JP6145048B2 - Al-containing heat-resistant ferritic stainless steel for fuel cells and method for producing the same - Google Patents

Description

  The present invention is suitable for a fuel cell high-temperature member having excellent oxidation resistance in a high-temperature steam-containing atmosphere using methane, natural gas, city gas, propane, kerosene, etc. as a fuel and having a good creep rupture life. The present invention relates to an Al-containing heat-resistant ferritic stainless steel and a method for producing the same.

From the viewpoints of depletion of fossil fuels such as petroleum, global warming due to CO ₂ emissions, and the like, there is a demand for the practical use and spread of new systems that replace conventional power generation systems. Among them, a clean polymer system such as a polymer electrolyte fuel cell (PEFC) and a solid oxide fuel cell (SOFC) are being widely used.

  In these fuel cells, hydrocarbon fuels such as methane, natural gas, city gas, propane, and kerosene are used as fuel for supplying hydrogen. When such fuel is used and mixed with air, parts such as fuel reformers, piping connected to these devices, and high-temperature heat exchangers are exposed to a high-temperature environment containing a large amount of water vapor and oxygen. . SUS310S, SUS316L series, SUS304 series austenitic stainless steel and high Ni heat-resistant austenitic stainless steel have been mainly used as the metal materials constituting these reformers, heat exchangers, gas pipes, and batteries.

  In recent years, the SOFC system has been remarkably improved, and high output operation is possible at a low temperature operation of 700 to 900 ° C. from the conventional vicinity of 1000 ° C. For the spread of these systems, it is desired to change from the above-mentioned austenitic stainless steel to an inexpensive and highly reliable metal material that can guarantee a device life of 90,000 to 100,000 hours or more. The creep rupture life is from the viewpoint of oxidation resistance in a high-temperature environment containing oxygen and equipment life, and heat-resistant ferritic stainless steel that can overcome these problems is a candidate.

From the above-mentioned viewpoint, ferritic stainless steel for fuel cell high-temperature equipment has been recently proposed. In the following Patent Document 1, Cr + 7 (Si + Al) ≧ 22, and trace elements such as Y and REM are added, and 800 ° C., 80% H ₂ O-20% CH ₄ , 950 ° C., 10% H ₂ O— A ferritic stainless steel is disclosed in which the mass increase of the oxide film is small in a high temperature environment typified by 90% air, the steam oxidation resistance is excellent, and the thermal fatigue characteristics are also good. The examples of the above publication are mainly Al-containing ferritic stainless steel, 0.01 ≦ C ≦ 0.05, and Ti and Nb are selectively added elements.

  Various studies have been made on Al-containing heat-resistant ferritic stainless steel as described above. For example, in the following Patent Document 2, Cr: 13 to 20%, Al: less than 1.5 to 2.5%, Si: 0.3 to 0.8%, Ti: 3 × (C + N) to 20 × ( An Al-containing heat-resistant ferritic stainless steel sheet excellent in workability and oxidation resistance characterized by C + N) and a method for producing the same are disclosed. In the examples of the above publication, Cr was fixed at 18%, and addition of Nb was not studied.

  Further, Patent Document 3 and Patent Document 4 below disclose high Al content ferritic stainless steel as compared with the above publication. Patent Document 3 includes Cr: 12-30%, Al: 2.5-8%, and further Ti: 0.02-0.2%, Zr: 0.02-0.2%. It is characterized by doing. The above publication does not consider addition of Nb. On the other hand, Patent Document 4 uses Nb: 0.3 to 0.7% as an essential additive element in the same component system as Patent Document 3 in order to obtain high strength and high impact resistance characteristics as a weight detection sensor-substrate. . In the example of the above publication, when Ti and Nb are added in combination, 0.4% or more of Nb is added to obtain the above characteristics.

  Up to now, the oxidation resistance of heat-resistant ferritic stainless steel has improved the protection of Al-based oxide films by increasing the amount of Al and Si, and also adding expensive rare earth elements. That is, it is aimed at high alloying with Al and Si and addition of expensive rare earth elements. On the other hand, regarding the oxidation resistance of the Al-based oxide film, there is no study of Ti or Nb that is utilized as a stabilizing element for ferritic stainless steel for improving corrosion resistance and workability. In other words, regarding the oxidation resistance and creep rupture life which are the problems for the fuel cell described above, heat resistant ferritic stainless steel utilizing trace elements such as Ti and Nb without resorting to high alloying or addition of expensive elements. The steel and its manufacturing method were not clarified.

JP 2008-156692 A JP 2004-307918 A JP-A-2005-232485 JP 2006-63380 A Heat treatment, 33, (1993), 251

  By controlling the solid solution state in steels such as Ti and Nb, the present invention controls the oxidation resistance and clearness required for fuel cells without relying on high alloying or addition of expensive elements. It is an object of the present invention to provide an Al-containing heat-resistant ferritic stainless steel having a fracture life and a method for producing the same.

(1) In mass%, C: less than 0.02%, Si: more than 0.15 to 0.7%, Mn: 0.3% or less, P: 0.035% or less, S: 0.003% Hereinafter, Cr: 13-20%, Al: 1.5-6%, N: 0.02% or less, Ti: 0.03-0.5% or less, Nb: 0.6% or less, the balance being Fe and Consisting of inevitable impurities,
The amount of solid solution Ti in the steel is [Ti] and the amount of solid solution Nb is [Nb]. When 13 ≦ Cr ≦ 16, the following equation (a) is satisfied, and when 16 <Cr ≦ 20, the following (b ) Satisfying the formula, any one or more of TiC, Ti ₄ C ₂ S ₂ , and FeTiP are precipitated , and 1050 ° C., 300 h in an oxidizing atmosphere of 20% water vapor, 20% oxygen, and the remaining nitrogen gas. After the accelerated oxidation test, the oxide film surface peeling rate was 10 pcs / cm ² when the heating / cooling heat cycle was performed 600 times after heating to 750 ° C. for 30 minutes in the air and then allowing to cool for 10 minutes. An Al-containing heat-resistant ferritic stainless steel for fuel cells used in a high temperature, high water vapor, high oxygen atmosphere characterized by:

0 ≦ [Ti] ≦ [Nb] +0.05, 0 <[Nb] ≦ 0.6 (a)
0 ≦ [Ti] ≦ 1/2 × [Nb] +0.15, 0 <[Nb] ≦ 0.6 (b)
(2) The steel is further mass%, Zr: 0.1% or less, La: 0.1% or less, Y: 0.1% or less, REM: 0.1% or less, B: 0.005 The Al-containing heat-resistant ferritic stainless steel for a fuel cell according to claim 1, characterized in that it contains at least one of the following:% or less, Mg: 0.005% or less, Ca: 0.005% or less .
(3) A method for producing an Al-containing heat-resistant ferritic stainless steel for a fuel cell according to any one of (1) or (2) , wherein the slab of the ferritic stainless steel is hot worked, and then cooled. In a manufacturing process combining hot working and annealing, heat treatment is performed at 800 to 1100 ° C. after hot working or after cold working, and an average cooling rate in a temperature range of 500 to 800 ° C. is set to 5 ° C./second or less. A method for producing an Al-containing heat-resistant ferritic stainless steel for a fuel cell. Hereinafter, the invention related to the steels (1) and (2) and the invention related to the manufacturing method (3) are referred to as the present invention. Further, the inventions of (1) to (3) may be collectively referred to as the present invention.

It is a figure which shows the surface FE-SEM observation result of an oxide film. It is a figure which shows the relationship (low Cr steel) of the amount of solid solution Ti which affects oxidation resistance, and the amount of solid solution Nb. It is a figure which shows the relationship (high Cr steel) of the amount of solid solution Ti which affects oxidation resistance, and the amount of solid solution Nb.

  In order to solve the above-mentioned problems, the present inventors have earnestly studied the influence of the solid solution state of Ti and Nb on the oxidation resistance and creep rupture life of Al-containing heat-resistant ferritic stainless steel. Research has been done to complete the present invention. The typical experimental results will be described below.

Table 1 shows typical test steel components. A and B steels were melted and subjected to hot-rolled sheet annealing and pickling to produce a cold-rolled sheet having a thickness of 1.2 mm. Cold-rolled sheet annealing was performed at 1000 ° C., and a part of Steel A was subsequently heat-treated at 650 ° C. for 1 hr in order to adjust the solid solution state of Ti and Nb.
The cold-rolled annealed sheet was subjected to precipitate extraction residue analysis, oxidation test under accelerated conditions, and creep test. In the extraction residue analysis of precipitates, the amount of Ti and Nb residues was measured, and the amount of solid solution Ti and the amount of solid solution Nb were determined by subtracting the amount of residue from the steel components.

酸化試験は、ＳＯＦＣの標準的な作動温度を想定した７５０℃で９万時間（１０年）の運転で生成する酸化物の生成量を実験室的に模擬する加速試験条件を検討した。供試鋼の酸化皮膜は主としてＡｌ₂Ｏ₃であり，酸化皮膜の成長はＡｌ₂Ｏ₃の成長速度に従う。
Ａｌ₂Ｏ₃の成長速度は、例えば，前記非特許文献１により求めることが可能であり，７５０℃，９万時間後の酸化増量は０．５〜０．６ｍｇ／ｃｍ²と予測される。これに相当する酸化増量を比較的短時間で模擬できる加速条件（温度，時間）として、１０５０℃，３００ｈｒを選定した。酸化雰囲気は、２０％水蒸気，２０％酸素，残部窒素ガスとした。試験片寸法は、１ｍｍ厚×２５ｍｍ×２０ｍｍとし，表面と端面はエメリー紙番手＃６００の湿式研磨とした。加速試験後、酸化増量を測定し，酸化皮膜表面のＦＥ−ＳＥＭ観察を行い，酸化増量の検証ならびに酸化皮膜の微細形態について検討した。引き続いて、酸化皮膜の健全性を評価するために、加速試験材を大気中で７５０℃に３０分加熱しその後１０分放冷とする、加熱・冷却の熱サイクルを繰り返し行い，酸化皮膜表面の剥離率を測定した。クリ−プ試験は、平行部６０ｍｍの板状試験片を作製し，７５０℃，初期応力１０ＭＰａの条件下で破断時間を求めた。

In the oxidation test, an accelerated test condition was simulated in which the amount of oxide produced in operation of 90,000 hours (10 years) at 750 ° C. assuming a standard operating temperature of SOFC was simulated in the laboratory. The oxide film of the test steel is mainly Al ₂ O ₃ and the growth of the oxide film follows the growth rate of Al ₂ O ₃ .
The growth rate of Al ₂ O ₃ can be determined by, for example, Non-Patent Document 1, and the increase in oxidation after 90,000 hours at 750 ° C. is predicted to be 0.5 to 0.6 mg / cm ² . 1050 ° C. and 300 hr were selected as acceleration conditions (temperature, time) that can simulate the corresponding increase in oxidation in a relatively short time. The oxidizing atmosphere was 20% water vapor, 20% oxygen, and the balance nitrogen gas. The test piece dimensions were 1 mm thick × 25 mm × 20 mm, and the surface and end surfaces were wet-polished with emery paper count # 600. After the acceleration test, the amount of increase in oxidation was measured, and the surface of the oxide film was observed by FE-SEM to verify the amount of increase in oxidation and to examine the fine form of the oxide film. Subsequently, in order to evaluate the integrity of the oxide film, the accelerated test material was heated in the atmosphere to 750 ° C. for 30 minutes and then allowed to cool for 10 minutes. The peel rate was measured. In the creep test, a plate-like test piece having a parallel part of 60 mm was prepared, and the fracture time was determined under the conditions of 750 ° C. and initial stress of 10 MPa.

The peel rate on the oxide film surface was measured as follows. The oxidation test is performed using four identical samples of 25 mm × 20 mm square. After the oxidation test, the photograph of the appearance of the front and back surfaces of the test piece is enlarged from the actual size to about 2 times. Therefore, the points which are traces of oxide scattering from the surface are counted. For these points, the actual size is 0.5 mm or more, and the size is sufficiently recognizable visually. The peel rate (ke / cm ² ) is the surface area (0.4 cm) of points counted from 4 samples and 8 surfaces.
Calculated by dividing by ² ). In a sample with remarkable peeling from the oxide film surface, a dot-like trace of 0.5 to 1 mm can be observed relatively easily. The criterion for determining that the oxide film hardly peeled was the case of one point or less of point-like traces per sample. That is, the peel rate ≦ 10 (4 / 0.4 cm ² ).

Table 2 shows the results obtained. The increase in oxidation largely reflected the predicted value, and the validity of the accelerated test could be verified. As can be seen from Table 2, the increase in oxidation is reduced by the decrease in the amount of solid solution Ti, and further by the solid solution Nb, and B follows the ideal growth law of the Al ₂ O ₃ film. Furthermore, in the accelerated oxidation test material, the peeling rate of the oxide film surface accompanying the heat cycle of heating / cooling is also greatly improved by the decrease in the amount of solid solution Ti and the presence of solid solution Nb.

上述した結果を説明するために行った，酸化皮膜表面の観察結果を図１に示す。Ｂは、０．１〜０．５μｍのＡｌ₂Ｏ₃粒子からなり，微視的にも均一性の高い酸化皮膜と言える。Ａ−２は概ねＢに近い状態である。他方，酸化増量が多めで皮膜表面の剥離率が高いＡ−１は、Ａｌ₂Ｏ₃粒子のサイズも大きめで，場所によっては図１に見られるような板状の酸化物粒子が入り混じった１０μｍにも及ぶ粗大な塊状領域が確認された。これら板状の酸化物粒子は、元素分析の結果から多量のＴｉが検出され，Ｔｉ含有酸化物であることを確認した。すなわち，Ａｌ₂Ｏ₃皮膜の健全性を阻害する要因は，Ｔｉ含有酸化物粒子の生成によると考えられる。このようなＴｉ含有酸化物粒子は、鋼の固溶Ｔｉ量が多い場合に生成し、固溶Ｎｂの存在により抑制されることが分かった。固溶Ｎｂの存在によりＴｉ含有酸化物の生成が抑制される理由は必ずしも明らかではないが、皮膜／鋼界面でのＴｉ酸化物の生成抑制に起因し、固溶Ｎｂが少なからずＡｌ₂Ｏ₃皮膜直下におけるＴｉの拡散と酸素との結合を抑制しているものと考えられる。

FIG. 1 shows an observation result of the oxide film surface, which was performed to explain the above-described results. B is made of Al ₂ O ₃ particles of 0.1 to 0.5 μm and can be said to be an oxide film having high uniformity even microscopically. A-2 is a state substantially close to B. On the other hand, A-1 with a large oxidation increase and a high peeling rate on the surface of the film has a large size of Al ₂ O ₃ particles, and plate-like oxide particles as shown in FIG. A coarse mass region as long as 10 μm was confirmed. From these elemental analysis results, a large amount of Ti was detected, and these plate-like oxide particles were confirmed to be Ti-containing oxides. That is, it is considered that the factor that hinders the soundness of the Al ₂ O ₃ film is due to the generation of Ti-containing oxide particles. It has been found that such Ti-containing oxide particles are generated when the amount of solid solution Ti in the steel is large and are suppressed by the presence of solid solution Nb. The reason why the generation of Ti-containing oxide is suppressed by the presence of solid solution Nb is not necessarily clear, but due to the suppression of the generation of Ti oxide at the film / steel interface, the amount of solid solution Nb is not limited to Al ₂ O _3. It is considered that the diffusion of Ti and the bond between oxygen immediately below the film is suppressed.

Further, as is clear from Table 2, it is effective to increase the amount of dissolved Nb in improving the creep rupture life. From the above examination, the soundness of the Al ₂ O ₃ coating produced under high temperature, high water vapor and high oxygen content atmosphere is greatly improved by increasing the solid solution Nb content while reducing the solid solution Ti content. It came to obtain a new knowledge. The soundness of the Al ₂ O ₃ film means microscopically uniform Al ₂ O ₃ particles, and almost no peeling of the oxide film surface occurs even after a long period of continuous oxidation and a thermal cycle accompanying heating and cooling. This means that there is not (10 / cm ² or less).

The present inventions (1) to (4) have been completed based on the knowledge obtained from the examination results represented by the above-described experiments.
Hereinafter, each requirement of the present invention will be described in detail. In addition, "%" display of the content of each element means "mass%".
First, the reasons for limiting the components will be described below.

C is an inevitable impurity element contained in the steel, and inhibits the soundness of the Al ₂ O ₃ film that is the target of the present invention. Therefore, the lower the amount of C, the better. However, excessive reduction leads to a significant increase in refining costs. Therefore, the upper limit is made less than 0.02%. From the viewpoint of oxidation resistance and manufacturability, the preferred range is 0.001 to less than 0.01%. More preferably, the content is 0.003 to 0.008%.
In addition to the deoxidizing action, Si is effective for enhancing the soundness of the Al ₂ O ₃ film targeted by the present invention. Therefore, the lower limit is over 0.15%. On the other hand, excessive addition causes deterioration of workability and weldability. Therefore, the upper limit is set to 0.7%. From the viewpoint of oxidation resistance and material, the preferred range is 0.3 to 0.6%.
Mn has a deoxidizing action, but lowers the target oxidation resistance of the present invention. In particular, it promotes the formation of spinel oxides that impair the soundness of the Al ₂ O ₃ film in a high water vapor and high oxygen atmosphere. Therefore, the upper limit is 0.3%. However, excessive reduction leads to an increase in refining costs. Therefore, the lower limit is preferably 0.01%. From the viewpoint of oxidation resistance and manufacturability, the preferred range is 0.05 to 0.25%. More preferably, the content is 0.1 to 0.2%.

P is an inevitable impurity element contained in the steel, and causes reduction in oxidation resistance and workability, which are the objects of the present invention. Therefore, the upper limit is 0.035%. However, excessive reduction leads to an increase in refining costs. Therefore, the lower limit is preferably 0.01%. From the viewpoint of oxidation resistance and manufacturability, the preferred range is 0.02 to 0.03%.
S is an inevitable impurity element contained in the steel, and lowers the oxidation resistance and hot workability that are the object of the present invention. In particular, it is feared that the presence of Mn inclusions and solute S will be the starting point for spinel oxides that destroy Al ₂ O ₃ coatings under high water vapor and high oxygen atmospheres. Therefore, the lower the amount of S, the better. However, excessive reduction leads to an increase in raw materials and refining costs. Therefore, the upper limit is made 0.003%. A preferable range is 0.0002 to 0.001% from the viewpoint of oxidation resistance, hot workability and manufacturing cost.

Cr is a basic constituent element in securing the oxidation resistance and creep rupture life which are the object of the present invention. In the present invention, if it is less than 13%, the desired characteristics are not sufficiently ensured. Therefore, the lower limit is 13%. However, excessive addition significantly reduces the toughness and ductility of the hot-rolled steel material, impairs manufacturability, and the upper limit is made 20% from the viewpoint of suppressing high alloying as a target of the present invention. From the standpoint of effects and manufacturability, the preferred range is 14.5 to 18.5%.
Al is an essential element for forming the Al ₂ O ₃ film of the Al-containing heat-resistant ferritic stainless steel that is the object of the present invention. In the present invention, if it is less than 1.5%, the soundness of the target Al ₂ O ₃ film is not ensured. Therefore, the lower limit is 1.5%. However, excessive addition leads to a significant decrease in the toughness and ductility of hot-rolled steel in addition to workability and weldability. Therefore, the upper limit is 6%. From the standpoint of effects and manufacturability, the preferred range is 1.8 to 5%.

N is an unavoidable impurity element contained in the steel, and inhibits the soundness of the Al ₂ O ₃ film targeted by the present invention. Therefore, the lower the amount of N, the better. However, excessive reduction leads to a significant increase in refining costs. Therefore, the upper limit is made 0.02%. From the viewpoint of oxidation resistance and manufacturability, the preferred range is 0.001 to 0.01%. More preferably, the content is 0.003 to 0.008%.

Ti fixes C and N as carbonitrides and has an effect of improving workability and oxidation resistance. Therefore, to express these effects, the lower limit is 0.03%. On the other hand, as the amount of Ti added increases, the amount of solid solution Ti also increases, promoting the formation of Ti-containing oxides under the high water vapor and high oxygen atmosphere, and impairing the soundness of the Al ₂ O ₃ film. . Therefore, the upper limit is set to 0.5% from the viewpoint of securing the oxidation resistance targeted by the present invention. From the viewpoint of effectively expressing the effect of improving the oxidation resistance by adding Ti, the preferred range is 0.1 to 0.3%. More preferably, it is 0.1 to 0.2%.
Nb, like Ti, fixes C and N as carbonitrides and has the effect of improving workability and oxidation resistance. Further, Nb is an effective element that improves the creep rupture life while suppressing the formation of the Ti-containing oxide in the high water vapor and high oxygen atmosphere. In order to obtain these effects, the lower limit is made 0.001%. Preferably it is 0.005% or more. Excessive addition hinders increase in raw material cost and processability. Therefore, the upper limit is 0.6%. From the viewpoint of cost effectiveness, the preferred range is 0.1 to 0.4%. More preferably, the content is 0.15 to 0.3%.

In the present invention, in order to improve the creep rupture life while ensuring the soundness of the Al ₂ O ₃ film, the amount of solid solution in steel is specified in addition to the addition amounts of Ti and Nb. . Hereinafter, the amount of solute Ti and the amount of solute Nb in steel are described as [Ti] and [Nb], respectively. Addition of Ti and Nb fixes C and N as carbonitrides as described above and contributes to the improvement in oxidation resistance which is the object of the present invention. In order to ensure the soundness of the Al ₂ O ₃ film under a high temperature, high water vapor, and high oxygen atmosphere, it is effective to decrease [Ti] and increase [Nb]. The allowable limit of [Ti] for ensuring oxidation resistance is different between a low Cr steel of 16% or less and a high Cr steel of over 16%.

  It has been found that in a low Cr steel of 16% or less, [Ti] ≦ 0.05%, and the allowable limit of [Ti] increases in proportion to [Nb]. Therefore, in the case of 13 ≦ Cr ≦ 16, 0 ≦ [Ti] ≦ [Nb] +0.05, 0 <[Nb] ≦ 0.6. From the viewpoint of improving the creep rupture life, [Ti] is preferably decreased by adding Nb or generating a Ti-based precipitate described later so that [Ti] ≦ 0.3. From the point of emphasizing improvement in oxidation resistance, the formation of Ti-based precipitates is preferably promoted to reduce [Ti] so that [Ti] ≦ 0.2.

  It was found that [Ti] ≦ 0.15% for high Cr steels exceeding 16%, and the allowable limit of [Ti] increases in proportion to 1/2 [Nb]. Therefore, in the case of 16 <Cr ≦ 20, 0 ≦ [Ti] ≦ 1/2 [Nb] +0.15, 0 <[Nb] ≦ 0.6. In the case of high Cr steel, the creep rupture life is higher than that of low Cr steel. From the point of emphasizing improvement in oxidation resistance, Nb addition or Ti precipitation is preferably performed so that [Ti] ≦ 0.2. [Ti] is reduced by the formation of the product.

Zr, La, Y, and REM can be selectively added in the present invention. These elements are expensive elements, although they have a remarkable effect on improving the soundness of the Al ₂ O ₃ film in addition to improving the hot workability. Therefore, when added, the upper limit is made 0.1%. A preferable range is 0.01 to 0.05% from the viewpoint of cost effectiveness.

  B, Mg, and Ca can be selectively added in the present invention. These elements are effective in improving the hot workability. However, excessive addition induces a decrease in manufacturability and surface flaws in hot working. Therefore, when added, the upper limit is made 0.005%. A preferable range is 0.0002 to 0.002% from the viewpoint of manufacturability and effects.

Next, the manufacturing method will be described below.
The Al-containing heat-resistant ferritic stainless steel of the present invention has the above-described components, and defines [Ti] and [Nb] from the viewpoint of improving oxidation resistance and creep rupture life. In order to satisfy [Ti] and [Nb] defined in the present invention, the following production conditions are preferable.

  The Al-containing heat-resistant ferritic stainless steel of the present invention is mainly a cold-rolled annealed sheet obtained by subjecting a hot-rolled steel strip to cold rolling after descaling by omitting annealing or annealing, followed by finish annealing and descaling. Is targeted. In some cases, a hot-rolled annealed plate that is not subjected to cold rolling may be used. Furthermore, for gas piping, a welding rod manufactured from a steel plate is also included. The pipe is not limited to a weld rod, and may be a seamless rod manufactured by hot working.

  The finish annealing of the steel described above is preferably 800 to 1100 ° C. If it is less than 800 ° C., the softening and recrystallization of the steel become insufficient, and predetermined material characteristics may not be obtained. On the other hand, if it exceeds 1100 ° C., it becomes coarse and may impair the toughness and ductility of the steel. More preferably, the temperature is set to 900 ° C. or higher in order to increase [Nb] and improve creep rupture life in addition to oxidation resistance. After these finish annealing, in order to efficiently form Ti-based precipitates and reduce [Ti], it is preferable to set the recrystallization temperature or lower to a slow cooling. Specifically, the average cooling rate in the range of 500 to 800 ° C. is preferably 5 ° C./second or less. The upper limit of the temperature for slow cooling is 800 ° C., more preferably 750 ° C. The lower limit is 500 ° C. Precipitation at less than 500 ° C. takes a long time and is practically difficult.

Note that Ti-based precipitates for reducing [Ti] are TiC, Ti ₄ C ₂ S ₂ , FeTiP, and the like. There are no particular restrictions on the size or dispersion state of these precipitated particles.

Examples where the steel of the present invention is a steel sheet will be described below.
Al-containing ferritic stainless steels having the components shown in Table 3 were melted and hot rolled to produce hot rolled sheets having a thickness of 4.0 to 5.0 mm. Steel No. 1-9 have the component range which this invention prescribes | regulates. Steel No. 10-17 are outside the component range which this invention prescribes | regulates.

鋼Ｎｏ．１〜４、１０〜１７は、熱延板焼鈍を省略して冷間圧延して１．２ｍｍ厚とし，仕上げ焼鈍は８５０〜９５０℃とし，焼鈍後の冷却はガス冷却（１０℃／秒）あるいは水冷（＞１００℃／秒）とした。鋼Ｎｏ．５〜９は、１０００℃で熱延板焼鈍を行い，冷間圧延して１．０ｍｍ厚とし，仕上げ焼鈍は９００〜１０００℃とした。焼鈍後の冷却はガス冷却（１０℃／秒），水冷（＞１００℃／秒），空冷（５℃／秒）とした。

Steel No. 1-4, 10-17 omit hot-rolled sheet annealing and cold-roll to 1.2 mm thickness, finish annealing is 850-950 ° C., cooling after annealing is gas cooling (10 ° C./second) Alternatively, water cooling (> 100 ° C./second) was performed. Steel No. 5-9 performed hot-rolled sheet annealing at 1000 degreeC, cold-rolled to 1.0 mm thickness, and finish annealing was 900-1000 degreeC. Cooling after annealing was gas cooling (10 ° C./second), water cooling (> 100 ° C./second), and air cooling (5 ° C./second).

The obtained steel sheet was measured at [Ti] and [Nb] as described above, 1050 ° C. in a high steam / high oxygen atmosphere, oxidization increase by 300 hr accelerated oxidation test, and 750 ° C. using an accelerated oxidation test material. The exfoliation rate on the surface of the oxide film was measured by heating and cooling thermal cycles. The target oxidation resistance of the present invention is that the oxide film surface hardly peels even after an accelerated oxidation test and a heat cycle accompanying heating and cooling (peeling rate of 10 / cm ² or less). Further, the creep rupture time under conditions of 750 ° C. and initial stress of 10 MPa was determined. The target of the creep life was 4000 hours as a guide.

  Table 4 shows the obtained results. Test Nos. 1 and 2 satisfy both the component range defined by the present invention and [Ti] and [Nb], have the target oxidation resistance of the present invention, and have a good creep rupture life. is there. From this, it can be seen that the production conditions defined in the present invention are not necessarily implemented as long as the component range of the present invention is satisfied.

試番４、５、８、１１、１３、１４、１６、１９、２０、２２、２３は本発明の規定する成分範囲と［Ｔｉ］，［Ｎｂ］を満たすものであり，本発明の規定する緩冷却を施したものである。これらは、本発明の目標とする耐酸化性を有し，クリ−プ破断寿命も良好である。これより，本発明の成分範囲を満たし，本発明で規定する製造条件を実施すれば本発明の目標とする耐酸化性とクリ−プ破断寿命を兼備することができる。

The trial numbers 4, 5, 8, 11, 13, 14, 16, 19, 20, 22, and 23 satisfy the component ranges defined by the present invention and [Ti] and [Nb], and are defined by the present invention. Slowly cooled. These have the oxidation resistance targeted by the present invention and a good creep rupture life. Thus, the oxidation resistance and creep rupture life targeted by the present invention can be achieved by satisfying the component range of the present invention and implementing the production conditions defined in the present invention.

  Although the trial numbers 3, 6, 7, 9, 10, 12, 15, 17, 18, and 21 satisfy the component ranges defined by the present invention, they are outside the production conditions defined by the present invention. The oxidation resistance was not obtained. These did not satisfy the relationship between [Ti] and [Nb] defined in the present invention, and good oxidation resistance could not be obtained. From this, it can be seen that it is important to satisfy both of the relations of [Ti] and [Nb] in addition to the component range defined in the present invention in order to obtain the target oxidation resistance of the present invention.

  The trial numbers 24 to 31 deviate from the components specified in the present invention, and these steel sheets include those satisfying the relationship between [Ti] and [Nb], but the target oxidation resistance in the present invention is obtained. I didn't. From this, it can be seen that it is necessary to satisfy the component range defined in the present invention in order to obtain the target characteristics.

  FIG. 2 shows the relationship between the amount of solute Ti and the amount of solute Nb affecting the oxidation resistance in a low Cr steel of 16% Cr or less. From this, it was confirmed that in order to obtain the target oxidation resistance of the present invention, 0 ≦ [Ti] ≦ [Nb] +0.05 was satisfied as described above.

  FIG. 3 shows the relationship between the amount of solute Ti and the amount of solute Nb affecting the oxidation resistance in high Cr steel exceeding 16% Cr. From this, it was confirmed that in order to obtain the target oxidation resistance of the present invention, 0 ≦ 1/2 × [Ti] ≦ [Nb] +0.15 was satisfied as described above.

  According to the present invention, by defining the solid solution state of steel components, Ti and Nb in steel, the oxidation resistance required for fuel cells can be achieved without resorting to high alloying or addition of expensive elements. It is possible to provide an Al-containing heat-resistant ferritic stainless steel for fuel cells and a method for producing the same, which have both properties and creep rupture life. The Al-containing heat-resistant ferritic stainless steel of the present invention can be industrially produced by subjecting it to slow cooling after finish annealing or further heat treatment, regardless of a special manufacturing method.

Claims (3)

In mass%
C: less than 0.02%,
Si: more than 0.15 to 0.7%,
Mn: 0.3% or less,
P: 0.035% or less,
S: 0.003% or less,
Cr: 13-20%,
Al: 1.5-6%,
N: 0.02% or less,
Ti: 0.03 to 0.5% or less,
Nb: 0.6% or less, the balance consisting of Fe and inevitable impurities,
The amount of solid solution Ti in the steel is [Ti] and the amount of solid solution Nb is [Nb]. When 13 ≦ Cr ≦ 16, the following equation (a) is satisfied, and when 16 <Cr ≦ 20, the following (b ) Satisfying the formula, any one or more of TiC, Ti ₄ C ₂ S ₂ , and FeTiP are precipitated , and 1050 ° C., 300 h in an oxidizing atmosphere of 20% steam, 20% oxygen, and the remaining nitrogen gas. After the accelerated oxidation test, the oxide film surface peeling rate was 10 pcs / cm ² when the heating / cooling heat cycle was performed 600 times after heating to 750 ° C. for 30 minutes in the air and then allowing to cool for 10 minutes. An Al-containing heat-resistant ferritic stainless steel for fuel cells used in a high temperature, high water vapor, high oxygen atmosphere characterized by:
0 ≦ [Ti] ≦ [Nb] +0.05, 0 <[Nb] ≦ 0.6 (a)
0 ≦ [Ti] ≦ 1/2 × [Nb] +0.15, 0 <[Nb] ≦ 0.6 (b)   The steel is further in mass%, Zr: 0.1% or less, La: 0.1% or less, Y: 0.1% or less, REM: 0.1% or less, B: 0.005% or less, 2. The Al-containing heat-resistant ferritic stainless steel for a fuel cell according to claim 1, comprising one or more of Mg: 0.005% or less and Ca: 0.005% or less. It is a manufacturing method of the Al content heat-resistant ferritic stainless steel for fuel cells as described in any one of Claim 1 or Claim 2 , Comprising: Hot-working the slab of the said ferritic stainless steel, and cold-working after that, In the manufacturing process combined with annealing, heat treatment is performed at 800 to 1100 ° C. after hot working or after cold working, and the average cooling rate in the temperature range of 500 to 800 ° C. is 5 ° C./second or less. A method for producing Al-containing heat-resistant ferritic stainless steel for fuel cells.

Images