Austenitic stainless steel sheet

Abstract

To provide an austenitic stainless steel sheet having low glossiness and excellent corrosion resistance.SOLUTION: An austenitic stainless steel sheet comprises a base material part and a passive film formed on the surface of the base material part, and has a predetermined chemical composition, and an arithmetic average roughness Ra of 0.30 μm or more, in which a difference between the maximum peak height Rp and the maximum valley depth Rv is 0.30 μm or more, the sum of the maximum peak height Rp and the maximum valley depth Rv is 1.00 μm or more, an average crystal grain size is 15 μm or more, and a ratio Cs / Cc between Cr concentration Cs of the passive film and Cr concentration Cc of the base material part is 0.7 or more.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an austenitic stainless steel sheet. [Background technology]

[0002] Due to its high corrosion resistance, oxidation resistance, and designability, stainless steel is used, for example, for building materials, kitchen utensils, general furniture and home appliances, automobile exhaust system parts, etc. Among these, some applications, such as exterior building materials such as roofing materials and exterior wall materials, and interior building materials that require designability, may require antiglare properties, and austenitic stainless steel sheets with low gloss are required.

[0003] For example, Patent Documents 1 to 3 disclose stainless steels that have low gloss and good anti-glare properties. The stainless steel disclosed in Patent Document 1 is an austenitic stainless steel strip in which the ten-point average roughness Rz of the interface between the base steel and oxide scale is 3 to 30 μm, and the surface layer of the base steel is a Cr-depleted layer.

[0004] Patent Document 2 also describes a steel sheet having a surface undulation with a roughness of Ra 1.8 μm or more, and a passive film on the surface in which the maximum Cr concentration is the average Cr concentration of the substrate + 3σ1 or more (σ1: standard deviation of the Cr concentration of the substrate), and in the pitting corrosion part, the area of ​​the Cu-enriched region (the region in which the Cu concentration is the average Cu concentration of the substrate + 3σ2 or more; σ2: standard deviation of the Cu concentration of the substrate) is 0.1 μm 2 The following austenitic stainless steel sheet is disclosed.

[0005] Patent Document 3 discloses an austenitic stainless steel sheet in which the gloss is reduced by forming a finely irregular surface through bright annealing and rolling with a dull roll. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-137538 [Patent Document 2] Japanese Patent Application Laid-Open No. 2008-274386 [Patent Document 3] Japanese Patent Application Publication No. 1-118301 Summary of the Invention [Problem to be solved by the invention]

[0007] However, even for such low-brightness austenitic stainless steels, corrosion resistance is an important property, but there is a concern that the corrosion resistance may be reduced if an attempt is made to reduce the brightness of the austenitic stainless steel.

[0008] In the production of austenitic stainless steel sheets or strips, the cold-rolled sheet is exposed to high temperatures in the air during the annealing process, resulting in the formation of an oxide scale, mainly composed of chromium oxides, on the surface. This oxide scale is removed in the pickling process following annealing by immersion in a pickling solution such as a nitric acid hydrofluoric acid solution or by electrolysis, resulting in an annealed and pickled sheet consisting of the stainless steel base surface and the passive film formed thereon.

[0009] However, when a scale consisting mainly of Cr oxides forms during manufacturing, the crystalline structure slows the diffusion rate of Cr, resulting in the formation of a Cr-depleted layer directly below the scale, particularly near the grain boundaries. During the subsequent pickling process using nitric acid and hydrofluoric acid, the Cr-depleted layer is preferentially dissolved, forming traces of dissolution at the grain boundaries known as microgrooves on the surface.

[0010] The presence of such microgrooves causes light to be diffusely reflected from the surface, resulting in a whitish, low-gloss surface. To achieve a lower gloss surface, deepening the microgrooves requires further reducing the Cr concentration in the Cr-depleted layer. If this Cr-depleted layer remains on the surface, corrosion resistance will be significantly reduced. Thus, achieving both low gloss and excellent corrosion resistance in austenitic stainless steels is difficult.

[0011] An object of the present invention is to provide an austenitic stainless steel sheet having low gloss and good corrosion resistance. [Means for solving the problem]

[0012] The present invention has been made to solve the above-mentioned problems, and the gist of the present invention is the following austenitic stainless steel sheet.

[0013] (1) A substrate and a passive film formed on the surface of the substrate, The chemical composition is, in mass%, C: 0.100% or less, Si: 0.10% or more and 2.00% or less, Mn: 0.1% or more and 1.5% or less, P: 0.050% or less, S: 0.0100% or less, Ni: 5.0% or more and 15.0% or less, Cr: 15.0% or more and 25.0% or less, Мо: 0.05% or more and 3.00% or less, N: 0.005% or more and 0.100% or less, Cu: Contains 0.05% or more and 3.00% or less, The balance is Fe and impurities. The arithmetic mean roughness Ra is 0.30 μm or more, The difference between the maximum peak height Rp and the maximum valley depth Rv is 0.30 μm or more, The sum of the maximum peak height Rp and the maximum valley depth Rv is 1.00 μm or more, The average crystal grain size is 15 μm or more, The austenitic stainless steel sheet has a ratio Cs / Cc of the Cr concentration Cs of the passive film to the Cr concentration Cc of the base material of 0.7 or more.

[0014] (2) The chemical composition further contains, in mass %, replacing a portion of the Fe, Al: 0.10% or less, Ti: 0.10% or less, Nb: 0.10% or less, V: 0.10% or less, Sn: 0.10% or less, W: 0.10% or less, Co: 0.10% or less, Sb: 0.10% or less, B: 0.005% or less, Ca: 0.01% or less, Mg: 0.01% or less, Ga: 0.01% or less, Zr: 0.05% or less, Hf: 0.05% or less, and REM: 0.05% or less, The austenitic stainless steel sheet according to (1) above, containing one or more selected from the group consisting of: [Effects of the Invention]

[0015] According to the present invention, an austenitic stainless steel sheet having low gloss and good corrosion resistance can be obtained. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a diagram showing the relationship between the arithmetic mean roughness Ra and the glossiness Gs (45°). [Figure 2] FIG. 2 is a diagram showing the relationship between the difference between the maximum peak height Rp and the maximum valley depth Rv and the gloss level Gs (45°). [Figure 3] FIG. 3 is a diagram showing the relationship between the sum of the maximum peak height Rp and the maximum valley depth Rv and the gloss level Gs (45°). DETAILED DESCRIPTION OF THE INVENTION

[0017] The present inventors have investigated the gloss and corrosion resistance of austenitic stainless steel sheets and have obtained the following findings.

[0018] By setting the heating rate of cold-rolled sheet annealing to 100°C / s or more, it is possible to pass through the temperature range of 600 to 800°C, where Fe oxides form on the base material surface, in a short time, quickly reach the temperature range of 800°C or above, where Cr oxides form and grow, and ensure a sufficient holding time.

[0019] As a result, (A) the Cr concentration in the oxide increases during oxide scale formation and growth, allowing for a lower Cr concentration in the Cr-depleted zone than before, and (B) grain growth is promoted, reducing the number of grain boundaries, further reducing the Cr concentration per unit length of each grain boundary. This increases the degree of preferential dissolution at grain boundaries during the pickling process described below, deepening the depth of the grain boundary dissolution marks, known as microgrooves, that form on the steel sheet surface. In addition, by extending the holding time during final annealing and the immersion time during pickling of the finish-annealed sheet, steel sheets with low surface gloss can be obtained.

[0020] In particular, we found that glossiness significantly decreased when the arithmetic mean roughness Ra, the most representative index of surface roughness, and the difference between the maximum peak height Rp and the maximum valley depth Rv increased, as well as when the maximum valley depth Rv, which is related to the depth of the microgrooves, was increased and the sum of this and the maximum peak height Rp increased.

[0021] Furthermore, the Cr content in the Cr-depleted layer directly below the scale, i.e., at the base metal interface, is lower than in conventional steels, improving scale removal in the pickling method described below and reducing the likelihood of residual scale residue. As a result, the Cr concentration on the base metal surface after pickling is higher than in conventional steels, and a passive film with a high Cr concentration is formed, resulting in a steel sheet with good corrosion resistance.

[0022] An embodiment of the present invention has been made based on the above findings. Hereinafter, each requirement of the austenitic stainless steel sheet of this embodiment will be described in detail.

[0023] 1. Base material and passive film The stainless steel sheet of this embodiment comprises a base material and a passive film formed on the surface of the base material. The passive film is typically a film having a thickness of 1 to 10 nm and made of a metal oxide and a metal hydroxide.

[0024] 2.Chemical composition The reasons for limiting the content of each element are as follows: In the following description, "%" in the content means "% by mass."

[0025] C: 0.100% or less C (carbon) reduces workability. Furthermore, it reduces the Cs / Cc ratio, which reduces corrosion resistance and intergranular corrosion resistance. For this reason, the C content is 0.100% or less. The C content is preferably 0.090% or less, and more preferably 0.080% or less. However, excessive reduction of the C content increases refining costs. For this reason, the C content is preferably 0.005% or more.

[0026] Si: 0.10% or more and 2.00% or less Silicon (Si) is an element that improves oxidation resistance. Therefore, the Si content is 0.10% or more. However, excessive Si content reduces the elongation of the steel, degrading workability. It also reduces scale removability. Therefore, the Si content is 2.00% or less. The Si content is preferably 0.20% or more and 1.95% or less, and more preferably 0.30% or more and 1.90% or less.

[0027] Mn: 0.1% or more and 1.5% or less Manganese (Mn) is used as a deoxidizing element. Therefore, the Mn content is 0.1% or more. However, if Mn is contained in excess, corrosion resistance decreases. Therefore, the Mn content is 1.5% or less. The Mn content is preferably 0.2% or more and 1.4% or less, and more preferably 0.3% or more and 1.3% or less.

[0028] P:0.050% or less P (phosphorus) is an element contained as an impurity in steel and reduces workability and weldability. For this reason, the P content is 0.050% or less. The P content is preferably 0.045% or less, and more preferably 0.040% or less. It is preferable to reduce P as much as possible, but excessive reduction of P increases refining costs. For this reason, the P content is preferably 0.005% or more.

[0029] S: 0.0100% or less S (sulfur) is an element contained as an impurity in steel and reduces corrosion resistance. For this reason, the S content is 0.0100% or less. The S content is preferably 0.0080% or less, and more preferably 0.0050% or less. It is preferable to reduce S as much as possible, but excessive reduction of S increases refining costs. For this reason, the S content is preferably 0.0001% or more.

[0030] Ni: 5.0% or more and 15.0% or less Ni (nickel) has the effect of improving corrosion resistance. Therefore, the Ni content is 5.0% or more. However, excessive Ni content reduces hot workability and weldability. Therefore, the Ni content is 15.0% or less. The Ni content is preferably 6.0% or more and 14.0% or less, and more preferably 7.0% or more and 13.0% or less.

[0031] Cr:15.0% or more and 25.0% or less Cr (chromium) has the effect of improving corrosion resistance. Therefore, the Cr content is 15.0% or more. However, if Cr is contained in excess, workability and scale removability decrease. Therefore, the Cr content is 25.0% or less. The Cr content is preferably 16.0% or more and 24.0% or less, and more preferably 17.0% or more and 23.0% or less.

[0032] Mo: 0.05% or more and 3.00% or less Mo (molybdenum) has the effect of improving corrosion resistance. Therefore, the Mo content is 0.05% or more. However, if Mo is contained in excess, workability and scale removability decrease. Furthermore, since Mo is an expensive element, the manufacturing cost increases. Therefore, the Mo content is 3.00% or less. The Mo content is preferably 0.08% or more and 2.50% or less, and more preferably 0.10% or more and 2.00% or less.

[0033] N: 0.005% or more and 0.100% or less N (nitrogen) has the effect of improving pitting corrosion resistance. Therefore, the N content is 0.005% or more. However, excessive N content reduces workability. Furthermore, it reduces the Cs / Cc ratio, and reduces corrosion resistance and intergranular corrosion resistance. Therefore, the N content is 0.100% or less. The N content is preferably 0.010% or more and 0.080% or less, and more preferably 0.015% or more and 0.050% or less.

[0034] Cu: 0.05% or more and 3.00% or less Copper (Cu) has the effect of improving corrosion resistance. Therefore, the Cu content is 0.05% or more. However, if Cu is contained in excess, hot workability decreases. Therefore, the Cu content is 3.00% or less. The Cu content is preferably 0.08% or more and 2.50% or less, and more preferably 0.10% or more and 2.00% or less.

[0035] In addition to the above elements, one or more elements selected from the group consisting of Al, Ti, Nb, V, Sn, W, Co, Sb, B, Ca, Mg, Ga, Zr, Hf, and REM may be contained in place of a portion of Fe, within the ranges shown below. In other words, the lower limit of the above elements is 0%. The reasons for limiting the amount of each element will be explained below.

[0036] Al: 0.10% or less Al (aluminum) has the effect of improving the oxidation resistance of stainless steel. Therefore, it may be added as needed. However, if an excessive amount of Al is added, workability decreases. Therefore, the Al content is 0.10% or less. On the other hand, to obtain the above effect, the Al content is preferably 0.001% or more. The Al content is preferably 0.002% or more and 0.08% or less, and more preferably 0.003% or more and 0.05% or less.

[0037] Ti: 0.10% or less Ti (titanium) has the effect of suppressing sensitization of stainless steel. Therefore, it may be contained as needed. However, if Ti is contained in excess, workability decreases. Therefore, the Ti content is 0.10% or less. On the other hand, to obtain the above effect, the Ti content is preferably 0.001% or more. The Ti content is preferably 0.002% or more and 0.08% or less, and more preferably 0.003% or more and 0.05% or less.

[0038] Nb: 0.10% or less Nb (niobium) has the effect of improving high-temperature strength. It also has the effect of improving the intergranular corrosion resistance of welds. Therefore, it may be contained as needed. However, if Nb is contained in excess, workability decreases. For this reason, the Nb content is 0.10% or less. On the other hand, in order to obtain the above effects, the Nb content is preferably 0.001% or more. The Nb content is preferably 0.002% or more and 0.08% or less, and more preferably 0.003% or more and 0.05% or less.

[0039] V: 0.10% or less V (vanadium) has the effect of improving corrosion resistance. Therefore, it may be contained as necessary. However, if excessive V is contained, workability decreases. Therefore, the V content is 0.10% or less. On the other hand, in order to obtain the above effect, the V content is preferably 0.001% or more. The V content is preferably 0.002% or more and 0.08% or less, and more preferably 0.003% or more and 0.05% or less.

[0040] Sn: 0.10% or less Sn (tin) has the effect of improving corrosion resistance. Therefore, it may be contained as needed. However, if Sn is contained in excess, workability decreases. Therefore, the Sn content is 0.10% or less. On the other hand, in order to obtain the above effect, the Sn content is preferably 0.01% or more. The Sn content is preferably 0.02% or more and 0.08% or less, and more preferably 0.03% or more and 0.05% or less.

[0041] W: 0.10% or less W (tungsten) has the effect of improving corrosion resistance. Therefore, it may be contained as needed. However, if W is contained in excess, workability decreases. Therefore, the W content is 0.10% or less. On the other hand, in order to obtain the above effect, the W content is preferably 0.01% or more. The W content is preferably 0.02% or more and 0.08% or less, and more preferably 0.03% or more and 0.05% or less.

[0042] Co:0.10% or less Co (cobalt) has the effect of improving secondary workability and toughness. Therefore, it may be contained as necessary. However, if Co is contained in excess, workability decreases. The Co content is 0.10% or less. On the other hand, to obtain the above effect, the Co content is preferably 0.01% or more. The Co content is preferably 0.02% or more and 0.08% or less, and more preferably 0.03% or more and 0.05% or less.

[0043] Sb: 0.10% or less Sb (antimony) has the effect of improving corrosion resistance. Therefore, it may be contained as needed. However, if Sb is contained in excess, workability decreases. Therefore, the Sb content is 0.10% or less. On the other hand, in order to obtain the above effect, the Sb content is preferably 0.01% or more. The Sb content is preferably 0.02% or more and 0.08% or less, and more preferably 0.03% or more and 0.05% or less.

[0044] B: 0.005% or less B (boron) has the effect of improving secondary workability. Therefore, it may be contained as necessary. However, if B is contained in excess, workability decreases. Therefore, the B content is 0.005% or less. On the other hand, in order to obtain the above effect, the B content is preferably 0.0001% or more. The B content is preferably 0.0005% or more and 0.004% or less, and more preferably 0.0010% or more and 0.003% or less.

[0045] Ca: 0.01% or less Ca (calcium) has a desulfurization effect. Therefore, it may be contained as needed. However, if an excessive amount of Ca is contained, corrosion resistance decreases. Therefore, the Ca content is 0.01% or less. On the other hand, in order to obtain the above effect, the Ca content is preferably 0.0001% or more. The Ca content is preferably 0.0005% or more and 0.008% or less, and more preferably 0.0010% or more and 0.005% or less.

[0046] Mg: 0.01% or less Mg (magnesium) has the effect of refining the structure and improving workability and toughness. Therefore, it may be added as needed. However, if Mg is added in excess, workability decreases. Therefore, the Mg content is 0.01% or less. On the other hand, to obtain the above effects, the Mg content is preferably 0.0001% or more. The Mg content is preferably 0.0005% or more and 0.008% or less, and more preferably 0.0010% or more and 0.005% or less.

[0047] Ga: 0.01% or less Ga (gallium) has the effect of improving corrosion resistance and hydrogen embrittlement resistance. Therefore, it may be contained as needed. However, if Ga is contained in excess, workability decreases. Therefore, the Ga content is 0.01% or less. On the other hand, in order to obtain the above effect, the Ga content is preferably 0.0001% or more. The Ga content is preferably 0.0005% or more and 0.008% or less, and more preferably 0.0010% or more and 0.005% or less.

[0048] Zr: 0.05% or less Zr (zirconium) has the effect of improving corrosion resistance. Therefore, it may be contained as necessary. However, if Zr is contained in excess, workability decreases. Therefore, the Zr content is 0.05% or less. On the other hand, in order to obtain the above effect, the Zr content is preferably 0.0001% or more. The Zr content is preferably 0.0005% or more and 0.01% or less, and more preferably 0.0010% or more and 0.005% or less.

[0049] Hf: 0.05% or less Hf (hafnium) has corrosion resistance. Therefore, it may be contained as needed. However, excessive Hf content reduces manufacturability. Therefore, the Hf content is 0.05% or less. On the other hand, in order to obtain the above effects, the Hf content is preferably 0.0001% or more. The Hf content is preferably 0.0005% or more and 0.01% or less, and more preferably 0.0010% or more and 0.005% or less.

[0050] REM: 0.05% or less REM (rare earth elements) have a deoxidizing effect. Therefore, they may be added as needed. However, excessive REM content reduces workability. Therefore, the REM content is 0.05% or less. On the other hand, to obtain the above effect, the REM content is preferably 0.0001% or more. The REM content is preferably 0.0005% or more and 0.01% or less, and more preferably 0.0010% or more and 0.005% or less.

[0051] REM refers to a total of 17 elements, including Sc, Y, and lanthanides, and the REM content above refers to the total content of these elements. In industry, REM is often added in the form of misch metal.

[0052] In the chemical composition of this embodiment, the balance is Fe and impurities. Here, "impurities" refer to components that are mixed in during the industrial production of stainless steel sheet due to various factors in raw materials such as ore and scrap, and in the manufacturing process, and are acceptable within a range that does not adversely affect this embodiment.

[0053] 3.Surface roughness Surface roughness is an important index when discussing gloss. Therefore, steel sheets, i.e., steel sheets that have been cold-rolled, annealed, and pickled, are evaluated using the arithmetic mean roughness Ra, maximum peak height Rp, and maximum valley depth Rv as specified in JIS B 0601:2013. While the surface roughness of stainless steel sheets is usually discussed only in the rolling direction, in this application, a high surface roughness is required regardless of the angle, since low gloss is required. Therefore, roughness is measured not only in the rolling direction but also in the direction perpendicular to the rolling direction. The specific measurement method will be described later.

[0054] 3-1.Arithmetic mean roughness Ra In the austenitic stainless steel sheet of this embodiment, the arithmetic mean roughness Ra is 0.30 μm or more. As shown in Figure 1, when the arithmetic mean roughness Ra is 0.30 μm or more, low gloss can be achieved. The arithmetic mean roughness Ra is preferably 0.40 μm or more, and more preferably 0.50 μm or more.

[0055] On the other hand, if the arithmetic mean roughness Ra is too rough, salt water, dust, etc. tend to accumulate. Therefore, the arithmetic mean roughness Ra is preferably 4.00 μm or less, more preferably 3.50 μm or less, and even more preferably 3.00 μm or less.

[0056] 3-2. Difference between maximum peak height Rp and maximum valley depth Rv In the austenitic stainless steel sheet of this embodiment, it is important that not only the arithmetic mean roughness Ra but also the difference between the maximum peak height Rp and the maximum valley depth Rv be within the following range. Basically, the maximum valley depth Rv is greater than the maximum peak height Rp. If the difference between the maximum peak height Rp and the maximum valley depth Rv is outside the following range, the desired low gloss cannot be achieved.

[0057] Specifically, the difference between the maximum peak height Rp and the maximum valley depth Rv is 0.30 μm or more. As shown in Figure 2, when the difference between the maximum peak height Rp and the maximum valley depth Rv is 0.30 μm or more, that is, the greater the value of the maximum valley depth Rv is compared to the value of the maximum peak height Rp, the more diffusely reflected incident light is by the surface irregularities, resulting in a whitish surface and low gloss. The difference between the maximum peak height Rp and the maximum valley depth Rv is preferably 0.40 μm or more, and more preferably 0.50 μm or more.

[0058] On the other hand, if the difference between the maximum peak height Rp and the maximum valley depth Rv is too large, salt water, dirt, etc. will be more likely to accumulate, so the difference between the maximum peak height Rp and the maximum valley depth Rv is preferably 2.00 μm or less, and more preferably 1.50 μm or less.

[0059] 3-3. Sum of maximum peak height Rp and maximum valley depth Rv Furthermore, the sum of the maximum peak height Rp and the maximum valley depth Rv is 1.00 μm or more. As shown in Figure 3, when the sum of the maximum peak height Rp and the maximum valley depth Rv is 1.00 μm or more, i.e., when the surface has very large values ​​for both, incident light is diffused by the surface irregularities, resulting in a whitish surface with low gloss. The sum of the maximum peak height Rp and the maximum valley depth Rv is preferably 1.20 μm or more, and more preferably 1.50 μm or more.

[0060] On the other hand, if the difference between the maximum peak height Rp and the maximum valley depth Rv is too large, salt water, dirt, etc. will be more likely to accumulate, so the sum of the maximum peak height Rp and the maximum valley depth Rv is preferably 3.00 μm or less, and more preferably 2.00 μm or less.

[0061] The maximum peak height Rp is preferably 0.30 μm or more. This is because light incident on the steel sheet surface is diffusely reflected, which tends to reduce gloss. The maximum peak height Rp is more preferably 0.40 μm or more. The maximum valley depth Rv is preferably 0.60 μm or more. This is because, like the maximum peak height Rp, diffuse reflection tends to reduce gloss. The maximum valley depth Rv is more preferably 0.80 μm or more.

[0062] The arithmetic mean roughness Ra, maximum peak height Rp, and maximum valley depth Rv are measured using the following procedure. A stylus surface roughness measuring instrument (SE3500 manufactured by Kosaka Laboratory Co., Ltd.) is used for the measurements, with a measurement length of 14.2 mm, a reference length of 0.835 mm, Gaussian filter characteristics, and linear leveling. The steel sheet surface (rolled surface) is measured three times in the rolling direction and three times in the direction perpendicular to the rolling direction, and the average values ​​of all the measured values ​​are used to calculate the arithmetic mean roughness Ra, maximum peak height Rp, and maximum valley depth Rv.

[0063] 4.Average grain size The larger the average grain size, the larger the grain boundary area per unit area, the greater the Cr deficiency per unit length of the grain boundaries, and the deeper the microgrooves after pickling. This results in low gloss. For this reason, the stainless steel sheet of this embodiment has an average grain size of 15 μm or more. If the average grain size is less than 15 μm, the Cr deficiency per unit grain boundary will be small, and the grain boundaries will dissolve shallowly during pickling, resulting in the arithmetic mean roughness Ra and maximum valley depth Rv not falling within the desired ranges. The average grain size is preferably 17 μm or more, and more preferably 20 μm or more. There is no particular upper limit to the average grain size, but considering the manufacturing conditions described below, the average grain size is preferably 35 μm or less.

[0064] The above-mentioned average crystal grain size is measured by the following procedure. The structure of a 300 μm × 300 μm area, centered at half the thickness, is observed in a cross section (also called an L-section) parallel to the rolling direction and thickness direction of the steel sheet. The observed structure is measured five times in each of the rolling direction and thickness direction using the cutting method specified in JIS G 0551:2020, and the average of all the measured values ​​is taken as the average crystal grain size.

[0065] 5.Cr concentration ratio As described above, when a cold-rolled sheet is annealed, a Cr-depleted layer forms directly below the scale. If poor pickling occurs during the subsequent pickling, the Cr-depleted layer may remain on the surface and reduce corrosion resistance. Therefore, in the stainless steel sheet of this embodiment, the ratio of the Cr concentration in the passive film to the Cr concentration in the base material is set within a certain range to ensure corrosion resistance. Specifically, the ratio Cs / Cc of the Cr concentration in the passive film to the Cr concentration in the base material is set to 0.7 or more.

[0066] Here, the lower limit of the Cr content in this embodiment is 15.0%, and 0.7 times that is theoretically 10.5%. Since stainless steel contains 10.5% or more Cr, if the Cs / Cc ratio is less than 0.7, it is believed that the outermost passive film does not have the corrosion resistance that general stainless steel has. For this reason, in the stainless steel sheet of this embodiment, the Cs / Cc is 0.7 or more so as not to leave a Cr-depleted layer with a Cr concentration of less than 10.5%.

[0067] The upper limit of the Cr content of the stainless steel sheet of this embodiment is 25.0%. 0.7 times 25.0% is 17.5%, which is close to the Cr content of SUS304, the most commonly used grade of stainless steel. In this case, even if the Cr content is near the upper limit, if the Cs / Cc ratio is less than 0.7, the corrosion resistance of the highly corrosion-resistant steel cannot be maintained. Therefore, by ensuring that the Cs / Cc ratio is 0.7 or more, a decrease in the corrosion resistance of the steel sheet surface can be suppressed. The Cs / Cc ratio is preferably 0.73 or more, and more preferably 0.75 or more.

[0068] The upper limit of the Cs / Cc ratio is not particularly limited, but is calculated to be 6.7. Cs / Cc is preferably 6.0 or less, and more preferably 5.0 or less.

[0069] Here, the Cr concentration Cs of the passive film is the average Cr concentration in the range from the surface to a depth of 5 nm, and the Cr concentration Cc of the base material is the average Cr concentration from a position 500 nm deep from the surface to a position 600 nm deep from the surface. Note that no Cr-depleted layer exists in the region from a position 500 nm deep from the surface to a position 600 nm deep from the surface, and the Cr concentration is equivalent to that of the alloy composition.

[0070] The Cs / Cc measurement is carried out as follows: Using a JEOL Ltd. Auger electron spectrometer (AES) JAMP-9510f, the intensities of the LMM peaks of Fe, Cr, Ni, Mo, Cu, Mn, and Si, and the KLL peaks of O, S, C, and N are measured for each depth in a 20 μm × 20 μm area at a 1000x magnification, while also using Ar ion sputtering, and the composition of each element is determined using the relative sensitivity factor. The degree of vacuum in the AES device is 5 × 10 -7 The electron gun acceleration voltage is 10 kV and the specimen current is 10 nA. For Ar ion sputtering, the ion gun acceleration voltage is 1 kV and the ion current is approximately 650 nA. The sputtering rate under these conditions is 0.09 nm s in SiO2 equivalent. -1 is.

[0071] 6. Glossiness In the present application, a glossiness Gs(45°) of 160 or less is determined to be low glossiness. For this reason, in the austenitic stainless steel sheet of this embodiment, the glossiness Gs(45°) is preferably 160 or less. Here, glossiness Gs(45°) refers to 45-degree specular gloss, that is, glossiness when the angle of incidence is 45°. The glossiness Gs(45°) is more preferably 150 or less, and even more preferably 140 or less.

[0072] The gloss level Gs(45°) is measured as follows: Using a gloss meter UGV-6P manufactured by Suga Test Instruments Co., Ltd., measurements are taken three times on the rolled surface in the rolling direction and in the direction perpendicular to the rolling direction, and the average of all the measurements is taken as the gloss level Gs(45°). The gloss level Gs(45°) is specified in JIS Z 8741:1997, and other measurement conditions can be adjusted based on this standard.

[0073] 7. Manufacturing method The stainless steel sheet of this embodiment can be stably produced, for example, by the following production method.

[0074] The method for manufacturing a stainless steel sheet according to this embodiment includes the following steps. Specifically, molten steel having the above-described chemical composition is produced in a converter or electric furnace, and then the molten steel is refined in an AOD furnace, VOD furnace, or the like. Subsequently, the molten steel is made into a material for hot rolling, such as a steel billet, by continuous casting or ingot casting. This material for hot rolling undergoes hot rolling, annealing of the hot-rolled sheet, pickling, cold rolling, finish annealing, and pickling to produce a stainless steel sheet. If necessary, the annealing of the hot-rolled sheet may be omitted, or the cold rolling, finish annealing, and pickling steps may be repeated. Surface grinding, alumina shot blasting, or the like may be performed between each step.

[0075] 7-1.Hot rolling process The material for hot rolling described above is heated and hot rolled. The heating temperature of the material for hot rolling, for example, a slab, is preferably 1100°C or higher and 1300°C or lower, and more preferably 1150°C or higher and 1250°C or lower. The heating temperature of the material for hot rolling refers to the surface temperature of the material at the outlet side of the heating furnace.

[0076] In hot rolling, multiple passes of rough rolling (several round trips through one stand) are performed, followed by finish rolling in one direction using multiple stands. It is important to increase the rough rolling end temperature and allow time for finish rolling to promote recrystallization of the steel sheet after rough rolling. Therefore, the end temperature of rough rolling is preferably 900°C or higher, and more preferably 950°C or higher. Rough rolling refers to rolling performed until the thickness of the hot rolling material is 30 mm.

[0077] The time from rough rolling to finish rolling is preferably 5 seconds or more, and more preferably 10 seconds or more. This is because recrystallization is promoted. The upper limit of the time from rough rolling to finish rolling is not particularly limited, but from the viewpoint of manufacturing efficiency, the upper limit is 150 seconds. After this, finish rolling is performed to obtain a hot-rolled sheet. In order to reduce the amount of strain in the hot-rolled sheet, the finish rolling end temperature is set to a relatively high temperature. Specifically, the finish rolling end temperature is preferably 800°C or more, and more preferably 850°C or more. The finish rolling end temperature refers to the surface temperature of the steel sheet immediately after passing through the final rolling roll in hot rolling.

[0078] The thickness of the hot-rolled sheet is preferably 1.0 mm or more and 6.0 mm or less, and more preferably 2.0 mm or more and 5.0 mm or less. The hot-rolled sheet is then wound into a coil. Here, the coiling temperature is preferably relatively low. Specifically, the coiling temperature is preferably 600°C or less, and more preferably 550°C or less. Furthermore, the hot-rolled sheet may be annealed as necessary.

[0079] 7-2.Cold rolling process Next, the hot-rolled sheet is cold-rolled to produce a cold-rolled sheet. In the cold-rolling process, which involves such processing, multi-pass rolling is generally performed using a Sendzimir rolling mill to produce a sheet with a predetermined thickness. In this process, mineral oil or water-soluble oil is used as a lubricant. Note that the conditions for cold-rolling, such as the reduction ratio, are not particularly limited. They may be adjusted appropriately depending on the desired sheet thickness, etc.

[0080] 7-3. Finish annealing process The cold-rolled sheet is then subjected to final annealing. In the final annealing, the temperature rise rate is 100°C / s or more. is.

[0081] If the heating rate in the final annealing is less than 100°C / s, the steel will pass through the temperature range of 600 to 800°C, where Fe oxides form, over a long period of time. By the time the steel reaches the high-temperature range of 800°C or higher, where Cr oxides form and grow, Fe oxides will have formed on the surface, resulting in low oxidation resistance. During subsequent holding in the high-temperature range, not only Cr but also Fe will be oxidized. For this reason, the heating rate in the final annealing is 100°C / s or higher. The heating rate is preferably 500°C / s or higher, and more preferably 1000°C / s or higher.

[0082] The temperature range for controlling the heating rate is at least 600 to 800°C, where Fe oxides are likely to precipitate. This suppresses the formation of Fe oxides, and in the subsequent annealing temperature holding step, Cr becomes preferentially oxidized to form scale, thereby achieving a low Cr content in the Cr-depleted zone.

[0083] The annealing temperature is 800°C or higher. If the annealing temperature is lower than 800°C, the Cr content in the Cr-depleted layer will not decrease due to the formation of Fe oxides, resulting in poor descaling. As a result, corrosion resistance will decrease and the desired gloss will not be achieved. If the annealing temperature exceeds 1250°C, the crystal grains may become coarse and the mechanical properties may be easily deteriorated. Therefore, the annealing temperature is preferably 1250°C or lower. The annealing temperature is preferably 850°C or higher and 1200°C or lower, and more preferably 900°C or higher and 1150°C or lower.

[0084] If the holding time at the annealing temperature is less than 30 seconds, the crystal grains do not grow sufficiently, and the Cr content in the Cr-depleted zone cannot be sufficiently reduced. As a result, the crystal grain boundaries are dissolved shallowly during pickling, and the desired glossiness is not achieved. In particular, the difference between the maximum peak height Rp and the maximum valley depth Rv and the sum of the maximum peak height Rp and the maximum valley depth Rv do not satisfy the range of this embodiment. Therefore, the holding time is 30 seconds or more, preferably 35 seconds or more, and more preferably 40 seconds or more. There is no particular upper limit to the holding time. However, from the viewpoint of manufacturability, the holding time is preferably 200 seconds or less.

[0085] By setting the heating rate in the 600-800°C temperature range to 100°C / s or more and ensuring a sufficient holding time at the annealing temperature, it is possible to ensure a sufficient residence time at high temperatures of 800°C or more, at which Cr oxidizes and the grain size coarsens. As a result, (A) the amount of Cr diffusing during oxide scale growth is increased, making the Cr-depleted zone Cr-free, and (B) by increasing the average grain size of 15 μm or more and increasing the number of grain boundaries, the Cr deficiency per unit grain boundary is increased, and the desired low gloss can be obtained by subsequent pickling.

[0086] By setting these annealing conditions, it is possible to produce a steel sheet with a desired surface roughness and low gloss. In addition, it is possible to achieve an average grain size of 15 μm or more, and further, a Cr-depleted layer develops in the surface layer, improving scale removability, which, combined with pickling described below, allows the Cr concentration ratio Cs / Cc to be 0.7 or more.

[0087] 7-4. Finish annealed sheet pickling process The steel sheet after finish annealing is pickled. The solution used for pickling is not particularly limited, but a solution containing one or more components such as nitric acid (HNO3), sulfuric acid (H2SO4), sodium sulfate (Na2SO4), hydrofluoric acid (HF), and ferric chloride (FeCl3) can be used. The solution immersion process may involve sequential immersion using multiple solutions. In other words, multiple immersions may be performed. Electrolysis may also be used, or a salt immersion process may be added before or after pickling. The solution temperature during pickling is preferably 30°C or higher, more preferably 40°C or higher, and even more preferably 45°C or higher.

[0088] By using the above solution, the Cr-depleted zone, which has a lower Cr content than conventional steels, is dissolved to remove scale, the grain boundaries are sufficiently dissolved, and the desired surface roughness is achieved. At the same time, the Cr-depleted zone, which can reduce Cs, is preferentially dissolved, resulting in a steel sheet with a Cr concentration ratio of 0.7 or more.

[0089] Furthermore, if the final immersion time in pickling is too short, the Cr-depleted layer will not be completely dissolved, and the desired low gloss will not be achieved. In particular, the difference between the maximum peak height Rp and the maximum valley depth Rv and the sum of the maximum peak height Rp and the maximum valley depth Rv will not satisfy the ranges of this embodiment. Therefore, the immersion time is 40 seconds or more, more preferably 50 seconds or more, and even more preferably 60 seconds or more. The total immersion time in pickling is preferably 200 seconds or less.

[0090] EXAMPLES The stainless steel sheet according to the present invention will be described in more detail below with reference to examples, but the present embodiment is not limited to these examples. [Example]

[0091] Steels having the chemical compositions shown in Table 1 were hot-rolled and hot-rolled sheet annealed under the manufacturing conditions shown in Table 2. Subsequently, cold-rolled steels were subjected to finish annealing and pickling under the manufacturing conditions shown in Table 2 to produce austenitic stainless steel sheets. The common manufacturing conditions other than those listed in Table 2 are described below. The thickness of the hot-rolled sheets was reduced from 200 mm to 35 mm for rough rolling and from 35 mm to 3.5 mm for finish rolling. After annealing the hot-rolled sheets, alumina shot was applied, followed by pickling by immersion in 300 g / L sulfuric acid for 120 seconds and immersion in 30 g / L hydrofluoric acid + 60 g / L nitric acid for 60 seconds. Cold-rolling was continued until the sheet thickness reached 1.0 mm. The pickling solution after finish annealing was 30 g / L hydrofluoric acid + 75 g / L nitric acid. The immersion time in the table is the immersion time for the final immersion. The temperature increase rate in the final annealing is a rate at which the temperature increases from 600 to 800° C. For other manufacturing conditions not specifically mentioned, the preferred manufacturing conditions in the specification were adopted.

[0092] [Table 1]

[0093] [Table 2]

[0094] The surface roughness, average grain size, Cr concentration ratio, and gloss of the resulting austenitic stainless steel sheets were measured by the following procedures.

[0095] (surface roughness) A 50mm square test piece was cut from an austenitic stainless steel plate, and the arithmetic mean roughness Ra, maximum peak height Rp, and maximum valley depth Rv were measured. The measurements were performed using an SE3500 manufactured by Kosaka Laboratory Co., Ltd. The measurement length was 14.2mm, the reference length was 0.835mm, the filter characteristics were Gaussian, and the leveling process was linear. Under the above conditions, the steel plate surface (rolled surface) was measured three times in the rolling direction and in the direction perpendicular to the rolling direction, and the arithmetic mean roughness Ra, maximum peak height Rp, and maximum valley depth Rv were calculated by averaging all the measured values.

[0096] (Average grain size) Two 20 mm square test pieces were cut from an austenitic stainless steel plate to prepare resin-embedded test pieces suitable for observation, which were then mirror-polished and etched. The test pieces were prepared so that the microstructure could be observed over a 300 μm x 300 μm area centered at half the thickness of the steel plate in a cross section (also called an L-section) parallel to the rolling direction and thickness direction of the steel plate. The average grain size was measured five times in each of the rolling direction and thickness direction using the cutting method specified in JIS G 0551:2020, and the average of all measurements was used as the average grain size.

[0097] (Cr concentration ratio) A 10mm x 13mm test piece was cut from the austenitic stainless steel plate, and the ratio of the Cr concentration Cs in the passive film to the Cr concentration Cc in the base material was measured using AES. The AES was performed using a JAMP-9510f manufactured by JEOL Ltd., and combined with Ar ion sputtering, the intensity of the LMM peaks of Fe, Cr, Ni, Mo, Cu, Mn, and Si, and the KLL peaks of O, S, C, and N were measured at each depth within a 20μm x 20μm area at a 1000x magnification, and the composition of each element was determined using the relative sensitivity factor.

[0098] The vacuum level inside the AES device is 5×10 -7 The electron gun acceleration voltage was 10 kV and the sample current was 10 nA. In Ar ion sputtering, the ion gun acceleration voltage was 1 kV and the ion current was approximately 650 nA. The sputtering rate under these conditions was 0.09 nm s in SiO2 equivalent. -1 The results are summarized in Table 3 below.

[0099] (glossiness) A 50 mm square test piece was cut from the austenitic stainless steel plate, and the gloss of the rolled surface was measured three times in the rolling direction and in the direction perpendicular to the rolling direction using a gloss meter UGV-6P manufactured by Suga Test Instruments Co., Ltd. The average of all the measured values ​​was taken as the glossiness Gs(45°). Glossiness Gs(45°) is specified in JIS Z 8741:1997, and other measurement conditions were adjusted based on this standard.

[0100] [Table 3]

[0101] Nos. C1 to C22, which satisfied the requirements of this embodiment, had good low gloss and corrosion resistance, whereas Nos. c1 to c14, which did not satisfy the requirements of this embodiment, did not achieve at least the desired gloss and had poor corrosion resistance.

[0102] Nos. C4, C9, C15, and C19 had a heating rate and holding time in finish annealing, and a pickling time that were within the ranges of more preferable conditions, and therefore had lower gloss than the other inventive examples. Nos. c1 to c3 had chemical compositions that did not satisfy the ranges of this embodiment, and scale removability was reduced. As a result, corrosion resistance was reduced. Furthermore, the difference between the maximum peak height Rp and the maximum valley depth Rv and the sum of the maximum peak height Rp and the maximum valley depth Rv did not satisfy the ranges of this embodiment, and low gloss was not achieved.

[0103] In Nos. c4, c8, and c12, the immersion time in pickling was short, so the difference between the maximum peak height Rp and the maximum valley depth Rv and the sum of the maximum peak height Rp and the maximum valley depth Rv did not satisfy the requirements of this embodiment, and low gloss was not achieved. In Nos. c5, c9, and c13, the temperature rise rate in finish annealing was slow, so the arithmetic mean roughness Ra was small, and low gloss was not achieved. In addition, corrosion resistance also deteriorated.

[0104] In Nos. c6, c10, and c14, the annealing temperature in the final annealing was too low, resulting in a small arithmetic mean roughness Ra and failure to achieve low gloss. In addition, the grain size was also small, resulting in a decrease in corrosion resistance.

[0105] For Nos. c7 and c11, the holding time in the finish annealing was too short, so the difference between the maximum peak height Rp and the maximum valley depth Rv and the sum of the maximum peak height Rp and the maximum valley depth Rv did not satisfy the range of this embodiment, and low gloss was not achieved.

Claims

1. A base material and a passivation film formed on a surface of the base material, The chemical composition, in mass%, is C: 0.100% or less, Si: 0.10% or more and 2.00% or less, Mn: 0.1% or more and 1.5% or less, P: 0.050% or less, S: 0.0100% or less, Ni: 5.0% or more and 15.0% or less, Cr: 15.0% or more and 25.0% or less, Мо: 0.05% or more and 3.00% or less, N: 0.005% or more and 0.100% or less, Cu: 0.05% or more and 3.00% or less; The balance is Fe and impurities. The arithmetic mean roughness Ra is 0.30 μm or more, The difference between the maximum peak height Rp and the maximum valley depth Rv is 0.30 μm or more, The sum of the maximum peak height Rp and the maximum valley depth Rv is 1.00 μm or more, The average crystal grain size is 15 μm or more, An austenitic stainless steel plate, wherein the ratio Cs / Cc of the Cr concentration Cs of the passive film to the Cr concentration Cc of the base material is 0.7 or more.

2. The chemical composition further contains, in mass %, replacing a portion of the Fe, Al: 0.10% or less, Ti: 0.10% or less, Nb: 0.10% or less, V: 0.10% or less, Sn: 0.10% or less, W: 0.10% or less, Co: 0.10% or less, Sb: 0.10% or less, B: 0.005% or less, Ca: 0.01% or less, Mg: 0.01% or less, Ga: 0.01% or less, Zr: 0.05% or less, Hf: 0.05% or less, and REM: 0.05% or less, The austenitic stainless steel sheet according to claim 1, comprising one or more selected from the group consisting of:

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