JP2000328200A - Austenitic stainless steel for conductive electric parts and fuel battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide austenitic stainless steel small in contact electric resistance and small in eluted metallic ions even in the case of being used in a pure state, particularly, in a separator environment of a solid high molecalar type fuel battery without executing expensive surface treatment and to provide a solid high molecular type fuel battery provided with a separator composed of the stainless steel. SOLUTION: This austenitic stainless steel for electric parts contains, by weight, <=0.03% C, 0.01 to 1.5% Si, 0.01 to 2.5% Mn, <=0.035% P, <=0.01% S, 17 to 30% Cr, 7 to 50% Ni, 0.0005 to 3.5% B, 0.001 to 0.2% Al, <=0.3% N, 0 to 7% Mo and 0 to 3% Cu, in which the contents of Cr, Mo and B also satisfy 17<=Cr+3Mo--2.5B, and the balance Fe, and B in the steel is precipitated as M2B type boride. The fuel battery provided with a separator is composed of the 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 a low contact electric resistance used as a current-carrying electrical part, and a 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 used for energization that requires a small 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 a low temperature around ℃ and the energy efficiency can be expected to be about 40%, it is expected to be used for emergency dispersed power sources such as small business establishments and telephone offices, small household distributed power sources using city gas as fuel, and hydrogen. It is expected to be put to practical use worldwide as a power source for low-pollution electric vehicles using gas, 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 there is a situation far from practical application.

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", has excellent electric conductivity in a battery environment, has low contact electric resistance and is excellent in corrosion resistance. It is no exaggeration to say that the practical application of stainless steel separators has succeeded in commercializing polymer electrolyte fuel cells and expanding their applications.

[0022]

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

[0023]

The gist of the present invention is as follows.

(1) By weight%, C: 0.03% or less, 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%, B: 0.0005-
3.5%, Al: 0.001 to 0.2%, N: 0.3%
Hereinafter, Mo: 0 to 7%, Cu: 0 to 3%, and the contents of Cr, Mo, and B satisfy the following formula. The balance consists of Fe and unavoidable impurities. ₂ B
Austenitic stainless steel for energized electrical components with low contact electrical resistance, precipitated as a type boride.

17 ≦ Cr + 3Mo−2.5B However, each element symbol in the formula indicates the content (% by weight). (2) The fuel electrode film and the oxidant electrode film are overlapped with the solid polymer electrolyte membrane at the center. In a polymer electrolyte fuel cell in which a fuel cell and an oxidizing gas are supplied to a stacked body in which a plurality of unit cells are stacked with a separator interposed between the unit cells to generate DC power, the separator (1) 20) A polymer electrolyte fuel cell comprising the austenitic stainless steel according to the above.

Here, “M” of M ₂ B represents a metal element. It is a metal element having a strong chemical affinity with B contained in steel instead of a specific metal element. M mainly contains Cr and Fe and a small amount of Ni and Mo because of the relationship with coexisting elements.

As specific examples, (Cr, Fe) ₂ B,
(Cr, Fe, Ni) ₂ B, (Cr, Fe, Mo) ₂ B, (C
r, Fe, Ni, Mo) ₂ B, Cr _1.2 Fe _0.76 Ni _0.04 B
There is something like that. In any case, “between Cr, Fe, Mo, Ni, and X (where X is a metal element in steel other than Cr, Fe, Mo, and Ni), which is a metal element in boride, and B content, "(Cr wt% / Cr atomic weight + Fe wt% / Fe atomic weight + Mo wt% / Mo atomic weight + Ni wt% /
Ni atomic weight + X weight% / X atomic weight) / (B weight% / B atomic weight) 2 is the reason for the notation of M ₂ B. This notation is not a special one but a very general notation. It is.

The separator has the above-mentioned four functions. 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 present inventors have found that in an environment of an austenitic stainless steel for a current-carrying electric component having a small contact electric resistance, particularly in a polymer electrolyte fuel cell separator environment, the amount of metal ions eluted from the steel surface is as small as possible, and for a long time. Various tests were conducted with the aim of developing a stainless steel that does not increase the contact electric resistance with graphite for electrodes even when used as a separator. 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, the corrosion product (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 inherent to stainless steel, and it is not easy to stably maintain the electric resistance at a value low enough to exhibit 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 secure corrosion resistance, the contact electric resistance increases as the film thickness increases,
Battery efficiency is significantly reduced.

F) M ₂ B-type boride precipitated on the surface of stainless steel acts as a contact point having excellent electric conductivity, and the contact electric resistance is determined by the number of contact points per contact unit area, the area, and the surface. It depends on the electric resistance of the passive film formed.

G) M containing mainly Fe and Cr in steel
_The stainless steel on which the 2B-type boride is precipitated can maintain a low contact electric resistance over time regardless of the passive film on the steel surface.

H) However, when the M ₂ B-type boride is positively precipitated, elements such as Cr, Mo, Fe and Ni, which are elements for improving corrosion resistance, are consumed. Is remarkable. Therefore, in order to increase the concentration of Cr and Mo in the steel, to strengthen the passive film, and to suppress metal elution in the separator environment, (Cr + 3
Mo-2.5B) needs to be 17% or more i) Corrosion resistance is ensured by positively adding Mo, but even if Mo is eluted, it is supported on the anode and cathode parts. The effect on catalyst performance is relatively minor.

[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 easily forms chromium carbides in steel and causes a decrease in corrosion resistance due to sensitization. Therefore,
The upper limit is set to 0.03% in order to minimize the precipitation of Cr carbide. In the steel of the present invention, the boride is positively precipitated, so that the main component is Cr and Fe, Ni,
(Cr, Fe) ₂ B, (Cr, F)
e, Ni) is precipitated as M ₂ B-type boride such as ₂ B,
Although the contact electric resistance of the stainless steel surface covered with the passivation film can be reduced, if C in the steel exceeds 0.03%, sensitization accompanying carbide precipitation in a cooling process from a high temperature is allowed. Is likely to become apparent, and a special heat treatment is required as a countermeasure for sensitization. Si: 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 deteriorates.
The i content was 0.01 to 1.5%.

Mn: Mn usually has the effect of fixing S in steel as Mn-based sulfide, and has the effect of improving hot workability. It is also a deoxidizing element or a Ni 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 is the most harmful impurity along with S and must be 0.035% or less. The lower the better.

S: S is the most harmful impurity like P and the upper limit is made 0.01%. Mn-based sulfides, Cr-based sulfides, Fe-based sulfides, or mostly non-metallic inclusions with these complex sulfides and oxides, depending on the coexisting elements in steel and the amount of S in the steel are doing. However, in a separator environment, non-metallic inclusions of any composition, although varying in degree, act as a starting point of corrosion and are harmful to maintaining passivation and suppressing corrosion elution.
S content in normal mass-produced steel exceeds 0.005% and 0.00
Although it is about 8%, it is desirable to reduce it to 0.004% or less in order to prevent the above harmful effects. Than,
The desirable S content in steel is 0.002% or less, and the most desirable S content level in steel is less than 0.001%, the lower the better. At less than 0.001% at the industrial mass production level, there is no problem 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 working, making production on a mass production scale difficult. On the other hand, if it is less than 17%, it is difficult to secure the corrosion resistance required for the separator even if other elements are changed. Due to the formation of the M ₂ B type boride, the amount of Cr in the steel contributing to the improvement of the corrosion resistance is reduced, and the corrosion resistance of the base metal is deteriorated. Therefore, in order to increase the concentration of Cr and Mo in the steel, to strengthen the passive film, and to suppress metal elution in the separator environment, (Cr + 3Mo−2.5
B) needs to be 17 or more.

Ni: Ni is an important element for making austenite based on metallography. Manufacturability, corrosion resistance, and moldability are ensured by using austenite. The lower limit is 7% and the upper limit is 50%. If it is less than 7%, it is difficult to form an austenitic structure, while
If it exceeds 0%, the cost becomes extremely high. Very little Ni is contained in the M ₂ B type boride.

B: B is the most important element in the present invention. (Cr, Fe) ₂ B, (Cr, Fe, Ni) ₂ mainly containing Cr and Fe and containing trace amounts of Ni and Mo
Precipitation as an M ₂ B-type boride such as B has the effect of lowering the contact electric resistance of the surface of the stainless steel covered with the passive film.

M ₂ B-type boride precipitation in stainless steel is a cause of deterioration of corrosion resistance and is generally treated as a troublesome person. In the present invention, the M ₂ B-type boride, which has been treated as a troublesome substance, is positively precipitated, and is used as an “electric path (bypass circuit)” for reducing the contact electric resistance which increases due to the formation of a passive film. It is.

The fact that borides have the effect of lowering the contact electric resistance means that these borides have metallic properties,
This can be explained by having better electrical conductivity than the passive film.

In general, an extremely thin passive film of about several tens of degrees is formed on the surface of stainless steel and shows excellent corrosion resistance. However, the passive film is more or less inferior in electric conductivity as compared with the base material and has a low contact electric resistance. Enhance. 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 thin in a corrosive environment. Particularly when used inside a polymer electrolyte fuel cell, it is not easy to keep the passivation film thin.

The fact that the boride having excellent electric conductivity is directly exposed on the surface without being covered with the passivation film is effective for stabilizing the good electric conductivity of the stainless steel surface for a long time. . That is, since the boride is stable in 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. Good electrical conductivity is ensured through the boride. As a result, it is possible to suppress an increase in the contact electric resistance of the steel surface. In other words, the fine boride exposed without being covered by the passivation film functions as an “electric path (bypass)”, so that the contact electric resistance can be kept low.

In general, when B is contained in a large amount, the austenitic stainless steel increases in strength and hardness, decreases ductility, and decreases in productivity. In order to ensure moldability as a separator for a polymer electrolyte fuel cell, it is necessary to precipitate B in steel as boride to reduce the amount of solid solution B. Precipitating B as a boride improves the formability of the steel. That is, it is necessary to precipitate B in steel as boride also from the viewpoint of ensuring formability.

In the present invention, B is contained in an amount of 0.0005% or more and 3.5% or less in order to positively precipitate boride. When the content exceeds 3.5%, production by a usual dissolution method becomes difficult. Further, the moldability at room temperature for a polymer electrolyte fuel cell separator cannot be ensured. Preferably 0.005 to 1.8%
More preferably, it is 0.05 to 1.20%. Most of B in steel is precipitated as boride by a usual production method. Even at 1125 ° C., it only forms a solid solution of about 0.01%. On the low temperature side, the amount of solid solution becomes even lower. Also in the steel of the present invention, a trace amount of solute B of about 0.01% or less exists depending on the temperature and the components in the steel.

Although the precipitation temperature depends on the content, it is near the solidification temperature, and once precipitated, it hardly forms a solid solution again. Since the boride itself has extremely poor deformability, the problem of cracking during production and processing increases as the B content increases and boride precipitation becomes more pronounced. As the content of B increases, the liquidus decreases, and there is a problem that the temperature range in which hot forging is possible is narrowed.
However, when the B content is more than 1.5% and up to 3.5%, which is the upper limit specified in the present invention, production on an industrial scale is possible with considerable difficulty. When the B content is about several tens ppm, the boride tends to precipitate at the crystal grain boundaries. In reducing the contact electrical resistance, it is judged that there is no significant effect on whether boride precipitates at the grain boundaries or within the grains. It is more preferable to do so. From this,
The B content is preferably 0.05% or more because the amount of boride precipitated in the grains becomes relatively large. Due to boride precipitation, the corrosion resistance of the base material decreases. The reason is that the precipitation of boride consumes a large amount of Cr around it. C previously consumed by boride formation
It is extremely effective to add Cr and Mo corresponding to the amounts of r and Mo at the stage of molten steel in order to reduce the reduction in corrosion resistance. The effect of the cooling rate is relatively small. Cr and M
It is necessary to adjust the o content so that Cr + 3Mo-2.5B% becomes 17% or more to ensure corrosion resistance inside the polymer electrolyte fuel cell main body. The coefficients for each element in the above formula are experimental rules.

For the determination of the amount of B in steel, a sample was prepared using an AA solution (10%).
% Acetylacetone-1% tetramethylammonium chloride-residual methanol) in the residue extracted by performing galvanostatic electrolysis in a non-aqueous solvent solution.
It is possible by analyzing the amount.

Further, the quantitative determination of the metal element bonded to B as a boride can be made by utilizing inductively coupled plasma emission spectroscopy. Qualitative analysis is possible by X-ray diffraction ^. Al: Al is added at the molten steel stage as a deoxidizing element,
The content is in the range of 001 to 0.2%. B added in the steel of the present invention is an element having a strong bonding force with oxygen in molten steel,
It is necessary to lower the oxygen concentration by Al deoxidation.

N: N is sometimes used as an austenite-forming element for adjusting the metallographic structure.
% Or less.

Mo: Mo has an effect of improving corrosion resistance in a small amount compared to Cr, and is contained at 7% or less as necessary. 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 was set to 7%. When the austenitic stainless steel of the present invention is used as a separator in a polymer electrolyte fuel cell, even if Mo in the steel elutes, the effect on the performance of the catalyst supported on the anode and the cathode is relatively small. It is. The effect on the cation conductivity of the fluorinated ion exchange resin membrane having hydrogen ion (proton) exchange groups is also small.

The Mo content is (Cr + 3Mo−2.5B) ≧ in relation to the Cr content in consideration of becoming a boride.
It is necessary to satisfy the relational expression of 17%.

Cu: Cu is contained in an amount of 3% or less as necessary. When an appropriate amount of Cu is contained, passivation is promoted, and there is an effect of preventing metal elution in a separator environment. When it is contained, the content is preferably 0.01% or more. On the other hand, when it exceeds 3%, workability during hot work is reduced, and mass production becomes difficult.

As other impurities, a small amount of V, REM
Including (rare earth element) or the like has no significant effect in the present invention.

The austenitic stainless steel of the present invention can be produced by ordinary melting, lumping, hot working and cold working. For annealing and pickling, a continuous annealing and pickling line used for ordinary mass production of stainless steel can be used as it is.

However, the B-containing stainless steel has poor hot workability, and both ends of the steel plate are cooled during hot working, so that ear cracks are likely to occur. The occurrence of the ear cracks can be prevented by reheating during the hot working or by providing the overlay welding layers on both side end surfaces of the slab and performing the hot working. By subjecting the boride having poor deformability to "crushing" and finely dispersing by hot rolling under high pressure, it is possible to improve the toughness which is deteriorated due to boride precipitation. Further, the coagulation and coarsening of the boride by heat treatment is also effective for further improving the processability. By holding for a long time in the heat treatment, the boride aggregates and becomes coarse.

As described above, the austenitic stainless steel of the present invention is suitable for use as a separator in a polymer electrolyte fuel cell, and has been described mainly with respect to the separator. Can be used for parts as required. For example, it is an electric component such as a connector for connecting an electric wire to an electric wire or an electric machine.

Hereinafter, the effects of the present invention will be described in detail with reference to specific examples.

[0063]

EXAMPLES Austenitic stainless steels of 21 chemical compositions shown in Table 1 were melted in a high-frequency induction heating type 150 kg 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. Commercially available Fe-B alloy iron was used for B addition.

[0064]

[Table 1]

The ingot thus formed into a round cross section was heated at 1220 ° C. for 3 hours in the atmosphere, and then hot forged with a press-type forging machine to obtain a thickness of 70 mm, a width of 380 mm and a length of 5 mm.
Finished to a 50 mm slab. The forged slab was processed into a slab having a thickness of 60 mm by cutting the slab surface to remove oxide scale on the surface and cracks on the end surface. This slab is heated to 1220 ° C. in the air and hot-rolled to a thickness of 4 m.
m, and then gradually cooled under heat insulating material winding conditions simulating the temperature history immediately after the end of hot rolling in mass production.

Although the M ₂ B type boride is a metal compound, its deformability is extremely poor at both the normal temperature range and the high temperature range, and it is likely to cause cracking of steel during hot working.
Forging and rolling were performed while repeating reheating in a temperature range of 0 ° C. Since the temperature at both ends of the coil is apt to decrease and ear cracks easily occur, hot rolling was performed while heating the coil end surface.

The hot-rolled material was annealed in the air atmosphere at 1080 ° C., air-cooled, pickled, and cold-rolled using a cold rolling mill. If necessary, the intermediate plate thickness is 1080
After softening and pickling at ℃, rolling was performed to a coil having a thickness of 3 mm, which is the target of the final thickness.

The cold-rolled coil was annealed at 1080 ° C. in the air, pickled, and tested. The surface of the coil after pickling also appeared to be no different from ordinary austenitic stainless steel in appearance.

The B content was determined as follows.

A sample taken from each steel slab was used as an AA solution (10
% Acetylacetone-1% tetramethylammonium chloride-remaining methanol) in a non-aqueous solvent, 20 m
A constant current electrolysis of about 3 hours was performed at a current density of A / cm ² to dissolve about 0.4 g equivalent, and the electrolysis test piece was ultrasonically cleaned immediately after electrolysis and used for electrolysis with AA non-aqueous solvent. The AA non-aqueous solvent solution was filtered using a Coster
Filtered off with Scientific Corporation Co., Ltd. "trade name Nuclepore", and analyzed the compound of M ₂ B on the filter.

The amount of boride in steel is small and the amount of residue is 40 μg.
If the amount is less than the following, it is separated by distillation and then curcumin absorptiometry. If it is 40 μg or more, it is dissolved in phosphoric acid (special grade phosphoric acid: special grade sulfuric acid: distilled water = 1: 1: 1). Was analyzed with an inductively coupled plasma emission spectrometer “ICPV-1014” manufactured by Shimadzu Corporation to determine the amount of B precipitated as boride. This is a common quantification method.

FIG. 2 shows the microstructure of the cold pressing plate (200 times).
Is shown. The dispersed phase is an M ₂ B type boride.

The contact electric resistance with the carbon plate was measured by a four-terminal method using a commercially available glassy carbon plate having a thickness of 0.6 mm and a contact area of a stainless steel test piece for evaluation of 1 cm ² . Immediately before the evaluation, the surface of the test piece for evaluation was subjected to wet-type No. 600 emery polishing, and the surface was subjected to evaluation after cleaning. The applied load was 12 kg / cm ² . Although the contact electric resistance changes depending on the applied load, an almost constant value is obtained at 12 kg / cm ² .

The details of the corrugated separator used in the performance evaluation in the fuel cell are as follows. However, for the test steel 5, it was difficult to form a thin plate at room temperature. Therefore, the gas flow path was cut by machining from a 4 mm thick hot-rolled coil and subjected to a test. Appearance is roughly Fig. 1.
As shown in FIG. Grooves were formed on both sides with a groove width of 2 mm and a groove depth of 1 mm, and were formed on the anode electrode side and the cathode electrode side.

(1) Shape of separator formed from a 0.3 mm thick steel plate: thickness 0.3 mm, length 80 mm, width 8
0mm gas flow path: height 0.8mm, gap between peaks 1.2mm
(Corrugating) (2) Separator surface finish: S for surface shot processing
Mechanically shot-polished using iC abrasives, 5%
Perform ultrasonic cleaning for 15 minutes in HNO ₃ + 3% HF at 40 ° C., perform alkali spray degreasing using a 6% aqueous sodium hydroxide solution immediately before the test, and perform simple water washing with running water. Distilled water immersion washing was performed three times, and distilled water spray washing 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 inside the battery was controlled 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 in the range of 0.04 to 0.20 bar. The cell performance evaluation was 500 ± 20 mA / cm ² −0.62 at a single cell voltage.
The measurement was performed successively from the state where ± 0.04 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. It is expected that the characteristics will change depending on the battery operating state, but this is a comparative evaluation under the same conditions.

Table 2 shows the evaluation results.

[0081]

[Table 2]

As is clear from Table 2, the voltage drop rate is less than 0.05 in all of the examples of the present invention, while the voltage drop rate is less than 0.05 in the comparative examples deviating from the chemical composition specified in the present invention. It was extremely large, 3 or more. In the case of the present invention, the contact electric resistance is as low as 0.12 Ω · cm ² or less, whereas in the comparative example, it is as high as 0.41 to 0.96 Ω · cm ² .

[0083]

The austenitic stainless steel of the present invention has excellent corrosion resistance, low contact electric resistance,
In particular, it is suitable for use as a separator in a polymer electrolyte fuel cell, and greatly contributes to the production of an inexpensive polymer electrolyte fuel cell.

[Brief description of the drawings]

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

FIG. 2 is a view showing a microphotograph showing a precipitation state of an M ₂ B-type boride.

[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 CC03 EE08 HH05

Claims (2)

[Claims] (1) In weight%, C: 0.03% or less, Si:
0.01 to 1.5%, Mn: 0.01 to 2.5%, P:
0.035% or less, S: 0.01% or less, Cr: 17 to
30%, Ni: 7 to 50%, B: 0.0005 to 3.5
%, Al: 0.001 to 0.2%, N: 0.3% or less,
Mo: 0 to 7%, Cu: 0 to 3%, and Cr,
The Mo and B contents satisfy the following equation, and the balance Fe
Austenitic stainless steel for current-carrying electric parts having low contact electric resistance, characterized in that B in the steel is precipitated as M ₂ B-type boride and is composed of unavoidable impurities. 17 ≦ Cr + 3Mo−2.5B However, each element symbol in the formula indicates the content (% by weight). 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.