食生成物を除去するとともに、不動態皮膜を生成させ、化成処理性を向上させる熱延鋼板の製造方法が提案されている。
【０００６】
【発明が解決しようとする課題】
しかしながら、特開昭59-64783号公報に記載された方法では、従来の熱延鋼板の酸洗処理設備に加え、新たに電解のための電源、電解槽、水洗槽等の設備が必要となり、電解処理工程が増え、経済的、生産効率的に問題を残していた。また、特開昭59-64783号公報に記載された技術で製造された熱延鋼板を用いた場合には、電着塗装、スプレー塗装時の塗料回り込みの悪い合わせ部では塗装が不十分となるため、合わせ部で酸洗、電解処理のままの鋼板が露出し、耐食性が著しく劣化するという問題もあった。
【０００７】
本発明は、上記した問題を有利に解決し、塗装性が良好でかつ塗装後の耐食性および合わせ部などの塗料の回り込みが少ない部分でも耐食性が良好であり、塗装性および耐食性に優れた熱延鋼板を提供することを目的とする。
【０００８】
【課題を解決するための手段】
本発明者らは、上記した課題を達成するために、黒皮付鋼板（熱間圧延のまま鋼板）に各種りん酸塩処理および各種塗料塗布処理を施し、塗装性と耐食性を調査した。その結果、りん酸鉄浴を用いてりん酸鉄被膜を形成し、ついでりん酸鉄被膜のうえに有機樹脂被膜を形成することにより、黒皮付鋼板であっても良好な塗装性と耐食性を有する熱延鋼板が得られることを知見した。
【０００９】
ここで、塗装性とは、自動車および家電メーカーで、加工やスポット溶接等の接合後、処理される塗装（電着塗装、スプレー塗装等）工程で問題となる特性、あるいは、塗装処理後評価される特性であり、各塗装後の外観、耐水二次密着性、電着塗装後耐食性、チッピング後の耐食性等を含む特性をいう。
本発明は上記した知見に基づいて構成されたものである。
【００１０】
すなわち、本発明は、黒皮付鋼板の少なくとも片面にりん酸鉄被膜を有し、該りん酸鉄被膜の上にさらに有機樹脂被膜を有することを特徴とする塗装性および耐食性に優れた熱延鋼板であり、前記りん酸鉄被膜の付着量を片面あたり0.05〜1.0g/m² とするのが好ましく、また、前記有機樹脂被膜の付着量を片面あたり0.3 〜4.0g/m² とするのが好ましい。
【００１１】
【発明の実施の形態】
本発明では、基板として、通常の熱間圧延工程で製造された表面にスケールを有する黒皮付鋼板を用いる。表層のスケール組成はとくに限定しない。通常、表層のスケールは、地鉄側からウスタイト（FeO ）、マグネタイト（Fe₃O₄）、ヘマタイト（Fe₂O₃）からなるものが好ましいが、熱延巻取り後徐冷しウスタイト（FeO ）が、共析変態してFe₃O₄と析出α−Feに変化したスケール、あるいは熱延巻取り後急冷しウスタイト（FeO ）が多量に残存したスケールとなってもよい。また、スケール厚みは、スケールと地鉄との密着性の観点からできるだけ薄く、10μm 未満とするのが望ましい。
【００１２】
本発明では、上記した黒皮付熱延鋼板表面の少なくとも片面に、まずりん酸鉄被膜を形成する。りん酸鉄被膜は、りん酸鉄のみを含有する水溶液（りん酸鉄浴）を用い、スプレー法、浸漬法、コーター法等により形成される。
りん酸鉄浴では、遊離りん酸の作用により、熱延鋼板表面のスケール上に均一な薄いりん酸鉄被膜が形成され、少ない付着量で熱延鋼板表面を均一に覆うことができる。従来のりん酸亜鉛系浴では、りん酸亜鉛結晶の核生成が局所的に行われ、さらに結晶成長の異方性が大きく、そのため、黒皮付熱延鋼板の表面を覆うには、りん酸亜鉛被膜の付着量を極めて多量とする必要があった。
【００１３】
熱延鋼板表面のスケール上に薄いりん酸鉄被膜を均一に形成させることにより、該りん酸鉄被膜の上に形成される有機樹脂被膜との密着性が確保される。
りん酸鉄被膜の付着量は、片面あたり0.05〜1.0g/m² とするのが好ましい。付着量が0.05g/m²未満では、スケール表面を完全に覆うことができず、耐食性が劣化する。一方、付着量が1.0g/m² を超えると、りん酸鉄被膜自体の凝集破壊が生じやすくなり、有機樹脂被膜との密着性が低下し、塗装後の塗膜密着性が劣化する。
【００１４】
本発明では、りん酸鉄被膜の上には、有機樹脂被膜が形成される。りん酸鉄被膜の上に有機樹脂被膜を形成することにより、塗装後の塗膜密着性や、塗装後の耐食性および塗装の回り込みにくい箇所の耐食性が向上する。
有機樹脂被膜は、りん酸鉄被膜の上に下記に示す塗料組成物をロールコータ法、バーコータ法、静電塗装法、カーテンフロー法等により塗布し形成される。有機樹脂被膜の付着量は、片面あたり0.3 〜4.0g/m²とするのが好ましい。付着量が片面あたり0.3g/m²未満では、十分な塗膜密着性、塗装後耐食性、塗装の回り込みにくい箇所の耐食性が不足する。一方、付着量が4.0g/m²超えると、有機樹脂被膜自体の凝集破壊が生じやすくなり、塗膜密着性が低下する傾向にあり、さらにスポット溶接性が劣化する。また付着量が多くなると経済的に高価となる。
【００１５】
有機樹脂被膜の形成に用いる塗料組成物の主体は、エポキシ樹脂、ウレタン樹脂、アクリル樹脂、アルキド樹脂、およびこれらの変性樹脂が好適である。変性樹脂としては、例えば、エピクロルヒドリン−ビスフェノールＡ型エポキシ樹脂100 重量部に対しイソシアネート化合物：10〜100 重量部を反応させたエポキシ当量1000〜5000のウレタン変性エポキシ樹脂、あるいはウレタン変性エポキシ樹脂のエポキシ基１当量に対し0.5 〜1.0 モルのジアルカノールアミンを付加した変性エポキシ樹脂、ウレタン樹脂 100重量部に対しエポキシ：５〜30重量部を反応させたエポキシ変性ウレタン樹脂などが好適に使用できる。
【００１６】
上記した有機樹脂を主体とする塗料組成物では、溶媒として有機溶剤あるいは水が好適に使用できる。有機溶剤としては、ブチルセロソルブ、ソルベッソ150 、PMアセテートが好ましい。また、塗料組成物には、必要に応じ、防錆顔料としてシリカ、クロム酸塩等を、あるいはさらに架橋剤を添加してもよい。
なお、熱延鋼板表面に形成されるりん酸鉄被膜および有機樹脂被膜は、鋼板の片面あるいは両面いずれであってもよいのはいうまでもない。
【００１７】
【実施例】
表１に示す組成の鋼を転炉で溶製し連続鋳造法でスラブとしたのち、熱間圧延により、2.3mm 厚の熱延板とした。熱延板は、巻取温度は620 ℃一定として、コイルに巻き取った。なお、熱間圧延の仕上げ圧延温度および仕上げ圧延後の冷却速度を変化し、熱延板のスケール厚を5.5 μm または3.0 μm に調整した。
【００１８】
これら熱延鋼板表面に、表２に示す付着量のりん酸鉄被膜を形成しついで該被膜の上に表２に示す付着量の有機樹脂被膜を形成し、各種特性を評価した。なお、りん酸鉄被膜を形成せず有機樹脂被膜のみを形成した例、あるいはりん酸鉄被膜および有機樹脂被膜いずれも形成しない例を比較例とした。
りん酸鉄被膜および有機樹脂被膜の形成条件は下記のとおりである。
（１）りん酸鉄被膜
下記に示すりん酸鉄浴を用い、スプレー処理でりん酸鉄被膜を形成した。
【００１９】
りん酸鉄浴：日本パーカライジング（株）製パルホス3456
浴温度：50℃
（２）有機樹脂被膜
下記Ａ〜Ｃの組成の有機複合塗料組成物をロールコータで塗布し、有機樹脂被膜を形成した。
【００２０】
▲１▼有機複合塗料組成物Ａ
主体有機樹脂：ウレタン変性エポキシ樹脂
溶媒：ブチルセロソルブ
防錆顔料：シリカ
▲２▼有機複合塗料組成物Ｂ
主体有機樹脂：エポキシ樹脂
溶媒：水
その他：架橋剤
▲３▼有機複合塗料組成物Ｃ
主体有機樹脂：エポキシ変性ウレタン樹脂
溶媒：水
防錆顔料：シリカ、クロム酸バリウム
鋼板表面に上記した各種被膜を形成された本発明例の熱延鋼板および比較例の熱延鋼板から、試験片を採取し、塗装後の外観、裸耐食性、合わせ目耐食性、塗装後の塗膜耐水二次密着性、電着塗装後の耐食性、塗装後の耐食性（チッピング後耐食性）、スポット溶接性を評価した。評価方法は、下記のとおりである。
【００２１】
(i) 塗装後の外観試験
試験片に、直接カチオン電着塗料を厚さ21μm 、中塗り塗料を厚さ30μm 、上塗り塗料を厚さ35μm 塗装し、外観を評価した。評価基準は、◎：著しく良好、○：良好、△：やや劣る、×：劣る、とした。
(ii) 裸耐食性試験
裸耐食性試験は、塗装の回り込みにくい部分の耐食性を評価する。
【００２２】
試験片を、塩水噴霧試験５hr（35℃）、乾燥試験５hr（60℃、相対湿度20％以下）、湿潤試験14hr（50℃、相対湿度95％以上）からなるサイクルを１日１サイクル繰り返し、鋼板厚さ0.8mm の腐食量が得られるまでの日数を求めた。評価基準は、◎：100 日以上（著しく良好）、○：81〜99日（良好）、△：51〜80日（やや劣る）、×：50日以下（劣る）、とした。
【００２３】
(iii) 合わせ目耐食性試験
合わせ目耐食性試験は、塗装ののらない合わせ目の耐食性を評価する。
同種の試験片をスポット溶接し合わせ目を作製し、（ii）裸耐食性試験と同様な試験を実施し、鋼板厚さ0.8mm の腐食量が得られるまでの日数を求めた。評価基準は、（ii）と同様に、◎：100 日以上（著しく良好）、○：100 日未満〜80日以上（良好）、△：80日未満〜50日以上（やや劣る）、×：50日未満（劣る）、とした。
【００２４】
(iv) 塗装後の耐食性試験（チッピング後耐食性）
塗装後の耐食性試験（チッピング後の耐食性）は、塗装面の耐食性を評価する。
表面にカチオン電着塗料を厚さ21μm 、中塗り塗料を厚さ30μm 、上塗り塗料を厚さ35μm 塗装した試験片について、３〜５ｇの小石20個を時速100km で塗膜面にショットするチッピング試験を行ったのち、ついで１週間に２回海水を散布する海岸暴露試験を行い塗膜傷部の腐食幅を測定した。評価基準は、◎：腐食幅が２mm以下（著しく良好）、○：腐食幅が２mm超え〜５mm以下（良好）、△：腐食幅が５mm超え〜10mm以下（やや劣る）、×：腐食幅が10mm超（劣る）、とした。
【００２５】
(v) 電着塗装後の耐食性試験
電着塗装後の耐食性試験は、カチオン電着塗膜面の耐食性を評価する。
表面にカチオン電着塗料を厚さ21μm した試験片について、塗膜面にスクラッチ傷を導入したのち、塩水噴霧試験を1200hr行い、スクラッチ傷部の腐食幅を測定した。評価基準は、◎：腐食幅が２mm以下（著しく良好）、○：腐食幅が２mm超え〜５mm以下（良好）、△：腐食幅が５mm超え〜10mm以上（やや劣る）、×：腐食幅が10mm超（劣る）、とした。
【００２６】
(vi) 塗装後の塗膜耐水二次密着性試験
塗膜耐水二次密着性試験は、劣化後の塗膜密着性を評価する。
表面にカチオン電着塗料を厚さ21μm 、中塗り塗料を厚さ30μm 、上塗り塗料を厚さ35μm 塗装した試験片について、50℃の蒸留水中に240hr 浸漬したのち、２mm方眼100 個の基盤目試験を行い、テーピング剥離状況で評価した。評価基準は、◎：剥離がない（著しく良好）、○：剥離が10％以下（良好）、△：剥離が10％超え50％以下（やや劣る）、×：剥離が50％超え（劣る）、とした。
【００２７】
(vii) スポット溶接性試験
スポット溶接性試験は速続打点数を評価する。
試験片に、先端６mmφのアルミナ分散銅合金製の溶接チップを用い、加圧力200kgf、溶接電流10kA、溶接時間10サイクル／50サイクルの連続溶接を行い、ナゲット径が基準径を下回るまでの連続打点数を測定した。評価基準は、◎：連続打点数が3000点以上（著しく良好）、○：連続打点数が2000点以上3000点未満（良好）、△：連続打点数が1000点以上2000点未満（やや劣る）、×：連続打点数が1000点未満（劣る）、とした。
【００２８】
各特性の評価結果を表２に示す。
【００２９】
【表１】

【００３０】
【表２】

【００３１】
表２から、本発明例（試験No.1〜No.6、No.11 〜No.14 ）は、塗装後の外観、裸耐食性、合わせ目耐食性、塗装後の塗膜耐水二次密着性、電着塗装後の耐食性、チッピング後の耐食性、スポット溶接性、いずれも良好（一部やや劣る場合もある）であるのに対し、本発明の範囲を外れる比較例（試験No.7〜No.10 ）は、いずれの特性もやや劣るかあるいは劣っており、本発明の熱延鋼板は、塗膜密着性、塗装後の耐食性に優れた鋼板であることがわかる。また、表２から、りん酸鉄被膜の付着量が0.05〜1.0g/m²の範囲、有機樹脂被膜の付着量が0.3 〜4.0g/m²の範囲を満足する場合にはとくに、優れた特性を示すことがわかる。
【００３２】
【発明の効果】
本発明によれば、裸耐食性、塗装性、塗装後耐食性、およびスポット溶接性に優れた熱延鋼板が得られ、黒皮付熱延鋼板（熱延のまま鋼板）の表面に直接塗装を施すことができ、しかも合わせ部等の塗料の回り込みにくい部分の耐食性に優れ、脱スケール工程を省略できる等の産業上格段の効果が期待できる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a highly corrosion-resistant hot-rolled steel sheet suitable for applications such as automobiles, home appliances, and building materials, and particularly relates to a hot-rolled steel sheet having excellent paintability.
[0002]
[Prior art]
A thin scale (black skin) is usually formed on the surface of the hot-rolled steel sheet. This scale is generally known to have a three-layer structure composed of wustite (FeO ₃ ), magnetite (Fe ₃ O ₄ ), and hematite (Fe ₂ O ₃ ) from the ground iron side.
[0003]
If this hot-rolled steel sheet is not subjected to descaling treatment and subjected to chemical conversion treatment, it is difficult to form a coating film due to chemical conversion treatment due to the scale. Corrosion resistance of the deteriorates.
For example, when a black skinned hot-rolled steel sheet is subjected to a chemical conversion treatment under the same conditions as a normal automobile coating process, that is, a treatment of immersing in a zinc phosphate bath at a liquid temperature of 43 ° C for 120 seconds, phosphoric acid adhering to the steel plate surface the amount of zinc coating is very small and less than 1.0 g / m ^2. Even if this steel sheet is subsequently subjected to electrodeposition coating, spray coating, etc., defects in electrodeposition coating, poor appearance after spray coating, etc. occur.
[0004]
For this reason, the hot-rolled steel sheet employs a process in which descaling is performed by pickling or the like, and then processing, joining such as spot welding is performed, chemical conversion treatment is performed, and coating is performed. However, the descaled hot-rolled steel sheet has the problem that the nucleation and growth of chemical conversion crystals are hindered due to the corrosion product after pickling, and the chemical conversion treatment property and coating film adhesion are inferior to those of cold-rolled steel sheets. It was. Possible causes of the generation of corrosion products after pickling are surface adsorption of pickling residue, water film after washing, and the like.
[0005]
In order to solve the above problems, JP-A-59-64783 discloses electrolysis using a descaled hot-rolled steel sheet as an anode, removes corrosion products on the surface of the hot-rolled steel sheet, and generates a passive film. A method of manufacturing a hot-rolled steel sheet that improves chemical conversion properties has been proposed.
[0006]
[Problems to be solved by the invention]
However, in the method described in JP-A-59-64783, in addition to the conventional pickling treatment equipment for hot-rolled steel sheets, a power source for electrolysis, an electrolytic bath, a washing bath and the like are newly required. The electrolytic treatment process has increased, leaving problems economically and in production efficiency. In addition, when using a hot-rolled steel sheet manufactured by the technique described in Japanese Patent Application Laid-Open No. 59-64783, the coating is insufficient at the joint where poor coating wrapping occurs during electrodeposition coating and spray coating. For this reason, there has been a problem that the steel plate as it is pickled and electrolytically treated is exposed at the mating portion, and the corrosion resistance is remarkably deteriorated.
[0007]
The present invention advantageously solves the above-mentioned problems, has good paintability, has good corrosion resistance after coating, and good corrosion resistance even in parts where the coating material wraps around little, such as a mating part, and has excellent paintability and corrosion resistance. An object is to provide a steel sheet.
[0008]
[Means for Solving the Problems]
In order to achieve the above-mentioned problems, the present inventors performed various phosphate treatments and various coating application treatments on a steel sheet with black skin (a steel sheet as hot-rolled), and investigated paintability and corrosion resistance. As a result, an iron phosphate coating is formed using an iron phosphate bath, and then an organic resin coating is formed on the iron phosphate coating. It has been found that a hot-rolled steel sheet is obtained.
[0009]
Here, paintability is a property that becomes a problem in the coating (electrodeposition coating, spray coating, etc.) process that is processed after joining by processing and spot welding, etc. by automobile and home appliance manufacturers, or evaluated after coating processing. These are properties including appearance after each coating, water secondary adhesion, corrosion resistance after electrodeposition coating, corrosion resistance after chipping, and the like.
The present invention is configured based on the above-described findings.
[0010]
That is, the present invention is a hot rolling excellent in paintability and corrosion resistance, characterized in that it has an iron phosphate coating on at least one side of a steel sheet with black skin, and further has an organic resin coating on the iron phosphate coating. a steel plate, wherein it is preferable to be per side 0.05 to 1.0 g / m ² adhesion amount of iron phosphate coating, also to the one surface per 0.3 to 4.0 g / m ² coating weight of the organic resin coating Is preferred.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
In this invention, the steel plate with a black skin which has a scale on the surface manufactured by the normal hot rolling process is used as a board | substrate. The scale composition of the surface layer is not particularly limited. Usually, the scale of the surface layer is preferably composed of wustite (FeO), magnetite (Fe ₃ O ₄ ), and hematite (Fe ₂ O ₃ ) from the side of the ground iron, but it is gradually cooled after hot rolling and wustite (FeO). However, it may be a scale in which eutectoid transformation has occurred and changed to Fe ₃ O ₄ and precipitated α-Fe, or a scale in which a large amount of wustite (FeO 2) remains after quenching by hot rolling. In addition, the scale thickness is preferably as thin as possible from the viewpoint of adhesion between the scale and the ground iron, and is preferably less than 10 μm.
[0012]
In the present invention, an iron phosphate coating is first formed on at least one surface of the above-described black-rolled hot-rolled steel sheet. The iron phosphate coating is formed by a spray method, a dipping method, a coater method or the like using an aqueous solution (iron phosphate bath) containing only iron phosphate.
In the iron phosphate bath, a uniform thin iron phosphate coating is formed on the scale of the hot rolled steel sheet surface by the action of free phosphoric acid, and the hot rolled steel sheet surface can be uniformly covered with a small amount of adhesion. In conventional zinc phosphate baths, nucleation of zinc phosphate crystals occurs locally, and the anisotropy of crystal growth is large. It was necessary to make the amount of zinc coating very large.
[0013]
By forming a thin iron phosphate coating uniformly on the scale on the surface of the hot-rolled steel sheet, adhesion with the organic resin coating formed on the iron phosphate coating is ensured.
The amount of iron phosphate coating deposited is preferably 0.05 to 1.0 g / m ² per side. If the adhesion amount is less than 0.05 g / m ² , the scale surface cannot be completely covered, and the corrosion resistance deteriorates. On the other hand, when the adhesion amount exceeds 1.0 g / m ² , the iron phosphate coating itself is liable to cohesive failure, the adhesion with the organic resin coating is lowered, and the coating adhesion after coating is deteriorated.
[0014]
In the present invention, an organic resin film is formed on the iron phosphate film. By forming the organic resin coating on the iron phosphate coating, the coating adhesion after coating, the corrosion resistance after coating, and the corrosion resistance of places where coating is difficult to wrap around are improved.
The organic resin film is formed by applying a coating composition shown below on the iron phosphate film by a roll coater method, a bar coater method, an electrostatic coating method, a curtain flow method, or the like. The adhesion amount of the organic resin film is preferably 0.3 to 4.0 g / m ² per side. If the amount of adhesion is less than 0.3 g / m ² per side, sufficient coating film adhesion, post-coating corrosion resistance, and corrosion resistance at places where coating is difficult to wrap around will be insufficient. On the other hand, when the adhesion amount exceeds 4.0 g / m ² , the organic resin coating itself tends to cohesive and breakage, and the coating adhesion tends to be lowered, and the spot weldability is further deteriorated. Moreover, when the amount of adhesion increases, it becomes economically expensive.
[0015]
The main component of the coating composition used for forming the organic resin film is preferably an epoxy resin, a urethane resin, an acrylic resin, an alkyd resin, or a modified resin thereof. Examples of the modified resin include a urethane-modified epoxy resin having an epoxy equivalent of 1000 to 5000 obtained by reacting an isocyanate compound: 10 to 100 parts by weight with 100 parts by weight of an epichlorohydrin-bisphenol A type epoxy resin, or an epoxy group of a urethane-modified epoxy resin. A modified epoxy resin to which 0.5 to 1.0 mole of dialkanolamine is added per equivalent, an epoxy modified urethane resin obtained by reacting 5 to 30 parts by weight of epoxy with 100 parts by weight of urethane resin, and the like can be suitably used.
[0016]
In the coating composition mainly composed of the organic resin described above, an organic solvent or water can be suitably used as the solvent. As the organic solvent, butyl cellosolve, Solvesso 150, and PM acetate are preferable. Further, if necessary, silica, chromate or the like as a rust preventive pigment, or a crosslinking agent may be added to the coating composition.
Needless to say, the iron phosphate coating and the organic resin coating formed on the surface of the hot-rolled steel plate may be on one side or both sides of the steel plate.
[0017]
【Example】
Steel with the composition shown in Table 1 was melted in a converter and made into a slab by a continuous casting method, and then hot rolled into a 2.3 mm thick hot rolled sheet. The hot-rolled sheet was wound around a coil at a constant winding temperature of 620 ° C. The scale thickness of the hot rolled sheet was adjusted to 5.5 μm or 3.0 μm by changing the finish rolling temperature of hot rolling and the cooling rate after finish rolling.
[0018]
An iron phosphate coating having an adhesion amount shown in Table 2 was formed on the surface of these hot-rolled steel sheets, and then an organic resin coating having an adhesion amount shown in Table 2 was formed on the coating, and various properties were evaluated. In addition, the example which formed only the organic resin film without forming an iron phosphate film, or the example which formed neither an iron phosphate film nor an organic resin film was made into the comparative example.
The conditions for forming the iron phosphate coating and the organic resin coating are as follows.
(1) Iron phosphate coating An iron phosphate coating was formed by spray treatment using the iron phosphate bath shown below.
[0019]
Iron phosphate bath: Parphos 3456 manufactured by Nihon Parkerizing Co., Ltd.
Bath temperature: 50 ℃
(2) Organic resin coating An organic composite coating composition having the following compositions A to C was applied with a roll coater to form an organic resin coating.
[0020]
(1) Organic composite coating composition A
Main organic resin: Urethane modified epoxy resin Solvent: Butyl cellosolv rust pigment: Silica (2) Organic composite coating composition B
Main organic resin: Epoxy resin Solvent: Water Other: Crosslinker (3) Organic composite coating composition C
Main organic resin: Epoxy-modified urethane resin Solvent: Water rust preventive pigment: Silica, Barium chromate Steel sheet of the present invention in which the various coating films described above were formed, and a specimen from the hot rolled steel sheet of the comparative example were collected. Then, the appearance after coating, bare corrosion resistance, joint corrosion resistance, water resistance secondary adhesion after coating, corrosion resistance after electrodeposition coating, corrosion resistance after coating (corrosion resistance after chipping), and spot weldability were evaluated. The evaluation method is as follows.
[0021]
(i) Appearance test after coating The test piece was directly coated with a cationic electrodeposition coating with a thickness of 21 μm, an intermediate coating with a thickness of 30 μm, and a top coating with a thickness of 35 μm. The evaluation criteria were ◎: remarkably good, ○: good, Δ: slightly inferior, x: inferior.
(ii) Bare corrosion resistance test The bare corrosion resistance test evaluates the corrosion resistance of parts that are difficult to wrap around.
[0022]
The test piece was subjected to a cycle consisting of a salt spray test 5 hr (35 ° C.), a drying test 5 hr (60 ° C., relative humidity 20% or less), and a wet test 14 hr (50 ° C., relative humidity 95% or more). The number of days until a corrosion amount of 0.8 mm was obtained. The evaluation criteria were ◎: 100 days or more (remarkably good), ○: 81-99 days (good), Δ: 51-80 days (slightly inferior), x: 50 days or less (poor).
[0023]
(iii) Joint corrosion resistance test The joint corrosion resistance test evaluates the corrosion resistance of joints without paint.
Spot joints of the same type of specimen were spot-welded to produce a joint, and (ii) a test similar to the bare corrosion resistance test was performed, and the number of days until a corrosion amount of 0.8 mm in thickness was obtained. Evaluation criteria are the same as (ii): ◎: 100 days or more (remarkably good), ○: less than 100 days to 80 days or more (good), △: less than 80 days to 50 days or more (slightly inferior), ×: Less than 50 days (inferior).
[0024]
(iv) Corrosion resistance test after painting (corrosion resistance after chipping)
The corrosion resistance test after coating (corrosion resistance after chipping) evaluates the corrosion resistance of the painted surface.
Chipping test in which 20 pieces of 3-5g pebbles are shot on the coating surface at a speed of 100km / h on a specimen coated with a 21μm thick cationic electrodeposition coating, 30μm thick intermediate coating, and 35μm thick top coating. After that, a coastal exposure test was performed in which seawater was sprayed twice a week, and the corrosion width of the scratches on the paint film was measured. Evaluation criteria are: A: Corrosion width is 2 mm or less (remarkably good), ○: Corrosion width exceeds 2 mm to 5 mm or less (good), Δ: Corrosion width exceeds 5 mm to 10 mm or less (slightly inferior), X: Corrosion width is Over 10 mm (inferior).
[0025]
(v) Corrosion resistance test after electrodeposition coating The corrosion resistance test after electrodeposition coating evaluates the corrosion resistance of the cationic electrodeposition coating surface.
A test piece having a surface of 21 μm thick cationic electrodeposition paint was introduced with scratches on the coating surface, and then subjected to a salt spray test for 1200 hours to measure the corrosion width of the scratches. Evaluation criteria are: ◎: Corrosion width is 2 mm or less (remarkably good), ○: Corrosion width exceeds 2 mm to 5 mm or less (good), △: Corrosion width exceeds 5 mm to 10 mm or more (slightly inferior), ×: Corrosion width is Over 10 mm (inferior).
[0026]
(vi) Water resistance secondary adhesion test after coating The film water resistance secondary adhesion test evaluates the coating adhesion after deterioration.
A test piece coated with 21μm of cationic electrodeposition coating, 30μm of intermediate coating, and 35μm of top coating on the surface was immersed in distilled water at 50 ° C for 240hrs, and then a base test of 100 pieces of 2mm squares. And evaluated by taping peeling condition. Evaluation criteria are: ◎: No peeling (remarkably good), ○: Peeling 10% or less (good), △: Peeling 10% to 50% or less (slightly inferior), X: Peeling 50% (poor) , And.
[0027]
(vii) Spot weldability test The spot weldability test evaluates the number of quick hit points.
The test piece was welded with an alumina-dispersed copper alloy with a tip of 6 mmφ, and welding was performed continuously at a pressure of 200 kgf, a welding current of 10 kA, and a welding time of 10 cycles / 50 cycles until the nugget diameter fell below the reference diameter. The score was measured. Evaluation criteria are: ◎: The number of continuous hits is 3000 points or more (remarkably good), ○: The number of continuous hits is 2000 points or more and less than 3000 points (good), Δ: The number of continuous hit points is 1000 points or more and less than 2000 points (slightly inferior) , X: The number of consecutive hit points was less than 1000 points (inferior).
[0028]
Table 2 shows the evaluation results of each characteristic.
[0029]
[Table 1]

[0030]
[Table 2]

[0031]
From Table 2, examples of the present invention (test No. 1 to No. 6, No. 11 to No. 14) are the appearance after coating, bare corrosion resistance, joint corrosion resistance, coating water resistance secondary adhesion after coating, Corrosion resistance after electrodeposition coating, corrosion resistance after chipping, and spot weldability are all good (some may be slightly inferior), but comparative examples out of the scope of the present invention (Test Nos. 7-No. 10) is slightly inferior or inferior in all properties, and it can be seen that the hot-rolled steel sheet of the present invention is a steel sheet excellent in coating film adhesion and corrosion resistance after coating. In addition, from Table 2, it is particularly excellent when the iron phosphate coating amount satisfies the range of 0.05 to 1.0 g / m ² and the organic resin coating amount satisfies the range of 0.3 to 4.0 g / m ² . It can be seen that the characteristics are shown.
[0032]
【The invention's effect】
According to the present invention, a hot-rolled steel sheet excellent in bare corrosion resistance, paintability, post-coating corrosion resistance, and spot weldability is obtained, and the surface of the hot-rolled steel sheet with black skin (the steel sheet as hot-rolled) is directly applied. In addition, it is excellent in the corrosion resistance of the portion where the coating material is difficult to go around, such as the mating portion, and it can be expected to have a significant industrial effect such as omitting the descaling step.

Claims

A hot-rolled steel sheet excellent in paintability and corrosion resistance, characterized by having an iron phosphate coating on at least one side of a black-coated hot-rolled steel plate and further having an organic resin coating on the iron phosphate coating.

The hot-rolled steel sheet according to claim 1, wherein the adhesion amount of the iron phosphate coating is 0.05 to 1.0 g / m ² per side.

The hot-rolled steel sheet according to claim 1 or 2, wherein the adhesion amount of the organic resin coating is 0.3 to 4.0 g / m ² per side.