JP7549563B2 - Aluminum alloy coated plate for tub - Google Patents

Description

The present invention relates to an aluminum alloy coated sheet for tubs.

As one type of packaging container used for beverages and food, beverage cans and food cans such as canned goods are widely distributed. Currently, the so-called pull-top type, which uses an easy-open end (EOE) that can be easily opened by hand without using a tool such as a can opener, is widely used for the lids (ends) of these cans. The stay-on tab (SOT) type, in which the tab does not come off the lid body, is becoming more popular due to safety and environmental concerns.

The tab material used to form the tabs is required to have not only formability but also high strength, repeated bending ability and high tab tear resistance. On the other hand, particularly in recent years, the thickness of tabs has been reduced in order to reduce costs, and a material strength higher than ever before is required to ensure the tab breaking strength. Furthermore, the reduction in thickness and the associated increase in strength have caused a significant decrease in tab tear resistance, and there is an increasing 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 crystallized particles and Mg-Si-based crystallized particles in the center of the sheet thickness, and the maximum sizes of these crystallized particles. 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 specifies 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. Tabs formed from such an aluminum alloy sheet are said to be resistant to tearing when opening a can, even when thinned, and to have excellent repeated bending properties.

For example, Patent Document 3 describes an aluminum alloy sheet for tabs that contains predetermined 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 random orientation sample over the entire range from the center of the sheet in the thickness direction to the sheet surface. Such an aluminum alloy sheet is said to be able to achieve both high strength and excellent bendability.

JP 2005-200754 A JP 2011-225977 A JP 2017-66458 A

However, in recent years, from the viewpoints of cost reduction, _CO2 reduction, resource saving, etc., the thickness of aluminum alloy sheets for tabs has been further reduced than before, and the bending stress on the surface layer of the bent part where it breaks due to repeated bending has been reduced, making it relatively easy to ensure the repeated bending property, which was a problem in the past. In addition, the tab breaking strength can be addressed by increasing the strength of the material. On the other hand, the progress in thinning and high strength has made it necessary to further improve the tab breaking property.

Therefore, the object of the present invention is to provide an aluminum alloy coated 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 0.05% by mass or more and 0.40% by mass or less of Si, 0.05% by mass or more and 0.50% by mass or less of Fe, 0.01% by mass or more and 0.30% by mass or less of Cu, 0.1% by mass or more and 0.6% by mass or less of Mn, 4.0% by mass or more and 6.0% by mass or less of Mg, with the balance being Al and inevitable impurities, and a resin layer. The coated aluminum alloy sheet for tabs has a total of 40 or more subgrains in an area of 205×10 ⁻¹² m ² in an image taken with a 50,000x transmission electron microscope when the structure of a region 50 nm thick in each of the thickness directions from the center of the sheet thickness is observed from the normal direction of the rolled surface.

The present invention provides an aluminum alloy coated sheet for tabs that has high resistance to tab cracking even when thinned.

1A and 1B are diagrams illustrating the appearance of a stay-on-tab type can lid and the can-opening operation, in which 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 explaining the can-opening operation. 1 is an example of a transmission electron microscope image of the structure of an aluminum alloy plate. FIG. 4 is a schematic diagram illustrating a method for measuring a tab tearing simulated load. 13 is an example of the measurement results of a tab tearing simulated load. 1 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 a 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 shown below is merely an example for embodying the technical idea of the present invention, and the present invention is not limited to the following embodiment. The size and positional relationship of the members shown in each drawing may be exaggerated for clarity of explanation. In this specification, the term "process" includes not only an independent process, but also a process that cannot be clearly distinguished from other processes, as long as the intended purpose of the process is achieved. Furthermore, the content of each component in the composition means the total amount of the multiple substances present in the composition, unless otherwise specified. Furthermore, the upper and lower limits of the numerical ranges described in this specification can be arbitrarily selected and combined.

As shown in FIG. 1A, the stay-on-tab type lid is composed of a lid material 2 and a tab 1. The rivet portion 21 formed at the approximate center of the disk-shaped lid material 2 (part of which is shown in FIG. 1A) is crimped into the rivet hole 11 of the tab 1, and the tab 1 is attached to the lid material 2. The rivet hole 11 is formed at a position closer to one end than the longitudinal center of the tab 1, and a ring-shaped hook portion 12 is formed at the other end so that a finger can be easily hooked. As shown in FIG. 1B, the tab 1 is made of a plate material (aluminum alloy plate) with a cut outer periphery and an inner periphery of the hook portion 12 bent downward to increase rigidity and ensure safety. On the other hand, the area where the rivet hole 11 and the U-shaped hole (slit)-like inner lance 14 are formed on the outer side of the plate is made into a flat plate, making it easily deformable. Generally, aluminum alloy plates such as 5182 alloy are widely used as the material for such tabs 1. The aluminum alloy plate is coated and baked on the surface, then cut and shaped into a specified shape to produce the tub 1, which is then attached by rivets to a lid 2 made by shaping another plate material. An opening area 23 for forming a drinking spout after opening the can is provided on the extension of the one end of the lid 2 from the tub 1, 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 tub 1 overlaps in FIG. 1A).

In opening the can, when the hook portion 12 of the tab 1 is pulled upward, as shown in FIG. 1C, this becomes the force point E, and the vicinity of the rivet portion 21 becomes the fulcrum F, and the tab 1 is raised, leaving the area fixed by the rivet portion 21. In detail, the tab 1 is divided by the inner lance 14, and the outer area such as the hook portion 12 is raised while its inner area (the area around the rivet hole 11) is fixed to the lid material 2. Then, one end of the tab 1 (the opposite side of the hook portion 12 across the rivet hole 11) becomes the action point L, and is pushed strongly downward by the lever action, together with pushing down a part of the opening area 23 of the lid material 2 directly below this one end. Then, the lid material 2 cracks along the score 25 from the vicinity of this pushed down part, and the opening area 23 is separated from the other part of the lid material 2 leaving a part and pushed downward (inside the can), forming a drinking spout (opening) in the lid material 2. The opening region 23 easily bends where the lid material 2 does not have the score 25 formed, maintaining the connection with the other parts of the lid material 2, and the tab 1 easily bends between both ends of the inner lance 14 around the rivet hole 11, and is connected to the rivet part 21 outside the opening region 23 of the lid material 2. Therefore, the opening region 23 and the tab 1 do not separate from the can body (can body).

Thus, when the can is opened, the tab acts as a lever and is subjected to a strong external force. For this reason, if the tab is not strong enough, it may bend at the thin part of the hook 12 near the center between the fulcrum F and the force point E shown in Figure 1C or at the thin part around the inner lance 14 (see bent part 4 in the lower right of Figure 1C), making it difficult to open the can (tab bend) or even tearing off. In addition, when the outer area of the inner lance 14, such as the hook 12, is raised, it may tear from the end of the inner lance 14 along the direction of movement (tab tear) (see tear part 5 in the lower right of Figure 1C).

In particular, in the case of a stay-on-tab end, if the opening area 23 is not pushed deep enough into the beverage can, it will be an obstacle when taking out (drinking) the beverage in the beverage can. For this reason, the tab 1 (hooking portion 12) is raised to nearly vertical, and then raised even more (tilted to the opposite side) to open the can. After opening the can, it is common to return (tilt) the tab 1 to its original position so that only the opening area 23 of the lid material 2 is pushed inward, as shown by the dashed line in FIG. 1C, so that the raised tab 1 does not interfere with drinking the beverage in the beverage can. 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, and then returned to its original position (bent back). In addition, if the opening area 23 is not sufficiently pushed in by bending and bending back the tab 1 once, it is necessary to repeat the bending and bending back operations again, so the tab 1 is required to have formability that can withstand at least four deformations and does not break at the bending point (see the broken portion 6 in the lower right of FIG. 1C). This property is called repeated bending property.

As described above, tab materials are required to have not only formability but also high strength, repeated bending ability and high tab tear resistance. On the other hand, particularly in recent years, the thickness of tabs has been reduced in order to reduce costs, and higher material strength than ever before is required to ensure the tab breaking strength. Furthermore, the reduction in thickness and the associated increase in strength have caused a significant decrease in tab tear resistance, and there is an increasing demand for materials with high tab tear resistance.

The coated aluminum alloy sheet for tabs according to one embodiment of the present invention 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 is subjected to a paint baking treatment and the structure of a region 50 nm thick from the center of the sheet thickness in both thickness directions (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 for tabs has a total of 40 or more subgrains in a region of 205× ^{10−12 m2} in an image taken with a 50,000x transmission electron microscope.

Composition of Aluminum Alloy The aluminum alloy plate constituting the aluminum alloy coated plate 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, and the remainder is Al and unavoidable impurities. The content of each component contained in the aluminum alloy plate and the reasons for the content limitations 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. If the Si content exceeds 0.40% by mass, the Si content is increased by 0.5% by mass, which is generated during casting and hot rolling. The Si content is preferably 0.30 mass% or less, and more preferably 0.50 mass% or less. The Si content is preferably 0.25 mass% or less, and more preferably 0.20 mass% or less. The Si content is preferably 0.06 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, which increases costs. On the other hand, if the Fe content exceeds 0.50% by mass, the Al-Fe-Mn-based metal The amount of intermetallic compounds increases, which promotes the generation and propagation of cracks, thereby decreasing the repeated bending property and tab crack resistance. The Fe content is preferably 0.10 mass % or more, and more preferably 0. The Fe content is 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 It is 0.25 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, and the tab folding strength is insufficient. On the other hand, if the Cu content exceeds 0.30% by mass, the strength becomes too high, and the tab folding strength is insufficient due to bending deformation such as repeated bending. The tab becomes easily torn off. The Cu content is preferably 0.02 mass% or more, and more preferably 0.03 mass% or more. The Cu content is preferably 0.25 mass% or less. The content is preferably 0.20% by mass or less, 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 mass %, the strength is insufficient, and the tab breaking strength is insufficient. On the other hand, if the Mn content exceeds 0.6 mass %, the strength becomes too high, and the Al-Fe-Mn system The amount of intermetallic compounds increases, and therefore the tab crack resistance decreases. The Mn content is preferably 0.2 mass% or more. The Mn content is preferably 0.55 mass% or less, and more preferably 0.55 mass% or less. is 0.45 mass % or less, particularly preferably 0.35 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 folding strength. On the other hand, if the Mg content exceeds 6.0% by mass, the strength becomes excessively high, resulting in reduced moldability. As the Mg content increases, the strength and work hardening ability improve, so that the material is more likely to break due to repeated bending. The Mg content is preferably 4.5 mass% or more. The Mg content is preferably is 5.5 mass % or less, more preferably 5.0 mass % or less.

(Ti: 0.1% by mass or less)
The aluminum alloy sheet may contain Ti. Ti is added as necessary for the purpose of refining the ingot structure, and this effect is obtained by adding 0.01 mass % or more. If the Ti content exceeds 1 mass %, coarse compounds are formed, which may reduce the tab breaking load. Therefore, the Ti content in the aluminum alloy may be limited to the above range. For example, an ingot refiner (Al-Ti-B) with a mass ratio of Ti and B of 5:1 is added. Since it is added to the molten metal before casting in the form of a waffle or rod, depending on the content ratio, Inevitably, B is also added. The Ti content may be preferably 0.08 mass % or less, and more preferably 0.06 mass % or less.

(balance: Al and inevitable impurities)
The aluminum alloy plate 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 other elements, for example, 0.05 mass% or less each and 0.15 mass% or less in total. If within the above range, the effects of the present invention are suppressed from being hindered even when the above elements are added, not limited to when they are contained as inevitable impurities.

The thickness of the aluminum alloy plate constituting 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-mentioned alloy composition, and is cold-rolled without intermediate annealing, and then heat-treated by paint baking. The aluminum alloy sheet has a structure having 40 or more subgrains 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 the problem that has been difficult to solve in the past, that is, the tab cracking load can be improved while maintaining strength. The subgrains of the aluminum alloy sheet are counted by observing 205×10 ⁻¹² m ² with a transmission electron microscope (TEM) at 50,000 times magnification when observing a 100 nm-thick structure in the center of the sheet thickness from the normal direction of the rolled surface.

Subgrains, also called subcrystal grains, are small, irregular grains that are formed when a material (structure) into which dislocations have been introduced by cold rolling or other methods undergoes recovery to a lower energy structure under a given temperature, time, and strain.

In other words, 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 and disappear and rearrange. When many dislocation-dense regions are formed, the probability of them coalescing and annihilat- ing with newly arriving dislocations increases, lowering the work hardening exponent (n value), but the formation of a subgrain structure improves the work hardening exponent (n value). When the work hardening exponent (n value) is improved, the plastic deformation region in the stress concentration area expands, 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 at 50,000x magnification. Dislocation-dense regions 100 and subgrains 200 are mixed together, and the subgrains 200 are identified within the crystal grains as small, individual, irregular grains with a sharp (clear and distinct) outer edge shape that is the boundary and few dislocations inside.

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

Therefore, in one embodiment, the size of the subgrain is, for example, 1000 nm or less, preferably 800 nm or less, more preferably 600 nm or less, in terms of the equivalent circle diameter. The lower limit of the equivalent circle diameter of the subgrain 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 aluminum alloy coated 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 the aluminum alloy plate, and the number of subgrains is as small as less than 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 plate for tabs has excellent strength and tab crack resistance. The plate strength of the aluminum alloy coated plate for tabs may be, for example, 300 MPa or more as 0.2% proof stress, preferably 320 MPa or more, more preferably 330 MPa or more, and even more preferably 340 MPa or more. The upper limit of the plate strength of the aluminum alloy coated plate for tabs is, for example, 390 MPa or less as 0.2% proof stress. The tab crack resistance of the aluminum alloy coated plate for tabs can be evaluated by a tab crack simulation load obtained by, for example, the method shown in FIG. 3. The tab crack simulation load by the method shown in FIG. 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. The upper limit of the tab crack simulation load is, for example, about 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 characteristics of the aluminum alloy plate described above are those of the aluminum alloy painted plate (precoated plate) after painting and paint baking treatments have been applied to the cold rolled plate (plate after cold rolling). Note that such structure and characteristics may also be those of the aluminum alloy plate after heat treatment under specific conditions simulating paint baking treatments has been applied to the cold rolled plate without painting and paint baking treatments or forming into a tab. These structures and characteristics are the same, or can be considered to be the same with only a slight difference, if the conditions for the paint baking treatment and the heat treatment are the same.

The aluminum alloy coated sheet for tabs may be a precoated sheet having 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 chloride-based, or polyester-based paint and then subjecting it to heat treatment. 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 temperature of the heat treatment may be, for example, a temperature at which the peak metal temperature (PMT) is about 200°C or higher and 290°C or lower. The thickness of the resin layer may be, for example, about 0.5 μm or higher and 15 μm or lower.

An example of a method for producing an aluminum alloy sheet constituting an aluminum alloy coated sheet for a tab will be described below. The method for producing 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, and these steps 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 a semi-continuous casting method, and the second step is to perform a homogenization heat treatment on the aluminum alloy ingot produced in the first step.

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, the area of the ingot surface that will become a non-uniform structure is removed by facing, and a homogenization heat treatment is performed. This homogenization heat treatment is performed, for example, 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 set by calculating backwards the total rolling reduction ratio by cold rolling from the thickness of the product plate obtained by cold rolling.

The winding temperature, which is the end temperature of hot rolling, is, for example, 300°C to 400°C, and preferably 320°C to 370°C. When the winding temperature is 300°C or higher, the recrystallization rate of the hot-rolled sheet is improved, and the tab tearing simulated load of the aluminum alloy sheet after paint baking is further improved. On the other hand, when the winding temperature is 400°C or lower, the occurrence of a surface defect called seizure on the sheet surface is suppressed, and the properties of the sheet surface are improved.

The fourth step is a step of 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 a specified thickness. Cold rolling is performed by setting multiple passes that result in a specified total rolling ratio so that the hot-rolled sheet is rolled to the thickness of the product sheet within an appropriate load range. Note that a pass refers to the sheet being rolled by passing it between a pair of work rolls once.

The total rolling ratio of the cold rolling 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 rolling ratio of the cold rolling is 85.0% or more and 95.0% or less, the strength of the aluminum alloy sheet is sufficient, the dislocation density is high, the formation of subgrains after the paint baking process is promoted, and the number of subgrains can be set within the range specified in the present invention. As a result, the tab crack resistance tends to be improved.

In addition to keeping the total rolling reduction ratio of the cold rolling within a specified range, it is preferable to keep the sheet passing interval between the final rolling pass of the cold rolling and the rolling pass immediately before it within 0.33 seconds. By keeping the sheet passing interval within 0.33 seconds, dynamic recovery in the final rolling pass is further promoted, and the formation of subgrains in the dislocation-dense region is further promoted, so that more subgrains than before are formed during the next paint baking process. The sheet passing interval is preferably within 0.32 seconds, and more preferably within 0.31 seconds.

The aluminum alloy plate produced by the above process is subjected to a chemical conversion treatment using a surface treatment agent such as a chromate or zircon type. After that, an organic paint containing an epoxy resin, a vinyl chloride sol type, or a polyester type is applied to form a coating. The coating is then painted and baked at a PMT (metal temperature) of 200°C to 290°C to produce a precoated aluminum alloy coated plate for tabs.

The present invention will be described in detail below with reference to 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 was subjected to facing and homogenization heat treatment by the methods shown as the first and second steps, and was hot rolled 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 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 rolling rate is the total rolling rate in cold rolling, and the sheet passing interval is the required time between the final rolling pass and the rolling pass immediately before it in cold rolling.

Then, the obtained aluminum alloy plate was subjected to a chemical conversion treatment, and then coated with an epoxy-based paint 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 produced aluminum alloy coated plate was used as a test material, and the number of subgrains, 0.2% yield strength, and tab tear resistance were measured using the following methods. The results are shown in Table 1.

Number of subgrains The structure of a region 50 nm thick in both thickness directions from the center of the sheet thickness of each test material 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 0.05 mm (0.1 mm) from the center of the sheet thickness in both thickness directions, and then a thin film of approximately 100 nm was formed by a twin-jet electrolytic polishing method. This thin film was photographed with a transmission electron microscope at a magnification of 50,000 times from the normal direction of the rolled surface in an area of 205 × 10 ^-12 m ^2. The number of subgrains measured within the photographed field of view was summed to calculate the number of subgrains. The upper limit of the number of subgrains is not particularly specified, but the upper limit is usually about 400 to 600.

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

Measurement of tab tearing simulated load Test pieces shown in FIG. 3A were prepared from each test material, and a tearing test simulating tab tearing was performed to obtain the tab tearing simulated load. As shown in FIG. 3B, the tearing test was performed by clamping the parts 26 and 27 of the test piece in one chuck of a tensile tester, and clamping and fixing the part 28 in the other chuck, and performing the tearing test at a tensile test chuck speed of 10 mm/min. The inner diameter of the holes of the test piece was 2.0 mm for both. By this tearing test, the peak loads at which cracks occurred in the holes 31 and 32 of the test piece were measured as shown in FIG. 4. The test was performed eight times using test pieces prepared from the same test material, and the minimum measurement results of the two peak loads measured in each test were extracted and averaged to obtain the tab tearing simulated load. Note that in some cases, the holes 31 and 32 may tear simultaneously in the tearing test, in which case the peak load obtained in one test becomes one and the peak load becomes high. 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 during the can opening operation, and the can has excellent tab tearing resistance.

As shown in Table 1, No. 1 to No. 3, in which the composition of the aluminum alloy coated plate is within the range specified by the present invention, correspond to examples because the number of subgrains is within the range specified by the present invention. In all of No. 1 to No. 3, the sheet passing interval between the final pass of cold rolling and the pass immediately before is within 0.33 seconds, and the tab cracking simulated load (tab cracking resistance) also reaches the acceptable value. As shown in Figure 5, the tab cracking simulated load and the sheet passing interval are approximately proportional to each other, and the slope of the linear approximation line indicates that the sheet passing interval at which the tab cracking simulated load is 65N or more is expected to be about 0.33 seconds. As shown in Figure 6, the tab cracking simulated load and the number of subgrains are approximately proportional to each other, and the slope of the linear approximation line indicates that the number of subgrains at which the tab cracking simulated load is 65N or more is expected to be about 40 or more.

1 Tab 2 Lid 11 Rivet hole 14 Inner lance 21 Rivet portion 100 Dislocation dense region 200 Subgrain

Claims (2)

An aluminum alloy plate comprising: 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; and the balance being Al and inevitable impurities; and a resin layer;
An aluminum alloy coated sheet for tabs having a total of 40 or more subgrains in an area of 205 x ^{10-12 m2} in an image taken with a transmission electron microscope at 50,000 magnification, when observing the structure of a region 50 nm thick from the center of the sheet thickness in both thickness directions from the normal direction of the rolled surface. The aluminum alloy coated sheet for tabs according to claim 1, wherein the aluminum alloy sheet has a Ti content of 0.1 mass% or less.