Aluminum alloy sheet and method for producing same

Abstract

To provide an aluminum alloy sheet excellent in rollability.SOLUTION: 0.05% by mass or more and 0.35% by mass or less of Si, 0.05% by mass or more and 0.50% by mass or less of Fe, 0.01% by mass or more and 0.35% by mass or less of Cu, 0.10% by mass or more and 0.80% by mass or less of Mn, 2.0% by mass or more and 6.0% by mass or less of Mg, 0.01% by mass or more and 0.15% by mass or less of Cr, and a balance of Al and inevitable impurities, in a cross section in a sheet thickness direction, a ratio of an area occupied by central grains is 1.15% or less, and in a sheet thickness Mg2Si region, a number-density ratio obtained by dividing a number-density of precipitates observed with a transmission-type electron microscope by a number-density in a reference aluminum alloy sheet, a Si-amount ratio obtained by dividing a Si content by a reference Si content, and a Mg-amount ratio obtained by dividing a Mg content by a reference Mg content satisfy the following formulas: Number density ratio ≥ 10.929 * Si amount ratio - 15.814 + 2.043 * (Mg amount ratio - 1) SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to an aluminum alloy sheet and a method for producing the same. [Background technology]

[0002] Two-piece aluminum cans, consisting of a cylindrical body with a bottom and a lid, are widely used as beverage packaging containers. The lid and pull tab of two-piece aluminum cans are primarily made of 5182 aluminum alloy (AA or JIS 5182 alloy), while the body is primarily made of 3104 alloy (AA or JIS 3104 alloy).

[0003] In recent years, growing environmental awareness has led to a demand for aluminum alloy sheets with low CO2 emissions. Although a large amount of CO2 is emitted during the refining process of aluminum from virgin metal, CO2 emissions are extremely low when aluminum scrap is blended, remelted, and cast, and improving the aluminum recycling rate will contribute to reducing CO2 emissions.

[0004] In two-piece aluminum cans, the can lid and can body are difficult to separate because they are seamed together. Therefore, the recycled UBC produced by remelting can scrap (UBC: Used Beverage Can) is a mixture of the material components of the can lid and can body, and contains more Si, Fe, Cu, Mn, and other elements found in 3104 alloy than 5182 alloy. However, 5182 alloy has lower upper limits for Si, Fe, Cu, Mn, and other elements than 3104 alloy, making it difficult to incorporate recycled UBC into the can lid and pull tab material, resulting in a low recycling rate.

[0005] As a measure to improve the recycling rate of aluminum alloy sheets for can ends and pull tabs, a technique has been proposed in which the material compositions of the aluminum alloy sheets for can ends and pull tabs are made closer to that of 3104 alloy, making it easier to blend recycled UBC ingots (see, for example, Patent Documents 1 to 4). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent No. 7420994 [Patent Document 2] Patent No. 7420995 [Patent Document 3] Patent No. 7420996 [Patent Document 4] Japanese Patent Publication No. 2023-131622 Summary of the Invention [Problem to be solved by the invention]

[0007] Problems associated with using aluminum alloy sheets for can lids and pull tabs with compositions similar to those of 3104 alloy include reduced formability and reduced pressure strength for can lids. Regarding pressure strength, it is possible to achieve a similar level to conventional alloys by increasing the amount of solute magnesium through annealing or other measures. However, achieving formability equivalent to that of 5182 alloy while maintaining pressure strength is difficult. In fact, Patent Documents 1 to 4 do not directly compare formability with 5182 alloy. When aluminum alloy sheets for can lids and pull tabs have compositions similar to those of 3104 alloy, the inferior formability compared to 5182 alloy must be compensated for by changing the shape of the can lids and pull tabs and by changing the forming method. However, changing the molds used in the can-making process increases costs, so there is a need for materials that can ensure both pressure strength and formability using the same molds and forming methods as conventional alloys.

[0008] When UBC recycled ingots are blended with 5182 alloy to a composition exceeding the upper limit of the 5182 alloy standard, it is known that cracks (edge ​​cracks) are more likely to occur at the widthwise edge of the plate during rolling, particularly as the Si content increases, causing the plate to break during rolling and resulting in major productivity issues.

[0009] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide an aluminum alloy sheet having excellent rollability even when recycled UBC blocks are blended therein and the Si content is high, and a manufacturing method thereof. [Means for solving the problem]

[0010] A first aspect is an aluminum alloy sheet containing from 0.05 to 0.35% by mass of Si, from 0.05 to 0.50% by mass of Fe, from 0.01 to 0.35% by mass of Cu, from 0.10 to 0.80% by mass of Mn, from 2.0 to 6.0% by mass of Mg, and from 0.01 to 0.15% by mass of Cr, with the balance being Al and unavoidable impurities. In the aluminum alloy sheet, the ratio of the area occupied by Mg2Si particles in a thickness central region of a cross section in the thickness direction parallel to the rolling direction is 1.15% or less. In the aluminum alloy sheet, in a region 50 nm thick from the center of the sheet thickness in a plane parallel to the rolled surface in both thickness directions, a number density ratio is a value obtained by dividing the number density of precipitates observed with a transmission electron microscope by a reference number density that is the number density of precipitates in a reference aluminum alloy sheet obtained with a Si content of 0.20 mass%, a Mg content of 4.8 mass%, and a hot rolling time from the start of rough hot rolling to the completion of finish hot rolling of 1242 seconds, a Si amount ratio is a value obtained by dividing the Si content of the aluminum alloy sheet by the reference Si content of 0.20 mass%, and a Mg amount ratio is a value obtained by dividing the Mg content of the aluminum alloy sheet by the reference Mg content of 4.8 mass%, which satisfy the following relational formula:

[0011] Number density ratio ≥ 10.929 × Si content ratio - 15.814 + 2.043 × (Mg content ratio - 1)

[0012] A second aspect is the method for producing an aluminum alloy sheet according to the first aspect, comprising the steps of: casting an ingot, homogenizing the ingot, hot-rolling the homogenized ingot, cold-rolling the ingot after the hot rolling, heat-treating the ingot during the cold rolling, and degreasing and chemical conversion treating the cold-rolled sheet material and baking a paint coating. In the method for producing an aluminum alloy sheet, a hot-rolling time ratio, which is a value obtained by dividing a hot-rolling time from the start of rough hot rolling to the completion of finish hot rolling by a standard hot-rolling time, an Si amount ratio, which is a value obtained by dividing a Si content of the aluminum alloy sheet by 0.13% by mass which is a standard Si content, and an Mg amount ratio, which is a value obtained by dividing a Mg content of the aluminum alloy sheet by 4.8% by mass which is a standard Mg content, satisfy the following relational expression:

[0013] Hot rolling time ratio≧1.02586×Si amount ratio-0.02586+0.19177×(Mg amount ratio-1)

[0014] A third aspect is a method for producing an aluminum alloy sheet, comprising the steps of: casting an ingot, homogenizing the ingot, hot-rolling the homogenized ingot, cold-rolling the hot-rolled ingot without any heat treatment therebetween, and degreasing and chemical conversion treating the cold-rolled sheet material, and baking a paint coating on the cold-rolled sheet material. In the method for producing an aluminum alloy sheet, a hot-rolling time ratio, which is a value obtained by dividing a hot-rolling time from the start of rough hot rolling to the completion of finish hot rolling by a standard hot-rolling time, an Si amount ratio, which is a value obtained by dividing a Si content of the aluminum alloy sheet by 0.13 mass% which is a standard Si content, and an Mg amount ratio, which is a value obtained by dividing a Mg content of the aluminum alloy sheet by 4.8 mass% which is a standard Mg content, satisfy the following relational expression:

[0015] Hot rolling time ratio≧1.02586×Si amount ratio-0.02586+0.19177×(Mg amount ratio-1) [Effects of the Invention]

[0016] According to the present invention, an aluminum alloy sheet having excellent rollability and a method for producing the same can be provided. [Brief explanation of the drawings]

[0017] [Figure 1] 1 is a graph showing the relationship between the Si content in an aluminum alloy plate and the ratio of the area occupied by Mg2Si particles detected in the central region of the plate thickness of the aluminum alloy plate. [Figure 2] 1 is a graph showing the relationship between the hot rolling time required for hot rolling during production of an aluminum alloy plate and the ratio of the area occupied by Mg2Si particles detected in the central region of the plate thickness of the aluminum alloy plate. [Figure 3] 1 is a graph showing the relationship between the hot rolling time required for hot rolling during production of an aluminum alloy plate and the number density of precipitates observed with a transmission electron microscope in a 100 nm-thick region at the center of the plate thickness of the aluminum alloy plate. [Figure 4] 1 is a graph showing the relationship between the Mg content in an aluminum alloy plate and the ratio of the area occupied by Mg2Si particles detected in the central region of the plate thickness of the aluminum alloy plate. [Figure 5] 1 is a graph showing the relationship between a Si amount ratio, which is a value obtained by dividing the Si content in an aluminum alloy plate by the Si content of a reference aluminum alloy plate, and a hot rolling time ratio, which is a value obtained by dividing a hot rolling time required for hot rolling in production of the aluminum alloy plate by the hot rolling time required for hot rolling in production of the reference aluminum alloy plate. [Figure 6] 1 is a graph showing the relationship between a Mg amount ratio, which is a value obtained by dividing the Mg content in an aluminum alloy plate by the Mg content of a reference aluminum alloy plate, and a hot rolling time ratio, which is a value obtained by dividing a hot rolling time required for hot rolling in production of the aluminum alloy plate by the hot rolling time required for hot rolling in production of the reference aluminum alloy plate. [Figure 7] This is an example of a transmission electron microscope image of a 100 nm thick region at the center of the thickness of an aluminum alloy plate. DETAILED DESCRIPTION OF THE INVENTION

[0018] In this specification, the term "process" refers not only to an independent process, but also to a process that cannot be clearly distinguished from other processes, as long as the intended purpose of the process is achieved. 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, embodiments of the present invention will be described in detail. However, the embodiments described below are intended to exemplify an aluminum alloy sheet and a method for manufacturing the same in order to embody the technical concept of the present invention, and the present invention is not limited to the aluminum alloy sheet and the method for manufacturing the same described below.

[0019] Aluminum alloy plate The aluminum alloy sheet has an alloy composition containing from 0.05 to 0.35% by mass of Si, from 0.05 to 0.50% by mass of Fe, from 0.01 to 0.35% by mass of Cu, from 0.10 to 0.80% by mass of Mn, from 2.0 to 6.0% by mass of Mg, and from 0.01 to 0.15% by mass of Cr, with the balance being Al and unavoidable impurities. The aluminum alloy sheet has an area ratio (hereinafter also referred to as "Mg2Si area ratio") of 1.15% or less in a thickness central region of a cross section in the thickness direction parallel to the rolling direction of the aluminum alloy sheet. Furthermore, in the aluminum alloy sheet, in a region 50 nm thick from the center of the sheet thickness in a plane parallel to the rolled surface in both thickness directions, a number density ratio which is a value obtained by dividing the number density of precipitates observed in the normal direction of the rolled surface with a transmission electron microscope by a reference number density which is the number density of precipitates in a reference aluminum alloy sheet obtained with an Si content of 0.20 mass%, an Mg content of 4.8 mass%, and a hot rolling time from the start of rough hot rolling to the completion of finish hot rolling of 1242 seconds, an Si amount ratio which is a value obtained by dividing the Si content of the aluminum alloy sheet by 0.20 mass%, which is the Si content of the reference aluminum alloy sheet, and an Mg amount ratio which is a value obtained by dividing the Mg content of the aluminum alloy sheet by 4.8 mass%, which is the Mg content of the reference aluminum alloy sheet, satisfy the following relational formula:

[0020] Number density ratio ≥ 10.929 × Si content ratio - 15.814 + 2.043 × (Mg content ratio - 1)

[0021] An aluminum alloy sheet having a specific alloy composition, in which the ratio of the area occupied by MgSi particles in the central thickness region is equal to or less than a predetermined value, and in which the number density ratio, Si content ratio, and Mg content ratio satisfy a predetermined relationship, has sufficient pressure resistance and, despite its high Si content, exhibits excellent rollability, and in particular, can suppress edge cracking during rolling. This allows the same molds and forming methods as conventional to be used when the aluminum alloy sheet is applied to can ends or pull tabs. The aluminum alloy sheet may be an aluminum alloy sheet for beverage cans, or may be an aluminum alloy sheet for can ends or pull tabs. Furthermore, the aluminum alloy sheet can satisfy the properties required for can ends and pull tabs, such as formability to withstand processing, pressure resistance to withstand internal pressure after filling with a beverage, openability for normal and easy can opening, tab bending strength, and tab tear strength.

[0022] Aluminum alloy composition The content of each component contained in the aluminum alloy plate and the reasons for limiting the content will be described below.

[0023] (Si: 0.05 mass% or more and 0.35 mass% or less) As is generally known, if the Si content is less than 0.05% by mass, high-purity aluminum ingots are required during casting, increasing costs and limiting the amount of recycled UBC ingots used. Furthermore, if the Si content exceeds 0.35% by mass, edge cracks occur at the edges in the sheet width direction during rolling, resulting in poor rollability. The Si content is preferably 0.33% by mass or less, and more preferably 0.30% by mass or less.

[0024] (Fe: 0.05 mass% or more and 0.50 mass% or less) As is generally known, if the Fe content is less than 0.05% by mass, high-purity aluminum ingots are required during casting, increasing costs and limiting the amount of recycled UBC ingots used. On the other hand, if the Fe content exceeds 0.50% by mass, the amount of Al-Fe-Mn intermetallic compounds increases, resulting in the formation of numerous intermetallic compounds during casting and hot rolling. This promotes the initiation and propagation of cracks, reducing the can-opening load and increasing the likelihood of unintended can opening, as well as reducing tab tearability. The Fe content is preferably 0.10% by mass or more, more preferably 0.15% by mass or more. The Fe content is also preferably 0.40% by mass or less, and even more preferably 0.35% by mass or less.

[0025] (Cu: 0.01 mass% or more and 0.35 mass% or less) If the Cu content is less than 0.01% by mass, the strength is insufficient, resulting in insufficient pressure resistance when used for can lids. On the other hand, if the Cu content exceeds 0.35% by mass, the strength becomes excessively high, resulting in reduced formability. The Cu content is preferably 0.02% by mass or more, more preferably 0.03% by mass or more. The Cu content is also preferably 0.30% by mass or less, and even more preferably 0.25% by mass or less.

[0026] (Mn: 0.10 mass% or more and 0.80 mass% or less) If the Mn content is less than 0.10% by mass, the strength is insufficient, resulting in insufficient pressure resistance and tab breakage strength when used as a can lid. On the other hand, if the Mn content exceeds 0.60% by mass, the strength becomes excessively high and the amount of Al-Fe-Mn intermetallic compounds increases, resulting in reduced formability and tab tear resistance. The Mn content is preferably 0.20% by mass or more. The Mn content is also preferably 0.70% by mass or less, and more preferably 0.60% by mass or less.

[0027] (Mg: 2.0 mass% or more and 6.0 mass% or less) If the Mg content is less than 2.0% by mass, even if a manufacturing process is carried out to increase the amount of dissolved Mg by heat treatment such as intermediate annealing, the strength will be insufficient, resulting in insufficient pressure resistance and tab bending strength of the lid. Furthermore, the work hardening ability of the aluminum alloy sheet will be insufficient, making it prone to necking during forming. On the other hand, if the Mg content exceeds 6.0% by mass, the strength will be excessively high and formability will be reduced. The Mg content is preferably 2.5% by mass or more, more preferably 3.0% by mass or more. Furthermore, the Mg content is preferably 5.5% by mass or less, more preferably 5.0% by mass or less.

[0028] (Cr: 0.01 mass% or more and 0.15 mass% or less) Since the addition of Cr promotes the precipitation of precipitates during hot rolling, if the Cr content is less than 0.01 mass%, precipitation sufficient to effectively reduce the MgSi area ratio is not obtained. On the other hand, if the Cr content exceeds 0.15 mass%, coarse crystallized particles are likely to be formed, which reduces the formability of the aluminum alloy sheet. Therefore, the Cr content is set to 0.01 mass% or more and 0.15 mass% or less. The Cr content is preferably 0.02 mass% or more, more preferably 0.03 mass% or more.

[0029] (Zn: 0.3% by mass or less) The aluminum alloy sheet may contain Zn. For example, scrap heat exchanger material can be used to cast the aluminum alloy sheet containing Zn. Zn does not affect the material properties of the aluminum alloy sheet if the content is 0.3% by mass or less. The Zn content may be 0.01% by mass or more. The Zn content may be preferably 0.27% by mass or less, more preferably 0.25% by mass or less.

[0030] (Ti: 0.1% by mass or less) The aluminum alloy sheet may contain Ti. Ti is added as needed to refine the ingot crystal grains. Refining the ingot structure during casting improves castability and enables high-speed casting. This effect is achieved by adding 0.01% by mass or more. On the other hand, if the Ti content exceeds 0.1% by mass, coarse compounds are formed, resulting in reduced rivet formability and a reduced tab tear load. Therefore, the Ti content in the aluminum alloy may be limited within the above range. When Ti is added, for example, an ingot refiner (Al-Ti-B) with a Ti to B mass ratio of 5:1 is added. Since Ti is added to the molten metal before casting in the form of waffles or rods, B is inevitably added according to the content ratio. The Ti content may be preferably 0.08% by mass or less, more preferably 0.06% by mass or less.

[0031] (Zr: 0.1% by mass or less) The aluminum alloy sheet may contain Zr. If the Zr content is 0.1% by mass or less, it does not affect the material properties of the aluminum alloy sheet. The Zr content may be 0.01% by mass or more. The Zr content may be preferably 0.08% by mass or less, and more preferably 0.06% by mass or less.

[0032] (B: 0.1% by mass or less) The aluminum alloy sheet may contain B. B is added as needed for the purpose of refining the ingot crystal grains. Refining the ingot structure during casting further improves formability. This effect is achieved by adding 0.01% by mass or more. The B content may be 0.01% by mass or more. The B content may be preferably 0.06% by mass or less, and more preferably 0.04% by mass or less.

[0033] (Be: 0.1% by mass or less) The aluminum alloy sheet may contain Be. The inclusion of Be provides antioxidant and inclusion prevention effects. The Be content may be 0.01% by mass or more. The Be content may be preferably 0.08% by mass or less, and more preferably 0.06% by mass or less.

[0034] As long as the contents of the above-mentioned Zn, Ti, Zr, B, and Be do not exceed the upper limit values, the effects of the present invention are naturally prevented from being hindered even when the aluminum alloy contains one or more of them, i.e., only one of them, or two or more of them. That is, the aluminum alloy sheet may contain all of Zn, Ti, Zr, B, and Be at or below the upper limit values, or may contain at least one of them at or below the upper limit values.

[0035] (balance: Al and unavoidable impurities) As is generally known, aluminum alloy sheets may contain inevitable impurities in addition to Al and the above alloy components. Examples of inevitable impurities include V, Na, Ca, Ni, In, Sn, and Ga. The allowable content of the inevitable impurities may be, for example, 0.05 mass% or less for each element and 0.15 mass% or less in total. Within the above ranges, the effects of the present invention are not hindered even when the elements are added, not just when they are contained as inevitable impurities.

[0036] Aluminum alloy sheet structure In the case of an aluminum alloy sheet, by setting the alloy composition as described above and then making the structure of the aluminum alloy sheet into a specific state, it is possible to achieve good rollability even when adding Si in an amount exceeding the upper limit of the Si standard for 5182 alloy, which has been difficult to do in the past.

[0037] In the aluminum alloy sheet, the amount of precipitation of Mg2Si particles in the alloy sheet after hot rolling is set to a predetermined range as the structure of the aluminum alloy sheet. This allows for achieving good pressure resistance and good rollability. In the aluminum alloy sheet after hot rolling, the ratio of the area occupied by Mg2Si particles in the central region of the thickness of a cross section in the thickness direction parallel to the rolling direction (Mg2Si area ratio) may be 1.15% or less, preferably 1% or less, and more preferably 0.8% or less. The Mg2Si area ratio may be, for example, 0.1% or more, preferably 0.15% or more. Specifically, a cross-section of an aluminum alloy plate parallel to the rolling direction and thickness direction is embedded in resin and polished to prepare a cross-section observation sample. Using a scanning electron microscope at an accelerating voltage of 15 kV and a magnification of 500x, COMPO images (composition images) of the region sandwiched between positions 1 / 4t above and below the center of the thickness of the aluminum alloy plate are taken, and the area of ​​Mg2Si particles is measured from the COMPO image and divided by the measured area to calculate the Mg2Si area ratio. Note that 1 / 4t means a thickness that is one-fourth of the thickness (t) of the aluminum alloy plate.

[0038] In addition, in the case of aluminum alloy sheets, the number of precipitated fine compounds in the alloy sheet after hot rolling is increased as the structure of the aluminum alloy sheet, thereby improving the rollability. Specifically, the structure of the central region of the sheet thickness, 50 mm from the center of the sheet thickness in both thickness directions (total thickness 100 nm) in a plane parallel to the rolled surface of the aluminum alloy sheet after hot rolling as a sample, was measured using a transmission electron microscope with a magnification of 20,000 times to measure the 523 μm 2 The area was photographed and the number density of precipitates (particles / μm 2 ) is calculated as the number density of precipitates in the standard aluminum alloy plate (standard number density; particles / μm 2 ) satisfies the following relational expression. Here, the reference aluminum alloy sheet is an aluminum alloy sheet with good rollability, which is obtained by setting the Si content to 0.20 mass%, the Mg content to 4.8 mass%, and setting the hot rolling time, which is the time required from the start of rough hot rolling to the completion of finish hot rolling, to 1242 seconds, and the reference number density is 0.703811 (pieces / μm 2) It is considered that even in an aluminum alloy sheet after hot rolling and then cold rolling to a product plate thickness, there is no significant change in the number density of fine compounds compared to the aluminum alloy sheet after hot rolling. Therefore, it is possible to regard the number density of fine compounds measured in an aluminum alloy sheet of the product plate thickness as the number density of fine compounds in the aluminum alloy sheet after hot rolling.

[0039] Number density ratio ≥ 10.929 × Si content ratio - 15.814 + 2.043 × (Mg content ratio - 1)

[0040] The Si content ratio in the relational expression is the value obtained by dividing the Si content in the target aluminum alloy sheet by 0.20 mass%, which is the Si content in the reference aluminum alloy sheet, and the Mg content ratio is the value obtained by dividing the Mg content in the target aluminum alloy sheet by 4.8 mass%, which is the Mg content in the reference aluminum alloy sheet.

[0041] Since the precipitates that precipitate during hot rolling contain Si, the Mg2Si area fraction can be reduced by allowing the precipitates to precipitate sufficiently. Therefore, if the number density ratio of the precipitates does not satisfy the above formula, the Mg2Si area fraction cannot be sufficiently reduced, resulting in insufficient rollability and causing fractures during rolling of the aluminum alloy sheet. Note that the composition of the Si-containing precipitates is not particularly limited, but the precipitates may contain Cr in addition to Si.

[0042] Manufacturing method A first aspect of the method for producing an aluminum alloy sheet includes a first step of casting an ingot, a second step of homogenizing the ingot, a third step of hot-rolling the homogenized ingot, a fourth step of cold-rolling (first cold-rolling) after the hot-rolling, a fifth step of heat-treating (intermediate annealing) during the cold-rolling, and a sixth step of cold-rolling (second cold-rolling) the heat-treated aluminum alloy sheet. The method for producing an aluminum alloy sheet of the first aspect may further include a seventh step of degreasing and chemical conversion treating the aluminum alloy sheet after the cold-rolling and baking a paint.

[0043] A second aspect of the method for producing an aluminum alloy sheet comprises a first step of casting an ingot, a second step of homogenizing the ingot, a third step of hot-rolling the homogenized ingot, and an eighth step of cold-rolling the hot-rolled ingot without any intermediate heat treatment. The method for producing an aluminum alloy sheet of the second aspect may further comprise a ninth step of degreasing, chemical conversion treating, and baking a paint onto the cold-rolled aluminum alloy sheet.

[0044] As will be described later, by appropriately controlling the hot rolling time, which is the time required from the start of rough hot rolling to the completion of finish hot rolling, in accordance with the Si content and the Mg content, an aluminum alloy sheet having sufficient pressure resistance and excellent rollability can be efficiently produced even when the Si content is relatively high.

[0045] The first step is a casting step in which an ingot having a desired alloy composition is produced by a semi-continuous casting method, and the second step is a homogenization heat treatment step in which the aluminum alloy ingot produced in the first step is subjected to homogenization heat treatment.

[0046] In the first step, an aluminum alloy is cast by a semi-continuous casting method (DC (direct chill) casting) to obtain an ingot. Next, in the second step, a process of removing regions of the ingot surface that would cause a non-uniform structure by facing and a process of performing a homogenization heat treatment are performed. This homogenization heat treatment is performed, for example, in the range of 400°C to 600°C, and also serves as preheating for the subsequent hot rolling.

[0047] The third process is the hot rolling process, in which the aluminum alloy ingot that has been subjected to the homogenization heat treatment in the second process is hot rolled. The hot rolling process is divided into a hot rough rolling process in which the ingot is rolled to a thickness of about several tens of millimeters using a reverse rolling mill, and a hot finish rolling process in which the ingot is rolled to a thickness of about several millimeters using, for example, a tandem mill and then wound into a coil.

[0048] The rough hot rolling process is usually transitioned from the finish hot rolling process without delay from the viewpoint of productivity, and therefore the time from the start of rough hot rolling to the end of finish hot rolling (hereinafter also referred to as "hot rolling time") is set to be as short as possible from the viewpoint of productivity. In this embodiment, the hot rolling time is controlled to satisfy the following relational expression according to the amount of Si and the amount of Mg added to the ingot, whereby precipitates are sufficiently precipitated and the ratio of the area of ​​Mg2Si particles in the cross section of the aluminum alloy sheet is reduced. If the hot rolling time does not satisfy the predetermined relational expression (if the hot rolling time is insufficient), a sufficient amount of precipitation is not obtained, and the ratio of the area of ​​Mg2Si particles increases, resulting in reduced rollability.

[0049] Hot rolling time ratio≧1.02586×Si amount ratio-0.02586+0.19177×(Mg amount ratio-1)

[0050] In the relational expressions, the hot rolling time ratio is a value obtained by dividing the hot rolling time of the aluminum alloy sheet to be produced by 800.2106 seconds, which is a standard hot rolling time. The Si content ratio is a value obtained by dividing the Si content of the aluminum alloy sheet to be produced by 0.13 mass%, which is a standard Si content. The Mg content ratio is a value obtained by dividing the Mg content of the aluminum alloy sheet to be produced by 4.8 mass%, which is a standard Mg content. The standard hot rolling time, standard Si content, and standard Mg content are set based on a standard aluminum alloy sheet that exhibits good rollability.

[0051] The coiling temperature, which is the temperature at which hot rolling is finished, may be, for example, 280°C or higher and 370°C or lower, and preferably 320°C or higher and 370°C or lower. When the coiling temperature is 280°C or higher, the recrystallization rate of the hot-rolled sheet is improved, and the formability of the aluminum alloy sheet is further improved. On the other hand, when the coiling temperature is 370°C or lower, the occurrence of a surface defect called seizure on the sheet surface is suppressed, and the properties of the sheet surface are improved.

[0052] The fourth step in the manufacturing method of the first embodiment is a cold rolling (first cold rolling) step in which the aluminum alloy sheet after hot rolling is cold rolled. In the fourth step, the total rolling reduction is preferably 50% or more to ensure that a fully recrystallized structure is obtained in the subsequent fifth step (intermediate annealing step). If the total rolling reduction in the first cold rolling is less than 50%, the accumulated strain due to rolling is insufficient, and there is a risk that a deformed structure will remain after the fifth step. In this case, there is a risk that the formability of the product sheet will be reduced. Note that the sheet may be finished to the product thickness without intermediate annealing. In this case, the rolling reduction is preferably 85% or more, more preferably 87% or more, to obtain the strength of the product sheet.

[0053] The fifth step in the manufacturing method of the first embodiment is an intermediate annealing step in which the cold-rolled sheet obtained in the fourth step is heat-treated. By performing the intermediate annealing step, the cold-rolled sheet is recrystallized and Cu, Mg, etc. are solid-solved, thereby increasing the strength of the aluminum alloy sheet. The intermediate annealing step preferably includes a heating step at 100°C / min or more, a holding step at 380°C to 550°C for 10 minutes or less, and a cooling step to 100°C at an average rate of 100°C / min or more. If the heating rate in the intermediate annealing step is less than 100°C / min, a holding temperature exceeding 550°C, a holding time exceeding 10 minutes, or an average cooling rate to 100°C less than 100°C / min, there is a concern that the recrystallized grains after heat treatment will become large, impairing the formability of the product sheet. Furthermore, if the holding temperature is less than 380°C, there is a risk that the deformed structure formed in the fourth step (first cold rolling) will remain in the aluminum alloy sheet after the intermediate annealing. In this case, the formability of the finished sheet may be reduced.

[0054] In the sixth step of the manufacturing method of the first embodiment, the intermediate-annealed aluminum alloy sheet obtained in the fifth step is cold-rolled (secondary cold rolling) to finish the sheet into an aluminum alloy sheet of a predetermined thickness. This step is performed by setting multiple passes to achieve a predetermined total rolling reduction. A "pass" refers to a single pass of rolling the sheet between a pair of work rolls. It is preferable that the total rolling reduction of the cold rolling in the sixth step is 50% to 90%, and the final coiling temperature after the final rolling pass is 135°C or higher. When the total rolling reduction of the secondary cold rolling is 50% or higher, sufficient accumulated strain due to rolling is obtained, resulting in sufficient strength of the product sheet. When the total rolling reduction of the secondary cold rolling is 90% or lower, the strength of the product sheet is not excessively high, and the formability of the product sheet is further improved. Furthermore, by setting the final coiling temperature after the final rolling pass to 135°C or higher, static recovery of dislocations after coiling is promoted, resulting in an improved work hardening exponent (n value), resulting in improved formability. The final coiling temperature range may preferably be 140°C or higher, more preferably 150°C or higher.

[0055] The seventh step in the manufacturing method of the first embodiment is a coating step for the aluminum alloy sheet after the secondary cold rolling. In the coating step, the aluminum alloy sheet is subjected to a chemical conversion treatment such as a chromate-based or zirconium-based treatment, and then the coating is baked. The coating baking may be performed by applying a coating such as an epoxy-based coating, a polyester-based coating, or a vinyl chloride-based coating to one or both sides of the aluminum alloy sheet, and baking the coating at a PMT (Peak Metal Temperature) of about 210°C or higher and 270°C or lower. By undergoing this coating step, an aluminum alloy sheet for can ends having a coating film formed on the surface can be obtained.

[0056] The first, second and third steps in the production method of the second embodiment are the same as those in the production method of the first embodiment.

[0057] The eighth step in the manufacturing method of the second embodiment is a step of cold rolling the hot-rolled sheet hot-rolled in the third step. In the eighth step, the hot-rolled sheet is cold-rolled without intermediate annealing to finish into an aluminum alloy sheet of a predetermined thickness. The cold rolling is performed by setting multiple passes to achieve a predetermined total rolling reduction so that the hot-rolled sheet is rolled to the thickness of the product sheet within an appropriate load range. The total rolling reduction of the cold rolling in the eighth step is preferably 85.0% or more, more preferably 90.0% or more. When the total rolling reduction of the cold rolling is 85.0% or more, sufficient strength of the aluminum alloy sheet can be obtained. Furthermore, by setting the final coiling temperature after the final rolling pass to 135°C or more, static recovery of dislocations after coiling is promoted, and formability is improved by increasing the work hardening index (n value). The final coiling temperature may preferably be in the range of 140°C or more, more preferably 150°C or more.

[0058] The ninth step in the manufacturing method of the second embodiment is a coating step for the aluminum alloy sheet after the cold rolling in the eighth step, and is the same as the seventh step in the manufacturing method of the first embodiment. [Example]

[0059] Although the embodiments of the present invention have been described above, examples in which the effects of the present invention have been confirmed will be specifically described below. However, the present invention is not limited to these examples.

[0060] Preparation of test materials Aluminum alloys having the compositions shown in Table 1 were cast using a semi-continuous casting method, and then subjected to facing and homogenization heat treatment using the methods shown as the first and second steps. The hot-rolled sheets were then hot-rolled without cooling. The hot-rolling time from the start of rough hot rolling to the end of finish hot rolling and the thickness of the hot-rolled sheets were as shown in Table 2. The coiling temperature for hot rolling was 300°C or higher and 370°C or lower. The resulting hot-rolled sheets were then cold-rolled. Samples 1 to 8 were cold-rolled to the intermediate annealing thickness shown in Table 2, then subjected to an intermediate annealing step under the conditions shown in Table 2, and then rolled to the product thickness using the cold-rolling reduction ratio shown in Table 2. Sample 9 was cold-rolled to the product thickness using a manufacturing method that did not perform an intermediate annealing step during cold rolling. Samples 10 to 12 were prepared to determine the slope of the change in Mg2Si area fraction when the Mg content was changed, and production was stopped at the hot-rolled sheet stage. The remainder of the composition shown in Table 1 is Al and unavoidable impurities.

[0061] [Table 1]

[0062] [Table 2]

[0063] Plate strength evaluation For Samples 1 to 9, JIS No. 5 tensile test specimens were prepared so that the tensile direction was parallel to the rolling direction. Tensile tests were then conducted in accordance with JIS Z2241 to determine the tensile strength (TS), 0.2% yield strength (YS), and elongation at break (EL). The results are shown in Table 2.

[0064] Evaluation of rollability For Samples 1 to 9 and Sample 13, the rollability was evaluated based on whether or not breakage occurred during rolling. Those in which breakage occurred during rolling were rated as having NG rollability, and those in which no breakage occurred were rated as having OK rollability. The results are shown in Table 2. As shown in Table 2, Sample 13 experienced breakage during cold rolling after hot rolling.

[0065] Mg2Si area ratio For each sample, the central part of the thickness direction cross section parallel to the rolling direction was photographed with a scanning electron microscope at 500x magnification, and the area ratio of Mg2Si particles (Mg2Si area ratio) was calculated. Specifically, a cross section parallel to the rolling direction of each aluminum alloy plate was embedded in resin and polished to prepare a sample for cross-sectional observation. COMPO images (composition images) were taken in 20 fields of view (approximately 0.8 mm) in the region sandwiched between positions 1 / 4t above and below the center of the thickness of the aluminum plate using a scanning electron microscope "JSM-7001F" manufactured by JEOL Ltd. under conditions of an accelerating voltage of 15 kV and a magnification of 500x. 2 ) were photographed. The area of ​​the Mg2Si particles was measured using the obtained COMPO image, and the Mg2Si area ratio was calculated by dividing it by the area of ​​the measurement field. Figure 1 shows the relationship between the Si content (Si amount) and the Mg2Si area ratio for each sample.

[0066] Note that, because Sample 13 broke during rolling, the Mg2Si area ratio was estimated using an approximate equation showing the relationship between the Si content and Mg2Si area ratio obtained from the evaluation results of other samples. Specifically, an approximate equation showing the relationship between the Si content and Mg2Si area ratio, shown in Figure 1, was established from the data of Samples 1 to 8. From the approximate equation, the Mg2Si area ratio of Sample 13, which broke during rolling, was estimated to be 1.24%, and is shown by a white circle in Figure 1. In addition, the upper limit of the Mg2Si area ratio that can ensure rollability was determined to be 1.15%.

[0067] Next, in order to exclude the influence of the Si content, the relationship between the hot rolling time and the Mg2Si area ratio obtained using the data of Samples 1 to 5, which have relatively similar Si contents, is shown in Figure 2.

[0068] Number of precipitates For samples 1, 4, and 5, the structure of a 50 nm thick region from the center of the sheet thickness in both thickness directions was observed using a transmission electron microscope (TEM) at a magnification of 20,000 times from the normal direction of the rolled surface, and the precipitates were counted and the number was calculated. Specifically, the test material was mechanically polished, and then a twin-jet electrolytic polishing method was used to make a thin film of approximately 100 nm, with a thickness of 50 nm from the center of the sheet thickness in both thickness directions. This thin film was then examined using a transmission electron microscope at a magnification of 20,000 times to obtain a 523 μm 2 The area was photographed. The number of precipitates detected within the photographed field of view was calculated. The number of precipitates was divided by the field area to calculate the number density of precipitates. The results are shown in Table 2. The relationship between the hot rolling time and the number density of precipitates is shown in Figure 3.

[0069] Next, to understand the influence of the Mg content (Mg amount) in the aluminum alloy sheet on the Mg2Si area ratio, the Mg2Si area ratio was measured for Samples 10 to 12, in which only the Mg amount was changed. The relationship between the Mg2Si area ratio and the Mg amount is shown in Figure 4.

[0070] Figure 5 shows the relationship between the Si content and hot rolling time for each sample. Figure 5 also shows the relationship between the Si content ratio (the Si content of each sample divided by the reference Si content) and the hot rolling time ratio (the hot rolling time divided by the reference hot rolling time). The dashed lines in Figure 5 indicate the hot rolling times at which the upper limit of the Mg2Si area fraction (1.15%) without fracture during rolling is reached for each Si content when the Mg content is 4.8% by mass, based on the approximation formulas obtained in Figures 1 to 4. The dashed lines in Figure 5 were specifically set as follows. In the relationship between hot rolling time and Mg2Si area fraction shown in Figure 2, the average Si content of the samples shown in Figure 2 is 0.13% by mass. Taking into account the relationship between Si content and Mg2Si area fraction shown in Figure 1, the correction term for the variation in Si content, with 0.13% by mass as the reference Si content, was set to 13.313 × (Si content - 0.13). Furthermore, taking into consideration that the average Mg content of the samples shown in Figure 2 is 4.8 mass%, and the relationship between the Mg content and the Mg2Si area ratio in Figure 4, a correction term for the Mg content fluctuation, with 4.8 mass% as the reference Mg content, was set to 0.0674 × (Mg content - 4.8). From the above, the following formula (1) was derived as the relationship between the Mg2Si area ratio, hot rolling time, Si content, and Mg content. (1) Mg2Si area ratio=-0.0021082675×hot rolling time+2.8370579336+13.313×(Si amount-0.13)+0.0674×(Mg amount-4.8)

[0071] Using equation (1), the hot rolling time at which the Mg2Si area ratio was 1.15% for the standard Si content and standard Mg content was calculated, and this was used as the standard hot rolling time. The standard hot rolling time was 800.2106 seconds. Since the Mg content of the sample shown in Figure 5 is approximately 4.8 mass%, equation (1) was used to set the Mg content to 4.8 mass%, and the hot rolling time at which the Mg2Si area ratio was 1.15% was calculated when the Si content was changed. The Si content and hot rolling time were normalized by the standard Si content and standard hot rolling time, respectively, to obtain an approximate equation showing the relationship between the Si content ratio and hot rolling time ratio, as shown in Figure 5.

[0072] Sample 13, which broke during rolling, is located to the right of the dashed line (the side where the break occurs), and it is believed that the break occurred because the hot rolling time was insufficient relative to the Si content.

[0073] Next, using equation (1), the Si content was set to 0.20 mass%, and the hot rolling time at which the MgSi area ratio became 1.15% was calculated when the MgSi content was varied. The Mg content and hot rolling time were normalized by the reference Mg content and reference hot rolling time, respectively, to obtain an approximate equation showing the relationship between the Mg content ratio and the hot rolling time ratio, as shown in Figure 6.

[0074] The approximate formula showing the relationship between the Si amount ratio and the hot rolling time ratio shown in FIG. 5 was combined with the approximate formula showing the relationship between the Mg amount ratio and the hot rolling time ratio shown in FIG. 6 to obtain the following approximate formula (2) showing the relationship between the hot rolling time ratio and the Si amount ratio and the Mg amount ratio. (2) Hot rolling time ratio≧1.02586×Si amount ratio-0.02586+0.19177×(Mg amount ratio-1)

[0075] As described above, an aluminum alloy sheet having an MgSi area fraction of 1.15% or less and having a precipitate number density ratio, a Si content ratio, and a Mg content ratio that satisfy a predetermined relationship has excellent rollability and can suppress cracks at the sheet width edge during rolling, even when the Si content is relatively high. Furthermore, by adjusting the hot rolling time so that the hot rolling time ratio, the Si content ratio, and the Mg content ratio satisfy a predetermined relationship, an aluminum alloy sheet having excellent rollability can be produced.

Claims

1. Si: 0.05 mass% or more and 0.35 mass% or less, Fe: 0.05 mass% or more and 0.50 mass% or less, Cu: 0.01 mass% or more and 0.35 mass% or less, Mn: 0.10 mass% 0.80% by mass or less, Mg: 2.0% by mass or more and 6.0% by mass or less, Cr: 0.01% by mass or more and 0.15% by mass or less, the remainder consisting of Al and inevitable impurities, In the central region of the thickness of the cross section in the thickness direction parallel to the rolling direction, Mg ２ The ratio of the area occupied by Si particles is 1.15% or less, a number density ratio which is a value obtained by dividing the number density of precipitates observed with a transmission electron microscope in a region 50 nm thick from the center of the sheet thickness in both thickness directions on a surface parallel to the rolled surface by a reference number density which is the number density of precipitates in a reference aluminum alloy sheet obtained with a Si content of 0.20 mass%, a Mg content of 4.8 mass%, and a hot rolling time from the start of rough hot rolling to the completion of finish hot rolling of 1242 seconds; a Si content ratio obtained by dividing the Si content by 0.20 mass% which is the reference Si content; An aluminum alloy plate in which the Mg content and the Mg quantity ratio, which is a value obtained by dividing the Mg content by 4.8 mass% which is a reference Mg content, satisfy the following relational expression: Number density ratio ≧ 10.929 × Si amount ratio − 15.814 + 2.043 × (Mg amount ratio − 1)

2. 2. The aluminum alloy plate according to claim 1, wherein the aluminum alloy plate satisfies at least one selected from the group consisting of: containing Zn and having a Zn content of 0.3% by mass or less; containing Ti and having a Ti content of 0.1% by mass or less; containing Zr and having a Zr content of 0.1% by mass or less; containing B and having a B content of 0.1% by mass or less; and containing Be and having a Be content of 0.1% by mass or less.

3. 3. A method for producing an aluminum alloy sheet according to claim 1, comprising the steps of: casting an ingot; homogenizing the ingot; hot-rolling the homogenized ingot; cold-rolling the ingot after the hot-rolling; and performing heat treatment during the cold-rolling. A hot rolling time ratio, which is the value obtained by dividing the hot rolling time from the start of hot rough rolling to the completion of hot finish rolling by the reference hot rolling time; a Si content ratio obtained by dividing the Si content by 0.13 mass% which is the reference Si content; and a Mg quantity ratio, which is a value obtained by dividing the Mg content by 4.8 mass% which is a reference Mg content, satisfy the following relational expression: Hot rolling time ratio≧1.02586×Si amount ratio-0.02586+0.19177×(Mg amount ratio-1)

4. 3. A method for producing an aluminum alloy sheet according to claim 1, comprising: a step of casting an ingot; a step of homogenizing the ingot; a step of hot-rolling the homogenized ingot; and a step of cold-rolling the ingot after the hot-rolling without performing heat treatment therebetween, A hot rolling time ratio, which is the value obtained by dividing the hot rolling time from the start of hot rough rolling to the completion of hot finish rolling by the reference hot rolling time; a Si content ratio obtained by dividing the Si content by 0.13 mass% which is the reference Si content; and a Mg quantity ratio, which is a value obtained by dividing the Mg content by 4.8 mass% which is a reference Mg content, satisfy the following relational expression: Hot rolling time ratio≧1.02586×Si amount ratio-0.02586+0.19177×(Mg amount ratio-1)

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