Ferritic stainless steel and method for manufacturing same

Abstract

The ferritic stainless steel according to the present invention comprises: 0.04 to 0.1% of C; 0.0005 to 0.03% of N; 0.01-1.0% of Si; 0.01 to 1.0% of Mn; 0.001 to 0.05% of P; 10.0 to 25.0% of Cr; 0.01 to 1.0% of Cu; 0.01 to 1.0% of Ni; 0.005 to 0.3% of Al; 0.02 to 0.5% of Ti; and the balance of Fe and inevitable impurities, and may satisfy formulas (1) and (2) below. Formula (1): 20 ≤ 420×[C]+23×[Ni]+9×[Cu]+7×[Mn]≤ 60, Formula (2): 0.5 ≤ [N] / ([Ti]+[Al])×100 ≤ 25 (where [C], [Ni], [Cu], [Mn], [N], [Ti], and [Al] represent wt% of 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] Ferritic stainless steel is used in various industrial fields, such as home appliances, kitchenware, and automotive parts.

[0003] Among them, 430-series ferritic stainless steel, which has high C and N content, exhibits high strength but faces the problem of surface quality issues, such as intergranular erosion after pickling, due to the high C and N content. Although methods such as adding large amounts of Ti or Nb exist to address this issue, they can lead to excessive increases in raw material costs or cause other quality problems. Therefore, further research is needed on manufacturing methods for ferritic stainless steel that offer excellent surface quality while reducing manufacturing costs.

[0004] One aspect of the present invention for solving the aforementioned problem is to provide a ferritic stainless steel and a method for manufacturing the same, which can secure material strength and excellent surface quality while reducing manufacturing costs by controlling the alloy composition and the distribution of precipitates on the surface and inside the steel.

[0005] 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.

[0006] To achieve the above objective, the ferritic stainless steel according to one embodiment of the present invention comprises C: 0.04 to 0.1%, N: 0.0005 to 0.03%, Si: 0.01 to 1.0%, Mn: 0.01 to 1.0%, P: 0.001 to 0.05%, Cr: 10.0 to 25.0%, Cu: 0.01 to 1.0%, Ni: 0.01 to 1.0%, Al: 0.005 to 0.3%, Ti: 0.02 to 0.5%, the remainder being Fe and unavoidable impurities, and may satisfy the following formulas (1) and (2).

[0007] Equation (1): 20 ≤ 420×[C]+23×[Ni]+9×[Cu]+7×[Mn] ≤ 60

[0008] Equation (2): 0.5 ≤ [N] / ([Ti]+[Al])×100 ≤ 25

[0009] (Here, [C], [Ni], [Cu], [Mn], [N], [Ti], and [Al] represent the weight percent of each element)

[0010] In one embodiment of the present invention, the stainless steel may have a total number of Cr carbides and Cr nitrides with a diameter of 100 nm or more per unit area in the thickness direction from the surface to a region of 0.002 t to 0.1 t (where t means plate thickness (mm)) that is 30% or less compared to the number of Cr carbides per unit area in the +1 / 4 t to -1 / 4 t region.

[0011] In one embodiment of the present invention, the stainless steel may have a number of Cr nitrides with a diameter of 100 nm or more per unit area in the +1 / 4t to -1 / 4t region that is 20% or less of the number of Cr carbides.

[0012] The stainless steel according to one embodiment of the present invention may have a room temperature tensile strength of 450 MPa or more.

[0013] A method for manufacturing ferritic stainless steel according to one embodiment of the present invention comprises the steps of: reheating a slab satisfying formulas (1) and (2) and containing C: 0.04 to 0.1%, N: 0.0005 to 0.03%, Si: 0.01 to 1.0%, Mn: 0.01 to 1.0%, P: 0.001 to 0.05%, Cr: 10.0 to 25.0%, Cu: 0.01 to 1.0%, Ni: 0.01 to 1.0%, Al: 0.005 to 0.3%, Ti: 0.02 to 0.5%, the remainder being Fe and unavoidable impurities; and hot rolling such that the temperature at the entry side of the finish rolling after reheating is 900℃ to 1150℃. It may include a step of hot rolling annealing at 800 to 1100°C for 1 to 10 minutes after the above hot rolling; and a step of cold rolling and cold rolling annealing after the above hot rolling annealing.

[0014] According to one embodiment of the present invention, the cold rolling annealing can be performed at 800 to 1050°C for 30 to 200 seconds.

[0015] The cold rolling according to one embodiment of the present invention can be performed with a final cold rolling reduction rate of 40% or more.

[0016] In one embodiment of the present invention, after the cold rolling annealing step, the stainless steel may have a sum of the number of Cr carbides and Cr nitrides with a diameter of 100 nm or more per unit area in the thickness direction from the surface to a region of 0.002 t to 0.1 t (where t means plate thickness (mm)) that is 30% or less compared to the number of Cr carbides per unit area in the +1 / 4 t to -1 / 4 t region.

[0017] In one embodiment of the present invention, after the cold rolling annealing step, the stainless steel may have a number of Cr nitrides with a diameter of 100 nm or more per unit area in the +1 / 4t to -1 / 4t region that is 20% or less of the number of Cr carbides.

[0018] According to the present invention, by controlling the alloy composition and the distribution of precipitates on the surface layer and inside the steel, it is possible to provide a ferritic stainless steel and a method for manufacturing the same that can secure the strength of the material and secure excellent surface quality while reducing manufacturing costs.

[0019] 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.

[0020] FIG. 1 is a diagram showing the tensile strength values ​​according to the values ​​of Equation (1) of the embodiments and comparative examples according to one embodiment of the present invention.

[0021] FIG. 2 is a diagram showing the number of Cr carbides and Cr nitrides per unit area in the center (+1 / 4t to -1 / 4t region) according to the values ​​of Equation (2) of the embodiments and comparative examples according to one embodiment of the present invention.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] The present invention aims to provide a ferritic stainless steel capable of ensuring excellent strength while also securing surface quality by controlling the alloy composition and the distribution of precipitates on the surface and inside the steel.

[0027] A ferritic stainless steel according to one embodiment of the present invention will be described in detail below.

[0028] A ferritic stainless steel according to one embodiment of the present invention may comprise C: 0.04 to 0.1%, N: 0.0005 to 0.03%, Si: 0.01 to 1.0%, Mn: 0.01 to 1.0%, P: 0.001 to 0.05%, Cr: 10.0 to 25.0%, Cu: 0.01 to 1.0%, Ni: 0.01 to 1.0%, Al: 0.005 to 0.3%, Ti: 0.02 to 0.5%, the remainder being Fe and unavoidable impurities.

[0029] The reasons for limiting the compositional range of each alloying element are described below. Unless otherwise noted, units are weight percent.

[0030] The content of C (carbon) can be 0.04~0.1%.

[0031] If C is less than 0.04%, there is a problem with reduced strength, and if it exceeds 0.1%, corrosion resistance and formability may be reduced.

[0032] The content of N (nitrogen) can be 0.0005~0.03%.

[0033] If N is less than 0.0005%, costs may increase due to excessive refining, and if it exceeds 0.03%, the amount of Cr nitrides may increase excessively, leading to a decrease in surface quality due to intergranular erosion.

[0034] The content of Si (silicon) can be 0.01 to 1.0%.

[0035] If Si is less than 0.01%, refining may be difficult, and if it exceeds 1.0%, surface defects may occur and formability may decrease.

[0036] The content of Mn (manganese) can be 0.01 to 1.0%.

[0037] If Mn is less than 0.01%, the refining cost may be high, and if it exceeds 1.0%, there is a problem of reduced moldability due to increased impurities.

[0038] The content of P (phosphorus) can be 0.001~0.05%.

[0039] If P is less than 0.001%, there is a problem with the refining cost becoming high, and if it exceeds 0.05%, there is a problem with the impurities increasing and the moldability decreasing.

[0040] The content of Cr (chromium) can be 10.0~25.0%.

[0041] If Cr is less than 10.0%, there is a problem with reduced corrosion resistance, and if it exceeds 25.0%, there is a problem with reduced formability.

[0042] The content of Cu (copper) can be 0.01 to 1.0%.

[0043] If the Cu content is less than 0.01%, there is a problem with the refining cost becoming high, and if it exceeds 1.0%, there is a problem with the impurities increasing and the formability decreasing.

[0044] The content of Ni (nickel) can be 0.01 to 1.0%.

[0045] When Ni is less than 0.01%, there is a problem with the refining cost becoming high, and when it exceeds 1.0%, there is a problem with the impurities increasing and formability decreasing.

[0046] The content of Al (aluminum) can be 0.005~0.3%.

[0047] If Al is less than 0.005%, deoxidation during steelmaking may not be sufficient, and if it exceeds 0.3%, there is a problem that a large amount of steelmaking inclusions are generated.

[0048] The titanium (Ti) content may be 0.02 to 0.5%.

[0049] If Ti is less than 0.02%, corrosion resistance and surface quality may be reduced, and if it exceeds 0.5%, there is a problem that a large amount of steelmaking inclusions are generated.

[0050] 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.

[0051] A ferritic stainless steel according to one embodiment of the present invention, comprising the alloy composition as described above, can satisfy the value of the following formula (1).

[0052] Equation (1): 20 ≤ 420×[C]+23×[Ni]+9×[Cu]+7×[Mn] ≤ 60

[0053] (Here, [C], [Ni], [Cu], and [Mn] represent the weight percent of each element)

[0054] The above formula (1) represents the compositional relationship of C, Ni, Cu, and Mn, which are elements that have an important influence on the strength of the material. It is preferable that the value of the above formula (1) be 20 or more and 60 or less. If the value of the above formula (1) is less than 20, there is a problem that the strength of the material is insufficient, and if it exceeds 60, there is a problem that the surface quality may be degraded due to reduced formability.

[0055] In addition, a ferritic stainless steel according to one embodiment of the present invention can satisfy the value of the following formula (2).

[0056] Equation (2): 0.5 ≤ [N] / ([Ti]+[Al])×100 ≤ 25

[0057] (Here, [N], [Ti], and [Al] represent the weight percent of each element)

[0058] The value of Equation (2) above represents the compositional relationship of N, Ti, and Al that affects the formation of Cr nitrides, and it is preferable that the value of Equation (2) be 0.5 or higher and 25 or lower. If the value of Equation (2) is less than 0.5, there is a problem of increased refining costs and inclusions because N is too low or Ti and Al are too high, and if it exceeds 25, there is a problem that Cr nitrides are excessively formed and intergranular erosion may occur.

[0059] In addition, the ferritic stainless steel according to one embodiment of the present invention can control the value of Equation (2), which affects the formation of Cr nitrides, to be 0.5 or higher and 25 or lower, so that the number of Cr nitrides with a diameter of 100 nm or more per unit area in the +1 / 4t to -1 / 4t (where t means plate thickness (mm)) region is 20% or less compared to the number of Cr carbides.

[0060] If the number of Cr nitrides exceeds 20% of the number of Cr carbides, the growth of Cr carbides may not be smooth, but if it is 20% or less, it is desirable in that the growth of Cr carbides can be promoted at high temperatures to improve elongation.

[0061] In addition, the ferritic stainless steel according to one embodiment of the present invention can control Cr carbides and Cr nitrides in the surface layer of the steel to prevent intergranular erosion, and specifically, the sum of the number of Cr carbides and Cr nitrides with a diameter of 100 nm or more per unit area in the thickness direction from the surface of the steel to the region of 0.002t to 0.1t (where t means plate thickness (mm)) can be made to be 30% or less compared to the number of Cr carbides per unit area in the central region of +1 / 4t to -1 / 4t.

[0062] If the sum of the number of Cr carbides and Cr nitrides in the surface layer exceeds 30% of the number of Cr carbides in the center, intergranular erosion may occur during pickling due to the Cr carbides and Cr nitrides in the surface layer.

[0063] In addition, the ferritic stainless steel according to one embodiment of the present invention may have a room temperature tensile strength of 450 MPa or more.

[0064] Hereinafter, a method for manufacturing ferritic stainless steel according to one embodiment of the present invention will be described.

[0065] A method for manufacturing ferritic stainless steel according to one embodiment of the present invention comprises the steps of: reheating a slab satisfying formulas (1) and (2) and containing C: 0.04 to 0.1%, N: 0.0005 to 0.03%, Si: 0.01 to 1.0%, Mn: 0.01 to 1.0%, P: 0.001 to 0.05%, Cr: 10.0 to 25.0%, Cu: 0.01 to 1.0%, Ni: 0.01 to 1.0%, Al: 0.005 to 0.3%, Ti: 0.02 to 0.5%, the remainder being Fe and unavoidable impurities; and hot rolling such that the temperature at the entry side of the finish rolling after reheating is 900℃ to 1150℃. It may include a step of hot rolling annealing at 800 to 1100°C for 1 to 10 minutes after the above hot rolling; and a step of cold rolling and cold rolling annealing after the above hot rolling annealing.

[0066] 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.

[0067] After manufacturing a slab satisfying the compositional components described above, a series of processes including reheating, hot rolling, hot rolling annealing, cold rolling, and cold rolling annealing may be performed.

[0068] First, the above slab can be reheated at 1100°C or higher, preferably 1000°C to 1300°C for 2 to 5 hours, and then hot-rolled and hot-rolled annealed to produce a hot-rolled material.

[0069] The above reheating temperature may be 1100°C or higher to reduce the hot rolling load, and may be limited to 1300°C or lower to prevent internal grain coarsening. If the above reheating time is less than 2 hours, the slab temperature may not be sufficiently secured, which may lead to an increase in rolling load and frequent surface defects, and if it exceeds 5 hours, sagging of the slab or edge cracks may occur inside the furnace.

[0070] The above hot rolling can be performed such that the finish rolling entry temperature is 900 to 1150°C, and the thickness of the hot-rolled steel sheet produced in this way can be 2 to 8 mm. In addition, after the above hot rolling, the steel is coiled at 500 to 800°C to control the distribution of Cr carbides and Cr nitrides.

[0071] When the finishing rolling entry temperature is less than 900℃ during the above hot rolling, the rolling load increases and shape defects increase, which may reduce productivity, and when it exceeds 1150℃, the increase in oxides due to excessive high-temperature operation may lead to a decrease in surface quality and a decline in material quality due to the deterioration of texture.

[0072] The above hot rolling annealing can be performed at 800 to 1100°C for 1 to 10 minutes.

[0073] If the above hot rolling annealing temperature is less than 800°C, recrystallization may not occur and thus a texture may not be formed, and if it exceeds 1100°C, the grains may coarsen and the strength of the steel sheet may be weakened. In addition, if the above hot rolling annealing time is less than 1 minute, recrystallization may not occur smoothly, and if it exceeds 10 minutes, the grains may coarsen and the strength of the steel sheet may be weakened.

[0074] The hot-rolled material that has undergone the above hot-rolled annealing can be cold-rolled and cold-rolled annealed one or more times, preferably 1 to 5 times.

[0075] Through the initial cold rolling and cold annealing, deformation can be induced in the steel of the hot-rolled material, thereby facilitating the smooth formation of precipitates during subsequent processes. Through subsequent cold rolling and cold annealing, a large number of precipitates can be formed, and recrystallization can be induced to secure fine grains. At this time, since manufacturing costs may increase as the number of cold rolling and cold annealing cycles increases, the cold rolling and cold annealing cycles can be performed five times or fewer.

[0076] The above cold rolling can be performed such that the final cold rolling reduction rate is 40% or more. If the final cold rolling reduction rate is less than 40%, the amount of deformation is insufficient, making it difficult to achieve fine grains.

[0077] The above cold rolling annealing can be performed at 800 to 1050°C for 30 to 200 seconds, and the thickness of the final cold-rolled product thus cold-rolled annealed can be 0.05 to 3 mm.

[0078] If the above cold rolling annealing is below 800℃, the rolled structure may not be sufficiently recrystallized, which may reduce workability, and if it exceeds 1050℃, the grain size may become coarsened and plate breakage may occur.

[0079] 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.

[0080] Examples

[0081] Vacuum induction slabs were manufactured to satisfy the various alloy compositions shown in Table 1 below. The units in Table 1 below are weight%.

[0082] Classification CSIMnPCrNiCuNTiT-Al Example 1 0.07 50.2 20.7 50.02 216.10.10.10.0 10.10.07 Example 2 0.07 10.2 10.5 50.02 116.10.10.10.0 10.10.09 Comparative Example 1 0.02 10.18 0.5 0.02 116.10.05 0.10.018 0.10.04 Comparative Example 2 0.03 20.2 10.5 0.02 216.10.05 0.10.014 0.08 0.06 Comparative Example 3 0.05 20.1 50.60.02116.10.050.10.0350.070.05Comparative Example 40.0620.220.70.01916.10.050.10.0270.040.06Comparative Example 50.0690.190.50.01816.10.050.10.0420.080.05Comparative Example 60.0810.170.80.02316.10.70.70.020.030.06Comparative Example 70.1150.210.50.02116.10.30.40.010.10.06

[0083] The manufactured slab was reheated in a furnace at 1200°C for 2 to 5 hours, then hot-rolled so that the finish rolling entry temperature reached 1000°C, coiled at 600°C, and then hot-rolled annealed at 1050°C for 5 minutes to produce a hot-rolled material with a thickness of 3 mm. The hot-rolled material was cold-rolled one or more times and cold-rolled annealed at 900°C for 100 seconds to produce a final cold-rolled product, a ferritic stainless steel with a thickness of 0.5 mm. At this time, the final reduction rate of each specimen was performed to be 40% or more.

[0084] The values ​​of Equation (1) and Equation (2), room temperature tensile strength, and Cr carbides and Cr nitrides in the surface and center of the cross-section of the ferritic stainless steel manufactured above were measured, and the results are shown in Table 2 below. In addition, the tensile strength according to the value of Equation (1) and the number of Cr carbides and Cr nitrides per unit area in the center (+1 / 4t to -1 / 4t region) according to the value of Equation (2) are shown in Figures 1 and 2.

[0085] For room temperature tensile strength, the ferritic stainless steel manufactured above was cut at a 90° angle to the rolling direction and processed into a plate-shaped test specimen according to JIS 13B standards, and then tested at room temperature (25±2℃) using a tensile testing machine (Zwick Roell, tensile speed 20 mm / min). The test was repeated at least three times for each condition, and the average value was taken as the tensile strength.

[0086] Cr carbides and Cr nitrides were observed using FE-SEM (acceleration voltage 15 kV, magnification 5,000x, observation area 50 x 50 μm) by analyzing at least 5 regions each in the surface layer (0.002t to 0.1t in the thickness direction from the surface (where t represents the plate thickness (mm))) and the center (+1 / 4t to -1 / 4t region). Cr carbides and Cr nitrides were identified by concurrent EDS component analysis, and the number per unit area was counted using image analysis software (ImageJ).

[0087] The occurrence of intergranular erosion was determined as 'occurred' if continuous cavities (≥1㎛ depth) were confirmed at the grain boundaries during surface FE-SEM observation.

[0088] Value of formula (1) Room temperature tensile strength (MPa) Value of formula (2) (Surface layer Cr carbide + Cr nitride) / (Core layer Cr carbide) × 100 (%) (Core layer Cr nitride / Cr carbide) × 100 (%) Grain boundary erosion Example 1 40 50 75.9 16.7 2.0 Not occurred Example 2 37 48 95.3 8.0 1.6 Not occurred Comparative Example 1 44 37 12.9 -- Not occurred Comparative Example 2 19 429 10.0 -- Not occurred Comparative Example 3 28 46 8 29.2 64.7 41.2 Occurred Comparative Example 4 33 47 127.0 42.1 26.3 Occurred Comparative Example 5 35 47 232.3 46.4 35.7 Occurred Comparative Example 6 6 25 48 22.2 37.8 16.2 Occurred Comparative Example 7 6 25 376.3 35.4 8.3 Occurred

[0089] As shown in Table 2 and Figures 1 and 2 above, Examples 1 and 2 satisfied the alloy composition presented in the present invention, satisfying the value of Equation (1) as 20 or more and 60 or less, and the value of Equation (2) as 0.5 or more and 25 or less. Therefore, in the case of Examples 1 and 2, it was confirmed that the sum of the number of Cr carbides and Cr nitrides in the surface layer satisfies 30% or less compared to the number of Cr carbides in the center, and the number of Cr nitrides satisfies 20% or less compared to the number of Cr carbides in the center. In addition, Examples 1 and 2 showed a high room temperature tensile strength of 450 MPa or more, and it was confirmed that no grain boundary erosion caused by Cr carbides and Cr nitrides in the surface layer occurred during pickling. On the other hand, in the case of Comparative Examples 1 and 2, where the value of Equation (1) was less than 20, it was confirmed that the room temperature tensile strength was low, less than 450 MPa. In addition, in the case of Comparative Examples 1 and 2, it was confirmed that Cr nitrides and Cr carbides were not formed due to the low C and N content, and as a result, intergranular erosion did not occur.

[0090] In addition, in Comparative Examples 3 to 5 where the value of Equation (2) exceeds 25, the sum of the number of Cr carbides and Cr nitrides in the surface layer was found to be high at 64.7%, 42.1%, and 46.4%, respectively, compared to the number of Cr carbides in the center, and the number of Cr nitrides was found to be high at 41.2%, 26.3%, and 35.7%, respectively, compared to the number of Cr carbides in the center, confirming that intergranular erosion caused by Cr carbides and Cr nitrides occurred.

[0091] In addition, in Comparative Examples 5 and 6, where the value of Equation (1) exceeds 60, the sum of the number of Cr carbides and Cr nitrides in the surface layer was 37.8% and 35.4%, respectively, compared to the number of Cr carbides in the center, confirming that intergranular erosion occurred due to Cr carbides and Cr nitrides in the surface layer.

[0092] 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. C: 0.04 to 0.1%, N: 0.0005 to 0.03%, Si: 0.01 to 1.0%, Mn: 0.01 to 1.0%, P: 0.001 to 0.05%, Cr: 10.0 to 25.0%, Cu: 0.01 to 1.0%, Ni: 0.01 to 1.0%, Al: 0.005 to 0.3%, Ti: 0.02 to 0.5%, the remainder being Fe and unavoidable impurities, Ferritic stainless steel satisfying the following formulas (1) and (2). Equation (1): 20 ≤ 420×[C]+23×[Ni]+9×[Cu]+7×[Mn] ≤ 60 Equation (2): 0.5 ≤ [N] / ([Ti]+[Al])×100 ≤ 25 (Here, [C], [Ni], [Cu], [Mn], [N], [Ti], and [Al] represent the weight percent of each element) 2. In Paragraph 1, The above stainless steel is a ferritic stainless steel in which the sum of the number of Cr carbides and Cr nitrides with a diameter of 100 nm or more per unit area in the thickness direction from the surface to the region of 0.002t to 0.1t (where t means plate thickness (mm)) is 30% or less compared to the number of Cr carbides per unit area in the region of +1 / 4t to -1 / 4t.

3. In Paragraph 1, The above stainless steel is a ferritic stainless steel in which the number of Cr nitrides with a diameter of 100 nm or more per unit area in the +1 / 4t to -1 / 4t region is 20% or less compared to the number of Cr carbides.

4. In Paragraph 1, The above stainless steel is a ferritic stainless steel having a room temperature tensile strength of 450 MPa or higher.

5. A step of reheating a slab comprising C: 0.04 to 0.1%, N: 0.0005 to 0.03%, Si: 0.01 to 1.0%, Mn: 0.01 to 1.0%, P: 0.001 to 0.05%, Cr: 10.0 to 25.0%, Cu: 0.01 to 1.0%, Ni: 0.01 to 1.0%, Al: 0.005 to 0.3%, Ti: 0.02 to 0.5%, the remainder being Fe and unavoidable impurities, satisfying the following formulas (1) and (2); A step of hot rolling such that the finish rolling entry temperature after the above reheating is 900℃ to 1150℃; A step of hot rolling annealing at 800~1100℃ for 1~10 minutes after the above hot rolling; and A method for manufacturing ferritic stainless steel comprising the steps of: cold rolling and cold rolling annealing after the above hot rolling annealing. Equation (1): 20 ≤ 420×[C]+23×[Ni]+9×[Cu]+7×[Mn] ≤ 60 Equation (2): 0.5 ≤ [N] / ([Ti]+[Al])×100 ≤ 25 (Here, [C], [Ni], [Cu], [Mn], [N], [Ti], and [Al] represent the weight percent of each element) 6. In Paragraph 5, A method for manufacturing ferritic stainless steel in which the above cold rolling annealing is performed at 800 to 1050°C for 30 to 200 seconds.

7. In Paragraph 5, A method for manufacturing ferritic stainless steel in which the above cold rolling is performed with a final cold rolling reduction rate of 40% or more.

8. In Paragraph 5, A method for manufacturing a ferritic stainless steel in which, after the above cold rolling and annealing step, the sum of the number of Cr carbides and Cr nitrides with a diameter of 100 nm or more per unit area in the thickness direction from the surface to the region of 0.002t to 0.1t (where t means plate thickness (mm)) is 30% or less compared to the number of Cr carbides per unit area in the +1 / 4t to -1 / 4t region.

9. In Paragraph 5, A method for manufacturing ferritic stainless steel in which, after the above cold rolling and annealing step, the number of Cr nitrides with a diameter of 100 nm or more per unit area in the +1 / 4t to -1 / 4t region is 20% or less compared to the number of Cr carbides.

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