JP6971184B2 - Ferritic stainless steel, its manufacturing method, and fuel cell components - Google Patents

Description

The present invention relates to ferritic stainless steel, a method for producing the same, and a member for a fuel cell.

Recently, due to problems such as the depletion of fossil fuels such as petroleum and the _{phenomenon of global warming due to CO 2} emissions, the spread of new systems that replace conventional power generation systems is accelerating. As one of them, "fuel cells", which have high practical value as distributed power sources and power sources for automobiles, are attracting attention. There are several types of fuel cells, but among them, polymer electrolyte fuel cells (PEFC) and solid oxide fuel cells (SOFC) are highly energy efficient, and their widespread use is expected in the future.

A fuel cell is a device that generates electric power through a reaction process opposite to the electrolysis of water, and requires hydrogen (fuel hydrogen) as a fuel. Fuel hydrogen is produced by reforming a hydrocarbon fuel such as city gas (LNG), methane, natural gas, propane, kerosene, and gasoline in the presence of a catalyst. Among them, a fuel cell using city gas as a raw material has an advantage that hydrogen can be produced in an area where city gas piping is installed.

The fuel reformer is usually operated at a high temperature of 200 to 900 ° C. in order to secure the amount of heat required for the hydrogen reforming reaction. In addition to the fuel reformer, the operating temperature of the combustor that heats the reformer, the heat exchanger, the battery body, and the like becomes extremely high.
Further, in such a fuel cell under high temperature operation, in addition to a large amount of water vapor, carbon dioxide, and carbon monoxide, an atmosphere containing a large amount of hydrogen and a trace amount of hydrogen sulfide derived from a hydrocarbon fuel (hereinafter, carburizing property). It will be exposed under (referred to as / reducing / sulfurizing environment). When, for example, a steel material is exposed to such an atmosphere, the surface of the material is carburized and corroded by sulfurization, which makes the operating environment harsh.

So far, austenitic stainless steel typified by SUS310S (25Cr-20Ni) has been used as a practical material for a fuel cell having sufficient durability in such a harsh environment. However, cost reduction is indispensable for the widespread use of fuel cell systems in the future, and reduction of alloy costs by optimizing the materials used is an important issue. Further, in the fuel cell system, when stainless steel containing a relatively high amount of Cr is applied, the operating temperature is very high, so that there is also a problem of preventing electrode poisoning due to evaporation of Cr.

From the above background, the high oxidation resistance of the Al-based oxide layer (Al-based oxide film) was utilized as the steel material constituting the fuel cell in order to exhibit good durability even in a harsh reformed gas environment. Various Al-containing ferritic stainless steels have been studied.

Patent Document 1 describes Cr: 13 to 20%, C: less than 0.02%, N: 0.02% or less, Si: more than 0.15 to 0.7%, Mn: 0.3% or less, Al. : 1.5 to 6%, Ti: 0.03 to 0.5%, Nb: 0.6% or less, and by adjusting the amount of solid-dissolved Ti and the amount of solid-dissolved Nb, oxidation resistance and creep break life Discloses a good Al-containing ferritic stainless steel for fuel cells. It has been shown that these stainless steels can obtain good oxidation resistance by an accelerated oxidation test in the atmosphere at 1050 ° C.

Patent Document 2 describes Cr: 11 to 25%, C: 0.03% or less, Si: 2% or less, Mn: 2% or less, Al: 0.5 to 4.0%, P: 0.05%. Hereinafter, S: 0.01% or less, N: 0.03% or less, Ti: 1% or less are contained, hydrogen gas is contained in an amount of 50% by volume or more, and Ti and Al are added to the steel surface in the oxide film and directly under the oxide film. A ferritic stainless steel for a fuel cell having improved oxidation resistance in a reformed gas environment by concentrating and adding a small amount of Mg, Ga, Sn, and Sb is disclosed.

Patent Document 3 describes Cr: 11.0 to 25.0%, C: 0.030% or less, Si: 2.00% or less, Mn: 2.00% or less, Al: 0.90 to 4.00. %, P: 0.050% or less, S: 0.0100% or less, N: 0.030% or less, Ti: 0.500% or less, with a small amount of B, Mg, Ca added and a combined addition with Sn. Discloses a ferritic stainless steel for fuel cells with improved oxidation resistance and creep resistance in a reformed gas environment.

Patent Document 4 describes Cr: 11 to 25%, C: 0.03% or less, Si: 2% or less, Mn: 2% or less, Al: 0.5 to 4.0%, P: 0.05%. Hereinafter, S: 0.01% or less, N: 0.03% or less, Ti: 0.5% or less, Ga: 0.1% or less, Mg: 0.01% or less, Zn: 0.05. % Or less, and by forming a surface film in which Ti and / or Al is concentrated by adding a small amount of Mg, Ga, Zn, and Sn, Sb, the oxidation resistance is improved. Ferritic stainless steel is disclosed.

Japanese Unexamined Patent Publication No. 2010-22638 Japanese Patent No. 6006893 Japanese Patent No. 6053994 Japanese Unexamined Patent Publication No. 2016-21176

The reformed gas of a fuel cell using the city gas or the like as a raw material may contain a large amount of hydrogen and a sulfide component added as an impurity or an odorant in addition to water vapor, carbon dioxide and carbon monoxide. be. However, conventionally, the oxidation resistance of ferritic stainless steel has been evaluated and examined only in an atmosphere containing water vapor and carbon dioxide as main components, an atmosphere containing water vapor and oxygen as main components, or an environment such as the atmosphere. .. That is, the oxidation characteristics of ferritic stainless steel under a harsh environment (carburizing / reducing / sulphurizing environment) containing carbon dioxide, carbon monoxide, a large amount of hydrogen, and a sulfide component are unknown.
Further, in the case of SOFC system or PEFC system, since the operating temperature of the fuel cell becomes high, further improvement of high temperature strength is required in addition to the above-mentioned oxidation characteristics.

Although the ferritic stainless steels of Patent Documents 1 to 4 have been studied for durability in an oxidizing environment, they are durable in a more severe environment such as carburizing / reducing / sulfurizing environment containing a large amount of hydrogen and hydrogen sulfide. No mention is made of sex.

The present invention has been devised to solve the above-mentioned problems, and is in an environment containing carbon dioxide, carbon monoxide, a large amount of hydrogen, and a sulfide component (carburizing / reducing / sulfide environment). However, it provides a ferrite-based stainless steel having both high oxidation resistance and excellent high-temperature strength, and a method for producing the same.

The gist of the present invention is as follows.
[1] By mass%
Cr: 12.0 to 16.0%,
C: 0.020% or less,
Si: 2.50% or less,
Mn: 1.00% or less,
P: 0.050% or less,
S: 0.0030% or less,
Al: 0.90% or more, 2.50% or less,
N: 0.030% or less,
Nb: 0.001-1.00%,
Ni: 0-1.0%,
Cu: 0-1.0%,
Mo: 0-1.0%,
Sb: 0-0.5%,
W: 0-1.0%,
Co: 0-0.5%,
V: 0-0.5%,
Ti: 0-0.5%,
Zr: 0-0.5%,
La: 0-0.1%,
Y: 0-0.1%,
Hf: 0-0.1%,
REM: 0-0.1%
Including,
B: 0.0200% or less,
Sn: 0.20% or less,
Ga: 0.0200% or less,
Mg: 0.0200% or less,
Ca: A ferritic stainless steel containing one or more of 0.0100% or less, satisfying the following formula (1), and having the balance composed of Fe and impurities.
10 (B + Ga) + Sn + Mg + Ca> 0.020 ... (1)
The element symbol in the formula (1) indicates the content (mass%) of each element in the steel.
[2] The ferrite-based stainless steel according to the above [1], wherein the B: 0.0002% or more in terms of mass%.
[3] The ferrite-based stainless steel according to the above [1] or [2], wherein the Nb concentration at the grain boundaries is in the range of 3.0 to 10% in terms of mass%.
[4] The above-mentioned [1] to [3], wherein the Sn is 0.005% or more in mass%, and the Sn concentration at the grain boundaries is 1.0 to 5.0%. The ferritic stainless steel according to any one of the items.
[5] Any of the above [1] to [4], wherein the Si: 0.5% or more, the Al: 1% or more, and the Nb: 0.15% or more in mass%. The ferritic stainless steel described in item 1.
[6] In terms of mass%, Ni: 0.10 to 1.0%, Cu: 0.10 to 1.0%, Mo: 0.10 to 1.0%, Sb: 0.01 to 0. .5%, W: 0.10 to 1.0%, Co: 0.10 to 0.5%, V: 0.10 to 0.5%, Ti: 0.01 to 0.5%, Zr: 0.01 to 0.5%, La: 0.001 to 0.1% or less, Y: 0.001 to 0.1%, Hf: 0.001 to 0.1%, REM: 0.001 to 0 The ferritic stainless steel according to any one of the above [1] to [5], which contains 1% or 2 or more of the above.
[7] The ferrite-based stainless steel according to any one of the above [1] to [6], which is applied to a fuel reformer, a heat exchanger or a fuel cell member.
[8] The ferrite-based stainless steel according to any one of the above [1] to [7], which is applied to a member of a combustor or a burner.

[9] After hot-working a stainless steel material having the composition according to any one of the above [1], [2], [5], or [6], heat treatment is omitted or heat treatment is performed at over 700 ° C. The method for producing a ferritic stainless steel according to any one of the above [1] to [6], wherein the cold working is then performed and the finish annealing is performed at a temperature of more than 700 ° C.
[10] The method for producing a ferritic stainless steel according to the above [9], wherein after the finish annealing, a heat treatment is performed in a temperature range of 600 to 700 ° C. for more than 1 minute and for 3 hours or less.
[11] The ferrite-based stainless steel according to the above [9], which has a holding time of more than 1 minute in a temperature range of 600 to 700 ° C. when heated and cooled to more than 700 ° C. in the finish annealing. Steel manufacturing method.

[12] A fuel cell member using the ferrite-based stainless steel according to any one of the above [1] to [6].

According to the present invention, high oxidation resistance and excellent high temperature strength are provided even in an environment containing carbon dioxide, carbon monoxide, a large amount of hydrogen, and a sulfide component (carburizing / reducing / sulphurizing environment). It is possible to provide a ferritic stainless steel having a combination, a method for manufacturing the same, and a member for a fuel cell.

In order to solve the above-mentioned problems, the present inventors have completed the present invention by repeating diligent experiments and studies on an Al-containing ferritic stainless steel having both high temperature strength and oxidation resistance. The "high temperature strength" in the present embodiment is a characteristic that can exhibit an excellent 0.2% proof stress even in a high temperature range of around 750 to 800 ° C., and the "oxidation resistance" is carbon dioxide and monoxide. It means the oxidation characteristics under a modified gas environment containing carbon, a large amount of hydrogen, and a sulfurizing component (hereinafter, also referred to as carburizing / reducing / sulfurizing environment).
The findings obtained in the present invention will be described below.

(A) In order to suppress deformation, which is usually a problem in structures operating in the high temperature range of around 750 to 800 ° C, the high temperature strength of the material ferrite stainless steel, especially 0.2% at around 750 ° C. It is effective to increase the proof stress and suppress the decrease of 0.2% proof stress at around 800 ° C.

(B) The improvement and suppression of the decrease in 0.2% proof stress in the high temperature range described above are not due to the excessive addition of Al or the addition of Mo, Cu, etc. that contribute to solid solution / precipitation strengthening, but B and Nb. , Sn, Mg, Ca, Ga were found to be significantly improved by adding a small amount and adjusting the amount of the addition. That is, in ferrite stainless steel, the property of increasing the 0.2% proof stress at around 750 ° C and suppressing the decrease in 0.2% proof stress at around 800 ° C can be achieved by adding these trace elements. Findings were obtained. There are still many unclear points about the action of improving the high temperature strength, but the mechanism of action described below is inferred based on the experimental facts.

(C) The addition of a small amount of B contributes not a little to the increase in proof stress and tensile strength at 750 to 800 ° C., and in particular has the effect of significantly improving the proof stress by 0.2%. The addition of a small amount of B suppresses the formation of cavities (nano-sized gaps) generated from the grain boundaries due to the segregation of B at the grain boundaries, delays the grain boundary slip, and causes the dislocation density in the crystal grains. It has the effect of increasing the internal stress associated with the rise of. In addition, we have found new findings that the action and effect of B are remarkable in Nb-added steel.

(D) It is well known that the addition of Nb is effective in increasing the strength in the temperature range up to 750 ° C. by strengthening the solid solution. Precipitation of Nb starts at 750 to 800 ° C. _{by forming an intermetallic compound called a Laves phase (Fe 2} Nb), but Nb and B are co-segregated at the grain boundaries to form B in (c) above. The action and effect can be manifested.

(E) Further, the above-mentioned action and effect of B, which is remarkable in the Nb-added steel, is superimposed by the combined addition of Mg, Ca, and Ga.

(F) Further, in order to further exert the effect of increasing the internal stress accompanying the increase in the dislocation density in the grain described in the above (c), the combined addition with Sn is effective. Although Sn is a grain boundary segregation element, when it is added in combination with B and Nb, its action as a solid solution strengthening element in the crystal grains is increased, and it is effective in increasing the high temperature strength with the increase in internal stress. ..

(G) Further, in order to enhance the oxidation resistance in the reformed gas environment containing the above-mentioned hydrogen and sulfide components, the content of Si, Al, Nb and Mn is adjusted within a predetermined range to improve the temperature and temperature. It is effective to promote the formation of an Al-based oxide film in a quality gas environment and to enhance the protective property of the film. Furthermore, the addition of B, Nb, Sn, Mg, Ca, and Ga in ferritic stainless steel does not impair the oxidation resistance in a modified gas environment. Rather, the addition of a small amount of Mg and Sn does not impair the oxidation resistance of the Al-based oxide film. It also enhances the protection of stainless steel and has the effect of oxidation resistance. In the present embodiment, the surface film before being exposed to the high temperature reformed gas environment is the "passive film", and the composition of the passive film is changed by various reactions when exposed to the high temperature reformed gas environment. The thing will be described separately from the "Al-based oxide film".

(H) The reformed gas environment (carburizing / reducing / sulphurizing environment) described above produces defects in the Al-based oxide film in the ferritic stainless steel as compared with the steam oxidation environment containing no atmosphere or hydrogen. easy. The reason why the reformed gas environment facilitates the formation of defects in the oxide film is not clear, but it is presumed that the sulfide produced under the reformed gas containing a sulfide component has some adverse effect on the oxide film. .. If a defect occurs in the Al-based oxide film in the reformed gas environment, the oxidation of Cr and Fe may proceed in the exposed base material. In response to the promotion of oxidation in such a reforming gas, Mg dissolves in an Al-based oxide film, and Sn delays the outward diffusion of Cr and Fe by segregation on the surface of the base metal, thereby causing an Al-based solution. The protective property of the oxide film can be further enhanced. As a result, the oxidation resistance of the ferritic stainless steel can be improved.

(I) Furthermore, regarding the precipitation (σ brittleness) of the intermetallic compound σ phase and brittleness at 475 ° C., which were the drawbacks of the conventional Al and Si-containing stainless steel, Cr, Nb, Si, in the component composition, It was found that adjusting the Al content was effective. The σ brittleness and the 475 ° C. brittleness are derived from the formation of intermetallic compounds containing Cr as the main component and Si and Al, and the formation sites are often grain boundaries. That is, in order to suppress σ brittleness and 475 ° C. brittleness, it can be said that it is effective to suppress the formation of the intermetallic compound itself and reduce the formation sites thereof. As a result of further studies by the present inventors on these, the formation of the intermetallic compound itself is suppressed by limiting the amount of Cr, and the structure is stabilized by suppressing the formation site by segregation of Nb into the grain boundaries. As a result, it was found that σ brittleness and 475 ° C. brittleness can be suppressed. Furthermore, since the formation of intermetallic compounds containing Si and Al can be suppressed by limiting the amount of Cr and adding Nb, the amounts of Si and Al that contribute to the oxidation resistance described in (h) above can be secured, and thus acid resistance. It is also possible to achieve both chemical properties and organizational stability.

As described above, in ferrite stainless steel, due to the combined addition of B, Nb, Sn, Mg, Ca and Ga, the high temperature strength and the reformed gas, which are important for durability in a carburizing / reducing / sulphurizing environment, are used. We have obtained a new finding that it can also have the oxidation resistance of stainless steel. Furthermore, in ferrite stainless steel, by optimizing the content of Cr, Nb, Si, and Al, it is possible to suppress σ brittleness and 475 ° C brittleness by improving the structure stability, and it is also possible to achieve both oxidation resistance. , Was also newly obtained.

Hereinafter, an embodiment of the ferritic stainless steel of the present invention will be described.

<Ingredient composition>
First, the reasons for limiting the components will be described below. In addition, "%" display of the content of each element means "mass%".

Cr is a constituent element that is basic for ensuring high-temperature strength in addition to corrosion resistance. In the present embodiment, if it is less than 12.0%, the target high-temperature strength and oxidation resistance cannot be sufficiently secured. Therefore, the lower limit of the Cr content is 12.0% or more. It is preferably 13.0% or more. However, excessive inclusion of Cr may promote the formation of the σ phase (intermetallic compound of Fe-Cr), which is an embrittlement phase, and promote cracking during production when exposed to a high temperature atmosphere. .. Therefore, the upper limit of the Cr content is 16.0% or less from the viewpoint of basic characteristics and manufacturability. It is preferably 15.0% or less.

C forms a solid solution or Cr carbide in the ferrite phase and inhibits oxidation resistance. Therefore, the upper limit of the amount of C is set to 0.020% or less. It is preferably 0.015% or less. However, since an excessive reduction in the amount of C leads to an increase in the refining cost, the lower limit is preferably 0.001% or more. From the viewpoint of oxidation resistance and manufacturability, it is more preferably 0.005% or more.

Si is an important element for ensuring oxidation resistance. Si slightly dissolves in the Al-based oxide film and also concentrates directly under the Al-based oxide film / at the steel interface, improving the oxidation resistance in a modified gas environment. In order to obtain these effects, the lower limit is preferably 0.50% or more. More preferably, it is 0.70% or more. On the other hand, if Si is excessively contained, the toughness and workability of the steel may be deteriorated and the formation of an Al-based oxide film may be hindered. Therefore, the upper limit is 2.50% or less. From the viewpoint of oxidation resistance and basic characteristics, 1.70% or less is preferable.

Mn can be dissolved together with Si in or directly under the Al-based oxide film in a reformed gas environment to enhance the protection of the film and contribute to the improvement of oxidation resistance. In order to obtain these effects, the lower limit is preferably 0.10% or more. More preferably, it is 0.20% or more. On the other hand, excessive inclusion of Mn inhibits the corrosion resistance of steel and the formation of Ti and Al oxide films, so the upper limit is set to 1.00% or less. From the viewpoint of oxidation resistance and basic characteristics, 0.90% or less is preferable.

In addition to being a deoxidizing element, Al is an essential element that forms an Al-based oxide film in the reformed gas and contributes to the improvement of oxidation resistance in the present embodiment. In the present embodiment, it is preferably 1.00% or more, and more preferably 1.50% or more in order to obtain good oxidation resistance. However, if Al is excessively contained, the toughness and weldability of the steel are lowered and the productivity is impaired, so that there is a problem in terms of economy as well as an increase in alloy cost. Therefore, the upper limit of the amount of Al is 2.50% or less from the viewpoint of basic characteristics and economic efficiency. More preferably, it is 2.30% or less.

P is an element that impairs manufacturability and weldability, and the smaller the content, the better, so the upper limit is 0.050% or less. However, since an excessive reduction of P leads to an increase in refining cost, the lower limit is preferably 0.003%. From the viewpoint of manufacturability and weldability, the preferable range is 0.005 to 0.040%, more preferably 0.010 to 0.030%.

S is an impurity element inevitably contained in steel, which lowers high temperature strength and oxidation resistance. In particular, the presence of grain boundary segregation of S, Mn-based inclusions, and solid solution S has an effect of lowering high-temperature strength and oxidation resistance. Therefore, the lower the amount of S, the better, so the upper limit is 0.0030% or less. However, since excessive reduction of S leads to an increase in raw materials and refining costs, the lower limit is preferably 0.0001% or more. From the viewpoint of manufacturability and oxidation resistance, a preferable range is 0.0001 to 0.0020%, and more preferably 0.0002 to 0.0010%.

N is an element that inhibits oxidation resistance like C. Therefore, the smaller the amount of N, the better, and the upper limit is 0.030% or less. However, since an excessive reduction of N leads to an increase in refining cost, the lower limit is preferably 0.002% or more. From the viewpoint of oxidation resistance and manufacturability, the preferable range of the amount of N is 0.005 to 0.020%.

Nb is a stabilizing element that fixes C and N, and oxidation resistance and corrosion resistance can be improved by increasing the purity of steel by this action. Further, in the present embodiment, it is an element that effectively acts to improve the high temperature strength due to the action and effect of co-segregation at the grain boundary with B. Furthermore, the intermetallic compound that causes σ brittleness and 475 ° C. brittleness proceeds to precipitate mainly at the grain boundaries, but this formation site is reduced by segregation of Nb into the grain boundaries. , The stability of the structure is increased, and as a result, σ brittleness and 475 ° C. brittleness can be suppressed. In order to obtain these effects, the lower limit of the amount of Nb is 0.001% or more, preferably 0.15% or more. On the other hand, since excessive inclusion of Nb leads to an increase in alloy cost and hindering manufacturability, the upper limit of the amount of Nb is set to 1.00% or less. It is preferably 0.60% or less.

B, Sn, Ga, Mg, and Ca are elements that can further exhibit the effect of increasing the high temperature strength, as described in the above findings (e) and (f). Further, these elements are also elements that promote the formation of an Al-based oxide film and contribute to the improvement of oxidation resistance. Therefore, in addition to the above-mentioned component composition, it contains one or more of B, Sn, Ga, Mg, and Ca.
B can delay grain boundary slip by segregating grain boundaries, increase internal stress due to an increase in dislocation density in the crystal grains, and improve 0.2% proof stress. Sn, Ga, Mg, and Ca have the effect of concentrating in the vicinity of the surface and promoting the selective oxidation of Al. In order to obtain such an effect, it is preferable that the lower limit of the content of each of B, Ga, Mg and Ca is 0.0002% or more, and the lower limit of Sn is 0.005% or more. On the other hand, excessive inclusion of these elements leads to an increase in steel refining cost and a decrease in manufacturability and corrosion resistance of steel. Therefore, the upper limit of the Ca content is 0.0100% or less, the upper limit of Sn is 0.20% or less, and the upper limit of B, Ga, and Mg is 0.0200% or less.

Further, the component composition of the present embodiment shall satisfy the following formula (1).
10 (B + Ga) + Sn + Mg + Ca> 0.020% ・ ・ ・ Equation (1)
The element symbol in the formula (1) indicates the content (mass%) of each element in the steel.

From the viewpoint of improving high temperature strength and oxidation resistance, the formula (1) is preferably 0.025% or more, more preferably 0.035% or more. The upper limit of the formula (1) is not particularly specified by the upper limit values of B, Sn, Ga, Mg, and Ca, but is preferably 0.2% from the viewpoint of high temperature strength and manufacturability.

Next, the concentration (mass%) of the segregating element at the grain boundary will be described.
As described in the above findings (c) and (d), in the present embodiment, the high temperature strength can be improved by co-segregating Nb and B at the grain boundaries. Similarly, by segregating Sn into the grain boundaries, it is possible to suppress the grain boundary slip and improve the high temperature strength. From these viewpoints, in the ferritic stainless steel according to the present embodiment, the concentration of Nb at the grain boundaries (grain boundary concentration) is preferably 3.0% or more. When Sn is contained, the grain boundary concentration of Sn is preferably 1.0% or more. On the other hand, excessive grain boundary segregation of Nb and Sn may cause a decrease in high-temperature strength in addition to impairing manufacturability with the crystal grain boundaries as the starting point of fracture. Therefore, the grain boundary concentration of Nb is preferably 10.0% or less, and the grain boundary concentration of Sn is preferably 5.0% or less.
The Nb concentration and Sn concentration at the grain boundaries can be adjusted by further performing heat treatment under predetermined conditions after finish annealing. Details will be described later.

The Nb concentration and Sn concentration at the grain boundaries can be measured by Auger electron spectroscopy (AES).
First, a notched test piece (0.8t × 4w × 20L (mm)) is collected from an arbitrary part of the matrix other than the oxide film. Next, the notched test piece is ^{cooled with liquid nitrogen in vacuum (vacuum degree: 10-6} MPa), and then the test piece is broken from the notch portion on the spot to expose the fracture surface. AES analysis is performed on the grain boundaries on the exposed fracture surface, Auger electron spectra are measured in the energy range of 0 to 1000 eV, and the detected elements are identified (qualitative analysis). Furthermore, the detected elements are quantified (quantitative analysis) using the obtained peak intensity ratio (relative sensitivity coefficient method). By the above method, the concentration of the element (Nb, Sn) segregated at the grain boundary can be obtained.

The ferrite-based stainless steel according to the present embodiment is composed of Fe and impurities other than the elements described above (residue), but can also contain arbitrary elements described later. Therefore, the lower limit of the content of Ni, Cu, Mo, Sb, W, Co, V, Ti, Zr, La, Y, Hf, and REM is 0% or more.
The "impurity" in the present embodiment is a component that is mixed by various factors in the manufacturing process including raw materials such as ore and scrap when industrially manufacturing steel, and is inevitably mixed. Also includes ingredients.

The ferrite-based stainless steel of the present embodiment has Ni: 1.0% or less, Cu: 1.0% or less, Mo: 1.0% or less, Sb: 0.5% or less, W: 1 as necessary. .0% or less, Co: 0.5% or less, V: 0.5% or less, Ti: 0.5% or less, Zr: 0.5% or less, La: 0.1% or less, Y: 0.1 % Or less, Hf: 0.1% or less, REM: 0.1% or less may be contained in one kind or two or more kinds.

Ni, Cu, Mo, Sb, W, Co, V, and Ti are elements effective for increasing the high temperature strength and corrosion resistance of steel, and may be contained as necessary. However, if it is contained in an excessive amount, the alloy cost will increase and the manufacturability will be hindered. Therefore, the upper limit of Ni, Cu and W is set to 1.0% or less. Since Mo is an element that is also effective in suppressing high-temperature deformation due to a decrease in the coefficient of thermal expansion, it is preferable that Mo is contained after setting the upper limit to 1.0% or less. Since Sb is an element that concentrates in the vicinity of the steel surface to promote selective oxidation of Al and has an effect of improving corrosion resistance, it is preferable that Sb is contained after setting the upper limit to 0.5% or less. The upper limit of Co, Ti, and V is 0.5% or less. The lower limit of the preferable content of any of the elements Ni, Cu, Mo, W, Co and V is 0.10% or more. The lower limit of the preferable contents of Sb and Ti is 0.01% or more.

Zr, La, Y, Hf, and REM are conventionally effective elements for improving hot workability, cleanliness of steel, and improving oxidation resistance, and may be contained as necessary. However, due to the technical idea of the present invention and the reduction of alloy cost, the effect of adding these elements cannot be relied on. When Zr, La, Y, Hf, and REM are contained, the upper limit of Zr is 0.5%, and the upper limit of La, Y, Hf, and REM is 0.1%, respectively. The more preferable lower limit of Zr is 0.01%, and the preferable lower limit of La, Y, Hf, and REM is 0.001%. Here, REM is an element and Sc (scandium) belonging to atomic numbers 58 to 71 excluding La and Y, and are, for example, Ce, Pr, Nd and the like. Further, the REM referred to in the present embodiment is composed of one or more selected from the elements belonging to atomic numbers 58 to 71 and Sc, and the REM amount is the total amount of these.

The ferritic stainless steel according to the present embodiment is composed of Fe and impurities (including unavoidable impurities) other than the elements described above, but in addition to the above-mentioned elements, the effect of the present invention is not impaired. Can be contained in. It is preferable to reduce Bi, Se and the like as much as possible, including the above-mentioned P and S, which are general impurity elements. On the other hand, the content ratio of these elements is controlled to the extent that the problem of the present invention is solved, and one or more of Bi ≦ 100 ppm and Se ≦ 100 ppm may be contained, if necessary.

The metal structure of the ferritic stainless steel of the present embodiment has a ferrite single-phase structure. This means that it does not contain the austenite phase or the martensite structure. When an austenite phase or a martensite structure is contained, the raw material cost is high and the yield such as ear cracks is likely to decrease during production. Therefore, the metal structure is a ferrite single-phase structure. It should be noted that although precipitates such as carbonitrides are present in the steel, they are not considered because they do not greatly affect the effect of the present invention, and the above description describes the structure of the main phase.

The shape of the ferrite-based stainless steel of the present embodiment is not particularly limited, and may be plate-shaped, tubular, rod-shaped, or the like, and may be appropriately determined according to the size, shape, and shape of the member to be applied.

<Manufacturing method>
Next, the above-mentioned method for producing a ferritic stainless steel according to the present embodiment can be produced by combining hot working, cold working, and each heat treatment (annealing), and is appropriately pickled as necessary. And descaling may be performed. As an example of the manufacturing method, a manufacturing method having each step of steelmaking-hot rolling-annealing-cold rolling-annealing (finish annealing) can be adopted, and the heat treatment after hot rolling is preferably over 700 ° C.
For example, a heat treatment after hot rolling may be performed at a temperature of over 700 ° C., a cold rolling may be performed after descaling, and then finish annealing and descaling at a temperature of over 700 ° C. may be performed to obtain a cold-rolled annealed plate. The rolling ratio for cold rolling is not particularly specified, but is preferably in the range of 30 to 80%.
Furthermore, when ferritic stainless steel is applied to gas piping applications, welded pipes manufactured from steel plates are also included, but the pipes are not limited to welded pipes, and seamless pipes manufactured by hot working are also included. good.

The temperature of heat treatment after hot working (hot-rolled sheet annealing) and finish annealing after cold rolling is set to over 700 ° C to recrystallize the steel and dissolve Nb and Sn, which are effective for increasing high-temperature strength. To make it happen. However, excessive temperature rise of each treatment temperature of heat treatment after hot working and finish annealing after cold rolling leads to coarsening of crystal grain size and deterioration of surface quality such as rough skin. Therefore, the upper limit of each treatment temperature of heat treatment after hot working and finish annealing after cold rolling is preferably 1050 ° C. or lower.

After finish annealing, it is desirable to further perform heat treatment (heat treatment after finish annealing) in order to adjust the concentrations of Nb and Sn at the grain boundaries within the above range to increase the high temperature strength. Specifically, after finish annealing, heat treatment may be performed by reheating to 600 to 700 ° C. and holding for more than 1 minute and for 3 hours or less. The term "holding" as used in the present embodiment may be a state in which a constant temperature is maintained as long as it is within the range of 600 to 700 ° C., or may be varied within the range. That is, as long as the time that the steel sheet exists between 600 and 700 ° C. is more than 1 minute and 3 hours or less, it does not matter whether or not the temperature of the steel sheet fluctuates during that time. Further, the temperature of the steel sheet in the present embodiment refers to the temperature of the surface of the steel sheet.

If the heat treatment temperature after finish annealing exceeds 700 ° C., or if the heat treatment holding time after finish annealing exceeds 3 hours, a coarse Laves phase (Fe ₂ Nb) having a long side of more than 1 μm is likely to precipitate, and the high temperature strength is high. May lead to a decline. Therefore, the upper limit of the heat treatment temperature after finish annealing is 700 ° C. or less, and the holding time is 3 hours or less. On the other hand, if the heat treatment temperature after finish annealing is less than 600 ° C., or if the heat treatment holding time after finish annealing is 1 minute or less, grain boundary segregation of Nb and Sn does not proceed and sufficient high-temperature strength cannot be obtained. In some cases, the grain boundary strength is lowered, σ phase precipitation due to structural instability, and brittleness at 475 ° C. occur. Therefore, the lower limit of the heat treatment temperature after finish annealing is 600 ° C. or higher, and the holding time is more than 1 minute.

The control of the grain boundary concentrations of Nb and Sn can also be adjusted in the process of finish annealing without performing the heat treatment after finish annealing. In that case, when cooling the finish annealing, that is, when cooling after heating to over 700 ° C., the passage time (cooling required time) in the temperature range of 600 to 700 ° C. is controlled to be over 1 minute. Specifically, the transit time between the temperatures can be controlled by adjusting the cooling rate between the temperatures of 600 to 700 ° C.

The atmosphere during heat treatment after hot working, finish annealing, and heat treatment after finish annealing is not particularly specified, but it is an atmosphere, an LNG fuel atmosphere, and a non-oxidizing atmosphere using hydrogen, nitrogen, argon, etc. (brilliant annealing). Is preferable.

The ferrite-based stainless steel according to the present embodiment can be obtained by the manufacturing method described above.

According to this embodiment, high oxidation resistance and excellent high temperature strength even in an environment containing carbon dioxide, carbon monoxide, a large amount of hydrogen, and a sulfide component (carburizing / reducing / sulphurizing environment). It is possible to provide a ferritic stainless steel that also has the above.
Furthermore, according to the present embodiment, by optimizing the composition of the components, it is possible to enjoy excellent tissue stability that can suppress σ phase precipitation and 475 ° C. brittleness.
Therefore, it is suitable for fuel cell members such as fuel reformers and heat exchangers used when reforming hydrocarbon fuels such as city gas, methane, natural gas, propane, kerosene, and gasoline into hydrogen. In particular, it is suitable for high temperature members of solid oxide fuel cells (SOFC) and solid polymer fuel cells (PEFC) whose operating temperature is high. Further, it can be suitably used for all peripheral members of a fuel cell, such as a burner and a combustor for storing the burner, which are in contact with a reforming gas and are used in a high temperature environment.

Next, an example of the present invention will be shown. The conditions in the examples are one condition example adopted for confirming the feasibility and effect of the present invention, and the present invention is used in the following examples. It is not limited to the conditions. The present invention can adopt various conditions as long as the gist of the present invention is not deviated and the object of the present invention is achieved.
The underlined lines in the table below indicate those outside the scope of the present invention.

Various ferritic stainless steels whose components are shown in Table 1 are melted (steels A to O), hot-rolled to a 4 mm thick hot-rolled plate, then heat-treated (hot-rolled plate annealing), pickled, and cold-rolled. To produce a cold-rolled steel sheet having a plate thickness of 0.8 mm. The hot-rolled plate was annealed in the range of 900 to 1000 ° C.
After cold rolling, finish annealing was performed at 900 to 1000 ° C. Some steel sheets (steels B, E, F and I) were subjected to heat treatment (heat treatment after finish annealing) under the conditions shown in Table 2 after finish annealing.
The obtained cold-rolled steel sheets (Nos. 1 to 19) were evaluated for grain boundary concentration measurement and various characteristics.

[Measurement of grain boundary concentration]
The Nb concentration and Sn concentration in the crystal grain system were measured by Auger electron spectroscopy.
First, a notched test piece (0.8t × 4w × 20L (mm)) was collected from a cold-rolled steel sheet. Next, the notched test piece is ^{cooled with liquid nitrogen in a vacuum (vacuum degree: 10-6} MPa), and then the notch portion is broken on the spot so that the fracture surface is not exposed to the atmosphere to expose the grain boundaries. I let you. AES analysis was performed on the exposed grain boundaries, Auger electron spectra were measured in the energy range of 0 to 1000 eV, and Nb elements and Sn elements were identified (qualitative analysis). Further, the amount of Nb and the amount of Sn were quantitatively analyzed using the obtained peak intensity ratio (relative sensitivity coefficient method), and the Nb concentration and Sn concentration (both mass%) segregated at the grain boundaries were determined. Table 2 shows the Nb concentration and Sn concentration obtained, and "<3.0" and "<1.0" in the table indicate that the detected values were less than 3.0% and less than 1.0%. Means.

[Oxidation resistance]
First, an oxidation test piece having a width of 20 mm and a length of 25 mm was cut out from a cold-rolled steel sheet and subjected to an oxidation test. Atmosphere oxidation test, assuming the reformed gas in which the city gas as fuel, 28 _vol% H 2 _O-10% by volume% CO-8 _{vol% CO 2 -0.01% H 2 S} -bal. Was an atmosphere of H _2. In the atmosphere, the oxidation test piece was heated to 650 ° C., held for 1000 hours, cooled to room temperature, and the oxidation increase ΔW (mg / cm ² ) was measured.
The evaluation of oxidation resistance was as follows.
⊚: Weight increase ΔW is less than ^{0.2 mg / cm 2.}
〇: Weight increase ΔW is 0.2 to 0.3 mg / cm ² .
X: Weight increase ΔW is more than ^{0.3 mg / cm 2.}
As for the oxidation resistance, the cases of "◎" and "○" were regarded as acceptable.

[High temperature strength]
Plate-shaped high-temperature tensile test pieces (plate thickness: 0.8 mm, parallel portion width: 10.5 mm, parallel portion length: 35 mm) were prepared from cold-rolled steel sheets in the longitudinal direction, and 750 ° C. and 800 ° C. A high-temperature tensile test was conducted with the strain rate set to 0.3% / min up to 0.2% proof stress and then 3 mm / min at each ° C, and 0.2% proof stress (750 ° C proof stress, 800 ° C proof stress) at each temperature. Was measured (according to JIS G 0567).
In the evaluation of high temperature strength, when the proof stress of 750 ° C is more than 120 MPa and the proof stress of 800 ° C is more than 40 MPa, it is evaluated as a pass (“〇”), and when any one of them is not satisfied, it is rejected (“×”). evaluated. When the proof stress of 750 ° C. was more than 150 MPa and the proof stress of 800 ° C. was more than 60 MPa, it was evaluated as having particularly excellent high-temperature strength (indicated by "◎" in Table 2).

[Structural stability (σ brittleness / 475 ° C brittleness)]
Two samples were taken from the cold-rolled steel sheet so that the center on the cross section perpendicular to the plate surface (center of plate thickness: near t / 2) could be observed, and one was heat-treated at 500 ° C for 1000 hours (500 ° C). Heat treatment), and the other was heat-treated at 650 ° C. × 1000 hours (heat treatment at 600 ° C.). The atmosphere of these heat treatments was set to the atmosphere. Next, after each sample after the heat treatment is embedded in a resin and polished, the Vickers hardness Hv 500 ° C after the heat treatment at _{500 ° C. and the Vickers hardness Hv 650 ° C.} after the heat treatment at 650 ° C. are loaded according to JIS Z 2244, respectively. _{The hardness increase amount ΔHv 500 ° C.} and ΔHv _{650 °} C. from the Vickers hardness before heat treatment, which were measured at 8.N and measured in advance before the heat treatment, were calculated.
For the evaluation of tissue stability (σ brittleness / 475 ° C brittleness), _{those with ΔHv 500 ° C} and ΔHv _{650 ° C} both less than 20 are evaluated as acceptable (“〇”), and if either one is 20 or more, it is evaluated. It was rejected (“x”) because the hardness increased significantly after the heat treatment and the structure was unstable.

The obtained evaluation results (measurement of grain boundary concentration, various characteristics) are shown in Table 2.
No. 1 to 13 satisfy the components specified in the present invention, and the evaluation of all the characteristics is "◯" or "◎". Above all, No. In the cases of 3, 7, 9 and 13, which satisfy the suitable grain boundary concentration of the present invention, a remarkable effect of improving the high temperature strength was exhibited, and the evaluation was “⊚”. In addition, No. Nos. 2, 3, 8, 9 and 11 are cases where the suitable component composition of the present invention (particularly Cr, Si, Al) is satisfied, and a remarkable effect of improving the oxidation resistance is exhibited, and the evaluation thereof is evaluated. It became "◎".

Steel No. Nos. 14 to 19 deviate from the steel components specified in the present invention, and the respective characteristics targeted by the present invention could not be compatible with each other, and one of the evaluations was "x".

Claims (12)

By mass%
Cr: 12.0 to 16.0%,
C: 0.020% or less,
Si: 2.50% or less,
Mn: 1.00% or less,
P: 0.050% or less,
S: 0.0030% or less,
Al: 0.90% or more, 2.50% or less,
N: 0.030% or less,
Nb: 0.001-1.00%,
Ni: 0-1.0%,
Cu: 0-1.0%,
Mo: 0-1.0%,
Sb: 0-0.5%,
W: 0-1.0%,
Co: 0-0.5%,
V: 0-0.5%,
Ti: 0-0.5%,
Zr: 0-0.5%,
La: 0-0.1%,
Y: 0-0.1%,
Hf: 0-0.1%,
REM: 0-0.1%
Including,
B: 0.0200% or less,
Sn: 0.20% or less,
Ga: 0.0200% or less,
Mg: 0.0200% or less,
Ca: A ferritic stainless steel containing one or more of 0.0100% or less, satisfying the following formula (1), and having the balance composed of Fe and impurities.
10 (B + Ga) + Sn + Mg + Ca> 0.020 ... (1)
The element symbol in the formula (1) indicates the content (mass%) of each element in the steel.
The ferrite-based stainless steel according to claim 1, wherein the B: 0.0002% or more in terms of mass%. The ferrite-based stainless steel according to claim 1 or 2, wherein the Nb concentration at the grain boundaries is in the range of 3.0 to 10% in terms of mass%. The invention according to any one of claims 1 to 3, wherein the Sn is 0.005% or more in terms of mass%, and the Sn concentration at the grain boundaries is 1.0 to 5.0%. Ferritic stainless steel. The ferrite according to any one of claims 1 to 4, wherein the Si: 0.5% or more, the Al: 1% or more, and the Nb: 0.15% or more in terms of mass%. Ferritic stainless steel. By mass%, Ni: 0.10 to 1.0%, Cu: 0.10 to 1.0%, Mo: 0.10 to 1.0%, Sb: 0.01 to 0.5%. , W: 0.10 to 1.0%, Co: 0.10 to 0.5%, V: 0.10 to 0.5%, Ti: 0.01 to 0.5%, Zr: 0.01 ~ 0.5%, La: 0.001 to 0.1% or less, Y: 0.001 to 0.1%, Hf: 0.001 to 0.1%, REM: 0.001 to 0.1% The ferrite-based stainless steel according to any one of claims 1 to 5, which contains one or more of the above. The ferritic stainless steel according to any one of claims 1 to 6, wherein the ferritic stainless steel is applied to a fuel reformer, a heat exchanger, or a fuel cell member. The ferrite-based stainless steel according to any one of claims 1 to 7, wherein the ferrite-based stainless steel is applied to a member of a combustor or a burner. A stainless steel material having the composition according to any one of claims 1, 2, 5, or 6 is hot-worked, then heat-treated at over 700 ° C., then cold-worked, and finish-annealed at over 700 ° C. The method for producing a ferritic stainless steel according to any one of claims 1 to 6, wherein the method is described in 1. The method for producing a ferritic stainless steel according to claim 9, wherein after the finish annealing, a heat treatment is performed in a temperature range of 600 to 700 ° C. for more than 1 minute and for 3 hours or less. The method for producing a ferritic stainless steel according to claim 9, wherein in the finish annealing, the holding time in the temperature range of 600 to 700 ° C. is set to more than 1 minute when heated to over 700 ° C. and cooled. .. A fuel cell member using the ferrite-based stainless steel according to any one of claims 1 to 6.