Ferritic stainless steel and manufacturing method therefor

Abstract

The present invention relates to a ferritic stainless steel and a manufacturing method therefor and, more specifically, to a ferritic stainless steel and a manufacturing method therefor, the ferritic stainless steel comprising: 0.003 to 0.02% of C, 0.003 to 0.02% of N, 0.05 to 0.2% of Si, 0.2 to 1.0% of Mn, 18.0 to 24.0% of Cr, 0.01 to 2.5% of Mo, 0.2 to 0.7% of Nb, 0.01 to 0.15% of Ti, and the balance being Fe and inevitable impurities, wherein the ferritic stainless steel satisfies expression (1) below. Expression (1): 60.0 ≤ ([Cr] + [Mo] + [Nb] + [Mn]) / ([Si]+[Ti]) ≤ 140.0 (where, [Cr], [Mo], [Nb], [Mn], [Si], and [Ti] denote wt% of each element.)

Description

Ferritic stainless steel and method of manufacturing the same

[0001] The present invention relates to ferritic stainless steel and a method for manufacturing the same.

[0002] Due to its excellent corrosion and oxidation resistance, stainless steel is applied in various fields ranging from room temperature to high temperatures. Among these, extensive research is being conducted to manufacture components such as fuel cell separators, which operate in high-temperature environments, using stainless steel.

[0003] To apply stainless steel to high-temperature fuel cells, the thickness of the scale formed on the surface of the stainless steel in a high-temperature oxidizing environment must not become excessive, and the integrity between the scale and the base material must be ensured. If the scale thickness exceeds a certain level, it may peel off and damage the material; furthermore, if the integrity between the scale and the base material is not ensured, electrical conductivity will be low, which can reduce the efficiency of the fuel cell.

[0004] Stainless steel used as a conventional linker for solid oxide fuel cells has secured high-temperature oxidation resistance and electrical conductivity by adding rare earth elements such as La, Zr, Y, or Ce. However, when rare earth elements are added, mass production is difficult and manufacturing costs increase excessively because the product must be manufactured using an ingot manufacturing method.

[0005] One aspect of the present disclosure provides a stainless steel having high electrical conductivity in a high-temperature environment without containing rare earth elements, and a method for manufacturing the same.

[0006] One aspect of the present disclosure provides a stainless steel having excellent oxidation resistance at high temperatures and a method for manufacturing the same.

[0007] One aspect of the present disclosure provides a stainless steel having high electrical conductivity by ensuring scale integrity in a high-temperature oxidizing environment, and a method for manufacturing the same.

[0008] The technical problems to be solved in this document are not limited to those mentioned above, and other technical problems not mentioned will be clearly understood by those skilled in the art to which this invention belongs from the description below.

[0009] A ferritic stainless steel according to one embodiment of the present disclosure may comprise, in weight percent, C: 0.003 to 0.02%, N: 0.003 to 0.02%, Si: 0.05 to 0.2%, Mn: 0.2 to 1.0%, Cr: 18.0 to 24.0%, Mo: 0.01 to 2.5%, Nb: 0.2 to 0.7%, Ti: 0.01 to 0.15%, and the remainder being Fe and unavoidable impurities. The stainless steel may satisfy Equation (1): 60.0 ≤ ([Cr] + [Mo] + [Nb] + [Mn]) / ([Si]+[Ti]) ≤ 140.0. Here, [Cr], [Mo], [Nb], [Mn], [Si], and [Ti] represent the weight percent of each element.

[0010] A method for manufacturing ferritic stainless steel according to one embodiment of the present disclosure may include the steps of reheating a slab, producing a hot-rolled material by hot rolling and hot-rolling annealing after reheating, and cold rolling and cold-rolling annealing the hot-rolled material. The slab may contain, in weight percent, C: 0.003 to 0.02%, N: 0.003 to 0.02%, Si: 0.05 to 0.2%, Mn: 0.2 to 1.0%, Cr: 18.0 to 24.0%, Mo: 0.01 to 2.5%, Nb: 0.2 to 0.7%, Ti: 0.01 to 0.15%, and the remainder being Fe and unavoidable impurities. The above slab may satisfy Equation (1): 60.0 ≤ ([Cr] + [Mo] + [Nb] + [Mn]) / ([Si]+[Ti]) ≤ 140.0. Here, [Cr], [Mo], [Nb], [Mn], [Si], and [Ti] represent the weight percent of each element. The above slab may be reheated at 1050 to 1280°C. The above hot rolling annealing may be performed at 900 to 1150°C. The above cold rolling annealing may be performed at 900 to 1150°C.

[0011] According to the present disclosure, a ferritic stainless steel capable of maintaining high electrical conductivity even in a high-temperature oxidizing environment while having excellent oxidation resistance characteristics, and a method for manufacturing the same can be provided.

[0012] According to the present disclosure, a ferritic stainless steel capable of maintaining high electrical conductivity over a long period of time even when applied as a solid oxide fuel cell connector and a method for manufacturing the same can be provided.

[0013] The effects obtainable from the present disclosure are not limited to those mentioned above, and other unmentioned effects will be clearly understood by those skilled in the art to which the present disclosure belongs from the description below.

[0014] Figure 1 is a high temperature (800°C) sheet resistance result according to the value of Equation (1) of ferritic stainless steel according to one embodiment of the present invention.

[0015] FIG. 2 is a TEM (Transmission Electron Microscope) image of a cross-sectional oxide layer of stainless steel after forming an oxide layer in an atmospheric atmosphere at a high temperature (800°C) for 2,000 hours according to one embodiment of the present invention.

[0016] Figure 3 is a TEM image of a cross-sectional oxide layer of stainless steel after forming an oxide layer in an atmospheric atmosphere at a high temperature (800°C) for 2,000 hours according to one comparative example of the present invention.

[0017] Hereinafter, embodiments of the present invention will be described in detail with reference to the attached drawings. In this specification, the same reference numerals denote the same elements throughout. Furthermore, various elements and areas in the drawings are depicted schematically. Accordingly, the technical concept of the present invention is not limited by the relative sizes or spacing depicted in the attached drawings. The terms used in this specification are intended to describe the present invention and are not intended to limit the present invention. Additionally, singular forms used in this specification include plural forms unless the relevant definitions clearly indicate otherwise.

[0018] Unless otherwise noted, units are weight percent. Furthermore, when a part is described as "containing" a certain component, this means that, unless specifically stated otherwise, it does not exclude other components but may include additional components.

[0019] Unless otherwise defined, all terms used herein, including technical and scientific terms, have the same meaning as generally understood by those skilled in the art to which the present invention pertains. Terms defined in advance are interpreted to have meanings consistent with relevant technical literature and the presently disclosed content.

[0020] Additionally, terms such as "about," "substantially," etc., in this specification are used to mean at or near the stated value when inherent manufacturing and material tolerances are presented in the said sense, and are used to prevent unscrupulous infringers from unfairly exploiting the disclosed content in which precise or absolute values ​​are mentioned to aid in understanding the invention.

[0021] Ferritic stainless steel

[0022] A ferritic stainless steel according to one embodiment of the present invention may comprise, in weight percent, C: 0.003 to 0.02%, N: 0.003 to 0.02%, Si: 0.05 to 0.2%, Mn: 0.2 to 1.0%, Cr: 18.0 to 24.0%, Mo: 0.01 to 2.5%, Nb: 0.2 to 0.7%, Ti: 0.01 to 0.15%, and the remainder being Fe and unavoidable impurities.

[0023] The reasons for limiting the composition of the above lecture are explained in detail below. Unless otherwise specified, all compositions below refer to weight percent.

[0024] Carbon (C): 0.003% to 0.02%

[0025] C is an essential element in the stainless steel manufacturing process. If the C content increases excessively, precipitates such as chromium carbides may form, which can adversely affect the composition and oxidation characteristics of the base material. Considering this, it is desirable to limit the upper limit of C to 0.02%. However, since controlling the C content to an extremely low level leads to an excessive increase in costs, it is desirable to limit the lower limit of C to 0.003%.

[0026] Nitrogen (N): 0.003% to 0.02%

[0027] If the content of N increases excessively, it can adversely affect quality by causing the precipitation of various nitrides or the formation of pores. Considering this, it is desirable to limit the upper limit of N to 0.02%. 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 of N to 0.003%.

[0028] Silicon (Si): 0.05% to 0.2%

[0029] Si is a component that must be strictly limited because, when the material is exposed to high temperatures, it can form film-like precipitates at the interface between the scale and the base material, thereby forming an insulating film and causing micropores (holes) at the interface. Considering this, it is desirable to limit the upper limit of Si to 0.2%. However, since reducing the Si content to 0.05% or less requires high-cost processes such as vacuum melting, it is desirable to limit the lower limit of Si to 0.05%.

[0030] Manganese (Mn): 0.2% to 1.0%

[0031] When stainless steel oxidizes at high temperatures, Mn diffuses rapidly to form a dense manganese / chromium oxide on the outer layer of the scale. Considering this, it is desirable to limit the lower limit of Mn to 0.2%. However, excessive addition of Mn may excessively promote scale growth and cause scale peeling; therefore, considering this, it is desirable to limit the upper limit of Mn to 1.0%.

[0032] Chrome (Cr): 18.0% to 24.0%

[0033] Cr is an essential element for ensuring the corrosion resistance of stainless steel. It is necessary to prevent the depletion of Cr due to oxidation over a long period in high-temperature oxidizing environments. Considering this, it is desirable to limit the lower limit of Cr to 18.0%. However, to prevent an increase in manufacturing costs and the precipitation of chromium carbides, intermetallic compounds, etc., it is desirable to limit the upper limit of Cr to 24.0%.

[0034] Molybdenum (Mo): 0.01% to 2.5%

[0035] Mo is an element that can increase the strength and oxidation resistance of materials in high-temperature environments. Therefore, it is desirable to limit the lower limit of Mo to 0.01%. However, since Mo is an expensive element, it is necessary to suppress the increase in manufacturing costs, and considering this, it is desirable to limit the upper limit of Mo to 2.5%.

[0036] Niobium (Nb): 0.2% to 0.7%

[0037] Due to its excellent oxidation properties, Nb oxidizes at the scale / base material interface to form oxides, thereby suppressing the formation of insulating silicon oxide and contributing to the improvement of the material's strength. Considering this, it is desirable to limit the lower limit of Nb to 0.2%. On the other hand, excessive addition of Nb impairs hot workability and leads to increased manufacturing costs; therefore, considering this, it is desirable to limit the upper limit of Nb to 0.7%.

[0038] Titanium (Ti): 0.010% to 0.15%

[0039] Ti is an element that inhibits scale peeling 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. Considering this, it is desirable to limit the lower limit of Ti to 0.010%. However, since excessive addition of Ti leads to increased manufacturing costs and forms titanium oxide on the outside of the scale, it is desirable to limit the upper limit of Ti to 0.15%.

[0040] The remaining component is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be incorporated during the ordinary manufacturing process, they cannot be excluded. As these impurities are known to any skilled person in the ordinary manufacturing process, all details thereof are not specifically mentioned in this specification.

[0041] A ferritic stainless steel according to one embodiment of the present invention can satisfy the following formula (1).

[0042] Equation (1): 60.0 ≤ ([Cr] + [Mo] + [Nb] + [Mn]) / ([Si]+[Ti]) ≤ 140.0

[0043] Here, [Cr], [Mo], [Nb], [Mn], [Si], and [Ti] represent the weight percent of each element.

[0044] The above equation (1) is an indicator of the soundness between the base material and the scale. Equation (1) limits the ratio of Si and Ti content to the sum of Cr, Mo, Nb, and Mn content to a certain range, representing a composition necessary to reduce the thickness growth rate of oxides formed at high temperatures and suppress voids in the form of micro-holes. As the ratio of Si and Ti content to the sum of Cr, Mo, Nb, and Mn content increases, the bonding strength between the base material and the oxides formed at high temperatures becomes stronger, and the growth of oxides is suppressed, thereby ensuring surface conductivity durability. In particular, Cr, Mo, Nb, and Mn are advantageous for preventing the formation of micro-holes that are formed due to inter-lattice shrinkage and expansion, which can adversely affect conductivity, as Si oxides and Ti oxides are formed at the interface. Considering the inhibition of oxide growth and the prevention of micro-hole formation between the base material and the oxide interface, the minimum content ratio of the sum of Si and Ti to the sum of Cr, Mo, Nb, and Mn is 60.0 or higher. However, since Nb, which can prevent the formation of micro-holes by forming Nb oxide or Laves phase (Fe2Nb or Nb2Si), and Mo, which can inhibit the oxide growth rate, are expensive components, economic feasibility must be considered, and excessive addition of Cr and Mn can actually inhibit the oxide growth rate, so the upper limit is preferably 140.0.

[0045] A ferritic stainless steel according to one embodiment of the present invention can satisfy the following formula (2).

[0046] Equation (2): 20.0 ≤ [Cr] / [Mn] ≤ 80.0

[0047] Here, [Cr] and [Mn] represent the weight percentage of each element.

[0048] Since Mn has a fast oxidation and diffusion rate at high temperatures, if its content is excessive, it can impair oxidation resistance due to excessive oxide growth, thereby causing scale peeling or reducing conductivity. Therefore, the upper limit of Equation (2) is preferably 80.0. Meanwhile, since it can reduce the formation of the second scale layer (CrMn) spinel that causes excessive growth of the first scale layer (Cr2O3) formed at high temperatures, the lower limit is preferably 20.0.

[0049] A ferritic stainless steel according to one embodiment of the present invention comprises a first scale layer containing chromium oxide at a base material interface in an atmospheric atmosphere of 600 to 850°C, a second scale layer containing chromium oxide and manganese oxide formed on the surface of the first scale layer, and an interface layer formed directly below the first scale layer, wherein the interface layer may include one or more of Ti oxide, Si oxide, Al oxide, or Laves phase. Regarding the prevention of the formation of micro-holes, it is more preferable to minimize Si oxide and form Ti oxide, Al oxide, or Laves phase among the phases formed in the interface layer.

[0050] The above oxide and Laves phase possess high atomic density and a stable crystal structure, which can contribute to strength and oxidation resistance. Here, the Laves phase is a typical Topologically Closed-Packed (TCP) phase containing Nb, having a hexagonal MgZn2-type crystal structure, and may contain one or more of Fe2Nb and Nb2Si.

[0051] In a ferritic stainless steel according to one embodiment of the present invention, the thickness of the first scale layer and the second scale layer may be 4 μm or less when exposed to an atmospheric atmosphere of 600 to 800°C for 2000 hours.

[0052] Referring to FIGS. 2 and FIGS. 3, it can be seen that a first scale layer is formed on the upper surface of the base material interface, and a second scale layer is formed thereon.

[0053] The interfacial layer consists of Ti, Si, and Al oxides. It can be confirmed that Cr oxide exists in a dense form in the first scale layer, and Cr and Mn oxides exist in a coarse form in the second scale layer.

[0054] The above "thickness of the first scale layer and the second scale layer" refers to the arithmetic mean thickness of the maximum and minimum thicknesses of the portion including the first scale layer and the second scale.

[0055] If the thickness of the scale exceeds a certain level, the scale may peel off and damage the material. In addition, as the thickness of the scale increases, the stress on the base material may increase, causing the effects of improved oxidation peelability and improved electrical conductivity to disappear. Therefore, in order to fully exert these effects, it is desirable to control the thickness of the first and second scale layers to 4 μm or less when exposed to a high-temperature operating environment for 2000 hours in order to secure surface conductivity.

[0056] A ferritic stainless steel according to one embodiment of the present invention may have a void area fraction included in the interface layer of less than 3%.

[0057] In order to ensure oxidation resistance and surface conductivity durability in a high-temperature oxidizing environment, integrity between the base material and the scale must be ensured. To this end, it is necessary to minimize the voids included at the base material / scale layer interface. To achieve the objective of the present invention, it is preferable that the area fraction of voids included in the interface layer be less than 3%.

[0058] A ferritic stainless steel according to one embodiment of the present invention may have a high-temperature surface resistance of 40 mΩ·cm2 or less even after exposure to a temperature of 600 to 800°C for 2000 hours.

[0059] Materials for solid oxide fuel cells must ensure electrical conductivity at high temperatures. Therefore, electrical conductivity can be ensured by controlling the sheet resistance to 40 mΩ·cm2 or less at a temperature of 600 to 800°C, where the fuel cell operates.

[0060] Next, a method for manufacturing ferritic stainless steel according to one embodiment of the present invention will be described in detail.

[0061] A method for manufacturing ferritic stainless steel according to one embodiment of the present invention may include the steps of reheating a slab, manufacturing a hot-rolled material by hot rolling and annealing after reheating, and cold rolling and cold annealing the hot-rolled material.

[0062] The above slab contains, in weight%, C: 0.003 to 0.02%, N: 0.003 to 0.02%, Si: 0.05 to 0.2%, Mn: 0.2 to 1.0%, Cr: 18.0 to 24.0%, Mo: 0.01 to 2.5%, Nb: 0.2 to 0.7%, Ti: 0.01 to 0.15%, the remainder being Fe and unavoidable impurities, and may satisfy the following formula (1).

[0063] Equation (1): 60.0 ≤ ([Cr] + [Mo] + [Nb] + [Mn]) / ([Si]+[Ti]) ≤ 140.0

[0064] Here, [Cr], [Mo], [Nb], [Mn], [Si], and [Ti] represent the weight percent of each element.

[0065] In addition, the above slab can satisfy the following equation (2).

[0066] Equation (2): 20.0 ≤ [Cr] / [Mn] ≤ 80.0

[0067] Here, [Cr] and [Mn] represent the weight percentage of each element.

[0068] The reason for setting the composition range of each alloy element, Equation (1) and Equation (2) above may be as described above.

[0069] First, the above slab can be reheated at 1050°C to 1280°C, and then hot-rolled and hot-rolled annealed to produce a hot-rolled material.

[0070] The above reheating temperature may be 1050°C or higher to reduce the hot rolling load, and may be limited to 1280°C or lower to prevent internal grain coarsening.

[0071] The finishing rolling temperature during the above hot rolling may be 700 to 950°C, and the thickness of the hot-rolled material produced in this way may be 2 to 6 mm.

[0072] When the finishing rolling temperature during the above hot rolling is below 700℃, the rolling load increases and shape defects increase, which may lower productivity, and when it exceeds 950℃, the surface quality may deteriorate due to an increase in oxides caused by excessive high-temperature operation.

[0073] The hot rolling annealing temperature of the above hot-rolled material may be 900 to 1150℃.

[0074] If the above hot rolling annealing temperature is less than 900℃, recrystallization does not occur and a texture may not be formed, and if it exceeds 1150℃, the grains may coarsen and the strength of the material may be weakened.

[0075] The above hot-rolled annealed material can be cold-rolled and cold-rolled annealed.

[0076] The above cold rolling annealing can be performed at 900 to 1150°C, and the thickness of the final cold-rolled product can be 0.1 to 2.5 mm.

[0077] If the above cold rolling annealing is below 900℃, the stress formed during rolling is not sufficiently removed, which may result in reduced workability, and if it exceeds 1150℃, the grain size may become coarsened and plate breakage may occur.

[0078] The present invention will be explained in more detail below through examples. However, the description of these examples is merely for illustrating the implementation of the present invention and does not limit the present invention. This is because the scope of the rights of the present invention is determined by the matters described in the patent claims and matters reasonably inferred therefrom.

[0079] {Example}

[0080] Slabs were manufactured to satisfy the various alloy compositions shown in Table 1 below. The unit is weight%.

[0081] Classification CNSiMnCrMoNbTiPS Comparative Example 1 0.00 50.03 80.15 90.24 17.6 31.08 0.01 0.22 70.02 0.003 Comparative Example 2 0.00 50.00 80.17 40.16 520.4 10.29 0.00 20.26 60.02 520.0006 Comparative Example 3 0.00 69 0.00 80.16 30.199 17.6 70.4 30.00 20.30 10.02 830.0004 Comparative Example 4 0.02 16 0.00 80.199 0.246220.010.0030.2940.02390.0015Comparative Example 50.01160.0080.2420.27717.9600.4830.1960.02460.0004Comparative Example 60.01330.0070.4430.14619.440.010.43600.02290.0051Comparative Example 70.00760.0080.4390.88418.241.780.4630.1250.02310.0006Comparative Example 80 .00770.0090.180.10821.410.010.0780.2330.02140.0025 Example 10.00570.0090.1810.24718.651.940.3430.0180.02270.0006 Example 20.00540.010.140.4922.20.1730.440.0790.02520.0006 Example 30.0060.020.160.35230.120.470.080.0265 0.0004 Example 40.020.0080.130.74200.050.420.120.02320.0015 Example 50.00650.0080.090.64240.120.390.110.02460.0004 Example 60.00740.0080.190.4321.32.20.550.060.02320.0006 Example 70.00640.0070.120.3521.800.420.090.02460.0025

[0082] The manufactured ingot was reheated to a temperature of 1250°C for 2 hours, hot-rolled to a finish rolling temperature of 900°C, and then hot-rolled annealed at a temperature of 1050°C for 30 minutes to produce a hot-rolled material with a thickness of 5.0 mm. The hot-rolled material was cold-rolled to a thickness of 2.0 mm, cold-rolled annealed at a temperature of 1050°C for 1 minute, and a 15 mm × 15 mm sample was produced to manufacture a ferritic stainless steel specimen.

[0083] Table 2 below shows the oxide thickness after discontinuous high-temperature oxidation of the ferritic stainless steel manufactured above at Equation (1) and Equation (2), repeating at 800°C and a heating / cooling rate of 100°C / hour every 250 hours for a total of 2000 hours, and the scale / base material interface integrity after discontinuous high-temperature oxidation at 800°C and a heating / cooling rate of 100°C / hour every 250 hours for a total of 2000 hours, and the high-temperature sheet resistance (mΩ·cm2) measured at 800°C for the high-temperature oxidized specimen.

[0084] Classification Formula (1) Formula (2) Oxide thickness (μm) Evaluation of oxide / base material interface integrity O (Good): Area fraction less than 3%, △ (Average): 3~4%, X (Poor): Exceeding 4% High-temperature sheet resistance (mΩcm) 2 Comparative Example 1 49.127 3.56.6X102 Comparative Example 2 47.43123.75.6X75 Comparative Example 3 39.4488.84.9△55 Comparative Example 4 45.1589.47.1△53 Comparative Example 5 42.746 4.84.3X92 Comparative Example 6 45.22133.23.8△52 Comparative Example 7 37.8820.66.6△57 Comparative Example 8 52.3119 8.25.6X77 Example 1 106.4375.52.8O35 Example 2 106.4145.33.0O21 Example 3 99.7565.72.7O17 Example 4 84.8427.03.2O18 Example 5 125.7537.53.5O17.5 Example 6 97.9249.53.2O24 Example 7 107.4862.32.9O29

[0085] For the steel grades of the comparative and exemplary examples, 15mm x 15mm (2mm thickness) specimens were prepared and maintained at 800°C in an atmospheric environment for 250 hours, followed by cooling and reheating at a heating and cooling rate of 200°C / hour for a total of 2000 hours. The thickness and composition of the scale layer were analyzed by analyzing the high-temperature oxide scale layer of the prepared specimens using a transmission electron microscope (TEM). To evaluate the integrity of the oxide / base material interface, 10 TEM images were obtained, and the interface layer area (4μm x 9μm) was image-analyzed to calculate the area fraction of micro-hole-shaped voids. The specimens were classified into good, average, and poor states based on the average area fraction. For the measurement of high-temperature sheet resistance, Pt paste was applied to both sides of the specimens oxidized for 2000 hours, pre-dried at 200°C, and then sintered at 800°C for 3 hours. For the prepared specimens with Pt sintered on both sides, a Pt mesh was laminated on both sides, a load of 15g was applied, and a Pt wire was drawn out at 4 terminals and placed in the furnace. After raising the temperature to 800 degrees, the resistance was measured using the DC 4-point probe method to calculate the sheet resistance. According to Table 2, if the composition of the present invention, Equation (1) and Equation (2) are satisfied, it was confirmed that scale growth at high temperatures can be reduced, and the thickness of the scale is 4μm or less. In addition, it was found that the integrity between the base material and the scale can be ensured, allowing the high-temperature sheet resistance to be controlled to 40 mΩ·cm2 or less.

[0086] FIG. 1 is the result of the high temperature (800°C) sheet resistance according to the value of Equation (1) of a ferritic stainless steel according to one embodiment of the present invention. Through this, it can be seen that excellent conductivity can be secured by ensuring that the high temperature sheet resistance is 40 mΩ·cm2 or less in the range where Equation (1) is 60.0 to 140.0.

[0087] Figure 2 is a TEM (Transmission Electron Microscopy) image of a cross-sectional oxide layer of stainless steel after forming an oxide layer in an atmospheric atmosphere at a high temperature (800°C) for 2,000 hours according to Example 1 of the present invention. In the case of Example 1, a small number of voids in the form of fine holes were observed in the interface layer, showing a structure favorable for conductivity. In addition, the thickness of the first and second scale layers is dense and thin, and Ti oxide, Si oxide, and Laves phase (Fe2Nb) could be observed in the interface layer.

[0088] Figure 3 is a TEM image of the cross-sectional oxide layer of stainless steel after forming an oxide layer in an atmospheric atmosphere at a high temperature (800°C) for 2,000 hours according to Comparative Example 5 of the present invention. In Comparative Example 5, it was confirmed that numerous voids in the form of fine holes were observed in the interface layer or directly below the interface layer, acting as an inhibitory factor for electron movement and exhibiting a structure unfavorable to durability.

[0089] Although embodiments of the invention disclosed above have been illustrated and described, the disclosed invention is not limited to the specific embodiments described above, and various modifications may be made by those skilled in the art to which the disclosed invention belongs without departing from the essence claimed in the claims.

Claims

1. In weight%, comprising C: 0.003 to 0.02%, N: 0.003 to 0.02%, Si: 0.05 to 0.2%, Mn: 0.2 to 1.0%, Cr: 18.0 to 24.0%, Mo: 0.01 to 2.5%, Nb: 0.2 to 0.7%, Ti: 0.01 to 0.15%, and the remainder being Fe and unavoidable impurities, Ferritic stainless steel satisfying the following formula (1). Equation (1): 60.0 ≤ ([Cr] + [Mo] + [Nb] + [Mn]) / ([Si]+[Ti]) ≤ 140.0 (Here, [Cr], [Mo], [Nb], [Mn], [Si], and [Ti] represent the weight percent of each element.) 2. In Claim 1, The above stainless steel is a ferritic stainless steel satisfying the following formula (2). Equation (2): 20.0 ≤ [Cr] / [Mn] ≤ 80.0 (Here, [Cr] and [Mn] represent the weight percentage of each element.) 3. In Claim 1, The above ferritic stainless steel comprises a first scale layer containing chromium oxide at the base material interface in an atmospheric atmosphere of 600 to 800°C; A second scale layer comprising chromium oxide and manganese oxide formed on the surface of the first scale layer; and Includes an interface layer formed directly below the first scale layer; The above interface layer is a ferritic stainless steel comprising one or more of Ti oxide, Si oxide, Al oxide, or Laves phase.

4. In Claim 3, The above Laves phase is a ferritic stainless steel containing one or more of Fe2Nb and Nb2Si.

5. In Claim 3, Ferritic stainless steel having a thickness of 4 μm or less of the first scale layer and the second scale layer when exposed to an atmospheric temperature of 600 to 800°C for 2000 hours.

6. In Claim 3, Ferritic stainless steel having a void area fraction of less than 3% in the above interface layer.

7. In Claim 1, Ferritic stainless steel having a high-temperature surface resistance of 40 mΩ·cm2 or less when exposed to an atmospheric temperature of 600 to 800°C for 2,000 hours.

8. A step of reheating a slab satisfying the following formula (1) at 1050 to 1280°C, comprising, in wt%, C: 0.003 to 0.02%, N: 0.003 to 0.02%, Si: 0.05 to 0.2%, Mn: 0.2 to 1.0%, Cr: 18.0 to 24.0%, Mo: 0.01 to 2.5%, Nb: 0.2 to 0.7%, Ti: 0.01 to 0.15%, and the remainder being Fe and unavoidable impurities; A step of manufacturing a hot-rolled material by hot rolling and hot rolling annealing at 900 to 1150°C after the above reheating; and A step of cold rolling the above hot-rolled material and cold rolling annealing at 900 to 1150°C; A method for manufacturing ferritic stainless steel including Equation (1): 60.0 ≤ ([Cr] + [Mo] + [Nb] + [Mn]) / ([Si]+[Ti]) ≤ 140.0 (Here, [Cr], [Mo], [Nb], [Mn], [Si], and [Ti] represent the weight percent of each element.) 9. In Claim 8, The above stainless steel is a method for manufacturing ferritic stainless steel that satisfies the following formula (2). Equation (2): 20.0 ≤ [Cr] / [Mn] ≤ 80.0 (Here, [Cr] and [Mn] represent the weight percentage of each element.) 10. In claim 8, A method for manufacturing ferritic stainless steel in which the finishing rolling temperature during the above hot rolling is 700 to 950℃.

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