JP4675066B2 - Ferritic stainless steel for solid oxide fuel cell separator - Google Patents

Description

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

In recent years, due to problems such as the depletion of fossil fuels typified by petroleum and the global warming phenomenon due to CO ₂ emissions, there has been a demand for practical use of a new system that replaces the conventional power generation system. As one of them, “fuel cell”, which has high practical value as a distributed power source and a power source for automobiles, is attracting attention.
There are several types of fuel cells. Among them, the solid oxide fuel cell (SOFC) has high energy efficiency and is the most promising fuel cell.

  The operating temperature of a solid oxide fuel cell has conventionally been as high as about 1000 ° C., and ceramics have been mainly used for the separator. However, in recent years, solid oxide fuel cells that operate at a low temperature of 600 to 800 ° C. have been developed by improving the solid electrolyte membrane (low temperature operation type SOFC). In this temperature range, metal materials can be applied. However, it is not sufficient that the metal material is simply excellent in high-temperature oxidation characteristics, and it is necessary to develop a material having the following characteristics.

First, it is necessary to have excellent “water vapor oxidation resistance” in a temperature range of 600 to 800 ° C. Also, it has a “thermal expansion coefficient” (about 13 × 10 ⁻⁶ (1 / K) from room temperature to 800 ° C.) equivalent to ceramic solid oxide. It must also have excellent thermal fatigue properties. In addition to these basic characteristics, it is desired to exhibit excellent electrical conductivity in a state of being in close contact with the ceramic solid oxide in a temperature range of 600 to 800 ° C.

  As a metal material excellent in steam oxidation resistance at high temperature, there is a high Cr high Ni type austenitic stainless steel. However, since this has a large coefficient of thermal expansion, thermal deformation and scale peeling occur due to repeated thermal expansion and contraction under conditions where starting and stopping are frequently performed and cannot be used. Ferritic stainless steel, on the other hand, has a thermal expansion coefficient comparable to that of ceramic solid oxides. It can be said that it is an optimal material.

  Patent Document 1 listed below discloses a ferritic steel for a solid oxide fuel cell separator in which an oxide film having good electrical conductivity is formed in a temperature range of 700 to 950 ° C.

JP 2003-105503 A

The ferritic steel of Patent Document 1 essentially contains Zr in order to improve the electrical conductivity of the oxide film and to improve the film adhesion. However, Zr is an expensive element, and it is expected that a steel having a component design that can remarkably improve electrical conductivity at high temperatures without adding such an element is expected. Further, in applications where there are many start / stop operations, further improvement of thermal fatigue characteristics is desired.
The present invention intends to develop and provide a steel for a solid oxide fuel cell separator having a basic composition that does not require an expensive element and simultaneously improving electrical conductivity and thermal fatigue characteristics at high temperatures.

  As a result of detailed studies, the inventors have found that it is possible to significantly improve the electrical conductivity in the range of 600 to 800 ° C. in the ferritic stainless steel to which Cu is added. It was also found that Cu addition is very effective for improving thermal fatigue characteristics. Furthermore, the steam oxidation resistance could be sufficiently secured by controlling the alloy composition. The present invention has been completed based on such findings.

That is, the steel provided by the present invention is, in mass%, C: 0.05% or less, Si: 1.5% or less, Mn: 1.5% or less, S: 0.01% or less, Cr: 15 to 15%. 40%, Cu: 0.4 to 4%, N: 0.05% or less, Mo: 0 to 4%, W: 0 to 4%, Nb: 0 to 0.8%, Ti: 0 to 0.5 %, Zr: 0 to 0.5%, V: 0 to 0.5%, Ta: 0 to 0.5%, Al: 0 to 0.5 %, Y: 0 to 0.1%, REM (rare earth) Element): 0 to 0.1%, Ca: 0 to 0.01%, B: 0 to 0.01%, ferritic stainless steel for solid oxide fuel cell separator having composition of balance Fe and inevitable impurities It is.

Here, Mo, W, Nb, Ti, Zr, V, Ta, Al, Y, REM, Ca, and B are arbitrarily added elements. The lower limit of the content of these elements of 0% means a case where the content is below the measurement limit in the analytical method performed in a normal steelmaking process.
As required, Mo: 0.1-4%, W: 0.1-4%, Nb: 0.05-0.8%, Ti: 0.03-0.5%, Zr: 0.05 The composition can satisfy 1 or 2 or more of -0.5%, V: 0.05-0.5%, Ta: 0.05-0.5%.
Furthermore, Al: 0.02~ 0.5%, Y : 0.0005~0.1%, REM: 0.0005~0.1%, Ca: 0.0005~0.01%, B: 0. It is good also as a composition which satisfy | fills 1 or 2 or more of 0002-0.01%.

Furthermore, the present invention provides the above steel having an electrical resistivity X at 750 ° C. of 30 mΩ · cm ² or less according to the test (A) below.
(A) A “sample disk” having a thickness of 1.5 mm and a diameter of 20 mm of double-sided # 400 wet-polishing finished from a cold-rolled annealed steel plate made of the steel, and a solid oxide made of yttria-stabilized zirconia (solid electrolyte) Two “zirconia disks” with a diameter of 20 mm and two “platinum electrode plates” with a diameter of 20 mm or more are prepared and stacked in order from the bottom: “lower platinum electrode plate—lower zirconia disk—sample disk” “Upper zirconia disk-upper platinum electrode plate” is configured, and a weight is placed on the upper platinum electrode plate so that the surface pressure between the sample disk and the upper zirconia disk is 1.9 kg / cm ^2. Then, an electric circuit was constructed so that a constant current could flow between the lower platinum electrode plate and the upper platinum electrode plate. In this state, the laminate was held at 750 ° C. in the atmosphere for 1000 hours, and then in that state 750 Measure the potential difference between the two platinum electrode plates when a constant current of 10 mA was passed between the two platinum electrode plates at ℃, and based on that value, calculate the area resistivity (mΩ · cm ² ) per one side of the sample disk This is the electric resistivity X.

  According to the present invention, a ferritic stainless steel exhibiting high electrical conductivity in the temperature range of 600 to 800 ° C. and excellent in thermal fatigue characteristics can be provided by an alloy design that does not require a special element. A special element can be further added based on a basic composition composed of relatively inexpensive elements. For this reason, a desired characteristic can be strengthened as needed, and the flexible response | compatibility according to a use is attained. In other words, an alloy having a cheaper composition than the conventional one can be provided for applications where it is sufficient to have the basic performance necessary for a solid oxide fuel cell separator. Therefore, the present invention contributes to the spread of solid oxide fuel cells.

  In general, when ferritic stainless steel is exposed to a high-temperature steam atmosphere (600 to 800 ° C.) of a fuel cell, oxidation proceeds easily and the electrical resistance of the conductive portion increases. When such a material is used for the separator, the function of the fuel cell is impaired. Further, when starting and stopping are frequently repeated, damage due to thermal fatigue becomes a problem. These have made it difficult to apply to ferritic stainless steel separators.

As a technique for improving the electrical conductivity of a ferritic stainless steel material at a high temperature at which steam oxidation is likely to occur, a second layer (for example, an oxide layer) having good conductivity is formed on the surface layer of the material, and the second layer is formed. It is considered to be effective to conduct electric conduction through. The steel of Patent Document 1 attempts to realize this by incorporating Zr. In the present invention, this is realized by a method of adding inexpensive Cu.
On the other hand, the addition of Cu is effective in improving the thermal fatigue characteristics by the action of solid solution strengthening or precipitation strengthening. That is, in the present invention, “improvement of electrical conductivity at high temperature” and “improvement of thermal fatigue characteristics” are achieved at once by utilizing the action of Cu. For this reason, a simple basic composition that does not require the inclusion of special elements could be constructed.

  Cu is known to exhibit an effect of increasing the electrical conductivity of the steel at room temperature when it exists in the steel as a metallic phase or a Cu-rich phase (these are sometimes called ε-Cu phases). . However, when such steel is exposed to a high temperature of 600 to 800 ° C., it is not clear how the electrical conductivity in that temperature range behaves. Detailed studies by the inventors have shown that Cu in ferritic stainless steel significantly improves electrical conductivity at high temperatures.

The mechanism is still unclear, but part of the metal Cu phase or Cu rich phase (hereinafter referred to as “Cu precipitation phase”) finely dispersed in the steel is exposed on the surface. The Cu precipitate phase is incorporated into the oxide film formed during holding at a high temperature, or the Cu precipitate phase is present in the oxide film in the form of a highly conductive oxide. It is possible. In the case of the former Cu precipitation phase, since it is originally excellent in conductivity, it can be considered that it contributes to the improvement of the electrical conductivity of the high temperature material. On the other hand, even in the case of the latter Cu oxide, for example, the electrical resistance of CuO at 700 ° C. is about 10 ⁻² Ω · m, and Cr ₂ O ₃ (10 ³ to 10 ² Ω · m) which is the main component of the oxide film. It is thought that it can contribute to the improvement of electrical conductivity at high temperatures.

As a result of the study by the inventors, it is desired that the solid oxide fuel cell separator material has high-temperature electrical conductivity at which the electrical resistivity X at 750 ° C. according to the test (A) is 30 mΩ · cm ² or less. I understood. In order to realize this, it is necessary to contain at least 0.4% by mass of Cu, but it is desirable to ensure a Cu content exceeding 0.5% by mass. It has also been found that the basic heat fatigue resistance desired for applications with many starting and stopping can be ensured by containing such an amount of Cu. More preferably, the Cu content exceeds 1% by mass. However, since excessive Cu content hardens the steel material excessively, the upper limit of the Cu content is limited to 4% by mass, and more preferably 3% by mass or less.

  In terms of metal structure, it is desirable that the Cu precipitation phase in the steel is contained by 0.05% or more by volume ratio. According to the study by the inventors, for example, in the case of ferritic stainless steel containing Cu exceeding 0.5% by mass, the operating temperature of the low-temperature operating SOFC is 600 to 800 ° C., which is an extremely short time of less than 1 hour. A Cu precipitation phase is generated by heating and holding. That is, it is possible to achieve both good electrical conductivity during high-temperature use and excellent thermal fatigue characteristics by normal startup and operation without performing special heat treatment before operation.

Hereinafter, constituent elements other than Cu will be described.
C and N are elements that improve high-temperature strength, particularly creep properties, but if added excessively to ferritic stainless steel, workability and low-temperature toughness may be significantly reduced. In addition, carbonitrides are likely to be generated by reaction with Ti and Nb, which causes a decrease in solid solution Ti and solid solution Nb effective in improving high temperature strength. Therefore, it is preferable that both C and N are small, and both are limited to 0.05% by mass or less. C is more preferably 0.03 mass% or less, and N is more preferably 0.03 mass% or less.

Si promotes stabilization of the Cr-based oxide and is effective in improving the steam oxidation resistance. However, if an excessive amount of Si is contained, a SiO ₂ oxide layer having a high electrical resistance is formed on the surface layer, and the electrical conductivity is lowered. Moreover, low temperature toughness is lowered, so that flaws are easily generated on the surface of the steel material, and the productivity is also lowered. For this reason, Si content is restrict | limited to 1.5 mass% or less, and it is preferable to set it as 1 mass% or less.

  Mn is an element that improves the scale peel resistance of ferritic stainless steel, but excessive Mn hardens the steel material and leads to a decrease in workability and low-temperature toughness. For this reason, Mn content is restrict | limited to 1.5 mass% or less.

  S is an element that adversely affects hot workability and weld hot cracking resistance, and also serves as a starting point for abnormal oxidation. For this reason, S is preferably reduced as much as possible, and is limited to 0.01% by mass or less.

  Cr is an alloy component necessary for imparting corrosion resistance, oxidation resistance, and electrical conductivity necessary for stainless steel. In order to ensure the steam oxidation resistance in the 600-800 degreeC range and favorable electrical conductivity, 15 mass% or more of Cr is required. In particular, when emphasizing excellent durability in a water vapor atmosphere, a Cr content of 20 mass% or more is preferable. However, if a large amount of Cr is contained, the workability and low-temperature toughness of ferritic stainless steel are lowered and the 475 ° C. embrittlement susceptibility is increased, so the content is limited to 40% by mass or less. Practically, it is preferably 30% by mass or less, and 25% by mass or less is often sufficient.

  Mo and W have the effect of improving high temperature strength and heat fatigue resistance by solid solution strengthening. In particular, the use of these elements is effective in applications that place importance on thermal fatigue characteristics. Mo and W may be added singly or in combination of two. The addition amount is preferably 0.1% by mass or more for both Mo and W. However, since excessive Mo and W harden the steel material excessively, these are all limited to a content of 4% by mass or less, and preferably 3% by mass or less.

  Nb, Ti, Zr, soot, and Ta have the effect of improving the high temperature strength of the ferritic stainless steel by solid solution or precipitation strengthening, and can be added as necessary. These may be added singly or in combination of two or more. In any case, the effect of addition becomes remarkable at 0.03 mass% or more. However, when these elements are contained in a large amount, the steel material is excessively hardened. Therefore, the upper limit of the Nb content is 0.8% by mass, and the upper limits of the contents of Ti, Zr, ∨, and Ta are all 0.5. Limited to mass%.

Al is an element that not only is used as a deoxidizing material but also remarkably improves high-temperature oxidation resistance, and is added as necessary. In order to fully exhibit these effects, it is desirable to contain 0.02% by mass or more of Al. However, a large amount of Al content deteriorates workability and weldability, and also impairs the electrical conductivity of the separator. In the present invention, the Al content is 0.5 mass% or less.

  Y, REM (rare earth elements; La, Ce, Nd, etc.) and Ca are elements that have the effect of strengthening the oxide film and further improving the oxidation resistance of ferritic stainless steel, and should be added as necessary. Can do. These may be added singly or in combination of two or more. Y and Ca are 0.0005 mass% or more, and the total content of REM is 0.0005 mass% or more, and the effect of addition becomes remarkable. However, if these elements are contained in a large amount, the steel material is excessively hardened, and surface flaws are likely to occur during the manufacture of the steel material. Therefore, the upper limit of the Y content is 0.1% by mass, and the upper limit of the REM content is the total content. The upper limit of Ca content is limited to 0.01% by mass.

  B is an element that improves the hot workability of stainless steel, and is added as necessary. In order to sufficiently exhibit the effect, it is desirable to secure a B content of 0.0002% by mass or more. However, if excessively added, the hot workability is deteriorated and the surface properties of the steel material are deteriorated. Therefore, when B is added, it is necessary to carry out within a range of 0.01% by mass or less.

  In addition, if necessary, Re is effective for improving strength, Sn is effective for improving free-cutting properties, Co, Hf, Sc and the like are effective for improving hot workability, for example, Re: 2 mass% or less, Sn 1% by mass or less, Co: 2% by mass or less, Hf: 1% by mass or less, and Sc: 0.1% by mass or less. It is preferable to reduce general impurity elements such as P, O, and Ni as much as possible. Usually, P is controlled to 0.1 mass% or less, O: 0.02 mass% or less, and Ni: 2 mass% or less. However, in order to ensure a high level of workability and weldability, these impurities It is desirable to regulate the content more strictly.

  The surface of the separator material is preferably mechanically polished so that the arithmetic average roughness Ra specified in JIS B 0601 is 0.05 to 50 μm. Thereby, the conductivity at the time of use can be improved at a high temperature by an action such as an improvement in adhesion between the solid electrolyte and the separator material.

  Ferritic stainless steel having the chemical composition shown in Table 1 was melted in a 30 kg vacuum melting furnace to obtain an ingot. A material for hot rolling was cut out from each ingot, and a cold-rolled annealed steel sheet having a thickness of 1.5 mm was manufactured in the steps of hot rolling → annealing → pickling → cold rolling → finish annealing.

A disk with a diameter of 20 mm was cut out from each cold-rolled annealed steel sheet, and both surfaces thereof were polished and finished with a # 400 count by wet polishing. The polished surface of each sample exhibited a surface roughness with Ra in the range of 0.2 to 0.6 μm. Using each of these disks (diameter 20 mm, plate thickness 1.5 mm) as a “sample disk”, the electrical resistivity X (mΩ · cm ² ) at 750 ° C. was determined in the following manner.

For each measurement sample, two “zirconia disks” with a diameter of 20 mm and a thickness of 3 mm made of yttria-stabilized zirconia solid oxide and two “platinum electrode plates” with a thickness of about 20 × 20 mm and a thickness of 0.5 mm were obtained. Prepared. A lead wire made of a platinum wire having a diameter of 0.5 mm was attached to the platinum electrode plate by spot welding. In an electric furnace, a platinum electrode plate, a zirconia disk, a sample disk as a material to be measured, a zirconia disk, and a platinum electrode plate were stacked in this order from the bottom. At that time, the entire surface of the zirconia disk on the side of the platinum electrode plate was in contact with the platinum electrode plate. Also, the center of the two zirconia disks and the sample disk was kept from shifting. By placing a weight on the upper platinum electrode plate of the laminate composed of “lower platinum electrode plate—lower zirconia disk—sample disk—upper zirconia disk—upper platinum electrode plate” thus configured, the sample disk and The surface pressure between the upper zirconia disk was set to 1.9 kg / cm ² . In addition, an electric circuit was configured through lead wires so that a constant current could flow between the upper platinum electrode plate and the lower platinum electrode plate. In this state, the temperature of the electric furnace was raised, and the laminate was held at 750 ° C. in the atmosphere for 1000 hours, and then both platinum electrode plates when a constant current of 10 mA was passed between both platinum electrode plates at 750 ° C. The potential difference between them was measured. Based on the value, the area resistivity (mΩ · cm ² ) per one side of the sample disk was obtained, and this was designated as electric resistivity X.

A sample having an electrical resistivity X at 750 ° C. of 30 mΩ · cm ² or less was evaluated as good (evaluation), and a sample having an electrical resistivity X exceeding 30 mΩ · cm ² was evaluated as poor (x evaluation). The results are shown in Table 2.

  As can be seen from Table 2, the steel according to the present invention containing a predetermined amount of Cu is significantly and stably improved in electrical conductivity at high temperatures compared to comparative steels with insufficient Cu content, and is solid oxide type. The conductivity desired for the fuel cell separator was satisfied.

  A material for hot forging was cut out from each ingot of ferritic stainless steel in Table 1 as in Example 1, and a round bar having a diameter of 30 mm was produced by hot forging. This was heat treated at 1000 ° C. for 0.5 hour, and then processed into a round bar test piece (diameter 10 mm, strain gauge 15 mm) for thermal fatigue test. Using a servo-type thermal fatigue tester, “Raise the temperature from 200 ° C. to 800 ° C. at 3 ° C./second → Hold at 800 ° C. for 30 seconds → Drop the temperature from 800 ° C. to 200 ° C. at 3 ° C./second → Hold at 200 ° C. for 30 seconds The heat pattern was repeated for 500 cycles in a state where the restraint rate was 100% (restrained so that expansion and contraction in the longitudinal direction of the test piece did not occur during the test).

  Those in which breakage did not occur in 500 cycles were evaluated as good (◯ evaluation), and those in which breakage occurred were evaluated as poor (× evaluation). The results are shown in Table 3.

  As can be seen from Table 3, all of the steels of the present invention containing a predetermined amount of Cu exhibited good thermal fatigue properties. These satisfied the thermal fatigue properties desired for solid oxide fuel cell separator materials used in many applications of starting and stopping. In contrast, Comparative Steel Nos. 11, 12, and 14 were insufficient in Cu content, so the thermal fatigue characteristics were not sufficiently improved. The comparative steels Nos. 13 and 15 showed good thermal fatigue characteristics, which is considered to be because Mo contained in about 2% improved the thermal fatigue characteristics instead of Cu. However, Nos. 13 and 15 have a low Cu content and therefore have insufficient electrical conductivity at high temperatures (Table 2) and are not suitable for solid oxide fuel cell separator materials.

With respect to the steels of the present invention shown in Table 1, the high temperature steam oxidation resistance was examined using the cold-rolled annealed steel sheet having a thickness of 1.5 mm manufactured in Example 1.
A polishing finish is prepared by polishing the surface of each cold-rolled annealed steel sheet with a # 400 count according to JIS R 6001, and this is put into a high-temperature steam atmosphere (750 ° C., 50 vol% H ₂ O + 50 vol% N ₂ ). Hold for 200 hours.
As a result, all the steels of the present invention had a mass increase of 0.5 mg / cm ² or less. From many experimental data of the inventors, it is judged that what satisfies this characteristic is a material exhibiting excellent durability that does not cause abnormal oxidation in a high-temperature steam atmosphere at 600 to 800 ° C.

Claims (4)

In mass%, C: 0.05% or less, Si: 1.5% or less, Mn: 1.5% or less, S: 0.01% or less, Cr: 15-40%, Cu: 0.4-4 %, N: 0.05% or less, Mo: 0-4%, W: 0-4%, Nb: 0-0.8%, Ti: 0-0.5%, Zr: 0-0.5% , V: 0 to 0.5%, Ta: 0 to 0.5%, Al: 0 to 0.5 %, Y: 0 to 0.1%, REM: 0 to 0.1%, Ca: 0 to 0% Ferritic stainless steel for solid oxide fuel cell separators having a composition of 0.01%, B: 0 to 0.01%, balance Fe and inevitable impurities. In mass%, C: 0.05% or less, Si: 1.5% or less, Mn: 1.5% or less, S: 0.01% or less, Cr: 15-40%, Cu: 0.4-4 %, N: 0.05% or less, Mo: 0-4%, W: 0-4%, Nb: 0-0.8%, Ti: 0-0.5%, Zr: 0-0.5% , V: 0 to 0.5%, Ta: 0 to 0.5%, Al: 0 to 0.5 %, Y: 0 to 0.1%, REM: 0 to 0.1%, Ca: 0 to 0% A solid having a composition of 0.01%, B: 0 to 0.01%, the balance Fe and inevitable impurities, and an electrical resistivity X at 750 ° C. of 30 mΩ · cm ² or less according to the test (A) below Ferritic stainless steel for oxide fuel cell separators.
(A) A “sample disk” having a thickness of 1.5 mm and a diameter of 20 mm on both sides # 400 wet-polished from a cold-rolled annealed steel sheet made of the steel and a diameter of 20 mm made of yttria-stabilized zirconia solid oxide Two “zirconia disks” and two “platinum electrode plates” with a diameter of 20 mm or more are prepared and stacked in order from the bottom: “lower platinum electrode plate—lower zirconia disk—sample disk—upper zirconia disk” -The upper platinum electrode plate "is configured, and a weight is placed on the upper platinum electrode plate so that the surface pressure between the sample disk and the upper zirconia disk is 1.9 kg / cm ² , An electric circuit is constructed so that a constant current can flow between the platinum electrode plate and the lower platinum electrode plate. In this state, the laminate is held in the atmosphere at 750 ° C. for 1000 hours, and then left in that state at 750 ° C. The potential difference between the two platinum electrode plates when a constant current of 10 mA was passed between the gold electrode plates was measured, and the area resistivity (mΩ · cm ² ) per one side of the sample disk was obtained based on the measured value. Let electric resistivity X.   In mass%, Mo: 0.1-4%, W: 0.1-4%, Nb: 0.05-0.8%, Ti: 0.03-0.5%, Zr: 0.05- The ferrite for a solid oxide fuel cell separator according to claim 1 or 2, satisfying one or more of 0.5%, V: 0.05-0.5%, Ta: 0.05-0.5%. Stainless steel. By mass%, Al: 0.02~ 0.5%, Y: 0.0005~0.1%, REM: 0.0005~0.1%, Ca: 0.0005~0.01%, B: The ferritic stainless steel for a solid oxide fuel cell separator according to claim 1 or 2, which satisfies 1 or 2 of 0.0002 to 0.01%.