JP2003187828A - Ferritic stainless steel for solid oxide type fuel cell member - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel material that can be used as a member for an inter-connector or the like, composing a solid oxide type fuel cell. <P>SOLUTION: The steel material contains by weight, ≤0.03% C, ≤2.0% Mn, ≤0.6 Ni, ≤0.03% N, 10.0 to 30.0% Cr, ≥1.5% in total of at least one kind of ≤2.0% Si and ≤6.0% Al, and Fe substantially as the remainder. The steel material preferably contains ≥0.8% of at least one kind of Si and Al, and ≥1.5% in total of them. Furthermore, the steel material may contains one or more kinds of 0.05 to 0.80% Nb, 0.05 to 0.50% Ti, 0.1 to 3.0% Mo, and 0.1 to 3.0% Cu, and/or 0.01 to 0.10% rare-earth element. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ferritic stainless steel suitable as a member such as an interconnector constituting a solid oxide fuel cell.

[0002]

2. Description of the Related Art Among fuel cells, solid oxide fuel cells have the highest operating temperature of about 1000 ° C., are easy to cogeneration, and have the highest power generation efficiency. Therefore, they have been developed as stationary generators. It has been advanced. Solid oxide fuel cells include vertical vertical stripe type, horizontal horizontal stripe type, and flat plate type, and each unit cell has a fuel electrode 1, a solid electrolyte 2, an air electrode 3, and an interconnector 4 combined as shown in FIG. The structure is as follows (refer to the text of the workshop "All about SOFC" held by the Fuel Cell Development Information Center in June 1999). Since the output of a single cell is small, for example, in a cylindrical vertical stripe type fuel cell, as shown in FIG. 2, a plurality of Ni felts 7 are interposed and combined.
Further, a required output is obtained by connecting a large number of single cells in series using the interconnector 4 according to the concept as shown in FIG. In the figure, 5 is a porous support tube, 6
Indicates a single cell composed of the fuel electrode 1, the solid electrolyte 2 and the air electrode 3, 11 indicates the flow of fuel, and 12 indicates the flow of air.

By the way, as described above, the solid oxide type fuel is used.
Since the operating temperature of the battery is around 1000 ° C,
From the viewpoint of the reliability of the
Is being used. That is, the fuel electrode material is N
i, Ni and ZrO₂Composite material, but also as cathode material
Is La with a part of La replaced by Sr._1-xSr_xMnO
₃The material of the system is used. Further interconnector
LaCrO as material₃The material of the system is used.

In recent years, the operating temperature of solid oxide fuel cells has been lowered due to the thinning of stabilized zirconia used as a solid electrolyte and the development of a low temperature solid electrolyte. There is a high possibility that metal materials can be used. Since the electrode material is required to have porosity, it is still advantageous to use a ceramic material, but a steel material is used as an interconnector material (separator material) that requires denseness, thermal conductivity, and electrical conductivity. There is a possibility that the material will be used. However, since it is used in combination with a solid oxide that is an electrolyte, and heating and cooling are frequently repeated while being exposed to high temperature steam, it is significantly different from the environment in which conventional heat resistant steel is used, A steel composition having sufficient durability in a fuel cell environment has not yet been clarified.

[0005]

The interconnector material (separator material) joined to the solid electrolyte has a function as an electrical interconnector for connecting the unit cells in series and a separator for separating the fuel and the air. Function is required. For this reason, it is required that the compactness, the thermal conductivity, and the electrical conductivity be excellent and the ionic conductivity be inferior. Further, it is required to be chemically stable to fuel, air and high temperature steam, to be hardly peeled off even if a scale is formed, to have a thermal expansion coefficient close to that of a solid electrolyte, and to be excellent in moldability. The present invention has been devised to solve such a problem, and an object thereof is to provide a steel material that can be applied as a member such as an interconnector that constitutes a solid oxide fuel cell.

[0006]

In order to achieve the object, the stainless steel for solid oxide fuel cell members of the present invention has a C content of 0.03% or less and a Mn content of 2.
0% or less, Ni: 0.6% or less, N: 0.03% or less,
Cr: 10.0 to 32.0% and Si:
2.0% or less or Al: 6.0% or less at least 1
1.5% or more of the total amount of seeds or more, and the balance is substantially F
It is characterized by consisting of e. Si and / or Al
0.8% or more for at least one of the above, and a total amount of 1.5
% Or more is preferable. Furthermore, in mass%, Nb: 0.05 to 0.80%, Ti: 0.05 to
0.50%, Mo: 0.1 to 3.0%, Cu: 0.1
3.0% of one or more kinds and / or rare earth elements: 0.
It is also possible to include 01 to 0.10%.

[0007]

The present inventors have found that the members such as the interconnector of the solid oxide fuel cell are repeatedly heated and cooled under the condition that
Since it is used in an environment exposed to high-temperature steam, we searched for a steel-based material that has a small difference in the coefficient of thermal expansion from solid electrolytes, especially yttria-stabilized zirconia, and that has excellent high-temperature oxidation resistance and scale debonding resistance. . From the viewpoint of corrosion resistance at high temperatures, we first narrowed down to stainless steel. Austenitic stainless steel has high high-temperature strength, but has a larger difference in thermal expansion coefficient from yttria-stabilized zirconia than ferritic stainless steel, and because the steel itself is inferior in scale peeling resistance, it is suitable for solid oxide fuel cells. Not suitable for parts.

In order to secure the oxidation resistance at around 800 ° C., a ferritic stainless steel having a Cr content of 10 mass% or more is sufficient. However, particularly in the fuel electrode, about 30 to 80% of water vapor is added to the fuel such as methane, so that more severe steam oxidation than in a normal atmosphere occurs. In order to secure the steam oxidation resistance in such an atmosphere, it is necessary to increase the Cr content to about 20% or more, or to add appropriate amounts of Al and Si. Furthermore, since the coefficient of thermal expansion is set to about 1.2 × 10 ⁻⁵ which is similar to that of yttria-stabilized zirconia, the Cr content, which decreases the coefficient of thermal expansion by addition, is set to around 20 mass% to improve high temperature oxidation resistance. Is effective for increasing the thermal expansion coefficient by the addition of Si and / or Al in an amount of C
It was adjusted according to the r content.

In order to meet the recent demand for lowering the operating temperature of fuel cells, the solid electrolyte itself is being improved or newly developed, and an interconnector material having a corresponding thermal expansion coefficient is required. By adjusting the contents of Cr, Al, and Si to obtain a ferritic stainless steel having a coefficient of thermal expansion around 1.2 × 10 ⁻⁵ , a material suitable for the interconnector material and the like can be obtained. Become. Since the interconnector and the like are used in parts where heating and cooling are repeated, naturally, thermal fatigue characteristics and scale peeling resistance are also required. For this improvement, it is preferable to add one or more of Nb, Ti, Mo and Cu.

The alloy components and contents contained in the ferritic stainless steel of the present invention will be described in detail below. In the following explanation, "%" indicating the content of each element
Indicates "mass%" unless otherwise specified. C: 0.03% or less C is generally considered to be an effective alloying component for high temperature characteristics such as high temperature strength, but if the content increases, corrosion resistance, oxidation resistance, workability, toughness, etc. will decrease. . Particularly when a large amount of C is contained, the amount of carbides increases and the formability is lowered, so the upper limit of the C content was set to 0.03%.

Mn: 2.0% or less It is an alloy component effective for improving the high-temperature oxidation characteristics of ferritic stainless steel, especially scale releasability. But,
Since Mn is an austenite phase stabilizing element, Mn
Due to the excessive addition of Al, a martensite phase is likely to be formed after cooling, which deteriorates the workability and causes a problem that the thermal expansion coefficient is largely changed due to the phase transformation. Therefore, the upper limit of the Mn content is set to 2.0%.

Ni: 0.6% or less Since it is an austenite phase stabilizing element, an excessive amount of Ni
Is added to ferritic stainless steel, a martensite phase is likely to be formed after cooling as in the case of Mn, which causes workability deterioration. In addition, since it is an expensive element, Ni
Excess addition of steel increases the cost of steel. Therefore, the upper limit of the Ni content is set to 0.6%.

N: 0.03% or less N, like C, is generally regarded as an alloy component effective for high temperature characteristics such as high temperature strength. However, when the content is large, corrosion resistance, oxidation resistance, workability, etc. Properties and toughness are reduced. Particularly, when N is contained in a large amount, the amount of nitrides increases and the formability is lowered, so the upper limit of the N content is set to 0.03%.

Cr: 10.0 to 32.0% Cr is an alloy component which is essential for stabilizing the ferrite phase and improving steam oxidation resistance and high temperature strength which are important in high temperature applications. It is also an alloy component that lowers the coefficient of thermal expansion depending on the amount added. Thermal expansion coefficient up to 800 ℃ 1.2
In order to suppress it to about 10 ^−5, its content is preferably about 20%, but by adjusting the Al and Si contents that have the effect of increasing the thermal expansion coefficient, the required thermal expansion coefficient In addition, the Cr content may be adjusted.
However, in order to secure the oxidation resistance at high temperature, the content of at least 10% is necessary. Further, as the amount of Cr increases, the heat resistance and the corrosion resistance improve, but the addition of an excessive amount of Cr embrittles the steel material and deteriorates the workability due to hardening. Therefore, the upper limit of the Cr content is set to 32.0%.

Si: 2.0% or less, Al: 6.0% or less
1.5% or more in total of at least one of Si and Al is a very effective alloy component for improving steam oxidation resistance by forming a complex oxide film with Cr on the surface of ferritic stainless steel. Moreover, the addition has the effect of increasing the thermal expansion coefficient of ferritic stainless steel. Therefore, the content of Cr is adjusted so that the coefficient of thermal expansion becomes necessary together with the content of Cr. Si, Al
Although each of the above effects is exhibited even if each of them is added alone, it is necessary to add 1.5% in total in order to improve the steam oxidation resistance. That is, for example, even if only Si corresponding to an unavoidable impurity is contained and Si is not positively added, the steam oxidation resistance can be improved by adding 1.5% or more of Al. Then, it is preferable to contain at least 0.8% of one of Si and Al. Also,
Excessive addition of Si and Al increases hardness and causes deterioration of workability and toughness. Therefore, the upper limits of the Si and Al contents are set to 2.0% and 6.0%, respectively.

Nb: 0.05 to 0.80%, Ti: 0.
05-0.50%, Mo: 0.1-3.0%, Cu:
0.1-3.0% Nb, Ti, Mo and Cu have the effect of improving the high temperature strength of ferritic stainless steel and improving the thermal fatigue property. Nb and Ti form carbonitrides with C and N, and improve the intergranular corrosion resistance, and also improve the strength as the amount of solid solution in the balance increases. In order to exert the effect, at least 0.05% of each must be contained. In addition, Ti has the effect of improving the high-temperature oxidation resistance and scale peeling resistance of the Al-containing ferritic stainless steel when added in an appropriate amount. However, the addition of excessive amounts of Nb and Ti is
Since a large amount of precipitates are generated, leading to a decrease in toughness, Nb
And the upper limits of the Ti contents were set to 0.80% and 0.50%, respectively. Mo and Cu form a solid solution in ferrite to improve the strength of the steel material. In order to obtain the effect, it is necessary to add at least 0.1%. However, addition of excessive amounts of Mo and Cu not only causes an increase in steel material cost, but also causes deterioration of hot workability, workability, toughness and the like. Therefore, when Mo or Cu is added, their upper limits are set to 3.0%.

Rare earth element: 0.01 to 0.10% The rare earth element including Y has the effect of significantly improving the steam oxidation resistance and scale peeling resistance of the ferritic stainless steel. In order to exert this effect, it is desirable to contain at least 0.01%. However, since excessive addition causes deterioration of workability, the upper limit is made 0.10% when adding a rare earth element.

In the ferritic stainless steel which is the object of the present invention, other alloying elements are not particularly specified, and they can be added as required. As an additive component of this kind, T which is effective in improving high temperature strength
a, W, V, Co, and Ca, Mg, B, etc. which are effective in improving hot workability and toughness, and Ta, W, V, Co are 3.
It is preferable to add 0 mass% or less and Ca, Mg, and B in 0.05 mass% or less. It is preferable to reduce P, S, O, etc., which are general impurity components, as much as possible.
Specifically, P: 0.04 mass% or less, S: 0.03 mass% or less, O: 0.02 mass% or less. Also,
In order to secure a higher level of workability and toughness, P,
Strictly regulate the upper limits of S and O contents.

[0019]

EXAMPLE Each ferritic stainless steel having the composition shown in Table 1 was melted in a 30 kg vacuum melting furnace, cut into a slab having a thickness of 40 mm, heated at 1250 ° C. for 2 hours, and then hot rolled to a plate thickness of 4.5 mm. did. After that, annealing, cold rolling and pickling were repeated to finally obtain a cold rolled annealed plate of 0.1 mm.

[0020]

As the test piece for the heat cycle test, 1
After cutting into a square of 00 mm and a square of 70 mm in the central portion, Ni solder was uniformly applied to the periphery of the vicinity of the cavity and bonded to an yttria-stabilized zirconia ceramic plate of 80 mm square so that no gap was formed. 1 in vacuum
It was carried out under the conditions of 130 ° C. × 30 minutes. The average coefficient of thermal expansion of the yttria-stabilized zirconia used in the test from room temperature to 800 ° C. was 1.20 × 10 ⁻⁵ . This test piece was subjected to a thermal cycle test in which the temperature was raised to 800 ° C. for 10 minutes in the atmosphere, soaked at 800 ° C. for 10 minutes, and then air-cooled at room temperature for 10 minutes, which was repeated 100 times. After the test, the sample and the ceramic plate were visually inspected for peeling.

In the steam oxidation test, the test material was 25 mm × 3
A 5 mm cutout was used as an oxidation test piece, and a steam oxidation test was performed at 900 ° C. for 200 hours in an electric furnace whose dew point was adjusted so that the steam concentration was 50% in the air atmosphere.
The oxidation weight gain was measured after the test, and the index of the weight gain of steel having good steam oxidation resistance was set to 2.0 mg / cm ² or less.

The scale releasability of the test material is 25 mm × 3
Create an oxidation test piece cut into 5 mm and
Electric furnace with adjusted dew point so that the water vapor concentration is 50%
Then, cycle of heating at 900 ° C for 25 minutes and then cooling for 5 minutes
An intermittent oxidation test was repeated 500 times. Acid after the test
Measure the weight loss and use the weight loss as the amount of peeling.
mg / cm ²The following was considered good.

The formability is evaluated based on this elongation value by processing a No. 13B test piece specified by JISZ2201 and then performing a tensile test in a direction parallel to the rolling direction according to JISZ2241 to measure elongation. did. As a measure of the elongation value that can be processed into solid oxide fuel cell members, the elongation is 2
A workability of 5% or more was evaluated as good.

[0025]

Steels of the test numbers (hereinafter, referred to as No.) 1 to 4 which are the steels of the present invention include peeling in the thermal cycle test, abnormal oxidation in the continuous steam oxidation test, and scale peeling in the intermittent steam oxidation test. Also, no problem was observed with respect to the decrease in elongation, and both of them satisfied the performance targeted by the present invention. This is because the steel component composition has a thermal expansion coefficient similar to that of the solid electrolyte and has sufficient steam oxidation resistance by optimizing the addition amounts of Cr, Si, and Al.

On the other hand, No. In the steel No. 5, since both Si and Al that increase the value of the coefficient of thermal expansion are less than the range of the composition of the present invention, the value of the coefficient of thermal expansion becomes less than 1.2 × 10 ^{−5 and} peeling occurs in the thermal cycle test. occured. In addition, Cr,
Since the addition amounts of Al and Si are both insufficient to secure the steam oxidation resistance, this No. Steel No. 5 showed abnormal oxidation in the steam oxidation test, and the amount of peeling increased in the scale peeling test.

No. The steels 6 and 7 have Cr and Si contents having a coefficient of thermal expansion of about 1.2 × 10 ⁻⁵ .
No peeling occurred in the heat cycle test. However, since the amounts of Al and Si were smaller than the range of the components of the present invention, abnormal oxidation occurred in the steam oxidation test, and the peeling amount was large in the scale peeling test.

No. Steel No. 8 contained about 6.5% Al, which exceeds the composition range of the present invention, and therefore showed good characteristics in both the steam oxidation test and the scale peeling test.
The thermal expansion coefficient exceeded 1.2 × 10 ⁻⁵ due to the excessive addition of 1 and peeling occurred in the thermal cycle test. In addition, A
The hardness was increased by the excessive addition of 1 and a good elongation value could not be secured, and the workability was not good.

[0030]

As described above, by adjusting the content of various alloy components in the ferritic stainless steel, it was possible to obtain a stainless steel having a thermal expansion coefficient similar to that of the solid electrolyte. Moreover, it is possible to improve the stability and thermal fatigue resistance in a high temperature steam resistant atmosphere by adjusting the alloy component content without impairing the compactness, thermal conductivity, and electrical conductivity that are the original properties of ferritic stainless steel. did it. Therefore, it was possible to provide not only an interconnector material to be joined to the solid electrolyte but also a material suitable as a metal member constituting the solid oxide fuel cell.

[Brief description of drawings]

FIG. 1 is a conceptual diagram showing the structure of a single cell of a solid oxide fuel cell, (a) vertical cylinder stripe type, (b) horizontal cylinder stripe type, (c) flat plate type

FIG. 2 is a conceptual diagram illustrating a connection state of single cells of a cylindrical vertical stripe solid oxide fuel cell.

FIG. 3 is a diagram conceptually illustrating a situation in which single cells are connected in series using an interconnector.

[Explanation of symbols]

1: Fuel electrode 2: Solid electrolyte 3: Air electrode 4: Interconnector 5: Porous support tube
6: Single cell 11: Fuel flow 12: Air flow

Continued front page    (72) Inventor Yukio Kawabata             4976 Nomura-Minami-cho, Shinnanyo-shi, Yamaguchi Nissin             Steel Business Division, Stainless Steel Company (72) Inventor Katsuyuki Shiotsuki             4976 Nomura-Minami-cho, Shinnanyo-shi, Yamaguchi Nissin             Steel Business Division, Stainless Steel Company F-term (reference) 5H026 AA06 CC03 CC06 CV05 EE08                       HH05

Claims (4)

[Claims] 1. In mass%, C: 0.03% or less,
Mn: 2.0% or less, Ni: 0.6% or less, N: 0.0
3% or less, Cr: 10.0 to 32.0%, Si: 2.0% or less or Al: 6.0% or less at least one type in a total amount of 1.5% or more, and the balance is A ferritic stainless steel for solid oxide fuel cell members, which is substantially composed of Fe. 2. A ferrite system for a solid oxide fuel cell member according to claim 1, which contains at least 0.8% of Si and / or Al and a total amount of at least 1.5%. Stainless steel. 3. Further, in mass%, Nb: 0.05
~ 0.80%, Ti: 0.05 to 0.50%, Mo:
The ferritic stainless steel for a solid oxide fuel cell member according to claim 1 or 2, which contains one or more of 0.1 to 3.0% and Cu: 0.1 to 3.0%. 4. Further, in mass%, the rare earth element:
Any one of Claims 1-3 containing 0.01-0.10%.
The ferritic stainless steel for a solid oxide fuel cell member as described in 1.