Ferritic stainless steel for denitration device

Abstract

The purpose of the present invention is to provide ferritic stainless steel for a denitration device having excellent corrosion resistance in a use environment of the denitration device.　The ferritic stainless steel for a denitration device has a component composition containing, in terms of mass%, 0.003%-0.030% of C, 0.030% or less of N, 0.20%-1.00% of Si, 0.05%-0.50% of Mn, 0.050% or less of P, 0.020% or less of S, 13.0%-21.0% of Cr, 0.001%-3.00% of Mo, 0.001%-0.50% of Al, 0.001%-0.60% of Nb, and 0.001%-0.15% of Ti, with the balance composed of Fe and inevitable impurities, wherein formulae (1) and (2) are satisfied. (1): (Ti + Nb × 47 / 97) / (8 × (C + N)) ≥ 1.0 (2): Cr - 12Nb - 2.0Mo - Si ≥ 10.0 In formulae (1) and (2), Ti, Nb, C, N, Cr, Mo, and Si indicate the contents (mass%) of the respective elements.

Description

Ferritic stainless steel for denitration equipment

[0001] The present invention relates to a ferritic stainless steel for use in denitration devices, and in particular to a ferritic stainless steel used in denitration devices such as urea SCR (Selective Catalytic Reduction) devices used to remove nitrogen oxides (NOx) contained in exhaust gases in automobiles, ships, thermal power plants, and the like.

[0002] In recent years, there has been a growing demand for improved exhaust gas purification in order to protect the global environment, and as a result, the use of denitration devices such as urea SCR has been expanding.

[0003] Here, urea SCR is an exhaust gas purification system that uses ammonia, a decomposition product of urea, as a reducing agent for NOx contained in exhaust gas. Ammonia reduces NOx through a catalyst to nitrogen (N ２ ) and water (H ２ However, there are safety issues in terms of flammability and toxicity, making it difficult to use directly in automobiles, etc. Urea SCR is also safe because it reduces NOx using ammonia obtained by decomposing urea as described above. For this reason, urea SCR is widely used in automobiles, thermal power plants, etc. The size of urea SCR varies depending on the application, but the principle of denitrification is the same for all.

[0004] In large-scale plants, anhydrous ammonia or aqueous ammonia may be used instead of urea. Anhydrous ammonia, in particular, is highly toxic, but can be used directly to reduce NOx. Ammonia water requires hydrolysis, but is safer than anhydrous ammonia. In the future, it will be used in thermal power plants, industrial furnaces, ships, and other places to reduce CO. ２ To reduce NOx emissions, it is expected that ammonia will be used as fuel, but even in this case, there is a possibility that NOx will be contained in the exhaust gas. Therefore, it is expected that the need for various types of denitration equipment will increase.

[0005] In urea-SCR, urea and ammonia rarely corrode stainless steel. However, during the process of decomposing urea-water solution at high temperatures to generate ammonia, and when unreacted urea-water solution evaporates, substances such as ammonia carbamate are generated that corrode stainless steel. Because urea-SCR is used in commercial vehicles such as buses and trucks that travel long distances, as well as in thermal power plants that operate for decades, high corrosion resistance to such corrosive substances is important.

[0006] For the reasons described above, stainless steel, which has excellent corrosion resistance, is used as a catalyst carrier for a denitration device such as urea SCR, and several techniques such as those described below have been disclosed.

[0007] For example, Patent Document 1 discloses a ferritic stainless steel as a material for urea SCR, in which the corrosion resistance of the base metal and weld metal is ensured by controlling the contents of C, N, Ti, Nb, Cr, and Mo in the base metal. Patent Document 2 discloses a ferritic stainless steel as a material for urea SCR, in which the corrosion resistance is ensured by controlling the total amount of Ni and Co in the base metal within a certain range. Patent Document 3 discloses a ferritic stainless steel as a material for urea SCR, in which the corrosion resistance is ensured by controlling the concentrations of C, N, Ti, and Nb in the base metal within a certain range. Patent Document 4 discloses a ferritic stainless steel as a material for urea SCR, in which the corrosion resistance is ensured by controlling the concentrations of Cr, Si, Al, Ti, Mn, and Fe in the surface layer of the base metal within a certain range. Patent Document 5 discloses a ferritic stainless steel as a material for urea SCR, in which the corrosion resistance is ensured by controlling the total amount of Cr, Si, and Mn in the base metal within a certain range.

[0008] JP 2022-115553 A JP 2018-168415 A International Publication No. 2016 / 035241 JP 2012-112025 A JP 2009-242933 A

[0009] However, the technologies described in Patent Documents 1 to 5 only describe the components of the base material and weld metal, and do not take into consideration the deterioration of corrosion resistance due to changes in the microstructure during high-temperature use. In recent years, the ambient temperature in which denitration devices are used has risen compared to the past, and for example, the ambient temperature in which urea-SCR devices are used has also risen due to rising exhaust gas temperatures in automobiles. As a result, there have been cases where a decrease in corrosion resistance has become a new problem after long-term use at high temperatures.

[0010] The present invention has been made in view of the above circumstances, and has as its object to provide a ferritic stainless steel for use in a denitration system that has excellent corrosion resistance in the environment in which the denitration system is used.

[0011] In this specification, excellent corrosion resistance in the operating environment of a denitration device means a corrosion rate of 0.5 g / (m) when a Huey test is carried out in accordance with JIS G 0573:1999 (65% nitric acid corrosion test method for stainless steel) after holding at 700°C for 1000 hours. ２ ・h) It means that it is the following.

[0012] The Huey test is commonly used to evaluate the corrosion resistance of stainless steel materials in the operating environment of denitration equipment. The inventors prepared prototype ferritic stainless steels containing various alloying elements, held them at 700°C for 1000 hours, and then performed the Huey test to investigate the influence of the alloying elements on corrosion resistance in the above-mentioned environment. As a result, they found that, in the above-mentioned environment, the corrosion resistance is better with increasing Cr content, while the corrosion resistance tends to be inferior with increasing Nb, Mo, and Si contents.

[0013] The present inventors have further discovered that when the amount of Laves phase, an intermetallic compound composed of Fe, Nb, Mo, Si, etc., precipitated at grain boundaries is small, corrosion resistance in the operating environment of a denitration device such as urea SCR is improved.

[0014] The present invention was made based on the above findings and further studies. That is, the gist of the present invention is as follows: [1] A ferritic stainless steel for a denitration system, containing, by mass%, C: 0.003 to 0.030%, N: 0.030% or less, Si: 0.20 to 1.00%, Mn: 0.05 to 0.50%, P: 0.050% or less, S: 0.020% or less, Cr: 13.0 to 21.0%, Mo: 0.001 to 3.00%, Al: 0.001 to 0.50%, Nb: 0.001 to 0.60%, and Ti: 0.001 to 0.15%, satisfying the following formulas (1) and (2), with the balance consisting of Fe and unavoidable impurities: (Ti+Nb×47 / 97) / (8×(C+N))≧1.0 (1) Cr-12Nb-2.0Mo-Si≧10.0 (2) where Ti, Nb, C, N, Cr, Mo, and Si in formulas (1) and (2) represent the contents (% by mass) of each element. [2] The ferritic stainless steel for a denitration apparatus according to [1], further containing, in mass%, one or more elements selected from the following Group A and Group B: Group A: One or more selected from Ni: 0.001 to 1.00%, Cu: 0.01 to 2.00%, Co: 0.01 to 1.00%, W: 0.01 to 2.00% Group B: One or more selected from V: 0.01 to 0.20%, Zr: 0.01 to 0.50%, Mg: 0.0005 to 0.0050%, Ca: 0.0005 to 0.0050%, B: 0.0005 to 0.0050%, REM (rare earth metal): 0.001 to 0.100%, Sn: 0.001 to 0.100%, Sb: 0.001 to 0.100% [3] A ferritic stainless steel for a denitration device according to [1] or [2], in which the grain boundary coverage of the Laves phase after holding at 700°C for 1000 hours is 20% or less.

[0015] According to the present invention, it is possible to provide a ferritic stainless steel for a denitration apparatus that has excellent corrosion resistance in the environment in which the denitration apparatus is used.

[0016] The present invention will be specifically described below.

[0017] First, the reason for limiting the chemical composition of the steel in the present invention will be explained. Note that the unit of the content of elements in the chemical composition of the steel is always "mass %", but hereinafter, unless otherwise specified, it will be simply expressed as "%".

[0018] C: 0.003 to 0.030% Increasing the C content improves strength, while decreasing it improves workability. Here, a C content of 0.003% or more is necessary to obtain sufficient strength. However, if the C content exceeds 0.030%, not only does workability decrease significantly, but Cr carbides precipitate at grain boundaries, causing sensitization and reducing corrosion resistance. Therefore, the C content is set to the range of 0.003 to 0.030%. The C content is preferably 0.004% or more. The C content is also preferably 0.025% or less, more preferably 0.020% or less, and even more preferably 0.010% or less.

[0019] N: 0.030% or less If the N content exceeds 0.030%, corrosion resistance and workability decrease. Therefore, the N content is set to 0.030% or less. Preferably, the N content is set to 0.025% or less. More preferably, the N content is set to 0.020% or less. Although there is no particular restriction on the lower limit of the N content, excessive reduction of the N content leads to an increase in costs, so the N content is preferably set to 0.003% or more.

[0020] Si: 0.20 to 1.00% Si is a useful element as a deoxidizer. This effect is obtained with a Si content of 0.20% or more. However, if the Si content exceeds 1.00%, excessive precipitation of the Laves phase occurs, reducing corrosion resistance. Therefore, the Si content is set to the range of 0.20 to 1.00%. The Si content is preferably 0.25% or more, more preferably 0.30% or more, and even more preferably 0.40% or more. Furthermore, the Si content is preferably 0.80% or less, more preferably 0.70% or less, and even more preferably 0.60% or less.

[0021] Mn: 0.05 to 0.50% Mn has a deoxidizing effect, and this effect is obtained with a Mn content of 0.05% or more. However, if the Mn content exceeds 0.50%, it promotes the precipitation of MnS, reducing corrosion resistance. Therefore, the Mn content is set to the range of 0.05 to 0.50%. The Mn content is preferably 0.10% or more, and more preferably 0.15% or more. Furthermore, the Mn content is preferably 0.40% or less, and more preferably 0.30% or less.

[0022] P: 0.050% or less P is an element that is inevitably contained in steel, and excessive content thereof makes intergranular corrosion more likely to occur. This tendency becomes more pronounced when the P content exceeds 0.050%. Therefore, the P content is set to 0.050% or less. Preferably, the P content is 0.030% or less. Note that there is no particular lower limit for the P content. However, since excessive dephosphorization leads to increased costs, the P content is preferably 0.005% or more.

[0023] S: 0.020% or less S is an element that is inevitably contained in steel, and an S content of more than 0.020% promotes the precipitation of MnS and reduces corrosion resistance. Therefore, the S content is set to 0.020% or less. Preferably, the S content is 0.010% or less. There is no particular lower limit for the S content. However, since excessive desulfurization increases costs, the S content is preferably 0.0005% or more.

[0024] Cr: 13.0 to 21.0% Cr is an important element for ensuring the corrosion resistance of stainless steel. If the Cr content is less than 13.0%, sufficient corrosion resistance cannot be obtained. On the other hand, if the Cr content exceeds 21.0%, workability decreases. The Cr content is preferably 15.0% or more, and more preferably 17.0% or more. In addition, the Cr content is preferably 20.0% or less, and more preferably 19.0% or less.

[0025] Mo: 0.001 to 3.00% Mo stabilizes the passivation film of stainless steel, improving corrosion resistance. This effect is achieved when the Mo content is 0.001% or more. However, if the Mo content exceeds 3.00%, excessive precipitation of the Laves phase occurs, reducing corrosion resistance in the operating environment of the denitration equipment. Therefore, the Mo content is set to the range of 0.001 to 3.00%. The Mo content is preferably 0.50% or more, and more preferably 1.00% or more. Furthermore, the Mo content is preferably 2.50% or less, and more preferably 2.00% or less.

[0026] Al: 0.001 to 0.50% Al is a useful element for deoxidation, and this effect is obtained with an Al content of 0.001% or more. However, Al is an element active against oxygen, and an Al content exceeding 0.50% reduces workability. Therefore, the Al content is set to the range of 0.001 to 0.50%. Preferably, the Al content is 0.015% or less.

[0027] Nb: 0.001 to 0.60% Nb is an element that combines with C and N to suppress the deterioration of corrosion resistance (sensitization) due to the precipitation of Cr carbonitrides. It is also an element that is effective in improving high-temperature strength. This effect is obtained when the Nb content is 0.001% or more. On the other hand, if the Nb content exceeds 0.60%, excessive precipitation of the Laves phase occurs, resulting in a decrease in corrosion resistance in the operating environment of the denitration equipment. Therefore, the Nb content is set to the range of 0.001 to 0.60%. The Nb content is preferably 0.10% or more, more preferably 0.30% or more. The Nb content is also preferably 0.50% or less, more preferably 0.40% or less.

[0028] Ti: 0.001 to 0.15% Ti bonds with the C and N contained in the steel and has the effect of preventing sensitization. This effect is obtained with a Ti content of 0.001% or more. On the other hand, excessive Ti content reduces workability, so the upper limit must be set at 0.15%. Therefore, the Ti content is set in the range of 0.001 to 0.15%. The Ti content is preferably 0.01% or more, and more preferably 0.05% or more. Furthermore, the Ti content is preferably 0.08% or less.

[0029] (Ti + Nb × 47 / 97) / (8 × (C + N)) ≧ 1.0 (1) In formula (1), Ti, Nb, C, and N represent the content (mass%) of each element. Formula (1) must be satisfied to ensure corrosion resistance in the operating environment of the denitration equipment. If the value of Ti + Nb × 47 / 97 is below the value of 8 × (C + N), Cr carbonitrides precipitate at grain boundaries, forming Cr-depleted layers around them, significantly reducing corrosion resistance. If the value of Ti + Nb × 47 / 97 is equal to or greater than the value of 8 × (C + N), i.e., if (Ti + Nb × 47 / 97) / (8 × (C + N)) ≧ 1.0, Ti and Nb will form carbonitrides instead of Cr, thereby suppressing the formation of a Cr-depleted layer. It is preferable that (Ti + Nb × 47 / 97) / (8 × (C + N)) be 1.25 or greater. Furthermore, the upper limit of (Ti+Nb×47 / 97) / (8×(C+N)) is not particularly limited, but as an example, (Ti+Nb×47 / 97) / (8×(C+N)) can be set to 3.00 or less.

[0030] Cr-12Nb-2.0Mo-Si ≧ 10.0 (2) In formula (2), Cr, Nb, Mo, and Si represent the content (mass%) of each element. The inventors have discovered the following: By adjusting the Cr content and the respective contents of Nb, Mo, and Si, which are elements that form the Laves phase, within a predetermined range, and further setting Cr-12Nb-2.0Mo-Si to 10.0 (mass%) or more, corrosion resistance in the operating environment of the denitration equipment is improved. Although the reason for this is unclear, it is known that intergranular corrosion occurs in the operating environment of the denitration equipment. On the other hand, since the Laves phase is easily dissolved in the operating environment of the denitration equipment and precipitates at grain boundaries, it is presumed that an increase in the amount of precipitation of the Laves phase promotes intergranular corrosion. By reducing the contents of Nb, Mo, and Si, the amount of precipitation of the Laves phase is suppressed, contributing to improved corrosion resistance. On the other hand, the inventors have discovered that Cr increases the amount of Laves phase precipitation, but also has the effect of improving the corrosion resistance of the Laves phase, and that a higher Cr content is preferable from the standpoint of corrosion resistance. Therefore, in the present invention, the Cr, Nb, Mo, and Si contents are each set within the above-mentioned ranges, and the Cr-12Nb-2.0Mo-Si is set to 10.0 or more. This suppresses the precipitation of Laves phases even when heated for a long period of time at the operating temperature range of 300 to 700°C of the denitration device, thereby maintaining excellent corrosion resistance. The Cr-12Nb-2.0Mo-Si is preferably 12.0 or more. Furthermore, the upper limit of Cr-12Nb-2.0Mo-Si is not particularly limited, but as an example, the Cr-12Nb-2.0Mo-Si can be set to 16.0 or less, or may be set to 15.0 or less.

[0031] The basic components (essential components) of the ferritic stainless steel for a denitration system of the present invention (hereinafter simply referred to as the ferritic stainless steel of the present invention) have been described above. The remainder of the component composition of the ferritic stainless steel of the present invention other than the above components can be Fe and unavoidable impurities.

[0032] Furthermore, in the present invention, the above-mentioned composition may further contain, as desired, one or more elements selected from the following Group A and Group B. Group A: One or more elements selected from Ni: 0.001 to 1.00%, Cu: 0.01 to 2.00%, Co: 0.01 to 1.00%, and W: 0.01 to 2.00%. Group B: One or more elements selected from V: 0.01 to 0.20%, Zr: 0.01 to 0.50%, Mg: 0.0005 to 0.0050%, Ca: 0.0005 to 0.0050%, B: 0.0005 to 0.0050%, REM (rare earth metal): 0.001 to 0.100%, Sn: 0.001 to 0.100%, and Sb: 0.001 to 0.100%.

[0033] Ni: 0.001 to 1.00% Although the reason is not clear, Ni is an element that effectively contributes to improving corrosion resistance when contained in a content of 0.001% or more. On the other hand, if the Ni content exceeds 1.00%, workability deteriorates. Therefore, when Ni is contained, the Ni content is set to the range of 0.001 to 1.00%. When Ni is contained, the Ni content is more preferably 0.05% or more. Furthermore, when Ni is contained, the Ni content is more preferably 0.70% or less.

[0034] Cu: 0.01 to 2.00% Cu is an element that enhances corrosion resistance. This effect is obtained when the Cu content is 0.01% or more. However, if the Cu content exceeds 2.00%, the precipitation of the epsilon phase will actually decrease the corrosion resistance. Therefore, when Cu is contained, the Cu content is set to the range of 0.01 to 2.00%.

[0035] Co: 0.01 to 1.00% Co is an element that enhances corrosion resistance. This effect is obtained when the Co content is 0.01% or more. However, if the Co content exceeds 1.00%, workability decreases. Therefore, when Co is contained, the Co content is set to the range of 0.01 to 1.00%. When Co is contained, the Co content is more preferably 0.05% or more. Furthermore, when Co is contained, the Co content is more preferably 0.70% or less.

[0036] W: 0.01 to 2.00% W is an element that enhances corrosion resistance. This effect is obtained when the W content is 0.01% or more. However, if the W content exceeds 2.00%, Laves phases precipitate excessively, reducing workability. Therefore, when W is contained, the W content is set to the range of 0.01 to 2.00%. When W is contained, the W content is more preferably 0.05% or more. Furthermore, when W is contained, the W content is more preferably 1.00% or less.

[0037] V: 0.01 to 0.20% Like Ti, V bonds with C and N contained in the steel and prevents sensitization. This effect is obtained when the V content is 0.01% or more. On the other hand, if the V content exceeds 0.20%, workability decreases. Therefore, when V is contained, the V content is set to the range of 0.01 to 0.20%. When V is contained, the V content is more preferably 0.15% or less, and even more preferably 0.10% or less.

[0038] Zr: 0.01 to 0.50% Like Ti and Nb, Zr is an element that bonds with C and N contained in steel and suppresses sensitization. This effect is obtained when the Zr content is 0.01% or more. On the other hand, if the Zr content exceeds 0.50%, workability decreases. Therefore, when Zr is contained, the Zr content is set to the range of 0.01 to 0.50%. When Zr is contained, the Zr content is more preferably 0.03% or more. Furthermore, when Zr is contained, the Zr content is more preferably 0.20% or less.

[0039] Mg: 0.0005 to 0.0050% Mg acts as a deoxidizer. This effect is obtained when the Mg content is 0.0005% or more. However, if the Mg content exceeds 0.0050%, the toughness of the steel decreases, resulting in a decrease in manufacturability. Therefore, if Mg is contained, the Mg content is set to the range of 0.0005 to 0.0050%. If Mg is contained, the Mg content is more preferably 0.0020% or less.

[0040] Ca: 0.0005 to 0.0050% Calcium improves the penetration of welds and improves weldability. This effect is achieved when the Ca content is 0.0005% or more. However, if the Ca content exceeds 0.0050%, it combines with S to form CaS, reducing corrosion resistance. Therefore, if Ca is contained, the Ca content is set to the range of 0.0005 to 0.0050%. If Ca is contained, the Ca content is more preferably 0.0010% or more. Furthermore, if Ca is contained, the Ca content is more preferably 0.0040% or less.

[0041] B: 0.0005 to 0.0050% B is an element that improves secondary work embrittlement. This effect is manifested when the B content is 0.0005% or more. However, if the B content exceeds 0.0050%, ductility decreases due to solid solution strengthening. Therefore, when B is contained, the B content is set to the range of 0.0005 to 0.0050%.

[0042] REM (Rare Earth Metals): 0.001 to 0.100% REM (Rare Earth Metals) are elements effective for deoxidation. This effect is achieved when the REM content is 0.001% or more. However, if the REM content exceeds 0.100%, hot workability decreases. Therefore, if REM is contained, the REM content is set to the range of 0.001 to 0.100%. If REM is contained, the REM content is more preferably 0.010% or more. Furthermore, if REM is contained, the REM content is more preferably 0.050% or less. Note that REM is a collective term for Sc, Y, and 15 elements ranging from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM content here refers to the total content of these elements.

[0043] Sn: 0.001 to 0.100% Sn is an element effective in suppressing roughness of the work surface. This effect is obtained when the Sn content is 0.001% or more. However, if the Sn content exceeds 0.100%, hot workability deteriorates. Therefore, when Sn is contained, the Sn content is set to the range of 0.001 to 0.100%. When Sn is contained, the Sn content is more preferably 0.050% or less.

[0044] Sb: 0.001 to 0.100% Like Sn, Sb is an element that is effective in suppressing roughness of the work surface. This effect is obtained when the Sb content is 0.001% or more. However, if the Sb content exceeds 0.100%, workability decreases. Therefore, when Sb is contained, the Sb content is set to the range of 0.001 to 0.100%. When Sb is contained, the Sb content is more preferably 0.050% or less.

[0045] In addition, when the content of Ni, Cu, Co, W, V, Zr, Mg, Ca, B, REM, Sn, or Sb described above as the optional components is less than the lower limit value, the component is considered to be contained as an unavoidable impurity.

[0046] Grain boundary coverage of Laves phase: 20% or less (preferred condition) The inventors have found that suppressing the amount of Laves phase precipitation at grain boundaries can improve corrosion resistance in the operating environment of a denitration system. In particular, they have found that a grain boundary coverage of Laves phase of 20% or less can more stably ensure excellent corrosion resistance in the operating environment of a denitration system. While the reason for this is unclear, it is known that intergranular corrosion occurs in the operating environment of a denitration system. Meanwhile, since Laves phases are easily dissolved in the operating environment of a denitration system and precipitate at grain boundaries, it is presumed that an increase in the amount of Laves phase precipitation promotes intergranular corrosion. Here, since the operating environment of a urea SCR component during operation reaches approximately 700°C in high-temperature portions, it is preferable that the grain boundary coverage of the Laves phase after holding the ferritic stainless steel at 700°C for 1,000 hours is 20% or less. The lower the grain boundary coverage, the better, and it may even be 0%.

[0047] Here, the grain boundary coverage of the Laves phase is evaluated by the following method. A sample is taken from a steel material after holding at 700°C for 1000 hours, with the longitudinal cross section as the observation surface, and is embedded in resin and mirror-polished. Then, backscattered electron images are observed with a scanning electron microscope (SEM) at a magnification of 1000x at five arbitrary locations (photographs are taken so that the field of view is 100µm x 100µm or more). In a central 100µm x 100µm region of each image obtained by SEM observation, the total length L of the grain boundary is measured. ＧＢ The total length L of the grain boundary covered by the Laves phase ＬａｖｅｓThe ratio of the Laves phase to the grain boundary coverage is defined as shown in formula (3). The average value of the Laves phase grain boundary coverages determined at the five locations is taken as the Laves phase grain boundary coverage (%) of the ferritic stainless steel. Laves phase grain boundary coverage (%) = (total length L of grain boundaries covered by the Laves phase) Ｌａｖｅｓ / total length L of the grain boundary ＧＢ ) × 100 ... (3)

[0048] Next, a preferred method for producing the ferritic stainless steel of the present invention will be described. The method for producing the ferritic stainless steel of the present invention is not particularly limited, but can be, for example, as follows. A steel slab having the above-described chemical composition is hot-rolled to form a hot-rolled sheet, which is then annealed as needed. The hot-rolled sheet or the hot-rolled annealed sheet is then cold-rolled to form a cold-rolled sheet of the desired thickness, which is then annealed as needed, thereby producing a ferritic stainless steel sheet having the above-described chemical composition. The conditions for hot rolling, cold rolling, hot-rolled sheet annealing, and cold-rolled sheet annealing are not particularly limited, and may be conventional.

[0049] In the steelmaking process for producing smelted steel, steel melted in a converter or electric furnace is preferably subjected to secondary refining using a method such as VOD (Vacuum Oxygen Decarburization) to produce steel containing the above-mentioned essential elements and optional elements that may be added as needed. The molten steel can be produced into a steel material by known methods, but continuous casting is preferred from the standpoint of productivity and quality. The steel material is then heated, preferably to 1050 to 1250°C, and hot-rolled to a desired thickness. Of course, hot processing can also be used to produce products other than plate materials. The hot-rolled sheet is then preferably subjected to continuous annealing at a temperature of 900 to 1150°C as needed, followed by descaling by pickling or the like to produce a hot-rolled product. If necessary, scale may be removed using shot blasting or a grinding brush before pickling.

[0050] Furthermore, the hot-rolled product (hot-rolled annealed sheet, etc.) may be subjected to a process such as cold rolling to produce a cold-rolled product. In this case, cold rolling may be performed once, but from the perspective of productivity and required quality, cold rolling may be performed twice or more with intermediate annealing in between. The total reduction ratio of one or more cold rolling passes is preferably 60% or more, more preferably 70% or more. The cold-rolled steel sheet is then preferably subjected to continuous annealing (finish annealing) at a temperature of preferably 900 to 1150°C, more preferably 950 to 1150°C, followed by pickling to produce a cold-rolled product. Note that continuous annealing may be performed as bright annealing, omitting pickling. Furthermore, after finish annealing, skin-pass rolling or the like may be performed to adjust the shape, surface roughness, and material properties of the steel sheet.

[0051] The ferritic stainless steel of the present invention described above is suitable for use in denitration devices such as urea SCR.

[0052] Steels having the chemical compositions shown in Table 1 were melted in a vacuum melting furnace, heated at 1150°C for 1 hour, and then hot-rolled to produce hot-rolled sheets with a thickness of 4.0 mm. The hot-rolled sheets were annealed by holding at 1080°C for 1 minute, and then the surface was ground to remove scale, followed by cold-rolling to a thickness of 1.0 mm to obtain cold-rolled sheets. The cold-rolled sheets were then finish-annealed by holding at 1040°C for 1 minute in an ammonia decomposition gas atmosphere to produce cold-rolled annealed sheets.

[0053] The cold-rolled annealed steel sheets obtained above were subjected to a heat treatment in which they were held in the atmosphere at 700°C for 1000 hours. Thereafter, they were cooled to room temperature, and their surfaces were polished with emery paper up to No. 600, followed by degreasing with acetone. The grain boundary coverage of the Laves phase and corrosion resistance in the operating environment of a denitration device were measured for the samples thus prepared, as described below. The results are shown in Table 2.

[0054] Grain boundary coverage of the Laves phase Samples were taken from the prepared samples using the longitudinal cross section as the observation surface, embedded in resin, and mirror-polished. Five randomly selected points were then observed at 1000x magnification using SEM backscattered electron imaging. In a 100 μm × 100 μm region in the center of each image, the total length L of the grain boundary was measured. ＧＢ The total length L of the grain boundary covered by the Laves phase ＬａｖｅｓThe grain boundary coverage of the Laves phase was calculated from the ratio of the Laves phase to the grain boundary by the above formula (3). The average value of the grain boundary coverage of the Laves phase calculated at each of the five locations was taken as the grain boundary coverage of the Laves phase for that sample. The grain boundary coverage of the Laves phase was evaluated according to the following criteria: ○: 20% or less ×: More than 20%

[0055] Evaluation of corrosion resistance in the operating environment of a denitration device (Huey test) Test pieces measuring 20 mm in width and 50 mm in length were cut from the prepared samples and subjected to a Huey test in accordance with JIS G 0573:1999 (65% nitric acid corrosion test method for stainless steel). A 65% by mass nitric acid solution was used, and the test was conducted in a boiling state. Up to three 48-hour tests were conducted. After each cycle, the weight was measured using an electronic balance, and the corrosion rate (g / (m)) was calculated from the change in weight before and after the test. ２ The corrosion rate of the sample was determined by the average of three cycles. The corrosion rate was evaluated according to the following criteria, with ◯ representing a pass and × representing a fail. ２ ・h) or less ×: 0.5g / (m ２ ・h) Super

[0056]

[0057]

[0058] As can be seen from Table 2, all of Inventive Examples No. 1 to 26 had good corrosion resistance in the operating environment of the denitration equipment. In contrast, Comparative Examples No. 27 to 35, which had component compositions outside the appropriate range, were unable to meet the target corrosion resistance in the operating environment of the denitration equipment.

[0059] ​​Specifically, in Comparative Example No. 27 (steel symbol B1), the Mo content exceeded the upper limit specified in the present invention, and therefore excellent corrosion resistance was not obtained in the operating environment of a denitration device. In Comparative Example No. 27, the grain boundary coverage of the Laves phase exceeded 20%. In Comparative Example No. 28 (steel symbol B2), the Nb content exceeded the upper limit specified in the present invention, and therefore excellent corrosion resistance was not obtained in the operating environment of a denitration device. In Comparative Example No. 28, the grain boundary coverage of the Laves phase exceeded 20%. In Comparative Example No. 29 (steel symbol B3), the Si content exceeded the upper limit specified in the present invention, and therefore excellent corrosion resistance was not obtained in the operating environment of a denitration device. In Comparative Example No. 29, the grain boundary coverage of the Laves phase exceeded 20%. In Comparative Example No. 30 (steel symbol B4), the Mn content exceeded the upper limit specified in the present invention, and therefore excellent corrosion resistance was not obtained in the operating environment of a denitration device. In Comparative Example No. 31 (steel symbol B5), the C content exceeded the upper limit of the present invention, so excellent corrosion resistance was not obtained in the operating environment of a denitration device. In Comparative Example No. 32 (steel symbol B6), the N content exceeded the upper limit of the present invention, so excellent corrosion resistance was not obtained in the operating environment of a denitration device. In Comparative Example No. 33 (steel symbol B7), the Cr content was below the lower limit of the present invention, so excellent corrosion resistance was not obtained in the operating environment of a denitration device. In Comparative Example No. 34 (steel symbol B8), all components were within the specified ranges, but formula (1) was not satisfied, so excellent corrosion resistance was not obtained in the operating environment of a denitration device. In Comparative Example No. 35 (steel symbol B9), all components were within the specified ranges, but formula (2) was not satisfied, so excellent corrosion resistance was not obtained in the operating environment of a denitration device. In Comparative Example No. 35, the grain boundary coverage of the Laves phase exceeded 20%.

[0060] The present invention can provide a ferritic stainless steel having excellent corrosion resistance in the environment in which a denitration device is used, and is therefore extremely useful industrially.

Claims

1. A ferritic stainless steel for a denitration system, containing, by mass%, C: 0.003 to 0.030%, N: 0.030% or less, Si: 0.20 to 1.00%, Mn: 0.05 to 0.50%, P: 0.050% or less, S: 0.020% or less, Cr: 13.0 to 21.0%, Mo: 0.001 to 3.00%, Al: 0.001 to 0.50%, Nb: 0.001 to 0.60%, and Ti: 0.001 to 0.15%, and satisfying the following formulas (1) and (2), with the balance being Fe and unavoidable impurities: (Ti+Nb×47 / 97) / (8×(C+N))≧1.0 (1) Cr-12Nb-2.0Mo-Si≧10.0 (2) In the formulas (1) and (2), Ti, Nb, C, N, Cr, Mo, and Si represent the content (mass%) of each element.

2. The ferritic stainless steel for a denitration system according to claim 1, further containing, by mass%, one or more elements selected from the following Group A and Group B. Group A: One or more elements selected from Ni: 0.001-1.00%, Cu: 0.01-2.00%, Co: 0.01-1.00%, and W: 0.01-2.00%; Group B: One or more elements selected from V: 0.01-0.20%, Zr: 0.01-0.50%, Mg: 0.0005-0.0050%, Ca: 0.0005-0.0050%, B: 0.0005-0.0050%, REM (rare earth metals): 0.001-0.100%, Sn: 0.001-0.100%, and Sb: 0.001-0.100%.

3. A ferritic stainless steel for a denitration system according to claim 1 or 2, in which the grain boundary coverage of the Laves phase after holding at 700°C for 1000 hours is 20% or less.

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