JP2009185387A - Ferritic stainless steel for separator of solid oxide fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a ferritic stainless steel for the separator of a solid oxide fuel cell, which presents excellent oxidation resistance in a high-temperature atmosphere containing a large amount of steam, has adequate electroconductivity and is inexpensive. <P>SOLUTION: The ferritic stainless steel has a composition comprising 0.03 mass% or less C, 1.0 mass% or less Si, 1.5 mass% or less Mn, 0.01 mass% or less S, 0.03% or less N, 11.0 to 20.0 mass% Cr, further one or more elements among 3.0 mass% or less Mo, 1.5 mass% or less Cu, 0.05 to 0.80 mass% Nb, 0.03 to 0.50 mass% Ti, 0.001 to 0.1 mass% Y, 0.001 to 0.1 mass% rare earth elements and 0.001 to 0.01 mass% Ca, as needed, and the balance substantially Fe; and has a mechanically polished surface with a surface roughness Ra of 0.05 to 50 μm measured by JIS B0601 through mechanical polishing. <P>COPYRIGHT: (C)2009,JPO&INPIT

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 new systems that replace conventional power generation systems. Fuel cells, which are clean power generation systems, are attracting attention as new power generation systems, distributed power sources, or automobile power sources. There are several types of fuel cells, such as phosphoric acid fuel cells, molten carbonate fuel cells, and polymer electrolyte fuel cells, depending on the electrolyte used. Among them, solid oxide fuel cells (SOFC) has the highest operating temperature and energy efficiency among fuel cells, and is the most promising fuel cell for practical use.

Conventionally, the operating temperature of a solid oxide fuel cell (SOFC) is as high as about 1000 ° C., and ceramics have been mainly used as its constituent members from the viewpoint of long-term reliability. It was difficult to use even high Cr and high Ni austenitic stainless steels with excellent high temperature oxidation properties. However, in recent years, the operating temperature can be lowered to about 600 to 800 ° C. by improving the solid electrolyte membrane. In order to apply a metal material to a separator constituting a low-temperature operation type solid oxide fuel cell (SOFC), good electrical conductivity of 50 mΩ · cm ² or less in a temperature range of 600 to 800 ° C. Excellent steam oxidation resistance even in a high-temperature atmosphere containing 1% water vapor, and furthermore, a thermal expansion coefficient equivalent to that of a ceramic solid electrolyte membrane (approximately 13 × 10 ⁻⁶ (1 / K) at room temperature to 800 ° C.) Satisfactory properties are required.

High Cr, high Ni austenitic stainless steel with excellent steam oxidation resistance at high temperatures has a thermal expansion coefficient about 1.5 times higher than that of solid electrolyte membranes. In the environment, thermal expansion and contraction occur, and thermal deformation and scale peeling occur, making it difficult to use. Ferritic stainless steel that has the same thermal expansion coefficient as that of the solid electrolyte membrane and does not cause thermal deformation or scale peeling even when subjected to thermal expansion or contraction is optimal as a member for a solid oxide fuel cell. . And, for the purpose of adjusting the coefficient of thermal expansion, improving the characteristics of the oxide film formed on the surface, etc., as related to the technology for adjusting the alloy composition of ferritic stainless steel, for example, as seen in Patent Documents 1 to 6, Various proposals have been made.

JP-A-9-157801 JP-A-10-280103 JP-A-8-277441 JP-A-7-145454 JP 2003-36868 A JP 2003-105503 A

The materials proposed in Patent Documents 1 to 6 and the like have characteristics that a thermal expansion coefficient approximates that of a stabilized zirconia that is a solid electrolyte, and an oxide film having good electrical conductivity is formed at about 700 to 950 ° C. It has. For this reason, the material suitable for the separator of the fuel cell which used stabilized zirconia as the electrolyte can be provided. However, the conductive portion of the separator in an actual fuel cell is exposed to an atmosphere containing several tens of percent or more of water vapor. For this reason, the separator is subjected to oxidation peculiar to the water vapor atmosphere (hereinafter referred to as “water vapor oxidation”), and is damaged or the electric resistance is lowered. In order not to cause such a problem, it is necessary to contain at least 20% by mass of Cr. On the other hand, when Cr exceeding 20 mass% is added, the 475 ° C. embrittlement sensitivity is remarkably lowered, and the separator may be cured and broken during use. In addition, the cost increases due to an increase in Cr.

The present invention has been devised to solve such a problem, and suppresses the Cr content to increase the 475 ° C. embrittlement susceptibility, and has excellent steam oxidation resistance, good electrical conductivity, and low thermal expansion. An object of the present invention is to provide a ferritic stainless steel for a solid oxide fuel cell separator that has a coefficient and is inexpensive and excellent in durability.

In order to achieve the object, the ferritic stainless steel for solid oxide fuel cell separator of the present invention has C: 0.03% by mass or less, Si: 1.0% by mass or less, Mn: 1.5% by mass or less , S: 0.01 mass% or less, N: 0.03% or less, Cr: 11.0-20.0 mass%, 3.0 mass% or less Mo, Ca: 0.001-0.01 mass% Or containing 1.5% by mass or less of Cu, with the balance being composed of Fe and inevitable impurities, and with a surface roughness Ra defined by JIS B0601, 0.05 to 50 μm mechanical polishing It has a finished surface. In the ferritic stainless steel of the present invention, Nb: 0.05 to 0.80 mass%, Ti: 0.03 to 0.50 mass%, Y: 0.001 to 0.1 mass%, rare earth element : 0.001 to 0.1% by mass, or one or more of them can also be included.

The ferritic stainless steel of the present invention is a conventional ferritic stainless steel based on the conventional ferritic stainless steel without adding excessive Cr, and the surface thereof is mechanically polished to reinforce the formed oxide film and resist steam oxidation. Can be improved. For this reason, ferritic stainless steel having improved steam oxidation resistance while maintaining good electrical conductivity and thermal expansion coefficient can be obtained at low cost. Therefore, according to the present invention, a separator material having excellent durability is provided at a low cost, so that the cost reduction, performance and durability of the solid oxide fuel cell can be expected. It can contribute to promotion.

In ferritic stainless steel with a Cr content of 20% by mass or less in order to reduce the cost and increase the 475 ° C. embrittlement susceptibility, steam oxidation proceeds easily when exposed to the high-temperature steam atmosphere of the fuel cell. The electrical resistance of the part increases, and the function of the fuel cell is impaired. This is a problem when using ferritic stainless steel. Steam oxidation in a high-temperature atmosphere is more damaging than oxidation in the air, and causes a problem such as deformation and hole opening by reducing the thickness of the steel substrate.

There are several theories regarding the mechanism of promoting oxidation by steam oxidation, such as the theory that steam dissociates into oxygen and hydrogen to promote the oxidation reaction, and the theory that steam reaches the steel substrate directly to promote oxidation. However, the current situation is not necessarily clear. However, the present inventors have speculated that this steam oxidation can be suppressed by stabilizing an oxide film mainly composed of Cr-based oxide formed on the surface of stainless steel. The mechanism and countermeasures will be described below.

In general, an oxide film formed on a stainless steel surface by heating imparts oxidation resistance to the stainless steel, and in a high temperature atmosphere, the oxidation resistance is improved by a Cr content of 11% by mass or more. Become prominent. However, in the case of ferritic stainless steel, when the substrate is exposed to a high temperature steam atmosphere at about 600 ° C., it has a Fe, Mn spinel structure before the Cr oxide in the oxide film is formed. Since a large amount of oxide is generated, the oxide film becomes porous. As a result, the amount of water vapor or oxygen molecules that permeate the oxide film and reach the base steel increases, and the steam oxidation of the base steel proceeds. This is to reduce not only the steam oxidation resistance but also the electrical conductivity. In other words, the Fe, Mn-based oxide layer has many defects such as voids, and the thickness of the oxide film is increased, resulting in poor adhesion to the substrate. As a result, the electrical conductivity is lowered.

In the course of various studies, the present inventors perform mechanical polishing finish before surface oxidation as a means to stabilize the oxide film mainly composed of Cr-based oxide formed on the stainless steel surface. Found that is effective. By performing mechanical polishing, a number of dislocations and slip bands are formed on the metal surface layer, and at a predetermined depth from the surface layer, it is affected by distortion due to polishing. The effect of strain promotes the diffusion of Cr in the surface layer with a predetermined thickness range to the surface layer. As a result, the dense layer of Cr is formed on the surface layer of stainless steel without forming a Fe, Mn-based porous oxide film at the very beginning of oxidation. An oxide film can be formed.

In addition, by applying mechanical polishing, not only the steam oxidation resistance can be improved, but also good electrical conductivity can be obtained. The reason is presumed as follows. That is, as described above, Cr oxide is formed at the very initial stage of oxidation, and the formation of an oxide having an Fe, Mn spinel structure in the outermost layer of steel is suppressed. By forming the Cr-based oxide film, the outward diffusion of Fe and Mn is suppressed, and the progress of the generation of Fe and Mn-based oxide is suppressed. As a result, a Cr-based oxide film that is dense, has good adhesion, and has few defects can not only improve steam oxidation resistance, but also reduce electrical resistance in a high temperature range and improve electrical conductivity.

In terms of pre-straining the stainless steel surface, a relatively coarse count is more advantageous for mechanical polishing. In order to reliably introduce polishing strain over a depth of about 10 μm to 100 μm, it is necessary to perform mechanical polishing so that the surface roughness Ra specified by JIS B0601 is 0.05 to 50 μm. If Ra is less than 0.05 μm, the introduction of polishing strain is insufficient. Conversely, when polishing finish exceeding 50 μm with Ra, the surface is too rough and the adhesion area with the electrolyte decreases,
Good electrical conductivity cannot be obtained. In addition, “mechanical polishing” described in this specification includes dry or wet methods such as belt polishing, manual polishing, and buffing that are performed manually or mechanically using an abrasive, a polishing grindstone, and a polishing cloth. In addition to mechanical polishing, grinding and shot blasting that can achieve the same effect are also included.

Next, the alloy components, content, etc. of ferritic stainless steel subjected to mechanical polishing finish will be described in detail. In the following description, “%” indicating the content of each element indicates “mass%” unless otherwise specified. C: 0.03% or less, N: 0.03% or less C is an alloy component that improves high-temperature strength, particularly creep characteristics. However, if it is excessively added to ferritic stainless steel, the workability, low temperature toughness, and the like are significantly reduced. In addition, carbonitrides are easily generated by reaction with Ti and Nb, and solid solution Ti and solid solution Nb effective in improving high temperature strength are reduced. Therefore, in this component system, the C and N contents are preferably as low as possible, and the upper limit is set to 0.03% for both.

Si: 1.0% or less An alloy component effective for stabilizing Cr-based oxides, and an effective component for improving steam oxidation resistance. However, if an excessive amount of Si exceeding 1.0% is contained, an oxide layer of SiO ₂ having a high electric resistance is formed on the surface layer, and the electric conductivity is lowered. Moreover, not only the low temperature toughness is lowered, but also the productivity is lowered, and soot is easily generated on the steel surface. For this reason, the upper limit of Si content was set to 1.0%.

Mn: 1.5% or less An alloy component effective for improving the scale peelability of ferritic stainless steel. However, if an excessive amount of Mn is contained, the steel material becomes hard and workability and low temperature toughness are lowered. Therefore, the upper limit of the Mn content is set to 1.5%. S: 0.01% or less A component that adversely affects hot workability and weld hot cracking resistance, and also serves as a starting point for abnormal oxidation. Therefore, the S content is desirably as low as possible, and the upper limit is set to 0.01%.

Cr: An alloy component indispensable for imparting corrosion resistance, oxidation resistance, and electrical conductivity necessary for 11.0 to 20.0% stainless steel. In order to ensure steam oxidation resistance at around 600 ° C. and good electrical conductivity, 11.0% or more of Cr is necessary. However, addition of Cr exceeding 20% lowers the workability, low temperature toughness and 475 ° C. embrittlement susceptibility of ferritic stainless steel. Therefore, the upper limit of the Cr content is set to 20.0%.

Furthermore, Mo and Cu are contained if necessary. Mo: 3.0% or less Mo is added particularly when thermal fatigue characteristics are required because it improves high-temperature strength and heat-resistant fatigue characteristics by solid solution strengthening. The addition of an excessive amount of Mo not only causes an increase in steel material cost, but also causes an excessive hardening, so the upper limit is set to 3.0%. Cu: 1.5% Cu is added particularly when high temperature strength is required in order to improve high temperature strength by solid solution or precipitation hardening. If an excessive amount of Cu is contained, the steel material becomes excessively hardened, so when added, the upper limit is 1.5%. More preferably, the upper limit is 0.5%.

Furthermore, you may contain the following component as needed. Nb: 0.05 to 0.80%, Ti: 0.03 to 0.50%, Nb and Ti have the effect of improving the high temperature strength of ferritic stainless steel by precipitation hardening and improving thermal fatigue properties. ing. In order to exert the effect, it is necessary that Nb is 0.05% or more and Ti is 0.03%. However, when an excessive amount of Nb and Ti is contained, the steel material is excessively hardened and leads to a decrease in toughness. Therefore, the upper limits of the Nb and Ti contents were set to 0.80% and 0.50%, respectively.

Y: 0.001 to 0.1% rare earth elements (La, Ce, Nd, etc.): 0.001 to 0.10% Ca: 0.001 to 0.01% are all dissolved in the oxide film and oxidized. Strengthens the film and further improves the corrosion resistance and oxidation resistance of ferritic stainless steel. In particular, Y improves the electrical conductivity in the oxide film. The effect becomes remarkable by adding 0.001% or more of Y, 0.001% or more of rare earth elements such as La, Ce, and Nd and 0.001% or more of Ca. However, if an excessive amount of Y, rare earth elements (La, Ce, Nd, etc.) and Ca are contained, the steel material is excessively hardened, and not only surface flaws are likely to occur during production, but also the production cost increases. Therefore, when they are added, the upper limit is 0.1% for Y and rare earth elements and 0.01% for Ca.

In the ferritic stainless steel targeted by the present invention, other alloy elements are not particularly defined, and it is preferable to reduce P, O, Ni, etc., which are general impurity components, as much as possible. Usually, P is controlled to 0.04 mass% or less, O: 0.02 mass% or less, and Ni: 0.6% or less. However, in order to ensure a higher level of workability and weldability, P , O, Ni content is more strictly regulated. In addition, elements such as Ta, W, V, and Zr effective for improving heat resistance and B, Mg, and Co effective for improving hot workability may be added as necessary.

Each ferritic stainless steel having the components and compositions shown in Table 1 was melted in a 30 kg vacuum melting furnace and forged into an ingot. After roughly rolling the ingot, a cold-rolled annealed plate having a thickness of 1.5 mm was manufactured through hot rolling, annealing pickling, cold rolling, and finish pickling. Steel type numbers 1, 7, and 8 are invention steels, and others are reference steels. A test piece was cut out from each ferritic stainless steel cold-rolled annealed plate. As the final finishing condition, the 2D finish specified in JIS G4305 is applied, and this cold-rolled annealed plate is subjected to wet polishing using Ra as specified in JIS B0601 as final finishing conditions of 0.03 μm, 0.10 μm, 25 μm, Five types of finishing materials with abrasives applied with a count of 68 μm were subjected to a high temperature steam oxidation test.

The high-temperature steam oxidation test was performed at 650 ° C. for 200 hours in an atmosphere of (20 vol% H ₂ O + 80 vol% H ₂ ) assuming an atmosphere to which the separator of the solid oxide fuel cell is exposed. The weight after the test was compared with the weight before the test, and when the weight change was 0.2 mg / cm ² or less, the steam change resistance was evaluated as ○, and the weight change exceeding that was evaluated as x. It can be said that the more non-oxidized, the stronger the environmental barrier function of the oxide film, and the better the steam oxidation resistance.

As can be seen from the results shown in Table 2, in the ferritic stainless steels of steel types 1 to 10 according to the present invention, both the 2D material and the Ra 0.03 μm polished finish have an oxidation increase of 0.2 mg / cm ² . The water vapor oxidation resistance was insufficient. On the other hand, the polishing finishes of Ra 0.10 μm, 25 μm, and 68 μm all had an oxidation increase of 0.2 mg / cm ² or less, and exhibited good steam oxidation resistance.

Next, using the same cold-rolled annealed plate, the final finishing conditions are 2D finishing material specified by JIS G4305, and Ra count specified by JIS B0601 is 0.03μm, 0.10μm, 25μm, 68μm. A disk having a radius of 10 mm and a material having a thickness of 0.7 mm was prepared. The polished finish disk is sandwiched from both sides by a solid oxide disk made of yttria stabilized zirconia having a radius of 10 mm, and platinum electrodes for supplying current are arranged above and below the sandwiched measurement sample, and the polished finish disk and zirconia A solid made of zirconia in which a weight is placed on a platinum electrode so that the contact pressure of the contact surface of the solid oxide disk is 1.9 kg / cm ² , a constant current is passed between the platinum electrodes, and a polished finish disk is sandwiched between them. Resistance measurement was performed by measuring the potential difference between the oxides. In the resistance measurement, the electrical resistance was measured after raising the temperature to 650 ° C. under the same atmosphere as in the above-mentioned example at 10 ° C./min and holding for 1 hour. The electrical conductivity was evaluated by assuming that the electrical resistance value was 50 mΩ · cm ² or less and that the electrical resistance value exceeded 50 mΩ · cm ² was x.

As can be seen from the results shown in Table 3, the ferritic stainless steel of the present invention, which has a Ra of 0.10 μm and 25 μm, has an electric resistance value at 650 ° C. of 50 mΩ · cm ² or less, which is good. It shows electrical conductivity. In contrast, Ra is a polishing finish of 68μm shows the electrical resistance value of more than 50mΩ · cm ^2, Ra is as large as the electric resistance value is 100 m [Omega · cm ² or more in a polishing finish and 2D finish of 0.03μm It was not suitable as a separator material for solid oxide fuel cells.

Claims (4)

C: 0.03 mass% or less, Si: 1.0 mass% or less, Mn: 1.5 mass% or less, S: 0.01 mass% or less, N: 0.03% or less, Cr: 11.0- 20.0% by mass, 3.0% by mass or less of Mo, Ca: 0.001 to 0.01% by mass, having a composition comprising the balance Fe and inevitable impurities, and having surface roughness specified by JIS B0601 A ferritic stainless steel for a solid oxide fuel cell separator, characterized by having a mechanically polished surface with a thickness Ra of 0.05 to 50 μm. Furthermore, the ferritic stainless steel for solid oxide fuel cell separators of Claim 1 containing 1 type or 2 types of Cu of 1.5 mass% or less. Furthermore, the ferrite for solid oxide fuel cell separators of Claim 1 or 2 containing 1 type or 2 types of Nb: 0.05-0.80 mass% and Ti: 0.03-0.50 mass% Stainless steel. Furthermore, the solid oxide type in any one of Claims 1-3 containing 1 type (s) or 2 or more types of Y: 0.001-0.1 mass% and rare earth elements: 0.001-0.1 mass%. Ferritic stainless steel for fuel cell separators.