JP7281929B2 - Stainless steel sheet and method for manufacturing stainless steel sheet - Google Patents

Description

TECHNICAL FIELD The present invention relates to a stainless steel plate, a method for manufacturing a stainless steel plate, and the like. More specifically, the present invention relates to a stainless steel plate suitable for use as a base material for a separator of a polymer electrolyte fuel cell, a method for producing the stainless steel plate, and the like.

Conventionally, a polymer electrolyte fuel cell (PEFC) is composed of a plurality of single cells, and the single cell is a membrane made by laminating an oxide electrode and a fuel electrode on both sides of a polymer electrolyte membrane. It has a structure in which an electrode assembly is sandwiched between two separators (conductive plates). Generally, a gas diffusion layer (eg, carbon paper) is arranged between the separator and the oxide electrode or fuel electrode.

The separator has various roles, and is required to have high durability and low surface contact resistance with the gas diffusion layer in order to increase the power generation efficiency of the PEFC.

In recent years, attempts have been made to use a metal material (eg, stainless steel plate or titanium foil) as the material for the separator. For example, a technique has been proposed to reduce the surface contact resistance of the separator by using a metal material whose surface has been electrochemically treated (see, for example, Patent Document 1).

Japanese Patent No. 6418364 (issued on November 7, 2018)

However, for example, in the technique described in Patent Document 1, since the separator base material is manufactured by further surface-treating an existing stainless steel plate, post-treatment equipment and work processes for the surface treatment are required, and the separator Substrate manufacturing costs can be high.

If a stainless steel plate that can be suitably used as a separator base material can be produced using a production line in general mass production equipment, the cost of the separator base material can be reduced.

One aspect of the present invention has been made in view of the above-described conventional problems, and an object thereof is to reduce manufacturing costs and to provide a stainless steel sheet that exhibits low surface contact resistance when a conductive thin film is formed on the surface. An object of the present invention is to provide a method for manufacturing a steel plate.

In order to solve the above problems, a stainless steel plate according to an aspect of the present invention has an arithmetic mean roughness Ra of 1 nm or more and 5 nm or less and a surface contact resistance of 200 mΩ·cm ² or less on the surface of the stainless steel plate. is characterized by

In order to solve the above problems, a method for manufacturing a stainless steel plate according to an aspect of the present invention provides a rolled material obtained by subjecting a material to be treated having a chemical composition of stainless steel to finish rolling, and adding H ₂ a bright annealing step of obtaining a stainless steel sheet by performing bright annealing at a temperature of 900° C. or higher and 1150° C. or lower in a reducing atmosphere having a partial pressure of 0.6 atm or more and a dew point of −30° C. or lower; (1) The reducing atmosphere is controlled so as to satisfy the formula.

DP≦(−7.43)×ln(%Mn)−55+12.5×P(H ₂ ) (1)
here,
DP: dew point (°C)
%Mn: Manganese concentration (% by mass) in the composition of the stainless steel
P(H ₂ ): the H ₂ partial pressure (atm).

According to one aspect of the present invention, there is provided a stainless steel sheet that can reduce manufacturing costs and has a clean surface that exhibits low surface contact resistance when a conductive thin film is formed on the surface, and a method for manufacturing the same. can do.

(a) is a flow chart showing an outline of a manufacturing process of a stainless steel plate, and (b) is a flow chart showing an outline of a process for manufacturing a separator using the stainless steel plate as a base material. (a) is an electron micrograph of the surface of the stainless steel sheet in the present embodiment after bright annealing, and (b) is a schematic representation of changes in the surface state of the material to be treated due to bright annealing and coating treatment in the present embodiment. 3 is a cross-sectional view shown in FIG. Graphs showing the results of surface analysis of samples using glow discharge optical emission spectroscopy (GDS), where (a) is a stainless steel plate in this embodiment, and (b) is a stainless steel plate in a comparative example. This is the result obtained by surface analysis as FIG. 4 is a diagram for explaining a method of measuring the surface contact resistance of a stainless steel plate in one example; Graphs showing the relationship between the dew point and hydrogen partial pressure in a reducing atmosphere during bright annealing and the presence or absence of products on the surface of a stainless steel sheet after bright annealing, wherein (a) to (d) are respectively stainless steel sheets. Mn concentration in the component composition of 0.5% by mass, 1.1% by mass, 1.5% by mass, and 1.8% by mass. (a) is a diagram showing an example of surface observation by AFM and an example of the results of measuring the arithmetic mean roughness Ra of the surface of the stainless steel plate of the example of the present invention, and (b) is a diagram showing an example of bright annealing under conditions other than the present invention. FIG. 2 is a diagram showing an example of surface observation by AFM and measurement results of surface arithmetic mean roughness Ra of a stainless steel plate of a comparative example obtained by applying heat treatment. (a) is an electron micrograph of the surface of a general stainless steel plate after bright annealing, and (b) schematically shows how the surface condition of the material to be treated changes due to general bright annealing and coating treatment. It is a cross-sectional view.

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below. The following description is for better understanding of the gist of the invention, and does not limit the invention unless otherwise specified. In addition, in the present application, "A to B" indicates A or more and B or less. Description of "%" in chemical composition means "% by mass" unless otherwise specified.

In this specification, the term "stainless steel plate" is used to include "stainless steel strip" and "stainless steel foil".

<Brief description of the findings of the invention>
In recent years, fuel cells (for example, PEFC) have been vigorously developed from the viewpoint of measures against environmental problems. On the other hand, at present, PEFCs are relatively expensive, and there is a demand for a reduction in manufacturing costs (reduction in the cost of component parts) for further spread. Therefore, it has been proposed to use a stainless steel plate for the separator base material.

A stainless steel plate has a relatively high surface contact resistance because its surface is covered with a passive film. For example, the surface contact resistance of the separator can be reduced by forming a single PEFC cell using a stainless steel plate whose surface has been roughened by subjecting it to electrolytic treatment or the like (see Patent Document 1). However, this type of technology imposes demands on the production equipment and the additional steps may lengthen the required time, thus limiting the cost reduction of the separator.

In order to reduce the surface contact resistance of the separator, a stainless steel plate (hereinafter sometimes referred to as "film-formed steel plate") having a conductive thin film (coating film) such as diamond-like carbon (DLC) formed on the surface is used. It is also known to manufacture separators by However, it has been reported that such separators may exhibit higher than expected surface contact resistance.

If a stainless steel sheet that exhibits low surface contact resistance when a coating film is formed on the surface can be manufactured using general mass production equipment, the manufacturing cost of the stainless steel sheet that can be suitably used as a separator base material can be suppressed. can do. The present inventors have made intensive studies on such a production method.

Here, an overview of a general stainless steel plate and separator manufacturing method (a series of manufacturing steps) will be described with reference to FIG. (a) of FIG. 1 is a flow chart showing an outline of a manufacturing process of a stainless steel plate. FIG. 1(b) is a flow chart showing an overview of the steps of manufacturing a separator using the above stainless steel plate as a base material. In addition, even in the method for manufacturing a stainless steel plate according to one embodiment of the present invention (hereinafter sometimes simply referred to as "this manufacturing method"), the outline of a series of manufacturing steps is the general manufacturing process for a stainless steel plate ( This is the same as (a) of FIG.

As shown in (a) of FIG. 1, in general, the method of manufacturing a stainless steel plate comprises a steelmaking process S1, a hot rolling process S2, a cold rolling process S3, an annealing and pickling process S4, a finish rolling process S5, a bright annealing process, and a (BA) including step S6.

The steelmaking step S1 is a step of melting and refining stainless steel raw materials to adjust the target chemical composition, and then pouring the molten steel into a mold to manufacture (steelmaking) a slab having the target chemical composition. In the hot rolling step S2, the slab is rolled (hot rolled) at a high temperature to form a steel plate (usually a steel strip, but hereinafter simply referred to as a steel plate in the sense of including a steel strip). ) is a step of manufacturing. In addition, the cold rolling step S3 is a step of rolling the steel sheet manufactured in the hot rolling step S2 even thinner. In the annealing/pickling step S4, the steel sheet that has been thinly rolled in the cold rolling step S3 is heated to remove strain in the material structure and soften it, and the scale on the surface of the steel sheet is removed with nitric acid and hydrofluoric acid. It is a step of pickling using a mixed solution or the like with.

The finish rolling step S5 is a step of further cold-rolling the steel plate after the annealing and pickling step S4 using smooth rolls. Thereby, the plate|board thickness of a steel plate is finally adjusted. The thickness of the steel sheet after the finish rolling step S5 is about 0.1 mm.

The bright annealing (BA) step S6 is a process of performing annealing while maintaining the surface properties of the steel plate (rolled material) obtained in the finish rolling step S5. For example, the steel sheet is annealed at a temperature of about 1000° C. in a reducing atmosphere (oxygen-free atmosphere) in which hydrogen and nitrogen are mixed.

Then, as shown in FIG. 2(b), the step of manufacturing a separator using the stainless steel plate after BA obtained in the bright annealing (BA) step S6 as a base material includes a processing step S11 and a coating step S12.

The processing step S11 is a step of processing the stainless steel plate (separator base material) after BA into a desired shape as a separator. For example, the shape of the separator may be a shape having a concave groove that serves as a channel for gas (oxygen or air), and a cross section perpendicular to the direction in which the channel extends has a rectangular wave shape.

The coating step S12 is a step of forming a conductive thin film (coating film) on the surface of the semi-finished product after the processing step S11. As this coating film, a known material can be applied, and for example, a thin film of DLC can be used. A separator is thus manufactured.

In the following description in this specification, the time after the finish rolling step S5 and before the bright annealing (BA) step S6 is before BA (time T1), and the time after the bright annealing (BA) step S6 and before the coating step S12 is BA. After (time T2), the time after coating step S12 may be referred to as after coating (time T3).

Conventionally, a film-formed steel sheet obtained by coating a stainless steel sheet after BA with, for example, DLC in the manufacturing process described above sometimes has a higher surface contact resistance than expected. The present inventors investigated this factor in detail and obtained the following findings. FIG. 7(a) is an electron micrograph of the surface of a general stainless steel plate 110 after BA.

As shown in (a) of FIG. 7, when the surface of a general stainless steel plate 110 after BA is observed using a high-resolution electron microscope, fine particulate products with a diameter of about 100 nm or less are present. However, the product was found to contain manganese (Mn) oxide. From this, it is considered that in the conventional separator manufacturing method, the surface state of the material to be treated changes as follows. FIG. 7(b) is a cross-sectional view schematically showing how the surface state of the object to be treated changes due to general BA and coating treatments.

As shown in FIG. 7B, unlike the rolled material 100 at time T1 (see FIG. 1) before BA, the stainless steel plate 110 at time T2 after BA has granular products 111 on its surface. are generating. The product 111 is generated by combining (i) Mn contained in the base material 101 of the rolled material 100 and (ii) oxygen present in a very small amount in the reducing atmosphere in the furnace of the BA. It is thought that

A conductive coating film 121 is formed so as to cover the product 111 on the film-formed steel plate 120 at time T3 after coating. The film-formed steel sheet 120 has a high surface contact resistance due to the presence of the product 111 with high electrical resistance. In addition, the coating film 121 of the film-formed steel plate 120 may have low durability. This is because the bonding strength at the interface between the coating film 121 and the product 111 may be low, and a high load may be applied to the convex portions of the coating film 121 during processing into PEFC single cells.

Based on the above findings, the present inventors conducted various studies on BA conditions, and found that it is possible to produce a stainless steel plate in which the above-described product 111 does not exist (or the product 111 has a very low density). The inventors have found the conditions under which this can be achieved, and conceived of the present manufacturing method. The conditions found by the inventors of the present invention will be described below.

<Manufacturing method of stainless steel plate>
The stainless steel sheet according to one aspect of the present invention is manufactured by controlling the conditions as follows in the bright annealing step S6 in the general stainless steel sheet manufacturing process (see the description above with reference to FIG. 1). be done. As for the other steps S1 to S5, the same processing as in the prior art may be performed, and detailed description thereof will be omitted.

In this manufacturing method, a rolled material (steel sheet at time T1) formed by performing finish rolling on a material to be treated (steel sheet after annealing and pickling step S4) having a chemical composition of stainless steel is subjected to bright annealing step S6. , the following processing is performed.

That is, in the bright annealing step S6 in this manufacturing method, the rolled material is subjected to a reduction atmosphere having a hydrogen (H ₂ ) partial pressure of 0.6 atm or more and a dew point of −30° C. or less and satisfying the following formula (1). , bright annealing is performed at a temperature of 900°C or higher and 1150°C or lower to obtain a stainless steel plate.

DP≦(−7.43)×ln(%Mn)−55+12.5×P(H ₂ ) (1)
Here, the reducing atmosphere is the atmosphere in the furnace in which bright annealing is performed. DP is the dew point (°C) in the reducing atmosphere, and "%Mn" is the manganese concentration (% by mass) in the material to be treated having the chemical composition of stainless steel. P(H ₂ ) is the hydrogen partial pressure (atm) in the reducing atmosphere.

The stainless steel plate obtained by this manufacturing method will be described below with reference to FIGS. 2(a) and 2(b). FIG. 2(a) is an electron micrograph of the surface of the stainless steel plate 10 after BA in one embodiment of the present invention.

As shown in FIG. 2(a), the stainless steel plate 10 of the present embodiment, which has been subjected to BA while satisfying the above conditions, has no particulate products containing Mn oxide on its surface, and is clean. It has a surface condition.

In this manufacturing method, the surface condition of the material to be treated changes as follows. FIG. 2(b) is a cross-sectional view schematically showing how the surface state of the object to be treated changes due to the BA and coating treatments in this embodiment.

As shown in FIG. 2(b), the rolled material 1 is manufactured at time T1 (see FIG. 1) before BA. The rolled material 1 is equivalent to the rolled material 100 in the conventional example described above. The material to be treated in the present manufacturing method (steel material manufactured in the steelmaking process S1 and supplied to each rolling process) has a chemical composition of stainless steel, specifically as follows.

The component composition of the stainless steel in the material to be treated is 11% by mass or more and 26% by mass or less of Cr, 0.1% by mass or more and 22% by mass or less of Ni, 0.1% by mass or more and 2.0% by mass or less of Mn, 0.005% by mass or more and 0.1% by mass or less of C, 0.1% by mass or more and 4.0% by mass or less of Si, 0.01% by mass or more and 5.0% by mass or less of Mo, 0.01% by mass 3.5% by mass or more of Cu, 0.001% by mass or more and 0.8% by mass or less of Nb, 0.005% by mass or more and 0.25% by mass or less of N, 0.001% by mass or more and 0.8% by mass % or less of Ti, and 0.001 mass % or more and 1.5 mass % or less of Al, with the balance being iron and unavoidable impurities.

That is, the material to be treated may have any component composition among austenitic stainless steel, ferritic stainless steel, martensitic stainless steel, and austenitic-ferritic (duplex) stainless steel.

The material to be treated may contain iron (Fe) and unavoidable impurities as the balance, and may contain other various additive elements as the balance.

The chemical composition of the material to be treated is hardly affected by various processes (rolling, annealing, working, coating, etc.), and the chemical composition of the stainless steel plate 10 is the same as the chemical composition of the material to be treated.

The rolled material 1 has a plate thickness of 0.005 mm or more and 0.2 mm or less. If the plate thickness is less than 0.005 mm, for example, the strength is insufficient to process the rolled material 1 for use as a separator, and the manufacturing cost increases, making it unsuitable for use as a separator. On the other hand, if the plate thickness exceeds 0.2 mm, the weight of the fuel cell is too heavy, and weight reduction of the fuel cell cannot be achieved. Therefore, it is unsuitable for use as a separator.

After subjecting the rolled material 1 to BA (time T2 after BA), the stainless steel plate 10 has no particulate products 111 (particulate products containing manganese oxide) on the surface of the base material 11 ( or the density of the product 111 is very low). Specifically, the surface of the stainless steel plate 10 has an arithmetic mean roughness Ra (JIS B 0601) of 1 nm or more and 5 nm or less, preferably 1 nm or more and 3 nm or less. Here, if the arithmetic mean roughness Ra is less than 1 nm, the adhesion between the base material 11 and the coating film 21 (described later) is insufficient. Also, if the arithmetic mean roughness Ra is too large, the surface contact resistance will not be sufficiently low when a DLC coating film is formed on the surface. Moreover, the durability of the coating is compromised. For the above reasons, the upper limit of the arithmetic mean roughness Ra was set to 5 nm.

The base material 11 of the stainless steel plate 10 is formed by annealing the base material 2 of the rolled material 1 with BA.

Since the stainless steel plate 10 does not have the product 111 on its surface, the surface contact resistance is lower than that of the conventional stainless steel plate 110 . Specifically, the stainless steel plate 10 has a surface contact resistance of 200 mΩ·cm ² or less, preferably 180 mΩ·cm ² or less. In addition, in this specification, the value of surface contact resistance is the value measured by the method described in the example mentioned later.

A coating film (coating layer) 21 is formed so as to cover the surface of the base material 11 in the film-formed steel plate 20 after the stainless steel plate 10 has been subjected to the coating process (time point T3). This coating film 21 is made of DLC, for example.

The process of coating the surface of the stainless steel plate 10 with DLC is performed, for example, as follows. That is, as a pretreatment, the surface of the stainless steel plate 10 is subjected to sputter cleaning using Ar ions. As a result, the passive film (thickness of about several nanometers) and fine deposits present on the surface of the base material 11 of the stainless steel plate 10 are removed to ensure the adhesion of the DLC coating. After the sputter cleaning, the stainless steel plate 10 is coated with DLC using, for example, a PVD (Physical Vapor Deposition) method. The film thickness of the DLC coated on the stainless steel plate 10 is, for example, 20 nm or more and 100 nm or less, preferably 40 nm or more and 60 nm or less.

Here, in the stainless steel plate 110 (see FIG. 7) on which particulate products 111 containing manganese oxide exist on the surface, the size of the products 111 is larger than that of the passive film. It is difficult to sufficiently remove 111. Therefore, as described above, in the film-formed steel plate 120, the DLC coating film is formed so as to cover the product 111, and the surface contact resistance increases.

According to this manufacturing method, the film-formed steel sheet 20 can form the coating film 21 flat on the base material 11 . Therefore, the adhesiveness between the base material 11 and the coating film 21 can be improved. As a result, durability of the coating film 21 can be enhanced. Also, the surface contact resistance of the film-formed steel sheet 20 can be reduced. Specifically, the film-formed steel sheet 20 has a surface contact resistance of 5 mΩ·cm ² or less, preferably 3 mΩ·cm ² or less.

FIG. 3(a) is a graph showing the results of analyzing the surface state of the stainless steel plate 10 using glow discharge optical emission spectroscopy (GDS). As shown in FIG. 3(a), the stainless steel plate 10 according to the present embodiment has a very low Mn concentration (a signal indicating the abundance of Mn element) on the surface.

On the other hand, FIG. 3(b) is a graph showing the results of analyzing the surface state of the stainless steel plate 110 in the comparative example, which was subjected to bright annealing that does not satisfy the above conditions, similarly using GDS. The Mn concentration on the surface of the stainless steel plate 110 is significantly high (Mn is concentrated on the surface).

Here, the relationship of the above-mentioned formula (1) in the bright annealing step S6 of the present manufacturing method, which the inventors found out as a result of earnest studies, will be described in more detail below.
DP≦(−7.43)×ln(%Mn)−55+12.5×P(H ₂ ) (1).

Hereinafter, the heating furnace used in the bright annealing step S6 will be referred to as a BA furnace, and the reducing atmosphere in the BA furnace will be referred to as a BA atmosphere.

The lower the dew point in the BA atmosphere, the lower the oxygen concentration in the BA atmosphere. Moreover, the higher the hydrogen concentration in the BA atmosphere, the stronger the reducing property of the BA atmosphere, and the hydrogen reacts with oxygen to lower the oxygen concentration. Therefore, if the dew point is made as low as possible and the hydrogen concentration is 100%, the generation of the product 111 after BA is suppressed. However, lowering the dew point more than necessary and increasing the hydrogen concentration increase the manufacturing cost of the stainless steel plate.

On the other hand, if BA is applied under general conditions, product 111 is readily formed. In addition, the higher the Mn concentration of the object to be treated, the more easily the product 111 is generated. Therefore, if the Mn concentration of the object to be treated is lowered, the generation of the product 111 after BA is suppressed. Therefore, it is ideal that the Mn concentration of the object to be treated is 0%. However, it is difficult to make the Mn concentration 0% for the following two reasons. (i) In general, the main raw material for stainless steel sheets is stainless steel scrap, and the scrap contains Mn. Therefore, Mn is inevitably mixed into the stainless steel plate. If the scrap is not used and only pure alloys are used as raw materials, the cost will be high. (ii) In order to ensure workability of the stainless steel sheet and to ensure manufacturability, it is usually required to contain about 1% of Mn.

Therefore, the present inventors have made various studies to specify the conditions for a BA atmosphere in which the product 111 is unlikely to occur while suppressing the additional cost that is additionally required from the production cost under general BA conditions as much as possible. , found the above equation (1).

If the dew point of the BA atmosphere becomes too high, the surface of the stainless steel plate is oxidized and colored. In this case, it is not suitable as a method for manufacturing stainless steel plates for separators. Therefore, the BA atmosphere preferably has a dew point of −30° C. or less. Also, if the hydrogen partial pressure is too low, the surface of the stainless steel sheet may be nitrided. The BA atmosphere preferably has a hydrogen partial pressure of 0.6 atm or more from the viewpoint of reducing the oxygen concentration and increasing the reducing property.

The temperature of the BA furnace is within the range of 900° C. or higher and 1150° C. or lower, for example, within the range of 950° C. or higher and 1000° C. or lower. Further, the time in the BA furnace is not particularly limited, and when the temperature of the BA furnace is the temperature described above, it may be, for example, 5 seconds to 30 seconds. It may be appropriately determined according to the processing speed in the production line of the stainless steel plate.

<Stainless steel plate>
A stainless steel plate in one aspect of the present invention is manufactured by the manufacturing method described above. The stainless steel plate has an arithmetic mean surface roughness Ra of 1 nm or more and 5 nm or less and a surface contact resistance of 200 mΩ·cm ² or less.

The chemical composition of the stainless steel plate is, for example, the components shown in Table 1 below.

C (carbon) is an element necessary for forming an austenitic structure and ensuring strength. Further, if the C content is excessively lowered, the refining cost increases, which is economically unfavorable. Therefore, the C content is made 0.005% or more. On the other hand, if the C concentration is too high, the material hardens more than necessary. Furthermore, since Cr carbide is formed and the corrosion resistance is lowered, the C content is set to 0.1% or less, preferably 0.07% or less.

Si (silicon) is an element added as a deoxidizing element in the refining process. It also has the effect of ensuring the heat resistance and high-temperature strength of the material. Therefore, the Si content should be 0.1% or more. On the other hand, if the Si concentration is too high, the material becomes brittle.

Mn (manganese) is sometimes added for the purpose of forming an austenitic structure. In addition, since stainless steel scrap is generally used as a raw material for stainless steel, Mn contained in the scrap is inevitably mixed. Since Mn in the stainless base material becomes the origin of particulate products containing MnO on the surface of the stainless steel in the BA process, a lower Mn concentration is advantageous for suppressing the products. However, in order to excessively lower the Mn concentration, it is necessary to limit the use of inexpensive scrap as a raw material, which increases raw material costs. Therefore, the Mn content is set to 0.1% or more. On the other hand, if the Mn content is excessively high, particulate products tend to form on the surface of the product.

Ni (nickel) is an element necessary for forming an austenitic structure and ensuring corrosion resistance. Therefore, the Ni content is set to 0.1%. On the other hand, since excessive addition of Ni leads to an increase in production cost, the Ni content is set to 22% or less, more preferably 10% or less.

Cr (chromium) is an element that improves corrosion resistance. Therefore, the Cr content is set to 11% or more. On the other hand, if the Cr concentration is too high, the material becomes hard and deteriorates workability. Therefore, the Cr content is set to 26% or less, more preferably 20% or less.

Mo (molybdenum) is an element that improves corrosion resistance. Therefore, Mo content shall be 0.01% or more. On the other hand, excessive addition of Mo increases the manufacturing cost. Therefore, the Mo content is 5.0% or less, preferably 2.0% or less.

Cu (copper) is an element effective in stabilizing the austenite structure and improving workability by softening the material, so the Cu content is made 0.01% or more. On the other hand, excessive addition of Cu not only increases manufacturing costs, but also leads to deterioration of manufacturability. Therefore, the Cu content is set to 3.5% or less, more preferably 1.0% or less.

Nb (niobium) combines with C and N and has the effect of improving corrosion resistance. Also, a small amount of Nb is inevitably mixed in from stainless steel scrap as a raw material. Therefore, the Nb content is set to 0.001% or more. On the other hand, excessive addition of Nb not only increases manufacturing cost but also leads to deterioration of manufacturability. Therefore, the Nb content is set to 0.8% or less, more preferably 0.4% or less.

N (nitrogen) is an element that contributes to the formation of an austenite structure and the strength improvement of the material. Moreover, if N is excessively lowered, the refining cost increases, which is not economically preferable. Therefore, the N content is set to 0.005% or more. On the other hand, if the N content is too high, the material becomes hard and the workability deteriorates. Therefore, the N content is set to 0.25% or less, more preferably 0.08% or less.

Ti (titanium) has the effect of improving corrosion resistance by combining with C and N. Also, a small amount of Ti is inevitably mixed in from stainless steel scrap as a raw material. Therefore, the Ti content should be 0.001% or more. On the other hand, excessive addition of Ti not only increases manufacturing costs, but also leads to deterioration of manufacturability. Therefore, the Ti content is set to 0.8% or less, more preferably 0.4% or less.

Al (aluminum) is added as a deoxidizing element in the refining process, and is an element that contributes to the improvement of high-temperature oxidation resistance. Also, a small amount of Al is inevitably mixed from reducing agents and the like used in the manufacturing process. Therefore, the Al content is set to 0.001% or more. On the other hand, excessive addition of Al not only increases manufacturing costs, but also leads to deterioration of manufacturability. Therefore, the Al content is set to 1.5% or less, more preferably 0.5% or less.

The arithmetic mean roughness Ra and the surface contact resistance on the surface of the stainless steel plate in one aspect of the present invention are values measured by the methods described in the examples below.

[Additional notes]
The present invention is not limited to the above-described embodiments, and can be modified in various ways within the scope of the claims. It is included in the technical scope of the invention.

A series of treatments (see (a) in FIG. 1) are performed under the conditions that satisfy and not satisfy the above formula (1) in this manufacturing method, and the stainless steel plate (after BA) having the composition and thickness shown in Table 2 below A stainless steel sheet at time T2) was obtained. Surface contact resistance measurement, deposit evaluation, and surface roughness (arithmetic mean roughness Ra) measurement were performed on each of the obtained test materials.

Further, after the surface of the test material was coated with DLC, the surface contact resistance was measured in the same manner as described below. The DLC coating treatment was carried out by subjecting the surface of the test material to a sputter cleaning treatment using Ar ions and then forming a DLC film with a film thickness of 50 to 100 nm using a general PVD method.

(Surface contact resistance measurement)
FIG. 4 is a diagram for explaining a method of measuring the surface contact resistance of a stainless steel plate in one example. As shown in FIG. 4, a stainless steel plate 50, which is a sample to be measured, is sandwiched between two sheets of carbon paper 52 and 53, and held and fixed by two holding jigs 51 from both sides. The two holding jigs 51 can apply a load F to the stainless steel plate 50 .

Each of the two holding jigs 51 is connected to a DC power supply 60 so that a current C can be passed from the carbon paper 52 to the carbon paper 53 via the stainless steel plate 50 . A voltmeter 70 is connected to the stainless steel plate 50 and the carbon paper 53 .

Using two holding jigs 51, the load F was adjusted so that the surface pressure between the stainless steel plate 50 and the carbon paper 53 was 1.0 MPa, and the electrical resistance between the stainless steel plate 50 and the carbon paper 53 was measured. bottom. The value obtained by multiplying this measured value by the area of the contact surface of the electrode was taken as the contact resistance value.

(Deposit evaluation)
The presence or absence of particles with a diameter of 10 nm or more on the surface of the stainless steel plate was determined by observation with a scanning electron microscope. The types of scanning electron microscopes and measurement conditions are described below.
・ Apparatus name: field emission scanning electron microscope SU5000 manufactured by Hitachi High-Technologies Corporation
・Measurement magnification: 50,000 times ・Acceleration voltage: 15 kV
- Field of view for measurement: 10 fields of view on the surface of the stainless steel plate were randomly selected and observed. When one or more particles with a diameter of 10 nm or more were observed in one visual field, the visual field was evaluated as "presence" of deposits. When the deposit was "presence" even in one of the ten fields of view, the sample was evaluated as "presence" of the deposit.

(Measurement of arithmetic mean roughness)
The arithmetic mean roughness Ra was measured using an atomic force microscope (AFM). The conditions of the measuring method are described below.
・Equipment name: NanoScopeIIIa manufactured by Digital Instruments
・Measurement conditions: Tapping AFM
・Scanning speed: 0.6 to 0.8 Hz
・Measurement points: 512 points ・Measurement range: 2 × 2 μm
Table 2 shows the results.

Table 2 shows the D value obtained by substituting the Mn concentration and the hydrogen partial pressure of the atmosphere in the BA furnace in each sample into the following equation (2). For each sample, the case of dew point≦D is indicated as “◯”, and the case of dew point>D is indicated as “x”.
D=(−7.43)×ln(%Mn)−55+12.5×P(H ₂ ) (2).

In all of the samples of the present invention evaluated as "good", no deposit was observed on the surface of the stainless steel plate, and the surface contact resistance was as low as 200 mΩ·cm ² or less. On the other hand, in the sample of the comparative example, which was evaluated as "x", deposits were observed on the surface of the stainless steel plate. Therefore, the sample of the comparative example had a surface contact resistance exceeding 200 mΩ·cm ² . Furthermore, the surface contact resistance of each stainless steel plate after applying the DLC coating was measured. As a result, the surface contact resistance was 3.8 or less for all the samples of the present invention that were judged to be "good".

The results shown in Table 2 are graphed and explained with reference to FIG. 5 as follows.

FIG. 5 is a graph showing the relationship between the dew point and hydrogen partial pressure in the reducing atmosphere during BA and the presence or absence of products on the surface of the stainless steel plate after BA, in which (a) to (d) are respectively The cases where the Mn concentration in the chemical composition of the stainless steel plate is 0.5% by mass, 1.1% by mass, 1.5% by mass, and 1.8% by mass are shown.

That is, (a) of FIG. Corresponding to samples 6 to 10, the dotted line indicated by F1 in the figure represents the following formula (3).
D[Mn:0.5]=(−7.43)×ln(0.5)−55+12.5×P(H ₂ ) (3).

When the Mn concentration is 0.5% by mass, if the dew point and hydrogen partial pressure of the BA atmosphere are adjusted so as to be included in the region below the dotted line indicated by F1 in the graph shown in FIG. good.

Then, as shown in (b) to (d) of FIG. 5, the higher the Mn concentration in the chemical composition of the stainless steel plate, the more appropriate the positions of the dotted lines indicated by F2 to F4 in the figure (that is, the dew point and hydrogen partial pressure). range) changes.

FIG. 6(a) is a view showing an example of surface observation by AFM and measurement results of surface arithmetic mean roughness Ra of a stainless steel plate of an example of the present invention. FIG. 6(b) shows an example of surface observation by AFM and measurement results of surface arithmetic mean roughness Ra of a comparative stainless steel sheet obtained by bright annealing under conditions outside the present invention. It is a diagram.

As shown in FIG. 6(a), the stainless steel plate of the example of the present invention has a very clean surface because the formation of deposits on the surface is greatly suppressed. In addition, since the stainless steel sheets of the present invention have such a surface state without being mechanically polished, they do not have polishing marks (polishing marks) on the surface. Further, for example, if electrolytic polishing treatment is performed, the value of the arithmetic mean roughness Ra can become larger than that of the stainless steel plate of the example of the present invention. On the other hand, as shown in FIG. 6(b), the stainless steel plate of the comparative example has a large amount of deposits on the surface, and the surface arithmetic mean roughness Ra greatly exceeds 5 nm.

As described above, by controlling the reducing atmosphere in the BA furnace so as to satisfy the relationship of formula (1) determined corresponding to the Mn concentration in the chemical composition of the stainless steel sheet, the surface is clean. A stainless steel plate having a low surface contact resistance can be produced when a conductive thin film is formed on the surface.
DP≦(−7.43)×ln(%Mn)−55+12.5×P(H ₂ ) (1).

In addition, according to this production method, the dew point and hydrogen partial pressure of the BA atmosphere may be determined within the range of formula (1), and excessively lowering the dew point and increasing the hydrogen partial pressure are suppressed. be done. As a result, it is possible to suppress the manufacturing cost and manufacture a stainless steel plate that can be suitably used as a separator base material for a PEFC (Polymer Electrolyte Fuel Cell).

1 Rolled material 10 Stainless steel plate 20 Film-formed steel plate 21 Coating film

Claims (5)

The arithmetic mean roughness Ra on the surface of the stainless steel plate is 1 nm or more and 5 nm or less,
The stainless steel plate is a bright annealed material,
The surface contact resistance was measured by fixing the stainless steel plate sandwiched between the first carbon paper and the second carbon paper and sandwiching it from both sides with two pressing jigs. A voltmeter is connected to the stainless steel plate and the second carbon paper while a load is applied using the two holding jigs so that the surface pressure between the stainless steel plate and the second carbon paper is 1.0 MPa. A value obtained by multiplying the measured value of the electrical resistance between the second carbon paper and the area of the contact surface of the electrode,
A stainless steel plate, wherein the surface contact resistance is 200 mΩ·cm ² or less. A rolled material obtained by performing finish rolling on a material to be treated having a chemical composition of stainless steel is subjected to a heat treatment at a temperature of 900°C or higher and 1150°C or lower in a reducing atmosphere having a _H2 partial pressure of 0.6 atm or higher and a dew point of -30°C or lower. including a bright annealing step of obtaining a stainless steel sheet by subjecting it to bright annealing at a temperature;
In the bright annealing step, the reducing atmosphere is controlled so as to satisfy the following formula (1) ,
No particulate product containing manganese oxide is formed on the surface of the stainless steel plate, or the arithmetic mean roughness Ra is 1 nm or more and 5 nm or less,
The surface contact resistance was measured by fixing the stainless steel plate sandwiched between the first carbon paper and the second carbon paper and sandwiching it from both sides with two pressing jigs. A voltmeter is connected to the stainless steel plate and the second carbon paper while a load is applied using the two holding jigs so that the surface pressure between the stainless steel plate and the second carbon paper is 1.0 MPa. A value obtained by multiplying the measured value of the electrical resistance between the second carbon paper and the area of the contact surface of the electrode,
A method for producing a stainless steel plate , wherein the surface contact resistance is 200 mΩ·cm ² or less .
DP≦(−7.43)×ln(%Mn)−55+12.5×P(H ₂ ) (1)
(here,
DP: dew point (°C)
%Mn: Manganese concentration (% by mass) in the composition of the stainless steel
P (H ₂ ): the H ₂ partial pressure (atm)) The chemical composition of the stainless steel is 11% by mass or more and 26% by mass or less of Cr, 0.1% by mass or more and 22% by mass or less of Ni, 0.1% by mass or more and 2.0% by mass or less of Mn, and 0.005 C, 0.1 mass % to 4.0 mass %, Mo, 0.01 mass % to 5.0 mass %, 0.01 mass % to 0.01 mass %3. 5 mass% or less of Cu, 0.001 mass% or more and 0.8 mass% or less of Nb, 0.005 mass% or more and 0.25 mass% or less of N, 0.001 mass% or more and 0.8 mass% or less of 3. The method for producing a stainless steel plate according to claim 2, comprising Ti and 0.001% by mass or more and 1.5% by mass or less of Al, with the balance being iron and unavoidable impurities. 4. The method for producing a stainless steel plate according to claim 2, wherein the rolled material has a plate thickness of 0.2 mm or less. The method for producing a stainless steel plate according to any one of claims 2 to 4 , wherein the stainless steel plate is a separator base material for polymer electrolyte fuel cells.

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