KR20180106143A - Ferritic stainless steel with high oxidation resistance and fuel cell interconnector using the same and fuel cell - Google Patents

Abstract

The present invention provides a ferritic stainless steel excellent in oxidation resistance, processability and chromium volatilization inhibiting ability. The ferritic stainless steel according to one embodiment of the present invention includes: 10-30 wt% of chromium (Cr); 0.1-2.0 wt% manganese (Mn); 0.01-0.5 wt% titanium (Ti); 0.001-0.3 wt% aluminum (Al); 0.3-0.7 wt% of lanthanum (La); 0.004-0.012 wt% of boron (B); 0.001-0.2 wt% carbon (C); 0.001-0.1 wt% nitrogen (N); and the remainder includes iron (Fe) and unavoidable impurities.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a ferritic stainless steel, and a fuel cell connector using the ferritic stainless steel and a fuel cell using the ferritic stainless steel,

TECHNICAL FIELD OF THE INVENTION The present invention relates to ferritic stainless steels, and more particularly, to oxidation resistant ferritic stainless steels, fuel cell interconnectors using the ferritic stainless steels, and fuel cells.

In general, ferritic stainless steel is a stainless steel containing 10 wt% or more of chromium (Cr), which is inexpensive as compared with austenitic stainless steel, and does not cause stress corrosion cracking due to chloride, Is the material that is increasing. Ferritic stainless steels can be used in high temperature environments such as power plant boilers and pipe materials, automobile exhaust system members, or fuel cell accessors. In this case, high heat resistance and oxidation resistance are required.

Particularly, when ferritic stainless steel is used for a fuel cell connector, it functions as a separator for separating the fuel and air supplied from the fuel cell and serves as a connecting material between the unit cells, Oxidative properties, similar thermal expansion rates to other parts of the fuel cell, and low cost. In the past, ceramic materials such as graphite and Cr-5Fe-1Y2O3-doped ceramic materials, which are highly stable at high temperatures, have been mainly considered as fuel cell interconnect materials, but metal materials are mainly considered due to problems such as processability. In the case of SOFC, most of Ni-based alloys such as Inconel were used, but due to price problems, it is difficult to commercialize them. Recently, the use of ferritic stainless steels has been actively studied.

A method of decreasing impurities such as C, N, O and the like and adding an alloy element which forms a stable oxidation layer such as Cr, Ni, Co, Zr and rare earth metals to improve oxidation resistance is mainly used in ferritic stainless steels. In order to increase the oxidation resistance and the corrosion resistance, a method of adding Mo, Al, a rare earth metal or the like or coating a coating layer or an alloy layer on the substrate has been proposed. However, these methods can increase the oxidation resistance of the ferritic stainless steel, The strength of the stainless steel may be lowered or the workability may be poor, and on the other hand, there is an economically disadvantageous aspect due to the addition of the oxidation-resistant element. In addition, when stainless steel having a high Cr content is used as an interconnector for a fuel cell, Cr is volatilized during operation of the fuel cell to stick to the anode side of the fuel cell, thereby deteriorating the performance of the fuel cell. Conventionally, such a problem has been solved through coating or the like, but there is a problem in that coating is not easy due to the nature of the connector, which is complicated in shape during the flow channel designing process.

1. Korean Patent No. 10-1042249 2. Korean Patent No. 10-1273936

The technical problem to be solved by the technical idea of the present invention is to provide a ferritic stainless steel excellent in oxidation resistance, processability and chromium volatilization inhibiting ability.

The technical problem to be solved by the technical idea of the present invention is to provide a fuel cell interconnector and a fuel cell including a ferritic stainless steel excellent in oxidation resistance, processability and chromium volatilization suppressing power.

However, these problems are illustrative, and the technical idea of the present invention is not limited thereto.

According to an aspect of the present invention, there is provided a ferritic stainless steel comprising 10 wt% to 30 wt% of chromium (Cr); 0.1 wt% to 2.0 wt% manganese (Mn); 0.01 wt% to 0.5 wt% titanium (Ti); 0.001 wt% to 0.3 wt% aluminum (Al); 0.3 wt% to 0.7 wt% of lanthanum (La); 0.004 wt% to 0.012 wt% boron (B); 0.001 wt% to 0.2 wt% carbon (C); 0.001 wt% to 0.1 wt% nitrogen (N); And the remainder includes iron (Fe) and unavoidable impurities.

In some embodiments of the present invention, the lanthanum (La) may range from 0.36 wt% to 0.65 wt%.

In some embodiments of the present invention, the boron (B) may range from 0.0048 wt% to 0.011 wt%.

In some embodiments of the present invention, the chromium (Cr) may range from 21.8 wt% to 23.1 wt%.

In some embodiments of the present invention, the manganese (Mn) may range from 0.3 wt% to 0.5 wt%.

In some embodiments of the present invention, the titanium (Ti) may range from 0.01 wt% to 0.06 wt%.

In some embodiments of the present invention, the aluminum (Al) may range from 0.01 wt% to 0.08 wt%.

In some embodiments of the present invention, the chromium (Cr) may range from 21.8 wt% to 23.1 wt%, the manganese (Mn) may range from 0.3 wt% to 0.5 wt% ) May be in the range of 0.01 wt% to 0.06 wt%, the aluminum (Al) may be in the range of 0.01 wt% to 0.08 wt%, the lanthanum may be in the range of 0.36 wt% to 0.65 wt% The boron (B) may range from 0.0048 wt% to 0.011 wt%.

According to an aspect of the present invention, there is provided a ferritic stainless steel comprising 10 wt% to 30 wt% of chromium (Cr); 0.1 wt% to 2.0 wt% manganese (Mn); 0.01 wt% to 0.5 wt% titanium (Ti); 0.001 wt% to 0.3 wt% aluminum (Al); 0.3 wt% to 0.7 wt% of lanthanum (La); 0.004 wt% to 0.012 wt% boron (B); And the remainder includes iron (Fe) and unavoidable impurities.

According to an aspect of the present invention, there is provided a fuel cell connector including the ferritic stainless steel described above.

According to an aspect of the present invention, there is provided a fuel cell comprising: a plurality of unit cells including an anode, an electrolyte, and a cathode; And a fuel cell connector connecting the unit cells and having the above-described contents of the elements.

The fuel cell connector serves not only as a connecting member for connecting each unit cell, but also as a separator for separating fuel and air supplied from the fuel cell. Therefore, such a fuel cell interconnector requires a high electrical conductivity and is required to have oxidation resistance at a high temperature of about 800 < 0 > C, in particular, and a thermal expansion rate similar to that of other parts of the fuel cell.

The oxidation resistant ferritic stainless steel according to the technical idea of the present invention forms a fuel cell interconnector with improved hot rolling processability, cold rolling processability, sheet resistance, high temperature strength, chromium volatilization inhibiting ability and formability by adding La and B Therefore, the cell performance and efficiency of the fuel cell can be increased, the fuel cell can be used for a long time at a high temperature, and the oxidation resistance can be increased, thereby providing a long life.

The effects of the present invention described above are exemplarily described, and the scope of the present invention is not limited by these effects.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a photograph showing hot rolling workability of an oxidation-resistant ferritic stainless steel according to an embodiment of the present invention. Fig.
2 is a photograph showing the cold rolling workability of the oxidation-resistant ferritic stainless steel according to one embodiment of the present invention.
3 is a graph showing the sheet resistance of the oxidation-resistant ferritic stainless steel according to one embodiment of the present invention.
4 is a graph showing the chrome volatility of the oxidation-resistant ferritic stainless steel according to one embodiment of the present invention.
5 is a graph showing strength and elongation of an oxidation-resistant ferritic stainless steel according to an embodiment of the present invention.
6 is a graph showing the average plastic strain ratio of the oxidation-resistant ferritic stainless steel according to one embodiment of the present invention.
7 is a schematic view showing a fuel cell according to an embodiment of the present invention.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. It will be apparent to those skilled in the art that the present invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the invention to those skilled in the art. The scope of technical thought is not limited to the following examples. Rather, these embodiments are provided so that this disclosure will be more thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the present specification, the same reference numerals denote the same elements. Further, various elements and regions in the drawings are schematically drawn. Accordingly, the technical spirit of the present invention is not limited by the relative size or spacing depicted in the accompanying drawings.

The technical idea of the present invention is to provide a ferritic stainless steel excellent in oxidation resistance, processability, and chrome volatilization inhibiting ability, and for example, to provide a ferritic stainless steel which can be applied to a fuel cell connector.

Such a fuel cell connector performs the function of a connecting member for connecting the unit cells, and functions as a separating member for separating the fuel and air supplied from the fuel cell. Therefore, the fuel cell interconnecting material is required to have high electrical conductivity, oxidation resistance, strength and chromium volatilization inhibiting ability at a moving temperature, processability capable of implementing various shapes, thermal expansion coefficient similar to other members constituting the fuel cell, Is required.

The ferritic stainless steel according to the technical idea of the present invention comprises 10 wt% to 30 wt% of chromium (Cr); 0.1 wt% to 2.0 wt% manganese (Mn); 0.01 wt% to 0.5 wt% titanium (Ti); 0.001 wt% to 0.3 wt% aluminum (Al); 0.3 wt% to 0.7 wt% of lanthanum (La); 0.004 wt% to 0.012 wt% boron (B); 0.001 wt% to 0.2 wt% carbon (C); 0.001 wt% to 0.1 wt% nitrogen (N); And the remainder includes iron (Fe) and unavoidable impurities.

In particular, the technical idea of the present invention is to increase the addition amount of lanthanum (La) so as to suppress the growth of the oxide film and improve the characteristics of the oxide film so that the ferritic stainless steel can be used as the material for fuel cell interconnectors. However, when the addition amount of La is increased, there is a problem that the workability is deteriorated. To overcome this deterioration of workability, B is further added. As a result, the concentration of La, which is a key element for controlling the oxidation characteristics, can be controlled over a wide range.

Hereinafter, the significance of the numerical content of each element constituting the ferritic stainless steel of the present invention will be described in detail.

The content of chromium (Cr) may range from 10 wt% to 30 wt%. Cr can increase the corrosion resistance. When the chromium content is within the above range, the corrosion resistance, processability, manufacturing cost, and manufacturing cost may be satisfactory.

The content of manganese (Mn) may range from 0.1 wt% to 2.0 wt%. Mn is an element that induces solid solution strengthening. When the Mn content is within the above range, the workability and manufacturing cost may be satisfactory.

The content of titanium (Ti) may range from 0.01 wt% to 0.5 wt%. Ti is an element for securing C and N by softening and improving elongation, and when the content of Ti is within the above range, the extensibility, workability and manufacturing cost may be satisfactory.

The content of aluminum (Al) may range from 0.001 wt% to 0.3 wt%. Al is a deoxidizing element, and when the content of Al is within the above range, workability and toughness may be satisfactory.

The content of lanthanum (La) may range from 0.3 wt% to 0.7 wt%. La is a lanthanoids rare earth element that increases oxidation resistance. When the content of lanthanum (La) is less than 0.3 wt%, the inhibition of oxide film formation may be restricted. On the other hand, when the content of lanthanum (La) exceeds 0.7 wt%, the oxidation may be promoted by La having high oxygen affinity or La-O inclusions may be formed, and the workability may be deteriorated.

The content of boron (B) may range from 0.004 wt% to 0.012 wt%. When the content of boron (B) is less than 0.004 wt%, the workability improving effect may not be conspicuous. On the other hand, when the content of boron (B) is more than 0.012 wt%, the uniformity of the surface oxide film may be deteriorated.

The content of carbon (C) may range from 0.001 wt% to 0.2 wt%. C is an element which deteriorates workability and rust resistance. When the content of C is within the above range, rustproofing and refining costs and the like may be satisfactory. C may correspond to undesired impurities, so the smaller the content, the better.

The content of nitrogen (N) may range from 0.001 wt% to 0.1 wt%. When the content of N is within the above range, the workability and the like may be satisfactory. N may correspond to undesired impurities, so the smaller the content, the better.

As described above, since carbon and nitrogen may not be intentionally added, they may be included in unavoidable impurities. In such a case, the ferritic stainless steel according to the technical idea of the present invention may contain 10 wt% to 30 wt% of chromium (Cr); 0.1 wt% to 2.0 wt% manganese (Mn); 0.01 wt% to 0.5 wt% titanium (Ti); 0.001 wt% to 0.3 wt% aluminum (Al); 0.3 wt% to 0.7 wt% of lanthanum (La); 0.004 wt% to 0.012 wt% boron (B); And the remainder includes iron (Fe) and unavoidable impurities.

The content of constituent elements can be limited based on the embodiments as described below. In this case, the ferritic stainless steel according to the technical idea of the present invention may contain chromium (Cr) of 21.8 wt% to 23.1 wt%; 0.3 wt% to 0.5 wt% manganese (Mn); 0.01 wt% to 0.06 wt% titanium (Ti); 0.01 wt% to 0.08 wt% aluminum (Al); 0.36 wt% to 0.65 wt% of lanthanum (La); 0.0048 wt% to 0.011 wt% boron (B); 0.003 wt% to 0.02 wt% carbon (C); And the remainder includes iron (Fe) and unavoidable impurities.

Hereinafter, embodiments are provided to facilitate understanding of the present invention. It should be understood, however, that the following examples are for the purpose of promoting understanding of the present invention, but the present invention is not limited by the following examples.

Alloy composition design

Table 1 shows the composition of the oxidation-resistant ferritic stainless steel according to one embodiment of the present invention.

wt% Cr Mn Ti Al La B C Fe Example 1 22.9 0.4 0.06 0.08 0.52 0.0109 0.003 Remainder Example 2 21.8 0.4 0.01 0.01 0.36 0.0048 0.003 Remainder Example 3 23.1 0.4 0.02 0.01 0.65 0.0092 0.02 Remainder Comparative Example 1 22.9 0.4 0.08 0.07 0.60 - 0.02 Remainder Comparative Example 2 22.4 0.4 0.05 0.04 0.16 0.011 0.01 Remainder Comparative Example 3
Crofer
22 APU) 20.0
~ 24.0 0.30
~ 0.80 0.03
~ 0.20 0.50 0.04
~ 0.20 - 0.03 Remainder Comparative Example 4 22.9 0.37 0.07 0.01 0.27 - 0.01 Remainder Comparative Example 5 23.1 0.4 0.06 0.03 0.86 0.012 0.02 Remainder

Specimen manufacturing

According to the contents of Examples and Comparative Examples in Table 1, the mixed elements were vacuum induction-melted to form alloys. The alloy was subjected to homogenization heat treatment at 1150 ° C for 2 hours. For reference, Comparative Example 3 was purchased from VDM Metals.

The homogenized heat-treated alloy was subjected to individual processes to produce a rear plate and a thin plate. In the case of the rear plate, after the homogenization heat treatment, the alloy was hot-rolled (at 80%) at 1150 ° C to prepare a rear plate of 18T. In the case of the thin plate, the alloy was subjected to hot rolling (88%) at 1150 占 폚 and recrystallization heat treatment at 900 占 폚 for 2 hours, followed by cold rolling to form a thin plate of 0.5T to 1T.

Characterization

For the sheet resistance analysis of the oxide films, the specimens were pre-oxidized at 800 ° C for 100 hours, and then the electrical resistance was measured at 800 ° C for 900 hours using the 4-terminal method.

To investigate the effect of chromium volatilization inhibition, the specimen was heated to 800 ℃ and air containing 60% moisture of saturated water vapor at room temperature was flowed to the heated specimen. At this time, volatilized chromium was injected through the Raschig ring And collected at intervals of 50 hours to 100 hours for 1000 hours. The collected chromium was dissolved in nitric acid and the solution was quantitatively analyzed by ICP method (Varian VISTA 720-ES Simultaneous ICP-OES equipment).

The high temperature tensile strength, yield strength, and elongation at 700 ° C of the specimens were measured to evaluate the mechanical properties at high temperature, which is the fuel cell operating temperature.

In the case of the thin plate, the average plastic deformation ratio was calculated by a tensile test to evaluate whether or not it has a formability suitable for press working.

Character analysis

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a photograph showing hot rolling workability of an oxidation-resistant ferritic stainless steel according to an embodiment of the present invention. Fig.

1, the surface states of the rear plate after hot rolling for Example 1, Example 2, Example 3, Comparative Example 1, Comparative Example 4, and Comparative Example 5 are shown. In Comparative Example 1 and Comparative Example 4, when 0.2 wt% or more of La was added and B was not added, the flowability of the molten metal was lowered and cracks were generated on the surface of the rear plate. In addition, in Comparative Example 5, when La was added at a high content of 0.86 wt%, cracking occurred even if B was added in an amount of 0.012 wt%. On the other hand, in Example 1, Example 2, and Example 3, since such cracks did not occur, it can be seen that the hot workability is excellent. Therefore, when the La content is increased, cracking is prevented by addition of B, and therefore it is analyzed that the hot workability is increased. However, as in Comparative Example 5, when the amount of La is too high as 0.86 wt%, it is understood that even if B is added, the crack preventing power is lowered.

2 is a photograph showing the cold rolling workability of the oxidation-resistant ferritic stainless steel according to one embodiment of the present invention.

Referring to FIG. 2, in Comparative Example 1 after cold rolling, a surface peeling phenomenon occurred. However, in Example 1, such a surface peeling phenomenon did not occur. Therefore, when the La content is increased, it is analyzed that the addition of B increases the cold rolling workability.

3 is a graph showing the sheet resistance of the oxidation-resistant ferritic stainless steel according to one embodiment of the present invention.

Referring to FIG. 3, it is a result of measuring the sheet resistance in real time for 900 hours at 800 ° C after pre-oxidizing for 100 hours. The higher the resistance of the oxide film formed on the fuel cell separator, the lower the efficiency of the fuel cell. Therefore, a material having a low sheet resistance of the oxide film is advantageous. As compared with Comparative Example 2 and Comparative Example 3, Example 1 is further reduced in sheet resistance over time, so that an oxide film having better conductivity is formed. Therefore, application to a fuel cell is analyzed to be more advantageous.

4 is a graph showing the chrome volatility of the oxidation-resistant ferritic stainless steel according to one embodiment of the present invention.

Referring to FIG. 4, cumulative chromium volatilization amounts in Examples 1, 2, and 3 increase with time. When the fuel cell is used at a high temperature for a long time, there is a problem that chromium volatilized in the interconnector contaminates the air electrode, so it is important to reduce the amount of chromium volatilization to the maximum. In Example 1, the amount of chrome volatilization is smaller than that in Comparative Example 2 and Comparative Example 3, and it is analyzed that an increase in La content has an effect of suppressing chromium volatilization.

5 is a graph showing strength and elongation of an oxidation-resistant ferritic stainless steel according to an embodiment of the present invention.

Table 2 shows tensile strength, yield strength, and elongation at 700 DEG C of the oxidation-resistant ferritic stainless steel according to one embodiment of the present invention.

Example 1 Comparative Example 1 Comparative Example 2 Comparative Example 3 Tensile Strength (MPa) 67 109 56 49 Yield strength (MPa) 51 102 37 38 Elongation (%) 75 - 88 94

5 and Table 2, it can be seen that increasing the content of La simply as in Comparative Example 1 can increase the tensile strength and yield strength, but the low processability characteristic of Comparative Example 1 as shown in FIGS. 1 and 2 As shown, the toughness is lowered. As shown in Example 1, when La and B were simultaneously added, tensile strength and yield strength were generally increased as compared with Comparative Example 2 and Comparative Example 3 in which the La content was low, and the elongation tended to decrease somewhat Of the total.

6 is a graph showing the average plastic strain ratio of the oxidation-resistant ferritic stainless steel according to one embodiment of the present invention.

Referring to FIG. 6, as a result of evaluation of formability of a material having a thickness of 0.5 T, the average plastic deformation ratio of Example 1 was higher than that of Comparative Example 2 and Comparative Example 3. The separating plate thin plate material is economical, not cutting, but press forming is important, and moldability is important. Therefore, when La and B are simultaneously added as in Example 1, it is analyzed that the formability of the thin plate is improved.

7 is a schematic view showing a fuel cell according to an embodiment of the present invention.

Referring to FIG. 7, the fuel cell has a stacked structure of a unit cell including an anode, an electrolyte, and a cathode, and a fuel cell connector. And the fuel cell connector connects the plurality of unit cells from both sides. The fuel cell connector serves as a connecting member for connecting each unit cell and as a separator for separating fuel and air supplied from the fuel cell. As described above, the fuel cell interconnector is formed of an oxidation-resistant ferritic stainless steel according to an embodiment of the present invention, and the oxidation-resistant ferritic stainless steel specifically includes Cr, Mn, Ti, Al, La, Fe and the like.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention as defined in the appended claims. Will be apparent to those of ordinary skill in the art.

Claims (11)

10 wt% to 30 wt% chromium (Cr);
0.1 wt% to 2.0 wt% manganese (Mn);
0.01 wt% to 0.5 wt% titanium (Ti);
0.001 wt% to 0.3 wt% aluminum (Al);
0.3 wt% to 0.7 wt% of lanthanum (La);
0.004 wt% to 0.012 wt% boron (B);
0.001 wt% to 0.2 wt% carbon (C);
0.001 wt% to 0.1 wt% nitrogen (N); And
The remainder is iron (Fe) and unavoidable impurities;
And a ferritic stainless steel. The method according to claim 1,
Wherein the lanthanum (La) is in the range of 0.36 wt% to 0.65 wt%. The method according to claim 1,
Wherein the boron (B) is in the range of 0.0048 wt% to 0.011 wt%. The method according to claim 1,
Wherein the chromium (Cr) is in the range of 21.8 wt% to 23.1 wt%. The method according to claim 1,
Wherein the manganese (Mn) is in the range of 0.3 wt% to 0.5 wt%. The method according to claim 1,
Wherein the titanium (Ti) is in the range of 0.01 wt% to 0.06 wt%. The method according to claim 1,
Wherein the aluminum (Al) is in the range of 0.01 wt% to 0.08 wt%. The method according to claim 1,
The chromium (Cr) ranges from 21.8 wt% to 23.1 wt%
The manganese (Mn) ranges from 0.3 wt% to 0.5 wt%
The titanium (Ti) ranges from 0.01 wt% to 0.06 wt%
The aluminum (Al) ranges from 0.01 wt% to 0.08 wt%
The lanthanum (La) ranges from 0.36 wt% to 0.65 wt%
Wherein the boron (B) is in the range of 0.0048 wt% to 0.011 wt%. 10 wt% to 30 wt% chromium (Cr);
0.1 wt% to 2.0 wt% manganese (Mn);
0.01 wt% to 0.5 wt% titanium (Ti);
0.001 wt% to 0.3 wt% aluminum (Al);
0.3 wt% to 0.7 wt% of lanthanum (La);
0.004 wt% to 0.012 wt% boron (B); And
The remainder is iron (Fe) and unavoidable impurities;
And a ferritic stainless steel. A fuel cell connector comprising a ferritic stainless steel according to any one of claims 1 to 9. A plurality of unit cells comprising an anode, an electrolyte and a cathode; And
A fuel cell connector according to claim 10 connecting the unit cells;
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