JP2010209405A - Stainless steel having excellent surface electric conductivity, and method for manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide stainless steel having high surface electric conductivity in a solid state, and mass-producible. <P>SOLUTION: The stainless steel contains Mo in a coating film on a surface of base steel. In the Mo content in the coating film, the ratio of Mo occupied in Cr, Al, Si, Fe and Mo by the X-ray photoelectron spectroscopy analytical method (XPS) is ≥10 atom%, the Mo content is the index indicating the ratio of Mo and MoO<SB>2</SB>out of Mo component present in the coating film. The stainless steel has excellent surface electric conductivity in which the value A by the formula: =(hMo+hMoO<SB>2</SB>)/(hMo+hMoO<SB>2</SB>+hMoO<SB>3</SB>) is ≥0.3, where hMo, hMoO<SB>2</SB>and hMoO<SB>3</SB>denote the peak intensity attributable to the bond energy of Mo, MoO<SB>2</SB>and MoO<SB>3</SB>in the XPS spectrum of the coating film, respectively. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a stainless steel material having a surface film with good conductivity on the surface of a steel substrate and a method for producing the same.

  Stainless steel is a material that maintains excellent corrosion resistance by having a thin protective oxide film (passive film) mainly composed of chromium oxide / hydroxide on the steel substrate surface. However, since this passive film has high electrical resistance, it has been difficult to apply stainless steel as it is for applications such as fuel cell-related parts and electrical contacts that require materials with low surface contact resistance. there were. In these conductive member applications, attempts have been made to apply stainless steel in which Sn, Ni, Au, Pt, carbon, etc. are coated on the surface of the steel substrate by means such as electroplating or physical vapor deposition. However, these methods involve complicated processes and increase in manufacturing costs, and there are various problems in industrial mass production.

  On the other hand, in order to improve the corrosion resistance of stainless steel, it is effective to contain Mo as a steel component, and Mo-containing steel types represented by SUS316 have been put into practical use. In the Mo-containing steel type, Mo is contained together with Cr in the passive film, and this type of film exhibits excellent protection in a corrosive environment. However, this film also has a high electrical resistance.

Patent Document 1 discloses a technique for improving corrosion resistance by anodic and cathodic electrolysis of stainless steel with an aqueous molybdate solution. However, the surface of the stainless steel obtained by this method is covered with MoO ₃ exhibiting semiconducting properties, and no improvement in surface electrical conductivity is observed.

  Patent Document 2 discloses a technique for improving the surface electrical conductivity of stainless steel by depositing a hydrophilic electrical conductive coating on the stainless steel substrate. An example is shown in which silicon dioxide is used as the hydrophilic substance and gold particles are used as the conductive particles. As other hydrophilic substances, many metal oxides including titanium dioxide are listed, and molybdenum dioxide is also described therein. However, the method of Patent Document 2 uses a sol-gel method or the like to deposit a hydrophilic substance on the surface of a stainless steel substrate, and does not modify the passive film itself. For this reason, the handling which does not peel off a surface deposit is requested | required. In addition, mass production using a continuous line is difficult and the manufacturing cost is high.

Japanese Patent Laid-Open No. 8-176891 JP 2008-21647 A

Stainless steel is composed of a metal matrix and a passive film formed on the surface of the metal matrix. In this specification, the portion of the metal matrix is called a “steel substrate”. Moreover, what consists of "steel substrate + the said film | membrane" which has a film formed by modifying a passive film or a passive film on the steel substrate may be called "solid stainless steel".
An object of the present invention is to provide a stainless steel material that can be mass-produced and exhibits high surface electrical conductivity while remaining pure.

The above object is achieved by a stainless steel material having a coating on which a Mo component is present in the form of Mo or MoO ₂ on a steel substrate. Mo and MoO ₂ have electrical conductivity, which greatly reduces the electrical resistance of the passive film which is inherently poor in electrical conductivity.

That is, in this invention, it is a stainless steel material containing Mo in the film on the surface of the steel substrate, and occupies the five elements of Cr, Al, Si, Fe, and Mo by X-ray photoelectron spectroscopy (XPS) in the film. The Mo content is 10 atomic% or more and is an index representing the relative abundance ratio of Mo and MoO _{2 in} the Mo components (Mo, MoO ₂ , MoO ₃ here) present in the film. A stainless steel material excellent in surface electrical conductivity having an A value in the formula (1) of 0.3 or more is provided.
A value = (hMo + hMoO ₂ ) / (hMo + hMoO ₂ + hMoO ₃ ) (1)
Here, hMo, hMoO _2, and hMoO ₃ are Mo, MoO _2, and MoO _{3 in} the XPS spectrum of the Mo3d5 / 2 orbit for the film (the horizontal axis indicates the binding energy and the vertical axis indicates the photoelectron intensity), respectively. It is a peak intensity resulting from the binding energy.

FIG. 1 schematically illustrates a typical MgKα ray-excited Mo3d5 / 2 orbital XPS spectrum of the coating observed in a stainless steel material having a Mo-containing coating. Here, the peaks corresponding to Mo, MoO ₂ and MoO ₃ are referred to as “Mo peak”, “MoO ₂ peak” and “MoO ₃ peak”, respectively. These peak positions vary slightly depending on the structure of the film. Note that the MoO ₃ peak on the high energy side of due to Mo3d5 / 2 orbital (the left side in the figure), a peak attributable to Mo3d3 / 2 orbital is observed.
The peak intensities hMo, hMoO ₂ and hMoO ₃ are obtained as follows.

Let P ₀ be a point on the XPS spectrum curve at a binding energy of 239 eV corresponding to the high energy side (left side of the figure) base of the peak due to the Mo3d3 / 2 orbit. Of the straight line passing through the point P _0, than MoO ₃ peak lower energy not intersect the XPS spectrum curve (hereinafter simply referred to as "curve") in (the right side in the figure), and than MoO ₃ peak on the low energy side (FIG. assuming a straight line on the right side) in contact with the curve, to the contact point between P _1. In the example of FIGS. 1B and 1C, a straight line having a contact at a valley portion that passes through the point P ₀ and is located on the right side of the MoO ₃ peak intersects with the curve on the right side. Such a straight line is not adopted as a straight line for determining P ₁ . If P ₁ is Sadamare, MoO ₃ peaks present in the range of binding energy Field of the line segment P ₀ P _1, the MoO ₂ peak and Mo peak, determine the peak height relative to the line segment P ₀ P _1, The values (cps) are hMoO ₃ , hMoO ₂ and hMo, respectively. In the example of FIG. 1A, hMoO ₃ is determined based on the line segment P ₀ P _1. Similarly, in the example of FIG. 1B, hMoO ₃ and hMoO ₂ are determined, and in the example of FIG. 1C, hMoO ₃ and hMo are determined. .

When the MoO ₂ peak or Mo peak is present on the lower energy side (right side in the figure) than the line segment P ₀ P ₁ , it has the contact Q ₀ in the valley to which the point P ₁ belongs, and the Mo peak (Mo peak). Is not observed, a straight line having a contact point Q ₁ is assumed in the valley on the low energy side (right side) of the MoO ₂ peak). Then, for the MoO ₂ peak and the Mo peak existing in the range of the binding energy to which the line segment Q ₀ Q ₁ belongs, the peak height with respect to the line segment Q ₀ Q ₁ is obtained, and the value (cps) is set to hMoO _2. And hMo. In the example of FIG. 1A, hMo is determined based on the line segment Q ₀ Q ₁ .

In the example of FIGS. 1A and 1C, no peak corresponding to MoO ₂ is observed, so hMoO ₂ is set to zero. Similarly, in the example of (b), no peak corresponding to Mo is observed, so hMo is set to zero. The value A is determined by substituting the values of hMo, hMoO ₂ , and hMoO ₃ thus obtained into the above equation (1). In the example of FIG. 1, the value A satisfies 0.3 or more in the cases (b) and (c).

“Stainless steel” is steel having a Cr content of 10.5% or more as described in JIS G0203: 2000, number 4201, and various steel types are applicable.
For example, the composition range of the ferritic steel grade is as follows: mass: C: 0.15% or less, Si: 1.2% or less, Mn: 1.2% or less, P: 0.04% or less, S : 0.03% or less, Ni: 0.6% or less, Cr: 10.5 to 35%, Mo: 0 to 3%, Cu: 0 to 1%, Nb: 0 to 1%, Ti: 0 to 1 %, Al: 0 to 0.2%, N: 0.025% or less, B: 0 to 0.01%, the balance Fe and unavoidable impurities.

  The composition range of the steel base of the austenitic steel grade is, in mass%, C: 0.15% or less, Si: 4% or less, Mn: 2.5% or less, P: 0.045% or less, S: 0 0.03% or less, Ni: 6 to 28%, Cr: 15 to 26%, Mo: 0 to 7%, Cu: 0 to 3.5%, Nb: 0 to 1%, Ti: 0 to 1%, Al : 0 to 0.1%, N: 0.3% or less, B: 0 to 0.01%, and a composition range in which the balance is Fe and inevitable impurities.

As a method of producing a stainless steel material having excellent surface electrical conductivity, a step of forming a MoO ₃ -containing film on the surface by cathodic electrolysis of a stainless steel material having a passive film in an aqueous solution containing molybdate ions (cathodic electrolysis) Process),
By heat-treating the stainless steel material having the MoO ₃ containing film in a reducing gas atmosphere such as hydrogen gas, a part of MoO _{3 in} the film was reduced, and one or more of Mo and MoO ₂ were contained. Process (reduction process) to form a film,
A manufacturing method is provided. In this case, it is preferable to use a reducing gas whose dew point D (° C.) and temperature T (° C.) satisfy the following formulas (2) and (3).
0.105T-115 ≦ D ≦ 30 (2)
250 ≦ T ≦ 1300 (3)

  ADVANTAGE OF THE INVENTION According to this invention, it became possible to provide the stainless steel material which improved the oxide film itself which exists in the steel base surface of stainless steel into what was excellent in electroconductivity. Since this stainless steel material exhibits excellent surface electrical conductivity as it is, it can be applied as a conductive member as it is after being formed into a predetermined shape in the same manner as a general stainless steel material. Further, this stainless steel material can be produced industrially at low cost and in large quantities using, for example, a stainless steel plate manufacturing facility. Various types of stainless steel can be selected depending on the application. Accordingly, the present invention provides a new metal material suitable for a current collector (such as a separator) and an electrical contact material of a polymer electrolyte fuel cell.

The figure which illustrated typically the XPS spectrum of Mo content coat formed on the stainless steel material surface. The figure which showed typically the process and film structure for obtaining the stainless steel material of this invention. The graph showing the relationship between A value and contact resistance.

FIG. 2 schematically shows a process and a film configuration for obtaining the stainless steel material of the present invention. (A) is the usual stainless steel which is a starting material. It has a passive film on the surface of the steel substrate. When this stainless steel is subjected to cathodic electrolysis in an aqueous solution containing molybdate ions, the Mo component in the liquid is deposited as MoO ₃ on the steel substrate (b). MoO ₃ has semiconducting properties and does not lead to an improvement in film conductivity at this stage. When the stainless steel having the MoO ₃ -containing film is heat-treated in a reducing gas atmosphere, a part of MoO ₃ is reduced to MoO ₂ or further Mo. Since Mo and MoO ₂ have electrical conductivity, stainless steel having a Mo or MoO ₂ -containing coating on the steel substrate remains pure and has surface electrical conductivity (c). In this way, the stainless steel material of the present invention is obtained.

[Mo content in film]
The passive film of stainless steel contains Cr and Fe as metal elements, and additionally contains Si, Al, Mo, etc. depending on the composition of the steel substrate. In the film obtained by applying MoO ₃ by cathodic electrolysis and then heat-treating in a reducing gas, a larger amount of Mo is present than in a passive film of a normal Mo-containing steel type. become. According to the study by the inventors, in order to realize a film excellent in surface electrical conductivity, the Mo content in the film in the five elements (Cr, Fe, Si, Al, Mo) is 10 atoms. % Or more is extremely effective. When the Mo content is less than that, the presence of an insulating oxide containing one or more of Cr, Fe, Si, and Al becomes dominant, and most of the Mo component is made conductive Mo or MoO ₂ by reduction treatment. Even when the film is constructed, the film has high insulation, and it is difficult to sufficiently improve the surface electrical conductivity. Here, the Mo content in the five elements is calculated from a quantitative value based on the integrated area of the spectrum of each element by XPS.

[Mo and MoO ₂ in the film]
In the present invention, the conductivity of the coating is improved by allowing at least one of conductive Mo and MoO ₂ to be present in the coating existing on the surface of the stainless steel. Therefore, it is necessary to ensure a sufficient amount of Mo and MoO ₂ in the film. According to the study by the inventors, the Mo element content in the film in Cr, Fe, Si, Al, and Mo as described above is set to 10 atomic% or more, and further, Mo or Mo among Mo components. It is important to increase the amount of MoO ₂ present. That is, the Mo component constituting the oxide film includes Mo, MoO ₂ , and MoO ₃ , and among them, it is necessary to reduce the existing ratio of MoO _{3 having} semiconducting properties as much as possible. As a result of detailed studies, the inventors have found that it is extremely effective to adopt the A value of the following formula (1) as an index for evaluating the relative proportion of Mo and MoO _{2 in} the Mo component present in the film. I found out.
A value = (hMo + hMoO ₂ ) / (hMo + hMoO ₂ + hMoO ₃ ) (1)
Here, hMo, hMoO ₂ and hMoO ₃ are determined from the XPS spectrum as described above.

In the case where the content of Mo element in the film in the five elements is 10 atomic% or more, when the A value is increased, the surface electrical conductivity rapidly increases when the A value approaches 0.3. In the region of 0.3 or more, excellent surface electrical conductivity is stably maintained.
In addition, the film thickness in the stainless steel material of the present invention exhibiting excellent surface electrical conductivity is about 5 to 50 nm.

[Stainless steel grade]
In the present invention, various stainless steels are applicable. Any of austenite, ferrite, martensite, ferrite + martensite, austenite + ferrite, high Mn austenite, etc. according to general stainless steel classification may be adopted.

The composition range in the case of adopting a ferrite system is, in mass%, C: 0.15% or less, Si: 1.2% or less, Mn: 1.2% or less, P: 0.04% or less, S: 0.03% or less, Ni: 0.6% or less, Cr: 10.5 to 35%, Mo: 0 to 3%, Cu: 0 to 1%, Nb: 0 to 1%, Ti: 0 to 1% , Al: 0 to 0.2%, N: 0.025% or less, B: 0 to 0.01%, remaining Fe and inevitable impurities.
The composition range of the steel base of the austenitic steel grade is, in mass%, C: 0.15% or less, Si: 4% or less, Mn: 2.5% or less, P: 0.045% or less, S: 0 0.03% or less, Ni: 6 to 28%, Cr: 15 to 26%, Mo: 0 to 7%, Cu: 0 to 3.5%, Nb: 0 to 1%, Ti: 0 to 1%, Al : 0 to 0.1%, N: 0.3% or less, B: 0 to 0.01%, and the balance is composed of Fe and inevitable impurities.
Examples of standard steel types include steel types defined in JIS G4305: 2005 and JIS G4312-1991.

[Cathode electrolysis process]
In the cathode electrolysis process, general stainless steel having the above chemical composition is used as a starting material. Cathodic electrolysis of the starting stainless steel in an aqueous solution containing molybdate ions causes MoO ₃ -based Mo oxide to precipitate on the stainless steel plate surface. As an electrolyte for supplying molybdate ions, molybdates such as ammonium molybdate and sodium molybdate can be used. The Mo content in the aqueous solution may be 0.01 to 0.5 mol / L. The adhesion amount of Mo oxide is preferably 5 × 10 ^{−3 to} 0.5 g / m ^{2 in} terms of Mo. When there is too little adhesion amount of Mo oxide, it will become difficult to make content rate of Mo element in the film | membrane occupied in the above-mentioned 5 element into 10 atomic% or more. If the amount of adhesion is too large, the amount of Mo consumed becomes excessive, which is uneconomical, and the reduction heat treatment time in the subsequent process becomes long, making it impractical. The amount of Mo oxide attached can be controlled by the amount of cathodic electrolysis.

[Reduction process]
The Mo oxide deposited on the surface by cathodic electrolysis is mainly composed of MoO ₃ having semiconducting properties, and the electrical resistance of the film remains high in this state. Therefore, by subjecting this material to heat treatment in a reducing gas atmosphere, the reduction reaction of MoO ₃ proceeds to generate conductive Mo or MoO ₂ . As the reducing gas, hydrogen gas, ammonia decomposition gas, a mixed gas of hydrogen and nitrogen, or the like can be used, and a gas of 100% hydrogen is more preferable. Although it is difficult to reduce all of MoO ₃ to Mo or MoO ₂ , it is sufficient that the A value in the formula (1) is 0.3 or more as described above.

When the dew point D (° C.) and the temperature T (° C.) satisfy the following formulas (2) and (3), a conductive film having good characteristics can be easily obtained.
0.105T-115 ≦ D ≦ 30 (2)
250 ≦ T ≦ 1300 (3)
When the dew point D is lowered, MoO ₃ is not easily reduced to Mo or MoO ₂ . When the temperature T is lowered, the progress of the reduction reaction is slow, and the heat treatment may take a long time. Further, when heat treatment is performed under conditions where the dew point D is lower than 0.105T-115, MoO ₃ is likely to sublimate, making it difficult to set the content of Mo element in the film in the above five elements to 10 atomic% or more. There is a case. It is more preferable to satisfy 0.105T-106.5 ≦ D. The heat treatment time (in-furnace time) depends on the temperature T, but the optimum condition can be found in the range of 3 to 120 seconds. In the case of a steel plate having a plate thickness of about 0.5 to 1 mm, the range may be 5 to 30 seconds.

A commercially available SUS430 ferritic stainless steel plate (Cr: 17% by mass, plate thickness 0.5 mm, No. 2D finish) was prepared, and cathodic electrolysis was performed under the following conditions.
(Cathode electrolysis conditions)
Electrolyte solution: 0.05 mol / L ammonium molybdate aqueous solution, 50 ° C.
・ Current density: 0.01 A / dm ²
・ Energization amount: 0.5 C / dm ²
Under these conditions, the Mo oxide was coated with an Mo conversion adhesion amount of 0.05 g / m ² . At this stage, most of the Mo component in the film was MoO ₃ , a slight amount of MoO ₂ was detected, and Mo was not detected (No. 17 in Table 1 described later).

Subsequently, the steel plate after the cathodic electrolysis was subjected to a reduction treatment under the following conditions (excluding No. 17).
(Reduction treatment conditions)
・ Atmospheric gas: 100% hydrogen
Dew point: -75 to 50 ° C (described in Table 1)
Temperature: 100-1300 ° C. (described in Table 1)
・ Heat treatment time: 10 seconds in furnace

The surface film of each sample thus obtained was analyzed with an X-ray source MgKα using a photoelectron spectroscopic analyzer (manufactured by CRATOS; ESCA-3400), and the binding energy of Cr, Al, Si, Fe, Mo The XPS spectrum resulting from was measured, and the Mo content (atomic%) in the five elements based on the integrated area of each spectrum was calculated. Further, from the XPS spectrum of the Mo3d5 / 2 orbit, the peak intensities hMo, hMoO ₂ and hMoO ₃ resulting from the binding energy of Mo, MoO ₂ and MoO ₃ are obtained by the above-described method, and A value = (hMo + hMoO ₂ ) / ( hMo + hMoO ₂ + hMoO ₃ ) was calculated. In addition, the value of hMo / hMoO _{3 and} the value of hMoO ₂ / hMoO ₃ were also calculated.

Further, a circular carbon paper having a diameter of 15 mm is brought into contact with the surface of the sample at a surface pressure of 1 MPa, and a DC voltage E is applied between the sample and the carbon paper so that the current density I at the contact surface is 1 A / cm ^2. The voltage E was measured by the four probe method, and the contact resistance R (mΩ · cm ² ) = E / I was determined.
These results are shown in Table 1.

As can be seen from Table 1, in all of the examples of the present invention, the Mo content in the five elements of Cr, Fe, Al, Si, and Mo in the film is 10 atomic% or more, and the A value for the film is A film containing 0.3 or more and one or more of conductive Mo and MoO ₂ among the Mo components was formed. As a result, the contact resistance stably showed a low value of 50 mΩ · cm ² or less.

In contrast, Comparative Examples No.10 is too low temperature of the reduction treatment, and because No.11 dew point reduction treatment was too high, both the A value is lowered by the residual many MoO ₃ unreduced The contact resistance was considerably high. In Nos. 12 to 16, since the dew point D is lower than 0.105T-115, the Al content in the film is high. Among these, in No. 12, although the Mo content in the film was 10 atomic% or more, the A value was lowered as a result, and the effect of reducing contact resistance was insufficient. In Nos. 13 to 16, the Mo content in the film was less than 10 atomic%, and the contact resistance was considerably high. No. 17 is a sample having a film as it is subjected to cathodic electrolysis, and it can be seen that almost all of the Mo component in the film is MoO ₃ at this stage.

  FIG. 3 shows the relationship between the A value and the contact resistance. It can be seen that when the A value is 0.3 or more, the contact resistance becomes extremely stable and low.

Claims (6)

A stainless steel material containing Mo in the coating on the surface of the steel substrate, and the Mo content in the five elements of Cr, Al, Si, Fe, and Mo by X-ray photoelectron spectroscopy (XPS) in the coating is 10 Surface electricity having an atomic value of not less than atomic% and an A value of the following formula (1) which is an index representing the relative abundance ratio of Mo and MoO _{2 in} the Mo component present in the film is 0.3 or more. Stainless steel material with excellent conductivity.
A value = (hMo + hMoO ₂ ) / (hMo + hMoO ₂ + hMoO ₃ ) (1)
Here, hMo, hMoO _2, and hMoO ₃ are Mo, MoO _2, and MoO ₃ , respectively, in the XPS spectrum of the Mo3d5 / 2 orbit of the coating (the horizontal axis represents the binding energy and the vertical axis represents the photoelectron intensity). It is a peak intensity resulting from the binding energy.   In the stainless steel material, the composition of the steel base is, by mass%, C: 0.15% or less, Si: 1.2% or less, Mn: 1.2% or less, P: 0.04% or less, S: 0.0. 03% or less, Ni: 0.6% or less, Cr: 10.5 to 35%, Mo: 0 to 3%, Cu: 0 to 1%, Nb: 0 to 1%, Ti: 0 to 1%, Al The surface electric conduction according to claim 1, which is a composition of a ferritic stainless steel type consisting of: 0 to 0.2%, N: 0.025% or less, B: 0 to 0.01%, the balance Fe and unavoidable impurities. Stainless steel with excellent properties.   The stainless steel material has a composition of steel body in mass%, C: 0.15% or less, Si: 4% or less, Mn: 2.5% or less, P: 0.045% or less, S: 0.03% Hereinafter, Ni: 6 to 28%, Cr: 15 to 26%, Mo: 0 to 7%, Cu: 0 to 3.5%, Nb: 0 to 1%, Ti: 0 to 1%, Al: 0 to 0% 2. The surface electrical conductivity according to claim 1, which is a composition of an austenitic stainless steel grade consisting of 0.1%, N: 0.3% or less, B: 0-0.01%, the balance being Fe and inevitable impurities. Excellent stainless steel material. Forming a MoO ₃ containing film on the surface by cathodic electrolysis of a stainless steel material having a passive film in a molybdate-containing aqueous solution (cathodic electrolysis process);
A process of reducing a part of MoO _{3 in} the film by heat-treating the stainless steel material having the MoO ₃ -containing film in a reducing gas atmosphere to form a film containing one or more of Mo and MoO ₂ ( Reduction process),
The manufacturing method of the stainless steel material excellent in surface electrical conductivity in any one of Claims 1-3 which has these. 5. The method for producing a stainless steel material according to claim 4, wherein in the reducing step, the reducing gas is such that a dew point D (° C.) and a temperature T (° C.) satisfy the following formulas (2) and (3).
0.105T-115 ≦ D ≦ 30 (2)
250 ≦ T ≦ 1300 (3)   The method for producing a stainless steel material according to claim 5, wherein the reducing gas is hydrogen gas.

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