Ferrite system stainless steel sheet and method for manufacturing ferrite system stainless steel sheet

Abstract

The ferritic stainless steel sheet contains, in mass%, C: more than 0% and 0.008% or less, Si: 0.01 to 2.50%, Mn: 0.01 to 0.50%, P: 0.0001 to 0.040%, S: 0.001 to 0.010%, Al: 0.001 to 2.500%, Cr: 10.0 to 25.0%, Nb: more than 0% and 0.80% or less, Ti: 0.05 to 0.50%, V: 0.01 to 0.15%, N: more than 0% and 0.050% or less, with the remainder including Fe and impurities, a composition parameter PS of a passive film represented by formula (1) is 0.03 to 0.15, and a surface coverage of Al-containing oxides having an equivalent circle diameter of 0.01 to 1.0 μm in the passive film is 15 to 40%. PS = Al / (Ti + Mn + Si + Cr) Formula (1). In formula (1), the element symbols are the contents (mass%) of each element at a depth position where the Al amount is the largest in the depth direction of the passive film.

Description

Technical Field

[0001] This invention relates to ferritic stainless steel sheets and a method for manufacturing ferritic stainless steel sheets, which are suitable for use as materials in devices that use urea solution to reduce NOx in exhaust gases, particularly automotive urea SCR (Selective Catalytic Reduction) systems in automotive exhaust gas treatment systems.

[0002] This application claims priority based on Japanese Patent Application No. 2021-151019 filed on September 16, 2021, the contents of which are incorporated herein by reference. Background Technology

[0003] In countermeasures against environmental problems such as global warming, regulations on exhaust gases emitted primarily from transportation equipment have been strengthened, with efforts underway to reduce carbon dioxide emissions. In the automotive sector, in addition to fuel-related efforts, measures include lightweighting the vehicle body, reusing exhaust heat to improve fuel efficiency, and incorporating exhaust gas recirculation (EGR), diesel particulate filters (DPF), and exhaust gas treatment systems such as urea-based selective reflux (SCR).

[0004] The urea-based SCR system is a purification device integrated into a series of exhaust systems: high-temperature exhaust gases from the engine pass through the exhaust manifold, catalytic converter, and are released into the atmosphere through the muffler. In the urea-based SCR system, urea is blown into the exhaust gases at approximately 500°C, where it decomposes through heat and moisture to produce ammonia. The ammonia then undergoes selective catalytic reduction of NOx to decompose it into harmless nitrogen. Since NOx reduction systems are relatively easy to implement, applications in automotive and stationary NOx treatment systems have been investigated.

[0005] In urea SCR systems, high-temperature exhaust gases pass through the system, thus requiring excellent oxidation resistance. It should be noted that, unlike the oxidation resistance required for normal atmospheric oxidation, the requirement in urea SCR systems is oxidation resistance in an atmosphere of water vapor purging urea solution.

[0006] In the case of general ferritic stainless steel requiring oxidation resistance, even after prolonged heating in an atmosphere at around 500°C to form a dense protective film mainly composed of Cr, there is almost no increase in oxidation or peeling of the oxide film.

[0007] However, even the aforementioned general ferritic stainless steels will oxidize more rapidly and experience oxide film peeling in the steam atmosphere present in areas where the high-temperature exhaust gas of a urea SCR system passes. Therefore, when using the aforementioned general ferritic stainless steel as a material in a urea SCR system, it can sometimes damage the catalyst in the urea SCR system.

[0008] Patent Document 1 discloses a ferritic stainless steel for use in urea SCR components. This steel not only satisfies certain steel composition requirements but also, as a component of the passivation film, satisfies a specified formula for the concentration ratio of Cr, Si, Al, Ti, Mn, and Fe within 20 nm of the surface. Patent Document 1 also investigated not only corrosion resistance in urea aqueous solutions but also oxidation resistance in a water vapor atmosphere.

[0009] As mentioned above, corrosion resistance and oxidation resistance in the urea environment are important for materials used in urea SCR systems. Regarding oxidation resistance, in addition to the composition of the raw materials, controlling the composition of the passivation film is also crucial. In Patent Document 1, a passivation film with a specified composition was obtained by a final pickling treatment of annealed cold-rolled steel sheets. That is, the pickling process removes the oxide scale generated in the preceding annealing process, thereby forming a passivation film. However, the pickling process is demanding, and excessive matrix dissolution caused by pickling can lead to reduced yield and uneven surface gloss, thus requiring further improvements in manufacturing cost and quality.

[0010] On the other hand, Patent Documents 2 and 3 disclose a ferritic stainless steel that improves weather resistance and corrosion resistance by controlling the passivation film during the annealing or pickling process.

[0011] However, in Patent Document 2, a passivation film is formed during pickling, raising concerns about the high load of the pickling process, similar to the case in Patent Document 1.

[0012] Furthermore, Patent Document 3 discloses a technology that improves the corrosion resistance of heat exchangers by forming a film at 900–1200°C in a special atmosphere. However, there are concerns about increased manufacturing costs due to the need for high-temperature processing.

[0013] Furthermore, of course, no research was conducted on water vapor oxidation resistance in the aforementioned patent document 3, leaving room for improvement in water vapor oxidation resistance.

[0014] As mentioned above, the stainless steel plates used in urea SCR systems are exposed to high-temperature exhaust gas atmospheres in a water vapor atmosphere, thus requiring excellent resistance to water vapor oxidation.

[0015] However, existing materials with suitable resistance to water vapor oxidation, as materials for urea SCR systems, involve a heavy pickling process, which is insufficient from the perspective of stable manufacturability and manufacturing cost. Therefore, there is a need for ferritic stainless steel sheets for urea SCR that meet the requirements of water vapor oxidation resistance and low cost.

[0016] Existing technical documents

[0017] Patent documents

[0018] Patent Document 1: Japanese Patent Application Publication No. 2012-112025

[0019] Patent Document 2: Japanese Patent Application Publication No. 05-271880

[0020] Patent Document 3: Japanese Patent Application Publication No. 2016-023341 Summary of the Invention

[0021] The problem that the invention aims to solve

[0022] The present invention is based on the above-mentioned facts, and the objective is to provide a ferritic stainless steel sheet with excellent resistance to water vapor oxidation and a method for manufacturing the same.

[0023] means for solving problems

[0024] The inventors of this invention conducted in-depth research to solve the aforementioned problems. They realized that, in order to exhibit excellent resistance to water vapor oxidation in an environment with urea-water blowing at approximately 500°C (a high-temperature water vapor environment), not only is the steel composition crucial, but the concentration distribution of the passivation film on the steel surface and the coverage rate of Al oxides in the passivation film are also very important. Furthermore, they recognized that optimizing the conditions in the heat treatment and pickling processes is essential to obtaining such a passivation film.

[0025] Traditionally, techniques to improve oxidation resistance by adjusting the element concentration in the passivation film of ferritic stainless steel sheets through pickling or similar processes have been achieved by controlling either the heat treatment or pickling process. However, a technique to improve water vapor oxidation resistance by adjusting the element concentration in the passivation film using both heat treatment and pickling processes is currently unknown.

[0026] Furthermore, it is known that Al and Si can be concentrated in the passivation film through a heat treatment process. However, these internal oxidizing elements, such as Al and Si, are removed in the subsequent pickling process. That is, a technique to ensure that Al and other elements concentrated during the heat treatment process remain in the final passivation film is not yet known. Additionally, Fe oxides concentrate on the surface of the steel in the heat-treated state without the pickling process. Such stainless steel, due to the concentrated Fe oxides on the steel surface, is prone to rusting during storage and transportation, accompanied by the dissolution of Al concentrated in the passivation film, leading to a deterioration in its resistance to water vapor oxidation.

[0027] The inventors of this invention have discovered that by controlling the concentration of Al among the elements concentrated in the passivation film on the steel surface during the heat treatment and pickling processes in ferritic stainless steel containing more than 10% Cr, a stable passivation film can be formed, resulting in good resistance to water vapor oxidation.

[0028] In addition, it is recognized that in order to obtain a passivation film that improves resistance to water vapor oxidation, it is important to adjust the temperature and atmosphere of the annealing process, which is a preceding step in the pickling process, and it is important to leave the oxide scale generated in the annealing process as a residue in the pickling process.

[0029] The main points of this invention are as follows.

[0030] [1] The chemical composition of the ferritic stainless steel plate according to one embodiment of the present invention, in mass %, contains:

[0031] C: More than 0% and less than 0.008%

[0032] Si: 0.01~2.50%

[0033] Mn: 0.01~0.50%

[0034] P: 0.0001~0.040%

[0035] S: 0.001~0.010%

[0036] Al: 0.001~2.500%

[0037] Cr: 10.0–25.0%

[0038] Nb: Above 0% and below 0.80%

[0039] Ti: 0.05–0.50%

[0040] V: 0.01~0.15%

[0041] N: More than 0% and less than 0.050%

[0042] Ni: 0-0.40%

[0043] Sn: 0~0.200%

[0044] Mo: 0–1.40%

[0045] Cu: 0–1.40%

[0046] B: 0~0.0020%

[0047] Sb: 0-0.5%

[0048] Zr: 0-0.5%

[0049] Co: 0-0.5%

[0050] W: 0-0.5%

[0051] Ta: 0~0.100%

[0052] Mg: 0–0.0050%

[0053] Ca: 0–0.0050%

[0054] Ga: 0-0.05%, and

[0055] REM: 0–0.1%,

[0056] The remaining portion contains Fe and impurities.

[0057] The passivation film existing on the surface of the steel plate has a composition parameter PS of 0.03 to 0.15, as expressed by equation (1).

[0058] The surface coverage of Al oxide-containing materials in the passivation film, with an equivalent circle diameter of 0.01–1.0 μm, is 15–40%.

[0059] PS = Al / (Ti + Mn + Si + Cr) Equation (1)

[0060] In Equation (1), the element symbols represent the content (mass%) of each element at the depth position where the Al content reaches its maximum in the depth direction of the passivation film.

[0061] [2] The ferritic stainless steel sheet according to [1] above may also contain, in the above chemical composition, one or more elements selected from the following elements by mass %:

[0062] Ni: 0.01~0.40%

[0063] Sn: 0.001~0.200%

[0064] Mo: 0.05–1.40%

[0065] Cu: 0.05–1.40%.

[0066] [3] The ferritic stainless steel sheet according to [1] or [2] above may also contain, in the above chemical composition, one or more elements selected from the following elements by mass %:

[0067] B: 0.0003~0.0020%

[0068] Sb: 0.005~0.5%

[0069] Zr: 0.005~0.5%

[0070] Co: 0.005-0.5%

[0071] W: 0.005~0.5%

[0072] Ta: 0.005~0.100%

[0073] Mg: 0.0001~0.0050%

[0074] Ca: 0.0001~0.0050%

[0075] Ga: 0.001~0.05%

[0076] REM: 0.001–0.1%.

[0077] [4] Another embodiment of the present invention provides a method for manufacturing a ferritic stainless steel plate, which is a method for manufacturing a ferritic stainless steel plate as described in any one of [1] to [3] above, wherein,

[0078] The final annealing process involves setting the dew point of the atmosphere to the range of -50 to -20°C, the soaking temperature to the range of 700 to 950°C, and the soaking time to the range of 5 seconds to 10 minutes for heat treatment, followed by electrolytic pickling.

[0079] [5] In the method for manufacturing ferritic stainless steel plates described in [4] above, the concentration of fluoride ions in the solution used in the electrolytic pickling can also be 1.0 to 10.0 g / L.

[0080] Invention Effects

[0081] According to the above embodiments of the present invention, a ferritic stainless steel plate with excellent resistance to water vapor oxidation and a method for manufacturing the same can be provided.

[0082] Furthermore, the ferritic stainless steel sheet of the above embodiments of the present invention can be used as a material suitable for devices (NOx reduction devices) that reduce NOx in exhaust gases using urea water in internal combustion engines, primarily diesel engines, and particularly for equipment such as automotive urea SCR systems. In particular, by using the ferritic stainless steel sheet of the above embodiments of the present invention as a material for components of urea SCR systems such as tanks, pipes, plates, rods, and springs, it is possible to contribute to the simplification and cost reduction of urea SCR systems. Attached Figure Description

[0083] Figure 1 This is a graph showing the relationship between the compositional parameter PS of the passivation film and the oxidation increment of ferritic stainless steel plates.

[0084] Figure 2 This is a graph showing the relationship between the compositional parameter PS of the passivation film, the surface coverage of Al oxides, and the oxidation increment of the steel plate. Detailed Implementation

[0085] The following describes a ferritic stainless steel sheet with excellent resistance to water vapor oxidation, as one embodiment of the present invention, and a method for manufacturing the ferritic stainless steel sheet.

[0086] The ferritic stainless steel plate of this embodiment has the following chemical composition (by mass%): C: more than 0% and less than 0.008%, Si: 0.01 to 2.50%, Mn: 0.01 to 0.50%, P: 0.0001 to 0.040%, S: 0.001 to 0.010%, Al: 0.001 to 2.500%, Cr: 10.0 to 25.0%, Nb: more than 0% and less than 0.80%, Ti: 0.05 to 0.50%, V: 0.01 to 0.15%, N: more than 0% and less than 0.050%, Ni: 0 to 0.40%, Sn: 0 to 0.200%, Mo: 0 to 0.40%. 1.40%, Cu: 0-1.40%, B: 0-0.0020%, Sb: 0-0.5%, Zr: 0-0.5%, Co: 0-0.5%, W: 0-0.5%, Ta: 0-0.100%, Mg: 0-0.0050%, Ca: 0-0.0050%, Ga: 0-0.05%, and REM: 0-0.1%, with the remainder containing Fe and impurities. The composition parameter PS of the passivation film on the surface of the steel plate, expressed by formula (1), is 0.03-0.15, and the surface coverage of Al oxide-containing components with an equivalent circle diameter of 0.01-1.0 μm in the passivation film meets the requirement of 15-40%.

[0087] PS = Al / (Ti + Mn + Si + Cr) Equation (1)

[0088] In Equation (1), the element symbols represent the content (mass%) of each element at the depth position where the Al content reaches its maximum in the depth direction of the passivation film.

[0089] The chemical composition of the ferritic stainless steel sheet (hereinafter sometimes simply referred to as steel sheet) of this embodiment will be described below.

[0090] <Chemical Composition>

[0091] C: More than 0% and less than 0.008%

[0092] Carbon (C) degrades the formability and corrosion resistance of steel sheets, therefore its content needs to be kept low, typically below 0.008%. A C content of 0% is also possible. However, setting the C content to 0% is difficult in practical steel sheet manufacturing; therefore, a C content exceeding 0% is also possible. However, excessive reduction in C content increases refining costs; therefore, if oxidation resistance is also considered, a C content of 0.001% or higher, or 0.002% or higher, is also possible. A C content of 0.007% or lower, or 0.006% or lower, is also possible.

[0093] Si: 0.01~2.50%

[0094] Si is useful as a deoxidizer and is effective in improving corrosion resistance, high-temperature strength, and oxidation resistance. However, Si degrades processability and pickling properties. Therefore, the Si content is set to 2.50% or less. On the other hand, excessively reducing the Si content increases refining costs; therefore, the lower limit for the Si content is set to 0.01% or more. The Si content can also be set to 0.03% or more, or 0.10% or more. The Si content can also be set to 1.50% or less, 1.00% or less, 0.50% or less, or 0.15% or less.

[0095] Mn: 0.01~0.50%

[0096] Mn is an element included as a deoxidizer, but if it is present in excess, corrosion resistance and oxidation resistance deteriorate. In particular, when Mn is used in a urea SCR system, Mn oxides are formed on the outer layer of the oxide scale, making it easier to form a red oxide scale. Therefore, the Mn content is set to 0.50% or less. The Mn content can also be set to 0.30% or less or 0.20% or less. Furthermore, considering oxide scale peeling properties, the Mn content is preferably 0.10% or less. On the other hand, when the Mn content is below 0.01%, the interface between the hot-rolled oxide scale formed during hot rolling and the steel substrate may become more uneven, resulting in a rougher surface on the ferritic stainless steel sheet. Therefore, the lower limit of the Mn content is set to 0.01% or more. The Mn content can also be set to 0.03% or more or 0.05% or more.

[0097] P: 0.0001~0.040%

[0098] Polymer (P) is an element that reduces the toughness of steel plates. Furthermore, P also reduces the toughness of the welded portion when steel plates are welded. Therefore, the upper limit for P content is set to 0.040% or less. While the lower limit for P content can include 0%, since the detection limit for chemical analysis is 0.0001%, the practical lower limit for P content in practical steel plates is 0.0001%. On the other hand, drastically reducing the P content in stainless steel plates would lead to increased manufacturing costs; therefore, the lower limit for P content is preferably 0.010% or more. P content can also be 0.030% or less, and even 0.020% or less.

[0099] S: 0.001~0.010%

[0100] Sulfur (S) is a harmful element that adversely affects corrosion resistance and high-temperature cracking of welded joints; therefore, the upper limit of S content is set below 0.010%. The improvement in corrosion resistance resulting from a decrease in S content saturates at 0.001%; ​​therefore, the lower limit of S content is set above 0.001%.

[0101] Al: 0.001~2.500%

[0102] Al is contained as a deoxygenating element. In addition, Al is an element that improves oxidation resistance.

[0103] When the Al content is below 0.001%, the Al-concentrated region containing Al oxides generated during the heat treatment process becomes difficult to remain in the passivation film, and insufficient resistance to water vapor oxidation in a water vapor atmosphere cannot be obtained. Therefore, the lower limit of the Al content is set to 0.001% or more. On the other hand, excessive Al content deteriorates the workability of the steel sheet, so the upper limit of the Al content is set to 2.500% or less. The Al content can also be 0.005% or more or 0.010% or more, and the Al content can also be 1.500% or less, 1.000% or less, 0.500% or less, 0.200% or less, or 0.150% or less.

[0104] Cr: 10.0–25.0%

[0105] Cr is an essential element for oxidation resistance in the steel of this embodiment. Cr is particularly necessary because it ensures good resistance to water vapor oxidation, especially in high-temperature environments with water vapor adhering to urea water. When the Cr content is below 10.0%, it becomes impossible to form a protective dense oxide scale when the steel sheet is used as a component in a urea SCR system. Therefore, the lower limit of the Cr content is set to 10.0% or more. On the other hand, excessive Cr content deteriorates the workability of the steel sheet; therefore, the upper limit of the Cr content is set to 25.0% or less. Furthermore, considering weldability as well, the Cr content is preferably 14.0% to 18.0%. The Cr content can also be 15.0% to 17.5%.

[0106] Nb: Above 0% and below 0.80%

[0107] Nitrogen (Nb) is an element that improves resistance to intergranular corrosion and high-temperature strength. If the Nb content exceeds 0.80%, the workability of the steel sheet deteriorates significantly; therefore, the upper limit for Nb content is set below 0.80%. Furthermore, considering manufacturability, oxidation resistance, and alloy cost, the Nb content can also be set above 0.003% or 0.005%, or below 0.30%.

[0108] Ti: 0.05–0.50%

[0109] Ti is an element that combines with C, N, and S to improve the corrosion resistance, intergranular corrosion resistance, room temperature ductility, and deep drawing properties of steel sheets. When the Ti content is below 0.05%, the improvement in these properties cannot be expected; therefore, the lower limit for Ti content is set to 0.05% or more. On the other hand, if the Ti content exceeds 0.50%, the workability of the steel sheet deteriorates significantly; therefore, the upper limit for Ti content is set to 0.50% or less. Furthermore, considering oxidation resistance and manufacturability, the Ti content is preferably 0.07% to 0.30%. The Ti content can also be 0.10% or more, 0.15% or more, or 0.25% or less.

[0110] V: 0.01~0.15%

[0111] V is an element that improves corrosion resistance. If the V content exceeds 0.15%, the corrosion resistance and workability of the steel plate deteriorate; therefore, the upper limit for V content is set at 0.15% or less. Furthermore, considering manufacturability, oxidation resistance, and alloy cost, the V content is preferably set at 0.01% or more. The V content can also be 0.03% or more, or 0.05% or more, or 0.14% or less, or 0.11% or less.

[0112] N: More than 0% and less than 0.050%

[0113] Like carbon, nitrogen (N) deteriorates the formability and corrosion resistance of steel sheets, therefore its content is set to 0.050% or less. On the other hand, excessively reducing the N content increases refining costs. If resistance to water vapor oxidation is also considered, the N content is preferably set to 0.002–0.045%. The N content can also be 0.005% or more, 0.040% or less, or 0.030% or less.

[0114] The ferritic stainless steel plate of this embodiment may contain one or more of the following chemical components: Ni: 0.01-0.40%, Sn: 0.001-0.20%, Mo: 0.05-1.40%, and Cu: 0.05-1.40%.

[0115] Ni: 0.01~0.40%

[0116] Ni is an element that further improves rust resistance and can be included as needed. When the Ni content is below 0.01%, no improvement in rust resistance can be expected; therefore, when Ni is present, the lower limit of the Ni content is preferably set to 0.01% or more. On the other hand, if the Ni content exceeds 0.40%, not only may the oxidation resistance of the steel plate deteriorate, but also its workability; therefore, the upper limit of the Ni content is set to 0.40% or less. Furthermore, considering manufacturability, oxidation resistance, and alloy cost, the Ni content is preferably set to 0.10% or more, and more preferably 0.30% or less.

[0117] Sn: 0.001~0.200%

[0118] Sn is an element that further improves corrosion resistance and high-temperature strength, and therefore can be included as needed. When the Sn content is below 0.001%, further improvements in corrosion resistance and high-temperature strength cannot be expected; therefore, when Sn is present, the lower limit of the Sn content is preferably set to 0.001% or more. On the other hand, if the Sn content exceeds 0.200%, manufacturability deteriorates significantly; therefore, the upper limit of the Sn content is set to 0.200% or less. Furthermore, considering processability, oxidation resistance, and alloy cost, the Sn content is preferably set to 0.010% or more, and more preferably 0.15% or less.

[0119] Mo: 0.05–1.40%

[0120] Mo is an element that improves the corrosion resistance of steel sheets, and therefore can be included as needed. When the Mo content is below 0.05%, further improvement in corrosion resistance cannot be expected; therefore, when Mo is present, the lower limit of the Mo content is preferably set to 0.05% or more. On the other hand, excessive Mo content deteriorates the workability and oxidation resistance of the steel sheet and leads to an increase in alloy costs; therefore, the upper limit of the Mo content is set to 1.40% or less. Furthermore, considering manufacturability, oxide scale adhesion, and alloy costs, the Mo content is preferably 0.30% or more, and more preferably 1.20% or less. The Mo content can also be 0.50% or more.

[0121] Cu: 0.05–1.40%

[0122] Cu is an element that improves the rust resistance and high-temperature strength of steel plates, and therefore can be included as needed. When the Cu content is below 0.05%, further improvement in corrosion resistance cannot be expected; therefore, when Cu is present, the lower limit of the Cu content is preferably set to 0.05% or more. On the other hand, if the Cu content exceeds 1.40%, the ductility of the steel plate deteriorates significantly; therefore, the upper limit of the Cu content is set to 1.40% or less. Furthermore, considering manufacturability and oxidation resistance, the Cu content is preferably 0.30% or more, and more preferably 1.20% or less.

[0123] The ferritic stainless steel sheet of this embodiment may, as needed, further contain the elements described below in the above chemical composition.

[0124] One or more of the following: B: 0.0003–0.0020%, Sb: 0.005–0.5%, Zr: 0.005–0.5%, Co: 0.005–0.5%, W: 0.005–0.5%, Ta: 0.005–0.100%, Mg: 0.0001–0.0050%, Ca: 0.0001–0.0050%, Ga: 0.001–0.05%, REM: 0.001–0.1%.

[0125] Boron (B) is an element that improves the secondary processing properties of parts during forming and can be included as needed. If the B content exceeds 0.0020%, manufacturability and resistance to intergranular corrosion deteriorate significantly; therefore, the upper limit for the B content is set to 0.0020% or less. Furthermore, considering processability, oxidation resistance, and alloy cost, when B is included, the B content is preferably set to 0.0003% or more, and preferably 0.0010% or less.

[0126] Sb, Zr, Co, and W are all elements that improve corrosion resistance, and one or more of them can be included as needed. When one or more of these elements are included, an effect is achieved by containing at least 0.005% of each. These elements are important for inhibiting corrosion rates, but excessive amounts deteriorate manufacturability and cost; therefore, the upper limit for their respective contents is set at 0.5% or less. More preferred ranges for these elements are 0.05% or more and 0.4% or less, respectively.

[0127] Ta (Ta) is an element that improves corrosion resistance by modifying inclusions. When Ta is present, the aforementioned effect is achieved if the Ta content is 0.005% or more. On the other hand, a Ta content exceeding 0.100% may lead to decreased ductility and toughness at room temperature. Therefore, the Ta content is preferably 0.100% or less, more preferably 0.050% or less.

[0128] Mg acts as a deoxidizer in molten steel by forming Mg oxide with Al, and also as a nucleation site for TiN crystals. TiN becomes a solidification nucleus for the ferrite phase during solidification. Therefore, by including Mg to promote TiN crystallization, a fine ferrite phase can be formed during solidification. This refinement of the solidification structure prevents surface defects such as ridging and rippling that are easily caused by coarse solidification structures when shaping steel sheets. Furthermore, a finer solidification structure improves workability. Therefore, Mg can be included as needed.

[0129] In the presence of Mg, it is preferable to contain 0.0001% or more to exhibit these effects. However, if the Mg content exceeds 0.0050%, manufacturability deteriorates, therefore the upper limit of the Mg content is set to 0.0050% or less. Preferably, the Mg content is set to 0.0003% or more, and more preferably 0.0020% or less, taking into account manufacturability.

[0130] Ca is an element that improves oxidation resistance when present in trace amounts, and can be contained in amounts up to 0.0050%. Preferably, it can be contained in amounts of 0.0001% or more.

[0131] Ga is an element that helps improve processability and can be contained in amounts up to 0.05%. Preferably, it can be contained in amounts of 0.001% or more.

[0132] REM (refined iron oxide) is an effective element for improving hot-rolling workability, steel purity, and corrosion resistance of steel sheets, and can be included as needed. When REM is included, it is preferable to set it to 0.001% or more to exhibit its effect. However, excessive REM content leads to increased alloy costs and reduced manufacturability; therefore, the upper limit is set to 0.1% or less. Preferably, considering effectiveness, economy, and manufacturability, the REM content is set to 0.001% or more, and 0.05% or less.

[0133] The term REM (Rare Earth Elements) here refers to scandium (Sc) and yttrium (Y), and the 15 elements from lanthanum (La) to lutetium (Lu) (lanthanides). In this embodiment, "REM" refers to a composition consisting of one or more of these elements. REM content refers to the total content of these elements. Examples of REM elements include La, Ce, and Nd.

[0134] In the ferritic stainless steel sheet of this embodiment, the remaining portion besides the aforementioned elements is Fe and impurities. However, other elements besides those mentioned above may also be included without impairing the effects of this embodiment. Furthermore, the term "impurities" here refers to components that are mixed in during the industrial manufacturing of the ferritic stainless steel sheet of this embodiment due to various factors such as raw materials like ore and scrap iron, and the manufacturing process; these are components that are permissible within the scope that do not adversely affect the present invention.

[0135] In the case of the steel plate of this embodiment, the composition parameter PS of the passivation film present on the surface of the steel plate, expressed by formula (1), is 0.03 to 0.15, and the surface coverage of Al-containing oxides with an equivalent circle diameter of 0.01 to 1.0 μm in the passivation film is set to 15% to 40%. In this case, an improvement in resistance to water vapor oxidation can be confirmed. It should be noted that the Al-containing oxides are included in the passivation film on the surface of the steel plate.

[0136] <Compositional parameters of the passivation film: PS (=Al / (Ti+Mn+Si+Cr)): 0.03~0.15>

[0137] In this embodiment, it was found that by keeping the amount of Al oxide contained in the passivation film within a certain range, the resistance to water vapor oxidation can be improved. As a parameter for evaluating the amount of Al oxide contained in the passivation film, the composition parameter PS is used in this embodiment. The composition parameter PS is an index conceived based on the following idea. First, focusing on the Al concentration in the passivation film, the depth position where the Al concentration reaches its maximum is determined in the thickness direction of the passivation film. Then, it was found that, from the viewpoint of ensuring the amount of Al oxide contained in the film, there exists an optimal range in the relationship between the concentrations of each constituent element (Al, Ti, Mn, Si, and Cr) of the passivation film at this specific depth position. Specifically, the composition parameter PS is obtained by importing the Al, Ti, Mn, Si, and Cr at the determined depth position into equation (1).

[0138] exist Figure 1 The figure shows the relationship between the oxidation increment of the steel plate and the composition parameter PS. Figure 1 The steel plate shown is the following steel plate, which underwent final annealing in the following conditions during the final annealing process: atmosphere: 1-50% H2 + remaining N2, atmosphere dew point: -40 to -20°C, soaking temperature: 814-1000°C, soaking time: 0-700 seconds, and then underwent electrolytic pickling in an acid solution containing 0.5-10.9 g / L of fluoride ions.

[0139] like Figure 1 As shown, when the composition parameter PS of the passivation film is below 0.03, the oxidation increment of the steel plate is 0.03 mg / cm³. 2 The above-mentioned components failed to exhibit excellent oxidation resistance when exposed to exhaust gases and urea solution. This is believed to be because the Al concentration in the oxide constituting the passivation film is low, resulting in a coarse oxide that easily allows water vapor and oxygen to permeate, thus failing to inhibit oxidation. Furthermore, when the composition parameter PS exceeds 0.15, the surface coverage of Al-containing oxides exceeds 40%, thus failing to exhibit excellent oxidation resistance. In other words, when the composition parameter PS of the passivation film is between 0.03 and 0.15, it becomes a dense oxide that is not easily permeable by water vapor and oxygen, exhibiting excellent oxidation resistance. Based on the above, the composition parameter PS of the passivation film is set to 0.03 to 0.15.

[0140] <Surface coverage of Al oxide-containing materials with an equivalent circle diameter of 0.01–1.0 μm: 15–40%>

[0141] (Equivalent circle diameter containing Al oxide: 0.01–1.0 μm)

[0142] When determining the surface coverage rate of Al oxides, the Al oxides used for measurement were defined as those with an equivalent circle diameter of 0.01 to 1.0 μm. When the surface coverage rate of Al oxides with an equivalent circle diameter of 0.01 to 1.0 μm was 15% to 40%, the steel sheet of this embodiment exhibited excellent resistance to water vapor oxidation when exposed to exhaust gases and urea solution. On the surface of the steel sheet not covered by Al oxides, oxidation was promoted, resulting in the formation of oxide scale. This oxide scale grew two-dimensionally, becoming a thick scale that was easily peeled off, and thus failed to exhibit excellent oxidation resistance. On the other hand, it is believed that Al oxides with an equivalent circle diameter of 0.01 to 1.0 μm can suppress the two-dimensional growth of oxide scale by pinning, exhibiting excellent oxidation resistance. When the equivalent circle diameter of the Al oxides was less than 0.01 μm, the pinning effect was small, and therefore did not contribute to improving oxidation resistance. Furthermore, regarding Al-containing oxides with an equivalent circle diameter exceeding 1.0 μm, these oxides also grow in the thickness direction, making them prone to peeling during the winding and processing of the steel strip, thus failing to exhibit excellent oxidation resistance. Based on the above, Al-containing oxides with an equivalent circle diameter in the range of 0.01–1.0 μm were selected as the subjects for measuring surface coverage.

[0143] (Surface coating rate containing Al oxides: 15-40%)

[0144] exist Figure 2The relationship between the composition parameter PS, the surface coverage of Al oxide, and the oxidation increment of the steel plate is shown. Even when the composition parameter PS of the passivation film is 0.03 to 0.15, excellent oxidation resistance cannot be exhibited when the surface coverage of the Al oxide-containing steel plate with an equivalent circle diameter of 0.01 to 1.0 μm is less than 15%. As mentioned above, oxidation is promoted on the surface not covered by Al oxide, resulting in the formation of oxide scale. When the surface coverage of Al oxide is less than 15%, this oxide scale grows two-dimensionally on the steel plate surface and then connects, thereby further growing into a thick oxide scale that is easily peeled off, failing to exhibit excellent oxidation resistance. On the other hand, it is believed that when the surface coverage of Al oxide is 15% or more, the two-dimensional growth of the aforementioned oxide scale is suppressed, such as by pinning, resulting in excellent oxidation resistance. Furthermore, when the surface coverage of the steel sheet containing Al oxide exceeds 40%, the Al oxide grows in the thickness direction, making it prone to peeling during the winding and processing of the finished steel strip, thus failing to exhibit excellent oxidation resistance. Based on the above, the surface coverage of the steel sheet containing Al oxide with an equivalent circle diameter of 0.01 to 1.0 μm is set in the range of 15% to 40%. From the viewpoint of further enhancing the pinning effect, the surface coverage of the steel sheet containing Al oxide with an equivalent circle diameter of 0.01 to 1.0 μm is preferably set to 20% or more. Furthermore, from the viewpoint of suppressing excessive growth of Al oxide in the thickness direction, the surface coverage of the steel sheet containing Al oxide with an equivalent circle diameter of 0.01 to 1.0 μm is preferably set to 35% or less.

[0145] The resistance of the steel plate to water vapor oxidation in this embodiment is determined not only by the increase in oxidation accompanying oxidation, but also by whether or not a red oxide scale is formed. Red oxide scale is a thick oxide scale formed when the steel plate is continuously oxidized in a water vapor atmosphere at a temperature range of approximately 500°C, resulting in an increase in the increase in oxidation. The peeling off of red oxide scale may degrade the NOx reduction performance in the urea SCR system, and is therefore undesirable.

[0146] It should be noted that, Figure 1 and Figure 2 The charts shown were obtained by measuring the compositional parameters PS of the passivation film of the steel plates, the equivalent circle diameter containing Al oxide, and the surface coverage in the following experiments, and measuring the resistance of each steel plate to water vapor oxidation.

[0147] (experiment)

[0148] As a sample, ferritic stainless steel sheets were manufactured through a series of processes including casting, hot rolling, pickling, cold rolling, annealing, and pickling. The inventors of this invention not only studied the steel composition in detail, but also investigated in detail the final annealing and pickling conditions during annealing and pickling of cold-rolled steel sheets, the composition of the passivation film, and the relationship between the oxidation increment and the presence or absence of red oxide scale in oxidation tests.

[0149] The steam oxidation test was conducted as follows: A 7.5% steam atmosphere was created by humidifying argon containing 10% O2, and the steel plate was continuously placed in the steam atmosphere at 500°C for 50 hours. For the test piece, a steel plate measuring 25mm in length and 20mm in width was used, with its surface and back surfaces in the final pickling state, and its end faces subjected to #600 grinding.

[0150] The test pieces for the steam oxidation test were obtained as follows: Cold-rolled steel sheets with a composition of 17.2% Cr, 1.1% Mo, 0.21% Ti, 0.08% Si, 0.12% Mn, 0.08% Al, 0.004% C, and 0.011% N were prepared. These sheets were then subjected to heat treatment (final annealing) in an atmosphere of 1–50% H₂ + the remainder N₂, with a dew point of -40°C to -20°C, an annealing temperature (soaking temperature) of 814°C to 1000°C, and a soaking time of 0 to 700 seconds. Following this, electrolytic pickling was performed in an acid solution containing 0.5–10.9 g / L of fluoride ions. Furthermore, the composition of the passivation film was analyzed before conducting the steam oxidation test on the test pieces.

[0151] The compositional parameter PS in the passivation film was measured as follows. A circular measurement area with a diameter of 4 mm was defined on the surface of the passivation film. For this measurement area, the passivation film was continuously sputtered from the film surface to a depth of 100 nm, and the elemental content within the measurement area was measured by glow discharge spectroscopy. The measurements using glow discharge spectroscopy were performed at 5 nm depths. Based on the measurement data, the elemental distribution from the film surface to a depth of 100 nm was measured. The elements to be measured were defined as those capable of generating cations. Focusing on the depth-direction distribution of Al among the detected elements, the depth position where the Al content is maximum was determined. Then, the contents (mass%) of Al, Ti, Mn, Si, and Cr at the determined depth positions were calculated. It should be noted that the content of each element is a percentage of the total amount of the target elements (Al, Ti, Mn, Si, and Cr) detected by glow discharge spectroscopy. The compositional parameter PS was obtained by importing the contents (mass%) of Al, Ti, Mn, Si, and Cr into equation (1). Glow discharge emission spectrophotometry was performed using a glow discharge emission spectrophotometer (RIGAKU Corporation, GDA750).

[0152] The surface coverage of Al oxides with an equivalent circle diameter of 0.01–1.0 μm was determined by elemental mapping analysis of the passivation film surface using an electron beam microanalyzer (EPMA, JXA-8530F, NEC Corporation) and observation of secondary electron images using FE-SEM. First, for the passivation film surface, the distribution of Al was determined by elemental mapping analysis at 4 μm square intervals within a 2 mm square measurement area. Regions with an intensity of 10% or more of the maximum intensity of the obtained Al distribution were defined as Al oxide-containing regions. That is, with the maximum Al intensity within the measurement area set to 100, regions with an Al intensity of 10 or more were defined as Al oxide-containing regions. The ratio of the area of ​​the Al oxide-containing region to the area of ​​the 2 mm square measurement area was defined as the Al oxide-containing surface coverage.

[0153] Furthermore, regions containing Al oxides were identified through elemental mapping analysis using secondary electron microscopy (EPMA) images, thus determining the morphology of the Al oxides. Specifically, five fields of view at 10,000x magnification were observed to identify Al oxides within these images. Their areas were measured using image processing, and the equivalent circle diameter was calculated assuming the Al oxides were circular. Al oxides within the range of 0.01–1.0 μm in equivalent circle diameter were identified.

[0154] <Manufacturing Method>

[0155] Next, the manufacturing method of the steel plate according to this embodiment will be described.

[0156] The steel plate manufacturing method of this embodiment includes the following steps: steelmaking, hot rolling, pickling, cold rolling, and annealing / pickling. In steelmaking, the following method is preferred: steel containing the aforementioned essential components and optional components as needed is smelted in a converter, followed by secondary refining. The molten steel is then used to form slabs using a known casting method (continuous casting). The slabs are heated to a specified temperature and continuously rolled to a specified thickness.

[0157] Pickling is performed on the hot-rolled steel sheet as needed. Furthermore, the hot-rolled sheet annealing process can be performed, taking into account production efficiency and material properties, or it can be omitted.

[0158] Regarding cold rolling conditions, the cold rolling of general stainless steel sheets is carried out using either reverse rolling on a Sendzimir mill with a roll diameter of approximately 50 to 100 mm, or unidirectional rolling on a tandem mill with a roll diameter of 400 mm or more. In the manufacturing method of this embodiment, either cold rolling method can be used. Furthermore, tandem rolling is superior in terms of productivity compared to Sendzimir rolling. To improve the r-value (Rankford value), an indicator of workability, cold rolling is preferably performed using a tandem mill with a roll diameter of 400 mm or more.

[0159] In the manufacturing method of this embodiment, a predetermined passivation film is obtained by performing a final annealing and pickling treatment on the cold-rolled steel sheet after annealing and pickling. Various methods can be listed as general heat treatment conditions (annealing conditions) for cold-rolled stainless steel sheets, but the inventors of the present invention have discovered that by performing a final annealing and pickling treatment on the cold-rolled steel sheet under the specific conditions described later, the Al oxides that have been concentrated during heat treatment remain on the surface of the steel sheet after pickling.

[0160] In their research on the conditions for final annealing and pickling, the inventors of this invention discovered that for cold-rolled stainless steel sheets with an Al-containing ferritic system, by performing heat treatment under the following conditions—annealing atmosphere of 3–20% H₂ + residual N₂, dew point of the atmosphere of -50–-20°C, soaking temperature of 700–950°C, and soaking time of 5 seconds–10 minutes—and then electrolytically pickling in an acid solution containing 1.0–10.0 g / L fluoride ions, Al oxides can also remain on the surface of the steel sheet. As a result, the composition parameter PS in the region from the surface of the steel sheet to within 100 nm becomes 0.03 or higher. Furthermore, it was found that the steel sheet obtained through final annealing and pickling, and the urea SCR system components such as tanks, pipes, plates, rods, and springs manufactured from this steel sheet, meet the requirements for resistance to water vapor oxidation.

[0161] The final annealing process is carried out in an atmosphere containing 3-20% hydrogen and the remainder nitrogen (3-20% H2 + remainder N2). If the hydrogen concentration in the atmosphere is less than 3%, nitriding on the steel surface is promoted, and the formation of Al oxides may be inhibited. Therefore, the hydrogen concentration in the annealing atmosphere is set to 3% or more. Preferably, the hydrogen concentration is 5% or more. On the other hand, if the hydrogen concentration in the atmosphere exceeds 20%, oxidation may be promoted due to hydrogen intrusion during the formation of Al-based oxide films or Al-based oxide films, which may adversely affect the formation of oxide films. Furthermore, if the hydrogen concentration in the atmosphere exceeds 20%, high costs may occur. Therefore, the hydrogen concentration in the annealing atmosphere is set to 20% or less. Preferably, the hydrogen concentration is 15% or less.

[0162] The dew point of the atmosphere in the final annealing process is set to be in the range of -50 to -20°C. When the dew point of the annealing atmosphere exceeds -20°C, the oxidation of Al becomes insufficient, and the composition of the passivation film does not satisfy equation (1). Therefore, the upper limit of the dew point is set to below -20°C. In addition, when the dew point of the annealing atmosphere is below -50°C, the formation effect of Al oxide becomes saturated, and the cost increases. Therefore, the lower limit of the dew point is set to above -50°C.

[0163] Furthermore, the homogenization temperature in the final annealing process is set in the range of 700–950°C. If the homogenization temperature during final annealing is below 700°C, the composition of the passivation film does not satisfy equation (1), recrystallization is not promoted, and the required ductility as a raw material cannot be obtained. Therefore, the lower limit of the homogenization temperature is set to 700°C or higher. On the other hand, if the homogenization temperature exceeds 950°C, excessive softening becomes a problem; therefore, the upper limit is set to 950°C or lower.

[0164] Therefore, the soaking time in the final annealing process is set to 5 seconds to 10 minutes. If the soaking time exceeds 10 minutes, excessive softening of the raw material becomes a problem; therefore, the soaking time is set to less than 10 minutes. There is no specific lower limit to the soaking time, but if the soaking time is less than 5 seconds, stable recrystallization will not occur, and the Al oxidation reaction will become incomplete, resulting in an Al oxide-containing surface coating rate of less than 15% after electrolytic pickling. Therefore, a soaking time of more than 5 seconds is set.

[0165] The final annealed electrolytic pickling is performed in a solution containing HCl, NaSO4, H2SO4, NaNO3, Na2SiF6, etc. Preferably, the electrolytic pickling is performed in a solution containing 1.0 to 10.0 g / L of fluoride ions. If the fluoride ion concentration in the solution during electrolytic pickling is less than 1.0 g / L, an excess of the Al-containing oxides used for surface coating may be present. Therefore, the fluoride ion concentration in the solution is preferably set to 1.0 g / L or more. More preferably, it is set to 2.0 g / L or more. On the other hand, if the fluoride ion concentration in the solution exceeds 10.0 g / L, the Al-containing oxides used for surface coating may be excessively removed and become non-existent. Therefore, the fluoride ion concentration in the solution is preferably set to 10.0 g / L or less. More preferably, it is set to 8.0 g / L or less.

[0166] It should be noted that if salt treatment is performed after the final annealing as described above, the surface coverage of the steel sheet containing Al oxides becomes less than 15%. Therefore, salt treatment is not performed after final annealing. From the viewpoint of ensuring the surface coverage of the steel sheet containing Al oxides, a combination of salt treatment and electrolytic pickling is also not preferred.

[0167] There are no specific regulations regarding the manufacturing methods for other processes, but appropriate selections can be made for hot rolling conditions and hot-rolled plate thickness. Alternatively, tempering and tension straightening can be applied after cold rolling / annealing. Furthermore, the plate thickness can be selected based on the required component thickness.

[0168] Example

[0169] As described in Examples 1 and 2 below, molten steel having the chemical composition shown in Table 1, adjusted to a specified composition, was melted and cast to obtain a billet. The obtained billet was hot-rolled, further pickled, and a 1.0 mm thick cold-rolled sheet was manufactured under the conditions of cold rolling, final annealing, and pickling shown in Table 3. Cold rolling was performed using a Sendzimir mill with a work roll diameter of 100 mm or a tandem mill with a work roll diameter of 450 mm. It should be noted that the underlines in Tables 1 to 3 indicate items outside the scope of the present invention. In addition, the symbol "-" in Table 1 indicates that the corresponding element content is 0% in the significant figures (up to the smallest digit) specified in the embodiments.

[0170] After cold rolling, final annealing was performed in a furnace with controlled atmosphere, dew point, soaking temperature, and soaking time. After final annealing, electrolytic pickling was carried out using a pickling solution containing NaSO4, H2SO4, NaNO3, and Na2SiF6 with the fluoride ion concentrations shown in Table 3.

[0171] Electrolytic pickling involves alternating anodic and cathodic electrolysis at a current density of 60 A / dm³. 2 The total electrolysis time was 10 seconds. Comparative Example c6 involved electrolytic pickling after salt immersion treatment. The salt treatment was performed by immersing the cold-rolled sheet in a mixture of NaNO3 and NaOH in molten salt heated to 470°C for 10 seconds. The resulting steel sheet was then subjected to a steam oxidation test.

[0172] The compositional parameter PS in the passivation film was determined as follows. A circular measurement area with a diameter of 4 mm was set on the surface of the passivation film. For this measurement area, the passivation film was continuously sputtered from the film surface to a depth of 100 nm, and the content of elements in the measurement area was determined by glow discharge spectroscopy. The measurement by glow discharge spectroscopy was performed at 5 nm depths. Based on the measurement data, the elemental distribution from the film surface to a depth of 100 nm was determined. The elements to be measured were set as those that can generate cations. Focusing on the depth direction distribution of Al among the detected elements, the depth position where the Al content is maximum was determined. Then, the content (mass%) of Al, Ti, Mn, Si, and Cr at the determined depth position was calculated. It should be noted that the content of each element is the percentage of the total amount of the elements to be measured detected by glow discharge spectroscopy. The compositional parameter PS was obtained by importing the content (mass%) of Al, Ti, Mn, Si, and Cr into Equation (1). The glow discharge emission spectrophotometry was performed using a glow discharge emission spectrophotometer (RIGAKU Corporation, GDA750).

[0173] The surface coating rate of Al oxides with an equivalent circle diameter of 0.01–1.0 μm was determined by elemental mapping analysis of the passivation film surface using an electron beam microanalyzer (EPMA, JXA-8530F, NEC Corporation) and observation of secondary electron images using a FE-SEM (JSM-7001F, NEC Corporation). First, for the passivation film surface, the Al distribution was determined by elemental mapping analysis at 4 μm square intervals within a 2 mm square measurement area. Regions with an intensity of 10% or more of the maximum intensity in the obtained Al distribution were defined as Al oxide-containing regions. The ratio of the area containing Al oxides to the area of ​​the 2 mm square measurement area was defined as the Al oxide-containing surface coating rate (referred to as "Steel Plate Surface Coating Rate" in Table 3).

[0174] Furthermore, regions containing Al oxides were identified through elemental mapping analysis using secondary electron microscopy (EPMA) images, thus determining the morphology of the Al oxides. Specifically, five fields of view at 10,000x magnification were observed to identify Al oxides within these images. Their areas were measured using image processing, and the equivalent circle diameter was calculated assuming the Al oxides were circular. Al oxides within the range of 0.01–1.0 μm in equivalent circle diameter were identified.

[0175] The steam oxidation test was conducted in a 7.5% steam atmosphere obtained by humidifying argon gas containing 10% O2. The steel plate was placed in this steam atmosphere at 500°C for 50 hours. The test pieces used in the steam oxidation test were 25mm long and 20mm wide, with the front and back surfaces in the final pickling state, and the end faces were ground to #600.

[0176] The determination of resistance to water vapor oxidation is based on the oxidized weight after the water vapor oxidation test. The oxidized weight is 0.01 mg / cm³. 2 In the following cases, the resistance to water vapor oxidation is considered superior, and the rating is deemed acceptable (very good) (recorded as "AA" in Tables 2 and 3). Additionally, when the oxidation weight exceeds 0.01 mg / cm³... 2 And it is 0.03 mg / cm³ 2 In the following cases, the resistance to water vapor oxidation is judged to be excellent, and the rating is deemed acceptable (good) (marked as "A" in Tables 2 and 3). On the other hand, when the oxidation weight is 0.03 mg / cm³... 2In the above situations, the oxide scale may easily peel off, damaging the equipment related to the urea SCR system, and therefore is judged as unqualified (poor) (marked as "B" in Tables 2 and 3). Furthermore, "oxidation resistance" in Tables 2 and 3 refers to resistance to water vapor oxidation.

[0177] <Example 1>

[0178] For the steels No. A1 to A28 and a1 to a11 shown in Table 1, stainless steel sheets were manufactured by performing a cold rolling process under the conditions of symbol C7 shown in Table 3.

[0179] As shown in Table 2, the steel composition of symbols B1 to B28, the composition parameter PS of the passivation film, and the coverage of Al oxides meet the conditions of the present invention, thus exhibiting good resistance to water vapor oxidation in the water vapor oxidation test.

[0180] The Al in symbol b1 is outside the lower limit range, the composition parameter PS is low, and the coverage area of ​​Al oxide becomes less than 15%, thus resulting in poor resistance to water vapor oxidation.

[0181] The Al in symbol b2 is outside the upper limit range, and the coverage of Al oxides exceeds 40%, thus it has poor resistance to water vapor oxidation.

[0182] The C in symbol b3 deviates from the upper limit range, resulting in abnormal oxidation and thus inhibiting the formation of Al oxides. The composition parameter PS becomes unsatisfactory within the scope of this invention, resulting in poor resistance to water vapor oxidation.

[0183] The Si and Mn in symbol b4 are outside the upper limit range, and Si and Mn are concentrated in the oxide. The composition parameter PS no longer meets the range of the present invention, and therefore the resistance to water vapor oxidation is poor.

[0184] The P and S values ​​of symbol b5 are outside the upper limit, and the composition parameter PS no longer meets the scope of this invention, thus resulting in poor resistance to water vapor oxidation.

[0185] The Cr content of symbol b6 exceeds the upper limit, resulting in excessively improved corrosion resistance. This makes it difficult to dissolve and remove the Cr from the oxide by acid washing, and the composition parameter PS becomes unsuitable for the scope of this invention. Therefore, it exhibits poor resistance to water vapor oxidation.

[0186] The Cr in symbol b7 deviates from the lower limit, and Fe is concentrated in the oxide to the point that it cannot be completely dissolved by acid washing. The composition parameter PS becomes unacceptable within the scope of this invention, resulting in poor resistance to water vapor oxidation.

[0187] The Mo and Cu in the b8 symbol exceed their upper limits, resulting in excessively increased corrosion resistance. As a result, during pickling, it becomes difficult to dissolve and remove the oxides of Fe and Cr, and the composition parameter PS fails to meet the required range, thus leading to poor resistance to water vapor oxidation.

[0188] In symbol b9, Mo and Cu deviate from their lower limits, leading to a deterioration in the adhesion of the oxide scale. Furthermore, the reduced corrosion resistance results in excessive acid pickling, accelerating the removal of Al-containing oxides, and reducing the Al-containing oxide coverage to below 15%. Consequently, resistance to water vapor oxidation is poor.

[0189] The Nb, Ti, V, and N values ​​of the symbol b10 deviate from the upper limit, resulting in reduced processability due to excessive Nb and V content. Furthermore, Ti concentrates in the oxides, causing the composition parameter PS to fall outside the scope of this invention. Additionally, the deterioration of corrosion resistance due to excessive N leads to excessive pickling of the steel sheet, accelerating the removal of Al oxides and reducing the Al oxide coverage to below 15%. Consequently, resistance to water vapor oxidation is poor.

[0190] The V content of symbol b11 deviates from the lower limit, increasing refining costs. Furthermore, insufficient V content leads to deterioration of corrosion resistance, resulting in excessive pickling of the steel plate. This accelerates the removal of Al oxides, reducing the Al oxide coating to below 15%. Consequently, resistance to water vapor oxidation is poor.

[0191] [Table 1]

[0192]

[0193] The underlined part indicates that it is outside the scope of this invention.

[0194] [Table 2]

[0195]

[0196] <Example 2>

[0197] Steel plates were manufactured by performing the following processes after cold rolling of steel Nos. A4-8, A14, 15, 17 and 18 shown in Table 1 under the conditions of symbols C1-C10 and c1-c6 shown in Table 3.

[0198] As shown in Table 3, the manufacturing process after cold rolling of symbols C1 to C10, the composition parameter PS of the passivation film, and the coverage of Al oxides meet the conditions of the present invention. Therefore, the resistance to water vapor oxidation in the water vapor oxidation test is good.

[0199] Because the symbol c1 was annealed in an atmosphere with a dew point exceeding -20°C as the upper limit, the composition parameter PS of the passivation film does not meet the scope of the invention, the coverage of Al oxides becomes less than 15%, and the resistance to water vapor oxidation is poor.

[0200] Symbol c2, due to annealing at 700°C, which is below the lower limit of the soaking temperature, has a passivation film composition parameter PS that does not meet the scope of the invention, and the coverage of Al oxides is less than 15%, thus exhibiting poor resistance to water vapor oxidation.

[0201] Symbol c3 exhibits poor resistance to water vapor oxidation because it is annealed at 950°C, which is above the upper limit of the soaking temperature, resulting in an Al oxide coating of over 40%.

[0202] Symbol c4 exhibits poor resistance to water vapor oxidation because it undergoes annealing at a time lower than the minimum soaking time of 5 seconds, resulting in an Al oxide coating of less than 15%. Furthermore, the "soaking time: 0 seconds" in symbol c4 indicates that the final annealing was completed instantaneously upon reaching the soaking temperature.

[0203] Symbol c5 exhibits poor resistance to water vapor oxidation because it is annealed for 600 seconds (10 minutes), which is above the upper limit of the soaking temperature, resulting in an Al oxide coating of over 40%.

[0204] Symbol c6, due to salt impregnation, has accelerated the dissolution and detachment of Al oxides, resulting in an Al oxide coverage of less than 15% and poor resistance to water vapor oxidation.

[0205] [Table 3]

[0206]

[0207] Industrial availability

[0208] As can be clearly seen from the above description, according to the present invention, a ferritic stainless steel sheet suitable for urea SCR systems and exhibiting excellent resistance to water vapor oxidation can be provided even without the addition of large amounts of expensive alloying elements. In particular, by applying the ferritic stainless steel sheet of the present invention to urea SCR systems in diesel vehicles, excellent components for urea SCR systems can be manufactured. Using these components to manufacture urea SCR systems can greatly contribute to environmental protection.

Claims

1. A ferritic stainless steel sheet, characterized by, The chemical composition, expressed as a percentage by mass, contains: C: more than 0% and less than 0.008%. Si: 0.01~2.50% Mn: 0.01~0.50% P：0.0001～0.040％、 S：0.001～0.010％、 Al：0.001～2.500％、 Cr：10.0～25.0％、 Nb: Above 0% and below 0.80% Ti: 0.05–0.50% V：0.01～0.15％、 N: More than 0% and less than 0.050% Ni: 0-0.40% Sn: 0~0.200% Mo: 0–1.40% Cu: 0–1.40% B：0～0.0020％、 Sb: 0-0.5% Zr：0～0.5％、 Co: 0-0.5% W：0～0.5％、 Ta: 0~0.100% Mg: 0–0.0050% Ca: 0–0.0050% Ga: 0-0.05%, and REM: 0–0.1%, The remaining portion contains Fe and impurities. The passivation film existing on the surface of the steel plate has a composition parameter PS of 0.03 to 0.15, as expressed by equation (1). The passivation film has an Al oxide-containing surface coverage rate of 15% to 40% with an equivalent circle diameter of 0.01 to 1.0 μm. PS = Al / (Ti + Mn + Si + Cr) Equation (1) In Equation (1), the element symbols represent the mass percentage of each element at the depth position where the Al content reaches its maximum in the depth direction of the passivation film.

2. The ferritic stainless steel sheet according to claim 1, characterized by, The chemical composition contains, by mass percent, one or more elements selected from the following: Ni: 0.01~0.40% Sn: 0.001~0.200% Mo: 0.05–1.40% Cu: 0.05–1.40%.

3. The ferritic stainless steel sheet according to claim 1 or 2, characterized by, The chemical composition contains, by mass percent, one or more elements selected from the following: B：0.0003～0.0020％、 Sb: 0.005~0.5% Zr：0.005～0.5％、 Co: 0.005-0.5% W：0.005～0.5％、 Ta: 0.005~0.100% Mg: 0.0001~0.0050% Ca: 0.0001~0.0050% Ga: 0.001~0.05% REM: 0.001–0.1%.

4. The method for manufacturing ferritic stainless steel sheet according to any one of claims 1 to 3, characterized in that, The final annealing process involves setting the dew point of the atmosphere to the range of -50 to -20°C, the soaking temperature to the range of 700 to 950°C, and the soaking time to the range of 5 seconds to 10 minutes for heat treatment, followed by electrolytic pickling.

5. The method of producing a ferrite system stainless steel sheet according to claim 4, characterized by The concentration of fluoride ions in the solution used in the electrolytic pickling is 1.0–10.0 g / L.

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