Ferritic stainless steel sheet

Abstract

To provide a ferritic stainless steel sheet capable of lowering the risk of productivity reduction and exhibiting superior surface quality after processing.SOLUTION: The ferritic stainless steel sheet comprises, in mass%, C: 0.030% or more and 0.060% 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: 0.20% or less, N: 0.010% or more and 0.050% or less, and Nb: 0.03% or more and 0.10% or less, the balance being Fe and impurities, wherein the γmax value is within 10 or more and 35 or less, the γH value is 5 or more, and the number density of Nb-containing precipitates having an average size of 1.0 μm or greater is 150 or more and 1000 or less per 1 mm2.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a ferritic stainless steel sheet. [Background technology]

[0002] Ferritic stainless steel sheets do not contain much Ni, so they can be produced more cheaply and have better price stability than austenitic stainless steel sheets. Furthermore, because they have excellent rust resistance, they are used in a variety of applications, including building materials, transportation equipment, home appliances, and cookware. When ferritic stainless steel sheets are used in home appliances and cookware, they are required to have high workability and a clean appearance. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 10-005812 [Patent Document 2] Japanese Patent Application Laid-Open No. 2005-230857 [Patent Document 3] Japanese Patent Application Publication No. 08-120340 [Patent Document 4] International Publication No. 2021 / 205876 [Non-patent literature]

[0004] [Non-Patent Document 1] Harase et al., Iron and Steel, 1990, Vol. 76, No. 9, pp. 126-133 [Non-patent document 2] Takagi, Journal of the Japan Institute of Metals, 2019, Vol. 83, No. 4, pp. 107-118 Summary of the Invention [Problem to be solved by the invention]

[0005] During press forming, ferritic stainless steel such as SUS430 undergoes a yield elongation of a few percent, which causes minute irregularities known as stretcher strain, deteriorating the surface quality.

[0006] Patent Documents 1 and 2 disclose methods for reducing yield point elongation by temper rolling. Patent Document 3 discloses a method for reducing yield point elongation by annealing a hot-rolled sheet for a long period of time before cold rolling.

[0007] Other methods that have been proposed include adding Al to reduce the amount of dissolved N and thereby reduce yield point elongation (see Non-Patent Document 1), and generating a trace martensite structure to reduce yield point elongation (see Patent Document 4).

[0008] However, the methods described in Patent Documents 1 to 3 increase the manufacturing costs due to the additional manufacturing steps. The method described in Non-Patent Document 1 reduces weld penetration by adding Al, and the method described in Patent Document 4 requires highly accurate temperature control, which carries the risk of reducing yield due to temperature being outside the specified range.

[0009] As a method for reducing yield point elongation while avoiding the above-mentioned disadvantages, a measure of coarsening the crystal grains can be used (see Non-Patent Document 2). However, in ferritic stainless steels such as SUS430, if the C content is reduced to coarsen the crystal grains, other problems arise, such as a deterioration in ridging properties, which leads to a deterioration in surface quality after processing, and the occurrence of surface defects during the hot rolling process. For this reason, the measure of coarsening the crystal grains has rarely been put into practical use.

[0010] An object of the present invention is to provide a ferritic stainless steel sheet that reduces the possibility of a decrease in productivity and has excellent surface quality after processing. [Means for solving the problem]

[0011] A ferritic stainless steel sheet according to one embodiment of the present invention contains, by mass%, C: 0.030% or more and 0.060% 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: 0.20% or less, N: 0.010% or more and 0.050% or less, Nb: 0.03% or more and 0.10% or less, with the balance being Fe and impurities, and has a γmax value represented by the following formula (1) of 10 or more and 35 or less, a γH value represented by the following formula (2) of 5 or more, and a number density of Nb-containing precipitates with an average size of 1.0 μm or more of 1 mm 2 150 or more and 1000 or less per unit; γmax=420C+470N+23Ni+9Cu+7Mn-11.5Cr-11.5Si-12Mo-23V-47Nb-49Ti-52Al+189... Formula (1) γH=506.0C+143.8N+2.3Mn+9.6Ni+5.4Cu-4.0Cr-4.3Si-48.1Al-13.5P-4.5S-3.8Mo-8.6Nb-6.8B-7.0V+52.18 ··· Formula (2). [Effects of the Invention]

[0012] According to one aspect of the present invention, it is possible to provide a ferritic stainless steel sheet that has excellent surface quality after processing while reducing the possibility of a decrease in productivity. DETAILED DESCRIPTION OF THE INVENTION

[0013] With the aim of reducing yield point elongation, which causes stretcher strain, and improving ridging resistance, the inventors investigated the relationship between steel components (chemical composition) and the metal structure (precipitation morphology of precipitates) after annealing of the cold-rolled sheet, and discovered a correspondence (relationship) between yield point elongation and the precipitation morphology of Nb-containing precipitates. They then discovered a method for realizing a ferritic stainless steel sheet that reduces yield point elongation while maintaining high ridging resistance, without increasing manufacturing costs.

[0014] Each requirement of the present invention will be explained in detail below. Note that the "%" in the content of each element means "% by mass."

[0015] (1) Chemical composition The reasons for limiting the chemical components are explained below.

[0016] <Basic composition> C: 0.030% or more and 0.060% or less C (carbon) is an element that increases the γmax and γH values, which are important in the present invention, and also forms carbides. Excessive addition of C leads to refinement of crystal grains due to an increase in carbides, which leads to an increase in yield point elongation, so the upper limit is set to 0.060%. From the viewpoint of reducing yield point elongation, the C content is preferably 0.055% or less.

[0017] However, if the C content is reduced too much, the γH value will decrease, causing surface defects such as scabs during hot rolling, and it will become necessary to add in excess other elements that have the effect of maintaining the γmax value within an appropriate range in order to improve ridging resistance. Therefore, the lower limit is set to 0.030%. From the viewpoint of improving ridging resistance, the C content is preferably 0.035% or more, and more preferably 0.040% or more.

[0018] Si:0.50% or less Silicon (Si) is an element that improves oxidation resistance, but excessive addition of Si reduces formability, so the upper limit is set to 0.50%. From the viewpoint of formability, the Si content is preferably 0.30% or less. However, excessive reduction of the Si content increases raw material costs. Therefore, the Si content is preferably 0.01% or more, and more preferably 0.03% or more.

[0019] Mn: 1.00% or less Like Si, the addition of a large amount of Mn (manganese) leads to a decrease in formability, so the upper limit is set to 1.00%. From the viewpoint of formability, the Mn content is preferably 0.50% or less. However, excessive reduction of the Mn content leads to an increase in raw material costs. Therefore, the Mn content is preferably 0.01% or more, and more preferably 0.05% or more.

[0020] P:0.100% or less Since P (phosphorus) is an element that reduces hot workability and formability, a lower P content is preferable, with the upper limit set to 0.100%. From the viewpoint of hot workability and formability, the P content is preferably 0.070% or less, and more preferably 0.050% or less. However, excessive reduction of the P content results in an increase in raw material costs. Therefore, from the viewpoint of raw material costs, the P content is preferably 0.010% or more, more preferably 0.015% or more, and even more preferably 0.020% or more.

[0021] S: 0.0100% or less S (sulfur) is an impurity element that deteriorates corrosion resistance and promotes cracking during manufacturing, so a low S content is preferable, with the upper limit set to 0.0100%. From the viewpoints of corrosion resistance and manufacturability, the S content is preferably 0.0050% or less, and more preferably 0.0030% or less. However, excessive reduction of the S content leads to an increase in refining costs. Therefore, the S content is preferably 0.0003% or more, and more preferably 0.0005% or more.

[0022] Cr:14.0% or more and 19.0% or less Cr (chromium) is an element that improves corrosion resistance, a basic property of stainless steel. If the Cr content is less than 14.0%, sufficient corrosion resistance for use as a cookware cannot be obtained. In addition, other elements such as Al and Si must be added to control the γmax value required for ridging resistance within an appropriate range. In this case, the stainless steel becomes stronger, which ultimately increases the yield point elongation. Therefore, the lower limit of the Cr content is set to 14.0%. From the viewpoint of yield point elongation, the Cr content is preferably 14.5% or more, and more preferably 15.0% or more.

[0023] On the other hand, excessive addition of Cr excessively reduces the γmax and γH values, making it necessary to add excess amounts of other elements to maintain the γmax and γH values within appropriate ranges. Furthermore, excessive addition of Cr promotes the formation of intermetallic compounds equivalent to the σ phase, which promotes cracking during manufacturing and reduced formability. Therefore, the upper limit of the Cr content is set to 19.0%. From the viewpoint of stable manufacturing (yield, rolling defects, etc.), the Cr content is preferably 18.5% or less, and more preferably 18.0% or less.

[0024] Al: 0.20% or less Although Al (aluminum) is an effective element for improving corrosion resistance or oxidation resistance, excessive addition of Al not only reduces workability, but also increases alloy costs and hinders manufacturability. Therefore, the upper limit of the Al content is set to 0.20%. From the viewpoint of manufacturability, the Al content is preferably 0.10% or less. From the viewpoint of cost, the Al content is preferably 0.002% or more, and more preferably 0.005% or more.

[0025] N: 0.010% or more and 0.050% or less Like C, N (nitrogen) is an element that increases the γH value and γmax value and also forms nitrides. Excessive addition of N causes grain refinement due to an increase in nitrides, leading to an increase in yield point elongation. Therefore, the upper limit of the N content is set to 0.050%. From the viewpoint of formability, the N content is preferably 0.040% or less.

[0026] However, if the N content is reduced too much, it becomes necessary to add in excess other elements that have the effect of maintaining the γmax value within an appropriate range in order to improve ridging resistance. Therefore, the lower limit of the N content is set to 0.010%. From the viewpoint of ridging resistance, the N content is preferably 0.015% or more, and more preferably 0.020% or more.

[0027] Nb: 0.03% or more and 0.10% or less Nb (niobium) forms Nb-containing precipitates. Nb-containing precipitates are precipitate phases containing Nb as a constituent element. In a ferritic stainless steel sheet according to one embodiment of the present invention, the Nb-containing precipitates are mainly Nb carbides (NbC), and specifically, the ratio of the number of Nb carbides to the total number of Nb-containing precipitates in which Nb is detected by EDS (Energy Dispersive X-ray Spectrometry) analysis may be 95% or more. Here, Nb carbide contains Nb and C as essential components and may further contain N as an additional component (additional component), and is defined as follows: That is, for individual precipitates in which Nb is detected by EDS analysis, the precipitates in which the ratio of C when Nb+C=100 in atomic ratio is 30 to 80 (in other words, the ratio of the number of C atoms to the total number of Nb and C atoms is 0.3 or more and 0.8 or less), and the number of C atoms is 5 times or more the number of N atoms (the value calculated by C / N is 5 or more; this includes the case where 0 is substituted for the value of N), are defined as Nb carbides.

[0028] In a ferritic stainless steel sheet according to one embodiment of the present invention, Nb-containing precipitates precipitate in the metal structure and fix carbon, thereby reducing the amount of carbides (e.g., Cr carbides) that finely precipitate during annealing of the cold-rolled sheet. Furthermore, Nb-containing precipitates precipitate in the metal structure and fix nitrogen, thereby reducing the amount of nitrides (e.g., Cr nitrides, Al nitrides) that finely precipitate during annealing of the cold-rolled sheet. Among the Nb-detected precipitates, those other than Nb carbides may be, for example, Nb nitrides (NbN), Nb carbonitrides (Nb(C,N)), or composite precipitates containing carbides, nitrides, and carbonitrides, or may be intermetallic compounds.

[0029] As described above, Nb is an element that fixes C by forming Nb carbide (NbC) and reduces yield point elongation. In one embodiment of the present invention, NbC is coarsely precipitated during hot rolling in the manufacturing process of a ferritic stainless steel sheet. This reduces the amount of carbides newly precipitated during annealing of the cold-rolled sheet, reduces the pinning effect produced by the carbides, and coarsens the crystal grains.

[0030] Excessive addition of Nb reduces the amount of dissolved C during slab heating, which can cause surface defects such as scabs during hot rolling. Furthermore, excessive precipitation of C as Nb-containing precipitates can reduce the martensite structure before cold rolling, thereby deteriorating ridging resistance. Therefore, the upper limit of the Nb content is set to 0.10%. From the viewpoints of manufacturability and ridging resistance, the Nb content is preferably 0.08% or less. However, excessive reduction of the Nb content prevents the effect of reducing yield point elongation by fixing C, so the lower limit of the Nb content is set to 0.03%. From the viewpoint of reducing yield point elongation, the Nb content is preferably 0.05% or more. Furthermore, Nb also has the effect of fixing N by forming nitrides (NbN) or Nb carbonitrides (Nb(C,N)). This reduces the amount of nitrides newly precipitated during cold-rolled sheet annealing, reduces the pinning effect caused by the nitrides, and coarsens the grains.

[0031] Remainder: Iron and impurities The ferritic stainless steel sheet according to one embodiment of the present invention may have a chemical composition containing the above-mentioned elements, with the balance being Fe (iron) and impurities. The term "impurities" refers to components that are unavoidable and are introduced during industrial production of stainless steel sheets due to various factors, such as raw materials like ores and scrap, or the manufacturing process, and are acceptable to the extent that they do not adversely affect the present invention. Impurities also include components that are difficult to remove, such as O (oxygen).

[0032] In one embodiment of the present invention, the chemical composition of a ferritic stainless steel sheet is "free of addition" of a certain element, meaning that the element is not artificially added during steelmaking. The ferritic stainless steel sheet in one embodiment of the present invention may be substantially free of various optional elements described below in its chemical composition, and "substantially free of" a certain optional element means that the optional element is not added, although trace amounts of the optional element may be present as impurities.

[0033] <Arbitrary element> The ferritic stainless steel sheet according to one embodiment of the present invention may contain optional elements in its chemical composition in addition to the basic composition described above. Specifically, the ferritic stainless steel sheet may contain one or more elements selected from the set consisting of the following groups A, B, C, and D, and even in this case, the remainder may be Fe and impurities.

[0034] [Group A] One or more selected from the group consisting 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 0.500% or less, and Sb: 0.005% or more and 0.500% or less Ni (nickel), Cu (copper), Mo (molybdenum), W (tungsten), Co (cobalt), Sn (tin), and Sb (antimony) are effective elements for improving corrosion resistance and oxidation resistance. One or more of these elements may be added as needed. However, excessive addition 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%, and for Mo to 1.00%. The upper limits for W, Co, Sn, and Sb are set to 0.50%. Since the effects of Sn and Sb are exhibited at 0.005% or more, the lower limits are set to 0.005%. The lower limits for the contents of Ni, Cu, Mo, W, and Co are preferably set to 0.05%.

[0035] [Group B] One or more 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 (boron), Ca (calcium), and Mg (magnesium) are elements that improve hot workability and secondary workability, and one or more of these elements are added as needed. Since the effect of B is exhibited at 0.0001% or more, the lower limit is set to 0.0001%. From the viewpoint of hot workability and secondary workability, B is preferably 0.0003% or more, and Ca and Mg are preferably 0.0002% or more. However, since excessive addition of B, Ca, and Mg can impair manufacturability, the upper limits for B are set to 0.0025%, and for Ca and Mg are set to 0.0050%. From the viewpoint of manufacturability, B is preferably 0.0012% or less, and Ca and Mg are preferably 0.0010% or less.

[0036] [Group C] One or two selected from the group consisting of Ti: 0.05% or less and V: 0.05% or less Ti (titanium) and V (vanadium) improve oxidation resistance. To achieve this effect, the lower limit is preferably 0.001%. From the viewpoint of oxidation resistance, the Ti content and V content are each more preferably 0.002% or more, and even more preferably 0.003% or more. On the other hand, excessive addition of Ti and V increases alloy costs and fixes C and N, resulting in a decrease in the γmax value and a deterioration in ridging properties, so the upper limit is set to 0.05%. From the viewpoints of alloy cost and manufacturability, the Ti content and V content are each preferably 0.03% or less, and even more preferably 0.01% or less.

[0037] [Group D] One or more 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 (yttrium), Hf (hafnium), and REM (rare earth metals) are elements effective in improving hot workability and steel cleanliness, as well as oxidation resistance, and one or more of these may be added as needed. If added, the upper limit is 0.05%. From the viewpoint of cleanliness and oxidation resistance, the preferred lower limit is 0.001%. Here, REM refers to elements with atomic numbers 57 to 71, such as La (lanthanum), Ce (cerium), Pr (praseodymium), and Nd (neodymium).

[0038] The ferritic stainless steel sheet according to one embodiment of the present invention may contain various elements in its chemical composition in addition to those described above, provided that the effects of the present invention are not impaired. For example, it is preferable to reduce the content of Bi (bismuth), Pb (lead), Se (selenium), H (hydrogen), Ta (tantalum), etc. as much as possible, but may contain one or more of Bi≦100 ppm, Pb≦100 ppm, Se≦100 ppm, H≦100 ppm, and Ta≦500 ppm, as needed, provided that the effects of the present invention are not adversely affected.

[0039] <Component index> γmax value: 10 to 35 γmax is a regression equation between the amount of γ (γmax) at the temperature at which the γ phase is most formed at high temperatures such as in the hot rolling temperature range and the components, and is expressed by the following equation (1): γmax=420C+470N+23Ni+9Cu+7Mn-11.5Cr-11.5Si-12Mo-23V-47Nb-49Ti-52Al+189... Formula (1) In addition, the content (mass%) of each element is substituted for the element symbol in formula (1), and 0 (zero) is substituted for elements that are not added. The hot rolling temperature range may be, for example, "Ac1+20°C or more" to "Ac1+120°C or less." In this embodiment, the temperature related to γmax is 950°C to 1100°C.

[0040] The higher the γmax value, the more the hard martensite structure fraction in the hot-rolled annealed sheet can be increased, and the more the coarse ferrite phase, which is the structure that causes ridging, can be broken down in the cold rolling process. As a result, the ridging resistance can be improved. Improving the ridging resistance leads to improving the surface quality after processing. In order to obtain sufficient ridging resistance, the lower limit of the γmax value is set to 10. From the viewpoint of ridging resistance, the γmax value is preferably 15 or more.

[0041] Increasing the C and N contents to improve the γmax value increases the amount of carbonitrides in the metal structure after annealing the cold-rolled sheet, which can lead to refinement of the grain size and increased yield point elongation. Increasing the Ni, Cu, and Mn contents to improve the γmax value may increase alloy costs and reduce workability due to solid solution strengthening. Therefore, the upper limit of the γmax value is set to 35. From the viewpoints of alloy cost and workability, the γmax value is preferably 30 or less.

[0042] γH value: 5 or higher γH is an index corresponding to the austenite phase ratio at a specific temperature in the slab heating temperature range before hot rolling, and is expressed by the following formula (2): γH=506.0C+143.8N+2.3Mn+9.6Ni+5.4Cu-4.0Cr-4.3Si-48.1Al-13.5P-4.5S-3.8Mo-8.6Nb-6.8B-7.0V+52.18 Equation (2) In addition, the content (mass%) of each element is substituted for the element symbol in formula (2), and 0 (zero) is substituted for elements that are not added. The slab heating temperature range before hot rolling may be, for example, 1100 to 1250°C or 1130 to 1230°C. The specific temperature for γH is 1180°C.

[0043] If the γH value is too low, the metal structure will become a single ferrite phase during slab heating, causing the crystal grains to coarsen, resulting in surface defects such as scabs during subsequent hot rolling. The lower limit of the γH value is set to 5 to prevent the occurrence of surface defects during hot rolling. From the viewpoint of preventing surface defects, the γH value is preferably 7 or more. Adding excessive C to increase the γH value increases the amount of carbides, resulting in an increase in yield point elongation, while adding excessive Ni or Cu increases the alloying cost. Therefore, the γH value is preferably 15 or less, and more preferably 12 or less.

[0044] (2) Metal structure <Precipitate> Number density of Nb-containing precipitates with an average size of 1.0 μm or more: 1 mm 2 150 to 1000 pieces per This specifies the number density of Nb-containing precipitates with an average size of 1.0 μm or more among the precipitates that precipitate in the metal structure.

[0045] Generally, carbon increases the austenite fraction during slab heating, preventing coarsening of ferrite grains and contributing to the prevention of surface defects such as scabs. However, it also precipitates fine carbides during cold-rolled sheet annealing. The refinement of grains due to the fine precipitated carbides leads to an increase in yield point elongation. In contrast, niobium (Nb) dissolves in the slab during heating and does not impede the role of carbon. It also precipitates coarsely as Nb-containing precipitates (mainly NbC) during the hot-rolling and hot-rolled sheet annealing processes, thereby reducing the amount of finely precipitated carbides during cold-rolled sheet annealing.

[0046] For precipitates with an average size of less than 1.0 μm, the crystal grains are refined according to the precipitate diameter. Therefore, in the ferritic stainless steel sheet of this embodiment, from the viewpoint of grain coarsening, Nb-containing precipitates are coarsely precipitated in the metallographic structure to an average size of 1.0 μm or more. Here, in this specification, the average size of precipitates is a value calculated for each precipitate in the metallographic structure. The average size of a certain precipitate is determined by the average value of the major axis and minor axis of the precipitate. The major axis means the length of the longest line segment connecting any two points on the periphery (outer edge) of the precipitate. The minor axis means the length of the longest line segment connecting the outer edges of the precipitates, which is perpendicular to the line segment of the major axis specified above.

[0047] The grain refinement effect is also influenced by the density of precipitates. In order to coarsen the grains, the number density of Nb-containing precipitates with an average size of 1.0 μm or more is set to 1 mm 2 The number density of Nb-containing precipitates having an average size of 1.0 μm or more is preferably 1000 or less per mm 2 If the number density of Nb-containing precipitates with an average size of 1.0 μm or more is excessively reduced, M 23 Carbides such as C6 precipitate finely, which in turn refines the grains, so the number density of Nb-containing precipitates with an average size of 1.0 μm or more is 1 mm2 The lower limit is 150 pieces per mm. 2 There are more than 200 per

[0048] The ferritic stainless steel sheet according to one embodiment of the present invention may contain Nb-containing precipitates having an average size of 2.0 μm or more. The larger the average size of the Nb-containing precipitates, the greater the effect of fixing C (and N). The number density may be calculated for Nb-containing precipitates having an average size of 1.0 μm or more and 10.0 μm or less, or for Nb-containing precipitates having an average size of 1.0 μm or more and 5.0 μm or less.

[0049] As described above, by controlling the precipitation morphology of Nb-containing precipitates, it is possible to appropriately coarsen the crystal grains, which results in a reduction in yield point elongation, suppresses the occurrence of stretcher strain during processing, and improves the surface quality after processing.

[0050] The ferritic stainless steel sheet according to one embodiment of the present invention may have an average grain size of 10 μm or more, preferably 12 μm or more, and more preferably 15 μm or more. There is no particular upper limit to the average grain size. The ferritic stainless steel sheet according to one embodiment of the present invention may have an average grain size of, for example, 10 μm or more and 30 μm or less.

[0051] (3) Manufacturing method A method for producing a ferritic stainless steel sheet according to one embodiment of the present invention will be described below.

[0052] <Hot rolling process> After heating the stainless steel slab, it is subjected to hot rolling (hot rolling) consisting of rough rolling and finish rolling to produce a hot-rolled sheet.

[0053] If the slab heating temperature exceeds 1250°C, the crystal grains will significantly enlarge during slab heating, forming coarse elongated grains that are long in the rolling direction during hot rolling. This can lead to surface defects such as scabs, edge cracks, and poor surface quality in the finished sheet. Furthermore, if the slab heating temperature is less than 1100°C, surface defects can occur, leading to poor corrosion resistance due to rusting from the defects. Therefore, the slab heating temperature is preferably 1100 to 1250°C. Furthermore, considering the reduction in productivity due to roll seizure, the slab heating temperature is preferably 1130 to 1230°C. After the above-described slab heating, hot rolling consisting of rough hot rolling and finish hot rolling is performed, and the slab is then coiled to form a hot roll (hot-rolled sheet). The hot rolling finish temperature (the surface temperature at the exit of the final rolling stand in finish hot rolling) is preferably 650°C or higher, more preferably 700°C or higher, because an excessively low temperature can cause surface defects such as seizure.

[0054] <Hot-rolled sheet annealing process> In the method for producing a ferritic stainless steel sheet according to this embodiment, after the hot rolling step, a hot-rolled sheet annealing step is performed in which the obtained hot-rolled sheet is annealed. When the hot-rolled sheet annealing step is performed by box annealing, the temperature increase and cooling are relatively slow, which can make it difficult to control the structure. For this reason, the hot-rolled sheet annealing is preferably performed by continuous annealing, which can improve productivity and also make it easier to control the temperature (i.e., the structure). By causing sufficient recrystallization in the hot-rolled sheet annealing step, the coarse hot-rolled structure is refined, improving ridging resistance.

[0055] Furthermore, in the method for producing a ferritic stainless steel sheet according to this embodiment, the hot-rolled sheet is annealed in the ferrite-austenite two-phase region, which allows a mixed structure of ferrite and martensite to be obtained after the hot-rolled sheet is annealed. In the subsequent cold-rolling process, the hard martensite structure disrupts the ferrite phase into random orientations, further improving ridging resistance.

[0056] Additionally, in the method for producing a ferritic stainless steel sheet according to this embodiment, NbC is coarsely precipitated in the hot-rolled sheet annealing step, which reduces the pinning force during the subsequent cold-rolled sheet annealing step, leading to coarsening of crystal grains.

[0057] In the hot-rolled sheet annealing process, if the annealing temperature is low, the hot-rolled sheet is likely to be non-recrystallized and the martensite structure ratio is likely to be low, making it difficult to obtain sufficient ridging resistance. Therefore, in the hot-rolled sheet annealing process, annealing is preferably performed at Ac1+20°C or higher.

[0058] On the other hand, if the annealing temperature of the hot-rolled sheet is too high, the martensite structure ratio tends to be low, and sufficient ridging resistance cannot be obtained. In addition, if the annealing temperature of the hot-rolled sheet is too high, NbC will form a solid solution, and during the subsequent annealing of the cold-rolled sheet, NbC will precipitate finely, resulting in finer grains and an increase in yield point elongation. Therefore, in the hot-rolled sheet annealing process, annealing is preferably performed at Ac1 + 120°C or less.

[0059] Ac1 is the temperature at which the phase transformation to the austenite phase begins upon heating, and is herein represented by the following formula (3): Ac1=310+35(Cr+1.72Mo+2.09Si+4.86Nb+8.29V+1.77Ti+21.4Al+40B-7.14C-8N-3.28Ni-1.89Mn-0.51Cu)... Formula (3) Here, the content (mass%) of each element is substituted for the element symbol in formula (3), and 0 (zero) is substituted for elements that are not added.

[0060] <Descaling process> In the case where surface scale is formed in the above-mentioned hot-rolled sheet annealing step and problems such as surface defects occur in subsequent steps, it is preferable to carry out a descaling treatment such as pickling as necessary.

[0061] <Cold rolling process> Following a descaling process that is performed as needed, cold rolling (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, the cold rolling reduction CR 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 of the cold rolling reduction CR is set to 85%. The cold rolling reduction CR is preferably 80% or less.

[0062] <Cold-rolled sheet annealing process> After the above-described cold rolling, cold-rolled sheet annealing (cold rolling annealing) for the purpose of recrystallization and grain growth is preferably performed in a temperature range of "Ac1-50°C" or more and Ac1 or less. If the annealing temperature in the cold-rolled sheet annealing process is less than "Ac1-50°C", not only is an unrecrystallized structure likely to result, but the martensite structure remains, making it impossible to obtain sufficient ductility. If the annealing temperature in the cold-rolled sheet annealing process is higher than Ac1, the two-phase region of ferrite + austenite is reached, resulting in a mixed structure of ferrite + martensite after cooling, making it impossible to obtain sufficient ductility.

[0063] Furthermore, in the cold-rolled sheet annealing step, from the viewpoint of ensuring ductility, the holding (soaking) time within the above temperature range is preferably 120 seconds or more. The holding time is more preferably 300 seconds or more. By performing the cold-rolled sheet annealing step, the ferritic stainless steel sheet of this embodiment is obtained. The cold-rolled sheet annealing step may be a final annealing step.

[0064] [Additional notes] The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in the above description are also included in the technical scope of the present invention. [Example]

[0065] An embodiment of the present invention will be described below.

[0066] The stainless steels shown in Table 1 were melted and hot-rolled, hot-rolled sheet annealed, and cold-rolled under the conditions shown in Table 2, followed by cold-rolled sheet annealing to produce product sheets. In Table 1, the values of γmaz and γH are the values calculated using the above-mentioned formulas (1) and (2), respectively (values rounded to one decimal place). In Table 1, * indicates that the value is outside the range specified in the present invention. In Table 2, underlines indicate that the value is outside the preferred range of manufacturing conditions.

[0067] [Table 1]

[0068] [Table 2]

[0069] <Organizational evaluation method> [Number density of Nb-containing precipitates with an average size of 1.0 μm or more] A cross section of a cold-rolled, annealed steel sheet (product 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. Then, five fields of view were photographed using an electron microscope at 7000x magnification. The number of Nb-containing precipitates with an average size of 1.0 μm or more was counted and divided by the photographed area to calculate the number density. Among the precipitates in the microstructure photograph, precipitates in which Nb was detected by EDS measurement were defined as Nb-containing precipitates. The average size of each of the multiple Nb-containing precipitate images in the microstructure photograph was calculated by averaging the major and minor axes, where the length of the longest line segment connecting any two points on the periphery (outer edge) of the Nb-containing precipitate was defined as the major axis, and the longest line segment connecting the outer edges of the Nb-containing precipitates perpendicular to the major axis was defined as the minor axis.

[0070] [Average grain size] A cross section of the steel sheet (product sheet) after cold-rolling and annealing was cut along a plane parallel to the rolling direction and perpendicular to the rolling width direction, and then pretreatment such as etching using aqua regia was performed. The central part of the sheet thickness was then observed using an optical microscope, and the grain size was determined using the cutting method specified in JIS G0551:2013, and this was taken as the average grain size.

[0071] <Characteristics evaluation method> [Ridging resistance] A No. 5 tensile test piece, as specified in JIS Z 2201:1998, was taken from the steel sheet (product sheet) after cold-rolling and annealing, with its longitudinal direction aligned with the rolling direction. A 16% tensile strain was applied to the No. 5 tensile test piece, with a gauge distance of 50 mm, so that the tensile direction was parallel to the rolling direction (a tensile test was performed). The surface texture of the tensile test piece (evaluation target material) after the tensile test was then measured using a surface roughness tester. Specifically, the waviness height was measured using a surface roughness tester for the portion between the gauge points of the evaluation target material, with a measurement length of 18 mm in the direction perpendicular to the rolling direction (in other words, the width direction of the evaluation target material). The waviness height is the average height of the waviness curve elements measured by the surface texture measurement specified in JIS B 0601:2001, etc. The cutoff values for the wavelength components were set at 5.0 mm for the upper limit and 0.8 mm for the lower limit (using a filter), and a waviness curve with wavelength components of 0.8 mm to 5.0 mm was determined. The reference length was set to 18 mm. The average height (μm) of the measured waviness curve elements was taken as the "ridging height." In assessing the ridging resistance, a ridging height of 12 μm or less, which does not significantly impair the appearance due to the uneven pattern caused by ridging after processing, was considered to be acceptable.

[0072] [Workability (uniform elongation)] Tensile test pieces of JIS No. 13 type B in the rolling direction were prepared. Furthermore, tensile tests were carried out on these tensile test pieces using a tensile testing machine to measure the uniform elongation (plastic elongation at maximum load). The evaluation of workability (uniform elongation) was deemed to pass if it was 15% or more, which is necessary for processing applications.

[0073] [Workability (yield elongation)] Tensile test pieces of JIS No. 13 type B in the rolling direction were prepared. Furthermore, tensile tests were carried out on these tensile test pieces using a tensile tester to measure the yield point elongation. A yield point elongation of 1.5 or less was considered to be acceptable.

[0074] <Evaluation results> The results of evaluating the structure and properties of each test material of the invention examples and comparative examples are shown in Table 3. In Table 3, * indicates that the value is outside the range specified by the present invention.

[0075] [Table 3]

[0076] In Examples C1 to C18, the ratio of the number of Nb carbides to the total number of Nb-containing precipitates in the microstructure photograph was 95% or more. The definition of Nb carbide is as described above. On the other hand, in comparative examples whose chemical compositions are outside the range specified in the present invention, the ratio of the number of Nb carbides to the total number of Nb-containing precipitates in the microstructure photograph may be less than 95%. In particular, comparative steels whose chemical compositions contain relatively high amounts of Nb, Ti, and V and relatively low amounts of C tend to have this ratio less than 95%.

[0077] As shown in Examples C1 to C18, by appropriately controlling the chemical components (chemical composition), γmax, γH, and the number density of Nb-containing precipitates with an average size of 1.0 μm or more, it was possible to control the uniform elongation in addition to the yield point elongation and ridging characteristics, and to obtain ferritic stainless steel sheets with excellent surface quality after processing without reducing productivity.

[0078] In Comparative Examples c1 and c13 (chemical compositions of Steel Nos. a1 and a13), the grains were refined and the yield point elongation was high because the C and Nb contents were outside the specified ranges. In Comparative Examples c2 and c12 (chemical compositions of Steel Nos. a2 and a12), the ridging resistance was deteriorated because the C and Nb contents were outside the specified ranges.

[0079] Comparative Examples c3, c4, c5, c9, and c10 had low elongation and large yield elongation because the Si, Mn, P, Al, and N contents were outside the specified ranges. Comparative Example c6 had excessive S, which caused the sheet to crack during hot rolling, making it impossible to produce a cold-rolled annealed steel sheet. Comparative Example c7 had a high Cr content and a low cold rolling rate, resulting in an unrecrystallized structure and low uniform elongation. Comparative Example c8 had a low Cr content and a high cold-rolled sheet annealing temperature, and Comparative Example c11 had low N content, resulting in a ferrite + martensite structure and low uniform elongation.

[0080] Comparative Example c14 had low uniform elongation and high yield point elongation due to its high γmax. Comparative Example c15 had low γmax and low ridging resistance. Comparative Example c16 had low ridging resistance due to its γH being outside the specified range, which caused surface defects such as scabs during hot rolling and also due to the hot-rolled sheet annealing temperature being outside the preferred range. [Industrial Applicability]

[0081] According to the present invention, a cold-rolled annealed ferritic stainless steel sheet (ferritic stainless steel sheet) that is less likely to develop stretcher strain or ridging patterns due to working can be provided without reducing productivity, and is therefore useful. Therefore, the ferritic stainless steel sheet of the present invention is suitably applied to applications requiring a good appearance after forming.

Claims

1. The steel sheet contains, in mass%, C: 0.030% or more and 0.060% 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: 0.20% or less, N: 0.010% or more and 0.050% or less, Nb: 0.03% or more and 0.10% or less, with the balance being Fe and impurities; The γmax value shown in the following formula (1) is 10 or more and 35 or less, The γH value shown in the following formula (2) is 5 or more, The number density of Nb-containing precipitates with an average size of 1.0 μm or more is 1 mm ２ 150 to 1000 defects per square meter. γmax=420C+470N+23Ni+9Cu+7Mn-11.5Cr-11.5Si-12Mo-23V-47Nb-49Ti-52Al+189... Formula (1) γH=506.0C+143.8N+2.3Mn+9.6Ni+5.4Cu-4.0Cr-4.3Si-48.1Al-13.5P-4.5S-3.8Mo-8.6Nb-6.8B-7.0V+52.18... Formula (2)

2. The ferritic stainless steel sheet according to claim 1, further comprising one or more elements selected from the group consisting of the following groups A, B, C, and D: [Group A] One or more selected from the group consisting 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 0.500% or less, and Sb: 0.005% or more and 0.500% or less. [Group B] One or more 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 [Group C] One or two selected from the group consisting of Ti: 0.05% or less and V: 0.05% or less [Group D] One or more 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.

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