Ferritic stainless steel sheet

Abstract

To provide a ferritic stainless steel sheet that has excellent corrosion resistance and an improved surface gloss. [Solution] The ferritic stainless steel sheet comprises a base portion which is a hot-rolled and annealed sheet, and a surface portion which has undergone a scale removal treatment. The surface portion has a plurality of directional pits and a plurality of stepped portions on its surface, and the number of stepped portions located at the boundary of each directional pit, as determined by the cutting method, is 2000 or less per cm, and the arithmetic mean height on the surface is 3.0 μm or less, and the root mean square height is 3.5 μm or less.

Description

Technical Field

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

Background Art

[0002] Conventionally, ferritic stainless steel has been applied to various uses. The ferritic stainless steel sheet has its steel properties adjusted and various finishing treatments are applied according to the use. For example, in uses where surface gloss is not required, such as structural members, a ferritic stainless steel sheet subjected to a finishing treatment generally called "No. 1 finish" may be used. The ferritic stainless steel sheet with No. 1 finish is obtained by subjecting the hot-rolled sheet after hot rolling to heat treatment and then pickling treatment.

[0003] Regarding pickling treatment, various techniques have been proposed so far (for example, see Patent Document 1).

[0004] Also, Patent Document 2 describes a technique for adjusting the surface state of a ferritic stainless steel sheet.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] Generally, stainless steel sheets with various finishes are manufactured by applying further treatment to steel sheets after pickling. However, ferritic stainless steel sheets with a No. 1 finish are obtained by removing the oxide film (scale) from the surface of the steel sheet by pickling after heat treatment, and then forming a new passive film on the surface of the steel sheet in the atmosphere. For ferritic stainless steel sheets with a No. 1 finish, further surface treatment after pickling is not expected.

[0007] The pickling techniques described in Patent Document 1, etc., aim to improve the efficiency of scale removal and shorten the processing time, and do not adequately consider the surface condition of the steel sheet after pickling. This is because, since further surface treatment can usually be applied to the steel sheet after pickling, the surface condition of the steel sheet after pickling is not considered important. For example, as in the technique described in Patent Document 2, the surface condition of the steel sheet can be adjusted by applying further surface treatment to the steel sheet, but this type of technique cannot be applied to ferritic stainless steel sheets with a No. 1 finish.

[0008] For ferritic stainless steel sheets with a No. 1 finish, the surface condition after pickling must exhibit excellent corrosion resistance. Furthermore, the glossiness of the steel sheet surface affects its aesthetic appeal. There is also a demand for improved glossiness in the surface condition after pickling, compared to other types of ferritic stainless steel sheets with a No. 1 finish. [Means for solving the problem]

[0009] To solve the above problems, a ferritic stainless steel sheet in one aspect of the present invention comprises a base portion which is a hot-rolled and annealed sheet, and a surface portion located on the base portion which has undergone a scale removal treatment, wherein the surface portion has a plurality of directional pits on its surface and a plurality of stepped portions located at the boundaries of each directional pit, the number of stepped portions determined by the cutting method is 2000 or less / cm, and the arithmetic mean height on the surface is 3.0 μm or less and the root mean square height is 3.5 μm or less. [Effects of the Invention]

[0010] According to one aspect of the present invention, it is possible to provide a ferritic stainless steel sheet having a surface state excellent in corrosion resistance and improved glossiness.

Brief Description of the Drawings

[0011] [Figure 1] It is a schematic diagram for explaining a ferritic stainless steel sheet in one embodiment of the present invention. [Figure 2] It is a figure showing an example of an electron microscope image of the surface layer part of a ferritic stainless steel sheet in one embodiment of the present invention. [Figure 3] It is a figure for explaining an example of image processing for an electron microscope image and an example of measurement of a stepped part by a cutting method. [Figure 4] It is a flowchart for explaining a process related to the surface state of a ferritic stainless steel sheet in one embodiment of the present invention. [Figure 5] It is a figure showing an example of an electron microscope image of the surface layer part of a ferritic stainless steel sheet in a comparative example. [Figure 6] It is a schematic diagram for explaining pickling treatment in examples and comparative examples.

Mode for Carrying Out the Invention

[0012] Hereinafter, one embodiment of the present invention will be described. In this specification, “%” regarding the content ratio of each element in the chemical composition of a ferritic stainless steel sheet (ferritic stainless steel) means “mass %”. The term “steel sheet” is used to mean including a steel strip unless otherwise specified. For numerical values X1 and X2 (where X1 < X2), “X1 to X2” means “X1 or more and X2 or less”.

[0013] 〔Ferritic Stainless Steel Sheet〕 Figure 1 is a schematic diagram illustrating a ferritic stainless steel sheet in one embodiment of the present invention. The figure indicated by reference numeral 1001 in Figure 1 is a schematic perspective view of the ferritic stainless steel sheet, and the figure indicated by reference numeral 1002 in Figure 1 is a schematic partial cross-sectional view of the ferritic stainless steel sheet.

[0014] As shown in Figure 1, the ferritic stainless steel sheet 1 in one embodiment of the present invention comprises a base portion 11 and a surface layer portion 12 located on the base portion 11. Since the surface layer portion 12 is a very thin portion, it is omitted from the illustration shown by reference numeral 1001 in Figure 1. The illustration shown by reference numeral 1002 in Figure 1 schematically shows the surface layer portion 12. The surface layer portion 12 will be described in more detail later.

[0015] The ferritic stainless steel sheet 1 is a so-called No. 1 finished ferritic stainless steel sheet obtained by annealing a hot-rolled sheet after hot rolling, followed by a scale removal treatment including pickling. Therefore, the base portion 11 is a hot-rolled and annealed sheet, and the surface portion 12 is a portion formed by the scale removal treatment including pickling.

[0016] The ferritic stainless steel sheet 1 has a first surface 1A and a second surface 1B located on the opposite side of the first surface 1A. The first surface 1A and the second surface 1B are the main surfaces of the ferritic stainless steel sheet 1. In the ferritic stainless steel sheet 1, the surface layer 12 may be located on both the first surface 1A side and the second surface 1B side. The surface layer 12 may be located on either the first surface 1A side or the second surface 1B side. For the sake of explanation, the surface layer 12 located on the first surface 1A side may be referred to as the first surface layer 12A, and the surface layer 12 located on the second surface 1B side may be referred to as the second surface layer 12B.

[0017] The surface layer portion 12 has a surface state that satisfies specific conditions (described later) both on a microscopic scale observable with an electron microscope and on a macroscopic scale measurable with a laser microscope. As a result, it is possible to provide the ferrite stainless steel sheet 1 with a No. 1 finish having a surface state excellent in corrosion resistance and improved glossiness.

[0018] In general, by applying various surface finishes, steel sheets having a highly excellent glossiness are known. The measure of the superiority or inferiority of glossiness can vary depending on the type of surface finish (classification of ferrite stainless steel sheets according thereto) and the use of the steel sheet. The ferrite stainless steel sheet 1 in one embodiment of the present invention has a surface state with improved glossiness in the category of ferrite stainless steel sheets with a No. 1 finish.

[0019] Hereinafter, first, the chemical composition of the ferrite stainless steel sheet 1 in the present embodiment will be described, and then, the surface state on a microscopic scale and the surface state on a macroscopic scale in the surface layer portion 12 of the ferrite stainless steel sheet 1 will be described respectively.

[0020] 〔Chemical Composition of Steel Sheet〕 The ferrite stainless steel sheet 1 in one embodiment of the present invention has a chemical composition of the SUS430 series. Specifically, the ferrite stainless steel sheet 1 may have a chemical composition containing, in mass %, C: 0.005% or more and 0.080% or less, Si: 0.02% or more and 0.50% or less, Mn: 0.05% or more and 0.50% or less, P: 0.030% or less, S: 0.0100% or less, Ni: 0.070% or more and 0.300% or less, Cr: 15.5% or more and 17.5% or less, N: 0.010% or more and 0.030% or less, Al: 0.05% or more and 0.20% or less. The ferrite stainless steel sheet 1 may have a chemical composition in which the balance consists of Fe and impurities. Each of the above elements will be described below.

[0021] (C: Carbon) Carbon (C) is an element that, by forming carbides with chromium (Cr) and other elements, creates interfaces that serve as sources of dislocations when ferritic stainless steel sheets deform. However, excessive addition of C reduces resistance to intergranular corrosion and workability, and increases the cost of refining. Therefore, the C content may be 0.005 to 0.080 mass%, or 0.010 to 0.070 mass%.

[0022] (Si: Silicon) Si acts as a deoxidizing agent during the melting process. However, if too much Si is added, the ferritic stainless steel sheet hardens and its ductility decreases. Therefore, the Si content may be 0.02 to 0.50 mass%, or 0.05 to 0.40 mass%.

[0023] (Mn: Manganese) Mn has an effect as a deoxidizing agent. However, if Mn is added in excess, the amount of MnS produced increases, reducing the corrosion resistance of ferritic stainless steel sheets. Therefore, the Mn content may be 0.05 to 0.50 mass%, or 0.10 to 0.40 mass%.

[0024] (P: Lin) Excessive phosphorus (P) content can degrade weldability, toughness of welded joints, and workability. Therefore, the P content may be 0.030% by mass or less. In ferritic stainless steel sheet 1, there is no particular lower limit to the P content. The P content may be 0 (including no additives) to 0.030% by mass. A P content of "0 (including no additives)" means that the presence of P as an impurity is permitted. Since it is difficult to reduce the P content to an extremely high degree in steelmaking, the P content may be adjusted to 0.001% by mass or more.

[0025] (S: Sulfur) S is an impurity atom that adversely affects hot workability, corrosion resistance, and oxidation resistance. Therefore, the S content may be 0.0100% by mass or less. In ferritic stainless steel sheet 1, there is no particular lower limit to the S content. The S content may be 0 (including no additive) to 0.0100% by mass. An S content of "0 (including no additive)" means that the presence of S as an impurity is permitted. Since it is difficult in steelmaking to reduce the S content to an extremely high degree, the S content may be adjusted to 0.0001% by mass or more.

[0026] (Ni: Nickel) Ni is an effective element for improving corrosion resistance. However, excessive Ni content destabilizes the ferrite phase and increases the raw material cost of ferritic stainless steel sheets. Therefore, the Ni content may be 0.070 to 0.300 mass%. The Ni content may also be 0.090 to 0.290 mass%.

[0027] (Cr: Chrome) Cr is necessary to form a passive film on the surface of steel sheets, thereby improving corrosion resistance. However, if Cr is added in excess, the ductility of ferritic stainless steel sheets decreases. Therefore, the Cr content may be 15.5 to 17.5 mass%, or 16.0 to 17.0 mass%.

[0028] (N: Nitrogen) Nitrogen (N) is an element that forms nitrides with Cr and other elements, creating interfaces that serve as dislocation sources when ferritic stainless steel sheets deform. However, excessive addition of N leads to a decrease in ductility due to solid solution strengthening. Therefore, the N content may be 0.010 to 0.030 mass%, or 0.015 to 0.027 mass%.

[0029] (Al: Aluminum) Al is an effective element for deoxidation and can reduce A2-type inclusions that negatively affect press workability. However, excessive addition of Al increases surface defects. Therefore, the Al content should be between 0.05 and 0.20 mass%.

[0030] <Other ingredients> The ferritic stainless steel sheet 1 may have a chemical composition that further contains one or two elements selected from the group consisting of Mo: 0.01% to 0.10% and Cu: 0.010% to 0.100% by mass.

[0031] (Mo: Molybdenum) Mo is an effective element for improving corrosion resistance. However, excessive addition of Mo increases the raw material cost of stainless steel. Therefore, when Mo is included in the chemical composition, the Mo content should be between 0.05 and 2.00 mass%.

[0032] (Cu: Copper) Cu is an effective element for improving corrosion resistance. However, excessive Cu content destabilizes the ferrite phase. Therefore, when Cu is included in the chemical composition, the Cu content should be between 0.010% and 0.100% by mass.

[0033] (impurities) The remaining portion of the ferritic stainless steel sheet 1 may contain impurities that are introduced from the raw materials and manufacturing process. The remaining portion of the ferritic stainless steel sheet 1 may contain impurities to the extent that they do not affect the properties of each of the above elements.

[0034] [Surface condition of the surface layer (microscopic scale)] Figure 2 shows an example of an electron microscope image of the surface layer of a ferritic stainless steel sheet in one embodiment of the present invention. As shown in Figure 2, the ferritic stainless steel sheet 1 has a plurality of orientation pits 2 and a plurality of stepped portions 3 on the surface of the surface layer 12. The stepped portions 3 may be located at the boundaries of adjacent orientation pits 2. In the example shown in Figure 2, the stepped portions 3 are shown as relatively white (high brightness) images.

[0035] In this embodiment, the ferritic stainless steel sheet 1 has a number of stepped portions 3 on the surface of the surface layer 12 that is 2000 or less, as determined by the cutting method. Specifically, the number of stepped portions 3 located on the surface of the surface layer 12 can be measured by applying the method specified in JIS G0551:2020 as a measurement method for crystal grains of the metal structure (the so-called cutting method).

[0036] Figure 3 illustrates an example of image processing for an electron microscope image and an example of measuring a step portion by sectioning. As shown in Figure 3, known image processing can be performed on an electron microscope image taken of the surface layer 12 of a ferritic stainless steel plate 1 using image processing software. By image processing, the portion of the color range corresponding to the step portion 3 (relatively white area) can be extracted, and hereafter, the extracted portion will be referred to as the extracted line 30. Then, four measurement lines ML that overlap with the processed image are drawn, for example, passing through the center of the image, and the number of intersections between each measurement line ML and the extracted line 30 in the processed image is measured. In the example shown in Figure 3, the four measurement lines ML passing through the center of the image are one vertical measurement line ML passing through the center of the image, one horizontal measurement line ML passing through the center of the image, and two diagonal measurement lines ML passing through the center of the image. The two diagonal measurement lines ML are measurement lines ML in directions that bisect the angle (90°) between the vertical and horizontal directions. By applying image processing to the electron microscope image, the number of stepped sections 3 can be easily measured.

[0037] In this embodiment, a 50 μm square region in the electron microscope image was used as one measurement region, and measurements were performed on each of the five measurement regions in the surface layer 12 using the method described above. Specifically, image processing was performed on each measurement region, four measurement lines ML were drawn passing through the center of the processed image, and the number of intersections between each measurement line ML and the extracted line 30 was measured. The total length of the 20 measurement lines ML (5 locations × 4 lines) and the total number of measured step portions 3 were used to calculate the number of step portions 3 per centimeter.

[0038] The number (density) of stepped portions 3 in the surface layer 12 can be changed depending on the conditions of the pickling treatment, as described later. In the ferritic stainless steel sheet 1, there is no particular lower limit to the number of stepped portions 3 (per cm) in the surface layer 12 that can be determined by the cutting method. However, even if the number of stepped portions 3 is less than 1000 / cm, the degree to which the effect produced by the surface condition of the ferritic stainless steel sheet 1 (improvement of corrosion resistance and glossiness) increases further is small. Therefore, from the viewpoint of efficiently manufacturing the ferritic stainless steel sheet 1 having a surface layer 12, the number of stepped portions 3 in the surface layer 12, as determined by the cutting method, may be between 1000 / cm and 2000 / cm.

[0039] As will be explained in more detail later, during the pickling process, the surface is gradually smoothed as the cutting progresses. This can be understood by comparing the comparative example shown in Figure 5, which will be described later, with the example of this embodiment shown in Figure 3. Therefore, in the electron microscope image of the surface layer 12 of the ferritic stainless steel sheet 1, there are parts where the image of the stepped portion 3 is not clearly shown at the boundary of adjacent directional pits 2. This is because, in the surface layer 12, a part of the stepped portion 3 can be cut to the extent that it does not clearly show a white (high brightness) image. As a result, in the image after image processing, there may be parts where the extracted lines 30 are not continuous (broken).

[0040] [Surface condition of the surface layer (macroscale)] Next, the macroscopic surface condition of the surface layer 12 will be described below, along with a description of the processing steps related to that surface condition.

[0041] Figure 4 is a flowchart illustrating the treatment related to the surface condition of a ferritic stainless steel sheet in one embodiment of the present invention. As shown in Figure 4, first, a steel slab having the aforementioned chemical composition is hot-rolled (S11) to obtain a hot-rolled sheet. Next, the hot-rolled sheet is continuously annealed (S12) to obtain a hot-rolled annealed sheet. The hot-rolled annealed sheet is subjected to a treatment to break up the surface scale (oxide scale) using a scale breaker (S13) and shot blasting (S14). In S13, a mechanical scale breaking method, for example, a repeated bending method may be used. Since steps S11 to S14 can be performed on the hot-rolled annealed sheet using known methods and conditions, a detailed explanation is omitted.

[0042] For example, the hot-rolled and annealed sheet, after the scale-crushing treatment, may be subjected to a grinding treatment (S15) using a brush or the like, or S15 may be omitted. Even if S15 is omitted, it is sufficient if the scale can be removed by pickling to obtain a ferritic stainless steel sheet 1.

[0043] Then, pickling treatment (S16) is performed on the hot-rolled and annealed sheet after S14 or S15. The surface layer 12 of the ferritic stainless steel sheet 1 obtained in this way has undulations on a macroscopic scale that are greater than the irregularities formed by the directional pits 2 and stepped portions 3. These undulations correspond, for example, to shot marks originating from S14.

[0044] In step S16 described above, the scale is removed by pickling, and relatively prominent directional pits 2 and stepped portions 3 are formed. Subsequently, further pickling promotes the erosion. This allows for the formation of a surface layer 12 with a moderately smooth surface. The overall surface irregularities of the surface layer 12 include shot marks, directional pits 2, and stepped portions 3. In this embodiment, the ferritic stainless steel sheet 1 has an arithmetic mean height of 3.0 μm or less on the surface of the surface layer 12 and a root mean square height of 3.5 μm or less.

[0045] The arithmetic mean height (Sa) on the surface of the surface layer 12 is the average value of the distance from the average plane of the surface, and is expressed by the following formula (1). The root mean square height (Sq) on the surface of the surface layer 12 is the standard deviation of the distance from the average plane of the surface, and is expressed by the following formula (2). The arithmetic mean height (Sa) and the root mean square height (Sq) can be calculated by known methods. In the ferritic stainless steel plate 1 in this embodiment, the values ​​of the arithmetic mean height (Sa) and the root mean square height (Sq) were obtained by measuring each of the five 10 mm square locations using a laser microscope and averaging the measured values.

[0046]

number

[0047]

number

[0048] The arithmetic mean height (Sa) and root mean square height (Sq) were measured in accordance with the ISO 25178 standard for surface roughness measurement. Image processing, including plane tilt correction and noise reduction, was performed on the images acquired using a laser microscope. The cutoff value λc used in the measurement was set to 360 μm.

[0049] The process in S13 described above is only necessary if it can crush the scale so that the scale removal by pickling in S16 is properly carried out in combination with S14 described above, and the specific means are not particularly limited. Various methods of mechanical scale breaking can be applied to the process in S13 described above. For example, the process in S13 may be carried out by the repeated bending method. In the process in S14 described above, for example, spherical iron-based abrasive material may be used as the abrasive material, and the average particle size of the abrasive material may be, for example, φ0.3 mm to 1.0 mm. The average particle size of the abrasive material may be, for example, φ0.4 mm to 0.8 mm or φ0.4 mm to 0.6 mm. The projection speed may be a general speed and may be set according to the type of abrasive material. For example, when spherical iron-based abrasive material is used, the projection speed may be 60 m / s to 90 m / s or 70 m / s to 80 m / s.

[0050] The process in S14 above can cause numerous cracks to form in the scale. In the process in S16 above, the pickling solution penetrates into the scale through the cracks. Performing the process in S14 after the process in S13 above can cause even more cracks to form in the scale, and in the process in S16 above, it becomes easier for the pickling solution to penetrate into the scale.

[0051] [Advantages] Typically, in the manufacturing process of general ferritic stainless steel sheets of the SUS430 series, after hot rolling, the hot-rolled sheets are wound into hot-rolled coils, and then batch annealing (box annealing) is performed to soften them. Performing batch annealing makes it easier to reduce the possibility of damage to the hot-rolled sheets during annealing.

[0052] While SUS430 series ferritic stainless steel sheets are generally manufactured by batch annealing, the ferritic stainless steel sheet 1 of this embodiment differs in that it is manufactured by continuous annealing as described in S12 above. By performing continuous annealing, unlike in the case of batch annealing, a Cr-deficient layer is formed on the surface of the hot-rolled sheet. The Cr-deficient layer is a portion of the surface of the base portion 11 located at the interface between the base portion 11 and the scale formed on the base portion 11, and has a chemical composition in which the amount of Cr is reduced compared to the chemical composition inside the base portion 11. In batch annealing, the surface of the hot-rolled sheet wound as a hot-rolled coil is heated in a low-oxygen state, so the amount of Cr oxide produced in the scale is relatively small, and a Cr-deficient layer is not formed between the scale and the base portion (base material).

[0053] In the manufacturing process of the ferritic stainless steel sheet 1 of this embodiment, a scale crushing treatment is performed after continuous annealing. Subsequently, during the pickling treatment after the scale crushing treatment, the pickling solution reaches the chromium-deficient layer through cracks formed in the scale. As a result, the dissolution of the chromium-deficient layer located at the joint between the substrate 11 and the scale by the pickling solution progresses. When the scale peels off as a result, numerous directional pits 2 and numerous stepped portions 3 are formed on the surface of the surface layer 12 on a microscopic scale.

[0054] Furthermore, by proceeding with the pickling process, the large undulations, directional pits 2, and stepped portions 3 formed by the scale crushing process can be smoothed out. As a result, a ferritic stainless steel sheet 1 can be produced in which the number of stepped portions 3 determined by the cutting method is 2000 or less, and the surface layer 12 has an arithmetic mean height of 3.0 μm or less and a root mean square height of 3.5 μm or less.

[0055] The surface condition of the surface layer 12 of the ferritic stainless steel sheet 1 will be further explained in comparison with the surface layer 120 of a comparative example ferritic stainless steel sheet that is not included in the scope of the present invention.

[0056] Figure 5 shows an example of an electron microscope image of the surface layer of a ferritic stainless steel sheet in the comparative example. The ferritic stainless steel sheet in the comparative example was manufactured by stopping the pickling treatment without further grinding after the scale had been removed from the surface of the substrate.

[0057] As shown in Figure 5, in the comparative example of the ferritic stainless steel sheet, the surface layer 120 has densely packed directional pits 2 and numerous stepped portions 3. Furthermore, in the surface layer 120, the irregularities (in other words, the heights) of the directional pits 2 are relatively large, and the stepped portions 3 are clearly visible.

[0058] Although further investigation is needed regarding the detailed mechanism, the ferritic stainless steel sheet 1 in this embodiment has a surface layer 12 that combines the following configurations (i) and (ii), thereby providing a ferritic stainless steel sheet with superior corrosion resistance and improved gloss compared to the comparative example ferritic stainless steel sheet having a surface layer 120, resulting in a No. 1 finish: (i) The number of stepped sections 3 determined by the cutting method is 2000 or less / cm (configuration relating to the surface condition on a microscopic scale); (ii) The arithmetic mean height on the surface is 3.0 μm or less, and the root mean square height is 3.5 μm or less (configuration concerning the surface state on a macroscopic scale).

[0059] Here, the ferritic stainless steel sheet 1 in this embodiment comprises a base portion 11 which is a hot-rolled and annealed sheet, and a surface portion 12 located on the base portion 11 which has undergone scale removal treatment. This configuration identifies it as a ferritic stainless steel sheet with a No. 1 finish. It is obvious to those skilled in the art that it is impossible or impractical to specifically (in detail) specify the surface condition of a ferritic stainless steel sheet in order to distinguish whether the surface of the ferritic stainless steel sheet is in its original state after pickling or has undergone further surface treatment after pickling. Identifying the ferritic stainless steel sheet 1 in this embodiment as a ferritic stainless steel sheet with a No. 1 finish by the above configuration actually facilitates understanding the orientation pits 2 and stepped portions 3 located on the surface of the surface portion 12.

[0060] The surface layer 12 may have a 60° gloss of 10 or higher. The 60° gloss value is obtained by measuring the surface layer 12 in accordance with JIS Z8741:1997. There is no particular upper limit to the 60° gloss of the surface layer 12. However, as mentioned above, it may be inefficient to proceed with the melting of the surface of the surface layer 12 to the extent that the number of stepped portions 3 is less than 1000 / cm in order to increase the 60° gloss value. Therefore, the surface layer 12 may have a 60° gloss of 10 or higher and 20 or lower.

[0061] Furthermore, even when a hot-rolled sheet that has undergone batch annealing (and therefore does not have a chromium-deficient layer on its surface) is subjected to pickling, the scale is relatively difficult to remove because it does not have a relatively easily dissolvable portion (chromium-deficient layer). Therefore, the surface layer 12 having both the components of (i) and (ii) above is not formed. In addition, the pickling time required to remove the scale becomes relatively long.

[0062] [Other components] Referring again to Figure 1, in this embodiment, the ferritic stainless steel sheet 1 may have a first surface 1A where the entire surface is the surface of the first surface layer 12A, and a second surface 1B where the entire surface is the surface of the second surface layer 12B. That is, the ferritic stainless steel sheet 1 may have the surface layer 12 covering the entire surface on both the first surface 1A and the second surface 1B. Furthermore, the ferritic stainless steel sheet 1 has side surfaces (end faces) connecting the outer edges of the first surface 1A and the second surface 1B, and the surface layer 12 may cover the entire surface on all end faces.

[0063] Furthermore, in this embodiment, the ferritic stainless steel sheet 1 may have the surface layer 12 covering the entire surface of either the first surface 1A or the second surface 1B. The first surface 1A or the second surface 1B having the surface layer 12 covering the entire surface has excellent corrosion resistance and improved gloss.

[0064] The ferritic stainless steel sheet 1 may have a first surface layer 12A located at the four corners and center of the first surface 1A, and a second surface layer 12B located at the four corners and center of the second surface 1B. In this case, it can be presumed that the first surface layer 12A is located over the entire surface of the first surface 1A and the second surface layer 12B is located over the entire surface of the second surface 1B. The ferritic stainless steel sheet 1 may have a first surface layer 12A located over the entire surface of the first surface 1A, and a second surface layer 12B located over the entire surface of the second surface 1B. As described above, the ferritic stainless steel sheet 1 may have a first surface layer 12A located at the four corners and center of the first surface 1A, while on the second surface 1B, the second surface layer 12B may be located locally, or the second surface layer 12B may not be located at all.

[0065] The ferritic stainless steel sheet 1 may have a thickness of, for example, 2 to 6 mm. The ferritic stainless steel sheet 1 does not need to have a surface layer 12 formed on the side surface (end face) connecting the outer edges of the first surface 1A and the second surface 1B.

[0066] [Additional notes] The present invention is not limited to the embodiments described above, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention. [Examples]

[0067] One embodiment of the present invention is described below.

[0068] Ferritic stainless steels having the chemical compositions of the SUS430 series shown in Table 1 were melted, and hot-rolled sheets of each type of ferritic stainless steel with a thickness of 3 mm were obtained by hot-rolling each of these chemical compositions.

[0069] [Table 1]

[0070] A hot-rolled annealed sheet was obtained by continuously annealing a hot-rolled sheet. As a scale-breaking treatment for the hot-rolled annealed sheet, the sheet was first subjected to repeated bending, and then shot blasting was performed. For the shot blasting, spherical iron-based abrasive material (SB3, manufactured by Shinto Kogyo Co., Ltd.) was used as the abrasive material. The average particle size of the abrasive material was φ0.45 mm, and the blasting speed was 76 m / s.

[0071] A test specimen was cut out for a hot-rolled, annealed sheet metal that had undergone shot blasting treatment, without any grinding using brushes or the like, and for the purpose of pickling. The size of the test specimen was 50 mm x 35 mm x thickness t. The thickness t was slightly thinner than 3 mm, as the hot-rolled sheet metal was 3 mm thick. Next, a beaker containing a rotor and an appropriate amount of pickling solution was placed on a magnetic stirrer with a temperature controller, and the test specimen was placed in the beaker. Specifically, the test specimen was immersed in the pickling solution using clamps or other fixing devices to fix its position in the beaker. The test specimen was then pickled while the rotor was rotated at a predetermined speed to create a steady flow in the pickling solution.

[0072] Figure 6 is a schematic diagram illustrating the pickling treatment in the examples and comparative examples (described later). Figure 6 shows a top-down plan view of the beaker opening. As shown in Figure 6, the center of beaker BK in the plan view (corresponding to the central axis of beaker BK with cylindrical side walls) is defined as center C. Beaker BK was placed on a magnetic stirrer so that the rotor RT was located at the center C at the bottom of beaker BK. In general, rotors come in various shapes. In the examples and comparative examples (described later), an oval-shaped rotor RT, which is elliptical in plan view, was used. The rotor RT has a longitudinal shape, and half of the major axis of the rotor RT is called the major axis radius R1. The major axis radius R1 can be said to be the radius of rotation when the rotor RT rotates.

[0073] The test specimen S was fixed in beaker BK such that the normal to the plate surface intersected with the center C, and the plate surface was oriented vertically. The distance D1 was defined as the distance between the plate surface on the center C side of the test specimen S (hereinafter referred to as the plate surface to be treated) and the center C. The distance D1 was made larger than the major axis radius R1. A protective seal was attached to the plate surface of the test specimen S opposite to the plate surface to be treated. For the specific pickling process, first, the test specimen S was positioned above beaker BK, and the rotor RT was rotated at a predetermined speed at the position of the center C inside beaker BK. Then, the test specimen S was lowered into beaker BK so that it was completely immersed in the pickling solution AC. This point was defined as 0 seconds, and after a predetermined immersion time had elapsed, the test specimen S was removed from beaker BK. The test material S was placed inside beaker BK such that the corner opposite to the surface to be treated was in contact with or close to the inner wall of beaker BK.

[0074] In the examples, an aqueous solution of sulfuric acid with a concentration of 200-400 [g / L] was used as the pickling solution AC, and the temperature of the pickling solution AC was set to 70-90 [°C]. The flow velocity of the pickling solution AC was set to 1-3.3 [m / s], and the immersion time was set to 30-40 [s]. Here, the flow velocity of the pickling solution AC is defined as follows: In beaker BK, forced vortices are generated within the range of the major axis radius R1 of the rotor RT in a plan view (the rotation range of the rotor RT), and free vortices are generated outside the rotation range of the rotor RT. The flow velocity of the pickling solution AC is the flow velocity of the free vortices and was calculated using the following equation (3); Flow velocity [m / s] = (2π × rotor radius [m] × rotational speed [rpm] × (rotor radius [m] / position of test material [m])) / 60 ... (3) In equation (3) above, the rotor radii of the two variables are the rotational radius of the rotor RT, and the position of the test material corresponds to the distance D1 mentioned above. The entire surface of the plate to be treated on the test material S is exposed to the free vortex of the pickling solution AC at the flow velocity determined by equation (3) above. The time the plate to be treated is exposed to the free vortex flow is defined as the immersion time.

[0075] Furthermore, the rotor RT is not limited to the above example, and rotors of various shapes (types) and materials can be used. To generate flow velocity in the beaker BK, a stirring blade or the like can be used instead of the rotor RT. In this case, it is easy to understand that the rotation radius of the stirring blade or the like can be substituted for the rotor radius in equation (3) above. Also, for example, the test material S can be used as follows. The pickling treatment can be applied to both sides of the board. That is, after the pickling treatment, the protective seal is peeled off from the board surface opposite to the pickled board surface (hereinafter referred to as the post-treatment board surface). Then, the protective seal is attached to the post-treatment board surface. After that, the board surface opposite to the post-treatment board surface is positioned so that it faces the center C side of beaker BK, and the pickling treatment can be applied to the board surface opposite to the post-treatment board surface in the same manner as described above.

[0076] The detailed conditions for each embodiment are shown in Table 2 below.

[0077] Furthermore, comparative specimens were prepared as follows and subjected to pickling treatment using the same apparatus as in the examples. First, hot-rolled sheets obtained by the same method as in the examples described above were subjected to continuous annealing or batch annealing to obtain comparative hot-rolled annealed sheets. Then, the comparative hot-rolled annealed sheets were subjected to repeated bending and shot blasting treatment in the same manner as in the examples described above. For some comparative specimens, both repeated bending and shot blasting treatment were omitted or omitted. Next, comparative specimens for pickling treatment were cut out without grinding treatment using brushes or the like, in the same manner as in the examples described above.

[0078] In the comparative examples, an aqueous solution with a sulfuric acid concentration of 200-500 g / L was used as the pickling solution, and the temperature of the pickling solution was set to 60-95°C. The flow rate of the pickling solution was set to 0-3.0 m / s, and the immersion time was set to 30-55 s. In addition, for some comparative examples, a slit plate was placed near the surface of the test material to locally obstruct the flow of the pickling solution to the surface of the test material, and the pickling treatment was performed. The detailed conditions for each comparative example are shown in Table 2 below.

[0079] [Testing and Evaluation Methods] The ferritic stainless steel sheets obtained in the examples and comparative examples were tested and evaluated as follows.

[0080] • Electron microscopy observation of the surface state Electron microscope (SEM) images were acquired using a scanning electron microscope (SEM) for five 50 μm square fields of view on the surface of the manufactured ferritic stainless steel sheet (the surface of the sheet after the above treatment). Then, image processing was performed using image processing software (software used: GIMP) to enhance the images of stepped areas included in the SEM images. In the image processing, relatively high-whiteness areas corresponding to stepped areas located at the boundaries of directional pits in the SEM image were extracted using the color range selection tool (the extraction threshold was set to 10 in GIMP's tool options).

[0081] Next, the number of stepped areas was measured from the processed images in accordance with the sectioning method specified in JIS G0551:2020. Specifically, for each SEM image, four measurement lines ML (see Figure 3) that overlap with the processed image were drawn passing through the center of the image, and the number of intersections between these measurement lines ML and the images of stepped areas enhanced by image processing (extracted lines 30, see Figure 3) was measured. Based on the total length of the measurement lines ML (20 lines in total, 5 fields × 4 lines) measured for each SEM image of the SEM-observed sample and the total number of stepped areas measured, the number of stepped areas per centimeter of the SEM-observed sample was converted.

[0082] • Laser microscopy observation of surface conditions Surface observation was performed on five 10mm square points on the surface of a manufactured ferritic stainless steel sheet using a laser microscope LEXT-OLS4100 (OLYMPUS). The arithmetic mean height (Sa) and root mean square height (Sq) on the surface were calculated using the instrument's built-in software.

[0083] • Corrosion resistance test The corrosion resistance of the manufactured ferritic stainless steel sheets was evaluated by performing a salt spray, drying, and wetting cycle test. The sample size was 50 mm wide x 100 mm long, and a combined cycle test machine CYP-90 (manufactured by Suga Test Instruments Co., Ltd.) was used. The material to be evaluated was placed in the test apparatus with the exposed surface (the surface of the sheet after the above treatment) facing upwards, and the inclination with respect to the horizontal plane (i.e., the installation angle) set to 70° (JIS Z2371). The salt spray and wetting cycle test was performed under the condition that one cycle consisted of spraying with a 5% NaCl aqueous solution (35°C x 15 minutes), drying (30% humidity, 60°C x 1 hour), and wetting (95% humidity, 50°C x 3 hours), and was performed for 10 cycles.

[0084] The corrosion resistance of the materials to be evaluated was assessed by washing and drying them after the corrosion resistance test, and then calculating the rusted area ratio. Specifically, a 40 mm x 20 mm area at the center of the surface of the material to be evaluated was photographed and binarized by image analysis. Then, the number of pixels in the rusted area was counted, and the rusted area ratio was calculated using the following formula (4); Rust area ratio [%] = Area of ​​rusted part [mm²] 2 ] / 800 [mm 2 ] × 100...(4).

[0085] If the rusted area ratio of the material being evaluated was 5% or less, the corrosion resistance of the ferritic stainless steel sheet was considered good (indicated as G in Table 2). If the rusted area ratio of the material being evaluated exceeded 5%, the corrosion resistance of the ferritic stainless steel sheet was considered poor (indicated as B in Table 2).

[0086] • Glossiness measurement The 60° gloss (60°Gs) of the manufactured ferritic stainless steel sheet was measured in accordance with JIS Z8741:1997.

[0087] The results are shown in Table 2.

[0088] [Table 2]

[0089] As shown in Table 2, the ferritic stainless steel sheets of Examples No. 1 to 13 all have the number of stepped sections, arithmetic mean height, and root mean square height within the predetermined range, and exhibit excellent corrosion resistance and a surface condition with improved gloss.

[0090] In contrast, for example, the ferritic stainless steel sheet of the comparative example, manufactured by continuous annealing, had the number of steps, arithmetic mean height, and root mean square height outside the specified range due to inadequate scaling or pickling. As a result, its corrosion resistance and gloss were insufficient.

[0091] Furthermore, for example, the ferritic stainless steel sheet of the comparative example, manufactured by batch annealing, exhibited the same scale crushing and pickling treatments as the example, but the number of steps, arithmetic mean height, and root mean square height were outside the specified range, resulting in insufficient corrosion resistance and gloss.

[0092] In the comparative example, a ferritic stainless steel sheet manufactured by partially applying the pickling solution during the pickling process had the number of stepped sections, arithmetic mean height, and root mean square height outside the specified range, resulting in insufficient corrosion resistance and gloss. [Explanation of symbols]

[0093] 1. Ferritic stainless steel sheet 1A 1st page 1B 2nd side 2. Directional pit 3 Stepped section 11 Base part 12 Surface layer

Claims

1. Ferritic stainless steel sheet, The base is a hot-rolled and annealed sheet, It comprises a surface layer located on the aforementioned base portion and subjected to scale removal treatment, The aforementioned surface layer is The surface has multiple directional pits and multiple stepped portions located at the boundaries of each directional pit. The number of stepped sections determined by the cutting method is 2000 or less per cm, and A ferritic stainless steel sheet having an arithmetic mean height of 3.0 μm or less and a root mean square height of 3.5 μm or less on the aforementioned surface.

2. The ferritic stainless steel sheet according to claim 1, wherein the surface layer has a 60° gloss of 10 or more.

3. The aforementioned base portion is, It contains, by mass%, C: 0.005% to 0.080%, Si: 0.02% to 0.50%, Mn: 0.05% to 0.50%, P: 0.030% or less, S: 0.0100% or less, Ni: 0.070% to 0.300%, Cr: 15.5% to 17.5%, N: 0.010% to 0.030%, and Al: 0.05% to 0.20%. A ferritic stainless steel sheet according to claim 1, having a chemical composition in which the remainder consists of Fe and impurities.

4. The ferritic stainless steel sheet according to claim 3, having a chemical composition further containing, in mass%, one or two selected from the group consisting of Mo: 0.01% to 0.10% and Cu: 0.010% to 0.100%.

5. It has a first surface and a second surface located on the opposite side of the first surface, The ferritic stainless steel sheet according to any one of claims 1 to 4, wherein the surface layer is located at least at the four corners and the center of the first surface and at the four corners and the center of the second surface.

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