Ferritic stainless steel plate

Abstract

This ferritic stainless steel sheet has excellent anti-glare properties, scratch inconspicuousness, roughness resistance, and corrosion resistance. Sa is 10.0 μm or more and 40.0 μm or less, and Spd is 1500 particles / mm 2 That's all.

Description

[Technical Field]

[0001] The present invention relates to a ferritic stainless steel sheet. [Background technology]

[0002] Stainless steel, particularly ferritic stainless steel sheets (hereinafter also simply referred to as steel sheets), are widely used in places that are easily seen and touched by people, such as sinks, commercial refrigerators, ice makers, showcases, roofs, exterior walls, escalators, elevators, etc. Therefore, in such applications, excellent design is required in addition to excellent corrosion resistance.

[0003] As a technique relating to stainless steel, for example, Patent Document 1 describes: "A steel plate to be processed into shutter components for building materials, made of ferritic stainless steel with a total Cr content and Mo content of 18% by weight or more but less than 27% by weight, with a surface roughness of Rz ≥ 2 μm formed by cold rolling, and a tensile strength of 600 N / mm 2 A ferritic stainless steel sheet for shutter members having excellent weather resistance and scratch resistance as follows: has been disclosed.

[0004] Patent Document 2 states: "A method for producing a ferritic stainless steel sheet having a thickness of 0.5 mm or less, comprising: a cold rolling step in which an intermediate material having a thickness of 3.0 mm or less is cold rolled at a reduction rate of 5 to 45% and a forward slip rate of 1 to 8%; and a shape correction step in which, after the cold rolling step, a shape correction is performed at an elongation rate of 0.1 to 0.8% to obtain a ferritic stainless steel sheet having a thickness of 0.5 mm or less." has been disclosed.

[0005] Patent Document 3 states: "A mirror-finished duplex stainless steel having a composition containing, by mass%, 0.01 to 0.2% C, 0.01 to 2.0% Si, 0.1 to 4.0% Mn, 0.05% or less P, 0.03% or less S, 10 to 20% Cr, 0.01 to 4.0% Ni, 0.15% or less N, 0.01% or less O, with the balance being Fe and unavoidable impurities; containing a ferrite phase and a martensite phase; having a hardness of 200 to 350 HV, a waviness of 2.0 μm or less, and a surface arithmetic mean roughness (Ra) of 0.1 μm or less; in any cross section, the area fraction of the martensite phase is 60 to 80%, the area fraction of carbides is 0.5 to 2.0%, and the major axis of each of the carbides is 1 μm or less." has been disclosed. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 10-168549 [Patent Document 2] Japanese Patent Publication No. 2020-163402 [Patent Document 3] Japanese Patent Publication No. 2022-151322 Summary of the Invention [Problem to be solved by the invention]

[0007] In recent years, the above-mentioned applications have required steel sheets that are excellent in design, particularly in anti-glare properties, scratch inconspicuousness, and roughness resistance.

[0008] Here, anti-glare property refers to the ability to suppress reflection of external light on the surface of a steel sheet (hereinafter also referred to as sheet surface). In the above applications, particularly in building materials such as roofs and exterior walls, as well as in the interior of commercial refrigerators, reflection of lighting, people, and objects on the sheet surface can be a problem. Therefore, steel sheets with excellent anti-glare properties are required.

[0009] Scratch conspicuousness refers to the ease with which scratches are made to the sheet surface. In the above applications, scratches often occur on the sheet surface. Therefore, there is a demand for steel sheets that are difficult to notice even when scratches are made to the sheet surface, in other words, steel sheets with excellent scratch conspicuousness.

[0010] Roughness resistance indicates the ease with which a sheet surface can be made to look fine without appearing rough. A fine-grained sheet surface gives a more subdued and luxurious impression. Therefore, steel sheets with excellent roughness resistance are required.

[0011] However, none of the techniques disclosed in Patent Documents 1 to 3 can be said to simultaneously provide excellent antiglare properties, scratch inconspicuousness, and roughness resistance. Therefore, particularly for the above-mentioned applications, there is currently a demand for the development of steel sheets that have excellent antiglare properties, scratch inconspicuousness, and roughness resistance in addition to excellent corrosion resistance.

[0012] The present invention was developed in consideration of the above-mentioned current situation, and aims to provide a ferritic stainless steel sheet having excellent antiglare properties, scratch inconspicuousness, roughness resistance, and corrosion resistance. In this disclosure, any numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits, respectively. [Means for solving the problem]

[0013] The inventors have conducted extensive research to achieve the above-mentioned object. Generally, glossiness is used as an index for evaluating design. Glossiness is an index of the intensity of light that causes specular reflection and the intensity of light that causes diffuse reflection, among the light incident on a plate surface. However, glossiness alone cannot adequately evaluate antiglare properties, scratch conspicuity, and roughness resistance. Furthermore, visual evaluation of appearance by humans can result in variations in results depending on the evaluator. Furthermore, Patent Documents 1 to 3 mentioned above do not disclose a method for quantitatively evaluating antiglare properties, scratch conspicuity, and roughness resistance.

[0014] As will be described later, generally, the higher the whiteness of the plate surface, the less noticeable scratches become. On the other hand, the rougher the surface appears. The reasons for this are as follows.

[0015] That is, the higher the whiteness, the more light is diffusely reflected. To improve whiteness, it is desirable to diffusely reflect more light. To achieve this, a surface roughening treatment is often performed to create irregularities on the surface. However, this roughening treatment lengthens the longest period of the fluctuations in the light reflection characteristics from one location to another, as described below, and the surface appears rough. Therefore, it is difficult to simultaneously improve anti-glare properties, scratch inconspicuousness, and roughness resistance, especially scratch inconspicuousness and roughness resistance.

[0016] Therefore, the inventors first investigated a method for quantitatively evaluating (i) antiglare property, (ii) scratch inconspicuousness, and (iii) roughness resistance, and developed the evaluation method described below. Based on the knowledge gained during the development of the evaluation method, the inventors then conducted further studies. Furthermore, the inventors produced various steel sheets and evaluated their antiglare property, scratch inconspicuousness, and roughness resistance based on the evaluation method described below. As a result, the inventors obtained the following findings. - A large number of fine irregularities (hereinafter referred to as fine irregularities) are provided on the plate surface, and the period of the fine irregularities is shortened. In particular, the surface properties of ferritic stainless steel sheets Sa is 10.0 μm or more and 40.0 μm or less, and Spd 1500 pieces / mm 2 That's all. Here, Sa and Spd are surface texture parameters defined in ISO25178, Sa is the arithmetic mean height, and Spd is the peak density. This ensures excellent corrosion resistance while simultaneously providing excellent anti-glare properties, scratch inconspicuousness, and roughness resistance.

[0017] The inventors have also discovered the following: The sheet surface is roughened by cold rolling and pickling to form fine irregularities, and then the sheet surface is ground with a brush. Specifically, the conditions described in [2] below are satisfied. This makes it possible to produce a steel sheet having the above-mentioned surface properties.

[0018] Next, the methods for evaluating (i) anti-glare properties, (ii) scratch inconspicuousness, and (iii) roughness resistance developed by the inventors will be described in detail.

[0019] (a) Anti-glare Generally, the lower the gloss level, the less light reflection there is and the better the anti-glare properties. Based on this, the inventors conducted extensive research and discovered that anti-glare properties can be quantitatively evaluated using GS(60°). Here, GS(60°) is the 60-degree specular gloss specified in JIS Z 8741:1997.

[0020] (b) Scratch inconspicuousness Scratches on a plate surface appear whitish. Therefore, the higher the whiteness of the plate surface, the less visible the scratches become. However, in reality, even if the whiteness is the same, the ease of scratch visibility, or the degree of scratch conspicuity, varies. The inventors investigated a method for quantitatively evaluating this scratch conspicuity. As a result, they discovered that in addition to L*, which is an index of whiteness, scratch conspicuity can be quantitatively evaluated using a scratch conspicuity index, specifically, the difference in luminance between the scratched and non-scratched areas in a grayscale image of the plate surface obtained by performing the following scratch application test and then photographing it under the following photographing conditions (hereinafter also referred to as the luminance difference between the scratched and non-scratched areas). Here, L* is the lightness index defined in JIS Z 8781-4:2013. [Scratch test] Grinding paper with abrasive grain size of #240 (hereinafter simply referred to as #240) was pressed against the surface of the ferritic stainless steel plate with a pressing load of 0.01 N / mm. 2The ferritic stainless steel plate is moved in the rolling direction at a speed of 10 mm / s, and scratches are created on the surface of the ferritic stainless steel plate. The area on the surface of the ferritic stainless steel plate other than the scratches is defined as the non-scratched area. The pressing load is set to 10 mm per unit area (1 mm 2 ) is the load per [Photography conditions] The background color reflected on the surface of the ferritic stainless steel plate is set to black, and light is irradiated from a direction perpendicular to the surface of the ferritic stainless steel plate.

[0021] That is, the appearance of a steel sheet changes significantly depending on how light hits the sheet surface, the color reflected on the sheet surface, etc. In light of this, the inventors have conducted extensive research and have discovered the following points. The whiter the surface of the board, the less noticeable the scratches will be, and the darker the surface of the board, the more noticeable the scratches will be. When illuminating the board surface with a light source, scratches are more noticeable if the light is installed directly above the board rather than on either side.

[0022] Therefore, by creating a scratch on the surface of the plate, setting the background (screen) reflected on the surface to black, and shining light perpendicular to the surface of the plate, the surface is photographed, and the brightness difference between the scratched and non-scratched areas in the resulting grayscale image can be used to quantitatively evaluate the scratch inconspicuousness.

[0023] As an example, as shown in Figure 1, a 30 mm diameter #240 grinding paper is attached to the tip of a robot arm (not shown) with a load control function via a rubber plug. Next, the grinding paper attached to the tip of the robot arm is pressed against a test piece taken from the steel plate, and a pressing load of 0.01 N / mm is applied. 2 The test piece was moved in the rolling direction of the steel plate at a speed of 10 mm / s to create scratches on the surface of the test piece. In the figure, reference numeral 1 denotes the test piece, 2 denotes the rubber stopper, and 3 denotes the abrasive paper.

[0024] Next, as shown in Figure 2, a light source is placed directly above the scratch on the test specimen, shining light onto the surface of the test specimen so that a black screen serving as the background is reflected on the surface of the test specimen. The surface of the test specimen is then photographed using a photographing device, such as a digital camera. In the figure, reference numeral 4 denotes the photographing device, 5 denotes the light source (lighting), and 6 denotes the background (screen). This results in a color image of the test specimen surface (five-dimensional information expressed as x: horizontal position, y: vertical position, R: brightness value (red), G: brightness value (green), and B: brightness value (blue)). The brightness value represents the brightness of each pixel (x, y) that makes up the image.

[0025] Next, as shown in Figure 3, 10 mm x 10 mm ROIs (regions of interest, target areas for image analysis) were set in the scratched and non-scratched areas of the obtained image. In the figure, reference numeral 7 denotes the scratched area, 8 denotes the non-scratched area, 9 denotes the ROI for the scratched area, and 10 denotes the ROI for the non-scratched area. As shown in Figure 2, under the above shooting conditions, the camera was tilted toward the test piece to project a black screen onto the surface of the test piece. This may result in slight differences in brightness between the sides closer to and further from the camera. Therefore, when taking the image, the test piece was positioned so that the shooting direction was approximately perpendicular to the direction in which the scratch extended (the rolling direction of the steel sheet). At the same time, ROIs were set in two non-scratched areas located on both sides of the scratch (the side closer to the camera and the side farther from the camera) in the shooting direction. Here, the shooting direction was the direction connecting the center of the camera and the center of the test piece, as viewed perpendicular to the surface of the test piece. The color image with the ROI set as described above is then grayscaled in accordance with ITU-R BT.601, where the luminance value L of each pixel is calculated using R x 0.299 + G x 0.587 + B x 0.114 to obtain a grayscale image. In the ROI of the scratched area in the obtained grayscale image, the luminance values ​​of the pixels in the top 10% with the highest luminance values ​​L are extracted, and their average value is used as the luminance of the scratched area. In addition, the average value of the luminance values ​​in the ROIs of the two non-scratched areas in the obtained grayscale image is used as the luminance of the non-scratched area. The difference in luminance between the scratched area and the non-scratched area is used as the scratch conspicuity index.

[0026] In addition, in the visual appearance evaluation by in-house evaluators other than the inventor, if L* was 50 or more and the scratch conspicuity index was 50 or less, all of the in-house evaluators rated the product as having low scratch visibility (a rating of second or higher on a five-point scale).

[0027] (c) Roughness resistance The inventors conducted extensive research and found that even when gloss and whiteness are the same, the impression given by the appearance often differs. After extensive investigation into the reasons for this, the inventors discovered that the difference in the impression given by the appearance is due to the difference in the degree of conspicuousness of the grain and streak patterns on the sheet surface. For example, when a dull texture is imparted to the sheet surface using rolls with different surface roughnesses, all of these steel sheets have equivalent gloss and whiteness values. However, the smaller the surface roughness of the sheet surface, the less conspicuous the grain pattern is, giving the impression that the sheet surface appears finer, in other words, a more subdued and luxurious impression. Conversely, the greater the surface roughness of the sheet surface, the more conspicuous the grain pattern is, giving the impression that the sheet surface appears rough. Hereinafter, grain and streak patterns are collectively referred to as "roughness." Furthermore, a sheet surface with inconspicuous roughness is referred to as "fine" and a sheet surface with noticeable roughness is referred to as "rough."

[0028] Even if indices such as glossiness and whiteness are equivalent, the resistance to roughness may differ. In other words, the reason why the resistance to roughness cannot be evaluated using these indices is that the degree of roughness depends on the fluctuations (differences) in the light reflection characteristics (e.g., the amount of specularly reflected light and the amount of diffusely reflected light) from location to location within a small area. Glossiness and whiteness are the average values ​​of specific light reflection characteristics of the plate surface in a specific measurement area. Therefore, no matter how much the light reflection characteristics of each small area fluctuate (in other words, no matter how much the light reflection characteristics vary from one small area to another), the fluctuations do not affect the characteristic values ​​of glossiness and whiteness.

[0029] Therefore, the inventors conducted extensive research into parameters that can quantitatively evaluate roughness resistance. As a result, they discovered the following: The degree of visibility of roughness on a plate surface is affected by the fluctuations in light reflection characteristics at each location within the above-mentioned microscopic region, in particular the period of the fluctuations within the plate surface (a value indicating how often, in the in-plane direction of the plate surface, an area with a high amount of specularly reflected light and an area with a low amount of specularly reflected light are repeated; or a value indicating how often, in the plate surface, an area with a high amount of diffusely reflected light and an area with a low amount of diffusely reflected light are repeated). In particular, when there are multiple types of "local fluctuations in light reflection characteristics" with different periods in various directions on the plate surface, the period of the "local fluctuations in light reflection characteristics" with the longest period (hereinafter also referred to as the longest period of local fluctuations in light reflection characteristics) significantly affects the roughness resistance of the plate surface. For example, the longer the longest period of the fluctuations in light reflection characteristics from place to place, the more noticeable the roughness of the plate surface will be, and conversely, the shorter the period, the less noticeable the roughness of the plate surface will be, and the plate surface will appear more uniform (in other words, finer).

[0030] Based on the above findings, the inventors thought that it might be possible to quantitatively evaluate the roughness resistance of a plate surface by quantifying the longest period of the location-by-location fluctuations in light reflection characteristics, and further investigated methods for doing so. As a result, the inventors discovered that it is possible to quantitatively evaluate the roughness resistance of a plate surface using the maximum autocorrelation length obtained by numerically analyzing a photographic image of the plate surface, and further discovered that, in particular, excellent roughness resistance (i.e., an appearance that gives a more subdued and luxurious impression) can be obtained when the maximum autocorrelation length is 50 μm or less.

[0031] In addition, in the visual appearance evaluation by in-house evaluators other than the inventors, all of the in-house evaluators rated the plate surface as giving a more tranquil and luxurious impression (ranking it second or higher on a five-point scale) when the maximum autocorrelation length was 50 μm or less.

[0032] Here, the maximum autocorrelation length is a value obtained by dividing the absolute maximum length of a region in an autocorrelation image obtained by image processing a microscopic image of the steel sheet surface obtained by coaxial epi-illumination, which includes the image center and in which the pixel brightness value is equal to or greater than a reference value, by 2. The maximum autocorrelation length is calculated, for example, as follows.

[0033] First, the plate surface is imaged using coaxial epi-illumination to obtain a microscopic image of the plate surface (hereinafter referred to as a plate surface image). Here, coaxial epi-illumination is a microscopic technique in which the optical axis of the illumination illuminating the sample to be observed is aligned with the optical axis of the objective lens, illumination is directed toward the sample from the direction of the objective lens, and the reflected light from the sample is focused by the objective lens to form an image. The pixel resolution of the plate surface image is 5 μm / pixel or less (one side of one pixel corresponds to the actual length of the object to be photographed of 5 μm or less), and the field of view is 4 mm square or more. Next, the obtained plate surface image is processed. The following description assumes that the plate surface image is acquired as a color image (five-dimensional information expressed as x: horizontal position, y: vertical position, R: brightness value (red), G: brightness value (green), and B: brightness value (blue)). If the board surface image is acquired as a monochrome image (3D information expressed as x: horizontal position, y: vertical position, L: brightness value), the grayscaling process described below can be omitted. Brightness refers to the brightness of each pixel (x, y) that makes up the image. First, the board surface image is grayscaled. In grayscaling, performed in accordance with ITU-R BT.601, the brightness value L at each (x, y) position in the color image is set to the value obtained by R × 0.299 + G × 0.587 + B × 0.114, and the color image is converted to a monochrome image. Next, a low-cut filter is used to remove image shading (brightness value differences between different locations within the image caused by the optical system of the imaging device. In microscope images formed using coaxial epi-illumination, brightness values ​​are generally higher in the center of the image and lower in the periphery). Next, the average brightness value of all pixels that make up the board surface image is subtracted from each pixel, making the average brightness value of all pixels that make up the board surface image 0. At this time, the brightness values ​​of some pixels become negative values. The converted image undergoes a fast Fourier transform (using the transform algorithm implemented in FFTPACK) and is then converted into a power spectrum image. The conversion to a power spectrum image is performed by multiplying the value of each pixel (which becomes a complex number after the fast Fourier transform) by the complex conjugate of that value, and then raising the resulting value to the 1 / 2 power.Next, the obtained power spectrum image is subjected to an inverse fast Fourier transform (using the transform algorithm implemented in FFTPACK), and the imaginary part of each pixel in the obtained image is removed, leaving only the real part. The value of each pixel is then divided by the value of the pixel with the highest value in the image, normalized, and created as an autocorrelation image. The obtained autocorrelation image has the highest brightness value in the center of the image. Next, only pixels with brightness values ​​of 0.02 or greater are extracted from the obtained autocorrelation image. Of the extracted pixels, only the region including the center of the image is further extracted using the four-connected method (a method in which pixels adjacent to each other on at least one of the above, below, left, and right sides are considered to be the same region), and this is used as the region to be analyzed. Next, the absolute maximum length of the region to be analyzed (the longest line segment cutting the region to be analyzed in two) is measured, and this value divided by 2 is used as the "maximum autocorrelation length." Then, for one steel plate having a size of 50 mm square or more, the above measurement is performed in 10 randomly selected fields of view, and the arithmetic mean value of the "maximum autocorrelation length" calculated for each field of view is defined as the maximum autocorrelation length of the steel plate.

[0034] The present invention has been completed based on the above findings and further investigations. That is, the gist and configuration of the present invention are as follows.

[0035] 1. Sa is 10.0 μm or more and 40.0 μm or less, Spd is 1500 pieces / mm 2 This is the ferritic stainless steel plate. Here, Sa and Spd are surface texture parameters defined in ISO25178, Sa is the arithmetic mean height, and Spd is the peak density.

[0036] 2.GS(60°) is 50 or less, L* is 50 or more, The scratch visibility index is 50 or less, 2. The ferritic stainless steel sheet according to 1 above, wherein the maximum autocorrelation length is 50 μm or less. Here, GS(60°), L*, the scratch conspicuousness index, and the maximum autocorrelation length are as follows: GS(60°): 60-degree mirror gloss as specified in JIS Z 8741:1997 L*: Lightness index specified in JIS Z 8781-4:2013 Scratch conspicuousness index: After the following scratching test, the difference in brightness between the scratched area and the non-scratched area in a grayscale image of the surface of a ferritic stainless steel plate obtained by photographing under the following photographing conditions. [Scratch test] Grit size: #240 grinding paper is pressed against the surface of the ferritic stainless steel plate with a pressing load of 0.01 N / mm 2 The ferritic stainless steel plate is moved in the rolling direction at a speed of 10 mm / s, and scratches are created on the surface of the ferritic stainless steel plate. The area on the surface of the ferritic stainless steel plate other than the scratches is defined as the non-scratched area. The pressing load is set to 10 mm per unit area (1 mm 2 ) is the load per [Photography conditions] The background color reflected on the surface of the ferritic stainless steel plate is set to black, and light is irradiated from a direction perpendicular to the surface of the ferritic stainless steel plate. Maximum autocorrelation length: In an autocorrelation image obtained by image processing a microscopic image of the surface of a ferritic stainless steel plate using a coaxial epi-illumination method, the absolute maximum length of the area that includes the center of the image and has pixel brightness values ​​equal to or greater than the reference value is divided by 2. [Effects of the Invention]

[0037] According to the present invention, a ferritic stainless steel sheet having excellent antiglare properties, scratch inconspicuousness, roughness resistance, and corrosion resistance can be obtained. [Brief explanation of the drawings]

[0038] [Figure 1] FIG. 2 is a schematic diagram showing the outline of a scratching test for evaluating scratch inconspicuousness. [Figure 2] FIG. 1 is a schematic diagram showing photographing conditions for a plate surface for evaluating scratch conspicuousness. [Figure 3] 10 is an example of an image of a board surface. DETAILED DESCRIPTION OF THE INVENTION

[0039] The present invention will be described based on the following embodiments. [1] Ferritic stainless steel plate The ferritic stainless steel sheet according to one embodiment of the present invention (also referred to as the steel sheet according to one embodiment of the present invention) has an Sa of 10.0 μm or more and 40.0 μm or less, and an Spd of 1500 pieces / mm 2 That's all.

[0040] Sa:10.0μm or more and 40.0μm or less In order to simultaneously obtain excellent anti-glare properties, scratch inconspicuousness, roughness resistance, and corrosion resistance, the Spd (to be described later) should be set to 1500 particles / mm 2 It is important to set Sa to 10.0 μm or more and 40.0 μm or less. In particular, by setting Sa to 10.0 μm or more, excellent antiglare properties and scratch inconspicuousness can be obtained. Therefore, Sa is 10.0 μm or more, preferably 15.0 μm or more, and more preferably 20.0 μm or more. However, if Sa exceeds 40.0 μm, water droplets and the like tend to accumulate, which may result in a decrease in corrosion resistance. Therefore, Sa is 40.0 μm or less, preferably 35.0 μm or less, and more preferably 30.0 μm or less.

[0041] Spd: 1500 pieces / mm 2 End In order to simultaneously obtain excellent anti-glare properties, scratch inconspicuousness, roughness resistance, and corrosion resistance, Sa should be set to 10.0 μm or more and 40.0 μm or less as described above, and Spd should be set to 1500 particles / mm 2 It is especially important to set Spd to 1500 particles / mm 2 By setting the Spd to 1500 particles / mm or more, it is possible to achieve both excellent scratch inconspicuousness and excellent roughness resistance. 2If the Spd is less than 1500 pieces / mm, the period of the minute irregularities on the plate surface will become longer, and the longest period of the fluctuations in the light reflection characteristics from place to place will also become longer. As a result, the roughness of the plate surface will become more noticeable, and excellent roughness resistance will not be achieved. Therefore, Spd should be set to 1500 pieces / mm 2 More than 2000 pieces / mm 2 More preferably, 2500 pieces / mm 2 The upper limit of Spd is not particularly limited. For example, Spd is 3000 particles / mm 2 The following would suffice.

[0042] Here, Sa and Spd can be determined by measuring the three-dimensional surface texture (surface roughness) in accordance with ISO 25178. A contact-type (stylus-type) roughness measuring instrument can be used as the measuring device. For example, the measurement field of view is 0.8 mm in the rolling direction × 2.0 mm in the direction perpendicular to the rolling direction (direction perpendicular to the rolling direction and the thickness direction), and the stylus scan speed is 0.5 mm / s. Furthermore, analysis software such as SURFCOMMap manufactured by Tokyo Seimitsu Co., Ltd. can be used, and analysis can be performed under a cutoff condition of 0.8 mm.

[0043] GS(60°):50 or less GS(60°), which is an index of antiglare properties, is preferably 50 or less, more preferably 40 or less, and even more preferably 30 or less. If GS(60°) is 50 or less, it can be said that the antiglare properties are excellent. The lower limit of GS(60°) is not particularly limited. For example, GS(60°) is preferably 10 or more.

[0044] Here, GS (60°) may be measured in accordance with JIS Z 8722: 2009. The measurement direction is the rolling direction (L direction) of the steel sheet.

[0045] L*: 50 or more, and scratch visibility index: 50 or less L*, which is one index of scratch conspicuity, is preferably 50 or more, more preferably 60 or more, and even more preferably 70 or more. The scratch conspicuity index, which is another index of scratch conspicuity, is preferably 50 or less, more preferably 40 or less, and even more preferably 30 or less. If L* is 50 or more and the scratch conspicuity index is 50 or less, it can be said that the scratch conspicuity is excellent. The upper limit of L* is not particularly limited. For example, L* is preferably 90 or less. The lower limit of the scratch conspicuity index is also not particularly limited. For example, the scratch conspicuity index may be 0. The scratch conspicuity index is preferably 5 or more.

[0046] Here, L* may be measured in accordance with JIS Z 8722:2009. For example, color measurement in accordance with JIS Z 8722:2009 is performed so that the rolling direction (L direction) of the steel sheet is the measurement direction. Note that the conditions are a 10-degree field of view, D65 as the light source, and CIELAB (L*a*b* system) as the color system. The measured value obtained by measurement under condition c (de:8°) (SCE condition) is defined as L*.

[0047] The scratch conspicuity index can be measured using the method described above. The abrasive grain size used in the scratch infliction test is in accordance with JIS R 6001-2:2017. Furthermore, #240 can also be said to have a maximum abrasive grain size of 127 μm or less, a particle size at the 3% point of cumulative height of 103 μm or less, a particle size at the 50% point of cumulative height of 57.0±3.0 μm or less, and a particle size at the 94% point of cumulative height of 40 μm or more, as measured by the electrical resistance test method in accordance with JIS R 6001-2:2017. The abrasive paper may be moved in the rolling direction in the scratch infliction test over the entire length of the test piece (the rolling direction of the steel plate) or over a portion of the test piece.

[0048] Furthermore, to irradiate light perpendicularly to the plate surface, for example, a light source, such as a surface-emitting light source, may be placed opposite the plate surface, and light may be irradiated from the light source toward the plate surface. A grayscale image is an image in which one pixel is represented by 8 bits and contains only brightness information without color information. In a grayscale image, shading, or brightness, is represented by 256 (2 to the power of 8) gradations from 0 to 255. Brightness: 0 is black, and brightness: 255 is white. To photograph the plate surface, for example, a digital camera may be used as the photographing device. For example, a surface-emitting white LED light may be used as the light source. For example, a blackout curtain for photography may be used as the screen. Other photographing conditions are not particularly limited. For example, by adjusting the tilt angle of the camera (the angle between the center of the camera and the center of the test piece relative to the vertical): 20 to 40°, the distance between the camera and the test piece: 20 to 40 cm, and the distance between the light source and the test piece: 40 to 60 cm, a black screen can be reflected onto the plate surface, making the background color reflected on the plate surface black. Furthermore, for example, the shutter speed (exposure time): 1 / 5, F-number (aperture value): 8.0, and ISO (sensitivity): 125 can be used.

[0049] Maximum autocorrelation length: 50 μm or less The maximum autocorrelation length, which is an index of roughness resistance, is preferably 50 μm or less, more preferably 40 μm or less, and even more preferably 30 μm or less. If the maximum autocorrelation length is 50 μm or less, it can be said that the roughness resistance is excellent. There is no particular limitation on the lower limit of the maximum autocorrelation length. For example, the maximum autocorrelation length is preferably 5 μm or more.

[0050] The maximum autocorrelation length may be measured by the method described above.

[0051] The above surface texture and characteristics may be present on at least one surface of the plate, but it is preferable that they be present on both surfaces of the plate.

[0052] Furthermore, ferritic refers to a structure mainly composed of ferrite phase, specifically, a structure consisting of ferrite phase occupying 80% or more, preferably 90% or more, and more preferably 95% or more of the area of ​​the entire structure, and a remaining structure (other than the ferrite phase). Examples of the remaining structure include martensite and retained austenite. It may also be a single ferrite phase (the area ratio of the ferrite phase to the entire structure is 100%).

[0053] The area ratio of the ferrite phase is measured as follows. Specifically, a test piece for cross-sectional observation is prepared from the stainless steel plate to be used as the test material. The test piece is then etched using aqua regia, and observed under an optical microscope at 200x magnification in 10 fields of view. The martensite phase, ferrite phase, and retained austenite phase are distinguished from each other based on the microstructural shape and etching intensity. The area ratio of the ferrite phase is then determined for each field of view using image processing, and the arithmetic average value for the 10 fields of view is calculated.

[0054] The thickness of the steel sheet according to one embodiment of the present invention (hereinafter also referred to as sheet thickness) is not particularly limited. From the viewpoint of manufacturability, the sheet thickness is preferably 0.1 mm or more, more preferably 0.5 mm or more, and even more preferably 1.0 mm or more. The sheet thickness is preferably 4.0 mm or less, more preferably 3.0 mm or less, and even more preferably 2.0 mm or less.

[0055] The composition of the steel sheet according to one embodiment of the present invention is not particularly limited, but for example, the following composition is preferable.

[0056] - The optimum chemical composition of the steel sheet (hereinafter also referred to as the optimum chemical composition) The preferred component composition is, in mass %, C: 0.001 to 0.150%, Si: 0.01 to 2.00%, Mn: 0.01 to 1.00%, P: 0.050% or less, S: 0.040% or less, Ni: 0.01 to 2.50%, Cr: 10.5 to 32.0%, Al: 0.001 to 6.5%, and N: 0.001 to 0.100%, Optionally, one or more groups selected from the following (Group A) to (Group C): The balance is Fe and unavoidable impurities. (Group A) One or more selected from Cu: 2.00% or less, Co: 2.00% or less, Mo: 3.00% or less, and W: 2.00% or less (Group B) One or more selected from Ti: 0.50% or less, Nb: 1.00% or less, V: 0.50% or less, and Zr: 0.50% or less (Group C) One or more selected from B: 0.0050% or less, Mg: 0.0050% or less, Ca: 0.0030% or less, Y: 0.20% or less, REM (rare earth metals): 0.20% or less, Sn: 0.50% or less, and Sb: 0.50% or less

[0057] The preferred component compositions will be described below. Note that the units for the preferred component compositions are all "% by mass," and hereinafter, unless otherwise specified, they will simply be represented by "%."

[0058] C: 0.001 to 0.150% Carbon dissolves in steel to increase the strength of the steel sheet, suppress scratches during manufacturing, and improve the manufacturability of the steel sheet. If the C content is less than 0.001%, this effect is not fully achieved. However, if the C content exceeds 0.150%, defects due to carbides are more likely to occur on the sheet surface, which may actually reduce the manufacturability of the steel sheet. Therefore, the C content is preferably in the range of 0.001 to 0.150%. The C content is more preferably 0.010% or more, and even more preferably 0.030% or more. The C content is more preferably 0.100% or less, and even more preferably 0.050% or less.

[0059] Si: 0.01 to 2.00% Si acts as a deoxidizer during steel smelting, reducing inclusions in steel that cause surface defects in steel sheets, thereby improving the manufacturability of steel sheets. Si also has the effect of increasing the strength of steel sheets and suppressing scratches during manufacturing, thereby improving the manufacturability of steel sheets. To achieve these effects, the Si content is preferably 0.01% or more. However, if the Si content exceeds 2.00%, defects caused by inclusions are more likely to occur on the sheet surface, which may actually reduce the manufacturability of the steel sheet. Therefore, the Si content is preferably in the range of 0.01 to 2.00%. The Si content is more preferably 0.10% or more, and even more preferably 0.20% or more. The Si content is more preferably 1.00% or less, and even more preferably 0.70% or less.

[0060] Mn: 0.01 to 1.00% Mn has the effect of increasing the strength of steel sheets, suppressing scratches during manufacturing, and improving the manufacturability of steel sheets. To achieve this effect, the Mn content is preferably 0.01% or more. However, if the Mn content exceeds 1.00%, MnS is likely to form in the steel, which may become the starting point for corrosion and reduce the corrosion resistance of the steel sheet. Therefore, the Mn content is preferably in the range of 0.01 to 1.00%.

[0061] P:0.050% or less P is an element that embrittles steel, causing cracks on the sheet surface and reducing the manufacturability of steel sheets. Therefore, it is desirable to reduce P as much as possible. Therefore, the P content is preferably 0.050% or less. The P content is more preferably 0.040% or less. There is no particular restriction on the lower limit of the P content. However, excessive de-P leads to an increase in manufacturing costs. Therefore, the P content is preferably 0.010% or more.

[0062] S: 0.040% or less S is present in steel as sulfide-based inclusions such as MnS, and is an element that causes the formation of surface defects due to inclusions, thereby reducing the manufacturability of steel sheets. Therefore, it is desirable to reduce S as much as possible. In particular, if the S content exceeds 0.040%, the effect becomes significant. Therefore, the S content is preferably 0.040% or less. The S content is more preferably 0.020% or less, and even more preferably 0.015% or less. There is no particular restriction on the lower limit of the S content. However, excessive desulfurization increases manufacturing costs. Therefore, the S content is preferably 0.0001% or more.

[0063] Ni: 0.01 to 2.50% Ni is an element that contributes to improving the toughness of steel sheets, suppresses breakage of steel sheets during the manufacturing process, and improves the manufacturability of steel sheets. To achieve this effect, the Ni content is preferably 0.01% or more. However, if the Ni content exceeds 2.50%, the descaling step in the manufacturing process becomes difficult, and the manufacturability of steel sheets may decrease. Therefore, the Ni content is preferably in the range of 0.01 to 2.50%. The Ni content is more preferably 0.05% or more. The Ni content is more preferably less than 1.00%, and even more preferably less than 0.30%.

[0064] Cr: 10.5 to 32.0% Cr is an element that contributes to improving the corrosion resistance of steel sheets. To achieve this effect, the Cr content is preferably 10.5% or more. However, if the Cr content exceeds 32.0%, roughness is likely to occur on the surface during hot rolling, which may reduce the manufacturability of the steel sheet. Therefore, the Cr content is preferably in the range of 10.5 to 32.0%. The Cr content is more preferably 12.0% or more, and even more preferably 16.0% or more. The Cr content is more preferably 22.0% or less, and even more preferably 18.0% or less.

[0065] Al: 0.001 to 6.5% Like Si, Al acts as a deoxidizer, reducing inclusions in steel that cause surface defects in steel sheets, thereby improving the manufacturability of steel sheets. To achieve this effect, the Al content is preferably 0.001% or more. However, if the Al content exceeds 6.5%, the steel becomes embrittled and prone to cracking, which may reduce the manufacturability of steel sheets. Therefore, the Al content is preferably in the range of 0.001 to 6.5%. The Al content is more preferably 0.600% or less, and even more preferably 0.060% or less.

[0066] N: 0.001 to 0.100% Like C, N dissolves in steel to increase the strength of the steel sheet, suppress scratches during manufacturing, and improve the manufacturability of the steel sheet. Here, if the N content is less than 0.001%, this effect cannot be fully achieved. However, if the N content exceeds 0.100%, defects tend to occur on the sheet surface. These defects themselves determine the degree of roughness on the sheet surface, which may make the roughness more noticeable. Therefore, the N content is preferably in the range of 0.001 to 0.100%. The N content is more preferably 0.005% or more, and even more preferably 0.010% or more. The N content is more preferably 0.080% or less, and even more preferably 0.050% or less.

[0067] The basic components of the preferred composition have been explained above, but the preferred composition may further contain any of the elements of (Group A) to (Group C) described above.

[0068] Cu:2.00% or less Cu has the effect of increasing the strength of steel sheet. This effect is achieved when the Cu content is preferably 0.01% or more. The Cu content is more preferably 0.05% or more, and even more preferably 0.10% or more. However, if the Cu content exceeds 2.00%, the steel will contain a large amount of ε-Cu phase, which will become the starting point for corrosion and reduce the corrosion resistance of the steel sheet. Therefore, when Cu is contained, the Cu content is preferably 2.00% or less. The Cu content is more preferably 0.50% or less, and even more preferably 0.20% or less.

[0069] Co:2.00% or less Co has the effect of increasing the strength of steel sheet. This effect is obtained when the Co content is preferably 0.01% or more. The Co content is more preferably 0.05% or more, and even more preferably 0.10% or more. However, if the Co content exceeds 2.00%, the steel sheet becomes embrittled. Therefore, when Co is contained, the Co content is preferably 2.00% or less. The Co content is more preferably 0.50% or less, and even more preferably 0.20% or less.

[0070] Mo: 3.00% or less Mo is an element that improves the corrosion resistance of steel sheets. This effect is achieved when the Mo content is preferably 0.01% or more. The Mo content is more preferably 0.05% or more, even more preferably 0.10% or more, and even more preferably 0.15% or more. However, if the Mo content exceeds 3.00%, the steel sheet becomes embrittled. Therefore, when Mo is contained, the Mo content is preferably 3.00% or less. The Mo content is more preferably 0.80% or less, even more preferably 0.60% or less, and even more preferably 0.45% or less.

[0071] W: 2.00% or less W is an element that improves the corrosion resistance of steel sheets. This effect is achieved when the W content is preferably 0.01% or more. The W content is more preferably 0.05% or more, and even more preferably 0.10% or more. However, if the W content exceeds 2.00%, the steel sheet becomes embrittled. Therefore, when W is contained, the W content is preferably 2.00% or less. The W content is more preferably 0.50% or less, and even more preferably 0.20% or less.

[0072] Ti: 0.50% or less Ti is an element that improves the corrosion resistance of steel sheets. This effect is achieved when the Ti content is preferably 0.01% or more. The Ti content is more preferably 0.02% or more, and even more preferably 0.03% or more. However, if the Ti content exceeds 0.50%, the steel sheet becomes embrittled. Therefore, when Ti is contained, the Ti content is preferably 0.50% or less. The Ti content is more preferably 0.30% or less, and even more preferably 0.10% or less.

[0073] Nb: 1.00% or less Like Ti, Nb has the effect of improving the corrosion resistance of steel sheets. This effect is obtained when the Nb content is preferably 0.01% or more. The Nb content is more preferably 0.02% or more, and even more preferably 0.03% or more. However, if the Nb content exceeds 1.00%, the steel sheet becomes embrittled. Therefore, when Nb is contained, the Nb content is preferably 1.00% or less. The Nb content is more preferably 0.50% or less, and even more preferably 0.20% or less.

[0074] V: 0.50% or less Like Ti and Nb, V has the effect of improving the corrosion resistance of steel sheets. This effect is obtained when the V content is preferably 0.01% or more. The V content is more preferably 0.02% or more, and even more preferably 0.03% or more. However, if the V content exceeds 0.50%, the steel sheet becomes embrittled. Therefore, when V is contained, the V content is preferably 0.50% or less. The V content is more preferably 0.20% or less, and even more preferably 0.10% or less.

[0075] Zr: 0.50% or less Like Ti and Nb, Zr has the effect of improving the corrosion resistance of steel sheets. This effect is obtained when the Zr content is preferably 0.01% or more. The Zr content is more preferably 0.02% or more, and even more preferably 0.03% or more. However, if the Zr content exceeds 0.50%, the steel sheet becomes embrittled. Therefore, when Zr is contained, the Zr content is preferably 0.50% or less. The Zr content is more preferably 0.20% or less, and even more preferably 0.10% or less.

[0076] B: 0.0050% or less B is an element that prevents edge cracking of steel sheets during hot rolling and improves the productivity of the steel sheets. This effect is obtained when the B content is preferably 0.0002% or more. The B content is more preferably 0.0003% or more, and even more preferably 0.0005% or more. However, if the B content exceeds 0.0050%, the hot workability deteriorates, leading to a decrease in the manufacturability of the steel sheets. Therefore, when B is contained, the B content is preferably 0.0050% or less. The B content is more preferably 0.0030% or less, and even more preferably 0.0020% or less.

[0077] Mg: 0.0050% or less Mg forms Mg oxide with Al in molten steel and acts as a deoxidizer. This effect is obtained when the Mg content is preferably 0.0005% or more. The Mg content is more preferably 0.0010% or more. On the other hand, if the Mg content exceeds 0.0050%, the steel sheet becomes embrittled. Therefore, when Mg is contained, the Mg content is preferably 0.0050% or less. The Mg content is more preferably 0.0030% or less.

[0078] Ca:0.0030% or less Ca forms oxides in molten steel and acts as a deoxidizer. This effect is achieved when the Ca content is preferably 0.0003% or more. The Ca content is more preferably 0.0005% or more, and even more preferably 0.0007% or more. However, if the Ca content exceeds 0.0030%, a large amount of CaS is formed in the steel, which becomes the starting point for corrosion and reduces the corrosion resistance of the steel sheet. Therefore, when Ca is contained, the Ca content is preferably 0.0030% or less. The Ca content is more preferably 0.0025% or less, and even more preferably 0.0015% or less.

[0079] Y: 0.20% or less Y is an element that prevents edge cracking of steel sheets during hot rolling and improves the productivity of steel sheets. This effect is obtained when the Y content is preferably 0.01% or more. The Y content is more preferably 0.02% or more. However, if the Y content exceeds 0.20%, the hot workability deteriorates, leading to a decrease in the manufacturability of the steel sheet. Therefore, when Y is contained, the Y content is preferably 0.20% or less. The Y content is more preferably 0.05% or less.

[0080] REM: 0.20% or less REM (Rare Earth Metals) are elements that prevent edge cracking of steel sheets during hot rolling and improve the productivity of steel sheets. This effect is achieved when the REM content is preferably 0.01% or more. The REM content is more preferably 0.02% or more. However, if the REM content exceeds 0.20%, the hot workability decreases, leading to a decrease in the manufacturability of the steel sheet. Therefore, when REM is contained, the REM content is preferably 0.20% or less. The REM content is more preferably 0.05% or less. Here, REM refers to elements belonging to Group 3 of the periodic table (excluding Y). Furthermore, the REM content refers to the total content of elements belonging to Group 3 of the periodic table (excluding Y).

[0081] Sn: 0.50% or less Sn is an element that prevents surface roughening of steel sheets during hot rolling and improves the productivity of steel sheets. This effect is achieved when the Sn content is preferably 0.01% or more. The Sn content is more preferably 0.03% or more. However, if the Sn content exceeds 0.50%, the steel sheet becomes embrittled. Therefore, when Sn is contained, the Sn content is preferably 0.50% or less. The Sn content is more preferably 0.20% or less.

[0082] Sb: 0.50% or less Sb is an element that prevents surface roughening of steel sheets during hot rolling and improves the productivity of steel sheets. This effect is achieved when the Sb content is preferably 0.01% or more. The Sb content is more preferably 0.03% or more. However, if the Sb content exceeds 0.50%, the steel sheet becomes embrittled. Therefore, when Sb is contained, the Sb content is preferably 0.50% or less. The Sb content is more preferably 0.20% or less.

[0083] The balance of the components other than those mentioned above is Fe and unavoidable impurities.

[0084] [2] Manufacturing method for ferritic stainless steel sheets Next, a preferred method for producing a ferritic stainless steel sheet according to one embodiment of the present invention will be described.

[0085] First, a material for cold rolling is prepared. Examples of materials for cold rolling include hot-rolled steel sheets and hot-rolled annealed steel sheets. The method for preparing the material for cold rolling is not particularly limited. In one embodiment, molten steel is produced in a melting furnace such as a converter, an electric furnace, or a vacuum melting furnace to obtain a predetermined composition, for example, molten steel adjusted to satisfy the above-mentioned range of the preferred composition. Next, the molten steel is formed into a steel material (steel slab) by a continuous casting method or an ingot-blooming method. Next, the steel material is hot-rolled to obtain a hot-rolled steel sheet. Next, the hot-rolled steel sheet is annealed to obtain a hot-rolled annealed steel sheet. The obtained hot-rolled annealed steel sheet is descaled as needed by pickling, shot blasting, surface grinding, etc., to obtain a material for cold rolling. Furthermore, the hot-rolled annealed steel sheet may be skin-pass rolled as needed.

[0086] The conditions for the hot rolling and hot-rolled sheet annealing are not particularly limited and may be conventional. For example, in hot rolling, the steel material is heated to 1050 to 1250°C and held at that temperature for 30 minutes to 24 hours, or if the steel material is in the temperature range immediately after casting, it is rolled as is without heating. The hot-rolling reduction ratio is not particularly limited and may be adjusted appropriately depending on the required thickness of the final product. Examples of conditions for hot-rolled sheet annealing include heating the hot-rolled steel sheet to a temperature range of 750 to 850°C and holding it at that temperature for 1 to 24 hours. Another example of conditions for hot-rolled sheet annealing is heating the hot-rolled steel sheet to a temperature range of 900 to 1100°C and holding it at that temperature for 1 second to 10 minutes.

[0087] (Cold rolling process) The cold-rolled material prepared as described above is cold-rolled to obtain a cold-rolled steel sheet. The dull roll used in the final pass is a dull roll having a predetermined surface roughness, specifically, a roughness Sa of 10.0 μm or more and 40.0 μm or less, and a roughness Spd of 1500 / mm 2 It is important to use a dull roll having the above-mentioned properties and to set the rolling reduction in the final pass to 0.80% or more.

[0088] Sa of the dull roll used in the final pass: 10.0 μm or more and 40.0 μm or less By setting the Sa of the dull roll used in the final pass (hereinafter also referred to as the final pass roll) to 10.0 μm or more and 40.0 μm or less, the height of the irregularities transferred to the surface of the cold-rolled steel sheet becomes appropriate, making it possible to simultaneously obtain excellent antiglare properties, scratch inconspicuousness, and corrosion resistance. Here, if the Sa of the final pass roll is less than 10.0 μm, the height of the irregularities transferred to the surface of the cold-rolled steel sheet will be too small. In this case, even if the subsequent steps are performed under appropriate conditions, excellent antiglare properties and scratch inconspicuousness will not be obtained. On the other hand, if the Sa of the final pass roll exceeds 40.0 μm, the height of the irregularities transferred to the surface of the cold-rolled steel sheet will be too large. In this case, even if the subsequent steps are performed under appropriate conditions, excellent corrosion resistance will not be obtained. Therefore, the Sa of the final dull roll is 10.0 μm or more, preferably 15.0 μm or more, and more preferably 20.0 μm or more. The final dull roll has an Sa of 40.0 μm or less, preferably 35.0 μm or less, and more preferably 30.0 μm or less.

[0089] Final pass roll Spd: 1500 pieces / mm 2 End Spd of the final pass roll is 1500 pieces / mm 2 By setting the Spd of the final pass roll to 1500 pieces / mm or more, the period of the irregularities transferred to the surface of the cold-rolled steel sheet becomes shorter, and excellent roughness resistance can be obtained. 2 If the Spd is less than 1500 / mm, the period of the irregularities transferred to the surface of the cold-rolled steel sheet becomes long, and excellent roughness resistance cannot be obtained even if the subsequent processes are carried out under appropriate conditions. 2 More than 2000 pieces / mm 2 More preferably, 2500 pieces / mm 2 The upper limit of Spd of the final pass roll is not particularly limited. Spd is 3000 pieces / mm 2 The following would suffice.

[0090] The Sa and Spd of the final pass roll are measured, for example, by the aforementioned method in accordance with ISO 25178 using a replica taken from the surface of the final pass roll. However, the shape data obtained from the surface shape of the replica may contain curved components due to the shape of the final pass roll or undulation components generated when the replica was taken. In this case, to eliminate the influence of these on Sa and Spd, F calculation or L filter is applied to the shape data to remove the curved components due to the shape of the final pass roll and the undulation components generated when the replica was taken. Furthermore, in accordance with ISO 25178, the Sa of the final pass roll is measured in a direction perpendicular to the circumferential direction of the surface of the final pass roll.

[0091] Final pass reduction: 0.80% or more If the reduction rate in the final pass is less than 0.80%, the irregularities will not be sufficiently transferred to the surface of the cold-rolled steel sheet, and the height of the irregularities formed on the surface of the cold-rolled steel sheet will be too small. In this case, even if the subsequent processes are performed under appropriate conditions, excellent antiglare properties and scratch inconspicuousness will not be obtained. Therefore, the reduction rate in the final pass is 0.80% or more, preferably 0.90% or more. There is no particular upper limit to the reduction rate in the final pass. The reduction rate in the final pass is preferably, for example, 1.50% or less.

[0092] Conditions other than those mentioned above are not particularly limited and may be conventional. For example, the total reduction rate of cold rolling and the reduction rates in the rolling passes other than the final pass may be set appropriately depending on the target thickness of the stainless steel sheet to be the final product. The number of rolling passes is also not particularly limited, and is preferably 4 to 6 passes, for example.

[0093] The final pass roll can be prepared by, for example, performing shot blasting or liquid honing on the roll surface. In these processes, if the processing time is insufficient or if the abrasive grains projected onto the roll are large, Spd will decrease. Also, if the projection velocity of the abrasive grains projected onto the roll is low, Sa will decrease. Therefore, by performing these processes for a sufficient time and adjusting the particle size and projection velocity of the abrasive grains projected onto the roll, the desired roll properties can be formed on the roll surface.

[0094] (Cold-rolled sheet annealing process) Next, the cold-rolled steel sheet is annealed to obtain a cold-rolled annealed steel sheet. The annealing conditions for the cold-rolled steel sheet are not particularly limited. In one embodiment, for example, a batch annealing furnace or a continuous annealing furnace is used, and the annealing conditions include holding the steel sheet at a temperature of 800°C or higher and 1050°C or lower for 5 seconds to 10 hours in an oxidizing atmosphere such as air or a non-oxidizing atmosphere such as nitrogen or ammonia decomposition gas. From the viewpoint of productivity, it is preferable to hold the steel sheet in a non-oxidizing atmosphere using a continuous annealing furnace for 3 minutes or shorter.

[0095] (pickling process) Next, the cold-rolled annealed steel sheet is pickled. In this pickling process, the treatment conditions are as follows: Treatment liquid: an aqueous nitric acid solution having a hydrochloric acid concentration of 0.40 to 1.00 mass % and a nitric acid concentration of 10.0 to 20.0 mass %; Processing temperature: 30 to 65°C, Processing time: 1.0 to 20.0 seconds Current density: 5.0~20.0A / dm 2 A positive electrolysis treatment is performed to obtain the above.

[0096] Hydrochloric acid concentration in treatment liquid: 0.40 to 1.00 mass% If the hydrochloric acid concentration in the treatment solution is less than 0.40% by mass, the formation of fine irregularities on the sheet surface will be insufficient, and excellent anti-glare properties and scratch inconspicuousness will not be achieved. On the other hand, if the hydrochloric acid concentration in the treatment solution is more than 1.00% by mass, the formation of fine irregularities on the sheet surface will be excessive, and corrosion resistance will be reduced. Therefore, the hydrochloric acid concentration in the treatment solution is set to 0.40 to 1.00% by mass.

[0097] Nitric acid concentration of treatment solution: 10.0 to 20.0 mass% If the nitric acid concentration of the treatment solution is less than 10.0% by mass, the formation of fine irregularities on the sheet surface will be insufficient, and excellent anti-glare properties and scratch inconspicuousness will not be achieved. On the other hand, if the nitric acid concentration of the treatment solution exceeds 20.0% by mass, the formation of fine irregularities on the sheet surface will be excessive, and corrosion resistance will be reduced. Therefore, the nitric acid concentration of the treatment solution is set to 10.0 to 20.0% by mass.

[0098] The aqueous solution (nitric acid aqueous solution) to be used as the treatment solution may contain unavoidable impurities, such as Fe ions and Cr ions, that are common in pickling treatment solutions, as long as the above-mentioned hydrochloric acid concentrations and nitric acid concentrations are ensured.

[0099] Processing temperature (temperature of processing liquid): 30 to 65°C If the treatment temperature is below 30°C, the formation of fine irregularities on the sheet surface will be insufficient, and excellent antiglare properties and scratch inconspicuousness will not be achieved. On the other hand, if the treatment temperature exceeds 65°C, the formation of fine irregularities on the sheet surface will be excessive, and corrosion resistance will decrease. Therefore, the treatment temperature is set to 30 to 65°C.

[0100] Treatment time (electrolysis time in treatment solution): 1.0 to 20.0 seconds If the treatment time is less than 1.0 second, the formation of fine irregularities on the plate surface will be insufficient, and excellent antiglare properties and scratch inconspicuousness will not be achieved. On the other hand, if the treatment time exceeds 20.0 seconds, the formation of fine irregularities on the plate surface will be excessive, and corrosion resistance will decrease. Therefore, the treatment time is set to 1.0 to 20.0 seconds.

[0101] Current density: 5.0~20.0A / dm 2 Current density in positive electrolysis is 5.0A / dm 2 If the current density in the positive electrolytic treatment is less than 20.0 A / dm, the formation of fine irregularities on the plate surface will be insufficient, and excellent antiglare properties and scratch inconspicuousness will not be obtained. 2 If the current density exceeds 5.0 to 20.0 A / dm, the formation of minute irregularities on the plate surface will be excessive, and the corrosion resistance will decrease. 2 Let's say.

[0102] The electrolytic treatment may be carried out in multiple steps. In the case of multiple steps, the hydrochloric acid and nitric acid concentrations, treatment temperature, and current density of the treatment solution are set within the above ranges in each treatment. The total treatment time for each treatment is set within the above range (1.0 to 20.0 seconds).

[0103] In addition, in a typical production line equipment configuration, the steel sheet is subjected to reverse electrolysis before and after the positive electrolysis. Here, reverse electrolysis may be performed, and there are no restrictions on the electrolysis current density of the reverse electrolysis. Note that the above treatment time does not include the treatment time for the reverse electrolysis.

[0104] Before or after the above-mentioned pickling by positive electrolysis, another pickling treatment may be performed separately. For example, when the cold-rolled sheet annealing is performed in an oxidizing atmosphere, an electrolytic treatment using a sodium sulfate aqueous solution may be performed before the above-mentioned pickling by positive electrolysis to remove scale. Furthermore, after the above-mentioned pickling by positive electrolysis, an immersion treatment in a nitric acid aqueous solution and an electrolytic treatment using a nitric acid aqueous solution may be performed for the purpose of passivation.

[0105] (Grinding process) Next, the pickled cold-rolled annealed steel sheet is ground with a grinding brush.

[0106] Grinding brush grit size: #500 or higher The surface of a cold-rolled annealed steel sheet is ground using a grinding brush with abrasive grain size of #500 or higher. This refines the irregularities on the sheet surface. As a result, Spd increases and roughness resistance is further improved. However, if the abrasive grains of the grinding brush are too coarse, sufficient Spd cannot be obtained. Furthermore, Sa becomes too large and corrosion resistance becomes insufficient. Therefore, the abrasive grain size of the grinding brush is #500 or higher, preferably #1000 or higher.

[0107] Here, the abrasive grain size is based on JIS R 6001-2:2017. Examples of #500 and above include #500, #600, #700, #800, #1000, #1200, #1500, #2000, #2500, #3000, #4000m, #6000, and #8000. #500 and above can also be defined as abrasive grains having a maximum particle size of 63 μm or less, a particle size at the 3% point of cumulative height of 50 μm or less, a particle size at the 50% point of cumulative height of 27.0 μm (25.0 μm + 2.0 μm) or less, and a particle size at the 94% point of cumulative height of 0.6 μm or more, as measured by an electrical resistance test method in accordance with JIS R 6001-2:2017. It can also be said that #1000 and above means that the maximum particle diameter of the abrasive grains measured using an electrical resistance test method in accordance with JIS R 6001-2:2017 is 32 μm or less, the particle diameter at the 3% point of cumulative height is 27 μm or less, the particle diameter at the 50% point of cumulative height is 12.5 μm (11.5 μm + 1.0 μm) or less, and the particle diameter at the 94% point of cumulative height is 0.6 μm or more.

[0108] Examples of abrasive grains include aluminum oxide (Al2O3), silicon carbide (SiC), and diamond. Before or after the above grinding, grinding with a brush that does not contain abrasive grains may be performed. This further improves the roughness resistance.

[0109] The grinding conditions other than those mentioned above are not particularly limited and may be in accordance with conventional methods. For example, from the viewpoint of miniaturizing the irregularities on the plate surface, the grinding amount (= [mass (g) of steel plate removed by grinding] / [area of ​​the plate surface to be ground (cm 2 )]) is 0.01 to 0.10 g / cm 2 is preferred. [Example]

[0110] Steels having the chemical compositions shown in Table 1 (the balance being Fe and unavoidable impurities) were melted into 100 kg steel ingots. The steel ingots were then heated at 1,150°C for 1 hour and then hot-rolled to produce hot-rolled steel sheets with a thickness of 3.5 mm. These hot-rolled steel sheets were subjected to hot-rolled annealing by holding them at 900°C for steel grades 1K and 1P, 1,040°C for steel grades 1Q and 1U, and 950°C for steel grades 1S and 1T for 20 seconds, respectively. The other steel grades were subjected to hot-rolled annealing by holding them at 800°C for 10 hours to produce hot-rolled annealed steel sheets. The front and back surfaces of these hot-rolled annealed steel sheets were then ground to remove scale, and materials for cold rolling were prepared.

[0111] Next, the prepared cold rolling materials were subjected to cold rolling under the conditions shown in Table 2 to obtain cold rolled steel sheets with a thickness of 1.0 mm. The number of rolling passes in the cold rolling was 9 to 11 passes in all cases.

[0112] The cold-rolled steel sheets were then annealed to obtain cold-rolled and annealed steel sheets. For steel types 1K, 1P, 1Q, 1S, 1T, and 1U, the conditions were a 3.5% by volume hydrogen-96.5% by volume nitrogen atmosphere at 980°C for 1 minute. For the other steel types, the conditions were a 3.5% by volume hydrogen-96.5% by volume nitrogen atmosphere at 850°C for 1 minute.

[0113] The cold-rolled and annealed steel sheets were then subjected to pickling under the conditions shown in Table 2. The cold-rolled and annealed steel sheets were then ground under the conditions shown in Table 2 to obtain ferritic stainless steel sheets.

[0114] The obtained steel sheets were cut into a length (rolling direction) of 300 mm x width (direction perpendicular to the rolling direction) of 200 mm, and the surface texture was measured as described above. The results of the surface texture measurements are also shown in Table 2. The evaluation of the surface texture and the following items (a) to (ii) were performed on both sides of the sheet, but since nearly identical results were obtained on both sides, the results of one side are shown here as a representative. The chemical compositions of the finally obtained steel sheets were substantially the same as those of the steel types listed in Table 1, and all of them satisfied the above-mentioned range of the preferred chemical composition. Furthermore, the steel structure was identified as described above, and it was confirmed that the structure of the obtained steel sheets all contained a ferrite phase with an area ratio of more than 95%.

[0115] The ferritic stainless steel sheets thus obtained were also measured for GS(60°), L*, scratch conspicuity index, and maximum autocorrelation length according to the procedures described above and evaluated for (a) antiglare properties, (b) scratch conspicuity, and (c) roughness resistance. The results are also shown in Table 2. (a) Anti-glare Pass A (pass, particularly excellent): GS (60°) is 40 or less Pass B (pass): GS (60°) is over 40 and less than 50 Fail: GS(60°) is over 50 (b) Scratch inconspicuousness Pass A (pass, particularly excellent): L* is 60 or more and the scratch visibility index is 30 or less Pass B (Pass): L* is 50 or more and the scratch visibility index is 50 or less (excluding Pass A) Fail: L* less than 50 and / or Scratch Conspicuousness Index greater than 50 (c) Roughness resistance Pass A (Pass, particularly excellent): Maximum autocorrelation length is 35 μm or less Pass B (Pass): Maximum autocorrelation length is over 35 μm and 50 μm or less Fail: Maximum autocorrelation length is greater than 50 μm

[0116] Here, a gloss meter GM-1 manufactured by Suga Test Instruments Co., Ltd. was used to measure GS (60°).

[0117] To measure L*, a spectrophotometer SD3000 manufactured by Nippon Denshoku Industries Co., Ltd. was used. To measure the scratch visibility index, a digital camera (Canon PowerShot G9XMarkII) was used as the photographing device, a surface-emitting white LED light (Nikki Co., Ltd. NLU20-AC) was used as the lighting, and a standard blackout curtain was used as the screen. The tilt angle of the photographing device was 30°, the distance between the photographing device and the test specimen was 30 cm, and the distance between the light source and the test specimen was 50 cm. The screen was reflected onto the plate surface, and the background color reflected on the plate surface was black. The shutter speed (exposure time) was 1 / 5, the F-number (aperture value) was 8.0, and the ISO (sensitivity) was 125.

[0118] To measure the maximum autocorrelation length, an Olympus DSX-510 optical microscope was used to capture images of the plate surface. Images were processed using Mitani Corporation's WinROOF2015 image analysis software and Python (open source). Images were captured using a 5x objective lens, the MPLFLN5XBDP, with a zoom magnification of 1x (total magnification of 69x), contrast correction on, exposure on auto, and light brightness set to 10,000. Bright-field images of a 4.1mm square area were captured using coaxial epi-illumination with a pixel count of 1194 x 1194. A low-cut filter was applied to the images using the background subtraction function in WinROOF2015, with the object size set to 1000 μm. The fast Fourier transform and inverse fast Fourier transform algorithms (implemented in FFTPACK) in Numpy, a Python numerical computation library, were used to generate the autocorrelation images. The reference value for binarizing the autocorrelation image was set to 0.02.,Furthermore, the shape feature measurement of WinROOF2015 was used to measure the,absolute maximum length of the region to be analyzed.

[0119] Test specimens taken from the steel sheets were subjected to a salt spray cyclic corrosion test according to JASO M609-91. Each test specimen underwent 15 cycles, consisting of a salt spray of 5% NaCl aqueous solution at 35°C for 2 hours, followed by a dry cycle at 60°C for 4 hours at 20% relative humidity, followed by a wet cycle at 50°C for 2 hours at 90% relative humidity. The test specimens were then assigned a rating number (RN) according to JIS G 0595:2004. The corrosion resistance was evaluated according to the following criteria. The results are shown in Table 2.

[0120] (2) Corrosion resistance Pass A (Pass, Excellent): RN is 6 or more Pass B (Pass): RN is 4 or more but less than 6 Fail: RN less than 4

[0121] [Table 1]

[0122] [Table 2] TIFF0007768469000003.tif233130

[0123] As shown in Table 2, all of the inventive examples satisfied all of the required characteristics (a), (b), (c) and (d) above.

[0124] On the other hand, none of the comparative examples satisfied at least one of the required properties (a), (b), (c) and (d).

[0125] In the comparative example of Test No. 1-42, the Sa of the final pass roll in cold rolling did not satisfy the appropriate range, and therefore the Sa of the steel sheet did not satisfy the appropriate range, and the desired antiglare properties and scratch inconspicuousness were not obtained. In the comparative example of Test No. 1-43, the Sa of the final pass roll in cold rolling exceeded the appropriate range, and therefore the Sa of the steel sheet exceeded the appropriate range, and the desired corrosion resistance was not obtained. In the comparative example of Test No. 1-44, the Spd of the final pass roll in cold rolling did not satisfy the appropriate range, and therefore the Spd of the steel sheet did not satisfy the appropriate range, and the desired roughness resistance was not obtained. In the comparative example of Test No. 1-45, the reduction ratio in the final pass of cold rolling was below the appropriate range, so the Sa of the steel sheet was below the appropriate range, and the desired antiglare properties and scratch visibility were not obtained. In addition, the Spd of the steel sheet was below the appropriate range, and the desired roughness resistance was not obtained. In the comparative example of Test No. 1-46, the hydrochloric acid concentration of the treatment liquid was below the appropriate range, so the Sa of the steel sheet was below the appropriate range, and the desired antiglare properties and scratch inconspicuousness were not obtained. In the comparative example of Test No. 1-47, the hydrochloric acid concentration of the treatment liquid exceeded the appropriate range, so the Sa of the steel sheet exceeded the appropriate range and the desired corrosion resistance was not obtained. In the comparative example of Test No. 1-48, the nitric acid concentration of the treatment liquid was below the appropriate range, so the Sa of the steel sheet was below the appropriate range, and the desired antiglare properties and scratch inconspicuousness were not obtained. In the comparative example of Test No. 1-49, the nitric acid concentration of the treatment solution exceeded the appropriate range, so the Sa of the steel sheet exceeded the appropriate range and the desired corrosion resistance was not obtained. In the comparative example of Test No. 1-50, the treatment temperature was below the appropriate range, so the Sa of the steel sheet did not fall within the appropriate range, and the desired antiglare properties and scratch inconspicuousness were not obtained. In the comparative example of Test No. 1-51, the treatment temperature exceeded the appropriate range, so that the Sa of the steel sheet exceeded the appropriate range, and the desired corrosion resistance was not obtained. In the comparative example of Test No. 1-52, the treatment time was below the appropriate range, so the Sa of the steel sheet did not fall within the appropriate range, and the desired antiglare properties and scratch inconspicuousness were not obtained. In the comparative example of Test No. 1-53, the treatment time exceeded the appropriate range, so that the Sa of the steel sheet exceeded the appropriate range, and the desired corrosion resistance was not obtained. In the comparative example of Test No. 1-54, the current density did not fall within the appropriate range, so the Sa of the steel sheet did not fall within the appropriate range, and the desired antiglare properties and scratch inconspicuousness were not obtained. In the comparative example of Test No. 1-55, the current density exceeded the appropriate range, so that the Sa of the steel sheet exceeded the appropriate range, and the desired corrosion resistance was not obtained. In the comparative example of Test No. 1-56, the abrasive grit size of the grinding brush was below the appropriate range, so the Sa of the steel plate exceeded the appropriate range and the desired corrosion resistance was not obtained. Also, the Spd of the steel plate was below the appropriate range and the desired roughness resistance was not obtained. [Industrial Applicability]

[0126] The ferritic stainless steel sheet of the present invention is particularly suitable for use as corrosion-resistant members where the appearance is important and scratches must be less noticeable, such as panels of commercial refrigerators, inner panels of elevators, top panels of sinks, components of home appliances, and interiors. [Explanation of symbols]

[0127] 1 test piece 2 rubber stoppers 3 Abrasive paper 4. Imaging equipment 5 Light source (lighting) 6 Background (screen) 7 Scratched area 8 Non-scratched area 9 ROI of the scratched area 10 ROI of non-scratch area

Claims

1. Sa is 10.0 μm or more and 40.0 μm or less, Spd is 1500 pieces / mm ２ This is the ferritic stainless steel plate. Here, Sa and Spd are surface texture parameters defined in ISO25178, Sa is the arithmetic mean height, and Spd is the peak density.

2. GS(60°) is 50 or less, L* is 50 or more, The scratch visibility index is 50 or less, The ferritic stainless steel sheet according to claim 1, wherein the maximum autocorrelation length is 50 μm or less. Here, GS(60°), L*, the scratch conspicuousness index, and the maximum autocorrelation length are as follows: GS (60°): 60-degree specular gloss as defined in JIS Z 8741:1997 L*: Lightness index as defined in JIS Z 8781-4:2013 Scratch conspicuousness index: After the following scratching test, the difference in brightness between the scratched area and the non-scratched area in a grayscale image of the surface of a ferritic stainless steel plate obtained by photographing under the following photographing conditions. [Scratch test] Grinding paper with abrasive grain size of #240 was pressed against the surface of the ferritic stainless steel plate with a pressing load of 0.01 N / mm. ２ The ferritic stainless steel sheet is moved in the rolling direction at a speed of 10 mm / s to form scratches on the surface of the ferritic stainless steel sheet. The area on the surface of the ferritic stainless steel sheet other than the scratches is defined as the non-scratched area. The pressing load is set per unit area (1 mm ２ ) is the load per [Shooting conditions] The background color reflected on the surface of the ferritic stainless steel plate is set to black, and light is irradiated from a direction perpendicular to the surface of the ferritic stainless steel plate. Maximum autocorrelation length: In an autocorrelation image obtained by image processing a microscopic image of the surface of a ferritic stainless steel plate using a coaxial epi-illumination method, the absolute maximum length of the area that includes the center of the image and has pixel brightness values ​​equal to or greater than a reference value is divided by 2.

Images