JP2009185387A - Ferritic stainless steel for separator of solid oxide fuel cell - Google Patents
Ferritic stainless steel for separator of solid oxide fuel cell Download PDFInfo
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本発明は、固体酸化物型燃料電池のセパレータに使用されるフェライト系ステンレス鋼に関する。 The present invention relates to a ferritic stainless steel used for a separator of a solid oxide fuel cell.
近年、石油を代表とする化石燃料の枯渇化,CO2排出による地球温暖化現象等の問題から、従来の発電システムに替わる新しいシステムの実用化が求められている。新しい発電システム,分散電源あるいは自動車の動力源として、クリーンな発電システムである燃料電池が注目を浴びている。 燃料電池には、用いられる電解質の違いによりリン酸型燃料電池,溶融炭酸塩型燃料電池,固体高分子型燃料電池等、いくつかの種類があるが、その中でも、固体酸化物型の燃料電池(SOFC)は作動温度,エネルギー効率ともに燃料電池の中では最も高く、実用化が最も有望視されている燃料電池である。 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.
従来、固体酸化物型燃料電池(SOFC)の作動温度は1000℃程度と高く、長期間の信頼性の観点から、その構成部材には主としてセラミックスが使用されてきた。高温酸化性に優れている高Cr,高Ni系のオーステナイト系ステンレス鋼でも使用が困難であった。 しかしながら、近年になって、固体電解質膜の改良により、600〜800℃程度まで作動温度の低下が可能となった。低温作動型の固体酸化物型燃料電池(SOFC)を構成するセパレータに金属材料を適用するためには、600〜800℃の温度域で50mΩ・cm2以下なる良好な電気伝導性や、数10%の水蒸気を含む高温雰囲気においても優れた耐水蒸気酸化性、さらにはセラミックス系固体電解質膜と同等の熱膨張係数(室温〜800℃で13×10-6(1/K)程度)を十分に満足できる特性が必要である。 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.
高温での耐水蒸気酸化性に優れている高Cr,高Ni系のオーステナイト系ステンレス鋼は、熱膨張係数が固体電解質膜の約1.5倍と高いため、起動・停止が頻繁に行われる使用環境では熱膨張・熱収縮が発生し、熱変形及びスケール剥離が発生するため使用し難い。 固体酸化物型燃料電池用の部材としては、熱膨張係数が固体電解質膜と同程度であり、熱膨張・熱収縮を受けても熱変形・スケール剥離が発生しないフェライト系ステンレス鋼が最適である。 そして、熱膨張係数の調整や、表面に形成される酸化皮膜の特性の改良等を目的として、フェライト系ステンレス鋼の合金組成を調整する技術に関して、例えば特許文献1〜6にみられるように、種々の提案がなされている。 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.
特許文献1〜6等で提案されている材料は、固体電解質である安定化ジルコニアと熱膨張係数が近似し、700〜950℃程度で良好な電気伝導性を有する酸化皮膜が形成されるという特性を備えている。このため、安定化ジルコニアを電解質とした燃料電池のセパレータに適した材料を提供できている。 しかしながら、実際の燃料電池におけるセパレータの導電部は数10%以上の水蒸気を含む雰囲気に曝されることになる。このため、セパレータは水蒸気雰囲気特有の酸化(以下「水蒸気酸化」と称す。)を受け、損傷されたり電気抵抗が低下したりする。このような問題を起させないためには、少なくとも20質量%を超えるCrを含有させる必要がある。 その一方で、20質量%を超えるCrを添加すると475℃脆化感受性が著しく低下し、使用中にセパレータが硬化し、破損することにも繋がることがある。加えて、Crの増加によりコストアップにもなっている。 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.
本発明は、このような問題を解消すべく案出されたものであり、Cr含有量を抑えて475℃脆化感受性を高め、かつ優れた耐水蒸気酸化性と良好な電気伝導性及び低熱膨張係数を有する、安価で耐久性に優れた固体酸化物型燃料電池セパレータ用フェライト系ステンレス鋼を提供することを目的とする。 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.
本発明の固体酸化物型燃料電池セパレータ用フェライト系ステンレス鋼は、その目的を達成するため、C:0.03質量%以下,Si:1.0質量%以下,Mn:1.5質量%以下,S:0.01質量%以下,N:0.03%以下,Cr:11.0〜20.0質量%、3.0質量%以下のMo,Ca:0.001〜0.01質量%を含有し、あるいは更に1.5質量%以下のCuを含み、残部がFeおよび不可避的不純物からなる組成を有するとともに、JIS B0601で規定される表面粗さRaで0.05〜50μmの機械研磨仕上面を有することを特徴とする。 本発明のフェライト系ステンレス鋼には、さらに、Nb:0.05〜0.80質量%,Ti:0.03〜0.50質量%,Y:0.001〜0.1質量%,希土類元素:0.001〜0.1質量%,の1種又は2種以上を含むこともできる。 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.
本発明のフェライト系ステンレス鋼は、過剰なCrを添加することなく、従来のフェライト系ステンレス鋼をベースにその表面に機械研磨仕上げを施すことにより、形成される酸化皮膜を強化して耐水蒸気酸化性を向上することができている。このため、良好な電気伝導性と熱膨張係数を維持したまま耐水蒸気酸化性が向上されたフェライト系ステンレス鋼が低コストで得られる。 したがって、本発明により、耐久性に優れたセパレータ用材料が低コストで提供されるので、固体酸化物型燃料電池のコストダウン,性能及び耐久性の向上が見込まれ、固体酸化物型燃料電池の普及促進に寄与できる。 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.
コストを抑え、475℃脆化感受性を高めるためにCr含有量を20質量%以下にしたフェライト系ステンレス鋼では、燃料電池の高温水蒸気雰囲気に曝されると水蒸気酸化が容易に進行するとともに、導電部の電気抵抗が増大し、燃料電池の機能が損なわれる。 この点が、フェライト系ステンレス鋼を用いる上で問題となっている。 高温雰囲気における水蒸気酸化は大気中における酸化よりも損傷が大きく、鋼素地を減肉させて変形,穴開き等のトラブルを発生させる原因となっている。 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.
水蒸気酸化による酸化促進機構については、水蒸気が酸素及び水素に解離して酸化反応を促進させるという説や、水蒸気が鋼素地に直接到達して酸化を促進させるという説等、いくつかの説があるものの、必ずしも明らかになっていないのが現状である。 しかし、本発明者等は、この水蒸気酸化は、ステンレス鋼表面に生成するCr系酸化物を主体とした酸化皮膜を安定化することにより抑制できると推測した。以下に、その機構と対策について説明する。 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.
一般的に、加熱によりステンレス鋼表面に生成する酸化皮膜は、ステンレス鋼に耐酸化性を付与するものであり、高温雰囲気にあっては11質量%以上のCr含有量で耐酸化性の向上が顕著になる。しかし、フェライト系ステンレス鋼の場合、600℃程度で素地が高温の水蒸気雰囲気に曝されると、酸化皮膜中でのCr系酸化物が形成される前に、Fe,Mn系のスピネル構造をもつ酸化物が多量に生成されるため、酸化皮膜がポーラスになる。その結果、酸化皮膜を透過して下地鋼に到達した水蒸気又は酸素分子が多くなり、下地鋼の水蒸気酸化が進行する。このことは、耐水蒸気酸化性のみでなく電気伝導度も低下させることにもなっている。すなわち、Fe,Mn系の酸化物層は、ボイド等の欠陥が多いこと、酸化皮膜の厚さが厚くなり加えて素地との密着性が乏しいことから、高温域における電気抵抗を増大させ、結果として電気伝導度の低下を招いている。 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.
本発明者等は、種々の検討を行っている過程で、ステンレス鋼表面に生成するCr系酸化物を主体とした酸化皮膜を安定化する手段として、表面酸化の前に機械研磨仕上げを施すことが有効であること見いだした。 機械研磨を施すことにより、金属表層に転位やすべり帯が多数形成され、表層から所定の深さまでは研磨による歪みの影響を受けることになる。歪みの影響により表層所定厚み範囲のCrの表層への拡散を促し、結果として、酸化のごく初期においてFe,Mn系のポーラスな酸化皮膜を生成させることなく、ステンレス鋼の表層にCrの緻密な酸化皮膜を形成することができる。 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.
また、機械研磨仕上げを施すことにより、耐水蒸気酸化性を向上させるだけでなく良好な電気伝導性を得ることもできる。その理由は、以下のように推測される。 すなわち、前述の通り、Cr酸化物を酸化のごく初期に形成させ、鋼の最表層でのFe,Mn系のスピネル構造をもつ酸化物の生成を抑止させることである。Cr系酸化物皮膜を形成させることにより、Fe,Mnの外方への拡散が抑制され、Fe,Mn系酸化物生成の進行が抑止される。その結果、緻密で密着性がよく、欠陥の少ないCr系酸化物皮膜が、耐水蒸気酸化性を向上させるだけでなく、高温域における電気抵抗を低減させ電気伝導度をも向上させることができる。 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.
ステンレス鋼表面に予め歪みを付与する意味から、機械研磨は比較的粗い番手の研磨の方が有利になる。表層数10μm〜100μm程度の深さにわたって確実に研磨歪みを導入するためには、JIS B0601で規定される表面粗さRaが0.05〜50μmになるように機械研磨を施す必要がある。Raで0.05μmに満たないと研磨歪みの導入が不充分である。逆にRaで50μmを超える研磨仕上げを施すと、表面が粗すぎて電解質との密着面積が減少し、
良好な電気伝導性が得られなくなる。 なお、本明細書中に記載している「機械研磨」には、研磨材,研磨砥石,研磨布を用いて人力で、あるいは機械力で行うベルト研磨,手研磨,バフ研磨などの乾式又は湿式の機械研磨はもちろん、同様な効果が得られる研削やショットブラストも含まれる。
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.
次に、機械研磨仕上げが施されるフェライト系ステンレス鋼の合金成分,含有量等について詳しく説明する。なお、以下の説明中、各元素の含有量を示す「%」は特に断りがない限り「質量%」を示す。C:0.03%以下,N:0.03%以下 Cは、高温強度、特にクリープ特性を改善する合金成分である。しかし、フェライト系ステンレス鋼に過剰添加すると加工性,低温靭性等が著しく低下する。また、TiやNbとの反応によって炭窒化物を生成しやすく、高温強度の改善に有効な固溶Tiや固溶Nbを減少させる。したがって、本成分系では、C,N含有量は少ない程好ましく、ともに上限を0.03%に設定した。 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%以下 Cr系酸化物の安定化に有効な合金成分であり、耐水蒸気酸化性の改善に有効な成分である。しかし、1.0%を超える過剰量のSiが含まれると、表層に電気抵抗が高いSiO2の酸化物層を形成し、電気伝導度を低下することになる。しかも、低温靭性を低下するばかりでなく、製造性も低下し、鋼表面に疵が生成しやすくなる。このため、Si含有量の上限は1.0%に設定した。 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%以下 フェライト系ステンレス鋼のスケール剥離性の改善に有効な合金成分である。しかし、過剰量のMnが含まれると鋼材が硬質化し、加工性及び低温靭性が低下する。したがって、Mn含有量の上限は1.5%に設定した。S:0.01%以下 熱間加工性,耐溶接高温割れ性に悪影響を及ぼす成分であり、異常酸化の起点にもなる。そのため、S含有量は可能な限り低くすることが望ましく、0.01%を上限とした。 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:11.0〜20.0% ステンレス鋼に必要な耐食性,耐酸化性,電気伝導性を付与する上で不可欠な合金成分である。600℃前後での耐水蒸気酸化性及び良好な電気伝導性を確保するためには、11.0%以上のCrが必要である。しかし、20%を超えるCrの添加は、フェライト系ステンレス鋼の加工性,低温靭性及び475℃脆化感受性を低下させる。したがって、Cr含有量の上限は20.0%に設定した。 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%.
さらに必要に応じてMoやCuも含有される。Mo:3.0%以下 Moは固溶強化により高温強度及び耐熱疲労特性を向上させるため、特に熱疲労特性が必要とされる場合に添加される。過剰量のMoの添加は、鋼材コストの上昇を招くばかりでなく,過度の硬質化を招くので、添加する場合は3.0%を上限とする。Cu:1.5% Cuは固溶又は析出硬化により高温強度を向上させるため、特に高温強度が必要とされる場合に添加される。過剰量のCuが含まれると鋼材が過度に硬質化するので、添加する場合は1.5%を上限とする。更に好ましくは、0.5%を上限とする。 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%.
さらに必要に応じて下記の成分を含有していてもよい。Nb:0.05〜0.80%,Ti:0.03〜0.50%, Nb,Tiは析出硬化によりフェライト系ステンレス鋼の高温強度を向上させ、熱疲労特性を改善する作用を有している。その効果を発揮させるには、Nbは0.05%以上、Tiは0.03%の含有が必要である。しかし、過剰量のNb,Tiが含まれると鋼材が過度に硬化して靭性低下に繋がるので、Nb及びTi含有量の上限はそれぞれ0.80%および0.50%に設定した。 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:0.001〜0.1% 希土類元素(La,Ce,Nd等):0.001〜0.10%Ca:0.001〜0.01% いずれも酸化皮膜中に固溶し、酸化皮膜を強化させ、フェライト系ステンレス鋼の耐食性及び耐酸化性をさらに向上させる。特に、Yは酸化皮膜中の電気伝導度を向上させる。0.001%以上のY,0.001%以上のLa,Ce,Nd等の希土類元素,0.001%以上のCaの添加でその効果が顕著になる。しかし、過剰量のY,希土類元素(La,Ce,Nd等),Caが含まれると鋼材が過度に硬化し、製造時に表面疵が生じやすくなるばかりでなく、製造コストの上昇を招く。したがって、それらを添加する場合は、Y及び希土類元素は0.1%を、Caは0.01%を上限とする。 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.
本発明が対象とするフェライト系ステンレス鋼では、他の合金元素に関しては特段規定されるものではなく、一般的な不純物成分であるP,O,Ni等は、可能な限り低減することが好ましい。通常はP:0.04質量%以下,O:0.02質量%以下,Ni:0.6%以下に規制すされるが、さらに高いレベルの加工性や溶接性を確保する場合には、P,O,Ni含有量をさらに厳しく規制する。 また、耐熱性の改善に有効なTa,W,V,Zrや、熱間加工性の改善に有効なB,Mg,Co等の元素も必要に応じて添加してもよい。 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.
表1の成分・組成をもつ各フェライト系ステンレス鋼を、30kg真空溶解炉で溶製し、インゴットに鍛造した。インゴットを粗圧延した後、熱延,焼鈍酸洗,冷延、仕上酸洗を経て板厚1.5mmの冷延焼鈍板を製造した。鋼種番号1,7,8が発明鋼、他は参考鋼である。 各フェライト系ステンレス鋼の冷延焼鈍板から試験片を切り出した。最終仕上条件として、JIS G4305で規定した2D仕上を施したままのもの、この冷延焼鈍板を最終仕上条件として湿式研磨によりJIS B0601で規定されるRaで0.03μm,0.10μm,25μm,68μmとなる番手で施した研磨材との5種類の仕上材で、高温水蒸気酸化試験に供した。 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.
高温水蒸気酸化試験は、固体酸化物型燃料電池のセパレータが曝される雰囲気を想定し、(20体積%H2O+80体積%H2)の雰囲気で650℃×200時間行った。 試験後の重量を試験前の重量と比較し、重量変化が0.2mg/cm2以下を○、それを超える重量変化があったものを×として、耐水蒸気酸化性を評価した。酸化が生じていないもの程、酸化皮膜の環境遮断機能が強く、耐水蒸気酸化性に優れているといえる。 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.
表2に示す結果からわかるように、本発明に従った鋼種番号1〜10のフェライト系ステンレス鋼では、2D材及びRa0.03μmの研磨仕上材はいずれも酸化増量が0.2mg/cm2を超えており、耐水蒸気酸化性が不充分であった。これに対して、Ra0.10μm,25μm及び68μmの研磨仕上材はいずれも酸化増量が0.2mg/cm2以下であり、良好な耐水蒸気酸化性を示していた。 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.
次に、同じ冷延焼鈍板を用い、最終仕上条件としてJIS G4305で規定される2D仕上材、及びJIS B0601で規定されるRaで0.03μm,0.10μm,25μm,68μmとなる番手で施した研磨仕上された、板厚0.7mm材で半径10mmの円板を作製した。 この研磨仕上円板を、半径10mmのイットリア安定化ジルコニア製固体酸化物の円板で両面から挟み込み、当該挟み込み測定試料の上下に電流供給用の白金電極を配置し、研磨仕上円板とジルコニア製固体酸化物円板の接触面の面圧が1.9kg/cm2となるように白金電極上に錘を乗せ、白金電極間に定電流を流して、研磨仕上円板を挟み込んだジルコニア製固体酸化物間の電位差を測定することにより抵抗測定を行った。 抵抗測定は、上記実施例と同じ雰囲気で10℃/分の条件で650℃に上昇させ、1時間保持した後に電気抵抗を測定した。電気抵抗値が50mΩ・cm2以下を○、50mΩ・cm2を超える電気抵抗値があったものを×として、電気伝導性を評価した。 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.
表3に示す結果からわかるように、本発明成分系のフェライト系ステンレス鋼でRaが0.10μm及び25μmの研磨仕上材は、650℃での電気抵抗値が50mΩ・cm2以下となり、良好な電気伝導性を示している。 これに対して、Raが68μmの研磨仕上材では50mΩ・cm2を超える電気抵抗値を示し、Raが0.03μmの研磨仕上材及び2D仕上材では電気抵抗値が100mΩ・cm2以上と大きく、固体酸化物型燃料電池のセパレータ用材料としては適していなかった。 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.
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JP2013016451A (en) * | 2011-07-06 | 2013-01-24 | Hyundai Motor Co Ltd | Fuel cell stack structure and method of manufacturing the same |
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