Method for manufacturing aluminum alloy drawn material

Abstract

Provided is a method for manufacturing an aluminum alloy rolled material, which can produce an aluminum alloy rolled material having excellent mechanical properties such as elongation and strength at a high recycling rate using various types of aluminum alloy scraps. 【Solution means】The method for manufacturing an aluminum alloy rolled material includes a casting step S12 of an ingot for an aluminum alloy part, a rolling step S32, and a final annealing step S33. The ingot for the aluminum alloy part contains 1.0 mass% or more and 7.5 mass% or less of Si, the contents of Fe and Mn are controlled, and the upper limit values of the contents of Cu, Mg, and Zn are regulated. When the contents of the above Si and the above Mg are [Si] and [Mg] in mass%, respectively, the value α calculated by the formula (1): α = [Si] - 2 × [Mg] is 0 or more, and the value β calculated by the formula (2): β = [Si] + 2.25 × [Mg] is 7.5 or less, and the final annealing temperature in the final annealing step is 265°C or higher.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing an aluminum alloy sheet.

Background Art

[0002] In recent years, achieving carbon neutrality has become an issue for society as a whole, and various methods for solving the above problems have been studied. For example, from the perspective of resource depletion, recycling of various materials has progressed, and recycling of metals that are consumed in large quantities has been carried out for a long time. For example, aluminum consumes a large amount of electricity and emits CO2 during the production of new ingots. Therefore, by reducing the amount of new ingots used through recycling, the amount of CO2 emissions during the production of aluminum alloy materials can be significantly reduced. However, due to the influence of impurities in the scraps used for recycling, there are various problems in applying recycling to sheets compared to casting materials. Patent Document 1 proposes a method for manufacturing an aluminum alloy for automotive members, in which aluminum alloy casting scraps are added with sash scraps, aluminum can scraps (UBC: Used Beverage Can) or ingots, melted to dilute impurities, and component adjustment is performed as necessary.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, even when using the manufacturing method described in Patent Document 1 above, it is necessary to make the Si content in the alloy after component adjustment, for example, 4.85% by mass or less. In addition, when the usage rate of casting parts scraps or automotive casting scraps is increased, the content of impurities increases, and the elongation and bendability of the obtained aluminum alloy material are significantly reduced.

[0005] The present invention has been made in view of such problems, and uses various types of aluminum alloy scraps to produce an aluminum alloy rolled material with excellent mechanical properties such as elongation and strength, and particularly high formability, at a high recycling rate. The purpose is to provide a method for manufacturing an aluminum alloy rolled material.

Means for Solving the Problems

[0006] The above object is achieved by the method for manufacturing an aluminum alloy rolled material according to the following [1] of the present invention.

[0007] [1] A method for manufacturing an aluminum alloy rolled material having an aluminum alloy part, A casting step of casting an ingot for an aluminum alloy part containing aluminum alloy scraps, A rolling step of rolling the ingot for the aluminum alloy part, A final annealing step of performing final annealing after the rolling step, and The ingot for the aluminum alloy part, with respect to the total mass of the ingot for the aluminum alloy part, Si: 1.0% by mass or more and 7.5% by mass or less, Fe: 0.15% by mass or more and 0.8% by mass or less, Mn: 0.3% by mass or more and 1.0% by mass or less, contains Cu: 0.5% by mass or less, Mg: 2.0% by mass or less, Zn: 0.7% by mass or less, and The balance consists of Al and unavoidable impurities, When the content of Si with respect to the total mass of the ingot for the aluminum alloy part is [Si] in mass%, and the content of Mg with respect to the total mass of the ingot for the aluminum alloy part is [Mg] in mass%, The value α calculated by the following formula (1) is 0 or more, and the value β calculated by the following formula (2) is 7.5 or less, The method for manufacturing an aluminum alloy rolled material, characterized in that the final annealing temperature in the final annealing step is 265°C or higher. Formula (1): α = [Si] - 2×[Mg] Formula (2): β = [Si] + 2.25×[Mg]

[0008] Moreover, the method for manufacturing the aluminum alloy drawn material of the present invention is preferably the following [2] to [4].

[0009] [2] The method for manufacturing an aluminum alloy drawn material according to [1], wherein the final annealing temperature in the final annealing step is 320°C or lower.

[0010] [3] The aluminum alloy scraps are scraps of automotive heat exchangers containing 0.50% by mass or more of Si and 0.10% by mass or more of Zn, scraps of aluminum cans containing 0.5% by mass or more of Mn and 0.8% by mass or more of Mg, sash scraps containing 0.20% by mass or more of Si, 0.35% by mass or more of Fe, and 0.2% by mass or more of Mg, and casting scraps containing 4.00% by mass or more of Si, including at least one selected from The method for manufacturing an aluminum alloy drawn material according to [1] or [2], wherein the total amount of the scraps is 50% by mass or more based on the total mass of the ingot for the aluminum alloy part.

[0011] [4] The method for manufacturing an aluminum alloy drawn material according to [3], wherein the aluminum alloy scraps include two or more selected from the scraps of automotive heat exchangers, the scraps of aluminum cans, the sash scraps, and the casting scraps.

Advantages of the Invention

[0012] According to the method for manufacturing an aluminum alloy drawn material according to the present invention, an aluminum alloy drawn material having excellent mechanical properties such as elongation and strength and particularly high formability can be manufactured at a high recycling rate using various types of aluminum alloy scraps.

Brief Description of the Drawings

[0013]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Modes for Carrying Out the Invention

[0014] It is expected that aluminum alloy scraps will become difficult to obtain from the market due to the expanding future demand. Therefore, the inventors of the present application have intensively studied a method for manufacturing an aluminum alloy sheet that allows for the blending of various alloy scraps without being limited to specific alloy scraps. In addition, the contents of Si and Mg and their ratios, etc. have a great influence on the formability of aluminum alloy materials. Therefore, the inventors of the present application have found a method for manufacturing an aluminum alloy sheet having a suitable range of the content of each component and a suitable balance between the Si content and the Mg content.

[0015] Hereinafter, a method for manufacturing an aluminum alloy sheet according to an embodiment of the present invention will be described. In this specification, the "aluminum alloy sheet" may be simply referred to as the "sheet".

[0016] [Method for Manufacturing Aluminum Alloy Sheet] The manufacturing method of the aluminum alloy flat product according to the present embodiment is a method for manufacturing an aluminum alloy flat product having an aluminum alloy part, and includes a casting process, a rolling process, and a final annealing process. The manufacturing method of the aluminum alloy flat product having an aluminum alloy part will be described in detail below.

[0017] 〔Casting process〕 The casting process is a process of casting an ingot for an aluminum alloy part containing aluminum alloy scraps. More specifically, using various aluminum alloy scraps so as to have the chemical composition described later, and adding ingots for adjustment as necessary, a material alloy is melted, and from the obtained molten metal, an ingot for an aluminum alloy part having a specific alloy composition is produced. The method of melting and casting the aluminum alloy is not particularly limited, and a conventional method or a known method may be used. The reasons for limiting the chemical components and their contents contained in the ingot for the aluminum alloy part will be specifically described below.

[0018] <Ingot for aluminum alloy part> (Si: 1.0 mass% or more and 7.5 mass% or less) Si is an element that easily generates Si compounds and Si grains having a size that can serve as recrystallization nuclei. On the other hand, when the content of Si becomes excessive, the Si compounds and Si grains are more likely to coarsen, resulting in a decrease in elongation and a deterioration in formability. Note that Si is an element contained in various aluminum alloy scraps. In the present embodiment, since the value calculated by a specific formula using the content of Si and the final annealing temperature are defined within an appropriate range, even with a high recycling rate, mechanical properties such as elongation and strength are good, and the formability of the aluminum alloy part can be improved.

[0019] If the Si content in the ingot part for the aluminum alloy is less than 1.0% by mass, there will be a shortage of compounds serving as recrystallization nuclei, making it difficult to obtain the desired elongation. Also, if the Si content in the ingot for the aluminum alloy part is less than 1.0% by mass, it is necessary to reduce the usage amount of aluminum alloy scraps with a high Si content, and the recycling rate cannot be improved. Therefore, the Si content in the ingot for the aluminum alloy part should be 1.0% by mass or more, preferably 1.5% by mass or more, and more preferably 2.0% by mass or more, based on the total mass of the ingot for the aluminum alloy part. On the other hand, if the Si content in the ingot for the aluminum alloy part exceeds 7.5% by mass, large Si compounds and Si grains are likely to be formed, resulting in a decrease in elongation and leading to a decrease in formability. Therefore, the Si content in the ingot for the aluminum alloy part should be 7.5% by mass or less, preferably 6.0% by mass or less, and more preferably 5.0% by mass or less, based on the total mass of the ingot for the aluminum alloy part.

[0020] (Fe: 0.15% by mass or more and 0.8% by mass or less) Fe is an element that easily forms intermetallic compounds of a size that can serve as recrystallization nuclei. Therefore, if the Fe content is less than 0.15% by mass, there will be a shortage of compounds serving as recrystallization nuclei, making it difficult to obtain the desired elongation. Also, since Fe is generally contained in aluminum alloy scraps, in order to make the Fe content in the ingot for the aluminum alloy part less than 0.15% by mass, it is necessary to use high-purity aluminum ingots, resulting in higher manufacturing costs. Therefore, the Fe content in the ingot for the aluminum alloy part should be 0.15% by mass or more, preferably 0.2% by mass or more, and more preferably 0.25% by mass or more, based on the total mass of the ingot for the aluminum alloy part. On the other hand, if the Fe content in the ingot for the aluminum alloy part exceeds 0.8% by mass, large intermetallic compounds are likely to be formed during casting, resulting in a decrease in elongation and leading to a decrease in formability. Therefore, the Fe content in the ingot for the aluminum alloy part should be 0.8% by mass or less, preferably 0.75% by mass or less, and more preferably 0.7% by mass or less, based on the total mass of the ingot for the aluminum alloy part.

[0021] (Mn: 0.3 mass% or more and 1.0 mass% or less) Mn is an element that has the effect of improving the strength of the aluminum alloy part. If the Mn content in the ingot for the aluminum alloy part is less than 0.3 mass%, the effect of improving the strength cannot be sufficiently obtained. Therefore, the Mn content with respect to the total mass of the ingot for the aluminum alloy part is 0.3 mass% or more, preferably 0.4 mass% or more, and more preferably 0.5 mass% or more. On the other hand, if the Mn content in the ingot for the aluminum alloy part exceeds 1.0 mass%, large intermetallic compounds are likely to be formed during casting, resulting in a decrease in elongation and leading to a decrease in formability. Therefore, the Mn content in the ingot for the aluminum alloy part is 1.0 mass% or less with respect to the total mass of the ingot for the aluminum alloy part, preferably 0.95 mass% or less, and more preferably 0.9 mass% or less.

[0022] (Cu: 0.5 mass% or less) Cu is an element that affects the strength and formability of the drawn material. Generally, Cu is contained in the aluminum alloy scraps. However, in this embodiment, the Cu contained in the ingot for the aluminum alloy part may be 0 mass%. However, when Cu is contained in the aluminum alloy part for the purpose of improving strength, the Cu content in the ingot for the aluminum alloy part is preferably 0.1 mass% or more, and more preferably 0.15 mass% or more with respect to the total mass of the ingot for the aluminum alloy part. On the other hand, if the Cu content in the ingot for the aluminum alloy part exceeds 0.5 mass%, the elongation decreases and leads to a decrease in formability. Therefore, the Cu content in the ingot for the aluminum alloy part is 0.5 mass% or less with respect to the total mass of the ingot for the aluminum alloy part, preferably 0.45 mass% or less, and more preferably 0.4 mass% or less.

[0023] (Mg: 2.0 mass% or less) Mg is an element that affects the strength and formability of the rolled material. In the present embodiment, the Mg contained in the ingot for the aluminum alloy part may be 0% by mass. However, when Mg is contained in the aluminum alloy part for the purpose of improving the strength, the Mg content in the ingot for the aluminum alloy part is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, based on the total mass of the ingot for the aluminum alloy part. On the other hand, when the Mg content in the ingot for the aluminum alloy part exceeds 2.0% by mass, large Mg compounds are likely to be formed, resulting in a decrease in elongation and a decrease in formability. Therefore, the Mg content in the ingot for the aluminum alloy part is 2.0% by mass or less, preferably 1.5% by mass or less, more preferably 1.0% by mass or less, based on the total mass of the ingot for the aluminum alloy part.

[0024] (Zn: 0.7% by mass or less) In the present embodiment, the Zn contained in the ingot for the aluminum alloy part may be 0% by mass. However, since Zn is generally contained in the aluminum alloy scraps, it is considered that the Zn content in the ingot for the aluminum alloy part is 0.1% by mass or more based on the total mass of the ingot for the aluminum alloy part. On the other hand, when the Zn content in the ingot for the aluminum alloy part exceeds 0.7% by mass, the elongation may decrease due to an increase in precipitates such as MgZn2, leading to a decrease in formability. Also, when the Zn content in the ingot for the aluminum alloy part exceeds 0.7% by mass, the pitting potential becomes lower and the corrosion rate increases. Therefore, the Zn content in the ingot for the aluminum alloy part is 0.7% by mass or less, preferably 0.65% by mass or less, more preferably 0.6% by mass or less, based on the total mass of the ingot for the aluminum alloy part.

[0025] (Other elements) In the aluminum alloy rolled material according to the present embodiment, the total of Al, Si, Fe, Mn, Cu, Mg, and Zn in the ingot for the aluminum alloy part is preferably 98.5% by mass or more, more preferably 99% by mass or more, still more preferably 99.5% by mass or more, based on the total mass of the ingot for the aluminum alloy part. In addition, examples of other elements of the above elements include Cr, Zr, Ni, Sn, Sr, Sb, and Ca. Since these elements may be contained in the aluminum alloy scraps, they may also be contained in the ingot for the aluminum alloy part obtained using these alloy scraps. In the present embodiment, the contents of Cr, Zr, Ni, Sn, Pb, Sr, Sb, and Ca in the ingot for the aluminum alloy part are not particularly limited. For example, the Cr content is preferably 0.1% by mass or less, the Zr content is preferably 0.1% by mass or less, the Ni content is preferably 0.2% by mass or less, the Sn content is preferably 0.1% by mass or less, the Sr content is preferably 0.18% by mass or less, the Sb content is preferably 0.5% by mass or less, and the Ca content is preferably 0.1% by mass or less.

[0026] (Balance: Al and inevitable impurities) The balance of the ingot for the aluminum alloy part in the present embodiment is Al and inevitable impurities. Examples of the inevitable impurities include Na. The Na content in the ingot for the aluminum alloy part is preferably 0.002% by mass or less based on the total mass of the ingot for the aluminum alloy part. Also, the Al content in the ingot for the aluminum alloy part is preferably 80% by mass or more based on the total mass of the ingot for the aluminum alloy part, and the total amount of the inevitable impurities in the ingot for the aluminum alloy part is preferably 0.15% by mass or less based on the total mass of the ingot for the aluminum alloy part.

[0027] Next, an equation using the Si content and the Mg content in the ingot for the aluminum alloy part will be described.

[0028] (Value α calculated by formula (1): 0 or more) When the Mg content increases too much with respect to the Si content, the strength increases due to solid solution strengthening or precipitation effect, and the elongation decreases. In the present embodiment, the effects of the Si content and the Mg content on the formability are expressed by the following formula (1). That is, when the value α calculated by the following formula (1) becomes smaller than 0, the elongation decreases and the formability of the aluminum alloy part deteriorates. Therefore, the above value α is 0 or more, preferably 0.5 or more, and more preferably 0.75 or more.

[0029] Formula (1): α = [Si] - 2×[Mg]

[0030] (Value β calculated by formula (2): 7.5 or less) When both the Si content and the Mg content are high, the precipitation of large Mg2Si precipitates increases, and the elongation decreases due to the influence of the precipitates. In the present embodiment, the influence of the precipitates is expressed by the following formula (2). That is, when the value β calculated by the following formula (2) exceeds 7.5, the elongation of the aluminum alloy part decreases. Therefore, the above value β is 7.5 or less, preferably 7.0 or less, and more preferably 6.5 or less.

[0031] Formula (2): β = [Si] + 2.25×[Mg]

[0032] However, in the above formulas (1) and (2), [Si] is the value representing the content of Si in mass % with respect to the total mass of the ingot for the aluminum alloy part, and [Mg] is the value representing the content of Mg in mass % with respect to the total mass of the ingot for the aluminum alloy part.

[0033] Note that the aluminum alloy part in this embodiment can be manufactured from various aluminum alloy scraps with a high recycling rate. To obtain excellent mechanical properties, the content of each component in the ingot for the aluminum alloy part, the value obtained from the formula, and the lower limit value of the final annealing temperature are adjusted. The recycling rate is the percentage of the mass of the used aluminum alloy scraps to the total mass of the obtained aluminum alloy part. If the recycling rate is 50% or more, an aluminum alloy part can be obtained using a sufficient amount of aluminum alloy scraps, and the reduction of manufacturing cost and CO2 emissions can be sufficiently achieved. Therefore, a high recycling rate is preferably 50% or more, more preferably 60% or more, further preferably 75% or more, and particularly preferably 90% or more.

[0034] <Aluminum alloy scraps> In the method for manufacturing an extended material according to this embodiment, the aluminum alloy scraps used when casting the ingot for the aluminum alloy part are not limited to specific alloy scraps, and various combinations of alloy scraps can be used. Specifically, the aluminum alloy scraps preferably contain at least one selected from various scraps described in the following (A) to (D), and more preferably contain two or more.

[0035] (A) Scrap of automotive heat exchangers The scrap of automotive heat exchangers includes the scrap collected from the heat exchangers equipped in commercial vehicles and the scrap that is the end material when manufacturing automotive heat exchangers. Therefore, this scrap of automotive heat exchangers includes brazing materials, core materials, and sacrificial anode materials. As the scrap of automotive heat exchangers, for example, those containing Si: 0.50 mass% or more and Zn: 0.10 mass% or more can be used.

[0036] (B) Scrap of aluminum cans (UBC: Used Beverage Can) As scrap of aluminum cans, it includes scrap of the aluminum can body and scrap of the aluminum can lid. The scrap of the aluminum can body and the scrap of the aluminum can lid differ in the contained components and their contents. For example, there are scraps collected only of the aluminum can body, scraps collected only of the aluminum can lid, and scraps in which these are mixed. In the present embodiment, all of these are grouped together and shown as scrap of aluminum cans. As the scrap of aluminum cans, for example, those containing Mn: 0.5% by mass or more and Mg: 0.8% by mass or more can be used.

[0037] (C) Sash scrap Sash scrap is scrap of a window frame made of an aluminum alloy. As sash scrap, for example, those containing Si: 0.20% by mass or more, Fe: 0.35% by mass or more, and Mg: 0.2% by mass or more can be used.

[0038] (D) Casting scrap Casting scrap includes scrap of members using the AC4C material specified in JIS H 5202, scrap of members using the ADC12 material specified in JIS H 5302, etc. As casting scrap, those containing Si: 4.00% by mass or more can all be used.

[0039] In the present embodiment, it is preferable that the total amount of the scrap described in the above (A) to (D) is 50% by mass or more with respect to the total mass of the ingot for the aluminum alloy part. That is, according to the method for manufacturing an aluminum alloy sheet according to the present embodiment, even when the total amount of the scrap described in the above (A) to (D) with respect to the ingot for the aluminum alloy part is high, an aluminum alloy sheet excellent in mechanical properties such as elongation and strength and having high formability can be obtained. The total amount of the scrap described in the above (A) to (D) is more preferably 60% by mass or more, and even more preferably 75% by mass or more with respect to the total mass of the ingot for the aluminum alloy part.

[0040] In this embodiment, in addition to the scraps described in the above (A) to (D), other aluminum alloy scraps and ingots can be used as raw materials for the ingots for aluminum alloy parts. Examples of other aluminum alloy scraps include aluminum alloy scraps contained in automobiles other than automotive heat exchangers, and aluminum alloy scraps contained in household electrical appliances.

[0041] According to the manufacturing method according to this embodiment, an aluminum alloy part having excellent mechanical properties can be obtained by manufacturing from the above aluminum alloy scraps with a high recycling rate. The recycling rate is a value expressed as a percentage of the mass of the used aluminum alloy scraps with respect to the total mass of the obtained aluminum alloy part. In this embodiment, when the total amount of the scraps described in the above (A) to (D) is 50% by mass or more with respect to the total mass of the ingot for aluminum alloy parts, the recycling rate also becomes 50% or more. For this reason, an aluminum alloy part can be obtained by using a sufficient amount of aluminum alloy scraps, and the reduction of manufacturing cost and the reduction of CO2 emissions can be sufficiently achieved. The recycling rate is preferably 50% or more, more preferably 60% or more, still more preferably 75% or more, and particularly preferably 90% or more.

[0042] 〔Homogenization heat treatment process〕 Next, the obtained ingot for aluminum alloy parts is subjected to homogenization heat treatment. The conditions of the homogenization heat treatment are not particularly limited, but for example, it is preferably 450°C to 620°C.

[0043] 〔Rolling process〕 The rolling process is a process of rolling the above ingot for aluminum alloy parts. Examples of the rolling process include a hot rolling process and a cold rolling process. Each process will be described below.

[0044] <Hot rolling> The aluminum alloy material after homogenization heat treatment is subjected to hot rolling. The starting temperature and the ending temperature of the hot rolling are not particularly limited. For example, the starting temperature is preferably 440°C to 610°C, and the ending temperature is preferably 440°C to 610°C.

[0045] <Cold rolling> The aluminum alloy material after the hot rolling is finished is subjected to cold rolling. The rolling reduction during cold rolling is not particularly limited. For example, it is preferably 50% or more and 98% or less.

[0046] 〔Final annealing process〕 The final annealing process is a process of performing final annealing after the above drawing process. If the temperature in the final annealing process is less than 265°C, the elongation decreases. Therefore, the final annealing temperature is 265°C or higher, preferably 280°C or higher, more preferably 290°C or higher, and even more preferably 300°C or higher. On the other hand, the upper limit of the final annealing temperature is not particularly limited. For example, when it is 330°C or lower, the elongation can be improved. Therefore, the final annealing temperature is preferably 330°C or lower, and more preferably 320°C or lower. The holding time at the final annealing temperature is not particularly limited and can be, for example, 1 hour or more and 5 hours or less. Also, the cooling rate after the final annealing is not particularly limited. Furthermore, the final annealing treatment may be performed in a batch annealing furnace or a continuous annealing furnace, but it is preferable to use a continuous annealing furnace.

[0047] The aluminum alloy flat product obtained by the manufacturing method according to this embodiment may be partially composed only of an aluminum alloy part, or may be entirely composed of an aluminum alloy part. Examples of a case where a part of the aluminum alloy flat product is composed of an aluminum alloy part include a clad material. Specifically, examples of the aluminum alloy flat product (clad material) include those having a core material composed of an aluminum alloy part and a skin material laminated on at least a part of the surface of this core material. Examples of the skin material include a sacrificial anode material described later. As the form of the clad material, the skin material may be laminated only on a part of the surface of the core material, or may be laminated on the entire surface of one or both surfaces. Further, a clad material obtained by extrusion may be used. For example, the skin material may be laminated on at least one of the inner surface and the outer surface of a hollow extruded material, or on the outer surface of a solid extruded material.

[0048] Next, the manufacturing method of the aluminum alloy flat product in the case where the aluminum alloy flat product is a clad material will be described in detail by taking the first to fourth embodiments as examples.

[0049] [First Embodiment] FIG. 1 is a flowchart showing a manufacturing method of an aluminum alloy flat product according to the first embodiment of the present invention. With reference to FIG. 1, the manufacturing method of the aluminum alloy flat product according to the first embodiment will be described.

[0050] [Manufacture of Ingot for Aluminum Alloy Part] [Melting and Casting Process] [Melting Step S11 of Raw Material for Aluminum Alloy Part] First, in the melting step S11 of the raw material for the aluminum alloy part, at least one type of scrap (specific scrap) among the scraps described in the above (A) to (D), and if necessary, other aluminum alloy scraps, new ingots, and ingots for adjustment are melted.

[0051] [Casting Step S12 of Ingot for Aluminum Alloy Part] Next, an ingot for the aluminum alloy part is cast. Specifically, the molten metal obtained by melting the raw material for the aluminum alloy part is poured into a mold and cooled to obtain an ingot for the aluminum alloy part. In this embodiment, the composition of the ingot for the aluminum alloy part is as described above.

[0052] <Homogenization treatment process> (Homogenization treatment process S13 of the ingot for the aluminum alloy part) Thereafter, the ingot for the aluminum alloy part is subjected to a homogenization treatment to obtain a homogenized ingot for the aluminum alloy part (a homogenized material for the aluminum alloy part).

[0053] [Manufacture of the ingot for the skin material] <Ingot casting process for the skin material> (Melting process S21 of the casting raw material for the skin material) Separate from the manufacturing process of the ingot for the aluminum alloy part, an ingot for the skin material is manufactured. First, in the melting process S21 of the casting raw material for the skin material, the casting raw material for the skin material is melted. Although the composition of the casting raw material for the skin material is not particularly limited, when producing a sacrificial anode material as the skin material, for example, the materials of the sacrificial anode material described later can be adopted.

[0054] (Casting process S22 of the ingot for the skin material) Next, an ingot for the skin material is cast. Specifically, the molten metal obtained by melting the casting raw material for the skin material is poured into a mold and cooled to obtain an ingot for the skin material.

[0055] <Homogenization treatment process> (Homogenization treatment process S23 of the ingot for the skin material) Thereafter, the ingot for the skin material is subjected to a homogenization treatment to obtain a homogenized ingot for the skin material (a homogenized material for the skin material).

[0056] [Stretching process] (Combination process S31) In the combination step S31, an ingot for an aluminum alloy part after homogenization treatment and an ingot for a skin material after homogenization treatment are combined to produce a combined material. As a method of combination, for example, the ingot for the skin material may be overlapped on a part or the entire surface of one main surface of the ingot for the aluminum alloy part, or the ingot for the skin material may be overlapped on a part or the entire surface of both main surfaces.

[0057] (Stretching step S32 of the combined material) Thereafter, the combined material produced in the above combination step S31 is stretched.

[0058] (Final annealing step S33) Thereafter, final annealing is performed on the stretched combined material at a temperature of 265°C or higher. Thereby, an aluminum alloy stretched material made of a clad material including a core material made of an aluminum alloy part and a skin material combined with at least a part of the surface of the core material can be manufactured.

[0059] [Second Embodiment] FIG. 2 is a flowchart showing a method for manufacturing an aluminum alloy stretched material according to the second embodiment of the present invention. With reference to FIG. 2, a method for manufacturing an aluminum alloy stretched material according to the second embodiment will be described.

[0060] [Manufacture of Ingot for Aluminum Alloy Part] Regarding the melting step S11 of the raw material for the aluminum alloy part, the casting step S12 of the ingot for the aluminum alloy part, and the homogenization treatment step S13 of the ingot for the aluminum alloy part, since they are the same as those in the first embodiment, the description thereof will be omitted in the second embodiment.

[0061] [Stretching Step] (Core Material Production Step S14) The ingot for the aluminum alloy part after the homogenization treatment step S13 is stretched to produce a core material.

[0062] [Manufacture of Ingot for Skin Material] For the melting process S21 of the casting raw material for the skin material, the casting process S22 of the skin material ingot, and the homogenization treatment process S23 of the skin material ingot, since they are the same as those in the first embodiment, the description thereof is omitted in the second embodiment.

[0063] (Skin material production process S24) The skin material is manufactured by slicing or stretching the skin material ingot after the homogenization treatment process S23. In the second embodiment, the melting process S21 of the casting raw material for the skin material ingot, the casting process S22 of the skin material ingot, the homogenization treatment process S23 of the skin material ingot, and the skin material production process S24 may be carried out before or after the production of the ingot for the aluminum alloy part, and pre-manufactured skin material ingots or skin materials may also be used. Also, in the skin material production process, as a method for producing a skin material with a desired thickness, either one or both of the methods of slicing the ingot or stretching it can be adopted.

[0064] (Combination process S41) In the combination process S41, the core material produced by the core material production process S14 and the skin material produced by the skin material production process S24 are combined to produce a combined material. The combination method is the same as that in the first embodiment.

[0065] (Stretching process S42 of the combined material) Thereafter, the combined material produced in the combination process S41 is stretched.

[0066] (Final annealing process S43) Thereafter, final annealing is carried out on the stretched combined material at a temperature of 265 °C or higher. Thereby, an aluminum alloy stretched material made of a clad material including a core material made of an aluminum alloy part and a skin material combined with at least a part of the surface of the core material can be manufactured.

[0067] [Third Embodiment] Figure 3 is a flowchart showing a method for manufacturing an aluminum alloy clad material according to the third embodiment of the present invention. With reference to Figure 3, the method for manufacturing an aluminum alloy clad material according to the third embodiment will be described.

[0068] [Manufacture of Ingot for Aluminum Alloy Part] For the melting step S11 of the raw material for the aluminum alloy part, the casting step S12 of the ingot for the aluminum alloy part, and the homogenization treatment step S13 of the ingot for the aluminum alloy part, since they are the same as those in the first embodiment, the description thereof is omitted in the third embodiment.

[0069] [Manufacture of Ingot for Skin Material] For the melting step S21 of the casting raw material for the skin material, the casting step S22 of the ingot for the skin material, the homogenization treatment step S23 of the ingot for the skin material, and the skin material production step S24, since they are the same as those in the second embodiment, the description thereof is omitted in the third embodiment.

[0070] [Cladding Process] (Combination Step S51) In the combination step S51, the ingot for the aluminum alloy part after the homogenization treatment and the skin material produced in the skin material production step S24 are combined to produce a combined material. The combination method is the same as that in the first embodiment.

[0071] (Cladding Process of Combined Material S52) Thereafter, the combined material produced in the combination step S51 is clad.

[0072] (Final Annealing Step S53) Thereafter, the clad combined material is subjected to final annealing at a temperature of 265°C or higher. Thereby, an aluminum alloy clad material composed of a core material made of an aluminum alloy part and a skin material combined with at least a part of the surface of the core material can be manufactured.

[0073] [Fourth Embodiment] FIG. 4 is a flowchart showing a method for manufacturing an aluminum alloy clad material according to the fourth embodiment of the present invention. With reference to FIG. 4, a method for manufacturing an aluminum alloy clad material according to the fourth embodiment will be described.

[0074] 〔Manufacture of Ingot for Aluminum Alloy Part〕 Regarding the melting step S11 of the raw material for the aluminum alloy part, the casting step S12 of the ingot for the aluminum alloy part, the homogenization treatment step S13 of the ingot for the aluminum alloy part, and the core material production step S14, since they are the same as those in the second embodiment described above, the description thereof is omitted in the fourth embodiment.

[0075] 〔Manufacture of Ingot for Skin Material〕 Regarding the melting step S21 of the casting raw material for the skin material, the casting step S22 of the ingot for the skin material, and the homogenization treatment step S23 of the ingot for the skin material, since they are the same as those in the first embodiment described above, the description thereof is omitted in the fourth embodiment.

[0076] (Combination Step S61) In the combination step S61, the core material after the core material production step S14 and the ingot for the skin material after the homogenization treatment step S23 are combined to produce a combined material. The combination method is the same as that in the first embodiment described above.

[0077] (Rolling Step S62 of Combined Material) Thereafter, the combined material produced in the combination step S61 is rolled.

[0078] (Final Annealing Step S63) Thereafter, final annealing is performed on the rolled combined material at a temperature of 265° C. or higher. Thereby, an aluminum alloy clad material including a core material made of an aluminum alloy part and a skin material combined with at least a part of the surface of the core material can be manufactured.

[0079] In the present invention, the conditions for casting and homogenization treatment of the ingot for the aluminum alloy part and the ingot for the skin material are not particularly limited, and general conditions can be applied. Further, as the method of spreading, for example, rolling can be used, and the conditions thereof can also be set as appropriate.

[0080] FIG. 5 is a schematic cross-sectional view showing an example of the shape of an aluminum alloy spread product manufactured by the manufacturing method according to each embodiment of the present invention. The aluminum alloy spread product 5 shown in FIG. 5 is a clad material obtained by extrusion. For example, an inner skin material 7 is laminated on the inner surface of a hollow core material 6, and an outer skin material 8 is laminated on the outer surface. Note that the skin material may be laminated only on one of the inner and outer surfaces, or the skin material may be laminated on the outer surface of a solid extruded material.

[0081] As the above skin material, in addition to the sacrificial anode material, a functional alloy material having a specific function can be adopted. The function and composition of the functional alloy material are not limited. As an example of the casting raw material for the skin material, the reasons for the numerical limitations of the components contained in the sacrificial anode material and their contents will be described below.

[0082] <Sacrificial anode material> (Zn: 0.50 mass% or more and 6.00 mass% or less) Zn in the sacrificial anode material is an element that has the effect of preventing pitting corrosion and crevice corrosion by lowering the potential of the base material and enhancing the sacrificial corrosion protection effect against the core material. If the Zn content in the sacrificial anode material is 0.50% by mass or more, a sufficient sacrificial corrosion protection effect can be obtained. Therefore, the Zn content in the sacrificial anode material is preferably 0.50% by mass or more, more preferably 0.60% by mass or more, and even more preferably 0.70% by mass or more with respect to the total mass of the sacrificial anode material. Also, when the Zn content in the sacrificial anode material is 6.00% by mass or less, it is possible to prevent the self-corrosiveness of the sacrificial anode material from increasing excessively and suppress the decrease in the corrosion resistance of the aluminum alloy extruded material. Therefore, the Zn content in the sacrificial anode material is preferably 6.00% by mass or less, more preferably 5.70% by mass or less, and even more preferably 5.50% by mass or less with respect to the total mass of the sacrificial anode material.

[0083] (Si: 0.05% by mass or more and 1.50% by mass or less) Si in the sacrificial anode material is an element that has the effect of improving the strength of the sacrificial anode material. If the Si content in the sacrificial anode material is 0.05% by mass or more, the effect of improving the strength can be obtained. Therefore, the Si content in the sacrificial anode material is preferably 0.05% by mass or more, more preferably 0.10% by mass or more, and even more preferably 0.15% by mass or more with respect to the total mass of the sacrificial anode material. Also, when the Si content in the sacrificial anode material is 1.50% by mass or less, the formation of Si-based compounds and Si particles can be suppressed, and the decrease in corrosion resistance can be prevented. Therefore, the Si content in the sacrificial anode material is preferably 1.50% by mass or less, more preferably 1.45% by mass or less, and even more preferably 1.40% by mass or less with respect to the total mass of the sacrificial anode material.

[0084] (Fe: 0.05% by mass or more and 2.00% by mass or less) Fe in the sacrificial anode material is an element that forms Al-Fe-Mn-Si-based compounds together with Si and Mn and has the effect of improving strength by dispersion strengthening. If the Fe content in the sacrificial anode material is 0.05% by mass or more, the effect of improving strength can be obtained. Therefore, the Fe content in the sacrificial anode material is preferably 0.05% by mass or more, more preferably 0.10% by mass or more, and even more preferably 0.12% by mass or more based on the total mass of the sacrificial anode material. Also, when the Fe content in the sacrificial anode material is 2.00% by mass or less, the formation of large intermetallic compounds during casting can be prevented, and the decrease in plastic workability can be suppressed. Therefore, the Fe content in the sacrificial anode material is preferably 2.00% by mass or less, more preferably 1.80% by mass or less, and even more preferably 1.60% by mass or less based on the total mass of the sacrificial anode material.

[0085] (Mg: 3.00% by mass or less) Mg in the sacrificial anode material is an element that has the effect of improving the strength of the sacrificial anode material itself by precipitating Mg2Si. However, in this embodiment, the Mg content in the sacrificial anode material may be 0% by mass. Also, when the Mg content in the sacrificial anode material is 3.00% by mass or less, it can be easily welded during hot clad rolling. Therefore, the Mg content in the sacrificial anode material is preferably 3.00% by mass or less, more preferably 2.80% by mass or less, and even more preferably 2.60% by mass or less based on the total mass of the sacrificial anode material.

[0086] (Mn: 1.80% by mass or less) Mn in the sacrificial anode material is an element that has the effect of improving strength by dissolving in the base material or forming an Al-Mn-Si-based intermetallic compound together with Si. However, in the present embodiment, the Mn content in the sacrificial anode material may be 0% by mass. Also, when the Mn content in the sacrificial anode material is 1.80% by mass or less, the formation of large intermetallic compounds during casting can be prevented, and a decrease in plastic workability can be suppressed. Therefore, the Mn content in the sacrificial anode material is preferably 1.80% by mass or less, more preferably 1.60% by mass or less, and even more preferably 1.40% by mass or less with respect to the total mass of the sacrificial anode material.

[0087] (Cu: 0.50% by mass or less) When the Cu content in the sacrificial anode material is 0.50% by mass or less, the pitting potential of the sacrificial anode material can be prevented from nobilizing, and the effect of sacrificial corrosion can be sufficiently obtained. Therefore, the Cu content in the sacrificial anode material is preferably 0.50% by mass or less, more preferably 0.40% by mass or less, and even more preferably 0.30% by mass or less with respect to the total mass of the sacrificial anode material.

[0088] (Cr: 0.30% by mass or less) Cr in the sacrificial anode material is an element that has the effect of improving strength by solid solution strengthening. However, in the present embodiment, the Cr content in the sacrificial anode material may be 0% by mass. Also, when the Cr content in the sacrificial anode material is 0.30% by mass or less, the formation of large intermetallic compounds during casting can be prevented, and a decrease in plastic workability can be suppressed. Therefore, the Cr content in the sacrificial anode material is preferably 0.30% by mass or less, more preferably 0.20% by mass or less, and even more preferably 0.10% by mass or less with respect to the total mass of the sacrificial anode material.

[0089] (Ti: 0.30% by mass or less) Ti in the sacrificial anode material is an element that has the effect of improving strength by solid solution strengthening. However, in this embodiment, the Ti content in the sacrificial anode material may be 0% by mass. Also, when the Ti content in the sacrificial anode material is 0.30% by mass or less, it is possible to prevent the formation of large intermetallic compounds during casting and suppress the decrease in plastic workability. Therefore, the Ti content in the sacrificial anode material is preferably 0.30% by mass or less, more preferably 0.20% by mass or less, and even more preferably 0.10% by mass or less with respect to the total mass of the sacrificial anode material.

[0090] (Zr: 0.30% by mass or less) Zr in the sacrificial anode material is an element that has the effect of improving strength by solid solution strengthening. However, in this embodiment, the Zr content in the sacrificial anode material may be 0% by mass. Also, when the Zr content in the sacrificial anode material is 0.30% by mass or less, it is possible to prevent the formation of large intermetallic compounds during casting and suppress the decrease in plastic workability. Therefore, the Zr content in the sacrificial anode material is preferably 0.30% by mass or less, more preferably 0.20% by mass or less, and even more preferably 0.10% by mass or less with respect to the total mass of the sacrificial anode material.

[0091] (V: 0.30% by mass or less) V in the sacrificial anode material is an element that has the effect of improving strength by solid solution strengthening. However, in this embodiment, the V content in the sacrificial anode material may be 0% by mass. Also, when the V content in the sacrificial anode material is 0.30% by mass or less, it is possible to prevent the formation of large intermetallic compounds during casting and suppress the decrease in plastic workability. Therefore, the V content in the sacrificial anode material is preferably 0.30% by mass or less, more preferably 0.20% by mass or less, and even more preferably 0.10% by mass or less with respect to the total mass of the sacrificial anode material.

[0092] (Balance: Al and unavoidable impurities) The remainder of the sacrificial anode material contained in the aluminum alloy drawn material is Al and inevitable impurities. The inevitable impurities include Ca, Be, Sb, rare earth elements, Li, etc. Specifically, it may be contained in the range of Ca: 0.05% by mass or less, Be: 0.01% by mass or less, and other elements: less than 0.01% by mass. Also, the total amount of inevitable impurities in the sacrificial anode material is preferably 0.05% by mass or less based on the total mass of the sacrificial anode material.

Examples

[0093] The present embodiment will be further specifically described below with reference to examples. However, the present invention is not limited to these examples, and modifications can be made and implemented within the scope that conforms to the gist of the present invention, and all of them are included in the technical scope of the present invention.

[0094] <Manufacture of Aluminum Alloy Drawn Material> Assuming that the raw materials for the aluminum alloy part shown in Table 1 below were used, these raw materials were melted at a temperature of 730°C. Then, by casting this at a temperature of 700°C, an ingot for the aluminum alloy part with a thickness of 50 mm, a width of 145 mm, and a length of 250 mm was manufactured. Regarding the scraps of aluminum cans, it is assumed that the can lids and can bodies are mixed, and as shown in Table 2 below, the mass ratio of the can lids to the can bodies was set to 25:75, and the composition of the scraps of aluminum cans was calculated. The ratios of the respective raw materials used when manufacturing the ingot for the aluminum alloy part and the recycling rate are shown in Table 3 below. The remainder of the components shown in Tables 1 and 2 is Al and inevitable impurities. Also, in Table 3, in addition to the respective raw materials in the ingot for the aluminum alloy part, 5% of the ingot for component adjustment was used, and the recycling rate was calculated. Therefore, in the case of a raw material that does not use any virgin ingot, the recycling rate is 95%.

[0095] Next, the obtained ingot for the aluminum alloy part was subjected to a homogenization treatment at an appropriate temperature in the range of 450°C to 620°C for 4 hours to obtain a homogenized material. Thereafter, hot rolling was performed on the homogenized material. In the hot rolling process, except for the non-uniform layer, the homogenized material with a height of 45 mm, a width of 145 mm, and a length of 100 mm was hot rolled until the thickness became 2.5 mm and the length became approximately 1.8 m. The starting temperature of the hot rolling was set to an appropriate temperature in the range of 440°C to 610°C. That is, the homogenized material after the homogenization treatment was taken out of the furnace, slowly cooled to an appropriate temperature in the range of 440°C to 610°C, and then hot rolling was started.

[0096] Thereafter, cold rolling was performed on the aluminum alloy plate subjected to hot rolling. In the cold rolling process, the aluminum alloy plate after hot rolling with a thickness of 2.5 mm and a length of approximately 1.8 m was cold rolled until the thickness became 0.15 mm. Thereafter, the aluminum alloy plate subjected to cold rolling was subjected to a final annealing treatment in a continuous annealing furnace. The temperature of this final annealing treatment was set to 240°C to 400°C and held for 1 hour to 5 hours. Thereby, a rolled material (aluminum alloy drawn material) of the above size made of an aluminum alloy part was obtained. The raw material No. used, the content of each component in the ingot for the aluminum alloy part constituting the aluminum alloy drawn material, and the values α and β calculated by formulas (1) and (2) are shown in Tables 4 and 5 below. The remainder of each component of the ingot for the aluminum alloy part shown in Table 1 is Al and unavoidable impurities.

[0097] <Evaluation of Aluminum Alloy Drawn Material> (Evaluation of Mechanical Properties) In accordance with the "Method of Tensile Testing for Metallic Materials" of JIS Z 2241:2023, a No. 13B test piece described in Appendix B was sampled from the obtained aluminum alloy drawn material, and a tensile test was carried out in the range of 10 to 35°C, and the tensile strength and elongation after fracture were measured. The measurement results are also shown in Tables 4 and 5 below. As evaluation criteria, those with a tensile strength of 120 MPa or more and an elongation of 14% or more were judged to have good mechanical properties. In addition, when the aluminum alloy sheet has excellent elongation, high formability can be obtained. Specifically, those with an elongation of 14% or more were judged to have particularly excellent formability.

[0098]

Table 1

[0099]

Table 2

[0100]

Table 3

[0101]

Table 4

[0102]

Table 5

[0103] <Evaluation Results of Aluminum Alloy Sheet As shown in Table 1 above, in Invention Examples No. 1 to 16, the content of components contained in the aluminum alloy sheet (ingot for aluminum alloy part), the value calculated by the formula, and the final annealing temperature are within the ranges defined in the present invention, and the mechanical properties such as elongation and strength are improved. In addition, in Invention Examples No. 5 to 7, the content of each component in the ingot for aluminum alloy part is the same, and only the final annealing temperature is changed. Among these invention examples, in particular, in Invention Examples No. 5 to 6, since the final annealing temperature is within a more preferable range defined in the present invention, the elongation is further improved.

[0104] In Comparative Example No. 1 and Comparative Example No. 2, the value α calculated from Formula (1) was smaller than the range defined in the present invention, and the solid solution amount and fine precipitates increased, so the strength increased and the elongation decreased.

[0105] In Comparative Example No. 3, since the Si content was less than the lower limit defined in the present invention, the elongation became small. In Comparative Example No. 4 and Comparative Example No. 5, since the value β calculated by Formula (2) was larger than the upper limit defined in the present invention, the large Mg2Si that was a factor in the decrease in elongation increased, and the elongation became small.

[0106] In Comparative Example No. 6, the Fe content was less than the lower limit defined in the present invention. In Comparative Example No. 7, the Fe content and the Mn content exceeded the upper limits defined in the present invention. Further, in Comparative Example No. 8, the Cu content exceeded the upper limit defined in the present invention. In Comparative Example No. 9, the Mn content exceeded the upper limit defined in the present invention. Furthermore, in Comparative Example No. 10, the Zn content exceeded the upper limit defined in the present invention. Therefore, in all of Comparative Examples No. 6 to 10, the elongation became smaller compared to the Invention Examples.

[0107] Comparative Example No. 11 is an example in which scrap of an automotive heat exchanger was used at a blending ratio of 100%. Since the Mn content exceeded the upper limit defined in the present invention, the elongation became small.

[0108] Comparative Example No. 12 is an example in which scrap of an aluminum can (UBC: Used Beverage Can) was used at a blending ratio of 100%. Since the Si content was less than the lower limit defined in the present invention and the value α calculated from Formula (1) was less than the lower limit defined in the present invention, the elongation became small.

[0109] Comparative Example No. 13 is an example in which sash scrap was used at a blending ratio of 100%. Since the Si content and the Mn content were less than the lower limits defined in the present invention and the value α calculated by Formula (1) was less than the lower limit defined in the present invention, the tensile strength became low.

[0110] Comparative Example No. 14 is an example in which an alloy symbol AC4C material specified in JIS H 5202:2010 was used at a blending ratio of 100%. Since the Mn content was less than the lower limit specified in the present invention and the value β calculated by formula (2) exceeded the upper limit of the range specified in the present invention, the tensile strength decreased.

[0111] In Comparative Examples No. 15 and 16, since the final annealing temperature in the final annealing process was less than the lower limit specified in the present invention, the elongation decreased.

[0112] As shown by these results, the method for manufacturing an aluminum alloy drawn material according to the present invention can blend various types of aluminum scraps at a high recycling rate, and the content of each component, the values α and β calculated from specific components, and the final annealing temperature are appropriately controlled. Therefore, mechanical properties such as elongation and strength are excellent, and particularly excellent elongation enables high formability to be obtained.

[0113] 5 Aluminum alloy drawn material 6 Core material 7 Inner skin material 8 Outer skin material

Claims

1. A method for manufacturing an aluminum alloy rolled material having an aluminum alloy part, comprising: a casting step of casting an ingot for an aluminum alloy part containing aluminum alloy scraps; a rolling step of rolling the ingot for an aluminum alloy part; a final annealing step of performing final annealing after the rolling step, wherein the ingot for an aluminum alloy part contains, based on the total mass of the ingot for an aluminum alloy part, Si: 1.0% by mass or more and 7.5% by mass or less, Fe: 0.15% by mass or more and 0.8% by mass or less, Mn: 0.3% by mass or more and 1.0% by mass or less, Cu: 0.5% by mass or less, Mg: 2.0% by mass or less, Zn: 0.7% by mass or less, the balance being composed of Al and unavoidable impurities, when the content of Si with respect to the total mass of the ingot for an aluminum alloy part is [Si] in mass% and the content of Mg with respect to the total mass of the ingot for an aluminum alloy part is [Mg] in mass%, the value α calculated by the following formula (1) is 0 or more and the value β calculated by the following formula (2) is 7.5 or less, wherein the aluminum alloy scraps contain Si: 0.50% by mass or more and Zn: 0.10% by mass or more, and are scraps of automotive heat exchangers, contain Mn: 0.5% by mass or more and Mg: 0.8% by mass or more, and are scraps of aluminum cans, contain Si: 0.20% by mass or more, Fe: 0.35% by mass or more and Mg: 0.2% by mass or more, and are sash scraps, and contain Si: 4.00% by mass or more, and are casting scraps, include at least one selected from the above, the total amount of the scraps is 50% by mass or more with respect to the total mass of the ingot for an aluminum alloy part, and the final annealing temperature in the final annealing step is 265°C or more. A method for manufacturing an aluminum alloy rolled material. Formula (1): α = [Si] - 2 × [Mg] Formula (2): β = [Si] + 2.25 × [Mg]

2. The method for manufacturing an aluminum alloy rolled material according to claim 1, wherein the aluminum alloy scraps include two or more selected from the scraps of automotive heat exchangers, the scraps of aluminum cans, the sash scraps, and the casting scraps.

3. The method for manufacturing an aluminum alloy rolled material according to claim 1 or 2, wherein the final annealing temperature in the final annealing step is 320°C or less.

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