JP7636311B2 - Aluminum alloy plate for can body - Google Patents

Description

The present invention relates to aluminum alloy sheets for can bodies.

The body of an aluminum beverage can is manufactured through processes such as drawing, redrawing, ironing, washing and drying, exterior printing and baking, interior painting and baking, and necking (mouth narrowing). The sidewall of the cup (drawn cup) after drawing and redrawing becomes thicker from the bottom to the opening compared to the plate thickness before processing. In this case, the degree of thickening varies in the circumferential direction due to the plastic anisotropy of the material, and as a result, a cup may be manufactured with a different wall thickness in the circumferential direction at the same height. If the wall thickness of the cup after drawing and redrawing is uneven in the circumferential direction, when the wall thickness is processed to a uniform thickness in the circumferential direction in the subsequent ironing process, the processing rate differs in the circumferential direction, so the tensile stress generated in the sidewall becomes uneven in the circumferential direction, making it more likely to break during processing. In recent years, there has been an increase in can bodies with smaller can body diameters, in other words, larger drawing ratios, than before. As the drawing ratio increases, the wall thickness difference in the circumferential direction of the cup tends to increase, so the problem of ironing workability may become more apparent.

In relation to the above, a method for producing an aluminum alloy sheet for a can body with good roundness of the drawn cup has been proposed, which specifies the chemical composition, ear ratio, soaking conditions, hot rough rolling conditions, hot finish rolling conditions, and cold rolling conditions (see, for example, Patent Document 1). Also, a method for producing an aluminum alloy sheet that can improve ironing formability has been proposed, which specifies the chemical composition, soaking conditions, hot rough rolling conditions, hot finish rolling conditions, cold rolling conditions, finish annealing conditions, and average crystal grain size of the hot rolled sheet (see, for example, Patent Document 2). Furthermore, a method for producing an aluminum alloy sheet for a DI can body with excellent high-speed ironing formability has been proposed, which specifies the chemical composition, soaking conditions, hot rough rolling conditions, hot finish rolling conditions, intermediate annealing conditions, cold rolling conditions, and finish annealing conditions (see, for example, Patent Document 3).

JP 2009-235475 A JP 2006-207006 A Japanese Patent Application Publication No. 10-121177

However, there is no known prior art that focuses on the sidewall thickness of the drawn cup, or even on the difference in thickness in the circumferential direction (anisotropy). In addition, there is a demand to ensure the ironing workability of can bodies with large drawing ratios, which have been increasing in recent years.

The present invention aims to provide an aluminum alloy sheet for can bodies that reduces the circumferential anisotropy of the wall thickness at the opening of the formed drawn cup.

The aluminum alloy sheet for can bodies according to the present invention contains Si: 0.16% by mass to 0.60% by mass, Fe: 0.3% by mass to 0.6% by mass, Cu: 0.10% by mass to 0.35% by mass, Mn: 0.5% by mass to 1.2% by mass, Mg: 0.7% by mass to 2.0% by mass, and the balance being Al and unavoidable impurities. In the aluminum alloy sheet for can bodies, the number density of intermetallic compounds having a circle equivalent diameter of 0.10 μm to less than 1.00 μm is 1.7×10 ⁵ pieces/mm ² to 4.0×10 ⁵ pieces/mm ² , and the number density of intermetallic compounds having a circle equivalent diameter of 0.10 μm to less than 1.00 μm, which become nuclei of recrystallization, is 1.0×10 ³ pieces/mm ² to 3.5×10 ³ pieces/mm ² .

The present invention provides an aluminum alloy sheet for can bodies that reduces the circumferential anisotropy of the wall thickness at the opening of the formed drawn cup.

In this specification, the term "process" includes not only an independent process, but also a process that cannot be clearly distinguished from other processes, as long as the intended purpose of the process is achieved. Furthermore, the content of each component in the composition means the total amount of the multiple substances present in the composition when multiple substances corresponding to each component are present in the composition, unless otherwise specified. Furthermore, the upper and lower limits of the numerical ranges described in this specification can be arbitrarily selected and combined from the numerical values exemplified as numerical ranges. Hereinafter, the embodiments of the present invention will be described in detail. However, the embodiments shown below are examples of aluminum alloy sheets for can bodies to embody the technical concept of the present invention, and the present invention is not limited to the aluminum alloy sheets for can bodies shown below.

The aluminum alloy sheet for can bodies according to one embodiment of the present invention is made of, for example, an Al-Mn-Mg alloy, an Al-Mg-Mn alloy, etc. Examples of the Al-Mn-Mg alloy, an Al-Mg-Mn alloy, etc. include general JIS alloys such as 3004 and 3104.

Specifically, the aluminum alloy sheet for can bodies contains Si: 0.16% by mass to 0.60% by mass, Fe: 0.3% by mass to 0.6% by mass, Cu: 0.10% by mass to 0.35% by mass, Mn: 0.5% by mass to 1.2% by mass, Mg: 0.7% by mass to 2.0% by mass, and the balance is Al and unavoidable impurities. In the aluminum alloy sheet for can bodies, the number density of intermetallic compounds having a circle equivalent diameter of 0.10 μm to less than 1.00 μm is 1.7×10 ⁵ pieces/mm ² to 4.0×10 ⁵ pieces/mm ² , and the number density of intermetallic compounds having a circle equivalent diameter of 0.10 μm to less than 1.00 μm, which become nuclei of recrystallization, is 1.0×10 ³ pieces/mm ² to 3.5×10 ³ pieces/mm ² .

The aluminum alloy sheet for the can body contains Si, Fe, Cu, Mn, and Mg in a predetermined range, and the number density of the intermetallic compounds whose circle equivalent diameters in the structure observation are in a predetermined range and the number density of the intermetallic compounds that become the nuclei of recrystallization are each in a predetermined range, so that the circumferential anisotropy of the wall thickness at the opening of the drawn cup to be formed is reduced. This is thought to be because, for example, the Cube orientation density, which affects the circumferential wall thickness difference of the drawn cup, is appropriately controlled, so that the circumferential wall thickness difference of the drawn cup is reduced. Specifically, for example, the Cube orientation density has a large effect on the circumferential wall thickness distribution of the side wall thickness of the drawn cup. For example, if the Cube orientation density is small, the drawn cup side wall will be thin in the 45° direction with respect to the rolling direction and thick in the 90° direction, so that the circumferential wall thickness difference will be large. Also, if the Cube orientation density is large, the drawn cup side wall will be thin in the 0° direction with respect to the rolling direction, so that the circumferential wall thickness difference will be large. Furthermore, by adjusting the Si content within a predetermined range, the number density of intermetallic compounds with a circle equivalent diameter within a predetermined range and the number density of intermetallic compounds with a circle equivalent diameter that serves as the nucleus of recrystallization are set within a predetermined range, which is thought to achieve an appropriate Cube orientation density and reduce the circumferential thickness difference in the side wall thickness of the drawn cup.

The content of each component in aluminum alloy plate for can bodies and the reasons for limiting the content are explained below.

Si: 0.16% by mass or more and 0.60% by mass or less When the Si content is less than 0.16% by mass, the number density of intermetallic compounds having a circle equivalent diameter of 0.10 μm or more and less than 1.00 μm becomes small, so the Cube orientation density becomes small, and in the produced drawn cup, the side wall thickness in the 45° direction relative to the rolling direction becomes thin, and the side wall thickness in the 90° direction becomes thick, so that the wall thickness difference in the circumferential direction of the drawn cup becomes large. Also, when it exceeds 0.60% by mass, the number density of intermetallic compounds having a circle equivalent diameter of 0.10 μm or more and less than 1.00 μm becomes large, so the Cube orientation density becomes large, and in the produced drawn cup, the side wall thickness in the 0° direction relative to the rolling direction becomes thin, so that the wall thickness difference in the circumferential direction of the drawn cup becomes large. The lower limit of the Si content is preferably 0.18% by mass or more, more preferably 0.20% by mass or more, even more preferably 0.24% by mass or more, and particularly preferably 0.3% by mass or more. The upper limit of the Si content may be preferably 0.56 mass % or less, more preferably 0.54 mass % or less, and further preferably 0.50 mass % or less.

Fe: 0.3% by mass or more and 0.6% by mass or less If the Fe content is less than 0.3% by mass, the intermetallic compounds (e.g., Al-Fe-Mn intermetallic compounds) are insufficient, and surface defects (seizure) may occur on the side wall during ironing. If the Fe content exceeds 0.6% by mass, the amount of large-sized intermetallic compounds increases, and breakage may occur during ironing. The lower limit of the Fe content is preferably 0.34% by mass or more, more preferably 0.36% by mass or more, and even more preferably 0.38% by mass or more. The upper limit of the Fe content is preferably 0.55% by mass or less, more preferably 0.5% by mass or less, and even more preferably 0.46% by mass or less.

Cu: 0.10% by mass or more and 0.35% by mass or less If the Cu content is less than 0.10% by mass, the strength (post-baking yield strength) is insufficient, and the pressure resistance of the can is insufficient. If the Cu content exceeds 0.35% by mass, the strength becomes too high and the formability may decrease. The lower limit of the Cu content is preferably 0.14% by mass or more, more preferably 0.16% by mass or more, and even more preferably 0.18% by mass or more. The upper limit of the Cu content is preferably 0.33% by mass or less, more preferably 0.30% by mass or less, and even more preferably 0.26% by mass or less.

Mn: 0.5% by mass or more and 1.2% by mass or less When the Mn content is less than 0.5% by mass, the strength (post-baking yield strength) is insufficient, and the pressure resistance of the can is insufficient. When the Mn content exceeds 1.2% by mass, the amount of large-sized intermetallic compounds increases, and breakage may occur during ironing. The lower limit of the Mn content is preferably 0.6% by mass or more, more preferably 0.8% by mass or more, and even more preferably 0.9% by mass or more. The upper limit of the Mn content is preferably 1.15% by mass or less, more preferably 1.10% by mass or less, even more preferably 1.08% by mass or less, and particularly preferably 1.06% by mass or less.

Mg: 0.7% by mass or more and 2.0% by mass or less When the Mg content is less than 0.7% by mass, the strength (post-baking yield strength) is insufficient, and the pressure resistance of the can is insufficient. When the Mg content exceeds 2.0% by mass, the strength becomes too high and the moldability may decrease. The lower limit of the Mg content is preferably 0.8% by mass or more, more preferably 0.9% by mass or more, and even more preferably 0.94% by mass or more. The upper limit of the Mg content is preferably 1.6% by mass or less, more preferably 1.4% by mass or less, and even more preferably 1.35% by mass or less.

Cr: 0.01% by mass or more and 0.10% by mass or less As is generally known, the aluminum alloy sheet may contain Cr. The Cr content of 0.01% by mass or more contributes to improving the strength of the material. In addition, if the Cr content is 0.10% by mass or less, it does not affect the material properties of the aluminum alloy sheet and the can properties after ironing. If the Cr content is 0.10% by mass or less, the generation of coarse intermetallic compounds is suppressed, and the deterioration of formability during ironing is suppressed. The upper limit of the Cr content may be preferably 0.05% by mass or less, or 0.03% by mass or less.

Zn: 0.01% by mass or more and 0.40% by mass or less As is generally known, the aluminum alloy sheet may contain Zn. If the content of Zn is 0.40% by mass or less, it does not significantly affect the material properties of the aluminum alloy sheet and the can properties after ironing. Although Zn is an inevitable impurity, Zn can be actively added within the above range. The upper limit of the Zn content is preferably 0.30% by mass or less, more preferably 0.26% by mass or less, more preferably 0.24% by mass or less, and even more preferably 0.20% by mass or less. In addition, the lower limit of the Zn content may be, for example, 0.01% by mass or more, preferably 0.10% by mass or more, and more preferably 0.16% by mass or more.

Ti: 0.005% by mass or more and 0.100% by mass or less As is generally known, the aluminum alloy plate may contain Ti. Ti is added as necessary for the purpose of refining the ingot crystal grains. Refining the ingot structure during casting improves castability and enables high-speed casting. This effect is obtained by adding 0.005% by mass or more. On the other hand, if the Ti content is 0.100% by mass or less, the generation of coarse intermetallic compounds can be suppressed. In addition, when Ti is added, for example, an ingot refiner (Al-Ti-B) may be added to the molten metal before casting. In this case, if the mass ratio of Ti to B is 5:1, for example, B according to the content ratio is also inevitably added, but this does not affect the characteristics. The upper limit of the Ti content is preferably 0.080% by mass or less, more preferably 0.060% by mass or less, and even more preferably 0.040% by mass or less. The lower limit of the Ti content may be, for example, 0.005 mass % or more.

The total content of Cr and Ti is 0.01 mass% or more and 0.150 mass% or less When the total content of Cr and Ti elements is 0.150 mass% or less, the generation of coarse intermetallic compounds is suppressed, and the deterioration of formability is suppressed. As long as the content of Cr and Ti elements does not exceed the upper limit value described above, not only when the aluminum alloy contains one or more kinds, i.e., only one kind, but also when the aluminum alloy contains two or more kinds, the effect of the present invention is naturally suppressed from being hindered.

(balance: Al and inevitable impurities)
The aluminum alloy sheet for can bodies may contain inevitable impurities in addition to Al and the above alloy components. Examples of inevitable impurities include V, Na, Zr, Ni, Ca, In, Sn, and Ga. The upper limit of the allowable content of inevitable impurities for Zr may be, for example, 0.3 mass% or less, preferably 0.1 mass% or less, and more preferably 0.05 mass% or less. For elements other than Zr, each may be, for example, 0.05 mass% or less and a total of 0.15 mass% or less. If within the above range, the effects of the present invention are suppressed from being hindered not only when the elements are contained as inevitable impurities, but also when the elements are added.

Number density of intermetallic compounds having a circle-equivalent diameter of 0.10 μm or more and less than 1.00 μm: 1.7×10 ⁵ /mm ² or more and 4.0×10 ⁵ /mm ² or less The number density of intermetallic compounds having a circle-equivalent diameter of 0.10 μm or more and less than 1.00 μm is calculated by observing the structure of a cross section parallel to the rolling direction of an aluminum alloy plate as a backscattered electron composition (COMPO) image. Specifically, the area of each intermetallic compound (e.g., Al-Fe-Mn, Al-Fe-Mn-Si, etc.) identified from the COMPO image is measured, and the number density of the intermetallic compound is calculated as the number per unit area of intermetallic compounds having a circle-equivalent diameter of 0.10 μm or more and less than 1.00 μm calculated from the area.

If the number density of the intermetallic compound having a circle equivalent diameter in a predetermined range is less than 1.7×10 ⁵ pieces/mm ² , the Cube orientation density is small during recrystallization, and in the produced drawn cup, the side wall thickness in the 45° direction relative to the rolling direction is thin and the side wall thickness in the 90° direction is thick, resulting in a large circumferential thickness difference in the drawn cup. If the number density exceeds 4.0×10 ⁵ pieces/mm ² , the Cube orientation density is large during recrystallization, and in the produced drawn cup, the side wall thickness in the 0° direction relative to the rolling direction is thin, resulting in a large circumferential thickness difference in the drawn cup. The lower limit of the number density of the intermetallic compound having a circle equivalent diameter in a predetermined range may be preferably 1.8×10 ⁵ pieces/mm ² or more, more preferably 1.85×10 ⁵ pieces/mm ² or more. The upper limit of the number density of the intermetallic compounds having a circle equivalent diameter in the predetermined range is preferably 3.8×10 ⁵ pieces/mm ² or less, more preferably 3.6×10 ⁵ pieces/mm ² or less, and further preferably 3.2×10 ⁵ pieces/mm ² or less.

Number density of intermetallic compounds having a critical circle equivalent diameter or more that become nuclei of recrystallization: 1.0 × ¹⁰³ particles/ ^mm2 or more and 3.5 × ¹⁰³ particles/mm2 or ^less. The critical circle equivalent diameter η _PSN that becomes nuclei of recrystallization is calculated by the following formula (for example, see HE Vatne, O. Engler, E. Nes: Materials Science and Technology, February, 1997, Vol. 13, pp. 93-102.) using the area fraction f and average radius r of intermetallic compounds having a circle equivalent diameter of 0.10 μm or more and less than 1.00 μm measured from the above COMPO image.
η _PSN = (4/3)/(10 ⁶ - (3f/2r))
In addition, when recrystallization occurs using compounds having a diameter equal to or larger than the critical circle equivalent diameter as nuclei, the crystal orientation of the recrystallized grains is said to be random.

The critical circle equivalent diameter of the intermetallic compound that serves as the nucleus for recrystallization of the aluminum alloy plate for the can body may be, for example, greater than 1.48 μm and not greater than 1.80 μm. The lower limit of the critical circle equivalent diameter of the intermetallic compound may preferably be 1.49 μm or more. The upper limit of the critical circle equivalent diameter of the intermetallic compound may preferably be 1.70 μm or less, more preferably 1.68 μm or less.

If the number density of the intermetallic compounds having a critical circle equivalent diameter or more, which become the nuclei of recrystallization, is less than 1.0 x 10 ³ / mm ² , the Cube orientation density increases during recrystallization, and in the produced drawn cup, the side wall thickness in the 0° direction relative to the rolling direction becomes thin, and the circumferential thickness difference of the drawn cup increases. Also, if it exceeds 3.5 x 10 ³ / mm ² , the Cube orientation density decreases during recrystallization, and in the produced drawn cup, the side wall thickness in the 45° direction relative to the rolling direction becomes thin, and the side wall thickness in the 90° direction becomes thick, and the circumferential thickness difference of the drawn cup increases. The lower limit of the number density of the intermetallic compounds having a critical circle equivalent diameter or more may be preferably 1.6 x 10 ³ / mm ² or more, more preferably 2.0 x 10 ³ / mm ² or more. The upper limit of the number density of intermetallic compounds having a diameter equal to or larger than the critical circle equivalent diameter may be preferably 3.2×10 ³ particles/mm ² or less.

In a drawn cup formed from an aluminum alloy plate for a can body, the variation in circumferential thickness near the opening is suppressed. The variation in circumferential thickness can be evaluated by circumferential anisotropy (%), which is calculated as a percentage by subtracting the maximum value from the minimum value of the thickness measured in the circumferential direction and dividing the result by the average value of the circumferential thickness. The upper limit of the circumferential anisotropy may be, for example, 10.8% or less, and preferably 10.7% or less. The lower limit of the circumferential anisotropy is 0%.

An example of a method for producing an aluminum alloy sheet for a can body will be described below. The method for producing an aluminum alloy sheet for a can body includes a casting step as a first step, a homogenization heat treatment step as a second step, a hot rolling step as a third step, and a cold rolling step as a fourth step, which are carried out in this order.

Steps 1 and 2: Casting and homogenization heat treatment Step 1 is a step of producing an ingot having a target composition by a semi-continuous casting method. Step 2 is a step of performing homogenization heat treatment on the aluminum alloy ingot produced in Step 1.

In the first step, an aluminum alloy is cast by a semi-continuous casting method (DC (direct chill) casting) to obtain an ingot. Next, in the second step, the ingot surface layer is subjected to surface grinding and homogenization heat treatment in the region that will become a non-uniform structure. In the homogenization heat treatment, it is preferable to hold the ingot at a temperature of about 480°C to 600°C for at least 2 hours. After the homogenization heat treatment is completed, the ingot may be cooled to the hot rolling start temperature and hot rolling, which is the third step, may be started, or the ingot may be cooled once to a temperature of 200°C or less, including room temperature, and then reheated to the hot rolling start temperature before starting hot rolling. Surface grinding may be performed before the homogenization heat treatment, or may be performed after the homogenization heat treatment when the ingot is cooled to a temperature of 200°C or less, including room temperature.

Step 3: Hot Rolling Step Step 3 is a step of hot rolling the aluminum alloy ingot that has been subjected to the homogenization heat treatment in Step 2. The thickness of the hot rolled plate obtained by hot rolling is usually set by back-calculating the total rolling reduction ratio by cold rolling from the thickness of the product plate obtained by cold rolling.

The starting temperature of hot rolling may be, for example, 450°C or higher, and preferably 480°C or higher. The lower limit of the coiling temperature, which is the end temperature of hot rolling, may be, for example, 300°C or higher, preferably 330°C or higher, and more preferably 340°C or higher. The upper limit of the coiling temperature may be, for example, 370°C or lower, and preferably 360°C or lower. When the coiling temperature is 370°C or lower, the occurrence of a surface defect called seizure on the surface of the hot rolled plate is suppressed, and the properties of the plate surface are improved.

Fourth step: cold rolling step The fourth step is a step of cold rolling the hot-rolled sheet hot-rolled in the third step. The fourth step may include a first cold rolling step, a first intermediate annealing step, a second cold rolling step, a second intermediate annealing step, and a final cold rolling step.

In the first cold rolling process, the hot rolled sheet after hot rolling is cold rolled to a predetermined sheet thickness. The processing rate (rolling rate) of the first cold rolling process is preferably 40% or more. If the processing rate is 40% or more, the Cube orientation density is prevented from becoming small during recrystallization. As a result, in the formed drawn cup, the side wall thickness in the 45° direction to the rolling direction is prevented from becoming thin, and the side wall thickness in the 90° direction is prevented from becoming thick, and the wall thickness difference in the circumferential direction of the drawn cup is prevented from becoming large. The lower limit of the processing rate of the first cold rolling process is preferably 50% or more, more preferably 60% or more, and even more preferably 70% or more.

In the first intermediate annealing process, intermediate annealing is performed on the first cold rolled sheet. The annealing temperature at this time is preferably in the range of 250°C to 290°C. Within this range, the Cube orientation density is prevented from decreasing during recrystallization. As a result, in the formed drawn cup, the side wall thickness in the 45° direction relative to the rolling direction is prevented from becoming thin, and the side wall thickness in the 90° direction is prevented from becoming thick, thereby preventing the thickness difference in the circumferential direction of the drawn cup from becoming large. The lower limit of the annealing temperature in the first intermediate annealing process may be preferably 260°C or higher. In addition, the upper limit of the annealing temperature in the first intermediate annealing process may be preferably 280°C or lower. It is believed that by performing the first intermediate annealing process at a predetermined annealing temperature, the Cube orientation density is improved in the second intermediate annealing process described later.

In the first intermediate annealing step, the first cold rolled sheet may be heated to the annealing temperature at a temperature increase rate of 30°C/hour or more and 50°C/hour or less. The lower limit of the temperature increase rate may be preferably 35°C/hour or more. The upper limit of the temperature increase rate may be preferably 45°C/hour or less. The time for holding the annealing temperature may be, for example, 2 hours or more and 6 hours or less. The lower limit of the time for holding the annealing temperature may be preferably 3 hours or more. The upper limit of the time for holding the annealing temperature may be preferably 5 hours or less. After holding the annealing temperature, the sheet may be cooled to a predetermined cooling temperature at a temperature decrease rate of, for example, 30°C/hour or more and 50°C/hour or less. The lower limit of the temperature decrease rate may be preferably 35°C/hour or more. The upper limit of the temperature decrease rate may be preferably 45°C/hour or less. The cooling temperature may be, for example, 220°C or less, preferably 210°C or less.

In the secondary cold rolling process, the primary cold rolled sheet after the primary intermediate annealing is cold rolled to a predetermined sheet thickness. The processing rate (rolling rate) of the secondary cold rolling process is preferably 20% or less. If the processing rate is 20% or less, the Cube orientation density is prevented from becoming small during recrystallization. As a result, in the formed drawn cup, the side wall thickness in the 45° direction with respect to the rolling direction is prevented from becoming thin, and the side wall thickness in the 90° direction is prevented from becoming thick, so that the wall thickness difference in the circumferential direction of the drawn cup is prevented from becoming large. The upper limit of the processing rate of the secondary cold rolling process is preferably 18% or less, more preferably 16% or less, and even more preferably 14% or less. The lower limit of the processing rate of the secondary cold rolling process may be, for example, 5% or more.

In the manufacturing method of aluminum alloy sheet for can bodies, if the first intermediate annealing process and the second cold rolling process are not performed, the Cube orientation density will not be sufficiently large in the subsequent second intermediate annealing process. As a result, the sidewall thickness in the 45° direction to the rolling direction of the formed drawn cup will be thin and the sidewall thickness in the 90° direction will be thick, resulting in a large difference in thickness in the circumferential direction of the drawn cup.

In the second intermediate annealing process, intermediate annealing is performed on the second cold rolled sheet. The annealing temperature at this time is preferably in the range of 300°C to 450°C. Within this range, the Cube orientation density is prevented from becoming small during recrystallization. As a result, in the formed drawn cup, the side wall thickness in the 45° direction to the rolling direction is prevented from becoming thin, and the side wall thickness in the 90° direction is prevented from becoming thick, and the wall thickness difference in the circumferential direction of the drawn cup is prevented from becoming large. The lower limit of the annealing temperature in the second intermediate annealing process may be preferably 300°C or more, more preferably 320°C or more. The upper limit of the annealing temperature in the second intermediate annealing process may be preferably 400°C or less, more preferably 380°C or less.

In the second intermediate annealing step, the second cold rolled sheet may be heated to the annealing temperature at a temperature increase rate of 30°C/hour or more and 50°C/hour or less. The lower limit of the temperature increase rate may be preferably 35°C/hour or more. The upper limit of the temperature increase rate may be preferably 45°C/hour or less. The time for holding the annealing temperature may be, for example, 2 hours or more and 6 hours or less. The lower limit of the time for holding the annealing temperature may be preferably 3 hours or more. The upper limit of the time for holding the annealing temperature may be preferably 5 hours or less. After holding the annealing temperature, the sheet may be cooled to a predetermined cooling temperature at a temperature decrease rate of, for example, 30°C/hour or more and 50°C/hour or less. The lower limit of the temperature decrease rate may be preferably 35°C/hour or more. The upper limit of the temperature decrease rate may be preferably 45°C/hour or less. The cooling temperature may be, for example, 240°C or less, preferably 210°C or less.

In the final cold rolling process, the secondary cold rolled sheet after the secondary intermediate annealing is cold rolled to a predetermined sheet thickness. The processing rate of the final cold rolling process is preferably 80% or more and 95% or less. If the processing rate is 80% or more, the strength of the aluminum alloy sheet is sufficient, and the pressure resistance of the can after DI forming and baking is sufficient. In addition, the Cube orientation density after the final cold rolling process is suppressed from becoming too large, and in the formed drawn cup, the side wall thickness in the 0° direction to the rolling direction is suppressed from becoming thin, and the wall thickness difference in the circumferential direction of the drawn cup is reduced. In addition, if the processing rate is 95% or less, the strength of the aluminum alloy sheet is suppressed from becoming too large, and the deterioration of formability is suppressed. In addition, the Cube orientation density after the final cold rolling process is prevented from becoming too small, and in the formed drawn cup, the side wall thickness in the 45° direction relative to the rolling direction is prevented from becoming thin, and the side wall thickness in the 90° direction is prevented from becoming thick, so that the wall thickness difference in the circumferential direction of the drawn cup is prevented from becoming large. The lower limit of the processing rate in the final cold rolling process may preferably be 85% or more. The upper limit of the processing rate in the final cold rolling process may preferably be 90% or less.

In the first cold rolling process, the second cold rolling process, and the final cold rolling process, multiple passes may be set to achieve a predetermined total rolling ratio so that the plate is rolled to the desired thickness within an appropriate load range. Note that a pass refers to the plate being rolled by passing it between a pair of work rolls once.

In the manufacturing method of aluminum alloy sheet for can bodies, a finish annealing process may be performed after cold rolling, if necessary. In the finish annealing process, the cold-rolled sheet after the final cold rolling process is held at a predetermined finish annealing temperature for a predetermined time. The finish annealing temperature may be, for example, 100°C or higher and 200°C or lower. The time for holding the finish annealing temperature may be, for example, 2 hours or higher and 4 hours or lower.

The above describes the embodiments of the present invention. Below, examples that confirm the effects of the present invention will be specifically described in comparison with comparative examples that do not satisfy the requirements of the present invention. Note that the present invention is not limited to these examples.

Preparation of aluminum alloy plate for can body Molten aluminum alloy (No. 1 to No. 8) having the composition shown in Table 1 was poured into a mold to prepare an ingot with a thickness of 50 mm. The surface layer of the prepared ingot was chamfered to a thickness of 45 mm. The ingot after chamfering was placed in an atmospheric furnace, heated from room temperature to 600°C at a heating rate of 40°C/hour, held at 600°C for 10 hours, cooled to 500°C at a cooling rate of 10°C/hour, and then held at 500°C for 10 hours. Immediately after completion of the holding, hot rolling was started to obtain a hot rolled plate with a thickness of 10 mm.

The obtained hot-rolled sheet was subjected to a first cold rolling process so that the sheet thickness was 2.7 mm (processing rate: 73%) to obtain a first cold-rolled sheet. The obtained first cold-rolled sheet was heated from room temperature to 270°C at a heating rate of 40°C/hour, held at 270°C for 4 hours, cooled to 200°C at a cooling rate of 40°C/hour, and then air-cooled.

Next, a secondary cold rolling process was carried out to obtain a secondary cold-rolled sheet with a thickness of 2.4 mm (processing rate: 11%). The obtained secondary cold-rolled sheet was heated from room temperature to 340°C at a heating rate of 40°C/hour, held at 340°C for 4 hours, cooled to 200°C at a cooling rate of 40°C/hour, and then air-cooled. Next, a final cold rolling process was carried out to obtain a cold-rolled sheet with a thickness of 0.3 mm (processing rate: 87.5%). The cold-rolled sheet was then placed in an atmospheric furnace heated to 150°C, held at 150°C for 3 hours, and a finish annealing process was carried out to produce an aluminum alloy sheet for can bodies with a thickness of 0.3 mm.

No. 1 to No. 5 correspond to examples, and No. 6 to No. 8 correspond to comparative examples.

Post-baking proof stress Each aluminum alloy plate prepared was subjected to a heat treatment (baking treatment) at 200°C for 20 minutes. A tensile test piece was taken from the heat-treated plate, with the width and length reduced to half that of the JIS No. 5 test piece specified in JIS Z 2241 (2011). At this time, the rolling direction was taken as the longitudinal direction of the test piece. A tensile test was performed using this test piece to determine the 0.2% proof stress, which was taken as the post-baking proof stress. A post-baking proof stress of 200 MPa or more and 300 MPa or less was rated A (pass), and a post-baking proof stress of less than 200 MPa was rated B (fail). The results are shown in Table 2.

Number density of intermetallic compounds with a circle-equivalent diameter of 0.10 μm or more and less than 1.00 μm A cross section parallel to the rolling direction of each of the produced aluminum alloy sheets was filled with resin and polished to prepare a sample for cross-sectional observation. COMPO images (composition images) were taken from 10 fields of view using a scanning electron microscope "JSM-7001F" manufactured by JEOL Ltd. under conditions of an acceleration voltage of 15 kV and a magnification of 5000 times. The area of each Al-Fe-Mn or Al-Fe-Mn-Si intermetallic compound was measured using the obtained COMPO images. The circle-equivalent diameter was calculated from the area of each intermetallic compound, and the number per unit area of intermetallic compounds with a circle-equivalent diameter of 0.10 μm or more and less than 1.00 μm was calculated to obtain the number density. Those having a number density of 1.7×10 ⁵ particles/mm ² or more and less than 4.0×10 ⁵ particles/mm ² with a circle equivalent diameter of 0.10 μm or more and less than 1.00 μm were rated as A (pass), and those having a number density of less than 1.7×10 ⁵ particles/mm ² or more than 4.0×10 ⁵ particles/mm ² were rated as B (fail). The results are shown in Table 2.

Number density of intermetallic compounds with a critical circle equivalent diameter or more that become nuclei of recrystallization The area ratio f and average radius r of the intermetallic compounds were calculated for the intermetallic compounds with a circle equivalent diameter of 0.10 μm or more and less than 1.00 μm obtained by the above-mentioned method. The area ratio f was calculated as the ratio of the total area of the intermetallic compounds to the area of the observation field. The average radius r was calculated as the arithmetic mean of the circle equivalent diameters of the intermetallic compounds. From the calculated area ratio f and average radius r of the intermetallic compounds, the critical circle equivalent diameter η _PSN of the intermetallic compounds that become nuclei of recrystallization was calculated for each sample by the following formula (see HE Vatne, O. Engler, E. Nes: Materials Science and Technology, February, 1997, Vol.13, pp.93-102.).
η _PSN = (4/3)/(10 ⁶ - (3f/2r))

Next, a cross section parallel to the rolling direction of each of the prepared aluminum alloy sheets was filled with resin and polished to prepare a sample for cross-sectional observation. COMPO images (composition images) were taken in 20 fields of view using a scanning electron microscope "JSM-7001F" manufactured by JEOL Ltd. under conditions of an acceleration voltage of 15 kV and a magnification of 500 times. The area of each Al-Fe-Mn or Al-Fe-Mn-Si intermetallic compound was measured using the obtained COMPO images. The circle equivalent diameter was calculated from the area of each intermetallic compound, and the number per unit area of intermetallic compounds with the previously obtained critical circle equivalent diameter η _PSN or more was calculated to obtain the number density. Samples with this number density of 1.0 × 10 ³ pieces/mm ² or more and 3.5 × 10 ³ pieces/mm ² or less were rated A (pass), and samples with a number density of less than 1.0 × 10 ³ pieces/mm ² or more and 3.5 × 10 ³ pieces/mm ² or less were rated B (fail). The results are shown in Table 2.

Circumferential anisotropy of the opening thickness of the drawn cup A blank with a diameter of 66.7 mm was punched out from the aluminum alloy plate with a thickness of 0.3 mm produced, and the blank was drawn under a pressure of 3 kN to produce a drawn cup with a diameter of 40 mm (hereinafter, also simply referred to as a "cup") (drawing ratio 1.67). Using the produced cup, the thickness near the cup opening (position 17 mm high from the bottom) was measured at 22.5° intervals in the circumferential direction. That is, the rolling direction was set to 0°, and the thickness was measured in the 0° direction, 22.5° direction, 45° direction, 67.5° direction, 90° direction, 112.5° direction, 135° direction, 157.5° direction, 180° direction, 202.5° direction, 225° direction, 247.5° direction, 270° direction, 292.5° direction, 315° direction, and 337.5° direction. From the data of a total of 16 points obtained, the average value of the thickness in the 0 ° direction and the 180 ° direction was calculated as the 0 ° average value; the average value of the thickness in the 22.5 ° direction, the 157.5 ° direction, the 202.5 ° direction and the 337.5 ° direction was calculated as the 22.5 ° average value; the average value of the thickness in the 45 ° direction, the 135 ° direction, the 225 ° direction and the 315 ° direction was calculated as the 45 ° average value; the average value of the thickness in the 67.5 ° direction, the 112.5 ° direction, the 247.5 ° direction and the 292.5 ° direction was calculated as the 67.5 ° average value; and the average value of the thickness in the 90 ° direction and the 270 ° direction was calculated as the 90 ° average value. Furthermore, from these five numerical values, the maximum value (maximum thickness), the minimum value (minimum thickness) and the average value (average thickness) were calculated, and the circumferential anisotropy (%) was calculated from the following formula.
Circumferential anisotropy = (maximum thickness - minimum thickness) / average thickness x 100 [%]
The samples with circumferential anisotropy calculated from the above formula of 10.8% or less were rated as A (pass), and the samples with circumferential anisotropy exceeding 10.8% were rated as B (fail). The results are shown in Table 2.

As shown in Tables 1 and 2, Nos. 1 to 5, in which the Si content is 0.16 mass% or more and 0.60 mass% or less, the number density of intermetallic compounds having a circle equivalent diameter of 0.10 μm or more and less than 1.00 μm is 1.7 × 10 ⁵ pieces / mm ² or more and 4.0 × 10 ⁵ pieces / mm ² or less, and the number density of intermetallic compounds having a critical circle equivalent diameter or more is 1.0 × 10 ³ pieces / mm ² or more and 3.5 × 10 ³ pieces / mm ² or less, had small circumferential anisotropy and were excellent.

Claims (3)

Si: 0.16 mass% or more and 0.60 mass% or less, Fe: 0.3 mass% or more and 0.6 mass% or less, Cu: 0.10 mass% or more and 0.35 mass% or less, Mn: 0.5 mass% or more and 1.2 mass% or less, Mg: 0.7 mass% or more and 2.0 mass% or less, and the remainder consists of Al and inevitable impurities,
In a structure observation, the number density of intermetallic compounds having an equivalent circle diameter of 0.10 μm or more and less than 1.00 μm is 1.7 × 10 ⁵ pieces/mm ² or more and 4.0 × 10 ⁵ pieces/mm ² or less,
An aluminum alloy sheet for can bodies, in which the number density of intermetallic compounds having a critical circle equivalent diameter or more, which act as nuclei for recrystallization, is 1.0 x ¹⁰³ particles/ ^mm2 or more and 3.5 x ¹⁰³ particles/mm2 or ^less . Further comprising at least one component selected from the group consisting of Cr, Zn and Ti;
2. The aluminum alloy sheet for can bodies according to claim 1, wherein the contents of Cr, Zn and Ti are, respectively, Cr: 0.01% by mass or more and 0.10% by mass or less, Zn: 0.01% by mass or more and 0.40% by mass or less, and Ti: 0.005% by mass or more and 0.100% by mass or less. Further comprising at least one component selected from the group consisting of Cr and Ti;
The Cr and Ti contents are Cr: 0.01 mass% or more and 0.10 mass% or less, and Ti: 0.005 mass% or more and 0.100 mass% or less, respectively;
2. The aluminum alloy sheet for can bodies according to claim 1, wherein the total content of Cr and Ti is 0.01% by mass or more and 0.150% by mass or less.