Uncoated aluminum alloy sheet for can lids

Abstract

Provided is an uncoated aluminum alloy sheet for can lids that achieves both high strength and high toughness while incorporating scrap raw material derived from cans.　According to one aspect of the present disclosure, an uncoated aluminum alloy sheet for can lids comprises: 0.20-0.39 mass% of Si; 0.30-0.59 mass% of Fe; 0.11-0.40 mass% of Cu; 0.75-0.98 mass% of Mn; and 1.4-3.1 mass% of Mg. The minimum evaluation value Smin of the evaluation value S is 360 MPa to 410 MPa both inclusive, where the evaluation value S is calculated according to expression (1) using 0.2% proof stress σ0.2, tensile strength σB, and the average σfm of the 0.2% proof stress and the tensile strength after heat treatment at 260°C for 25 seconds in each of 0°, 45°, and 90° directions with respect to the rolling direction.　Expression (1): S=σfm / (σ0.2 / σB)

Description

Unpainted aluminum alloy sheet for can lids CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This international application claims the benefit of Japanese Patent Application Nos. 2023-223496, 2023-223497, 2023-223498, and 2023-223499, filed with the Japan Patent Office on December 28, 2023, the entire disclosures of which are incorporated herein by reference.

[0002] The present disclosure relates to an unpainted aluminum alloy sheet for can ends.

[0003] In recent years, with the rise of environmental awareness, CO ２ There is a demand for aluminum alloy sheets with low CO emissions. ２ The compounding of new aluminum ingots in the casting process indirectly contributes significantly to these emissions.

[0004] The production of primary aluminum ingots uses a large amount of electricity during the smelting process and produces a large amount of CO ２ Therefore, reducing the amount of virgin aluminum used and increasing the horizontal recycling rate will reduce CO emissions in the manufacture of aluminum alloy sheets. ２ This will lead to reduced emissions.

[0005] Generally, CO generated when aluminum scrap is remelted and cast ２ It is said that waste emissions can be reduced to about one-thirtieth of that when producing virgin aluminum ingots. In particular, the production volume of aluminum alloy sheets for beverage cans used worldwide is extremely large, and further improving the horizontal recycling rate for these cans will have a significant impact on reducing the environmental impact.

[0006] Among these, can ends made of 5182 aluminum alloy (AA5182 alloy) have lower upper limits for the compositional specifications of Si, Fe, Cu, Mn, etc. than can bodies made of 3104 aluminum alloy (AA3104 alloy), making it difficult to incorporate scrap derived from can stock containing 3104 aluminum alloy.

[0007] For example, if can scrap (UBC: Used Beverage Can) generated in the market is blended as is, the weight ratio of the can body to the can lid will result in a higher content of 3104 aluminum alloy components, which will likely exceed the upper limit of the content of 5182 aluminum alloy, making it necessary to dilute the components with virgin metal.

[0008] Therefore, compared with the aluminum alloy sheet for can bodies, the aluminum alloy sheet for can ends uses a larger amount of virgin metal to adjust the composition to that of 5182 aluminum alloy, resulting in a low recyclability. Therefore, by changing the can ends to an alloy whose composition is easily blended with 3104 aluminum alloy, the virgin metal usage rate for can ends can be significantly reduced.

[0009] Patent Documents 1 to 5 disclose aluminum alloy sheets for can lids that have compositions relatively close to those of 3104 aluminum alloy, which has excellent recyclability.

[0010] Japanese Patent Application Laid-Open No. 2001-73106 Japanese Patent Application Laid-Open No. 9-070925 Japanese Patent Application Laid-Open No. 11-269594 Japanese Patent Application Laid-Open No. 2000-160273 Japanese Patent Application Laid-Open No. 2016-160511

[0011] The problem of using an alloy for can lids with a composition similar to that of 3104 aluminum alloy is that it reduces the pressure resistance of the can lid and the toughness of the material. The pressure resistance of a can lid is the internal pressure value when the can lid is inverted against the pressure inside the can, and is the resistance value when the internal pressure of the can increases unexpectedly due to a change in the external environment.

[0012] In particular, positively pressurized cans for beer and carbonated beverages require high pressure resistance. Generally, the stronger the material and the thicker the plate, the greater the pressure resistance. For this reason, the lids of positively pressurized cans are made of high-strength 5182 aluminum alloy, which contains a large amount of magnesium, a component that contributes to increased strength.

[0013] In contrast, if conventional 3104 aluminum alloy is used for can lids, the pressure resistance is significantly reduced, and there is a high risk that the lid will invert and leak contents if the internal pressure of the can suddenly increases. Furthermore, if the plate thickness is increased to increase the pressure resistance, this will result in an increase in the weight and cost of the lid.

[0014] Furthermore, the toughness of the material affects the formability and openability of the lid. If the material has low toughness, cracks may occur during molding, especially at the rivet and countersink areas of the lid. Also, if the internal pressure of the can suddenly increases, cracks may occur at the score area, increasing the risk of the can contents leaking. These cracks occur particularly along the rolling direction. Therefore, the material must have toughness against tensile stress and bending stress in directions perpendicular to the rolling direction.

[0015] However, aluminum alloy sheets for can lids, which have chemical compositions relatively close to those of the conventional 3104 aluminum alloy, do not satisfy either or both of the above-mentioned two problems, i.e., material strength (i.e., pressure resistance of the lid) and toughness (i.e., formability and openability).

[0016] One aspect of the present disclosure is to provide an unpainted aluminum alloy sheet for can ends that contains scrap raw materials derived from can stock and yet has both high strength and high toughness after paint baking.

[0017] In one embodiment of the present disclosure, 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, and the balance is aluminum (Al) and unavoidable impurities or contains 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 ° direction, 45 ° direction, and 90 ° direction with respect to the rolling direction ０．２ , tensile strength σ Ｂ , and the average value σ of 0.2% proof stress and tensile strength ｆｍ Among the evaluation values ​​S calculated by the following formula (1) using ｍｉｎ is 360 MPa or more and 410 MPa or less, and the solidus temperature is Mg ２The temperature difference minus the solid solution temperature of Si is 30°C or more, the crystallization temperature of the primary crystal is lower than the solidification start temperature of aluminum, and the area of ​​the L-ST cross section in the width direction center part after heat treatment at 260°C for 25 seconds is 0.3 μm ２ More than Mg ２ In the unpainted aluminum alloy sheet for can ends, the ratio of the total area of ​​Si particles in the L-ST cross section is 0.2% or less.

[0018] S = σ ｆｍ / (σ ０．２ / σ Ｂ ) ... (1) According to this configuration, it is possible to achieve both high strength and high toughness in the aluminum alloy sheet after paint baking while blending scrap raw material derived from can stock. That is, it is possible to blend a certain amount of 3104 aluminum alloy scrap for can bodies, reduce the usage rate of new metal, and reduce CO ２ Furthermore, it is possible to obtain an unpainted aluminum alloy sheet for can ends that has high formability and can be used for positive pressure can ends, which require high pressure resistance.

[0019] The "unpainted aluminum alloy sheet for can ends" of the present disclosure is an alloy sheet (i.e., a bare material) that is to be used as an aluminum alloy sheet for can ends after painting. That is, the "aluminum alloy sheet for can ends" is obtained by painting the "unpainted aluminum alloy sheet for can ends" of the present disclosure.

[0020] Fig. 1 is an explanatory diagram of an L-ST cross section, and Fig. 2 is a graph showing an example of the relationship between the cold rolling rate and the strength anisotropy.

[0021] Hereinafter, embodiments to which the present disclosure is applied will be described with reference to the drawings.

[0022] [1. First embodiment] [1-1. Configuration] The unpainted aluminum alloy sheet for can ends of the present disclosure may be continuously painted as a coil on a painting line, or may be cut into sheets and then painted or laminated to form into lids. The unpainted aluminum alloy sheet for can ends of the present disclosure can be used for any application. Furthermore, the unpainted aluminum alloy sheet for can ends is not limited to applications as beverage can lids, and can be widely used for food can lids, etc.

[0023] <Composition> The unpainted aluminum alloy sheet for can ends (hereinafter also simply referred to as "alloy sheet") of the present disclosure contains aluminum (Al), silicon (Si), iron (Fe), copper (Cu), manganese (Mn), and magnesium (Mg).

[0024] The lower limit of the Si content is 0.20 mass%, preferably 0.30 mass%. If the Si content is less than 0.20 mass%, the amount of Si precipitated in the processing heat of hot rolling and cold rolling after solution treatment may decrease, and the strength of the alloy sheet after paint baking may be insufficient.

[0025] The average Si content of 3104 aluminum alloy specified in 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 3104 aluminum alloy scrap can be blended.

[0026] The upper limit of the Si content is 0.39 mass%, preferably 0.35 mass%. If the Si content exceeds 0.39 mass%, Mg ２ The amount of Si particles increases, and the toughness of the alloy sheet after paint baking decreases.

[0027] The lower limit of the Fe content is 0.30% by mass, preferably 0.40% by mass. The average Fe content 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 3104 aluminum alloy scrap can be blended.

[0028] The upper limit of the Fe content is 0.59% by mass. If the Fe content exceeds 0.59% by mass, the amount of Al-Fe-Mn or Al-Fe-Mn-Si intermetallic compounds (i.e., second phase particles) increases. As a result, crack propagation paths are created, and the toughness of the alloy sheet after paint baking decreases.

[0029] The lower limit of the Cu content is 0.11 mass%, preferably 0.15 mass%, and more preferably 0.20 mass%. If the Cu content is less than 0.11 mass%, there will be insufficient Cu, which increases strength by solid solution or precipitation, and the strength of the alloy sheet after paint baking will decrease. Note that by precipitating Cu during hot rolling and cold rolling after solution treatment, the strength of the alloy sheet after paint baking will be significantly increased.

[0030] The average Cu content of the 3104 aluminum alloy is 0.15% by mass, so by making the Cu content 0.15% by mass or more, it is possible to blend a large amount of 3104 aluminum alloy scrap.

[0031] The upper limit of the Cu content is 0.40 mass%, preferably 0.25 mass%. If the Cu content exceeds 0.40 mass%, the toughness of the alloy sheet after paint baking decreases.

[0032] The lower limit of the Mn content is 0.75% by mass. If the Mn content is less than 0.75% by mass, there will be insufficient Mn, which increases strength through solid solution or precipitation, and the average strength of the alloy sheet after paint baking will decrease.

[0033] The average Mn content of the 3104 aluminum alloy is 1.1 mass% and the average Mn content of the 5182 aluminum alloy is 0.35 mass%. Therefore, by setting the Mn content to 0.75 mass% or more, it is possible to blend more 3104 aluminum alloy scrap than in the conventional 5182 aluminum alloy.

[0034] The upper limit of the Mn content is 0.98% by mass, preferably 0.90% by mass. If the Mn content exceeds 0.98% by mass, the amount of Al-Fe-Mn or Al-Fe-Mn-Si intermetallic compounds (i.e., second phase particles) increases. As a result, crack propagation paths are created, and the toughness of the alloy sheet after paint baking decreases.

[0035] The lower limit of the Mg content is 1.4% by mass. If the Mg content is less than 1.4% by mass, there will be insufficient Mg, which increases strength through solid solution, and the average strength of the alloy sheet will decrease. The lower limit of the Mg content is preferably 2.3% by mass. This increases the average strength of the alloy sheet. Note that by precipitating Mg during hot rolling and cold rolling after solution treatment, the strength of the alloy sheet after paint baking will be significantly increased.

[0036] The upper limit of the Mg content is 3.1% by mass. The average Mg content standard for 3104 aluminum alloy is 1.05% by mass, and the average Mg content standard for 5182 aluminum alloy is 4.5% by mass. Therefore, by setting the Mg content to 3.1% by mass or less, it is possible to incorporate a large amount of 3104 aluminum alloy scrap while reducing the amount of Mg-containing raw material to be added.

[0037] The alloy plate may contain titanium (Ti). The upper limit of the Ti content is preferably 0.10 mass%. By including Ti, the ingot structure of the alloy plate is refined. The alloy plate may also contain zinc (Zn). The upper limit of the Zn content is preferably 0.25 mass%. Furthermore, the alloy plate may also contain chromium (Cr). The upper limit of the Cr content is preferably 0.10 mass%.

[0038] The alloy plate may contain unavoidable impurities to the extent that 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 in the respective ranges described above, with the balance consisting of aluminum and unavoidable impurities or including aluminum and unavoidable impurities. The upper limit of the total amount of unavoidable impurities is preferably 0.15 mass%. The balance may contain substances other than aluminum and unavoidable impurities.

[0039] <Material strength and pressure resistance> Rolled aluminum alloy sheets have material anisotropy, and their strength varies in the 0°, 45°, and 90° directions relative to the rolling direction. When the pressure inside the can increases, deformation begins in the direction with the lowest strength.

[0040] Therefore, the alloy sheet of the present disclosure has a 0.2% yield strength σ after heat treatment at 260°C for 25 seconds in each of the 0°, 45°, and 90° directions relative to the rolling direction. ０．２ , tensile strength σ Ｂ , and the average value σ of 0.2% proof stress and tensile strength ｆｍ The evaluation value S (S ０° , S ４５° , and S ９０° ), the minimum evaluation value S ｍｉｎ (=min(S ０° , S ４５° , S ９０° ) is 360 MPa or more and 410 MPa or less.

[0041] S = σ ｆｍ / (σ ０．２ / σ Ｂ ) ... (1) The pressure resistance value of the lid formed of an aluminum alloy plate is empirically the minimum evaluation value S after painting and baking. ｍｉｎ and the plate thickness t, there is a strong positive correlation with the value V of the following formula (2).

[0042] V = t ２．２７ ×S ｍｉｎ ... (2) Therefore, the minimum evaluation value S after the alloy plate is subjected to heat treatment at 260 ° C for 25 seconds ｍｉｎ By setting the pressure to 360 MPa or more, a lid having sufficient pressure resistance can be formed without increasing the plate thickness.

[0043] The minimum evaluation value S after heat treatment at 260° C. for 25 seconds ｍｉｎ If the stress exceeds 410 MPa, the material strength becomes excessively high, and the toughness of the material decreases. In other words, shear bands are likely to occur due to the tensile stress and bending stress that occur in the material during molding, and molding cracks are likely to occur. The minimum evaluation value S after heat treatment at 260°C for 25 seconds ｍｉｎ By setting the compressive strength to 410 MPa or less, it is possible to achieve both strength of the material (i.e., pressure resistance of the lid) and toughness (i.e., formability and openability).

[0044] 0.2% proof stress σ in formula (1) ０．２ and tensile strength σ Ｂis measured by the method specified in JIS-Z-2241: 2011. The plate thickness t is measured, for example, with a microgauge.

[0045] The withstand pressure of an aluminum alloy plate is measured, for example, by the following procedure. First, a shell formed from a baked aluminum alloy plate is fixed to a jig and internal pressure is applied. Next, this internal pressure is gradually increased, and the internal pressure value when the shell inverts (i.e., buckles) is taken as the withstand pressure value.

[0046] Specifically, a φ204 full-form (B64) shell mold is used to mold the shell. Internal pressure values ​​are measured using a Versatile Technology DV036E buckle and missile measuring machine. Specifically, after fixing the molded shell in place with a dedicated jig, the internal pressure is increased using a program, and the internal pressure value is read when the shell inverts. For example, the internal pressure is increased at a rate of approximately 175 kPa / s, and when it reaches approximately 350 kPa to 400 kPa, the internal pressure is increased at a rate of 10 kPa / s.

[0047] <Toughness> It is known that the formability of the lid and the force required to open the score (i.e., opening force) are affected by the toughness of the aluminum alloy plate after paint baking.

[0048] (Number of repeated bending) A repeated bending test is one of the evaluation indices for the toughness of an aluminum alloy sheet. For a given sheet thickness, the more repeated bending times an aluminum alloy sheet can be subjected to, the more excellent its toughness.

[0049] The repeated bending test is carried out as follows. For example, a test piece is heat-treated at 260°C for 25 seconds and cut into a strip shape with a width of 12.5 mm and a length of 200 mm. The test piece is placed so that the bending ridgeline is parallel to the rolling direction of the alloy plate. Both ends of the test piece are fixed with chucks, and a tension of 200 N is applied.

[0050] In this state, the test piece is repeatedly bent by rotating the other chuck 90° left and right, using a jig with a bending radius of 2.0 mm as a fulcrum, which is positioned 150 mm in the longitudinal direction of the test piece from the end of the test piece fixed to one of the immovable chucks, and the number of bends until the test piece breaks is measured.

[0051] The number of bending times is counted as one bending operation to either the left or right by 90° and one operation returning to the original position. If the wire breaks during bending, the angle Θ (0°-90°) is read and the number of repeated bending times N is calculated using the following formula (3). In formula (3), N ０ is the total number of times that the test piece was bent 90° to either the left or right and returned to the original 0° position from the 90° bent position until it broke.

[0052] N=N ０ +Θ / 90 ... (3) Repeated bending evaluation is more disadvantageous as the plate thickness increases, so it is necessary to consider it by correcting it with a standard plate thickness. Therefore, the normalized number of repeated bending times N is normalized by the following formula (4) using a plate thickness of 0.235 mm as the standard. ｓ where t (mm) is the thickness of the test piece.

[0053] N ｓ = N × t / 0.235 (4) The normalized number of repeated bending times N for the unpainted aluminum alloy sheet of the present disclosure ｓ Preferably, the number is 18 or more.

[0054] (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 are high, the distribution of Mg ２ As a result, Mg ２ The Si particles act as crack initiation points and propagation paths, which reduces toughness.

[0055] In the unpainted aluminum alloy sheet of the present disclosure, the area of ​​the L-ST cross section in the width direction center part after heat treatment at 260 ° C. for 25 seconds is 0.3 μm ２ More than Mg ２ The ratio of the total area of ​​Si particles 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.

[0056] Mg ２The area ratio of Si particles can be measured, for example, by the following method. First, a 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 The image is taken as is, and a COMPO (backscattered electron composition) image is obtained.

[0057] The captured COMPO image is analyzed using the image analysis software "ImageJ." Specifically, the most frequent value of the image brightness in 256 gradations is defined as the background brightness, and particles with brightness lower than the value obtained by subtracting 30 from the most frequent brightness are defined as Mg particles. ２ It is determined to be a Si particle.

[0058] Determined Mg ２ Among the Si particles, 0.3 μm ２ The total area of ​​particles with an area of ​​0.3 μm or more was calculated and divided by the photographed area of ​​10 fields of view (i.e., the total photographed area), resulting in an area of ​​0.3 μm ２ More than Mg ２ The percentage of the total area of ​​the Si particles in the L-ST cross section is calculated.

[0059] <Calculation of phase diagram by computer software> In the alloy plate of the present disclosure, in order to increase the toughness of the material, Mg, which becomes the initiation point and propagation path of cracks and affects the decrease in toughness, is used. ２ It is preferable to redissolve the Si particles in the ingot during a homogenization heat treatment step.

[0060] While avoiding local melting of the aluminum matrix, Mg ２ In order to redissolve Si, the solidus temperature must be higher than Mg ２ It is preferable that the temperature is higher than the solid solution temperature of Si, and that the temperature is higher than the solidus temperature of Mg ２ The temperature difference minus the solid solution temperature of Si is preferably 30° C. or more.

[0061] Furthermore, in materials in which the crystallization temperature of Al6(Mn,Fe) is higher than the solidification start temperature of aluminum, Al6(Mn,Fe) is generated as coarse crystals during casting, which can lead 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 solidification start temperature of aluminum.

[0062] The Mg mentioned here ２ The solid solution temperature of Si is the same as that of Mg in the equilibrium diagram. ２ The temperature at which Si can exist is the highest temperature at which Si can exist, and the lowest temperature at which a liquid phase can exist. The solidification start temperature of aluminum is the highest temperature at which solid Al can exist in the equilibrium diagram, and the crystallization temperature of the primary crystal is the highest temperature at which Al6(Mn,Fe) can exist.

[0063] Mg ２ The solution temperature and solidus temperature of Si, the solidification start temperature of aluminum, and the crystallization temperature of the primary crystal can be obtained from an equilibrium phase diagram of the aluminum alloy calculated using thermodynamic calculation software.

[0064] Mg ２ The solution temperature and solidus temperature of Si, the solidification start temperature of aluminum, and the crystallization temperature of the primary crystal are uniquely determined by the composition of the aluminum alloy. One method for determining these boundary temperatures from the alloy composition is to calculate the thermodynamic quantities required for each calculation using the CALPHAD method.

[0065] Such thermodynamic calculations for multi-component alloys can be performed using commercially available system software (e.g., "JMatPro" developed by Sente Software) that combines the thermodynamic database, interface, and phase diagram creation functions required for the calculations.

[0066] In the unpainted aluminum alloy plate of the present disclosure, Mg ２ Since the solid solution temperature of Si is low and the crystallization temperature of the primary crystal is lower than the solidification start temperature of Al, no coarse crystals are generated during casting, and the performance degradation caused by the coarse crystals can be avoided.

[0067] Also, below the solidus temperature and Mg ２By homogenizing the heat treatment at a temperature higher than the solid solution temperature of Si, Mg ２ Mg redissolves Si, becoming the starting point and propagation path for cracks, which affects the decrease in toughness. ２ The amount of Si particles can be reduced.

[0068] <Strength anisotropy> It is known that materials with a low cold rolling reduction (hereinafter abbreviated as cold rolling reduction) have high toughness. For example, a material in which a rolled sheet is annealed in a continuous annealing furnace (CAL) during cold rolling to reduce the final cold rolling reduction (i.e., the cold rolling reduction after annealing) has high toughness.

[0069] In addition, the higher the cold rolling rate, the higher the 0.2% yield strength in the direction at 90° to the rolling direction. σ０．２＿９０° , 0.2% proof stress σ in the 0° direction ０．２＿０° Therefore, the difference in 0.2% yield strength 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.

[0070] The alloy sheet of the present disclosure has a 0.2% yield strength σ in the direction of 0° to the rolling direction after heat treatment at 260°C for 25 seconds, which is calculated by formula (5). ０．２＿０° From the above, after heat treatment at 260 ° C for 25 seconds, the 0.2% proof stress σ in the direction at 90 ° to the rolling direction ０．２＿９０° It is preferable that the value D obtained by subtracting the above is −13 MPa or more and 13 MPa or less.

[0071] D = σ０．２＿０° -σ ０．２＿９０° ... (5) 0.2% proof stress σ in the 0° direction relative to the rolling direction ０．２＿０° From this, 0.2% proof stress σ in the direction 90° to the rolling direction ０．２＿９０° The material structure meaning of the strength anisotropy minus can be explained as follows.

[0072] After hot rolling or annealing, the material is in a recrystallized state, with a high concentration of isotropic Cube orientation. From this point, plastic deformation by cold rolling transforms the Cube orientation into a rolling texture with anisotropy in the rolling direction. Furthermore, the larger the cold rolling rate, the more the crystal grains are elongated in the rolling direction, so the diameter of the crystal grains along the 0° direction relative to the rolling direction increases, while the change in the diameter of the crystal grains along the 90° direction relative to the rolling direction becomes smaller than in the 0° direction.

[0073] The structural changes caused by these rolling processes and the 0.2% yield strength σ ０．２ The relationship between these two is expressed by the Hall-Petch equation as shown in Equation (6): In Equation (6), κ is the resistance to sliding of the grain boundary, and d is the grain size.

[0074] σ ０．２ ∝κ×d−½ (6) The resistance κ has different values ​​when tension is applied in the 0° direction or 90° direction relative to the rolling direction. This is because the degree of integration of the rolling texture, which is anisotropic in the rolling direction, increases with an increase in the cold reduction ratio, and the resistance to sliding of the grain boundaries changes depending on the tension direction.

[0075] In addition, in the direction of 0° from the rolling direction, the grains elongate and their diameters increase with increasing cold rolling, whereas in the direction of 90° from the rolling direction, the change in grain size with increasing cold rolling is relatively small. The cumulative effect of these effects results in strength anisotropy with increasing cold rolling.

[0076] <Method for producing aluminum alloy sheet> The unpainted aluminum alloy sheet of the present disclosure can be produced, 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.

[0077] Next, the four sides of the ingot, excluding the front and rear ends, are chamfered. The ingot is then placed in a soaking furnace for homogenization. The temperature in the homogenization is preferably, for example, 470°C or higher and 620°C or lower. The time for the homogenization is preferably, for example, 1 hour or higher and 20 hours or lower.

[0078] When the homogenization temperature is 400°C or higher, segregation of the ingot structure is easily eliminated. Furthermore, when the homogenization temperature is 450°C or higher, Mg ２ The Si particles are redissolved, and the strength and toughness of the alloy plate can be improved. ２ When the temperature is higher than the solid solution temperature of Si, Mg ２The re-dissolution of Si 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, more preferably the solidus temperature or lower, local melting of the aluminum alloy is unlikely to occur.

[0079] When the homogenization treatment time is 1 hour or more, the temperature of the entire slab becomes uniform, segregation of the ingot structure is easily eliminated, and Mg ２ The longer the homogenization time, the easier it is to redissolve the Si particles. ２ The Si particles can be redissolved. However, if the homogenization treatment time exceeds 20 hours, the effect of the homogenization treatment becomes saturated.

[0080] 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 having a thickness of about several tens of mm by reverse rolling. In the finish rolling process, the thickness of the plate material is reduced to about several mm by, for example, tandem rolling, and the plate material is wound into a coil to form a hot-rolled coil.

[0081] If the total reduction rate of the finish rolling is high, a recrystallized structure is formed after coiling, and the concentration of the isotropic Cube orientation can be increased. If the coiling temperature of the finish rolling is high, a recrystallized structure is formed after coiling, and the concentration of the Cube orientation can be increased.

[0082] Following the hot rolling, the plate material is cold rolled. In the cold rolling, the hot rolled coil is rolled until the product plate thickness is reached. The cold rolling may be either single rolling or tandem rolling. In the cold rolling by single rolling, it is preferable to perform the rolling in two or more passes.

[0083] Furthermore, by subjecting the coil to solution treatment during cold rolling to redissolve Mg and other elements, it is possible to obtain an alloy sheet with high strength and reduced final cold rolling rate, suppressing the anisotropy of the material. For example, by performing heat treatment (i.e., annealing) at a target solid temperature of 440°C or higher using a continuous annealing furnace (CAL), followed by forced cooling by air cooling or the like, it is possible to effectively increase the strength of the alloy sheet.

[0084] Furthermore, by setting the finishing temperature of cold rolling in intermediate passes other than the final pass to 120°C or higher, Si, Cu, and Mg are finely precipitated and age-hardened, thereby increasing the strength of the alloy sheet. Furthermore, by setting the finishing temperature to 130°C or higher, the strength of the alloy sheet can be further increased.

[0085] When no solution treatment is performed during cold rolling, the cold rolling rate is preferably 80% or more. When the cold rolling rate is 80% or more, the strength of the alloy sheet can be increased. Furthermore, the lower the cold rolling rate, the more isotropic Cube orientation remains, so the cold rolling rate is preferably 92% or less.

[0086] When solution treatment is performed during cold rolling, the cold rolling rate after solution treatment (i.e., annealing) is preferably 50% or more. By resolving Mg and other elements through solution treatment, the strength of the alloy sheet can be increased even if the cold rolling rate is low. Furthermore, the lower the cold rolling rate, the more isotropic Cube orientation remains, so the cold rolling rate is preferably 80% or less.

[0087] The cold rolling rate R (%) is the thickness t after hot rolling or solution treatment. ０ (mm), product thickness after cold rolling t １ (mm) is calculated using the following formula (7).

[0088] R = (t ０ -t １ ) / t ０ × 100 (7) 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, the unpainted aluminum alloy plate of the present disclosure can suppress an increase in plate thickness to maintain high pressure resistance.

[0089] By such a rolling treatment, an unpainted aluminum alloy sheet is obtained. The obtained unpainted aluminum alloy sheet (i.e., a coil cold-rolled to a product thickness) is subjected to painting including pre-coating in a painting line or the like to obtain a painted aluminum alloy sheet. The surface of the cold-rolled coil is degreased, washed, and subjected to a chemical conversion treatment, and then painted, followed by a paint baking treatment.

[0090] Chemical solutions such as chromate-based and zirconium-based are used in chemical conversion treatment. Epoxy-based and polyester-based paints are used. These can be selected according to the application. In the paint baking process, the actual temperature of the coil (PM Ｔ The alloy plate is heated to a peak metal temperature (PMT) of 220°C to 270°C for approximately 30 seconds or less. The lower the PMT, the more the material recovery is suppressed, and the higher the strength of the alloy plate can be maintained.

[0091] [1-2. Effects] According to the embodiment described above in detail, the following effects can be obtained.

[0092] (1a) While blending scrap raw materials derived from can stock, it is possible to achieve both high strength and high toughness of the aluminum alloy sheet after paint baking. That is, it is possible to blend a certain amount of 3104 aluminum alloy scrap for can bodies, reduce the usage rate of virgin metal, and reduce CO ２ Furthermore, it is possible to obtain an aluminum alloy sheet for can ends after baking that has high formability and can be used for positive pressure can ends, which require high pressure resistance.

[0093] [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-described embodiments and can take on various forms.

[0094] (2a) In addition to the unpainted aluminum alloy plate of the above embodiment, the present disclosure also includes various forms such as a member made of this unpainted aluminum alloy plate and a method for manufacturing this unpainted aluminum alloy plate.

[0095] (2b) The function of one component in the above embodiments may be distributed among multiple components, or the functions of multiple components may be integrated into one component. Also, part of the configuration of the above embodiments may be omitted. Furthermore, at least part of the configuration of the above embodiments may be added to, replaced with, or the like, the configuration of another of the above embodiments. Note that all aspects included in the technical idea identified by the wording of the claims are embodiments of the present disclosure.

[0096] 3. Examples The following describes the details of tests conducted to confirm the effects of the present disclosure and the evaluation results thereof.

[0097] <Production of Unpainted Aluminum Alloy Sheets> As examples and comparative examples, unpainted aluminum alloy sheets S1 to S17 shown in Tables 1 and 2 were produced. Specific production procedures are described below.

[0098] First, an ingot containing the components (% by mass) of Alloy Nos. 1-9 shown in Table 3, with the balance being aluminum and unavoidable impurities, was produced by semi-continuous casting. The ingot contained 0.10% by mass or less of Ti, 0.25% by mass or less of Zn, 0.10% by mass or less of Cr, and 0.15% by mass or less of unavoidable impurities.

[0099] Next, all four sides of the ingot, excluding the front and rear ends, were chamfered. The ingot was then placed in a furnace and subjected to homogenization treatment. The homogenization treatment temperature is shown in Table 1. After homogenization treatment, the ingot was removed from the furnace and immediately hot-rolled to produce a rolled sheet.

[0100] Furthermore, for S1-S7, S14, S15, and S17, the rolled sheets after hot rolling were cold rolled until they reached the CAL thickness shown in Table 1. Thereafter, the rolled sheets having the CAL thickness were annealed in a continuous annealing furnace (CAL). The CAL temperature during annealing is as shown in Table 1. After annealing, the rolled sheets were cooled to room temperature by air cooling. After cooling, the rolled sheets were cold rolled again. The target cold reduction ratios in the cold rolling after annealing are as shown in Table 1.

[0101] For S8-S13 and S16, the hot-rolled rolled sheets were subjected to cold rolling without annealing. The target cold rolling reduction ratios are shown in Table 1.

[0102] The unpainted aluminum alloy sheets S1-S17 were obtained by the above-mentioned treatment. The product thickness after cold rolling in S1-S17 (i.e., t １ ) was in the range of approximately 0.235±0.03 mm. Table 1 shows the plate thickness (i.e., product plate thickness) measured by a microgauge for the unpainted aluminum alloy plates S1-S17.

[0103]

[0104]

[0105] <Evaluation of unpainted aluminum alloy sheets> (Tensile properties) Three No. 5 test pieces specified in JIS-Z-2241: 2011 were prepared from each of the unpainted aluminum alloy sheets S1 to S17 by milling. The longitudinal directions of the three test pieces extended in directions forming angles of 0°, 45°, and 90° with respect to the rolling direction, respectively.

[0106] These test pieces were subjected to a heat treatment at 260°C for 25 seconds, which is a heat treatment simulating paint baking, and then a tensile test was carried out in accordance with JIS-Z-2241:2011 to measure the 0.2% proof stress and tensile strength. ０．２ and tensile strength σ Ｂ The measurement results and the average value σ of 0.2% yield strength and tensile strength ｆｍ The results are shown in Tables 1 and 2.

[0107] Furthermore, three evaluation values ​​S were calculated from the measurement results of the tensile tests in the 0° direction, the 45° direction, and the 90° direction relative to the rolling direction and from the formula (1). The minimum evaluation value S was the minimum of these evaluation values ​​S. ｍｉｎ is shown in Table 2.

[0108] (Toughness) In the unpainted aluminum alloy plates of S1-S17, the area after heat treatment at 260°C for 25 seconds was 0.3 μm by the measurement method described in the embodiment. ２More than Mg ２ The ratio (area ratio) of the total area of ​​the Si particles in the L-ST cross section was calculated. The measurement results are shown in Table 2.

[0109] For the unpainted aluminum alloy plates S1-S17, the number of repeated bendings and the normalized number of repeated bendings after heat treatment at 260°C for 25 seconds were calculated using the measurement method described in the embodiment and equations (3) and (4). The results are shown in Table 2.

[0110] (Strength anisotropy) For the unpainted aluminum alloy plates S1-S17, the strength anisotropy (i.e., value D) when heat treatment was performed at 260°C for 25 seconds was calculated from the formula (5) described in the embodiment. The results are shown in Table 2.

[0111] (Phase diagram calculation) Mg calculated based on the components (mass%) of Alloy Nos. 1-9 ２ Table 3 shows the solution temperature and solidus temperature of Si, the solidification start temperature of aluminum, and the crystallization temperature of the primary crystal (i.e., Al6(Mn,Fe)).

[0112] Each boundary temperature was calculated based on an equilibrium diagram calculated using "JMatPro" for the five main components (Si, Fe, Cu, Mn, Mg). The effects of Ti, Zn, Cr, and unavoidable impurities were not taken into consideration.

[0113] (Scrap Blending Ratio) With respect to the compositions of the unpainted aluminum alloy plates S1 to 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.

[0114] In Table 2, aluminum alloy plates designated as "≧50" can contain 50 mass% or more of 3104 aluminum alloy. The possible blending ratio of 3104 aluminum alloy scrap is determined based on Table 4.

[0115] Table 4 shows the relationship between the blending ratios of 3104 aluminum alloy and 5182 aluminum alloy and the average values ​​of the component specifications. The first row of Table 4 shows the average values ​​of the component specifications of 3104 aluminum alloy, and the second row shows the average values ​​of the component specifications of 5182 aluminum alloy.

[0116] For example, when the compounding 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%.

[0117] Therefore, when the proportions of each component of the aluminum alloy sheet are equal to or greater than the above-mentioned values ​​of Si, Fe, Cu, Mn, and Mg, the possible blending ratio of 3104 aluminum alloy sheet is 50% by mass or more. As the blending ratio of 3104 aluminum alloy increases, the contents of Si, Fe, Cu, and Mn increase and the content of Mg decreases. Unpainted aluminum alloy sheets S1-S15 can blend 50% by mass or more of 3104 aluminum alloy scrap.

[0118] (Evaluation) The unpainted aluminum alloy sheets S1-S7 and S12-S14 have higher strength (i.e., S16) than the unpainted aluminum alloy sheet S16. ｍｉｎ In addition, the unpainted aluminum alloy sheets S1-S7 and S12-S14 were able to achieve the same strength as the conventional 5182 aluminum alloy for can ends shown in S17, despite having a lower Mg content.

[0119] Generally, the higher the strength of a material, the lower the number of repeated bending times, i.e., the toughness. However, by homogenizing the material at high temperatures, ２ S14, in which Si is redissolved, is Mg ２ Compared to S15, which has insufficient redissolution of Si, S14 can be repeatedly bent more times despite having the same alloy components and equivalent strength. In other words, S14 has both high toughness and high strength.

[0120] In S1-S7, the cold rolling rate is reduced by the intermediate annealing process. Therefore, S1-S7 have smaller strength anisotropy than S8-S13, which have cold rolling rates exceeding 80%. In other words, it can be seen that strength anisotropy corresponds to the cold rolling rate. S1-S7, which have had their cold rolling rate reduced by the intermediate annealing process, have both high strength and high toughness (i.e., a high number of repeated bending times) compared to S8-S13, which have cold rolling rates exceeding 80%.

[0121] As shown in Figure 2, when comparing S1, S7, and S8, which have the same alloy components but different cold rolling rates, there is a negative correlation between strength anisotropy and cold rolling rate. In highly tough materials in which the cold rolling rate is reduced to 50% or more and 80% or less by intermediate annealing or the like, the absolute value of strength anisotropy is thought to be maximum in the negative direction at a cold rolling rate of 80%. Here, based on the trend in Figure 2, it is estimated that the strength anisotropy at a cold rolling rate of 80% will be -13 MPa, so -13 MPa can be said to be the lower limit of strength anisotropy.

[0122] Similarly, the absolute value of strength anisotropy is considered to be maximum in the positive direction at a cold rolling rate of 50%, and from the trend in Figure 2, the strength anisotropy at a cold rolling rate of 50% is estimated to be 7 MPa. On the other hand, when looking at the strength anisotropy of S3, S5, etc., it is clear that some degree of variation in strength anisotropy is unavoidable. Therefore, a strength anisotropy of about 13 MPa at a cold rolling rate of 50% is considered appropriate.

Claims

1. The content of silicon (Si) is 0.20% by mass or more and 0.39% by mass or less, the content of iron (Fe) is 0.30% by mass or more and 0.59% by mass or less, the content of copper (Cu) is 0.11% by mass or more and 0.40% by mass or less, the content of manganese (Mn) is 0.75% by mass or more and 0.98% by mass or less, the content of magnesium (Mg) is 1.4% by mass or more and 3.1% by mass or less, and the balance consists of aluminum (Al) and inevitable impurities or contains aluminum (Al) and inevitable impurities. After heat treatment at 260 °C for 25 seconds in the 0° direction, 45° direction, and 90° direction 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 Among the evaluation values S calculated by the following formula (1), the minimum evaluation value S ｍｉｎ which is the minimum value is 360 MPa or more and 410 MPa or less, the temperature difference obtained by subtracting the solid solution temperature of Mg ２ Si from the solidus temperature is 30 °C or more, the primary crystal crystallization temperature 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 or more, and the ratio of the total area of Mg ２ 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. The aluminum alloy sheet for an uncoated can lid according to claim 1, wherein 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 a value of -13 MPa or more and 13 MPa or less, and the aluminum alloy sheet for an uncoated can lid.

3. An aluminum alloy sheet for an uncoated can lid according to claim 1 or 2, which has been heat-treated at 260°C for 25 seconds, and for a test piece cut out in a strip shape with a width of 12.5 mm and a length of 200 mm, when a bending operation of bending 90° and returning to the 0° position is repeated in a direction in which the bending ridge line is parallel to the rolling direction, the number of repeated bending operations N, which is the number of bending operations until the test piece breaks, is normalized by the plate thickness t of the test piece and the following formula (2) to obtain the normalized number of repeated bending operations N ｓ is 18 or more, an aluminum alloy sheet for an uncoated can lid. N ｓ = N × t / 0.235... (2)

Images