JP7570243B2 - Stainless steel welded components for urea SCR systems - Google Patents

Description

The present invention relates to stainless steel welded components used in exhaust system components for passenger cars, motorcycles, commercial vehicles, construction machinery, ships, etc., and in exhaust gas treatment equipment for power plants, as well as in urea SCR (Selective Catalytic Reduction) systems that use urea water as a reducing agent for nitrogen oxides (NOx).

Ferritic stainless steels are widely used in exhaust system components for passenger cars, motorcycles, commercial vehicles, construction machinery, etc. In particular, for downstream exhaust system components known as the cold end, from the standpoint of corrosion resistance, workability, weldability, etc., SUH409L steel, which contains 11% Cr and has C and N fixed with Ti, SUS430LX, which contains approximately 17% Cr and has C and N fixed with Ti, and SUS436J1L and SUS436L, which also contain Mo, are often used.

In recent years, exhaust gas regulations and fuel efficiency regulations have become stricter every year in light of global environmental issues, and automobile and parts manufacturers have been considering and implementing many countermeasures. Among these, the urea SCR system uses ammonia, a decomposition product of urea, as a NOx reducing agent to reduce the NOx contained in exhaust gas. This system uses urea, which is safer than ammonia, and has been used in commercial vehicles such as buses and trucks, but recently, with the strengthening of exhaust gas regulations, it is being applied to passenger cars and construction machinery.

Generally, a urea SCR system consists of a urea tank that stores and supplies urea water solution to the exhaust system, an exhaust pipe section into which the urea water is sprayed, and an exhaust pipe section in which a catalyst such as SCR is installed. Of these, the exhaust system components into which the urea water is sprayed require corrosion resistance in a high-temperature urea environment. Since exhaust system components in this area are usually assembled by welding, corrosion resistance is required, including at the welds. Furthermore, the severity of the corrosive environment may differ depending on the part of the exhaust system component into which the urea water is sprayed. In such cases, a combination of materials with different levels of corrosion resistance may be used, but the welds require corrosion resistance similar to that of the base material on the more corrosion-resistant side.

In addition to the above, urea SCR system components are required to be corrosion resistant in a high-temperature urea environment, as well as corrosion resistant to exhaust gas condensate on the inner surface and to be corrosion resistant to salt damage on the outer surface. Furthermore, they must have fatigue properties, high-temperature strength, workability, etc., so SUS436L is often used.

Several technologies have been proposed to address these issues.

Patent Document 1 discloses a ferritic stainless steel that contains C: 0.05% or less, N: 0.05% or less, Si: 0.02-1.5%, Cr: 10-22, Nb: 0.03-1%, S: 0.0012% or less, and satisfies Cr+4Si-2Mn≧10, and has excellent corrosion resistance in urea water.

Patent Document 2 discloses a ferritic stainless steel sheet for urea SCR system components, which contains C: 0.010% or less, N: 0.020% or less, Si: 0.5% or less, Mn: 0.5% or less, Cr: 10.0-20.0%, Ti: 0.05-0.30%, and Al: 0.03-0.5%, and the maximum concentration ratio of Cr, Si, Al, Ti, Mn, and Fe within 20 nm from the surface satisfies the relationship (Cr+Ti+Al)/(Fe+Si+Mn)>0.35.

In Patent Document 3, C: 0.03% or less, N: 0.009-0.03%, Si: 0.2-1%, Mn: 0.2-1%, P: 0.04% or less, S: 0.01% or less, Cu: 0.5% or less, Ni: 0.5% or less, Cr: 15-22%, Mo: 2% or less, Ti: 0.16-1%, Nb: 0.2-1%, Al: 0.02-1%, V: 0.2% or less, Co: 0.2 %, Sn: 0.05% or less, REM: 0.1% or less, Zr: 0.01% or less, Al+30REM: 0.15% or more, 1/{Nb+(7/4)Ti-7(C+N)}: 3 or less, and an average crystal grain size of 25 to 65 μm are disclosed, and it is described that the plate can be used in a urea SCR system. However, Patent Documents 1 to 3 do not take into consideration corrosion resistance in a high-temperature urea environment.

Patent Document 4 discloses a ferritic stainless steel sheet for urea SCR housings, which contains C: 0.020% or less, N: 0.020% or less, Si: 0.01-0.50%, Mn: 0.01-0.50%, P: 0.040% or less, S: 0.010% or less, Cu: 0.40-0.80%, Ni: 0.05-0.6%, Cr: 20.5-24.0%, and one or both of Ti and Nb in the ranges of Ti: 0.01-0.40% and Nb: 0.01-0.55%, and satisfies Ti+Nb×48/93≧8(C+N). Patent Document 4 aims to improve corrosion resistance in a high-temperature urea environment, but does not consider the corrosion resistance of welded members, which is necessary for practical use.

Patent Document 5 discloses a ferritic stainless steel containing C: 0.015% or less, N: 0.020% or less, Si: 0.15-1.00%, Mn: 0.50% or less, P: 0.04% or less, S: 0.01% or less, Cr: 20.50-30.50%, Ti: 0.03-0.35%, etc., and satisfying at least one of 0.20<Ni, 0.02≦Co, and 0.20<Ni+1.2×Co. Patent Document 5 aims to improve corrosion resistance in a high-temperature urea environment, but does not take into account the corrosion resistance of welded members, which is necessary for practical use.

Patent Document 6 discloses an exhaust purification device that adds urea water to the exhaust gas and supplies an appropriate amount of ammonia to the NOx catalyst. In addition, Patent Document 7 discloses a urea water spray structure that suppresses the production of by-products derived from urea, such as ammonium sulfate, acid ammonium sulfate, and cyanuric acid, and prevents corrosion and clogging of the exhaust passage. Neither document discloses the material of the exhaust pipe.

Patent document 8 discloses an exhaust gas purification device for an internal combustion engine that can effectively agitate exhaust gas to eliminate bias in reducing agent concentration and uniformly distribute the exhaust gas flow over a reduction catalyst, thereby enabling all parts of the reduction catalyst to function effectively and exhibit good purification performance. One example is a urea SCR system, and stainless steel is listed as a material used, but there is no detailed description of stainless steel.

Patent Document 9 discloses a method for improving the durability of an exhaust pipe and an exhaust purification device, which is characterized by performing a shot peening treatment on the welded portion of an exhaust pipe made of a ferritic stainless steel plate, through which exhaust gas containing ammonia and hydrogen passes. SUS436L is disclosed as an example of a ferritic stainless steel used in the exhaust pipe, but the detailed chemical composition of the ferritic stainless steel is not disclosed.

Patent Document 10 discloses a stainless steel for a urea water injector that has sufficient corrosion resistance even against urea water at higher temperatures and is highly wear-resistant, the stainless steel having C: 0.2% or less, Ni: 3 to 11%, Cr: 12% or more, HRC hardness of 40 or more, and satisfying Cr- ^10C +2Ni≧2.18× ^10-3t2-1.87 × ^10-1t +9 (where t is the maximum temperature of the urea water (°C)). Patent Document 10 mentions improving corrosion resistance in a high-temperature urea environment as one of its problems, but does not consider the corrosion resistance of welded members.

Patent Document 11 discloses stainless steel for urea synthesis units that contains C: 0.030% or less, N: 0.10-0.20%, Si: 1.00% or less, Mn: 1.50% or less, P: 0.030% or less, S: 0.030% or less, Ni: 8.00-9.00%, Cr: 26.00-27.50%, and Mo: 0.50-1.50%. Patent Document 11 contains large amounts of Ni and Mo and is expensive.

Patent No. 5563203 Patent No. 5684547 Special Publication No. 2015-532681 International Publication No. 2016/035241 JP 2018-164815 A JP 2007-162488 A International Publication No. 2013/088850 JP 2008-128093 A International Publication No. 2013/179435 JP 2015-197085 A Japanese Patent Application Publication No. 10-226852

The installation of urea SCR systems is spreading from commercial vehicles such as buses and trucks to passenger cars and construction machinery. In passenger cars, the exhaust gas temperature near the urea water injection section tends to rise, and in construction machinery, the amount of urea water injected tends to increase, so the exhaust system components downstream of the urea water injection section are required to have even better corrosion resistance against urea water. Furthermore, the severity of the corrosive environment may vary depending on the part of the exhaust pipe into which the urea water is injected. In such cases, a combination of materials with different levels of corrosion resistance may be used, and the welded parts are required to have the same corrosion resistance as the base material on the more corrosion-resistant side.

The present invention was proposed in light of these issues, and aims to provide stainless steel welded components that can be suitably used in urea SCR systems.

The gist of the present invention, which aims to solve the above problems, is as follows:

(1) A stainless steel welded component for a urea SCR system, comprising a base metal and a weld metal, containing, by mass%, C: 0.020% or less, N: 0.020% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.040% or less, S: 0.010% or less, Cr: 20.50 to 30.50%, and Mo: 2.20% or less, with one or both of Ti and Nb in the range of Ti: 0.35% or less and Nb: 0.60% or less, with the balance being Fe and unavoidable impurities, and wherein the contents of C, N, Ti, Nb, Cr, and Mo in the base metal and the weld metal satisfy (Ti + Nb) / (C + N) ≧ 10.00 and Cr - 4.2Mo ≧ 17.00 mass%.

(2) The stainless steel welded component for a urea SCR system according to (1), characterized in that the base material contains, in mass %, one or more of the following elements in place of a portion of Fe: Ni: 0-0.80%, Cu: 0-0.40%, W: 0-1.20%, V: 0-0.50%, and Co: 0-1.50%.

(3) The stainless steel welded component for a urea SCR system according to (1) or (2), characterized in that the base material contains, in mass %, one or more of the following elements in place of a portion of Fe: Zr: 0-0.50%, Sn: 0-0.30%, Sb: 0-0.20%, Al: 0-0.200%, Ca: 0-0.0020%, Mg: 0-0.0020%, B: 0-0.0050%, REM: 0-0.010%, Ga: 0-0.0100%, and Ta: 0-0.500%.

The stainless steel welded components for urea SCR systems of the present invention are suitable for urea SCR systems in passenger cars, motorcycles, commercial vehicles, construction machinery, ships, etc. The stainless steel welded components of the present invention have excellent corrosion resistance in high-temperature urea environments, and can contribute to the high-temperature, long-life, and lightweight construction of urea SCR system components through thinning.

FIG. 1 is a graph showing the relationship between the average corrosion rate and Cr-4.2Mo of steel.

The following describes in detail the embodiments of the present invention.

First, the inventors' investigations will be described. In investigating corrosion resistance in a high-temperature urea environment, the inventors applied the Huey test (JIS G0573), which is used to evaluate materials for urea synthesis plants. The Huey test is an immersion test in a boiling nitric acid solution, but there is insufficient knowledge about ferritic stainless steels in this environment. Therefore, the inventors conducted the Huey test on ferritic stainless steels with different amounts of alloying elements, with Cr ranging from 11 to 24% and Mo ranging from 0 to 2%.

As a result, it was found that Cr is very effective in the environment, while Mo, which is usually effective in improving corrosion resistance, does not work effectively in the environment. The Huey test is performed by repeating a 48-hour test five times to determine the corrosion rate for each test, and the corrosion rate was evaluated as the average corrosion rate. When the average corrosion rate is arranged with respect to the Cr and Mo amounts of the steel, it is found to have a good correspondence with Cr-4.2Mo as shown in Figure 1, and when this value is 17.00 mass% or more, the average corrosion rate, which is the standard for excellent corrosion resistance in this patent, is 0.50 g m ^-2 h ^-1 or less. Here, each element symbol in the formula represents the content (mass%) of each element.

In addition, detailed observation of the corrosion pattern in the environment confirmed that the corrosion pattern was global, accompanied by intergranular corrosion, and that it is important to ensure intergranular corrosion resistance. Intergranular corrosion is likely to occur due to the precipitation of Cr carbides at the grain boundaries and the resulting generation of Cr-deficient layers, so it is necessary to suppress the precipitation of Cr carbides. For this purpose, it is effective to add elements such as Nb and Ti, which easily bond with C and N, so in order to ensure good intergranular corrosion resistance, it is necessary to make (Ti + Nb) / (C + N) ≥ 10.00. Each element symbol in the formula represents the content (mass %) of each element. Particular care is required in welded parts, as C and N may enter from the outside during welding work.

He said that the severity of the corrosive environment may vary depending on the location of the exhaust system components into which the urea solution is injected, and in such cases, materials with different levels of corrosion resistance may be used in combination. This means that SUS436L steel, which is commonly used as a urea SCR system component, is used in combination with ferritic stainless steel, which has higher corrosion resistance than SUS436L steel. Austenitic welding materials are often used to weld the two together.

Since the Cr content of SUS436L steel is approximately 17% and the Mo content is 1%, in order to ensure the corrosion resistance of the weld, it is necessary to select welding materials with a high Cr content and low C content so that the base metal and weld metal parts satisfy Cr-4.2Mo≧17.00 mass% and (Ti+Nb)/(C+N)≧10.00. Examples include 308L and 309L, but the 309L series, which has a higher Cr content, is preferable.

Regardless of whether welding materials are used, it is necessary to prevent C and N from entering from the outside during welding work and to adjust the dilution rate of the base metal so that the weld metal part satisfies Cr-4.2Mo ≧ 17.00 mass% and (Ti + Nb) / (C + N) ≧ 10.00.

The effects of the alloying elements in the base metal of the stainless steel welded components of the present invention and the reasons for limiting their content are described in detail below. % means mass % unless otherwise specified.

(C: 0.020% or less)
C reduces intergranular corrosion resistance and workability, so its content must be kept low. For this reason, the upper limit of the C content is set to 0.020%, and preferably 0.015%. C does not need to be contained in the base material, and the lower limit of the C content is 0. If the C content is excessively reduced, the refining cost will increase, so the lower limit of the C content is set at 0.002%. , or 0.003%.

(N: 0.020% or less)
N is an element useful for pitting corrosion resistance, but it also reduces intergranular corrosion resistance and workability, so its content must be kept low. For this reason, the upper limit of the N content is set at 0.020. %, and preferably 0.018%. N does not need to be contained in the base metal, and the lower limit of the content is 0. Excessively lowering the N content increases the refining cost. The lower limit of the N content may be set to 0.002% or 0.003%.

(Si: 1.00% or less)
Silicon is not an essential element and the lower limit of its content is 0. However, it is effective in improving oxidation resistance and has the effect of improving urea corrosion resistance at high temperatures. Therefore, silicon may be contained in the base material as necessary. In order to obtain the effect of improving the urea corrosion resistance at high temperatures, the content of 0.05% or more is preferable, 0.10% or more is more preferable, and 0.15% or more is even more preferable. However, excessive The addition of Si reduces workability and toughness, so the upper limit of the Si content is set to 1.00%. The upper limit of Si is preferably 0.95%, more preferably 0.90%, and further preferably 0.85%. %.

(Mn: 1.00% or less)
Since Mn deteriorates the corrosion resistance, the upper limit of the Mn content is set to 1.00%, and preferably 0.90%. Mn does not need to be contained in the base metal, and the lower limit of the Mn content is 0. Since an excessively low Mn content leads to increased costs, the lower limit of the Mn content may be set to 0.03% or 0.05%.

(P: 0.040% or less)
Since P is an element that deteriorates workability and weldability, the upper limit of the P content is set to 0.040%, preferably 0.035% or less. The lower limit of the P content is 0.

(S: 0.010% or less)
Since S is an element that deteriorates corrosion resistance, the upper limit of the S content is set to 0.010%, preferably 0.003% or less, and more preferably 0.002% or less. is 0.

(Cr: 20.50% or more, 30.50% or less)
Since Cr is the most basic and important element for ensuring urea corrosion resistance at high temperatures, the lower limit of the Cr content must be set to 20.50%, preferably 22.00% or more, and more preferably 10.50% or more. The content of Cr is preferably 22.50% or more, and more preferably 23.00% or more. On the other hand, the corrosion resistance can be improved as the Cr content is increased, but the excessive addition of Cr reduces the workability and manufacturability. For this reason, the Cr content is set to 30.50% or less, preferably 30.00% or less, more preferably 29.50% or less, and further preferably 29.00% or less.

(Mo: 2.20% or less)
Mo is effective for improving general corrosion resistance, but does not work effectively in a high-temperature urea environment. Excessive addition of Mo reduces workability and is expensive, leading to increased costs. Therefore, the Mo content is set to 2.20% or less, preferably 2.00% or less, more preferably 1.80% or less, and even more preferably 1.60% or less. On the other hand, when Mo is added to improve corrosion resistance, it is preferable to set it to 0.20% or more. When Mo is added, the Mo content is limited in relation to the Cr content, as described above. The inclusion of Mo is not essential, and the lower limit of the content is 0.

(Ti: 0.35% or less)
Ti has the effect of fixing C and N as carbonitrides to suppress intergranular corrosion, and also has the effect of fixing S as sulfides or carbosulfides to improve corrosion resistance. The lower limit is preferably set to 0.03%, more preferably 0.05%, and further preferably 0.07%. Excessive addition of Nb has a detrimental effect on workability and manufacturability, so the upper limit is set to 0.03%. The Ti content is 0.35%, preferably 0.32%, and more preferably 0.30%. Ti is not an essential element, and the lower limit of the Ti content is 0.

(Nb: 0.60% or less)
Like Ti, Nb has the effect of fixing C and N as carbonitrides to suppress intergranular corrosion. In addition, Nb has the effect of improving high-temperature strength, so when it is contained, 0.03% or more is set. It is preferable that the content of Nb is 0.05% or more, more preferably 0.07% or more. Since excessive addition of Nb has an adverse effect on workability, it is preferable that the upper limit of Nb be 0.60%. The Nb content is more preferably 0.55%, and further preferably 0.50%. Nb is not an essential element, and the lower limit of the Nb content is 0.

However, the Ti and Nb contents must satisfy the following relationship in relation to the C and N contents:

((Ti+Nb)/(C+N)≧10.00)
As described above, in order to ensure intergranular corrosion resistance, it is necessary to satisfy (Ti+Nb)/(C+N)≧10.00 in both the base metal and the weld metal. When the value is below 10.00, precipitation of Cr carbides and formation of Cr-depleted layers occur at the grain boundaries, increasing the susceptibility to intergranular corrosion. In particular, in a high-temperature urea environment, Cr is a very effective element for corrosion resistance, so It is necessary to avoid the formation of a Cr-deficient layer. The larger the (Ti+Nb)/(C+N) value, the better the intergranular corrosion resistance. However, if it is excessive, it leads to an increase in costs. is preferably 50.00 or less.

(Cr-4.2Mo≧17.00% by mass)
As mentioned above, the Cr-4.2Mo value is an important index for high-temperature urea corrosion resistance. In order to ensure urea corrosion resistance at high temperatures, the Cr-4.2Mo value must be 17.00 mass% or more. is preferably 18.00% by mass or more, and more preferably 19.00% by mass or more.

The remainder of the chemical composition of the base material is Fe and unavoidable impurities. The stainless steel of the present invention may further contain one or more of Ni, Cu, W, V, and Co in place of a portion of the Fe.

(Ni: 0-0.80%)
Ni is one of the elements that improves high-temperature urea corrosion resistance. The inclusion of Ni is not essential, and the lower limit of the Ni content is 0. The effect of improving high-temperature urea corrosion resistance can be obtained even with a small amount of Ni inclusion, but it is necessary to ensure the effect. In order to obtain the above, it is preferable to add Ni at 0.15% or more. It is more preferable to add Ni at 0.20% or more, and more preferable to add Ni at 0.25% or more. However, the excessive addition of Ni increases the cost. Therefore, the content is preferably in the range of 0.80% or less, more preferably 0.70% or less, and further preferably 0.60% or less.

(Cu: 0-0.40%)
Cu is generally an effective element for improving corrosion resistance. Cu content is not essential, and the lower limit of the Cu content is 0. If added in excess, Cu will not effectively improve corrosion resistance in a high-temperature urea environment. Therefore, the effect of improving corrosion resistance can be obtained even with a small amount of content, but in order to reliably obtain the effect, it is necessary to add it in the range of 0.40% or less. On the other hand, when it is contained, it is preferable that it is 0.10% or more.

(W: 0-1.20%)
W is one of the elements that improves corrosion resistance. The inclusion of W is not essential, and the lower limit of the content is 0. The effect of improving corrosion resistance can be obtained even with a small amount of W, but in order to ensure the effect, The content of W is preferably 0.20% or more, and more preferably 0.30% or more. Excessive addition of W deteriorates the workability and is expensive, leading to increased costs. For this reason, the content of W is The content is preferably equal to or less than 1.20%, and more preferably equal to or less than 1.00%.

(V: 0-0.50%)
V is one of the elements that improves corrosion resistance. The inclusion of V is not essential, and the lower limit of the content is 0. The effect of improving corrosion resistance can be obtained even with a small amount of V inclusion, but in order to ensure the effect, The V content is preferably 0.05% or more. Also, the V content is preferably 0.10% or more. However, excessive addition of V deteriorates the workability and increases the cost because of its high price. For this reason, the V content is preferably 0.50% or less, and more preferably 0.45% or less.

(Co: 0-1.50%)
Co is one of the elements that improves high-temperature urea corrosion resistance. It also has the effect of improving secondary workability and toughness. The inclusion of Co is not essential, and the lower limit of the content is 0. High-temperature urea corrosion resistance The effects of improving the hardness, secondary workability and toughness can be obtained even with a small amount of Co, but to ensure the effects, it is preferable to contain 0.02% or more of Co. Furthermore, it is preferable to contain 0.05% or more of Co. It is preferable to add Co in an amount of 0.08% or more, and more preferable to add Co in an amount of 0.08% or more. However, since excessive addition of Co leads to an increase in costs, it is preferable to add Co in an amount of 1.50% or less. More preferably, it is 1. It is preferably 20% or less, and more preferably 1.00% or less.

The stainless steel of the present invention may further contain one or more of Zr, Sn, Sb, Al, Ca, Mg, B, REM, Ga, and Ta in place of a portion of the Fe.

(Zr: 0-0.50%)
Zr is an element that improves corrosion resistance, particularly intergranular corrosion resistance. The inclusion of Zr is not essential, and the lower limit of the content is 0. The effect of improving intergranular corrosion resistance can be obtained even with a small amount of Zr. However, in order to reliably obtain the effect, it is preferable to add 0.03% or more, and more preferably 0.05% or more. Excessive addition of Zr deteriorates the workability and is expensive. Therefore, the Zr content is preferably 0.50% or less, and more preferably 0.40% or less.

(Sn: 0-0.30%)
Sn is an element that improves corrosion resistance. The inclusion of Sn is not essential, and the lower limit of the content is 0. Although the effect of improving corrosion resistance can be obtained even with a small amount of Sn, in order to reliably obtain the effect, 0 It is preferable to add Sn at least 0.01%, and more preferably at least 0.03%. Excessive addition of Sn deteriorates workability and manufacturability. For this reason, the Sn content is set to 0.30% or less. It is preferable that the content of the SiC layer is 0.25% or less, and more preferable that the content of the SiC layer is 0.25% or less.

(Sb: 0-0.20%)
Sb is an element that improves corrosion resistance. The inclusion of Sb is not essential, and the lower limit of the content is 0. Although the effect of improving corrosion resistance can be obtained even with a small amount of Sb, in order to reliably obtain the effect, 0 It is preferable to add Sb at least 0.01%, and more preferably at least 0.03%. However, excessive addition of Sb reduces workability and manufacturability. For this reason, the Sb content is set to 0.20% or less. % or less, and more preferably 0.15% or less.

(Al: 0 to 0.200% or less)
Al is a useful element as a deoxidizing element. There is no need for Al to be contained in the steel after deoxidization, and deoxidization can also be performed with Ca or the like, so the lower limit of the content is 0. When used for deoxidization, extremely lowering the content leads to increased costs, so the lower limit may be set at 0.002% or 0.004%. Excessive addition of Al deteriorates toughness and manufacturability. For this reason, the Al content is preferably 0.200% or less, and more preferably 0.150% or less.

(Ca: 0-0.0020%)
Ca is a useful element in refining because it has a deoxidizing effect. There is no need for Ca to be contained in the steel after deoxidization, and deoxidation can also be performed with Al, etc., so the lower limit of the content is 0. When Ca is used for deoxidization, extremely lowering the content leads to increased costs, so the lower limit may be set at 0.0002% or 0.0004%. Ca forms sulfides. Since Ca deteriorates the corrosion resistance, the Ca content is preferably set to 0.0020% or less, and more preferably set to 0.0015% or less.

(Mg: 0-0.0020%)
Mg is a useful element for refining because it has a deoxidizing effect, and is also effective in refining the structure and improving workability and toughness. The inclusion of Mg is not essential, and the lower limit of the content is 0. Although the effect of improving workability and toughness can be obtained even with a small amount of content, in order to reliably obtain the effect, it is preferable to contain 0.0002% or more, and more preferably 0.0005% or more. Since excessive addition of Mg deteriorates the corrosion resistance, the Mg content is preferably set to 0.0020% or less, and more preferably set to 0.0015% or less.

(B: 0-0.0050%)
B is an element that improves workability, particularly secondary workability. The inclusion of B is not essential, and the lower limit of the content is 0. The effect of improving workability, particularly secondary workability, is enhanced by a small amount of B. However, to ensure the effect, it is preferable to add 0.0002% or more. It is more preferable to add 0.0003% or more. Excessive addition of B reduces intergranular corrosion resistance. In order to reduce the B content, the B content is preferably 0.0050% or less, and more preferably 0.0030% or less.

(REM: 0-0.010%)
REM is the sum of Sc, Y, and elements with atomic numbers of 57 to 71, such as La, Ce, Pr, and Nd. REM is a useful element for refining because it has a deoxidizing effect. There is no need for REM to be contained in the steel after deoxidization, and the lower limit of the content is 0. When REM is used for deoxidization, extremely lowering the content leads to increased costs, so the lower limit is 0. The lower limit of REM may be 0.001% or 0.002%. Excessive addition of REM leads to increased costs, so the REM content is preferably 0.010% or less, and more preferably 0.008% or less.

(Ga: 0-0.0100%)
Ga is an element that forms stable sulfides to improve corrosion resistance and hydrogen embrittlement resistance. Ga is not an essential element, and the lower limit of the Ga content is 0. Although the effect of improving chemical resistance can be obtained even with a small amount of Nb, in order to reliably obtain the effect, it is preferable to add Nb at 0.0002% or more, and more preferably at 0.0004% or more. However, excessive addition Since this leads to an increase in costs, the Ga content is preferably 0.0100% or less, and more preferably 0.0050% or less.

(Ta: 0-0.500%)
Ta is an element that improves corrosion resistance. Ta is not an essential element, and the lower limit of the Ta content is 0. Although the effect of improving corrosion resistance can be obtained even with a small amount of Ta, 0 is required to reliably obtain the effect. It is preferable that the content of Cr is 0.010% or more. It is more preferable that the content of Cr is 0.050% or more, and further more preferable that the content of Cr is 0.100% or more. Excessive addition of Cr reduces toughness and leads to an increase in costs. Therefore, the Ta content is preferably 0.500% or less, and more preferably 0.400% or less.

The stainless steel base material of the present invention is manufactured by a general method for manufacturing ferritic stainless steel. For example, molten steel having the above-mentioned chemical composition is produced in a converter or electric furnace, refined in an AOD furnace or VOD furnace, etc., and made into steel billets by continuous casting or ingot casting. It is then manufactured through the steps of hot rolling - annealing the hot-rolled sheet - pickling - cold rolling - finish annealing - pickling. If necessary, the annealing of the hot-rolled sheet may be omitted, or the cold rolling - finish annealing - pickling may be repeated.

In addition, the stainless steel of the present invention can be used to manufacture welded pipes using conventional methods for manufacturing stainless steel pipes for exhaust system components, such as electric resistance welding, TIG welding, and laser welding.

Welded components are made from the stainless steel plates or pipes produced by the above method. Generally, the ends of the material are processed for welding or formed into the required shape before welding. Examples of welding methods include electric resistance welding, TIG welding, and laser welding.

The present invention will be explained in more detail based on examples.

The stainless steel with the composition shown in Tables 1-1 and 1-2 was melted in a 180 kg vacuum melting furnace and cast into a 45 kg steel ingot, which was then subjected to the processes of hot rolling, hot-rolled sheet annealing, shot blasting, cold rolling, finish annealing, and pickling to produce a cold-rolled steel sheet with a thickness of 2 mm. The hot-rolled sheet was produced by rolling the material to a thickness of 50 mm at a heating temperature of 1200°C to a thickness of 5 mm and air cooling. The hot-rolled sheet annealing and finish annealing conditions were 850 to 1050°C x 1 minute, air cooling.

The cold-rolled steel plate thus prepared, having a thickness of 2 mm, and a cold-rolled SUS436L steel plate, also having a thickness of 2 mm, were butt-welded by TIG welding. YS309L was used as the welding material. After welding, the weld metal part was cut out for component analysis, and the boundary between the weld metal part and the SUS436L was cut, and the welded part including the cold-rolled steel plate base material and the weld metal part was subjected to a corrosion test.

<Component analysis of weld metal>
The weld metal was cut perpendicular to the plate thickness direction based on the boundary between the surface weld metal and the base metal, and the entire surface was mechanically polished to remove any scale from welding. The weld metal was then subjected to component analysis. The C content of the weld metal was determined by the combustion-infrared absorption method, the N content by the thermal conductivity method, and the Cr, Mo, Ti and Nb contents by ICP atomic emission spectrometry.

<Nitric acid corrosion test>
Two corrosion test pieces of 20 mm x 40 mm were cut out from the above welded members so that the weld length in the test pieces was 20 mm, and the entire surfaces were wet-polished with emery paper up to #320.

The corrosion test was performed in accordance with JIS G0573. A 65% nitric acid solution was used, and the test was performed in a boiling state. The test was repeated five times for 48 hours each, and the specimens were weighed after each test to determine the corrosion rate (g·m ⁻² ·h ⁻¹ ) from the change in mass before and after the test. The corrosion rate was then evaluated based on the average value of the five corrosion rates obtained.

The results of the component analysis of the weld metal and the corrosion rate in the nitric acid corrosion test are shown in Table 2. Here, the corrosion rate is the average value of two test pieces. Note that for No. 3 and No. 16, the cold-rolled steel sheets prepared did not contain Ti, but the SUS436L welded to the prepared cold-rolled steel sheets contained Ti, so the weld metal contained Ti.

As shown in Table 2, in Examples 1 to 19, the average corrosion rate was 0.50 g m ^-2 h ^-1 or less, and the corrosion resistance was excellent. Comparative Examples 20 and 21 in which the Cr content of the base metal and the Cr-4.2Mo values of the base metal and the weld metal did not satisfy the present invention, Comparative Example 22 in which the base metal and the weld metal did not satisfy (Ti + Nb) / (C + N) ≧ 10.00, Comparative Example 23 in which the Cr-4.2Mo value of the base metal did not satisfy the present invention, Comparative Example 24 in which the Cr content of the base metal and the Cr-4.2Mo value of the weld metal did not satisfy, Comparative Example 25 in which the Cr-4.2Mo value of the weld metal did not satisfy, and Comparative Example 26 in which the weld metal did not satisfy (Ti + Nb) / (C + N) ≧ 10.00 had a corrosion rate exceeding 0.50 g m ^-2 h ^-1 and were inferior in corrosion resistance.

The ferritic stainless steel of the present invention is suitable as a welding component for urea SCR systems used in the exhaust systems of passenger cars, motorcycles, commercial vehicles, construction machinery, etc. It is also suitable as a welding component for exhaust gas treatment equipment that uses urea water as a reducing agent for nitrogen oxides (NOx) in power plants, etc.

Claims (3)

In mass percent,
C: 0.020% or less,
N: 0.020% or less,
Si: 1.00% or less,
Mn: 1.00% or less,
P: 0.040% or less,
S: 0.010% or less,
Cr: 20.50 to 30.50%; and Mo: 2.20% or less;
The steel sheet comprises a base metal containing one or both of Ti and Nb in the ranges of Ti: 0.35% or less and Nb: 0.60% or less, with the balance being Fe and unavoidable impurities, and a weld metal;
A stainless steel welded component for a urea SCR system, characterized in that the contents of C, N, Ti, Nb, Cr, and Mo in the base metal and the weld metal satisfy (Ti + Nb) / (C + N) ≧ 10.00 and Cr - 4.2Mo ≧ 17.00 mass%. The stainless steel welded component for a urea SCR system according to claim 1, characterized in that the base material contains, in mass %, one or more of the following elements in place of a portion of Fe: Ni: 0-0.80%, Cu: 0-0.40%, W: 0-1.20%, V: 0-0.50%, and Co: 0-1.50%. The stainless steel welded component for a urea SCR system according to claim 1 or 2, characterized in that the base material contains, in mass %, one or more of Zr: 0-0.50%, Sn: 0-0.30%, Sb: 0-0.20%, Al: 0-0.200%, Ca: 0-0.0020%, Mg: 0-0.0020%, B: 0-0.0050%, REM: 0-0.010%, Ga: 0-0.0100%, and Ta: 0-0.500% in place of a portion of Fe.

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