Ferritic stainless steel and exhaust component

Abstract

The ferritic stainless steel has a prescribed chemical composition, at a surface, at least one of an oxide containing 5 mass% or more of Al and an oxide containing 5 mass% or more of Si is present, and the number of the oxides of which the diameter D expressed by D = (Dmax + Dmin) / 2 is 0.1 μm to 2.0 μm is 10 or more per 93 μm 2 of the surface, where Dmax is set to the maximum diameter of the oxides at the surface, and Dmin is set to the minimum diameter of the oxides at the surface.

Description

Technical Field

[0001] This invention relates to ferritic stainless steel and exhaust components. More specifically, it relates to ferritic stainless steel with excellent resistance to red oxide scale in a high-temperature steam atmosphere, and exhaust components obtained using this ferritic stainless steel as a raw material.

[0002] This application claims priority based on Japanese Patent Application No. 2020-178302 filed on October 23, 2020, the contents of which are incorporated herein by reference. Background Technology

[0003] Stainless steel is typically heated to high temperatures of 300–900°C when used in applications such as exhaust path components, furnace combustion components, fuel cell components, or plant equipment components. Furthermore, in these applications, the use of stainless steel in environments containing water vapor can potentially lead to the formation of a red oxide scale (Fe-based oxide). This red oxide scale can not only adhere to other components and cause adverse effects in certain situations, but it can also reduce high-temperature strength due to wall shrinkage caused by oxidation.

[0004] Therefore, ferritic stainless steels with resistance to red oxide scale under high-temperature steam atmospheres are desirable. Various methods have been known to improve resistance to red oxide scale in the past.

[0005] Patent Documents 1 and 2 describe how adding Si promotes Cr diffusion, thereby increasing the formation of Cr-based oxides and strengthening the oxide film. As a result, the inventions described in Patent Documents 1 and 2 improve resistance to water vapor oxidation and red oxide scale.

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: Japanese Patent Application Publication No. 2003-160844

[0009] Patent Document 2: Japanese Patent Application Publication No. 2003-160842 Summary of the Invention

[0010] The problem that the invention aims to solve

[0011] The aforementioned existing technologies focus on optimizing the Cr and Si content in steel. However, such control through the addition of alloying elements leads to deterioration in manufacturability and increased costs. Therefore, the inventors of this invention have investigated methods other than adding alloying elements to improve resistance to red oxide scale. Specifically, to improve resistance to red oxide scale, the inventors of this invention have focused on oxides with a specific composition on the surface of stainless steel.

[0012] The purpose of this invention is to provide a ferritic stainless steel with excellent resistance to red oxide scale and an exhaust component with excellent resistance to red oxide scale obtained from the ferritic stainless steel as a raw material.

[0013] Methods for solving problems

[0014] The gist of the present invention for solving the above-mentioned problems is as follows.

[0015] [1] In one embodiment of the present invention, the ferritic stainless steel contains, as a chemical composition, 0.05% to 2.50% by mass of Si, 0.05% to 1.50% by mass of Mn, less than 0.025% by mass of C, less than 0.040% by mass of P, less than 0.003% by mass of S, less than 0.025% by mass of N, 0.01% to 0.50% by mass of Ni, 10.50% to 25.00% by mass of Cr, 0.01% to 1.80% by mass of Cu, 0.002% to 0.200% by mass of Al, 0.001% to 1.00% by mass of Nb, 0% to 2.5% by mass of W, 0% to 3.00% by mass of Mo, 0% to 0.500% by mass of Ti, 0% to 0.0100% by mass of B, 0% to 0% by mass of ... The composition comprises 0.0030% by mass Ca, 0% to 0.50% by mass Hf, 0% to 0.40% by mass Zr, 0% to 0.50% by mass Sb, 0% to 0.30% by mass Co, 0% to 1.0% by mass Ta, 0% to 1.00% by mass Sn, 0% to 0.30% by mass Ga, 0% to 0.50% by mass V, 0% to 0.003% by mass Mg, and 0% to 0.20% by mass REM, with the remainder containing Fe and impurities. At the surface, at least one oxide is present, consisting of an oxide containing 5% or more Al and an oxide containing 5% or more Si. Furthermore, the number of oxides with a diameter D of 0.1 μm to 2.0 μm, as expressed by formula (1), present at the surface is 93 μm. 2 There are more than 10.

[0016] D=(Dmax+Dmin) / 2 (1)

[0017] (In equation (1) above, Dmax is the maximum diameter of the oxide at the surface, and Dmin is the minimum diameter of the oxide at the surface.)

[0018] Based on the above composition, ferritic stainless steel with excellent resistance to red oxide scale can be achieved.

[0019] [2] According to the ferritic stainless steel described in [1], for the above-mentioned surface, by using a D65 light source in a diffused illumination manner, the surface is illuminated at an angle of 8° relative to the normal of the above-mentioned surface, and the CIE 1976 luminance L is measured with a field of view of 10° and a measurement time of 1 second. * It can also be 60 or higher.

[0020] Based on the above composition, ferritic stainless steel with excellent pattern design capabilities can be achieved.

[0021] [3] According to the ferritic stainless steel described in [1] or [2], the above chemical composition may also contain one or more of the following elements: 0.01% to 2.5% by mass of W, 0.01% to 3.00% by mass of Mo, 0.001% to 0.500% by mass of Ti, 0.0002% to 0.0100% by mass of B, 0.0002% to 0.0030% by mass of Ca, 0.001% to 0.50% by mass of Hf, 0.0 1% to 0.40% by mass of Zr, 0.005% to 0.50% by mass of Sb, 0.01% to 0.30% by mass of Co, 0.001% to 1.0% by mass of Ta, 0.002% to 1.00% by mass of Sn, 0.0002% to 0.30% by mass of Ga, 0.01% to 0.50% by mass of V, 0.0003% to 0.003% by mass of Mg, and 0.001% to 0.20% by mass of REM.

[0022] Based on the above composition, the machinability, high-temperature strength, corrosion resistance, oxidation resistance of steel plates, and secondary machinability of formed products made using ferritic stainless steel can be improved.

[0023] [4] The ferritic stainless steel according to any one of [1] to [3], wherein, after being kept in an atmosphere of 300 to 900°C for more than 100 hours, the Cr content is set to [Cr], the Si content is set to [Si], and the Al content is set to [Al] in units of mass% from the surface to 1.0 μm, the following formula (2) can also be satisfied.

[0024] [Cr]+[Si]+[Al]≥18.0 (2)

[0025] [5] In the case of the ferritic stainless steel of another aspect of the present invention, when the Cr content is set to [Cr], the Si content is set to [Si] and the Al content is set to [Al] in units of mass % from the surface to 1.0 μm, the following formula (2) is satisfied.

[0026] [Cr]+[Si]+[Al]≥18.0 (2)

[0027] [6] One embodiment of the exhaust component of the present invention comprises ferritic stainless steel as described in any one of [1] to [5].

[0028] Invention Effects

[0029] According to the above-described solution of the present invention, it is possible to provide ferritic stainless steel with excellent resistance to red oxide scale and exhaust components with excellent resistance to red oxide scale. Attached Figure Description

[0030] Figure 1 This is a flowchart illustrating an example of a method for manufacturing ferritic stainless steel according to one embodiment of the present invention.

[0031] Figure 2A This is an example of a SEM image of the surface of ferritic stainless steel, steel No. 6, as shown in the embodiment.

[0032] Figure 2B This is an example of a SEM image of the surface of ferritic stainless steel, steel No. 8, as shown in the embodiment. Detailed Implementation

[0033] Hereinafter, a ferritic stainless steel according to one embodiment of the present invention (ferritic stainless steel of this embodiment), a method for manufacturing the ferritic stainless steel of this embodiment, and an exhaust component obtained using the ferritic stainless steel of this embodiment as raw material (exhaust component of this embodiment) will be described. The following description is provided to better understand the gist of the invention, and unless otherwise specified, it does not limit the invention. Furthermore, in this specification, "A to B" means A or higher and B or lower.

[0034] Furthermore, in this specification, the term "stainless steel" refers to stainless steel materials without specifying a particular shape. Examples of such stainless steel materials include steel plates, steel pipes, and steel bars.

[0035] <<Ferritic Stainless Steel>>

[0036] <Chemical Composition>

[0037] The composition (chemical composition) of the ferritic stainless steel in one embodiment of the present invention is as follows. In addition to the components shown below, the ferritic stainless steel also contains iron (Fe) and / or small amounts of impurities that may be introduced from the raw materials or during the manufacturing process.

[0038] (Chromium: Cr)

[0039] Cr is an essential element for forming a passive coating and ensuring corrosion resistance. Furthermore, it is also effective in ensuring resistance to red oxide scale. To achieve this effect, the Cr content is 10.50% by mass or more. Preferably, the Cr content is 12.50% by mass or more.

[0040] On the other hand, excessive Cr content increases material costs and reduces toughness. Therefore, the Cr content is 25.00% by mass or less. Preferably, the Cr content is 23.00% by mass or less.

[0041] (Silicon: Si)

[0042] Si is an effective element for improving resistance to red oxide scale. To achieve this effect, the Si content is 0.05% by mass or more. Preferably, the Si content is 0.10% by mass or more.

[0043] On the other hand, excessive Si content can lead to reduced toughness and processability. Therefore, the Si content is 2.50% by mass or less. Preferably, the Si content is 2.00% by mass or less.

[0044] (Copper: Cu)

[0045] Cu is an element included to ensure high-temperature strength. To achieve this effect, the Cu content is 0.01% by mass or more. Preferably, the Cu content is 0.02% by mass or more.

[0046] On the other hand, if the Cu content is excessive, the ferrite phase becomes unstable, and the material cost increases. Therefore, the Cu content is 1.80% by mass or less. Preferably, the Cu content is 1.60% by mass or less.

[0047] (Niobium: Nb)

[0048] Nitrogen (Nb) is an element included to ensure high-temperature strength. To achieve this effect, the Nb content is 0.001% or more. Preferably, the Nb content is 0.05% by mass or more, and more preferably 0.10% by mass or more.

[0049] On the other hand, excessive Nb content may degrade processability and toughness. Therefore, the Nb content is 1.00% by mass or less. Preferably, the Nb content is 0.70% by mass or less, and more preferably 0.45% by mass or less.

[0050] (Manganese: Mn)

[0051] In ferritic stainless steel, manganese (Mn) is an element that improves the adhesion of oxide scale. To achieve this effect, the Mn content is 0.05% by mass or more. Preferably, the Mn content is 0.10% by mass or more.

[0052] On the other hand, if the Mn content is excessive, the ferrite phase becomes unstable and promotes the formation of MnS, which becomes the corrosion initiation point. Therefore, the Mn content is set to 1.50% by mass or less. The Mn content is preferably 1.20% by mass or less.

[0053] (Nickel: Ni)

[0054] Ni is an element that improves the corrosion resistance of ferritic stainless steel. To achieve this effect, the Ni content is 0.01% by mass or more. Preferably, the Ni content is 0.05% by mass or more.

[0055] On the other hand, if the Ni content is excessive, the ferrite phase becomes unstable, and the material cost increases. Therefore, the Ni content is 0.50% by mass or less. Preferably, the Ni content is 0.30% by mass or less.

[0056] (Carbon: C)

[0057] If carbon (C) is present in excess, the amount of carbides in ferritic stainless steel increases, and the steel's corrosion resistance decreases. Therefore, the C content should be 0.025% by mass or less. Preferably, the C content is 0.020% by mass or less.

[0058] The lower the C content, the better. It can also be 0%, but if the C content is reduced beyond what is necessary, the cost will increase. Therefore, the C content can also be set to 0.002% by mass or higher.

[0059] (Phosphorus: P)

[0060] If phosphorus (P) is present in excess, the workability of ferritic stainless steel decreases. Therefore, the P content is 0.040% by mass or less. The P content is preferably 0.030% by mass or less. Lower P content is preferred, and 0% is also acceptable, but if the P content is reduced beyond this, costs increase; therefore, the P content can also be set to 0.001% by mass or more.

[0061] (Sulfur: S)

[0062] Excessive sulfur (S) content promotes the formation of corrosion initiation sites in ferritic stainless steels. Therefore, the S content is 0.003% by mass or less. The S content is preferably 0.002% by mass or less. Lower S content is preferred, and 0% is also acceptable, but if the S content is reduced beyond this, costs increase; therefore, the S content can also be set to 0.0001% by mass or more.

[0063] (Nitrogen: N)

[0064] If nitrogen (N) is present in excess, it will form nitrides with other elements, leading to hardening of ferritic stainless steel. Therefore, the N content should be 0.025% by mass or less. The N content is preferably 0.020% by mass or less. Lower N content is preferred, and 0% is also acceptable, but if the N content is reduced beyond this, costs will increase; therefore, the N content can also be set to 0.003% by mass or more.

[0065] (Aluminum: Al)

[0066] Al is an effective element for improving the corrosion resistance of ferritic stainless steels and enhancing their resistance to red oxide scale. Furthermore, Al is an effective deoxidizer during steelmaking. To achieve these effects, the Al content is 0.002% by mass or more. Preferably, the Al content is 0.008% by mass or more.

[0067] On the other hand, excessive Al content can lead to deterioration of surface quality. Therefore, the Al content should be below 0.200% by mass.

[0068] (Other ingredients)

[0069] The ferritic stainless steel of one embodiment of the present invention may further contain one or more of the following elements: 0.01% to 2.5% by mass of W, 0.01% to 3.00% by mass of Mo, 0.001% to 0.500% by mass of Ti, 0.0002% to 0.0100% by mass of B, 0.0002% to 0.0030% by mass of Ca, 0.001% to 0.50% by mass of Hf, and 0.01% to The ferritic stainless steel of this embodiment may contain 0.40% by mass of Zr, 0.005% to 0.50% by mass of Sb, 0.01% to 0.30% by mass of Co, 0.001% to 1.0% by mass of Ta, 0.002% to 1.00% by mass of Sn, 0.0002% to 0.30% by mass of Ga, 0.01% to 0.50% by mass of V, 0.001% to 0.20% by mass of REM, and 0.0003% to 0.003% by mass of Mg. Furthermore, the ferritic stainless steel of this embodiment may also contain 0.20% or less by mass of La, preferably 0.10% or less by mass, or 0.20% or less by mass of Ce, preferably 0.05% or less by mass, as REM.

[0070] However, the presence of these elements is not essential, so their content can be 0% or lower than the range described below.

[0071] (Tungsten: W)

[0072] W is an element included to ensure high-temperature strength. To achieve this effect, the W content is preferably 0.01% by mass or more. More preferably, it is 0.1% by mass or more.

[0073] On the other hand, excessive W content increases material costs. Therefore, in the ferritic stainless steel of this embodiment, the W content is 2.5% by mass or less. The W content is preferably 1.5% by mass or less, and more preferably 1.3% by mass or less.

[0074] (Molybdenum: Mo)

[0075] Mo is an element that can be included to ensure high-temperature strength and resistance to red oxide scale. To achieve this effect, the Mo content is preferably 0.01% by mass or more.

[0076] On the other hand, if the Mo content is excessive, hardening occurs, processability decreases, and material costs increase. Therefore, the Mo content is 3.00% by mass or less. Preferably, the Mo content is 2.50% by mass or less.

[0077] (Ti)

[0078] Ti is an element that, through reaction with C and / or N, can convert ferritic stainless steel into a single-phase ferritic system at 900–1000°C, thereby improving its resistance to red oxide scale and its workability. Therefore, Ti may be included. To achieve this effect, the Ti content is preferably 0.001% by mass or more. The Ti content is preferably 0.010% by mass or more, and more preferably 0.050% by mass or more.

[0079] On the other hand, excessive Ti content may degrade processability and surface quality. Therefore, the Ti content is 0.500% by mass or less. Preferably, the Ti content is 0.300% by mass or less, and more preferably 0.250% by mass or less.

[0080] (Boron: B)

[0081] Boron (B) is an element that improves the secondary workability of formed articles manufactured using ferritic stainless steel. To achieve this effect, the B content is preferably 0.0002% by mass or more.

[0082] On the other hand, if the content of B is excessive, it may easily form compounds such as Cr2B, leading to deterioration of resistance to red oxide scale. Therefore, the B content is 0.0100% by mass or less. The B content is preferably 0.0080% by mass or less, or 0.0030% by mass or less.

[0083] (Calcium: Ca)

[0084] Ca is an element that promotes resistance to high-temperature oxidation. Therefore, it can be included as needed. To achieve this effect, the Ca content is preferably 0.0002% by mass or more.

[0085] On the other hand, excessive Ca content can lead to a decrease in corrosion resistance. Therefore, the Ca content should be below 0.0030% by mass.

[0086] (Hf)

[0087] Hf is an element that improves corrosion resistance, high-temperature strength, and oxidation resistance. It may also be included as needed. To achieve this effect, the Hf content is preferably 0.001% by mass or more. More preferably, the Hf content is 0.01% by mass or more.

[0088] On the other hand, excessive Hf content may lead to reduced processability and manufacturability. Therefore, the Hf content should be below 0.50% by mass.

[0089] (Zirconium: Zr)

[0090] Zr is an element that improves high-temperature strength, corrosion resistance and high-temperature oxidation resistance. Therefore, Zr may be optionally contained. When this effect is to be obtained, the Zr content is preferably 0.01 mass% or more.

[0091] On the other hand, excessive content of Zr leads to a decrease in workability and manufacturability. Therefore, the Zr content is 0.40 mass% or less.

[0092] (Antimony: Sb)

[0093] Sb is an element that improves high-temperature strength. Therefore, Sb may be optionally contained. When this effect is to be obtained, the Sb content is preferably 0.005 mass% or more. The Sb content is more preferably 0.01 mass% or more.

[0094] On the other hand, excessive content of Sb reduces weldability and toughness. Therefore, the Sb content is 0.50 mass% or less.

[0095] (Cobalt: Co)

[0096] Co is an element that improves high-temperature strength. Therefore, Co may be optionally contained. When this effect is to be obtained, the Co content is preferably 0.01 mass% or more.

[0097] On the other hand, excessive content of Co reduces toughness, thereby decreasing manufacturability. Therefore, the Co content is 0.30 mass% or less.

[0098] (Tantalum: Ta)

[0099] Ta is an element that improves high-temperature strength. Therefore, Ta may be optionally contained. When this effect is to be obtained, the Ta content is preferably 0.001 mass% or more. The Ta content is more preferably 0.01 mass% or more, and still more preferably 0.1 mass% or more.

[0100] On the other hand, excessive content of Ta reduces weldability and toughness. Therefore, the Ta content is 1.0 mass% or less.

[0101] (Tin: Sn)

[0102] Sn is an element that improves corrosion resistance and high-temperature strength. Therefore, Sn may be optionally contained. When this effect is to be obtained, the Sn content is preferably 0.002 mass% or more. The Sn content is more preferably 0.01 mass% or more.

[0103] On the other hand, excessive content of Sn may lead to a decrease in toughness and manufacturability. Therefore, the Sn content is 1.00 mass% or less.

[0104] (Gallium: Ga)

[0105] Ga is an element that improves corrosion resistance and resistance to hydrogen embrittlement. Therefore, it may be included as needed. To achieve this effect, the Ga content is preferably 0.0002% by mass or more. More preferably, the Ga content is 0.01% by mass or more.

[0106] On the other hand, excessive Ga content reduces weldability and toughness. Therefore, the Ga content should be 0.30% by mass or less.

[0107] (vanadium: V)

[0108] V is an element that fixes dissolved C and N in steel as compounds, thereby improving the ductility and workability of the steel. Therefore, V can be included as needed. To achieve this effect, the V content is preferably 0.01% by mass or more.

[0109] On the other hand, excessive V content reduces the workability of steel. Therefore, the V content should be 0.50% by mass or less.

[0110] (Magnesium: Mg)

[0111] Mg is not only a deoxidizing element, but also an element that refines the microstructure of the slab, thereby improving its formability. Therefore, Mg can be included as needed. To achieve this effect, the Mg content is preferably 0.0003% by mass or more.

[0112] On the other hand, excessive Mg content can lead to a decrease in corrosion resistance, weldability, and surface quality; therefore, the Ca content should be below 0.003% by mass.

[0113] (Rare earth elements: REM)

[0114] REM refers to scandium (Sc) and a group of 15 elements (lanthanides) from lanthanum (La) to lutetium (Lu). A REM may contain only one lanthanide element or two or more. When a REM contains only one lanthanide element, for example, as described below, it may contain either La or Ce, or it may contain only a lanthanide element other than La or Ce. Furthermore, when a REM contains two or more lanthanides, there are no particular restrictions on the combination of elements; however, as an example, it may contain La and Ce, or it may contain multiple lanthanides found in a mixed rare earth alloy by adding them.

[0115] REM is an element that improves the cleanliness of stainless steel and also enhances its resistance to high-temperature oxidation. Therefore, REM may be included as needed. To obtain these effects, the REM content is preferably 0.001% by mass or more. The REM content is more preferably 0.01% by mass or more.

[0116] On the other hand, excessive REM content increases alloy costs and reduces manufacturability. Therefore, the REM content is 0.20% by mass or less.

[0117] Lanthanum: La

[0118] As a REM (Refined Metal), it may also contain La. La is an element that improves the cleanliness of stainless steel, enhances its resistance to high-temperature oxidation, and also improves its resistance to red oxide scale and oxide scale peeling. When La is included to achieve this effect (using metallic La, etc.), the La content is preferably set to 0.001% by mass or more. The La content is more preferably 0.01% by mass or more.

[0119] On the other hand, if the content of La is excessive, the material cost will increase. Therefore, the La content is 0.20% by mass or less. The La content is preferably 0.10% by mass or less, and considering cost, the La content is more preferably 0.05% by mass or less, and even more preferably 0.03% by mass or less.

[0120] (Cerium: Ce)

[0121] As a REM (Residual Oil), it may also contain Ce. Ce is an element that improves the cleanliness of stainless steel and its resistance to high-temperature oxidation, as well as its resistance to red oxide scale and its resistance to oxide scale peeling. When Ce is included to obtain this effect (using metallic Ce, etc.), the Ce content is preferably 0.001% by mass or more. The Ce content is more preferably 0.01% by mass or more.

[0122] On the other hand, excessive Ce content increases material costs. Therefore, the Ce content is 0.20% by mass or less. Preferably, the Ce content is 0.05% by mass or less.

[0123] The chemical composition of the ferritic stainless steel of this embodiment can be obtained by elemental analysis using conventional methods such as ICP-AES, at a depth of 1 / 4 of the plate thickness from the surface (any distance from the surface in the thickness direction ranging from 1 / 8 to 3 / 8 of the thickness is permissible). Furthermore, C and S can be determined using the combustion-infrared absorption method, N can be determined using the inert gas melting-thermal conductivity method, and O can be determined using the inert gas melting-infrared absorption method.

[0124] <Oxides containing Al or Si>

[0125] The ferritic stainless steel of this embodiment has at least one oxide selected from oxides containing 5% by mass of Al and oxides containing 5% by mass of Si at its surface. Furthermore, the number of oxides present on the surface with a diameter D of 0.1 μm to 2.0 μm (hereinafter referred to as "Al / Si oxides") expressed by the following formula (1) is 93 μm. 2 There are more than 10.

[0126] This oxide improves resistance to red oxide scale.

[0127] D=(Dmax+Dmin) / 2 (1)

[0128] (In equation (1) above, Dmax is the maximum diameter of each oxide at the surface, and Dmin is the minimum diameter of each oxide at the surface.)

[0129] The size of oxides on the surface of ferritic stainless steel can be measured, for example, by scanning electron microscopy (SEM).

[0130] Specifically, scanning electron microscopy (SEM) was used to capture SEM images of the steel surface. The area of ​​one field of view was set to 93 μm. 2 The maximum and minimum diameters of the oxides were calculated from the SEM image using image analysis software such as Photoshop (registered trademark) (manufactured by Adobe Photoshop Inc.).

[0131] Here, the Al and Si content in the oxides on the surface of ferritic stainless steel can be determined, for example, by energy dispersive X-ray spectroscopy (EDS). That is, EDS can determine whether the oxides are statistically significant oxides (oxides containing more than 5% by mass of Al or oxides containing more than 5% by mass of Si).

[0132] In this embodiment, the "maximum diameter" of the oxide refers to the maximum width between two parallel lines when the oxide is held between them in a top-view view. Furthermore, in this specification, the "minimum diameter" of the oxide refers to the minimum width between two parallel lines when the oxide is held between them in a top-view view.

[0133] The reasons why oxides containing 5% or more Al or 5% or more Si (sometimes referred to as Al / Si oxides) improve resistance to red oxide scale are believed to be as follows: First, Al / Si oxides act as a protective coating. Second, heating causes the growth of Al / Si oxides, which lowers the oxygen partial pressure around them. Al, Si, Cr, and Fe are readily oxidized in that order; therefore, Al, Si, and Cr are preferentially oxidized compared to Fe. Thus, the growth of Al / Si oxides can reduce the formation of Fe oxides, i.e., red oxide scale.

[0134] However, when excessive Al / Si oxides are present on the surface of ferritic stainless steel, the surface gloss may decrease due to the presence of Al / Si oxides, thus deteriorating the pattern designability of the ferritic stainless steel. Therefore, the preferred number of Al / Si oxides on the surface of ferritic stainless steel is per 93 μm. 2 The number of patterns is 25 or less, more preferably 22 or less. In this case, the gloss level at the surface of the ferritic stainless steel can be improved, and the pattern design can be well maintained. In this embodiment, "gloss level" refers to the CIE1976 gloss level L measured by using a D65 light source in a diffused illumination manner, illuminating the surface of the ferritic stainless steel at an 8° angle to the normal, with a field of view of 10° and a measurement time of 1 second. * .

[0135] When the diameter D of oxides containing 5% by mass or more Al or Si on the surface of ferritic stainless steel is less than 0.1 μm, the improvement effect of the oxides on resistance to red oxide scale is low. Therefore, in this embodiment, oxides with a diameter D of 0.1 μm or more are considered.

[0136] On the other hand, if oxides with a diameter D exceeding 2.0 μm are present in oxides containing 5% or more Al or Si, the surface gloss of the ferritic stainless steel may decrease, and the pattern designability of the ferritic stainless steel may deteriorate. Therefore, it is preferable to control the number of oxides containing 5% or more Al or Si and with a diameter D of 0.1 μm to 2.0 μm on the surface of ferritic stainless steel within a specified range.

[0137] Furthermore, on the surface of ferritic stainless steel, oxides containing more than 5% by mass of Al or Si and with a diameter D exceeding 2.0 μm are preferably minimal (e.g., per 93 μm). 2 (5 or fewer), the most preferred option is none. In this case, the surface gloss of the ferritic stainless steel can be improved, and the pattern designability of the ferritic stainless steel can be enhanced.

[0138] The inventors of this invention focused on Al / Si oxides on the surface of ferritic stainless steel and realized that by controlling the number of Al / Si oxides within a specified range, ferritic stainless steel with excellent resistance to red oxide scale can be achieved.

[0139] In this embodiment, the surface of the ferritic stainless steel has a passive film with a thickness of 2.0 to 8.0 nm, in addition to Al / Si oxides. The passive film refers to a very dense and highly adhesive film formed by hydrated chromium hydroxyl oxide and chromium oxide, mainly composed of Cr.

[0140] The thickness of the passive film can be determined using a high-frequency glow discharge luminescence analyzer (GDS). Specifically, using a GDS analyzer (such as the HORIBA GD-Profiler2 or an equivalent device), the oxygen concentration is analyzed along the thickness direction from the surface at a 2.5 nm interval. The area from the surface to the position where the oxygen concentration shows half of the peak value is taken as the passive film, and its thickness is determined by measuring this value.

[0141] For example, other GDS measurement conditions are described below.

[0142] Gas replacement time: 200 seconds

[0143] Pre-splashing time: 30 seconds

[0144] Background: 5 seconds

[0145] Depth: 1.01μm

[0146] Pressure: 600Pa

[0147] Output power: 35W

[0148] RMS value: 8.75W

[0149] Module: 8V

[0150] Phase: 4V

[0151] Frequency: 100Hz; Duty Cycle: 0.25.

[0152] In the case of the ferritic stainless steel of this embodiment, when used at high temperatures containing water vapor, Al / Si oxides grow, reducing the oxygen partial pressure around the Al / Si oxides. As a result, unlike conventional ferritic stainless steel, it exhibits excellent resistance to red oxide scale due to a surface morphology characterized by reduced formation of Fe oxides, which constitute the main body of the red oxide scale, and increased formation of Cr, Al, and Si oxides.

[0153] For example, after being kept in an atmosphere of 300 to 900°C for more than 100 hours, within the range from the surface to 1.0 μm (surface layer), when the Cr content is set to [Cr], the Si content is set to [Si], and the Al content is set to [Al] in units of mass %, the following formula (2) is satisfied.

[0154] [Cr]+[Si]+[Al]≥18.0 (2)

[0155] It is believed that when equation (2) is not satisfied, Fe oxides dominate the surface, resulting in an excessive amount of red oxide scale.

[0156] The [Cr]+[Si]+[Al] content is preferably 20.0 (mass%) or more.

[0157] It is not necessary to separately limit the Cr content [Cr], Si content [Si], and Al content [Al] from the surface to 1.0 μm, but considering the effect of suppressing red oxide scale, the Si content and / or Al content are preferably 3.0% by mass or more.

[0158] It is believed that Al / Si oxides grow slowly in a high-temperature atmosphere for a certain period of time, but if the time exceeds 100 hours, there is no significant change. Therefore, the state of Cr, Si, and Al also does not change much.

[0159] That is, it is believed that when using the ferritic stainless steel of this embodiment as a component such as an exhaust path component, a furnace combustion engine component, a fuel cell component or a plant equipment related component, the content of Cr, Al and Si in the surface layer satisfies formula (2).

[0160] On the other hand, even with the above-mentioned maintenance, the chemical composition at the 1 / 4 depth location remained unchanged.

[0161] <<Exhaust Components>>

[0162] The exhaust component of this embodiment is obtained by using ferritic stainless steel as the raw material and processing it. Therefore, the exhaust component of this embodiment, in the stage obtained through processing or the like (before use as a component), contains the following chemical composition: 0.05% to 2.50% by mass of Si, 0.05% to 1.50% by mass of Mn, less than 0.025% by mass of C, less than 0.040% by mass of P, less than 0.003% by mass of S, less than 0.025% by mass of N, 0.01% to 0.50% by mass of Ni, 10.50% to 25.00% by mass of Cr, 0.01% to 1.80% by mass of Cu, 0.002% to 0.200% by mass of Al, 0.001% to 1.00% by mass of Nb, 0% to 2.5% by mass of W, 0% to 3.00% by mass of Mo, 0% to 0.500% by mass of Ti, and 0% to 0.0100% by mass of... B, 0% to 0.0030% Ca, 0% to 0.50% Hf, 0% to 0.40% Zr, 0% to 0.50% Sb, 0% to 0.30% Co, 0% to 1.0% Ta, 0% to 1.00% Sn, 0% to 0.30% Ga, 0% to 0.50% V, 0% to 0.003% Mg, and 0% to 0.20% REM, with the remainder containing Fe and impurities. At the surface, at least one oxide containing 5% or more Al and 5% or more Si is present. Furthermore, the number of oxides with a diameter D of 0.1 μm to 2.0 μm, as expressed by formula (1), present at the surface is 93 μm. 2 There are more than 10.

[0163] D=(Dmax+Dmin) / 2 (1)

[0164] In the above formula (1), Dmax is the maximum diameter of the oxide on the surface and Dmin is the minimum diameter of the oxide on the surface.

[0165] Furthermore, for the aforementioned surface, it is also possible to use a D65 light source in a diffused illumination manner, receiving light at an 8° angle relative to the surface normal, thereby obtaining the CIE1976 luminance L measured at a field of view of 10° and a measurement time of 1 second. * It is above 60.

[0166] Furthermore, when the exhaust component of this embodiment is kept in an atmosphere of 300 to 900°C for more than 100 hours, the following formula (2) is satisfied in the range from the surface to 1.0 μm, in terms of mass %: Cr content is set to [Cr], Si content is set to [Si], and Al content is set to [Al].

[0167] [Cr]+[Si]+[Al]≥18.0 (2)

[0168] That is, when the exhaust component of this embodiment is, for example, an exhaust path component, a furnace combustion component, a fuel cell component, or a factory equipment related component, and when used under normal conditions for a certain period of time (after use) in such an application, the following formula (2) is satisfied when the Cr content is set to [Cr], the Si content is set to [Si], and the Al content is set to [Al] in the range from the surface to 1.0 μm, in units of mass %.

[0169] [Cr]+[Si]+[Al]≥18.0 (2)

[0170] In either the ferritic stainless steel of this embodiment or the exhaust component of this embodiment, the Cr content [Cr], Si content [Si], and Al content [Al] in the range from the surface to 1.0 μm can be measured using GDS.

[0171] Specifically, the analysis area was set to φ4mm, and all elements other than C and N in each steel were measured at 2.5nm intervals up to a depth of 1.0μm. From the measurement results, the contents of Cr, Al, and Si at the positions where Cr, Al, and Si showed peaks within the range up to a depth of 1.0μm were calculated.

[0172] <<Manufacturing Methods of Ferritic Stainless Steel>>

[0173] In the past, methods for improving resistance to red oxide scale included: promoting Cr diffusion in steel and promoting the formation of Cr oxides by performing surface grinding as a finishing process; or forming a hot-dip coating on the surface.

[0174] The inventors of this invention have discovered that, for example, by the following manufacturing method, it is possible to obtain an Al / Si oxide layer with a surface area of ​​93 μm. 2 It is a ferritic stainless steel with more than 10 components and excellent resistance to red oxide scale.

[0175] In one embodiment of the present invention, the ferritic stainless steel is obtained, for example, by means of ferritic stainless steel tape. Figure 1This is a flowchart illustrating an example of a method for manufacturing ferritic stainless steel according to this embodiment. For example... Figure 1 As shown, the manufacturing method of ferritic stainless steel strip in this embodiment includes: a pretreatment process S1, a hot rolling process S2, an annealing process S3, a first pickling process S4, a cold rolling process S5, a final annealing process S6, a nitric acid electrolysis process S7, and a final pickling process S8.

[0176] The optimal conditions for each process are explained below. For conditions not explicitly stated below, generally known conditions may be used.

[0177] <Pre-treatment process>

[0178] In the pretreatment step S1, firstly, steel with its chemical composition adjusted according to the aforementioned scope of the present invention is melted in a melting furnace under vacuum or argon atmosphere, and then cast to produce a slab. Afterwards, hot-rolled slab sheets are cut from the slab. Then, the slab sheets are heated in atmospheric atmosphere to a temperature range of 1100°C to 1300°C. The heating and holding time of the slab sheets is not limited. When the pretreatment step is performed industrially, the casting can be continuous casting.

[0179] <Hot Rolling Process>

[0180] Hot rolling process S2 is a process of manufacturing hot-rolled steel strip of a specified thickness by hot rolling the slab (steel ingot) obtained in the pretreatment process S1. There are no restrictions on the conditions for hot rolling; they can be adjusted according to the required mechanical properties, etc.

[0181] Annealing process

[0182] Annealing process S3 is a process that softens the hot-rolled steel strip obtained in hot rolling process S2 by heating it. This annealing process S3 is performed as needed, or it may be omitted.

[0183] <First pickling process>

[0184] The first pickling process S4 is a process in which the oxide scale adhering to the surface of the steel strip is washed away using pickling solutions such as hydrochloric acid or a mixture of nitric acid and hydrofluoric acid.

[0185] <Cold rolling process>

[0186] The cold rolling process S5 is a process that further thins the steel strip that has had its oxide scale removed in the first pickling process S4.

[0187] <Final Annealing Process>

[0188] The final annealing process S6 is a process of removing strain and softening the steel strip by heating the steel strip that has been thinned in the cold rolling process S5. In addition, it is a process of forming an outer oxide such as (Fe,Cr)3O4 or Cr2O3 and an inner oxide such as Al or Si.

[0189] Regarding the final annealing process S6, for the aforementioned purposes, the annealing is carried out at a temperature of approximately 900–1100°C for a duration of 30–90 seconds, in an atmosphere such as air or liquefied gas (LNG), depending on the alloy composition.

[0190] <Nitric acid electrolysis process>

[0191] The nitric acid electrolysis step S7 is a process in which the steel strip obtained in the final annealing step S6 is electrolyzed in an aqueous nitric acid solution. In the nitric acid electrolysis step S7, the oxides adhering to the surface of the steel strip are partially removed.

[0192] Specifically, the surface of the steel strip obtained in the final annealing process S6 is formed with an outer oxide layer, such as (Fe,Cr)3O4 or Cr2O3. Furthermore, between this outer oxide layer and the base material, an inner oxide layer, primarily composed of oxides such as Al or Si, is formed. In the nitric acid electrolysis process S7, nitric acid electrolysis is performed under conditions where most of the outer oxide layer is removed, while most of the inner oxide layer remains. Preferably, the condition is set such that although most of the inner oxide layer remains, a portion of it is slightly peeled off and easily removed by the final pickling process S8.

[0193] The nitric acid concentration in the nitric acid electrolysis step S7 is preferably below 150 g / L. Under these conditions, after the final pickling step S8, it is easy for residues to remain on the surface of the steel strip at a rate of 93 μm. 2 It consists of more than 10 Al / Si oxides.

[0194] On the other hand, if more Al / Si oxides remain than necessary, the surface gloss decreases. Therefore, in order to set the amount of Al / Si oxides on the surface after the final pickling process to a preferred range, the nitric acid concentration in the nitric acid electrolysis step S7 is preferably 100 g / L or more. Furthermore, in order to effectively remove the outer oxide layer in a short time, the nitric acid concentration in the nitric acid electrolysis step S7 is preferably 130 g / L or more.

[0195] The liquid temperature in the nitric acid electrolysis step S7 is preferably below 70°C, more preferably below 60°C. Under these conditions, after the final pickling step S8, it is easy for residues to remain on the surface of the steel strip at a rate of 93 μm. 2 It consists of more than 10 Al / Si oxides.

[0196] On the other hand, the liquid temperature in the nitric acid electrolysis process S7 is preferably 50°C or higher, more preferably 60°C or higher. In this case, the outer oxide layer can be effectively removed in a short time.

[0197] The preferred current density in the nitric acid electrolysis process S7 is 150 mA / cm². 2 The following applies. In this case, after the final pickling process S8, it is easy for residues to remain on the surface of the steel strip every 93μm. 2 It consists of more than 10 Al / Si oxides.

[0198] On the other hand, the current density in the nitric acid electrolysis process S7 is preferably 100 mA / cm². 2 The above is preferred, and more preferably is 120 mA / cm. 2 The above is further optimized to 130 mA / cm. 2 That's all. In this case, the outer oxide layer can be effectively removed in a short time.

[0199] The electrolysis time in the nitric acid electrolysis step S7 is more preferably 120 seconds or less. In this case, after the final pickling step S8, it is possible to leave 93 μm of residue on the surface of the steel strip. 2 It consists of more than 10 Al / Si oxides.

[0200] On the other hand, the electrolysis time in the nitric acid electrolysis step S7 is preferably 60 seconds or more. In this case, it is possible to set a state in which most of the outer oxide layer is reliably removed and a portion of the inner oxide layer is easily peeled off, and the residual amount of Al / Si oxide after the final pickling process is set to a preferred range.

[0201] <Final pickling process>

[0202] The final pickling process S8 involves immersing the steel strip, after the nitric acid electrolysis process S7, in a pickling solution such as a mixture of nitric acid and hydrofluoric acid. In the final pickling process S8, the internal oxides that were slightly peeled off during the nitric acid electrolysis process S7 are removed. This ensures that the steel strip surface is clean and free of oxides, with a thickness of 93 μm. 2 The brightness is improved by removing more than 10 Al / Si oxides and removing any excess Al / Si oxides.

[0203] As mentioned above, in the existing technology, further finishing processes such as fine grinding or coating are performed to improve resistance to red oxide scale. However, such finishing processes require the introduction of new equipment, which increases manufacturing costs. From this perspective, there is a need for a method to manufacture ferritic stainless steel with excellent resistance to red oxide scale without increasing manufacturing costs.

[0204] In the manufacturing method described in this embodiment, the apparatus commonly used in the pickling process can be used in the nitric acid electrolysis step S7 and the final pickling step S8, thus enabling the production of ferritic stainless steel with excellent resistance to red oxide scale without increasing manufacturing costs.

[0205] <Manufacturing Method for Exhaust Components>

[0206] The exhaust component of this embodiment can be obtained by processing the ferritic stainless steel of this embodiment into a specified component shape using a known processing method.

[0207] <Example>

[0208] The embodiments of the present invention will be described below. First, ferritic stainless steel was manufactured by using the components shown in Table 1 below as raw materials and proceeding under the same conditions up to the final annealing step S6 of the above manufacturing method, then performing a nitric acid electrolysis step S7 with the electrolysis time shown in Table 2, and then performing a final pickling step S8.

[0209] In this embodiment, the composition of each stainless steel shown in Table 1 is expressed as mass % . Furthermore, the remainder besides the components shown in Table 1 consists of Fe and impurities. Additionally, the underlines in Table 1 indicate that the range of components contained in the stainless steels of the comparative examples of the present invention is outside the scope of the present invention.

[0210] [Table 1]

[0211]

[0212] As shown in Table 1, ferritic stainless steels produced in a manner whose chemical composition falls within the scope of this invention are designated as steel grades A1 to A10. Furthermore, ferritic stainless steels produced in a manner whose chemical composition falls outside the scope of this invention are designated as steel grades B1 to B3.

[0213] Table 2 shows the conditions used to manufacture steel products No. 1 to 46 using steel grades A1 to A10 and steel grades B1 to B3, and the evaluation results for each steel product. The conditions used in manufacturing each steel product shown in Table 2 are as follows.

[0214] • Vacuum atmosphere in the melting furnace during pretreatment step S1

[0215] • The mass of the slab produced in pretreatment step S1 is 30 kg.

[0216] • The heating temperature of the slab in pretreatment step S1 is 1230℃

[0217] • The heating time for the slab in pretreatment step S1 is 2 hours.

[0218] · Plate thickness after hot rolling step S2: 4 mm

[0219] · Annealing step S3: Not implemented

[0220] · Pickling solution used in first pickling step S4: Nitric acid-hydrofluoric acid solution (aqueous solution with hydrofluoric acid concentration: 30 g / L and nitric acid concentration: 100 g / L)

[0221] · Liquid temperature in first pickling step S4: 40 to 50°C

[0222] · Plate thickness after cold rolling step S5: 1.5 mm

[0223] · Annealing temperature in final annealing step S6: 900 to 1100°C (changed according to alloy composition)

[0224] · Annealing time in final annealing step S6: 60 seconds

[0225] · Annealing atmosphere in final annealing step S6: Atmosphere

[0226] · Nitric acid concentration in nitric acid electrolysis step S7: 150 g / L

[0227] · Liquid temperature in nitric acid electrolysis step S7: 50 to 70°C

[0228] · Current density in nitric acid electrolysis step S7: 150 mA / cm 2

[0229] · Electrolysis time in nitric acid electrolysis step S7: 30 to 180 seconds (shown in Table 2)

[0230] · Pickling solution used in final pickling step S8: Nitric acid-hydrofluoric acid solution (aqueous solution with hydrofluoric acid concentration: 20 g / L and nitric acid concentration: 70 to 80 g / L)

[0231] · Liquid temperature in final pickling step S8: 40 to 50°C

[0232] [Table 2]

[0233]

[0234] For Steel Nos. 1 to 46, the number of Al / Si-based oxides was measured by the method detailed below. The measurement results are shown in Table 2.

[0235] <Number of Al / Si-based oxides>

[0236] The number of Al / Si oxides on the surface of the steel was determined as follows. First, SEM images of the steel surface were taken at 10,000x magnification using a scanning electron microscope (SEM) SU5000 (manufactured by Hitachi High-Technologies Corporation). The dimensions of one field of view were 8.34 μm in length × 11.2 μm in width, and the area of ​​one field of view was 93 μm². 2 Furthermore, the composition of the oxides was analyzed using an energy dispersive X-ray spectroscopy (EDS) instrument (manufactured by Horiba Corporation) with an accelerating voltage of 15 kV and an analysis time of 60 seconds.

[0237] By analyzing the images taken by SEM using the image analysis software "Photoshop (registered trademark)" (manufactured by Adobe Photoshop Co., Ltd.), the number of oxides containing more than 5% by mass of Al or Si and with a diameter D of 0.1 μm to 2.0 μm can be calculated, i.e., the number of Al / Si oxides.

[0238] Furthermore, the thickness of the passive film was determined using the method described above. Although not shown in the table, the thickness of the passive film ranged from 2.0 to 8.0 nm.

[0239] Figure 2A , Figure 2B Examples of SEM photos taken using the methods described above. For example... Figure 2A As shown, the number of Al / Si oxides present on the surface of steel No. 6, which is an example of the invention, is more than 10 in one field of view. In contrast, as... Figure 2B As shown, the number of Al / Si oxides present on the surface of steel No. 8, used as a comparative example, is less than 10 in one field of view. In the SEM image of steel No. 6, it appears that there are more than 30 oxides in one field of view. However, the number of oxides containing more than 5% by mass of Al or Si, and with a diameter D of 0.1 μm to 2.0 μm, i.e., Al / Si oxides, is 19 in one field of view.

[0240] As shown in Table 2, for steel grades A1 to A10 with electrolysis times of 30 to 120 seconds, the number of Al / Si oxides with diameters D of 0.1 μm to 2.0 μm in grades A1 to A10 and electrolysis times of 30 to 120 seconds is as follows: 2 There are more than 10.

[0241] On the other hand, even for steel grades A1 to A9, but with an electrolysis time of 180 seconds, the number of Al / Si oxides in steels No. 4, 8, 12, 16, 20, 24, 28, 31, and 34 was [missing information - likely a number] per 93 μm. 2 Less than 10.

[0242] On the other hand, for steel grades B1 or B2, in any of steel grades No. 37 to 42, the number of Al / Si oxides is 93 μm. 2 Less than 10.

[0243] For steel grade B3, the number of Al / Si oxides in steels No. 43 and 44, with an electrolysis time of 40–60 seconds, is [number] per 93 μm. 2 For steel grades B3 and steels No. 45 and 46 with an electrolysis time of 120–180 seconds, the number of Al / Si oxides per 93 μm is 10 or more. 2 Less than 10.

[0244] <Oxidation Increment (Evaluation of Resistance to Red Oxidation Scale)>

[0245] In addition, for steels No. 1 to 46, in order to evaluate resistance to red oxide scale, the oxidation increment of the steel was measured according to JIS Z 2281:1993 (Metallic Materials - Test Method for Continuous Oxidation at High Temperature) as follows.

[0246] First, 20mm × 25mm test pieces were cut from each steel sample. For each test piece, under conditions simulating the combustion of petroleum-based fuels, it was continuously heated at 600°C for 100 hours in an atmospheric environment with a water vapor concentration of 10% by volume. The oxidation increment was calculated from the mass change before and after the test.

[0247] As a criterion for evaluating resistance to red oxide scale, if the oxidation increment is 0.20 mg / cm² 2 The following are judged to have excellent resistance to red oxide scale.

[0248] As shown in Table 2, for steel grades A1 to A10, the number of Al / Si oxides per 93 μm is... 2 For steel grades No. 1-3, 5-7, 9-11, 13-15, 17-19, 21-23, 25-27, 29-30, 32-33, 35, and 36 with more than 10 grades, the oxidation increment is 0.20 mg / cm³. 2 Therefore, these steels exhibit good resistance to red oxide scale.

[0249] On the other hand, even for steel grades A1 to A9, the number of Al / Si oxides per 93 μm 2For steel grades No. 4, 8, 12, 16, 20, 24, 28, 31, and 34 with an oxidation increment of less than 10, the increase exceeded 0.20 mg / cm³. 2 Therefore, these steels exhibit low resistance to red oxide scale.

[0250] In addition, for steel grades B1 or B2, the number of Al / Si oxides per 93 μm is... 2 For steel grades No. 37-42 with fewer than 10 oxidation increments, the oxidation increase exceeds 0.20 mg / cm³. 2 Therefore, these steels exhibit low resistance to red oxide scale.

[0251] For steel grade B3, regardless of the number of Al / Si oxides, the oxidation increment in all steel grades No. 43 to 46 exceeds 0.20 mg / cm³. 2 Therefore, these steels exhibit low resistance to red oxide scale. It is speculated that, for steel grade B3, due to its low Al content of only 0.001% by mass, even with more than 10 Al / Si oxides, the resistance to red oxide scale is still low.

[0252] <Surface morphology after long-term testing>

[0253] To evaluate the surface oxide morphology after long-term testing, the Cr, Si, and Al contents of any part of the test pieces of steel No. 1–4, 13–16, 32–34, and 37–39, which were evaluated for the aforementioned oxidation increment (red oxide scale resistance evaluation), were determined using a glow discharge spectrophotometer (GDS) (HORIBA GD-Profiler2). The analysis area was set to φ4 mm, and measurements were taken at 2.5 nm intervals from the surface to a depth of 1.0 μm.

[0254] Elemental analysis involves determining all elements other than C and N contained in any part of the steel.

[0255] Other GDS measurement conditions are set as follows.

[0256] Gas replacement time: 200 seconds

[0257] Pre-splashing time: 30 seconds

[0258] Background: 5 seconds

[0259] Depth: 1.01μm

[0260] Pressure: 600Pa

[0261] Output power: 35W

[0262] RMS value: 8.75W

[0263] Module: 8V

[0264] Phase: 4V

[0265] Frequency: 100Hz; Duty cycle: 0.25.

[0266] After measurement, the contents of Cr, Al, and Si at the peak positions up to a depth of 1.0 μm were substituted into equation (2) for calculation to confirm whether the condition was met. The table shows the value of ([Cr]+[Si]+[Al]) on the left side of equation (2).

[0267] As shown in Table 2, the oxidation increment is 0.20 mg / cm³. 2 The following, No.1~3, 13~15 and 32~33, which have high resistance to red oxide scale, satisfy formula (2).

[0268] On the other hand, the oxidation increment became greater than 0.20 mg / cm³. 2 Nos. 4, 16, 34 and 37-39, which have low resistance to red oxide scale, do not satisfy formula (2).

[0269] <Luminosity>

[0270] To evaluate the pattern designability of steel, the gloss level of steel No. 1 to 46 is measured according to the following procedure.

[0271] Using a spectrophotometer (model: CM-700d, manufactured by KONICA MINOLTA), after white calibration at room temperature (23°C), the gloss level L at the surface of the steel was measured at this temperature. * As the evaluation criterion, the luminance L is used... * A score of 60 or above indicates excellent pattern design.

[0272] As shown in Table 2, the brightness L of steel grades A1 to A10 with electrolysis times of 60 to 180 seconds is as follows: No. 2 to 4, 6 to 8, 10 to 12, 14 to 16, 18 to 20, 22 to 24, 26 to 28, 29 to 31, and 33 to 34. * The value is above 60. Therefore, for these steels, they exhibit good pattern design potential.

[0273] On the other hand, the brightness L of steel grades A1 to A9 and electrolysis times of 30 to 40 seconds for steels No. 1, 5, 9, 13, 17, 21, 25, and 32 is also noteworthy. * Below 60.

[0274] (Additional Notes)

[0275] This invention is not limited to the embodiments described above. Various modifications can be made within the scope of the claims. Embodiments obtained by appropriately combining the technical solutions disclosed in different embodiments are also included within the technical scope of this invention.

Claims

1. A ferritic stainless steel, wherein, As a chemical component, it contains: 0.05% to 2.50% by mass of Si Mn, 0.05% to 1.50% by mass C, less than 0.025% by mass P less than 0.040% by mass S content below 0.003% by mass N less than 0.025% by mass Ni, 0.01% to 0.50% by mass Cr, 10.50% by mass to 25.00% by mass Cu from 0.01% to 1.80% by mass Al from 0.002% to 0.200% by mass 0.001% by mass to 1.00% by mass of Nb W, 0% to 2.5% by mass Mo from 0% to 3.00% by mass Ti with a mass percentage of 0% to 0.500% B, 0% by mass to 0.0100% by mass 0% by mass to 0.0030% by mass of Ca, Hf of 0% to 0.50% by mass Zr, 0% by mass to 0.40% by mass Sb, 0% to 0.50% by mass Co, 0% to 0.30% by mass Ta from 0% to 1.0% by mass Sn, 0% by mass to 1.00% by mass Ga from 0% to 0.30% by mass V, 0% by mass to 0.50% by mass 0% by mass to 0.003% by mass of Mg, and REMs ranging from 0% to 0.20% by mass The remaining portion contains Fe and impurities. At the surface, there is at least one oxide selected from oxides containing 5% by mass or more Al and oxides containing 5% by mass or more Si, and the number of oxides present at the surface with a diameter D of 0.1 μm to 2.0 μm, as expressed by the following formula (1), is 93 μm. 2 More than 10 D=(Dmax+Dmin) / 2 (1), In equation (1), Dmax is the maximum diameter of the oxide at the surface, and Dmin is the minimum diameter of the oxide at the surface.

2. The ferritic stainless steel according to claim 1, wherein, For the surface, a D65 light source was used in a diffuse illumination manner, illuminating it at an angle of 8° relative to the surface normal, thereby measuring the CIE 1976 luminance L at a field of view of 10° and a measurement time of 1 second. * It is above 60.

3. The ferritic stainless steel according to claim 1, wherein, The chemical composition contains one or more of the following elements: W, 0.01% to 2.5% by mass Mo from 0.01% to 3.00% by mass Ti, 0.001% to 0.500% by mass B, ranging from 0.0002% to 0.0100% by mass 0.0002% by mass to 0.0030% by mass of Ca, 0.001% to 0.50% by mass of Hf Zr, 0.01% to 0.40% by mass 0.005% to 0.50% by mass of Sb 0.01% to 0.30% by mass of Co Ta from 0.001% to 1.0% by mass Sn, 0.002% by mass to 1.00% by mass Ga from 0.0002% to 0.30% by mass V, 0.01% by mass to 0.50% by mass 0.0003% by mass to 0.003% by mass of Mg, and REM concentrations ranging from 0.001% to 0.20% by mass.

4. The ferritic stainless steel according to any one of claims 1 to 3, wherein, After being kept in an atmosphere of 300 to 900°C for more than 100 hours, within the range from the surface to 1.0 μm, when the Cr content is set to [Cr], the Si content is set to [Si], and the Al content is set to [Al] in terms of mass %, the following equation (2) is satisfied. [Cr]+[Si]+[Al]≥18.0 (2).

5. A ferritic stainless steel having the chemical composition of claim 1 or 3, wherein, in the range from the surface to 1.0 μm, the Cr content is set to [Cr], the Si content is set to [Si], and the Al content is set to [Al], in units of mass % and the following formula (2) is satisfied. 36.7≥[Cr]+[Si]+[Al]≥18.0 (2).

6. An exhaust component comprising ferritic stainless steel as claimed in any one of claims 1 to 5.

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