Coated steel sheet

Abstract

[Problem] To provide a coated steel sheet using a Zn-based plated steel sheet for which both the ability to conceal discoloration originating from the base sheet and the adhesivity of a colored coating film during processing are achieved. [Solution] A coated steel sheet according to an embodiment of the present invention comprises a Zn-based plated steel sheet and a colored coating layer. The thickness of the colored coating layer is 3.0-10.0 μm. The area ratio of a coloring pigment in the colored coating layer is 10-50%. Virtual grid lines are set at intervals of 1 μm in a cross-section obtained by cutting the colored coating layer in the thickness direction, and the area ratio of the coloring pigment within 1 μm × 1 μm regions partitioned by the grid lines is measured for at least 500 regions. In the measurement results, the abundance ratio of regions in which the area ratio is 0-10% is within the range of 10-40% with respect to all measured regions, and the abundance ratio of regions in which the area ratio is 60-100% is within the range of 10-40% with respect to all measured regions.

Description

Painted steel plate

[0001] This invention relates to painted steel sheets.

[0002] For applications such as home appliances, building materials, and automobiles, pre-coated steel sheets, which have a colored coating applied beforehand, are increasingly being used as steel sheet materials, replacing conventional post-painted steel sheets that were painted after forming. Such pre-coated steel sheets are made by coating the surface of a rust-preventively treated or plated steel sheet with a colored coating. Pre-coated steel sheets have the characteristics of having a beautiful appearance, sufficient workability, and good corrosion resistance.

[0003] In relation to the pre-coated steel sheets described above, thinning the colored coating has been considered from the perspective of rationalizing the painting process for forming the colored coating and conserving resources. However, thinning the colored coating can make defects that may exist on the surface of the base steel sheet or plated steel sheet, as well as areas with reduced aesthetic appeal that occur when processing the pre-coated steel sheet into the desired shape, more noticeable, potentially affecting the beautiful appearance that the pre-coated steel sheet originally possesses.

[0004] Therefore, as disclosed in Patent Documents 1 and 2 below, for example, a technique has been proposed to maintain the beautiful appearance of pre-coated steel sheets by incorporating a pigment, such as carbon black, into the colored coating to make the above-mentioned defects less noticeable.

[0005] Japanese Patent Publication No. 2007-177608 Japanese Patent Publication No. 2015-217907

[0006] The inventors of the present invention have been diligently studying the use of a Zn-plated steel sheet having a Zn-plated layer as a base material for the pre-coated steel sheet described above, and have found that such a Zn-plated steel sheet blackens over time when exposed to a high-humidity environment. Pre-coated steel sheets using Zn-plated steel sheets as a base material are often used as housings for products installed outdoors, such as outdoor units for air conditioners and distribution boards, and these housings are generally given a white-based coloring. However, because the surface of the Zn-plated steel sheet, which is the base material, is easily visible through the white-based coloring film, the blackening of the plating layer described above becomes easily visible.

[0007] Faced with the phenomena described above, the inventors diligently investigated methods to make the blackening less noticeable, even when the colored coating is a thin film. As one approach, the inventors first considered increasing the content of coloring pigments contained in the colored coating, not limited to carbon black. However, it was found that increasing the content of colored coating in the film too much made the blackening less noticeable, but also made the colored coating itself brittle, causing it to peel off during processing of the pre-coated steel sheet. Therefore, it became clear that it is necessary to establish a technology that can achieve both the ability to conceal such discoloration and the ability of the coating to adhere to processing.

[0008] Therefore, the present invention has been made in view of the above problems, and the object of the present invention is to provide a painted steel sheet using a Zn-plated steel sheet that can achieve both the ability to conceal discoloration originating from the substrate and the ability to adhere the colored film to the workpiece while ensuring corrosion resistance.

[0009] In order to solve the above problems, the inventors of this invention conducted diligent research and came up with the idea that by deliberately introducing a certain variation in the distribution of coloring pigments in the colored film, and intentionally creating areas with different film hardnesses within the colored film, it may be possible to achieve both the above-mentioned discoloration concealment and process adhesion. The gist of the present invention, completed based on this idea, is as follows.

[0010] (1) A Zn-plated steel sheet having a Zn-based plating layer containing at least Zn on at least one surface of a steel sheet that serves as a base material, and a colored film layer located on the Zn-based plating layer and containing at least a resin component and a colored pigment, wherein the thickness of the colored film layer is 3.0 to 10.0 μm, and the colored pigment in the colored film layer occupies 10 to 50% of the film area in the cross-section in the thickness direction of the colored film layer, and the cross-section in the thickness direction of the colored film layer, (2) A painted steel sheet, wherein, in each direction perpendicular to the thickness direction, virtual grid lines are set at 1 μm intervals, and the area ratio of the colored pigment in a 1 μm × 1 μm area partitioned by the virtual grid lines is measured for 500 areas, and the proportion of areas with an area ratio of 0% or more and 10% or less is within the range of 10 to 40% of the total measured areas, and the proportion of areas with an area ratio of 60% or more and 100% or less is within the range of 10 to 40% of the total measured areas. (2) The painted steel sheet according to (1), wherein, in the cross-section, the proportion of areas with an area ratio of more than 10% and less than 60% is within the range of 30 to 70% of the total measured areas, and the sum of the proportion of areas with an area ratio of 0% or more and 10% or less, the proportion of areas with an area ratio of 60% or more and 100% or less, and the proportion of areas with an area ratio of more than 10% and less than 60% is 100%. (3) The painted steel sheet according to (1) or (2), wherein, with respect to the cross section, a range is set at an arbitrary position in the thickness direction of the colored film layer, with the length along the thickness direction being 1 μm centered on that arbitrary position, and virtual grid lines are set at 1 μm intervals in a direction perpendicular to the thickness direction, and in a region of size 1 μm × 1 μm defined by the range with a length of 1 μm being demarcated by the virtual grid lines, the area ratio of the colored pigment is measured in 50 regions, and in the measurement results, there are at least 5 regions where the area ratio is 0% or more and 10% or less, and there are at least 5 regions where the area ratio is 60% or more and 100% or less.(4) The painted steel sheet according to (3), wherein in the cross-section, there are at least 10 areas where the area ratio is greater than 10% and less than 60%. (5) The painted steel sheet according to (1) or (2), wherein the colored film layer further contains resin particles. (6) The painted steel sheet according to (5), wherein the average particle size of the resin particles is 3 to 10 μm, and the resin particles in the colored film layer occupy 3 to 25% of the film area in the cross-section of the colored film layer. (7) The colored film layer is L as defined in JIS Z 8781-4:2013. ＊ (1) or (2) a painted steel sheet according to (1) or (2), wherein the value is 50 or more. (8) A painted steel sheet according to (1) or (2), wherein the coloring pigment is a white pigment of at least one of titanium oxide, zinc oxide, barium sulfate, or zinc sulfide. (9) A painted steel sheet according to (1) or (2), wherein the thickness of the Zn-based plated steel sheet is 0.2 to 5.0 mm. (10) A painted steel sheet according to (1) or (2), wherein the Zn-based plating layer is a plating layer containing, by mass%, Al: 4 to 22%, Mg: 1 to 10%, Si: 0.0001 to 2.0000%, with the remainder being Zn and impurities.

[0011] As described above, according to the present invention, in a painted steel sheet using a Zn-plated steel sheet, it is possible to achieve both the ability to conceal discoloration originating from the substrate and the workability of the colored film while ensuring corrosion resistance.

[0012] This is a schematic explanatory diagram showing an example of the structure of a painted steel sheet according to an embodiment of the present invention. This is a schematic explanatory diagram showing an example of the structure of a painted steel sheet according to the same embodiment. This is an explanatory diagram for explaining the colored film layer having of the painted steel sheet according to the same embodiment. This is an explanatory diagram for explaining the distribution state of colored pigments in the colored film layer according to the same embodiment. This is an explanatory diagram for explaining the distribution state of colored pigments in the colored film layer according to the same embodiment.

[0013] Preferred embodiments of the present invention will be described in detail below with reference to the attached drawings. In this specification and the drawings, components having substantially the same functional configuration are denoted by the same reference numerals, and redundant explanations will be omitted. In the following description, the notation "numerical value A to numerical value B" indicates that the values ​​are greater than or equal to numerical value A and less than or equal to numerical value B.

[0014] (Regarding painted steel sheets) In the following, painted steel sheets according to embodiments of the present invention will be described in detail with reference to the drawings. For convenience, the coordinate system shown in the drawings may be referred to as appropriate in the following description.

[0015] <Overall Structure of Painted Steel Sheet> Figures 1A and 1B are schematic explanatory diagrams showing an example of the structure of a painted steel sheet according to an embodiment of the present invention. As schematically shown in Figure 1A, the painted steel sheet 1 according to this embodiment has a Zn-based plated steel sheet 10 and a colored film layer 20 located on one surface of the Zn-based plated steel sheet 10. The Zn-based plated steel sheet 10 also has a base steel sheet 11 and a Zn-based plating layer 13 provided on the base steel sheet 11. The Zn-based plating layer 13 and the colored film layer 20 may also be provided on both sides of the base steel sheet 11, as schematically shown in Figure 1B.

[0016] The various components of the painted steel sheet 1 according to this embodiment will be described in detail below.

[0017] <Regarding the base steel sheet 11> Various types of steel sheets can be used as the base steel sheet 11, such as Al-killed steel, ultra-low carbon steel containing Ti, Nb, etc., and high-strength steel in which reinforcing elements such as P, Si, Mn are further added to ultra-low carbon steel.

[0018] Furthermore, the thickness of the base steel sheet 11 can be appropriately set according to the mechanical strength (e.g., tensile strength), processability, and ease of manufacture required for the painted steel sheet 1 according to this embodiment. For example, the thickness of the base steel sheet 11 can be, for example, about 0.2 to 5.0 mm.

[0019] <About the Zn-based plating layer 13> A Zn-based plating layer 13 is located on the surface of the base steel sheet 11 as described above. This Zn-based plating layer 13 is composed of various zinc-based platings. Examples of such zinc-based platings include hot-dip galvanizing, electroplating, zinc-nickel alloy plating, alloyed hot-dip galvanizing, aluminum-zinc alloy plating, zinc-aluminum-magnesium alloy plating, zinc-vanadium composite plating, zinc-zirconium composite plating, and the like.

[0020] Among such zinc-based platings, it is particularly preferable to use a zinc-aluminum-magnesium alloy plating as the Zn-based plating layer 13, and more preferably to use a zinc-aluminum-magnesium-silicon alloy plating containing Al: 4 to 22% by mass, Mg: 1 to 10% by mass, Si: 0.0001 to 2.0000% by mass, with the remainder being Zn and impurities.

[0021] [Al: 4-22% by mass] By setting the Al content to 4% by mass or more, it is possible to further improve the corrosion resistance of the steel sheet. The Al content is more preferably 5% by mass or more. On the other hand, by setting the Al content to 22% by mass or less, it is possible to further improve the corrosion resistance of the steel sheet while suppressing the saturation of the corrosion resistance improvement effect described above. The Al content is more preferably 16% by mass or less.

[0022] [Mg: 1-10% by mass] By setting the Mg content to 1% by mass or more, it is possible to further improve the corrosion resistance of the steel sheet. The Mg content is more preferably 2% by mass or more. On the other hand, by adjusting the Mg concentration in the plating bath used to form the plating layer to such that the Mg content in the Zn-containing plating layer 20 after production is 10% by mass or less, it is possible to stabilize the generation of dross in the plating bath and stably manufacture plated steel sheets. It is more preferable to adjust the Mg concentration in the plating bath used to form the Zn-containing plating layer 20 to such that the Mg content in the Zn-containing plating layer 20 after production is 5% by mass or less.

[0023] [Si: 0.0001 to 2.0000 mass%] By setting the Si content to 0.0001 mass% or more, it is possible to further improve the adhesion of the Zn-based plating layer 13 (more specifically, the adhesion between the base steel plate 11 and the Zn-based plating layer 13). On the other hand, by setting the Si content to 2.0000 mass% or less, it is possible to further improve the adhesion of the Zn-based plating layer 13 while suppressing the saturation of the adhesion improvement effect of the Zn-based plating layer 13. The Si content is more preferably 1.6000 mass% or less.

[0024] Furthermore, in the Zn-based plating layer 13 according to this embodiment, elements such as Fe, Sb, and Pb may be included individually or in combination in an amount of 1% by mass or less, in place of a portion of the remaining Zn.

[0025] Examples of zinc-plated steel sheets provided with a Zn-based plating layer 13 having the above-described chemical composition include hot-dip zinc-aluminum-magnesium alloy plated steel sheets having a Zn-6%Al-3%Mg alloy plating layer, and plated steel sheets having a Zn-11%Al-3%Mg-0.2%Si alloy plating layer, such as hot-dip zinc-aluminum-magnesium-silicon alloy plated steel sheets (for example, "SuperDyma®" manufactured by Nippon Steel Corporation).

[0026] The amount of Zn-based plating layer 13 deposited on both sides of the steel sheet is 30 g / m² in total. ２ That is all (i.e., 15 g / m per side). ２ (The above is preferable.) The amount of adhesion is 30 g / m ２ By doing so, it becomes possible to reliably ensure the corrosion resistance of the zinc-plated steel sheet. More preferably, the amount of plating applied is 40 g / m² in total on both sides of the steel sheet. ２ That concludes the explanation. On the other hand, the total amount of plating deposited on both sides of the steel sheet was 600 g / m². ２ The following applies (i.e., 300 g / m² per side). ２ The following is preferable: Adhesion amount of 600 g / m ２By setting the following, it becomes possible to further improve the corrosion resistance while ensuring the smoothness of the surface of the Zn-based plating layer 13. The total plating adhesion amount on both sides of the steel sheet is more preferably 550 g / m ２ is as follows.

[0027] <Regarding the thickness of the Zn-based plated steel sheet 10> In the coated steel sheet 1 according to the present embodiment, the thickness of the Zn-based plated steel sheet 10 having the base steel sheet 11 and the Zn-based plating layer 13 as described above (for example, the thickness d1 in FIGS. 1A and 1B) is preferably within the range of 0.2 to 5.0 mm. If the thickness of the Zn-based plated steel sheet 10 is within the above range, it becomes easy to perform forming while ensuring designability. The thickness of the Zn-based plated steel sheet according to the present embodiment is more preferably 0.4 mm or more. Further, the thickness of the Zn-based plated steel sheet according to the present embodiment is more preferably 3.5 mm or less.

[0028] <Regarding the colored film layer 20> The colored film layer 20 according to the present embodiment is a film layer colored in a desired color by having a coloring pigment, and is also a film layer that contributes to the improvement of the corrosion resistance of the coated steel sheet 1 according to the present embodiment. Such a colored film layer 20 is composed of a film-forming component 201 and a coloring pigment 203, as schematically shown in FIG. 2A. Further, the colored film layer 20 according to the present embodiment may further contain resin particles 205 in the film-forming component 201, as schematically shown in FIG. 2B.

[0029] [Regarding the film-forming component 201] The film-forming component 201 of the colored film layer 20 according to the present embodiment is not particularly limited, and any known various materials can be used as long as they contribute to the improvement of the corrosion resistance of the coated steel sheet 1 according to the present embodiment and function as a binder for the coloring pigment 203.

[0030] Considering the above points, from the viewpoints of manufacturing simplicity and cost performance, it is preferable to use various organic resins as the film-forming component 201 Such film-forming components 201 include, for example, acrylic resins, polyester resins, urethane resins, fluorine resins, and the like. Also, in the colored film layer 20 according to the present embodiment, it is also possible to use such various organic resins in combination.

[0031] Regarding the state of the organic resin as described above, there is no particular limitation, and it may be a water-dispersed aqueous resin or a solvent-soluble resin. However, from the perspective of manufacturing simplicity, it is more preferable that it is a water-dispersed aqueous resin.

[0032] When the resin particles 205 are contained in the film-forming component 201, it is preferable to select a resin of the same type as the resin particles 205 as the film-forming component 201. Thereby, the affinity between the film-forming component 201 and the resin particles 205 is improved, and it is possible to further improve the adhesion of the colored film layer 20.

[0033] Here, in the colored film layer 20 according to the present embodiment, the glass transition point Tg of the film-forming component 201 composed of the above components is preferably 30°C or higher. When the glass transition point Tg of the film-forming component 201 is 30°C or higher, the colored film layer 20 exhibits better hardness, and it is possible to improve the scratch resistance. The glass transition point Tg of such a film-forming component 201 is more preferably 35°C or higher, and still more preferably 40°C or higher.

[0034] On the other hand, the glass transition point Tg of such a film-forming component 201 is preferably 70°C or lower. When the glass transition point Tg of the film-forming component 201 is 70°C or lower, the colored film layer 20 has better processability. The glass transition point Tg of such a film-forming component 201 is more preferably 65°C or lower, and still more preferably 60°C or lower.

[0035] In the present embodiment, in order to set the glass transition point Tg of the film-forming component 201 to a desired temperature, a plurality of organic resins having different glass transition points Tg may be used in combination.

[0036] Here, the glass transition point Tg can be determined, for example, by thermomechanical analysis (TMA), in which a needle is inserted from the surface of the film to be measured in the direction of film thickness, and a constant temperature change is applied to measure the change in thermal expansion of the object to be measured, or by dynamic mechanical analysis (DMA), in which the film to be measured, which has been peeled off from the substrate, is subjected to periodic deformation while a constant temperature change is applied to analyze its viscoelasticity. It is also possible to measure the glass transition point by scraping off the film and subjecting the obtained sample to differential scanning calorimetry (DSC).

[0037] [Regarding the coloring pigment 203] In the colored film layer 20 according to this embodiment, the coloring pigment 203 is included in order to give the colored film layer 20 a desired color tone. The type of coloring pigment 203 is not particularly limited, and various known pigments can be used as appropriate depending on the color desired for the colored film layer 20. Examples of such coloring pigments include aluminum pigment, carbon black, titanium dioxide, zinc oxide, and the like.

[0038] In particular, in the colored film layer 20 according to this embodiment, if a white film is desired, the formed colored film layer 20 shall be treated according to L as defined in JIS Z 8781-4:2013. ＊ When the value is measured, the L ＊ It is preferable to use a combination of white coloring pigments so that the value is 50 or more. Examples of such white coloring pigments include titanium dioxide, zinc oxide, barium sulfate, zinc sulfide, etc., and it is also possible to use a combination of such white coloring pigments as appropriate. When aiming for a white film, the above L ＊ There is no specific upper limit specified for the value, L ＊ The value can also be 100.

[0039] ◇Area ratio of colored pigment 203 in the colored film layer 20 In the colored film layer 20 according to this embodiment, the colored pigment 203 described above occupies 10 to 50% of the film area when the cross-section in the thickness direction of the colored film layer 20 is observed.

[0040] If the area ratio of the coloring pigment 203 is less than 10%, the content of the coloring pigment 203 is too low to conceal the discoloration that occurs in the substrate Zn-plated steel sheet 10. When the area ratio of the coloring pigment 203 is 10% or more, it becomes possible to conceal the discoloration even if discoloration occurs in the substrate Zn-plated steel sheet 10. The area ratio of the coloring pigment 203 in the colored film layer 20 is preferably 12% or more, and more preferably 15% or more. Furthermore, in order to achieve such an area ratio, the content of the coloring pigment 203 is preferably 15% by mass or more, more preferably 20% by mass or more, and even more preferably 25% by mass or more.

[0041] On the other hand, if the area ratio of the coloring pigment 203 exceeds 50%, it is possible to conceal the discoloration occurring on the Zn-plated steel sheet 10, but the colored film layer 20 becomes too brittle, making it impossible to ensure workability. By making the area ratio of the coloring pigment 203 50% or less, it is possible to conceal the discoloration occurring on the Zn-plated steel sheet 10 while improving the workability of the colored film layer 20. The area ratio of the coloring pigment 203 in the colored film layer 20 is preferably 40% or less, and more preferably 30% or less. Furthermore, in order to achieve such an area ratio, the content of the coloring pigment 203 is preferably 55% by mass or less, more preferably 50% by mass or less, and even more preferably 45% by mass or less.

[0042] Here, the content of film-forming component 201 and coloring pigment 203 in the colored film layer 20 is approximately the same as the content of film-forming component and coloring pigment contained in the colored paint used to form the colored film layer 20.

[0043] The area ratio of the colored pigment 203 described above can be measured by performing cross-sectional observation of the colored film layer 20. Specifically, the cross-section is processed using a focused ion beam (FIB), and the resulting cross-section is observed with a scanning electron microscope (SEM). In this process, the cross-section of the colored film layer 20 is measured so that the total area of ​​the observed cross-sections is 500 μm². ２ Observe until this occurs, calculate the area ratio of the coloring pigment 203 to the total area of ​​the colored film layer 20, and use this as the area ratio of the coloring pigment 203. Alternatively, subtract the above area ratio of the coloring pigment 203 from 100% to obtain the value that can be used as the area ratio of the film-forming component 201.

[0044] The distribution state of the colored pigment 203 realized in the colored film layer 20 according to this embodiment will be described in detail below.

[0045] [Regarding the resin particles 205] The resin particles 205 that may be contained in the colored film layer 20 according to this embodiment have the role of mitigating the load applied to the colored film layer 20 when a load is applied to the colored film layer 20 due to the toughness and ductility exhibited by the resin particles 205, thereby further improving the scratch resistance of the colored film layer 20.

[0046] Various known organic resin particles can be used as such resin particles 205. Examples of such organic resin particles include acrylic resin particles, polyester resin particles, urethane resin particles, fluororesin particles, silicone resin particles, and polyolefin resin particles. It is more preferable to use acrylic resin particles as the above-mentioned organic resin particles. Furthermore, the coloring pigment contained in the colored film layer 20 may itself function as the above-mentioned resin particles 205.

[0047] ◇Average particle size of resin particles 205 Here, when the colored film layer 20 according to this embodiment contains resin particles 205, the average particle size of the resin particles 205 is preferably in the range of 3 to 10 μm. By making the average particle size of the resin particles 205 3 μm or more, it becomes possible to enjoy the improved scratch resistance due to the toughness and ductility of the resin as described above. The average particle size of the resin particles 205 is more preferably 4 μm or more.

[0048] On the other hand, by setting the average particle size of the resin particles 205 to 10 μm or less, it is possible to further improve the scratch resistance of the painted steel sheet 1 according to this embodiment while maintaining its aesthetic appeal. More preferably, the average particle size of the resin particles 205 is 8 μm or less.

[0049] Depending on the thickness of the colored film layer 20, as described later, some of the resin particles 205 may protrude from the colored film layer 20. However, even if some of the resin particles 205 protrude from the colored film layer 20, this will not affect the aesthetic appearance as long as the content of the resin particles 205 according to this embodiment is within the range described below. However, the average particle size of the resin particles 205 is preferably 0.3 to 2.0 times the average thickness of the colored film layer 20. By keeping it within this range, it is possible to further improve scratch resistance without worrying about the resin particles 205 falling off the colored film layer 20. The average particle size of the resin particles 205 is more preferably 0.4 times or more the average thickness of the colored film layer 20. Furthermore, the average particle size of the resin particles 205 is more preferably 1.5 times or less the average thickness of the colored film layer 20.

[0050] Here, the average particle size of the resin particles 205 can be measured by direct observation from the cross-section. Specifically, the coated steel plate 1 is embedded perpendicular to the thickness direction of the coating in a room-temperature drying epoxy resin, and the embedded surface is mechanically polished before being observed with a scanning electron microscope (SEM). At that time, the particle size of the resin particles 205 observed at any multiple locations (for example, 10 locations) is measured, and the average value of the obtained multiple particle sizes is taken as the average particle diameter of the resin particles 205.

[0051] ◇Area ratio of resin particles 205 in the colored film layer 20 Furthermore, in the colored film layer 20 according to this embodiment, it is preferable that the resin particles 205 described above occupy 3 to 25% of the film area when the cross-section of the colored film layer 20 is observed. By having an area ratio of 3% or more of resin particles 205, it is possible to enjoy the above-mentioned effect of improved scratch resistance. The area ratio of resin particles 205 in the colored film layer 20 is more preferably 7% or more. In addition, in order to achieve such an area ratio, it is preferable that the content of resin particles 205 is 3% by mass or more, and more preferably 5% by mass or more.

[0052] On the other hand, by reducing the area ratio of the resin particles 205 to 25% or less, it becomes possible to further improve the resistance to blackening and workability of the painted steel sheet 1 according to this embodiment while maintaining its aesthetic appeal. The area ratio of the resin particles 205 in the colored film layer 20 is more preferably 20% or less. Furthermore, in order to achieve such an area ratio, the content of the resin particles 205 is preferably 20% by mass or less, and more preferably 15% by mass or less.

[0053] The area ratio of the resin particles 205 described above can be measured by observing the cross-section of the colored film layer 20. Specifically, a painted steel plate 1 is embedded perpendicular to the thickness direction of the film in a room-temperature drying epoxy resin, and the embedded surface is mechanically polished before being observed with a scanning electron microscope (SEM). At that time, the cross-section of the colored film layer 20 is observed, and the total area of ​​the observed cross-section is 3000 μm². ２ Observe until this occurs, calculate the area ratio of the resin particles 205 to the total area of ​​the colored film layer 20, and use this as the area ratio of the resin particles 205. Alternatively, subtract the area ratio of the colored pigment 203 and the area ratio of the resin particles 205 from 100% to obtain the value that will be used as the area ratio of the film-forming component 201.

[0054] In this embodiment, the colored film layer 20, when the area ratio of the film-forming component 201, the area ratio of the colored pigment 203, and the area ratio of the resin particles 205 are added together, equals the total area of ​​the colored film layer 20 (100%).

[0055] [Regarding the average thickness of the colored film layer 20] In this embodiment, the colored film layer 20 having the above-described components has an average thickness in the range of 3.0 to 10.0 μm. If the average thickness of the colored film layer 20 is less than 3.0 μm, it becomes difficult to clearly recognize the color tone of the colored film layer 20. By setting the average thickness of the colored film layer 20 to 3.0 μm or more, it becomes possible to clearly recognize the color tone of the colored film layer 20 along with the design properties of the surface of the Zn-plated steel sheet 10, thereby improving the design properties of the painted steel sheet 1 according to this embodiment. The average thickness of the colored film layer 20 is preferably 3.5 μm or more, and more preferably 4.0 μm or more.

[0056] On the other hand, if the average thickness of the colored film layer 20 exceeds 10.0 μm, it is costly, and coating defects such as blotches may occur, making it difficult to obtain a stable appearance. The average thickness of the colored film layer 20 is preferably 9.0 μm or less, and more preferably 8.0 μm or less.

[0057] The thickness of the colored film layer 20 can be measured by cross-sectional observation. The thickness can be measured at any number of locations (for example, 10 locations), and the average of the obtained thicknesses can be taken as the thickness of the colored film layer 20.

[0058] The method for preparing the sample for the cross-sectional observation described above is the same as the method for preparing the sample for measuring the average particle size of the resin particles 205 described above, so a detailed explanation will be omitted.

[0059] [Regarding the distribution of colored pigment 203] In the colored film layer 20 according to this embodiment, the colored pigment 203 is not uniformly dispersed. Rather, a predetermined variation is intentionally introduced in the distribution of the colored pigment within the colored film layer 20, thereby intentionally creating areas within the colored film layer 20 with different film hardnesses. The distribution of colored pigment 203 within the colored film layer 20 will be described in detail below.

[0060] Figures 3 and 4 are explanatory diagrams for explaining the distribution state of the colored pigment 203 in the colored film layer 20 according to this embodiment. Below, first, as shown in Figure 3, in order to observe the cross-section obtained by cutting the colored film layer 20 in the thickness direction (z-axis direction in the figure), the cross-section is processed using a focused ion beam (FIB), and when the obtained cross-section is observed with a scanning electron microscope (SEM), virtual grid lines are set at 1 μm intervals in both the thickness direction and the direction perpendicular to the thickness direction (y-axis direction in the figure), and attention is focused on the region of size 1 μm × 1 μm that is partitioned by these virtual grid lines.

[0061] In the colored film layer 20 according to this embodiment, when 500 regions of the size 1 μm × 1 μm are observed, attention is paid to the area ratio occupied by the colored pigment 203 in each of the 1 μm × 1 μm regions. In this case, in the colored film layer 20 according to this embodiment, regions where the area ratio of the colored pigment 203 is 0% or more and 10% or less exist within the range of 10 to 40% of the total measured regions, and regions where the area ratio is 60% or more and 100% or less exist within the range of 10 to 40% of the total measured regions. That is, in the colored film layer 20 according to this embodiment, the proportion of regions where the above area ratio is 0% or more and 10% or less is within the range of 10 to 40% of the 500 measured regions, and the proportion of regions where the above area ratio is 60% or more and 100% or less is within the range of 10 to 40% of the 500 measured regions.

[0062] Furthermore, in the distribution of the colored pigment 203, it is preferable that the proportion of areas with an area ratio of more than 10% but less than 60% is within the range of 30 to 70% of the total measured area, and that the sum of the proportion of areas with an area ratio of 0% or more and 10% or less, the proportion of areas with an area ratio of 60% or more, and the proportion of areas with an area ratio of more than 10% but less than 60% equals 100%.

[0063] In the colored film layer 20 according to this embodiment, the proportion of areas where the area ratio of the colored pigment 203 is 0% or more and 10% or less is within the range of 10% to 40%, and the proportion of areas where such an area ratio is 60% or more is within the range of 10% to 40% of the measured area, which causes variation in the hardness of the colored film layer 20. As a result, in the painted steel sheet 1 according to this embodiment, the distortion caused by processing can be mitigated in the softer parts, making it possible to achieve both discoloration concealment and processing adhesion.

[0064] Here, in the colored film layer 20 according to this embodiment, the proportion of regions with an area ratio of 0% or more and 10% or less is preferably 15% or more, and more preferably 20% or more. Furthermore, the proportion of regions with an area ratio of 0% or more and 10% or less is preferably 35% or less, and more preferably 30% or less. Furthermore, the proportion of regions with an area ratio of 60% or more is preferably 15% or more, and more preferably 20% or more. Furthermore, the proportion of regions with an area ratio of 60% or more is preferably 35% or less, and more preferably 30% or less. Furthermore, the proportion of regions with an area ratio of more than 10% and less than 60% is preferably 35% or more, and more preferably 40% or more. Furthermore, the proportion of regions with an area ratio of more than 10% and less than 60% is preferably 60% or less, and more preferably 50% or less.

[0065] Furthermore, in order to observe the cross-section obtained by cutting the colored film layer 20 in the thickness direction (z-axis direction in the figure), the cross-section is processed using a focused ion beam (FIB), and attention is focused on an arbitrary position in the thickness direction (z-axis direction) of the colored film layer 20 (for example, position z1 in Figure 4), as schematically shown in Figure 4. At this time, as shown in Figure 4, a range with a length of 1 μm along the thickness direction centered on position z1 (range from z1-0.5 μm to z1+0.5 μm) is set, and virtual grid lines are set at 1 μm intervals in a direction perpendicular to the thickness direction (can be considered as the y-axis direction or horizontal direction). Then, the area ratio of the colored pigment is measured in 50 regions of a size of 1 μm × 1 μm defined by the above-mentioned virtual grid lines within the range with a length of 1 μm.

[0066] In such cases, it is preferable that the colored film layer 20 according to this embodiment has at least five regions where the area ratio of the colored pigment 203 is 0% or more and 10% or less, and at least five regions where the area ratio is 60% or more and 100% or less. Furthermore, in such observation, it is preferable that there are at least ten regions where the area ratio is greater than 10% and less than 60%. By having the colored pigment 203 present throughout the entire colored film layer 20 in a manner that satisfies the above conditions, the colored film layer 20 according to this embodiment exhibits better processing adhesion while ensuring discoloration concealment.

[0067] Furthermore, there are no specific limits on the number of areas with an area ratio of 0% or more and 10% or less, the number of areas with an area ratio of 60% or more and 100% or less, and the number of areas with an area ratio of more than 10% but less than 60%. However, in practice, the upper limits for such areas are approximately 30, 30, and 40, respectively.

[0068] Furthermore, it is more preferable that the number of regions with an area ratio of 0% or more and 10% or less is 10 or more, the number of regions with an area ratio of 60% or more and 100% or less is 10 or more, and the number of regions with an area ratio of more than 10% and less than 60% is 15 or more. By having these regions within the above ranges, the colored film layer 20 according to this embodiment will exhibit better processing adhesion while ensuring discoloration concealment.

[0069] Figures 3 and 4 illustrate an example where the colored film layer 20 does not contain resin particles 205, and the above explanation has been given with reference to these figures. However, even when the colored film layer 20 contains resin particles 205, it can be handled in the same manner as described above. In this case, a person skilled in the art can clearly distinguish between the colored pigment 203 and the resin particles 205 in the cross-sectional observation described above.

[0070] Furthermore, the colored film layer 20 according to this embodiment may, as necessary, contain rust-preventive pigments, surface-modified metal powders or glass powders, dispersants, leveling agents, waxes, aggregates, and other additives, as well as diluting solvents, within a range that does not impair the effects described above.

[0071] In this case, when a rust-preventive pigment is included, it is preferable that its content occupies 1 to 15% of the film area when, for example, a cross-section of the colored film layer 20 is observed. Furthermore, various known rust-preventive pigments can be used. In addition, when the colored film layer 20 according to this embodiment contains the rust-preventive pigment as described above and other additives, the sum of the area ratio of the film-forming component 201, the area ratio of the colored pigment 203, the area ratio of the resin particles 205, and the area ratio of other additives equals the total area of ​​the colored film layer 20 (100%).

[0072] Furthermore, in the painted steel sheet 1 according to this embodiment, a chemical conversion treatment layer (not shown) using various chemical conversion treatment agents may be provided to further improve the adhesion between the Zn-based plated steel sheet 10 and the colored film layer 20.

[0073] The painted steel sheet 1 according to this embodiment has been described in detail above with reference to Figures 1A to 4.

[0074] (Regarding the manufacturing method of painted steel sheets) Below, an example of a manufacturing method of painted steel sheets according to this embodiment will be briefly described. First, the base steel sheet is subjected to various pretreatments, including alkaline degreasing, water washing, and pickling, to obtain a clean steel sheet surface. Then, a Zn-based plating layer is formed on the surface of the base steel sheet. In this plating process, a plating bath having components capable of achieving the desired plating composition is prepared, and the Zn-based plating layer is formed by various known methods.

[0075] A colored film layer 20 is formed by applying a coating solution for forming a colored film layer to the surface of the Zn-based plating layer formed as described above, and then heating and drying it. The coating solution can be applied using generally known coating methods, such as roll coating, curtain flow coating, air spray, airless spray, immersion, bar coating, or brush application.

[0076] Furthermore, the plate temperature when applying the coating solution should be between 10°C and 50°C. If the plate temperature is below 10°C, the amount of heat input during the heat drying process described later may be insufficient, and if the plate temperature is above 50°C, the diffusion of the colored pigment 203 during the heat drying process described later may be excessively promoted. The plate temperature when applying the coating solution is preferably 15°C or higher. Also, the plate temperature when applying the coating solution is preferably 45°C or lower.

[0077] In this embodiment, the painted steel sheet 1 is configured to intentionally create variations in the distribution of the colored pigment 203. This is achieved by setting the heating and drying time (so-called baking time) to 20 seconds or less, and by performing cooling within 10 seconds of reaching the peak metal temperature (PMT). This prevents the colored pigment 203 from diffusing uniformly within the colored film layer 20, resulting in the aforementioned distribution of the colored pigment 203. The baking time is preferably 15 seconds or less, and more preferably 10 seconds or less. The time from reaching the peak metal temperature to performing cooling should be as short as possible. While there is no specific lower limit, it is practically around 2 seconds.

[0078] By going through the manufacturing process described above, a painted steel sheet 1 can be obtained in which the distribution of the colored pigment 203 as described earlier is achieved. The manufacturing method of the surface-treated steel sheet according to this embodiment has been briefly described above.

[0079] The painted steel sheet according to this embodiment will be described in detail below with reference to examples and comparative examples. Note that the following examples are merely examples of painted steel sheets according to this embodiment, and the painted steel sheet according to this embodiment is not limited to the examples below.

[0080] (1) Metal plates Four types of metal plates, A1 to A4, as shown in Table 1 below, were prepared. Here, for the various plated steel plates shown A1 to A3, the amount of plating layer attached to one side was 70 g / m². ２ Furthermore, these metal plates are treated with a chromate-free chemical conversion treatment (CT-E300 / manufactured by Nippon Parkerizing Co., Ltd.) at a rate of 60 mg / m². ２ The treated materials were also prepared. The treatment solution used for the chemical conversion treatment contains a silane coupling agent as a component, and the film layer formed by this chemical conversion treatment functions as a chemical conversion treatment film layer. The presence or absence of chemical conversion treatment on the metal plates is summarized in Table 3 below.

[0081]

[0082] (2) Preparation of colored paints Colored paints were prepared for use in forming a colored film layer. The resins shown in Table 2 below were prepared as binder resins. To each resin solution, a melamine-based curing agent was added at a solid content ratio of 15% by mass. Furthermore, the resin particles shown in Table 3 below were prepared and added in the predetermined particle size and amount shown in Table 5 below. In addition, titanium dioxide and a mixture of titanium dioxide and carbon black shown in Table 4 below were prepared as coloring pigments and added in the predetermined amounts shown in Tables 5-1 to 5-3 below to prepare the colored paints.

[0083]

[0084]

[0085]

[0086] (3) Sample preparation The colored paint prepared as described above was applied to a metal plate using a bar coater so that the dry film thickness was as shown in Tables 5-1 to 5-3 below. The plate was then heated for the baking time shown in Tables 5-1 to 5-3 so that the maximum plate temperature (PMT) reached was 200°C to form a colored film layer.

[0087]

[0088]

[0089]

[0090] (4) Evaluation of Samples Each sample prepared using the above method was evaluated for performance based on the following criteria. The evaluation results obtained are shown in Tables 6-1 to 6-3 below.

[0091] <L ＊ Value > As a reference index for the whiteness of the colored film, L ＊ The values ​​were measured and evaluated based on the following criteria: [Evaluation Criteria] Score 3: L ＊ Value 85 or higher 2: L ＊ Value between 75 and 85: 1: L ＊ The value is between 50 and 75.

[0092] <Blackening Resistance> The blackening resistance of painted steel sheets was evaluated based on the following criteria. Samples were left standing for 6 days in an atmosphere of 70°C and 80% RH, and the color difference △E before and after the test was measured. ＊ The following was measured. The following evaluation criteria were used, and a score of 2 or higher was considered a pass. [Evaluation Criteria] Score 5: △E ＊ is less than 1. 4: △E ＊ The value is 1 or greater but less than 3. 3: △E ＊ 3 or more and less than 4. 2: △E ＊ The value is 4 or greater but less than 5. 1: △E ＊ 5 or more.

[0093] <Processing Adhesion> The processing adhesion of the painted steel sheet was evaluated based on the following criteria. The prepared sample was bent 90° in a 20°C atmosphere with an inner radius of 1 mm, and then a tape peel test was performed on the outside of the bent portion. The appearance of the peeled portion was evaluated according to the following evaluation criteria, and a score of 2 or higher was considered a pass. [Evaluation Criteria] Score 5: Almost no paint peeling is observed (peeling area less than 1% from the bent portion). 4: Very slight paint peeling is observed (peeling area 1% or more but less than 3% from the bent portion). 3: Slight paint peeling is observed (peeling area 3% or more but less than 5% from the bent portion). 2: Partial paint peeling is observed (peeling area 5% or more but less than 20% from the bent portion). 1: Paint peeling is observed overall (peeling area 20% or more from the bent portion).

[0094] <Corrosion Resistance> The corrosion resistance of the painted steel sheet was evaluated based on the following criteria. After sealing the edges of the test sheet with tape, a salt spray test (SST) in accordance with JIS Z 2371:2015 was performed for 72 hours, and the rust formation was observed. The sheet was evaluated according to the following evaluation criteria, with a score of 2 or higher being considered a pass. [Evaluation Criteria] 4: Rust formation area is less than 1%. 3: Rust formation area is 1% or more and less than 3%. 2: Rust formation area is 3% or more and less than 5%. 1: Rust formation area is 5% or more.

[0095] <Scratch Resistance> As a reference performance, the scratch resistance of the painted steel sheet was evaluated by the following coin scratch test. A coin was placed in contact with the prepared sample at a 45-degree angle and scratched with a load of 500g. The scratches made at each load were evaluated according to the following criteria. [Evaluation Criteria] Score 5: Almost no paint peeling or change in gloss is observed (less than 2% paint peeling at the coin contact area). 4: Almost no paint peeling is observed, but a slight change in gloss is observed (less than 2% paint peeling at the coin contact area). 3: Slight paint peeling is observed (2% or more but less than 5% paint peeling at the coin contact area). 2: Partial paint peeling is observed (5% or more but less than 20% paint peeling at the coin contact area). 1: Paint peeling is observed overall (20% or more paint peeling at the coin contact area).

[0096]

[0097]

[0098]

[0099] As is clear from Tables 6-1 to 6-3 above, the samples corresponding to the embodiments of the present invention showed excellent resistance to blackening, work adhesion, and corrosion resistance, as well as excellent scratch resistance. On the other hand, the samples corresponding to the comparative examples of the present invention were inferior in any one of the evaluations of resistance to blackening, work adhesion, or corrosion resistance, and did not achieve a balance between resistance to blackening, work adhesion, and corrosion resistance.

[0100] Although preferred embodiments of the present invention have been described in detail above with reference to the attached drawings, the present invention is not limited to these examples. It is clear to any person with ordinary skill in the art to which the present invention belongs that various modifications or alterations can be conceived within the scope of the technical idea described in the claims, and these are also understood to fall within the technical scope of the present invention.

[0101] The embodiments disclosed herein are illustrative and not restrictive in all respects. The embodiments described above may be omitted, replaced, or modified in various ways without departing from the appended claims, the technical scope of the invention as described later, and the spirit thereof. For example, the constituent elements of the embodiments described above can be combined in any way without impairing their effects. Furthermore, such any combination will naturally yield the effects and benefits of each constituent element in the combination, as well as other effects and benefits that will be obvious to those skilled in the art from the description herein.

[0102] Furthermore, the effects described herein are merely descriptive or illustrative, and not limiting. In other words, the technology according to the present invention may produce other effects that will be apparent to those skilled in the art from the description herein, in addition to or instead of the effects described above.

[0103] Furthermore, the following configuration also falls within the technical scope of the present invention: (1) A Zn-plated steel sheet having a Zn-based plating layer containing at least Zn on at least one surface of a steel sheet that serves as a base material, and a colored film layer located on the Zn-based plating layer and containing at least a resin component and a colored pigment, wherein the thickness of the colored film layer is 3.0 to 10.0 μm, and the colored pigment in the colored film layer occupies 10 to 50% of the film area in the cross-section in the thickness direction of the colored film layer. Painted steel sheet, wherein, in the cross-section of the colored film layer in the thickness direction, virtual grid lines are set at 1 μm intervals in both the thickness direction and the direction perpendicular to the thickness direction, and the area ratio of the colored pigment in the 1 μm × 1 μm area partitioned by the virtual grid lines is measured for 500 areas, and the proportion of areas with an area ratio of 0% or more and 10% or less is within the range of 10 to 40% of the total measured areas, and the proportion of areas with an area ratio of 60% or more and 100% or less is within the range of 10 to 40% of the total measured areas. (2) The painted steel sheet according to (1), wherein the proportion of the area ratio of the region in the cross-section that is greater than 10% but less than 60% is within the range of 30 to 70% of the total area measured, and the sum of the proportion of the area ratio of the region that is 0% or more but less than 10%, the proportion of the area ratio of the region that is 60% or more but less than 100%, and the proportion of the area ratio of the region that is greater than 10% but less than 60% is 100%. (3) The painted steel sheet according to (1) or (2), wherein, with respect to the cross section, a range is set at an arbitrary position in the thickness direction of the colored film layer, with the length along the thickness direction being 1 μm centered on that arbitrary position, and virtual grid lines are set at 1 μm intervals in a direction perpendicular to the thickness direction, and in a region of size 1 μm × 1 μm defined by the range with a length of 1 μm being demarcated by the virtual grid lines, the area ratio of the colored pigment is measured in 50 regions, and in the measurement results, there are at least 5 regions where the area ratio is 0% or more and 10% or less, and there are at least 5 regions where the area ratio is 60% or more and 100% or less.(4) The painted steel sheet according to (3), wherein in the cross-section, there are at least 10 areas in which the area ratio is greater than 10% and less than 60%. (5) The painted steel sheet according to any one of (1) to (4), wherein the colored film layer further contains resin particles. (6) The painted steel sheet according to (5), wherein the average particle size of the resin particles is 3 to 10 μm, and the resin particles in the colored film layer occupy 3 to 25% of the film area in the cross-section of the colored film layer. (7) The colored film layer is L as defined in JIS Z 8781-4:2013. ＊ (1) to (2) The painted steel sheet according to any one of (1) to (2) wherein the value is 50 or more. (8) The painted steel sheet according to any one of (1) to (2) wherein the coloring pigment is a white pigment of at least one of titanium oxide, zinc oxide, barium sulfate, or zinc sulfide. (9) The painted steel sheet according to any one of (1) to (2) wherein the thickness of the Zn-plated steel sheet is 0.2 to 5.0 mm. (10) The painted steel sheet according to any one of (1) to (2) wherein the Zn-plated layer is a plating layer containing, by mass%, Al: 4 to 22%, Mg: 1 to 10%, Si: 0.0001 to 2.0000%, with the remainder being Zn and impurities.

[0104] 1 Painted steel sheet 10 Zn-plated steel sheet 11 Base steel sheet 13 Zn-plated layer 20 Colored film layer 201 Film-forming component 203 Coloring pigment 205 Resin particles

Claims

1. A Zn-plated steel sheet having a Zn-based plating layer containing at least Zn on at least one surface of a base steel sheet, and a colored film layer located on the Zn-based plating layer and containing at least a resin component and a colored pigment, wherein the thickness of the colored film layer is 3.0 to 10.0 μm, the colored pigment in the colored film layer occupies 10 to 50% of the film area in the cross-section in the thickness direction of the colored film layer, and in the cross-section in the thickness direction of the colored film layer, virtual grid lines are set at 1 μm intervals in both the thickness direction and the direction perpendicular to the thickness direction, and the area ratio of the colored pigment in a region of size 1 μm × 1 μm partitioned by the virtual grid lines is measured for 500 regions, and the measurement results are as follows: A painted steel sheet in which the proportion of the area ratio of the region having an area ratio of 0% or more and 10% or less is within the range of 10 to 40% of the total measured area, and the proportion of the area ratio of the region having an area ratio of 60% or more and 100% or less is within the range of 10 to 40% of the total measured area.

2. The painted steel sheet according to claim 1, wherein, in the cross-section, the proportion of the area having a ratio of more than 10% but less than 60% is within the range of 30 to 70% of the total area measured, and the sum of the proportion of the area having a ratio of 0% or more but less than 10%, the proportion of the area having a ratio of 60% or more but less than 100%, and the proportion of the area having a ratio of more than 10% but less than 60% is 100%.

3. With respect to the cross section, a range is set at an arbitrary position in the thickness direction of the colored film layer, with the length along the thickness direction being 1 μm centered on that arbitrary position, and virtual grid lines are set at 1 μm intervals in a direction perpendicular to the thickness direction, and in a region of size 1 μm × 1 μm defined by the range with a length of 1 μm being demarcated by the virtual grid lines, the measurement results of measuring the area ratio of the colored pigment in 50 regions show that at least 5 regions have an area ratio of 0% or more and 10% or less, and at least 5 regions have an area ratio of 60% or more and 100% or less, as described in claim 1 or 2.

4. The painted steel sheet according to claim 3, wherein in the cross-section, there are at least 10 or more regions where the area ratio is greater than 10% and less than 60%.

5. The painted steel sheet according to claim 1 or 2, wherein the colored film layer further contains resin particles.

6. The painted steel sheet according to claim 5, wherein the average particle size of the resin particles is 3 to 10 μm, and the resin particles in the colored film layer occupy 3 to 25% of the film area in the cross-section of the colored film layer.

7. The colored film layer is L as defined in JIS Z 8781-4:2013. ＊ A painted steel sheet according to claim 1 or 2, wherein the value is 50 or more.

8. The painted steel sheet according to claim 1 or 2, wherein the coloring pigment is a white pigment of at least one of titanium dioxide, zinc oxide, barium sulfate, or zinc sulfide.

9. The painted steel sheet according to claim 1 or 2, wherein the thickness of the Zn-plated steel sheet is 0.2 to 5.0 mm.

10. The coated steel sheet according to claim 1 or 2, wherein the Zn-based plating layer contains, by mass%, Al: 4-22%, Mg: 1-10%, Si: 0.0001-2.0000%, with the remainder being Zn and impurities.

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