JP2020111806A - Stainless steel sheet and method for producing the same, separator for fuel battery, fuel battery cell, and fuel battery stack - Google Patents

Abstract

To provide a stainless steel sheet which has excellent corrosion resistance, low contact resistance and excellent press workability even when a thickness is thinned.SOLUTION: The stainless steel sheet has a chemical composition containing, by mass%, Cr of 20-26%, N of 0.1% or less, Si of 2.0% or less and the like, has a thickness of 30-110 μm, has a ratio of an austenite phase of 80 vol.% or more, has an average crystal particle diameter of austenite crystal grains of 1/2 or less the thickness of the steel sheet, has a Cr nitride precipitated to a surface layer, has contents of Cr, Fe and N obtained by a glow-discharge optical emission spectrometry satisfying the following expressions (1) and (2), and has a maximum deviation value CNp at a depth of 3/8 t to 5/8 t and an average value CNa at a depth of 1/8 t to 3/8 t satisfying the following expression (3). Expression (1): 0.1≤Cr/Fe≤0.5. Expression (2): Cr/N≤1.8. Expression (3): 0.8≤CNp/CNa≤1.6.SELECTED DRAWING: Figure 2

Description

The present invention relates to a stainless steel plate, a method for manufacturing the same, a fuel cell separator, a fuel cell, and a fuel cell stack.

As a material having conductivity, metal-based materials are used for various purposes. One of such applications is, for example, a fuel cell separator. A fuel cell uses the energy generated during the binding reaction between hydrogen and oxygen to generate electricity. Therefore, there are expectations for both energy conservation and environmental measures. Fuel cells are classified into a solid oxide type, a molten carbonate type, a phosphoric acid type, a solid polymer type, and the like depending on the type of electrolyte. The polymer electrolyte fuel cell has a high output density and can be miniaturized. It also operates at a lower temperature than other types of fuel cells and is easier to start and stop. For this reason, it is expected to be used for electric vehicles and small cogeneration for home use.

For example, the main functions required for a polymer electrolyte fuel cell separator (hereinafter, sometimes simply referred to as “separator”) are as follows.
(1) Function as a "flow path" for uniformly supplying fuel gas and oxidizing gas within the electrode surface,
(2) Function as a "flow path" for efficiently discharging the water generated on the cathode side from the fuel cell together with the carrier gas such as air and oxygen after the reaction,
(3) The function of contacting the electrode film and forming a path for electricity, and further serving as an electrical "connector" between the unit cells,
(4) A function as a "partition wall" between adjacent cells and an anode chamber of one cell and a cathode chamber of the other cell, and (5) In the water-cooled fuel cell, a cooling water flow path is formed. Functions as a "partition wall" between adjacent cells.

The base material of the separator required to have such a function is roughly classified into a metal-based material and a carbon-based material.

Examples of the metallic material include stainless steel, titanium, carbon steel and the like. A separator made of a metal-based material (hereinafter referred to as “metal separator”) is manufactured by press working or the like. Since the metal separator has excellent workability peculiar to metal, it has an advantage that the thickness can be reduced and the weight can be reduced. On the other hand, there is a concern that the electrical conductivity may decrease due to the elution of metal ions due to corrosion and the oxidation of the metal surface. Therefore, the metal separator has a problem that the contact resistance with the gas diffusion layer may increase.

The environment to which the separator is exposed is very harsh. The operating temperature of the battery fluctuates from room temperature to 80° C. or higher, and a potential is applied to the separator during power generation. The potential range is different between the anode and the cathode, but ranges from about 0 to about 1 V vs SHE. Since the separator is used in a humid environment, it is required to reduce the elution of harmful metal ions and suppress the occurrence of corrosion such as pitting corrosion by surface treatment or alloy design.

As means for solving the above problem, Japanese Patent Laid-Open No. 2004-296381 proposes that the surface of the metal separator in contact with the electrode is plated with gold.

Japanese Patent No. 5120799 discloses a separator based on austenitic stainless steel having an N content of more than 1.00% by mass and 2.00% by mass or less.

Japanese Patent No. 3676679 discloses a stainless steel separator for a fuel cell in which a conductive ceramic layer such as TiN is formed on the surface of the fuel cell facing the fuel electrode.

Japanese Unexamined Patent Publication No. 2010-49980 discloses a fuel cell separator including a nitriding layer that is obtained by nitriding the surface of a base material made of stainless steel and that is continuously present in the depth direction from the surface of the base material. It is disclosed.

In JP-A-2006-316338, a ferritic stainless steel containing Cr: 18 to 24% and Mo: 0 to 4% by mass is contacted with an inert gas containing nitrogen gas at 800° C. or higher. A method for producing a stainless steel product, which is subjected to a nitrogen absorption treatment, is described.

In WO 2013/018322, it is described that a stainless steel plate is subjected to an electrolytic treatment and an acid treatment of being immersed in a pickling solution to impart fine irregularities to the surface to stabilize the contact resistance.

Japanese Patent No. 6315158 discloses a method in which a Fe-Cr-based stainless steel having a low Ni content is formed into a steel sheet, and then nitrogen is absorbed to austenite-phase the structure, so that an expensive raw material such as Mo is not used. It is described that corrosion resistance can be improved and good press workability can be secured.

JP 2004-296381 A Japanese Patent No. 5120799 Japanese Patent No. 3667679 JP, 2010-49980, A JP 2006-316338 A International publication 2013/018322 Japanese Patent No. 6315158

In Japanese Patent Laid-Open No. 2004-296381, the surface of stainless steel is plated with gold. However, since gold is a rare and expensive metal, there are problems from the viewpoint of economic efficiency and resource limitation.

In Japanese Patent No. 5120799, steel is melted using a pressurized ESR (electro slag remelting) method in which nitrogen is pressurized to 1 atm or more, and high N content stainless steel is produced at the stage of ingot (slab). .. High N content stainless steel has high strength and requires strong working, so it is difficult to make it thin. In particular, it is very difficult to realize a thickness of around 100 μm, which is preferable as a steel plate for a separator. In Japanese Patent No. 5120799, a separator having a thickness of 5 mm is machined into a separator shape.

The method of manufacturing a separator from a slab or thick plate of high-strength and difficult-to-process high-strength stainless steel by mechanical cutting has poor mass productivity and is not an economical manufacturing method. Even if a thin steel plate is obtained by cold rolling, nitride precipitates in the annealing process for releasing the processing strain caused by cold rolling, and ductility decreases. Therefore, it is difficult to process the obtained steel sheet into a separator shape by press forming.

Regarding the contact resistance, it has been proposed to form a conductive nitride layer on the surface of a stainless steel plate in a post-process, as in the techniques described in Japanese Patent No. 3667679 and Japanese Patent Laid-Open No. 2010-49980. However, since the vapor deposition method and the plasma nitriding treatment, which are high-cost processes, are performed, there is a problem in productivity.

Further, in the operating state of the fuel cell (low output load state), the separator may be exposed to a high potential that may cause the passive passivation of stainless steel. The hyperpassive corrosion occurs when Cr ₂ O ₃ forming the passivation film is dissolved by the reaction of the following formula.
Cr ₂ O ₃ +5H ₂ O → 2HCrO ₄ ⁻ +8H ⁺ +6e ⁻

In order to reduce the passivation corrosion, it is effective to contain Si in stainless steel. On the other hand, Si deteriorates the workability of stainless steel, and if it is contained in a large amount, it becomes difficult to austenite the stainless steel. Therefore, it is difficult to increase the Si content and reduce the excessive passive corrosion in a separator that requires precision processing or a separator that austenites the structure to achieve high corrosion resistance.

Japanese Unexamined Patent Publication No. 2006-316338 describes a treatment method in which ferritic stainless steel containing 18 to 24% Cr is absorbed in a high-temperature gas atmosphere containing nitrogen. However, there is no disclosure of a method for reducing the contact resistance of the surface. Further, as the thickness is reduced, the workability deteriorates, but measures against it are not disclosed.

The stainless steel sheet described in Japanese Patent No. 6315158 is obtained by absorbing nitrogen in a ferritic stainless steel containing 20 to 26% Cr and then pickling the surface to expose the conductive nitride, The contact resistance with the gas diffusion layer is reduced. However, the separator using this stainless steel plate may increase the contact resistance with the carbon paper used for the gas diffusion layer by repeatedly applying a load.

An object of the present invention is to provide a stainless steel sheet having excellent corrosion resistance, low contact resistance, and excellent press workability even if the thickness is reduced, and a method for producing the same.

The stainless steel sheet disclosed herein has a chemical composition, in mass %, of Cr: 20 to 26%, N: 0.6 to 2.0%, Si: 2.0% or less, C: 0.040% or less, P: 0.030% or less, S: 0.030% or less, Mn: 1.5% or less, Cu: 0.50% or less, Mo: 0.50% or less, Ni: 0.10% or less, Ca: Less than 50 ppm, sol. Al: less than 300 ppm, balance: Fe and impurities, the stainless steel sheet has a thickness of 30 to 110 μm, and the microstructure is a structure in which the proportion of the austenite phase is 80% by volume or more. The average crystal grain size is ½ or less of the thickness of the stainless steel sheet, the stainless steel sheet has Cr nitride deposited on the surface layer, and is obtained by glow discharge emission spectroscopy from the surface of the stainless steel sheet. The Cr, Fe, and N contents to be satisfied satisfy the following formulas (1) and (2) in the depth direction analysis in the range of the sputtering depth of 0 to 0.05 μm, and the depth from the surface of the stainless steel plate is satisfied. In the profile of the N content obtained by glow discharge emission spectral analysis in the horizontal direction, with the thickness of the stainless steel plate being t, the maximum deviation value CNp in the range of depth 3/8t to 5/8t and the depth 1/8t to The average value CNa in the range of 3/8 t satisfies the following expression (3).
0.1≦Cr/Fe≦0.5 (1)
Cr/N≦1.8 (2)
0.8≦CNp/CNa≦1.6 (3)
The Cr, Fe, and N contents obtained by glow discharge emission spectroscopy from the surface of the stainless steel plate are substituted for Cr, Fe, and N in the formulas (1) and (2) in atomic %. The units of CNp and CNa in the formula (3) are atomic %. The maximum deviation value CNp is the maximum value CNp1 in the depth range of 3/8t to 5/8t and the minimum value CNp2 in the same range, whichever has a larger difference from CNa.

The method for producing a stainless steel sheet disclosed herein is the method for producing a stainless steel sheet as described above, wherein the chemical composition is% by mass, Cr: 20 to 26%, N: 0.1% or less, Si: 2.0. % Or less, C: 0.040% or less, P: 0.030% or less, S: 0.030% or less, Mn: 1.5% or less, Cu: 0.50% or less, Mo: 0.50% or less. , Ni: 0.10% or less, Ca: less than 50 ppm, sol. Al: less than 300 ppm, the balance: a step of preparing a slab which is Fe and impurities, a step of obtaining a rolled steel sheet having a thickness of 30 to 110 μm by hot rolling and cold rolling the slab, and the rolled steel sheet, An annealing step of annealing and cooling in a gas atmosphere containing nitrogen, and a step of pickling the rolled steel sheet after the annealing step with a solution containing a non-oxidizing acid, the holding time HoT of the annealing step is The following formula (4) is satisfied.
t ² /31≦HoT≦t ² /15 (4)
In the formula (4), the thickness of the rolled steel plate is substituted for t in μm. The unit of HoT is seconds.

According to the present invention, a stainless steel sheet having excellent corrosion resistance, low contact resistance, and excellent press workability can be obtained even if the thickness is reduced.

FIG. 1 is a flow chart showing a method for manufacturing a stainless steel sheet according to an embodiment of the present invention. FIG. 2 is an exploded perspective view showing an example of the fuel cell unit. FIG. 3 is a perspective view of a fuel cell which is an assembly of a plurality of cells. FIG. 4 is a schematic diagram of an apparatus used for measuring contact resistance.

The present inventors conducted various studies and obtained the following findings.

(1) In the stainless steel sheet described in Japanese Patent No. 6315158, after ferritic stainless steel containing 20 to 26% Cr absorbs nitrogen, the surface is pickled to expose the conductive nitride. As a result, the contact resistance with the gas diffusion layer is reduced. However, the separator using this stainless steel plate may increase the contact resistance with the carbon paper used for the gas diffusion layer by repeatedly applying a load.

(2) In order to suppress an increase in contact resistance due to repeated loading, it is effective to form a conductive layer containing a conductive carbonaceous material on the surface of the stainless steel plate. When forming the conductive layer, it is necessary to make the stainless steel plate thinner than before.

(3) If the thickness of the above-mentioned stainless steel plate is 110 μm or less, the workability is significantly reduced. This is considered to be related to the formation of nitride inside the steel sheet. Although the cause is unknown, when the stainless steel plate is thinned, the nitrogen absorbed from the front and back surfaces may be concentrated near the center of the plate thickness. The effect of nitrogen enrichment is mitigated by diffusion when the stainless steel sheet is relatively thick, but becomes apparent when the thickness becomes thin.

The present inventors further examined the manufacturing conditions of the stainless steel sheet in detail. As a result, it was found that (4) when the thickness of the stainless steel sheet was 110 μm or less, the heat treatment conditions had to be strictly controlled in order to obtain an appropriate structure. Specifically, it has been found that the annealing time for absorbing nitrogen needs to be controlled in relation to the plate thickness.

The present invention has been completed based on the above findings. Hereinafter, a stainless steel plate, a fuel cell separator, a fuel cell and a fuel cell stack according to an embodiment of the present invention will be described in detail.

[Stainless steel plate]
[Chemical composition]
A stainless steel sheet according to an embodiment of the present invention has a chemical composition described below. In the following description, “%” of the content of an element means mass% unless otherwise specified.

Cr: 20-26%
Chromium (Cr) has a function of forming a Cr ₂ O ₃ passivation film on the surface of stainless steel to improve the corrosion resistance. The lower limit of the Cr content is 20%. If it is less than 20%, a large amount of martensite phase may be contained in the structure transformed into austenite by nitrogen absorption. On the other hand, the deformation resistance increases as the Cr content increases. The upper limit of the Cr content is set to 26% in order to ensure stable manufacturability of a steel sheet having a thin sheet thickness of up to about 30 μm by cold rolling or the like in an industrial production process. The lower limit of the Cr content is preferably 23%. If it is less than 23%, there is a possibility that a processing-induced martensite phase will be generated when strong processing is performed. The upper limit of the Cr content is preferably 25%. This is to ensure more stable manufacturability (in particular, flatness of a thin steel plate).

N: 0.6-2.0%
Nitrogen (N) has a function of improving the corrosion resistance by being added in a range not forming a nitride in stainless steel. It is also an element that promotes austenitization of stainless steel. The lower limit of the N content is 0.6%. This is to secure the minimum nitrogen amount necessary to obtain the austenite phase in the Fe-Cr-N system. The upper limit of the N content is 2.0%. This is to suppress the formation of nitrides such as Cr ₂ N and CrN in the crystal grains. The lower limit of the N content is preferably 0.8%. The upper limit of the N content is preferably 1.6%.

Si: 2.0% or less Silicon (Si) may not be added. Since Si is an element that deteriorates the workability of stainless steel, it is usually not an element that is positively added. On the other hand, when it is used in a strong oxidizing environment such as concentrated nitric acid, it may be added. Si produces a SiO ₂ on the surface when the stainless steel is exposed to a hyperpassive corrosive environment, and exerts an effect of covering and protecting the Cr ₂ O ₃ passivation film. Therefore, it may be added when there is a possibility that it will be exposed to an environment in which excessive passivation corrosion is of particular concern. Even if it is added, the upper limit of the Si content is 2.0%. When the Si content exceeds 2.0%, workability is deteriorated, and brittle σ phase is likely to be precipitated during manufacturing, cracks may occur in the process of working the steel plate, or flatness is poor and press working is difficult. The shape may not be suitable. The upper limit of the Si content is preferably 1.5%.

C: 0.040% or less Carbon (C) may not be added. C is a solid solution strengthening element and contributes to improving the strength of stainless steel. However, in the stainless steel sheet of this embodiment, N is contained in a certain amount or more, so solid solution strengthening by N is sufficient, and it is not necessary to add C for strength. If the C content increases, a large number of carbides are generated in the manufacturing process, and these carbides become the starting points of fracture, deteriorating the formability of steel. Therefore, the C content is 0.040% or less. The upper limit of the C content is preferably 0.030%.

P: 0.030% or less Phosphorus (P) is an impurity. P is segregated at the grain boundaries during solidification and increases solidification cracking susceptibility. Therefore, the P content is preferably as low as possible. Therefore, the P content is 0.030% or less.

S: 0.030% or less Sulfur (S) is an impurity. S segregates at the grain boundaries during solidification, increasing the solidification cracking susceptibility. Therefore, the S content is preferably as low as possible. Therefore, the S content is 0.030% or less.

Mn: 1.5% or less Manganese (Mn) may not be added. Mn suppresses the deterioration of hot workability due to S. Mn also deoxidizes stainless steel. However, when the Mn content increases, precipitation of an intermetallic compound phase such as a σ phase is promoted. The precipitation of the σ phase lowers the structural stability and also reduces the toughness and ductility of the stainless steel. Therefore, the Mn content is set to 1.5% or less. The upper limit of the Mn content is preferably 1.0%, more preferably 0.5%.

Cu: 0.50% or less Copper (Cu) may not be added. Cu is easily segregated at the grain boundaries and is an austenite stabilizing element. When the Cu content increases, ferrite formation is suppressed during solidification during casting, and solidification cracking susceptibility increases. Further, if the Cu content is high, hot workability may be deteriorated. Therefore, the Cu content is 0.50% or less.

Mo: 0.50% or less Molybdenum (Mo) may not be added. Mo has the effect of increasing the corrosion resistance of stainless steel. However, Mo is an expensive element that is classified as a rare metal and is not preferable in providing a material having excellent economical efficiency. Moreover, when the Mo content is high, a structure mainly composed of an austenite phase may not be obtained. Therefore, the Mo content is 0.50% or less. The upper limit of the Mo content is preferably 0.30%.

Ni: 0.10% or less Nickel (Ni) may not be added. Ni is an element that promotes austenitization of stainless steel. However, Ni is an element belonging to rare metals and is not preferable in providing a material with excellent economical efficiency. Further, the elution of Ni ions may reduce the oxygen reduction reaction rate at the interface between the platinum catalyst and the polymer electrolyte membrane. Therefore, the Ni content is 0.10% or less.

Ca: less than 50 ppm Calcium (Ca) is an impurity. CaS and MnS are generally known as non-metallic inclusions that can be a starting point of corrosion of stainless steel. The Ca content is set to less than 50 ppm in order not to generate a large amount of CaS which is a corrosion starting point.

sol. Al: less than 300 ppm Aluminum (Al) may not be added. Al deoxidizes stainless steel. However, if the Al content is too high, the cleanliness of the steel decreases, and the workability and ductility of stainless steel decrease. Therefore, the Al content is less than 300 ppm. In addition, in this specification, Al content means content of acid-soluble Al (sol.Al).

The balance of the chemical composition of the stainless steel sheet according to the present embodiment is Fe and impurities. The impurities referred to here are elements mixed from ores and scraps used as raw materials for steel, or elements mixed from the environment of the manufacturing process.

[Thickness]
The stainless steel sheet according to the present embodiment has a thickness of 30 to 110 μm. The thickness is the average thickness.

The lower limit of the plate thickness is set to 30 μm, because if the plate thickness is less than 30 μm, it becomes difficult to economically manufacture by rolling while maintaining the good shape, and the penetration pits due to inclusions or the like. This is because the risk of defects such as is high. Since the separator is also a partition, through pits are not allowed. The lower limit of the plate thickness is preferably 50 μm.

The upper limit of the plate thickness is 110 μm in order to prevent the total thickness from becoming too thick when the conductive layer is provided on the stainless steel plate. The upper limit of the plate thickness is preferably 100 μm.

[Microstructure and average grain size of austenite grains]
The structure of the stainless steel sheet according to the present embodiment mainly has an austenite phase. Specifically, the proportion of iron phases other than the austenite phase such as ferrite phase and martensite phase, and the proportion of compound phases such as Cr ₂ N and CrN are low, and austenite for all phases detectable by X-ray diffraction (XRD) The ratio of the phases (hereinafter referred to as “γ phase fraction”) is 80% by volume or more. The γ phase fraction is preferably 90% by volume or more, and ideally 100% by volume (the structure is an austenite single phase). Good press workability (ductility) can be obtained by increasing the γ phase fraction.

In the stainless steel sheet according to the present embodiment, the average grain size of austenite crystal grains (hereinafter referred to as “γ grain size”) is ½ or less of the thickness of the stainless steel sheet. If the γ particle size becomes coarse, good press workability may not be obtained. For example, in the case of one crystal grain in the plate thickness direction, uniform deformation during press working cannot be obtained, resulting in poor press workability.

The γ particle size is measured as follows. A line of a specified length is drawn on the microstructure photograph of which the magnification is known. Count the number of austenite grains on the line. The crystal grains at both ends of the line are counted as 1/2. The grain length of the austenite crystal grain in the microstructure photograph is obtained by dividing the line length by the number of crystal grains straddling the line. Five L-section and five T-section photographs of a microstructure with a magnification of 200 were taken, and the grain size of the austenite crystal grains was measured by the above method for a total of 10 microstructure photographs, and the average value was taken as the γ grain size. To do.

[Cr, Fe, and N contents near the surface]
In the stainless steel sheet according to the present embodiment, the Cr, Fe, and N contents obtained by the glow discharge optical emission spectroscopy from the surface have the following formula (in the depth direction analysis in the sputtering depth range of 0 to 0.05 μm). 1) and the formula (2) are satisfied.
0.1≦Cr/Fe≦0.5 (1)
Cr/N≦1.8 (2)
The Cr, Fe, and N contents obtained by glow discharge emission spectroscopy from the surface of the stainless steel plate are substituted for Cr, Fe, and N in the formulas (1) and (2) in atomic %.

Here, "in the depth direction analysis in the range of the sputtering depth of 0 to 0.05 µm, the following expressions (1) and (2) are satisfied" means that all the measurement points in the range of 0 to 0.05 µm. In, the expression (1) and the expression (2) are satisfied. 0 to 0.05 μm is a portion that directly affects the contact resistance and corrosion resistance of the separator. Hereinafter, the region of 0 to 0.05 μm from the surface will be referred to as “the vicinity of the surface”.

The distribution of Cr, Fe, and N contents in the vicinity of the surface is obtained by performing analysis in the depth direction from the surface using glow discharge emission spectroscopy (GD-OES; Glow Discharge-Optical Emission Spectroscopy). For the glow discharge emission spectroscopic analysis, for example, a Marcus type high frequency glow discharge emission analysis device (GD-Profiler2) manufactured by Horiba Ltd. can be used. The obtained distribution of each element in the depth direction is converted into an atomic ratio, and Cr/Fe and Cr/N are calculated.

Cr/Fe in the formula (1) is an index of Cr concentration. The fact that Cr/Fe near the surface is larger than the value (0.26 to 0.39) in the bulk composition of the stainless steel sheet means that the vicinity of the surface is Cr-rich. If Cr/Fe near the surface is too large or too small, good corrosion resistance cannot be obtained. Therefore, Cr/Fe near the surface is set to 0.1 to 0.5. The lower limit of Cr/Fe near the surface is preferably 0.2. The upper limit of Cr/Fe near the surface is preferably 0.4.

Cr/N in the equation (2) is an index showing the ratio of CrN and Cr ₂ N. The more Cr ₂ N existing near the surface, the larger the Cr/N. CrN is excellent in corrosion resistance, whereas Cr ₂ N is inferior in corrosion resistance. Therefore, it is preferable that Cr ₂ N is small. Therefore, Cr/N near the surface is set to 1.8 or less. Cr/N in the vicinity of the surface is preferably 1.5 or less.

[Distribution of N content in the plate thickness direction]
In the N content profile obtained by glow discharge emission spectroscopy in the depth direction from the surface, the stainless steel sheet according to the present embodiment has a maximum deviation in the range of 3/8t to 5/8t, where t is the thickness of the stainless steel sheet. The value CNp and the average value CNa in the depth range of 1/8t to 3/8t satisfy the following formula (3).
0.8≦CNp/CNa≦1.6 (3)
Here, the maximum deviation value CNp is a maximum value CNp1 in the range of depths 3/8t to 5/8t and a minimum value CNp2 in the same range, whichever has a larger difference from CNa. That is, if |CNp1-CNa|-|CNa-CNp2| is positive, CNp=CNp1 and if it is negative, CNp=CNp2.

The stainless steel sheet according to the present embodiment has a thin thickness of 30 to 110 μm and a high N content of 0.6 to 2.0%. In such a stainless steel plate, in order to obtain good press workability, it is necessary to make the distribution of the N content flat in the plate thickness direction. In particular, it is desirable that the N content near the center in the plate thickness direction (near 1/2t) be approximately the same as the N content near 1/4t.

The mechanism by which good press workability is obtained by flattening the distribution of the N content in the plate thickness direction is not necessarily clear, but it is presumed as follows. When the N content in the plate thickness center is too much higher than in other parts, nitride is formed in the plate thickness center or the structure has a high concentration of the last minute nitride is formed, so that nitriding is performed during processing. It is possible that the nitrides are forming a substance and the nitrides thereof adversely affect the workability. If the N content in the plate thickness center is too small compared to the other parts, the ferrite that has not been transformed into austenite remains, or even if it is transformed into austenite, the concentration is at the most limit to retain austenite. However, there is a possibility that ferrite and martensite are generated during processing, and these ferrite and martensite adversely affect workability. Further, since the strength of nitrogen-absorbing stainless steel depends on the N content, a large change in the distribution of the N content in the thickness direction results in a large change in the strength distribution in the thickness direction, resulting in stress concentration during deformation. There is a possibility that it has happened and the workability has deteriorated.

CNp/CNa is an index of the distribution of N content in the plate thickness direction. The closer this value is to 1, the flatter the distribution. CNp/CNa of the stainless steel sheet according to the present embodiment is 0.8 to 1.6. The lower limit of CNp/CNa is preferably 0.85, more preferably 0.9. The upper limit of CNp/CNa is preferably 1.5, more preferably 1.4.

CNp, CNp1, CNp2, and CNa, which are the sources thereof, can be measured and calculated by the following method. The data used as the basis for calculation is the same as the method for measuring the distribution of Cr, Fe, and N contents in the vicinity of the surface, and GD-OES is used. The measurement is performed while performing the sputtering at a measurement interval such that one-point measurement data is obtained at a depth interval of 0.01 μm or less when converted to the depth. Measure from the surface to half of the plate thickness t up to 5/8t. After converting the obtained raw data into content data (unit is atomic %), the average is taken so that one data point is obtained at 0.05 to 0.1 μm. Based on these data, the maximum value of N content in the depth range of 3/8t to 5/8t is CNp1, the minimum value is CNp2, and the average value of N content in the depth range of 1/8t to 3/8t. Is CNa. The accuracy of the depth range is about 0.1 μm depending on the accuracy of the data. The maximum deviation CNp takes a value of CNp1 and CNp2 that has a larger difference from CNa. That is, if |CNp1-CNa|-|CNa-CNp2| is positive, CNp=CNp1 and if it is negative, CNp=CNp2. Here, | of |x| is a symbol indicating the absolute value of the expression inside, and |x| indicates the absolute value of x.

[Mechanical properties]
The stainless steel sheet according to the present embodiment has good press workability. The stainless steel sheet according to the present embodiment preferably has an elongation (elongation at break) in a tensile test of 15% or more, more preferably 20% or more. Here, the tensile test refers to a tensile test based on ASTM A370 and performed at a strain rate of 1×10 ⁻³ /sec.

[Conductive layer containing conductive carbonaceous material]
The stainless steel sheet according to the present embodiment can be provided with a conductive layer containing a conductive carbonaceous material on at least one surface. Thereby, the contact resistance with the gas diffusion layer becomes lower.

Examples of the electrically conductive carbonaceous material include graphite and carbon black such as acetylene black and Ketjen black. As the Ketjen black, for example, commercially available products such as Ketjen Black EC, Ketjen Black EC600JD, Carbon ECP, Carbon ECP600JD manufactured by Lion Co., Ltd. can be used. As the acetylene black, Denka Black (registered trademark) manufactured by Denki Kagaku Kogyo Co., Ltd. may be mentioned.

The fact that the conductive layer contains graphite can be confirmed by, for example, the Raman spectrum of the conductive layer showing a peak of graphite. Specifically, when Raman spectroscopy gives the peaks of the D band and the G band for the conductive layer and the half-value width of the G band is 100 cm ⁻¹ or less, the conductive layer contains sufficient graphite. You can judge. When the full width at half maximum of the G band exceeds 100 cm ^-1 , it can be determined that the conductive layer does not contain graphite in a sufficient amount.

The conductive layer may be a carbon-resin composite layer having a carbonaceous material dispersed in a matrix resin.

The carbon-resin composite layer contains a carbonaceous material and a matrix resin made of a thermoplastic resin or a thermosetting resin. The carbon-resin composite layer preferably has a volume ratio (C/R) of the carbonaceous material (C) and the matrix resin (R) of 6/4 to 9/1. If the volume ratio (C/R) is smaller than 6/4, the conductivity may decrease, and if it is larger than 9/1, the flexibility, flexibility and corrosion resistance may be poor.

The thickness of the carbon-resin composite layer is preferably 0.02 to 5.0 mm. If the thickness is less than 0.02 mm, corrosion may start from slight cracks in the carbon-resin composite layer, and if it is 5.0 mm or more, flexibility may be adversely affected. The thickness of the carbon-resin composite layer is more preferably 0.05 to 2.0 mm.

The carbonaceous material forming the carbon-resin composite layer is not limited to this, but may be one or more of powders such as natural graphite, artificial graphite, expanded graphite, expanded graphite, scaly graphite, and spherical graphite. Mixtures can be used. From the viewpoint of flexibility and conductivity, it is preferable to include expanded graphite or expanded graphite powder.

The matrix resin forming the carbon-resin composite layer may be a thermoplastic resin or a thermosetting resin.

The thermoplastic resin includes, but is not limited to, polypropylene resin (PP), polyethylene resin (PE), polyamide resin (PA), polyphenylene sulfide resin (PPS), polymethylpentene resin (PMP), polyether ether ketone resin. One or two of (PEEK), polyphenylene ether resin (PPE), liquid crystal polymer resin (LCP), polyamideimide resin (PAI), polysulfone resin (PSU), polyethylene terephthalate resin (PET) and polybutylene terephthalate resin (PBT). Mixtures of one or more can be used.

With regard to polyolefin resins such as PP, PE and PMP, it is preferable to use a modified polyolefin resin in which a part or all of the polyolefin resin is graft-modified with an unsaturated carboxylic acid or a derivative thereof. By using such a modified polyolefin resin, it is expected that the flexibility and the adhesion with the stainless steel plate are improved, and thereby the contact resistance is also expected to be reduced.

The thermosetting resin can be, but is not limited to, one or a mixture of two or more phenolic resins and epoxy resins.

[Adhesive layer]
The stainless steel plate according to the present embodiment may further include an adhesive layer between the surface of the stainless steel plate and the conductive layer.

The adhesive composition forming the adhesive layer is not limited to this, but an adhesive composition containing an adhesive polyolefin resin (for example, Admer (trade name) manufactured by Mitsui Chemicals, Inc.), graft modified with an unsaturated carboxylic acid. Adhesive composition containing modified polyolefin resin (for example, Unistall (trade name) manufactured by Mitsui Chemicals, Inc.), adhesive composition containing modified polyolefin resin graft-modified with halogen (for example, Toyo Co., Ltd. Tuck (trade name), phenol resin adhesive composition (for example, AH-1148 (trade name) manufactured by Lignite Co., Ltd.), epoxy resin adhesive composition (for example, YSLV-80XY manufactured by Nippon Steel & Sumikin Chemical Co., Ltd. (trade name) )) can be used. The adhesive composition preferably contains a modified polyolefin resin in which a part or all of the polyolefin resin is graft-modified with an unsaturated carboxylic acid or a derivative thereof (for example, see JP-A-2005-146178).

The thickness of the adhesive layer is preferably 0.1 to 10 μm. If the thickness of the adhesive layer is less than 0.1 μm, the adhesive strength may be insufficient. When the thickness of the adhesive layer exceeds 10 μm, the conductivity may be insufficient.

[Stainless steel sheet manufacturing method]
FIG. 1 is a flow chart showing a method for manufacturing a stainless steel sheet according to an embodiment of the present invention. This manufacturing method includes a step of preparing a slab (step S1), a step of obtaining a rolled steel sheet having a thickness of 30 to 110 μm by hot rolling and cold rolling the slab (step S2), and nitrogen of the rolled steel sheet. It is provided with a step of annealing and cooling in a gas atmosphere containing (step S3), and a pickling step of pickling with a solution containing a non-oxidizing acid (step S4). Hereinafter, each step will be described in detail.

[Slab preparation process]
The chemical composition is% by mass, Cr: 20 to 26%, N: 0.1% or less, Si: 2.0% or less, C: 0.040% or less, P: 0.030% or less, S:0. 0.030% or less, Mn: 1.5% or less, Cu: 0.50% or less, Mo: 0.50% or less, Ni: 0.10% or less, Ca: less than 50 ppm, sol. Al: less than 300 ppm, balance: Fe and a slab that is an impurity are prepared (step S1).

The chemical composition of this slab is the same as that of the above-mentioned stainless steel sheet except N content. The N content of the slab is set to 0.1% or less because if the N content exceeds 0.1%, the deformation resistance increases and it becomes difficult to form a steel sheet by rolling. The upper limit of the N content of the slab is preferably 0.05%.

The step of preparing the slab is not limited to this, but can be performed as follows, for example.

Dissolve the raw materials. Ferrochrome and ferrosilicon for producing stainless steel, cast iron, scrap of ferritic stainless steel and the like can be used as the raw material. Melting is mainly performed in an electric furnace. At the laboratory level, it can also be carried out in a vacuum induction heating furnace. Refining is performed to reduce carbon content, gas components, and metal inclusions. For refining, an AOD (Argon-Oxygen-Decarburization) method, a VOD (Vacuum-Oxygen-Decarburization) method, a V-AOD method, or the like can be applied. Then, it is cast into a continuous casting device or a case to form a slab having a shape suitable for rolling. The chemical composition of the slab can be adjusted by blending the raw materials and refining conditions.

[Rolling process]
The slab is hot-rolled and cold-rolled to obtain a rolled steel sheet having a thickness of 30 to 110 μm (step S2). The hot rolling and the cold rolling may be repeatedly performed, and if necessary, intermediate heat treatment such as annealing or pickling may be performed. Further, in addition to hot rolling and cold rolling, hot forging and cutting may be further performed if necessary.

The rolling process is not limited to this, but can be performed as follows, for example.

Hot rolling with a tandem mill or Steckel mill turns the slab into a hot coil. This hot coil is annealed and pickled. Further, it is cold-rolled by a multi-roll cold rolling mill to obtain a rolled steel sheet having a thickness of 30 to 110 μm.

The rolling ratio of the cold rolling immediately before the annealing process (step S3), more specifically, the rolling ratio from the final intermediate annealing during the cold rolling process to the final sheet thickness after the cold rolling (cold rolling (cold If there is no intermediate annealing during the hot rolling process, the rolling ratio from the hot rolling to the final cold-rolled sheet thickness (hereinafter referred to as "heat treatment cold rolling ratio") is performed by the annealing process (step S3). The susceptibility to nitrogen absorption in is different. Specifically, the higher the cold rolling rate after heat treatment, the more difficult it is for nitrogen to be absorbed (the N content after annealing tends to decrease). Further, the higher the cold rolling rate after heat treatment, the more uniform the N content distribution tends to be. Therefore, the cold rolling rate after heat treatment is preferably 95% or more, more preferably 97% or more. Here, the rolling rate is ((original thickness)-(finished thickness))/(original thickness) (x100 in the case of% display).

For example, when a steel plate having a plate thickness of 5 mm is manufactured by hot rolling, the plate is ground to a plate thickness of 4 mm, and the plate is cold rolled to a plate thickness of 0.08 mm without intermediate annealing, the cold rolling ratio after heat treatment is It becomes 98% by (4-0.08)÷4 (×100). Similarly, when cold rolling a material ground to a plate thickness of 4 mm, cold rolling to a plate thickness of 0.5 mm is followed by intermediate annealing, and then cold rolling to a plate thickness of 0.08 mm. The post-cold rolling rate is (0.5-0.08)÷0.5 (×100), which is 84%.

[Annealing process]
The rolled steel sheet is annealed and cooled in a gas atmosphere containing nitrogen (step S3). By this step, nitrogen is absorbed from the surface of the steel sheet to convert the structure of the steel sheet into an austenite phase.

The ratio of the partial pressure of nitrogen to the total pressure of the processing gas is preferably 0.4 to 0.8. If the ratio of the partial pressure of nitrogen to the total pressure is less than 0.4, nitrogen is not sufficiently supplied from the surface, and it is difficult to make the γ phase fraction 80% by volume or more over the entire thickness when the steel sheet is thick. become. On the other hand, if the ratio of the partial pressure of nitrogen to the total pressure of the processing gas is higher than 0.8, Cr nitride may be excessively generated on the surface, which may be a starting point of cracking during processing. As the gas mixed with nitrogen, hydrogen is preferably used so as not to oxidize the steel sheet. Argon may be used instead of hydrogen or in addition to hydrogen.

The annealing temperature is preferably 950 to 1200°C. If the annealing temperature is lower than 950° C., not only the austenite phase but also the Cr ₂ N phase exists in the equilibrium state, so that the γ phase fraction may not be 80% by volume or more. On the other hand, if the annealing temperature exceeds 1200° C., especially when Si is contained, a liquid phase is generated near the grain boundary, and there is a possibility that the liquid phase is melted and embrittled. The annealing temperature varies depending on the Cr content, but is preferably 1050 to 1150°C.

The appropriate annealing holding time HoT depends on the thickness t of the steel sheet. In a thin steel plate having a plate thickness of 30 to 110 μm, the holding time needs to be strictly controlled in order for the distribution of the N content in the plate thickness direction to satisfy the above formula (3). If the holding time is too short, the γ phase fraction cannot be 80% by volume or more over the entire thickness. On the other hand, when the holding time is too long, the γ grains are coarsened, and Cr nitride may be excessively precipitated on the surface, and an excessive peak of N content occurs at the center of the plate thickness, resulting in poor workability. descend.

In order for the distribution of the N content in the plate thickness direction to satisfy the above formula (3), the annealing holding time HoT needs to satisfy the following formula (4).
t ² /31≦HoT≦t ² /15 (4)
Here, the thickness of the stainless steel plate is substituted for t in μm. The unit of HoT is seconds.

The lower limit of the holding time HoT of annealing is preferably t ² /28. The upper limit of the holding time HoT of annealing is preferably t ² /19.

Cool the annealed steel sheet. The steel sheet after annealing is preferably cooled immediately. When the annealed steel sheet is gradually cooled, there is a possibility that nitrides will be excessively precipitated in the intermediate temperature range. However, the steel sheet according to the present embodiment has a thickness of 30 to 110 μm and has a small heat capacity with respect to the heat radiation area. Therefore, if it is left to cool outside the furnace, it can be cooled sufficiently quickly. Water cooling or the like is not preferable because it is deformed by quenching strain.

The annealing step can be carried out, for example, by passing the steel sheet through an annealing line called a continuous bright annealing line.

By this annealing step, a steel sheet having a γ phase fraction of 80% by volume or more is obtained. The steel sheet after the annealing step has an N content of 0.6 to 2.0 mass%, a γ phase fraction of 80% by volume or more, and a γ grain size of ½ or less of the thickness of the stainless steel sheet. Is adjusted so that

The N content of the steel sheet after the annealing step can be adjusted by the N content of the slab and the annealing conditions. Specifically, the N content of the steel sheet after the annealing step is increased by increasing the N content of the slab, increasing the nitrogen partial pressure during annealing, increasing the temperature of annealing, or lengthening the holding time of annealing. The content can be increased.

The γ grain size can be adjusted by the annealing conditions. Specifically, if the annealing temperature is lowered or the holding time is shortened, the γ grain size can be reduced.

In the annealing process, the austenite phase conversion due to nitrogen absorption simultaneously progresses from the front and back surfaces of the steel sheet. The γ grains grown from the front and back surfaces collide with each other, so that grain growth temporarily stops. The γ grain size at this point is approximately one half of the plate thickness.

[Pickling step]
The steel sheet after the annealing step is pickled with a solution containing a non-oxidizing acid (step S4). For pickling, a non-oxidizing acid is used because it does not oxidize the surface of the steel sheet. Acids that can be used are, for example, (1) hydrofluoric acid, (2) sulfuric acid, (3) hydrochloric acid, and mixed acids of these acids.

(1) Hydrofluoric acid The concentration of hydrofluoric acid is preferably 1 to 5 mass %. The processing temperature is preferably 35 to 75°C. If the temperature is lower than 35°C, the treatment may take a long time. Further, in summer, the temperature rise due to heat generation during pickling cannot be controlled completely, and there is a possibility that stable processing cannot be performed depending on the outside temperature. On the other hand, if the temperature is higher than 75°C, corrosive fumes may be generated from the treatment liquid. The treatment temperature is more preferably 40 to 55°C. The treatment time is preferably 2 to 10 minutes.

(2) Sulfuric acid The concentration of sulfuric acid is preferably 10 to 40% by mass. The processing temperature is preferably 35 to 75°C. If the temperature is lower than 35°C, the treatment may take a long time. Further, in summer, the temperature rise due to heat generation during pickling cannot be controlled completely, and there is a possibility that stable processing cannot be performed depending on the outside temperature. On the other hand, if the temperature is higher than 75° C., harmful SO _x gas may be generated from the treatment liquid. The concentration is more preferably 15 to 30% by mass. The treatment temperature is more preferably 50 to 60°C. The treatment time is preferably 2 to 10 minutes.

(3) Hydrochloric acid The concentration of hydrochloric acid is preferably 4 to 15% by mass. The processing temperature is preferably 35 to 75°C. If the temperature is lower than 35°C, the treatment may take a long time. Further, in summer, the temperature rise due to heat generation during pickling cannot be controlled completely, and there is a possibility that stable processing cannot be performed depending on the outside temperature. On the other hand, if the temperature is higher than 75°C, corrosive fumes may be generated from the treatment liquid. The concentration is more preferably 4 to 12% by mass. The treatment temperature is more preferably 40 to 55°C. The processing time is preferably 2 to 15 minutes.

The method for manufacturing the stainless steel sheet according to the embodiment of the present invention has been described above. In the present embodiment, after Fe—Cr stainless steel is formed into a steel sheet, nitrogen is absorbed to convert the structure into an austenite phase. Thereby, the corrosion resistance of the stainless steel plate can be improved without using an expensive raw material such as Mo. Further, by setting the γ phase fraction to 80% by volume or more and further making the N content distribution in the plate thickness direction uniform, good press workability can be secured.

In this embodiment, the N content of the slab is 0.1% or less, so that a rolled steel sheet can be obtained relatively easily. Further, since the steel sheet having a thickness of 30 to 110 μm is formed and nitrogen is absorbed, the austenite phase can be formed in a relatively short time.

According to this embodiment, the steel plate after the annealing step is further pickled with a solution containing a non-oxidizing acid, whereby a Cr nitride layer having both conductivity and corrosion resistance can be formed on the surface.

In the present embodiment, 80% by volume or more of the structure is the austenite phase, but since the outermost surface is the entrance for N absorption, the N content is high and Cr ₂ N or CrN is generated. These precipitate in lamella form and have higher corrosion resistance than base iron, so they are exposed in terraces by pickling.

Table 1 shows the physical properties of CrN, Cr ₂ N, and other substances. As shown in Table 1, Cr ₂ N and CrN have an overwhelmingly smaller electric resistance than Cr ₂ O ₃ . Further, CrN existing on the outermost surface is considered to be resistant to excessive passive corrosion, which also contributes to reduction of excessive passive corrosion.

By providing such a Cr nitride layer, it is possible to obtain a stainless steel sheet having both conductivity and corrosion resistance. Such a form of Cr nitride cannot be realized by a material obtained by casting in a pressurized nitrogen atmosphere.

Cr/Fe and Cr/N in the formulas (1) and (2) are also indexes of the morphology of the Cr nitride layer. That is, as the amount of Cr nitrides (Cr ₂ N and CrN) increases, the surface becomes richer in Cr and the value of Cr/Fe near the surface increases. Further, the smaller the amount of Cr ₂ N relative to CrN, the smaller the value of Cr/N.

Therefore, it is possible to increase the value of Cr/Fe by increasing the N content of the slab, increasing the nitrogen partial pressure during annealing, increasing the annealing temperature, or increasing the annealing holding time. it can. Further, for example, the value of Cr/N can be reduced by increasing the pickling treatment temperature, lengthening the pickling treatment time, and the like.

[Formation of conductive layer]
When forming a conductive layer having a conductive carbonaceous material on a stainless steel plate, the following method can be used.

A lump-shaped (block-shaped or the like) conductive carbonaceous material is slid against the Cr nitride film on the surface of the stainless steel plate. The conductive carbonaceous material is preferably graphite. Graphite has a weak bond between faces of a six-membered ring composed of carbon atoms. Therefore, when the graphite is slid on the Cr nitride film, the graphite becomes scaly particles and is oriented substantially parallel to the surface of the Cr nitride film. Thereby, the surface of the Cr nitride film can be efficiently covered with graphite.

When forming a carbon-resin composite layer in which a carbonaceous material is dispersed in a matrix resin as the conductive layer, the following method can be used.

A mixture containing a carbonaceous material and a matrix resin is directly hot pressed onto the surface of a stainless steel plate. Alternatively, a slurry in which a carbonaceous material and a matrix resin are dispersed in a solvent may be applied to the surface of a stainless steel plate using a doctor blade or the like, dried and then hot pressed.

When forming a carbon-resin composite layer, a powder mixture containing a carbonaceous material powder and a matrix resin powder is hot pressed to previously form a carbon-resin composite layer, and the obtained carbon-resin composite layer is obtained. Is particularly preferable to be laminated on the surface of the stainless steel plate by hot pressing. Prior to this laminating step, the adhesive composition having a stainless steel plate surface may be applied. In this case, the stainless steel plate and the carbon-resin composite layer are laminated via the adhesive layer.

[Fuel cell separator]
A fuel cell separator according to an embodiment of the present invention includes the stainless steel plate according to the present embodiment. More specifically, the fuel cell separator according to the present embodiment is the stainless steel plate according to the present embodiment having irregularities or the like that function as flow paths. The fuel cell separator according to the present embodiment can be manufactured by pressing the stainless steel plate according to the present embodiment.

[Fuel cell and fuel cell stack]
A fuel cell according to an embodiment of the present invention includes the fuel cell separator according to the present embodiment. A fuel cell stack according to an embodiment of the present invention includes a plurality of fuel cell units according to the present embodiment.

FIG. 2 is an exploded perspective view showing the configuration of a cell 10 which is an example of a polymer electrolyte fuel cell. FIG. 3 is a perspective view of a polymer electrolyte fuel cell 1 which is an assembly (stack) of a plurality of cells 10. The cell 10 of FIG. 2 and the polymer electrolyte fuel cell 1 of FIG. 3 are merely examples, and the configurations of the fuel cell and the fuel cell stack according to the present embodiment are not limited to these.

As shown in FIG. 2, the cell 10 has an anode (anode-side gas diffusion electrode layer or fuel electrode film) 3 on one surface of the solid polymer electrolyte membrane 2 and a cathode (cathode-side gas diffusion electrode layer or oxidation) on the other surface. The agent electrode film) 4 is laminated, and the separators 5a and 5b are laminated on both surfaces of the laminated body.

The fuel cell separator according to the present embodiment may be a separator (water separator) having a flow path of cooling water. The fuel cell stack according to the present embodiment may be a water-cooled fuel cell in which a water separator is arranged between cells or every several cells.

As the solid polymer electrolyte membrane 2, a fluorine-based proton conductive membrane having a hydrogen ion exchange group can be used. The anode 3 and the cathode 4 may be provided with a catalyst layer made of a particulate platinum catalyst, graphite powder, and, if necessary, a fluororesin having a hydrogen ion exchange group. In this case, the fuel gas or the oxidizing gas comes into contact with the catalyst layer to promote the reaction.

A flow path 6a is provided in the separator 5a. A fuel gas (hydrogen or hydrogen-containing gas) A is flown through the flow path 6a to supply hydrogen to the anode 3. The flow path 6b is provided in the separator 5b. Oxidizing gas B such as air is caused to flow through the flow path 6b, and oxygen is supplied to the cathode 4. The hydrogen and oxygen thus supplied cause an electrochemical reaction to generate DC power.

Hereinafter, the present invention will be described more specifically with reference to Examples. The invention is not limited to these examples.

[Example 1] (without conductive layer)
[Manufacture of rolled steel sheets]
Steels of eight kinds of chemical compositions shown in Table 2 were melted in a high-frequency induction heating type 30 kg vacuum melting furnace to produce a substantially frustoconical casting ingot having a diameter Φ125 to 115 mm and a height of 320 mm.

These casting ingots were ground with a grinder on the surface black skin on the side of the truncated cone. After grinding, according to the manufacturing conditions shown in Table 3, hot forging, cutting, hot rolling, grinding (black skin shaving) and cold rolling are performed, and rolling with a thickness t1 (30 to 110 μm) shown in Table 6 is performed. A steel plate was manufactured. No intermediate annealing was performed during cold rolling. All materials showed good productivity without causing edge cracking in cold rolling.

[Annealing treatment]
A raw material having a width of 70 mm and a length of 200 mm was cut out from each rolled steel sheet, subjected to bright annealing treatment and continuously subjected to nitrogen absorption treatment in a solid state (hereinafter referred to as “annealing treatment”) by a continuous annealing simulator device.

For the annealing treatment, a gas having a partial pressure ratio shown in Table 4 was used. The total pressure was 1 atm.

The heat pattern for the annealing treatment was selected according to the thickness of the material, and is shown in Table 6 below. After holding at the temperature shown in the "holding temperature" column of Table 6 for the time shown in the "holding time" column, a heat treatment of quenching was performed. The cooling rate was 8 to 10°C/sec with an average cooling rate of up to 500°C.

[Pickling treatment]
The annealed material was pickled under the conditions shown in Table 5.

The rolled steel plates of materials 1 to 8 (see Tables 2 and 3) were annealed and pickled under the conditions shown in Table 6 to produce stainless steel plates.

[Survey of organization, etc.]
A sample was taken from the central portion of the thickness of each stainless steel plate, and the N content was measured.

The γ particle size was measured from the cross-sectional micrograph of each stainless steel plate by the method described in the embodiment.

From the surface of each stainless steel plate, by GD-OES, distribution of Cr, Fe, and N contents in the range of sputter depth 0 to 0.05 μm, and N content in the range of sputter depth 0 to 5/8 t. Was measured. A Marcus type high-frequency glow discharge emission spectrometer (GD-Profiler2) manufactured by Horiba Ltd. was used for the measurement.

The surface of each stainless steel plate was observed by SEM to investigate the surface morphology.

The structure of each stainless steel plate was determined by the X-ray diffraction method described below.

The results of these investigations are shown in Table 7.

In the column of "γ grain size" in Table 7, the average grain size of the austenite crystal grains of each stainless steel is described. No. 28, which did not become an austenite phase, cannot be measured, and was therefore given as "-".

In the column of "GD-OES" in Table 7, from the data of the Cr/Fe range, the Cr/N range, and the sputter depths 1/8t to 5/8t at the sputter depths of 0 to 0.05 μm. The calculated CNp/CNa is listed.

In the column of “X-ray diffraction” in Table 7, the γ phase fraction (γ%) identified and calculated by X-ray diffraction by the θ-2θ method is described.

The X-ray diffraction measurement was performed on the surface and the surface ground and polished to t/2, and for the calculation of the γ phase fraction, the γ phase fraction from each surface was calculated and averaged. For the sake of convenience, the γ phase fraction is easy to separate from each of the other phases of “γ”, “α′ or α”, and “Cr ₂ N”, and the two diffraction lines each having a relatively high intensity. Was calculated. Selected diffraction lines are shown in Table 7. Since each phase is slightly deviated from the diffraction angle of the standard substance due to nitrogen absorption, the diffraction line having a peak in the angular range shown in Table 8 was used for calculation. Specifically, for each diffraction line intensity, a value obtained by dividing the intensity obtained by subtracting the background by the calculated intensity ratio of each diffraction line was calculated, and the value summed for each phase was taken as the intensity of each phase. The ratio of the strength of the γ phase to the total strength of each phase was defined as the γ phase fraction. The CrN phase is extremely weak in the measurement by the θ-2θ method, which is not the thin film method, and is therefore excluded in this calculation.

The method of calculating the strength of each phase is described below.

I_meas ^k：k番目の選択回折線の測定強度（バックグランド含む）
BG^k：k番目の選択回折線のバックグランド強度
I_calc ^k：ｋ番目の選択回折線の計算強度比
γ相分率は、
γ相分率＝[γ相強度]／（[γ相強度]＋[α’あるいはα相強度]＋[Ｃｒ_２Ｎ相強度]）

I _meas ^k : Measured intensity of the kth selected diffraction line (including background)
BG ^k : Background intensity of the kth selected diffraction line
I _calc ^k : calculated intensity ratio of the kth selected diffraction line γ phase fraction is
γ phase fraction = [γ phase strength]/([γ phase strength] + [α'or α phase strength] + [Cr ₂ N phase strength])

[Measurement of elongation]
From each stainless steel plate, an ASTM half size test piece having an original thickness and a length of 100 mm was cut out. According to ASTM A370, a tensile test was carried out at a strain rate of 1×10 ⁻³ /sec, and proof stress, tensile strength and elongation (elongation at break) were measured. When the elongation was 15% or more, the press workability was evaluated as good.

[Measurement of contact resistance]
The resistance value immediately after the production of the stainless steel plate and the resistance value after the durability test simulating the battery environment were measured. Specifically, the durability test was performed by immersing a stainless steel plate in H ₂ SO ₄ at 90° C. and pH 2 for 96 hours, and then thoroughly washing with water and drying. When the corrosion resistance of the stainless steel sheet is not good, the contact resistance increases due to the development of the passivation film on the surface. A contact resistance of 10 mΩ·cm ² or less before and after the durability test was evaluated as low contact resistance.

The contact resistance was measured by Jun Suzuki, 3 others, “Reduction of contact resistance by heat treatment after pickling of titanium alloy containing precious metal element”, Japan Titanium Association magazine “Titanium” Vol. 54 (2006), No. 4, p. It was carried out according to the method reported in No. 259. Specifically, it was carried out using the apparatus schematically shown in FIG. A stainless steel plate 11 was sandwiched between carbon papers 12 (TGP-H-90, manufactured by Toray Industries, Inc.) having an area of 1 cm ² used for a gas diffusion layer, and further sandwiched by electrodes 13 plated with gold. A constant current was applied between the electrodes while applying a load of 10 kgf/cm ^{2 to} both ends of the electrode 13, and the voltage drop between the carbon paper 12 and the stainless steel plate 11 was measured. The contact resistance was calculated based on this result. Since the value of the obtained contact resistance is the sum of the contact resistances of the sandwiched surfaces (penetration resistance), this was divided by 2 to obtain the contact resistance per one surface of the gas diffusion layer. A digital multimeter (KEITHLEY 2001 manufactured by Toyo Technica Co., Ltd.) was used for measuring the current and the voltage drop.

[Measurement of pitting potential]
The pitting potential was measured according to JIS G0577. The test bath solution was a 3.5% NaCl aqueous solution. The stainless steel plate was completely immersed in the degassed test bath solution, left for 10 minutes, and then subjected to anodic polarization by a potentiodynamic method with a potential sweep rate of 20 mV/min from the natural potential. The potential corresponding to the current density of 100 μAcm ⁻² was defined as the pitting potential. When the pitting corrosion potential was higher than 0.75 V vs SCE, it was evaluated as excellent in pitting corrosion resistance.

[Evaluation of Passivation Corrosion Resistance]
The passivation corrosion resistance was evaluated by immersing a stainless steel plate in a H ₂ SO ₄ solution having a pH of 3 at 80° C. simulating a battery environment, blowing Ar gas into a degassed state, and holding it in a self-potential state for 10 minutes. Anodic polarization was performed from the spontaneous potential to 1.4 V vs SHE at a sweep rate of 20 mV/min. In the stainless steel plate, an increase in the current density due to the passivation corrosion is observed from about 0.9 V vs SHE. The maximum current density at 0.9 V or higher, which is considered to be within the passivation region, was used as an index of the passivation corrosion resistance. When the maximum current density at 0.9 V or more was less than 100 μA/cm ² , it was evaluated that the anti-passivation corrosion resistance was excellent.

The results are shown in Table 9.

As shown in Tables 7 and 9, the stainless steel plates of Nos. 2 to 14, 17 to 19, 21 and 23 to 26 have a γ phase fraction of 80% by volume or more and a γ particle diameter of each plate thickness. It was less than half. These stainless steel sheets had an elongation of 15% or more and showed good press workability.

The stainless steel sheets of Nos. 2 to 14, 17 to 19, 21 and 23 to 26 have Cr/Fe in the vicinity of the surface in the range of 0.1 to 0.5 and Cr/N in the vicinity of the surface of 1. It was 8 or less. These stainless steel sheets had a terrace-like structure surface.

The stainless steel sheets of Nos. 2 to 14, 17 to 19, 21 and 23 to 26 showed excellent results in any evaluation of contact resistance, pitting corrosion resistance and transpassive corrosion resistance. In particular, no pitting corrosion occurred in the stainless steel plates of Nos. 2 to 14, 23, 25 and 26 in which the annealing condition was in the suitable range.

The stainless steel plate of No. 1 had a small amount of nitrogen absorption because the Cr content of Material 1 was too low, and the γ phase fraction was less than 80% by volume. The surface did not have a terrace-like structure, and corrosion hole recesses generated during pickling were observed. The stainless steel plate of No. 1 had a low elongation, was not suitable for processing applications, and had poor contact resistance and corrosion resistance after a durability test.

The stainless steel plate of No. 15 had a low elongation because the Cr content of the material 7 was too high.

In the stainless steel plate of No. 16, the Si content of the material 8 was too high, so the α′ phase was mixed in the structure, the elongation was low, and the corrosion resistance was also poor.

The stainless steel plate of No. 20 had a small value of CNp/CNa because the annealing time was too short, and the variation in the distribution of the N content in the depth direction was large, so the elongation was low.

The stainless steel plate of No. 22 had a large value of CNp/CNa because the annealing time was too long, and the variation in the distribution of the N content in the depth direction was large, so the elongation was low.

Since the stainless steel plate of No. 26 was pickled with a solution containing an oxidizing acid, it did not have a terrace structure on the surface. The contact resistance was high and the corrosion resistance was poor.

Since the stainless steel plate of No. 28 was not annealed, its structure did not become an austenite phase. Therefore, the elongation was low and the corrosion resistance was poor.

Since the stainless steel plate of No. 29 was not subjected to pickling treatment, its surface did not have a terrace structure. Since the passivation film Cr ₂ O ₃ remained on the surface layer, the contact resistance was high.

[Example 2] (with conductive layer)
Using the stainless steel plates of Nos. 7 to 9 and 11 shown in Table 6, the conductive layers were attached to both surfaces by the following method after the pickling treatment. The thickness of the conductive layer per one side was as shown in Table 10.

[Adhesive layer forming step]
A modified polyolefin resin adhesive (UNISTOL (trade name) manufactured by Mitsui Chemicals, Inc.) was used as the adhesive composition for forming the adhesive layer. The surface of the stainless steel plate after pickling was coated with a modified polyolefin resin adhesive to a coating thickness of 5 μm using a table coater and dried at room temperature for 10 minutes to form an adhesive layer. An adhesive layer was similarly formed on the back surface.

[Formation of conductive layer]
As the conductive carbonaceous material, spherical graphite powder (SG-BH (trade name) manufactured by Ito Graphite Industry Co., Ltd., average particle diameter: 20 μm) and expanded graphite powder (EC100 (trade name) manufactured by Ito Graphite Industry Co., Ltd.), The average particle diameter: 160 μm) was used. As the matrix resin, polypropylene resin (PP) powder (Sumitomo Seika Chemical Co., Ltd., Flowbren HP-8522 (trade name)) was used. The spherical graphite powder was mixed at 60% by volume, the expanded graphite powder at 10% by volume, and the polypropylene resin powder at 30% by volume to prepare a powder mixture. 0.2 g or 0.06 g of the powder mixture is evenly charged into a female die having a volume of 50×50×20 mm in a pressing device (tabletop hot press MP-SCL manufactured by Toyo Seiki Seisakusho Co., Ltd.) and hot as a prepress. Pressing (pressure: 2 MPa, temperature: 180° C.) was performed to form a sheet (thickness: 50 μm or 15 μm). The obtained sheet was placed on both sides of the substrate with the adhesive layer prepared above and pressed at a heating temperature of 180° C. and a pressure of 5 MPa (main pressing, molding time 10 minutes).

[Various evaluations]
The above contact resistance, durability test, pitting potential measurement, and passivation current measurement were also performed on the stainless steel sheet after the formation of the conductive layer. The results are shown in Table 10.

As shown in Table 10, by providing the conductive layer, the contact resistance was lower than that in the case where the conductive layer was not provided. Furthermore, except for No. 54, the contact resistance after the durability test was lower than that before the durability test. None of the pitting potentials were detected, and the passivation current was significantly suppressed by the conductive layer.

Only the number 54 had a slightly increased contact resistance after the durability test. The passivation current was also suppressed as compared with the case without the conductive layer, but the maximum current value was higher than that of the other samples having the conductive layer. From this, it can be seen that if the conductive layer is too thin, the effect of the conductive layer decreases.

Since the material of No. 11 in Table 6 has a slightly large rolled finish thickness of 110 μm, it is necessary to thin the conductive layer when severe conditions such as a total thickness of 150 μm or less after the conductive layer is formed are imposed. Since a material having a small total thickness may be required, it is preferable that the thickness of the stainless steel plate is thinner, as shown in Nos. 7 and 9 of Table 6.

The embodiments of the present invention have been described above. The above-described embodiments are merely examples for implementing the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment without departing from the spirit thereof.

1 Solid Polymer Fuel Cell 10 Cell 2 Solid Polymer Electrolyte Membrane 3 Anode 4 Cathode 5a, 5b Separator 6a, 6b Flow Path

Claims (7)

A stainless steel plate,
The chemical composition is% by mass,
Cr: 20-26%,
N: 0.6 to 2.0%,
Si: 2.0% or less,
C: 0.040% or less,
P: 0.030% or less,
S: 0.030% or less,
Mn: 1.5% or less,
Cu: 0.50% or less,
Mo: 0.50% or less,
Ni: 0.10% or less,
Ca: less than 50 ppm,
sol. Al: less than 300 ppm,
Remainder: Fe and impurities,
The stainless steel plate has a thickness of 30 to 110 μm,
The microstructure is a structure in which the proportion of the austenite phase is 80% by volume or more,
The average crystal grain size of the austenite crystal grains is ½ or less of the thickness of the stainless steel plate,
The Cr, Fe, and N contents obtained by glow discharge optical emission spectroscopy from the surface of the stainless steel plate are the following formula (1) and formula in the depth direction analysis in the range of the sputtering depth of 0 to 0.05 μm. Satisfy (2),
In the N content profile obtained by glow discharge emission spectroscopy in the depth direction from the surface of the stainless steel plate, the maximum deviation value CNp in the depth range of 3/8t to 5/8t, where t is the thickness of the stainless steel plate. And the average value CNa in the range of the depth 1/8t to 3/8t satisfy the following formula (3).
0.1≦Cr/Fe≦0.5 (1)
Cr/N≦1.8 (2)
0.8≦CNp/CNa≦1.6 (3)
The Cr, Fe, and N contents obtained by glow discharge emission spectroscopy from the surface of the stainless steel plate are substituted for Cr, Fe, and N in the formulas (1) and (2) in atomic %. The units of CNp and CNa in the formula (3) are atomic %. The maximum deviation value CNp is the maximum value CNp1 in the range of depths 3/8t to 5/8t and the minimum value CNp2 in the same range, whichever has the larger difference from CNa. The stainless steel plate according to claim 1,
A stainless steel plate further comprising a conductive layer containing a conductive carbonaceous material on at least one surface. The stainless steel plate according to claim 2,
The said electrically conductive layer is a stainless steel plate which is a carbon-resin composite layer which has the said carbonaceous material disperse|distributed in matrix resin. A fuel cell separator comprising the stainless steel plate according to claim 1. A fuel cell, comprising the fuel cell separator according to claim 4. A fuel cell stack comprising a plurality of the fuel battery cells according to claim 5. The method for manufacturing a stainless steel sheet according to claim 1,
The chemical composition is% by mass, Cr: 20 to 26%, N: 0.1% or less, Si: 2.0% or less, C: 0.040% or less, P: 0.030% or less, S:0. 0.030% or less, Mn: 1.5% or less, Cu: 0.50% or less, Mo: 0.50% or less, Ni: 0.10% or less, Ca: less than 50 ppm, sol. Al: less than 300 ppm, balance: Fe and a step of preparing a slab that is an impurity,
Hot rolling and cold rolling the slab to obtain a rolled steel sheet having a thickness of 30 to 110 μm;
An annealing step of cooling the rolled steel sheet by annealing in a gas atmosphere containing nitrogen,
The rolled steel sheet after the annealing step, a step of pickling with a solution containing a non-oxidizing acid,
A manufacturing method in which the holding time HoT of the annealing step satisfies the following formula (4).
t ² /31≦HoT≦t ² /15 (4)
In the formula (4), the thickness of the rolled steel plate is substituted for t in μm. The unit of HoT is seconds.

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