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EP-4741153-A1 - COATED STEEL SHEET

EP4741153A1EP 4741153 A1EP4741153 A1EP 4741153A1EP-4741153-A1

Abstract

[PROBLEM] To provide a painted steel sheet which can be further improved in corrosion resistance and productivity while having a low-gloss paint film layer having a particle or a thin film portion. [SOLUTION] Provided is a painted steel sheet including two or more paint film layers including a rust preventive paint film layer on at least one side of a steel sheet having a Zn-containing plating layer, wherein: a 60-degree specular glossiness Gs (60°) specified in JIS Z 8741:1997 of the paint film layer on at least one face of the steel sheet is 20% or less; an average film thickness of the rust preventive paint film layer is 5 µm or more and 15 µm or less; the rust preventive paint film layer contains a binder resin, a cross-linking agent, and a rust preventive pigment; and in a cross section obtained by cutting the paint film layer in a thickness direction at an arbitrary position, a coverage ratio of the rust preventive pigment to a total area of the rust preventive paint film layer is 30% or more and 60% or less in terms of area ratio.

Inventors

  • KAWAMURA, YASUAKI
  • FUJII, TAKASHI
  • SHIBAO, FUMIO

Assignees

  • Nippon Steel Corporation

Dates

Publication Date
20260513
Application Date
20240703

Claims (10)

  1. A painted steel sheet comprising two or more paint film layers including a rust preventive paint film layer on at least one side of a steel sheet having a Zn-containing plating layer, wherein: a 60-degree specular glossiness Gs (60°) specified in JIS Z 8741:1997 of the paint film layer on at least one face of the steel sheet is 20% or less; an average film thickness of the rust preventive paint film layer is 5 µm or more and 15 µm or less; the rust preventive paint film layer contains a binder resin, a cross-linking agent, and a rust preventive pigment; and in a cross section obtained by cutting the paint film layer in a thickness direction at an arbitrary position, a coverage ratio of the rust preventive pigment to a total area of the rust preventive paint film layer is 30% or more and 60% or less in terms of area ratio.
  2. The painted steel sheet according to claim 1, wherein the rust preventive pigment contains at least one or more of Si, P, Mg, and V.
  3. The painted steel sheet according to claim 2, wherein the Si contains any one or more of calcium ion-exchanged silica and magnesium ion-exchanged silica, the P contains any one or more of zinc phosphate, magnesium dihydrogen phosphate, and aluminium dihydrogen triphosphate, and the Mg contains any one or more of magnesium dihydrogen phosphate, magnesium oxide, and magnesium hydroxide.
  4. The painted steel sheet according to claim 1, wherein: on the face of the steel sheet where the 60-degree specular glossiness Gs (60°) is 20% or less, the two or more paint film layers have the rust preventive paint film layer at a lower layer and have a colored paint film layer at an outermost layer located on a farthest side from the steel sheet, and the colored paint film layer contains a resin particle, and a binder resin, a cross-linking agent, and a color pigment as a film-forming component; and in the cross section, an average particle diameter of the resin particle is two times or more and five times or less an average film thickness at a portion not having the resin particle of the colored paint film layer. Where the average particle diameter is a value obtained by measuring, in one observation visual field region in the cross section, circle-equivalent diameters of the resin particles existing in the observation visual field region, calculating an average value of upper two circle-equivalent diameters in descending order of the circle-equivalent diameters, and further averaging the averaged values similarly obtained for the observation visual field regions at ten arbitrary locations, by a number of fields of view.
  5. The painted steel sheet according to claim 4, wherein the coverage ratio of the resin particles to a total area of the colored paint film layer is 5% or more and 30% or less in terms of area ratio.
  6. The painted steel sheet according to claim 4, wherein in the cross section, a projecting portion of the resin particle projecting from a position on a surface at the portion not containing the resin particle of the colored paint film layer is covered, at half or more of a length along an outer peripheral portion of the projecting portion, with the film-forming component having a thickness of 0.5 µm or more.
  7. The painted steel sheet according to any one of claims 4 to 6, wherein the resin particle is a resin particle made of an acrylic resin.
  8. The painted steel sheet according to any one of claims 4 to 6, wherein the binder resin is a binder resin made of an acrylic resin or a polyester resin.
  9. The painted steel sheet according to claim 7, wherein the binder resin is a binder resin made of an acrylic resin or a polyester resin.
  10. The painted steel sheet according to claim 1 or 2, having a characteristic selected from any one or two of a surface roughness of the steel sheet being 2 µm or more in Ra on the face of the steel sheet where the 60-degree specular glossiness Gs (60°) is 20% or less, and the paint film layer containing 15% or more of an aggregate composed of a pigment containing silica.

Description

TECHNICAL FIELD The present invention relates to a painted steel sheet. BACKGROUND ART Steel sheets with high design are increasingly used for applications for building material use, home appliance use, automobile use, and so on. A steel sheet has a matte appearance with suppressed gloss as an example of the high design. Examples of the steel sheet having the matte appearance include a low-gloss painted steel sheet with an organic coating layer containing resin particles which is provided on a steel sheet surface, and so on. Further, as a steel sheet having a paint film layer on the surface, there is a known steel sheet provided with a rust preventive paint film layer containing a rust preventive pigment for improving the corrosion resistance of the steel sheet. For example, Patent Document 1 below proposes a technique of improving, in a zinc-based plated steel sheet having an organic coating, the abrasion resistance of the organic coating by containing a resin particle having a particle diameter larger than the film thickness of the organic coating (hereinafter, also referred to as a "large-diameter particle") in the organic coating. PRIOR ART DOCUMENT PATENT DOCUMENT Patent Document 1: International Publication Pamphlet No. WO 2014/112544 SUMMARY OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION The steel sheet having the matte appearance is subjected to low-gloss painting or the like and thus has large recesses and projections on a steel sheet surface to suppress the reflection of light by the recesses and projections and realize the low gloss. Accordingly, a layer constituting the low-gloss painting (hereinafter, also called a "low-gloss paint film layer") can contain a thin film portion. If a flaw or the like occurs at the thin film portion, a corrosion factor in an environment such as rain water is likely to intrude through the flaw or the like, and can be a factor causing corrosion on the steel sheet surface. Besides, if the organic coating containing the large-diameter particle as proposed in Patent Document 1 above is employed as the low-gloss paint film layer, the large-diameter particle is largely exposed from the binder resin layer constituting the organic coating. The corrosion factor is likely to intrude through the interface between the large-diameter particle and the binder resin layer, and can be a factor causing corrosion on the steel sheet surface. Besides, if the zinc-based plated steel sheet having the organic coating containing the large-diameter particle is wound into a coil form or subjected to predetermined press processing, the large-diameter particle falls off the organic coating, resulting in a decrease in design of the organic coating in some cases. Further, the large-diameter particle that has fallen off the organic coating may contaminate various facilities for winding the steel sheet into the coil form or performing the press processing to hinder the processing. From the above, in the case of handling the organic coating containing the large-diameter particle, it is necessary to sufficiently pay attention on handling it and there is room for further improvement from the viewpoint of improving the productivity. Hence, the present invention has been made in view of the above problems, and an object of the present invention is to provide a painted steel sheet which can be further improved in corrosion resistance and productivity while having a low-gloss paint film layer having a particle or a thin film portion. MEANS FOR SOLVING THE PROBLEMS As a result of earnest studies for solving the above-described problems, the present inventors obtained the following findings. Specifically, if the surface of a particle having a particle diameter larger than an average film thickness contained in an organic resin coating layer can be covered by an organic resin, the interface between the large-diameter particle and the binder resin layer can be lengthened. Note that owing to the "interface between the large-diameter particle and the binder resin layer is long", a predetermined portion of the particle projecting from the binder resin layer is covered by a film-forming component, so that an exposed portion of the particle can be decreased. Further, lengthening the interface between the large-diameter particle and the binder resin layer as above can suppress the intrusion of the corrosion factor to suppress the corrosion. Further, it is possible to prevent the fall-off and deformation of the particle. Based on the idea, the present inventors discussed in detail coating conditions of a paint containing the particle having the particle diameter larger than the average film thickness. As a result of this, the present inventors have found that when the average particle diameter of the particle, the average film thickness of the organic resin coating layer, the thickness of the organic resin covering the surface of the particle, and so on satisfy specific conditions, it is possible to preven