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 disclosed contains 0.20 mass% or more and 0.39 mass% or less of Si, 0.30 mass% or more and 0.59 mass% or less of Fe, 0.11 mass% or more and 0.40 mass% or less of Cu, 0.75 mass% or more and 0.98 mass% or less of Mn, and 1.4 mass% or more and 3.1 mass% or less of Mg, wherein in 0°, 45°, and 90° directions relative to the rolling direction, after heat treatment at 260°C for 25 seconds, the 0.2% yield strength σ0.2, the tensile strength σB, and the average value σfm of the 0.2% yield strength and tensile strength are calculated by formula (1), and the minimum evaluation value Smin is 360 MPa or more and 410 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, an aluminum alloy sheet with a low CO2 emission in the manufacturing process has been demanded. In the aluminum manufacturing process, the composition of the new aluminum ingot in the casting process contributes greatly indirectly to the CO2 emission.

[0003] The production of new 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 new aluminum ingot and increasing the horizontal recycling rate lead to a reduction in the CO2 emission in the production of aluminum alloy sheets.

[0004] Generally, it is said that the CO2 emission when aluminum scrap is remelted and cast can be suppressed to about 1 / 30 of that when producing new 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 burden.

[0005] Among them, a can lid formed of a 5182 aluminum alloy (AA5182 alloy) has lower upper limits of component specifications such as Si, Fe, Cu, and Mn than a can body formed of a 3104 aluminum alloy (AA3104 alloy), and it is difficult to blend scrap derived from a can material mixed with the 3104 aluminum alloy.

[0006] For example, if used beverage can (UBC) scrap generated in the market is directly blended, since it contains more components of the 3104 aluminum alloy due to the weight ratio between the can body and the can lid, it is easy to exceed the upper limit of the components of the 5182 aluminum alloy, and it becomes necessary to dilute the components with new 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 virgin metal usage rate of the can lid can be greatly 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 include a decrease in the pressure resistance of the can lid and the toughness of the material. 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 increases unexpectedly 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 a conventional 3104 aluminum alloy is used for the can lid, the pressure resistance is greatly reduced, and when the internal pressure of the can suddenly increases, there is a high risk that the lid will invert and the contents will leak. Also, increasing the plate thickness to increase the pressure resistance will lead to an increase in the lid weight and an increase in 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 particularly in the rivet part or countersink part of the lid. Also, when the internal pressure of the can suddenly increases, 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 with respect to tensile stress and bending stress in a direction perpendicular to the rolling direction is required.

[0014] However, an aluminum alloy sheet for a can lid that is relatively close to the composition of a conventional 3104 aluminum alloy does 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 unpainted aluminum alloy sheet for a can lid that can achieve both high strength and high toughness after painting and baking while incorporating scrap raw materials derived from can materials.

Means for Solving the Problems

[0016] One aspect of the present disclosure has a silicon (Si) content of 0.20 mass% or more and 0.39 mass% or less, an iron (Fe) content of 0.30 mass% or more and 0.59 mass% or less, a copper (Cu) content of 0.11 mass% or more and 0.40 mass% or less, a manganese (Mn) content of 0.75 mass% or more and 0.98 mass% or less, a magnesium (Mg) content of 1.4 mass% or more and 3.1 mass% or less, with the balance being aluminum (Al) and unavoidable impurities, and the 0.2% proof stress σ after heat treatment at 260°C for 25 seconds in each of the 0°, 45°, and 90° directions with respect to the rolling direction 0.2, 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 following formula (1) using the above, the minimum evaluation value S min is 360 MPa or more and 410 MPa or less, the temperature difference obtained by subtracting the solid solution temperature of Mg2Si from the solidus temperature is 30 °C or more, the crystallization temperature of the primary crystal is lower than the aluminum solidification start temperature, and in the L-ST cross section at the center of the width direction after heat treatment at 260 °C for 25 seconds, the area is 0.3 μm 2 The ratio of the total area of Mg2Si particles of the above or more to the L-ST cross section is 0.2% or less, which is an aluminum alloy sheet for an uncoated can lid. S = σ fm / (σ 0.2 / σ B ) ···(1)

[0017] According to such a configuration, while blending 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 the 3104 aluminum alloy for can bodies can be blended, reducing the new metal usage rate and the CO2 emission amount. Furthermore, an aluminum alloy sheet for an uncoated can lid with high formability that can be used for positive pressure can lid applications requiring high pressure resistance can be obtained.

[0018] The "aluminum alloy sheet for an uncoated can lid" of the present disclosure is an alloy sheet (that is, a bare material) that is used as an aluminum alloy sheet for a can lid after being painted. That is, by performing a painting and baking process on the "aluminum alloy sheet for an uncoated can lid" of the present disclosure, an "aluminum alloy sheet for a can lid" can be obtained.

Brief Description of the Drawings

[0019]

Figure 1

Figure 2

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 unpainted aluminum alloy sheet for can lids of the present disclosure may be continuously painted in coil form on a painting line, or may be cut into sheet form, painted, laminated, and formed into lids. The unpainted aluminum alloy sheet for can lids of the present disclosure can be used for any application. Further, the unpainted aluminum alloy sheet for can lids is not limited to beverage can lid applications and can be widely used for food can lid applications and the like.

[0021] [Composition] The unpainted aluminum alloy sheet for can lids 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 0.20% by mass, and preferably 0.30% by mass. If the Si content is less than 0.20% 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] Also, 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 0.30% 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 0.30% 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, more scrap of the 3104 aluminum alloy can be blended.

[0026] The upper limit of the Fe content is 0.59% by mass. When the Fe content exceeds 0.59% by mass, the intermetallic compounds (i.e., the second-phase particles) of the Al-Fe-Mn system or the Al-Fe-Mn-Si system increase. As a result, the crack propagation path is generated, and the toughness of the alloy plate after painting baking decreases.

[0027] The lower limit of the Cu content is 0.11% by mass, preferably 0.15% by mass, and more preferably 0.20% by mass. When the Cu content is less than 0.11% by mass, there is insufficient Cu to increase the strength by solid solution or precipitation, and the strength of the alloy plate after painting baking decreases. Note that by precipitating Cu in the cold rolling process after hot rolling and solution treatment, the strength of the alloy plate 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 0.15% by mass or more, more scrap of the 3104 aluminum alloy can be blended.

[0029] The upper limit of the Cu content is 0.40% by mass, and preferably 0.25% by mass. When the Cu content exceeds 0.40% by mass, the toughness of the alloy plate after painting baking decreases.

[0030] The lower limit of the Mn content is 0.75% by mass. 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 plate after painting baking decreases.

[0031] In addition, 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 0.98% by mass, and 0.90% by mass is preferable. When the Mn content exceeds 0.98% by mass, the intermetallic compounds (i.e., the second-phase particles) of the Al-Fe-Mn system or the Al-Fe-Mn-Si system increase. As a result, a crack propagation path is generated, and the toughness of the alloy plate after painting baking decreases.

[0033] The lower limit of the Mg content is 1.4% by mass. When the Mg content is less than 1.4% by mass, there is insufficient Mg to increase the strength by solid solution, and the average strength of the alloy plate decreases. The lower limit of the Mg content is preferably 2.3% by mass. Thereby, the average strength of the alloy plate is increased. In addition, by precipitating Mg in the cold rolling process after hot rolling and solution treatment, the strength of the alloy plate after painting baking increases significantly.

[0034] The upper limit of the Mg content is 3.1% 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 3.1% by mass or less, while blending more scrap of the 3104 aluminum alloy, the additional blending amount of the Mg-containing raw material can be reduced.

[0035] The alloy plate 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 plate is refined. In addition, the alloy plate may contain zinc (Zn). The upper limit of the Zn content is preferably 0.25% by mass. Furthermore, the alloy plate may contain chromium (Cr). The upper limit of the Cr content is preferably 0.10% by mass.

[0036] The alloy plate may contain inevitable impurities as long as the performance of the alloy plate is not significantly impaired. That is, the alloy plate 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, for the alloy plate of the present disclosure, in each of the 0°, 45°, and 90° directions with respect to the rolling direction, the 0.2% proof stress σ 0.2 , tensile strength σ B , and the average value σ fm of the 0.2% proof stress and the tensile strength are used to calculate the evaluation value S (S 0° , S 45° , and S 90° ) by the following formula (1), and the minimum evaluation value S min (= min(S 0° , S 45° , S 90° )) is 360 MPa or more and 410 MPa or less. S = σ fm / (σ 0.2 / σ B ) ···(1)

[0039] 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) expressed by the minimum evaluation value S min after painting and baking and the plate thickness t. V = t 2.27 × S min ···(2)

[0040] Therefore, the minimum evaluation value S minBy setting it to 360 MPa or more, a lid with sufficient pressure resistance can be formed without increasing the plate thickness.

[0041] Also, the minimum evaluation value S after heat treatment at 260 °C for 25 seconds min If it exceeds 410 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. The minimum evaluation value S after heat treatment at 260 °C for 25 seconds min By setting it to 410 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).

[0042] The 0.2% proof stress σ in formula (1) 0.2 and the tensile strength σ B are measured by the method specified in JIS-Z-2241:2011. The plate thickness t is measured with a micro gauge, for example.

[0043] 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.

[0044] Specifically, a φ204Fullform (B64) shape shell mold is used for forming the shell. For measuring the internal pressure value, a buckling & missile measuring machine DV036E from VERSATILE TECHNOLOGY is used. Specifically, after fixing a shell formed 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.

[0045] <Toughness> It is known that the formability of the lid and the force required to open the score part (i.e., the opening force) are affected by the toughness of the aluminum alloy plate after painting and baking.

[0046] (Number of repeated bends) As one of the evaluation indexes of the toughness of the aluminum alloy plate, there is a 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.

[0047] 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.

[0048] In this state, starting from the end of the test piece fixed to one immovable chuck, using a bending jig with a bending radius of R2.0 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.

[0049] The number of bends is counted as one each for the operation of bending 90° to either the left or right and the operation of returning to the original position. If it breaks halfway, the angle Θ (0° - 90°) is read, and the number of repeated bends N is calculated by 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)

[0050] Since the repeated bending evaluation is more disadvantageous as the plate thickness increases, it is necessary to correct and consider it with a reference plate thickness. Therefore, the normalized number of repeated bends N s normalized by the following formula (4) is obtained with a reference plate thickness of 0.235 mm. Here, t (mm) is the plate thickness of the test piece. N s = N × t / 0.235 ···(4)

[0051] The above-mentioned normalized repeated bending times N in the unpainted aluminum alloy plate of the present disclosure s is preferably 18 times or more.

[0052] (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 points and propagation paths of cracks and affect the reduction of toughness.

[0053] In the L-ST cross-section of the central portion in the width direction after heat treatment at 260°C for 25 seconds of the unpainted aluminum alloy plate of the present disclosure, which is indicated by hatching in FIG. 1, the area is 0.3 μm 2 The ratio of the total area of Mg2Si particles with an area of 0.3 μm or more in the L-ST cross-section is 0.2% or less. In FIG. 1, L indicates the longitudinal direction, ST indicates the plate thickness direction, and LT indicates the width direction.

[0054] The area ratio of Mg2Si particles can be measured, for example, by the following method. First, the measurement sample is cut, and the surface for measurement (that is, the L-ST cross-section) is mechanically polished to a mirror finish. Next, the polished surface (that is, the L-ST cross-section) is observed using a SEM (scanning electron microscope) to obtain 10 fields of view 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 photography is performed to obtain a COMPO (reflected electron composition) image.

[0055] The photographed COMPO image is analyzed by image analysis software "ImageJ". Specifically, the most frequent value of the brightness of the image in 256 gradations is used as the background brightness, and particles with a brightness lower than the value obtained by subtracting 30 from the brightness of the most frequent value are determined as Mg2Si particles.

[0056] Among the determined Mg2Si particles, 0.3 μm 2Calculate the total area of the particles with the above area and divide it by the imaging area of 10 fields (i.e., the total imaged area) to obtain the ratio of the total area in the L-ST cross-section of Mg2Si particles with an area of 0.3 μm 2 or more.

[0057] <Calculation of State Diagram by Computer Software> In the alloy plate of the present disclosure, for the purpose of enhancing the toughness of the material, it is preferable to re-dissolve Mg2Si particles that serve as crack initiation points and propagation paths and affect the reduction of toughness in the homogenization heat treatment process of the ingot.

[0058] In order to re-dissolve Mg2Si while avoiding local melting of the aluminum matrix, it is preferable that the solidus temperature is higher than the solution temperature of Mg2Si, and further preferably, the temperature difference obtained by subtracting the solution temperature of Mg2Si from the solidus temperature is 30 °C or more.

[0059] Also, in materials where the crystallization temperature of Al6(Mn,Fe) is higher than the aluminum solidification start temperature, Al6(Mn,Fe) forms as coarse crystals during casting, leading to molding defects such as pinholes. Therefore, it is preferable that the crystallization temperature of the primary crystal (i.e., Al6(Mn,Fe)) is lower than the aluminum solidification start temperature.

[0060] Here, the solution temperature of Mg2Si refers to the highest temperature at which Mg2Si can exist in the equilibrium phase diagram and is the lowest temperature at which a liquid phase can exist. Also, the aluminum solidification start temperature refers to the highest temperature at which solid Al can exist in the equilibrium phase diagram, and the primary crystal crystallization temperature is the highest temperature at which Al6(Mn,Fe) can exist.

[0061] The solution temperature of Mg2Si, the solidus temperature, the aluminum solidification start temperature, and the primary crystal crystallization temperature are obtained from the equilibrium phase diagram of the aluminum alloy calculated using thermodynamic calculation software.

[0062] The solution temperature of Mg2Si, solidus temperature, aluminum solidification start temperature, and primary crystal crystallization temperature are uniquely determined by the composition of the aluminum alloy. As a method for obtaining these boundary temperatures from the alloy composition, there is a method of calculating the thermodynamic quantities required for each calculation by the CALPHAD method.

[0063] Such thermodynamic calculations for multi-component alloys can be performed using commercially available system software that combines the thermodynamic database, interface, and phase diagram creation function required for the calculation (for example, "JMatPro" developed by Sente Software).

[0064] In the uncoated aluminum alloy sheet of the present disclosure, since the solution temperature of Mg2Si is lower than the solidus temperature and the primary crystal crystallization temperature is lower than the Al solidification start temperature, coarse crystals are not generated during casting, and performance degradation caused by coarse crystals can be avoided.

[0065] Also, by performing homogenization heat treatment at a temperature below the solidus temperature and above the solution temperature of Mg2Si, Mg2Si can be redissolved, and Mg2Si particles that become the starting point and propagation path of cracks and affect the reduction of toughness can be reduced.

[0066] <Strength anisotropy> Materials with a low cold rolling ratio (hereinafter abbreviated as cold rolling ratio) are known to have high toughness. For example, materials obtained by annealing a rolled sheet during cold rolling in a continuous annealing furnace (CAL) to lower the final cold rolling ratio (that is, the cold rolling ratio after annealing) have high toughness.

[0067] Also, the higher the cold rolling ratio, 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 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 ratio of the material.

[0068] The alloy plate of the present disclosure has a 0.2% proof stress σ in the 0° direction with respect to the rolling direction after heat treatment at 260°C for 25 seconds, which is obtained by the formula (5). 0.2_0° The value D obtained by subtracting 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 preferably -13 MPa or more and 13 MPa or less. D = σ 0.2_0° - σ 0.2_90° ···(5)

[0069] The 0.2% proof stress σ in the 0° direction with respect to the rolling direction 0.2_0° subtracted by the 0.2% proof stress σ in the 90° direction with respect to the rolling direction 0.2_90° can be explained as follows in terms of the material structure of the strength anisotropy.

[0070] The material after hot rolling or annealing is in a recrystallized state with a high degree of isotropy of 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 rate, 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 becomes smaller compared to the 0° direction.

[0071] The relationship between these microstructural changes caused by rolling and the 0.2% proof stress σ 0.2 is shown by the relationship of formula (6) referring to the Hall-Petch formula. In formula (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)

[0072] For tension in the 0° direction or 90° direction with respect to the rolling direction, the resistance κ takes different values. This is because as the cold rolling rate 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.

[0073] Also, in the direction of 0° with respect to the rolling direction, the crystal grains elongate and the diameter increases with the increase in the cold rolling ratio, while in the direction of 90° with respect to the rolling direction, the change in the crystal grain size with respect to the cold rolling ratio is relatively small. The accumulation of these effects causes strength anisotropy with respect to the increase in the cold rolling ratio.

[0074] <Manufacturing method of aluminum alloy plate> The unpainted aluminum alloy plate of the present disclosure can be manufactured, for example, as follows. First, a semi-continuous casting method (that is, DC casting) is performed on an aluminum alloy having the composition of the unpainted aluminum alloy plate of the present disclosure according to a conventional method to produce an ingot.

[0075] Next, the four surfaces of the ingot excluding the front and rear ends are face-milled. Thereafter, the ingot is put into a soaking furnace and subjected to a 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.

[0076] When the temperature in the homogenization treatment is 400°C or higher, it is easy to eliminate the segregation of the ingot structure. Further, when the temperature in the homogenization treatment is 450°C or higher, the Mg2Si particles can be re-dissolved to improve the strength and toughness of the alloy plate. Furthermore, when the temperature in the homogenization treatment is 470°C or higher, more preferably, at or above the solid solution temperature of Mg2Si, the re-dissolution of the Mg2Si particles is promoted, and the strength and toughness of the alloy plate can be further improved. On the other hand, when the temperature in the homogenization treatment is 620°C or lower, more preferably, below the solidus temperature, local melting of the aluminum alloy hardly occurs.

[0077] When the time of the homogenization treatment is 1 hour or longer, the temperature of the entire slab becomes uniform, the segregation of the ingot structure is also easily eliminated, and the Mg2Si particles are easily re-dissolved. The longer the homogenization treatment time, the more the Mg2Si particles can be re-dissolved. However, when the time of the homogenization treatment exceeds 20 hours, the effect of the homogenization treatment saturates.

[0078] After homogenization treatment, the ingot is subjected to hot rolling. The hot rolling process has 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.

[0079] When the total reduction ratio of the 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 the finish rolling is high, a recrystallized structure is formed after winding, and the degree of aggregation of the cube orientation can be increased.

[0080] Subsequent to hot rolling, cold rolling of the plate material is performed. In cold rolling, the hot rolled coil is rolled until it reaches the product plate thickness. The cold rolling may be either single rolling or tandem rolling. In cold rolling by single rolling, it is preferable to perform rolling in a plurality of passes of 2 passes or more.

[0081] Also, by subjecting the coil to solution treatment during cold rolling and re-dissolving Mg or the like, an alloy plate can be obtained in which the material is strengthened while reducing the final cold rolling ratio to suppress the anisotropy of the material. For example, by performing heat treatment (i.e., annealing) with a target bulk temperature of 440°C or higher using a continuous annealing furnace (CAL), and then performing forced cooling by air cooling or the like, it is possible to effectively increase the strength of the alloy plate.

[0082] Also, by setting the rise 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 plate can be increased. Furthermore, by setting the rise temperature to 130°C or higher, the strength of the alloy plate can be further increased.

[0083] 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 plate can be increased. Also, since the isotropic cube orientation remains more as the cold rolling ratio is lower, the cold rolling ratio is preferably 92% or less.

[0084] When solution heat treatment is performed during cold rolling, the cold rolling reduction after solution heat treatment (i.e., annealing) is preferably 50% or more. By re-dissolving Mg or the like by solution heat treatment, the strength of the alloy sheet can be increased even if the cold rolling reduction is low. Also, since more isotropic cube orientations remain as the cold rolling reduction is lower, the cold rolling reduction is preferably 80% or less.

[0085] The cold rolling reduction R (%) is determined by the following formula (7) using the plate thickness t0 (mm) after hot rolling or solution heat treatment and the product plate thickness t1 (mm) after cold rolling. R = (t0 - t1) / t0 × 100 ···(7)

[0086] 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 in formula (2), and it is preferable 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 sheet of the present disclosure, an increase in plate thickness for maintaining high pressure resistance can be suppressed.

[0087] By such rolling treatment, an uncoated aluminum alloy sheet is obtained. By performing coating including precoat on the obtained uncoated aluminum alloy sheet (i.e., the coil cold rolled to the product plate thickness) on a coating line or the like, a coated aluminum alloy sheet 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.

[0088] 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, heating is performed at a peak metal temperature (PMT) of the coil within the range of 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 sheet can be maintained high.

[0089] [1-2. Effects] According to the embodiments described in detail above, the following effects can be obtained. (1a) While blending scrap raw materials derived from can materials, it is possible to achieve both high strength and high toughness of the aluminum alloy plate after painting and baking. That is, a certain amount of scrap of 3104 aluminum alloy for can bodies can be blended, reducing the new metal usage rate and reducing CO2 emissions. Furthermore, an aluminum alloy plate for can lids with high formability that can be used for positive pressure can lid applications requiring high pressure resistance can be obtained after painting and baking.

[0090] [2. Other Embodiments] Although the embodiments of the present disclosure have been described above, it goes without saying that the present disclosure is not limited to the above embodiments and can take various forms.

[0091] (2a) In addition to the unpainted aluminum alloy plate of the above embodiment, the present disclosure also includes various forms such as members composed of this unpainted aluminum alloy plate and manufacturing methods of this unpainted aluminum alloy plate.

[0092] (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. Also, at least a part of the configuration of the above embodiment may be added, replaced, etc. with respect to 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.

[0093] [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.

[0094] ><Manufacture of Unpainted Aluminum Alloy Plate> As examples and comparative examples, unpainted aluminum alloy plates of S1 - S17 shown in Tables 1 and 2 were manufactured. The specific manufacturing procedure will be described below.

[0095] First, ingots containing the components (mass %) of alloy numbers 1 - 9 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.

[0096] Next, four sides 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.

[0097] Furthermore, for S1 - S7, S14, S15, and S17, cold rolling was carried out on the rolled plate after hot rolling until it reached the CAL plate thickness shown in Table 1. Then, annealing was carried out on the rolled plate with the CAL plate thickness in a continuous annealing furnace (CAL). The CAL temperature during annealing is as shown in Table 1. After annealing, the rolled plate was air - cooled to room temperature. After cooling, cold rolling was carried out on the rolled plate again. The target cold rolling rate in the cold rolling after annealing is as shown in Table 1.

[0098] For S8 - S13 and S16, cold rolling was carried out on the rolled plate after hot rolling without annealing. The target cold rolling rate in the cold rolling is as shown in Table 1.

[0099] By the above - mentioned treatment, unpainted aluminum alloy plates of S1 - S17 were obtained. Also, the product plate thickness after cold rolling in S1 - S17 (that is, t1 in Equation (7)) was set 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 - S17 is shown in Table 1.

[0100] [Table 1]

[0101] [Table 2]

[0102] [Table 3]

[0103] [Evaluation of Unpainted Aluminum Alloy Sheet] [Tensile Properties] Three No. 5 test pieces specified in JIS-Z-2241:2011 were produced from the unpainted aluminum alloy sheets of S1 - S17 by milling, three at a time. The longitudinal directions of the three test pieces extend in directions making angles of 0°, 45°, and 90° with respect to the rolling direction, respectively.

[0104] 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 conducted in accordance with JIS-Z-2241:2011, and the 0.2% proof stress and tensile strength were measured. The 0.2% proof stress σ 0.2 and the tensile strength σ B measurement results, and the average value σ fm of the 0.2% proof stress and tensile strength are shown in Table 1 and Table 2.

[0105] Also, three evaluation values S were calculated from the measurement results of the tensile tests in the 0°, 45°, and 90° directions with respect to the rolling direction and Equation (1). The minimum evaluation value S min which is the minimum value of these evaluation values S is shown in Table 2.

[0106] [Toughness] In the unpainted aluminum alloy sheets of S1 - S17, the ratio (area ratio) of the total area in the L-ST cross-section of Mg2Si particles with an area of 0.3 μm 2 or more after a 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.

[0107] In the unpainted aluminum alloy plates of S1 - S17, 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 from the measurement method described in the embodiment, Equation (3), and Equation (4). The results are shown in Table 2.

[0108] (Strength anisotropy) In the unpainted aluminum alloy plates of S1 - S17, the strength anisotropy (i.e., value D) when heat treated at 260°C for 25 seconds was calculated from Equation (5) described in the embodiment. The results are shown in Table 2.

[0109] (Phase diagram calculation) Table 3 shows the solution temperature of Mg2Si, solidus temperature, aluminum solidification start temperature, and crystallization temperature of primary crystals (i.e., Al6(Mn,Fe)) calculated based on the components (mass %) of alloy numbers 1 - 9.

[0110] Each boundary temperature was calculated based on the equilibrium phase diagram obtained using "JMatPro" based on the main five components (Si, Fe, Cu, Mn, Mg). Here, the effects of Ti, Zn, Cr, and inevitable impurities are not considered.

[0111] (Scrap blending ratio) Regarding the composition of the unpainted aluminum alloy plates of S1 - S17, it was determined whether the possible blending ratio of 3104 aluminum alloy scrap was 50 mass% or more. The results are shown in Table 2.

[0112] In Table 2, the aluminum alloy plates marked "≧50" can be blended with 3104 aluminum alloy at 50 mass% or more. The possible blending ratio of 3104 aluminum alloy scrap is determined based on Table 4.

[0113] 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.

[0114] For example, when the blending ratio of 3104 aluminum alloy is 50% by mass, the average value of Si is 0.20% by mass, the average value of Fe is 0.29% by mass, the average value of Cu is 0.11% by mass, the average value of Mn is 0.7% by mass, and the average value of Mg is 2.8% by mass.

[0115] Therefore, when the ratio of each component of the aluminum alloy sheet is equal to or higher than the above numerical values of Si, Fe, Cu, Mn, and Mg, the possible blending ratio of the 3104 aluminum alloy sheet becomes 50% by mass or more. The higher the blending 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 - S15 can be blended with 50% by mass or more of the scrap of 3104 aluminum alloy.

[0116]

Table 4

[0117] (Evaluation) The unpainted aluminum alloy sheets of S1 - S7 and S12 - S14 have higher strength (that is, S min ) than the unpainted aluminum alloy sheet of S16. Also, among the unpainted aluminum alloy sheets of S1 - S7 and S12 - S14, equivalent strength could be achieved compared to the conventional 5182 aluminum alloy for can lids shown in S17, despite having a lower Mg content.

[0118] Generally, the higher the strength of a material, the more likely the number of repeated bending cycles, that is, the toughness, tends to decrease. However, S14, which was homogenized at a high temperature to redissolve Mg2Si, has more repeated bending cycles than S15 with insufficient redissolution of Mg2Si, even though they have the same alloy components and equivalent strength. That is, S14 combines high toughness and high strength.

[0119] In S1 - S7, the cold rolling rate is reduced by an intermediate annealing process. Therefore, compared with S8 - S13 where the cold rolling rate exceeds 80%, S1 - S7 has a smaller strength anisotropy. That is, it can be seen that the strength anisotropy and the cold rolling rate are correlated. S1 - S7 with the cold rolling rate reduced by the intermediate annealing process has both high strength and high toughness (i.e., a large number of repeated bending times) compared with S8 - S13 where the cold rolling rate exceeds 80%.

[0120] As shown in Fig. 2, when comparing S1, S7, and S8 with the same alloy composition but different cold rolling rates, there is a negative correlation between the strength anisotropy and the cold rolling rate. For high - toughness materials with the cold rolling rate suppressed to 50% or more and 80% or less by intermediate annealing or the like, it is considered that the absolute value of the strength anisotropy is maximized at a cold rolling rate of 80% in the negative direction. Here, since it is estimated from the trend in Fig. 2 that the strength anisotropy at a cold rolling rate of 80% is - 13 MPa, it can be said that - 13 MPa is the lower limit of the strength anisotropy.

[0121] Similarly, it is considered that the absolute value of the strength anisotropy is maximized at a cold rolling rate of 50% in the positive direction, and it is estimated from the trend in Fig. 2 that the strength anisotropy at a cold rolling rate of 50% is 7 MPa. On the other hand, referring to the strength anisotropy of S3, S5, etc., it can be seen that some variation in the strength anisotropy is inevitable. Therefore, it is considered appropriate that the strength anisotropy at a cold rolling rate of 50% is about 13 MPa.

Claims

1. The silicon (Si) content is 0.20% by mass or more and 0.39% by mass or less, the iron (Fe) content is 0.30% by mass or more and 0.59% by mass or less, the copper (Cu) content is 0.11% by mass or more and 0.40% by mass or less, the manganese (Mn) content is 0.75% by mass or more and 0.98% by mass or less, the magnesium (Mg) content is 1.4% by mass or more and 3.1% by mass or less, the balance consists of aluminum (Al) and inevitable impurities, The 0.2% proof stress σ, the tensile strength σ, and the average value σ of the 0.2% proof stress and the tensile strength after heat treatment at 260°C for 25 seconds in the directions of 0°, 45°, and 90° with respect to the rolling direction ０．２ , the tensile strength σ Ｂ , and the minimum evaluation value S, which is the minimum value among the evaluation values S calculated by the following formula (1) using the average value σ of the 0.2% proof stress and the tensile strength ｆｍ is 360 MPa or more and 410 MPa or less, ｍｉｎ and The temperature difference obtained by subtracting the solid solubility temperature of Mg ２ Si from the solidus temperature is 30 °C or higher, the crystallization temperature of the primary crystal is lower than the solidification start temperature of aluminum, In the L-ST cross-section of the central portion in the width direction after heat treatment at 260°C for 25 seconds, the area is 0.3 μm ２ or more of Mg ２ The ratio of the total area of Si particles in the L-ST cross-section is 0.2% or less. An aluminum alloy sheet for an unpainted can lid. S = σ ｆｍ / (σ ０．２ / σ Ｂ )...(1)

2. An aluminum alloy sheet for an unpainted can lid according to Claim 1, 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° is in the range of -13 MPa or more and 13 MPa or less, an aluminum alloy sheet for an unpainted can lid.

3. An aluminum alloy sheet for an unpainted can lid according to Claim 1 or 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 breakage of the test piece, is normalized by the plate thickness t of the test piece and the following formula (2). ｓ An unpainted aluminum alloy sheet for can lids, where ｓ is 18 or more times. N ｓ = N × t / 0.235 ··· (2)

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