Unpainted aluminum alloy sheet for can lid

Abstract

To provide an unpainted aluminum alloy sheet for a can lid capable of achieving both high strength and high toughness while blending a scrap raw material derived from a can material.SOLUTION: An uncoated aluminum alloy sheet for a can lid according to an aspect of the present disclosure contains 0.27 mass% or more and 0.39 mass% or less of Si, 0.35 mass% or more and 0.55 mass% or less of Fe, 0.17 mass% or more and 0.25 mass% or less of Cu, 0.75 mass% or more and 0.95 mass% or less of Mn, and 2.2 mass% or more and 2.8 mass% or less of Mg, wherein in a 0° direction to a rolling direction, an evaluation value S calculated by the following formula (1) using the 0.2% yield strength σ0.2, the tensile strength σB, and the average value σfm of the 0.2% yield strength and the tensile strength is 380 MPa or more and 440 MPa or less. S=σfm / (σ0.2 / σB) (1)SELECTED DRAWING: None

Description

Technical Field

[0001] The present disclosure relates to an aluminum alloy sheet for an unpainted can lid.

Background Art

[0002] In recent years, due to the increasing environmental awareness, aluminum alloy sheets with low CO2 emissions in the manufacturing process have been demanded. In the aluminum manufacturing process, the composition of primary aluminum ingots in the casting process contributes significantly to CO2 emissions indirectly.

[0003] The production of primary aluminum ingots uses a large amount of electricity in the smelting process, leading to a large amount of CO2 emissions. Therefore, reducing the amount of primary aluminum ingots and increasing the horizontal recycling rate can lead to a reduction in CO2 emissions in the production of aluminum alloy sheets.

[0004] Generally, it is said that the CO2 emissions when recycling aluminum scrap and casting it are reduced to about one-thirtieth compared to the production of primary aluminum ingots. In particular, the production volume of aluminum alloy sheets for beverage cans used worldwide is very large, and further improving the horizontal recycling rate has great significance for reducing the environmental impact.

[0005] Among them, the can lids formed of 5182 aluminum alloy (AA5182 alloy) have lower upper limits of component specifications such as Si, Fe, Cu, and Mn compared to the can bodies formed of 3104 aluminum alloy (AA3104 alloy), and it is difficult to incorporate scrap derived from can materials mixed with 3104 aluminum alloy.

[0006] For example, if the can scrap (UBC: Used Beverage Can) generated in the market is directly incorporated, since it contains more components of 3104 aluminum alloy due to the weight ratio between the can body and the can lid, it is likely to exceed the upper limit of the components of 5182 aluminum alloy, and it becomes necessary to dilute the components with primary ingots.

[0007] Therefore, the aluminum alloy sheet for can lids uses more virgin metal than the aluminum alloy sheet for can bodies and is adjusted to the composition of 5182 aluminum alloy, resulting in a low recycling rate. Therefore, by changing the alloy of the can lid to an alloy with a composition that is easy to blend with 3104 aluminum alloy, the usage rate of virgin metal for the can lid can be significantly reduced.

[0008] Patent Documents 1-5 disclose an aluminum alloy sheet for can lids that is relatively close to the composition of 3104 aluminum alloy, which is excellent in recyclability.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Summary of the Invention

Problems to be Solved by the Invention

[0010] When the alloy for the can lid is made to have a composition close to that of 3104 aluminum alloy, problems such as a decrease in the pressure resistance of the can lid and the toughness of the material can be cited. The pressure resistance of the can lid is the internal pressure value when the can lid is inverted against the pressure inside the can, and it becomes the resistance value when the internal pressure of the can unexpectedly increases due to changes in the external environment.

[0011] In particular, positive pressure cans for beer and carbonated beverages require high pressure resistance. Generally, the higher the strength of the material and the thicker the plate thickness, the higher the pressure resistance. Therefore, a high-strength 5182 aluminum alloy containing a large amount of Mg, which is a component that contributes to an increase in strength, is used for the lids of positive pressure cans.

[0012] On the other hand, when using the conventional 3104 aluminum alloy for the can lid, the pressure resistance is greatly reduced, and when the internal pressure of the can increases unexpectedly, the lid is likely to reverse and the contents may leak. Also, increasing the plate thickness to increase the pressure resistance will lead to an increase in the lid weight and the lid cost.

[0013] Furthermore, the toughness of the material affects the formability and openability of the lid. When the toughness of the material is low, forming cracks may occur especially in the rivet part and countersink part of the lid. Also, when the internal pressure of the can increases unexpectedly, cracks may occur in the score part, increasing the risk of leakage of the can contents. In particular, these cracks occur along the rolling direction. Therefore, toughness against tensile stress and bending stress in a direction perpendicular to the rolling direction is required.

[0014] However, aluminum alloy sheets for can lids that are relatively close to the composition of the conventional 3104 aluminum alloy do not satisfy either or both of the above two problems, namely, the strength of the material (i.e., the pressure resistance of the lid) and the toughness (i.e., formability and openability).

[0015] One aspect of the present disclosure aims to provide an uncoated aluminum alloy sheet for can lids that can achieve both high strength and high toughness after painting and baking while blending scrap raw materials derived from can materials.

Means for Solving the Problems

[0016] One embodiment of the present disclosure has a silicon (Si) content of 0.27 mass% or more and 0.39 mass% or less, an iron (Fe) content of 0.35 mass% or more and 0.55 mass% or less, a copper (Cu) content of 0.17 mass% or more and 0.25 mass% or less, a manganese (Mn) content of 0.75 mass% or more and 0.95 mass% or less, a magnesium (Mg) content of 2.2 mass% or more and 2.8 mass% or less, with the balance being aluminum (Al) and unavoidable impurities, and in the 0° direction with respect to the rolling direction, the 0.2% proof stress σ 0.2 , the tensile strength σ Band the average value σ of 0.2% proof stress and tensile strength fm An unpainted aluminum alloy sheet for can lids, wherein the evaluation value S calculated by the following formula (1) using fm is 380 MPa or more and 440 MPa or less. S = σ fm / (σ 0.2 / σ B ) ···(1)

[0017] According to such a configuration, while incorporating scrap raw materials derived from can materials, high strength and high toughness can be achieved simultaneously in the aluminum alloy sheet after painting and baking. That is, a certain amount of scrap of 3104 aluminum alloy for can bodies can be incorporated, reducing the new ingot usage rate and the CO2 emission amount. Furthermore, an unpainted aluminum alloy sheet for can lids with high formability that can be used for positive pressure can lid applications requiring high pressure resistance can be obtained.

[0018] Note that the "unpainted aluminum alloy sheet for can lids" of the present disclosure is an alloy sheet (i.e., the bare material) that is used as an aluminum alloy sheet for can lids after being painted. That is, by performing a baking process on the "unpainted aluminum alloy sheet for can lids" of the present disclosure, an "aluminum alloy sheet for can lids" can be obtained.

Brief Description of the Drawings

[0019]

Figure 1

Embodiments for Carrying Out the Invention

[0020] Hereinafter, embodiments to which the present disclosure is applied will be described with reference to the drawings. [1. First Embodiment] [1-1. Configuration] The aluminum alloy sheet for an unpainted can lid of the present disclosure may be continuously painted in a coil state on a painting line, or may be cut into sheets, painted or laminated, and formed into a lid. The aluminum alloy sheet for an unpainted can lid of the present disclosure can be used for any application. Further, the aluminum alloy sheet for an unpainted can lid is not limited to beverage can lid applications and can be widely used for food can lid applications and the like.

[0021] <Composition> The aluminum alloy sheet for an unpainted can lid of the present disclosure (hereinafter, also simply referred to as "alloy sheet") contains aluminum (Al), silicon (Si), iron (Fe), copper (Cu), manganese (Mn), and magnesium (Mg).

[0022] The lower limit of the Si content is 0.27% by mass, and preferably 0.30% by mass. If the Si content is less than 0.27% by mass, the precipitation amount of Si in the processing heat of cold rolling after hot rolling and solution treatment decreases, and the strength of the alloy sheet after painting baking may be insufficient.

[0023] Further, the average value of the Si component standard of the 3104 aluminum alloy standardized by JIS-H-4000:2014 is 0.30% by mass. Therefore, by setting the Si content to 0.27% by mass or more, preferably 0.30% by mass or more, a large amount of scrap of the 3104 aluminum alloy can be blended.

[0024] The upper limit of the Si content is 0.39% by mass, and preferably 0.35% by mass. If the Si content exceeds 0.39% by mass, the Mg2Si particles increase, and the toughness of the alloy sheet after painting baking decreases.

[0025] The lower limit of the Fe content is 0.35% by mass, and preferably 0.40% by mass. The average value of the Fe component standard of the 3104 aluminum alloy is 0.40% by mass. Therefore, by setting the Fe content to 0.40% by mass or more, a large amount of scrap of the 3104 aluminum alloy can be blended.

[0026] The upper limit of the Fe content is 0.55% by mass. When the Fe content exceeds 0.55% by mass, intermetallic compounds (i.e., second-phase particles) in the Al-Fe-Mn system or Al-Fe-Mn-Si system increase. As a result, crack propagation paths are generated, and the toughness of the alloy sheet after painting baking decreases.

[0027] The lower limit of the Cu content is 0.17% by mass, and 0.20% by mass is preferred. When the Cu content is less than 0.17% by mass, there is insufficient Cu to increase the strength by solid solution or precipitation, and the strength of the alloy sheet decreases. By precipitating Cu during the cold rolling process after hot rolling and solution treatment, the strength of the alloy sheet after painting baking increases significantly.

[0028] Also, the average value of the Cu component standard of the 3104 aluminum alloy is 0.15% by mass. Therefore, by setting the Cu content to 0.17% by mass or more, more scrap of the 3104 aluminum alloy can be blended.

[0029] The upper limit of the Cu content is 0.25% by mass. When the Cu content exceeds 0.25% by mass, the toughness of the alloy sheet after painting baking decreases.

[0030] The lower limit of the Mn content is 0.75% by mass, and 0.80% by mass is preferred. When the Mn content is less than 0.75% by mass, there is insufficient Mn to increase the strength by solid solution or precipitation, and the average strength of the alloy sheet after painting baking decreases.

[0031] Also, the average value of the Mn component standard of the 3104 aluminum alloy is 1.1% by mass, and the average value of the Mn component standard of the 5182 aluminum alloy is 0.35% by mass. Therefore, by setting the Mn content to 0.75% by mass or more, more scrap of the 3104 aluminum alloy can be blended compared to the conventional 5182 aluminum alloy.

[0032] The upper limit of the Mn content is 0.95% by mass, and preferably 0.90% by mass. When the Mn content exceeds 0.95% by mass, the intermetallic compounds (i.e., the second-phase particles) in the Al-Fe-Mn system or the Al-Fe-Mn-Si system increase. As a result, crack propagation paths are generated, and the toughness of the alloy sheet after painting baking decreases.

[0033] The lower limit of the Mg content is 2.2% by mass. When the Mg content is less than 2.2% by mass, there is insufficient Mg to increase the strength by solid solution, and the average strength of the alloy sheet decreases. Incidentally, by precipitating Mg in the cold rolling process after hot rolling and solution treatment, the strength of the alloy sheet after painting baking increases significantly.

[0034] The upper limit of the Mg content is 2.8% by mass. The average value of the Mg component standard of the 3104 aluminum alloy is 1.05% by mass, and the average value of the Mg component standard of the 5182 aluminum alloy is 4.5% by mass. Therefore, by setting the Mg content to 2.8% by mass or less, it is possible to reduce the additional blending amount of the Mg-containing raw material while blending a large amount of the 3104 aluminum alloy scrap.

[0035] The alloy sheet may contain titanium (Ti). The upper limit of the Ti content is preferably 0.10% by mass. By containing Ti, the ingot structure of the alloy sheet is refined. Further, the alloy sheet may contain zinc (Zn). The upper limit of the Zn content is preferably 0.25% by mass. Furthermore, the alloy sheet may contain chromium (Cr). The upper limit of the Cr content is preferably 0.10% by mass.

[0036] The alloy sheet may contain inevitable impurities within a range that does not significantly impair the performance of the alloy sheet. That is, the alloy sheet contains Si, Fe, Cu, Mn, Mg, Ti, Zn, and Cr within the above ranges respectively, and the balance consists of aluminum and inevitable impurities. The upper limit of the total amount of inevitable impurities is preferably 0.15% by mass.

[0037] <Material Strength and Pressure Resistance> The rolled plate of the aluminum alloy has material anisotropy, and the strength shows different values in the 0°, 45°, and 90° directions with respect to the rolling direction. The deformation when the pressure inside the can increases starts from the direction with the lowest strength.

[0038] Therefore, the alloy plate of the present disclosure has a 0.2% proof stress σ 0.2 , tensile strength σ B , and an evaluation value S fm calculated by the following formula (1) using the average value σ 0° of the 0.2% proof stress and the tensile strength is 380 MPa or more and 440 MPa or less. S = σ fm / (σ 0.2 / σ B ) ···(1)

[0039] Furthermore, the alloy plate of the present disclosure has a 0.2% proof stress σ 0.2 , tensile strength σ B , and an average value σ fm of the 0.2% proof stress and the tensile strength in each of the 0°, 45°, and 90° directions with respect to the rolling direction. After heat treatment at 260 °C for 25 seconds, the evaluation value S (S 0° , S 45° , and S 90° ) calculated by the above formula (1) is preferably such that the minimum evaluation value S min (= min(S 0° , S 45° , S 90° )) is 330 MPa or more and 390 MPa or less.

[0040] The pressure resistance value of the lid formed of the aluminum alloy plate has a strong positive correlation with the value V of the following formula (2) represented by the minimum evaluation value S min after painting and baking and the plate thickness t. V = t 2.27 × S min ···(2)

[0041] Therefore, the evaluation value S 0°By setting it to 380 MPa or more, a lid with sufficient pressure resistance can be formed without increasing the plate thickness. Further, the minimum evaluation value S of the alloy plate after heat treatment at 260 °C for 25 seconds min is 330 MPa or more, which enhances the effect of improving the pressure resistance. Also, the minimum evaluation value S after heat treatment at 260 °C for 25 seconds min is preferably 360 MPa or more. By setting the minimum evaluation value S min to 360 MPa or more, the pressure resistance of the lid can be further enhanced.

[0042] Also, when the evaluation value S 0° exceeds 440 MPa, the toughness of the material decreases because the material strength becomes excessively high. That is, shear bands are likely to occur with respect to the tensile stress and bending stress generated in the material during forming, and forming cracks are likely to occur. By setting the evaluation value S 0° to 440 MPa or less, it is possible to balance the strength of the material (i.e., the pressure resistance of the lid) and the toughness (i.e., formability and opening property). Further, the minimum evaluation value S of the alloy plate after heat treatment at 260 °C for 25 seconds min is 390 MPa or less, which enhances the compatibility between strength and toughness.

[0043] The 0.2% proof stress σ 0.2 and the tensile strength σ B in Equation (1) are measured by the method specified in JIS-Z-2241:2011. The plate thickness t is measured with, for example, a micro gauge.

[0044] The pressure resistance of the aluminum alloy plate is measured, for example, by the following procedure. First, a shell formed from a painted and baked aluminum alloy plate is fixed to a jig, and an internal pressure is applied. Next, this internal pressure is gradually increased, and the internal pressure value when the shell is inverted (i.e., buckled) is taken as the pressure resistance value.

[0045] Specifically, a φ204 Fullform (B64) shape shell mold is used for shell molding. For measuring the internal pressure value, a buckle & missile measuring machine DV036E from VERSATILE TECHNOLOGY is used. Specifically, after fixing the shell molded with a dedicated jig, the internal pressure is increased by a program, and the internal pressure value when the shell is inverted is read. For example, the internal pressure is increased at a speed of approximately 175 kPa / s, and when it reaches approximately 350 kPa to 400 kPa, the internal pressure is increased at a speed of 10 kPa / s.

[0046] <Toughness> It is known that the toughness of the aluminum alloy plate after painting and baking affects the formability of the lid and the force required for opening the score part (i.e., the opening force).

[0047] (Number of repeated bends) One of the evaluation indicators of the toughness of the aluminum alloy plate is the repeated bending test. If the plate thickness is the same, the more the number of repeated bends, the better the toughness of the aluminum alloy plate.

[0048] The repeated bending test is carried out according to the following procedure. For example, a test piece cut into a strip shape with a width of 12.5 mm and a length of 200 mm, which has been heat-treated at 260°C for 25 seconds, is arranged in a direction where the bending ridge line is parallel to the rolling direction of the alloy plate. Both ends of this test piece are fixed with chucks, and a tension is applied with a load of 200 N.

[0049] In this state, starting from the end of the test piece fixed to one stationary chuck, using a bending jig with an R of 2.0 mm arranged at a position 150 mm in the longitudinal direction of the test piece as a fulcrum, the other chuck is rotated 90° to the left and right to perform repeated bending, and the number of bends until the test piece breaks is measured.

[0050] The number of bending cycles is counted as one for each operation of bending 90° to either the left or right and one for each operation of returning to the original position. If it breaks during the process, the angle Θ (0° - 90°) is read, and the repeated bending cycle number N is calculated using the following formula (3). In formula (3), N0 is the total number of times the operation of bending 90° to either the left or right and the operation of returning from the 90° - bent position to the original 0° position are performed until the test piece breaks. N = N0 + Θ / 90 ···(3)

[0051] Since the repeated bending evaluation becomes more disadvantageous as the plate thickness increases, it is necessary to correct and consider it with a reference plate thickness. Therefore, the normalized repeated bending cycle number N s normalized by the following formula (4) is obtained based on a plate thickness of 0.235 mm. Here, t (mm) is the plate thickness of the test piece. N s = N × t / 0.235 ···(4)

[0052] Regarding the above - mentioned normalized repeated bending cycle number N s in the uncoated aluminum alloy plate of the present disclosure, it is preferably 17 or more.

[0053] (Second - phase particles) Toughness is affected by strength and the distribution of second - phase particles. That is, the higher the strength and the higher the density of second - phase particles, the lower the toughness. In particular, when the contents of Mg and Si increase, Mg2Si particles are likely to be formed. As a result, Mg2Si particles become the starting point and propagation path of cracks, affecting the reduction of toughness.

[0054] In the L - ST cross - section of the central part in the width direction after heat treatment at 260°C for 25 seconds, which is shown by hatching in FIG. 1, the uncoated aluminum alloy plate of the present disclosure preferably has a ratio of the total area of Mg2Si particles with an area of 0.3 μm 2 or more in the L - ST cross - section of 0.2% or less. In FIG. 1, L represents the longitudinal direction, ST represents the plate - thickness direction, and LT represents the width direction.

[0055] The area ratio of Mg2Si particles can be measured, for example, by the following method. First, the measurement sample is cut, and the surface to be measured (i.e., the L-ST cross-section) is mechanically polished to a mirror finish. Next, the polished surface (i.e., the L-ST cross-section) is observed using an SEM (scanning electron microscope), and 10 fields of view are obtained in the central region of the plate thickness. The acceleration voltage of the SEM is 15 kV, the magnification is 1000 times, and the range of one field of view is 0.012 mm 2 and imaging is performed to obtain a COMPO (backscattered electron composition) image.

[0056] The captured COMPO image is analyzed using the image analysis software "ImageJ". Specifically, the mode value of the brightness of the image in 256 gradations is taken as the background brightness, and particles with a brightness lower than the value obtained by subtracting 30 from the mode brightness are determined to be Mg2Si particles.

[0057] Among the determined Mg2Si particles, the total area of particles with an area of 0.3 μm 2 or more is calculated and divided by the photographed area of 10 fields of view (i.e., the total photographed area), whereby the ratio of the total area of Mg2Si particles with an area of 0.3 μm 2 or more in the L-ST cross-section is calculated.

[0058] <Strength anisotropy> It is known that materials with a low cold rolling ratio (hereinafter abbreviated as cold rolling rate) have high toughness. Also, the higher the cold rolling rate, the greater the 0.2% proof stress σ 0.2_90° in the direction 90° to the rolling direction compared to the 0.2% proof stress σ 0.2_0° in the 0° direction. Therefore, the difference in the 0.2% proof stress between the 0° direction and the 90° direction with respect to the rolling direction, that is, the strength anisotropy, can be associated with the cold rolling rate of the material.

[0059] The alloy plate of the present disclosure has a value D obtained by subtracting the 0.2% proof stress σ 0.2_0° in the 0° direction with respect to the rolling direction after heat treatment at 260°C for 25 seconds, which is obtained by formula (5), from the 0.2% proof stress σ 0.2_90° in the 90° direction with respect to the rolling direction after heat treatment at 260°C for 25 seconds, preferably in the range of -12 MPa or more and 12 MPa or less. D = σ0.2_0° -σ 0.2_90° ···(5)

[0060] The 0.2% proof stress σ in the 0° direction with respect to the rolling direction after heat treatment at 260°C for 25 seconds 0.2_0° minus the 0.2% proof stress σ in the 90° direction with respect to the rolling direction after heat treatment at 260°C for 25 seconds 0.2_90° The material texture - related meaning of the strength anisotropy can be explained as follows.

[0061] The material after hot rolling or annealing is in a recrystallized state with a high degree of isotropy in the cube orientation. From this, due to plastic deformation by cold rolling, the cube orientation deforms into a rolled texture with anisotropy in the rolling direction. Furthermore, the greater the cold rolling ratio, the more elongated the crystal grains are in the rolling direction. Therefore, the diameter of the crystal grains along the 0° direction with respect to the rolling direction increases, while the change in the diameter of the crystal grains along the 90° direction with respect to the rolling direction is smaller compared to the 0° direction.

[0062] The relationship between these microstructural changes caused by rolling and the 0.2% proof stress σ 0.2 is shown by the relationship of Equation (6) referring to the Hall - Petch equation. In Equation (6), κ is the resistance to slip at the grain boundaries and d is the grain diameter. σ 0.2 ∝κ×d -1 / 2 ···(6)

[0063] For tension in the 0° or 90° direction with respect to the rolling direction, the resistance κ takes different values. This is because as the cold rolling ratio increases, the degree of the rolled texture with anisotropy in the rolling direction increases, and the resistance to slip at the grain boundaries changes depending on the tensile direction.

[0064] Also, in the 0° direction with respect to the rolling direction, as the cold rolling ratio increases, the crystal grains elongate and the diameter increases, while in the 90° direction with respect to the rolling direction, the change in the grain diameter with respect to the cold rolling ratio is relatively small. By the accumulation of these effects, strength anisotropy occurs with the increase in the cold rolling ratio.

[0065] <Manufacturing method of aluminum alloy sheet> The unpainted aluminum alloy sheet of the present disclosure can be manufactured, for example, as follows. First, an aluminum alloy having the composition of the unpainted aluminum alloy sheet of the present disclosure is subjected to a semi-continuous casting method (i.e., DC casting) according to a conventional method to produce an ingot.

[0066] Next, four sides of the ingot except for the front and rear ends are faced. Thereafter, the ingot is put into a soaking furnace for homogenization treatment. The temperature in the homogenization treatment is preferably, for example, 470°C or higher and 620°C or lower. The time of the homogenization treatment is preferably, for example, 1 hour or longer and 20 hours or shorter.

[0067] When the temperature in the homogenization treatment is 400°C or higher, segregation of the ingot structure is likely to be eliminated. Further, when the temperature in the homogenization treatment is 450°C or higher, Mg2Si particles can be redissolved to improve the strength and toughness of the alloy sheet. Further, when the temperature in the homogenization treatment is 470°C or higher, more preferably 550°C or higher, redissolution of Mg2Si particles is promoted, and the strength and toughness of the alloy sheet can be further improved. On the other hand, when the temperature in the homogenization treatment is 620°C or lower, local melting of the aluminum alloy hardly occurs.

[0068] When the time of the homogenization treatment is 1 hour or longer, the temperature of the entire slab becomes uniform, segregation of the ingot structure is likely to be eliminated, and Mg2Si particles are likely to be redissolved. The longer the homogenization treatment time, the more Mg2Si particles can be redissolved. However, when the time of the homogenization treatment exceeds 20 hours, the effect of the homogenization treatment saturates.

[0069] After the homogenization treatment, the ingot is subjected to hot rolling. The hot rolling process includes a rough rolling process and a finish rolling process. In the rough rolling process, the ingot is processed into a plate material with a thickness of about several tens of mm by reverse rolling. In the finish rolling process, for example, by tandem rolling or the like, the thickness of the plate material is reduced to about several mm, and a hot rolled coil in which the plate material is wound into a coil shape is formed.

[0070] When the total reduction ratio of finish rolling is high, a recrystallized structure is formed after winding, and the degree of aggregation of the isotropic cube orientation can be increased. When the winding temperature of finish rolling is high, a recrystallized structure is formed after winding, and the degree of aggregation of the cube orientation can be increased.

[0071] Cold rolling of the sheet material is carried out following hot rolling. In cold rolling, the hot rolled coil is rolled until it reaches the product sheet thickness. Cold rolling may be either single rolling or tandem rolling. In cold rolling by single rolling, it is preferable to carry out rolling in multiple passes of two or more.

[0072] Also, by performing solution treatment on the coil during cold rolling and redissolving Mg etc., it is possible to obtain an alloy sheet that increases the strength of the material while suppressing the anisotropy of the material by reducing the final cold rolling ratio. For example, by performing heat treatment (i.e., annealing) at a target bulk temperature of 440 °C or higher using a continuous annealing furnace (CAL), and then performing forced cooling such as air cooling, it is possible to effectively increase the strength of the alloy sheet.

[0073] Also, by setting the exit temperature of cold rolling in the intermediate passes other than the final pass to 120 °C or higher, Si, Cu, and Mg are finely precipitated and age hardened, so that the strength of the alloy sheet can be increased. Furthermore, by setting the exit temperature to 130 °C or higher, the strength of the alloy sheet can be further increased.

[0074] When solution treatment is not performed during cold rolling, the cold rolling ratio is preferably 80% or more. When the cold rolling ratio is 80% or more, the strength of the alloy sheet can be increased. Also, since more isotropic cube orientation remains as the cold rolling ratio is lower, the cold rolling ratio is preferably 92% or less.

[0075] When solution treatment is performed during cold rolling, the cold rolling ratio after solution treatment (i.e., annealing) is preferably 50% or more. By redissolving Mg etc. by solution treatment, the strength of the alloy sheet can be increased even if the cold rolling ratio is low. Also, since more isotropic cube orientation remains as the cold rolling ratio is lower, the cold rolling ratio is preferably 80% or less.

[0076] The cold rolling reduction rate R(%) is obtained by the following formula (7) using the plate thickness t0 (mm) after hot rolling or solution treatment and the product plate thickness t1 (mm) after cold rolling. R = (t0 - t1) / t0 × 100 ···(7)

[0077] The product plate thickness can be appropriately selected so as to obtain a desired pressure resistance. As shown in the above formula (2), the pressure resistance improves as the plate thickness increases. The product plate thickness can be selected according to the value V of formula (2), and it is preferably under the condition that the value V is 13.0 or more, preferably 14.0 or more after heat treatment at 260 °C for 25 seconds. As described above, according to the uncoated aluminum alloy plate of the present disclosure, an increase in the plate thickness for maintaining high pressure resistance can be suppressed.

[0078] By such rolling treatment, an uncoated aluminum alloy plate is obtained. By performing coating including pre-coating on the obtained uncoated aluminum alloy plate (that is, a coil cold-rolled to the product plate thickness) on a coating line or the like, a coated aluminum alloy plate is obtained. The cold-rolled coil is subjected to degreasing, washing, and chemical conversion treatment on the surface, and after further applying a paint, it is subjected to a coating baking treatment.

[0079] In the chemical conversion treatment, chemical solutions such as chromate-based and zirconium-based are used. As the paint, epoxy-based, polyester-based, etc. are used. These can be selected according to the application. In the coating baking treatment, it is heated at the peak metal temperature (PMT) of the coil at 220 °C or more and 270 °C or less for approximately 30 seconds or less. At this time, the lower the PMT, the more the recovery of the material is suppressed, and the strength of the alloy plate can be maintained high.

[0080] [1-2. Effects] According to the embodiments described in detail above, the following effects can be obtained. (1a) It is possible to achieve both high strength and high toughness of the aluminum alloy plate after painting and baking while blending scrap raw materials derived from can materials. That is, a certain amount of scrap of 3104 aluminum alloy for can bodies can be blended, reducing the usage rate of virgin metal and the CO2 emissions. Furthermore, a highly formable aluminum alloy plate for can lids after painting and baking that can be used for the purpose of positive pressure can lids requiring high pressure resistance can be obtained.

[0081] [2. Other Embodiments] As described above, the embodiments of the present disclosure have been explained. Needless to say, the present disclosure is not limited to the above embodiments and can take various forms.

[0082] (2a) The present disclosure includes, in addition to the unpainted aluminum alloy plate of the above embodiment, members composed of this unpainted aluminum alloy plate, and various forms such as a manufacturing method of this unpainted aluminum alloy plate.

[0083] (2b) The function of one component in the above embodiment may be dispersed as a plurality of components, or the functions of a plurality of components may be integrated into one component. Also, a part of the configuration of the above embodiment may be omitted. Further, at least a part of the configuration of the above embodiment may be added to, replaced with, etc. the configuration of other above embodiments. Note that all aspects included in the technical idea specified from the language described in the claims are embodiments of the present disclosure.

[0084] [3. Examples] The content of the tests conducted to confirm the effects of the present disclosure and the evaluation results thereof will be described below.

[0085] <Manufacture of Unpainted Aluminum Alloy Plate> As examples and comparative examples, unpainted aluminum alloy plates of S1 - S7 shown in Tables 1 and 2 were manufactured. The specific manufacturing procedures will be described below.

[0086] First, ingots containing the components (mass %) of alloy numbers 1 - 4 shown in Table 3, with the balance being aluminum and inevitable impurities, were produced by the semi - continuous casting method. The ingots contain Ti of 0.10 mass % or less, Zn of 0.25 mass % or less, Cr of 0.10 mass % or less, and inevitable impurities of 0.15 mass % or less.

[0087] Next, four surfaces of the ingot excluding the front and rear ends were face - milled. Then, the ingot was put into a furnace and homogenization treatment was carried out. The temperature of the homogenization treatment is as shown in Table 1. After the homogenization treatment, the ingot was taken out of the furnace and hot rolling was immediately started to obtain a rolled plate.

[0088] Furthermore, for S1 and S2, the rolled plates after hot rolling were annealed in a continuous annealing furnace (CAL) at the CAL temperature shown in Table 1. After annealing, the rolled plates were air - cooled to room temperature. After cooling, cold rolling was carried out on the rolled plates. The target cold rolling ratio in the cold rolling after annealing is as shown in Table 1.

[0089] For S4, S5, and S7, cold rolling was carried out on the rolled plates after hot rolling until the CAL plate thickness shown in Table 1 was reached. Then, the rolled plates with the CAL plate thickness were annealed in a continuous annealing furnace (CAL). The CAL temperature during annealing is as shown in Table 1. After annealing, the rolled plates were air - cooled to room temperature. After cooling, cold rolling was carried out on the rolled plates again. The target cold rolling ratio in the cold rolling after annealing is as shown in Table 1.

[0090] For S3 and S6, cold rolling was carried out on the rolled plates after hot rolling without annealing. The target cold rolling ratio in the cold rolling is as shown in Table 1.

[0091] By the above - mentioned treatment, unpainted aluminum alloy plates of S1 - S7 were obtained. Also, the product plate thickness after cold rolling in S1 - S7 (that is, t1 in Equation (7)) was in the range of approximately 0.235 ± 0.03 mm. The plate thickness measured by a micro - gauge (that is, the product plate thickness) of the unpainted aluminum alloy plates of S1 - S7 is shown in Table 1.

[0092]

Table 1

[0093]

Table 2

[0094]

Table 3

[0095] <Evaluation of Unpainted Aluminum Alloy Plate> (Tensile Properties) Three No. 5 test pieces specified in JIS-Z-2241:2011 were produced from the unpainted aluminum alloy plates of S1 - S7 by milling, three pieces for each. The longitudinal directions of the three test pieces extend in the directions forming angles of 0°, 45°, and 90° with respect to the rolling direction, respectively.

[0096] These test pieces were subjected to a heat treatment at 260°C for 25 seconds, which is a heat treatment assuming painting baking, and then a tensile test was performed in accordance with JIS-Z-2241:2011, and the 0.2% proof stress and tensile strength were measured. For the test pieces in the 0° direction with respect to the rolling direction, the 0.2% proof stress and tensile strength were also measured in the state before the heat treatment. Also, the measurement results of the 0.2% proof stress σ 0.2 and the tensile strength σ B , the evaluation value S (S 0° , S 45° , and S 90° ) calculated from Equation (1), and the minimum evaluation value S min , which is the minimum value of the evaluation value S, are shown in Table 1 and Table 2.

[0097] (Toughness) In the unpainted aluminum alloy plates of S1 - S7, the ratio (area ratio) of the total area of Mg2Si particles with an area of 0.3 μm 2 or more in the L-ST cross-section after the heat treatment at 260°C for 25 seconds was calculated by the measurement method described in the embodiment. The measurement results are shown in Table 2.

[0098] In the unpainted aluminum alloy plates of S1 - S7, from the measurement method described in the embodiment, and formulas (3) and (4), the number of repeated bending times and the normalized number of repeated bending times after heat treatment at 260°C for 25 seconds were calculated. The results are shown in Table 2.

[0099] (Strength anisotropy) In the unpainted aluminum alloy plates of S1 - S7, from formula (5) described in the embodiment, the strength anisotropy (i.e., value D) after heat treatment at 260°C for 25 seconds was calculated. The results are shown in Table 2.

[0100] (Scrap blending ratio) Regarding the composition of the unpainted aluminum alloy plates of S1 - S7, it was determined whether the possible blending ratio of 3104 aluminum alloy scrap is 50 mass% or more. The results are shown in Table 2.

[0101] In Table 2, the unpainted aluminum alloy plates marked with "≧50" can be blended with 50 mass% or more of 3104 aluminum alloy. The possible blending ratio of 3104 aluminum alloy scrap is determined based on Table 4.

[0102] Table 4 shows the correspondence between the blending ratio of 3104 aluminum alloy and 5182 aluminum alloy and the average value of the component specifications. The first row of Table 4 is the average value of the component specifications of 3104 aluminum alloy, and the second row is the average value of the component specifications of 5182 aluminum alloy.

[0103] For example, when the blending ratio of 3104 aluminum alloy is 50 mass%, the average value of Si is 0.20 mass%, the average value of Fe is 0.29 mass%, the average value of Cu is 0.11 mass%, the average value of Mn is 0.7 mass%, and the average value of Mg is 2.8 mass%.

[0104] Therefore, when the proportion of each component of the unpainted aluminum alloy sheet is equal to or greater than the above numerical values of Si, Fe, Cu, Mn, and Mg, the possible compounding ratio of the 3104 aluminum alloy sheet is 50% by mass or more. The higher the compounding ratio of the 3104 aluminum alloy, the higher the contents of Si, Fe, Cu, and Mn, and the lower the content of Mg. The unpainted aluminum alloy sheets of S1 - S5 can contain 50% by mass or more of the scrap of the 3104 aluminum alloy.

[0105]

Table 4

[0106] (Evaluation) The unpainted aluminum alloy sheets of S1 - S5 have a lower Mg content than the unpainted aluminum alloy sheet of S7, but have high strength (i.e., S min ). That is, the unpainted aluminum alloy sheets of S1 - S5 can achieve both high strength and high toughness compared with S7.

[0107] The unpainted aluminum alloys of S1, S3 - S5 have a small area ratio of Mg2Si particles, which is 0.2% or less, after heat treatment at 260°C for 25 seconds. Also, for example, when comparing S1 with S4 - S5, although S4 - S5 with a smaller area ratio of Mg2Si particles has the same strength as S1, the number of repeated bending times after heat treatment at 260°C for 25 seconds is larger, showing high strength and high toughness.

[0108] Furthermore, the number of repeated bending times also varies depending on the anisotropy of the material structure. For example, when comparing S4 - S5 with an anisotropy of strength of - 12 MPa or more and 12 MPa or less after heat treatment at 260°C for 25 seconds with S1 - S2 with an anisotropy of strength exceeding 12 MPa, although the strengths are relatively close, the number of repeated bending times of S4 - S5 is much higher than that of S1 - S2.

Claims

1. The silicon (Si) content is 0.27% by mass or more and 0.39% by mass or less, the iron (Fe) content is 0.35% by mass or more and 0.55% by mass or less, the copper (Cu) content is 0.17% by mass or more and 0.25% by mass or less, the manganese (Mn) content is 0.75% by mass or more and 0.95% by mass or less, the magnesium (Mg) content is 2.2% by mass or more and 2.8% by mass or less, the balance consists of aluminum (Al) and unavoidable impurities, In the direction of 0° with respect to the rolling direction, the 0.2% proof stress σ ０．２ , the tensile strength σ Ｂ , and the average value σ of the 0.2% proof stress and the tensile strength ｆｍ An aluminum alloy sheet for an unpainted can lid, wherein the evaluation value S calculated by the following formula (1) using these is 380 MPa or more and 440 MPa or less. S = σ ｆｍ / (σ ０．２ / σ Ｂ )...(1)

2. An aluminum alloy sheet for an uncoated can lid according to Claim 1, After heat treatment at 260°C for 25 seconds in the directions of 0°, 45°, and 90° with respect to the extension direction, the 0.2% proof stress σ ０．２ , the tensile strength σ Ｂ , and the average value σ of the 0.2% proof stress and the tensile strength ｆｍ Among the evaluation values S calculated by the above formula (1) using, the minimum evaluation value S ｍｉｎ is 330 MPa or more and 390 MPa or less, and In the L-ST cross-section of the central part in the width direction after heat treatment at 260 °C for 25 seconds, the proportion of the total area of Mg ２ Si particles with an area of 0.3 μm ２ or more in the L-ST cross-section is 0.2% or less. An aluminum alloy sheet for an unpainted can lid.

3. An aluminum alloy sheet for an uncoated can lid according to Claim 1 or Claim 2, the minimum evaluation value S ｍｉｎ is 360 MPa or more and 390 MPa or less, The 0.2% proof stress σ in the 0° direction with respect to the rolling direction after heat treatment at 260°C for 25 seconds ０．２＿０° from the 0.2% proof stress σ in the 90° direction with respect to the rolling direction after heat treatment at 260°C for 25 seconds ０．２＿９０° is an unpainted aluminum alloy sheet for can lids with a value obtained by subtracting of not less than -12 MPa and not more than 12 MPa.

4. An aluminum alloy sheet for an uncoated can lid according to Claim 2, For a test piece cut into a strip shape with a width of 12.5 mm and a length of 200 mm, which has been heat-treated at 260°C for 25 seconds, when a bending operation of bending 90° and returning to the 0° position is repeated in a direction where the bending ridge line is parallel to the rolling direction, the normalized number of repeated bending times N, which is the number of times of the bending operation until the test piece breaks, is normalized by the plate thickness t of the test piece and the following formula (2). ｓ An aluminum alloy sheet for an unpainted can lid, wherein ｓ is 17 or more times. N ｓ = N × t / 0.235... (2)

5. An aluminum alloy sheet for an uncoated can lid according to Claim 3, For a test piece cut into a strip shape with a width of 12.5 mm and a length of 200 mm, which has been heat-treated at 260°C for 25 seconds, when a bending operation of bending 90° and returning to the 0° position is repeated in a direction where the bending ridge line is parallel to the rolling direction, the normalized number of repeated bending times N, which is the number of times of the bending operation until the breakage of the test piece, is normalized by the plate thickness t of the test piece and the following formula (2). ｓ An aluminum alloy sheet for an unpainted can lid, wherein ｓ is 17 or more. N ｓ = N × t / 0.235...(2)

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