WO2011053041A2

WO2011053041A2 - Ferritic stainless steel for solid oxide fuel cells, and connection material using same

Info

Publication number: WO2011053041A2
Application number: PCT/KR2010/007520
Authority: WO
Inventors: 김규영; 김도형; 전재호; 서형석; 윤대원
Original assignee: 포항공과대학교 산학협력단
Priority date: 2009-10-30
Filing date: 2010-10-29
Publication date: 2011-05-05
Also published as: WO2011053041A3

Abstract

The present invention relates to stainless steel, comprises no expensive rare earth elements which do not enable the easy manufacture of steel, wherein the stainless steel of the present invention has high oxidation resistance and high electrical conductivity, and simultaneously has a low speed of volatilization of chromium (Cr) at a high temperature, and therefore is suitable for a connection material for fuel cells. The stainless steel according to the present invention comprises: 20 wt % to 35 wt % of Cr; 1 wt % or less of Mn; 0.3 wt % to 5 wt % of Nb, with the remainder being Fe and inevitable impurities.

Description

Ferritic stainless steel for solid oxide fuel cell and connecting material using same

The present invention relates to stainless steel for fuel cells operating at high temperatures. More specifically, a solid oxide fuel cell that operates at high temperatures because it can obtain excellent oxidation resistance and electrical conductivity without containing rare earth elements such as La and Y, which are expensive and difficult in steelmaking, and have low high temperature Cr volatility. It can be used as a material for (SOFC: Solid Oxide Fuel Cell), and in particular, it relates to a ferritic stainless steel (ferritic stainless steel) suitable for the connection material of Planar Solid Oxide Fuel Cells (PSOFC).

In general, a fuel cell is a power generation device that generates electrical energy from hydrogen energy. Such fuel cells are phosphate type (PAFC; Phosphoric Acid Fuel Cell), molten carbonate type (MCFC; Molten Carbonate Fuel Cell), solid oxide type (SOFC), and polymer electrolyte membrane fuel cells. PEMFC; Polymer Electrolyte Membrane Fuel Cell (PEMFC), and its operating temperature varies depending on the type of fuel cell.The solid oxide type is about 1,000 ℃, the molten carbonate type is about 650 ℃, the phosphate type is about 200 ℃, and the polymer electrolyte type. It is about 100 degrees C or less.

The dual SOFC has the advantage of high power generation efficiency by operating at the highest temperature, and is divided into tubular SOFC and flat SOFC. In the case of the dual flat SOFC, the manufacturing process is simpler than the tubular SOFC, which is advantageous for commercialization and large capacity.

On the other hand, the main components of the plate-type SOFC is composed of a unit cell consisting of a solid oxide electrolyte and an electrode and a connecting material for forming a stack by connecting the unit cells.

The SOFC connection material is a material that electrically connects and separates each unit cell and serves as a passage between fuel and air supplied to each unit cell, and has excellent stability at high temperature, high electrical conductivity, and a unit of thermal expansion coefficient. Properties that must be similar to the coefficient of thermal expansion of a cell are required. The connecting material may also be termed 'bipolar plate' or 'separator', depending on the function of the fuel cell.

As a material of the SOFC connector, conventionally, a solid oxide having excellent stability at high temperature was used, but the solid oxide connector has a weak mechanical strength, a high production cost, and difficult processing.

In order to overcome such drawbacks of the solid oxide connecting material, attempts have been made to utilize a metal material having excellent workability and low production cost as an SOFC connecting material.

Among metal materials, ferritic stainless steel is cheaper than other metal materials, and its coefficient of thermal expansion is similar to that of unit cells.However, due to the formation of an oxide layer at high temperatures, the increase of electrical resistance and the volatilization of chromium (Cr) on the surface. There is a disadvantage of contaminating the electrode.

In order to solve this problem, the ferritic stainless steel developed for the SOFC connector has been added to the rare earth element such as lanthanum (La) and yttrium (Y) to reduce the growth rate of the oxide layer and improve the electrical conductivity. However, rare earth elements such as lanthanum (La) and yttrium (Y) have been a cause for lowering the productivity and increasing the price of SOFC coupling materials because the materials themselves are expensive and rare and make steelmaking difficult.

The present invention is to solve the above-mentioned problems of the prior art, and is excellent in oxidation resistance and electrical conductivity and at high temperature without adding rare earth elements such as lanthanum (La) or yttrium (Y), which make steelmaking difficult and expensive. An object of the present invention is to provide a ferritic stainless steel for fuel cell connection material having a low volatilization rate of chromium (Cr).

Another object of the present invention is to provide a fuel cell connection material having a low cost and excellent performance compared to the existing connection material.

In order to achieve the above object, the present invention, Cr: more than 20% by weight to 35% by weight, Mn: 1% by weight or less, Nb: 0.3% to 5% by weight, for the fuel cell consisting of the remaining Fe and unavoidable impurities Provide ferritic stainless steel.

In addition, in the ferritic stainless steel for fuel cells, the content of Nb is characterized in that it is contained in more than 0.8% to 1.5% by weight.

In addition, the ferritic stainless steel for fuel cells may be further selected from C: 0.03% by weight, N: 0.03% by weight, Mo: 5.0% by weight, Cu: 3.0% by weight, or less selected from Ti, V, or Zr. The above can be contained in 0.5 weight% or less.

The present invention also provides a connecting material made of a ferritic stainless steel of the above composition. In the present invention, the 'connector' refers to a device for connecting one part to another part (electrically, mechanically, or electrically and mechanically). In addition, the connector can perform several functions in the fuel cell, for example, isolation and containment of reactant gas, and electrical connection of the cells in series by providing a low resistance path to current. The connecting material may also be referred to as a bipolar plate or a separator, depending on the function of the fuel cell.

The reason for limiting the components of the ferritic stainless steel for fuel cells according to the present invention as described above is as follows.

Cr: over 20 wt% to 35 wt%

Cr is an element necessary for securing basic corrosion resistance as ferritic stainless steel, and when Cr content is 20% by weight or less, the corrosion resistance required as a fuel cell connecting material is not sufficient. In addition, when Cr content exceeds 35 weight%, the sigma phase which is a secondary phase precipitates, and the characteristic of a material falls. Therefore, it is necessary to maintain Cr content in the range of 20 weight%-35 weight%.

Mn: 1.O weight% or less

Mn is a component having the effect of reducing S that is mixed with inevitable impurities and solid solutioned to ferritic stainless steel, and suppresses grain boundary segregation of S and is produced by S during hot rolling of stainless steel. It is an effective element to prevent cracking and can be added as necessary. On the other hand, such an effect can be exerted when added in an amount of 0.001% by weight or more, and when Mn exceeds 1.0% by weight, since the oxidation rate is greatly increased, Mn is most preferably added in the range of 0.001 to 1.0% by weight. desirable.

Nb: 0.3-5 wt%

Since Nb reacts with C and N in stainless steel to form carbonitrides to fix C and N, it is known to prevent deterioration of corrosion resistance due to Cr carbonitride precipitation and to improve the press formability of ferritic stainless steel. It is an ingredient.

However, the present inventors confirmed that when a large amount of Nb is added, an effect of improving oxidation resistance and lowering of high temperature Cr volatilization speed, which is not known in the art, can be obtained. This effect is not clear, but the presence of Nb precipitated or segregated in the form of carbonitride at the interface between the oxide layer and the alloy, thereby slowing down the volatilization of Cr while making the oxidation mechanism of Cr slow. It is presumably because it acts to block.

When the content of Nb is less than 0.3% by weight, it is difficult to sufficiently obtain the above effects, and when the content of Nb is more than 5% by weight, the effect is saturated, so it is preferably contained in the range of 0.3% by weight to 5% by weight.

In addition, in order to increase the oxidation resistance and electrical conductivity and lower the high temperature Cr volatilization rate, it is most preferable that Nb is added in the range of more than 0.8% by weight to 1.5% by weight, which is not the case when Nb is added in excess of 0.8% by weight. Compared to the case, the electrical conductivity may be further improved, and when Nb is added in an amount of 1.5 wt% or less, the high temperature Cr volatilization rate may be lower than that in the case where it is added in excess of 1.5 wt%. This may be due to the phenomenon that the concentration of Nb that precipitates or segregates at the interface between the oxide layer and the alloy is rather reduced compared to the case where Nb is added in excess of 1.5% by weight.

C: 0.03 wt% or less, N: 0.03 wt% or less

C and N react with Cr in the ferritic stainless steel for fuel cells to precipitate as Cr carbonitride at grain boundaries, thereby degrading the corrosion resistance of the stainless steel. Therefore, it is preferable to keep content of C and N low at 0.03 weight% or less, and it is more preferable to keep it at 0.015 weight% or less, respectively.

Mo: 5.0 weight% or less

Mo is an effective element for suppressing the corrosion of ferritic stainless steel and can be added as necessary. However, when added in excess of 5.0% by weight, the brittleness of the stainless steel is sharply increased, making steelmaking difficult, so it is added in less than 5.0% by weight, more preferably in the range of 0.1 to 3.0% by weight.

Cu: 3.0 wt% or less

Cu may be optionally added to improve the corrosion resistance of the ferritic stainless steel, and when added in excess of 3.0% by weight, the hot workability is lowered, so the amount may be added in an amount of 3.0% by weight or less, and more preferably in an amount of 2.0% by weight or less. do.

At least one selected from Ti, V or Zr: 0.5 wt% or less

Ti, V, and Zr react with C and N in stainless steel to form carbonitrides, thereby improving the formability of stainless steel. Therefore, if necessary, Ti, V, and Zr may be added so that the sum of the three components is 0.5% by weight or less. More preferably at most 0.3 wt.

At least one selected from rare earth elements or elements such as Si, Ti, Al, Mg, and Ca having higher oxygen affinity than Cr: 0.1 wt% or less

If the rare earth element or the element having higher oxygen affinity than Cr is added at 0.1 wt% or less, it may be added when additional oxidation resistance is required by increasing the oxidation resistance of the material.

Inevitable impurities

In the stainless steel according to the present invention, impurities that may be inevitably incorporated in the steelmaking process may exist regardless of the improvement in oxidation resistance and electrical conductivity required for fuel cell ferritic stainless steel and a decrease in high temperature Cr volatilization rate. For example, Si used for deoxidation in the solvent step of stainless steel may contain 0.1 wt% or less. Other S may also be contained in an amount of 0.1 wt% or less, more preferably 0.01 wt% or less.

Stainless steel for fuel cells and a fuel cell connecting material using the same according to the present invention can be expected the following effects.

First, since the ferrite stainless steel is used, it is possible to enjoy the unique effects of the ferritic stainless steel, that is, the manufacturing is easy, the manufacturing cost is low, and the difference in coefficient of thermal expansion with the ceramic constituting the unit cell is not great.

Second, the ferritic stainless steel according to the present invention is expensive and does not use rare earth elements such as lanthanum (La) or yttrium (Y), which makes steelmaking difficult, so that mass production is easier and can be manufactured at a lower cost than before. do.

Third, the ferritic stainless steel according to the present invention can not only obtain excellent oxidation resistance and electrical conductivity comparable to that of expensive lanthanum (La) or yttrium (Y), but can also lower the volatilization of Cr at high temperatures. .

1 is an evaluation of the oxidation resistance of the stainless steels prepared according to the Examples and Comparative Examples of the present invention shows the mass change amount of the steel in 800 ℃ air.

FIG. 2 is an evaluation of electrical resistance in order to compare the electrical conductivity of stainless steels manufactured according to Examples and Comparative Examples of the present invention, and the surface contact resistance (ASR) after oxidizing for 100 hours in 800 ° C. air. It shows the result of measuring.

Figure 3 shows the results of measuring the amount of Cr volatilized for 24 hours in 800 ℃ air as evaluating the Cr volatilization rate of the stainless steel prepared according to the Examples and Comparative Examples of the present invention.

Hereinafter, with reference to the accompanying drawings will be described in detail with respect to the manufacturing process of the ferritic stainless steel for fuel cells according to the embodiment of the present invention, and the results of the characteristics evaluation of the manufactured stainless steel, the present invention is limited to the following examples no. Therefore, it will be apparent to those skilled in the art that the present invention may be variously modified without departing from the technical spirit of the present invention.

Manufacture of Ferritic Stainless Steel for Fuel Cell

As a method of manufacturing the ferritic stainless steel according to the embodiment of the present invention, all known stainless steel manufacturing methods may be used, and are not limited to a specific method.

The present inventors prepared a stainless steel having a composition as shown in Table 1, by comparing each characteristic, and evaluated whether the stainless steel according to the embodiment of the present invention is suitable for the fuel cell connecting material, the specific manufacturing method Same as

Table 1

	Cr (wt.%)	Mn (wt.%)	Nb (wt.%)	Ti (wt.%)	La (wt.%)	Y (wt.%)
Example 1	21	0.5	One	-	-	-
Example 2	21	0.5	4
Example 3	21	0.5	0.3
Comparative Example 1	21	0.5	-	-	-	-
Comparative Example 2	21	0.5	-	One	-	-
Comparative Example 3	21	0.5	-	-	0.06	-
Comparative Example 4	21	0.5	-	-	-	0.05

Example 1

In Example 1 of the present invention, by using a vacuum induction melting method, an alloy of the composition shown in Table 1 was made, the prepared alloy was subjected to homogenization treatment at 1200 ℃ for at least 24 hours and then quenched and cut to a thickness of 2 mm A stainless steel sheet was prepared.

Example 2

Prepared in the same manner as in Example 1, while maintaining the content of Cr and Mn in the same manner as in Example 1, 4% by weight of Nb was added.

Example 3

Prepared in the same manner as in Example 1, while maintaining the same content of Cr and Mn, 0.3% by weight of Nb was added.

Comparative Example 1

Prepared in the same manner as in Example 1, in order to compare with the addition of Nb, while maintaining the same content of Cr and Mn, a stainless steel sheet was prepared without adding Nb.

Comparative Example 2

Prepared by the same process as in Example 1 of Comparative Example 2, in order to compare with the addition of Nb, to maintain the same content of Cr and Mn, by adding 1% by weight of Ti instead of Nb stainless steel sheet Prepared.

Comparative Example 3

Prepared in the same manner as in Example 1, in order to compare with the addition of Nb, the contents of Cr and Mn were kept the same and 0.06% by weight of the rare earth element La was added.

[Comparative Example 4]

Prepared in the same manner as in Example 1, in order to compare with the addition of Nb, the contents of Cr and Mn were kept the same, and the rare earth element Y was added by 0.05% by weight.

Oxidation Resistance Evaluation

The oxidation resistance evaluation of the stainless steel specimens prepared as described above was performed by measuring the change in weight while oxidizing in an air atmosphere of 800 ° C. similar to the operating environment of the SOFC fuel cell, and the result was as shown in FIG. 1.

As confirmed in FIG. 1, when 0.3 to 4% by weight of Nb was added to the ferritic stainless steel according to the embodiment of the present invention, it was found that oxidation resistance was greatly improved as compared with Comparative Example 1 without adding Nb. Can be.

In addition, compared with Example 1, in the case of Comparative Example 2 in which 1% by weight of Ti was added instead of Nb, oxidation resistance was inferior to that of Examples of the present invention.

And the Example of this invention shows the oxidation resistance equivalent to the comparative example 3 which adds the rare earth element La to stainless steel, and the comparative example 4 which added Y.

High Temperature Electrical Conductivity Evaluation

In addition, in order to evaluate the high-temperature electrical conductivity of the prepared specimens, the specimens of the above composition were oxidized in air at 800 ° C. for 100 hours, similar to the operating environment of the fuel cell, and then sampled using a two-probe galvanodynamic polarization method. The contact resistance of the alloy was measured under a condition of 1 cm 2 and the scan speed of 10 mV, and the result was as shown in FIG. 2.

FIG. 2 is a comparison of the area contact resistance (ASR) measured in order to compare the electrical conductivity of the examples of the present invention and the comparative example. When Nb is added to ferritic stainless steel by 1% by weight, Nb is Compared with Comparative Example 1 not added, the electrical conductivity was significantly increased, and Example 1 with Nb showed similar electrical conductivity with Comparative Example 3 and Comparative Example 4 with La and Y added.

In contrast, in Comparative Example 2 in which Ti was added instead of Nb, the electrical conductivity was significantly reduced. Example 2, in which 4% by weight of Nb was added, and Example 3, in which 0.3% by weight of Nb was added, showed similar electrical conductivity to Comparative Example 1.

High Temperature Cr Volatilization Rate Evaluation

In order to evaluate the high temperature Cr volatilization rate of the prepared specimens, the above specimens were oxidized in air at 800 ° C. for 24 hours, and the Cr volatilization was measured by collecting Cr gas, and wet air of 98% RH was tested. It was used in, the result was as shown in FIG.

Comparing the Cr volatilization rate of the example of FIG. 3 with the comparative example, when Nb was added to the ferritic stainless steel, the Cr volatilization rate was reduced compared to Comparative Example 1 without adding Nb.

In addition, in Examples 1, 2, and 3 to which Nb was added, Cr volatilization rates were lower than those of Comparative Example 3 and Comparative Example 4, which were added with La and Y.

In addition, Example 1, in which 1% by weight of Nb was added, showed a Cr volatilization rate similar to that of Comparative Example 2 in which Ti was added instead of Nb.

As confirmed by the above evaluation results, when 0.3 to 4% by weight of Nb was added to the ferritic stainless steel according to the present invention, oxidation resistance was improved and Cr volatilization rate was decreased.

In particular, when 1% by weight of Nb was added, not only did they exhibit oxidation resistance and electrical conductivity similar to those of 0.06% by weight of La or 0.05% by weight of Y, but the Cr volatilization rate at high temperature was La. Or lower than the addition of Y.

On the other hand, when Ti is added up to 1% by weight instead of Nb, the volatilization rate of Cr is similar but the electrical conductivity and oxidation resistance are deteriorated.

Claims

Cr: over 20% to 35% by weight,

Mn: 1 wt% or less,

Nb: 0.3% to 5% by weight,

Ferritic stainless steel for fuel cells, consisting of the remaining Fe and unavoidable impurities.
The method of claim 1,

Nb is more than 0.8% by weight to ferrite-based stainless steel for fuel cells, characterized in that it comprises 1.5% by weight.
The method according to claim 1 or 2,

Add to

C: 0.03% by weight or less,

N: 0.03% by weight or less,

Mo: 5.0 weight% or less,

Cu: 3.0 wt% or less,

0.5 wt% or less of at least one selected from Ti, V or Zr,

At least one selected from the group consisting of Si, Ti, Al, Mg, and Ca having an oxygen affinity higher than that of rare earth elements or Cr: 0.1 wt% or less of ferritic stainless steel for fuel cells.
A connecting material for a solid oxide fuel cell made of the steel according to claim 1.