JP2000309854A - Austenitic stainless steel for current-carrying electric parts, and fuel cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce an austenitic stainless steel for current-carrying electric parts, minimal in contact electric resistance, and to provide a separator composed of the stainless steel, and a solid-electrolyte-type fuel cell having the separator. SOLUTION: The austenitic stainless steel has a composition containing, by weight, 0.015-0.2% C. 0.01-1.5% Si. 0.01-2.5% Mn, 17-30% Cr, 7-50% Ni, 0-3% Cu, 0-0.3% N, 0-7% Mo, and 0-0.2% Al and satisfying Cr+3Mo=17 to 50%, and further, the content of C precipitating as a Cr carbide and the total content of C in the steel satisfy inequality (content (wt.%) of C precipitating as Cr carbide)×100/(total C content (wt.%) in the steel-0.012%)>=85. A fuel cell 1 is provided with a separator 5 composed of the stainless steel.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a separator (bipolar) made of austenitic stainless steel having low contact electric resistance used as a current-carrying electric part, and the above-mentioned stainless steel used as a small distributed power source for use in automobiles and homes. (Sometimes referred to as a plate).

[0002]

2. Description of the Related Art Austenitic stainless steel is excellent in corrosion resistance because a passive film is formed on its surface. However, since the passive film on the surface has a large electric resistance, it is not suitable for an electric component which is used by being energized and requires a low contact electric resistance. The thicker the passive layer film, the better the corrosion resistance, but the higher the electrical resistance.

[0003] If the contact electric resistance of austenitic stainless steel can be reduced, it becomes possible to use the austenitic stainless steel as a current-carrying electrical component requiring corrosion resistance. One of the current-carrying electrical components requiring excellent corrosion resistance and small contact electric resistance is a separator of a polymer electrolyte fuel cell.

A fuel cell is a battery that generates DC power using hydrogen and oxygen, and is a solid oxide fuel cell,
There are a molten carbonate fuel cell, a phosphoric acid fuel cell, and a polymer electrolyte fuel cell. The name of the fuel cell is derived from the constituent materials of the "electrolyte" part that forms the basis of the cell.

At present, the fuel cells that have reached the commercial stage include a phosphoric acid fuel cell and a molten carbonate fuel cell.
Approximate operating temperatures of the fuel cell are 1000 ° C. for a solid oxide fuel cell, 650 ° C. for a molten carbonate fuel cell, 200 ° C. for a phosphoric acid fuel cell, and 80 ° C. for a polymer electrolyte fuel cell.

A polymer electrolyte fuel cell has an operating temperature of 80
Since it is easy to start and stop at low temperatures around ℃ and the energy efficiency can be expected to be about 40%, it can be used as an emergency dispersed power source for small business establishments and telephone offices, a small household dispersed power source using city gas as fuel, and hydrogen gas. It is expected to be put to practical use worldwide as a power source for low-pollution electric vehicles using methanol or gasoline as fuel.

[0007] The above-mentioned various fuel cells are called by a common name of "fuel cell", but when considering the constituent materials of the respective cells, it is necessary to consider them completely different. The performance required by the presence or absence of corrosion of the constituent materials due to the electrolyte used, the presence or absence of high-temperature oxidation that begins to become apparent at around 380 ° C, the presence or absence of sublimation and reprecipitation of the electrolyte, the presence of condensation, etc. Because it is completely different. In fact, the materials used are also various, ranging from graphite-based materials to Ni clad materials, high alloys, and stainless steels.

[0008] It is hardly conceivable to apply the materials used in commercially available phosphoric acid fuel cells and molten carbonate fuel cells to the constituent materials of solid polymer fuel cells.

FIG. 1 shows the structure of a polymer electrolyte fuel cell. FIG. 1 (a) is an exploded view of a fuel cell (single cell), and FIG. 1 (b) is a perspective view of the whole fuel cell. It is. As shown in FIG. 1, the fuel cell 1 is an aggregate of single cells. In the single cell, as shown in FIG. 1A, a solid polymer electrolyte membrane 2 has a fuel electrode membrane (anode) 3 laminated on one surface and an oxidant electrode membrane (cathode) 4 laminated on the other surface. It has a structure in which separators 5a and 5b are overlapped on both surfaces.

A typical solid polymer electrolyte membrane 2 includes:
There is a fluorine ion exchange resin membrane having a hydrogen ion (proton) exchange group.

The fuel electrode film 3 and the oxidant electrode film 4 include
A catalyst layer made of a particulate platinum catalyst, graphite powder, and, if necessary, a fluororesin having a hydrogen ion (proton) exchange group is provided so as to come into contact with a fuel gas or an oxidizing gas.

The flow path 6a provided in the separator 5a
From the fuel gas (hydrogen or hydrogen-containing gas) A to supply hydrogen to the fuel electrode film 3. Also, the separator 5
An oxidizing gas B such as air is flown from a flow path 6b provided in b, and oxygen is supplied. The supply of these gases causes an electrochemical reaction to generate DC power.

The functions required of a polymer electrolyte fuel cell separator include (1) a function as a "flow path" for uniformly supplying fuel gas in a plane on the fuel electrode side, and (2) water generated on the cathode side. As a "flow path" for efficiently discharging the fuel and the carrier gas such as air and oxygen from the fuel cell to the outside of the system, and (3) simply maintaining low electrical resistance and good conductivity as an electrode for a long time. The function as an electrical "connector" between cells, and (4) the function as a "partition wall" between an anode chamber of one cell and a cathode chamber of an adjacent cell in an adjacent cell.

So far, the use of a carbon plate as a separator material has been intensively studied. However, the carbon plate has a problem that it is easily cracked, and furthermore, the machining cost for flattening the surface and the gas flow path are reduced. There is a problem that the machining cost for forming is very high. Each is a fatal issue, and there are situations that can make commercialization of fuel cells themselves difficult.

[0015] Among carbons, a heat-expandable graphite processed product has been receiving the most attention as a material for a polymer electrolyte fuel cell separator because it is extremely inexpensive. However, in order to reduce the gas permeability and impart the function as the partition wall, the resin impregnation and firing must be performed “a plurality of times”. In addition, there are many issues to be solved in the future, such as machining costs for securing flatness and forming grooves,
It has not been put to practical use.

As a move against the study of the use of graphite-based materials, attempts have been made to apply stainless steel to the separator for the purpose of cost reduction.

Japanese Patent Application Laid-Open No. 10-228914 discloses a fuel cell separator made of a metal member and having a gold plating directly applied to a contact surface of a unit cell with an electrode. Stainless steel, aluminum, and Ni-iron alloy are listed as metal members, and SUS304 is used as stainless steel. In the present invention,
It is said that since the separator is plated with gold, the contact resistance between the separator and the electrode is reduced, and the conduction of electrons from the separator to the electrode is improved, so that the output voltage of the fuel cell is increased.

JP-A-8-180883 discloses a solid polymer electrolyte fuel cell in which a passive film formed on the surface uses a separator made of a metal material which is easily formed by the atmosphere. I have. Stainless steel and titanium alloy are mentioned as metal materials. In the present invention, a passivation film is always present on the surface of the metal used for the separator, and the degree of ionization of water generated in the fuel cell is reduced due to the fact that the surface of the metal is hardly chemically attacked. The fuel cell is said to be reduced, and a decrease in the electrochemical reactivity of the fuel cell is suppressed. Further, it is said that the electrical contact resistance value is reduced by removing the passive body film at the portion of the separator that contacts the electrode film or the like and forming a noble metal layer.

However, even if a metal material such as stainless steel provided with a passivation film on the surface disclosed in the above-mentioned publication is used for the separator as it is, the corrosion resistance is not sufficient and the metal is eluted, and the metal eluted. The performance of the supported catalyst is degraded by the ions (hereinafter, referred to as poisoning of the supported catalyst).
In addition, corrosion products such as Cr-OH and Fe-OH generated after elution have a problem that the contact resistance of the separator increases. At present, noble metal plating is applied.

The application of metal materials to separators so far has only been applied, and is far from practical use.

It is very strongly desired to develop a stainless steel which can be used as a separator without any expensive surface treatment and which is "pure" and has low contact electric resistance in a battery environment and excellent corrosion resistance. It is no exaggeration to say that the practical use of separators has succeeded in commercializing polymer electrolyte fuel cells and expanding their applications.

[0022]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a low solid contact resistance and excellent corrosion resistance, particularly in a separator environment of a polymer electrolyte fuel cell without expensive surface treatment. An object of the present invention is to provide an austenitic stainless steel with less eluting metal ions even when used in the above, a polymer electrolyte fuel cell separator made of the stainless steel, and a polymer electrolyte fuel cell using the separator.

[0023]

The gist of the present invention is as follows.

(1) C: 0.015 to 0.2% by weight
%, Si: 0.01-1.5%, Mn: 0.01-2.
5%, P: 0.035% or less, S: 0.01% or less, C
r: 17 to 30%, Ni: 7 to 50%, Cu: 0 to 3
%, N: 0 to 0.3%, Mo: 0 to 7%, Al: 0 to 0%
0.2%, Cr + 3Mo is 17-50%, and the balance of Fe and inevitable impurities, the amount of C precipitated as Cr-based carbide and the total amount of C in steel are:
Austenitic stainless steel with low contact electrical resistance for current-carrying electrical components that satisfies the following formula.

(C weight% precipitated as Cr-based carbide)
× 100 / (total C weight% in steel-0.012%) ≧ 85 (2) A plurality of unit cells in which a fuel electrode membrane and an oxidant electrode membrane are overlapped with the solid polymer electrolyte membrane at the center, between the unit cells In a polymer electrolyte fuel cell in which a fuel gas and an oxidizing gas are supplied to a stacked body laminated with a separator interposed therebetween to generate DC power, the separator is made of the austenitic stainless steel described in (1) above. Molecular fuel cell.

The separator has the four functions described above. That is, a) a function as a "flow path" for uniformly supplying a fuel gas in-plane on the fuel electrode side, b) air generated on the cathode side by reacting air from the fuel cell,
Function as a "flow path" to efficiently discharge the carrier gas together with a carrier gas such as oxygen, c) Function as an electrical "connector" between single cells that maintains low electrical resistance and good electrical conductivity as an electrode for a long time And d) adjacent cells have a function as a "partition wall" between the anode chamber of one cell and the cathode chamber of the adjacent cell. In some cases, the functions are shared by a plurality of plates. As used herein, the term "separator" refers to a plate having at least the function of C).

The Cr-based carbide refers to a carbide containing Cr represented by C ₂₇ C ₆ . Fe, Mo in carbides
In some cases, a small amount of an element having a strong chemical affinity with C, such as Cr, is contained.

The present inventors have found that in an environment of an austenitic stainless steel for a current-carrying electric component having a low contact electric resistance, particularly a solid oxide fuel cell separator environment, the metal ions eluted from the steel surface are as small as possible and can be used as a separator for a long time. Various tests were conducted with the aim of developing a stainless steel that does not increase the contact electrical resistance with graphite for electrodes even when used. As a result, the present inventors have obtained the following findings and completed the present invention.

A) Austenitic stainless steel exhibits relatively good corrosion resistance in a separator environment, but metal elution occurs in general austenitic stainless steel.

B) When metal elution occurs, corrosion products (F
e-based hydroxide) is produced, resulting in an increase in contact electric resistance and a significant adverse effect on supported catalyst performance. Battery performance represented by electromotive force deteriorates in a short time, and proton conductivity of a fluorine-based ion exchange resin membrane having a hydrogen ion (proton) exchange group deteriorates. c) The electric resistance of the passive film formed on the surface is specific to stainless steel, and it is not easy to stably maintain the electric resistance of the passive film small enough to secure battery performance.

D) On the other hand, in order to secure corrosion resistance as a separator in the environment inside the battery body, a passive film is indispensable.

E) Even if the passive film is strengthened to ensure corrosion resistance, the contact electric resistance increases as the film thickness increases,
Battery efficiency is significantly reduced.

F) The Cr-based carbide on the stainless steel surface
It acts as a contact point with excellent electrical conductivity, and the contact electric resistance, which becomes a problem when stainless steel is used as a separator, depends on the number and area of contact points per unit area of contact, and the passive film formed on the surface. Depends on electrical resistance.

G) The stainless steel in which carbides mainly containing Cr are precipitated in the steel, irrespective of the passive film on the steel surface,
The contact electric resistance can be kept low over time. However, the amount of C precipitated as Cr-containing carbide and the total amount of C in the steel must satisfy the following formula.

(C weight% precipitated as Cr-based carbide)
× 100 / (total C weight% in steel-0.012%) ≧ 85 h) In order to strengthen the passivation film and suppress the elution of metal in the separator environment, the content of Cr and Mo should be (Cr
+ 3Mo) must be in the range of 17 to 50%.

J) Corrosion resistance is ensured by actively adding Mo. Even if Mo is eluted, the effect on the performance of the catalyst supported on the anode and the cathode is relatively small. Having.

[0037]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for defining the chemical composition of the austenitic stainless steel of the present invention will be described in detail below. In addition, the following% display shows weight%.

C: C is the most important element in the present invention. Dispersion and precipitation as a Cr-based carbide has an effect of lowering the contact electric resistance of the surface of the stainless steel covered with the passive film.

In general, it is said that the precipitation of chromium carbides in stainless steel is the main cause of deterioration of corrosion resistance known as “sensitization”, and the smaller the precipitation, the better. However, in the present invention, chromium-based carbides are positively precipitated and used.

On the surface of stainless steel, a few tens of ultra-thin passivation films are formed and exhibit excellent corrosion resistance.
It is inferior in electric conductivity to the base material and increases the contact electric resistance. Although it is possible to reduce the electric resistance by making the passivation film thinner, it is not easy to stably maintain the passivation film in a thin state especially in the polymer electrolyte fuel cell.

It is extremely effective that the Cr-based carbide having excellent electric conductivity is directly exposed to the surface without being covered with the passivation film, in order to stabilize the electric conductivity of the stainless steel surface for a long time. That is, since the Cr-based carbide is stable in terms of corrosion resistance and does not form a passive film on the surface, even if the passive film on the surface becomes thick inside the polymer electrolyte fuel cell, it is exposed to the steel surface. C
Good electrical conductivity is ensured through the r-based carbide,
An increase in the contact electric resistance of the steel surface can be suppressed. In other words, the fine chromium-based carbide exposed without being covered by the passivation film functions as an “electrical passage (bypass)”, so that the contact electric resistance can be kept low.

In general, when a large amount of C is contained in austenitic stainless steel, strength and hardness increase,
Ductility also decreases. In addition, the reduction in hot workability also increases.
In order to secure the formability as a separator material for a polymer electrolyte fuel cell, it is necessary to precipitate C in steel as carbide to reduce the amount of solid solution C. By precipitating C as a carbide, the formability of steel is improved. That is, it is necessary to precipitate C in steel as carbide from the viewpoint of ensuring formability. Further, the heat treatment of the carbides to make them coarse and coarse is also effective for further improving the workability. By holding for a long time, the carbides are aggregated and coarsened.

In the steel of the present invention, C is contained in an amount of 0.015% or more and 0.2% or less in order to positively precipitate carbides. If the content exceeds 0.2%, the moldability for a polymer electrolyte fuel cell separator cannot be ensured.

(C weight% precipitated as Cr-based carbide)
× 100 / (total C weight% in steel-0.012%) ≧ 85: When austenitic stainless steel having a passivation film on the surface is used as a separator, in order to reduce the contact electrical resistance with the electrode, use a Cr-based It is necessary to adjust the C weight precipitated as carbide and the total C weight in steel so as to satisfy the following formula.

(C weight% precipitated as Cr-based carbide)
× 100 / (total C weight% in steel-0.012%) ≧ 85 or less, this (C weight% precipitated as Cr-containing material) × 100 / (total C weight% in steel)
-0.012) is called an I value.

When the I value is less than 85, there is no effect of the precipitation of Cr-based carbide on the steel surface, and it is almost equal to the contact electric resistance of ordinary austenitic stainless steel.

In order to make the I value 85 or more, a heat treatment for maintaining the temperature in a temperature range of 500 to 950 ° C. may be performed as a final heat treatment. In a temperature range exceeding 950 ° C., the Cr-based carbide becomes thermally unstable and re-dissolves. On the other hand, if the temperature is lower than 500 ° C., the diffusion rates of C and Cr in the steel are low, and the precipitation treatment time in mass production becomes long, which is not preferable from an industrial viewpoint. The processing temperature range suitable for Cr-based carbide precipitation is 600 ° C or more,
It is below ° C.

Although Cr-based carbides are finely dispersed and precipitated in steel, they tend to preferentially precipitate at crystal grain boundaries where they are easily precipitated.
In order to lower the contact electric resistance, chromium-based carbides
There is not much effect on where the particles are precipitated, but from the viewpoint of uniform dispersion, it is desirable that the particles are also dispersed and precipitated within the particles.

In order to disperse the particles in the grains, a working strain is imparted by hot rolling or cold rolling within a temperature range and time in which the Cr-based carbide does not re-dissolve in a state where the Cr-based carbide is once precipitated. After that, the temperature may be maintained again in the Cr-based carbide precipitation temperature range of 500 ° C. or more and 950 ° C. or less. The C in the re-dissolved steel precipitates again using the remaining carbide as a nucleus without forming a solid solution in the grain boundary or grain, and the carbide is also precipitated in the grain by forming a new grain boundary. Become.

As is well known, in the Cr-based carbide precipitation treatment, there is a possibility that the base material corrosion resistance may be reduced due to sensitization. Sensitization is a reduction in corrosion resistance caused by the formation of a Cr-deficient layer around the precipitation of Cr-based carbides. 500 ° C or more, 950 ° C with Cr-based carbide precipitation
By maintaining the temperature in the following temperature range for a long period of time and slowly cooling, sensitization can be avoided or the degree of sensitization can be reduced.
Generally, a lower cooling rate is more desirable. However, the heat treatment time for suppressing sensitization has a phenomenon that varies depending on the C content in steel and the history of the material. That is, it is difficult to unconditionally define the heat treatment for suppressing sensitization based on the precipitation state of the carbide, the amount of residual working strain, the holding temperature, and the like before the heat treatment of the carbide precipitation treatment. An example is furnace cooling at 830 ° C. for 6 hours. Immediately after performing the precipitation heat treatment, the treatment may be continuously performed without cooling. Also,
After cooling once, heating may be performed again in a temperature range of 500 ° C. or more and 950 ° C. or less, and cooling may be performed slowly.

Next, the amount of (C weight% precipitated as Cr-based carbide) was determined by, for example, using a test material in an AA solution (10%).
% Acetylacetone-1% tetramethylammonium chloride-residual methanol) in a non-aqueous solvent solution, and the quantitative analysis of Cr in the "extraction residue" obtained by dissolving the metal, Cr is all C
It can be performed by equivalent calculation assuming that it is r ₂₃ C ₆ .

Further, the determination of (total C weight% in steel) can be performed by using an infrared absorption method. That is, the test piece is heated and melted in an oxygen gas stream, and the carbon in the steel is sufficiently heated to be carbon dioxide, sent to an infrared absorption cell together with oxygen, and quantitatively analyzed by the amount of infrared absorption by carbon dioxide. This method is currently the most common method for determining C in steel.

Si: Si is contained in the range of 0.01 to 1.5%. Si is a deoxidizing element as effective as Al in mass-produced steel. If it is less than 0.01%, the deoxidation becomes insufficient, while if it exceeds 1.5%, the moldability decreases, so the Si content was made 0.01 to 1.5%.

Mn: Mn content is 0.01 to 2.5%
Must be within the range. Usually, Mn has an effect of fixing S in steel as Mn-based sulfide, and has an effect of improving hot workability. Also, deoxidizing element or N
It is also an i-balance adjusting element. To obtain these effects, 0.01% or more is required. On the other hand, if it exceeds 2.5%, the amount of surface scale generated during production increases and the surface properties of the steel sheet deteriorate, so the upper limit was made 2.5%.

P: P in steel must be 0.035% or less. In the present invention, like S, it is the most harmful impurity. The lower the better.

S: The amount of S in steel must be 0.01% or less. In the present invention, like P, it is the most harmful impurity. The lower the better, the better. Depending on the coexisting element and S content in steel, Mn-based sulfide, Cr
Almost all precipitate as sulfides, Fe sulfides, or composite nonmetallic inclusions with these composite sulfides and oxides. However, in a separator environment, the nonmetallic inclusions of any composition, although varying in degree, act as a starting point of corrosion and are harmful to the maintenance of the passive film and the suppression of corrosion elution. The amount of S in normal mass-produced steel is
Although it is more than 0.005% and about 0.008%, it is desirable to reduce it to 0.004% or less in order to prevent the above harmful effects. It is more preferably 0.002% or less, most preferably less than 0.001%, the lower the better. The level of less than 0.001% at the industrial mass production level is sufficiently possible with the current refining technology. .

Cr: Cr is a very important basic alloy element for ensuring the corrosion resistance of the base material. The higher the content, the higher the corrosion resistance. If it exceeds 30%, cracks are likely to occur during hot work, making production on a mass production scale difficult. Also, 17
If it is less than%, it is difficult to secure the corrosion resistance required as a separator even if other elements are changed.

Ni: Ni is an important element for making the steel of the present invention metallographically austenitic. By using an austenitic material, hot workability, corrosion resistance, and moldability are ensured. If it is less than 7%, it is difficult to form an austenitic structure, while if it exceeds 50%, it becomes extremely expensive in terms of cost. Therefore, the lower limit is 7%,
The upper limit was set to 50%.

Cu: Cu has a function of passivating the steel surface, and is contained at 3% or less as necessary. When it is contained, the content is preferably 0.01% or more. On the other hand, when it exceeds 3%, hot workability is deteriorated and mass production becomes difficult.

N: N is an austenite-forming element and is contained at 0.3% or less as necessary. If it exceeds 0.3%, the formability of the sheet at normal temperature deteriorates, so the upper limit is 0.3%.
And

Mo: Mo is contained as required. C
An effect of improving the corrosion resistance is obtained with a small amount as compared with r. However, if the content exceeds 7%, it is difficult to avoid precipitation of an intermetallic compound such as a sigma phase, and production becomes difficult due to the problem of embrittlement of steel. Therefore, the upper limit is set to 7%.

As a feature of Mo, even if Mo elutes, the effect on the performance of the catalyst supported on the anode and the cathode is relatively small. The effect on the cation conductivity of the fluorinated ion exchange resin membrane having a hydrogen ion (proton) exchange group is small.

(Cr + 3Mo): As a stainless steel for a separator, it is in a passivated state in an environment of 70 ° C. to 100 ° C. at most, which is the operating temperature of a polymer electrolyte fuel cell, and is continuously in contact. It is necessary that the electric resistance value is low. It is necessary to suppress the increase in the thickness of the passive film and the formation of corrosion products within a practical range. As a necessary condition for this, at least the content of Cr and Mo must be such that the corrosion index (Cr + 3Mo) is in the range of 17 to 50%.

As other impurities, a small amount of B, V, RE
The inclusion of M (rare earth element) has no significant effect in the present invention.

As described above, the austenitic stainless steel of the present invention is suitable for use as a separator in a polymer electrolyte fuel cell, but it is suitable for a component that is required to have a low contact electric resistance in an electric component to be energized. Can be used. For example, it is an electric component such as a connector for connecting an electric wire to an electric wire or an electric machine.

[0066]

EXAMPLES The effects of the present invention will be described below with reference to examples.

An austenitic stainless steel having the 21 chemical compositions shown in Table 1 was subjected to a high-frequency induction heating method of 150 kg.
It melted in a vacuum melting furnace. A commercially available melting raw material was used as the melting raw material, and the amount of impurities in the steel was adjusted.

[0068]

[Table 1]

The ingot having a round cross section was heated at 1280 ° C. for 3 hours in the atmosphere, and then hot forged with a press-type forging machine, and each ingot was turned into a test slab having the following two dimensions. Finished.

A slab having a thickness of 30 mm, a width of 100 mm, and a length of 120 mm A slab having a thickness of 70 mm, a width of 380 mm, and a length of 550 mm is hot-rolled into a hot-rolled steel sheet having a thickness of 6 mm, and immediately after the end of hot rolling in actual production. Was gradually cooled under the heat insulating material wrapping condition simulating the temperature history. Next, the hot-rolled steel sheet is annealed in a heating furnace maintained at a constant temperature of 800 ° C. in the air atmosphere for 16 hours and then cooled in a furnace for 48 hours to give a test material (hereinafter referred to as “material A”). And

In the slab, the slab surface is cut by machining to remove oxide scale on the surface, and the slab has a thickness of 6 mm.
It was finished into a 2 mm slab. This slab is air
After heating to 00 ° C. and finishing by hot rolling to a thickness of 4 mm, as in the case of the actual production, the material was gradually cooled under heat insulating material winding conditions simulating the temperature history immediately after the end of hot rolling. Next, the hot-rolled steel sheet was annealed in a heating furnace maintained at a constant temperature of 800 ° C. in the air atmosphere for 16 hours, and then cooled for 48 hours. Next, after pickling, cold rolling was performed using a cold rolling mill at a rolling reduction of 80% to a thickness of 0.3 mm (+
-0.02) cold rolled steel sheet (hereinafter, referred to as material B).

From the above-mentioned hot-rolled steel sheet material A and cold-rolled steel sheet material B, a test piece for measuring contact electric resistance, a separator for a polymer electrolyte fuel cell, and a grain boundary corrosion test piece were prepared. Each of these test pieces was subjected to a Cr carbide precipitation treatment under the conditions shown in the final heat treatment column in Tables 2 and 3,
Each test was used. The test pieces and test conditions were as shown below. The separator prepared from the material B was subjected to a Cr carbide precipitation treatment before being cold-formed into the shape of the separator.

Contact Electric Resistance Measurement Test Dimension of test piece: thickness: 3 mm or 0.3 mm, width: 4
0 mm, length: 40 mm The contact electric resistance was such that a commercially available glassy carbon plate having a thickness of 0.6 mm was used as an electrode, and the test piece was brought into contact with the carbon plate with a contact area of 1 cm ² . The contact resistance was measured by a four-terminal method. The surface of the test piece for evaluation was subjected to wet-type No. 600 emery polishing immediately before evaluation, and the surface was subjected to evaluation after cleaning. The applied load was 12 kg / cm ² . The contact electric resistance changes with the applied load,
At g / cm ² , an almost constant value is obtained.

Evaluation of Characteristics in a State in which a Separator is Loaded in a Polymer Electrolyte Fuel Cell A performance evaluation by loading a polymer electrolyte fuel cell was performed using a material B having a thickness of 0.3 mm. The details of the corrugated shape are as follows.

A) Separator shape: Material A Thickness 5 mm, length 80 mm, width 80 mm Gas flow path: height 0.8 mm, gap between peaks 1.2 mm
(Machining) Material B Thickness 0.3 mm, height 80 mm, width 80 mm Gas flow path: height 0.8 mm, gap between peaks 1.2 mm
(Corrugated processing) b) Separator surface finish: Si for surface shot processing
Mechanically shot-polished using C abrasives, 5% H
Ultrasonic cleaning was performed for 15 minutes in NO ₃ + 3% HF at 40 ° C. Further, immediately before the test, alkali spray degreasing treatment using 6% sodium hydroxide aqueous solution was performed. Distilled water immersion cleaning was performed three times, and distilled water spray cleaning was further performed for 4 minutes to dry with a cool air dryer, and then subjected to each test.

In the evaluation of the characteristics in a state where the separator was loaded inside the solid polymer type single cell battery, the voltage of the single cell battery was measured one hour after the fuel gas was flown into the battery, and the voltage was compared with the initial voltage. Then, the rate of voltage decrease was checked. The rate of decrease was determined by 1− (voltage V after one hour has elapsed / initial voltage v).

As the solid polymer fuel single cell battery used for the evaluation, a commercially available battery cell FC50 manufactured by Electrochem, USA, was modified and used.

The gas for fuel on the anode side is 99.9.
999% hydrogen gas was used, and air was used as the cathode electrode side gas. The entire body of the battery was kept at 78 ± 2 ° C., and the humidity control inside the battery was adjusted on the entry side based on the measurement of the exhaust gas moisture concentration on the cell exit side. The pressure inside the battery is
One atmosphere. The introduction gas pressure of hydrogen gas and air into the battery was adjusted at 0.04 to 0.20 bar. The cell performance was evaluated at a single cell voltage of 500 (+ −100) mA / cm ² −0.
Measurement was carried out successively from the state where 62 (+ -0.03) V was confirmed.

As a single cell performance measuring system, a modified fuel cell measuring system based on the 890 series manufactured by Scribner, USA was used. Although the characteristics are expected to change depending on the battery operating state, comparative evaluation was performed under the same conditions.

Intergranular Corrosion Test Specimen The intergranular corrosion resistance was examined by a Strauss test. IJS G
A sulfuric acid-copper sulfate corrosion test was performed according to the provisions of 0575.

The test results are shown in Tables 2 and 3.

[0082]

[Table 2]

[0083]

[Table 3]

Each of the heat treatments was carried out to precipitate Cr carbide, and an analytical test piece was cut out from a part of the test piece for measuring the contact electric resistance, and C deposited as a Cr-containing carbide by the following method.
The amount and the total C content in the steel were determined. Tables 2 and 3 show I values obtained from the analysis results.

For the determination of the (C weight% precipitated as Cr-containing carbide) value, the test material was measured using an AA solution (10% acetylacetone-1% tetramethylammonium chloride-
(Residual methanol) was determined based on the result of quantitative analysis of Cr in “extraction residue” obtained by performing a constant current electrolysis in a non-aqueous solvent solution using the same. That is, in an AA non-aqueous solvent solution, constant current electrolysis is performed for about 3 hours at a current density of 20 mA / cm ² to dissolve about 0.4 g equivalent. The AA non-aqueous solvent solution used for washing and the AA non-aqueous solvent solution used for electrolysis are filtered off with a filter diameter of 0.2 μm (trade name: Nuclepore, manufactured by Coster Scientific Corporation), and the residue on the filter is phosphoric acid sulfate. (Special grade phosphoric acid: special grade sulfuric acid: distilled water = 1: 1: 1
In 1), the metal component was analyzed using an inductively coupled plasma emission spectrometer “ICPV-1014” manufactured by Shimadzu Corporation to quantify the Cr concentration. Cr is all Cr ₂₃ C ₆
And the amount of C was quantified by equivalent calculation.

Further, the determination of (total C weight% in steel) was determined by a generally used infrared absorption method.

The precipitation state of carbides differs depending on the hot rolling conditions, the cooling conditions after hot rolling, and the heat treatment conditions for precipitation of Cr carbide, even in steel sheets having the same chemical composition. Corrosion resistance and contact electric resistance depend on the degree of chromium deficiency layer formation. Values and the performance inside the polymer electrolyte fuel cell.

In the examples of the present invention, the contact electric resistance was 0.2
Ω · cm ² or less, but 0.41 to 0.96 in the comparative example.
Very high Ω · cm ² . Steel numbers 13, 14, and 15 having a low C content in steel of less than 0.01% have a particularly large contact resistance value due to a small amount of carbide precipitation.

The higher the I value, the higher the proportion of C in the steel which precipitates as Cr-based carbides. However, if it is less than 85, the contact electric resistance with the carbon plate and the performance in the polymer electrolyte fuel cell are not sufficient. . This tendency is greatly affected by the I value as the C content in the steel is lower, and the performance is inferior when the C content is lower.

The characteristics of the polymer electrolyte fuel cell with the separator loaded are such that the voltage drop rate is less than 0.05 in all of the present invention examples, but the chemical composition specified in the present invention is out of the range. In the comparative example, the voltage drop rate is 0.2 to> 0.
8 was extremely large.

In Table 3, steel Nos. 20 and 21 cracked from the end faces called edge cracks during hot rolling. As a countermeasure against hot ear cracks, it is known that it is effective to keep the S content in steel low.
In both cases, the S content in steel is about 0.001%, which is extremely low industrially. Therefore, the high content of P or Si in the steel is due to hot cracking.

[0092] Steel No. 19 is a duplex stainless steel having both a ferrite and austenitic structure. The duplex stainless steel structure has relatively good corrosion resistance, but has strong directivity in workability at room temperature, and it is difficult to work on a separator that requires thin plate formability. 830 ° C shown in Table 3
In the steel subjected to air-cooling heat treatment after holding for 24 hours, the corrosion resistance was ensured, but embrittlement due to σ phase precipitation was remarkable. Hardness was high, and cutting and cutting were extremely difficult.

[0093]

The austenitic stainless steel of the present invention has a low contact electric resistance and exhibits extremely excellent electric properties especially as a separator for a polymer electrolyte fuel cell. Further, a polymer electrolyte fuel cell using this separator is inexpensive and has excellent electrical characteristics.

[Brief description of the drawings]

FIG. 1 is a diagram showing a structure of a polymer electrolyte fuel cell.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Fuel cell 2 Solid molecular electrolyte membrane 3 Fuel electrode membrane 4 Oxidizer electrode membrane 5a, 5b Separator

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Norifumi Doi 4-5-33 Kitahama, Chuo-ku, Osaka-shi, Osaka Sumitomo Metal Industries, Ltd. F-term (reference) 5H026 AA06 CC05 EE08 HH05

Claims (2)

[Claims] (1) C: 0.015 to 0.2% by weight, S
i: 0.01 to 1.5%, Mn: 0.01 to 2.5%,
P: 0.035% or less, S: 0.01% or less, Cr: 1
7-30%, Ni: 7-50%, Cu: 0-3%, N:
0.3% or less, Mo: 0 to 7%, Al: 0 to 0.2%, and Cr + 3Mo is 17 to 50%, the balance being Fe and unavoidable impurities, which precipitate as Cr-based carbides. An austenitic stainless steel for a current-carrying electric part having a low contact electric resistance, wherein the amount of C and the total amount of C in the steel satisfy the following expression. (C weight% precipitated as Cr-based carbide) x 100 / (total C weight% in steel-
(0.012%) ≧ 85 2. A laminate in which a plurality of unit cells in which a fuel electrode membrane and an oxidant electrode membrane are overlapped with the solid polymer electrolyte membrane in the center, and a separator is interposed between the unit cells,
In a polymer electrolyte fuel cell that generates a DC power by supplying a fuel gas and an oxidant gas, the separator is a separator.
A polymer electrolyte fuel cell comprising the austenitic stainless steel according to any one of the preceding claims.