JP7747838B2 - Aluminum alloy coated sheet for tubs - Google Patents

Description

The present invention relates to a coated aluminum alloy sheet for use in tubs.

Beverage cans, canned food cans, and other food cans are widely distributed as packaging containers used for beverages and food. Currently, the so-called pull-top system, which uses an easy-open end (EOE) that allows the can to be easily opened by hand without the use of a can opener or other tool, is widely used for the lids (ends). However, the stay-on tab (SOT) system, in which the tab does not detach from the lid itself, is becoming more popular due to safety and environmental concerns.

The tab material used to form the tabs must be easy to form, as well as have high strength, repeated bending resistance, and high tab tear resistance. Meanwhile, particularly in recent years, tabs have become thinner to reduce costs, and higher material strength than ever before is required to ensure tab breakage strength. Furthermore, thinner tabs and the resulting higher strength have significantly reduced tab tear resistance, leading to an increased demand for materials with high tab tear resistance.

For example, Patent Document 1 describes an aluminum alloy sheet for packaging container tabs that contains specified amounts of Si, Fe, Cu, Mn, and Mg, where the Fe and Mn contents satisfy a specified relationship, and further specifies the sum of the area ratios of Al-Fe-Mn-based crystals and Mg-Si-based crystals in the center of the sheet thickness and the maximum size of these crystals. Tabs formed from such an aluminum alloy sheet are said to be free from tearing or cracking when the can is opened.

For example, Patent Document 2 describes an aluminum alloy sheet for packaging container tabs that contains specified amounts of Mg, Cu, Fe, Si, and Mn, where the Fe and Mn contents satisfy a specified relationship, and where the total area ratio of Al-Fe-Mn intermetallic compounds and Mg-Si intermetallic compounds with a maximum length of 1 μm or more at the center of the sheet thickness is specified. Tabs formed from such aluminum alloy sheet are said to be resistant to tearing when opening cans, even when thinned, and to have excellent repeated bending properties.

For example, Patent Document 3 describes an aluminum alloy sheet for tabs that contains specified amounts of Mg, Cu, Fe, Si, Mn, and Cr and has a texture in which the Cube orientation density is 1.5 times or more that of a randomly oriented sample throughout the entire thickness direction, from the center of the sheet to the surface. Such an aluminum alloy sheet is said to be able to achieve both high strength and excellent bendability.

Japanese Patent Application Laid-Open No. 2005-200754 JP 2011-225977 A JP 2017-66458 A

However, in recent years, in order to reduce costs, _CO2 emissions, and resource conservation, aluminum alloy sheets for tabs have become thinner than ever before. As a result, the bending stress on the surface of the bent part, which breaks during repeated bending, has decreased, making it relatively easy to ensure repeated bending resistance, which was a previous challenge. Furthermore, the tab breaking strength can be addressed by increasing the strength of the material. On the other hand, with the advances in thinner and stronger sheets, further improvements in tab tear resistance are required.

Therefore, the object of the present invention is to provide a coated aluminum alloy sheet for tabs that has high resistance to tab cracking even when thinned.

The coated aluminum alloy sheet for tabs according to the present invention comprises an aluminum alloy sheet containing from 0.05 to 0.40% by mass of Si, from 0.05 to 0.50% by mass of Fe, from 0.01 to 0.30% by mass of Cu, from 0.1 to 0.6% by mass of Mn, and from 4.0 to 6.0% by mass of Mg, with the balance being Al and inevitable impurities, and a resin layer. When the structure of the coated aluminum alloy sheet for tabs is observed from the normal direction of the rolled surface in a region 50 nm thick from the center of the sheet thickness in both thickness directions, the coated aluminum alloy sheet has a total of 40 or more subgrains in an area of 205 × ^{10-12 m2} in an image taken with a transmission electron microscope at 50,000 magnification.

The present invention makes it possible to provide a coated aluminum alloy sheet for tabs that has high resistance to tab cracking even when thinned.

1A and 1B are diagrams illustrating the appearance of the lid of a stay-on-tab type can and the can-opening operation, where FIG. 1A is a plan view, FIG. 1B is a cross-sectional view taken along the line A-A in FIG. 1A, and FIG. 1C is a cross-sectional view and a plan view of the main parts illustrating the can-opening operation. 1 is an example of a transmission electron microscope image of the structure of an aluminum alloy plate. FIG. 10 is a schematic diagram illustrating a method for measuring a tab tearing simulated load. 10 is an example of the measurement results of a simulated tab tear load. 10 is a graph showing the relationship between a tab cracking simulated load and a plate threading interval. 1 is a graph showing the relationship between the tab tearing simulated load and the number of subgrains.

The following describes an aluminum alloy coated sheet for tabs according to one embodiment of the present invention. However, the embodiment described below is merely an example for embodying the technical concept of the present invention, and the present invention is not limited to the following embodiment. The sizes and relative positions of components shown in the drawings may be exaggerated for clarity. In this specification, the term "process" refers not only to an independent process, but also to processes that cannot be clearly distinguished from other processes, as long as the intended purpose of the process is achieved. Furthermore, unless otherwise specified, the content of each component in a composition refers to the total amount of the multiple substances present in the composition. Furthermore, the upper and lower limits of the numerical ranges described in this specification can be arbitrarily selected and combined.

As shown in Figure 1A, a stay-on-tab lid consists of a lid material 2 and a tab 1. The tab 1 is attached to the lid material 2 by crimping a rivet 21 formed approximately at the center of the disk-shaped lid material 2 (partially removed in Figure 1A) into a rivet hole 11 in the tab 1. The rivet hole 11 is located toward one end of the tab 1 from the longitudinal center, and a ring-shaped hook 12 is formed at the other end for easy finger hooking. As shown in Figure 1B, the outer periphery of the cut aluminum alloy plate and the inner periphery of the hook 12 are bent downward to increase rigidity and ensure safety. The rivet hole 11 and the area surrounding the U-shaped inner lance 14 are formed as a single flat plate, allowing for easy deformation. Aluminum alloy plates, such as 5182 alloy, are commonly used as the material for such tabs 1. The aluminum alloy plate is coated and baked on the surface, then cut and formed into the specified shape to produce the tub 1, which is then rivet-attached to a lid 2 made from another formed plate. An opening area 23, which will form a drinking spout after the can is opened, is provided on the lid 2 on the extension of the one end of the tab 1 and is surrounded by scores 25. This score 25 does not go completely around the perimeter of the opening area 23, but is discontinuous at one point (the area where the tab 1 overlaps in Figure 1A).

When opening the can, the hooking portion 12 of the tab 1 is pulled upward, as shown in Figure 1C. This acts as a force point E, and the vicinity of the rivet portion 21 acts as a fulcrum F, causing the tab 1 to lift, leaving the area secured by the rivet portion 21. Specifically, the tab 1 is split by the inner lance 14, and its outer area, including the hooking portion 12, is lifted, while its inner area (the area around the rivet hole 11) remains secured to the lid 2. One end of the tab 1 (the opposite side of the hooking portion 12 across the rivet hole 11) then acts as a lever, forcing it downward with a force, depressing a portion of the opening area 23 of the lid 2 directly below this end. The lid 2 then cracks along the score 25 near the depressed portion, and the opening area 23 is separated from the rest of the lid 2, leaving only a portion, and pushed downward (into the interior of the can), forming a drinking spout (opening) in the lid 2. The opening region 23 allows the lid material 2 to easily bend where the score 25 is not formed, maintaining its connection to the rest of the lid material 2, and the tab 1 easily bends between both ends of the inner lance 14 around the rivet hole 11, connecting to the rivet portion 21 outside the opening region 23 of the lid material 2. As a result, the opening region 23 and tab 1 do not separate from the can body (can barrel).

As such, the tab acts as a lever and is subjected to a strong external force when opening the can. For this reason, if the tab is not strong enough, it may bend at the narrow part of the hooking portion 12 near the center between fulcrum F and force point E shown in Figure 1C or at the narrow part around the inner lance 14 (see bent portion 4 in the lower right of Figure 1C), making it difficult to open the can (tab bend) or even causing it to tear. Furthermore, when the outer area of the inner lance 14, such as the hooking portion 12, is raised, there is a risk that the end of the inner lance 14 will tear in the direction of movement (tab tear) (see torn portion 5 in the lower right of Figure 1C).

Furthermore, particularly with stay-on-tab ends, if the opening region 23 is not pushed deep enough into the beverage can, it will interfere with removing (drinking) the beverage inside. For this reason, the tab 1 (hooking portion 12) is raised nearly vertically and then raised even further (tilted in the opposite direction) to open the can. After opening the can, to prevent the raised tab 1 from interfering with drinking the beverage inside, it is common to return (tilt) the tab 1 to its original position (as shown by the dashed line in Figure 1C ) so that only the opening region 23 of the lid material 2 is pushed inward. In other words, the tab 1 is bent to an angle of nearly 90° or more near both ends of the inner lance 14 around the rivet hole 11 before being returned to its original position (bent back). Furthermore, if the opening region 23 is not sufficiently pushed in by a single bending and unbending operation, the tab 1 must be bent and unbent again. Therefore, the tab 1 must be moldable enough to withstand at least four deformations and not break at the bending point (see fracture point 6 in the lower right corner of Figure 1C ). This property is called repeated bending resistance.

As described above, tab materials are required to be not only easily moldable, but also to have high strength, repeated bending resistance, and high tab tear resistance. Meanwhile, particularly in recent years, tabs have become thinner in order to reduce costs, and higher material strength than ever before is required to ensure tab breakage strength. Furthermore, thinner tabs and the resulting higher strength have significantly reduced tab tear resistance, leading to an increased demand for materials with high tab tear resistance.

1. Coated aluminum alloy sheet for tabs According to one embodiment of the present invention, the coated aluminum alloy sheet for tabs comprises an aluminum alloy sheet and a resin layer provided on one or both sides of the aluminum alloy sheet. When the coated aluminum alloy sheet for tabs has been paint-baked and the structure of a 50-nm-thick region in each of the thickness directions from the center of the sheet thickness (hereinafter sometimes referred to as the center of the sheet thickness) is observed from the normal direction of the rolled surface, the coated aluminum alloy sheet has a total of 40 or more subgrains in an area of 205 × 10 ^{-12 m2} in an image taken with a transmission electron microscope at 50,000 magnification.

Aluminum Alloy Composition The aluminum alloy sheet constituting the coated aluminum alloy sheet for tabs is made of, for example, an Al-Mg alloy. Examples of the Al-Mg alloy include general JIS alloys such as 5182.

Specifically, the aluminum alloy plate contains Si: 0.05% to 0.40% by mass, Fe: 0.05% to 0.50% by mass, Cu: 0.01% to 0.30% by mass, Mn: 0.1% to 0.6% by mass, Mg: 4.0% to 6.0% by mass, with the balance being Al and unavoidable impurities. The content of each component contained in the aluminum alloy plate and the reasons for limiting the content are explained below.

(Si: 0.05% by mass or more and 0.40% by mass or less)
If the Si content is less than 0.05% by mass, high-purity aluminum ingots are required during casting, which increases costs. Furthermore, if the Si content exceeds 0.40% by mass, numerous intermetallic compounds are formed during casting and hot rolling, accelerating the initiation and propagation of cracks, thereby reducing repeated bending properties and tab crack resistance. The Si content is preferably 0.30% by mass or less, more preferably 0.25% by mass or less, and even more preferably 0.20% by mass or less. Furthermore, the Si content is preferably 0.06% by mass or more.

(Fe: 0.05% by mass or more and 0.50% by mass or less)
If the Fe content is less than 0.05% by mass, high-purity aluminum ingots are required during casting, increasing costs. On the other hand, if the Fe content exceeds 0.50% by mass, the amount of Al-Fe-Mn intermetallic compounds increases, promoting the initiation and propagation of cracks, thereby reducing repeated bending properties and tab crack resistance. The Fe content is preferably 0.10% by mass or more, more preferably 0.15% by mass or more. The Fe content is also preferably 0.40% by mass or less, more preferably 0.35% by mass or less, even more preferably 0.30% by mass or less, and particularly preferably 0.25% by mass or less.

(Cu: 0.01% by mass or more and 0.30% by mass or less)
If the Cu content is less than 0.01% by mass, the strength is insufficient, resulting in insufficient tab folding strength. On the other hand, if the Cu content exceeds 0.30% by mass, the strength becomes excessive, making the tab more susceptible to tearing due to repeated bending deformation. The Cu content is preferably 0.02% by mass or more, more preferably 0.03% by mass or more. The Cu content is also preferably 0.25% by mass or less, more preferably 0.20% by mass or less, even more preferably 0.15% by mass or less, and particularly preferably 0.10% by mass or less.

(Mn: 0.1% by mass or more and 0.6% by mass or less)
If the Mn content is less than 0.1% by mass, the strength is insufficient, resulting in insufficient tab breaking strength. On the other hand, if the Mn content exceeds 0.6% by mass, the strength becomes excessive and the amount of Al-Fe-Mn intermetallic compounds increases, resulting in reduced tab crack resistance. The Mn content is preferably 0.2% by mass or more. The Mn content is also preferably 0.55% by mass or less, more preferably 0.45% by mass or less, and particularly preferably 0.35% by mass or less.

(Mg: 4.0 mass% or more and 6.0 mass% or less)
If the Mg content is less than 4.0% by mass, the strength is insufficient, resulting in insufficient tab breaking strength. On the other hand, if the Mg content exceeds 6.0% by mass, the strength becomes excessive and formability deteriorates. Furthermore, as the Mg content increases, the strength and work hardening ability improve, making the sheet more susceptible to tearing due to repeated bending. The Mg content is preferably 4.5% by mass or more. Furthermore, the Mg content is preferably 5.5% by mass or less, more preferably 5.0% by mass or less.

(Ti: 0.1% by mass or less)
The aluminum alloy sheet may contain Ti. Ti is added as needed to refine the ingot structure, and this effect is achieved by adding 0.01% by mass or more. If the Ti content exceeds 0.1% by mass, coarse compounds may be formed, which may reduce the tab tear load. Therefore, the Ti content in the aluminum alloy may be limited within the above range. When Ti is added, for example, an ingot refiner (Al-Ti-B) with a Ti to B mass ratio of 5:1 is added. Since Ti is added to the molten metal before casting in the form of waffles or rods, B is inevitably added according to the content ratio. The Ti content may be preferably 0.08% by mass or less, more preferably 0.06% by mass or less.

(balance: Al and inevitable impurities)
The aluminum alloy sheet may contain inevitable impurities in addition to Al and the above alloy components. Examples of inevitable impurities include Cr, Zn, Zr, B, V, Na, Ca, Ni, In, Sn, and Ga. The allowable content of inevitable impurities is, for example, 0.1 mass% or less for Cr, preferably 0.05 mass% or less. For example, 0.3 mass% or less for Zr, preferably 0.1 mass% or less, more preferably 0.05 mass% or less. For example, the other elements may each be 0.05 mass% or less and a total of 0.15 mass% or less. Within the above ranges, the effects of the present invention are not hindered even when the elements are added, not just when they are contained as inevitable impurities.

The thickness of the aluminum alloy plate that makes up the aluminum alloy coated plate for the tab may be, for example, 0.18 mm or more and 0.40 mm or less.

In one embodiment of the present invention, the aluminum alloy sheet has the above-described alloy composition, and is cold-rolled without intermediate annealing, and then heat-treated by paint baking. The aluminum alloy sheet has a structure in which 40 or more subgrains are present in a predetermined region in the center of the sheet thickness. By making the aluminum alloy sheet have a predetermined number of subgrains or more, it is possible to solve a problem that has been difficult to solve in the past: that is, the tab tear load can be improved while maintaining strength. The subgrains present in the aluminum alloy sheet are counted by observing a 100 nm-thick structure in the center of the sheet thickness from the normal direction of the rolled surface using a transmission electron microscope (TEM) at 50,000 magnification, at 205 × 10 ^-12 m ² .

Subgrains, also known as subcrystal grains, are small, irregularly shaped particles that form when a material (structure) into which dislocations have been introduced by cold rolling or other processes undergoes recovery to form a lower-energy structure under given temperature, time, and strain.

That is, in the case of coated aluminum alloy sheets for tabs, subgrains with sharp boundaries are formed when dislocations in dislocation-dense regions such as dislocation cell walls and deformation bands introduced by cold rolling coalesce, annihilate, and rearrange. When numerous dislocation-dense regions are formed, the probability of them coalescing and annihilat- ing with newly arriving dislocations increases, reducing the work hardening exponent (n value). However, the formation of a subgrain structure improves the work hardening exponent (n value). An improved work hardening exponent (n value) expands the plastic deformation region in the stress concentration area, improving tab crack resistance. For this reason, the more subgrains there are, the higher the tab crack load.

An example of an observed subgrain is shown in Figure 2. Figure 2 is an example of a transmission electron microscope (TEM) image taken at 50,000x magnification. Dislocation-dense regions 100 and subgrains 200 are mixed together, and the subgrains 200 are distinguishable within the crystal grains as small, individual, irregularly shaped grains with sharp (clear and distinct) outer edge shapes that form their boundaries and few internal dislocations.

In this way, it is possible to count the number of individual subgrains within the observation field of a transmission electron microscope. It is also possible to measure the subgrain area ratio instead of the number of subgrains present within a given observation field, but in this case the size of the subgrains is not taken into account, so a high subgrain area ratio can be obtained even with a coarse subgrain structure. Because a coarse subgrain structure has a lower dislocation density than a fine subgrain structure, the strength is lower and the strength required for a tab cannot be obtained.

Therefore, in one embodiment, the size of the subgrains is, for example, 1000 nm or less, preferably 800 nm or less, and more preferably 600 nm or less in terms of equivalent circle diameter. The lower limit of the equivalent circle diameter of the subgrains is, for example, 50 nm or more. The equivalent circle diameter is calculated by measuring the boundary length of the subgrain and calculating it as the diameter of a circle having a circumference length equal to the boundary length.

In one embodiment, the number of subgrains in a predetermined region in the center of the thickness of the aluminum alloy plate constituting the coated aluminum alloy plate for tabs may be 40 or more, preferably 50 or more, and more preferably 60 or more. The upper limit of the number of subgrains is, for example, 600 or less, or 400 or less.

If subgrain formation is not advanced in a specific region in the center of the plate thickness of an aluminum alloy plate, or if the number of subgrains is as few as 40, the aluminum alloy plate will have high strength, but on the other hand, it may not be possible to achieve both excellent tab crack resistance and tab breakage strength (high strength). In other words, it may not be possible to improve the tab crack resistance while maintaining the high strength of the aluminum alloy coated plate for tabs.

The aluminum alloy coated sheet for tabs has excellent strength and tab crack resistance. The sheet strength of the aluminum alloy coated sheet for tabs may be, for example, 300 MPa or more, preferably 320 MPa or more, more preferably 330 MPa or more, and even more preferably 340 MPa or more, as 0.2% proof stress. The upper limit of the sheet strength of the aluminum alloy coated sheet for tabs is, for example, 390 MPa or less, as 0.2% proof stress. Furthermore, the tab crack resistance of the aluminum alloy coated sheet for tabs can be evaluated, for example, by the tab crack simulation load determined by the method schematically shown in Figure 3. The tab crack simulation load according to the method shown in Figure 3 may be, for example, 65 N or more, preferably 67 N or more, more preferably 68 N or more, and even more preferably 70 N. Furthermore, the upper limit of the tab crack simulation load is, for example, approximately 100 N or less. If the 0.2% yield strength is 300 MPa or more and the tab tearing simulated load is 65 N or more, the can can be opened without breaking or tearing the tab while maintaining sufficient strength. Details of the method for evaluating the tab tearing simulated load will be described later.

The structure and properties of the aluminum alloy sheet described above are those of a painted aluminum alloy sheet (pre-coated sheet) after painting and paint baking processes have been applied to a cold-rolled sheet (sheet after cold rolling). Note that these structure and properties may also be those of an aluminum alloy sheet after heat treatment under specific conditions simulating paint baking processes has been applied to a cold-rolled sheet, without painting and paint baking processes or forming into a tab. These structure and properties will be the same, or can be considered to be the same with only slight differences, if the conditions for the paint baking process and the heat treatment are the same.

The coated aluminum alloy sheet for tabs may be a precoated sheet with a resin layer provided on one or both sides of the aluminum alloy sheet. The resin layer may be a baked resin layer formed by applying an organic paint such as an epoxy-based, vinyl vinyl sol-based, or polyester-based paint and then heat treating it. The aluminum alloy sheet to which the organic paint is applied may be a surface-treated aluminum alloy sheet that has been surface-treated or chemically treated with a surface treatment agent such as a chromate-based or zircon-based agent. The heat treatment temperature may be, for example, a temperature at which the peak metal temperature (PMT) is approximately 200°C or higher and 290°C or lower. The thickness of the resin layer may be, for example, approximately 0.5 μm or higher and 15 μm or lower.

[0023] An example of a method for manufacturing an aluminum alloy sheet for forming an aluminum alloy coated sheet for a tab will now be described. The method for manufacturing an aluminum alloy sheet includes a casting step as a first step, a homogenization heat treatment step as a second step, a hot rolling step as a third step, and a cold rolling step as a fourth step, which are carried out in this order.

(First to third steps: casting, homogenization heat treatment, hot rolling)
The first step is to produce an ingot having a target composition by semi-continuous casting, and the second step is to subject the aluminum alloy ingot produced in the first step to homogenization heat treatment.

In the first step, an aluminum alloy is cast using a semi-continuous casting method (DC (direct chill) casting) to obtain an ingot. Next, in the second step, areas of the ingot surface that would otherwise have a non-uniform structure are removed by facing, and a homogenization heat treatment is performed. This homogenization heat treatment is carried out, for example, at a temperature in the range of 400°C to 570°C, and also serves as preheating for the subsequent hot rolling.

The third step is hot rolling the aluminum alloy ingot that has been subjected to the homogenization heat treatment in the second step. The thickness of the hot-rolled plate obtained by hot rolling is usually determined by back-calculating the total rolling reduction ratio for cold rolling from the thickness of the product plate obtained by cold rolling.

The coiling temperature, which is the temperature at which hot rolling ends, is, for example, 300°C or higher and 400°C or lower, and preferably 320°C or higher and 370°C or lower. A coiling temperature of 300°C or higher improves the recrystallization rate of the hot-rolled sheet, further improving the tab tearing simulated load of the aluminum alloy sheet after paint baking. On the other hand, a coiling temperature of 400°C or lower suppresses the occurrence of a surface defect known as seizure on the sheet surface, improving the sheet surface properties.

The fourth step is cold rolling the hot-rolled sheet obtained in the third step. In the fourth step, the hot-rolled sheet is cold-rolled without intermediate annealing to finish it into an aluminum alloy sheet of the specified thickness. Cold rolling is performed by setting multiple passes to achieve a specified total rolling reduction so that the hot-rolled sheet is rolled to the thickness of the product sheet within an appropriate load range. A pass refers to the sheet being rolled by passing it between a pair of work rolls once.

The total cold rolling reduction is preferably 85.0% or more and 95.0% or less, and more preferably 87.0% or more and 93.0% or less. When the total cold rolling reduction is 85.0% or more and 95.0% or less, sufficient strength is obtained for the aluminum alloy sheet, and the dislocation density is increased, promoting the formation of subgrains after paint baking, making it possible to achieve a subgrain number within the range specified in the present invention. As a result, tab crack resistance tends to be improved.

In addition to keeping the total cold rolling reduction within a specified range, it is preferable to keep the sheet threading interval between the final cold rolling pass and the immediately preceding rolling pass within 0.33 seconds. Keeping the sheet threading interval within 0.33 seconds further promotes dynamic recovery in the final rolling pass, further promoting the formation of subgrains in the dislocation-dense regions, and resulting in the formation of more subgrains than conventionally during the subsequent paint baking process. The sheet threading interval is preferably within 0.32 seconds, and more preferably within 0.31 seconds.

The aluminum alloy sheet produced through the above process is subjected to a chemical conversion treatment using a surface treatment agent such as a chromate or zirconium-based agent. An organic paint containing an epoxy resin, vinyl chloride sol, or polyester is then applied to form a coating. The resulting coating is then baked at a PMT (Pass-to-Metal Temperature) of approximately 200°C to 290°C, producing a pre-coated aluminum alloy coated sheet for use in tabs.

The present invention will be explained in more detail below using examples, but the present invention is not limited to these examples.

(Preparation of test material)
An aluminum alloy having the composition shown in Table 1 was cast by a semi-continuous casting method, and then subjected to facing and homogenization heat treatment by the methods shown as the first and second steps, followed by hot rolling without cooling. The end temperature of the hot rolling was set to 300°C or more and 370°C or less as the coiling temperature. The obtained hot-rolled sheet was then cold-rolled under the conditions shown in Table 1 without intermediate annealing to obtain an aluminum alloy sheet as a cold-rolled sheet having a sheet thickness of 0.25 mm. The balance of the composition shown in Table 1 is Al and unavoidable impurities. The cold reduction ratio is the total rolling ratio in cold rolling, and the sheet threading interval is the time required between the final rolling pass and the immediately preceding rolling pass in cold rolling.

The resulting aluminum alloy plate was then subjected to a chemical conversion treatment, after which an epoxy-based paint was applied and baked in a continuous baking furnace at a PMT of 200°C to 290°C. The baking temperature and baking time were the same for all manufacturing examples No. 1 to No. 3.

The manufactured aluminum alloy coated sheets were used as test materials, and the number of subgrains, 0.2% yield strength, and tab tear simulated load (tab tear resistance) were measured using the following methods. The results are shown in Table 1.

Number of subgrains The structure of each test material in a region 50 nm thick in both thickness directions from the center of the sheet thickness was observed with a transmission electron microscope (TEM) at a magnification of 50,000 times from the normal direction of the rolled surface, and the number of subgrains was counted to calculate the number of subgrains.

Specifically, the test material was mechanically polished to a thickness of 0.05 mm (0.1 mm) in both thickness directions from the center of the plate thickness, and then a thin film approximately 100 nm thick was formed using a twin-jet electrolytic polishing method. This thin film was then photographed using a transmission electron microscope at a magnification of 50,000 times from the normal direction of the rolled surface, over an area of 205 × 10 ⁻¹² m ^2. The number of subgrains measured within the photographed field of view was totaled to calculate the number of subgrains. There is no particular upper limit to the number of subgrains, but the upper limit is usually around 400 to 600.

Measurement of 0.2% Yield Strength For each test material, a JIS No. 5 tensile test piece was prepared with the tensile direction parallel to the rolling direction, and a tensile test was performed in accordance with the provisions of JIS Z2241. The plate thickness was calculated by dividing the coating thickness to determine the 0.2% yield strength. The appropriate range for 0.2% yield strength is 300 MPa or more, and within this range, even thin-walled tabs will satisfy the tab breaking strength. There is no specific upper limit for 0.2% yield strength, but considering the adverse effects on formability, etc., the upper limit is usually around 390 MPa.

Measurement of Tab Breaking Simulated Load: Test specimens shown in FIG. 3A were prepared from each test material, and a tear test simulating tab breakage was performed to determine the tab breakage simulated load. As shown in FIG. 3B, the test specimens were clamped between portions 26 and 27 of the test specimen in one chuck of a tensile tester, and portion 28 was clamped and fixed in the other chuck. The tear test was performed at a tensile test chucking speed of 10 mm/min. The inner diameter of each hole in the test specimen was 2.0 mm. This tear test measured the peak loads at which cracks occurred in holes 31 and 32 of the test specimen, as shown in FIG. 4. The test was performed eight times using test specimens prepared from the same test material, and the minimum measurement result of the two peak loads measured in each test was extracted and averaged to determine the tab breakage simulated load. Note that in some cases, holes 31 and 32 may tear simultaneously during the tear test. In such cases, a single peak load was obtained in one test, resulting in a higher peak load. In the present invention, cases where only one peak load was obtained in the tear test were excluded from the test results. The appropriate range of the tab tearing simulated load was set to 65 N or more. If the tab tearing simulated load is 65 N or more, the can opening operation can be completed without causing tab tearing, and the can has excellent tab tearing resistance.

As shown in Table 1, Nos. 1 to 3, whose aluminum alloy coated sheet compositions are within the range specified by the present invention, qualify as examples because the number of subgrains is also within the range specified by the present invention. For all of Nos. 1 to 3, the sheet threading interval between the final cold rolling pass and the pass immediately preceding it was within 0.33 seconds, and the tab tearing simulated load (tab tear resistance) also met the acceptable value. Furthermore, as shown in Figure 5, the tab tearing simulated load and the sheet threading interval are roughly proportional to each other. Based on the slope of the linear approximation line, the sheet threading interval at which the tab tearing simulated load is 65 N or greater is expected to be approximately 0.33 seconds. Furthermore, as shown in Figure 6, the tab tearing simulated load and the number of subgrains are roughly proportional to each other. Based on the slope of the linear approximation line, the number of subgrains at which the tab tearing simulated load is 65 N or greater is expected to be approximately 40 or greater.

1 tab 2 lid material 11 rivet hole 14 inner lance 21 rivet portion 100 dislocation dense region 200 subgrain

Claims (3)

an aluminum alloy plate containing Si: 0.05% by mass or more and 0.40% by mass or less, Fe: 0.05% by mass or more and 0.50% by mass or less, Cu: 0.01% by mass or more and 0.30% by mass or less, Mn: 0.1% by mass or more and 0.6% by mass or less, Mg: 4.0% by mass or more and 6.0% by mass or less, with the balance being Al and inevitable impurities; and a resin layer;
A coated aluminum alloy sheet for tabs having a total of 50 or more subgrains in an area of 205 × 10 ⁻¹² m2 in an image taken with a transmission electron microscope at 50,000 magnification, when the structure of a region 50 nm thick from the center of the sheet thickness in ^both thickness directions is observed from the normal direction of the rolled surface. The coated aluminum alloy sheet for tabs according to claim 1, wherein the total number of subgrains in the region is 60 or more. The coated aluminum alloy sheet for tabs according to claim 1 or 2, wherein the aluminum alloy sheet has a Ti content of 0.1% by mass or less.