Method for producing ferritic stainless steel and cold-rolled ferritic stainless steel sheet

Abstract

Provided is ferrite stainless steel which demonstrates excellent brazing properties when brazing is performed at a high temperature using a Ni-containing brazing material, which has excellent corrosion resistance, and in which embrittlement at 475°C is suppressed.　This ferrite stainless steel has a component composition including, in terms of mass%, 0.003%-0.030% of C, 0.01%-1.00% of Si, 0.05%-0.50% of Mn, 0.050% or less of P, 0.020% or less of S, 15.0%-25.0% of Cr, 1.00%-2.50% of Mo, 0.001%-0.020% of Al, 0.25%-0.60% of Nb, 0.030% or less of N, 0.01%-2.50% of Ni, and Fe and unavoidable impurities as the balance, wherein the Nb solid solution amount is 0.20% by mass or more.

Description

Technical Field

[0001] The present invention relates to ferritic stainless steel, and more particularly to ferritic stainless steel having excellent corrosion resistance for use in the exhaust gas condensate environment of automobiles and suppressing 475°C embrittlement.

Background Art

[0002] In recent years, from the perspective of global environmental protection, there has been a demand for further improvement in fuel efficiency and strengthening of exhaust gas purification for automobiles. For this reason, the application of automotive heat exchangers such as exhaust heat recovery devices and EGR (Exhaust Gas Recirculation) coolers is expanding.

[0003] Here, an exhaust heat recovery device is a device that improves fuel efficiency by heating the engine cooling water with the heat of exhaust gas to shorten the warm-up time at the start of the engine, and its application is expanding mainly for hybrid vehicles. Generally, an exhaust heat recovery device is installed between a catalytic converter and a muffler, and is composed of a heat exchanger portion that combines pipes, plates, fins, side plates, etc., and an inlet / outlet pipe portion. Then, the exhaust gas enters the heat exchanger portion from the inlet pipe, where the heat is transferred to the cooling water through a heat transfer surface such as fins, and is discharged from the outlet pipe. In addition, for the adhesion and assembly of the plates and fins that make up the heat exchanger of such an exhaust heat recovery device, brazing with a Ni-containing brazing material is mainly used.

[0004] In addition, an EGR cooler is a device for suppressing nitrogen oxides (NOx) that are likely to be generated at high temperatures by recirculating exhaust gas with a low oxygen concentration to the intake side of the engine to lower the combustion temperature of the fuel. If the high-temperature exhaust gas is directly recirculated to the engine, the fuel will burn at an inappropriate timing, resulting in abnormal vibrations called knocking. Therefore, the EGR cooler is composed of a pipe portion that takes in a part of the exhaust gas, a heat exchanger portion that cools the intake exhaust gas, and a pipe portion that returns the cooled exhaust gas to the intake side of the engine. The heat exchanger of the EGR cooler is configured by stacking thin plates in a fin shape for reasons such as weight reduction, compactification, and cost reduction. For the adhesion and assembly of these, brazing with a Ni-containing brazing material is mainly used.

[0005] As described above, since the heat exchangers of the exhaust heat recovery device and the EGR cooler are adhered and assembled by brazing using a Ni-containing brazing material, the materials used for these heat exchangers are required to have good brazing properties with respect to the Ni-containing brazing material. Furthermore, automobile exhaust gas contains sulfur oxides (SOx) and hydrocarbons (HC) in addition to NOx. When these condense inside the heat exchanger, they become highly corrosive acidic condensate. Similar condensate may also occur in the muffler. Therefore, the materials used for these heat exchangers are also required to have corrosion resistance. In particular, since the temperature becomes high during the brazing heat treatment, it is necessary to prevent so-called sensitization, in which Cr at the grain boundaries reacts with C and N to form Cr carbonitrides, and a Cr-deficient layer with poor corrosion resistance forms around the Cr carbonitrides, and ensure corrosion resistance.

[0006] For the above reasons, austenitic stainless steels such as SUS316L or SUS304L with a reduced carbon content to prevent sensitization have usually been used for exhaust heat recovery devices and EGR coolers. However, austenitic stainless steels have problems in that they are costly because they contain a large amount of Ni, and their thermal expansion is large, so their fatigue characteristics, that is, their thermal fatigue characteristics at high temperatures, are low in a use environment where they are subjected to restraint force due to intense vibrations at high temperatures.

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

[0008] For example, Patent Document 1 discloses a ferritic stainless steel that ensures corrosion resistance by forming an oxide film containing 16% or more of Nb in terms of cation fraction after brazing, as a material for exhaust heat recovery devices and EGR coolers. Patent Document 2 discloses a ferritic stainless steel that ensures corrosion resistance by controlling the addition amounts of Al, Ti, and Si, as a material for exhaust heat recovery devices and EGR coolers. Patent Document 3 discloses a ferritic stainless steel that ensures corrosion resistance by controlling the contents of Cr, Si, and Al and the film thickness of the oxide film in the oxide film after brazing, as a material for heat exchangers and fuel supply system members. Further, Patent Document 4 discloses a ferritic stainless steel that ensures brazing properties by adding components such as Cr, Cu, Al, and Ti in a certain relational expression and suppressing the addition amounts of Al and Ti, as a material for EGR coolers. In addition, Patent Document 5 discloses a ferritic stainless steel that ensures brazing properties by suppressing the addition amounts of Al, Ti, and Zr, as an EGR cooler member having a structure joined by Ni brazing. Furthermore, Patent Document 6 discloses a ferritic stainless steel that ensures corrosion resistance against condensed water by suppressing the surface roughness, as a ferritic stainless steel material for brazing.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

[0010] However, in the techniques described in Patent Documents 1 to 6, the corrosion resistance against condensed water may be insufficient. Further, when applying a ferritic stainless steel with a high Cr content to heat exchangers such as exhaust heat recovery devices and EGR coolers, and mufflers, there is a problem of 475°C embrittlement. 475°C embrittlement means that when a ferritic stainless steel is held near 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, resulting in an increase in the hardness of the steel and embrittlement. When the increase in hardness due to 475°C embrittlement exceeds 20 HV, the ferritic stainless steel becomes significantly brittle and may be damaged when applied to the above-mentioned uses.

[0011] The present invention has been developed in view of the above situation, and aims to provide a ferritic stainless steel having excellent brazing properties when performing brazing at high temperatures using a Ni-containing brazing filler metal, excellent corrosion resistance, and suppression of 475°C embrittlement.

[0012] In this specification, "excellent brazing properties" means that after performing a brazing process in which a steel sheet coated with a Ni-containing brazing filler having a composition of Ni-29 mass% Cr-4 mass% Si-6 mass% P is heated at 1080°C for 10 minutes in a nitrogen carrier gas atmosphere of 1 Torr and cooled to room temperature, the ratio of the equivalent diameter of the brazing filler after heating to the equivalent diameter of the brazing filler before heating (the spreading rate of the brazing filler) is 150% or more.

[0013] In addition, in this specification, "excellent corrosion resistance" means that, using the steel sheet after the brazing treatment, a test piece with a size of 20 mm square is taken from a part where the brazing material is not attached, a measurement surface with a size of 11 mm square is left and covered with a sealing material, and then this test piece is immersed in a 3.5 mass% NaCl solution at 30°C. The pitting potential Vc'100 measured in accordance with JIS G 0577:2014 except for the concentration of the NaCl solution is 300 mV (vs SCE) or more.

[0014] Furthermore, in this specification, "suppressing 475°C embrittlement" means that, using the steel sheet after the brazing treatment, it refers to the case where the increase in Vickers hardness after holding a test piece taken from a part where the brazing material is not attached at 475°C for 1000 hours is 20 HV or less. The measurement of Vickers hardness is carried out in accordance with JIS Z 2244:2020, and five points are measured at the center of the plate thickness of the surface parallel to the rolling direction, and the average value is obtained.

Means for Solving the Problems

[0015] The inventors of the present invention found that when a ferritic stainless steel was held at 475°C for 1000 hours, the increase in Vickers hardness due to 475°C embrittlement differed depending on the steel type. As a result of further investigation, it was found that even for the same steel type, the increase in Vickers hardness due to 475°C embrittlement differed depending on the manufacturing method. As a result of intensive investigation into various factors affecting 475°C embrittlement, it was clarified that the more the amount of Nb in solid solution in the steel, the more 475°C embrittlement is suppressed, and when the amount of Nb in solid solution is controlled in the range of 0.20 mass% or more, the increase in Vickers hardness when held can be reduced to 20 HV or less. The present invention was completed after further investigation based on the above findings.

[0016] That is, the gist configuration of the present invention is as follows. [1] By 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 - 25.0%, Mo: 1.00 - 2.50%, Al: 0.001 - 0.020%, Nb: 0.25 - 0.60%, N: 0.030% or less, containing Ni: 0.01 - 2.50%, and having a component composition consisting of the balance Fe and inevitable impurities, a ferritic stainless steel with a solid solution Nb amount of 0.20 mass% or more. [2] In the said component 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 solid solution Nb amount is 0.26 mass% or more, the ferritic stainless steel according to [1]. [3] The said component composition further contains, by mass, one or more groups selected from the following Group A and Group B, the ferritic stainless steel according to [1] or [2]. Group A: one or two or more selected from Cu: 1.00% or less, Co: 1.00% or less, W: 2.00% or less Group B: one or two or more selected from 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, Sb: 0.100% or less [4] The increase in Vickers hardness after holding at 475°C for 1000 hours is 20 HV or less, the ferritic stainless steel according to any one of [1] to [3]. [5] A method for manufacturing the ferritic stainless steel according to any one of [1] to [4], comprising the step of preparing a cold-rolled steel sheet having the said component composition, and a step of performing finish annealing on the said cold-rolled steel sheet, which is held in the temperature range of 1040 - 1150°C for 5 seconds or more and then cooled in the temperature range of 900 - 600°C at an average cooling rate of 15°C / second or more, a method for manufacturing a ferritic stainless steel.

[0017] According to the present invention, it is possible to obtain a ferritic stainless steel having excellent brazing properties when brazing at a high temperature using a Ni-containing brazing filler metal, excellent corrosion resistance, and suppressing 475°C embrittlement.

Embodiments for Carrying Out the Invention

[0018] Hereinafter, the present invention will be specifically described.

[0019] First, in the present invention, the reason for limiting the component composition of the steel will be described. In the component composition of the steel, the unit of the content of each element is “mass%”, but hereinafter, unless otherwise specified, it is simply indicated by “%”.

[0020] C: 0.003 to 0.030% When the C content increases, the strength improves, and when it decreases, the workability improves. Here, C needs to be contained at 0.003% or more in order to obtain sufficient strength. However, when the C content exceeds 0.030%, the workability decreases significantly, and Cr carbides precipitate at the grain boundaries, causing sensitization and reducing the corrosion resistance. Therefore, the C content is in the range of 0.003 to 0.030%. The C content is preferably 0.004% or more. Also, the C content is preferably 0.025% or less, more preferably 0.020% or less, and even more preferably 0.010% or less.

[0021] Si: 0.01 to 1.00% Si is an element useful as a deoxidizer. Its effect can be obtained with a Si content of 0.01% or more. However, when the Si content exceeds 1.00%, oxides are formed during brazing, reducing the brazing properties. Therefore, the Si content is 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. Also, the Si content is preferably 0.90% or less, more preferably 0.80% or less, and even more preferably 0.70% or less.

[0022] Mn: 0.05 to 0.50% Mn has a deoxidizing effect, and its effect can be obtained with a Mn content of 0.05% or more. However, when the Mn content exceeds 0.50%, MnS is generated and the corrosion resistance is reduced. Therefore, the Mn content is in the range of 0.05 to 0.50%. The Mn content is preferably 0.10% or more, more preferably 0.15% or more. Also, the Mn content is preferably 0.40% or less, more preferably 0.30% or less.

[0023] P: 0.050% or less P is an element inevitably contained in steel, and excessive content tends to cause intergranular corrosion. This tendency becomes prominent with a P content exceeding 0.050%. Therefore, the P content is set to 0.050% or less. Preferably, the P content is 0.040% or less. Note that the lower limit of the P content is not particularly limited. However, excessive P removal leads to an increase in cost, so the P content is preferably 0.005% or more.

[0024] S: 0.020% or less S is an element inevitably contained in steel, and a S content exceeding 0.020% promotes the precipitation of MnS and reduces the corrosion resistance. Therefore, the S content is set to 0.020% or less. The S content is preferably 0.015% or less, more preferably 0.010% or less. Note that the lower limit of the S content is not particularly limited. However, excessive S removal leads to an increase in cost, so the S content is preferably 0.0005% or more.

[0025] Cr: 15.0 to 25.0% Cr is an important element for ensuring the corrosion resistance of stainless steel. On the other hand, when Cr holds the ferrite phase near 475°C, it decomposes into two phases: an α solid solution with a low Cr concentration and an α' solid solution with a high Cr concentration, which causes 475°C embrittlement. If the Cr content is less than 15.0%, sufficient corrosion resistance cannot be obtained. On the other hand, when the Cr content exceeds 25.0%, 475°C embrittlement becomes prominent. The Cr content is preferably 17.0% or more, more preferably 18.0% or more. Also, the Cr content is preferably 23.0% or less, more preferably 21.0% or less.

[0026] Mo: 1.00 - 2.50% 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 exceeds 2.50%, the precipitation of the Laves phase during cooling after finish annealing is promoted, the amount of solid-solved Nb decreases, and it becomes difficult to suppress 475°C embrittlement. Therefore, the Mo content is in the range of 1.00 - 2.50%. The Mo content is preferably 1.30% or more, more preferably 1.60% or more. Also, the Mo content is preferably 2.25% or less, more preferably 2.00% or less.

[0027] Al: 0.001 - 0.020% Al is an element useful for deoxidation, and its effect is obtained with an Al content of 0.001% or more. However, Al is an element active with respect to oxygen, and when the Al content exceeds 0.020%, oxides are formed and the brazing property deteriorates. Therefore, the Al content is in the range of 0.001 - 0.020%. Preferably, the Al content is 0.015% or less.

[0028] Nb: 0.25 - 0.60% Nb is an element that suppresses the decrease in corrosion resistance (sensitization) due to 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 exceeds 0.60%, it becomes hardened and the workability deteriorates. Therefore, the Nb content is in the range of 0.25 to 0.60%. The Nb content is preferably 0.275% or more, more preferably 0.30% or more. Also, the Nb content is preferably 0.50% or less, more preferably 0.40% or less.

[0029] N: 0.030% or less When the N content exceeds 0.030%, the corrosion resistance and workability deteriorate. Therefore, the N content is set to 0.030% or less. The N content is preferably 0.025% or less, more preferably 0.020% or less. Note that the lower limit of the N content is not particularly limited, but excessive reduction of the N content causes an increase in cost, so the N content is preferably 0.003% or more.

[0030] Ni: 0.01 - 2.50% Ni is an element that effectively contributes to the improvement of corrosion resistance when the content is 0.01% or more. On the other hand, when the Ni content exceeds 2.50%, the stress corrosion cracking susceptibility increases. Therefore, the Ni content is in the range of 0.01 to 2.50%. The Ni content is preferably 0.80% or more, more preferably 1.00% or more. Also, the Ni content is preferably 2.00% or less, more preferably 1.50% or less, and even more preferably 1.20% or less.

[0031] The basic components (essential components) in the ferritic stainless steel of the present invention have been described above. Among the component compositions of the present invention, the components (remainder) other than the above are Fe and inevitable impurities.

[0032] From the viewpoint of obtaining more excellent corrosion resistance, in the above component composition, the contents of Mo, Nb, and Ni are preferably Mo: 1.30 - 2.50%, Nb: 0.30 - 0.60%, and Ni: 0.80 - 2.50%.

[0033] The ferritic stainless steel of the present invention can further contain one or more selected from Cu, Co, and W within the following ranges, respectively.

[0034] Cu: 1.00% or less Cu is an element that enhances corrosion resistance. To obtain this effect, it is preferable that the Cu content is 0.01% or more. The Cu content is more preferably 0.10% or more. However, when the Cu content exceeds 1.00%, the hot workability deteriorates. Therefore, when Cu is contained, the Cu content should be 1.00% or less. The Cu content is preferably 0.80% or less, and more preferably 0.60% or less.

[0035] Co: 1.00% or less Co is an element that enhances corrosion resistance. To obtain this effect, it is preferable that the Co content is 0.01% or more. The Co content is more preferably 0.10% or more. However, when the Co content exceeds 1.00%, the workability deteriorates. Therefore, when Co is contained, the Co content should be 1.00% or less. The Co content is preferably 0.80% or less, and more preferably 0.60% or less.

[0036] W: 2.00% or less W is an element that enhances high-temperature strength and corrosion resistance. To obtain this effect, it is preferable that the W content is 0.01% or more. The W content is more preferably 0.10% or more. However, when the W content exceeds 2.00%, the workability deteriorates. Therefore, when W is contained, the W content should be 2.00% or less. The W content is preferably 1.50% or less, and more preferably 1.00% or less.

[0037] The ferritic stainless steel of the present invention can further contain one or more selected from Ti, V, Zr, Mg, Ca, B, REM, Sn, and Sb within the following ranges, respectively.

[0038] Ti: 0.10% or less Ti combines with C and N contained in the steel and has the effect of preventing sensitization. To obtain this effect, the Ti content is preferably 0.01% or more. The Ti content is more preferably 0.02% or more. On the other hand, Ti is an element active with respect to oxygen. When Ti is contained in an amount exceeding 0.10%, oxides mainly composed of Ti are formed on the surface of the steel during brazing treatment. This oxide significantly reduces the brazability. Therefore, when Ti is contained, the Ti content should be 0.10% or less. The Ti content is preferably 0.08% or less, and more preferably 0.06% or less.

[0039] V: 0.20% or less Similar to Ti, V combines with C and N contained in the steel and prevents sensitization. To obtain this effect, the V content is preferably 0.01% or more. The V content is more preferably 0.02% or more. On the other hand, when the V content exceeds 0.20%, the workability deteriorates. Therefore, when V is contained, the V content should be 0.20% or less. The V content is preferably 0.18% or less, and more preferably 0.15% or less.

[0040] Zr: 0.10% or less Similar to Ti and Nb, Zr is an element that combines with C and N contained in the steel and suppresses sensitization. To obtain this effect, the Zr content is preferably 0.01% or more. The Zr content is more preferably 0.02% or more. On the other hand, when the Zr content exceeds 0.10%, the workability deteriorates. Therefore, when Zr is contained, the Zr content should be 0.10% or less. The Zr content is preferably 0.08% or less, and more preferably 0.06% or less.

[0041] Mg: 0.0050% or less Mg acts as a deoxidizer. To obtain this effect, the Mg content is preferably 0.0003% or more. The Mg content is more preferably 0.0008% or more. However, when the Mg content exceeds 0.0050%, the toughness of the steel decreases and the manufacturability deteriorates. Therefore, when Mg is contained, the Mg content should be 0.0050% or less. The Mg content is preferably 0.0025% or less, and more preferably 0.0020% or less.

[0042] Ca: 0.0050% or less Ca improves the weldability by improving the penetration of the welded part. To obtain this effect, the Ca content is preferably 0.0003% or more. The Ca content is more preferably 0.0010% or more. However, when the Ca content exceeds 0.0050%, it combines with S to form CaS and the corrosion resistance decreases. Therefore, when Ca is contained, the Ca content should be 0.0050% or less. The Ca content is more preferably 0.0025% or less, and more preferably 0.0020% or less.

[0043] B: 0.0050% or less B is an element that improves secondary processing brittleness. To obtain this effect, the B content is preferably 0.0003% or more. The B content is more preferably 0.0010% or more. However, when the B content exceeds 0.0050%, the ductility decreases due to solid solution strengthening. Therefore, when B is contained, the B content should be 0.0050% or less. Preferably it is 0.0040% or less, and more preferably 0.0030% or less.

[0044] REM (rare earth metal): 0.100% or less REM (rare earth metals: elements with atomic numbers 57 to 71 such as La, Ce, Nd) is an element effective for deoxidation. To obtain this effect, it is preferable that the REM content is 0.005% or more. The REM content is more preferably 0.010% or more. However, when the REM content exceeds 0.100%, the hot workability deteriorates. Therefore, when REM is contained, the REM content should be 0.100% or less. Preferably it is 0.080% or less, and more preferably 0.060% or less. Note that REM is a general term for Sc, Y, and the 15 elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, and the REM content referred to here is the total content of these elements.

[0045] Sn: 0.100% or less Sn is an element effective for suppressing rough surface during processing. To obtain this effect, it is preferable that the Sn content is 0.001% or more. The Sn content is more preferably 0.005% or more. However, when the Sn content exceeds 0.100%, the hot workability deteriorates. Therefore, when Sn is contained, the Sn content should be 0.100% or less. The Sn content is preferably 0.080% or less, and more preferably 0.060% or less.

[0046] Sb: 0.100% or less Sb, like Sn, is an element effective for suppressing rough surface during processing. To obtain this effect, it is preferable that the Sb content is 0.001% or more. The Sb content is more preferably 0.005% or more. However, when the Sb content exceeds 0.100%, the workability deteriorates. Therefore, when Sb is contained, the Sb content should be 0.100% or less. The Sb content is preferably 0.080% or less, and more preferably 0.060% or less.

[0047] Solid solution Nb amount: 0.20 mass% or more The inventors have found that by setting the amount of Nb dissolved in steel to 0.20% by mass or more, the increase in Vickers hardness due to 475°C embrittlement can be suppressed to 20 HV or less. Although the reason for this is not clear, it is presumed that when Nb is dissolved in steel, Cr becomes less likely to diffuse, and as a result, 475°C embrittlement is suppressed. The amount of Nb in solid solution is preferably 0.26% by mass or more. The upper limit of the amount of Nb in solid solution is not limited. As an example, the amount of Nb in solid solution is 0.50% or less. The amount of Nb in solid solution can be determined in detail by the method described in the examples.

[0048] In addition, from the viewpoint of obtaining more excellent corrosion resistance and a more excellent effect of suppressing 475°C embrittlement, in the above component composition, the contents of Mo, Nb, and Ni are Mo: 1.30 to 2.50%, Nb: 0.30 to 0.60%, Ni: 0.80 to 2.50%, and the amount of Nb in solid solution in the steel is preferably 0.26% by mass or more.

[0049] The ferritic stainless steel of the present invention preferably has an increase in Vickers hardness of 20 HV or less, more preferably 15 HV or less, after being held at 475°C for 1000 hours. The smaller the increase in the above Vickers hardness, the more preferable it is, and it may be 0 HV. The increase in the above Vickers hardness can be determined in detail by the method described in the examples.

[0050] Next, a preferred manufacturing method of the ferritic stainless steel of the present invention will be described. The manufacturing method of the ferritic stainless steel of the present invention includes a step of preparing a cold-rolled steel sheet having the above-described component composition, and a step of subjecting the cold-rolled steel sheet to finish annealing in which it is held in a temperature range of 1040 to 1150°C for 5 seconds or more and then cooled in a temperature range of 900 to 600°C at an average cooling rate of 15°C / second or more. The step of preparing the cold-rolled steel sheet is not particularly limited. For example, a steel material (steel slab) having the above-described component composition is hot-rolled into a hot-rolled steel sheet, the hot-rolled steel sheet is subjected to hot-rolled sheet annealing as necessary, and then cold-rolled to prepare a cold-rolled steel sheet having a desired plate thickness. Then, the thus-prepared cold-rolled steel sheet is held in a temperature range of 1040 to 1150°C for 5 seconds or more and then subjected to finish annealing in which it is cooled in a temperature range of 900 to 600°C at an average cooling rate of 15°C / second or more, whereby the ferritic stainless steel of the present invention can be manufactured. Note that conditions such as hot rolling, cold rolling, and hot-rolled sheet annealing are not particularly limited and may follow conventional methods.

[0051] In the steelmaking process of melting steel, it is preferable to subject the steel melted in a converter or an electric furnace or the like to secondary refining by the VOD (Vacuum Oxygen Decarburization) method or the like to obtain a steel containing the above essential components and components added as necessary. The melted molten steel can be made into a steel material by a known method, but from the viewpoints of productivity and quality, it is preferably made by the continuous casting method. The steel material is then preferably held at a temperature of 1050 to 1250°C for 30 minutes or more and then hot-rolled into a hot-rolled steel sheet having a desired plate thickness. Of course, hot working other than sheet materials can also be performed. The above hot-rolled steel sheet is preferably subjected to hot-rolled sheet annealing in which it is held at a temperature of 900 to 1150°C for 30 seconds or more and then descaled by pickling or the like to obtain a hot-rolled product. Note that, if necessary, scale removal may be performed by shot blasting or a grinding brush before pickling.

[0052] Furthermore, the above hot-rolled product (such as hot-rolled annealed steel sheet) is made into a cold-rolled product through processes such as cold rolling. In this case, the cold rolling may be carried out once, but from the viewpoints of productivity and required quality, it may also be two or more times of cold rolling with intermediate annealing. The total reduction ratio of one or two or more times of cold rolling is preferably 60% or more, more preferably 70% or more. In this way, a cold-rolled steel sheet having the above component composition is prepared.

[0053] Subsequently, the cold-rolled steel sheet prepared as described above is subjected to finish annealing by holding it at a temperature (finish annealing temperature) in the range of 1040 to 1150 °C for 5 seconds or more and then cooling it in the temperature range of 900 to 600 °C at an average cooling rate of 15 °C / second or more. If the finish annealing temperature is less than 1040 °C, recrystallization is insufficient, and if it exceeds 1150 °C, the coarsening of crystal grains becomes remarkable. Therefore, the finish annealing temperature is set in the temperature range of 1040 °C to 1150 °C. Also, if the holding time in the above temperature range is less than 5 seconds, recrystallization is insufficient, so the holding time in the above temperature range is 5 seconds or more. Although the upper limit of the holding time in the above temperature range is not limited, from the viewpoint of preventing remarkable coarsening of crystal grains, it is preferably 100 seconds or less.

[0054] Furthermore, the inventors of the present invention have clarified that by holding at a temperature in the range of 1040 to 1150 °C for 5 seconds or more and then cooling in the temperature range of 900 to 600 °C at an average cooling rate of 15 °C / second or more, the reduction in the amount of dissolved Nb due to the precipitation of the Laves phase (intermetallic compound containing Mo and Nb mainly composed of Fe) during cooling is suppressed, and the amount of dissolved Nb can be 0.20 mass% or more. As a reason for this, in the range of the component composition of the present invention, the Laves phase precipitates at 900 °C or lower. Therefore, it is considered that by making the average cooling rate at 900 °C or lower sufficiently fast, the precipitation of the Laves phase can be suppressed. Since atomic diffusion is slow at 600 °C or lower and the Laves phase hardly precipitates at a normal average cooling rate, the average cooling rate in the temperature range of 900 to 600 °C may be 15 °C / second or more. The upper limit of the average cooling rate in the above 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. In addition, when oxide scale can be formed by finish annealing, the cold-rolled product (cold-rolled annealed steel sheet) obtained as described above may be pickled. Further, in order not to form oxide scale, finish annealing may be 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. Through the above steps, the ferritic stainless steel of the present invention is obtained.

[0055] The ferritic stainless steel of the present invention described above is suitably used for automotive parts exposed to a condensed water environment derived from exhaust gas, in which one or more joints are assembled by brazing. Examples of automotive parts exposed to a condensed water environment derived from exhaust gas include exhaust heat recovery devices, EGR coolers, etc. The ferritic stainless steel of the present invention is particularly suitably used as a heat exchanger for an exhaust heat recovery device or an EGR cooler, or as a member of a muffler.

Examples

[0056] Steel having the component composition shown in Table 1 was melted in a vacuum melting furnace, heated at 1150 °C for 1 hour, and then a hot-rolled steel sheet with a thickness of 4.0 mm was produced by hot rolling. After performing hot-rolled sheet annealing by holding at 1080 °C for 1 minute, the surface was ground to remove scale and cold-rolled to a thickness of 1.0 mm to prepare a cold-rolled steel sheet. The cold-rolled annealed steel sheet obtained by subjecting the cold-rolled steel sheet to finish annealing under the conditions shown in Table 2 in an ammonia-decomposed gas atmosphere was polished to 600 grit with emery abrasive paper on its surface, degreased with acetone, and subjected to testing.

[0057] Regarding this cold-rolled annealed steel sheet, the amount of Nb in solid solution was measured as follows, and evaluations of (1) brazing property, (2) corrosion resistance, and (3) suppression of 475 °C brittleness were carried out, and the results are shown in Table 2.

[0058] (Amount of Nb in solid solution) Regarding the above cold-rolled annealed steel sheet, the amount of Nb contained in precipitates such as Laves phase and carbonitrides was measured by extraction residue analysis, and the difference between the Nb content in the steel and the amount of Nb in the precipitates was defined as the amount of dissolved Nb. Regarding the method of extraction residue analysis, constant current electrolysis was carried out with a 10% AA-based electrolyte (10 Vol% acetylacetone - 1 mass% tetramethylammonium chloride - methanol electrolyte), and precipitates in the steel were extracted using a membrane filter with a pore size of 0.2 μm, and the amount of Nb in the precipitates was analyzed by ICP emission spectrometry to determine the amount of Nb in the precipitates.

[0059] (1) Evaluation of brazing property From the prepared cold-rolled annealed steel sheet, a test piece with a width of 50 mm and a length of 50 mm was cut out, and a Ni brazing filler metal with a diameter of 10 mm and a thickness of 1 mm (composition, Ni: balance, Cr: 29 mass%, Si: 4 mass%, P: 6 mass%) was applied to the surface of the horizontally placed plate. Then, with the surface coated with the Ni brazing filler metal facing up, the test piece was heated in a nitrogen carrier gas atmosphere of 1080 °C and 1 Torr for 10 minutes in a horizontal state, and then cooled to room temperature for brazing treatment. After that, the equivalent diameter of the circle of the brazing filler metal on the surface of the test piece (equivalent diameter of the circle of the brazing filler metal after heating) was measured. And the ratio of the equivalent diameter of the circle of the brazing filler metal after heating to the diameter of the brazing filler metal before heating (10 mm, the equivalent diameter of the circle is also the same) (spreading rate of the brazing filler metal) was obtained and evaluated according to the following criteria. Spreading rate of the brazing filler metal after heating relative to before heating = (equivalent diameter of the circle of the brazing filler metal after heating / diameter of the brazing filler metal before heating (10 mm)) × 100 (%) ○ (Pass): 150% or more × (Fail): Less than 150%

[0060] (2) Evaluation of corrosion resistance After the brazing process (after the brazing process performed in the evaluation of brazing properties), test pieces were taken from each cold-rolled annealed steel sheet, with a size of 20 mm square, from the part where the brazing material was not adhered. After covering this test piece with a silicone resin sealing material, leaving a measurement surface of 11 mm square, the measurement surface was polished with emery paper (#600). Then, this test piece was immersed in a 3.5 mass% NaCl solution at 30 °C, and pitting potential measurement was carried out in accordance with JIS G 0577:2014 except for the concentration of the said NaCl solution. After holding at the natural potential for 10 minutes, measurement was performed at a scanning rate of 20 mV / min until the anodic current density reached 1.1 mA / cm 2 and the potential when the current density reached 100 μA / cm 2 was defined as the pitting potential Vc'100. Considering the usage conditions of the heat exchanger parts of the waste heat recovery device and the EGR cooler, if the pitting potential Vc'100 is 300 mV (vs SCE) or more, it can be determined that the corrosion resistance is excellent. Based on the following criteria, ◎ and 〇 were considered as passing. ◎ (Passing, more excellent): 350 mV (vs SCE) or more 〇 (Passing): 300 mV (vs SCE) or more and less than 350 mV (vs SCE) × (Failing): Less than 300 mV (vs SCE)

[0061] (3) Evaluation of suppression of 475 °C brittleness Using the part where the brazing material was not adhered on the test piece of each cold-rolled annealed steel sheet after the brazing process (after the brazing process performed in the evaluation of brazing properties), before and after holding at 475 °C for 1000 hours in an electric furnace, Vickers hardness measurement was carried out in accordance with JIS Z 2244:2020. The load was 300 gf, and 5 points were measured at the center of the plate thickness on the surface parallel to the rolling direction, and the average value was obtained. If the increase in Vickers hardness due to 475 °C embrittlement (Vickers hardness after holding at 475 °C for 1000 hours - Vickers hardness before holding at 475 °C for 1000 hours) is 20 HV or less, it is judged that 475 °C brittleness is suppressed. Based on the following criteria, ◎ and 〇 were considered as passing. ◎ (Passing, more excellent): 15 HV or less 〇 (Passing): More than 15 HV and 20 HV or less × (Non-conforming): Exceeding 20 HV

[0062]

Table 1

[0063]

Table 2

[0064] From Table 2, in Invention Examples No. 1 to 24 and 35 to 38, all showed good brazing property, corrosion resistance, and suppression of 475°C embrittlement. Furthermore, among the invention examples, in No. 1 to No. 8, No. 10, No. 11, No. 14 to No. 23, and No. 35 to 38 that satisfied all of Mo: 1.30 to 2.50%, Nb: 0.30 to 0.60%, and Ni: 0.80 to 2.50%, the pitting potential Vc’100 was 350 mV or more, showing more excellent corrosion resistance. Also, among the invention examples, in No. 2, No. 3, No. 7, No. 10 to No. 12, No. 14 to No. 16, No. 18, No. 23, and No. 24 where the amount of solid solution Nb was 0.26 mass% or more, the increase in Vickers hardness after holding at 475°C for 1000 hours was 15 HV or less, showing a more excellent effect of suppressing 475°C brittleness. In contrast, in Comparative Examples No. 25 to 34 where the component composition and manufacturing method were outside the appropriate range, it was not possible to simultaneously satisfy all of brazing property, corrosion resistance, and suppression of 475°C embrittlement.

[0065] More specifically, in Comparative Example No. 25 (steel symbol B1), since the Cr content exceeded the upper limit value of the present invention, the target suppression of 475°C embrittlement could not be obtained. In Comparative Example No. 26 (steel symbol B2), since the Mo content exceeded the upper limit value of the present invention, the target suppression of 475°C embrittlement could not be obtained. In Comparative Example No. 27 (steel symbol B3), since the Al content exceeded the upper limit value of the present invention, the target brazing property could not be obtained. In Comparative Example No. 28 (steel symbol B4), since the Si content exceeded the upper limit value of the present invention, the target brazing property could not be obtained. In Comparative Example No. 29 (steel symbol B5), since the Mn content exceeded the upper limit of the present invention, the target corrosion resistance could not be obtained. In Comparative Example No. 30 (steel symbol B6), since the Cr content was less than the lower limit of the present invention, the target corrosion resistance could not be obtained. In Comparative Example No. 31 (steel symbol B7), since the Mo content was less than the lower limit of the present invention, the target corrosion resistance could not be obtained. In Comparative Example No. 32 (steel symbol B8), since the Nb content was less than the lower limit of the present invention, the target corrosion resistance could not be obtained, and the amount of Nb in solid solution was lower than 0.20% by mass, so the suppression of 475°C embrittlement could not be achieved. In Comparative Examples No. 33 and No. 34, since the average cooling rate in the temperature range of 900 to 600°C after holding at the finish annealing temperature was less than 15°C / second, the amount of Nb in solid solution was lower than 0.20% by mass, and the suppression of 475°C embrittlement could not be achieved.

Industrial Applicability

[0066] According to the present invention, a ferritic stainless steel that can be suitably used for heat exchangers such as exhaust heat recovery devices and EGR coolers and members of mufflers used in the exhaust gas condensate environment of automobiles can be obtained, which is extremely useful industrially.

Claims

1. by 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: 17.0 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, containing Ni: 0.01 to 2.50%, having a component composition in which the balance consists of Fe and unavoidable impurities, A ferritic stainless steel in which the amount of Nb in solid solution is 0.20% by mass or more.

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

3. The ferritic stainless steel according to claim 1, wherein the above component composition further contains one or more groups selected from the following Group A and Group B by mass%. 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 Ti: 0.10% or less, V: 0.20% or less, Mg: 0.0050% or less, Ca: 0.0050% or less, REM: 0.100% or less, and Sn: 0.100% or less

4. The ferritic stainless steel according to claim 2, wherein the above component composition further contains one or more groups selected from the following Group A and Group B by mass%. Group A: One or more selected from among Cu: 1.00% or less, Co: 1.00% or less, and W: 2.00% or less Group B: One or more selected from among 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

5. The ferritic stainless steel according to any one of Claims 1 to 4, wherein the increase in Vickers hardness after holding at 475°C for 1000 hours is 20 HV or less.

6. A method for manufacturing a ferritic stainless cold-rolled steel sheet made of the ferritic stainless steel according to any one of Claims 1 to 4, comprising: a step of preparing a cold-rolled steel sheet having the above-described component composition; a step of performing finish annealing on the cold-rolled steel sheet by holding it in a temperature range of 1040 to 1150°C for 5 seconds or more and then cooling it in a temperature range of 900 to 600°C at an average cooling rate of 15°C / second or more.

7. A method for manufacturing a ferritic stainless cold-rolled steel sheet made of the ferritic stainless steel according to Claim 5, comprising: a step of preparing a cold-rolled steel sheet having the above-described component composition; a step of performing finish annealing on the cold-rolled steel sheet by holding it in a temperature range of 1040 to 1150°C for 5 seconds or more and then cooling it in a temperature range of 900 to 600°C at an average cooling rate of 15°C / second or more.

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