Ferritic stainless steel for denitration equipment

Abstract

An object of the present invention is to provide a ferritic stainless steel for a denitration apparatus having excellent corrosion resistance in the use environment of the denitration apparatus. In 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%, Ti: 0.001 to 0.15%, satisfying the following formulas (1) and (2), and the balance being Fe and inevitable impurities, a ferritic stainless steel for a denitration apparatus having a component composition. (Ti + Nb × 47 / 97) / (8 × (C + N)) ≥ 1.0 ···(1) Cr - 12Nb - 2.0Mo - Si ≥ 10.0 ···(2) In formulas (1) and (2), Ti, Nb, C, N, Cr, Mo and Si represent the content (mass %) of each element.

Description

Technical Field

[0001] The present invention relates to a ferritic stainless steel for a denitration device, and particularly to a ferritic stainless steel used in a denitration device such as urea SCR (Selective Catalytic Reduction) used to remove nitrogen oxides (NOx) contained in exhaust gas in automobiles, ships, thermal power plants, etc.

Background Art

[0002] In recent years, from the perspective of global environmental protection, strengthening of exhaust gas purification has been demanded. For this reason, the application of denitration devices such as urea SCR is expanding.

[0003] Here, urea SCR is an exhaust gas purification system that uses ammonia, which is a decomposition product of urea, as a reducing agent for NOx contained in exhaust gas. Ammonia decomposes NOx into nitrogen (N2) and water (H2O) through a catalyst, but there are problems with safety in terms of flammability, toxicity, etc., and it is difficult to directly use it in automobiles, etc. In urea SCR, ammonia obtained by decomposing urea as described above is used to reduce NOx, so it is also excellent in safety. For this reason, urea SCR is widely applied to automobiles, thermal power plants, etc. The sizes of urea SCR vary depending on the application, but the denitration principle is the same in all cases.

[0004] In large plants, anhydrous ammonia or aqueous ammonia may be used instead of urea. In particular, anhydrous ammonia, although highly toxic, can be directly used for reducing NOx. Aqueous ammonia requires hydrolysis, but is safer than anhydrous ammonia. In the future, it is assumed that ammonia will be used as a fuel for CO2 reduction in thermal power plants, industrial furnaces, ships, etc., and in that case, NOx may also be contained in the exhaust gas. Therefore, it is expected that the needs for various denitration devices will increase.

[0005] In urea SCR, urea and ammonia hardly corrode stainless steel. However, in the process of decomposing urea water at high temperature to generate ammonia, or when unreacted urea water volatilizes, substances such as ammonia carbamate are generated and corrode stainless steel. Since urea SCR is used not only in commercial vehicles such as buses and trucks with long driving distances but also in thermal power plants that operate for several decades, high corrosion resistance to such corrosive substances is important.

[0006] For the above reasons, for example, stainless steel with excellent corrosion resistance is applied to the catalyst carrier for denitration devices such as urea SCR, and several technologies as follows have been disclosed.

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

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

[0009] However, the techniques described in Patent Documents 1 to 5 only describe the components of the base material and the weld metal, and do not consider the deterioration of corrosion resistance due to changes in the microstructure during high-temperature use. In recent years, the operating environment temperature of the denitration device has been rising compared to the past. For example, in automobiles, the operating environment temperature of urea SCR has also increased due to the rising exhaust gas temperature. For this reason, a new problem of corrosion resistance deterioration has emerged after long-term use at high temperatures.

[0010] The present invention has been made in view of the above circumstances, and an object thereof is to provide a ferritic stainless steel for a denitration device having excellent corrosion resistance in the operating environment of the denitration device.

[0011] In this specification, excellent corrosion resistance in the operating environment of the denitration device means that after holding at 700 ° C for 1000 hours, the corrosion rate when performing a Huey test in accordance with JIS G 0573: 1999 (Method for Corrosion Test of Stainless Steel in 65% Nitric Acid) is 0.5 g / (m 2 ·h) or less. [Means for Solving the Problems]

[0012] The Huey test is generally used to evaluate the corrosion resistance of stainless steel materials in the operating environment of a denitration device. The inventors of the present invention fabricated ferritic stainless steels having various alloy components, held them at 700 °C for 1000 hours, and then conducted the Huey test to investigate the influence of alloy components on the corrosion resistance in the above environment. As a result, it was found that in the above environment, the higher the Cr content, the better the corrosion resistance, while the corrosion resistance tends to be inferior as the contents of Nb, Mo, and Si increase.

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

[0014] The present invention has been further studied based on the above findings. That is, the gist configuration of the present invention is as follows. [1] 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%, containing a ferritic stainless steel for a denitration device having a component composition that satisfies the following formulas (1) and (2) and the balance consisting of Fe and inevitable impurities. (Ti + Nb × 47 / 97) / (8 × (C + N)) ≥ 1.0 ···(1) Cr - 12Nb - 2.0Mo - Si ≥ 10.0 ···(2) However, in formulas (1) and (2), Ti, Nb, C, N, Cr, Mo, and Si represent the content (% by mass) of each element. [2] Further, the ferritic stainless steel for a denitration apparatus according to [1], containing at least one group selected from the following Group A and Group B in mass %. 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%, and 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%, and Sb: 0.001 to 0.100% [3] The ferritic stainless steel for a denitration apparatus according to [1] or [2], wherein the grain boundary coverage rate of the Laves phase after holding at 700°C for 1000 hours is 20% or less.

Advantages of the Invention

[0015] According to the present invention, it is possible to provide a ferritic stainless steel for a denitration apparatus having excellent corrosion resistance in the use environment of the denitration apparatus.

Embodiments for Carrying Out the Invention

[0016] Hereinafter, the present invention will be specifically described.

[0017] First, in the present invention, the reasons for limiting the component composition of the steel will be described. In the component composition of the steel, the unit of the content of each element is “% by mass” in all cases, but hereinafter, it will be simply indicated as “%” unless otherwise specified.

[0018] C: 0.003 to 0.030% As the C content increases, the strength improves, and as it decreases, the workability improves. Here, C needs to be contained at 0.003% or more to obtain sufficient strength. However, when the C content exceeds 0.030%, the workability decreases significantly, and Cr carbides precipitate at grain boundaries, causing sensitization and reducing corrosion resistance. Therefore, the C content is in the range of 0.003 - 0.030%. The C content is preferably 0.004% or more. Also, the C content is 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 When the N content exceeds 0.030%, the corrosion resistance and workability decrease. Therefore, the N content is 0.030% or less. Preferably, the N content is 0.025% or less. More preferably, the N content is 0.020% or less. Note that the lower limit of the N content is not particularly limited, but excessive reduction of the N content causes an increase in cost, so the N content is preferably 0.003% or more.

[0020] Si: 0.20 - 1.00% Si is an element useful as a deoxidizer. Its effect can be obtained with a Si content of 0.20% or more. However, when the Si content exceeds 1.00%, the precipitation of the Laves phase becomes excessive and the corrosion resistance decreases. Therefore, the Si content is in the range of 0.20 - 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. Also, 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 - 0.50% Mn has a deoxidizing effect, and its effect can be obtained with a Mn content of 0.05% or more. However, when the Mn content exceeds 0.50%, the precipitation of MnS is promoted and the corrosion resistance decreases. Therefore, the Mn content is in the range of 0.05 - 0.50%. The Mn content is preferably 0.10% or more, and more preferably 0.15% or more. Also, 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 inevitably contained in steel, and excessive content tends to cause intergranular corrosion. This tendency becomes significant with a P content exceeding 0.050%. Therefore, the P content is 0.050% or less. Preferably, the P content is 0.030% or less. Note that the lower limit of the P content is not particularly limited. However, excessive P removal leads to an increase in cost, so the P content is preferably 0.005% or more.

[0023] S: 0.020% or less S is an element inevitably contained in steel, and a S content exceeding 0.020% promotes the precipitation of MnS and reduces the corrosion resistance. Thus, the S content is 0.020% or less. Preferably, the S content is 0.010% or less. Note that the lower limit of the S content is not particularly limited. However, excessive S removal leads to an increase in cost, so the S content is preferably 0.0005% or more.

[0024] Cr: 13.0 - 21.0% Cr is an important element to ensure the corrosion resistance of stainless steel. When the Cr content is less than 13.0%, sufficient corrosion resistance cannot be obtained. On the other hand, when the Cr content exceeds 21.0%, the workability decreases. The Cr content is preferably 15.0% or more, and more preferably 17.0% or more. Also, the Cr content is preferably 20.0% or less, and more preferably 19.0% or less.

[0025] Mo: 0.001 - 3.00% Mo stabilizes the passive film of stainless steel to improve corrosion resistance. This effect can be obtained when the Mo content is 0.001% or more. However, when the Mo content exceeds 3.00%, the precipitation of the Laves phase becomes excessive and the corrosion resistance in the use environment of the denitration device decreases. Therefore, the Mo content shall be in the range of 0.001 - 3.00%. The Mo content is preferably 0.50% or more, and more preferably 1.00% or more. Also, the Mo content is preferably 2.50% or less, and more preferably 2.00% or less.

[0026] Al: 0.001 - 0.50% Al is an element useful for deoxidation, and its effect can be obtained with an Al content of 0.001% or more. However, Al is an element active with respect to oxygen, and when the Al content exceeds 0.50%, the workability deteriorates. Therefore, the Al content shall be in the range of 0.001 - 0.50%. Preferably, the Al content is 0.015% or less.

[0027] Nb: 0.001 - 0.60% Nb is an element that suppresses the decrease in corrosion resistance (sensitization) due to the precipitation of Cr carbonitride by binding with C and N. It is also an element effective for improving high-temperature strength. This effect can be obtained when the Nb content is 0.001% or more. On the other hand, when the Nb content exceeds 0.60%, the precipitation of the Laves phase becomes excessive and the corrosion resistance in the use environment of the denitration device decreases. Therefore, the Nb content shall be in the range of 0.001 - 0.60%. The Nb content is preferably 0.10% or more, and more preferably 0.30% or more. Also, the Nb content is preferably 0.50% or less, and more preferably 0.40% or less.

[0028] Ti: 0.001 - 0.15% Ti combines with C and N contained in steel and has the effect of preventing sensitization. This effect can be obtained with a Ti content of 0.001% or more. On the other hand, excessive Ti content reduces workability, so the upper limit needs to be set at 0.15%. Therefore, the Ti content should be in the range of 0.001 to 0.15%. The Ti content is preferably 0.01% or more, more preferably 0.05% or more. Also, 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 (% by mass) of each element. In order to ensure corrosion resistance in the use environment of the denitration device, it is necessary to satisfy the above formula (1). When the value of Ti + Nb×47 / 97 is lower than the value of 8×(C + N), Cr carbonitrides precipitate at the grain boundaries and a Cr-depleted layer is formed around them, resulting in a significant reduction in corrosion resistance. If the value of Ti + Nb×47 / 97 is equal to or greater than the value of 8×(C + N), that is, (Ti + Nb×47 / 97) / (8×(C + N))≧1.0, Ti or Nb can form carbonitrides instead of Cr, suppressing the formation of the Cr-depleted layer. (Ti + Nb×47 / 97) / (8×(C + N)) is preferably 1.25 or more. Also, 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 (% by mass) of each element. The inventors have found the following. The content of Cr and the content of each of Nb, Mo, and Si, which are Laves phase forming elements, are adjusted to a predetermined range, and further, Cr-12Nb-2.0Mo-Si is made 10.0 (mass%) or more. Thereby, the corrosion resistance in the use environment of the denitration apparatus becomes good. The reason for this is not clear, but it is known that intergranular corrosion occurs in the use environment of the denitration apparatus. On the other hand, the Laves phase is easily dissolved in the use environment of the denitration apparatus and precipitates at the grain boundaries. Therefore, it is presumed that an increase in the precipitation amount of the Laves phase promotes intergranular corrosion. By reducing the contents of Nb, Mo, and Si, the precipitation amount of the Laves phase can be suppressed, contributing to the improvement of corrosion resistance. On the other hand, the inventors have found that Cr increases the precipitation amount of the Laves phase but also has an effect of improving the corrosion resistance of the Laves phase, and a higher content of Cr is preferable from the viewpoint of corrosion resistance. For this reason, in the present invention, after setting the contents of Cr, Nb, Mo, and Si to the above-described ranges, Cr-12Nb-2.0Mo-Si is made 10.0 or more. Thereby, precipitation of the Laves phase is suppressed even when heated for a long time in the use temperature range of the denitration apparatus, 300 to 700 °C, and excellent corrosion resistance can be maintained. Cr-12Nb-2.0Mo-Si is preferably 12.0 or more. Further, the upper limit of Cr-12Nb-2.0Mo-Si is not particularly limited, but as an example, Cr-12Nb-2.0Mo-Si can be 16.0 or less, and may be 15.0 or less.

[0031] As described above, the basic components (essential components) in the ferritic stainless steel for a denitration apparatus of the present invention (hereinafter, also simply referred to as the ferritic stainless steel of the present invention) have been described. Among the component compositions of the ferritic stainless steel of the present invention, the balance other than the above components can be Fe and inevitable impurities.

[0032] Further, in the present invention, one or more groups selected from the following Group A and Group B can be optionally contained in the above component composition. 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%, and W: 0.01 to 2.00% Group B: One or more 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%, Sb: 0.001 - 0.100%

[0033] Ni: 0.001 - 1.00% Ni is an element that effectively contributes to the improvement of corrosion resistance when its content is 0.001% or more, although the reason is not clear. On the other hand, when the Ni content exceeds 1.00%, the workability deteriorates. Therefore, when Ni is contained, the Ni content should be in the range of 0.001 - 1.00%. When Ni is contained, the Ni content is more preferably 0.05% or more. Also, when Ni is contained, the Ni content is more preferably 0.70% or less.

[0034] Cu: 0.01 - 2.00% Cu is an element that enhances corrosion resistance. This effect can be obtained when the Cu content is 0.01% or more. However, when the Cu content exceeds 2.00%, the corrosion resistance decreases due to the precipitation of the epsilon phase. Therefore, when Cu is contained, the Cu content should be in the range of 0.01 - 2.00%.

[0035] Co: 0.01 - 1.00% Co is an element that enhances corrosion resistance. This effect can be obtained when the Co content is 0.01% or more. However, when the Co content exceeds 1.00%, the workability deteriorates. Therefore, when Co is contained, the Co content should be in the range of 0.01 - 1.00%. When Co is contained, the Co content is more preferably 0.05% or more. Also, when Co is contained, the Co content is more preferably 0.70% or less.

[0036] W: 0.01 - 2.00% W is an element that enhances corrosion resistance. This effect can be obtained when the W content is 0.01% or more. However, when the W content exceeds 2.00%, the Laves phase precipitates excessively and the workability deteriorates. Therefore, when W is contained, the W content should be in the range of 0.01 - 2.00%. When W is contained, the W content is more preferably 0.05% or more. Also, when W is contained, the W content is more preferably 1.00% or less.

[0037] V: 0.01 - 0.20% Similar to Ti, V combines with C and N contained in the steel to prevent sensitization. This effect can be obtained when the V content is 0.01% or more. On the other hand, when the V content exceeds 0.20%, the workability deteriorates. Therefore, when V is contained, the V content should be in the range of 0.01 - 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 - 0.50% Similar to Ti and Nb, Zr is an element that combines with C and N contained in the steel to suppress sensitization. This effect can be obtained when the Zr content is 0.01% or more. On the other hand, when the Zr content exceeds 0.50%, the workability deteriorates. Therefore, when Zr is contained, the Zr content should be in the range of 0.01 - 0.50%. When Zr is contained, the Zr content is more preferably 0.03% or more. Also, when Zr is contained, the Zr content is more preferably 0.20% or less.

[0039] Mg: 0.0005 - 0.0050% Mg acts as a deoxidizer. This effect can be obtained when the Mg content is 0.0005% or more. However, when the Mg content exceeds 0.0050%, the toughness of the steel decreases and the manufacturability deteriorates. Therefore, when Mg is contained, the Mg content should be in the range of 0.0005 - 0.0050%. When Mg is contained, the Mg content is more preferably 0.0020% or less.

[0040] Ca: 0.0005 - 0.0050% Ca improves the weldability by enhancing the fusion ability of the welded part. This effect can be obtained when the Ca content is 0.0005% or more. However, when the Ca content exceeds 0.0050%, it combines with S to form CaS, resulting in a decrease in corrosion resistance. Therefore, when Ca is contained, the Ca content should be in the range of 0.0005 - 0.0050%. When Ca is contained, the Ca content is more preferably 0.0010% or more. Also, when Ca is contained, the Ca content is more preferably 0.0040% or less.

[0041] B: 0.0005 - 0.0050% B is an element that improves secondary processing brittleness. This effect is manifested when the B content is 0.0005% or more. However, when the B content exceeds 0.0050%, the ductility decreases due to solid solution strengthening. Therefore, when B is contained, the B content should be in the range of 0.0005 - 0.0050%.

[0042] REM (rare earth metals): 0.001 - 0.100% REM (rare earth metals) is an element effective for deoxidation. This effect can be obtained when the REM content is 0.001% or more. However, when the REM content exceeds 0.100%, the hot workability decreases. Therefore, when REM is contained, the REM content should be in the range of 0.001 - 0.100%. When REM is contained, the REM content is more preferably 0.010% or more. Also, when REM is contained, the REM content is more preferably 0.050% or less. Note that REM is a general term for Sc, Y, and the 15 elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, and the REM content referred to here is the total content of these elements.

[0043] Sn: 0.001 - 0.100% Sn is an element effective in suppressing work surface roughness. Its effect can be obtained when the Sn content is 0.001% or more. However, when the Sn content exceeds 0.100%, the hot workability deteriorates. Therefore, when containing Sn, the Sn content should be in the range of 0.001 to 0.100%. When containing Sn, more preferably, the Sn content is 0.050% or less.

[0044] Sb: 0.001 - 0.100% Similar to Sn, Sb is an element effective in suppressing work surface roughness. Its effect can be obtained when the Sb content is 0.001% or more. However, when the Sb content exceeds 0.100%, the workability deteriorates. Therefore, when containing Sb, the Sb content should be in the range of 0.001 to 0.100%. When containing Sb, more preferably, the Sb content is 0.050% or less.

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

[0046] Grain boundary coverage rate of Laves phase: 20% or less (preferred condition) The inventors have found that by suppressing the precipitation amount of the Laves phase at the grain boundary, the corrosion resistance in the use environment of the denitration device can be improved. In particular, it has been found that when the grain boundary coverage rate of the Laves phase is 20% or less, excellent corrosion resistance in the use environment of the denitration device can be more stably ensured. The reason for this is not clear, but it is known that intergranular corrosion occurs in the use environment of the denitration device. On the other hand, the Laves phase is easily dissolved in the use environment of the denitration device and precipitates at the grain boundary. Therefore, it is presumed that when the precipitation amount of the Laves phase increases, intergranular corrosion is promoted. Here, since the use environment of the urea SCR member during operation is about 700°C in the high-temperature part, it is preferable that the grain boundary coverage rate of the Laves phase after holding ferritic stainless steel at 700°C for 1000 hours is 20% or less. The lower the grain boundary coverage rate, the more preferable it is, and it may be 0%.

[0047] Here, the grain boundary coverage rate of the Laves phase is evaluated by the following method. After holding at 700°C for 1000 hours, a sample is taken from the steel material with its longitudinal cross-section as the observation surface, and it is resin-embedded and mirror-polished. Then, five arbitrary locations are observed at a magnification of 1000 times by observing the backscattered electron image of SEM (scanning electron microscope) (photographed so that the field of view range is 100 μm × 100 μm or more). In the region of 100 μm × 100 μm at the center of each image obtained from the SEM observation, the total length L of the grain boundaries GB and the total length L of the grain boundaries covered by the Laves phase with respect to L Laves The ratio is defined as the grain boundary coverage rate of the Laves phase as shown in formula (3). The average value of the grain boundary coverage rates of the Laves phase obtained at the above five locations is taken as the grain boundary coverage rate (%) of the Laves phase of the ferritic stainless steel. Grain boundary coverage rate of Laves phase (%) = (Total length L of grain boundaries covered by Laves phase Laves / Total length L of the grain boundaries GB ) × 100 ···(3)

[0048] Next, a preferred manufacturing method of the ferritic stainless steel of the present invention will be described. The manufacturing method of the ferritic stainless steel of the present invention is not particularly limited. For example, it can be manufactured as follows. A steel slab having the above component composition is hot-rolled into a hot-rolled sheet, and the hot-rolled sheet is annealed if necessary. Then, the hot-rolled sheet or the hot-rolled annealed sheet is cold-rolled into a cold-rolled sheet having a desired plate thickness, and further, if necessary, the cold-rolled sheet is annealed to manufacture a ferritic stainless steel sheet having the above component composition. Note that the conditions for hot rolling, cold rolling, hot-rolled sheet annealing, cold-rolled sheet annealing, etc. are not particularly limited, and may follow conventional methods.

[0049] In the steelmaking process of melting steel, it is preferable to subject the molten steel melted in a converter or an electric furnace to secondary refining by the VOD (Vacuum Oxygen Decarburization) method or the like to obtain steel containing the above essential components and optional components contained as required. The molten steel can be made into a steel material by a known method, but from the viewpoints of productivity and quality, it is preferably by the continuous casting method. The steel material is then preferably heated to 1050 to 1250 °C and hot-rolled into a hot-rolled sheet with a desired thickness. Of course, hot working other than sheet materials can also be carried out. The above hot-rolled sheet is preferably continuously annealed at a temperature of 900 to 1150 °C as required, and then descaled by pickling or the like to obtain a hot-rolled product. Incidentally, if necessary, scale removal may be carried out by shot blasting or grinding brush before pickling.

[0050] Furthermore, the above hot-rolled product (hot-rolled annealed sheet, etc.) may be made into a cold-rolled product through processes such as cold rolling. In this case, the cold rolling may be carried out once, but from the viewpoints of productivity and required quality, it may also be two or more times of cold rolling with intermediate annealing. The total reduction ratio of one or two or more times of cold rolling is preferably 60% or more, more preferably 70% or more. The cold-rolled steel sheet is then preferably continuously annealed (finish annealing) at a temperature of 900 to 1150 °C, more preferably 950 to 1150 °C, pickled, and preferably made into a cold-rolled product. Incidentally, the continuous annealing may be carried out by bright annealing to omit pickling. Furthermore, after finish annealing, skin pass rolling or the like may be carried out to adjust the shape, surface roughness, and material of the steel sheet.

[0051] The ferritic stainless steel of the present invention described above is suitably used in a denitration device such as urea SCR.

Examples

[0052] Steel having the component composition shown in Table 1 was melted in a vacuum melting furnace, heated at 1150 °C for 1 hour, and then a hot-rolled sheet with a thickness of 4.0 mm was produced by hot rolling. After performing hot-rolled sheet annealing by holding at 1080 °C for 1 minute, the surface was ground to remove scale and cold-rolled to a thickness of 1.0 mm to obtain a cold-rolled sheet. The cold-rolled sheet was subjected to finish annealing by holding at 1040 °C for 1 minute in an ammonia-decomposed gas atmosphere to produce a cold-rolled annealed sheet.

[0053] The cold-rolled annealed sheet obtained above was subjected to a heat treatment of holding at 700 °C for 1000 hours in the atmosphere. Then, it was cooled to room temperature, its surface was polished to 600 grit with emery paper, and degreasing was performed with acetone. For the samples prepared in this way, the measurement of the grain boundary coverage rate of the Laves phase and the corrosion resistance evaluation in the use environment of the denitration device were carried out as follows. The results are shown in Table 2.

[0054] Grain boundary coverage rate of Laves phase After collecting samples with the longitudinal cross-section as the observation surface from the prepared samples, resin embedding and mirror polishing were performed, and then five arbitrary locations were observed at a magnification of 1000 times by SEM backscattered electron image observation. In the region of 100 μm × 100 μm at the center of each image, the total length L of the grain boundaries GB and the total length L of the grain boundaries covered by the Laves phase Laves The grain boundary coverage rate of the Laves phase was determined by the above formula (3) from the ratio. And the average value of the grain boundary coverage rate of the Laves phase obtained at each of the above five locations was taken as the grain boundary coverage rate of the Laves phase of the sample. The grain boundary coverage rate of the Laves phase was evaluated according to the following criteria. ○: 20% or less ×: More than 20%

[0055] Corrosion resistance evaluation (Huey test) in the use environment of the denitration device From the prepared samples, test pieces with a width of 20 mm and a length of 50 mm were cut out, and a Huey test was conducted in accordance with JIS G 0573:1999 (Method for 65% Nitric Acid Corrosion Test of Stainless Steel). 65 mass% nitric acid was used as the solution, and the test was carried out in a boiling state. The test for 48 hours per cycle was carried out up to 3 cycles. After each cycle, the weight was measured with an electronic balance, and the corrosion rate (g / (m 2 ·h)) was determined from the weight change before and after the test, and the average value of 3 cycles was taken as the corrosion rate of that sample. The corrosion rate was evaluated according to the following criteria, and was considered qualified in the case of ○ and unqualified in the case of ×. ○: 0.5 g / (m 2 ·h) or less ×: More than 0.5 g / (m 2 ·h)

[0056]

Table 1

[0057]

Table 2

[0058] From Table 2, in all of Invention Examples Nos. 1 to 26, the corrosion resistance in the use environment of the denitration device was good. On the other hand, in Comparative Examples Nos. 27 to 35 where the component composition was outside the appropriate range, the target corrosion resistance in the use environment of the denitration device could not be satisfied.

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

Industrial Applicability

[0060] According to the present invention, a ferritic stainless steel having excellent corrosion resistance in the use environment of a denitration device can be provided, which is extremely useful industrially.

Claims

1. In mass percent, C: 0.003-0.030%, N: 0.030% or less, Si: 0.20-1.00%, Mn: 0.05-0.50%, P: 0.050% or less, S: 0.020% or less, Cr: 13.0-21.0%, Mo: 0.001 to 3.00%, Al: 0.001-0.50%, Nb: 0.001 to 0.60%, and Ti: 0.001 to 0.15%; A ferritic stainless steel for a denitration system, which has a composition that satisfies 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 contents (mass%) of each element.

2. The ferritic stainless steel for a denitration system according to claim 1 , further comprising, by mass %, one or more elements selected from the following Group A and Group B: Group A: One or more selected from Ni: 0.001-1.00%, Cu: 0.01-2.00%, Co: 0.01-1.00%, W: 0.01-2.00% Group B: One or more 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 metal): 0.001-0.100%, Sn: 0.001-0.100%, Sb: 0.001-0.100%

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

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