JP2019173072A - Ferritic stainless steel and manufacturing method therefor, and fuel battery member - Google Patents

Abstract

To provide a ferritic stainless steel having high oxidation resistance and excellent high temperature strength, and excellent structure stability capable of suppressing σ phase deposition or 475°C brittleness even under reduction/carburization/sulfidation environment.SOLUTION: There is provided a ferritic stainless steel containing, 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:2.50% or less, N:0.030% or less, Nb:0.001 to 1.00%, and further one or more kinds of B:0.0200% or less, Sn:0.20% or less, Ga:0.0200% or less, Mg:0.0200% or less, Ca:0.0100% or less, with satisfying the following formula (1), and the balance Fe with impurities. 10(B+Ga)+Sn+Mg+Ca>0.020 (1)SELECTED DRAWING: None

Description

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

Recently, the spread of new systems replacing conventional power generation systems is accelerating due to problems such as depletion of fossil fuels such as petroleum and global warming due to CO ₂ emissions. As one of them, “fuel cell”, which has high practical value as a distributed power source and a power source for automobiles, is attracting attention. There are several types of fuel cells. Among them, polymer electrolyte fuel cells (PEFC) and solid oxide fuel cells (SOFC) have high energy efficiency, and are expected to expand in the future.

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

The fuel reformer is usually operated at a high temperature of 200 to 900 ° C. in order to ensure the amount of heat necessary for the hydrogen reforming reaction. In addition to the fuel reformer, the operating temperature of the combustor, the heat exchanger, the battery body, and the like that heat the reformer is very high.
Further, in such a fuel cell under high temperature operation, an atmosphere containing a large amount of hydrogen and a small amount of hydrogen sulfide derived from a hydrocarbon fuel (hereinafter referred to as carburizing property) in addition to a large amount of water vapor, carbon dioxide, and carbon monoxide. / Reducing / sulfiding environment). When a steel material is exposed to such an atmosphere, for example, the surface of the material is corroded by carburization and sulfidation, and the operating environment is severe.

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

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

  In Patent Document 1, 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 : 1.5-6%, Ti: 0.03-0.5%, Nb: 0.6% or less, oxidation resistance and creep rupture life by adjusting the amount of solid solution Ti and the amount of solid solution Nb Discloses a good Al-containing ferritic stainless steel for fuel cells. These stainless steels show that good oxidation resistance is obtained by an accelerated oxidation test in air at 1050 ° C.

  In Patent Document 2, 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, hydrogen gas is contained by 50% by volume or more, and Ti or Al is added to the steel surface in the oxide film and immediately below the oxide film. There is disclosed a ferritic stainless steel for fuel cells that is enriched and added with a slight amount of Mg, Ga, Sn, and Sb to improve oxidation resistance in a reformed gas environment.

  In Patent Document 3, 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, and a small amount of B, Mg, Ca and combined addition with Sn Discloses a ferritic stainless steel for a fuel cell with improved oxidation resistance and creep resistance in a reformed gas environment.

  In Patent Document 4, 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, Mg, Ga, Zn, and also by adding a trace amount of Sn, Sb to form a surface film enriched Ti and / or Al, to improve the oxidation resistance Ferritic stainless steel is disclosed.

JP 2010-222638 A Japanese Patent No. 6006893 Japanese Patent No. 6053994 JP 2016-211076 A

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

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

  The present invention has been devised to solve the above-described problems, and is under an environment (carburizing / reducing / sulfiding environment) containing carbon dioxide, carbon monoxide, a large amount of hydrogen, and a sulfur component. However, the present invention provides a ferritic 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: 2.50% or less,
N: 0.030% or less,
Nb: 0.001 to 1.00%,
Ni: 0 to 1.0%,
Cu: 0 to 1.0%
Mo: 0 to 1.0%,
Sb: 0 to 0.5%,
W: 0 to 1.0%
Co: 0 to 0.5%,
V: 0 to 0.5%
Ti: 0 to 0.5%,
Zr: 0 to 0.5%,
La: 0 to 0.1%
Y: 0 to 0.1%
Hf: 0 to 0.1%,
REM: 0 to 0.1%
Including,
B: 0.0200% or less,
Sn: 0.20% or less,
Ga: 0.0200% or less,
Mg: 0.0200% or less,
Ca: Ferritic stainless steel containing one or more of 0.0100% or less, satisfying the following formula (1), and the balance consisting of Fe and impurities.
10 (B + Ga) + Sn + Mg + Ca> 0.020 (1)
In addition, each element symbol in Formula (1) shows content (mass%) of each element in steel.
[2] Ferritic stainless steel according to the above [1], wherein the B is 0.0002% or more in mass%.
[3] The ferritic stainless steel as described in [1] or [2] above, wherein the Nb concentration at the grain boundaries is in the range of 3.0 to 10% by mass.
[4] In the above [1] to [3], the Sn is 0.005% or more by mass%, and the Sn concentration of the crystal grain boundary is 1.0 to 5.0%. The ferritic stainless steel according to any one of the above.
[5] Any one of [1] to [4] above, wherein, in mass%, the Si: 0.5% or more, the Al: 1% or more, and the Nb: 0.15% or more. The ferritic stainless steel according to one item.
[6] In mass%, Ni: 0.10 to 1.0%, Cu: 0.10 to 1.0%, Mo: 0.10 to 1.0%, Sb: 0.01 to 0 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: 0.001-0.1% or less, Y: 0.001-0.1%, Hf: 0.001-0.1%, REM: 0.001-0 The ferritic stainless steel according to any one of the above [1] to [5], wherein the ferritic stainless steel contains 1% or two or more of 1%.
[7] The ferritic stainless steel according to any one of [1] to [6], which is applied to a fuel reformer, a heat exchanger, or a fuel cell member.
[8] The ferritic stainless steel according to any one of [1] to [7], which is applied to a combustor or a burner member.

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

[12] A fuel cell member using the ferritic stainless steel according to any one of [1] to [6].

  According to the present invention, high oxidation resistance and excellent high-temperature strength can be obtained even in an environment containing carbon dioxide, carbon monoxide, a large amount of hydrogen, and a sulfur component (carburizing / reducing / sulfiding environment). A ferritic stainless steel, a manufacturing method thereof, and a fuel cell member can be provided.

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

(A) Usually, in order to suppress deformation which becomes a problem in a structure operating in a high temperature range near 750 to 800 ° C., the high temperature strength of the ferritic stainless steel as a material, particularly 0.2% at around 750 ° C. It is effective to increase the proof stress and suppress the decrease in 0.2% proof stress in the vicinity of 800 ° C.

(B) Improvement of 0.2% proof stress and suppression of decrease in the above-described high temperature range are not caused by excessive addition of Al, addition of Mo, Cu, etc. contributing to solid solution / precipitation strengthening, B, Nb , Sn, Mg, Ca, Ga was found to be remarkably improved by adding a small amount and adjusting the amount of addition. That is, in the ferritic stainless steel, a new property that the 0.2% proof stress near 750 ° C. and the reduction of the 0.2% proof strength near 800 ° C. can be achieved by adding these trace elements. Knowledge was obtained. Although there are still many unclear points regarding the action of improving the high-temperature strength, the action mechanism described below is inferred based on experimental facts.

(C) The addition of a small amount of B contributes to the increase in yield strength and tensile strength at 750 to 800 ° C., and has the effect of significantly improving the 0.2% yield strength. The addition of a small amount of B suppresses the formation of cavities (nano-sized gaps) generated from the grain boundaries as a result of B segregating at the grain boundaries, delaying the grain boundary sliding, and dislocation density within the crystal grains. This has the effect of increasing the internal stress associated with the rise of. Moreover, the novel effect which the effect of these B becomes remarkable with Nb addition steel was discovered.

(D) It is well known that the addition of Nb is effective for increasing the strength in the temperature range up to 750 ° C. by solid solution strengthening. Precipitation of Nb starts at 750 to 800 ° C. by forming an intermetallic compound called a Laves phase (Fe ₂ Nb), and Nb and B co-segregate at the grain boundary to cause B of (c) above. The effect can be made obvious.

(E) Moreover, the effect of B which becomes remarkable with Nb addition steel mentioned above is superimposed by composite addition of Mg, Ca, and Ga.

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

(G) Further, in order to improve the oxidation resistance in the reformed gas environment containing hydrogen and sulfide components as described above, the content of Si, Al, Nb, and Mn is adjusted within a predetermined range, so that the temperature is improved. It is effective to promote the formation of an Al-based oxide film in a gas environment and to enhance the protection of the film. Furthermore, the addition of B, Nb, Sn, Mg, Ca, and Ga in ferritic stainless steel does not impair the oxidation resistance under the reformed gas environment. Rather, the addition of a small amount of Mg and Sn is an Al-based oxide film. The protective property is further increased and the effect of oxidation resistance is also exhibited. In this embodiment, the surface film before being exposed to a high-temperature reformed gas environment is a “passive film”, and the composition of the passive film exposed to a high-temperature reformed gas environment is changed by various reactions. This is distinguished from “Al-based oxide film”.

(H) The above-described reformed gas environment (carburizing / reducing / sulfiding environment) produces defects in the Al-based oxide film in ferritic stainless steel compared to the steam oxidation environment that does not contain air or hydrogen. easy. The reason why the reformed gas environment facilitates the generation of defects in the oxide film is not clear, but it is speculated that sulfides generated under the reformed gas containing sulfide components have some adverse effects on the oxide film. . If a defect occurs in the Al-based oxide film in the reformed gas environment, oxidation of Cr and Fe may proceed in the exposed base material. For such oxidation promotion in the reformed gas, Mg is dissolved in the Al-based oxide film, and Sn is segregated on the surface of the base material to delay the outward diffusion of Cr and Fe, so that the Al-based The protective property of the oxide film can be further increased. As a result, the oxidation resistance of the ferritic stainless steel can be improved.

(I) Furthermore, regarding the precipitation of the intermetallic compound σ phase at high temperatures (σ brittleness) and 475 ° C. brittleness, which were disadvantages of conventional Al and Si-containing stainless steels, in the component composition, Cr, Nb, Si, It has been found that adjusting the Al content is effective. σ brittleness and 475 ° C. brittleness are derived from the formation of intermetallic compounds mainly containing Cr and containing Si and Al, and the production sites are often grain boundaries. That is, in order to suppress the σ brittleness and the 475 ° C. brittleness, it can be said that it is effective to suppress the generation of the intermetallic compound itself and to reduce the generation site. As a result of further investigation by the present inventors, the formation of intermetallic compounds is suppressed by limiting the amount of Cr, and the structure is stabilized by suppressing generation sites by segregation to the crystal grain boundaries of Nb. As a result, it was found that σ brittleness and 475 ° C brittleness can be suppressed. Furthermore, since the production of intermetallic compounds containing Si and Al can be suppressed by limiting the Cr amount and adding Nb, the amount of Si and Al contributing to the oxidation resistance described in the above (h) can be ensured. It is also possible to achieve both chemical properties and tissue stability.

  As described above, in ferritic stainless steel, the combined addition of B, Nb, Sn, Mg, Ca, and Ga increases the high-temperature strength and the reformed gas important as durability in a carburizing / reducing / sulfiding environment. The new knowledge that it can combine the oxidation resistance of was obtained. Furthermore, in ferritic stainless steel, by optimizing the contents of Cr, Nb, Si, and Al, it is possible to suppress σ brittleness and 475 ° C brittleness by improving the structural stability, and also achieve both oxidation resistance. The new knowledge was also 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 fundamental in securing high-temperature strength in addition to corrosion resistance. In this embodiment, if it is less than 12.0%, the target high-temperature strength and oxidation resistance are not sufficiently ensured. Therefore, the lower limit of the Cr content is 12.0% or more. Preferably it is 13.0% or more. However, excessively containing Cr sometimes promotes the formation of σ phase (an intermetallic compound of Fe—Cr), which is an embrittlement phase, and promotes 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. Preferably it is 15.0% or less.

  C inhibits oxidation resistance by forming a solid solution or Cr carbide in the ferrite phase. For this reason, the upper limit of the C amount is set to 0.020% or less. Preferably it is 0.015% or less. However, excessive reduction of the amount of C leads to an increase in refining costs, so the lower limit is preferably made 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 in securing oxidation resistance. Si slightly dissolves in the Al-based oxide film, and also concentrates directly under the Al-based oxide film / steel interface to improve the oxidation resistance under the reformed 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, excessive inclusion of Si may hinder the toughness and workability of steel and the formation of an Al-based oxide film, so the upper limit is made 2.50% or less. From the viewpoint of oxidation resistance and basic characteristics, 1.70% or less is preferable.

  Mn can form a solid solution in or directly under the Al-based oxide film together with Si in the reformed gas environment, thereby increasing the protection of the film and contributing to the improvement of oxidation resistance. In order to obtain these effects, the lower limit is preferably set to 0.10% or more. More preferably, it is 0.20% or more. On the other hand, when Mn is excessively contained, the corrosion resistance of steel and the formation of Ti and Al-based oxide films are inhibited, so the upper limit is made 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 contributes to improving oxidation resistance by forming an Al-based oxide film in the reformed gas in the present embodiment. In this embodiment, in order to obtain good oxidation resistance, the content is preferably 1.00% or more, more preferably 1.50% or more. However, excessively containing Al causes a decrease in the toughness and weldability of the steel and hinders productivity, so that there is a problem in economic efficiency along with an increase in alloy cost. Therefore, the upper limit of the Al amount is 2.50% or less from the viewpoint of basic characteristics and economy. More preferably, it is 2.30% or less.

  P is an element that hinders manufacturability and weldability, and the lower the content, the better. Therefore, the upper limit is made 0.050% or less. However, since excessive reduction of P leads to an increase in refining costs, the lower limit is preferably set to 0.003%. From the viewpoint of manufacturability and weldability, the preferred range is 0.005 to 0.040%, more preferably 0.010 to 0.030%.

  S is an impurity element inevitably contained in steel, and lowers the high temperature strength and oxidation resistance. In particular, the segregation of S grain boundaries, the presence of Mn inclusions and solute S has the effect of reducing the high temperature strength and oxidation resistance. Therefore, the lower the amount of S, the better. Therefore, the upper limit is made 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 set to 0.0001% or more. From the viewpoint of manufacturability and oxidation resistance, the preferred range is 0.0001 to 0.0020%, more preferably 0.0002 to 0.0010%.

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

  Nb is a stabilizing element that fixes C and N, and can improve oxidation resistance and corrosion resistance through high purity of steel by this action. Moreover, in this embodiment, it is an element which acts effectively also at the improvement of high temperature strength by the effect of the co-segregation in the grain boundary with B. Further, intermetallic compounds that cause σ brittleness and 475 ° C. brittleness mainly precipitate with the grain boundary as a production site, but this production site is reduced by the segregation of Nb to the crystal grain boundary. 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 Nb content is 0.001% or more, preferably 0.15% or more. On the other hand, excessively containing Nb leads to an increase in alloy costs and obstructs manufacturability, so the upper limit of the Nb amount is 1.00% or less. Preferably it is 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). Furthermore, these elements are elements that contribute to the improvement of oxidation resistance by promoting the formation of an Al-based oxide film. Therefore, in addition to the above component composition, one or more of B, Sn, Ga, Mg, and Ca are contained.
B segregates at the grain boundary due to segregation at the grain boundary, and can increase the internal stress accompanying the increase in the dislocation density in the crystal grains and improve the 0.2% yield strength. Sn, Ga, Mg, and Ca have an action of concentrating near the surface and promoting selective oxidation of Al. In order to obtain such effects, it is preferable that the lower limit of each content 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, containing these elements excessively increases the refining cost of the steel and decreases the productivity and the corrosion resistance of the steel. For this reason, 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 limits of B, Ga, and Mg are all 0.0200% or less.

Furthermore, the component composition of this embodiment shall satisfy the following formula (1).
10 (B + Ga) + Sn + Mg + Ca> 0.020% Formula (1)
In addition, each element symbol in Formula (1) shows content (mass%) of each element in steel.

  From the viewpoint of improving the high temperature strength and oxidation resistance, the formula (1) is preferably 0.025% or more, more preferably 0.035% or more. In addition, although the upper limit of Formula (1) is not specifically prescribed | regulated by the upper limit of B, Sn, Ga, Mg, Ca, it is preferable to set it as 0.2% from a viewpoint of high temperature strength and manufacturability.

Next, the density | concentration (mass%) of the segregation element in a crystal grain boundary is demonstrated.
As described in the above findings (c) and (d), in this embodiment, the high temperature strength can be improved by co-segregating Nb and B at the grain boundaries. Similarly, Sn segregates to the grain boundary, thereby suppressing the grain boundary sliding and improving 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 inhibiting the manufacturability in addition to the grain boundary becoming a 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 the Sn concentration at the crystal grain boundary can be adjusted by performing a heat treatment under predetermined conditions after the finish annealing. Details will be described later.

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

The ferritic stainless steel according to the present embodiment is composed of Fe and impurities other than the above-described elements (remainder), but can also contain any element 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 “impurities” in the present embodiment are components that are mixed due to various factors in the manufacturing process including raw materials such as ores and scraps when industrially manufacturing steel, and are inevitably mixed. Including ingredients.

  The ferritic stainless steel according to the present embodiment, if necessary, Ni: 1.0% or less, Cu: 1.0% or less, Mo: 1.0% or less, Sb: 0.5% or less, W: 1 0.0% 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, and REM: 0.1% or less may be contained.

  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 excessively contained, it will lead to an increase in alloy costs and obstruct manufacturability, so the upper limit of Ni, Cu, W is 1.0% or less. Since Mo is an element effective for suppressing high-temperature deformation due to a decrease in the thermal expansion coefficient, it is preferable to contain Mo 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, the upper limit is preferably set 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 element of Ni, Cu, Mo, W, Co, and V is 0.10% or more. The minimum of preferable content of Sb and Ti shall be 0.01% or more.

  Zr, La, Y, Hf, and REM are elements that have been conventionally effective for improving hot workability and steel cleanliness and improving oxidation resistance, and may be contained as necessary. However, from the technical idea of the present invention and the reduction of alloy costs, it does not depend on the effect of addition of these elements. 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%. A more preferable lower limit of Zr is 0.01%, and a preferable lower limit of La, Y, Hf, and REM is 0.001%. Here, REM is an element belonging to atomic numbers 58 to 71 excluding La and Y and Sc (scandium), and is, for example, Ce, Pr, Nd, or the like. Moreover, REM as used in this embodiment is comprised by 1 or more types selected from the element which belongs to atomic number 58-71, and Sc, and REM amount is these total amounts.

  The ferritic stainless steel according to the present embodiment is composed of Fe and impurities (including inevitable impurities) other than the elements described above. However, in addition to the elements described above, the effects of the present invention are not impaired. It can be made to contain. It is preferable to reduce as much as possible Bi, Se, etc. as well as the aforementioned 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 may include one or more of Bi ≦ 100 ppm and Se ≦ 100 ppm as necessary.

  The metal structure of the ferritic stainless steel of the present embodiment is a ferrite single phase structure. This means that an austenite phase and a martensite structure are not included. When an austenite phase or a martensite structure is included, in addition to an increase in raw material cost, a yield reduction such as an ear crack is likely to occur at the time of manufacture, so the metal structure is a ferrite single phase structure. Although precipitates such as carbonitrides are present in the steel, they do not take account of the effects of the present invention, so these are not considered, and the above describes the structure of the main phase.

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

<Manufacturing method>
Next, although it is the manufacturing method of the ferritic stainless steel of this embodiment mentioned above, it can manufacture by combining hot processing, cold processing, and each heat processing (annealing), and it pickles suitably as needed. Or descaling. As an example of a manufacturing method, the manufacturing method which has each process of steelmaking-hot rolling-annealing-cold rolling-annealing (finish annealing) can be employ | adopted, and it is preferable that the heat processing after hot rolling shall be over 700 degreeC.
For example, the heat treatment after hot rolling may be performed at a temperature exceeding 700 ° C., cold rolling after descaling, and then finish annealing and descaling exceeding 700 ° C. may be performed to form a cold-rolled annealed plate. Further, the rolling rate of cold rolling is not particularly specified, but is preferably in the range of 30 to 80%.
In addition, when ferritic stainless steel is applied to gas piping applications, it includes welded rods manufactured from steel plates, but the piping is not limited to welded rods and even seamless rods manufactured by hot working. Good.

  The temperature of heat treatment after hot working (hot rolled sheet annealing) and finish annealing after cold rolling exceeds 700 ° C. is a solution of Nb and Sn effective for increasing the high-temperature strength by recrystallizing the steel. This is to make it happen. However, an excessive increase in the processing temperature of the heat treatment after hot working or the finish annealing after cold rolling causes the crystal grain size to become coarse, leading to deterioration of the surface quality such as rough skin. Accordingly, the upper limit is preferably set to 1050 ° C. or less for each processing temperature of heat treatment after hot working and finish annealing after cold rolling.

  After finish annealing, it is desirable to further perform heat treatment (heat treatment after finish annealing) in order to increase the high-temperature strength by adjusting the concentration of Nb and Sn at the grain boundaries within the above-mentioned range. Specifically, after finish annealing, a heat treatment may be performed by reheating to 600 to 700 ° C. and holding for more than 1 minute and 3 hours or less. In addition, as long as it is in the range of 600 to 700 ° C., “holding” in the present embodiment may be a state in which a constant temperature is maintained, or may be fluctuated within the range. That is, if the time for which the steel sheet exists between 600 and 700 ° C. is more than 1 minute and 3 hours or less, the presence or absence of fluctuations in the steel sheet temperature during that time does not matter. Moreover, the temperature of the steel plate as used in this embodiment refers to the temperature of the steel plate surface.

When the heat treatment temperature after finish annealing exceeds 700 ° C. or the holding time of the heat treatment after finish annealing exceeds 3 hours, a coarse Laves phase (Fe ₂ Nb) having a long side exceeding 1 μm is likely to precipitate, It may cause a decrease. 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, the segregation of grain boundaries of Nb and Sn does not proceed and sufficient high-temperature strength cannot be obtained. In some cases, sigma phase precipitation and 475 ° C. brittleness may occur due to a decrease in grain boundary strength, structural instability. Therefore, the lower limit of the heat treatment temperature after finish annealing is 600 ° C. or more, and the holding time is more than 1 minute.

  Note that the control of the grain boundary concentration of Nb and Sn can be adjusted in the final annealing step without performing the heat treatment after the final annealing. In that case, when finishing annealing is cooled, that is, when cooling is performed after heating to over 700 ° C., control is performed so that the passing time (required cooling time) in the temperature range of 600 to 700 ° C. exceeds 1 minute. Specifically, the passage 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 in the air, LNG fuel atmosphere, non-oxidizing atmosphere using hydrogen, nitrogen, argon, etc. (bright annealing) It is preferable that

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

According to the present 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 sulfur component (carburizing / reducing / sulfiding environment). Can be provided.
Furthermore, according to the present embodiment, by optimizing the component composition, it is possible to enjoy excellent structure stability that can suppress σ phase precipitation and 475 ° C. brittleness.
Therefore, it is suitable for fuel cell members such as city gas, methane, natural gas, propane, kerosene, gasoline and other fuels such as fuel reformers and heat exchangers used when reforming hydrocarbon fuels to hydrogen, In particular, it is suitable for a high temperature member of a solid oxide fuel cell (SOFC) or a polymer electrolyte fuel cell (PEFC) in which the operating temperature is high. Furthermore, it can be suitably used for all members that are in contact with the reformed gas and used in a high-temperature environment, such as fuel cell peripheral members such as a burner and a combustor that stores the burner.

Next, examples of the present invention will be described. The conditions in the examples are one example of conditions used to confirm the feasibility and effects of the present invention, and the present invention was used in the following examples. It is not limited to the conditions. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.
In addition, the underline in the table | surface shown below shows what has remove | deviated from the scope of the present invention.

Various ferritic stainless steels having the components shown in Table 1 are melted (steel A to O) and hot rolled into a hot rolled sheet having a thickness of 4 mm, followed by heat treatment (hot rolled sheet annealing), pickling, and cold rolling. A cold-rolled steel sheet having a thickness of 0.8 mm was manufactured. In addition, hot-rolled sheet annealing was performed in the range of 900-1000 degreeC.
After cold rolling, finish annealing was performed at 900 to 1000 ° C. Some steel plates (steel 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.
About the obtained cold-rolled steel plate (No. 1-19), the measurement of the grain boundary concentration and various characteristics were evaluated.

[Measurement of grain boundary concentration]
The Nb concentration and Sn concentration in the crystal grain system were measured by Auger electron spectroscopy.
First, a test piece with a notch (0.8 t × 4 w × 20 L (mm)) was collected from the cold rolled steel sheet. Next, after the notched specimen was cooled with liquid nitrogen in a vacuum (vacuum degree: 10 ⁻⁶ MPa), the notched portion was broken in situ so that the fracture surface was not exposed to the atmosphere, and the grain boundaries were exposed. I let you. The exposed grain boundary was subjected to AES analysis, an Auger electron spectrum was measured in an energy range of 0 to 1000 eV, and Nb element and Sn element were identified (qualitative analysis). Furthermore, using the obtained peak intensity ratio (relative sensitivity coefficient method), the Nb amount and the Sn amount were quantitatively analyzed, and the Nb concentration and the Sn concentration (both mass%) segregated at the crystal grain boundaries were obtained. The Nb concentration and Sn concentration obtained in Table 2 are shown. In the table, “<3.0” and “<1.0” 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 the cold-rolled steel sheet and subjected to an oxidation test. The atmosphere of the oxidation test is assumed to be a reformed gas using city gas as a fuel, and 28 volume% H ₂ O—10% volume% CO—8 volume% CO ₂ —0.01% H ₂ S-bal. Was an atmosphere of H _2. In this atmosphere, the oxidation test piece was heated to 650 ° C., held for 1000 hours, then cooled to room temperature, and the oxidation increase ΔW (mg / cm ² ) was measured.
The evaluation of oxidation resistance was as follows.
A: Weight increase ΔW is less than 0.2 mg / cm ² .
A: Weight increase ΔW is 0.2 to 0.3 mg / cm ² .
X: Weight increase ΔW exceeds 0.3 mg / cm ² .
In addition, as for oxidation resistance, the case of "(double-circle)" and "(circle)" was set as the pass.

[High temperature strength]
A plate-shaped high-temperature tensile test piece (plate thickness: 0.8 mm, parallel part width: 10.5 mm, parallel part length: 35 mm) with the rolling direction as the longitudinal direction is produced from the cold-rolled steel sheet, and is 750 ° C. and 800 At each temperature, the strain rate is 0.3% / min up to 0.2% proof stress, and then a high temperature tensile test is performed at 3 mm / min, and 0.2% proof stress at each temperature (750 ° C proof strength, 800 ° C proof strength). Was measured (according to JIS G 0567).
Evaluation of high-temperature strength is evaluated as a pass (“◯”) when the 750 ° C. yield strength is over 120 MPa and the 800 ° C. yield strength is over 40 MPa, and is rejected (“×”) when neither is satisfied. evaluated. When the 750 ° C. yield strength exceeded 150 MPa and the 800 ° C. yield strength exceeded 60 MPa, the high-temperature strength was evaluated as being particularly excellent (indicated by “◎” in Table 2).

[Structure stability (σ brittleness / 475 ° C brittleness)]
Two samples were taken from the cold-rolled steel plate so that the center on the cross section perpendicular to the plate surface (plate thickness center: around t / 2) could be observed, one of which was heat-treated at 500 ° C. for 1000 hours (500 ° C. Heat treatment), and the other was heat treated at 650 ° C. for 1000 hours (heat treatment at 600 ° C.). The atmosphere for these heat treatments was in the air. Next, after each heat-treated sample was buried in a resin and polished, the Vickers hardness Hv after 500 ° C. heat treatment was _{500 ° C.} , and the Vickers hardness after heat treatment at _{650 ° C.} was Hv _{650 ° C. according} to JIS Z 2244. Measured at .8N, the amount of increase in hardness ΔHv _{500 ° C.} and ΔHv _{650 ° C.} from pre-heat-treatment Vickers hardness measured in advance before heat treatment was calculated.
The evaluation of the structural stability (σ brittleness / 475 ° C brittleness) was evaluated as a pass (“◯”) when both ΔHv _{500 ° C} and ΔHv _{650 ° C} were less than 20, and when either was 20 or more It was rejected ("x") because the hardness increase after heat treatment was large and the structure was unstable.

The obtained evaluation results (measurement of grain boundary concentration, various characteristics) are as shown in Table 2.
No. 1-13 satisfy | fill the component prescribed | regulated by this invention, and evaluation of all the characteristics became "(circle)" or "(double-circle)". Among these, No. Nos. 3, 7, 9, and 13 are cases where the preferred grain boundary concentration of the present invention was satisfied, and a significant high-temperature strength improvement effect was exhibited. The evaluation was “◎”. No. 2, 3, 8, 9, and 11 are cases where the preferred component composition of the present invention (particularly Cr, Si, Al) is satisfied, and a significant effect of improving oxidation resistance is exhibited. “◎”.

  Steel No. Nos. 14 to 19 deviate from the steel components defined in the present invention, and the respective characteristics targeted by the present invention could not be achieved, and either evaluation was “x”.

Claims (12)

In 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: 2.50% or less,
N: 0.030% or less,
Nb: 0.001 to 1.00%,
Ni: 0 to 1.0%,
Cu: 0 to 1.0%
Mo: 0 to 1.0%,
Sb: 0 to 0.5%,
W: 0 to 1.0%
Co: 0 to 0.5%,
V: 0 to 0.5%
Ti: 0 to 0.5%,
Zr: 0 to 0.5%,
La: 0 to 0.1%
Y: 0 to 0.1%
Hf: 0 to 0.1%,
REM: 0 to 0.1%
Including,
B: 0.0200% or less,
Sn: 0.20% or less,
Ga: 0.0200% or less,
Mg: 0.0200% or less,
Ca: Ferritic stainless steel containing one or more of 0.0100% or less, satisfying the following formula (1), and the balance consisting of Fe and impurities.
10 (B + Ga) + Sn + Mg + Ca> 0.020 (1)
In addition, each element symbol in Formula (1) shows content (mass%) of each element in steel.   2. The ferritic stainless steel according to claim 1, wherein the B is 0.0002% or more in mass%.   The ferritic stainless steel according to claim 1 or 2, wherein the Nb concentration at a grain boundary is in a range of 3.0 to 10% by mass%.   The Sn is 0.005% or more in mass%, and the Sn concentration at the crystal grain boundary is 1.0 to 5.0%. Ferritic stainless steel.   5. The ferrite according to claim 1, wherein the Si content is 0.5% or more, the Al content is 1% or more, and the Nb content is 0.15% or more. Stainless steel.   In 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: 0.001-0.1% or less, Y: 0.001-0.1%, Hf: 0.001-0.1%, REM: 0.001-0.1% The ferritic stainless steel according to any one of claims 1 to 5, wherein one or more of these are contained.   The ferritic stainless steel according to any one of claims 1 to 6, which is applied to a fuel reformer, a heat exchanger, or a fuel cell member.   The ferritic stainless steel according to any one of claims 1 to 7, wherein the ferritic stainless steel is applied to a combustor or a burner member.   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 a temperature exceeding 700 ° C, then cold-worked, and finish annealing at a temperature exceeding 700 ° C. The method for producing a ferritic stainless steel according to any one of claims 1 to 6, wherein:   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.   10. The method for producing a ferritic stainless steel according to claim 9, wherein, in the finish annealing, when heating to 700 ° C. and cooling, a holding time in a temperature range of 600 to 700 ° C. is set to more than 1 minute. .   The member for fuel cells using the ferritic stainless steel as described in any one of Claims 1-6.