Ferritic stainless steel sheet

Abstract

A ferritic stainless steel sheet which is characterized by containing, in mass%, 0.02% or less of C, 0.01% to 1.00% of Si, 0.01% to 1.00% of Mn, 0.040% or less of P, 0.006% or less of S, 12.5% to 30.0% of Cr, 0.30% to 2.50% of Mo, 0.02% to 0.20% of Al, 0.35% or less of Ti, 0.35% or less of Nb and 0.050% or less of N, and which is also characterized in that, with respect to the steel sheet surface, the reflectance which is defined as the difference between the tristimulus value Y10 of di:8° (including the specular reflection component from the sample) and the tristimulus value Y10 of de:8° (excluding the specular reflection component from the sample) as specified in the geometric conditions C of JIS Z 8722 is 8 to 15 and the specular gloss at 75 degrees Gs(75°) as determined by the method 2 of JIS Z 8741 is 30 to 80.

Description

Technical Field

[0001] The present invention relates to a ferritic stainless steel sheet that exhibits excellent anti-glare properties when used as exterior building materials typified by the roofs and outer walls of buildings or as vehicle steel sheets.

Background Art

[0002] Due to the demand for maintenance-free in the industry, stainless steel sheets are being used as building materials such as exterior wall materials and roof materials, and vehicle materials. Recently, the amount of ferritic stainless steel used as exterior materials for large buildings has been increasing. This is because ferritic stainless steel has less thermal expansion than austenitic stainless steel and does not cause much change due to repeated thermal expansion and contraction.

[0003] As characteristics required for exterior steel sheets including roof materials, while ensuring corrosion resistance in the atmospheric environment to which the steel sheet is exposed, anti-glare properties that suppress the reflection of sunlight in consideration of the surrounding environment are being emphasized, and the required level of anti-glare properties is also increasing.

[0004] <000002I>When imparting unevenness to the surface to impart anti-glare properties, various methods for enhancing anti-glare properties by devising the state of the surface formed by dull finishing have been disclosed.

[0005] For example, Patent Document 1 discloses a technique for enhancing anti-glare properties by controlling unevenness by dull finish roll rolling. On the other hand, when imparting unevenness to the surface to enhance anti-glare properties, it is necessary to pay attention to the lack of a stainless steel-like luster (increase in whiteness). However, the above documents do not disclose a technique for controlling luster.

[0006] Patent Document 2 discloses a manufacturing technique in which a steel sheet roughened by shot blasting to enhance anti-glare properties is then given luster in a cold rolling process, but the specific surface shape is not disclosed.

[0007] Patent Document 3 discloses a technique for producing a ferritic stainless steel sheet with low gloss and low whiteness by controlling the arithmetic mean roughness Sa, recess area ratio, and developed area ratio Sdr of the surface after dull finishing. However, the above technique has the problem that it cannot satisfy the need for even higher anti-glare properties due to the limitation of low whiteness.

[0008] Patent Document 4 discloses a technology relating to ferritic stainless steel that possesses both high whiteness and high image quality. However, this document does not disclose any technology related to anti-glare properties. Anti-glare properties depend on the total amount of light observed reflected from the surface of the steel plate; that is, it is necessary to consider the light reflected not only from the raised areas of the surface shape but also from the valleys, and this technology is not disclosed.

[0009] Patent document 5 discloses technology for a ferritic stainless steel sheet that achieves both aesthetic appeal and anti-glare properties. Aesthetic appeal is defined by whiteness, and no technology related to glossiness, which is an indicator of the glossiness of the steel sheet, is disclosed. [Prior art documents] [Patent Documents]

[0010] [Patent Document 1] Japanese Patent Application Publication No. 11-244904 [Patent Document 2] Special Publication No. 2018-524175 [Patent Document 3] Japanese Patent Publication No. 2021-038431 [Patent Document 4] International Publication No. WO2023-032377 [Patent Document 5] Japanese Patent Publication No. 2022-116519 [Overview of the Initiative] [Problems that the invention aims to solve]

[0011] In view of the problems of the conventional technology described above, the object of the present invention is to provide a ferritic stainless steel sheet that combines high anti-glare properties and high gloss. [Means for solving the problem]

[0012] In order to solve the above problems, this invention investigated the relationship between the surface morphology, gloss, and reflectivity of stainless steel sheets. To achieve high anti-glare properties, surface whitening due to light diffused by surface depressions is unavoidable. In contrast, this invention has found that by appropriately dispersing smooth areas without depressions, it is possible to achieve a surface that combines high gloss and high anti-glare properties.

[0013] The gist of the ferritic stainless steel sheet with excellent anti-glare properties in this invention is as follows: [1] Containing, by mass%, C: 0.020% or less, Si: 0.01~1.00%, Mn: 0.01~1.00%, P: 0.040% or less, S: 0.0060% or less, Cr: 12.5~30.0%, Mo: 0.30~2.50%, Al: 0.020~0.200%, Ti: 0.35% or less, Nb: 0.35% or less, N: 0.050% or less, with the remainder being Fe and impurities, the reflectance on the steel plate surface is defined as the difference between the tristimulus value Y10 when di: 8° (when including the component that causes specular reflection from the sample) and the tristimulus value Y10 when de: 8° (when excluding the component that causes specular reflection from the sample), as specified in geometric condition C of JIS Z 8722, and JIS Z A ferritic stainless steel sheet characterized by having a 75-degree specular gloss Gs(75°) of 30 to 80 as defined in Method 2 of 8741.

[0014] [2] The ferritic stainless steel sheet according to [1], characterized in that the root mean square gradient Sdq of the contour curved surface as defined in JIS B 0681-2:2018 is 0.4 or more and 1.0 or less on the surface of the steel sheet, the area ratio S of the smooth portion without depressions is 50% or more and 85% or less, and the relationship between the area ratio S of the smooth portion and the root mean square gradient Sdq of the contour curved surface satisfies equation (1). 24<5.4-27.0×Sdq+0.9×S<70 ··· Formula (1)

[0015] [3] Further, in place of a part of the Fe, in mass %, one or more of the following are contained: Ni: 0.5% or less, Cu: 0.5% or less, V: 1.00% or less, Zr: 0.005 - 0.500%, Ga: 0.030% or less, REM: 0.10% or less, Ta: 0.10% or less, Sn: 0.005 - 0.500%, W: 1.00% or less, Co: 1.00% or less, Sb: 0.30% or less. The ferritic stainless steel sheet according to [1] or [2] is characterized by this.

[0016] [4] The ferritic stainless steel sheet according to [1] or [2], which is characterized by being used for exterior building materials. [5] The ferritic stainless steel sheet according to [3], which is characterized by being used for exterior building materials.

[0017] According to the present invention, it is possible to provide a stainless steel sheet having both high antiglare property and high gloss.

Brief Description of the Drawings

[0018] [Figure 1] It is a white interference microscope image explaining the recessed part and the smooth part.

Modes for Carrying Out the Invention

[0019] The various conditions limited in the present invention will be described below.

[0020] <Surface Form> Anti-glare properties are achieved by diffusing the light incident on the steel plate surface, that is, by suppressing the light that is specularly reflected by the steel plate surface. Suppressing specular reflection is effective by roughening the steel plate surface, especially when it has depressions that diffuse light. On the other hand, gloss is achieved on surfaces where specular reflection is abundant, and the smoother the surface, the higher the gloss. Therefore, if a roughened surface with depressions is made to achieve a highly anti-glare surface, the surface becomes low-gloss, and furthermore, the diffused light causes the surface to whiten. As described above, anti-glare and gloss are inversely related. The inventors have found a solution to this problem by dispersing highly anti-glare areas and highly glossy areas on the steel plate surface. In other words, by appropriately dispersing depressions that suppress specular reflection and smooth areas that increase specular reflection, they have achieved both high anti-glare and high gloss.

[0021] Figure 1 is a white-light interference microscope image illustrating recessed and smooth areas. Recesses occur on the surface of steel plates due to shot blasting and dull roll rolling. The surface of the steel plate is observed using a white-light interference microscope with a 50x objective lens and a 0.55x internal lens, and the distribution of areas with a depth of 0.5 μm or more is identified from the resulting three-dimensional image. In this invention, areas with a depth of 0.5 μm or more are defined as recesses. The boundary between areas with a depth of 0.5 μm or more and areas with a depth of less than 0.5 μm is defined as the contour. Within the contour of the recess, areas with an equivalent circular diameter of 10 μm or more are defined as recessed areas, and the surfaces other than the recessed areas are defined as smooth areas.

[0022] While there are no particular limitations on the degree of smoothness of the smooth areas, a Ra of less than 1.0 μm as defined in JIS B 0601 is preferable to ensure specular reflectivity.

[0023] <Reflectance> Reflectance is defined as the difference between the tristimulus value Y10 at di:8° (when the component resulting in specular reflection from the sample is included) and the tristimulus value Y10 at de:8° (when the component resulting in specular reflection from the sample is excluded), as specified in geometric condition C of JIS Z 8722. Y10 is one of the tristimulus values ​​that represents the spectral sensitivity corresponding to the human eye and indicates luminance. The difference between the tristimulus value Y10 at di:8° and the tristimulus value Y10 at de:8°, which is the defined reflectance, is the luminance of the specularly reflected light reflected from the sample surface. If this reflectance is low, glare is reduced but the appearance becomes whiter. If the reflectance is high, glare increases. If the reflectance is less than 8, the appearance of the stainless steel becomes whiter, resulting in a poor appearance. Therefore, the reflectance should be 8 or higher. If the reflectance exceeds 15, specular reflection from the surface increases and the anti-glare properties are impaired. Therefore, the reflectance should be 15 or lower.

[0024] <Glossiness> The gloss was evaluated using the 75-degree specular gloss Gs(75°) specified in JIS Z 8741. As described above, the steel plate surface according to the present invention has areas of low gloss and areas of high gloss dispersed, and the gloss of the steel plate surface represents the sum of the gloss values ​​of both areas. Generally, to measure the gloss of a non-glossy surface, an incident (receiving) angle greater than 60° is preferable, so the 75-degree specular gloss Gs(75°) was adopted to reflect the gloss of the low-gloss areas in the evaluation. Hereafter, it will also be abbreviated as "Gs(75°)". In this invention, Gs(75°) is set to 30 to 80. If Gs(75°) is less than 30, the gloss is impaired. Therefore, Gs(75°) is set to 30 or higher. If Gs(75°) exceeds 80, the difference in gloss between the low-gloss recessed areas and the high-gloss smooth areas becomes large, exceeding 100, giving the surface a glaring impression. Therefore, the gloss is set to 80 or lower.

[0025] <Root mean square gradient Sdq of a contour surface> To impart anti-glare properties to a steel plate, it is necessary to form recesses to scatter incident light. These recesses are responsible for light scattering, and the reflectance (anti-glare properties) and glossiness are evaluated based on the light diffused by the numerous recesses present in the surface. Therefore, it is necessary to evaluate the entire set of recesses present in the surface. The inventors have found a correlation between the root mean square gradient Sdq of the contour surface, which indicates the degree of unevenness in the surface as defined in JIS B 0681, and the reflectance.

[0026] The root mean square gradient Sdq of a contour surface can be evaluated using a white light interferometry microscope. Refer to Figure 1 for explanation. The height Z is evaluated for all areas of width X and depth Y within the field of view observed with the white light interferometry microscope, and the evaluated surface (XYZ) data constitutes the contour surface. The contour surface includes both concave and smooth areas. The root mean square gradient Sdq of the contour surface is an index indicating the severity of the surface irregularities, obtained by differentiating the gradient of the surface irregularities. In the following formula, A is the reference area, which in this invention refers to the entire measurement surface. Z(X,Y) is a function representing the height Z at coordinate (X,Y). Sdq = {(1 / A)∫∫ A [(∂Z(X,Y) / ∂X) 2 +(∂Z(X,Y) / ∂Y) 2 ]dXdY} 0.5

[0027] If the root mean square gradient Sdq of the contour surface is less than 0.4, in-plane light scattering is insufficient and it is difficult to obtain a reflectance of 15 or less; therefore, the root mean square gradient Sdq of the contour surface is preferably set to 0.4 or higher. If the root mean square gradient Sdq of the contour surface exceeds 1.0, light scattering increases and it becomes difficult to obtain a reflectance of 8 or higher; therefore, the root mean square gradient Sdq of the contour surface is preferably set to 1.0 or lower.

[0028] <Area ratio S of the smooth area> In a white-light interferometry microscope observation using a 50x objective lens and a 0.55x internal lens for observing the surface morphology, the captured image is binarized to separate the area inside and outside the contour of the recessed portion, and the area ratio S of the smooth portion is calculated from the image analysis. The area ratio S of the smooth portion is the average value of 20 fields of view of the image.

[0029] If the area ratio S of the smooth area is less than 50%, the diffuse reflection component of the light reflected from the surface of the stainless steel plate increases, causing the steel plate surface to appear white and impairing its appearance. Therefore, the lower limit of the area ratio S of the smooth area is preferably set to 50%. On the other hand, if the area ratio S of the smooth area exceeds 85%, the specular reflection component of the light reflected from the surface of the stainless steel plate increases, degrading the anti-glare properties of the steel plate. Therefore, the upper limit of the area ratio S is preferably set to 85%.

[0030] Furthermore, in order to impart a suitable luster to a steel plate, it is necessary to form smooth areas that do not easily scatter incident light. The visual characteristics of luster depend on the area of ​​smooth areas present within a surface. From the definition of smooth areas described above, we found that a stainless steel-like luster can be ensured by controlling the ratio of smooth areas S within the surface. That is, when the ratio of smooth areas S and the root mean square gradient Sdq of the contour curved surface described above preferably satisfy equation (1), it is possible to impart a high level of anti-glare and a high luster characteristic of stainless steel. 24<5.4-27.0×Sdq+0.9×S<70 ··· Formula (1)

[0031] Furthermore, even if the root mean square gradient Sdq falls outside the range of 0.4 to 1.0, if the root mean square gradient Sdq is within the range of 0.3 to 1.1 and satisfies equation (1), and the area ratio S of the smooth portion is 50% to 85%, the reflectance and gloss required by the present invention can be obtained. Also, even if the area ratio S of the smooth portion exceeds 80% and does not satisfy equation (1), if the root mean square gradient Sdq is within the range of 0.4 to 1.0 and the value of the middle side of equation (1) is 72 or less, the reflectance and gloss required by the present invention can be obtained.

[0032] <Ferritic stainless steel> This invention relates to ferritic stainless steel. As described later, when an acid pickling step is added to the manufacturing process, a mixed acid of nitric acid and hydrofluoric acid is preferably used. In this process, ferritic stainless steel dissolves more uniformly on the surface than austenitic stainless steel, and can be manufactured without damaging the uneven surface created by shot blasting or dulling. On the other hand, austenitic stainless steel has the characteristic that the grain boundaries dissolve preferentially in nitric acid and hydrofluoric acid pickling, and the uneven surface created by shot blasting or dulling cannot be maintained after pickling. For this reason, this invention relates to ferritic stainless steel.

[0033] <About the steel composition> As described above, the present invention relates to the control of the root mean square gradient Sdq and the smooth area ratio S of the contour curved surface of a stainless steel sheet. The following shows the range of stainless steel components to which the solution for forming the surface properties is applied. In this specification, "%" refers to "mass%" unless otherwise specified.

[0034] C: 0.020% or less Since carbon (C) degrades the formability and corrosion resistance of steel sheets, its content must be kept low, and was set at 0.020% or less. Excessive reduction of the C content increases refining costs, so considering corrosion resistance, a C content of 0.002-0.008% is preferable.

[0035] Si: 0.01~1.00% Si is useful as a deoxidizing agent and is also an effective element for corrosion resistance, high-temperature strength, and oxidation resistance, and these effects can be obtained at concentrations of 0.01% or higher. On the other hand, adding more than 1.00% Si deteriorates processability and pickling properties. Therefore, the Si content should be 0.01 to 1.00%, preferably 0.04 to 0.60%.

[0036] Mn: 0.01~1.00% Mn is an element added as a deoxidizing agent, and its effect is obtained at a concentration of 0.01% or higher. Adding too much Mn will degrade corrosion resistance and oxidation resistance, so the Mn content should be between 0.01% and 1.00%. Preferably, it should be between 0.04% and 0.60%.

[0037] P:0.040% or less Since phosphorus (P) impairs the toughness of the base material and weld, the upper limit of the P content was set at 0.040%. On the other hand, drastically reducing the P content of stainless steel leads to increased manufacturing costs, so a lower limit of 0.010% for the P content is preferable.

[0038] S:0.0060% or less S is a harmful element that adversely affects corrosion resistance and high-temperature cracking of welded joints. Therefore, the upper limit for S content is set at 0.0060%. Preferably, it is 0.0030% or less.

[0039] Cr: 12.5~30.0% Cr is a key component for maintaining corrosion resistance, and corrosion resistance improves with increasing Cr content. A Cr content of 12.5% ​​or more is necessary to ensure the desired corrosion resistance in steel. Since exceeding 30.0% Cr content degrades manufacturability and increases costs, the upper limit for Cr content is set at 30.0%. Preferably, it is between 17.0% and 25.0%.

[0040] Mo: 0.30~2.50% Mo, when combined with Cr, significantly improves the corrosion resistance of steel. However, if its content is less than 0.30%, the corrosion resistance is insufficient, while if it exceeds 2.50%, the toughness and workability of the steel deteriorate. Therefore, the Mo content should be between 0.30% and 2.50%. Preferably, it is between 0.50% and 1.80%.

[0041] Al: 0.020~0.200% Al is added as a deoxidizing element, and it also improves oxidation resistance. If the Al content is less than 0.020%, the deoxidizing effect cannot be fully obtained. On the other hand, excessive addition of Al deteriorates processability. Therefore, the Al content should be between 0.020% and 0.200%. Preferably, it is between 0.040% and 0.120%.

[0042] Ti: 0.35% or less By adding Ti, carbon (C) and nitrogen (N), which negatively affect corrosion resistance and workability, can be fixed. While Ti is added for this purpose, excessive Ti content increases surface defects and degrades manufacturability. Therefore, the upper limit of Ti content is set at 0.35%. Preferably, it is 0.28% or less. The preferred lower limit of Ti content is 0.05%. Ti may not be included at all.

[0043] Nb: 0.35% or less Like Ti, Nb can fix C and N, which negatively affect corrosion resistance and workability. Nb is added for this purpose, but excessive Nb content hardens the steel and deteriorates its workability. Therefore, the upper limit for Nb content is set at 0.35%. Preferably, it is 0.28% or less. The preferred lower limit for Nb content is 0.04%. Nb may not be included at all.

[0044] N: 0.050% or less Similar to carbon (C), nitrogen (N) degrades moldability and corrosion resistance, so the N content was kept below 0.050%. Furthermore, excessive reduction of the N content increases refining costs, so considering oxidation resistance, a content of 0.002-0.025% is preferable.

[0045] The remainder of the above steel components consists of Fe and unavoidable impurities. Here, unavoidable impurities refer to components that are mixed in during the industrial production of steel due to various factors in the manufacturing process, including raw materials such as ore and scrap, and are acceptable as long as they do not adversely affect the present invention.

[0046] Furthermore, in place of some of the Fe, one or more of the following may be included: Ni: 0.5% or less, Cu: 0.5% or less, V: 1.00% or less, Zr: 0.005-0.50%, Ga: 0.030% or less, REM: 0.10% or less, Ta: 0.10% or less, Sn: 0.005-0.50%, W: 1.00% or less, Co: 1.00% or less, and Sb: 0.30% or less.

[0047] Ni: 0.5% or less Ni is an element that improves rust resistance and can be added as needed. Improved rust resistance can be expected when the Ni content is 0.05% or higher. When the Ni content exceeds 0.5%, oxidation resistance deteriorates, as does processability; therefore, the upper limit for Ni content is preferably 0.5%, and more preferably 0.3% or less.

[0048] Cu: 0.5% or less Cu is an element that improves rust resistance and high-temperature strength, and can be added as needed. Since ductility deteriorates when the Cu content exceeds 0.5%, the upper limit is preferably 0.5%. More preferably, it is 0.30% or less.

[0049] V: 1.00% or less Since V is an element that improves corrosion resistance, it can be added as needed. Because corrosion resistance and processability deteriorate when the V content exceeds 1.00%, the upper limit is preferably 1.00%. More preferably, it is 0.50% or less.

[0050] Zr: 0.005~0.500% Zr can be added as needed to improve corrosion resistance, and its effect is observed at a Zr content of 0.005% or higher. While Zr is an important element for suppressing the corrosion rate, excessive addition worsens manufacturability and cost. Therefore, the Zr content range is preferably 0.005 to 0.500%, and more preferably 0.050 to 0.400%.

[0051] Ga: 0.030% or less Ga may be included as needed because it has the effect of improving corrosion resistance. However, if Ga is included in excess, the hot workability will decrease, so the Ga content is preferably 0.030% or less. More preferably 0.010% or less.

[0052] REM: 0.10% or less REM is an element that improves hot workability and the cleanliness of steel, and is effective in improving corrosion resistance in the present invention, and may be added as needed. When REM is added, it is preferable that it is 0.001% or more, which is the amount at which its effect is exhibited. However, excessive addition leads to an increase in alloy cost and a decrease in manufacturability, so the upper limit of REM content is preferably 0.10%. More preferably, considering the effect, economy, and manufacturability, it is 0.001 to 0.10% of one or more types. REM is an element belonging to atomic numbers 57 to 71, and examples include La, Ce, and Nd.

[0053] Ta: 0.10% or less Excessive Ta content reduces toughness, so a Ta content of 0.10% or less is preferable. Furthermore, since Ta has the effect of improving corrosion resistance, it may be included at 0.01% or more as needed. More preferably, it is 0.03 to 0.08%.

[0054] Sn: 0.005~0.500% Adding sn (Sn) further enhances the high corrosion resistance of stainless steel. To achieve this effect, it is preferable to contain 0.005% or more sn, more preferably 0.010% or more, and even more preferably 0.020% or more. On the other hand, excessive addition leads to a decrease in workability, so the upper limit of the sn content is preferably 0.500%, and more preferably 0.300% or less.

[0055] W: 1.00% or less W may be included as needed because it has the effect of improving corrosion resistance. However, if W is included in excess, the toughness will decrease due to coarse carbonitrides, etc., so the W content is preferably 1.00% or less. More preferably 0.60% or less.

[0056] Co: 1.00% or less Co may be included as needed because it has the effect of improving the rust resistance of steel. However, if Co is included in excess, the toughness will decrease, so the Co content is preferably 1.00% or less. More preferably 0.60% or less.

[0057] Sb: 0.30% or less Sb can be added as needed to improve corrosion resistance, and its effect is observed when the Sb content is 0.005% or higher. Although Sb is an important element for suppressing the corrosion rate, excessive addition worsens manufacturability and cost, so the Sb content should be 0.30% or less. The Sb content range is preferably 0.010 to 0.25%, and more preferably 0.020 to 0.20%.

[0058] <Manufacturing method> The processes that can control the recesses and smooth surfaces of the final stainless steel sheet product are limited, making the cold rolling process and the final annealing and pickling process crucial.

[0059] To appropriately impart both recessed and smooth areas to the surface of the final stainless steel sheet product, dull rolling in the cold rolling process and shot blasting before pickling in the final annealing and pickling process are suitable.

[0060] Dull rolling is a rolling method in which steel sheets are rolled using generally roughened rolling rolls, thereby transferring the roughened surface of the rolls to the steel sheets. Generally used dull rolling rolls have a roughened surface across their entire surface, making it difficult to impart high gloss by retaining smooth areas, which is one of the features of the present invention. Therefore, in order to impart smooth areas to the steel sheets, it is necessary to form smooth areas on the dull rolls themselves.

[0061] For example, a method can be used to randomly roughen a dull roll by shot blasting, while simultaneously imparting smooth areas with minimal irregularities to the surface of the dull roll even when shot particles are projected onto it. The following describes the preferred conditions for shot blasting a dull roll. The shot particles are spherical, and a mixture of relatively small (200-400 μm) and relatively large (600-800 μm) particles is used. The mixing ratio should be adjusted so that the average particle size of 600-800 μm is 10% or less by weight, with the remainder being mainly shot particles with an average particle size of 200-400 μm. The mixture of large and small shot particles disperses the impact energy of each individual shot particle, allowing for a random dispersion of roughened areas with large irregularities and smooth areas. Even 0.5% of the 600-800 μm shot particles is effective. The projection density should be 20-60 kg / m³. 2 The projection speed is preferably controlled to 35-85 m / s. If the number of rolling cycles using the dull roll whose surface shape has been adjusted by the method described above is large, the area ratio S of the smooth portion of the stainless steel sheet will fall outside the range of the present invention, so it is preferable that the number of rolling cycles using the dull roll be 3 or less.

[0062] By using the aforementioned dull rolls and rolling the steel sheet for three or fewer passes, the resulting surface shape of the steel sheet, with its transferred irregularities, satisfies the root mean square gradient Sdq of the contour curved surface: 0.4 to 1.0, the area ratio of the smooth part S: 50 to 85%, and equation (1). The surface reflectivity characteristics are a reflectance of 8 to 15 and a gloss (Gs(75°)): 30 to 80, resulting in a stainless steel sheet with high anti-glare properties and gloss. The final annealing pickling after dull roll rolling is not particularly limited to the general stainless steel manufacturing process, except for steps that include shot blasting.

[0063] This document describes preferred conditions for imparting high anti-glare properties and high gloss to steel plates using shot blasting. The shot particles are spherical, and a mixture of relatively small (200-400 μm) and relatively large (600-800 μm) average particle sizes is used. The mixing ratio should be adjusted so that the 600-800 μm particles constitute 10% or less by weight, with the remainder being primarily 200-400 μm particles. The mixture of large and small particles disperses the impact energy of each individual particle, allowing for a disordered dispersion of roughened and smooth areas. Even 0.5% of the 600-800 μm particles is effective. The projection density should be 10-40 kg / m³. 2 The projection speed is preferably controlled to 30-80 m / s. By applying such shot blasting to a steel plate, the shape of the steel plate surface satisfies the root mean square gradient Sdq of the contour curved surface: 0.4-1.0, the area ratio of the smooth part S: 50-85%, and equation (1), and the surface reflection characteristics are a reflectance of 8-15 and gloss (Gs(75°)): 30-80, resulting in a stainless steel plate with high anti-glare properties and gloss. The material of the shot particles is preferably metal or ceramic. The pickling process after shot blasting is not mandatory and may be bypassed if necessary.

[0064] If an acid pickling process is added, a mixed acid of nitric acid and hydrofluoric acid is preferable. If an acid solution other than a mixed acid of nitric acid and hydrofluoric acid is used, the entire surface of the steel sheet will dissolve unevenly, increasing the unevenness of the smooth areas other than the depressions created by shot blasting or dull roll rolling, and reducing the gloss. Furthermore, the concentrations of each acid solution are preferably HF: 20-50 g / L, HNO3: 50-110 g / L, the acid solution temperature is preferably 30-60°C, and the pickling time is preferably 10-120 s.

[0065] Aside from the cold rolling and final annealing / pickling processes mentioned above, the other steps are not particularly limited and can be carried out using general stainless steel manufacturing processes.

[0066] <Used in exterior building materials> The ferritic stainless steel sheet of the present invention is suitable for use in exterior building materials because it is a stainless steel sheet that combines high glare resistance and high gloss. [Examples]

[0067] Molten steel having the chemical composition shown in Table 1, adjusted to the specified components, was produced, and this molten steel was cast into slabs. The resulting slabs were hot-rolled, and then subjected to hot-rolled sheet annealing and pickling, cold-rolling, and final annealing and pickling to produce 2.0 mm thick cold-rolled sheets.

[0068] Recesses were formed on the surface of the cold-rolled sheet by dull roll rolling in the final cold-rolling process or by shot blasting in the final annealing and pickling process. Shot blasting was not performed in the final annealing and pickling process after dull roll rolling. The shot blasting conditions in the final annealing and pickling process and the dull roll rolling conditions in the final cold-rolling process are shown in Table 2. The weight ratio (%) of shot particles with an average particle size of 600 to 800 μm is shown as the "Shot particle size A ratio".

[0069] After intermediate and final cold rolling, annealing was performed in a furnace with controlled soaking temperature and time. If pickling was not performed in the final annealing pickling process, the final annealing was performed using bright annealing with controllable dew point. After final annealing, immersion pickling was performed using a pickling solution containing NaNO3 and HF. Immersion pickling was performed with HF: 40 g / L, HNO3: 90 g / L, acid solution temperature at 50°C, and pickling time of 30 s.

[0070] Surface optical property analysis was performed on the surface of steel sheets after dull roll rolling or shot blasting, followed by nitrite-hydrofluoric acid pickling. Reflectance was defined as the difference between the tristimulus value Y10 with di:8° (including the component that results in specular reflection from the sample) and the tristimulus value Y10 with de:8° (excluding the component that results in specular reflection from the sample), as specified in geometric condition C of JIS Z 8722. Y10 used in the calculation of reflectance was measured using a spectrophotometer (Konica Minolta CM-700d).

[0071] Glossiness was evaluated using a gloss meter (Suga Test Instruments Co., Ltd.: Gloss Meter UGV-6P) according to the 75-degree specular gloss Gs(75°) specified in JIS Z 8741.

[0072] Furthermore, the surface shape of the steel plate surface was analyzed. The root mean square gradient Sdq of the contour curve was measured using a white light interference microscope (Bruker Contour GT-I Elite). Objective lens magnification: 50x, internal lens magnification: 0.55x, measurement range: 3mm x 3mm, noise reduction: median filter with filter size 3x3 was applied.

[0073] The area ratio S of the smooth area was evaluated using a white light interferometry microscope with a 50x objective lens and a 0.55x internal lens. In the observation field, areas with a depth of 0.5 μm or more were defined as depressions based on the average height information, and the boundary between areas with a depth of 0.5 μm or more and areas with a depth of less than 0.5 μm was defined as the contour. The area inside the contour of the depression with an equivalent circle diameter of 10 μm or more was defined as the depression, and the surface outside the depression was defined as the smooth area (Figure 1). The area ratio S of the smooth area was calculated by image analysis of the relief image obtained by the white light interferometry microscope. The captured image was binarized into areas inside and outside the contour of the depression, and the area ratio S of the smooth area was calculated from the image analysis. The area ratio S of the smooth area was taken as the average value of the five fields of view described above. The root mean square gradient Sdq of the contour surface was also evaluated using a white light interferometry microscope.

[0074] Anti-glare properties were considered acceptable if the reflectance was between 8 and 15. If the reflectance was less than 8, the stainless steel would appear white, resulting in an undesirable appearance and therefore it was deemed unacceptable. If the reflectance exceeded 15, specular reflection on the surface increased, impairing the anti-glare properties and therefore it was deemed unacceptable. Gloss was considered acceptable if the 75-degree specular gloss Gs(75°) (hereinafter simply referred to as "Gs(75°)") was between 30 and 80. If Gs(75°) was less than 30, the gloss was impaired, so a Gs(75°) of 30 or higher was considered acceptable. If Gs(75°) exceeded 80, the difference in gloss between the low-gloss recessed areas and the surface became large, giving the surface a glaring impression, so a Gs(75°) of 80 or lower was considered acceptable.

[0075] <Example 1> The steels No. A1 to A16 shown in Table 1 were manufactured under the conditions indicated by symbol B6 in Table 2, following the final cold rolling, final annealing, and pickling processes.

[0076] Anti-glare properties were evaluated as follows: reflectance of 8-11 was particularly good (S), reflectance of 12-15 was good (G), and reflectance outside the range of 8-15 was poor (X). Gloss was evaluated as follows: Gs(75°) of 30-60 was particularly good (S), Gs(75°) of 61-80 was good (G), and Gs(75°) outside the range of 30-80 was poor (X).

[0077] [Table 1]

[0078] [Table 2]

[0079] [Table 3]

[0080] As shown in Table 3, symbols C1 to C16 satisfied the conditions of the present invention in terms of steel composition, reflectance, and gloss, and therefore exhibited particularly good (S) or good (G) anti-glare surface properties that combined reflectance and gloss.

[0081] Steel No. a1, with designation c1, has a carbon content exceeding the upper limit of the content range, resulting in the formation of coarse Ti and Nb carbides. During shot blasting and dull roll rolling, these carbides could not follow the deformation of the base material, promoting fine cracking, resulting in poor reflectivity and gloss (X). In steel No. a2, the Si and Mn content exceeded the upper limit of the content range, resulting in a hard base material. Consequently, the base material could not undergo sufficient plastic deformation during shot blasting or dull roll rolling, resulting in a gloss level exceeding the upper limit and being poor (X).

[0082] In steel No. a3, the P and S content exceeded the upper limit of the content range, promoting the formation of MnS and the intermetallic compound FeTiP. As a result, during pickling, the areas around MnS and intermetallic compounds dissolved preferentially, creating numerous fine depressions, resulting in poor reflectivity and gloss (X).

[0083] Steel No. a4, designated c4, had an Al content exceeding the upper limit of the content range, resulting in the formation of a large amount of intermetallic compounds such as AlN. During pickling, the area around the intermetallic compounds dissolved preferentially, creating numerous fine depressions, and resulting in poor reflectivity and gloss (X). In steel No. a5, the Al content was outside the lower limit of the content range, resulting in insufficient deoxidation. A large amount of oxide remained in the substrate, which, during shot blasting and dull roll rolling, could not follow the deformation of the substrate, promoting fine cracks and resulting in poor reflectivity and gloss (X).

[0084] In steel No. a6, the Cr content exceeded the upper limit of the content range, resulting in a hard base material. Consequently, the base material could not undergo sufficient plastic deformation during shot blasting or dull roll rolling, causing the reflectivity to exceed the upper limit and resulting in a poor (X) rating. In steel No. a7, the Cr content fell outside the lower limit of the content range, resulting in a softened base material. Consequently, the base material deformed excessively during shot blasting and dull roll rolling, causing the reflectivity to exceed the lower limit and resulting in a poor (X) rating.

[0085] In steel No. a8, the Mo content exceeded the upper limit of the content range, resulting in a hard base material. Consequently, the base material could not undergo sufficient plastic deformation during shot blasting or dull roll rolling, causing the reflectivity to exceed the upper limit and resulting in a poor (X) rating. Steel No. a9, designated c9, had a Mo content outside the lower limit of the content range, resulting in insufficient corrosion resistance. During pickling, the substrate was excessively dissolved across its entire surface, creating numerous fine depressions, and resulting in poor reflectivity and gloss (X).

[0086] Steel No. a10, with designation c10, has Ti, Nb, and N content that exceeds the upper limit of the content range, resulting in the formation of coarse Ti nitrides and Nb nitrides. During shot blasting and dull roll rolling, these cannot follow the deformation of the base material, promoting fine cracking and resulting in poor reflectivity and gloss (X).

[0087] <Example 2> For steels No. A1-11, A13, A15, and A16 shown in Table 1, the processes from final cold rolling and final annealing and pickling were manufactured under the conditions indicated by symbols B1-B12 and b1-b9 in Table 2. Except for the conditions shown in Table 2, the process was the same as in Example 1 above. The results are shown in Table 4.

[0088] [Table 4]

[0089] As shown in Table 4, for symbols D2, D4, D6-D9, D11, and D12, the processes after the final cold rolling and final annealing pickling were within the preferred range of the present invention, and the root mean square gradient Sdq of the contour curved surface and the area ratio S of the smooth portion were within the most preferred range of the present invention. As a result, the reflectance and glossiness satisfied the conditions of the present invention, and thus the surface possessed both reflectance and glossiness, and the anti-glare surface properties were particularly good (S) or good (G).

[0090] The symbols D1, D3, D5, and D10 indicate that the processes from the final cold rolling and final annealing pickling onwards were within the preferred range of the present invention, and the root mean square gradient Sdq of the contour curved surface and the area ratio S of the smooth portion were within the preferred range of the present invention. As a result, the reflectance and glossiness satisfied the conditions of the present invention, and thus the anti-glare surface properties, which combined reflectance and glossiness, were good (G).

[0091] In the case of symbol d1 (manufacturing method No. b1), the shot blast particle size A ratio fell outside the upper limit of the preferred range, resulting in a high frequency of coarse shot particles impacting the steel plate, and the reflectivity and gloss fell below the lower limit of the preferred range, thus resulting in a defective product (X). In the case of symbol d2 (manufacturing method No. b2), the shot blasting speed and density were outside the upper limit of the suitable range, resulting in a large area of ​​impact between the shot particles and the steel plate, and the reflectivity and gloss were below the lower limit, thus it was a defective product (X). In the case of symbol d3 (manufacturing method No. b3), the shot blasting speed and density were outside the lower limit of the optimal range, resulting in a small impact area between the shot particles and the steel plate, and shallow indentations were formed, causing the reflectivity and gloss to exceed the upper limit, resulting in a defective product (X). In the case of symbol d4 (manufacturing method No. b4), the shot blast particle size A ratio fell outside the lower limit of the suitable range, resulting in a small impact area between the shot particles and the steel plate, and shallow indentations. Consequently, the reflectivity and gloss exceeded the upper limit, resulting in a defective product (X).

[0092] In the case of symbol d5 (manufacturing method No. b5), the ratio of shot blast particle size A to the dull roll fell outside the upper limit of the suitable range, resulting in a high frequency of coarse shot particles colliding with the dull roll, and the reflectivity and gloss fell below the lower limit, resulting in a defective product (X). In the case of symbol d6 (manufacturing method No. b6), the projection speed and projection density of the shot blast onto the dull roll fell outside the lower limit of the optimal range, resulting in a small impact area between the shot particles and the dull roll, and forming shallow indentations, which exceeded the upper limit in reflectivity and gloss, making it a defective product (X). In the case of symbol d7 (manufacturing method No. b7), the projection speed and projection density of the shot blast onto the dull roll fell outside the upper limit of the preferred range, resulting in a large area of ​​impact between the shot particles and the dull roll, and the reflectivity and gloss fell below the lower limit of the preferred range, thus resulting in a defective product (X). In the case of symbol d8 (manufacturing method No. b8), the ratio of shot blast particle size A to the dull roll fell outside the lower limit of the suitable range, the area in which the shot particles collided with the steel plate was small, and the resulting in shallow indentations caused the reflectivity and gloss to exceed the upper limit, resulting in a defective product (X).

[0093] In the case of symbol d9 (manufacturing method No. b9), the number of dull roll rolling cycles exceeded the upper limit of the suitable range, resulting in excessive transfer of the irregularities formed on the dull roll, and the reflectivity and gloss fell below the lower limit, resulting in a defective product (X). [Industrial applicability]

[0094] According to the present invention, a stainless steel sheet with high anti-glare properties and gloss, suitable for use on the roofs and exteriors of buildings, can be obtained.

Claims

1. In mass%, it contains C: 0.020% or less, Si: 0.01-1.00%, Mn: 0.01-1.00%, P: 0.040% or less, S: 0.0060% or less, Cr: 12.5-30.0%, Mo: 0.30-2.50%, Al: 0.020-0.200%, Ti: 0.35% or less, Nb: 0.35% or less, N: 0.050% or less, with the remainder being Fe and impurities. On the surface of the steel plate, the reflectance defined by the difference between the tristimulus value Y10 with di: 8° (when the component resulting in specular reflection from the sample is included) and the tristimulus value Y10 with de: 8° (when the component resulting in specular reflection from the sample is excluded), as specified in geometric condition C of JIS Z 8722, is between 8 and 15. Furthermore, a ferritic stainless steel sheet characterized by having a 75-degree specular gloss Gs (75°) of 30 to 80 as defined in JIS Z 8741 Method 2.

2. On the surface of the steel plate, the root mean square gradient Sdq of the contour curved surface as defined in JIS B 0681-2:2018 is 0.4 or more and 1.0 or less. The area ratio S of the smooth area without depressions is 50% or more and 85% or less. The ferritic stainless steel sheet according to claim 1, characterized in that the relationship between the area ratio S of the smooth portion and the root mean square gradient Sdq of the contour curved surface satisfies equation (1). 24<5.4-27.0×Sdq+0.9×S<70... Formula (1) The area ratio S of the smooth area is evaluated using a white light interference microscope with a 50x objective lens and a 0.55x internal lens. In the observation field, areas with a depth of 0.5 μm or more are defined as depressions based on the average value of the height information, the boundary between areas with a depth of 0.5 μm or more and areas with a depth of less than 0.5 μm is defined as the contour, and the area inside the contour of the depression with an equivalent circle diameter of 10 μm or more is defined as the depression, while the surface other than the depression is defined as the smooth area.

3. Furthermore, the ferritic stainless steel sheet according to claim 1 or 2 is characterized in that, in place of a portion of the Fe, it contains one or more of the following in mass percent: Ni: 0.5% or less, Cu: 0.5% or less, V: 1.00% or less, Zr: 0.005 to 0.500%, Ga: 0.030% or less, REM: 0.10% or less, Ta: 0.10% or less, Sn: 0.005 to 0.500%, W: 1.00% or less, Co: 1.00% or less, and Sb: 0.30% or less.

4. A ferritic stainless steel sheet according to claim 1 or 2, characterized in that it is used as an exterior building material.

5. The ferritic stainless steel sheet according to claim 3, characterized in that it is used as an exterior building material.

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