SE530155C2 - Ferritic chromium stainless steel for fuel cells, contains preset amount of carbon, silicon, manganese, chromium, nickel, molybdenum, niobium, titanium, zirconium, rare earth metals, aluminum and nitrogen - Google Patents

Abstract

The ferritic chromium steel contains carbon (in wt.%) (0.1 or less), silicon (0.1-1), manganese (0.6 or less), chromium (20-25), nickel (2 or less), molybdenum (0.5-2), niobium (0.3-1.5), titanium (0.5 or less), zirconium (0.5 or less), rare earth metals (0.3 or less), aluminum (0.1 or less) and nitrogen (0.07 or less) and remainder of iron and normally occurring impurities. The total of zirconium and titanium content is at least 0.2%. The ferritic chromium steel contains carbon (in wt.%) (0.1 or less), silicon (0.1-1), manganese (0.6 or less), chromium (20-25), nickel (2 or less), molybdenum (0.5-2), niobium (0.3-1.5), titanium (Ti) (0.5 or less), zirconium (Zr) (0.5 or less), rare earth metals (0.3 or less), aluminum (0.1 or less) and nitrogen (0.07 or less) and remainder of iron and normally occurring impurities. The total of zirconium and titanium content (Zr+Ti) is at least 0.2%. The zirconium in ferritic chromium stainless is substituted by hafnium. The molybdenum in ferritic chromium stainless is partly substituted by tungsten. The niobium in ferritic chromium stainless is partly substituted by tantalum and/or vanadium.

Description

5300155 which is the common material used as the electrolyte fuel cell, and is therefore considered a suitable choice for this application.

It is desirable that the oxide layer formed on the steel interconnect material not be cracked or cracked due to thermal cycling, as this can cause unwanted, catastrophic corrosion of the steel. This means that the oxide layer formed on the surface of the material should have good adhesion to the material. The oxide layer should also have good electrical conductivity and not grow too thick during the life of the fuel cell, as thicker oxide layers will lead to an increased electrical resistance.

The oxide formed should also be chemically resistant to the gases used as fuels in an SOFC, i.e. no volatile metal-containing substances such as chromoxohydroxides should be formed. Volatiles, such as chromoxohydroxides, will contaminate the electroactive ceramics in an SOFC stack, which in turn will lead to a reduction in fuel cell efficiency.

The most commercially available ferritic chrome steels are alloyed with aluminum and / or silicon. These alloying elements form AIZO; and / or SiO 2 at the operating temperature of the SOFC. Both of these oxides are electrically insulating oxides that will increase the electrical resistance of the cell and reduce the efficiency of the fuel cell. This has led to the development of ferritic steels with low Al and Si contents to ensure good conductivity of the oxide layers formed. These newly developed steels are usually also alloyed with manganese. The addition of Mn to the steel will cause the formation of chromium oxide-based spinel structures in the oxide layer formed. In general, however, Mn has little effect on the corrosion resistance of the steel and it is therefore desirable that the Mn content of the steel is closely monitored at low levels. Excessive concentration of Mn l steel will lead to the growth of thick oxide layers, due to significant high temperature corrosion.

In addition to Mn, fl era of these newly developed steels are alloyed with group lll elements, i.e. Sc, La and Y and / or other rare earth metals (REM). The addition of 10 15 20 25 30 530 'P55 La, Y or REM is done to increase the life of the material at high temperatures. Strong oxide formers, such as La, Y and REM, are said to reduce the oxygen ion mobility in the formed Cr 2 O 3 layer, which will lead to a decrease in the growth rate of the oxide layer.

An example of a chromium steel for use in SOFCs is described in patent application US 2003/0059335, in which the steel contains 12-28% Cr, 0.01-0.4% La, 0.2-1.0% Mn, 0.05 -0.4% Ti, less than 0.2% Si and less than 0.2% Al.

EP 1 600 520 A1 describes another example of a chromium steel for use in SOFCs containing 20-25% Cr, up to 0.5% Mn, Zr + Hf 0.001-0.1%, up to 0.4% Si and up to 0.4% Al. The patent application EP 1 298 228 A2 also describes a steel for use in SOFC.

This steel contains 15-30% Cr, not more than 1.0% Mn, up to 1% Si and at least one of Y up to 0.5%, REM up to 0.2 and Zr up to 1%.

Another example of a ferritic steel for SOFC is described in US 6,294,131 B1.

The steel contains 18-28.5% V Cr, 0.005-0.10% Mn, up to 0.1% Si and 0.005-0.50% REM.

In addition, ferritic steels for use in SOFCs are described in Leszek Niewolak et al., "Development of High Strength Ferritic Steel for Interconnect Application in SOFCs", 7 "" European SOFC Forum, Session B08, Wednesday 5 July 2006, at 16:45 , Act No. B084 It was found that additions of Nb and W to high-Cr containing ferritic steels could develop dispersed precipitates of lava phase sediment.

The purpose of the invention is to provide an alternative steel suitable for use in solid oxide fuel cells. 10 15 20 25 30 530 155 SUMMARY The object of the invention is achieved by a ferritic chromium steel as defined in claim 1, which provides a material suitable for use in solid oxide fuel cells, in particular as an interconnect. The composition of the ferritic chromium steel enables a very good corrosion resistance, a suitable thermal expansion, a good adhesion of the oxide formed on the surface of the material and in particular a very low contact resistance.

The composition of the steel makes it possible for the Si contained in the steel to be bound into particles rich in Si. More specifically, the steel comprises Si-rich particles comprising Mo and Nb. The presence of these particles minimizes the risk of Si diffusing to the surface and forming silicon oxides. By forming these silica-rich particles, the formation of a silica-rich portion in the oxide layer under the chromium oxide is avoided. The reduced formation of silicon oxides on the oxide layer is considered to be the main reason for the low rate of deterioration of the contact resistance of these alloys. Consequently, the composition, in accordance with the present invention, also enables a much higher Si content than previously considered suitable for the application of solid oxide fuel cells, which also enables a more cost-effective manufacturing process since there is no need to suppress the Si content in the melt. However, not only is the addition of Mo and Nb needed to form silicon-rich particles, but it is also necessary, for use as an interconnect in solid oxide fuel cells, to additionally add Zr and / or Ti to the alloy of the invention. Zr and / or Ti will first of all ensure a well-adhering oxide layer, but it can also improve the formation of silicon-rich particles inside the steel matrix. Although the ferritic chromium steel, according to the present invention, is primarily developed for use in solid oxide fuel cells, it is expected that it may also be used in other types of fuel cells such as polymer electrolyte membrane cells (PEM). 10 15 20 25 30 530 155 ti? BRIEF DESCRIPTION OF THE FIGURES Figure 1 illustrates the area-specific resistance as a function of time for the three melts A, B & C of the alloy according to the invention compared to model alloy 5.

Figure 2 illustrates the SEM / EDS mapping of Mo, Nb and Si of melt C of the alloy of the invention.

Figure 3 illustrates the SEM / EDS mapping of a commercial ferritic 22% chromium steel without the addition of Mo and Nb.

DETAILED DESCRIPTION The contribution from the various elements is described below. All percentages are by weight.

Carbon (C) when added can increase the high temperature strength by forming carbides with certain metals, such as Mo. In this application, however, the carbon content must be kept low so that the added, generally known, carbide formers such as Mo, Nb, Ti and Zr are not bound in the metal matrix. The carbon content should preferably be a maximum of 0.1% by weight, preferably a maximum of 0.05% by weight, more preferably a maximum of 0.03% by weight.

Silicon (Si) is usually kept very low in ferritic steels for use as interconnects in solid oxide fuel cells, to avoid the formation of silica. In a conventional steelmaking process, however, there is always a small amount of silicon in the steel. To reduce the silicon content to below 0.25%, advanced vacuum melting processes are usually needed, or scrap with a low silicon content is needed for the melting. For the present invention none of these are needed, instead the silicon content should be 0.1-1%, preferably 0.18-0.5%, more preferably a maximum of 0.4%. However, the Si content can even be found in contents exceeding 0.2%, as long as the majority of the silicon is bound in particles inside the steel matrix and is not oxidized on the surface of the steel. This can be achieved by a suitable addition of the elements Mo and Nb, which are discussed further below.

Manganese (Mn) in excessively high levels will lead to a higher oxidation rate of the steel. However, a small amount of Mn is needed to form the top layer of manganese chromium spinel on the oxide layer, which will reduce the evaporation of chromium as well as increase the conductivity of the layer. Consequently, the manganese content should be a maximum of 0.6%, preferably a maximum of 0.4%. Preferably at least 0.2% Mn is present in the steel.

Chromium (Cr) is added above all to give a good resistance to high temperature corrosion by forming a chromium oxide layer. The amount of chromium can also be varied to adjust the thermal expansion of the ferritic alloy. For working temperatures above 700 ° C, however, the chromium content should be 15-25% to avoid depletion of chromium from the steel core. According to an embodiment of the invention, the chromium content is at least 20%, preferably at least 21%, more preferably at least 21.5%.

According to another embodiment of the invention, the chromium content is a maximum of 24%, preferably a maximum of 23.5%.

Nickel (Ni) can be added to the steel to adjust the thermal expansion to fit the ceramic components inside the fuel cell. However, excessive Ni levels should be avoided to reduce the risk of austenite particles forming inside the steel.

Consequently, the nickel content should be a maximum of 2%. According to an embodiment of the invention, the nickel content is a maximum of 1%, preferably a maximum of 0.5%.

Molybdenum (Mo) is added to increase mechanical strength but also to form silicon-rich particles. The formation of silicon-rich particles by adding Mo to the steel will reduce the electrical resistance of the oxide layer on the steel, which in turn will lead to a slower rate of deterioration of the fuel cell itself. Furthermore, it is expected that Mo can be partially replaced by W and still achieve a similar result, in accordance with what is generally known.

Accordingly, the molybdenum content should be 0.5-2%. According to one embodiment, the molybdenum content is a maximum of 1.8%, preferably a maximum of 1.5%, more preferably a maximum of 1.2%. The most preferred molybdenum content is at least 0.6%.

Niobium (Nb) is added to promote the formation of silicon-rich particles inside the steel matrix, which will reduce the electrical resistance of the oxide layer on the steel, which in turn leads to a slower rate of deterioration of the fuel cell itself.

It is also expected that Nb can be replaced by Ta and / or V, and still achieve a similar result, in accordance with what is generally known.

Accordingly, the content of niobium (or tantalum and / or vanadium) should be 0.2-1.5%, preferably a maximum of 1.0%. According to one embodiment, the niob content is at least 0.3%, preferably at least 0.4%.

Titanium (Ti) is added to steel to improve the adhesion of the oxide layer formed.

In addition, it is considered likely that the addition of titanium will dop the formed oxide layer, which will increase the conductivity of the chromium oxide. However, no additional effect has been observed for levels above 0.5% Ti. In order to achieve cost efficiency, the titanium content should thus be a maximum of 0.5%, preferably a maximum of 0.3%, more preferably a maximum of 0.1%.

The same effect as with Ti can be achieved by adding zirconium to the alloy.

The content of Zr + Ti should always be at least 0.2% to achieve the desired well-adhered, electrically conductive oxide layer.

Zirconium (Zr) is added to steel to improve the wind adhesion of the oxide layer formed. This is to avoid cleavage and cracking in the oxide layer.

Furthermore, it is expected that Zr can be exchanged for Hf, in accordance with what is generally known, while a similar result is still obtained, with respect to the adhesion. Zr can also improve the formation of the silicon-rich particles inside the steel matrix. Consequently, the zirconium content should be a maximum of 0.5%. According to one embodiment, the zirconium content is 0.2-0.35%, preferably 0.2-0.3%.

Rare earth metals (REM) are usually added to materials that should have good resistance to high temperature corrosion, such as alumina formers, and are said to block the diffusion at the grain boundaries and thereby reduce the oxidation rate of the material. In this context, REM is considered to be any metal from the lanthanoid elements (element number 57 up to 71) as well as group-III elements in the periodic table, i.e. Scandium (element 21) and Yttrium (element 39). There is no need to add REM to the present alloy, however, it can be added to further enhance the resistance to high temperature corrosion.

The content of REM should therefore be a maximum of 0.3%. According to one embodiment, the ferritic chromium steel, according to the present invention, contains no additive of REM.

Aluminum (Al) is often added to high-temperature-resistant alloys because it forms a well-protecting alumina layer on the steel surface. However, if the application is for the steel to act as a current collector, it is absolutely necessary that the oxide layer formed is conductive and not electrically insulating. Consequently, the aluminum content should be a maximum of 0.1%, preferably a maximum of 0.05%.

Nitrogen (N) should be kept low as it will form metal nitrides with the necessary alloying elements such as niobium, titanium and zirconium. If Nb, Ti or Zr are bound in nitrides, they will not have the beneficial effect on the adhesion of the oxide layer. Consequently, the nitrogen content should be a maximum of 0.07%, preferably a maximum of 0.05%, more preferably a maximum of 0.03%.

Common contaminants such as S and P should be kept as low as possible to promote the formation of cleaner oxide layers. Excessive contaminant content can also lead to problems with cleavage of the oxide layer. Accordingly, S and P are preferably kept below 0.008% of each. The alloy may also contain other impurities as a result of the raw material used as well as the manufacturing process.

However, the impurities are in a content that does not significantly affect the properties of the ferritic chromium steel when used in the intended application.

According to the most preferred embodiment of the present invention, the ferritic steel comprises additions of Mo, Nb and Zr simultaneously. Thus, silicon-rich particles containing Mo and Nb are formed, which prevents the formation of a surface oxide containing silica and the oxide on the surface has an improved conductivity due to the addition of Zr. This achieves a superior steel with the desired properties, above all a very good electrical surface conductivity.

An example of the most preferred embodiment is a steel with an approximate composition of (in weight percent): Si 0.2 Mn 0.3 Cr 22 Mo 1 Nb 0.4 Zr 0.3 Ti 0.05 residue Fe and normally occurring impurities .

Example 1 The contact resistance and the ability to trap the silicon inside the alloy matrix, in the form of silicon-rich Nb-Mo-Si particles, in 3 different melts A, B & C of the alloy according to the invention were compared with 6 model alloys with chemical compositions near the alloy of the invention. The chemical compositions, by weight of the alloying elements in these alloys, are given in the table below. The residue is iron and normally occurring contaminants.

Table 1 Chemical composition of your melts of the alloy according to the invention, as well as some model alloys. Alloy Si Mn Cr Ni Mo Nb Ti Zr Ce N Alloy according to 0.24 0.35 22.08 0.06 1.03 0.90 0.043 0.22 0.019 invention meltA Alloy according to 0.18 0.38 22.16 1.03 1.02 0.42 0.047 0.28 0.018 invention meltB Alloy according to 0.36 0.37 22.22 0.06 0.64 0.44 0.06 0.29 0.015 invention meltC Model | alloy10, 09 0.32 21.87 0.07 0.62 0.29 0.010 0.005 0.022 Model alloy2 0.19 0.34 21.85 0.06 <0.01 0.33 0.020 0.016 0.023 Model alloy3 0.19 0.39 22 .13 0.06 1.04 0.46 0.036 0.056 0.081 Model alloy4 0.20 0.25 22.13 0.06 1.05 0.46 0.042 0.055 Model alloy5 0.16 0.39 22.0 0.06 0, 0.03 0.02 <0.02 0.03 Model alloy 0.16 0.39 22.09 0.06 1.04 0.35 0.018 0.040 0.06 0.022 In all the melts of the alloy according to the invention, i.e. melt A, B & C, Nb-Mo-Si-rich particles were formed inside the alloy matrix. By forming silicon-rich particles inside the matrix, silicon is prevented from diffusing out to the surface of the alloy and oxidizing below the chromium oxide layer. Only the melt C of the alloy according to the invention, with the highest amount of added silicon, i.e. 0.36%, showed the formation of small amounts of silica under the chromium layer. This particular melt of the alloy of the invention had the lowest levels of added Mo and Nb compared to the other melts of the alloy of the invention. All these melts of the alloy according to the invention also had more than 0.2% Zr added. 10 15 20 25 30 530 0155 ll Model alloy 1 with low silicon content, i.e. 0.09%, showed only a slight formation of Nb-Mo-Si particles inside the alloy matrix. The reason for this is the low content of Nb, Si and Mo added to this model alloy. Despite the relatively low Si content, it also showed some Si under the chromium oxide layer formed. It should also be pointed out that this model alloy 1 also had a very low addition of Zr. in Model Alloy 2, without added Mo, showed no formation of silicon-rich particles within the alloy matrix and it also showed enrichment of silicon below the chromium oxide layer.

This shows that the addition of only Nb to the alloy will not form silicon-rich particles inside the matrix.

Model alloy 3, with a high N content, shows the formation of a small amount of particles, but the addition of nitrogen caused the layer to split off and crack.

The reason for the cleavage is the formation of metal nitrides of the necessary added metal, such as Zr.

Model alloy 4 with the addition of a small amount of Al, i.e. 0.32%, showed the formation of a thin but well-adhering alumina layer. The alloy also formed Nb-Mo-Si-rich particles inside the matrix. However, the well-electrically insulating alumina will reduce the efficiency of the fuel cell if this alloy is used as an interconnect.

Model alloy 5, without any addition of Mo and Nb, showed no formation of particles within the matrix, and in addition it also showed an enrichment of silicon below the formed chromium oxide layer. In model alloy 6, a small amount of silicon-enriched particles was observed, but the amounts of Nb, Si and Zr added to this alloy are too low. In this specific model alloy, Ce was also added to see if the addition of REM to the alloy would have any beneficial effect.

In addition, the electrical contact resistance of the alloys was tested and the area-specific resistance (ASR) was measured for all alloys. The electrical resistances at the interface between (La, Sr) MnO 2 (LSM) plates and the alloys were measured by a DC four-point method in air at 750 ° C for a period of 1000 hours. A contact layer of (La, Sr) (Mn, Co) O3 was applied between the alloys and the LSM plates. The model alloy with the largest increase in ASR was model alloy 4. The ASR for this model alloy was twice as high as for the other alloys. The second largest increase in ASR was observed for the model alloy without any added Mo or Nb.

In Fig. 1, ASR is presented as a function of time and it can be clearly seen that the model alloy 5, without any addition of the necessary alloy metals, Mo, Nb & Zr, has the largest increase in ASR over a 1000 hour period.

Furthermore, this model alloy has a lower silicon content than the 3 melts A, B & C of the alloy according to the invention, but still the electrical deterioration of this alloy is much higher than the melts of the alloy according to the invention. The mn content in model alloy 5 is the same as in the A, B and C melts of the alloy according to the invention.

The very lowest ASR increases were measured for the three melts A, B & C of the alloy according to the invention, with the highest content added Zr and also these are presented in Fig. 1. Although these alloys had the highest levels of added silicon, they gave the lowest area specific electrical resistance (ASR).

Example 2 Samples of the various melts A, B and C, of the alloy of the invention, measuring 40 x 30 x 0.2 mm, and a sample of a commercially available ferritic 22% chromium steel, were oxidized. in air at 850 ° C for 1008 hours. The commercial steel has a nominal chemical composition in weight% of 20-24% Cr, 0.30-0.80% Mn, <0.50% Si, 0.03-0.20% Ti and 0.04-0 , 20% La. For the chemical composition of the 3 test pieces, see Table 1. The oxidized test pieces were halved after oxidation and polished and inspected by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS).

Figure 2 shows an SEM photomicrograph of the cross section of the melt C of the alloy according to the invention together with an EDS analysis with mapping of the elements Mo, Nb and Si. In the EDS classification scheme for each element, the lighter areas represent a higher concentration of the specific element. Here it can be clearly seen that the elements Mo, Nb and Si are found again in particles inside the alloy matrix. In SEM photomicrograph, these particles appear brighter due to the higher concentration of heavy elements, such as Mo and Nb. An Nb-Mo-Si particle has been circled in Figure 2 to more clearly show the effect of the Nb and Mo capture of silicon in particles within the alloy matrix.

The chemical analysis of these particles was performed by point EDS analysis and the chemical composition of these particles was compared with the chemical composition of the respective melt of the alloy of the invention. The results from these experiments were summarized in Table 2 below.

It can be clearly seen that the particles contain much more silicon than the alloy itself. However, the molybdenum and niobium levels are also greatly elevated in these particles. The enrichment of Si in these particles is increased by a factor of 10.

From this, it can be concluded that by adding the appropriate amount of both Mo and Nb to a silicon-containing ferritic alloy, silicon can be bound inside Nb-Mo-Si-rich particles which will prevent the silicon from diffusing out to the surface and oxidizing. Table 2: Chemical composition (in% by weight) of particles compared to the composition of the alloy.

Cr Mo Nb 16 1.02 I 0.42 0.18 74.5 Particle 12.36 6.03 26.81 1.94 52.86 The alloy of the invention melts C 22.22 0.64 0.44 0.36 75.5 Particle 8.93 6.45 36.21 3.51 44.9 The oxidized sample of commercially available ferritic 22% chromium steel was halved after oxidation and polished and inspected by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Figure 3 shows the cross-sectional SEM photomicrograph of the commercial ferritic 22% chromium steel together with an EDS analysis with mapping of the elements Cr, Si and Fe. In the EDS mapping scheme for each element, the lighter areas represent a higher concentration of the specific element. This specific commercial alloy contains no Mo and Nb additives. Here it is clear that the small amount of silicon present in the steel is found in particle strands just below the formed chromium oxide shell. In the SEM photomicrograph in Figure 3, a black arrow has been added to show exactly where the silicon has been enriched under the chromium oxide shell. l The EDS mapping of Si is a white arrow added as a guide for the eye to illuminate the observed enrichment of silicon. The formation of silica on the surface will lead to an increase in the electrical resistance of the steel surface. In an interconnect in a fuel cell application, this will lead to a deterioration of the fuel cell efficiency. Furthermore, no silicon-rich particles are found inside the alloy matrix.

Claims (25)

10 15 20 25 30 PATENT REQUIREMENTS 530 »155 Ferritic chromium steel comprising by weight: C Si Mn Cr Ni Mo Nb Ti Zr REM Al N max 0.1 0.1 - 1 max 0.6 20 - 25 max 2 0.5 - 2 0.3 - 1.0 max 0.5 max 0.5 max 0.3 max 0.1 max 0.07 the residue Fe and normally occurring impurities, where the content of Zr + Ti is at least 0.2%. Ferritic chromium steel according to claim 1, comprising 21-24% Cr, preferably 21, 5-23.5%. Ferritic chromium steel according to claim 1 or 2, comprising 0.5-1.8% Mo, preferably 0.5-1.5% Mo. Ferritic chromium steel according to claim 3, comprising 0.6-1.2% Mo. Ferritic chromium steel according to claim 1, comprising 0.4-1.0% Nb. Ferritic chromium steel according to any one of the preceding claims, comprising 0.18-0.5% Si. 10 15 20 25 30 530 155 lip Ferritic chromium steel according to claim 6, comprising more than 0.2 to 0.4% Si. Ferritic chromium steel according to any one of the preceding claims, comprising 0.20-0.35% Zr + Ti. Ferritic chromium steel according to any one of the preceding claims, comprising a maximum of 0.3% Ti, preferably a maximum of 0.1% Ti. Ferritic chromium steel according to claim 8 or 9, comprising 0.2-0.3% Zr. 11. 1 1. Ferritic chromium steel according to any one of the preceding claims, wherein Zr is at least partially replaced by Hf. Ferritic chromium steel according to any one of the preceding claims, wherein Mo is partially replaced by W. Ferritic chromium steel according to any one of the preceding claims, wherein Nb is at least partially replaced by Ta and / or V. Ferritic chromium steel according to any one of the preceding claims, wherein it does not contain any additive of REM. Ferritic chromium steel according to any one of the preceding claims, comprising a maximum of 0.05% N, preferably a maximum of 0.03% N. Ferritic chromium steel according to any one of the preceding claims, comprising particles rich in Si. The ferritic chromium steel of claim 16, wherein the acidic particles further comprise Mo and Nb. 10 15 20 25 30 530 155 »___ l '' r Ferritic chromium steel according to any one of the preceding claims, comprising a maximum of 1% Ni, preferably a maximum of 0.5% Ni. IN Ferritic chromium steel according to any one of the preceding claims, comprising a maximum of 0.05% C, preferably a maximum of 0.03% C. Ferritic chromium steel according to any one of the preceding claims, comprising a maximum of 0.4% Mn. Ferritic chromium steel according to any one of the preceding claims, comprising at least 0.2% Mn. Ferritic chromium steel according to any one of the preceding claims, comprising a maximum of 0.05% Al. 23. Ferritic chromium steel having the following approximate composition by weight: Si 0,2 Mn 0,3 Cr 22 Mo 1 Nb 0,4 Zr 0,3 Ti 0,05 The residue Fe and normally occurring impurities. Use of a steel according to any one of the preceding claims in a fuel cell, such as a solid oxide fuel cell. A fuel cell, such as a solid oxide fuel cell, comprising an interconnect element made of a steel according to any one of claims 1 to 23.