Ferritic stainless steel sheet with excellent corrosion resistance and workability.

Abstract

To provide a ferritic stainless steel sheet excellent in corrosion resistance and workability.SOLUTION: A ferritic stainless steel sheet excellent in corrosion resistance and workability contains C, Si, Mn, P, S, Cr, Mo, N and Al, further contains either or both of 0.40% or less Nb and 0.40% or less Ti, and the balance Fe with inevitable impurities, wherein a first surface that is a surface corresponding to thickness of 1 / 8 of sheet thickness (t) from the surface of the steel sheet and a second surface that is a surface corresponding to thickness of 1 / 4 of the sheet thickness (t) from the surface of the steel sheet are exposed, when area ratios of crystal grains having an orientation having an angle difference between a normal direction of the surface of the steel sheet and a {111} plane direction measured by the first surface and the second surface of ±10° or less are represented by S1 and S2, and average crystal particle sizes (μm) on the first surface and the second surface are represented by G1 and G2, the following relations are satisfied. Expression (1): S2≥0.3 and S1 / S2≥1.2. Expression (2): G1≤30 μm and G1 / G2≤1.0.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a ferritic stainless steel sheet excellent in corrosion resistance and workability.

Background Art

[0002] Conventionally, ferritic stainless steel sheets have been widely applied to building materials, kitchen materials, materials for electric parts, etc. because they do not contain Ni or contain only a trace amount of Ni and are inexpensive. However, conventional ferritic stainless steel sheets have inferior corrosion resistance compared to austenitic stainless steel sheets represented by SUS304. For this reason, recently, high-purity ferritic stainless steel sheets with improved corrosion resistance, particularly pitting corrosion resistance, have been developed by adding Si to ferritic stainless steel sheets with reduced impurity elements such as C and N due to the improvement of manufacturing technology. By adding Si in an amount exceeding 1.0% by mass, the corrosion resistance can be improved and the tensile strength can be increased. However, by containing Si in an amount exceeding 1.0% by mass, the internal stress is increased, and particularly the workability in deep drawing is deteriorated. Therefore, there is a problem that the strain remaining after deep drawing becomes large, easily inducing wavy unevenness called ears or processing cracks at the edge of the drawn cylinder.

[0003] So far, as an evaluation index for deep drawing property, there is a Lankford value (hereinafter referred to as "r value"). The r value is represented by the following formula as the ratio of the true strain in the sheet width direction to the true strain in the sheet thickness direction. r value = ln(w / w0) / ln(t / t0) (Here, w0 represents the sheet width before the test, w represents the sheet width after the test, t0 represents the sheet thickness before the test, and t represents the sheet thickness after the test.) It is known that a material with a large r value mainly undergoes deformation in the sheet width direction during deep drawing forming, and the amount of deformation in the sheet thickness direction is small, so the reduction in sheet thickness is small and the fracture limit is high.

[0004] Therefore, regarding the processability of ferritic stainless steel sheets, for example, Patent Document 1 states that the base material of the steel pipe contains C: 0.001~0.02%, Si: 0.1~1.5%, etc., and 0.1~0.8% of one or more Nb or Ti, and is measured by the EBSP method over the entire thickness of the sheet {111} <112> and {111} <011> The stainless steel plate has a total area ratio of 40% or more in all directions, an average r value of 1.5 or more, and an r value of 1.0 or more in the 45° direction. The steel pipe has a YR (tensile strength / 0.2% yield strength) of 1.1 or more, and is measured by the EBSP method across the entire plate thickness of the welded steel pipe butt joint. <011> and {211} <011> A ferritic stainless steel pipe for exhaust components with excellent workability is disclosed, in which the sum of the area ratios of the orientations is 30% or more.

[0005] Furthermore, Patent Document 2 contains Cr, Sn, Si, Mn, etc., and when the plate thickness is t, {100} from the surface to t / 4 <012> A ferritic stainless steel sheet with excellent workability and an X-ray diffraction intensity of 2 or higher in a given direction is disclosed. Furthermore, Patent Document 3 discloses a ferritic stainless steel sheet containing Si, Mn, Cr, Cu, Ti, B, etc., with an average r value of ≥ 1.3, which exhibits excellent heat resistance and workability. Furthermore, Patent Document 4 discloses a ferritic stainless steel sheet with excellent workability, which contains Si, Mn, Ni, Cr, Nb, etc., has a precipitate with a particle size of 2 μm or less of 0.5% by mass or less, and the crystal orientation of the rolled surface at a depth of 1 / 4 of the sheet thickness is 1.2 or more in terms of the integral intensity ratio defined by a predetermined formula. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Patent No. 6261640 [Patent Document 2] Patent No. 5843982 [Patent Document 3] Patent No. 5546911 [Patent Document 4] Patent No. 4562281 [Overview of the project] [Problems that the invention aims to solve]

[0007] While the ferritic stainless steel sheets described in Patent Documents 1 to 4 all possess excellent workability, their corrosion resistance has not been evaluated, and therefore, they are not intended to improve both workability and corrosion resistance in a balanced manner. In particular, ferritic stainless steel is increasingly required to possess corrosion resistance, especially pitting corrosion resistance, equivalent to that of austenitic stainless steel, as a substitute for austenitic stainless steel. Furthermore, while elements such as Cr and Mo are known to improve the pitting corrosion resistance of stainless steel sheets, Cr and Mo are expensive, and excessive inclusion in steel would impair economic efficiency and, in addition, worsen manufacturability during refining, for example.

[0008] Therefore, the present invention has been made in view of the above problems, and aims to provide a ferritic stainless steel sheet that contains more than 1.0 mass% Si and is excellent in both corrosion resistance (especially pitting corrosion resistance) and workability (especially deep drawing workability). [Means for solving the problem]

[0009] Therefore, the features of the present invention are listed below. (1) In mass%, C: 0.020% or less, Si: more than 1.0% and less than 2.5%, Mn: 1.0% or less, P: 0.040% or less, S: 0.0030% or less, Cr: 13.0% or more and 23.0% or less, Mo: 0.50% or less N: 0.020% or less, and Al: Contains 0.20% or less, and further, A ferritic stainless steel sheet having a chemical composition containing Nb: 0.40% or less and Ti: 0.40% or less, or both, with the remainder being Fe and unavoidable impurities, Let the thickness of the steel plate be t. From the surface of the steel plate, a surface parallel to the surface is removed in the thickness direction so that a first surface corresponding to 1 / 8 of the thickness (t) is exposed from the surface of the steel plate, and a second surface corresponding to 1 / 4 of the thickness (t) is exposed sequentially. The total area ratios of crystal grains having orientations where the angular difference between the normal direction of the surface of the steel plate and the {111} plane orientation measured on the first and second surfaces is ±10° or less are defined as S1 and S2, respectively. When the average grain size (μm) on the first and second surfaces is G1 and G2, respectively, S1 and S2, and G1 and G2 are ferritic stainless steel sheets with excellent corrosion resistance and workability, satisfying the relationships shown in equations (1) and (2) below. Equation (1): S2 ≥ 0.3 and S1 / S2 ≥ 1.2 Formula (2): G1≦30μm, G1 / G2≦1.0

[0010] (2) A ferritic stainless steel sheet with excellent corrosion resistance and workability as described in (1), wherein the content (mass%) of Cr, Si, and Mo in the chemical composition is expressed as [Cr], [Si], and [Mo], respectively, and satisfies the relationship of the pitting index (PI) represented by the following formula (3). Formula (3): PI=[Cr]+2[Si]+3[Mo]≧19.0

[0011] (3) The chemical composition is further expressed in mass% as follows: B: 0.0050% or less, Ni: 1.0% or less, Cu: 1.0% or less, V: 0.50% or less, W: 0.50% or less, Sn: 0.10 or less, Ca: 0.0100% or less, Mg: 0.010% or less, Zr: 0.50% or less, Co: 0.50% or less, Ga: 0.10% or less, La: 0.10% or less, Y: 0.10% or less, Hf: 0.10% or less, and REM: containing at least one selected from the group consisting of 0.10% or less, the ferritic stainless steel sheet excellent in corrosion resistance and workability described in (1) or (2).

Advantages of the Invention

[0012] The present invention can achieve a specific effect of providing a ferritic stainless steel sheet containing more than 1.0% by mass of Si content and excellent in both corrosion resistance (especially pitting corrosion resistance) and workability (especially deep drawing workability).

Brief Description of the Drawings

[0013] [Figure 1] It is a figure which shows an example of the crystal orientation map by the measurement of EBSD.

Modes for Carrying Out the Invention

[0014] [[ID=�5]]Hereinafter, embodiments of the present invention will be described. The following description is an example of an embodiment of this invention and does not limit the scope of this patent claim.

[0015] In order to solve the above-mentioned problems, the present inventors have earnestly studied the action effects of additive elements that improve both corrosion resistance (especially pitting corrosion resistance) and workability (especially deep drawing workability) in a ferritic stainless steel sheet, and the influence of heat treatment on the workability after deep drawing, and have obtained the following new findings and completed the present invention.

[0016] The ferritic stainless steel sheet of the present invention has a chemical composition, in mass%, containing C: 0.020% or less, Si: greater than 1.0% and 2.5% or less, Mn: 1.0% or less, P: 0.040% or less, S: 0.0030% or less, Cr: 13.0% to 23.0%, Mo: 0.50% or less, N: 0.020% or less, and Al: 0.20% or less. Furthermore, it contains either or both of Nb: 0.40% or less and Ti: 0.40% or less as essential additive elements, with the remainder being Fe and unavoidable impurities.

[0017] (chemical composition) The reasons for the limitations on each essential additive element are explained below. Note that in the following descriptions of each component of the chemical composition, "mass%" is simply represented as "%". (C: 0.020% or less) Carbon (C) reduces processability and corrosion resistance, and solid-solution carbon inhibits the development of the {111} texture. Furthermore, solid-solution carbon reduces the r value, leading to decreased processability. Therefore, a lower carbon content is preferable, and the upper limit is set to 0.020% or less. Preferably, reducing it to less than 0.010% further improves corrosion resistance. On the other hand, C is an interstitial solid-solution element and has a strong tendency to segregate at grain boundaries, thus contributing to the strengthening of grain boundaries. To obtain these effects of C, it is preferable to set the lower limit to 0.001% or more. A preferred range, considering refining costs, is 0.003% or more and less than 0.015%.

[0018] (Si: more than 1.0% and less than 2.5%) Silicon (Si) is effective as a deoxidizing element, improving oxidation resistance. It also contributes to finer grain formation, increases tensile strength and hardness, and improves mechanical strength. Furthermore, it can improve corrosion resistance, especially pitting corrosion resistance, so the lower limit should be at least 1.0%. On the other hand, it acts as a solid solution strengthening element, lowering the r value and leading to reduced processability, so the upper limit should be 2.5% or less. Considering the respective effects and manufacturability, the range should be between 1.0% and 2.5%. The preferred range, considering the respective effects and manufacturability, is between 1.1% and 2.0%.

[0019] (Mn:1.0% or less) Manganese (Mn) is an effective element for deoxidation and sulfur fixation. However, because it can lead to a decrease in corrosion resistance and oxidation resistance, the upper limit should be 1.0% or less. To ensure the deoxidation and sulfur fixation effects, it is preferable to set the lower limit at 0.01% or more. The preferred range is 0.05% to 0.5%, considering the respective effects and manufacturing costs.

[0020] (P:0.040% or less) Phosphorus (P) is an element that inhibits manufacturability and weldability, and is the main cause of the decrease in processability targeted by this invention. Therefore, the lower the content, the better, and the upper limit is set to 0.040% or less. However, excessive reduction leads to an increase in refining costs, so it is preferable to set the lower limit to 0.005% or more. A more preferable range, considering manufacturing costs, is 0.010% to 0.030%.

[0021] (S:0.0030% or less) S (sulfur) segregates at grain boundaries, leading to a decrease in hot workability and the workability targeted by this invention. In particular, it acts as a solid solution strengthening element, lowering the r value and thus reducing workability, so a lower content is preferable, and the upper limit is set to 0.0030% or less. However, excessive reduction leads to an increase in raw material and refining costs, so it is preferable to set the lower limit to 0.0001% or more. A more preferable range is 0.0002% to 0.0015%, taking into account the suppression of embrittlement and manufacturing costs.

[0022] (Cr: 13.0~23.0%) Cr (chromium) is a fundamental element of the ferritic stainless steel of this invention and is an essential element for ensuring corrosion resistance and heat resistance. The lower limit is set at 13.0% or more to ensure corrosion resistance and heat resistance suitable for electric kettle applications. The upper limit is set at 23.0% or less from the perspective of workability and manufacturability, as chromium acts as a solid solution strengthening element and lowers the r value, leading to a decrease in workability. However, from the perspective of economic efficiency compared to SUS430J1L (19Cr) and SUS443J1 (21Cr), the preferred range is 15.0% to 19.0%. Considering performance and alloy cost, the more preferred range is 16.0% to 18.0%.

[0023] (Mo: 0.50% or less) Mo (molybdenum), like Ni and Cu, is an effective element for obtaining the workability targeted by this invention, in addition to its corrosion resistance. For each effect to manifest, the Mo content is preferably 0.02% or more. However, excessive content increases alloy costs and acts as a solid solution strengthening element, lowering the r value and thus reducing workability, thus hindering the manufacturability of both hot and cold working. Therefore, the upper limit of the Mo content is 0.50% or less. A more preferable range, considering manufacturability and performance, is 0.05% to 0.30%.

[0024] (N:0.020% or less) Nitrogen (N), like carbon (C), reduces processability and corrosion resistance, so a lower N content is preferable; therefore, the upper limit should be 0.020% or less. However, excessive reduction leads to increased refining costs, so a lower limit of 0.001% is preferable. Although N, like carbon, is an interstitial solid solution element, it has a small tendency to segregate at grain boundaries and contributes little to strengthening the grain boundaries, leading to a decrease in processability by lowering the r value. Therefore, considering performance and manufacturing costs, the preferred range is 0.005% to 0.015%.

[0025] (Al: 0.20% or less) Aluminum (Al) is an extremely effective deoxidizing element. However, because it can reduce the toughness and weldability of steel, its upper limit should be 0.20% or less. The lower limit is preferably 0.005% or more, taking into account the deoxidizing effect. A more preferable range, considering manufacturability and performance, is 0.010% to 0.070%.

[0026] (Nb: 0.40% or less, and Ti: 0.40% or less, or both) Nb and Ti, through their action as stabilizing elements that fix C and N, are effective elements not only in improving workability and corrosion resistance but also in improving workability, which is the target of this invention. Nb and Ti combine with C and N to form carbides, nitrides, and carbonitrides, developing the {111} orientation and promoting an improvement in the r value. However, since Nb and Ti act separately and independently on C and N, it is preferable that the Nb and Ti content be 0.01% or more, which is the amount at which their respective effects manifest. However, if the Nb and Ti content exceeds 0.40% each, the development of the {111} orientation will be inhibited due to the increase in alloy cost and the increase in recrystallization temperature, so the upper limit should be 0.40% or less. The preferred range is 0.03% to 0.30%, taking into account the respective effects, alloy cost, and manufacturability. A more preferred range is 0.05% to 0.20%.

[0027] Furthermore, the ferritic stainless steel sheet of the present invention may contain the following optional additive elements as needed. (B:0.0050% or less) Boron (B) is an element that segregates along the grain diameter, increasing grain boundary strength and particularly improving workability in terms of toughness. Therefore, its addition to ferritic stainless steel is effective. The B content is preferably 0.0003% or more to exhibit these effects. However, if the B content exceeds 0.0050%, it will cause a decrease in elongation, so the upper limit is set at 0.0050%. Preferably, considering material cost and workability, the content is between 0.0005% and 0.0020%.

[0028] (Ni: 1.0% or less) Nickel (Ni) is an effective element for corrosion resistance and is a suitable element for obtaining workability. To delay P segregation after heat treatment, which is the subject of this invention, and to obtain workability, it is preferable that the Ni content be greater than 0.03%. On the other hand, if the Ni content exceeds 1.0%, it leads to an increase in alloy cost and an increase in material strength, which lowers the r value and reduces workability, so the upper limit of the Ni content is set to 1.0%. The preferred range for the Ni content is 0.05% to 0.5%, taking into consideration performance and alloy cost.

[0029] (Cu:1.0% or less) Copper (Cu) is an effective element for corrosion resistance and is a suitable element for obtaining workability. To delay P segregation after heat treatment, which is the subject of this invention, and to obtain workability, it is preferable that the Cu content be greater than 0.03%. On the other hand, if the Cu content exceeds 1.0%, it leads to an increase in alloy cost and a decrease in workability due to an increase in material strength, so the upper limit of the Cu content is set to 1.0%. The preferred range for the Cu content is 0.05% to 0.5%, taking into consideration performance and alloy cost.

[0030] (V:0.50% or less) Vanadium (V) is an effective element for improving corrosion resistance and the workability targeted by this invention. In particular, it contributes to improving corrosion resistance and workability by reducing solid-solution C and N through the formation of carbonitrides, and is therefore added as needed. The V content is preferably 0.01% or more, which is necessary for its effect to manifest. If the V content exceeds 0.50%, it leads to an increase in alloy costs and a decrease in manufacturability, and causes hardening and a decrease in elongation due to solid-solution strengthening and precipitation strengthening, so the upper limit of the V content should be 0.50% or less. The preferred range for the V content is 0.02% to 0.30%, taking into consideration workability, manufacturability, and alloy costs.

[0031] (W: 0.50% or less) W (tungsten) is an element effective in improving corrosion resistance and the workability targeted by this invention. Since it contributes to corrosion resistance and workability by solid-solving in steel, it is added as needed. Preferably, the W content is 0.01% or more, which is the amount at which the respective effects manifest. If the W content exceeds 0.50%, it leads to an increase in alloy costs and a decrease in manufacturability, and causes hardening and a decrease in elongation due to solid-solution strengthening and precipitation strengthening. Therefore, the upper limit of the W content is 0.50% or less. The preferred range for the W content is 0.02% to 0.30%, taking into consideration performance, manufacturability, and alloy costs.

[0032] (Sn:0.10% or less) Tin (Sn) is an element that is effective in improving workability by providing corrosion resistance and suppressing grain boundary segregation of phosphorus (P), and is added as needed. For these effects to manifest, the lower limit of the Sn content is preferably 0.01% or higher. However, if the Sn content exceeds 0.10%, the alloy cost increases and the manufacturability of hot and cold working is hindered; therefore, the upper limit of the Sn content should be 0.10% or less.

[0033] (Ca:0.0100% or less) Calcium (Ca) is an element that improves hot workability and the cleanliness of steel, and is added as needed. The Ca content is preferably 0.0003% or more to achieve these effects. However, if the Ca content exceeds 0.10%, it can lead to decreased manufacturability and reduced corrosion resistance due to water-soluble inclusions such as CaS; therefore, the upper limit for Ca content is set at 0.0100%. Preferably, considering manufacturability and oxidation resistance, the Ca content is between 0.0003% and 0.0050%.

[0034] (Mg:0.010% or less) Magnesium (Mg) acts as a deoxidizing agent in molten steel by forming Mg oxide with Al, and also acts as a crystallization nucleation site for TiN. TiN acts as a solidification nucleation site for the ferrite phase during the solidification process, promoting TiN crystallization and enabling the formation of a fine ferrite phase during solidification. By refining the solidification structure, surface defects caused by coarse solidification structures such as rising in the product can be prevented, and workability is improved, so it is added as needed. The Mg content is preferably 0.0001% or more to exhibit each effect. However, if the Mg content exceeds 0.010%, the manufacturability deteriorates, so the upper limit of the Mg content should be 0.010% or less. Preferably, considering manufacturability, it should be between 0.0003% and 0.0020%.

[0035] (Zr:0.50% or less) Zr (zirconium) is an effective element for improving the cleanliness of steel and obtaining workability, and is added as needed. The content is preferably 0.01% or more, which is necessary for the desired effect to manifest. Since a Zr content exceeding 0.50% leads to increased alloy costs and decreased manufacturability, the upper limit for Zr content is set at 0.50%. The preferred range, considering performance, manufacturability, and alloy cost, is 0.02% to 0.30%.

[0036] (Co:0.50% or less) Cobalt (Co) is an effective element for obtaining workability by solid-solving in steel, and is added as needed. The content is preferably 0.01% or more, which is necessary for the desired effect to manifest. Since a Co content exceeding 0.50% leads to increased alloy costs and decreased manufacturability, the upper limit for Co content should be 0.50% or less. The preferred range, considering performance, manufacturability, and alloy cost, is 0.02% to 0.30%.

[0037] (Ga:0.10% or less) Gallium (Ga) is an element that improves hot workability and the cleanliness of steel, and is added as needed. It is preferable that its content be 0.0003% or more to achieve the desired effects. However, if the Ga content exceeds 0.10%, it can lead to a decrease in manufacturability and corrosion resistance, so the upper limit of the Ga content should be 0.10% or less. Preferably, considering manufacturability and oxidation resistance, the content should be between 0.01% and 0.05%.

[0038] (La, Y, Hf, REM: 0.10% or less each) La (lanthanum), Y (yttrium), Hf (hafnium), and REM (rare earth elements) can be added as needed because they improve hot workability and the cleanliness of steel, and significantly enhance oxidation resistance and hot workability. Their content is preferably 0.001% or more, which is the amount at which their effects manifest. However, excessive addition of La, Y, Hf, and REM only leads to increased alloy costs and decreased manufacturability; therefore, the upper limit for the content of each of La, Y, Hf, and REM should be 0.10% or less. Preferably, considering the effects, economy, and manufacturability, at least one of them should be present in an amount of 0.001% to 0.050%. REM refers to rare earth metals of the lanthanide series, actinide series, and composite metals thereof, such as Ce, Pr, and Sm.

[0039] (The remainder is Fe and unavoidable impurities) The remainder consists of Fe and unavoidable impurities, such as As and Sb. Here, unavoidable impurities refer to components that are mixed in during the industrial production of stainless steel due to various factors in the raw materials such as ore and scrap, and the manufacturing process, and are acceptable within a range that does not adversely affect the present invention.

[0040] (total area ratio, area ratio) Furthermore, the ferritic stainless steel sheet of the present invention (hereinafter simply referred to as "stainless steel sheet") has a thickness of t, and the sheet is shaved in the thickness direction from the surface of the sheet so that a surface parallel to the surface is exposed, thereby sequentially exposing a first surface which is equivalent to 1 / 8 of the sheet thickness (t) from the surface of the sheet and a second surface which is equivalent to 1 / 4 of the sheet thickness (t) from the surface of the sheet. The total area ratio (hereinafter simply referred to as "area ratio") of crystal grains having an orientation where the angular difference between the normal direction of the surface of the sheet and the {111} plane orientation measured on the first and second surfaces is ±10° or less is denoted as S1 and S2, respectively, and the relationship in equation (1) is satisfied. Equation (1): S2 ≥ 0.3 and S1 / S2 ≥ 1.2

[0041] The stainless steel sheet of the present invention is a ferritic stainless steel that reduces impurity elements such as C and N and contains 1% or more Si, thereby improving corrosion resistance. However, Si is a solid solution strengthening element, and it is known that ferritic stainless steel sheets with added Si exhibit reduced workability, particularly deep drawing workability, when manufactured using a conventional cold rolling and annealing process. This is because the accumulation of {111} planes with high r values ​​is slowed down in the surface layer of the steel sheet where shear deformation due to cold working acts. Ferritic stainless steel has a body-centered cubic crystal structure, which has lower symmetry compared to the face-centered cubic crystal structure of austenitic stainless steel sheets. However, even with a body-centered cubic crystal structure with low symmetry, the accumulation of {111} planes, which have small interatomic distances, increases the r value and improves deep drawing workability. The {111} plane is a typical crystal plane that enhances the workability of ferritic stainless steel and has the effect of increasing the r value. A crystal grain with a {111} plane orientation refers to a crystal grain having a crystal orientation where the angular difference between the normal direction of a plane parallel to the surface of the steel plate and the direction perpendicular to the {111} plane orientation is 0°. Furthermore, a crystal grain having an orientation where the angular difference between the normal direction of a plane parallel to the surface of the steel plate and the {111} plane orientation is 10° or less refers to a crystal grain having an orientation where the angular difference between the normal direction on the surface of the stainless steel plate and the {111} plane orientation on the surface of the steel plate is 10° or less (hereinafter referred to as "crystal orientation grain {111}±10°").

[0042] The surface of stainless steel has numerous planes such as {111} planes, {110} planes, and {100} planes with respect to the direction normal to the surface. However, in the body-centered cubic structure of stainless steel sheets, {111} planes have high symmetry and excellent workability, resulting in a large r value. As the planes become {110} or {100}, the workability decreases and the r value decreases. Therefore, if the area ratio S2 of {111}±10° crystal orientation grains at deep positions in the thickness direction of the stainless steel sheet is 0.3 or more, the r value will be large, and excellent deep drawing workability can be achieved. For this reason, the stainless steel sheet of the present invention is designed to have a large amount of {111} plane texture at deep positions in the thickness direction. If the area ratio S2 is less than 0.3, the surface of the stainless steel sheet will have a large amount of {110} planes, {100} planes, and {211} planes with low r values, resulting in an r value of less than 1.0, which does not reach the average r value of the present invention.

[0043] Furthermore, in the stainless steel sheet of the present invention, the area ratio S1 of crystal orientation grains {111}±10° on the first surface, in relation to the area ratio S2 on the second surface, satisfies the area ratio (S1 / S2) shown in equation (1) of 1.2 or more. This results in the stainless steel sheet of the present invention having a greater amount of {111} plane texture at deeper positions in the thickness direction of the sheet. The deep drawing process of stainless steel sheets in the present invention causes displacement not only on the surface of the stainless steel sheet but also in the thickness direction. Therefore, the r value varies depending on the area ratio of crystal orientation grains {111}, {110}, and {100} in the metal structure in the thickness direction. In the stainless steel sheet of the present invention, by setting the area ratio (S1 / S2) to 1.2 or more, the difference in deformation between the first surface, which is closer to the surface of the stainless steel sheet after deep drawing, can be reduced even if there is greater stress or strain in the second surface, which is deeper in the thickness direction of the stainless steel sheet. Conversely, if the area ratio (S1 / S2) is less than 1.2, the reduction in sheet thickness after deep drawing becomes larger, and the possibility of cracks occurring on the surface of the stainless steel sheet increases.

[0044] The area ratio can be measured by EBSD (Electron Beam Backscatter Diffraction). EBSD is a method for rapidly measuring and analyzing the crystal orientation of each grain on the surface of a sample. The area ratio of crystal orientation grains can be quantified by displaying a crystal orientation map divided into two regions: {111}±10° and the other regions. On the steel plate surface of the first and second faces, EBSD measurements are performed with a magnification of 100 in a measurement area of ​​850 μm in the plate width direction and 2250 μm in the rolling direction. For example, as shown in Figure 1, the crystal orientation map for {111}±10°, where the angle difference between the normal direction of the plane parallel to the plate surface and the {111} plane orientation is 10° or less, is displayed and its area ratio is determined. In the crystal orientation map shown in Figure 1, crystal grains with the orientation of {111}±10° are shown as black areas, and crystal grains with orientations other than {111}±10° are shown as white areas. Here, Figure 1 shows an example of a crystal orientation map obtained by EBSD measurement. In the t / 8 plane in the thickness direction of the plate, the black areas indicate crystal grains with an orientation of {111} ± 10° or less, and the total area ratio is 0.5.

[0045] (Average grain size, grain size ratio) Furthermore, the stainless steel sheet of the present invention satisfies equation (2) when the average grain size (μm) on the first surface and the average grain size (μm) on the second surface are G1 and G2, respectively. Formula (2): G1≦30μm, G1 / G2≦1.0

[0046] In the stainless steel sheet of the present invention, the average grain size G1 on the first surface near the surface is set to 30 μm or less. Generally, the larger the average grain size, the larger the r value, but in the surface layer, the r value may decrease due to the growth of grains with low r values, and finer grain size is preferable for improving deep drawing workability. Furthermore, it has been found that in the microstructure of ferritic stainless steel sheets, not only the texture of the ferrite matrix but also the distribution state of ferrite grains with an average grain size of 30 μm or less greatly affects the microstructure. Therefore, the average grain size G1 near the surface of the steel sheet is set to 30 μm or less. Preferably, it is 20 μm or less. Since the r value does not increase as the average grain size becomes finer, there is no particular lower limit to the average grain size, but 5 μm is preferred. For this reason, it is preferable that the average ferrite grain size G1 at a depth of 1 / 4 of the sheet thickness from the surface of the steel sheet is 5 μm or more.

[0047] Furthermore, in the stainless steel sheet of the present invention, when the average grain size (μm) on the first and second surfaces is G1 and G2, respectively, the grain size ratio (G1 / G2) in equation (2) satisfies 1.0 or less. This suppresses the progression of grain growth near the surface of the steel sheet and makes the grain size finer compared to the interior of the steel sheet, thereby obtaining excellent deep-drawability. As a result, a stainless steel sheet with a gradient structure in which the grain size decreases from the center of the sheet thickness to near the surface of the steel sheet is obtained, and a texture that improves the r value and reduces the in-plane anisotropy of the r value can be sufficiently developed not only in the center of the sheet thickness but also on the surface of the steel sheet, thereby significantly improving the deep-drawability of the stainless steel sheet as a whole.

[0048] For measuring the average grain size on the surface of the stainless steel plate, the surface of the steel plate is wet-polished or electrolytically to a predetermined depth, and the grain size is measured at five arbitrary points on the exposed surface in accordance with the cutting method in JIS G 0551:2013 (Microscopic Testing Method for Grain Size). The average value is then calculated to determine the average grain size.

[0049] (pitting corrosion index) Furthermore, the stainless steel sheet of the present invention satisfies the relationship shown in the following formula (3) for the pitting index (PI), which is expressed with the respective contents of Cr, Si, and Mo as variables [Cr], [Si], and [Mo]. Formula (3): PI=[Cr]+2[Si]+3[Mo]≧19.0

[0050] Stainless steel containing Cr is susceptible to corrosion in environments containing halogen ions such as chloride ions. The passive film is locally destroyed by the action of chloride ions, and pitting corrosion progresses as these areas preferentially fail. The progression of this pitting corrosion varies greatly depending on the elements contained in the stainless steel. Generally, the pitting index (PI = Cr + 3Mo) is known as an indicator of the pitting corrosion resistance of stainless steel. The stainless steel sheet of the present invention satisfies equation (3), which was obtained by newly discovering the effect of Si as the pitting index (PI).

[0051] The stainless steel sheet of this invention employs pitting potential measurement as a simple method for evaluating the order of corrosion resistance. Many of the additive elements were measured, and in addition to Cr and Mo, Si, which showed a particularly strong effect, was focused on. The influence of the respective content of [Cr], [Si], and [Mo] was investigated. Pitting potential measurement was performed in accordance with JIS G 0577 (Method for measuring pitting potential of stainless steel), with a current value of 100 μA / cm² in a 3.5 mass% NaCl aqueous solution at 30°C. 2 A potential exceeding a certain value was defined as the pitting potential V'c100. In the stainless steel sheet of the present invention, when the [Si] content exceeds 1.0%, it was found that the pitting potential improves not only with an increase in [Cr] but also with an increase in [Si], and that the effect is twice that of [Cr]. Although this effect is not entirely clear, analysis of the passivation film suggests that the Si oxides formed in the inner layer of the film and at the steel interface exert an effect of suppressing the destruction of the passivation film by halogen ions. Furthermore, it was found that even when Si is added in amounts exceeding 1.0%, the effect of including [Mo] is three times that of [Cr]. Therefore, the pitting index (PI) of the stainless steel sheet of the present invention is expressed as PI = [Cr] + 2[Si] + 3[Mo].

[0052] Furthermore, the pitting potential V'c100 was compared with that of SUS430J1L (19Cr ferritic stainless steel), SUS443J1 (21Cr ferritic stainless steel), and SUS304 (18Cr-8Ni austenitic stainless steel). By setting the PI of formula (1) for the stainless steel sheet of the present invention to 19.0, a pitting potential of 0.25V, equivalent to or better than that of SUS430J1L (19Cr ferritic stainless steel), was achieved. Also, by setting the PI of the stainless steel sheet of the present invention to 21.0, a pitting potential of 0.35V, equivalent to that of SUS443J1 (21Cr ferritic stainless steel) and SUS304 (18Cr-8Ni austenitic stainless steel), was achieved. Therefore, the pitting index (PI) of the stainless steel sheet of the present invention was set to 19.0 or higher. Furthermore, if the pitting index (PI) of the stainless steel sheet of the present invention exceeds 25.0, the Si content [Si] in the steel tends to become too high, resulting in excessively high tensile strength and hardness, reduced workability, and increased manufacturing costs. Similarly, if the Mo content [Mo] in the steel increases, raw material costs increase. For this reason, it is preferable to set the upper limit of the pitting index (PI) to 25.0.

[0053] (Average r-value, Δr-value) In the stainless steel sheet of the present invention, it is important to control the r value. Furthermore, the r value in the rolling direction (r0) is controlled within an appropriate range, and the average r value of the r value in the rolling direction (r0), the r value at 45° to the rolling direction (r45), and the r value at 90° to the rolling direction (r90), obtained by applying a tensile strain of 14.4% elongation, is 1.5 or higher, and the in-plane anisotropy Δr value is 0.5 or lower. The mean r-value and Δr-value are expressed by the following equations (4) and (5). Equation (4): Mean r value = (r0 + 2r45 + r90) / 4 Equation (5): Δr=(r0+r90-2r45) / 2 The meaning of each symbol is shown below. r0: r value in the rolling direction r45: r value at a 45° angle to the rolling direction r90: r value at a 90° angle to the rolling direction.

[0054] A minimum average r value of 1.50 is necessary to prevent cracking even with high deep drawing, and this was set as the lower limit. Cracking occurs when wavy irregularities called "ears" form on the edge of the drawn cylinder. To ensure stable displacement of the steel sheet and further suppress cracking even with high deep drawing, it is preferable to set the average r value to 1.90 or higher. There is no specific upper limit for the average r value; it may be determined appropriately by considering the average r value of ferritic stainless steel sheets that can be manufactured with general manufacturing equipment and devices, for example, an upper limit of 2.50 or less may be set.

[0055] Furthermore, in order to achieve an average r value of 1.50 or higher, the r value (r0) must be 1.00 or higher, so this was set as the lower limit. Even with a high degree of deep drawing, it is preferable to set the r value (r0) to 1.50 or higher in order for the steel plate to displace stably and for processing cracks to be further suppressed. There is no particular upper limit set for the r value (r0), but in order to prevent the anisotropy Δr from becoming extremely large, it is preferable to set the upper limit of the r value (r0) to 3.00 or lower. More preferably, the r value (r0) is in the range of 1.50 to 2.50.

[0056] Furthermore, in the stainless steel of the present invention, the Δr value is controlled to 0.5 or less. Generally, if the r value in the rolling direction (r0) and the r value in the 90-degree rolling direction (r90) are high, the steel sheet will displace stably even with a high degree of deep drawing, and processing cracks will be further suppressed. However, if the r value in the rolling direction (r0) and the r value in the 90-degree rolling direction (r90) are extremely large compared to the r value in the 45-degree rolling direction (r45), and the Δr value exceeds 0.5, the length of the rim on the edge of the cylinder will increase, making it difficult to obtain a forming height that does not produce a rim in deep drawing. Therefore, the Δr value is reduced to suppress the occurrence of a rim. In other words, even with a high degree of deep drawing, the steel sheet can be processed stably. Consequently, in deep drawing, in order to suppress the occurrence of a rim and prevent processing cracks, it is not enough to simply increase the average r value; it is necessary to balance r0, r45, and r90 to prevent the Δr value from becoming large and to keep it below 0.5.

[0057] The r value (r0) in the rolling direction, the average r value, and the Δr value can be controlled by adjusting the various conditions in the steel sheet manufacturing process. For example, by appropriately setting the manufacturing conditions, such as setting the reduction ratio in cold rolling and the final annealing temperature to appropriate values, it is possible to manufacture steel sheets that satisfy equations (4) and (5).

[0058] The r-value, average r-value, and Δr-value can be measured and determined according to JIS Z 2254:2008 (Test method for plastic strain ratio of thin metal sheets). Specifically, the r-value is calculated from the width and thickness of a JIS No. 13B tensile test specimen before and after tensile deformation, when a uniform plastic strain is applied to a predetermined level by tensile testing. Furthermore, after applying a 14.4% strain to the tensile test specimen in the rolling direction, the width and thickness are measured from the parallel, 45°, and 90° directions, and the r-values ​​(r0, r45, r90) for each direction are determined. These values ​​are then substituted into the formulas for the average r-value and Δr-value to calculate them.

[0059] (Manufacturing method) The following describes preferred manufacturing methods and conditions for the stainless steel sheet of the present invention, but the present invention is not limited thereto. Ferritic stainless steel sheets can be manufactured, for example, by the following process: slab heating → hot rolling → (hot-rolled sheet annealing) → cold rolling → intermediate annealing → finish cold rolling → finish annealing. Although the "pickling" step is omitted from the process description, pickling is carried out as appropriate according to conventional methods. Furthermore, conventionally, the "cold rolling → intermediate annealing" process was performed multiple times as needed. However, in relation to the manufacturing method of the stainless steel sheet of the present invention, the influence of cold rolling conditions and cold-rolled sheet annealing conditions was investigated. Therefore, for controlling the metal structure of the stainless steel sheet of the present invention, cold rolling with a combination of large-diameter and small-diameter rolls is suitable, and in particular, it is desirable to perform continuous cold rolling with large-diameter and small-diameter rolls without intermediate annealing in the cold rolling process, and to perform finish annealing at a low annealing temperature. This will be explained below.

[0060] The stainless steel sheet of the present invention is manufactured by hot-rolling a slab to a predetermined thickness, followed by hot-rolling annealing. After annealing, cold rolling and cold-rolled annealing are performed. The sheet is manufactured in a single cold-rolling process using a combination of large-diameter and small-diameter rolls, without intermediate annealing during the cold-rolling process. Cold rolling using a combination of large-diameter and small-diameter rolls is preferred. In particular, the metal structure is controlled by combining large-diameter and small-diameter rolls during cold rolling. Furthermore, it is desirable to continuously roll with large-diameter and small-diameter rolls without intermediate annealing during cold rolling. This promotes development in the {111} direction not only near the center of the sheet thickness but also on the surface. The large-diameter rolls for cold rolling are preferably 400 mm and the small-diameter rolls are preferably 100 mm. Any of the following types of rolling mills can be used: cluster mills (multi-stage rolling mills), planetary rolling mills, Zenzimir rolling mills, etc. In conventional manufacturing methods, the reduction ratio in the cold rolling process is increased to improve workability. In the stainless steel sheet of the present invention, the reduction ratio is set to less than 80% in order to keep the Δr value at 0.5 or less. Furthermore, during annealing after cold rolling, the heating temperature is set to less than 950°C to promote the development of a texture that improves grain refinement and workability. Considering the development of grain refinement and recrystallized texture, a finish cold rolling reduction ratio of 60-80% and an annealing temperature of 880-930°C are desirable.

[0061] Furthermore, considering productivity, it is desirable to manufacture in a single cold-rolling process as mentioned above. However, the stainless steel sheet of the present invention can be manufactured in a two-step cold-rolling process that includes intermediate annealing. In the two-step cold-rolling process, either a large-diameter roll or a small-diameter roller is cold-rolled twice, followed by finish annealing. In the first (preliminary) cold-rolling process before intermediate annealing, the reduction ratio is less than 50%, and in the second (later) cold-rolling process (finish cold-rolling) after intermediate annealing, the reduction ratio is less than 75%. This is because if the cold-rolling reduction ratio is excessively high, the Δr value will increase due to the rise in the r values ​​of r0 and r90. Preferably, in the first (preliminary) cold-rolling process, the reduction ratio is 30-50%, and in the second (later) finish cold-rolling process after intermediate annealing, the reduction ratio is 60-75%. In this case, it is preferable to set the annealing temperature during cold rolling of the cold-rolled sheet to less than 950°C to suppress grain growth and refine the grain, thereby controlling the microstructure such as the area ratio of crystal orientations and average grain size, as well as the average r value. Similarly, it is preferable to set the annealing temperature after cold rolling to 880-930°C to suppress grain growth and refine the grain, thereby controlling the microstructure such as the area ratio of crystal orientations and average grain size, as well as the average r value.

[0062] The stainless steel sheet of the present invention is within the range of a predetermined chemical composition defined in the present invention, and by the manufacturing method described above, a ferritic stainless steel sheet can be obtained in which both the values ​​of formula (1) and formula (2) satisfy the appropriate range of the present invention. By suppressing shear deformation during finish cold rolling, it is manufactured in a cold rolling process combining large-diameter and small-diameter rolls without intermediate annealing. This makes it possible to refine the average grain size by omitting intermediate annealing and preventing recrystallization. Furthermore, in rolling with rolls of a normal diameter of about 100 mm used for rolling stainless steel, the suppression of shear deformation is insufficient. By combining it with a rolling mill having a roll diameter of 400 mm or more, the area ratio of the {111} plane orientation can be 0.3 or more, the area ratio can be 1.2 or more, and furthermore, the average grain size on the t / 4 plane near the surface can be 30 μm or less, and the grain size ratio can be 1.0 or less.

[0063] The slab thickness, hot-rolled sheet thickness, etc., can be selected as appropriate. In cold rolling, the reduction ratio, roll roughness, roll diameter, rolling oil, number of rolling passes, rolling speed, and rolling temperature can be selected as appropriate. Annealing can be performed as bright annealing in a non-oxidizing atmosphere such as hydrogen gas or nitrogen gas, or in air, if necessary. Furthermore, after annealing, temper rolling or a tension leveling process for shape correction can be selected. [Examples]

[0064] The present invention will be described in detail based on the following examples. However, the present invention is not limited to the examples shown below.

[0065] Table 1 shows the content of essential additive elements and, in some cases, optional additive elements in steel grades A to P. For steel grades A to H, the essential additive elements are within the scope of the present invention. For steel grades I to P, the essential additive elements are outside the scope of the present invention. Note that for steel grade C, Nb is 0%, but since it may be irreversibly present as an impurity in practice, "0%" here represents the actual content, and a content of less than 0.01% is permissible. Similarly, for steel grade E, Ti is 0%, but since it may be irreversibly present as an impurity in practice, "0%" here represents the actual content, and a content of less than 0.01% is permissible.

[0066] [Table 1]

[0067] The manufacturing methods for Invention Examples 1 to 12 and Comparative Examples 1 to 13 are shown below. Steel with the component composition shown in Table 1 was melted and cast into slabs, which were then hot-rolled to produce hot-rolled sheets with a thickness of 4.0 mm or 6.0 mm. Subsequently, the hot-rolled sheets were continuously annealed, then pickled, cold-rolled, annealed, and pickled again to produce sample sheets for Invention Examples 1-12 and Comparative Examples 1-13. The cold-rolling process is shown in Table 2.

[0068] [Table 2]

[0069] Cold rolling involves using a combination of large-diameter rolls (400 mm in diameter) and small-diameter rolls (100 mm in diameter) to control the microstructure of the cold-rolled sheet. For a single cold rolling cycle, a 4.0 mm thick hot-rolled sheet was annealed (HA: Heat Anealing) at 870-950°C, then cold-rolled to a thickness of 0.8 mm using a combination of large and small diameter rolls, or a small diameter roll alone, followed by final heating (FA: Final Heating) to produce the final product. In addition, if the cold rolling process involves two passes, a 6.0 mm thick hot-rolled sheet is annealed (HA: Heat Anealing) at 870-950°C, followed by the first intermediate cold rolling using large-diameter or small-diameter rolls, and then intermediate annealing (IA: Internal Heating). Finally, for the second finish rolling, the sheet is cold-rolled to a thickness of 0.8 mm using a combination of large-diameter and small-diameter rolls, or small-diameter rolls alone, followed by final annealing (FA: Final Heating) to produce the final product.

[0070] The values ​​shown in Table 2 represent the reduction ratio by the rolling mill. When the thickness of the material before and after rolling is h1 and h2, respectively, the value is calculated using the formula (h1-h2) / h1 and is expressed as a percentage. The numbers in parentheses () in Table 2 indicate whether the reduction occurred before (1) or after (2) IA.

[0071] Next, the total area fraction, area ratio, average grain size, grain size ratio, average r value, and Δr value were measured for the samples of Invention Examples 1-12 and Comparative Examples 1-13, with grain orientation {111}±10°. The calculated value of PI in equation (3) and the measured pitting potential are also shown. The results are shown in Table 3. Furthermore, the following describes the measurement methods for the total area fraction, area ratio, average grain size, grain size ratio, average r value and Δr value, and pitting potential of grains with crystal orientation {111}±10°.

[0072] <Measurement of area fraction of crystal orientation grains {111}±10°> The surface of a stainless steel plate, exposed by wet or electrolytic polishing to a predetermined depth, was measured using EBSD (electron beam backscatter diffraction). In the EBSD measurement, a crystal orientation analysis system (PEGASUS2300, manufactured by TSL Corporation) was used to create mapping images for each crystal grain with different crystal orientations. For these mapping images, the contour tracking algorithm of image analysis software was used to process the total area ratio of regions enclosed by a single closed contour line. Furthermore, the area ratio was calculated from the total area ratios at each predetermined depth.

[0073] <Measurement of crystal grain size> For measuring the average grain size on the surface of stainless steel sheets, the surface of the steel sheet was wet-polished or electrolytically to a predetermined depth, and the grain size was measured at five arbitrary points on the exposed surface in accordance with the cutting method in JIS G 0551:2013 (Microscopic Testing Method for Grain Size). The average value was then calculated to determine the average grain size. Furthermore, the grain size ratio was calculated from the average grain sizes at each predetermined depth.

[0074] <Measurement of average r-value and Δr-value> The average r-value and Δr-value were measured according to JIS Z 2254:2008 (Test method for plastic strain ratio of thin metal sheets). The r-value can be calculated from the width and thickness of a JIS No. 13B tensile test specimen before and after tensile deformation when a uniform plastic strain is applied to a predetermined level. Furthermore, after applying a 14.4% strain to the tensile test specimen in the rolling direction, the width and thickness were measured from the parallel, 45°, and 90° directions, and the r-values ​​(r0, r45, r90) for each direction were determined. These values ​​were then substituted into the formulas for the average r-value and Δr-value to calculate them.

[0075] (Measurement of pitting potential) Pitting potential was measured in accordance with JIS G 0577, and the pitting potential V'c100 was determined by a current value of 100 μA / cm². 2The potential (V) was measured when it exceeded a certain value. At this time, if the pitting potential was less than 0.30V, it was assumed that sufficient pitting resistance could not be obtained. If the pitting potential was between 0.30V and 0.35V, it was assumed that pitting resistance equivalent to that of 19Cr-containing ferritic stainless steel (SUS304J1L) was obtained. If the pitting potential was 0.35V or higher, the pitting resistance was evaluated as equivalent to that of 21Cr-containing ferritic stainless steel and 18-8 austenitic stainless steel (SUS443J1, SUS304).

[0076] [Table 3]

[0077] Next, we will evaluate the corrosion resistance and workability of the stainless steel of the present invention. The evaluation method is described below. The results are also shown in Table 4.

[0078] (processability) The processability was evaluated using the following procedure. First, blank discs (blanks) with a thickness of 0.8 mm and diameters of 80 mm and 90 mm were prepared for each of the inventive example and comparative example shown in Table 2. Using a punch diameter of 40 mm and drawing ratios (blank diameter ÷ punch diameter: 80 ÷ 40 = 2.00, 90 ÷ 40 = 2.25) of 2.00 and 2.25, two cup-shaped deep-drawn cylindrical products with inner diameters of 20 mm and 22.5 mm were produced. Next, the prepared deep-drawn cylindrical products were cooled to -45°C in a freezer. Then, a drop weight test was performed on the body of the deep-drawn cylindrical product removed from the freezer by dropping a 1 kg weight from a height of 1 m to evaluate whether or not longitudinal cracks occurred. In this drop-weight test, the processability was evaluated as follows: if a cylindrical deep-drawn part made with a drawing ratio of 2.00 developed a longitudinal crack, it was marked as "× (unacceptable)"; if no longitudinal crack occurred in the cylindrical deep-drawn part made with a drawing ratio of 2.00, but a longitudinal crack occurred in the cylindrical deep-drawn part made with a drawing ratio of 2.25, it was marked as "〇 (good)"; and if no longitudinal crack occurred in either the cylindrical deep-drawn part made with a drawing ratio of 2.00 or 2.25, it was marked as "◎ (excellent)". In this case, the larger the drawing ratio, the stricter the evaluation, indicating superior processability.

[0079] (corrosion resistance) The method for evaluating corrosion resistance is described in detail below. A test specimen measuring 50 mm in width and 100 mm in the rolling direction was cut out, and one side surface of the test specimen was wet-polished with #600 grit. Next, three sides of the test specimen (excluding one side in the width direction) were coated with resin (one-component condensation type RTV rubber KE44 manufactured by Shin-Etsu Silicone Co., Ltd.). Then, two polyethylene tubes measuring 20 mmφ × 10 mm were bonded onto a 70 mm × 150 mm bakelite plate, and the unpolished surface of the test specimen was placed on top and bonded. A combined salt-dry-wet cycle test (CCT) was performed on the sample obtained in this manner. The sample was placed in the CCT apparatus with the surface of the test specimen at a 75° angle to the horizontal plane and the side of the test specimen not coated with resin facing downwards. Three cycles were performed, each consisting of 5% salt spray (35°C, 2 hours), drying (60°C, 25% RH, 4 hours), and wetting (50°C, 95% RH, 2 hours). After that, the sample was washed with water and dried, and the rust area ratio on the surface of the test specimen was evaluated. Furthermore, in this evaluation, if the rating number (RN) is less than 8 (corresponding to a rust area ratio exceeding 0.25%), it is marked as "×" indicating poor corrosion resistance; if the RN is 8 or higher (corresponding to a rust area ratio of 0.25% or less), it is marked as "〇" indicating excellent corrosion resistance; and if the RN is 9.5 or higher (corresponding to a rust area ratio of 0.05% or less), it is marked as "◎" indicating extremely excellent corrosion resistance. Furthermore, the evaluation of this rust-affected area ratio conforms to the "Annex JC (Normative) Rating Number Method" of JIS Z 2371: Salt spray test method.

[0080] [Table 4]

[0081] As shown in Table 4, the ferritic stainless steel sheets of Invention Examples 1 to 12 all satisfy the appropriate range for the present invention in terms of chemical composition and the values ​​of formulas (1), (2), and (3). They also exhibit excellent corrosion resistance with a pitting potential of 0.25V or higher, all rated "○" or higher, and excellent workability with all rated "○" or higher.

[0082] In contrast, the ferritic stainless steel sheets of Comparative Examples 1 to 13 are described below. Comparative Example 1 had a chemical composition within the scope of the present invention, but in terms of manufacturing method, it was cold-rolled using only small-diameter rolls. The grain size G1 and G1 / G2 were within the scope of the present invention, and S2 in the area ratio was 0.3, which is within the scope of the present invention. However, the S1 / S2 ratio was low at 1.1, and the average r value was low at "1.45", resulting in poor processability ("×"). On the other hand, the pitting potential was 0.35V, and the corrosion resistance was excellent ("◎").

[0083] Comparative Example 2 had a chemical composition within the scope of the present invention, but in terms of manufacturing method, it was cold-rolled twice using only small-diameter rolls. The grain size G1 was 25 μm, G1 / G2 was 0.90, which is within the scope of the present invention, and S2 in the area ratio was 0.6, which is within the scope of the present invention. However, the S1 / S2 ratio was low at 1.1, resulting in a high Δr value of "0.90" and poor processability ("×"). The pitting potential was 0.35 V, and the corrosion resistance was excellent ("◎").

[0084] Comparative Example 3 had a chemical composition within the scope of the present invention, but in terms of manufacturing method, it was cold-rolled using only small-diameter rolls. The grain size G1 was 25 μm, G1 / G2 was 0.75, which is within the scope of the present invention, and S2 in the area ratio was 0.35, which is within the scope of the present invention. However, the S1 / S2 ratio was low at 1.1, resulting in a high Δr value of "0.90" and poor processability ("×"). The pitting potential was 0.35 V, and the corrosion resistance was excellent ("◎").

[0085] Comparative Example 4 had a high carbon content of 0.021% by mass in the steel, which is outside the scope of the present invention. However, its S2 content was low at 0.25 within the area ratio, and its average r value was low at 1.40, resulting in poor workability ("×").

[0086] Comparative Example 5 had a high Si content of 2.6% in the steel, which was outside the scope of the present invention. As a result, the S2 ratio within the area was low at 0.25, and the average r value was low at 1.35, resulting in poor workability ("×").

[0087] Comparative Example 6 had a high Cr content of 23.2% in the steel, a low S2 content of 0.25 within the area ratio, and a low average r value of 1.35, resulting in poor workability ("×").

[0088] Comparative Example 7 had a high Mn content of 1.05% in the steel, a low S2 ratio of 0.25 within the area ratio, and a low average r value of 1.45, resulting in poor workability ("×").

[0089] Comparative Example 8 had a high P content of 0.042% in the steel, a low S1 / S2 ratio of 1.0 within the area ratio, and a large grain size of G1 at 31 μm, resulting in poor workability ("×").

[0090] Comparative Example 9 had a high N content of 0.021% in the steel, a low S2 content of 0.25 within the area ratio, and a low average r value of 1.35, resulting in poor workability ("×").

[0091] Comparative Example 10 had a high Nb content of 0.41% in the steel, a low S1 / S2 ratio of 1.1 within the area ratio, and a high Δr value of "0.65," resulting in poor workability ("×").

[0092] Comparative Example 11 had a high Ti content of 0.41% in the steel, a low S1 / S2 ratio of 1.1 in the area ratio, a large grain size G1 of 35 μm, a large G1 / G2 ratio of 1.05, and a high Δr value of "0.75," resulting in poor workability ("×").

[0093] Comparative Example 12 has a low Si content of 0.9% in the steel, which is outside the scope of the present invention. The pitting index was 19.0, but the average grain size G1 was 31 μm, which is outside the scope of the present invention, and the pitting potential was 0.20 V, resulting in a "fail" rating for corrosion resistance.

[0094] Comparative Example 13 had a low chemical composition with a Cr content of 12.8% in the steel, and although it contained a large amount of Si and Mo to achieve a PI of 19.0, its workability was poor ("×") due to its low average r value.

[0095] From the results of these Invention Examples 1-12 and Comparative Examples 1-13, it can be seen that in order to obtain the corrosion resistance and processability targeted by the present invention, it is important that the chemical composition range, total area ratio, area ratio, average grain size, grain size ratio, average r value and Δr value, and pitting index (PI) are within the range specified in the present invention. Furthermore, the addition of trace elements such as B, Ni, Cu, V, W, Sn, Ca, Mg, Zr, Co, Ga, La, Y, Hf, and REM, as well as the preferred manufacturing method specified in the present invention, are effective in improving corrosion resistance and processability.

Claims

1. In mass percent, C: 0.020% or less, Si: more than 1.0% and less than 2.5%, Mn: 1.0% or less, P: 0.040% or less, S: 0.0030% or less, Cr: 13.0% or more and 23.0% or less, Mo: 0.50% or less N: 0.020% or less, and Al: Contains 0.20% or less, and further, A ferritic stainless steel sheet having a chemical composition containing Nb: 0.40% or less and Ti: 0.40% or less, or both, with the remainder being Fe and unavoidable impurities, Let the thickness of the steel plate be t. From the surface of the steel plate, a surface parallel to the surface is removed in the thickness direction so that a first surface corresponding to 1 / 8 of the thickness (t) is exposed from the surface of the steel plate, and a second surface corresponding to 1 / 4 of the thickness (t) is exposed sequentially. The total area ratios of crystal grains having orientations where the angular difference between the normal direction of the surface of the steel plate and the {111} plane orientation measured on the first and second surfaces is ±10° or less are defined as S1 and S2, respectively. When the average grain size (μm) on the first and second surfaces is G1 and G2, respectively, S1 and S2, and G1 and G2, satisfy the relationships shown in equations (1) and (2) below, respectively. Equation (1): S2 ≥ 0.3 and S1 / S2 ≥ 1.2 Formula (2): G1≦30μm, G1 / G2≦1.0 and, A ferritic stainless steel sheet with excellent corrosion resistance and workability, satisfying the relationship for the pitting index (PI) represented by the following formula (3), where the respective content (mass%) of Cr, Si, and Mo in the aforementioned chemical composition are expressed as [Cr], [Si], and [Mo], respectively. Formula (3): PI=[Cr]+2[Si]+3[Mo]≧19.0

2. The aforementioned chemical composition is further expressed in mass%, B: 0.0050% or less, Ni: 1.0% or less, Cu: 1.0% or less, V: 0.50% or less, W: 0.50% or less, Sn: 0.10 or less, Ca: 0.0100% or less, Mg: 0.010% or less, Zr: 0.50% or less, Co: 0.50% or less, Ga: 0.10% or less, La: 0.10% or less, Y: 0.10% or less, Hf: 0.10% or less, A ferritic stainless steel sheet with excellent corrosion resistance and workability according to claim 1, containing at least one selected from the group consisting of REM (rare earth metals of the lanthanide series, actinide series, excluding La, or composite metals thereof) in an amount of 0.10% or less.

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