Ferritic stainless steel plate

Abstract

To provide a ferritic stainless steel sheet excellent in processability and anti-ridging characteristics.SOLUTION: A ferritic stainless steel sheet comprises 0.010% or more and 0.035% or less of C, 0.50% or less of Si, 1.00% or less of Mn, 0.100% or less of P, 0.0100% or less of S, 14.0% or more and 19.0% or less of Cr, 0.50% or less of Al, 0.010% or more and 0.050% or less of N, and the balance of Fe and impurities, where the γ max of Formula (1) is 10 or more and 40 or less, and the number density of a precipitate having a circle-equivalent diameter of 0.05 μm or more and 0.5 μm or less is 5.0×104 or more and 2.0×105 or less per 1 mm2. Formula (1) γ max=420 C+470 N+23 Ni+9 Cu+7 Mn-11.5 Cr-11.5 Si-12 Mo-23 V-47 Nb-49 Ti-52 Al+189.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a ferritic stainless steel sheet, and more particularly to a ferritic stainless steel sheet having excellent workability and ridging resistance. [Background technology]

[0002] Ferritic stainless steel sheets do not contain much Ni, making them cheaper to manufacture and more cost-stable than austenitic stainless steel sheets. Furthermore, ferritic stainless steel sheets have excellent rust resistance, making them suitable for a variety of applications, including building materials, transportation equipment, and home appliances. In particular, unlike austenitic stainless steel sheets, ferritic stainless steel sheets are magnetic, making them suitable for use in induction-heated cookware. Many cookware, such as pots and pans, are formed using stretching. Therefore, sufficient elongation is required to form them into the desired shape.

[0003] On the other hand, ferritic stainless steel sheets have a problem in that surface irregularities (ridging) parallel to the rolling direction often occur on the surface during forming, which mars the surface's appearance. In cookware, where surface appearance significantly affects commercial value, if ridging occurs, it is necessary to remove the irregularities by polishing after forming. In other words, the occurrence of significant ridging poses a problem of increased manufacturing costs. In general, the greater the strain applied to ferritic stainless steel sheets, i.e., the more severe the processing, the greater the tendency for ridging to occur.

[0004] In recent years, with the diversification of shapes of household cooking utensils, there is a demand for ferritic stainless steel sheets that can be subjected to more severe processing than before. That is, there is a demand for ferritic stainless steel sheets with higher elongation. On the other hand, there is also a demand for lower manufacturing costs for household cooking utensils. That is, there is a demand for ferritic stainless steel sheets with reduced ridging, which increases manufacturing costs. For these reasons, there is a demand for ferritic stainless steel sheets that have higher elongation and that exhibit sufficiently small ridging even when subjected to larger strains than before.

[0005] To address the above-mentioned problems, for example, Patent Document 1 discloses a manufacturing method in which ridging is reduced by cold rolling a hot-rolled steel sheet into a mixed structure of ferrite and martensite, but the method does not take ductility into consideration, and sufficient workability is not obtained.

[0006] Patent Document 2 discloses a method of reducing ridging by forming a mixed structure consisting of a ferrite phase and a martensite structure through hot-rolled sheet annealing and then cold-rolling the resulting structure, and of suppressing a decrease in ductility by setting the martensite structure ratio to 20% or less, thereby ensuring workability. However, this method requires a hot-rolled sheet annealing step, and the manufacturing cost is not sufficiently reduced. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 1-111816 [Patent Document 2] International Publication No. 2017 / 002148 Summary of the Invention [Problem to be solved by the invention]

[0008] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a ferritic stainless steel sheet that is excellent in workability and ridging resistance. [Means for solving the problem]

[0009] In order to solve the above problems, the present invention employs the following configuration. (1) In mass%, C: 0.010% or more and 0.035% or less, Si: 0.50% or less, Mn: 1.00% or less, P: 0.100% or less, S: 0.0100% or less, Cr: 14.0% or more and 19.0% or less, Al: 0.50% or less, N: Contains 0.010% or more and 0.050% or less, The balance is Fe and impurities, γmax shown in formula (1) is 10 or more and 40 or less, The number density of precipitates with a circle equivalent diameter of 0.05 μm or more and 0.5 μm or less is 1 mm 2 5.0 x 10 per 4 pcs or more, 2.0×10 5 Ferritic stainless steel plate having a thickness of 1000 or less. γmax=420C+470N+23Ni+9Cu+7Mn-11.5Cr-11.5Si-12Mo-23V-47Nb-49Ti-52Al+189... Formula (1) In formula (1), C, N, Ni, Cu, Mn, Cr, Si, Mo, V, Nb, Ti, and Al represent the content (mass%) of each element in the steel sheet, and 0 is substituted for elements that are not contained. (2) A ferritic stainless steel sheet according to (1), further containing, in place of a portion of Fe, one or more of the following groups A to D: [Group A] One or more of the following: Sn: 0.005% or more and 0.50% or less, Ni: 2.00% or less, Cu: 2.00% or less, Mo: 1.00% or less, W: 0.50% or less, Co: 0.50% or less, Sb: 0.005% or more and 0.50% or less. [Group B] One or more of the following: B: 0.0001% or more and 0.0025% or less, Ca: 0.0050% or less, Mg: 0.0050% or less. [Group C] One or more of Ti: 0.05% or less, Nb: 0.05% or less, V: 0.05% or less. [Group D] One or more of the following: Y: 0.05% or less, Zr: 0.05% or less, Hf: 0.05% or less, REM: 0.05% or less. [Effects of the Invention]

[0010] According to the present invention, a ferritic stainless steel sheet having excellent workability and ridging resistance can be provided. DETAILED DESCRIPTION OF THE INVENTION

[0011] The inventors investigated the relationship between the morphology of the mixed structure of ferrite and martensite during cold rolling, which is intended to improve ridging resistance, and the elongation after finish annealing, and found a correspondence between the precipitation size of carbonitrides and the elongation after finish annealing. As a result, they discovered a ferritic stainless steel sheet that achieves high elongation while maintaining high ridging resistance, without impairing productivity or increasing manufacturing costs.

[0012] Each requirement of the present invention will be explained in detail below. Note that the "%" in the content of each element means "% by mass."

[0013] (A) Chemical components The reasons for limiting the chemical components are explained below.

[0014] Cr:14.0% or more and 19.0% or less Cr is an element that improves corrosion resistance, a basic characteristic of stainless steel. A Cr content of less than 14.0% not only fails to provide sufficient corrosion resistance for use as a cookware, but also requires the inclusion of excess amounts of other elements, such as Al and Si, to control γmax within an appropriate range, which is necessary for ridging resistance, resulting in reduced workability (elongation). Therefore, the lower limit of the Cr content is set to 14.0% or more. From the viewpoint of workability, the Cr content is preferably 14.5% or more, more preferably 15.0% or more. On the other hand, excessive Cr content excessively reduces the value of γmax, which requires the inclusion of excess amounts of other elements to maintain γmax within an appropriate range, and promotes the formation of intermetallic compounds equivalent to σ, which promotes cracking during manufacturing and reduced formability. Therefore, the upper limit of the Cr content is set to 19.0% or less. From the viewpoint of stable manufacturing (yield, rolling defects, etc.), the Cr content is preferably 18.5% or less, more preferably 18.0% or less.

[0015] C: 0.010% or more and 0.035% or less C is an element that increases γmax, which is important in the present invention, and also forms carbides. On the other hand, excessive C content leads to refinement of crystal grains due to an increase in the number density of carbides, leading to a decrease in elongation. Therefore, the upper limit of the C content is set to 0.035% or less. From the viewpoint of formability, the C content is preferably 0.030% or less. However, if the C content is excessively reduced, it becomes necessary to excessively add other elements that are effective in maintaining γmax within an appropriate range in order to improve ridging resistance, so the lower limit of the C content is set to 0.010% or more. From the viewpoint of ridging resistance, the C content is preferably 0.020% or more, more preferably 0.025% or more.

[0016] Si:0.50% or less Although Si is an element that improves oxidation resistance, excessive content leads to a decrease in formability. Therefore, the Si content is set to 0.50% or less. From the viewpoint of formability, the Si content is preferably 0.30% or less. However, since an excessive decrease in Si content leads to an increase in raw material costs, the Si content is preferably 0.01% or more, and more preferably 0.03% or more.

[0017] Mn: 1.00% or less Like Si, a large amount of Mn reduces formability, so the upper limit of the Mn content is set to 1.00%. From the viewpoint of formability, the Mn content is preferably 0.50% or less. However, since an excessive decrease in Mn content increases raw material costs, the Mn content is preferably 0.01% or more, and more preferably 0.05% or more.

[0018] P:0.100% or less Since P is an element that reduces workability, a low content is preferable, with the upper limit set to 0.100% or less. From the viewpoint of workability, the P content is preferably 0.070% or less, and more preferably 0.05% or less. However, since an excessive reduction in P content increases raw material costs, from the viewpoint of raw material costs, the P content may be set to 0.005% or more, 0.010% or more, or 0.015% or more.

[0019] S: 0.0100% or less S is an unavoidable impurity element that deteriorates corrosion resistance and promotes cracking during manufacturing, so a lower content is preferable, with the upper limit set to 0.0100%. From the viewpoints of corrosion resistance and manufacturability, the content is preferably 0.0030% or less, and more preferably 0.0020% or less. However, since an excessive decrease in S content leads to an increase in refining costs, it may be set to 0.0003% or more, or even 0.0004% or more.

[0020] Al: 0.50% or less Al is an element effective in improving corrosion resistance or oxidation resistance. However, excessive Al content not only reduces workability, but also increases alloy costs and hinders manufacturability. Therefore, the upper limit of the Al content is set to 0.50% or less. From the viewpoint of manufacturability, the preferred upper limit is 0.20% or less. To improve corrosion resistance or oxidation resistance and reduce costs, the Al content may be 0.002% or more, or even 0.005% or more.

[0021] N: 0.010% or more and 0.050% or less Like C, N is an element that increases γmax and also forms nitrides. On the other hand, excessive N content leads to an increase in the number density of nitrides. Therefore, the upper limit of the N content is set to 0.050% or less. From the viewpoint of formability, it is preferably set to 0.04% or less. However, if the N content is excessively reduced, it becomes necessary to add in excess other elements that have the effect of maintaining γmax within an appropriate range in order to improve ridging resistance, so the lower limit of the N content is set to 0.010% or more. From the viewpoint of ridging resistance, it is preferably 0.015% or more, more preferably 0.020% or more.

[0022] In addition to the above basic composition, elements contained in one or more of the following groups A to D may be selectively contained.

[0023] [Group A] Group A includes one or more elements of Ni: 2.00% or less, Cu: 2.00% or less, Mo: 1.00% or less, W: 0.50% or less, Co: 0.50% or less, Sn: 0.005% or more and 0.50% or less, and Sb: 0.005% or more and 0.50% or less. Ni, Cu, Mo, W, Co, Sn, and Sb are elements effective in improving corrosion resistance or oxidation resistance, and one or more of these elements may be contained as needed. However, excessive content of these elements not only reduces formability, but also increases alloy costs and hinders manufacturability. Therefore, the upper limits for Ni and Cu are set to 2.00% or less. The upper limit for Mo is set to 1.00% or less. The upper limits for W, Co, Sn, and Sb are set to 0.50% or less. Since the effects of Sn and Sb are exhibited at 0.005% or more, the lower limits are set to 0.005% or more. A more preferable lower limit for the content of Ni, Cu, Mo, W, and Co is set to 0.05% or more.

[0024] [Group B] The B group includes one or more elements selected from the group consisting of B: 0.0001% or more and 0.0025% or less, Ca: 0.0050% or less, and Mg: 0.0050% or less. B, Ca, and Mg are elements that improve hot workability and secondary workability, and one or more of them may be contained as needed. Since the effect of B is exhibited at 0.0001% or more, this is the lower limit. From the viewpoint of hot workability and secondary workability, B is preferably 0.0003% or more. Similarly, from the viewpoint of hot workability and secondary workability, Ca and Mg are preferably 0.0002% or more. However, excessive content of these elements leads to impaired manufacturability. Therefore, the upper limits of B are set to 0.0025% or less, and the upper limits of Ca and Mg are set to 0.0050% or less. From the viewpoint of manufacturability, B is preferably 0.0012% or less, and Ca and Mg are preferably 0.0010% or less.

[0025] [Group C] The C group includes one or more elements selected from the group consisting of Ti: 0.05% or less, Nb: 0.05% or less, and V: 0.05% or less. Ti, Nb, and V improve oxidation resistance. To achieve this effect, the lower limit of these elements may be set to 0.001% or more. From the viewpoint of oxidation resistance, the lower limit of these elements is preferably 0.002% or more, and more preferably 0.003% or more. On the other hand, excessive content of these elements increases alloy costs and fixes C and N, resulting in a decrease in γmax and a deterioration in ridging properties, so the upper limit is set to 0.05% or less. From the viewpoint of alloy cost and manufacturability, the upper limit of these elements is preferably 0.03% or less, and more preferably 0.01% or less.

[0026] [Group D] The D group includes one or more elements selected from the group consisting of Y: 0.05% or less, Zr: 0.05% or less, Hf: 0.05% or less, and REM: 0.05% or less. Y, Hf, and REM are elements that improve hot workability and steel cleanliness, as well as being effective in improving oxidation resistance, and one or more of these may be contained as needed. When these elements are contained, the upper limit is 0.05% or less. From the viewpoint of cleanliness and oxidation resistance, when these elements are contained, the preferred lower limit is 0.001% or more. Here, REM refers to rare earth elements belonging to atomic numbers 57 to 71, such as La, Ce, Pr, and Nd. REM refers to one or more elements selected from these rare earth elements, and the amount of REM refers to the total amount of rare earth elements.

[0027] In addition to the elements described above, other elements may be contained within a range that does not impair the effects of the present invention. Although it is preferable to reduce Bi, Pb, Se, H, Ta, etc. as much as possible, one or more of Bi≦100 ppm, Pb≦100 ppm, Se≦100 ppm, H≦100 ppm, and Ta≦500 ppm may be contained as necessary within a range that does not affect the effects of the present invention.

[0028] γmax: 10% or more and 40% or less γmax can be calculated from the following formula (1). γmax is an index corresponding to the maximum austenite phase fraction at high temperatures such as the hot rolling temperature range. The higher γmax, the more the hard martensite structure fraction can be increased in the hot-rolled sheet, and ridging can be reduced by breaking up the coarse ferrite phase, which is the structure that causes ridging, more in the cold rolling process. In order to obtain sufficient ridging resistance, the lower limit of γmax is set to 10% or more. From the viewpoint of ridging resistance, γmax is preferably 15% or more, and more preferably 20% or more.

[0029] Increasing C and N to improve γmax increases the amount of carbonitrides after finish annealing, resulting in finer grain sizes and higher strength. Increasing Ni, Cu, and Mn to improve γmax can increase alloy costs and reduce workability due to solid solution strengthening. Therefore, the upper limit of γmax is set to 40%. From the viewpoints of alloy cost and workability, γmax is preferably 37% or less, and more preferably 35% or less.

[0030] γmax=420C+470N+23Ni+9Cu+7Mn-11.5Cr-11.5Si-12Mo-23V-47Nb-49Ti-52Al+189... Formula (1) In formula (1), C, N, Ni, Cu, Mn, Cr, Si, Mo, V, Nb, Ti, and Al represent the content (mass%) of each element in the steel sheet, and 0 is substituted for elements that are not contained.

[0031] (B) Metal structure <Precipitate> Number density of precipitates between 0.05 μm and 0.5 μm: 1 mm 2 5.0 x 10 per 4 pcs or more, 2.0×10 5 less than pcs In the present invention, it is important to specify the number density of precipitates having a size of 0.05 μm or more and 0.5 μm or less. The reason for this will be explained below.

[0032] In order to suppress deterioration of ridging properties, in the present invention, a hot-rolled steel sheet containing a mixed structure of ferrite and martensite is cold-rolled and then finish-annealed. C and N contained in martensite are likely to precipitate finely as carbides or nitrides during finish-annealing. The fine precipitates that precipitate during finish-annealing are mainly precipitates with a grain size in the range of 0.05 to 0.5 μm, and such precipitates refine the crystal grain size and reduce workability. Therefore, in the present invention, it is necessary to limit the number density of precipitates with a grain size in the range of 0.05 to 0.5 μm.

[0033] In the present invention, precipitates with a grain size of less than 0.05 μm and precipitates with a grain size of more than 0.5 μm are not observed for the following reasons.

[0034] To improve ridging resistance, it is necessary to ensure the amount of martensite in the steel sheet after hot rolling. Therefore, hot rolling is performed under conditions that minimize the precipitation of precipitates. Even so, a small amount of fine precipitates may precipitate during hot rolling. However, the precipitates precipitated during hot rolling coarsen with a grain size of more than 0.5 μm during the subsequent finish annealing. Such coarse precipitates do not affect the workability or ridging properties, so there is no need to limit their number density. Therefore, precipitates larger than 0.5 μm are excluded.

[0035] Fine precipitates less than 0.05 μm are unlikely to precipitate during hot rolling, and even if they do precipitate during finish annealing, they coarsen during annealing and are therefore almost absent from the final product. Therefore, it is sufficient to observe precipitates of 0.05 μm or more.

[0036] For these reasons, in the present invention, the number density of precipitates having a size of 0.05 μm or more and 0.5 μm or less is limited.

[0037] In this embodiment, in order to improve workability, the upper limit of the number density of fine precipitates of 0.05 μm or more and 0.5 μm or less is set to 1 mm 2 2.0 x 10 per 5 From the viewpoint of workability, the number density is 1 mm 2 Per 1.5 x 10 4 Preferably, it is 1.2 × 10 or less, and more preferably, it is 1.2 × 10 5 There are less than 100 pieces.

[0038] Furthermore, precipitates of 0.05 μm or more and 0.5 μm or less are formed by the precipitation of C and N in martensite during finish annealing, which are utilized to reduce ridging. Therefore, excessive reduction of these precipitates leads to deterioration of ridging resistance. Therefore, the lower limit of the number density of precipitates is set to 1 mm. 2 5.0 x 10 per 4 From the viewpoint of anti-ridging characteristics, 1 mm 2 7.0 x 10 per 4 Preferably, it is 1.0 × 10 or more, and more preferably 1.0 × 10 5 There are more than one.

[0039] In order to satisfy the number density of precipitates, it is more preferable to set the contents of C and N to appropriate amounts and further adjust the contents of elements other than C and N so as to satisfy the specified range of γmax.

[0040] The precipitates are mainly carbonitrides, specifically (Fe, Cr) 23 Since they precipitate as C6, AlN, and Cr2N, the amount of precipitation can be controlled by limiting the contents of the elements that form them. Since AlN precipitates more easily than Cr2N, N precipitates first as AlN, and the remaining N precipitates as Cr2N. For this reason, it is preferable that the following formula (2-1) or formula (3-1) is satisfied. It is even more preferable that the following formula (2-2) or formula (3-2) is satisfied.

[0041] When [Al] / 27 ≥ [N] / 14, 0.2%≦20[C]+2.9[N]≦0.8%…Formula (2-1) 0.3%≦20[C]+2.9[N]≦0.7%…Formula (2-2)

[0042] When [Al] / 27<[N] / 14, 0.2%≦20[C]+1.5[Al]+8.4([N]-0.52[Al])≦0.8% …Formula (3-1) 0.3%≦20[C]+1.5[Al]+8.4([N]-0.52[Al])≦0.7% …Formula (3-2)

[0043] <Crystal grain size> To improve workability (elongation), the average crystal grain size is preferably 14 μm or more, more preferably 15 μm or more, and even more preferably 20 μm or more. On the other hand, excessive coarsening of the crystal grain size of the ferrite phase in which carbides and nitrides are dispersed leads to a longer finish annealing time, which increases manufacturing costs and reduces productivity. Therefore, the crystal grain size is preferably 40 μm or less, and even more preferably 30 μm or less.

[0044] (C) Manufacturing method Next, a preferred method for producing the ferritic stainless steel sheet of the present invention will be described.

[0045] <Hot rolling> First, a slab made of stainless steel having the above chemical composition is heated and then hot-rolled to form a hot-rolled sheet. The hot-rolling process includes rough rolling and finish rolling.

[0046] [Slab heating] If the slab heating temperature exceeds 1250°C, the crystal grains will significantly enlarge during slab heating, resulting in the formation of coarse extended grains during the hot rolling process, which will cause edge cracks in the hot-rolled sheet and deterioration of the surface quality of the finished sheet. Furthermore, if the slab heating temperature is less than 1100°C, this will cause surface defects, leading to deterioration of corrosion resistance due to rusting from the defects. Therefore, the slab heating temperature is preferably 1100 to 1250°C. Furthermore, considering factors such as reduced productivity due to rolling roll seizure, the slab heating temperature is preferably 1130 to 1230°C.

[0047] [Rough rolling] In the manufacturing method of this embodiment, it is necessary to increase the austenite fraction in the steel between the end of rough rolling and the start of finish rolling. In order to ensure a sufficient austenite fraction in the steel at this stage, it is preferable to hold the steel at a temperature of 1000°C or higher and 1100°C or lower for 30 seconds or longer between the end of rough rolling and the start of finish rolling. A more preferable holding time is 1 minute or longer.

[0048] If the holding temperature is higher than 1100°C or lower than 1000°C, the equilibrium austenite fraction is low and the austenite fraction remains low even after long-term holding, so the holding temperature is set to 1000°C or higher and 1100°C or lower, more preferably 1020°C or higher and 1060°C or lower.

[0049] Furthermore, if the plate thickness after rough rolling is less than 30 mm, a temperature drop occurs between the end of rough rolling and the start of finish rolling, resulting in a decrease in the austenite phase fraction, so the plate thickness of the rough-rolled plate is preferably 30 mm or more, and more preferably 34 mm or more.

[0050] As described above, by controlling the holding temperature and holding time to increase the austenite phase ratio before the start of finish rolling, it is possible to increase the martensite structure ratio of the hot-rolled sheet, which leads to a reduction in ridging and also reduces the amount of carbonitrides that precipitate in the high-temperature hot rolling process, with fine carbonitrides that precipitate during finish annealing becoming the main precipitates.

[0051] [Finishing rolling] The finish rolling end temperature FT is set to 700 to 950°C. If the finish rolling end temperature FT exceeds 950°C, the austenite phase fraction decreases during and after finish rolling, resulting in a decrease in the martensite structure fraction of the hot-rolled sheet. This makes it impossible to sufficiently separate the coarse ferrite structure that causes ridging during the cold rolling process, leading to a deterioration in ridging properties. Therefore, the upper limit is set to 950°C. From the viewpoint of ridging properties, a temperature of 920°C or less is preferable.

[0052] On the other hand, an excessive decrease in the finish rolling finish temperature FT can cause defects in the sheet shape and defects on the steel sheet surface due to an increase in the rolling reaction force, so the lower limit is set to 700°C or higher. From the viewpoints of manufacturability and surface quality, it is preferably 750°C or higher.

[0053] Furthermore, if the steel sheet is exposed to high temperatures for a long time from the start of finish rolling to the completion of coiling, the martensite structure ratio of the hot-rolled sheet decreases due to the phase transformation of the austenite phase into the ferrite phase and carbonitrides, and the coarse ferrite phase that causes ridging cannot be sufficiently separated in the cold rolling process, leading to deterioration of the ridging properties. Therefore, it is preferable to perform rolling and cooling to a temperature of 600°C or less within 30 seconds after the start of finish rolling. More preferably, it is preferable to perform finish rolling and cooling to a temperature of 600°C or less within 20 seconds.

[0054] Furthermore, from the viewpoint of suppressing the decrease in the martensite structure ratio of the hot-rolled sheet, the PT shown in Equation (4), which is the relational expression between the time from the start of finish rolling to 600°C and the average temperature, is set to 1.8 × 10 3 It is preferable to satisfy the following:

[0055] PT={273+(F0T+600) / 2}×{Log(t+20)} …Equation (4) Where, F0T: finish rolling start temperature, t: time (seconds) from the start of finish rolling to 600°C

[0056] [Winding] If the coiling temperature is high, the austenite phase transforms into ferrite and carbonitrides as the hot-rolled coil gradually cools, reducing the martensite structure ratio of the hot-rolled sheet and leading to deterioration of ridging properties. Therefore, the upper limit is set to 600°C or less. From the viewpoint of recrystallization, it is more preferably 550°C or less. Furthermore, if carbonitrides are precipitated in the coiling process, the precipitate size increases to exceed 0.5 μm. Therefore, the coiling temperature is set to 600°C or less, and carbonitrides are precipitated to 0.5 μm or less during finish annealing.

[0057] [Hot-rolled sheet annealing omitted] Hot-rolled sheet annealing is omitted because it increases manufacturing costs regardless of whether box annealing or continuous annealing is used. Omitting hot-rolled sheet annealing leads to deterioration of ridging resistance and reduction in elongation, but according to the present invention, it is possible to suppress deterioration of ridging resistance and reduction in elongation even when hot-rolled sheet annealing is omitted. Furthermore, if hot-rolled sheet annealing is performed to precipitate carbonitrides as precipitates, the size will increase and the circle equivalent diameter will be 0.5 μm or more. Therefore, hot-rolled sheet annealing is omitted and the precipitates are precipitated to a diameter of 0.5 μm or less during finish annealing.

[0058] <Descaling process> If surface scale is formed and problems such as surface defects occur in subsequent processes, it is preferable to carry out a descaling treatment such as pickling as necessary.

[0059] <Cold rolling> Following descaling, which is performed as needed, cold rolling is performed to a predetermined plate thickness. In order to accumulate sufficient strain for recrystallization in the subsequent annealing process, the cold rolling reduction CR is set to 40% or more. From the viewpoint of recrystallization, it is preferably 50% or more. Since an excessive increase in the cold rolling reduction CR reduces productivity and leads to an increase in manufacturing costs, the upper limit is set to 85%, and it is preferably 80% or less.

[0060] <Finishing annealing (final annealing)> After the above-described cold rolling, the steel sheet is subjected to finish annealing for the purpose of recrystallization and grain growth at a temperature of 800°C to 950°C for a time of 5 seconds to 15 minutes. If the temperature is less than 800°C, the cold-rolled sheet cannot be sufficiently recrystallized, and the martensite structure present in the hot-rolled sheet cannot be sufficiently transformed into ferrite and carbonitride, leading to a decrease in elongation. Therefore, a temperature of 800°C or higher is preferable. A temperature of 820°C or higher is even more preferable.

[0061] Furthermore, if the final annealing temperature exceeds 950°C, new austenite phase will be generated, resulting in a structure with a large amount of martensite structure formed after final annealing, which will lead to a decrease in elongation, so the temperature is preferably 950°C or less. More preferably, it is 930°C or less. Note that if the martensite structure is contained in the structure after final annealing at 10% or less, elongation will not decrease, so the martensite structure fraction is preferably 10% by area or less.

[0062] Furthermore, the holding time for the finish annealing is preferably 5 seconds or more to ensure sufficient time for recrystallization and transformation. It is more preferably 10 seconds or more. However, an excessively long holding time reduces productivity and increases manufacturing costs, so it is preferably 15 minutes or less. It is even more preferably 10 minutes or less. As described above, low-temperature coiling at 600°C or less is performed in the coiling process, and hot-rolled sheet annealing is omitted, so carbonitrides precipitate in the finish annealing process. The strain accumulated by cold rolling makes it possible to precipitate precipitates with a size of 0.05 μm to 0.5 μm. In addition, if the composition system simultaneously satisfies formula (2) or formula (3), the number density of precipitates can be reliably controlled within the specified range. [Example]

[0063] Next, an embodiment of the present invention will be described. The stainless steels shown in Tables 1A to 1C were melted and hot-rolled and cold-rolled under the conditions shown in Table 2, and then final annealed to produce product sheets. The sheet thickness after rough rolling was set to 30 mm or more. Table 3 shows the results of property evaluation.

[0064] [Table 1A]

[0065] [Table 1B]

[0066] [Table 1C]

[0067] [Table 2]

[0068] [Table 3]

[0069] <Characteristics evaluation method> [Number density of precipitates with a circle equivalent diameter of 0.05 to 0.5 μm] A cross section of the steel sheet was cut along a plane parallel to the rolling direction and perpendicular to the rolling width direction, and then etching was performed using the SPEED method. After that, five fields of view were photographed using an electron microscope at a magnification of 7000x. The number of precipitates was counted from the microstructure photographs, and the count was divided by the photographed area to calculate the density. The 2.0 × 106 precipitates, which are the grain sizes of 14 μm or more (described later), were used. 5 pieces / mm 2 The following were considered successful:

[0070] [Martensite structure ratio] The martensite structure fraction (area ratio) was measured using EBSD (electron backscattering diffraction) crystal orientation analysis. Specifically, an EBSD pattern of the cross section of the stainless steel was first obtained using an EBSD detector mounted on a scanning electron microscope (SEM). The EBSD pattern acquisition conditions were as follows:

[0071] Measurement surface: A cross section cut by a plane parallel to the rolling direction and perpendicular to the rolling width direction ·Measurement magnification: 500x ·Measurement area: 200μm square Measurement pitch (step size): 0.5 μm

[0072] The acquired EBSD pattern was converted into an IQ (Image Quality) image using OIM analysis software. Here, an IQ image is an image analysis that shows clarity. The martensite structure has a more complex internal structure than the ferrite phase and is less clear, so it appears dark in the IQ image. On the other hand, the ferrite phase has a simpler internal structure than the martensite structure and is more clear, so it appears bright in the IQ image. The IQ image was binarized, and the martensite structure fraction (area ratio) was calculated by dividing the area of ​​the martensite structure by the area of ​​the entire stainless steel.

[0073] [Average grain size] The average grain size was measured as follows: The center of the sheet thickness of a cross section cut along a plane parallel to the rolling direction and perpendicular to the rolling width direction was observed using an optical microscope, and the grain size number was determined using the cutting method specified in JIS G 0551:2013, which was used as the average grain size. A grain size of 14 μm or more, which is necessary to ensure elongation, was considered to be acceptable.

[0074] [Workability (uniform elongation)] Tensile test pieces in the rolling direction of JIS No. 13 Type B were prepared. Furthermore, tensile tests were carried out on these test pieces using a tensile testing machine to measure the uniform elongation. A uniform elongation of 17.0% or more, which is necessary for processing complex shapes, was considered to be acceptable.

[0075] [Ridging resistance] Ridging resistance was measured by taking a No. 5 tensile test piece specified in JIS Z 2241:2011 with its longitudinal direction in the rolling direction, applying a 16% tensile strain, and then measuring the height (μm) of the wrinkles (waviness) on the surface with a roughness meter to determine the ridging height. A value of 12.0 μm or less was considered acceptable, as this minimizes the effort required to polish and remove the unevenness caused by ridging that occurs after processing complex shapes.

[0076] <Characteristics evaluation> As shown in Table 3, in Examples C1 to C18, by appropriately controlling the chemical composition and the number density of precipitates with a circle-equivalent diameter of 0.05 to 0.5 μm, it was possible to control the crystal grain size, elongation, and ridging properties, and it was possible to achieve both workability and ridging resistance without reducing productivity.

[0077] In Comparative Example c1, since the C content was outside the specified range, other elements were added in large amounts to make γmax fall within the specified range, resulting in a decrease in elongation.

[0078] In Comparative Example c2, the C content was outside the specified range, the number density of precipitates was high, and the crystal grain size was refined, resulting in a decrease in elongation.

[0079] In Comparative Example c3, since the Si content was outside the specified range, other elements were added in large amounts to bring γmax within the specified range, resulting in a decrease in elongation.

[0080] In Comparative Example c4, since the Mn content was outside the specified range, other elements were added in large amounts to make γmax fall within the specified range, resulting in a decrease in elongation.

[0081] In Comparative Example c5, the P content was excessive, and therefore the elongation was reduced.

[0082] In Comparative Example c6, the sheet cracked during hot rolling due to excessive S, and a finish-annealed steel sheet could not be produced.

[0083] In Comparative Example c7, since the Cr content was outside the specified range, other elements were added in large amounts to bring γmax within the specified range, resulting in a decrease in elongation.

[0084] In Comparative Example c8, the Cr content was outside the specified range, the number density of precipitates was high, and the crystal grain size was refined, resulting in a decrease in elongation.

[0085] In Comparative Example c9, since the Al content was outside the specified range, other elements were added in large amounts to bring γmax within the specified range, resulting in reduced elongation. In addition, the manufacturing conditions were outside the preferred range, resulting in poor ridging properties.

[0086] In Comparative Example c10, since the N content was outside the specified range, other elements were added in large amounts to bring γmax within the specified range, resulting in reduced elongation. In addition, the manufacturing conditions were outside the preferred range, resulting in poor ridging properties.

[0087] In Comparative Example c11, the N content was outside the specified range, the number density of precipitates was high, and the crystal grain size was refined, resulting in a decrease in elongation.

[0088] In Comparative Example c12, the γmax was lower than the predetermined lower limit, and therefore the ridging resistance was deteriorated.

[0089] In comparative example c13, γmax was higher than the specified upper limit, and γmax was particularly increased by C and N, so the number density of carbonitride precipitates was higher than the specified upper limit, the crystal grain size was fine, and the elongation was low. [Industrial Applicability]

[0090] According to the present invention, a cold-rolled ferritic stainless steel sheet having excellent formability and being less likely to develop ridging patterns due to forming can be provided without reducing productivity, and is therefore useful. Therefore, the cold-rolled ferritic stainless steel sheet of the present invention is suitably applied to applications requiring forming.

Claims

1. In mass%, C: 0.010% or more and 0.035% or less, Si: 0.50% or less, Mn: 1.00% or less, P: 0.100% or less, S: 0.0100% or less, Cr: 14.0% or more and 19.0% or less, Al: 0.50% or less, N: Contains 0.010% or more and 0.050% or less, with the balance being Fe and impurities; γmax shown in formula (1) is 10% or more and 40% or less, The number density of precipitates with a circle equivalent diameter of 0.05 μm or more and 0.5 μm or less is 1 mm ２ 5.0 x 10 per ４ pcs or more, 2.0×10 ５ Ferritic stainless steel plate having a thickness of 1000 μm or less. γmax=420C+470N+23Ni+9Cu+7Mn-11.5Cr-11.5Si-12Mo-23V-47Nb-49Ti-52Al+189... Formula (1) In the formula (1), C, N, Ni, Cu, Mn, Cr, Si, Mo, V, Nb, Ti, and Al represent the content (mass%) of each element in the steel sheet, and 0 is substituted for elements that are not contained.

2. The ferritic stainless steel sheet according to claim 1, further containing, in place of a portion of Fe, one or more of the following Groups A to D: [Group A] One or more of Ni: 2.00% or less, Cu: 2.00% or less, Mo: 1.00% or less, W: 0.50% or less, Co: 0.50% or less, Sn: 0.005% or more and 0.50% or less, and Sb: 0.005% or more and 0.50% or less. [Group B] One or more of B: 0.0001% or more and 0.0025% or less, Ca: 0.0050% or less, and Mg: 0.0050% or less. [Group C] One or more of Ti: 0.05% or less, Nb: 0.05% or less, and V: 0.05% or less. [Group D] One or more of Y: 0.05% or less, Zr: 0.05% or less, Hf: 0.05% or less, and REM: 0.05% or less.

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