JP2007016297A - Steel for solid-oxide fuel cell separator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide steel for a solid-oxide fuel cell separator on which an oxide film having satisfactory electric conductivity at about 700 to 950°C is formed and which has satisfactory oxidation resistance particularly even in use at a high temperature for a long period of time. <P>SOLUTION: The steel for a solid-oxide fuel cell separator has a composition which is composed of, by mass, ≤0.1% C, <0.2% (not including 0%) Si, 0.1 to 1.0% Mn, 17 to 26% Cr, ≤2% Ni, 0.01 to 0.8% Zr, one or more kinds among 0.01 to 0.3% Y and 0.005 to 0.2% rare earth elements (REM) and the balance essentially Fe and in which Al is controlled to ≤0.08% and further, as inevitable impurities, the contents of S, O, N and B are limited to ≤0.015%, ≤0.010%, ≤0.040% and ≤0.0030%, respectively, and also the following expression is satisfied: (O)/(0.028Si+0.003Mn+0.024Al+0.023Zr+0.14Y+0.08REM)≤1.0 ...(1). <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a steel for a solid oxide fuel cell separator used for a separator of a solid oxide fuel cell.

Fuel cells have excellent characteristics such as high power generation efficiency, low generation amount of SOx, NOx, CO ₂ , good responsiveness to load fluctuations, and compactness. It is expected to be applied to wide-scale power generation systems such as large-scale centralized alternatives, distributed suburban areas, and private power generation.
Depending on the electrolyte used, fuel cell types are classified into phosphoric acid type, molten carbonate type, solid oxide type, and solid polymer type. Among them, solid oxide type fuel cells include electrolytes such as stabilized zirconia. It uses ceramics and is operated at a fairly high temperature around 1000 ° C.
The solid oxide fuel cell is operated at a high temperature, so there is no need to use a catalyst for the electrode reaction, fossil fuel can be internally reformed at a high temperature, and various fuels such as coal gas can be used, Combined with gas turbine or steam turbine using high-temperature exhaust heat, so-called combined cycle power generation enables high-efficiency power generation, and is compact because all components are solid. And is very promising as a next-generation power supply source.

However, many problems remain to be solved for the practical application of solid oxide fuel cells. In particular, in the case of a flat plate fuel cell capable of high power density, a separator is an important component. .
This separator supports three layers of an electrolyte, a fuel electrode, and an air electrode, and has a function of forming a gas flow path and flowing current. Therefore, since separators are required to have properties such as electrical conductivity at high temperatures, oxidation resistance, and a small difference in thermal expansion from the electrolyte, in view of these required properties, conductive ceramics have conventionally been used. Many have been used.
However, since ceramics have poor processability and are expensive, there remains a problem in terms of enlargement and practical use of fuel cells.

In recent years, solid oxide fuel cells have been remarkably improved and the operating temperature can be lowered from around 1000 ° C. to about 700 to 950 ° C. Therefore, a separator made of an inexpensive and reliable metal material Material development is required.
Therefore, the present inventors have disclosed an alloy disclosed in Japanese Patent Application Laid-Open No. 2003-173895 (see Patent Document 1) as a solid oxide fuel cell separator steel of a ferritic metal material considering characteristics at about 700 to 950 ° C. Developed.
The alloy shown in Japanese Patent Application Laid-Open No. 2003-173895 forms an oxide film having good electrical conductivity in the vicinity of 700 to 950 ° C. when used for a separator of a solid oxide fuel cell, and is used for a long time. In addition, since it has good oxidation resistance, in particular, peeling resistance, has a small difference in thermal expansion from the electrolyte, and can reduce the cost and performance of the fuel cell, it has a relatively low temperature of 700 to 950. This has the effect of greatly contributing to the practical use, high efficiency, and large size of the solid oxide fuel cell that operates at about ° C.

JP 2003-173895 A

In order to put a solid oxide fuel cell into practical use, an apparatus life of approximately 80 to 100,000 hours or more is required, and separator steel also has long-term oxidation resistance at a use temperature of 700 to 950 ° C. It has been demanded. Therefore, in order to further improve the reliability and long-term durability of the solid oxide fuel cell, it is necessary to further improve the long-term oxidation resistance of the separator steel at the above operating temperature.
An object of the present invention is to form a solid oxide fuel cell separator steel that forms an oxide film having good electrical conductivity at about 700 to 950 ° C. and has good oxidation resistance even when used at a high temperature for a long time. Is to provide.

Based on the alloy disclosed in Japanese Patent Application Laid-Open No. 2003-173895, the present inventors have conducted various studies on additive elements and contents in order to further improve long-term oxidation resistance at a use temperature of 700 to 950 ° C. .
As a result, when a small amount of Al added as a deoxidizing element is used as a separator at a high temperature, the protective film Cr ₂ O ₃ coating is partially formed by the pressure when it is oxidized and volume-expanded in the vicinity of and inside the base material. It was found that the oxidation resistance deteriorates. Thus, it has been found that long-term oxidation resistance can be drastically improved by regulating Al below a very low specific level and reducing the internal oxidation of Al.
Furthermore, Zr, Y, and rare earth elements, which are effective elements for obtaining good oxidation resistance, tend to form oxides by forming oxides when O in the alloy is large. If Zr, Y, and rare earth elements are fixed as inclusions, the amount of Zr, Y, and rare earth elements dissolved in the base material will decrease, contributing to the suppression of oxide film growth, densification, and improved adhesion. The effective amount that can be reduced. Therefore, in order to make the added Zr, Y, and rare earth elements act effectively, it is effective to reduce the inclusions of these elements as much as possible. For that purpose, not only can O be kept low as an impurity element. The present inventors have found that it is necessary to simultaneously control the amounts of Si, Al and Zr, Y, and rare earth elements having a deoxidizing effect in order to maintain good oxidation resistance.

That is, the present invention, in mass%, C: 0.1% or less, Si: less than 0.2% (excluding 0%), Mn: 0.1-1.0%, Cr: 17-26% Ni: 2% or less, Zr: 0.01-0.8%, Y: 0.01-0.3% and rare earth element (REM): 0.005-0.2% More than seeds, the balance is substantially Fe, Al is regulated to 0.08% or less, and further, as inevitable impurities, S: 0.015% or less, O: 0.010% or less, N: 0 0.040% or less, B: Steel for a solid oxide fuel cell separator that is limited to 0.0030% or less and satisfies the formula (1).
(O) / (0.028Si + 0.003Mn + 0.024Al + 0.023Zr + 0.14Y + 0.08REM) ≦ 1.0 ... (1)

  The separator steel of the solid oxide fuel cell according to the present invention is an alloy shown in Japanese Patent Application Laid-Open No. 2003-173895. By suppressing Al as an impurity element and further limiting the amount of O and other alloy elements more optimally, In addition to forming an oxide film having good electrical conductivity in the vicinity of 700 to 950 ° C. of the alloy shown in Japanese Patent Application Laid-Open No. 2003-173895, the characteristic that the difference in thermal expansion from the electrolyte is small is maintained as it is, especially for a long time. As a result, it was possible to reduce the cost and improve the performance of the fuel cell, and to put the solid oxide fuel cell to practical use and higher efficiency. It can greatly contribute to enlargement.

In the steel for solid oxide fuel cell separator of the present invention, the reason why each chemical composition is specified in the following range is as follows. Unless otherwise specified, the mass% is indicated.
Al: 0.08% or less Al is one of the deoxidizing elements disclosed in Japanese Patent Application Laid-Open No. 2003-173895 proposed by the present inventors, and its effect was mainly aimed at reducing O by deoxidation. .
As a result of various investigations of additive elements and contents to further improve long-term oxidation resistance, stable and good oxidation resistance is ensured by keeping Al low as an element that should be specifically regulated. It was found that there is an effect that can be done.
And as a result of carrying out the experiment in detail about the range which can exhibit the effect which further improves favorable oxidation resistance in the long time use, if it is 0.08% or less, the said effect can be exhibited. Preferably it is 0.05% or less, More preferably, it is 0.03% or less.
C: 0.1% or less C has the effect of increasing the high temperature strength by forming carbides, but conversely deteriorates workability and reduces the amount of Cr effective in oxidation resistance by combining with Cr. . Therefore, it is limited to 0.1% or less. Desirably, it is 0.08% or less. A preferable lower limit is 0.001%.

Si: Less than 0.2% Si forms a thin SiO ₂ film in the vicinity of the interface between the Cr ₂ O ₃ oxide film and the base material to increase the thickness of the oxide film, and lowers the electrical conductivity of the oxide film. Less than 0.2%. A preferable Si range is 0.1% or less, and a more desirable range is 0.05% or less. In addition, when Al which is a deoxidizing element is lowered, Si, which is also a deoxidizing element, needs to be added for the purpose of deoxidation. Therefore, the preferable lower limit is preferably 0.01%.

Mn: 0.1 to 1.0%
Mn forms a spinel oxide together with Fe and Cr. The spinel type oxide layer containing Mn is formed outside the Cr ₂ O ₃ oxide layer. This spinel oxide layer has a protective effect of preventing Cr, which degrades the ceramic electrolyte of the solid oxide fuel cell, from evaporating from the separator steel. In addition, since this spinel type oxide usually has a higher oxidation rate than Cr ₂ O ₃ , it works against the oxidation resistance itself, while maintaining the smoothness of the oxide film and reducing the contact resistance. It has the effect of preventing the decrease and evaporation of Cr harmful to the electrolyte.
On the other hand, if added excessively, as described above, the oxidation resistance of the Mn-containing spinel-type oxide itself is insufficient, resulting in poor oxidation resistance. Therefore, Mn is limited to 0.1 to 1.0%. A preferable upper limit of Mn is 0.8%. Moreover, the minimum with preferable Mn is 0.2%.

Cr: 17-26%
Cr is an important element for maintaining oxidation resistance and electrical conductivity by producing a Cr ₂ O ₃ coating in the present invention. Therefore, a minimum of 17% is required. However, excessive addition is not so effective in improving oxidation resistance, but also causes deterioration of workability, so it is limited to 17 to 26%. A preferable upper limit of Cr is 25%, and a more preferable upper limit is 24%. A preferable lower limit of Cr is 18%, and a more preferable lower limit is 20%.
Ni: 2% or less Ni is effective in improving toughness by adding a small amount to the steel of the present invention. However, Ni is an austenite-forming element, and excessive addition becomes a ferrite-austenite two-phase structure, resulting in an increase in thermal expansion coefficient and an increase in cost. Further, excessive addition of Ni deteriorates the oxidation resistance. Therefore, Ni is limited to 2% or less. Preferably it is 1% or less. A preferred lower limit is 0.1%.

Zr: 0.01 to 0.8% or less, Y: 0.01 to 0.3% and rare earth element (REM): 0.005 to 0.2%, one or more kinds Zr, Y, rare earth elements ( REM) has the effect of greatly improving the oxidation resistance and the electrical conductivity of the oxide film when added in a small amount. This is considered to be mainly due to improving the adhesion of the oxide film.
In the present invention, good oxidation resistance is maintained mainly with a Cr-based oxide film, but in order to improve the adhesion of the Cr-based oxide film, Zr is essential, and Y and REM are used alone or Compound addition is essential. However, excessive addition deteriorates hot workability, so Zr is limited to 0.01 to 0.8%, Y is limited to 0.01 to 0.3%, and REM is limited to 0.005 to 0.20%. Preferably, they are Zr: 0.01-0.6%, Y: 0.01-0.2%, REM: 0.005-0.10%.
More preferably, it is a composite addition of Zr: 0.01 to 0.6% and REM: 0.005 to 0.20%, and the adhesion of the oxide film is further improved. Separation can be prevented. Among rare earth elements, since La is most effective in improving the adhesion of the oxide film, the combined addition of Zr: 0.01 to 0.6% and La: 0.005 to 0.20% is most preferable.

Next, the reasons for limiting the inevitable impurity elements will be described.
S: 0.015% or less S not only lowers the oxidation resistance by forming sulfide inclusions with Mn, La, etc., but reducing the amount of effective rare earth elements effective in oxidation resistance. In order to deteriorate hot workability and surface skin, the content is limited to 0.015% or less. Desirably, it is 0.008% or less.
O: 0.010% or less O forms oxide inclusions with Al, Si, Mn, Cr, La, Zr, etc., and not only harms hot workability and cold workability, but also oxidation resistance. In order to reduce the solid solution amount of La, Zr, etc., which are elements that greatly contribute to the improvement of the property, the effect of improving the oxidation resistance by these elements is reduced. Therefore, it is limited to 0.010% or less. Preferably, it is 0.008% or less.
N: 0.040% or less N is an austenite-generating element. Therefore, when excessively added to the Fe—Cr ferritic steel of the present invention, N not only generates an austenite phase and cannot maintain a ferrite single phase, but also Al. In order to form nitride inclusions with Cr and the like and impair hot workability and cold workability, the content is limited to 0.040% or less. Preferably it is 0.030% or less, More preferably, 0.020% or less is good.

B: 0.0030% or less B not only deteriorates the oxidation resistance by increasing the growth rate of the oxide film at a high temperature of about 700 ° C. or higher, but also increases the surface roughness of the oxide film, In order to degrade the contact resistance by reducing the contact area with the electrode, it is better to limit it to 0.0030% or less as an impurity and reduce it to 0% as much as possible. The upper limit is preferably 0.0020% or less, and more preferably less than 0.0010%.
The balance is substantially Fe
The balance is substantially Fe, but unavoidable elements are included. In addition to the impurity elements described above, the present invention steel may contain the following elements that basically do not affect the properties of the steel of the present invention within the following ranges as long as the amount is small.
P ≦ 0.04%, Cu ≦ 0.30%, Mg ≦ 0.02%, Ca ≦ 0.02%, Co ≦ 2%, Ti ≦ 0.1%

(1) Formula: (O) / (0.028Si + 0.003Mn + 0.024Al + 0.023Zr + 0.14Y + 0.08REM) ≦ 1.0
In the steel of the present invention, in order for Zr, Y and rare earth elements (REM), which have a great effect in improving the oxidation resistance and the electrical conductivity of the oxide film, to exhibit a sufficient effect, these elements are oxide inclusions. It is necessary to prevent it from being completely fixed to etc.
For that purpose, it is effective to keep the ratio of the amount of O to the amount of Al, Si and Zr, Y, REM having a deoxidizing effect low as shown in the formula (1). However, one of the features of the steel of the present invention is that Al having a deoxidizing effect is limited to a very low level, and the amount of Si is also limited, so that it is difficult to control the amount of O. Therefore, in order to sufficiently exhibit the effects of Zr, Y, and REM, it is important to comprehensively manage the amounts of Al, Si, Zr, Y, REM, and O using this equation (1).
If the value of the formula (1) exceeds 1.0, Zr, Y, and REM are fixed to the inclusions and do not contribute to the improvement of the oxidation resistance and the electric conductivity of the oxide film. Was 1.0 or less. In addition, it calculates as 0 about an additive-free element among Y and REM.

The following examples further illustrate the present invention.
The steel of the present invention and the comparative steel were melted in a vacuum induction furnace to produce a 10 kg ingot. At the time of vacuum melting, in order to keep Al and impurity elements low within the specified range, raw materials with high purity were selected, and melting was performed by controlling operating conditions such as furnace atmosphere.
In particular, O was strictly managed as follows. Originally, in order to keep the amount of O low, it is common to add a large amount of Al, which is a strong deoxidizing element. However, in the steel of the present invention, it is necessary to reduce Al, so deoxidation is insufficient. There was a possibility. Therefore, in order to keep the amount of O low while reducing Al, the amount of Al added is kept to the minimum necessary to the extent that the deoxidation effect can be obtained, and the operating conditions such as selection of raw materials and degree of vacuum are managed very strictly. To dissolve. However, some of the comparative steels were not considered because of the influence of impurity elements.
Then, it heated to 1100 degreeC and forge-stretched to the 30-mm square bar | burr, and annealed at 780 degreeC for 1 hour. Table 1 shows the steel No. of the present invention. 1-9, comparative steel no. The chemical composition of 11-14 is shown. In Table 1, comparative steel No. 11 to 13 are alloys disclosed in Japanese Patent Application Laid-Open No. 2003-173795.

Test pieces were cut out from these materials and subjected to various tests.
First, using a cylindrical test piece having a diameter of 10 mm and a length of 20 mm, heat treatment was performed at 950 ° C. for 1000 hours and 750 ° C. for 1000 hours in the atmosphere, and then the oxidation increase and the surface oxide scale peeling amount were measured. Further, using a plate-shaped test piece of 10 mm ^W × 10 mm ^L × 3 mm ^t , heating was performed at 950 ° C. for 1000 hours in the air to form an oxide film on the surface, and then the electrical resistance at 950 ° C. was measured. Further, after heating at 750 ° C. in the atmosphere for 1000 hours to form an oxide film on the surface, the electrical resistance at 750 ° C. was measured.
The electrical resistance was measured by a four-terminal method with a Pt mesh fixed to the surface of the test piece with a Pt paste, and expressed as a sheet resistance (mΩ · cm ² ). These test results are summarized in Table 2.

From Table 2, comparative steel No. 1 with a large amount of Al. Compared to 11, the steel of the present invention has less oxidation increase of the cylindrical specimen after heating at 950 ° C. × 1000 Hr and 750 ° C. for 1000 Hr in the atmosphere. This is considered to be because the internal oxidation was reduced and the Cr ₂ O ₃ coating of the protective film was stabilized by sufficiently reducing Al.
Comparative steel No. No. 12 is not sufficient in oxidation resistance because of the large amount of Si. Comparative steel No. No. 13, the value of the formula (1) is large, the effects of Zr and REM which are effective in oxidation resistance cannot be sufficiently exhibited, peeling of the scale is observed, and the electric resistance is also high.
Further, comparative steel No. 1 containing a large amount of Al. No. 14 had good oxidation resistance, but the electric resistance of the oxide film was extremely high, and only properties not desirable for application to a solid oxide fuel cell separator were obtained.
On the other hand, the steel according to the present invention has an electric resistance value measured at 950 ° C. after forming an oxide film on the surface by heating at 950 ° C. in the atmosphere for 1000 hours, and an oxide film on the surface by heating at 750 ° C. for 1000 hours. The values of the electrical resistance measured at 750 ° C. after forming the film are sufficiently small.

  The solid oxide fuel cell separator steel of the present invention forms an oxide film having good electrical conductivity in the vicinity of 700 to 950 ° C., and also has good oxidation resistance, especially peel resistance, even when heated for a long time. In addition, since it has the property that the difference in thermal expansion from the electrolyte is small, such as steel bars, wire rods, powders, powder sintered bodies, porous bodies, steel foils, etc. It can be used after being processed into various shapes.

Claims (1)

In mass%, C: 0.1% or less, Si: less than 0.2% (not including 0%), Mn: 0.1 to 1.0%, Cr: 17 to 26%, Ni: 2% Hereinafter, Zr: 0.01 to 0.8%, including Y: 0.01 to 0.3% and rare earth element (REM): 0.005 to 0.2%, or two or more kinds, The balance is substantially Fe, Al is regulated to 0.08% or less, and further, as inevitable impurities, S: 0.015% or less, O: 0.010% or less, N: 0.040% or less, B: Steel for solid oxide fuel cell separator, limited to 0.0030% or less and satisfying the formula (1).
(O) / (0.028Si + 0.003Mn + 0.024Al + 0.023Zr + 0.14Y + 0.08REM) ≦ 1.0 ... (1)