Austenitic stainless cold-rolled steel sheet and method for producing same

Abstract

Provided is an austenitic stainless cold-rolled steel sheet having excellent moldability and fatigue properties. The austenitic stainless cold-rolled steel sheet has a chemical composition containing 0.001-0.100 mass% of C, 0.01-2.00 mass% of Si, 0.01-10.00 mass% of Mn, 0.010-0.050 mass% of P, 0.0001-0.0100 mass% of S, 3.00-15.00 mass% of Ni, 13.0-25.0 mass% of Cr, and 0.001-0.300 mass% of N, and the X-ray diffraction intensities of the orientations ｛211｝<111>, ｛110｝<111>, ｛110｝<322>, and ｛110｝<211> are all 2 or less.

Description

Austenitic stainless steel cold rolled sheet and method for manufacturing the same

[0001] The present invention relates to a cold-rolled austenitic stainless steel sheet and a method for producing the same.

[0002] In recent years, improving the fuel efficiency of transportation equipment has become an essential issue from the perspective of environmental issues. Transportation equipment refers to equipment used to transport luggage or passengers, such as automobiles, motorcycles, tricycles, bicycles, buses, and railroad cars. To improve the fuel efficiency of transportation equipment, efforts to reduce the weight of transportation equipment bodies have been actively promoted. Weight reduction of vehicle bodies is largely achieved by reducing the weight of the materials used in the components that make up the body, for example, by reducing the thickness of the materials. However, reducing the thickness of the materials reduces the rigidity and crash absorption performance of the components. To address this issue, increasing the strength of the materials that make up the components is an effective solution, and high-strength ordinary steel (high-tensile steel) is being used.

[0003] However, because ordinary steel has low corrosion resistance, it is assumed that it will be heavily painted. Therefore, ordinary steel cannot be used for unpainted or lightly painted parts, and heavy painting increases costs. On the other hand, Cr-containing stainless steel has superior corrosion resistance compared to ordinary steel, which is expected to reduce rust allowance (thickness that allows for rust formation), thereby reducing weight and simplifying painting. Furthermore, austenitic stainless steel has superior hydrogen embrittlement resistance compared to ordinary steel, making it particularly effective for components exposed to hydrogen environments and improving the safety of transportation and energy equipment. Therefore, the use of austenitic stainless steel offers significant benefits, such as weight reduction, simplified painting, and improved safety.

[0004] On the other hand, components that make up hydrogen fuel cells, such as fuel components such as pipes, tanks, valves, and nozzles, are often manufactured by pressing steel sheets using techniques such as deep drawing and stretch forming. If the steel sheet has large in-plane anisotropy during this process, earrings can form, resulting in shape defects and reduced yields. Since shape precision is particularly strict for components that make up hydrogen fuel cells, applying austenitic stainless steel sheets poses a major challenge.

[0005] For example, Patent Document 1 discloses a technique for reducing in-plane anisotropy by using a stable austenitic stainless steel that is resistant to the formation of work-induced martensite due to cold working strain, and by performing a double cold rolling and annealing process.

[0006] Patent Document 2 discloses a technique for reducing in-plane anisotropy by adding Al and cold rolling the steel using large diameter rolls.

[0007] Patent Document 3 discloses a technique in which hot rolling is performed under predetermined conditions, and then cold rolling and annealing are performed without annealing the hot-rolled sheet.

[0008] Patent Document 4 discloses a technique for reducing in-plane anisotropy by setting the hot rolling reduction to 98.7% or more and performing hot-rolled sheet annealing, cold rolling, and finish annealing. In the examples, a |Δr| of 0.03 to 0.07 is obtained.

[0009] Patent Document 5 discloses a technique for hot rolling austenitic stainless steel under predetermined conditions to reduce |Δr| after annealing the hot-rolled sheet to 0.2 or less. This technique involves specifying the slab heating temperature, finish rolling temperature, and hot-rolled sheet annealing temperature to reduce Δr, which is an index of in-plane anisotropy.

[0010] Patent Document 6 also discloses a technique for omitting the annealing of the hot-rolled sheet to set |Δr| to 0.15 or less.

[0011] Patent Document 7 discloses an austenitic stainless steel sheet for press work that contains Cu and Al and has small earrings.

[0012] JP 8-165524, JP 9-263829, JP 10-128409, JP 10-158736, JP 8-41550, JP 8-260044, JP 9-87742

[0013] Patent Documents 5 and 6 are based on processes that do not involve cold rolling, making them difficult to apply to parts with strict thickness precision. The technologies of Patent Documents 1 to 4 are based on cold rolling, which tends to develop specific crystal orientations and increase in-plane anisotropy. Furthermore, even when |Δr| is small, the differences between the maximum and minimum r values ​​can become large, making them difficult to apply to precision parts. Furthermore, while Patent Document 7 lists the earring rate of the invention steel as 1.9 to 3.9%, there are cases where an even lower earring rate is required.

[0014] When considering the application of austenitic stainless steel sheets to precision parts, specifically, d obtained by the following formula (i) ｒ is required to be 0.31 or less. ｒ =r ｍａｘ -r ｍｉｎ ...(i) where the meanings of the symbols in formula (i) are as follows: ｍａｘ r: Maximum Lankford value in the directions of 0°, 45°, and 90° relative to the rolling direction ｍｉｎ : Lankford value in the direction at 45° to the rolling direction

[0015] On the other hand, when materials are made thinner to reduce the weight of parts, bending fatigue becomes an issue. Specifically, the plane bending fatigue limit is required to be 250 MPa or more.

[0016] An object of the present invention is to provide an austenitic cold-rolled stainless steel sheet having excellent formability and fatigue properties, and a method for producing the same.

[0017] The present inventors produced various cold-rolled austenitic stainless steel sheets and conducted detailed analyses and studies.

[0018] Reducing the in-plane anisotropy of a material is effective in improving its press formability. Materials with small in-plane anisotropy have a low earring rate during forming, improving yield. In order to reduce the in-plane anisotropy of steel sheets, it is necessary to control the crystal orientation of the steel sheets. However, since strength differs depending on the crystal orientation, it is necessary to weaken the strength of specific crystal orientations to form a random texture.

[0019] In the case of austenitic stainless steel cold rolled sheets, specific crystal orientations tend to develop during the cold rolling process, making it difficult to obtain a random texture. ｒ It has been difficult to reduce the fatigue strength to 0.31 or less. Furthermore, as mentioned above, high fatigue strength is required for thin-walled and lightweight structures.

[0020] The inventors identified the crystal orientation that increases the in-plane anisotropy of austenitic cold-rolled stainless steel material, and by controlling the chemical composition and manufacturing conditions, they were able to complete an austenitic cold-rolled stainless steel sheet that has small in-plane anisotropy and high fatigue strength.

[0021] The present invention relates to the following cold-rolled austenitic stainless steel sheet and a method for producing the same.

[0022] [1] Chemical composition, in mass%, C: 0.001 to 0.100%, Si: 0.01 to 2.00%, Mn: 0.01 to 10.00%, P: 0.010 to 0.050%, S: 0.0001 to 0.0100%, Ni: 3.00 to 15.00%, Cr: 13.0 to 25.0%, N: 0.001 to 0.300%, Ti: 0 to 0.50%, Nb: 0 to 0.60%, Al: 0 to 1.000%, Cu: 0 to 2.00%, Mo: 0 to 3.00%, V: 0 to 0.50%, Zr: 0 to 0.50%, B: 0 to 0.0050%, 1. An austenitic stainless cold rolled steel sheet comprising: Ca: 0 to 0.0100%, W: 0 to 3.00%, Sn: 0 to 0.50%, Co: 0 to 0.30%, Mg: 0 to 0.0100%, Sb: 0 to 0.300%, REM: 0 to 0.200%, Ga: 0 to 0.3000%, Ta: 0 to 1.000%, Hf: 0 to 1.000%, Bi: 0 to 0.020%, and the balance: Fe and impurities; and 2. X-ray diffraction intensities of the {211}<111> orientation, the {110}<111> orientation, the {110}<322> orientation, and the {110}<211> orientation are all 2 or less.

[0023] [2] The chemical composition is, in mass%, Ti: 0.01 to 0.50%, Nb: 0.01 to 0.60%, Al: 0.001 to 1.000%, Cu: 0.01 to 2.00%, Mo: 0.01 to 3.00%, V: 0.01 to 0.50%, Zr: 0.01 to 0.50%, B: 0.0002 to 0.0050%, Ca: 0.0005 to 0.0100%, W: 0.10 to 3.00%, Sn: 0.01 to 0.50%, Co: 0.03 to 0.30%, Mg: 0.0002 to 0.0100%, Sb: 0.005 to 0.300%, The austenitic stainless cold rolled steel sheet according to [1] above, containing one or more selected from REM: 0.002 to 0.200%, Ga: 0.0002 to 0.3000%, Ta: 0.001 to 1.000%, Hf: 0.001 to 1.000%, and Bi: 0.001 to 0.020%.

[0024] [3] d obtained by the following formula (i) ｒThe austenitic stainless cold-rolled steel sheet according to the above [1] or [2], wherein d is less than 0.30. ｒ =r ｍａｘ -r ｍｉｎ ...(i) where the meanings of the symbols in formula (i) are as follows: ｍａｘ r: Maximum Lankford value in the directions of 0°, 45°, and 90° relative to the rolling direction ｍｉｎ : Lankford value in the direction at 45° to the rolling direction

[0025] [4] The austenitic stainless cold-rolled steel sheet according to any one of [1] to [3] above, having a plane bending fatigue limit of 250 MPa or more at room temperature.

[0026] [5] A method for producing an austenitic stainless cold-rolled steel sheet according to any one of [1] to [4] above, comprising rolling the sheet under conditions of a reduction ratio of 5 to 60%, a rolling temperature of 50 to 200°C, and a work roll diameter of 150 mm or less, followed by annealing at a heating rate of 10°C / sec or more at 800 to 1000°C and a holding temperature of 1050 to 1150°C.

[0027] According to the present invention, it is possible to provide an austenitic stainless cold-rolled steel sheet having excellent formability and fatigue properties.

[0028] FIG. 1 is a diagram showing the textures of a steel according to the present invention and a comparative steel.

[0029] Hereinafter, embodiments of the present invention will be described in detail. The present invention is not limited to the following embodiments, and modifications and improvements can be made to the following embodiments as appropriate based on the ordinary knowledge of those skilled in the art, without departing from the spirit of the present invention. In the following description, "%" regarding the content of each element means "% by mass" unless otherwise specified.

[0030] The reasons for limiting the content of each element will be explained in detail below.

[0031] <C: 0.001 to 0.100%> C is an element that increases the structural stability of the austenite phase, so the lower limit of the C content is set to 0.001%. However, excessive C content reduces corrosion resistance and heat resistance, and C is an element that promotes the development of texture, so the upper limit of the C content is set to 0.100%. From the viewpoint of stretchability and deep drawability, the lower limit of the C content is preferably 0.003%. From the viewpoint of weldability, the upper limit of the C content is preferably 0.090% or 0.080%. Furthermore, from the viewpoint of manufacturability, the lower limit of the C content is more preferably 0.005%, and from the viewpoint of fatigue strength, the upper limit of the C content is more preferably 0.070% or 0.060%.

[0032] <Si: 0.01 to 2.00%> Si is a deoxidizing element. Furthermore, Si is a solid-solution strengthening element, and is an effective element for improving fatigue strength and reducing in-plane anisotropy. Therefore, the lower limit of the Si content is set to 0.01%. However, excessive Si content actually reduces fatigue strength and increases in-plane anisotropy. Furthermore, since it leads to a rapid decrease in ductility, the upper limit of the Si content is set to 2.00%. Considering corrosion resistance, the lower limit of the Si content is preferably 0.05%, and considering manufacturability, the upper limit of the Si content is preferably 1.80%, 1.60%, 1.40%, 1.20%, 1.00%, or 0.90%. Furthermore, in consideration of weldability, the lower limit of the Si content is more preferably 0.10%, 0.20%, or 0.30%, and in consideration of toughness, the upper limit of the Si content is more preferably 0.80%, 0.70%, or 0.60%.

[0033] <Mn: 0.01 to 10.00%> Mn is a deoxidizing element. Furthermore, Mn promotes the formation of deformation twins and is an effective element for improving fatigue strength and reducing in-plane anisotropy. Therefore, the lower limit of the Mn content is set to 0.01%. However, excessive Mn content actually reduces fatigue strength and increases in-plane anisotropy. Furthermore, since it leads to a decrease in corrosion resistance, the upper limit of the Mn content is set to 10.00%. Considering oxidation resistance, the lower limit of the Mn content is preferably 0.10%, 0.50%, 1.00%, 1.50%, 2.00%, 2.50%, or 3.00%, and considering in-plane anisotropy, the upper limit of the Mn content is preferably 9.50%. Furthermore, in consideration of manufacturability, the lower limit of the Mn content is more preferably 3.50%, 4.50%, or 5.00%, and in consideration of weldability, the upper limit of the Mn content is more preferably 9.30%.

[0034] <P: 0.010 to 0.050%> P is an element present as an impurity in steel. Since P reduces formability, corrosion resistance, manufacturability, and other properties, the lower the P content, the better. Too much P generates coarse phosphides, which become the starting point for void formation during processing. It also reduces fatigue strength and increases in-plane anisotropy. Therefore, the upper limit of the P content is set to 0.050%. On the other hand, excessive reduction of P leads to increased manufacturing costs, so the lower limit of the P content is set to 0.010%. Considering refining costs, the lower limit of the P content is preferably 0.015%. Considering fatigue strength, the upper limit of the P content is preferably 0.040% or even 0.030%.

[0035] <S: 0.0001 to 0.0100%> S is an element present as an impurity in steel. Since S reduces formability, corrosion resistance, manufacturability, and other properties, the lower the content, the better. Too much S generates coarse sulfides, which become the starting point for void formation during the fatigue process. It also reduces fatigue strength and increases in-plane anisotropy. Therefore, the upper limit of the S content is set to 0.0100%. On the other hand, excessive reduction of S leads to increased manufacturing costs, so the lower limit of the S content is set to 0.0001%. Considering refining costs, the lower limit of the S content is preferably 0.0005%. Considering weldability, the upper limit of the S content is preferably 0.0080%, 0.0060%, 0.0040%, or 0.0020%.

[0036] <Ni: 3.00 to 15.00%> Ni contributes to high strength and improves formability through the stability of the austenite phase. Ni is also an element effective in improving fatigue strength and reducing in-plane anisotropy. Therefore, the lower limit of the Ni content is set to 3.00%. However, since excessive Ni content increases costs, the upper limit of the Ni content is set to 15.00%. In consideration of corrosion resistance and suppression of texture development, the lower limit of the Ni content is preferably 4.00%. The upper limit of the Ni content is preferably 14.00%, 13.00%, 12.00%, 11.00%, 10.00%, 9.00%, or 8.00%. In consideration of formability and fatigue strength, the lower limit of the Ni content is more preferably 5.00%, and in consideration of weldability, the upper limit of the Ni content is more preferably 7.00% or 7.50%.

[0037] <Cr: 13.0 to 25.0%> Cr is an element contained to improve corrosion resistance. Cr is also an element effective in improving fatigue strength and reducing in-plane anisotropy. In particular, the lower limit of the Cr content is set to 13.0% to eliminate painting and improve high-temperature rigidity. On the other hand, if the Cr content is too high, toughness will be significantly reduced due to the formation of intermetallic compounds, so the upper limit of the Cr content is set to 25.0%. In consideration of suppressing texture development, the lower limit of the Cr content is preferably 14.0%, while in consideration of manufacturability, cost, and corrosion resistance of welds, the upper limit of the Cr content is preferably 23.0%, 20.0%, 17.0%, or 16.0%.

[0038] <N: 0.001 to 0.300%> N is an element effective in improving corrosion resistance and suppressing hydrogen embrittlement. Furthermore, the present inventors discovered that N is extremely important for suppressing texture development, and that random texture is formed from planar dislocation structures, etc., formed during the hot rolling stage. Therefore, the lower limit of the N content is set to 0.001%. On the other hand, excessive N content significantly hardens the steel and reduces fatigue strength. It also increases in-plane anisotropy. Therefore, the upper limit of the N content is set to 0.300%. From the viewpoint of fatigue strength, the lower limit of the N content is preferably 0.010%, 0.030%, or 0.050%. From the viewpoints of stretchability and deep drawability, the upper limit of the N content is preferably 0.250% or 0.200%. Furthermore, considering weldability, the lower limit of the N content is more preferably 0.060%, and from the viewpoint of manufacturability, the upper limit of the N content is more preferably 0.180%.

[0039] The chemical composition of the steel sheet of the present invention may further contain one or more elements selected from Ti, Nb, Al, Cu, Mo, V, Zr, B, Ca, W, Sn, Co, Mg, Sb, REM, Ga, Ta, Hf, and Bi within the ranges shown below. Note that these elements are not necessarily essential for the steel of the present invention, and therefore the lower limit of their content is 0%. The reasons for limiting each element will be explained below.

[0040] <Ti: 0 to 0.50%> <Nb: 0 to 0.60%> Ti and Nb bond with C and N to suppress the formation of coarse Cr carbonitrides, thereby preventing intergranular corrosion and contributing to improved formability (e.g., deep drawability). Therefore, Ti and / or Nb may be added as needed, with the lower limit of the Ti content preferably being 0.01%, and the lower limit of the Nb content preferably being 0.01%. However, excessive Ti and Nb content causes coarse precipitation of Ti-based precipitates and Nb-based precipitates, developing a texture and increasing in-plane anisotropy. It also reduces fatigue strength. Therefore, the upper limit of the Ti content is set to 0.50%, and the upper limit of the Nb content is set to 0.60%. From the viewpoints of formability and fatigue strength, the lower limit of the Ti content and the lower limit of the Nb content are preferably 0.05% or 0.10%, respectively, and from the viewpoints of manufacturability and cost, the upper limit of the Ti content and the upper limit of the Nb content are preferably 0.50%, 0.40%, or 0.30%, respectively. Furthermore, from the viewpoint of manufacturing cost, the upper limit of the Ti content and the upper limit of the Nb content are more preferably 0.20%.

[0041] <Al: 0 to 1.000%> Al is an element contained as a deoxidizing element. Furthermore, Al is an element effective in improving formability by forming nitrides, increasing strength through solid-solution strengthening, and improving oxidation resistance. To efficiently achieve these effects, it is preferable to contain 0.001% or more of Al as needed. However, excessive Al content can lead to reduced high-temperature rigidity, surface flaws, reduced weldability, and reduced fatigue strength due to coarse AlN. Furthermore, the precipitation of coarse AlN develops texture and increases in-plane anisotropy. Therefore, the upper limit of the Al content is set to 1.000%. Considering deoxidation efficiency, the lower limit of the Al content is preferably 0.020%, while considering toughness, the upper limit of the Al content is preferably 0.900%, 0.800%, 0.700%, 0.600%, or 0.500%. In addition, in consideration of weldability, the lower limit of the Al content is preferably 0.020%, the upper limit of the Al content is more preferably 0.400%, 0.300%, or 0.200%, and the Al content is further preferably less than 0.100%.

[0042] <Cu: 0 to 2.00%> Cu contributes to improving corrosion resistance and improving high-temperature rigidity and fatigue strength through the precipitation of ε-Cu, so it is preferable to include 0.01% or more of Cu as needed. However, if the Cu content is too high, ductility decreases significantly and in-plane anisotropy increases. It also decreases fatigue strength. Therefore, the upper limit of the Cu content is set to 2.00%. In consideration of oxidation resistance, the lower limit of the Cu content is preferably 0.05%, and in consideration of cost, the upper limit of the Cu content is preferably 1.80%, 1.50%, or 1.30%. In consideration of toughness, the lower limit of the Cu content is preferably 0.10%, and in consideration of weldability, the upper limit of the Cu content is more preferably 1.00% or 0.50%.

[0043] <Mo: 0 to 3.00%> Mo is an element that improves corrosion resistance. Furthermore, Mo is a solid-solution strengthening element and is effective in increasing fatigue strength. Therefore, Mo is preferably contained in an amount of 0.01% or more as needed. However, excessive Mo content reduces formability, increases in planar anisotropy, and increases costs. Furthermore, fatigue strength and toughness decrease significantly, so the upper limit of the Mo content is set to 3.00%. From the viewpoint of high-temperature rigidity, the lower limit of the Mo content is preferably 0.05% or 0.10%, and from the viewpoint of oxidation resistance, the upper limit of the Mo content is preferably 2.50% or 2.10%. Furthermore, from the viewpoint of cost reduction, the upper limit of the Mo content is more preferably 1.50%, 1.00%, 0.50%, or 0.20%.

[0044] <V: 0 to 0.50%> <Zr: 0 to 0.50%> Like Ti and Nb, V and Zr are elements that bond with C and N and suppress the formation of coarse Cr carbonitrides. Furthermore, V and Zr are also elements that are effective in improving fatigue strength by forming fine precipitates (Zr(C,N), V(C,N)). Therefore, it is preferable to include 0.01% or more of V and Zr, respectively, as needed. However, excessive V and Zr content reduces toughness and increases in-plane anisotropy. It also reduces fatigue strength. Therefore, the upper limits of the V content and the Zr content are each set to 0.50%. From the viewpoint of formability, the lower limits of the V content and the Zr content are each preferably 0.05%, and the upper limits of the V content and the Zr content are each preferably 0.30%. Furthermore, in consideration of the impact absorption characteristics of the welded portion during high-speed deformation, the lower limit of the V content and the lower limit of the Zr content are each preferably 0.10%, and in consideration of bendability, the upper limit of the V content and the upper limit of the Zr content are each more preferably 0.20%.

[0045] <B: 0 to 0.0050%> B is an element effective in increasing strength and suppressing secondary processing cracking. B can also be effective in improving hot formability through grain boundary segregation. Therefore, it is preferable to add 0.0002% or more of B as needed. However, excessive B content can act as a starting point for void formation during the fatigue process, resulting in reduced formability and increased in-plane anisotropy. It also reduces fatigue strength. Therefore, the upper limit of the B content is set to 0.0050%. Considering corrosion resistance, the upper limit of the B content is preferably 0.0040% or 0.0030%. Considering weldability, the lower limit of the B content is preferably 0.0005%, and the upper limit of the B content is more preferably 0.0020%.

[0046] <Ca: 0 to 0.0100%> Ca fixes S and improves hot formability. Therefore, it is preferable to include 0.0005% or more of Ca as needed. However, excessive Ca content not only reduces corrosion resistance, but also leads to increased in-plane anisotropy and reduced fatigue strength due to coarse CaS. Therefore, the upper limit of the Ca content is set to 0.0100%. From the viewpoint of manufacturability, the upper limit of the Ca content is preferably 0.0080%, 0.0050%, or 0.0030%. Furthermore, the lower limit of the Ca content is more preferably 0.0010%.

[0047] <W: 0 to 3.00%> W is an element that improves corrosion resistance and also acts as a solid-solution strengthening element. Therefore, it is preferable to include 0.10% or more of W depending on the corrosion resistance level required in the usage environment. However, excessive W content leads to reduced formability and toughness, increased costs, and increased in-plane anisotropy. It also leads to the formation of intermetallic compounds such as σ phases that reduce fatigue strength. Therefore, the upper limit of the W content is set to 3.00%. The upper limit of the W content is preferably 2.50%, 2.00%, or 1.50%, and the lower limit of the W content is more preferably 0.50% or 1.00%.

[0048] <Sn: 0 to 0.50%> Sn contributes to improving corrosion resistance and high-temperature strength, so it is preferable to add 0.01% or more as needed. However, excessive Sn content may cause slab cracking during manufacturing and grain boundary cracking during processing (e.g., hole expansion). In addition, grain boundary segregation develops texture, increasing in-plane anisotropy and leading to a decrease in fatigue strength. Therefore, the upper limit of the Sn content is set to 0.50%. Considering refining costs and manufacturability, the upper limit of the Sn content is preferably 0.40% or 0.30%. Furthermore, the lower limit of the Sn content is more preferably 0.05%, 0.10%, or 0.15%.

[0049] <Co: 0 to 0.30%> Co contributes to improving high-temperature strength, so it is preferable to add 0.03% or more as needed. However, excessive Co content leads to reduced toughness during manufacturing, increased costs, and increased in-plane anisotropy. It also reduces fatigue strength. Therefore, the upper limit of the Co content is set to 0.30%. In consideration of refining costs and manufacturability, the upper limit of the Co content is preferably 0.20% or 0.10%. Furthermore, the lower limit of the Co content is more preferably 0.05%.

[0050] <Mg: 0 to 0.0100%> Mg is an element that is optionally contained as a deoxidizing element. Furthermore, Mg contributes to improving manufacturability by refining the structure, improving hot formability, and improving the formability of welds. To achieve these effects, it is preferable to contain 0.0002% or more of Mg. However, excessive Mg content significantly reduces corrosion resistance and increases in-plane anisotropy due to coarse MgO. It also reduces fatigue strength. Therefore, the upper limit of the Mg content is set to 0.0100%. Considering manufacturability, the upper limit of the Mg content is preferably 0.0080%, 0.0060%, 0.0040%, or 0.0020%. Furthermore, the lower limit of the Mg content is more preferably 0.0010%.

[0051] <Sb: 0 to 0.300%> Sb is an element that segregates at grain boundaries to increase high-temperature strength. To achieve this effect, it is preferable to add 0.005% or more Sb as needed. However, excessive Sb content can lead to grain boundary cracking during processing (e.g., hole expansion), cracking during welding, and increased in-plane anisotropy due to Sb segregation. It also reduces fatigue strength. Therefore, the upper limit of the Sb content is set to 0.300%. In consideration of high-temperature properties, the lower limit of the Sb content is preferably 0.010%, 0.020%, or 0.030%. In consideration of manufacturing cost and toughness, the upper limit of the Sb content is preferably 0.200%. Furthermore, the lower limit of the Sb content is preferably 0.040% or 0.050%, and the upper limit of the Sb content is more preferably 0.100%.

[0052] <REM: 0 to 0.200%> REM (rare earth elements) are effective in improving oxidation resistance, so it is preferable to add 0.002% or more of REM as needed. However, even if REM is added excessively, the effect saturates, and REM granulation reduces corrosion resistance and hole expandability, and increases in-plane anisotropy. It also reduces fatigue strength. Therefore, the upper limit of the REM content is set to 0.200%. Considering formability and manufacturing costs, the upper limit of the REM content is preferably 0.100%. Furthermore, the lower limit of the REM content is more preferably 0.050%.

[0053] The term REM follows the general definition. That is, REM refers to two elements, scandium (Sc) and yttrium (Y), and 15 elements (lanthanoids) from lanthanum (La) to lutetium (Lu). REM may be contained alone or in the form of a mixture.

[0054] <Ga: 0 to 0.3000%> Ga is an element that is optionally contained to improve corrosion resistance and suppress hydrogen embrittlement. In particular, from the viewpoint of forming sulfides and hydrides that are effective in achieving these effects, the lower limit of the Ga content is preferably set to 0.0002%. However, excessive Ga content generates coarse sulfides, which leads to an increase in in-plane anisotropy. It also reduces fatigue strength. Therefore, the upper limit of the Ga content is set to 0.3000%. The upper limit of the Ga content is more preferably 0.2000% or 0.1000%. Furthermore, from the viewpoint of manufacturability, the lower limit of the Ga content is preferably 0.0010% or 0.0020%.

[0055] <Ta: 0 to 1.000%> <Hf: 0 to 1.000%> Ta and Hf are elements effective in improving high-temperature strength and are contained as needed. To achieve this effect, it is preferable that Ta and Hf are each contained in an amount of 0.001% or more. However, from the viewpoint of cost, the upper limit of the Ta content and the upper limit of the Hf content are each set to 1.000%. From the viewpoint of improving high-temperature strength, the upper limit of the Ta content and the upper limit of the Hf content are more preferably 0.500%, 0.100%, 0.050%, or 0.010%, respectively. Furthermore, the lower limit of the Ta content and the lower limit of the Hf content are more preferably 0.005%.

[0056] <Bi: 0 to 0.020%> Bi is an element effective in improving machinability and is contained as needed. To achieve this effect, it is preferable to contain 0.001% or more of Bi. However, from the viewpoint of cost, the upper limit of the Bi content is set to 0.020%. From the viewpoint of improving machinability, the upper limit of the Bi content is preferably 0.010%. Furthermore, the lower limit of the Bi content is more preferably 0.005%.

[0057] The balance of the chemical composition of the steel sheet of the present invention is Fe and impurities, which refer to components that are mixed in from raw materials such as ores and scraps during industrial steel production and other factors, and are acceptable within a range that does not adversely affect the properties of the steel sheet of the present invention.

[0058] <X-ray diffraction intensity> In the austenitic stainless cold-rolled steel sheet according to an embodiment of the present invention, the X-ray diffraction intensities of the {211}<111> orientation, {110}<111> orientation, {110}<322> orientation, and {110}<211> orientation obtained by X-ray diffraction are all 2 or less. Preferably, at least one of the X-ray diffraction intensities is 1 or less, more preferably, at least two of the X-ray diffraction intensities are 1 or less, and more preferably, at least three of the X-ray diffraction intensities are 1 or less. Most preferably, all of the X-ray diffraction intensities are 1 or less.

[0059] The strength of the crystal orientation of austenitic stainless steel varies depending on the chemical composition, rolling conditions, and heat treatment conditions, affecting its properties. While various methods for measuring crystal orientation strength exist, the present invention specifies the crystal orientation strength obtained by X-ray diffraction. Regarding the texture, an X-ray diffractometer (manufactured by Rigaku Corporation) was used to obtain (200), (111), (220), and (311) pole figures in the region of 1 / 4 to 1 / 2 of the plate thickness (obtained by a combination of mechanical polishing and electrolytic polishing) using Mo-Kα radiation. From these, three-dimensional crystal orientation density functions were calculated using the spherical harmonic method, and then compared at cross sections (φ2 = 45° cross sections) where the crystal orientation strength (the intensity ratio to a random sample) can be seen as contour lines using a three-dimensional texture representation known as the Bunge method.

[0060] As can be seen from FIG. 1, the orientation strength of Comparative Steel 1 is high at 2 or more, and the d ｒ On the other hand, in the steel of the present invention, the strength of each of the above crystal orientations is small, being less than 2, and ｒ is extremely small at 0.06.

[0061] <Lankford Value> According to JIS Z 2254:2021, a JIS 13B tensile test specimen (hereinafter also referred to as "test specimen A") is taken from the cold-rolled sheet so that the length direction of the test specimen coincides with the rolling direction of the steel sheet. Similarly, a JIS 13B tensile test specimen (hereinafter also referred to as "test specimen B") is taken so that the length direction of the test specimen coincides with the 45° direction of the rolling direction of the steel sheet, and a JIS 13B tensile test specimen (hereinafter also referred to as "test specimen C") is taken so that the length direction of the test specimen coincides with the 90° direction of the rolling direction of the steel sheet. Then, after imparting a 28% strain to each of test specimen A, test specimen B, and test specimen C, the Lankford value in each direction is calculated using the following formula (a). The gauge lengths of test specimen A, test specimen B, and test specimen C are 20 mm. r = ln(W ０ / W) / ln(t ０ / t) ...(a) where W ０ is the plate width before tension (mm), W is the plate width after tension (mm), t ０ is the plate thickness (mm) before tension, and t is the plate thickness (mm) after tension.

[0062] Then, the Lankford values ​​for each direction are substituted into the following equation (i) to obtain d ｒ was calculated. ｒ =r ｍａｘ -r ｍｉｎ ...(i) where the meanings of the symbols in formula (i) are as follows: ｍａｘ r: Maximum Lankford value in the directions of 0°, 45°, and 90° relative to the rolling direction ｍｉｎ : Lankford value in the direction at 45° to the rolling direction

[0063] In the present invention, the d obtained using the above formula (i) ｒ The preferred range of d is less than 0.30. As described above, the steel sheet of the present invention has a random texture with extremely weak texture development. ｒ It is possible to obtain a cold-rolled steel sheet having a preferable d ｒ is 0.20 or less, and more preferably 0.10 or less.

[0064] <Plane Bending Fatigue Limit> In accordance with JIS Z 2275:1978, JIS No. 1 test specimens were taken so that the rolling direction of the steel plate coincided with the longitudinal direction of the test specimen. A plane bending fatigue test was performed at room temperature under fully reversed conditions at 1500 rpm, with the amplitude stress varied, to prepare an S-N (amplitude stress-fracture cycle) diagram. From this S-N diagram, the stress when the S-N line becomes horizontal is determined as the fatigue limit. In the present invention, the fatigue limit is preferably 250 MPa or more. The width of the parallel portion of the test specimen is 20 mm.

[0065] <Thickness> The thickness of the austenitic stainless cold-rolled steel sheet according to the embodiment of the present invention is not particularly limited and may be appropriately set depending on the characteristics of the product to be used, etc. In consideration of reducing the weight of parts, the thickness of the austenitic stainless cold-rolled steel sheet is preferably 5.0 mm or less, and more preferably 4.0 mm or less.

[0066] The above manufacturing method will be described in detail below.

[0067] Stainless steel having the above composition is melted and forged or cast, and then hot-rolled to obtain a hot-rolled steel sheet. The hot-rolled steel sheet is then annealed and pickled in sequence. Furthermore, after cold-rolling to a predetermined thickness, annealing and pickling are performed in sequence to obtain a finished steel sheet. Each step can be performed using existing equipment, and the conditions are not particularly limited and can be adjusted appropriately depending on the composition of the stainless steel, etc. Furthermore, if necessary, existing treatments (e.g., surface polishing, temper rolling, treatment using a tension leveler, etc.) may be performed after the final pickling.

[0068] The hot rolling may be performed as usual, and the temperature, reduction, etc. may be adjusted as appropriate. For cold rolling, the reduction is set to 5 to 60%, and the rolling temperature to 50 to 200°C. If the reduction exceeds 60%, the cold rolling texture develops, and the recrystallization texture develops during subsequent annealing, increasing the in-plane anisotropy. On the other hand, if the reduction is less than 5%, the thickness accuracy and shape deteriorate. Therefore, the reduction is set to 5 to 60%. Furthermore, considering the suppression of texture development, the reduction is preferably 50% or less. Furthermore, considering productivity, the reduction is preferably 10% or more.

[0069] Regarding the rolling temperature, if it is less than 50°C, deformation-induced martensitic transformation occurs, leading to the development of cold-rolled texture and recrystallization texture, resulting in increased in-plane anisotropy. Conversely, if it exceeds 200°C, manufacturability significantly decreases. Therefore, the rolling temperature is set to 50 to 200°C. Furthermore, considering the sheet thickness accuracy and shape, a temperature of 60°C or higher is preferable, and 150°C or lower is preferable. In addition, the work roll diameter used for cold rolling is 150 mm or less. If the work roll diameter exceeds 150 mm, it is difficult to introduce shear deformation, making it difficult for texture randomization to occur in the subsequent annealing process. On the other hand, if the work roll diameter is excessively small, deterioration of the sheet shape and reduced productivity will occur, so a work roll diameter of 50 mm or more is preferable. Furthermore, considering workability, a work roll diameter of 80 mm or more is preferable, and 120 mm or less is preferable.

[0070] The cold-rolled sheet thus obtained is annealed by heating and holding at 1050 to 1150°C to obtain a recrystallized structure, thereby controlling the grain size number specified in JIS G 0551:2020 to 7 to 10, preferably 8 to 9. This ensures ductility and fatigue strength. Holding temperatures below 1050°C result in an unrecrystallized structure, resulting in poor material properties. Furthermore, in-plane anisotropy increases. Furthermore, fatigue strength decreases. Therefore, the holding temperature is set to 1050°C or higher. On the other hand, temperatures above 1150°C cause the crystal grains to coarsen, making the surface prone to roughening during processing, and the texture develops significantly, increasing in-plane anisotropy. Therefore, the upper limit of the holding temperature is set to 1150°C. Furthermore, considering pickling properties, the holding temperature is preferably 1050°C or higher and 1120°C or lower.

[0071] Furthermore, the heating rate (hereinafter simply referred to as "heating rate") from 800 to 1000°C during the process of heating to the holding temperature of 1050 to 1150°C is set to 10°C / s or more. If the heating rate is less than 10°C / s, the development of texture cannot be suppressed, and in-plane anisotropy increases. This temperature range is the initial stage of recrystallization of the texture, and is the temperature range where many nuclei of the {110} orientation are formed. By passing through this temperature range quickly, the generation of the {110} orientation is suppressed. Therefore, the heating rate is set to 10°C / s or more. Considering improved productivity, reduced in-plane anisotropy, and reduced surface roughness due to fine grains, a heating rate of 20°C / s or more is preferable. There is no particular upper limit to the heating rate, but considering that the product is in a plate shape, it should be set to 500°C / s or less.

[0072] The present invention will be specifically explained below with reference to examples, but the present invention should not be construed as being limited to these examples.

[0073] Austenitic stainless steel sheets were produced according to the following procedure. Stainless steels having the chemical compositions shown in Tables 1 and 2 were melted and hot-rolled to obtain 5.0 mm thick hot-rolled steel sheets. These cold-rolled steel sheets were then annealed and pickled, followed by cold rolling, annealing, and pickling to obtain cold-rolled steel sheets. The strength of each crystal orientation and the Lankford value in each direction were measured for these cold-rolled steel sheets using the methods described above. Furthermore, formed products were produced by cylindrical drawing using a 40 mm diameter punch, and the ear shape was evaluated for pass / fail. Earring ratios of 1.5% or less were evaluated as C, those even smaller than 1.0% as B, those extremely small as 0.5% or less as A, and those greater than 1.5% as D. Furthermore, plane bending fatigue tests were conducted, and fatigue limits of 250 MPa or greater were evaluated as A, and those less than 250 MPa as B.

[0074]

[0075]

[0076]

[0077]

[0078] As shown in Tables 1 and 3, the examples of the present invention have weaker development of each crystal orientation than the comparative examples (i.e., each orientation is 2 or less), and d ｒ The fatigue limit for plane bending fatigue was 250 MPa or more, confirming high fatigue strength.

[0079] As shown in Tables 2 and 4, the comparative examples have intensities of more than 3 in at least one crystal orientation, and ｒ exceeded 0.31. Thus, in the comparative example, the in-plane anisotropy was large and the earing was large.

[0080] According to the present invention, an austenitic stainless steel sheet having excellent formability and fatigue properties can be provided. Specifically, according to the present invention, the above-mentioned steel sheet can be efficiently produced without requiring any special new equipment. The steel sheet of the present invention is suitable for use in transportation equipment, which requires particular strength, and fuel cells, hydrogen fuel components, and the like, which require hydrogen embrittlement resistance.

Claims

1. Chemical composition, in mass%, is: C: 0.001 to 0.100%, Si: 0.01 to 2.00%, Mn: 0.01 to 10.00%, P: 0.010 to 0.050%, S: 0.0001 to 0.0100%, Ni: 3.00 to 15.00%, Cr: 13.0 to 25.0%, N: 0.001 to 0.300%, Ti: 0 to 0.50%, Nb: 0 to 0.60%, Al: 0 to 1.000%, Cu: 0 to 2.00%, Mo: 0 to 3.00%, V: 0 to 0.50%, Zr: 0 to 0.50%, B: 0 to 0.0050%, 1. An austenitic stainless cold rolled steel sheet comprising: Ca: 0 to 0.0100%, W: 0 to 3.00%, Sn: 0 to 0.50%, Co: 0 to 0.30%, Mg: 0 to 0.0100%, Sb: 0 to 0.300%, REM: 0 to 0.200%, Ga: 0 to 0.3000%, Ta: 0 to 1.000%, Hf: 0 to 1.000%, Bi: 0 to 0.020%, and the balance: Fe and impurities; and 2. X-ray diffraction intensities of the {211}<111> orientation, the {110}<111> orientation, the {110}<322> orientation, and the {110}<211> orientation are all 2 or less.

2. The chemical composition is, in mass%, Ti: 0.01 to 0.50%, Nb: 0.01 to 0.60%, Al: 0.001 to 1.000%, Cu: 0.01 to 2.00%, Mo: 0.01 to 3.00%, V: 0.01 to 0.50%, Zr: 0.01 to 0.50%, B: 0.0002 to 0.0050%, Ca: 0.0005 to 0.0100%, W: 0.10 to 3.00%, Sn: 0.01 to 0.50%, Co: 0.03 to 0.30%, Mg: 0.0002 to 0.0100%, Sb: 0.005 to 0.300%, 2. The austenitic stainless cold rolled steel sheet according to claim 1, containing one or more selected from the group consisting of REM: 0.002 to 0.200%, Ga: 0.0002 to 0.3000%, Ta: 0.001 to 1.000%, Hf: 0.001 to 1.000%, and Bi: 0.001 to 0.020%.

3. d obtained by the following formula (i) ｒ The austenitic stainless cold rolled steel sheet according to claim 1 or 2, wherein d is less than 0.

30. ｒ =r ｍａｘ -r ｍｉｎ ...(i) where the meanings of the symbols in formula (i) are as follows: ｍａｘ r: Maximum Lankford value in the directions of 0°, 45°, and 90° relative to the rolling direction ｍｉｎ : Lankford value in the direction at 45° to the rolling direction 4. The austenitic stainless steel cold rolled sheet according to claim 1 or 2, having a plane bending fatigue limit of 250 MPa or more at room temperature.

5. A method for producing austenitic stainless steel cold rolled sheet according to claim 1 or 2, comprising rolling under conditions of a reduction ratio of 5 to 60%, a rolling temperature of 50 to 200°C, and a work roll diameter of 150 mm or less, followed by annealing at 800 to 1000°C at a heating rate of 10°C / sec or more and a holding temperature of 1050 to 1150°C.

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