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

Abstract

This invention provides a ferritic stainless steel sheet with excellent rignification resistance and low yield elongation. [Solution] The ferritic stainless steel sheet contains, by 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 remainder being Fe and impurities. The γp value is 15~30, the Cr equivalent is 17.0~19.5, the C index is less than 17.5, the average grain size is 7 μm or more, the proportion of grains with a grain size of 8 μm or less is less than 50%, and the proportion of grains with a grain size of 15 μm or more is 7% or more.

Description

Technical Field

[0001] The present invention relates to ferritic stainless steel sheets and the like.

Background Art

[0002] Ferritic stainless steel is excellent in corrosion resistance and heat resistance, and is used in various fields such as home appliances, cooking utensils, and construction applications. Generally, it is known that ridging is likely to occur during the forming process of ferritic stainless steel sheets. Conventionally, methods for improving the ridging characteristics of ferritic stainless steel sheets have been reported (see Patent Documents 1 to 3).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, for example, in the technologies described in Patent Documents 1 to 3, C (carbon) and N (nitrogen) that dissolve are likely to increase. When the yield elongation increases due to such dissolved C and N, there is a problem that stretcher strain is likely to occur.

[0005] One aspect of the present invention aims to provide a ferritic stainless steel sheet or the like that is excellent in ridging resistance and has a small yield elongation.

Means for Solving the Problems

[0006] To solve the above problems, a ferritic stainless steel sheet according to one aspect of the present invention contains, by mass%, C: 0.010% to 0.100%, Si: 0.10% to 1.00%, Mn: 0.10% to 1.00%, P: 0.005% to 0.050%, S: 0.0100%, Cr: 12.0% to 18.0%, N: 0.010% to 0.100%, and Al: 0.050% to 0.250%, with the remainder being Fe and impurities, and is a ferritic stainless steel sheet with the following formula (1 The chemical composition has a γp value represented by the following formula (2) of 15 to 30, a Cr equivalent represented by the following formula (2) of 17.0 to 19.5, and a C index represented by the following formula (6) of less than 17.5, and in a cross section parallel to the rolling direction and perpendicular to the rolling surface which is also the surface, the average grain size is 7 μm or more, and 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. 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) C index=Cr-11.6C...(6) Here, the elemental symbols in equations (1), (2), and (6) are replaced with the content (mass%) of each element, and 0 is substituted for elements that are not added.

[0007] 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 elemental symbols in equations (1) and (2) above are replaced with the values ​​of the content (mass%) of each element, and 0 is substituted for elements that are not added.

[0008] To solve the aforementioned problems, an annealing temperature determination apparatus according to one aspect of the present invention is used for hot-rolled steel slabs having a chemical composition in mass%, containing C: 0.010% to 0.100%, Si: 0.10% to 1.0%, Mn: 0.1% to 1.0%, P: 0.005% to 0.050%, S: 0.0100%, Cr: 12.0% to 18.0%, N: 0.010% to 0.100%, and Al: 0.050% to 0.250%, with the remainder being Fe and impurities, and having a γp value represented by formula (1) below of 15 to 30, a Cr equivalent represented by formula (2) below of 17.0 to 19.5, and a C index represented by formula (6) below of less than 17.5, and for hot-rolled steel slabs, and for hot-rolled steel slabs, and after the hot-rolled steel slab annealing... An annealing temperature determination device for determining the annealing temperature range for the final annealing performed on a cold-rolled sheet obtained by cold-rolling a hot-rolled annealed sheet, comprising: 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 the first annealing temperature range for the hot-rolled sheet annealing; and a second temperature determination unit that determines the second annealing temperature range for the final annealing, wherein the first temperature determination unit uses the content (mass%) of each element and the thickness to determine the first annealing temperature range such 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 such that it is 800°C or higher and less than or equal to the value of Ac1 represented by the following formula (4) - 70°C. 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 × the above Cr equivalent + 310...(4) M = {0.003071 × γp + 0.000522 × (AT - AC1) - 0.00887 × Ht + 0.000051 × (AT - AC1 - 7.699) × (γp - 21.7) + 0.005513} × 100 ···(5) C index=Cr-11.6C...(6) Here, the elemental symbols in equations (1), (2), and (6) are replaced with the content (mass%) of each element, and 0 is replaced for elements that are not added. In equation (5), AT is the annealing temperature (°C) in the hot-rolled sheet annealing, and Ht is the thickness (unit: mm) of the hot-rolled sheet. [Effects of the Invention]

[0009] According to one aspect of the present invention, it is possible to provide a ferritic stainless steel sheet or the like that has excellent resistance to rising and low yield elongation. [Brief explanation of the drawing]

[0010] [Figure 1] This is a schematic diagram illustrating a cross-section of a ferritic stainless steel sheet according to one embodiment of the present invention. [Figure 2] This flowchart shows an example of a method for manufacturing a ferritic stainless steel sheet according to one embodiment of the present invention. [Figure 3] This is a block diagram showing a schematic configuration of an annealing temperature determination device according to one embodiment of the present invention. [Modes for carrying out the invention]

[0011] 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 to include a steel strip unless otherwise specified. Each JIS standard referred to in this specification is intended to be the latest enacted version or revised version at the time of filing this application unless otherwise specified. For numerical values X1 and X2 (where X1 < X2), "X1~X2" means "X1 or more and X2 or less". 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".

[0012] Ridging is a surface defect that occurs, for example, on the surface of a ferritic stainless steel sheet after forming. Specifically, it refers to streak-like or rib-like undulations that occur in a direction parallel to the processing direction on the surface of the ferritic stainless steel sheet. The "processing direction" is the direction in which the ferritic stainless steel sheet is stretched by forming. Examples of the forming process include press working, stretching, or drawing.

[0013] Conventionally, for example, by dispersing a certain amount of martensite phase in the metal structure after annealing of a hot-rolled sheet, the colony structure (a structure formed by the aggregation of crystal grains having similar crystal orientations) is pulverized during subsequent cold rolling to improve the ridging resistance. However, in the conventional general method, yield elongation is likely to occur, which may cause problems with the occurrence of stretcher strain on the surface.

[0014] That is, conventionally, ferritic stainless steel was produced by performing final annealing after cold rolling, but in this final annealing, the amount of C and N in solid solution sometimes increased. When there is a large amount of dissolved C and N, the yield elongation tends to increase, which causes stretcher strain on the surface. The increase in the amount of C and N in solid solution during final annealing becomes particularly prominent when the final annealing is performed by continuous annealing, which tends to have high-temperature conditions. Note that stretcher strain refers to minute irregularities formed on the surface of a stainless steel sheet, which are caused by the yield elongation during processing.

[0015] As a result of intensive studies, the inventors of the present invention have found a method of defining an appropriate hot-rolled sheet annealing temperature according to changes in the content of each element in the chemical composition (i.e., changes in components) to appropriately control the amount of martensite. In addition, a method of appropriately controlling the final annealing conditions has been found, leading to the realization of a ferritic stainless steel sheet having excellent anti-ridge characteristics and a small yield elongation, and a method for producing the same.

[0016] [Ferritic Stainless Steel Sheet] [Chemical Composition] The chemical composition of the ferritic stainless steel sheet in this embodiment will be described below. Hereinafter, the ferritic stainless steel sheet according to an embodiment of the present invention may be abbreviated as "this stainless steel sheet" for convenience of explanation. The chemical composition of this stainless steel sheet is defined within a range that is likely to reduce the possibility of increasing manufacturing costs and raw material costs within the composition range required for ferritic stainless steel. Such a chemical composition is a prerequisite for setting the hot-rolled sheet annealing temperature to control the amount of martensite in the hot-rolled annealed sheet within an appropriate range.

[0017] This stainless steel plate has a chemical composition containing, in 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. The balance in the chemical composition of this stainless steel plate may consist of Fe and impurities.

[0018] And this stainless steel plate has a chemical composition in which the γp value represented by the following formula (1) is 15 or more and 30 or less, the Cr equivalent represented by the following formula (2) is 17.0 or more and 19.5 or less, and the C index represented by the following formula (6) is less than 17.5: 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) C index = Cr - 11.6C ··· (6) Here, the values of the contents (mass %) of each element are substituted in the places of the element symbols in the above formulas (1), (2), and (6), and 0 is substituted for elements without addition.

[0019] The γp value of this stainless steel plate may be 17 or more and 28 or less, and the Cr equivalent may be 17.5 or more and 19.0 or less. Each of the above elements will be described below.

[0020] (C: Carbon) Carbon (C) is an austenite-forming element that facilitates the formation of the austenite phase and is an important element for generating the martensite phase during hot-rolled sheet annealing. However, if carbon is added in excess, the amount of dissolved carbon after final annealing tends to increase, which is a factor in increasing the yield elongation of the stainless steel sheet. Furthermore, significantly reducing the carbon content increases manufacturing costs (refining costs). Therefore, the carbon content may be between 0.010% and 0.100%, and between 0.030% and 0.060%.

[0021] (Si: Silicon) Si has a deoxidizing effect during the melting process. However, if Si is added in excess, the stainless steel sheet may harden and its ductility may decrease. Therefore, the Si content may be between 0.10% and 1.00%, or between 0.15% and 0.40%.

[0022] (Mn: Manganese) Mn is an austenite-forming element and is effective in generating martensite during hot-rolled sheet annealing. However, if Mn is added in excess, the amount of MnS produced increases, which can reduce the corrosion resistance of the stainless steel sheet. Therefore, the Mn content may be between 0.10% and 1.00%, or between 0.25% and 0.45%.

[0023] (P: Lin) P is an element that reduces hot workability; therefore, the upper limit for P content is 0.050%. From the standpoint of workability, the P content may be 0.035% or less. However, excessively reducing the P content will lead to an increase in raw material costs. Therefore, the P content may be 0.005% or more.

[0024] (S: Sulfur) S degrades corrosion resistance and promotes cracking during manufacturing. Therefore, the upper limit for S content is 0.0100%. From the viewpoint of corrosion resistance and manufacturability, the S content may be 0.0070% or less. However, excessively reducing the S content will lead to an increase in refining costs. Therefore, the S content may be 0.0003% or more.

[0025] (Cr: Chrome) Cr forms a passive film on the surface of the stainless steel sheet, thereby improving its corrosion resistance. However, if Cr is added in excess, the ductility of the stainless steel sheet decreases. Therefore, the Cr content may be between 12.0% and 18.0%, or between 15.0% and 17.0%.

[0026] (N: Nitrogen) N is an austenite-forming element and is an important element for the formation of martensite during hot-rolled sheet annealing. However, if N is added in excess, the amount of dissolved N after final annealing tends to increase, which is a factor in the increased yield elongation of the stainless steel sheet and reduces the ductility of the stainless steel sheet due to solid solution strengthening. Therefore, the N content may be between 0.010% and 0.100%, or between 0.010% and 0.050%.

[0027] (Al: Aluminum) Al is an effective element for deoxidation and also precipitates as AlN, reducing the amount of dissolved nitrogen. However, Al is a strong ferrite-forming element, and excessive addition can reduce the amount of martensite generated during hot-rolled sheet annealing more than necessary. Therefore, the Al content may be between 0.050% and 0.250%, or between 0.050% and 0.150%.

[0028] (Other ingredients) This stainless steel sheet may have a chemical composition that further contains 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).

[0029] Copper (Cu) is an effective element for improving corrosion resistance. When Cu is included in the chemical composition, the Cu content may be between 0.01% and 0.50%.

[0030] Nickel (Ni) is an austenite-forming element and affects the amount of martensite and the strength of the stainless steel sheet. On the other hand, if Ni is added in excess, the raw material cost increases, and the austenite phase becomes more stable than necessary, 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 between 0.01% and 0.20%.

[0031] Mo (molybdenum) is an effective element for improving corrosion resistance. However, if Mo is added in excess, the raw material cost of this stainless steel sheet will increase. When Mo is included in the chemical composition, the Mo content may be between 0.001% and 0.100%.

[0032] Niobium (Nb) can combine with carbon or nitrogen and be fixed as a carbonitride, thereby increasing the purity of the stainless steel sheet, improving its ductility, and reducing stretcher strain. 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 between 0.001% and 0.100%.

[0033] V (vanadium) combines with C or N and is fixed as a carbonitride, thereby increasing the purity of the stainless steel sheet, improving its ductility, and reducing stretcher strain. However, if V is added in excess, the raw material cost of the stainless steel sheet increases. When V is included in the chemical composition, the V content may be between 0.01% and 0.15%.

[0034] Titanium (Ti), like Nb, is an element that forms carbonitrides, which can reduce stretcher strain. It also reduces grain boundary precipitation of Cr carbonitrides during heat treatment, improving the corrosion resistance of this stainless steel sheet. However, since Ti is an expensive element, excessive addition of Ti will increase the raw material cost of this stainless steel sheet. When Ti is included in the chemical composition, the Ti content may be between 0.001% and 0.100%.

[0035] Boron (B) is an element effective in improving toughness. When B is included in the chemical composition, the B content may be between 0.0001% and 0.0025%.

[0036] Tin (Sn) is an effective element for improving corrosion resistance. However, if Sn is added in excess, the hot workability and toughness of the stainless steel sheet will decrease. When Sn is included in the chemical composition, the Sn content may be between 0.005% and 0.500%.

[0037] Cobalt (Co) is an effective element for improving corrosion resistance and heat resistance. However, if Co is added in excess, the raw material cost of this stainless steel sheet will increase. When Co is included in the chemical composition, the Co content may be between 0.05% and 0.50%.

[0038] Tungsten (W) is an effective element for improving high-temperature strength. However, if W is added in excess, the raw material cost of this stainless steel sheet will increase. When W is included in the chemical composition, the W content may be between 0.05% and 1.00%.

[0039] Antimony (Sb) is effective in improving processability by promoting the formation of deformation strips during rolling. However, if Sb is added in excess, this effect saturates, and processability may decrease. When Sb is included in the chemical composition, the Sb content may be between 0.005% and 0.500%.

[0040] Zr (zirconium) is an effective element 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 between 0.05% and 0.50%.

[0041] Yttrium (Y) is an effective element for improving hot workability and oxidation resistance. However, this effect saturates when Y is added in excess. When Y is included in the chemical composition, the Y content may be between 0.001% and 0.100%.

[0042] Magnesium (Mg) forms Mg oxide with Al in molten steel and acts as a deoxidizing agent. However, if Mg is added in excess, the toughness of the stainless steel sheet may decrease. When Mg is included in the chemical composition, the Mg content may be between 0.0001% and 0.0050%.

[0043] Calcium (Ca) is an effective element for degassing. When Ca is included in the chemical composition, the Ca content may be between 0.0001% and 0.0050%.

[0044] Rare Earth Metals (REMs) such as Sc (scandium) and La (lanthanum) are effective in improving hot workability and oxidation resistance, similar to Y. However, these effects saturate above 0.100%. When REMs are included in the chemical composition, the total REM content may be between 0.001% and 0.100%.

[0045] (Regarding remaining material and impurities) In this stainless steel sheet, the remainder other than the components described above may consist of Fe (iron) and impurities. "Impurities" refers to components that are mixed in during the industrial manufacture of stainless steel sheets due to various factors such as raw materials like ore and scrap, or the manufacturing process, and are acceptable as long as they do not adversely affect the present invention. For example, O (oxygen) can reduce the impact value and fatigue life of this stainless steel sheet because it creates nonmetallic inclusions. The content of O as an impurity may be 0.01% or less. Such impurities may be unavoidable impurities that are inevitably mixed in due to raw materials or the manufacturing process, or they may be other impurities.

[0046] In the chemical composition of this stainless steel sheet, "additive-free" means that the element in question was not artificially added during the steelmaking process. This stainless steel sheet does not need to substantially contain any of the aforementioned arbitrary elements in its chemical composition. "Substantially free" of an arbitrary element means that the element is not added, but it is permissible for it to be present in trace amounts as an impurity.

[0047] For example, since the carbon content of this stainless steel sheet is not significantly reduced, there is no need to add nickel to ensure sufficient martensite formation during hot-rolled sheet annealing, and therefore, it does not need to contain substantially any nickel. Furthermore, this stainless steel sheet does not need to contain substantially any titanium.

[0048] Furthermore, in this stainless steel sheet, the C index represented by formula (6) above is less than 17.5. This stainless steel sheet achieves a reduction in yield elongation by reducing the amount of dissolved carbon and dissolved nitrogen after final annealing. The inventors have newly discovered that reducing the amount of dissolved carbon is particularly effective in reducing yield elongation. The inventors have also discovered that when the C index is 17.5 or higher, it becomes difficult to sufficiently reduce the amount of dissolved carbon after final annealing even if Cr carbides are precipitated by the manufacturing method of this stainless steel sheet described later, thus completing the present invention.

[0049] This stainless steel sheet may have a C index of 10.84 or higher and 17.0 or lower.

[0050] <Metal structure and properties of this stainless steel sheet> This stainless steel sheet has the chemical composition described above and is a cold-rolled annealed steel sheet manufactured by controlling the manufacturing conditions (especially the hot-rolled sheet annealing temperature and the final annealing temperature), and has the following metal structure. Specifically, this stainless steel sheet has a mixed grain structure in which the average grain size is 7 μm or more, 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. Cold-rolled annealed steel sheet (cold-rolled annealed sheet) refers to a steel sheet obtained by cold-rolling a hot-rolled annealed sheet that has undergone an appropriate pickling process, and then performing a final annealing on the resulting cold-rolled sheet. The manufacturing process of this stainless steel sheet will be described later.

[0051] Figure 1 is a schematic diagram illustrating the cross-section of the stainless steel sheet. As shown in Figure 1, the cross-section 12 is a cross-section (a so-called L-section) that is parallel to the rolling direction of the stainless steel sheet 1 and perpendicular to the rolling surface 11. The rolling surface 11 is also the surface of the stainless steel sheet 1.

[0052] The cross section 12 may be an L-shaped cross section at the center of the plate width. Let the thickness of the stainless steel plate 1 be t and the width be w. The thickness t of the 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. Also, the width w of the 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.

[0053] The average grain size and the ratio of the first and second grains in this stainless steel sheet can be calculated as follows. Specifically, the cross-section 12 can be calculated using the electron backscattered diffraction (EBSD) method. For example, the EBSD pattern of the cross-section 12 is acquired using an EBSD detector mounted on a scanning electron microscope (SEM).

[0054] Next, using OIM (Orientation Imaging Microscopy) analysis software, the acquired EBSD pattern is analyzed to identify grains at interfaces with an orientation difference of 15° or more, defining them as grain boundaries. This means that regions enclosed by large-angle grain boundaries, which are generally defined as grain boundaries, are considered grains. The average grain size can then be calculated using the Area Method (Average by Area Fraction Method). Furthermore, grains with a grain size of 8 μm or less are designated as first grains, and grains with a grain size of 15 μm or more are designated as second grains, and the proportions occupied by the first and second grains can be calculated. The grain size may be calculated using the diameter of a circle with an area equal to the area of ​​each individual grain.

[0055] This stainless steel sheet, having the aforementioned mixed grain structure, can have a yield strength of 390 MPa or less, a yield elongation of 3.5% or less, and a surface rhinestone height of 15 μm or less when a tensile strain of 16% is applied in the rolling direction.

[0056] The yield strength is measured by the following method. Specifically, for this stainless steel sheet, for example, a JIS No. 13B tensile test specimen as specified in JIS Z 2201 is taken so that the tensile direction is parallel to the rolling direction. The 0.2% yield strength (MPa) measured by performing a tensile test as specified in JIS Z 2241 on the tensile test specimen is defined as the yield strength of this stainless steel sheet.

[0057] Yield elongation, like proof strength, can be measured by a tensile test specified in JIS Z 2241. Specifically, for this stainless steel sheet, for example, a JIS No. 13B tensile test specimen, as specified in JIS Z 2241, is taken so that the tensile direction is parallel to the rolling direction. The yield elongation (%) measured by performing a tensile test on the tensile test specimen as specified in JIS Z 2241 is defined as the yield elongation of this stainless steel sheet.

[0058] The ridging height is measured by the following method. Specifically, for the stainless steel sheet, a JIS No. 5 tensile test specimen, as specified in JIS Z 2201, is taken so that it is in a tensile direction parallel to the rolling direction. Next, a tensile strain of 16% is applied to the tensile test specimen, 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 specified in JIS B 0601. A cutoff value was set (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 for wavelength components of 0.8 to 5.0 mm was defined. The reference length was set to 18 mm. The waviness height (μm) measured in this way is defined as the ridging height.

[0059] Furthermore, in the cross-section 12 of this stainless steel sheet, the area ratio 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 ratio of the martensite phase remaining after final annealing, the better the workability. If the area ratio is 1.0% or less, the workability of this stainless steel sheet will be good.

[0060] The area ratio of the martensite phase can be calculated as follows: The EBSD pattern acquired for 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 and lower clarity compared to the ferrite phase, so the image of the martensite phase is darker than the image of the ferrite phase. By binarizing the IQ map and dividing the area of ​​the martensite phase in the IQ map by the total area of ​​the IQ map, the area ratio of the martensite phase can be calculated.

[0061] [Manufacturing method for this stainless steel sheet] Figure 2 is a flowchart showing an example of a method for manufacturing a ferritic stainless steel sheet according to one embodiment of the present invention. As shown in Figure 2, the method for manufacturing this stainless steel sheet includes the following steps: melting step S1, hot rolling step S2, hot rolled sheet annealing step S3, cold rolling step S4, and final annealing step S5. Each step will be described below.

[0062] <Smelting process S1 and hot rolling process S2> In the melting process S1, a steel slab having the aforementioned chemical composition is melted. In the melting process S1, a general melting apparatus can be used and general melting conditions can be set. Next, in the hot rolling process S2, a hot-rolled steel sheet (hot-rolled steel strip) is produced by hot-rolling the steel slab manufactured in the melting process S1. In this specification, a hot-rolled steel sheet may be abbreviated as a hot-rolled strip. In the hot rolling process S2, a general hot-rolling apparatus and hot-rolling conditions can be used.

[0063] For example, as hot rolling conditions, the heating temperature of the steel slab may be 1150 to 1250°C. Also, the finish rolling temperature in the hot rolling process may be 800 to 1100°C or 810 to 1060°C. After the hot-rolled sheet is cooled in a cooling zone after finish rolling, the winding temperature when winding it into a coil may be set to 600 to 850°C or 650 to 800°C.

[0064] <Hot-rolled sheet annealing process S3> Next, in the hot-rolled sheet annealing process S3, the hot-rolled sheet produced in the hot-rolling process 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 annealed steel sheet: Hot-rolled sheet annealing temperature = Ac1 + (M - 0.2678 × γp + 0.888 × Ht - 1.403) / (-0.05847 + 0.0051 × γp) ... (3) In equation (3) above, M is an index representing the area ratio of the martensite phase in the L section of the hot-rolled annealed steel sheet (corresponding to the aforementioned section 12 (see Figure 1)), and Ht is the thickness of the hot-rolled sheet (unit: mm). γp is the value obtained by equation (1) above.

[0065] Furthermore, Ac1 is a value obtained by the following formula (4): Ac1=35×Cr equivalent+310...(4) Here, the Cr equivalent is the value obtained by equation (2) mentioned above.

[0066] In this specification, hot-rolled annealed steel sheets may be abbreviated as hot-rolled annealed sheets. Equation (3) above is a relational expression found by the inventors based on numerous experiments, taking into account the effects of chemical composition and the thickness of the hot-rolled sheet, regarding the relationship between the amount of martensite in a hot-rolled annealed sheet and the annealing temperature of the hot-rolled sheet. Since martensite is generated from interfaces in the metal structure of the hot-rolled sheet, if 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 easily generated. This tendency is also reflected in equation (3) above. 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.

[0067] Here, the γp value of this stainless steel sheet is between 15 and 30. If the γp value is less than 15, martensite is less likely to form in the hot-rolled sheet annealing process S3, making it difficult to improve the rignification properties. Also, if the γp value is extremely small, it becomes difficult to apply the above equation (3). Furthermore, if the γp value is greater than 30, excessive martensite is formed in the hot-rolled sheet annealing process S3. In this case, the microstructure of this stainless steel sheet becomes finer, the yield strength increases, and the yield elongation becomes larger.

[0068] By annealing the hot-rolled sheet at a hot-rolled sheet annealing temperature such that the value of M in equation (3) is in the range of 2.0 to less than 10.0, the area ratio of the martensite phase in the L section (the same section as section 12 in Figure 1) of the hot-rolled annealed sheet can be adjusted to 2.0% to less than 10.0%. By setting the area ratio of the martensite phase in the hot-rolled annealed sheet to 2.0% to less than 10.0%, the rising resistance of the stainless steel sheet after the subsequent cold-rolling process S4 and the final annealing process S5 can be improved, and the occurrence of stretcher strain due to yield elongation can be reduced.

[0069] Specifically, when a tensile strain of 16% is applied in the rolling direction, a stainless steel sheet can be obtained in which the surface rhinestone height is 15 μm or less and the yield elongation is 3.5% or less. Furthermore, by the method described above, the workability of the stainless steel sheet can also be improved, and specifically, the yield strength of the stainless steel sheet can be reduced to 390 MPa or less.

[0070] When the martensite content of hot-rolled and annealed stainless steel exceeds 10.0%, the microstructure of the stainless steel becomes finer, increasing its yield strength and yield elongation.

[0071] If the martensite content of the hot-rolled and annealed sheet is less than 2.0%, the crushing of the colony structure in the cold-rolling process S4 will be insufficient, resulting in insufficient improvement in rising resistance. Furthermore, in the cold-rolling process S4, local strains accumulated around the martensite phase become recrystallization nucleation sites, making recrystallization less likely in the final annealing process S5, thus reducing workability.

[0072] As mentioned above, conventionally, it has been difficult to appropriately adjust the amount of martensite in hot-rolled annealed sheets, which fluctuates due to various factors, and there have been no guidelines for properly adjusting the amount of martensite in hot-rolled annealed sheets. In contrast, in the manufacturing method of this stainless steel sheet, by using the aforementioned equation (3), a relational expression discovered by the inventors, in a component system having the aforementioned chemical composition, the amount of martensite in the hot-rolled annealed sheet can be adjusted to an appropriate range in which the properties of this stainless steel sheet can be obtained.

[0073] In the hot-rolled sheet annealing process S3, general conditions other than the hot-rolled sheet annealing temperature can be used. For example, the heating rate during the heating 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 process S3 means that the material is cooled immediately after the temperature at the center of the sheet thickness reaches the predetermined temperature. In addition, in the hot-rolled sheet annealing process 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.

[0074] <Cold rolling process S4> Next, in the cold rolling process S4, a cold-rolled steel sheet (cold-rolled steel strip) is produced by cold-rolling the hot-rolled annealed sheet manufactured in the hot-rolled sheet annealing process S3. In this specification, cold-rolled steel sheets may be abbreviated as cold-rolled sheets. A pickling process may be appropriately included between the hot-rolled sheet annealing process S3 and the cold rolling process S4. In the cold rolling process S4, the cold rolling conditions are set so that the total cold-rolling ratio after the completion of the cold rolling process S4 is 60% or more. The total cold-rolling ratio after the completion of the cold rolling process S4 may be 70% or more, and the upper limit may be 90%.

[0075] <Final annealing process S5> Next, in the final annealing step S5, the cold-rolled sheet produced in the cold-rolling step S4 is annealed. In the final annealing step S5, the ferrite phase in the cold-rolled sheet is recrystallized, and the final 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 final annealing step S5 is 1.0% or less. The final annealing step S5 according to one embodiment of the present invention is also called the finish annealing step.

[0076] In the final annealing step S5, the final annealing temperature is set to be between 800°C and Ac1-70°C. Ac1 is a value that varies depending on the chemical composition and is determined by the aforementioned equation (4). Ac1 is an indicator of the temperature at which the austenite phase formation begins. If the final annealing temperature is above Ac1-70°C, which is the precipitation temperature range for carbonitrides, the amount of solid solution of C and N in the final annealing increases. By setting the upper limit of the final annealing temperature to Ac1-70°C, the amount of solid solution of C and N in the stainless steel after the final annealing step S5 can be reduced, thereby reducing the yield elongation of the stainless steel. Furthermore, by setting the final annealing temperature to 800°C or higher, recrystallization can be promoted in the final annealing step S5.

[0077] In the chemical composition of this stainless steel sheet, the Cr equivalent value used to calculate Ac1 is between 17.0 and 19.5. Therefore, the Ac1 value based on the chemical composition of this stainless steel sheet is between 905.0 and 992.5, and consequently, the upper limit of the final annealing temperature is between 835.0°C and 922.5°C.

[0078] In the final annealing step S5, the cold-rolled sheet is heated at a heating rate of, for example, 100°C / s or less during the heating process to reach the final annealing temperature.

[0079] In the final annealing step S5, the soaking time at the final annealing temperature shall be 0 seconds or more and 60 seconds or less. A soaking time of 0 seconds in the final annealing step S5 means that the material is cooled immediately after the temperature at the center of the plate thickness reaches the predetermined final annealing temperature. In addition, in the final annealing step S5, after soaking at the final annealing temperature, the cooling rate in the temperature range from the final annealing temperature to 500°C may be 5 to 100°C / second.

[0080] The atmosphere used for the final annealing process S5 is preferably one with a dew point of -40°C or lower and containing hydrogen gas (H2) and nitrogen gas (N2) at a pressure ratio of 0% to 30%. Performing the final annealing process S5 in the aforementioned atmosphere can improve the appearance of the stainless steel sheet. Thus, the final annealing process S5 may be performed, for example, under conditions corresponding to bright annealing.

[0081] Upon completion of the final annealing process S5, the stainless steel sheet as the final product is obtained. It is preferable that the area ratio of the martensite phase in the stainless steel sheet becomes 1.0% or less, more preferably 0.5% or less, and even more preferably 0.2% or less after the final annealing process S5. Most preferably, the martensite phase disappears completely, becoming 0%.

[0082] [Annealing temperature determination device] The annealing temperature determination device that determines the range of annealing temperatures for the hot-rolled sheet in the aforementioned hot-rolled sheet annealing process S3 and the range of final annealing temperatures for the aforementioned final annealing process S5 also falls within the scope of the present invention. Figure 3 is a block diagram showing a schematic configuration of the 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 sheets 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 final annealing obtained by cold-rolling a hot-rolled sheet after hot-rolled annealing.

[0083] As shown in Figure 3, the annealing temperature determination device 2 comprises 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.

[0084] Furthermore, the annealing temperature determination device 2 may include an input unit 30, an output unit 40, and a communication unit 50. Known devices can be used as the input unit 30, the output unit 40, and the communication unit 50, and the specific configuration is not particularly limited. Each part of the annealing temperature determination device 2 may be connected to a system bus SB, and configured to exchange data with each other via the system bus SB.

[0085] The control unit 10 is a component that comprehensively controls the operation of each part of the annealing temperature determination device 2, and is, for example, a CPU (Central Processing Unit). Each part of the control unit 10 may be implemented as software operated by the CPU, for example. The storage unit 20 is a volatile or non-volatile storage device (for example, a hard disk or flash memory) that stores various data used in the control unit 10.

[0086] The component data 21 is data relating to the content of each element in the chemical composition of the steel slab, and is obtained, for example, by performing a component analysis on the steel slab obtained by the melting process S1 or the hot-rolled sheet after the hot-rolling process S2. The sheet thickness data 22 is the sheet thickness value of the hot-rolled sheet after the hot-rolling process S2 that is subjected to the hot-rolled sheet annealing process S3. The component data 21 and the sheet thickness data 22 may be stored in the storage unit 20, for example, via the input unit 30 or the communication unit 50.

[0087] The information acquisition unit 15 acquires the content (mass%) of each element in the chemical composition of the steel slab and the thickness of the hot-rolled sheet. The information acquisition unit 15 may also acquire this information by reading the component data 21 and thickness data 22 stored in the storage unit 20. Alternatively, the information acquisition unit 15 may acquire the content of each element in the chemical composition of the steel slab and the thickness of the hot-rolled sheet via the communication unit 50.

[0088] The first temperature determination unit 16 calculates the values ​​of γp and Ac1 using the elemental content and hot-rolled sheet thickness values ​​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 process S3 such that M in the following equation (5) is 2.0 or more and less than 10.0: M = {0.003071 × γp + 0.000522 × (AT - AC1) - 0.00887 × Ht + 0.000051 × (AT - AC1 - 7.699) × (γp - 21.7) + 0.005513} × 100 ···(5) In equation (5) above, γp is the value obtained by equation (1) above, AT is the annealing temperature (°C) in the hot-rolled sheet annealing process S3, and Ht is the thickness of the hot-rolled sheet (unit: mm). Also, Ac1 is the value obtained by equation (4) above. Note that equation (3) above can be derived by rearranging equation (5).

[0089] The first temperature determination unit 16 may set AT(2.0), which is the value of AT when M is 2.0 in equation (5), as the lower limit of the first annealing temperature range. Alternatively, the first temperature determination unit 16 may set AT(10.0), which is the value of AT when M is 10.0 in equation (5), as the upper limit of the first annealing temperature range. In this case, the first temperature determination unit 16 determines the first annealing temperature range to be between AT(2.0)°C and AT(10.0)°C.

[0090] The first annealing temperature range determined by the first temperature determination unit 16 may be displayed on a screen by the output unit 40, or output to an external device via the communication unit 50.

[0091] Conventionally, it has been extremely difficult to empirically control the amount of martensite in hot-rolled and annealed sheets, which is greatly influenced by chemical composition and other factors. The annealing temperature determination device 2 uses equation (5) above to calculate the values ​​of γp and Ac1 corresponding to various chemical compositions and thicknesses of hot-rolled sheets, and can calculate the first annealing temperature range in the hot-rolled sheet annealing process S3 such that the amount of martensite in the hot-rolled and annealed sheet is 2.0% or more and less than 10.0%.

[0092] The second temperature determination unit 17 calculates the value of Ac1 using the content values ​​of each element acquired by the information acquisition unit 15, and determines the second annealing temperature range in the final annealing process S5 to be between 800°C and the value of Ac1 - 70°C. Specifically, the second temperature determination unit 17 determines the temperature to be the upper limit of the second annealing temperature range, which is the value of Ac1 - 70°C. The second temperature determination unit 17 may use the value of Ac1 calculated by the first temperature determination unit 16, or it may calculate it using the aforementioned formula (4).

[0093] The second annealing temperature range determined by the second temperature determination unit 17 may be displayed on a screen by the output unit 40, or output to an external device via the communication unit 50.

[0094] The first temperature determination unit 16 and the second temperature determination unit 17 may determine a specific temperature from the calculated annealing temperature range as the annealing temperature. The process in this case will be explained below using the process of the first temperature determination unit 16 as an example.

[0095] Generally, in batch annealing, multiple coils are annealed simultaneously. Even in continuous annealing, from the viewpoint of productivity, it is preferable to anneal a group of coils without changing (maintaining) the annealing temperature. The annealing temperature determination device 2 may store, for example, the component data 21 and thickness data 22 of the multiple coils that are to be processed in the hot-rolled sheet annealing process S3 in the storage unit 20.

[0096] The first temperature determination unit 16 (i) calculates the values ​​of γp and Ac1 for each of the multiple coils based on the information acquired by the information acquisition unit 15. The first temperature determination unit 16 can also (ii) calculate the first annealing temperature range in the hot-rolled sheet annealing process S3 for each of the multiple coils such that M in equation (5) is 2.0 or more and less than 10.0.

[0097] For example, a batch (one processing unit) is defined as a set of multiple coils that are annealed simultaneously in batch annealing, or a set of multiple coils that are annealed without changing the hot-rolled sheet annealing temperature in continuous annealing. The first temperature determination unit 16 uses a first annealing temperature range for each of the multiple coils included in one processing unit, in which M in equation (5) is 2.0 or more and less than 10.0, to determine whether there is an overlapping temperature range for all of the multiple coils.

[0098] Next, if there are overlapping temperature ranges for a single 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 communication unit 50 may output the determined temperature range. Alternatively, the output unit 40 or communication unit 50 may output the overlapping temperature ranges.

[0099] Furthermore, the first temperature determination unit 16 may generate a determination result indicating that it cannot determine an appropriate annealing temperature if there are no overlapping temperature ranges for a given processing unit. The output unit 40 or the communication unit 50 may output this determination result.

[0100] [Examples of implementation using software] The function of the annealing temperature determination device 2 (hereinafter referred to as "the device") is a program (information processing program) that causes the device to function as a computer, and can be realized by a program that causes the computer to function as each control block of the device (particularly each part included in the control unit 10).

[0101] 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., memory) as hardware for executing the program. By executing the program using this control device and storage device, each of the functions described in the above embodiment is realized.

[0102] The program may be recorded on one or more computer-readable recording media, not temporary ones. These recording media may or may not be provided by the device. In the latter case, the program may be supplied to the device via any wired or wireless transmission medium.

[0103] Furthermore, some or all of the functions of each of the control blocks can also be realized by logic circuits. For example, an integrated circuit in which logic circuits that function as each of the control blocks are formed is also included in the scope of the present invention.

[0104] 〔summary〕 The ferritic stainless steel sheet in Embodiment 1 of the present invention contains, by mass%, C: 0.010% to 0.100%, Si: 0.10% to 1.00%, Mn: 0.10% to 1.00%, P: 0.005% to 0.050%, S: 0.0100%, Cr: 12.0% to 18.0%, N: 0.010% to 0.100%, and Al: 0.050% to 0.250%, with the remainder being Fe and impurities. The 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, the Cr equivalent represented by the following formula (2) is 17.0 or more and 19.5 or less, and the C index represented by the following formula (6) is less than 17.5, and in a cross section parallel to the rolling direction and perpendicular to the rolling surface which is also the surface, the average grain size is 7 μm or more, and 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. 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) C index=Cr-11.6C...(6) Here, the elemental symbols in equations (1), (2), and (6) are replaced with the content (mass%) of each element, and 0 is substituted for elements that are not added.

[0105] The ferritic stainless steel sheet in embodiment 2 of the present invention may contain, in 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: 0.010% to 0.050%, and Al: 0.050% to 0.150%.

[0106] The ferritic stainless steel sheet in embodiment 3 of the present invention is, in embodiment 1 or 2, composed of, 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 elements selected from the group consisting of: 0.05% to 0.50%, W: 0.05% to 1.00%, Sb: 0.005% to 0.500%, Zr: 0.05% to 0.50%, Y: 0.001% to 0.100%, Mg: 0.0001% to 0.0050%, Ca: 0.0001% to 0.0050%, and REM (rare earth metals): totaling 0.001% to 0.100%.

[0107] In embodiment 4 of the present invention, the ferritic stainless steel sheet may, in any one embodiment of embodiments 1 to 3, have a yield strength of 390 MPa or less, a yield elongation of 3.5% or less, and a surface rigning height of 15 μm or less when a tensile strain of 16% is applied in the rolling direction.

[0108] In embodiment 5 of the present invention, the ferritic stainless steel sheet may have a martensite phase area ratio of 1.0% or less in the cross-section, in any one embodiment of embodiments 1 to 4.

[0109] A method for manufacturing a ferritic stainless steel sheet according to embodiment 6 of the present invention is a method for manufacturing a ferritic stainless steel sheet according to any one embodiment of embodiments 1 to 5, comprising: a hot-rolled sheet annealing step 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 final annealing step in which a cold-rolled sheet obtained by cold-rolling the hot-rolled sheet annealed sheet obtained in the hot-rolled sheet annealing step is annealed under the conditions that the final annealing temperature is 800°C or higher, the value of Ac1 represented by the following formula (4) is -70°C or lower, and the soaking time at the final annealing temperature is 0 seconds or more and 60 seconds or less.

[0110] Hot-rolled sheet annealing temperature = Ac1 + (M - 0.2678 × γp + 0.888 × Ht - 1.403) / (-0.05847 + 0.0051 × γp) ... (3) Ac1 = 35 × the above Cr equivalent + 310...(4) Here, in formula (3), M is an index representing the area ratio of the martensitic phase in the hot-rolled annealed sheet in a cross section parallel to the rolling direction and perpendicular to the rolling surface, and Ht is the thickness of the hot-rolled sheet (unit: mm).

[0111] The annealing temperature determination apparatus in embodiment 7 of the present invention is used to perform hot-rolled annealing of a hot-rolled sheet obtained by hot-rolling a steel slab having a chemical composition in mass%, containing C: 0.010% to 0.100%, Si: 0.10% to 1.00%, Mn: 0.10% to 1.00%, P: 0.005% to 0.050%, S: 0.0100%, Cr: 12.0% to 18.0%, N: 0.010% to 0.100%, and Al: 0.050% to 0.250%, with the remainder being Fe and impurities, and having a γp value represented by formula (1) of 15 to 30, a Cr equivalent represented by formula (2) of 17.0 to 19.5, and a C index represented by formula (6) of less than 17.5, and cold-rolling the said hot-rolled sheet. An annealing temperature determination device for determining the annealing temperature range for the final annealing performed on a rolled cold-rolled sheet comprises: 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 the first annealing temperature range for the hot-rolled sheet annealing; and a second temperature determination unit that determines the second annealing temperature range for the final annealing. The first temperature determination unit uses the content (mass%) of each element and the thickness to determine the first annealing temperature range such 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 such that it is 800°C or higher and less than or equal to the value of Ac1 represented by the following formula (4) - 70°C.

[0112] 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 × the above Cr equivalent + 310...(4) M = {0.003071 × γp + 0.000522 × (AT - AC1) - 0.00887 × Ht + 0.000051 × (AT - AC1 - 7.699) × (γp - 21.7) + 0.005513} × 100 ···(5) C index=Cr-11.6C...(6) Here, the elemental symbols in equations (1), (2), and (6) are replaced with the content (mass%) of each element, and 0 is replaced for elements that are not added. In equation (5), AT is the annealing temperature (°C) in the hot-rolled sheet annealing, and Ht is the thickness (unit: mm) of the hot-rolled sheet.

[0113] The information processing program in embodiment 8 of the present invention is for causing a computer to function as the annealing temperature determination device of embodiment 7.

[0114] [Additional Notes] The present invention is not limited to these embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in each embodiment are also included within the technical scope of the present invention. [Examples]

[0115] Examples of the present invention are described below.

[0116] [Manufacturing conditions] Ferritic stainless steel sheets within the range defined in this invention are referred to as "examples of invention," and ferritic stainless steel sheets outside the range defined in this invention are referred to as "comparative examples." In this example, first, steel slabs with the compositions shown in Table 1 below were manufactured by melting them on an actual production line. In Table 1, the values ​​of Cr equivalent, Ac1, and γp are the values ​​calculated by the formulas described above. Also in Table 1, values ​​outside the range defined in this invention are underlined.

[0117] [Table 1]

[0118] Using steel slabs of each steel number listed in Table 1, hot rolling, hot-rolled sheet annealing, and cold rolling (sheet thickness 0.8 mm) were performed, followed by final annealing to produce cold-rolled annealed sheets (ferritic stainless steel sheets). The sheet thickness of the hot-rolled sheets, the hot-rolled sheet annealing temperature, and the final annealing temperature (FA temperature) were as specified in Table 2 below. The soaking time at the final annealing temperature during the final annealing was 30 seconds.

[0119] Other specific conditions for hot rolling, cold rolling, and final annealing were general conditions. These general conditions are as exemplified in the "Modes for Carrying Out the Invention" section above. Each test material was evaluated as follows.

[0120] [Evaluation Method] <Martensite amount> For the L-sections of hot-rolled and cold-rolled annealed sheets, EBSD patterns were acquired using an EBSD detector mounted on a scanning electron microscope (SEM) under the following acquisition conditions: • Magnification: 100-800x ·Measurement area: 100~1000μm square • Measurement pitch (step size): 0.3~0.8μm From the acquired EBSD patterns, IQ images were generated using OIM analysis software (OIM Analysis7). The IQ images were binarized, and the martensite area ratio was calculated by dividing the total area of ​​the martensite phase region by the area of ​​the measurement surface.

[0121] <Rising height> From the cold-rolled annealed sheet after final annealing, a JIS No. 5 tensile test specimen (first tensile test specimen) as specified in JIS Z 2201 was taken parallel to the rolling direction. Next, using an Instron type tensile testing machine, a tensile test was performed on the first tensile test specimen with a gauge length of 50 mm and the tensile direction parallel to the rolling direction, applying a tensile strain of 16%.

[0122] Next, using a surface roughness measuring instrument, the average height of the waviness curve elements was measured in the intergauge portion of the first tensile test specimen, in the surface texture measurement method specified in JIS B 0601:2013, etc., with a measurement length of 18 mm in the direction perpendicular to the rolling direction. A cutoff value was set (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 for wavelength components of 0.8 to 5.0 mm was defined. The value of the average height of the waviness curve elements was adopted as the rigning height value. For evaluation of rigning resistance, a rigning height of 15 μm or less was considered acceptable.

[0123] <Yield strength and yield elongation> A tensile test specimen (second tensile test specimen) of type JIS 13B was taken from a cold-rolled and annealed sheet in the rolling direction. A tensile test specified in JIS Z 2241 was performed on the second tensile test specimen using a tensile testing machine, and the 0.2% proof stress and yield elongation were measured. For the evaluation of workability, a proof stress of 390 MPa or less was considered acceptable. For the evaluation of yield elongation, a yield elongation of 3.5% or less was considered acceptable.

[0124] <Mixed grain structure> For the cold-rolled and annealed sheets, EBSD patterns of the L-section were obtained, similar to the martensite content measurement described above. Using OIM analysis software, grains were identified by defining interfaces with orientation differences of 15° or more as grain boundaries. The average grain size was calculated using the Area method. The area ratios of the first grain (grain size 8 μm or less) and the second grain (grain size 15 μm or more) were also calculated. The grain size was calculated using the diameter of a circle with an area equal to the area of ​​each individual grain.

[0125] [Evaluation Results] Table 2 shows the results of evaluating the structure and properties of each test material in the inventive example and comparative example. In Table 2, compositions and values ​​that fall outside the range defined in the present invention are underlined.

[0126] [Table 2]

[0127] As shown in Invention Examples No. 2 and 4 and Comparative Examples No. 3 and 5, by appropriately adjusting the hot-rolled sheet annealing temperature based on the chemical composition and hot-rolled sheet thickness, it was possible to achieve a martensite content of 2.0% or more and less than 10.0% in the hot-rolled annealed sheet.

[0128] Furthermore, as shown in Invention Examples No. 2, 4, and 8 and Comparative Examples No. 3, 5, and 7, by appropriately adjusting the hot-rolled sheet annealing temperature and the final annealing temperature, a cold-rolled annealed sheet having a mixed grain structure in which the average grain size and the ratio of first and second grains are within the specified range of the present invention was obtained. In contrast, in the cold-rolled annealed sheet of Comparative Example No. 5, recrystallization did not occur in the final annealing process, and a mixed grain structure was not obtained.

[0129] The cold-rolled annealed sheets in Invention Examples No. 1, 2, 4, 6, 7, and 9-25 all possessed excellent rising resistance and low yield elongation. Furthermore, the cold-rolled annealed sheets in Invention Examples also exhibited relatively good processability. All of these cold-rolled annealed sheets in Invention Examples 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 amount of martensite is so low that the martensite phase region cannot be distinguished in the binarized IQ image.

[0130] In contrast, Comparative Examples No. 3, 5, 8, and 26-33, in which at least one of the chemical composition, hot-rolled sheet annealing conditions, and final annealing conditions was outside the specified range of the present invention, failed to satisfy at least one of the predetermined properties. In particular, as shown in Comparative Example No. 33, it was demonstrated that when the C index in the chemical composition was outside the specified range of the present invention, the yield elongation was not sufficiently reduced. [Explanation of symbols]

[0131] 1. Stainless steel sheet (ferritic stainless steel sheet) 2. Annealing temperature determination device 11. Rolled surface (surface) 12 Cross-section 15 Information acquisition department 16 1st temperature determination section 17 Second temperature determination section S3 Hot-rolled sheet annealing process S5 Final annealing process

Claims

1. A ferritic stainless steel sheet containing, by mass%, C: 0.010% to 0.100%, Si: 0.10% to 1.00%, Mn: 0.10% to 1.00%, P: 0.005% to 0.050%, S: 0.0100%, Cr: 12.0% to 18.0%, N: 0.010% to 0.100%, and Al: 0.050% to 0.250%, with the remainder 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, the Cr equivalent represented by the following formula (2) is 17.0 or more and 19.5 or less, and the C index represented by the following formula (6) is less than 17.

5. In a cross-section parallel to the rolling direction and perpendicular to the rolling surface which is also the aforementioned surface, The average crystal grain size is 7 μm or more, and A ferritic stainless steel sheet in which the proportion of first crystal grains with a grain size of 8 μm or less is less than 50%, and the proportion of second crystal grains with a 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) C index=Cr-11.6C...(6) Here, the elemental symbols in equations (1), (2), and (6) are replaced with the content (mass%) of each element, and 0 is substituted for elements that are not added.

2. A ferritic stainless steel sheet according to claim 1, containing, 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: 0.010% to 0.050%, and Al: 0.050% to 0.150%.

3. In mass percent, 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: 0.05% to 0.50%, W: 0.05% to 1.00% The ferritic stainless steel sheet according to claim 1, further containing one or more elements selected from the group consisting of Sb: 0.005% to 0.500%, Zr: 0.05% to 0.50%, Y: 0.001% to 0.100%, Mg: 0.0001% to 0.0050%, Ca: 0.0001% to 0.0050%, and REM (rare earth metals): 0.001% to 0.100% in total.

4. The bearing capacity is 390 MPa or less. Yield growth is 3.5% or less, and, The ferritic stainless steel sheet according to claim 1, wherein the rigning height of the surface when a tensile strain of 16% is applied in the rolling direction is 15 μm or less.

5. 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 manufacturing a ferritic stainless steel sheet according to any one of claims 1 to 5, A hot-rolled sheet annealing step is performed 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, A method for manufacturing a ferritic stainless steel sheet, comprising: a final annealing step in which a cold-rolled sheet obtained by cold-rolling a hot-rolled annealed sheet obtained by the hot-rolled sheet annealing step is annealed at a final annealing temperature of 800°C or higher, and at or below the value of Ac1 represented by the following formula (4) - 70°C, and the soaking time at the final annealing temperature is 0 seconds or more and 60 seconds or less. Hot-rolled sheet annealing temperature = Ac1 + (M - 0.2678 × γp + 0.888 × Ht - 1.403) / (-0.05847 + 0.0051 × γp) ... (3) Ac1 = 35 × the above Cr equivalent + 310 ... (4) Here, in equation (3) above, The aforementioned M is an index representing the area ratio of the martensite phase in the hot-rolled annealed sheet in a cross-section parallel to the rolling direction and perpendicular to the rolling surface. Ht is the thickness of the hot-rolled sheet (in mm).

7. In mass%, it contains C: 0.010% to 0.100%, Si: 0.10% to 1.00%, Mn: 0.10% to 1.00%, P: 0.005% to 0.050%, S: 0.0100%, Cr: 12.0% to 18.0%, N: 0.010% to 0.100%, and Al: 0.050% to 0.250%, with the remainder being Fe and impurities, and the following formula An annealing temperature determination device for determining the annealing temperature range for hot-rolled sheets annealed on hot-rolled sheets obtained by hot-rolling a steel slab having a chemical composition in which the γp value represented by (1) is 15 or more and 30 or less, the Cr equivalent represented by the following formula (2) is 17.0 or more and 19.5 or less, and the C index represented by the following formula (6) is less than 17.5, and for determining the annealing temperature range for final annealing performed on cold-rolled sheets obtained by cold-rolling the hot-rolled annealed sheets 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 for determining the first annealing temperature range in the hot-rolled sheet annealing, The system includes a second temperature determination unit that determines the second annealing temperature range in the final annealing, The first temperature determination unit determines the first annealing temperature range using the content (mass%) of each element and the plate thickness value such 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 such that it is 800°C or higher and less than or equal to the value of Ac1 represented by the following formula (4) minus 70°C, and is an 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 × the above Cr equivalent + 310 ... (4) M = {0.003071 × γp + 0.000522 × (AT - AC1) - 0.00887 × Ht + 0.000051 × (AT - AC1 - 7.699) × (γp - 21.7) + 0.005513} × 100 ... (5) C index=Cr-11.6C...(6) Here, the elemental symbols in equations (1), (2), and (6) are replaced with the content (mass%) of each element, and 0 is substituted for elements that are not added. In the above formula (5), The aforementioned AT is the annealing temperature (°C) in the hot-rolled sheet annealing process. Ht is the thickness of the hot-rolled sheet (in mm).

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

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