ALUMINUM ALLOY SHEET FOR CAN LID

CROSS-REFERENCE TO RELATED APPLICATION

[0001]This international application claims the benefit of Japanese Patent Application No. 2023-067373 filed on Apr. 17, 2023 with the Japan Patent Office, the entire disclosure of Japanese Patent Application No. 2023-067373 is incorporated herein by reference.

TECHNICAL FIELD

[0002]The present disclosure relates to an aluminum alloy sheet for a can lid.

BACKGROUND ART

[0003]In recent years, increasing environmental awareness calls for an aluminum alloy sheet that produces low CO₂ emissions in its manufacturing process. In the manufacturing process of aluminum, a major and indirect cause of CO₂ emissions is to blend primary aluminum in casting process.

[0004]Production of the primary aluminum consumes a large amount of electricity in refining process, which leads to a large amount of CO₂ emissions. Thus, a reduction of a blending amount of the primary aluminum and an increase in closed recycling rate will lead to a reduction of CO₂ emissions in the production of the aluminum alloy sheet.

[0005]In general, it is said that CO₂ emissions can be reduced to about one-thirtieth in a case where aluminum scraps are re-melted for casting compared with a case where the primary aluminum is produced. In particular, since the amount of production of aluminum alloy sheets for beverage cans used around the world is very large, a further improvement in its closed recycling rate is significantly meaningful in the reduction of environmental load.

[0006]Among those beverage cans, a can lid made of 5182 aluminum alloy (AA5182 alloy) has low upper limits on compositional standards of Si, Fe, Cu, Mn, and the like compared with a can body made of 3104 aluminum alloy (AA3104 alloy). Thus, it is difficult to blend scraps of can materials containing 3104 aluminum alloy with such a can lid made of 5182 aluminum alloy (AA5182 alloy).

[0007]For example, if can scraps (UBC: Used Beverage Can) gathered in a city are blended as they are, the resultant contains more compositions of the 3104 aluminum alloy than compositions of the 5182 aluminum alloy due to the weight ratio between a can body and can lids, and thus the compositional upper limits of the 5182 aluminum alloy are easily exceeded. As a result, it becomes necessary to dilute the resultant composition with primary metal.

[0008]Thus, compared with an aluminum alloy sheet for a can body, an aluminum alloy sheet for a can lid is prepared by using a large amount of the primary metal to adjust to the compositions of the 5182 aluminum alloy, which makes its recycling rate low. Accordingly, a usage rate of the primary metal for can lids can be significantly reduced by changing the alloy for can lids to an alloy including compositions that can be easily blended with the 3104 aluminum alloy.

[0009]Patent Documents 1 to 5 disclose aluminum alloy sheets for a can lid that include compositions relatively close to that of the 3104 aluminum alloy, which has good recyclability.

PRIOR ART DOCUMENTS

Patent Documents

SUMMARY OF THE INVENTION

Problems to be Solved by the Invention

[0015]Declines in a buckling pressure (pressure resistance) of a can lid and in toughness of a material are raised as problems when making the alloy for can lids include compositions close to the compositions of a 3104 aluminum alloy. The buckling pressure of the can lid is an internal pressure value when the can lid is reversely deformed (buckled) against a pressure inside the can, and also is a resistance value when the internal pressure of the can accidentally increases due to a change in the external environment.

[0016]Particularly, positive pressure cans used for beer and carbonated beverages require a high buckling pressure. In general, the buckling pressure increases as the strength of the material increases and as a sheet thickness increases. Thus, a high-strength 5182 aluminum alloy containing a large amount of Mg is used for lids of the positive pressure cans.

[0017]In contrast, if the conventional 3104 aluminum alloy is used for the can lid, the buckling pressure is largely reduced, which increases a risk of the lid being reversely deformed to cause the content to leak when the internal pressure of the can is unexpectedly increased. Also, if the sheet thickness is increased to increase the buckling pressure, the weight of the lid and the cost of the lid are increased.

[0018]Furthermore, toughness of the material affects formability and openability of the lid. If the toughness of the material is low, a molding crack may occur particularly in a rivet part and a countersink part of the lid. In addition, a crack may occur in a score part when the internal pressure of the can is unexpectedly increased, which may increase a risk of a leakage of the content of the can. These cracks occur particularly in a rolling direction. Thus, toughness against a tensile stress and a bending stress in a direction perpendicular to the rolling direction is required.

[0019]However, an aluminum alloy sheet for a can lid that includes compositions relatively close to that of the conventional 3104 aluminum alloy do not solve either one or both of the aforementioned two problems. In other words, they do not satisfy either one or both of the strength of the material (or the buckling pressure of the lid) and the toughness of the material (or formability and openability).

[0020]In one aspect of the present disclosure, it is desirable to provide an aluminum alloy sheet for a can lid that can achieve both high strength and high toughness while containing scrap materials derived from can materials.

Means for Solving the Problems

[0021]One mode of the present disclosure is an aluminum alloy sheet for a can lid that includes a silicon (Si) content of 0.20 mass % or more and 0.47 mass % or less; an iron (Fe) content of 0.30 mass % or more and 0.70 mass % or less; a copper (Cu) content of 0.11 mass % or more and 0.40 mass % or less; a manganese (Mn) content of 0.70 mass % or more and 1.20 mass % or less; a magnesium (Mg) content of 1.1 mass % or more and 3.7 mass % or less; and a balance consisting of or including aluminum (Al) and inevitable impurities. The aluminum alloy sheet has a solidus temperature higher than a solid solutionizing temperature of Mg₂Si.

[0022]According to such a configuration, the solidus temperature of the aluminum alloy can be brought higher than the solid solutionizing temperature of Mg₂Si crystals. Thus, an aluminum alloy ingot undergoes a soaking treatment in such a temperature range. Consequently, a reduction of Mg₂Si crystals, which cause cracks and unexpected opening at the time of molding, can be expected.

[0023]As a result, the aluminum alloy sheet can achieve both high strength and high toughness while containing scrap materials derived from can materials. In other words, it is possible to blend a certain amount of scraps of the 3104 aluminum alloy for a can body into the aluminum alloy sheet and while reducing a usage rate of primary metal to reduce CO₂ emissions. The aluminum alloy sheet for a can lid can also achieve a high formability to be able to be used for a can lid of a positive pressure can that requires a high buckling pressure.

MODE FOR CARRYING OUT THE INVENTION

[0024]Hereinafter, embodiments in which the present disclosure is implemented will be explained.

1. First Embodiment

[1-1. Configuration]

<Composition>

[0025]An aluminum alloy sheet for a can lid of the present disclosure (hereinafter also simply referred to as alloy sheet) includes aluminum (Al), silicon (Si), iron (Fe), copper (Cu), manganese (Mn), and magnesium (Mg).

[0026]The lower limit of the Si content is 0.20 mass %. The average value of compositional standard of Si in a 3104 aluminum alloy specified in JIS-H-4000: 2014 is 0.30 mass %. The average value of compositional standard of Si in a 5182 aluminum alloy specified in JIS-H-4000: 2014 is 0.10 mass %. Thus, by arranging the Si content to be 0.20 mass % or more, a large amount of scraps of the 3104 aluminum alloy can be blended.

[0027]The upper limit of the Si content is 0.47 mass %, and preferably 0.39 mass %. If the Si content exceeds 0.47 mass %, the difference between a solid solutionizing temperature of Mg₂Si and a solidus temperature of an Al matrix becomes small, which makes it difficult to have Mg₂Si existing in an ingot formed into a solid solution in a homogenizing treatment process. Also, a coarse Mg₂Si newly precipitates in a hot rolling process, which decreases strength and toughness of the alloy.

[0028]In addition, by arranging the Si content to be 0.39 mass % or less, Mg₂Si can be easily formed into a solid solution in the homogenizing treatment process. Furthermore, precipitation of a coarse Mg₂Si in the hot rolling process is inhibited, which makes it possible to obtain excellent strength and toughness without performing a heat treatment process after the hot rolling process.

[0029]The lower limit of the Fe content is 0.30 mass %. The average value of compositional standard of Fe in the 3104 aluminum alloy is 0.40 mass %. The average value of compositional standard of Fe in the 5182 aluminum alloy is 0.18 mass %. Thus, by arranging the Fe content to be 0.30 mass % or more, a large amount of the scraps of the 3104 aluminum alloy can be blended.

[0030]The upper limit of the Fe content is 0.70 mass %, preferably 0.59 mass %, and more preferably 0.51 mass %. If the Fe content exceeds 0.70 mass %, unusually coarse Al—Fe—Mn-based or Al—Fe—Mn—Si-based intermetallic compounds (in other words, giant compounds) increase. As a result, a crack propagation path is generated and the toughness of the alloy sheet is decreased. If the Fe content is 0.51 mass % or less, the strength and toughness of the alloy can be supplemented while inhibiting crystallization of the aforementioned coarse intermetallic compounds.

[0031]The lower limit of the Cu content is 0.11 mass %, preferably 0.17 mass %, and more preferably 0.20 mass %. If the Cu content is less than 0.11 mass %, due to a lack of Cu, which increases the strength of the alloy sheet by forming a solid solution or precipitation, the strength of the alloy sheet decreases. The average value of compositional standard of Cu in the 3104 aluminum alloy is 0.15 mass %. The average value of compositional standard of Cu in the 5182 aluminum alloy is 0.075 mass %. Thus, by arranging the Cu content to be 0.11 mass % or more, a large amount of the scraps of the 3104 aluminum alloy can be blended.

[0032]The upper limit of the Cu content is 0.40 mass %, and preferably 0.25 mass %. If the Cu content exceeds 4.0 mass %, coarse precipitates increase which decreases the toughness of the alloy sheet. By arranging the Cu content to be 0.25 mass % or less, the strength of the alloy sheet can be increased without having its toughness largely impaired.

[0033]The lower limit of the Mn content is 0.70 mass %. If the Mn content is less than 0.70 mass %, due to a lack of Mn, which increases the strength of the alloy sheet by forming a solid solution or precipitation, the average strength of the alloy sheet decreases. The average value of compositional standard of Mn in the 3104 aluminum alloy is 1.1 mass %. The average value of compositional standard of Mn in the 5182 aluminum alloy is 0.35 mass %. Thus, by arranging the Mn content to be 0.70 mass % or more, a large amount of the scraps of the 3104 aluminum alloy can be blended compared with the conventional 5182 aluminum alloy.

[0034]The upper limit of the Mn content is 1.20 mass %, preferably 0.98 mass %, and more preferably 0.92 mass %. If the Mn content exceeds 1.20 mass %, unusually coarse Al—Fe—Mn-based or Al—Fe—Mn—Si-based intermetallic compounds increase. As a result, a crack propagation path is generated and the toughness of the alloy sheet is decreased.

[0035]The lower limit of the Mg content is 1.1 mass %. If the Mg content is less than 1.1 mass %, due to a lack of Mg, which increases the strength of the alloy sheet by forming a solid solution, the average strength of the alloy sheet decreases. The strength of the alloy sheet significantly increases by having Mg precipitate in cold rolling process after the hot rolling process and solution treatment.

[0036]The upper limit of the Mg content is 3.7 mass %, preferably 3.3 mass %, more preferably 3.1 mass %, and yet more preferably 2.9 mass %. If the Mg content exceeds 3.7 mass %, the solidus temperature of the Al matrix decreases and the solid solutionizing temperature of Mg₂Si increases. Thus, it becomes difficult to have Mg₂Si existing in the ingot formed into a solid solution in the homogenizing treatment process. Furthermore, due to a decrease in the solidus temperature of the Al matrix, coarse Al—Fe—Mn-based or Al—Fe—Mn—Si-based intermetallic compounds increase. Accordingly, the strength and toughness of the alloy sheet are impaired.

[0037]The alloy sheet may contain titanium (Ti). The upper limit of the Ti content is preferably 0.10 mass %. An ingot structure of the alloy sheet is micronized by containing Ti. Meanwhile, if Ti is excessively contained, it causes inclusions, and the toughness of the alloy sheet is decreased. The alloy sheet may also contain zinc (Zn). The upper limit of the Zn content is preferably 0.25 mass %. The alloy sheet may also contain chromium (Cr). The upper limit of the Cr content is preferably 0.10 mass %.

[0038]The alloy sheet may contain inevitable impurities to the extent that the performance of the alloy sheet is not significantly impaired. In other words, the alloy sheet contains Si, Fe, Cu, Mn, Mg, Ti, Zn, and Cr to the extent respectively described above, and the balance consists of or includes Al and inevitable impurities. The upper limit of the total amount of the inevitable impurities is preferably 0.15 mass %. The balance may contain a substance other than Al and the inevitable impurities.

<Calculation of Phase Diagram by Computer Software>

[0039]As regards the alloy sheet of the present disclosure, it is preferable to re-solid solutionize Mg₂Si particles, which become a starting point and a propagation path of a crack and have an effect on a decrease in toughness, in a homogenizing heat treatment process for the ingot to increase the toughness of the materials.

[0040]In order to re-solid solutionize Mg₂Si while avoiding local melting of the Al matrix, it is necessary that the solidus temperature is higher than the solid solutionizing temperature of Mg₂Si. Furthermore, it is preferable that the difference in the temperature obtained by deducting the solid solutionizing temperature of Mg₂Si from the solidus temperature is 30° C. or more. By having the solid solutionizing temperature of Mg₂Si lower than the solidus temperature, coarse crystals are not generated at the time of casting, and therefore, degradation of performance due to the coarse crystals can be avoided.

[0041]As regards a material whose crystallization temperature of Al6 (Mn, Fe) is higher than a solidification initiating temperature of Al, Al6 (Mn, Fe) is formed as a coarse crystal at the time of casting, which becomes a cause of a molding failure such as a pinhole. Thus, it is preferable that the solidification initiating temperature of Al is higher than the crystallization temperature of a primary crystal (that is, Al6 (Mn, Fe)). It is even more preferable that the difference in temperature obtained by deducting the crystallization temperature of the primary crystal from the solidification initiating temperature of Al is 1° C. or more.

[0042]When Al6 (Mn, Fe) is crystallized, and liquid phase Al is surrounding the crystallized Al6 (Mn, Fe), Al6 (Mn, Fe) tends to grow into a huge crystal. Such a huge crystal can become a starting point of propagation of a crack in the alloy and causes a decrease in toughness. In a case where the solidification initiating temperature of Al is higher than the crystallization temperature of Al6 (Mn, Fe), the surrounding Al has already initiated solidification when Al6 (Mn, Fe) is crystallized. Therefore, a generation of a huge crystal is inhibited, and an aluminum alloy sheet for a can lid having higher toughness can be obtained.

[0043]The solid solutionizing temperature of Mg₂Si described herein indicates the highest temperature at which Mg₂Si can exist in an equilibrium diagram, which is the lowest temperature at which the liquid phase can exist. The solidification initiating temperature of Al indicates the highest temperature at which solid phase Al can exist in the equilibrium diagram. The crystallization temperature of the primary crystal is the highest temperature at which Al6 (Mn, Fe) can exist.

[0044]The solid solutionizing temperature of Mg₂Si, the solidus temperature of the aluminum alloy, the solidification initiating temperature of Al, and the crystallization temperature of the primary crystal are obtained from the equilibrium diagram of the aluminum alloy calculated by using a thermodynamic calculation software.

[0045]The solid solutionizing temperature of Mg₂Si, the solidus temperature of the aluminum alloy, the solidification initiating temperature of Al, and the crystallization temperature of the primary crystal are uniquely determined depending on the composition of the aluminum alloy. Methods of obtaining these boundary temperatures from the alloy composition include a method of calculating thermodynamic quantities necessary in each arithmetic operation by CALPHAD method.

[0046]Such thermodynamic calculations on a multicomponent alloy can be performed by using a commercially available system software having thermodynamic database necessary for calculation, an interface, and a function to create phase diagrams (for example, “JMatPro” developed by Sente Software Ltd.).

<Method of Producing Aluminum Alloy Sheet>

[0047]The aluminum alloy sheet of the present disclosure can be produced as below, for example. Firstly, in accordance with a usual method, an ingot is produced by performing a semi-continuous casting method (that is, DC casting) on an aluminum alloy having the composition of the aluminum alloy sheet of the present disclosure.

[0048]Then, four surfaces of the ingot, excluding a front end surface and a rear end surface, are ground. After the grinding, the ingot is placed in a soaking furnace and undergoes a homogenizing treatment. The temperature of the homogenizing treatment is preferably 470° C. or higher and 620° C. or lower, for example. The duration of time of the homogenizing treatment is preferably one hour or more and 20 hours or less, for example.

[0049]When the temperature of the homogenizing treatment is 400° C. or higher, segregation in the ingot structure is easily reduced. When the temperature of the homogenizing treatment is 450° C. or higher, the Mg₂Si particles can be re-solid solutionized to improve strength and toughness of the alloy sheet. When the temperature of the homogenizing treatment is 490° C. or higher, preferably 550° C. or higher, and more preferably, equal to or higher than the solid solutionizing temperature of Mg₂Si, re-solid solutionization of the Mg₂Si particles is facilitated, and strength and toughness of the alloy sheet can be further improved. When the temperature of the homogenizing treatment is 620° C. or lower, and more preferably, equal to or higher than the solidus temperature, local melting of the aluminum alloy does not easily occur.

[0050]When the duration of time for the homogenizing treatment is one hour or longer, the temperature of the entire ingot becomes uniform, and the segregation in the ingot structure is easily reduced. Thus, it becomes easier to re-solid solutionize the Mg₂Si particles. The longer the duration of time for the homogenizing treatment, the more the Mg₂Si particles can be re-solid solutionized. However, if the duration of time for the homogenizing treatment exceeds 20 hours, the effect of the homogenizing treatment is saturated.

[0051]After the homogenizing treatment, the ingot is submitted to a hot rolling process. The hot rolling process includes a rough rolling process and a finish rolling process. In the rough rolling process, the ingot is processed into a plate material having a thickness of about tens of millimeters by reverse rolling. In the finish rolling process, the thickness of the plate material is reduced to about a few millimeters by tandem rolling, for example, and a hot-rolled coil is formed by winding the plate material into a form of a coil.

[0052]If a total reduction rate is high in the finish rolling process, then recrystallized structures are formed after winding, which can increase a degree of integration of isotropic cube orientation. If the temperature is high during winding in the finish rolling process, then the recrystallized structures are formed after winding, which can increase the degree of integration of the cube orientation. The toughness of the aluminum alloy sheet is improved by increasing the degree of integration of the cube orientation of the aluminum alloy sheet.

[0053]After the hot rolling process, cold rolling is performed on the plate material. In this cold rolling process, the hot-rolled coil is rolled until the thickness of the sheet reaches a product sheet thickness. The cold rolling may be either single rolling or tandem rolling. In the cold rolling in a case of the single rolling, it is preferable that the rolling is performed in two or more divided passes.

[0054]By setting the finish temperature in intermediate passes, excluding the final pass, at 120° C. or higher in the cold rolling process, Si, Cu, Mg and the like are finely precipitated and subjected to age-hardening. Accordingly, the strength of the alloy sheet can be increased. Furthermore, by setting the finish temperature at 130° C. or higher, the strength of the alloy sheet can be further increased.

[0055]A cold rolling ratio (that is, target total reduction rate) is preferably 80% or more. In a case where the cold rolling ratio is 80% or more, the strength of the alloy sheet can be increased. Meanwhile, the cold rolling ratio is preferably 92% or less. By setting the cold rolling ratio at 92% or less, anisotropy of a crystal grain structure is reduced, and the toughness of the alloy sheet against a tensile stress and a bending stress in a direction perpendicular to the rolling direction is improved.

[0056]The cold rolling ratio R (%) is obtained through the following formula (1) by using a sheet thickness t₀ (mm) of the hot rolled sheet after the hot finish rolling and the product sheet thickness t₁ (mm) after the cold rolling.

$\begin{array}{cc}R=\left(t_0-t_1\right)/t_0\times 100& \left(1\right)\end{array}$

[0057]The product sheet thickness can be appropriately selected so as to obtain a desired buckling pressure. The buckling pressure improves as the sheet thickness increases, however, according to the aluminum alloy sheet of the present disclosure, it is possible to inhibit an increase in the sheet thickness for a purpose of keeping the buckling pressure high.

[0058]As long as the effects of the aluminum alloy sheet of the present disclosure are exerted, annealing may be included, for example, before or after the cold rolling process or between the passes in the aforementioned method of producing the aluminum alloy sheet.

[0059]Pre-coating is performed on a coating line or the like on the coil that had undergone the cold rolling until the product sheet thickness was achieved. The coil that had undergone the cold rolling is subjected to degreasing on a front surface, cleaning, chemical conversion coating, and coating and baking treatment after being coated with a coating material.

[0060]In the chemical conversion coating, chemicals such as a chromate-based chemical and a zirconium-based chemical are used. As the coating material, materials such as an epoxy-based material and a polyester-based material are used. These chemicals and coating materials can be selected in accordance with applications. In the coating and baking treatment, the coil is heated within about 30 seconds at 220° C. or higher and 270° C. or lower in an actual temperature of the coil (PMT: Peak Metal Temperature). In this stage, the lower the PMT is, the more the recovery of the material is inhibited, which makes it possible to keep the strength of the alloy sheet high.

[1-2. Effect]

[0061]According to the embodiment explained in detail above, the following effect can be obtained.

[0062](1a) The solidus temperature of the aluminum alloy can be brought higher than the solid solutionizing temperature of Mg₂Si crystals. The aluminum alloy ingot therefore undergoes a soaking treatment in such a temperature range. Consequently, a reduction of Mg₂Si crystals, which cause cracks and unexpected opening at the time of forming, can be expected.

[0063]As a result, the aluminum alloy sheet can achieve both high strength and high toughness while containing scrap materials derived from can materials. In other words, it is possible to blend a certain amount of scraps of the 3104 aluminum alloy for a can body into raw materials and to reduce a usage rate of primary metal to reduce CO₂ emissions. It is also possible to obtain the aluminum alloy sheet for a can lid that has high formability and thus is able to be used for a can lid of a positive pressure can that requires a high buckling pressure.

2. Other Embodiments

[0064]The embodiment of the present disclosure has been explained above; however, it is needless to say that the present disclosure is not limited to the aforementioned embodiment and can be implemented in various forms.

[0065](2a) In addition to the aluminum alloy sheet of the aforementioned embodiment, the present disclosure also includes various other forms, such as a member including this aluminum alloy sheet and a method of producing this aluminum alloy sheet.

[0066](2b) Functions of one element in the aforementioned embodiments may be distributed to two or more elements, and functions of two or more elements may be integrated into one element. A part of the configuration of the aforementioned embodiments may be omitted. In addition, at least a part of the configuration of the aforementioned embodiments may be added to or replaced with the configuration of other embodiments. Any and all modes included in the technical idea specified by the languages used in the claims are embodiments of the present disclosure.

3. Examples

[0067]Hereinafter, details of tests conducted to confirm the effects of the present disclosure and evaluation results of the tests will be explained.

<Calculation of Equilibrium Diagram>

[0068]As examples and comparative examples, equilibrium diagrams of aluminum alloys S1 to S14 shown in Table 1 were calculated through a calculation method described in the embodiment using “JMatPro”. The results are shown in Table 1.

<Evaluation of Aluminum Alloy Sheet>

(Various Temperatures)

[0069]In S2 to S9, and S11 to S14, the solidus temperature is higher than the solid solutionizing temperature of Mg₂Si. Therefore, these alloys can undergo soaking at a temperature at which Mg₂Si can be formed into a solid solution.

[0070]In S1, the Si content is relatively large. Thus, the solid solutionizing temperature of Mg₂Si is higher than the solidus temperature. As a result, in Si, Al melts locally before Mg₂Si is formed into a solid solution if the temperature increases. Accordingly, it is difficult to perform soaking to have Mg₂Si formed into a solid solution.

[0071]Meanwhile, in S2, the Si content is less than the Si content in S1. Thus, there is a temperature range in which no liquid phase appears even though Mg₂Si can be formed into a solid solution. In S3, the Si content is even less than the Si content in S2. Thus, the difference in temperature between the solidus temperature and the solid solutionizing temperature of Mg₂Si is more than 30° C., which is a large difference. As a result, in S3, there is a temperature range in which soaking is possible despite variations caused in production.

[0072]In S4, the Fe content is relatively large. Thus, the crystallization temperature of Al6 (Mn, Fe) is higher than the solidification initiating temperature of Al. Accordingly, in S4, Al6 (Mn, Fe) crystallizes in the liquid phase of Al as the alloy is cooled, which may result in a huge crystal.

[0073]Meanwhile, in S5, the Fe content is less than the Fe content in F4. Thus, the solidification initiating temperature of Al is higher than the crystallization temperature of Al6 (Mn, Fe). Accordingly, in S5, generation of a huge crystal can be inhibited. In S6, the Fe content is even less than the Fe content in F5. Thus, the difference between the solidification initiating temperature of Al and the crystallization temperature of Al6 (Mn, Fe) becomes large. Accordingly, in S6, a decrease in strength and toughness due to a huge crystal is less likely to occur.

[0074]In S7, the Mn content is relatively large. Thus, the crystallization temperature of Al6 (Mn, Fe) is higher than the solidification initiating temperature of Al. Accordingly, in S7, Al6 (Mn, Fe) crystallizes in the liquid phase of Al as the alloy is cooled, which may result in a huge crystal.

[0075]Meanwhile, in S8, the Mn content is less than the Mn content in S7. Thus, the solidification initiating temperature of Al is higher than the crystallization temperature of Al6 (Mn, Fe). Accordingly, in S8, generation of a huge crystal is inhibited. In S9, the Mn content is even less than the Mn content in S8. Thus, the difference between the solidification initiating temperature of Al and the crystallization temperature of Al6 (Mn, Fe) becomes large. Accordingly, in S9, a decrease in strength and toughness due to a huge crystal is less likely to occur.

[0076]In S10, the Mg content is relatively large. Thus, the solid solutionizing temperature of Mg₂Si is higher than the solidus temperature. Accordingly, in S10, if the temperature increases, Al locally melts before Mg₂Si is formed into a solid solution, which makes it difficult to perform soaking and homogenizing treatment to have Mg₂Si formed into a solid solution. In S10, the crystallization temperature of Al6 (Mn, Fe) is higher than the solidification initiating temperature of Al. Thus, in S10, Al6 (Mn, Fe) crystallizes in the liquid phase of Al as the alloy is cooled, which results in a huge crystal that causes a decrease in strength and toughness.

[0077]Meanwhile, in S11, the Mg content is less than the Mg content in S10. Thus, there is a temperature range in which no liquid phase appears even though Mg₂Si can be formed into a solid solution. In S12, the Mg content is even less than the Mg content in S11. Thus, the solidification initiating temperature of Al is higher than the crystallization temperature of Al6 (Mn, Fe). Accordingly, in S12, generation of a huge crystal is inhibited.

[0078]In S13, the Mg content is even less than the Mg content in S12. Thus, the difference between the solidus temperature and the solid solutionizing temperature of Mg₂Si is more than 30° C., which is a large difference. As a result, in S13, there is a temperature range in which soaking is possible despite variations caused in production.

[0079]In S14, the Mg content is even less than the Mg content in S13. Thus, the difference between the solidification initiating temperature of Al and the crystallization temperature of Al6 (Mn, Fe) is even larger. Accordingly, in S14, a decrease in strength and toughness due to a huge crystal is less likely to occur.

(Scrap Blending Ratio)

[0080]Table 2 shows blending ratios of the 3104 aluminum alloy and the 5182 aluminum alloy in correspondence with the average values of the compositional standard. The first line of Table 2 shows the average value of the compositional standard of the components of the 3104 aluminum alloy; and the second line of Table 2 shows the average value of the compositional standard of the components of the 5182 aluminum alloy.

[0081]For example, when the blending ratio of the 3104 aluminum alloy is 50 mass %, the average value of the Si content is 0.20 mass %, the average value of the Fe content is 0.29 mass %, the average value of the Cu content is 0.11 mass %, the average value of the Mn content is 0.7 mass %, and the average value of the Mg content is 2.8 mass %.

[0082]Accordingly, if the ratio of each of the components Si, Fe, Cu, Mn, and Mg of the aluminum alloy sheet is more than the aforementioned values, a possible blending ratio of the 3104 aluminum alloy sheet is more than 50 mass %. The more the blending ratio of the 3104 aluminum alloy increases, the more the Si, Fe, Cu, and Mn contents increase, but the more the Mg content decreases. The aluminum alloy sheets of S1 to S14 can include 50 mass % or more of scraps of the 3104 aluminum alloy.