AU2013367319B2

AU2013367319B2 - Aluminum alloy sheet for DI can body

Info

Publication number: AU2013367319B2
Application number: AU2013367319A
Authority: AU
Inventors: Yasuhiro Aruga
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 2012-12-27
Filing date: 2013-12-20
Publication date: 2016-11-03
Anticipated expiration: 2033-12-20
Also published as: CN104903481B; KR20150087419A; AU2013367319A1; JP5848694B2; JP2014125677A; KR101718264B1; CN104903481A; WO2014103924A1

Abstract

In this aluminum alloy sheet, the density of a specific set of atoms is regulated in a cold rolling composition manufactured by controlling isothermal conditions and cold rolling conditions for an ingot when manufacturing the aluminum alloy sheet for a 3000 series DI can with a specific composition, thereby promoting subgraining in a can body composition during a coating baking treatment (heat treatment) after can body formation. Thus, the piercing resistance of a can is made superior even in a severe usage environment with lower internal pressure for the can body which is made thin.

Description

DESCRIPTION

Title of Invention: ALUMINUM ALLOY SHEET FOR DI CAN BODY

Technical Field [0001]

The present invention relates to aluminum alloy sheets that are drawn and wall-ironed into bodies of cans as packaging containers for beverages and foodstuffs. Specifically; the present invention relates to an aluminum alloy sheet for drawn and ironed can bodies, where the aluminum alloy sheet is drawn and wall-ironed into beverage can bodies.

Background Art [00021

Two-piece cans are now known as one of packaging containers for beverages or foodstuffs. The two-piece cans each include a bottomed tubular body (can body) and a disc-like lid (can lid or can end), where the body includes a bottom and a side wall as an integral structure, and the lid is sealed to an opening of the body and forms a top lace of the can. The cans often employ, as materials, aluminum alloy sheets typically of Aluminum Association (AA) Standard or Japanese Industrial Standard (JIS) 3xxx series, from the viewpoints typically of formability, corrosion resistance, and strength. Of the two-piece cans produced from aluminum alloy sheets, a body of a tall tubular can such as a beverage can is often formed or shaped by a drawing-ironing multistep process called drawing and wall ironing (DI) forming. The formed body is subjected to coating (painting), baking, necking to reduce the diameter of an opening; flanging to extend an edge of the opening outward, and thereby yields a can body. Finally the body is filled with a content such as beverage or foodstuff and the lid is seamed to the opening to seal the can Cans produced by the method are called drawn and ironed cans (DI cans) and are widely distributed. The “drawn and ironed can(s)’ is hereinafter also referred to as a “can(s)” as appropriate.

[0003]

The aluminum alloy cans serving as packaging containers have been designed to have a smaller and smaller wall thickness so as to reduce the weights of the cans and the amounts of raw materials (aluminum alloys) to form the cans. This is required for cost reduction of beverages packaged in the cans. As a result, current aluminum alloy cans have a thickness of side wall (thinnest portion) of about 0.105 to about 0.110 mm, where the thickness is determined as excluding the coating. Disadvantageously, however, the resulting thin-walled cans, when a protrusion comes in contact with and is forced into the side wall (circumferential wall) having a particularly small thickness, may undergo leakage 1 of the content because the tip of the protrusion may penetrate the side wall to open a hole (pinhole). The contact of the protrusion occurs typically when a hard foreign substance comes in contact with the side wall from the outside upon production (contents filling lid seaming and passage through a transfer system in production process), distribution, and handling by a consumer. In addition, crack (flange crack) may occur at an edge ofthe opening upon flanging to expand the opening edge.

[0004]

Under these circumstances, improvements in material aluminum alloy sheets have been advanced so as to prevent side-wall pinhole generation and opening flange crack of such thinned cans, namely, to offer better side-wall puncture resistance and better flangeability (can expansibility).

[0005]

Typically, Patent Literature (PTL) 1 discloses a method for designing a can body that is formed from a cold-rolled aluminum alloy sheet having a 3xxx series chemical composition by dawningand-ironing (DI forming) or draw forming. Specifically, the can body is designed so that the can body has a thickness of 0.07 mm to 0.14 mm after undergoing a heat treatment representing (simulating) coat baking has a tensile strength of300 MPa to 500 MPa and an elongation of 3% to 8% in a can axis direction of a wall thereof and offers puncture resistance in terms of a puncture strength of 35 N or more.

The puncture strength is determined by removing a surface film such as a painting and converting a puncture strength of the resulting wall having a wall thickness (t) into a puncture strength of a can (wall) having a wall thickness of 0.105 mm. The method therefore determines the wall thickness so as to provide the puncture strength by specifying a Mg content, or determines the Mg content with respect to a given wall thickness by specifying a desired puncture strength.

[0006]

Various techniques have been proposed to control intermetallic compounds of cold-rolled aluminum alloy sheets having 3xxx series chemical compositions so as to offer better puncture resistance. Typically, PTL 2 discloses a technique of allowing a cold-rolled aluminum alloy sheet having a 3xxx series chemical composition to include intermetallic compounds distributed in the surface in a specific density and a specific area percentage. The literature describes that the cold-rolled aluminum alloy sheet gives, by drawing and ironing, a can body having excellent puncture resistance and having an elongation of 3% to less than 6% and a tensile strength greater than 290 MPa to 330 MPa in a can axis direction of a side wall of the can body when the can body has a side wall thickness of 0.110 to 0.130 mm including external and internal coatings. 2 [0007]

Likewise, PTL 3 and 4 disclose techniques of controlling the distribution density and area percentage of inter-metallic compounds having predetermined sizes. The techniques are intended to provide a higher strength (better puncture resistance) and better toughness of cold-rolled aluminum alloy sheets having 3xxx series chemical compositions. PTL 5 discloses a technique of dawning and ironing a cold-rolled aluminum alloy sheet having a 3xxx series chemical composition at a predetermined working rate (reduction rate) and subjecting the resulting workpiece to a heat treatment at 210°C to 250°C. The technique is intended to control the tensile strength and work hardening by dawning and ironing and to thereby provide better puncture resistance.

[0008]

Independently, in the can-use cold-rolled aluminum alloy sheets having 3xxx series chemical compositions, there have been proposed various techniques of specifying amounts of solutes (solid-solution) of elements such as Si, Cu, Mn, and Fe so as to provide better properties such as drawing-and-ironing formability (DI formability) and strength upon reduction in wall thickness.

Citation List Patent Literature [0009] PTL 1' Japanese Patent No. 4667722 PTL 2 Japanese Unexamined Patent Application Publication (JPA) No. 2004-68061 PTL 3 JP-ANo. 2007-197815 PTL 4: JP-ANo. 2009-270192 PTL 5 JP-ANo. 2007-169767

Summary of Invention Technical Problem [0010]

Recently the can body of a drawn and ironed can requires more and more severe (better) puncture resistance (higher puncture strength) because the drawn and ironed can is handled and used under severer conditions. Typically, the difference in pressure between the inside and outside of the can becomes larger, and the can body becomes more liable to deform. The related techniques, however, are still susceptible to improvements so as to offer such better puncture resistance under severer conditions.

[0011] 3

For example, the Mg content control alone as in PTL1 has a ceiling in puncture strength up to the required level because the puncture strength is also significantly affected by the presence of compounds in the microstructure. The technique disclosed in PTL 2 Mis to meet the reduction requirement in side wall thickness of a can. This is because the technique provides better puncture resistance by allowing the can to have a large side wall thickness greater than 0.110 mm. The technique disclosed in PTL 5 specifies a baking temperature in can coating within a relatively high temperature range and thereby foils to meet a requirement to perform a heat treatment at a lower temperature in can production The control of intermetallic compounds as disclosed in PTL 3 to 5 is certainly effective for better puncture resistance, but is still susceptible to improvements so as to meet the severer requirement in puncture resistance.

[0012]

The present invention has been made in consideration of the problems, and it is an object of the present invention to provide an aluminum alloy sheet that is used to give drawn and ironed can bodies having thinner walls and meeting the severer requirement in puncture resistance (puncture strength).

Solution to Problem [0013]

To achieve the object, the present invention provides an aluminum alloy sheet for drawn and ironed can bodies. The aluminum alloy sheet contains, in a chemical composition in mass percent, Mn in a content of 0.3% to 1.3%, Mg in a content of 0.7% to 3.0%, Si in a content of 0.1% to 0.5%, Fe in a content of 0.1% to 0.8%, and Cu in a content of 0.01% to 0.4%, with the remainder consisting of A1 and inevitable impurities. The aluminum alloy sheet includes atomic clusters as measured with a three-dimensional atom probe field ion microscope. The atomic clusters include a specific atomic cluster that meets both conditions (i) and (ii). Specifically, (0 the specific atomic cluster includes at least one of Mg atom and Cu atom in a total number of 5 or more; and (ii) the specific atomic cluster has a distance between a reference atom and any of adjacent atoms adjacent to the reference atom of 0.80 mn or less even when the reference atom is any one of the Mg atom and Cu atom contained in the atomic cluster. The average number density of the specific atomic cluster meeting the conditions (0 and (ii) is controlled to lx 1024 or less per cubic meter.

[0014]

The aluminum alloy sheet may further contain at least one element selected from the group consisting of Cr in a content of 0.001% to 0.1% and Zn in a content of 0.05% to 0.5%. In an embodiment, the aluminum alloy sheet preferably has such strength properties that, give a 0.2% yield strength of from 280 MPa to 350 MPa in a can axis 4 direction of a side wall of the can body. The 0.2% yield strength is determined by subjecting the aluminum alloy sheet to drawing and wall ironing to give a can body having a thickness of0.085 to 0.110 mm in a thinnest portion of a side wall of the can body, subjecting the can body to a heat treatment at 200°C for 20 minutes, and measuring the 0.2% yield strength of the can body after the heat treatment. In another embodiment, the aluminum alloy sheet preferably has such puncture resistance that gives a maximum load of 35 N or more. The maximum load is determined by subjecting the aluminum alloy sheet to drawing and wall ironing to give a can body having a thickness of0.085 to 0.110 mm in a thinnest portion of a side wall of the can body, subjecting the can body to a heat treatment at 200°C for 20 minutes, applying an internal pressure of 1.7 kgfcm2 (= 166.6 kPa) to the can body after the heat treatment, pushing a puncture needle perpendicularly to the can body side wall at a rate of 50 mm/minute, where the pushing is performed at a position of the can body side wall at a distance L of 60 mm from a can bottom in a can axis direction, and the puncture needle has a hemispherical tip with a radius of 0.5 mm, measuring loads until the puncture needle penetrates the can body side wall, and defining a maximum value among the measured loads as the maximum load.

Advantageous Effects of Invention [0016]

Assume that an aluminum alloy sheet having a 3xxx series chemical composition and acting as a material for drawn and ironed can bodies is formed (drawn and wall-ironed) into a can body, and the can body is subjected to coat baking (heat treatment). In this case, the can body microstructure has better puncture resistance when subgrain formation is accelerated. However, it is difficult to quantitatively determine the magnitude of subgrain formation in the can body microstructure by microstructural factors such as dislocation density and grain shapes.

[0016]

In contrast the present inventors have found that Cu-containing 3xxx series aluminum alloy sheets have a significant, difference in puncture resistance level when they include clusters of specific atoms that are present in different states. The clusters of specific atoms can be identified by analysis using a three-dimensional atom probe field ion microscope. Specifically, the present inventors have found that the can body more undergoes subgrain formation in the microstructure and has better puncture resistance with a decreasing amount of the cluster of specific atoms as specified in the present invention (hereinafter also briefly referred to as “specific atomic cluster”; and that in contrast to this, the can body less undergoes subgrain formation in the microstructure and has inferior puncture resistance with an increasing amount of the specific atomic cluster. 5 [0017]

Accordingly, the state (average number density) of the specific atomic cluster specified in the present invention can act as an index for indicating the puncture resistance when a Cu-containing 3xxx series aluminum alloy sheet is formed into a can body. Based on this, the present invention controls the state (average number density) of the specific atomic cluster in a Cu-containing 3xxx series aluminum alloy sheet and can allow the aluminum alloy sheet to have better puncture resistance at such a level as to meet the requirement of severer puncture resistance (higher puncture strength) required of a thin-walled can body.

Brief Description of Drawings [0018] IHg. l] Fig. 1 is a cross-sectional view schematically illustrating how to measure the puncture strength of a can body.

Description of Embodiments [0019]

An aluminum alloy sheet for can bodies according to embodiments of the present invention will be illustrated. The aluminum alloy sheet for can bodies is hereinafter simply referred to as an ‘‘aluminum alloy sheet’.

[0020]

Aluminum Alloy Chemical Composition

The aluminum alloy sheet for drawn and ironed can bodies according to the present invention contains, in a chemical composition in mass percent, Mn in a content of 0.3% to 1.3%, Mg in a content ofO. 7% to 3.0%, Siinacontentof0.1%to0.5%, Fein a content of 0.1% to 0.8%, and Cu in a content of 0.01% to 0.4%, with the remainder including A1 and inevitable impurities. The aluminum alloy chemical composition may further contain at least one element selected from the group consisting of Cr in a content of 0.001% to 0.1% and Zn in a content of 0.05% to 0.5%. All percentages in the chemical composition (contents of elements) are in mass percent.

[0021]

Manganese (Mn): 0.3% to 1.3%

Manganese (Mn) effectively allows the aluminum alloy to have higher strengths and allows the aluminum alloy sheet to be formed into a can body having a higher side wall strength and thereby having buckling strength and puncture resistance at certain levels.

In addition, Mn forms Al-Mn-Fe intermetallic compounds in the aluminum alloy and is 6 thereby appropriately dispersed to accelerate recrystallization after hot rolling. Thus, the element contributes to better workability of the aluminum alloy sheet. Mn, if contained in a content less than 0.3%, may have these effects insufficiently. To prevent this, the Mn content may be controlled to 0.3% or more, and preferably 0.4% or more. In contrast, Mn, if contained in a content greater than 1.3%, may form larger amounts of coarse Al-Mn-Fe intermetallic compounds and may cause the can body to have inferior puncture resistance. To prevent this, the Mn content may be controlled to 1.3% or less, preferably 1.1% or less, and more preferably 1.0% or less in terms of upper limit.

[0022]

Magnesium (Mg): 0.7% to 3.0%

Magnesium (Mg) effectively allows the aluminum alloy to have higher strengths.

Mg, if contained in a content less than 0.7%, may cause the aluminum alloy sheet to give a can body that has a low side wall strength and has insufficient puncture resistance. In contrast, Mg if contained in a content greater than 3.0%, may cause the aluminum alloy sheet to undergo excess work hardening and to readily suffer from defects. The defects are exemplified by cracking such as tear off (body crack) upon ironing; and wrinkles and streaks upon necking. To prevent this, the Mg content may be controlled within a range of 0.7% to 3.0%, preferably 1.0% to 2.6%, and more preferably 1.2% to 2.2%.

[0023]

Silicon (Si): 0.1% to 0.5%

Silicon (Si) forms Al-Fe-Mh-Si intermetallic compounds. The aluminum alloy sheet may have better formability with more appropriate distribution of the intermetallic compounds. For this reason, the Si content may be controlled to 0.1% or more, and preferably 0.2% or more. In contrast, Si, if contained in excess, may form large numbers of coarse Al-Mn-Fe-Si intermetallic compounds and Mg-Si intermetallic compounds and may cause the can body to have inferior puncture resistance. To prevent this, the Si content maybe controlled to 0.5% or less, and preferably 0.4% or less in terms of upper limit.

[0024]

Iron (Fe): 0.1% to 0.8%

Iron (Fe) is contained as a base metal impurity in the aluminum alloy, forms Al-Mn-Fe intermetallic compounds in the aluminum alloy, and is dispersed appropriately. This accelerates recrystallization after hot rolling and allows the aluminum alloy sheet to have better workability. In addition, Fe accelerates the crystallization and/or precipitation of Mn and effectively contributes to the control of the average amount of solute Mn and the dispersion of Mn-containing intermetallic compounds in the aluminum matrix. For this reason, the Fe content may be controlled to 0.1% or more, and preferably 0.3% or more. In 7 contrast, Fe, if contained in excess, may readily form veiy coarse proeutectic intermetaUic compounds and may cause the aluminum alloy sheet to offer inferior drawing-and-ironing formability and puncture resistance. To prevent this, the Fe content may be controlled to 0.8% or less, and preferably 0.7% or less in terms of upper limit.

[0025]

Copper (Cu): 0.01% to 0.4%

Copper (Cu) causes solute strengthening and allows the aluminum alloy sheet to have higher strengths. For this reason, Cu is contained essentially. The Cu content may be controlled to 0.01% or more, and preferably 0.05% or more in terms of lower limit. In contrast, Cu, if contained in excess, may cause the aluminum alloy sheet to be excessively hard although it may allow the aluminum alloy sheet to readily have higher strengths.

This may cause the aluminum alloy sheet to have inferior formability and inferior corrosion resistance. To prevent this, the Cu content may be controlled to 0.4% or less, and preferably 0.3% or less in terms of upper limit.

[0026]

Chromium (Cr): 0.001% to 0.1% and/or zinc (Zn): 0.05% to 0.5%

Chromium (Cr) and zinc (Zn) act as strengthening elements as with Cu. In addition to Cu, at least one selected from the group consisting of Cr in a content of 0.001% to 0.1% and Zn in a content of 0.05% to 0.5% can be contained selectively. Upon the selective use, the Cr content may be controlled to 0.001% or more, and preferably 0.002% or more. In contrast, Cr, if contained in excess, may cause the formation of very coarse precipitates and may cause the aluminum alloy sheet to have inferior formability. To prevent this, the Cr content may be controlled to 0.1% or less, and preferably about 0.05% or less in terms of upper limit. Also upon the selective use, the Zn content may be controlled to 0.05% or more, and preferably 0.06% or more. In contrast, Zn, if contained in excess, may cause the aluminum alloy sheet to have inferior corrosion resistance. To prevent this, the Zn content maybe controlled to 0.5% or less, and preferably about 0.45% or less in terms of upper limit.

[0027]

The aluminum alley sheet according to the present invention contains inevitable impurities in addition to these elements. Typically, the aluminum alloy sheet may contain, as the inevitable impurities, Zr in a content of 0.10% or less, Ti in a content of 0.2% or less (and preferably 0.1% or less), and B in a content of 0.05% or less (and preferably 0.01%). Within the ranges, the impurities do not affect the properties of the aluminum alloy sheet and may be contained therein. Among them, Ti effectively contributes to grain refinement and, when in coexistence with a trace amount of boron (B), further effectively contributes to 8 grain refinement. However, these elements, if contained in excess, may form very coarse Al-Ti intermetaUic compounds and coarse Ti-B particles and may adversely affect the formability of the aluminum alloy sheet.

[0028]

Microstructure of Aluminum Alloy Sheet for Drawn and Ironed Can Bodies

Atomic Cluster and Puncture Resistance

On the assumption that the aluminum alloy sheet for drawn and ironed can bodies has the above-mentioned aluminum alloy chemical composition, fine atomic clusters present in the microstructure of the aluminum alloy sheet are controlled according to the present invention This is performed for better puncture resistance (higher puncture strength).

[0029]

Assume that a material aluminum alloy sheet having a 3xxx series chemical composition is formed (drawn and wall-ironed) into a can body, and the can body is subjected to coat baking (baking of the coating). In this case, the can body has better puncture resistance by allowing the can body to undergo subgrain formation in the microstructure. The “subgrain” is also called “sub-structure” and refer to a fine phase formed in a grain. Each subgrain includes a partial dislocation-free region and allows a slip plane to act upon the application of deformation. This allows the can body to have better puncture resistance even when so-called puncture occurs at the can body upon use or handling as a can The puncture refers to the application of external force at a pinpoint of the can body. The puncture resistance is improved by development of work hardening due to accumulation of new dislocations at the puncture portion [0030]

Assume that microstructures of can bodies significantly differing in effective puncture resistance from each other are compared in photomicrographs using a transmission electron microscope. The can body microstructures differ in magnitude of dislocation and can be qualitatively distinguished in degree of subgrain formation relatively easily. Specifically, the microstructure of a can body having inferior puncture resistance includes a large number of streaky or linear dislocations and can be easily distinguished from the microstructure of the other can body having a smaller number of the dislocations and having superior puncture resistance. However, it is difficult under present circumstances to quantitatively distinguish these microstructures by microstructural factors that can be observed with a scanning electron microscope (SEM) or transmission electron microscope (TEM). Such microstructural factors are exemplified by the degree (magnitude) of dislocation and the degree (magnitude) of subgrain formation. 9 [0031]

In contrast, the present inventors have found that the can body microstructure undergoes subgrain formation in a different magnitude depending on the state of a specific atomic duster, where the state is determined by analysis using a three-dimensional atom probe field ion microscope; and that the can body microstructure undergoes more subgrain formation to offer better puncture resistance with a decreasing number density of a specific atomic duster specified in the present invention [0032]

The present inventors have also found that, in contrast to the above, the can body microstructure less undergoes subgrain formation to offer inferior puncture resistance with an increasing density of the specific atomic duster.

[0033]

Control of the average number density of the specific atomic duster in a Cu-containing 3xxx series aluminum alloy sheet enables control of the magnitude of subgrain formation in the can body microstructure and the puncture resistance of the aluminum alloy sheet, where the specific atomic duster is measured by a three-dimensional atom probe field ion microscope and indudes at least one of Mg atom and Cu atom. This allows the Cu-containing 3xxx series aluminum alloy sheet to have better puncture resistance at higher level that is required of a thin-walled can body.

[0034]

Definition of Specific Atomic Cluster

The present invention specifies a specific atomic duster, out of atomic dusters measured using a three-dimensional atom probe field ion microscope in the microstructure of the Cu-containing 3xxx series aluminum alloy sheet for drawn and ironed can bodies. The control of the specific atomic cluster contributes to the microstructure subgrain formation and the puncture resistance in the can body.

[0035]

The “specific atomic duster” refers to an atomic duster that meets both the following conditions (i) and (ii). (i) The specific atomic cluster includes either one or both of Mg atom and Cu atom in a total number of 5 or more, (ii) The specific atomic duster has a distance between a reference atom and any of adjacent atoms adjacent to the reference atom of 0.80 nm or less even when the reference atom is any one of the Mg atom and Cu atom contained in the atomic duster.

[0036]

The specific atomic cluster specified in the present invention may include at least one of the two atoms, ie., Mg atom and Cu atom, alone, but may often further indude A1 10 atom derived from the matrix. The specific duster may also further indude one or more atoms of other alloy elements such as Mn, Si, and Fe. Depending on the chemical composition of the material 3xxx series aluminum alloy, the specific atomic cluster may indude atoms typically of Cr, Zn, V, and Ti contained as selective elements or impurities. The other atoms may be inevitably counted by the three-dimensional atom probe (3DAP) analysis in some cases.

[0037]

However, the other atoms derived from alloy elements or impurities, even when contained in the specific atomic cluster specified in the present invention, are contained in lower levels as compared with the total number of the Mg atom and Cu atom.

Accordingly, a duster, even when containing one or more of such other atoms, but meeting the conditions relating to the specific distance and the specific total number of the Mg atom and Cu atom, can act as a specific atomic duster specified in the present invention, as with one inducting at least one of Mg atom and Cu atom alone. A duster, when meeting the conditions regarding the number of atoms and the distance between adjacent atoms, is counted as a specific atomic cluster specified in the present invention even when inducting one or more other atoms. In contrast, a cluster, when not meeting at least one of the conditions regarding the number of atoms and the distance between adjacent atoms, is not counted as a specific atomic cluster specified in the present invention.

[0038]

In this connection, the “adjacent atoms ' in the specific atomic cluster specified in the present invention may be not only a pair of different atoms, i.e., Mg atom and Cu atom, but also a pair of Mg atoms or a pair of Cu atoms. Typically, assume that a atomic cluster indudes either one of Mg atom and Cu atom in a number of zero, i.e., indudes the other one of Mg atom and Cu atom alone. In this case, the atomic duster, when meeting the conditions regarding distance of adjacent atoms (0.80 mn or less) and the number of atoms (5 or more) among Mg atoms or among Cu atoms, is defined as, and counted in the average number density, a specific atomic duster specified in the present invention. In contrast, assume that a duster meets the condition regarding the number of adjacent atoms within the specific distance as measured by 3DAP analysis. Even in this case, the atomic cluster, if inducting neither Mg atom nor Cu atom, is not defined as, and not counted as, a specific atomic cluster specified in the present invention. Specifically, the “specific atomic clusteff as specified in the present invention essentially indudes Mg atom, or Cu atom, or both [0089]

The condition regarding the distance between adjacent atoms in an atomic duster is met when the distance between a reference atom and any one of adjacent atoms is 0.80 nm 11 or less, where the reference atom is either one of Mg atom and Cu atom contained in the atomic cluster, and the one of adjacent atoms is Mg atom, Cu atom, or an atom of another element. Specifically, all the distances between the reference Mg atom or Cu atom and all adjacent atoms may be 0.80 nm or less. The specific atomic cluster may include an adjacent atom at a distance out of the specific range, as long as the cluster includes at least one another adjacent atom at a distance within the range with respect to the reference atom. The specific atomic cluster includes Mg atom(s) and/or Cu atom(s) in a total number of 5 or more, where all the Mg atom(s) and/or Cu atom(s) meet the condition regarding the distance between the reference atom and another adjacent atom.

[0040]

Average Number Density of Specific Atomic Cluster

The present invention controls the average number density of the specific atomic cluster to lx 1024 or less per cubic meter in the microstructure of the aluminum alloy sheet to be used as a material for drawn and ironed can bodies, where the specific atomic cluster is measured by 3DAP analysis. This accelerates the subgrain formation in the can body microstructure upon coat baking and allows the can body to have better puncture resistance, where the can body is formed from the material aluminum alloy sheet.

[0041]

Thus, the control of the specific atomic cluster accelerates the subgrain formation and allows a slip plane to act in a partial dislocation-free region upon coat baking of a can body, where the can body is formed from a Cu-containing 3xxx series aluminum alloy sheet. This allows the can body to have better puncture resistance even when so-called puncture occurs at the can body upon use or handling as a can. The puncture refers to the application of external force at a pinpoint of the can body. The puncture resistance is improved by development of work hardening due to accumulation of new dislocations at the puncture portion [0042]

In contrast an aluminum alloy sheet having an average number density of the specific atomic cluster greater than the upper limit lx 1024 per cubic meter includes atomic clusters in excess. Even when this material aluminum alloy sheet is formed into a can body and undergoes coat baking, the resulting can body less undergoes subgrain formation in the microstructure. When puncture occurs by external force upon use or handling of the can, the can may foil to have better puncture resistance, because work hardening due to accumulation of new dislocations less develops in the puncture portion [0043] 12

The condition, in terms ofupper limit, for the average number density of the specific atomic cluster herein includes an average number density of zero where the specific atomic cluster is neither detected nor measured. The number of the specific atomic cluster does not have to be controlled to zero, as long as the average number density is controlled to lx 1024 or less per cubic meter. The material aluminum alloy sheet may include the specific atomic cluster in an average number density of lx 1022 or more per cubic meter in terms of preferred lower limit. The lower limit is preferred from the viewpoint of satisfactory productivity of the material aluminum alloy sheet.

[0044]

Three-Dimensional Atom Probe Field Ion Microscope

In the field of aluminum alloy materials, the average number density of the specific atomic cluster may be measured with a three-dimensional atom probe field ion microscope typically by a technique described in JP-A No. 2011-184795. According to the technique in the literature, atom clusters that contribute to better press formability are measured with a three-dimensional atom probe field ion microscope in a Zn-containing 5xxx series aluminum alloy sheet. The aluminum alloy sheet herein is specified to include atomic clusters each including at least one of Mg atom and Cu atom in a total number of 20 or more and to have a distance of 0.80 nm or less between a reference atom and any one of other adjacent atoms, where the reference atom is selected from the Mg atoms and Cu atoms contained in one atomic cluster. The aluminum alloy sheet proposed in the technique is specified to include atomic clusters meeting the conditions in an average number density of lx 104 or more per cubic micrometer and thereby less suffers from the generation of stretcher-strain marks.

[0045]

The analysis using the three-dimensional atom probe field ion microscope is generally used typically in analysis of microstructures or atom clusters in the fields of high-density magnetic recording layers, electronic devices, aluminum alloys, steels, and copper alloys.

[0046]

The three-dimensional atom probe field ion microscope (3D atom probe field ion microscope; hereinafter also briefly referred to as “3DAP’) includes a field ion microscope (FTM) in combination with a time-of-flight mass spectrometer. The 3DAP having the structure acts as a local analyzer that observes individual atoms in a metal surface with the field ion microscope and identifies the atoms by time-of-flight mass spectrometry. In addition, the 3DAP enables simultaneous analysis of type and position of an atom emitted 13 from the sample and acts as a device very effective for structural analysis of atomic clusters.

[0047]

The 3DAP utilizes “field evaporation”. The field evaporation refers to such a phenomenon that an atom of a sample is ionized in itself in a high electrical filed. When the sample is applied with such a high voltage necessary for the field evaporation of the atom of the sample, the atom is ionized from the sample surface, travels through a probe hole, and reaches a detector.

[0048]

The detector is a position-sensitive detector and is configured not only to determine or analyze the mass of an individual ion (to identify the element as the atom species), but also to determine the position (atomic position), as detected, of the individual ion by measuring the time of flight of the ion to the detector. Advantageously, the 3DAP therefore enables simultaneous measurement of the positions and species of atoms at the tip of the sample and enables three-dimensional reconstruction and observation ofthe atomic structure ofthe sample tip. The 3DAP also enables examination ofthe distribution of atoms in the depth direction from the sample tip with atomic-level resolution, because the field evaporation sequentially occurs from the sample tip surface.

[0049]

The 3DAP utilizes a high electrical field. Accordingly, a sample to be analyzed requires high conductivity as in a metal and generally requires such a shape as to be a very thin needle shape having a tip diameter of about 100 nm or less. Such a sample having a very thin needle-shaped tip to be analyzed may be prepared by sampling a specimen typically from the central part in the thickness direction ofthe aluminum alloy sheet and subjecting the specimen sequentially to cutting with a precise cutting machine and electrcpofishing. The measurement may be performed typically by applying a pulsed high voltage on the order of 1 kV to the aluminum alloy sheet sample having the needle-shaped tip using the LEAP3000 (supplied by Imago Scientific Instruments Corporation). Thus, several millions of atoms are ionized and emitted from the sample tip. Each ofthe ions is detected by a position-sensitive detector to measure a time of flight for each ion between the emission from the sample tip upon the application of the pulsed voltage and the arrival at the detector. Based on the time of flight data, the mass spectrometry (identification ofthe element as the atomic species) of each ion is performed.

[0050]

Thus, a two-dimensional map indicating positions at which individual ions arrive is plotted. In addition, a coordinate in the depth direction is added to the two-dimensional 14 map as appropriate. This utilizes such a property that the field evaporation occurs sequentially and regularly from the sample tip surface. The resulting map is subjected to three-dimensional mapping (three-dimensional atomic structure; construction of an atomic map) using an analysis software IVAS. This gives a three-dimensional atomic map of the sample tip.

[0051]

The three-dimensional atomic map is further analyzed on atomic clusters using the maximum separation method that is a method for defining or characterizing atoms belonging to a precipitate or a duster. The method gives dm® and NUn as parameters, where dmax represents a maximum distance between specified solute atoms; and Nnm represents a minimum number of atoms constituting the duster. In the analysis by the method, a “specific cluster ' is defined while setting the adjacent maximum distance dm® of Mg and Cu atoms to 0.80 nm, and setting the minimum number Nmin of Mg and Cu atoms in total to 5. Based on the result, how specific clusters are dispersed is evaluated, and the number density of the specific clusters is quantitatively determined. The number density of the specific dusters refers to the specific average number density at a number of measured samples of 3 or more.

[0052]

Efficiency of Atomic Detedion by 3DAP

For the atomic detection effidency, the current 3DAP technique, however, can detect at most about 50% of the ionized atoms and tails to detect the remainder atoms. If the atomic detection efficiency by 3DAP significantly varies typically by improvement in fixture, the measured result of average number density (per cubic meter) of the specific atomic cluster specified in the present invention by 3DAP may possibly vaxy. For the reproducibility in the measurement of the average number density of the specific atomic cluster, the atomic detection efficiency by 3DAP is preferably set at approximately constant level of about 50%.

[0053]

Production Method

Next, a method for producing an alixminxxm alloy sheet for drawn and ironed can bodies according to the present invention will be illustrated. The alixminum alloy sheet according to the present invention may be produced by the steps of casting, soaking, hot rolling, and cold rolling. In the casting step, an alixminum alloy having the chemical composition is melted and cast to give an ingot. In the soaking step, the ingot is subjected to a heat treatment for homogenization. In the hot rolling step, the ingot after homogenization is subjected to hot rolling to give a hot-rolled sheet. In the cold rolling step, 15 the hot-rolled sheet is subjected to cold rolling without annealing. In the production method, the soaking of the ingot is performed two times under after-mentioned conditions, and the cold rolling is performed under after-mentioned specific conditions. This allows the aluminum alloy sheet after cold rolling to have a microstructure as specified in the present invention [0054]

Melting and Casting

Initially, the aluminum alloy is melted, cast by a known semi-continuous casting technique such as direct chill casting (DC casting), cooled down to a temperature lower than the solidus temperature of the aluminum alloy, and thereby yields an ingot. The casting, if performed at an excessively low casting rate less than 40 mm/min, or at an excessively low cooling rate less than 0.5°C/sec, may cause the formation oflarge amounts of coarse intermetallic compounds in the ingot. In contrast, the casting, if performed at an excessively high casting rate greater than 65 mm/min, or at an excessively high cooling rate greater than 1.5°C/sec, may readily cause ingot crack, “porosity", or “shrinkage cavity7", resulting in inferior casting yield. To prevent this, the casting may be performed at a casting rate of 40 to 65 nrni/min and a cooling rate of 0.5°C/sec to 1.5°C/sec. The cooling rate herein refers to one relating to the temperature at the central part of the ingot, namely, the temperature at the central part in a plane perpendicular to the casting direction, and refers to the rate in cooling of the aluminum alloy from the liquidus temperature down to solidus temperature.

[0055]

Soaking

The ingot before rolling is essentially subjected to a homogenization heat treatment (soaking) at a predetermined temperature. The heat treatment removes internal stress, homogenizes solute elements segregated upon casting, and diffuses and dissolves intermetallic compounds formed upon casting to homogenize the microstructure of the ingot.

[0056]

It should be noted, however, that the soaking operation in the present invention is performed two times as “double soakingf. The “double soaking” is distinguished from two-stage soaking. In the two-stage soaking, a workpiece after first soaking is cooled not down to 200°C or lower, but down to a cooling end temperature of higher than 200°C, held at that temperature, and subjected to hot rolling as being held at that temperature or as being reheated to a higher temperature. In contrast, in the double soaking herein, a workpiece after first soaking is once cooled down to a temperature of200°C or lower 16 (including room temperature), reheated, held at the reheating temperature for a predetermined time, and subjected to hot rolling.

[0057]

Specifically, the first soaking is performed at a soaking temperature of from 580°C to lower than the melting-point temperature. The first soaking temperature is set to 580°C or higher so as to dissolve coarse Al-Fe-Mn compounds formed upon casting. The first soaking, if performed at a soaking temperature of lower than 580°C, may foil to dissolve coarse Al-Fe-Mn compounds and may cause the cold-rolled sheet to have inferior formability into a can body.

[0058]

After the first soaking, the ingot is once cooled down to a temperature of200°C or lower including room temperature. In this process, the ingot may be cooled at an average cooling rate of 80°C/hr or more in the temperature range of from 500°C down to 200°C.

The cooling, if performed at an average cooling rate less than 80°C/hr in the temperature range, may not only cause Al-Fe-Mn compounds to be formed in larger amounts during cooling, but also cause the specific atomic cluster including Mg and/or Cu specified in the present invention to be formed in forger amounts. Assume that the cooling is stopped in the midway at a high temperature (higher than 200°C), and the workpiece is immediately subjected to second soaking as in the two-stage soaking. In this case, already-dispersed Al-Fe-Mn compounds act as nuclei and grow in amounts. To prevent this, cooling down to a temperature of200°C or lower is performed. The soaking (cooling), if performed under conditions out of the specific conditions, may cause the cold-rolled sheet for drawn and ironed cans to foil to have such a microstructure that gives a can body having excellent puncture resistance, where the microstructure is of a portion in the width direction and/or in the thickness direction of the sheet.

[0059]

The second snaking is performed at a soaking temperature of from450°C to 550°C. The second soaking maybe performed at an average heating rate of the ingot of 30°C/hr or more, and preferably greater than 30°C/hr in the temperature range of from 200°C up to 400°C. This condition is specified for the following reason During temperature rise in the second snaking Mg-Si compounds are formed. The control of the average heating rate of the ingot to greater than 30°C/hr particularly in the temperature range of from 200°C up to 400°C may reduce the amount of the specific atomic duster inducting Mg and/or Cu to be formed. The heating, if performed at an excessively low rate, may foil to restrain the amount of the specific atomic cluster including Mg and/or Cu specified in the present invention and may cause the amount to exceed the upper limit. 17 [0060]

The first and second soakings, if performed each for a time shorter than 2 hours, may foil to complete ingpt homogenization. In contrast, the soakings, if performed each for a time longer than 8 hours, may foil to have better effects and may cause inferior productivity. To prevent these, the first and second soakings are preferably, but not limitatively performed each for a time of from 2 to 8 hours.

[0061]

Hot Rolling

The ingot homogenized in the soaking step is subjected to hot rolling. The hot rolling may be performed under conditions within the ranges for common or regular conditions. Typically, the ingot is initially roughly rolled and then finish-rolled to give a hot-rolled aluminum alloy sheet having a predetermined thickness.

[0062]

Cold Rolling

The hot-rolled sheet is cold-rolled without preliminary annealing and without process annealing between passes and finally yields an aluminum alloy sheet having a predetermined thickness. The cold rolling is preferably performed so as to give a total rolling reduction (cold reduction ratio) of 77% to 90% and to allow cold-rolled sheet after the cold rolling to have a thickness of 0.25 to 0.33 mm The total rolling reduction in the cold rolling is naturally determined in relation with the desired thickness of the cold-rolled sheet. However, the total rolling reduction is preferably controlled within the range so as to provide a preferred coiling temperature range in order to control the average number density of the specific atomic cluster including Mg and/or Cu within the range as specified in the present invention.

[0063]

Coiling after the cold rolling is preferably performed at a coiling temperature in the range of from 120°C to 160°C. The coiling if performed at a temperature out of the temperature range (warm-working temperature range), may highly possibly feil to allow the cold-rolled sheet to have a microstructure including the specific atomic cluster including Mg and/or Cu in an average number density within the range specified in the present invention. The coiling, if performed at a temperature of higher than 160°C, may cause the specific atomic cluster including Mg and/or Cu specified in the present invention to be formed in an excessively high average number density greater than lx 1024 per cubic meter. This aluminum alloy sheet, if formed into a can body and subjected to a heat treatment at 200°C for 20 minutes, may undergo less subgrain formation and may have inferior puncture resistance. In contrast, the coiling if performed at a temperature lower than 18 120°C (e.g, at room temperature) as in regular cold rolling may cause the coiled sheet immediately after the coiling to have excessively high strengths and a low elongation and to have inferior cup formability before drawing and wall ironing.

[0064]

In regular cold rolling conditions, a sheet (coil) is coiled at a temperature around room temperature. This is because of ensuring a sufficient rolling reduction and a sufficient amount of a lubricating oil and/or a coolant to be used from the viewpoints of lubricating and control of processing heat generation. In contrast, according to the present invention, the workpiece after the cold rolling is coiled at a temperature in a higher temperature range (warm working temperature range) of from 120°C to 160°C, and preferably from 120°C to 145°C. This is performed so as to accelerate processing heat generation contrarily.

[0065]

Method for Producing Drawn and Ironed Can

The material aluminum alloy sheet (cold-rolled sheet) according to the present invention may be formed into a can body of a drawn and ironed can typically by a method as described below as an embodiment. Initially, the aluminum alloy sheet according to the present invention is blanked into a disc (blanking), drawn into a shallow cup (cupping), and then subjected to drawing and wall ironing. These drawing operations and further ironing operations are repeated multiple times to allow the workpiece to gradually have a higher side wall and to have a bottomed tubular shape having a predetermined bottom shape and a predetermined side-wall height. These processing operations are preferably performed to a reduction in thickness (ironing reduction rate) of the can body side wall of from 60% to 70%. The workpiece is then subjected to trimming in which peripheral (opening) of the side wall is cut out and trimmed. The drawing and wall ironing is performed so that the can body in the state has a small wall thickness as side wall thickness in the range of from 0.085 to 0.110 mm in the thinnest portion [0066]

Next, the can body is sequentially subjected to degreasing coating on outer and inner surfaces, and baking of the coating The resulting can body has a high strength as a 0.2% yield strength of from about 280 MPa to about 350 MPa, where the 0.2% yield strength is a strength in a can axis direction of the side wall thinnest portion As the strength herein, a strength of the formed can body after a heat treatment at 200°C for 20 minutes can be used in place of a strength after actual coat baking The heat treatment is a “heat treatment of the can body corresponding to coat baking” as specified in the present invention and is performed at a temperature for a time each corresponding to coat baking. 19

The coat baking (heat treatment), as accelerating the subgrain formation in the can body microstructure, allows the can body to have better puncture resistance.

[0067]

The can body after coating and baking is subjected to necking to reduce the opening in diameter, further subjected to flanging to extend or flange the edge of the opening outward, and yields a final can body. When used for a beverage or a foodstuff the can body is filled with the content (beverage or foodstuff) charged through the opening, and the opening is seamed and sealed with a can lid, where the can lid has been produced in another process.

Examples [0068]

While has been described with reference to preferred embodiments thereof the present invention will be illustrated in further detail with reference to several examples below which demonstrate advantageous effects of the present invention, in comparison with comparative examples which do not meet at least one of conditions specified in the present invention It should be noted, however, that the examples are never construed to limit the scope of the invention.

[0069]

Aluminum Alloy Sheet Test Samples

Aluminum alloys having chemical compositions given in Table 1 were melted, cast by semi-continuous casting at a casting rate and a cooling rate within the preferred ranges in common among the samples, and yielded ingots.

[0070]

The ingots were subjected to the double soaking. Specifically, the ingots were subjected to first soaking at a soaking temperature of600°C for 4 hours in common among the samples and once cooled down to room temperature at different average cooling rates (°C/hr) as given in Table 2 in the temperature range from 500°C down to 200°C. The ingots were then subjected to second soaking. Specifically, the ingots were reheated from room temperature at different average heating rates (°C/hr) as given in Table 2 in the temperature range from 200°C up to 400°C and subjected to second soaking at a soaking temperature of500°C for 4 hours in common among the samples.

[0071]

The samples were subjected to hot rolling that was started at the temperature of 500°C and completed at an end temperature of330°C in common among the samples and yielded hot-rolled sheets having a thickness of 2.0 to 3.0 mm.

[0072] 20

The hot-rolled sheets were subjected to cold rolling without annealing and without process annealing in the midway in common among the samples and yielded coiled long aluminum alloy sheets each having a thickness of 0.28 mm and a width of2000 mm. This cold rolling was performed at different total rolling reductions (%) and different coiling temperatures (°C) as given in Table 2.

[0073]

The symbol regarding an element in the aluminum alloy sheet chemical composition of Table 1 indicates that the material aluminum alloy did not approximately contain the element (contained the element in a content of approximately 0%).

[0074]

Can Body

The resulting coiled aluminum alloy sheets were subjected to cupping drawing and wall ironing (at an ironing reduction rate of from 65% to 70%), trinnning of the opening and yielded bottomed tubular can bodies having an outer diameter of about 66 mm, a height (can axial length) of 124 mm, and a side wall thickness of0.090 mm. The can bodies were further subjected sequentially to degreasing and a heat treatment, and yielded can body test samples. The heat treatment simulated baking in coating and was performed at 200°C for 20 minutes.

[0075]

Evaluations

The cold-rolled aluminum alloy sheets were examined on microstructure to measure the average number density of the specific atomic cluster specified in the present invention by the measuring method using a three-dimensional atom probe field ion microscope and an analysis software. The cold-rolled aluminum alloy sheets were also examined to measure the drawing-and-ironing formability into can bodies and the 0.2% yield strength. In addition, the resulting can bodies after the heat treatment were examined to measure or evaluate the puncture resistance and the 0.2% yield strength.

The heat treatment simulated the coat baking. The results are indicated in Table 2 continued from Table 1. Numbers are in common with each other in Tables 1 and 2.

[0076]

Microstructure Measurement by 3DAP

The measurement by the 3DAP technique was performed as follows. Three test specimens having a length of 30 mm and a width of 1 mm were cut out from each of the cold-rolled sheets in the width direction at 1-mm intervals using cutting equipment. The test specimens were subjected to electropolishing to be thinned and yielded needle-shaped test specimens having a tip radius of about 50 nm. The measurement was therefore 21 performed in a portion adjacent to the thickness central part. The three test specimens each having a needle-shaped tip were each subjected to 3DAP measurement using the LEAP3000 to measure the number densities (per cubic meter) of the specific atomic duster specified in the present invention The measured number densities were averaged to give an average number density. The volume as measured by the 3DAP technique was about l.O 1024 to l.OxlO'21 m3.

[0077]

Formability

The dawning and ironing was performed as follows. Each 1000 blanks were cut out (blanked) from each cold-rolled aluminum alloy sheet coil at three points in total, ie., at one point in the longitudinal direction central part adjacent to the width direction central part and at two points in the both ends on a line extending in the width direction through the central part. The blanks were subjected to continuous forming procedures (cupping dawning and ironing) at an ironing reduction rate of 65% to produce can bodies. A sample not suffering from a defect (e. g, tear off or pinhole) was evaluated as having excellent formability and indicated with O’; whereas a sample suffering from a defect were evaluated as having poor formability and indicated with “ X ”.

[0078]

Puncture Resistance

Whether the can bodies obtained from each sample (each cold-rolled sheet) had better puncture resistance was investigated. In particular, investigated was whether the can bodies obtained from regions ranging in the width direction and thickness direction of each cold-rolled sheet had better puncture resistance generally or uniformly. Accordingly, all the formed ten can bodies per each sample were subjected to a puncture test to evaluate puncture resistance so that can bodies were uniformly included in the test samples, where the can bodies were produced from each cold-rolled aluminum alloy sheet coil at the three points, ie., at one point in the central part and at two points in both ends corresponding to the central part, each in the width direction [0079]

The puncture resistance test was performed in the following manner as illustrated in Fig. 1. The sample can body was fixed, to which an internal pressure of 1.7 kgfcm2 (= 166.6 kPa) was applied. The side wall ofthe can body was punctured (pierced) with a puncture needle having a hemispherical tip with a radius of 0.5 mm at a rate of 50 mm/minute. In this process, the puncture needle was pushed vertically with respect to the side wall at a position at a distance L from the can bottom in the can axis direction of 60 mm, where the can axis direction agreed with the rolling direction ofthe aluminum alloy 22 sheet. Loads (LD until the puncture needle penetrated the side wall were measured. Of the measured loads, the maximum load was defined as a puncture strength [0080]

The results of the puncture resistance test were evaluated as follows. A sample having a maximum load of 40 N or more on average in all the tested can bodies was evaluated as having excellent puncture resistance in the entire width direction of the cold-rolled aluminum alloy sheet and indicated with “(§}’; and a sample having a maximum load of 35 N or more on average was evaluated as having good puncture resistance and indicated with “O’. In contrast, a sample having a maximum load less than 35 N on average in all the tested can bodies was evaluated as having poor puncture resistance in the entire width direction and thickness direction of the cold-rolled aluminum alloy sheet and indicated with “x”.

[0081]

In the embodiment of the present invention, a drawn and ironed can is assumed to be handled and used at an internal pressure of 1.7 kgficm2 (= 166.6 kPa) as in the puncture test, where the internal pressure is lower as compared with conventional puncture tests. Under such conditions of lower internal pressure, the difference in pressure between the inside and outside of the can becomes larger, the can body more deforms, and a more severe (better) puncture resistance is required. Rupture of a can body upon puncture actually occurs by collision of articles having various shapes, but it is difficult to evaluate puncture resistance with respect to all such articles with various shapes. A severer evaluation method to evaluate puncture resistance has therefore been demanded. The puncture resistance test in the present invention is therefore performed under such condition of a lower internal pressure as to cause the can body sample to deform more. Under the severe condition, the can body sample may difficultly exhibit a higher puncture strength.

[0082]

Puncture resistance evaluation in conventional techniques is generally performed at a higher internal pressure of 2.0 kgficm2 (= 196 kPa). Accordingly, even an identical test sample material offers a lower puncture strength in the test method employed in the embodiment of the present invention as compared with the test under the conventional conditions, because the former test is performed under severer conditions. Specifically, it can be said that the material (aluminum alloy sheet or can body) according to the present invention has superior puncture resistance to conventional equivalents even when the material has a puncture strength (N) as determined by the test method in the present invention equal to or somewhat lower than a puncture strength (N) determined by the conventional test performed at an internal pressure of 2.0 kgfcm2. In other words, it 23 cannot be said that a sample having excellent puncture resistance in the test at an internal pressure of 2.0 kgfcm2 always offers excellent puncture resistance in the test specified in the present invention performed at a lower internal pressure of 1.7 kgficm2.

[0083] 0.2% Yield Strength

Tensile tests were performed as follows to measure a 0.2% yield strength of each of the cold-rolled sheets and the can body side walls. Test specimens were sampled from the cold-rolled sheet and the side wall of can body (after the heat treatment simulating coat baking) and subjected to tensile tests according to JIS Z 2201. The test samples were prepared each as a JIS No. 5 test specimen so that the longitudinal direction of the test specimen agreed with the rolling direction (can axis direction). The tests were performed at a constant crosshead speed of 5 mm/minute until the test specimen broken.

[0084]

As is indicated in Tables 1 and 2, Examples 1 to 10 (samples according to the present invention) employed aluminum alloys having chemical compositions within the ranges specified in the present invention and were produced under preferred conditions.

In these examples, the cold-rolled sheets each had an average number density of the specific atomic cluster within the range specified in the present invention, as indicated in Table 2.

[0085]

The examples had good drawing-and-ironing formability and still offered excellent puncture resistance. The puncture resistance was determined after the aluminum alloy sheet was drawn and ironed to give thin-wall can bodies having a side wall thickness of 0.090 mm in a thinnest portion, where the formed can bodies were designed to have a high strength in terms of 0.2% yield strength of280 MPa to 350 MPa in the can axis direction in the side wall after the heat treatment corresponding to coat baking. In addition, the examples offered such excellent puncture resistance in terms of puncture strength of 35 N or more or 40 N or more even evaluated under severe conditions where an internal pressure of 1.7 kgficm2 (= 166.6 kPa) was applied to the can body. Specifically, the results demonstrate that the can bodies designed to have a thinned wall thickness in side wall and to have a high strength had good formability and excellent puncture resistance even under severer conditions.

[0086]

In contrast, Comparative Examples 11 to 20 in Tables 1 and 2 employed aluminum alloys having chemical compositions out of the range specified in the present invention, or were produced under a condition in the soaking or cold rolling not meeting the preferred 24 condition in the present invention. In the comparative examples, the cold-rolled sheet had an average number density of the specific atomic cluster out of the range specified in the present invention, had an insufficient 0.2% yield strength, and/or had inferior drawing-and-ironing formabifity. In addition, all these comparative examples had inferior puncture resistance.

[0087]

Comparative Example 11 underwent cooling after first soaking down to room temperature at an excessively low average cooling rate less than 70°C/hr in the temperature range of from 500°C down to 200°C. The sample therefore included the specific atomic cluster specified in the present invention in a large amount with an average number density greater than the upper limit and had inferior puncture resistance, where the specific atomic cluster was formed during cooling.

[0088]

Comparative Example 12 underwent second soaking at an excessively low average heating rate less than 30°C/hr in the temperature range of from 200°C up to 400°C. The sample therefore included the specific atomic cluster specified in the present invention in a large amount with an average number density greater than the upper limit and had inferior puncture resistance, where the specific atomic cluster was formed during cooling.

[0089]

Comparative Example 13 underwent cold rolling at an excessively low total rolling reduction The sample therefore had excessively low strengths of the cold-rolled sheet and of the can body after bake hardening (BH) and had inferior puncture resistance.

[0090]

Comparative Example 14 underwent cold rolling at an excessively high coiling temperature. The sample therefore included an increased amount of the specific atomic cluster specified in the present invention in the cold-rolled sheet with an average number density greater than the upper limit and offered inferior puncture resistance.

[0091]

Comparative Examples 15 to 20 in Tables 1 and 2 had any of the contents of Cu,

Mn, Mg, Si, and Fe out of the ranges specified in the present invention.

[0092]

Comparative Example 15 had an excessively low Mg content and an excessively low solute Mg content. Comparative Example 16 had an excessively high Mn content.

These comparative examples therefore had inferior puncture resistance over the width direction when determined under the severe internal pressure condition.

[0093] 25

Comparative Example 17 had an excessively low Mn content. Comparative Example 18 had an excessively high Si content. Comparative Example 19 had an excessively high Fe content. These comparative examples therefore suffered from defects upon drawing and wall ironing could not be practically formed into can bodies, and were not subjected to the subsequent puncture test, because the testing is meaningless to perform.

[0094]

Comparative Example 20 did not contain Cu. The sample therefore had a low strength of the can body and inferior puncture resistance over the width direction when determined under the severe internal pressure condition.

[0095] 26 ο Ο) ω ω η Ε Chemical composition of aluminum alloy sheet in mass percent, with the remainder consisting of aluminum 05 Ο ζ Si Fe Cu Mn Mg Cr Zn 1 0.33 0,45 0.22 0.86 1.5 - - 2 0.27 0.44 0,14 0.85 2.7 - -- 3 0.20 0.25 0.21 1.1 2.3 - - ω ω Ω. μ 4 0.45 0.68 0.40 0.87 0.80 - - 5 0.15 0.43 0,20 0.87 1.2 - 0,22 03 X 6 0.26 0,43 0,25 0.40 1.2 0.06 0.22 LU 7 0.27 0.43 0.18 1.2 18 0.02 0.05 8 0.27 0.15 0.07 0.90 1.3 - 0.45 9 0.24 0.42 0.22 13 10 0.02 - 10 0.28 0,44 0.30 0.65 1.2 0.02 0.21 11 0.27 0.50 0.07 1.1 13 - - (0 12 0.40 0.43 0.35 1.20 10 - 0,22 ω Ο. 13 0.39 0.74 0.15 1.0 10 0.02 - Ε 05 14 0.33 0.45 0.22 0.86 15 - - LU 15 0.27 0,45 0.10 0.87 0.60 - - 1 ie 0.26 0.43 0.30 ~rr 0.05 05 i_ 05 17 0.26 0,43 0.20 0,20 12 0.02 0.22 Ω_ Ε 18 0.55 0.43 0.20 0,87 12 0.02 0.22 Ο 19 0.26 0.84 0.20 0.87 1.2 0.02 0.22 20 0.26 0.43 - 0.87 1.0 0.02 0.22 [Table l] to 00 [0097]

(continued from Table 1) Sheet producing conditions (condition control) Microstructure and properties of sheet Properties after can making and baking at 200X for 20 min. Category ω _Ω E 3 z First soaking average heating rate [X/hr] Second soaking average heating rate [X/hr] Total cold rolling reduction [%] Coiling temperature after cold rolling m Average number density of atomic cluster x 1022 [number per cubic meter] 0.2% Yield strength [MPa] Drawing- and- ironing formability 0.2% Yield strength [MPa] Average puncture strength [N] Evaluated puncture resistance of entire sheet 1 100 50 86 130 7.3 29Θ o 314 39 o 2 100 60 79 125 2.4 300 o 3*3 42 @ 3 100 50 82 140 40 307 o 328 38 (Λ to 4 100 40 88 150 75 263 o 281 36 0 Q_ E 5 _ϊ ao 40 82 135 28 272 o 291 38 o cd X LU 5 100 30 Si 140 16 284 o 303 38 o 7 100 50 88 155 80 306 o 325 36 o a 120 50 80 140 2.8 285 o 302 41 3 80 60 80 150 if 284 o 31? 35 o 10 too 50 86 125 8.9 288 o 305 40 ¢-) 11 70 40 82 150 290 o 310 34 X (Λ 12 100 20 88 145 107 285 o 302 34 X CD Q. 13 100 40 73 130 :u 252 0 269 34 X E CD 14 100 50 86 170 12S 285 o 309 33 X LU CD > IS 120 60 84 140 7.7 244 0 255 32 X 14 100 40 88 ISO 40 290 o 310 33 X CD i_ CD 17 100 50 86 140 15 265 X Q_ E 18 100 ' 50 80 130 9.6 268 x o O IS 100 50 82 140 12 275 X ?0 so 50 78 155 0 257 X 268 34 X

[Table 2]

While the present invention has been described in detail with reference to specific embodiments thereof those skilled in the art will recognize that various changes, modifications, and alternations are possible without departing from the spirit and scope of the invention.

The present invention is based on Japanese Patent Application No. 2012-285870 filed on December 27, 2012, the entire contents of which are incorporated herein by reference.

Industrial Applicability [0098]

As is described above, the aluminum alloy sheet (cold-rolled sheet) for drawn and ironed can bodies according to the present invention can be formed into can bodies having better puncture resistance at target level and ensures satisfactory puncture resistance of the can bodies. The cold-rolled aluminum alloy sheet is therefore most suitable for a cold-rolled aluminum alloy sheet for use in the production of drawn and ironed can bodies that have a thin wall thickness and a high strength and require puncture resistance under severer use conditions. 29

Claims

CLAIMS [Claim l] An aluminum alloy sheet for drawn and ironed can bodies, the aluminum alloy sheet comprising: in a chemical composition in mass percent, Mn in a content of 0.3% to 1.3%; Mg in a content of 0.7% to 3.0%; Si in a content of 0.1% to 0.5%; Fe in a content of 0.1% to 0.8%; and Cu in a content of 0.01% to 0.4%, with the remainder consisting of A1 and inevitable impurities, the aluminum alloy sheet comprising atomic clusters as measured with a three-dimensional atom probe field ion microscope, the atomic clusters comprising a specific atomic cluster meeting both following conditions (0 and (ii): (i) the specific atomic cluster comprising at least one of Mg atom and Cu atom in a total number of 5 or more; and (ii) the specific atomic cluster having a distance between a reference atom and any of adjacent atoms adjacent to the reference atom of 0.80 nm or less even when the reference atom is any one of the Mg atom and Cu atom contained in the atomic cluster, an average number density of the specific atomic cluster meeting the conditions (i) and (ii) being controlled to lx 1024 or less per cubic meter. [Claim
2] The aluminum alloy sheet for drawn and ironed can bodies according to claim 1, further comprising at least one element selected from the group consisting of Cr in a content of 0.001% to 0.1%; and Zn in a content of 0.05% to 0.5%. [Claim
3] The aluminum alloy sheet for drawn and ironed can bodies according to one of claims 1 and 2, wherein the aluminum alloy sheet has such strength properties that give a 0.2% yield strength of280 MPa to 350 MPa in a can axis direction of a can body side wall where the 0.2% yield strength is determined by subjecting the aluminum alloy sheet to drawing and wall ironing to give a can body having a thickness of0.085 to 0.110 mm in a thinnest portion of a side wall of the can body subjecting the can body to a heat treatment at 200°C for 20 minutes; and measuring the 0.2% yield strength of the can body after the heat treatment. [Claim
4] The aluminum alloy sheet for drawn and ironed can bodies according to claim 1, wherein the aluminum alloy sheet has such puncture resistance that gives a maximum load of 35 N or more, where the maximum load is determined by: subjecting the aluminum alloy sheet to drawing and wall ironing to give a can body having a thickness of0.085 to 0.110 mm in a thinnest portion of a side wall of the can body; subjecting the can body to a heat treatment at 200°C for 20 minutes; applying an internal pressure of 1.7 kgftcm2 (= 166.6 kPa) to the can body after the heat treatment; pushing a puncture needle perpendicularly to the can body side wall at a rate of 50 mm/minute, where the pushing is performed at a position of the can body side wall at a distance L of 60 mm from a can bottom in a can axis direction, and the puncture needle has a hemispherical tip with a radius of 0.5 mm; measuring loads until the puncture needle penetrates the can body side wall and defining a maximum value among the measured loads as the maximum load.