Tab aluminum alloy coated sheet

Abstract

To provide a tab aluminum alloy coated sheet capable of achieving high resistance to tab tearing even under reduced thickness.SOLUTION: A tab aluminum alloy coated sheet comprises an aluminum alloy sheet containing Si: 0.05 mass% or more and 0.40 mass% or less, Fe: 0.05 mass% or more and 0.50 mass% or less, Cu: 0.01 mass% or more and 0.30 mass% or less, Mn: 0.1 mass% or more and 0.6 mass% or less, and Mg: 4.0 mass% or more and 6.0 mass% or less, with the balance being Al and unavoidable impurities, and a resin layer. When the tab aluminum alloy coated sheet is observed from a normal direction to a rolling surface with respect to structures in regions each having a thickness of 50 nm in both thickness directions from the sheet-thickness center, the sheet has, in an image captured by a 50,000×transmission electron microscope, 50 or more subgrains in a region of 205×10-12 m2, and the equivalent circle diameter of the subgrains is 50 nm or more and 1000 nm or less.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a coated aluminum alloy sheet for a tab. [Background technology]

[0002] 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 type, which uses an easy-open end (EOE) that can be easily opened by hand without using a can opener or other tool, is widely used for the lid (end). Stay-on tab (SOT), in which the tab does not come off the lid body, is becoming more popular due to safety and environmental concerns.

[0003] Tab materials for forming tabs are required to have not only formability but also high strength, repeated bending resistance, and high tab tear resistance. Meanwhile, particularly in recent years, tabs have been made thinner to reduce costs, and higher material strength than ever before is required to ensure tab breaking strength. Furthermore, the reduction in thickness and the resulting increase in strength have significantly reduced tab tear resistance, and there is an increasing demand for materials with high tab tear resistance.

[0004] For example, Patent Document 1 describes an aluminum alloy sheet for packaging container tabs, which contains predetermined amounts of Si, Fe, Cu, Mn, and Mg, the contents of Fe and Mn satisfying a predetermined relationship, and further defines 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 sizes of these crystals. Tabs formed from such an aluminum alloy sheet are said to be free from tearing and cracking when the can is opened.

[0005] For example, Patent Document 2 describes an aluminum alloy sheet for packaging container tabs, which contains predetermined amounts of Mg, Cu, Fe, Si, and Mn, the contents of Fe and Mn satisfy a predetermined relationship, and the total area ratio of Al-Fe-Mn intermetallic compounds and M-Si intermetallic compounds having a maximum length of 1 μm or more at the center of the sheet thickness is specified. Tabs formed from such an aluminum alloy sheet are said to be resistant to tearing when opening cans even when thinned, and to have excellent repeated bending properties.

[0006] 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 randomly oriented sample over the entire thickness direction from the center to the surface of the sheet. Such an aluminum alloy sheet is said to be able to achieve both high strength and excellent bendability. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-200754 [Patent Document 2] Japanese Patent Application Laid-Open No. 2011-225977 [Patent Document 3] Japanese Patent Application Laid-Open No. 2017-66458 Summary of the Invention [Problem to be solved by the invention]

[0008] However, in recent years, in order to reduce costs, CO2 emissions, and conserve resources, aluminum alloy sheets for tabs have become thinner than ever before. This has reduced the bending stress on the surface of the bent part, which is where the sheet breaks during repeated bending. This has made it relatively easy to ensure the repeated bending properties that were previously an issue. Furthermore, the tab breaking strength can now be addressed by increasing the strength of the material. However, with the advances in thinner and stronger sheets, further improvements in tab tear resistance are required.

[0009] SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a coated aluminum alloy sheet for tabs which has high resistance to tab cracking even when it is thinned. [Means for solving the problem]

[0010] 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, from 4.0 to 6.0% by mass of Mg, with the remainder being Al and unavoidable impurities, and a resin layer. When the structure of the coated aluminum alloy sheet for tabs is observed in 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 coated aluminum alloy sheet has a size of 205 × 10 in an image taken with a transmission electron microscope at 50,000 magnifications. -12 m 2 It has a total of 40 or more subgrains in the area. [Effects of the Invention]

[0011] According to the present invention, it is possible to provide a coated aluminum alloy sheet for tabs that has high resistance to tab cracking even when made thin. [Brief explanation of the drawings]

[0012] [Figure 1] 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 AA in FIG. 1A, and FIG. 1C is a cross-sectional view and a plan view of the main parts that explain the can-opening operation. [Figure 2] 1 is an example of a transmission electron microscope image of the structure of an aluminum alloy plate. [Figure 3] FIG. 10 is a schematic diagram illustrating a method for measuring a tab tearing simulated load. [Figure 4] 10 is an example of the measurement results of a simulated tab tear load. [Figure 5] 10 is a graph showing the relationship between a tab cracking simulated load and a plate threading interval. [Figure 6]1 is a graph showing the relationship between the tab tearing simulated load and the number of subgrains. DETAILED DESCRIPTION OF THE INVENTION

[0013] An aluminum alloy coated sheet for a tab according to one embodiment of the present invention will be described below. However, the embodiment shown 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 size and positional relationship of components shown in each drawing may be exaggerated for clarity. In this specification, the term "process" refers not only to an independent process, but also to a process 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.

[0014] As shown in FIG. 1A, a stay-on-tab lid is composed of a lid member 2 and a tab 1. The tab 1 is attached to the lid member 2 by crimping a rivet 21 formed approximately at the center of the disk-shaped lid member 2 (partially removed in FIG. 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 FIG. 1B, the tab 1 is formed by bending the outer periphery of the cut aluminum alloy plate and the inner periphery of the hook 12 downward to increase rigidity and ensure safety. The rivet hole 11 and the area where the U-shaped inner lance 14 is formed 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 a predetermined shape to manufacture tab 1, which is then attached with rivets to lid member 2, which is formed by forming another plate material. On the extension of the one end side of tab 1, lid member 2 has opening region 23, which will form a drinking spout after the can is opened, surrounded by scores 25. This score 25 does not go completely around the perimeter of opening region 23, but is formed so as to be discontinuous at one point (the area where tab 1 overlaps in Figure 1A).

[0015] When opening the can, as shown in FIG. 1C , when the hooking portion 12 of the tab 1 is pulled upward, this becomes a force point E, and the vicinity of the rivet portion 21 becomes a fulcrum F, and the tab 1 is raised, leaving the area fixed by the rivet portion 21. More specifically, the tab 1 is divided by the inner lance 14, and the outer area, such as the hooking portion 12, is raised, while its inner area (the area around the rivet hole 11) remains fixed to the lid member 2. Then, one end of the tab 1 (the opposite side of the hooking portion 12 across the rivet hole 11) becomes a force point L, and is pushed downward by leverage, pushing down a part of the opening area 23 of the lid member 2 directly below this end. Then, the lid member 2 cracks along the score 25 from the vicinity of the pushed-down part, and the opening area 23 is separated from the rest of the lid member 2, leaving only a portion, and is pushed downward (into the interior of the can), forming a drinking spout (opening) in the lid member 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 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 portion 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 barrel).

[0016] Thus, the tab acts as a lever and is subjected to a strong external force when opening the can. Therefore, if the tab is not strong enough, it may bend at the narrow part of the hooking portion 12 near the center between the fulcrum F and the force point E shown in Fig. 1C or at the narrow part around the inner lance 14 (see bent portion 4 in the lower right of Fig. 1C), making it difficult to open the can (tab bend) or even tearing off. Furthermore, when the outer area of ​​the inner lance 14, such as the hooking portion 12, is raised, the end of the inner lance 14 may tear along the direction of the movement (tab tear) (see tear portion 5 in the lower right of Fig. 1C).

[0017] Furthermore, particularly in 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 from the can. For this reason, the tab 1 (the latching portion 12) is raised nearly vertically and then raised even further (tilted backward) to open the can. After opening the can, to prevent the raised tab 1 from interfering with drinking the beverage from the can, it is common to return (tilt) the tab 1 to its original position (tilt) so that only the opening region 23 of the lid member 2 is pushed inward, as shown by the dashed line in FIG. 1C . That is, 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). 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 FIG. 1C ). This property is called repeated bending property.

[0018] As described above, tab materials are required to have not only formability but also high strength, repeated bending resistance, and high tab tear resistance. Meanwhile, particularly in recent years, tabs have been made thinner in order to reduce costs, and higher material strength than ever before is required to ensure tab breaking strength. Furthermore, the thinner walls and the resulting higher strength have significantly reduced tab tear resistance, and there is an increasing demand for materials with high tab tear resistance.

[0019] Aluminum alloy coated sheet for tubs 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 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 region has a thickness of 205 × 10 in an image taken with a transmission electron microscope at 50,000 magnifications. -12 m 2 It has a total of 40 or more subgrains in the area.

[0020] Aluminum alloy composition The aluminum alloy sheet constituting the coated aluminum alloy sheet for tabs is made of, for example, an Al-Mg alloy, such as a general JIS alloy such as 5182.

[0021] Specifically, the aluminum alloy plate contains Si: 0.05% by mass to 0.40% by mass, Fe: 0.05% by mass to 0.50% by mass, Cu: 0.01% by mass to 0.30% by mass, Mn: 0.1% by mass to 0.6% by mass, Mg: 4.0% by mass to 6.0% by mass, and the balance being Al and inevitable impurities. The content of each component contained in the aluminum alloy plate and the reasons for limiting the content will be described below.

[0022] (Si: 0.05 mass% or more and 0.40 mass% or less) If the Si content is less than 0.05% by mass, high-purity aluminum ingots are required during casting, increasing 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, resulting in reduced 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.

[0023] (Fe: 0.05 mass% or more and 0.50 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.

[0024] (Cu: 0.01 mass% or more and 0.30 mass% or less) If the Cu content is less than 0.01% by mass, the strength is insufficient, resulting in insufficient tab breaking 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.

[0025] (Mn: 0.1 mass% or more and 0.6 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.

[0026] (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.

[0027] (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.

[0028] (balance: Al and unavoidable 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, and preferably 0.05 mass% or less for Zr. For example, 0.3 mass% or less, preferably 0.1 mass% or less, and more preferably 0.05 mass% or less for Zr. The other elements may each be, for example, 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 not only when the elements are contained as inevitable impurities but also when the elements are added.

[0029] The thickness of the aluminum alloy plate constituting the coated aluminum alloy plate for tabs may be, for example, 0.18 mm or more and 0.40 mm or less.

[0030] Aluminum alloy sheet structure 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, and the structure of the aluminum alloy sheet has 40 or more subgrains in a predetermined region in the center of the sheet thickness. By making the structure of 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, to improve the tab breaking load while maintaining strength. The subgrains in the aluminum alloy sheet have a density of 205 × 10 when a 100 nm thick structure in the center of the sheet thickness is observed from the normal direction of the rolled surface. -12 m 2 are counted by observing them under a transmission electron microscope (TEM) at 50,000 magnification.

[0031] Subgrains, also known as subcrystal grains, are small, irregular grains that are formed when a material (structure) that has had dislocations introduced by cold rolling or other processes undergoes recovery to form a lower-energy structure under given temperature, time, and strain.

[0032] 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 many 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. Therefore, the more subgrains there are, the higher the tab crack load.

[0033] 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 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.

[0034] In this way, the number of individual subgrains can be counted within the observation field of the transmission electron microscope. It is possible to measure the area ratio of subgrains instead of the number of subgrains present within a given observation field. However, this does not take into account the size of the subgrains, and therefore 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, resulting in a lower strength and making it impossible to obtain the strength required for a tab.

[0035] 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.

[0036] In one embodiment, the number of subgrains in a predetermined region in the center of the sheet thickness of the aluminum alloy sheet constituting the coated aluminum alloy sheet 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.

[0037] When subgrain formation is not advanced in a predetermined region in the center of the sheet thickness of an aluminum alloy sheet, or when the number of subgrains is as small as less than 40, the aluminum alloy sheet has high strength, but on the other hand, it may not be possible to achieve both excellent tab crack resistance and tab breaking strength (high strength). That is, it may not be possible to improve the tab crack resistance while maintaining the high strength of the aluminum alloy coated sheet for tabs.

[0038] The coated aluminum alloy sheet for a tab has excellent strength and tab crack resistance. The sheet strength of the coated aluminum alloy sheet for a tab 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, in terms of 0.2% yield strength. The upper limit of the sheet strength of the coated aluminum alloy sheet for a tab is, for example, 390 MPa or less, in terms of 0.2% yield strength. The tab crack resistance of the coated aluminum alloy sheet for a tab can be evaluated, for example, by a tab crack simulation load determined by the method schematically shown in FIG. 3. The tab crack simulation load according to 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 while maintaining sufficient strength without causing tab bending, tab tearing, etc. Details of the method for evaluating the tab tearing simulated load will be described later.

[0039] The structure and properties of the aluminum alloy sheet described above are those of a coated aluminum alloy sheet (precoated sheet) obtained after painting and paint baking treatments on a cold-rolled sheet (sheet after cold rolling). These structure and properties may also be those of an aluminum alloy sheet obtained after heat treatment under specific conditions simulating paint baking treatments on a cold-rolled sheet without painting and paint baking treatments or forming into a tab. These structure and properties are the same, or can be considered to be the same with only slight differences, if the conditions for the paint baking treatment and the heat treatment are the same.

[0040] The coated aluminum alloy 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 coating such as an epoxy-based, vinyl vinyl sol-based, or polyester-based coating and then heat treating it. The aluminum alloy sheet to which the organic coating 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 coating agent. The heat treatment temperature may be, for example, a temperature at which the peak metal temperature (PMT) reaches 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.

[0041] Manufacturing method An example of a method for manufacturing an aluminum alloy sheet constituting an aluminum alloy coated sheet for a tab will be described below. 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, and these steps are carried out in this order.

[0042] (Processes 1 to 3: 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.

[0043] In the first step, an aluminum alloy is cast by a semi-continuous casting method (DC (direct chill) casting) to obtain an ingot. Next, in the second step, a process of removing regions of the ingot surface that would cause a non-uniform structure by facing and a process of performing a homogenization heat treatment are 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.

[0044] 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 back-calculating the total rolling reduction ratio by cold rolling from the plate thickness of the product plate obtained by cold rolling.

[0045] The coiling temperature, which is the temperature at which hot rolling is finished, is, for example, 300°C or higher and 400°C or lower, and preferably 320°C or higher and 370°C or lower. When the coiling 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 coiling temperature is 400°C or lower, the occurrence of a surface defect called seizure on the sheet surface is suppressed, and the sheet surface properties are improved.

[0046] The fourth step is a step of cold rolling the hot-rolled sheet hot-rolled 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 predetermined thickness. The cold rolling is performed by setting multiple passes to achieve a predetermined total rolling reduction 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 a single passage of the sheet between a pair of work rolls.

[0047] The total cold rolling reduction is preferably 85.0% or more and 95.0% or less, 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, the aluminum alloy sheet has sufficient strength, and the dislocation density is increased, which promotes the formation of subgrains after paint baking treatment, 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.

[0048] In addition to keeping the total rolling reduction within a predetermined 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. By keeping the sheet threading interval within 0.33 seconds, dynamic recovery in the final rolling pass is further promoted, and subgrain formation in the dislocation-dense region is further promoted, 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, more preferably within 0.31 seconds.

[0049] The aluminum alloy sheet manufactured through the above process is subjected to a chemical conversion treatment using a surface treatment agent such as a chromate-based or zircon-based agent. Then, an organic coating containing an epoxy-based resin, a vinyl chloride sol-based agent, or a polyester-based agent is applied to form a coating film. The formed coating film is then subjected to a paint baking process at a PMT (metal temperature) of approximately 200°C to 290°C, resulting in the production of a pre-coated aluminum alloy coated sheet for tabs. [Example]

[0050] The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples.

[0051] (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 remainder of the composition shown in Table 1 is Al and unavoidable impurities. The cold reduction ratio is the total reduction 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.

[0052] Subsequently, 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 of Production Examples No. 1 to No. 3.

[0053] [Table 1]

[0054] The produced coated aluminum alloy sheets were used as test materials, and the number of subgrains, 0.2% proof stress, and tab tear resistance were measured by the following methods. The results are shown in Table 1.

[0055] Number of subgrains The structure of each test material in a 50 nm thick region in both thickness directions from the center of the sheet thickness was observed using 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.

[0056] Specifically, the test material was mechanically polished to a thickness of 0.05 mm (0.1 mm) from the center of the plate thickness in both thickness directions, and then a twin-jet electrolytic polishing method was used to make a thin film with a thickness of approximately 100 nm. This thin film was then examined with a transmission electron microscope at a magnification of 50,000 times, from the normal direction of the rolled surface, to obtain a 205 × 10 -12 m 2 The area was photographed. 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 on the number of subgrains, but the upper limit is usually around 400 to 600.

[0057] Measurement of 0.2% proof stress For each test material, a JIS No. 5 tensile test piece was prepared with the tensile direction parallel to the rolling direction. A tensile test was conducted in accordance with the JIS Z2241 standard. The plate thickness was calculated by subtracting the coating thickness from the plate thickness to determine the 0.2% yield strength. The optimum range for 0.2% yield strength is 300 MPa or higher, and within this range, even thin-walled tabs will meet the tab breaking strength requirement. 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.

[0058] Tab tearing simulated load measurement Test specimens were prepared from each test material, as shown in Figure 3A. A tear test simulating tab tearing was performed to determine the simulated tab tear load. As shown in Figure 3B, the test specimens were clamped in one chuck of a tensile testing machine (regions 26 and 27) and clamped in the other chuck (region 28). 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. The peak loads at which cracks occurred in holes 31 and 32 of the test specimen were measured, as shown in Figure 4. The test was performed eight times using test specimens prepared from the same test material. The minimum of the two peak loads measured in each test was extracted and averaged to determine the simulated tab tear load. Note that simultaneous tearing of holes 31 and 32 may occur during the tear test. In such cases, a single peak load is obtained in a single 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 for 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.

[0059] As shown in Table 1, Nos. 1 to 3, whose aluminum alloy coated sheets have compositions within the range specified by the present invention, are examples because the number of subgrains is within the range specified by the present invention. In 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. [Explanation of symbols]

[0060] 1 tab 2 Lid material 11 Rivet holes 14 Inner Lance 21 Rivet section 100 Dislocation-dense region 200 subgrains

Claims

1. 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; When observing the structure of a 50 nm thick region from the center of the plate thickness in both thickness directions from the normal direction of the rolled surface, the image taken by a 50,000-magnification transmission electron microscope shows a 205 × 10 －１２ m ２ a total of 50 or more subgrains in the region of The coated aluminum alloy sheet for tabs, wherein the subgrains have an equivalent circle diameter of 50 nm or more and 1000 nm or less.

2. 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; When observing the structure of a 50 nm thick region from the center of the plate thickness in both thickness directions from the normal direction of the rolled surface, the image taken by a 50,000-magnification transmission electron microscope shows a 205 × 10 －１２ m ２ a total of 50 or more subgrains in the region of The aluminum alloy sheet is a coated aluminum alloy sheet for tabs having a 0.2% yield strength of 300 MPa or more.

3. 3. The coated aluminum alloy sheet for tabs according to claim 1, wherein the aluminum alloy sheet contains at least one element selected from the group consisting of Cr, Zn, Zr, B, V, Na, Ca, Ni, In, Sn, and Ga in an amount of 0.05% by mass or less each and 0.15% by mass or less in total.

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