Ferritic stainless steel cold-rolled annealed steel sheet, raw material cold-rolled steel sheet, and method for manufacturing said cold-rolled annealed steel sheet

Abstract

To provide a ferritic stainless cold-rolled annealed steel plate that has superior oxidation resistance and high-temperature fatigue resistance, as well as superior toughness.SOLUTION: A ferritic stainless cold-rolled annealed steel plate comprises, in mass%, C: 0.020% or less, Si: 0.05-1.00%, Mn: 0.05-1.00%, P: 0.050% or less, S: 0.010% or less, Al: 1.00-3.00%, N: 0.020% or less, Cr: 15.0-22.0%, Nb: 0.30-0.80%, Mo: 1.00-3.00%, and Ni: 0.01-1.00%, with the balance being Fe and inevitable impurities, characterized by a plate thickness of 1.8 mm or more and 2.8 mm or less, and DBTT≤20+20(t-1.8), where the DBTT denotes ductile-brittle transition temperature (°C) and t denotes plate thickness (mm).SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a ferritic stainless steel cold-rolled annealed steel sheet, and more particularly to a Cr-containing steel, particularly a ferritic stainless steel cold-rolled annealed steel sheet having excellent oxidation resistance, high-temperature fatigue resistance, and toughness, which is suitable for use in exhaust system components used under high temperatures, such as exhaust pipes and converter cases for automobiles and motorcycles, and in thermal power plants, as well as to a cold-rolled steel sheet that is the raw material for the steel sheet, and a method for manufacturing the cold-rolled annealed steel sheet. [Background technology]

[0002] Automotive exhaust system components, such as exhaust manifolds, exhaust pipes, converter cases, and mufflers (hereinafter sometimes referred to as automotive exhaust system components), require excellent oxidation resistance and high-temperature fatigue resistance (hereinafter sometimes collectively referred to as heat resistance). High-temperature fatigue is a high-cycle fatigue phenomenon that occurs when automotive exhaust system components are continuously subjected to vibrations while the engine is running. Specifically, automotive exhaust system components are heated to high temperatures by the engine's operation, and when they are continuously subjected to vibrations generated by driving the vehicle, strain accumulates, resulting in high-cycle fatigue. Furthermore, because automotive exhaust system components are processed into complex shapes, they also require excellent workability at room temperature.

[0003] As a material for such automobile exhaust system members, Cr-containing steels such as Type 429 (14 mass% Cr-0.9 mass% Si-0.4 mass% Nb system) to which Nb and Si are added in combination are often used.

[0004] In addition, SUS444 (19 mass% Cr-0.4 mass% Nb-2 mass% Mo) specified in JIS G4305, Cr-containing steel with improved high-temperature yield strength due to the combined addition of Nb and Mo, and ferritic stainless steel with the combined addition of Nb, Mo, and W (hereinafter also referred to as SUS444, etc.) have also been developed.

[0005] However, in response to recent tightening of automobile exhaust gas regulations and the need to improve fuel efficiency, exhaust gas temperatures are trending upwards to over 900°C, and as a result, even the above-mentioned SUS444 and other materials may not be able to provide sufficient heat resistance, and there is a demand for the development of materials that offer better heat resistance than the above-mentioned SUS444 and other materials.

[0006] As such a material, for example, Patent Document 1 discloses "a ferritic stainless steel sheet having excellent heat resistance and workability, containing, by mass%, C: less than 0.010%, N: 0.020% or less, Si: more than 0.1% and 2.0% or less, Mn: 2.0% or less, Cr: 12.0 to 25.0%, Cu: more than 0.9% and 2.0%, Ti: 0.05 to 0.3%, Nb: 0.001 to 0.1%, Al: 1% or less, B: 0.0003 to 0.003% or less, with Cu / (Ti+Nb) being 5 or more, and the balance being Fe and unavoidable impurities."

[0007] Patent Document 2 discloses "a ferritic stainless steel sheet having excellent heat resistance and workability, containing, by mass%, C: 0.02% or less, N: 0.02% or less, Si: over 0.1% and 1.0% or less, Mn: 0.5% or less, P: 0.020 to 0.100%, Cr: 13.0 to 20.0%, Nb: 0.5 to 1.0%, Cu: 1.0 to 3.0%, Mo: 1.5 to 3.5% or less, W: 2.0% or less, B: 0.0001 to 0.0010%, Al: 0.01 to 1.0% or less, the balance being Fe and unavoidable impurities, and wherein Mo+W is 2.0 to 3.5%."

[0008] Patent Document 3 describes a ferritic stainless steel containing, by mass%, C: 0.020% or less, Si: 3.0% or less, Mn: 1.0% or less, P: 0.040% or less, S: 0.030% or less, Cr: 10.0% or more but less than 16.0%, N: 0.020% or less, Al: 1.4 to 4.0%, Ti: more than 0.15% but 0.5%, Ni: 0.05 to 0.5%, with the balance being Fe and unavoidable impurities, and satisfying the following formula (1): Al% / Cr%≧0.14 (1) In the formula, Al% and Cr% represent the contents (mass%) of Al and Cr, respectively."

[0009] Patent Document 4 discloses "a ferritic stainless steel characterized by containing, by mass%, C: 0.020% or less, Si: more than 0.1% and up to 3.0%, Mn: 2.0% or less, P: 0.050% or less, S: 0.010% or less, Al: 0.05 to 6.0%, N: 0.020% or less, Cr: 12 to 30%, Cu: 0.4 to 4.0%, Nb: 0.02 to 1.0%, Ti: 0.01 to 1.0%, Mo: 0.1 to 6.0%, Co: 0.01 to 3.0%, Ni: 0.02 to 1.0%, with the total of Si and Al being ≧0.50, and the balance being Fe and unavoidable impurities."

[0010] Patent Document 5 describes a ferritic stainless steel containing, by mass%, C: 0.020% or less, Si: more than 0.1% and not more than 3.0%, Mn: 0.05 to 2.0%, P: 0.050% or less, S: 0.010% or less, Al: 0.3 to 6.0%, N: 0.020% or less, Cr: 12 to 30%, Nb: more than 0.3% and not more than 1.0%, Ti: 0.01 to 0.5%, Mo: 0.3 to 6.0%, Co: 0.01 to 3.0%, and Ni: 0.02 to 1.0%, and containing the ingredients in amounts that satisfy the following formulas (1) to (3), with the balance being Fe and unavoidable impurities. Si+Al>1.0% (1) Al-Mn>0% (2) Nb-Ti>0% (3) (In the formulas (1) to (3), Si, Al, Mn, Nb, and Ti represent the content (mass %) of each element.)" is disclosed.

[0011] Patent Document 6 discloses "a ferritic stainless steel having a composition containing, by mass%, C: 0.020% or less, Si: 0.05 to 2.00%, Mn: 0.05 to 2.00%, P: 0.050% or less, S: 0.010% or less, Al: 1.0 to 4.0%, N: 0.020% or less, Cr: 12.0 to 25.0%, Nb: 0.40 to 0.80%, Ti: less than 0.01%, Mo: 1.0 to 4.0%, Co: 0.01 to 1.00%, Ni: 0.01 to 1.00%, and the balance being Fe and unavoidable impurities." [Prior art documents] [Patent documents]

[0012] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-202257 [Patent Document 2] Japanese Patent Application Laid-Open No. 2012-207252 [Patent Document 3] International Publication No. 2014 / 050016 [Patent Document 4] Japanese Patent Application Laid-Open No. 2015-96648 [Patent Document 5] Patent No. 6123964 [Patent Document 6] International Publication No. 2020 / 080104 Summary of the Invention [Problem to be solved by the invention]

[0013] Automotive exhaust system components are processed into complex shapes by press working, etc. Therefore, the materials for automotive exhaust system components are required to have excellent workability at room temperature, particularly excellent toughness, in addition to excellent heat resistance.

[0014] However, the materials disclosed in Patent Documents 1 to 6 contain large amounts of elements such as Nb, Mo, W, and Al in order to obtain excellent heat resistance, which significantly reduces toughness. As a result, there is concern about the occurrence of brittle cracking during press working, etc. Therefore, there is currently a demand for the development of materials that have excellent oxidation resistance and high-temperature fatigue resistance, as well as excellent toughness.

[0015] The present invention has been made in view of the above-mentioned current situation, and an object of the present invention is to provide a ferritic stainless cold-rolled annealed steel sheet having excellent oxidation resistance and high-temperature fatigue resistance, as well as excellent toughness.

[0016] Another object of the present invention is to provide a cold-rolled steel sheet that is a raw material for the above-mentioned ferritic stainless steel cold-rolled annealed steel sheet, and a method for producing the above-mentioned ferritic stainless steel cold-rolled annealed steel sheet.

[0017] The term "excellent oxidation resistance" means having both excellent continuous oxidation resistance and excellent cyclic oxidation resistance.

[0018] Here, "excellent continuous oxidation resistance" means that when a test piece is held at 1100°C in an air atmosphere for 200 hours, the test piece has an oxidation weight gain of 50 g / m or more. 2 This means that there is no abnormal oxidation and no peeling of oxide scale. The oxidation weight gain is calculated using the following formula. [Oxidation gain (g / m 2 )] = [Increase in mass of test piece before and after holding (g)] ÷ [Surface area of test piece (m 2 )]

[0019] Furthermore, "excellent resistance to repeated oxidation" means that when the cycle consisting of the following steps (a) to (d) is repeated 400 times in an air atmosphere, the test piece has an oxidation weight gain of 50 g / m or more. 2 This means that there is no abnormal oxidation, which results in the formation of flaking oxide scale, and no peeling of oxide scale. (a) 200°C for 1 minute (b) Temperature increase from 200°C to 1100°C (average temperature increase rate 30°C / min) (c) 1100℃ for 20 minutes (d) Cooling from 1100°C to 200°C (average cooling rate 90°C / min) The oxidation weight gain is calculated using the following formula. [Oxidation gain (g / m 2 )] = [Increase in mass of test piece (g) after 400 cycles] ÷ [Surface area of test piece (m 2 )]

[0020] "Excellent high-temperature fatigue resistance" means that the material exhibits properties superior to those of SUS444. More specifically, "excellent high-temperature fatigue resistance" means that the number of cycles to fracture is 1.0 x 10 when a test piece is repeatedly subjected to a bending stress of 50 MPa at 950°C. 6 This means more than one cycle.

[0021] "Excellent toughness" means that the plate thickness is 1.8 mm or more and 2.8 mm or less, and DBTT≦20+20(t-1.8) is satisfied. Here, DBTT is the ductile-brittle transition temperature (°C), and t is the plate thickness (mm). DBTT is determined by conducting a V-notch Charpy impact test in accordance with JIS Z2242:2018, and is the temperature at which the brittle fracture surface ratio is 50%. [Means for solving the problem]

[0022] The inventors have conducted extensive research to achieve the above object.

[0023] the result, (1) The composition of the elements is appropriately controlled, in particular, by simultaneously containing appropriate amounts of Nb, Mo, and Al in addition to Cr, and (2) The amount of precipitated Nb in the cold-rolled steel sheet used as the raw material for the cold-rolled annealed steel sheet is 0.15 mass% or less. By doing so, it has been found that excellent toughness can be achieved at the same time as excellent oxidation resistance and high-temperature fatigue resistance.

[0024] Specifically, by including 0.30 to 0.80 mass% Nb, 1.00 to 3.00 mass% Mo, and 1.00 to 3.00 mass% Al, high-temperature strength is increased over a wide temperature range, and excellent high-temperature fatigue resistance is obtained. Furthermore, a protective continuous Al-based oxide scale forms on the steel sheet surface at high temperatures, improving oxidation resistance. Furthermore, the precipitation of a second phase (σ phase) that adversely affects properties due to the inclusion of Mo is suppressed by including Al.

[0025] Furthermore, the present inventors have further investigated the improvement of toughness. When ferritic stainless steel containing 0.30 mass% or more of Nb is heat-treated in the temperature range of 700 to 1100°C, an intermetallic processed product called a Laves phase with a composition of FeNb is precipitated. Therefore, many steels with different compositions were produced under various conditions, and the relationship between the precipitation of this Laves phase and the toughness of cold-rolled and annealed steel sheets was investigated intensively.

[0026] As a result, it was found that when hot-rolled sheet annealing is omitted (i.e., hot-rolled sheet annealing is not performed) for steel with appropriate chemical composition, and cold rolling and cold-rolled sheet annealing are performed under appropriate conditions, the precipitation of coarse Laves phase during hot-rolled sheet annealing can be avoided, and at the same time, the grain coarsening can be suppressed and toughness can be improved by the grain boundary pinning effect caused by the precipitation of fine Laves phase during cold-rolled sheet annealing.

[0027] This is thought to be due to the following mechanism. By omitting hot-rolled sheet annealing, the precipitation of coarse Laves phases, which adversely affect toughness, is avoided during hot-rolled sheet annealing, and the amount of solute Nb in the steel can be secured in the cold-rolled steel sheet after cold rolling. This allows fine Laves phases to precipitate during cold-rolled sheet annealing. Furthermore, by performing cold rolling at an appropriate reduction, a large number of dislocations accumulate in the cold-rolled steel sheet after cold rolling. By annealing the cold-rolled sheet under appropriate conditions, using these large number of dislocations as precipitation sites, fine and dispersed Laves phases precipitate during cold-rolled sheet annealing. The pinning effect of the fine and dispersed precipitated Laves phases suppresses the coarsening of crystal grains in the cold-rolled and annealed steel sheet, improving toughness.

[0028] In contrast, when a cold-rolled and annealed stainless steel sheet was produced from a steel containing the components of the present invention by a conventional manufacturing process for the cold-rolled and annealed stainless steel sheet, i.e., by hot-rolling, cold-rolling, followed by hot-rolled sheet annealing, and cold-rolled sheet annealing, respectively, the cold-rolled and annealed steel sheet did not have good toughness.

[0029] This is thought to be due to the following reasons. When hot-rolled sheet annealing is performed, most of the Nb contained in the steel precipitates as Laves phases. However, because dynamic recovery occurs during hot rolling and dislocations are not accumulated in the hot-rolled steel sheet, these Laves phases precipitate coarsely. These coarse Laves phases do not dissolve in the steel even during cold-rolled sheet annealing, remaining coarse, reducing the toughness of the cold-rolled and annealed steel sheet. Furthermore, because the precipitated Laves phases are coarse, they do not exert a pinning effect on the grain boundaries, making it impossible to suppress coarsening of crystal grains during cold-rolled sheet annealing. Furthermore, because the amount of solute Nb in the steel decreases in the cold-rolled steel sheet after cold rolling, the amount of fine Laves phases newly precipitated during cold-rolled sheet annealing also decreases. Therefore, hot-rolled sheet annealing easily coarsens the crystal grains of the cold-rolled and annealed steel sheet, leading to a decrease in toughness.

[0030] Based on the above, the inventors discovered that in the manufacturing process of ferritic stainless cold-rolled and annealed steel sheet containing 0.30 mass% or more of Nb, it is important to omit the hot-rolled sheet annealing after hot rolling, to cold-roll with a total reduction of 40% or more, and to further perform cold-rolled sheet annealing at an annealing temperature of 950 to 1080°C for an annealing time of 5 seconds to 10 minutes as a post-cold-rolling step. The inventors also discovered that manufacturing using these steps results in a cold-rolled and annealed steel sheet with excellent oxidation resistance, high-temperature fatigue resistance, and excellent toughness.

[0031] The present invention was completed based on the above findings and further investigations.

[0032] That is, the gist and configuration of the present invention are as follows. [1] In mass%, C: 0.020% or less, Si: 0.05 to 1.00%, Mn: 0.05 to 1.00%, P: 0.050% or less, S: 0.010% or less, Al: 1.00~3.00%, N: 0.020% or less, Cr: 15.0~22.0%, Nb: 0.30 to 0.80% Mo: 1.00 to 3.00%, and Ni: 0.01 to 1.00% and the balance being Fe and unavoidable impurities, A ferritic stainless steel cold-rolled annealed steel sheet with a thickness of 1.8 mm or more and 2.8 mm or less, and satisfying DBTT≦20+20(t-1.8). Here, DBTT is the ductile-brittle transition temperature (°C) and t is the plate thickness (mm). [2] The component composition further comprises, in mass%, Co: 1.00% or less, Cu: 2.00% or less, and W: 2.00% or less The ferritic stainless steel cold-rolled annealed steel sheet according to [1], containing one or more selected from the following: [3] The component composition further comprises, in mass%, Ti: 0.20% or less, Zr: 0.50% or less, and V: 0.50% or less The ferritic stainless steel cold-rolled annealed sheet according to [1] or [2], containing one or more selected from the following: [4] The component composition further comprises, in mass%, Sn: 0.50% or less, Sb: 0.50% or less, B: 0.0050% or less, Ca: 0.0050% or less, and Mg: 0.0050% or less The ferritic stainless cold-rolled annealed steel sheet according to any one of [1] to [3], containing one or more selected from the following: [5] A cold-rolled steel sheet that is a raw material for the ferritic stainless steel cold-rolled annealed steel sheet according to any one of [1] to [4], A cold-rolled steel sheet having the above-mentioned composition and containing 0.15 mass % or less of precipitated Nb. [6] A method for producing a ferritic stainless steel cold-rolled annealed steel sheet according to any one of [1] to [4], preparing a hot-rolled steel sheet having the above-mentioned composition; A step of cold rolling the hot-rolled steel sheet at a total reduction rate of 40% or more without hot-rolled sheet annealing to obtain a cold-rolled steel sheet; a step of subjecting the cold-rolled steel sheet to cold-rolled annealing under conditions of an annealing temperature of 950 to 1080°C and an annealing time of 5 seconds to 10 minutes to obtain a cold-rolled annealed steel sheet; A method for producing a ferritic stainless steel cold-rolled annealed steel sheet, comprising: [Effects of the Invention]

[0033] According to the present invention, it is possible to obtain a cold-rolled and annealed ferritic stainless steel sheet having excellent oxidation resistance and high-temperature fatigue resistance as well as excellent toughness.

[0034] Therefore, the cold-rolled annealed ferritic stainless steel sheet of the present invention can be suitably used for various complex-shaped automotive exhaust system components even when exhaust gas temperatures are high, and is of great industrial value. [Brief explanation of the drawings]

[0035] [Figure 1] FIG. 1 is a schematic diagram of a test piece used in a high-temperature fatigue test. DETAILED DESCRIPTION OF THE INVENTION

[0036] The present invention will be described based on the following embodiments.

[0037] First, the chemical composition of a ferritic stainless cold-rolled annealed steel sheet according to one embodiment of the present invention will be described. Hereinafter, "%" indicating the chemical composition is "% by mass."

[0038] C: 0.020% or less C is an element effective in increasing the strength of steel. However, if the C content exceeds 0.020%, toughness and formability are significantly reduced, so the C content is set to 0.020% or less. From the viewpoint of ensuring toughness and formability, the C content is preferably set to 0.010% or less. The C content is more preferably set to 0.008% or less. Furthermore, from the viewpoint of ensuring the strength required for an automobile exhaust system member, the C content is preferably set to 0.001% or more. The C content is more preferably set to 0.003% or more.

[0039] Si: 0.05 to 1.00% Si is an important element necessary for improving oxidation resistance. To ensure oxidation resistance in high-temperature exhaust gas, the Si content is set to 0.05% or more. The Si content is preferably 0.10% or more. On the other hand, if the Si content exceeds 1.00%, not only does toughness decrease but also oxide scale becomes more likely to peel off. Therefore, the Si content is set to 1.00% or less. The Si content is preferably 0.80% or less, more preferably 0.60% or less, and even more preferably 0.25% or less.

[0040] Mn: 0.05 to 1.00% Mn has the effect of increasing the resistance to exfoliation of oxide scale. To achieve this effect, the Mn content is set to 0.05% or more. The Mn content is preferably 0.10% or more, and more preferably 0.20% or more. On the other hand, if the Mn content exceeds 1.00%, oxide scale grows abnormally and oxidation resistance decreases. Therefore, the Mn content is set to 1.00% or less. The Mn content is preferably 0.80% or less, more preferably 0.60% or less, and even more preferably 0.25% or less.

[0041] P:0.050% or less P is a harmful element that reduces the toughness of steel. Therefore, it is desirable to reduce the P content as much as possible. Therefore, the P content is set to 0.050% or less. The P content is preferably 0.030% or less. There is no particular lower limit for the P content. However, since excessive dephosphorization increases costs, the P content is preferably 0.005% or more.

[0042] S: 0.010% or less Sulfur reduces the toughness, elongation, and r-value of steel, adversely affecting formability, and is also a harmful element that reduces corrosion resistance, a basic property of stainless steel. Therefore, it is desirable to reduce the S content as much as possible. Therefore, the S content is set to 0.010% or less. The S content is preferably 0.005% or less. There is no particular lower limit for the S content. However, because excessive desulfurization increases costs, the S content is preferably 0.0005% or more.

[0043] Al: 1.00-3.00% Al is an element necessary for suppressing high-temperature deformation through solid-solution strengthening and improving high-temperature fatigue resistance. The higher the operating temperature, the more the high-temperature fatigue resistance of steel tends to decrease due to high-temperature deformation. Therefore, Al is an important factor in the trend toward higher exhaust gas temperatures. Al also has the effect of forming a protective continuous oxide scale at high temperatures, thereby improving the oxidation resistance of steel. Furthermore, when Mo is added to steel, Al also has the effect of suppressing the precipitation of a second phase (σ phase) containing Mo. The precipitation of the second phase (σ phase) reduces the amount of solute Mo, thereby preventing the solid-solution strengthening effect of Mo. Furthermore, the second phase coarsens in a short time, becoming the starting point for crack initiation and degrading high-temperature fatigue resistance. To achieve these effects, the Al content is set to 1.00% or more. The Al content is preferably 1.50% or more, more preferably 2.00% or more. On the other hand, an Al content exceeding 3.00% significantly hardens the steel and reduces its toughness. Therefore, the Al content is set to 3.00% or less, preferably 2.80% or less, and more preferably 2.50% or less.

[0044] N: 0.020% or less N is an element that reduces the toughness and formability of steel. Therefore, it is desirable to reduce the N content as much as possible. Therefore, the N content is set to 0.020% or less. From the viewpoint of ensuring toughness and formability, the N content is preferably set to 0.015% or less. There is no particular lower limit for the N content. However, since excessive denitrification increases costs, the N content is preferably 0.001% or more.

[0045] Cr: 15.0 to 22.0% Cr is an element necessary to ensure corrosion resistance, a basic characteristic of stainless steel, and is also effective in improving oxidation resistance. If the Cr content is less than 15.0%, sufficient oxidation resistance cannot be obtained. Furthermore, an increase in oxide scale makes cracks more likely to occur, which reduces high-temperature fatigue resistance. Therefore, the Cr content is set to 15.0% or more. The Cr content is preferably 16.0% or more, and more preferably 17.0% or more. On the other hand, Cr hardens steel at room temperature through solid solution strengthening, which can reduce toughness. In particular, if the Cr content exceeds 22.0%, this problem becomes more pronounced. Therefore, the Cr content is set to 22.0% or less. The Cr content is preferably 21.0% or less, and more preferably 20.0% or less.

[0046] Nb: 0.30 to 0.80% Nb forms carbonitrides with C and N and fixes them, making it an effective element for improving corrosion resistance, formability, and intergranular corrosion resistance of welds. Nb also increases high-temperature strength through solid-solution strengthening and improves high-temperature fatigue resistance. Furthermore, Nb precipitates finely as a Laves phase having a composition of FeNb during cold-rolled sheet annealing. The grain boundary pinning effect of this Laves phase inhibits grain coarsening in cold-rolled and annealed steel sheets, thereby improving toughness. This effect can be achieved by setting the Nb content to 0.30% or more. If the Nb content is less than 0.30%, the toughness improvement effect due to the pinning effect of the fine Laves phase cannot be fully achieved. Therefore, the Nb content is set to 0.30% or more. The Nb content is preferably 0.35% or more, and more preferably 0.40% or more. However, if the Nb content exceeds 0.80%, the Laves phase precipitates coarsely, even when using the manufacturing method of the present invention, leading to embrittlement of the steel. Therefore, the Nb content is set to 0.80% or less, preferably 0.70% or less, and more preferably 0.60% or less.

[0047] Mo: 1.00-3.00% Mo is an element that improves the high-temperature strength of steel through solid-solution strengthening and improves high-temperature fatigue resistance. Such effects can be achieved by setting the Mo content to 1.00% or more. Furthermore, if the Mo content is less than 1.00%, the high-temperature strength becomes insufficient and excellent high-temperature fatigue resistance cannot be obtained. Therefore, the Mo content is set to 1.00% or more. The Mo content is preferably 1.50% or more, more preferably 1.80% or more, and even more preferably 2.20% or more. On the other hand, if the Mo content exceeds 3.00%, the steel becomes hardened and the toughness decreases. Furthermore, coarse intermetallic compounds such as σ phases are likely to form, which actually reduces the high-temperature fatigue resistance. Therefore, the Mo content is set to 3.00% or less. The Mo content is preferably 2.80% or less, more preferably 2.50% or less.

[0048] Ni: 0.01 to 1.00% Ni is an element that improves the toughness of steel. To achieve this effect, the Ni content is set to 0.01% or more. The Ni content is preferably 0.05% or more, and more preferably 0.10% or more. However, Ni is a strong γ-phase forming element, and generates γ-phase at high temperatures, which reduces oxidation resistance. Therefore, the Ni content is set to 1.00% or less. The Ni content is preferably 0.80% or less, and more preferably 0.50% or less.

[0049] The basic components (essential components) of a ferritic stainless steel sheet according to one embodiment of the present invention have been described above. The balance other than the basic components can be Fe and unavoidable impurities.

[0050] The chemical composition of the cold-rolled annealed ferritic stainless steel sheet according to one embodiment of the present invention may further contain one or more elements selected from the following groups A to C.

[0051] (Group A) One or more selected from Co: 1.00% or less, Cu: 2.00% or less, and W: 2.00% or less

[0052] Co: 1.00% or less Co is an element effective in improving the toughness of steel. To achieve this effect, the Co content is preferably 0.01% or more. Therefore, when Co is contained, the Co content is preferably 0.01% or more. On the other hand, if Co is contained in excess, the toughness of the steel decreases. Therefore, when Co is contained, the Co content is set to 1.00% or less. When Co is contained, the Co content is preferably 0.30% or less, and more preferably 0.10% or less.

[0053] Cu:2.00% or less Cu is an element that has the effect of improving the corrosion resistance of steel. Cu is also an element that has the effect of improving high-temperature strength at around 600°C through precipitation strengthening. To achieve this effect, the Cu content is preferably 0.01% or more. Therefore, when Cu is contained, the Cu content is preferably 0.01% or more. When Cu is contained, the Cu content is more preferably 0.30% or more, and even more preferably 1.00% or more. On the other hand, when the Cu content exceeds 2.00%, oxide scale becomes more likely to peel off, and cyclic oxidation resistance deteriorates. Therefore, when Cu is contained, the Cu content is set to 2.00% or less. When Cu is contained, the Cu content is preferably 1.50% or less, and more preferably 1.35% or less.

[0054] W: 2.00% or less Like Mo, W is an element that improves the high-temperature strength of steel through solid solution strengthening and improves high-temperature fatigue resistance. To achieve this effect, the W content is preferably 0.01% or more. Therefore, when W is contained, the W content is preferably 0.01% or more. When W is contained, the W content is more preferably 0.10% or more, and even more preferably 1.00% or more. On the other hand, if W is contained in an excessive amount, not only does the steel harden, but strong scale is formed in the annealing process, making descaling during pickling difficult. Therefore, when W is contained, the W content is set to 2.00% or less. When W is contained, the W content is preferably 1.80% or less, and more preferably 1.50% or less.

[0055] (Group B) One or more selected from Ti: 0.20% or less, Zr: 0.50% or less, and V: 0.50% or less

[0056] Ti: 0.20% or less Like Nb, Ti forms carbonitrides with C and N and fixes them, making it an effective element for improving corrosion resistance, formability, and intergranular corrosion resistance of welds. Furthermore, the inclusion of Ti allows Ti to bond preferentially with C and N over Nb. This ensures a solid-solution Nb content in the steel that is effective for high-temperature strength, effectively improving high-temperature fatigue resistance. Furthermore, Ti is also effective for improving oxidation resistance. To achieve these effects, the Ti content is preferably 0.01% or more. Therefore, when Ti is contained, the Ti content is preferably 0.01% or more. When Ti is contained, the Ti content is more preferably 0.05% or more. On the other hand, when the Ti content exceeds 0.20%, coarse Ti nitrides precipitate, reducing toughness. Therefore, when Ti is contained, the Ti content is limited to 0.20% or less. When Ti is contained, the Ti content is preferably 0.15% or less, more preferably 0.12% or less.

[0057] Zr: 0.50% or less Zr is an element effective in improving oxidation resistance. To achieve this effect, the Zr content is preferably 0.01% or more. Therefore, when Zr is contained, the Zr content is preferably 0.01% or more. However, when the Zr content exceeds 0.50%, coarse Zr carbonitrides precipitate, reducing toughness. Therefore, when Zr is contained, the Zr content is set to 0.50% or less.

[0058] V: 0.50% or less V is an element that improves the toughness and oxidation resistance of steel. To achieve this effect, the V content is preferably 0.01% or more. Therefore, when V is contained, the V content is preferably 0.01% or more. When V is contained, the V content is preferably 0.03% or more, and more preferably 0.05% or more. However, when the V content exceeds 0.50%, coarse V carbonitrides precipitate, reducing toughness. Furthermore, the surface properties are also reduced. Therefore, when V is contained, the V content is set to 0.50% or less. When V is contained, the V content is preferably 0.30% or less, and more preferably 0.20% or less.

[0059] (Group C) One or more selected from Sn: 0.50% or less, Sb: 0.50% or less, B: 0.0050% or less, Ca: 0.0050% or less, and Mg: 0.0050% or less

[0060] Sn: 0.50% or less Sn is an element that has the effect of improving the corrosion resistance of steel. Sn is also an element that has the effect of improving high-temperature strength. In order to obtain such an effect, the Sn content is preferably 0.01% or more. Therefore, when Sn is contained, the Sn content is preferably 0.01% or more. When Sn is contained, the Sn content is preferably 0.03% or more. On the other hand, if Sn is contained in excess, the toughness of the steel decreases. Therefore, when Sn is contained, the Sn content is set to 0.50% or less. The Sn content is preferably 0.30% or less, and more preferably 0.20% or less.

[0061] Sb: 0.50% or less Sb is an element effective in improving the toughness of steel. To achieve this effect, the Sb content is preferably 0.01% or more. Therefore, when Sb is contained, the Sb content is preferably 0.01% or more. When Sb is contained, the Sb content is preferably 0.03% or more. On the other hand, if Sb is contained in excess, the toughness decreases. Therefore, when Sb is contained, the Sb content is set to 0.50% or less. When Sb is contained, the Sb content is preferably 0.30% or less, and more preferably 0.10% or less.

[0062] B: 0.0050% or less B is an element effective in improving the workability of steel, particularly its resistance to secondary work embrittlement. To achieve this effect, the B content is preferably 0.0002% or more. Therefore, when B is contained, the B content is preferably 0.0002% or more. When B is contained, the B content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. On the other hand, excessive B content leads to the formation of BN, resulting in a decrease in toughness. Therefore, when B is contained, the B content is set to 0.0050% or less. When B is contained, the B content is preferably 0.0030% or less, and more preferably 0.0020% or less.

[0063] Ca: 0.0050% or less Ca is an element effective in preventing nozzle clogging due to the precipitation of inclusions that are likely to occur during continuous casting. To achieve this effect, the Ca content is preferably 0.0002% or more. Therefore, when Ca is contained, the Ca content is preferably 0.0002% or more. When Ca is contained, the Ca content is more preferably 0.0005% or more. On the other hand, excessive Ca content causes surface defects and adversely affects surface properties. Therefore, when Ca is contained, the Ca content is 0.0050% or less. When Ca is contained, the Ca content is preferably 0.0030% or less, more preferably 0.0020% or less.

[0064] Mg: 0.0050% or less Mg is an element that improves the equiaxed crystal ratio of a slab and improves toughness. Furthermore, Mg also has the effect of suppressing coarsening of Nb and Ti carbonitrides. Coarsening of Nb carbonitrides reduces the amount of solute Nb in the steel, which may result in a deterioration of high-temperature fatigue resistance. Coarsening of Ti carbonitrides may serve as the initiation point for brittle cracking, resulting in a decrease in toughness. To achieve these effects, the Mg content is preferably 0.0002% or more. Therefore, when Mg is contained, the Mg content is preferably 0.0002% or more. When Mg is contained, the Mg content is more preferably 0.0004% or more. On the other hand, when the Mg content exceeds 0.0050%, the surface quality of the steel deteriorates. Therefore, when Mg is contained, the Mg content is set to 0.0050% or less. When Mg is contained, the Mg content is preferably 0.0030% or less, more preferably 0.0015% or less.

[0065] When the content of any of the above-mentioned optional added elements is less than the preferable lower limit, the optional added element is considered to be contained as an unavoidable impurity.

[0066] Next, a cold-rolled steel sheet that is a raw material for the cold-rolled annealed ferritic stainless steel sheet according to one embodiment of the present invention will be described.

[0067] The cold-rolled steel sheet according to one embodiment of the present invention has the same composition as that of the cold-rolled annealed ferritic stainless steel sheet described above. The ranges of the contents of the elements in the cold-rolled steel sheet and the reasons for limiting them are the same as those described above, and therefore will not be described here. Furthermore, in the cold-rolled steel sheet according to one embodiment of the present invention, it is important that the amount of precipitated Nb is 0.15 mass % or less.

[0068] Precipitated Nb content in cold-rolled steel sheet: 0.15% by mass or less As described above, in order to obtain excellent toughness, it is important to avoid the precipitation of coarse Laves phases during hot-rolled sheet annealing and to keep the amount of precipitated Nb in the cold-rolled steel sheet at 0.15% by mass or less. The low amount of coarse Laves phases in the cold-rolled steel sheet allows a large amount of fine Laves phases to precipitate during cold-rolled sheet annealing, suppressing the coarsening of crystal grains during cold-rolled sheet annealing, thereby obtaining excellent toughness in the cold-rolled and annealed steel sheet. Therefore, the amount of precipitated Nb in the cold-rolled steel sheet is set to 0.15% by mass or less. The amount of precipitated Nb in the cold-rolled steel sheet is preferably set to 0.10% by mass or less. The lower limit of the amount of precipitated Nb in the cold-rolled steel sheet is not particularly limited. However, it is difficult to avoid the occurrence of precipitated Nb due to the precipitation of Laves phases during steelmaking, hot rolling, etc., and excessive reduction of the amount of precipitated Nb increases costs. Therefore, the amount of precipitated Nb in the cold-rolled steel sheet is preferably 0.05% by mass or more.

[0069] Here, the amount of precipitated Nb in the cold-rolled steel sheet is determined as follows. Test specimens measuring 10 mm long and 50 mm wide were cut from the cold-rolled steel sheets used as test materials. Precipitates were then extracted by constant-current electrolysis using 10 vol% acetylacetone-1 mass% tetramethylammonium chloride-methanol. A cellulose acetate membrane filter (pore size 0.2 μm, 47 mm diameter) was used to filter the extracted residue. The collected residue, along with the filter, was placed in a platinum crucible and incinerated at 580°C. 0.75 g Na2O2 + 0.75 g LiBO4 was added and melted using a gas burner to form a melt. The melt was dissolved in 25 mL of 0.8 mass% tartaric acid / 10 vol% sulfuric acid, and the volume was adjusted to 100 mL with pure water. ICP atomic emission spectroscopy was used to evaluate the amount of precipitated Nb (mass%) in the steel.

[0070] Next, a method for producing a cold-rolled annealed ferritic stainless steel sheet according to one embodiment of the present invention will be described.

[0071] A method for producing a ferritic stainless steel cold-rolled annealed steel sheet according to one embodiment of the present invention comprises the steps of: preparing a hot-rolled steel sheet having the above-described chemical composition (hot-rolled steel sheet preparation step); cold-rolling the hot-rolled steel sheet at a total reduction of 40% or more without hot-rolled sheet annealing (without hot-rolled sheet annealing) to obtain a cold-rolled steel sheet (cold-rolling step); and cold-rolling the cold-rolled steel sheet at an annealing temperature of 950 to 1080°C for an annealing time of 5 seconds to 10 minutes to obtain a cold-rolled annealed steel sheet (cold-rolled sheet annealing step). In the following description of the manufacturing method, the temperature refers to the surface temperature of a slab, cold-rolled steel sheet, or the like, unless otherwise specified.

[0072] [Preparation process for hot-rolled steel sheets] In the hot-rolled steel sheet preparation step, a hot-rolled steel sheet having the above-described chemical composition is prepared. The method for preparing the hot-rolled steel sheet is not particularly limited, and conventional methods can be used. For example, a steel having the above-described chemical composition is melted through any secondary refining process in a known melting furnace, such as a converter or electric furnace, and then formed into a slab by continuous casting or an ingot-blooming process. Examples of secondary refining include ladle refining and vacuum refining, with the VOD (Vacuum Oxygen Decarburization) process being particularly preferred. Furthermore, from the viewpoints of productivity and quality, continuous casting is preferred. The slab is then heated to 1100 to 1250°C and hot-rolled to prepare a hot-rolled steel sheet having the above-described chemical composition. The hot-rolling conditions are not particularly limited, and conventional methods can be used. From the viewpoint of manufacturability, the thickness of the hot-rolled steel sheet to be prepared is preferably 3.0 mm or more and 5.0 mm or less.

[0073] Next, the hot-rolled steel sheet is optionally subjected to descaling by pickling or the like. Before pickling, the hot-rolled steel sheet may be subjected to shot blasting to remove scales.

[0074] Hot-rolled sheet annealing: omitted As described above, since coarse Laves phases precipitate during hot-rolled sheet annealing, the amount of precipitated Nb in the cold-rolled steel sheet does not reach 0.15 mass% or less, and the toughness of the cold-rolled and annealed steel sheet cannot be improved. Therefore, hot-rolled sheet annealing is omitted (hot-rolled sheet annealing is not performed).

[0075] [Cold rolling process] In the cold rolling step, the hot rolled steel sheet is subjected to cold rolling at a total reduction rate of 40% or more to obtain a cold rolled steel sheet.

[0076] Cold rolling total reduction: 40% or more As described above, cold rolling can introduce a large amount of dislocations, which has the effect of finely dispersing the Laves phase that precipitates during annealing of the cold-rolled sheet after cold rolling. In order to obtain excellent toughness in a cold-rolled annealed steel sheet, the cold rolling reduction must be 40% or more. Therefore, the total cold rolling reduction is set to 40% or more. The total cold rolling reduction is preferably 50% or more. On the other hand, although there is no particular upper limit for the total cold rolling reduction, the total cold rolling reduction is preferably 70% or less in order to prevent the cold rolling load from becoming excessive.

[0077] The conditions for the cold rolling step other than those mentioned above are not particularly limited and may be the same as those in the ordinary method, except that intermediate annealing is not performed in order to maintain the strain introduced by the cold rolling. Through the above steps, a cold-rolled steel sheet that is the raw material for the cold-rolled annealed steel sheet of the present invention is obtained.

[0078] [Cold-rolled sheet annealing process] Next, the cold-rolled steel sheet obtained in the above step is annealed under the conditions of an annealing temperature of 950 to 1080°C and an annealing time of 5 seconds to 10 minutes.

[0079] Annealing temperature: 950~1080℃ In this cold-rolled sheet annealing process, it is important to finely precipitate the Laves phase in the cold-rolled steel sheet. Fine precipitation of the Laves phase in the steel sheet and its grain boundary pinning effect can suppress coarsening of crystal grains. As a result, excellent toughness can be achieved in the obtained cold-rolled annealed steel sheet (product sheet). Here, if the annealing temperature is less than 950°C, coarse unrecrystallized grains may remain in the product sheet, and good toughness may not be obtained. Furthermore, if the annealing temperature is more than 1080°C, the Laves phase precipitated during the temperature rise will re-dissolve in the steel, the above-mentioned pinning effect will not be obtained, and the toughness will decrease due to coarsening of crystal grains. Therefore, the annealing temperature is set to a range of 950 to 1080°C. The annealing temperature is preferably 980°C or higher. Furthermore, the annealing temperature is preferably 1060°C or lower.

[0080] Annealing time: 5s to 10min As mentioned above, in this cold-rolled sheet annealing process, it is important to finely precipitate the Laves phase in the cold-rolled steel sheet. Here, if the annealing time is less than 5 seconds, coarse unrecrystallized grains remain in the steel sheet, and good toughness may not be obtained. Furthermore, if the annealing time exceeds 10 minutes, the Laves phase becomes coarse, which causes the crystal grains to become coarse and the toughness to deteriorate. Therefore, the annealing time is set to the range of 5 seconds to 10 minutes (5 seconds to 600 seconds). The annealing time is preferably 20 seconds or more. Furthermore, the annealing time is preferably 5 minutes or less.

[0081] The annealing temperature is the maximum temperature reached during annealing of the cold-rolled sheet. The annealing time is the holding time in the temperature range from (annealing temperature -10°C) to the annealing temperature, and the temperature does not have to be constant during the holding time.

[0082] After the above-mentioned cold-rolled sheet annealing, the sheet is optionally subjected to pickling or the like to be descaled.

[0083] The cold-rolled and annealed steel sheet thus obtained is then subjected to processes such as cutting, bending, pressing, stretching, drawing, etc. depending on the intended use, and is formed into automobile or motorcycle exhaust pipes, catalyst outer casing materials, etc. The method for welding these members is not particularly limited, and applicable methods include ordinary arc welding such as MIG (Metal Inert Gas), MAG (Metal Active Gas), and TIG (Tungsten Inert Gas), resistance welding such as spot welding and seam welding, high-frequency resistance welding such as electric resistance welding, and high-frequency induction welding.

[0084] The manufacturing conditions other than those mentioned above are not particularly limited, and may be those according to conventional methods.

[0085] The thickness of the cold-rolled annealed ferritic stainless steel sheet of the present invention is 1.8 mm or more and 2.8 mm or less, and preferably 2.5 mm or less.

[0086] The properties (oxidation resistance, high-temperature fatigue resistance, and toughness) of the cold-rolled annealed ferritic stainless steel sheet of the present invention are as described above. [Example]

[0087] Steel having the chemical composition shown in Table 1 (the balance being Fe and unavoidable impurities) was melted in a vacuum melting furnace and cast into a 50 kg steel ingot. The 50 kg steel ingot was then forged. This steel ingot was heated to 1170°C and then hot-rolled to prepare hot-rolled steel sheets. The thickness of each hot-rolled steel sheet is shown in Table 2. For comparison, Nos. 18 and 19 were hot-rolled under the conditions shown in Table 2.

[0088] The front and back surfaces of these hot-rolled steel sheets and hot-rolled annealed steel sheets were polished and descaled. Next, the hot-rolled steel sheets (except Nos. 18 and 19 which were hot-rolled annealed steel sheets) were cold-rolled to obtain cold-rolled steel sheets. The total cold-rolling reduction and the thickness of each cold-rolled steel sheet are shown in Table 2. The amounts of precipitated Nb were measured for the cold-rolled steel sheets thus obtained by the method described above. The results are shown in Table 2. Next, the cold-rolled steel sheets were subjected to cold-rolled annealing under the conditions shown in Table 2, and then the steel sheet surfaces were polished and descaled to obtain cold-rolled annealed steel sheets as the final products.

[0089] The cold-rolled and annealed steel sheets were evaluated for (1) oxidation resistance, (2) high-temperature fatigue resistance, and (3) toughness according to the following test methods. The results are also shown in Table 2.

[0090] (1) Oxidation resistance <Evaluation test for continuous oxidation resistance> A 30 mm x 20 mm test piece was cut out from the cold-rolled annealed steel sheet obtained as described above, and a 4 mm diameter hole was drilled in the top of the test piece. The surface and end faces of the test piece were then polished with #320 emery paper and degreased. The test piece was then suspended in a furnace heated to 1100°C and held in an air atmosphere, and held in this state for 200 hours. After holding, the oxidation weight gain (g / m) of the test piece was calculated using the following formula: 2 ) was calculated. [Oxidation gain (g / m 2 )] = [Increase in mass of test piece before and after holding (g)] ÷ [Surface area of test piece (m 2 )] Here, the mass increase (g) of the test specimen before and after holding was calculated by subtracting the mass of the test specimen before holding from the mass of the test specimen after holding, excluding the mass of the spalled oxide scale. The test was carried out twice for each cold-rolled and annealed steel sheet. The presence or absence of oxide scale spalling was visually confirmed for the test pieces from both tests, and if oxide scale spalling was observed in at least one test, it was deemed to have occurred. Furthermore, for the presence or absence of abnormal oxidation, the oxidation weight gain was 50 g / m2 in both tests. 2 Less than (oxidation gain <50g / m 2) in the case of no abnormal oxidation, and the oxidation gain is 50g / m in at least one test. 2 or more (oxidation gain ≧ 50g / m 2 ) was judged to have abnormal oxidation. The continuous oxidation resistance was evaluated according to the following criteria. 〇 (Pass): No abnormal oxidation and no peeling of oxide scale △ (Fail): No abnormal oxidation and oxide scale peeling × (Fail): Abnormal oxidation

[0091] <Evaluation test for cyclic oxidation resistance> Test pieces were prepared in the same manner as in the evaluation test for continuous oxidation resistance. Then, the test pieces were suspended in a furnace in an air atmosphere, and the following cycle consisting of (a) to (d) was repeated 400 times. (a) 200°C for 1 minute (b) Temperature increase from 200°C to 1100°C (average temperature increase rate 30°C / min) (c) 1100℃ for 20 minutes (d) Cooling from 1100°C to 200°C (average cooling rate 90°C / min) After repeating the cycle consisting of (a) to (d) above 400 times, the presence or absence of oxide scale spalling and abnormal oxidation were determined in the same manner as in the evaluation test for continuous oxidation resistance.Then, the repeated oxidation resistance was evaluated according to the following criteria. 〇 (Pass): No abnormal oxidation and no peeling of oxide scale △ (Fail): No abnormal oxidation and oxide scale peeling × (Fail): Abnormal oxidation

[0092] (2) High-temperature fatigue resistance <High temperature fatigue test> Test specimens with the shape shown in Figure 1 were machined from the cold-rolled and annealed steel sheets obtained as described above. The test was conducted at 950°C, and the test was started after heating to 950°C and holding for 30 minutes. A bending stress of 50 MPa in amplitude was repeatedly applied to the test specimens at a stress ratio of -1, and the number of bending cycles until fracture was measured. The rotation speed was 1300 rpm (22 Hz). The number of bending cycles until breakage (number of breakage cycles) was measured and evaluated according to the following criteria. ○ (Pass): 1.0 x 10 6 More than a cycle ×(Fail):1.0×10 6 Less than a cycle

[0093] (3) Toughness <Charpy impact test> Charpy test specimens were prepared from the cold-rolled annealed steel sheets obtained as described above by machining. They were 55 mm long, 10 mm wide, and 10 mm thick, with a 2 mm deep V-notch cut in the longitudinal center. The longitudinal direction of the Charpy test specimens was aligned with the rolling direction. Using these specimens, Charpy impact tests were conducted in accordance with JIS Z2242:2018 at temperatures ranging from -50 to 50°C with temperature intervals of 10°C. After fracture, the brittle fracture surface ratio of the fractured specimens was measured. Three tests were conducted at each test temperature, and the average brittle fracture surface ratio was calculated. The temperature at which the brittle fracture surface ratio reached 50% was determined from a curve connecting the average brittle fracture surface ratios for each test temperature. This temperature was designated the DBTT (ductile-brittle transition temperature (°C)). The DBTT was then evaluated according to the following criteria. ◎ (Pass, Excellent): 20+20(t-1.8)-DBTT≧10 ○(Pass):20+20(t-1.8)-DBTT≧0 ×(Fail):20+20(t-1.8)-DBTT<0

[0094] [Table 1]

[0095] [Table 2]

[0096] As shown in Table 2, all of the inventive examples had excellent oxidation resistance and high-temperature fatigue resistance, as well as excellent toughness.

[0097] On the other hand, in the comparative examples, at least one of oxidation resistance, high-temperature fatigue resistance, and toughness was not sufficiently obtained.

[0098] That is, No. 14 had an Nb content below the appropriate range, and failed in high-temperature fatigue resistance and toughness. No. 15 had a Cr content below the appropriate range, and failed in oxidation resistance (continuous oxidation resistance and cyclic oxidation resistance) and high-temperature fatigue resistance. No. 16 had a Mo content below the appropriate range and failed to pass the high-temperature fatigue resistance test. No. 17 had an Al content below the appropriate range, and failed in oxidation resistance (continuous oxidation resistance and cyclic oxidation resistance) and high-temperature fatigue resistance. In Nos. 18 and 19, the amount of precipitated Nb in the cold-rolled steel sheet exceeded the appropriate range due to the hot-rolled sheet annealing, and the toughness was unacceptable. Nos. 20 to 22 failed in toughness because the cold-rolled sheet annealing conditions were outside the appropriate range. No. 23 failed in toughness because the total reduction rate in cold rolling was outside the appropriate range. [Industrial Applicability]

[0099] The cold-rolled annealed ferritic stainless steel sheet according to one embodiment of the present invention is suitable for use as an exhaust system member for automobiles, etc. The cold-rolled annealed ferritic stainless steel sheet according to one embodiment of the present invention can also be suitably used as an exhaust system member for thermal power generation systems and as a member for solid oxide fuel cells (separators, interconnectors, reformers, etc.).

Claims

1. In mass%, C: 0.020% or less, Si: 0.05-1.00%, Mn: 0.05-1.00%, P: 0.050% or less, S: 0.010% or less, Al: 1.00-3.00%, N: 0.020% or less, Cr: 16.0-22.0%, Nb: 0.30-0.80%, Mo: 1.00 to 3.00%, and Ni: 0.01-1.00% and the balance being Fe and unavoidable impurities, A ferritic stainless steel cold-rolled annealed steel sheet having a thickness of 1.8 mm or more and 2.8 mm or less and satisfying DBTT≦20+20(t−1.8). Here, DBTT is the ductile-brittle transition temperature (°C), and t is the plate thickness (mm).

2. The ferritic stainless cold-rolled annealed steel sheet according to claim 1, wherein the component composition further contains, in mass%, one or more selected from the following groups A to C: Group A: One or two selected from Cu: 2.00% or less and W: 0.35% or less Group B: one or two selected from Ti: 0.15% or less and Zr: 0.05% or less Group C: One or more selected from Sn: 0.06% or less, Sb: 0.14% or less, B: 0.0050% or less, Ca: 0.0050% or less, and Mg: 0.0050% or less

3. A cold-rolled steel sheet that is a raw material for the ferritic stainless steel cold-rolled annealed steel sheet according to claim 1 or 2, A cold-rolled steel sheet having the above-described composition and having a precipitated Nb content of 0.15 mass% or less.

4. In mass %, C: 0.020% or less, Si: 0.05-1.00%, Mn: 0.05-1.00%, P: 0.050% or less, S: 0.010% or less, Al: 1.00-3.00%, N: 0.020% or less, Cr: 15.0-22.0%, Nb: 0.30-0.80%, Mo: 1.00 to 3.00%, and Ni: 0.01~1.00% Contains Further, optionally, Contains one or more selected from the following groups A to C: Group A: one or more selected from Co: 0.10% or less, Cu: 2.00% or less, and W: 0.35% or less Group B: one or more selected from Ti: 0.15% or less, Zr: 0.05% or less, and V: 0.20% or less Group C: One or more selected from Sn: 0.06% or less, Sb: 0.14% or less, B: 0.0050% or less, Ca: 0.0050% or less, and Mg: 0.0050% or less The balance has a composition consisting of Fe and unavoidable impurities, A method for producing a ferritic stainless steel cold-rolled annealed steel sheet having a sheet thickness of 1.8 mm or more and 2.8 mm or less and satisfying DBTT≦20+20(t−1.8), where DBTT is the ductile-brittle transition temperature (°C), and t is the sheet thickness (mm), comprising: preparing a hot-rolled steel sheet having the above-mentioned composition; A step of cold rolling the hot-rolled steel sheet at a total reduction rate of 40% or more without hot-rolled sheet annealing to obtain a cold-rolled steel sheet; A step of subjecting the cold-rolled steel sheet to cold-rolled annealing under conditions of an annealing temperature of 950 to 1080°C and an annealing time of 5 seconds to 10 minutes to obtain a cold-rolled annealed steel sheet; A method for producing a ferritic stainless steel cold-rolled annealed steel sheet, comprising:

Images