JP2009167502A - Austenitic stainless steel for fuel cell separator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an austenitic stainless steel which is inexpensive, is excellent in corrosion resistance and electric conductivity, and also is excellent in workability. <P>SOLUTION: The austenitic stainless steel has a composition containing, by mass, ≤0.08% C, 0.01 to 3.00% Si, 0.01 to 10.00% Mn, 0.01 to 3.00% Cu, 15.0 to 40.0% Ni, 20.0 to 35.0% Cr, 0.1 to 4.0% Mo and 0.005 to 0.300% N, and the balance Fe with inevitable impurities. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an austenitic stainless steel for a fuel cell separator.

The solid polymer fuel cell has a basic unit of a single cell in which both sides of a membrane electrode assembly (MEA) in which electrodes are joined to both sides of a solid polymer electrolyte membrane are sandwiched by a separator having a gas flow path. A structure in which single cells are stacked and connected in series to obtain a high voltage is called a cell stack.
Since the separator has a function of electrically connecting each single cell, it is required to have good electrical conductivity. The solid polymer electrolyte membrane is made of a polymer having a large number of sulfonic acid groups and exhibits strong acidity in a wet state. Therefore, the separator is also required to have high corrosion resistance. Conventionally, carbon having both good electric conductivity and corrosion resistance has been generally used for a fuel cell separator, but a carbon separator is expensive. For this reason, in recent years, the use of metal separators such as stainless steel and Ti has been studied. However, the metal separator generally has a problem that the corrosion resistance is inferior to the carbon separator.

In order to solve this problem, various proposals have heretofore been made.
For example, in Patent Document 1, a gold coating layer is formed on the surface of a stainless steel plate, a void is formed on the surface of the stainless steel plate by intergranular corrosion, and a gold coating layer is formed in a state of being embedded in the void. A fuel cell separator is disclosed.
In the same document,
(1) The corrosion resistance is improved by the gold coating layer, and
(2) Since the gold coating layer is embedded in the voids on the surface of the stainless steel plate, it is possible to prevent peeling and cracking of the gold coating layer at the bent portion during pressing.
Is described.

Patent Documents 2 to 4 disclose fuel cell separators obtained by plasma nitriding both surfaces of austenitic stainless steel, Inconel (registered trademark) 600, or industrial pure titanium.
In Patent Document 2,
(1) When the surface of stainless steel is nitrided at a temperature of 590 ° C. or lower, a nitride layer having an M ₄ N type crystal structure can be formed.
(2) Since the M ₄ N-type nitride layer does not contain CrN as a main component, Cr effective for corrosion resistance does not decrease, and corrosion resistance is maintained even after nitriding,
Is described.

JP 2003-92117 A JP 2006-134855 A JP 2007-39786 A JP 2007-73440 A

Coating the surface of the metal separator with gold has the advantage that not only the corrosion resistance is improved, but also the contact resistance with the electrode is reduced. However, gold coating is expensive.
On the other hand, M ₄ N-type nitrides can be formed by nitriding austenitic stainless steel having a face-centered cubic structure, so that the cost is low. Further, M ₄ N type nitrides, can be the constituent element an element other than Cr, be formed M ₄ N type nitride layer on the surface of the stainless steel, the Cr just below the nitride layer A deficient layer does not form (ie, the corrosion resistance of the base does not decrease). Furthermore, since M ₄ N-type nitride has high chemical stability in an oxidizing environment, the contact resistance with the electrode can be kept low.
In order to further improve the corrosion resistance of the austenitic stainless steel provided with such a nitride layer, it is preferable to increase the Cr content of the nitride layer, and for this purpose, the Cr content in the base is increased. It is effective to make it. However, since Cr is a ferrite phase forming element, it is difficult to maintain an austenite single phase with a high Cr composition. Usually, excessive addition of Cr increases the hardness of the steel, raises the transition temperature, and increases the impact resistance value. Furthermore, if the σ phase is precipitated during the manufacturing process, the formability is remarkably deteriorated. Therefore, the high Cr steel has remarkably poor formability regardless of whether it is hot or cold.

  The problem to be solved by the present invention is to provide an austenitic stainless steel for a fuel cell separator that is low in cost, excellent in corrosion resistance and conductivity, and excellent in workability.

In order to solve the above problems, an austenitic stainless steel for a fuel cell separator according to the present invention,
C ≦ 0.08 mass%,
0.01 ≦ Si ≦ 3.00 mass%,
0.01 ≦ Mn ≦ 10.00 mass%,
0.01 ≦ Cu ≦ 3.00 mass%,
15.0 ≦ Ni ≦ 40.0 mass%,
20.0 ≦ Cr ≦ 35.0 mass%,
0.1 ≦ Mo ≦ 4.0 mass%, and
0.005 ≦ N ≦ 0.300 mass%
The balance is composed of Fe and inevitable impurities.
In the austenitic stainless steel for fuel cells, α defined by the formula (1) is preferably −9 ≦ α.
α = [Ni] + 30 * [C] + 30 * [N] + 0.5 * [Mn] + 0.16 * [Cu]-[Cr]-[Mo] -1.5 * [Si] (1)

When the amount of addition of Ni, C, Mo, etc. is optimized in a high Cr composition, it is possible to maintain an austenite single phase while maintaining high Cr. Moreover, since it is an austenite single phase, it is excellent also in hot and cold workability. Further, when such austenitic stainless steel is nitrided under predetermined conditions, layered cubic M ₄ N type nitride or M ₄ N type + M _2-3 N type nitride may be formed on the surface. it can. Since the obtained nitride layer has a high Cr content, it exhibits excellent conductivity and corrosion resistance in an acidic atmosphere.

Hereinafter, an embodiment of the present invention will be described in detail.
[1. Austenitic stainless steel for fuel cell separator]
[1.1. Cr amount and M ₄ N type nitride]
When the austenite (γ) phase is nitrided, M ₄ N-type nitride is generated on the surface. Here, the “M ₄ N-type nitride” has a structure in which nitrogen is arranged in an octahedral space at the center of a unit cell of a face-centered cubic lattice made of a transition metal element (M). Examples of the transition metal element (M) constituting the M ₄ N type nitride include Fe, Cr, Ni, and Mo. The M ₄ N-type nitride is considered to be a fcc or fct structure nitride in which nitrogen is supersaturated and has high density of dislocations and twins, and has high hardness. Further, since Cr nitride is not a main component, even if a nitride layer is formed on the surface, Cr effective for corrosion resistance does not decrease in the base immediately below the nitride layer, and the corrosion resistance of the base is maintained. In addition, since M ₄ N-type nitride forms a strong covalent bond between the transition metal element and the nitrogen element while maintaining the metal bond between the transition metal elements, the M ₄ N-type nitride maintains the high conductivity while maintaining the high conductivity. Reactivity to oxidation is reduced. Furthermore, since the M ₄ N-type nitride has the same face-centered cubic structure as the base (γ phase), the alignment with the base is good and the movement of electrons between the bases is easy.

Upon further progress of the nitridation of the γ-phase, M ₄ dissolved nitrogen of the N-type nitride becomes supersaturated, M ₄ high nitrogen of M _2-3 N type nitrides than N type nitrides M ₄ N type Precipitates on nitride stacking faults. That is, a nitride having a nano-level stacked crystal structure of M ₄ N-type nitride and M _2-3 N-type nitride is obtained. Here, “M _2-3 N-type nitride” means a structure in which nitrogen atoms are arranged between lattices of a close-packed hexagonal lattice made of a transition metal element (M). The transition metal element constituting the M _2-3 N type nitride (M), there Fe, Cr, Ni, Mo and the like.
When the γ-phase nitridation further proceeds, a hexagonal Cr ₂ N, CrN and M _2-3 N-type nitride and a cubic M ₄ are formed on the nitride layer having a nano-level stacked crystal structure. A nitride layer containing at least one N-type nitride is formed.

Both the M ₄ N-type nitride and the M _2-3 N-type nitride can contain transition metal elements other than Cr as constituent elements. Further, even if the nitride layer consisting of M ₄ N type nitrides and M _2-3 N type nitride is formed on the surface of the γ-phase, Cr-depleted zone is not generated immediately under the nitride layer. That is, both the M ₄ N-type nitride and the M _2-3 N-type nitride have a composition that substantially corresponds to the composition of the matrix. Therefore, the higher the Cr concentration in the base, the higher the Cr concentration in the nitride layer.
In order to form a nitrided layer of transition metal nitride that achieves both high conductivity and corrosion resistance, it is preferable that the Cr concentration in the substrate is high. For that purpose, the Cr content in the austenitic stainless steel is preferably 20 mass% or more. The Cr content is more preferably 25 mass% or more, and further preferably 26 mass% or more.

The austenitic stainless steel for a fuel cell separator according to the present invention comprises an austenite single phase. In the present invention, the “austenite single phase” means that the magnetic permeability measured under an external magnetic force of 200 (Oe) is 1.005 or less.
Usually, since Cr is a ferrite-forming element, excessive addition of Cr alone forms a ferrite phase. However, if the austenite single phase is maintained, nitridation is uniformly performed, and a uniform nitride layer having conductivity that is more excellent in corrosion resistance can be formed.

  An austenitic stainless steel having a Cr content in a predetermined range can be produced by holding for a predetermined time at a temperature at which it becomes an austenitic region. At this time, since Cr is a ferrite forming element, if it is excessively added, the austenite region is narrowed. Therefore, ideally, an austenitic stainless steel can be produced even with a steel containing 20 mass% or more of Cr, but in order to obtain an austenitic single phase structure containing a predetermined amount of Cr using a general steel, In actual manufacturing processes, unrealistic (high temperature, long time, etc.) heat treatment is often required. In order to easily produce austenitic stainless steel containing a predetermined amount of Cr, it is preferable to use steel having a specific composition.

[2. Concrete example]
[2.1. First Specific Example]
A first specific example of austenitic stainless steel suitable as a material for a fuel cell separator includes the following elements, with the balance being Fe and inevitable impurities. The kind of additive element, its component range, and the reason for limitation are as follows.

(1) C ≦ 0.08 mass%.
C is an austenite generating element and contributes to stabilization of the austenite phase. Moreover, since it is an interstitial element, it contributes to the improvement of strength.
On the other hand, when the C content is excessive, the formation of Cr carbides causes a Cr-deficient phase in which the solid solution Cr concentration of the matrix phase is reduced, thereby deteriorating the corrosion resistance of the stainless steel itself. In addition, a non-uniform nitride layer is generated during nitriding. Therefore, the C content is preferably 0.08 mass% or less. The C content is more preferably 0.04 mass% or less, and still more preferably 0.02 mass% or less.

(2) 0.01 ≦ Si ≦ 3.00 mass%.
Si is a deoxidizing element and contributes to the cleanliness of steel. In order to obtain such an effect, the Si content is preferably 0.01 mass% or more.
On the other hand, Si is a ferrite-forming element, and excessive addition stabilizes the ferrite phase. Furthermore, excess Si forms oxides such as SiO ₂ in the steel and causes a non-uniform nitrided layer to form during nitriding of stainless steel. Accordingly, the Si content is preferably 3.0 mass% or less. The Si content is more preferably 2.00 mass% or less, and still more preferably 1.00 mass% or less.

(3) 0.01 ≦ Mn ≦ 10.00 mass%.
Mn is an austenite generating element and contributes to stabilization of the austenite phase. In order to obtain such an effect, the Mn content is preferably 0.01 mass% or more.
On the other hand, if the Mn content is excessive, the corrosion resistance of the stainless steel in an acidic atmosphere is significantly deteriorated. Therefore, the Mn content is preferably 10.00 mass% or less. The Mn content is more preferably 5.00 mass% or less, and still more preferably 3.00 mass% or less.

(4) 0.01 ≦ Cu ≦ 3.00 mass%.
Cu is an austenite generating element and contributes to stabilization of the austenite phase. In order to obtain such effects, the Cu content is preferably 0.01 mass% or more.
On the other hand, steel types containing a large amount of Cu cannot be recycled as scrap (Cu is a trap element), and in addition, the corrosion resistance of stainless steel in an acidic atmosphere may be significantly reduced. Therefore, the Cu content is preferably 3.00 mass% or less. The Cu content is more preferably 2.00 mass% or less, and still more preferably 1.00 mass% or less.

(5) 15.0 ≦ Ni ≦ 40.0 mass%.
Ni is an austenite-forming element and is a useful element that strongly contributes to stabilization of the austenite phase. In order to obtain such an effect, the Ni content is preferably 15.0 mass% or more. The Ni content is more preferably 18.0 mass% or more, and further preferably 21.0 mass% or more.
On the other hand, when the Ni content is excessive, the cost is increased. Therefore, the Ni content is preferably 40.0 mass% or less. The Ni content is more preferably 35.0 mass% or less.

(6) 20.0 ≦ Cr ≦ 35.0 mass%.
Cr is a ferrite forming element and strongly contributes to stabilization of the ferrite phase. However, it is an important element that contributes to the improvement of corrosion resistance and strength. In order to obtain such an effect, the Cr content is preferably 20.0 mass% or more. The Cr content is more preferably 25.0 mass% or more, and further preferably 26.0 mass% or more.
On the other hand, if the Cr content is excessive, the residual amount of undissolved Cr carbonitride during the solution heat treatment is increased, and the corrosion resistance of the stainless steel itself is significantly reduced. Further, undissolved Cr carbonitride causes a non-uniform nitrided layer to be generated during nitriding. Therefore, the Cr content is preferably 35.0 mass% or less. The Cr content is more preferably 33.0 mass% or less.

(7) 0.1 ≦ Mo ≦ 4.0 mass%.
Mo is a ferrite-forming element and contributes to stabilization of the ferrite phase. However, it is an element that greatly contributes to the improvement of corrosion resistance. In order to obtain such an effect, the Mo content is preferably 0.1 mass% or more. The Mo content is more preferably 0.5 mass% or more.
On the other hand, if the Mo content is excessive, Mo carbonitrides are formed in the steel, which causes a non-uniform nitrided layer to be generated during nitriding. Therefore, the Mo content is preferably 4.0 mass% or less. The Mo content is more preferably 3.5 mass% or less, and even more preferably 2.5 mass% or less.

(8) 0.005 ≦ N ≦ 0.300 mass%.
N is an austenite generating element and contributes to stabilization of the austenite phase. It is also an element that contributes to improving corrosion resistance and strength. In order to obtain such an effect, the N content is preferably 0.005 mass% or more.
On the other hand, when the N content is excessive, the residual amount of undissolved Cr nitride during the solution heat treatment is increased, resulting in a deficient phase in which the solid solution Cr concentration of the parent phase is lowered, and the corrosion resistance of the stainless steel itself is remarkably increased. Reduce. In addition, undissolved Cr nitride also causes a non-uniform nitride layer to be formed during nitriding. Therefore, the N content is preferably 0.300 mass% or less. The N content is more preferably 0.250 mass% or less, and still more preferably 0.200 mass% or less.

In the austenitic stainless steel for a fuel cell separator according to the present invention, it is preferable that S, O, and P, which are unavoidable impurities, are in the following ranges in addition to the additive elements being in the above-mentioned range.
(14) S ≦ 0.050 mass%.
(15) P ≦ 0.060 mass%.
(16) O ≦ 0.060 mass%.
These elements affect the conductivity and corrosion resistance of the nitride layer when plasma nitriding is performed. In order to suppress a decrease in the conductivity and corrosion resistance of the nitride layer, the smaller the number of these elements, the better.
Specifically, the S content is preferably 0.050 mass% or less. The S content is more preferably 0.030 mass% or less, and still more preferably 0.020 mass% or less.
Moreover, specifically, P content is preferably 0.060 mass% or less. The P content is more preferably 0.040 mass% or less, and still more preferably 0.030 mass% or less.
Furthermore, specifically, the O content is preferably 0.060 mass% or less. The O content is more preferably 0.040 mass% or less, and still more preferably 0.030 mass% or less.

In the austenitic stainless steel for a fuel cell separator according to the present invention, in addition to the additive element being in the above range, α defined by the following formula (1) is preferably −9 ≦ α.

α = [Ni] + 30 * [C] + 30 * [N] + 0.5 * [Mn] + 0.16 * [Cu]-[Cr]-[Mo] -1.5 * [Si] (1)
However, [Ni] etc. represents mass% of each element.

Usually, excessive addition of Cr increases the hardness of the steel, raises the transition temperature, and raises the impact resistance value. Therefore, the high Cr steel has extremely poor formability regardless of whether it is hot or cold. It has been known. Furthermore, since Cr is a ferrite phase generating element, it is difficult to maintain an austenite single phase with a high Cr composition.
On the other hand, when the content of the additive element affecting the formability is optimized so as to satisfy the formula (1), the nitride Cr concentration of the M ₄ N-type crystal structure is increased, and the austenite single phase is reduced. In addition to being able to be maintained, good moldability can be obtained. In order to obtain such an effect, α is preferably −6 <α, and more preferably −2 <α.

Furthermore, the austenitic stainless steel for a fuel cell separator according to the present invention is solidified at a temperature not less than b value and not more than (b + c) value after cold rolling in addition to the component elements and the component balance being in the above-mentioned ranges. Those obtained by heat treatment are preferred.
However,
α = [Ni] + 30 * [C] + 30 * [N] + 0.5 * [Mn] + 0.16 * [Cu]-[Cr]-[Mo] -1.5 * [Si] (1)
b = (α + 10) × 22 + 850 (2)
c = (5-α) × 19.3 (3)

A metal separator is generally manufactured by cold rolling a steel sheet to a thickness of about 0.1 mm, subjecting it to a solution heat treatment, and then press-molding. The solution heat treatment is performed to impart press formability to the cold-rolled steel sheet. When the σ phase, which is a hard intermetallic compound, precipitates during the solution heat treatment, the formability of stainless steel is significantly deteriorated regardless of whether it is hot or cold. In order to maintain the austenite single phase and suppress the precipitation of the σ phase, the solution heat treatment temperature is preferably not less than the b value defined by the formula (2).
On the other hand, if the solution heat treatment temperature is too high, the crystal grains become coarse. A 0.1 mm-thick austenitic stainless steel plate made of coarse crystal grains does not have sufficient workability during press molding, and is liable to cause defects such as cracks and chips. Furthermore, steel with excessive addition of Cr has increased hardness, increased elasticity limit, and increased impact resistance, so that high Cr steel has poor formability regardless of whether it is hot or cold. Therefore, the solution heat treatment temperature is preferably not more than the (b + c) value defined by the equations (2) and (3).
The c value preferably further satisfies the following expression (3 ′).
c = (5-α) × 8.8 (3 ′)

The solution heat treatment temperature affects the material elongation and grain size. Generally, when the solution heat treatment temperature is too low, the σ phase is precipitated and the elongation is lowered. In order to obtain good workability, the elongation during the tensile test (JIS Z 2241) is preferably 10% or more.
On the other hand, when the solution heat treatment temperature is too high, the crystal grains become coarse and the workability deteriorates. In order to obtain good processability, the crystal grain size is preferably 4 or more in terms of crystal grain size number (JIS G 0551).
Stainless steel having such elongation and crystal grain size can be obtained by optimizing the solution heat treatment temperature. The higher the Cr composition, the narrower the suitable solution heat treatment temperature range.

[2.2. Second specific example]
The austenitic stainless steel for a fuel cell separator according to the present invention has the above-described composition,
C ≦ 0.04 mass%,
0.01 ≦ Si ≦ 2.00 mass%
0.01 ≦ Mn ≦ 5.00 mass%
0.01 ≦ Cu ≦ 2.00 mass%
18.0 ≦ Ni ≦ 40.0 mass%
25.0 ≦ Cr ≦ 35.0 mass%
0.1 ≦ Mo ≦ 3.5 mass%, and
0.005 ≦ N ≦ 0.250 mass%
In which the balance is Fe and inevitable impurities.
Details of reasons for limiting each additive element, points where S, P, and O are preferably less than a predetermined amount, points where α is preferably within a predetermined range, and solution heat treatment at a predetermined solution heat treatment temperature Since it is the same as that of the 1st specific example that it is preferable to do, description is abbreviate | omitted.

[2.3. Third specific example]
The austenitic stainless steel for a fuel cell separator according to the present invention has the above-described composition,
C ≦ 0.02 mass%,
0.01 ≦ Si ≦ 1.00 mass%
0.01 ≦ Mn ≦ 3.00 mass%
0.01 ≦ Cu ≦ 1.00 mass%
20.0 ≦ Ni ≦ 35.0 mass%
26.0 ≦ Cr ≦ 33.0 mass%
0.5 ≦ Mo ≦ 2.5 mass%, and
0.005 ≦ N ≦ 0.200 mass%
In particular, the balance being Fe and inevitable impurities.
Details of reasons for limiting each additive element, points where S, P, and O are preferably less than a predetermined amount, points where α is preferably within a predetermined range, and solution heat treatment at a predetermined solution heat treatment temperature Since it is the same as that of the 1st specific example that it is preferable to do, description is abbreviate | omitted.

[3. Manufacturing method of fuel cell separator]
The fuel cell separator can be manufactured by processing austenitic stainless steel containing a predetermined amount of Cr into a predetermined shape and nitriding the surface.
The nitriding method is not particularly limited, and various methods can be used. Specifically, as the nitriding method,
(1) Using an object to be processed as a cathode, applying a DC voltage to generate glow discharge (ie, low temperature non-equilibrium plasma), ionizing a part of the gas component, and covering the ionized gas component in the non-equilibrium plasma A plasma nitriding method in which nitriding is performed by high-speed collision with the surface of the workpiece
(2) Gas nitriding method in which nitriding proceeds at atmospheric pressure and by an equilibrium reaction,
and so on.
In particular, the plasma nitriding method is
(1) It is easy to remove the passive film formed on the surface of the workpiece.
(2) Simultaneously with the formation of the nitride layer, it is easy to remove the oxide film formed on the surface of the workpiece.
(3) A nitride layer containing M ₄ N-type nitride having a high nitrogen concentration, which is rich in conductivity and corrosion resistance in the depth direction from the surface of the workpiece, can be obtained quickly.
There is an advantage.

  Specifically, plasma nitriding of austenitic stainless steel is performed by the following procedure. That is, first, an object made of austenitic stainless steel is placed in a nitriding furnace and evacuated. Next, a mixed gas of hydrogen gas and argon gas is introduced into the furnace, and a voltage is applied at a degree of vacuum of several to several tens of Torr (665 to 2128 Pa) using the workpiece as a cathode and the furnace wall as an anode. When a voltage is applied, a glow discharge is generated on the object to be processed as a cathode, and nitrogen ionized by the glow discharge collides, enters, and diffuses on the surface of the object to be processed. As a result, at the same time as the workpiece is heated, a nitride layer is formed on the surface of the workpiece.

[4. Action of austenitic stainless steel for fuel cell separator according to the present invention]
When the γ phase is nitrided, cubic M ₄ N type nitride is formed on the surface. Since the γ phase has a face-centered cubic structure, nitrogen atoms invade into octahedral voids at the center of the unit cell of the face-centered cubic lattice composed of transition metal elements by nitriding to form M ₄ N-type nitrides. Cheap. When the γ-phase nitridation further proceeds, M _2-3 N-type nitride is deposited on the stacking faults of the M ₄ N-type nitride, resulting in a nitride having a nano-level stacked structure. Both the M ₄ N type nitride and the M ₄ N type + M _2-3 N type nitride can use elements other than Cr as constituent elements, so the Cr in the base is not consumed. Corrosion resistance is not reduced.

Further, in order to achieve both high conductivity and corrosion resistance, it is preferable that the Cr concentration in the nitride layer is high, and for that purpose, the Cr concentration in the matrix is preferably high. However, in general, excessive addition of Cr has a problem in that it is difficult to maintain an austenite single phase, and formability is significantly reduced.
On the other hand, if the amount of addition of Ni, C, Mo, etc. is optimized in a high Cr composition, an austenite phase single phase can be maintained while being high Cr. Moreover, since it is an austenite single phase, it is excellent also in hot and cold workability. In particular, when the additive element is optimized to satisfy the formula (1), high formability can be maintained while maintaining the austenite single phase.
Furthermore, when such austenitic stainless steel is nitrided under predetermined conditions, cubic M ₄ N type nitride or M ₄ N type + M _2-3 N type nitride can be formed on the surface. Since the obtained nitride layer has a high Cr content, it exhibits excellent conductivity and corrosion resistance in an acidic atmosphere.

(Examples 1-12, Comparative Examples 1-11)
[1. Preparation of sample]
An alloy having chemical components shown in Table 1 was melted and cast using a high frequency induction furnace to obtain a 50 kg steel ingot. Next, this steel ingot was uniformly heated and then made into a 60-square plate by hot forging. The plate material was made into a thin plate having a thickness of 0.1 mm by cold rolling. The obtained thin plate was heat-treated in a reducing atmosphere. The heat treatment temperature was 800 to 1300 ° C., and the heat treatment time was 60 s. Table 1 shows the alloy composition. Tables 2 and 3 show α, heat treatment conditions, b, c, and b + c.
Furthermore, the thin plate surface was nitrided using a plasma nitriding method. Plasma nitriding was performed according to the following procedure. That is, after the separator obtained by press molding was pickled, both surfaces of the heat treatment material were subjected to plasma nitriding treatment by micropulse direct current glow discharge.
The plasma nitriding conditions were nitriding temperature: 425 ° C., nitriding time: 60 minutes, gas mixing ratio during nitriding: N ₂ : H ₂ = 7: 3, and processing pressure: 3 Torr (300 Pa).

[2. Test method]
[2.1. Contact resistance value]
Contact resistance values before and after the corrosion resistance test were measured using a pressure load contact electrical resistance measuring device TRS-2000SS manufactured by ULVAC-RIKO. Carbon paper was interposed between the sample cut out to 30 mm × 30 mm × and the electrode, and the electrode / carbon paper / sample / carbon paper / electrode were stacked in this order. The electrical resistance when a current of 1 A / cm ² was passed at a measurement surface pressure of 1.0 MPa was measured twice, and the average value of each electrical resistance was determined to obtain the contact resistance value.
Carbon paper coated with carbon black carrying a platinum catalyst (carbon paper TGP-H-090 manufactured by Toray Industries, Inc., thickness 0.26 [mm], bulk density 0.49 [g / cm ^3] , Porosity 73 [%], thickness direction volume resistivity 0.07 [o · cm ² ]). As the electrode, a Cu electrode having a diameter of f20 was used. Further, a sample subjected to heat treatment at 1100 ° C. × 60 s was used as the contact resistance value measurement sample.

[2.2. Ion elution amount]
In the fuel cell, a potential of about 1 [V vs SHE] is applied to the oxygen electrode side in comparison with the hydrogen electrode side. Further, the solid polymer electrolyte membrane has a proton exchange group such as a sulfonic acid group in the molecule, and is used in a water-containing state, and thus exhibits a strong acidity. For this reason, the corrosion resistance was evaluated in an acid environment simulating the environment where the fuel cell separator is exposed in the fuel cell stack, using a constant potential electrolysis test method which is an electrochemical technique.
Specifically, first, a sample having a size of 30 mm × 30 mm was cut out from the central portion of the thin plate and immersed in a sulfuric acid aqueous solution having a pH of 4 and a temperature of 80 ° C. for 100 hours. The atmosphere at that time was N ₂ gas deaeration simulating the anode electrode environment, or an open air state simulating the cathode environment. After the corrosion resistance test, the ion elution amounts of Fe, Cr, and Ni dissolved in the sulfuric acid aqueous solution were measured by fluorescent X-ray analysis. Moreover, what performed the heat processing of 1100 degreeC x 60 s was used for the sample for ion elution amount measurement.

[2.3. Hot workability]
The extent of occurrence of flaws during hot forging was visually evaluated. The hot workability was evaluated in five stages (A>B>C>D> E) from class A (no flaws generated) to class E (large cracks generated: not forged).
[2.4. Hot formability]
A 6φ × 55 L cylindrical test piece was cut out from the plate material after hot forging, and a high-temperature high-speed tensile test (greeble test) was performed. That is, the temperature of the test piece was raised to 1200 ° C. by energization heating and held for 60 seconds, and then the test piece was broken at a tensile rate of 50.8 mm / s. Using the specimen after fracture, the fracture drawing (degree of constriction) was measured. The higher the fracture drawing value (%), the better the hot formability.

[2.5. Cold workability]
The occurrence frequency of defects such as ear cracks during cold rolling was visually evaluated. The cold workability was evaluated in five stages (A>B>C>D> E) from class A (no flaw generation) to class E (large crack generation: unrollable).
[2.6. Cold formability]
A 15φ × 22.5 L cylindrical test piece was cut out from the plate material after hot forging, and subjected to 75% compression (end face constrained compression test (room temperature)) by a 600 t press in the length direction. Such a compression test was performed five times, and the deformation resistance value during the compression test and the number of test pieces in which cracks occurred on the side surfaces of the test piece were measured. The smaller the deformation resistance value and the smaller the crack generation rate, the better the cold formability.

[2.7. Permeability]
Using a VSM (vibrating sample magnetometer), the magnetic permeability was measured under the condition of an external magnetic force of 200 (Oe). As the sample, a material subjected to heat treatment after cold rolling was used.

[2.8 Particle size]
The crystal grain size was measured according to JIS G 0551 by observing the structure of a longitudinal section of a 0.1 mm thick plate.
[2.9 Hardness]
A vertical section of a 0.1 mm thin plate was measured using a Vickers hardness meter after mirror polishing.
[2.10 Elongation]
A test piece (JIS No. 13B) was cut out along the rolling direction of the 0.1 mm thin plate, a tensile test (JIS Z 2241) was performed, and the elongation (%) at that time was measured.

[3. result]
Tables 4 to 7 show the test results. In Comparative Examples 1 to 11, since the content of the component elements is not appropriate, the contact resistance value after the corrosion resistance test is relatively high, and the ion elution amount is relatively large. Moreover, depending on the composition, there are steel types (Comparative Examples 2, 5, 7, 9 to 11) that cannot maintain the austenite single phase.
In contrast, in Examples 1 to 12, since the content of the component elements is appropriate, the contact resistance value after the corrosion resistance test is low, the ion elution amount is small, and the austenite single phase is maintained in all steel types. I was able to. In addition, there is a correlation between α represented by formula (1), the contact resistance value, and the ion elution amount, and by optimizing α, the characteristics as a separator are improved while maintaining the austenite single phase. I understood it.

However, even in Examples 1 to 12 in which the content of the component elements is appropriate, mechanical properties (hardness, elongation), hot workability, and cold workability differ greatly due to fluctuations in the solution heat treatment temperature during production. I understood it. This is because the σ phase, which is a hard intermetallic compound, precipitates when the solution heat treatment temperature is low. In order to prevent sigma phase precipitation, the solution heat treatment temperature must be higher than the b value.
On the other hand, if the solution heat treatment temperature is too high, the crystal grains become coarse. A 0.1 mm-thick austenitic stainless steel plate made of coarse crystal grains does not have sufficient workability during press molding, and is liable to cause defects such as cracks and chips. Furthermore, steel with excessive addition of Cr has increased hardness, increased elastic limit, and increased impact resistance, so high Cr steel has formability regardless of whether it is hot or cold. It is known to get worse. Therefore, the solution heat treatment temperature must be set to a certain temperature ((b + c) value) or less so that the crystal grains can be maintained in a fine state (for example, a crystal grain size number of 4 or more).
Any of the examples produced by the production process satisfying these conditions is excellent in hot and cold workability, and is preferable from the viewpoint of actual operation. Furthermore, there is a correlation between the b and c values represented by the formulas (2) and (3) and the hot and cold workability, and by optimizing the b and c values, It was found that the workability between hot and cold is improved.

  The embodiment of the present invention has been described in detail above, but the present invention is not limited to the above embodiment, and various modifications can be made without departing from the scope of the present invention.

  The austenitic stainless steel for fuel cell separators according to the present invention can be used as a material for fuel cell separators.

Claims (6)

C ≦ 0.08 mass%,
0.01 ≦ Si ≦ 3.00 mass%,
0.01 ≦ Mn ≦ 10.00 mass%,
0.01 ≦ Cu ≦ 3.00 mass%,
15.0 ≦ Ni ≦ 40.0 mass%,
20.0 ≦ Cr ≦ 35.0 mass%,
0.1 ≦ Mo ≦ 4.0 mass%, and
0.005 ≦ N ≦ 0.300 mass%
An austenitic stainless steel for fuel cell separators, the balance being Fe and inevitable impurities. The austenitic stainless steel for a fuel cell separator according to claim 1, wherein α defined by the following formula (1) is −9 ≦ α.
α = [Ni] + 30 * [C] + 30 * [N] + 0.5 * [Mn] + 0.16 * [Cu]-[Cr]-[Mo] -1.5 * [Si] (1) S ≦ 0.050 mass%,
P ≦ 0.060 mass%, and
O ≦ 0.060 mass%
The austenitic stainless steel for fuel cell separators according to claim 1 or 2, further comprising: The austenitic stainless steel for a fuel cell separator according to any one of claims 1 to 3, which is obtained by performing a solution heat treatment at a temperature not lower than b value and not higher than (b + c) value after cold rolling.
However,
α = [Ni] + 30 * [C] + 30 * [N] + 0.5 * [Mn] + 0.16 * [Cu]-[Cr]-[Mo] -1.5 * [Si] (1)
b = (α + 10) × 22 + 850 (2)
c = (5-α) × 19.3 (3) The said c value is stainless steel for fuel cell separators of Claim 4 which satisfy | fills following (3 ') Formula.
c = (5-α) × 8.8 (3 ′)   The stainless steel for a fuel cell separator according to any one of claims 1 to 5, wherein the grain size number is 4 or more and the elongation at the time of the tensile test is 10% or more.