ferritic stainless steel

Abstract

This invention provides a ferritic stainless steel that exhibits good brazing properties and excellent corrosion resistance after brazing when using Ni-containing solder for high-temperature brazing. The ferritic stainless steel has the following composition: by mass % containing C: 0.003–0.030%, Si: 0.01–1.00%, Mn: 0.05–0.30%, P: less than 0.050%, S: less than 0.020%, Cr: 24.0–30.0%, Ni: 1.50–3.00%, Mo: 1.00–3.00%, Al: 0.001–0.020%, Nb: 0.20–0.80%, N: less than 0.030%, satisfying the following formulas (1) and (2), and the remainder consisting of Fe and unavoidable impurities. Ni-2(Si+Mn)≥0.00%···(1), Cr+1.5Mo+Si+1.5Nb-2.5Ni≤25.0%···(2) (Ni, Si, Mn, Cr, Mo, Nb in formulas (1) and (2) represent the content of each element (mass%)).

Description

Technical Field

[0001] This invention relates to ferritic stainless steel, and particularly to ferritic stainless steel that exhibits good brazing properties and excellent corrosion resistance when brazed at high temperatures using Ni-containing filler metal. Background Technology

[0002] In recent years, from the perspective of global environmental protection, there has been a demand for automobiles to further improve fuel efficiency and enhance exhaust gas purification. As a result, the application of automotive heat exchangers such as waste heat recovery units and EGR (Exhaust Gas Recirculation) coolers has been expanding.

[0003] Here, a waste heat recovery unit is a device that improves fuel efficiency by using the heat from the engine cooling water for heating or by using the heat from exhaust gases to heat the engine cooling water, thereby shortening the engine warm-up time during startup. Typically, the waste heat recovery unit is located between the catalytic converter and the muffler. The waste heat recovery unit consists of a heat exchanger section (composed of combined pipes, plates, fins, side plates, etc.), an inlet pipe section, and an outlet pipe section. Exhaust gases then enter the heat exchanger section through the inlet pipe, where the heat is transferred to the cooling water via heat transfer surfaces such as fins, and discharged through the outlet pipe. Furthermore, the bonding and assembly of the plates and fins of the heat exchanger section, which constitutes such a waste heat recovery unit, mainly utilizes brazing with Ni-containing solder.

[0004] Furthermore, the EGR cooler consists of a pipe that draws in a portion of exhaust gas from the exhaust manifold, a heat exchanger that cools the drawn-in exhaust gas, and a pipe that returns the cooled exhaust gas to the intake side of the engine. Specifically, the EGR cooler has a structure that includes both a water flow path and an exhaust gas path along the path of exhaust gas returning from the exhaust manifold to the intake side of the engine. By employing this structure, the high-temperature exhaust gas on the exhaust side is cooled by the heat exchanger, and the cooled exhaust gas returns to the intake side of the engine, reducing the engine's combustion temperature and suppressing NO that is easily generated at high temperatures. X In addition, due to reasons such as lightweighting, compactness, and cost reduction, the heat exchanger part of the EGR cooler is constructed by overlapping thin plates into fins, and their bonding and assembly still mainly use brazing with Ni-containing solder.

[0005] Thus, the heat exchanger sections of the waste heat recovery unit and EGR cooler are bonded and assembled using brazing with Ni-containing solder. Therefore, the materials used in these heat exchanger sections require good brazing properties with Ni-containing solder. Furthermore, since the exhaust gas contains nitrogen oxides (NOx)... X ), sulfur oxides (SO XHydrocarbons (HC) condense in heat exchangers, forming highly corrosive acidic condensate. Therefore, the materials used in these heat exchanger sections also require corrosion resistance. Especially during the high temperatures reached in brazing heat treatment, it is necessary to prevent Cr at grain boundaries from reacting with C and N to form Cr carbonitrides and creating a poorly corrosion-resistant Cr-deficient layer around them – a process known as sensitization – thus ensuring corrosion resistance.

[0006] For the reasons mentioned above, the heat exchanger sections of waste heat recovery units and EGR coolers typically use austenitic stainless steels such as SUS316L or SUS304L, which have reduced carbon content to prevent sensitization. However, there are problems with austenitic stainless steels in the following aspects: austenitic stainless steels are expensive due to their high Ni content, and they have low fatigue characteristics, i.e., low thermal fatigue characteristics at high temperatures, in environments where they are subjected to high temperatures and intense vibrations due to their large thermal expansion.

[0007] Therefore, research is also underway on using steels other than austenitic stainless steel for the heat exchanger parts of waste heat recovery units and EGR coolers.

[0008] For example, Patent Document 1 discloses a ferritic stainless steel that ensures corrosion resistance by generating an oxide film containing more than 16.0% Nb in terms of cation fraction after brazing, as a material for waste heat recovery units and EGR coolers.

[0009] Patent document 2 discloses a ferritic stainless steel system that ensures corrosion resistance by controlling the amount of Al, Ti, and Si added, as a material for waste heat recovery units and EGR coolers.

[0010] Patent document 3 discloses a ferritic stainless steel that ensures corrosion resistance by controlling the content of Cr, Si and Al in the oxide film after brazing and the thickness of the oxide film as a material for heat exchangers and fuel supply system components.

[0011] In addition, Patent Document 4 discloses a ferritic stainless steel for use in EGR coolers, which ensures brazing properties by adding components such as Cr, Cu, Al, and Ti in a certain formula and suppressing the addition of Al and Ti.

[0012] Furthermore, Patent Document 5 discloses a ferritic stainless steel system that ensures brazing properties by suppressing the addition of Al, Ti, and Zr as an EGR cooler component having a structure joined by Ni brazing.

[0013] Furthermore, Patent Document 6 discloses a ferritic stainless steel for brazing that ensures brazing performance by suppressing the addition of Ti and Zr.

[0014] Existing technical documents

[0015] Patent documents

[0016] Patent Document 1: Japanese Patent No. 6157664

[0017] Patent Document 2: Japanese Patent No. 6159775

[0018] Patent Document 3: Japanese Patent No. 6270821

[0019] Patent Document 4: Japanese Patent Application Publication No. 2010-121208

[0020] Patent Document 5: Japanese Patent Application Publication No. 2009-174040

[0021] Patent Document 6: Japanese Patent Application Publication No. 2010-285683 Summary of the Invention

[0022] However, in the technologies described in Patent Documents 1 to 6, the solderability is sometimes insufficient, depending on the solder used and the soldering conditions. In particular, in the prior art, it cannot be said that it is possible to obtain the good corrosion resistance described above while also ensuring sufficient solderability at high temperatures using Ni-containing solders.

[0023] The present invention was developed in view of the above-mentioned situation, and its purpose is to provide a ferritic stainless steel that has good brazing properties when brazed at high temperatures using Ni-containing brazing filler metal and excellent corrosion resistance after brazing.

[0024] It should be noted that, in this specification, good brazing performance means that, after brazing a steel plate coated with Ni-containing filler metal (JIS standard: BNi-5) in a nitrogen carrier gas atmosphere at 1170°C and 1 Torr for 10 minutes and then cooling to room temperature, the ratio of the equivalent circle diameter of the filler metal after heating to the equivalent circle diameter of the filler metal before heating (filler metal spreading rate) is 150% or more.

[0025] In addition, excellent corrosion resistance refers to the following: using the above-mentioned steel plate brazed with Ni-containing solder, a 20 mm square test piece is taken from the part without solder, leaving an 11 mm square measurement surface. The test piece is covered with a sealing material and then immersed in a 3.5% NaCl aqueous solution at 30°C. Except for the concentration of the NaCl aqueous solution, the pitting potential Vc'100 measured according to JIS G 0577 is 300 mV or higher (vs SCE).

[0026] When using Ni-containing brazing filler metal for high-temperature brazing, the inventors conducted an in-depth study on the relationship between the elemental composition of various stainless steels and their brazing properties.

[0027] The results showed that suppressing the Al content in stainless steel, and further suppressing the Si and Mn content relative to the Ni content, improved the wettability with Ni-containing solders.

[0028] Furthermore, the relationship between the elemental composition of various stainless steels and their corrosion resistance after brazing was investigated in depth. The results showed that the high-temperature heat treatment following brazing caused the precipitation of the σ phase, leading to a decrease in corrosion resistance. Here, the σ phase is an intermetallic compound containing large amounts of Cr and Mo, forming a Cr and Mo-deficient layer around it, thus reducing corrosion resistance after brazing. Further research revealed that by including an appropriate amount of Ni and further suppressing the contents of Cr, Mo, Si, and Nb relative to the Ni content, the precipitation of the σ phase during the high-temperature heat treatment of brazing could be inhibited.

[0029] This invention was completed based on the above insights and through further research. Specifically, the main structure of this invention is as follows.

[0030] [1] A ferritic stainless steel having the following composition:

[0031] It contains, by mass%:

[0032] C: 0.003~0.030%

[0033] Si: 0.01~1.00%

[0034] Mn: 0.05~0.30%

[0035] P: below 0.050%

[0036] S: Below 0.020%

[0037] Cr: 24.0–30.0%

[0038] Ni: 1.50–3.00%

[0039] Mo: 1.00–3.00%

[0040] Al: 0.001~0.020%

[0041] Nb: 0.20–0.80%

[0042] N: below 0.030%,

[0043] It satisfies the following equations (1) and (2), and the remainder consists of Fe and unavoidable impurities.

[0044] Ni-2(Si+Mn)≥0.00%···(1)

[0045] Cr+1.5Mo+Si+1.5Nb-2.5Ni≤25.0%···(2)

[0046] (In equations (1) and (2), Ni, Si, Mn, Cr, Mo, and Nb represent the content (mass%) of each element.)

[0047] [2] The ferritic stainless steel according to [1] above, wherein, by mass percent, it further contains, selected from:

[0048] Cu: 0.01~1.00%

[0049] Co: 0.01~1.00%

[0050] W: 0.01~2.00%

[0051] One or more of them.

[0052] [3] The ferritic stainless steel according to [1] or [2], wherein, by mass percent, it further contains, selected from:

[0053] Ti: 0.01~0.10%

[0054] V: 0.01~0.20%

[0055] Zr: 0.01~0.10%

[0056] Mg: 0.0005~0.0050%

[0057] Ca: 0.0005~0.0050%

[0058] B: 0.0005~0.0050%

[0059] REM (Rare Earth Metals): 0.001~0.100%

[0060] Sn: 0.001~0.100%

[0061] Sb: 0.001~0.100%

[0062] One or more of them.

[0063] [4] Ferritic stainless steel according to [1] or [2], which is used for waste heat recovery devices or waste gas recirculation devices for assembling one or more joints by brazing.

[0064] [5] Ferritic stainless steel according to [3], used for waste heat recovery devices or waste gas recirculation devices that assemble one or more joints by brazing.

[0065] According to the present invention, a ferritic stainless steel with good brazing properties when brazed at high temperatures using Ni-containing filler metal and excellent corrosion resistance after brazing can be provided. Detailed Implementation

[0066] The present invention will now be described in detail.

[0067] First, the reasons for limiting the composition of steel to the above-mentioned range in this invention will be explained. It should be noted that the unit of the content of elements in the composition of steel is "mass%", but unless otherwise specified, it will be expressed as "%" below.

[0068] C: 0.003~0.030%

[0069] A higher carbon (C) content increases strength, while a lower content increases processability. To achieve sufficient strength, a C content of 0.003% or more is required. However, if the C content exceeds 0.030%, the decrease in processability becomes significant, and Cr carbides precipitate at grain boundaries, causing sensitization and reducing corrosion resistance. Therefore, the C content is in the range of 0.003% to 0.030%. A C content of 0.004% or more is preferred. Furthermore, a C content of 0.025% or less is preferred, more preferably 0.020% or less, and even more preferably 0.010% or less.

[0070] Si: 0.01~1.00%

[0071] Si is an element used as a deoxidizing material. This effect is achieved when the Si content is 0.01% or more. However, if the Si content exceeds 1.00%, Si-concentrated substances such as Si oxides and Si nitrides are formed on the surface of the steel plate during brazing heat treatment, reducing brazability. Furthermore, during high-temperature brazing using Ni-containing filler metal, σ phase precipitation occurs, reducing corrosion resistance. Therefore, the Si content is in the range of 0.01% to 1.00%. The Si content is preferably 0.50% or more, more preferably 0.60% or more, and even more preferably 0.70% or more. Additionally, the Si content is preferably 0.85% or less, more preferably 0.80% or less.

[0072] Mn: 0.05~0.30%

[0073] Mn has a deoxidizing effect, which is achieved when the Mn content is 0.05% or more. However, if the Mn content exceeds 0.30%, Mn concentrates form on the steel plate surface during brazing heat treatment, reducing brazability. Therefore, the Mn content is in the range of 0.05% to 0.30%. The Mn content is preferably 0.10% or more. Furthermore, the Mn content is preferably 0.25% or less, more preferably 0.20% or less, and even more preferably 0.15% or less.

[0074] P: below 0.050%

[0075] Phosphorus (P) is an unavoidable element in steel, but excessive amounts can easily lead to intergranular corrosion. This tendency becomes significant when the P content exceeds 0.050%. Therefore, the P content is 0.050% or less. Preferably, the P content is 0.040% or less, more preferably 0.030% or less. It should be noted that there is no particular limitation on the lower limit of the P content. However, excessive P removal leads to increased costs, so the P content is preferably 0.005% or more.

[0076] S: below 0.020%

[0077] Sulfur (S) is an unavoidable element in steel. However, S content exceeding 0.020% promotes the precipitation of MnS, reducing corrosion resistance. Therefore, the S content should be 0.020% or less. Preferably, the S content should be 0.010% or less. It should be noted that there is no particular limitation on the lower limit of the S content. However, excessive S removal leads to increased costs; therefore, the S content is preferably 0.0005% or more.

[0078] Cr: 24.0–30.0%

[0079] Cr is an important element for ensuring the corrosion resistance of stainless steel. Sufficient corrosion resistance cannot be obtained when the Cr content is less than 24.0%. On the other hand, if the Cr content exceeds 30.0%, the σ phase precipitates during brazing, reducing corrosion resistance. Therefore, the Cr content is in the range of 24.0% to 30.0%. The Cr content is preferably 24.5% or more, more preferably 25.0% or more. Furthermore, the Cr content is preferably 28.0% or less, more preferably 26.5% or less.

[0080] Ni: 1.50–3.00%

[0081] Ni is one of the important elements in this invention. The brazing properties with Ni-containing solders are improved by containing 1.50% or more Ni. The mechanism by which Ni improves brazing properties is not yet clear, but it is believed that when the base material contains an appropriate amount of Ni, wettability is improved through interaction with Ni contained in the solder. Furthermore, the precipitation of σ phase during brazing can be suppressed. However, if the Ni content exceeds 3.00%, the susceptibility to stress corrosion cracking increases. Therefore, the Ni content is in the range of 1.50% to 3.00%. The Ni content is preferably 1.75% or more, more preferably 2.00% or more. Furthermore, the Ni content is preferably 2.75% or less, more preferably 2.50% or less.

[0082] Mo: 1.00–3.00%

[0083] Mo stabilizes the passivation film on stainless steel, thereby improving its corrosion resistance. This effect is achieved when the Mo content is 1.00% or higher. However, if the Mo content exceeds 3.00%, σ phase precipitation occurs during brazing, reducing corrosion resistance. Therefore, the Mo content is preferably in the range of 1.00% to 3.00%. The Mo content is preferably 1.25% or higher, more preferably 1.50% or higher. Furthermore, the Mo content is preferably 2.50% or lower, more preferably 2.00% or lower.

[0084] Al: 0.001~0.020%

[0085] Al is a useful element for deoxidation, and this effect is achieved when the Al content is 0.001% or higher. However, if the Al content exceeds 0.020%, Al-concentrated compounds such as Al oxides and Al nitrides are formed on the steel surface during brazing, reducing the wettability, spreadability, and adhesion of the brazing filler metal, making brazing difficult. Therefore, the Al content is in the range of 0.001% to 0.020%. Preferably, the Al content is 0.015% or less.

[0086] Nb: 0.20–0.80%

[0087] Nitrogen (Nb) is an element that suppresses the decrease in corrosion resistance (sensitization) caused by the precipitation of Cr carbonitrides through bonding with C and N. This effect is achieved when the Nb content is 0.20% or higher. On the other hand, if the Nb content exceeds 0.80%, σ phase precipitation occurs during brazing, reducing corrosion resistance. Therefore, the Nb content is in the range of 0.20% to 0.80%. The Nb content is preferably 0.25% or higher, more preferably 0.30% or higher. Furthermore, the Nb content is preferably 0.60% or lower, more preferably 0.35% or lower.

[0088] N: below 0.030%

[0089] If the nitrogen (N) content exceeds 0.030%, corrosion resistance and processability 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. It should be noted that there is no particular limitation on the lower limit of the N content, but excessively reducing the N content will lead to an increase in cost; therefore, the N content is preferably 0.003% or more.

[0090] Ni-2(Si+Mn)≥0.00%···(1)

[0091] In formula (1), Ni, Si, and Mn represent the content (mass%) of each element.

[0092] In this invention, to improve solderability, Ni, Si, and Mn are each set to a specified content. Furthermore, the inventors conducted in-depth research and discovered that if Ni₂(Si+Mn) (Ni content minus twice the total of Si and Mn content) is less than 0.00%, the desired solderability cannot be obtained. The reasoning is that Ni improves solderability, but on the other hand, Si and Mn hinder solderability; therefore, the balance of these elements has a significant impact on solderability. Therefore, in this invention, while setting the Ni, Si, and Mn contents to the aforementioned ranges, Ni₂(Si+Mn) is set to 0.00% or more. Ni₂(Si+Mn) is preferably 0.50% or more. In particular, by setting the Cr content to less than 26.0%, the Al content to less than 0.015%, and Ni₂(Si+Mn) to 0.50% or more, even better solderability can be obtained.

[0093] Cr+1.5Mo+Si+1.5Nb-2.5Ni≤25.0%···(2)

[0094] In formula (2), Cr, Mo, Si, Nb and Ni represent the content (mass%) of each element.

[0095] In this invention, to suppress the precipitation of the σ phase during brazing, the contents of Cr, Mo, Si, Nb, and Ni are set to specified values. Furthermore, the inventors conducted in-depth research and discovered that if the content of Cr + 1.5Mo + Si + 1.5Nb - 2.5Ni is greater than 25.0%, the σ phase precipitates during brazing, reducing corrosion resistance. The rationale is that Ni inhibits the precipitation of the σ phase, but on the other hand, Cr, Mo, Si, and Nb promote its precipitation; therefore, the balance of these elements has a significant impact on the precipitation of the σ phase. Therefore, in this invention, the contents of Cr, Mo, Si, Nb, and Ni are set to the aforementioned ranges, while the content of Cr + 1.5Mo + Si + 1.5Nb - 2.5Ni is set to 25.0% or less.

[0096] The basic components (essential components) of the ferritic stainless steel of the present invention have been described above. In the composition of the present invention, the components other than those described above (the remaining portion) are Fe and unavoidable impurities.

[0097] The ferritic stainless steel of the present invention may further contain one or more of Cu, Co, and W within the following ranges.

[0098] Cu: 0.01~1.00%

[0099] Cu is an element that improves corrosion resistance. This effect is achieved when the Cu content is 0.01% or more. However, if the Cu content exceeds 1.00%, the hot workability decreases. Therefore, when Cu is present, the Cu content is in the range of 0.01% to 1.00%. When Cu is present, the Cu content is more preferably 0.10% or more. Furthermore, when Cu is present, the Cu content is more preferably 0.80% or less, and even more preferably 0.60% or less.

[0100] Co: 0.01~1.00%

[0101] Co is an element that improves corrosion resistance. This effect is achieved when the Co content is 0.01% or more. However, if the Co content exceeds 1.00%, processability decreases. Therefore, when Co is present, the Co content is in the range of 0.01% to 1.00%. When Co is present, the Co content is more preferably 0.05% or more. Furthermore, when Co is present, the Co content is more preferably 0.70% or less.

[0102] W: 0.01~2.00%

[0103] W is an element that improves corrosion resistance. This effect is achieved when the W content is 0.01% or more. However, if the W content exceeds 2.00%, σ phase precipitates during brazing. Therefore, when W is present, the W content is in the range of 0.01% to 2.00%. When W is present, the W content is more preferably 0.05% or more. Furthermore, when W is present, the W content is more preferably 1.00% or less.

[0104] The ferritic stainless steel of the present invention may further contain one or more of the following elements selected from Ti, V, Zr, Mg, Ca, B, REM, Sn, and Sb, respectively, within the range described below.

[0105] Ti: 0.01~0.10%

[0106] Ti has the effect of bonding with C and N contained in steel, thus preventing sensitization. This effect is achieved when the Ti content is 0.01% or more. On the other hand, Ti is an oxygen-reactive element, and when Ti content exceeds 0.10%, a Ti oxide film forms on the surface of the steel during brazing, reducing brazability. Therefore, when Ti is present, the Ti content is in the range of 0.01% to 0.10%. More preferably, the Ti content is 0.05% or less.

[0107] V: 0.01~0.20%

[0108] Similar to Ti, V bonds with the C and N contained in the steel, thus preventing sensitization. These effects are achieved when the V content is 0.01% or more. On the other hand, if the V content exceeds 0.20%, the workability decreases. Therefore, when V is present, the V content is in the range of 0.01% to 0.20%. When V is present, the V content is more preferably 0.15% or less, and even more preferably 0.10% or less.

[0109] Zr: 0.01~0.10%

[0110] Like Ti and Nb, Zr is an element that bonds with C and N contained in steel, thus suppressing sensitization. This effect is achieved when the Zr content is 0.01% or more. On the other hand, if the Zr content exceeds 0.10%, the workability decreases. Therefore, when Zr is present, the Zr content is in the range of 0.01% to 0.10%. When Zr is present, the Zr content is more preferably 0.03% or more. Furthermore, when Zr is present, the Zr content is more preferably 0.05% or less.

[0111] Mg: 0.0005~0.0050%

[0112] Mg acts as a deoxidizer. This effect is achieved when the Mg content is 0.0005% or higher. However, if the Mg content exceeds 0.0050%, the toughness and manufacturability of the steel decrease. Therefore, when Mg is present, the Mg content is in the range of 0.0005% to 0.0050%. More preferably, the Mg content is 0.0020% or less.

[0113] Ca: 0.0005~0.0050%

[0114] Ca improves the weld penetration of the welded part, thus enhancing weldability. This effect is achieved when the Ca content is 0.0005% or higher. However, if the Ca content exceeds 0.0050%, it bonds with S to form CaS, reducing corrosion resistance. Therefore, when Ca is present, the Ca content is in the range of 0.0005% to 0.0050%. When Ca is present, the Ca content is more preferably 0.0010% or higher. Furthermore, when Ca is present, the Ca content is more preferably 0.0040% or lower.

[0115] B: 0.0005~0.0050%

[0116] Boron (B) is an element that improves brittleness during secondary processing. This effect is achieved when the B content is 0.0005% or higher. However, if the B content exceeds 0.0050%, ductility decreases due to solid solution strengthening. Therefore, when B is present, the B content should be in the range of 0.0005% to 0.0050%.

[0117] REM (Rare Earth Metals): 0.001~0.100%

[0118] Rare earth metals (REMs: elements with atomic numbers 57-71 such as La, Ce, and Nd) are effective for deoxidation. This effect is achieved when the REM content is 0.001% or higher. However, if the REM content exceeds 0.100%, hot workability decreases. Therefore, when containing REM, the REM content is in the range of 0.001% to 0.100%. More preferably, the REM content is 0.010% or higher. Furthermore, more preferably, the REM content is 0.050% or lower. It should be noted that REM is a collective term for 15 elements, including Sc, Y, and lanthanum (La) from atomic number 57 to lutetium (Lu) from atomic number 71; the REM content mentioned here refers to the total content of these elements.

[0119] Sn: 0.001~0.100%

[0120] Sn is an effective element for suppressing surface roughness during processing. This effect is achieved when the Sn content is 0.001% or higher. However, if the Sn content exceeds 0.100%, hot workability decreases. Therefore, when Sn is present, the Sn content is in the range of 0.001% to 0.100%. When Sn is present, a Sn content of 0.050% or less is more preferred.

[0121] Sb: 0.001~0.100%

[0122] Similar to Sn, Sb is an effective element for suppressing surface roughness during processing. This effect is achieved when the Sb content is 0.001% or higher. However, if the Sb content exceeds 0.100%, processability decreases. Therefore, when Sb is present, the Sb content is in the range of 0.001% to 0.100%. When Sb is present, an Sb content of 0.050% or less is more preferred.

[0123] Next, a preferred manufacturing method for the ferritic stainless steel of the present invention will be described.

[0124] The manufacturing method of the ferritic stainless steel of the present invention is not particularly limited. For example, steel can be smelted in a furnace such as a converter or electric furnace, or further refined by secondary refining such as ladle refining or vacuum refining to produce steel with the composition of the present invention described above. Then, steel sheets (billets) are produced by continuous casting or ingot-bill rolling, and the billets are hot-rolled to produce hot-rolled plates. Alternatively, the hot-rolled plates can be hot-rolled and annealed to produce hot-rolled and annealed plates as needed. Then, the hot-rolled plates or hot-rolled and annealed plates are cold-rolled to produce cold-rolled plates of the desired thickness. Alternatively, the cold-rolled plates can be further cold-rolled and annealed to produce cold-rolled and annealed plates as needed.

[0125] It should be noted that there are no special restrictions on the conditions for hot rolling, cold rolling, hot rolling plate annealing, and cold rolling plate annealing; conventional methods can be followed.

[0126] The preferred steelmaking process involves secondary refining of steel molten in a converter or electric furnace using methods such as VOD (Vacuum Oxygen Decarburization) to produce steel containing the aforementioned essential components and any additional components added as needed. The molten steel can be produced into steel slabs (billets) using known methods, but continuous casting is preferred for both productivity and quality. The steel slabs are then preferably heated to 1050–1250°C and hot-rolled to produce hot-rolled plates of the desired thickness. Alternatively, they can be hot-worked into shapes other than plates. Preferably, the hot-rolled plates are then continuously annealed at 900–1150°C as needed, followed by descaling using pickling or similar methods to produce hot-rolled products. It should be noted that, if necessary, the oxide scale can also be removed before pickling by shot peening, brushing, or similar methods.

[0127] Furthermore, the aforementioned hot-rolled products (hot-rolled annealed sheets, etc.) can also be processed into cold-rolled products through processes such as cold rolling. In this case, the cold rolling can be done in one pass, but from the viewpoint of productivity and quality requirements, it can also be set as two or more cold rolling passes with intermediate annealing. The total reduction rate of one or more cold rolling passes is preferably 60% or more, more preferably 70% or more. Then, the cold-rolled steel sheet is preferably continuously annealed (finishing annealing) at a temperature of 900 to 1150°C, more preferably 950 to 1150°C, and pickled to produce cold-rolled products. It should be noted that continuous annealing can also be performed by bright annealing, thus omitting pickling. Furthermore, depending on the application, surface finishing rolling or other processes can be performed after finishing annealing to adjust the shape, surface roughness, and material of the steel sheet.

[0128] The ferritic stainless steel of the present invention described above is well suited for use in waste heat recovery units and waste gas recirculation devices where one or more joints are assembled by brazing. It is particularly well suited for heat exchanger components of the aforementioned waste heat recovery units and waste gas recirculation devices.

[0129] Example

[0130] Steel with the composition shown in Table 1 was melted in a vacuum melting furnace and heated at 1150°C for 1 hour. Hot-rolled sheets with a thickness of 4.0 mm were then produced by hot rolling. After annealing the hot-rolled sheets at 1080°C for 1 minute, the surface was ground to remove oxide scale, and the sheets were cold-rolled to a thickness of 1.0 mm. Cold-rolled annealed sheets were obtained by finishing annealing at 1040°C for 1 minute in an ammonia decomposition gas atmosphere. The surface of these sheets was ground to 600 mesh with diamond abrasive paper and degreased with acetone for testing.

[0131] For the cold-rolled annealed sheet, brazing was performed using Ni-containing brazing filler metal as described below, and (1) brazing performance was evaluated, and (2) the amount of σ phase precipitation and (3) corrosion resistance were evaluated on the cold-rolled annealed sheet after brazing treatment. The results are shown in Table 2.

[0132] (1) Evaluation of solderability

[0133] Test pieces with a width of 50 mm and a length of 50 mm were cut from the prepared cold-rolled annealed sheet and brazed as follows: A Ni-containing solder (JIS standard: BNi-5) with a diameter of 10 mm and a thickness of 1 mm was coated onto the surface of the horizontal test piece. The test piece coated with the Ni-containing solder was then heated for 10 minutes at 1170°C and 1 Torr in a nitrogen carrier gas atmosphere with the solder-coated side facing upwards and placed horizontally. It was then cooled to room temperature. The equivalent circle diameter of the solder on the surface of the test piece was then measured (the equivalent circle diameter of the solder after heating). The ratio of the equivalent circle diameter of the solder after heating to the diameter of the solder before heating (10 mm, with the same equivalent circle diameter) (solder spread rate) was then calculated and evaluated according to the following criteria.

[0134] The spreading rate of the solder after heating relative to that before heating = (equivalent circle diameter of the solder after heating / diameter of the solder before heating (10mm)) × 100 (%)

[0135] ◎(Qualified, Excellent): 160% or higher

[0136] ○ (Pass): 150% or more but less than 160%

[0137] × (Unacceptable): Less than 150%

[0138] (2) Determination of σ phase precipitation

[0139] Test pieces of each cold-rolled annealed sheet after brazing were used. A cross-section (L-shaped) specimen was taken from the portion without brazing filler metal. After electrolytic polishing and aqua regia etching, the precipitation amount (area %) of the σ phase was determined by point counting method according to ASTM E 562, within a 500x magnification field of view using an optical microscope. It should be noted that the observation position was the center of the sample's sheet thickness.

[0140] ○: 1.0% or less

[0141] ×: Greater than 1.0%

[0142] (3) Evaluation of corrosion resistance

[0143] Test pieces were taken from the brazed portion of each cold-rolled annealed sheet, with a 20 mm square section removed from the area without brazing filler metal. An 11 mm square measurement surface was left on this section and coated with a silicone sealant. The test piece was then immersed in a 3.5% NaCl aqueous solution at 30°C. The pitting potential was measured according to JIS G0577, except for the concentration of the NaCl solution. After holding at spontaneous potential for 10 minutes, the current density was increased to 100 μA / cm² using potentiodynamic method with a potential scan rate of 20 mV / min. 2 The potential at that time is set as the pitting potential Vc'100, and its value is shown in Table 2. It should be noted that, considering the operating conditions of the heat exchanger section of the waste heat recovery unit and EGR cooler, if the pitting potential Vc'100 is above 300mV (vs SCE) equivalent to SUS304L with a good track record in the heat exchanger section of the waste heat recovery unit and EGR cooler, it can be judged as having excellent corrosion resistance.

[0144] ○ (Pass): 300mV (vs SCE) or above

[0145] × (Unacceptable): Less than 300mV (vs SCE)

[0146] [Table 1]

[0147]

[0148] The components other than those listed above consist of Fe and unavoidable impurities.

[0149] • The underlined part is outside the scope of this invention.

[0150] Table 2:

[0151]

[0152] As shown in Table 2, all invention examples No. 1 to 27 exhibit good brazing properties and excellent corrosion resistance. In particular, Nos. 6, 9, 10, 12, 13, 16, 18, 19, and 21, with Cr content less than 26.0%, Al content less than 0.015%, and Ni-2(Si+Mn) ≥ 0.50%, show exceptionally good brazing properties.

[0153] In contrast, in Comparative Examples No. 28 to 38, whose composition is outside the appropriate range, good solderability and excellent corrosion resistance cannot be simultaneously satisfied.

[0154] More specifically, in Comparative Example No. 28 (steel designation B1), since the Cr content exceeds the upper limit of the present invention, the σ phase precipitation in the microstructure after brazing exceeds 1.0%, and excellent corrosion resistance cannot be obtained.

[0155] In Comparative Example No. 29 (steel designation B2), since the Mo content exceeds the upper limit of the present invention, the σ phase precipitation in the microstructure after brazing exceeds 1.0%, and excellent corrosion resistance cannot be obtained.

[0156] In Comparative Example No. 30 (steel mark B3), good solderability could not be obtained because the Al content exceeded the upper limit of the present invention.

[0157] In Comparative Example No. 31 (steel designation B4), good brazing properties could not be obtained because the Si content exceeded the upper limit of the present invention. Furthermore, the σ phase precipitation in the microstructure after brazing exceeded 1.0%, resulting in poor corrosion resistance.

[0158] In Comparative Example No. 32 (steel designation B5), good brazing properties could not be obtained because the Mn content exceeded the upper limit of the present invention. Furthermore, excellent corrosion resistance could not be achieved.

[0159] In Comparative Example No. 33 (steel designation B6), since the Nb content exceeds the upper limit of the present invention, the σ phase precipitation in the microstructure after brazing exceeds 1.0%, and excellent corrosion resistance cannot be obtained.

[0160] In Comparative Example No. 34 (steel designation B7), excellent corrosion resistance could not be obtained because the Cr content was less than the lower limit of the present invention.

[0161] In Comparative Example No. 35 (steel designation B8), excellent corrosion resistance could not be obtained because the Mo content was less than the lower limit of the present invention.

[0162] In Comparative Example No. 36 (steel designation B9), good brazing properties could not be obtained because the Ni content was less than the lower limit of the present invention. Furthermore, the σ phase precipitation in the microstructure after brazing exceeded 1.0%, resulting in poor corrosion resistance.

[0163] In Comparative Example No. 37 (steel designation B10), all components are within the specified range, but formula (2) is not satisfied, and the σ phase precipitation exceeds 1.0%, so excellent corrosion resistance cannot be obtained.

[0164] In Comparative Example No. 38 (steel mark B11), all components are within the specified range, but Equation (1) is not satisfied, and good brazing properties cannot be obtained.

[0165] Industrial availability

[0166] According to the present invention, ferritic stainless steel suitable for use in waste gas recirculation devices such as heat exchanger components of waste heat recovery units and EGR coolers assembled by brazing can be obtained, and is therefore extremely useful in industry.

Claims

1. A ferritic stainless steel having the following composition: It contains, by mass%: C：0.003～0.030％、 Si: 0.01~1.00% Mn: 0.05~0.30% P: below 0.050% S: Below 0.020% Cr：24.0～30.0％、 Ni: 1.50–3.00% Mo: 1.00–3.00% Al：0.001～0.014％、 Nb: 0.20–0.80% N: below 0.030%, Furthermore, satisfying the following equations (1) and (2), the remaining part consists of Fe and unavoidable impurities; Ni-2(Si+Mn)≥0.00%···(1) Cr+1.5Mo+Si+1.5Nb-2.5Ni≤25.0%···(2) In equations (1) and (2), Ni, Si, Mn, Cr, Mo, and Nb represent the content of each element in terms of mass%.

2. The ferritic stainless steel according to claim 1, wherein, Further contains, by weight percent, a selection from: Cu: 0.01~1.00% Co: 0.01~1.00% W：0.01～2.00％ One or more of them.

3. The ferritic stainless steel according to claim 1 or 2, wherein, Further contains, by weight percent, a selection from: Ti: 0.01~0.10% V：0.01～0.20％、 Zr：0.01～0.10％、 Mg: 0.0005~0.0050% Ca: 0.0005~0.0050% B：0.0005～0.0050％、 REM stands for rare earth metals: 0.001–0.100%. Sn: 0.001~0.100% Sb: 0.001~0.100% One or more of them.

4. The ferritic stainless steel according to claim 1 or 2, used for a waste heat recovery unit or waste gas recirculation device for assembling one or more joints by brazing.

5. The ferritic stainless steel according to claim 3, used for a waste heat recovery unit or waste gas recirculation device for assembling one or more joints by brazing.

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