Ferritic stainless steel and method for producing same

Abstract

Provided is a ferritic stainless steel which has excellent brazability and excellent corrosion resistance when brazed at high temperatures using a Ni-containing brazing filler metal, and which is suppressed in 475 DEG C embrittlement. A ferritic stainless steel having a component composition containing, in mass%, C: 0.003 to 0.030%, Si: 0.01 to 1.00%, Mn: 0.05 to 0.50%, P: 0.050% or less, S: 0.020% or less, Cr: 15.0 to 25.0%, Mo: 1.00 to 2.50%, Al: 0.001 to 0.020%, Nb: 0.25 to 0.60%, N: 0.030% or less, and Ni: 0.01 to 2.50%, with the remainder being Fe and unavoidable impurities, the amount of solid-solution Nb being 0.20% by mass or more.

Description

Technical Field

[0001] The present invention relates to a ferritic stainless steel, and more particularly to a ferritic stainless steel having excellent corrosion resistance and suppressed 475°C embrittlement for use in an environment containing exhaust gas condensate in automobiles. Background Art

[0002] In recent years, demands for improved fuel efficiency and enhanced exhaust gas purification have been growing for automobiles to protect the global environment. Consequently, the use of automotive heat exchangers such as exhaust heat recovery units and EGR (Exhaust Gas Recirculation) coolers has been expanding.

[0003] Here, a waste heat recovery device refers to a device that shortens the warm-up time during engine startup and improves fuel efficiency by utilizing the heat of exhaust gas to heat the engine's cooling water. Its application, centered on hybrid vehicles, is expanding. Typically, a waste heat recovery device is located between the catalytic converter and the muffler and consists of a heat exchanger section composed of a combination of tubes, plates, fins, side plates, etc., as well as inlet and outlet pipe sections. Furthermore, exhaust gas enters the heat exchanger section from the inlet pipe, where its heat is transferred to the cooling water via heat transfer surfaces such as the fins, and then discharged from the outlet pipe. Furthermore, brazing using a Ni-containing brazing filler metal is primarily used for bonding and assembling the plates and fins of the heat exchanger that constitutes such a waste heat recovery device.

[0004] In addition, the EGR cooler is a device used to suppress nitrogen oxides (NOx) that are easily generated at high temperatures by returning exhaust gas with a low oxygen concentration to the intake side of the engine to reduce the combustion temperature of the fuel. If the high-temperature exhaust gas is directly returned to the engine, the fuel will burn at an inappropriate time, and abnormal vibrations called knocking will occur. Therefore, the EGR cooler is composed of a pipe part that introduces a part of the exhaust gas, a heat exchanger part that cools the introduced exhaust gas, and a pipe part that returns the cooled exhaust gas to the intake side of the engine. Regarding the heat exchanger of the EGR cooler, for reasons such as lightweighting, compactness, and cost reduction, thin plates are stacked into a fin shape, and brazing using Ni-containing brazing filler metal is mainly used for their bonding and assembly.

[0005] In this way, the heat exchangers of the waste heat recovery device and the EGR cooler are bonded and assembled by brazing using a Ni-containing brazing filler metal. Therefore, the raw materials used in these heat exchangers require good brazing properties for the Ni-containing brazing filler metal. In addition, the exhaust gas of the automobile contains sulfur oxides (SOx) and hydrocarbons (HC) in addition to NOx. Therefore, they condense inside the heat exchanger to form highly corrosive acidic condensed water. The same condensed water is also produced in the muffler. Therefore, corrosion resistance is also required for the raw materials used in these heat exchangers. In particular, high temperatures are reached during the brazing heat treatment. Therefore, it is necessary to prevent the Cr at the grain boundary from reacting with C and N to form Cr carbonitrides, thereby forming a Cr-deficient layer with poor corrosion resistance around the Cr carbonitrides, which is the so-called sensitization, to ensure corrosion resistance.

[0006] For these reasons, 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, austenitic stainless steels have the following problems: high costs due to the high nickel content, and low fatigue properties (i.e., thermal fatigue resistance) in environments where they are subjected to high temperatures, severe vibration, and constraints due to their high thermal expansion.

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

[0008] For example, Patent Document 1 discloses a ferritic stainless steel as a material for an exhaust heat recovery device or an EGR cooler, wherein corrosion resistance is ensured by forming an oxide film containing 16% or more Nb in terms of cation fraction after brazing.

[0009] Patent Document 2 discloses a ferritic stainless steel having corrosion resistance secured by controlling the addition amounts of Al, Ti, and Si as a material for an exhaust heat recovery device or an EGR cooler.

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

[0011] Patent Document 4 discloses, as a material for an EGR cooler, a ferritic stainless steel having brazing properties ensured by adding components such as Cr, Cu, Al, and Ti according to a certain relationship and suppressing the amounts of Al and Ti added.

[0012] Furthermore, Patent Document 5 discloses a ferritic stainless steel having a structure bonded by Ni brazing, in which brazing properties are ensured by suppressing the addition amounts of Al, Ti, and Zr.

[0013] Patent Document 6 discloses, as a ferritic stainless steel material for brazing, a ferritic stainless steel having reduced surface roughness to thereby ensure corrosion resistance to condensed water.

[0014] Prior art literature

[0015] Patent Literature

[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 No. 5462583

[0020] Patent Document 5: Japanese Patent No. 5264199

[0021] Patent Document 6: Japanese Patent Application Laid-Open No. 2020-12128 Summary of the Invention

[0022] Problems to be solved by the invention

[0023] However, the technologies described in Patent Documents 1 to 6 sometimes lack sufficient corrosion resistance against condensed water. Furthermore, when ferritic stainless steel with a high Cr content is used in heat exchangers and mufflers, such as exhaust heat recovery units and EGR coolers, 475°C embrittlement can occur. 475°C embrittlement refers to the phenomenon in which, when ferritic stainless steel is held at approximately 475°C, the ferrite phase decomposes into two phases: an α solid solution with a low Cr concentration and an α' solid solution with a high Cr concentration, increasing the steel's hardness and causing it to become brittle.

[0024] Ferritic stainless steel becomes significantly brittle when its hardness increases by more than 20 HV due to 475°C embrittlement, and there is a concern that damage may occur when used in the above-mentioned applications.

[0025] The present invention has been developed in view of the above-mentioned current situation, and an object of the present invention is to provide a ferritic stainless steel having excellent brazing properties and excellent corrosion resistance when brazing is performed at high temperatures using a Ni-containing brazing filler metal, while suppressing 475°C embrittlement.

[0026] It should be noted that, in this specification, "excellent brazing properties" means that after a steel plate coated with a Ni-containing brazing filler metal having a composition of Ni-29 mass % Cr-4 mass % Si-6 mass % P is brazed at 1080°C and 1 torr in a nitrogen carrier gas atmosphere for 10 minutes and cooled to room temperature, the ratio of the equivalent circular diameter of the brazing filler metal after heating to the equivalent circular diameter of the brazing filler metal before heating (the spreading rate of the brazing filler metal) is 150% or more.

[0027] In this specification, "excellent corrosion resistance" means that, using the brazed steel sheet described above, a 20 mm square test piece is cut from the portion to which the brazing filler metal is not attached, the test piece is covered with a sealing material after leaving an 11 mm square measurement surface, and the test piece is immersed in a 3.5 mass % NaCl solution at 30°C. The pitting potential Vc'100 measured in accordance with JIS G0577:2014, excluding the concentration of the NaCl solution described above, is 300 mV (vs SCE) or higher.

[0028] In this specification, "475°C embrittlement is suppressed" means that, after brazing the steel plate, a test piece cut from a portion where the brazing filler metal was not attached exhibits a Vickers hardness increase of 20 HV or less after being held at 475°C for 1000 hours. Vickers hardness is measured in accordance with JIS Z 2244:2020, with the average value of the hardness measured at five points along the center of the plate thickness on a surface parallel to the rolling direction.

[0029] Methods used to solve problems

[0030] The inventors discovered that when ferritic stainless steel is kept at 475°C for 1000 hours, the increase in Vickers hardness due to 475°C embrittlement varies depending on the type of steel. As a result of further research, the following insight was obtained: even for the same type of steel, the increase in Vickers hardness due to 475°C embrittlement varies depending on the manufacturing method. The results of in-depth research on various factors affecting 475°C embrittlement showed that the greater the amount of dissolved Nb in the steel, the more suppressed the 475°C embrittlement. When the amount of dissolved Nb is controlled to a range of 0.20% by mass or more, the increase in Vickers hardness during the above-mentioned holding period can be reduced to less than 20HV. The present invention was completed after further research based on the above-mentioned insights.

[0031] That is, the gist of the present invention is as follows.

[0032] [1] A ferritic stainless steel having a composition comprising, in mass%, C: 0.003-0.030%, Si: 0.01-1.00%, Mn: 0.05-0.50%, P: 0.050% or less, S: 0.020% or less, Cr: 15.0-25.0%, Mo: 1.00-2.50%, Al: 0.001-0.020%, Nb: 0.25-0.60%, N: 0.030% or less, Ni: 0.01-2.50%, the balance being Fe and unavoidable impurities, wherein the amount of dissolved Nb is 0.20 mass% or more.

[0033] [2] The ferritic stainless steel according to [1], wherein the contents of Mo, Nb, and Ni in the above-mentioned composition are Mo: 1.30-2.50%, Nb: 0.30-0.60%, and Ni: 0.80-2.50%, and the amount of dissolved Nb is 0.26 mass % or more.

[0034] [3] The ferritic stainless steel according to [1] or [2], wherein the above composition further contains, in mass %, one or more selected from the following groups A and B:

[0035] Group A: one or more selected from Cu: 1.00% or less, Co: 1.00% or less, and W: 2.00% or less;

[0036] Group B: one or more selected from the group consisting of Ti: 0.10% or less, V: 0.20% or less, Zr: 0.10% or less, Mg: 0.0050% or less, Ca: 0.0050% or less, B: 0.0050% or less, REM: 0.100% or less, Sn: 0.100% or less, and Sb: 0.100% or less.

[0037] [4] The ferritic stainless steel according to any one of [1] to [3], wherein the increase in Vickers hardness after being held at 475°C for 1000 hours is 20 HV or less.

[0038] [5] A method for producing ferritic stainless steel, which is the method for producing ferritic stainless steel according to any one of [1] to [4] above, comprising:

[0039] A process for preparing a cold-rolled steel sheet having the above-mentioned composition; and

[0040] The cold-rolled steel sheet is subjected to a final annealing step of holding the steel sheet in a temperature range of 1040-1150°C for 5 seconds or longer and then cooling the steel sheet in a temperature range of 900-600°C at an average cooling rate of 15°C / s or higher.

[0041] According to the present invention, a ferritic stainless steel can be obtained which has excellent brazing properties when brazing is performed at high temperatures using a Ni-containing brazing filler metal, has excellent corrosion resistance, and has suppressed 475° C. embrittlement. DETAILED DESCRIPTION

[0042] The present invention will be described in detail below.

[0043] First, the reasons for limiting the steel composition in the present invention will be explained. It should be noted that the unit of the content of each element in the steel composition is "mass %". Hereinafter, unless otherwise specified, it will be expressed simply as "%.

[0044] C: 0.003~0.030%

[0045] When the C content increases, the strength increases, and when it decreases, the workability improves. Here, in order to obtain sufficient strength, it is necessary to contain 0.003% or more of C. However, when the C content exceeds 0.030%, the reduction in workability becomes significant, and Cr carbides precipitate at the grain boundaries, causing sensitization and reducing corrosion resistance. Therefore, the C content is set to the range of 0.003 to 0.030%. The C content is preferably 0.004% or more. In addition, the C content is preferably 0.025% or less, more preferably 0.020% or less, and even more preferably 0.010% or less.

[0046] Si: 0.01~1.00%

[0047] Si is an element useful as a deoxidizing material. This effect is achieved when it contains 0.01% or more Si. However, when the Si content exceeds 1.00%, oxides are formed during brazing, which reduces the brazing properties. Therefore, the Si content is set in the range of 0.01 to 1.00%. The Si content is preferably 0.20% or more, more preferably 0.30% or more, and even more preferably 0.40% or more. In addition, the Si content is preferably 0.90% or less, more preferably 0.80% or less, and even more preferably 0.70% or less.

[0048] Mn: 0.05~0.50%

[0049] Mn has a deoxidizing effect, which is achieved when the content is 0.05% or more. However, when the Mn content exceeds 0.50%, MnS is generated, which reduces corrosion resistance. Therefore, the Mn content is set to a range of 0.05-0.50%. The Mn content is preferably 0.10% or more, and more preferably 0.15% or more. In addition, the Mn content is preferably 0.40% or less, and more preferably 0.30% or less.

[0050] P: 0.050% or less

[0051] Phosphorus (P) is an unavoidable element in steel, and excessive P content can easily lead to intergranular corrosion. This tendency becomes pronounced when the P content exceeds 0.050%. Therefore, the P content is limited to 0.050% or less. The P content is preferably 0.040% or less. It should be noted that there is no particular lower limit for the P content. However, excessive P removal increases costs, so the P content is preferably 0.005% or more.

[0052] S: 0.020% or less

[0053] S is an element inevitably contained in steel. S content exceeding 0.020% promotes the precipitation of MnS, reducing corrosion resistance. Therefore, the S content is set to 0.020% or less. The S content is preferably 0.015% or less, and more preferably 0.010% or less. It should be noted that there is no particular lower limit for the S content. However, excessive desulfurization leads to increased costs, so the S content is preferably 0.0005% or more.

[0054] Cr: 15.0~25.0%

[0055] Cr is an important element for ensuring the corrosion resistance of stainless steel. On the other hand, Cr is an element that causes 475°C embrittlement by decomposing the ferrite phase into two phases, an α solid solution with a low Cr concentration and an α' solid solution with a high Cr concentration, when the ferrite phase is maintained near 475°C. When the Cr content is less than 15.0%, sufficient corrosion resistance cannot be obtained. On the other hand, when the Cr content is greater than 25.0%, 475°C embrittlement becomes significant. The Cr content is preferably 17.0% or more, more preferably 18.0% or more. In addition, the Cr content is preferably 23.0% or less, more preferably 21.0% or less.

[0056] Mo: 1.00~2.50%

[0057] Mo stabilizes the passive film of stainless steel and improves corrosion resistance. This effect is obtained when the Mo content is 1.00% or more. However, when the Mo content is greater than 2.50%, the precipitation of the Laves phase during cooling after the final annealing is promoted and the amount of solid-solution Nb is reduced, making it difficult to suppress 475°C embrittlement. Therefore, the Mo content is set in the range of 1.00 to 2.50%. The Mo content is preferably greater than 1.30%, more preferably greater than 1.60%. In addition, the Mo content is preferably less than 2.25%, more preferably less than 2.00%.

[0058] Al: 0.001~0.020%

[0059] Al is an element useful for deoxidation, and this effect is achieved when the content is 0.001% or more. However, Al is an element active toward oxygen, and when the Al content exceeds 0.020%, it forms oxides, which degrade brazing properties. Therefore, the Al content is set within the range of 0.001% to 0.020%. Preferably, the Al content is 0.015% or less.

[0060] Nb: 0.25~0.60%

[0061] Nb is an element that suppresses the reduction (sensitization) of corrosion resistance caused by the precipitation of Cr carbonitrides by combining with C and N. This effect is obtained when the Nb content is 0.25% or more. On the other hand, when the Nb content is greater than 0.60%, hardening occurs and workability decreases. Therefore, the Nb content is set to the range of 0.25 to 0.60%. The Nb content is preferably 0.275% or more, more preferably 0.30% or more. In addition, the Nb content is preferably 0.50% or less, more preferably 0.40% or less.

[0062] N: 0.030% or less

[0063] When the N content exceeds 0.030%, corrosion resistance and workability decrease. Therefore, the N content is set to 0.030% or less. The N content is preferably 0.025% or less, and more preferably 0.020% or less. It should be noted that there is no particular lower limit for the N content, but excessive reduction in the N content will lead to increased costs, so the N content is preferably 0.003% or more.

[0064] Ni: 0.01~2.50%

[0065] Ni is an element that effectively contributes to the improvement of corrosion resistance when it is contained at 0.01% or more. On the other hand, when the Ni content exceeds 2.50%, the stress corrosion cracking sensitivity increases. Therefore, the Ni content is set to the range of 0.01 to 2.50%. The Ni content is preferably 0.80% or more, more preferably 1.00% or more. In addition, the Ni content is preferably 2.00% or less, more preferably 1.50% or less, and even more preferably 1.20% or less.

[0066] 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, components other than the above (the balance) are Fe and unavoidable impurities.

[0067] From the viewpoint of obtaining better corrosion resistance, the contents of Mo, Nb, and Ni in the above composition are preferably Mo: 1.30-2.50%, Nb: 0.30-0.60%, and Ni: 0.80-2.50%.

[0068] The ferritic stainless steel of the present invention may further contain one or two or more selected from Cu, Co, and W within the following ranges.

[0069] Cu: 1.00% or less

[0070] Cu is an element that improves corrosion resistance. To achieve this effect, the Cu content is preferably set to 0.01% or more. The Cu content is more preferably 0.10% or more. However, when the Cu content exceeds 1.00%, hot workability decreases. Therefore, when Cu is contained, the Cu content is set to 1.00% or less. The Cu content is preferably 0.80% or less, and more preferably 0.60% or less.

[0071] Co: less than 1.00%

[0072] Co is an element that improves corrosion resistance. To achieve this effect, the Co content is preferably set to 0.01% or more. The Co content is more preferably 0.10% or more. However, when the Co content exceeds 1.00%, workability decreases. Therefore, when Co is contained, the Co content is set to 1.00% or less. The Co content is preferably 0.80% or less, and more preferably 0.60% or less.

[0073] W: 2.00% or less

[0074] W is an element that improves high-temperature strength and corrosion resistance. To achieve this effect, the W content is preferably set to 0.01% or more. A W content of 0.10% or more is more preferred. However, a W content exceeding 2.00% reduces workability. Therefore, when W is included, the W content is set to 2.00% or less. The W content is preferably 1.50% or less, and more preferably 1.00% or less.

[0075] The ferritic stainless steel of the present invention may further contain one or two or more selected from Ti, V, Zr, Mg, Ca, B, REM, Sn, and Sb within the following ranges.

[0076] Ti: less than 0.10%

[0077] Ti combines with the carbon and nitrogen contained in steel to prevent sensitization. To achieve this effect, the Ti content is preferably set to 0.01% or higher. A more preferred Ti content is 0.02% or higher. On the other hand, Ti is an element reactive toward oxygen. When the Ti content exceeds 0.10%, oxides primarily composed of Ti form on the steel surface during brazing. These oxides significantly reduce brazing properties. Therefore, when Ti is present, the Ti content is set to 0.10% or lower. The Ti content is preferably 0.08% or lower, and more preferably 0.06% or lower.

[0078] V: 0.20% or less

[0079] Like Ti, V combines with C and N contained in steel to prevent sensitization. To achieve this effect, the V content is preferably set to 0.01% or higher. A V content of 0.02% or higher is more preferred. On the other hand, a V content exceeding 0.20% reduces workability. Therefore, when V is included, the V content is set to 0.20% or lower. The V content is preferably 0.18% or lower, and more preferably 0.15% or lower.

[0080] Zr: 0.10% or less

[0081] Like Ti and Nb, Zr is an element that combines with C and N contained in steel to suppress sensitization. To achieve this effect, the Zr content is preferably set to 0.01% or higher. A Zr content of 0.02% or higher is more preferred. On the other hand, a Zr content exceeding 0.10% reduces workability. Therefore, when Zr is included, the Zr content is set to 0.10% or lower. The Zr content is preferably 0.08% or lower, and more preferably 0.06% or lower.

[0082] Mg: 0.0050% or less

[0083] Mg acts as a deoxidizing agent. To achieve this effect, the Mg content is preferably set to 0.0003% or more. More preferably, the Mg content is 0.0008% or more. However, if the Mg content exceeds 0.0050%, the toughness of the steel decreases, impairing manufacturability. Therefore, when Mg is included, the Mg content is set to 0.0050% or less. The Mg content is preferably 0.0025% or less, and more preferably 0.0020% or less.

[0084] Ca: 0.0050% or less

[0085] Ca improves weld penetration and weldability. To achieve this effect, the Ca content is preferably set to 0.0003% or more. A more preferred Ca content is 0.0010% or more. However, if the Ca content exceeds 0.0050%, it combines with sulfur to form CaS, which reduces corrosion resistance. Therefore, when Ca is included, the Ca content is set to 0.0050% or less. The Ca content is more preferably 0.0025% or less, and even more preferably 0.0020% or less.

[0086] B: 0.0050% or less

[0087] Boron (B) is an element that improves secondary processing brittleness. To achieve this effect, the B content is preferably set to 0.0003% or more. More preferably, the B content is 0.0010% or more. However, when the B content exceeds 0.0050%, ductility decreases due to solid solution strengthening. Therefore, when B is included, the B content is set to 0.0050% or less. It is preferably 0.0040% or less, and more preferably 0.0030% or less.

[0088] REM (rare earth metals): 0.100% or less

[0089] REM (rare earth metals: elements with atomic numbers 57 to 71 such as La, Ce, and Nd) are elements effective for deoxidation. In order to achieve this effect, it is preferred to set the REM content to 0.005% or more. The REM content is more preferably 0.010% or more. However, when the REM content is greater than 0.100%, hot workability decreases. Therefore, when REM is contained, the REM content is set to 0.100% or less. It is preferably 0.080% or less, and more preferably 0.060% or less. It should be noted that REM is a general term for Sc, Y, and 15 elements ranging from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, and the REM content mentioned here refers to the total content of these elements.

[0090] Sn: 0.100% or less

[0091] Sn is an element that is effective in suppressing surface roughness during machining. To achieve this effect, the Sn content is preferably set to 0.001% or more. The Sn content is more preferably 0.005% or more. However, when the Sn content exceeds 0.100%, hot workability decreases. Therefore, when Sn is contained, the Sn content is set to 0.100% or less. The Sn content is preferably 0.080% or less, and more preferably 0.060% or less.

[0092] Sb: 0.100% or less

[0093] Sb, like Sn, is an element effective in suppressing surface roughness during machining. To achieve this effect, the Sb content is preferably set to 0.001% or more. The Sb content is more preferably 0.005% or more. However, when the Sb content exceeds 0.100%, workability decreases. Therefore, when Sb is contained, the Sb content is set to 0.100% or less. The Sb content is preferably 0.080% or less, and more preferably 0.060% or less.

[0094] Solid solution Nb content: 0.20 mass% or more

[0095] The inventors have found the following insight: by making the amount of dissolved Nb in the steel 0.20% by mass or more, the increase in Vickers hardness caused by 475°C embrittlement can be suppressed to less than 20HV. The reason is unclear, but it is speculated that when Nb is dissolved in the steel, Cr becomes difficult to diffuse, resulting in the suppression of 475°C embrittlement. The amount of dissolved Nb is preferably 0.26% by mass or more. It should be noted that there is no upper limit to the amount of dissolved Nb. As an example, the amount of dissolved Nb is 0.50% or less. It should be noted that the amount of dissolved Nb can be determined in detail by the method described in the examples.

[0096] It should be noted that from the viewpoint of obtaining better corrosion resistance and better suppression of 475°C embrittlement, the contents of Mo, Nb and Ni in the above-mentioned composition are preferably Mo: 1.30-2.50%, Nb: 0.30-0.60%, Ni: 0.80-2.50%, and the amount of dissolved Nb in the steel is 0.26% by mass or more.

[0097] The increase in Vickers hardness of the ferritic stainless steel of the present invention after being held at 475°C for 1000 hours is preferably 20 HV or less, more preferably 15 HV or less. A smaller increase in Vickers hardness is preferred, and 0 HV is also possible. The increase in Vickers hardness can be determined in detail by the method described in the Examples.

[0098] Next, a preferred method for producing the ferritic stainless steel of the present invention will be described. The method for producing the ferritic stainless steel of the present invention comprises: preparing a cold-rolled steel sheet having the above-described composition; and subjecting the cold-rolled steel sheet to a final annealing step of holding the cold-rolled steel sheet in a temperature range of 1040-1150°C for at least 5 seconds, followed by cooling in a temperature range of 900-600°C at an average cooling rate of at least 15°C / second. The step of preparing the cold-rolled steel sheet is not particularly limited. For example, a cold-rolled steel sheet having the above-described composition can be prepared by hot-rolling a steel material (slab) to produce a hot-rolled steel sheet, annealing the hot-rolled steel sheet as needed, and then cold-rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet of the desired thickness. The ferritic stainless steel of the present invention can then be produced by subjecting the thus-prepared cold-rolled steel sheet to a final annealing step of holding the cold-rolled steel sheet in a temperature range of 1040-1150°C for at least 5 seconds, followed by cooling in a temperature range of 900-600°C at an average cooling rate of at least 15°C / second. It should be noted that the conditions for hot rolling, cold rolling, hot-rolled sheet annealing, etc. are not particularly limited and may be carried out according to conventional methods.

[0099] In the steelmaking process, steel melted in a converter or electric furnace is preferably subjected to secondary refining using a VOD (Vacuum Oxygen Decarburization) method to produce steel containing the aforementioned essential components and components added as needed. The molten steel can be made into raw steel using known methods, but continuous casting is preferred for productivity and quality reasons. The raw steel is then preferably held at a temperature of 1050-1250°C for at least 30 minutes and then hot-rolled to produce hot-rolled steel sheets of the desired thickness. Of course, hot working into materials other than sheet materials is also possible. The hot-rolled steel sheets are preferably then subjected to hot-rolled sheet annealing at a temperature of 900-1150°C for at least 30 seconds and then descaled by pickling or the like to produce hot-rolled products. It should be noted that, if necessary, the scale can be removed by shot blasting, brush grinding, or the like before pickling.

[0100] The hot-rolled product (hot-rolled annealed steel sheet, etc.) is then subjected to cold rolling and other processes to produce a cold-rolled product. This cold rolling process can be performed once, but from the perspective of productivity and required quality, it can be performed twice or more with intermediate annealing. The total reduction ratio for the one or more cold rolling processes is preferably 60% or more, more preferably 70% or more. The above-described cold-rolled steel sheet is prepared.

[0101] The cold-rolled steel sheet prepared as described above is then subjected to a final annealing process in which the steel is held in a temperature range of 1040°C to 1150°C (final annealing temperature) for at least 5 seconds and then cooled in a temperature range of 900°C to 600°C at an average cooling rate of at least 15°C / second. If the final annealing temperature is lower than 1040°C, recrystallization is insufficient, while if it is higher than 1150°C, grain coarsening becomes significant. Therefore, the final annealing temperature is set to a temperature range of 1040°C to 1150°C. Furthermore, if the holding time in the above temperature range is less than 5 seconds, recrystallization is insufficient. Therefore, the holding time in the above temperature range is set to at least 5 seconds. It should be noted that there is no upper limit to the holding time in the above temperature range, but from the perspective of preventing significant grain coarsening, it is preferably 100 seconds or less.

[0102] Moreover, the inventors have made it clear that by keeping the temperature range of 1040-1150°C for more than 5 seconds and cooling at an average cooling rate of more than 15°C / second in the temperature range of 900-600°C, the precipitation of the Laves phase (an intermetallic compound mainly composed of Fe and containing Mo and Nb) during cooling can be suppressed, and the amount of solid-solution Nb can be made to be more than 0.20% by mass. As the reason for this, it is believed that: within the scope of the composition of the present invention, the Laves phase precipitates when it is below 900°C, and therefore, by making the average cooling rate below 900°C fast enough, the precipitation of the Laves phase can be suppressed. It should be noted that when it is below 600°C, the diffusion of atoms is slow, and the Laves phase is hardly precipitated at the usual average cooling rate. Therefore, the average cooling rate in the temperature range of 900-600°C is set to more than 15°C / second. It should be noted that the upper limit of the average cooling rate in the above-mentioned temperature range is not particularly limited. As an example, the average cooling rate in the above temperature range may be 40° C. / second or less.

[0103] It should be noted that if scale is generated during the final annealing, the cold-rolled product (cold-rolled annealed steel sheet) obtained above may be pickled. Alternatively, to avoid scale formation, the final annealing may be performed as bright annealing and pickling may be omitted. Furthermore, depending on the intended use, the shape, surface roughness, and material quality of the steel sheet may be adjusted by performing skin pass rolling after the final annealing.

[0104] The ferritic stainless steel of the present invention can be obtained through the above steps.

[0105] The ferritic stainless steel of the present invention described above is suitable for use in automotive parts assembled by brazing at one or more joints and exposed to condensed water from exhaust gas. Examples of automotive parts exposed to condensed water from exhaust gas include waste heat recovery units and EGR coolers. The ferritic stainless steel of the present invention is particularly suitable for use as heat exchangers and muffler components in waste heat recovery units and EGR coolers.

[0106] Example

[0107] Steel having the chemical composition shown in Table 1 was melted in a vacuum melting furnace, heated at 1150°C for 1 hour, and then hot-rolled to produce a 4.0 mm thick hot-rolled steel sheet. After annealing the hot-rolled sheet by holding at 1080°C for 1 minute, the surface was polished to remove scale, and the sheet was cold-rolled to a thickness of 1.0 mm to prepare a cold-rolled steel sheet. The cold-rolled steel sheet was final annealed in an ammonia decomposition gas atmosphere under the conditions shown in Table 2. The surface of the resulting cold-rolled annealed steel sheet was polished to 600 grit using corundum paper, degreased with acetone, and used for testing.

[0108] The cold-rolled annealed steel sheets were evaluated for (1) brazing properties, (2) corrosion resistance, and (3) suppression of 475° C. embrittlement by measuring the amount of dissolved Nb as follows. The results are shown in Table 2.

[0109] (solid solution Nb content)

[0110] The amount of Nb contained in precipitates such as Laves phase and carbonitrides was determined by extractive residue analysis of the cold-rolled annealed steel sheets described above. The difference between the Nb content in the steel and the Nb content in the precipitates was taken as the amount of dissolved Nb. The extractive residue analysis method involved extracting the precipitates from the steel using constant current electrolysis using a 10% AA electrolyte (10% by volume acetylacetone-1% by mass tetramethylammonium chloride-methanol electrolyte) and a membrane filter with a pore size of 0.2 μm. The Nb content in the precipitates was then analyzed by ICP emission spectroscopy to determine the Nb content in the precipitates.

[0111] (1) Evaluation of brazing properties

[0112] A test piece with a width of 50 mm and a length of 50 mm was cut out from the produced cold-rolled annealed steel plate, and a Ni brazing material with a diameter of 10 mm and a thickness of 1 mm was applied to the surface of the horizontal plate (composition: Ni: balance, Cr: 29% by mass, Si: 4% by mass, P: 6% by mass). The surface coated with the Ni brazing material was then placed horizontally with the surface facing upward. In this state, a brazing process was performed by heating the test piece at 1080°C and 1 torr in a nitrogen carrier gas atmosphere for 10 minutes and then cooling the brazing piece to room temperature. Then, the equivalent circular diameter of the brazing material on the surface of the test piece was measured (the equivalent circular diameter of the brazing material after heating). Then, the ratio of the equivalent circular diameter of the brazing material after heating to the diameter of the brazing material before heating (10 mm, the equivalent circular diameter is also the same) (the spreading rate of the brazing material) was calculated and evaluated according to the following benchmarks.

[0113] 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 (10 mm)) × 100 (%)

[0114] ○(Qualified): 150% or more

[0115] ×(Unqualified): less than 150%

[0116] (2) Evaluation of corrosion resistance

[0117] Using test pieces of each cold-rolled annealed steel sheet after brazing treatment (after brazing treatment performed in (1) Evaluation of brazing properties), a 20 mm square test piece was cut from the portion where the brazing material was not attached. For the test piece, an 11 mm square measurement surface was left out and then covered with a silicone resin sealing material, and then the measurement surface was polished with emery grinding paper (#600). Next, the test piece was immersed in a 3.5 mass % NaCl solution at 30°C, and the pitting potential was measured in accordance with JIS G0577:2014 except for the concentration of the above-mentioned NaCl solution. After maintaining the natural potential for 10 minutes, the measurement was performed at a scanning rate of 20 mV / min until the anode current density reached 1.1 mA / cm 2 Until the current density reaches 100μA / cm 2 The potential at this point is defined as the pitting potential Vc'100. It should be noted that, considering the operating conditions of the heat exchanger components of the exhaust heat recovery unit and EGR cooler, a pitting potential Vc'100 of 300 mV (vs SCE) or higher is considered excellent corrosion resistance. According to the following criteria, ◎ and ○ are considered acceptable.

[0118] ◎(Qualified, better): 350mV (vs SCE) or above

[0119] ○ (Pass): 300mV (vs SCE) or higher and less than 350mV (vs SCE)

[0120] × (Unqualified): Less than 300mV (vs SCE)

[0121] (3) Evaluation of 475℃ Brittleness Suppression

[0122] The Vickers hardness of the portion of each cold-rolled annealed steel sheet not attached to the brazing filler metal was measured before and after being held at 475°C for 1000 hours in an electric furnace in accordance with JIS Z2244:2020. The load was set to 300 gf, and five points were measured at the center of the plate thickness on a surface parallel to the rolling direction, and the average value was calculated. When the increase in Vickers hardness due to 475°C embrittlement (Vickers hardness after holding at 475°C for 1000 hours minus Vickers hardness before holding at 475°C for 1000 hours) was 20 HV or less, it was determined that 475°C embrittlement was suppressed. ◎ and ○ were marked as acceptable according to the following criteria.

[0123] ◎(Qualified, better): 15HV or less

[0124] ○ (Qualified): Greater than 15HV and less than 20HV

[0125] ×(Unqualified): Greater than 20HV

[0126]

[0127]

[0128] Table 2 shows that Inventive Examples Nos. 1 to 24 and 35 to 38 all exhibit excellent brazing properties, corrosion resistance, and suppression of 475°C embrittlement. Furthermore, among the Inventive Examples, Examples Nos. 1 to 8, 10, 11, 14 to 23, and 35 to 38, which meet all the conditions of Mo: 1.30 to 2.50%, Nb: 0.30 to 0.60%, and Ni: 0.80 to 2.50%, exhibited pitting potentials Vc'100 exceeding 350 mV, demonstrating even superior corrosion resistance. Furthermore, among the inventive examples, No. 2, No. 3, No. 7, No. 10-No. 12, No. 14-No. 16, No. 18, No. 23, and No. 24, which had a dissolved Nb content of 0.26 mass % or greater, showed an increase in Vickers hardness of 15 HV or less after being held at 475°C for 1000 hours, demonstrating a superior suppression of 475°C embrittlement. In contrast, Comparative Examples Nos. 25-34, whose component compositions and manufacturing methods fell outside the appropriate ranges, failed to simultaneously satisfy all of the requirements for brazing properties, corrosion resistance, and suppression of 475°C embrittlement.

[0129] More specifically, in Comparative Example No. 25 (Steel No. B1), the Cr content exceeded the upper limit of the present invention, and thus the targeted suppression of 475° C. embrittlement could not be achieved.

[0130] Comparative Example No. 26 (Steel No. B2) failed to achieve the targeted suppression of 475° C. embrittlement because the Mo content exceeded the upper limit of the present invention.

[0131] In Comparative Example No. 27 (Steel No. B3), the Al content exceeded the upper limit of the present invention, and thus the target brazeability could not be obtained.

[0132] In Comparative Example No. 28 (Steel No. B4), the Si content exceeded the upper limit of the present invention, and thus the target brazing properties could not be obtained.

[0133] Comparative Example No. 29 (Steel No. B5) failed to achieve the target corrosion resistance because the Mn content exceeded the upper limit of the present invention.

[0134] Comparative Example No. 30 (Steel No. B6) failed to obtain the target corrosion resistance because the Cr content was below the lower limit of the present invention.

[0135] Comparative Example No. 31 (Steel No. B7) failed to obtain the target corrosion resistance because the Mo content was below the lower limit of the present invention.

[0136] Comparative Example No. 32 (Steel No. B8) failed to achieve the target corrosion resistance because the Nb content was below the lower limit of the present invention, and the amount of dissolved Nb was less than 0.20 mass %, failing to achieve the target suppression of 475° C. embrittlement.

[0137] Comparative Examples No. 33 and No. 34 had an average cooling rate of less than 15°C / s in the temperature range of 900-600°C after holding at the final annealing temperature, resulting in a dissolved Nb content of less than 0.20 mass %, and thus failed to achieve the desired suppression of 475°C embrittlement.

[0138] Industrial applicability

[0139] According to the present invention, a ferritic stainless steel suitable for use in exhaust heat recovery devices, heat exchangers of EGR coolers, and muffler components used in an exhaust gas condensate environment of automobiles can be obtained, and therefore the present invention is extremely useful industrially.

Claims

1. A ferritic stainless steel having a composition comprising, in mass%, 0.003% to 0.030% C, 0.01% to 1.00% Si, 0.05% to 0.50% Mn, 0.050% or less P, 0.020% or less S, 15.0% to 25.0% Cr, 1.00% to 2.50% Mo, 0.001% to 0.020% Al, 0.25% to 0.60% Nb, 0.030% or less N, 0.01% to 2.50% Ni, with the balance being Fe and unavoidable impurities, wherein the amount of dissolved Nb is 0.20% by mass or more.

2. The ferritic stainless steel according to claim 1, wherein In the above composition, the contents of Mo, Nb and Ni are Mo: 1.30-2.50%, Nb: 0.30-0.60%, Ni: 0.80-2.50%, and the amount of solid-dissolved Nb is 0.26 mass % or more.

3. The ferritic stainless steel according to claim 1, wherein The composition further contains, in mass %, one or more selected from the following groups A and B: Group A: one or more selected from Cu: 1.00% or less, Co: 1.00% or less, and W: 2.00% or less; Group B: one or more selected from the group consisting of Ti: 0.10% or less, V: 0.20% or less, Zr: 0.10% or less, Mg: 0.0050% or less, Ca: 0.0050% or less, B: 0.0050% or less, REM: 0.100% or less, Sn: 0.100% or less, and Sb: 0.100% or less.

4. The ferritic stainless steel according to claim 2, wherein The composition further contains, in mass %, one or more selected from the following groups A and B: Group A: one or more selected from Cu: 1.00% or less, Co: 1.00% or less, and W: 2.00% or less; Group B: one or more selected from the group consisting of Ti: 0.10% or less, V: 0.20% or less, Zr: 0.10% or less, Mg: 0.0050% or less, Ca: 0.0050% or less, B: 0.0050% or less, REM: 0.100% or less, Sn: 0.100% or less, and Sb: 0.100% or less. The ferritic stainless steel according to any one of claims 1 to 4, wherein the increase in Vickers hardness after being held at 475°C for 1000 hours is 20 HV or less.

6. A method for producing ferritic stainless steel, the method for producing ferritic stainless steel according to any one of claims 1 to 4, comprising: a step of preparing a cold-rolled steel sheet having the aforementioned composition; and The cold-rolled steel sheet is subjected to a final annealing step of holding the steel sheet in a temperature range of 1040 to 1150° C. for 5 seconds or longer and then cooling the steel sheet in a temperature range of 900 to 600° C. at an average cooling rate of 15° C. / s or higher.

7. A method for producing ferritic stainless steel, which is the method for producing ferritic stainless steel according to claim 5, comprising: a step of preparing a cold-rolled steel sheet having the aforementioned composition; and The cold-rolled steel sheet is subjected to a final annealing step of holding the steel sheet in a temperature range of 1040 to 1150° C. for 5 seconds or longer and then cooling the steel sheet in a temperature range of 900 to 600° C. at an average cooling rate of 15° C. / s or higher.

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