Ferritic stainless steel sheet and method for producing ferritic stainless steel

Abstract

To realize a ferritic stainless steel sheet and a method for producing the same that achieve high corrosion resistance and high formability without increasing the alloy addition amount.SOLUTION: A ferritic stainless steel sheet according to one aspect of the present invention contains, in mass%, C: 0.0010% to 0.010%, Si: 0.01% to 1.20%, Mn: 0.01% to 1.50%, P: 0.035% or less, S: 0.010% or less, Cr: 10.5% to 23.0%, Mo: 0.01% to 1.50%, Ni: 0.01% to 0.60%, Cu: 0.01% to 1.60%, Al: 0.002% to 0.150%, Ti: 0.10% to 0.30%, Nb: 0.001% to 0.20%, and N: 0.0020% to 0.030%, with the balance comprising Fe and impurities, and has an average nitrogen concentration of 0.03 mass% to 0.12 mass% at a position of 50 μm from a surface of the steel sheet.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a ferritic stainless steel sheet used for parts that require high corrosion resistance and high formability. [Background technology]

[0002] As a measure to improve fuel efficiency in automobiles, which has been prompted by global environmental issues, efforts are being made to reduce the weight of automobile bodies. To reduce the weight of automobile bodies, efforts are being made to increase the strength of the steel plates used in automobiles and reduce the thickness of the plates, or to replace them with aluminum or resin materials to reduce weight. The need for weight reduction is not limited to automobile bodies, but is also required for various parts, one of which is the exhaust system parts that treat engine exhaust gases.

[0003] Exhaust system components exposed to high-temperature engine exhaust gases require high-temperature strength, oxidation resistance, and corrosion resistance. Stainless steels such as heat-resistant steel SUH409L, high-purity ferritic stainless steel SUS436J1L, AISI439, AISI429, and AISI441 are used, and exhaust system components weigh 20–30 kg per vehicle. Reducing the weight of exhaust system components requires reducing the steel plate thickness, but the durability required for exhaust system components is significantly affected by the steel plate thickness, strength, and corrosion resistance. For example, increasing the material's strength reduces its workability when fabricated into components. Furthermore, steels with high Cr and Mo content are typically used to improve corrosion resistance, but this increases alloy costs and reduces workability due to solid-solution strengthening. For this reason, attempts have been made to absorb nitrogen into the surface layer of steel to increase the surface strength and improve corrosion resistance.

[0004] In the technology described in Patent Document 1, a ferritic stainless steel material with high surface hardness has an average chemical composition, in mass %, of C≦0.1%, Si≦1.0%, Mn≦1.0%, P≦0.04%, S≦0.03%, Cr: 14-18%, N: 0.02-0.05%, Al: 0-0.2%, Ti: 0-0.05%, an N concentration in the surface layer of 0.05-0.15%, and nitrides with an average particle size of 0.2 μm or less precipitated in the surface layer at a precipitated N concentration of 0.05 mass % or more, and is then final cold-rolled and then bright annealed at 750-950°C in a mixed gas containing N2 and H2 (volume ratio of N2: 20-50%). The technology described in Patent Document 1 is said to provide a ferritic stainless steel material with high surface hardness, excellent corrosion resistance, and press workability by performing aging treatment at 300 to 700°C as needed to increase surface hardness after bright annealing. However, the ferritic stainless steel material described in Patent Document 1 has low amounts of the stabilizing elements Ti and Nb and high amounts of solute C and N, resulting in low Lankford value and elongation, making it difficult to process by press forming.

[0005] Patent Document 2 discloses an Fe-based alloy for solid-phase nitrogen absorption, which contains C≦0.020 mass%, N≦0.030 mass%, and 5.0≦Cr≦18.0 mass%, and further contains one or more of the elements 0.5≦Si≦3.0 mass%, 0.1≦Al≦3.0 mass%, and 0.05≦Ti≦3.0 mass%, with the balance being Fe and unavoidable impurities. The alloy has a ferrite phase area ratio of 95% or more at 23°C, a nitrogen absorption layer formed on the surface, and the hardness of the nitrogen absorption layer is 400 HV or more. The area ratio of the ferrite phase in the base at 23°C is 95% or more. This alloy achieves high magnetic permeability, high electrical resistivity, high saturation magnetic flux density, and high hardness. However, the Fe-based alloy for solid-phase nitrogen absorption described in Patent Document 1 has a problem in that the surface layer has too high hardness, making it difficult to process by press molding.

[0006] Patent Document 3 discloses a ferritic stainless steel sheet having a chemical composition, in mass %, of C: 0.01% or less, Si: 0.05 to 1.2%, Mn: 0.05 to 1.5%, P: 0.035% or less, S: 0.01% or less, Cr: 10.5 to 20.0%, Ni: 0.01 to 0.60%, Mo: 0.01 to 2.0%, Cu: 0.01 to 1.6%, Al: 0.001 to 0.10%, N: 0.001 to 0.02%, Nb: 0.20 to 0.60%, optional elements, balance: Fe and impurities, and an average nitrogen concentration in a region from the surface to 10 μm in the sheet thickness direction is 0.05 to 0.10% or less. Patent Document 3 discloses a method for producing the above-mentioned ferritic stainless steel sheet, in which the sheet is rapidly heated to 1000 to 1100°C and annealed for 4 to 60 seconds in a mixed atmosphere of nitrogen and hydrogen, thereby producing a ferritic stainless steel sheet having high wear resistance without impairing workability, corrosion resistance, and high-temperature strength. However, rapid heating causes large temperature variations within the steel sheet, which leads to the problem of non-uniformity in the material properties.

[0007] The technology described in Patent Document 4 describes a ferritic stainless steel that contains, by mass%, C: 0.003 to 0.020%, Si: 0.05 to 1.00%, Mn: 0.10 to 0.50%, P: 0.05% or less, S: 0.01% or less, Cr: 16.0 to 25.0%, Ti: 0.05 to 0.35%, Al: 0.005 to 0.05%, and N: 0.005 to 0.025%, with the remainder being Fe and unavoidable impurities, and that generates a nitrogen-enriched layer with a peak nitrogen concentration of 0.05 to 0.30 mass% from the surface to a depth of 0.05 μm, thereby providing a ferritic stainless steel that exhibits good brazeability when brazing is performed at high temperatures using a Ni-containing brazing filler metal and also has excellent corrosion resistance. However, the ferritic stainless steel described in Patent Document 4 has a problem in that the nitrogen-enriched phase formed from the surface layer to a depth of 0.05 μm does not provide sufficient corrosion resistance.

[0008] The technology described in Patent Document 5 has a base material and a nitride layer formed on the surface of the base material, and the chemical composition of the base material is, in mass %, C: 0.001 to 0.020%, Si: 0.01 to 1.50%, Mn: 0.01 to 1.50%, P: 0.010 to 0.050%, S: 0.0001 to 0.010%, Cr: 16.0 to 25.0%, N: 0.001 to 0.030%, Ti: 0.01 to 0.30%, optional elements, the balance: Fe and unavoidable impurities, The metal structure of the material contains 95% or more ferrite phase by volume, the nitride layer is a layer in a region from the surface of the rolled surface to a depth of 0.05 μm in the sheet thickness direction, and the average nitrogen concentration in the nitride layer is 0.80% or more by mass. This ferritic stainless steel sheet is annealed at 850 to 1000°C for 30 to 300 seconds in a non-oxidizing atmosphere of a mixture of nitrogen and hydrogen, resulting in a ferritic stainless steel sheet that combines corrosion resistance and workability. However, it is industrially difficult to increase the nitrogen content in the surface layer to 0.80% or more, and even if this is achieved, there is the problem that it is difficult to provide ductility sufficient to withstand working. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Publication No. 11-350088 [Patent Document 2] Japanese Patent Publication No. 2022-186017 [Patent Document 3] Japanese Patent Application Publication No. 2023-144403 [Patent Document 4] International Publication No. 2015 / 141145 [Patent Document 5] International Publication No. 2021 / 100687 Summary of the Invention [Problem to be solved by the invention]

[0010] As described above, it is difficult to stably produce ferritic stainless steel sheets that have high corrosion resistance and high workability without requiring increased alloy additions using conventional techniques, and new techniques are therefore needed.

[0011] An object of one aspect of the present invention is to provide a ferritic stainless steel sheet having high corrosion resistance and high workability without increasing the amount of alloy added, and a method for manufacturing the same. [Means for solving the problem]

[0012] In order to solve the above problems, a ferritic stainless steel sheet according to one embodiment of the present invention contains, in mass%, C: 0.0010 to 0.010%, Si: 0.01 to 1.20%, Mn: 0.01 to 1.50%, P: 0.035% or less, S: 0.010% or less, Cr: 10.5 to 23.0%, Mo: 0.01 to 1.50%, Ni: 0.01 to 0.60%, Cu: 0.01 to 1.60%, Al: 0.002 to 0.150%, Ti: 0.10 to 0.30%, Nb: 0.001 to 0.20%, N: 0.0020 to 0.030%, with the remainder being Fe and impurities, and the steel sheet has an average nitrogen concentration of 0.03 to 0.12 mass% at a position 50 μm from the surface. [Effects of the Invention]

[0013] According to one aspect of the present invention, a ferritic stainless steel sheet having high corrosion resistance and high workability and a method for manufacturing the same can be realized without increasing the amount of alloy added. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a graph plotting the annealing time in finish annealing on the horizontal axis and the average nitrogen concentration at a position 50 μm from the surface on the vertical axis, according to Example 1 of the present invention, for each test material. [Figure 2] 1 is a graph plotting the annealing time in the final annealing on the horizontal axis and the total elongation at fracture on the vertical axis, according to Example 1 of the present invention, for each test material. [Figure 3]1 is a graph plotting the annealing time in finish annealing on the horizontal axis and the pitting potential difference on the vertical axis, according to Example 1 of the present invention, for each test material. DETAILED DESCRIPTION OF THE INVENTION

[0015] An embodiment of the present invention will be described in detail below. In this specification, the unit of content of each component element, "%", means "mass %" unless otherwise specified. Furthermore, in this application, "A to B" indicates A or more and B or less. A ferritic stainless steel sheet and a manufacturing method thereof according to this embodiment will be described in detail below.

[0016] <Ferritic stainless steel plate> (Chemical composition) The ferritic stainless steel sheet in this embodiment may contain, by mass%, C: 0.0010-0.010%, Si: 0.01-1.20%, Mn: 0.01-1.50%, P: 0.035% or less, S: 0.010% or less, Cr: 10.5-23.0%, Mo: 0.01-1.50%, Ni: 0.01-0.60%, Cu: 0.01-1.60%, Al: 0.036-0.150%, Ti: 0.10-0.30%, Nb: 0.001-0.20%, N: 0.0020-0.030%, and the balance being Fe and impurities. The reasons for specifying the content of each component are explained below.

[0017] C: 0.0010% to 0.010% C forms Cr carbides and causes sensitization, which reduces corrosion resistance. Therefore, it is necessary to add stabilizing elements Nb, Ti, V, and Zr in proportion to the amount of C, which increases alloy costs. Therefore, the C content is set to 0.010% or less. The C content is preferably 0.008% or less.

[0018] On the other hand, carbon is incorporated into molten iron during the reduction of iron ore. While it is possible to remove most of the carbon during converter refining and degassing, this increases the refining time, reducing productivity, and also results in the oxidation of chromium, reducing alloy yield. Therefore, the carbon content is set to 0.0010% or more. The carbon content is preferably 0.0020% or more.

[0019] Si: 0.01 to 1.20% Silicon is effective for deoxidation during melting and refining, and is also effective in suppressing the formation of oxide scale during hot rolling heating, so the Si content is set to 0.01% or more. To ensure the oxidation resistance required for automobile exhaust system parts, the Si content is preferably set to 0.10% or more.

[0020] On the other hand, since Si reduces the ductility of steel sheets through solid solution strengthening, the content is set to 1.20% or less. Furthermore, since Si forms an Si oxide film during finish annealing of cold-rolled steel sheets and reduces pickling properties, the content is preferably set to 1.10% or less.

[0021] Mn: 0.01 to 1.50% Mn is an element added as a deoxidizer and also contributes to increasing high-temperature strength in the medium temperature range. During long-term use, Mn-based oxides form on the surface, contributing to the adhesion of scale (oxides) and the suppression of abnormal oxidation. Therefore, the content is set at 0.01% or more.

[0022] On the other hand, excessive addition of Mn reduces the toughness of the hot-rolled sheet due to the precipitation of the γ phase (austenite phase) and also reduces the corrosion resistance by forming MnS, so the upper limit of the Mn content is set to 1.50%.

[0023] In addition, in consideration of high-temperature ductility, scale adhesion, and suppression of abnormal oxidation, the Mn content is preferably 0.20 to 1.00%.

[0024] P:0.035% or less P is an element contained as an impurity in raw materials such as molten pig iron and alloys such as ferrochrome. Since P is an element that is detrimental to hot workability and toughness, its content is set to 0.035% or less. Since P also reduces workability, its content is preferably set to 0.030% or less. Furthermore, excessive reduction of P content requires the use of high-purity raw materials, which leads to increased costs, so the lower limit of the P content is preferably set to 0.010%.

[0025] S: 0.010% or less S is an element that combines with Ti, Nb, C, etc. to form precipitates, reducing the amount of solute C and increasing the susceptibility to secondary work cracking. The effect becomes significant when the S content exceeds 0.010%, so the S content is set to 0.010% or less. The lower the S content, the fewer sulfide-based inclusions there are, improving corrosion resistance. However, reducing S content increases the desulfurization load and increases manufacturing costs. Therefore, it is preferable to set the lower limit of the S content to 0.001%. The S content is preferably 0.001% to 0.008%.

[0026] Cr: 10.5 to 23.0% Cr is an essential element for ensuring oxidation resistance and corrosion resistance. If the Cr content is less than 10.5%, these effects are not achieved, while if the C content exceeds 23.0%, workability and toughness are reduced. Therefore, the Cr content is set to 10.5 to 23.0%. In addition, considering the reduction in crevice corrosion resistance and high-temperature strength due to oxidation caused by the structure of exhaust system parts, the Cr content is preferably 13.5 to 19.0%.

[0027] Mo: 0.01 to 1.50% Mo, like Cu, has the effect of suppressing active dissolution and inhibiting the progression of pitting corrosion, making it an effective element for improving corrosion resistance. To achieve this effect, the Mo content is set to 0.01% or more. To obtain even higher corrosion resistance, the Mo content is preferably 0.03% or more. On the other hand, excessive Mo increases strength through solid solution strengthening and impairs press formability, so the upper limit of the Mo content is set to 1.50% or less. To achieve both corrosion resistance and workability, the Mo content is preferably 1.30% or less.

[0028] Ni: 0.01 to 0.60% Ni is an element effective in suppressing the progression of pitting corrosion, and this effect is stably exhibited when added at 0.01% or more, so the lower limit of the content is set to 0.01%. On the other hand, adding a large amount of Ni may lead to material hardening due to solid solution strengthening, so the upper limit of the content is set to 0.60%. In consideration of alloy cost, the Ni content is preferably 0.05 to 0.40%.

[0029] Cu: 0.01 to 1.60% Cu is often contained in the steel due to scrap during melting. However, reducing Cu by using high-purity raw materials can promote active dissolution during pit growth, impairing corrosion resistance, so the lower limit of the Cu content is set to 0.01% or more. To further improve corrosion resistance, the Cu content is preferably 0.03% or more. Furthermore, when Cu is added to increase high-temperature strength, excessive Cu content reduces hot workability and corrosion resistance, so the Cu content is set to 1.60% or less. However, because corrosion resistance can be reduced due to Cu precipitation, the Cu content is preferably 1.30% or less.

[0030] Al: 0.036 to 0.150% Al is an effective element for deoxidation, and controlling the composition of inclusions also has an effect on suppressing nozzle clogging. This effect is manifested at an Al content of 0.036% or more, so the lower limit of the Al content is set to 0.036%. Note that, in order to suppress the growth of oxide films during annealing of the cold-rolled sheet and promote nitrogen absorption, it is preferable that the Al content be 0.040% or more.

[0031] On the other hand, Al increases the basicity of the slag, precipitates the water-soluble inclusion CaS in the steel, and may reduce corrosion resistance, so the upper limit of the Al content is set at 0.150%. Also, considering the reduction in elongation due to alumina-based non-metallic inclusions, it is preferable that the Al content be 0.070% or less.

[0032] Ti:0.10~0.30% or less Ti, together with Nb, is an element that combines with C, N, or S to improve corrosion resistance, intergranular corrosion resistance, room-temperature ductility, and deep drawability. Therefore, the Ti content is determined based on the economically achievable reduction amounts of C, N, and S and the amount of Nb added. However, excessive Ti addition impairs room-temperature workability due to solid solution strengthening, so the upper limit is set to 0.30%. However, since Nb is a more expensive element than Ti, it is preferable to add Ti as a supplement to Nb rather than adding Nb alone as a stabilizing element, so the Ti content is set to 0.10% or more. Furthermore, Ti has better sulfide system properties than Nb and is an effective element for suppressing pitting corrosion, so it is preferable to add 0.12% or more.

[0033] Nb: 0.001 to 0.20% Like Ti, Nb forms carbonitrides with C and N in steel, preventing sensitization and improving corrosion resistance, while also improving high-temperature strength and thermal fatigue properties. When added in combination with Ti, Nb also has the effect of increasing the Lankford value. To obtain the Lankford value required for exhaust system parts, 0.001% or more of Nb is added. From the viewpoint of high-temperature strength and corrosion resistance, it is preferable to set the Nb content to 0.01% or more.

[0034] On the other hand, excessive addition of Nb is undesirable because it generates excessive Fe3Nb3C and Laves phase (Fe2B) during use in exhaust system parts, impairing the solid solution strengthening ability of Nb at high temperatures. Therefore, the Nb content is set to 0.20% or less. To achieve high-temperature strength, corrosion resistance, and workability at the same time, it is preferable to set the Nb content to 0.1% or less.

[0035] N: 0.0020% to 0.030% N is an element that forms Cr nitrides at grain boundaries, causing sensitization and resulting in a decrease in corrosion resistance. While it can be reduced by degassing in a vacuum, prolonged degassing is difficult due to the lowered molten steel temperature. Therefore, there is an industrial limit to how much N can be reduced, so the lower limit for the N content is set at 0.0020%. On the other hand, a large amount of nitrogen reduces workability and corrosion resistance, and requires the addition of large amounts of stabilizing elements. Therefore, the upper limit for the N content is set at 0.030%. From the viewpoint of achieving both workability and corrosion resistance, it is preferable to set the N content to 0.008 to 0.025%. The above nitrogen content (nitrogen concentration) is the N amount averaged over the entire thickness, and is the average concentration from the nitrogen-absorbed layer in the surface layer to the center of the sheet thickness, which is not affected by nitrogen absorption during annealing.

[0036] Furthermore, the ferritic stainless steel sheet according to one embodiment of the present invention may contain, in addition to the above elements, one or more of the following elements: Sn: 0.001-0.20%, Co: 0.001-0.10%, V: 0.005-0.10%, Zr: 0.005-0.10%, B: 0.0003-0.0030%, Ca: 0.0001-0.0010%, Mg: 0.0001-0.0010%, and REM: 0.0001-0.0010%. Furthermore, the ferritic stainless steel sheet according to one embodiment of the present invention may be made using high-purity raw materials, and the upper limits of the concentrations of the above-mentioned added elements may be set.

[0037] Sn: 0.001 to 0.20% Like Mo and Cu, Sn is an element that enhances corrosion resistance by suppressing the progression of pitting corrosion, and is therefore preferably added as needed. To achieve this effect, the Sn content is preferably 0.001% or more. On the other hand, it is known that Sn concentrates below the oxide scale and can cause hot rolling cracks and defects. Furthermore, long-term aging at 400 to 700°C can reduce the toughness of the steel, so it is preferable to reduce the Sn content as much as possible. Therefore, it is preferable to add Sn in an amount of 0.20% or less.

[0038] Co: 0.001 to 0.10% Co, like Nb, Mo, and Cu, has the effect of increasing high-temperature strength, but since it is a relatively expensive element, it is preferable to add it as needed. To achieve this effect, the Co content is preferably 0.001% or more. On the other hand, since Co reduces room-temperature workability due to solid-solution strengthening in high-purity ferritic stainless steel sheets, it is preferable to add 0.10% or less.

[0039] V: 0.005 to 0.10% V functions as a stabilizing element for forming carbonitrides, and is therefore preferably added as needed. To achieve this effect, the V content is preferably 0.005% or more. On the other hand, the addition of a large amount of V may promote the formation of coarse carbides due to solidification segregation, which may reduce ductility and toughness, so the V content is preferably 0.10% or less.

[0040] Zr: 0.005% to 0.10% Zr functions as a stabilizing element for forming carbonitrides, so it is preferable to add it as needed. To achieve this effect, the Zr content is preferably 0.005% or more. On the other hand, adding a large amount of Zr may promote the formation of coarse carbides due to solidification segregation, which may reduce ductility and toughness, so the Zr content is preferably 0.10% or less.

[0041] B: 0.0001% to 0.0030% B has the effect of increasing grain boundary strength through grain boundary segregation and improving secondary workability, so it may be added as needed, and in order to exert this effect, it is preferable to set the lower limit of the content to 0.0001%. However, excessive addition of B can cause problems such as Cr2B, (Cr, Fe) 23 Precipitation of (C, B)6 impairs toughness and corrosion resistance, so the upper limit of the B content is preferably set to 0.0030%.

[0042] Ca: 0.0001 to 0.0010% Ca is an element added for desulfurization and is also mixed in through refractory corrosion and / or slag entrainment. To prevent defects and deterioration of corrosion resistance caused by sulfides, the Ca content is preferably 0.0001% or more. However, excessive addition of Ca increases the amount of Ca-containing oxides, which can lead to nozzle clogging and defects, so the upper limit of the Ca content is preferably 0.0010%.

[0043] Mg: 0.0001 to 0.0010% Mg is an element added for desulfurization and is also introduced through refractory corrosion and / or slag entrainment. To prevent defects and deterioration of corrosion resistance caused by sulfides, the Mg content is preferably 0.0001% or more. However, excessive addition of Mg increases the amount of Mg-containing oxides, which can lead to nozzle clogging and defects. Therefore, the upper limit of the Mg content is preferably 0.0010%.

[0044] REM: 0.0001 to 0.0010% REM (Rare Earth Metals) refers to lanthanoid elements (elements with atomic numbers 57 to 71, such as La, Ce, Pr, Nd, and Sm). REM are added for desulfurization and to fix P in steel to prevent temper embrittlement. REM may also be added for the purpose of refining the solidification structure and / or improving oxidation resistance. To achieve these combined effects, the REM content is preferably 0.0001% or more. However, excessive addition of REM can lead to the formation of coarse oxides and / or sulfides, which can cause nozzle clogging and flaws. Therefore, the upper limit of the REM content is preferably 0.0010%.

[0045] In a ferritic stainless steel according to one embodiment of the present invention, the balance other than the above-mentioned components is Fe and impurities. The impurities are components other than the above-mentioned components that are mixed in due to the raw materials and the manufacturing process, and may be mixed in to a degree that does not affect the properties of the above-mentioned components. The impurities may be inevitable impurities that are inevitably mixed in due to the raw materials and the manufacturing process, or other impurities.

[0046] (Average nitrogen concentration at 50 μm from the surface) The nitrogen concentration in the surface layer of the stainless steel sheet strengthens the passive film and improves corrosion resistance. This is thought to be because N concentrated in the passive film combines with hydrogen ions in the environment to form ammonium ions, suppressing the decrease in pH at the early stage of pitting corrosion. However, if the stainless steel sheet surface is polished or oxidized by automobile exhaust gases, controlling the composition at the outermost surface of the stainless steel sheet becomes meaningless, and it becomes necessary to control the composition down to a certain depth. On the other hand, increasing the nitrogen concentration deep into the surface layer of the stainless steel sheet significantly reduces workability, so the thickness of the nitrogen-enriched phase, which is a phase with a high nitrogen concentration, must not be too thick. The ferritic stainless steel sheet of this embodiment has an average nitrogen concentration of 0.03 to 0.12 mass% at a depth of 50 μm from the surface of the steel sheet. This allows the ferritic stainless steel sheet of this embodiment to have high corrosion resistance and high workability. If the average nitrogen concentration is less than 0.03 mass%, the effect of improving corrosion resistance cannot be obtained. On the other hand, if the average nitrogen concentration exceeds 0.12 mass%, workability will be significantly reduced.

[0047] To ensure corrosion resistance in more severe environments, the average nitrogen concentration is preferably 0.05% or more, and to ensure workability in press molding, the average nitrogen concentration is preferably 0.10% or less.

[0048] The nitrogen concentration distribution in the surface layer of a ferritic stainless steel sheet can be evaluated, for example, by glow discharge optical emission spectrometry (GD-OES), an electron probe microanalyzer (EPMA), or a method combining a microscope test with gas analysis. However, because extreme fluctuations in the nitrogen concentration occur at a microscopic level due to the precipitation of nitrides in the nitrogen-absorbing layer at the surface, the method of averaging the EPMA line analysis values ​​provides the highest measurement accuracy. Therefore, in this specification, the average nitrogen concentration at a position 50 μm from the surface of the steel sheet is the average value of a 1000 μm line analysis performed by EPMA at a position 50 μm from the surface parallel to the steel sheet surface.

[0049] <Method of manufacturing ferritic stainless steel sheet> A method for producing a ferritic stainless steel sheet according to one embodiment of the present invention includes a hot rolling process in which a heated slab is hot rolled to obtain a hot-rolled steel sheet, a pickling process in which the hot-rolled steel sheet is pickled to obtain a hot-rolled pickled sheet, a cold rolling process in which the hot-rolled pickled sheet is cold-rolled to obtain a cold-rolled steel sheet, a finish annealing process in which the cold-rolled steel sheet is finish-annealed to obtain a finish-annealed sheet, and a temper rolling process in which the finish-annealed sheet is temper-rolled to obtain a tempered steel sheet.

[0050] In the method for producing a ferritic stainless steel sheet according to this embodiment, in the cold rolling step, the surface roughness after final cold rolling is set to 0.3 to 0.8 μm in terms of Ra. In the method for producing a ferritic stainless steel sheet according to this embodiment, in the finish annealing step, the holding temperature for finish annealing is 900 to 1000°C, the holding time is 300 to 500 seconds, and the average cooling rate from the holding temperature to 500°C is 5 to 15°C / s. In the method for producing a ferritic stainless steel sheet according to this embodiment, in the temper rolling step, rolling is performed with an elongation in temper rolling of 0.3 to 2.0%.

[0051] In the manufacturing method of the ferritic stainless steel sheet according to this embodiment, known manufacturing methods may be used for the steps other than the cold rolling step, the finish annealing step, and the temper rolling step. For example, under known conditions, a slab having a thickness of 250 to 150 mm is cast by a continuous casting method, and the slab is kept at or heated to maintain a temperature of 150°C or less, and the surface of the slab may be ground with a grinder as needed. Subsequently, in the hot rolling step, the slab is heated to 1050 to 1250°C in a hot rolling furnace, and hot-rolled to produce a hot-rolled steel sheet. The hot-rolled steel sheet may be, for example, a hot-rolled steel strip or a hot-rolled coil.

[0052] The hot-rolled steel sheet may be coiled at 550°C or less, and then, if necessary, subjected to a hot-rolled sheet annealing process. Subsequently, the hot-rolled steel sheet is pickled in a pickling process and cold-rolled in a cold-rolling process. In the finish annealing process, finish annealing is performed in an annealing atmosphere of a mixed gas of 95% nitrogen and 5% hydrogen with a dew point of -30°C or less, at a holding temperature of 900 to 1000°C for a holding time of 300 seconds or more and 500 seconds or less. Thereafter, cooling is performed from the holding temperature to 500°C at an average cooling rate of 5°C / s or more and 15°C / s or less. Thereafter, in the temper rolling process, temper rolling is performed with an elongation of 0.3 to 2.0%. Thereafter, each of the processes of electrolytic pickling using a mixed acid containing one or more of sulfuric acid, nitric acid, and fluorine ions may be performed. Note that temper rolling may be performed before electrolytic pickling.

[0053] The above-mentioned manufacturing method can provide a ferritic stainless steel sheet having excellent corrosion resistance and workability, with a thickness of 0.8 to 2.0 mm and an average nitrogen concentration of 0.03 to 0.12 mass% at a position 50 μm from the surface. The ferritic stainless steel sheet manufactured by the manufacturing method according to one embodiment of the present invention can be welded and made into a stainless steel pipe for use in exhaust system parts.

[0054] The method for producing a ferritic stainless steel sheet according to this embodiment will be described in detail below.

[0055] [Hot rolling process] The hot rolling process is a process in which a heated slab is hot rolled to obtain a hot-rolled steel sheet. In the hot rolling process, the slab is heated and the hot-rolled steel sheet is coiled.

[0056] (slab heating) Slabs are heated for hot rolling. This reduces deformation resistance and allows hot rolling from the original slab thickness to a hot-rolled strip with a thickness of 3 to 8 mm. If the heating temperature is too low, the oxide scale on the ferritic stainless steel sheet becomes thin, making it more susceptible to seizure marks on the work rolls during hot rolling. Furthermore, recrystallization between rough hot rolling and finish hot rolling develops the texture necessary to improve the workability of the product after finish hot rolling. However, if the heating temperature is too low, recrystallization between rough hot rolling and finish hot rolling is insufficient, resulting in reduced workability.

[0057] Considering these effects, the heating temperature for hot rolling is set to 1050°C or higher. Since an increase in the Nb content delays recrystallization, the heating temperature for hot rolling is preferably 1150°C or higher. On the other hand, if the heating temperature is too high, the strength decreases, causing the slab to sag in the hot rolling heating furnace, resulting in poor transport. Furthermore, the skid may push the slab into the furnace, making it more susceptible to defects. Therefore, the heating temperature for hot rolling is set to 1250°C or lower. Since a higher heating temperature can coarsen recrystallized grains between rough hot rolling and finish hot rolling, inhibiting the desired development of the hot-rolled sheet texture, the heating temperature for hot rolling is preferably 1200°C or lower.

[0058] (Hot rolling / coiling) In the hot rolling, the slab is rolled to a thickness of 25 to 40 mm in the rough hot rolling, with the rough hot rolling finish temperature preferably being 1000 to 1100°C, and then rolled to a thickness of 3 to 8 mm in the finish hot rolling. The finish hot rolling temperature is preferably 800 to 950°C, and the cooling rate is preferably 20 to 200°C / s. The coiling temperature is preferably 350 to 630°C to avoid the formation of non-uniform precipitates in the longitudinal direction of the hot rolled coil and to prevent the growth of scale and increase the efficiency of pickling.

[0059] [Pickling process] The pickling process is a process in which a hot-rolled steel sheet is pickled to obtain a hot-rolled pickled sheet. Specifically, in the pickling process, the hot-rolled coil obtained in the hot rolling process is mechanically descaled using shot blasting, a hot skin pass, bending rolls, a grinding brush, etc., and then chemically descaled by immersion in acid. Pickling is preferably performed using a bath containing sulfuric acid as the main component at 50 to 90°C, and smut is further removed using nitric acid or a mixed acid of nitric acid and hydrofluoric acid, from the viewpoint of the resulting surface quality. Furthermore, for applications where ridging of the product is a problem, it is preferable to anneal the hot-rolled sheet at 850 to 1100°C before pickling. If the heating temperature for the hot-rolled sheet annealing is less than 850°C, recrystallization is insufficient, and the ridging reduction effect cannot be obtained. On the other hand, if the heating temperature exceeds 1100°C, the scale grows thick, increasing the load of the pickling process.

[0060] [Cold rolling process] The cold rolling process is a process in which the hot-rolled pickled sheet is cold-rolled to obtain a cold-rolled steel sheet. In the cold rolling process, the hot-rolled coil that has been pickled in the pickling process is cold-rolled to obtain a cold-rolled coil (cold-rolled steel sheet) with a thickness of 0.8 to 2.0 mm. In order to develop a texture that improves the workability of the final product, it is preferable that the work roll diameter for cold rolling be 400 mm or more. Furthermore, while the thickness required for each application of automotive exhaust system parts is achieved by cold rolling, it is preferable that the thickness be 1.5 mm or less to obtain a desirable rolling texture such as {111}ND after cold rolling. In order to ensure the strength required for exhaust system parts, a thickness of 1.0 mm or more is preferable. In the cold rolling process, the roughness of the work roll used for cold rolling is adjusted to make the surface roughness of the steel sheet after cold rolling (i.e., after final cold rolling) 0.3 to 0.8 μm in terms of Ra. If the surface roughness Ra is less than 0.3 μm, nitrogen absorption is delayed, and if the surface roughness Ra exceeds 0.8 μm, covering defects are generated after temper rolling, making crevice corrosion more likely to occur.

[0061] [Finishing annealing process] The final annealing step is a step in which the cold-rolled steel sheet obtained in the cold rolling step is final-annealed to obtain a final-annealed steel sheet. In the final annealing step, annealing is performed in a temperature range of 900 to 1000°C in an atmosphere containing 90% or more nitrogen and the remainder mainly hydrogen, with a dew point of -30°C or lower. If the holding temperature in the final annealing is less than 900°C, recrystallization and / or grain growth does not proceed sufficiently within the steel composition range of the ferritic stainless steel sheet according to one embodiment of the present invention, resulting in the formation of a partial unrecrystallized structure or a fine-grained structure, which significantly reduces workability. If the holding temperature in the final annealing is more than 1000°C, problems such as rough surface during press forming due to coarsening of crystal grains and reduction in material strength resulting in the steel sheet being drawn during annealing may occur.

[0062] In the final annealing process, the holding time at 900 to 1000°C is set to 300 seconds or more and 500 seconds or less. By setting the holding time to 300 seconds or more, a nitrogen absorption layer can be grown to 50 μm or more from the surface after recrystallization and grain growth, and the average nitrogen concentration at a position 50 μm from the surface can be set to 0.03 mass% or more and 0.12 mass% or less. For thick materials, the sheet passing speed is slow and the heating and cooling times are long, so the holding time is preferably 400 seconds or more. On the other hand, if the holding time exceeds 500 seconds, abnormal grain growth may occur in some areas, resulting in a non-uniform metal structure. The holding time is preferably 480 seconds or less.

[0063] Cooling after finish annealing is performed from the finish annealing holding temperature to 500°C at an average cooling rate of 5°C / s or more and 15°C / s or less. By cooling at 5°C / s or more, coarse precipitation of nitrides in the surface nitrogen absorption layer can be suppressed, resulting in solid solution of nitrogen atoms or fine precipitates segregating at grain boundaries. This improves corrosion resistance. When a large amount of B and / or Cu is contained, there is a concern that corrosion resistance and workability may be degraded due to precipitation of boride and / or metallic Cu. Therefore, the average cooling rate is preferably 6°C / s or more. On the other hand, if the cooling rate is too fast, it becomes difficult to control the dew point and / or temperature of the atmospheric gas, making it difficult to control the thickness of the nitrogen absorption layer. Therefore, since grain boundary segregation of grain boundary strengthening elements does not occur, the average cooling rate is set to 15°C / s or less. To ensure the segregation of grain boundary strengthening elements as a countermeasure against sheet warpage due to cooling, the average cooling rate is preferably 10°C / s or less.

[0064] As described above, a metal structure in which a nitrogen absorption layer is appropriately formed in the surface layer of a steel sheet has a V´c100 measured using the pitting corrosion potential measurement method for stainless steel specified in JIS G 0577 (2014) that is 30 mV or more higher than a steel sheet with the same composition except for nitrogen that does not have a nitrogen absorption layer, demonstrating good corrosion resistance.

[0065] [Temper rolling process] The temper rolling process is a process in which the finish-annealed sheet obtained in the finish annealing process is temper-rolled to obtain a temper steel sheet. In the temper rolling process, rolling is performed with an elongation of 0.3 to 2.0%. If the elongation is less than 0.3%, the roughness of the work roll is not sufficiently transferred, reducing the surface roughness of the steel sheet and decreasing lubrication performance. If the elongation exceeds 2.0%, work hardening occurs, resulting in reduced workability.

[0066] Pickling may be performed before or after the temper rolling process. When pickling is performed before the temper rolling process to remove the oxide film formed during finish annealing, pickling is performed with an acid solution containing one or more of sulfate ions, nitrate ions, and fluoride ions. Before pickling, the oxide film may be modified by neutral salt electrolysis or immersion in a salt bath.

[0067] The method for producing a ferritic stainless steel sheet according to this embodiment has the above-described configuration, and therefore can produce a ferritic stainless steel sheet having an average nitrogen concentration of 0.03 to 0.12 mass% at a position 50 μm from the surface of the steel sheet. Furthermore, because the method for producing a ferritic stainless steel sheet according to this embodiment is a manufacturing technique that utilizes nitrogen in the annealing atmosphere during annealing of the cold-rolled sheet, there is no need to increase the amount of alloy added, and production costs can be reduced.

[0068] The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention. [Example]

[0069] An embodiment of the present invention will now be described.

[0070] In this example, a 200 mm thick slab having a composition (by mass%) of 17.3%Cr-0.1%Si-0.1%Mn-0.03%P-0.001%S-0.0050%C-0.0080%N-0.005%Nb-0.15%Ti-0.02%Mo-0.08%Ni-0.02%Cu-0.03%Al-0.0004%B was heated to 1200°C. The slab was hot-rolled to a thickness of 5 mm at a finish hot rolling temperature of 850°C, cooled with steam and water, coiled at 500°C, and subsequently pickled in a tandem cold rolling mill with a work roll diameter of 800 mm to produce a ferritic stainless steel cold-rolled steel sheet having a thickness of 1.5 mm and a surface roughness Ra of 0.4 μm.

[0071] To vary the average nitrogen concentration at a position 50 μm from the surface of the steel sheet, the steel sheet was finish-annealed at 900 to 1000°C for 30 to 500 seconds. The finish-annealing atmosphere was a mixture of 95% nitrogen and 5% hydrogen. The dew point was set to -50°C. After finish-annealing, the steel sheet was cooled to 500°C at an average cooling rate of 5°C / s to produce a cold-rolled, finish-annealed sheet. The resulting cold-rolled, finish-annealed sheet was then temper-rolled. The elongation of the temper-rolling was set to 1.0%. The steel sheet was then electrolytically pickled in a mixed acid of sulfuric acid and sodium nitrate to prepare the test material.

[0072] The average nitrogen concentration was measured at a position 50 μm from the surface of each of the obtained test materials. The average nitrogen concentration was measured by line analysis using an EPMA, where a 1000 μm long line was analyzed parallel to the steel sheet surface at a position 50 μm from the surface, and the average value was taken as the average value. In addition, each of the obtained test materials was evaluated for workability. The workability evaluation was performed according to JIS Z 2241, Tensile Test Method for Metallic Materials: 2022. In addition, each of the obtained test materials was evaluated for corrosion resistance. The corrosion resistance was evaluated using the pitting potential measurement method for stainless steel specified in JIS G 0577:2014. For the pitting potential measurement, the surface of the steel sheet was polished as specified.

[0073] Figure 1 is a graph in which the horizontal axis represents the annealing time in the final annealing and the vertical axis represents the average nitrogen concentration at a position 50 μm from the surface, and the results of each test material are plotted. As shown in Figure 1, the average nitrogen concentration increased as the annealing time increased. When the annealing time was 300 seconds, the average nitrogen concentration was 0.03 mass%, and when the annealing time was 500 seconds, the average nitrogen concentration was 0.12 mass%.

[0074] Figure 2 is a graph plotting the values ​​of each test material, with the horizontal axis representing the annealing time in the final annealing and the vertical axis representing the total elongation at fracture. As shown in Figure 2, when the annealing time was between 300 seconds and 500 seconds, there was no significant decrease in total elongation, but when the annealing time exceeded 500 seconds, the total elongation dropped sharply.

[0075] Figure 3 is a graph plotting the annealing time in the final annealing on the horizontal axis and the pitting potential difference on the vertical axis for each test material. As shown in Figure 3, the pitting potential difference was high when the annealing time was 300 seconds or more and 500 seconds or less. [Example]

[0076] Other examples of the present invention will be described below. In these examples, ferritic stainless steel sheets S1 to S35 as examples and ferritic stainless steel sheets R1 to R19 as comparative examples were produced as follows.

[0077] First, a 250 mm thick slab having the steel composition shown in Table 1 was cast and heated to 1200°C. After hot-rolling to a thickness of 4 mm at a finish hot-rolling temperature of 850°C, it was cooled by steam and water, and then wound into a hot-rolled coil at 550°C and air-cooled. The hot-rolled sheet annealing was omitted, and mechanical descaling was performed by shot blasting, followed by sulfuric acid pickling to remove scale. Cold-rolling was performed using a work roll with a roll diameter of 400 mm to obtain a cold-rolled steel sheet with a thickness of 1.5 mm. The surface roughness of the cold-rolled steel sheet was set to the values ​​shown in Table 2. Next, the steel sheet was finish-annealed in a 95% nitrogen and 5% hydrogen gas atmosphere with a dew point of -45°C under the annealing conditions shown in Table 2. Subsequently, temper rolling was performed under the conditions shown in Table 2. Subsequently, electrolytic pickling was performed in a mixed acid of sulfuric acid and nitric acid to remove the oxide film, thereby producing each ferritic stainless steel sheet. [Table 1] [Table 2] In Tables 1 and 2, configurations that fall outside the scope of the present invention are underlined.

[0078] <Characteristics evaluation> (Average nitrogen concentration at a position 50 μm from the surface of the steel plate) For each of the prepared ferritic stainless steel sheets, the average nitrogen concentration at a position 50 μm from the surface of the steel sheet was measured using an EPMA. Specifically, quantitative line analysis of nitrogen at a position 50 μm from the surface of the steel sheet was performed over a length of 1000 μm, and the average nitrogen concentration at a position 50 μm from the surface of the steel sheet was calculated. The calculated average nitrogen concentrations are shown in Table 2.

[0079] (Corrosion resistance evaluation) The pitting potential of each ferritic stainless steel sheet was measured based on the "Method for measuring pitting potential of stainless steel" specified in JIS G 0577:2014. The measured pitting potential is defined as V'c100s. The evaluation surface of each ferritic stainless steel sheet is the ND surface, which is parallel to the rolling direction and the sheet width direction. In addition, the pitting potential was measured at a location where the surface of the steel sheet was polished 200 μm. This value is defined as V'c100s 200 Pitting potential difference ΔV´c100=V´c100s-V´c100s 200 The effect of nitrogen absorption on corrosion resistance was evaluated as follows: The calculated pitting potential difference is shown in Table 2. In this example, specimens that showed a pitting potential difference ΔV'c100 of 30 mV or more were deemed to be acceptable.

[0080] (Processability evaluation) For each of the prepared ferritic stainless steel sheets, test pieces were prepared in a direction parallel to the rolling direction of the steel sheet, and tensile tests were performed based on the "Method for tensile testing of metallic materials" specified in JIS Z 2241:2022. The total elongation at break was measured. The measured total elongation at break is shown in Table 2. In this example, a total elongation of 35% or more was considered acceptable.

[0081] As shown in Figure 2, the ferritic stainless steel sheets R1 to R35 of the examples had an average nitrogen concentration of 0.03 to 0.12 mass% from the surface of the steel sheet after temper rolling. The ferritic stainless steel sheets R1 to R35 had high workability with a total elongation at fracture of 32% or more in a tensile test, and also had high corrosion resistance with a pitting potential difference ΔV'c100 of 30 mV or more between the surface of the steel sheet and a position 200 μm from the surface.

[0082] On the other hand, the ferritic stainless steel sheet R1 had a low holding temperature during final annealing, so the nitrogen absorption layer did not grow, resulting in a fine-grained structure with low elongation and no improvement in corrosion resistance.

[0083] The ferritic stainless steel sheet R2 had a high holding temperature during final annealing, which resulted in an excessively high nitrogen concentration in the nitrogen absorption layer, resulting in low elongation and low corrosion resistance.

[0084] The ferritic stainless steel sheet R3 had a short holding time during final annealing, so the nitrogen absorption layer did not grow, resulting in a fine-grained structure with low elongation and no improvement in corrosion resistance.

[0085] The ferritic stainless steel sheet R4 had a long holding time during final annealing, which resulted in an excessively high nitrogen concentration in the nitrogen absorption layer, resulting in low elongation and low corrosion resistance.

[0086] The ferritic stainless steel sheet R5 had a small surface roughness after final cold rolling, which delayed nitrogen absorption and resulted in low corrosion resistance.

[0087] The ferritic stainless steel sheet R6 had high surface roughness after final cold rolling and a large elongation rate during temper rolling, which resulted in reduced elongation and poor workability.

[0088] The R7 ferritic stainless steel sheet had a high surface roughness Ra after final cold rolling and a low elongation rate after temper rolling, which made it prone to crevice corrosion due to surface covering defects, resulting in reduced corrosion resistance.

[0089] The R8 ferritic stainless steel sheet contains less than 0.0010% C and less than 0.0020% N, which results in a coarse solidification structure, resulting in a duplex structure containing coarse grains even after cold rolling and annealing, which causes roughness in the processed surface and reduces elongation.

[0090] In the R9 ferritic stainless steel sheet, the C content exceeded 0.010%, which caused the grain boundaries to become sensitized during annealing and cooling, resulting in intergranular corrosion, and the corrosion resistance of both the nitrogen absorption layer and the inner layer in the surface layer was reduced.

[0091] The R10 ferritic stainless steel sheet had a Si content of less than 0.05%, which resulted in an increase in oxide inclusions due to insufficient deoxidation during refining, and a decrease in elongation due to solid solution strengthening caused by P, which was due to the P content exceeding 0.035%.

[0092] The ferritic stainless steel sheet R11 contains more than 1.20% Si, and therefore the elongation at room temperature is reduced due to solid solution strengthening by Si.

[0093] The R12 ferritic stainless steel sheet had a manganese content of less than 0.05%, which resulted in insufficient deoxidation and the presence of many oxide-based inclusions, and the N content exceeded 0.020%, which resulted in solid solution strengthening, resulting in reduced elongation.

[0094] The R13 ferritic stainless steel sheet contains more than 1.50% Mn, and therefore the elongation at room temperature is reduced due to solid solution strengthening by Mn.

[0095] The ferritic stainless steel sheet R14 had a Cr content of less than 10.5%, which resulted in a fine-grained structure due to the precipitation of the gamma phase during final annealing, resulting in reduced elongation. Furthermore, in the nitrogen absorption layer, the loss of dissolved Cr due to the formation of nitrides could not be compensated for by the strengthening of the passive film by N, resulting in reduced corrosion resistance in both the surface and inner layers.

[0096] The R15 ferritic stainless steel sheet contains more than 23.0% Cr and more than 1.50% Mo, and as a result, the elongation at room temperature is reduced due to the solid solution strengthening of Cr and Mo.

[0097] The R16 ferritic stainless steel sheet contains more than 0.60% Ni, more than 0.20% Nb, and less than 0.10% Ti, so the elongation decreased due to solid solution strengthening by Ni and Nb.

[0098] The R17 ferritic stainless steel sheet contains less than 0.01% Ni, less than 0.01% Mo, and less than 0.01% Cu, so the mechanism for inhibiting active dissolution is lost, resulting in a decrease in corrosion resistance in both the surface and inner layers.

[0099] Since the R18 ferritic stainless steel sheet contains more than 1.60% Cu, fine Cu-rich clusters are formed during the cooling process after the final annealing of the cold-rolled steel sheet, resulting in reduced elongation.

[0100] The R19 ferritic stainless steel sheet contains more than 0.0030% B, which causes precipitation of chromium-containing borides, making it susceptible to intergranular corrosion, resulting in a decrease in corrosion resistance in both the surface and inner layers.

Claims

1. The alloy contains, in mass%, C: 0.0010 to 0.010%, Si: 0.01 to 1.20%, Mn: 0.01 to 1.50%, P: 0.035% or less, S: 0.010% or less, Cr: 10.5 to 23.0%, Mo: 0.01 to 1.50%, Ni: 0.01 to 0.60%, Cu: 0.01 to 1.60%, Al: 0.002 to 0.150%, Ti: 0.10 to 0.30%, Nb: 0.001 to 0.20%, N: 0.0020 to 0.030%, and the balance including Fe and impurities; A ferritic stainless steel sheet having an average nitrogen concentration of 0.03 to 0.12 mass % at a position 50 μm from the surface of the steel sheet.

2. The ferritic stainless steel sheet according to claim 1, further containing one or more elements selected from the group consisting of Sn: 0.001 to 0.20%, Co: 0.001 to 0.10%, V: 0.005 to 0.10%, Zr: 0.005 to 0.10%, B: 0.0003% to 0.0030%, Ca: 0.0001 to 0.0010%, Mg: 0.0001 to 0.0010%, and REM: 0.0001 to 0.0010%.

3. A method for producing the ferritic stainless steel sheet according to claim 1 or 2, comprising: a hot rolling step of hot rolling the heated slab to obtain a hot-rolled steel sheet; a pickling step of pickling the hot-rolled steel sheet to obtain a hot-rolled pickled sheet; a cold rolling step of cold rolling the hot-rolled pickled steel sheet to obtain a cold-rolled steel sheet; a finish annealing step of finish annealing the cold-rolled steel sheet to obtain a finish annealed sheet; a temper rolling step of temper rolling the finish annealed sheet to obtain a temper steel sheet, In the cold rolling step, the surface roughness after final cold rolling is set to 0.3 to 0.8 μm in Ra, In the final annealing step, the holding temperature of the final annealing is 900 to 1000 ° C., the holding time is 300 seconds or more and 500 seconds or less, and the average cooling rate from the holding temperature to 500 ° C. is 5 ° C. / s or more and 15 ° C. / s or less, In the temper rolling step, the steel is rolled at an elongation rate of 0.3 to 2.0%.

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