Ferritic Stainless Steel

Abstract

To provide a ferritic stainless steel that excels in oxidation resistance, thermal fatigue characteristics (thermal fatigue life and thermal fatigue fracture life), high-temperature fatigue characteristics, and toughness.SOLUTION: A ferritic stainless steel contains, in mass%, C: 0.015% or less, Si: 0.50-0.90%, Mn: 1.00% or less, P: 0.040% or less, S: 0.010% or less, Al: 0.25-0.38%, N: 0.015% or less, Cr: 16.0-17.5%, Cu: 1.00-1.30%, Nb: 0.48-0.65%, Ni: 0.50% or less, V: 0.005-0.080%, and Ti: 0.005-0.30%, where the ratio of Si content to Al content (Si / Al) satisfies Si / Al≥2.0, with the balance being Fe and inevitable impurities.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to ferritic stainless steels, and more particularly to Cr-containing steels, and particularly to ferritic stainless steels having excellent oxidation resistance, thermal fatigue properties (thermal fatigue life and thermal fatigue rupture life), high-temperature fatigue properties, and toughness that are suitable for use in exhaust system components used at high temperatures, such as exhaust pipes and converter cases of automobiles and motorcycles, and exhaust ducts of thermal power plants. [Background technology]

[0002] Exhaust system components such as exhaust manifolds, exhaust pipes, converter cases, and mufflers of automobiles are required to have excellent oxidation resistance, thermal fatigue properties, and high-temperature fatigue properties (hereinafter, these are sometimes collectively referred to as heat resistance). Thermal fatigue refers to a low-cycle fatigue phenomenon caused by thermal strain that occurs in the material itself when exhaust system components are repeatedly heated and cooled with the start and stop of the engine, as the exhaust system components are in a restrained state in relation to surrounding parts, restricting the thermal expansion and contraction of the exhaust system components. On the other hand, high-temperature fatigue refers to a high-cycle fatigue phenomenon in which cracks occur due to continued vibration in a heated state, and is a phenomenon completely different from the above-mentioned thermal fatigue.

[0003] Currently, Cr-containing steels such as Type 429 (14 mass% Cr-0.9 mass% Si-0.4 mass% Nb) that contain Nb and Si are widely used as materials for components that require the above-mentioned oxidation resistance, thermal fatigue properties, and high-temperature fatigue properties. However, with improvements in engine performance, as exhaust gas temperatures rise to over 800°C, Type 429 in particular is no longer able to fully satisfy the thermal fatigue properties.

[0004] To address this issue, for example, Cr-containing steel containing Nb and Mo to improve high-temperature strength, such as SUS444 (19mass%Cr-0.5mass%Nb-2mass%Mo) specified in JIS G4305, has been developed. Ferritic stainless steels containing Nb, Mo, and W have also been developed. However, rare metals such as Mo and W are subject to the risk of resource depletion and price hikes due to limited areas where they can be mined, and so there is a demand for ferritic stainless steels that have high heat resistance without containing Mo or W.

[0005] For example, Patent Documents 1 to 3 disclose ferritic stainless steels that contain Cu and Al to improve oxidation resistance, thermal fatigue properties, and high-temperature fatigue properties as ferritic stainless steels that do not contain Mo or W. On the other hand, when various alloy elements are added to improve heat resistance, the steel becomes hard and brittle, and there is a risk of brittle cracks occurring during part molding. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent No. 4386144 [Patent Document 2] Patent No. 4702493 [Patent Document 3] Patent No. 5900715 Summary of the Invention [Problem to be solved by the invention]

[0007] According to the research of the present inventors, Patent Documents 1 to 3 disclose steels that contain Cu and Al to improve oxidation resistance, thermal fatigue properties, and high-temperature fatigue properties, but there is a growing demand for even higher thermal fatigue properties, which are the most important property. Furthermore, Patent Documents 1 to 3 do not take into account the toughness required during part molding.

[0008] Details are described below.

[0009] Traditionally, in thermal fatigue testing, the life (thermal fatigue life) was defined and evaluated as the number of cycles at which the stress value dropped to a certain percentage (for example, 70% or 75%) of the initial value. However, in reality, no breakage occurs at that point, and conventional thermal fatigue testing does not take into account the behavior thereafter until breakage. When evaluating the durability of actual parts, it is often evaluated based on whether a through crack occurs and whether exhaust gas leaks occur, so it is also necessary to consider the time (cycles) required from the end of the life in the thermal fatigue test until the test piece breaks.

[0010] For example, even if there are steels with the same thermal fatigue life defined as the number of cycles at which the stress value drops to a certain percentage, if the number of cycles from reaching the thermal fatigue life to fracture is longer, it will take more time for a through crack to appear in an actual part. In other words, a steel with a longer number of cycles from reaching the thermal fatigue life to fracture will exhibit superior durability. The ferritic stainless steel of the present invention was developed taking into consideration not only the thermal fatigue life defined in conventional tests, but also the number of cycles from the thermal fatigue life to fracture. Hereinafter, the number of cycles from the thermal fatigue life to fracture will be referred to as the thermal fatigue fracture life.

[0011] In general, increasing high-temperature yield strength (strength) is effective in extending thermal fatigue life. This is because increasing high-temperature yield strength increases the stress required before deformation begins. On the other hand, increasing high-temperature elongation increases the strain (time) required from the start of deformation until rupture. Furthermore, increasing not only high-temperature elongation but also high-temperature yield strength is effective in extending thermal fatigue rupture life. This is because it reduces the plastic strain that occurs during each thermal cycle. Generally, as yield strength (strength) increases, elongation decreases, but if elongation can be increased without reducing yield strength (strength), it is possible to extend both the thermal fatigue life and the thermal fatigue rupture life, thereby improving the overall durability of the actual part.

[0012] On the other hand, press working is often performed when forming parts. In general, if the amount of alloy contained is large to increase heat resistance, the toughness of the steel is deteriorated, and there is a risk of brittle cracking occurring during press forming, especially at low temperatures in winter. It is required to increase heat resistance, including the thermal fatigue properties as described above, while at the same time not decreasing toughness.

[0013] Therefore, the present invention was made with a view to increasing the thermal fatigue rupture life by focusing on the yield strength (high-temperature yield strength) and elongation (high-temperature elongation) in high-temperature tensile tests (hereinafter, high-temperature yield strength and high-temperature elongation may be collectively referred to as high-temperature tensile properties), while also focusing on simultaneously satisfying excellent toughness.

[0014] An object of the present invention is to provide a ferritic stainless steel that is excellent in oxidation resistance, thermal fatigue properties (thermal fatigue life and thermal fatigue rupture life), high-temperature fatigue properties, and toughness.

[0015] In the present invention, "excellent oxidation resistance" means that there is no abnormal oxidation (oxidation weight increase ≧ 50 g / m) even when held in air at 1000°C for 400 hours. 2 ) and peeling of oxide scale. Even when held in a steam atmosphere at 950°C for 200 hours, there is no abnormal oxidation (oxidation weight increase ≧ 50g / m 2) and oxidation scale peeling. If abnormal oxidation or oxidation scale peeling occurs, abnormally grown oxides and peeled oxide scale flow through the exhaust pipe, and there is a risk of causing trouble such as clogging the catalytic converter located downstream. In the present invention, the thermal fatigue test is performed assuming an environment with a maximum temperature of 850°C as described below, but the evaluation of oxidation resistance is performed as an accelerated test at a higher temperature to prevent such trouble. The exhaust gas flowing through the exhaust pipe contains gasoline-derived components (NOx, SOx, etc.) and water vapor, and in the present invention, it is particularly important to evaluate in an atmosphere containing water vapor, which has a large effect on oxidation resistance. When the atmosphere contains water vapor, the oxide scale formed by combining oxygen in the water vapor and Cr in the steel is less dense than the oxide scale formed by combining oxygen in the air and Cr in the steel. Therefore, abnormal oxidation is more likely to occur in a water vapor atmosphere, and oxide scale peeling is also more likely to occur.

[0016] In addition, "excellent thermal fatigue properties" means that the thermal fatigue life is equal to or greater than that of SUS444, and that the thermal fatigue rupture life is superior to that of SUS444. Specifically, the thermal fatigue life is equal to or greater than that of SUS444 when the temperature is repeatedly increased and decreased between 200 and 850°C, and the thermal fatigue rupture life is longer than that of SUS444. Here, the thermal fatigue life is defined as the number of cycles at which the maximum tensile stress (stress at 200°C) at the beginning of the test (the 5th cycle when the behavior of stress and strain becomes stable) drops to 70%. In addition, the thermal fatigue rupture life is defined as the number of cycles from the thermal fatigue life to the rupture of the test piece by continuing the test after the above-mentioned thermal fatigue life is reached. In other words, the thermal fatigue life + thermal fatigue rupture life = the total number of test cycles until the test piece ruptures.

[0017] In addition, "excellent high-temperature fatigue properties" means that the material has properties equal to or greater than those of SUS444. Specifically, the number of cycles to fracture when repeatedly subjected to a bending stress of 75 MPa at 850°C is 1.0 x 10 6 cycle or more.

[0018] Moreover, "excellent toughness" means that in a Charpy impact test, the brittle fracture rate is 50% or less at -30°C. The test specimens used in the Charpy impact test are steel plates with a thickness of 2.0 mm, with the rolling direction as the longitudinal direction and a V-notch as defined in JIS Z 2242:2018 machined perpendicular to the rolling direction. [Means for solving the problem]

[0019] The inventors conducted extensive research into the effects of various elements on oxidation resistance, thermal fatigue properties (thermal fatigue life), high-temperature fatigue properties, and toughness in order to develop a ferritic stainless steel that has oxidation resistance, thermal fatigue properties (thermal fatigue life), and high-temperature fatigue properties all equal to or better than those of SUS444, and that has thermal fatigue properties (thermal fatigue rupture life) superior to SUS444 and also has excellent toughness.

[0020] As a result, it was found that by containing 0.48-0.65% Nb and 1.00-1.30% Cu in mass%, high temperature yield strength increases in a wide temperature range without decreasing toughness, and excellent thermal fatigue properties (thermal fatigue life) and high temperature fatigue properties can be obtained. In addition, it was found that by containing an appropriate amount of Al (0.25-0.38%), not only can the decrease in oxidation resistance due to the inclusion of Cu be suppressed without decreasing toughness, but also the yield strength can be increased in a temperature range where the effect of containing Cu cannot be obtained, thereby improving the thermal fatigue properties (thermal fatigue life). Furthermore, it was found that by containing an appropriate amount of Si (0.50-0.90%), the continuous oxidation resistance in water vapor is greatly improved without decreasing toughness, and high temperature fatigue properties are also improved by containing more Si than Al. Furthermore, it was found that by containing an appropriate amount of Ti, Ti preferentially forms carbides and nitrides instead of Nb, and the amount of solid-solubilized Nb in the steel increases, thereby increasing the high temperature yield strength and improving the thermal fatigue life.

[0021] In addition to the above, it has been newly discovered that a steel having an excellent thermal fatigue rupture life as described above can be obtained by containing, by mass%, V in the range of 0.005 to 0.080%, Al in the range of 0.25 to 0.38%, and further containing the ratio of Si content to Al content (Si / Al) such that Si / Al ≧ 2.0.

[0022] Based on the above findings, the present invention has been completed by containing appropriate amounts of all of Nb, Cu, Al, V, Si and Ti. If the appropriate amount of any one of the above elements is not contained, the excellent oxidation resistance, thermal fatigue properties (thermal fatigue life and thermal fatigue rupture life), high temperature fatigue properties and toughness expected by the present invention cannot all be obtained at the same time.

[0023] The present invention relates to the following. [1] In mass percent, C: 0.015% or less, Si: 0.50-0.90%, Mn: 1.00% or less, P: 0.040% or less, S: 0.010% or less, Al: 0.25-0.38%, N: 0.015% or less, Cr: 16.0-17.5%, Cu: 1.00-1.30%, Nb: 0.48-0.65%, Ni: 0.50% or less, V: 0.005~0.080%, Ti: 0.005 to 0.30% A ferritic stainless steel having a composition in which the ratio of Si content to Al content (Si / Al) satisfies Si / Al ≧ 2.0, with the remainder consisting of Fe and unavoidable impurities. [2]Further, in mass percent: B: 0.0050% or less, REM: 0.08% or less, Zr: 0.50% or less, Co: 0.50% or less, Sn: 0.50% or less, Sb: 0.50% or less The ferritic stainless steel according to [1], which contains one or more selected from the following: [3] Furthermore, in mass percent: Ca: 0.0050% or less, Mg: 0.0050% or less The ferritic stainless steel according to [1] or [2], which contains one or two selected from the following: Effect of the Invention

[0024] According to the present invention, it is possible to provide a ferritic stainless steel having excellent oxidation resistance, thermal fatigue properties (thermal fatigue life and thermal fatigue rupture life), high-temperature fatigue properties, and toughness. Therefore, the ferritic stainless steel of the present invention can be suitably used for exhaust system members of automobiles and the like. [Brief description of the drawings]

[0025] [Figure 1] FIG. 1 is a diagram illustrating a high-temperature plane bending fatigue test piece. [Diagram 2] FIG. 2 is a diagram illustrating a high-temperature tensile test piece. [Diagram 3] FIG. 3 is a diagram illustrating a thermal fatigue test piece. [Figure 4] FIG. 4 is a diagram illustrating the temperature and restraint conditions in the thermal fatigue test. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] The present invention will be described in detail below.

[0027] The ferritic stainless steel of the present invention contains, by mass%, C: 0.015% or less, Si: 0.50 to 0.90%, Mn: 1.00% or less, P: 0.040% or less, S: 0.010% or less, Al: 0.25 to 0.38%, N: 0.015% or less, Cr: 16.0 to 17.5%, Cu: 1.00 to 1.30%, Nb: 0.48 to 0.65%, Ni: 0.50% or less, V: 0.005 to 0.080%, and Ti: 0.005 to 0.30%, with the ratio of Si content to Al content (Si / Al) satisfying Si / Al ≧ 2.0, and the balance consisting of Fe and unavoidable impurities.

[0028] In the present invention, the balance of the components is very important, and by using such a combination of components, it is possible to obtain a ferritic stainless steel that has oxidation resistance, thermal fatigue properties (thermal fatigue life), and high-temperature fatigue properties equal to or better than those of SUS444, thermal fatigue properties (thermal fatigue rupture life) that are better than those of SUS444, and excellent toughness with a brittle fracture rate of 50% or less at -30°C in a Charpy impact test, without using rare metals such as Mo or W. If even one of the components in the above composition is outside its content range, the desired oxidation resistance, thermal fatigue properties (thermal fatigue life, thermal fatigue rupture life), high-temperature fatigue properties, and toughness cannot all be obtained simultaneously.

[0029] Next, the composition of the ferritic stainless steel of the present invention will be described. Hereinafter, % indicating the content of steel components means mass % unless otherwise specified.

[0030] C: 0.015% or less C is an element effective in increasing the strength of steel, but if the content exceeds 0.015%, the toughness and formability are significantly reduced. Therefore, in the present invention, the C content is set to 0.015% or less. From the viewpoint of ensuring formability, the C content is preferably 0.010% or less. From the viewpoint of ensuring strength as an exhaust system member, the C content is preferably 0.003% or more. The C content is more preferably 0.004% or more. The C content is more preferably 0.008% or less.

[0031] Silicon: 0.50 to 0.90% Silicon is an important element necessary for improving oxidation resistance in a steam-containing atmosphere. In order to ensure continuous oxidation resistance in steam equal to or greater than that of SUS444, a silicon content of 0.50% or more is necessary. On the other hand, an excessive silicon content of more than 0.90% not only makes the oxide scale more likely to peel off, but also reduces toughness. Therefore, the silicon content is set to 0.50-0.90%. The silicon content is preferably 0.60% or more.

[0032] Si / Al≧2.0 Furthermore, Si is an important element for effectively utilizing the solid solution strengthening effect of Al and increasing the thermal fatigue rupture life. As described later, Al has a solid solution strengthening effect at high temperatures, and is the only element that improves high-temperature fatigue properties and increases the thermal fatigue rupture life. This effect can only be obtained when Al is in a solid solution state in the steel. However, on the other hand, Al is likely to form oxides on the surface layer of the steel and nitrides inside the steel. If the Al content is greater than the Si content, Al preferentially forms oxides or nitrides at high temperatures, and the amount of solid-solubilized Al decreases, so that it cannot fully contribute to solid solution strengthening, and excellent high-temperature fatigue properties and thermal fatigue rupture life cannot be obtained. In order to obtain the desired high-temperature fatigue properties, it is necessary to contain more Si than Al, and further, in order to obtain the desired thermal fatigue rupture life, the ratio of the Si content to the Al content (Si / Al) must be Si / Al≧2.0. By doing so, Si is preferentially oxidized to continuously form a dense oxide layer on the steel sheet surface, and this oxide layer has the effect of suppressing the inward diffusion of oxygen and nitrogen from the outside, so that Al is maintained in a solid solution state without being oxidized or nitridized. As a result, the solid solution state of Al is stably secured, and the effect of greatly increasing high-temperature fatigue properties and thermal fatigue rupture life is exhibited. In order to fully exhibit the effect of Al, it is important to keep Al in a solid solution state in the steel without oxidizing it at high temperatures as much as possible. In order to obtain the above effect, in the present invention, Si is contained so as to satisfy Si / Al≧2.0. Moreover, Si / Al is preferably 2.6 or more.

[0033] Mn: 1.00% or less Mn is an element contained as a deoxidizer and to increase the strength of steel. To obtain this effect, it is preferable to contain 0.05% or more of Mn. However, excessive Mn content tends to generate γ phase at high temperatures, reducing heat resistance, especially oxidation resistance. It also reduces toughness. Therefore, the Mn content is set to 1.00% or less. The Mn content is more preferably 0.10% or more. The Mn content is also preferably 0.60% or less, more preferably 0.20% or less.

[0034] P:0.040% or less P is a harmful element that reduces the toughness of steel, and it is desirable to reduce it as much as possible. Therefore, in the present invention, the P content is set to 0.040% or less. The P content is preferably 0.030% or less. There is no particular lower limit for the P content. However, since excessive de-P leads to an increase in costs, the P content is preferably 0.005% or more.

[0035] S: 0.010% or less S reduces elongation and r-value, adversely affects formability, and is also a harmful element that reduces corrosion resistance, a basic property of stainless steel, so it is desirable to reduce it as much as possible. Therefore, in the present invention, the S content is set to 0.010% or less. The S content is preferably 0.003% or less. There is no particular lower limit for the S content. However, since excessive de-S leads to increased costs, the S content is preferably 0.0005% or more.

[0036] Al: 0.25-0.38% Al is an important element that improves the oxidation resistance and high-temperature fatigue properties of steel containing Cu, and in the present invention, greatly increases the thermal fatigue rupture life. To obtain this effect, it is necessary to contain 0.25% or more Al, and then contain Si to satisfy Si / Al≧2.0 as described above, and contain V in the range of 0.005 to 0.080% as described below to dissolve Al in the steel. When Al is dissolved in the steel, the high-temperature strength (yield strength) of the steel increases due to solid solution strengthening, while during deformation at high temperatures, the stress is not concentrated locally and deformation in the thickness direction (thickness reduction) progresses, but the stress is distributed throughout the steel, and deformation in the thickness direction (thickness reduction) is delayed. When thickness reduction occurs locally, stress is increasingly concentrated in the thinned portion, and the thickness reduction progresses at an accelerated rate, but the ferritic stainless steel of the present invention, which deforms overall, dramatically increases the time (cycles) until fracture. Other solid solution strengthening elements such as Nb, Mo, and W do not have the effect of dispersing stress and relieving stress concentration, and this effect is unique to Al. On the other hand, if the Al content exceeds 0.38%, Al is likely to precipitate as AlN, and not only will the expected effect of increasing the thermal fatigue rupture life of Al not be obtained, but the toughness will decrease significantly. Therefore, the Al content is set to a range of 0.25 to 0.38%. The Al content is preferably 0.30% or more. The Al content is preferably 0.34% or less, and more preferably 0.32% or less.

[0037] N: 0.015% or less N is an element that reduces the toughness and formability of steel, and if the content exceeds 0.015%, the above-mentioned reduction becomes significant. Therefore, the N content is set to 0.015% or less. From the viewpoint of ensuring toughness and formability, it is preferable to reduce N as much as possible, and it is desirable to make it less than 0.010%. There is no particular lower limit for the N content. However, since excessive denitrification leads to an increase in costs, the N content is preferably 0.004% or more.

[0038] Cr: 16.0~17.5% Cr is an important element that is effective in improving the corrosion resistance and oxidation resistance that are characteristic of stainless steel, but if the Cr content is less than 16.0%, sufficient oxidation resistance cannot be obtained. On the other hand, Cr is an element that hardens steel at room temperature and reduces toughness, and if it is contained in excess of 17.5%, the above-mentioned adverse effects become significant. Therefore, the Cr content is set to the range of 16.0 to 17.5%.

[0039] Ni: 0.50% or less Ni is an element that improves the toughness of steel. To obtain this effect, it is preferable to contain 0.02% or more of Ni. However, Ni is expensive and is also a strong γ-phase forming element, and if Ni is contained in an amount exceeding 0.50%, γ-phase will be generated at high temperatures, lowering the oxidation resistance. Therefore, the Ni content is set to 0.50% or less. The Ni content is preferably 0.05% or more. The Ni content is preferably 0.30% or less, and more preferably 0.20% or less.

[0040] Cu: 1.00-1.30% Cu is an element very effective in improving thermal fatigue properties, and by finely precipitating as ε-Cu at around 600°C, it greatly increases the strength (yield strength) of the steel and greatly improves the thermal fatigue properties. In order to obtain thermal fatigue properties equal to or better than those of SUS444, it is necessary to contain Cu at 1.00% or more. However, a Cu content exceeding 1.30% not only promotes the coarsening of ε-Cu, thereby reducing the strengthening effect, but also reduces the high-temperature elongation. If the high-temperature elongation is reduced, an excellent thermal fatigue rupture life cannot be obtained. Furthermore, a Cu content exceeding 1.30% reduces the toughness. For this reason, the Cu content is set to the range of 1.00 to 1.30%. The Cu content is preferably 1.10% or more. Furthermore, the Cu content is preferably 1.20% or less.

[0041] Nb: 0.48-0.65% Nb fixes C and N as carbonitrides, improving corrosion resistance, formability, and intergranular corrosion resistance of welds. In addition, Nb dissolved in steel increases high-temperature yield strength and improves thermal fatigue properties (thermal fatigue life, thermal fatigue rupture life) and high-temperature fatigue properties. This effect can be achieved with a Nb content of 0.48% or more. However, a Nb content of more than 0.65% can cause the formation of Laves phase (Fe 2 This makes it easier for Nb to precipitate, promoting embrittlement. Therefore, the Nb content is set to the range of 0.48 to 0.65%. The Nb content is preferably 0.50% or more. Moreover, the Nb content is preferably 0.55% or less.

[0042] V: 0.005~0.080% V is an element effective in improving the workability of steel and is also effective in improving oxidation resistance. Furthermore, in the present invention, it is an important element for Al to exhibit the effect of dissolving in steel and increasing thermal fatigue rupture life. By containing V, V combines with N in the steel and precipitates as fine nitrides (VN). This suppresses Al from combining with N to form coarse nitrides. If Al forms coarse AlN, the amount of solute Al in the steel decreases, and the above-mentioned effect of increasing thermal fatigue rupture life due to the inclusion of Al cannot be sufficiently obtained. Ti, which will be described later, is also an element that easily combines with N to form nitrides, but since TiN is coarse, when Ti forms coarse TiN, it leads to a decrease in high-temperature fatigue properties. Such an effect of securing the amount of solute Al by precipitation of fine nitrides becomes significant when the V content is 0.005% or more. Therefore, the V content is set to 0.005% or more. However, excessive V content exceeding 0.080% leads to the precipitation of coarse VN, which not only reduces toughness but also makes it difficult to obtain an excellent thermal fatigue rupture life. Therefore, the V content is set to 0.080% or less. The V content is preferably 0.010% or more. Moreover, the V content is preferably 0.050% or less, and more preferably 0.030% or less.

[0043] Ti: 0.005~0.30% Ti combines with C and N preferentially over Nb, and precipitates as TiC or TiN, thereby enhancing corrosion resistance, formability, and intergranular corrosion resistance of welded parts. Ti further reduces the amount of Nb carbonitride precipitation and ensures the amount of Nb in solid solution in steel, thereby enhancing the effect of improving high-temperature yield strength, thermal fatigue properties, and high-temperature fatigue properties. This effect can be obtained by including 0.005% or more of Ti. However, an excessive Ti content of more than 0.30% not only saturates the effect of improving oxidation resistance, but also leads to a decrease in toughness due to the precipitation of coarse TiN, making it easier for cracks to occur during press working. Furthermore, for example, repeated bending and unbending in a hot-rolled sheet annealing line can cause breakage, adversely affecting manufacturability. Therefore, the Ti content is set to 0.005 to 0.30%. In order to obtain good toughness, the Ti content is preferably 0.100% or less, more preferably 0.050% or less.

[0044] In the ferritic stainless steel of the present invention, the balance consists of Fe and unavoidable impurities.

[0045] The ferritic stainless steel of the present invention may further contain, in addition to the above essential components, one or more selected from B, REM, Zr, Co, Sn and Sb within the following ranges.

[0046] B: 0.0050% or less B is an element effective in improving the workability of steel, particularly the secondary workability. This effect can be obtained with a B content of 0.0002% or more. Therefore, when B is contained, the B content is preferably 0.0002% or more. However, a B content exceeding 0.0050% generates coarse BN, which reduces the workability. Therefore, when B is contained, the B content is set to 0.0050% or less. The B content is more preferably 0.0005% or more. Moreover, the B content is preferably 0.0020% or less.

[0047] REM: 0.08% or less REM (rare earth elements) are elements that improve oxidation resistance. This effect is obtained with an REM content of 0.01% or more. Therefore, when REM is contained, the REM content is preferably 0.01% or more. However, an REM content exceeding 0.08% embrittles the steel. Therefore, when REM is contained, the REM content should be 0.08% or less. Note that REM is a collective term for Sc, Y, and 15 elements ranging from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, and the REM content referred to here is the total content of these elements.

[0048] Zr: 0.50% or less Zr is an element that improves oxidation resistance. This effect is obtained with a Zr content of 0.01% or more. Therefore, when Zr is contained, the Zr content is preferably 0.01% or more. However, when the Zr content exceeds 0.50%, Zr intermetallic compounds precipitate, embrittling the steel. Therefore, when Zr is contained, the Zr content is set to 0.50% or less. The Zr content is more preferably 0.10% or less.

[0049] Co:0.50% or less Co is an element effective in improving the toughness of steel. To obtain this effect, it is preferable to contain 0.01% or more of Co. Therefore, when Co is contained, the Co content is preferably 0.01% or more. However, Co is an expensive element, and even if it is contained in excess of 0.50%, the above effect is saturated. Therefore, when Co is contained, the Co content is set to 0.50% or less. The Co content is more preferably 0.02% or more, and further preferably 0.03% or more. Moreover, the Co content is preferably 0.20% or less, and more preferably 0.10% or less.

[0050] Sn: 0.50% or less Sn is an element that has the effect of improving the corrosion resistance and high-temperature strength of steel, and is contained as necessary. This effect is obtained with a Sn content of 0.01% or more. Therefore, when Sn is contained, the Sn content is preferably 0.01% or more. On the other hand, an excessive Sn content reduces the workability of steel, so the Sn content is set to an upper limit of 0.50%. Therefore, when Sn is contained, the Sn content is set to 0.50% or less. The Sn content is more preferably 0.03% or more. Moreover, the Sn content is preferably 0.30% or less, more preferably 0.10% or less.

[0051] Sb: 0.50% or less Sb is an element that has the effect of improving the toughness of steel, and is contained as necessary. This effect can be obtained with a content of 0.01% or more. Therefore, when Sb is contained, the Sb content is preferably 0.01% or more. On the other hand, an excessive content of Sb rather reduces toughness, so the upper limit of the Sb content is set to 0.50%. Therefore, when Sb is contained, the Sb content is set to 0.50% or less. The Sb content is more preferably 0.03% or more. Moreover, the Sb content is preferably 0.30% or less, and more preferably 0.20% or less.

[0052] The ferritic stainless steel of the present invention may further contain, in addition to the above components, one or two elements selected from Ca and Mg within the following ranges.

[0053] Ca: 0.0050% or less Ca is an effective component for preventing nozzle clogging due to inclusion precipitation that is likely to occur during continuous casting. This effect can be obtained when the Ca content is 0.0002% or more. Therefore, when Ca is contained, the Ca content is preferably 0.0002% or more. On the other hand, in order to obtain good surface properties without generating surface defects, the Ca content needs to be 0.0050% or less. Therefore, when Ca is contained, the Ca content is 0.0050% or less. The Ca content is more preferably 0.0005% or more. Moreover, the Ca content is preferably 0.0030% or less, more preferably 0.0020% or less.

[0054] Mg: 0.0050% or less Mg is an element that improves the equiaxed crystal ratio of a slab and is effective in improving workability and toughness. In steels containing Nb as in the present invention, Mg also has the effect of suppressing the coarsening of Nb carbonitrides. This effect is obtained with a Mg content of 0.0002% or more. If Nb carbonitrides become coarse, the amount of Nb dissolved in the steel decreases, leading to a decrease in thermal fatigue properties. Therefore, when Mg is contained, the Mg content is preferably 0.0002% or more. On the other hand, if the Mg content exceeds 0.0050%, the surface properties of the steel are deteriorated. Therefore, when Mg is contained, the Mg content is set to 0.0050% or less. The Mg content is more preferably 0.0004% or more. Moreover, the Mg content is preferably 0.0030% or less, more preferably 0.0020% or less.

[0055] In addition, when the content of B, REM, Zr, Co, Sn, Sb, Ca, and Mg described above as optional components is less than the lower limit, the component is considered to be included as an inevitable impurity. In addition, in accordance with the purpose of the present invention, Mo and W, both of which are rare metals, are not actively contained. In other words, it is preferable that the ferritic stainless steel of the present invention does not contain Mo or W. However, both elements may be mixed in at 0.1% or less from the raw material scrap, etc.

[0056] Next, a method for producing the ferritic stainless steel of the present invention will be described.

[0057] The method for producing the ferritic stainless steel of the present invention can be suitably used as long as it is a normal method for producing ferritic stainless steel, and is not particularly limited. For example, steel is melted in a known melting furnace such as a converter or an electric furnace, or further subjected to secondary refining such as ladle refining or vacuum refining to produce a steel having the above-mentioned composition of the present invention, and a steel billet (slab) is produced by a continuous casting method or an ingot casting-blooming rolling method. Then, the steel can be produced by a manufacturing process in which the steel is subjected to each process such as hot rolling, hot-rolled sheet annealing, pickling, cold rolling, finish annealing, and pickling to produce a cold-rolled annealed sheet. The above cold rolling may be performed once or two or more times with intermediate annealing in between, and each process of cold rolling, finish annealing, and pickling may be performed repeatedly. Furthermore, the hot-rolled sheet annealing may be omitted, and when adjustment of the surface gloss or roughness of the steel sheet is required, skin pass rolling may be performed after cold rolling or finish annealing. Depending on the application, the hot-rolled annealed sheet can be used as it is.

[0058] Preferred production conditions for the above production method will now be described.

[0059] In the steelmaking process for producing steel, it is preferable to perform secondary refining of steel melted in a converter or electric furnace by the VOD method or the like to produce steel containing the above essential components and components that are contained as necessary. The produced molten steel can be made into a steel material by a known method, but from the viewpoint of productivity and quality, it is preferable to use a continuous casting method. The steel material is then preferably heated to 1050 to 1250°C and hot-rolled into a hot-rolled sheet of a desired thickness (3 mm to 6 mm). Of course, it can also be hot-worked into other products besides sheet materials. The hot-rolled sheet is then subjected to continuous annealing at a temperature of 900 to 1150°C as necessary to produce a hot-rolled annealed sheet, and then descaled by pickling or the like to produce a hot-rolled product. The annealing may be omitted. Note that, if necessary, scale may be removed by shot blasting or brush grinding before pickling.

[0060] Furthermore, the hot-rolled annealed sheet or hot-rolled sheet may be subjected to a process such as cold rolling to produce a cold-rolled product. In this case, cold rolling may be performed once, but may be performed twice or more with intermediate annealing in between in terms of productivity and required quality. The total reduction ratio of one or more cold rollings is preferably 60% or more, more preferably 65% ​​or more, and even more preferably 70% or more. The cold-rolled steel sheet is then preferably subjected to continuous annealing (finish annealing) at a temperature of 900 to 1150°C, more preferably 950 to 1100°C, pickled, and produced as a cold-rolled product. Furthermore, depending on the application, after the finish annealing, skin pass rolling or the like may be performed to adjust the shape, surface roughness, and material of the steel sheet. Brush grinding may be performed before the pickling.

[0061] The hot-rolled or cold-rolled product obtained as described above is then processed and shaped according to the respective applications. For example, it is cut, bent, stretched, drawn, and other processes to be shaped into automobile or motorcycle exhaust pipes, catalyst outer cylinders, exhaust ducts for thermal power plants, or fuel cell-related components, such as separators, interconnectors, or reformers. The method of welding these components is not particularly limited. For example, ordinary arc welding such as MIG (Metal Inert Gas), MAG (Metal Active Gas), or TIG (Tungsten Inert Gas), resistance welding such as spot welding or seam welding, high-frequency resistance welding such as electric resistance welding, high-frequency induction welding, and the like can be applied. EXAMPLES

[0062] The present invention will now be described in detail with reference to examples.

[0063] Steels having compositions of No. 1 to 36 shown in Table 1 were melted in a vacuum melting furnace, cast into 50 kg steel ingots, heated to 1100 ° C, and hot-rolled into 35 mm thick sheet bars. Two pieces of 200 mm length were cut out from the sheet bar, one of which was heated to 1050 ° C, hot-rolled into a 5 mm thick hot-rolled sheet, annealed at a temperature in the range of 950 to 1100 ° C, and ground to obtain a hot-rolled annealed sheet. Subsequently, cold rolling was performed at a rolling reduction of 60%, and finish annealing was performed at a temperature of 1000 to 1100 ° C, and the cold-rolled annealed sheet was 2.0 mm thick, and subjected to oxidation tests (continuous oxidation tests in air and continuous oxidation tests in water vapor), high-temperature fatigue tests, high-temperature tensile tests, and Charpy impact tests. The other sheet bar was heated to 1100 ° C, and hot-forged into a 30 mm square bar. From this square bar, a 150 mm length was cut out, finish annealed in the range of 1000 to 1100°C, and subjected to a thermal fatigue test. For reference, a cold-rolled annealed sheet was also prepared for SUS444 (No. 21) in the same manner as above and subjected to an oxidation test, a high-temperature fatigue test, and a high-temperature tensile test, and a square bar was also prepared in the same manner and subjected to a thermal fatigue test. The annealing temperature was determined for each steel while checking the structure within the above temperature range.

[0064] <Continuous oxidation test in air> Samples of 30 mm x 20 mm were cut out from each of the cold-rolled annealed sheets obtained as described above, a 4 mm diameter hole was drilled in the top of the sample, the surface and end faces were polished with #320 emery paper, degreased, and then the sample was hung in a furnace in an air atmosphere heated to 1000°C and held there for 400 hours. After the test, the mass of the sample was measured, and the difference from the mass before the test, which had been measured in advance, was calculated to obtain the oxidation weight gain (g / m 2 ) was calculated. If spalling of the oxide scale occurred, the mass of the spalled oxide scale was added to the oxidation weight gain mentioned above. Each test was performed twice, and the larger oxidation weight gain was taken as the oxidation weight gain of that steel. If spalling of the oxide scale occurred in even one of the two samples, it was judged that spalling had occurred. A similar test was also performed on SUS444 (No. 21), and the results showed that no abnormal oxidation occurred, but spalling of the oxide scale was observed. The continuous oxidation properties in air were judged as follows:

[0065] [Criteria for determining continuous oxidation characteristics in air] ○ (Pass): No abnormal oxidation or scale peeling occurred △ (Fail): No abnormal oxidation occurred, but scale peeling occurred. × (Failed): Abnormal oxidation (oxidation increase ≧ 50g / m 2 ) occurred The results obtained are shown in Table 1.

[0066] <Continuous oxidation test in water vapor> Samples of 30 mm × 20 mm were cut out from each of the cold-rolled annealed sheets obtained as described above, a hole of 4 mm diameter was drilled in the top of the sample, and the surface and end faces were polished with #320 emery paper and degreased. Then, the samples were immersed in 20 vol% H 2 O-Remainder N 2 The samples were subjected to a continuous oxidation test in water vapor, in which the samples were held in a furnace heated to 950°C for 200 hours, with a mixed gas of the above gas flowing at 0.5 L / min / sheet, creating a water vapor-containing atmosphere. After the test, the mass of the sample was measured, and the difference from the mass measured before the test was calculated to determine the oxidation weight gain (g / m 2 ) was calculated. If spalling of oxide scale occurred, the mass of the spalled oxide scale was added to the oxidation weight increase mentioned above. The same test was also conducted on SUS444 (No. 21), and as a result, abnormal oxidation occurred. The continuous oxidation characteristics in steam were judged as follows.

[0067] [Continuous oxidation characteristics in water vapor] ○: No abnormal oxidation or peeling of oxide scale occurred (passed) △: No abnormal oxidation occurred, but peeling of oxide scale occurred (failed) ×: Abnormal oxidation (oxidation increase ≧ 50g / m 2 ) occurred (failed) The results obtained are shown in Table 1. In the evaluation of the continuous oxidation test in air and the continuous oxidation test in water vapor, those marked with ◯ were judged to pass (excellent oxidation resistance), and those marked with the other marks were judged to fail.

[0068] <High temperature fatigue test> From the various cold-rolled annealed sheets obtained as described above, test pieces with the shape and dimensions shown in Figure 1 were cut out and subjected to high-temperature plane bending fatigue tests. After heating to 850°C and holding at a constant temperature for 30 minutes, a Schenk-type high-temperature plane bending fatigue test was performed in which a bending stress of 75 MPa was applied to the steel sheet surface at 22 Hz (1300 rpm) with a stress ratio of -1, and the number of vibration cycles until fracture (fatigue life) was measured. A similar test was also performed on SUS444 (No. 21), and the high-temperature fatigue life was 1.0 x 10 6 The high temperature fatigue properties were evaluated as follows.

[0069] [High temperature fatigue property evaluation criteria] ○:1.0×10 6 Cycle or more (pass) ×:1.0×10 6 Less than a cycle (fail) The results obtained are shown in Table 1.

[0070] <High temperature tensile test> From the various cold-rolled annealed sheets prepared as described above, test pieces with the shape shown in Figure 2 were prepared by machining. The gauge length was 50 mm. The test was started after heating to 850°C and holding for 15 minutes. The tensile speed was 0.2 mm / min. After the tensile test, the fractured parts were butted together, the gauge length was measured, and the difference from the gauge length of 50 mm before the test was divided by 50 mm to obtain the elongation value El (%), which was recorded together with the 0.2% yield strength (MPa). The same test was also performed on SUS444 (No. 21), and the high-temperature 0.2% yield strength was 29 MPa and the high-temperature elongation was 34%. The high-temperature 0.2% yield strength (PS) and high-temperature elongation (El) were determined as follows.

[0071] [High temperature 0.2% proof stress (PS) judgment criteria] ○:30MPa or more ×: Less than 30MPa

[0072] [High temperature elongation (El) criteria] ○: 70% or more ×: Less than 70% The results obtained are shown in Table 1.

[0073] <Thermal fatigue test> The 30 mm square bars prepared above were machined to produce test pieces with the shape shown in FIG. 3, and subjected to the following thermal fatigue test. As shown in Figure 4, the thermal fatigue test was performed by repeatedly heating and cooling the test piece between 200°C and 850°C while restraining it at a restraint ratio of 0.4. The heating and cooling rates were 5°C / sec, respectively, and the holding time at 200°C was 2 min, and the holding time at 850°C was 2 min. The above restraint ratio can be expressed as η=a / (a+b) as shown in Figure 4, where a is (free thermal expansion strain amount - controlled strain amount) / 2, and b is controlled strain amount / 2. The free thermal expansion strain amount is the strain amount when the temperature is raised without applying any mechanical stress, and the controlled strain amount is the absolute value of the strain amount generated during the test. The actual restraint strain amount generated in the material due to the restraint is (free thermal expansion strain amount - controlled strain amount). The thermal fatigue life was calculated by dividing the load detected at 200°C by the cross-sectional area of ​​the uniformly heated parallel part of the test piece (see Figure 3), and the number of cycles at which the stress value dropped to 70% of the initial value (5th cycle) was evaluated as the thermal fatigue life. The test was continued until the test piece broke, and the number of cycles from the thermal fatigue life to break was evaluated as the thermal fatigue rupture life. The same test was also conducted on SUS444 (No. 21), and the thermal fatigue life was 980 cycles and the thermal fatigue rupture life was 140 cycles. The thermal fatigue properties were evaluated as follows:

[0074] [Thermal fatigue criteria (thermal fatigue life)] ○: 980 cycles or more (pass) ×: Less than 980 cycles (failed)

[0075] [Thermal fatigue judgment criteria (thermal fatigue rupture life)] ○: 270 cycles or more (pass) ×: Less than 270 cycles (failed) The results obtained are shown in Table 1.

[0076] <Toughness (Charpy impact test)> From the various cold-rolled annealed sheets prepared as described above, V-notch test pieces as specified in JIS Z2242:2018 were prepared by machining. The test pieces were longitudinally aligned along the rolling direction, and V-notches were machined in the direction perpendicular to the rolling direction. Charpy tests were performed on three pieces at -30°C in accordance with JIS Z2242:2018. The fracture surface after the test was observed, and the brittle fracture rate was calculated and evaluated as follows using the average value of the three pieces.

[0077] [Toughness Criteria] 〇: Brittle fracture rate ≦50% (pass) ×: Brittle fracture rate >50% (fail) The results obtained are shown in Table 1.

[0078] [Table 1]

[0079] As shown in Table 1, in the steels No. 1 to 20 of the present invention, no abnormal oxidation or spalling of oxide scale occurred in either of the two oxidation tests (continuous oxidation test in air and continuous oxidation test in steam), and they were excellent in oxidation resistance. They also had excellent thermal fatigue life and high-temperature fatigue properties equal to or better than SUS444 (steel No. 21), and had a thermal fatigue rupture life superior to SUS444. Furthermore, their toughness was superior to SUS444.

[0080] On the other hand, steel No. 22 failed the oxidation resistance (resistance to continuous oxidation in water vapor) and thermal fatigue rupture life because the Si content was less than 0.50% and the ratio of Si content to Al content (Si / Al) was Si / Al<2.0. Steel No. 23 had a Si content exceeding 0.90%, and failed the oxidation resistance (resistance to continuous oxidation in air) and toughness tests. Steel No. 24 had a Mn content exceeding 1.00%, and failed the oxidation resistance (resistance to continuous oxidation in water vapor) and toughness tests. Steel No. 25 had an Al content of less than 0.25%, and failed in terms of thermal fatigue properties (thermal fatigue rupture life). Steel No. 26 had an Al content exceeding 0.38%, and failed in toughness. Steel No. 27 had a Ni content exceeding 0.50%, and failed the oxidation resistance (resistance to continuous oxidation in the atmosphere). Steel No. 28 had a Cr content of less than 16.0%, and failed the oxidation resistance (resistance to continuous oxidation in air and continuous oxidation resistance in water vapor). Steel No. 29 had a Cr content exceeding 17.5%, and failed to pass the toughness test. Steel No. 30 had a Cu content of less than 1.00%, and failed in terms of thermal fatigue properties (thermal fatigue life and thermal fatigue rupture life). Steel No. 31 had a Cu content exceeding 1.30%, and failed in thermal fatigue properties (thermal fatigue rupture life) and toughness. Steel No. 32 had a Nb content of less than 0.48%, and failed in terms of thermal fatigue properties (thermal fatigue life). Steel No. 33 had a V content of less than 0.005%, and failed in terms of thermal fatigue properties (thermal fatigue rupture life). Steel No. 34 had a V content exceeding 0.080%, and failed to pass the thermal fatigue properties (thermal fatigue rupture life) and toughness tests. Steel No. 35 had a Si / Al ratio of <2.0, and failed in terms of thermal fatigue properties (thermal fatigue rupture life). Steel No. 36 had a Ti content of less than 0.005%, and failed in terms of thermal fatigue properties (thermal fatigue life). [Industrial Applicability]

[0081] The ferritic stainless steel of the present invention is suitable for use as exhaust system members for automobiles, etc. Furthermore, it can also be suitably used as exhaust system members for thermal power generation systems and members for solid oxide fuel cells.

Claims

1. In mass percent, C: 0.015% or less, Si: 0.50-0.90%, Mn: 1.00% or less, P: 0.040% or less, S: 0.010% or less, Al: 0.25-0.38%, N: 0.015% or less, Cr: 16.0-17.5%, Cu: 1.00-1.30%, Nb: 0.48-0.65%, Ni: 0.50% or less, V: 0.005-0.080%, Ti: 0.005 to 0.30%; A ferritic stainless steel having a composition in which the ratio of Si content to Al content (Si / Al) satisfies Si / Al≧2.0, with the remainder consisting of Fe and unavoidable impurities.

2. Further, in mass%, B: 0.0050% or less, REM: 0.08% or less, Zr: 0.50% or less, Co: 0.50% or less, Sn: 0.50% or less, Sb: 0.50% or less The ferritic stainless steel according to claim 1, which contains one or more selected from the following:

3. Further, in mass%, Ca: 0.0050% or less, Mg: 0.0050% or less The ferritic stainless steel according to claim 1 or 2, which contains one or two selected from the following:

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