JP7648979B2 - Galvanized Steel Sheet - Google Patents

Description

The present disclosure relates to plated steel sheets, and more specifically, to plated steel sheets having a plating layer with a hairline formed on the surface.

Corrosion resistance and design are sometimes required for articles such as electrical equipment, building materials, and automobiles. As a material suitable for such applications, plated steel sheets with a hairline texture, a type of texture, formed on the surface of the plating layer have been proposed.

For example, in JP 2010-509495 A (Patent Document 1) and JP 2006-124824 A (Patent Document 2), a hairline is formed on the surface of a galvanized layer of a plated steel sheet having a galvanized layer to enhance the design of the plated steel sheet.

In addition, in JP 2017-136645 A (Patent Document 3) and JP 2013-536901 A (Patent Document 4), in a plated steel sheet having an electrolytic zinc-plated layer formed thereon, a hairline is formed on the surface of the electrolytic zinc-plated layer to enhance the design of the plated steel sheet.

Special Publication No. 2010-509495 JP 2006-124824 A JP 2017-136645 A Special Publication No. 2013-536901

In the above-mentioned patent document, the design of the plated steel sheet is improved by adjusting the surface roughness of the plating layer on which the hairline is formed.

The required visibility of the hairline varies depending on the application. For example, in the case of plated steel sheets for use in electrical equipment, it is required that the hairline be visible when the plated steel sheet is viewed from a close distance of less than 1 m. In this specification, a hairline that is visible is also referred to as having a high design quality.

The object of the present disclosure is to provide a plated steel sheet that can enhance the aesthetic appeal of the hairline when viewed from a close distance.

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. On the surface of the plating layer, a surface roughness profile is measured by a laser microscope in a measurement range of 5000 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 micro-areas n (n is an integer from 1 to 100) arranged at a pitch of 50 μm from the end of the measurement range, and a peak count RPc with a dead zone width of 360 nm is obtained for each micro-area n. The total number of first contrast parts in the measurement range where the absolute difference between the peak counts RPc of adjacent micro-areas n is 6 or more is defined as N. The total number of second contrast sections in which the absolute difference between the peak count RPc of microregion m (m is an integer from 1 to 98) and the peak count RPc of microregion m+1 is less than 6 and the absolute difference between the peak count RPc of microregion m and the peak count RPc of microregion m+2 is 6 or more is defined as M. The total number of third contrast sections in which the absolute difference between the peak count RPc of microregion k (k is an integer from 1 to 98) and the peak count RPc of microregion k+1 is 6 or more and the absolute difference between the peak count RPc of microregion k+1 and the peak count RPc of microregion k+2 is 6 or more is defined as K. In this case, the total number J of contrast sections defined by formula (1) is greater than 7.
J=N+M-K (1)

The plated steel sheet disclosed herein can enhance the aesthetic appeal of the hairline when viewed from a close distance.

FIG. 1 is a photographic image showing an example of a hairline formed on the surface of a zinc-based plating layer of a plated steel sheet. FIG. 2 is an SEM image of the surface of the plating layer on which hairlines are formed. FIG. 3A is a schematic diagram showing an example of the surface roughness of a plating layer in a cross section perpendicular to the extension direction of a hairline. FIG. 3B 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 hair line, which is different from FIG. 3A. FIG. 4 is a cross-sectional view of the plated steel sheet of the present embodiment. FIG. 5 is a plan view of the plated steel sheet shown in FIG. 4 as viewed from above the plating layer. FIG. 6 is a diagram showing an example of a surface roughness profile measured by a laser microscope. FIG. 7 is a schematic diagram for explaining a method for measuring the peak count RPc in one micro area n (n is an integer from 1 to 100). FIG. 8 is a diagram showing an example of the peak count RPc in the measurement range. FIG. 9 is a diagram showing an example of the peak count RPc in the measurement range, which is different from FIG.

The present inventors have investigated means for improving the design of a hairline formed on a plated steel sheet when viewed from a close distance. In this specification, close distance means a distance of less than 1 m from the hairline steel sheet. Also, high design quality means that the hairline is visible.

In conventional plated steel sheets with hairlines, the surface roughness (e.g., arithmetic mean roughness Ra) of the plating layer on which the hairlines are formed is adjusted. 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 depth of these grinding marks is increased to improve the visibility of the hairlines, thereby improving the visibility of the hairlines on the surface of the plating layer, as shown in Figure 1.

The above technical concept is premised on the premise that the contrast of the hairline correlates with the depth of the grinding marks (the height difference of the unevenness formed on the surface). In this case, it is believed that the contrast in the macroscopic region of the hairline visible to the naked eye correlates with the contrast of the unevenness in the microscopic region of the surface where the hairline is visible. Therefore, the inventors confirmed the contrast of the unevenness in the microscopic region of the surface of the plating layer where a hairline visible to the naked eye is formed, using the following method.

Specifically, the micro-area of the surface of the plating layer on which the hairline was formed was observed using a scanning electron microscope. FIG. 2 is an SEM image (secondary electron image: field area is 750 μm × 750 μm) of a micro-area of the surface of the plating layer on which the hairline was formed. Referring to FIG. 2, in the micro-area of the surface where the hairline was visible, grinding marks (concave and convex) extending in one direction were formed, and the contrast due to the depth of the grinding marks (height difference between the concave and convex parts) was confirmed. However, it was found that the macro contrast of the hairline visible to the naked eye may not necessarily correlate with the contrast of the unevenness in the micro-area shown in FIG. 2. In other words, the depth of the grinding marks (unevenness) on the surface of the plating layer in the plated steel sheet did not necessarily correlate with the design of the hairline when the plated steel sheet was viewed from a close distance.

Based on these new findings, the inventors considered adopting other means to improve the aesthetic appeal of the hairline when viewed at close range, rather than increasing the average height difference of the surface irregularities by deepening the grinding marks as has been done in the past.

As a result of the study, the inventors found that the visibility of the hairline is more significantly correlated with the distribution state of the grinding marks (density difference of the unevenness) than with the depth of the grinding marks (average height difference of the unevenness). Specifically, unevenness having a height difference that is visible as a contrast is defined as a "visible unevenness", and when the direction perpendicular to the extension direction of the hairline is divided into a plurality of micro-areas of a predetermined unit, if the density difference of the visible unevenness in adjacent micro-areas is large, the visibility of the contrast at the boundary between adjacent micro-areas is increased. As a result, the visibility of the hairline is increased. If the density difference of the visible unevenness in adjacent micro-areas is small, the visibility of the boundary between adjacent micro-areas is decreased. As a result, the visibility of the hairline is decreased. In the following explanation, the boundary between adjacent micro-areas where the density difference of the visible unevenness in adjacent micro-areas is large is referred to as a "contrast part". If there are many contrast parts, the visibility of the hairline is increased. In other words, by adjusting the density distribution of the visual irregularities, the visibility of the hairline can be improved.

Figure 3A is a schematic diagram showing the surface roughness of the plating layer in a cross section perpendicular to the extension direction of the hairline. With reference to Figure 3A, it is assumed that the surface of the plating layer is divided into a plurality of micro-regions 1 to 8 of a predetermined length L in a direction perpendicular to the extension direction of the hairline. Each of the micro-regions 1 to 8 has the same length. In each of the micro-regions 1 to 8, the density (pieces/L) of irregularities that are visible (visible irregularities) and have a predetermined height difference or more is obtained. In Figure 3A, it is assumed that the visible irregularity density in micro-region 1 is 10 pieces/L, the visible irregularity density in micro-region 2 is 2 pieces/L, and the visible irregularity density in micro-region 3 is 1 piece/L. In Figure 3A, the boundary where the difference in visible irregularity density between adjacent micro-regions is 6 pieces/L or more is sufficiently visible. Therefore, the boundary is recognized as a "contrast part". 3A , the visually recognizable irregularity density difference at the boundary between microregion 1 and microregion 2, the visually recognizable irregularity density difference at the boundary between microregion 3 and microregion 4, and the visually recognizable irregularity density difference at the boundary between microregion 7 and microregion 8 are each 6 pieces/L or more. Therefore, these boundaries correspond to contrast areas.

The inventors further found that when the hairline is observed from a close distance, if the interval between adjacent contrast parts is too narrow, it cannot be regarded as a visible unevenness, and the design of the hairline is reduced. For example, when the surface of the plating layer is as shown in FIG. 3B, the visible unevenness density difference at the boundary between micro-area 1 and micro-area 2, the visible unevenness density difference at the boundary between micro-area 2 and micro-area 3, the visible unevenness density difference at the boundary between micro-area 3 and micro-area 4, and the visible unevenness density difference at the boundary between micro-area 7 and micro-area 8 are each 6 pieces/L or more. Therefore, these boundaries correspond to contrast parts. In FIG. 3B, contrast parts exist continuously at adjacent boundaries (the boundary between micro-area 1 and micro-area 2, the boundary between micro-area 2 and micro-area 3, and the boundary between micro-area 3 and micro-area 4). When the hairline is viewed from a close distance, a thinner hairline can be viewed compared to when the hairline is viewed from a long distance. However, even at close range, if contrast portions are present consecutively, it is difficult to distinguish adjacent consecutive contrast portions. Therefore, when the hairline is observed at close range, multiple contrast portions are close to each other in the regions of microregions 1 to 4, and these contrast portions are recognized as one contrast portion. On the other hand, when the surface of the plating layer is as shown in FIG. 3A, the multiple contrast portions are arranged with gaps between each other and are not arranged consecutively. Therefore, when the hairline is observed at close range, each contrast portion is visible, enhancing the design of the hairline.

Based on the above findings, the inventors further investigated means for improving the aesthetic appeal of the hairline when viewed from a close distance. As a result, they found that the aesthetic appeal of the hairline when viewed from a close distance can be improved if the surface roughness profile of the measurement range, obtained by measuring the surface roughness profile of the surface of the zinc-based plating layer with a laser microscope over a 5,000 μm length in the direction perpendicular to the extension direction of the hairline, satisfies the following requirements:

Specifically, in the surface roughness profile of the measurement range, the measurement range is divided into 100 microregions n (n is an integer from 1 to 100) arranged at a length pitch of 50 μm from the end of the measurement range. For each microregion n, the peak count RPc is determined so that the dead zone width is 360 nm. The total number of first contrast parts in the measurement range where the absolute difference between the peak count RPc of adjacent microregions n is 6 or more is defined as N. The total number of second contrast parts where the absolute difference between the peak count RPc of microregion m (m is an integer from 1 to 98) and the peak count RPc of microregion m+1 is less than 6 and the absolute difference between the peak count RPc of microregion m and the peak count RPc of microregion m+2 is 6 or more is defined as M. The total number of third contrast portions in which the absolute difference between the peak count RPc of microregion k (k is an integer from 1 to 98) and the peak count RPc of microregion k+1 is 6 or more and the absolute difference between the peak count RPc of microregion k+1 and the peak count RPc of microregion k+2 is 6 or more is K. In this case, the total number J of contrast portions defined by formula (1) is greater than 7.
J=N+M-K (1)

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

[1]
A plated steel sheet,
A base steel plate;
A plating layer is formed on the base steel sheet, and has a hairline 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 having a length of 5000 μm in a direction perpendicular to the extension direction of the hairline, and the surface roughness profile in the measurement range is obtained;
The measurement range is divided into 100 micro-areas n (n is an integer from 1 to 100) arranged at a length pitch of 50 μm from the end of the measurement range, and a peak count RPc with a dead zone width of 360 nm is obtained for each of the micro-areas n;
In the measurement range,
The total number of first contrast portions in which the absolute difference between the peak counts RPc of adjacent micro regions n is 6 or more is defined as N,
a total number of second contrast portions in which the absolute difference between the peak count RPc of the microregion m (m is an integer from 1 to 98) and the peak count RPc of the microregion m+1 is less than 6 and the absolute difference between the peak count RPc of the microregion m and the peak count RPc of the microregion m+2 is 6 or more is defined as M;
the total number of third contrast portions in which the absolute difference between the peak count RPc of the microregion k (k is an integer from 1 to 98) and the peak count RPc of the microregion k+1 is 6 or more and the absolute difference between the peak count RPc of the microregion k+1 and the peak count RPc of the microregion k+2 is 6 or more is K, the total number J of contrast portions defined by formula (1) is greater than 7,
Galvanized steel sheet.
J=N+M-K (1)

[2]
The plated steel sheet according to [1],
The exposure rate of the base steel sheet when the plating layer is viewed in plan is less than 5.0%.
Galvanized steel sheet.

[3]
The plated steel sheet according to [1] or [2],
The plating layer is a zinc-based plating layer.
Galvanized steel sheet.

The plated steel sheet of this embodiment will be described below with reference to the drawings. In each drawing, the same or equivalent components are given the same reference symbols, and the same description will not be repeated.

[Regarding 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 sheet thickness direction of the plated steel sheet 1 is defined as TD. The sheet width direction perpendicular to the rolling direction RD and the sheet thickness direction TD of the plated steel sheet 1 is defined as WD.

With reference to FIG. 4, the plated steel sheet 1 comprises 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 only on one side of the base steel sheet 100. However, the plated steel sheet 1 may have the plating layer 10 formed on both sides of the base steel sheet 100.

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 plated 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 plated layer 10. The plated steel sheet 1 may include a chemical conversion coating formed on the plated layer 10, and a resin layer formed on the chemical conversion coating.

Figure 5 is a plan view of the plated steel sheet 1 shown in Figure 4, viewed from above the plating layer 10. With reference to Figure 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.

[Regarding base steel sheet 100]
As the base steel sheet 100, a known steel sheet applicable to a known plated steel sheet (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 each mechanical property (e.g., tensile strength, workability, etc.) required for 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 material use may be used. The base steel sheet 100 may be a hot-rolled steel sheet or a cold-rolled steel sheet.

The chemical composition of the base steel plate is, for example, in mass %, C: 0.01-0.25%, Si: 0.001-1.200%, Mn: 0.01-2.50%, P: 0.001-0.200%, S: 0.001-0.050%, sol. Al: 0.015-0.060%, and the balance is Fe and impurities.

[Regarding plating layer 10]
The plating layer 10 is formed on the surface of the base steel sheet 100. The type of the plating layer 10 is not particularly limited. The plating layer 10 is, for example, 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) means a plating layer mainly composed of X. Mainly composed of X means that the content of X, which is the main component element in the plating layer, is at least 50.0% or more. For example, a zinc-based plating layer means a plating layer with a Zn content of 50.0% or more.

From the viewpoint of the aesthetics of the formed hairline HL, the preferred plating layer 10 is 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 highly durable.

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 has a composition that is excellent in corrosion resistance even when the hairline HL is formed. Therefore, from the viewpoint of achieving both aesthetics and corrosion resistance, the plating layer 10 that is preferable is a zinc-based plating layer that has a sacrificial anticorrosion function and is excellent in corrosion resistance.

When the plating layer 10 is a zinc-based plating layer, as described above, it means that the Zn content in the plating layer is at least 50.0% or more. For example, the chemical composition of the zinc-based plating layer contains 50.0% or more Zn by mass%, and contains 0 to 50.0% in total of one or more 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 as optional elements, and may further contain 0 to 5.0% C as an optional element. The chemical composition of the zinc-based plating layer does not have to contain 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.

The preferred lower limit of the Zn content in the zinc-based plating layer is 80.0%, and more preferably 85.0%. In other words, 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 zinc alloy plating. The zinc-based plating layer is also 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.

[Method of measuring the chemical composition of plating layer 10]
The chemical composition of the above-mentioned plating layer 10 consisting of the X-based plating layer can be measured by the following method. A sample including a cross section parallel to the plate thickness direction TD of the plated steel sheet 1 and including the plating layer 10 is taken. The cross section parallel to the plate thickness direction TD is called the observation surface. In the observation surface of the sample, a line analysis based on the EPMA (Electron Probe Micro Analyzer) method is performed from the surface of the plated steel sheet 1 to the plate thickness direction TD to obtain 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 specified as the X-based plating layer. In the sample observation surface, a line analysis is performed at any 10 points, and the content (mass%) of each element in the range (line) of the X-based plating layer 10 specified by the line analysis is obtained. The arithmetic average value of the content of each element in the range of the X-based plating layer 10 obtained by the line analysis at 10 points is obtained. The chemical composition of the X-based plating layer 10 is specified based on the calculated arithmetic mean value of each element content.

[Thickness of plating layer 10]
There are no particular limitations on the thickness of the plating layer 10, 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.

[Method for measuring thickness of plating layer 10]
The thickness of the plating layer 10 can be obtained by the following method. A sample including a cross section parallel to the plated steel sheet 1 in the thickness direction TD and including the plating layer 10 is taken. The cross section parallel to the thickness direction is called the observation surface. The observation surface of the sample is mirror-polished. Using a scanning electron microscope, the observation surface after mirror polishing is observed at a magnification (500 to 3000 times) at which the entire length (thickness) of the plating layer 10 in the thickness direction TD can 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 any 10 points. The arithmetic average value of the thicknesses measured at the 10 points is defined as the thickness (μm) of the plating layer 10.

[About 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 plate width direction WD. As described in Fig. 2, in a minute region on the surface of the plating layer 10 on which the hairline HL is formed, a concave portion and a convex portion extending in one direction are formed.

[Contrast in microscopic areas]
As described above, the visibility of the hairline HL correlates with the density of the contrast portion, which is the boundary between adjacent micro-areas where the density difference between the visible irregularities is large 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 micro-areas. Furthermore, the design of the hairline HL when viewed from a close distance is also affected by the spacing between adjacent contrast portions. A method for identifying the contrast portion will be described below.

[Method of identifying contrast areas]
The contrast area is defined in the following way.
5, an arbitrary range of 5000 μm in length in a direction perpendicular to the extension direction of the hairline HL on the surface of the plating layer 10 is defined as a 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.
・Use a laser confocal type laser microscope.
The light source wavelength is 410 nm or less.
・Height display resolution is 1 nm or less.
- Width accuracy is within ±5% of measured value.
- The accuracy of the height measurement can be guaranteed when observing a length of 5000 μm in one field of view.
An example of a laser microscope that satisfies the above conditions is a shape analysis laser microscope VK-X250 manufactured by Keyence Corporation. If the accuracy of the measured value in the height direction (Z-axis direction) cannot be guaranteed when observing a length of 5000 μm in one field of view, multiple consecutive images observed at a magnification that can guarantee the accuracy of the measured value in the Z-axis direction may be linked together and measured as one profile. An example of the obtained profile is shown in FIG. 6.

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. An example of the stripping agent is Neo River S-701, a product name manufactured by Sansai Kako Co., Ltd.

A laser microscope measures the surface shape optically. The 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 roughness profile may decrease. 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.

Using the surface roughness profile obtained in the measurement range MR, and with reference to JIS B0601:2013, the peak count RPc is determined in the following manner. In the surface roughness profile, the measurement range MR is divided into 100 microregions n (n is an integer from 1 to 100) arranged at a pitch of 50 μm from the end P of the measurement range MR. Then, the peak count RPc is determined for each of the 100 microregions n.

The peak count RPc is obtained by the following method. Figure 7 is a schematic diagram for explaining the method of measuring the peak count RPc in one micro area n. As shown in Figure 7, the measured cross-sectional curve is processed with a cutoff wavelength λ = 0.08 mm to obtain a surface roughness profile C1. A dead zone DB with a width of 360 nm is provided with the mean line X of the obtained surface roughness profile C1 as the center. 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 goes above the dead zone DB to the point where the surface roughness profile goes below the dead zone DB again after going above the dead zone DB. In the example of Figure 7, the peak count is six. The dead zone DB is set to 360 nm because 360 nm is the minimum wavelength of visible light. In other words, the unevenness counted as the peak count RPc is a visible unevenness. An unevenness with a height difference larger than the dead zone DB is defined as a "visible unevenness". The peak count RPc for each micro area n is obtained by the above method. The peak count of the micro-area n means the density of the visible irregularities in the micro-area n.

Based on the peak count RPc of each micro-area obtained, a contrast portion is identified, which is a boundary portion where the visual irregularities of adjacent micro-areas n have a predetermined density difference. Specifically, the total number N of first contrast portions, the total number M of second contrast portions, and the total number K of third contrast portions are identified by the following method.

(1) In the measurement range MR, the absolute difference in the peak count RPc of adjacent micro areas n means the density difference of the visible irregularities of adjacent micro areas n and n+1. Therefore, a region where the absolute difference in the peak count RPc of adjacent micro areas n is 6 or more is defined as a "first contrast portion." The first contrast portion is a region that is visible from a close distance. The total number of first contrast portions in the measurement range MR is N.
(2) In the measurement range MR, a region where the absolute difference between the peak count RPc of microregion m (m is an integer from 1 to 98) and the peak count RPc of microregion m+1 is less than 6 and the absolute difference between the peak count RPc of microregion m and the peak count RPc of microregion m+2 is 6 or more is defined as a "second contrast region." The second contrast region is a region that is visible from a close distance, similar to the first contrast region. The total number of second contrast regions in the measurement range MR is M.
(3) A region where the absolute difference between the peak count RPc of microregion k (k is an integer from 1 to 98) and the peak count RPc of microregion k+1 is 6 or more, and the absolute difference between the peak count RPc of microregion k+1 and the peak count RPc of microregion k+2 is 6 or more is defined as a "third contrast region." The third contrast is a region where two contrast regions are adjacent to each other and therefore are visually recognized as one contrast. The total number of third contrast regions in the measurement range MR is K.

Using the total number N of first contrast sections, the total number M of second contrast sections, and the total number K of third contrast sections obtained by the above methods, the total number J of contrast sections in the measurement range MR is defined by equation (1).
J=N+M-K (1)

The total number J of contrast parts means the density of the boundaries (contrast parts) of the micro-areas visible in the measurement range MR, and means the density of the visible hairlines HL. When the total number J of contrast parts is greater than 7, the density of contrast visible from a close distance is sufficiently high. Therefore, the design quality of the hairlines is enhanced when viewed from a close distance.

Figure 8 is an example of a peak count RPc in the measurement range MR. In the measurement range MR in Figure 8, the total number N of first contrast sections (denoted by N in Figure 8) is 10, the total number M of second contrast sections (denoted by M in Figure 8) is 8, and the total number K of third contrast sections (denoted by K in Figure 8) is 1. Therefore, the total number J of contrast sections in the measurement range MR is 17. Therefore, in the case of Figure 8, the design of the hairline is enhanced when viewed from a close distance.

Figure 9 is an example of peak count RPc in the measurement range MR, different from Figure 8. In the measurement range MR in Figure 9, the total number N of first contrast sections (denoted by N in Figure 9) is 5, the total number M of second contrast sections (denoted by M in Figure 9) is 3, and the total number K of third contrast sections (denoted by K in Figure 9) is 1. Therefore, the total number J of contrast sections in the measurement range MR is 7. Therefore, in the case of Figure 9, the design quality of the hairline is low when viewed from a close distance.

As described above, in the plated steel sheet 1 of this embodiment, the total number J of contrast parts derived from the peak count RPc in the micro-areas divided at 50 μm pitch within the measurement range MR is greater than 7. In this case, the density of contrast parts on the surface of the plating layer 10 is sufficiently high. Therefore, the design of the hairline is improved when viewed from a close distance.

The preferred lower limit of the total number J is 10, and more preferably 14.

[Preferable Exposure Rate of Base Steel Sheet 100 When Plated Steel Sheet 1 is Viewed in Plane]
A preferable exposure rate of the base steel sheet 100 when the plating layer 10 of the plated steel sheet 1 is viewed in plan 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) is obtained in the plated steel sheet 1.

In the plated steel sheet 1 of this embodiment, unlike conventional plated steel sheets, the visibility and design of the hairline HL are not improved by increasing the average height difference (depth of grinding marks, etc.) of the unevenness of the surface of the plated layer 10, but by increasing the density of the contrast portion described above, the design of the hairline HL is improved when viewed from a close distance. Therefore, unlike conventional steel sheets with hairlines, the plated steel sheet 1 of this embodiment does not need to grind the plated layer 10 more than necessary to improve the design of the hairline HL. As a result, the plated steel sheet 1 is easy to suppress the exposure rate of the base steel sheet 100. In particular, when the plated steel sheet 1 is an electroplated layer, the electroplated layer may be formed thin, with a thickness of 1.5 to 6.0 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 close distance can be improved while ensuring long-term corrosion resistance.

[Exposure rate measurement method]
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-mentioned [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 is 50.0% or more by mass% is defined as the main component element X. Next, using the plated steel sheet 1 from which the sample of [Method for measuring the chemical composition of the plated layer 10] is taken, five arbitrary rectangular areas of 1 mm x 1 mm are selected when the plated steel sheet 1 is viewed from above the plated layer 10 (when the plated steel sheet 1 is viewed in plan). EPMA analysis (area analysis) is performed on the selected rectangular areas. By image analysis, a region in each rectangular area where the main component element X of the plating is not detected (plating undetected region) is identified. In this embodiment, a region where the detection intensity of the plating main component element X is 1/16 or less of the detection intensity when a standard sample (pure metal sample) is measured is recognized as a plating main component element undetected region. The ratio (%) of the total area of the regions where the plating main component elements are not detected to the total area of the five rectangular regions is defined as the exposure rate (%) of the base steel sheet 100.

[Another embodiment of the plated steel sheet 1 according to the present embodiment]
The plated steel sheet 1 of the present embodiment is not limited to the above-mentioned embodiment. As described above, in the plated steel sheet 1 of the present embodiment, one or more resin layers may be formed on the surface of the plating layer 10. In the plated steel sheet 1 of the present embodiment, a chemical conversion coating may also be formed on the surface of the plating layer 10. In the plated steel sheet 1 of the present embodiment, a chemical conversion coating may also be formed on the surface of the plating layer 10, and a resin layer may also be formed on the chemical conversion coating.

[Method of 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.

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) Plating layer formation step (Step 3) Hairline formation step Each step will be described below.

[(Process 1) Base material steel plate preparation process]
In the base steel sheet preparation step, there is prepared the above-mentioned base steel sheet 100. As described above, the base steel sheet 100 may be a hot-rolled steel sheet or a cold-rolled steel sheet.

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

[Electrolytic zinc plating method]
When the zinc-based plating layer 10 is formed by electrogalvanizing, the electrogalvanizing may be performed by a known method. In this specification, the electrogalvanizing includes electrogalvanizing of a zinc alloy. The electrogalvanizing may be performed using a known plating solution. The electrogalvanizing solution may be, for example, a sulfate bath, a chloride bath, a zincate bath, a cyanide bath, a pyrophosphate bath, a boric acid bath, a citric acid bath, a complex bath, or a combination 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. In addition, an organic additive may be added to the plating solution to obtain a desired effect such as a leveling effect or an increase in hardness.

[Hot-dip galvanizing method]
When forming the zinc-based coating layer 10 by hot-dip galvanizing, the hot-dip galvanizing may be performed by a known method. A known coating bath may be used as the coating bath for the hot-dip galvanizing. For example, a hot-dip galvanizing bath containing Al and the balance consisting of Zn and impurities may be used. The impurities may be Fe. 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, Ca, Y, La, Ce, Sn, Bi, In, Cr, Ti, Ni, Co, V, Nb, Cu, Mn, Sr, Sb, Pb, and B.

[(Step 3) Hairline formation step]
In the hairline forming step, a 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 .

The grinding device used for hairline processing includes a conveying roll, a pair of contact rolls, and a ring-shaped abrasive belt. The pair of contact rolls hold the ring-shaped abrasive belt. As the pair of contact rolls rotate, the ring-shaped abrasive belt moves between the pair of contact rolls. The base steel sheet 10 having a plating layer 10 on its surface is conveyed between the conveying roll and the ring-shaped abrasive belt by the conveying roll. The ring-shaped abrasive belt is pressed against the surface of the plating layer 10. As the pair of contact rolls rotate, the ring-shaped 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.

In the grinding process, the speed at which the plated steel sheet 1 passes 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 plating layer 10 ground in the grinding 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.

Preferably, the grain size and/or density of the abrasive grains in the width direction of the abrasive belt or grinding brush are distributed differently. Here, the width direction of the abrasive belt and grinding brush corresponds to the plate width direction of the base steel plate 100.

The abrasive belt and grinding brush used in conventional hairline processing are set so that the grain size and density of the abrasive grains are uniform in the width direction. Conventionally, the grain size of the abrasive grains was increased to improve the visibility of the hairline. In contrast, in the manufacturing method of this embodiment, the grain size and/or density of the abrasive grains in the width direction of the abrasive belt or grinding brush are distributed differently. This makes it possible to adjust the density of the visible unevenness in each micro-area n in the direction perpendicular to the extension direction of the hairline HL, and to adjust the density of the contrast area. The grain size and density distribution of the abrasive grains in the width direction of the abrasive belt and grinding brush can be adjusted as appropriate depending on the desired appearance.

When a grinding brush is used for hairline processing, the ring-shaped grinding brush includes a central shaft portion and a plurality of brush bristles extending radially from the central shaft portion. The brush bristles are arranged in the axial direction of the grinding brush. A plurality of brush bristles with different abrasive grain densities are arranged in the axial direction. Hairline processing is performed using such a grinding brush. By changing the abrasive grain density of the brush bristles in the axial 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 brush bristles of the grinding brush is adjusted so that the total number J of contrast portions defined by formula (1) on the surface of the plating layer is greater than 7.

When an abrasive belt is used for hairline processing, the abrasive grain density is changed in the width direction of the abrasive belt. This makes it possible to change the density of the grinding marks (unevenness) formed by the abrasive belt on the surface of the plating layer. The abrasive grain density of the abrasive belt is adjusted so that the total number J of contrast areas on the surface of the plating layer, defined by formula (1), is greater than 7.

The effects of one aspect of the present invention will be explained in more detail below using 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.

Plated steel sheets with the test numbers listed 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 process was carried out on each base steel sheet to obtain various plating thicknesses. Specifically, an electrolytic zinc plating process was carried out to form an electrolytic zinc plating layer as the plating layer. The thickness of the plating layer was 0.5 to 25.0 μm. The thickness of the plating layer was measured in accordance with the above-mentioned [Method for measuring the thickness of plating layer 10].

A hairline formation process was carried out on the surface of the plating layer on the base steel sheet of each test number. In the hairline formation process, multiple abrasive belts with different distributions of abrasive grain density in the width direction of the abrasive belt were prepared and hairlines were formed. Through the above manufacturing process, plated steel sheets with hairlines were manufactured.

The plated steel sheet on which the hairline had been formed 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

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 time to produce an organosilicon compound. The produced organosilicon compound was further mixed with phosphoric acid, a phosphoric acid compound, to produce a treatment liquid.

The treatment liquid was picked up by a roll and transferred onto the plating layer so that the deposition amount of the chemical conversion coating after baking and drying was 0.3 g/ ^m2 .

The steel sheet with the treatment liquid transferred onto the plating layer was baked and dried. Specifically, the steel sheet with the treatment liquid transferred onto the plating layer was placed in a furnace maintained at 180°C, and the steel sheet was 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 the above process, a chemical conversion coating was formed on the plating layer.

A resin layer was formed on the steel plate on which the chemical conversion coating had been formed. A urethane-based resin (manufactured by ADEKA Corporation under the product name HUX-232) was used as the binder resin for the resin coating. Polyethylene-based resin particles (manufactured by Mitsui Chemicals Inc. under the product name Chemipearl) were used as the resin particles. The above-mentioned binder resin and resin particles were dispersed in water to prepare a paint.

The prepared paint 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 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. Through the above steps, a resin layer was formed.

[Evaluation test]
The following evaluation tests were carried out on the plated steel sheets having each test number.
(Test 1) Measurement of the total number J of contrast parts (Test 2) Measurement of grinding mark depth (Test 3) Measurement of base steel plate exposure rate (Test 4) Evaluation test of hairline design at close range (Test 5) Evaluation test of corrosion resistance Tests 1 to 5 are described below.

[(Test 1) Measurement test of total number J of contrast parts]
For the plated steel sheet of each test number, the total number N of first contrast parts, the total number M of second contrast parts, and the total number K of third contrast parts were determined according to the method described in [Method of identifying contrast parts] above, and the total number J of contrast parts was determined. Note that as the laser microscope, a shape analysis laser microscope VK-X250 manufactured by Keyence Corporation was used. In addition, the resin layer and the chemical conversion coating were removed with a stripping agent. As the stripping agent, Neo River S-701 manufactured by Sansai Kako Co., Ltd. was used. The determined N, M, K, and J are shown in Table 1.

[(Test 2) Measurement of grinding mark depth]
The difference between the maximum height and the minimum height of the surface roughness profile C1 obtained by the above-mentioned [Method for identifying contrast parts] for the plated steel sheet of each test number was defined as the grinding mark depth (μm). The grinding mark depth (μm) for the plated steel sheet of each test number is shown in Table 1.

[(Test 3) Base Steel Sheet Exposure Rate Measurement Test]
For each plated steel sheet with each test number, the exposed ratio of the base steel sheet was measured according to the method described in the above [Method of measuring exposed ratio]. The main plating component element was Zn. The measured exposed ratios (%) are shown in Table 1.

[(Test 4) Hairline design evaluation test at close range]
The design of the hairline at a close distance was evaluated for each plated steel sheet of each test number by the following method. First, the plated steel sheet of each test number was installed indoors at an angle of 60° to the horizontal. At this time, the plated steel sheet was installed so that the plating layer was facing the ceiling side of the room. Next, an observer visually observed the plated steel sheet from a position 50 cm away from the position where the plated steel sheet was installed on the floor of the room, facing the plated steel sheet. At this time, the observer's line of sight was at the same height as the plated steel sheet. A total of 10 observers visually observed and performed a sensory evaluation to determine whether the design was excellent. Specifically, if the hairline was clearly visible by visual observation, it was judged to be excellent in design. A score was given based on the number of people who judged the design to be excellent, and a score of B or higher was judged to be a pass. The evaluation results (A to C) are shown in the "Design" column of the "Evaluation Results" column in Table 1.
Eight or more judges the design to be excellent: A
Five or more but less than eight judged the design to be excellent: Rating B
Less than 5 people judged the design to be excellent: Rating C

[(Test 5) Corrosion Resistance Evaluation Test]
The corrosion resistance (long-term corrosion resistance) of the plated steel sheets of each test number was evaluated by the following method. A test piece of 75 mm x 100 mm x sheet thickness was taken from the plated steel sheets of each test number. The surface of the test piece of 75 mm x 100 mm was used as the measurement surface. The end face and the back face of the test piece were protected with tape seal. Then, a salt spray test of 5% NaCl held at 35 ° C was carried out in accordance with JIS Z 2371 (2015). The test was carried out for 240 hours, and the rust occurrence rate after the test was obtained. Specifically, the area where rust occurred on the measurement surface after the test was visually obtained. The rust occurrence rate (%), which is the area ratio where rust occurred, was obtained 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 evaluated as passing (indicated as "E (Excellent)" in the "Corrosion Resistance" column of the "Evaluation Results" column in Table 1). On the other hand, when the rust occurrence rate was 5% or more, the specimen was evaluated as failing (shown as a rating of "B (Bad)" in the "Corrosion Resistance" column of the "Evaluation Results" column in Table 1).

[Evaluation Results]
The evaluation results are shown in Table 1. In the plated steel sheets of test numbers 1 to 10, the total number J of contrast portions was greater than 7. Therefore, the plated steel sheets of these test numbers had excellent design properties. Furthermore, in test numbers 1 to 7, the exposed rate was less than 5.0%. Therefore, the plated steel sheets of these test numbers not only had excellent design properties but also excellent corrosion resistance.

On the other hand, in the plated steel sheets of test numbers 11 to 13, the total number J of contrast areas was 7 or less. Therefore, excellent design was not achieved.

As described above, it was found that by increasing the total number J of contrast parts to more than 7, excellent design can be obtained even if the grinding mark depth is relatively shallow.

The above describes an embodiment of the present disclosure. However, the above-described embodiment is merely an example for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment within the scope of the spirit of the present disclosure.

1 Plated steel sheet 10 Plated layer HL Hairline

Claims (3)

A plated steel sheet,
A base steel plate;
A plating layer is formed on the base steel sheet, and has a hairline 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 having a length of 5000 μm in a direction perpendicular to the extension direction of the hairline, and the surface roughness profile in the measurement range is obtained;
The measurement range is divided into 100 micro-areas n (n is an integer from 1 to 100) arranged at a length pitch of 50 μm from the end of the measurement range, and a peak count RPc with a dead zone width of 360 nm is obtained for each of the micro-areas n;
In the measurement range,
The total number of first contrast portions in which the absolute difference between the peak counts RPc of adjacent micro regions n is 6 or more is defined as N,
a total number of second contrast portions in which the absolute difference between the peak count RPc of the microregion m (m is an integer from 1 to 98) and the peak count RPc of the microregion m+1 is less than 6 and the absolute difference between the peak count RPc of the microregion m and the peak count RPc of the microregion m+2 is 6 or more is defined as M;
the total number of third contrast portions in which the absolute difference between the peak count RPc of the microregion k (k is an integer from 1 to 98) and the peak count RPc of the microregion k+1 is 6 or more and the absolute difference between the peak count RPc of the microregion k+1 and the peak count RPc of the microregion k+2 is 6 or more is K, the total number J of contrast portions defined by formula (1) is greater than 7,
Galvanized steel sheet.
J=N+M-K (1) The plated steel sheet according to claim 1,
The exposure rate of the base steel sheet when the plating layer is viewed in plan is less than 5.0%.
Galvanized steel sheet. The plated steel sheet according to claim 1 or 2,
The plating layer is a zinc-based plating layer.
Galvanized steel sheet.

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