JP2001214286A - Method for producing stainless steel for conductive part - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing stainless steel for conductive pars excellent in corrosion resistance and small in the increase of contact electric resistance even in the case of being used for a long time. SOLUTION: In this method for producing stainless steel, stainless steel is corroded in an acidic aqueous solution to expose one or more kinds among M23C6 type, M4C type, M2C type and MC type carbide metallic inclusions and M2B type boride metallic inclusions having electric conductivity to the surface, next, neutralizing treatment is performed in an alkaline aqueous solution of pH 7 or more, and, after that, water washing and drying are furthermore performed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a stainless steel material for a current-carrying part having a low contact electric resistance.
In particular, the stainless steel material produced by the method of the present invention is required to have good corrosion resistance and small contact electric resistance, and is suitable for a polymer electrolyte fuel cell separator which is required to have little performance deterioration over a long period of time. is there.

[0002]

2. Description of the Related Art Stainless steel has excellent corrosion resistance because a passive film is formed on its surface. However, since the passivation film on the surface has a large electric resistance, it is not suitable for an electric component used for conducting electricity which requires a small contact electric resistance. The thicker the passivation film, the better the corrosion resistance, but the higher the electrical resistance tends to be.

[0003] If the contact electric resistance of stainless steel can be reduced, it becomes possible to use stainless steel as a current-carrying electric component requiring corrosion resistance. One of the current-carrying electrical components requiring excellent corrosion resistance and low contact electrical resistance is a polymer electrolyte fuel cell separator (sometimes called a bipolar plate), a terminal for an electrical component, and the like.

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 cannot be considered at all that the material used for the phosphoric acid type fuel cell and the molten carbonate type fuel cell which are commercially available is directly applied to the constituent material of the polymer electrolyte fuel cell.

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-based proton conductive 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 the polymer electrolyte fuel cell separator are (1) a function as a "flow path" for uniformly supplying fuel gas in the plane on the fuel electrode side, and (2) a function as a "flow path" on the cathode side.
(3) The water generated on the cathode side is efficiently discharged out of the system together with the carrier gas such as air and oxygen after reaction from the fuel cell. (4) function as an electrical "connector" between single cells maintaining low electrical resistance and good electrical conductivity as an electrode for a long time; and (5) one of adjacent cells And the like as a "partition wall" between the anode chamber of the cell and the cathode chamber of the adjacent cell.

So far, the use of a carbon plate as a separator material in a polymer electrolyte fuel cell has been intensively studied. However, the carbon plate has a problem that it is easily cracked. There is a problem that the machining cost and the machining cost for forming the gas flow path are extremely 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 that is easily formed by the atmosphere. 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. It is said that the fuel cell is reduced in electrochemical reaction. 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 metal elution occurs, and 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.

JP-A-11-112018 discloses a low-temperature fuel cell separator material having improved conductivity by dispersing and pressing carbon powder on a stainless steel substrate having good acid resistance. It is described that a large amount of Joule heat generated in a contact portion is reduced by lowering surface resistance and contact resistance. As a method of forming carbon particles dispersed in stripes on the surface of a base material, it is disclosed that carbon particles such as carbon black and graphite powder are attached to the surface of a steel sheet and then pressed by a rolling method. Furthermore, by heat-treating the stainless steel substrate after pressing the carbon powder, the carbon particles can be more firmly joined to the surface of the substrate through the diffusion layer formed between the substrate and the stainless steel substrate. It is shown.

In Japanese Patent Application Laid-Open No. 11-144744, a stainless steel base material is subjected to a heat treatment of a carbon-based particle-dispersed coating film to decompose and eliminate the coating film components, so that the surface of the base material is made of carbon. A separator for a low-temperature fuel cell in which a bonding layer of system particles is formed and attached is disclosed.

[0021]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for producing a stainless steel material having good corrosion resistance, low contact electric resistance, and an extremely small increase in contact electric resistance even when used for a long time. It is in.

[0022]

The gist of the present invention is as follows.

(1) Immersion of stainless steel in acidic aqueous solution
And the surface has a conductive M _{twenty three}C₆Type, M_FourC type,
M_TwoC-type and MC-type carbide-based metal inclusions and M_TwoType B boride
One or more of the metal-based inclusions
Neutralize with an alkaline aqueous solution having H of 7 or more.
No stainless steel for current-carrying parts
Method of producing steelless material.

(2) The average corrosion weight loss due to the acidic aqueous solution is 5
The method for producing a stainless steel material for a current-carrying part according to the above (1), wherein the production amount is set to 60 g / m ² .

(3) When the stainless steel material is expressed in mass%, C:
0.15% or less, Si: 0.01 to 1.5%, Mn:
0.01 to 1.5%, P: 0.04% or less, S: 0.0
1% or less, Cr: 15 to 36%, Al: 0.001 to 6
%, B: 0 to 3.5%, N: 0.035% or less, Ni:
0-5%, Mo: 0-7%, Cu: 0-1%, Ti: 0
-25 × (C% + N%), Nb: 0-25 × (C% + N
%), And the contents of Cr, Mo and B satisfy the following formula, and the energization according to any one of the above (1) or (2), which is a ferritic stainless steel material comprising the balance of Fe and impurities. Manufacturing method of stainless steel for parts. 17% ≦ Cr + 3 × Mo−2.5 × B However, each element symbol in the formula indicates the content (% by mass).

(4) When the stainless steel material is expressed in mass%, C:
0.005-0.2%, Si: 0.01-1.5%, M
n: 0.01 to 2.5%, P: 0.040% or less, S:
0.01% or less, Cr: 17 to 30%, Ni: 7 to 50
%, B: 0 to 3.5%, N: 0 to 0.4%, Cu: 0 to 0%
It is an austenitic stainless steel material containing 2%, Al: 0 to 6%, Mo: 0 to 7%, and the contents of Cr, Mo and B satisfy the following formula, and the balance is Fe and impurities ( The method for producing a stainless steel material for a current-carrying part according to 1) or 2). 17% ≦ Cr + 3 × Mo−2.5 × B However, each element symbol in the formula indicates the content (% by mass).

Here, M ₂₃ C ₆ type, M ₄ C type, M ₂ C type,
MC type carbide-based metal inclusions or M ₂ B-type boride-based metal inclusions and "M" shows a metal element, instead of a specific metal element, exhibit strong metal element having chemical affinity with C or B . Generally, M is based on the relationship with coexisting elements in steel.
It is mainly composed of r and Fe, and often contains trace amounts of Ni and Mo. In the case of carbides, B also has an effect as "M". As the M ₂₃ C ₆ type, Cr ₂₃ C ₆ , (Cr, Fe) ₂₃
C _6, as the M ₂ C type as Mo ₂ C, WC as MC type carbide-based metal inclusions or M ₂ B-type boride-based metal _{inclusions,, Cr 2 B, (Cr} , Fe) 2 B, (Cr , Fe, N
i) ₂ B, (Cr, Fe, Mo) ₂ B, (Cr, Fe, Ni,
Mo) ₂ B and Cr _1.2 Fe _0.76 Ni _0.04 B. The M ₄ C type includes B ₄ C and the like. The above M ₂₃ C ₆
Type, M ₄ C type, M ₂ C type, MC type carbide-based metal inclusion or M ₂ B type boride-based metal inclusion,
M ₂₃ a portion is replaced with a B (C, B) ₆ type, M ₄ (C, B)
-Type, M ₂ (C, B) type, M (C, B) type carbide-based metal inclusions or M ₂ (C, B) metal inclusions such boride-based metal inclusions also is often deposited, The above notation shall include these. Basically, it is presumed that similar performance can be expected with a metal-based dispersion having good electric conductivity.

The subscript index “ ₂ ” of the “M ₂ B” type notation is
"Cr, Fe, Mo, Ni, which are metal elements in boride,
Between X (where X is a metal element in steel other than Cr, Fe, Mo, and Ni) and the amount of B, "(Cr mass% / Cr
Atomic weight + Fe mass% / Fe atomic weight + Mo mass% / Mo atomic weight + Ni mass% / Ni atomic weight + X mass% / X atomic weight) /
(B mass% / B atomic weight) means that a stoichiometric relationship of about 2 ″ is established. This notation is not a special one but an extremely general notation.

The separator generally has the five functions described above. That is, a) a function as a "flow path" for uniformly supplying the fuel gas in the plane on the fuel electrode side, b) a function as a "flow path" for uniformly supplying the oxidizing gas in the plane on the cathode side, c) A function as a "flow path" for efficiently discharging the water generated on the cathode side from the fuel cell together with a carrier gas such as air and oxygen after the reaction from the fuel cell, d) Low contact electric resistance as an electrode for a long time. And e) a function as an electrical "connector" between the single cells maintaining good electrical conductivity, and e) a function as a "partition wall" between an anode chamber of one cell and a cathode chamber of an adjacent cell in an adjacent cell. Things. 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 d).

In the present invention, the steel material is a material produced by hot working or cold working a steel ingot, and refers to a steel plate, a wire rod, a small-diameter steel bar, a steel pipe, and the like that can be used as a current-carrying part.

The present inventors have found that even if a stainless steel material having low contact electric resistance and excellent corrosion resistance is used, particularly as a separator of a polymer electrolyte fuel cell for a long time, the contact electric resistance with graphite for electrodes does not increase. Various tests were conducted to develop stainless steel. As a result, the following findings were obtained.

A) The electric resistance of the passive film formed on the surface of stainless steel is specific to stainless steel, and is sufficient for using the separator as a separator in a polymer electrolyte fuel cell to exhibit the electric resistance. It is not easy to maintain a low value stably.

B) The contact electric resistance depends on the contact area per unit area. That is, it depends on the number of contact points per unit area, the total area of contact points, and the electrical resistance of each contact point.

C) Therefore, if conductive stainless steel or carbide or boride metal inclusions are dispersed and exposed on the surface of the stainless steel so as to penetrate the passive film, and the surface roughness is set to a predetermined range, the contact electric resistance is reduced. Can be greatly reduced,
The contact electric resistance can be kept low over time. The carbide or boride-based metal inclusion functions as an “electric path”.

D) In a separator environment, stainless steel exhibits relatively good corrosion resistance, but metal elution and corrosion occur, and corrosion products (mostly hydroxide mainly composed of Fe) are generated. Causes an increase in contact electric resistance, and
Significantly adversely affects supported catalyst performance. As a result, battery performance represented by electromotive force deteriorates in a short time, and the proton conductivity of the fluorine-based ion exchange resin membrane having a hydrogen ion (proton) exchange group deteriorates.

E) On the other hand, in order to ensure the corrosion resistance of stainless steel in the environment inside the battery main body, a passive film is indispensable. The resistance increases and the battery efficiency drops significantly. The generation of Joule heat increases.

F) In order to strengthen the passive film and suppress the elution of metal in the separator environment, the contents of Cr and Mo are (Cr% + 3 × Mo%), and when B is contained, (Cr% + 3 × Mo% −2.5B%) must be 17% or more.

G) Corrosion resistance is ensured by positively adding Mo. Even if Mo is eluted by corrosion,
The effect on the performance of the catalyst supported on the anode and cathode portions is relatively minor. This is presumably because the eluted Mo exists as molybdate ions, which are anions, and thus has little effect on inhibiting the proton conductivity of the fluorine-based ion exchange resin membrane having hydrogen ion (proton) exchange groups. . W also exists as a tungstate ion, and the same behavior is observed. V
Similar behavior can be expected for.

H) To expose the conductive metal inclusions on the surface of the stainless steel, it is preferable to perform a pickling treatment in an acidic aqueous solution.

I) After pickling, washing with water and drying to produce a current-carrying part and using it in the air or in a separator environment,
The contact electric resistance tends to increase with time.

J) Pickling-The main reason that the contact electric resistance increases as time passes after water washing is surface oxidation by oxygen in the atmosphere, which remains in minute gaps formed on the pickling surface. This is because the acid solution is corroded by drying, concentration, and ejection. Immediately after pickling, the surface of the stainless steel has a very thin passive film and is in a state where many hydrated water molecules and hydroxynium ions are bound together. The water molecules come off until they reach a state where the water molecules are dried, and oxygen is bonded to the places where the water molecules come off, so that corrosion products are increased in volume and the contact electric resistance is increased. Such a phenomenon sufficiently progresses in a unit of several hours, so that the contact electric resistance becomes high before using as a current-carrying component.

K) However, by performing a neutralization treatment with an aqueous alkali solution after pickling, it is possible to prevent a deterioration in performance as a current-carrying part even when used for a long time in a corrosive environment, especially in a separator environment, and particularly an increase in contact electric resistance. can do.

L) The amount of exposure and protrusion height of the conductive metal inclusion which determines the electrical contact resistance value is the average corrosion amount (abrasion amount).
Therefore, there is an appropriate range of average corrosion weight loss that can be industrially controlled and has the lowest contact resistance. This is related to the number of contact points and the roughness that govern the contact electric resistance.

[0044]

Embodiments of the present invention will be described below in detail. The percentages shown below indicate mass%.

[0045] Metal inclusions: on the surface of stainless steel, M ₂₃ C ₆ type having conductivity, M ₄ C type, M ₂ C type, MC type carbide-based metal inclusions and M ₂ B-type boride-based metal inclusions One of things
Disperse and expose more than seeds. The term “exposed” means that at least a part of the metal inclusions is exposed to the outer surface without being covered with the passive film formed on the surface of the stainless steel, and is projected. The reason for dispersing, exposing, and projecting the metal inclusion is to reduce the contact electric resistance by allowing the metal inclusion to function as a path for electricity (a bypass).

In general, M ₂₃ C ₆ type carbide-based metal inclusions in stainless steel cause a reduction in corrosion resistance.
Often viewed as harmful. In the present invention, M ₂₃ C ₆ type carbide-based metal inclusions, which have been regarded as harmful, are also positively precipitated, and are used as an “electric path” for reducing the contact electric resistance which increases due to the formation of a passive film. .

There are the following methods for dispersing and exposing metal inclusions.

(1) Precipitating carbon in steel as chromium carbide etc. (2) Precipitating boron in steel as chromium boride (3) Conductivity such as B ₄ C, WC, Mo ₂ C During metal processing such as projection (hereinafter, referred to as shot) of metal fine powder having polishing, grinding and grinding, etc., "submerged in the steel surface and left behind" (4) called surface modification such as vapor deposition and ion implantation Combination of surface treatment method and heat treatment for precipitation treatment Among these, (4) is expensive as an industrial method, but has the characteristic that the characteristics in the thickness direction hardly change except for the outermost layer. ,It is valid.

Here, M ₂₃ C ₆ type, M ₂ C type, MC type carbide-based metal inclusions and M ₂ B type boride-based metal inclusions
The reason that it is desirable to mainly contain at least one metal element of chromium, molybdenum, and tungsten is that all of these carbides and borides are thermodynamically stable as metal inclusions, This is because it has excellent corrosion resistance as well as excellent base material. In addition, each of them has high hardness, which is desirable for the purpose of the present invention in which a function such as “cut into the steel surface and retained” by using mechanical processing such as shot, polishing, and grinding is performed. Furthermore, even if these metal inclusions are eluted, all of the eluted metal ions are corrosion resistance improving elements that promote passivation of stainless steel, and molybdenum and tungsten are molybdate ions while being metal ions. This is because they form an anion such as tungstate ion and have a small effect on the proton conductivity as an electrolyte membrane.

In order to expose the metal inclusions precipitated in the steel to the outer surface of the passive film formed on the stainless steel surface,
(1) Pickling stainless steel, including combinations,
Alternatively, it is possible by (2) rolling with a rolling roll (commonly referred to as a “dal roll”) having minute irregularities obtained by “shot processing” or “etching” the roll surface, or (3) directly performing shot processing on a stainless steel plate. . In particular, the pickling process is extremely effective in macroscopically dissolving the surface of a large area and slightly dissolving the surface, and protruding while leaving metal inclusions excellent in corrosion resistance, suitable for mass production. Industrial means.

Pickling: Pickling is carried out in a stainless steel matrix (Matr
It may be immersed in an acidic solution capable of dissolving ix), or electrolyzed in an acidic solution.

Desirably, the acidic solution is a solution which selectively corrodes only the mother phase while leaving the conductive metal inclusions uniform and uniform.

Examples of the acidic solution include a nitric hydrofluoric acid aqueous solution, a sulfuric acid aqueous solution, and a hydrochloric acid aqueous solution which are the most commonly used pickling solutions in a stainless steel mass production line. It is also possible to use a commercially available acidic aqueous solution for etching stainless steel, which is mainly composed of a commercially available aqueous ferric chloride solution used for etching stainless steel. If necessary, it is also possible to add an organic or inorganic additive for reducing the deterioration of the acidic solution or smoothing the corroded surface.

The concentration of the acidic aqueous solution required for pickling differs depending on the type of stainless steel and the processing temperature. The pickling temperature may range from room temperature to a temperature not higher than the boiling point, and the concentration and temperature are preferably determined while observing the state of corrosion. It is preferable to adjust the average corrosion weight loss in an acidic aqueous solution to 5 to 60 g / m ² and to adjust the surface roughness to a center line average roughness Ra of 0.06 to 5 μm. The surface roughness “center line average roughness Ra” is measured according to JIS.
B 0601-1982 is a value representing the degree of two-dimensional surface roughness.

A preferred acidic aqueous solution has a hydrofluoric acid concentration of 2 to 20% by mass and a nitric acid concentration of 5 to 20% by mass in a nitric hydrofluoric acid aqueous solution, and a temperature of the aqueous solution of 30 to 20% by mass.
90 ° C. If the concentration of hydrofluoric acid or nitric acid is less than the above lower limit, the pickling efficiency is inferior, and if it exceeds the upper limit, the surface roughness of stainless steel deteriorates and the contact electric resistance increases. When a nitric hydrofluoric acid-based aqueous solution is used, it is possible to simultaneously perform the exposure treatment of the conductive metal inclusions and the strengthening of the passive film on the surface of the matrix.

When a sulfuric acid-based aqueous solution is used, the corrosion product adheres to the surface after pickling, and therefore, a treatment for removing the corrosion product using an acid solution such as a hydrochloric acid aqueous solution that dissolves the corrosion product. It is necessary to perform. Specifically, there is a 5 to 25% by mass aqueous sulfuric acid solution.

When hydrochloric acid is used, the passivation film cannot be strengthened on the surface of the matrix phase as in the case of nitric acid.
A 5% by mass aqueous hydrochloric acid solution is preferred. It is desirable that the liquid temperature be controlled between room temperature and 85 ° C.

The pickling solution may be a mixed solution thereof,
If necessary, a commercially available inhibitor for suppressing the corrosion rate may be added.

In pickling, it is efficient to immerse stainless steel in an acidic aqueous solution as described above, and it is efficient to stir and flow the acidic solution. Further, a method of spraying, showering, or spraying an acidic solution on a stainless steel surface with a nozzle may be used.

FIG. 2 is a cross-sectional view showing a case where the surface of the stainless steel with the metal inclusions exposed is brought into contact with another conductive body surface.

In this figure, a passivation film 12 is present on the surface of stainless steel 10, and the deposited boride-based metal inclusions 13, carbide-based metal inclusions 15, and projected metal inclusions 14 Each of them is exposed from the surface and is in contact with the carbon plate 11.

Surface roughness: Since surface roughness affects contact electric resistance, if the center line average roughness Ra is less than 0.06 μm, the surface is too smooth and conductive metal inclusions are present near the steel surface. Even the improvement effect is small. If it is too smooth, the number of contact points will decrease. On the other hand, if it exceeds 5 μm, the number of contact points per unit area is significantly reduced, and the contact electric resistance tends to decrease. Therefore, Ra is desirably set to 0.06 to 5 μm. More preferably, it is 0.06 to 2.5 μm.

The surface roughness “center line average roughness Ra” is a value representing the degree of two-dimensional surface roughness defined in JIS B 0601-1982.

Neutralization treatment: In the production method of the present invention, it is important to perform a neutralization treatment after the pickling treatment. Even after the pickled surface has been thoroughly washed with water, the acid component remains in the fine irregularities on the surface, in the gap between the metal inclusion and the parent phase, and in the grain boundaries. In many cases. The corrosion of the surface progresses due to the concentration and ejection of the acid component, which causes a problem that the contact resistance of the surface increases with time. By immersing in an alkaline aqueous solution having a pH of 7 or more, or by performing a neutralization treatment of spraying an alkaline aqueous solution having a pH of 7 or more on the surface, characteristic deterioration can be remarkably improved. The term “spray” used herein includes spraying and showering an alkaline aqueous solution from a nozzle.

As the alkaline aqueous solution, one that satisfies that it is water-soluble, that it has excellent water washability after treatment, that it is industrially easy to treat waste liquid, that it is easily available, and that it is inexpensive. . As a preferred example, 3-1
There is an aqueous solution of sodium hydroxide having a concentration of 0% by mass.

Chemical composition of stainless steel: C: C is an important element along with B in the present invention.
By dispersing and precipitating as an r-based carbide, there is an effect of reducing the contact electric resistance of the surface of the stainless steel covered with the passive film. This can be explained by the fact that the Cr-based carbide has metallic properties and has better electrical conductivity than the passive film. In the case of stainless steel containing B, a large amount of boride precipitates, so that it may be incorporated into boride and precipitate.

In general, a very thin passive film of about several tens of millimeters 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. It is possible to reduce the electrical resistance by thinning the passive film, but especially when used inside a polymer electrolyte fuel cell,
It is not easy to maintain a stable thin passivation film. Directly exposing the Cr-based carbide having excellent electrical conductivity to the surface without being covered with the passivation film is extremely effective for stabilizing the electrical conductivity of the stainless steel surface for a long time. That is, the Cr-based carbide is stable in terms of corrosion resistance and does not form a passive film on the surface. Therefore, even if the passivation film on the surface becomes thick inside the polymer electrolyte fuel cell, good electrical conductivity is ensured via the Cr-based carbide exposed on the steel surface, and the contact of the steel surface with the steel surface is ensured. An increase in electric resistance can be suppressed. In other words, the fine chromium-based carbides that are exposed without being covered by the passivation film function as “paths for electricity”, thereby maintaining low contact electric resistance.

In general, when a large amount of C is contained, the strength and hardness of stainless steel are increased, ductility is reduced, and manufacturability is reduced. In order to ensure moldability as a separator material for a polymer electrolyte fuel cell, it is preferable that carbon in steel is precipitated as a carbide to reduce the amount of solid solution C. Precipitation of C as a carbide improves the formability of the steel. That is, from the viewpoint of ensuring formability, it is effective to precipitate C in steel as carbide. 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. Before the heat treatment for agglomeration and coarsening, if the residual strain is added by cold rolling, shot of the metal fine powder, grinding or grinding in advance, the precipitation time and the aggregation time of the carbide can be shortened. In addition, the time required for the corrosion resistance to recover to the same level as that of the base metal is shortened even for the sensitization that becomes apparent due to carbide precipitation.

For ferritic stainless steel, 0.15
%, The C-formability cannot be ensured for a polymer electrolyte fuel cell separator. Therefore, the C content is preferably 0.15% or less. Further, in order to prevent sensitization caused by carbide precipitation, the value of (C mass% precipitated as Cr-containing carbide) × 100 / [(total C mass% in steel) −0.0015%] is set to 80 or more. It is desirable to do.

In order to precipitate carbides and to disperse and expose them on the surface of stainless steel, the content is preferably 0.02% or more, more preferably 0.04% or more. Further, in order to promote the precipitation of carbides, 500 to 950 ° C.
It is preferable to perform a heat treatment for maintaining the temperature in the above range. In a temperature range exceeding 950 ° C., the Cr-based carbide becomes thermally unstable and re-dissolves. On the other hand, at temperatures below 500 ° C., C and C
The diffusion rate of r becomes slow, and the precipitation treatment time in mass production becomes long, which is not preferable from an industrial viewpoint. A more preferred treatment temperature range for precipitating Cr-based carbides is 650 to 920 ° C.
The most desirable temperature range is 800 to 900 ° C.

In austenitic stainless steel, 0.005 to 0.2%
It is good to contain C in the range of. When the content exceeds 0.2%, the moldability for a polymer electrolyte fuel cell separator cannot be ensured. In order to precipitate more carbide and disperse and expose it on the stainless steel surface, it is preferable to contain 0.06% or more. In order to prevent sensitization due to carbide precipitation, (C
C mass% precipitated as r-containing carbide) × 100 /
((Total C mass% in steel) -0.012%) It is desirable to set the value to 85 or more.

To promote the precipitation of carbides,
It is preferable to perform a heat treatment for maintaining the temperature in a temperature range of 950 ° C. However, in a temperature range exceeding 950 ° C., the Cr-based carbide becomes thermally unstable and forms a solid solution again. On the other hand, when the temperature is lower than 500 ° C., the diffusion rates of C and Cr in the steel are slow, and the precipitation treatment time in mass production becomes long, which is not preferable from an industrial viewpoint. A processing temperature range suitable for the precipitation of a Cr-based carbide is 600 to 900 ° C.

Although Cr-based carbides are finely dispersed and precipitated in steel, they tend to preferentially precipitate at crystal grain boundaries where precipitation is likely. In lowering the contact electric resistance, whether the chromium carbide precipitates at the grain boundary or in the grains is not so much affected,
From the viewpoint of uniformly dispersing, it is presumed that it is desirable that the particles are dispersed and precipitated in the grains.

In order to disperse in the grains, hot rolling is performed within a temperature range and time in which a part of the Cr-based carbide is dissolved in a state where the Cr-based carbide is once precipitated and is not completely re-dissolved. Alternatively, after applying a working strain by cold rolling, 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 effect can be confirmed from several percent of the degree of cold work to be applied, but is preferably about 20 to 30% or more. It is also possible to impart processing strain only to the vicinity of the surface layer and precipitate carbide only near the surface layer. In any case, 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 in the grain, and a new grain boundary is formed, so that the carbide in the grain is also formed. Will be precipitated.

When a processing strain is applied only to the vicinity of the surface layer and a precipitation treatment is performed, in the ferrite system described above, (C mass% precipitated as Cr-containing carbide) × 100 /
((Total C mass% in steel)-0.0015%) A value of 80 or more, in the austenitic system, (C mass% precipitated as Cr-containing carbide) x 100 / ((Total C mass% in steel) (0.012%), it is difficult to establish a relationship of 85 or more, but qualitatively, it is desirable to satisfy these relationships on an exposed surface.

As is well known, in the Cr-based carbide precipitation treatment, there is a possibility that the corrosion resistance of the base metal may be reduced by sensitization. Sensitization is a reduction in corrosion resistance caused by the formation of a Cr-deficient layer around the precipitation of Cr-based carbides. The sensitization can be avoided or improved by maintaining the temperature in a temperature range of 500 ° C. or more and 950 ° C. or less for a long time and slowly cooling. Generally, a lower cooling rate is more desirable.

However, the heat treatment time for suppressing sensitization is a phenomenon that varies depending on the C content in steel and the history of the material, and it is difficult to specify the conditions. That is, the heat treatment for suppressing the sensitization varies depending on the state of carbide precipitation, the amount of residual working strain, the holding temperature, and the like before the heat treatment of the carbide precipitation treatment, so that it is difficult to unconditionally define the heat treatment.

As an example, furnace cooling at 830 ° C. for 6 hours can be mentioned. Immediately after the precipitation heat treatment is performed, the heat treatment for suppressing the sensitization may be continued without cooling. Also, after once cooled, heated again to a temperature range of 500 ° C. or more and 950 ° C. or less,
Sensitization may be avoided or improved by slow cooling. As one guide, the ferrite type has a relationship of (C mass% precipitated as Cr-containing carbide) × 100 / ((total C mass% in steel) −0.0015%) value of 80 or more. (C mass% precipitated as Cr-containing carbide) × 100 / ((total C mass% in steel) −
It is preferable to satisfy a relationship of 85 or more in terms of (0.012%) value. Whether or not corrosion resistance is ensured without sensitization can be easily confirmed by a grain boundary corrosion detection method such as "sulfuric acid-copper sulfate corrosion test" specified in JIS G-0575.

When quantifying the (C mass% precipitated as Cr-containing carbide) value, a round bar having a diameter of 8 mm was processed from the test material, and an AA solution (10% acetylacetone-1% tetramethylammonium chloride-remaining methanol) was used. )
From the results of quantitative analysis of Cr in “extraction residue” obtained by performing galvanostatic electrolysis in a non-aqueous solvent solution using
The amount of C can be quantitatively evaluated by equivalent calculation assuming that all Cr is Cr ₂₃ C ₆ .

That is, in an AA non-aqueous solvent solution, 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 through a filter having a filter diameter of 0.2 μm (for example, “Nuclepore” (trade name) manufactured by Coster Scientific Corporation), and the residue on the filter was subjected to phosphoric acid sulfuric acid (special grade phosphoric acid: special grade sulfuric acid: distilled water). = 1
1: 1), and by analyzing the metal component with an inductively coupled plasma emission spectrometer (for example, ICPV-1014 (trade name, Shimadzu Corporation)), the Cr concentration can be determined.

Further, the determination of (total C mass% in steel) can be performed by an infrared absorption method. That is, the test piece was heated and melted in an oxygen stream, and the carbon in the steel was sufficiently heated to form carbon dioxide, which was sent to an infrared absorption cell together with oxygen, and quantitatively analyzed at the end of the infrared class with carbon dioxide. . At present, it is the most common method for determining C in steel.

Si: The amount of Si in the steel is 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%, deoxidation will be insufficient, while if it exceeds 1.5%, moldability will be reduced.

Mn: In ferritic stainless steel, it is preferable to contain Mn in the range of 0.01 to 1.5%. Usually, Mn has the effect of fixing S in steel as Mn-based sulfide, and has the effect of improving hot workability. In austenitic stainless steel, Mn is 0.01 to
It is contained at 2.5%. Mn is an effective austenite phase stabilizing element. However, it is not necessary to contain more than 2.5%.

P: The P content in steel is preferably set to 0.04% or less. In the present invention, P is the most harmful impurity along with S. The lower the better.

S: S content in steel is preferably 0.01% or less. In the present invention, S is the most harmful impurity along with P. The lower the better, the better. Depending on the coexisting element in steel and the amount of S in steel, Mn-based sulfide, C
Almost all precipitate as r-based sulfides, Fe-based sulfides, or composite nonmetallic inclusions with these composite sulfides and oxides. However, in the separator environment of a polymer electrolyte fuel cell, nonmetallic inclusions of any composition, although varying in degree, act as a starting point of corrosion, and are harmful to maintenance of a passive film and suppression of corrosion elution. . The S content in normal mass-produced steel is more than 0.005% and about 0.008%.
It is desirable to reduce it to 04% or less. The more desirable S content in steel is 0.002% or less, and the most desirable level of S content in steel is less than 0.001%, the lower the better. If it is less than 0.001% at the level of industrial mass production, there is no problem at all with a slight increase in production cost 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. 36% Cr content in ferrite
If it exceeds, production on a mass production scale becomes difficult. In an austenitic system, if it exceeds 30%, the austenitic phase becomes unstable even by adjusting other alloy components. With a Cr content of less than 15% in a ferrite system and less than 17% in an austenitic system, it becomes difficult to secure the corrosion resistance required for a separator even if other elements are changed.

Precipitation of borides and carbides may lower the amount of Cr in steel, which contributes to the improvement of corrosion resistance, as compared with the amount of Cr in the molten steel stage, thereby deteriorating the base metal corrosion resistance. In order to ensure corrosion resistance inside the polymer electrolyte fuel cell, in the case of the steel of the present invention in which M ₂ B type boride is precipitated,
At least (Cr% + 3 × Mo% −2.5 × B%) ≧ 1
It is desirable that the amount of Cr in steel satisfy the relational expression of 7.0%. Also, when M ₂₃ C ₆ type carbide is precipitated,
Since the solid solution amount of Cr commensurate to carbide precipitation amount is decreased, C precipitated as Cr% -M ₂₃ C ₆ type carbide during the (steel
r%) + 3 × Mo% ≧ 17%
It is desirable that the amount is r.

Al: Al is added as a deoxidizing element in the molten steel stage. When B is contained in the steel of the present invention, since B is an element having a strong bonding force with oxygen in molten steel, it is necessary to lower the oxygen concentration by Al deoxidation. Therefore, 0.0
The content is preferably in the range of 01 to 6%.

B: B is contained as needed, but when it is contained, it exerts an important effect. When it is contained, the content is preferably 3.5% or less. In B-containing steel, Cr, composed mainly of Fe, Ni, Mo and contains trace amounts _{(Cr, Fe) 2 B,} (Cr, Fe, Ni) by precipitating the M ₂ B-type boride such as ₂ B, It has the effect of reducing the contact electric resistance of the stainless steel surface covered with the passive film. On the other hand, the contact electric resistance can be kept low over time by causing the fine particle powder such as B ₄ C to be “cut in and remain” when shot, polished, or ground.

This can be explained by the fact that these borides have metallic properties and have better electrical conductivity than passive films.

By exposing boride having excellent electric conductivity directly to the surface without being covered with the passive film,
It is extremely effective to stabilize the electric conductivity of the stainless steel surface for a long time.

That is, the boride is stable in terms of corrosion resistance like the carbide, and does not form a passive film on the surface. Therefore, even if the passivation film on the surface becomes thick inside the polymer electrolyte fuel cell, good electrical conductivity is ensured through the boride exposed on the steel surface, and the contact electric power on the steel surface is ensured. An increase in resistance can be suppressed.
In other words, the boride having fine metallic properties, which is exposed without being covered by the passivation film, functions as an “electric path”, so that the contact electric resistance can be kept low.

In general, when B is contained in a large amount, the strength and hardness of the stainless steel are increased, the ductility is reduced, and the decrease in productivity is increased. In order to ensure moldability as a separator material for a polymer electrolyte fuel cell, it is preferable 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, from the viewpoint of ensuring formability, B in steel
Is effective as a boride. further,
By holding at a temperature around 1200 ° C. for a long time, boride can be aggregated and coarsened, which is effective for further improving workability. However, there is a disadvantage that the material is easily deformed due to a high holding temperature.

Further, in the manufacturing process, by increasing the hot forging ratio (forging ratio) and carrying out strong working, it is possible to finely disperse the boride, which is the cause of the decrease in the deformability, into “crushed”. is there. Fine crushing can reduce toughness degradation. Increasing the forging ratio (forging ratio) in the cold and performing strong working is also effective for finely disintegrating the "broken" boride, which is the cause of the decrease in deformability.
Fine crushing can reduce toughness degradation.

If B is contained in an amount exceeding 3.5%, it becomes difficult to produce B by a usual dissolution method. In addition, the moldability at room temperature for a polymer electrolyte fuel cell separator cannot be ensured. Therefore, when B is contained, the content is preferably set to 3.5% or less.

As is well known, most of B in steel precipitates as boride. 0.01 at 1125 ° C
% Or less. On the low temperature side, the amount of solid solution becomes even lower.

Although the boride precipitation temperature depends on the content, it is near the solidification temperature of stainless steel, and once precipitated, exhibits a behavior of hardly re-dissolving. There is also a problem that the liquidus line decreases with the B content, and the temperature range where hot forging is possible is narrowed. Since boride itself has extremely poor deformability, B
The greater the content and the more remarkable boride precipitation, the greater the problem of cracking during production and processing, and the worse the mass productivity. However, when the B content is up to 3.5%, production on an industrial scale is possible with considerable difficulty.

The M ₂ B type boride having almost no deformability is rolled while being dispersed in such a manner as to “crush in the rolling direction”. The state of dispersion of the boride affects the formability.
The control of the boride dispersion state in the steel is possible by devising forging conditions and hot rolling conditions. Especially hot,
Cold rolling under cold conditions is effective.

When the B content is about several tens of ppm, the boride tends to precipitate at the crystal grain boundaries. In lowering the contact electric resistance, whether the boride precipitates at the grain boundary or in the grains is not so much affected, but it is preferable that the boride is uniformly dispersed from the viewpoint of workability at room temperature and avoiding cracking problems. Needless to say.

There is a possibility that the corrosion resistance of the base material may be reduced by boride precipitation. The decrease in corrosion resistance of the base material due to boride precipitation
This occurs because Cr and Mo in the base material are consumed by precipitation of boride. It is extremely important to contain in advance the amount of Cr and the amount of Mo consumed by boride formation at the stage of molten steel in order to reduce the decrease in corrosion resistance. The effect of the cooling rate is relatively small.

In order to ensure corrosion resistance inside the polymer electrolyte fuel cell body, Cr% + 3 × Mo% −2.5 × B%
It is preferable to satisfy ≧ 17%. Such coefficients for each element are specified by experimental rules.

When quantifying the amount of B in steel, the amount of B
A round bar having a diameter of mm was processed, and B was extracted from the “extraction residue” obtained by performing galvanostatic electrolysis in a non-aqueous solvent solution using an AA solution (10% acetylacetone-1% tetramethylammonium chloride—remaining methanol). It can be evaluated by quantitative analysis. That is, in an AA non-aqueous solvent solution, about 0.4 g equivalent was dissolved by performing constant current electrolysis for about 3 hours at a current density of 20 mA / cm ² ,
Immediately after the electrolysis, the AA non-aqueous solvent solution obtained by ultrasonically cleaning the electrolytic test piece and the AA non-aqueous solvent solution used for the electrolysis are filtered with a filter having a filter diameter of 0.2 μm (for example, Coster Scientific
The solution can be filtered off using "Nuclepore" (trade name, manufactured by Corporation) and quantified using the residue on the filter.
When the amount of boride in the steel is small and the amount of residue is as small as 40 μg or less, it is separated by distillation and then subjected to curcumin absorption spectrophotometry, and when the amount is 40 μg or more, phosphoric acid (special phosphoric acid: special grade sulfuric acid: distilled water = 1) : 1: 1) and then dissolved in an inductively coupled plasma emission spectrometer (for example, Shimadzu Corp., trade name ICPV-1014) to analyze the metal component and determine the amount of B precipitated as boride. can do.

The residue obtained by the galvanostatic electrolysis in the AA solution is collected on a filter as a compound of M ₂ B. The quantification of the metal element bonded to B as boride is possible according to the above-mentioned procedure utilizing inductively coupled plasma emission spectroscopy. M ₂ B qualitative analysis is possible by X-ray diffraction.

N: N in ferritic stainless steel
Is an impurity. Since N deteriorates room temperature toughness, the upper limit is preferably set to 0.035%. The lower the better. Industrially, the content is most preferably 0.007% or less. In austenitic stainless steel, N
Is an element effective for adjusting the austenite phase balance as an austenite-forming element. However, the upper limit is preferably set to 0.4% in order not to deteriorate workability.

Ni: In austenitic stainless steel, Ni is an extremely important alloy element for austenite phase stability. Ferritic stainless steel also has the effect of improving corrosion resistance and toughness. In austenitic stainless steel, the lower limit is preferably set to 7%, and the upper limit is preferably set to 50%. If the content is less than 7%, it is difficult to stabilize the austenite phase, while if it is contained more than 50%, the production becomes difficult. The upper limit of the ferritic stainless steel is preferably set to 7%. If it is 7% or more, it is difficult to form a ferrite-based structure, and although it is affected by other elements, it becomes a ferrite and austenite two-phase structure. In the two-phase structure, the formability of a thin plate is directional, and it becomes impossible to secure sufficient workability as a material for a polymer electrolyte fuel cell separator.

Mo: Mo has the effect of improving the corrosion resistance with a small amount compared to Cr. It may be contained as needed in an amount of 7% or less. 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 steel embrittlement. Therefore, the upper limit is preferably set to 7%.

[0108] Further, as a feature of Mo, even if Mo is dissolved out of steel due to corrosion inside the polymer electrolyte fuel cell, the effect on the performance of the catalyst supported on the anode and the cathode is compared. There is a characteristic that is minor. Mo is not present as a metal cation, but is present as a molybdate ion, which is an anion. Therefore, the effect on the cation conductivity of a fluorine-based ion exchange resin membrane having a hydrogen ion (proton) exchange group is small. it can.

Mo is an extremely important additive element for maintaining corrosion resistance, and is expressed by (Cr% + 3 × Mo% −2.5 ×
It is desirable that the amount of Mo in steel satisfy the relational expression of (B%) ≧ 17%.

It can be present as Mo ₂ C-type carbide in steel or on the steel surface, as a residue after shot processing, mechanical processing such as polishing or grinding, or as a precipitate. By dispersing and exposing the conductive metal on the surface of stainless steel, there is an effect of reducing the contact electric resistance of the stainless steel surface covered with the passive film. This can be explained by the fact that the Mo-based carbide has metallic properties and has better electrical conductivity than the passive film. By exposing Mo-based carbide, which has excellent electrical conductivity, directly to the surface without being covered by the passivation film, the fine Mo-based carbide functions as a "path of electricity" to maintain low contact electric resistance. Can be.

In addition, although the cost is industrially high, the Mo concentration in the stainless steel surface layer is increased by ion implantation or the like, and is reacted with C in the steel by a subsequent heat treatment.
It is also possible to form ₂ C.

Cu: In ferritic stainless steel, C:
u is preferably contained at 1% or less. If the content exceeds 1%, the hot workability is reduced, and it is difficult to secure mass productivity. In austenitic stainless steel, the content is preferably 2% or less.
Cu is an effective austenite phase stabilizing element and has an effective function in maintaining a passive state.

Ti, Nb: Ti and Nb have the effect of reducing the deterioration of toughness of ferritic stainless steel. If necessary, it is contained at 25 × (C% + N%) or less. T
The effect of improving toughness can be obtained even if it is contained in a composite of i and Nb.

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

[0115]

EXAMPLE 1 Stainless steels having 16 chemical compositions shown in Table 1 were melted in a 150 kg vacuum melting furnace of a high frequency induction heating system and formed into ingots. 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. Steel symbols a to i and p are ferritic stainless steels, and j to o are austenitic stainless steels.

[0116]

[Table 1] A cold-rolled steel sheet was manufactured from each ingot by the following steps.

Incott → forging → cutting → hot rolling → annealing → cooling → descaling by shot-lasting → pickling → cold rolling (intermediate annealing) → annealing → pickling. Details of each manufacturing process were as follows. Ingot: [Steel symbol] [Ingot shape] af, p: Round gi: Square flat jm: Round n, o: Square flat. Forging (Pressure method, heating in air): [Steel symbol] [Heating temperature] [Heating time] [Thickness Width Length (mm)] af, p: 1220 ° C 3h 70 400 600 gi: 1180 ° C 3h 50 400 600 jm: 1260 ° C 3h 70 400 600 n, o: 1180 ° C 3h 50 400 600.

Cutting: The above slab is cut to remove oxide scale on the surface and cracks on the end face [Steel symbol] [Finished thickness after cutting (mm)] af, p: 60 gi: 42 jm: 60 n, o: 42. Hot rolling: [Steel symbol] [Sluff ゛ heating temperature, in air] [Finishing thickness (mm)] af, p: 1220 ° C 3.8 gi: 1180 ° C 2.6 jm: 1260 ° C 3.8 n, o: 1180 ° C 2.6.

In each case, after the hot rolling, a heat insulating material was wound and gradually cooled to simulate the temperature history immediately after the end of hot rolling in mass production.

Each of the steel symbols g to i and n and o is a stainless steel containing 0.6% or more of B. B in steel precipitates at around 1200 ° C. below the solidus line. Since M ₂ B-type boride is an intermetallic compound, its deformability is extremely poor both in a normal temperature range and a high temperature range.
Forging and rolling were performed while repeating reheating in a temperature range of 1000 ° C. or more and 1200 ° C. or less. Further, since the temperature of the coil end face is easily lowered and ear cracks are easily generated, hot rolling was performed while maintaining the heat of the coil end face and, if necessary, heating.

Annealing after hot rolling (in air): [Steel symbol] [Annealing temperature ° C] [Cooling] a, e, f, g, h, i, p: 840 Air cooling b: 925 Water cooling c, d: 1000 Water cooling j, k, l, m, n, o: 1080 air cooling.

The holding time was 20 minutes in each case. The high-temperature oxidation scale on the surface was removed by shots, pickling and cold rolling: all were finished to 0.3 mm.

If necessary, during the cold rolling, soft annealing was performed at 810 ° C., and pickling was performed in a 7% by mass nitric acid, a 4% by mass hydrofluoric acid aqueous solution simulating ordinary mass production line conditions, and at 60 ° C.

The cold-rolled coil was subjected to final annealing at the same temperature as the hot-rolled material, and was used as a test material. Metal inclusions were dispersed and exposed on the surface of the stainless steel sheet by the following method. Metallic inclusions were dispersed and exposed on the surface of the stainless steel plate to perform a neutralization treatment.

The dispersion, exposure and surface roughness of the metal inclusions on the surface of the cold-rolled steel sheet were performed by changing the conditions of the pickling liquid (liquid composition, temperature) and the pickling time. The following four kinds of pickling liquids were used.

Nitric acid: 15%, hydrofluoric acid: 3%, water: balance Nitric acid: 10%, hydrofluoric acid: 8%, water: balance Nitric acid: 10%, hydrofluoric acid: 4%, water: balance Nitric acid: 8%, Hydrofluoric acid: 3%, water: balance.

The temperature of the pickling solution was changed from room temperature to boiling, and the temperature was set at 60 ° C.

As another method of dispersing and exposing metal inclusions on the surface of a stainless steel plate, a method of shot of hard fine powder having conductivity of WC, Mo ₂ C and B ₄ C was used.
The shot conditions were as follows.

Each of the shot fine powders was a powder manufactured industrially, and had an average particle size of about 200 μm. 99% or more in WC, 90% in Mo ₂ C, 95 in B ₄ C
% Or more, MC type, M ₂ C type, M ₄ C
Corresponds to type carbide. The projection conditions were as follows.

Projection pressure: 5 kg / cm ² Projection distance: 200 mm Projection amount: 5 kg / min Projection angle: 80 degrees Using the test pieces prepared by the above method, the following (1) to
The test of (4) was performed. (1) Test of stainless steel sheet in which metal inclusions are dispersed and exposed by projecting conductive powder on the cold-rolled steel sheet. Each cold-rolled steel sheet having the steel symbol shown in Table 2 was pickled under the pickling conditions shown in the same table. After pickling, washing, and drying, a hard fine powder having conductivity of WC, Mo ₂ C, and B ₄ C is shot, and then immersed in a 6% by mass sodium hydroxide solution having a liquid temperature of 25 ° C. for 3 minutes. Then, ultrasonic cleaning was performed, and further ultrasonic cleaning was performed in distilled water for 15 minutes. Some of the comparative examples were not subjected to the neutralization treatment. The neutralization and the washing may be performed after the pickling before the shot treatment. After drying with a cool air dryer, the surface roughness (center average roughness Ra and maximum height Rmax) specified in JIS B 0601-1982, the contact electric resistance immediately after neutralization, and the temperature after leaving in air for 500 hours The contact electrical resistance was measured.

Table 2 shows the results.

[0132]

[Table 2] Nos. 1 to 13 shown in Table 2 are cases where shots were applied, and Nos. 14 to 19 are examples where no shots were applied, and are comparative examples in which metal inclusions were not exposed on the surface.

Nos. 5 to 7 of the present invention and 16 of the comparative example
No. 19 to No. 19 used steel having a low carbon content in order to avoid the influence of the M ₂₃ C ₆ type conductive metal precipitate.

In Table 2, the presence or absence of the M ₂₃ C ₆ and M ₂ B type precipitates is indicated as “precipitated” if it can be easily visually confirmed that the particles are macroscopically and uniformly dispersed on the surface with an optical microscope. . The identification was performed from a diffraction image of a transmission electron microscope.

The contact electric resistance was measured using a 0.3 mm-thick cold-rolled steel plate after pickling and a commercially available glassy carbon plate having a thickness of 0.6 mm (manufactured by Showa Denko, trade name “SG3”). Stainless steel test piece contact area is 1cm ² ,
It was measured by a four-terminal method. Immediately before the evaluation, the surface of the test piece for evaluation was subjected to evaluation after washing the surface with water. When the pickling treatment is not performed and the shot is not performed, the surface is a wet 600th emery paper polished surface. Load load is 11.2kg / c
It was m ^2. Although the contact electric resistance changes depending on the applied load, it has been confirmed that a substantially constant value can be obtained at 10 kg / cm ² or more.

As is evident from Table 2, when the conductive powder was “cut into the stainless steel sheet surface and left behind” by shot processing, the contact resistance was 8.45 mΩ · cm ^2.
It was as follows. On the other hand, in the comparative example where no shot is given,
The contact electric resistance was as high as 107.9 mΩ · cm ² or more. Here, in Nos. 14 and 15, the contact resistance is high even though M ₂₃ C ₆ which is a conductive metal inclusion is precipitated in the steel. This is because No. 14 has a center line average roughness of 0.03 μm
And the conductive inclusions did not protrude from the surface because the surface was extremely smooth.
It is considered that the conductive inclusions did not sufficiently function as "paths of electricity" because the proportion of conductive inclusions at the contact points was low due to the large surface roughness of 54 μm and the maximum roughness of 56.2 μm. . The surface roughness of the stainless steel material is desirably 0.06 to 5 μm in center line average roughness Ra. In the shot processing, the conductive powder is "dipped into the surface and left", and the surface roughness is determined by the center line average roughness R
It is clear that this is one of extremely effective industrial means that satisfies both the adjustment of a to 0.05 to 5 μm. In addition, it is determined that the function of the "electric path" of the conductive particles "inlaid and left behind" by the shot processing does not deteriorate over time, but the surface of the steel sheet actually shot It can be confirmed that the contact resistance of Example 3 shows substantially the same contact resistance even after 300 hours.

In the case where conductive metal precipitates such as M ₂₃ C ₆ and M ₂ B types are precipitated in steel, No. 1-4, 8-1
Good contact resistance was obtained as indicated by 0, 12, and 13. It is determined that the two improvement effects are in a superimposed relationship.

As is clear from Comparative Examples 15 to 19, when the neutralization treatment was not performed, the contact electric resistance after 500 hours had elapsed was large, and the effect of the neutralization treatment was remarkable. (2) Test of stainless steel sheet in which metal inclusions are precipitated and dispersed and exposed In order to confirm the effect of only the deposited metal inclusions without projecting the electrically conductive fine powder, the carbon content is relatively high. Pickling was carried out under various pickling conditions shown in Table 3 using stainless steel in which ₂₃ C ₆ type carbide was precipitated and stainless steel in which the boron content was high and M ₂ B type boride was precipitated. The metal inclusions were dispersed and exposed.

After pickling, the substrate was immersed in a 6% by mass sodium hydroxide solution having a liquid temperature of 25 ° C. for 3 minutes, ultrasonically washed, further ultrasonically washed in distilled water for 15 minutes, and washed in running water for 15 minutes. And ultrasonic cleaning for 5 minutes in distilled water. Some of the comparative examples were not subjected to the neutralization treatment. As a comparative example, stainless steel having a low carbon content and a low or no boron content was also pickled under the same conditions. The surface roughness and contact electrical resistance of these pickled cold-rolled steel sheets were measured by the same methods as described above. Further, the contact electric resistance was measured even after being left in the air for 500 hours. Table 3 shows the measurement results.

[0140]

[Table 3] As is clear from Table 3, all of the present invention examples in which M ₂₃ C ₆ and M ₂ B type precipitates are dispersed and exposed have a contact electric resistance of 15.
6mΩ · cm ² or less, but the precipitate of the comparative example is dispersed and not exposed on the surface of the steel sheet, even if the surface roughness is within the range specified in the present invention is 101.6mΩ · cm ² or more. It is clear that it is high. The effect of dispersion precipitation is remarkable. M ₂₃
The improvement effects of C ₆ -type carbide and M ₂ B-type boride are determined to be in a superimposed relationship.

(3) Test of a stainless steel sheet in which metal inclusions are dispersed and exposed and the surface roughness is changed As shown in Table 4, pickling was carried out under various pickling conditions to obtain a cold rolled steel sheet. Was changed in the surface roughness.

After pickling, the substrate was immersed in a 6% by mass sodium hydroxide solution at a liquid temperature of 25 ° C. for 3 minutes, ultrasonically washed, further ultrasonically washed in distilled water for 15 minutes, and washed in running water for 15 minutes. And ultrasonic cleaning for 5 minutes in distilled water. Some of the comparative examples were not subjected to the neutralization treatment. The surface roughness and the contact electric resistance of each test piece were measured immediately after the neutralization treatment. The contact electric resistance was measured after the test piece was left in the air for 500 hours. Table 4 shows the results.

[0143]

[Table 4] As is clear from Table _{4, M 23 C 6, M 2} B -type precipitates in the steel dispersed stainless steel surface, even if it is exposed, there is a case where the contact resistance by the surface roughness becomes high. Contact does not occur on the surface, but only on a few points, even if the surface is very smooth, in other words, if the surface is very smooth, the number of contact points is small Therefore, it is considered that a sufficient opportunity for contact via the conductive metal inclusions exposed on the steel sheet surface cannot be obtained, and as a result, the contact resistance value increases.

FIGS. 3 and 4 show the surface roughness of a steel symbol n (Table 1) polished by wet No. 600 emery paper after pickling in 10% nitric acid-3% hydrofluoric acid for 5 minutes. FIG. 3 shows a measurement result in two dimensions, and FIG. 4 shows a measurement result in three dimensions.
Ra was 0.2133 and 0.2147 μm. For these measurements, a commercially available roughness meter was used.

FIG. 5 is a microphotograph (× 1000) of steel symbol n in Table 1. What is surrounded by a white line is the dispersed phase M ₂ B-type boride. The cross section is obtained by SEM observation. From the top, it is possible to determine the situation where M ₂ B protrudes “out of the head” on the surface and is dispersed.

FIG. 6 shows an electron micrograph (× 2000) of steel symbol O in Table 1. It can be seen that the M ₂₃ C ₆ type carbide and the M ₂ B type boride are deposited in an overlapping manner.

(4) Test in which a separator made of stainless steel in which metal inclusions were dispersed and exposed was incorporated into a polymer electrolyte fuel cell. The stainless steel of the present invention was loaded into a polymer electrolyte fuel cell as a separator. For performance evaluation, a corrugated separator plate was produced from the finally annealed cold-rolled steel plate of each steel symbol shown in Table 5.

The separator plate is formed in the shape shown in FIG. 1 on both surfaces (anode electrode side and cathode electrode side) by machining and discharging a gas flow path having a groove width of 2 mm and a groove depth of 1 mm by machining.
The evaluation was performed in a state where the separator was loaded inside the solid polymer type single cell battery. The characteristics were evaluated by measuring the voltage of the single-cell battery one hour after the fuel gas was flowed into the battery, and comparing the voltage with the initial voltage to determine the rate of voltage decrease. The rate of decrease is 1- (voltage V /
It was determined by the 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 fuel gas for 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 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. Depending on the battery operation status,
It is expected that there will be changes in the characteristics, but this is a comparative evaluation under the same conditions. Table 5 shows the results.

[0152]

[Table 5] As is clear from Table 5, the stainless steel specified in the present invention was able to maintain a low contact electric resistance even in a state of being loaded in the polymer electrolyte fuel cell.

[0153]

According to the present invention, it is possible to produce a stainless steel material which is excellent in corrosion resistance and can maintain a low contact electric resistance for a long period of time. 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 state in which the surface of the stainless steel in which metal inclusions are exposed is brought into contact with another conductive body surface.

FIG. 3 is a diagram showing a two-dimensional surface roughness state of a stainless steel plate after pickling.

FIG. 4 is a diagram showing a three-dimensional surface roughness state of a stainless steel plate after pickling.

FIG. 5 is a diagram showing dispersed M ₂ B-type borides.

FIG. 6 is a diagram showing dispersed M ₂₃ C ₆ type carbide and 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

Continuation of the front page (72) Inventor Norihi Doi 4-5-33 Kitahama, Chuo-ku, Osaka-shi, Osaka Inside Sumitomo Metal Industries, Ltd. (72) Inventor Akira Seki 4-5-33 Kitahama, Chuo-ku, Osaka-shi, Osaka Sumitomo Metal Industries, Ltd. F-term (reference) 4K057 WA04 WA20 WB02 WB03 WB11 WE02 WE03 WE07 WE08 WF01 WF06 WG02 WG03 WJ10 WK10 WM03 WM04 WM13 WN01 WN02 5H026 AA06 BB00 BB03 BB10 EE08H00

Claims (4)

[Claims] 1. The surface of a stainless steel material is corroded by an acidic aqueous solution, and the surface thereof is electrically conductive M ₂₃ C ₆ type, M ₄
C type, M ₂ C type, MC type carbide-based metal inclusions and M ₂ B
A stainless steel material for a current-carrying part, characterized by exposing at least one of the boride-type metal inclusions, neutralizing with an aqueous alkaline solution having a pH of 7 or more, and then further washing with water and drying. Production method. 2. The average corrosion weight loss due to an acidic aqueous solution is 5 to 60.
method for producing a live parts stainless steel according to claim 1, characterized in that the g / m ^2. 3. The method according to claim 2, wherein the stainless steel material is C: 0.1% by mass.
5% or less, Si: 0.01 to 1.5%, Mn: 0.01
1.5%, P: 0.04% or less, S: 0.01% or less, Cr: 15 to 36%, Al: 0.001 to 6%,
B: 0 to 3.5%, N: 0.035% or less, Ni: 0 to 0%
5%, Mo: 0 to 7%, Cu: 0 to 1%, Ti: 0 to 2
5 × (C% + N%), Nb: 0 to 25 × (C% + N%)
And a Cr, Mo and B content satisfying the following formula, and is a ferritic stainless steel material consisting of a balance of Fe and impurities. Method of manufacturing steel. 17% ≦ Cr + 3 × Mo−2.5 × B However, each element symbol in the formula indicates the content (% by mass). 4. A stainless steel material containing C: 0.0
05 to 0.2%, Si: 0.01 to 1.5%, Mn:
0.01-2.5%, P: 0.040% or less, S: 0.
01% or less, Cr: 17 to 30%, Ni: 7 to 50%,
B: 0 to 3.5%, N: 0 to 0.4%, Cu: 0 to 2
%, Al: 0 to 6%, Mo: 0 to 7%, and C
The method for producing a stainless steel material for a current-carrying part according to claim 1, wherein the content of r, Mo and B satisfies the following equation, and the austenitic stainless steel material is composed of a balance of Fe and impurities. 17% ≦ Cr + 3 × Mo−2.5 × B However, each element symbol in the formula indicates the content (% by mass).