US20180171451A1

US20180171451A1 - Aluminum alloy sheet

Info

Publication number: US20180171451A1
Application number: US15/797,346
Authority: US
Inventors: Katsushi Matsumoto; Masahiro Yamaguchi; Yasuhiro Aruga
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 2016-12-20
Filing date: 2017-10-30
Publication date: 2018-06-21
Also published as: JP2018100435A; CN108203779A

Abstract

Provided is a 6000-series aluminum alloy sheet excellent in strength and bendability. The alloy sheet is an Al—Mg—Si aluminum alloy sheet including Mg: 0.5 to 1.3% by mass, and Si: 0.7 to 1.5% by mass; one or more elements selected from Mn: 0.05 to 0.5% by mass, Zr: 0.04 to 0.20% by mass, and Cr: 0.04 to 0.20% by mass; and Al and inevitable impurities as the remainder of the alloy sheet. Transition-element-based dispersed particles which are present on grain boundaries of the alloy sheet and which have a size of 0.05 μm or more have a number density of 0.001 nm⁻¹or less. The grain boundaries show a PFZ width of 60 nm or less after the aluminum alloy sheet is subjected to an artificial aging of holding the aluminum alloy sheet at 200 to 250° C. for 10 to 30 minutes.

Description

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to an aluminum alloy sheet, more specifically, an Al—Mg—Si aluminum alloy sheet suitable for a member for constituting a skeleton structure of an automobile, a railroad vehicle or the like.

Description of Related Art

In recent years, against global environmental problems based on exhaust gas and others, transportation machines, such as automobiles, have been required to be improved in gasoline mileage by making their machine body light. In order to meet this requirement, as the raw material of automobiles and others, the use of aluminum alloy material, which is light, has been increasing instead of that of a steel such as a sheet-form steel.
For example, for panels (such as outer panels and inner panels) used in panel structural bodies such as hoods, fenders, doors, roofs and trunk grids of automobiles, thin and highly strong aluminum panel sheets, out of aluminum alloy sheets, have been adopted. In particular, Al—Mg—Si aluminum alloy sheets (such as 6000-series alloy sheets prescribed in JIS) have been used. Aluminum alloys for outer panels and inner panels are required to have a high workability. In order to meet such a requirement, various methods have been so far suggested. For example, Patent Literature 1 (JP 2003-105473 A) discloses an aluminum alloy sheet improved in bendability by controlling the cooling rate of an aluminum alloy for this sheet after the alloy is solutionized.
Also as raw materials used in front pillows, center pillows, roof rails, dashboards, side members, cross-members, side sills, inner sills, and other skeleton structures (the so-called white bodies) of automobiles, instead of steels, thin and highly strong Al—Mg—Si alloy sheets are required to be used. It has been required to develop, as a raw material used for a skeleton structure of a transportation machine, a raw material having such an excellent bendability that the material can sufficiently absorb energy at the time of a collision of the machine.
Under actual situations, however, as a raw material used for skeleton structures of vehicles, developments of an aluminum alloy sufficiently good in strength and bendability have not yet been advanced.

CITATION LIST

Patent literature

Patent Literature 1: JP 2003-105473 A

SUMMARY OF THE INVENTION

In order to meet such requirements, the present invention has been made. An object thereof is to provide a 6000-series aluminum alloy sheet excellent in strength and bendability.
[1] An aspect of the present invention is an Al—Mg—Si aluminum alloy sheet, including: Mg: 0.5 to 1.3% by mass, and Si: 0.7 to 1.5% by mass; including one or more elements selected from the group consisting of the following: Mn: 0.05 to 0.5% by mass, Zr: 0.04 to 0.20% by mass, and Cr: 0.04 to 0.20% by mass; and further including: Al and inevitable impurities as the remainder of the aluminum alloy sheet; the aluminum alloy sheet having grains, and grain boundaries therebetween; transition-element-based dispersed particles which are present on the grain boundaries and have a size of 0.05 μm or more having a number density of 0.001 nm⁻¹or less; and the grain boundaries showing a PFZ width of 60 nm or less after the aluminum alloy sheet is subjected to an artificial aging of holding the aluminum alloy sheet at 200 to 250° C. for 10 to 30 minutes.
[2] The Al—Mg—Si aluminum alloy sheet is preferably the aluminum alloy sheet according to item [1] further including Cu: more than 0% by mass, and 0.5% or less by mass.
[3] The Al—Mg—Si aluminum alloy sheet is preferably the aluminum alloy sheet according to item [1] further including Sc: 0.02 to 0.1% by mass.
[4] The Al—Mg—Si aluminum alloy sheet is preferably the aluminum alloy sheet according to item [2], further including Sc: 0.02 to 0.1% by mass.
[5] The Al—Mg—Si aluminum alloy sheet is preferably the aluminum alloy sheet according to item [1] further including one or more elements selected from the group consisting of the following: Ag: 0.01 to 0.2% by mass, and Sn: 0.001 to 0.1% by mass.
[6] The Al—Mg—Si aluminum alloy sheet is preferably the aluminum alloy sheet according to item [2], further including one or more elements selected from the group consisting of the following: Ag: 0.01 to 0.2% by mass, and Sn: 0.001 to 0.1% by mass.
[7] The Al—Mg—Si aluminum alloy sheet is preferably the aluminum alloy sheet according to item [3], further including one or more elements selected from the group consisting of the following: Ag: 0.01 to 0.2% by mass, and Sn: 0.001 to 0.1% by mass.
[8] The Al—Mg—Si aluminum alloy sheet is preferably the aluminum alloy sheet according to any one of items [1] to [7] which shows a 0.2% yield strength of 250 MPa or more, and a VDA bending angle of 60° or more after the aluminum alloy sheet is subjected to the artificial aging of holding the aluminum alloy sheet at 200 to 250° C. for 10 to 30 minutes.
According to the present invention, a 6000-series aluminum alloy sheet excellent in strength and bendability can be provided.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a perspective view illustrating an embodiment of a VDA bending test for evaluating the impact absorption of a specimen.

BEST MODE OF THE INVENTION

The inventors have made various investigations to realize a 6000-series aluminum alloy sheet sufficiently good in strength and bendability for a member for constituting skeleton structures of automobiles and other transportation machines.
When a 6000-series aluminum alloy sheet is subjected to an artificial aging such as paint-baking, precipitate free zones (PFZs), where precipitates of Si, Mg₂Si, and others are not present, are formed near crystal grain boundaries in this sheet. It is known that the PFZs are low in strength and bendability to turn to starting points of cracks when the alloy sheet is bent.
The inventors have paid attention to the fact that an aluminum alloy can be improved in strength and bendability by decreasing the width of its PFZs, which make the strength and the bendability of the alloy low.
The inventors have made eager investigations to find out that the number density of transition-element-based dispersed particles precipitated in crystal grain boundaries in an aluminum alloy sheet is decreased, thereby making it possible to decrease the width of PFZs making their appearance near the crystal grain boundaries after the alloy sheet is artificially aged, so as to provide the aluminum alloy sheet with an excellent strength and bendability.
When such transition-element-based dispersed particles are present in a predetermined number density or more in crystal grain boundaries of an aluminum alloy sheet, made is a promotion of the composite precipitation of Mg and Si diffused into the grain boundaries at the time of cooling the alloy sheet just after the solutionizing of the alloy sheet, this precipitation being a precipitation onto the surface of the dispersed particles. As a result, the diffusion of atoms of Mg and Si from the matrix of the alloy into the grain boundaries is promoted, and further the concentration of the Mg and Si atoms is lowered near the grain boundaries. Thus, regions of depletion layers of these elements are enlarged so that the alloy sheet is decreased in PFZ width to be deteriorated in bendability.
However, in the aluminum alloy sheet of the present invention, transition-element-based dispersed particles present in its crystal grain boundaries are decreased in number density, thereby making it possible to restrain the composite precipitation of Mg and Si into the dispersed particles. As a result, depletion regions of Mg and Si can be made small so that the alloy sheet can be decreased in PFZ width to be improved in strength and bendability.
Hereinafter, a description will be made in detail about an aluminum alloy sheet according to an embodiment of the present invention.
The “aluminum alloy sheet” referred to in the embodiment of the invention denotes an aluminum alloy sheet which has been yielded by subjecting a rolled sheet, such as a hot-rolled sheet or a cold-rolled sheet, to a thermal treatment, such as solubilizing or hardening, and which has not been subjected to any artificial aging treatment, such as paint-baking.

1. Microstructure

(1) The Number Density of Transition-Element-Based Dispersed Particles Which are Present on Grain Boundaries in the Aluminum Alloy Sheet and have a Size of 0.05 μm or More: 0.001 nm⁻¹or Less.
The aluminum alloy sheet according to the embodiment of the present invention has a characteristic that the number density of transition-element-based dispersed particles precipitated on grain boundaries in the alloy sheet is controlled into a predetermined value or less.
If the number density of the transition-element-based dispersed particles present on the grain boundaries is large, the composite precipitation of Mg and Si atoms into the dispersed particles is promoted, these Mn and Si atoms having been diffusing into the grain boundaries when the aluminum alloy is cooled just after solutionized in the process for producing the aluminum alloy sheet. As a result, the diffusion of Mg and Si from the matrix into the grain boundaries is promoted so that the Mg and Si concentrations are lowered near the grain boundaries. Thus, depletion regions of Mg and Si are enlarged so that the aluminum alloy sheet can be increased in PFZ width to be deteriorated in strength and bendability.
In the aluminum alloy sheet according to the embodiment of the present invention, the number density of the transition-element-based dispersed particles present on the grain boundaries is decreased, whereby the composite precipitation of Mg and Si into the dispersed particles is restrained. As a result, depletion regions of Mg and Si can be made small so that the aluminum alloy sheet can be decreased in PFZ width to be improved in strength and bendability.
When the transition-element-based dispersed particles which are present on the grain boundaries and have a size of 0.05 μm or more have a number density of 0.001 nm⁻¹or less, the aluminum alloy sheet can gain the above-mentioned bendability-improving effect based on the decrease in the PFZ width. The number density of the transition-element-based dispersed particles is preferably 0.0005 nm⁻¹or less.
In the embodiment of the present invention, target ones out of the transition-element-based dispersed particles on the grain boundaries, the number of these target particles being required to be specified, are only dispersed particles having an equivalent circle diameter of 0.05 μm or more. This is because dispersed particles having an equivalent circle diameter less than 0.05 μm are so small that an effect of the particles onto an increase in the PFZ width may be ignored.
In the embodiment of the present invention, the “transition-element-based dispersed particles” denote one or more intermetallic compounds including Mn, Cr, Zr, Fe, Cu and/or some other elements.

(2) The PFZ Width of the Grain Boundaries After the Aluminum Alloy is Artificially Aged: 60 nm or Less

6000-series aluminum alloy sheets, examples thereof including the aluminum alloy sheet according to the embodiment of the present invention, are each subjected to an artificial aging such as paint-baking, thereby forming, near crystal grain boundaries thereof, precipitate free zones (PFZs), in which precipitates of Si, Mg₂Si and the like are not present.
As described above, in the aluminum alloy sheet according to the embodiment of the present invention, by decreasing the number density of the transition-element-based dispersed particles on the crystal grain boundaries, the composite precipitation of Mg and Si into the dispersed particles can be restrained, so that depletion regions of Mg and Si can be made small to make the PFZ width sufficiently small. As a result, the aluminum alloy sheet can be provided with compatibility between excellent strength and bendability.
The aluminum alloy sheet according to the embodiment of the present invention is an aluminum alloy sheet in which the PFZ width near crystal grain boundaries is sufficiently decreased. The property that the PFZ width is sufficiently decreased can be especially remarkably expressed by subjecting a solutionized aluminum alloy sheet (i.e., a T4 material) according to the invention to an artificial aging, that is, by treating the T4 material to be converted to a T6 material.
Thus, in the embodiment of the invention, the aluminum alloy sheet thereof is specified by aging an aluminum alloy sheet as a T4 material artificially, and then measuring the PFZ width of the artificially aged aluminum alloy sheet, that is, the aluminum alloy sheet treated thermally to a T6 material.
For example, the aluminum alloy sheet according to the embodiment of the invention is subjected to an artificial aging of holding the alloy sheet at 200 to 250° C. for 10 to 30 minutes; in this way, it can be verified that the resultant alloy sheet has a sufficiently decreased PFZ width.
As described above, the aluminum alloy sheet according to the embodiment of the present invention is artificially aged, whereby the alloy sheet can remarkably express a sufficiently decreased PFZ width. About the aluminum alloy sheet according to the embodiment of the invention, the PFZ width of its grain boundaries after the alloy sheet is artificially aged is 60 nm or less, preferably 40 nm or less.
When the PFZ width is in such a range after the artificial aging, the aluminum alloy sheet according to the embodiment of the invention can exhibit an excellent collision resistance in the case of using this sheet for a member for skeleton structures for vehicles and others.
About the number density of transition-element-based dispersed particles which are present on grain boundaries in an aluminum alloy sheet and which have a size of 0.05 μm or more, and the PFZ width of the grain boundaries, measurements can be made as described below, using, for example, a transmission electron microscope (TEM).
About the number density of the transition-element-based dispersed particles, which are present on the grain boundaries and have a size of 0.05 μm or more, for example, from an aluminum alloy sheet treated thermally into a T4 material, this sheet being an alloy sheet solutionized, samples are collected at 5 sites selected at random, and each of the samples is prepared as a TEM-observing specimen to render a sheet-thickness-central portion thereof an observing surface. An electron beam is then adjusted to make the incident direction thereof parallel to the (100) plane of each of the samples. About each of the samples, its grain boundary region is then photographed within three viewing fields at a magnification of 100000 or more. About each of the viewing fields, energy dispersive X-ray spectroscopy (EDX) is used to analyze precipitates on the grain boundary. Out of the precipitations, transition-element-based precipitates are then identified except Mg—Si based precipitates, Mg—Si—Cu based precipitates, and Si precipitates. Out of the identified precipitates, precipitates having an equivalent circle diameter of 0.05 μm or more are counted. The number density (nm⁻¹) of the precipitates per length of the grain boundary is then calculated. The average of the respective number densities in the three viewing fields in each of the samples is calculated. The number density is measurable by calculating the average of the respective number densities in the three viewing fields in each of the samples, and then gaining the average of the respective average number densities of the five-site samples.
About the PFZ width, for example, from an artificially aged aluminum alloy sheet, i.e., an aluminum alloy sheet treated thermally into a T6 material, samples are collected in the same manner as in the above-mentioned TEM observation after the solutionizing. An electron beam is then adjusted to make the incident direction thereof parallel to the (100) plane of each of the samples. About each of the samples, its grain boundary region is then observed within three viewing fields at a magnification of 100000 or more. About each of the viewing fields, the PFZ width is measured at a site having the largest PFZ width out of the observed sites. The average value of the respective PFZ widths at the 15 sites in total is then calculated. In this way, the PFZ width of the aluminum alloy sheet is measurable.
A PFZ of an aluminum alloy is, in a TEM photograph thereof, a region surrounded by an interface between a precipitation region (dark color region in the TEM photograph) of one crystal grain and a precipitation diluted region (thin color region in the TEM photograph), and a crystal grain boundary therein. The PFZ width is, in the TEM photograph, the distance between the following: the interface between the precipitation region of the crystal grain and the precipitation diluted region; and the crystal grain boundary.

2. Chemical Component Composition

The following will describe the composition of the aluminum alloy sheet according to the embodiment of the present invention. This aluminum alloy sheet is made of a 6000-series aluminum alloy. It is sufficient for the component composition thereof to be equivalent to an ordinary chemical component composition of a 6000-series aluminum alloy.

(1) Si: 0.7 to 1.5% by Mass Both Inclusive

Si, as well as Mg, is an element contributing to solute strengthening of an aluminum alloy. Moreover, Si is an element essential for forming an aged precipitate contributing to an improvement of the alloy in strength when the alloy is subjected to an artificial aging such as paint-baking, so as to cause the aluminum alloy sheet according to the embodiment to exhibit age hardening power and gain a strength (yield strength) necessary for automobile structural materials.
The addition of Si in a predetermined amount proportion or more makes small the thickness of concentration-depletion layers of Si atoms that are each formed by the diffusion of the Si atoms into the grain boundaries when the aluminum alloy is solutionized and cooled. Thus, the addition makes it possible to decrease the aluminum alloy sheet in PFZ width to improve the alloy sheet in bendability.
If the Si content is less than 0.7% by mass, the aluminum alloy sheet is short in strength. Thus, the Si content is 0.7% or more by mass, preferably 0.8% or more by mass.
In the meantime, if the Si content is more than 1.5% by mass, coarse compound particles are produced to deteriorate the alloy sheet in ductility. Thus, the Si content is 1.5% or less by mass, preferably 1.4% or less by mass.

(2) Mg 0.5 to 1.3% by Mass Both Inclusive

Mg, as well as Si, is an element contributing to solute strengthening of an aluminum alloy. Moreover, Mg is an element essential for producing, together with Si, an aged precipitate contributing to an improvement of the aluminum alloy in strength when the alloy is subjected to an artificial aging such as paint-baking, so as to cause the aluminum alloy sheet according to the embodiment to exhibit age hardening power and gain a yield strength necessary for automobile structural materials. Moreover, the addition of Mg in a predetermined amount proportion or more decreases the thickness of concentration-depletion layers of Mg atoms that are each formed by the diffusion of Mg into the grain boundaries when the aluminum alloy is solutionized and cooled. Thus, the addition makes it possible to decrease the aluminum alloy sheet in PFZ width to improve the alloy sheet in bendability.
If the Mg content is less than 0.5% by mass, the aluminum alloy sheet is short in strength. Thus, the Mg content is 0.5% or more by mass, preferably 0.6% or more by mass.
In the meantime, if the Mg content is more than 1.3% by mass, shear zones are easily formed in the aluminum alloy when the alloy is cold-rolled, so that the alloy is cracked when rolled. Thus, the Mg content is 1.3% or less by mass, preferably 1.2% or less by mass.
(3) At Least One Selected from Mn: 0.05 to 0.5% by Mass Both Inclusive; Zr: 0.04 to 0.20% by Mass Both Inclusive; and Cr: 0.04 to 0.20% by Mass Both Inclusive
Mn, Zr and Cr are each present in the form of dispersed particles, and contribute to making the crystal grains finer to improve the aluminum alloy sheet in shapability. If the addition amount proportion of these elements is too small, the number density of the dispersed particles is lowered. Thus, when the aluminum alloy sheet is stretched, an increase quantity of the yield strength thereof is lowered so that the alloy sheet may be unfavorably lowered in strength after subjected to paint-baking. In the meantime, if the content of these elements is too large, coarse compound particles are produced to deteriorate the alloy sheet unfavorably in ductility.
Accordingly, the content of Mn is from 0.05 to 0.5% by mass both inclusive; that of Zr is from 0.04 to 0.20% by mass both inclusive; and that of Cr is from 0.04 to 0.20% by mass both inclusive.
The aluminum alloy sheet may contain only one of Mn, Zr and Cr, or may contain any combination of two or more thereof.

(4) Remainder

The remainder of the alloy sheet is preferably made of Al and inevitable impurities. It is supposed that as the inevitable impurities, trace elements are incorporated into the alloy sheet. The trace elements are taken thereinto in accordance with the raw materials, the producing facilities and other situations, and are, for example, Ti, B, Fe, Zn, V, Na, Ca, In, Be and Sr.
The composition of the aluminum alloy sheet according to the embodiment of the present invention is not limited to the above-mentioned composition. The aluminum alloy sheet may optionally contain other elements as far as the alloy sheet can maintain the characteristics thereof. Examples of such optionally-incorporable elements are given below.
(5) Cu: More than 0% by Mass, and 0.5% or Less by Mass
Cu undergoes solute strengthening to contribute to an improvement of the alloy sheet in strength, and further has an effect that when the alloy sheet is aged, age hardening for the final product is promoted. In order to gain such an effect of Cu, the alloy sheet preferably contains Cu in a proportion more than 0% by mass. In the meantime, if the content of Cu is too large, the alloy sheet may unfavorably be deteriorated in corrosion resistance. Thus, the content of Cu is preferably 0.5% or less by mass.

(6) Sc: 0.02 to 0.1% by Mass Both Inclusive

Sc is present in the form of dispersed particles, and contributes to making the crystal grains finer to improve the alloy sheet in shapability. If the addition amount thereof is too small, the number density of the dispersed particles is lowered. Thus, when stretched, the alloy sheet may be unfavorably lowered in yield-strength-increased quantity to be lowered in strength after subjected to paint-baking. In the meantime, if the content is too large, coarse compound particles may be unfavorably produced to deteriorate the alloy sheet in ductility.
Thus, the content of Sc is preferably from 0.02 to 0.1% by mass both inclusive.
(7) One or More Selected from Ag: 0.01 to 0.2% by Mass Both Inclusive, and Sn: 0.001 to 0.1% by Mass Both Inclusive
Ag has an effect of precipitating, tightly and finely, an aged precipitate contributing to an improvement of the aluminum alloy sheet in strength according to artificial aging thermal treatment after the aluminum alloy is worked and shaped into a structural material, thereby promoting an enhancement of the strength. Moreover, the addition of Sn restrains clusters from being produced at room temperature. Thus, Sn has an effect of holding, over a long term, an excellent shaping-workability of the solutionized and quenched sheet. Thereafter, at the time of subjecting the alloy sheet further to an artificial aging such as paint-baking, the alloy sheet is improved in strength.
If the addition amount of these elements is too small, the above-mentioned effects cannot be gained. In the meantime, if the addition amount of Ag is too large, Ag may reversely lower the rollability, the weldability and various other properties of the alloy sheet, the strength-improving effect may also be saturated, and further costs may increase. If the addition amount of Sn is too large, it is feared that the effect thereof is saturated and the alloy sheet rather undergoes hot brittlement to be remarkably deteriorated in hot workability (heat rollability).
Thus, the content of Ag is preferably from 0.01 to 0.2% by mass both inclusive, and that of Sn is preferably from 0.001 to 0.1% by mass both inclusive.
The alloy sheet may contain only one of Ag and Sn, or may contain a combination of the two.

3. Mechanical Properties

As described, the aluminum alloy sheet according to the embodiment of the present invention is lowered in PFZ width, which makes the strength and bendability lower. Thus, the alloy sheet can have not only an excellent strength but also an excellent bendability. For this reason, when the aluminum alloy sheet is applied to a skeleton structure of a transportation machine such as an automobile, the alloy sheet can exhibit an excellent energy-absorbing performance when the machine collides.
The aluminum alloy sheet according to the embodiment of the present invention can especially remarkably express an excellent strength and bendability by subjecting this alloy sheet to an artificial aging to be thermally treated into a T6 material. For example, excellent mechanical properties of the alloy sheet can be verified by subjecting the alloy sheet to an artificial aging of holding the alloy sheet at 200 to 250° C. for 10 to 30 minutes.

(1) Strength

The aluminum alloy sheet according to the embodiment of the present invention shows a 0.2% yield strength of 250 MPa or more, preferably 270 MPa or more, this yield strength being measured in a tensile test after the alloy sheet is artificially aged, that is, after thermally treated into a T6 material. In this case, the alloy sheet can ensure a sufficient strength.

(2) Bendability

The bendability of any alloy sheet can be estimated, using the bending angle (referred to also as the VDA bending angle in the specification) (°) thereof that is measurable by making, for example, a VDA bending test based on a VDA standard “VDA 238-100 Plate Bending Test for Metallic Materials” prescribed in Verband der Automobilindustrie e.V.
The aluminum alloy sheet according to the embodiment of the present invention shows a bending angle of 60° or more, preferably 70° or more, this bending angle being measured in a VDA bending test after the alloy sheet is artificially aged, that is, after thermally treated into a T6 material. In this case, the alloy sheet can ensure a sufficiently good energy-absorbing performance.

4. Producing Method

The following will describe a method for producing the aluminum alloy sheet according to the embodiment of the present invention.
As described above, in the aluminum alloy sheet according to the embodiment of the invention, transition-element-based dispersed particles present on its crystal grain boundaries are sufficiently decreased in number density so that the precipitation of Si, Mg₂Si and others into the grain boundaries can be restrained. Thus, the PFZ width can be sufficiently decreased.
As can be understood from producing-steps that will be described below, such properties can be attained by making a strict control of, particularly, the temperature-raising rate of an alloy for the aluminum alloy sheet in a homogenizing thermal treatment thereof, and further making a control of the temperature for solutionizing the alloy and the cooling rate at the time of cooling just after the solutionizing.
Hereinafter, each of the steps will be described.

(1) Melting and Casting

In melting and casting steps, an Al alloy molten and adjusted into the above-mentioned 6000-series component standard range is initially cast into an aluminum alloy ingot (slab) by a method selected appropriately from ordinary melting and casting methods such as a continuous casting and rolling method and a semi-continuous casting method (DC casting method).

(2) Homogenizing Thermal Treatment

Next, the cast aluminum alloy ingot is subjected to homogenizing thermal treatment.
The temperature of the ingot for the homogenizing thermal treatment is a homogenizing temperature (holding temperature) of 450° C. or higher and lower than the melting point of the ingot. This homogenizing thermal treatment (thermal homogenization) makes it possible to homogenize the microstructure of the ingot, that is, to lose segregations inside crystal grains in the ingot microstructure. If this homogenizing temperature is lower than 450° C., the segregations inside the crystal grains cannot be sufficiently lost. Thus, the remaining segregations act as fracture origins, so that the aluminum alloy is lowered in bendability.
In the embodiment of the present invention, the finally obtained aluminum alloy sheet can be decreased in PFZ width, and can be improved in bendability by controlling heating and temperature-raising conditions in the homogenizing thermal treatment. Specifically, the heating and temperature-raising conditions are controlled as follows:
[Temperatures from 200 to 450° C.: 80° C./Hour, or More]
In the embodiment of the present invention, the aluminum alloy ingot is heated at an average heating rate of 80° C./hour or more in a temperature range from 200 to 450° C. The average heating rate in the temperature range from 200 to 450° C. is preferably 90° C./hour or more, more preferably 100° C./hour or more.
If the average heating rate in this temperature range is less than 80° C./hour, transition-element-based dispersed particles precipitate excessively in the crystal grain boundaries. At the time of cooling the alloy just after a subsequent solutionizing, the following is promoted: the composite precipitation of Mg and Si diffused into the grain boundaries onto the surface of the dispersed particles. Thus, the diffusion of Mg and Si from the matrix is promoted. In this way, near the crystal grain boundaries, the atomic concentration of Mg and Si is lowered. Accordingly, near the crystal grain boundaries, regions of depletion layers of Mg and Si elements are enlarged so that the aluminum alloy is increased in PFZ width to be lowered in bendability.
When the average heating rate in the temperature range is 80° C./hour or more, an excessive precipitation of the transition-element-based dispersed particles can be restrained in the crystal grain boundaries to restrain the above-mentioned composite precipitation of Mg and Si. Consequently, the aluminum alloy can be decreased in PFZ width to be improved in bendability.
[Temperatures from 450° C. to Homogenizing Temperature: 40° C./Hour or Less]
If the average heating temperature in a temperature range from 450° C. to the homogenizing temperature of the alloy is more than 40° C./hour, the transition-element-based dispersed particles precipitated on the grain boundaries cannot be made coarse so that the number density of the dispersed particles on the grain boundaries becomes large. Consequently, the aluminum alloy sheet is increased in PFZ width after artificially aged. For this reason, in the temperature range from 450° C. to the homogenizing temperature, the alloy is heated at an average heating rate of 40° C./hour or less. By heating the alloy in this temperature range at an average heating rate of 40° C./hour or less, the transition-element-based dispersed particles precipitated on the grain boundaries become more coarse to make it possible to promote a decrease in the number density of the transition-element-based dispersed particles precipitated on the grain boundaries. Thus after the artificial aging, the PFZ width can be decreased.

[Holding Time After Arrival at Homogenizing Temperature: 4 Hours or Longer]

It is preferred to heat the aluminum alloy ingot at the above-mentioned average heating rate to cause the temperature thereof to arrive at the homogenizing temperature lower than the melting point, and subsequently hold the ingot at the homogenizing temperature for 4 hours or longer. This manner makes it possible to make the transition-element-based dispersed particles precipitated in the grain boundaries more coarse to decrease the number density thereof.
The homogenizing thermal treatment may be conducted under any one of a one-time thermally-homogenizing condition, a second-time thermally-homogenizing condition, and a two-step thermally-homogenizing condition. The two-stage thermally-homogenizing condition is preferred, and the one-time thermally-homogenizing condition is more preferred.
Under the one-time thermally-homogenizing condition, the ingot is held at the homogenizing temperature and subsequently cooled to a hot rolling starting temperature, or held at or near a hot rolling starting temperature to start hot rolling. Under the two-time thermally-homogenizing condition, the ingot is subjected to a first thermal-homogenization; the ingot is subsequently cooled once to a temperature of 200° C. or lower, which includes room temperature, further re-heated and held at the temperature for a predetermined period; and then hot rolling of the ingot is started. In contrast, under the two-stage thermally-homogenizing condition, the ingot is cooled after the first thermal homogenization; however, the ingot is not cooled to 200° C. or lower; thus, the cooling is stopped at a higher temperature, and the ingot is held at the temperature; and then hot rolling of the ingot is started at the same temperature, or started after the ingot is reheated to a higher temperature.
Under any one of these conditions, in the first temperature-raising (the temperature-raising under the one-time thermally-homogenizing condition, the first temperature-raising under the two-time thermally-homogenizing condition, or the temperature-raising under the two-stage thermally-homogenizing condition) in the homogenizing thermal treatment, the average heating speed is controlled into 80° C./hour or more from 200 to 450° C., thereby making it possible to restrain an excessive precipitation of the transition-element-based dispersed particles in the grain boundaries. Consequently, the finally obtained aluminum alloy sheet can be decreased in PFZ width to be improved in bendability.

(3) Hot Rolling

The ingot (slab) is then hot-rolled. Correspondingly to a sheet thickness to be attained by the rolling, the hot rolling is composed of a rough rolling step thereof, and a finish rolling step thereof. In each of the rough rolling step and the finish rolling step, for example, a reverse type or tandem type rolling machine is appropriately used.
If the hot rolling starting (i.e., rough rolling starting) temperature exceeds the solid phase line temperature of the slab, the slab undergoes burning (i.e., partial melting) so that the hot rolling itself may become difficult to attain. If the hot rolling starting temperature is lower than 350° C., load at the hot rolling time becomes too high so that the hot rolling itself may become difficult to attain. Thus, the hot rolling (rough rolling) starting temperature is preferably from 350° C. to the solid phase line temperature, more preferably from 400° C. to the solid phase line temperature.

(4) Hot Rolling Annealing Treatment

Before subjected to cold rolling, which will be detailed later, the hot-rolled sheet may be optionally annealed (thermally treated). By annealing the hot-rolled sheet, the crystal grains are made finer and the aggregate microstructures are made proper, whereby a further improvement of the sheet can be made in shapability and other properties.

(5) Cold Rolling

The resultant hot-rolled sheet is cold-rolled. In the cold rolling, the sheet is rolled to produce a cold-rolled sheet (examples thereof including a coil) having a desired final thickness. In order to make the crystal grains finer, the cold roll reduction is preferably 40% or more. For the same purpose as the annealing step has, the sheet may be subjected to intermediate annealing in the middle of a path of the cold rolling.

(6) Solutionizing and Quenching

[Solutionizing Temperature: 500° C. or Higher]

The resultant cold-rolled sheet is solutionized and then quenched to room temperature. This solutionizing makes it possible to solid-solutionize Mg and Si sufficiently to improve the alloy sheet in strength. If the solutionizing temperature (holding temperature) is lower than 500° C., the element of each of Mg and Si cannot be sufficiently solid-solutionized so that the alloy sheet cannot gain a high strength. Thus, the solutionizing temperature (holding temperature) is 500° C. or higher, preferably 550° C. or higher.
[Average Cooling Rate from 500 to 300° C.: 50° C./Second or More]
As described above, according to the method for producing the aluminum alloy sheet according to the embodiment of the present invention, in the homogenizing thermal treatment step, the slab is heated at a large average heating rate of 80° C./second or more in the temperature range from 200 to 450° C., and further heated at an average heating rate of 40° C./hour or less in the temperature range from 450° C. to the homogenizing temperature. In this way, about the cold-rolled sheet, the transition-element-based dispersed particles on its crystal grain boundaries are sufficiently decreased in number density by solutionizing the sheet.
In the method for producing the aluminum alloy sheet according to the embodiment of the present invention, the cold-rolled sheet is solutionized, and then the average cooling rate is strictly controlled after the sheet is held at the solutionizing temperature, as described above. In this way, the transition-element-based dispersed particles present on the crystal grain boundaries of the finally obtained aluminum alloy sheet are sufficiently decreased in number density. Thus, the alloy sheet can be decreased in PFZ width to be improved in bendability.
Specifically, in the method for producing the aluminum alloy sheet according to the embodiment of the present invention, after solutionized, the alloy sheet is cooled at an average cooling rate of 50° C./second or more in the temperature range from 500 to 300° C. The cooling at such a large average cooling rate can restrain grain boundary diffusion of Mg and Si during the cooling to restrain the composite precipitation of Mg and Si on the surface of the transition-element-based dispersed particles precipitated in the grain boundaries. It is therefore possible to restrain the formation of depletion layers of Mg atoms and Si atoms near the crystal grain boundaries. For this reason, when the aluminum alloy sheet according to the embodiment of the invention is artificially aged to be thermally treated into a T6 material, the PFZ width of the grain boundaries can be decreased into 60 nm or less, so that the alloy sheet can exhibit an excellent strength and bendability.
In order to ensure such an average cooling rate of 50° C./second or more in the quenching, it is allowable to select an appropriate means from an air cooling means using a fan, water cooling means such as mist, spraying and immersion, and other means, and use the selected means.

(7) Pre-Aging

In the producing method according to the embodiment, after the quenching, the alloy sheet may be optionally subjected to pre-aging to make higher the age hardenability gained in the artificial aging, which is, for example, paint-baking. In the pre-aging, preferably, the alloy sheet is held in a temperature range from 60 to 150° C., preferably from 70 to 120° C. for 1 to 24 hours. After the artificial pre-aging, the cooling rate is preferably 1° C./hour or less.

(8) Preliminary Distorting

In the method for producing the aluminum alloy sheet according to the present invention, after the above-mentioned solutionizing and quenching or, if the pre-aging is performed, after six hours or longer elapse from the completion of the cooling after this pre-aging, a distortion of about 5 to 20% may be optionally applied to the cold-rolled sheet. In this way, dislocations are introduced into the cold-rolled sheet. After the quenching, or after six hours or longer elapse from the end of the pre-aging, clusters are present in the alloy sheet. Consequently, at the time of the subsequent artificial aging or paint-baking, the clusters hinder the shift which follows the restoration of the dislocations so that the dislocations are not easily restored. As a result, not only the conventional precipitation strengthening but also the dislocation strengthening can heighten the strength of the alloy sheet after the artificial aging or the paint-baking. Furthermore, the introduction of the distortions makes it possible to precipitate Mg and Si preferentially on the dislocations present in the PFZs in the grain boundaries at the time of the paint-baking or the artificial aging. Consequently, the alloy sheet can be decreased in PFZ width to be further improved in bendability.
In the preliminary distorting step, the manner of applying the distortion to the cold-rolled sheet may be any manner. The manner is, for example, a manner of applying the distortion thereto, using, for example, a tensile tester, cold rolling or a leveler.
Those skilled in the art who have contacted the above-detailed method for producing the aluminum alloy sheet according to the embodiment of the present invention could gain the aluminum alloy sheet according to the invention by a method different from the above-detailed producing method through trial and error.

EXAMPLES

Hereinafter, the present invention will be more specifically by way of working examples thereof. The invention is not limited by the working examples. Thus, the working examples may be carried out in the state of being each modified into a scope conforming to the subjects of the invention that have been described above and will be described below. Any one of the examples modified in such a way is included in the technical scope of the invention.

1. Specimen Production

Aluminum alloy ingots each having a composition shown in Table 1 were produced by a DC casting method.
In Tables 1 to 3, any underlined numerical value shows that the value is outside the range concerned that is defined by the present invention.

	TABLE 1

	Chemical composition (% by mass) of aluminum alloy sheet (remainder: Al)

Alloy No.

Mg

Si

Fe

Cu

Mn

Zr

Cr

Sc

Sn

Ag

Ti

1	0.7	1.2	0.14	—	0.1	—	—	—	—	—	0.02
2	0.7	1.1	0.14	0.3	0.15	—	—	—	—	—	0.02
3	0.5	1.2	0.14	—	0.20	—	—	—	—	—	0.02
4	0.8	0.9	0.14	—	0.15	—	—	—	—	—	0.02
5	1.3	0.7	0.12	0.30	0.12	—	—	—	—	—	—
8	0.5	1.5	0.13	0.08	0.48	—	—	—	0.04	—	—
7	0.6	0.8	—	—	—	0.14	0.15	—	—	—	—
8	0.6	1.1	0.05	0.15	0.05	0.10	0.06	—	—	—	—
9	0.6	0.8	0.25	—	—	0.17	0.08	0.07	—	0.12	0.02
10	0.4	0.7	0.12	—	0.08	—	—	—	—	—	0.02
11	0.5	0.6	0.12	—	0.08	—	—	—	—	—	0.02

Subsequently, in each of the examples, the resultant ingots were each subjected to homogenizing thermal treatment and hot rolling under conditions shown in Table 2 to yield hot-rolled sheets each having a thickness from 3 to 25 mm. Each of the hot-rolled sheets was cold-rolled to yield a cold-rolled sheet having a thickens of 2.0 mm. The resultant cold-rolled sheet was solutionized under conditions shown in Table 2, and then pre-aged at 100° C. for 8 hours to yield an aluminum alloy sheet treated thermally into a T4 material. The holding period at the arrival temperature in the solutionizing was set to 30 minutes. Predetermined ones out of the entire resultant samples in the individual examples were subjected to preliminary distorting treatment under conditions shown in Table 2. About the individual average heating rates shown in Table 2 in the homogenizing thermal treatment, table-cells for specimens about which the thermally-homogenizing pattern was a two-time thermally-homogenizing condition pattern, out of the entire specimens, each show the average heating rate in the first temperature-raising.
Subsequently, all of the specimens were artificially aged under conditions shown in Table 2 to yield aluminum alloy sheets thermally treated to T6 materials.

	TABLE 2

	Producing conditions

Homogenizing thermal treatment

					Average
				Average	heating rate			Hot rolling
				heating rate	(° C./hr.) from			Hot rolling
				(° C./hr.)	450° C. to	Homogenizing	Holding	starting
	Specimen	Alloy No.	Soaking	from 200 to	homogenizing	temperaturere	period	temperature
Classification	No.	in Table 1	pattern	450° C.	temperature	(° C.)	(hr.)	(° C.)

Invention	1	1	One time	90	40	545	8	505
examples	2	2	One time	80	20	540	6	500
	3	3	One time	95	30	550	6	510
	4	4	One time	85	25	545	8	495
	5	5	Two times	100	40	550	6	400
	6	6	Two times	80	35	540	6	405
	7	7	Two times	95	25	565	4	395
	8	8	Two times	105	15	555	6	410
	9	9	One time	90	30	560	6	505
Comparative	10	1	One time	40	40	540	4	500
examples	11	2	One time	80	60	540	4	500
	12	3	Two times	50	50	540	4	400
	13	4	One time	60	60	545	5	495
	14	4	One time	80	40	520	4	480
	15	10	Two times	80	40	540	4	400
	16	11	Two times	80	40	540	4	400

Producing conditions

Preliminary distorting

Solutionizing

treatment

			Average	Period until
		Holding	cooling rate	application of	Preliminary	Artificial aging
	Specimen	temperature	(° C./s) from	preliminary	distortion	Artificial aging
Classification	No.	(° C.)	500 to 300° C.	distortion	(%)	conditions

Invention	1	550	60	—	—	230° C. × 20 min

	examples	2	545	55	6	hours	6	250° C. × 10 min
		3	550	70	24	hours	9	210° C. × 20 min

4

555

50

—

200° C. × 30 min

5

550

65

120

hours

5

215° C. × 25 min

6

550

80

—

200° C. × 20 min

	7	545	55	36	hours	6	205° C. × 30 min
	8	560	90	192	hours	10	210° C. × 30 min

		9	550	75	—	—	220° C. × 20 min
	Comparative	10	540	50	—	—	200° C. × 20 min

examples

11

545

50

6

hours

5

205° C. × 20 min

12

540

50

—

230° C. × 20 min

13	545	50	One	minute	5	250° C. × 20 min
14	490	50	10	hours	3	225° C. × 20 min
15	550	50	24	hours	5	225° C. × 30 min
16	550	50	72	hours	6	230° C. × 20 min

2. Microstructure Observation

In each of the examples, measurements were made about the number density of transition-element-based dispersed particles which were present on grain boundaries of the aluminum alloy sheet and had a size of 0.05 μm or more, and the PFZ width of the grain boundaries. These results are shown in Table 3.

[Number Density of Dispersed Particles]

Any one of the T4 materials in the example, which were materials before the artificial aging, was observed through an TEM to measure the number density of the transition-element-based dispersed particles which were present on the grain boundaries and had a size of 0.05 μm or more. Specifically, from five sites of the T4 material specimen, samples were collected, and the specimen was adjusted to permit a sheet-thickness central portion thereof to be TEM-observed. An electron beam was then adjusted to make the incident direction thereof parallel to the (100) plane. In each of the samples, a region of the grain boundaries was observed at a magnification of 100000 or more within three viewing fields. In each of the viewing fields, precipitates on the grain boundaries were analyzed by an energy dispersive X-ray spectroscopy, so as to identify transition-element-based precipitates other than Mg—Si based precipitates, Mg—Si—Cu based precipitates, and Si precipitates. Out of these identified precipitates, precipitates each having an equivalent circle diameter of 0.05 μm or more were counted, and then the number density (nm⁻¹) thereof per grain boundary length was calculated. The average of the respective number densities in the three viewing fields in each of the samples was calculated, and then the respective averages of the five-site samples were averaged. The resultant value is used as the number density of the transition-element-based dispersed particles which were present on the grain boundaries of the specimen and had a size of 0.05 μm or more. This value is shown in the column “Number density of grain boundary precipitates” in Table 3.

[PFZ Width of Grain Boundaries]

The PFZ width of the grain boundaries was measured by TEM-observing a T6 material of the example, which was yielded by aging any one of the T6 materials artificially. Specifically, from five sites of the T6 material specimen, samples were collected, and the specimen was adjusted to permit a sheet-thickness central portion thereof to be TEM-observed. An electron beam was then adjusted to make the incident direction thereof parallel to the (100) plane. About each of the samples, its grain boundary region was then observed within three viewing fields at a magnification of 100000 or more. About each of the viewing fields, the PFZ width was measured at a site having the largest PFZ width out of the observed sites. The average value of the respective PFZ widths at the 15 sites in total was then calculated. The resultant value was used as the PFZ width of the grain boundaries of the specimen. Table 3 shows the value in the column “PFZ width”.

3. Mechanical Properties

Mechanical properties of each of the examples were measured as described below. The results are shown in Table 3.

[Strength]

The strength of the example was evaluated by making a tensile test of any one of the T6 materials, which had been artificially aged, and then measuring the 0.2% yield strength (MPa) thereof. The tensile test was made by working the T6 material into a JIS No. 5 specimen to make the tensile direction perpendicular to the rolled direction of the material, and then pulling the material at a between-scoring-point distance of 50 mm and a tensile rate of 5 mm/minute on the basis of JIS Z 2241 (revised in 2011) until the specimen was fractured.
In the present invention, any case where the 0.2% yield strength is 250 MPa or more is judged to be acceptable.

[Bendability]

The bendability of the example was evaluated in accordance with the bending angle (° C.) thereof which was measured by a VDA bending test of each of three of the T6 materials, which had been artificially aged, on the basis of the VDA Standard “VDA 238-100 Plate Bending Test for Metallic Materials” prescribed by Verband der Automobilindustrie e.V. This testing method is illustrated in FIG. 1 as a perspective view.
As represented by a dotted line in FIG. 1, a tabular specimen of the T6 material is initially put horizontally onto two rolls to make right and left parts thereof equal in length to each other, these rolls being positioned to be in parallel to each other and have a roll gap therebetween. Specifically, the T6 material tabular specimen is put horizontally onto two rolls to make right and left parts thereof equal in length to each other, and position the center of the specimen at the center of the roll gap in such a manner that the rolled direction of the specimen is made perpendicular to the extended direction of a pushing and bending tabular tool located vertically above the specimen. From the above, the pushing and bending tool is pushed onto the center of the planer specimen to impose a load onto the specimen. In this way, this tabular specimen is pushed and bent (thrusted and bent) toward the narrow roll gap to push the center of the bent tabular specimen into the narrow roll gap.
In this case, at the time when the load F from the pushing and bending tool shifted from the above becomes maximum, the angle between the bent outside surfaces of a central portion of the tabular specimen is measured as the bend angle (°) thereof to evaluate the impact absorption of the specimen. It can be mentioned that as the bend angle is larger, the tabular specimen is not fractured by the pressure in the middle, so as to keep a sustaining bend deformation so that the specimen is higher in impact absorption (pressure-fracturing resistance) and is better in bendability.
Test conditions for this VDA bending test are demonstrated hereinafter, using reference signs represented in FIG. 1. The tabular specimen is rendered a square of 60 mm in width “b” and 60 mm in length “l”. The roll diameter D of each of the two rolls is 30 mm. The roll gap L is set to 4 mm, which is two times the tabular specimen thickness. The sign “s” is the pushed-in depth of the center of the tabular specimen into the roll gap when the load F becomes maximum. As has been illustrated in FIG. 1, about the tabular pushing and bending tool, the side at the lower end side of this tool, which can be pushed to come into contact with the center of the tabular specimen, is made into a tapered form. This tapered form is such a sharply-curved form that the radius of the tip (lower end) of the side is 0.2 mm. The bending test was made about the three planer specimens (i.e., made three times). As the bending angle (°) of the example, the average of the resultant values was adopted.
In the present invention, any case where the VDA bending angle is 60° or more is judged to be acceptable.

	TABLE 3

	Microstructure

Number density (nm⁻¹) of

Mechanical properties

		grain boundary		0.2% Yield
Classification	Specimen No.	precipitates	PFZ width (nm)	strength (MPa)	VDA bending angle (°)

Invention	1	0.00036	37	286	71
examples	2	0.00082	31	297	75
	3	0.00031	29	299	78
	4	0.00096	45	289	67
	5	0.00053	40	285	84
	6	0.00061	43	293	70
	7	0.00048	52	284	74
	8	0.00067	48	306	63
	9	0.00028	47	311	66
Comparative	10	0.0017	87	278	59
Examples	11	0.00114	61	259	57
	12	0.00135	79	256	38
	13	0.00129	62	268	47
	14	0.00211	83	234	65
	15	0.00093	73	229	70
	16	0.00087	79	218	72

4. Conclusion

As shown in Tables 1 to 3, specimens Nos. 1 to 9, which are invention examples, are examples satisfying all the requirements (about the composition, the producing conditions and the microstructure) specified by the present invention. These specimens can each attain a 0.2% yield strength of 250 MPa or more, and a VDA bending angle of 60° or more.
By contrast, about No. 10, the average heating rate is small in the temperature range from 200 to 450° C. in the homogenizing thermal treatment. Thus, the dispersed particles are excessively precipitated on the grain boundaries to increase the PFZ width. Consequently, the bendability is insufficient.
About No. 11, the average heating rate is large in the temperature range from 450° C. to the homogenizing temperature in the homogenizing thermal treatment. Thus, the dispersed particles on the grain boundaries are increased in number density to increase the PFZ width. Consequently, the bendability is insufficient.
About No. 12, the average heating rate is small in the temperature range from 200 to 450° C. in the homogenizing thermal treatment, and is large in the temperature range from 450° C. to the homogenizing temperature therein. Thus, the dispersed particles are excessively precipitated on the grain boundaries to increase the PFZ width. Consequently, the bendability is insufficient.
About No. 13, the average heating rate is small in the temperature range from 200 to 450° C. in the homogenizing thermal treatment, and is large in the temperature range from 450° C. to the homogenizing temperature therein. Thus, the dispersed particles on the grain boundaries are increased in number density to increase the PFZ width. Consequently, the bendability is insufficient.
No. 14 is low in holding temperature in the solutionizing so that the dispersed particles are excessively precipitated on the grain boundaries to increase the PFZ width. Consequently, the strength is insufficient.
No. 15 is small in Mg content by percentage to be increased in PFZ width to be insufficient in strength.
No. 16 is small in Si content by percentage to be increased in PFZ width to be insufficient in strength.

Claims

1. An Al—Mg—Si aluminum alloy sheet, comprising:

Mg: 0.5 to 1.3% by mass;

Si: 0.7 to 1.5% by mass;

one or more elements selected from the group consisting of the following:

Mn: 0.05 to 0.5% by mass,

Zr: 0.04 to 0.20% by mass, and

Cr: 0.04 to 0.20% by mass; and

Al and inevitable impurities,

wherein:

the aluminum alloy sheet has grains, and grain boundaries therebetween;

transition-element-based dispersed particles are present on the grain boundaries and have a size of 0.05 μm or more and have a number density of 0.001 nm⁻¹or less; and

the grain boundaries show a PFZ width of 60 nm or less after the aluminum alloy sheet is subjected to an artificial aging of holding the aluminum alloy sheet at 200 to 250° C. for 10 to 30 minutes.

2. The aluminum alloy sheet according to claim 1, further comprising:

Cu: more than 0% by mass, and 0.5% or less by mass.

3. The aluminum alloy sheet according to claim 1, further comprising:

Sc: 0.02 to 0.1% by mass.

4. The aluminum alloy sheet according to claim 2, further comprising:

Sc: 0.02 to 0.1% by mass.

5. The aluminum alloy sheet according to claim 1, further comprising one or more elements selected from the group consisting of:

Ag: 0.01 to 0.2% by mass, and

Sn: 0.001 to 0.1% by mass.

6. The aluminum alloy sheet according to claim 2, further comprising one or more elements selected from the group consisting of:

Ag: 0.01 to 0.2% by mass, and

Sn: 0.001 to 0.1% by mass.

7. The aluminum alloy sheet according to claim 3, further comprising one or more elements selected from the group consisting of:

Ag: 0.01 to 0.2% by mass, and

Sn: 0.001 to 0.1% by mass.

8. The aluminum alloy sheet according to claim 1, showing a 0.2% yield strength of 250 MPa or more, and a VDA bending angle of 60° or more after the aluminum alloy sheet is subjected to the artificial aging of holding the aluminum alloy sheet at 200 to 250° C. for 10 to 30 minutes.