Ferritic stainless steel sheet and method for producing the same

Abstract

To reduce secondary processing cracks in ferritic stainless steel sheets.SOLUTION: A ferritic stainless steel sheet contains, in mass%, C: 0.0010-0.010%, Si: 0.05-1.20%, Mn: 0.05-1.50%, P: 0.035% or less, S: 0.010% or less, Cr: 10.5-23.0%, Mo: 0.01-1.5%, Ni: 0.01-0.60%, Cu: 0.01-1.60%, Al: 0.002-0.50%, Ti: 0.005-0.30%, Nb: 0.001-0.50%, B: 0.0003-0.0030%, and N: 0.0020-0.020%, the balance being Fe and impurities, wherein a TN value is 10-18, a crystal grain size number G is 5.0-6.0, a surface roughness Ra is 0.2-1.0 μm, and in an oxalic acid etching test, a crystal grain boundary condition is a step structure and a pit condition exhibits pit structure I or pit structure II.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a stainless steel sheet for a member that requires corrosion resistance and workability, such as being used for exhaust system parts of automobiles.

Background Art

[0002] As one of the measures to improve the fuel efficiency of automobiles, which starts from global environmental problems, the weight reduction of the vehicle body is being promoted. For example, the steel sheet used in automobiles is made as high-strength as possible to reduce the sheet thickness, or it is replaced with aluminum or resin materials to reduce the weight. The need for weight reduction is required not only for the automobile body but also for various parts. One of them is the exhaust system parts that process the exhaust gas of the engine.

[0003] Exhaust system parts exposed to high-temperature engine exhaust gas are required to have high high-temperature strength, oxidation resistance, and corrosion resistance. Therefore, stainless steels such as heat-resistant steel SUH409L, high-purity ferritic stainless steels SUS436J1L, AISI439, AISI429, AISI441, etc. are used, and the weight of the exhaust system parts is 20 to 30 kg per vehicle. For parts such as exhaust manifolds manufactured by welding and bending pipes, it is common to process converter and muffler parts by multi-stage forming of stainless steel sheets. However, when these high-purity ferritic stainless steel sheets are press-worked, secondary processing cracks are likely to occur. Therefore, measures such as increasing the sheet thickness and assembling by welding, reducing the degree of processing in the part design, increasing the number of multi-stage forming processes to reduce the degree of processing in one time, and performing warm forming are being taken. In addition, in stainless steel sheets, technical developments have been carried out to improve secondary processing cracks, such as adding B, refining grains, and increasing the r-value.

[0004] In Patent Document 1, in SUS436J1L steel, by adding B in an amount of 0.0005 to 0.0015% by mass, setting the cooling rate after annealing to 15°C / s or more, and making the solid-solution B 0.0004% by mass or more, a technique for improving secondary processing cracking and enhancing corrosion resistance has been reported. However, due to the high cooling rate, B may not segregate at grain boundaries, and there were cases where the grain boundaries could not be sufficiently strengthened. Also, since SUS436J1L steel contains Mo, even a small amount of B segregation may enable grain boundary strengthening, but it was difficult to prevent secondary processing cracking in other steel grades.

[0005] In Patent Document 2, in ferritic stainless steel, with B: 0.0005 to 0.0035% by mass, the mass ratio of Nb / Ti of 0.9 or more, and performing intermediate annealing by heating in the temperature range of (recrystallization temperature - 100°C) to the recrystallization completion temperature for 1 minute or less after intermediate cold rolling, and performing finish cold rolling at a rolling rate of 80% or more and making the minimum value of the r-value 1.8 or more, a technique for enhancing secondary processability has been reported. However, applying the two-pass cold rolling method is not preferable because it increases the manufacturing cost. Also, when the second cold rolling rate is 80% or more, the product sheet thickness becomes extremely thin, so it is not suitable for manufacturing automotive exhaust system parts. Further, increasing the hot rolled sheet thickness to increase the cold rolling rate had a risk of sheet breakage in the pickling and cold rolling processes due to the decrease in toughness of the hot rolled sheet.

[0006] In Patent Document 3, 5 to 100 ppm of B is added to high-purity ferritic stainless steel, 18 ≦ Nb / (C + N) + 2Ti / (C + N) ≦ 60, the crystal grain size is 40 μm or less, and the surface roughness Ra ≦ 0.3 μm or less. Further, a manufacturing technique for a stainless steel sheet excellent in deep drawability with a resin film, resistance to secondary processing brittleness, and corrosion resistance has been reported. Since annealing is performed with the crystal grain size of 40 μm or less, the added B also precipitates as borides with Cr, and it was difficult to improve secondary processing brittleness. Also, due to the small surface roughness, oil separation during press working is likely to occur, and it is necessary to use high-viscosity oil, resulting in problems such as a load on the removal of press oil after forming.

[0007] In Patent Document 4, in order to avoid reducing the grain boundary strength and increasing the secondary processing brittleness by the grain boundary segregation of P, in order to precipitate P as a phosphide, after annealing, the temperature is maintained at 650 to 750 °C for more than 5 minutes and 3 hours or less, so that the extraction residue amount of P is 0.01% by mass or more, and the longitudinal size of the precipitated phosphorus compound is 1 μm or less. A manufacturing technique for a high-purity ferritic stainless steel sheet for deep drawing forming with excellent secondary processing brittleness has been reported. However, since the grain boundary segregation of P does not occur unless it is held at around 500 °C for a long time, it is a phenomenon that can be ignored when manufacturing with ordinary continuous annealing equipment. Therefore, it has been industrially difficult to perform inefficient production such as performing a long-time heat treatment at 650 to 750 °C.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0009] Thus, with the technologies disclosed so far, it is difficult to prevent secondary processing cracks in high-purity ferritic stainless steel sheets in multi-stage forming such as automotive exhaust system parts, and new technology development is required.

[0010] Therefore, an aspect of the present invention aims to provide a ferritic stainless steel sheet excellent in secondary processing brittleness and a method for manufacturing the same without impairing corrosion resistance and productivity.

Means for Solving the Problems

[0011] The gist of the present invention is as follows.

[0012] 〔1〕In terms of mass%, C: 0.0010 to 0.010%, Si: 0.05 to 1.20%, Mn: 0.05 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: 0.005 to 0.30%, Nb: 0.001 to 0.50%, B: 0.0003 to 0.0030%, N: 0.0020 to 0.020%, the balance being composed of Fe and impurities, and the TN value represented by the following formula (1) is 10 or more and 18 or less, the crystal grain size number G defined in JIS G 0551 is 5.0 or more and 6.0 or less, the surface roughness Ra defined in JIS B 0601 is 0.2 μm or more and 1.0 μm or less, and in the nitric acid etching test of stainless steel defined in JIS G 0571, the state of the crystal grain boundaries is a stepped structure and the state of the pits exhibits pit structure I or pit structure II. A ferritic stainless steel sheet characterized by this. TN value = (Ti + 0.5Nb) / (C + 0.9N + 1.2S + 1.5B) ··· Formula (1)

[0013] 〔2〕Furthermore, the ferritic stainless steel sheet according to 〔1〕, characterized by containing at least one group or more of elements shown in at least one group selected from the following group A to group C. Group A elements: In terms of mass%, one or two of Sn: 0.001 to 0.20% and Co: 0.001 to 0.10%, Group B elements: In terms of mass%, one or two of V: 0.005 to 0.10% and Zr: 0.005 to 0.10%, Group C elements: In terms of mass%, one or more of Ca: 0.0001 to 0.0010%, Mg: 0.0001 to 0.0010%, and REM: 0.0001 to 0.0010%.

[0014] A method for manufacturing a ferritic stainless steel sheet according to [3], [1], or [2], comprising a hot rolling step of subjecting a heated slab to hot rolling to obtain a hot rolled steel sheet, a pickling step of pickling the hot rolled steel sheet to obtain a pickled hot rolled sheet, a cold rolling step of cold rolling the pickled hot rolled 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, and a temper rolling step of temper rolling the finish annealed sheet to obtain a temper rolled steel sheet. In the method for manufacturing a ferritic stainless steel sheet, in the finish annealing step, the holding temperature of the finish annealing is set to 880 to 1100 °C, the holding time is set to 20 seconds or more and 300 seconds or less, and the average cooling rate from the holding temperature to 500 °C is set to 5 °C / s or more and 15 °C / s or less. In the temper rolling step, the work roll roughness of the temper rolling is set to 1.0 to 3.0 μm in terms of Ra, and the elongation rate of the temper rolling is set to 0.3 to 2.0% and rolled. A method for manufacturing a ferritic stainless steel sheet, characterized by the above.

Advantages of the Invention

[0015] According to one aspect of the present invention, it is possible to provide a ferritic stainless steel sheet excellent in corrosion resistance and workability, which imparts high secondary workability at room temperature without impairing the corrosion resistance and productivity of a ferritic stainless steel sheet containing Ti or Nb, and a method for manufacturing the same.

Brief Description of the Drawings

[0016]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0017] Hereinafter, one aspect of the present invention will be described. In this specification, "A~B" indicating a numerical range is intended to mean A or more and B or less.

[0018] In the finish annealing of a ferritic stainless steel sheet containing Ti and Nb which are 10 times or more of C+N in mass% in order to avoid the reduction of corrosion resistance due to sensitization during the manufacturing process and welding of a stainless steel thin sheet, by appropriately controlling the annealing conditions, a part of C is dissolved with the grain size number G of the crystal being 5.0 or more and 6.0 or less, and by setting the cooling rate after finish annealing to 5°C / s or more and 15°C / s or less, C is segregated at the grain boundary, the grain boundary strength can be increased, and the susceptibility to secondary processing cracks can be reduced.

[0019] On the other hand, when the crystal grain size is reduced, surface roughness called orange peel is likely to occur, and the frictional force between the material and the mold becomes strong during forming, impairing the workability. However, by setting the surface roughness Ra to 0.2 μm or more and 1.0 μm or less, it was possible to solve the problem by increasing the initial introduction amount of the lubricating oil.

[0020] Based on the above findings, the present invention has found an ideal crystal grain size, grain boundary properties, and control method as a ferritic stainless steel sheet for such applications.

[0021] <Ferritic stainless steel sheet> [Chemical composition] The specific reasons for the content of each component will be described below. In the following description, "%" indicating the content of each element indicates "mass%" unless otherwise specified.

[0022] The ferrite stainless steel sheet according to an embodiment of the present invention contains, by mass%, C: 0.0010 to 0.010%, Si: 0.05 to 1.20%, Mn: 0.05 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: 0.005 to 0.30%, Nb: 0.001 to 0.50%, B: 0.0003 to 0.0030%, N: 0.0020 to 0.020%, and the balance consists of Fe and impurities.

[0023] C: 0.0010% to 0.010% Since C forms Cr carbides and causes sensitization, which reduces corrosion resistance, it is necessary to add stabilizing elements Nb, Ti, V, Zr corresponding to the amount of C, increasing the alloy cost. Therefore, the amount of C is set to 0.010% or less. Preferably, it is 0.008% or less.

[0024] On the other hand, C is taken into the hot metal during the process of reducing iron ore. Although most of it can be removed in the converter refining and degassing processes, it not only prolongs the refining time and impairs productivity, but also secondarily oxidizes Cr and reduces the alloy yield. Therefore, the amount of C is set to 0.0010% or more. Preferably, it is 0.0020% or more.

[0025] Si: 0.05 to 1.20% Si is effective for deoxidation during melting refining and is also effective for suppressing the formation of oxide scale during hot rolling heating. Therefore, the amount of Si is set to 0.05% or more. To ensure oxidation resistance as automotive exhaust system parts, it is preferably 0.10% or more.

[0026] On the other hand, since Si reduces the ductility of the thin steel sheet by solid solution strengthening, the amount of Si is set to 1.20% or less. Also, since Si forms a Si oxide film during finish annealing of the cold rolled steel sheet and reduces pickling properties, it is preferably 1.10% or less.

[0027] Mn: 0.05 to 1.50% Mn is an element added as a deoxidizer and also contributes to the increase in high-temperature strength in the medium-temperature range. In addition, during long-term use, Mn-based oxides are formed on the surface layer, contributing to the adhesion of scale (oxide) and the suppression effect of abnormal oxidation, so it should be 0.05% or more.

[0028] On the other hand, excessive addition causes a decrease in the toughness of the hot-rolled sheet due to the precipitation of the γ phase (austenite phase), and also forms MnS, reducing the corrosion resistance, so the upper limit is set at 1.50%.

[0029] In consideration of high-temperature ductility, scale adhesion, and suppression of abnormal oxidation, 0.20 - 1.00% is preferable.

[0030] P: 0.035% or less P is an element contained as an impurity in alloys such as hot metal and ferrochrome, which are raw materials. Since it is harmful to hot workability and toughness, it should be 0.035% or less. Because P is also an element that reduces workability, it is preferably 0.030% or less. In addition, excessive reduction leads to an increase in cost, such as the need to use high-purity raw materials, so the lower limit of P is preferably set at 0.010%.

[0031] S: 0.010% or less S combines with Ti, Nb, and C to form precipitates, reducing the amount of dissolved C and increasing the sensitivity to secondary processing cracks. When it exceeds 0.010%, the effect becomes significant, so it should be 0.010% or less. The lower the S content, the fewer sulfide-based inclusions and the better the corrosion resistance. However, reducing S increases the desulfurization load and manufacturing cost, so the lower limit is preferably set at 0.001%. Preferably, it is 0.001% - 0.008%.

[0032] Cr: 10.5 - 23.0% Cr is an essential element for ensuring oxidation resistance and corrosion resistance. If it is less than 10.5%, these effects will not be manifested. If it exceeds 23.0%, it will lead to a decrease in workability and deterioration of toughness. Therefore, it is set to 10.5 - 23.0%. Considering the intergranular corrosion resistance caused by the structure of the exhaust system parts and the decrease in high-temperature strength due to oxidation, 13.5% - 19.0% is preferable.

[0033] Mo: 0.01 - 1.50% Mo, like Cu, has the effect of suppressing active dissolution and suppressing the progress of pitting corrosion. Therefore, it is an element effective for improving corrosion resistance. To exhibit its effect, it should be 0.01% or more. To obtain higher corrosion resistance, 0.03% or more is preferable. On the other hand, excessive Mo increases the strength by solid solution strengthening and impairs the press formability. Therefore, its upper limit is set to 1.50% or less. For the balance between corrosion resistance and workability, 1.30% or less is preferable.

[0034] Ni: 0.01 - 0.60% Ni is an element effective for suppressing the progress of pitting corrosion. Its effect is stably exerted with the addition of 0.01% or more, so the lower limit is set to 0.01%. On the other hand, adding a large amount may cause hardening of the material due to solid solution strengthening. Therefore, its upper limit is set to 0.60%. Considering the alloy cost, 0.05 - 0.40% is preferable.

[0035] Cu: 0.01 - 1.60% Cu is often contained due to scraps during melting. However, if high-purity raw materials are used to reduce Cu, it may promote active dissolution during the growth of pitting corrosion and impair corrosion resistance. Therefore, the lower limit is set to 0.01% or more. To enhance corrosion resistance more, 0.03% or more is preferable. Also, when added to increase high-temperature strength, excessive content will reduce hot workability and corrosion resistance. Therefore, it is set to 1.60% or less. Since there may be a case of a decrease in corrosion resistance due to Cu precipitation, 1.30% or less is preferable.

[0036] Al: 0.002 - 0.150% Al is an element effective for deoxidation, and since its effect is manifested at 0.002% or more, the lower limit is set at 0.002%. In addition, in order to obtain a deoxidation effect by combining with Si and Mn, it is preferably 0.005% or more.

[0037] On the other hand, since Al may increase the basicity of the slag, precipitate water-soluble inclusion CaS in the steel, and reduce the corrosion resistance, the upper limit is set at 0.150%. Also, considering the reduction in elongation due to alumina-based non-metallic inclusions, it is preferably 0.050% or less.

[0038] Ti: 0.005 to 0.30% or less Ti, together with Nb, is an element that combines with C, N, and S to improve corrosion resistance, intergranular corrosion resistance, normal temperature ductility, and deep drawing properties. Therefore, the content of Ti is determined from the amount that can economically reduce C, N, and S and the amount of Nb added. However, excessive addition of Ti impairs the workability at normal temperature due to solid solution strengthening, so the upper limit is set at 0.30% or less. However, since Nb is a more expensive element than Ti when adding Nb alone as a stabilizing element, it is preferably 0.005% or more because it is preferable to add Ti as an auxiliary to Nb. Also, since Ti has higher sulfide-based performance than Nb and is effective for suppressing the occurrence of pitting corrosion, it is preferably added at 0.12% or more.

[0039] Nb: 0.001 to 0.50% Nb, like Ti, forms carbonitrides with C and N in the steel, is an element that prevents sensitization and improves corrosion resistance, and is also an element that improves high-temperature strength and thermal fatigue characteristics. When added in combination with Ti, it also has the effect of increasing the Rankford value. In order to obtain the required Rankford value for exhaust system parts, it is added at 0.001% or more. From the viewpoints of high-temperature strength and corrosion resistance, it is preferably 0.01% or more.

[0040] On the one hand, excessive addition is not preferable because it will cause excessive formation of Fe3Nb3C and Laves phase (Fe2b) during use as exhaust system components, impairing the solid solution strengthening ability at high temperatures by Nb. Therefore, it should be 0.50% or less. For the sake of achieving both high-temperature strength and corrosion resistance and workability, it is preferably 0.45% or less.

[0041] B: 0.0003% - 0.0030% B can increase the grain boundary strength by grain boundary segregation and has the effect of improving the secondary workability. Therefore, it can be added as needed. In order to exert its effect, it is preferably 0.0003% or more as the lower limit. However, excessive addition will cause precipitation of Cr2B and (Cr, Fe)(C, B)6, impairing toughness and corrosion resistance. Therefore, it is preferably 0.0030% or less as the upper limit. 23 (C, B)6 precipitation will impair toughness and corrosion resistance. Therefore, it is preferably 0.0030% or less as the upper limit.

[0042] N: 0.0020% - 0.020% N is an element that forms Cr nitrides at grain boundaries and causes a decrease in corrosion resistance due to sensitization. Although it can be reduced by degassing treatment in a vacuum, long-term degassing treatment is difficult because the molten steel temperature drops. There are industrial limitations in reducing N, so the lower limit of N is 0.0020% or more. On the other hand, when containing a large amount of nitrogen, workability and corrosion resistance will decrease, and a large amount of addition of stabilizing elements will be required. Therefore, the upper limit of N is 0.020%. From the perspective of achieving both workability and corrosion resistance, it is preferably 0.008 - 0.013%.

[0043] Ti and Nb are typical stabilizing elements. They are added according to the amounts of C and N to prevent sensitization caused by the formation of Cr carbonitrides. However, excessive addition will extremely reduce the solid solution C and N, and secondary processing embrittlement will occur no matter how the heat treatment conditions are controlled. Also, since they also exist as sulfides and borides, it is necessary to optimize the addition amount according to the amounts of S and B.

[0044] Therefore, the optimal range of Ti and Nb as stabilizing elements, formulated in mass% in relation to the amounts of C, N, S, and B, is the TN value shown in the following formula (1). To obtain the corrosion resistance required for automotive exhaust system components, the TN value should be 10 or more. To ensure the corrosion resistance of the welded part, it is preferable that the TN value be 12 or more. On the other hand, when the TN value exceeds 18, the solid-solved C, N, B, and S, which are grain boundary strengthening elements, are extremely reduced, making secondary processing embrittlement likely to occur. Therefore, the TN value should be 18 or less. To improve secondary processing embrittlement at low temperatures, it is preferable that the TN value be 16 or less.

[0045] TN value = (Ti + 0.5Nb) / (C + 0.9N + 1.2S + 1.5B) ··· Formula (1) In addition, the ferritic stainless steel sheet according to an embodiment of the present invention may contain, in addition to the above elements, as Group A elements, one or two of Sn: 0.001 to 0.20% and Co: 0.001 to 0.10%, as Group B elements, one or two of V: 0.005 to 0.10% and Zr: 0.005 to 0.10%, and as Group C elements, one or more of Ca: 0.0001 to 0.0010%, Mg: 0.0001 to 0.0010%, and REM: 0.0001 to 0.0010%, selected from at least one group shown in the group consisting of Groups A to C, and an upper limit regulation may be performed using high-purity raw materials.

[0046] Group A elements Sn: 0.001 to 0.20% Since Sn, like Mo and Cu, is an element that enhances corrosion resistance by suppressing the progress of pitting corrosion, it is preferably added as needed. To exhibit its effect, 0.001% or more is preferable. However, Sn is known to concentrate under the oxide scale and cause hot rolling cracks and defects. Also, when aging at 400 to 700°C for a long time, it may reduce the toughness of the steel. Since it is preferably reduced as much as possible, an addition of 0.20% or less is preferable.

[0047] 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, 0.001% or more is preferable. However, in high-purity ferritic stainless steel sheets, Co reduces room-temperature workability due to solid solution strengthening, so it is preferable to add 0.10% or less.

[0048] Group B elements V: 0.005 to 0.10% V functions as a stabilizing element for the formation of carbonitrides, so it is preferable to add it as needed. To achieve this effect, it is preferable to add 0.005% or more. On the other hand, adding a large amount of V may promote the formation of coarse carbides due to solidification segregation, which may reduce ductility and toughness, so it is preferable to add 0.10% or less.

[0049] Zr: 0.005% to 0.10% Zr functions as a stabilizing element for the formation of carbonitrides, so it is preferable to add it as needed. To achieve this effect, it is preferable to add 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 it is preferable to add 0.10% or less.

[0050] Group C elements Ca: 0.0001 to 0.0010% Ca is an element added for desulfurization, and is also mixed in through refractory corrosion and slag entrainment. To prevent defects and deterioration of corrosion resistance caused by sulfides, a Ca content of 0.0001% or more is preferred. However, excessive addition can increase Ca-containing oxides, leading to nozzle clogging and defects, so the upper limit is preferably set at 0.0010%.

[0051] Mg: 0.0001 to 0.0010% Mg is an element added for desulfurization and is also an element that gets mixed in due to the erosion of refractories and the entrainment of slag. In order to prevent defects caused by sulfides and deterioration of corrosion resistance, it is preferably 0.0001% or more. However, excessive addition may cause nozzle clogging due to an increase in Mg-containing oxides and the occurrence of defects, so it is preferably set with an upper limit of 0.0010%.

[0052] REM: 0.0001~0.0010% REM (Rare Earth Metals, rare earth elements) refers to lanthanoid series elements (elements with atomic numbers 57 to 71 such as La, Ce, Pr, Nd, Sm, etc.). REM is an element added to desulfurize and fix P in steel to prevent temper embrittlement, and may also be added for the purpose of refining the solidification structure and improving oxidation resistance. In order to exert these effects in a composite manner, it is preferably 0.0001% or more. However, excessive addition may cause nozzle clogging and the occurrence of defects due to the formation of coarse oxides and sulfides, so it is preferably set with an upper limit of 0.0010%.

[0053] In the ferritic stainless steel according to one embodiment of the present invention, the balance other than the above-described respective components is Fe and impurities. Impurities are components other than the above-described respective components that are mixed in from raw materials and the manufacturing process, and may be mixed in as long as they do not affect the characteristics of the above-described respective components. The impurities may be inevitable impurities that are inevitably mixed in from raw materials and the manufacturing process, or other impurities.

[0054] [Crystal grain size number G specified in JIS G 0551: 5.0 or more and 6.0 or less] The crystal grain size strongly affects secondary processing embrittlement accompanied by intergranular fracture. The intergranular strength can be strengthened by segregating a certain amount of solid solution C, N, etc. at the grain boundaries, but due to stabilizing elements such as Ti, Nb, etc., the amount of solid solution C, N is limited, so the crystal grain size number G (hereinafter referred to as GSN) specified in JIS G 0551: 2020 is set to coarse grains of 6.0 or less.

[0055] Considering the case where the amounts of B and S are small, it is preferable to set GSN to 5.8 or less. On the other hand, when GSN is small, the grain boundaries decrease and stress concentration occurs at specific grain boundaries, promoting secondary processing embrittlement, so GSN should be 5.0 or more. In steel grades with a high Cr content, this tendency becomes stronger, so it is preferable to set GSN to 5.3 or more.

[0056] [Surface roughness Ra as defined in JIS G 0601: 0.2 μm or more and 1.0 μm or less] Since the ferritic stainless steel sheet according to one embodiment of the present invention has relatively coarse grains, it tends to be slightly rough during processing. Therefore, in order to prevent necking from occurring at the uneven portions due to roughness during processing, it is necessary to hold a large amount of lubricating oil, so the surface roughness Ra defined in JIS G 0601:2012 is set to 0.2 μm or more. In order to withstand high-degree forming, it is preferable to set the surface roughness Ra to 0.3 μm or more.

[0057] On the other hand, if the surface roughness is made too large, when the formed parts are used in the actual environment for a long time, the snow melting salts flying from the road will accumulate and intergranular corrosion is likely to occur, so the surface roughness Ra should be 1.0 μm or less. Considering the corrosion resistance in areas where a large amount of snow melting salts accumulate, it is preferable to set the surface roughness Ra to 0.7 μm or less.

[0058] [In the oxalic acid etching test of stainless steel defined in JIS G 0571, the state of the grain boundaries is a stepped structure, and the state of the pits exhibits pit structure I or pit structure II]

[0059] In the ferritic stainless steel sheet according to one embodiment of the present invention, in order to increase the grain boundary strength and improve secondary processing embrittlement, the composition and heat treatment conditions are optimized to segregate C, N, B, and S at the grain boundaries. However, since these grain boundary segregation elements segregate and some form precipitates, it is almost impossible to accurately quantify only the grain boundary segregation amount.

[0060] Therefore, the segregation state is distinguished by utilizing the nitric acid etching test for stainless steel specified in JIS G 0571:2003. The nitric acid etching test for stainless steel is a test method targeting austenitic stainless steel. However, it is also effective for the evaluation of ferritic stainless steel sheets according to an embodiment of the present invention, and the as-received material can be etched to evaluate the state of crystal grain boundaries and pits.

[0061] In the classification indicating the state of crystal grain boundaries, by forming a stepped structure, corrosion resistance and secondary processing brittleness can be achieved simultaneously. The grooved structure shows insufficient corrosion resistance due to sensitization and is not acceptable. In the mixed structure, corrosion resistance can be ensured, but it is insufficient for grain boundary strengthening. On the other hand, showing a free ferrite structure or dendritic intergranular grooved structure is not acceptable because it cannot occur in thin plate products of ferritic stainless steel.

[0062] Also, in the classification indicating the state of pits, when showing either pit structure I or pit structure II, since segregation occurs at the grain boundaries and carbonitrides are also precipitated, the grain boundary strengthening and stabilization effects are achieved simultaneously.

[0063] The ferritic stainless steel sheet according to an embodiment of the present invention has a stepped structure in the state of crystal grain boundaries and exhibits either pit structure I or pit structure II in the nitric acid etching test.

[0064] <Manufacturing method of ferritic stainless steel sheet> The manufacturing method of the ferritic stainless steel sheet according to an embodiment of the present invention is a method for manufacturing the ferritic stainless steel sheet according to an aspect of the present invention. The manufacturing method includes a hot rolling step of performing hot rolling on a heated slab to obtain a hot rolled steel sheet, a pickling step of pickling the hot rolled steel sheet to obtain a pickled hot rolled sheet, a cold rolling step of cold rolling the pickled hot rolled 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, and a temper rolling step of temper rolling the finish annealed sheet to obtain a temper rolled steel sheet.

[0065] In the finish annealing process, the holding temperature for finish annealing is set to 880 - 1100 °C, the holding time is set to 20 seconds or more and 300 seconds or less, and the average cooling rate from the holding temperature to 500 °C is set to 5 °C / s or more and 15 °C / s or less. In the quenching and tempering rolling process, the work roll roughness for quenching and tempering rolling is set to 1.0 - 3.0 μm in terms of Ra, and the elongation rate for quenching and tempering rolling is set to 0.3 - 2.0% for rolling.

[0066] For processes other than the finish annealing process and the quenching and tempering rolling process in the manufacturing method, known manufacturing methods may be used. For example, under known conditions, it is cast into a slab with a thickness of 250 - 150 mm by a continuous casting method, heat insulation or heating and holding is carried out so that the slab does not drop below 150 °C, and the surface layer of the slab is ground as necessary. Subsequently, in the hot rolling process, it is heated to 1250 - 1050 °C in a hot rolling heating furnace and hot rolled into a hot rolled steel plate. The hot rolled steel plate may be, for example, a hot rolled steel strip or a hot rolled coil.

[0067] The hot rolled steel plate is coiled at 550 °C or lower, and then a hot rolled plate annealing process may be carried out on the hot rolled steel plate as necessary. Subsequently, pickling is carried out in the pickling process and cold rolling is carried out in the cold rolling process. In the finish annealing process, finish annealing is carried out at a holding temperature of 880 - 1100 °C for a holding time of 20 seconds or more and 300 seconds or less, and the average cooling rate from the holding temperature to 500 °C is set to 5 °C / s or more and 15 °C / s or less. Then, in the quenching and tempering rolling process, the work roll roughness for quenching and tempering rolling is set to 1.0 - 3.0 μm in terms of Ra and the elongation rate is set to 0.3 - 2.0%. Then, it may go through each process of electrolytic pickling with a mixed acid containing one or more of sulfuric acid, nitric acid, and fluoride ions.

[0068] By such a manufacturing method, a ferritic stainless steel plate with a thickness of 0.5 - 2.0 mm, a crystal grain size of 5.0 or more and 6.0 or less as defined in JIS G 0551, a surface roughness Ra of 0.2 μm or more and 1.0 μm or less as defined in JIS B 0601, and in the sulfuric acid etching test of stainless steel as defined in JIS G 0571, the state of the grain boundaries is a stepped structure and the state of the pits exhibits pit structure I or pit structure II can be obtained.

[0069] However, continuous pits caused by the precipitation of borides in a banded form within the crystal grains due to excessive addition of B serve as the starting points for corrosion. Therefore, the pit structure exhibited by the ferritic stainless steel sheet in one embodiment of the present invention is intended to be a discontinuous pit structure I or pit structure II that occurs in a dot-like manner.

[0070] According to the above configuration, a ferritic stainless steel sheet excellent in secondary workability can be obtained. Note that the temper rolling may be performed before pickling. The ferritic stainless steel sheet manufactured by the manufacturing method according to one embodiment of the present invention may be welded and formed into a pipe to be used as an exhaust system component.

[0071] The manufacturing method will be described in detail below.

[0072] [After slab casting, insert into the hot rolling heating furnace] The casting thickness of the slab is set to 150 mm or more in order to obtain the hot rolling ratio required for forming the structure by hot rolling. Considering the productivity during continuous casting, it is preferably 200 mm or more. On the other hand, if the slab becomes too thick, the solidification structure becomes particularly coarse in the range of 1 / 4 to the center of the plate thickness, and even if the hot rolling ratio is increased, it becomes impossible to form the hot rolled sheet structure necessary for improving workability. Therefore, it is set to 250 mm or less. In order to reduce the product's rigging, it is preferably 240 mm or less.

[0073] If necessary, depending on the surface condition of the slab and the intended use of the final product, the cast slab is preferably surface-treated with a grinder or the like. Also, unlike carbon steel, the target steel has no phase transformation and remains in a coarse solidification structure, and there is a risk of cracking due to a decrease in toughness caused by coarse grains. Therefore, it is preferable to keep the temperature at 150°C or more until it is inserted into the heating furnace for hot rolling after casting.

[0074] [Hot rolling process] (Slab heating) The slab is heated for hot rolling. This is to reduce the deformation resistance and perform hot rolling from the slab thickness to a hot-rolled steel strip with a thickness of 3 to 8 mm. If the heating temperature is too low, the oxide scale of the ferritic stainless steel plate becomes thin, making it easy for sticking defects to occur between the work rolls during hot rolling. Furthermore, by recrystallizing between rough hot rolling and finish hot rolling, a grain structure necessary for improving the workability of the product develops after finish hot rolling. However, if the heating temperature is too low, the recrystallization during this period becomes insufficient, and the workability may decrease.

[0075] Considering these effects, the heating temperature for hot rolling should be 1050°C or higher. As the Nb content increases, recrystallization is delayed, so the heating temperature is preferably 1150°C or higher. On the other hand, if the heating temperature is too high, due to a decrease in strength, the slab may sag in the hot rolling heating furnace, causing transportation problems. Also, skids may be pushed into the slab, making it easy for defects to occur, so it should be 1250°C or lower. An increase in the heating temperature may inhibit the desirable development of the hot-rolled sheet grain structure due to coarsening of the recrystallized grains between rough hot rolling and finish hot rolling, so it is preferably 1200°C or lower.

[0076] (Hot Rolling and Coiling) In hot rolling, in rough hot rolling, the rough hot rolling finish temperature is preferably 1000 to 1100°C, and it is rolled to a thickness of 25 to 40 mm. In finish hot rolling, it is rolled to a thickness of 3 to 8 mm. The finish hot rolling temperature is preferably 800 to 950°C, the cooling rate is preferably 20 to 200°C / s, and the coiling temperature is preferably 350 to 630°C to avoid non-uniform precipitate formation in the longitudinal direction of the hot-rolled coil, prevent scale growth, and improve pickling efficiency.

[0077] The hot-rolled coil is mechanically descaled using shot blasting, bending rolls, grinding brushes, etc., and then immersed in acid for chemical descaling. Pickling is preferably carried out using a bath with sulfuric acid as the main component at 50 to 90 °C, and further removing the smut with nitric acid or a mixed acid of nitric acid and hydrofluoric acid from the perspective of the resulting surface properties. Also, in applications where product ridging is a problem, it is preferable to perform hot-rolled sheet annealing at 850 to 1100 °C before pickling. If the heating temperature of the hot-rolled sheet annealing is less than 850 °C, recrystallization is insufficient and the effect of reducing ridging cannot be obtained. On the other hand, if the heating temperature exceeds 1100 °C, the scale grows thick, increasing the load on the pickling process.

[0078] [Cold rolling process] The pickled hot-rolled coil is cold-rolled to obtain a cold-rolled coil (cold-rolled steel sheet) with a thickness of 0.5 to 3.0 mm. To develop a grain structure that enhances the workability of the final product, it is preferable that the work roll diameter of the cold rolling is 400 mm or more. Also, although the thickness required for each application of automotive exhaust system parts is achieved by cold rolling, to obtain a desirable rolled grain structure such as {111}ND after cold rolling, it is preferably 2.0 mm or less. To ensure the strength required for exhaust system parts, 0.8 mm or more is preferable.

[0079] [Final annealing process] When annealing the cold-rolled steel sheet, it is annealed in a temperature range of 880 to 1100 °C. If the holding temperature in the final annealing is lower than this, i.e., less than 880 °C, within the steel composition range of the ferritic stainless steel sheet according to an embodiment of the present invention, due to the pinning effect of precipitates such as carbonitrides, phosphides, sulfides, and intermetallic compounds, it is difficult to make the crystal grain size number G (GSN) 6.0 or less. Also, if it exceeds 1100 °C, the GSN becomes less than 5.0, causing surface roughness called orange peel during processing, resulting in a decrease in local ductility due to this unevenness and galling with the die, reducing the workability. To ensure that the GSN is 5.0 or more, it is preferable to set the holding temperature to 1080 °C or less.

[0080] Also, the holding time at 880 to 1100°C is set to 20 seconds or more and 300 seconds or less. To grow the crystal grain size so that the GSN is 6.0 or less and 5.0 or more after recrystallization, a holding time of 20 seconds or more is required. For thick materials, since the passing speed of the plate is slow and the heating and cooling times are long, the holding time is preferably 60 seconds or more. On the other hand, when the holding time is long and exceeds 300 seconds, abnormal grain growth may occur in part, resulting in a non-uniform metal structure. Therefore, the holding time is preferably 250 seconds or less.

[0081] Also, for the cooling after finish annealing, the temperature is cooled from the holding temperature of finish annealing to 500°C at an average cooling rate of 5°C / s or more and 15°C / s or less. By setting it to 5°C / s or more, the grain boundary strength can be increased while keeping C, N, S, and B segregated at the grain boundaries. When a large amount of B is contained, etc., 6°C / s or more is preferable. On the other hand, if the cooling rate is too fast, segregation of grain boundary strengthening elements at the grain boundaries will not occur, so the cooling rate is set to 15°C / s or less. To ensure segregation of grain boundary strengthening elements, it is preferable to set the cooling rate to 10°C / s or less.

[0082] The metal structure with appropriate grain boundary strengthening like this exhibits pit structure I or pit structure II when an oxalic acid etching test is performed. If the cooling rate is less than 5°C / s, a part of B becomes boride and exhibits a groove-like structure. Also, when the cooling rate exceeds 15°C / s, B does not segregate at the grain boundaries or around Ti carbonitrides, so while it exhibits a stepped structure, it does not become pit structure I or pit structure II.

[0083] [Quenching and tempering rolling process and pickling process] After finish annealing, quenching and tempering rolling is performed. Also, pickling may be performed after quenching and tempering rolling, or quenching and tempering rolling may be performed after pickling.

[0084] In temper rolling, the surface roughness Ra of the steel sheet is set to 0.2 to 1.0 μm by setting the work roll roughness to Ra of 1.0 to 3.0 μm and the elongation rate of temper rolling to 0.3 to 2.0%. When the Ra of the work roll roughness is less than 1.0 μm, the steel sheet roughness becomes low and the lubrication performance during press working deteriorates. Also, when the Ra of the work roll roughness exceeds 3.0 μm, the convex portions are likely to become covering scratches and the surface quality of the product deteriorates.

[0085] Also, it is carried out at an elongation rate of temper rolling of 0.3 to 2.0%. When the elongation rate is less than 0.3%, the work roll roughness is not sufficiently transferred, the surface roughness of the steel sheet decreases, and the lubrication performance deteriorates. Also, when the elongation rate exceeds 2.0%, the workability deteriorates due to work hardening. Further, when removing the oxide film formed during finish annealing by pickling, pickling is carried out with an acid solution containing one or more of sulfuric acid, nitric acid, and fluoride ions. Modification of the oxide film by neutral salt electrolysis or salt bath immersion may be carried out before pickling.

[0086] <Experiment> A 200 mm thick slab having a composition of 17.3% Cr - 0.1% Si - 0.3% Mn - 0.03% P - 0.001% S - 0.0050% C - 0.0080% N - 0.005% Nb - 0.011 to 0.280% Ti - 0.02% Mo - 0.08% Ni - 0.02% Cu - 0.03% Al - 0.0004% B by mass was heated to 1200°C. It was hot-rolled to a plate thickness of 5 mm at a finish hot-rolling temperature of 850°C for the slab, coiled at 500°C after air-water cooling, subsequently pickled, and made into a ferritic stainless steel cold-rolled steel sheet with a plate thickness of 1.2 mm using a tandem cold rolling mill with a work roll diameter of 800 mm.

[0087] In order to make the grain size number G (GSN) of the crystal 5.0 to 6.0, finish annealing was carried out at 880 to 1080 °C for 60 to 300 seconds. For 0.15% Ti steel, samples were prepared by changing the holding temperature in finish annealing from 850 to 1100 °C to vary the grain size number G (GSN) of the crystal from 4.0 to 9.0, and changing the average cooling rate after annealing to 1 to 20 °C / s up to 500 °C. The normalized rolling of the obtained samples was carried out with a work roll roughness of Ra 2.6 μm and an elongation of 1.0%, and electrolytic pickling was performed with a mixed acid of sulfuric acid and sodium nitrate to obtain test materials, and a part of them was used for the evaluation of secondary processing brittleness. The other test materials were cooled to 500 °C or below at a cooling rate of 8 °C / s continuously after finish annealing, and then cooled to room temperature. Subsequently, they were pickled and normalized rolled to obtain test materials.

[0088] For the evaluation of secondary processability brittleness, after punching out a thin plate of the test material into a blank of 85 mm φ, cylindrical drawing was performed with a punch of 40 mm φ. A frustum-shaped fitting was placed in the cup after drawing, and a 5 kg weight was dropped from 1 m above at room temperature to check whether brittle fracture occurred in the cup. For the L cross-section (the plane parallel to the rolling direction and the plate thickness direction) of the test material, a sulfuric acid etching test of stainless steel specified in JIS G 0571 was carried out.

[0089] Figure 1 shows the influence of the chemical composition represented by the TN value on the corrosion resistance evaluated by secondary processing cracks and sulfuric acid etching. It can be seen that when the TN value is 10 or more and 18 or less, the corrosion resistance and secondary processing brittleness are excellent. On the other hand, when the TN value is less than 10, intergranular corrosion occurred in the sulfuric acid etching test, and it was found that the corrosion resistance was poor. Also, when the TN value exceeded 18, it was confirmed that the secondary processing brittleness deteriorated.

[0090] Figure 2 shows the influence of the average cooling rate up to 500 °C after annealing and the grain size number G (GSN) of the crystal on the secondary processing brittleness in 0.15% Ti steel. It was found that secondary processing cracks did not occur under the conditions that the grain size number G (GSN) of the crystal was 5.0 or more and 6.0 or less, and the average cooling rate up to 500 °C after finish annealing was 5 °C / s or more and 15 °C / s or less.

[0091] The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope indicated in 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

[0092] Next, the present invention will be described in more detail with reference to examples.

[0093] <Manufacturing method> A 250-mm thick slab having the steel composition shown in Table 1 was cast, heated to 1200°C, hot-rolled to a plate thickness of 5 mm with a finish hot-rolling temperature of 850°C, air-water cooled, and coiled at 550°C to obtain a hot-rolled coil, and then air-cooled thereafter. Hot-rolling plate annealing was omitted, and after mechanical descaling by shot blasting, sulfuric acid pickling was performed to remove scale. Cold rolling was performed using a work roll with a roll diameter of 400 mmφ to obtain a cold-rolled steel sheet with a plate thickness of 1.2 mm. Finish annealing was performed under the annealing conditions shown in Table 2, and temper rolling was performed with a work roll roughness of Ra of 2.6 μm and an elongation rate of 1.0%. Electrolytic pickling was performed in a mixed acid of sulfuric acid and nitric acid to remove the oxide film.

[0094] Table 2 shows the results of evaluating the properties of the ferrite stainless steel sheet thus obtained. In Tables 1 and 2, the configurations outside the specified range of the present invention are indicated with underlines.

[0095]

Table 1

[0096]

Table 2

[0097] <Property evaluation> [Crystal grain size] Based on JIS G 0551:2020 "Steel - Microscopic test method for crystal grain size", the crystal grain size number G (GSN) was measured. A grain size number G of 5.0 or more and 6.0 or less was considered acceptable.

[0098] [Grain boundary and pit properties] Based on the "Oxalic acid etching test for stainless steel" specified in JIS G 0571:2003, the state of grain boundaries and pits was evaluated. The evaluation surface of the test material was an L-section parallel to the rolling direction and the plate thickness direction.

[0099] Those presenting a stepped structure rather than a grooved structure (synonymous with intergranular corrosion) or a mixed structure, and presenting pit structure I or pit structure II were considered qualified.

[0100] [Surface roughness] Based on the "Geometrical Product Specifications (GPS) - Surface texture: Profile method - Terms, definitions and surface texture parameters" specified in JIS B 0601:2012, the arithmetic mean roughness Ra was measured in the direction perpendicular to the rolling direction on the surface of the test material. A surface roughness Ra of 0.2 - 1.0 μm was considered qualified.

[0101] [Secondary processing brittleness] After applying a general-purpose press working oil to the test material, a disc with a diameter of 85 mm was punched out, and cylindrical drawing was performed using a punch with a diameter of 40 mm and a punch shoulder R of 4 mm and a die with a diameter of 44 mm and a shoulder R of 4 mm to form a cup shape. Those with ductile fracture during cup forming were evaluated as having poor workability. After cooling the cup without cracks to -60 °C, a frustum-shaped fitting was inserted, and a 5 kg weight was dropped from a position 1 m above to evaluate secondary processing cracking based on whether brittle fracture occurred in the cup.

[0102] [Evaluation results] In the present invention, examples S1 to S35 were subjected to finish annealing, pickling, and temper rolling. When the L-section of the specimen was evaluated for the state of grain boundaries and pits using an oxalic acid etching test, only discontinuous pits were observed, confirming that corrosion resistance and grain boundary strength were maintained. Furthermore, the surface roughness Ra was 0.2 μm or more and 1.0 μm or less, and evaluation of secondary work embrittlement showed no cracking during cylindrical drawing. Furthermore, since the grain size number G (GSN) was in the range of 5.0 to 6.0, and the cooling rate after finish annealing was 5°C / s or more and 15°C / s or less, no secondary work cracking occurred.

[0103] On the other hand, the comparative examples R1 to R6 had temperature histories during finish annealing outside the range specified in the present invention, and those with low holding temperatures or short holding times had fine-grained structures, low elongation, and cracks during deep drawing. Those with high holding temperatures or long holding times had GSNs of less than 5.0, which resulted in poor formability due to rough surfaces during deep drawing and cracks during deep drawing. Furthermore, those with cooling rates after finish annealing outside the range specified in the present invention had insufficient grain boundary segregation of grain boundary strengthening elements, resulting in brittle cracks due to secondary work cracking.

[0104] For R7, the work roll roughness during temper rolling was less than 1.0 μm in Ra, so the surface roughness of the steel sheet was low at 0.1 μm in Ra, and it cracked during deep drawing.

[0105] For R8, the work roll roughness during temper rolling exceeded 3.0 μm in Ra, resulting in a large surface roughness of 1.2 μm in Ra on the steel sheet, which caused cracks during deep drawing due to adhesion of the convex parts to the die.

[0106] In R9, the C content was less than 0.0010%, the N content was less than 0.0020%, and the TN value was more than 18.0, so the grain boundary segregation of the grain boundary strengthening elements was insufficient, causing secondary work cracking.

[0107] In R10, the C content was over 0.010% and the TN value was less than 10.0, which caused the grain boundaries to become sensitized during annealing and cooling, resulting in intergranular corrosion. In addition, the elongation rate in temper rolling exceeded 2.0%, which reduced the elongation due to work hardening, resulting in cracking during deep drawing.

[0108] In R11, since Si is less than 0.05%, due to insufficient deoxidation during refining, the amount of oxide inclusions increased. Also, since P is more than 0.035%, the elongation decreased due to solid solution strengthening of P. Moreover, since the elongation rate of quenching and tempering rolling is less than 0.3%, the transfer of work roll roughness was insufficient, and the inflow of processing oil also decreased, resulting in cracking during deep drawing.

[0109] In R12, since Si is more than 1.20%, the elongation at room temperature decreased due to solid solution strengthening of Si, resulting in cracking during deep drawing.

[0110] In R13, since Mn is less than 0.05%, deoxidation was insufficient and there were many oxide inclusions. Furthermore, since N is more than 0.020%, the elongation decreased due to solid solution strengthening, resulting in cracking during deep drawing.

[0111] In R14, since Mn is more than 1.50%, the elongation at room temperature decreased due to solid solution strengthening of Mn, resulting in cracking during deep drawing.

[0112] In R15, since Cr is less than 10.5%, due to the precipitation of γ phase during finish annealing, the grain boundary strengthening elements decreased from ferrite grains, and the grain boundaries were not strengthened during cooling, resulting in secondary processing cracking.

[0113] In R16, since Cr is more than 23.0% and Mo is more than 1.50%, the elongation at room temperature decreased due to solid solution strengthening of Cr and Mo, resulting in cracking during deep drawing.

[0114] In R17, since Ni is more than 0.60%, Nb is more than 0.50%, and Ti is less than 0.005%, the elongation decreased due to solid solution strengthening of Ni and Nb, resulting in cracking during deep drawing.

[0115] In R18, since Ni is less than 0.01%, Mo is less than 0.01%, Cu is less than 0.01%, and the TN value is less than 10.0, the difference in corrosion resistance between the matrix and grain boundaries is small. In the oxalic acid etching test, intergranular corrosion and pitting corrosion did not occur, but secondary processing cracking occurred.

[0116] Since Cu in R19 exceeds 1.60%, fine Cu-rich clusters are formed during the cooling process after finish annealing of the cold-rolled steel sheet, resulting in a decrease in elongation and cracking during deep drawing.

[0117] Since B in R20 exceeds 0.0030%, borides containing chromium are precipitated, showing poor corrosion resistance such as intergranular corrosion and continuous pitting corrosion in the oxalic acid etching test.

Industrial Applicability

[0118] According to one embodiment of the present invention, it is possible to produce a ferritic stainless steel sheet with high corrosion resistance and workability and excellent secondary processing brittleness with high productivity. Therefore, the present invention contributes to weight reduction and long life of exhaust system parts, fuel system parts, structural members, etc. of automobiles.

Claims

1. By mass%, C: 0.0010 to 0.010%, Si: 0.05 to 1.20%, Mn: 0.05 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: 0.005 to 0.30%, Nb: 0.001 to 0.50%, B: 0.0003 to 0.0030%, N: 0.0020 to 0.020%, the balance being composed of Fe and impurities, and the TN value represented by the following formula (1) is 10 or more and 18 or less, the grain size number G of the crystal defined in JIS G 0551 is 5.0 or more and 6.0 or less, the surface roughness Ra defined in JIS B 0601 is 0.2 μm or more and 1.0 μm or less, in the sulfuric acid etching test of stainless steel defined in JIS G 0571, the state of the grain boundaries is a stepped structure, and the state of the pits exhibits pit structure I or pit structure II, a ferritic stainless steel sheet characterized thereby. TN value = (Ti + 0.5Nb) / (C + 0.9N + 1.2S + 1.5B) ··· Formula (1)

2. Furthermore, it contains at least one group or more of elements shown in at least one group selected from the following Group A to Group C, the ferritic stainless steel sheet according to Claim 1. Group A elements: By mass%, Sn: 0.001 to 0.20%, Co: 0.001 to 0.10%, one or two of them, Group B elements: By mass%, V: 0.005 to 0.10%, Zr: 0.005 to 0.10%, one or two of them, Group C elements: By mass%, Ca: 0.0001 to 0.0010%, Mg: 0.0001 to 0.0010%, REM: 0.0001 to 0.0010%, one or more of them.

3. A method for manufacturing the ferritic stainless steel sheet according to Claim 1 or 2, a hot rolling step of subjecting a heated slab to hot rolling to obtain a hot rolled steel sheet, a pickling step of pickling the hot rolled steel sheet to obtain a pickled hot rolled sheet, a cold rolling step of cold rolling the pickled hot rolled 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 rolled steel sheet, in the method for manufacturing a ferritic stainless steel sheet having. In the finish annealing process, the holding temperature of the finish annealing is set to 880 to 1100 °C, the holding time is set to 20 seconds or more and 300 seconds or less, and the average cooling rate from the holding temperature to 500 °C is set to 5 °C / s or more and 15 °C / s or less. In the temper rolling process, the ferrite stainless steel sheet manufacturing method is characterized in that rolling is performed with the work roll roughness of the temper rolling being 1.0 to 3.0 μm in terms of Ra and the elongation rate of the temper rolling being 0.3 to 2.0%.

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