Ferrite stainless for fuel cell seperator

Abstract

The present invention relates to a ferritic stainless steel for a solid oxide fuel cell separator, comprising, in weight percent, C: 0.002~0.013%, N: 0.002~0.015%, Si: 0.05~0.2%, Mn: 0.2~0.9%, Cr: 19~25%, Mo: 0.4% or less, Nb: 0.1~0.7%, Ti: 0.1% or less, Al: 0.01~0.15%, and the remainder being Fe and unavoidable impurities, wherein the weight fraction of Si in a range of 0.1 μm above and 0.1 μm below the interface between the scale formed on the base material and the base material is 2% or less.

Description

Technology Field

[0001] The present invention relates to stainless steel for solid oxide fuel cell separators, and more specifically, to ferritic stainless steel capable of maintaining high conductivity in a solid oxide fuel cell environment. Background Technology

[0002] The present invention is a result derived through the following research tasks.

[0003] (National R&D project that supported this invention)

[0004] (Assignment No.) 20203030030020

[0005] (Ministry Name) Ministry of Trade, Industry and Energy

[0006] (Name of Project Management (Specialized) Agency) Korea Institute of Energy Technology Evaluation and Planning

[0007] (Research Project Name) Industrial Technology Innovation Project

[0008] (Project Title) Development of New Special Steel Materials and Mass Production Technology for Solid Oxide Fuel Cell Separators

[0009] (Contribution rate) 1 / 1

[0010] (Name of Project Performing Organization) POSCO Corp.

[0011] (Research Period) 2020.05.01 ~ 2024.04.30

[0012] Currently, stainless steel is primarily used as a separator material for solid oxide fuel cells because the scale formed on its surface possesses excellent high-temperature oxidation resistance. However, the scale generated within the solid oxide fuel cell environment must possess not only oxidation resistance but also excellent high-temperature conductivity.

[0013] Scale formed on the surface can be damaged by peeling, which can lead to poor conductivity and reduce fuel cell efficiency; therefore, the characteristics of the scale are very important for application as a separator material.

[0014] Generally, the high-temperature scale formed on the surface of stainless steel consists primarily of Cr2O3, which provides oxidation resistance. However, while Cr2O3 exhibits excellent oxidation resistance, its low electrical conductivity can act as a weakness when used as a separator material. Additionally, besides Cr2O3, Al and Si can also easily form oxides at high temperatures; although Al and Si oxides offer superior corrosion resistance, they reduce the electrical conductivity of the scale, so their formation must be suppressed as much as possible.

[0015] Since Al and Si are primarily used during the deoxidation process to remove oxygen in the stainless steelmaking process, it is difficult to completely remove Al and Si from stainless steel.

[0016] To solve these problems, a method of adding rare earth elements has been proposed, but it is difficult to apply to the conventional stainless steel manufacturing process, which leads to an excessive increase in material manufacturing costs. The problem to be solved

[0017] Considering the limitations and problems of the prior art, the present invention aims to provide a ferritic stainless steel with excellent scale conductivity by maximizing the content of Cr-Mn oxide, which has superior conductivity compared to Cr oxide in the scale, and minimizing the content of Al and Si. means of solving the problem

[0018] The ferritic stainless steel according to an embodiment of the present invention comprises a base material containing, in weight%, C: 0.002~0.013%, N: 0.002~0.015%, Si: 0.05~0.2%, Mn: 0.2~0.9%, Cr: 19~25%, Mo: 0.4% or less (excluding 0), Nb: 0.1~0.7%, Ti: 0.1% or less (excluding 0), Al: 0.01~0.15%, and the remainder being Fe and unavoidable impurities, wherein the weight fraction of Si in a range of 0.1㎛ above and 0.1㎛ below the interface between the scale formed on the base material and the base material is 2% or less.

[0019] In addition, the ferritic stainless steel according to an embodiment of the present invention can satisfy the following equation (1):

[0020] Equation (1): (Cr-Mn oxide thickness) / (Cr oxide thickness+Si oxide thickness) ≥ 2.0 Effects of the invention

[0021] According to the ferritic stainless steel of the present invention, it is possible to manufacture a component that maintains high electrical conductivity even when applied to a separator plate of a fuel cell in a high-temperature oxidation environment. Brief explanation of the drawing

[0022] Figure 1 is a cross-sectional photograph showing the oxide layer formed in Example 3. Figure 2 is a cross-sectional photograph showing the oxide layer formed in Comparative Example 2. Figure 3 is a cross-sectional photograph showing the oxide layer formed in Comparative Example 4. Figure 4 is an EDS graph showing the composition of the Cr-Mn composite oxide in the oxide layer formed in Example 3. Figure 5 is an EDS graph showing the composition of Cr oxide in the oxide layer formed in Example 3. Figure 6 is an EDS graph showing the composition of Si oxide in the oxide layer formed in Example 3. Specific details for implementing the invention

[0023] A ferritic stainless steel according to one embodiment of the present invention comprises a base material containing, in weight percent, C: 0.002~0.013%, N: 0.002~0.015%, Si: 0.05~0.2%, Mn: 0.2~0.9%, Cr: 19~25%, Mo: 0.4% or less (excluding 0), Nb: 0.1~0.7%, Ti: 0.1% or less (excluding 0), Al: 0.01~0.15%, and the remainder being iron and unavoidable impurities, wherein the weight fraction of Si within a 0.2㎛ range, which is 0.1㎛ above and 0.1㎛ below the interface between the scale formed on the base material and the base material, satisfies 2% or less.

[0024] In addition, the content of Cr-Mn oxide is higher than that of Cr oxide, Si, and Al oxides, which have an adverse effect on electrical conductivity within the scale formed at high temperatures, and by limiting (Cr-Mn oxide thickness) / (Cr oxide thickness+Si oxide thickness) ≥ 2.0, it can help improve the electrical conductivity of the scale.

[0025] Preferred embodiments of the present invention are described below. However, embodiments of the present invention may be modified in various other forms, and the technical concept of the present invention is not limited to the embodiments described below. Furthermore, the embodiments of the present invention are provided to more completely explain the present invention to those with average knowledge in the relevant technical field.

[0026] The terms used in this application are used merely to describe specific examples. For this reason, singular expressions include plural expressions unless the context clearly requires them to be singular. Additionally, it should be noted that terms such as “comprising” or “comprising” used in this application are used to clearly indicate the presence of features, steps, functions, components, or combinations thereof described in the specification, and are not used to preliminarily exclude the existence of other features, steps, functions, components, or combinations thereof.

[0027] Meanwhile, unless otherwise defined, all terms used in this specification shall be understood to have the same meaning as generally understood by those skilled in the art to which the present invention pertains. Accordingly, unless explicitly defined in this specification, specific terms should not be interpreted in an overly ideal or formal sense. Furthermore, terms such as “about,” “substantially,” etc., in this specification are used to mean at or near the numerical value when inherent manufacturing and material tolerances are presented in the stated meaning, and are used to prevent unscrupulous infringers from unfairly exploiting the disclosure in which precise or absolute numerical values ​​are mentioned to aid in understanding the present invention.

[0029] (Ingredient System)

[0030] Unless otherwise noted, units are weight percent.

[0031] C: 0.002~0.013%

[0032] It is an essential element in the stainless steel manufacturing process. Since an excessive increase in C content can lead to the formation of precipitates such as Cr carbides, which can adversely affect the composition and oxidation characteristics of the base material, the upper limit is restricted to 0.013%. However, since controlling the C content to an extremely low level results in an excessive increase in costs, it is desirable to limit the lower limit to 0.002%.

[0033] N: 0.002~0.015%

[0034] If the content increases excessively, it adversely affects quality due to the precipitation of various nitrides or the occurrence of pores, so the upper limit is limited to 0.015%. However, since controlling the N content to an extremely low level leads to an excessive increase in costs, it is desirable to limit the lower limit to 0.002% or higher.

[0035] Si: 0.05~0.2%

[0036] When the material is exposed to high temperatures, it forms an insulating film by forming a film-like oxide at the interface between the scale and the base material, so it is a component that must be strictly limited, and thus its upper limit is limited to 0.2%. However, since reducing the Si content to 0.05% or less requires high-cost processes such as vacuum melting, the lower limit in this invention is limited to 0.05%.

[0037] Mn: 0.2~0.9%

[0038] When stainless steel oxidizes at high temperatures, it diffuses rapidly to form dense Mn / Cr oxide on the outer layer of the scale, so at least 0.2% must be added. However, excessive addition of Mn promotes excessive growth of the scale, which may cause scale peeling, so the upper limit is restricted to 0.9%.

[0039] Cr: 19~25%

[0040] It is an essential element for ensuring the corrosion resistance of stainless steel. At least 19% must be added to prevent Cr depletion due to long-term oxidation in high-temperature oxidizing environments. However, to prevent increased manufacturing costs and the precipitation of Cr carbides and intermetallic compounds, it is desirable to limit the upper limit to 25%.

[0041] Mo: 0.4% or less (excluding 0)

[0042] It is an element that can increase the strength of the material in high-temperature environments. Since it is an expensive element, it is desirable to limit the upper limit to 0.4% to suppress the increase in manufacturing costs.

[0043] Nb: 0.1~0.7%

[0044] Due to its excellent oxidation properties, it oxidizes at the scale / base material interface to form oxides, which can suppress the formation of insulating Si oxides, so the lower limit is limited to 0.1%. On the other hand, if added excessively, it impairs hot workability and leads to increased manufacturing costs, so it is desirable to limit the upper limit to 0.7%.

[0045] Ti: 0.1% or less (excluding 0)

[0046] Although it has the effect of increasing the strength of the material by forming an internal oxide just below the interface between the base material and the scale at high temperatures, that is, near the surface of the base material, excessive addition leads to increased manufacturing costs and forms Ti oxide outside the scale, so it is desirable to limit the upper limit to 0.1%.

[0047] Al: 0.01~0.15%

[0048] Al oxide is an element primarily used in high-temperature materials due to its excellent oxidation resistance, but it is an element prone to forming internal oxides at high temperatures, such as at grain boundaries below the interface between the scale and the base material. Since internal oxides have an adverse effect on electrical conductivity, the Al content is limited to 0.15% or less. However, because reducing Al to 0.01% or less requires additional processing, which can lead to an increase in process costs, the lower limit in this invention is limited to 0.01%.

[0049] In addition, “Cr oxide” in this specification refers to all Cr oxides in oxide form that are obviously recognizable by a person skilled in the art, such as Cr3O4, Cr2O3, CrO2, and CrO3.

[0050] In addition, “Si oxide” in this specification refers to all Si oxides in oxide form that are obviously recognizable by a person skilled in the art, such as SiO2, SiO, and Si2O3.

[0051] In addition, “Cr-Mn oxide” in this specification refers to a complex oxide of chromium and manganese, and Cr-Mn oxide with high electrical conductivity is produced by the addition of Mn.

[0052] The ratio of (Cr-Mn oxide thickness) / (Cr oxide thickness + Si oxide thickness) must satisfy 2.0 or higher to have the electrical conductivity required for a fuel cell separator.

[0053] It is desirable that the Si weight fraction within the range of 0.1 μm above and 0.1 μm below the base material interface be 2% or less. Specifically, when using ferritic stainless steel as a material for a solid oxide fuel cell separator, the electrical conductivity of the scale formed on the surface of the stainless steel is important, and in particular, since Si oxides formed at the interface between the scale and the base material are a major cause of reduced electrical conductivity, if the Si weight fraction within the range of 0.1 μm above and 0.1 μm below the base material interface is 2% or less, the reduction in electrical conductivity due to the scale is not significant, making it possible to use it as a material for a solid oxide fuel cell separator. However, if the Si weight fraction within the range of 0.1 μm above and 0.1 μm below the base material interface is 2% or more, the reduction in electrical conductivity due to the scale caused by Si oxides occurs significantly, leading to a decrease in fuel cell efficiency due to the high resistance of the separator and making it impossible to use it as a separator material.

[0054] The present invention will be explained in more detail below through examples. However, it should be noted that the following examples are intended merely to illustrate and explain the invention in more detail, and are not intended to limit the scope of the invention. This is because the scope of the invention is determined by the matters described in the patent claims and matters reasonably inferred therefrom.

[0056] (Example)

[0057] Table 1 below shows the alloy composition including comparative and inventive examples of ferritic stainless steel, the Si weight fraction within a 0.2 μm range (0.1 μm above and 0.1 μm below) based on the interface between the scale and the base material, and (Cr-Mn oxide thickness) / (Cr oxide thickness + Si oxide thickness) within the scale.

[0058] When ferritic stainless steel is exposed to an oxidizing environment of 300 to 900°C, a Cr oxide is formed in the portion immediately above the base material of the ferritic stainless steel, a Cr-Mn composite oxide is formed on top of the oxide, a Si oxide is formed at the interface between the base material and the scale, and an Al oxide is formed in the portion of the base material below the scale interface. According to the example, the Si weight fraction within a 0.2 μm section, which is a range of 0.1 μm above and 0.1 μm below the interface between the scale formed on the base material and the base material, satisfies 2% or less, and satisfies (Cr-Mn oxide thickness) / (Cr oxide thickness + Si oxide thickness) ≥ 2.0.

[0059] The Si weight fraction was determined by measuring the weight fraction of metal atoms in the scale and the base material using GDS (Glow Discharge Spectroscopy) on the surface of the ferritic stainless steel on which scale had formed. The weight fraction of metal atoms was measured not only within the scale but also at the scale-base material interface and up to the base material. After plotting the depth profile (relationship between weight fraction and penetration depth) from the outermost layer of the scale to the base material, the Si weight fraction was measured within a 0.2 μm range, which is 0.1 μm above and 0.1 μm below the base material interface.

[0060] The oxide thickness was measured by observing the cross-section of the scale-formed ferritic stainless steel using TEM BFI (Bright Field Image). After taking a TEM BFI including a cross-section of 7 µm length, the oxide scale thickness was measured at 10 locations at 0.7 µm intervals, and the average value was used.

[0061] In addition, the ferritic stainless steel for a fuel cell separator according to the present embodiment has an area resistivity of 20 mΩ·cm 2 It may be less than.

[0062] Area Specific Resistance (ASR) was measured using the following method.

[0063] A ferritic stainless steel with an area of ​​2 cm x 2 cm was prepared and exposed to 800°C in the atmosphere for 500 hours to form an oxide scale on the surface. Subsequently, a platinum (Pt) mesh was attached to the scale surface formed on a 1 cm x 1 cm area of ​​the stainless steel using platinum (Pt) paste, and the system was prepared for resistance measurement using a platinum wire. Then, a constant current (100 mA) was applied through the platinum wire using a current source device capable of applying current, and the interface area (1 cm²) of the measured resistance was measured using a voltage meter capable of measuring voltage drop. 2Multiply by ) and divide by half to obtain the area resistivity (ASR, mΩ·cm) of the scale 2 ) calculated.

[0064] Looking at Examples 1 to 7 in Table 1, the addition of 0.1% or more of Nb suppressed the formation of Si oxide at the interface between the base material and the scale, resulting in a low Si oxide fraction. Additionally, the addition of 0.2% or more of Mn resulted in the formation of Cr-Mn oxide, and through the suppression of Si oxide, a (Cr-Mn oxide thickness) / (Cr oxide thickness + Si oxide thickness) of 2.0 or higher was observed.

[0065] Cr-Mn oxide, which has higher electrical conductivity than Cr oxide, is produced by the addition of Mn, but it must be produced at least 2.0 times thicker than the thickness of Cr oxide and Si oxide to have the electrical conductivity required as a fuel cell separator.

[0066] In contrast, Comparative Example 1 has less than 0.1% Nb added and exhibits a (Cr-Mn oxide thickness) / (Cr oxide thickness + Si oxide thickness) of less than 2.0, with an area resistivity of 20 mΩ·cm 2 It was confirmed that it exceeded. In addition, Comparative Examples 2 and 4 had Si added in excess of 0.2% and exhibited (Cr-Mn oxide thickness) / (Cr oxide thickness + Si oxide thickness) of less than 2.0, with an area resistivity of 20 mΩ·cm 2 It was confirmed that it exceeded. Furthermore, Comparative Example 3 had less than 0.2% Mn added and exhibited (Cr-Mn oxide thickness) / (Cr oxide thickness + Si oxide thickness) of less than 2.0, with an area resistivity of 20 mΩ·cm 2 It was possible to confirm that it exceeded.

[0067] FIG. 1 is a drawing relating to Example 3, FIG. 2 is a drawing relating to Comparative Example 2, FIG. 3 is a drawing relating to Comparative Example 4, FIG. 4 is an EDS graph showing the composition of Cr-Mn composite oxide in the oxide layer formed in Example 3, FIG. 5 is an EDS graph showing the composition of Cr oxide in the oxide layer formed in Example 3, and FIG. 6 is an EDS graph showing the composition of Si oxide in the oxide layer formed in Example 3.

[0068] To secure high-temperature electrical conductivity as a separator material, it is necessary to suppress the formation of Si oxide at the interface between the base material and the scale, and the formation of Al oxide in the base material portion beneath the scale. To achieve this, Nb and Ti are added to form Nb oxide at the interface between the scale and the base material and to form Ti internal oxide in the base material beneath the scale, thereby suppressing Si oxide and Al oxide.

[0069] Gangjong C N Si Mn Cr Mo Nb Ti Al Si weight fraction (wt%) Cr-Mn oxide thickness / (Cr oxide + Si oxide thickness) Area resistivity (ASR, mΩ·cm) 2 ) Example 1 0.003 0.005 0.11 0.5 19.6 0.2 0.13 0.08 0.08 1.7 2.1 19.7 Example 2 0.005 0.006 0.09 0.5 21.5 0.3 0.52 0.07 0.09 0.2 3.1 13.8 Example 3 0.005 0.007 0.08 0.6 22.4 0.1 0.62 0.04 0.07 <0.1 3.3 10.3 Example 4 0.011 0.005 0.12 0.3 23.6 0.2 0.25 0.05 0.12 1.2 2.7 17.1 Example 5 0.005 0.005 0.18 0.7 22.1 0.2 0.43 0.06 0.09 1.0 3.0 16.5 Example 6 0.007 0.009 0.08 0.5 22.0 0.3 0.38 0.05 0.09 0.7 2.9 15.4 Example 7 0.005 0.012 0.11 0.4 24.2 0.1 0.19 0.05 0.07 1.4 2.3 18.2 Comparative Example 1 0.005 0.005 0.12 0.5 21.9 0.2 0.07 0.05 0.09 3.5 1.1 53.1 Comparative Example 2 0.006 0.007 0.32 0.4 21.8 0.3 0.26 0.04 0.08 2.9 1.6 36.9 Comparative Example 3 0.007 0.006 0.11 0.1 22.3 0.3 0.31 0.06 0.09 1.1 0.5 45.7 Comparative Example 4 0.005 0.006 0.22 0.3 20.9 0.2 0.38 0.05 0.20 4.8 0.9 85.5

[0070] Although exemplary embodiments of the present invention have been described above, the present invention is not limited thereto, and those skilled in the art will understand that various changes and modifications are possible within the scope and concept of the claims set forth below.

Claims

Claim 1 A ferritic stainless steel for a fuel cell separator, comprising, in weight%, C: 0.002~0.013%, N: 0.002~0.015%, Si: 0.05~0.2%, Mn: 0.2~0.9%, Cr: 19~25%, Mo: 0.4% or less (excluding 0), Nb: 0.1~0.7%, Ti: 0.1% or less (excluding 0), Al: 0.01~0.15%, and the remainder being Fe and unavoidable impurities, wherein the Si weight fraction in a range of 0.1㎛ above and 0.1㎛ below the interface between the scale formed on the base material and the base material is 2% or less. Claim 2 The ferritic stainless steel for a fuel cell separator according to claim 1, satisfying the following formula (1): Formula (1): (Cr-Mn oxide thickness) / (Cr oxide thickness + Si oxide thickness) ≥ 2.0 Claim 3 In claim 1, the area resistivity is 20 mΩ·cm 2 Ferritic stainless steel for fuel cell separator plates.

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