Plated steel sheet

Abstract

The present invention provides a plated steel sheet which is capable of enhancing designability of hairlines when viewed from a medium distance of about 1-3 m. A plated steel sheet according to the present disclosure comprises: a base steel plate; and a plating layer which is formed on the base steel plate and has hairlines that are formed on the surface thereof. On the surface of the plating layer, a surface roughness profile is measured by a laser microscope in a measurement range that has a length of 60910 µm in a direction that is perpendicular to the extending direction of the hairlines, thereby obtaining the surface roughness profile of the measurement range, the measurement range is divided into 100 minute areas that are arranged at a pitch of 609.1 µm from an end of the measurement range, a peak count RPc having a dead zone width of 360 nm is obtained for each minute area, and two or more contrast parts, in which the absolute difference of peak count RPc between two adjacent minute areas is 16 or more, are present in the measurement range.

Description

Plated steel sheet

[0001] The present disclosure relates to a plated steel sheet, and more particularly to a plated steel sheet including a plating layer having a hairline formed on the surface.

[0002] Corrosion resistance and design are often required for products such as electrical appliances, furniture, building materials, and automobiles. As a material suitable for such applications, plated steel sheets with a hairline texture, which is a type of texture, formed on the surface of the plating layer have been proposed.

[0003] For example, in JP-A No. 2010-509495 (Patent Document 1) and JP-A No. 2006-124824 (Patent Document 2), in a plated steel sheet having a hot-dip galvanized layer formed thereon, a hairline is formed on the surface of the hot-dip galvanized layer to enhance the design of the plated steel sheet.

[0004] Furthermore, in Japanese Patent Laid-Open Publication No. 2017-136645 (Patent Document 3) and Japanese Patent Laid-Open Publication No. 2013-536901 (Patent Document 4), in a plated steel sheet having an electrogalvanized layer formed thereon, a hairline is formed on the surface of the electrogalvanized layer to enhance the design of the plated steel sheet.

[0005] JP-A-2010-509495 JP-A-2006-124824 JP-A-2017-136645 JP-A-2013-536901

[0006] In the above-mentioned patent documents, the surface roughness of the plating layer on which hairlines are formed is adjusted to improve the design properties of the plated steel sheet.

[0007] The desired aesthetic quality of the hairline varies depending on the application. For example, in the case of plated steel sheets for indoor or outdoor building materials, furniture, and automotive applications, the hairline is required to be visible when the plated steel sheet is viewed from a medium distance of about 1 to 3 m. In this specification, a hairline that is visible when the plated steel sheet is viewed from a medium distance of about 1 to 3 m is also referred to as having a high aesthetic quality of the hairline at a medium distance.

[0008] An object of the present disclosure is to provide a plated steel sheet that can improve the design of hairlines over a medium distance.

[0009] The plated steel sheet according to the present disclosure includes a base steel sheet and a plating layer. The plating layer is formed on the base steel sheet, and a hairline is formed on the surface. A surface roughness profile of the plating layer is measured using a laser microscope over a measurement range of 60910 μm in a direction perpendicular to the extension direction of the hairline, and the surface roughness profile of the measurement range is obtained. The measurement range is divided into 100 microregions arranged at a length pitch of 609.1 μm from the end of the measurement range, and a peak count RPc is obtained for each microregion with a dead zone width of 360 nm. Two or more contrast areas are present within the measurement range where the absolute difference between the peak counts RPc of two adjacent microregions is 16 or more.

[0010] The plated steel sheet of the present disclosure can improve the design of hairlines over medium distances.

[0011] FIG. 1 is a photographic image showing an example of a hairline formed on the surface of a plating layer of a plated steel sheet. FIG. 2 is an SEM image of the surface of the plating layer on which a hairline is formed. FIG. 3 is a schematic diagram showing an example of the surface roughness of the plating layer in a cross section perpendicular to the extension direction of the hairline. FIG. 4 is a cross-sectional view of a plated steel sheet of this embodiment. FIG. 5 is a plan view of the plated steel sheet shown in FIG. 4, viewed from above the plating layer. FIG. 6 is a diagram showing an example of a surface roughness profile measured with a laser microscope. FIG. 7 is a schematic diagram for explaining a method for measuring peak counts RPc in one microregion n (n is an integer from 1 to 100). FIG. 8 is a diagram showing an example of peak counts RPc in a measurement range.

[0012] The present inventors have investigated means for improving the aesthetic appeal of a hairline when viewed from a medium distance in a plated steel sheet having a hairline as shown in Fig. 1. In this specification, "medium distance" means that the distance from the surface of the plating layer on which the hairline is formed is 1 to 3 m in a hairline steel sheet. Furthermore, "high aesthetic appeal" means that the hairline is visible.

[0013] In conventional plated steel sheets on which hairlines are formed, the surface roughness (e.g., arithmetic mean roughness Ra) of the plating layer on which the hairlines are formed is adjusted. Specifically, when forming hairlines on the surface of the plating layer of a plated steel sheet, a grinding belt or the like is used to form multiple grinding marks extending in one direction on the surface of the plating layer. In conventional plated steel sheets, the visibility of the hairlines is improved by increasing the depth of the grinding marks.

[0014] The above-described technical concept is premised on the premise that the contrast of the hairline correlates with the depth of the grinding marks (the difference in elevation of the irregularities formed on the surface). This technical concept is based on the idea that the contrast of the macroscopic region of the hairline visible to the naked eye correlates with the contrast of the irregularities in the microscopic region of the surface where the hairline is visible. Therefore, the inventors confirmed the contrast of the irregularities in the microscopic region of the surface of the plating layer where a visible hairline was formed, using the following method.

[0015] Specifically, a microscopic region on the surface of the plating layer on which a hairline was formed was observed using a scanning electron microscope. Figure 2 is an SEM image (secondary electron image: field area 750 μm × 750 μm) of a microscopic region on the surface of the plating layer on which a hairline was formed. Referring to Figure 2 , grinding marks (concave and convex portions) extending in one direction were formed in the microscopic region on the surface where the hairline was visible, and contrast due to the depth of the grinding marks (height difference between the concave and convex portions) was confirmed. However, it was found that the macroscopic contrast of the visible hairline when the plated steel sheet was viewed from a medium distance did not necessarily correlate with the contrast of the concave and convex portions in the microscopic region shown in Figure 2 . In other words, the depth of the microscopic grinding marks (concave and convex portions) on the surface of the plating layer of the plated steel sheet did not necessarily correlate with the design quality of the macroscopic hairline when the plated steel sheet was viewed from a medium distance.

[0016] Based on the above new findings, the present inventors considered adopting a different method to enhance the design of the hairline when viewed from a medium distance, rather than increasing the average height difference of the surface irregularities by deepening the grinding marks as in the past.

[0017] As a result of investigation, the inventors found that the visibility of hairlines is significantly correlated with the distribution of grinding marks (difference in density of irregularities) rather than with the depth of the grinding marks (average height difference of irregularities). In a cross section perpendicular to the extension direction of the hairlines, irregularities caused by the hairlines are formed on the plating surface. Irregularities having a height difference that is visible as contrast are defined as "visible irregularities."

[0018] The direction perpendicular to the extension direction of the hairline is divided into multiple micro-regions of a predetermined unit length. If the difference in density of the visible asperities between adjacent micro-regions is large, the contrast at the boundary between the adjacent micro-regions becomes prominent. As a result, the visibility of the hairline increases. If the difference in density of the visible asperities between adjacent micro-regions is small, the contrast at the boundary between the adjacent micro-regions becomes unclear and inconspicuous. As a result, the visibility of the hairline decreases.

[0019] In the following description, the boundary between adjacent microscopic areas where there is a large difference in density of the visible asperities between the adjacent microscopic areas will be referred to as a "contrast area." The more contrast areas there are, the greater the visibility of the hairline. In other words, by adjusting the density distribution of the visible asperities, the visibility of the hairline can be improved.

[0020] The above content will be explained with reference to a specific example. Fig. 3 is a schematic diagram showing the surface roughness of the plating layer in a cross section perpendicular to the extension direction of the hairline. Referring to Fig. 3, it is assumed that the surface of the plating layer is divided into a plurality of microregions 1 to 8, each having a predetermined length L, in a direction perpendicular to the extension direction of the hairline. Each of the microregions 1 to 8 has the same length L. In each of the microregions 1 to 8, the density (number / L) of visible irregularities (visible irregularities) that have a predetermined height difference or more is determined.

[0021] In Figure 3, the visual irregularity density of microregion 1 is significantly high (30 counts / L), while the visual irregularity density of microregion 2 is low (5 counts / L). In this case, the difference in visual irregularity density between adjacent microregions 1 and 2 is significantly large. Therefore, the contrast between microregions 1 and 2 makes the boundary between microregions 1 and 2 clearly visible.

[0022] Based on the above findings, the inventors considered that the design quality of the hairline when viewed from a medium distance could be improved by adjusting the length L of the microregion to an appropriate value and adjusting the difference in visual irregularity density between two adjacent microregions to an appropriate range. Therefore, the inventors further investigated the length L of each microregion and the difference in visual irregularity density between two adjacent microregions that can improve the design quality of the hairline when viewed from a medium distance.

[0023] As a result of their investigations, the inventors found that when the surface roughness profile of the surface of the plating layer was measured using a laser microscope over a measurement range 60910 μm long in the direction perpendicular to the extension direction of the hairline, the surface roughness profile of the measurement range was determined, the measurement range was divided into 100 micro-areas arranged at a pitch of 609.1 μm long from the end of the measurement range, and the peak count RPc was determined for each micro-area with a dead zone width of 360 nm, if there were two or more contrast areas in the measurement range where the absolute difference between the peak count RPc of two adjacent micro-areas was 16 or more, the design quality of the hairline would be significantly enhanced when viewed from a medium distance.

[0024] The gist of the plated steel sheet of this embodiment, which was completed based on the above findings, is as follows.

[0025] A plated steel sheet according to a first embodiment includes a base steel sheet and a plating layer formed on the base steel sheet and having a hairline formed on its surface. A surface roughness profile of the plating layer is measured using a laser microscope over a measurement range of 60910 μm in length in a direction perpendicular to the extension direction of the hairline, the surface roughness profile of the measurement range is determined, the measurement range is divided into 100 microregions arranged at a pitch of 609.1 μm from the end of the measurement range, and a peak count RPc is determined for each microregion with a dead zone width of 360 nm. Two or more contrast areas are present within the measurement range where the absolute difference between the peak counts RPc of two adjacent microregions is 16 or more.

[0026] The plated steel sheet of the second embodiment is the plated steel sheet of the first embodiment, in which the exposed ratio of the base steel sheet when the plating layer is viewed in plan is less than 5.0%.

[0027] The plated steel sheet of the third embodiment is the plated steel sheet of the first or second embodiment, in which the plating layer is a zinc-based plating layer.

[0028] A plated steel sheet according to a fourth embodiment is the plated steel sheet according to any one of the first to third embodiments, in which the thickness of the plating layer is less than 5.0 μm.

[0029] Hereinafter, the plated steel sheet of the present embodiment will be described with reference to the drawings. In each drawing, the same or corresponding components are designated by the same reference numerals, and the same description will not be repeated. Furthermore, % in the chemical composition of the plating layer means % by mass unless otherwise specified.

[0030] [Regarding the plated steel sheet 1] Fig. 4 is a cross-sectional view of the plated steel sheet 1 of this embodiment. In Fig. 4, the rolling direction of the plated steel sheet 1 is defined as RD. The thickness direction of the plated steel sheet 1 is defined as TD. The width direction of the plated steel sheet 1, which is perpendicular to the rolling direction RD and the thickness direction TD, is defined as WD.

[0031] Referring to Fig. 4 , the plated steel sheet 1 includes a base steel sheet 100 and a plating layer 10. The plating layer 10 is formed on the surface of the base steel sheet 100. In Fig. 4 , the plating layer 10 is formed on only one side of the base steel sheet 100. However, in the plated steel sheet 1, the plating layer 10 may be formed on both sides of the base steel sheet 100.

[0032] The plated steel sheet 1 may further include a resin layer (not shown). When the plated steel sheet 1 includes a resin layer, the resin layer is formed on the surface of the plating layer 10. The plated steel sheet 1 may also include a chemical conversion coating (not shown). The chemical conversion coating is formed on the surface of the plating layer 10. The plated steel sheet 1 may include a chemical conversion coating formed on the plating layer 10 and a resin layer formed on the chemical conversion coating.

[0033] Fig. 5 is a plan view of the plated steel sheet 1 shown in Fig. 4, viewed from above the plating layer 10. Referring to Fig. 5, a hairline HL is formed on the surface of the plating layer 10 of the plated steel sheet 1. The hairline HL is composed of a plurality of fine irregularities extending in the rolling direction RD. The fine irregularities are, for example, grinding marks. The base steel sheet 100 and the plating layer 10 will be described below.

[0034] [Regarding the base steel sheet 100] As the base steel sheet 100, a well-known steel sheet applicable to well-known plated steel sheets (e.g., electrogalvanized steel sheet, electrogalvanized alloy plated steel sheet, hot-dip galvanized steel sheet, alloyed hot-dip galvanized steel sheet, etc.) may be used depending on the mechanical properties (e.g., tensile strength, workability, etc.) required of the plated steel sheet 1 to be manufactured. For example, as the base steel sheet 100, a steel sheet for electrical equipment use may be used, or a steel sheet for building materials use may be used. The base steel sheet 100 may be a hot-rolled steel sheet or a cold-rolled steel sheet.

[0035] The chemical composition of the base steel plate 100 contains, for example, in mass %, C: 0.01 to 0.25%, Si: 0.001 to 1.200%, Mn: 0.01 to 2.50%, P: 0.001 to 0.200%, S: 0.001 to 0.050%, and sol. Al: 0.015 to 0.060%. The Fe content of the base steel plate 100 is, for example, 90.0% or more. However, the chemical composition of the base steel plate 100 is not limited to the above-mentioned chemical composition.

[0036] [Regarding the Plating Layer 10] The plating layer 10 is formed on the surface of the base steel sheet 100. The type of plating layer 10 is not particularly limited. Examples of the plating layer 10 include a Ni-based plating layer, a Cu-based plating layer, a zinc-based plating layer, an Au-based plating layer, a Sn-based plating layer, an Al-based plating layer, and an alloy plating layer containing two or more of Ni, Cu, Zn, Au, Sn, and Al. The above-mentioned X-based plating layer (X is one of Ni, Cu, Zn, Au, Sn, and Al) refers to a plating layer consisting mainly of X. "Consisting mainly of X" means that the content of X, which is the main component element in the plating layer, is at least 50.0% by mass. For example, a zinc-based plating layer refers to a plating layer having a Zn content of 50.0% by mass or more.

[0037] From the viewpoint of the aesthetic appearance of the formed hairline HL, the plating layer 10 is preferably a Ni-based plating layer, a Cu-based plating layer, an Au-based plating layer, a Sn-based plating layer, an Al-based plating layer, or an alloy plating layer containing two or more of Ni, Cu, Au, Sn, and Al, which are excellent in durability.

[0038] On the other hand, in consideration of damage to the plating layer 10 when the hairline HL is formed on the surface of the plating layer 10, it is preferable that the plating layer 10 have a composition that is excellent in corrosion resistance even when the hairline HL is formed. Therefore, from the viewpoint of achieving both aesthetic appearance and corrosion resistance, a zinc-based plating layer is a preferable plating layer 10. The zinc-based plating layer has a sacrificial corrosion protection function and is excellent in corrosion resistance.

[0039] When the plating layer 10 is a zinc-based plating layer, as described above, this means that the Zn content in the plating layer is at least 50.0% by mass. The chemical composition of the zinc-based plating layer may, for example, contain 50.0% or more by mass of Zn, and 0 to 50.0% total of one or more optional elements selected from the group consisting of Al, Mg, Si, Ni, Fe, Co, Cr, Ca, Y, La, Ce, Sn, Bi, In, Ti, V, Nb, Cu, Mn, Sr, Sb, Pb, and B, and may further contain 0 to 5.0% C as an optional element. The chemical composition of the zinc-based plating layer does not necessarily include Al, Mg, Si, Ni, Fe, Co, Cr, Ca, Y, La, Ce, Sn, Bi, In, Ti, V, Nb, Cu, Mn, Sr, Sb, Pb, B, and C.

[0040] The preferred lower limit of the Zn content in the zinc-based plating layer is 80.0% by mass, and more preferably 85.0%. That is, the zinc-based plating layer is made of Zn or a Zn alloy, with the remainder being impurities. The zinc-based plating layer is a layer made of zinc plating or a zinc alloy plating. The zinc-based plating layer is an electrolytic zinc-based plating layer or a hot-dip zinc-based plating layer. As mentioned above, the chemical composition of the zinc-based plating layer is well known.

[0041] [Method for measuring the chemical composition of the plating layer 10] The chemical composition of the above-mentioned plating layer 10 consisting of an X-based plating layer can be measured by the following method. A sample including the plating layer 10 and a cross section parallel to the thickness direction TD of the plated steel sheet 1 is taken. The cross section parallel to the thickness direction TD is used as the observation surface.

[0042] On the observation surface of the sample, the plating layer 10 and the base steel sheet 100 are clearly distinguishable by visual inspection. Line analysis based on EPMA (Electron Probe Micro Analyzer) is performed from the surface of the plated steel sheet 1 in the sheet thickness direction TD to determine the content (mass%) of each element on the line to be measured. In the line analysis, a region (range) containing 50.0 mass% or more of the main component element X is identified as the X-based plating layer. Line analysis is performed at 10 arbitrary locations on the observation surface of the sample, and the content (mass%) of each element in the range (line) of the X-based plating layer 10 identified by the line analysis is determined. The arithmetic mean value of the content of each element in the range of the X-based plating layer 10 obtained by the line analysis at the 10 locations is determined. The chemical composition of the X-based plating layer 10 is determined based on the arithmetic mean value of the content of each element determined. In the EPMA line analysis, the acceleration voltage is 15 kV, the irradiation current is 30 nA, and the electron beam diameter is 0.1 μm.

[0043] [Thickness of the plating layer 10] The thickness of the plating layer 10 is not particularly limited, and any known thickness will suffice. When the plating layer 10 is a zinc-based plating layer, the thickness of the plating layer 10 is, for example, 0.5 to 25.0 μm. The preferred upper limit of the plating layer thickness is 20.0 μm, more preferably 15.0 μm, even more preferably 10.0 μm, even more preferably 8.0 μm, even more preferably less than 5.0 μm, even more preferably 4.8 μm, and even more preferably 4.5 μm. The preferred lower limit of the plating layer thickness is 0.8 μm, even more preferably 1.0 μm, even more preferably 1.3 μm, even more preferably 1.4 μm, and even more preferably 1.5 μm.

[0044] [Method for Measuring the Thickness of the Plating Layer 10] The thickness of the plating layer 10 can be determined by the following method. A sample including a cross section parallel to the thickness direction TD of the plated steel sheet 1 and including the plating layer 10 is taken. The cross section parallel to the thickness direction is used as the observation surface. The observation surface of the sample is mirror-polished. Using a scanning electron microscope, the mirror-polished observation surface is observed at a magnification (500 to 3000 times) that allows the entire length (thickness) of the plating layer 10 in the thickness direction TD to be observed, and a backscattered electron image is generated. In the backscattered electron image, the base steel sheet 100 and the plating layer 10 can be easily distinguished by contrast. Therefore, the plating layer 10 is identified based on the contrast. The length (thickness) of the identified plating layer 10 in the thickness direction TD is measured at 10 arbitrary locations. The arithmetic mean of the thicknesses measured at the 10 locations is defined as the thickness (μm) of the plating layer 10. The thickness is rounded to one decimal place.

[0045] [Regarding the hairline HL] As shown in Fig. 5, a hairline HL is formed on the surface of the plating layer 10. In Fig. 5, the extension direction of the hairline HL is the rolling direction RD. However, the hairline HL does not have to extend in the rolling direction RD, and may extend in another direction. The hairline HL is arranged in a direction perpendicular to the extension direction of the hairline HL. In Fig. 5, the hairline HL is arranged in the sheet width direction WD. As described in Fig. 2, in a small region on the surface of the plating layer 10 on which the hairline HL is formed, concave and convex portions extending in one direction are formed.

[0046] [Regarding the contrast portion CT] When the multiple irregularities formed in the direction perpendicular to the extension direction of the hairline HL on the surface of the plating layer 10 are divided into multiple microscopic regions, the boundary where the density difference of the visible irregularities between adjacent microscopic regions is large is referred to as the contrast portion CT. As described above, the design quality of the hairline HL when viewed from a medium distance correlates with the density of the contrast portion CT.

[0047] [Method for Identifying Contrast Region CT] The contrast region CT is defined as follows. As shown in FIG. 5 , an arbitrary range of 60910 μm in length perpendicular to the extension direction of the hairline HL on the surface of the plating layer 10 is defined as the measurement range MR. The surface roughness profile of the measurement range MR is measured using a laser microscope. The laser microscope satisfies the following conditions: A laser confocal laser microscope is used. The light source wavelength is 408 nm. The height display resolution is 1 nm or less. The width accuracy is within ±5% of the measured value. The accuracy of the height direction measurement can be guaranteed when observing a 60910 μm length in a single field of view. An example of a laser microscope that satisfies the above conditions is the VK-X250 Shape Analysis Laser Microscope manufactured by Keyence Corporation. Note that if the accuracy of the height direction (TD axis) measurement cannot be guaranteed when observing a 60910 μm length in a single field of view, multiple consecutive images observed at a magnification that can guarantee the accuracy of the TD axis measurement can be connected and measured as a single profile. An example of the obtained profile is shown in FIG.

[0048] If a resin layer and / or a chemical conversion coating is formed on the surface of the plating layer 10, the resin layer and the chemical conversion coating are removed using a stripping agent such as a solvent or remover that does not attack the plating layer 10. After removing the resin layer and the chemical conversion coating, the surface roughness profile of the measurement range MR of the plating layer 10 is measured. For example, the stripping agent is Neo River S-701, a product name manufactured by Sansai Kako Co., Ltd.

[0049] A laser microscope optically measures the surface shape. A resin layer formed on the plating layer 10 transmits light. Therefore, even if a resin layer is formed on the plating layer 10, the surface shape of the plating layer 10 can be measured using a laser microscope. However, the laser emitted from the laser microscope may be refracted or scattered by the resin layer. In this case, the accuracy of the surface roughness profile may be reduced. Therefore, if a resin layer is formed on the plating layer 10, the resin layer is removed before measuring the surface roughness profile of the plating layer 10.

[0050] Using the surface roughness profile obtained in the measurement range MR and with reference to JIS B0601:2013, the peak count RPc is determined as follows. In the surface roughness profile, the measurement range MR is divided into 100 microregions n (n is an integer between 1 and 100) arranged at a length pitch of 609.1 μm from the end P of the measurement range MR (see FIG. 5 ). The peak count RPc is then determined for each of the 100 microregions n. The length of the microregion is set to 609.1 μm for the following reason: The average limiting resolution (visual limit) of the human eye is approximately 1.0 arcminute with a visual acuity of 1.0, and the required resolution at a distance of 1.5 m is approximately 41.7 dpi. Converting 41.7 dpi to μm gives 609.1 μm. Therefore, the length of the microregion n is set to 609.1 μm.

[0051] The peak count RPc is determined using the following method. Figure 7 is a schematic diagram illustrating the method for measuring the peak count RPc in one microregion n. As shown in Figure 7, the measured cross-sectional curve is processed at a cutoff wavelength λ = 0.08 mm to obtain a surface roughness profile C1. A dead zone DB having a width of 360 nm is set around the mean line X of the obtained surface roughness profile C1. In the surface roughness profile C1, a point below the dead zone DB is counted as one peak, from the point where the surface roughness profile C1 rises above the dead zone DB until it again rises below the dead zone DB. In the example of Figure 7, the peak count RPc is six. The dead zone DB is set to 360 nm because 360 ​​nm is the minimum wavelength of visible light.

[0052] The irregularities counted as the peak count RPc are visible irregularities. Irregularities with a height difference greater than the dead zone DB are defined as "visible irregularities." Using the above method, the peak count RPc for each micro-area n is determined. The peak count RPc for the micro-area n indicates the density of visible irregularities within the micro-area n.

[0053] Based on the peak count RPc obtained for each microregion, a boundary where the absolute difference in the peak count RPc of the visually recognizable irregularities between adjacent microregions is large is identified as a contrast region CT. Specifically, the absolute difference ΔRPc in the peak count RPc between adjacent microregions k and k+1 (k is 1 to 99) is calculated using the following formula: ΔRPc = |(peak count RPc of microregion k) - (peak count RPc of microregion k+1)| If the absolute difference ΔRPc is 16 or greater, the boundary between microregion k and microregion k+1 can be visually recognized as a striking contrast when viewed from a medium distance. Therefore, the boundary is recognized as a contrast region CT.

[0054] In the measurement range MR, the absolute difference ΔRPc is calculated for all adjacent microregions k and k+1 to identify the contrast portions CT. When two or more contrast portions CT exist in the measurement range MR, there are a sufficiently large number of contrast portions CT that are visible on the plated steel sheet 1 when viewed from a medium distance. This improves the aesthetic appeal of the hairline when viewed from a medium distance.

[0055] 8 is a diagram showing an example of the peak count RPc in the measurement range MR. In the measurement range MR in FIG. 8, three contrast areas CT exist within the measurement range MR. Therefore, the design quality of the hairline is sufficiently high when viewed from a medium distance.

[0056] As described above, in the plated steel sheet 1 of this embodiment, there are two or more contrast portions CT derived from the peak count RPc in 100 microscopic regions separated at a length pitch of 609.1 μm within the measurement range MR. In this case, when the plated steel sheet 1 is viewed from a medium distance, the number density of the contrast portions CT on the surface of the plating layer 10 is sufficiently high. Therefore, the design quality of the hairline HL is sufficiently improved when the plated steel sheet 1 is viewed from a medium distance.

[0057] The lower limit of the total number of contrast portions CT is preferably 2, more preferably 4, even more preferably 5, even more preferably 6, even more preferably 7, even more preferably 8, and even more preferably 10.

[0058] [Preferable Exposure Rate of Base Steel Sheet 100 When Plated Steel Sheet 1 is Viewed in Plan] When the plating layer 10 of the plated steel sheet 1 is viewed in plan, a preferable exposure rate of the base steel sheet 100 is less than 5.0%. A more preferable exposure rate of the base steel sheet 100 is 0%. In this case, sufficient corrosion resistance (long-term corrosion resistance) can be obtained in the plated steel sheet 1.

[0059] Unlike conventional plated steel sheets, the plated steel sheet 1 of this embodiment does not improve the visibility and design of the hairline HL by increasing the average height difference (e.g., the depth of grinding marks) of the surface irregularities of the plated layer 10, but rather improves the design of the hairline HL when viewed from a medium distance by increasing the number density of the contrast portions CT. Therefore, unlike conventional steel sheets with hairlines, the plated steel sheet 1 of this embodiment does not require excessive grinding of the plated layer 10 to improve the design of the hairline HL. As a result, the plated steel sheet 1 is more likely to suppress the exposure rate of the base steel sheet 100. In particular, when the plated steel sheet 1 has an electroplated layer, the plated steel sheet 10 may be formed as thin as less than 5.0 μm. Even in such a case, the plated steel sheet 1 of this embodiment can suppress the exposure rate of the base steel sheet 100 to less than 5.0%. As a result, the visibility of the hairline HL when viewed from a medium distance can be improved while ensuring long-term corrosion resistance. In order to further improve the corrosion resistance of the plated steel sheet 1, the lower limit of the thickness of the plated layer 10 is preferably more than 1.0 μm, more preferably 1.1 μm, even more preferably more than 1.2 μm, and even more preferably 1.5 μm.

[0060] [Method for Measuring Exposure Rate] The exposure rate of the base steel sheet 100 in the plated steel sheet 1 is measured by the following method. First, the chemical composition of the plated layer is identified based on the above-described [Method for Measuring the Chemical Composition of the Plated Layer 10], and the main component element X of the plated layer is identified. In the obtained chemical composition, an element that accounts for 50.0% or more by mass is designated as the main component element X. Next, using the plated steel sheet 1 from which the sample was taken in the [Method for Measuring the Chemical Composition of the Plated Layer 10], five rectangular regions measuring 1 mm x 1 mm are arbitrarily selected when viewing the plated steel sheet 1 from above the plated layer 10 (i.e., when viewing the plated steel sheet 1 from above). EPMA analysis (area analysis) is performed on the selected rectangular regions. In the EPMA area analysis, the acceleration voltage is 15 kV, the probe current is 30 nA, the electron beam diameter is 1.0 μm, and the measurement pitch is 1.95 μm. Image analysis is used to identify regions within each rectangular region where the main component element X of the plated layer is not detected (plating-undetected regions). In this embodiment, a region where the detection intensity of the main component element X in the plating layer is 1 / 16 or less of the detection intensity when a standard sample (a pure metal sample of X) is measured is recognized as a plating main component element undetected region. The ratio (%) of the total area of ​​the plating main component element undetected regions to the total area of ​​the five rectangular regions is defined as the exposure rate (%) of the base steel sheet 100. The exposure rate of the base steel sheet 100 is defined as the value obtained by rounding the obtained value to one decimal place.

[0061] [Other embodiments of the plated steel sheet 1 of this embodiment] The plated steel sheet 1 of this embodiment is not limited to the above-described embodiment. As described above, the plated steel sheet 1 of this embodiment may have one or more resin layers formed on the surface of the plating layer 10. The plated steel sheet 1 of this embodiment may also have a chemical conversion coating formed on the surface of the plating layer 10. The plated steel sheet 1 of this embodiment may also have a chemical conversion coating formed on the surface of the plating layer 10, and a resin layer further formed on the chemical conversion coating.

[0062] [Method for manufacturing plated steel sheet 1] An example of a method for manufacturing the plated steel sheet 1 of the present embodiment will be described below. The plated steel sheet 1 may be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of the method for manufacturing the plated steel sheet 1 of the present embodiment.

[0063] An example of a method for manufacturing the plated steel sheet 1 includes the following steps: (Step 1) Base steel sheet preparation step (Step 2) Plated layer formation step (Step 3) Hairline formation step Each step will be described below.

[0064] [(Step 1) Base Steel Plate Preparation Step] In the base steel plate preparation step, the above-described base steel plate 100 is prepared. As described above, the base steel plate 100 may be a hot-rolled steel plate or a cold-rolled steel plate.

[0065] [(Step 2) Plating Layer Forming Step] In the plating layer forming step, the plating layer 10 is formed on the surface of the base steel sheet 100 by a well-known electroplating method or hot-dip galvanizing method. The method for forming the plating layer 10 may be an electroplating method or a hot-dip galvanizing method. When the plating layer 10 is a zinc-based plating layer, for example, the following electrogalvanizing method or hot-dip galvanizing method may be carried out.

[0066] [Electrogalvanizing Method] When forming the zinc-based plating layer 10 by electrogalvanizing, the electrogalvanizing method may be performed by a known method. In this specification, the term "electrogalvanizing method" also includes electrogalvanizing of zinc alloys. The plating solution used in the electrogalvanizing method may be any known plating solution. Examples of electrogalvanizing solutions include sulfate baths, chloride baths, zincate baths, cyanide baths, pyrophosphate baths, boric acid baths, citric acid baths, other complex baths, and combinations thereof. The electrogalvanizing solution may contain, in addition to Zn ions, one or more single ions or complex ions selected from the group consisting of Fe, Ni, Co, Cr, and C. Furthermore, organic additives may be added to the plating solution to achieve desired effects such as leveling or increased hardness.

[0067] [Hot-dip galvanizing method] When forming the zinc-based coating layer 10 by hot-dip galvanizing, the hot-dip galvanizing method may be performed by a known method. A known coating bath may be used as the coating bath for the hot-dip galvanizing method. For example, a hot-dip galvanizing bath containing Al with the balance being Zn and impurities may be used. The impurities may be Fe, for example. In addition to Zn, Al, and Fe, the hot-dip galvanizing bath may contain one or more elements selected from the group consisting of Mg, Si, Ni, Co, Cr, Ca, Y, La, Ce, Sn, Bi, In, Ti, V, Nb, Cu, Mn, Sr, Sb, Pb, B, and C.

[0068] (Step 3) Hairline Forming Step In the hairline forming step, hairline processing is performed on the surface of the plating layer 10 to form a hairline HL on the surface of the plating layer 10 .

[0069] A grinding device used for hairline processing includes, for example, a conveying roll, a pair of contact rolls, and a circular abrasive belt. The pair of contact rolls hold the circular abrasive belt. As the pair of contact rolls rotate, the circular abrasive belt moves between the pair of contact rolls. A base steel sheet 10 having a plating layer 10 on its surface is conveyed between the conveying roll and the circular abrasive belt by the conveying roll. The circular abrasive belt is pressed against the surface of the plating layer 10. As the pair of contact rolls rotate, the circular abrasive belt grinds the surface of the plating layer 10 of the base steel sheet 100. As a result, a hairline HL is formed on the surface of the plating layer 10. The grinding device may include a grinding brush instead of the abrasive belt.

[0070] In the hairline forming process, the speed (travel speed) of the plated steel sheet 1 passing through the grinding device, the rotational speed of the contact roll, and the pressing force of the contact roll against the plated steel sheet 1 can be adjusted as appropriate. The amount of the plating layer ground in the hairline forming process can be adjusted by adjusting the rotational speed of the contact roll and the pressing force of the contact roll against the plating layer 10. When the grinding device is equipped with a grinding brush, the amount of grinding can be adjusted, for example, by adjusting the rotational speed of the grinding brush and / or the pressing force of the grinding brush against the plating layer 10.

[0071] Preferably, the grain size and / or density of the abrasive grains are distributed differently across the width of the abrasive belt or abrasive brush, which corresponds to the width of the base steel plate 100.

[0072] The abrasive belts and grinding brushes used in conventional hairline processing are designed so that the abrasive grain size and density are uniform across the width. Conventionally, to improve the visibility of the hairline, the abrasive grain size was increased. In contrast, the manufacturing method of this embodiment uses a different distribution of abrasive grain size and / or density across the width of the abrasive belt or grinding brush. This allows for adjustment of the density of the visible irregularities in each micro-area n in the direction perpendicular to the extension direction of the hairline HL, thereby adjusting the density of the contrast area CT. The distribution of abrasive grain size and density across the width of the abrasive belt and grinding brush can be adjusted as needed depending on the desired appearance.

[0073] When a grinding brush is used for hairline processing, the ring-shaped grinding brush includes a central shaft and multiple brush bristles extending radially from the central shaft. The brush bristles are arranged in the axial direction of the grinding brush (the width direction of the grinding brush). Multiple brush bristles with different abrasive grain densities are arranged in the width direction of the grinding brush. Hairline processing is performed using such a grinding brush. By changing the abrasive grain density of the brush bristles in the width direction of the grinding brush, it is possible to change the density of the grinding marks (unevenness) formed on the surface of the plating layer by the grinding brush. The abrasive grain density of the polishing belt is adjusted so that the contrast area CT within the measurement range MR on the surface of the plating layer is 2 or more.

[0074] The effects of one aspect of the present invention will be described more specifically below with reference to examples. The conditions in the following examples are examples of conditions adopted to confirm the feasibility and effects of the plated steel sheet of this embodiment.

[0075] Plated steel sheets with the test numbers shown in Table 1 were prepared. The base steel sheet of each plated steel sheet was SPCC as specified in JIS G 3141:2017, and had a thickness of 0.6 mm. A plating layer formation step was carried out on each base steel sheet so as to achieve various plating thicknesses.

[0076]

[0077] Specifically, in test numbers 1 to 5 and 17 to 20, a Zn plating layer was formed as the plating layer by a well-known electroplating method (indicated as "Zn plating" in the "Plating type" column in Table 1). The chemical composition of the Zn plating layer was measured based on the above-mentioned [Method for measuring the chemical composition of plating layer 10]. As a result, the plating layers of these test numbers all had a Zn content of 99.0% or more and had a chemical composition substantially consisting of Zn.

[0078] In test numbers 6 to 8, a Zn—Ni plating layer was formed as the plating layer by a well-known electroplating method (indicated as "Zn—Ni plating" in the "Plating type" column in Table 1). The chemical composition of the Zn—Ni plating layer was measured based on the above-mentioned [Method for measuring the chemical composition of plating layer 10]. As a result, the plating layers of these test numbers all had a chemical composition in which the Ni content was 10 to 15% by mass, with the remainder being Zn.

[0079] In Test No. 9, a Zn—Co plating layer was formed as the plating layer by a well-known electroplating method (denoted as "Zn—Co plating" in the "Plating type" column in Table 1). The chemical composition of the Zn—Co plating layer was measured based on the above-mentioned [Method for measuring the chemical composition of plating layer 10]. As a result, the plating layer of Test No. 9 had a chemical composition in which the Co content was 2% by mass, with the remainder being Zn.

[0080] In test numbers 10 and 11, a Zn—Fe plating layer was formed as the plating layer by a well-known electroplating method (denoted as "Zn—Fe plating" in the "Plating type" column in Table 1). The chemical composition of the Zn—Fe plating layer was measured based on the above-mentioned [Method for measuring the chemical composition of plating layer 10]. As a result, the plating layers of test numbers 10 and 11 both had a chemical composition in which the Fe content was 10 to 14% by mass, with the remainder being Zn.

[0081] In Test No. 12, a Zn—Al plating layer was formed as the plating layer by a well-known hot-dip plating method (shown as "Zn—Al plating" in the "Plating type" column in Table 1). The chemical composition of the Zn—Al plating layer was measured based on the above-mentioned [Method for measuring the chemical composition of plating layer 10]. As a result, the plating layer of Test No. 12 had a chemical composition in which the Al content was 0.2% by mass, with the remainder being Zn.

[0082] For test numbers 13 to 16, a Zn—Al—Mg coating layer was formed as the coating layer by a well-known hot-dip coating method (denoted as "Zn—Al—Mg coating" in the "Coating type" column in Table 1). The chemical composition of the Zn—Al—Mg coating layer was measured based on the above-mentioned [Method for measuring the chemical composition of coating layer 10]. As a result, the coating layers for these test numbers all had a chemical composition in which the Al content was 5 to 19% by mass, the Mg content was 2 to 6% by mass, and the balance was Zn and trace elements (B, Bi, Ca, Ce, In, La, P, Si, Sn, Sr, and Y) with a total content of less than 1%.

[0083] The thickness of the zinc plating layer of each test piece was measured according to the above-mentioned "Method for measuring the thickness of the plating layer 10." The measurement results are shown in the "Plating layer thickness (μm)" column in Table 1.

[0084] A hairline formation process was carried out on the surface of the plating layer on the base steel sheets of test numbers 1 to 16 and 18 to 20. In the hairline formation process, a plurality of polishing belts with different abrasive grain density distributions in the width direction of the polishing belt were prepared, and hairlines were formed. Plated steel sheets with hairlines were produced by the above manufacturing process. Note that the hairline formation process was not carried out for test number 17.

[0085] The plated steel sheet with the hairline formed thereon was subjected to a chemical conversion treatment to form a chemical conversion coating on the plated layer. Specifically, the following silane coupling agent A and silane coupling agent B were prepared. Silane coupling agent A: 3-aminopropyltrimethoxysilane Silane coupling agent B: 3-glycidoxypropyltrimethoxysilane

[0086] Silane coupling agent A and silane coupling agent B were added to water adjusted to pH 4, with a solids mass ratio (silane coupling agent A / silane coupling agent B) of 1.0. The mixture was then stirred for a predetermined period of time to produce an organosilicon compound. The produced organosilicon compound was further mixed with phosphoric acid, a phosphate compound, to produce a treatment liquid.

[0087] The treatment solution was picked up by a roll and transferred onto the plating layer. At this time, the deposition weight of the chemical conversion coating after baking and drying was 0.3 g / m ２ The treatment solution was transferred onto the plating layer so that

[0088] The steel sheet with the treatment solution transferred onto the plating layer was subjected to bake drying. Specifically, the steel sheet with the treatment solution transferred onto the plating layer was placed in a furnace maintained at 180°C and held in the furnace until the temperature of the steel sheet reached 130°C. After the temperature of the steel sheet reached 130°C, the steel sheet was removed from the furnace and air-cooled to room temperature. Through these steps, a chemical conversion coating was formed on the plating layer.

[0089] A resin layer was formed on the steel sheet on which the chemical conversion coating had been formed. A urethane-based resin (trade name: HUX-232, manufactured by ADEKA Corporation) was used as the binder resin for the resin coating. Polyethylene-based resin particles (trade name: Chemipearl, manufactured by Mitsui Chemicals, Inc.) were used as the resin particles. The binder resin and resin particles were dispersed in water to prepare a paint.

[0090] The prepared coating material was scooped up with a roll and transferred onto a steel plate. At this time, the amount of treatment liquid applied was adjusted so that the average film thickness of the resin coating after baking and drying would be 5 μm. The steel plate onto which the treatment liquid had been transferred was placed in a furnace maintained at 250°C. The steel plate was held in the furnace until the temperature of the steel plate reached 180°C. After the temperature of the steel plate reached 180°C, the steel plate was removed from the furnace and air-cooled to room temperature. A resin layer was formed by the above steps.

[0091] [Evaluation Tests] The following evaluation tests were carried out on the plated steel sheets with each test number: (Test 1) Measurement of the total number of contrast areas (Test 2) Measurement of the base steel sheet exposure rate (Test 3) Evaluation of hairline design at medium distance (Test 4) Evaluation of corrosion resistance Tests 1 to 4 are explained below.

[0092] [(Test 1) Test for Measuring the Total Number of Contrast Portions] For each plated steel sheet with each test number, the total number of contrast portions CT within the measurement range MR was counted according to the method described above in [Method for Identifying Contrast Portions CT]. The laser microscope used was a VK-X250 shape analysis laser microscope manufactured by Keyence Corporation. The resin layer and chemical conversion coating were removed using a stripping agent. Neo River S-701, manufactured by Sansai Kako Co., Ltd., was used as the stripping agent. The resulting total number of contrast portions CT is shown in Table 1.

[0093] [(Test 2) Test for Measuring Exposure Ratio of Base Steel Sheet] For each plated steel sheet with each test number, the exposure ratio of the base steel sheet was measured according to the method described above in [Method for Measuring Exposure Ratio]. Zn was used as the main plating component element. The measured exposure ratios (%) are shown in Table 1.

[0094] [(Test 3) Hairline Design Evaluation Test at Medium Distance] The hairline design at medium distance was evaluated for each plated steel sheet with each test number using the following method. First, the plated steel sheet with each test number was placed indoors at an angle of 60° to the horizontal. The plated steel sheet was placed so that the plating layer faced the ceiling of the room. Next, an observer visually observed the plated steel sheet from a location 1.5 m away from the plated steel sheet's placement on the floor of the room, facing the plated steel sheet. The observer's line of sight was at the same height as the plated steel sheet. A total of 10 observers visually observed the plated steel sheet and performed a sensory evaluation to determine whether the plated steel sheet had excellent design. Specifically, if the hairline was clearly visible upon visual observation, it was judged to have excellent design. A score was assigned based on the number of observers who judged the design to be excellent, and a score of B or higher was deemed to have passed. The evaluation results (A to C) are shown in the "Design" column under the "Evaluation Results" column in Table 1. Eight or more judges judge the design to be excellent: Rating A Five to eight judges judge the design to be excellent: Rating B Less than five judges judge the design to be excellent: Rating C

[0095] [(Test 4) Corrosion Resistance Evaluation Test] The corrosion resistance (long-term corrosion resistance) of each plated steel sheet was evaluated using the following method. A test specimen measuring 75 mm × 100 mm × thickness was taken from each plated steel sheet. The 75 mm × 100 mm surface of the test specimen was used as the measurement surface. The end faces and back faces of the test specimen were protected with tape seals. A 5% NaCl salt spray test maintained at 35°C was then conducted in accordance with JIS Z 2371:2015. The test was conducted for 240 hours, and the rust occurrence rate after the test was determined. Specifically, the area where rust occurred on the measurement surface after the test was determined visually. The rust occurrence rate (%), which is the area ratio of rust occurrence, was calculated based on the area where rust occurred and the area of ​​the measurement surface. If the rust occurrence rate was less than 5%, it was marked with "E (Excellent)" in the "Corrosion Resistance" column of the "Evaluation Results" column in Table 1. On the other hand, if the rust occurrence rate is 5% or more, the evaluation is indicated as "B (Bad)" in the "Corrosion Resistance" column of the "Evaluation Results" column in Table 1.

[0096] [Evaluation Results] The evaluation results are shown in Table 1. Referring to Table 1, the plated steel sheets of test numbers 1 to 16 had two or more contrast portions where the absolute difference in peak count RPc was 16 or more. Therefore, the plated steel sheets of these test numbers had excellent design properties when viewed from a medium distance.

[0097] Furthermore, in test numbers 1 to 3, 5, 6, and 8 to 12, the contrast areas were 6 or more. Therefore, the plated steel sheets with these test numbers had even better design properties than the plated steel sheets with test numbers 4, 7, and 13 to 16, which had contrast areas of less than 6.

[0098] Furthermore, in test numbers 3 to 16, the thickness of the plating layer was 1.3 μm or more and the exposed rate of the base steel sheet was less than 5.0%. Therefore, these test numbers exhibited even better corrosion resistance than test numbers 1 and 2. In the corrosion resistance evaluation test, the plated steel sheets of test numbers 1 and 2 exhibited a rust occurrence rate of 5% or more, but less than 10%, and thus exhibited a certain degree of corrosion resistance.

[0099] On the other hand, in test number 17, the hairline forming step was not carried out. Therefore, the total number of contrast portions CT was less than 2. As a result, sufficient design properties were not obtained.

[0100] In test number 18, there were too few grinding marks in the hairline forming step. Therefore, the total number of contrast portions CT was less than 2. As a result, sufficient design properties were not obtained.

[0101] In test number 19, the grinding marks were too shallow in the hairline forming step. Therefore, the total number of contrast portions CT was less than 2. As a result, sufficient design properties were not obtained.

[0102] In test number 20, the grinding marks were excessively fine during the hairline formation process. Therefore, the total number of contrast areas CT was less than 2. As a result, sufficient design was not obtained. Furthermore, the exposed rate of the base steel sheet exceeded 5.0%. As a result, sufficient corrosion resistance was not obtained.

[0103] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments within the scope of the present disclosure.

[0104] 1 Plated steel sheet 10 Plated layer HL Hairline

Claims

1. A plated steel sheet comprising: a base steel sheet; and a plating layer formed on the base steel sheet and having a hairline formed on its surface; wherein a surface roughness profile of the surface of the plating layer is measured using a laser microscope within a measurement range of 60910 μm in length in a direction perpendicular to the extension direction of the hairline to determine the surface roughness profile of the measurement range; the measurement range is divided into 100 micro-areas arranged at a pitch of 609.1 μm in length from the end of the measurement range, and a peak count RPc with a dead zone width of 360 nm is determined for each micro-area; and within the measurement range, there are two or more contrast areas where the absolute difference between the peak count RPc of two adjacent micro-areas is 16 or more.

2. The plated steel sheet according to claim 1, wherein the exposed ratio of the base steel sheet when the plated layer is viewed in plan is less than 5.0%.

3. The plated steel sheet according to claim 1, wherein the plated layer is a zinc-based plated layer.

4. The plated steel sheet according to claim 1, wherein the thickness of the plated layer is less than 5.0 μm.

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