Fe-cr-ni steel sheet and method for producing same

Abstract

The present invention addresses the problem of providing an Fe-Cr-Ni steel sheet having improved heat resistance. A Fe-Cr-Ni steel sheet according to the present invention has a prescribed chemical composition. The value of 0.5 × Cr + 1.4 × Ni + 3.2 × Nb + 0.1 × Mo + 3.6 × Cu − 3.4 × Si − 0.6 × Co (the element symbols in the formula represent the contained amounts (mass%) of the corresponding elements, and zero is substituted when the element is not contained) is 50 or less. The corresponding grain boundary frequency of corresponding grain boundaries having a Σ value of 3-29 is 50% or greater in total. KAM representing the proportion for which KAM values are 1-2°, and HAGB representing the ratio between crystal grain boundary lengths for which the relative orientation difference is 15° or greater and the total grain boundary length for which the relative orientation difference is 2° or greater, satisfy KAM / HAGB ≤ 0.010.

Description

Fe-Cr-Ni steel sheet and manufacturing method thereof

[0001] The present invention relates to an Fe-Cr-Ni steel sheet that is used as a material for heat-resistant parts that require heat resistance, and is particularly applicable to exhaust manifolds, converters, and turbocharger parts for automobiles and motorcycles, as well as boilers, power plants, chemical plants, etc. The Fe-Cr-Ni steel sheet of the present invention is an austenitic stainless steel sheet or a superalloy.

[0002] Automotive exhaust manifolds, front pipes, center pipes, mufflers, and other environmentally friendly parts for purifying exhaust gases use materials with excellent heat resistance, such as oxidation resistance, high-temperature strength, and thermal fatigue properties, in order to ensure stable passage of high-temperature exhaust gases. Furthermore, because these parts are exposed to corrosive environments such as condensed water, they are also required to have excellent corrosion resistance.

[0003] Stainless steel is widely used in these components due to the need for stricter exhaust gas regulations, improved engine performance, and lighter vehicle weight. Furthermore, in recent years, exhaust gas regulations have become even stricter, and the trend toward improved fuel economy and downsizing has led to an upward trend in the temperature of exhaust gases, particularly those passing through the exhaust manifold directly below the engine. In addition, the number of vehicles equipped with superchargers such as turbochargers is increasing, requiring even greater heat resistance from the stainless steel used in exhaust manifolds and turbochargers. With regard to the rise in exhaust gas temperature, it is expected that the temperature will rise from the conventional level of around 900°C to around 1000°C.

[0004] On the other hand, the internal structure of a turbocharger is complex, and it is important to improve supercharging efficiency while ensuring heat-resistant reliability, so the use of heat-resistant austenitic stainless steel has been primarily disclosed. In addition to typical heat-resistant austenitic stainless steels such as SUS310S (25% Cr-20% Ni) and Ni-based alloys, Patent Document 1 discloses high-Cr and Mo-added steel. Furthermore, Patent Document 2 discloses an exhaust guide part for a nozzle vane-type turbocharger that uses austenitic stainless steel with 2 to 4% added Si.

[0005] In Patent Document 2, the steel composition is specified taking into consideration hot workability during steel manufacturing, but it cannot be said that it fully satisfies the high-temperature properties required for the above-mentioned parts. Furthermore, while it is considered important to maintain the hole expandability of punched holes, the steel composition specified based on hot workability was unable to provide sufficient hole expandability. Furthermore, stainless steel cast steel is used for turbocharger housings, but due to their thick walls, there is a need for thinner and lighter materials.

[0006] Patent Document 3 discloses that the high-temperature strength and creep properties of heat-resistant austenitic stainless steel sheet are improved by determining the optimum ranges of the contents of Nb, V, C, N, Al, and Ti and optimizing the manufacturing process. However, the technical problem with the invention disclosed in Patent Document 3 is the improvement of high-temperature strength and creep properties at 800°C, and the invention disclosed in Patent Document 3 is insufficient for use with exhaust gases exceeding 900°C.

[0007] Furthermore, Patent Document 4 discloses a heat-resistant austenitic stainless steel in which the material composition and processing conditions are optimized to have a hardness of 40 HRC or more at room temperature after heat treatment at 700°C for 400 hours. However, the problem with the invention disclosed in Patent Document 4 is to have high-temperature strength that can withstand a usage environment of 550°C or more, and Patent Document 4 only discloses high-temperature strength at 700°C, so the heat-resistant austenitic stainless steel according to the invention disclosed in Patent Document 4 is insufficient to withstand exhaust gases exceeding 900°C.

[0008] Furthermore, Patent Document 5 discloses that it is possible to achieve improved intergranular corrosion resistance and improved high-temperature strength with a small-grain-size material by controlling the low ΣCSL grain boundary frequency, the average crystal grain size, etc. However, the "high-temperature strength" in Patent Document 5 refers to high-temperature strength in water, and does not disclose any specific means for achieving strength against exhaust gases exceeding 900°C.

[0009] Furthermore, the stainless steel for nuclear power plants disclosed in Patent Document 6 is characterized by ensuring excellent intergranular corrosion resistance in high-temperature water by increasing the twin grain boundary ratio in the steel. However, Patent Document 6 does not disclose the high-temperature strength of the stainless steel for nuclear power plants, nor does Patent Document 6 disclose any specific means for achieving strength against exhaust gases exceeding 900°C.

[0010] Furthermore, the corrosion-resistant austenitic alloy disclosed in Patent Document 7 is characterized in that it is obtained by subjecting an austenitic alloy to more than 30% cold working and heat treatment, thereby forming twin boundaries within the austenite grains and dispersing precipitates on the austenite grain boundaries and / or twin boundaries. These characteristics suppress grain boundary sliding and increase grain boundary strength, so that the corrosion-resistant austenitic alloy has higher stress corrosion cracking resistance. However, the stress corrosion cracking resistance disclosed in Patent Document 7 is a characteristic in high-temperature water, and Patent Document 7 does not disclose any specific solution for achieving strength against exhaust gases at temperatures above 900°C.

[0011] The improvement in properties achieved by increasing the twin boundary ratio is primarily aimed at improving corrosion resistance. Patent documents 8 to 14 also describe various steel compositions and manufacturing conditions for this purpose. The steel compositions are targeted at SUS304 and SUS316-based austenitic stainless steels, but there is no mention of superalloy Fe-Cr-Ni steels or materials containing Si, Nb, and Co, as described in the present application. Regarding rolling conditions, a low reduction ratio is the norm, e.g., 2 to 15% in Patent document 9, and 2 to 5% in Patent documents 12 and 13. Furthermore, the subsequent heat treatment conditions call for a relatively long heat treatment time, e.g., 900 to 1000°C for 5 hours or more in Patent document 9, 927 to 1227°C for 1 to 60 minutes in Patent document 13, and 900 to 950°C for 10 to 48 hours in Patent document 11. Meanwhile, Patent document 12 describes a temperature of 1052°C or higher for no more than 2 minutes. When recrystallization is promoted and many high-angle boundaries are generated, the frequency of twin boundaries decreases. Therefore, in order to increase the frequency of twin boundaries, it is thought that heat treatment at a relatively low temperature for a long time as described above is carried out to prevent recrystallization from progressing and to allow the originally existing high-angle boundaries to migrate.

[0012] Furthermore, the austenitic stainless steel sheet disclosed in Patent Document 15 is a steel sheet invented based on the finding that a higher frequency of annealing twins results in higher high-temperature strength at 900°C, and discloses that a high-strength material of 70 MPa or more can be obtained by increasing the frequency of annealing twins in a corrosion-resistant austenitic stainless steel sheet to 40% or more. However, the strength against exhaust gases above 900°C is insufficient.

[0013] Furthermore, Patent Documents 16 and 17 disclose that the austenitic stainless steel sheets disclosed have a frequency of 70% or more or 80% or more of coincidence grain boundaries by performing cold rolling and annealing processes once or twice, and that austenitic stainless steel sheets with good creep properties at 900 to 950°C can be obtained by holding for a time of 120 seconds to 20 minutes. However, although Nb, Mo, Cu, and Co, which are characteristic of the present invention, are listed as essential or optional elements, there is no description of their effects on coincidence grain boundaries or stacking fault energy, making them unclear. Also, there is room for improvement in high-temperature strength.

[0014] International Publication No. 2014 / 157655 Japanese Patent No. 4937277 JP 2013-209730 A JP 2005-281855 A JP 2011-168819 A JP 2005-15896 A JP 2008-63602 A JP 11-80905 A JP 2003- JP 253401 A JP 2009-161802 A JP 2009-191341 A JP 2009-287104 A JP 2010-275569 A JP 2014-5509 A International Publication No. 2017 / 164344 Japanese Patent No. 6879877 Japanese Patent No. 7166082 A

[0015] When heat-resistant parts are exposed to high-temperature environments, they suffer from excessive deformation and, in extreme cases, fracture due to insufficient high-temperature strength and rigidity. In conventional Fe—Cr—Ni steel sheets, including austenitic stainless steel sheets, adding alloy elements to increase high-temperature strength not only results in insufficient room-temperature ductility but also leads to increased costs. The object of the present invention is to solve the above-mentioned problems and improve the heat resistance of Fe—Cr—Ni steel sheets, including austenitic stainless steel sheets, used in heat-resistant parts by controlling their structure. More specifically, the object is to provide an Fe—Cr—Ni steel sheet that has excellent 0.2% yield strength and creep properties at 900°C, as well as excellent room-temperature ductility.

[0016] The problem to be solved by this application is addressed by heat-resistant components such as exhaust components for transportation vehicles such as automobiles and motorcycles, and components for chemical plants. Among these, automotive exhaust system components, such as exhaust manifolds, turbochargers, wastegate valves, and turbo housings, are particularly targeted. For turbochargers, these include the housing that forms the outer frame and precision components inside nozzle vane turbochargers (e.g., back plates, oil deflectors, compressor wheels, nozzle mounts, nozzle plates, nozzle vanes, drive rings, and drive levers). The type of automotive fuel in this case may be any of gasoline, diesel, synthetic fuel, hydrogen, and the like, and is not limited thereto.

[0017] In Fe-Cr-Ni steels, including austenitic stainless steels, grain boundaries form after cold rolling and annealing, resulting in polycrystalline bodies. The atomic arrangement at grain boundaries can be regular or irregular. When the atomic arrangement at a grain boundary is regular with few gaps, it results in a low-energy structure, making it less susceptible to grain boundary degradation. A representative example of this special grain boundary is the correspondence boundary. In contrast, when the atomic arrangement at a grain boundary is irregular, it is called a random grain boundary and has a high-energy structure. While many geometric combinations of correspondence orientation relationships occur, grain boundaries formed by correspondence orientation relationships between Σ3 and Σ29 are called correspondence boundaries. This Σ value is an odd number, and the smaller this value, the higher the density of correspondence lattice points, making it a more low-energy grain boundary.

[0018] Most of the above-mentioned conventional knowledge improves corrosion resistance by increasing the frequency of coincidence boundaries. For example, Patent Document 9 suppresses intergranular corrosion by increasing the frequency of coincidence boundaries of Σ29 or less to 75% or more for SUS304 with a Si content of 0.59% and unknown Nb and Co contents. However, Patent Document 9 requires heat treatment at 900 to 1000°C for 5 hours or more to increase the frequency of coincidence boundaries, which is inefficient for industrial mass production. Furthermore, the invention disclosed in Patent Document 9 increases the frequency of coincidence boundaries to suppress precipitation at grain boundaries and improve corrosion resistance, but the effect on mechanical properties at high temperatures is unknown.

[0019] In order to solve the above problems, the present inventors conducted detailed research into the relationship between the metallographic structure and high-temperature properties of Fe-Cr-Ni steel sheets. As a result, they discovered that for materials that require heat resistance in components exposed to extremely severe thermal environments, such as turbochargers, by ensuring heat resistance through the steel composition and by controlling the characteristics of the grain boundaries in the metallographic structure through a cold rolling process and a subsequent annealing process, remarkably excellent high-temperature strength can be obtained.

[0020] Furthermore, the inventors have found that the annealing time for increasing the frequency of coincidence grain boundaries can be shortened by using steel compositions that reduce the stacking fault energy (SFE) calculated by the following formula described in D. DULIEU and J. NUTTING, Iron and Steel Inst. Special Report, 86 (1964) 140:

[0021] SFE (mJm -2 ) = 0.5 × Cr + 1.4 × Ni + 3.2 × Nb + 0.1 × Mo + 3.6 × Cu - 3.4 × Si - 0.6 × Co Where, the element symbol in the formula means the content (mass%) of the element, and 0 is substituted when the element is not contained.

[0022] Furthermore, when the average value of the crystal orientation difference between the target measurement point and its surrounding measurement points is taken as the KAM value, the KAM has a ratio (area ratio) of 1 to 2 degrees, and the proportion of high-angle boundaries (HAGB) is calculated by dividing the length of grain boundaries with a relative orientation difference of 15 degrees or more by the total grain boundary length, i.e., the length of grain boundaries with a relative orientation difference of 2 degrees or more, so that KAM / HAGB≦0.01 is satisfied. In addition, it has been found that high-temperature strength and creep strength can be improved by creating a structure in which the total frequency of coincidence boundaries with Σ values ​​of 3 to 29 is 50% or more.

[0023] The present invention has been made based on the above findings to solve the above problems, and includes the following aspects.

[0024] [1] In mass%, C: 0.300% or less, Si: 5.00% or less, Mn: 10.00% or less, P: 0.050% or less, S: 0.0100% or less, Ni: 2.00 to 40.00%, Cr: 14.00 to 30.00%, N: 0.400% or less, Al: 0.001 to 1.000%, V: 0.02 to 1.00%, Nb: 0.001 to 1.000%, Co: 0.001 to 1.000%, Cu: 0 to 4.00%, Mo: 0 to 5.00%, Ti: 0 to 1.000 %, B: 0 to 0.0100%, Ca: 0 to 0.0100%, W: 0 to 3.00%, Zr: 0 to 0.30%, Sn: 0 to 0.50%, Mg: 0 to 0.0100%, Sb: 0 to 0.300%, REM: 0 to 0.200%, Ga: 0 to 0.3000%, Ta: 0 to 1.00%, Hf: 0 to 1.00%, Bi: 0 to 0.020%, and the balance being Fe and unavoidable impurities, and the value of stacking fault energy (SFE) calculated by the following formula (1) is 50 mJm -2 The Fe-Cr-Ni steel sheet is characterized in that the frequency of coincidence boundaries with a Σ value of 3 to 29 is 50% or more in total, the KAM indicating the proportion of grain boundaries with a KAM value of 1 to 2 degrees, and the HAGB indicating the ratio of the length of grain boundaries with a relative misorientation of 15 degrees or more to the length of all grain boundaries with a relative misorientation of 2 degrees or more satisfy KAM / HAGB≦0.010. -2 ) = 0.5 × Cr + 1.4 × Ni + 3.2 × Nb + 0.1 × Mo + 3.6 × Cu - 3.4 × Si - 0.6 × Co (1) In the formula, the element symbol indicates the content (mass%) of the element, and 0 is substituted when the element is not contained.

[0025] [2] Cu: 0.01-4.00%, Mo: 0.01-5.00%, Ti: 0.001-1.000%, B: 0.0001-0.0100%, Ca: 0 .0001-0.0100%, W: 0.01-3.00%, Zr: 0.01-0.30%, Sn: 0.01-0.50%, Mg: 0.0001-0.01 The Fe-Cr-Ni steel plate according to [1], characterized in that it contains one or more of: Sb: 0.001 to 0.300%, REM: 0.001 to 0.200%, Ga: 0.0001 to 0.3000%, Ta: 0.01 to 1.00%, Hf: 0.01 to 1.00%, and Bi: 0.001 to 0.020%.

[0026] [3] An exhaust part, a precision internal part of a turbocharger, a wastegate valve, or a turbo housing using the Fe-Cr-Ni steel plate according to [1] or [2].

[0027] [4] A method for producing the Fe-Cr-Ni steel sheet according to [1], comprising the steps of: C: 0.300% or less, Si: 5.00% or less, Mn: 10.00% or less, P: 0.050% or less, S: 0.0100% or less, Ni: 2.00 to 40.00%, Cr: 14.00 to 30.00%, N: 0.400% or less, Al: 0.001 to 1.000%, V: 0.02 to 1.00%, Nb: 0.001 to 1.000%, Co: 0.001 to 1.000%, %, Cu: 0 to 4.00%, Mo: 0 to 5.00%, Ti: 0 to 1.000%, B: 0 to 0.0100%, Ca: 0 to 0.0100%, W: 0 to 3.00%, Zr: 0 to 0.30%, Sn: 0 to 0.50%, Mg: 0 to 0.0100%, Sb: 0 to 0.300%, REM: 0 to 0.200%, Ga: 0 to 0.3000%, Ta: 0 to 1.00%, Hf: 0 to 1.00%, Bi: 0 to 0.020%, and the balance being Fe and unavoidable impurities. A hot rolling process in which a pure slab is hot rolled to obtain a hot rolled steel sheet; a first cold rolling process in which the hot rolled steel sheet is rolled at a reduction rate of 80% or less to obtain a cold rolled steel sheet; a first annealing and pickling process in which the cold rolled steel sheet is annealed at a temperature range of 950 to 1300°C for a holding time of 120 seconds or less at 950°C or higher, and then pickled; a second cold rolling process in which the cold rolled steel sheet after the first annealing is rolled at a reduction rate of 10% or less; A method for producing an Fe-Cr-Ni steel sheet, comprising a second annealing / pickling step of annealing a steel sheet to a maximum temperature of 900 to 1000°C, heating it up to 900°C at a rate of 10°C / sec or more, and holding it at the maximum temperature for less than 60 seconds, followed by annealing it to a maximum temperature of 1000 to 1200°C, heating it up to the maximum temperature at a rate of less than 10°C / sec, and holding it from 1000°C to the maximum temperature for 60 seconds or more but less than 50 hours, and then pickling it.

[0028] According to the present invention, it is possible to provide an Fe-Cr-Ni steel sheet having excellent high-temperature properties, and when applied to automobile exhaust parts in particular, it contributes greatly to weight reduction and high exhaust temperature.

[0029] The Fe-Cr-Ni steel sheet of the present invention will be described in detail below.

[0030] <Chemical Composition> First, the chemical composition of the Fe-Cr-Ni steel of the present invention will be described.

[0031] (C: 0.300% or less) C may be added to ensure austenite structure formation and improve high-temperature strength and creep life. C is not an essential element, and the C content may be 0%. To achieve the above effects, the C content is preferably 0.001% or more. The C content may be 0.005% or more, 0.010% or more, 0.015% or more, 0.030% or more, 0.045% or more, or 0.050% or more. On the other hand, excessive addition not only leads to hardening, but also deteriorates corrosion resistance, particularly intergranular corrosion resistance of welds, due to the formation of Cr carbides, deteriorates high-temperature sliding properties due to the carbides, and increases surface roughness due to the formation of intergranular erosion grooves during pickling of cold-rolled annealed sheets. Therefore, the C content is set to 0.300% or less. The C content may be 0.270% or less, 0.250% or less, 0.210% or less, 0.170% or less, or 0.150% or less.

[0032] (Si: 5.00% or less) Si acts as a deoxidizing element. It may be added to improve oxidation resistance, steam oxidation resistance, and high-temperature sliding properties through the internal oxidation of Si, and to improve high-temperature strength and creep life by increasing the frequency of corresponding grain boundaries. Deoxidation can also be achieved with other elements, and Si is not an essential element, so the Si content may be 0%. To achieve the above effect, the Si content is preferably 0.01% or more. In consideration of hot workability during steel sheet production, the Si content may be 0.20% or more, or 0.50% or more. In terms of reducing stacking fault energy, the Si content may be 1.00% or more, 1.20% or more, or 1.50% or more. In consideration of high-temperature sliding properties, the Si content may be 1.80% or more, 1.90% or more, or 2.00% or more. On the other hand, if the Si content is high, the steel becomes hard and coarse Si-based oxides are generated, significantly reducing the machining accuracy of parts, so the Si content is set to 5.00% or less. In consideration of pickling properties during steel sheet production, the Si content may be 4.80% or less, 4.50% or less, or 4.00% or less. In consideration of precipitate suppression and solidification cracking properties during welding, the Si content may be 3.50% or less, 3.40% or less, or 3.30% or less.

[0033] (Mn: 10.00% or less) Mn acts as a deoxidizing element and may be added to ensure austenite structure formation and improve scale adhesion. Deoxidation can also be performed with other elements, and Mn is not an essential element, so the Mn content may be 0%. To obtain the above effect, the Mn content is preferably 0.01% or more. The Mn content may be 0.10% or more, 0.40% or more, 0.70% or more, 1.50% or more, 2.00% or more, or 2.50% or more. On the other hand, if the Mn content is high, the inclusion cleanliness significantly deteriorates, workability and hole expandability decrease, and pickling ability significantly deteriorates, resulting in a rough surface of the Fe—Cr—Ni steel sheet. Therefore, the Mn content is set to 10.00% or less. From the viewpoint of effectively reducing stacking fault energy, the Mn content may be 9.40% or less, 8.80% or less, 8.00% or less, 7.50% or less, 7.00% or less, or 6.50% or less. In consideration of pickling properties and oxidation characteristics during steel sheet production, the Mn content may be 6.00% or less, or 5.00% or less.

[0034] (P: 0.050% or less) P is an impurity and an element that promotes hot workability and solidification cracking during manufacturing, as well as hardening and reducing ductility, so the P content is set to 0.050% or less. The P content may be 0.045% or less, 0.040% or less, 0.035% or less, or 0.030% or less. The lower the P content, the better, and it may be 0%. In consideration of refining costs, the P content may be 0.001% or more, 0.002% or more, 0.005% or more, 0.010% or more, 0.015% or more, or 0.020% or more.

[0035] (S: 0.0100% or less) S is an impurity and an element that reduces hot workability during manufacturing and also deteriorates corrosion resistance. Furthermore, when coarse sulfides (MnS) are formed, the inclusion cleanliness significantly deteriorates and room-temperature ductility deteriorates, so the S content is set to 0.0100% or less. The S content may be 0.0090% or less, 0.0080% or less, 0.0065% or less, 0.0050% or less, 0.0040% or less, or 0.0030% or less. The lower the S content, the better, and it may be 0%. Taking refining costs into consideration, the S content may be 0.0001% or more, 0.0002% or more, 0.0003% or more, 0.0004% or more, or 0.0005% or more.

[0036] (Ni: 2.00 to 40.00%) Ni is added to form an austenite structure and ensure corrosion resistance and oxidation resistance. Since a low Ni content significantly coarsens crystal grains, the Ni content is set to 2.00% or more. Considering manufacturability, room-temperature ductility, and corrosion resistance, the Ni content may be 3.20% or more, 3.50% or more, 4.50% or more, 5.00% or more, 8.00% or more, or 10.00% or more. Since a high Ni content increases costs and also increases stacking fault energy, the Ni content is set to 40.00% or less. From the viewpoint of reducing stacking fault energy and effectively increasing the frequency of coincidence grain boundaries, the Ni content may be 37.50% or less, or 35.00% or less. From the standpoint of cost, the Ni content may be 30.00% or less, 25.00% or less, 22.00% or less, 20.00% or less, 16.00% or less, or 14.00% or less.

[0037] (Cr: 14.00 to 30.00%) Cr is an element that improves corrosion resistance, oxidation resistance, and high-temperature sliding properties, and is an element necessary from the viewpoint of suppressing abnormal oxidation in consideration of the exhaust part environment. Cr is also an element that is effective in improving high-temperature strength. To achieve this effect, the Cr content is set to 14.00% or more. From the viewpoint of steel sheet manufacturability and processability, the Cr content may be 14.50% or more, 15.00% or more, 16.00% or more, 17.00% or more, 18.50% or more, or 20.00% or more. If the Cr content is high, the steel becomes hard, which deteriorates formability and leads to increased costs. The Cr content is set to 30.00% or less. In consideration of the manufacturing cost and stacking fault energy, the Cr content may be 29.00% or less, 28.00% or less, 26.00% or less, 25.00% or less, or 24.00% or less.

[0038] (N: 0.400% or less) N, like C, is an effective element for forming austenite structure and ensuring high-temperature strength, creep, and high-temperature sliding properties, and may be added. N is not an essential element, and the N content may be 0%. N is known as a solid-solution strengthening element with respect to high-temperature strength, and N is also effective in twin formation. To achieve this effect, the N content is preferably 0.001% or more. In addition to its independent effect, N also exerts the effect of improving high-temperature strength by forming clusters with Cr, so the N content may be 0.005% or more, or 0.010% or more. From the viewpoint of suppressing softening of the steel, the N content may be 0.020% or more, or 0.030% or more. From the viewpoints of high-temperature strength, creep properties, and high-temperature sliding properties, the N content may be 0.040% or more, 0.050% or more, 0.070% or more, 0.100% or more, or 0.150% or more. A high N content significantly hardens the room-temperature material, degrading cold workability during steel plate manufacturing and also impairing formability and part precision during part processing, so the N content is set to 0.400% or less. From the viewpoints of suppressing pinholes during welding and intergranular corrosion of welds, the N content may be 0.370% or less or 0.360% or less. From the viewpoint of room-temperature ductility, the N content may be 0.350% or less, 0.345% or less, 0.340% or less, 0.320% or less, 0.300% or less, 0.280% or less, 0.250% or less, or 0.200% or less.

[0039] (Al: 0.001 to 1.000%) Al acts as a deoxidizing element and is added to improve inclusion cleanliness, thereby improving corrosion resistance, oxidation resistance, and hole expandability. It also has the effect of suppressing oxide scale spalling and contributing to improved high-temperature sliding properties through slight internal oxidation. To achieve this effect, the Al content is set to 0.001% or more. Taking refining costs into consideration, the Al content may be 0.007% or more, 0.010% or more, 0.030% or more, 0.050% or more, 0.100% or more, or 0.150% or more. Since Al is a ferrite-forming element, a high Al content reduces the stability of the austenite structure and leads to increased surface roughness due to reduced pickling properties. Therefore, the Al content is set to 1.000% or less. In view of surface defects, the Al content may be 0.900% or less, 0.850% or less, 0.700% or less, 0.600% or less, 0.550% or less, or 0.500% or less. In view of weldability, the Al content may be 0.450% or less, 0.400% or less, 0.350% or less, or 0.300% or less.

[0040] (V: 0.02 to 1.00%) V is an element that improves corrosion resistance, and also promotes the formation of V carbides and σ phases, thereby improving high-temperature strength. To achieve this effect, the V content is set to 0.02% or more. In consideration of manufacturability, the V content may be 0.04% or more, 0.07% or more, 0.10% or more, 0.12% or more, 0.15% or more, 0.18% or more, or 0.25% or more. Since a high V content increases alloy costs and decreases the abnormal oxidation limit temperature, the V content is set to 1.00% or less. In consideration of inclusion cleanliness, the V content may be 0.90% or less, 0.80% or less, 0.70% or less, 0.60% or less, or 0.50% or less.

[0041] (Nb: 0.001 to 1.000%) Nb is added to improve corrosion resistance and intergranular corrosion resistance by bonding with C and N, as with Ti, and to retard the progress of recrystallization and improve high-temperature strength. In addition to its C and N fixing function, the Nb content is set to 0.001% or more to obtain the effects of increasing high-temperature strength due to solute Nb and high strength due to twin boundary precipitation of Laves phases. Considering high-temperature strength and intergranular corrosion resistance of welds, the Nb content may be 0.002% or more, 0.003% or more, 0.005% or more, 0.007% or more, or 0.010% or more. Considering creep properties, the Nb content may be 0.011% or more, 0.015% or more, 0.020% or more, or 0.030% or more. A high Nb content significantly deteriorates hot workability during steel sheet production, and also leads to deterioration of ductility and fatigue properties due to coarse Nb carbonitrides and an increase in stacking fault energy. Furthermore, the frequency of coincidence grain boundaries decreases, so the Nb content is set to 1.000% or less. Taking alloy cost into consideration, the Nb content may be 0.900% or less, 0.800% or less, 0.700% or less, 0.600% or less, or 0.500% or less. Taking stacking fault energy into consideration, the Nb content may be 0.400% or less, 0.300% or less, 0.200% or less, 0.100% or less, 0.080% or less, or 0.050% or less.

[0042] (Co: 0.001 to 1.000%) Co is added because it contributes to improving high-temperature strength and increasing the frequency of twinning by reducing stacking fault energy, thereby contributing to improving high-temperature properties. From the viewpoint of improving stacking fault energy and high-temperature properties, the Co content is set to 0.001% or more. In consideration of refining costs, the Co content may be 0.010% or more, 0.030% or more, 0.050% or more, 0.070% or more, or 0.100% or more. A high Co content leads to hardening, deterioration of toughness during steel plate production, and increased costs, so the Co content is set to 1.000% or less. In consideration of manufacturability, the Co content may be 0.900% or less, 0.800% or less, 0.650% or less, 0.500% or less, 0.400% or less, 0.300% or less, or 0.250% or less.

[0043] The Fe-Cr-Ni steel sheet of the present invention may contain the following elements in addition to the elements described above.

[0044] (Cu: 0 to 4.00%) Cu is an element effective for stabilizing and softening the austenite phase, and therefore may be added as needed. Since Cu is not an essential element, the Cu content may be 0%. To achieve the above effect, the Cu content is preferably 0.001% or more. To stabilize the austenite phase, the Cu content may be 0.005% or more, 0.01% or more, 0.02% or more, 0.03% or more, or 0.05% or more. In consideration of corrosion resistance, the Cu content may be 0.08% or more, 0.10% or more, 0.15% or more, or 0.20% or more. A high Cu content leads to deterioration in oxidation resistance and manufacturability, and also to deterioration in elongation. Furthermore, since stacking fault energy increases and the frequency of annealing twins decreases, the Cu content is set to 4.00% or less. In consideration of manufacturability, the Cu content may be 3.75% or less, 3.50% or less, 3.00% or less, 2.50% or less, 2.00% or less, 1.50% or less, or 1.00% or less.

[0045] (Mo: 0 to 5.00%) Mo may be added as needed to improve corrosion resistance and high-temperature strength. Since Mo is not an essential element, the Mo content may be 0%. While solid solution strengthening is the primary factor in improving high-temperature strength, Mo also contributes to fine precipitation strengthening at twin boundaries of Mo carbides because it is an element that promotes precipitation of σ phases and the like. To achieve this effect, the Mo content is preferably 0.001% or more. Considering the strengthening stability due to precipitation, the Mo content may be 0.005% or more, 0.01% or more, 0.05% or more, 0.10% or more, 0.20% or more, 0.30% or more, 0.40% or more, or 0.50% or more. Since a high Mo content increases stacking fault energy and reduces the frequency of annealing twins and also reduces the frequency of coincidence grain boundaries, the Mo content is set to 5.00% or less. In consideration of inclusion cleanliness, the Mo content may be 4.50% or less, 4.00% or less, 3.50% or less, or 3.00% or less. In consideration of abnormal oxidation characteristics, the Mo content may be 2.50% or less, 2.00% or less, or 1.50% or less.

[0046] (Ti: 0 to 1.000%) Ti may be added as needed to improve corrosion resistance and intergranular corrosion resistance by bonding with C and N. Since Ti is not an essential element, the Ti content may be 0%. Although the effect of Ti inclusion can be obtained even in small amounts, if Ti is included, the Ti content is preferably 0.001% or more. In consideration of high-temperature strength and intergranular corrosion resistance of welds, the Ti content may be 0.005% or more, 0.010% or more, 0.015% or more, 0.020% or more, or 0.025% or more. In terms of creep properties, the Ti content may be 0.030% or more, 0.035% or more, or 0.040% or more. A high Ti content makes nozzle clogging more likely to occur during the casting stage, significantly degrading manufacturability, and also leads to deterioration of ductility and fatigue properties due to the formation of coarse Ti carbonitrides, so the Ti content is set to 1.000% or less. In consideration of alloy cost, the Ti content may be 0.900% or less, 0.800% or less, 0.650% or less, 0.500% or less, 0.400% or less, or 0.300% or less.

[0047] (B: 0 to 0.0100%) B is an element that improves hot workability in the steel sheet manufacturing stage and may be added as needed. Since B is not an essential element, the B content may be 0%. Furthermore, high strength is also achieved through twin boundary segregation of B. The effect of containing B can be obtained even in small amounts, but when B is contained, the B content is preferably 0.0001% or more. In consideration of refining costs, the B content may be 0.0002% or more, 0.0003% or more, or 0.0005% or more. A high B content leads to the formation of borocarbides, which reduces cleanliness and ductility and deteriorates intergranular corrosion resistance, so the B content is set to 0.0100% or less. In consideration of the decrease in ductility, the B content may be 0.0090% or less, 0.0080% or less, 0.0060% or less, 0.0030% or less, 0.0020% or less, or 0.0010% or less.

[0048] (Ca: 0 to 0.0100%) Ca is added as needed for desulfurization. Since Ca is not an essential element, the Ca content may be 0%. Although the effect of Ca inclusion can be obtained even with a small amount, if Ca is added, the Ca content is preferably 0.0001% or more. From the viewpoint of manufacturability, the Ca content may be 0.0002% or more, 0.004% or more, 0.006% or more, or 0.0010% or more. A high Ca content generates water-soluble inclusions CaS, which leads to a decrease in cleanliness and a significant decrease in corrosion resistance, so the Ca content is set to 0.0100% or less. From the viewpoint of surface quality, the Ca content may be 0.0080% or less, 0.0060% or less, 0.0050% or less, 0.0040% or less, 0.0030% or less, or 0.0020% or less.

[0049] (W: 0 to 3.00%) W contributes to improving corrosion resistance and high-temperature strength, so it may be added as needed. Since W is not an essential element, the W content may be 0%. Although the effects of containing W can be obtained even in small amounts, if W is added, the W content is preferably 0.001% or more. Furthermore, in consideration of refining costs, the W content may be 0.005% or more, 0.01% or more, 0.02% or more, 0.05% or more, 0.08% or more, or 0.10% or more. Since a high W content leads to hardening, deterioration of toughness during steel plate production, and increased costs, the W content is set to 3.00% or less. In consideration of manufacturability, the W content may be 2.80% or less, 2.50% or less, or 2.00% or less. In consideration of abnormal oxidation properties, the W content may be 1.50% or less, 1.40% or less, 1.20% or less, or 1.00% or less.

[0050] (Zr: 0 to 0.30%) Zr may be added as needed to improve the intergranular corrosion resistance and oxidation resistance of welds by bonding with C and N. Since Zr is not an essential element, the Zr content may be 0%. Although the effects of containing Zr can be obtained even in small amounts, if Zr is contained, the Zr content is preferably 0.001% or more. In consideration of refining costs, the Zr content may be 0.005% or more, 0.01% or more, 0.02% or more, 0.05% or more, 0.08% or more, or 0.10% or more. Since a high Zr content not only increases costs but also significantly deteriorates manufacturability and hole expandability, the Zr content is set to 0.30% or less. In consideration of manufacturability, the Zr content may be 0.28% or less, 0.26% or less, 0.23% or less, 0.20% or less, or 0.18% or less.

[0051] (Sn: 0 to 0.50%) Sn contributes to improving corrosion resistance and high-temperature strength, so it may be added as needed. Since Sn is not an essential element, the Sn content may be 0%. Although the effect of containing Sn can be obtained even in small amounts, when Sn is contained, the Sn content is preferably 0.001% or more. In consideration of refining costs, the Sn content may be 0.005% or more, 0.01% or more, 0.02% or more, 0.03% or more, 0.05% or more, or 0.08% or more. Since a high Sn content may cause slab cracking during steel plate production, the Sn content is set to 0.50% or less. In consideration of manufacturability, the Sn content may be 0.45% or less, 0.40% or less, 0.35% or less, 0.30% or less, or 0.25% or less.

[0052] (Mg: 0 to 0.0100%) Mg is sometimes added as a deoxidizing element, and also contributes to improving the cleanliness of inclusions and refining the structure of the slab by refining and dispersing oxides. Since Mg is not an essential element, the Mg content may be 0%. Although the effect of containing Mg can be obtained even in small amounts, when Mg is contained, the Mg content is preferably 0.0001% or more. In consideration of refining costs, the Mg content may be 0.0002% or more, 0.0003% or more, 0.0004% or more, 0.0005% or more, or 0.0007% or more. Since a high Mg content leads to deterioration of weldability and corrosion resistance and reduction of hole expandability due to coarse inclusions, the Mg content is set to 0.0100% or less. The Mg content may be 0.0090% or less, 0.0080% or less, 0.0065% or less, 0.0050% or less, 0.0035% or less, or 0.0025% or less.

[0053] (Sb: 0 to 0.300%) Sb is an element that segregates at grain boundaries to increase high-temperature strength and may be added as needed. Since Sb is not an essential element, the Sb content may be 0%. Although the effect of containing Sb can be obtained even in small amounts, if Sb is contained, the Sb content is preferably 0.001% or more. In consideration of high-temperature properties, the Sb content may be 0.002% or more, 0.005% or more, 0.010% or more, 0.020% or more, 0.030% or more, or 0.035% or more. Since a high Sb content causes Sb segregation and cracks during welding, the Sb content is set to 0.300% or less. In consideration of manufacturing cost and toughness, the Sb content may be 0.280% or less, 0.250% or less, 0.200% or less, 0.150% or less, or 0.100% or less.

[0054] (REM: 0 to 0.200%) REM (rare earth elements) are effective in improving oxidation resistance and high-temperature sliding properties, and may be added as needed. REM is not an essential element, so the REM content may be 0%. The effect of containing REM can be obtained even in small amounts, but if REM is contained, the REM content is preferably 0.001% or more. In consideration of workability, the REM content may be 0.002% or more, 0.003% or more, 0.005% or more, 0.010% or more, 0.015% or more, or 0.020% or more. If the REM content increases, the effect saturates and REM granules cause a decrease in corrosion resistance, so the REM content is set to 0.200% or less. Taking production costs into consideration, the content may be 0.180% or less, 0.160% or less, 0.130% or less, 0.100% or less, 0.080% or less, or 0.060% or less. Note that, according to the general definition, REM (rare earth element) refers to two elements, scandium (Sc) and yttrium (Y), and 15 elements (lanthanoids) from lanthanum (La) to lutetium (Lu). They may be added alone or as a mixture.

[0055] (Ga: 0 to 0.3000%) Ga may be added as needed to improve corrosion resistance and suppress hydrogen embrittlement. Since Ga is not an essential element, the Ga content may be 0%. The effect of containing Ga can be obtained even with a small amount, but if it is contained, the Ga content is preferably 0.0001% or more from the viewpoint of sulfide and hydride formation. The Ga content may be 0.0002% or more, 0.0005% or more, 0.0010% or more, 0.0020% or more, 0.0050% or more, 0.1000% or more, or 0.1500% or more. If the Ga content is high, coarse sulfides are generated and the r-value deteriorates, so the Ga content is set to 0.3000% or less. The Ga content may be 0.2900% or less, 0.2800% or less, 0.2600% or less, 0.2500% or less, or 0.2400% or less.

[0056] The present invention does not particularly specify other components. For example, Ta and Hf may be added in amounts of 0.01% to 1.00% to improve high-temperature strength. Since Ta and Hf are not essential elements, their contents may be 0%. The Ta and Hf contents may be 0.02% or more, 0.05% or more, 0.10% or more, or 0.20% or more, respectively. The Ta and Hf contents may be 0.80% or less, 0.60% or less, 0.50% or less, or 0.40% or less, respectively. Furthermore, Bi may be contained in an amount of 0.001 to 0.020% as needed. The Bi content may be 0.002% or more, 0.003% or more, or 0.005% or more. The Bi content may be 0.018% or less, 0.016% or less, or 0.015% or less.

[0057] It is preferable to reduce the contents of common harmful elements and impurity elements such as As and Pb as much as possible, and the contents of As and Pb may be 0%. The contents of As and Pb may be 0.010% or less, 0.008% or less, 0.006% or less, 0.004% or less, or 0.002% or less, respectively.

[0058] The balance of the chemical composition other than the above elements is Fe and inevitable impurities. Here, inevitable impurities refer to components that are mixed in due to various factors in the manufacturing process, including raw materials such as ores and scraps, when industrially manufacturing steel sheets, and that do not adversely affect the properties of the hot-rolled steel material of the present invention, i.e., are contained in a range that allows the hot-rolled steel sheet of the present invention to obtain desired properties such as workability.

[0059] <Stacking fault energy (SFE) value is 50 (mJm -2 ) or less> The chemical composition further has a stacking fault energy (SFE) value calculated by the following formula (1) of 50 (mJm -2 ) or less. This makes it easier to generate annealing twins even with short-term heat treatment, and makes it possible to increase the frequency of coincidence grain boundaries.

[0060] SFE (mJm -2) = 0.5 × Cr + 1.4 × Ni + 3.2 × Nb + 0.1 × Mo + 3.6 × Cu - 3.4 × Si - 0.6 × Co (1) In the formula, the element symbol indicates the content (mass%) of the element, and 0 is substituted when the element is not contained.

[0061] In order to lower the SFE value, it is effective to decrease the Cr, Ni, Nb, Mo, and Cu and increase the Si and Co, but an excessive decrease in the SFE value significantly deteriorates manufacturability, so the SFE value is preferably -50 or more. The SFE value may be -35 or more, -20 or more, -5 or more, 0 or more, 5 or more, 10 or more, or 20 or more. Taking into consideration creep properties, oxidation properties, and the like, the SFE value may be 45 or less, 40 or less, 35 or less, or 30 or less.

[0062] <The total frequency of coincidence boundaries with Σ values ​​of 3 to 29 is 50% or more>

[0063] Important properties of Fe-Cr-Ni steel sheets used for heat-resistant applications are high-temperature strength and creep properties. In particular, when a complex shape, such as a turbocharger housing, is used in a high-temperature environment, excessive deformation can cause contact between parts and poor gas flow, leading to breakage and reduced thermal efficiency, and thus to a decrease in the reliability of part performance. In Fe-Cr-Ni steel sheets, the precipitation of Cr carbides and other precipitates at random grain boundaries and the propagation of creep voids along random grain boundaries to agglomerate and coarsen are considered to be grain boundary degradation, both of which cause deterioration of creep properties and high-temperature properties.

[0064] In the Fe-Cr-Ni of the present invention, in order to achieve excellent heat resistance, the frequency of coincidence boundaries with a Σ value of 3 to 29 is set to 50% or more in total, taking into account the relationship between KAM and HAGB described later.

[0065] In Fe-Cr-Ni steel, grain boundaries are formed after cold rolling and annealing, resulting in a polycrystalline body. As mentioned above, when the relative orientation difference between two adjacent grains is 15 degrees or more and the grain boundary is a coincidence boundary, the grain boundary has a low-energy structure with a regular atomic arrangement and few gaps, making it less susceptible to grain boundary degradation. The Σ value of the coincidence lattice formed by two adjacent grains is 3 to 29, and grain boundaries formed in coincidence lattices within this range are considered to be coincidence grain boundaries. Furthermore, the smaller the Σ value, the higher the density of coincidence lattice points, and the stronger the characteristics of a low-energy grain boundary.

[0066] On the other hand, random grain boundaries, where the relative misorientation between two adjacent crystal grains is 15 degrees or more and the atomic arrangement is irregular and high energy, are prone to grain boundary degradation. Here, a grain boundary where the relative misorientation between two adjacent crystal grains is less than 15 degrees is defined as a low-angle grain boundary, a grain boundary where the relative misorientation between two adjacent crystal grains is 15 degrees or more is defined as a high-angle grain boundary, and a high-angle grain boundary other than a coincidence boundary is defined as a random grain boundary. Since the sum of the low-angle grain boundary frequency and the high-angle grain boundary frequency is 100%, in order to increase the coincidence boundary frequency, it is preferable to reduce the low-angle grain boundary frequency and increase the high-angle grain boundary frequency to reduce the random grain boundary frequency.

[0067] The Σ3 grain boundary, which is the lowest Σ grain boundary among the correspondence grain boundaries, is made up of annealing twins. Annealing twins are generated during heat treatment and are closely related to stacking fault energy caused by steel components. Therefore, from the perspective of preventing grain boundary degradation, it is preferable that the proportion of correspondence grain boundaries, especially the proportion of annealing twins, among the crystal grain boundaries of the entire material is high.

[0068] The higher the coincidence boundary frequency, the higher the proportion of grain boundaries with a regular atomic arrangement and a low-energy structure with few gaps, making it less likely that grain boundary degradation will occur even at high temperatures, resulting in improved high-temperature strength and creep life. Furthermore, the lower the random grain boundary frequency, the lower the proportion of grain boundaries with an irregular atomic arrangement and a high-energy structure, making it less likely that grain boundary degradation will occur. In particular, considering percolation theory, from the perspective of interrupting the random grain boundary network with coincidence boundaries and suppressing the progression of continuous grain boundary degradation, the random grain boundary network can be interrupted by setting the coincidence boundary frequency to 50% or more.

[0069] The coincidence boundary frequency of the coincidence boundaries having a Σ value of 3 to 29 may be 55% or more, 60% or more, 65% or more, 70% or more, or 75% or more in total. The random boundary frequency is preferably 50% or less, and may be 45% or less, 40% or less, 35% or less, 30% or less, or 25% or less.

[0070] (Method for measuring the frequency of coincidence grain boundaries with a Σ value of 3 to 29) The frequency of coincidence grain boundaries is determined as the ratio of the length of the coincidence grain boundaries to the total length of the grain boundaries in the cross section of the material. Specifically, a length x thickness cross section of a plate-shaped sample with a size of approximately 10 mm long x 5 mm wide x thickness is colloidally finished, and a FE-SEM (manufactured by JEOL) / EBSD (Electron Back-Scattering Diffraction pattern) / OIM (manufactured by TSL) is used to perform crystal orientation analysis on a 100 μm long x 300 μm thick region within a range of approximately 1 / 4 to 1 / 2 of the plate thickness from the center of the plate thickness of the material at an acceleration voltage of 25 kV, a working distance of 14 mm, a measurement magnification of 500 times, and a measurement interval of 1.0 μm, and the total length of the grain boundaries and the length of the coincidence grain boundaries present in the observed range are measured. The thickness of the plate-shaped sample may be, for example, 2 mm or less depending on the thickness of the Fe—Cr—Ni steel plate. Even when the thickness of the Fe-Cr-Ni steel plate is 0.3 μm or less, the sample may be taken so that the total measurement area of ​​a plurality of fields is 100 μm long×300 μm thick.

[0071] <KAM / HAGB≦0.010> The structure of the Fe—Cr—Ni steel sheet of the present invention satisfies KAM / HAGB≦0.010. Here, the average value of the crystal orientation misorientation between the target measurement point and its surrounding measurement points is referred to as the KAM value, and KAM in the above formula is the proportion of KAM values ​​of 1 to 2 degrees relative to all measurement points. Furthermore, HAGB in the above formula is the proportion of high-angle grain boundaries, and is a value obtained by dividing the length of grain boundaries with a relative misorientation of 15 degrees or more by the total grain boundary length, i.e., the length of grain boundaries with a relative misorientation of 2 degrees or more. When the coincidence boundary frequency of coincidence boundaries with a Σ value of 3 to 29 is 50% or more in total and the structure satisfies KAM / HAGB≦0.010, high-temperature strength can be increased.

[0072] A small KAM means that there is little residual strain and that recrystallization has been completed normally. Therefore, it is thought that high-temperature deformation is less likely to occur and high-temperature strength is higher. The smaller the residual strain, the larger the HAGB value, approaching 1.0 (=100%). In other words, the smaller the KAM / HAGB ratio, the less residual strain there is in the Fe-Cr-Ni steel sheet and the higher the high-temperature strength. Furthermore, if KAM / HAGB is large and recrystallization has not been completed normally, the steel may harden and its elongation may deteriorate. By satisfying KAM / HAGB≦0.010, good elongation properties can be obtained.

[0073] (Method for measuring KAM and HAGB) KAM and HAGB are measured by the following procedure. A cross section (L cross section) parallel to the rolling direction and thickness direction of an Fe-Cr-Ni steel sheet is used as the observation surface, and mechanical polishing and mechanical-chemical polishing using a colloidal silica suspension or the like are performed to remove surface strain. After such polishing is performed to obtain a mirror-like observation surface, the surface is observed and measured using a field emission scanning electron microscope (manufactured by JEOL, hereinafter referred to as "FE-SEM") equipped with an EBSD (Electron Back-Scattering Diffraction pattern) measuring device. KAM and HAGB are then determined by analysis using an OIM (Orientation Imaging Microscopy, manufactured by TSL). The software used is "OIM Analysis." The measurement range is the region from the center of the sheet thickness to 1 / 4 of the sheet thickness, with one field of view up to 1 mm 2 The measurement is carried out in 10 fields of view at a measurement magnification of 200 times and a measurement interval of 1.5 μm.

[0074] <Method for Manufacturing Fe—Cr—Ni Steel Sheet> Next, a method for manufacturing the Fe—Cr—Ni steel sheet of the present invention will be described.

[0075] The method for producing a steel sheet of the present invention comprises steelmaking - hot rolling - annealing and pickling - first cold rolling - first annealing and pickling - second cold rolling - second annealing and pickling. Annealing after hot rolling may be omitted if necessary.

[0076] In steelmaking, a suitable method is to melt steel containing the essential elements and optional additive elements in an electric furnace or converter, followed by secondary refining. The resulting molten steel is formed into a slab using a known casting method (continuous casting), and the slab is heated to a predetermined temperature and hot-rolled to a predetermined thickness using a known hot-rolling method. As described above, for the components to which the present invention is applied, manufacturing conditions are set in the processes after hot rolling according to known methods to ensure the desired grain size, cross-sectional hardness, and surface roughness. In this application, the following manufacturing conditions are specified to ensure that the coincidence boundary frequency of coincidence boundaries with a Σ value of 3 to 29 is 50% or more and that KAM / HAGB is ≦0.010.

[0077] After hot rolling or after annealing and pickling following the hot rolling step, the manufacturing method of the present invention includes two cold rolling steps and an annealing and pickling step.

[0078] (First Cold Rolling Step) In the first cold rolling step, the hot-rolled steel sheet obtained in the hot rolling step is cold-rolled at a rolling reduction of 80% or less to obtain a cold-rolled steel sheet. If the rolling reduction in this step exceeds 80%, grain growth in the subsequent recrystallization is suppressed, and the frequency of coincidence boundaries with a Σ value of 3 to 29 is less likely to increase. In addition, the strain introduced during processing increases, and elongation decreases. On the other hand, if the rolling reduction is too low, recrystallization is less likely to occur, so the rolling reduction is preferably 3% or more. Taking into account the material and sheet shape, the rolling reduction may be 50% or more, 60% or more, 65% or more, 70% or more, or 75% or more.

[0079] (First Annealing and Pickling Process) Next, in the first annealing and pickling process, in order to increase the frequency of correspondence grain boundaries, particularly annealing twins, after the second cold rolling, annealing, and pickling process, the cold-rolled steel sheet is annealed in the temperature range of 950 to 1300°C, with a holding time of 120 seconds or less at 950°C or higher. To obtain a metal structure with a high frequency of correspondence grain boundaries after the second cold rolling, annealing, and pickling process, it is preferable that the metal structure after the first cold rolling, annealing, and pickling process be as fine-grained as possible. Therefore, grain growth is suppressed by setting the holding time at 950°C or higher, at which recrystallization occurs and grain growth occurs, to 120 seconds or less. If grain growth occurs excessively, new correspondence grain boundaries are not formed, and the number of correspondence grain boundaries is unlikely to increase even when performing the second cold rolling, etc., as described below. Furthermore, the holding time is set to 120 seconds or less to prevent deterioration of the elongation of the steel.

[0080] Since an excessively short holding time results in poor recrystallization, a holding time of 1 second or more at 950°C or higher is preferred. Considering the stability of the material, the holding time at 950°C or higher may be 5 seconds or more, 10 seconds or more, 15 seconds or more, or 20 seconds or more. Also, considering the stability of the material, the holding time at 950°C or higher may be 110 seconds or less, 100 seconds or less, 90 seconds or less, 80 seconds or less, or 70 seconds or less.

[0081] If the annealing temperature exceeds 1300°C, excessive grain growth occurs, resulting in material degradation, particularly deterioration in elongation and orange peel during processing. In the case of turbo components, the occurrence of orange peel leads to poor high-temperature sliding properties and exhaust gas flow. Therefore, the annealing temperature is set to 1300°C or lower. On the other hand, if the annealing temperature is lower than 950°C, recrystallization does not complete normally, resulting in increased residual strain, increasing KAM and increasing KAM / HAGB. Therefore, the annealing temperature is set to 950°C or higher. Considering the stability of the material, the annealing temperature may be 1000°C or higher, 1050°C or higher, or 1100°C or higher. Furthermore, considering the stability of the material, the annealing temperature may be 1250°C or lower, 1230°C or lower, or 1180°C or lower.

[0082] (Second Cold Rolling Step) In the second cold rolling step, the reduction ratio in cold rolling is set to 10% or less. If the reduction ratio in this step exceeds 10%, recrystallization will proceed in the subsequent annealing step, and new random grain boundaries will be formed, thereby reducing the frequency of coincidence grain boundaries. Since an excessive reduction in the reduction ratio will result in a poor steel sheet shape, the reduction ratio is preferably 1% or more. In consideration of manufacturability and the formation of annealing twins, the reduction ratio may be 2% or more, or 3% or more. In consideration of manufacturability and the formation of annealing twins, the reduction ratio may be 8% or less, or 7% or less.

[0083] (Second Annealing and Pickling Process) In the second annealing and pickling process, a two-stage heat treatment is carried out in order to increase the frequency of coincidence grain boundaries, particularly annealing twins.

[0084] The first stage heat treatment is performed at a maximum temperature of 900 to 1000°C, with a heating rate up to 900°C of 10°C / sec or more, and a holding time at the maximum temperature of less than 60 seconds. The second stage heat treatment is performed at a maximum temperature of 1000 to 1200°C, with a heating rate up to the maximum temperature of less than 10°C / sec, and a holding time from 1000°C to the maximum temperature of 60 seconds or more but less than 50 hours. The first and second stage heat treatments are performed consecutively.

[0085] The first stage heat treatment in the second annealing after cold rolling is intended to suppress the formation of precipitates that inhibit the formation of coincidence boundaries and to raise the temperature to the temperature range where coincidence boundaries begin to form. The subsequent second stage heat treatment is intended to promote the formation of coincidence boundaries by grain growth, suppress the formation of random grain boundaries due to new recrystallization, and obtain a metal structure with a high frequency of coincidence boundaries. Each heat treatment is explained below.

[0086] [First-stage heat treatment] In the first-stage heat treatment, the heating rate is increased in the temperature range up to 900°C to suppress the formation of σ phase and Cr carbonitrides, which precipitate at relatively low temperatures. These precipitates not only inhibit the formation of coincidence grain boundaries but also serve as nuclei for newly formed random grain boundaries. Furthermore, if the number of precipitates becomes too large, elongation deteriorates. From the above technical viewpoint, the heating rate is set to 10°C / sec or more. From the viewpoint of manufacturability, a heating rate of 100°C / sec or less is preferable. The heating rate may be 15°C / sec or more, or 20°C / sec or more. The heating rate may also be 90°C / sec or less, 80°C / sec or less, or 70°C / sec or less.

[0087] Furthermore, by setting the maximum temperature to 900 to 1000°C and the holding time at the maximum temperature to 60 seconds or less, coarsening of precipitates is suppressed before the start of recrystallization. If the maximum temperature is less than 900°C, recrystallization will not be completed normally, residual strain will increase, KAM will increase, and KAM / HAGB will increase. Furthermore, during the temperature rise in the subsequent second-stage heat treatment, precipitates that hinder the improvement of the frequency of corresponding grain boundaries will be generated. The maximum temperature may be 950°C or higher. If the maximum temperature is more than 1000°C, the generation and coarsening of precipitates will be promoted during the first-stage heat treatment.

[0088] A long holding time at the maximum temperature promotes coarsening of precipitates and inhibits grain growth during the second heat treatment, so a shorter holding time is preferable. The holding time at the maximum temperature may be 0.5 seconds or more, 1 second or more, 5 seconds or more, 7 seconds or more, or 10 seconds or more. The holding time at the maximum temperature may be 50 seconds or less, 40 seconds or less, or 30 seconds or less.

[0089] [Second-Stage Heat Treatment] In the second-stage heat treatment, the heating rate from the temperature of the first-stage heat treatment to the maximum temperature is slowed down to promote the movement of dislocations during the temperature rise, leading to the movement of random grain boundaries and the formation of coincidence grain boundaries. Therefore, the heating rate to the maximum temperature is set to less than 10°C / sec. From the viewpoint of manufacturability, the heating temperature is preferably 0.1°C / sec or higher, and may be 0.5°C / sec or higher, 1°C / sec or higher, 2°C / sec or higher, 3°C / sec or higher, or 5°C / sec or higher. Furthermore, during the holding period from 1000°C to the maximum temperature, random grain boundaries migrate and grain growth occurs, reducing the random grain boundary frequency, while coincidence grain boundaries such as annealing twins are formed, increasing the coincidence grain boundary frequency.

[0090] If the maximum temperature of the heat treatment is excessively high, recrystallization with new random grain boundaries occurs, reducing the frequency of coincidence boundaries. Furthermore, if the holding temperature of the heat treatment is excessively high or the holding time is excessively long, the crystal grains become coarse, which may lead to deterioration of the material, particularly elongation, and the occurrence of orange peel. If the maximum temperature of the heat treatment is low, recrystallization will not be completed normally, resulting in a large amount of residual strain, an increase in KAM, and an increase in KAM / HAGB. Furthermore, elongation will deteriorate. Therefore, the maximum temperature of the heat treatment is set to 1000 to 1200°C, and from the viewpoint of material properties, the maximum temperature is preferably 1000 to 1150°C. The maximum temperature may be 1120°C or less, 1100°C or less, or 1080°C or less.

[0091] The holding time at 1000°C to the maximum temperature is 60 seconds or more and less than 50 hours. If the holding time is too long or too short, the elongation of the steel deteriorates. Furthermore, if the holding time is too short, the formation of correspondence grain boundaries is not promoted, resulting in a low frequency of correspondence grain boundaries. From the viewpoint of productivity, the holding time at 1000°C to the maximum temperature may be less than 30 hours, less than 24 hours, less than 18 hours, less than 12 hours, less than 10 hours, or less than 5 hours. From the viewpoint of promoting the formation of correspondence grain boundaries and improving the material properties by softening, the holding time may be 90 seconds or more, 120 seconds or more, 180 seconds or more, 300 seconds or more, or 600 seconds or more.

[0092] In the manufacturing method of the present invention, grain growth during annealing after the second cold rolling promotes the formation of coincidence boundaries, and the formation of random grain boundaries due to new recrystallization is suppressed, thereby reducing the frequency of random grain boundaries, resulting in a metal structure with a high frequency of coincidence boundaries. In order to suppress new recrystallization, it is effective to reduce the σ phase and Cr carbonitrides that serve as nuclei for recrystallization, as well as to add elements that form a metal structure that is difficult to recrystallize. In the Fe-Cr-Ni steel sheet of the present invention, Nb is added as described above. In addition, in order to promote grain growth and the formation of coincidence boundaries, it is effective to reduce the σ phase and Cr carbonitrides that have a pinning effect on grain boundaries, anneal at an appropriate temperature, and promote the formation of annealing twins as an alloy component with low stacking fault energy. In the present invention, the SFE value is 50 mJm -2 The alloy composition is as follows:

[0093] In the present invention, a smoother surface can be obtained by cold rolling after hot-rolled sheet annealing and pickling, followed by cold-rolled sheet annealing and pickling. The cold rolling process may be performed by tandem rolling, Sendzimir rolling, cluster rolling, or the like. For functional applications such as automotive exhaust parts, 2B or 2D products are generally used, but when high surface smoothness and gloss are required, bright annealing may be performed after cold rolling to produce a BA product.

[0094] For example, the slab thickness and hot-rolled sheet thickness may be appropriately designed. In cold rolling, the roll roughness, roll diameter, rolling oil, number of rolling passes, rolling speed, rolling temperature, and the like may be appropriately selected. Intermediate annealing may be performed during cold rolling, and either batch annealing or continuous annealing may be used. Furthermore, as a pretreatment for pickling, neutral salt electrolysis or salt bath immersion may be performed or omitted. The pickling process may involve treatment using nitric acid, nitric acid electrolytic pickling, sulfuric acid, or hydrochloric acid. After annealing and pickling of the cold-rolled sheet, shape and material adjustment may be performed using temper rolling, a tension leveler, or the like. Furthermore, a lubricating coating may be applied to the Fe—Cr—Ni steel sheet to further improve press formability, and the type of lubricating film may be appropriately selected. In addition, special surface treatments such as nitriding and carburizing may be performed after part processing to further improve heat resistance.

[0095] The Fe-Cr-Ni steel sheet thus obtained can be suitably used for, for example, exhaust parts, precision internal parts of turbochargers, wastegate valves, or turbo housings.

[0096] Steel was melted and cast into a slab having the chemical composition shown in Table 1, followed by hot rolling, hot-rolled sheet annealing, and pickling. Then, cold rolling and final annealing were performed under the conditions shown in Table 2, and pickling was further performed to obtain a 2.0 mm thick Fe—Cr—Ni steel sheet. The SFE in Table 1 is stacking fault energy, and is the value calculated by the above-mentioned formula (1). In Comparative Example 10, the second cold rolling, annealing, and pickling were omitted, and in Comparative Example 11, the second annealing and pickling were performed, but the second-stage annealing was omitted. In Comparative Example 12, the annealing equivalent to the first annealing and pickling was omitted, and only annealing under conditions equivalent to the second-stage annealing was performed.

[0097] The frequency (%) of coincidence grain boundaries and the frequency (%) of random grain boundaries were measured for each of the obtained Fe-Cr-Ni steel sheets by the methods described above, and a creep test and a high-temperature tensile test were also carried out at 900°C.

[0098] For the creep test, flanged test specimens with a test width of 8 mm and a distance between grades of 35 mm were cut out from 2.0 mm thick Fe-Cr-Ni steel plates so that a load was applied in the rolling direction, and a constant load creep test was carried out at 900°C and 20 MPa using a creep testing machine equipped with a heating furnace. At this time, the no-load time at the test temperature was set to 1 hour, and a load was applied after 1 hour.

[0099] In addition, JIS No. 13B test pieces were cut out from 2.0 mm thick Fe-Cr-Ni steel plates so that tension was applied in the rolling direction, and tensile tests were carried out at room temperature in accordance with JIS Z2241:2011 to determine the fracture elongation.

[0100] For the high-temperature tensile test, a flanged test piece with a test width of 10 mm and a gauge length of 35 mm was cut out from a 2.0 mm thick Fe-Cr-Ni steel plate so that a load was applied in the rolling direction, and a tensile test was performed in accordance with JIS G 0567: 2020 to determine the 0.2% yield strength. For the test, an Autograph AG-100kNX manufactured by Shimadzu Corporation, equipped with an infrared heating furnace, was used.

[0101] In addition, the item "Correspondence boundary frequency" in the table indicates the total frequency (%) of coincidence boundaries with Σ values ​​of 3 to 29, and the item "Random grain boundary frequency" indicates the frequency (%) of random grain boundaries, that is, the difference (%) between the frequency of high-angle grain boundaries, which are grain boundaries where the relative misorientation between adjacent crystal grains is 15 degrees or more, and the frequency of correspondence boundaries with Σ of 3 to 29. The item "KAM / HAGB" indicates the ratio between the proportion of high-angle grain boundaries and the proportion of KAM values ​​of 1 to 2 degrees.

[0102] In Table 3, materials marked with a symbol "◯" in the column for "high temperature strength" have a 0.2% yield strength of 40 MPa or more at 900°C and are judged to have excellent 0.2% yield strength at 900°C. Materials marked with a symbol "X" have a 0.2% yield strength of less than 40 MPa and are judged to have poor 0.2% yield strength.

[0103] In the "Creep test" column of Table 3, materials marked with a "○" have a rupture life of 100 hours or more and are judged to have excellent creep properties at 900°C. Materials marked with an "×" have a rupture life of less than 100 hours and are judged to have poor creep properties at 900°C.

[0104] In Table 3, materials marked with a symbol "○" in the "Elongation at break" column indicate that the elongation at break is 30% or more and that the materials are judged to have excellent room-temperature ductility. Materials marked with a symbol "×" indicate that the elongation at break is less than 30% and that the materials are judged to have poor room-temperature ductility.

[0105] When the evaluations of the high temperature strength, creep test and elongation at break were all "good", it was determined that the object of the present invention was achieved.

[0106]

[0107]

[0108]

[0109] It was confirmed that steels manufactured using the chemical composition and manufacturing conditions specified in the present invention had long creep rupture lives and extremely excellent heat resistance. In addition, they also had high fracture elongation and excellent formability. In contrast, comparative examples whose chemical composition or manufacturing conditions did not satisfy the specifications of the present invention had short creep rupture lives. It was also found that the comparative steels with long creep rupture lives were unsuitable for use as heat-resistant parts because of their inferior fracture elongation and extremely poor formability.

[0110] Specifically, Comparative Example 1 had a high Cr content, which hardened the steel, resulting in poor fracture elongation. It is also believed that the stacking fault energy was large, resulting in little annealing twin formation. As a result, the frequency of coincidence boundaries was low, making it more susceptible to grain boundary degradation at high temperatures, resulting in a shorter creep rupture life.

[0111] Comparative Example 2 is an example with a low Cr content. It is also believed that the maximum temperature of the first annealing stage after the second cold rolling was low, preventing recrystallization from completing normally, resulting in a large amount of residual strain. This resulted in a large KAM and a large KAM / HAGB ratio. As a result, the high-temperature strength was poor.

[0112] Comparative Example 3 had a high Ni content, which resulted in reduced ductility and poor fracture elongation. It is also believed that the stacking fault energy was large, resulting in little annealing twin formation. As a result, the frequency of coincidence grain boundaries was low, making it more susceptible to grain boundary degradation at high temperatures, resulting in a shorter creep rupture life.

[0113] Comparative Example 4 had a high Nb content, which resulted in reduced ductility and poor fracture elongation. It is also believed that the stacking fault energy was large, resulting in little annealing twin formation. As a result, the frequency of coincidence grain boundaries was low, making it more likely that grain boundary degradation would occur at high temperatures, resulting in a shorter creep rupture life.

[0114] Comparative Example 5 had a high Cu content and resulted in poor fracture elongation. It is believed that the stacking fault energy was large and the formation of annealing twins was small. As a result, the frequency of coincidence grain boundaries was low, which made the grain boundary degradation phenomenon at high temperatures more likely to occur, resulting in a shorter creep rupture life.

[0115] Comparative Example 6 had a high Mo content, resulting in excessively high strength and poor fracture elongation. It is believed that the stacking fault energy was large, resulting in little annealing twin formation. As a result, the frequency of coincidence grain boundaries was low, making it more susceptible to grain boundary degradation at high temperatures, resulting in low high-temperature strength and a short creep rupture life.

[0116] It is believed that the high Si content in Comparative Example 7 caused the steel to become hard, resulting in poor breaking elongation.

[0117] Comparative Example 8 had a high Co content, which is thought to have hardened the steel, resulting in poor elongation at break.

[0118] In Comparative Example 9, the contents of the individual elements were within the ranges of the present invention, but the balance of the contents of the elements was unfavorable, resulting in a large stacking fault energy. As a result, the frequency of coincidence grain boundaries was low, making the grain boundary degradation phenomenon at high temperatures more likely to occur, resulting in a low high-temperature strength and a short creep rupture life.

[0119] Comparative Examples 10 to 25 are examples in which the steel compositions were within the range of the present invention, but the manufacturing method was inappropriate, and the Fe-Cr-Ni steel sheets of the present invention could not be obtained.

[0120] Comparative Example 10 is an example that has undergone the combination of cold rolling and annealing only once. As a result, the frequency of coincidence grain boundaries became low, and grain boundary degradation at high temperatures became more likely to occur, resulting in a shorter creep rupture life.

[0121] Comparative Example 11 is an example in which only annealing was performed at a maximum temperature in the range of 900 to 1000°C after the second cold rolling. Therefore, it is thought that random grain boundaries migrated and grain growth occurred, preventing the formation of coincidence boundaries such as annealing twins. As a result, the frequency of coincidence boundaries decreased, making it easier for grain boundary degradation to occur at high temperatures, resulting in a shorter creep rupture life.

[0122] Comparative Example 12 is an example in which only annealing was performed after the second cold rolling, with the maximum temperature being in the range of 1000 to 1200°C. Therefore, it is thought that precipitates were formed before annealing, which inhibited the improvement of the coincidence boundary frequency. As a result, the coincidence boundary frequency decreased, making it easier for grain boundary degradation to occur at high temperatures, and therefore the creep rupture life was shortened.

[0123] In Comparative Example 13, the reduction ratio in the first cold rolling was high, which is thought to have made it difficult for recrystallization to occur during the subsequent annealing. It is also thought that the strain introduced during rolling was large. As a result, the frequency of coincidence grain boundaries decreased, making it easier for grain boundary degradation to occur at high temperatures, resulting in a shorter creep rupture life.

[0124] In Comparative Example 14, the reduction ratio of the second cold rolling was high, and it is thought that recrystallization progressed during the subsequent annealing, leading to the formation of new random grain boundaries. As a result, the frequency of coincidence grain boundaries decreased, making it easier for grain boundary degradation to occur at high temperatures, and therefore shortening the creep rupture life.

[0125] In Comparative Example 15, the annealing temperature after the first cold rolling was low, and recrystallization was not completed normally, resulting in a large amount of residual strain and a large KAM. Therefore, it is believed that KAM / HAGB increased, and the frequency of coincidence grain boundaries did not increase after the second cold rolling and annealing. As a result, the frequency of coincidence grain boundaries decreased, making it easier for grain boundary degradation to occur at high temperatures, resulting in low high-temperature strength and a short creep rupture life. Furthermore, the fracture elongation was poor.

[0126] In Comparative Example 16, the annealing temperature after the first cold rolling was high, causing excessive grain growth, making it difficult for new coincidence boundaries to be formed, and as a result, the number of coincidence boundaries did not increase even after the second cold rolling, reducing the frequency of coincidence boundaries and making it easier for grain boundary degradation to occur at high temperatures, resulting in a shorter creep rupture life and poorer fracture elongation.

[0127] In Comparative Example 17, the holding time at 950°C or higher in the annealing after the first cold rolling was long, which is thought to have caused poor recrystallization. As a result, the frequency of coincidence grain boundaries decreased, making the grain boundary degradation phenomenon at high temperatures more likely to occur, resulting in a shorter creep rupture life. In addition, the fracture elongation was poor.

[0128] In Comparative Example 18, the heating rate in the first annealing stage after the second cold rolling was low, and the heating rate in the second annealing stage was high. This is thought to have prevented the formation of σ-phase and Cr carbonitrides, which precipitate at relatively low temperatures, from being suppressed. Furthermore, dislocation movement during heating was not promoted, preventing random grain boundary movement and the formation of coincidence boundaries. As a result, the frequency of coincidence boundaries was low, making it easier for grain boundary degradation to occur at high temperatures, resulting in a shortened creep rupture life. Furthermore, the amount of Cr carbonitrides precipitated was too high, resulting in poor fracture elongation.

[0129] In Comparative Example 19, the maximum temperature in the first annealing stage after the second cold rolling was low, and recrystallization was not completed normally, resulting in a large amount of residual strain and a large KAM. Therefore, KAM / HAGB increased, and precipitates that inhibited the improvement of the corresponding grain boundary frequency were formed during the subsequent temperature rise in the second heat treatment. As a result, the corresponding grain boundary frequency decreased, making it easier for grain boundary degradation to occur at high temperatures, resulting in low high-temperature strength and a short creep rupture life. Furthermore, the fracture elongation was poor.

[0130] In Comparative Example 20, the maximum temperature in the first annealing stage after the second cold rolling was high, which is thought to have promoted the formation and coarsening of precipitates during the first heat treatment stage, inhibiting grain growth during the second heat treatment stage. As a result, the frequency of coincidence grain boundaries decreased, making it easier for grain boundary degradation to occur at high temperatures, resulting in a shorter creep rupture life. Furthermore, the elongation at break was poor.

[0131] In Comparative Example 21, the holding time at the maximum temperature in the first annealing stage after the second cold rolling was long, which is thought to have promoted the coarsening of precipitates and inhibited grain growth during the second heat treatment stage. As a result, the frequency of coincidence grain boundaries was reduced, making it easier for grain boundary degradation to occur at high temperatures, resulting in a shorter creep rupture life. Furthermore, the fracture elongation was poor.

[0132] In Comparative Example 22, the maximum temperature in the second annealing stage after the second cold rolling was low, and recrystallization was not completed normally, resulting in a large amount of residual strain and a large KAM. Therefore, KAM / HAGB increased, and random grain boundary migration and grain growth did not occur, resulting in a small random grain boundary frequency. As a result, the coincidence boundary frequency decreased, making it easier for grain boundary degradation to occur at high temperatures, resulting in low high-temperature strength and a short creep rupture life. Furthermore, the fracture elongation was poor.

[0133] In Comparative Example 23, the maximum temperature in the second annealing stage after the second cold rolling was high, which is thought to have caused recrystallization with new random grain boundaries. As a result, the frequency of coincidence grain boundaries decreased, making the grain boundary degradation phenomenon at high temperatures more likely to occur, resulting in a shorter creep rupture life. In addition, the fracture elongation was poor.

[0134] In Comparative Example 24, the holding time between 1000°C and the maximum temperature in the second annealing stage after the second cold rolling was short, which is thought to have prevented recrystallization from completing normally, resulting in a large amount of residual strain and a large KAM. This is thought to have resulted in a large KAM / HAGB and insufficient formation of coincidence grain boundaries. As a result, the frequency of coincidence grain boundaries decreased, making it easier for grain boundary degradation to occur at high temperatures, resulting in low high-temperature strength and a short creep rupture life. Furthermore, the fracture elongation was poor.

[0135] In Comparative Example 25, the holding time between 1000°C and the maximum temperature in the second annealing stage after the second cold rolling was long, which is thought to have caused the crystal grains to become coarse. As a result, the frequency of coincidence grain boundaries decreased, making it easier for grain boundary degradation to occur at high temperatures, resulting in a shorter creep rupture life. Furthermore, the fracture elongation was poor.

[0136] The present invention provides an Fe—Cr—Ni steel sheet with excellent properties for exhaust parts requiring heat resistance. By using materials incorporating the present invention, particularly for automotive exhaust manifolds and turbochargers, significant weight reductions can be achieved compared to conventional castings, leading to compliance with exhaust gas regulations, weight reduction, and improved fuel economy. Furthermore, the elimination of cutting and grinding processes and surface treatments for parts also contributes significantly to cost reduction. When used for turbochargers, the present invention can be applied to any of these parts. Specifically, the present invention includes the housing that constitutes the outer frame of the turbocharger and precision parts inside nozzle vane-type turbochargers (e.g., back plates, oil deflectors, compressor wheels, nozzle mounts, nozzle plates, nozzle vanes, drive rings, drive levers, etc.). In the case of exhaust manifolds, any of press-formed sheet metal parts, pipe parts, and double-pipe parts may be used. Furthermore, the fuel for automobiles equipped with turbochargers may be any of gasoline, diesel, synthetic fuels such as ethanol or methanol, or hydrogen, or a mixture of these, and is not limited to these.Furthermore, the present invention is not limited to automobiles and motorcycles, and can also be applied to parts used in high-temperature environments such as various boilers (e.g., incinerator boilers, next-generation boilers, sheath heaters, combustion burners, etc.), fuel cell systems, and chemical plants (e.g., ethylene plants, hydrogen production plants, polysilicon production plants, steel plants, reduction iron-making facilities, nuclear power plants, thermal power plants, oil mining plants, etc.), making it extremely useful for industry.

Claims

1. In mass%, C: 0.300% or less, Si: 5.00% or less, Mn: 10.00% or less, P: 0.050% or less, S: 0.0100% or less, Ni: 2.00 to 40.00%, Cr: 14.00 to 30.00%, N: 0.400% or less, Al: 0.001 to 1.000%, V: 0.02 to 1.00%, Nb: 0.001 to 1.000%, Co: 0.001 to 1.000%, Cu: 0 to 4.00%, Mo: 0 to 5.00%, Ti: 0 to 1.000%, B: 0 to 0.0100%, Ca: 0 to 0.0100%, W : 0 to 3.00%, Zr: 0 to 0.30%, Sn: 0 to 0.50%, Mg: 0 to 0.0100%, Sb: 0 to 0.300%, REM: 0 to 0.200%, Ga: 0 to 0.3000%, Ta: 0 to 1.00%, Hf: 0 to 1.00%, Bi: 0 to 0.020%, and the balance being Fe and unavoidable impurities, and the value of stacking fault energy (SFE) calculated by the following formula (1) is 50 mJm -2 The Fe-Cr-Ni steel sheet is characterized in that the frequency of coincidence boundaries with a Σ value of 3 to 29 is 50% or more in total, and the KAM, which indicates the proportion of grain boundaries with a KAM value of 1 to 2 degrees, and the HAGB, which indicates the ratio of the length of grain boundaries with a relative misorientation of 15 degrees or more to the length of all grain boundaries with a relative misorientation of 2 degrees or more, satisfy KAM / HAGB≦0.

010. -2 ) = 0.5 × Cr + 1.4 × Ni + 3.2 × Nb + 0.1 × Mo + 3.6 × Cu - 3.4 × Si - 0.6 × Co (1) In the formula, the element symbol indicates the content (mass%) of the element, and 0 is substituted when the element is not contained.

2. Cu: 0.01-4.00%, Mo: 0.01-5.00%, Ti: 0.001-1.000%, B: 0.0001-0.0100%, Ca: 0.0001-0.0100%, W: 0.01-3.00%, Zr: 0.01-0.30%, Sn: 0.01-0.50%, Mg: 0.0001-0.0100%, Sb: 0.001-0.300%, REM: 0.001-0.200%, Ga: 0.0001-0.3000%, Ta: 0.01~1.00%, Hf: 0.01~1.00%, The Fe-Cr-Ni steel sheet according to claim 1, characterized in that it contains one or more of Bi: 0.001 to 0.020%.

3. Exhaust parts, precision internal parts of turbochargers, wastegate valves, or turbo housings made from the Fe-Cr-Ni steel sheet according to claim 1 or 2.

4. A method for producing an Fe-Cr-Ni steel sheet according to claim 1, comprising, in mass %, C: 0.300% or less, Si: 5.00% or less, Mn: 10.00% or less, P: 0.050% or less, S: 0.0100% or less, Ni: 2.00 to 40.00%, Cr: 14.00 to 30.00%, N: 0.400% or less, Al: 0.001 to 1.000%, V: 0.02 to 1.00%, Nb: 0.001 to 1.000%, Co: 0.001 to 1.000%, Cu: 0 to 4.00%, Mo: 0 to 5.00%, Ti: 0 to 1.000%, B: 0 to 0.0100%, a hot rolling step of hot rolling a slab containing Ca: 0 to 0.0100%, W: 0 to 3.00%, Zr: 0 to 0.30%, Sn: 0 to 0.50%, Mg: 0 to 0.0100%, Sb: 0 to 0.300%, REM: 0 to 0.200%, Ga: 0 to 0.3000%, Ta: 0 to 1.00%, Hf: 0 to 1.00%, Bi: 0 to 0.020%, and the balance being Fe and unavoidable impurities, to obtain a hot-rolled steel sheet; a first cold rolling step of rolling the hot-rolled steel sheet at a reduction of 80% or less to obtain a cold-rolled steel sheet; a first annealing / pickling step of annealing the cold-rolled steel sheet in a temperature range of 950 to 1300°C with a holding time of 120 seconds or less at 950°C or higher, followed by pickling; a second cold rolling step of rolling the cold-rolled steel sheet after the first annealing at a reduction of 10% or less; and a second annealing / pickling step of annealing the cold-rolled steel sheet after the second cold rolling step at a maximum temperature of 900 to 1000°C, at a heating rate up to 900°C of 10°C / sec or higher, and for a holding time at the maximum temperature of less than 60 seconds, followed by annealing at a maximum temperature of 1000 to 1200°C, at a heating rate up to the maximum temperature of less than 10°C / sec, and for a holding time from 1000°C to the maximum temperature of 60 seconds or more but less than 50 hours, followed by pickling.

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