Ferritic stainless steel plate

Abstract

A ferritic stainless steel sheet is provided which has excellent press formability and brazability when brazing heat treatment is performed at high temperatures, and which also has excellent corrosion resistance after the brazing heat treatment. A ferritic stainless steel sheet having a chemical composition containing, by mass%, C: 0.003 to 0.030%, Si: 0.01 to 1.30%, Mn: 0.05 to 0.30%, P: 0.050% or less, S: 0.020% or less, Cr: 24.0 to 28.0%, Ni: 1.60 to 2.50%, Mo: 1.50 to 3.00%, Al: 0.001 to 0.150%, Ti: 0.01 to 0.35%, and N: 0.030% or less, with the balance being Fe and unavoidable impurities, and having an average r-value of 1.2 or more.

Description

[Technical Field]

[0001] The present invention relates to a ferritic stainless steel sheet, and in particular to a ferritic stainless steel sheet that has excellent press formability and brazability when subjected to brazing heat treatment at high temperatures, and also has excellent corrosion resistance after the brazing heat treatment. [Background technology]

[0002] In recent years, in order to comply with stricter exhaust gas regulations in the automotive field, there has been a demand for improved exhaust gas purification performance and fuel efficiency. As a result, the application of automotive heat exchangers such as exhaust heat recovery units and EGR (Exhaust Gas Recirculation) coolers is expanding.

[0003] An exhaust heat recovery system uses the heat of exhaust gas to heat the coolant through a heat exchanger, improving fuel economy and heating performance. An EGR cooler has a mechanism for recirculating exhaust gas, cools the hot exhaust gas on the exhaust side through a heat exchanger, and then re-inhales the cooled exhaust gas into the engine, lowering the combustion temperature and suppressing the generation of nitrogen oxides.

[0004] Many heat exchangers in exhaust heat recovery devices and EGR coolers have both water passages and exhaust gas passages on the path that returns exhaust gas to the engine's intake side, and are constructed with components such as pipes, plates, fins, tubes, and side plates. Of these, the fins are formed by press working. Brazing using Ni-containing brazing filler metals is used to join and assemble these components. For this reason, the materials used in heat exchangers must have good processability during press working and good brazing properties with Ni-containing brazing filler metals.

[0005] Furthermore, exhaust gas contains nitrogen oxides (NOx) and sulfur oxides (SOx), which condense inside the heat exchanger, forming highly corrosive, acidic condensate. Therefore, materials used in heat exchanger components must have high corrosion resistance. Furthermore, brazing heat treatment is performed at high temperatures, which can cause chromium carbonitrides to precipitate at grain boundaries, resulting in the formation of chromium-depleted zones in the vicinity, which can lead to a decrease in corrosion resistance—a phenomenon known as sensitization. Therefore, to ensure corrosion resistance after brazing heat treatment, it is necessary to prevent sensitization.

[0006] For these reasons, austenitic stainless steels such as SUS304L and SUS316L, which have a regulated carbon content to prevent sensitization, have been used as materials for the heat exchanger parts of exhaust heat recovery devices and EGR coolers. However, austenitic stainless steels are expensive because they contain a lot of Ni, and because they have a large thermal expansion coefficient, they can be prone to fracture due to thermal fatigue when subjected to constraints that involve severe vibration at high temperatures.

[0007] Therefore, the use of steels other than austenitic stainless steels in the heat exchanger parts of exhaust heat recovery devices and EGR coolers is being considered.

[0008] For example, Patent Document 1 discloses a ferritic stainless steel as a material for exhaust heat recovery devices and EGR coolers, which ensures corrosion resistance by forming an oxide film containing 16.0% or more Nb in cationic fraction after brazing.

[0009] Patent Document 2 discloses a ferritic stainless steel as a material for exhaust heat recovery devices and EGR coolers, in which corrosion resistance is ensured by controlling the amounts of Al, Ti, and Si added.

[0010] Patent Document 3 discloses a ferritic stainless steel as a material for exhaust heat recovery devices and EGR coolers, in which corrosion resistance is ensured by controlling the Al, Si, and Cr concentrations in the oxide film after an annealing process.

[0011] Patent Documents 4 and 5 disclose ferritic stainless steels that have excellent condensed water corrosion resistance and brazability by adding elements such as Cr and Mo according to a certain relationship and by suppressing the amounts of Al and Ti added.

[0012] Patent Documents 6 and 7 disclose ferritic stainless steels that have excellent condensed water corrosion resistance and brazability by adding components such as Si and Ni in a certain relationship and by suppressing the amount of Al added. [Prior art documents] [Patent documents]

[0013] [Patent Document 1] Patent No. 6157664 [Patent Document 2] Patent No. 6159775 [Patent Document 3] Japanese Patent Application Publication No. 2020-63499 [Patent Document 4] Patent No. 6517371 [Patent Document 5] Patent No. 6583517 [Patent Document 6] Patent No. 6699670 [Patent Document 7] Patent Publication No. 2023-146183 Summary of the Invention [Problem to be solved by the invention]

[0014] However, as the shapes of exhaust system components become more complex, even stainless steels that are difficult to process but have excellent corrosion resistance are now required to reduce gaps between parts and have good brazeability when brazing contact points between parts. To reduce the gaps between parts, further improvements in press forming accuracy are required. Therefore, ferritic stainless steels with better press formability, corrosion resistance, and brazeability are in demand.

[0015] The present invention has been made in view of the above circumstances, and has an object to provide a ferritic stainless steel sheet that has excellent press formability and brazeability when brazing heat treatment is performed at high temperatures, and that has excellent corrosion resistance after the brazing heat treatment.

[0016] In this specification, high-temperature brazing heat treatment (hereinafter simply referred to as brazing heat treatment) refers to the following heat treatment: In a nitrogen carrier gas atmosphere of 0.2 Torr, the material is heated from room temperature to 970°C, held at 970°C for 30 minutes, then heated from 970°C to 1180°C, and held at 1180°C for 10 minutes. The material is then cooled to 200°C or below in the same atmosphere. Details of the brazing heat treatment are described in the examples.

[0017] Furthermore, in this specification, "excellent brazeability" when performing brazing heat treatment at high temperatures means that the brazing filler metal has a spreading ratio of 120% or more, as shown below. A Ni-containing brazing filler metal is applied to the surface of a steel sheet, and the brazing heat treatment is performed. The spreading ratio of the brazing filler metal is defined as the ratio of the average diameter (average value of spreading length) of the Ni-containing brazing filler metal after the brazing heat treatment to the diameter of the Ni-containing brazing filler metal before the brazing heat treatment. Details of the brazeability evaluation are described in the Examples.

[0018] "Excellent corrosion resistance after brazing heat treatment" means that test specimens are taken from steel sheets that have been subjected to the above brazing heat treatment, and immersed in a 6 mass % FeCl3 + 1 mass % HCl mixed aqueous solution at temperatures that are changed in 5°C increments from 20°C to 50°C and then held constant (20°C, 25°C, 30°C, 35°C, 40°C, 45°C, 50°C) according to ASTM G48 Method C for 72 hours, and then the test specimen surface is observed, and the minimum temperature at which pitting corrosion occurs on the test specimen surface (critical pitting corrosion initiation temperature) is 35°C or higher. Details of the corrosion resistance evaluation are described in the Examples.

[0019] "Excellent press formability" means that the average r-value of the steel sheet is 1.2 or more. Details of the evaluation of press formability will be described in the Examples. [Means for solving the problem]

[0020] The present inventors have conducted extensive research into the relationship between the constituent elements of various stainless steels and their press formability. In addition to Cr, Mo, Ni, and other elements are sometimes added to ferritic stainless steels to improve corrosion resistance. However, the addition of these elements has the effect of solid-solution strengthening and can be a factor in reducing the r-value. Meanwhile, Ti, a stabilizing element, is expected to lower the recrystallization temperature, promoting recrystallization and increasing the r-value. However, Ti, when added in large amounts, can reduce brazability. Meanwhile, Nb, another stabilizing element, does not reduce brazability, but significantly increases the recrystallization temperature, potentially suppressing recrystallization and reducing the r-value. Taking the above into consideration, the present inventors have discovered the following: Specifically, the contents of constituent elements such as Cr, Mo, Ni, and Ti are appropriately controlled. Furthermore, when Nb is added, the contents of Ti and Nb are appropriately controlled. This ensures brazability and corrosion resistance, suppresses the recrystallization temperature, and facilitates recrystallization of the annealed steel sheet. This improves the r value, and in turn ensures sufficient press formability.

[0021] The present invention has been completed based on the above findings and after further investigation. That is, the gist and configuration of the present invention are as follows. [1] In mass%, C: 0.003 to 0.030%, Si: 0.01 to 1.30%, Mn: 0.05 to 0.30%, P: 0.050% or less, S: 0.020% or less, Cr: 24.0~28.0%, Ni: 1.60~2.50% Mo: 1.50-3.00%, Al: 0.001 to 0.150%, Ti: 0.01 to 0.35%, and N: 0.030% or less, the balance being Fe and unavoidable impurities, A ferritic stainless steel plate with an average r-value of 1.2 or more. [2] The component composition further includes, in mass%, Cu: 0.01 to 1.00%, Co: 0.01 to 1.00%, W: 0.01 to 2.00%, and Nb: 0.20% or less and, when Nb is contained, satisfies the following formula (1): Ti / 48-Nb / 93≧0 (1) Here, Ti and Nb in formula (1) represent the contents (mass %) of each element. [3] The component composition further includes, in mass%, V: 0.01 to 0.20%, Zr: 0.01 to 0.10%, Mg: 0.0005 to 0.0050%, Ca: 0.0005 to 0.0050%, B: 0.0005~0.0050%, REM (rare earth metal): 0.001~0.100%, Sn: 0.001 to 0.100%, and Sb: 0.001 to 0.100% The ferritic stainless steel sheet according to [1] or [2], containing one or more selected from the following: [4] The ferritic stainless steel sheet according to any one of [1] to [3], which is used as a member for an exhaust heat recovery device or an EGR cooler. [Effects of the Invention]

[0022] According to the present invention, a ferritic stainless steel sheet can be obtained which has excellent press formability and brazeability when brazing heat treatment is carried out at high temperatures, and which also has excellent corrosion resistance after the brazing heat treatment. DETAILED DESCRIPTION OF THE INVENTION

[0023] The present invention will be specifically described below.

[0024] First, the reasons for limiting the composition of the ferritic stainless steel sheet to the above ranges in the present invention will be explained. Note that the unit of the content of elements in the composition is "% by mass," but hereinafter, unless otherwise specified, it will be simply expressed as "%."

[0025] C: 0.003 to 0.030% Increasing the C content improves strength, while decreasing it improves workability. Here, a C content of 0.003% or more is required to obtain sufficient strength. However, if the C content exceeds 0.030%, not only does workability decrease significantly, but Cr carbides precipitate at grain boundaries during brazing heat treatment, causing sensitization and reducing corrosion resistance after brazing heat treatment. Therefore, the C content is set to the range of 0.003 to 0.030%. The C content is preferably 0.005% or more. The C content is also preferably 0.025% or less, more preferably 0.020% or less, and even more preferably 0.010% or less.

[0026] Si: 0.01 to 1.30% Si is a useful element as a deoxidizer. This effect can be achieved with a Si content of 0.01% or more. However, if the Si content exceeds 1.30%, Si oxides are formed on the steel sheet surface during brazing heat treatment, resulting in a decrease in brazability. Therefore, the Si content is set to the range of 0.01 to 1.30%. The Si content is preferably 0.50% or more, and more preferably 0.60% or more. Furthermore, the Si content is preferably 1.00% or less, and more preferably 0.80% or less.

[0027] Mn: 0.05 to 0.30% Mn has a deoxidizing effect, and this effect is obtained when the Mn content is 0.05% or more. However, if the Mn content exceeds 0.30%, corrosion resistance deteriorates. Therefore, the Mn content is set to the range of 0.05 to 0.30%. The Mn content is preferably 0.10% or more, and more preferably 0.15% or more. Furthermore, the Mn content is preferably 0.25% or less, and more preferably 0.20% or less.

[0028] P:0.050% or less P is an element that is inevitably contained in steel, and excessive content thereof makes intergranular corrosion more likely to occur. This tendency becomes more pronounced when the P content exceeds 0.050%. Therefore, the P content is set to 0.050% or less. Preferably, the P content is 0.030% or less. There is no particular lower limit for the P content. However, since excessive dephosphorization increases costs, the P content is preferably 0.005% or more.

[0029] S: 0.020% or less S is an element that is inevitably contained in steel, and an S content of more than 0.020% promotes the precipitation of MnS and reduces corrosion resistance. Therefore, the S content is set to 0.020% or less. Preferably, the S content is 0.015% or less. There is no particular lower limit for the S content. However, since excessive desulfurization increases costs, the S content is preferably 0.0005% or more.

[0030] Cr: 24.0~28.0% Cr is an important element for ensuring the corrosion resistance of stainless steel. If the Cr content is less than 24.0%, sufficient corrosion resistance cannot be obtained. On the other hand, if the Cr content exceeds 28.0%, the steel becomes hard and workability decreases. Therefore, the Cr content is set to the range of 24.0 to 28.0%. The Cr content is preferably 25.0% or more. Furthermore, from the viewpoint of workability, the Cr content is preferably 27.0% or less, and more preferably 26.0% or less.

[0031] Ni: 1.60-2.50% By including 1.60% or more Ni, it is possible to suppress the precipitation of the σ phase during brazing heat treatment, which causes a decrease in corrosion resistance after brazing heat treatment. On the other hand, if the Ni content exceeds 2.50%, the stress corrosion cracking susceptibility increases. Therefore, the Ni content is set to the range of 1.60 to 2.50%. The Ni content is preferably 1.70% or more, and more preferably 1.80% or more. Furthermore, the Ni content is preferably 2.30% or less, and more preferably 2.20% or less.

[0032] Mo: 1.50-3.00% Mo stabilizes the passive film of stainless steel, improving its corrosion resistance. This effect is achieved when the Mo content is 1.50% or more. However, if the Mo content exceeds 3.00%, a σ phase precipitates during brazing heat treatment, reducing the corrosion resistance after brazing heat treatment. Therefore, the Mo content is set to the range of 1.50 to 3.00%. The Mo content is preferably 1.70% or more, and more preferably 1.80% or more. Furthermore, the Mo content is preferably 2.50% or less, and more preferably 2.00% or less.

[0033] Al: 0.001 to 0.150% Al is a useful element for deoxidation, and this effect can be achieved with an Al content of 0.001% or more. However, Al is an element active against oxygen, and if the Al content exceeds 0.150%, oxides mainly composed of Al will form on the steel surface during brazing heat treatment. These oxides significantly reduce brazability. Therefore, the Al content is set to the range of 0.001 to 0.150%. The Al content is preferably 0.100% or less, and more preferably 0.050% or less.

[0034] Ti: 0.01 to 0.35% Ti bonds with the C and N contained in steel and has the effect of preventing sensitization during welding. This effect is achieved with a Ti content of 0.01% or more. Furthermore, among stabilizing elements, Ti has the effect of lowering the recrystallization temperature, which facilitates recrystallization during annealing, thereby increasing the r-value and improving workability during press forming. On the other hand, Ti is an oxygen-active element, and if the Ti content exceeds 0.35%, Ti-based oxides are formed on the steel surface during brazing heat treatment. These oxides reduce brazeability. Therefore, the Ti content is limited to a range of 0.01 to 0.35%. The Ti content is preferably 0.05% or more, more preferably 0.10% or more. Furthermore, from the viewpoint of brazeability, the Ti content is preferably 0.20% or less, more preferably 0.15% or less.

[0035] N: 0.030% or less If the N content exceeds 0.030%, corrosion resistance and workability during press forming will decrease. Therefore, the N content is set to 0.030% or less. The N content is preferably 0.025% or less, and more preferably 0.015% or less. Although there is no particular restriction on the lower limit of the N content, excessive reduction of the N content will lead to an increase in costs, so the N content is preferably set to 0.003% or more.

[0036] The basic components (essential components) of the ferritic stainless steel sheet of the present invention have been described above. The balance other than the above basic components can be Fe and inevitable impurities. Incidentally, inevitable impurities are impurities that are inevitably mixed in from raw materials, the manufacturing process, manufacturing equipment, etc., and are allowed to be included to the extent that they do not impair the object of the present invention. Examples of raw materials include iron ore, reduced iron, scrap, etc. Examples of impurities include O, H, and Zn.

[0037] Furthermore, in the present invention, one or more elements selected from Cu, Co, W, and Nb may be further contained within the following ranges.

[0038] Cu: 0.01 to 1.00% Cu is an element that improves corrosion resistance. This effect is obtained when the Cu content is 0.01% or more. However, if the Cu content exceeds 1.00%, hot workability decreases. Therefore, when Cu is contained, the Cu content is set to a range of 0.01 to 1.00%. When Cu is contained, the Cu content is preferably 0.10% or more. Furthermore, when Cu is contained, the Cu content is preferably 0.80% or less, and more preferably 0.60% or less.

[0039] Co: 0.01 to 1.00% Co is an element that improves corrosion resistance. This effect is obtained when the Co content is 0.01% or more. However, if the Co content exceeds 1.00%, workability decreases. Therefore, when Co is contained, the Co content is set to a range of 0.01 to 1.00%. When Co is contained, the Co content is preferably 0.05% or more. Furthermore, when Co is contained, the Co content is preferably 0.70% or less.

[0040] W: 0.01 to 2.00% W is an element that improves corrosion resistance. This effect is obtained when the W content is 0.01% or more. However, if the W content exceeds 2.00%, workability decreases. Therefore, when W is contained, the W content is set to a range of 0.01 to 2.00%. When W is contained, the W content is preferably 0.05% or more. Furthermore, when W is contained, the W content is preferably 1.00% or less.

[0041] Nb: 0.20% or less Nb, like Ti, is an element that combines with C and N to suppress the deterioration of corrosion resistance (sensitization) due to the precipitation of Cr carbonitrides during brazing heat treatment. Furthermore, by adding Nb in addition to Ti, good brazability can be more stably obtained. On the other hand, if the Nb content exceeds 0.20%, the r-value decreases and workability during press forming deteriorates. Therefore, the Nb content is set to 0.20% or less. The Nb content is preferably 0.15% or less, and more preferably 0.10% or less.

[0042] Ti / 48-Nb / 93≧0 (1) In formula (1), Ti and Nb represent the content (mass %) of each element. Nb has the effect of raising the recrystallization temperature more than Ti. Therefore, if the Nb content exceeds the Ti content in terms of atomic ratio, recrystallization becomes difficult to proceed, and unrecrystallized extended grains tend to remain after annealing heat treatment, reducing press formability. Therefore, when Nb is contained, the Ti content and Nb content should be set within the above-mentioned ranges, and formula (1) should be satisfied.

[0043] In the present invention, one or more elements selected from V, Zr, Mg, Ca, B, REM, Sn, and Sb may further be contained within the following ranges.

[0044] V: 0.01 to 0.20% Like Ti, V bonds with C and N contained in the steel to prevent sensitization. This effect is obtained when the V content is 0.01% or more. On the other hand, if the V content exceeds 0.20%, workability decreases. Therefore, when V is contained, the V content is set to a range of 0.01 to 0.20%. When V is contained, the V content is preferably 0.15% or less, and more preferably 0.10% or less.

[0045] Zr: 0.01 to 0.10% Like Ti and Nb, Zr is an element that bonds with C and N contained in steel and suppresses sensitization. This effect is obtained when the Zr content is 0.01% or more. On the other hand, if the Zr content exceeds 0.10%, workability decreases. Therefore, when Zr is contained, the Zr content is set to a range of 0.01 to 0.10%. When Zr is contained, the Zr content is preferably 0.03% or more. Furthermore, when Zr is contained, the Zr content is preferably 0.05% or less.

[0046] Mg: 0.0005 to 0.0050% Mg acts as a deoxidizer. This effect is obtained when the Mg content is 0.0005% or more. However, if the Mg content exceeds 0.0050%, the toughness of the steel decreases, resulting in poor manufacturability. Therefore, if Mg is contained, the Mg content is set to a range of 0.0005 to 0.0050%. If Mg is contained, the Mg content is preferably 0.0020% or less.

[0047] Ca: 0.0005 to 0.0050% Ca improves the penetration of welds and improves weldability. This effect is achieved when the Ca content is 0.0005% or more. However, if the Ca content exceeds 0.0050%, it bonds with S to form CaS, which reduces corrosion resistance. Therefore, when Ca is contained, the Ca content is set to a range of 0.0005 to 0.0050%. When Ca is contained, the Ca content is preferably 0.0010% or more. Furthermore, when Ca is contained, the Ca content is preferably 0.0025% or less.

[0048] B: 0.0005 to 0.0050% B is an element that improves secondary work embrittlement. This effect is manifested when the B content is 0.0005% or more. However, if the B content exceeds 0.0050%, ductility decreases due to solid solution strengthening. Therefore, when B is contained, the B content is set to a range of 0.0005 to 0.0050%. When B is contained, the B content is preferably 0.0025% or less.

[0049] REM (rare earth metal): 0.001~0.100% REM is an element effective for deoxidation. This effect is obtained when the REM content is 0.001% or more. However, if the REM content exceeds 0.100%, hot workability decreases. Therefore, when REM is contained, the REM content is set to a range of 0.001 to 0.100%. When REM is contained, the REM content is preferably 0.005% or more. Furthermore, when REM is contained, the REM content is preferably 0.050% or less. Note that REM (rare earth metal) is a collective term for a total of 17 elements, including Sc, Y, and lanthanoid elements. One or more of these 17 elements can be contained as REM. Note that the REM content here refers to the total content of these elements.

[0050] Sn: 0.001 to 0.100% Sn is an element effective in suppressing surface roughness during processing. This effect is obtained when the Sn content is 0.001% or more. However, if the Sn content exceeds 0.100%, hot workability decreases. Therefore, when Sn is contained, the Sn content is set to a range of 0.001 to 0.100%. When Sn is contained, the Sn content is preferably 0.050% or less.

[0051] Sb: 0.001 to 0.100% Like Sn, Sb is an element that is effective in suppressing surface roughness during processing. This effect is obtained when the Sb content is 0.001% or more. However, if the Sb content exceeds 0.100%, hot workability decreases. Therefore, when Sb is contained, the Sb content is set to a range of 0.001 to 0.100%. When Sb is contained, the Sb content is preferably 0.050% or less.

[0052] The chemical composition of the ferritic stainless steel sheet of the present invention has been described above. Note that when the content of Cu, Co, W, V, Zr, Mg, Ca, B, REM, Sn, or Sb described above as optional components is less than the lower limit, the component is considered to be present as an unavoidable impurity.

[0053] Furthermore, in the present invention, the r-value is increased to improve press moldability. In particular, it is important for materials for exhaust heat recovery devices and EGR coolers to have sufficient processability during press molding.

[0054] Average r value: 1.2 or higher To ensure excellent press moldability, it is necessary to sufficiently increase the r-value (Lankford value). Therefore, in the present invention, the average r-value is set to 1.2 or more. The average r-value is preferably 1.4 or more. The average r-value is measured by the method described in the Examples. There is no particular upper limit to the average r-value, but as an example, the average r-value may be 2.0 or less.

[0055] Next, a preferred method for producing the ferritic stainless steel sheet of the present invention will be described.

[0056] In the steelmaking process, steel melted in a converter or electric furnace is preferably subjected to secondary refining using a vacuum oxygen decarburization (VOD) method or the like to produce steel containing the above-mentioned essential elements and optional elements added as needed. The molten steel can be produced into a steel material by known methods, but from the standpoint of productivity and quality, continuous casting is preferred. The steel material is then heated, preferably to 1050 to 1200°C, and hot-rolled to a desired thickness. The hot-rolled sheet is then preferably subjected to continuous annealing at a temperature of 900 to 1000°C as needed, followed by descaling by pickling or the like to produce a hot-rolled product. Within the range of the chemical composition of the ferritic stainless steel sheet of the present invention, annealing the hot-rolled sheet at a temperature exceeding 1000°C can easily cause coarsening of the structure, which can lead to a decrease in toughness. Furthermore, annealing the hot-rolled sheet at a temperature below 900°C can result in no recrystallization, leaving extended grains, which can lead to a decrease in the r-value. If necessary, scale may be removed by shot blasting or abrasive brushing before pickling.

[0057] Furthermore, the hot-rolled product (hot-rolled annealed sheet, etc.) is subjected to a process such as cold rolling to produce a cold-rolled product. In this case, cold rolling may be performed once, but from the viewpoint of productivity and required quality, cold rolling may be performed twice or more times with intermediate annealing in between. The total reduction ratio of the one or more cold rolling times is preferably 60% or more, more preferably 70% or more. The cold-rolled steel sheet (cold-rolled sheet) is then preferably subjected to continuous annealing (finish annealing) at a temperature of preferably 800 to 950°C, more preferably 820 to 920°C, followed by pickling to produce a cold-rolled product. Within the range of the chemical composition of the ferritic stainless steel sheet of the present invention, if the annealing temperature of the cold-rolled sheet exceeds 950°C, the structure is likely to become coarse, which may result in a decrease in toughness. Furthermore, if the annealing temperature of the cold-rolled sheet is less than 800°C, recrystallization does not occur, and extended grains remain, which may result in a decrease in the r-value. Furthermore, continuous annealing may be performed as bright annealing, and pickling may be omitted. Furthermore, depending on the application, after finish annealing, skin pass rolling or the like may be performed to adjust the shape, surface roughness, and material properties of the steel sheet.

[0058] The ferritic stainless steel sheet of the present invention described above is suitable for use as a component of an exhaust heat recovery device or an EGR cooler (exhaust gas recirculation device) in which one or more joints are assembled by brazing heat treatment or welding, and is particularly suitable for use as a heat exchanger component of the exhaust heat recovery device or EGR cooler. [Example]

[0059] Steel having the chemical composition shown in Table 1 was melted in a vacuum melting furnace and heated at 1150°C for 1 hour, after which a hot-rolled sheet having a thickness of 4.2 mm was produced by hot rolling, and the hot-rolled sheet was annealed by holding it at 975°C for 30 seconds or more. Scale was then removed from the surface of the steel sheet by grinding, and the sheet was cold-rolled to a thickness of 1.2 mm to produce a cold-rolled sheet. The cold-rolled sheet was then finish-annealed by holding it at 890°C for 60 seconds to produce a cold-rolled annealed sheet (ferritic stainless steel sheet).

[0060] (1) Brazeability evaluation A 50 × 50 mm test piece was cut from the prepared cold-rolled annealed sheet. The surface was polished with #600 emery paper and then degreased with acetone. A 5 mm diameter (5 mm diameter circle) Ni-containing brazing filler metal (main composition: Ni-29 mass% Cr-6 mass% P-4 mass% Si) with a thickness of 1 mm was placed on the polished surface. The Ni-containing brazing filler metal was then placed on the horizontal steel sheet surface with the Ni-containing brazing filler metal side facing up, and the brazing heat treatment described below was performed. The test piece surface was then photographed, and the spread length of the Ni-containing brazing filler metal in directions parallel to and perpendicular to the rolling direction was measured using image analysis. The ratio of the average diameter (average spread length) of the Ni-containing brazing filler metal after brazing heat treatment to the diameter (5 mm) of the Ni-containing brazing filler metal before brazing heat treatment (the brazing filler metal spread ratio) was calculated and evaluated according to the following criteria. The average diameter of the Ni-containing brazing filler metal after the brazing heat treatment is the average value (arithmetic mean) of the maximum spread length (mm) of the Ni-containing brazing filler metal in a direction parallel to the rolling direction of the steel sheet after the brazing heat treatment and the maximum spread length (mm) of the Ni-containing brazing filler metal in a direction perpendicular to the rolling direction of the steel sheet.

[0061] Brazing filler metal spread rate (%) = (average diameter of brazing filler metal after brazing heat treatment / diameter of brazing filler metal before brazing heat treatment) x 100 ○(Pass): 120% or more × (Fail): Less than 120% In this test, a brazing filler metal with a spreading rate of 120% or more was evaluated as having excellent brazing properties when performing brazing heat treatment at high temperatures.

[0062] <Brazing heat treatment> In a 0.2 Torr nitrogen carrier gas atmosphere, the sample was heated from room temperature to 970°C at an average heating rate of 18°C / min, held at 970°C for 30 minutes, then heated from 970°C to 1180°C at an average heating rate of 18°C / min, and held at 1180°C for 10 minutes. Subsequently, in the same atmosphere as above, the sample was cooled at an average cooling rate of 11°C / min from 1180°C to 800°C, an average cooling rate of 6°C / min from 800°C to 600°C, an average cooling rate of 4°C / min from 600°C to 400°C, and an average cooling rate of 2°C / min from 400°C to 200°C. The sample was then cooled from 200°C to room temperature in an air atmosphere.

[0063] (2) Evaluation of corrosion resistance The prepared cold-rolled annealed sheet was subjected to the brazing heat treatment. Then, 50 mm × 30 mm test pieces were taken from the cold-rolled annealed sheet subjected to the brazing heat treatment. The surfaces were polished with #600 emery paper and then degreased with acetone to prepare multiple test pieces for corrosion resistance evaluation. Then, in accordance with ASTM G48 Method C, each test piece for corrosion resistance evaluation was immersed for 72 hours in a 6 mass% FeCl3 + 1 mass% HCl mixed aqueous solution at each temperature (20 °C, 25 °C, 30 °C, 35 °C, 40 °C, 45 °C, 50 °C) that was changed in 5 °C increments from 20 °C to 50 °C and maintained at a constant temperature. The surface of each test piece was then observed with a magnifying glass to confirm the occurrence of pitting corrosion. The lowest temperature at which pitting corrosion occurred was defined as the critical pitting corrosion initiation temperature, and the temperature was evaluated according to the following criteria. ◎ (Pass, particularly good): Critical pitting temperature is 45°C or higher ○ (Pass): Critical pitting temperature is 35℃ or higher and less than 45℃ × (Fail): Critical pitting temperature is less than 35°C In this test, materials with a minimum temperature at which pitting corrosion occurred (critical pitting corrosion temperature) of 35°C or higher were evaluated as having excellent corrosion resistance after brazing heat treatment.

[0064] (3) Press formability evaluation For each cold-rolled and annealed sheet thus produced, JIS 13B test pieces were taken in the direction parallel to the rolling direction, the direction perpendicular to the rolling direction, and the direction at a 45° angle to the rolling direction, in accordance with JIS Z 2241:2011. Using the taken test pieces, a tensile test was performed at room temperature at a test speed of 10 mm / min with a strain of 15%, and the r-value in each direction was determined. The average r-value was then calculated using the following formula and evaluated according to the following criteria. Average r-value = (r-value parallel to the rolling direction + 2 × r-value at an angle of 45° to the rolling direction + r-value perpendicular to the rolling direction) / 4 ◎ (pass, particularly good): 1.4 or higher ○ (Pass): 1.2 or more, less than 1.4 × (Fail): Less than 1.2 In this test, an average r value of 1.2 or more was evaluated as having excellent press formability.

[0065] [Table 1]

[0066] [Table 2]

[0067] As shown in Tables 1 and 2, all of the ferritic stainless steel sheets obtained in Examples 1 to 29 exhibited excellent press formability, brazability when subjected to high-temperature brazing heat treatment, and corrosion resistance after the brazing heat treatment. Examples 4 and 12, which had high Cr contents, exhibited particularly good corrosion resistance. Example 19, which had a high Ti content, also exhibited particularly good press formability.

[0068] On the other hand, in Comparative Examples 30 to 39, whose component compositions were outside the appropriate range, it was not possible to simultaneously satisfy the targets for press formability, brazeability when performing brazing heat treatment at high temperatures, and corrosion resistance after the brazing heat treatment.

[0069] In Comparative Example No. 30, the Al content exceeded the upper limit of the present invention, and therefore excellent brazability was not obtained. In Comparative Example No. 31, the Si content exceeded the upper limit of the present invention, and therefore excellent brazability was not obtained. In Comparative Example No. 32, the Mn content exceeded the upper limit of the present invention, and therefore excellent corrosion resistance was not obtained. In Comparative Example No. 33, the Mo content exceeded the upper limit of the present invention, so that the σ phase precipitated significantly during the brazing heat treatment, and excellent corrosion resistance was not obtained. In Comparative Example No. 34, the Cr content was below the lower limit of the present invention, and therefore excellent corrosion resistance was not obtained. In Comparative Example No. 35, the Ni content was below the lower limit of the present invention, and therefore excellent corrosion resistance was not obtained. In Comparative Example No. 36, the Mo content was below the lower limit of the present invention, and therefore excellent corrosion resistance was not obtained. In Comparative Example No. 37, the Ti content exceeded the upper limit of the present invention, and therefore excellent brazability was not obtained. In Comparative Example No. 38, the C content exceeded the upper limit of the present invention, and therefore excellent corrosion resistance was not obtained. In Comparative Example No. 39, the N content exceeded the upper limit of the present invention, and therefore excellent press formability and excellent corrosion resistance were not obtained. [Industrial Applicability]

[0070] The present invention provides a ferritic stainless steel sheet that is suitable as a material for heat exchangers such as heat recovery devices and EGR coolers, particularly heat exchangers such as exhaust heat recovery devices and EGR coolers that have one or more joints assembled by brazing heat treatment or welding, and is therefore extremely useful industrially.

Claims

1. In mass percent, C: 0.003-0.030%, Si: 0.01 to 1.30%, Mn: 0.05-0.30%, P: 0.050% or less, S: 0.020% or less, Cr: 24.0-28.0%, Ni: 1.60-2.50%, Mo: 1.50-3.00%, Al: 0.001-0.150%, Ti: 0.01–0.35%, and N: Contains 0.030% or less, with the remainder consisting of Fe and unavoidable impurities, A ferritic stainless steel sheet with an average r-value of 1.2 or higher.

2. The aforementioned component composition is further expressed in mass%, Cu: 0.01 to 1.00%, Co: 0.01 to 1.00%, W: 0.01–2.00%, and Nb: 0.20% or less A ferritic stainless steel sheet according to claim 1, which contains one or more selected from among, and if it contains Nb, satisfies the following formula (1). Ti / 48-Nb / 93≧0...(1) Here, Ti and Nb in equation (1) represent the content (mass%) of each element.

3. The aforementioned component composition is further expressed in mass%, V: 0.01-0.20%, Zr: 0.01 to 0.10%, 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%, and Sb: 0.001-0.100% A ferritic stainless steel sheet according to claim 1 or 2, containing one or more selected from among them.

4. A ferritic stainless steel sheet according to claim 1 or 2, used as a component of a heat recovery unit or EGR cooler.

5. The ferritic stainless steel sheet according to claim 3, used as a component of a heat recovery unit or an EGR cooler.