JPH06293941A - Metallic material for solid electrolyte type fuel cell - Google Patents

Abstract

PURPOSE:To produce an austenitic stainless steel for solid electrolyte type fuel cell, having superior oxidation resistance and electric conductivity even in a high temp. oxidizing atmosphere. CONSTITUTION:The austenitic stainless steel has a composition consisting of <=0.15% C, 0.375-4.55% Si, <=3.0% Mn, 15-30% Cr, 20-60% Ni, 2.5-6.0% Al, and the balance Fe with inevitable impurities or further containing <=0.5%, in total, of one or more kinds among Y and rare earth elements and satisfying the inequality {Al(%)-1}/4<=Si(%)<=0.8Al(%)-0.25. If necessary, the content of S as an inevitable impurity is regulated to <=0.002% and also the total content of S and O is regulated to <=0.005%.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an austenitic stainless steel for solid oxide fuel cells, which exhibits excellent oxidation resistance and electric conductivity even in a high temperature oxidizing atmosphere of 600 ° C. or higher.

[0002]

2. Description of the Related Art In recent years, from the viewpoint of environmental problems including depletion of petroleum resources and air pollution, which are expected in the future, there has been proposed a fuel cell that can use reformed coal gas and has high energy conversion efficiency. It has been in the spotlight as a next-generation power supply source, and "Research and development of fuel cell power generation technology" has been promoted as part of the Moonlight Project since 1981.

Fuel cells are classified into "phosphoric acid type", "molten carbonate type", "solid electrolyte type", etc. according to the type of electrolyte that generates electromotive force, and their operating temperatures are different. The basic technical development of the phosphoric acid fuel cell, which has an operating temperature of about 200 ° C., has been completed and is nearing practical use.

Further, the solid oxide fuel cell has some unsolved problems because the operating temperature reaches 1000 ° C., but a) its energy efficiency is higher than that of other power generation systems, b ) Various fuel gases such as coal reformed gas can be used, c) NO _X generation is small and environmental impact is small, d) Structure is compact because liquid and melt are not used, etc. It is considered to be the most promising power generation system for the next generation because it has many

However, in these fuel cells, the reduction of manufacturing cost, the prolongation of life, the solution of problems of reliability and maintainability are extremely important issues for practical use in the future. There is a strong demand for the development of inexpensive and high-performance materials to satisfy the above requirements.

In particular, the solid oxide fuel cell described above can be said to be developed including the cell structure, but the "plate type" among them is excellent in mass productivity and can obtain high output density. Expectations are high, and the development of materials applicable to this is under pressure.

That is, a "plate type" solid oxide fuel cell is composed of a flat power generation membrane composed of an electrolyte, a thin film of a fuel electrode and an air electrode and an interconnector, and between the power generation membrane and the interconnector. The corrugated support layer has a structure that forms a gas flow path, but the components such as the interconnector and corrugated support layer that are being studied in the laboratory are resistant to oxidation and electrical properties at high temperatures. Conventionally, conductive ceramics have been used because of their conductivity and compatibility with the difference in thermal expansion from the electrolyte. However, such a material has a problem that it cannot cope with an increase in the size of a system because it has poor workability, its strength at high temperature is smaller than that of a metal material, and it is expensive. In addition, due to the large area, there is a problem that it is difficult to overcome the reliability for stable operation for a long time.
Therefore, there has been an urgent need to develop inexpensive and high-performance metal materials.

By the way, "stainless steel" comes to mind when it comes to a relatively inexpensive metal material that is excellent in strength and durability at high temperatures. However, the above-mentioned interconnector and corrugated support of the solid oxide fuel cell are considered. When stainless steel is applied as a component such as a layer, it is different from conventional stainless steels in terms of ensuring oxidation resistance in an extremely harsh oxidizing environment of 1000 ° C and good electrical conductivity at high temperatures. It is necessary to give the image quality.

It is known, however, that the oxidation resistance is improved to some extent by adding an appropriate amount of an oxidation resistance improving element such as Al or Si to steel. For example, in Fe-based alloys, in the case of ferrite type,
If 2.0% or more of Al is added after adding Cr (hereinafter,% representing the component ratio is% by weight), a uniform Al-based oxide scale will be stably generated on the surface, and oxidation resistance will be improved. Since it is a material with excellent properties, it is used as a support member for heating wires and automobile exhaust gas reforming catalysts. Further, in an austenitic Fe-based alloy, for example, Japanese Patent Publication No.
As disclosed in Japanese Patent Publication No. 8, by adding 15% or more of Cr and then 4.0% or more of Al, a uniform Al-based oxide scale is stably formed on the surface. And becomes a material having excellent oxidation resistance.

However, since the oxide formed on the surface of a metal material which is excellent in oxidation resistance generally has low electric conductivity, such a heat resistant material is directly applied to a battery interconnector, a corrugated support layer or the like. Is considered difficult,
When applying it, it is necessary to devise to secure electric conductivity at high temperature. In particular, α-Al ₂ O _{3, which} is considered to be the most desirable for improving the oxidation resistance of stainless steel under conditions exceeding 1000 ° C, is an insulating oxide, and has a conductivity of about 10 ^-6 S / cm at around 1000 ° C. Only show.

However, since the stainless steel containing the oxidation resistance improving element, especially the austenitic stainless steel, has good workability and excellent high temperature strength and oxidation resistance at high temperature, its basic It was considered to be extremely promising as a material for a constituent member of a solid oxide fuel cell if it could secure electric conductivity in a high temperature oxidizing atmosphere without deteriorating the characteristic features.

Therefore, the object of the present invention is to improve the workability and mechanical strength at high temperature of the austenitic stainless steel, which is less expensive than the ceramic material. An object of the present invention is to provide an austenitic stainless steel for a solid oxide fuel cell, which has characteristics and advantages as they are, and which has excellent oxidation resistance and electrical conductivity even in a high temperature oxidizing atmosphere.

[0013]

Means for Solving the Problems As a result of intensive studies to achieve the above object, the present inventors were able to obtain the following knowledge. That is, when Cr, Al, Si, etc. are added to the austenitic stainless steel as alloying elements for improving the oxidation resistance, the oxidation rate is 900 ° C or higher from the viewpoint of the oxidation rate (the oxidation rate decreases). Although it can be used to some extent, the electrical conductivity at high temperature decreases as the oxide scale grows.

In particular, in the case of austenitic stainless steel, it is most effective to add Al in an amount of 4.0% or more from the viewpoint of securing oxidation resistance when applied at temperatures above 1000 ° C.
As a result of the addition, an insulative Al-based oxide scale is formed on the steel surface, resulting in a marked decrease in electrical conductivity. Therefore, when such a high Al-added austenitic stainless steel is applied to a member such as an interconnector of a solid oxide fuel cell,
Since the electrical resistance becomes high, the electrical characteristics may drop sharply.

However, if proper amounts of Al and Si are not added, a spinel type oxide mainly composed of Fe-Cr is generated in a high temperature oxidizing atmosphere of about 1000 ° C, and the oxidation resistance of the base material is rapidly deteriorated. .

On the other hand, in the Cr-containing steel, by adjusting the composition, the surface of "Cr ₂ O ₃ having a relatively good electric conductivity at 1000 ° C. of about 2 × 10 ^-3 S / cm" in a high temperature oxidizing atmosphere was added. A scale can be formed, but in this case the scale is
The growth rate of silicon is high, and abnormal oxidation occurs due to the formation of Fe-Cr spinel oxide, which accelerates the deterioration of the material. Therefore, in the case of the Cr-based oxide scale, not only the scale becomes thick and the electric conductivity is lowered, but also the electric conduction becomes very unstable due to the scale peeling during heating.

However, depending on the compositional adjustment of steel such as Si, the surface oxide scale produced in a high temperature oxidizing atmosphere is mainly composed of an Al-based oxide and contains a Cr-based oxide. It has been found that an austenitic stainless steel excellent in both oxidation resistance and electrical conductivity can be realized.

That is, the following matters were clarified. a) Cr 15% or more, Ni 20% or more, and Al 4.0%
The stainless steel containing the above does not deteriorate the excellent oxidation resistance even in the temperature range from 850 ° C. to over 950 ° C., although it produces an Al-based oxide scale, but it has a problem in electrical conductivity. b) However, when Al is adjusted to 2.5% or more in the above alloy-based austenitic stainless steel and the Si content is adjusted to "a specific region related to Al content", in a high temperature oxidizing atmosphere. In the oxide-based scale containing Al-based oxide mainly containing Cr-based oxide,
Although such steels are inferior in oxidation resistance to heat-resistant alloys that produce Al-based oxide scales, they are superior in oxidation resistance to Fe-Cr-Ni-based alloys and have a higher thermal resistance than alloys that produce Al-based oxide scales. Shows electrical conductivity. c) Moreover, when 0.5% or less in total of Y, rare earth elements or Ca is further added to the above stainless steel, the deterioration life of the material can be extended without impairing the electrical conductivity. d) Further, when the contents of the impurity elements S and O in these stainless steels are reduced to a specific region, abnormal oxidation is further suppressed and excellent oxidation resistance is obtained for a long time. It becomes a material to maintain.

The present invention has been completed after further studies based on the above findings and the like. "The austenitic stainless steel is C: 0.15% or less, Si: 0.375.
~ 4.55%, Mn: 3.0% or less, Cr: 15-30%, N
i: 20 to 60%, Al: 2.5 to 6.0%, or
Alternatively, one or more of Y, rare earth elements or Ca: including 0.5% or less in total, and having the formula {Al (%) -1} / 4 ≤ Si (%) ≤ 0.8 Al (%)-0.25 The balance is satisfied and the balance consists of Fe and unavoidable impurities, and the content of S, which is an unavoidable impurity, is 0.0 if necessary.
By making the composition such that the total content of S and O is 0.005% or less at 02% or less, excellent oxidation resistance in a high temperature oxidizing atmosphere, which can be sufficiently satisfied even for a solid oxide fuel cell, The point is that it has electrical conductivity. "

In the present invention, the reason why the component ratio of steel is numerically limited as described above will be explained below.

[Action]

C: Although the C has an effect of ensuring the mechanical strength of the steel, deteriorates the workability by forming a Cr ₂₃ C ₆ type carbide in or HAZ during use at high temperature, oxidation by Cr It is an element that not only significantly reduces the effect of improving the chemical conversion property but also facilitates scale peeling, so the content exceeding 0.15% must be avoided. If the mechanical strength is important, it may be contained up to the upper limit, but C
It can be said that the lower the content, the better. However, it can be said that about 0.0001% is actually the lower limit because of the problem of steelmaking costs.

Si: Si is an important component in the steel of the present invention, and its content needs to be adjusted within the shaded region in FIG. That is, when the Si content is less than 0.375% or less than the value indicated by the straight line “Si (%) = {Al (%) −1} / 4”, it is uniform in the high temperature oxidizing atmosphere.
Since only Al-based oxide scale is produced, electrical conductivity becomes a problem. On the other hand, if the Si content exceeds 4.55%,
(%) = 0.8Al (%) − 0.25 ”is exceeded, a spinel type oxide mainly composed of Fe—Cr is formed and the oxidation resistance sharply decreases. Therefore, the Si content is 0.375-4.
The value was determined to be 55% and to satisfy the formula {Al (%)-1} / 4 ≤ Si (%) ≤ 0.8 Al (%)-0.25.

Mn: Mn is an element that is inevitably accompanied in steel and is an effective component for stabilizing the austenite phase.
It may be positively added for the purpose of ensuring strength at high temperatures. However, if the content exceeds 3.0%, the oxidation resistance deteriorates rapidly, so the Mn content was defined as 3.0% or less, but it is desirable to adjust it in the range of 0.01 to 3.0%.

Cr: Cr is a basic element necessary for obtaining oxidation resistance at high temperature together with Al. That is, it is heated to a temperature of over 600 ° C., “the electrical conductivity is not significantly impaired.
In order to generate an Al-based and Cr-based oxide scale "and maintain the oxidation resistance, a Cr content of 15% or more is necessary.
If Cr is contained in an amount of more than 0%, not only the effect of improving the oxidation resistance cannot be obtained, but also the formability and workability are adversely affected. Therefore, the Cr content is defined as 15 to 30%. It was

Ni: Ni is an important element not only for providing the basic properties of austenitic steel but also for forming a composite oxide scale containing a Cr-based oxide in an Al-based oxide. However, if the content is less than 20%, the austenite phase becomes unstable and only a single scale of Cr type or Al type is produced. On the other hand, the steel containing more than 60% is difficult to practically use in terms of cost, so the Ni content is 20 to 60%.
I decided.

Al: Al is an important basic element in the steel of the present invention, and it is necessary to contain Al in an amount of 2.5% or more in order to form a composite oxide scale mainly containing an Al oxide and containing a Cr oxide. is there. However, if 6.0% is included, Si, Ni
Regardless of the amount, only the Al-based oxide scale will be uniformly formed, and the toughness decrease at room temperature will be extremely remarkable, so the Al content was set to 2.5 to 6.0%.

Y, rare earth element (REM) and Ca: these components have the same effect of improving the oxidation resistance of the steel,
Further, since it also has an effect of fixing S in the steel and improving hot workability, one or more kinds are added if necessary, but if it is contained in excess, coarse oxides are formed. On the contrary, since the oxidation resistance deteriorates, the upper limit of the total content is the total amount.
Although it was set to 0.5%, it is desirable to adjust the total amount to 0.1% or less.

S and O: S which is an unavoidable impurity element
Is preferably regulated to 0.002% or less. Because S
This is because when the content exceeds 0.002%, the formation of MnS becomes remarkable. In other words, if MnS is formed in the steel, it decomposes in a high temperature atmosphere and the free S is concentrated in the grain boundaries, which prevents the diffusion of Cr, Al, etc. to the surface of the base metal and forms a protective film. Repair becomes difficult.

Further, S existing at the grain boundary is combined with oxygen (O) to serve as a starting point of oxidation, which causes a thin profit factor of the oxide scale. Therefore, in order to prevent this adverse effect, a rare earth element or a rare earth element that forms a more stable sulfide at a higher temperature than Mn as necessary.
Immobilization of S is achieved by adding Ca or the like. However, in order to enhance these effects, it is preferable that the O concentration in the steel is low. This is because these additional elements are likely to form oxides, are consumed as oxides before they function as S-fixing elements in steel, and the effective amount decreases. Therefore, it is preferable that the "S (%) + O (%)" value in the steel is low, but in order to sufficiently suppress the above-mentioned adverse effects, this value should be 0.005% or less.

Other typical unavoidable impurity elements in the steel of the present invention include P, Cu and N. The contents of these elements will be described below. P: P as an impurity is preferably regulated to 0.03% or less. Cu: Cu may accompany the steel as an impurity from the Ni source, but up to about 1.5% is allowed. N: N combines with Cr and Al in steel to form a nitride,
It is an element that reduces the high temperature oxidation resistance of Al, and is preferably regulated to 0.10% or less. For the purpose of reducing the adverse effects of C and N in the steel, if necessary, Cr or
A stabilizing element such as Ti, Nb, or Zr, which has a stronger affinity for C and N than Al, may be added. When adding this stabilizing element, it is necessary to keep the total amount to 2.0% or less due to the problem of material embrittlement.

Further, since Mo also has a function of ensuring strength at high temperatures and a function of improving corrosion resistance, it may be added if necessary, but even if it is added in excess of 10.0%, a further performance is required. The improvement effect cannot be obtained.

By the way, the austenitic stainless steel according to the present invention exhibits excellent oxidation resistance and electrical conductivity in a high temperature oxidizing atmosphere because the scale formed is mainly composed of Al-based oxide and Cr-based. This is because the oxide is included in an appropriate ratio. In other words, in a scale composed of Al-based oxides and Cr-based oxides, a) Electrical conductivity is increased by forming Cr-based oxides into insulating-based Al-based oxides. , B) There is a tendency that the oxidation resistance decreases due to the increase of Cr-based oxides, and the electrical conductivity rapidly decreases due to the increase of the scale thickness. Content is important, but when adjusted to the composition according to the steel of the present invention, it is 15 to 60 vol% with respect to Al-based oxide in a high temperature oxidizing atmosphere.
A scale containing an oxide mainly composed of Cr is formed at a ratio of, and excellent oxidation resistance and electrical conductivity are compatible with each other.

Next, the present invention will be described more specifically by way of examples.

[Examples] Steels having the compositions shown in Tables 1 and 2 were melted in a vacuum melting furnace, and then forged, hot-rolled and cold-rolled to obtain a plate material having a thickness of 2 mm.

[0033]

[Table 1]

[0034]

[Table 2]

Next, various test materials were cut out from each of the above steel plates, and the characteristics (oxidation resistance, conductivity) in a high temperature oxidizing atmosphere were investigated. First, each test material was subjected to a continuous oxidation test in the air at 1000 ° C., and after 200 hours, it was taken out and the kind of oxide formed on the surface was examined. The investigation on the type of oxide was carried out by the method of identifying the oxide formed on the surface by X-ray diffraction (target: Cu, incident angle α = 0.3 °). The results of this identification are shown in Table 3.

[0036]

[Table 3]

The results shown in Table 3 also show that the oxide produced in the steel of the present invention was "a main oxide (α-Al ₂ O ₃ )."
It can be confirmed that the oxide is a composite of Cr-based oxide (Cr ₂ O ₃ ). In contrast, comparative steels 25-27, 31 and
33 is an Al ₂ O ₃ scale, and comparative steels 23 to 24, 28 to 30 and 32 are all FeCr ₂ O ₄ , (FeCr ₂ O ₄ , Cr ₂ O ₃ , Al ₂ O ₃ )
It can be seen that a multi-layer scale is formed. Further, when the scale cross section of Comparative Steel 28 was observed and EPME analysis was performed, an internal oxide layer of Al ₂ O ₃ grew deep in the base metal,
It was confirmed that AlN was generated at the tip.

FIG. 2 shows the results of conducting the above continuous oxidation test on the austenitic stainless steel sheets containing various ratios of Si and Al, identifying the types of the produced oxides, and arranging them. Only when the contents of Si, Si and Al are adjusted to fall within the range specified in the present invention, the produced oxide is “Al-based oxide (α-Al ₂ O ₃ ), Cr-based oxide (Cr ₂ O ₃ ). It is understood that it becomes a compound thing.

By the way, FIG. 3 shows the steel of the present invention and the comparative steel.
It is the graph which compared the change with time of the oxidation increase of 23,24,27,30 in 1000 ℃ atmosphere, and arranged to evaluate the oxidation resistance {Oxidation increase is the sample and the peeling scale. Measure the weight including the change amount per unit area (mg / cm ² )
Are evaluated in}. Although the steel of the present invention is inferior to the comparative steel 27 (where the Al ₂ O ₃ scale is formed) from the time course of the increase in oxidation shown in FIG. 3, it is an existing Fe-Cr steel (comparative steel 24).
It can be seen that the oxidation resistance is superior to that of Fe-Cr-Ni steel (Comparative Steel 23). It was also confirmed that the ones containing Y, rare earth elements, and Ca did not show scale peeling and were more excellent in oxidation resistance.

On the other hand, FIG. 4 is a graph showing changes with time of the electric resistances of the inventive steels 5 and 14 and the comparative steels 27 and 30 in the atmosphere at 1000 ° C. In addition, the measurement of this "time-dependent change in electrical resistance" was performed in the thickness direction of the steel sheet, and the results were arranged in terms of area resistance (Ω · cm ² ).

From the results shown in FIG. 4, the steels 5 and 14 of the present invention maintain a low electric resistance of about 0.10 Ω · cm ² and have an extremely high electric conductivity, whereas the FeCr ₂ O ₄ scale -The electrical resistance of the steel 30
In the case of the l ₂ O ₃ scale forming steel 27, it can be confirmed that the electric resistance sharply increased to an extremely high value in a short time. The tendency of the electric resistance to change with time was the same in the other steels of the present invention. Further, it is an important required performance when applied to a solid oxide fuel cell member (interconnector, etc.) that it is excellent in electrical conductivity as well as oxidation resistance, and the steel of the present invention is a solid oxide fuel cell member. You can see that it is very good.

[0042]

[Summary of Effects] As described above, according to the present invention, a comparative example showing excellent oxidation resistance and electric conductivity even in a high temperature oxidizing atmosphere and exhibiting excellent performance as a constituent member of a solid oxide fuel cell. It is possible to provide austenitic stainless steel that is economically inexpensive, and it is possible to greatly contribute to the practical application of the solid oxide fuel cell, and it is possible to bring about an extremely excellent effect in industry.

[Brief description of drawings]

FIG. 1 is a graph showing Si and Al content ranges in the steel of the present invention.

FIG. 2 is a graph showing the results of identifying and arranging the types of oxides produced in the continuous oxidation test of austenitic stainless steel sheets containing Si and Al in various proportions.

FIG. 3 is a graph comparing the high temperature oxidation resistance of the steels produced in the examples.

FIG. 4 is a graph showing changes in electric resistance of steels produced in Examples in a high temperature oxidizing atmosphere.

Claims (4)

[Claims] 1. A weight ratio of C: 0.15% or less, Si:
0.375 to 4.55%, Mn: 3.0% or less, Cr: 15 to 30%,
Ni: 20 to 60%, Al: 2.5 to 6.0%, and the formula {Al (%) -1} / 4 ≤ Si (%) ≤ 0.8 Al (%)-0.25 is satisfied and the balance is Fe and An austenitic stainless steel for a solid oxide fuel cell, characterized by comprising unavoidable impurities. 2. A weight ratio of C: 0.15% or less, Si:
0.375 to 4.55%, Mn: 3.0% or less, Cr: 15 to 30%,
Ni: 20-60%, Al: 2.5-6.0% Y, one or more of rare earth elements or Ca: 0.5 in total
% Or less, and satisfies the formula {Al (%) -1} / 4 ≤ Si (%) ≤ 0.8 Al (%)-0.25, the balance being Fe and unavoidable impurities. Austenitic stainless steel for electrolyte fuel cells. 3. A weight ratio of C: 0.15% or less, Si:
0.375 to 4.55%, Mn: 3.0% or less, Cr: 15 to 30%,
Ni: 20 to 60%, Al: 2.5 to 6.0%, and the formula {Al (%) -1} / 4 ≤ Si (%) ≤ 0.8 Al (%)-0.25 is satisfied and the balance is Fe and It consists of unavoidable impurities, and the content of S, an unavoidable impurity, is 0.002% or less,
An austenitic stainless steel for a solid oxide fuel cell, characterized in that the total content of S and O is 0.005% or less. 4. A weight ratio of C: 0.15% or less, Si:
0.375 to 4.55%, Mn: 3.0% or less, Cr: 15 to 30%,
Ni: 20-60%, Al: 2.5-6.0% Y, one or more of rare earth elements or Ca: 0.5 in total
% Or less, and satisfies the formula {Al (%) -1} / 4 ≤ Si (%) ≤ 0.8 Al (%)-0.25 with the balance being Fe and unavoidable impurities, and S which is an unavoidable impurity. Content of 0.002% or less,
An austenitic stainless steel for a solid oxide fuel cell, characterized in that the total content of S and O is 0.005% or less.