Ferritic stainless steel sheet and method for producing the same

Abstract

This ferritic stainless steel sheet has a chemical composition comprising, in % by mass, more than 0% and 0.008% or less of C, 0.01 to 2.50% of Si, 0.01 to 0.50% of Mn, 0.0001 to 0.040% of P, 0.001 to 0.010% of S, 0.001 to 2.500% of Al, 10.0 to 25.0% of Cr, more than 0% and 0.80% or less of Nb, 0.05 to 0.50% of Ti, 0.01 to 0.15% of V, more than 0% and 0.050% or less of N and a remainder made up by Fe and impurities, in which a passivation film has a compositional parameter PS represented by formula (1) of 0.03 to 0.15 inclusive, and the surface cover ratio by an Al-containing oxide having an equivalent circle diameter of 0.01 to 1.0 μm inclusive in the passivation film is 15 to 40%.　PS = Al / (Ti+Mn+Si+Cr) ... formula (1) In formula (1), the element symbols respectively represent the contents (% by mass) of the elements at a depth position at which the amount of Al is largest in the depth direction of the passivation film.

Description

[Technical Field]

[0001] The present invention relates to a ferritic stainless steel sheet suitable as a material for use in devices that use urea water to reduce NOx in exhaust gases, particularly automotive urea SCR (Selective Catalytic Reduction) systems in automotive exhaust gas treatment systems, and a method for manufacturing the ferritic stainless steel sheet. 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]

[0002] As part of measures to combat environmental issues such as global warming, regulations on exhaust gases emitted primarily from transportation equipment have been strengthened, and efforts to curb carbon dioxide emissions are underway. In the case of automobiles, in addition to fuel-related efforts, measures are being taken to improve fuel efficiency by reducing the vehicle body weight and reusing exhaust heat, as well as by installing exhaust gas treatment devices such as EGR (Exhaust Gas Recirculation), DPF (Diesel Particulate Filter), and urea SCR.

[0003] Among these, the urea SCR system is a purification device installed in a series of exhaust systems in which high-temperature exhaust gas emitted from the engine passes through the exhaust manifold and catalytic converter, and is released into the atmosphere through the muffler. In a urea SCR system, urea is sprayed onto high-temperature exhaust gas at around 500°C, where it is decomposed by heat and moisture to produce ammonia, and the ammonia and NOx are selectively reduced on a catalyst to decompose them into harmless nitrogen. Treatment systems aimed at reducing NOx are relatively easy to handle, so their application to stationary NOx treatment systems as well as automotive applications is being considered.

[0004] Urea SCR systems require excellent oxidation resistance because high-temperature exhaust gas passes through them. Note that, unlike oxidation resistance in normal air, urea SCR systems require oxidation resistance in the water vapor atmosphere in which the urea solution is sprayed.

[0005] In the case of general ferritic stainless steels, which require oxidation resistance, even when heated in air at around 500°C for a long period of time, a dense protective film mainly composed of Cr is formed, and there is almost no oxidation weight gain or peeling of the oxide film.

[0006] However, even with the above-mentioned common ferritic stainless steels, oxidation is accelerated and the oxide film peels off in a steam atmosphere, such as in the areas where high-temperature exhaust gas passes through in urea SCR systems. For this reason, when the above-mentioned common ferritic stainless steels are used as materials for urea SCR systems, the catalyst in the urea SCR system can be damaged.

[0007] Patent Document 1 discloses a ferritic stainless steel for use in urea SCR parts, which, in addition to the steel components, has a passive film composition that satisfies a specific formula for the concentration ratio of Cr, Si, Al, Ti, Mn, and Fe within 20 nm from the surface. Patent Document 1 also examines the corrosion resistance in a urea aqueous solution and the oxidation resistance in a water vapor atmosphere.

[0008] As described above, corrosion resistance and oxidation resistance in a urea environment have been considered important for materials used in urea SCR systems. Regarding oxidation resistance, controlling the composition of the passive film, in addition to the component composition of the material, is important. In the above-mentioned Patent Document 1, a passive film with a predetermined composition is obtained by a final pickling process in which cold-rolled steel sheets after annealing are pickled. That is, the passive film is formed by removing the oxide scale formed in the preceding annealing process in the pickling process. However, the pickling process is burdensome, and excessive dissolution of the substrate during pickling can lead to reduced yields and uneven surface gloss. Therefore, further improvements in manufacturing costs and quality are required.

[0009] On the other hand, Patent Documents 2 and 3 disclose ferritic stainless steels in which the weather resistance and corrosion resistance are improved by controlling the passive film in the annealing process or pickling process.

[0010] However, in Patent Document 2, a passive film is formed during pickling, and as in the case of Patent Document 1, there is a concern that the pickling process may impose a high load.

[0011] Furthermore, Patent Document 3 discloses a technology for improving the corrosion resistance of heat exchangers by using a film formed in a special atmosphere at 900 to 1200°C. However, since a high-temperature process is required, there are concerns that the manufacturing costs will increase.

[0012] In addition, the above Patent Document 3 does not naturally consider steam oxidation resistance, and there is room for improvement in steam oxidation resistance.

[0013] As described above, the stainless steel sheets used in urea SCR systems are required to have excellent steam oxidation resistance because they are exposed to high-temperature exhaust gases in a steam atmosphere.

[0014] However, conventional materials with good steam oxidation resistance are often insufficient for use in urea SCR systems due to the large load imposed on the pickling process and in terms of stable manufacturing and manufacturing costs. In other words, there has been a demand for a low-cost ferritic stainless steel sheet for urea SCR that satisfies the steam oxidation resistance requirements. [Prior art documents] [Patent documents]

[0015] [Patent Document 1] Japanese Patent Application Publication No. 2012-112025 [Patent Document 2] Japanese Patent Publication No. 05-271880 [Patent Document 3] Japanese Patent Application Publication No. 2016-023341 Summary of the Invention [Problem to be solved by the invention]

[0016] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a ferritic stainless steel sheet having excellent steam oxidation resistance and a method for producing the same. [Means for solving the problem]

[0017] The present inventors have conducted extensive research to solve the above-mentioned problems. As a result, they have found that in order to achieve excellent steam oxidation resistance in a high-temperature environment of about 500°C where urea water is sprayed (high-temperature steam environment), not only the steel composition but also the concentration distribution in the passive film on the steel surface and the coverage of Al-containing oxides in the passive film are extremely important. They have also found that in order to obtain such a passive film, it is important to optimize the conditions in the heat treatment and pickling processes.

[0018] Conventionally, techniques for adjusting the element concentrations in the passive film of a ferritic stainless steel sheet by pickling or the like to improve oxidation resistance have been realized by controlling either the heat treatment process or the pickling process, but no technique for improving steam oxidation resistance by adjusting the element concentrations in the passive film using both the heat treatment process and the pickling process has been known.

[0019] It has been known for some time that heat treatment can enrich Al and Si in the passive film. However, internally oxidized elements such as Al and Si are removed in the subsequent pickling process. In other words, no technology is known for leaving Al and other elements enriched in the heat treatment process in the final passive film. Furthermore, Fe oxides are enriched on the as-heat-treated steel surface obtained by omitting the pickling process. Such stainless steels can rust during storage or transportation due to the enriched Fe oxide on the steel surface. This can lead to the elution of Al that has been enriched in the passive film, potentially degrading steam oxidation resistance.

[0020] The inventors have discovered that in ferritic stainless steels containing 10% or more Cr, by controlling the concentration of Al, one of the elements that concentrate in the passive film on the steel surface, in the heat treatment and pickling processes, a healthy passive film can be formed and good steam oxidation resistance can be obtained.

[0021] Furthermore, the inventors have found that in order to obtain a passive film for improving steam oxidation resistance, it is important to adjust the temperature and atmosphere of the annealing step, which is a step preceding the pickling step, and that it is also important to leave the oxide scale formed in the annealing step without completely removing it in the pickling step.

[0022] The gist of the present invention is as follows. [1] A ferritic stainless steel sheet according to one embodiment of the present invention has a chemical composition, in mass%, C: more than 0%, 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%, less than 0.80%, Ti: 0.05 to 0.50% V: 0.01 to 0.15%, N: More than 0%, 0.050%, Ni: 0 to 0.40% Sn: 0 to 0.200%, Mo: 0-1.40% Cu: 0-1.40% B: 0~0.0020%, Sb: 0 to 0.5% Zr: 0-0.5% Co: 0-0.5%, W: 0-0.5%, Ta: 0 to 0.100%, Mg: 0 to 0.0050%, Ca: 0 to 0.0050%, Ga: 0 to 0.05%, and REM: 0 to 0.1% with the remainder being Fe and impurities, The passive film present on the steel sheet surface has a composition parameter PS represented by formula (1) of 0.03 or more and 0.15 or less, The surface coverage of the passive film with Al-containing oxides having an equivalent circle diameter of 0.01 to 1.0 μm is 15 to 40%. PS=Al / (Ti+Mn+Si+Cr) Formula (1) Here, the element symbols in formula (1) represent the content (mass %) of each element at the depth position where the Al content is maximum in the depth direction of the passive film. [2] The ferritic stainless steel sheet according to the above [1], wherein the chemical composition is, in mass%, Ni: 0.01 to 0.40% Sn: 0.001 to 0.200%, Mo: 0.05 to 1.40% Cu: 0.05 to 1.40% The resin composition may contain one or more selected from the following: [3] The ferritic stainless steel sheet according to the above [1] or [2], wherein the chemical composition is, in mass%, B: 0.0003~0.0020%, Sb: 0.005 to 0.5% Zr: 0.005~0.5% Co: 0.005 to 0.5%, W: 0.005 to 0.5%, Ta: 0.005 to 0.100%, Mg: 0.0001 to 0.0050%, Ca: 0.0001 to 0.0050%, Ga: 0.001 to 0.05%, REM: 0.001~0.1%, may contain one or more of the above. [4] A method for producing a ferritic stainless steel sheet according to another embodiment of the present invention is a method for producing a ferritic stainless steel sheet according to any one of the above [1] to [3], The heat treatment is performed in the finish annealing step with an atmospheric dew point in the range of -50 to -20°C, a soaking temperature in the range of 700 to 950°C, and a soaking time in the range of 5 seconds to 10 minutes, followed by electrolytic pickling. [5] In the method for producing a ferritic stainless steel sheet according to the above item [4], the fluorine ion concentration in the solution used in the electrolytic pickling may be 1.0 to 10.0 g / L. [Effects of the Invention]

[0023] According to the above-described embodiment of the present invention, a ferritic stainless steel sheet having excellent steam oxidation resistance and a method for manufacturing the same can be provided. Furthermore, the ferritic stainless steel sheet of the above embodiment according to the present invention can be used as a suitable material for devices (NOx reduction devices) that use urea water to reduce NOx in exhaust gas from internal combustion engines, mainly diesel engines, particularly for equipment used in automotive urea SCR systems, etc. In particular, using the ferritic stainless steel sheet of the above embodiment according to the present invention as a material for urea SCR system parts such as tanks, pipes, plates, rods, and springs can contribute to simplifying and reducing the cost of urea SCR systems. [Brief explanation of the drawings]

[0024] [Figure 1] FIG. 1 is a graph showing the relationship between the composition parameter PS of the passive film and the oxidation weight gain of a ferritic stainless steel sheet. [Figure 2] FIG. 2 is a diagram showing the relationship between the composition parameter PS of the passive film, the surface coverage of the Al-containing oxide, and the oxidation weight gain of the steel sheet. DETAILED DESCRIPTION OF THE INVENTION

[0025] Hereinafter, a ferritic stainless steel sheet having excellent steam oxidation resistance according to one embodiment of the present invention and a method for producing the ferritic stainless steel sheet will be described.

[0026] The ferritic stainless steel sheet of this embodiment has a chemical composition, in mass%, of C: more than 0.008% and not more 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 0.80%, Ti: 0.05 to 0.50%, V: 0.01 to 0.15%, N: more than 0.050% and not more than 0.050%, Ni: 0 to 0.40%, Sn: 0 to 0.200%, Mo: 0 to 1.40%, Cu: 0 to 1.40%, and B: 0 to 0.000%. The ferritic stainless steel sheet contains 20%, 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 being Fe and impurities, and the passive film present on the steel sheet surface has a composition parameter PS, as shown in formula (1), of 0.03 to 0.15, and the passive film has a surface coverage of 15-40% by weight of Al-containing oxides with a circle equivalent diameter of 0.01-1.0 μm.

[0027] PS=Al / (Ti+Mn+Si+Cr) Formula (1)

[0028] In the formula (1), the element symbols represent the content (mass %) of each element at the depth position where the Al content is maximum in the depth direction of the passive film.

[0029] The chemical composition of the ferritic stainless steel sheet (hereinafter sometimes simply referred to as steel sheet) of this embodiment will be described below.

[0030] <Chemical composition> C: More than 0%, 0.008% Since C deteriorates the formability and corrosion resistance of steel sheet, its content must be kept low, and the C content is set to 0.008% or less. The C content may be 0%. However, since it is difficult to achieve a C content of 0% in practical steel sheet from a manufacturing perspective, the C content may be set to more than 0%. However, since an excessive reduction in the C content increases refining costs, taking oxidation resistance into consideration, the C content may be set to 0.001% or more or 0.002% or more. The C content may be set to 0.007% or less or 0.006% or less.

[0031] Si: 0.01 to 2.50% Silicon is useful as a deoxidizer and is an effective element for improving corrosion resistance, high-temperature strength, and oxidation resistance. However, silicon also deteriorates workability and pickling properties. Therefore, the silicon content is set to 2.50% or less. However, excessive reduction of the silicon content increases refining costs, so the lower limit of the silicon content is set to 0.01% or more. The silicon content may be set to 0.03% or more, or 0.10% or more. The silicon content may also be set to 1.50% or less, 1.00% or less, 0.50% or less, or 0.15% or less.

[0032] Mn: 0.01 to 0.50% Mn is an element added as a deoxidizer, but excessive addition reduces corrosion resistance and oxidation resistance. In particular, when Mn is used in a urea SCR system, Mn oxides form on the outer layer of the scale, making red scale more likely to form. Therefore, the Mn content is set to 0.50% or less. The Mn content may be set to 0.30% or less or 0.20% or less. Furthermore, considering scale peelability, the Mn content is preferably set to 0.10% or less. On the other hand, if the Mn content is less than 0.01%, the interface between the hot-rolled scale formed during hot rolling and the steel sheet substrate becomes uneven, which may result in a rough surface for the ferritic stainless steel sheet. Therefore, the lower limit of the Mn content is set to 0.01% or more. The Mn content may be set to 0.03% or more, or 0.05% or more.

[0033] P: 0.0001 to 0.040% P is an element that reduces the toughness of steel sheets. Furthermore, P also reduces the toughness of welds when steel sheets are welded. Therefore, the upper limit of the P content is set to 0.040% or less. Although the lower limit of the P content may include 0%, since the detection limit of chemical analysis is 0.0001%, the practical lower limit of the P content in practical steel sheets is 0.0001%. On the other hand, since excessively reducing the P content of stainless steel sheets increases manufacturing costs, the lower limit of the P content is preferably 0.010% or more. The P content may be 0.030% or less, or 0.020% or less.

[0034] S: 0.001 to 0.010% Since S is a harmful element that adversely affects corrosion resistance and hot cracking of welds, the upper limit of S content is set to 0.010%. Since the improvement in corrosion resistance due to a decrease in S content saturates at 0.001%, the lower limit of S content is set to 0.001%.

[0035] Al: 0.001 to 2.500% Al is contained as a deoxidizing element and is also an element that improves oxidation resistance. If the Al content is less than 0.001%, it becomes difficult to retain the Al-enriched region containing the Al-containing oxide formed during the heat treatment process in the passivation film, and sufficient steam oxidation resistance in a steam atmosphere cannot be obtained. Therefore, the lower limit of the Al content is set to 0.001% or more. On the other hand, since excessive Al content deteriorates the workability of the steel sheet, the upper limit of the Al content is set to 2.500% or less. The Al content may be 0.005% or more or 0.010% or more, but may also be 1.500% or less, 1.000% or less, 0.500% or less, 0.200% or less, or 0.150% or less.

[0036] Cr: 10.0 to 25.0% Cr is an essential element for the oxidation resistance of the steel sheet of this embodiment. Cr is an element necessary for ensuring good steam oxidation resistance, particularly in a high-temperature, steam-containing environment where urea water adheres. If the Cr content is less than 10.0%, a protective, dense oxide scale will not form when the steel sheet is used as a component of a urea SCR system. Therefore, the lower limit of the Cr content is set to 10.0% or more. On the other hand, an excessive Cr content deteriorates the workability of the steel sheet, so the upper limit of the Cr content is set to 25.0% or less. Furthermore, taking weldability into consideration, the Cr content is preferably 14.0 to 18.0%. The Cr content may be 15.0 to 17.5%.

[0037] Nb: More than 0%, less than 0.80% Nb is an element that improves intergranular corrosion resistance and high-temperature strength. If the Nb content exceeds 0.80%, the workability of the steel sheet deteriorates significantly, so the upper limit of the Nb content is set to 0.80%. Furthermore, taking into consideration manufacturability, oxidation resistance, and alloy cost, the Nb content may be 0.003% or more, or 0.005% or more, or 0.30% or less.

[0038] Ti: 0.05 to 0.50% Ti is an element that combines with C, N, and S to improve the corrosion resistance, intergranular corrosion resistance, room-temperature ductility, and deep drawability of steel sheet. If the Ti content is less than 0.05%, the above properties are not expected to improve, so the lower limit of the 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 significantly deteriorates, so the upper limit of the Ti content is set to 0.50% or less. Furthermore, in consideration of oxidation resistance and manufacturability, the Ti content is preferably 0.07 to 0.30%. The Ti content may be 0.10% or more, or 0.15% or more, or 0.25% or less.

[0039] V: 0.01 to 0.15% V is an element that improves corrosion resistance. If the V content exceeds 0.15%, the corrosion resistance and workability of the steel sheet deteriorate, so the upper limit of the V content is set to 0.15% or less. Furthermore, in consideration of manufacturability, oxidation resistance, and alloy costs, the V content is preferably set to 0.01% or more. The V content may be 0.03% or more or 0.05% or more, or may be 0.14% or less or 0.11% or less.

[0040] N: More than 0%, less than 0.050% Like C, N deteriorates the formability and corrosion resistance of steel sheets, so its content is set to 0.050% or less. On the other hand, excessive reduction of the N content increases refining costs. Taking steam oxidation resistance into consideration, the N content is preferably set to 0.002 to 0.045%. The N content may be 0.005% or more, 0.040% or less, or 0.030% or less.

[0041] The ferritic stainless steel sheet of this embodiment may contain one or more of the following in the above chemical composition: Ni: 0.01 to 0.40%, Sn: 0.001 to 0.20%, Mo: 0.05 to 1.40%, and Cu: 0.05 to 1.40%.

[0042] Ni: 0.01 to 0.40% Ni is an element that further improves rust resistance and can be contained as needed. If the Ni content is less than 0.01%, the effect of improving rust resistance is not expected, so if Ni is contained, the lower limit of the Ni content is preferably 0.01% or more. On the other hand, if the Ni content exceeds 0.40%, the oxidation resistance of the steel sheet may deteriorate and the workability may also deteriorate, so 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 desirably 0.10% or more, and preferably 0.30% or less.

[0043] Sn: 0.001 to 0.200% Sn is an element that further improves corrosion resistance and high-temperature strength, so it can be contained as needed. If the Sn content is less than 0.001%, further improvements in corrosion resistance and high-temperature strength are not expected, so if Sn is contained, the lower limit of the Sn content is preferably 0.001% or more. On the other hand, if the Sn content exceeds 0.200%, manufacturability is significantly deteriorated, so the upper limit of the Sn content is set to 0.200% or less. Furthermore, considering workability, oxidation resistance, and alloy cost, the Sn content is preferably 0.010% or more and 0.15% or less.

[0044] Mo: 0.05 to 1.40% Mo is an element that improves the corrosion resistance of steel sheets, so it can be added as needed. If the Mo content is less than 0.05%, further improvement in corrosion resistance is not expected. Therefore, when Mo is added, the lower limit of the Mo content is preferably 0.05% or more. On the other hand, excessive Mo content deteriorates the workability and oxidation resistance of steel sheets and leads to increased alloy costs, so the upper limit of the Mo content is 1.40% or less. Furthermore, considering manufacturability, scale adhesion, and alloy costs, the Mo content is preferably 0.30% or more and 1.20% or less. The Mo content may be 0.50% or more.

[0045] Cu: 0.05 to 1.40% Cu is an element that improves the rust resistance and high-temperature strength of steel sheets, and can be added as needed. If the Cu content is less than 0.05%, further improvement in corrosion resistance is not expected, so if Cu is added, the lower limit of the Cu content is preferably 0.05% or more. On the other hand, if the Cu content exceeds 1.40%, the ductility of the steel sheet is significantly deteriorated, so the upper limit of the Cu content is set to 1.40% or less. Furthermore, in consideration of manufacturability and oxidation resistance, the Cu content is preferably 0.30% or more and 1.20% or less.

[0046] The ferritic stainless steel sheet of this embodiment may further contain the following elements in addition to the above chemical composition, as necessary.

[0047] One or more of B: 0.0003 to 0.0020%, Sb: 0.005 to 0.5%, Zr: 0.005 to 0.5%, Co: 0.005 to 0.5%, W: 0.005 to 0.5%, Ta: 0.005 to 0.100%, Mg: 0.0001 to 0.0050%, Ca: 0.0001 to 0.0050%, Ga: 0.001 to 0.05%, and REM: 0.001 to 0.1%.

[0048] B is an element that improves secondary workability during part molding and can be added as needed. If the B content exceeds 0.0020%, manufacturability and intergranular corrosion resistance will be significantly deteriorated, so the upper limit of the B content is set to 0.0020% or less. Furthermore, considering workability, oxidation resistance, and alloy cost, if B is included, the B content is preferably set to 0.0003% or more and 0.0010% or less.

[0049] Sb, Zr, Co, and W are all elements that improve corrosion resistance, and one or more of them can be added as needed. When one or more of these elements are added, the effect is achieved by adding 0.005% or more of each. These elements are important for suppressing the corrosion rate, but excessive content reduces manufacturability and costs, so the upper limit of each content is set to 0.5% or less. The more desirable ranges for these elements are 0.05% or more and 0.4% or less, respectively.

[0050] Ta is an element that improves corrosion resistance by modifying inclusions. When Ta is contained, the above effect is exhibited if the Ta content is 0.005% or more. On the other hand, if the Ta content exceeds 0.100%, it may lead to a decrease in ductility and toughness at room temperature. Therefore, the Ta content is preferably 0.100% or less, and more preferably 0.050% or less.

[0051] Mg is an element that acts as a deoxidizer by forming Mg oxide with Al in molten steel, and also acts as a nucleus for TiN crystallization. TiN acts as a nucleus for the ferrite phase during the solidification process. Therefore, the inclusion of Mg promotes the crystallization of TiN, thereby enabling the formation of a fine ferrite phase during solidification. By refining the solidification structure, surface defects caused by coarse solidification structures, such as ridging and roping, which tend to occur when steel sheets are formed into product shapes, can be prevented. In addition, refining the solidification structure improves workability. Therefore, Mg may be added as needed.

[0052] When Mg is contained, it is preferable to contain 0.0001% or more to exert these effects. However, if the Mg content exceeds 0.0050%, manufacturability deteriorates, so the upper limit of the Mg content is set to 0.0050% or less. In consideration of manufacturability, the Mg content is preferably set to 0.0003% or more, and more preferably 0.0020% or less.

[0053] Ca is an element that improves oxidation resistance when contained in trace amounts, and may be contained in the range of 0.0050% or less, preferably 0.0001% or more.

[0054] Ga is an element that contributes to improving workability, and may be contained in the range of 0.05% or less, preferably 0.001% or more.

[0055] REM is an element that improves hot workability and steel cleanliness, and is effective in improving the corrosion resistance of steel sheets, and may be added as needed. When REM is added, it is preferable that the REM content be 0.001% or more, at which point the effect is manifested. However, since an excessive REM content increases alloy costs and reduces manufacturability, the upper limit is set to 0.1% or less. Preferably, taking into consideration the effect, economy, and manufacturability, the REM content is set to 0.001% or more and 0.05% or less. Here, REM (rare earth elements) refers to two elements, scandium (Sc) and yttrium (Y), and 15 elements (lanthanoids) from lanthanum (La) to lutetium (Lu). In this embodiment, "REM" refers to a material composed of one or more of these elements. The REM content refers to the total content of these elements. Examples of REM include La, Ce, and Nd.

[0056] In the ferritic stainless steel sheet of this embodiment, the balance other than the above-mentioned elements is Fe and impurities. However, elements other than the above-mentioned elements may also be contained within a range that does not impair the effects of this embodiment. Note that the impurities referred to here refer to components that are mixed in during industrial production of the ferritic stainless steel sheet of this embodiment due to raw materials such as ore and scrap, or various factors in the manufacturing process, and are acceptable within a range that does not adversely affect the present invention.

[0057] In the steel sheet of this embodiment, the passive film present on the steel sheet surface has a composition parameter PS represented by formula (1) of 0.03 to 0.15, and the surface coverage of the passive film with Al-containing oxides having a circle-equivalent diameter of 0.01 to 1.0 μm is 15 to 40%. In this case, improvement in steam oxidation resistance is observed. The Al-containing oxides are contained in the passive film on the steel sheet surface.

[0058] <Composition parameter of passive film PS (= Al / (Ti + Mn + Si + Cr)): 0.03 to 0.15> In this embodiment, it has been found that the steam oxidation resistance is improved when the amount of Al-containing oxide contained in the passive film is within a certain range. In this embodiment, a composition parameter PS is used as a parameter for evaluating the amount of Al-containing oxide contained in the passive film. The composition parameter PS is an index devised based on the following idea. First, the Al concentration in the passive film is focused on, and the depth position in the thickness direction of the passive film at which the Al concentration is maximum is identified. Then, it has been found that there is an optimal range for the relationship between the concentrations of the constituent elements (Al, Ti, Mn, Si, and Cr) of the passive film at the identified depth position from the perspective of ensuring the Al-containing oxide contained in the film. Specifically, the composition parameter PS can be calculated by incorporating Al, Ti, Mn, Si, and Cr at the identified depth position into Equation (1).

[0059] The relationship between the oxidation weight gain of a steel sheet and the composition parameter PS is shown in Figure 1. The steel sheet shown in Figure 1 is a steel sheet that has been subjected to finish annealing in a finish annealing process under the following conditions: atmosphere: 1 to 50% H2 + balance N2, atmospheric dew point: -40 to -20°C, soaking temperature: 814 to 1000°C, soaking time: 0 to 700 seconds, and then electrolytic pickling in an acid solution containing 0.5 to 10.9 g / L of fluoride ions.

[0060] As shown in Figure 1, when the composition parameter PS of the passive film is less than 0.03, the oxidation weight gain of the steel sheet is 0.03 mg / cm 2 As a result, the passive film does not exhibit excellent oxidation resistance in an environment exposed to exhaust gases and urea water. This is thought to be because the Al concentration in the oxides that make up the passive film is low, resulting in coarse oxides that easily permeate water vapor and oxygen, making it difficult to suppress oxidation. On the other hand, when the composition parameter PS exceeds 0.15, the surface coverage of the Al-containing oxide exceeds 40%, resulting in poor oxidation resistance. In other words, when the composition parameter PS of the passive film is 0.03 or more and 0.15 or less, the passive film becomes a dense oxide that is difficult for water vapor and oxygen to permeate, and excellent oxidation resistance can be exhibited. For these reasons, the composition parameter PS of the passive film is set to 0.03 or more and 0.15 or less.

[0061] <Surface coverage of Al-containing oxides with a circle equivalent diameter of 0.01 to 1.0 μm: 15 to 40%> (Circle equivalent diameter of Al-containing oxide: 0.01 to 1.0 μm) When determining the surface coverage of the Al-containing oxide, the Al-containing oxide to be measured is an Al-containing oxide having an equivalent circle diameter of 0.01 to 1.0 μm. When the surface coverage of the Al-containing oxide having an equivalent circle diameter of 0.01 to 1.0 μm is 15 to 40%, the steel sheet of this embodiment can exhibit excellent steam oxidation resistance in an environment exposed to exhaust gas and urea water. On the steel sheet surface not covered with the Al-containing oxide, oxidation is promoted and oxide scale is formed. This oxide scale grows two-dimensionally to become thick and easily peels off, resulting in poor oxidation resistance. On the other hand, an Al-containing oxide having an equivalent circle diameter of 0.01 to 1.0 μm can suppress the two-dimensional growth of oxide scale in a pinning-like manner, and is therefore thought to exhibit excellent oxidation resistance. When the equivalent circle diameter of the Al-containing oxide is less than 0.01 μm, the pinning effect is small and does not contribute to improving oxidation resistance. Furthermore, Al-containing oxides with an equivalent circle diameter of more than 1.0 μm grow in the thickness direction and easily peel off during winding or processing of the product steel strip, resulting in a loss of excellent oxidation resistance. For these reasons, Al-containing oxides with an equivalent circle diameter in the range of 0.01 to 1.0 μm are used as the object of measurement for the surface coverage rate.

[0062] (Surface coverage of Al-containing oxide: 15-40%) Figure 2 shows the relationship between the composition parameter PS, the surface coverage of Al-containing oxides, and the oxidation mass gain of steel sheets. Even when the composition parameter PS of the passive film satisfies the range of 0.03 to 0.15, excellent oxidation resistance cannot be achieved if the surface coverage of Al-containing oxides with a circle-equivalent diameter of 0.01 to 1.0 μm is less than 15%. As mentioned above, oxidation is accelerated on surfaces not covered by Al-containing oxides, resulting in the formation of oxide scale. When the surface coverage of Al-containing oxides is less than 15%, this oxide scale grows two-dimensionally on the steel sheet surface and further connects to form a thick oxide scale that easily peels off, resulting in poor oxidation resistance. On the other hand, when the surface coverage of Al-containing oxides is 15% or more, the two-dimensional growth of the oxide scale is suppressed like pinning, resulting in excellent oxidation resistance. Furthermore, when the surface coverage of Al-containing oxides exceeds 40%, the Al-containing oxides grow in the thickness direction and easily peel off during coiling and processing of the steel strip, resulting in poor oxidation resistance. For these reasons, the steel sheet surface coverage rate of Al-containing oxides having a circle-equivalent diameter of 0.01 to 1.0 μm is set to a range of 15 to 40%. From the viewpoint of further exerting the pinning effect, the steel sheet surface coverage rate of Al-containing oxides having a circle-equivalent diameter of 0.01 to 1.0 μm is preferably 20% or more. Furthermore, from the viewpoint of suppressing excessive growth of Al-containing oxides in the thickness direction, the steel sheet surface coverage rate of Al-containing oxides having a circle-equivalent diameter of 0.01 to 1.0 μm is preferably 35% or less.

[0063] The steam oxidation resistance of the steel sheet of this embodiment is determined by whether or not a scale called red scale is formed in addition to the oxidation mass gain associated with oxidation. Red scale is a thick oxide scale that forms when the steel sheet is continuously oxidized in a steam atmosphere at a temperature of about 500°C due to an increase in oxidation mass gain. Peeling of red scale is undesirable because it may deteriorate the NOx reduction performance in a urea SCR system.

[0064] The graphs shown in Figures 1 and 2 were obtained by measuring the composition parameter PS of the passive film of the steel sheets, the circle equivalent diameter of the Al-containing oxides, and the surface coverage rate, as well as measuring the steam oxidation resistance of each steel sheet in the following experiment.

[0065] (experiment) Ferritic stainless steel sheets were produced as samples through the steps of casting, hot rolling, pickling, cold rolling, annealing, and pickling. The inventors conducted a detailed study of the steel composition, as well as the relationship between the final annealing and pickling conditions when annealing and pickling the cold-rolled steel sheets, the composition of the passive film, and the oxidation weight gain and the occurrence of red scale in oxidation tests.

[0066] The steam oxidation test was carried out by humidifying argon containing 10% O2 to create a 7.5% water vapor atmosphere, and leaving the steel sheet in this water vapor atmosphere at 500°C for 50 hours. The test specimens used for the steam oxidation test were steel sheets measuring 25 mm in length and 20 mm in width, with the front and back surfaces left as they were after the final pickling, and the edges polished with a #600 grit polishing.

[0067] The test specimens for the steam oxidation tests were 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-0.011%N. These cold-rolled steel sheets were heat-treated (finish annealed) in an atmosphere of 1-50% H2 + balance N2, with a dew point of -40°C to -20°C, annealing temperatures (soaking temperatures) of 814°C to 1000°C, and soaking times of 0 to 700 seconds. They were then electrolytically pickled in an acid solution containing 0.5 to 10.9 g / L of fluoride ions. The composition of the passive film on the test specimens was analyzed before the steam oxidation tests.

[0068] The composition parameter PS in the passive film is measured as follows: A circular measurement area with a diameter of 4 mm is set on the surface of the passive film. The passive film is continuously sputtered from the surface to a depth of 100 nm within this measurement area, while the element content within the measurement area is measured by glow discharge optical emission spectroscopy. Measurements by glow discharge optical emission spectroscopy are performed every 5 nm. Based on the measurement data, the element distribution from the surface to a depth of 100 nm is measured. The elements to be measured are elements that can generate cations. Focusing on the depth distribution of Al among the detected elements, the depth position where the Al content is maximum is identified. The contents (mass%) of Al, Ti, Mn, Si, and Cr at the identified depth position are then calculated. The contents of each element are expressed as a percentage of the total amount (Al, Ti, Mn, Si, and Cr) of the elements to be measured detected by glow discharge optical emission spectroscopy. The composition parameter PS is obtained by inserting the contents (mass%) of Al, Ti, Mn, Si, and Cr into equation (1). The glow discharge optical emission spectroscopy is carried out using a glow discharge optical emission spectroscopy analyzer (GDA750, manufactured by Rigaku Corporation).

[0069] The surface coverage of Al-containing oxides with a circle-equivalent diameter of 0.01 to 1.0 μm is determined by combining elemental mapping analysis of the passive film surface using an electron probe microanalyzer (EPMA (JXA-8530F, manufactured by JEOL Ltd.)) and observation of secondary electron images using an FE-SEM. First, elemental mapping analysis is performed on the passive film surface within a 2 mm square measurement area at a 4 μm square pitch to measure the Al distribution. Within the obtained Al distribution, the region with an intensity of 10% or more of the maximum intensity is defined as the Al-containing oxide region. In other words, if the maximum Al intensity within the measurement area is taken as 100, the region with an Al intensity of 10 or more is defined as the Al-containing oxide presence region. The surface coverage of Al-containing oxides is defined as the ratio of the area of ​​the Al-containing oxide presence region to the area of ​​the 2 mm square measurement area.

[0070] In addition, the region where the Al-containing oxide was found by the elemental mapping analysis is observed in the secondary electron image of the EPMA, and the morphology of the Al-containing oxide is identified. Specifically, five fields of view of the secondary electron image at a magnification of 10,000 times are observed, and the Al-containing oxide is identified in this secondary electron image. The area is measured by image processing, and the circle-equivalent diameter is calculated assuming that the Al-containing oxide is circular. The Al-containing oxide with a circle-equivalent diameter in the range of 0.01 to 1.0 μm is identified.

[0071] <Manufacturing method> Next, a method for manufacturing the steel sheet according to this embodiment will be described. The method for producing a steel sheet according to this embodiment includes the steps of steelmaking, hot rolling, pickling, cold rolling, and annealing / pickling. In steelmaking, a method is preferred in which a steel having a composition containing the essential components and optional components, as required, is melted in a converter, followed by secondary refining. The melted steel is formed into a slab by a known casting method (continuous casting). The slab is heated to a predetermined temperature and hot-rolled to a predetermined thickness by continuous rolling.

[0072] The hot-rolled steel sheet is pickled as needed. Hot-rolled steel sheet annealing may be performed or omitted depending on productivity and material properties.

[0073] Regarding the cold rolling conditions, typical cold rolling of stainless steel sheets is either reverse rolling using a Sendzimir rolling mill with a roll diameter of about 50 to 100 mm, or unidirectional rolling using a tandem rolling mill with a roll diameter of 400 mm or more. Either cold rolling method may be adopted in the manufacturing method of this embodiment. Tandem rolling is also superior to Sendzimir rolling in terms of productivity. In order to increase the r-value (Lankford value), which is an index of workability, it is preferable to perform cold rolling using a tandem rolling mill with a roll diameter of 400 mm or more.

[0074] In the manufacturing method of this embodiment, a predetermined passivation film is obtained by final annealing and pickling, in which the cold-rolled steel sheet is annealed and pickled after cold rolling. Various methods are known as general heat treatment conditions (annealing conditions) for stainless steel sheets after cold rolling, but the inventors have found that by subjecting the cold-rolled steel sheet to final annealing and pickling under the specific conditions described below, it is possible to cause Al oxides that have been concentrated by the heat treatment to remain on the steel sheet surface even after pickling.

[0075] The inventors, while investigating the conditions for final annealing and pickling, found that by subjecting Al-containing cold-rolled ferritic stainless steel sheet to a final annealing process under conditions of an annealing atmosphere of 3 to 20% H2 + balance N2, an atmospheric dew point of -50 to -20°C, a soaking temperature of 700 to 950°C, and a soaking time of 5 seconds to 10 minutes, followed by electrolytic pickling in an acid solution containing 1.0 to 10.0 g / L of fluoride ions, Al oxides can also be retained on the steel sheet surface, resulting in a composition parameter PS of 0.03 or greater in the region within 100 nm from the steel sheet surface. They also found that the steel sheet obtained by the final annealing and pickling process, and urea SCR system components manufactured from this steel sheet, such as tanks, pipes, plates, rods, and springs, have satisfactory steam oxidation resistance.

[0076] The final annealing step is performed in an atmosphere containing 3 to 20% hydrogen gas and the remainder being nitrogen gas (3 to 20% H2 + remainder N2). If the hydrogen gas concentration in the atmosphere is less than 3%, nitriding of the steel sheet surface may be promoted and the generation of Al oxides may be suppressed. Therefore, the hydrogen gas concentration in the annealing atmosphere is set to 3% or more. The hydrogen gas concentration is preferably 5% or more. On the other hand, if the hydrogen gas concentration in the atmosphere exceeds 20%, hydrogen penetration during the formation of an Al-based or Al-based oxide film may promote oxidation, which may have an adverse effect on the formation of the oxide film. In addition, if the hydrogen gas concentration in the atmosphere exceeds 20%, there is a risk of increased costs. Therefore, the hydrogen gas concentration in the annealing atmosphere is set to 20% or less. The hydrogen gas concentration is preferably 15% or less. The dew point of the atmosphere in the final annealing step is set to a range of -50 to -20°C. If the dew point of the annealing atmosphere exceeds -20°C, the oxidation of Al becomes insufficient and the composition of the passive film does not satisfy formula (1), so the upper limit of the dew point is set to -20°C or less. Furthermore, if the dew point of the annealing atmosphere is less than -50°C, the effect of forming Al oxides saturates and costs increase, so the lower limit of the dew point is set to -50°C or more.

[0077] The soaking temperature in the finish annealing step is set to a range of 700 to 950°C. If the soaking temperature during finish annealing is less than 700°C, the composition of the passive film will not satisfy formula (1), recrystallization will not be promoted, and the necessary ductility for the material will not be obtained. Therefore, the lower limit of the soaking temperature is set to 700°C or higher. On the other hand, if the soaking temperature exceeds 950°C, excessive softening will become a problem, so the upper limit is set to 950°C or lower.

[0078] Furthermore, the soaking time in the finish annealing step is set to 5 seconds or more and 10 minutes or less. If the soaking time exceeds 10 minutes, excessive softening of the material becomes a problem, so the soaking time is set to 10 minutes or less. There is no particular lower limit to the soaking time, but if the soaking time is less than 5 seconds, stable recrystallization does not occur, and the oxidation reaction of Al becomes insufficient, resulting in a surface coverage of Al-containing oxides of less than 15% after electrolytic pickling. Therefore, the soaking time is set to 5 seconds or more.

[0079] Electrolytic pickling after finish annealing is performed in a solution containing HCl, NaSO4, H2SO4, NaNO3, Na2SiF6, etc. Preferably, electrolytic pickling is performed in a solution containing 1.0 to 10.0 g / L of fluorine ions. If the fluorine ion concentration in the solution used in electrolytic pickling is less than 1.0 g / L, there is a risk that the Al-containing oxide that coats the surface will be present in excess. Therefore, the fluorine ion concentration in the solution is preferably 1.0 g / L or more. It is more preferable that the fluorine ion concentration is 2.0 g / L or more. On the other hand, if the fluorine ion concentration in the solution exceeds 10.0 g / L, there is a risk that the Al-containing oxide that coats the surface will be excessively removed and will no longer be present. Therefore, the fluorine ion concentration in the solution is preferably 10.0 g / L or less. It is more preferable that the fluorine ion concentration is 8.0 g / L or less. If salt treatment is performed after the above-described final annealing, the surface coverage of the steel sheet with Al-containing oxides will be less than 15%. Therefore, salt treatment is not performed after final annealing. A combination of salt treatment and electrolytic pickling is also not preferable from the viewpoint of ensuring the surface coverage of the steel sheet with Al-containing oxides.

[0080] There are no particular restrictions on the manufacturing methods for other processes, but the hot rolling conditions, hot rolled sheet thickness, etc. may be selected as appropriate. Temper rolling and tension leveling may also be applied after cold rolling and annealing. Furthermore, the product sheet thickness may also be selected according to the required thickness of the component. [Example]

[0081] As explained in Examples 1 and 2 below, molten steel having the chemical composition shown in Table 1, adjusted to the specified composition, was produced, and this molten steel was cast to obtain a slab. The resulting slab was hot-rolled and further pickled, and a 1.0 mm-thick cold-rolled sheet was produced under the cold-rolling, finish-annealing, and pickling conditions shown in Table 3. The 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. Note that underlines in Tables 1 to 3 indicate values ​​outside the scope of the present invention. Furthermore, the symbol "-" in Table 1 means that the content of the corresponding element is 0% in significant figures (numbers down to the least significant digit) specified in the embodiment.

[0082] After cold rolling, the steel sheets were subjected to finish annealing in a furnace with controlled atmosphere, dew point, soaking temperature, and soaking time. After finish annealing, electrolytic pickling was carried out using a pickling solution containing NaSO, HSO, NaNO, and NaSiF, with the fluorine ion concentration shown in Table 3.

[0083] Electrolytic pickling is performed by alternating electrolysis, which alternates between anodic and cathodic electrolysis, at a current density of 60A / dm 2 The total electrolysis time was 10 seconds. Comparative Example c6 is a case where electrolytic pickling was performed after salt immersion treatment. The salt treatment was performed by immersing the cold-rolled steel sheet in a molten salt mixture of NaNO3 and NaOH heated to 470°C for 10 seconds. The obtained steel sheet was subjected to a steam oxidation test.

[0084] The composition parameter PS in the passive film was measured as follows. A circular measurement area with a diameter of 4 mm was set on the surface of the passive film. The passive film was continuously sputtered from the film surface to a depth of 100 nm within this measurement area, while the element contents within the measurement area were measured by glow discharge optical emission spectroscopy. Measurements by glow discharge optical emission spectroscopy were performed every 5 nm. Based on the measurement data, the element distribution from the film surface to a depth of 100 nm was measured. The elements to be measured were those capable of generating cations. Focusing on the depth distribution of Al among the detected elements, the depth position at which the Al content was maximum was identified. The contents (mass%) of Al, Ti, Mn, Si, and Cr at the identified depth position were then calculated. The contents of each element are expressed as a percentage of the total amount of the target element detected by glow discharge optical emission spectroscopy. The composition parameter PS was obtained by incorporating the contents (mass%) of Al, Ti, Mn, Si, and Cr into equation (1). The glow discharge optical emission spectroscopy was performed using a glow discharge optical emission spectroscopy analyzer (GDA750, manufactured by Rigaku Corporation).

[0085] The surface coverage of Al-containing oxides with a circle-equivalent diameter of 0.01 to 1.0 μm was determined by combining elemental mapping analysis of the passive film surface using an electron probe microanalyzer (EPMA, manufactured by JEOL Ltd., JXA-8530F) and secondary electron image observation using a field emission scanning electron microscope (FE-SEM, manufactured by JEOL Ltd., JSM-7001F). First, elemental mapping analysis was performed on the passive film surface within a 2 mm square measurement area at a 4 μm square pitch to measure the Al distribution. Within the obtained Al distribution, the region with an intensity of 10% or more of the maximum intensity was defined as the Al-containing oxide region. The surface coverage of Al-containing oxides was determined as the ratio of the area of ​​the Al-containing oxide region to the area of ​​the 2 mm square measurement area (referred to as "Steel sheet surface coverage" in Table 3).

[0086] In addition, the regions where Al-containing oxides were identified by elemental mapping analysis were observed in secondary electron images of an EPMA to identify the morphology of the Al-containing oxides. Specifically, five secondary electron images at 10,000x magnification were observed, and the Al-containing oxides were identified in these secondary electron images. Their areas were measured by image processing, and the equivalent circle diameters were calculated assuming that the Al-containing oxides were circular. Al-containing oxides with equivalent circle diameters in the range of 0.01 to 1.0 μm were identified.

[0087] The steam oxidation test was carried out in a 7.5% water vapor atmosphere, which was prepared by humidifying argon containing 10% O2. The steel plate was left in this water vapor atmosphere at 500°C for 50 hours. The test specimens for the steam oxidation test were 25mm long and 20mm wide, with the front and back surfaces left as they were after the final pickling, and the edges polished with a #600 grit polishing.

[0088] The steam oxidation resistance was evaluated using the oxidation weight after the steam oxidation test as an index. 2 In the following cases, the resistance to steam oxidation was judged to be excellent and the sample was rated as "pass" (very good) (denoted as "AA" in Tables 2 and 3). 2 Ultra, 0.03mg / cm 2 In the following cases, the resistance to steam oxidation was judged to be excellent and was judged to be good (denoted as "A" in Tables 2 and 3). On the other hand, if the oxidation weight was 0.03 mg / cm 2 In the above cases, the oxide scale easily peels off and there is a risk of damaging equipment related to the urea SCR system, so the product is judged to be unacceptable (denoted as "B" in Tables 2 and 3). Note that "oxidation resistance" in Tables 2 and 3 refers to resistance to steam oxidation.

[0089] Example 1 For steel Nos. A1 to A28 and a1 to a11 shown in Table 1, the steps after cold rolling were carried out under the condition of symbol C7 shown in Table 3 to produce stainless steel sheets.

[0090] As shown in Table 2, the steel components, composition parameter PS of the passive film, and coverage of the Al-containing oxide in the samples B1 to B28 satisfied the conditions of the present invention, and therefore had good steam oxidation resistance in the steam oxidation test.

[0091] In the specimen with symbol b1, Al was outside the lower limit range, the composition parameter PS was low, and the coverage area ratio of the Al-containing oxide was less than 15%, so the steam oxidation resistance was poor.

[0092] In the specimen b2, the Al content was outside the upper limit range, and the coverage of the Al-containing oxide exceeded 40%, so the steam oxidation resistance was poor.

[0093] In the specimen b3, C was outside the upper limit range, abnormal oxidation occurred, suppressing the generation of Al oxides, and the composition parameter PS did not satisfy the range of the present invention, resulting in poor steam oxidation resistance.

[0094] In the specimen b4, the Si and Mn contents were outside the upper limit ranges, and Si and Mn were concentrated in the oxide, so that the composition parameter PS did not satisfy the range of the present invention, and therefore the steam oxidation resistance was poor.

[0095] In the specimen b5, P and S were outside the upper limits, and the composition parameter PS did not satisfy the range of the present invention, so the steam oxidation resistance was poor.

[0096] In the case of symbol b6, the Cr content was outside the upper limit, which resulted in excessive improvement in corrosion resistance, making it difficult to dissolve and remove the Cr in the oxide by pickling, and the composition parameter PS no longer satisfied the range of the present invention, resulting in poor steam oxidation resistance.

[0097] In the specimen b7, the Cr content was below the lower limit, Fe was concentrated in the oxide to the extent that it could not be completely dissolved by pickling, and the composition parameter PS did not satisfy the range of the present invention, resulting in poor steam oxidation resistance.

[0098] In the case of b8, the Mo and Cu contents exceeded the upper limits, resulting in excessively improved corrosion resistance. As a result, it became difficult to dissolve and remove the oxides of Fe and Cr during pickling, and the range of the composition parameter PS was no longer satisfied, resulting in poor steam oxidation resistance.

[0099] In the specimen b9, the Mo and Cu contents were below the lower limits, resulting in poor scale adhesion. Furthermore, excessive pickling was performed due to the deterioration of corrosion resistance, which promoted the removal of Al-containing oxides and resulted in a coating rate of less than 15%. As a result, the steam oxidation resistance was poor.

[0100] In the case of b10, the contents of Nb, Ti, V, and N were outside the upper limits, resulting in a decrease in workability due to the excessive Nb and V contents. In addition, Ti was concentrated in the oxides, and the composition parameter PS did not satisfy the range of the present invention. Furthermore, the deterioration of corrosion resistance due to the excessive N content led to excessive pickling of the steel sheet, which promoted the removal of Al-containing oxides and resulted in a coverage of Al-containing oxides of less than 15%. As a result, the steam oxidation resistance was poor.

[0101] In the case of b11, the V content was below the lower limit, resulting in an increase in smelting costs. In addition, the corrosion resistance was deteriorated due to the insufficient V content, which resulted in excessive pickling of the steel sheet, accelerating the removal of Al-containing oxides and resulting in a coverage of Al-containing oxides of less than 15%. As a result, the steam oxidation resistance was poor.

[0102] [Table 1]

[0103] [Table 2]

[0104] <Example 2> The steps after cold rolling of steel Nos. A4 to A8, A14, 15, 17, and 18 shown in Table 1 were carried out under the conditions indicated by symbols C1 to C10 and c1 to c6 shown in Table 3 to produce steel sheets.

[0105] As shown in Table 3, the materials C1 to C10 had good steam oxidation resistance in the steam oxidation test because the manufacturing process after cold rolling, the composition parameter PS of the passive film, and the coverage rate of the Al-containing oxide satisfied the conditions of the present invention.

[0106] Since the specimen c1 was annealed in an atmosphere with a dew point above the upper limit of -20°C, the composition parameter PS of the passive film did not meet the range of the invention, the coverage of the Al-containing oxide was less than 15%, and the resistance to steam oxidation was poor.

[0107] Reference symbol c2 was annealed at a temperature below 700°C, which is the lower limit of the soaking temperature, and the composition parameter PS of the passive film did not meet the range of the invention, and the coverage of the Al-containing oxide was less than 15%, so the steam oxidation resistance was poor.

[0108] The specimen c3 was annealed at a temperature exceeding 950°C, which is the upper limit of the soaking temperature, and the coverage of the Al-containing oxide exceeded 40%, resulting in poor steam oxidation resistance.

[0109] Reference symbol c4 was annealed for less than 5 seconds, which is the lower limit of the soaking time, and the coverage of the Al-containing oxide was less than 15%, so the steam oxidation resistance was poor. Note that the "soaking time: 0 seconds" for reference symbol c4 means that the finish annealing was completed the moment the soaking temperature was reached.

[0110] The specimen c5 was annealed for more than 600 seconds (10 minutes), which is the upper limit of the soaking temperature, and the coverage of the Al-containing oxide exceeded 40%, resulting in poor steam oxidation resistance.

[0111] In the case of the specimen c6, the salt immersion accelerated the dissolution and detachment of the Al-containing oxide, resulting in a coverage rate of the Al-containing oxide of less than 15%, and the steam oxidation resistance was poor.

[0112] [Table 3] [Industrial Applicability]

[0113] As is clear from the above explanation, the present invention provides a ferritic stainless steel sheet that is suitable for urea SCR systems and has excellent steam oxidation resistance, without the need to add large amounts of expensive alloying elements. In particular, by applying the ferritic stainless steel sheet of the present invention to the urea SCR systems of diesel vehicles, excellent urea SCR system components can be manufactured. Manufacturing urea SCR systems using these components can significantly contribute to environmental protection.

Claims

1. The chemical composition, in mass%, is C: more than 0%, less than 0.008%, Si: 0.01-2.50%, Mn: 0.01 to 0.50%, P: 0.0001-0.040%, S: 0.001-0.010%, Al: 0.001-2.500%, Cr: 10.0-25.0%, Nb: more than 0%, less than 0.80%, Ti: 0.05-0.50%, V: 0.01-0.15%, N: more than 0%, less than 0.050%, Ni: 0 to 0.40%, Sn: 0-0.200%, Mo: 0 to 1.40%, Cu: 0 to 1.40%, B: 0 to 0.0020%, Sb: 0 to 0.5%, Zr: 0 to 0.5%, Co: 0 to 0.5%, W: 0 to 0.5%, Ta: 0-0.100%, Mg: 0 to 0.0050%, Ca: 0-0.0050%, Ga: 0 to 0.05%, and REM: 0~0.1% with the remainder being Fe and impurities, With respect to the passive film present on the steel sheet surface, the composition parameter PS represented by formula (1) is 0.03 or more and 0.15 or less, A ferritic stainless steel sheet characterized in that the surface coverage of the passive film with Al-containing oxides having an equivalent circle diameter of 0.01 to 1.0 μm is 15 to 40%. PS=Al / (Ti+Mn+Si+Cr)... Formula (1) Here, the element symbols in formula (1) represent the content (mass %) of each element at the depth position where the Al content is maximum in the depth direction of the passive film.

2. In the chemical composition, in mass %, Ni: 0.01 to 0.40%, Sn: 0.001-0.200%, Mo: 0.05-1.40%, Cu: 0.05-1.40% 2. The ferritic stainless steel sheet according to claim 1, further comprising one or more selected from the following:

3. In the chemical composition, in mass %, B: 0.0003 to 0.0020%, Sb: 0.005 to 0.5%, Zr: 0.005 to 0.5%, Co: 0.005 to 0.5%, W: 0.005-0.5%, Ta: 0.005-0.100%, Mg: 0.0001 to 0.0050%, Ca: 0.0001-0.0050%, Ga: 0.001-0.05%, REM: 0.001-0.1%, 3. The ferritic stainless steel sheet according to claim 1, further comprising one or more of the following:

4. The method for producing a ferritic stainless steel sheet according to claim 1 or 2, A method for producing a ferritic stainless steel sheet, characterized in that a heat treatment is performed in a finish annealing step in which the dew point of the atmosphere is set in the range of -50 to -20°C, the soaking temperature is set in the range of 700 to 950°C, and the soaking time is set in the range of 5 seconds to 10 minutes, followed by electrolytic pickling.

5. 5. The method for producing a ferritic stainless steel sheet according to claim 4, wherein the fluorine ion concentration in the solution used in the electrolytic pickling is 1.0 to 10.0 g / L.

6. A method for producing the ferritic stainless steel sheet according to claim 3, comprising: A method for producing a ferritic stainless steel sheet, characterized in that a heat treatment is performed in a finish annealing step in which the dew point of the atmosphere is set in the range of -50 to -20°C, the soaking temperature is set in the range of 700 to 950°C, and the soaking time is set in the range of 5 seconds to 10 minutes, followed by electrolytic pickling.

7. A method for manufacturing a ferritic stainless steel sheet as described in Claim 6, characterized in that the fluorine ion concentration in the solution used in the electrolytic pickling is 1.0 to 10.0 g / L.

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