Ferritic stainless steel

Abstract

The objective is to provide a ferritic stainless steel with excellent high-temperature conductivity by controlling the ratio of Cr to Mn and / or the thickness of the oxide scale. [Solution] The ferritic stainless steel of the present invention is characterized by comprising, in weight percent, C: 0.003-0.02, N: 0.003-0.02, Si: 0.05-0.5, Mn: 0.1-1.5, Cr: 19.0-25.0, Mo: 0.01-2.0, Nb: 0.05-0.7, Ti: 0.01-0.2, with the balance being Fe and other unavoidable impurities, and satisfying formula (1): 35≦Cr / Mn≦60. Here, Cr and Mn refer to the weight percent of each element.

Description

[Technical Field]

[0001] The present invention relates to a ferritic stainless steel, and more particularly to a ferritic stainless steel having excellent high-temperature conductivity by controlling the ratio of Cr to Mn or the thickness of oxide scale. [Background technology]

[0002] Stainless steel has excellent corrosion and oxidation resistance and is used in a variety of fields, from room temperature to high temperatures. Among these, much research is being conducted to produce stainless steel components, such as separators for fuel cells, which operate in high-temperature environments. To apply stainless steel to high-temperature fuel cells, the scale that forms on the surface of the stainless steel in a high-temperature oxidizing environment must not become too thick or its electrical conductivity must not decrease. If the scale becomes too thick, it may peel off and damage the material, and if the electrical conductivity is low, it may reduce the efficiency of the fuel cell.

[0003] When stainless steel is oxidized, chromium oxide (Cr2O3) forms on the surface, and this oxide scale made of chromium oxide provides corrosion resistance. However, while the scale formed in this process has excellent corrosion resistance, it also has low electrical conductivity. Stainless steel with excellent electrical conductivity is required. Summary of the Invention [Problem to be solved by the invention]

[0004] In order to solve the above-mentioned problems, an object of the present invention is to provide a ferritic stainless steel having excellent high-temperature conductivity by controlling the ratio of Cr to Mn and / or the thickness of the oxide scale.

[0005] The problems to be solved by the present invention are not limited to the above problems, and other problems not mentioned will be clearly understood by those skilled in the art from the following description. [Means for solving the problem]

[0006] As a means for achieving the above object, one example of the present invention provides a ferritic stainless steel characterized by being composed, by weight percent, of C: 0.003-0.02, N: 0.003-0.02, Si: 0.05-0.5, Mn: 0.1-1.5, Cr: 19.0-25.0, Mo: 0.01-2.0, Nb: 0.05-0.7, Ti: 0.01-0.2, with the remainder being Fe and other unavoidable impurities, and satisfying formula (1): 35≦Cr / Mn≦60. Here, Cr and Mn refer to the weight percent of each element.

[0007] The ferritic stainless steel of the present invention has a high-temperature interfacial conductivity of 40 mΩcm at 800°C. 2 The following is the result.

[0008] The ferritic stainless steel of the present invention has an oxide scale thickness of 3.5 μm or less.

[0009] The ferritic stainless steel of the present invention has oxide scale with a diameter of 3.0 μm or less.

[0010] In the ferritic stainless steel of the present invention, the oxide scale contains Cr, Mn, and O as main components. [Effects of the Invention]

[0011] According to the present invention, by taking into consideration the correlation between the Cr and Mn ratio and electrical conductivity, it is possible to provide a ferritic stainless steel that can form a fine and uniform oxide scale in a high-temperature oxidizing environment and ensure high-temperature interfacial conductivity. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 2 is a diagram showing the component composition of oxide scale in Example 1 of the present invention. [Figure 2] FIG. 2 is a diagram showing the shape of oxide scales in Examples 1 and 2 of the present invention. [Figure 3] FIG. 1 is a view showing the shape of oxide scales in Comparative Examples 2 and 3. DETAILED DESCRIPTION OF THE INVENTION

[0013] The following describes preferred embodiments of the present invention. However, the embodiments of the present invention can be modified into 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 art.

[0014] The terms used in this application are merely used to describe specific examples. Thus, for example, singular expressions include plural expressions unless the context clearly dictates otherwise. Note that the terms "comprise" or "include" 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 intended to preclude the presence of other features, steps, functions, components, or combinations thereof.

[0015] On the other hand, unless otherwise defined, all terms used herein should be considered to have the same meaning as commonly understood by a person of ordinary skill in the art to which the present invention pertains. Therefore, unless clearly defined herein, specific terms should not be construed in an overly ideal or formal sense.

[0016] Furthermore, in this specification, the terms "about," "substantially," and the like are used to mean from or near a numerical value when manufacturing and material tolerances inherent in the stated meaning are given, and are used to prevent unscrupulous infringers from unfairly taking advantage of disclosures in which precise or absolute numerical values ​​are stated to aid in the understanding of the present invention.

[0017] As a means for achieving the above-mentioned object, a ferritic stainless steel according to one example of the present invention can consist, in weight percent, of C: 0.003-0.02, N: 0.003-0.02, Si: 0.05-0.5, Mn: 0.1-1.5, Cr: 19.0-25.0, Mo: 0.01-2.0, Nb: 0.05-0.7, Ti: 0.01-0.2, with the remainder being Fe and other unavoidable impurities.

[0018] The reasons for limiting the range of each alloying element are as follows.

[0019] The C content may be 0.003 to 0.02% by weight.

[0020] Carbon is an essential element in the stainless steel manufacturing process. Excessive carbon content can lead to the formation of precipitates such as chromium carbides, which can adversely affect the composition and oxidation properties of the base material. Taking this into account, carbon content can be set to 0.02% or less. However, controlling the carbon content to an extremely low level can result in excessive cost increases. Taking this into account, carbon content can be set to 0.003% or more. The N content may be 0.003 to 0.02% by weight.

[0021] If the N content is excessive, various nitrides may precipitate or pores may be generated, adversely affecting quality. Taking this into consideration, the N content can be set to 0.02% or less. However, controlling the N content to an extremely low level may result in excessive cost increases. Taking this into consideration, the N content can be set to 0.003% or more.

[0022] The Si content may be 0.05 to 0.5% by weight.

[0023] Silicon is an element that must be strictly limited because it forms a film-like precipitate at the interface between the scale and the base material when the material is exposed to high temperatures, forming an insulating film. Taking this into consideration, the Si content can be limited to 0.5% or less. However, reducing the Si content to less than 0.05% may require expensive processes such as vacuum melting. Taking this into consideration, the Si content can be limited to 0.05% or more.

[0024] The Mn content may be 0.1 to 1.5% by weight.

[0025] When stainless steel is oxidized at high temperatures, Mn rapidly diffuses and forms dense manganese / chromium oxides in the outer layer of the scale. Taking this into consideration, Mn can be added at 0.1% or more. However, excessive addition of Mn can excessively promote scale growth and cause scale spalling. Taking this into consideration, Mn can be limited to 1.5% or less. In the present invention, Mn, together with Cr, is correlated with electrical conductivity, and electrical conductivity can be ensured by controlling Mn together with Cr.

[0026] The Cr content may be 19.0 to 25.0 wt %.

[0027] Cr is an essential element for ensuring the corrosion resistance of stainless steel. It is necessary to prevent Cr depletion due to prolonged oxidation in a high-temperature oxidizing environment. Taking this into consideration, 19.0% or more of Cr can be added. However, it is necessary to prevent an increase in manufacturing costs and the precipitation of chromium carbides, intermetallic compounds, etc. Taking this into consideration, the Cr content can be limited to 25.0% or less. In the present invention, it is ensured that Cr, together with Mn, has a correlation with electrical conductivity, and electrical conductivity can be ensured by controlling Cr together with Mn.

[0028] The Mo content may be 0.01 to 2.0% by weight.

[0029] Mo is an element that can increase the strength of materials in high-temperature environments. Therefore, Mo can be added at 0.01% or more. However, since Mo is an expensive element, it is necessary to suppress increases in manufacturing costs. Taking this into consideration, Mo can be limited to 2.0% or less.

[0030] The Nb content may be 0.05 to 0.7 wt %.

[0031] Nb has excellent oxidation properties, so it is oxidized at the scale / base metal interface to form oxides, which can suppress the formation of insulating silicon oxides. Taking this into consideration, the Nb content can be set to 0.05% or more. However, excessive addition of Nb can impair hot workability and increase manufacturing costs. Taking this into consideration, the Nb content can be set to 0.7% or less.

[0032] The Ti content may be 0.01 to 0.2% by weight.

[0033] At high temperatures, Ti forms an internal oxide just below the interface between the base metal and the scale, i.e., near the surface of the base metal, thereby increasing the strength of the material. Taking this into consideration, Ti content can be set to 0.01% or more. However, adding too much Ti increases manufacturing costs and can form titanium oxide outside the scale. Taking this into consideration, Ti content can be set to 0.2% or less. The remaining component is iron (Fe). However, in the normal manufacturing process, unintentional impurities may be inevitably mixed in from the raw materials or the surrounding environment, and this cannot be excluded. Since these impurities are known to anyone skilled in the normal manufacturing process, not all of their contents will be specifically mentioned in this specification.

[0034] A ferritic stainless steel according to one example of the present invention may be a ferritic stainless steel that satisfies the formula (1): 35≦Cr / Mn≦60, where Cr and Mn represent the weight percentages of each element.

[0035] The ferritic stainless steel according to one example of the present invention has a high temperature interface conductivity of 40 mΩcm at 800°C. 2 It may be a ferritic stainless steel, which is:

[0036] When the ratio of Cr to Mn is between 35 and 60, the high-temperature interfacial conductivity at 800°C is 40mΩcm. 2 This shows that the ratio of Cr to Mn is correlated with electrical conductivity, and by controlling the ratio of Cr to Mn to between 35 and 60, the high-temperature interfacial conductivity at 800°C is 40mΩcm. 2 The above results in excellent electrical conductivity.

[0037] The ferritic stainless steel according to one example of the present invention may be a ferritic stainless steel having an oxide scale thickness of 3.5 μm or less, or a ferritic stainless steel having an oxide scale diameter of 3.0 μm or less. The main components of the oxide scale of the present invention may be Cr, Mn, and O.

[0038] The oxide scale of the present invention may be CrMn oxide, which has a thick Cr oxide-independent spinel structure that is commonly formed on the surface of stainless steel, ensuring excellent electrical conductivity.

[0039] In the present invention, the oxide scale may have a thickness of 3.5 μm or less and a diameter of 3.0 μm or less. Here, the oxide scale can be considered as the thickness and diameter of CrMn oxide by controlling the ratio of Cr to Mn.

[0040] When the thickness of the oxide scale is 3.5 μm or less and the diameter is 3.0 μm or less, this corresponds to the uniform formation of plate-shaped CrMn oxide on the surface layer of the ferritic stainless steel. When CrMn oxide is uniformly formed, excellent electrical conductivity can be ensured. Even if the thickness of the CrMn oxide is 3.5 μm or less and / or the diameter is 3.0 μm or less and the scale is uniform, if the Cr to Mn ratio does not satisfy the range of 35 to 60, the high-temperature interfacial conductivity at 800°C is 40 mΩ cm or less, which is considered to be excellent electrical conductivity. 2 The following cannot be guaranteed:

[0041] When ferritic stainless steel is exposed to an oxidizing environment at 300 to 900°C, CrMn oxides can form on the surface of the ferritic stainless steel. If the composition of the ferritic stainless steel falls within the range of the present invention and the Cr / Mn ratio is 35 to 60, fine and uniform CrMn oxides can be formed on the surface. By controlling this, the high-temperature interfacial conductivity at 800°C can be reduced to 40 mΩcm. 2 The more the thickness is secured below this, the better the electrical conductivity can be obtained at high temperatures. Also, the fine and uniform CrMn oxide can have an oxide scale with a thickness of 3.5 μm or less and a diameter of 3.0 μm or less.

[0042] The ferritic stainless steel of the present invention can be produced by referring to the conventional method for producing ferritic stainless steel, and is not limited to the production method described in the following examples. For example, the annealing temperature, rolling reduction, etc. in the following examples can be changed as necessary.

[0043] The present invention will be described in more detail below with reference to examples and drawings. However, such description is for the purpose of explaining the implementation of the present invention, and the present invention is not limited by such description of the embodiments. The scope of the present invention is determined by the matters described in the claims and matters reasonably inferred therefrom. [Example]

[0044] Table 1 below shows the alloy compositions of the invention examples and comparative examples, and whether or not ferritic stainless steels having the values ​​of formula (1) can ensure high-temperature interfacial conductivity. The alloy compositions shown in Table 1 below were produced by casting 50 kg ingots, which were then reheated at approximately 1240°C and hot-rolled to a thickness of approximately 5.5 mm. The hot-rolled material was annealed at approximately 1050°C and cold-rolled to a thickness of approximately 2.0 mm. The cold-rolled material was then annealed at approximately 1050°C to prepare 15 mm x 15 mm samples, which were then used to produce ferritic stainless steel test pieces.

[0045] High temperature interface conductivity is 40mΩcm 2 The following cases are indicated by "○" and are 40mΩcm 2 If the temperature exceeded the limit, the conductivity was classified as "x." Conductivity was determined by measuring the ASR (Area Specific Resistance) for 500 hours at 800°C for samples that had been pre-heat treated at 800°C for 100 hours.

[0046] [Table 1]

[0047] In Examples 1 and 2, the alloy composition and the value of formula (1) satisfy the range of the present invention. The high-temperature interfacial conductivity at 800°C is 40 mΩcm. 2 The results are as follows, confirming that high-temperature interfacial conductivity was ensured. In Comparative Examples 1 to 4, the alloy compositions satisfy the range of the present invention, but the value of formula (1) is less than 35 or exceeds 60. Specifically, in Comparative Examples 1 and 2, the values ​​of formula (1) are 74.5 and 63.7, respectively, and Cr was added in excess relative to Mn.

[0048] In Comparative Examples 3 and 4, the values ​​of formula (1) were 34.1 and 32.1, respectively, and Cr was added in a small amount compared to Mn. This confirms that even if the alloy composition satisfies the range of the present invention, if the Cr / Mn ratio is outside the range of the present invention, the high-temperature interfacial conductivity at 800°C cannot be ensured, i.e., high-temperature conductivity cannot be ensured.

[0049] Table 2 below shows the thickness and diameter of the CrMn oxide of Inventive Examples 1 and 2 and Comparative Examples 2 and 3. The thickness of the CrMn oxide of the present invention was measured by processing the steel to a width of approximately 30 μm and a depth of 10 μm using a focused ion beam (FIB) and observing it using a scanning electron microscope (SEM). The thickness was measured three times at random positions on the processed steel, and it was confirmed whether the average value was 3.5 μm or less. The diameter of the CrMn oxide of the present invention was measured by observation using the SEM. CrMn oxide with a diameter of 3.0 μm or less was observed as plate-like CrMn oxide, and CrMn oxide with a diameter of more than 3.0 μm was observed as single CrMn oxide.

[0050] [Table 2]

[0051] Inventive Examples 1 and 2, whose alloy compositions and values ​​of formula (1) fall within the ranges of the present invention, the thicknesses of the CrMn oxide are 2.12 μm and 2.24 μm, respectively, and are 3.5 μm or less. The diameter of the CrMn oxide is 3.0 μm or less, and the CrMn oxide layer is observed to be plate-like. Comparative Example 2, whose value of formula (1) is 63.7 (exceeding 60), has a thickness of 3.67 μm. Furthermore, the diameter of the CrMn oxide exceeds 3.0 μm, and single-type CrMn oxide is present. As shown in Tables 1 and 2, it can be seen that when formula (1) is not satisfied and the thickness of the CrMn oxide exceeds 3.5 μm, conductivity cannot be ensured at high temperatures.

[0052] In Comparative Example 3, where the value of formula (1) is 34.1, which is less than 35, the thickness of the CrMn oxide is 2.74 μm. Furthermore, the diameter of the CrMn oxide exceeds 3.0 μm, resulting in a single-type CrMn oxide. As shown in Tables 1 and 2, Comparative Example 3 is formed to a thickness of 3.5 μm or less, which corresponds to the thickness range of the CrMn oxide of the present invention. However, because the Cr / Mn ratio is less than 35, the CrMn oxide layer is not dense and sound, and internal pores are formed. As a result, Comparative Example 3 cannot ensure conductivity at high temperatures. Therefore, when the value of formula (1) is less than 35, high-temperature conductivity cannot be ensured, even if the CrMn oxide is relatively thin, i.e., 3.5 μm or less. Therefore, in the present invention, the thickness of CrMn oxide can be controlled by whether or not formula (1) is satisfied, which controls the ratio of Cr and Mn simultaneously with the alloy composition. It can be confirmed that whether or not formula (1) is satisfied is the most important factor for high temperature conductivity.

[0053] The present invention will be described in more detail below with reference to Tables 1 and 2 and the accompanying drawings.

[0054] FIG. 1 shows the chemical composition of the CrMn oxide scale of Example 1. FIG. 2 shows the surface and cross-section of the CrMn oxide scale of Examples 1 and 2. FIG. 3 shows the surface and cross-section of the CrMn oxide scale of Comparative Examples 2 and 3.

[0055] The components of the CrMn oxide of the present invention were analyzed using FIB SEM / EDS (energy dispersive spectroscopy).

[0056] As shown in FIG. 1, the oxide scale of the present invention is not Cr oxide but CrMn oxide that can ensure conductivity, and it can be seen that the main components are Cr, Mn, and O.

[0057] The thickness of the CrMn oxide of the present invention was measured using an FIB to obtain a width of approximately 30 μm and a depth of 10 μm, and the surface and cross-sectional shapes of the CrMn oxide were also observed using an SEM.

[0058] The shapes of the surface and cross section of the CrMn oxide of the inventive and comparative examples can be seen in Figures 2 and 3. Figure 2 shows SEM photographs of the surface and cross section of a stainless steel sample having an alloy composition corresponding to the inventive example after high-temperature oxidation at 800°C for 500 hours, and Figure 3 shows SEM photographs of the surface and cross section of a stainless steel sample having an alloy composition corresponding to the inventive example after high-temperature oxidation at 800°C for 500 hours.

[0059] In Figure 2, in the case of Inventive Examples 1 and 2, which satisfy the range of formula (1) of the present invention, CrMn oxide particles having a diameter of 3.0 μm or less are formed in the surface layer, and it can be observed that the CrMn oxide particles are formed very uniformly and are plate-like. Furthermore, it can be seen that the thickness of the CrMn oxide particles is 3.5 μm or less. This confirms that in order to ensure excellent high-temperature electrical conductivity, the value of formula (1) must satisfy the range of the present invention, and that formula (1) must satisfy the range of the present invention so that CrMn oxide particles are finely and uniformly distributed.

[0060] 3, in Comparative Examples 2 and 3, which do not satisfy the range of formula (1) of the present invention, CrMn oxide particles having a diameter exceeding 3.0 μm are sometimes formed in the surface layer, and single CrMn oxide particles formed very irregularly can be observed. However, in Comparative Example 3, although the thickness of the CrMn oxide is 3.5 μm or less, it can be seen that the CrMn oxide particles were not densely and soundly formed, which prevented high-temperature electrical conductivity from being ensured.

[0061] The ferritic stainless steel of the present invention is exposed to a high-temperature oxidizing environment at 300 to 900°C, and CrMn oxide is formed on the surface. When the Cr to Mn ratio is 35 to 60, fine and uniform CrMn oxide is formed on the surface, and the high-temperature interface conductivity at 800°C is 40mΩcm. 2 The results are as follows, and it can be confirmed that high-temperature interfacial conductivity was ensured.

Claims

1. In weight percent, C: 0.003 to 0.02, N: 0.003 to 0.02, Si: 0.05 to 0.5, Mn: 0.1 to 1.5, Cr: 19.0 to 25.0, Mo: 0.01 to 2.0, Nb: 0.05 to 0.7, Ti: 0.01 to 0.2, the balance being Fe and other unavoidable impurities, A ferritic stainless steel characterized by satisfying the following formula (1): Formula (1): 35≦Cr / Mn≦60 (Here, Cr and Mn refer to the weight percentage of each element.)

2. High temperature interface conductivity at 800°C is 40mΩcm ２ 2. The ferritic stainless steel according to claim 1, wherein:

3. 2. The ferritic stainless steel according to claim 1, wherein the thickness of the oxide scale is 3.5 μm or less.

4. 2. The ferritic stainless steel according to claim 1, wherein the diameter of the oxide scale is 3.0 μm or less.

5. 5. The ferritic stainless steel according to claim 3, wherein the oxide scale contains Cr, Mn, and O as main components.

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