Ferritic stainless steel sheet and method for producing the same, annealing temperature determination apparatus, and information processing program

Abstract

To provide a ferritic stainless steel sheet that exhibits superior ridging resistance and corrosion resistance.SOLUTION: The ferritic stainless steel sheet contains, in mass%, C: 0.01-0.10%, Si: 0.1-1.0%, Mn: 0.1-1.0%, P: 0.005-0.050%, S: 0.01% or less, Cr: 12.0-18.0%, N: 0.01-0.10%, and Al: 0.05-0.25%, with the balance being Fe and impurities, and has a γp value of 15-30, a Cr equivalent of 17.0-19.5, a surface roughness Ra of 0.1 μm or less, an average crystal grain size of 7 μm or more, a proportion of crystal grains having a grain size of 8 μm or less of less than 50%, and a proportion of crystal grains having a grain size of 15 μm or more of 7% or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] Ferritic stainless steel has excellent corrosion resistance and heat resistance and is used in a variety of fields, including home appliances, cookware, and construction. Generally, ferritic stainless steel sheets are prone to ridging during forming, and in applications where a beautiful surface is required, a polishing process may be performed to remove the ridging after forming. Alternatively, a glossy surface can be achieved by bright annealing the ferritic stainless steel sheet instead of polishing it. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 61-253323 [Patent Document 2] Patent Application No. Hei 4-160117 [Patent Document 3] Japanese Patent Application Laid-Open No. 2008-1945 Summary of the Invention [Problem to be solved by the invention]

[0004] When a ferritic stainless steel sheet is bright annealed, its corrosion resistance may decrease. Various measures have been investigated to improve the corrosion resistance of bright annealed ferritic stainless steel (see Patent Documents 1 to 3).

[0005] However, for example, in the techniques described in Patent Documents 1 to 3, good corrosion resistance may not be obtained depending on the production conditions, and there is also room for improvement in terms of raw material costs and anti-ridging properties.

[0006] An object of one aspect of the present invention is to provide a ferritic stainless steel sheet or the like that has both excellent ridging resistance and corrosion resistance. [Means for solving the problem]

[0007] In order to achieve the above object, a ferritic stainless steel sheet according to one aspect of the present invention contains, by mass%, C: 0.01% to 0.10%, Si: 0.1% to 1.0%, Mn: 0.1% to 1.0%, P: 0.005% to 0.050%, S: 0.01% or less, Cr: 12.0% to 18.0%, N: 0.01% to 0.10%, and Al: 0.05% to 0.25%, with the remainder being Fe and impurities. and has a chemical composition in which the γp value represented by the following formula (1) is 15 or more and 30 or less, and the Cr equivalent represented by the following formula (2) is 17.0 or more and 19.5 or less, the arithmetic mean roughness Ra value on the surface is 0.1 μm or less, and in a cross section parallel to the rolling direction and perpendicular to the rolling surface which is also the surface, the average crystal grain size is 7 μm or more, and the proportion of first crystal grains having a crystal grain size of 8 μm or less is less than 50%, and the proportion of second crystal grains having a crystal grain size of 15 μm or more is 7% or more.

[0008] γp=420C+470N+23Ni+9Cu+7Mn-11.5Cr-11.5Si-12Mo-23V-47Nb-49Ti-52Al+189...(1) Cr equivalent=Cr+1.72Mo+2.09Si+4.86Nb+8.29V+1.77Ti+21.4Al+40B-7.14C-8.0N-3.28Ni-1.89Mn-0.51Cu...(2) Here, the content (mass %) of each element is substituted for the element symbol in the formulas (1) and (2), and 0 is substituted for elements that are not added.

[0009] In order to solve the above-mentioned problems, an annealing temperature determination device according to one aspect of the present invention is a device for determining an annealing temperature for a hot-rolled sheet obtained by hot-rolling a steel slab having a chemical composition containing, by mass%, C: 0.01% or more and 0.10% or less, Si: 0.1% or more and 1.0% or less, Mn: 0.1% or more and 1.0% or less, P: 0.005% or more and 0.050% or less, S: 0.01% or less, Cr: 12.0% or more and 18.0% or less, N: 0.01% or more and 0.10% or less, and Al: 0.05% or more and 0.25% or less, with the balance consisting of Fe and impurities, and in which a γp value represented by the following formula (1) is 15 or more and 30 or less, and a Cr equivalent represented by the following formula (2) is 17.0 or more and 19.5 or less, and a cold-rolled sheet obtained by cold-rolling the hot-rolled annealed sheet after the hot-rolled sheet annealing. an information acquisition unit that acquires values ​​of the content (mass%) of each element in the chemical composition of the steel slab and the thickness of the hot-rolled sheet; a first temperature determination unit that determines a first annealing temperature range for the hot-rolled sheet annealing; and a second temperature determination unit that determines a second annealing temperature range for the bright annealing, wherein the first temperature determination unit determines the first annealing temperature range using the content (mass%) of each element and the thickness so that the value of M in the following formula (5) is 2.0 or more and less than 10.0, and the second temperature determination unit determines the second annealing temperature range so that the value of Ac1 expressed by the following formula (4) is 800°C or more and is −50°C or less.

[0010] γp=420C+470N+23Ni+9Cu+7Mn-11.5Cr-11.5Si-12Mo-23V-47Nb-49Ti-52Al+189...(1) Cr equivalent=Cr+1.72Mo+2.09Si+4.86Nb+8.29V+1.77Ti+21.4Al+40B-7.14C-8.0N-3.28Ni-1.89Mn-0.51Cu...(2) Ac1 = 35 × Cr equivalent + 310 (4) M = {0.0033 × γp + 0.000355(AT - Ac1) - 0.0121 × Ht - 0.00256} × 100 (5) Here, the values of the content (% by mass) of each element are substituted in the positions of the element symbols in the formulas (1) and (2), 0 is substituted for the elements without addition, and in the formula (5), the AT is the annealing temperature (°C) in the annealing of the hot-rolled sheet, and the Ht is the sheet thickness (unit: mm) of the hot-rolled sheet.

Advantages of the Invention

[0011] According to one aspect of the present invention, it is possible to provide a ferritic stainless steel sheet or the like having excellent anti-ridge formation characteristics and corrosion resistance.

Brief Description of the Drawings

[0012] [Figure 1] It is a schematic diagram for explaining the cross section of a ferritic stainless steel sheet according to an embodiment of the present invention. [Figure 2] It is a flowchart showing an example of a manufacturing method of a ferritic stainless steel sheet according to an embodiment of the present invention. [Figure 3] It is a block diagram showing a schematic configuration of an annealing temperature determination device according to an embodiment of the present invention.

Modes for Carrying Out the Invention

[0013] Hereinafter, an embodiment of the present invention will be described in detail. In this specification, “%” regarding the content ratio of each element in the chemical composition of the ferritic stainless steel (steel sheet) means “% by mass”. The term “steel sheet” is used in a meaning including a steel strip unless otherwise specified. Each JIS standard referred to in this specification intends the latest enacted version or revised version at the time of filing of the present application unless otherwise specified. For numerical values X1 and X2 (where X1 < X2), “X1 to X2” means “not less than X1 and not more than X2”. Also, in this specification, for the convenience of explanation, the area ratio of the martensite phase in the metal structure calculated (evaluated) by the method described later may be simply abbreviated as “martensite amount”.

[0014] Ridging is a surface defect that occurs on the surface of a ferritic stainless steel sheet after, for example, forming. Specifically, it refers to striped or streak-like undulations that occur on the surface of the ferritic stainless steel sheet in a direction parallel to the forming direction. The "forming direction" refers to the direction in which the ferritic stainless steel sheet is stretched by forming. Examples of forming processes include pressing, pulling, and drawing.

[0015] A known method for improving ridging resistance is to disperse a certain amount of martensite in the metal structure of a hot-rolled sheet after annealing, thereby pulverizing the colony structure (a structure formed by the aggregation of crystal grains with similar crystal orientations) during subsequent cold rolling. However, when a ferritic stainless steel sheet is obtained by bright annealing a cold-rolled sheet, the presence of a large amount of martensite in the metal structure of the hot-rolled annealed sheet can reduce corrosion resistance.

[0016] That is, an increase in the C concentration in the martensite phase of the hot-rolled annealed sheet makes it more likely that localized two-phase region heating will occur during bright annealing. When two-phase region heating occurs during bright annealing, an austenite phase is formed. During cooling after bright annealing, Cr carbonitrides are formed at the grain boundaries of the formed austenite phase in the surface layer of the cold-rolled annealed sheet, causing sensitization. This reduces the corrosion resistance of the ferritic stainless steel sheet and makes it more susceptible to corrosion, such as rust, on the surface.

[0017] Thus, in bright annealed ferritic stainless steel sheets, there is a trade-off between ridging resistance and corrosion resistance. It is expected that there is an appropriate amount of martensite (the amount of martensite in a hot-rolled annealed sheet) that will provide both good ridging resistance and good corrosion resistance. However, the amount of martensite produced during annealing of a hot-rolled sheet varies significantly depending on slight differences in the chemical composition and the annealing temperature. Therefore, it has been difficult to appropriately adjust the amount of martensite in a hot-rolled annealed sheet.

[0018] As a result of extensive research, the inventors have discovered a method for appropriately controlling the amount of martensite by specifying an appropriate annealing temperature for the hot-rolled sheet in accordance with changes in the content of each element in the chemical composition (i.e., changes in composition). In addition, they have also discovered a method for appropriately controlling the bright annealing conditions, resulting in the realization of a ferritic stainless steel sheet having both excellent ridging resistance and corrosion resistance, and a method for manufacturing the same.

[0019] [Ferritic stainless steel plate] <Chemical composition> The chemical composition of the ferritic stainless steel sheet according to this embodiment will be described below. Hereinafter, for convenience of explanation, the ferritic stainless steel sheet according to one embodiment of the present invention may be abbreviated as "the present stainless steel sheet." The chemical composition of the present stainless steel sheet is specified within the composition range required for ferritic stainless steel, within a range that easily reduces the possibility of increases in manufacturing costs and raw material costs. This chemical composition is the premise for setting the annealing temperature of the hot-rolled sheet to control the amount of martensite in the hot-rolled annealed sheet within an appropriate range.

[0020] The stainless steel sheet has a chemical composition containing, by mass%, C: 0.01% to 0.10%, Si: 0.1% to 1.0%, Mn: 0.1% to 1.0%, P: 0.005% to 0.050%, S: 0.01% or less, Cr: 12.0% to 18.0%, N: 0.01% to 0.10%, and Al: 0.05% to 0.25%. The balance of the chemical composition of the stainless steel sheet may consist of Fe and impurities.

[0021] The stainless steel sheet has a chemical composition in which the γp value represented by the following formula (1) is 15 or more and 30 or less, and the Cr equivalent represented by the following formula (2) is 17.0 or more and 19.5 or less: γp=420C+470N+23Ni+9Cu+7Mn-11.5Cr-11.5Si-12Mo-23V-47Nb-49Ti-52Al+189...(1) Cr equivalent=Cr+1.72Mo+2.09Si+4.86Nb+8.29V+1.77Ti+21.4Al+40B-7.14C-8.0N-3.28Ni-1.89Mn-0.51Cu...(2) Here, the content (mass %) of each element is substituted for the element symbol in the formulas (1) and (2), and 0 is substituted for elements that are not added.

[0022] The stainless steel sheet may have a γp value of 17 or more and 28 or less, and a Cr equivalent of 17.5 or more and 19.0 or less. Each of the above elements will be explained below.

[0023] (C: carbon) Carbon (C) is an austenite-forming element that facilitates the formation of the austenite phase and is an important element for forming the martensite phase during hot-rolled sheet annealing. However, if excessive C is added, the martensite phase will be excessively formed, which may reduce the corrosion resistance of the stainless steel sheet after bright annealing. Furthermore, significantly reducing the C content increases manufacturing costs (refining costs). Therefore, the C content may be 0.01% or more and 0.10% or less, or 0.030% or more and 0.060% or less.

[0024] (Si: Silicon) Silicon acts as a deoxidizer during the melting process. However, excessive silicon addition can harden the stainless steel sheet and reduce its ductility. Therefore, the silicon content may be 0.1% to 1.0%, or 0.15% to 0.40%.

[0025] (Mn: Manganese) Mn is an austenite-forming element and is effective in forming martensite during annealing of hot-rolled steel sheets. However, excessive addition of Mn increases the amount of MnS formed, which can reduce the corrosion resistance of the stainless steel sheet. Therefore, the Mn content may be 0.1% to 1.0%, or 0.25% to 0.45%.

[0026] (P: Rin) P is an element that reduces hot workability, so the upper limit of the P content is set to 0.050%. From the viewpoint of workability, the P content may be 0.035% or less. However, excessive reduction of the P content results in an increase in raw material costs. Therefore, the P content may be 0.005% or more.

[0027] (S: sulfur) S deteriorates corrosion resistance and promotes cracking during manufacturing. Therefore, the upper limit of the S content is set to 0.0100%. From the viewpoints of corrosion resistance and manufacturability, the S content may be 0.0070% or less. However, excessive reduction of the S content leads to an increase in refining costs. Therefore, the S content may be 0.0003% or more.

[0028] (Cr: chromium) Cr forms a passive film on the surface of the stainless steel sheet, improving corrosion resistance. However, excessive Cr addition reduces the ductility of the stainless steel sheet. Therefore, the Cr content may be 12.0% to 18.0%, or 15.0% to 17.0%.

[0029] (N: nitrogen) N is an austenite-forming element and is important for forming martensite during annealing of hot-rolled sheets. However, if excessive N is added, the ductility of the stainless steel sheet decreases due to solid solution strengthening. Therefore, the N content may be 0.01% or more and 0.10% or less, or 0.01% or more and 0.05% or less.

[0030] (Al: Aluminum) Al is an effective deoxidizing element and reduces Al2-based inclusions that adversely affect press workability. However, Al is a strong ferrite-forming element, and excessive addition can unnecessarily reduce the amount of martensite formed during hot-rolled sheet annealing. Therefore, the Al content may be 0.05% or more and 0.25% or less, or 0.05% or more and 0.15% or less.

[0031] (Other ingredients) The present stainless steel sheet may have a chemical composition further containing one or more elements selected from the group consisting of Cu, Ni, Mo, Nb, V, Ti, B, Sn, Co, W, Sb, Zr, Y, Mg, Ca, and REM (rare earth metals).

[0032] Copper (Cu) is an element effective in improving corrosion resistance. When Cu is contained in the chemical composition, the Cu content may be 0.01% or more and 0.50% or less.

[0033] Ni (nickel) is an austenite-forming element that affects the amount of martensite and the strength of the stainless steel sheet. However, excessive Ni addition increases raw material costs and over-stabilizes the austenite phase, making it difficult to adjust the amount of martensite during hot-rolled sheet annealing. When Ni is included in the chemical composition, the Ni content may be 0.01% or more and 0.20% or less.

[0034] Mo (molybdenum) is an element effective in improving corrosion resistance. However, excessive addition of Mo increases the raw material cost of the stainless steel sheet. When Mo is included in the chemical composition, the Mo content may be 0.001% or more and 0.100% or less.

[0035] Niobium (Nb) combines with carbon or nitrogen and forms carbonitrides, thereby purifying the stainless steel sheet, improving its ductility, and reducing stretcher strain. Stretcher strain refers to minute irregularities formed on the surface of a stainless steel sheet due to yield elongation during processing. However, since Nb is an expensive element, excessive addition of Nb increases the raw material cost of the stainless steel sheet. When Nb is included in the chemical composition, the Nb content may be 0.001% or more and 0.100% or less.

[0036] Vanadium (V) is an element that effectively purifies the stainless steel sheet by bonding with C or N and fixing it as a carbonitride, thereby improving the ductility of the stainless steel sheet. However, excessive V addition increases the raw material cost of the stainless steel sheet. When V is included in the chemical composition, the V content may be 0.01% or more and 0.15% or less.

[0037] Like Nb, Ti (titanium) is an element that forms carbonitrides, and it reduces the grain boundary precipitation of Cr carbonitrides during heat treatment, thereby improving the corrosion resistance of the stainless steel sheet. However, because Ti is an expensive element, adding excessive Ti increases the raw material cost of the stainless steel sheet. When Ti is included in the chemical composition, the Ti content may be 0.001% or more and 0.100% or less.

[0038] B (boron) is an element effective in improving toughness. When B is contained in the chemical composition, the B content may be 0.0001% or more and 0.0025% or less.

[0039] Sn (tin) is an element effective in improving corrosion resistance. However, excessive addition of Sn reduces the hot workability and toughness of the stainless steel sheet. When Sn is included in the chemical composition, the Sn content may be 0.005% or more and 0.500% or less.

[0040] Cobalt (Co) is an element that is effective in improving corrosion resistance and heat resistance. However, excessive Co addition increases the raw material cost of the stainless steel sheet. When Co is included in the chemical composition, the Co content may be 0.05% or more and 0.50% or less.

[0041] W (tungsten) is an element that is effective in improving high-temperature strength. However, if excessive W is added, the raw material cost of this stainless steel sheet increases. When W is included in the chemical composition, the W content may be 0.05% or more and 1.00% or less.

[0042] Sb (antimony) is effective in improving workability by promoting the formation of deformation bands during rolling. However, if excessive Sb is added, this effect saturates and workability may decrease. When Sb is included in the chemical composition, the Sb content may be 0.005% or more and 0.500% or less.

[0043] Zr (zirconium) is an element effective for denitrification, deoxidation, and desulfurization. However, excessive addition of Zr increases the raw material cost of this stainless steel sheet. When Zr is included in the chemical composition, the Zr content may be 0.05% or more and 0.50% or less.

[0044] Y (yttrium) is an element that is effective in improving hot workability and oxidation resistance. However, if Y is added in excess, this effect saturates. When Y is included in the chemical composition, the Y content may be 0.001% or more and 0.100% or less.

[0045] Magnesium (Mg) acts as a deoxidizer by forming Mg oxide with Al in molten steel. However, excessive Mg addition can reduce the toughness of the stainless steel plate. When Mg is included in the chemical composition, the Mg content may be 0.0001% or more and 0.0050% or less.

[0046] Ca (calcium) is an element effective for degassing. When the chemical composition contains Ca, the Ca content may be 0.0001% or more and 0.0050% or less.

[0047] Like Y, REMs (Rare Earth Metals) such as Sc (Scandium) and La (Lanthanum) are effective in improving hot workability and oxidation resistance. However, these effects saturate when the content exceeds 0.100%. When REMs are included in the chemical composition, the total REM content may be 0.001% or more and 0.100% or less.

[0048] (Remainder / Impurities) In the present stainless steel sheet, the remainder other than the aforementioned components may be Fe (iron) and impurities. "Impurities" refer to components that are mixed in during industrial production of stainless steel sheet due to various factors, such as raw materials such as ore and scrap, or the manufacturing process, and are acceptable within a range that does not adversely affect the present invention. For example, O (oxygen) generates nonmetallic inclusions, which can reduce the impact value and fatigue life of the present stainless steel sheet. The content of O as an impurity may be 0.01% or less. Such impurities may be inevitable impurities derived from raw materials or the manufacturing process, or other impurities.

[0049] In the chemical composition of the present stainless steel sheet, "no addition" of a certain element means that the element was not artificially added during steelmaking. The present stainless steel sheet may be substantially free of the various optional elements described above in its chemical composition. "Substantially free" of a certain optional element means that the optional element is not added, and the inclusion of trace amounts as impurities is permitted.

[0050] For example, since the C content of the present stainless steel sheet is not significantly reduced, there is no need to add Ni to ensure the amount of martensite formed during annealing of the hot-rolled sheet, and therefore the present stainless steel sheet may be substantially free of Ni.Furthermore, the present stainless steel sheet may be substantially free of Ti.

[0051] <Metal structure and properties of this stainless steel plate> This stainless steel sheet is a cold-rolled and annealed steel sheet having the chemical composition described above and produced by controlling the production conditions (particularly the hot-rolled sheet annealing temperature and bright annealing temperature), and has the following metallographic structure. That is, this stainless steel sheet has an average grain size of 7 μm or more, and a duplex grain structure in which the proportion of first grains with a grain size of 8 μm or less is less than 50%, and the proportion of second grains with a grain size of 15 μm or more is 7% or more. The cold-rolled and annealed steel sheet (cold-rolled and annealed sheet) refers to a steel sheet obtained by bright annealing a cold-rolled sheet obtained by cold-rolling a hot-rolled and annealed sheet that has been appropriately subjected to a pickling process. The production process of this stainless steel sheet will be described later.

[0052] Fig. 1 is a schematic diagram illustrating the cross section of the present stainless steel sheet. As shown in Fig. 1, cross section 12 is a cross section (a so-called L cross section) parallel to the rolling direction of the present stainless steel sheet 1 and perpendicular to the rolled surface 11. The rolled surface 11 is also the surface of the present stainless steel sheet 1.

[0053] The cross section 12 may be an L-shaped cross section at the center of the plate width. The plate thickness of the present stainless steel plate 1 is t, and the plate width is w. The plate thickness t of the present stainless steel plate 1 may be 0.3 mm or more and 3.0 mm or less, or 0.4 mm or more and 2.0 mm or less. The plate width w of the present stainless steel plate 1 may be 900 mm or more and 1300 mm or less, or 950 mm or more and 1260 mm or less.

[0054] The average crystal grain size and the proportions of the first and second crystal grains in this stainless steel sheet can be calculated as follows: That is, they can be calculated using an electron backscattered diffraction pattern (hereinafter sometimes abbreviated as "EBSD") method for the cross section 12. Specifically, for example, an EBSD pattern of the cross section 12 is obtained using an EBSD detector mounted on a scanning electron microscope (SEM).

[0055] Next, using OIM (Orientation Imaging Microscopy) analysis software, the acquired EBSD pattern is analyzed to identify grain boundaries at interfaces with a misorientation of 15° or more. This means that the regions surrounded by high-angle grain boundaries, which are generally defined as grain boundaries, are identified as grains. The average grain size can then be calculated using the Area Method (Average by Area Fraction Method). Furthermore, the proportions of the first and second grains can be calculated by defining grains with a grain size of 8 μm or less as first grains and grains with a grain size of 15 μm or more as second grains. The grain size can be calculated as the diameter of a circle having an area equal to the area of ​​each individual grain.

[0056] By having the duplex grain structure described above, this stainless steel sheet has a yield strength of 390 MPa or less and a surface ridging height of 15 μm or less when subjected to a tensile strain of 16% in the rolling direction. In addition to such good formability and ridging resistance, this stainless steel sheet also has high corrosion resistance, with no surface corrosion in the neutral salt spray test specified in JIS Z 2371.

[0057] The yield strength is measured by the following method. That is, for example, a JIS No. 13B tensile test piece specified in JIS Z 2201 is taken from the stainless steel sheet, with the tensile direction parallel to the rolling direction. The tensile test piece is subjected to a tensile test specified in JIS Z 2241, and the 0.2% yield strength (MPa) measured is defined as the yield strength of the stainless steel sheet.

[0058] The corrosion resistance of this stainless steel plate is evaluated by a neutral salt spray test specified in JIS Z 2371. In this salt spray test, the appearance of the test piece surface 96 hours after the start of spraying is evaluated for the presence or absence of visually observable corrosion. The corrosion to be observed may be rust such as red rust. Corrosion that cannot be observed visually or corrosion that is difficult to determine visually is not considered to be corrosion.

[0059] The ridging height is measured by the following method. Specifically, for example, a JIS No. 5 tensile test piece, as specified in JIS Z 2201, is taken from the stainless steel sheet, with the tensile direction parallel to the rolling direction. Next, a 16% tensile strain is applied to the tensile test piece, and the waviness height is measured using a surface roughness measuring instrument, with a measurement length of 18 mm in the direction perpendicular to the rolling direction. The waviness height is the average height of the waviness curve elements measured by the surface texture measurement method specified in JIS B 0601. Cutoff values ​​were determined (using a filter) with an upper limit of 5.0 mm and a lower limit of 0.8 mm for the wavelength component, and a waviness curve with wavelength components of 0.8 to 5.0 mm was determined. The reference length was set to 18 mm. The waviness height (μm) measured in this manner is defined as the ridging height.

[0060] The present stainless steel sheet has an arithmetic mean roughness Ra of the surface of 0.1 μm or less. Such a present stainless steel sheet may be a ferritic stainless steel sheet that has been given a bright annealed finish. The bright annealed finish refers to the BA finish specified in JIS G 4305. The present stainless steel sheet that has been given a bright annealed finish has an arithmetic mean roughness Ra of the surface of 0.1 or less and exhibits a good appearance.

[0061] The arithmetic mean roughness Ra of the surface of this stainless steel sheet is a value calculated from a roughness curve measured by the surface texture measurement specified in JIS B0601.

[0062] Furthermore, in the cross section 12 of the present stainless steel sheet, the area fraction of the martensite phase is preferably 1.0% or less, more preferably 0.5% or less, even more preferably 0.2% or less, and most preferably 0%. The smaller the area fraction of the martensite phase remaining after bright annealing, the better the corrosion resistance and workability. If the area fraction is 1.0% or less, the corrosion resistance and workability of the present stainless steel sheet will be good.

[0063] The area ratio of the martensite phase can be calculated as follows. That is, the EBSD pattern acquired for the cross section 12 is converted into an IQ (Image Quality) image using OIM analysis software. In the IQ image (IQ map), the martensite phase has a more complex internal structure than the ferrite phase and is less clear, so the image of the martensite phase is darker than the image of the ferrite phase. The IQ map is binarized, and the area of ​​the martensite phase in the IQ map is divided by the total area of ​​the IQ map to calculate the area ratio of the martensite phase.

[0064] [Method for manufacturing the present stainless steel sheet] Figure 2 is a flowchart showing an example of a method for producing a ferritic stainless steel sheet according to one embodiment of the present invention. As shown in Figure 2, this method for producing a stainless steel sheet includes the steps of an ingot refining step S1, a hot rolling step S2, a hot-rolled sheet annealing step S3, a cold rolling step S4, and a bright annealing step S5. Each step will be described below.

[0065] <Smelting process S1 and hot rolling process S2> In the smelting step S1, a steel slab having the above-mentioned chemical composition is smelted. In the smelting step S1, a general smelting apparatus can be used, and general smelting conditions can be set. Next, in the hot rolling step S2, the steel slab produced in the smelting step S1 is hot rolled to produce a hot-rolled steel sheet (hot-rolled steel strip). In this specification, the hot-rolled steel sheet may be abbreviated as a hot-rolled sheet. In the hot rolling step S2, a general hot-rolling apparatus and hot-rolling conditions can be used.

[0066] For example, as a hot rolling condition, the heating temperature of the steel slab may be 1150 to 1250°C. The finish rolling temperature in the hot rolling step may be 800 to 1100°C, or 810 to 1060°C. After the finish rolling, the hot-rolled sheet is cooled in a cooling zone and then wound into a coil, and the winding temperature may be set to 600 to 850°C, or 650 to 800°C.

[0067] <Hot-rolled sheet annealing process S3> Next, in the hot-rolled sheet annealing step S3, the hot-rolled sheet produced in the hot rolling step S2 is annealed at a hot-rolled sheet annealing temperature in which the value of M in the following formula (3) is in the range of 2.0 or more and less than 10.0, thereby producing a hot-rolled and annealed steel sheet: Hot-rolled sheet annealing temperature = Ac1 + 28.17 × M - 9.3 × γp + 34.1 × Ht + 7.21 (3) In the formula (3), M is an index representing the area ratio of the martensite phase in the L cross section (corresponding to the cross section 12 (see FIG. 1)) of the hot-rolled annealed steel sheet, Ht is the thickness of the hot-rolled sheet (unit: mm), and γp is a value calculated by the formula (1) above.

[0068] Furthermore, Ac1 is a value calculated by the following formula (4): Ac1=35×Cr equivalent+310...(4) Here, the Cr equivalent is a value calculated by the above-mentioned formula (2).

[0069] In this specification, hot-rolled annealed steel sheet may be abbreviated as hot-rolled annealed sheet. The above formula (3) is a relational expression discovered by the present inventors regarding the relationship between the amount of martensite in a hot-rolled annealed sheet and the annealing temperature of the hot-rolled sheet, based on numerous experiments and taking into consideration the effects of the chemical composition and the thickness of the hot-rolled sheet. Since martensite forms at interfaces in the metal structure of a hot-rolled sheet, when the thickness of the hot-rolled sheet is thin, the metal structure becomes finer and the number of interfaces increases. As a result, martensite is more likely to form. This tendency is also reflected in the above formula (3). The thickness of the hot-rolled sheet may be 2 mm or more and 6 mm or less, or 3 mm or more and 5 mm or less.

[0070] By annealing at a hot-rolled sheet annealing temperature where the value of M in formula (3) is in the range of 2.0 or more and less than 10.0, the area fraction of the martensite phase in the L cross section (the same cross section as cross section 12 in Figure 1) of the hot-rolled annealed sheet can be adjusted to 2.0% or more and less than 10.0%. By adjusting the area fraction of the martensite phase in the hot-rolled annealed sheet to 2.0% or more and less than 10.0%, the ridging resistance and corrosion resistance of the stainless steel sheet can be improved after the subsequent cold rolling step S4 and bright annealing step S5.

[0071] Specifically, it is possible to obtain a stainless steel sheet in which the surface ridging height is 15 μm or less when a 16% tensile strain is applied in the rolling direction and which does not exhibit corrosion in the neutral salt spray test described above. Furthermore, the above-mentioned method can also improve the workability of the stainless steel sheet, and specifically, the yield strength of the stainless steel sheet can be made 390 MPa or less.

[0072] If the martensite content of the hot-rolled annealed sheet exceeds 10.0%, the C concentration in the martensite phase increases, causing localized heating in the two-phase region during heating in the bright annealing step S5, making it easier for the austenite phase to form. Cr carbonitrides form in the surface layer of the cold-rolled annealed sheet at the grain boundaries of the formed austenite phase during cooling in the bright annealing step S5, causing sensitization and reducing the corrosion resistance of the stainless steel sheet.

[0073] If the martensite content of the hot-rolled annealed sheet is less than 2.0%, the colony structure will not be crushed sufficiently in the cold rolling step S4, resulting in insufficient improvement in ridging resistance. Furthermore, in the cold rolling step S4, local strain accumulated around the martensite phase becomes a recrystallization nucleation site, making it difficult for recrystallization to occur in the bright annealing step S5, resulting in reduced corrosion resistance and workability.

[0074] As mentioned above, it has been difficult to appropriately adjust the amount of martensite in a hot-rolled annealed sheet, which varies due to various factors, and there have been no guidelines for appropriately adjusting the amount of martensite in a hot-rolled annealed sheet. In contrast, in the manufacturing method of the present stainless steel sheet, by using the above-mentioned formula (3), which is the relational formula discovered by the inventors, in a component system having the above-mentioned chemical composition, the amount of martensite in the hot-rolled annealed sheet can be adjusted to an appropriate range that obtains the properties of the present stainless steel sheet.

[0075] In the hot-rolled sheet annealing step S3, general conditions can be used as conditions other than the hot-rolled sheet annealing temperature. For example, the temperature rise rate during the temperature rise process may be 5 to 20°C / second. The soaking time at the hot-rolled sheet annealing temperature may be, for example, 0 to 90 seconds. A soaking time of 0 seconds in the hot-rolled sheet annealing step S3 means that the material is cooled immediately after the temperature at the center of the sheet thickness reaches a predetermined temperature. Furthermore, in the hot-rolled sheet annealing step S3, after soaking at the hot-rolled sheet annealing temperature, the cooling rate in the temperature range from the hot-rolled sheet annealing temperature to 400°C may be 5 to 100°C / second.

[0076] <Cold rolling process S4> Next, in the cold rolling step S4, the hot-rolled annealed sheet produced in the hot-rolled sheet annealing step S3 is cold-rolled to produce a cold-rolled steel sheet (cold-rolled steel strip). In this specification, the cold-rolled steel sheet may be abbreviated as cold-rolled sheet. A pickling step may be appropriately included between the hot-rolled sheet annealing step S3 and the cold rolling step S4. In the cold rolling step S4, the cold rolling conditions include setting the total cold rolling reduction ratio after completion of the cold rolling step S4 to 60% or more. The total cold rolling reduction ratio after completion of the cold rolling step S4 may be 70% or more, with the upper limit being 90%.

[0077] <Bright annealing process S5> Next, in the bright annealing step S5, the cold-rolled sheet produced in the cold rolling step S4 is annealed. In the bright annealing step S5, the ferrite phase in the cold-rolled sheet is recrystallized, and bright annealing is performed so that the area ratio of the martensite phase in the cold-rolled annealed sheet (corresponding to the present stainless steel sheet) after the bright annealing step S5 is 1.0% or less. The bright annealing step S5 according to one embodiment of the present invention is also referred to as a final annealing step or a finish annealing step.

[0078] In the bright annealing step S5, the bright annealing temperature is set to 800°C or higher and Ac1-50°C or lower. Ac1 is a value that varies depending on the chemical composition and is calculated using the aforementioned formula (4). Ac1 is a guideline for the temperature at which austenite phase formation begins. If the bright annealing temperature reaches the two-phase region temperature of ferrite and austenite, which is equal to or higher than Ac1, the austenite phase will form, which will cause a decrease in corrosion resistance. By setting the upper limit of the bright annealing temperature to Ac1-50°C, even if there are areas in the cold-rolled sheet where C segregates and Ac1 becomes locally small, the austenite phase is less likely to form. Furthermore, by setting the bright annealing temperature to 800°C or higher, recrystallization can be promoted in the bright annealing step S5.

[0079] In the chemical composition of this stainless steel sheet, the Cr equivalent value used to calculate Ac1 is 17.0 to 19.5. Therefore, the Ac1 value based on the chemical composition of this stainless steel sheet is 905.0 to 992.5, and therefore the upper limit of the bright annealing temperature is 855.0°C to 942.5°C.

[0080] In the bright annealing step S5, in the process of increasing the temperature to the bright annealing temperature, the cold-rolled sheet is heated at a temperature increase rate of, for example, 100° C. / s or less.

[0081] In the bright annealing step S5, the soaking time at the bright annealing temperature is 0 seconds or more and 60 seconds or less. A soaking time of 0 seconds in the bright annealing step S5 means that the material is cooled immediately after the temperature at the center of the plate thickness reaches the predetermined bright annealing temperature. In addition, in the bright annealing step S5, after soaking at the bright annealing temperature, the cooling rate in the temperature range from the bright annealing temperature to 500°C may be 5 to 100°C / second.

[0082] The atmosphere in which the bright annealing step S5 is performed has a dew point of -40°C or lower and contains hydrogen gas (H2) and nitrogen gas (N2) at a pressure ratio of 0% to 30%. If the dew point of the atmosphere is -40°C or lower, the deposition of oxide scale on the surface of the stainless steel sheet can be reduced in the bright annealing step S5. Furthermore, if the pressure ratio of nitrogen gas in the atmosphere is 30% or lower, the precipitation of Cr nitrides on the surface of the stainless steel sheet can be reduced. Therefore, by performing the bright annealing step S5 in the above-mentioned atmosphere, the appearance of the stainless steel sheet can be improved.

[0083] By completing the bright annealing step S5, the present stainless steel sheet is obtained as a final product. By the bright annealing step S5, the area ratio of the martensite phase in the present stainless steel sheet is preferably 1.0% or less, more preferably 0.5% or less, and even more preferably 0.2% or less. It is most preferable that the martensite phase disappears and becomes 0%.

[0084] [Annealing temperature determination device] The present invention also encompasses an annealing temperature determination device that determines the annealing temperature range of the hot-rolled sheet in the aforementioned hot-rolled sheet annealing step S3 and the bright annealing temperature range in the aforementioned bright annealing step S5. FIG. 3 is a block diagram showing a schematic configuration of an annealing temperature determination device 2 according to one embodiment of the present invention. The annealing temperature determination device 2 determines a first annealing temperature range, which is the annealing temperature range for hot-rolled sheet annealing performed on a hot-rolled sheet obtained by hot-rolling a steel slab having the aforementioned chemical composition. The annealing temperature determination device 2 also determines a second annealing temperature range, which is the annealing temperature range for bright annealing performed on a cold-rolled sheet obtained by cold-rolling the hot-rolled annealed sheet after hot-rolling annealing.

[0085] 3, the annealing temperature determination device 2 includes a control unit 10 and a storage unit 20. The control unit 10 includes an information acquisition unit 15, a first temperature determination unit 16, and a second temperature determination unit 17. The storage unit 20 may store at least component data 21 and plate thickness data 22.

[0086] The annealing temperature determination device 2 may also include an input unit 30, an output unit 40, and a communication unit 50. Known devices may be used as the input unit 30, the output unit 40, and the communication unit 50, and the specific configurations are not particularly limited. Each unit of the annealing temperature determination device 2 may be connected to a system bus SB, and is configured to be able to exchange data with each other via the system bus SB.

[0087] The control unit 10 is a component that comprehensively controls the operation of each unit of the annealing temperature determination device 2, and is, for example, a CPU (Central Processing Unit). Each unit included in the control unit 10 may be realized, for example, as software operated by the CPU. The storage unit 20 is a volatile or non-volatile storage device (for example, a hard disk or a flash memory) that stores various data used in the control unit 10.

[0088] The component data 21 is data relating to the content of each element in the chemical composition of the steel slab, and is, for example, data obtained by performing component analysis on the steel slab obtained in the smelting process S1 or the hot-rolled sheet after the hot-rolling process S2. The thickness data 22 is the thickness value of the hot-rolled sheet after the hot-rolling process S2 and to be subjected to the hot-rolled sheet annealing process S3. The component data 21 and the thickness data 22 may each be stored in the memory unit 20 via, for example, the input unit 30 or the communication unit 50.

[0089] The information acquiring unit 15 acquires the content (mass%) of each element in the chemical composition of the steel slab and the thickness value of the hot-rolled sheet. The information acquiring unit 15 may acquire this information by reading out the component data 21 and the thickness data 22 stored in the memory unit 20. The information acquiring unit 15 may also acquire the content of each element in the chemical composition of the steel slab and the thickness value of the hot-rolled sheet via the communication unit 50.

[0090] The first temperature determination unit 16 calculates the values ​​of γp and Ac1 using the values ​​of the content of each element and the thickness of the hot-rolled sheet acquired by the information acquisition unit 15. Then, the first temperature determination unit 16 determines the first annealing temperature range in the hot-rolled sheet annealing step S3 so that M in the following formula (5) is 2.0 or more and less than 10.0: M={0.0033×γp+0.000355(AT-Ac1)-0.0121×Ht-0.00256}×100...(5) In the formula (5), γp is a value calculated by the formula (1), AT is the annealing temperature (°C) in the hot-rolled sheet annealing step S3, and Ht is the thickness of the hot-rolled sheet (unit: mm). Ac1 is a value calculated by the formula (4). The formula (5) can be transformed to obtain the formula (3).

[0091] The first temperature determining unit 16 may set AT(2.0), which is the value of AT when M is 2.0 in the formula (5), as the lower limit of the first annealing temperature range. Alternatively, the first temperature determining unit 16 may set AT(10.0), which is the value of AT when M is 10.0 in the formula (5), as the upper limit of the first annealing temperature range. In this case, the first temperature determining unit 16 determines the first annealing temperature range as not less than AT(2.0)°C and less than AT(10.0)°C.

[0092] The first annealing temperature range determined by the first temperature determining unit 16 may be displayed on a screen by the output unit 40, or may be output to an external device via the communication unit 50.

[0093] Conventionally, it has been extremely difficult to empirically control the amount of martensite in a hot-rolled annealed sheet, which is greatly affected by the chemical composition, etc. The annealing temperature determination device 2 uses the above formula (5) to calculate the values ​​of γp and Ac1 corresponding to various chemical compositions and thicknesses of the hot-rolled sheet, and can calculate the first annealing temperature range in the hot-rolled sheet annealing process S3 so that the amount of martensite in the hot-rolled annealed sheet is 2.0% or more and less than 10.0%.

[0094] Second temperature determination unit 17 calculates the value of Ac1 using the content values ​​of each element acquired by information acquisition unit 15, and determines the second annealing temperature range in the bright annealing step S5 to be 800°C or higher and Ac1 value -50°C or lower. Specifically, second temperature determination unit 17 determines a temperature that is the Ac1 value -50°C, which is the upper limit of the second annealing temperature range. Second temperature determination unit 17 may use the value calculated by first temperature determination unit 16 for the value of Ac1, or may calculate it using the above-mentioned formula (4).

[0095] The second annealing temperature range determined by the second temperature determining unit 17 may be displayed on a screen by the output unit 40, or may be output to an external device via the communication unit 50.

[0096] The first temperature determination unit 16 and the second temperature determination unit 17 may determine a specific temperature within the calculated annealing temperature range as the annealing temperature. The process in this case will be described below using the process by the first temperature determination unit 16 as an example.

[0097] Generally, in batch annealing, multiple coils are annealed simultaneously. In continuous annealing, from the viewpoint of productivity, it is preferable to anneal a group of multiple coils without changing (maintaining) the annealing temperature. The annealing temperature determination device 2 may have, for example, a memory unit 20 stored with component data 21 and sheet thickness data 22 of multiple coils to be processed in the hot-rolled sheet annealing process S3.

[0098] The first temperature determination unit 16 (i) calculates the values ​​of γp and Ac1 for each of the plurality of coils based on the information acquired by the information acquisition unit 15. Furthermore, the first temperature determination unit 16 (ii) can calculate the first annealing temperature range in the hot-rolled sheet annealing step S3 for each of the plurality of coils so that M in the formula (5) is equal to or greater than 2.0 and less than 10.0.

[0099] For example, a set of multiple coils that are annealed simultaneously in batch annealing, or a set of multiple coils that are annealed without changing the annealing temperature of the hot-rolled sheet in continuous annealing, is defined as one batch (one processing unit).The first temperature determination unit 16 determines whether there are overlapping temperature ranges for all of the multiple coils, using the first annealing temperature range for each of the multiple coils included in one processing unit, in which M in formula (5) is 2.0 or more and less than 10.0.

[0100] Next, when overlapping temperature ranges exist for one processing unit, the first temperature determination unit 16 may determine, for example, the median value of the overlapping temperature ranges as the annealing temperature in the hot-rolled sheet annealing process S3. The output unit 40 or the communication unit 50 may output the determined temperature range. Furthermore, the output unit 40 or the communication unit 50 may output the overlapping temperature ranges.

[0101] If there are no overlapping temperature ranges for one processing unit, the first temperature determination unit 16 may generate a determination result that an appropriate annealing temperature cannot be determined. The output unit 40 or the communication unit 50 may output the determination result.

[0102] [Software implementation example] The functions of the annealing temperature determination device 2 (hereinafter referred to as the "device") can be realized by a program (information processing program) for causing a computer to function as the device, and a program for causing a computer to function as each control block of the device (particularly each part included in the control unit 10).

[0103] In this case, the device includes a computer having at least one control device (e.g., a processor) and at least one storage device (e.g., a memory) as hardware for executing the program. The control device and storage device execute the program to realize each function described in the embodiment.

[0104] The program may be non-transitory and may be recorded on one or more computer-readable recording media. The recording media may or may not be included in the device. In the latter case, the program may be supplied to the device via any wired or wireless transmission medium.

[0105] Furthermore, some or all of the functions of the control blocks can be realized by logic circuits. For example, an integrated circuit in which a logic circuit that functions as each of the control blocks is formed is also included in the scope of the present invention.

[0106] 〔summary〕 A ferritic stainless steel sheet according to a first aspect of the present invention contains, by mass%, C: 0.01% to 0.10%, Si: 0.1% to 1.0%, Mn: 0.1% to 1.0%, P: 0.005% to 0.050%, S: 0.01% or less, Cr: 12.0% to 18.0%, N: 0.01% to 0.10%, and Al: 0.05% to 0.25%, with the balance being Fe and impurities, and The steel has a chemical composition in which the γp value represented by the formula (1) is 15 or more and 30 or less, and the Cr equivalent represented by the formula (2) is 17.0 or more and 19.5 or less, the arithmetic mean roughness Ra value on the surface is 0.1 μm or less, and in a cross section parallel to the rolling direction and perpendicular to the rolling surface which is also the surface, the average crystal grain size is 7 μm or more, and the proportion of first crystal grains having a crystal grain size of 8 μm or less is less than 50%, and the proportion of second crystal grains having a crystal grain size of 15 μm or more is 7% or more.

[0107] γp=420C+470N+23Ni+9Cu+7Mn-11.5Cr-11.5Si-12Mo-23V-47Nb-49Ti-52Al+189...(1) Cr equivalent=Cr+1.72Mo+2.09Si+4.86Nb+8.29V+1.77Ti+21.4Al+40B-7.14C-8.0N-3.28Ni-1.89Mn-0.51Cu...(2) Here, the content (mass %) of each element is substituted for the element symbol in the formulas (1) and (2), and 0 is substituted for elements that are not added.

[0108] The ferritic stainless steel sheet of aspect 2 of the present invention may be the same as that of aspect 1, and may contain, by mass%, C: 0.030% to 0.060%, Si: 0.15% to 0.40%, Mn: 0.25% to 0.45%, P: 0.035% or less, S: 0.0070% or less, Cr: 15.0% to 17.0%, N: 0.01% to 0.05%, and Al: 0.05% to 0.15%.

[0109] The ferritic stainless steel sheet according to aspect 3 of the present invention is the same as that according to aspect 1 or 2, except that it contains, by mass%, Cu: 0.01% to 0.50%, Ni: 0.01% to 0.20%, Mo: 0.001% to 0.100%, Nb: 0.001% to 0.100%, V: 0.01% to 0.15%, Ti: 0.001% to 0.100%, B: 0.0001% to 0.0025%, Sn: 0.005% to 0.500%, Co: It may further contain one or more selected from the group consisting of 0.05% or more and 0.50% or less, W: 0.05% or more and 1.00% or less, Sb: 0.005% or more and 0.500% or less, Zr: 0.05% or more and 0.50% or less, Y: 0.001% or more and 0.100% or less, Mg: 0.0001% or more and 0.0050% or less, Ca: 0.0001% or more and 0.0050% or less, and REM (rare earth metals): 0.001% or more and 0.100% or less in total.

[0110] The ferritic stainless steel sheet according to aspect 4 of the present invention may be any one of aspects 1 to 3, in which the proof stress is 390 MPa or less, the surface is free from corrosion in a neutral salt spray test specified in JIS Z 2371, and the height of ridging on the surface when a tensile strain of 16% is applied in the rolling direction is 15 μm or less.

[0111] In a fifth aspect of the present invention, the ferritic stainless steel sheet of any one of the first to fourth aspects may have an area ratio of martensite phase of 1.0% or less in the cross section.

[0112] A method for producing a ferritic stainless steel sheet in a sixth aspect of the present invention is the method for producing a ferritic stainless steel sheet of any one of the first to fifth aspects, comprising: a hot-rolled sheet annealing step of annealing a hot-rolled sheet obtained by hot-rolling a steel slab at a hot-rolled sheet annealing temperature such that the value of M in formula (3) below is in the range of 2.0 or more and less than 10.0; and a bright annealing step of cold-rolling the hot-rolled annealed sheet obtained in the hot-rolled sheet annealing step, under conditions where the bright annealing temperature is 800°C or more, the value of Ac1 expressed by formula (4) below is −50°C or less, the soaking time at the bright annealing temperature is 0 seconds or more and 60 seconds or less, and the bright annealing atmosphere has a dew point of −40°C or less and contains hydrogen gas and 0% or more and 30% or less nitrogen gas.

[0113] Hot-rolled sheet annealing temperature = Ac1 + 28.17 × M - 9.3 × γp + 34.1 × Ht + 7.21 (3) Ac1 = 35 × Cr equivalent + 310 (4) Here, in the formula (3), M is an index representing the area ratio of the martensite phase in a cross section of the hot-rolled annealed sheet parallel to the rolling direction and perpendicular to the rolling surface, and Ht is the thickness (unit: mm) of the hot-rolled sheet.

[0114] The annealing temperature determination device in a seventh aspect of the present invention is a device for determining the annealing temperature of a hot-rolled sheet obtained by hot-rolling a steel slab having a chemical composition containing, by mass%, C: 0.01% or more and 0.10% or less, Si: 0.1% or more and 1.0% or less, Mn: 0.1% or more and 1.0% or less, P: 0.005% or more and 0.050% or less, S: 0.01% or less, Cr: 12.0% or more and 18.0% or less, N: 0.01% or more and 0.10% or less, and Al: 0.05% or more and 0.25% or less, with the balance consisting of Fe and impurities, and having a γp value represented by the following formula (1) of 15 or more and 30 or less, and a Cr equivalent represented by the following formula (2) of 17.0 or more and 19.5 or less, and a device for determining the annealing temperature of a cold-rolled sheet obtained by cold-rolling the hot-rolled sheet. An annealing temperature determination device for determining an annealing temperature range for annealing includes an information acquisition unit that acquires values ​​of the content (mass%) of each element in the chemical composition of the steel slab and the thickness of the hot-rolled sheet, a first temperature determination unit that determines a first annealing temperature range for the annealing of the hot-rolled sheet, and a second temperature determination unit that determines a second annealing temperature range for the bright annealing, wherein the first temperature determination unit determines the first annealing temperature range using the content (mass%) of each element and the thickness so that the value of M in the following formula (5) is 2.0 or more and less than 10.0, and the second temperature determination unit determines the second annealing temperature range so that the value of Ac1 expressed by the following formula (4) is 800°C or more and is −50°C or less.

[0115] γp=420C+470N+23Ni+9Cu+7Mn-11.5Cr-11.5Si-12Mo-23V-47Nb-49Ti-52Al+189...(1) Cr equivalent=Cr+1.72Mo+2.09Si+4.86Nb+8.29V+1.77Ti+21.4Al+40B-7.14C-8.0N-3.28Ni-1.89Mn-0.51Cu...(2) Ac1 = 35 × Cr equivalent + 310 (4) M = {0.0033 × γp + 0.000355(AT - Ac1) - 0.0121 × Ht - 0.00256} × 100 (5) Here, the element symbols in the formulas (1) and (2) are substituted with the content (mass%) of each element, and 0 is substituted for elements that are not added. In the formula (5), the AT is the annealing temperature (°C) in the annealing of the hot-rolled sheet, and the Ht is the thickness (unit: mm) of the hot-rolled sheet.

[0116] The information processing program according to the eighth aspect of the present invention is for causing a computer to function as the annealing temperature determining device according to the seventh aspect.

[0117] [Additional Notes] The present invention is not limited to the present embodiment, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in the present embodiment are also included in the technical scope of the present invention. [Example]

[0118] An embodiment of the present invention will be described below.

[0119] [Manufacturing conditions] Ferritic stainless steel sheets within the ranges specified in the present invention are referred to as "invention examples," and ferritic stainless steel sheets outside the ranges specified in the present invention are referred to as "comparative examples." In these examples, steel slabs having the compositions shown in Table 1 below were first produced by melting them in an actual production line. In Table 1, the values ​​of Cr equivalent, Ac1, and γp are each calculated using the above-mentioned formula. In Table 1, values ​​outside the ranges specified in the present invention are underlined.

[0120] [Table 1]

[0121] Steel slabs of each steel No. listed in Table 1 were hot-rolled, hot-rolled sheet annealed, and cold-rolled (to a sheet thickness of 0.8 mm), followed by bright annealing to produce cold-rolled annealed sheets (ferritic stainless steel sheets). The hot-rolled sheet thickness, hot-rolled sheet annealing temperature, and bright annealing temperature (BA temperature) were set as shown in Table 2 below. The bright annealing soaking time at the bright annealing temperature was 30 seconds, and the atmosphere was a hydrogen gas atmosphere (dew point: -50°C) containing 5% nitrogen gas at a pressure ratio. Comparative Example No. 26 listed in Table 2 was not bright-annealed under the conditions specified in the present invention, but was instead air-annealed at 870°C in an air atmosphere. This Comparative Example No. 26 corresponds to the No. 2B finish specified in JIS G 4305.

[0122] The specific conditions for hot rolling, cold rolling, and bright annealing other than those described above were general conditions. These general conditions are as exemplified in the above-mentioned "Description of the Invention." Each test material was evaluated as follows.

[0123] [Evaluation method] <Martensite content> EBSD patterns were obtained for the L cross sections of the hot-rolled and annealed sheets using an EBSD detector mounted on a scanning electron microscope (SEM) under the following acquisition conditions: ·Measurement magnification: 100~800x ·Measurement area: 100~1000μm square Measurement pitch (step size): 0.3~0.8μm An IQ image was generated from the acquired EBSD pattern using OIM analysis software (OIM Analysis 7). The IQ image was binarized, and the martensite area fraction was calculated by dividing the total area of ​​the martensite phase region by the area of ​​the measurement surface.

[0124] <Riding height> From the cold-rolled and annealed sheet after bright annealing, a JIS No. 5 tensile test piece (first tensile test piece) specified in JIS Z 2201 was taken parallel to the rolling direction. Next, using an Instron tensile testing machine, a tensile test was performed on the first tensile test piece with a gauge length of 50 mm and a tensile direction parallel to the rolling direction, and a tensile strain of 16% was applied.

[0125] Next, a surface roughness measuring instrument was used to measure surface texture as specified in JIS B 0601:2013, etc., to measure the average height of the waviness curve elements in the gage length of 18 mm in the direction perpendicular to the rolling direction in the portion between the gauge points of the first tensile test piece. Cutoff values ​​were determined (using a filter) with an upper limit of 5.0 mm and a lower limit of 0.8 mm for the wavelength components, and a waviness curve with wavelength components of 0.8 to 5.0 mm was defined. The value of the average height of the waviness curve elements was used as the value of the ridging height. In evaluating the ridging resistance, a ridging height of 15 μm or less was considered acceptable.

[0126] <Yield strength> A JIS No. 13 type B tensile test specimen (second tensile test specimen) was taken from the cold-rolled annealed sheet in the rolling direction. A tensile test specified in JIS Z 2241 was carried out on the second tensile test specimen using a tensile tester, and the 0.2% proof stress was measured. In assessing workability, a proof stress of 390 MPa or less was considered acceptable.

[0127] <Mixed grain structure> For the cold-rolled annealed steel sheets, an EBSD pattern was obtained from the L-section, similar to the martensite content measurement described above. OIM analysis software was used to identify grain boundaries, with interfaces with a misorientation of 15° or more. The average grain size was calculated using the area method. The area ratios of the primary grains with grain sizes of 8 μm or less and the secondary grains with grain sizes of 15 μm or more were also calculated. The grain size was calculated as the diameter of a circle with an area equal to the area of ​​each individual grain.

[0128] <Surface condition> The arithmetic mean roughness Ra (μm) of the surface of the cold-rolled annealed sheet was measured in the direction perpendicular to the rolling direction according to JIS B0601: 2013. The surface condition was evaluated as passing when the arithmetic mean roughness Ra was 0.1 μm or less.

[0129] [Evaluation results] The results of evaluating the structure and properties of each test material in the invention examples and comparative examples are shown in Table 2. In Table 2, compositions and values ​​outside the ranges specified in the present invention are underlined.

[0130] [Table 2]

[0131] As shown by Examples 1 and 4 and Comparative Examples 2 and 5, by appropriately adjusting the annealing temperature of the hot-rolled sheet based on the chemical composition and thickness of the hot-rolled sheet, the martensite content of the hot-rolled annealed sheet could be made 2.0% or more and less than 10.0%.

[0132] Furthermore, as shown in Examples 1, 4, and 15 and Comparative Examples 2, 5, and 16, by appropriately adjusting the hot-rolled sheet annealing temperature and the bright annealing temperature, a cold-rolled annealed sheet having a duplex grain structure in which the average grain size and the proportions of the first and second grains are within the ranges specified in the present invention was obtained. Note that in the cold-rolled annealed sheet of Comparative Example 5, recrystallization did not occur in the bright annealing step, and a duplex grain structure was not obtained.

[0133] The cold-rolled and annealed sheets of Examples 1, 3, 4, 6 to 15, and 17 to 25 all had excellent ridging resistance and corrosion resistance. Furthermore, the cold-rolled and annealed sheets of the Examples also had relatively excellent workability. The cold-rolled and annealed sheets of the Examples all had a martensite content of 1.0% or less, and in some cases the martensite content was approximately 0.0%. A martensite content of approximately 0.0% means that the martensite content was so small that the martensite phase region was indistinguishable in the binarized IQ image.

[0134] In contrast, in Comparative Examples Nos. 2, 5, 16, and 26 to 33, at least one of the chemical composition, hot-rolled sheet annealing conditions, and bright annealing conditions was outside the range specified in the present invention, and at least one of the predetermined properties did not satisfy the criteria of the present invention. [Explanation of symbols]

[0135] 1 Stainless steel plate (ferritic stainless steel plate) 2 Annealing temperature determination device 11 Rolled surface (surface) 12 cross sections 15 Information acquisition department 16 1st temperature determination section 17 Second temperature determination section S3 Hot-rolled sheet annealing process S5 Bright annealing process

Claims

1. A ferritic stainless steel plate containing, by mass%, C: 0.01% or more and 0.10% or less, Si: 0.1% or more and 1.0% or less, Mn: 0.1% or more and 1.0% or less, P: 0.005% or more and 0.050% or less, S: 0.01% or less, Cr: 12.0% or more and 18.0% or less, N: 0.01% or more and 0.10% or less, and Al: 0.05% or more and 0.25% or less, with the balance being Fe and impurities, It has a chemical composition in which the γp value represented by the following formula (1) is 15 or more and 30 or less, and the Cr equivalent represented by the following formula (2) is 17.0 or more and 19.5 or less, The arithmetic mean roughness Ra of the surface is 0.1 μm or less, In a cross section parallel to the rolling direction and perpendicular to the rolling surface, The average crystal grain size is 7 μm or more, and A ferritic stainless steel plate in which the proportion of first crystal grains having a crystal grain size of 8 μm or less is less than 50%, and the proportion of second crystal grains having a crystal grain size of 15 μm or more is 7% or more. γp=420C+470N+23Ni+9Cu+7Mn-11.5Cr-11.5Si-12Mo-23V-47Nb-49Ti-52Al+189...(1) Cr equivalent = Cr+1.72Mo+2.09Si+4.86Nb+8.29V+1.77Ti+21.4Al+40B-7.14C-8.0N-3.28Ni-1.89Mn-0.51Cu... (2) Here, the content (mass %) of each element is substituted for the element symbol in the formulas (1) and (2), and 0 is substituted for elements that are not added.

2. 2. The ferritic stainless steel sheet according to claim 1, containing, in mass%, C: 0.030% or more and 0.060% or less, Si: 0.15% or more and 0.40% or less, Mn: 0.25% or more and 0.45% or less, P: 0.035% or less, S: 0.0070% or less, Cr: 15.0% or more and 17.0% or less, N: 0.01% or more and 0.05% or less, and Al: 0.05% or more and 0.15% or less.

3. In mass%, Cu: 0.01% or more and 0.50% or less, Ni: 0.01% or more and 0.20% or less, Mo: 0.001% or more and 0.100% or less, Nb: 0.001% or more and 0.100% or less, V: 0.01% or more and 0.15% or less, Ti: 0.001% or more and 0.100% or less, B: 0.0001% or more and 0.0025% or less, Sn: 0.005% or more and 0.500% or less, Co: 0.05% or more and 0.50% or less, W: 0.05% or more and 1.00% 2. The ferritic stainless steel sheet according to claim 1, further comprising one or more elements selected from the group consisting of Sb: 0.005% or more and 0.500% or less, Zr: 0.05% or more and 0.50% or less, Y: 0.001% or more and 0.100% or less, Mg: 0.0001% or more and 0.0050% or less, Ca: 0.0001% or more and 0.0050% or less, and REM (rare earth metals): 0.001% or more and 0.100% or less in total.

4. The yield strength is 390 MPa or less, No corrosion occurs on the surface in the neutral salt spray test specified in JIS Z 2371, and 2. The ferritic stainless steel sheet according to claim 1, wherein the height of ridging on the surface when a tensile strain of 16% is applied in the rolling direction is 15 μm or less.

5. 2. The ferritic stainless steel sheet according to claim 1, wherein the area ratio of the martensite phase in the cross section is 1.0% or less.

6. A method for producing a ferritic stainless steel sheet according to any one of claims 1 to 5, A hot-rolled sheet annealing process in which a hot-rolled sheet obtained by hot-rolling a steel slab is annealed at a hot-rolled sheet annealing temperature in which the value of M in the following formula (3) is in the range of 2.0 or more and less than 10.0; and a bright annealing step of annealing a cold-rolled sheet obtained by cold-rolling the hot-rolled annealed sheet obtained by the hot-rolled sheet annealing step under conditions where the bright annealing temperature is 800°C or higher, the value of Ac1 expressed by the following formula (4) is −50°C or lower, the soaking time at the bright annealing temperature is 0 seconds or longer and 60 seconds or shorter, and the bright annealing atmosphere has a dew point of −40°C or lower and contains hydrogen gas and 0% to 30% nitrogen gas. Hot-rolled sheet annealing temperature = Ac1 + 28.17 × M - 9.3 × γp + 34.1 × Ht + 7.21 (3) Ac1 = 35 × Cr equivalent + 310 (4) Here, in the formula (3), The M is an index representing the area ratio of the martensite phase in a cross section parallel to the rolling direction and perpendicular to the rolling surface in the hot-rolled annealed sheet, The Ht is the thickness (unit: mm) of the hot-rolled sheet.

7. An annealing temperature determination device for determining an annealing temperature range for hot-rolled sheet annealing performed on a hot-rolled sheet obtained by hot-rolling a steel slab having a chemical composition containing, by mass%, C: 0.01% or more and 0.10% or less, Si: 0.1% or more and 1.0% or less, Mn: 0.1% or more and 1.0% or less, P: 0.005% or more and 0.050% or less, S: 0.01% or less, Cr: 12.0% or more and 18.0% or less, N: 0.01% or more and 0.10% or less, and Al: 0.05% or more and 0.25% or less, with the balance consisting of Fe and impurities, and having a γp value represented by the following formula (1) of 15 or more and 30 or less, and a Cr equivalent represented by the following formula (2) of 17.0 or more and 19.5 or less, and for bright annealing performed on a cold-rolled sheet obtained by cold-rolling the hot-rolled annealed sheet after the hot-rolled sheet annealing, an information acquisition unit that acquires the content (mass%) of each element in the chemical composition of the steel slab and the thickness of the hot-rolled sheet; A first temperature determination unit that determines a first annealing temperature range in the hot-rolled sheet annealing; a second temperature determination unit that determines a second annealing temperature range in the bright annealing, The first temperature determination unit determines the first annealing temperature range using the content (mass%) of each element and the value of the plate thickness so that the value of M in the following formula (5) is 2.0 or more and less than 10.0, The second temperature determination unit determines the second annealing temperature range so that the second annealing temperature range is 800 ° C. or higher and is equal to or lower than the value of Ac1 represented by the following formula (4) −50 ° C. Annealing temperature determination device. γp=420C+470N+23Ni+9Cu+7Mn-11.5Cr-11.5Si-12Mo-23V-47Nb-49Ti-52Al+189...(1) Cr equivalent = Cr+1.72Mo+2.09Si+4.86Nb+8.29V+1.77Ti+21.4Al+40B-7.14C-8.0N-3.28Ni-1.89Mn-0.51Cu... (2) Ac1 = 35 × Cr equivalent + 310 (4) M = {0.0033 × γp + 0.000355 (AT - Ac1) - 0.0121 × Ht - 0.00256} × 100 ... (5) Here, the content (mass%) of each element is substituted for the element symbol in the formulas (1) and (2), and 0 is substituted for non-added elements. In the formula (5), AT is the annealing temperature (°C) in the hot-rolled sheet annealing, The Ht is the thickness (unit: mm) of the hot-rolled sheet.

8. An information processing program for causing a computer to function as the annealing temperature determining device according to claim 7.

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