Ferritic stainless steel sheet and method for manufacturing the same

Abstract

To provide a ferritic stainless steel sheet excellent in temper coloring resistance.SOLUTION: A ferritic stainless steel sheet contains C, N, Si, Mn, P, S, Cr, Ni, Cu, Mo, Al, V, B, and O and further contains one or both of Ti: 0.40% or less and Nb: 0.80% or less, and the balance of Fe and impurities. The steel sheet has an oxide film on the surface thereof and satisfies following expressions: [Ti]+[Nb]≥0.03, 18.0≤[*Cr]+[*Si]×[Si]≤35.0, 0<[*Cu]-[Cu]≤5.00, surface Ra≤1.20 μnm, and color difference L*≥55.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a ferritic stainless steel sheet and a method for manufacturing the same, and more particularly to a ferritic stainless steel sheet with excellent temper color resistance and a method for manufacturing the same. [Background technology]

[0002] Cast iron and carbon steel were traditionally used for exhaust system components in automobiles, such as exhaust manifolds, converters, front pipes, center pipes, and mufflers. However, with stricter exhaust gas regulations, improved engine performance, and the trend towards lighter vehicle bodies, stainless steel has come into use due to its heat resistance and corrosion resistance. Ferritic stainless steel has a lower coefficient of thermal expansion and superior thermal fatigue properties compared to austenitic stainless steel, and is also widely used due to its lower material cost.

[0003] When cast iron or carbon steel was used for exhaust system components, rusting was inevitable. However, the introduction of stainless steel has improved both functionality and aesthetics. In recent years, car owners have begun to visually inspect the underside of their vehicles during delivery, inspections, and vehicle inspections, and even slight rust that does not affect functionality is now being noticed. Furthermore, even the temper color that forms on exhaust system components heated during driving has begun to be discussed.

[0004] Temper color is a phenomenon that occurs due to light interference when the oxide film formed on the surface of steel is approximately 1 μm or less thick. As the thickness of the oxide film increases, the metallic luster changes from yellow to red and then to blue. In automotive exhaust system components, it is important to avoid reddish temper color, which can be mistaken for rust.

[0005] Furthermore, automotive exhaust system components are exposed to a high-temperature, salty environment where they are heated by the accumulation of de-icing salt sprayed to prevent road freezing and salt derived from seawater. As a result, oxide films tend to grow easily, and the temper color progresses to the red range.

[0006] There are several methods to prevent temper coloration in stainless steel. Patent document 1 describes a technique to improve temper color resistance by bright annealing of stainless steel to enrich the surface with Si or Al and form an oxide film. However, bright annealing reduces productivity and increases manufacturing costs. Furthermore, this technique is intended for applications such as microwave ovens and gas ranges, and has not been studied for use in high-temperature, salt-damaged environments such as automotive exhaust system components.

[0007] Patent document 2 describes a technique for improving temper color resistance by polishing stainless steel or by polishing and bright annealing to form a surface oxide film with a high ratio of Cr, Si, and Al to Fe. However, polishing reduces productivity and increases manufacturing costs. Furthermore, the technique is intended for cooking utensils and heating appliances, and has not been studied for use in high-temperature, salt-damaged environments such as automotive exhaust system components.

[0008] Patent document 3 describes a technique for improving temper color resistance by adding tungsten (Sn) to high-purity ferritic stainless steel. However, the addition of Sn leads to an increase in alloy costs. Furthermore, studies have not been conducted on high-temperature, salt-damaged environments such as those found in automotive exhaust system components.

[0009] On the other hand, there is a constant demand for price reduction in automotive exhaust system components, and production volumes are high, making the application of the above technology impractical from an economic standpoint. Regarding temper color, while automotive exhaust system components are more tolerant of temper color than kitchen equipment and heating equipment in terms of avoiding reddish temper color that can be mistaken for rust, they are used in harsh environments with high temperatures and salt damage. For these reasons, it became necessary to develop technology to improve temper color resistance suitable for automotive exhaust system components. [Prior art documents] [Patent Documents]

[0010] [Patent Document 1] Japanese Patent Application Laid-Open No. 62-156254 [Patent Document 2] Japanese Patent Application Laid-Open No. 8-295999 [Patent Document 3] Japanese Patent No. 6106450 [Summary of the Invention] [Problems to be Solved by the Invention]

[0011] The present invention has been made in view of the above circumstances, and an object thereof is to provide a ferritic stainless steel sheet excellent in temper color resistance. Another object of the present invention is to provide a method for manufacturing a ferritic stainless steel sheet excellent in temper color resistance, which does not require bright annealing finish or polishing finish that is costly in manufacturing cost. [Means for Solving the Problems]

[0012] In order to solve the above problems, the present inventors have intensively studied various factors such as the content of each element of the ferritic stainless steel sheet and the surface state that affect the temper color resistance. As a result, it has been found that in addition to the cation concentration of Cr and Si in the surface layer generated in the manufacturing process, the cation concentration of Cu also affects the temper color resistance. Further, it has been found that the brightness index roughness of the stainless steel surface also affects it, and it is presumed that this is caused by the adhesion of C and S during the manufacturing process. Based on these findings, the inventors have invented a ferritic stainless steel sheet excellent in temper color resistance and a method for manufacturing the same.

[0013] That is, the gist of the present invention aimed at solving the above problems is as follows. [1] The chemical composition of the steel is, in mass%, C: 0.030% or less, N: 0.030% or less, Si: 0.01% or more and 5.00% or less, Mn: 0.01% or more and 3.00% or less, P: 0.050% or less, S: 0.0100% or less, Cr: 8.0% or more, 25.0% or less, Ni: 0.001% or more, 2.00% or less, Cu: 0.001% or more, 2.00% or less, Mo: 0.001% or more, 4.00% or less, Al: 0.001% or more, 0.80% or less, V: 0.01% or more, 0.30% or less, B: 0.0001% or more, 0.0050% or less, It contains O: 0.0001% or more and 0.0050% or less, and further contains one or two types of Ti: 0.40% or less and Nb: 0.80% or less, with the remainder being Fe and impurities. The surface of the steel has an oxide film, A ferritic stainless steel sheet that satisfies the following equations (i) to (v). [Ti] + [Nb] ≥ 0.03 ... Equation (i) 18.0≦[*Cr]+[*Si]×[Si]≦35.0 ··· Formula (ii) 0<[*Cu]-[Cu]≦5.00...Equation (iii) Surface Ra≦1.20 μm ...Equation (iv) L * ≧55...Equation (v) However, in formulas (i) to (iii), [element symbol] represents the content (mass%) of the element in the steel, and [*element symbol] represents the average content (mass%) of the element in the cation concentration excluding C, N, S, P, and O from the surface of the oxide film to a depth of 40 nm. In formula (iv), the surface Ra is the arithmetic mean roughness Ra of the steel sheet surface. In formula (v) L * This is the brightness index. [2] The chemical composition of steel is, in mass%, C: Less than 0.020% N: Less than 0.020% Si: more than 0.10%, less than 3.00%, Mn: 0.01% or more, less than 0.40% P: 0.050% or less, S: Less than 0.0020% Cr: more than 10.0%, less than 21.0%, Ni: 0.001% or more, less than 0.60% Cu: 0.001% or more, less than 0.30% Mo: 0.001% or more, 2.20% or less, Al: 0.001% or more, 0.80% or less, V: 0.01% or more, 0.30% or less, B: 0.0001% or more, 0.0050% or less, It contains O: 0.0001% or more and 0.0050% or less, and further contains one or two of the following: Ti: less than 0.24% and Nb: 0.55% or less, with the remainder being Fe and impurities. The surface of the steel has an oxide film, A ferritic stainless steel sheet that satisfies the following equations (i) to (v). [Ti] + [Nb] ≥ 0.08 ... Equation (i) 19.5≦[*Cr]+[*Si]×[Si]≦33.0 ··· Formula (ii) 0.02≦[*Cu]-[Cu]≦4.00...Equation (iii) Surface Ra≦1.20μm...Formula (iv) L * ≧60 ···Formula (v) However, in formulas (i) to (iii), [element symbol] represents the content (mass%) of the element in the steel, and [*element symbol] represents the average content (mass%) of the element in the cation concentration excluding C, N, S, P, and O from the surface of the oxide film to a depth of 40 nm. In formula (iv), the surface Ra is the arithmetic mean roughness Ra of the steel sheet surface. In formula (v) L * This is the brightness index. [3] A ferritic stainless steel sheet as described in [1] or [2], wherein the surface of the steel sheet is pickled. [4] A ferritic stainless steel sheet according to any one of [1] to [3], wherein the average S content in the area from the surface of the oxide film to a depth of 40 nm is 0.3 mass% or less, and the average C content is 6 mass% or less. [5] In mass%, in place of a portion of Fe, W: 0.001% or more, 0.50% or less, Y: 0.001% or more, 0.50% or less, REM: 0.001% or more, 0.50% or less, Ca: 0.0001% or more, 0.0050% or less, Zr: 0.001% or more, 0.50% or less, Hf: 0.001% or more, 1.0% or less, Sn: 0.001% or more, less than 0.05% Mg: 0.0001% or more, 0.0050% or less, Co: 0.001% or more, 1.0% or less, Sb: 0.001% or more, 1.0% or less, Bi: 0.001% or more, 1.0% or less, Ta: 0.001% or more, 1.0% or less, Ga: 0.0001% or more, 0.50% or less, A ferritic stainless steel sheet according to any one of items [1] to [4], containing one or more of the above. A method for manufacturing a ferritic stainless steel sheet as described in any one of the items [6] [1] to [5], The process comprises at least a degreasing step before final annealing, a final annealing step after the degreasing step, and a final electrolytic pickling step after the final annealing step. The aforementioned final annealing step is characterized by an annealing atmosphere of 2 to 15 volume percent hydrogen and the remainder nitrogen, an annealing temperature of 800 to 1200°C, a heating time of 5 seconds to 200 seconds in the range of 400°C to 700°C, a holding time of 20 seconds to 200 seconds in the range of 800°C to 1200°C, and a heating time in the range of 400°C to 700°C being shorter than the holding time in the range of 800°C to 1200°C. A method for manufacturing ferritic stainless steel sheets, wherein the final pickling step is electrolytic pickling for 3 seconds to 60 seconds using a pickling solution containing sulfuric acid in a concentration of 50 g / L to 300 g / L and Cu ions in a concentration of 200 mg / L or less. [Effects of the Invention]

[0014] The present invention provides a ferritic stainless steel sheet with excellent temper color resistance. Furthermore, the present invention provides a method for manufacturing a ferritic stainless steel sheet with excellent temper color resistance that does not require high-cost bright annealing or polishing finishes. The ferritic stainless steel sheet according to the present invention is a ferritic stainless steel sheet with excellent temper color resistance that is ideal for use in exhaust system components and their accessory parts that are exposed in the underside of the vehicle body of transportation equipment such as automobiles and motorcycles. In particular, it exhibits excellent temper color resistance even in high-temperature salt damage environments where de-icing salts spread to prevent road freezing or salts derived from seawater adhere to and are heated. [Modes for carrying out the invention]

[0015] The present invention will be described in detail below.

[0016] First, the reason for limiting the chemical composition of the steel in the ferritic stainless steel sheet of the present invention will be explained. Here, "%" in relation to the chemical composition of the steel refers to mass percent.

[0017] (C: 0.030% or less) Since carbon (C) is an element that reduces oxidation resistance and thus reduces temper color resistance, the C content should be 0.030% or less. To further suppress the deterioration of the above properties, it is preferable that the C content be less than 0.020%. Considering moldability and corrosion resistance, it is more preferable that it be 0.015% or less. Even more preferable is 0.010% or less. However, excessive reduction leads to an increase in refining costs, so it is preferable that the C content be 0.0001% or more. More preferably, it is 0.0010%, and even more preferably, 0.0020% or more.

[0018] (N:0.030% or less) Like carbon (C), nitrogen (N) is an element that reduces oxidation resistance and thus reduces temper color resistance; therefore, the N content should be 0.030% or less. To further suppress the deterioration of the above properties, it is preferable that the N content be less than 0.020%. Considering moldability and corrosion resistance, it is more preferable that the N content be 0.015% or less. Even more preferable is 0.010% or less. However, excessive reduction leads to an increase in refining costs, so it is preferable that the N content be 0.0001% or more. More preferably, it is 0.0010%, and even more preferably, 0.0020% or more.

[0019] (Si: 0.01% or more, 5.00% or less) Si is an element included as a deoxidizing agent and also improves oxidation resistance and temper color resistance, so the Si content should be 0.01% or more. To further improve the above properties, the Si content should preferably be more than 0.10%, and more preferably 0.30% or more. To consider corrosion resistance, it is more preferable to have a Si content of 0.50% or more. However, excessive Si content leads to a decrease in processability, so the Si content should be 5.00% or less. To consider economics from the standpoint of raw material costs and manufacturing costs, it is preferable to have a Si content of 3.00% or less. To consider manufacturability, it is preferable to have a Si content of 1.60% or less. More preferably, it is 1.20% or less, and even more preferably, 0.80% or less.

[0020] (Mn: 0.01% or more, 3.00% or less) Mn is an element included as a deoxidizing agent, and the Mn content should be 0.01% or more. Considering refining costs, a Mn content of 0.05% or more is preferable. More preferably, it should be 0.10% or more, and even more preferably, 0.15% or more. However, excessive Mn content leads to a decrease in oxidation resistance and reduces temper color resistance, so the Mn content should be 3.00% or less. Considering corrosion resistance, a Mn content of 1.10% or less is preferable. To further suppress the decrease in temper color resistance, a Mn content of less than 0.40% is even more preferable.

[0021] (P:0.050% or less) P is an impurity that is mainly introduced from the raw materials during steelmaking and refining. When its content is high, toughness and weldability decrease, so a lower content is better. For this reason, the P content should be 0.050% or less. Furthermore, considering manufacturability and manufacturing costs, the P content should preferably be 0.040% or less. Considering workability, the P content should be more preferably 0.035% or less. Even more preferably, it should be 0.030% or less. However, excessive reduction leads to an increase in refining costs, so the P content should preferably be 0.001% or more. More preferably, it should be 0.005% or more, and even more preferably, 0.010% or more.

[0022] (S:0.0100% or less) S is an impurity that is mainly introduced from raw materials during steelmaking and refining. It reduces oxidation resistance and temper color resistance, so the S content should be 0.0100% or less. To further suppress the deterioration of the above properties, it is preferable that the S content be less than 0.0020%. Considering manufacturability and corrosion resistance, it is even more preferable that the S content be 0.0014% or less. However, excessive reduction leads to increased refining costs, so it is preferable that the S content be 0.0001% or more. More preferably, the S content is 0.0003% or more.

[0023] (Cr: 8.0% or more, 25.0% or less) Cr is an element that improves oxidation resistance and temper color resistance, so the Cr content should be 8.0% or more. To further improve the above properties, it is preferable to have a Cr content of more than 10.0%. To consider corrosion resistance, it is more preferable to have a Cr content of 10.5% or more. Even more preferable is 15.0% or more. However, excessive Cr content leads to a decrease in manufacturability and processability, so the Cr content should be 25.0% or less. From the perspective of raw material costs, it is preferable to have a Cr content of 21.0% or less. More preferably, it is less than 18.0%.

[0024] (Ni: 0.001% or more, 2.00% or less) Ni is an element that improves corrosion resistance, and the Ni content should be 0.001% or more. Preferably, it should be 0.01% or more, and more preferably 0.05% or more. However, excessive Ni content leads to a decrease in oxidation resistance and reduces temper color resistance, so the Ni content should be 2.00% or less. To further suppress the decrease in the above properties, it is preferable that the Ni content be less than 0.60%. Considering raw material costs and manufacturability, it is more preferable that the Ni content be 0.50% or less. Even more preferable is 0.40% or less.

[0025] (Cu: 0.001% or more, 2.00% or less) Cu is an element that improves corrosion resistance and high-temperature strength, and the Cu content should be 0.001% or more. Preferably, it should be 0.005% or more, and more preferably 0.01% or more. However, excessive Cu content leads to a decrease in oxidation resistance and reduces temper color resistance, so the Cu content should be 2.00% or less. To further suppress the decrease in the above properties, it is preferable that the Cu content be less than 0.30%. More preferably, it should be 0.20% or less.

[0026] (Mo: 0.001% or more, 4.00% or less) Mo is an element that improves corrosion resistance and high-temperature strength, and the Mo content should be 0.001% or more. Preferably, it should be 0.005% or more, and more preferably 0.01% or more. However, excessive Mo content leads to a decrease in manufacturability and processability, so the Mo content should be 4.00% or less. From the perspective of raw material costs, it is economically feasible to have a Mo content of 2.20% or less. More preferably, it should be 1.20% or less, and even more preferably less than 0.20%.

[0027] (Al: 0.001% or more, 0.80% or less) Al is an element included as a deoxidizing agent, and the Al content should be 0.001% or more. Preferably, it should be 0.005% or more, and more preferably 0.01% or more. However, excessive Al content leads to a decrease in manufacturability, so the Al content should be 0.80% or less. Considering workability and weldability, it is preferable that the Al content be 0.60% or less. More preferably, it should be 0.30% or less, and even more preferably 0.20% or less.

[0028] (V: 0.01% or more, 0.30% or less) V is an element that improves oxidation resistance and temper color resistance, so the V content should be 0.01% or more. Considering corrosion resistance and high-temperature strength, a V content of 0.02% or more is preferable. More preferably, it should be 0.03% or more. However, excessive V content leads to a decrease in high-temperature strength due to the coarsening of precipitates, so the V content should be 0.30% or less. Considering the surface properties and manufacturability of the steel, a V content of 0.20% or less is preferable. More preferably, it should be less than 0.15%, and even more preferably, 0.10% or less.

[0029] (B: 0.0001% or more, 0.0050% or less) B is an element that improves oxidation resistance and temper color resistance, so the B content should be 0.0001% or more. Considering manufacturability and high-temperature strength, a B content of 0.0002% or more is preferable. More preferably, it should be 0.0003% or more, and even more preferably, 0.001% or more. However, excessive B content leads to a decrease in hot workability and a deterioration in the surface properties of the steel surface. Therefore, the B content should be 0.0050% or less. Furthermore, considering manufacturability and formability, a B content of 0.0030% or less is preferable. More preferably, it should be 0.0015% or less.

[0030] (O: 0.0001% or more, 0.0050% or less) Oxygen (O) is an unavoidable impurity that causes surface defects due to bubbles and inclusions. It also leads to a decrease in oxidation resistance and reduces temper color resistance, so the O content should be 0.0050% or less. Furthermore, considering manufacturability, the O content is preferably 0.0040% or less. More preferably, it is 0.0035% or less. However, excessive reduction of O leads to an increase in refining costs. In addition, the surface layer of steel contains internal oxides of Si and Al in addition to the oxide film, which are thought to contribute to improved temper color resistance. Therefore, the O content should be 0.0001% or more. Preferably, it is 0.0003% or more, and even more preferably, 0.0005% or more. Here, the O content refers to the total content including oxygen dissolved in the steel and oxygen from oxides interposed in the steel.

[0031] The ferritic stainless steel sheet according to this embodiment contains either Ti or Nb, or both, to improve temper color resistance. Since Ti and Nb have overlapping effects in improving temper color resistance, it is sufficient to include at least one of them. As described in the explanation of formula (i), the total content of Ti and Nb is 0.03% or more.

[0032] (Ti:0.40% or less) Ti is an element that combines with C, N, and S to improve oxidation resistance and also improves temper color resistance. Considering corrosion resistance, intergranular corrosion resistance, and deep drawability, the Ti content is preferably 0.09% or more. More preferably 0.120% or more. However, excessive Ti content leads to a decrease in oxidation resistance and reduces temper color resistance, so the Ti content should be 0.40% or less. Considering further suppression of the decrease in the above properties, the Ti content is preferably less than 0.24%. Considering reduction of raw material costs, uniform elongation, hole-expanding processability, and manufacturability, the Ti content is preferably 0.23% or less. More preferably 0.22% or less. The lower limit for no Ti content may be 0%.

[0033] (Nb:0.80% or less) Nb is an element that combines with C, N, and S to improve oxidation resistance and temper color resistance. Considering corrosion resistance, intergranular corrosion resistance, and high-temperature strength, the Nb content is preferably 0.01% or more. More preferably 0.05% or more, and even more preferably more than 0.05%. However, excessive Nb content leads to a decrease in manufacturability and processability, so the Nb content should be 0.80% or less. Considering economic efficiency from the standpoint of raw material costs, the Nb content is preferably 0.55% or less. More preferably less than 0.35%, and even more preferably 0.25% or less. The lower limit for the absence of Nb may be 0%.

[0034] In the ferritic stainless steel sheet according to this embodiment, the remainder of the elements other than those mentioned above and the selectively included elements described later is Fe and impurities. However, other elements other than those mentioned above may also be included to the extent that they do not impair the effects of this embodiment. Here, impurities refer to components that are mixed in during the industrial production of the ferritic stainless steel according to the present invention due to raw materials such as ore and scrap, and various factors in the manufacturing process, and are acceptable to the extent that they do not adversely affect the present invention.

[0035] Next, we will explain equations (i) to (v).

[0036] As described above, Ti and Nb are elements that improve oxidation resistance by bonding with C, N, and S, and also improve temper color resistance. Therefore, the material should contain one or two of Ti and Nb, totaling 0.03% or more. That is, it should satisfy formula (i). Considering corrosion resistance and workability, it is preferable that the value on the left side of formula (i) be 0.08% or more.

[0037] [Ti] + [Nb] ≥ 0.03 ... Equation (i)

[0038] However, the [element symbol] in formula (i) represents the content (mass%) of the element in the steel.

[0039] Furthermore, while investigating various effects on temper color resistance, the inventors discovered that the concentration of Cr, Si, and Cu cationic elements in the surface layer generated during the manufacturing process affects temper color resistance. They then found that satisfying formulas (ii) and (iii) is necessary to improve temper color resistance.

[0040] 18.0≦[*Cr]+[*Si]×[Si]≦35.0 ··· Formula (ii)

[0041] 0<[*Cu]-[Cu]≦5.00...Equation (iii)

[0042] However, in formulas (ii) and (iii), [element symbol] represents the content (mass%) of the element in the steel. Also, [*element symbol] represents the average content (mass%) of the element in the cation concentration excluding C, N, S, P, and O in the range from the surface of the oxide film to a depth of 40 nm. Note that a very thin oxide film is formed on the surface of the steel sheet according to this embodiment. As will be described later, the concentrations of C, Si, and Cu in the oxide film and the steel substrate directly beneath the oxide film affect the temper resistance. Therefore, the measurement range for [*Cr], [*Si], and [*Cu] is the range from the surface of the oxide film to a depth of 40 nm, which is the surface layer of the steel sheet including the oxide film and the steel substrate.

[0043] The average mass %) content of a given element, represented by [*element symbol], in the cation concentration excluding C, N, S, P, and O from the surface of the oxide film to a depth of 40 nm, is measured by glow discharge emission spectrometry (GDS). Specifically, the oxide film is sputtered with an argon plasma to a depth exceeding 40 nm from the surface, and the emission lines of the sputtered elements within the argon plasma are continuously spectrally analyzed, integrating the emission intensities up to a depth of 40 nm. Then, the concentrations of Cr, Si, and Cu are determined when the total cation concentration excluding C, N, S, P, and O is set to 100 mass%, and these are designated as [*Cr], [*Si], and [*Cu].

[0044] Cr becomes concentrated in the oxide film, while Cr deficiency occurs in the steel substrate as the oxide film forms. The less Cr deficiency there is, the better the oxidation resistance and temper color resistance. Therefore, it is desirable to have a high average cation concentration not only in the oxide film but also in the surface layer, including the steel substrate.

[0045] Similar to Cr, it is desirable for Si to have a high average cation concentration in the surface layer, including both the oxide film and the steel substrate. However, Si has a low content relative to steel, making it difficult to detect Si deficiency within the steel substrate even if Si enrichment in the oxide film is detected. After careful consideration, we concluded that both the cation concentration in the surface layer and the Si content in the steel are important for Si.

[0046] If the value of the middle side of formula (ii) is less than 18.0, the temper color resistance will decrease, so the value of the middle side of formula (ii) should be 18.0 or higher. To further improve the above properties, it is preferable to set the value of the middle side of formula (ii) to 19.5 or higher. If the value of the middle side of formula (ii) is excessively high, it is possible that the total content of Cr and Si is high or that the oxide film is thick, both of which lead to a decrease in processability. For this reason, the value of the middle side of formula (ii) should be 35.0 or lower. Also, considering economics from the standpoint of raw material costs and manufacturing costs, it is preferable to set the value of the middle side of formula (ii) to 33.0 or lower.

[0047] Furthermore, in addition to the concentration of elements contained in steel on the surface during annealing and pickling, copper can also be concentrated due to copper adhering to the surface during the manufacturing process. That is, if the pickling solution contains copper ions during the pickling process, these copper ions will adhere to the surface of the steel sheet and precipitate as copper, thus increasing its concentration. It has been found that this concentration of copper leads to a decrease in oxidation resistance and a decrease in temper color resistance. Therefore, it is desirable to minimize the concentration of copper on the surface of the steel sheet as much as possible.

[0048] If the value of the middle side in formula (iii) exceeds 5.00, the temper color resistance will decrease. Therefore, the value of the middle side in formula (iii) should be 5.00 or less. Considering further suppressing the decrease of the above characteristics, the value of the middle side in formula (iii) is preferably 4.00 or less. Also, if the steel has a pickled surface, Cu is concentrated on the surface layer, so the value of the middle side in formula (iii) is greater than 0. Considering economy from the viewpoints of raw material cost and manufacturing cost, the value of the middle side in formula (iii) is preferably 0.02 or more.

[0049] In addition, the inventors of the present invention brightness index have found that the roughness also has an impact, and have found that it is necessary to satisfy formula (iv) and formula (v) in order to improve the temper color resistance.

[0050] Surface Ra ≤ 1.20 μm ··· Formula (iv)

[0051] L * ≥ 55 ··· Formula (v)

[0052] The surface Ra in formula (iv) is the arithmetic mean roughness Ra of the steel plate surface, and the L * [[ID=...]] This is the brightness index. lightness index L * is the L defined in JIS Z 8781-4:2013 * a * b * in the L * color system.

[0053] It is considered that when the surface Ra (arithmetic mean roughness Ra) is large and the steel plate surface is rough, the temper color is promoted by diffuse reflection of light. Therefore, the arithmetic mean roughness of the steel plate surface should be 1.2 μm or less. Also, brightness index L * means that the larger the value, the brighter the steel plate surface. As will be described later, brightness index L * if it is small, the temper color resistance will decrease. Therefore, it should be 55 or more, and more preferably 60 or more.

[0054] Next, we will discuss the relationship between the surface condition of the steel sheet and the manufacturing conditions. If the degreasing process before the final annealing is omitted, rolling oil and other substances that adhered to the surface of the steel sheet during cold rolling will remain. If rolling oil and other substances remain on the surface of the steel sheet, the carbon content contained in the rolling oil and other substances will react with chromium in the steel at the grain boundaries during annealing to form chromium carbides, which are thought to promote the dissolution of the grain boundaries during subsequent pickling. In other words, if the degreasing process before annealing is omitted, there is a risk that the grain boundaries will be deeply dissolved during pickling, increasing the arithmetic surface roughness Ra. Therefore, in order to satisfy the above equation (iv), it is advisable to perform a degreasing process before the annealing process when manufacturing steel sheets.

[0055] Also, brightness index L * This value decreases when the steel plate surface is rough or when sulfur is present on the steel plate surface. During the pickling process, sulfur from the pickling solution may adhere to the steel plate surface. When sulfur from the pickling solution adheres to the steel plate surface along with carbon from the rolling oil, it is thought that oxidation resistance decreases, and temper color resistance decreases. Therefore, after pickling, it is advisable to lower the sulfuric acid concentration of the pickling solution used in the pickling process in order to minimize the residue of the pickling solution.

[0056] Therefore, it is preferable that the steel sheet of this embodiment has an average S content of 0.3 mass% or less from the surface of the oxide film to a depth of 40 nm, and an average C content of 6 mass% or less. This reduces the arithmetic surface roughness of the steel sheet, and L * The value can be increased.

[0057] The average sulfur (S) and carbon (C) content from the surface of the oxide film to a depth of 40 nm is measured by glow discharge emission spectrometry (GDS). Specifically, the oxide film is sputtered with an argon plasma to a depth exceeding 40 nm from the surface, and the emission lines of the sputtered elements within the argon plasma are continuously spectrally analyzed, and the total emission intensity up to a depth of 40 nm is integrated. Then, the content of C and S is calculated, assuming the total of all detected elements is 100 mass%, and these are used as the average content of S and C.

[0058] Furthermore, the properties of the ferritic stainless steel sheet according to this embodiment can be further improved by selectively including one or more of the following elements as needed: W, Y, REM, Ca, Zr, Hf, Sn, Mg, Co, Sb, Bi, Ta, and Ga. These elements are described below. Note that these elements do not need to be included, so the lower limit of the content of each of these elements is 0%.

[0059] (W: 0.001% or more, 0.50% or less) W is an element that improves high-temperature strength, corrosion resistance, and oxidation resistance, and is preferably contained in an amount of 0.001% or more as needed. More preferably, it is 0.01% or more, and even more preferably, 0.05% or more. However, excessive W content may lead to increased raw material costs and a decrease in processability, toughness, and manufacturability, so the W content is preferably 0.50% or less. More preferably, it is 0.40% or less, and even more preferably, 0.30% or less.

[0060] (Y: 0.001% or more, 0.50% or less) Y is an element that improves rust resistance, hot workability, and oxidation resistance, and is preferably present in an amount of 0.001% or more as needed. More preferably, it is 0.003% or more, and even more preferably, 0.01% or more. However, excessive Y content may lead to increased raw material costs and decreased manufacturability, so it is preferable to keep the Y content at 0.50% or less. More preferably, it is 0.20% or less, and even more preferably, 0.10% or less.

[0061] (REM: 0.001% or more, 0.50% or less) Rare earth metals (REMs) are elements that improve rust resistance, hot workability, and oxidation resistance, and it is preferable to include 0.001% or more as needed. More preferably, it is 0.003% or more, and even more preferably, 0.01% or more. However, excessive REM content may lead to increased raw material costs and decreased manufacturability, so it is preferable to keep the REM content at 0.50% or less. More preferably, it is 0.20% or less, and even more preferably, 0.10% or less. REM refers to the collective term for 15 elements (lanthanides) from scandium (Sc) and lanthanum (La) to lutetium (Lu). REM may include one of the above elements alone, or two or more. When two or more of the above elements are included as REM, the REM content is the total content of those elements.

[0062] (Ca: 0.0001% or more, 0.0050% or less) Ca is an element that improves corrosion resistance, oxidation resistance, and manufacturability, and it is preferable to contain 0.0001% or more as needed. More preferably, it is 0.0002% or more, and even more preferably, 0.0003% or more. However, excessive Ca content may lead to a decrease in corrosion resistance and manufacturability, so it is preferable to keep the Ca content at 0.0050% or less. More preferably, it is 0.0030% or less, and even more preferably, 0.0020% or less.

[0063] (Zr: 0.001% or more, 0.50% or less) Zr is an element that improves corrosion resistance, intergranular corrosion resistance, high-temperature strength, and oxidation resistance, and it is preferable to contain 0.001% or more as needed. More preferably, it is 0.01% or more, and even more preferably, 0.03% or more. However, excessive Zr content may lead to increased raw material costs and decreased manufacturability, so it is preferable to keep the Zr content at 0.50% or less. More preferably, it is 0.30% or less, and even more preferably, 0.20% or less.

[0064] (Hf: 0.001% or more, 1.0% or less) Hf is an element that improves corrosion resistance, intergranular corrosion resistance, high-temperature strength, and oxidation resistance, and is preferably present in an amount of 0.001% or more as needed. More preferably, it is 0.003% or more, and even more preferably, 0.01% or more. However, excessive Hf content may lead to increased raw material costs and decreased manufacturability, so it is preferable to keep the Hf content at 1.0% or less. More preferably, it is 0.50% or less, and even more preferably, 0.30% or less.

[0065] (Sn: 0.001% or more, less than 0.05%) Sn is an element that improves corrosion resistance and high-temperature strength, and it is preferable to contain 0.001% or more as needed. More preferably, it is 0.003% or more, and even more preferably, 0.01% or more. However, excessive Sn content may lead to increased raw material costs and increased manufacturing costs due to decreased toughness and manufacturability, so it is preferable to keep the Sn content below 0.05%. More preferably, it is below 0.04%.

[0066] (Mg: 0.0001% or more, 0.0050% or less) Mg may be included as a deoxidizing element, and it is also an element that improves moldability and oxidation resistance, and it is preferable to include 0.0001% or more as needed. More preferably, it is 0.0003% or more, and even more preferably, 0.0005% or more. However, excessive Mg content may lead to a decrease in corrosion resistance, weldability, and surface quality, so it is preferable to keep the Mg content at 0.0050% or less. More preferably, it is 0.0030% or less, and even more preferably, 0.0020% or less.

[0067] (Co: 0.001% or more, 1.0% or less) Co is an element that improves high-temperature strength, and it is preferable to include 0.001% or more as needed. More preferably, it is 0.01% or more, and even more preferably, 0.03% or more. However, excessive Co content may lead to increased raw material costs and a decrease in processability, toughness, and manufacturability, so it is preferable to keep the Co content at 1.0% or less. More preferably, it is 0.50% or less, and even more preferably, less than 0.30%.

[0068] (Sb: 0.001% or more, 1.0% or less) Sb is an element that improves high-temperature strength, and it is preferable to have a content of 0.001% or more as needed. More preferably, it is 0.005% or more, and even more preferably, 0.01% or more. However, excessive Sb content may lead to a decrease in weldability and toughness, so it is preferable to have an Sb content of 1.0% or less. More preferably, it is 0.50% or less, and even more preferably, 0.40% or less.

[0069] (Bi:0.001% or more, 1.0% or less) Bi is an element that suppresses roping that occurs during cold rolling and improves manufacturability, and it is preferable to contain 0.001% or more as needed. More preferably, it is 0.003% or more, and even more preferably, 0.01% or more. However, excessive Bi content may lead to increased raw material costs and a decrease in processability and hot workability, so it is preferable to keep the Bi content at 1.0% or less. More preferably, it is 0.50% or less, and even more preferably, 0.30% or less.

[0070] (Ta: 0.001% or more, 1.0% or less) Ta is an element that improves high-temperature strength, and it is preferable to include 0.001% or more as needed. More preferably, it is 0.003% or more, and even more preferably, 0.01% or more. However, excessive Ta content may lead to increased raw material costs and a decrease in toughness and manufacturability, so it is preferable to keep the Ta content at 1.0% or less. More preferably, it is 0.50% or less, and even more preferably, 0.30% or less.

[0071] (Ga: 0.0001% or more, 0.50% or less) Ga is an element that improves corrosion resistance and hydrogen embrittlement resistance, and it is preferable to include 0.0001% or more as needed. More preferably, it is 0.0003% or more, and even more preferably, 0.001% or more. However, excessive Ga content may lead to increased raw material costs and decreased manufacturability, so it is preferable to keep the Ga content at 0.50% or less. More preferably, it is 0.30% or less, and even more preferably, 0.20% or less.

[0072] Next, a method for manufacturing a ferritic stainless steel sheet according to this embodiment will be described. The ferritic stainless steel sheet according to this embodiment may be manufactured by any method, but for example, it can be manufactured by the following method.

[0073] The method for manufacturing ferritic stainless steel sheets in this embodiment can employ the general process for manufacturing ferritic stainless steel. Generally, the steel is molten in a converter or electric furnace, refined in an AOD furnace or VOD furnace, and then formed into steel billets by continuous casting or ingot forming. The process then involves hot rolling, annealing of the hot-rolled sheet, pickling, cold rolling, finish annealing, and pickling. If necessary, the annealing of the hot-rolled sheet may be omitted, or the cold rolling, finish annealing, and pickling may be repeated.

[0074] The conditions for each of these processes can be general conditions, for example, a hot rolling heating temperature of 1000-1300°C, a hot-rolled sheet annealing temperature of 900-1200°C, and a cold-rolled sheet annealing temperature of 800-1200°C. The hot rolling conditions, whether or not hot-rolled sheet annealing is performed, and the cold rolling conditions can be selected as appropriate.

[0075] Furthermore, the treatment before final pickling can be a general process, such as mechanical treatments like shot blasting or grinding brushes, or chemical treatments like molten salt treatment or neutral salt electrolysis. Temper rolling or tension leveling may also be applied after cold rolling and annealing. The thickness of the finished sheet can also be selected according to the required component thickness. Additionally, this steel sheet can be used as a material to manufacture welded pipes using conventional methods for manufacturing stainless steel pipes for exhaust systems, such as electric resistance welding, TIG welding, or laser welding.

[0076] However, the manufacturing method of this embodiment must include at least a degreasing step before the final annealing, a final annealing step after the degreasing step, and a final electrolytic pickling step after the final annealing step.

[0077] As explained in the description of arithmetic mean roughness Ra, if the degreasing process is omitted before the final annealing, it will lead to a decrease in temper color resistance due to the adhesion of carbon to the surface of the steel sheet. Therefore, it is necessary to perform the degreasing process before the final annealing. By performing the degreasing process, rolling oil and other substances that adhered during cold rolling etc. prior to the degreasing process are removed, thereby suppressing the formation of chromium carbides at the grain boundaries of the steel during annealing, and suppressing surface roughening of the steel sheet in the subsequent pickling process. There are no particular restrictions on the conditions of the degreasing process.

[0078] The final annealing process is carried out under the following conditions: the annealing atmosphere is 2-15 volume% hydrogen with the remainder being nitrogen; the annealing temperature is 800-1200°C; the heating time in the range of 400°C to 700°C is 5 seconds to 200 seconds; the holding time in the range of 800°C to 1200°C is 20 seconds to 200 seconds; and the heating time in the range of 400°C to 700°C is shorter than the holding time in the range of 800°C to 1200°C.

[0079] To suppress the decrease in temper color resistance due to chromium deficiency in the surface layer of the steel, the hydrogen concentration in the annealing atmosphere should be 2% by volume or more. Preferably, it should be 3% by volume or more. The upper limit of the hydrogen concentration should be 15% by volume or less. To suppress chromium deficiency in the surface layer, a hydrogen concentration of 15% by volume or less in the annealing atmosphere is sufficient. In general bright annealing, the hydrogen concentration is 25% by volume or 100% by volume, which is expensive to manufacture. Therefore, in this embodiment, considering economics, annealing is performed with a hydrogen concentration of 15% by volume or less instead of general bright annealing. Preferably, the hydrogen concentration should be 10% by volume or less.

[0080] The remaining portion of the annealing atmosphere is nitrogen. This allows the annealing atmosphere to be reduced, thereby suppressing oxidation of the steel sheet surface during annealing.

[0081] The annealing temperature is preferably in the range of 800 to 1200°C, taking productivity into consideration. However, if production costs are a priority, the annealing temperature may be set to 800 to 980°C.

[0082] Furthermore, even if the annealing atmosphere for the final annealing is a reducing atmosphere consisting of hydrogen and nitrogen, Cr can oxidize up to a certain temperature, and from temperatures close to the final annealing temperature, oxidation of Cr is suppressed or reduced. Taking this into consideration, in order to suppress the decrease in temper color resistance due to Cr deficiency in the surface layer of the steel, the heating time in the range of 400°C to 700°C is 5 seconds to 200 seconds, and the holding time in the range of 800°C to 1200°C is 20 seconds to 200 seconds, with the heating time in the range of 400°C to 700°C being shorter than the holding time in the range of 800°C to 1200°C. If the final annealing temperature is set to 980°C or lower for economic reasons from the perspective of manufacturing costs, it is preferable to shorten the heating time in the range of 400°C to 700°C compared to the holding time in the range of 800°C to 980°C.

[0083] In the final pickling process after the final annealing process, electrolytic pickling with a pickling solution containing sulfuric acid is included, taking productivity into consideration. Since an excessively low sulfuric acid concentration in the pickling solution will lead to poor descaling, the sulfuric acid concentration in the pickling solution should be 50 g / L or higher. Preferably, it should be 70 g / L or higher. Since an excessively high sulfuric acid concentration in the pickling solution will lead to an increase in the surface Ra of the steel and a decrease in temper color resistance due to sulfur deposition, the sulfuric acid concentration in the pickling solution should be 300 g / L or lower. Preferably, it should be 150 g / L or lower.

[0084] Furthermore, the pickling solution contains Cu ions that have been dissolved from the steel that has been pickled up to that point. To excessively reduce the Cu ion concentration of the pickling solution would require frequent replacement of the pickling solution and cleaning of the pickling tank, which would lead to decreased productivity and increased manufacturing costs. Therefore, the Cu ion concentration of the pickling solution may be 0.1 mg / L or higher. Preferably, it is 1 mg / L or higher. If the Cu ion concentration of the pickling solution is excessively high, it will lead to a decrease in temper color resistance due to the adhesion and deposition of Cu ions on the surface of the steel. Therefore, the Cu ion concentration of the pickling solution should be 200 mg / L or lower. Preferably, it is 100 mg / L or lower.

[0085] If the electrolytic pickling time is too short, it will lead to poor desizing, so the electrolytic pickling time should be 3 seconds or more. Preferably, it should be 4 seconds or more. If the electrolytic pickling time is too long, it will lead to an increase in the surface Ra of the steel and a decrease in temper color resistance due to the adhesion of S and Cu, so the electrolytic pickling time should be 60 seconds or less. Preferably, it should be 25 seconds or less.

[0086] Other conditions for electrolytic pickling do not need to be particularly limited. The temperature of the pickling solution can be, for example, 10-50°C, and the electrolytic current density can be 1-15 A / dm². 2 It's fine to be approximate, but it doesn't have to be limited to these conditions.

[0087] Furthermore, after electrolytic pickling, nitric acid immersion treatment may be performed to the extent that it does not affect the surface condition.

[0088] As described above, the ferritic stainless steel sheet of this embodiment can improve temper color resistance. As a result, the ferritic stainless steel sheet of this embodiment can be suitably used as a material for automobile exhaust system components. [Examples]

[0089] The effects of the present invention will be made clearer by the following examples. However, the present invention is not limited to the following examples and can be implemented with appropriate modifications without altering its essence.

[0090] Steels having the component compositions and the values ​​on the left side of formula (i) shown in Tables 1A and 1B (Examples A-O of the present invention, Comparative Examples a-r) were melted in a vacuum melting furnace and cast into 150 kg ingots to form steel billets. These steel billets were then hot-rolled to form 4.0 mm thick steel plates. Next, the hot-rolled steel plates were pickled, cold-rolled to a thickness of 1.0 mm, and after final annealing, electrolytic pickling with a pickling solution containing sulfuric acid and immersion in nitric acid were performed to obtain ferritic stainless steel plates (product plates).

[0091] If a degreasing process was performed, it was carried out after the final cold rolling and before the final annealing. The final annealing temperature was in the range of 800 to 1200°C. Nitric acid immersion was performed by immersing the material in a nitric acid aqueous solution with a concentration of 20 to 100 g / L for 3 to 10 seconds. In the manufacturing process of the finished product plates, the presence or absence of a degreasing process before final annealing, the hydrogen concentration (the remainder being nitrogen) in the atmosphere during final annealing, the heating time in the range of 400°C to 700°C, the holding time in the range of 800°C to 1200°C, the sulfuric acid concentration of the pickling solution for electrolytic pickling, the Cu ion concentration, and the time were carried out under the conditions shown in Tables 2A and 2B.

[0092] [Table 1A]

[0093] [Table 1B]

[0094] [Table 2A]

[0095] [Table 2B]

[0096] The arithmetic mean roughness (roughness Ra) of the product board obtained above, brightness index L * The following measurements were taken. In addition, the average content (mass%) of Cr, Si, and Cu in the cation concentration excluding C, N, S, P, and O in the steel surface, including the oxide film and steel substrate of the product plate, from the surface down to a depth of 40 nm was measured by glow discharge emission spectrometry (GDS), denoted as [*Cr], [*Si], and [*Cu] respectively. Furthermore, by integrating all the emission intensities obtained by GDS measurement and determining the C and S content when the total of all detected elements is set to 100% by mass, the average S content and the average C content were calculated.

[0097] Temper color resistance was assessed by a high-temperature salt damage test, which involved heat treatment in a high-temperature salt damage environment. The surface of the steel plate after the high-temperature salt damage test was then examined. a * value and b * value We measured and evaluated it. * and b * This is L as defined in JIS Z 8781-4:2013. * a * b * a in color system * and b * That is the case.

[0098] For the high-temperature salt damage test, a 20mm wide x 50mm long test specimen taken from a product plate was used. The high-temperature salt damage test involved heating the test specimen, cooling, immersion in salt water, and drying, repeated four times. The heating conditions were 350°C for a holding time of 150 minutes. The cooling conditions were 25°C for a holding time of 30 minutes. For the salt water immersion conditions, a saturated NaCl aqueous solution was used as the salt water, with a solution temperature of 25°C and an immersion time of 30 minutes. In other words, a 26% by mass NaCl aqueous solution was used as the salt water. For the drying conditions, after removal from the salt water, the temperature was set to 50°C for a holding time of 30 minutes. Air was used for heating, cooling, and drying.

[0099] Temper color resistance was determined by measuring the color difference after a high-temperature salt damage test, a * Value and b * The value was evaluated. The oxide film showing temper color increases in thickness as a * As the value increases, it changes from colorless to yellowish, and from yellowish to reddish. Furthermore, as the oxide film thickens, it changes from reddish to blueish, a * The value drops sharply and at the same time b * The value also decreases. In this evaluation, a * The value is greater than 11.5 or b * Values ​​less than 5.0 are marked as "× (defective)", b * When the value is 5.0 or higher, a * Values ​​between 10.5 and 12.0 were marked "● (acceptable)", values ​​between 9.0 and 10.5 were marked "○ (good)", and values ​​of 9.0 or less were marked "◎ (even better)".

[0100] Tables 3A and 3B show the Ra (μm) and L of the product plates for Examples A-O of the present invention and Comparative Examples a-r. * Ra, which is the value of the middle side of equations (ii) and (iii), and the value of the left side of equation (iv), and L, which is the value of the left side of equation (v). * The results of the temper color resistance evaluation are shown below.

[0101] [Table 3A]

[0102] [Table 3B]

[0103] As is clear from Tables 1A to 3B, steel sheets with the component composition specified in the present invention and satisfying formulas (i) to (v) exhibit superior temper color resistance compared to the steel sheets of the comparative examples. Furthermore, it can be seen that formulas (ii) to (v) are satisfied if the manufacturing conditions specified in the present invention are met. In addition, the steel sheets of the present invention had an average S content of 0.3% or less from the surface to a depth of 40 nm, and an average C content of 6% or less.

[0104] Furthermore, instead of high-temperature salt damage tests, we also conducted multiple tests using continuous atmospheric oxidation tests without saltwater immersion, varying the heating temperature from 200 to 500°C, and changing the total heating time from 1 to 100 hours. As a result, under conditions where the oxide film was approximately 1 μm or less and temper color was produced, a * value, b * Although the values ​​varied, the temper color of the steel of the present invention was on the side of fading from reddish to bluish, indicating superior temper color resistance. Therefore, it is considered that the present invention exhibits excellent temper color resistance in various environments.

[0105] From the above, it can be seen that steel sheets having the chemical composition specified in the present invention and satisfying formulas (i) to (v) have excellent temper color resistance. [Industrial applicability]

[0106] According to the present invention, it is possible to provide a ferritic stainless steel sheet with excellent temper color resistance through pickling, without the need for high-cost bright annealing or polishing finishes, and particularly a ferritic stainless steel sheet with excellent temper color resistance in high-temperature salt damage environments where de-icing salts spread to prevent road freezing and salts derived from seawater adhere and heat up, for use as exhaust system parts in transportation equipment such as automobiles and motorcycles. Specific applications include exhaust manifolds, converters, front pipes, center pipes, mufflers, insulators, EGRs, EGR coolers, turbo components, and fastening components for automobile exhaust systems.

Claims

1. The chemical composition of steel, in mass percent, C: 0.030% or less, N: 0.030% or less, Si: 0.01% or more, 5.00% or less, Mn: 0.01% or more, 3.00% or less, P: 0.050% or less, S: 0.0100% or less, Cr: 8.0% or more, 25.0% or less, Ni: 0.001% or more, 2.00% or less, Cu: 0.001% or more, 2.00% or less, Mo: 0.001% or more, 4.00% or less, Al: 0.001% or more, 0.80% or less, V: 0.01% or more, 0.30% or less, B: 0.0001% or more, 0.0050% or less, It contains O: 0.0001% or more and 0.0050% or less, and further contains one or two types of Ti: 0.40% or less and Nb: 0.80% or less, with the remainder being Fe and impurities. The surface of the steel has an oxide film, A ferritic stainless steel sheet that satisfies the following equations (i) to (v). [Ti]+[Nb]≧0.03...Formula (i) 18.0≦[*Cr]+[*Si]×[Si]≦35.0...Formula (ii) 0<[*Cu]-[Cu]≦5.00...Formula (iii) Surface Ra≦1.20μm...Formula (iv) L*≧55...Formula (v) However, in formulas (i) to (iii), [element symbol] represents the content (mass%) of the element in the steel, and [*element symbol] represents the average content (mass%) of the element in the cation concentration excluding C, N, S, P, and O from the surface of the oxide film to a depth of 40 nm. In formula (iv), surface Ra is the arithmetic mean roughness Ra of the steel plate surface. In formula (v), L* is the lightness index.

2. The chemical composition of steel, in mass percent, C: Less than 0.020% N: Less than 0.020% Si: more than 0.10%, 3.00% or less, Mn: 0.01% or more, less than 0.40% P: 0.050% or less, S: Less than 0.0020%, Cr: more than 10.0%, 21.0% or less, Ni: 0.001% or more, less than 0.60% Cu: 0.001% or more, less than 0.30% Mo: 0.001% or more, 2.20% or less, Al: 0.001% or more, 0.80% or less, V: 0.01% or more, 0.30% or less, B: 0.0001% or more, 0.0050% or less, It contains O: 0.0001% or more and 0.0050% or less, and further contains one or two of the following: Ti: less than 0.24%, Nb: 0.55% or less, with the remainder being Fe and impurities. The surface of the steel has an oxide film, A ferritic stainless steel sheet that satisfies the following equations (i) to (v). [Ti]+[Nb]≧0.08...Formula (i) 19.5≦[*Cr]+[*Si]×[Si]≦33.0...Formula (ii) 0.02≦[*Cu]−[Cu]≦4.00...Formula (iii) Surface Ra≦1.20μm...Formula (iv) L*≧60...Formula (v) However, in formulas (i) to (iii), [element symbol] represents the content (mass%) of the element in the steel, and [*element symbol] represents the average content (mass%) of the element in the cation concentration excluding C, N, S, P, and O from the surface of the oxide film to a depth of 40 nm. In formula (iv), surface Ra is the arithmetic mean roughness Ra of the steel plate surface. In formula (v), L* is the lightness index.

3. A ferritic stainless steel sheet according to claim 1 or claim 2, wherein the surface of the steel sheet is pickled.

4. A ferritic stainless steel sheet according to any one of claims 1 to 3, wherein the average S content in the area from the surface of the oxide film to a depth of 40 nm is 0.3% by mass or less, and the average C content is 6% by mass or less.

5. In mass percent, substitute for a portion of Fe, W: 0.001% or more, 0.50% or less, Y: 0.001% or more, 0.50% or less, REM: 0.001% or more, 0.50% or less, Ca: 0.0001% or more, 0.0050% or less, Zr: 0.001% or more, 0.50% or less, Hf: 0.001% or more, 1.0% or less, Sn: 0.001% or more, less than 0.05% Mg: 0.0001% or more, 0.0050% or less, Co: 0.001% or more, 1.0% or less, Sb: 0.001% or more, 1.0% or less, Bi: 0.001% or more, 1.0% or less, Ta: 0.001% or more, 1.0% or less, Ga: 0.0001% or more, 0.50% or less, A ferritic stainless steel sheet according to any one of claims 1 to 4, comprising one or more of the above.

6. A method for manufacturing a ferritic stainless steel sheet according to any one of claims 1 to 5, The process comprises at least a degreasing step before final annealing, a final annealing step after the degreasing step, and a final electrolytic pickling step after the final annealing step. The final annealing step is characterized by an annealing atmosphere of 2 to 15 volume percent hydrogen and the remainder nitrogen, an annealing temperature of 800 to 1200°C, a heating time of 5 seconds to 200 seconds in the range of 400°C to 700°C, a holding time of 20 seconds to 200 seconds in the range of 800°C to 1200°C, and a heating time in the range of 400°C to 700°C being shorter than the holding time in the range of 800°C to 1200°C. A method for manufacturing ferritic stainless steel sheets, wherein the final pickling step is electrolytic pickling for 3 seconds to 60 seconds using a pickling solution containing sulfuric acid in a concentration of 50 g / L to 300 g / L and Cu ions in a concentration of 200 mg / L or less.

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