JP7584533B2 - Ferritic stainless steel and exhaust gas parts - Google Patents

Description

The present invention relates to a ferritic stainless steel and an exhaust gas part, more specifically to a ferritic stainless steel having excellent resistance to red scale in a high-temperature steam atmosphere, and an exhaust gas part obtained by using the ferritic stainless steel as a raw material.
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.

When stainless steel is used for applications such as exhaust gas route components, stove combustion equipment, fuel cell components, or plant-related materials, it is usually heated to high temperatures of 300 to 900°C. Furthermore, in the above applications, the stainless steel is used in an environment containing water vapor, which can result in the formation of red scale (iron-based oxides). In some cases, this red scale can fly off and adhere to other parts, causing adverse effects, and there is also the risk of reducing high-temperature strength due to thinning caused by oxidation.

Therefore, ferritic stainless steels that are resistant to red scale formation in high-temperature steam atmospheres are desirable. Various methods have been known to improve red scale resistance.

Patent Document 1 and Patent Document 2 state that the addition of Si promotes the diffusion of Cr, increases the amount of Cr-based oxides produced, and strengthens the oxide film. As a result, the inventions described in Patent Document 1 and Patent Document 2 improve steam oxidation resistance and red scale resistance.

Japanese Patent Application Publication No. 2003-160844 Japanese Patent Application Publication No. 2003-160842

The above-mentioned conventional techniques focus on Cr and Si in steel and optimize the Cr and Si contents in the steel. However, such control by adding alloy elements leads to poor manufacturability and increased costs. Therefore, the inventors have investigated ways to improve red scale resistance other than by adding alloy elements. Specifically, the inventors have focused on oxides of a specific composition on the surface of stainless steel to improve red scale resistance.

The present invention aims to provide a ferritic stainless steel having excellent red scale resistance and an exhaust gas part made from this ferritic stainless steel having excellent red scale resistance.

In order to solve the above problems, the gist of the present invention is as follows.
[1] The ferritic stainless steel according to one embodiment of the present invention has a chemical composition of 0.05% by mass or more and 2.50% by mass or less of Si, 0.05% by mass or more and 1.50% by mass or less of Mn, 0.025% by mass or less of C, 0.040% by mass or less of P, 0.003% by mass or less of S, 0.025% by mass or less of N, 0.01% by mass or more and 0.50% by mass or less of Ni, 10.50% by mass or more and 25.0 ... Cr below 0.01 mass% and 1.80 mass% and less, Cu from 0.002 mass% to 0.200 mass% and less, Nb from 0.001 mass% to 1.00 mass% and less, W from 0 mass% to 2.5 mass% and less, Mo from 0 mass% to 3.00 mass% and less, Ti from 0 mass% to 0.500 mass% and less, B from 0 mass% to 0.0100 mass% and less, Ca from 0 mass% to 0.0030 mass% and less, 0 The oxide contains at least one of oxides containing 5% or more by mass of Al and oxides containing 5% or more by mass of Si on the surface, and the number of oxides having a diameter D of 0.1 μm or more and 2.0 μm or less, represented by the following formula (1), among the oxides present on the surface in an observation field of view , is 10 or more per 93 ^μm2 .
D=(Dmax+Dmin)/2...(1)
(In the 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.)

According to the above configuration, it is possible to realize a ferritic stainless steel having excellent red scale resistance.

[2] The ferritic stainless steel according to [1] may have a CIE 1976 lightness L* of 60 or more when the surface is measured using a ^D65 light source in a diffuse lighting system with light received at an angle of 8° to the normal to the surface, with a viewing angle of 10° and a measurement time of 1 second.

The above configuration makes it possible to realize a ferritic stainless steel with excellent design properties.

[3] The ferritic stainless steel described in [1] or [2] has a chemical composition of 0.01% by mass or more and 2.5% by mass or less of W, 0.01% by mass or more and 3.00% by mass or less of Mo, 0.001% by mass or more and 0.500% by mass or less of Ti, 0.0002% by mass or more and 0.0100% by mass or less of B, 0.0002% by mass or more and 0.0030% by mass or less of Ca, 0.001% by mass or more and 0.50% by mass or less of Hf, 0.01% by mass or more and 0.40% by mass or less of Zr, 0.00 The alloy may contain one or more of 5 mass % or more and 0.50 mass % or less of Sb, 0.01 mass % or more and 0.30 mass % or less of Co, 0.001 mass % or more and 1.0 mass % or less of Ta, 0.002 mass % or more and 1.00 mass % or less of Sn, 0.0002 mass % or more and 0.30 mass % or less of Ga, 0.01 mass % or more and 0.50 mass % or less of V, 0.0003 mass % or more and 0.003 mass % or less of Mg, and 0.001 mass % or more and 0.20 mass % or less of REM.

According to the above configuration, it is possible to improve the workability, high-temperature strength, corrosion resistance, oxidation resistance of the steel plate, and secondary workability of molded products manufactured using ferritic stainless steel.

[4] The ferritic stainless steel according to any one of [1] to [3] may satisfy the following formula (2) in a range from the surface to 1.0 μm after being held in an atmosphere of 300 to 900° C. for 100 hours or more, where the Cr content is [Cr], the Si content is [Si], and the Al content is [Al], all in mass %:
[Cr]+[Si]+[Al]≧18.0…(2)

[5] In the ferritic stainless steel according to another embodiment of the present invention, the following formula (2) is satisfied in the range from the surface to 1.0 μm, where the Cr content is [Cr], the Si content is [Si], and the Al content is [Al], all in unit mass %:
36.7≧ [Cr]+[Si]+[Al]≧18.0…(2)

[6] An exhaust gas part according to one embodiment of the present invention comprises a ferritic stainless steel according to any one of [1] to [5].

According to the above aspect of the present invention, it is possible to provide a ferritic stainless steel having excellent red scale resistance and an exhaust gas component having excellent red scale resistance.

1 is a flowchart showing an example of a method for producing a ferritic stainless steel according to an embodiment of the present invention. 1 is a diagram showing an example of a SEM photograph of the surface of ferritic stainless steel of Example Steel No. 6. 1 is a diagram showing an example of a SEM photograph of the surface of ferritic stainless steel of Example Steel No. 8.

Below, we will explain a ferritic stainless steel according to one embodiment of the present invention (the ferritic stainless steel according to this embodiment), a manufacturing method for the ferritic stainless steel according to this embodiment, and an exhaust gas part obtained using the ferritic stainless steel according to this embodiment as a material (the exhaust gas part according to this embodiment). The following description is intended to provide a better understanding of the gist of the invention, and does not limit the present invention unless otherwise specified. Also, in this specification, "A to B" indicates A or more and B or less.

In this specification, the term "stainless steel" refers to a stainless steel material whose specific shape is not limited. Examples of stainless steel materials include steel plates, steel pipes, and bar steel.

<<Ferritic stainless steel>>
<Chemical composition>
The composition (chemical composition) of components contained in the ferritic stainless steel according to one embodiment of the present invention is as follows. In addition to the components shown below, the ferritic stainless steel is made up of iron (Fe) and/or small amounts of impurities (impurities) that may be mixed in from the raw materials or during the manufacturing process.

(Chromium: Cr)
Cr is an essential element for forming a passive film and ensuring corrosion resistance. It is also effective for ensuring red scale resistance. To obtain this effect, the Cr content is 10.50 mass% or more. The Cr content is preferably 12.50 mass% or more.
On the other hand, an excessive Cr content increases the material cost and reduces the toughness, so the Cr content is set to 25.00 mass% or less, and preferably 23.00 mass% or less.

(Silicon: Si)
Silicon is an element effective in improving red scale resistance. To obtain this effect, the silicon content is 0.05 mass% or more. The silicon content is preferably 0.10 mass% or more.
On the other hand, if the Si content is excessive, it causes a decrease in toughness and workability. Therefore, the Si content is 2.50 mass% or less, and preferably 2.00 mass% or less.

(Copper: Cu)
Cu is an element contained to ensure high-temperature strength. To obtain this effect, the Cu content is 0.01 mass % or more, and preferably 0.02 mass % or more.
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 mass% or less, and preferably 1.60 mass% or less.

(Niobium: Nb)
Nb is an element contained to ensure high-temperature strength. To obtain this effect, the Nb content is 0.001% or more. The Nb content is preferably 0.05% by mass or more, and more preferably 0.10% by mass or more.
On the other hand, if Nb is contained excessively, there is a possibility that workability and toughness may be deteriorated. Therefore, the Nb content is 1.00 mass% or less. The Nb content is preferably 0.70 mass% or less, and more preferably 0.45 mass% or less.

(Manganese: Mn)
Mn is an element that improves the adhesion of scale in ferritic stainless steel. To obtain this effect, the Mn content is 0.05 mass% or more, and preferably 0.10 mass% or more.
On the other hand, excessive Mn content destabilizes the ferrite phase and promotes the generation of MnS, which is the starting point of corrosion, so the Mn content is set to 1.50 mass% or less, and preferably 1.20 mass% or less.

(Nickel: Ni)
Ni is an element that improves the corrosion resistance of ferritic stainless steel. To obtain this effect, the Ni content is 0.01 mass% or more, and preferably 0.05 mass% or more.
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 mass% or less, and preferably 0.30 mass% or less.

(Carbon: C)
If C is contained excessively, the amount of carbides in the ferritic stainless steel increases, and the corrosion resistance of the steel decreases. Therefore, the C content is 0.025 mass% or less. The C content is preferably 0.020 mass% or less.
The lower the C content, the more preferable, and 0% is acceptable; however, reducing the C content more than necessary increases the cost, so the C content may be 0.002 mass% or more.

(Rin: P)
If P is contained excessively, the workability of the ferritic stainless steel is reduced. Therefore, the P content is 0.040 mass% or less. The P content is preferably 0.030 mass% or less. The lower the P content, the more preferable it is, and 0% is also acceptable. However, if the P content is reduced more than necessary, the cost increases, so the P content may be 0.001 mass% or more.

(Sulfur: S)
If S is contained excessively, the generation of corrosion starting points is promoted in ferritic stainless steel. Therefore, the S content is 0.003 mass% or less. The S content is preferably 0.002 mass% or less. The lower the S content, the more preferable it is, and 0% is also acceptable. However, if the S content is reduced more than necessary, the cost increases, so the S content may be 0.0001 mass% or more.

(Nitrogen: N)
If N is contained excessively, N will form nitrides with other elements, which will lead to hardening of the ferritic stainless steel. Therefore, the N content is 0.025 mass% or less. The N content is preferably 0.020 mass% or less. The lower the N content, the better, and 0% is acceptable, but if the N content is reduced more than necessary, the cost will increase, so the N content may be 0.003 mass% or more.

(Aluminum: Al)
Al is an element effective for improving the corrosion resistance of ferritic stainless steel and for improving red scale resistance. Furthermore, Al is an element effective as a deoxidizer during steelmaking. To obtain these effects, the Al content is 0.002 mass% or more. The Al content is preferably 0.008 mass% or more.
On the other hand, if an excessive amount of Al is contained, there is a possibility that the surface quality may deteriorate. Therefore, the Al content is set to 0.200 mass% or less.

(Other ingredients)
The ferritic stainless steel according to one embodiment of the present invention contains 0.01 mass% or more and 2.5 mass% or less of W, 0.01 mass% or more and 3.00 mass% or less of Mo, 0.001 mass% or more and 0.500 mass% or less of Ti, 0.0002 mass% or more and 0.0100 mass% or less of B, 0.0002 mass% or more and 0.0030 mass% or less of Ca, 0.001 mass% or more and 0.50 mass% or less of Hf, 0.01 mass% or more and 0.40 mass% or less of Zr, 0.005 mass% or more and 0.006 mass% or less of Cr, and 0.005 mass% or more and 0.008 mass% or less of Ni. It may further contain one or more of 50 mass% or less Sb, 0.01 mass% or more and 0.30 mass% or less Co, 0.001 mass% or more and 1.0 mass% or less Ta, 0.002 mass% or more and 1.00 mass% or less Sn, 0.0002 mass% or more and 0.30 mass% or less Ga, 0.01 mass% or more and 0.50 mass% or less V, 0.001 mass% or more and 0.20 mass% or less REM, and 0.0003 mass% or more and 0.003 mass% or less Mg. In addition, the ferritic stainless steel according to this embodiment may contain 0.20 mass% or less, preferably 0.10 mass% or less La, or 0.20 mass% or less, preferably 0.05 mass% or less Ce as REM.
However, since the inclusion of these elements is not essential, the content may be 0% or may be less than the range described below.

(Tungsten: W)
W is an element that may be contained to ensure high-temperature strength. To obtain this effect, the W content is preferably 0.01 mass % or more, and more preferably 0.1 mass % or more.
On the other hand, excessive W content increases material costs. Therefore, in the ferritic stainless steel according to the present embodiment, the W content is 2.5 mass% or less. The W content is preferably 1.5 mass% or less, and more preferably 1.3 mass% or less.

(Molybdenum: Mo)
Mo is an element that may be contained to ensure high-temperature strength and red scale resistance. To obtain this effect, the Mo content is preferably 0.01 mass % or more.
On the other hand, excessive Mo content hardens the steel, reduces workability, and increases material costs. Therefore, the Mo content is 3.00 mass% or less, and preferably 2.50 mass% or less.

(Titanium: Ti)
Ti is an element that can convert ferritic stainless steel into a ferritic single phase at 900 to 1000°C by reacting with C and/or N, improving red scale resistance and workability. Therefore, Ti may be contained. To obtain this effect, the Ti content is preferably 0.001 mass% or more. The Ti content is preferably 0.010 mass% or more, and more preferably 0.050 mass% or more.
On the other hand, if Ti is contained excessively, the workability and surface quality may be deteriorated. Therefore, the Ti content is 0.500 mass% or less. The Ti content is preferably 0.300 mass% or less, and more preferably 0.250 mass% or less.

(Boron: B)
B is an element that improves the secondary workability of molded articles produced using ferritic stainless steel. To obtain this effect, the B content is preferably 0.0002 mass % or more.
On the other hand, if B is contained in excess, compounds such as Cr ₂ B are likely to be formed, and red scale resistance may deteriorate. Therefore, the B content is 0.0100 mass% or less. The B content is preferably 0.0080 mass% or less, and more preferably 0.0030 mass% or less.

(Calcium: Ca)
Ca is an element that promotes high-temperature oxidation resistance. Therefore, Ca may be contained as necessary. To obtain this effect, the Ca content is preferably 0.0002 mass% or more.
On the other hand, an excessive Ca content leads to a decrease in corrosion resistance, so the Ca content is set to 0.0030 mass% or less.

(Hafnium: Hf)
Hf is an element that improves corrosion resistance, high-temperature strength, and oxidation resistance. Hf may be contained as necessary. To obtain this effect, the Hf content is preferably 0.001 mass% or more. The Hf content is more preferably 0.01 mass% or more.
On the other hand, an excessive Hf content may deteriorate workability and manufacturability, so the Hf content is set to 0.50 mass% or less.

(Zirconium: Zr)
Zr is an element that improves high-temperature strength, corrosion resistance, and high-temperature oxidation resistance. Therefore, Zr may be contained as necessary. To obtain this effect, the Zr content is preferably 0.01 mass% or more.
On the other hand, an excessive Zr content leads to deterioration of workability and manufacturability, so the Zr content is set to 0.40 mass% or less.

(Antimony: Sb)
Sb is an element that improves high-temperature strength. Therefore, Sb may be contained as necessary. To obtain this effect, the Sb content is preferably 0.005 mass% or more. The Sb content is more preferably 0.01 mass% or more.
On the other hand, an excessive Sb content reduces weldability and toughness, so the Sb content is set to 0.50 mass % or less.

(Cobalt: Co)
Co is an element that improves high-temperature strength. Therefore, Co may be contained as necessary. To obtain this effect, the Co content is preferably 0.01 mass % or more.
On the other hand, an excessive Co content reduces toughness and hence manufacturability, so the Co content is set to 0.30 mass% or less.

(Tantalum: Ta)
Ta is an element that improves high-temperature strength. Therefore, Ta may be contained as necessary. To obtain this effect, the Ta content is preferably 0.001 mass% or more. The Ta content is more preferably 0.01 mass% or more, and further preferably 0.1 mass% or more.
On the other hand, an excessive Ta content reduces weldability and toughness, so the Ta content is set to 1.0 mass % or less.

(Tin: Sn)
Sn is an element that improves corrosion resistance and high-temperature strength. Therefore, Sn may be contained as necessary. To obtain this effect, the Sn content is preferably 0.002 mass% or more. The Sn content is more preferably 0.01 mass% or more.
On the other hand, an excessive Sn content may cause a decrease in toughness and manufacturability, so the Sn content is set to 1.00 mass% or less.

(Gallium: Ga)
Ga is an element that improves corrosion resistance and hydrogen embrittlement resistance. Therefore, Ga may be contained as necessary. To obtain this effect, the Ga content is preferably 0.0002 mass% or more. The Ga content is more preferably 0.01 mass% or more.
On the other hand, an excessive Ga content reduces weldability and toughness, so the Ga content is set to 0.30 mass% or less.

(Vanadium: V)
V is an element that fixes solute C and N in steel as compounds and improves the ductility and workability of the steel. Therefore, V may be contained as necessary. To obtain this effect, the V content is preferably 0.01 mass% or more.
On the other hand, an excessive V content reduces the workability of the steel, so the V content is set to 0.50 mass % or less.

(Magnesium: Mg)
Mg is a deoxidizing element and also an element that refines the structure of the slab and improves the formability. Therefore, Mg may be contained as necessary. To obtain this effect, the Mg content is preferably 0.0003 mass% or more.
On the other hand, since an excessive Mg content leads to deterioration of corrosion resistance, weldability, and surface quality, the Ca content is 0.003 mass % or less.

(Rare earth element: REM)
REM is a collective term for scandium (Sc) and 15 elements from lanthanum (La) to lutetium (Lu) (lanthanoid elements). REM may contain any one of the lanthanoid elements alone. In the case where any one of the lanthanoid elements is contained as the REM, for example, either La or Ce may be contained as described later, and La and Ce may be contained. In addition, when two or more lanthanoid elements are contained as REM, the combination of elements is not particularly limited. For example, La and Ce may be contained. Preferably, a misch metal may be added so that a plurality of lanthanoid elements contained in the misch metal are contained as REM.
REM is an element that improves the cleanliness of stainless steel and also improves high-temperature oxidation resistance. Therefore, REM may be contained as necessary. To obtain these effects, the REM content is 0. The REM content is preferably 0.001% by mass or more. The REM content is more preferably 0.01% by mass or more.
On the other hand, an excessive REM content increases the alloy cost and reduces manufacturability, so the REM content is set to 0.20 mass% or less.

(Lantern: La)
La may be contained as the REM. La is an element that improves the cleanliness of stainless steel and also improves high-temperature oxidation resistance, and further improves red scale resistance and scale spalling resistance. When La is contained (using metallic La or the like) to obtain this effect, the La content is preferably 0.001 mass% or more. The La content is more preferably 0.01 mass% or more.
On the other hand, excessive La content increases material costs. Therefore, the La content is 0.20 mass% or less. The La content is preferably 0.10 mass% or less, and in consideration of costs, the La content is more preferably 0.05 mass% or less, and further preferably 0.03 mass% or less.

(Cerium: Ce)
Ce may be contained as the REM. Ce is an element that improves the cleanliness of stainless steel and also improves high-temperature oxidation resistance, and further improves red scale resistance and scale spalling resistance. When Ce is contained (using metallic Ce or the like) to obtain this effect, the Ce content is preferably 0.001 mass% or more. The Ce content is more preferably 0.01 mass% or more.
On the other hand, excessive Ce content increases material costs, so the Ce content is 0.20 mass% or less, and preferably 0.05 mass% or less.

The chemical composition of the ferritic stainless steel according to this embodiment can be obtained by performing elemental analysis using a general method such as ICP-AES from a depth position of 1/4 of the plate thickness from the surface (a range of 1/8 to 3/8 of the thickness from the surface in the thickness direction is acceptable). C and S can be measured using the combustion-infrared absorption method, N using the inert gas fusion-thermal conductivity method, and O using the inert gas fusion-infrared absorption method.

<Oxide containing Al or Si>
The ferritic stainless steel according to this embodiment has at least one of oxides containing 5% by mass or more of Al and oxides containing 5% by mass or more of Si on the surface, and the number of oxides having a diameter D represented by the following formula (1) of 0.1 μm or more and 2.0 μm or less (hereinafter referred to as "Al/Si-based oxides") on the surface is 10 or more per 93 ^μm2 .
This oxide improves red scale resistance.
D=(Dmax+Dmin)/2...(1)
(In the above formula (1), Dmax is the maximum diameter of each oxide on the surface, and Dmin is the minimum diameter of each oxide on the surface.)

The size of the oxides on the surface of the ferritic stainless steel can be measured, for example, by a scanning electron microscope (SEM).
Specifically, a scanning electron microscope (SEM) is used to take an SEM photograph of the surface of the steel material. The area of one field of view is 93 ^μm2 . From this SEM photograph, the maximum and minimum diameters of the oxides are calculated using image analysis software, for example, "Photoshop (registered trademark)" (manufactured by Adobe Co., Ltd.).
Here, the Al and Si contents in the oxides on the surface of the ferritic stainless steel can be measured, for example, by energy dispersive X-ray spectroscopy (EDS). That is, by EDS, it can be determined whether the oxides are oxides whose number is counted (oxides containing 5% by mass or more of Al or oxides containing 5% by mass or more of Si).
In this embodiment, the "maximum diameter" of an oxide means the maximum width between two parallel lines when the oxide is sandwiched between the two parallel lines when the oxide is viewed in a plan view. Also, in this specification, the "minimum diameter" of an oxide means the minimum width between two parallel lines when the oxide is sandwiched between the two parallel lines when the oxide is viewed in a plan view.

The reasons why oxides containing 5% or more by mass of Al or oxides containing 5% or more by mass of Si (sometimes called Al/Si-based oxides) improve red scale resistance are believed to be as follows. First, Al/Si-based oxides act as a protective coating. Second, Al/Si-based oxides grow when heated, lowering the oxygen partial pressure around the Al/Si-based oxides. Since Al, Si, Cr, and Fe are more likely to be oxidized in that order, Al, Si, and Cr are oxidized preferentially over Fe. Therefore, the growth of Al/Si-based oxides can reduce the formation of red scale, which is an Fe-based oxide.

However, when an excessive amount of Al/Si oxides is present on the surface of the ferritic stainless steel, the Al/Si oxides may reduce the brightness of the surface of the ferritic stainless steel, and the design of the ferritic stainless steel may be deteriorated. Therefore, the number of Al/Si oxides on the surface of the ferritic stainless steel is preferably 25 or less per 93 ^μm2 , and more preferably 22 or less. In this case, the brightness of the surface of the ferritic stainless steel can be improved and the design can be maintained well. In this embodiment, "brightness" means CIE 1976 brightness L* measured by using a D65 light source in a diffuse lighting method, receiving light in a direction of 8° with respect to the normal line of the surface of the ferritic stainless steel, with a viewing angle of 10° and a measurement time of 1 ^second .

When the diameter D of oxides containing 5 mass% or more of Al or Si on the surface of ferritic stainless steel is less than 0.1 μm, the effect of the oxides in improving red scale resistance is low. Therefore, in this embodiment, the target is oxides having a diameter D of 0.1 μm or more.
On the other hand, among oxides containing 5% or more by mass of Al or Si, the presence of oxides having a diameter D exceeding 2.0 μm may reduce the brightness of the surface of the ferritic stainless steel, which may deteriorate the design properties of the ferritic stainless steel. Therefore, it is preferable to control the number of oxides containing 5% or more by mass of Al or Si and having a diameter D of 0.1 μm or more and 2.0 μm or less on the surface of the ferritic stainless steel to within a predetermined range.

Furthermore, it is preferable that the surface of the ferritic stainless steel contains 5 mass% or more of Al or Si and has a small number of oxides (e.g., 5 or less per 93 ^μm2 ) with a diameter D exceeding 2.0 μm, and it is most preferable that no oxides are present. In this case, the brightness of the surface of the ferritic stainless steel can be improved, and the design of the ferritic stainless steel can be improved.

The inventors focused on Al/Si oxides on the surface of ferritic stainless steel and discovered that by controlling the number of Al/Si oxides within a predetermined range, it is possible to realize a ferritic stainless steel with excellent red scale resistance.

In addition to the Al/Si oxides, a passive film with a thickness of 2.0 to 8.0 nm exists on the surface of the ferritic stainless steel according to this embodiment. The passive film is a very dense and highly adhesive film consisting of hydrated chromium oxyhydroxide and chromium oxide, mainly composed of Cr.

The thickness of the passive film can be determined by using a high-frequency glow discharge optical emission spectrometer (GDS). Specifically, a GDS analyzer (e.g., a GD-Profiler 2 manufactured by HORIBA or an equivalent device) is used to analyze the oxygen concentration at a pitch of 2.5 nm in the thickness direction from the surface, and the portion from the surface to the position where the oxygen concentration is half the peak value is defined as the passive film, and the thickness of the passive film is measured.
For example, other GDS measurement conditions are as follows:
Gas replacement time: 200 seconds,
Pre-sputtering time: 30 seconds,
Background: 5 seconds,
Depth: 1.01 μm,
Pressure: 600 Pa,
Output: 35W,
Effective value: 8.75 W,
Module: 8V,
Phase: 4V,
Frequency: 100Hz Duty cycle: 0.25.

In the ferritic stainless steel according to this embodiment, when used at high temperatures in the presence of water vapor, Al/Si oxides grow and reduce the oxygen partial pressure around the Al/Si oxides. As a result, unlike ordinary ferritic stainless steel, the surface morphology is such that the generation of Fe oxides, which are the main component of red scale, is reduced and the generation of Cr, Al, and Si oxides is increased, resulting in excellent red scale resistance.
For example, after being held in an atmosphere of 300 to 900° C. for 100 hours or more, the following formula (2) is satisfied in the range from the surface to 1.0 μm (surface layer portion), where the Cr content is [Cr], the Si content is [Si], and the Al content is [Al], all in unit mass %.
[Cr]+[Si]+[Al]≧18.0…(2)
When formula (2) is not satisfied, it is believed that the surface is mainly composed of Fe-based oxides, that is, red scale is excessively formed.
[Cr] + [Si] + [Al] is preferably 20.0 (mass %) or more.

There is no need to limit the Cr content [Cr], Si content [Si], and Al content [Al] within the range from the surface to 1.0 μm, but in terms of the effect of suppressing red scale, it is preferable that the Si content and/or Al content are each 3.0 mass% or more.

In a high-temperature atmosphere, Al/Si-based oxides grow gradually for a certain period of time, but once the period exceeds 100 hours, there is no significant change, and it is believed that the states of Cr, Si, and Al also do not change significantly.
In other words, when the ferritic stainless steel according to this embodiment is used as a component such as an exhaust gas path component, a stove combustion equipment component, a fuel cell component, or a plant-related material, it is considered that the contents of Cr, Al, and Si in the surface layer satisfy formula (2).
On the other hand, even if the above-mentioned holding is performed, the chemical composition at the 1/4 depth position does not change.

＜＜Exhaust gas parts＞＞
The exhaust gas component according to this embodiment is obtained by processing the ferritic stainless steel according to this embodiment as a raw material. Therefore, the exhaust gas component according to this embodiment has a chemical composition at the stage obtained by processing or the like (before being used as a component) of 0.05% to 2.50% by mass of Si, 0.05% to 1.50% by mass of Mn, 0.025% by mass or less of C, 0.040% by mass or less of P, 0.003% by mass or less of S, 0.025% by mass or less of N, 0.01% to 0.50% by mass of Ni, 1.0% by mass or less of Cr, 0.01% by mass or less of Ni, 0.02% by mass or less of Ni ... 0.50 mass% or more and 25.00 mass% or less of Cr, 0.01 mass% or more and 1.80 mass% or less of Cu, 0.002 mass% or more and 0.200 mass% or less of Al, 0.001 mass% or more and 1.00 mass% or less of Nb, 0 mass% or more and 2.5 mass% or less of W, 0 mass% or more and 3.00 mass% or less of Mo, 0 mass% or more and 0.500 mass% or less of Ti, 0 mass% or more and 0.0100 mass% or less of B, 0 mass% or more and 0.05 mass% or less of C, 0 mass% or more and 0.2 mass% or less of C, 0 mass% or more and 0.4 mass% or less of C The oxide contains 0% by mass or more of Ca and 0.50% by mass or less of Hf, 0% by mass or more of Zr and 0.40% by mass or less of Sb, 0% by mass or more of Sb and 0.50% by mass or less of Co, 0% by mass or more of Ta and 1.0% by mass or less of Ta, 0% by mass or more of Sn and 0.30% by mass or less of Ga, 0% by mass or more of V and 0.50% by mass or less of Mg and 0% by mass or more of REM, with the balance being Fe and impurities. At least one of an oxide containing 5% by mass or more of Al and an oxide containing 5% by mass or more of Si is present on the surface, and the number of oxides present on the surface having a diameter D represented by the following formula (1) of 0.1 μm or more and 2.0 ^μm or less is 10 or more per 93 μm2.
D=(Dmax+Dmin)/2...(1)
In the 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.
Furthermore, the surface may have a CIE 1976 lightness L* of 60 or greater, measured using a D65 light source in a diffuse illumination mode, receiving light at an angle of 8° to the normal to the surface, with a viewing ^angle of 10° and a measurement time of 1 second.
In addition, the exhaust gas part according to this embodiment satisfies the following formula (2) in a range from the surface to 1.0 μm after being held in an atmosphere of 300 to 900° C. for 100 hours or more, where the Cr content is [Cr], the Si content is [Si], and the Al content is [Al], all in unit mass %:
[Cr]+[Si]+[Al]≧18.0…(2)
In other words, the exhaust gas part according to this embodiment is, for example, an exhaust gas path component, a stove combustion equipment, a fuel cell component, or a plant-related material, and when used for a certain period of time under normal conditions for such applications (after use), in the range from the surface to 1.0 μm, the following formula (2) is satisfied, where the Cr content is [Cr], the Si content is [Si], and the Al content is [Al], all in unit mass %:
[Cr]+[Si]+[Al]≧18.0…(2)

In both the ferritic stainless steel according to this embodiment and the exhaust gas component according to 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.
Specifically, the analysis region is φ4 mm, and all elements contained in each steel are selected and measured at 2.5 nm pitches up to a depth of 1.0 μm, except for C and N. From the measurement results, the contents of Cr, Al, and Si are calculated at the positions where Cr, Al, and Si each show a peak within the range up to a depth of 1.0 μm.

<<Method of manufacturing ferritic stainless steel>>
Conventionally, methods for improving red scale resistance have been used, such as a method of promoting Cr diffusion in the steel by surface polishing as a finishing process and encouraging the generation of Cr oxides, or a method of forming a hot-dip plating layer on the surface.

The present inventors have found that, for example, by the following manufacturing method, it is possible to obtain a ferritic stainless steel having excellent red scale resistance and in which the number of Al/Si-based oxides on the surface is 10 or more per 93 ^μm2 .

The ferritic stainless steel according to one embodiment of the present invention is obtained, for example, as a ferritic stainless steel strip. Fig. 1 is a flow chart showing an example of a method for producing a ferritic stainless steel according to this embodiment. As shown in Fig. 1, the method for producing a ferritic stainless steel strip according to this embodiment includes a pretreatment step S1, a hot rolling step S2, an annealing step S3, a first pickling step S4, a cold rolling step S5, a final annealing step S6, a nitric acid electrolysis step S7, and a final pickling step S8.
The preferred conditions for each step will be described below. For conditions not described below, known conditions can be used.

<Pretreatment process>
In the pretreatment step S1, first, a melting furnace in a vacuum or argon atmosphere is used to melt steel having a chemical composition adjusted to fall within the range of the present invention described above, and the steel is cast to produce a slab. Then, a slab piece for hot rolling is cut out from the slab. The slab piece is then heated to a temperature range of 1100°C to 1300°C in an air atmosphere. The time for which the slab piece is heated and held is not limited. When the pretreatment step is carried out industrially, the casting may be continuous casting.

<Hot rolling process>
The hot rolling step S2 is a step of producing a hot rolled steel strip having a predetermined thickness by hot rolling the slab (steel ingot) obtained in the pretreatment step S1. The hot rolling conditions are not limited and may be adjusted according to the required mechanical properties, etc.

<Annealing process>
The annealing step S3 is a step for softening the hot-rolled steel strip obtained in the hot rolling step S2 by heating the steel strip. This annealing step S3 is a step that is performed as necessary, and does not necessarily have to be performed.

<First pickling process>
The first pickling step S4 is a step of washing off scale adhering to the surface of the steel strip with a pickling solution such as hydrochloric acid or a mixture of nitric acid and hydrofluoric acid.

<Cold rolling process>
The cold rolling step S5 is a step of further rolling the steel strip that has been descaled in the first pickling step S4 to make it thinner.

<Final annealing process>
The final annealing step S6 is a step for removing distortion by heating the steel strip thinly rolled in the cold rolling step S5, thereby softening the steel strip, and for forming an inner oxide, which is an oxide of Al or Si, etc., together with an outer oxide, such as (Fe, Cr) ₃ O ₄ or Cr ₂ O ₃ .
For the above-mentioned purpose, the annealing in the final annealing step S6 is performed at a temperature of about 900 to 1100° C. for a time period of 30 to 90 seconds depending on the alloy components in an atmosphere of a combustion gas such as air or liquefied combustion gas (LNG).

<Nitric acid electrolysis process>
The nitric acid electrolysis step S7 is a step 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, oxides adhering to the surface of the steel strip are partially removed.
Specifically, an outer oxide layer, such as (Fe, Cr) ₃ O ₄ or Cr ₂ O ₃ , is formed on the surface of the steel strip obtained in the final annealing step S6. In addition, an inner oxide, which is mainly an oxide of Al or Si, is formed between this outer oxide layer and the base metal. In the nitric acid electrolysis step S7, nitric acid electrolysis is performed under conditions such that most of the outer oxide layer is removed while most of the inner oxide remains. Although most of the inner oxide remains, it is preferable that some of the inner oxide is slightly peeled off and made into a state that is easily removed in the final pickling step S8.

The nitric acid concentration in the nitric acid electrolysis step S7 is preferably 150 g/L or less. In this case, it is easy to leave 10 or more Al/Si-based oxides per 93 ^μm2 on the surface of the steel strip after the final pickling step S8.
On the other hand, if more Al/Si-based oxides than necessary remain, the brightness of the surface decreases. Therefore, in order to keep the amount of Al/Si-based oxides on the surface after the final pickling step within a preferred range, the nitric acid concentration in the nitric acid electrolysis step S7 is preferably 100 g/L or more. In order to efficiently remove the outer layer oxides in a short time, the nitric acid concentration in the nitric acid electrolysis step S7 is preferably 130 g/L or more.

The solution temperature in the nitric acid electrolysis step S7 is preferably 70° C. or less, and more preferably 60° C. or less. In this case, it is easy to leave 10 or more Al/Si-based oxides per 93 μm ² on the surface of the steel strip after the final pickling step S8.
On the other hand, the solution temperature in the nitric acid electrolysis step S7 is preferably 50° C. or higher, and more preferably 60° C. or higher. In this case, the outer oxide layer can be removed efficiently in a short time.

The current density in the nitric acid electrolysis step S7 is preferably 150 mA/ ^cm2 or less. In this case, it is easy to leave 10 or more Al/Si-based oxides per 93 ^μm2 on the surface of the steel strip after the final pickling step S8.
On the other hand, the current density in the nitric acid electrolysis step S7 is preferably 100 mA/cm2 or ^more , more preferably 120 mA/cm2 or ^more , and even more preferably 130 mA/cm2 or ^more . In this case, the outer oxide layer can be efficiently removed in a short time.

The electrolysis time in the nitric acid electrolysis step S7 is more preferably 120 seconds or less. In this case, 10 or more Al/Si-based oxide particles per 93 ^μm2 can be left on the surface of the steel strip after the final pickling step S8.
On the other hand, the electrolysis time in the nitric acid electrolysis step S7 is preferably 60 seconds or more, which can reliably remove most of the outer oxide layer, while making part of the inner oxide easily peelable, and can keep the amount of remaining Al/Si-based oxide after the final pickling step within a preferred range.

<Final pickling process>
The final pickling step S8 is a step in which the steel strip after the nitric acid electrolysis step S7 is immersed in a pickling solution such as a mixture of nitric acid and hydrofluoric acid. In the final pickling step S8, the inner oxides that have peeled off slightly in the nitric acid electrolysis step ^S7 are removed. This makes it possible to remove more than necessary Al/Si-based oxides while securing 10 or more Al/Si-based oxides per 93 μm2 on the surface of the steel strip, thereby improving the lightness.

As described above, in the conventional technology, a finishing process such as polishing or forming a plating layer is further carried out as a finishing process to improve red scale resistance. However, such a finishing process requires the introduction of new equipment for the finishing process, which increases the manufacturing cost. From this perspective, there is a demand for a manufacturing method for manufacturing ferritic stainless steel with excellent red scale resistance without increasing the manufacturing cost.

In the above-described manufacturing method according to this embodiment, the nitric acid electrolysis step S7 and the final pickling step S8 can use equipment commonly used in pickling steps, making it possible to produce a ferritic stainless steel with excellent red scale resistance without increasing manufacturing costs.

<Manufacturing method for exhaust gas parts>
The exhaust gas component according to this embodiment is obtained by processing the above-mentioned ferritic stainless steel according to this embodiment into a predetermined component shape by a known processing method.

<Example>
Examples of the present invention will be described below. First, using the components shown in Table 1 below as raw materials, the steps up to the final annealing step S6 of the above-mentioned manufacturing method were all carried out under the same conditions, followed by the nitric acid electrolysis step S7 for the electrolysis time shown in Table 2, and then the final pickling step S8, to manufacture ferritic stainless steel.

In this example, the composition of each stainless steel shown in Table 1 is shown in mass %. The remainder other than each component shown in Table 1 is Fe and impurities. The underlines in Table 1 indicate that the range of each component contained in each stainless steel according to the comparative example of the present invention is outside the range of the present invention.

As shown in Table 1, ferritic stainless steels manufactured so that their chemical compositions were within the range of the present invention were designated as steel types A1 to A10. Ferritic stainless steels manufactured so that their chemical compositions were outside the range of the present invention were designated as steel types B1 to B3.

Table 2 shows the conditions used to manufacture steel products No. 1 to 46 using steel types A1 to A10 and steel types B1 to B3, and the evaluation results of each steel product. The conditions used to manufacture each steel product shown in Table 2 are as follows.

Atmosphere of melting furnace in pretreatment step S1: vacuum Mass of slab piece produced in pretreatment step S1: 30 kg
Heating temperature of slab piece in pretreatment step S1: 1230° C.
Heating time of the slab in the pretreatment process S1: 2 hours; Plate thickness after the hot rolling process S2: 4 mm
Annealing step S3: Not performed. Pickling solution used in first pickling step S4: Nitric hydrofluoric acid solution (aqueous solution with hydrofluoric acid concentration of 30 g/L and nitric acid concentration of 100 g/L).
- Solution temperature in the first pickling process S4: 40 to 50°C
・ Sheet thickness after cold rolling process S5 1.5 mm
Annealing temperature in the final annealing step S6: 900 to 1100°C (changes depending on the alloy composition)
Annealing time in the final annealing step S6: 60 seconds; Annealing atmosphere in the final annealing step S6: air; Nitric acid concentration in the nitric acid electrolysis step S7: 150 g/L
・Liquid temperature in nitric acid electrolysis step S7: 50 to 70°C
Current density in nitric acid electrolysis step S7: 150 mA/cm ²
Electrolysis time in the nitric acid electrolysis step S7: 30 to 180 seconds (shown in Table 2)
Pickling solution used in the final pickling process S8 Nitric hydrofluoric acid solution (aqueous solution with hydrofluoric acid concentration of 20 g/L and nitric acid concentration of 70 to 80 g/L)
・Liquid temperature in the final pickling process S8: 40 to 50°C

The number of Al/Si-based oxides was measured for steel materials No. 1 to 46 using the method described in detail below. The measurement results are shown in Table 2.

<Number of Al/Si-based oxides>
The number of Al/Si-based oxides on the surface of the steel material was measured as follows. First, a scanning electron microscope (SEM) SU5000 (manufactured by Hitachi High-Technologies Corporation) was used to take an SEM photograph of the surface of the steel material at a magnification of 10,000 times. The dimensions of one field of view were 8.34 μm vertical x 11.2 μm horizontal, and the area of one field of view ^was 93 μm2. In addition, elemental analysis was performed on the composition of the oxides using an energy dispersive X-ray spectroscopy (EDS) for electron microscopes (manufactured by Horiba, Ltd.) at an acceleration voltage of 15 kV and an analysis time of 60 seconds.

The SEM photographs were analyzed using the image analysis software "Photoshop (registered trademark)" (Adobe Inc.) to calculate the number of oxides containing 5 mass% or more of Al or Si and having a diameter D of 0.1 μm or more and 2.0 μm or less, i.e., the number of Al/Si-based oxides.

The thickness of the passive film was also measured using the method described above. Although not shown in the table, the thickness of the passive film was 2.0 to 8.0 nm.

2A and 2B show examples of SEM photographs taken by the above method. As shown in FIG. 2A, the number of Al/Si-based oxides present on the surface of steel No. 6, which is an example of the invention, was 10 or more per visual field. In contrast, as shown in FIG. 2B, the number of Al/Si-based oxides present on the surface of steel No. 8, which is a comparative example, was less than 10 per visual field. In the SEM photograph of steel No. 6, it appears that there are 30 or more oxides in one visual field. However, the number of oxides containing 5 mass% or more of Al or Si and having a diameter D of 0.1 μm or more and 2.0 μm or less, i.e., Al/Si-based oxides, was 19 per visual field.

As shown in Table 2, in the steel materials Nos. 1 to 3, 5 to 7, 9 to 11, 13 to 15, 17 to 19, 21 to 23, 25 to 27, 29 to 30, 32 to 33, 35, and 36, which were steel types A1 to A10 and had electrolysis times of 30 to 120 seconds, the number of Al/Si-based oxides having a diameter D of 0.1 μm or more and 2.0 ^μm or less was 10 or more per 93 μm2.
On the other hand, even among the steel types A1 to A9, in the steel materials Nos. 4, 8, 12, 16, 20, 24, 28, 31 and 34 in which the electrolysis time was 180 seconds, the number of Al/Si-based oxides was less than 10 per 93 ^μm2 .

On the other hand, in the case of steel type B1 or B2, the number of Al/Si-based oxides was less than 10 per 93 μm ² in all of steel materials Nos. 37 to 42.

In the case of steel type B3, steel materials No. 43 and 44 in which the electrolysis time was 40 to 60 seconds had 10 or more Al/Si-based oxide particles per 93 ^μm2 , but in the case of steel materials No. 45 and 46 in which the electrolysis time was 120 to 180 seconds, the number of Al/Si-based oxide particles was less ^than 10 per 93 μm2.

<Oxidation Increase (Red Scale Resistance Evaluation)>
In addition, for Steel Nos. 1 to 46, in order to evaluate the red scale resistance, the oxidation weight gain of the steel was measured as follows in accordance with JIS Z 2281:1993 (High-temperature continuous oxidation test method for metallic materials).
First, a test piece measuring 20 mm x 25 mm was cut out from each steel material. The test piece was continuously heated at 600°C for 100 hours in an atmospheric environment with a water vapor concentration of 10 vol%, simulating a situation in which a petroleum fuel was burned. The oxidation weight gain was calculated from the change in mass before and after the test.
As a criterion for evaluating red scale resistance, an oxidation weight increase of 0.20 mg/ ^cm2 or less was determined to be excellent in red scale resistance.

As shown in Table 2, among the steel types A1 to A10, the steel materials Nos. 1 to 3, 5 to 7, 9 to 11, 13 to 15, 17 to 19, 21 to 23, 25 to 27, 29 to 30, 32 to 33, 35, and 36 in which the number of Al/ ^Si -based oxides was 10 or more per 93 μm2 had an oxidation weight gain of 0.20 mg/ ^cm2 or less. Therefore, it was shown that these steel materials had good red scale resistance.

On the other hand, even among the steel types A1 to A9, the oxidation weight gain exceeded 0.20 mg/ ^cm2 in steel materials Nos. 4, 8, 12, 16, 20, 24, 28, 31 and 34, in which the number of Al/Si-based oxides was less than 10 per ⁹³ μm2. Therefore, it was shown that these steel materials had low red scale resistance.

In addition, in the steel materials Nos. 37 to 42, which were steel types B1 or B2 and had less than 10 Al/Si-based oxides per ⁹³ μm2, the oxidation weight gain exceeded 0.20 mg/ ^cm2 . Therefore, it was shown that these steel materials had low red scale resistance.

In steel type B3, the oxidation weight gain exceeded 0.20 mg/ ^cm2 in all steel materials Nos. 43 to 46, regardless of the number of Al/Si-based oxides. This indicates that these steel materials have low red scale resistance. It is presumed that steel type B3 had low red scale resistance even when the number of Al/Si-based oxides was 10 or more, because the Al content was low at 0.001 mass%.

<Surface morphology after long-term testing>
In order to evaluate the oxide morphology on the surface after the long-term test, among the test pieces for which the above-mentioned oxidation increase (red scale resistance evaluation) was evaluated, the Cr, Si, and Al contents at arbitrary points of the test pieces were measured using a glow discharge spectroscopy (GDS) (GD-Profiler 2 manufactured by HORIBA) for steel materials Nos. 1 to 4, 13 to 16, 32 to 34, and 37 to 39. The analysis area was φ4 mm, and measurements were made at 2.5 nm pitches from the surface to a depth of 1.0 μm.
The elements analyzed were all elements other than C and N contained in each steel at random locations and were measured.
Other GDS measurement conditions were as follows:
Gas replacement time: 200 seconds,
Pre-sputtering time: 30 seconds,
Background: 5 seconds,
Depth: 1.01 μm,
Pressure: 600 Pa,
Output: 35W,
Effective value: 8.75 W,
Module: 8V,
Phase: 4V,
Frequency: 100Hz Duty cycle: 0.25.
After the measurement, the contents of Cr, Al, and Si at the positions where they each showed a peak up to a depth of 1.0 μm were substituted into formula (2) to perform calculations and confirm whether they were satisfactory. The value of the left side of formula (2) ([Cr] + [Si] + [Al]) is shown in the table.

As shown in Table 2, Nos. 1 to 3, 13 to 15, and 32 to 33, which had an oxidation weight gain of 0.20 mg/ ^cm2 or less and had high red scale resistance, satisfied formula (2).
On the other hand, the oxidation weight gain exceeded 0.20 mg/ ^cm2 , and Nos. 4, 16, 34, and 37 to 39 had low red scale resistance and did not satisfy formula (2).

<Lightness>
In order to evaluate the design of the steel materials, the brightness of the surface of steel materials No. 1 to 46 was measured in the following manner.
Using a spectrophotometer (model: CM-700d, manufactured by Konica Minolta), white calibration was performed at room temperature of 23° C., and then the lightness L ^* of the surface of the steel material was measured at the same temperature. As a criterion for evaluation, a lightness L ^* of 60 or more was judged to be excellent in design.

As shown in Table 2, the steel materials Nos. 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, which were steel types A1 to A10 and had electrolysis times of 60 to 180 seconds, had a lightness L ^* of 60 or more. Therefore, it was shown that these steel materials had good design properties.

On the other hand, in the steel materials Nos. 1, 5, 9, 13, 17, 21, 25, and 32 of the steel types A1 to A9 in which the electrolysis time was 30 to 40 seconds, the lightness L ^* was less than 60.

(Additional Notes)
The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention.

Claims (6)

The chemical composition is:
0.05% by mass or more and 2.50% by mass or less of Si;
0.05% by mass or more and 1.50% by mass or less of Mn,
0.025% by weight or less of C,
0.040% by weight or less of P,
0.003% by weight or less of S;
0.025% by weight or less of N,
0.01% by mass or more and 0.50% by mass or less of Ni;
10.50% by mass or more and 25.00% by mass or less of Cr;
0.01% by mass or more and 1.80% by mass or less of Cu,
0.002% by mass or more and 0.200% by mass or less of Al;
0.001% by mass or more and 1.00% by mass or less of Nb;
0% by mass or more and 2.5% by mass or less of W;
Mo, from 0% by mass to 3.00% by mass
Ti, from 0% by mass to 0.500% by mass;
B,
0% by mass or more and 0.0030% by mass or less of Ca,
Hf of 0 mass% or more and 0.50 mass% or less,
0% by mass or more and 0.40% by mass or less of Zr;
Sb,
0% by mass or more and 0.30% by mass or less of Co,
Ta, from 0% by mass to 1.0% by mass
0% by mass or more and 1.00% by mass or less of Sn,
Ga, from 0% by mass to 0.30% by mass;
V, 0% by mass or more and 0.50% by mass or less;
0% by mass or more and 0.003% by mass or less of Mg, and 0% by mass or more and 0.20% by mass or less of REM
The balance includes Fe and impurities,
At least one of an oxide containing 5% by mass or more of Al and an oxide containing 5% by mass or more of Si is present on the surface, and the number of oxides having a diameter D represented by the following formula (1) of 0.1 μm or more and 2.0 μm or less among the oxides present on the surface in an observation field of view is 10 or more per 93 ^μm2 .
Ferritic stainless steel.
D=(Dmax+Dmin)/2...(1)
Here, in the 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. The surface has a CIE 1976 lightness L ^* of 60 or more, measured using a D65 light source in a diffuse illumination mode, receiving light at an angle of 8° to the normal to the surface, with a viewing angle of 10° and a measurement time of 1 second.
The ferritic stainless steel according to claim 1. The chemical composition is
0.01% by mass or more and 2.5% by mass or less of W;
0.01% by mass or more and 3.00% by mass or less of Mo;
0.001% by mass or more and 0.500% by mass or less of Ti;
0.0002% by mass or more and 0.0100% by mass or less of B;
0.0002% by mass or more and 0.0030% by mass or less of Ca;
Hf of 0.001% by mass or more and 0.50% by mass or less,
0.01% by mass or more and 0.40% by mass or less of Zr;
0.005% by mass or more and 0.50% by mass or less of Sb;
0.01% by mass or more and 0.30% by mass or less of Co;
Ta of 0.001% by mass or more and 1.0% by mass or less,
0.002% by mass or more and 1.00% by mass or less of Sn,
0.0002% by mass or more and 0.30% by mass or less of Ga;
0.01% by mass or more and 0.50% by mass or less of V;
0.0003% by mass or more and 0.003% by mass or less of Mg, and 0.001% by mass or more and 0.20% by mass or less of REM
Contains one or more of the following:
The ferritic stainless steel according to claim 1 or 2. After being held in an atmosphere of 300 to 900 ° C. for 100 hours or more, in the range from the surface to 1.0 μm, the following formula (2) is satisfied, where the Cr content is [Cr], the Si content is [Si], and the Al content is [Al], in unit mass%,
The ferritic stainless steel according to any one of claims 1 to 3.
[Cr]+[Si]+[Al]≧18.0…(2) In the range from the surface to 1.0 μm, when the Cr content is [Cr], the Si content is [Si], and the Al content is [Al], the following formula (2) is satisfied:
Ferritic stainless steel.
36.7≧ [Cr]+[Si]+[Al]≧18.0…(2) An exhaust gas component comprising the ferritic stainless steel according to any one of claims 1 to 5.

Images