Ferritic stainless steel and manufacturing method therefor

Abstract

A ferritic stainless steel according to the present invention comprises a base material and an oxide scale layer formed on the outermost surface portion of the base material, wherein the base material includes an interface portion, which is in contact with the oxide scale layer and comprises a Ti-Al composite oxide, and comprises, by wt%, 0.0010-0.0200% of C, 0.0010-0.0200% of N, 0.010-0.400% of Si, 0.10-1.50% of Mn, 18.0-25.0% of Cr, 0.01-2.00% of Mo, 0.05-1.00% of Nb, 0.010-0.200% of Ti, 0.010-0.200% of Al, and the balance of Fe and other inevitable impurities, and the value of relation (1) satisfies 390 or less. Relation (1): 1000×[Si] / (0.65×[Nb]+[Ti]+[Al]) ≤ 390 (where [Nb], [Ti] and [Al] mean the wt% of the respective elements)

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, from 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, nor should electrical conductivity decrease. Furthermore, if the scale thickness exceeds a certain level, it may peel off and damage the material, and if electrical conductivity is low, it can reduce the efficiency of the fuel cell.

[0004] In addition, when manufacturing parts for fuel cell separators using ferritic stainless steel, if the formability is poor, cracks may occur during the forming of the microchannels, which are the gas flow paths of the separator, or an orange peel phenomenon may occur in the channel bends due to coarse grains, which can reduce the efficiency of the fuel cell.

[0005] When stainless steel oxidizes, chromium oxide (Cr2O3) forms on the surface, and this oxide scale composed of chromium oxide provides corrosion resistance. However, while the resulting scale exhibits excellent corrosion resistance, it has low electrical conductivity; therefore, to be applied as a fuel cell component, it is necessary to form a conductive oxide scale at high temperatures to reduce electrical resistance as much as possible.

[0006] One aspect of the present invention for solving the above-mentioned problem is to provide a ferritic stainless steel and a method for manufacturing the same, which can maintain high electrical conductivity even in a high-temperature oxidizing environment by forming an oxide scale with excellent conductivity in a high-temperature oxidizing environment and by adding alloying elements such as Ti and Al, which have a high affinity for oxygen, to form a Ti-Al composite oxide instead of an insulating Si oxide, thereby suppressing the formation of Si oxide.

[0007] The technical problems intended 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.

[0008] To achieve the above objective, a ferritic stainless steel according to one embodiment of the present invention comprises a base material and an oxide scale layer formed on the outermost surface of the base material, wherein the base material comprises an interface portion in contact with the oxide scale layer and comprising a Ti-Al composite oxide, and comprises, in weight%, C: 0.0010% to 0.0200%, N: 0.0010% to 0.0200%, Si: 0.010% to 0.400%, Mn: 0.10% to 1.50%, Cr: 18.0% to 25.0%, Mo: 0.01% to 2.00%, Nb: 0.05% to 1.00%, Ti: 0.010% to 0.200%, Al: 0.010% to 0.200%, the remainder being Fe and other unavoidable impurities, and wherein the value of the following formula (1) is 390 The following can be satisfied.

[0009] Equation (1): 1000×[Si] / (0.65×[Nb]+[Ti]+[Al]) ≤ 390

[0010] Here, [Nb], [Ti], and [Al] represent the weight percent of each element.

[0011] In addition, the interface portion according to one embodiment of the present invention may include Ti oxide, and the Ti concentration measured by transmission electron microscopy energy-dispersive X-ray spectroscopy (TEM-EDS) analysis may be 0.01 at.% to 70.0 at.%.

[0012] In addition, the interface portion according to one embodiment of the present invention may include Al oxide, and the Al concentration measured by transmission electron microscopy energy-dispersive X-ray spectroscopy may be 0.01 at.% to 70.0 at.%.

[0013] In addition, the interface portion according to one embodiment of the present invention includes an Nb-based Laves phase, and the Nb concentration measured by transmission electron microscopy energy-dispersive X-ray spectroscopy may be 0.01 at.% to 70.0 at.%.

[0014] In addition, the Laves phase according to one embodiment of the present invention may include one selected from Nb2Si, Fe2Nb, and mixtures thereof.

[0015] In addition, the interface portion according to one embodiment of the present invention may have an O concentration of 0.1 at.% to 80.0 at.% as measured by transmission electron microscope energy-dispersive X-ray spectroscopy.

[0016] In addition, the oxide scale layer according to one embodiment of the present invention may include one or more selected from manganese oxide, chromium oxide, and manganese-chromium oxide.

[0017] In addition, the oxide scale layer according to one embodiment of the present invention may have a thickness of 4 μm or less.

[0018] In addition, the ferritic stainless steel according to one embodiment of the present invention may have an interfacial contact resistance of 40 mΩ cm² or less at 300°C to 900°C.

[0019] In addition, a method for manufacturing a ferritic stainless steel according to one embodiment of the present invention comprises the steps of: reheating a steel material comprising, in weight%, C: 0.0010% to 0.0200%, N: 0.0010% to 0.0200%, Si: 0.010% to 0.400%, Mn: 0.10% to 1.50%, Cr: 18.0% to 25.0%, Mo: 0.01% to 2.00%, Nb: 0.05% to 1.00%, Ti: 0.010% to 0.200%, Al: 0.010 to 0.200%, the remainder being Fe and other unavoidable impurities, satisfying that the value of Formula (1) is 390 or less; and manufacturing a hot-rolled material by hot rolling and hot-rolling annealing after reheating. and may include the step of manufacturing a steel plate by cold rolling and cold annealing the hot-rolled material.

[0020] In addition, the ferritic stainless steel according to one embodiment of the present invention comprises a base material and an oxide scale layer formed on the outermost surface of the base material, and the base material may include an interface portion in contact with the oxide scale layer and comprising a Ti-Al composite oxide.

[0021] In addition, the interface portion according to one embodiment of the present invention may include Ti oxide, and the Ti concentration measured by transmission electron microscopy energy-dispersive X-ray spectroscopy may be 0.01 at.% to 70.0 at.%.

[0022] In addition, the interface portion according to one embodiment of the present invention may include Al oxide, and the Al concentration measured by transmission electron microscopy energy-dispersive X-ray spectroscopy may be 0.01 at.% to 70.0 at.%.

[0023] In addition, the interface portion according to one embodiment of the present invention includes an Nb-based Laves phase, and the Nb concentration measured by transmission electron microscopy energy-dispersive X-ray spectroscopy may be 0.01 at.% to 70.0 at.%.

[0024] In addition, the Laves phase according to one embodiment of the present invention may include one selected from Nb2Si, Fe2Nb, and mixtures thereof.

[0025] In addition, the interface portion according to one embodiment of the present invention may have an O concentration of 0.1 at.% to 80.0 at.% as measured by transmission electron microscope energy-dispersive X-ray spectroscopy.

[0026] In addition, the oxide scale layer according to one embodiment of the present invention may include one or more selected from manganese oxide, chromium oxide, and manganese-chromium oxide.

[0027] In addition, the oxide scale layer according to one embodiment of the present invention may have a thickness of 4 μm or less.

[0028] In addition, the ferritic stainless steel according to one embodiment of the present invention may have an interfacial contact resistance of 40 mΩ cm² or less at 300°C to 900°C.

[0029] According to the present invention, a ferritic stainless steel and a method for manufacturing the same can be provided, which can maintain high electrical conductivity even in a high-temperature oxidizing environment by forming an oxide scale with excellent conductivity in a high-temperature oxidizing environment and forming a Ti-Al composite oxide to suppress the formation of insulating Si oxide.

[0030] The effects obtainable from the present invention 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 invention belongs from the description below.

[0031] FIG. 1 is a diagram showing cross-sectional TEM / FIB images and EDS mapping analysis results after heat-treating a ferritic stainless steel according to one embodiment of the present invention in an 800°C atmospheric atmosphere for 500 hours.

[0032] Figure 2 is a diagram showing the cross-sectional TEM / FIB image and EDS mapping analysis results after heat-treating a ferritic stainless steel of a comparative example in an 800°C atmospheric environment for 500 hours.

[0033] 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.

[0034] 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.

[0035] 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. For instance, singular expressions in this specification include plural expressions unless the context clearly indicates an exception.

[0036] 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.

[0037] In order to apply stainless steel as a fuel cell separator, it is necessary to ensure that an oxide scale with excellent conductivity is formed in a high-temperature oxidation environment, and that insulating Si oxide is not formed on the surface layer due to Si, which is inevitably included as an alloying element. Accordingly, the present invention aims to provide a ferritic stainless steel capable of maintaining high electrical conductivity even in a high-temperature oxidation environment by adding alloying elements such as Ti and Al, which have a high affinity for oxygen, to the interface of the base material to form a Ti-Al composite oxide instead of Si oxide, and by adding Nb, etc., to form a Laves phase such as Nb2Si to suppress the formation of Si oxide, and by forming an oxide scale with excellent conductivity in a high-temperature oxidation environment on the outermost surface of the base material.

[0038] A ferritic stainless steel according to one embodiment of the present invention will be described in detail below. Furthermore, unless otherwise specifically stated in this specification, "%" regarding chemical composition means "weight%".

[0039] A ferritic stainless steel according to one embodiment of the present invention may include a base material and an oxide scale layer formed on the outermost surface of the base material. In addition, the base material includes an interface portion that is in contact with the oxide scale layer and includes a Ti-Al composite oxide, wherein the interface portion refers to a region extending from a point where the maximum oxygen concentration in the base material is present to a point where the maximum oxygen concentration in the base material is 50% in the thickness direction of the base material.

[0040] The ferritic stainless steel according to one embodiment of the present invention comprises, in weight percent, C: 0.0010% to 0.0200%, N: 0.0010% to 0.0200%, Si: 0.010% to 0.400%, Mn: 0.10% to 1.50%, Cr: 18.0% to 25.0%, Mo: 0.01% to 2.00%, Nb: 0.05% to 1.00%, Ti: 0.010% to 0.200%, Al: 0.010% to 0.200%, the remainder being Fe and other unavoidable impurities.

[0041] The reasons for limiting the compositional range of each alloying element are described below. Unless otherwise noted, units are weight percent.

[0042] The carbon (C) content may be 0.0010% to 0.0200%.

[0043] 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, the upper limit of C is restricted to 0.0200%, preferably 0.0150%. However, since controlling the C content to an extremely low level leads to an excessive increase in costs, the lower limit of C is restricted to 0.0010%, preferably 0.0030%, and more preferably 0.0040%, taking this into account.

[0044] The nitrogen (N) content may be 0.0010% to 0.0200%.

[0045] If the content of N increases excessively, it can adversely affect quality by causing the precipitation of various nitrides or the occurrence of pores. Considering this, the upper limit of N is limited to 0.0200%, preferably 0.0150%. However, since controlling the N content to an extremely low level leads to an excessive increase in costs, the lower limit of N is limited to 0.0010%, preferably 0.0030%, and more preferably 0.0050%.

[0046] The silicon (Si) content may be 0.010% to 0.400%.

[0047] Si is a component that must be strictly limited because it forms an insulating film by forming film-like precipitates at the interface between the scale and the base material when the material is exposed to high temperatures. Considering this, the upper limit of Si is limited to 0.400%, preferably 0.350%, and more preferably 0.300%. However, since high-cost processes such as vacuum melting are required to reduce the Si content to 0.010% or less, the lower limit of Si is limited to 0.010%, preferably 0.040%, and more preferably 0.050%.

[0048] The manganese (Mn) content may be 0.10% to 1.50%.

[0049] Mn rapidly diffuses when stainless steel oxidizes at high temperatures, forming a dense manganese / chromium oxide on the outer layer of the scale. Considering this, the lower limit of Mn is limited to 0.10%, preferably 0.15%, and more preferably 0.20%. However, excessive addition of Mn excessively promotes scale growth, which may lead to scale delamination; therefore, considering this, the upper limit of Mn is limited to 1.50%, preferably 1.20%, and more preferably 1.00%.

[0050] The chromium (Cr) content may be 18.0% to 25.0%.

[0051] Cr is an essential element for ensuring the corrosion resistance of stainless steel. Therefore, it is necessary to prevent the depletion of Cr due to oxidation over a long period in a high-temperature oxidizing environment. Considering this, the lower limit of Cr is limited to 18.0%, preferably 18.5%, and more preferably 19.0%. However, in order to prevent an increase in manufacturing costs and the precipitation of Cr carbides, intermetallic compounds, etc., the upper limit of Cr is limited to 25.0%, preferably 24.5%, and more preferably 24.0%.

[0052] The molybdenum (Mo) content may be 0.01% to 2.00%.

[0053] Mo is an element that can increase the strength of the material in a high-temperature environment. Therefore, the lower limit of Mo is limited to 0.01%. However, since Mo is an expensive element, in order to suppress the increase in manufacturing costs, the upper limit of Mo is limited to 2.00%, preferably 1.50%.

[0054] The niobium (Nb) content may be 0.05% to 1.00%.

[0055] Due to its excellent oxidation properties, Nb is an element that oxidizes at the scale / substrate interface to form oxides, thereby suppressing the formation of insulating silicon oxide and contributing to the improvement of the material's strength. Considering this, the lower limit of Nb is limited to 0.05%, preferably 0.07%, and more preferably 0.10%. However, since excessive addition of Nb impairs hot workability and leads to an increase in manufacturing costs, the upper limit of Nb is limited to 1.00%, preferably 0.90%, and more preferably 0.70%.

[0056] The titanium (Ti) content may be 0.010% to 0.200%.

[0057] Ti is an element that increases 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. Considering this, the lower limit of Ti is limited to 0.010%, preferably 0.015%, and more preferably 0.020%. However, if Ti is added excessively, it leads to an increase in manufacturing costs and forms Ti oxide outside the scale; therefore, considering this, the upper limit of Ti is limited to 0.200%, preferably 0.150%, and more preferably 0.120%.

[0058] The aluminum (Al) content may be 0.010% to 0.200%.

[0059] Al is an element that increases the strength and oxidation resistance of the material by forming internal oxides, such as Ti, near the surface of the base material just below the interface between the base material and the scale at high temperatures. Considering this, the lower limit of Al is limited to 0.010%, preferably 0.015%, and more preferably 0.020%. However, since excessive addition of Al leads to increased manufacturing costs or the formation of Al oxides outside the scale, the upper limit of Al is limited to 0.200%, preferably 0.180%, and more preferably 0.150%.

[0060] 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.

[0061] A ferritic stainless steel according to one embodiment of the present invention can satisfy the following formula (1).

[0062] Equation (1): 1000[Si] / (0.65[Nb]+[Ti]+[Al]) ≤ 390

[0063] Here, [Nb], [Ti], and [Al] represent the weight percent of each element.

[0064] Equation (1) represents a composition necessary to suppress the formation of insulating Si oxide by controlling the content ranges of Si, Nb, Ti, and Al to a certain range and forming a Ti-Al composite oxide in a high-temperature oxidizing atmosphere. Generally, when a certain amount of Si is contained in the base material, insulating Si oxide is easily formed in a high-temperature oxidizing atmosphere. Accordingly, in the present invention, by adding Ti and Al, which have a high affinity for oxygen, a Ti-Al composite oxide is formed, thereby suppressing Si from bonding with oxygen. In addition, in the present invention, Nb is added to form a Laves phase in the form of Nb2Si through bonding with Si, thereby preventing the formation of insulating Si oxide.

[0065] More specifically, the present invention satisfies the alloy composition and must satisfy the value of Equation (1) in order to cause the formation of Ti-Al composite oxide and Laves phase as described above, thereby suppressing the formation of insulating Si oxide. Accordingly, it is preferable that the value of Equation (1) be 390 or less, more preferably 385 or less, and most preferably 380 or less.

[0066] In addition, a ferritic stainless steel according to one embodiment of the present invention may include a base material having an interface portion that contacts an oxide scale layer and includes a Ti-Al composite oxide, and an oxide scale layer formed on the outermost surface portion of the base material. Here, the interface portion refers to a region extending from a point where the maximum oxygen concentration in the base material is present to a point in the thickness direction of the base material that is 50% of the maximum oxygen concentration in the base material.

[0067] The interface of the above base material includes a Ti-Al composite oxide, and in addition to the Ti-Al composite oxide, it may include Ti oxide, Al oxide, Nb oxide, Laves phase, etc.

[0068] Ti and Al, which have a high affinity for oxygen in the base material, combine with oxygen at the interface of the base material to form a Ti-Al composite oxide, and can form the remaining amount of Ti or Al oxides that are not formed after the Ti-Al composite oxide is formed.

[0069] More specifically, when measured by Transmission Electron Microscopy Energy-Dispersive X-ray Spectroscopy (TEM-EDS) analysis, the above interface may have a Ti concentration of 0.01 at.% to 70.0 at.% and an Al concentration of 0.01 at.% to 70.0 at.%. A higher Ti concentration in the above interface helps improve strength and heat resistance, but if it is less than 0.01 at.%, the reaction with oxygen may not be smooth, and if it exceeds 70.0 at.%, it may actually lower oxidation resistance. A higher Al concentration in the above interface increases oxidation resistance, but if it is less than 0.01 at.%, it may have a negative effect on oxide stability at high temperatures, and if it exceeds 70.0 at.%, it may lower oxidation resistance.

[0070] The above interface may contain one or more Nb-based Laves phases selected from Nb2Si, Fe2Nb, and mixtures thereof, and the Nb concentration measured by TEM-EDS analysis may be 0.01 at.% to 70.0 at.%. If the Nb concentration included in the above interface is less than 0.01 at.%, the formation of the Laves phase may not be smooth, and if it exceeds 70.0 at.%, the ductility and machinability of the steel may be reduced.

[0071] In addition, the O concentration measured by TEM-EDS analysis of the above interface may be between 0.1 at.% and 80.0 at.%. O included in the above interface is an element essential for oxide formation; if the concentration is less than 0.1 at.%, oxide formation may not proceed smoothly, and if it exceeds 80.0 at.%, it may reduce the ductility of the steel and thereby reduce its mechanical properties.

[0072] On the outermost surface of a base material including an interface portion in which the above-mentioned Ti-Al composite oxide, etc. is formed, an oxide scale layer containing an oxide scale with excellent conductivity in a high-temperature oxidation environment may be formed.

[0073] The above oxide scale layer may include manganese oxide, chromium oxide, manganese-chromium oxide, etc.

[0074] If the thickness of the oxide scale layer exceeds a certain level, the effects of improved oxidation peel resistance and electrical conductivity may be lost due to increased stress on the base material, and the scale may peel off, potentially damaging the material. Considering this, it is desirable to control the thickness of the oxide scale layer to 4㎛ or less in an atmosphere of 300℃ to 900℃, more preferably to 3.8㎛ or less, and most preferably to 3.5㎛ or less.

[0075] In addition, the ferritic stainless steel according to one embodiment of the present invention can maintain high electrical conductivity in a high-temperature oxidation environment by adding Ti and Al, which have a high affinity for oxygen, to the alloy components to form a Ti-Al composite oxide at the interface of the base material, and adding Nb to form a Laves phase such as Nb2Si or Fe2Nb to suppress the formation of Si oxides, and by forming an oxide scale with excellent conductivity even in a high-temperature oxidation environment on the outermost surface of the base material. Accordingly, the ferritic stainless steel according to one embodiment of the present invention can satisfy an interfacial contact resistance of 40 mΩ㎠ or less at 300°C to 900°C.

[0076] Next, a method for manufacturing ferritic stainless steel according to one embodiment of the present invention will be described.

[0077] A method for manufacturing a ferritic stainless steel according to one embodiment of the present invention comprises the steps of: reheating a steel material comprising, in weight%, C: 0.0010% to 0.0200%, N: 0.0010% to 0.0200%, Si: 0.010% to 0.400%, Mn: 0.10% to 1.50%, Cr: 18.0% to 25.0%, Mo: 0.01% to 2.00%, Nb: 0.05% to 1.00%, Ti: 0.010% to 0.200%, Al: 0.010 to 0.200%, the remainder being Fe and other unavoidable impurities, and satisfying that the value of the following formula (1) is 390 or less; and manufacturing a hot-rolled material by hot rolling and hot-rolling annealing after reheating. and may include the step of manufacturing a steel plate by cold rolling and cold annealing the hot-rolled material.

[0078] Equation (1): 1000×[Si] / (0.65×[Nb]+[Ti]+[Al]) ≤ 390

[0079] Here, [Nb], [Ti], and [Al] represent the weight percent of each element.

[0080] The reason for limiting the component range of each alloy composition above may be the same as described above, and each manufacturing step will be explained in more detail below.

[0081] First, the above steel can be reheated at 1050°C to 1280°C, and then hot-rolled and hot-rolled annealed to produce a hot-rolled material.

[0082] 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.

[0083] The finishing rolling temperature during the above hot rolling may be 700°C to 950°C, and the thickness of the hot-rolled material produced in this way may be 2 mm to 6 mm.

[0084] 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.

[0085] The hot rolling annealing temperature of the above hot-rolled material may be 900℃ to 1150℃.

[0086] 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.

[0087] The above hot-rolled annealed material can be cold-rolled and cold-rolled annealed.

[0088] The above cold rolling annealing can be performed at 900°C to 1150°C, and the thickness of the final cold-rolled product can be 0.05 mm to 2.5 mm.

[0089] 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.

[0090] The present invention will be explained in more detail below through the following examples. However, the following examples are merely illustrative of the present invention, and the scope of the present invention is not limited thereto.

[0091] Examples

[0092] Steel was manufactured to satisfy the various alloy compositions shown in Table 1 below. The unit is weight%.

[0093] Classification CsiMnCrNbMoTiAlN Example 1 0.00470.1260.4922.40.420.170.1040.1300.0106 Example 2 0.00570.1280.4922.40.430.170.0990.0870.0099 Example 3 0.01070.1500.4921.90.420.180.0730.0500.0102 Example 4 0.00400.1300.5022.20.510.2 10.0200.0200.0056 Comparative Example 10.00520.1900.5022.20.000.200.05100.0057 Comparative Example 20.00610.1700.5022.10.520.000.03000.0063 Comparative Example 30.00800.0680.4822.30.000.000.15000.0084 Comparative Example 40.00570.1300.5022.20.510.210.00000.0056

[0094] The manufactured steel was reheated to a temperature of 1250°C, hot-rolled to a finish rolling temperature of 900°C, and then hot-rolled annealed at a temperature of 1050°C 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, and a 15 mm × 15 mm sample was produced to manufacture a ferritic stainless steel specimen. The Equation (1) and high-temperature interface contact resistance of the ferritic stainless steel produced above were measured, and the results are shown in Table 2 below.

[0095] The high-temperature interface contact resistance was measured as follows. Pt paste was applied to both sides of a high-temperature oxidized specimen, pre-dried at 200°C, and then sintered at 800°C for 3 hours. After stacking Pt mesh on both sides of the specimen sintered with Pt on both sides, a load of 15g was applied, and a 4-terminal Pt wire was drawn out and placed inside the furnace. After raising the temperature to 800°C, the resistance was measured at 500 hours using the DC 4-point probe method to calculate the resistance value.

[0096] Classification 1000×[Si] / (0.65×[Nb]+[Ti]+[Al]) High-temperature interface contact resistance (mΩcm)2 Example 1 248.55.23 Example 2 275.06.37 Example 3 378.89.48 Example 4 349.98.67 Comparative Example 1 3725.550.2 Comparative Example 2 462.043.5 Comparative Example 3 453.341.6 Comparative Example 4 392.240.8

[0097] As shown in Table 2 above, in the case of Examples 1 to 4 satisfying that the value of Equation (1) is 390 or less, the high-temperature interface contact resistance is 40 mΩcm 2 Although the results were satisfactory as shown below, in the case of Comparative Examples 1 to 4, which do not satisfy Equation (1), the high-temperature interface resistance value was 40 mΩcm 2 It was confirmed that it appeared to be inferior to the above. In addition, cross-sectional TEM / FIB images and EDS mapping analysis results after heat-treating the steel of Example 1 and Comparative Example 1 in an 800°C atmospheric environment for 500 hours are shown in Figures 1 and 2.

[0098] The above TEM / FIB images and EDS mapping analysis were performed according to the specimen preparation and analysis procedures generally used by ordinary skilled technicians. Specifically, cross-sectional thin film specimens were collected using FIB after forming a surface protective layer, and the cross-sectional structure and compositional distribution were observed using a Transmission Electron Microscope (TEM). In addition, the distribution of Ti, Al, Nb, O, etc., was confirmed using an EDS analysis device attached to the TEM, and the composition of the interface was qualitatively determined from the EDS mapping results.

[0099] As shown in FIG. 1, in the case of Example 1 satisfying the value of Equation (1), it was confirmed that an Mn-Cr oxide layer is formed on the outermost surface of the base material, and that a Ti-Al composite oxide is formed on the interface of the base material and on the base material, and that a Laves phase in the form of Nb2Si is formed on the interface of the base material. That is, according to the present invention, it was found that by adding Ti and Al, which have high affinity for oxygen, a Ti-Al composite oxide is formed and a Laves phase in the form of Nb2Si is formed, thereby preventing the formation of an insulating Si oxide.

[0100] On the other hand, as shown in FIG. 2, in the case of Comparative Example 1, which does not satisfy the value of Equation (1), it can be confirmed that an insulating Si oxide exists in the form of a film at the interface between the outermost oxide layer and the base material. As a result, the high-temperature interfacial electrical conductivity measurement result is 40 mΩcm, as shown in Table 2 above. 2 It can be seen that it appears inferior due to excess.

[0101] 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. A ferritic stainless steel comprising a base material and an oxide scale layer formed on the outermost surface of the base material, The above base material includes an interface portion that is in contact with the oxide scale layer and includes a Ti-Al composite oxide, and The above base material is a ferritic stainless steel comprising, in weight%, C: 0.0010% to 0.0200%, N: 0.0010% to 0.0200%, Si: 0.010% to 0.400%, Mn: 0.10% to 1.50%, Cr: 18.0% to 25.0%, Mo: 0.01% to 2.00%, Nb: 0.05% to 1.00%, Ti: 0.010% to 0.200%, Al: 0.010% to 0.200%, the remainder being Fe and other unavoidable impurities, and satisfying the value of the following formula (1) being 390 or less. Equation (1): 1000×[Si] / (0.65×[Nb]+[Ti]+[Al]) ≤ 390 (Here, [Nb], [Ti], and [Al] represent the weight percent of each element) 2. In Paragraph 1, The above interface comprises Ti oxide, and the ferritic stainless steel has a Ti concentration of 0.01 at.% to 70.0 at.% as measured by Transmission Electron Microscopy Energy-Dispersive X-ray Spectroscopy (TEM-EDS) analysis.

3. In Paragraph 1, The above interface comprises Al oxide, and the ferritic stainless steel has an Al concentration of 0.01 at.% to 70.0 at.% as measured by transmission electron microscopy energy-dispersive X-ray spectroscopy.

4. In Paragraph 1, The above interface comprises an Nb-based Laves phase, and the ferritic stainless steel has an Nb concentration of 0.01 at.% to 70.0 at.% as measured by transmission electron microscopy energy-dispersive X-ray spectroscopy.

5. In Paragraph 4, The above Laves phase is a ferritic stainless steel comprising one or more selected from Nb2Si, Fe2Nb, and mixtures thereof.

6. In Paragraph 1, The above interface is a ferritic stainless steel having an O concentration of 0.1 at.% to 80.0 at.% as measured by transmission electron microscopy energy-dispersive X-ray spectroscopy.

7. In Paragraph 1, The above oxide scale layer is a ferritic stainless steel comprising one or more selected from manganese oxide, chromium oxide, and manganese-chromium oxide.

8. In Paragraph 1, The above oxide scale layer is a ferritic stainless steel with a thickness of 4㎛ or less.

9. In Paragraph 1, The above ferritic stainless steel is a ferritic stainless steel having an interfacial contact resistance of 40 mΩ㎠ or less at 300℃ to 900℃.

10. A step of reheating a steel material comprising, in wt%, C: 0.0010% to 0.0200%, N: 0.0010% to 0.0200%, Si: 0.010% to 0.400%, Mn: 0.10% to 1.50%, Cr: 18.0% to 25.0%, Mo: 0.01% to 2.00%, Nb: 0.05% to 1.00%, Ti: 0.010% to 0.200%, Al: 0.010% to 0.200%, the remainder being Fe and other unavoidable impurities, and satisfying that the value of the following formula (1) is 390 or less; A step of manufacturing a hot-rolled material by hot rolling and hot-rolled annealing after the above reheating; and A step of manufacturing a steel plate by cold rolling and cold rolling annealing the above hot-rolled material; A method for manufacturing ferritic stainless steel including Equation (1): 1000×[Si] / (0.65×[Nb]+[Ti]+[Al]) ≤ 390 (Here, [Nb], [Ti], and [Al] represent the weight percent of each element) 11. In Paragraph 10, The above ferritic stainless steel comprises a base material and an oxide scale layer formed on the outermost surface of the base material, and A method for manufacturing ferritic stainless steel, wherein the above-mentioned base material is in contact with the oxide scale layer and includes an interface portion comprising a Ti-Al composite oxide.

12. In Paragraph 10, A method for manufacturing ferritic stainless steel in which the above interface comprises Ti oxide and the Ti concentration measured by transmission electron microscopy energy-dispersive X-ray spectroscopy is 0.01 at.% to 70.0 at.%.

13. In Paragraph 10, A method for manufacturing ferritic stainless steel in which the above interface comprises Al oxide and the Al concentration measured by transmission electron microscopy energy-dispersive X-ray spectroscopy is 0.01 at.% to 70.0 at.%.

14. In Paragraph 10, A method for manufacturing ferritic stainless steel in which the above interface comprises an Nb-based Laves phase and the Nb concentration measured by transmission electron microscopy energy-dispersive X-ray spectroscopy is 0.01 at.% to 70.0 at.%.

15. In Paragraph 14, A method for manufacturing ferritic stainless steel comprising one selected from Nb2Si, Fe2Nb, and mixtures thereof, wherein the Laves phase is the above.

16. In Paragraph 10, A method for manufacturing ferritic stainless steel in which the above interface has an O concentration of 0.1 at.% to 80.0 at.% as measured by transmission electron microscopy energy-dispersive X-ray spectroscopy.

17. In Paragraph 10, A method for manufacturing ferritic stainless steel in which the oxide scale layer comprises one or more selected from manganese oxide, chromium oxide, and manganese-chromium oxide.

18. In Paragraph 10, A method for manufacturing ferritic stainless steel in which the above oxide scale layer has a thickness of 4 μm or less.

19. In Paragraph 10, The above ferritic stainless steel is a method for manufacturing ferritic stainless steel having an interfacial contact resistance of 40 mΩ㎠ or less at 300℃ to 900℃.

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