Ferritic stainless steel sheet, method for producing the same, ferritic stainless hot-rolled annealed steel sheet, annealing temperature determination device, and information processing program

Abstract

To provide a ferritic stainless steel sheet that exhibits superior resistance to ridging and high formability.SOLUTION: A ferritic stainless steel sheet contains, in 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. The ferritic stainless steel sheet has a γp value of 15 to 30, a Cr equivalent of 17.0 to 19.5 and an average crystal grain size of 9 μm or more, with less than 35% of the crystal grains having a crystal grain size of 8 μm or less, and 20% or more of the crystal grains having a crystal grain size of 15 μm or more.SELECTED DRAWING: Figure 1

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 household appliances, cooking utensils, and architectural applications. On the other hand, ferritic stainless steel is inferior in ductility compared to austenitic stainless steel. Further, generally, in the case of ferritic stainless steel, ridging is likely to occur during forming, and in applications where surface aesthetics are required, a polishing process for removing ridging after forming is necessary.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0004] Various measures have been studied to improve the ridging resistance characteristics of ferritic stainless steel (see Patent Documents 1 to 4).

[0005] However, for example, in the technologies described in Patent Documents 1 to 4, there is room for improvement in terms of manufacturing cost or raw material cost for realizing a specific chemical composition, or manufacturability or processability.

[0006] One aspect of the present invention is to provide a ferritic stainless steel sheet having excellent anti-ridge characteristics and workability.

Means for Solving the Problems

[0007] In order to solve the above problems, a ferritic stainless steel sheet according to one aspect of the present invention contains, 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, has 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. In a cross-section parallel to the rolling direction and perpendicular to the rolling surface, the average crystal grain size is 9 μm or more, and the proportion of the first crystal grains with a crystal grain size of 8 μm or less is less than 35%, and the proportion of the second crystal grains with a crystal grain size of 15 μm or more is 20% 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 values of the contents (mass%) of the respective elements are substituted in the positions of the element symbols in the above formulas (1) and (2), and 0 is substituted for the elements without addition.

Advantages of the Invention

[0008] According to one aspect of the present invention, it is possible to provide a ferritic stainless steel sheet having excellent anti-ridge characteristics and workability.

Brief Description of the Drawings

[0009]

Figure 1

Figure 2

Figure 3

Mode for Carrying Out the Invention

[0010] Hereinafter, an embodiment of the present invention will be described in detail. In this specification, “%” regarding the content rate of each element in the chemical composition of ferritic stainless steel (steel plate) means “mass %”. The term “steel plate” is used to include a steel strip unless otherwise specified. For numerical values X1 and X2 (where X1 < X2), “X1 to X2” means “X1 or more and X2 or less”. Further, 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”.

[0011] Ridging is a surface defect that occurs, for example, on the surface of a ferritic stainless steel plate after forming processing. 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 plate. The “processing direction” is the direction in which the ferritic stainless steel plate is stretched by forming processing. Examples of the forming processing include press processing, drawing processing, and ironing processing.

[0012] Conventionally, for example, a method of improving the ridging resistance by dispersing a certain amount of martensite phase in the metal structure after annealing of a hot-rolled sheet and pulverizing the colony structure (a structure formed by aggregation of crystal grains having similar crystal orientations) during subsequent cold rolling is known.

[0013] However, if a large amount of martensite phase exists in the metallographic structure of the hot-rolled annealed sheet, the toughness of the hot-rolled annealed sheet will be significantly reduced, and the manufacturability may deteriorate. In addition, the location where the martensite phase exists in the metallographic structure of the hot-rolled annealed sheet decomposes into fine ferrite grains during subsequent cold-rolled sheet annealing. Therefore, the metallographic structure of the cold-rolled annealed steel sheet (hereinafter sometimes referred to as the cold-rolled annealed sheet) is a mixed grain structure in which fine ferrite grains are dispersed in the ferrite structure. When a large amount of martensite phase is generated by hot-rolled sheet annealing, the average crystal grain size of the cold-rolled annealed sheet (ferritic stainless steel sheet) becomes fine, the yield strength of the cold-rolled annealed sheet increases, and the workability deteriorates. Thus, there has been a certain so-called trade-off relationship between the improvement of the anti-ridge property and the manufacturability and workability.

[0014] It is expected that there is an appropriate amount of martensite (the amount of martensite in the hot-rolled annealed sheet) that can obtain good anti-ridge properties without impairing manufacturability and workability. However, the amount of martensite generated during hot-rolled sheet annealing varies greatly depending on slight differences in the components in the chemical composition and the annealing temperature. Therefore, it has been difficult to appropriately adjust the amount of martensite in the hot-rolled annealed sheet.

[0015] As a result of intensive studies, the inventors of the present invention have found a method of appropriately controlling the amount of martensite by defining an appropriate hot-rolled sheet annealing temperature according to the change in the content of each element in the chemical composition (that is, the change in components), and have realized a method for manufacturing a ferritic stainless steel sheet having excellent anti-ridge properties and workability. Excellent workability means that the load and springback during processing are small.

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

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

[0018] And this 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 values of the contents (mass%) of the respective elements are substituted in the places of the element symbols in the above formulas (1) and (2), and 0 is substituted for the elements not added.

[0019] This 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 described below.

[0020] <C: Carbon> C is an austenite-forming element and is an important element for generating martensite during hot-rolled sheet annealing. However, if C is added in excess, an excessive amount of martensite phase will be produced, which may reduce the workability. Also, highly reducing the C content will increase the manufacturing cost (refining cost). Therefore, the C content may be 0.01% or more and 0.10% or less, and may be 0.030% or more and 0.060% or less.

[0021] <Si: Silicon> Si has an effect as a deoxidizer in the melting stage. However, if Si is added in excess, this stainless steel sheet may be hardened and its ductility may decrease. Therefore, the Si content may be 0.1% or more and 1.0% or less, and may be 0.15% or more and 0.40% or less.

[0022] <Mn: Manganese> Mn is an austenite-forming element and is an effective element for generating martensite during hot-rolled sheet annealing. However, if Mn is added in excess, the amount of MnS generated will increase and the corrosion resistance of this stainless steel sheet may decrease. Therefore, the Mn content may be 0.1% or more and 1.0% or less, and may be 0.25% or more and 0.45% or less.

[0023] <P: Phosphorus> P is an element that reduces hot workability. Therefore, the upper limit of the P content is set at 0.050%. From the perspective 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 cost. Therefore, the P content may be 0.005% or more.

[0024] <S: Sulfur> S deteriorates the corrosion resistance and promotes cracking during manufacturing. Therefore, the upper limit of the S content is set at 0.0100%. From the perspectives of corrosion resistance and manufacturability, the S content may be 0.0070% or less. However, excessively reducing the S content will cause an increase in refining cost. Therefore, the S content may be 0.0003% or more.

[0025] <Cr: Chromium> Cr forms a passive film on the surface of this stainless steel plate to enhance corrosion resistance. However, if Cr is added in excess, the ductility of this stainless steel plate will decrease. Therefore, the Cr content may be 12.0% or more and 18.0% or less, and may be 15.0% or more and 17.0% or less.

[0026] <N: Nitrogen> N is an austenite-forming element and is an important element for generating martensite during hot-rolled plate annealing. However, if N is added in excess, the ductility of this stainless steel plate will decrease due to solid solution strengthening. Therefore, the N content may be 0.01% or more and 0.10% or less, and may be 0.01% or more and 0.05% or less.

[0027] <Al: Aluminum> Al is an element effective for deoxidation and reduces A2-type inclusions that have an adverse effect on press formability. However, Al is a strong ferrite-forming element and excessive addition can reduce the amount of martensite generated during hot-rolled plate annealing more than necessary. Therefore, the Al content may be 0.05% or more and 0.25% or less, and may be 0.05% or more and 0.15% or less.

[0028] <Other components> This stainless steel plate may have a chemical composition further containing one or more 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] Cu (copper) is an element effective for improving corrosion resistance. When the chemical composition contains Cu, the Cu content may be 0.01% or more and 0.50% or less.

[0030] Ni (Nickel) is an austenite-forming element and an element that affects the amount of martensite and the strength of this stainless steel plate. On the other hand, if Ni is added in excess, the raw material cost increases, and also, the austenite phase is stabilized more than necessary, resulting in a decrease in the ductility of this stainless steel plate. When the chemical composition contains Ni, the content of Ni may be 0.01% or more and 0.20% or less.

[0031] Mo (Molybdenum) is an element effective in improving corrosion resistance. However, if Mo is added in excess, the raw material cost of this stainless steel plate increases. When the chemical composition contains Mo, the content of Mo may be 0.001% or more and 0.100% or less.

[0032] Nb (Niobium) combines with C or N and is fixed as a carbonitride, thereby so-called purifying this stainless steel plate, improving the ductility of this stainless steel plate, and reducing stretcher strain (minute unevenness formed on the surface of the stainless steel plate caused by yield elongation during processing). However, since Nb is an expensive element, if Nb is added in excess, the raw material cost of this stainless steel plate increases. When the chemical composition contains Nb, the content of Nb may be 0.001% or more and 0.100% or less.

[0033] V (Vanadium) is an element effective in so-called purifying this stainless steel plate and improving the ductility of this stainless steel plate by combining with C or N and being fixed as a carbonitride. However, if V is added in excess, the raw material cost of this stainless steel plate increases. When the chemical composition contains V, the content of V may be 0.01% or more and 0.15% or less.

[0034] Ti (Titanium), like Nb, is an element that forms carbonitrides and suppresses the grain boundary precipitation of Cr carbonitrides during heat treatment to improve the corrosion resistance of this stainless steel plate. However, since Ti is an expensive element, if Ti is added in excess, the raw material cost of this stainless steel plate increases. When the chemical composition contains Ti, the content of Ti may be 0.001% or more and 0.100% or less.

[0035] B (boron) is an element effective for improving toughness. When B is included in the chemical composition, the content of B may be 0.0001% or more and 0.0025% or less.

[0036] Sn (tin) is an element effective for improving corrosion resistance. However, when Sn is added in excess, the hot workability and strength of adhesion of this stainless steel sheet decrease. When Sn is included in the chemical composition, the content of Sn may be 0.005% or more and 0.500% or less.

[0037] Co (cobalt) is an element effective for improving corrosion resistance and heat resistance. However, when Co is added in excess, the raw material cost of this stainless steel sheet increases. When Co is included in the chemical composition, the content of Co may be 0.05% or more and 0.50% or less.

[0038] W (tungsten) is an element effective for improving high-temperature strength. However, when W is added in excess, the raw material cost of this stainless steel sheet increases. When W is included in the chemical composition, the content of W may be 0.05% or more and 1.00% or less.

[0039] Sb (antimony) is effective for improving workability by promoting the generation of deformation bands during rolling. However, when Sb is added in excess, this effect saturates and the workability may decrease. When Sb is included in the chemical composition, the content of Sb may be 0.005% or more and 0.500% or less.

[0040] Zr (zirconium) is an element effective for denitrification, deoxidation, and desulfurization. However, when Zr is added in excess, the raw material cost of this stainless steel sheet increases. When Zr is included in the chemical composition, the content of Zr may be 0.05% or more and 0.50% or less.

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

[0042] Mg (magnesium) forms magnesium oxide together with Al in molten steel and acts as a deoxidizer. However, if Mg is added in excess, the toughness of this stainless steel sheet may decrease. When the chemical composition contains Mg, the content of Mg may be 0.0001% or more and 0.0050% or less.

[0043] Ca (calcium) is an element effective for degassing. When the chemical composition contains Ca, the content of Ca may be 0.0001% or more and 0.0050% or less.

[0044] REM (Rare Earth Metal) such as Sc (scandium) and La (lanthanum) is effective for improving hot workability and oxidation resistance, similar to Y. However, these effects saturate when exceeding 0.10%. When the chemical composition contains REM, the total content of REM may be 0.001% or more and 0.100% or less.

[0045] <Regarding the balance and impurities> In this stainless steel sheet, the balance other than the above-described components may be Fe and impurities. "Impurities" means components that are mixed in due to raw materials such as ores and scraps and various factors in the manufacturing process when industrially manufacturing the stainless steel sheet, and are allowed within a range that does 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 generates non-metallic inclusions. The content of O as an impurity may be 0.01% or less.

[0046] In the chemical composition of this stainless steel sheet, that an element is "not added" means that the element is not artificially added during steelmaking. This stainless steel sheet may not substantially contain the above-described various optional elements in the chemical composition. That an optional element is "substantially not contained" means that it is sufficient that the optional element is not added, and it is allowed to contain a trace amount as an impurity.

[0047] For example, in this stainless steel plate, since the C content is not highly reduced, there is no need to add Ni to ensure the amount of martensite formed during hot-rolled plate annealing. Therefore, it may not substantially contain Ni. Also, this stainless steel plate may not substantially contain Ti.

[0048] 〔Properties of this stainless steel plate〕 This stainless steel plate is a cold-rolled annealed steel plate having the chemical composition as described above and manufactured by controlling manufacturing conditions (particularly, hot-rolled plate annealing temperature), and has the following metallographic structure. That is, this stainless steel plate has a mixed grain structure in which the average crystal grain size is 9 μm or more, and the proportion of the first crystal grains with a crystal grain size of 8 μm or less is less than 35%, and the proportion of the second crystal grains with a crystal grain size of 15 μm or more is 20% or more. The cold-rolled annealed steel plate (cold-rolled annealed sheet) means a steel plate obtained by subjecting a cold-rolled steel plate obtained by cold rolling a hot-rolled annealed plate to which a pickling process has been appropriately performed to finish annealing (final annealing). The manufacturing process of this stainless steel plate will be described later.

[0049] FIG. 1 is a schematic diagram for explaining a cross-section of a ferritic stainless steel plate according to an embodiment of the present invention. As shown in FIG. 1, the cross-section 12 is a cross-section (so-called L cross-section) parallel to the rolling direction of this stainless steel plate 1 and perpendicular to the rolling surface 11. The cross-section 12 may be an L cross-section at the center of the plate width. Let the plate thickness of this stainless steel plate 1 be t and the plate width be w. This stainless steel plate 1 may have a plate thickness t of 0.3 mm or more and 3.0 mm or less, and may be 0.4 mm or more and 2.0 mm or less. Also, this stainless steel plate 1 may have a plate width w of 900 mm or more and 1300 mm or less, and may be 950 mm or more and 1260 mm or less.

[0050] The average crystal grain size and the ratios of the first and second crystal grains in this stainless steel plate can be calculated as follows. That is, for cross-section 12, it can be calculated using the Electron Back Scattered Diffraction Pattern (hereinafter sometimes abbreviated as "EBSD") method. Specifically, for example, an EBSD detector mounted on a scanning electron microscope (SEM) is used to obtain the EBSD pattern of cross-section 12.

[0051] Next, for the obtained EBSD pattern, using OIM (Orientation Imaging Microscopy) analysis software, crystal grains are identified with grain boundaries being the interfaces where the orientation difference is 15° or more. This means that generally, the region surrounded by large-angle grain boundaries defined as grain boundaries is regarded as a crystal grain. And the average crystal grain size can be calculated using the Area method (Average by Area Fraction Method). Also, crystal grains with a crystal grain size of 8 μm or less are regarded as the first crystal grains, and crystal grains with a crystal grain size of 15 μm or more are regarded as the second crystal grains, and the ratios occupied by the first and second crystal grains respectively can be calculated. The crystal grain size uses the value obtained as the diameter of a circle having an area equal to the area of each individual crystal grain by calculation.

[0052] By having the mixed grain structure as described above, this stainless steel plate can have a proof stress of 370 MPa or less and a ridging height on the surface of 15 μm or less when a tensile strain of 16% is applied in the rolling direction. The ridging height is measured as follows.

[0053] That is, for this stainless steel plate, for example, a JIS No. 5 tensile test piece defined in JIS Z 2201 is taken so that the tensile direction is parallel to the rolling direction. Next, a tensile strain of 16% is applied to the tensile test piece, and using a surface roughness measuring machine, the waviness height is measured with a measuring length of 18 mm in the direction perpendicular to the rolling direction. The waviness height is the average height of the waviness curve element measured by the surface property measurement defined in JIS B 0601:2001 or the like. The cut-off value is determined with the upper limit of the wavelength component being 5.0 mm and the lower limit being 0.8 mm (using a filter), and the waviness curve with a wavelength component of 0.8 mm to 5.0 mm is defined. The reference length is 18 mm. The waviness height (μm) measured in this way is defined as the ridging height.

[0054] Also, in cross-section 12 of this stainless steel plate, the area ratio of the martensite phase may be 2.0% or less. The area ratio of the martensite phase can be calculated as follows. That is, the EBSD pattern obtained for cross-section 12 is IQ (Image Quality) imaged by OIM analysis software. In the IQ image (IQ map), the internal structure of the martensite phase is more complex and the sharpness is lower than that of the ferrite phase, so the image of the martensite phase is darker than the image of the ferrite phase. The IQ map is binarized, and the area ratio of the martensite phase can be calculated by dividing the area of the martensite phase in the IQ map by the total area of the IQ map.

[0055] 〔Manufacturing method of this stainless steel plate〕 Figure 2 is a flowchart showing an example of a manufacturing method of a ferritic stainless steel plate in an embodiment of the present invention. As shown in Figure 2, in the manufacturing method of this stainless steel plate, each process of a melting process S1, a hot rolling process S2, a hot rolled sheet annealing process S3, a cold rolling process S4, and a final annealing process S5 is performed. Hereinafter, each process will be described.

[0056] <Melting process S1 and hot rolling process S2> In the melting process S1, a steel slab containing each of the above-described components 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 manufactured by hot-rolling the steel slab manufactured in the melting process S1. In this specification, the hot-rolled steel sheet may be abbreviated as a hot-rolled sheet. In the hot rolling process S2, a general hot rolling apparatus and hot rolling conditions can be used.

[0057] For example, as the hot rolling conditions, the heating temperature of the steel slab may be 1150°C to 1250°C. Further, the finish rolling temperature in the hot rolling process may be 950°C to 1100°C, and may be 980°C to 1060°C. After cooling the hot-rolled sheet after finish rolling in the cooling zone, the coiling temperature when coiling into a coil may be set to 600°C to 850°C, and may be set to 650°C to 800°C.

[0058] <Hot-rolled sheet annealing process S3> Next, in the hot-rolled sheet annealing process S3, a hot-rolled annealed steel sheet is manufactured by annealing the hot-rolled sheet manufactured in the hot rolling process S2 at a hot-rolled sheet annealing temperature in the range where the M value is 2 or more and less than 5 in the following formula (3): 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) described above) in the hot-rolled annealed steel sheet, and Ht is the thickness of the hot-rolled sheet (unit: mm).

[0059] Also, Ac1 is a value obtained by the following formula: Ac1 = 35×Cr equivalent + 310 Here, the Cr equivalent is a value obtained by the above-described formula (2).

[0060] In this specification, the hot-rolled annealed steel sheet may be abbreviated as the hot-rolled annealed plate. The formula (3) is a relational expression found by the inventors based on a number of experiments, considering the influence of the chemical composition and the thickness of the hot-rolled plate, regarding the relationship between the amount of martensite in the hot-rolled annealed plate and the annealing temperature of the hot-rolled plate. Since martensite is generated from the interfaces in the metal structure of the hot-rolled plate, when the thickness of the hot-rolled plate is thin, the metal structure becomes fine and the number of interfaces increases. As a result, martensite is likely to be generated. This tendency is also reflected in the formula (3). The thickness of the hot-rolled plate may be 2 mm or more and 6 mm or less, and may be 3 mm or more and 5 mm or less.

[0061] In the method for manufacturing this stainless steel plate, by annealing at an annealing temperature of the hot-rolled plate in the range where the M value in the formula (3) is 2 or more and less than 5, the area ratio of the martensite phase in the L cross-section (the same cross-section as cross-section 12 in Fig. 1) of the hot-rolled annealed plate can be adjusted to 2.0 to 5.0%. By setting the area ratio of the martensite phase in the hot-rolled annealed plate to 2.0 to 5.0%, the anti-ridgeability and workability of this stainless steel plate after the subsequent cold rolling process S4 and the final annealing process S5 can be enhanced. Specifically, a stainless steel plate can be obtained in which the yield strength is 370 MPa or less and the ridge height on the surface when a tensile strain of 16% is applied in the rolling direction is 15 μm or less.

[0062] In the hot-rolled plate annealing process S3, as conditions other than the hot-rolled plate annealing temperature, general conditions 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 plate annealing temperature may be, for example, 0 to 90 seconds. That the soaking time in the hot-rolled plate annealing process S3 is 0 seconds means that after the temperature at the center of the plate thickness of the material reaches the predetermined temperature, it is immediately cooled. Also, in the hot-rolled plate annealing process S3, after soaking at the hot-rolled plate annealing temperature, the cooling rate in the temperature range from the hot-rolled plate annealing temperature to 400 °C may be 5 to 100 °C / second.

[0063] If the area ratio of the martensite phase in the hot-rolled annealed sheet is outside the range of 2.0 to 5.0%, the following can be said. That is, a hot-rolled annealed sheet having the above-described chemical composition and having an area ratio of the martensite phase of less than 2.0% or exceeding 5.0% is referred to as a "comparative hot-rolled annealed sheet" for convenience of explanation. When using a comparative hot-rolled annealed sheet, it is difficult to obtain the characteristics of this stainless steel sheet even if the conditions in the final annealing step S5 described later are changed. Generally, when the final annealing temperature is increased or the final annealing time is prolonged, the cold-rolled annealed sheet has a reduced yield strength due to coarsening of the crystal grain size. However, in the component system of this stainless steel sheet, a relatively large amount of carbides are generated in the final annealing step S5, resulting in a pinning effect and making it difficult for grain growth to occur. Therefore, the influence of the final annealing temperature and the final annealing time is relatively small. Therefore, when using a comparative hot-rolled annealed sheet, it is difficult to coarsen the crystal grains by final annealing and reduce the yield strength. When the amount of martensite in the comparative hot-rolled annealed sheet is large, the workability deteriorates due to a decrease in the average crystal grain size of the cold-rolled annealed sheet even if the conditions in the final annealing step S5 are changed. Also, when the amount of martensite in the comparative hot-rolled annealed sheet is small, the improvement in the anti-ridge property is insufficient even if the conditions in the final annealing step S5 are changed. Incidentally, when the total cold rolling rate is 60% or more in the cold rolling step S4 described later, the influence of the conditions of the cold rolling step S4 on the characteristics of the cold-rolled annealed sheet is small, so the influence of the cold rolling step S4 can be disregarded. Also, the influence of a pickling step that may be appropriately performed in the steps after the hot-rolled sheet annealing step S3 can be disregarded.

[0064] As described above, conventionally, it has been difficult to appropriately adjust the amount of martensite in the hot-rolled annealed sheet that varies due to various factors, and there has been no guideline for appropriately adjusting the amount of martensite in the hot-rolled annealed sheet. In contrast, in the manufacturing method of this stainless steel sheet, by using the relational expression (the above formula (3)) found by the present inventors in the component system having the above-described chemical composition, the amount of martensite in the hot-rolled annealed sheet can be adjusted to an appropriate range in which the characteristics of this stainless steel sheet can be obtained.

[0065] The hot-rolled annealed plate (ferritic stainless hot-rolled annealed steel plate) as an intermediate product of this stainless steel plate has the same chemical composition as this stainless steel plate, and the area ratio of the martensite phase in the L cross-section (the same cross-section as cross-section 12 in FIG. 1) may be 2.0% or more and less than 5.0%. The area ratio of the martensite phase in the hot-rolled annealed plate may be measured in the same manner as the area ratio of the martensite phase in the aforementioned stainless steel plate.

[0066] In the ferritic stainless hot-rolled annealed steel plate in this embodiment, the area ratio of the martensite phase is 2.0% or more and less than 5.0%, whereby the toughness can be increased. Specifically, the Charpy impact test value at 25°C can be 15 J / cm 2 or more. The Charpy impact test value of the ferritic stainless hot-rolled annealed steel plate in this embodiment may be 15 to 50 J / cm 2 and may be 20 to 50 J / cm 2 . The Charpy impact test value changes depending on the amount of martensite. By using the ferritic stainless hot-rolled annealed steel plate in this embodiment, the manufacturability of this stainless steel plate can be improved.

[0067] <Cold rolling process S4> Next, in the cold rolling process S4, a cold-rolled steel plate (cold-rolled steel strip) is manufactured by cold-rolling the hot-rolled annealed plate manufactured in the hot-rolled plate annealing process S3. An acid pickling process may be appropriately included between the hot-rolled plate annealing process S3 and the cold rolling process S4. In the cold rolling process S4, as the cold rolling condition, the total cold rolling rate after the completion of the cold rolling process S4 is set to 60% or more. The total cold rolling rate after the completion of the cold rolling process S4 may be 70% or more, and the upper limit may be 90%.

[0068] <Final annealing process S5> Next, in the final annealing step S5, the cold-rolled steel sheet produced in the cold rolling step S4 is annealed at a temperature equal to or higher than the recrystallization start temperature and lower than Ac1. In the final annealing step S5, recrystallization of the ferrite phase in the cold-rolled steel strip is performed, and finish annealing is carried out so that the area ratio of the martensite phase in the cold-rolled annealed sheet (corresponding to this stainless steel sheet) after the final annealing step S5 is 2.0% or less.

[0069] In the final annealing step S5, during the heating-up process, the cold-rolled steel sheet is heated at a heating rate of 100 °C / s or less to a temperature equal to or higher than the recrystallization start temperature and lower than Ac1. The recrystallization start temperature and Ac1 vary depending on the chemical composition. In the final annealing step S5, for example, when the recrystallization start temperature of a cold-rolled steel sheet with a certain chemical composition is lower than 800 °C and Ac1 is higher than 930 °C, the annealing temperature for the cold-rolled steel sheet may be 800 - 930 °C. In the final annealing step S5, the soaking time at the annealing temperature may be, for example, 0 - 60 seconds. The soaking time in the final annealing step S5 being 0 seconds means that after the temperature at the center of the plate thickness of the material reaches the predetermined temperature, it is immediately cooled. Also, in the final annealing step S5, after soaking at the annealing temperature, the cooling rate in the temperature range from the annealing temperature to 500 °C may be 5 - 100 °C / second.

[0070] By the end of the final annealing step S5, this stainless steel sheet as the final product is obtained. In this stainless steel sheet, the martensite phase may disappear, that is, the area ratio of the martensite phase may be 0%. This stainless steel sheet only needs to have an area ratio of the martensite phase of 2.0% or less, and may be less than 1.0%. If the area ratio of the remaining martensite phase is 2.0% or less, the workability of this stainless steel sheet can be further improved.

[0071] 〔Annealing Temperature Determination Device in One Embodiment〕 The annealing temperature determination device for determining the range of the hot-rolled sheet annealing temperature in the above-described hot-rolled sheet annealing step S3 also falls within the scope of the present invention. FIG. 3 is a block diagram showing a schematic configuration of the annealing temperature determination device in one embodiment of the present invention.

[0072] As shown in FIG. 3, the annealing temperature determination device 1 includes a control unit 10 and a storage unit 20. The control unit 10 includes an information acquisition unit 15 and a temperature determination unit 16. Component data 21 and plate thickness data 22 are stored in the storage unit 20. The annealing temperature determination device 1 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 1 may be connected to a system bus SB, and configured to be able to exchange data with each other via the system bus SB.

[0073] The control unit 10 is, for example, a CPU (Central Processing Unit) that controls the overall operation of the annealing temperature determination device 1. Each part included in the control unit 10 may be realized as software that operates by, for example, the CPU. The storage unit 20 is a volatile or non-volatile storage device (such as a hard disk, a flash memory, etc.) that stores various data used in the control unit 10.

[0074] The component data 21 is data regarding 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 melting process S1 or the hot-rolled sheet after the hot rolling process S2. The plate thickness data 22 is the hot-rolled sheet after the hot rolling process S2, and is the value of the plate thickness of the hot-rolled sheet to be subjected to the hot-rolled sheet annealing process S3. The component data 21 and the plate thickness data 22 may be stored in the storage unit 20 via, for example, the input unit 30 or the communication unit 50. The information acquisition unit 15 may acquire the content of each element (mass%) and the value of the plate thickness of the hot-rolled sheet in the chemical composition of the steel slab via the communication unit 50.

[0075] The temperature determination unit 16 calculates the values of γp and Ac1 using the content (mass%) of each element and the value of the hot-rolled sheet thickness acquired by the information acquisition unit 15, and determines the annealing temperature range in the hot-rolled sheet annealing process S3 so that M in the following formula (4) is 2 or more and less than 5: M = {0.0033×γp + 0.000355(AT - Ac1) - 0.0121×Ht - 0.00256}×100 ···(4) In the above formula (4), γp is the value obtained by the above formula (1), 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). When the above formula (4) is transformed, the above formula (3) is derived.

[0076] Also, Ac1 is the value obtained by the following formula: Ac1 = 35×Cr equivalent + 310 Here, the Cr equivalent is the value obtained by the above formula (2).

[0077] The annealing temperature range determined by the temperature determination unit 16 may be displayed on a screen or the like by the output unit 40, or may be output to an external device via the communication unit 50.

[0078] Conventionally, it has been extremely difficult to empirically control the amount of martensite in hot-rolled annealed sheets that is greatly affected by chemical composition and the like. However, in the annealing temperature determination device 1, using the above formula (4), the values of γp and Ac1 are calculated corresponding to various chemical compositions and the thickness of the hot-rolled sheet, and the annealing temperature range in the hot-rolled sheet annealing process S3 can be quickly calculated so that the amount of martensite in the hot-rolled annealed sheet is 2.0% or more and less than 5.0%.

[0079] The temperature determination unit 16 may determine a specific temperature within the calculated annealing temperature range as the annealing temperature. The processing in this case will be described as follows.

[0080] Generally, in batch annealing, a plurality of coils are annealed simultaneously. Even in continuous annealing, from the viewpoint of productivity, it is preferable to perform annealing on a plurality of coils as a set without changing (maintaining) the annealing temperature. The annealing temperature determination device 1 may store, for example, in the storage unit 20, component data 21 and plate thickness data 22 of a plurality of coils to be processed in the hot-rolled sheet annealing step S3. The temperature determination unit 16 can calculate the values of γp and Ac1 for each of the plurality of coils based on the information acquired by the information acquisition unit 15, and can calculate the annealing temperature range in the hot-rolled sheet annealing step S3 such that M in the above formula (4) is 2 or more and less than 5 for each of the plurality of coils.

[0081] A set of a plurality of coils annealed simultaneously in batch annealing or a set of a plurality of coils annealed without changing the hot-rolled sheet annealing temperature in continuous annealing is defined as one batch (one processing unit). The temperature determination unit 16 determines whether there is a temperature range that overlaps all of the plurality of coils using the annealing temperature range in which M in the above formula (4) is 2 or more and less than 5 for each of the plurality of coils included in one processing unit. For one processing unit, when there is an overlapping temperature range, the temperature determination unit 16 may determine, for example, the central value of the overlapping temperature range as the annealing temperature in the hot-rolled sheet annealing step S3. The determined temperature range may be output by the output unit 40 or the communication unit 50. The above overlapping range may be output by the output unit 40 or the communication unit 50. For one processing unit, when there is no overlapping range, the temperature determination unit 16 may output a determination result indicating that an appropriate annealing temperature cannot be determined by the output unit 40 or the communication unit 50.

[0082] 〔Example of implementation by software〕 The functions of the annealing temperature determination device 1 (hereinafter referred to as the "device") can be realized by a program for causing a computer to function as the device and by programs for causing a computer to function as each control block of the device (especially each part included in the control unit 10).

[0083] In this case, as hardware for executing the above program, the above device includes a computer having at least one control device (e.g., a processor) and at least one storage device (e.g., a memory). By executing the above program with this control device and storage device, each function described in the above embodiment is realized.

[0084] The above program may be recorded on one or more computer-readable recording media, rather than temporarily. This recording medium may or may not be included in the above device. In the latter case, the above program may be supplied to the above device via any wired or wireless transmission medium.

[0085] Also, part or all of the functions of the above control blocks can also be realized by a logic circuit. For example, an integrated circuit in which a logic circuit functioning as each of the above control blocks is formed is also included in the scope of the present invention.

[0086] 〔Summary〕 The ferritic stainless steel sheet according to Aspect 1 of the present invention contains, 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, with the balance being Fe and impurities. It is a ferritic stainless steel sheet having 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. In a cross section parallel to the rolling direction and perpendicular to the rolling surface, the average crystal grain size is 9 μm or more, and the proportion of the first crystal grains with a crystal grain size of 8 μm or less is less than 35%, and the proportion of the second crystal grains with a crystal grain size of 15 μm or more is 20% 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 values of the contents (mass%) of the respective elements are substituted in the positions of the element symbols in the formulas (1) and (2), and 0 is substituted for the elements not added.

[0087] The ferritic stainless steel sheet according to Embodiment 2 of the present invention, in the above Embodiment 1, in mass%, has a chemical composition containing 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.

[0088] The ferritic stainless steel sheet according to Embodiment 3 of the present invention, in the above Embodiment 1 or 2, in mass%, further contains one or more selected from the group consisting of 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% 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.

[0089] The ferritic stainless steel sheet according to Embodiment 4 of the present invention, in any one of the above Embodiments 1 to 3, has a yield strength of 370 MPa or less and a surface ridging height of 15 μm or less when a tensile strain of 16% is applied in the rolling direction.

[0090] In the ferritic stainless steel sheet according to Aspect 5 of the present invention, in any one of Aspects 1 to 4, in the cross section, the area ratio of the martensite phase is 2.0% or less.

[0091] The method for manufacturing a ferritic stainless steel sheet according to Aspect 6 of the present invention is the method for manufacturing a ferritic stainless steel sheet described in any one of Aspects 1 to 5, and includes 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 in a range where the M value is 2 or more and less than 5 in the following formula (3), and a final annealing step of annealing a cold-rolled sheet obtained by cold-rolling the hot-rolled annealed sheet obtained in the hot-rolled sheet annealing step so that the area ratio of the martensite phase in the cross section is 2.0% or less: Hot-rolled sheet annealing temperature = Ac1 + 28.17×M - 9.3×γp + 34.1×Ht + 7.21 ··· (3) Here, 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 plane in the hot-rolled annealed sheet, Ht is the plate thickness (unit: mm) of the hot-rolled sheet, and Ac1 = 35×Cr equivalent + 310.

[0092] The ferritic stainless steel hot-rolled annealed sheet according to Aspect 7 of the present invention contains, 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, the balance being composed of Fe and impurities, 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, and in a cross section parallel to the rolling direction and perpendicular to the rolling plane, the area ratio of the martensite phase is 2.0% or more and less than 5.0%, and the Charpy impact test value at 25°C is 15 J / cm 2 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 values of the contents (mass %) of the respective elements are substituted in the positions of the element symbols in the formulas (1) and (2), and 0 is substituted for the elements not added.

[0093] In the ferritic stainless hot-rolled annealed steel sheet according to Embodiment 8 of the present invention, in the above Embodiment 7, by 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% 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): in total 0.001% or more and 0.100% or less, further contains one or more selected from the group consisting of.

[0094] The annealing temperature determination device according to aspect 9 of the present invention is for a hot-rolled plate obtained by hot-rolling a steel slab having 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, with the balance being 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. The annealing temperature determination device determines the annealing temperature range of the hot-rolled plate annealing performed on the hot-rolled plate, and includes an information acquisition unit that acquires the content (mass%) of each element in the chemical composition of the steel slab and the value of the plate thickness of the hot-rolled plate, and a temperature determination unit that determines the annealing temperature range. The temperature determination unit determines the annealing temperature range so that M in the following formula (4) is 2 or more and less than 5 using the content (mass%) of each element and the value of the plate thickness: 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) M = {0.0033×γp + 0.000355(AT - Ac1) - 0.0121×Ht - 0.00256}×100 ··· (4) Here, the value of the content (mass%) of each element is substituted into the position of the element symbol in the above formula (1) and formula (2), 0 is substituted for the element without addition. In the above formula (4), AT is the annealing temperature (°C) in the hot-rolled plate annealing, Ht is the plate thickness (mm) of the hot-rolled plate, and Ac1 = 35×Cr equivalent + 310.

[0095] The information processing program according to aspect 10 of the present invention is for causing a computer to function as the annealing temperature determination device according to claim 9.

[0096] [Supplementary Notes] The present invention is not limited to the present embodiment, and various modifications are possible within the scope indicated in 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. [Examples]

[0097] Examples of the present invention will be described below.

[0098] The ferritic stainless steel sheet within the range defined in the present invention is referred to as an "invention example", and the ferritic stainless steel sheet outside the range defined in the present invention is referred to as a "comparative example". In this example, first, a steel slab having the composition shown in Table 1 below was produced by melting in an actual operation line. In Table 1, the Cr equivalent, Ac1, and γp are values calculated by the aforementioned formulas, respectively. Also, in Table 1, values outside the range defined in the present invention are underlined.

[0099] [Table 1]

[0100] Using the steel slabs of each steel No. described in Table 1, hot rolling was performed to a plate thickness in the range of 3.0 to 5.0 mm, followed by hot-rolled plate annealing (see Table 2 described later for the annealing temperature) and cold rolling (plate thickness 0.8 mm), and finally, final annealing (annealing temperature: 850 °C) was performed to produce product plates. Conditions other than the annealing temperature in the hot-rolled plate annealing, as well as the specific conditions of hot rolling, cold rolling, and final annealing, were set as general conditions. These general conditions are as exemplified in the aforementioned "Mode for Carrying Out the Invention". Each test material was evaluated as follows.

[0101] [Amount of Martensite] Regarding the L cross-section of the hot-rolled annealed plate, an EBSD pattern was acquired under the following acquisition conditions using an EBSD detector mounted on a scanning electron microscope (SEM): ·Measurement magnification: 100 to 800 times · Measurement area: 100 - 1000 μm square · Measurement pitch (step size): 0.3 - 0.8 μm.

[0102] From the obtained 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.

[0103] 〔Charpy impact test value〕 To evaluate the toughness of the hot-rolled annealed plate, a Charpy impact test was conducted. Specifically, first, a Charpy impact test specimen with a V-notch in the plate width direction (C direction) was taken from the hot-rolled annealed plate. For the Charpy impact test specimen, the Charpy impact test was carried out 3 times at 25°C in accordance with JIS Z 2242:2018, and the average value was calculated as the Charpy impact test value. For the evaluation of the toughness of the hot-rolled annealed plate, a Charpy impact test value at 25°C of 15 J / cm 2 or more was considered qualified.

[0104] 〔Ridging height〕 From the product plate (this stainless steel plate) after final annealing, a JIS No. 5 tensile test specimen (the first tensile test specimen) specified in JIS Z 2201 was taken parallel to the rolling direction. Next, using an Instron-type tensile testing machine, with a gauge length of 50 mm, a tensile test was conducted on the first tensile test specimen in the tensile direction parallel to the rolling direction, and a tensile strain of 16% was applied. Next, using a surface roughness measuring machine, the average height of the waviness curve elements in the direction perpendicular to the rolling direction in the portion between the gauges of the first tensile test specimen was measured with a measurement length of 18 mm in accordance with surface property measurements specified in JIS B 0601:2001, etc. A cut-off value was determined with an upper limit of the wavelength component of 5.0 mm and a lower limit of 0.8 mm (using a filter), and a waviness curve with a wavelength component of 0.8 mm - 5.0 mm was defined. The value of the average height of the waviness curve elements was adopted as the value of the ridging height. For the evaluation of the ridging resistance property, a ridging height of 15 μm or less was considered qualified.

[0105] 〔Yield Strength〕 From the product plate (this stainless steel plate) after final annealing, a JIS No. 13 Type B tensile test piece (the second tensile test piece) in the rolling direction was taken. For the second tensile test piece, a tensile test defined in JIS Z 2241 was carried out using a tensile testing machine, and the 0.2% yield strength was measured. As an evaluation of workability, a yield strength of 370 MPa or less was considered acceptable.

[0106] 〔Grain Mixed Structure〕 For the product plate (this stainless steel plate) after final annealing, in the same manner as the measurement of the amount of martensite described above, an EBSD pattern of the L cross-section was obtained, and using OIM analysis software, grains were identified with interfaces having an orientation difference of 15° or more as grain boundaries. The average grain size was calculated using the Area method. Also, the proportion of each of the first grains with a grain size of 8 μm or less and the second grains with a grain size of 15 μm or more was calculated. The grain size was the value obtained by calculation using the diameter of a circle having an area equal to the area of each individual grain.

[0107] <Evaluation Results> Table 2 shows the results of evaluating the microstructure and properties of each test material of the inventive examples and comparative examples. In Table 2, compositions and values outside the ranges defined in the present invention are underlined.

[0108]

Table 2

[0109] As shown in Invention Examples Nos. 1, 3, 4, 6 to 8, 10, 12 to 17, 19 to 29, by appropriately controlling the annealing temperature of the hot-rolled sheet based on the chemical composition and the thickness of the hot-rolled sheet, the amount of martensite in the hot-rolled annealed sheet can be made 2.0% or more and less than 5.0%. Then, by performing cold rolling and final annealing, a ferritic stainless steel sheet having a mixed grain structure in which the average crystal grain size and the ratios of the first and second crystal grains are within the specified ranges of the present invention were obtained. The ferritic stainless steel sheets of the invention examples have excellent anti-rigging properties and workability. Further, due to the relatively high toughness of the hot-rolled sheet, they had high manufacturability. The ferritic stainless steel sheets of the invention examples had a martensite amount of 0% or substantially 0% measured using the same measurement method as for the hot-rolled annealed sheet. The fact that the amount of martensite is substantially 0% means that the amount of martensite is small to the extent that the martensite phase region cannot be discriminated in the binarized IQ image.

[0110] On the other hand, in Comparative Examples Nos. 2, 5, 9, 11, 18, 30 to 38 where the chemical composition or the annealing conditions (temperature conditions) of the hot-rolled sheet were outside the specified ranges of the present invention, at least one of the predetermined characteristics did not meet the criteria.

Explanation of Signs

[0111] S1 Melting process S2 Hot rolling process S3 Hot-rolled sheet annealing process S4 Cold rolling process S5 Final annealing process 1 This stainless steel sheet 11 Rolling surface 12 Cross section

Claims

1. A ferritic stainless steel sheet 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, having 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, in a cross-section parallel to the rolling direction and perpendicular to the rolling surface, the average crystal grain size is 9 μm or more, and a ferritic stainless steel sheet in which the proportion of the first crystal grains having a crystal grain size of 8 μm or less is less than 35%, and the proportion of the second crystal grains having a crystal grain size of 15 μm or more is 20% 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 values of the contents (mass%) of the respective elements are substituted in the places of the element symbols in the above formulas (1) and (2), and 0 is substituted for the elements not added.

2. The ferritic stainless steel sheet according to Claim 1, having a chemical composition containing, by 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% 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): in total 0.001% or more and 0.100% or less, further containing one or more selected from the group consisting of, the ferritic stainless steel sheet according to claim 1.

4. The yield strength is 370 MPa or less, and When a tensile strain of 16% is applied in the rolling direction, the surface ridging height is 15 μm or less, the ferritic stainless steel sheet according to claim 1.

5. In the cross section, the area ratio of the martensite phase is 2.0% or less, the ferritic stainless steel sheet according to claim 1.

6. A method for manufacturing the ferritic stainless steel sheet according to any one of claims 1 to 5, A hot-rolled plate obtained by hot-rolling a steel slab is annealed at a hot-rolled plate annealing temperature in the range where the M value is 2 or more and less than 5 in the following formula (3), a hot-rolled plate annealing step; A cold-rolled plate obtained by cold-rolling the hot-rolled annealed plate obtained by the hot-rolled plate annealing step is annealed so that the area ratio of the martensite phase in the cross section is 2.0% or less, a final annealing step, a method for manufacturing a ferritic stainless steel sheet including. Hot-rolled plate annealing temperature = Ac1 + 28.17 × M - 9.3 × γp + 34.1 × Ht + 7.21... (3) (Here, 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 plate, The Ht is the plate thickness of the hot-rolled plate (unit: mm), Ac1 = 35 × Cr equivalent + 310 is.)

7. 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 composed of Fe and impurities, 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. In a cross-section parallel to the rolling direction and perpendicular to the rolling surface, the area ratio of the martensite phase is 2.0% or more and less than 5.0%. The Charpy impact test value at 25°C is 15 J / cm ２ or more, a ferritic stainless hot-rolled annealed steel sheet. 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 values of the contents (mass%) of the respective elements are substituted in the positions of the element symbols in the above formulas (1) and (2), and 0 is substituted for the elements without addition.

8. By 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% 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): in total 0.001% or more and 0.100% or less, further containing one or more selected from the group consisting of. The ferritic stainless hot-rolled annealed steel sheet according to claim 7.

9. 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, with the balance being composed 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, an annealing temperature determination device for determining the annealing temperature range of hot-rolled sheet performed on a hot-rolled sheet obtained by hot-rolling a steel slab having such a chemical composition, an information acquisition unit that acquires the content (mass %) of each element in the chemical composition of the steel slab and the value of the sheet thickness of the hot-rolled sheet, and a temperature determination unit that determines the annealing temperature range in the hot-rolled sheet annealing, wherein the temperature determination unit determines the annealing temperature range such that M in the following formula (4) is 2 or more and less than 5, using the content (mass %) of each element and the value of the sheet thickness, 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) M = {0.0033 × γp + 0.000355(AT - Ac1) - 0.0121 × Ht - 0.00256} × 100... (4) (wherein, the value of the content (mass %) of each element is substituted in the positions of the element symbols in the above formulas (1) and (2), and 0 is substituted for elements without addition, in the above formula (4), the AT is the annealing temperature (°C) in the hot-rolled sheet annealing, the Ht is the sheet thickness (mm) of the hot-rolled sheet, Ac1 = 35 × Cr equivalent + 310 is.)

10. An information processing program for causing a computer to function as the annealing temperature determination device according to Claim 9.

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