Aluminum alloy plate for can lids

Abstract

This invention provides an aluminum alloy plate for can lids that can achieve excellent stress relaxation properties even when using recycled materials. [Solution] The aluminum alloy plate for the can lid contains, with respect to the total mass of the aluminum alloy plate for the can lid, Si: 0.05% to 0.50% by mass, Fe: 0.05% to 0.50% by mass, Cu: 0.05% to 0.40% by mass, Mn: 0.50% to 0.80% by mass, Mg: 2.0% to 4.0% by mass, and Zn: 0.10% to 0.40% by mass, as well as at least one selected from Ti, Cr, Ca, B, and Be, in the range of Ti: 0.15% by mass or less, Cr: 0.15% by mass or less, Ca: 0.05% by mass or less, B: 0.05% by mass or less, and Be: 0.05% by mass or less, with the remainder being Al and unavoidable impurities.

Description

[Technical Field]

[0001] The present invention relates to an aluminum alloy sheet for can ends having excellent recyclability. [Background Art]

[0002] Conventionally, as food packaging containers, two-piece aluminum cans consisting of a bottomed cylindrical body and a can end have been widely used. In particular, excellent stress relaxation properties are required for aluminum alloy sheets used for can ends of food cans. The stress relaxation property of a can end refers to the property that, even when the shape is deformed, the shape returns to its original steady state as heat is removed. For example, during the production of food cans, when heat sterilization is performed after filling the contents, a phenomenon in which the end bulges due to an increase in internal pressure may occur. In addition, if sterilization is insufficient during the heat sterilization treatment, bacteria remaining in the contents will proliferate and generate corrosive gas, resulting in bulging of the end similar to that during the heat sterilization treatment.

[0003] Here, if the stress relaxation property of the can end is low, even if the internal pressure decreases after the heat sterilization treatment, the can end remains deformed and may not return to its original shape. As a result, for a food can with a bulging end, it is difficult to determine whether the bulging is simply caused by heating or by bacterial proliferation. Therefore, the food can is excluded in the inspection after sterilization treatment, causing problems such as decreased productivity.

[0004] In view of this, Patent Document 1 discloses an aluminum alloy sheet for a packaging container end that is less likely to break even when the packaging container is exposed to high temperature after filling the contents, the internal pressure of the packaging container rises and buckling occurs, and that even when the shape is deformed, the shape returns to its original steady state as heat is removed, that is, has excellent stress relaxation properties. The aluminum alloy sheet contains 0.05 to 0.3 mass% of Cu, 0.2 to 0.6 mass% of Mn, and 2.0 to 5.5 mass% of Mg, with the contents of Si and Fe regulated within predetermined ranges.

[0005] Incidentally, in recent years, achieving carbon neutrality has become a challenge for society as a whole, and the development of aluminum alloy materials that produce less CO2 during manufacturing is being considered. Aluminum consumes a large amount of electricity and emits a large amount of CO2 in the refining process of new ingots, but when aluminum scrap is blended and remelted for casting, the amount of new ingots used can be reduced, so CO2 emissions become extremely low. Thus, improving the recycling rate of aluminum will greatly contribute to reducing CO2 emissions. For this reason, there is a demand for development of aluminum alloy sheets for can lids that are closer in composition to A3104, which has excellent recyclability. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Patent No. 4829988 [Overview of the project] [Problems that the invention aims to solve]

[0007] The aluminum alloy plate for packaging container lids described in Patent Document 1 obtains excellent stress relaxation properties by specifying the components (Cu, Mn, Mg, Si, Fe) in the aluminum alloy plate within a specific range. Therefore, if one attempts to bring the composition of the aluminum alloy plate described in Patent Document 1 closer to the composition of A3104 while maintaining excellent stress relaxation properties, it will fall outside the specified component range in Patent Document 1, resulting in a deterioration of the stress relaxation properties of the can lid.

[0008] This invention has been made in view of the above problems, and aims to provide an aluminum alloy plate for can lids that can obtain excellent stress relaxation characteristics even when recycled materials are used. [Means for solving the problem]

[0009] The above objective is achieved by the aluminum alloy plate for can lids described in [1] below according to the present invention.

[0010] [1] With respect to the total mass of the aluminum alloy plate for the can lid, Si: 0.05% by mass or more and 0.50% by mass or less, Fe: 0.05% by mass or more and 0.50% by mass or less, Cu: 0.05% by mass or more and 0.40% by mass or less, Mn: 0.50 mass% or more and 0.80 mass% or less, Mg: 2.0% by mass or more and 4.0% by mass or less, It contains Zn: 0.10% by mass or more and 0.40% by mass or less, At least one selected from Ti, Cr, Ca, B, and Be, Ti: 0.15% by mass or less, Cr: 0.15% by mass or less, Ca: 0.05% by mass or less, B: 0.05% by mass or less, Be: Contains in the range of 0.05% by mass or less. An aluminum alloy plate for can lids, characterized in that the remainder consists of Al and unavoidable impurities.

[0011] Furthermore, the aluminum alloy plate for the can lid of the present invention is preferably as described in [2] to [5] below.

[0012] [2] In the aforementioned unavoidable impurities, Na: 0.05% by mass or less, V: 0.05% by mass or less, Ni: 0.05% by mass or less, In: 0.05% by mass or less, Sn: 0.05% by mass or less, The aluminum alloy plate for can lids according to [1], characterized in that Ga: is restricted to 0.05% by mass or less.

[0013] [3] Solid solution Cu content: 0.05% by mass or more and 0.40% by mass or less, Solid solution Mg amount: 1.5% by mass or more and 4.0% by mass or less, and The aluminum alloy sheet for can ends according to [1] or [2], characterized in that a solid solution Zn content is 0.10 mass% or more and 0.40 mass% or less.

[0014] [4] The aluminum alloy sheet for can ends according to [1] or [2], characterized in that a stress relaxation rate measured in accordance with a stress relaxation test specified in EMAS-1011 is 52% or less. [5] The aluminum alloy sheet for can ends according to [3], characterized in that a stress relaxation rate measured in accordance with a stress relaxation test specified in EMAS-1011 is 52% or less. Effects of the Invention

[0015] According to the present invention, an aluminum alloy sheet for can ends that can obtain excellent stress relaxation properties even when a recycled raw material is used can be provided. Brief Description of Drawings

[0016] [Figure 1] FIG. 1 is a plan view showing the dimensions of a test piece used for measuring the stress relaxation rate. [Figure 2] FIG. 2 is a schematic diagram showing a method for measuring the stress relaxation rate. [Figure 3] FIG. 3 is a schematic diagram showing a calculation method for the stress relaxation rate. Mode for Carrying Out the Invention

[0017] The inventors of the present application conducted various studies on aluminum alloy sheets for can ends capable of achieving both excellent recyclability and stress relaxation properties for the purpose of reducing environmental load. First, the relationship between stress relaxation properties was investigated for Cu and Mg, which are considered to affect the stress relaxation properties of aluminum alloy sheets. As a result, for both Cu and Mg in the aluminum alloy sheet, the stress relaxation rate decreases as the content of each increases, leading to improved stress relaxation properties.

[0018] However, when recycled materials are used as the material for aluminum alloy sheets for can lids, the Cu content increases but the Mg content decreases, resulting in a decline in stress relaxation properties. Therefore, the inventors of this invention conducted further research and found that by including Zn within a predetermined range in the aluminum alloy sheet, it is possible to prevent the decline in stress relaxation properties due to the decrease in Mg. This invention is based on the above findings.

[0019] [Aluminum alloy plate for can lids] The aluminum alloy sheet for can lids according to the embodiment of the present invention is an aluminum alloy sheet used for the lid portion of cans such as food cans and beverage cans. The can lid portion includes the can lid and the pull tab. The content of each component contained in the aluminum alloy sheet for can lids according to the embodiment of the present invention and the reasons for the numerical limitations will be described in detail below. In this specification, the aluminum alloy sheet for can lids may be simply referred to as an aluminum alloy sheet.

[0020] (Si: 0.05 mass% or more and 0.50 mass% or less) Si is an element commonly found in recycled aluminum alloy materials. If the Si content in the aluminum alloy sheet is less than 0.05% by mass, high-purity aluminum ingots are required during casting, which increases costs and reduces recyclability. Therefore, the Si content in the aluminum alloy sheet should be 0.05% by mass or more, preferably 0.15% by mass or more, and more preferably 0.20% by mass or more, relative to the total mass of the aluminum alloy sheet for can lids.

[0021] On the other hand, if the Si content exceeds 0.50 mass%, the amount of Mg2Si particles increases, and the amount of solid-solution Mg decreases, resulting in a decrease in stress relaxation properties. Therefore, the Si content in the aluminum alloy sheet should be 0.50 mass% or less, preferably 0.45 mass% or less, and more preferably 0.35 mass% or less, relative to the total mass of the aluminum alloy sheet.

[0022] (Fe: 0.05 mass% or more and 0.50 mass% or less) Fe is an element commonly found in recycled aluminum alloys. If the Fe content in an aluminum alloy sheet is less than 0.05% by mass, high-purity aluminum ingots are required during casting, increasing costs and reducing recyclability. Therefore, the Fe content in an aluminum alloy sheet should be 0.05% by mass or more, preferably 0.15% by mass or more, and more preferably 0.20% by mass or more, relative to the total mass of the aluminum alloy sheet.

[0023] On the other hand, if the Fe content exceeds 0.50 mass%, the amount of Al-Fe-Mn intermetallic compounds increases, and a large number of intermetallic compounds are formed during casting and hot rolling. As a result, crack initiation and propagation are promoted, reducing the bendability necessary for lid forming. Therefore, the Fe content in the aluminum alloy sheet should be 0.50 mass% or less, preferably 0.45 mass% or less, and more preferably 0.35 mass% or less, relative to the total mass of the aluminum alloy sheet.

[0024] (Cu: 0.05 mass% or more and 0.40 mass% or less) Cu is an element that has the effect of improving the stress relaxation properties of aluminum alloy sheets. If the Cu content is less than 0.05 mass%, even if the conditions are set to increase the amount of dissolved Cu during heat treatment processes such as soaking and intermediate annealing, the amount of dissolved Cu will be insufficient, and the stress relaxation properties will deteriorate. Therefore, the Cu content in aluminum alloy sheets should be 0.05 mass% or more, preferably 0.10 mass% or more, and more preferably 0.15 mass% or more, relative to the total mass of the aluminum alloy sheet.

[0025] On the other hand, if the Cu content exceeds 0.40 mass%, excess Cu precipitates before recrystallization occurs during hot rolling, significantly reducing the strength of the aluminum alloy sheet. Therefore, the Cu content in the aluminum alloy sheet should be 0.40 mass% or less, preferably 0.35 mass% or less, and more preferably 0.30 mass% or less, relative to the total mass of the aluminum alloy sheet.

[0026] (Mn: 0.50 mass% or more and 0.80 mass% or less) Mn is an element that improves the strength of aluminum alloy sheets. If the Mn content is less than 0.50 mass%, the strength will be insufficient, resulting in insufficient pressure resistance when applied to can lids. Furthermore, it will deviate from the target alloy composition of A3104, thus reducing recyclability. Therefore, the Mn content in the aluminum alloy sheet should be 0.50 mass% or more, preferably 0.52 mass% or more, and more preferably 0.55 mass% or more, relative to the total mass of the aluminum alloy sheet.

[0027] On the other hand, if the Mn content exceeds 0.80 mass%, the amount of Al-Fe-Mn intermetallic compounds increases, and a large number of intermetallic compounds are formed during casting and hot rolling. As a result, crack initiation and propagation are promoted, reducing the bendability necessary for lid forming. Therefore, the Mn content in the aluminum alloy sheet should be 0.80 mass% or less, preferably 0.70 mass% or less, and more preferably 0.65 mass% or less, relative to the total mass of the aluminum alloy sheet.

[0028] (Mg: 2.0 mass% or more and 4.0 mass% or less) Mg is an element that has the effect of improving the stress relaxation properties of aluminum alloy sheets. If the Mg content is less than 2.0 mass%, even if the conditions are set to increase the amount of solid-solution Mg during heat treatment processes such as soaking and intermediate annealing, the amount of solid-solution Mg will be insufficient, and the stress relaxation properties will deteriorate. Therefore, the Mg content in aluminum alloy sheets should be 2.0 mass% or more, preferably 2.2 mass% or more, and more preferably 2.4 mass% or more, relative to the total mass of the aluminum alloy sheet.

[0029] On the other hand, if the Mg content exceeds 4.0 mass%, the cold rolling processability becomes insufficient, and fracture occurs during the rolling of the aluminum alloy sheet. Furthermore, the alloy composition deviates from the target A3104, reducing its recyclability. Therefore, the Mg content in the aluminum alloy sheet should be 4.0 mass% or less, preferably 3.8 mass% or less, and more preferably 3.6 mass% or less, relative to the total mass of the aluminum alloy sheet.

[0030] (Zn: 0.10 mass% or more and 0.40 mass% or less) Zn is an element that has the effect of improving the stress relaxation properties of aluminum alloy sheets. In particular, when the content of Si, Fe, and Mn is specified as described above so that recycled raw materials can be used, excellent stress relaxation properties can be obtained by incorporating Zn into the aluminum alloy sheet to a predetermined content. If the Zn content is less than 0.10 mass%, even if the conditions are set to increase the amount of solid-solution Zn in heat treatment processes such as soaking and intermediate annealing, the amount of solid-solution Zn will be insufficient, and the stress relaxation properties will deteriorate. Therefore, the Zn content in the aluminum alloy sheet should be 0.10 mass% or more, preferably 0.12 mass% or more, and more preferably 0.15 mass% or more, relative to the total mass of the aluminum alloy sheet.

[0031] On the other hand, even if the Zn content in the aluminum alloy exceeds 0.40 mass%, Al-Cu-Mg-Zn precipitates segregate at grain boundaries during processes such as cooling after hot rolling, and the amount of dissolved Zn atoms stops increasing. Therefore, no further improvement in stress relaxation properties can be obtained. Accordingly, the Zn content in the aluminum alloy sheet should be 0.40 mass% or less, preferably 0.38 mass% or less, and more preferably 0.35 mass% or less, relative to the total mass of the aluminum alloy sheet.

[0032] The aluminum alloy sheet according to this embodiment contains, in addition to Si, Fe, Cu, Mn, Mg, and Zn, at least one element selected from Ti, Cr, Ca, B, and Be, within the range shown below. The content of each of the above elements selectively included in the aluminum alloy sheet and the reasons for the limitations will be explained below.

[0033] (Ti: 0.01 mass% or more and 0.15 mass% or less) Ti has the effect of refining the ingot crystal grains, and refining the ingot structure during casting improves castability and enables high-speed casting. Therefore, aluminum alloy sheets may contain Ti as needed, but may also contain 0% by mass. However, if the Ti content in the aluminum alloy sheet is 0.01% by mass or more, the effect of refining the structure can be obtained. Therefore, from the viewpoint of castability, when Ti is included in an aluminum alloy sheet from among Ti, Cr, Ca, B, and Be, the Ti content is preferably 0.01% by mass or more relative to the total mass of the aluminum alloy sheet.

[0034] On the other hand, if the Ti content in the aluminum alloy sheet exceeds 0.15 mass%, coarse compounds are formed, reducing the bendability necessary for lid forming. Therefore, among Ti, Cr, Ca, B, and Be, when Ti is included in the aluminum alloy sheet, the Ti content in the aluminum alloy sheet should be 0.15 mass% or less, preferably 0.12 mass% or less, and more preferably 0.10 mass% or less, relative to the total mass of the aluminum alloy sheet.

[0035] (Cr: 0.01 mass% or more and 0.15 mass% or less) Cr is an element that promotes the precipitation of intermetallic compounds during hot rolling, thereby improving rollability. Therefore, aluminum alloy sheets may contain Cr as needed, but may also contain 0% by mass. However, if the Cr content in the aluminum alloy sheet is 0.01% by mass or more, the effect of improving rollability can be obtained. Accordingly, among Ti, Cr, Ca, B, and Be, from the viewpoint of rollability, when Cr is included in an aluminum alloy sheet, it is preferable that the Cr content be 0.01% by mass or more relative to the total mass of the aluminum alloy sheet.

[0036] On the other hand, if the Cr content exceeds 0.15 mass%, coarse crystals tend to form, reducing the formability of the aluminum alloy sheet. Therefore, when Cr is included in an aluminum alloy sheet from among Ti, Cr, Ca, B, and Be, the Cr content is preferably 0.15 mass% or less, 0.12 mass% or less, and more preferably 0.10 mass% or less, relative to the total mass of the aluminum alloy sheet.

[0037] (Ca: 0.01 mass% or more and 0.05 mass% or less) Ca is an element that promotes the solid solution of Mg during casting, and the strength of aluminum alloy sheets can be improved by solid solution strengthening. Therefore, aluminum alloy sheets may contain Ca as needed, but may also contain 0% by mass. However, if the Ca content in the aluminum alloy sheet is 0.01% by mass or more, the effect of improving strength can be obtained. Therefore, from the viewpoint of adjusting the strength of aluminum alloy sheets, among Ti, Cr, Ca, B, and Be, when Ca is included in the aluminum alloy sheet, it is preferable that the Ca content be 0.01% by mass or more relative to the total mass of the aluminum alloy sheet.

[0038] On the other hand, if the Ca content exceeds 0.05 mass%, brittle fracture may occur during hot rolling. Therefore, when Ca is included in an aluminum alloy sheet from among Ti, Cr, Ca, B, and Be, the Ca content is preferably 0.05 mass% or less, 0.04 mass% or less, and more preferably 0.03 mass% or less, relative to the total mass of the aluminum alloy sheet.

[0039] (B: 0.01 mass% or more and 0.05 mass% or less) B has the effect of refining the ingot crystal grains, and refining the ingot structure during casting improves castability and enables high-speed casting. Therefore, aluminum alloy sheets may contain B as needed, but may also contain 0% by mass. However, if the B content in the aluminum alloy sheet is 0.01% by mass or more, the effect of refining the structure can be obtained. Therefore, from the viewpoint of castability of aluminum alloy sheets, among Ti, Cr, Ca, B, and Be, when B is included in an aluminum alloy sheet, it is preferable that the B content be 0.01% by mass or more relative to the total mass of the aluminum alloy sheet.

[0040] On the other hand, if the B content exceeds 0.05% by mass, coarse compounds are formed, reducing the bendability necessary for lid forming. Therefore, when B is included in an aluminum alloy sheet from among Ti, Cr, Ca, B, and Be, the B content is preferably 0.05% by mass or less, preferably 0.04% by mass or less, and more preferably 0.03% by mass or less, relative to the total mass of the aluminum alloy sheet.

[0041] (Be: 0.01 mass% or more and 0.05 mass% or less) Be is an element that has the effect of preventing oxidation of aluminum alloy sheets and preventing the formation of inclusions. Therefore, aluminum alloy sheets may contain Be as needed, but may also contain 0% by mass. However, if the Be content in the aluminum alloy sheet is 0.01% by mass or more, the oxidation prevention effect and the inclusion prevention effect can be obtained. Therefore, among Ti, Cr, Ca, B, and Be, from the viewpoint of preventing oxidation and the formation of inclusions in the aluminum alloy sheet, when Be is included in the aluminum alloy sheet, it is preferable that the Be content be 0.01% by mass or more relative to the total mass of the aluminum alloy sheet.

[0042] On the other hand, if the Be content in the aluminum alloy sheet exceeds 0.05% by mass, the Be-containing compounds become coarse, which may reduce the rollability. Therefore, when Be is included in the aluminum alloy sheet from among Ti, Cr, Ca, B, and Be, the Be content in the aluminum alloy sheet should be 0.05% by mass or less, preferably 0.04% by mass or less, and more preferably 0.03% by mass or less, relative to the total mass of the aluminum alloy sheet.

[0043] (Remainder: Al and inevitable impurities) The remainder of the aluminum alloy sheet according to this embodiment consists of Al and unavoidable impurities. Examples of unavoidable impurities include Na, V, Ni, In, Sn, and Ga. The content of Na, V, Ni, In, Sn, and Ga relative to the total mass of the aluminum alloy sheet is preferably 0.05% by mass or less for each, and the total content of these unavoidable impurities is preferably 0.15% by mass or less. If the content of unavoidable impurities is within the above range, it does not hinder the effects of the present invention.

[0044] In the aluminum alloy sheet according to this embodiment, the above-mentioned Si, Fe, Cu, Mn, Mg, and Zn are specified to have appropriate content, thereby obtaining excellent recyclability and stress relaxation properties. Furthermore, specifying the amount of solid-solution Cu, solid-solution Mg, and solid-solution Zn will allow for even better stress relaxation properties. The preferred ranges for the content of solid-solution Cu, solid-solution Mg, and solid-solution Zn will be described below.

[0045] (Solved Cu amount: 0.05 mass% or more and 0.40 mass% or less) Solid-solution Cu atoms are elements that inhibit dislocation movement and improve the stress relaxation properties of aluminum alloy sheets. In particular, adjusting the amount of solid-solution Cu in the aluminum alloy sheet can further improve the stress relaxation properties. Setting the amount of solid-solution Cu in the aluminum alloy sheet to 0.05 mass% or more allows the effect of solid-solution Cu atoms to be obtained, further improving the stress relaxation properties. Therefore, the amount of solid-solution Cu in the aluminum alloy sheet is preferably 0.05 mass% or more, more preferably 0.10 mass% or more, and even more preferably 0.12 mass% or more, relative to the total mass of the aluminum alloy sheet.

[0046] On the other hand, if the amount of dissolved Cu is 0.40 mass% or less, it is possible to suppress grain boundary segregation of Al-Cu-Cu-Zn precipitates during processes such as cooling after hot rolling, and to fully obtain the effect of improving stress relaxation properties due to the increase in dissolved Cu atoms. Therefore, the amount of dissolved Cu in the aluminum alloy sheet is preferably 0.40 mass% or less, more preferably 0.35 mass% or less, and even more preferably 0.30 mass% or less, relative to the total mass of the aluminum alloy sheet.

[0047] (Solved Mg amount: 1.5% by mass or more and 4.0% by mass or less) Solid-solution Mg atoms are elements that inhibit dislocation movement and improve the stress relaxation properties of aluminum alloy sheets. In particular, adjusting the amount of solid-solution Mg in the aluminum alloy sheet can further improve the stress relaxation properties. Setting the amount of solid-solution Mg in the aluminum alloy sheet to 1.5% by mass or more allows the effect of solid-solution Mg atoms to be obtained, further improving the stress relaxation properties. Therefore, the amount of solid-solution Mg in the aluminum alloy sheet is preferably 1.5% by mass or more, more preferably 1.7% by mass or more, and even more preferably 1.9% by mass or more, relative to the total mass of the aluminum alloy sheet.

[0048] On the other hand, if the amount of solid-solution Mg is 4.0% by mass or less, it is possible to suppress grain boundary segregation of Al-Cu-Mg-Zn precipitates during processes such as cooling after hot rolling, and to fully obtain the effect of improving stress relaxation properties due to the increase in solid-solution Mg atoms. Therefore, the amount of solid-solution Mg in the aluminum alloy sheet is preferably 4.0% by mass or less, more preferably 3.8% by mass or less, and even more preferably 3.7% by mass or less, relative to the total mass of the aluminum alloy sheet.

[0049] (Amount of solid solution Zn: 0.10 mass% or more and 0.40 mass% or less) Solid-solution zinc atoms are elements that inhibit dislocation movement and improve the stress relaxation properties of aluminum alloy sheets. In particular, adjusting the amount of solid-solution zinc in the aluminum alloy sheet can further improve the stress relaxation properties. Setting the amount of solid-solution zinc in the aluminum alloy sheet to 0.10 mass% or more allows the effect of solid-solution zinc atoms to be obtained, further improving the stress relaxation properties. Therefore, the amount of solid-solution zinc in the aluminum alloy sheet is preferably 0.10 mass% or more, more preferably 0.12 mass% or more, and even more preferably 0.15 mass% or more, relative to the total mass of the aluminum alloy sheet.

[0050] On the other hand, if the amount of dissolved Zn is 0.40% by mass or less, it is possible to suppress grain boundary segregation of Al-Cu-Mg-Zn precipitates during processes such as cooling after hot rolling, and to fully obtain the effect of improving stress relaxation properties due to the increase in dissolved Zn atoms. Therefore, the amount of dissolved Zn in the aluminum alloy sheet is preferably 0.40% by mass or less, more preferably 0.38% by mass or less, and even more preferably 0.35% by mass or less, relative to the total mass of the aluminum alloy sheet.

[0051] (Stress relaxation rate: 52% or less) In aluminum alloy plates for can lids, if the stress relaxation rate is 52% or less, the shape of the can lid will easily return to its original state when the internal pressure decreases after it has bulged due to an increase in internal pressure. Therefore, the stress relaxation rate is preferably 52% or less, and more preferably 50% or less. The lower limit of the stress relaxation rate is not particularly limited, but a lower value is preferable.

[0052] The stress relaxation rate can be measured in accordance with EMAS-1011 "Stress Relaxation Test," standardized by the Japan Electronic Materials Manufacturers Association. The specific measurement method is described below.

[0053] Figure 1 is a plan view showing the size of the test specimen used to measure the stress relaxation rate. Figure 2 is a schematic diagram showing the method for measuring the stress relaxation rate, and Figure 3 is a schematic diagram showing the method for calculating the stress relaxation rate. First, the rolling direction of the aluminum alloy sheet to be measured is set to be the longitudinal direction of the test specimen, and a test specimen 1 with a thickness of 0.3 mm, a width of 10 mm, and a length of 100 mm is taken from this aluminum alloy sheet as shown in Figure 1. Next, as shown in Figure 2, one end of the test specimen 1 is fixed to the holding block 2 using a holding jig 3, in accordance with the cantilever type described in the Japan Electronic Materials Manufacturers Association standard EMAS-1011. Then, a load is applied to the test specimen 1 by placing a position adjustment block 4 under the other end of the test specimen 1. The loading conditions are as follows: the height of the position adjustment block 4 from the reference surface of the holding block 2 (deflection height) is set to 10 mm, and the horizontal distance from the position where the deflection of the test piece 1 begins to the point where the test piece 1 contacts the position adjustment block (deflection distance) is set according to the Young's modulus of the sample so that the loaded stress is 80% of the yield strength after stretching. Then, with the load applied to the test piece 1, the test piece 1 is held in an atmosphere at a test temperature of 120°C for a holding time of 3 hours, and then cooled to remove the load. The stress relaxation rate can then be calculated by dividing the height (h) from the reference surface of the holding block 2 to the tip position P2 of the test piece 1 after stress removal by the height (H) from the reference surface of the holding block 2 to the tip position P1 of the test piece 1 before stress removal. Note that a larger stress relaxation rate indicates that the shape of the aluminum alloy plate is less likely to return to its original shape, and can be judged as having inferior stress relaxation characteristics.

[0054] [Manufacturing method for aluminum alloy sheets] Next, a method for manufacturing an aluminum alloy sheet according to an embodiment of the present invention will be described. The manufacturing method shown below is just one example, and in the present invention, the manufacturing method is not particularly limited as long as the content of each element contained in the aluminum alloy sheet is within the above range.

[0055] [Manufacturing method for aluminum alloy sheets] The method for manufacturing an aluminum alloy sheet according to this embodiment includes steps 1 to 6, and if a coating film is to be formed, it further includes step 7. Each step will be described in detail below.

[0056] (1st step) In the first step, an aluminum alloy ingot having the desired composition is produced by a semi-continuous casting method (DC (Direct Chill) casting).

[0057] (2nd process) In the second step, the areas with an uneven structure on the surface of the aluminum alloy ingot produced in the first step are removed by surface machining, and then a homogenization heat treatment is performed. This homogenization heat treatment is carried out in a range of, for example, 400°C to 600°C, and also serves as preheating for the subsequent hot rolling. The temperature of the homogenization heat treatment is preferably 450°C or higher, and more preferably 500°C or higher. Performing the homogenization heat treatment as described above can increase the amount of solid-solution Zn.

[0058] (3rd step) In the third step, the homogenized material that has undergone homogenization heat treatment in the second step is hot-rolled. The hot-rolling process includes a hot rough-rolling step in which the homogenized material is rolled using a reverse rolling mill until the plate thickness is several tens of millimeters, and a hot finish-rolling step in which the material is rolled using, for example, a tandem mill until the plate thickness is several millimeters and then wound into a coil.

[0059] The winding temperature, which is the end temperature of hot rolling, is, for example, 280°C to 370°C, preferably 320°C to 370°C. If the winding temperature is 280°C or higher, the recrystallization rate of the hot-rolled sheet improves, and the formability of the aluminum alloy painted sheet after paint baking is further improved. On the other hand, if the winding temperature is 370°C or lower, the occurrence of surface defects called scorching on the sheet surface is suppressed, and the properties of the sheet surface are improved.

[0060] (4th step) In the fourth step, the hot-rolled sheet obtained in the third step is subjected to cold rolling (primary cold rolling). In order to reliably obtain a completely recrystallized structure in the fifth step (intermediate annealing) after the fourth step, it is preferable that the total rolling ratio of the cold rolling in the fourth step be 50% or more. If the total rolling ratio is less than 50%, the accumulated strain from rolling will be insufficient, and a deformed structure may remain after the fifth step (intermediate annealing). In that case, the formability of the product sheet may be reduced. Alternatively, the sheet may be finished to the product thickness without performing intermediate annealing. In that case, it is preferable to have a rolling ratio of 85% or more, and more preferably 87% or more, in order to obtain the strength of the product sheet.

[0061] (5th step) In the fifth step, intermediate annealing is performed on the cold-rolled sheet obtained in the fourth step. Intermediate annealing recrystallizes the cold-rolled sheet and solid-solves Cu and Mg, thereby increasing the strength of the aluminum alloy sheet after painting and baking. Intermediate annealing preferably includes a step of heating at a heating rate of 100°C / min or more, a step of holding at 380 to 550°C for 10 minutes or less, and a step of cooling to 100°C at an average cooling rate of 100°C / min or more. If the heating rate of intermediate annealing is less than 100°C / min, if the holding temperature exceeds 550°C, if the holding time exceeds 10 minutes, or if the average cooling rate to 100°C is less than 100°C / min, there is a concern that the recrystallized grains after annealing will become larger, impairing the formability of the product sheet. Furthermore, if the holding temperature is less than 380°C, the deformation structure formed by the fourth step (primary cold rolling) may remain in the aluminum alloy sheet after annealing. As a result, the formability of the product sheet may decrease. Therefore, the holding temperature for intermediate annealing is preferably 380°C or higher, more preferably 400°C or higher, even more preferably 450°C or higher, and particularly preferably 500°C or higher. Performing intermediate annealing as described above can increase the amount of solid-solution Zn.

[0062] (6th step) In the sixth step, the intermediate annealed sheet obtained in the fifth step is cold-rolled (second cold-rolled) to finish it into an aluminum alloy sheet of a predetermined thickness. In this sixth step, multiple passes are set to achieve a predetermined total rolling ratio. A pass refers to the process of the sheet passing between a pair of work rolls once for rolling. Preferably, the total rolling ratio of the cold rolling in the sixth step is 50-90%, and the final winding temperature after the final rolling pass is 135°C or higher. If the total rolling ratio of the secondary cold rolling is less than 50%, the accumulated strain due to rolling will be insufficient, resulting in a decrease in the strength of the product sheet. On the other hand, if the total rolling ratio of the secondary cold rolling exceeds 90%, the strength of the product sheet will become too high, which may reduce the formability of the product sheet. Furthermore, by setting the final winding temperature after the final rolling pass to 120°C or higher, the static recovery of dislocations after winding is promoted, resulting in improved formability due to an improvement in the work hardening index (n value). The range of the final winding temperature is preferably 135°C or higher, and more preferably 150°C or higher.

[0063] (7th step) In the seventh step, painting is performed on the aluminum alloy sheet after secondary cold rolling. The painting process involves applying a chemical conversion treatment such as chromate or zirconium to the aluminum alloy sheet, followed by paint baking. Preferably, the paint baking process involves applying epoxy paint, polyester paint, or vinyl chloride sol paint to one or both sides of the aluminum alloy sheet and baking it at a Peak Metal Temperature (PMT) of approximately 210 to 270°C. Through this painting process, a painted sheet can be obtained that has an aluminum alloy sheet and a paint film formed on its surface.

[0064] Furthermore, the aluminum alloy sheet according to this embodiment includes all of the cold-rolled sheet obtained in the fourth step, the intermediate-annealed sheet obtained in the fifth step, and the aluminum alloy sheet obtained in the sixth step. [Examples]

[0065] The embodiments will be described in more detail below with reference to examples, but the present invention is not limited to these examples, and can be implemented with modifications within the scope that is consistent with the spirit of the present invention, and all such modifications are included within the technical scope of the present invention.

[0066] <Manufacturing of aluminum alloy sheets for can lids> First, as shown in the first step above, an aluminum alloy raw material having the composition shown in Table 1 below was cast by a semi-continuous casting method. Next, surface milling and homogenization heat treatment were performed using the method shown in the second step above, and hot rolling was performed using the method shown in the third step above without cooling. The coiling temperature for hot rolling was set to 300°C or higher and 370°C or lower. Subsequently, the obtained hot-rolled sheet was subjected to cold rolling (primary cold rolling) and intermediate annealing using the methods shown in the fourth and fifth steps above. Then, as shown in the sixth step above, cold rolling (secondary cold rolling) was performed on all samples until a sheet thickness of 0.3 mm was achieved. For Invention Examples No. 1 to 3 shown in Tables 1 and 2 below, the total rolling ratio of the cold rolling in the sixth step was set to 76%, for Invention Example No. 4 and Comparative Example No. 2, the total rolling ratio was set to 77%, and for Comparative Example No. 1, the total rolling ratio was set to 67%. Subsequently, as shown in step 7 above, the aluminum alloy sheet was subjected to degreasing, chemical conversion treatment, and paint baking, or an equivalent heat treatment, to produce the aluminum alloy sheet for the can lid of the inventive example and comparative example.

[0067] <Measurement of solid solubility> For each aluminum alloy plate obtained, the amount of solid-soluble Cu, solid-soluble Mg, and solid-soluble Zn was measured by quantitative analysis using the thermal phenol method. The specific methods for measuring the amount of solid-soluble Cu, solid-soluble Mg, and solid-soluble Zn are as follows: First, 5 g samples were taken from Invention Example 1, Invention Example 3, and Comparative Example 1, and each sample was dissolved in phenol to obtain a solution. Next, the obtained solution was subjected to quantitative analysis of Zn by ICP (Inductively Coupled Plasma) emission spectrometry, and the quantitative analysis value was taken as the total Zn amount. Then, the above solution was separated into extraction residue and extract using a filter (pore size: 0.1 μm). Subsequently, the obtained extract residue was dissolved, and quantitative analysis of Zn was performed by ICP emission spectrometry, and the quantitative analysis value was taken as the amount of residual Zn (intermetallic compound Zn). Finally, the value obtained by subtracting the amount of residual Zn from the total Zn amount was taken as the amount of solid-soluble Zn. Furthermore, quantitative analysis of Cu and Mg was performed on the above extract by ICP emission spectrometry, and the quantitative analysis values ​​were defined as the amount of solid-soluble Cu and solid-soluble Mg. Note that for Invention Example 2, Invention Example 4, and Comparative Example 2, the amounts of solid-soluble Cu, solid-soluble Mg, and solid-soluble Zn were not measured, so the solid-solution amount column is marked with "-".

[0068] <Measurement of stress relaxation rate> The stress relaxation rate of the obtained aluminum alloy sheet was measured using the method shown in Figures 2 and 3. Specifically, the aluminum alloy sheet was subjected to a baking process equivalent to a painting and baking process, followed by a 2.5% stretch. Next, the rolling direction of the obtained treated material was set to be the longitudinal direction of the test piece, and a test piece 1 with dimensions of 0.3 mm thickness, 10 mm width, and 100 mm length, as shown in Figure 1, was taken from this treated material. The baking conditions were a temperature of 255°C and a heating time of 20 seconds.

[0069] Next, in accordance with the cantilever type described in the Japan Electronic Materials Manufacturers Association standard EMAS-1011, one end of the test specimen 1 was fixed to the holding block 2 using the holding jig 3. Then, a load was applied to the test specimen 1 by placing the position adjustment block 4 under the other end of the test specimen 1. The load conditions were set such that the height of the position adjustment block 4 from the reference surface of the holding block 2 (deflection height) was 10 mm, and the horizontal distance from the position where the deflection of the test specimen 1 begins to the point where the test specimen 1 contacts the position adjustment block (deflection distance) was set so that the applied stress corresponded to 80% of the yield strength after stretching, in accordance with the Young's modulus of the sample. After that, the test specimen 1 was held in an atmosphere with a test temperature of 120°C and a holding time of 3 hours while the load was applied to it.

[0070] Subsequently, the blocks used to apply load to the specimen were removed, and as shown in Figure 3, the stress relaxation rate was calculated by dividing the height (h) to the tip position P2 of specimen 1 after stress removal by the height (H) to the tip position P1 of specimen 1 before stress removal. Specimens with a stress relaxation rate of 52% or less were evaluated as having a superior stress relaxation rate, while those with a stress relaxation rate exceeding 52% were evaluated as having a poor stress relaxation rate.

[0071] <Evaluation of recyclability> Aluminum alloy sheets made without recycled materials have a low Mn content, while aluminum alloy sheets made with recycled materials have an increased Mn content. Therefore, aluminum alloy sheets with a Mn content of less than 0.50% by mass were evaluated as having low recyclability (×), and aluminum alloy sheets with a Mn content of 0.50% by mass or more were evaluated as having excellent recyclability (○).

[0072] Table 1 below shows the content of each component in the aluminum alloy sheet. Table 2 below shows the evaluation results for the solid solution amount, stress relaxation rate, and recyclability of specific elements.

[0073] [Table 1]

[0074] [Table 2]

[0075] As shown in Tables 1 and 2 above, Invention Examples No. 1 to 4 all exhibited excellent stress relaxation characteristics, with stress relaxation rates of 52% or less, which is smaller than that of the comparative examples. This is thought to be because increasing the Zn content in the aluminum alloy plate and controlling it within the range defined in the present invention increased the amount of solid-solution Zn, thereby inhibiting the movement of dislocations.

[0076] In contrast, Comparative Example No. 1 is an aluminum alloy sheet corresponding to Reference Document 1, and its Mn and Zn content is below the lower limit specified in the present invention. Therefore, its stress relaxation properties are reduced, and because it is not possible to use a large amount of recycled material when manufacturing the aluminum alloy sheet, it was evaluated as having low recyclability. Comparative Example No. 2 also had a Zn content below the lower limit specified in the present invention, resulting in poor stress relaxation properties. [Explanation of symbols]

[0077] 1 Test specimen 2 Holding Blocks 3 Holding Jig 4 Position adjustment block

Claims

1. With respect to the total mass of the aluminum alloy plate for the can lid, Si: 0.05% by mass or more and 0.50% by mass or less, Fe: 0.05% by mass or more and 0.50% by mass or less, Cu: 0.05% by mass or more and 0.40% by mass or less, Mn: 0.50% by mass or more and 0.80% by mass or less, Mg: 2.2% by mass or more and 4.0% by mass or less, It contains Zn: 0.10% by mass or more and 0.40% by mass or less, It contains at least one selected from Ti, Cr, Ca, B, and Be, each of which is Ti: 0.15% by mass or less, Cr: 0.15% by mass or less, Ca: 0.05% by mass or less, B: 0.05% by mass or less, Be: 0.05% by mass or less, The remainder consists of Al and unavoidable impurities. An aluminum alloy plate for can lids, characterized in that the stress relaxation rate, measured in accordance with the stress relaxation test specified in EMAS-1011, is 52% or less.

2. In the aforementioned unavoidable impurities, Na: 0.05% by mass or less, V: 0.05% by mass or less, Ni: 0.05% by mass or less, In: 0.05% by mass or less, Sn: 0.05% by mass or less, The aluminum alloy plate for can lids according to claim 1, characterized in that Ga is restricted to 0.05% by mass or less.

3. Solid solution Cu amount: 0.05% by mass or more and 0.40% by mass or less, Solid solution Mg amount: 1.5% by mass or more and 4.0% by mass or less, and The aluminum alloy plate for can lids according to claim 1 or 2, characterized in that the solid solution Zn content is 0.10% by mass or more and 0.40% by mass or less.

4. With respect to the total mass of the aluminum alloy plate for the can lid, Si: 0.05% by mass or more and 0.50% by mass or less, Fe: 0.05% by mass or more and 0.50% by mass or less, Cu: 0.05% by mass or more and 0.40% by mass or less, Mn: 0.50% by mass or more and 0.80% by mass or less, Mg: 2.2% by mass or more and 4.0% by mass or less, It contains Zn: 0.10% by mass or more and 0.40% by mass or less, It contains at least one selected from Ti, Cr, Ca, B, and Be, each of which is Ti: 0.15% by mass or less, Cr: 0.15% by mass or less, Ca: 0.05% by mass or less, B: 0.05% by mass or less, Be: Contains in the range of 0.05% by mass or less. The remainder consists of Al and unavoidable impurities. Solid solution Cu amount: 0.05% by mass or more and 0.40% by mass or less, Solid solution Mg amount: 1.5% by mass or more and 4.0% by mass or less, and An aluminum alloy plate for can lids, characterized by having a solid solution Zn content of 0.10% by mass or more and 0.40% by mass or less.

5. In the unavoidable impurity, Na: 0.05% by mass or less, V: 0.05% by mass or less, Ni: 0.05% by mass or less, In: 0.05% by mass or less, Sn: 0.05% by mass or less, The aluminum alloy plate for can lids according to claim 4, characterized in that Ga is restricted to 0.05% by mass or less.

6. The aluminum alloy plate for can lids according to claim 4 or 5, characterized in that the stress relaxation rate measured in accordance with the stress relaxation test specified in EMAS-1011 is 52% or less.

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