JP2018080371A - Ferritic stainless steel and method for producing the same, and fuel cell member - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a ferritic stainless steel for fuel cells that has markedly excellent creep properties to be an index of durability under a high temperature environment including a modified gas.SOLUTION: A ferritic stainless steel contains, in mass%, Cr: 11-25%, C: 0.001% or more and 0.03% or less, Si: 0.01% or more and 2.0% or less, Mn: 0.01% or more and 2.0% or less, Al: 0.5% or more and 4.0% or less, P: 0.05% or less, S: 0.01% or less, N: 0.03% or less; and further contains, optionally, at least one of Ti: 1% or less and Nb: 1% or less; and contains at least one of B: 0.0005% or more and 0.0025% or less and Sn: 0.005% or more and 0.5% or less, with the balance being Fe and inevitable impurities. The aggregate structure of it has a random intensity ratio in {211} <011> orientation of 2.5 or more.SELECTED DRAWING: Figure 1

Description

  The present invention is suitable for high-temperature members of fuel cells such as reformers and heat exchangers used when reforming hydrocarbon fuels such as city gas, methane, natural gas, propane, kerosene, and gasoline into hydrogen. The present invention relates to a ferritic stainless steel and a manufacturing method thereof. In particular, it is suitable for a high-temperature member of a solid oxide fuel cell (SOFC) that requires creep resistance with suppressed material damage and oxidation resistance in a high-temperature environment including a reformed gas environment.

Recently, the spread of new systems replacing conventional power generation systems is accelerating due to problems such as depletion of fossil fuels such as petroleum and global warming due to CO ₂ emissions. 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, polymer electrolyte fuel cells (PEFC) and solid oxide fuel cells (SOFC) have high energy efficiency, and are expected to expand in the future.

  A fuel cell is a device that generates electric power through a reaction process opposite to that of water electrolysis, and requires hydrogen. Hydrogen is produced by reforming a hydrocarbon fuel such as city gas (LNG), methane, natural gas, propane, kerosene, and gasoline in the presence of a catalyst. Above all, a fuel cell using city gas as a raw fuel has an advantage that hydrogen can be produced in an area where city gas piping is provided.

  The fuel reformer is usually operated at a high temperature of 200 to 900 ° C. in order to ensure the amount of heat necessary for the hydrogen reforming reaction. Further, under such a high temperature operation, it is exposed to an oxidizing atmosphere containing a large amount of water vapor, carbon dioxide, carbon monoxide and the like, and the heating / cooling cycle by starting and stopping is repeated according to the demand for hydrogen. Until now, austenitic stainless steel represented by SUS310S (25Cr-20Ni) has been used as a practical material having sufficient durability under such a severe environment. In the future, cost reduction is indispensable for the spread of fuel cell systems, and reduction of alloy costs by optimizing the materials used is an important issue.

  From the background described above, application of Al-containing ferritic stainless steel having high oxidation resistance to alumina to a fuel reformer is disclosed.

In Patent Document 1, Cr: 8-35%, C: 0.03% or less, N: 0.03% or less, Mn: 1.5% or less, Si: 0.8-2.5% and / or Al: 0.6 to 6.0%, Nb: 0.05 to 0.80%, Ti: 0.03 to 0.50%, Mo: 0.1 to 4%, Cu: 0.1 A ferritic stainless steel for a petroleum fuel reformer is disclosed that contains one or two or more of -4% and has a composition in which the total amount of Si and Al is adjusted to 1.5% or more. In this stainless steel, in a thermal fatigue test in which a material is repeatedly heated and cooled in a temperature range of 200 to 900 ° C. (restraint rate: 50%), the repetition of damage at which the initial maximum tensile stress is reduced to 3/4 is 500 cyc or more. It is characterized by that. Further, the oxidation resistance is evaluated in 50 volume% H ₂ O + 20 volume% CO ₂ and 50 volume% H ₂ O + 10 ppm SO ₂ assuming an atmosphere to which the petroleum fuel reformer is exposed.

In Patent Document 2, Cr: 8 to 25%, C: 0.03% or less, N: 0.03% or less, Si: 0.1 to 2.5%, Mn: 1.5% or less, Al: Including 0.1 to 4%, Nb: 0.05 to 0.80%, Ti: 0.03 to 0.5%, Mo: 0.1 to 4%, Cu: 0.1 to 4% Ferritic stainless steel for alcohol-based fuel reformers containing one or more types is disclosed. In this stainless steel, in a thermal fatigue test in which a material is repeatedly heated and cooled in a temperature range of 200 to 900 ° C. (restraint rate: 100%), the repetition of breakage in which the initial maximum tensile stress is reduced to 3/4 is 1000 cyc or more. It is characterized by that. Further, the oxidation resistance is evaluated in 50 volume% H ₂ O + 20 volume% CO ₂ assuming an atmosphere to which the alcohol fuel reformer is exposed.

In Patent Document 3, Cr: 12 to 20%, C: 0.03% or less, N: 0.03% or less, Si: 0.1 to 1.5%, Mn: 0.95 to 1.5% Al: 1.5% or less, including Nb: 0.1 to 0.8, Mo: 0.1 to 4%, Cu: 0.1 to 4.0 or more, A = A ferritic stainless steel for a hydrocarbon fuel reformer in which an A value defined by Cr + Mn + 5 (Si + Al) is adjusted to a range of 15 to 25 is disclosed. In this stainless steel, in a thermal fatigue test in which a material is repeatedly heated and cooled in a temperature range of 200 to 900 ° C. (constraint rate 100%), the repeated repetition of failure in which the initial maximum tensile stress is reduced to 3/4 is 800 cyc or more. It is characterized by that. Further, the oxidation resistance is evaluated in 50 volume% H ₂ O + 20 volume% CO ₂ assuming an atmosphere to which the hydrocarbon fuel reformer is exposed.

In Patent Document 4, C: less than 0.02%, Si: 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: 0.03-0.5%, Nb: 0.001-0.1% or less, solid in steel When the amount of dissolved Ti is [Ti] and the amount of dissolved Nb in the steel is [Nb], and 13 ≦ Cr ≦ 16, 0 ≦ [Ti] ≦ [Nb] +0.05, 0 <[Nb] ≦ 0. When 10 is satisfied and 16 <Cr ≦ 20, 0 ≦ [Ti] ≦ 1/2 × [Nb] +0.15, [Ti] ≦ 0.12, and 0 <[Nb] ≦ 0.1 are satisfied. An Al-containing ferritic stainless steel for fuel cells is disclosed. This stainless steel is characterized by a creep rupture time of 4000 h or more at 750 ° C. and an initial stress of 10 MPa. The oxidation resistance is evaluated at 1050 ° C. in 20% by volume H ₂ O + 20% by volume O ₂ (remainder nitrogen).

  In Patent Document 5, C: 0.001 to 0.03%, Si: 0.01 to 2%, Mn: 0.01 to 1.5%, P: 0.005 to 0.05%, S: 0.0001-0.01%, Cr: 16-30%, N: 0.001-0.03%, Al: 0.8-3%, Sn: 0.01-1% A high-purity ferritic stainless steel sheet excellent in oxidation resistance and high-temperature strength is disclosed, in which the 0.2% proof stress is 40 MPa or more and the tensile strength is 60 MPa or more. The oxidation resistance of this stainless steel is evaluated at 1050 ° C. in the atmosphere.

  In order to apply ferritic stainless steel to a high temperature member of a fuel reformer or a fuel cell system, it is necessary that deformation is small at the time of high temperature use. From this point of view, Patent Documents 1 to 3 show the number of cycles in which the material is damaged by the thermal fatigue test, Patent Document 4 shows the time for the material to break in the creep test, and Patent Document 5 shows the high temperature strength measured by the high temperature tensile test. , Each of them is subject to evaluation. Since it is used for a long time at a high temperature in an actual use environment, it is predicted that the creep characteristic is optimal as an evaluation index. However, only Patent Document 4 considers from such a viewpoint.

  Many techniques have been disclosed so far for improving the creep characteristics of ferritic heat resistant steel, but it is known that B is effective as an additive element.

  In Patent Document 6, C: 0.01 to 0.05%, Si: 0.01 to 0.8%, Mn: 2% or less, P: 0.05% or less, S: 0.01% or less, Cr: 8 to 13%, Ni: 0.1 to 2.0%, W alone or a combination of W and Mo is added to 0.50 to 2.5%, V: 0.05 to 0.30%, Nb: Welding characterized by containing 0.02-0.20%, B: 0.001-0.01%, Al: 0.005-0.20%, N: 0.01-0.06%. Ferritic heat-resistant steel having excellent toughness of the part is disclosed.

  In Patent Document 7, C: 0.001 to 0.03%, Si: 0.01 to 2%, Mn: 0.01 to 1.5%, P: 0.005 to 0.05%, S: 0.0001 to 0.01%, Cr: 16 to 30%, N: 0.001 to 0.03%, Al: more than 0.8% to 3%, Sn: 0.01 to 1%, the balance being Fe And high-purity ferritic stainless steel excellent in oxidation resistance and high-temperature strength, characterized by comprising 0.2% proof stress at 800 ° C. of 40 MPa or more and tensile strength of 60 MPa or more. A steel sheet and a manufacturing method thereof are disclosed.

In Patent Document 8, C: 0.01 or more and less than 0.08%, N: 0.01 to 0.10%, Si: 0.50% or less, Mn: 0.05 to 0.50%, Cr: 8.00 to 13.00%, W: more than 1.50% to 3.50%, Mo: 0.50% or less, V: 0.10 to 0.30%, Nb: 0.01 to 0.15 %, Ni: 0.20% or less, Co: 0.20% or less, Cu: 0.20% or less, B: 0.0010 to 0.0100% A high chromium ferritic heat resistant steel is disclosed.
In addition, the influence of Sn on creep properties in Al-containing ferritic stainless steel has not been clarified so far.

Japanese Patent No. 3886785 Japanese Patent No. 3910419 Japanese Patent No. 3942876 Japanese Patent No. 5544106 Japanese Patent No. 5709570 Japanese Patent No. 3475621 Japanese Patent No. 5709570 Japanese Patent No. 3869908

  In recent years, in the case of an SOFC system that is expected to spread widely, components such as a fuel reformer and a heat exchanger are continuously operated in a temperature range of 500 to 800 ° C. Therefore, it is important for Al-containing ferritic stainless steel used in these parts to suppress creep deformation during high-temperature operation, particularly slight deformation around 700 ° C. from the viewpoint of improving durability as a structure. It is positioned as.

  The steel materials disclosed in Patent Documents 1 to 5 have an increased life against damage / destruction of the material, and the effectiveness against slight creep deformation during high temperature operation has not been studied at all. Furthermore, no mention is made of the effect of trace elements effective on such creep strength. On the other hand, the object of the technique disclosed in Patent Document 6 and Patent Document 7 is that the metal structure is a tempered martensite structure, and in such a metal structure, the trace elements are stabilized in carbides and strengthened grain boundaries. It is thought that it contributes to improve creep characteristics.

  However, Al-containing ferritic stainless steel used for SOFC has a ferrite single-phase structure that does not cause martensitic transformation, and there is no description regarding a technique that suggests improvement of creep characteristics in Al-containing stainless steel.

On the other hand, it is known that the creep characteristics are affected by the crystal orientation (texture) of the steel material. The cause of this is considered to be an active sliding system and the angle of the tensile direction as important factors (see, for example, LAdel Valle et al .: Acta Materialia, 55 (2007), p455-466).
Since the deformation stress (critical decomposition shear stress) of each active slip system varies depending on the additive element, when the creep characteristics are improved by texture control, it is predicted that the optimum texture for the creep characteristics varies depending on the steel composition.

As described above, the ferritic stainless steel for fuel cells realized by controlling the texture with respect to creep characteristics that are important as durability in a high temperature environment containing reformed gas has not yet appeared.
The present invention was made for the purpose of providing an Al-containing ferritic stainless steel having good creep characteristics, which is suitable as a structural material for various members for fuel cells. Is the gist.

(1) In mass%, Cr: 11-25%, C: 0.001% to 0.03%, Si: 0.01% to 2.0%, Mn: 0.01% to 2. 0% or less, Al: 0.5% or more and 4.0% or less, P: 0.05% or less, S: 0.01% or less, N: 0.03% or less, and further if necessary Ti: 1% or less, Nb: 1% or less, including one or more, B: 0.0005% or more and 0.0025% or less, Sn: 0.005% or more and 0.5% or less A ferritic stainless steel characterized in that it has a texture with a random intensity ratio of 2.5 or more in {211} <011> orientation, with the balance consisting of Fe and inevitable impurities. steel.
(2) In mass%, Ni: 1% or less, Cu: 1% or less, Mo: 2% or less, W: 1% or less, Co: 0.5% or less, V: 0.5% or less, Ca: 0.005% or less, Mg: 0.005% or less, Zr: 0.5% or less, Sb: 0.5% or less, La: 0.1% or less, Y: 0.1% or less, Hf: The ferritic stainless steel according to the above (1), which contains one or more of 0.1% or less and REM: 0.1% or less.
(3) When manufacturing the steel having the composition according to any one of the above (1) to (2), after the hot rolling, the heat treatment performed at over 700 ° C. is omitted, and then the rolling rate is 25 to 70%. A method for producing ferritic stainless steel, characterized by combining cold rolling and heat treatment.
(4) A fuel cell member using the ferritic stainless steel according to any one of (1) to (2).

It is a graph which shows the relationship between {211} <011> random strength ratio of two types of steel, and the minimum creep speed | rate (epsilon) '(% / h).

  In order to solve the above-described problems, the present inventors have conducted intensive experiments and studies on the relationship between the composition of the Al-containing ferritic stainless steel, the texture and the creep characteristics, and completed the present invention. The knowledge obtained by the present invention will be described below.

(A) As a result of investigating the relationship between the creep characteristics and crystal orientation of Al-containing ferritic stainless steel cold-rolled annealed plates, {211} <011> ({} is the direction perpendicular to the plate surface (ND), <> is rolled. It has been found for the first time that there is a good correlation with the random intensity ratio in the direction parallel to the direction (RD). FIG. 1 shows the relationship between the {211} <011> random strength ratio and the minimum creep rate ε ′ (% / h) of 18% Cr steel and 18% Cr-2% Al based steel. The creep test piece was collected in a direction parallel to the rolling direction, and the creep test was carried out using a plate-like test piece in accordance with JIS Z 2271 at a temperature of 750 ° C. and a stress of 10 MPa. The random intensity ratio is the intensity at the {211} <011> position in the φ2 = 45 ° section by creating a three-dimensional display diagram (ODF) from the reflection X-ray analysis results of the (200), (110), and (111) planes. Calculated from From the figure, only in 18% Cr-2% Al steel, when the {211} <011> strength ratio exceeds 2.5, the creep rate is remarkably lowered, that is, the creep characteristics are improved.
(B) The random intensity ratio of {211} <011> varies depending on the manufacturing process (conditions), but particularly varies greatly depending on the heat treatment conditions and the cold rolling rate after hot rolling.

  Hereinafter, each requirement of the present invention will be described in detail. In addition, "%" display of the content of each element means "mass%".

  The reason for limiting the component (I) will be described below.

  Cr is an element that improves corrosion resistance, creep characteristics, and oxidation resistance. In the present invention, if it is less than 11%, the target creep characteristics and oxidation resistance are not sufficiently ensured. Therefore, the lower limit is 11%. However, excessive addition of Cr not only promotes the formation of the σ phase which is an embrittlement phase when exposed to a high temperature atmosphere, but also increases the alloy cost, so the upper limit is made 25%. From the viewpoint of basic characteristics, oxidation resistance and manufacturability, the preferred range is 13 to 22%. A more preferable range is 16 to 20%.

  Since C deteriorates corrosion resistance, it is preferably as small as possible, and the upper limit is made 0.03%. However, excessive reduction leads to an increase in refining costs, so the lower limit is made 0.001%. From the viewpoint of creep characteristics, oxidation resistance and manufacturability, the preferred range is 0.002 to 0.02%.

  Si is an element that improves oxidation resistance. In order to obtain this effect, the lower limit is made 0.01%. On the other hand, excessive addition may impair the toughness and workability of the steel, and the formation of the Al-based oxide film targeted by the present invention, so the upper limit is made 2%. From the viewpoint of oxidation resistance, manufacturability and moldability, 1% or less is preferable. When actively utilizing the effect of Si, it is preferable to set the content within a range of 0.3 to 1%.

  Mn is dissolved in the oxide film together with Si in a reformed gas environment to enhance the protective property. In order to obtain this effect, the lower limit is made 0.01%. On the other hand, excessive addition inhibits the corrosion resistance of steel and the formation of Ti and Al-based oxide films targeted by the present invention, so the upper limit is made 2% or less. From the viewpoint of oxidation resistance and basic characteristics, 1% or less is preferable. When actively utilizing the effect of Mn, the content is preferably set in the range of 0.2 to 1%.

  In addition to the deoxidizing element, Al is an additive element essential for forming an Al-based oxide film targeted by the present invention and suppressing Cr evaporation. In the present invention, it is an important element that ensures creep characteristics in combination with the texture. If it is less than 0.5%, the target Cr evaporation suppression effect cannot be obtained. Therefore, the lower limit is 0.5%. However, excessive addition of Al not only exhibits the effect of improving the creep characteristics by the combination with the texture, but also reduces the toughness and weldability of the steel and inhibits the productivity, so the upper limit is made 4.0%. . From the viewpoint of inhibiting Cr evaporation and creep characteristics of the present invention, the preferred range is 1.0 to 3.5%. A more preferable range for production is 1.5 to 2.5%.

  P is an element that inhibits manufacturability and weldability, and the lower the content, the better. Therefore, the upper limit is made 0.05%. However, excessive reduction leads to an increase in refining costs, so the lower limit is preferably 0.003%. From the viewpoint of manufacturability and weldability, the preferred range is 0.005 to 0.04%, more preferably 0.01 to 0.03%.

  S is an unavoidable impurity element contained in the steel, and lowers the protective property of the Al-based film targeted by the present invention. In particular, the presence of Mn-based inclusions and solute S also acts as a fracture starting point for Al-based oxide films when used at high temperatures for long periods of time. Therefore, the lower the amount of S, the better, so the upper limit is made 0.01%. However, excessive reduction leads to an increase in raw materials and refining costs, so the lower limit is made 0.0001%. From the viewpoint of manufacturability and oxidation resistance, the preferred range is 0.0001 to 0.002%, more preferably 0.0002 to 0.001%.

  N, like C, inhibits the target oxidation resistance of the present invention. For this reason, the smaller the amount of N, the better. The upper limit is made 0.03%. However, excessive reduction leads to cost increase such as longer refining time, so the lower limit is preferably made 0.002%. From the viewpoint of oxidation resistance and manufacturability, the preferred range is 0.005 to 0.02%.

One or two of Ti and Nb are added as follows.
Ti improves creep properties and oxidation resistance through high purity of steel by the action of a stabilizing element that fixes C and N. In order to obtain these effects, the lower limit is preferably 0.01%. On the other hand, excessive addition leads to a decrease in manufacturability and a decrease in oxidation resistance accompanying an increase in alloy costs and a recrystallization temperature, so the upper limit is made 1.0%. From the viewpoint of alloy cost, manufacturability and oxidation resistance, the preferred range is 0.05 to 0.5%. Furthermore, the suitable range which utilizes the effect of Ti actively is 0.1 to 0.4%.
Nb improves creep properties and oxidation resistance through high purity of steel by the action of a stabilizing element that fixes C and N. In order to obtain these effects, the lower limit is preferably 0.01%. On the other hand, excessive addition leads to a decrease in manufacturability and a decrease in oxidation resistance accompanying an increase in alloy costs and a recrystallization temperature, so the upper limit is made 1.0%. From the viewpoint of alloy cost, manufacturability and oxidation resistance, the preferred range is 0.05 to 0.5%. Furthermore, the suitable range which utilizes the effect of Nb actively is 0.2 to 0.6%.

One or two of B and Sn are added as follows.
B has an effect of improving secondary workability in addition to creep characteristics. Since the improvement effect is exerted by addition of 0.0005% or more, this is the lower limit. On the other hand, excessive addition causes deterioration of manufacturability, particularly productivity at the time of continuous casting. In addition, the effect of improving creep characteristics is saturated, so 0.0025% is made the upper limit. A preferable range is 0.0005 to 0.0012%.
Sn is an element that has an effect of improving creep characteristics and has an effect of improving corrosion resistance depending on the environment. Since these effects are exhibited at 0.005% or more, this is the lower limit. On the other hand, addition of a large amount causes deterioration of manufacturability, so 0.5% is made the upper limit. In consideration of manufacturability, the preferable range is 0.008 to 0.12%.

  Ni, Cu, Mo, W, Co, and V are effective elements for increasing the high temperature strength and corrosion resistance of the member, and are added as necessary. However, excessive addition leads to an increase in alloy cost and obstructs manufacturability, so the upper limit of Ni, Cu and W is 1%. Since Mo is an element effective for suppressing high-temperature deformation due to a decrease in the thermal expansion coefficient, the upper limit is made 2%. The upper limit of Co and V is 0.5%. The lower limit of the more preferable content of any element is 0.1%.

  Ca and Mg are elements that improve hot workability and secondary workability, and are added as necessary. However, excessive addition leads to a decrease in manufacturability, so the upper limit is 0.005%. Preferable lower limits are each 0.0001%.

  Zr, Sb, La, Y, Hf, and REM are effective elements for improving hot workability and steel cleanliness and improving oxidation resistance, and may be added as necessary. However, from the technical idea of the present invention and the reduction of alloy costs, it does not depend on the effect of addition of these elements. When added, the upper limit of Zr and Sb is 0.5%, and the upper limit of La, Y, Hf, and REM is 0.1%. A more preferable lower limit of Zr is 0.01%, and a preferable lower limit of La, Y, Hf, and REM is 0.001%. Here, REM is an element belonging to atomic numbers 57 to 71, such as Ce, Pr, and Nd.

  In addition to the elements described above, the elements of the present invention can be contained within a range not impairing the effects of the present invention. Bi, Pb, Se, H, Ta and the like are preferably reduced as much as possible. On the other hand, the content ratio of these elements is controlled within the limits to solve the problems of the present invention, and if necessary, Bi ≦ 100 ppm, Pb ≦ 100 ppm, Se ≦ 100 ppm, H ≦ 100 ppm, Ta ≦ 500 ppm. It may contain seeds or more.

Next, the texture will be described.
The random intensity ratio of {211} <011> is set to 2.5 or more. Random intensity ratio is measured at the center (t / 2), 3t / 8, t / 4, and t / 8 of the thickness (t) by using reflected X-rays to create ODF (3D representation of texture) Then, the random intensity ratio at φ1 = 0 ° and Φ = 30 ° in the ODF φ2 = 45 ° cross section may be read and averaged. Alternatively, measure the crystal orientation in the cross section (L cross section (plane perpendicular to the TD direction) or C cross section (plane perpendicular to the RD direction)) using EBSD, and make ODF on the analysis software. You may obtain | require by said method. However, in the EBSD method, unlike the X-ray, since the measurement range is generally narrow, it is necessary to measure a wide range of 3 mm or more in the width or the longitudinal direction of the rolling with the total thickness and obtain an average value. .
The {211} <011> orientation random strength ratio is 0.5 or less for a normal annealed plate. In order to set this value to 2.5 or more, it is necessary to devise a manufacturing method described later.

Next, the manufacturing method will be described below.
The ferritic stainless steel of the present invention is manufactured by a process of combining cold rolling and heat treatment after hot rolling.
In the present invention, “heat treatment” refers to an operation in which a target material is artificially heated and held for a certain period of time. However, the material is heated and heated to around 100 ° C. by processing heat generated during cold rolling, but this heating and heating by cold rolling is not included in the heat treatment.
Conventionally, the heat treatment performed at over 700 ° C. after hot rolling is omitted. When the heat treatment is performed at a temperature higher than 700 ° C., not only an important texture is not obtained in the present invention, but also the toughness is deteriorated in the cold rolling process, and the ear cracks and the plate breakage occur.

  Before cold rolling, the surface scale is removed by pickling or coil grinder. The cold rolling rate is 25 to 70%. If the cold rolling rate is less than 25%, the random intensity ratio of {211} <011> is less than 2.5, so this was set as the lower limit. Further, even when it is 70% or more, the random intensity ratio of {211} <011> is not satisfied. Considering cold rolling productivity and surface characteristics, the cold rolling rate is preferably in the range of 33% to 67%. The heat treatment temperature after cold rolling is not particularly specified, but is preferably in the range of 850 ° C to 1000 ° C. The atmosphere during the heat treatment is not particularly specified, but considering the surface pickling property after the heat treatment, a BA (bright annealing) atmosphere is preferable.

The reason why a steel excellent in creep characteristics can be obtained by the present invention is under intensive study, but at present, it is considered as follows.
The relationship between the texture and the creep properties is as described above. However, in the steel containing a relatively large amount of Al as in the present invention, the active slip system in the steel and the accumulation form of dislocations are not added to Al. And are expected to be different. It is known that when a large amount of Si is added to steel, the active slip system changes (for example, Barrett et al: Trans. ASM 25 (1937), 702).
Al is located next to Si in the periodic table, and is highly likely to exhibit an effect close to Si. Therefore, it is surmised that the activity slip system changed when Al was added to the steel. That is, in the present invention, the active slip system is changed by containing Al, and it is considered that good creep characteristics are obtained particularly in the {211} <011> orientation.

  In the present invention, in the ferritic stainless steel containing Al as described above, the important point is that the creep characteristics are improved by controlling the specific crystal orientation (texture).

  Examples of the present invention will be described below.

  Various ferritic stainless steels having the components shown in Table 1 are melted and subjected to hot rolling, annealing pickling, and cold rolling, and cold rolled steel sheets having a thickness of 0.8 to 2.0 mm under the conditions shown in Table 2. Manufactured. From the L cross section of the obtained steel plate, the crystal orientation was measured in the range of total plate thickness × length 3 mm using EBSD, and the random strength ratio of {211} <011> was obtained.

The creep test was a constant load test in accordance with JIS Z 2271, and a plate-shaped test piece having a parallel portion of 10 mm and a width of 35 mm was used. The test conditions were set to 750 ° C. and initial stress of 10 MPa, and the minimum creep rate was evaluated in order to evaluate the creep resistance strength related to slight high temperature deformation, which is the subject of the present invention. When the minimum creep rate was smaller than 5.0 × 10 ⁻³ (% / h), the creep characteristics were evaluated as good.

  The obtained results are also shown in Table 2. The examples of the present invention satisfying the components specified in the present invention and the metal structure satisfy high creep characteristics. No. in Table 2 In Nos. 1 to 27, steel components are within the scope of the present invention, and manufacturing conditions (hot rolled sheet annealing temperature and cold rolling rate) satisfy the present invention (No. 2, 4, 6, 8, 11, 15, 17, 20, 23 and 25) show good creep characteristics. No. In 28 to 39, the steel component is outside the scope of the present invention. That is, no. Nos. 28, 29, 37, and 38 have a texture within the range of the present invention, but the Al content is outside the range of the present invention, so that the creep characteristics are poor. No. Nos. 31, 34, and 35 have B or Sn outside the scope of the present invention, so that the texture is within the scope of the present invention, but the creep characteristics are poor.

  According to the present invention, an Al-containing ferritic stainless steel having good creep characteristics can be provided. This steel material is a material suitable for a fuel cell reformer.

Claims (4)

  In mass%, Cr: 11-25%, C: 0.001% to 0.03%, Si: 0.01% to 2.0%, Mn: 0.01% to 2.0% Al: 0.5% or more and 4.0% or less, P: 0.05% or less, S: 0.01% or less, N: 0.03% or less, and if necessary, Ti: 1 type or less, Nb: 1% or less including 1% or less, and B: 0.0005% or more and 0.0025% or less, Sn: 0.005% or more and 0.5% or less Alternatively, a ferritic stainless steel including two or more types, the balance being Fe and inevitable impurities, and having a texture with a random intensity ratio of {211} <011> orientation of 2.5 or more.   Further, by mass%, Ni: 1% or less, Cu: 1% or less, Mo: 2% or less, W: 1% or less, Co: 0.5% or less, V: 0.5% or less, Ca: 0 0.005% or less, Mg: 0.005% or less, Zr: 0.5% or less, Sb: 0.5% or less, La: 0.1% or less, Y: 0.1% or less, Hf: 0.1 % Or less, REM: 0.1% or less of 1 type or 2 types or more are contained, The ferritic stainless steel of Claim 1 characterized by the above-mentioned.   In manufacturing the steel having the composition according to any one of claims 1 to 2, after the hot rolling, the heat treatment performed at over 700 ° C is omitted, and then the cold rolling at a rolling rate of 25 to 70%. And ferritic stainless steel production method characterized by combining heat treatment.   The member for fuel cells using the ferritic stainless steel of any one of Claims 1-2.

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