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EP-4741154-A1 - PRECOATED STEEL SHEET

EP4741154A1EP 4741154 A1EP4741154 A1EP 4741154A1EP-4741154-A1

Abstract

[PROBLEM] To further improve design properties and productivity of a pre-painted steel sheet by preventing resin particles from dropping out of a painting layer and deformation. [SOLUTION] The pre-painted steel sheet according to the present invention includes: a zinc-based plated steel sheet; and a painting layer located on a surface of a zinc-based plating layer, wherein the painting layer contains resin particles, and a binder resin and a color pigment as film-forming components, and in a cross-section cut at any position of the painting layer in a thickness direction, an average value of area ratios occupied by the resin particles is within a range of 5 to 40%, an average thickness dA at each portion of the painting layer not containing the resin particle is within a range of 10 to 40 µm, an average particle size ø of the resin particles is 4.0 times or less than the average thickness dA, an average substantial thickness dB of the painting layer at each portion containing the resin particle is within a range of 4 to 30 µm, and an average value of number ratios of exposed resin particles is 5 to 50%.

Inventors

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

Assignees

  • Nippon Steel Corporation

Dates

Publication Date
20260513
Application Date
20240703

Claims (9)

  1. A pre-painted steel sheet comprising: a zinc-based plated steel sheet having a zinc-based plating layer, which is a plating layer containing at least zinc, located on a surface of a base steel sheet; and a painting layer located on a surface of the zinc-based plating layer, wherein the painting layer contains resin particles, and a binder resin and a color pigment as film-forming components, in a cross-section cut at any position of the painting layer in a thickness direction, an average value of area ratios, each of which is an area percentage occupied by the resin particles, is within a range of 5 to 40%, an average thickness dA at each portion of the painting layer not containing the resin particle is within a range of 10 to 40 µm, an average particle size ø of the resin particles is 4.0 times or less than the average thickness dA, at a portion of the painting layer containing the resin particle, when a thickness obtained by subtracting a thickness of a portion corresponding to the resin particle from an overall thickness of the painting layer including the resin particle is referred to as a substantial thickness of the painting layer at a portion containing the resin particle, an average substantial thickness dB, which is an average value of the substantial thicknesses at the portions each containing the resin particle, is within a range of 4 to 30 µm, and when the resin particle having a portion whose outer periphery is not covered with the film-forming components with a thickness of 1 µm or more is referred to as an exposed resin particle, an average value of number ratios of the exposed resin particles is 5 to 50%.
  2. The pre-painted steel sheet according to claim 1, wherein in the cross-section, 80% or more of a length along the outer periphery of the exposed resin particle is covered with the film-forming components.
  3. The pre-painted steel sheet according to claim 1 or 2, wherein when a 60-degree specular gloss G S (60°) specified in JIS Z 8741:1997 is measured on a surface of the painting layer before and after tensile elongation of 30% is applied to the pre-painted steel sheet, a rate of change of the 60-degree specular gloss G S (60°) before and after applying the tensile elongation is 60% or less.
  4. The pre-painted steel sheet according to claim 1 or 2, wherein a compressive elastic modulus of the binder resin at 25°C is within a range of 60 to 150 MPa, a compressive elastic modulus of the resin particle at 25°C is within a range of 60 to 250 MPa, and the compressive elastic modulus of the binder resin at 25°C is equal to or less than the compressive elastic modulus of the resin particle at 25°C.
  5. The pre-painted steel sheet according to claim 1 or 2, wherein when a particle size distribution is measured, the painting layer contains, small-size resin particles with an average particle size within a range of 9 to 26 µm, having a peak within a range of 17 ± 8 µm in particle size and large-size resin particles with an average particle size within a range of 40 to 70 µm, having a peak within a range of 55 ± 10 µm in particle size, as the resin particles.
  6. The pre-painted steel sheet according to claim 5, wherein a content percentage CL of the small-size resin particles is within a range of 5 to 30 mass% relative to a total solid content mass of the painting layer, and a content percentage CH of the large-size resin particles is within a range of 5 to 30 mass% relative to the total solid content mass of the painting layer.
  7. The pre-painted steel sheet according to claim 5, wherein a ratio (CL/CH) of the content percentage CL of the small-size resin particles to the content percentage CH of the large-size resin particles is within a range of 0.5 to 1.5.
  8. The pre-painted steel sheet according to claim 5, wherein a ratio (øL/øH) of an average particle size øL of the small-size resin particles to an average particle size øH of the large-size resin particles is within a range of 0.14 to 0.58.
  9. The pre-painted steel sheet according to claim 5, wherein a compressive elastic modulus of the small-size resin particle at 25°C is within a range of 60 to 250 MPa, and a compressive elastic modulus of the large-size resin particle at 25°C is within a range of 60 to 250 MPa.

Description

TECHNICAL FIELD The present invention relates to a pre-painted steel sheet. BACKGROUND ART Metal sheets with high design properties are increasingly used for home appliances, building materials, automobiles, and other applications. Examples of such metal sheets with high design properties include, for example, metal sheets with colored organic coatings, metal sheets with highly glossy surfaces, and other metal sheets. Among the metal sheets with such design properties, when a zinc-based plated steel sheet provided with various types of zinc-based plating layers is used as a raw material of the metal sheets, it is possible to achieve steel sheets with both excellent corrosion resistance and excellent design properties resulting from zinc-based plated steel sheets. In the metal sheets and steel sheets with high design properties as described above, a scratch resistance of an outermost surface of the metal sheets and steel sheets with the design properties is important to maintain the excellent design properties. For this reason, many proposals have been conventionally made regarding the scratch resistance of the metal sheets and steel sheets with high design properties. For example, Patent Document 1 listed below has proposed a technology to improve an abrasion resistance of an organic film on a zinc-based plated steel sheet having the organic film by making the organic film contain beads having a particle size larger than a thickness of the organic film (hereinafter also referred to as "large-size beads"). 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 However, when the zinc-based plated steel sheet having the organic film containing large-size beads as proposed in Patent Document 1 listed above is coiled or subjected to a predetermined press-working process, the large-size beads may drop out of the organic film, and the design properties of the organic film may be degraded. In addition, there is concern that the large-size beads that drop out of the organic film may contaminate various coiling and press-working facilities, thereby interfering with the processes. For these reasons, it is necessary to take sufficient care when handling the organic film containing large-size beads, and there is room for further improvement from a viewpoint of improving productivity. In addition, when coiled or press-worked, the large-size beads may be deformed by a coiling tension of the coil, a pressure during the press-working process, or the like even if the large-size beads do not drop out. When such deformation occurs, the organic film at the deformed portion becomes relatively flat, resulting in a partial change in gloss, which may degrade the desired design properties. This degradation of design properties due to gloss level change is also referred to as pressure marks. Thus, there is room for further improvement in organic films containing large-size beads in terms of maintaining the design properties. The present invention has been made in view of the above problems, and an object thereof is to provide a pre-painted steel sheet that can further improve design properties and productivity while having a painting layer containing resin particles with a particle size larger than a thickness. MEANS FOR SOLVING THE PROBLEMS As a result of earnest studies for solving the above-described problems, the present inventors obtained ideas that if surfaces of resin particles contained in the painting layer, each having a particle size larger than an average thickness, can be covered to some extent with a binder resin, the resin particles may be prevented from dropping out or being deformed. Based on this idea, the inventors examined in detail conditions for resin particles having the particle size larger than the average thickness. As a result, the inventors found that it is possible to prevent the resin particles from dropping out of the painting layer and deformation, thereby further improving the design properties and productivity of the pre-painted steel sheet when the average particle size of the resin particles, the average thickness of the painting layer, and a thickness of the binder resin covering the surface of the resin particle satisfy certain conditions,. The gist of the invention completed based on such findings is as follows. (1) A pre-painted steel sheet including: a zinc-based plated steel sheet having a zinc-based plating layer, which is a plating layer containing at least zinc, located on a surface of a base steel sheet; and a painting layer located on a surface of the zinc-based plating layer, wherein the painting layer contains resin particles, and a binder resin and a color pigment as film-forming components, in a cross-section cut at any position of the painting layer in a thickness direction, an average value of area ratios, each of which is an area percentage occupied by the resin par