Ferritic stainless steel having excellent high-temperature conductivity

Abstract

A ferritic stainless steel according to an embodiment of the present invention comprises: a base material; and Cr-Mn oxide scales formed on the outermost surface of the base material, wherein the base material contains, in wt%, 0.0030-0.0200% of carbon (C), 0.0030-0.0200% of nitrogen (N), 0.05-0.50% of silicon (Si), 0.10-1.50% of manganese (Mn), 19.0-25.0% of chromium (Cr), 0.010-2.000% of molybdenum (Mo), 0.050-0.700% of niobium (Nb), and 0.040-0.100% of titanium (Ti), with the remainder comprising iron (Fe) and inevitable impurities, and satisfies expression (1). Expression (1): 30.0 ≤ ([Cr] / [Mn]) x ([Nb] + [Mo] + 2x[Ti]) ≤ 40.0 (where, [Cr], [Mn], [Nb], [Mo], and [Ti] refer to the content of the respective elements in wt%)

Description

Ferritic stainless steel with excellent high-temperature conductivity

[0001] The present invention relates to a ferritic stainless steel with excellent high-temperature conductivity, and more specifically, to a ferritic stainless steel with excellent high-temperature conductivity in which the thickness of a fine and uniform CrMn oxide scale on the surface is controlled in a high-temperature oxidizing environment.

[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] When stainless steel oxidizes, chromium oxide (Cr2O) inevitably forms on the surface, and due to the oxide scale composed of chromium oxide, the stainless steel possesses corrosion resistance. However, this oxide scale has a negative effect on electrical conductivity.

[0004] To apply stainless steel to high-temperature fuel cells, the thickness of the scale formed on the surface in high-temperature oxidizing environments must not become excessive, nor should electrical conductivity be degraded by the scale. If the scale thickness exceeds a certain level, it may peel off, damaging the material and reducing electrical conductivity. This leads to a decrease in fuel cell efficiency. Therefore, to apply stainless steel to fuel cell components, it must possess characteristics such as controlled scale thickness and conductivity.

[0005] The objective of the present invention to solve the aforementioned problem is to provide a ferritic stainless steel capable of maintaining high electrical conductivity even in a high-temperature oxidizing environment.

[0006] The problems that the present invention aims to solve are not limited to those mentioned above, and other unmentioned problems will be clearly understood by those skilled in the art from the description below.

[0007] A ferritic stainless steel according to one embodiment of the present invention comprises a base material and a Cr-Mn oxide scale formed on the outermost surface of the base material, wherein the base material comprises, in weight%, carbon (C): 0.0030~0.0200%, nitrogen (N): 0.0030~0.0200%, silicon (Si): 0.05~0.50%, manganese (Mn): 0.10~1.50%, chromium (Cr): 19.0~25.0%, molybdenum (Mo): 0.010~2.000%, niobium (Nb): 0.050~0.700%, titanium (Ti): 0.040~0.100%, the remainder being iron (Fe) and unavoidable impurities, and satisfies the following formula (1).

[0008] Equation (1): 30.0 ≤ ([Cr] / [Mn]) x ([Nb] + [Mo] + 2x[Ti]) ≤ 40.0

[0009] (Here, [Cr], [Mn], [Nb], [Mo], and [Ti] represent the weight% content of each element)

[0010] In addition, the ferritic stainless steel according to one embodiment of the present invention is a ferritic stainless steel in which the base material comprises a Cr oxide layer in the thickness direction of the base material from the outermost surface portion.

[0011] In addition, the ferritic stainless steel according to one embodiment of the present invention may include a Ti oxide layer in the thickness direction of the base material below the Cr oxide layer.

[0012] In addition, the ferritic stainless steel according to one embodiment of the present invention has an interfacial conductivity of 40 mΩcm at a temperature of 800°C. 2 It may be less than.

[0013] In addition, in the ferritic stainless steel according to one embodiment of the present invention, the oxide scale may have a thickness of 2.0 μm or less.

[0014] A method for manufacturing ferritic stainless steel according to another embodiment of the present invention comprises the steps of: preparing a steel material satisfying the following formula (1), comprising, in weight%, carbon (C): 0.0030~0.0200%, nitrogen (N): 0.0030~0.0200%, silicon (Si): 0.05~0.50%, manganese (Mn): 0.10~1.50%, chromium (Cr): 19.0~25.0%, molybdenum (Mo): 0.010~2.000%, niobium (Nb): 0.050~0.700%, titanium (Ti): 0.040~0.100%, the remainder being iron (Fe) and unavoidable impurities; and reheating the steel material at 1050℃~1280℃ to form a Cr-Mn oxide scale formed on the outermost surface of the steel material. The method includes the step of hot rolling the reheated steel at a finishing rolling temperature of 700℃ to 950℃; and the step of hot rolling annealing the hot-rolled steel at 900℃ to 1150℃.

[0015] Equation (1): 30.0 ≤ ([Cr] / [Mn]) x ([Nb] + [Mo] + 2x[Ti]) ≤ 40.0

[0016] (Here, [Cr], [Mn], [Nb], [Mo], and [Ti] represent the weight% content of each element)

[0017] In addition, a method for manufacturing ferritic stainless steel according to one embodiment of the present invention may include the step of cold rolling and cold rolling annealing the hot-rolled annealed material at 900°C to 1150°C.

[0018] In addition, in the method for manufacturing ferritic stainless steel according to one embodiment of the present invention, the thickness of the Cr-Mn oxide scale formed on the steel material can be controlled to be 2.0 μm or less.

[0019] In addition, in the method for manufacturing ferritic stainless steel according to one embodiment of the present invention, in the step of forming the Cr-Mn oxide scale, a Cr oxide layer may be formed from the outermost surface of the steel material in the thickness direction of the base material.

[0020] In addition, in the method for manufacturing ferritic stainless steel according to one embodiment of the present invention, a Ti oxide layer may be formed in the thickness direction of the base material below the Cr oxide layer.

[0021] According to the present invention, a ferritic stainless steel with excellent high-temperature conductivity can be provided by controlling the composition and oxide scale thickness.

[0022] Figure 1 is a graph showing the relationship between Ti content and Equation (1).

[0023] Figure 2 is a cross-sectional image of the high-temperature oxidation scale structure of an embodiment according to the present invention taken with Scanning Electron Microscopy (SEM).

[0024] Figure 3 is a photograph of the high-temperature oxidation scale structure of an embodiment according to the present invention taken with SEM-EDS.

[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.

[0028] 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.

[0029] Unless otherwise specifically stated in this specification, the % indicating the content of each element is based on weight.

[0030] First, a ferritic stainless steel according to one aspect of the present invention will be described.

[0031] A ferritic stainless steel according to one embodiment of the present invention comprises a base material and a Cr-Mn oxide scale formed on the outermost surface portion of the base material. Here, the oxide scale refers to a layer in which oxide is formed, and may be used interchangeably with an oxide layer.

[0032] The above base material comprises, in weight%, carbon (C): 0.0030~0.0200%, nitrogen (N): 0.0030~0.0200%, silicon (Si): 0.05~0.50%, manganese (Mn): 0.10~1.50%, chromium (Cr): 19.0~25.0%, molybdenum (Mo): 0.010~2.000%, niobium (Nb): 0.050~0.700%, titanium (Ti): 0.040~0.100%, the remainder being iron (Fe) and unavoidable impurities, and satisfies the following formula (1).

[0033] Equation (1): 30.0 ≤ ([Cr] / [Mn]) x ([Nb] + [Mo] + 2x[Ti]) ≤ 40.0

[0034] (Here, [Cr], [Mn], [Nb], [Mo], and [Ti] represent the weight% content of each element)

[0035] The reasons for limiting the scope of each component are described below.

[0036] Carbon (C): 0.0030~0.0200%

[0037] Carbon (C) is an essential element in the stainless steel manufacturing process. Since an excessive increase in carbon content can lead to the formation of precipitates such as chromium carbides, which can adversely affect the composition and oxidation characteristics of the base material, the upper limit is restricted to 0.0200%. However, since controlling the carbon content to an extremely low level results in an excessive increase in costs, the lower limit may be restricted to 0.0030%, preferably 0.0050 to 0.0200%, and more preferably 0.0050 to 0.069%.

[0038] Nitrogen (N): 0.0030~0.0200%

[0039] Since an excessive increase in nitrogen (N) content can adversely affect quality by causing the precipitation of various nitrides or the occurrence of pores, the upper limit is limited to 0.0200%. However, since controlling the nitrogen content to an extremely low level leads to an excessive increase in costs, the lower limit may be limited to 0.0030% or higher, preferably 0.0048 to 0.0200%, and more preferably 0.0048 to 0.102%.

[0040] Silicon (Si): 0.05~0.50%

[0041] Silicon (Si) is a component that must be strictly limited because it forms an insulating film by forming a film-like precipitate at the interface between the scale and the base material when the material is exposed to high temperatures, so its upper limit is limited to 0.50%. However, since reducing the silicon content to 0.05% or less requires high-cost processes such as vacuum melting, the lower limit in the present invention may be limited to 0.05%, preferably 0.11 to 0.50%, and more preferably 0.11 to 0.14%.

[0042] Manganese (Mn): 0.10~1.50%

[0043] Manganese (Mn) must be added in an amount of at least 0.10% because it rapidly diffuses when stainless steel oxidizes at high temperatures to form dense manganese / chromium oxide on the outer layer of the scale. However, excessive addition of manganese may excessively promote scale growth and cause scale peeling, so the upper limit may be restricted to 1.50%, preferably 0.29 to 1.50%, and more preferably 0.29 to 0.67%.

[0044] Chrome (Cr): 19.0~25.0%

[0045] Chromium (Cr) is an essential element for ensuring the corrosion resistance of stainless steel. At least 19.0% must be added to prevent chromium depletion due to oxidation over a long period in a high-temperature oxidizing environment. However, to prevent an increase in manufacturing costs and the precipitation of chromium carbides, intermetallic compounds, etc., the upper limit may be restricted to 25.0%, preferably 21.5 to 25.0%, and more preferably 21.5 to 22.7%.

[0046] Molybdenum (Mo): 0.010~2.000%

[0047] Molybdenum (Mo) is an element that can increase the strength of materials in high-temperature environments. Therefore, it is necessary to add at least 0.010%, but since it is an expensive element, the upper limit can be restricted to 2.000% to prevent an increase in manufacturing costs.

[0048] Niobium (Nb): 0.050~0.700%

[0049] Niobium (Nb) is added at a concentration of 0.050% or more because, due to its excellent oxidation properties, it oxidizes at the scale / substrate interface to form oxides, thereby suppressing the formation of insulating silicon oxides and contributing to the improvement of the material's strength. On the other hand, excessive addition reduces hot workability and increases manufacturing costs, so it is desirable to limit the upper limit to 0.700%.

[0050] Titanium (Ti): 0.040~0.100%

[0051] Titanium (Ti) is required to have a content of at least 0.040% because it forms 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, thereby increasing the strength of the material and contributing to increased conductivity through Ti doping. However, excessive addition leads to increased manufacturing costs and forms titanium oxide on the outside of the scale, making the scale thickness thicker than 2.0 μm, so it is desirable to limit the upper limit to 0.100%.

[0052] The remaining component of the present invention 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 person skilled in the ordinary manufacturing process, all details thereof are not specifically mentioned in this specification.

[0053] The base material according to the present invention satisfies the following equation (1).

[0054] Equation (1): 30.0 ≤ ([Cr] / [Mn]) x ([Nb] + [Mo] + 2x[Ti]) ≤ 40.0

[0055] (Here, [Cr], [Mn], [Nb], [Mo], and [Ti] represent the weight% content of each element)

[0056] When Equation (1) satisfies 30.0 to 40.0, a fine and uniform CrMn oxide layer of 2.0 μm or less can be formed on the surface of the base material. Compared to the thick Cr oxide formed on the surface of general stainless steel, the CrMn oxide has a spinel structure, which can secure excellent electrical conductivity, and thus the interfacial conductivity at a temperature of 800°C is 40 mΩcm 2 The following may be possible. In particular, the present invention is characterized by securing excellent high-temperature interfacial conductivity by controlling not only the ratio of Cr and Mn but also the relationship with elements such as Nb, Mo, and Ti that have excellent oxidation properties. Formula (1) may preferably have a value of 33.8 to 38.8.

[0057] FIG. 2 is a cross-sectional image of the high-temperature oxide scale structure of an embodiment according to the present invention taken using Scanning Electron Microscopy (SEM), and FIG. 3 is a cross-sectional image of the high-temperature oxide scale structure of an embodiment according to the present invention taken using SEM-EDS. According to FIG. 2, a Cr oxide layer may be included in the thickness direction of the base material from the outermost surface of the base material, and a Ti oxide layer may be included in the thickness direction of the base material below the Cr oxide layer of the base material. Additionally, according to FIG. 3, an oxide (O) is formed on an Fe base material, and the oxide exists in the form of manganese oxide, chromium oxide, and titanium oxide. A Ti oxide layer is formed on the base material in the form shown in FIG. 2, a Cr oxide layer is formed thereon, and a Cr-Mn oxide scale is formed on the Cr oxide layer. When such an oxide scale structure is formed as in the present invention, the Cr oxide layer is formed, ensuring high corrosion resistance of the stainless steel, and the Ti oxide layer increases the strength of the material and increases conductivity due to Ti doping.

[0058] In addition, in a ferritic stainless steel according to one embodiment of the present invention, the average thickness of the Cr-Mn oxide scale may be 2.0 μm or less. When the average thickness of the oxide scale is 2.0 μm or less, a plate-shaped CrMn oxide layer can be uniformly formed on the surface of the ferritic stainless steel, and excellent electrical conductivity can be secured through this uniform CrMn oxide layer. The average thickness of the oxide scale is calculated as the average of any 10 point regions of the oxide scale measured through SEM images. If the average thickness of the oxide scale formed on the base material exceeds 2.0 μm, electrical conductivity decreases due to excessive oxide; therefore, the thickness of the oxide scale is controlled to be 2.0 μm or less. The oxide scale contains Cr, Mn, and O as main components.

[0059] The following describes a method for manufacturing ferritic stainless steel, which is another aspect of the present invention.

[0060] A method for manufacturing ferritic stainless steel according to another embodiment of the present invention comprises the steps of: preparing a steel material satisfying the following formula (1), comprising, in weight%, carbon (C): 0.0030~0.0200%, nitrogen (N): 0.0030~0.0200%, silicon (Si): 0.05~0.50%, manganese (Mn): 0.10~1.50%, chromium (Cr): 19.0~25.0%, molybdenum (Mo): 0.010~2.000%, niobium (Nb): 0.050~0.700%, titanium (Ti): 0.040~0.100%, the remainder being iron (Fe) and unavoidable impurities; and reheating the steel material at 1050℃~1280℃ to form a Cr-Mn oxide scale formed on the outermost surface of the steel material. The method includes the step of hot rolling the reheated steel at a finishing rolling temperature of 700℃ to 950℃; and the step of hot rolling annealing the hot-rolled steel at 900℃ to 1150℃.

[0061] Equation (1): 30.0 ≤ ([Cr] / [Mn]) x ([Nb] + [Mo] + 2x[Ti]) ≤ 40.0

[0062] (Here, [Cr], [Mn], [Nb], [Mo], and [Ti] represent the weight% content of each element)

[0063] The effects of controlling the composition of the steel and Equation (1) are as described above, and the steel may be a slab or an ingot, but is not limited thereto.

[0064] In addition, a method for manufacturing ferritic stainless steel according to one embodiment of the present invention may include the step of cold rolling and cold rolling annealing the hot-rolled annealed material at 900°C to 1150°C. When the ferritic stainless steel satisfying the composition and formula (1) is reheated to a high temperature of 900°C or higher, a Cr-Mn oxide containing Cr and Mn may be formed on the surface of the ferritic stainless steel.

[0065] In addition, in the method for manufacturing ferritic stainless steel according to one embodiment of the present invention, the thickness of the Cr-Mn oxide scale formed on the steel material can be controlled to be 2.0 μm or less. Since electrical conductivity decreases due to excessive oxide when the average thickness of the oxide scale formed on the base material exceeds 2.0 μm, it is desirable to control the thickness of the oxide scale to be 2.0 μm or less.

[0066] In addition, in the step of forming the Cr-Mn oxide scale according to one embodiment of the present invention, a Cr oxide layer may be formed in the thickness direction of the base material from the outermost surface of the steel material, and the Cr oxide layer may include the form of Cr oxides such as Cr2O3 and CrO3.

[0067] In addition, in the method for manufacturing ferritic stainless steel according to one embodiment of the present invention, a Ti oxide layer may be formed in the thickness direction of the base material below the Cr oxide layer. Since titanium (Ti) can increase 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, and increase conductivity through Ti doping, a Ti internal oxide layer may be formed near the surface of the base material, and the Ti oxide layer may include the form of Ti oxides such as TiO2 and Ti2O3.

[0068] The method may include the step of hot rolling the reheated steel at a finishing rolling temperature of 700°C to 950°C and the step of hot rolling annealing the hot-rolled steel at 900°C to 1150°C, and subsequently cold rolling and cold rolling annealing the hot-rolled annealed steel at 900°C to 1150°C. However, it is not limited thereto, and the steps of conventional hot working and cold working may also be performed.

[0069] The present invention will be described in more detail below through examples and drawings. However, such description is merely for illustrating the implementation of the present invention and does not limit the present invention. This is because the scope of the present invention is determined by the matters described in the claims and matters reasonably inferred therefrom.

[0070] {Example}

[0071] Steel grades 1 to 11 having the compositions of Table 1 above were reheated at 1050°C to 1280°C, followed by hot rolling and hot rolling annealing to produce plates with a thickness of 2 to 6 mm. The finishing temperature for hot rolling was in the range of 700°C to 950°C, and the hot rolling annealing temperature was performed in the range of 900°C to 1150°C. Subsequently, the hot-rolled material was cold-rolled and cold-rolled annealed at 900°C to 1150°C to produce a final material with a thickness of 0.05 to 2 mm.

[0072] For the manufactured material, the equation (1), oxide scale thickness, and high temperature interface contact resistance at 800°C were measured and are shown in Table 1 below.

[0073] The high-temperature interfacial conductivity is 40 mΩcm. 2 If less than or equal to 40 mΩcm, mark "O". 2 Conductivity was distinguished by marking it as "X" when it exceeded the limit. Conductivity was determined by measuring the Area-Specific Resistance (ASR) of a sample that had undergone pre-heat treatment at 800°C for 100 hours for 500 hours at a temperature of 800°C.

[0074] Interfacial conductivity equation at 800°C (1) Average thickness of oxide scale (μm) Remarks 10.00 600.1 10.4 622.4 0.2 000.04 80.5 000.00 72O38.81.8 Invention Example 1 20.00 500.1 40.4 921.5 0.2 000.06 90.5 100.00 60O37.21.5 Invention Example 2 30.00 530.1 30.5 321.9 0.01 0. 0530.70.0058O33.80.8Invention Example 340.00500.130.4021.90.2000.0500.4000.0089O38.31.1Invention Example 450.00550.120.5021.80.0050.0300.5800.0048X28.12.8Comparative Example 160.00600.120.2921.6 0.2000.0350.5000.0049X57.43.6Comparative Example 270.00690.130.3622.70.2100.0670.0010.0102X21.85.1Comparative Example 380.00500.110.6622.50.2000.0500.4900.0056X26.93.1Comparative Example 490.00500.1 30.67 22.00.00 40.04 30.5 100.00 60X19.7 3.5 Comparative Example 5 100.00 510.1 30.47 21.6 0.2 000.048 0.00 20.00 57X13.7 4.2 Comparative Example 6 110.00 600.110.30 22.00.2 000.15 00.00 10.01 01X36.7 6.3 Comparative Example 7

[0075] Invention Examples 1 to 4, which satisfy the compositional range of the present invention and the range of Formula (1), can be confirmed to have excellent high-temperature interfacial conductivity at 800°C by including a Cr-Mn oxide scale formed on the outermost surface of the base material as shown in FIG. 2 at a depth of 2.0 μm or less.

[0076] On the other hand, in the case of Comparative Examples 1 and 2, in which the Ti content falls short of the scope of the present invention, it was confirmed that the high-temperature interfacial conductivity at 800°C was inferior because the formation of Ti oxide inside the base material was not sufficient, and the thickness of the Cr-Mn oxide scale formed on the outermost surface of the base material exceeded 2.0 μm.

[0077] In addition, in the case of Comparative Examples 3 and 6, in which the Nb content is less than 0.050%, the range of Equation (1) is not satisfied, so an excessive amount of oxide is formed, and the thickness of the Cr-Mn oxide scale formed on the outermost surface of the base material exceeds 2.0 μm, which can be confirmed to have inferior high-temperature interfacial conductivity at 800°C.

[0078] In addition, according to Comparative Example 4, even if all compositions are satisfied, if the range of Equation (1) is not controlled, it was confirmed that the thickness of the Cr-Mn oxide scale formed on the outermost surface of the base material exceeds 2.0 μm, and thus the high-temperature interfacial conductivity at 800°C is inferior.

[0079] In addition, Comparative Example 5, which contains less than 0.010% Mo, did not satisfy the range of Equation (1), so excessive oxide was formed, and the thickness of the Cr-Mn oxide scale formed on the outermost surface of the base material exceeded 2.0㎛, which was confirmed to have inferior high-temperature interfacial conductivity at 800℃.

[0080] In addition, Comparative Example 7, in which the Ti content exceeds 0.100%, was found to have inferior high-temperature interfacial conductivity at 800°C because the thickness of the oxide scale exceeded 2.0 μm, which formed an oxide scale on the outside of the base material due to the excessive Ti content.

[0081] 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

1. A ferritic stainless steel comprising a base material and a Cr-Mn oxide scale formed on the outermost surface of the base material, The above base material comprises, in weight%, carbon (C): 0.0030~0.0200%, nitrogen (N): 0.0030~0.0200%, silicon (Si): 0.05~0.50%, manganese (Mn): 0.10~1.50%, chromium (Cr): 19.0~25.0%, molybdenum (Mo): 0.010~2.000%, niobium (Nb): 0.050~0.700%, titanium (Ti): 0.040~0.100%, the remainder being iron (Fe) and unavoidable impurities, and is a ferritic stainless steel satisfying the following formula (1). Equation (1): 30.0 ≤ ([Cr] / [Mn]) x ([Nb] + [Mo] + 2x[Ti]) ≤ 40.0 (Here, [Cr], [Mn], [Nb], [Mo], and [Ti] represent the weight% content of each element) 2. In Claim 1, The above base material is a ferritic stainless steel containing a Cr oxide layer in the thickness direction of the base material from the outermost surface.

3. In Claim 2, The above base material is a ferritic stainless steel comprising a Ti oxide layer in the thickness direction of the base material below the Cr oxide layer.

4. In Claim 1, The interfacial conductivity at a temperature of 800℃ is 40 mΩcm 2 Lee Ha-in, ferritic stainless steel.

5. In Claim 1, The above oxide scale is a ferritic stainless steel with an average thickness of 2.0 μm or less.

6. A step of preparing a steel material satisfying the following formula (1), comprising, in weight%, carbon (C): 0.0030~0.0200%, nitrogen (N): 0.0030~0.0200%, silicon (Si): 0.05~0.50%, manganese (Mn): 0.10~1.50%, chromium (Cr): 19.0~25.0%, molybdenum (Mo): 0.010~2.000%, niobium (Nb): 0.050~0.700%, titanium (Ti): 0.040~0.100%, and the remainder being iron (Fe) and unavoidable impurities; A step of reheating the steel at 1050℃ to 1280℃ to form a Cr-Mn oxide scale on the outermost surface of the steel; A step of hot rolling the above-mentioned reheated steel at a finishing rolling temperature of 700℃ to 950℃; and A method for manufacturing ferritic stainless steel, comprising the step of hot-rolling and annealing the above hot-rolled steel at 900℃ to 1150℃. Equation (1): 30.0 ≤ ([Cr] / [Mn]) x ([Nb] + [Mo] + 2x[Ti]) ≤ 40.0 (Here, [Cr], [Mn], [Nb], [Mo], and [Ti] represent the weight% content of each element) 7. In Claim 6, A method for manufacturing ferritic stainless steel, comprising the step of cold rolling and cold rolling annealing the hot-rolled annealed material at 900℃ to 1150℃.

8. In Claim 6, A method for manufacturing ferritic stainless steel, wherein the oxide scale formed on the steel material has an average thickness of 2.0 μm or less.

9. In Claim 6, A method for manufacturing ferritic stainless steel in which, in the step of forming the above Cr-Mn oxide scale, a Cr oxide layer is formed from the outermost surface of the steel material in the thickness direction of the base material.

10. In Claim 9, A method for manufacturing ferritic stainless steel in which a Ti oxide layer is formed in the thickness direction of the base material below the above Cr oxide layer.

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