JP4310723B2 - Steel for solid oxide fuel cell separator - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to steel used for a separator of a solid oxide fuel cell.
[0002]
[Prior art]
Fuel cells have high power generation efficiency, SOx, NOx, Co₂Because it has excellent features such as low generation amount, good response to load fluctuations, compactness, etc., it is a large-scale centralized type as an alternative to thermal power generation, urban suburban distributed type, and private power generation It is expected to be applied to a wide range of power generation systems.
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 has been conventionally operated at a fairly high temperature around 1000 ° C.
[0003]
The above-mentioned solid oxide fuel cell is operated at a high temperature, so that it is not necessary 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 are used. It can be used, combined with gas turbines or steam turbines using high-temperature exhaust heat, and so-called combined cycle power generation enables high-efficiency power generation, and it is compact because all components are solid. Therefore, it is very promising as a next-generation power supply source.
[0004]
However, many problems remain to be solved for the practical application of solid oxide fuel cells. 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. Although many have been used, ceramics have poor workability and are expensive, so that problems remain in terms of the enlargement and practical use of fuel cells.
[0005]
Therefore, development of a separator made of an inexpensive and reliable metal material is required. When a normal metal material is used at 1000 ° C., the surface is oxidized to produce an oxide film, but for use as a separator material, this oxide film is stable and oxidation does not proceed, and the oxide film has electrical conductivity. is necessary.
In order to satisfy such required characteristics, Japanese Patent Laid-Open No. 6-264193 discloses a metal material for a solid oxide fuel cell, C: 0.1% or less, Si: 0.5 to 3.0%, Mn: 3.0% or less, Cr An austenitic stainless steel composed of: 15-30%, Ni: 20-60%, Al: 2.5-5.5%, and the balance Fe has been proposed.
[0006]
In addition, in JP-A-7-163011, as a separator for a solid electrolyte fuel cell, Fe: 60 to 82% and Cr: 18 to 40%, an additive element that reduces the contact resistance between the air electrode of the unit cell ( It has been proposed to use an alloy comprising La, Y, Ce or Al alone. Furthermore, Japanese Patent Laid-Open No. 7-145454 proposes a material comprising Cr: 5 to 30%, Co: 3 to 45%, La: 1% or less, and the balance Fe as a metal material for a solid oxide fuel cell. .
[0007]
[Problems to be solved by the invention]
Recently, solid oxide fuel cells have been remarkably improved, and it has become possible to lower the operating temperature from around 1000 ° C. to about 700 to 950 ° C. For this reason, it is expected that the practical use will be accelerated.
Since the material disclosed in JP-A-6-264193 mentioned above contains a considerable amount of Al and Cr, the surface oxide film is mainly composed of an Al-based oxide, which contains a Cr-based oxide. However, as will be described later, Al-based oxides have low electrical conductivity and are not necessarily sufficient for solid electrolyte fuel cell separators, and austenitic stainless steels are compared to stabilized zirconia in electrolytes. In addition, since the coefficient of thermal expansion is large, the battery performance is liable to be deteriorated due to the cracking of the electrolyte due to the thermal cycle that accompanies the start and stop of the battery, and there is a problem in stability during long-term use. Furthermore, since it contains a lot of expensive Ni, the price is high, which is insufficient for the practical use of fuel cells.
[0008]
On the other hand, the materials disclosed in JP-A-7-16301 and JP-A-7-145454 have a low thermal expansion coefficient compared to austenitic stainless steel and are close to the thermal expansion coefficient of stabilized zirconia of the electrolyte. Therefore, it is advantageous for stability in long-term use and also has good electrical conductivity. However, the oxidation resistance after long-term use is insufficient, especially the exfoliation phenomenon accompanying the increase of the oxide layer is promoted, and the groove provided in the separator that becomes the gas flow path in the battery is narrowed, and the battery function is deteriorated There is a problem to make.
In addition, the materials disclosed in JP-A-8-35042 and JP-A-8-277441 have a low coefficient of thermal expansion compared to austenitic stainless steel and are close to the coefficient of thermal expansion of stabilized zirconia in the electrolyte. Although it is advantageous for stability in use, no consideration is given to electrical conductivity that is important as a property of the separator material.
[0009]
In addition, the materials disclosed in JP-A-915701 and JP-A-10-280103 also have a low coefficient of thermal expansion up to 1000 ° C. In addition, it has good oxidation resistance at 1000 ° C. and electrical conductivity.
However, the above-described known materials are all developed for the purpose of obtaining good characteristics as a separator for a solid oxide fuel cell operating at 1000 ° C. The characteristics at the operating temperature of about 700 to 950 ° C. are not considered at all.
[0010]
Further, since the solid oxide fuel cell is repeatedly operated and stopped, the stress due to the thermal cycle acts on the separator of the component parts. Because of this thermal stress, there is a concern that fracture will occur particularly when the impact properties at room temperature are low. Therefore, the separator steel must have impact properties sufficient to withstand the thermal stress. Does not consider any impact properties at room temperature.
The object of the present invention is to form an oxide film having good electrical conductivity at about 700 to 950 ° C., and also to have good oxidation resistance, especially peel resistance, even at long term use, and at room temperature. An object of the present invention is to provide an inexpensive solid oxide fuel cell separator steel having excellent impact characteristics and a small difference in thermal expansion from an electrolyte.
[0011]
[Means for Solving the Problems]
As a result of various studies, the present inventor first made the metal material of interest ferrite. The first reason for this is that the average thermal expansion coefficient of stabilized zirconia, which is an electrolyte of solid oxide, from room temperature to about 750 ° C. is about 11 × 10.^-6For normal austenitic metal materials, approximately 16 x 10^-6This is because the difference in thermal expansion between the two is large and the stability during use for a long time is considered to be problematic.
The second reason is that austenite is generally expensive because it contains expensive Ni, whereas ferrite is based on Fe and does not contain Ni, or even if it contains a small amount, it is inexpensive.
[0012]
Next, the inventor conducted various studies on the electric conductivity of the oxide film to be formed.
Al oxide and Cr oxide are known as representative oxide films having protective properties. Generally, when the temperature reaches around 700-950 ° C, Al₂O_ThreeIs more advantageous in terms of protection, but Al₂O_ThreeWhen the electrical resistance of the film forming material was measured, it was found to be very large and could not be used as a separator.
Meanwhile, Cr₂O_ThreeIt has been found that the electrical resistance of the film forming material is sufficiently small and can be used for a separator. Therefore, in the present invention, a ferrite metal material that forms an oxide film mainly composed of a Cr-based oxide on the surface, that is, a Fe—Cr-based material is used as a basis.
[0013]
Next, although it is oxidation resistance which becomes a problem when used for a long time, the oxidation resistance of the Cr-based oxide film is usually inferior to that of the Al-based oxide film in the vicinity of 700 to 950 ° C. as described above. Even when a Cr-based oxide film is the main component, an Fe-based alloy (for example, a Fe-Cr alloy such as SUS430) is more acid-resistant than a Ni-based alloy (for example, a Ni-Cr alloy represented by JIS NCF600). The conversion is inferior.
Therefore, it is very difficult to satisfy the oxidation resistance when the Fe—Cr system is used as a basis in accordance with the above policy.
[0014]
As a result of various studies to solve this problem, the present inventor, in addition to adding one or more of Y, rare earth elements, and Zr to the Fe-Cr system, suppressing Al low, and adding a small amount of Si and Mn As a result, it has been found that good oxidation resistance, in particular, peeling resistance can be obtained while mainly using a Cr-based oxide film, and that the film formation state is stable even after heating for a long time.
In particular, when Zr is added in combination with Y and / or rare earth elements, the peel resistance is most improved. It was also found that even when the above elements were added, the oxide film formed was mainly composed of a Cr-based oxide film, so that the electrical resistance did not increase so much.
[0015]
Furthermore, Y, rare earth elements, and Zr, which are effective elements for obtaining good oxidation resistance, tend to form sulfides and oxides when they contain a large amount of S and O in the alloy. If Y, rare earth elements, and Zr are fixed as inclusions, the amount of Y, rare earth elements, and Zr that dissolve in the matrix decreases, contributing to the suppression of oxide film growth, densification, and improved adhesion. The effective amount decreases. Therefore, in order to make the added Y, rare earth element, and Zr act effectively, it is effective to reduce inclusions of these elements as much as possible, and it is good oxidation resistance to keep impurity elements such as S and O low. I found that it is necessary to maintain sex.
Also, N and B have been found to be effective to suppress as impurities because they may form a compound with a part of elements effective for maintaining oxidation resistance, such as La. In particular, B has been newly found to reduce the contact resistance by deteriorating the smoothness of the oxide film surface. From the viewpoint of contact resistance, it is necessary to keep B in the separator steel low as an impurity.
Further, since Ti is an element that lowers oxidation resistance, it needs to be kept low as an impurity.
[0016]
Moreover, in order to process the steel of the steel material for a separator into a separator, it is necessary to reduce the hardness. For that purpose, it is necessary to perform an appropriate annealing. This annealing is effective not only for adjusting the hardness but also for adjusting the ferrite crystal grain size, and the impact characteristics can be improved by using an appropriate fine grain structure.
As described above, the present invention has been achieved by adjusting the alloy components and optimizing the heat treatment conditions, structure, and mechanical properties.
[0017]
  That is, according to the first invention of the present invention, in mass%, C: 0.2% or less, Si: less than 0.2% (not including 0%), Mn: 1.0% or less (not including 0%) ), Ni: 2% or less, Cr: 15-30%, Al: 1% or less, La: 0.005-0.1%, Zr: 1% or less, the balance is Fe and inevitable impurities, As unavoidable impurities, Ti: 0.1% or less, S: 0.015% or less, O: 0.010% or less, N: 0.050% or less, B: 0.0030% or less, and ( 1) It is made of steel satisfying the formula, the hardness is 280 HV or less, and the average ferrite grain size is ASTM No. It is a steel for a solid oxide fuel cell separator that is two or more fine particles.
(O + 2S) /(0. 035Zr + 0.16La) ≦ 2.0 ... (1)
The element of the formula (1) is mass%.
[0018]
  The second invention of the present invention, in mass%, C: 0.1% or less,Si: Less than 0.2% (excluding 0%)Mn: 1.0% or less (excluding 0%), Cr: 17-26%, Ni: 1% or less, Al: 0.001% or more and less than 0.5%, Zr: 0.01-0. 8%, La: 0.005 to 0.8%Including the restIs FeAnd inevitable impuritiesAs an inevitable impurity,Ti: 0.1% or less,S: 0.015% or less, O: 0.010% or less, N: 0.020% or less, B: 0.0030% or less, and made of steel satisfying the formula (1). 280HV or less, average ferrite grain size is ASTM No. It is a steel for a solid oxide fuel cell separator that is two or more fine particles.
[0019]
  The third invention of the present invention is, in mass%, C: 0.1% or less, Si: less than 0.2% (not including 0%), Mn: less than 0.2% (not including 0%) Cr: 17 to 26%, Ni: 1% or less, Al: 0.001% or more and less than 0.5%, Zr: 0.01 to 0.8%, La: 0.005 to 0.8%Including the restIs FeAnd inevitable impuritiesAs an inevitable impurity,Ti: 0.1% or less,S: 0.015% or less, O: 0.010% or less, N: 0.020% or less, B: 0.0030% or less, and made of steel satisfying the formula (1). 280HV or less, average ferrite grain size is ASTM No. It is a steel for a solid oxide fuel cell separator that is two or more fine particles.
[0020]
  The fourth invention of the present invention is the above-mentioned alloy,Further, Y: 0.01 to 0.3% is satisfied, and the formula (2) is satisfiedSteel for solid oxide fuel cell separator.
(O + 2S) / (0.27Y + 0.035Zr + 0.16La) ≦ 2.0 (2)
[0021]
  The fifth invention of the present invention is, in mass%, C: 0.08% or less, Si: less than 0.2% (not including 0%), Mn: 0.5% or less (not including 0%) Cr: 18-25%, Ni: 0.5% or less, Al: 0.001% or more and less than 0.5%, La: 0.005-0.1%, Zr: 0.01-0.6% The balance consists of Fe and unavoidable impurities. Ti: 0.1% or less, S: 0.008% or less, O: 0.008% or less, N: 0.020% or less, B: Limited to 0.0020% or less, and (1)), The hardness is 280 HV or less, and the average ferrite grain size is ASTM No. It is a steel for solid oxide fuel cell separators that is two or more fine grains.
[0022]
  The sixth invention of the present invention is, in mass%, C: 0.08% or less, Si: less than 0.2% (not including 0%), Mn: 0.5% or less (not including 0%) Cr: 18-25%, Ni: 0.5% or less, Al: 0.001% or more and less than 0.5%, La: 0.005-0.1%, Zr: 0.01-0.6% The balance consists of Fe and unavoidable impurities. Ti: 0.1% or less, S: 0.008% or less, O: 0.008% or less, N: 0.020% or less, B: Limited to less than 0.0010% and (1)), The hardness is 280 HV or less, and the average ferrite grain size is ASTM No. It is a steel for solid oxide fuel cell separators with 3 or more fine grains.
[0023]
  Preferably, it is a steel for a solid oxide fuel cell separator containing Mo + 1 / 2W ≦ 5.0% by mass%, Mo alone or two kinds of Mo and W.
  More preferably, in mass%, V, Nb, HA solid oxide fuel cell separator steel containing 0.01 to 1.0% of one or more of f in total.
[0024]
  More preferably, the 2 mmV notch Charpy impact value at 20 ° C. is 8 J / cm.²This is steel for solid oxide fuel cell separators as described above.
  More preferably, the 2 mmV notch Charpy impact value at 20 ° C. is 10 J / cm.²This is steel for solid oxide fuel cell separators as described above.
  More preferably, the electrical resistance of the oxide film at 750 ° C. after heating at 750 ° C. for 1000 hours is 100 mΩ · cm.²The surface oxide scale was peeled after heating at 850 ° C for 100 hours.DepartIt is a solid oxide fuel cell separator steel that does not grow.
  More preferably, the electric resistance of the oxide film at 750 ° C. after heating at 750 ° C. for 1000 hours is 50 mΩ · cm.²This is a steel for a solid oxide fuel cell separator in which peeling of the surface oxide scale does not substantially occur after heating at 850 ° C. for 100 hours.
  In addition, the present invention further includes 0.005 to 0.1% of La and a rare earth element other than La (REM) in any of the compositions of the first to third, fifth and sixth inventions described above. And a steel for a solid oxide fuel cell separator that satisfies the formula (3).
(O + 2S) / (0.035Zr + 0.16REM) ≦ 2.0 (3)
[0025]
DETAILED DESCRIPTION OF THE INVENTION
The reasons for limiting the components in the present invention will be described below.
C has the effect of increasing the high temperature strength by forming carbides, but conversely deteriorates the workability and reduces the amount of Cr effective for oxidation resistance by combining with Cr. Therefore, it is limited to 0.2% or less. Desirably, it is 0.1% or less, and more desirably 0.08% or less.
[0026]
Si is involved in the formation of a film mainly composed of a Cr-based oxide layer on the inner surface of the groove that forms the flow path of the high-temperature gas provided in the separator. Or an element that has the effect of preventing the peeling phenomenon.
One of the effects of Si is probably Cr₂O_ThreeThin discontinuous SiO near the interface between oxide film and base metal₂It is considered that a film is formed to improve the oxidation resistance.
[0027]
  In addition, the SiO₂Coating is made of base material and Cr₂O₃At the coating interface, the base material, Cr₂O₃Coating, SiO₂Forms a state where the coating is finely entangled, thereby improving the adhesion with the base material, and Cr₂O₃There is an effect of preventing peeling of the film.
  Such an effect is particularly great at a high temperature of 1000 ° C. or higher, and not necessarily great at 700 to 950 ° C. However, in order to obtain the above effect, it is necessary to add a small amount of Si.
On the other hand, excessive addition leads to deterioration of workability and toughness and SiO 2₂Since the film becomes too thick, it causes problems such as continuous peeling of the oxide film and a decrease in the electrical conductivity of the film.Is 0. Less than 2% (excluding 0%)And
[0028]
Mn forms a spinel oxide together with Fe and Cr. The spinel oxide layer containing Mn is Cr₂O_ThreeFormed outside the oxide layer. This spinel oxide layer has a protective effect of preventing Cr, which deteriorates the ceramic electrolyte of the solid oxide fuel cell, from evaporating from the separator steel. Also, this spinel type oxide is usually Cr₂O_ThreeCompared to, the oxidation rate is high, which is disadvantageous to the oxidation resistance itself, while maintaining the smoothness of the oxide film to reduce contact resistance and prevent the evaporation of Cr harmful to the electrolyte Has an effect.
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 1% or less (not including 0%). When the operating temperature is as low as 700 to 950 ° C, it may be 0.5% or less (not including 0%), and when the operating temperature is lower, it may be less than 0.2% (not including 0%). .
[0029]
In the present invention, Cr is Cr₂O_ThreeIt is an important element for maintaining oxidation resistance and electrical conductivity by forming a coating. Therefore, a minimum of 15% is required. However, excessive addition is not so effective in improving oxidation resistance but also causes deterioration of workability, so it is limited to 15 to 30%. A desirable Cr range is 17 to 26%, and a more desirable Cr range is 18 to 25%.
[0030]
  Y, rare earth element (REM), and Zr have the effect of greatly improving the oxidation resistance and the electrical conductivity of the oxide film when added in small amounts. In particular, the effect of improving the oxidation resistance when combined with a small amount of Si and Mn is large, which is considered to be mainly due to the improvement of the adhesion of the oxide film.
  In the present invention, only the Cr-based oxide film has oxidation resistance, but in order to improve the adhesion of the Cr-based oxide film., Among the rare earth elements (REM), it is essential to add La and Zr, and if necessary, rare earth elements other than La and La,YIf necessary, 0.005 to 0.1% may be contained.. However, excessive addition degrades hot workability, so Y is 0.3% Or less, rare earth element (REM) is 0.1% Or less, Zr is limited to 1% or less. Desirably, Y: 0.01 to 0.3%, rare earth element (REM): 0.005 to 0.10%, Zr: 0.01% to 0.8%.
[0031]
  Furthermore, Y: 0.01 to 0.3%, La requiredWhen one or more of rare earth elements (REM): 0.005 to 0.10% and Zr: 0.01% to 0.8% are added in combination, the adhesion of the oxide film is further improved for a long time. Even after heating, peeling of the oxide film can be prevented. More preferably,Require LaRare earth elements (REM): 0.005 to 0.10% and Zr: 0.01% to 0.8%. Among rare earth elements, La is most effective in improving the adhesion of the oxide film,In the present invention, La: 0.005 to 0.1% and Zr: 1% or less are essential and added in combination.The combined addition of La: 0.005 to 0.10% and Zr: 0.01% to 0.6% is most desirable.
  Zr, like V, Nb, Ta, and Hf, which will be described later, is combined with C to form carbides, improves workability by C fixation, and contributes to strength improvement.
[0032]
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 cost increase. Further, excessive addition of Ni deteriorates the oxidation resistance. Therefore, Ni is limited to 2% or less. Desirably, it is 1% or less, and more desirably 0.5% or less.
[0033]
Al is usually added as a deoxidizer. If you add a lot of Al, Al₂O_ThreeA film is formed, but as mentioned above, Al₂O_ThreeThe coating is effective for oxidation resistance, but increases the electrical resistance of the oxide coating. Therefore, in the case of the present invention, Al₂O_ThreeIn order to avoid the formation of a film, Al is limited to 1% or less. Desirably, it is 0.001% or more and less than 0.5%.
[0034]
Mo has an effect of increasing the high temperature strength, and therefore may be added when high temperature strength is important. W also has the same effect as Mo, but in order to exert the same effect as Mo, addition of Mo twice as much by mass is necessary. Since a large amount of W impairs hot workability, W must be combined with Mo to keep the total amount of Mo and W low. Thus, excessive addition not only deteriorates workability but also reduces oxidation resistance, so Mo + 1 / 2W is limited to 5% or less. Desirably, it is 3% or less.
[0035]
  V, Nb, Hf is combined with C to form a carbide, and improves workability by fixing C. While contributing to strength improvement, in the vicinity of 700 to 900 ° C., except for Hf, an oxide having little protection property is formed to deteriorate the oxidation resistance.
  The above Hf is most preferable among these elements because it has an effect on oxidation resistance, but is selected as necessary because it is expensive. V, Nb, HExcessive addition of the element f forms a large amount of primary carbides and degrades workability. Therefore, considering workability, strength, and oxidation resistance, V, Nb, HYou may add 1 type or 2 types or more in the range of 0.01 to 1.0% of f. Desirably, it is 0.03 to 0.6%.
[0036]
Next, the reasons for limiting the inevitable impurity elements will be described.
Since Ti is an element that deteriorates oxidation resistance by forming an internal oxidation phase, it is limited to 0.1% or less as an impurity.
S forms sulfide inclusions with Mn, rare earth elements (REM), etc., reducing the amount of effective rare earth elements effective in oxidation resistance, not only reducing oxidation resistance, but also hot In order to deteriorate the workability and surface skin, the content is limited to 0.015% or less. Desirably, it is 0.008% or less.
O forms oxide inclusions with Al, Si, Mn, Cr, Y, rare earth elements (REM), Zr, etc., not only harming hot workability and cold workability, but also oxidation resistance In order to reduce the amount of solid solution of elements such as Y, rare earth elements (REM), and Zr that greatly contribute to the improvement, the effect of improving the oxidation resistance by these elements is reduced. Therefore, it is limited to 0.010% or less. Desirably, it is 0.008% or less.
[0037]
N is an austenite-generating element, so when added excessively to the Fe-Cr ferritic steel of the present invention, not only does the austenite phase form and the ferrite single phase cannot be maintained, but also Al, Cr, etc. In order to form inclusions and impair hot and cold workability, the content is limited to 0.050% or less. Desirably, it is 0.020% or less, and more desirably 0.010% 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 to reduce the contact area between the oxide film and the electrode. In order to degrade the contact resistance, it is better to limit the impurities to 0.0030% or less and reduce them to 0% as much as possible. A desirable upper limit is 0.0020% or less, more desirably less than 0.0010%.
[0038]
  In the steel of the present invention, Y, rare earth element (REM), and Zr, which have a great effect on improving the oxidation resistance and the electrical conductivity of the oxide film, are particularly effective for the sulfide inclusions. It is necessary to prevent it from being completely fixed to the oxide inclusions. To that end, (1)~ (3)As shown in the formulaZr and La, orY, Zr,La or Zr and La are requiredIt is effective to keep the ratio of the amount of S and O to the amount of rare earth element (REM) added low.
  (1)~ (3)When the value of the formula exceeds 2.0, Y, Zr, and rare earth elements (REM) are fixed to inclusions and do not contribute to the improvement of oxidation resistance and the electrical conductivity of the oxide film. (1)~ (3)The value of the formula was 2.0 or less.
[0039]
The following elements may be included in the steel of the present invention within the following range.
P ≤ 0.04%, Cu ≤ 0.30%, Mg ≤ 0.02%, Ca ≤ 0.02%, Co ≤ 2%
[0040]
Next, in order to use the steel having the above-described composition to produce a solid oxide fuel cell separator steel, it is necessary to form a gas flow path by cold plastic working or machining. It is important to have impact characteristics that can be easily machined and that the steel does not crack due to the tensile stress associated with the thermal cycle.
In the present invention, this index is defined using hardness, grain size and impact characteristics.
Hardness, crystal grain size, and impact characteristics, which are characteristics required for the steel for solid oxide fuel cell separators of the present invention, are not determined only by the alloy composition, but depend on the plastic processing method of the material, heat treatment conditions such as annealing, etc. In order to use it as a steel for solid oxide fuel cell separators, it is important to satisfy not only the chemical components but also the hardness, crystal grain size, impact characteristics, etc., to meet the specified range of the present invention. It is.
[0041]
In order to satisfy the above-described hardness, crystal grain size, and impact characteristics, annealing is preferably performed in order to remove processing strain generated in the raw material manufacturing process and remaining in the material. When the annealing temperature is higher than 950 ° C, the crystal grains described later become coarse, and when the annealing temperature is lower than 650 ° C, it takes a long time for softening. Since there is a possibility of embrittlement, the annealing temperature is preferably in the range of 650 to 950 ° C.
Note that the annealing holding time should be short in order not to coarsen the crystal grains at high temperatures, while it takes a long time to reduce the hardness at low temperatures. It can be appropriately selected from the relationship.
[0042]
If the hardness after annealing is higher than 280HV, it takes time for these processes or the shape accuracy is lowered. Therefore, the hardness is set to 280HV or less. Desirably, it is 200HV or less. Moreover, if it is in the range of this hardness, in order to correct the deformation | transformation by the thermal strain after annealing, you may perform cold processing, such as cold rolling, after annealing.
[0043]
The solid oxide fuel cell separator steel of the present invention is used at a working temperature of about 700 ° C. or higher. However, since the fuel cell is repeatedly operated and stopped, the separator steel has a working temperature and a normal temperature. Repeatedly undergoes a heat cycle between heating and cooling. In particular, since tensile stress acts during cooling, it is sufficient that the impact characteristics at normal temperature are good in order to prevent cracking during cooling.
In order to obtain good impact characteristics in a steel having a ferrite structure such as the solid oxide fuel cell separator steel of the present invention, it is effective to refine the ferrite crystal grains. In order to obtain impact characteristics described later, it is necessary to make the average ferrite grain size finer than ASTM No. 2. Desirably, fine grains having an average ferrite grain size of ASTM No. 3 or more are preferable.
Impact characteristics can be evaluated by 2mmV notch Charpy impact value at 20 ℃, impact value is 8J / cm²More than that, preferably 10J / cm²The above is good.
[0044]
Next, the solid oxide fuel cell separator steel of the present invention has excellent electrical resistance particularly in the temperature range of 700 to 950 ° C. Therefore, it was defined as follows as an index indicating the excellent electrical resistance.
As an evaluation means for evaluating electrical conductivity, the electrical resistance of the oxide film at 750 ° C after heating for 1000 hours at 750 ° C is 100 mΩ · cm²Below, preferably 50mΩ ・ cm²It is important that:
In addition, as a means of evaluating the phenomenon that the oxidation of the formed Cr-based oxide film progresses and becomes a surface oxide scale after long-term use, the surface oxide scale peels substantially after heating at 850 ° C. for 100 hours. It is important that it does not occur. Note that “the surface oxide scale does not peel substantially” means that there is no natural peeling of the scale, and means that no external impact is applied.
[0045]
The steel of the present invention is a material suitable for a solid oxide fuel cell separator, and is often processed into a steel plate or steel strip. It can be used by processing into various shapes such as steel bars, wire rods, powders, powder sintered bodies, porous bodies, steel foils, etc., for parts for other uses that can make the best use of.
[0046]
【Example】
Example 1
  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 the impurity element low within the specified range, raw materials with high purity were selected and operation conditions such as the atmosphere in the furnace were controlled to suppress mixing and remaining of Ti, S, O, N, B, etc. . 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. Tables 1 and 2 show the steel Nos. 1-23, Comparative Alloy No. The chemical composition of 31-40 is shown. In Table 2, the comparative steel No. Reference numeral 40 denotes an austenitic alloy known as JIS NCF600.
  The values of the formulas shown in the table are as follows: Zr and La composite additive alloy (1) formula, Y, Zr, La composite additive alloy (2) formula, rare earth element (REM) essentially including Zr and La The value of the composite additive alloy is the value calculated by equation (3).
  Moreover, about the alloy which does not add La essentially in the table | surface, it calculated by the formula shown below.
(O + 2S) / (0.27Y + 0.035Zr + 0.16REM) ≦ 2.0
The element of the formula is% by mass, and the calculation was performed with 0 for the additive-free element among Y, Zr, and rare earth element (REM) in the above formula.
[0047]
[Table 1]
[0048]
[Table 2]
[0049]
Test pieces were cut out from these materials and subjected to various tests.
First, the optical cross-sectional structure observation of the longitudinal section of these steels was performed, and the average ferrite crystal grain size was measured. In addition, a 2 mm V notch Charpy test piece was collected and a 2 mm V notch Charpy impact test was performed at 20 ° C. to determine the impact value. Further, using a cylindrical test piece having a diameter of 10 mm and a length of 20 mm, heat treatment was performed at 850 ° C. × 100 Hr and 750 ° C. and 1000 Hr in the air, and then the amount of peeling of the surface oxide scale was measured.
10mm^W× 10mm^L× 3mm^tThe plate-shaped sample was heated at 1000 ° C. in the air at 750 ° C. to form an oxide film on the surface, and then the electrical resistance at 750 ° C. was measured. The electrical resistance is area resistance (mΩ ・ cm²)
Further, the average thermal expansion coefficient from 30 ° C. to 750 ° C. was measured. The test results are summarized in Table 3 and Table 4.
[0050]
[Table 3]
[0051]
[Table 4]
[0052]
From Table 3, the steels of the present invention all have a hardness after annealing of 280 HV or less, an average ferrite grain size of ASTM No. 2 or more, and a 2 mmV notch Charpy impact value of 8 J / cm.²It has the above value, Charpy impact value is 10J / cm²There are more.
From Table 3, the steel of the present invention shows no separation of scale after heating in the atmosphere at 750 ° C. × 1000 Hr and 850 ° C. × 100 Hr.
Further, the steel of the present invention has a sufficiently small electric resistance value measured at 750 ° C. after an oxide film is formed on the surface by heating at 750 ° C. in air for 1000 hours. This is mainly due to the thin dense Cr on the surface₂O_ThreeThis is probably because a film is formed.
Furthermore, the steel according to the present invention has an average coefficient of thermal expansion of about 11 × 10 5 up to 30 to 750 ° C.^-6It is close to stabilized zirconia, a solid electrolyte, as small as / ° C.
[0053]
On the other hand, it can be seen from Table 4 that Comparative Steel No. 31 has a small amount of Cr and therefore has a large amount of peeling and cannot withstand long-term use. Since Comparative Steel No. 32 has a large amount of Cr, scale peeling is not observed at 750 ° C, but a small amount of scale peeling is observed at 850 ° C.
Since comparative steel No.33 contains more than 3% Al, Al₂O_ThreeA film is formed and the value of electrical resistance is much greater than that of the steel of the present invention. Also, comparative steel No. 34 has high Si and probably thick SiO₂It seems that a film is formed and the value of electric resistance is high.
In comparison steel Nos. 35 and 36, the amount of O is large and the amount of Zr, Y and rare earth elements (REM) added is small, so the value of formula (1) is large, which is effective for oxidation resistance. The effects of Zr, Y, and rare earth elements (REM) with a certain amount cannot be sufficiently exhibited, scale peeling is observed, and the electrical resistance is high.
[0054]
  Comparative steel No. Since 37 contains a large amount of B, a dense oxide scale is not formed, and a part of the scale is peeled off and the electrical resistance is high. Comparative steel No. 38 contains a large amount of S, so that the value of the formula (1) becomes large, the effect of rare earth elements (REM) having an effect on oxidation resistance cannot be sufficiently exhibited, peeling of the scale is observed, and electric resistance Is also high.
  Comparative steel No. 39 contains a lot of Ti, andOf the above formulaSince the value is large, peeling of the scale is observed and the electric resistance is high. Comparative steel No. No oxidation scale peeling was observed for 40, and the electrical resistance was low, but since it was an austenitic Ni-based alloy, the thermal expansion coefficient was very large.
[0055]
  Reference Example No. 3,Invention steel No. 5, 23 and B, comparative steel No. 1-4 are cross-sectional micrographs and schematic views of the oxide film after heating 37 at 1000 ° C. for 100 hours.
  As shown in FIGS. 1 to 3, the steel according to the present invention has a smooth surface of the oxide film (1) and a good adhesion because the matrix (2) has entered the oxide film. Comparative steel No. 4 containing B shown in FIG. It can be seen that the surface of the oxide film (1) 37 is not smooth and uneven, and the matrix (2) does not bite into the oxide film. For this reason, since it is thought that the comparative steel containing B not only has a high contact resistance but also a poor adhesion of the oxide film, it is important to keep B as low as possible as an impurity.
[0056]
(Example 2)
  Reference Example No. 1-3Invention steel No. 4, 5After hot forging, it was finished into a plate shape having a thickness of 5 mm by hot rolling and annealed at 780 ° C. for 1 hour. Further, cold working and annealing were repeated to finish a plate shape having a thickness of 1 mm and 0.3 mm, followed by annealing at 850 ° C. for 3 minutes and 2 minutes, respectively. The optical cross-sectional observation of the longitudinal section of these steels was performed, and the grain size was measured.
  Moreover, the 2 mmV notch Charpy impact test was done at 20 degreeC about the 30 mm square forging material and the hot rolled material of thickness 5mm which annealed by changing annealing temperature, and the impact value was calculated | required.
  These results are summarized in Table 5.
[0057]
[Table 5]
[0058]
In either case, the hardness is 280HV or less and the average ferrite grain size is ASTM No. 3 or more. The degree and very fine. The 2mmV notch Charpy impact value of a 5mm thick plate is 10J / cm.²As described above, the impact characteristics are also good.
[0059]
【The invention's effect】
As described above, the steel according to the present invention is used for a separator of a solid oxide fuel cell, thereby forming an oxide film having good electrical conductivity in the vicinity of 700 to 950 ° C., and also having good acid resistance even when used for a long time. It operates at a relatively low temperature of about 700 to 950 ° C because it has low diffusivity, particularly peel resistance, has a small difference in thermal expansion from the electrolyte, and can reduce the cost and performance of the fuel cell. This can greatly contribute to the practical application, high efficiency, and enlargement of solid oxide fuel cells.
[Brief description of the drawings]
[Figure 1]Reference exampleIt is a cross-sectional microscope picture of the steel for solid oxide type fuel cell separators.
FIG. 2 is a cross-sectional micrograph of steel for a solid oxide fuel cell separator of the present invention.
FIG. 3 is a cross-sectional micrograph of steel for a solid oxide fuel cell separator of the present invention.
FIG. 4 is a cross-sectional micrograph of a comparative steel.

Claims (13)

In mass%, C: 0.2% or less, Si: less than 0.2% (not including 0%), Mn: 1.0% or less (not including 0%), Ni: 2% or less, Cr 15 to 30%, Al: 1% or less, La: 0.005 to 0.1%, Zr: 1% or less, and the balance is Fe and inevitable impurities. 1% or less, S: 0.015% or less, O: 0.010% or less, N: 0.050% or less, B: 0.0030% or less, and satisfying the formula (1) , The hardness is 280 HV or less, the average ferrite grain size is ASTM No. A steel for a solid oxide fuel cell separator, wherein the steel is a fine particle of 2 or more.
(O + 2S) / ( 0.035Zr + 0.16 La ) ≦ 2.0 (1)
The element of the formula (1) is mass%. In mass%, C: 0.1% or less, Si: less than 0.2% (not including 0%), Mn: 1.0% or less (not including 0%), Cr: 17 to 26%, Ni: 1% or less, Al: 0.001% or more and less than 0.5%, Zr: 0.01 to 0.8%, La: 0.005 to 0.8%, the balance being Fe and inevitable impurities As unavoidable impurities, Ti is limited to 0.1% or less, S: 0.015% or less, O: 0.010% or less, N: 0.020% or less, B: 0.0030% or less. And having a hardness of 280HV or less and an average ferrite grain size of ASTM No. A steel for a solid oxide fuel cell separator, wherein the steel is a fine particle of 2 or more.
(O + 2S) / ( 0.035Zr + 0.16 La ) ≦ 2.0 (1)
The element of the formula (1) is mass%. In mass%, C: 0.1% or less, Si: less than 0.2% (not including 0%), Mn: less than 0.2% (not including 0%), Cr: 17 to 26%, Ni: 1% or less, Al: 0.001% or more and less than 0.5%, Zr: 0.01 to 0.8%, La: 0.005 to 0.8%, the balance being Fe and inevitable impurities As unavoidable impurities, Ti is limited to 0.1% or less, S: 0.015% or less, O: 0.010% or less, N: 0.020% or less, B: 0.0030% or less. And having a hardness of 280HV or less and an average ferrite grain size of ASTM No. A steel for a solid oxide fuel cell separator, wherein the steel is a fine particle of 2 or more.
(O + 2S) / ( 0.035Zr + 0.16 La ) ≦ 2.0 (1)
The element of the formula (1) is mass%. The steel for a solid oxide fuel cell separator according to any one of claims 1 to 3, further comprising Y: 0.01 to 0.3% and satisfying the formula (2) .
(O + 2S) / (0.27Y + 0.035Zr + 0.16La) ≦ 2.0 (2) In mass%, C: 0.08% or less, Si: less than 0.2% (not including 0%), Mn: 0.5% or less (not including 0%), Cr: 18 to 25%, Ni: 0.5% or less, Al: 0.001% or more and less than 0.5%, La: 0.005 to 0.1%, Zr: 0.01 to 0.6%, the balance being Fe and inevitable impurities Inevitable impurities are limited to Ti: 0.1% or less, S: 0.008% or less, O: 0.008% or less, N: 0.020% or less, B: 0.0020% or less. and (1) made of steel which satisfies the equation, hardness is 280HV or less, the average ferrite crystal grain size is ASTM No. A steel for a solid oxide fuel cell separator, wherein the steel is a fine particle of 2 or more.
(O + 2S) / (0.035Zr + 0.16La) ≦ 2.0 ( 1 ) In mass%, C: 0.08% or less, Si: less than 0.2% (not including 0%), Mn: 0.5% or less (not including 0%), Cr: 18 to 25%, Ni: 0.5% or less, Al: 0.001% or more and less than 0.5%, La: 0.005 to 0.1%, Zr: 0.01 to 0.6%, the balance being Fe and inevitable impurities Inevitable impurities are limited to Ti: 0.1% or less, S: 0.008% or less, O: 0.008% or less, N: 0.020% or less, B: less than 0.0010%. and (1) made of steel which satisfies the equation, hardness is 280HV or less, the average ferrite crystal grain size is ASTM No. A steel for a solid oxide fuel cell separator, characterized by being 3 or more fine grains.
(O + 2S) / (0.035Zr + 0.16La) ≦ 2.0 ( 1 )   The solid oxide fuel cell separator according to any one of claims 1 to 6, characterized by containing Mo + 1 / 2W ≤ 5.0% in terms of mass%, including Mo alone or Mo and W. steel. At mass%, V, Nb, solid oxide fuel as claimed in any one of claims 1 to 7, characterized in that it comprises 0.01% to 1.0% in total of one or two or more H f Steel for battery separator. 9. The steel for a solid oxide fuel cell separator according to claim 1, wherein a ² mmV notch Charpy impact value at 20 ° C. is 8 J / cm ² or more. 9. The steel for a solid oxide fuel cell separator according to claim 1, wherein a ² mmV notch Charpy impact value at 20 ° C. is 10 J / cm ² or more. And 1000Hr the electrical resistance of the oxide film at 750 ° C. after heating is 100 m [Omega · cm ² or less at 750 ° C., to claim 1 further exfoliation of surface oxide scale after 100Hr heated at 850 ° C. is characterized in that it does not occur The solid oxide fuel cell separator steel according to any one of 10. And 1000Hr the electrical resistance of the oxide film at 750 ° C. after heating is 50 m [Omega · cm ² or less at 750 ° C., to claim 1 further exfoliation of surface oxide scale after 100Hr heated at 850 ° C. is characterized in that it does not occur The steel for a solid oxide fuel cell separator according to any one of 11. A total of 0.005 to 0.1% of La and rare earth elements other than La (REM) are further satisfied, and the formula (3) is satisfied. Steel for solid oxide fuel cell separator.
(O + 2S) / (0.035Zr + 0.16REM) ≦ 2.0 (3)