Copper-plated steel sheet

Abstract

A copper-plated steel sheet 10 comprises: a base steel plate 20; and a dendritic copper plating film 30 formed on at least one surface of the base steel plate 20. When the copper-plated steel sheet 10 is immersed in 0.5 mass% dilute sulfuric acid at 25°C for 5 seconds, and is subsequently washed with water and dried, the lightness L* of the copper plating film 30 measured within one hour after drying is 50-71.

Description

[Technical Field]

[0001] The present invention relates to a copper-plated steel sheet. [Background technology]

[0002] Copper-plated steel sheets, which have a copper-plated layer formed on a base steel sheet, have good brazing and soldering properties and are therefore widely used in a variety of products, such as double-wrapped pipes (automobile brake pipes, oil supply pipes, etc.), welded pipes (earth rods, etc.), oil coolers, etc. Copper-plated steel sheets are also used as the base material for sliding parts, such as cylindrical wound bushings (slide bearings) used in transmissions, etc. For example, Patent Document 1 proposes a sliding member (copper-plated steel sheet) that includes a sliding substrate such as a steel sheet, a pure copper plating layer formed on the surface of the sliding substrate, and a composite copper plating layer formed on the pure copper plating layer, which contains a plurality of copper and graphite-containing agglomerates and has a Vickers hardness of 120 or less. However, this sliding member is costly to form the composite copper plating layer, and may not be able to maintain its sliding properties (the length of time the coefficient of friction is maintained) sufficiently when a large load is applied.

[0003] Furthermore, a sliding resin film formed from a lubricating resin or the like is known as a film with excellent sliding properties. For example, Patent Document 2 proposes a sliding resin film (coated object) in which a sliding property-improving paint containing, as main components, 95 to 50 wt % of a matrix resin whose film-forming temperature is lower than the melting point of polytetrafluoroethylene resin and 5 to 50 wt % of a polytetrafluoroethylene resin having an average particle size of 2 to 40 μm is applied to the surface of a substrate such as a metal plate and baked. However, such a sliding resin film has poor adhesion to substrates such as metal plates and is easily peeled off from the substrate in a sliding environment, so it may not be able to maintain sliding properties sufficiently.

[0004] On the other hand, steel sheets that have been painted to improve durability, functionality, and design are widely used in a variety of applications, including building materials and home appliances. When painting bare steel sheets, phosphate coating is commonly performed as a base treatment to improve the adhesion and corrosion resistance of the paint film. However, the phosphate coating formed by this treatment may not have sufficient heat resistance, making it unsuitable as a base for painting in some applications, and the coating may peel off from the base steel sheet under high-temperature conditions. Copper plating films have excellent heat resistance and may be used as a base film instead of phosphate coatings, but typical copper plating films have poor adhesion to paint.

[0005] Furthermore, as a technique for reducing the unevenness of the surface of a copper plating film, Patent Document 3 describes a method in which, when a copper plating film is formed on an insulating base material having a metal film for power supply formed on its surface while gradually increasing the current density Dk, the stirring speed V of the plating solution is increased in accordance with the increase in current density Dk. Also, Non-Patent Document 1 discloses a technique for forming a dendritic copper plating film on a Zn-Al substrate. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 2018-197387 [Patent Document 2] Patent No. 4339960 [Patent Document 3] Japanese Patent Application Laid-Open No. 2013-95968 [Non-patent literature]

[0007] [Non-Patent Document 1] H Tanabe and 7 others, “Cu Dendrite Crystal Ball Formation on a Zn-Al Substrate by Electroplating Method”, e-Journal of Surface Science and Nanotechnology, Vol. 20, No. 4, pp. 232-236, 2022 Summary of the Invention [Problem to be solved by the invention]

[0008] An object of the present invention is to provide a copper-plated steel sheet in which a coating film is formed on a copper-plated film and the coating film is not easily peeled off. [Means for solving the problem]

[0009] As a result of extensive research into copper-plated steel sheets, the inventors have found that the lightness L * The present inventors have found that a dendritic copper plating film having a specific range of dendritic structure exhibits excellent adhesion to a coating film when the coating film is formed on the copper plating film, and have completed the present invention.

[0010] That is, the present invention provides a copper-plated steel sheet comprising a base steel sheet and a dendritic copper plating film formed on at least one surface of the base steel sheet, The copper-plated steel sheet was immersed in 0.5% by mass of dilute sulfuric acid at 25°C for 5 seconds, then rinsed with water and dried, and the lightness L of the copper plating film was measured within 1 hour. * It is a copper-plated steel sheet having a viscosity of 50 to 71. [Effects of the Invention]

[0011] According to the present invention, it is possible to provide a copper-plated steel sheet in which, when a coating film is formed on a copper-plated film, the coating film is not easily peeled off. [Brief explanation of the drawings]

[0012] [Figure 1]1 is a schematic cross-sectional view of a copper-plated steel sheet according to an embodiment of the present invention. FIG. [Figure 2] FIG. 1 is a schematic cross-sectional view of a copper-plated steel sheet according to an embodiment of the present invention, further comprising a coating film. [Figure 3] This is an example of a three-dimensional image of a protrusion on a copper-plated steel sheet measured by X-ray CT. [Figure 4] FIG. 2 is a schematic diagram illustrating the process of forming a copper plating film. [Figure 5] FIG. 10 is a schematic diagram for explaining pin-on-disk evaluation. DETAILED DESCRIPTION OF THE INVENTION

[0013] The following is a detailed description of the embodiments of the present invention. The present invention is not limited to the following embodiments, and it should be understood that modifications and improvements made to the following embodiments based on the ordinary knowledge of those skilled in the art without departing from the spirit of the present invention are also within the scope of the present invention. In this specification, the "%" designation for components means "% by mass" unless otherwise specified.

[0014] FIG. 1 is a schematic cross-sectional view of a copper-plated steel sheet according to an embodiment of the present invention. As shown in Fig. 1, a copper-plated steel sheet 10 according to an embodiment of the present invention comprises a base steel sheet 20 and a copper plating film 30 formed on one surface of the base steel sheet 20. Note that Fig. 1 shows an example in which the copper plating film 30 is formed on one surface of the base steel sheet 20, but the copper plating film 30 may be formed on both surfaces of the base steel sheet 20.

[0015] The copper-plated steel sheet 10 according to an embodiment of the present invention may further comprise a coating film. A schematic cross-sectional view of a copper-plated steel sheet according to an embodiment of the present invention further comprising a coating film is shown in FIG. As shown in FIG. 2, the coating film 40 is provided on the copper plating film 30 .

[0016] Here, in this specification, the term "steel plate" refers to a steel material in the form of a plate (including a strip). There are no particular limitations on the base steel sheet 20, and various steel sheets such as hot-rolled steel sheets and cold-rolled steel sheets can be used. Among these, cold-rolled steel sheets are preferably used as the base steel sheet 20. The composition of the base steel sheet 20 is not particularly limited either, and may be selected appropriately depending on the application.

[0017] The copper plating film 30 is a film that has good adhesion to the base steel sheet 20 and the coating film 40. Therefore, by providing the copper plating film 30, the coating film 40 can be made less likely to peel off from the copper-plated steel sheet 10. The copper plating film 30 is dendritic. In this specification, the term "dendritic copper plating film 30" refers to a copper plating film 30 that has dendritic protrusions 31 when viewed in a cross section parallel to the thickness direction, as shown in Fig. 1. If the copper plating film 30 is dendritic, when a coating film 40 is provided on the copper plating film 30, an anchor effect occurs between the copper plating film 30 and the coating film 40, making the coating film 40 less likely to peel off. Whether or not the copper plating film 30 is dendritic can be determined by observing a cross section of the copper-plated steel sheet 10 parallel to the thickness direction using a microscope (for example, SEM).

[0018] The copper plating film 30 was measured within one hour after the copper-plated steel sheet 10 was immersed in 0.5 mass % diluted sulfuric acid at 25°C for 5 seconds, rinsed with water, and dried. * The lightness L is 50 to 71, preferably 52 to 65. * is related to the state of formation of dendritic protrusions 31 in the copper plating film 30. * If this is the case, it can be considered that the dendritic protrusions 31 are sufficiently formed in the copper plating film 30, and therefore the anchoring effect can be enhanced. Here, the lightness L * L * a * b *It is the brightness in the color space (CIE 1976) and can be measured in accordance with JIS K5600-4-4: 1999. After rinsing, the sample is dried using a dryer with hot air at 40°C for 3 minutes. In addition, when the coating film 40 is provided on the copper plating film 30, the lightness L * is measured under the above conditions after removing the coating film 40. The method for removing the coating film 40 is not particularly limited, and the coating film 40 may be dissolved and removed using a solvent or the like that can remove the coating film 40. This also applies when a discoloration prevention treatment layer, which will be described below, is provided on the copper plating film 30.

[0019] When viewed in a cross section parallel to the thickness direction, the copper plating film 30 preferably continuously covers the base steel sheet 20 to a thickness T of 1.5 μm or more. The thickness T of the copper plating film 30 refers to the minimum thickness of the copper plating film 30 that covers the base steel sheet 20. Here, in this specification, "the copper plating film 30 continuously covers the base steel sheet 20" means that there is no part of the base steel sheet 20 where the copper plating film 30 is not formed and is exposed.

[0020] By continuously coating the base steel sheet 20 with the copper plating film 30 having the above-described thickness T, it is easy to obtain a dendritic copper plating film 30. Therefore, when a coating film 40 is provided on the copper plating film 30, the effect of suppressing peeling of the coating film 40 can be reliably ensured. The thickness T of the copper plating film 30 is preferably 1.7 μm or more. Note that there is no particular upper limit to the thickness T of the copper plating film 30, but in consideration of production costs and the like, it is generally 10.0 μm or less, preferably 8.0 μm or less. The thickness T of the copper plating film 30 can be measured by observing a cross section of the copper-plated steel sheet 10 parallel to the thickness direction using a microscope (for example, SEM).

[0021] The dendritic protrusions 31 formed on the portion of the base steel sheet 20 continuously coated with a thickness T of 1.5 μm or more (hereinafter referred to as the "base coating portion") create an anchoring effect between the copper plating film 30 and the coating film 40 when the coating film 40 is formed on the copper plating film 30, thereby preventing the coating film 40 from peeling even under a sliding environment with a heavy load. Therefore, from the viewpoint of ensuring this effect, it is preferable that the volume fraction of the protrusions 31 in the region up to a height of 10 μm from the base coating portion is 0.10% or more. By controlling the volume fraction of the protrusions 31 within this range, it is possible to ensure that there are enough protrusions 31 to obtain the anchoring effect. From the viewpoint of stably ensuring this effect, the volume fraction of the protrusions 31 is more preferably 0.20% or more, and even more preferably 0.30% or more. The volume fraction of the protrusions 31 is not particularly limited, as the anchoring effect is more easily obtained as the volume fraction increases, but it is generally 50.0% or less.

[0022] The volume ratio of the protrusions 31 in the region up to 10 μm in height from the base coating portion can be measured by X-ray CT. X-ray CT irradiates the object to be measured (copper-plated steel sheet 10) while rotating it to obtain a transmission image, and then performs a reconstruction process on the obtained image, thereby making it possible to observe the internal structure of the copper-plated steel sheet 10 in three dimensions. Here, an example of a three-dimensional image of the protrusions 31 on the copper-plated steel sheet 10 measured by X-ray CT is shown in FIG. The base steel sheet 20 and the base film portion (continuously coated portion) of the copper-plated film 30 are uniformly flat, and when the measurement object (copper-plated steel sheet 10) is rotated, the X-ray absorption contrast changes uniformly with the change in thickness. Therefore, the base steel sheet 20 and the base film portion of the copper-plated film 30 have the same X-ray absorption contrast and are indistinguishable. On the other hand, the region where the protrusions 31 are formed has an uneven surface, and when the measurement object (copper-plated steel sheet 10) is rotated, the X-ray absorption contrast changes in the region with uneven surface depending on the shape. Therefore, by setting a threshold value for the X-ray absorption contrast from the three-dimensional image of the entire copper-plated steel sheet 10 obtained by X-ray CT, it is possible to extract only the three-dimensional image of the protrusions 31. That is, the brightness values ​​of the X-ray CT differ between the protrusion 31 and the base film portion of the base steel sheet 20 and copper plating film 30 due to differences in X-ray absorption contrast, so as shown in Figure 3, it is possible to easily extract only the three-dimensional image of the protrusion 31. The volume fraction of the protrusions 31 in the region up to 10 μm in height from the base coating can be calculated from the three-dimensional image (Fig. 3) obtained by X-ray CT measurement. In Fig. 3, only the region where the protrusions 31 are formed is extracted, so the starting point (the surface of the base coating) of the height of the region where the volume fraction of the protrusions 31 is measured is the lowest point. Even when the coating film 40 is present, a three-dimensional image of only the protrusion 31 can be extracted by performing X-ray CT measurement in the same manner as above.

[0023] The copper coating weight of the copper plating film 30 is not particularly limited, but is preferably 10 to 150 g / m 2 , more preferably 13 to 120 g / m 2 , and more preferably 15 to 60 g / m 2 By controlling the copper deposition amount within this range, it is possible to stably obtain a copper plating film 30 that has good adhesion to the base steel sheet 20 and the coating film 40.

[0024] A discoloration prevention treatment layer may be formed on the copper plating film 30 to prevent discoloration of the copper plating film 30. Providing the discoloration prevention treatment layer is effective in preventing discoloration of the copper plating film 30 over time, and is particularly effective in preventing discoloration of the copper plating film 30 for several days after its formation. The anti-tarnish treatment layer is not particularly limited as long as it does not impair the adhesion of the coating film 40 that will be formed subsequently. For example, the anti-tarnish treatment layer can be formed by immersing the base steel sheet 20 in an aqueous solution / dispersion of a commercially available anti-tarnish agent and then drying it. Examples of commercially available anti-tarnish agents include VERZONE See-U Guard D manufactured by Daiwa Kasei Co., Ltd., benzotriazole group-containing silane coupling agents manufactured by Shin-Etsu Chemical Co., Ltd., Gospel (C-30, C-70, or C-220) manufactured by Gospel Chemical Co., Ltd., Top Rinse CU-5 manufactured by Okuno Chemical Industries Co., Ltd., KPC-2003 manufactured by Metal Chemical Engineering Research Institute Co., Ltd., CU-5600 manufactured by Meltex Inc., and BTZ-M manufactured by Kyodo Pharmaceutical Co., Ltd. The discoloration prevention treatment layer may be formed on the entire surface of the copper plating film 30, or may be formed on a portion of the surface.

[0025] The coating film 40 is not particularly limited, and may be a coating film 40 that provides the required properties depending on the application. For example, when the copper-plated steel sheet 10 is used as a building material or home appliance, it can be coated with a coating film 40 that provides durability and a predetermined design. Such a coating film 40 can be formed using a commercially available paint. Furthermore, when the copper-plated steel sheet 10 is used as a sliding member, it can be coated with a coating film 40 that provides sliding properties. An example of such a coating film 40 is a sliding resin film formed from a resin that has sliding properties. Here, the term "sliding resin film" as used herein refers to a resin film that has a dynamic friction coefficient of 0.2 or less when a test piece is prepared by taking a flat portion from a plated steel sheet on which a target resin film has been formed and conducting a friction and wear test using a surface property tester (HEIDON-TYPE14) manufactured by Shinto Scientific Co., Ltd. under the following conditions: a load of 1 N, a 10 mm diameter SUS ball as the counter material, a sliding (moving) speed of 150 mm / min, and a test piece temperature of 20 to 30°C. The sliding resin film is not particularly limited, and any known film in the art can be used. For example, a resin film containing a lubricant or a self-lubricating resin film can be used as the sliding resin film. Among these, a film in which polytetrafluoroethylene (PTFE) resin is dispersed in a matrix resin is preferred. The matrix resin is not particularly limited, but examples include polyester resin, linear polymer polyester resin, acrylic resin, epoxy resin, polyurethane resin, phenoxy resin, phenolic resin, polyvinylidene fluoride (PVdF) / acrylic resin, and vinyl chloride resin.

[0026] The thickness of the coating film 40 is not particularly limited, but is preferably 3 to 40 μm. By controlling the thickness of the coating film 40 to fall within this range, peeling of the coating film 40 can be stably suppressed.

[0027] The aforementioned anti-tarnish treatment layer may be formed between the copper plating film 30 and the coating film 40 .

[0028] The method for producing the copper-plated steel sheet 10 according to the embodiment of the present invention is not particularly limited as long as it is a method capable of producing the copper-plated steel sheet 10 having the above-described characteristics, and can be carried out in accordance with a known method. An example of the method for producing the copper-plated steel sheet 10 according to the embodiment of the present invention will be described below.

[0029] The method for producing a copper-plated steel sheet 10 according to an embodiment of the present invention includes an electroplating step in which a copper plating film 30 is formed on at least one surface of a base steel sheet 20 by electroplating. In the electroplating step, copper pyrophosphate plating is preferably performed followed by copper sulfate plating.

[0030] Copper pyrophosphate plating is performed by immersing the base steel sheet 20 in a copper pyrophosphate plating solution. The conditions for copper pyrophosphate plating are not particularly limited and can be appropriately set in accordance with known methods. Typical conditions are as follows: Copper pyrophosphate plating solution composition: Copper pyrophosphate 50g / L, potassium pyrophosphate 250g / L, oxalic acid 10g / L Copper pyrophosphate plating solution pH: 9.2 Copper pyrophosphate plating solution temperature: 20 to 60°C Current density: 1~10A / dm 2 Time: 5 to 100 seconds The copper pyrophosphate plating may be carried out once or multiple times under different conditions.

[0031] The copper sulfate plating is preferably carried out by immersing the base steel sheet 20 in a copper sulfate plating solution and changing the flow rate of the copper sulfate plating solution. More specifically, it is more preferable to alternately and repeatedly perform copper sulfate plating at a low flow rate (e.g., a flow rate of 0.30 m / sec or less) and copper sulfate plating at a high flow rate (e.g., a flow rate of 0.50 m / sec or more). Here, a schematic diagram for explaining the process of forming the copper plating film 30 is shown in FIG. When copper sulfate plating is performed at a low flow rate, the plated surface is likely to be supplied with insufficient copper ions, and the deposited copper is likely to be destroyed by hydrogen bubbles generated during plating. As a result, particulate copper adheres to the surface (State A). On the other hand, when copper sulfate plating is performed at a high flow rate, copper ions are likely to be sufficiently supplied to the plated surface. As a result, the plating grows along the surface, and a copper plating film 30 is formed that covers the entire surface with the particulate copper attached (State B). Next, when copper sulfate plating is performed again at a low flow rate, particulate copper adheres as in State A, and the copper grows as dendritic crystals (dendrites) (State C). Next, when copper sulfate plating is performed again at a high flow rate, a copper plating film 30 is formed that covers the entire surface, reinforcing the dendritic copper and increasing the thickness of the copper plating film 30 (State D). By alternately performing copper sulfate plating at a low flow rate and copper sulfate plating at a high flow rate in this manner, a copper plating film 30 having the protrusions 31 described above can be formed.

[0032] The number of times copper sulfate plating at a low flow rate and copper sulfate plating at a high flow rate are repeated and the current application time per cycle may be adjusted appropriately depending on the composition of the copper sulfate plating solution used, the current density, the plating amount to be obtained, etc. Typically, when the current density is 20 to 60 A / dm 2 The current application time per cycle (current application time for plating at a low flow rate and for plating at a high flow rate) is 1 to 30 seconds, and the number of repetitions is 5 to 50. Copper sulfate plating can be carried out using an electroplating device (such as a flow cell) that is capable of causing the plating solution to flow.

[0033] The conditions for copper sulfate plating are not particularly limited and can be appropriately set in accordance with known methods, but typical conditions are as follows. Copper sulfate plating solution composition: copper sulfate 220g / L, sulfuric acid 45g / L Copper sulfate plating solution pH: 1.0 Copper sulfate plating solution temperature: 20 to 50°C

[0034] The method for producing the copper-plated steel sheet 10 according to the embodiment of the present invention may further include a coating film forming step of forming a coating film 40 on the copper plating film 30 . The coating film forming step is not particularly limited as long as it can form the coating film 40, and can be carried out in accordance with a known method. In a typical coating film forming step, the coating film 40 can be formed by applying a paint onto the copper plating film 30 and drying it. The method for applying the paint is not particularly limited, and known methods such as dipping, bar coating, roll coating, spin coating, and spraying can be used. The paint can be applied once or multiple times so that the coating film 40 formed has a predetermined thickness. The drying temperature is not particularly limited and can be set appropriately depending on the composition of the paint, etc.

[0035] When a discoloration prevention treatment layer is provided on the copper plating film 30 or between the copper plating film 30 and the coating film 40, the discoloration prevention treatment step can be performed after the electroplating step or between the electroplating step and the coating film formation step. The discoloration prevention treatment step may be carried out by applying a solution containing a discoloration prevention agent such as benzotriazole onto the copper plating film 30 and then drying it. The method for applying the solution containing the discoloration inhibitor is not particularly limited, and the above-mentioned known methods can be used. The drying temperature is not particularly limited and may be set appropriately depending on the type of solution. [Example]

[0036] The present invention will be described in detail below with reference to examples, but the present invention should not be construed as being limited to these examples.

[0037] Examples 1 to 7 A cold-rolled steel sheet with a thickness of 0.5 mm was prepared as the base steel sheet. This cold-rolled steel sheet was subjected to electrolytic degreasing, water washing, and pickling in this order. The electrolytic degreasing was carried out by immersing the cold-rolled steel sheet as the anode in a sodium-based degreasing agent (temperature: 60°C) with a sodium concentration of 5%, and applying a current density of 2 A / dm 2 The pickling was carried out for 10 seconds. The cold-rolled steel sheet was then immersed in a 2% hydrochloric acid aqueous solution (temperature: 20°C) for 30 seconds. Next, the cold-rolled steel sheet was immersed in a copper pyrophosphate plating solution at 60°C, and the current density was set to 5 A / dm 2 Copper pyrophosphate plating was performed for 15 seconds. Next, the cold-rolled steel sheet that had been subjected to copper pyrophosphate plating was immersed in a copper sulfate plating solution at 35°C, and copper sulfate plating at a low flow rate and copper sulfate plating at a high flow rate were alternately performed under the current density and flow rate conditions of the copper sulfate plating solution shown in Table 1, for a total of 21 cycles (11 cycles at a high flow rate and 10 cycles at a low flow rate). In this way, a copper-plated steel sheet with a copper plating film formed thereon was obtained. The compositions and pH of the pyrophosphate plating solution and copper sulfate plating solution used are as follows: Copper pyrophosphate plating solution composition: Copper pyrophosphate 50g / L, potassium pyrophosphate 250g / L, oxalic acid 10g / L Copper pyrophosphate plating solution pH: 9.2 Copper sulfate plating solution composition: copper sulfate 220g / L, sulfuric acid 45g / L Copper sulfate plating solution pH: 1.0 In addition, the flow velocity in this example refers to the velocity of the plating solution flowing in the direction along the cold-rolled steel sheet between the electrodes (cold-rolled steel sheet - anode).

[0038] Next, the copper-plated steel sheet on which the copper plating film had been formed was immersed in a 2 g / L aqueous benzotriazole solution (60°C) for 3 seconds and then dried to form a discoloration-resistant treatment layer. Next, a paint (resin composition) containing 25% PTFE particles with an average particle size of 10 μm in a polyester resin (matrix resin) was applied to the copper-plated steel sheet on which the discoloration-resistant treatment layer had been formed, and then dried at 200°C to form a 20 μm-thick coating film (slidable resin film). For some copper-plated steel sheets (Example 7), the slidable resin film was formed without forming the discoloration-resistant treatment layer. Here, the average particle size refers to the particle size at 50% of the cumulative value in the particle size distribution determined by laser diffraction / scattering.

[0039] (Comparative Examples 1 to 6) Instead of alternately plating copper sulfate at a low flow rate and copper sulfate at a high flow rate 21 times in total, copper sulfate plating at either a low flow rate or a high flow rate was performed 21 times. Next, a discoloration prevention treatment layer and a coating film were formed in the same manner as in the above examples.

[0040] The copper-plated steel sheets obtained in the above examples were evaluated as follows.

[0041] <Copper coverage of copper plating film> A copper-plated steel sheet on which only a copper plating film was formed was immersed in a mixed aqueous solution of ammonia (ammonia concentration 28%) and hydrogen peroxide to dissolve the copper plating film, and the mass difference [g] before and after dissolution of the copper plating film was measured. The mass difference was calculated as the area [m 2 ] to obtain the copper coating weight [g / m 2 ] was calculated.

[0042] <Copper plating film thickness T (minimum thickness)> A copper-plated steel sheet on which only the copper plating film was formed was cut so that a cross section parallel to the thickness direction could be observed, and resin was embedded so that the cut surface would be the observation surface. Next, the resin-embedded measurement specimen was subjected to a mirror finish by wet polishing. The mirror-finished surface was observed using an SEM (SU6600 model, manufactured by Hitachi High-Tech Corporation). In the SEM image at 2,000x magnification, the minimum thickness [μm] at a length of 1,000 μm in the direction perpendicular to the thickness direction was taken as the thickness T of the copper plating film. In the cross-sectional observation for this evaluation, it was also confirmed whether the copper plating film was dendritic or not.

[0043] <Volume ratio of protrusions> A copper-plated steel sheet with only the copper plating layer formed was cut into a φ10 mm disk-shaped specimen and X-ray CT was performed using a Carl Zeiss Xradia 520 versa. The pixel size of the X-ray transmission image was 1 μm / pixel, the observation magnification was 28x (4x objective lens), and the observation area was 1.0 mm x 1.0 mm x 0.5 mm. The tube voltage was set to 140 kV. The obtained X-ray CT images were then reconstructed to obtain three-dimensional images of the protrusions. Next, using image analysis software Avizo (ver. 2022.2) manufactured by FEI, the protrusions in the region up to 10 μm in height from the base coating (1000 μm × 1000 μm × 10 μm) were identified, and their volume fraction was calculated.

[0044] <Lightness of copper plating film L * > Copper-plated steel sheets with only a copper plating film formed were immersed in 0.5% by mass diluted sulfuric acid at 25°C for 5 seconds, then rinsed with water and dried, and the lightness L of the copper plating film was measured within 1 hour. * After washing, the fabric was dried using a dryer with hot air at 40°C for 3 minutes. * The measurement was carried out under the following conditions. Measurement equipment: Spectrophotometer (TC-1800, manufactured by Tokyo Denshoku Co., Ltd.) Optical conditions: d / 8° method (double beam optical system) Field of view: 2 degrees Measurement method: Reflected light measurement Standard light: C Color system: CIELAB Measurement wavelength: 380~780nm Measurement wavelength interval: 5nm Spectrometer: Diffraction grating 1200 / mm Lighting: Halogen lamp (voltage 12V, power 50W, rated life 2000 hours) Measurement area: 7.25mmφ Detector: Photomultiplier tube (R928 manufactured by Hamamatsu Photonics K.K.) Reflectance: 0~150% Measurement temperature: 23℃ Standard plate: white

[0045] <Evaluation of coating adhesion> The copper-plated steel sheet on which the coating had been formed was punched into a disk with a diameter of 6 mm. The clearance was 0.1 mm. Next, a cross section parallel to the thickness direction was cut from the punched copper-plated steel sheet so that it could be observed, and resin was embedded so that the cut surface would be the observation surface. Next, the resin-embedded measurement test piece was mirror-finished by wet polishing. The length of peeling of the coating from the punched edge of the mirror-finished surface was measured. The evaluation criteria are as follows: Peeling length less than 20 μm: Excellent adhesion of the coating Peeling length 20-100 μm: Good adhesion of coating film (passed) Peeling length exceeds 100 μm: Poor adhesion of coating (failure)

[0046] <Pin-on-disk evaluation> A 6 mm diameter disc-shaped test piece was obtained by punching from a copper-plated steel sheet on which a coating (slidable resin film) had been formed. This test piece was evaluated using a pin-on-disc friction and wear tester (Rhesca Corporation, Model FPR-2100). Specifically, as shown in Figure 5, the test piece was attached to the tip of the pin (14 mm diameter) of the tester, and the disc (material: SKD11) was rotated to slide, and the time until the copper plating film on the test piece was exposed was measured. The applied load during sliding was 3 kgf, and the rotation speed was 32 rpm. The evaluation criteria were as follows: The copper plating film is exposed for over 200 hours: Excellent in maintaining sliding properties The time until the copper plating film is exposed is between 100 and 200 hours: Good maintenance of sliding properties (passed) If the time until the copper plating film is exposed is less than 100 hours: The effect of maintaining the sliding properties is poor (failed)

[0047] The evaluation results are shown in Table 1. It was confirmed that in Examples 1 to 7, the volume ratio of the protrusions in the region from the base coating portion to a height of 10 μm was 0.10% or more.

[0048] [Table 1]

[0049] Cross-sectional observation of copper-plated steel sheets on which only a copper plating film was formed confirmed that the copper plating films of Examples 1 to 7 were dendritic. In contrast, the copper plating films of Comparative Examples 1 to 3 were smooth films that grew along the surface, and the copper plating films of Comparative Examples 4 to 6 were films in which particulate copper adhered to the surface. Furthermore, as shown in Table 1, the copper-plated steel sheets of Examples 1 to 7 had a lightness L * In contrast, the copper-plated steel sheets of Comparative Examples 1 to 6 had good lightness L * was outside the range of 50 to 71, the results of each characteristic evaluation were poor.

[0050] As can be seen from the above results, the present invention can provide a copper-plated steel sheet in which a coating film is formed on a copper-plated film and the coating film is not easily peeled off. [Explanation of symbols]

[0051] 10 Copper-plated steel sheet 20 Base steel sheet 30 Copper plating film 31 Protrusion 40 Paint film

Claims

1. A copper-plated steel sheet comprising a base steel sheet and a dendritic copper plating film formed on at least one surface of the base steel sheet, The copper-plated steel sheet was immersed in 0.5 mass % dilute sulfuric acid at 25°C for 5 seconds, rinsed with water, dried, and the lightness L of the copper plating film was measured within 1 hour. * Copper-plated steel sheet having a hardness of 50 to 71.

2. The copper-plated steel sheet according to claim 1 , further comprising a coating film provided on the copper plating film.

3. The copper-plated steel sheet according to claim 2 , wherein the coating film is a slidable resin film.

4. The copper-plated steel sheet according to claim 1 , further comprising a discoloration prevention treatment layer provided on the copper plating film.

5. The copper-plated steel sheet according to claim 2 or 3, further comprising a discoloration prevention treatment layer provided between the copper plating film and the coating film.

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