Aluminum alloy for storage tank and manufacturing method therefor

Abstract

Disclosed in the present invention is an aluminum alloy for a storage tank. The aluminum alloy comprises Al and inevitable impurity elements, and further comprises the following chemical elements in percentages by mass: 0.02-0.09% of Si, 4.0-4.6% of Mg, 0.10-0.30% of Fe, 0.2-0.8% of Mn, 0.002-0.012% of Cu, and 0.06-0.12% of Cr. Further disclosed in the present invention is a method for manufacturing an aluminum alloy for a storage tank. The method comprises the steps of: smelting and casting, so as to obtain an ingot, wherein the flow rate of cooling water during casting is 190-220 L / s; face milling; ingot heating; rough rolling, wherein the total reduction rate of rough rolling is controlled to be 95% or more; finish rolling; and coiling, wherein the coiling temperature is controlled to be 350-380°C.

Description

Aluminum alloy for storage tank and manufacturing method thereof TECHNICAL FIELD

[0001] The present application relates to an aluminum alloy and a manufacturing method thereof, in particular to an aluminum alloy for a tank and a manufacturing method thereof. BACKGROUND

[0002] The multi-purpose aluminum alloy material for liquefied natural gas is used as the outer shell of the storage tank, and needs to go through the processes of flaw detection, water pressure test and rust prevention, so that the product quality is required to be high.

[0003] Since the aluminum alloy for the storage tank is required to have high strength and be widely used, the strength of the aluminum alloy needs to be improved while the production cost is low, and in the prior art:

[0004] For example, the Chinese patent document with the publication number CN108977706A and the publication date of December 11, 2018 and the name of "aluminum alloy plate for liquefied gas storage tank and preparation method thereof" discloses a product for a liquefied gas storage tank, the strength of the material is improved by increasing the Mg content of the alloy, and in the preparation method, a separate soaking treatment process is adopted, the chemical composition of the ingot is uniformly distributed by better control of the soaking process, and the strength and toughness of the aluminum alloy plate are effectively improved.

[0005] For another example, the Chinese patent document with the publication number CN103981411A and the publication date of August 13, 2014 and the name of "low-temperature-resistant aluminum alloy profile and preparation method thereof" discloses a production method of a low-temperature-resistant profile, the aluminum alloy profile has excellent low-temperature resistance, still has high strength, plasticity and toughness in a 20K low-temperature environment, is not prone to cold brittle low-temperature transformation, and can be used for manufacturing a liquid hydrogen and liquid oxygen storage tank of a spacecraft and a rocket power device and a structural support of a low-temperature superconducting magnet. SUMMARY

[0006] One of the purposes of the present application is to provide an aluminum alloy for a storage tank, which can guarantee the strength and welding performance of the aluminum alloy by combining the material design scheme with the process improvement under the premise of guaranteeing the basic characteristics of the aluminum alloy.

[0007] In order to achieve the above-mentioned purpose, the present application provides an aluminum alloy for a storage tank, which contains Al and inevitable impurity elements, and further contains the following chemical elements in mass percentage:

[0008] Si: 0.02-0.09%, Mg: 4.0-4.6%, Fe: 0.10-0.30%, Mn: 0.2-0.8%, Cu: 0.002-0.012%, Cr: 0.06-0.12%.

[0009] Further, in the aluminum alloy for storage tanks according to the present application, the content of each chemical element is as follows:

[0010] Si: 0.02-0.09%, Mg: 4.0-4.6%, Fe: 0.10-0.30%, Mn: 0.2-0.8%, Cu: 0.002-0.012%, Cr: 0.06-0.12%; the balance being Al and inevitable impurities.

[0011] The design principle of each chemical component in the aluminum alloy for storage tanks according to the present application is as follows:

[0012] Si: In the aluminum alloy for storage tanks according to the present application, Si is the main component for improving the flow performance of the aluminum alloy, and can greatly improve the casting performance of the aluminum alloy. Adding a certain content of Si can improve the tensile strength, hardness and corrosion resistance of the aluminum alloy; but when the content of Si is too high, hard particles containing Si will appear in the aluminum alloy, leading to increased brittleness of the alloy and deteriorated welding performance. Therefore, in the aluminum alloy for storage tanks according to the present application, in order to improve the welding performance of the aluminum alloy, the content of Si is controlled to be between 0.02% and 0.09%. In some embodiments, the content of Si is between 0.03% and 0.07%.

[0013] Mg: In the aluminum alloy for storage tanks according to the present application, Mg can improve the strength, hardness, heat resistance, corrosion resistance and cutting performance of the aluminum alloy. For the aluminum alloy for storage tanks, the strength improvement mainly utilizes the solid solution strengthening effect of Mg, and hot rolling is required for plate rolling, which will make the alloy brittle due to the formation of Mg2Si. Therefore, in the aluminum alloy for storage tanks according to the present application, the content of Mg is controlled to be between 4.0% and 4.6%.

[0014] Fe: In the aluminum alloy for storage tanks according to the present application, Fe will form FeAl3, Fe2Al or α-Al-Si-Fe flaky or acicular structure in the aluminum alloy, and the hard and brittle Fe-rich cathode phase is easy to segregate at the grain boundaries, which will destroy the continuity of the structure and reduce the forming performance of the material. During welding, this Fe impurity phase will also reduce the fluidity of the alloy, leading to increased tendency of welding hot cracking. Therefore, in the aluminum alloy for storage tanks according to the present application, the content of Fe is controlled to be between 0.10% and 0.30%.

[0015] Mn: In the aluminum alloy for storage tanks described in the present application, the limiting solid solubility of Mn element in the aluminum matrix is 1.82% (658°C), except for a small amount of Mn solid solution in the matrix, the rest forms Al6Mn second phase with Al. Under the same content, the strengthening effect of Mn element on aluminum alloy is almost 2 times of Mg, while it can ensure that the alloy has better stability. The optimization of Mn on aluminum alloy mainly includes: (1) uniform β phase precipitation, improve the stability of alloy organization; (2) enhance the corrosion resistance, reduce stress corrosion cracking; (3) increase the recrystallization temperature, prevent grain coarsening; (4) can play a certain solid solution strengthening effect; (5) combined with Fe, Si and other impurity atoms to form Al6(FeMn), Al6(FeMnSi), Al 12 (FeMn)3Si second phase, offset the negative impact of impurity elements such as Fe, Si on the alloy. However, when the mass percentage content of Mn element is too high, not only can not continue to improve the strength of the alloy, but also can greatly reduce its plastic deformation ability, and in the hot rolling process, it is easy to cause the occurrence of "sodium brittleness" phenomenon with Na. Therefore, in the aluminum alloy for storage tanks described in the present application, in order to ensure the formability of the plate during processing and use, the mass percentage content of Mn element is controlled between 0.2-0.8%. In some embodiments, the mass percentage content of Mn element is 0.3-0.7%.

[0016] Cu: In the aluminum alloy for storage tanks described in the present application, Cu element can improve the mechanical properties of aluminum alloy cutting and grinding, and also can reduce pitting corrosion. A small amount of Cu element can effectively prevent the softening of the alloy in the heat affected zone of welding. In addition, Cu element has a certain solid solution strengthening effect, and with the increase of the content of Mg element, the limiting solubility of Cu in Al matrix gradually decreases. When Cu element exists in the form of Al2CuMg or Cu2FeAl7 intermetallic compound, it is beneficial to reduce the stress corrosion cracking tendency. When the mass percentage content of Cu element is too high, it may increase the stress corrosion and intergranular corrosion sensitivity of the alloy. Therefore, in the aluminum alloy for storage tanks described in the present application, in order to ensure the quality of the weld, the mass percentage content of Cu element is controlled between 0.002-0.012%.

[0017] Cr: In the aluminum alloy for storage tanks described in the present application, Cr element forms (CrFe)Al7, (CrMn)Al 12 intermetallic compounds in aluminum. Cr element can also hinder the nucleation and growth process of recrystallization, and has a certain strengthening effect on the alloy, improving the high temperature performance. In terms of welding performance, Cr element can also combine with Fe to reduce the harmful effect of impurity Fe, improve the stress corrosion resistance of the alloy and reduce the hot cracking tendency. Therefore, in the aluminum alloy for storage tanks described in the present application, the mass percentage content of Cr element is controlled between 0.06-0.12%.

[0018] Further, in the aluminum alloy for storage tanks described in the present application, the total amount of inevitable impurity elements is ≤0.15%, and the content of each impurity is ≤0.05%.

[0019] Further, the microstructure of the aluminum alloy for storage tanks described in the present application has at least one of the second phases Al6(FeMn), Al6(FeMnSi), Al 12 (FeMn)3Si, (CrFe)Al7.

[0020] In the present application, through component design, the added Mn and Cr elements can both inhibit the network precipitation of Fe elements. Among them, Mn can combine with Fe to generate blocky second phases Al6(FeMn), Al6(FeMnSi), Al 12 (FeMn)3Si; Cr can combine with Fe to generate the second phase (CrFe)Al7, which has the effect of hindering the growth of coarse grains after welding. Through the SNIF stirring method during smelting, it can be fully combined to minimize the precipitation of elemental Fe or aluminum-iron phases.

[0021] Further, the mechanical properties of the aluminum alloy for storage tanks described in the present application satisfy: yield strength ≥ 160 MPa, tensile strength ≥ 290 MPa, and elongation ≥ 28%. In some embodiments, the yield strength of the aluminum alloy for storage tanks described in the present application is 160-170 MPa. In some embodiments, the tensile strength of the aluminum alloy for storage tanks described in the present application is 290-305 MPa. In some embodiments, the elongation of the aluminum alloy for storage tanks described in the present application is 28-32%.

[0022] Further, in the aluminum alloy for storage tanks described in the present application, the mechanical properties of the weld obtained after welding satisfy: yield strength ≥ 155 MPa, tensile strength ≥ 295 MPa, and elongation ≥ 28%. In some embodiments, the yield strength of the weld is ≥ 160 MPa. In some embodiments, the yield strength of the weld is 155-170 MPa, such as 160-170 MPa. In some embodiments, the tensile strength of the weld is ≥ 300 MPa. In some embodiments, the tensile strength of the weld is 295-305 MPa, such as 300-305 MPa. In some embodiments, the elongation of the weld is 28-31%. In some embodiments, the performance of the weld is tested according to GB / T 2651-2023 Metallic Materials - Method of Tensile Test of Welded Joint.

[0023] Another object of the present application is to provide a manufacturing method of an aluminum alloy for storage tanks, which can manufacture an aluminum alloy plate strip with high strength.

[0024] To achieve the above object, the present application provides a manufacturing method of an aluminum alloy for storage tanks, comprising the steps of:

[0025] melting and casting to obtain an ingot: wherein the cooling water flow rate during casting is controlled to be 190-220 L / s;

[0026] milling the surface;

[0027] heating the ingot;

[0028] rough rolling: controlling the total reduction rate of rough rolling to be more than 95%;

[0029] finish rolling;

[0030] coiling: controlling the coiling temperature to be 350-380℃.

[0031] In the present application, the cooling water flow rate during casting is controlled to be 190-220 L / s to rapidly solidify the structure, the dendrite grows rapidly at a large cooling rate but the width size is controllable; at the same time, the rapid cooling can concentrate the impurities and segregation to spread to the surface of the ingot, and the milling of the surface can reduce the content of impurities and segregation in the product as much as possible; during the hot rolling process, the original grains are transformed into flat and elongated deformed structures by severe plastic deformation, and the large reduction amount rolling process is used as much as possible in the finish rolling to crush the coarse grains in the ingot, weld the casting defects, which plays an important role in improving the use uniformity of the hot rolled product of the aluminum alloy for storage tanks, and can greatly increase the strength of the hot rolled product of the aluminum alloy for storage tanks. At the same time, the deformation energy obtained by rolling is the driving force for recrystallization of the alloy, and the appearance of recrystallized structure and large-angle grain boundaries can balance the strength and plasticity of the aluminum alloy, so that the high-strength finished product of the aluminum alloy for storage tanks can be stably realized by using the process parameters of the present application, and the welding performance is improved.

[0032] In addition, in the coiling process of the present application, high-temperature coiling is used, i.e. the coiling temperature is controlled to be 350-380℃, which can utilize the temperature of the material itself and the slow heat dissipation effect after coiling to produce a "self-annealing" effect. The high-temperature coiling not only provides a driving force for recrystallization of the material, but also has the effect of reducing dislocation entanglement, thereby increasing the proportion of recrystallized structure and balancing the strength and plasticity of the material.

[0033] Further, in the melting step of the manufacturing method of the present application, the melting temperature is controlled to be 680-750℃.

[0034] In this invention, if the melting temperature is too low, impurities will not be completely removed, and if the melting temperature is too high, the melt will be overburned and oxides will increase. Therefore, in this invention, the melting temperature is controlled at 680-750°C.

[0035] Furthermore, during the smelting process, argon blowing is used to protect the melt, and the SNIF rotor runs at a low speed to reduce the mixing of surface oxide slag into the melt.

[0036] In some implementations, the ingot thickness is controlled to be 540–640 mm during smelting and casting.

[0037] In some implementations, after the ingot is removed from the casting machine, a milling operation is performed, controlling the side milling (double-sided) amount to be ≥20mm, such as 20-30mm; the thickness milling (double-sided) amount is controlled between 20-30mm.

[0038] Furthermore, during the ingot heating process, the ingots are first wiped: each ingot is wiped with D40 to ensure that there is no oil, aluminum shavings, or dust residue on the surface of the ingot.

[0039] Furthermore, the process for heating the ingot can be controlled as follows: differential heating to increase the heating rate, with the furnace gas temperature at 520–540°C and the heating time at 2–4 hours.

[0040] Furthermore, in the ingot heating step of the manufacturing method described in this invention, the ingot heating temperature is controlled at 475-485°C, and the temperature is maintained for 3-6 hours after reaching the set temperature.

[0041] Furthermore, in the roughing step of the manufacturing method described in this invention, 2 to 4 passes of vertical rollers are used before the head and tail are cut off in the roughing, with each pass having a rolling edge amount of 12 to 20 mm.

[0042] In this invention, 2 to 4 passes of vertical rolls are used before the roughing and cutting of the head and tail, and the edge rolling amount of each pass can be controlled to be 12 to 20 mm, which can further ensure that edge cracks do not occur during the rolling process.

[0043] Furthermore, in the roughing step of the manufacturing method described in this invention, the thickness of the intermediate billet obtained by roughing is 16 to 32 mm.

[0044] During the rolling process of this invention, the temperature of the sheet rises due to deformation. At high temperatures, the aluminum alloy sheet is relatively soft. To prevent the sheet from collapsing, the thickness of the intermediate billet can be controlled to be 16-32 mm.

[0045] In some implementations, the thickness of the intermediate billet is 16–30 mm.

[0046] Furthermore, in the finishing rolling step, the reduction per rolling pass is controlled at 40-43%, and the reduction per final rolling pass is increased to 50%.

[0047] Further, in the coiling step of the manufacturing method, the coiling tension is controlled to be 12-18 N / mm 2 .

[0048] In the present application, the coiling tension can be controlled at a lower level, so as to avoid the cold shrinkage of the plate surface during the heat cooling process.

[0049] The aluminum alloy for storage tanks and the manufacturing method thereof have the following characteristics and beneficial effects:

[0050] The aluminum alloy for storage tanks can ensure the strength and welding performance of the aluminum alloy under the premise of ensuring the basic characteristics of the aluminum alloy through the material design scheme combined with process improvement.

[0051] In some embodiments, the mechanical properties of the aluminum alloy for storage tanks satisfy: yield strength ≥ 160 MPa, tensile strength ≥ 290 MPa, and elongation ≥ 28%; and the mechanical properties of the weld obtained after welding satisfy: yield strength ≥ 155 MPa, tensile strength ≥ 295 MPa, and elongation ≥ 28%.

[0052] The finished product of the aluminum alloy for storage tanks can be widely used in low-temperature storage tanks, such as LNG liquid and liquid oxygen. The high-strength, low-temperature-resistant and easily-welded aluminum plate has a wide application space and high economic value.

[0053] The aluminum alloy for storage tanks and the manufacturing method thereof have low cost and low energy consumption, without the need to increase equipment and consume materials, and can obtain an aluminum alloy for storage tanks with uniform structure and few defects. BRIEF DESCRIPTION OF DRAWINGS

[0054] Figure 1 shows the process flow diagram of the manufacturing method of the aluminum alloy for storage tanks.

[0055] Figure 2 shows the microsecond phase in the embodiment of the present application. Top: Example 2; bottom: Example 11. DETAILED DESCRIPTION

[0056] The aluminum alloy for storage tanks and the manufacturing method thereof will be further explained and described below in conjunction with the drawings and specific embodiments of the present application, but the explanation and description do not constitute an improper limitation on the technical solutions of the present application.

[0057] Examples 1-12

[0058] Figure 1 shows the process flow diagram of the hot rolling step.

[0059] As shown in FIG. 1, the aluminum alloy for storage tanks of embodiments 1-12 of the present application is prepared using the following steps:

[0060] (1) Melting: During the smelting process, the melt is protected by argon blowing, the SNIF rotor is operated at a low speed, and the surface oxidized slag is reduced to prevent mixing into the melt; the melting temperature is controlled at 680-750°C.

[0061] (2) Casting: The cooling water flow rate during casting is 190-220 L / s; in some embodiments, the ingot thickness can be controlled at 540-640 mm.

[0062] (3) Milling: After the aluminum alloy ingot is extracted from the casting machine, the side surface milling (double side) amount is controlled to be greater than or equal to 20 mm, and the thickness milling (double side) amount is controlled to be between 20-30 mm.

[0063] (4) Ingot heating: First, the ingot is wiped: D40 is used to wipe the upper and lower surfaces of each ingot to ensure that the ingot surface is free of oil stains, aluminum chips, and dust residues; the process during ingot heating can be controlled as follows: differential temperature heating, increasing the heating rate, the furnace gas temperature can be 535°C, the ingot heating temperature is 475-485°C, the heating time is 3 h, and the temperature is maintained for 3-6 h after heating.

[0064] (5) Rough rolling: To ensure that edge cracking does not occur during rolling, 2-4 passes of vertical rolls are used before rough rolling; the edge amount of each pass is 12-20 mm; the total reduction rate of rough rolling is controlled to be greater than 95%; the intermediate blank thickness is controlled to be 16-32 mm; then the intermediate blank is subjected to reciprocating rolling in the rough rolling mill, during which the 120 mm thick intermediate blank is cut at the heavy shear to remove the head and tail, and the 30 mm thick intermediate blank is cut at the light shear to remove the head.

[0065] In some embodiments, before alloy rolling, the rough rolling emulsion concentration needs to be raised for a period of 24 h, and the concentration is raised for a period of 24 h, and the concentration is raised for a period of 24 h.

[0066] (6) Finish rolling: the rolling pass reduction amount is controlled to be 40-43%, and the final rolling pass reduction amount is increased to 50%.

[0067] (7) Coiling: the coiling temperature is controlled to be 350-380°C, and the coiling tension can be 12-18 / N / mm 2 .

[0068] Table 1 lists the mass percentage of each chemical element in the aluminum alloy for storage tanks of embodiments 1-12.

[0069] Table 1. (wt%, the balance is Al)

[0070] Table 2-1 and Table 2-2 list the specific process parameters of the aluminum alloy for storage tanks manufactured in Examples 1-12.

[0071] Table 2-1.

[0072] Table 2-2.

[0073] The aluminum alloy for storage tanks prepared in Examples 1-12 were sampled, and their mechanical properties were tested and microstructure was observed, and the test results are listed in Table 3. Among them:

[0074] Tensile test: The sample was prepared and tensile tested according to GB / T 228.1-2021 Metal Materials Tensile Test Part 1: Room Temperature Test Method.

[0075] Microstructure observation: The microstructure in the plate was observed by transmission electron microscopy.

[0076] Table 3 lists the performance test results and microstructure observation results of the aluminum alloy for storage tanks of Examples 1-12.

[0077] Table 3.

[0078] As can be seen from the above Table 3, the microstructure of the aluminum alloy for storage tanks of Examples 1-12 prepared by the manufacturing method described in the present application all have at least one of the second phases Al6(FeMn), Al6(FeMnSi), Al 12 (CrFe)Al7. At the same time, the aluminum alloy for storage tanks of Examples 1-12 also has very excellent mechanical properties, with a yield strength greater than 160 MPa, a tensile strength greater than 290 MPa, and an elongation greater than 28%.

[0079] Figure 2 shows the second phase of Examples 2 and 11 of the present application. As shown in Figure 2, these second phases are uniformly distributed in the material matrix, and there is no aggregation phenomenon.

[0080] In addition, the aluminum alloy for storage tanks of Examples 1-12 were tested for welding performance, the welding method was MIG welding, Ar was used as the protective gas, and ER5356 was used as the welding wire. After welding, the sample plate was annealed, and the annealing parameters were 3h furnace temperature rise to 420℃, and 10h holding after temperature. The aluminum alloy for storage tanks of Examples 1-12 after welding were sampled, and their mechanical properties were tested, and the performance test results are listed in Table 4. Among them:

[0081] Tensile test: The sample was prepared and tensile test was carried out according to GB / T 2651-2023 Metal materials - Wrought-steel failure test - Transverse tensile test method

[0082] Table 4 lists the results of the welding performance test of the aluminum alloy for storage tanks of examples 1-12.

[0083] Table 4.

[0084] As can be seen from the above table 4, the aluminum alloy for storage tanks of examples 1-12 prepared by the manufacturing method of the present application still has very excellent mechanical properties after welding, the yield strength is greater than 155 MPa, the tensile strength is greater than 295 MPa, and the elongation is greater than 28%.

[0085] It should be noted that the combination of the technical features in the case is not limited to the combination mode recorded in the claims of the case or the combination mode recorded in the specific embodiments. All the technical features recorded in the case can be freely combined or combined in any way, unless contradictory to each other.

[0086] It should also be noted that the above examples are only specific embodiments of the present application. Obviously, the present application is not limited to the above examples, and similar changes or modifications that can be directly derived or easily conceived by those skilled in the art from the disclosure of the present application should all fall within the scope of protection of the present application.

Claims

1. An aluminum alloy for a storage tank, containing Al and unavoidable impurity elements, characterized by comprising, in mass %, It also contains the following chemical elements in the following mass percentages: Si: 0.02-0.09%, Mg: 4.0-4.6%, Fe: 0.10-0.30%, Mn: 0.2-0.8%, Cu: 0.002-0.012%, Cr: 0.06-0.12%.

2. The aluminum alloy for storage tanks according to claim 1, characterized by The mass percentages of the chemical elements are: Si: 0.02-0.09%, Mg: 4.0-4.6%, Fe: 0.10-0.30%, Mn: 0.2-0.8%, Cu: 0.002-0.012%, Cr: 0.06-0.12%; the balance being Al and inevitable impurity elements.

3. The aluminum alloy for a storage tank according to claim 1 or 2, characterized in that, The total amount of inevitable impurity elements is ≤0.15%.

4. The aluminum alloy for storage tanks according to claim 1 or 2, characterized by, The mass percentage of Si is 0.03-0.07%, and / or the mass percentage of Mn is 0.3-0.7%.

5. The aluminum alloy for storage tanks according to claim 1 or 2, characterized by, The microstructure thereof has at least one of Al6(FeMn), Al6(FeMnSi), Al 12 (FeMn)3Si, (CrFe)Al7.

6. The aluminum alloy for storage tanks according to claim 1 or 2, wherein The mechanical properties meet the following requirements: yield strength ≥ 160 MPa, tensile strength ≥ 290 MPa, and elongation ≥ 28%.

7. The aluminum alloy for storage tanks according to claim 6, wherein The mechanical properties meet the following requirements: yield strength is 160-170 MPa, tensile strength is 290-305 MPa, and elongation is 28-32%.

8. The aluminum alloy for storage tanks according to claim 1 or 2, wherein The mechanical properties of the weld obtained after welding meet the following requirements: yield strength ≥ 155 MPa, tensile strength ≥ 295 MPa, and elongation ≥ 28%.

9. The aluminum alloy for storage tanks according to claim 8, wherein The mechanical properties of the weld obtained after welding meet the following requirements: yield strength ≥ 160 MPa, tensile strength ≥ 300 MPa.

10. The method of producing an aluminum alloy for a storage tank according to any one of claims 1 to 9, characterized in that, The steps include: melting and casting to obtain an ingot, wherein the cooling water flow rate during casting is 190-220 L / s; face milling; ingot heating; rough rolling: the total reduction rate of rough rolling is controlled to be more than 95%; finish rolling; coiling: the coiling temperature is controlled to be 350-380°C.

11. The production method according to claim 10, wherein In the melting step, the melting temperature is controlled to be 680-750°C.

12. The production method according to claim 10, wherein In the ingot heating step, the ingot heating temperature is controlled to be 475-485°C, and the temperature is maintained for 3-6 h after reaching the temperature.

13. The production method according to claim 10, wherein In the rough rolling step, 2-4 stands of vertical rolls are used before the head and tail of the rough rolling are cut, and the edge amount of each stand is 12-20 mm.

14. The production method according to claim 10, wherein In the rough rolling step, the thickness of the intermediate billet obtained by rough rolling is 16-32 mm.

15. The production method according to claim 10, wherein In the winding step, the winding tension is controlled to be 12-18 N / mm 2 .

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