Ferritic stainless steel hot rolled sheet and its manufacturing method

Abstract

To provide a ferritic stainless steel sheet having good workability and surface properties.SOLUTION: There is provided a ferritic stainless hot steel sheet which has a chemical composition comprising, by mass%, 0.030% or less of C, 1.00% or less of Si, 2.00% or less of Mn, 11.0 to 30.0% of Cr, 0.005 to 0.100% of P. 0.0100% or less of S. 0.005 to 1.00% of Al. 0.030% or less of N, one or more selected from Ti and Nb, an arbitrary element and the balance Fe with inevitable impurities, wherein in the metal structure, the recrystallization rate is 95% or more, the precipitation amount Pp of P is 0.005% or more and the maximum size of the P-containing precipitates is 0.2 to 1.0 μm.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a hot-rolled ferritic stainless steel sheet, a method for producing the same, and a cold-rolled ferritic steel sheet. [Background technology]

[0002] Ferritic stainless steels have excellent corrosion resistance and are therefore used in applications in corrosive environments, such as automobile exhaust parts. In particular, in recent years, they have been used in products such as kitchen utensils and utensils, and home appliances, where rust prevention is essential for product appearance.

[0003] These products often have complex shapes, so the ferritic stainless steel used as the material must have not only corrosion resistance but also high formability.

[0004] As a ferritic stainless steel sheet with improved formability, Patent Document 1 discloses a ferritic stainless steel sheet with controlled crystal grain size, etc. Non-Patent Document 1 also discloses an IF stainless steel sheet, which is a type of ferritic stainless steel, that has improved formability by containing Ti and / or Nb to fix C and N. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2019 / 188094 [Non-patent literature]

[0006] [Non-Patent Document 1] Sawatani et al., and three others, "Workability of Ti-added low C, N-17%Cr stainless steel sheet", Iron and Steel, No. 5, 63rd year, (1977), pp. 832-842 Summary of the Invention [Problem to be solved by the invention]

[0007] The IF stainless steel sheet disclosed in Non-Patent Document 1 is annealed at a high temperature during cold-rolled steel sheet annealing to improve formability. While this method is not performed on steel sheets with fine grains, as in Patent Document 1, it is a commonly used method that can improve the r-value by growing the grains. High-temperature annealing reduces the high-temperature strength in the annealing furnace, and plastic deformation may occur due to tension applied to the sheet in the furnace. This may result in poor surface quality, such as the appearance of patterns on the surface. This problem is particularly likely to occur when high-temperature annealing is performed in a highly productive vertical furnace, or with thin steel sheets, which are more likely to achieve a high r-value.

[0008] On the other hand, if the annealing temperature is reduced in order to ensure good surface quality, the annealing time becomes long, which reduces manufacturability. Thus, there is a problem in that it is difficult to improve the r-value and formability while also ensuring good surface quality.

[0009] An object of the present invention is to solve the above problems and to provide a ferritic stainless steel sheet that is excellent in manufacturability and has good formability and surface properties. [Means for solving the problem]

[0010] The present invention has been made to solve the above-mentioned problems, and is summarized as the following ferritic stainless steel sheet and method for producing the same.

[0011] (1) Chemical composition, in mass%, C: 0.030% or less, Si: 1.00% or less, Mn: 2.00% or less, Cr: 11.0~30.0%, P: 0.005~0.100%, S: 0.0100% or less, Al: 0.005 to 1.00%, N: 0.030% or less, and Ti and Nb and one or more selected from Ti: 0 to 0.50% Nb: 0 to 1.00%, Sn: 0 to 0.50% Ni: 0 to 1.00% Cu: 0-1.00% Mo: 0-2.00%, W: 0~1.00%, Co: 0 to 0.50% V: 0~0.50%, Zr: 0 to 0.50% Sb: 0 to 0.50% B: 0~0.0025%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, Y: 0~0.20%, Hf: 0~0.20%, REM: 0~0.10%, The balance is Fe and impurities. In metal structure, The recrystallization rate is 95% or more, The amount of precipitated P (Pp) is 0.005% or more, A ferritic stainless hot-rolled steel sheet in which the maximum size of phosphorus-containing precipitates is 0.2 to 1.0 μm.

[0012] (2) The chemical composition is in mass%: Sn: 0.005 to 0.50% Ni: 0.05 to 1.00%, Cu: 0.05 to 1.00%, Mo: 0.05 to 2.00%, W: 0.05 to 1.00%, Co: 0.05 to 0.50%, V: 0.05 to 0.50%, Zr: 0.05 to 0.50%, and Sb: 0.005 to 0.50% The hot-rolled ferritic stainless steel sheet according to (1) above, which contains one or more selected from the following and satisfies the following formula (i): 0.03≦Nb+Ti (i) However, each element symbol in the above formula represents the content (mass%) of each element contained in the steel, and if the element is not contained, it is set to zero.

[0013] (3) The chemical composition is in mass%: B: 0.0001~0.0025%, Ca: 0.0002 to 0.0050%, and Mg: 0.0002 to 0.0050%, The hot-rolled ferritic stainless steel sheet according to (1) or (2) above, containing one or more selected from the following:

[0014] (4) The chemical composition is in mass%: Y: 0.001 to 0.20%, Hf: 0.001 to 0.20%, and REM: 0.001~0.10%, The hot-rolled ferritic stainless steel sheet according to any one of (1) to (3) above, which contains one or more selected from the following:

[0015] (5) A method for producing a ferritic stainless steel hot-rolled steel sheet according to any one of (1) to (4) above, A step of subjecting a slab having the chemical composition described in any one of (1) to (4) above to hot rolling including rough rolling and finish rolling; After the finish rolling in the hot rolling, a step of performing heat treatment at a heat treatment temperature T in the range of 700 to 830 ° C. for a heat treatment time t that satisfies the following formulas (ii) and (iii): and cooling the heat-treated steel sheet to 400°C or less within 30 minutes after the heat treatment. 1410≦A≦1450 (ii) A = T × log(20 + t) (iii) In the above formula, each symbol is defined as follows: A: Constant T: Heat treatment temperature (K) t: Heat treatment time (hr)

[0016] (6) A cold-rolled steel sheet using the hot-rolled steel sheet according to any one of (1) to (4) above, The average r value is 1.4 or more, A ferritic stainless steel cold-rolled sheet having an arithmetic mean waviness Wa of 0.30 μm or less. [Effects of the Invention]

[0017] According to the present invention, a ferritic stainless steel sheet can be obtained which is excellent in manufacturability and has good formability and surface properties. DETAILED DESCRIPTION OF THE INVENTION

[0018] The present inventors have carefully investigated the relationship between the annealing temperature in the annealing process after cold rolling of ferritic stainless steel sheet, the state of surface pattern generation, and the metal structure, and have obtained the following findings (a) to (c).

[0019] (a) The state of occurrence of surface patterns is strongly dependent not only on the annealing temperature but also on the state of phosphide precipitation before cold rolling. The present inventors have found that phosphide precipitation is effective for high-temperature annealing.

[0020] The above findings are significant, as phosphides have traditionally been thought to contribute little to improving strength at high temperatures. In contrast to precipitates containing elements such as Nb and Mo, which contribute particularly effectively at high temperatures, phosphides precipitate, dissolve, and grow into grains earlier. As a result, when soaked at high temperatures for approximately 10 minutes, phosphides undergo the above process to become coarse precipitates, which no longer contribute to strengthening the steel. Therefore, phosphides can maintain their strength even at high temperatures during short-term heat treatments, such as annealing cold-rolled steel sheets.

[0021] Therefore, by allowing the phosphide to be sufficiently precipitated in each step before cold rolling, high-temperature strength can be ensured during annealing performed after cold rolling, and the occurrence of surface patterns can be suppressed even when annealing is performed at a high temperature.

[0022] (b) As described above, it is effective to sufficiently promote the precipitation of phosphides in processes prior to cold rolling, such as the heat treatment and coiling process of the hot-rolled steel sheet. However, if the phosphides become coarse, the toughness of the cold-rolled steel sheet will decrease. On the other hand, if the phosphides are too fine, they will pin the movement of grain boundaries and suppress grain growth during annealing after cold rolling. As a result, recrystallization is suppressed and the surface properties deteriorate. For this reason, it is necessary to control the size of the phosphides within a certain range. Therefore, it is desirable to precipitate the phosphides while controlling their shape during an appropriate treatment time.

[0023] (c) At this time, it is effective not only to simply precipitate phosphides but also to sufficiently promote recrystallization. In other words, it is effective to perform a heat treatment that promotes recrystallization while precipitating phosphides of an appropriate size, followed by cold rolling and annealing after cold rolling. This method improves productivity and makes it possible to produce ferritic stainless steel sheets with excellent surface properties and workability.

[0024] The present invention has been made based on the above findings. Each feature of one embodiment of the present invention will be described in detail below.

[0025] The ferritic stainless steel sheet of this embodiment includes both a hot-rolled steel sheet and a cold-rolled steel sheet.

[0026] 1.Hot rolled steel plate 1-1. Chemical composition of hot-rolled steel sheets The reasons for limiting each element are as follows. In the following description, "%" for the content means "% by mass." The chemical composition of the hot-rolled steel sheet remains the same even after it has been subjected to a cold-rolling process or the like to become a cold-rolled steel sheet.

[0027] C: 0.030% or less Since C is an element that reduces the r-value, which is an index of formability, it is preferable to reduce its content. Therefore, the C content is set to 0.030% or less. From the viewpoint of formability, the C content is preferably set to 0.018% or less. However, excessive reduction of C leads to an increase in refining costs, so the C content is preferably set to 0.001% or more, and more preferably 0.002% or more.

[0028] Si: 1.00% or less Although Si is an element that improves oxidation resistance, excessive inclusion of Si reduces formability. Therefore, the Si content is set to 1.00% or less. From the viewpoint of formability, the Si content is preferably set to 0.30% or less. However, excessive reduction of Si increases raw material costs. Therefore, the Si content is preferably set to 0.01% or more, and more preferably set to 0.05% or more.

[0029] Mn: 2.00% or less Like Si, Mn, when contained in large amounts, reduces formability. Therefore, the Mn content is set to 2.00% or less. From the viewpoint of formability, the Mn content is preferably set to 0.30% or less. On the other hand, excessive reduction of Mn increases raw material costs. Therefore, the Mn content is preferably set to 0.01% or more, and more preferably set to 0.05% or more.

[0030] Cr: 11.0 to 30.0% Cr is an element that improves corrosion resistance, which is a basic property of stainless steel. If the Cr content is less than 11.0%, sufficient corrosion resistance cannot be obtained. Therefore, the Cr content is set to 11.0% or more. From the viewpoint of corrosion resistance, the Cr content is more preferably set to 14.0% or more, and even more preferably set to 16.0% or more. moreover However, excessive Cr content promotes the formation of intermetallic compounds such as the σ phase, which leads to cracking during manufacturing and reduced formability. Therefore, the Cr content is set to 30.0% or less. From the viewpoint of stable manufacturing (yield, rolling defects, etc.), the Cr content is preferably set to 25.0% or less, and more preferably 20.0% or less.

[0031] P: 0.005 to 0.100% P is an element that constitutes phosphide. In order to obtain good surface properties by precipitation of phosphide, the P content is set to 0.005% or more. From the viewpoint of raw material costs, the P content is preferably set to 0.010% or more, and more preferably set to 0.020% or more.

[0032] However, excessive P content reduces formability (r-value and product elongation). Therefore, the P content is set to 0.100% or less. From the viewpoint of formability, the P content is preferably set to 0.070% or less, and more preferably set to 0.050% or less.

[0033] S: 0.0100% or less S is an impurity element that promotes cracking during manufacturing. Therefore, the S content is set to 0.0100% or less. From the viewpoint of manufacturability, the S content is preferably set to 0.0030% or less, and more preferably set to 0.0020% or less. However, excessive reduction of S increases refining costs. Therefore, the S content is preferably set to 0.0003% or more, and more preferably set to 0.0004% or more.

[0034] Al: 0.005 to 1.00% Al is an element effective in improving corrosion resistance or oxidation resistance. Therefore, the Al content is set to 0.005% or more. The Al content is preferably set to 0.010% or more. However, excessive Al content not only reduces formability but also increases alloy costs and reduces manufacturability. Therefore, the Al content is set to 1.00% or less.

[0035] N: 0.030% or less Like C, N is an element that reduces formability (r-value). For this reason, the N content is set to 0.030% or less. From the viewpoint of formability, the N content is preferably set to 0.015% or less. On the other hand, excessive reduction of N increases refining costs. For this reason, the N content is preferably set to 0.002% or more. From the viewpoint of refining costs, the N content is more preferably set to 0.005% or more.

[0036] Ti and Nb One or more selected from The ferritic stainless hot-rolled steel sheet of this embodiment contains Ti and Nb and one or more selected from the following: Preferably, the following formula (i) is satisfied:

[0037] Ti: 0 to 0.50% Ti has the effect of fixing C and N as precipitates and purifying the steel, thereby improving the r-value and product elongation, and improving formability. It also has the effect of fixing P as precipitates and improving high-temperature strength in the annealing process after cold rolling. Therefore, Ti may be added as needed.

[0038] However, excessive Ti content increases alloy costs and reduces manufacturability due to an increase in the recrystallization temperature. Therefore, the Ti content is set to 0.50% or less. From the viewpoints of alloy costs and manufacturability, the Ti content is preferably set to 0.40% or less, and more preferably set to 0.30% or less.

[0039] On the other hand, in order to obtain the above effects, the Ti content is preferably 0.03% or more. From the viewpoint of formability and ensuring high-temperature strength in the annealing process after cold rolling, the Ti content is more preferably 0.05% or more, and even more preferably 0.10% or more. Note that Ti, together with Nb described later, preferably satisfies the following formula (i):

[0040] Nb: 0 to 1.00% Like Ti, Nb also acts as a stabilizing element that fixes C and N, and by purifying the steel, it improves the r-value and product elongation, thereby improving formability. It also fixes P as precipitates, ensuring high-temperature strength in the annealing process after cold rolling. Therefore, Nb may be added as needed.

[0041] However, excessive Nb content increases alloy costs and reduces manufacturability due to an increase in the recrystallization temperature. Therefore, the Nb content is set to 1.00% or less. From the viewpoints of alloy costs and manufacturability, the Nb content is preferably set to 0.50% or less, and more preferably set to 0.30% or less.

[0042] On the other hand, in order to obtain the above effects, the Nb content is preferably 0.03% or more. From the viewpoint of formability and ensuring high-temperature strength in the annealing process after cold rolling, the Nb content is preferably 0.04% or more. It is preferable that Nb, together with Ti, satisfy the following formula (i):

[0043] 0.03≦Nb+Ti (i) However, each element symbol in the above formula represents the content (mass%) of each element contained in the steel, and if the element is not contained, it is set to zero.

[0044] In addition to the above elements, one or more elements (A group elements) selected from Sn, Ni, Cu, Mo, W, Co, V, Zr, and Sb may be contained within the ranges shown below. The reasons for limiting each element will be explained below.

[0045] Sn: 0 to 0.50% Sn has the effect of enhancing corrosion resistance and oxidation resistance. Therefore, it may be contained as needed. However, excessive Sn content not only leads to a decrease in formability, but also increases alloy costs and decreases manufacturability. Therefore, the Sn content is set to 0.50% or less. On the other hand, to obtain the above effects, the Sn content is preferably set to 0.005% or more.

[0046] Ni: 0 to 1.00% Ni has the effect of enhancing corrosion resistance and oxidation resistance. Therefore, it may be contained as needed. However, excessive Ni content not only leads to a decrease in formability, but also increases alloy costs and reduces manufacturability. Therefore, the Ni content is set to 1.00% or less. On the other hand, to obtain the above effects, the Ni content is preferably set to 0.05% or more.

[0047] Cu: 0 to 1.00% Cu has the effect of enhancing corrosion resistance and oxidation resistance. Therefore, it may be contained as needed. However, excessive Cu content not only reduces formability, but also increases alloy costs and reduces manufacturability. Therefore, the Cu content is set to 1.00% or less. On the other hand, to obtain the above effects, the Cu content is preferably set to 0.05% or more.

[0048] Mo: 0 to 2.00% Mo has the effect of enhancing corrosion resistance and oxidation resistance. Therefore, it may be added as needed. However, excessive Mo content not only reduces formability, but also increases alloy costs and reduces manufacturability. Therefore, the Mo content is set to 2.00% or less. On the other hand, to obtain the above effects, the Mo content is preferably set to 0.05% or more.

[0049] W: 0 to 1.00% W has the effect of enhancing corrosion resistance and oxidation resistance. Therefore, it may be added as needed. However, excessive W content not only reduces formability, but also increases alloy costs and reduces manufacturability. Therefore, the W content is set to 1.00% or less. On the other hand, to obtain the above effects, the W content is preferably set to 0.05% or more.

[0050] Co: 0 to 0.50% Co has the effect of enhancing corrosion resistance and oxidation resistance. Therefore, it may be contained as needed. However, excessive Co content not only leads to a decrease in formability, but also increases alloy costs and decreases manufacturability. Therefore, the Co content is set to 0.50% or less. On the other hand, to obtain the above effects, the Co content is preferably set to 0.05% or more.

[0051] V: 0 to 0.50% V has the effect of enhancing corrosion resistance and oxidation resistance. Therefore, it may be contained as needed. However, excessive V content not only leads to a decrease in formability, but also increases alloy costs and decreases manufacturability. Therefore, the V content is set to 0.50% or less. On the other hand, to obtain the above effects, the V content is preferably set to 0.05% or more.

[0052] Zr: 0 to 0.50% Zr has the effect of enhancing corrosion resistance and oxidation resistance. Therefore, it may be contained as needed. However, excessive Zr content not only leads to a decrease in formability, but also increases alloy costs and decreases manufacturability. Therefore, the Zr content is set to 0.50% or less. On the other hand, to obtain the above effects, the Zr content is preferably set to 0.05% or more.

[0053] Sb: 0 to 0.50% Sb has the effect of enhancing corrosion resistance and oxidation resistance. Therefore, it may be contained as needed. However, excessive Sb content not only reduces formability, but also increases alloy costs and reduces manufacturability. Therefore, the Sb content is set to 0.50% or less. On the other hand, to obtain the above effects, the Sb content is preferably set to 0.005% or more.

[0054] In addition to the above elements, one or more elements selected from B, Ca, and Mg (B group elements) may be contained within the ranges shown below. The reasons for limiting the amount of each element will be explained below.

[0055] B: 0 to 0.0025% B has the effect of improving hot workability and secondary workability, and may be added as necessary. However, excessive B content reduces manufacturability. Therefore, the B content is set to 0.0025% or less. From the viewpoint of manufacturability, the B content is preferably set to 0.0012% or less. On the other hand, in order to obtain the above effects, the B content is preferably set to 0.0001% or more, and more preferably set to 0.0003% or more.

[0056] Ca: 0 to 0.0050% Ca has the effect of improving hot workability and secondary workability, so it may be contained as needed. However, excessive Ca content reduces manufacturability. Therefore, the Ca content is set to 0.0050% or less. From the viewpoint of manufacturability, the Ca content is preferably set to 0.0010% or less. On the other hand, in order to obtain the above effects, the Ca content is preferably set to 0.0002% or more.

[0057] Mg: 0 to 0.0050% Mg has the effect of improving hot workability and secondary workability, so it may be added as needed. However, excessive Mg content reduces manufacturability. Therefore, the Mg content is set to 0.0050% or less. From the viewpoint of manufacturability, the Mg content is preferably set to 0.0010% or less. On the other hand, to obtain the above effects, the Mg content is preferably set to 0.0002% or more.

[0058] In addition to the above elements, one or more elements selected from Y, Hf, and REM (C group elements) may be contained within the ranges shown below. The reasons for limiting the amount of each element will be explained below.

[0059] Y: 0 to 0.20% Y has the effect of improving hot workability, cleanliness, and oxidation resistance of steel. Therefore, it may be added as needed. However, excessive Y content increases alloy costs. Therefore, the Y content is set to 0.20% or less. On the other hand, to obtain the above effects, the Y content is preferably set to 0.001% or more.

[0060] Hf: 0 to 0.20% Hf has the effect of improving hot workability, cleanliness, and oxidation resistance of steel. Therefore, it may be added as needed. However, excessive Hf content increases alloy costs. Therefore, the Hf content is set to 0.20% or less. On the other hand, to obtain the above effects, the Hf content is preferably set to 0.001% or more.

[0061] REM: 0 to 0.10% REM has the effect of improving hot workability, cleanliness, and oxidation resistance of steel. Therefore, it may be added as needed. However, excessive REM content increases alloy costs. Therefore, the REM content is set to 0.10% or less. On the other hand, to obtain the above effects, the REM content is preferably set to 0.001% or more.

[0062] REM refers to a total of 17 elements, including Sc, Y, and lanthanides, and the REM content above refers to the total content of these elements. In industry, REM is often added in the form of misch metal.

[0063] In the chemical composition of the hot-rolled steel sheet according to this embodiment, the balance is Fe and impurities. Here, "impurities" refer to components that are mixed in during industrial steel production due to various factors in raw materials such as ores and scrap, or in the manufacturing process, and are acceptable within a range that does not impair the steel's effectiveness. Examples of impurities include Bi, Pb, Se, H, and Ta, and these elements may be contained within the ranges of Bi≦100 ppm, Pb≦100 ppm, Se≦100 ppm, H≦100 ppm, and Ta≦500 ppm.

[0064] 1-2. Metal structure of hot-rolled steel sheets The metal structure of the hot-rolled steel sheet of this embodiment is a ferrite structure. That is, by using a ferritic stainless steel hot-rolled steel sheet, raw material costs can be reduced, the r-value can be improved, and formability can be ensured. In addition, a decrease in yield due to edge cracks or the like that occur during manufacturing can be suppressed.

[0065] 1-2-1. Recrystallization rate If hot-rolled steel sheets are cold-rolled without sufficient recrystallization, unevenness parallel to the rolling direction, called roping, will be generated during cold rolling. Then, in subsequent processes, the convex portions will rub against the rolls of the manufacturing equipment, creating patterns. Table The surface quality will be further deteriorated.

[0066] Therefore, by performing a recrystallization treatment to promote recrystallization of the hot-rolled steel sheet and increasing the recrystallization rate, the occurrence of roping can be reduced and the surface quality can be improved. For this reason, the recrystallization rate of the hot-rolled steel sheet is set to 95% or more. To further suppress the deterioration of surface quality due to roping, the recrystallization rate of the hot-rolled steel sheet is preferably set to 97% or more.

[0067] Here, the recrystallization rate refers to the ratio (%) of the area of recrystallized grains to the measured area. When calculating the recrystallization rate, a cross section (L cross section) parallel to the rolling direction of the hot-rolled steel sheet is etched, and then the structure is observed using an optical microscope at a magnification of 50 times. 2 The above measurement areas are set, and the area of elongated crystal grains with an aspect ratio of 5 or more is defined as the unrecrystallized area, while the area of regular crystal grains with an aspect ratio of less than 5 is defined as the recrystallization rate, with the area ratio (%) of the recrystallized area being defined as the major axis / minor axis.

[0068] 1-2-2.P-containing precipitates The hot-rolled steel sheet of this embodiment contains phosphides, i.e., P-containing precipitates. As described above, it is effective to allow the P-containing precipitates to precipitate sufficiently before cold rolling. It is also desirable to control the size of the P-containing precipitates within an appropriate range. This improves the strength and improves the surface properties of the cold-rolled steel sheet during annealing after cold rolling. Therefore, the precipitation amount and size of the P-containing precipitates are controlled within the following ranges.

[0069] (a) Amount of precipitation The amount of precipitated P Pp in a hot-rolled steel sheet is an index showing the amount of P present as precipitates containing P. That is, the greater the amount of precipitated P Pp, the greater the amount of precipitated P.

[0070] Therefore, in the hot-rolled steel sheet of this embodiment, the amount of precipitated P, Pp, is set to 0.005% or more by mass. If the amount of precipitated P, Pp, is less than 0.005%, the effect of improving the surface quality of the P-containing precipitates cannot be sufficiently obtained. Therefore, the amount of precipitated P, Pp, is set to 0.005% or more. The amount of precipitated P, Pp, is preferably set to 0.007% or more, and more preferably set to 0.010% or more.

[0071] The amount of P precipitated, Pp, is measured using the following procedure. Steel plates cut into approximately 30 mm squares are wet-polished on all sides using #600 polishing. 1 g of the plate is then electrolyzed at a constant potential of -100 mV in a methanol solution of 10% maleic anhydride and 2% tetramethylammonium chloride. This dissolves the stainless steel base material, leaving other precipitates behind. The remaining undissolved precipitates are captured using a 200 μm mesh filter, washed with pure water, and dried. Then, they are dissolved using aqua regia and perchloric acid and subjected to elemental analysis using ICP. Pp is calculated by dividing the resulting amount of P by the mass change of the sample due to electrolysis, i.e., the total weight of the dissolved base material and the precipitates contained therein (1 g electrolyzed).

[0072] (b) Maximum size of P-containing precipitates As described above, it is necessary to control the size of the P-containing precipitates within an appropriate range. Specifically, if the maximum size of the P-containing precipitates is less than 0.2 μm, they will dissolve in the annealing process after cold rolling, and will not contribute to improving the strength during annealing. In addition, the number of precipitates increases, which makes it easier to suppress the growth of crystal grains and reduces the r-value. Therefore, the maximum size of the P-containing precipitates is set to 0.2 μm or more. It is preferable that the maximum size of the P-containing precipitates is set to 0.3 μm or more.

[0073] On the other hand, if the maximum size of the P-containing precipitates exceeds 1.0 μm, the P-containing precipitates become fracture initiation points or crack propagation paths, deteriorating the toughness of the hot-rolled steel sheet. Therefore, the maximum size of the P-containing precipitates is set to 1.0 μm or less. The maximum size of the P-containing precipitates is preferably set to 0.8 μm or less.

[0074] The maximum size of the P-containing precipitates is calculated by the following procedure. Specifically, TEM observation is performed, and 20 fields of view, each about 10 μm × 10 μm in size, are observed. The largest average size of the long and short sides of the P-containing precipitates in the observed fields is determined as the maximum size of the P-containing precipitates. Whether or not a precipitate is a P-containing precipitate is determined using EDX.

[0075] 2.Cold rolled steel plate A cold-rolled steel sheet can be obtained by cold-rolling the above-mentioned hot-rolled steel sheet and annealing after cold rolling. This cold-rolled steel sheet has the same chemical composition as the above-mentioned hot-rolled steel sheet. The average r-value of this cold-rolled steel sheet is set to 1.4 or more. This is because an average r-value of 1.4 or more enables deep drawing with a drawing ratio of 2.3 or more. The average r-value is an index of workability and is calculated by the following formula (a).

[0076] Average r value = (r0 + 2r 45 +r 90 ) / 4 (a) However, each symbol in the above formula (a) is defined as follows. r0: r value in the rolling direction r 90 : r value in the direction perpendicular to the rolling direction r45 : r value in the 45-degree rolling direction

[0077] The average r-value is calculated using the plastic strain ratio test method in accordance with JIS Z 2254:2008.

[0078] Furthermore, this cold-rolled steel sheet has an arithmetic mean waviness Wa of 0.30 μm or less. If the arithmetic mean waviness Wa is 0.30 μm or less, the uneven pattern parallel to the rolling direction due to roping cannot be confirmed by visual observation. Here, the arithmetic mean waviness is an index that represents the degree of surface unevenness, and is calculated in accordance with JIS B 0601:2013.

[0079] Specifically, the surface profile of a reference length of 20 mm is measured on each measurement line, and a waviness curve with wavelength components of 0.8 to 2.5 mm is determined with cutoff values λf = 2.5 mm and λc = 0.8 mm. The arithmetic mean waviness Wa (μm) is calculated from the waviness curve for each of the five measurement lines. This test is performed on three test pieces, and the average value is used as the evaluation index.

[0080] The thickness of the cold-rolled steel sheet according to this embodiment is preferably 1.2 mm or less. If the thickness is 1.2 mm or less, the surface properties tend to deteriorate during annealing after cold rolling, particularly due to a decrease in high-temperature strength during annealing.

[0081] 3. Manufacturing method A preferred method for producing the ferritic stainless steel sheet of this embodiment will now be described. The ferritic stainless steel sheet of this embodiment can achieve the effects described above regardless of the production method, but can be stably produced, for example, by the following production method.

[0082] 3-1.Hot rolling A slab having the above chemical composition is heated and then subjected to hot rolling, which includes rough rolling and finish rolling, to produce a hot-rolled steel sheet. The heating temperature of the slab during hot-rolling is preferably 1100 to 1250°C. If the slab heating temperature is less than 1100°C, the low temperature makes surface defects more likely to occur. Furthermore, corrosion resistance deteriorates due to rusting from the defects. For this reason, the heating temperature of the slab is preferably 1100°C or higher. On the other hand, if the heating temperature of the slab exceeds 1250°C, Ti carbosulfide (Ti4C2S2) dissolves during heating, increasing the amount of solute carbon, which then reprecipitates during the hot rolling process, delaying recrystallization. As a result, roping is more likely to occur due to poor recrystallization after finish rolling.

[0083] Furthermore, the crystal grains grow significantly during slab heating, resulting in the formation of coarse elongated grains during the hot rolling process, which leads to poor workability and roping in the product sheet. Therefore, the slab heating temperature is preferably 1250°C or less. Considering the reduction in productivity due to roll seizure, the slab heating temperature is more preferably in the range of 1130 to 1230°C.

[0084] The total reduction rate in hot rolling is not particularly defined, but when the cold-rolled steel sheet according to this embodiment is to be produced, it is usually in the range of 95 to 99%.

[0085] 3-2.Heat treatment In the manufacturing method of this embodiment, after the completion of hot rolling, i.e., after finish rolling, it is preferable to maintain the temperature in a certain range to promote recrystallization of the hot-rolled steel sheet and to perform a precipitation treatment to sufficiently precipitate phosphides. After the completion of hot rolling, it is preferable to perform heat treatment at a heat treatment temperature T in the range of 700 to 830°C without decreasing the temperature below 650°C, for a heat treatment time t that satisfies the following formulas (ii) and (iii). That is, it is preferable to maintain the heat treatment temperature T in the range of 700 to 830°C for a time t so that the following A satisfies 1410 to 1450. Thereafter, it is preferable to cool to 400°C or less within 30 minutes. Note that the logarithm in the following formula (iii) is a common logarithm.

[0086] 1410≦A≦1450 (ii) A = T × log(20 + t) (iii) In the above formula, each symbol is defined as follows: A: Constant T: Heat treatment temperature (K) t: Heat treatment time (hr)

[0087] Here, if the steel is cooled to 650°C or below after the completion of finish hot rolling and then heated again for the purpose of recrystallization, fine phosphorus-containing precipitates will precipitate. Furthermore, the fine precipitation of phosphorus-containing precipitates will inhibit the progress of recrystallization. As a result, it becomes impossible to achieve a recrystallization rate of 95% or more. Therefore, before the temperature is lowered to 650°C or below, the steel is heated or held (soaked) and then heat-treated. As a result, the maximum size of the phosphorus-containing precipitates can be controlled to 0.2 μm or more.

[0088] Furthermore, if the heat treatment temperature T is less than 700°C, not only will recrystallization be insufficient, but fine P-containing precipitates will precipitate, inhibiting recrystallization. For this reason, the heat treatment temperature T is preferably 700°C or higher. This will enable a recrystallization rate of 95% or higher. The heat treatment temperature T is more preferably 750°C or higher. On the other hand, if the heat treatment temperature T is higher than 830°C, it will be difficult to achieve a P precipitation amount Pp of 0.005% or higher. For this reason, the heat treatment temperature T is preferably 830°C or lower. The heat treatment temperature T is more preferably 800°C or lower. Note that the temperature may be lowered once before holding, but as mentioned above, it is necessary to ensure that it does not fall below 650°C.

[0089] The appropriate range for the heat treatment time t varies depending on the temperature, so it is preferable that the heat treatment time t satisfies the above formulas (ii) and (iii). If A shown in formula (ii) is less than 1410, not only will the recrystallization rate of 95% or more not be satisfied, but the P content in the precipitates will be 0.005% or less. Therefore, A is preferably 1410 or more. On the other hand, if A exceeds 1450, the P-containing precipitates will grow coarsely, with the maximum size reaching 1.0 μm or more. Therefore, A is preferably 1450 or more.

[0090] By setting the heat treatment conditions as described above, the maximum size of the P-containing precipitates can be set within a predetermined range, and the amount of precipitated P Pp can also be adjusted.

[0091] After heat treatment, it is preferable to cool to 400°C or below within 30 minutes. If the cooling rate is slow in the temperature range above 400°C, the precipitates will become coarse and the hot-rolled sheet will become brittle at 475°C, resulting in a deterioration in toughness. The cooling method is not important, but since the center of the coil takes longer to cool, it is preferable to cool the entire coil to 400°C or below within the specified time by a method such as rewinding the coil and cooling it.

[0092] In addition, if surface scale is formed during the heat treatment and problems such as surface defects occur in subsequent processes, it is preferable to carry out a descaling treatment such as pickling as necessary.

[0093] 3-3.Cold rolling Next, the obtained heat-treated steel sheet is cold-rolled. In the annealing process described below, it is desirable to promote the recrystallization nucleation of {111} oriented grains, which have a high r-value. For this reason, it is preferable to set the cold-rolling reduction to 65% or more.

[0094] 3-4. Annealing After the cold rolling described above is performed to obtain a cold-rolled steel sheet, annealing is performed for the purpose of recrystallization and grain growth. The annealing temperature is not particularly specified, but it is preferable to perform annealing in the range of Ts + 50 to Ts + 120°C, where Ts is the recrystallization temperature. This is because if the annealing temperature is less than Ts + 50°C, recrystallization is not sufficiently promoted. On the other hand, if annealing is performed at a temperature exceeding Ts + 120°C, the crystal grain size becomes too large, resulting in significant surface roughness after processing. In addition, considering productivity in industrial production, it is preferable to keep the annealing time to 3 minutes or less. As a result, a good r-value can be obtained.

[0095] Ts refers to the lowest temperature at which the recrystallization rate is 98% or more when the material is held at the annealing temperature for 1 minute. To check Ts, for example, the annealing temperature can be changed in 10°C increments.

[0096] The ferritic stainless steel sheets (hot-rolled steel sheets and cold-rolled steel sheets) of this embodiment will be described in more detail below with reference to examples, but the present invention is not limited to these examples. [Example]

[0097] Stainless steels having the chemical compositions shown in Table 1 were produced by melting.

[0098] [Table 1]

[0099] Under the conditions shown in Table 2, the reduction ratio was 98%, and the hot rolling pressure Immediately after rolling, the steel sheets were continuously cooled, and then kept at the heat treatment temperature and time shown in Table 2, after which they were cooled to produce 4.0 mm thick hot-rolled ferritic stainless steel sheets. Subsequently, each hot-rolled steel sheet obtained in Table 2 was cold-rolled to produce a cold-rolled steel sheet, which was then annealed for 1 minute at the temperature shown in Table 3 to produce a cold-rolled ferritic stainless steel sheet. The properties of the obtained hot-rolled steel sheets and cold-rolled steel sheets were measured by the following procedures.

[0100] (Recrystallization rate of hot-rolled steel sheet) The recrystallization rate was calculated using the following procedure. When calculating the recrystallization rate, a cross section (L cross section) parallel to the rolling direction of the hot-rolled steel sheet was etched, and then the structure was observed using an optical microscope at a magnification of 50 times. 2 The above measurement areas were set, and the area of elongated crystal grains with an aspect ratio of 5 or more was defined as the unrecrystallized area, and the area of regular crystal grains with an aspect ratio of less than 5 was defined as the recrystallization rate, with the area ratio (%) of the recrystallized area being defined as the recrystallization rate.

[0101] (P precipitate amount Pp in hot-rolled steel sheet) The amount of Pp in the precipitate was determined by cutting steel sheets approximately 30 mm square, wet-polishing the entire surface with #600 polishing paper, and then electrolyzing 1 g of the precipitate in a methanol solution of 10% maleic anhydride and 2% tetramethylammonium chloride at a constant potential of -100 mV. The precipitate that remained undissolved was captured using a 200 μm mesh filter, washed with pure water, dried, and then dissolved in aqua regia and perchloric acid for elemental analysis using ICP. Pp was calculated by dividing the resulting P amount by the mass change of the sample due to electrolysis (1 g electrolyzed).

[0102] (Maximum size of phosphorus-containing precipitates in hot-rolled steel sheets) The maximum size of the P-containing precipitates was determined by TEM observation, in which 20 fields of approximately 10 μm × 10 μm were observed. The largest average size of the long and short sides of the P-containing precipitates in the observed fields was determined as the maximum size of the P-containing precipitates. Whether or not the precipitates were P-containing precipitates was determined using EDX.

[0103] (Charpy impact value of hot-rolled steel sheet) The Charpy impact values, which serve as an index of manufacturability for hot-rolled steel sheets and cold-rolled steel sheets made from them, were measured. For the measurements, V-notch sub-size Charpy impact test specimens were prepared from the hot-rolled steel sheets with the same thickness as the hot-rolled steel sheets so that the longitudinal direction of the test specimens was parallel to the rolling direction, and five specimens were tested for each test specimen according to the test method described in JIS Z 2242:2018. The impact value was calculated by dividing the measured absorbed energy by the cross-sectional area, and the average value of the five specimens was calculated. From the viewpoint of being able to unwind the hot-rolled coil without any problems and having good manufacturability, a Charpy impact value of 15 J / cm was selected. 2 When the results were equal to or greater than this, the characteristics were deemed to be good and the sample was judged to be acceptable.

[0104] [Table 2]

[0105] (average r value of cold-rolled steel sheets) The average r-value of the cold-rolled steel sheet was calculated by the plastic strain ratio test method in accordance with JIS Z 2254: 2008. The following formula (a) is also defined in accordance with the above standard. Average r value = (r0 + 2r 45 +r 90 ) / 4 (a) However, each symbol in the above formula (a) is defined as follows. r0: r value in the rolling direction r 90 : r value in the direction perpendicular to the rolling direction r 45 : r value in the 45-degree rolling direction

[0106] (Arithmetic mean waviness Wa (μm)) The arithmetic mean waviness was calculated in accordance with JIS B 0601:2013. Specifically, a surface profile with a reference length of 20 mm was measured on each measurement line, and a waviness curve with wavelength components of 0.8 to 2.5 mm was determined with cutoff values λf = 2.5 mm and λc = 0.8 mm. The arithmetic mean waviness Wa (μm) was calculated from the waviness curve for each of the five measurement lines. This test was performed on three test pieces, and the average value was used as the evaluation index.

[0107] [Table 3]

[0108] As shown in the above examples, by performing heat treatment at a predetermined temperature and time immediately after hot rolling and appropriately controlling the recrystallization rate and the P precipitation amount Pp and size at the time of hot-rolled steel sheet production, it is possible to ensure the toughness of the hot-rolled steel sheet without any manufacturing problems. Furthermore, by cold-rolling this hot-rolled steel sheet and annealing the resulting cold-rolled steel sheet at a high temperature, a high r-value can be obtained. At the same time, the pattern caused by the decrease in high-temperature strength during annealing of the cold-rolled steel sheet does not occur, and the roping after annealing is low, at 0.30 μm or less, making it possible to obtain a steel sheet with excellent surface quality. [Industrial Applicability]

[0109] The present invention provides a hot-rolled steel sheet and a manufacturing method thereof that enable highly productive production of a ferritic stainless steel cold-rolled sheet having excellent formability and surface properties, and is therefore useful. Therefore, the ferritic stainless steel hot-rolled steel sheet of the present invention is suitably applied to the production of ferritic stainless steel cold-rolled sheets for forming applications.

Claims

1. The chemical composition, in mass%, is C: 0.030% or less, Si: 1.00% or less, Mn: 2.00% or less, Cr: 11.0-30.0%, P: 0.005-0.100%, S: 0.0100% or less, Al: 0.005-1.00%, N: 0.030% or less, and One or more selected from Ti and Nb, Ti: 0 to 0.50%, Nb: 0 to 1.00%, Sn: 0 to 0.50%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, Mo: 0-2.00%, W: 0-1.00%, Co: 0 to 0.50%, V: 0-0.50%, Zr: 0 to 0.50%, Sb: 0 to 0.50%, B: 0 to 0.0025%, Ca: 0-0.0050%, Mg: 0 to 0.0050%, Y: 0-0.20%, Hf: 0-0.20%, REM: 0-0.10%, The balance is Fe and impurities. In metal structure, The recrystallization rate is 95% or more, The amount of precipitated Pp is 0.005% or more, A ferritic stainless hot-rolled steel sheet in which the maximum size of phosphorus-containing precipitates is 0.2 to 1.0 μm.

2. The chemical composition is, in mass %, Sn: 0.005-0.50%, Ni: 0.05-1.00%, Cu: 0.05-1.00%, Mo: 0.05-2.00%, W: 0.05-1.00%, Co: 0.05-0.50%, V: 0.05-0.50%, Zr: 0.05 to 0.50%, and Sb: 0.005 to 0.50%, The ferritic stainless steel hot rolled sheet according to claim 1, which contains one or more selected from the following and satisfies the following formula (i): 0.03≦Nb+Ti...(i) In the above formula, each element symbol represents the content (mass%) of each element contained in the steel, and if the element is not contained, it is set to zero.

3. The chemical composition is, in mass %, B: 0.0001 to 0.0025%, Ca: 0.0002 to 0.0050%, and Mg: 0.0002 to 0.0050%, The ferritic stainless steel hot rolled sheet according to claim 1 or 2, comprising one or more selected from the following:

4. The chemical composition is, in mass %, Y: 0.001-0.20%, Hf: 0.001 to 0.20%, and REM: 0.001-0.10%, The ferritic stainless steel hot rolled sheet according to any one of claims 1 to 3, comprising one or more selected from the following:

5. The method for producing a ferritic stainless steel hot-rolled steel sheet according to any one of claims 1 to 4, A step of hot rolling a slab having the chemical composition according to any one of claims 1 to 4, comprising rough rolling and finish rolling; After the finish rolling in the hot rolling, a step of performing heat treatment at a heat treatment temperature T in the range of 700 to 830 ° C. for a heat treatment time t that satisfies the following formulas (ii) and (iii): After the heat treatment, cooling to 400°C or less within 30 minutes, a method for producing a hot-rolled ferritic stainless steel sheet, wherein the heat treatment is carried out after the finish rolling of the hot rolling and before the temperature is reduced to 650°C or less. 1410≦A≦1450...(ii) A=T×log(20+t)...(iii) In the above formula, each symbol is defined as follows: A: Constant T: Heat treatment temperature (K) t: heat treatment time (hr)

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