ALUMINUM ALLOY SHEET FOR FORMING

Abstract

The present invention pertains to an Al—Mg—Si alloy sheet for forming, that contains 0.2-2.0% of Mg, 0.3-2.0% of Si, and 0.005-0.3% of Si (all amounts given with respect to mass), the balance comprising Al and unavoidable impurities, wherein the aluminum alloy sheet for forming is characterized in that the structure of the aluminum alloy sheet is such that the average number density of compounds having a circle-equivalent diameter within a range of 0.3-20 μm as measured by SEM at 500 times magnification is more than 0/mm2 but not more than 5,000/mm2, and of the compounds measured by SEM, the average count ratio of compounds that contain 0.5% by mass or more of Sn as identified using an X-ray spectrograph, is 0% or more but less than 50%. This aluminum alloy sheet for forming exhibits high BH response and good formability.

Description

TECHNICAL FIELD

The present invention relates to an Al—Mg—Si alloy sheet for forming. The term “aluminum alloy sheet” used in the present invention means an aluminum alloy sheet that is a rolled sheet, such as a hot-rolled sheet or cold-rolled sheet, and that has undergone refining, such as a solution heat treatment and a quenching treatment, and has not undergone a bake hardening treatment. Hereinafter, aluminum is referred to also as Al.

BACKGROUND ART

In recent years, the social request for weight reduction in vehicles including automobiles is increasing more and more due to considerations to the global environment or the like. In order to meet the request, aluminum alloy materials which are excellent in terms of formability and bake hardenability and are more lightweight are coming to be increasingly used as materials for automotive panels, in particular, large body panels such as hood, door and roof (outer panels and inner panels), in place of steel materials such as steel sheets.

Among those, as panels such as the outer panels (outer sheets) and inner panels (inner sheets) of panel structures, such as hoods, fenders, doors, roofs, and trunk lids of automobile, use of Al—Mg—Si-based AA or JIS 6000-series (hereinafter simply referred to also as “6000-series”) aluminum alloy sheets which are thin high-strength aluminum alloy sheets is being investigated.

The 6000-series aluminum alloy sheets contain Si and Mg as essential components. In particular, a 6000-series aluminum alloy with excess Si has a composition in which the Si/Mg mass ratio is 1 or greater, and has excellent age hardenability. Because of this, this sheet not only shows reduced proof stress during press forming and bending to ensure formability, but also has such bake hardenability (hereinafter referred to also as BH response) that the sheet undergoes age hardening upon heating in an artificial aging (hardening) treatment at a relatively low temperature, such as a paint baking treatment of panels after being formed, to have an increased proof stress, thereby ensuring the strength required of panels.

Furthermore, the 6000-series aluminum alloy sheets are relatively low in alloying-element content as compared with other 5000-series aluminum alloys and the like having a high alloy content, e.g., Mg content. Because of this, in cases when scraps of these aluminum alloy sheets are reused as an aluminum-alloy material to be melted (raw material to be melted), slabs of the original 6000-series aluminum alloys are easy to obtain. They thus have excellent recyclability.

Meanwhile, as is known well, an automotive outer panel is produced by subjecting an aluminum alloy sheet to combined formings such as stretch forming in press forming and bending forming. For example, in the case of a large outer panel such as a hood or door, the shape of a formed product as the outer panel is imparted by press forming such as stretching and then the peripheral edge part of this outer panel is subjected to hem work (hemming) to form a flat hem or the like and thereby joining with an inner panel is performed. Thus, a panel structure is obtained.

The 6000-series aluminum alloys have the advantage of having excellent BH response but have room-temperature aging properties. There has hence been a problem in that they, when held at room temperature for several months after a solution quenching treatment, undergo age hardening and increase in strength, thereby deteriorating in formability into panels, in particular, bendability. For example, in the case where a 6000-series aluminum alloy sheet is for use in automotive panel applications, it is subjected to a solution heat treatment and a quenching treatment in the aluminum manufacturer and thereafter (after the production) placed at room temperature (allowed to stand at room temperature) usually for about 1-4 months until it is formed into panels at an automobile manufacturer. During this period, age hardening (room-temperature aging) proceeds considerably. Especially in the case of outer panels for which severe bending is performed, even when it can be formed without any problem at one month after the production, there has been a problem in that cracking occurs or the like in hem working at three months after the production. Consequently, in 6000-series aluminum alloy sheets for automotive panels, in particular, for outer panels, it is necessary to suppress room-temperature aging over a relatively long period of about 1-4 months.

Moreover, in the case where such room-temperature aging is great, a problem also arises in that the BH response deteriorates and the heating during an artificial aging (hardening) treatment at a relatively low temperature, such as a paint baking treatment of the panel after being formed, does not improve the proof stress to such a degree that the panel comes to have the required strength.

Hitherto, from the standpoint of the microstructure of 6000-series aluminum alloy sheets, in particular, the compounds (crystals or precipitates) formed by elements contained therein, various proposals have been made on property improvements such as attempting improvements in formability or BH response and inhibition of room-temperature aging. Recently, in particular, it has been proposed to make an attempt to directly examine and control clusters (aggregates of atoms) which affect the BH response and room-temperature aging properties of a 6000-series aluminum alloy sheet.

Furthermore, in conventional patent documents which are relevant to the addition of Sn according to the present invention, many methods have been proposed in which Sn is positively added to a 6000-series aluminum alloy sheet to suppress room-temperature aging and improve bake hardenability. For example, Patent Document 1 proposes a method in which Sn, which has the effect of suppressing change-with time, is added in an appropriate amount and a solution heat treatment and subsequent preliminary aging are performed to thereby obtain both of suppressed room-temperature aging properties and bake hardenability. Patent Document 2 proposes a method in which Sn, which has the effect of suppressing change-with-time, and Cu, which improves formability, are added to improve formability, bake hardenability and corrosion resistance.

PRIOR ART DOCUMENTS

Patent Documents

Patent Document 1: JP-A-09-249950

Patent Document 2: JP-A-10-226894

SUMMARY OF THE INVENTION

Problem that the Invention is to Solve

However, even in a conventional Al—Mg—Si alloy sheets to which Sn has been positively added, there has still been room for improvement for obtaining both satisfactory formability and high BH response after long-term room-temperature aging.

In view of that problem, an object of the present invention is to provide an Al—Mg—Si alloy sheet for forming which contains Sn and can exhibit high BH response and satisfactory workability even when a car body paint baking treatment is performed after long-term room-temperature aging.

Means for Solving the Problem

For achieving the object, the gist of the aluminum alloy sheet for forming of the present invention is an Al—Mg—Si alloy sheet containing, in terms of mass %, 0.2-2.0% of Mg, 0.3-2.0% of Si and 0.005-0.3% of Sn, with the remainder being Al and unavoidable impurities, the aluminum alloy sheet having a microstructure in which compounds each having an equivalent circular diameter in a range of 0.3-20 μm, as examined with an SEM having a magnification of 500 times, have an average number density of more than 0 count/mm² and 5,000 counts/mm² or less, and among the compounds examined with the SEM, compounds containing Sn in an amount of 0.5 mass % or more as identified with an X-ray spectrometer have an average proportion in number of 0% or more and less than 50%.

Effects of the Invention

In the microstructure of the Al—Mg—Si alloy sheet, the Sn serves at room temperature to capture (trap) atomic holes to thereby have the effect of inhibiting room-temperature diffusion of Mg and Si to inhibit the strength from increasing at room temperature, and during the forming of the sheet into panels, to improve the press formability including hem workability, drawability and punch stretch formability (hereinafter, this press formability is referred to also as hem workability as a representative). During an artificial aging treatment of the panels, such as a paint baking treatment, the Sn releases the captured holes and hence in turn enhances the diffusion of Mg and Si, thereby enhancing the BH response.

However, the present inventors have found that the addition of such Sn involves a great restriction due to peculiar properties of Sn. The Sn's effect of capturing and releasing atomic holes is exhibited only when the Sn forms a solid solution in the matrix. However, the amount in which Sn forms a solid solution in the matrix is so small that even when the addition amount of Sn is reduced to equal to or below a theoretical solute amount in usual sheet production processes, a large proportion thereof does not form a solid solution and undesirably crystallizes out or precipitates as compounds. The Sn which has thus crystallized out or precipitated as compounds does not have the effect of capturing and releasing atomic holes.

Because of this, in the present invention, the present inventors have ventured to reconsider sheet production processes and contrived production conditions concerning, for example, process annealing to control the state of the Sn contained, as will be described later. The Sn is thereby inhibited from precipitating as compounds and the formation of a solid solution of the Sn in the matrix is enhanced, thereby ensuring a solute Sn amount. Thus, aging is suppressed by the Sn's effect of capturing and releasing atomic holes and the effect of improving the hem workability and BH response is sufficiently exhibited.

As a result, it is possible to provide an Sn-containing Al—Mg—Si alloy sheet which, even when having undergone long-term room-temperature aging for, for example, 100 days after the sheet production, can exhibit higher formability and BH response.

In a conventional Sn-containing Al—Mg—Si alloy sheets, such an effect of Sn has been unable to be sufficiently exhibited.

The reasons for this are presumed to be because, although the formation of solid solution and the precipitation of Mg and Si, which are major elements, have always been attracting attention hitherto, the existence state of the solid-solution or precipitate of Sn, which merely is one of selectively used additive elements, has been attracting little attention. In the sheets produced by ordinary methods, the Sn is present in the form of compounds formed by crystallization or precipitation (hereinafter simply referred to as precipitation). Unlike this, and because causing Sn to form a solid solution is difficult in itself and the solid solution state of Sn is rare existence state, it is presumed that the effect produced by Sn present in a solid-solution state has been less apt to be found out.

MODES FOR CARRYING OUT THE INVENTION

Modes for carrying out the present invention will be explained below in detail with respect to each requirement.

(Chemical Component Composition)

First, the chemical component composition of the Al—Mg—Si (hereinafter referred to also as 6000-series) alloy sheet of the present invention is explained below. As sheets for automotive panels or the like, 6000-seiries aluminum alloy sheets to which the present invention relates are required to have various properties including excellent formability, BH response, strength, weldability, and corrosion resistance. Namely, even the sheets which have undergone a refining treatment and thereafter undergone long-term room-temperature aging for 100 days are required to be excellent in terms of press formability into automotive panels or the like and hem workability, and also excellent in BH response, so that the As proof stress is 110 MPa or less, BH (bake hardening) response of 100 MPa or greater in terms of proof stress difference, and hem workability of 2 or greater according to the evaluation criteria which will be described later in Examples.

In order to satisfy such requirements, the aluminum alloy sheet has, as a prerequisite, a composition containing, in terms of mass %, 0.2-2.0% of Mg, 0.3-2.0% of Si and 0.005-0.3% of Sn, with the remainder being Al and unavoidable impurities. All indications by % of the each element content mean mass %. In this description, percentage on mass basis (mass %) is the same as percentage on weight basis (wt %). With respect to the content of a chemical component, there are cases where “X % or less (exclusive of 0%)” is expressed by “more than 0% and X % or less”.

It is preferable that 6000-series aluminum alloy sheets to which the present invention relates should be 6000-series aluminum alloy sheets with excess Si in which the mass ratio of Si to Mg, Si/Mg, is 1 or greater and which have better BH response. 6000-series aluminum alloy sheets not only show reduced proof stress during press forming and bending to ensure formability, but also have such excellent age hardenability (BH response) that the sheets undergo age hardening upon heating in an artificial aging treatment at a relatively low temperature, such as a paint baking treatment of panels after being formed, to have an increased proof stress, thereby ensuring necessary strength. Of these sheets, the 6000-series aluminum alloy sheets with excess Si are superior in the BH response to the 6000-series aluminum alloy sheets in which the mass ratio Si/Mg is less than 1.

In the present invention, elements other than Mg and Si are impurities or elements which may be contained, and are regulated to contents (permissible amounts) on elements levels according to the AA or JIS standards, etc.

Namely, in the present invention also, in the cases where not only high-purity Al base metal but also 6000-series alloys containing elements other than Mg and Si as additive elements (alloying elements) in large amounts, other aluminum alloy scrap materials, low-purity Al base metal, and the like are used in large quantities as melted raw materials for the alloy, from the standpoint of resource recycling, other elements such as those shown below are inevitably included in substantial amounts. Since refining performed for intentionally diminishing these elements itself leads to an increase in cost, it is necessary to accept some degree of inclusion. There are useful content ranges which do not inhibit the object or effects of the present invention, even when such elements are included in substantial amounts.

Consequently, in the present invention, inclusion of each of such elements shown below is permissible within the range not beyond the upper limit specified below, which is in accordance with the AA or JIS standards or the like.

Specifically, there may be further contained one kind or two or more kinds selected from the group consisting of 1.0% or less (exclusive of 0%) of Mn, 1.0% or less (exclusive of 0%) of Cu, 1.0% or less (exclusive of 0%) of Fe, 0.3% or less (exclusive of 0%) of Cr, 0.3% or less (exclusive of 0%) of Zr, 0.3% or less (exclusive of 0%) of V, 0.05% or less (exclusive of 0%) of Ti, 1.0% or less (exclusive of 0%) of Zn, and 0.2% or less (exclusive of 0%) of Ag, within those ranges in addition to the basic composition shown above.

In the case where these elements are contained, the content of Cu is preferably 0.7% or less, more preferably 0.3% or less, because Cu is prone to impair the corrosion resistance when contained in a large amount. Meanwhile, Mn, Fe, Cr, Zr, and V are prone to yield relatively coarse compounds when contained in large amounts, and are prone to impair the hem workability (hem bendability), which is addressed by the present invention. Consequently, the content of Mn is preferably 0.6% or less, more preferably 0.3% or less, and the content of each of Cr, Zr, and V is preferably 0.2% or less, more preferably 0.1% or less.

The content range and the purposes of each element or permissible amount thereof in the 6000-series aluminum alloy are explained below in order.

Si: 0.3-2.0%

Si, as a major element, is an essential element for contributing to solid-solution strengthening, and for forming Mg—Si precipitates which contribute to an improvement in strength, during an artificial aging treatment such as a paint baking treatment, thus exhibiting age hardenability and thereby obtaining the strength (proof stress) required of automotive outer panels. From the standpoint of exhibiting excellent age hardenability in a paint baking treatment after forming into panels, it is preferable that the 6000-series aluminum alloy is made to have a composition which has an Si/Mg mass ratio of 1.0 or greater and in which Si has been incorporated in a larger amount, relative to Mg, than in the so-called excess Si type. In the case where the content of Si is too low, Mg—Si precipitates are yielded in an insufficient amount, resulting in a considerable decrease in BH response. Meanwhile, in the case where the content of Si is too high, coarse crystals and precipitates are formed, resulting in a considerable decrease in bendability. Consequently, the Si is in the range of 0.3-2.0%. A more preferred lower limit thereof is 0.6%, and a more preferred upper limit thereof is 1.4%.

Mg: 0.2-2.0%

Mg also, as a major element, is an essential element for contributing to solid-solution strengthening, and for forming Mg—Si precipitates which contribute to an improvement in strength, during an artificial aging treatment such as a paint baking treatment, thus exhibiting age hardenability and thereby obtaining the proof stress required of panels. In the case where the content of Mg is too low, Mg—Si precipitates are yielded in an insufficient amount, resulting in a considerable decrease in BH response. Consequently, the proof stress required of panels is not obtained. Meanwhile, in the case where the content of Mg is too high, coarse crystals and precipitates are formed, resulting in a considerable decrease in bendability. Consequently, the content of Mg is in the range of 0.2-2.0%. A more preferred lower limit thereof is 0.3%, and a more preferred upper limit thereof is 1.0%.

Sn: 0.005-0.3%

Sn is an essential element and at room temperature, it has the effects of capturing atomic holes to thereby inhibit room-temperature diffusion of Mg and Si and inhibit a room-temperature increase in strength (room-temperature age hardening) from occurring over a prolonged period, and of improving the press formability, in particular hem workability, of the sheet when the sheet which has undergone room-temperature aging is press-formed into panels. Meanwhile, during an artificial aging treatment of the formed panels, such as a paint baking treatment, the Sn releases the captured holes and hence in turn enhances the diffusion of Mg and Si, thereby enhancing the BH response.

In the case where the content of Sn is too low, not only the room-temperature increase in strength cannot be inhibited, resulting in an increase in proof stress and impaired hem workability, but also Mg—Si precipitates are prone to be yielded in a reduced amount during a BH treatment, resulting in a tendency to a decrease in BH response. Consequently, the content of Sn is in the range of 0.005-0.3%. A more preferred lower limit thereof is 0.01%, and a more preferred upper limit thereof is 0.2%.

It is, however, noted that these effects of Sn are exhibited only when the Sn forms a solid solution. Consequently, in the present invention, a necessary solute Sn amount is ensured by specifying the proportion in number of compounds containing Sn in a certain amount or larger among compounds each having a size within a specific range, as will be described later.

Therefore, the Sn-containing Al—Mg—Si alloy sheet of the present invention considerably differs from Al—Mg—Si alloy sheets free from Sn in both microstructure and property because of the formation of solid solution of the Sn. Even in the case of Al—Mg—Si alloy sheets into which Sn has been incorporated similarly (in the same amount), they come to differ in solute Sn amount when production conditions concerning, for example, process annealing differ therebetween. Under ordinary sheet production conditions (ordinary processes), the Sn is prone to precipitate as compounds, resulting in a considerably reduced solute amount. The sheets hence come to differ considerably from each other in microstructure thereof. Because of this, even if Sn has been incorporated similarly (in the same amount), a microstructure which, as in the present invention, is effective in highly suppressing room-temperature aging properties and in improving the BH response and hem workability is not always obtained.

(Microstructure)

The microstructure of the 6000-series aluminum alloy sheet of the present invention is explained below.

Average Number Density of Compounds:

First, the sheet has a microstructure in which compounds each having an equivalent circular diameter in a range of 0.3-20 μm, as examined with an SEM having a magnification of 500 times, have an average number density of 5,000 counts/mm² or less (exclusive of 0 count/mm²), i.e., more than 0 count/mm² and 5,000 counts/mm² or less, preferably 4,500 counts/mm² or less (exclusive of 0 count/mm²), more preferably 4,000 counts/mm² or less (exclusive of 0 count/mm²).

In the cases when the number density of compounds having an equivalent circular diameter of 0.3 μm or larger is reduced to 5,000 counts/mm² or less, the number of points from which fractures start in the microstructure of the sheet during forming decreases to improve the hem workability. In addition, increases in solute Mg amount and solute Si amount are brought about, and Mg—Si precipitates, which are yielded during a BH treatment and contribute to strength enhancement, are yielded in a larger amount, thereby improving the BH response as well. Meanwhile, in the case where the number density of compounds having an equivalent circular diameter of 0.3 μm or larger exceeds 5,000 counts/mm², the number of points from which fractures start in the microstructure of sheet during forming increases, resulting in decreases in hem workability and press formability such as drawability and stretch formability. In addition, decreases in solute Mg amount and solute Si amount result, and Mg—Si precipitates, which are yielded during a BH treatment and contribute to strength enhancement, are yielded in an insufficient amount, resulting in a decrease in BH response.

Whether a compound can be a starting point from which a fracture starts during hem working depends on the size thereof, and does not depend on the composition thereof. The larger the equivalent circular diameter beyond 0.3 μm the more the compound is prone to be a starting point for fracture. However, coarse compounds having an equivalent circular diameter exceeding the specified upper limit of 20 μm considerably impair the basic mechanical properties and quality of the sheet. Because of this, according to ordinary sheet production processes and quality control, production is performed while minimizing the presence of such coarse compounds. Such a measuring range with SEM is hence meaningless. Consequently, the equivalent circular diameter of compounds specified in the present invention is in the range of 0.3-20 μm, and the compounds are not limited in composition. Here, the equivalent circular diameter of a compound according to the present invention is the diameter of a circle having the same area as the compound, which has an indefinite shape. It has conventionally been in wide use as a method for precisely measuring or specifying the size of a compound with satisfactory reproducibility.

The lower the average number density of compounds each having an equivalent circular diameter in the range of 0.3-20 μm, the more preferred from the standpoints of diminishing starting points for fractures during hem working and ensuring a solute Sn amount. In actual production, however, it is impossible to reduce it completely to 0 count/mm² because of production limits of efficient sheet production method. Consequently, a lower limit, as specified in the present invention, of the average number density of compounds each having an equivalent circular diameter in the range of 0.3-20 μm does not include 0 count/mm². In view of limitations due to efficient sheet production, the lower limit is about 100 counts/mm².

In an observation of a black-and-white image with an SEM having a magnification of 500 times, compounds observed in the sheet microstructure are various compounds (precipitates or crystals) observed as white grains scatteringly present in the microstructure, such as Al—Fe, Al—Fe—Mn, and Al—Fe—Mn—Si compounds and Sn-containing compounds including Al—Si—Sn compounds. There are also cases where a small number of Mg—Si compounds are sporadically observed as black grains scatteringly present in the microstructure. Thus, the compounds have various compositions depending on the compositions of the aluminum alloy sheets, and it is difficult to limit the compositions thereof to specific ones. This also is a reason why the compounds specified in the present invention are not limited in composition.

Measure of Solute Sn Amount:

The present invention is characterized in that a solute Sn amount necessary for exhibiting the effects of Sn is ensured. As a measure (criterion) of ensuring the solute Sn amount, among the compounds examined with the SEM and having an equivalent circular diameter in the range of 0.3-20 μm, compounds containing Sn in an amount of 0.5 mass % or more which have been identified with an X-ray spectrometer have an average proportion in number of less than 50% (inclusive of 0%), i.e., 0% or higher and less than 50%, preferably less than 40% (inclusive of 0%), more preferably less than 30% (inclusive of 0%). Any compound having an Sn content less than 0.5% is not included in the examination object of Sn-containing compounds, as a measure of solute Sn amount. Here, in the case where compounds which contain an extremely small amount of Sn to have a content of Sn less than 0.5 mass % are also included in the examination object, there is a possibility that even compounds containing Sn in a smaller amount than a measurement error in the X-ray spectrometer might be detected and all the compounds within the size range shown above might be included in the examination object undesirably. In such a case, the solute Sn amount cannot be accurately reflected. Consequently, a lower limit of 0.5 mass % or more was set on the Sn contents of the compounds, from the standpoints of correlation and reproducibility.

That the proportion of the number (average number) of compounds containing Sn in an amount of 0.5 mass % or more to the number of all the compounds observed with the SEM and having an equivalent circular diameter in the range of 0.3-20 μm is less than 50% indicates that the amount of Sn which has precipitated is small and the solute Sn amount suffices for the added Sn to exhibit the effects thereof as described above. Meanwhile, in the case where the proportion of the number (average number) of the compounds containing Sn in an amount of 0.5 mass % or more is 50% or higher, this indicates that the amount of Sn which has precipitated is large and the solute Sn amount is too small for the added Sn to exhibit the effects thereof.

The indirect measurement of solute Sn amount in the present invention, which is the proportion in number of compounds containing Sn in an amount not less than a specific value among compounds having a specific size, renders the evaluation of solute Sn amount easy with satisfactory reproducibility.

Furthermore, the evaluation of solute Sn amount by means of the proportion in number of Sn-containing compounds, although being an indirect measuring method, correlates well with the effects exhibited by solute Sn, so long as the compounds which have an equivalent circular diameter in the range of 0.3-20 μm and contain Sn in an amount of 0.5 mass % or more are the examination object. Namely, the effects exhibited by solute Sn correlates well with the proportion of the number (average number) of Sn-containing compounds in which whether Sn is contained in an amount of 0.5 mass % or more or not is determined with an X-ray spectrometer. This feature is supported by the Examples which will be given later.

The effects of inhibiting the room-temperature diffusion of Mg and Si and inhibiting a room-temperature increase in strength (room-temperature age hardening) from occurring over a prolonged period, which are due to the capture of atomic holes at room temperature, are exhibited only when the proportion of the average number of compounds containing Sn in an amount of 0.5 mass % or more is regulated so as to be less than 50% as specified above to thereby ensure a solute Sn amount. As a result, the sheet which has undergone room-temperature aging shows improved press formability, in particular, improved hem workability, when press-formed into panels. In addition, during an artificial aging treatment of the formed panels, such as a paint baking treatment, the effect of releasing the captured holes is exhibited, and the diffusion of Mg and Si can hence be enhanced to heighten the BH response.

Theoretically, a lower limit of the proportion of the number (average number) of compounds containing Sn in an amount of 0.5 mass % or more is the case of 0%, which is the case where all the Sn has formed a solid solution and the number of Sn-containing compound is zero. It is, however, noted that in ordinary processes, Sn is prone to precipitate and the Sn which has once precipitated is very difficult to bring into a solid-solution state again, as in sheet production processes which will be described later. Consequently, though the proportion in number of Sn-containing compounds can be made 0% if the efficiency of production is ignored, in view of limitations in efficient (industrial) production, a lower limit of the proportion of the number (average number) of Sn-containing compounds is about 0.1%.

Examination of Compounds:

The measurement for the number density of compounds having an equivalent circular diameter in the range of 0.3-20 μm with an SEM having a magnification of 500 times is made with respect to ten portions arbitrarily selected at a depth corresponding to ¼ the sheet thickness direction from a surface of a test sheet (ten specimens are collected). The number densities determined with respect to these specimens are averaged to obtain an average number density (counts/mm²). A measurement for the proportion in number of compounds containing Sn in an amount of 0.5 mass % or more, which will be described later, is also made along with this measurement with the SEM, and the proportions in number for the specimens are averaged to obtain an average proportion in number (%) in the same manner. Specifically, as for a cross-section perpendicular to the sheet thickness direction of a test sheet which has just undergone a refining treatment, with respect to a plane which passes through arbitrarily selected points located at a depth corresponding to ¼ the sheet thickness direction from a surface and which is parallel with the sheet surface, an examination is made with an SEM (scanning electron microscope) having a magnification of 500 times. Specimens are prepared in the following manner. Surfaces of ten sheet cross-section specimens obtained by sampling the above-described part are mechanically ground to remove a layer of about 0.25 mm from each sheet surface by the mechanical grinding. The surfaces are then regulated by buffing to prepare the specimens. Next, the number of compounds having an equivalent circular diameter within the range shown above is counted with an automatic analyzer while utilizing reflected-electron images, and a number density is calculated therefrom. The parts to be examined are the polished specimen surfaces, and the examination region in each specimen is 240 μm×180 μm.

The X-ray spectrometer to be used for determining the proportion in number of Sn-containing compounds is well known as an analyzer based on energy dispersive X-ray spectroscopy, and is usually called EDX. This X-ray spectrometer usually belongs to the SEM to be used in the present invention, and is generally used for quantitative analysis for the compositions or the like of compounds observed. With this X-ray spectrometer, the number of compounds identified to contain Sn among all number of the compounds having an equivalent circular diameter in the range of 0.3-20 μm observed with the SEM is counted, and the results of the measurements on the ten specimens are averaged to calculate an average proportion in number.

(Production Process)

Next, a process for producing the aluminum alloy sheet of the present invention is explained below.

Production steps of the aluminum alloy sheet of the present invention are themselves ordinary method or known method. It may be produced by forming, by casting, a slab of an aluminum alloy having the 6000-series component composition, thereafter performing a homogenizing heat treatment, hot rolling, and cold rolling to obtain a given sheet thickness, and then further performing a refining treatment such as a solution quenching treatment.

However, for making the Sn form a solid solution during these production steps to obtain the specified proportion of the number (average number) of Sn-containing compounds, not only the average cooling rate during the casting is controlled but also use is made of specified preferred conditions for process annealing during the cold rolling. In the case where such process annealing conditions are not used, it is difficult to make the Sn form a solid solution and obtain the specified proportion of the number (average number) of Sn-containing compounds.

(Melting and Casting Cooling Rate)

First, in melting and casting steps, an aluminum alloy melt that has been melted and regulated so as to have a component composition within the 6000-series composition range is cast by a suitably selected ordinary melting and casting method, such as a continuous casting method or a semi-continuous casting method (DC casting method). Here, from the standpoint of obtaining the number density of compounds having an equivalent circular diameter of 0.3 μm or greater specified in the present invention and the proportion of the number (average number) of Sn-containing compounds specified in the present invention, it is preferable that the average rate of cooling from the liquidus temperature to the solidus temperature during the casting should be as high (quick) as possible at 30° C./min or greater.

In the case where such temperature (cooling rate) control in a high-temperature region during the casting is not performed, the cooling rate in this high-temperature region is inevitably low. Such a reduced average cooling rate in the high-temperature region results in a larger amount of coarsely yielded crystals in the temperature range of the high-temperature region and gives a slab having increased unevenness in crystal size or amount along the sheet width direction and thickness direction. As a result, it becomes highly probable that the number density of compounds having an equivalent circular diameter of 0.3 μm or greater and the proportion of the number (average number) of Sn-containing compounds cannot be regulated so as to be within the ranges specified in the present invention.

(Homogenizing Heat Treatment)

Next, the aluminum alloy slab obtained by casting is subjected to a homogenizing heat treatment prior to hot rolling. The purpose of this homogenizing heat treatment (soaking treatment) is to homogenize the microstructure, that is, to eliminate segregation within the grains in the microstructure of the slab. The conditions are not particularly limited so long as the purpose is achieved therewith, and the treatment may be an ordinary one conducted once or in one stage.

A homogenizing heat treatment temperature is suitably selected from the range of 500° C. or higher and lower than the melting point, and a homogenizing time is suitably selected from the range of 4 hours or more. In the case where the homogenizing temperature is too low, the segregation within grains cannot be sufficiently eliminated and they act as starting points for fractures, resulting in decreases in stretch flangeability and bendability. Thereafter, hot rolling may be started immediately. Alternatively, hot rolling may be started after holding and cooling to an appropriate temperature.

After the homogenizing heat treatment has been performed, cooling may be performed to room temperature at an average cooling rate of 20-100° C./h between 300° C. and 500° C., then reheating may be performed to 350° C.-450° C. at an average heating rate of 20-100° C./h, and then hot rolling can be started in this temperature range.

In the case where the average rate of cooling after the homogenizing heat treatment and the rate of reheating to be conducted thereafter do not satisfy those conditions, the possibility of forming coarse Mg—Si compounds increases, resulting in decreases in the basic mechanical properties, such as strength and elongation, that are a prerequisite which the 6000-series aluminum alloy sheet must satisfy before making the Sn exhibit the effects thereof.

(Hot Rolling)

The hot rolling is constituted of a slab rough rolling step and a finish rolling step in accordance with the thickness of the sheet to be rolled. In the rough rolling step and finish rolling step, rolling mills such as a reverse type and a tandem type are suitably used.

In such conditions that the hot-rolling (rough-rolling) start temperature exceeds the solidus temperature, burning occurs and, hence, the hot rolling itself is difficult to carry out. Meanwhile, in the case where the hot-rolling start temperature is lower than 350° C., the load during hot-rolling is too high, rendering the hot rolling itself difficult. Consequently, the hot-rolling start temperature is preferably in the range of 350° C. to the solidus temperature, more preferably in the range of 400° C. to the solidus temperature.

(Annealing of Hot-Rolled Sheet)

Annealing (rough annealing) before cold rolling is not always necessary for the hot-rolled sheet. However, it may be performed in order to further improve properties such as formability by making the grains smaller and optimizing the texture.

(Cold Rolling)

In cold rolling, the hot-rolled sheet is rolled to produce a cold-rolled sheet (including a coil) having a desired final sheet thickness. However, from the standpoint of making the grains even smaller, it is desirable that the total cold rolling ratio should be 60% or greater regardless of the number of passes.

(Process Annealing)

It is preferable that before this cold rolling (after the hot rolling) or during the cold rolling (between passes), process annealing should be repeatedly performed two or more times to bring the Sn, which has formed as compounds in the preceding steps including the hot rolling step, into a solid-solution state. In the process annealing, the sheet is held for 0.1-10 seconds at a high temperature of 480° C. or higher but not higher than the melting point and then forcedly cooled (rapidly cooled) to room temperature at an average cooling rate of 3° C./sec or higher. In ordinary processes, the Sn is prone to precipitate and the Sn which has once precipitated is considerably difficult to bring into a solid-solution state again. For regulating the proportion of the average number of Sn-containing compounds to the number of compounds having the specific size to a value less than 50%, it is necessary that such a high-temperature short-time heat treatment should be performed multiple times. So long as those condition ranges are satisfied, the conditions for the multiple heat treatments need not be the same and may differ.

With respect to the conditions for this process annealing, in the case where the sheet temperature is lower than 480° C., an insufficient solute Sn amount results, even when the process annealing is performed two or more times. This applies also in the case where process annealing in which the annealing temperature and the rapid-cooling conditions are within the ranges is performed only once. Although the holding time may be a short period including a moment, e.g., 0.1 second, holding for a period exceeding 10 seconds considerably impairs the mechanical properties of the sheet. Meanwhile, in the case where the cooling after the annealing is not the forced cooling (rapid cooling) to room temperature at an average cooling rate of 3° C./sec or higher by air cooling, mist or water cooling, or the like, that is, in the case where the average cooling rate is less than 3° C./sec, the Sn which has once formed a solid solution undesirably precipitates again to form compounds.

Annealing under such conditions, including the rapid cooling, is impossible with a batch type furnace, and a continuous heat treatment furnace is necessary in which the sheet is passed, while being unwound, through the furnace and wound up. The present inventors have found that even in the case of using such continuous annealing capable of rapid cooling, the continuous annealing performed only once always results in an insufficient solute Sn amount. Because of this, process annealing by continuous annealing is repeatedly performed two or more times. However, the larger the number of repetitions of continuous annealing, the more the efficiency of the production steps is reduced considerably. It is therefore preferable that the number of repetitions should be about 2.

(Solution and Quenching Treatments)

After the cold rolling, solution and quenching treatments are performed. The solution treatment and the quenching treatment may be heating and cooling which are performed on an ordinary continuous heat treatment line, and are not particularly limited. However, from the standpoint of obtaining a sufficient solid-solution amount of each element and because it is desirable that the grains of the microstructure of the sheet should be finer, it is preferred to conduct the treatments under such conditions that heating is performed at a heating rate of 5° C./sec or higher to a solution treatment temperature of 520° C. or higher and not higher than the melting temperature, followed by holding for 0-10 seconds.

The average rate of cooling from the solution treatment temperature to a quenching stop temperature is preferably regulated to 3° C./sec or higher. In the case where the cooling rate is too low, Mg—Si compounds and the like are prone to precipitate during the cooling and they are prone to act as starting points for cracks during press forming or bending, resulting in a decrease in the formability. In order to secure that cooling rate, means such as air cooling with fans or water cooling with mist or spray or by immersion, etc. and conditions therefor are selected and used for the quenching treatment.

(Reheating Treatment)

Subsequently, in order to form aggregates of atoms (clusters) serving as nuclei for Mg—Si compounds to be yielded in a BH treatment, a preliminary aging treatment (reheating treatment) is conducted after the solution and quenching treatments. It is desirable that the reaching temperature (actual temperature) of the sheet should be in the temperature range of 80-150° C. and the holding time should be in the range of 3-50 hours. Cooling to room temperature after the reheating treatment may be standing to cool or may be conducted by forcedly cooling by using the cooling means used in the quenching, in order to heighten the efficiency of the production.

The present invention will be explained below in more detail by reference to Examples. However, the present invention should not, of course, be construed as being limited by the following Examples, and can be suitably modified and performed as long as the modifications conform to the gist of the present invention described hereinabove and hereinafter. All such modifications are included in the technical range of the present invention.

EXAMPLES

Examples of the present invention are explained. 6000-series aluminum alloy sheets were individually produced so as to differ in the solute Sn amount specified in the present invention, by the conditions for process annealing. The number density of the compounds and solute Sn amount determined therefrom were examined. These sheets were held at room temperature for 100 days and then also evaluated for BH response (bake hardenability) and hem workability. The results thereof are shown in Table 2.

Specific conditions used for producing the aluminum alloy sheets were as follows. Slabs of aluminum alloys respectively having the compositions shown in Table 1 were commonly produced through casting by the DC casting method. Here, the average rate of cooling from the liquidus temperature to the solidus temperature in the casting was set at 50° C./min in common with all the Examples. With respect to the indications of the contents of elements in Table 1, which show the compositions of the 6000-series aluminum alloy sheets of the Examples, the indications using blanks as the values of elements each indicate that the content thereof is below a detection limit and that the element is not contained, i.e., 0%.

Subsequently, the slabs were subjected to a soaking treatment of 540° C.×4 hours and hot rough rolling was then initiated, in common with all the Examples. Thereafter, in the succeeding finish rolling, hot rolling to a thickness of 2.5 mm is performed to obtain hot-rolled sheets, in common with all the Examples. The hot-rolled aluminum alloy sheets were subjected to rough annealing of 500° C.×1 minute in common with all the Examples and then to process annealing with a continuous annealing furnace, during cold-rolling passes (between passes), under various conditions while changing the number of repetitions, temperature, average cooling rate, etc. as shown in Table 2. Thus, cold-rolled sheets (product sheets) having a thickness of 1.0 mm were finally obtained.

Furthermore, the cold-rolled sheets were subjected to a solution heat treatment with a 560° C.-niter furnace, held for 10 seconds after a target temperature had been reached, and then quenched by water cooling, in common with all the Examples. Immediately after the quenching, a preliminary aging treatment was conducted in which holding is performed at 100° C. for 5 hours (after the holding, gradually cooling is performed at a cooling rate of 0.6° C./hour).

From the sheets which had just undergone these refining treatments, test sheets (blanks) were cut out. The test sheets were examined for microstructure (the number density of compounds and the proportion in number of Sn-containing compounds). Furthermore, test sheets (blanks) were cut out from the sheets which had been allowed to stand at room temperature for 100 days after the refining treatments, and the test sheets were examined for strength (AS proof stress) and BH response. The results thereof are shown in Table 2.

(Microstructure of Test Sheets)

As the microstructure of each test sheet which had just undergone the refining treatments, the average number density (counts/mm²) of compounds having an equivalent circular diameter in the range of 0.3-20 μm was examined with an SEM with a magnification of 500 times and among those compounds examined, the average proportion in number (%) of compounds containing Sn in an amount of 0.5% or more which had been identified with an X-ray spectrometer was examined, by the measuring methods described above.

(Tensile Test)

A tensile test was conducted in the following manner. No. 5 specimens (25 mm ×50 mmGL×sheet thickness) according to JIS Z2201 were sampled from each test sheet which had been allowed to stand at room temperature for 100 days after the refining treatments, and subjected to the tensile test at room temperature. Here, the tensile direction of each specimen was set so as to be perpendicular to the rolling direction. The tensile rate was set at 5 mm/min until the 0.2% proof stress and at 20 mm/min after the proof stress. The number N of examinations for mechanical property was 5, and an average value therefor was calculated. With respect to the specimens to be examined for proof stress after BH, a 2% pre-strain as a simulation of sheet press forming was given to the specimens by the tensile tester, and the BH treatment was then performed.

(BH Response)

The test sheets were subjected to the 100-day room-temperature aging and then to an artificial age hardening treatment (BH) of 185° C.×20 min, and were thereafter examined for 0.2% proof stress (proof stress after BH) through the tensile test, in common with the test sheets. The BH response of each test sheet was evaluated on the basis of the difference between these 0.2% proof stresses (increase amount in proof stress). The case where the increase amount of 0.2% proof stress was 100 MPa or greater was rated as acceptable.

(Hem Workability)

Hem workability was evaluated with respect to the test sheets which had undergone the 100-day room-temperature standing. In the test, strip-shaped specimens having a width of 30 mm were used and subjected to 90° bending at an inward bending radius of 1.0 mm with a down flange. Thereafter, an inner having a thickness of 1.0 mm was interposed, and the specimen was subjected, in order, to pre-hem working in which the bent part was further bent inward to approximately 130° and flat-hem working in which the bent part was further bent inward to 180° and the end portion was brought into close contact with the inner.

The surface state, such as the occurrence of rough surface, a minute crack or a large crack, of the bent part (edge bent part) of the flat hem was visually examined and visually evaluated on the basis of the following criteria. Ratings of 0 to 2 were acceptable.

0, no crack and no rough surface; 1, slight rough surface; 2, deep rough surface; 3, minute surface crack; 4, linearly continued surface crack; 5, fracture.

Invention Examples shown as Nos. 1 to 4 and 12 to 23 in Table 2 are within the component composition range according to the present invention (alloys Nos. 1 to 13 in Table 1), and have been produced under conditions within the preferred ranges including those for process annealing. Because of this, these Invention Examples each satisfy the average number density of compounds specified in the present invention and the average proportion in number of compounds containing Sn in an amount of 0.5 mass % or more, are inhibited from suffering the precipitation of the Sn contained, and have a large solute Sn amount, as shown in Table 2.

As a result, the Invention Examples each have a feature in which the As proof stress, even after having undergone long-term room-temperature aging for 100 days after the refining treatments, is on the level of 90-110 MPa and, despite this, the proof stress after BH (bake hardening) is on the level of 190 MPa, as shown in Table 2. Namely, the difference in proof stress is 100 MPa or greater, showing that they have excellent BH response. Furthermore, since the As proof stress is relatively low even after the long-term room-temperature aging after the refining treatments, they are excellent in terms of press formability into automotive panels or the like and also excellent in terms of hem workability.

Moreover, as can be seen from Table 2, even when the same alloy No. 1 shown in Table 1 is used, the average number density of compounds or the solid-solution state of Sn (average proportion in number of Sn-containing compounds) differ considerably depending on differences in process annealing conditions, to show considerably different properties. Specifically, of Invention Examples 1 to 4, Invention Examples 3 and 4, for which relatively high process annealing temperatures and relatively high average cooling rates were used, each have a low average number density of compounds but have a low average proportion in number of Sn-containing compounds, were inhibited from suffering the precipitation of the Sn contained, and have a large solute Sn amount, as compared with Invention Examples 1 and 2, for which relatively low process annealing temperatures and relatively low average cooling rates were used. As a result, Invention Examples 3 and 4, even after long-term room-temperature aging for 100 days after the refining treatments, attain a larger difference in proof stress through BH and show better BH response, as compared with Invention Examples 1 and 2.

In contrast, Comparative Examples 5 to 11 in Table 2, which use alloy No. 1 in Table 1 like those Invention Examples, are Examples in which the process annealing conditions were outside the preferred ranges. Because of this, these Comparative Examples are ones in which the compounds specified in the present invention are contained in too large an amount and the average number density thereof is too high beyond the upper limit. Even when the average number density of the compounds is within the range specified in the present invention, the average proportion in number of compounds containing Sn in an amount of 0.5 mass % or more is too high beyond 50%, and therefore the precipitation of the contained Sn has not been inhibited, and the solute Sn amounts are small. Because of this, they are poor in press formability into automotive panels or the like and in hem workability and are poor also in BH response, with the proof stress difference being less than 100 MPa, as compared with the Invention Examples, which have the same alloy composition.

In Comparative Example 5, process annealing was not performed.

In Comparative Example 6, process annealing was conducted only once, although the requirements thereof concerning temperature, holding time and average cooling rate are satisfied.

In Comparative Example 7, the temperature in the first process annealing was as low as below 480° C., although the second process annealing satisfied the requirements concerning temperature, holding time and average cooling rate.

In Comparative Example 8, the temperature in the second process annealing was as low as below 480° C., although the first process annealing satisfied the requirements concerning temperature, holding time and average cooling rate.

In Comparative Example 9, both of the temperatures in the first and second process annealings were as low as below 480° C.

In Comparative Examples 10 and 11, the temperature and holding time in the first and second process annealings satisfied the requirements, but the average cooling rate in the first or second process annealing was too low.

Meanwhile, Comparative Examples 24 to 29 in Table 2 have been produced within the preferred ranges including conditions for process annealing. However, they use alloys Nos. 14 to 19 in Table 1, and the contents of Mg, Si and Sn, which are essential elements, are outside the ranges according to the present invention. Because of this, Comparative Examples 24 to 29 are relatively too high especially in the As proof stress after 100-day room-temperature holding and are hence poor in press formability into automotive panels or the like and in hem workability or poor in BH response, as compared with the Invention Examples as shown in Table 2. Furthermore, in Comparative Example 27, the Sn content was too high and cracks generated during the hot rolling, making the production of a hot-rolled sheet itself impossible.

Comparative Example 24 is alloy 14 in Table 1, in which the Si content is too low.

Comparative Example 25 is alloy 15 in Table 1, in which the Si content is too high.

Comparative Example 26 is alloy 16 in Table 1, in which the Sn content is too low.

Comparative Example 27 is alloy 17 in Table 1, in which the Sn content is too high.

Comparative Example 28 is alloy 18 in Table 1, in which the Mg content is too low.

Comparative Example 29 is alloy 19 in Table 1, in which the Mg content is too high.

Those results of the Examples establish the critical significance or effects of the composition or chemical structure specified in the present invention and of the preferred production conditions including the process annealing conditions, with respect to improvements in the hem workability and BH response of Sn-containing 6000-series aluminum alloy sheets after long-term room-temperature aging.

TABLE 1
Alloy	Chemical components of aluminum alloy sheet (mass %; remainder Al)
No.	Mg	Si	Sn	Fe	Mn	Cr	Zr	V	Ti	Cu	Zn	Ag
1	0.55	0.95	0.05
2	0.55	0.95	0.05	0.2
3	0.40	0.80	0.05	0.2	0.12					0.3
4	0.40	1.20	0.09	0.2				0.21	0.01
5	0.30	0.50	0.05	0.2						0.8
6	0.55	1.30	0.05	0.2	0.7		0.05
7	0.55	0.80	0.21	0.2	0.07
8	0.55	0.90	0.05	0.2		0.22
9	0.55	1.20	0.02	0.2		0.05		0.05
10	1.50	1.00	0.10	0.2						0.1	0.01
11	0.70	0.95	0.05	0.2					0.05
12	0.55	1.20	0.01	0.7							0.6
13	0.55	0.90	0.05	0.2			0.2				0.1	0.1
14	1.50	0.20	0.05	0.2
15	0.40	2.10	0.05	0.2
16	0.60	1.20	0.002	0.2
17	0.60	1.10	0.40	0.2
18	0.10	0.80	0.05	0.2
19	2.20	0.95	0.05	0.2

TABLE 2
			Conditions for process annealing between
			cold-rolling passes
			First continuous annealing	Second continuous annealing
		Alloy	Temperature ×	Average	Temperature ×	Average
		No. in	time	cooling rate	time	cooling rate
Classification	No.	Table 1	° C. × 5 sec	° C./sec	° C. × 5 sec	° C./sec
Inv. Ex.	1	1	480	5	480	5
Inv. Ex.	2	1	490	10	500	10
Inv. Ex.	3	1	520	50	510	100
Inv. Ex.	4	1	520	100	520	100
Comp. Ex.	5	1	—	—	—	—
Comp. Ex.	6	1	490	10	—	—
Comp. Ex.	7	1	420	50	510	100
Comp. Ex.	8	1	520	50	420	100
Comp. Ex.	9	1	460	50	460	100
Comp. Ex.	10	1	510	1	510	1
Comp. Ex.	11	1	510	50	510	1
Inv. Ex.	12	2	520	50	510	100
Inv. Ex.	13	3	520	50	510	100
Inv. Ex.	14	4	520	50	510	100
Inv. Ex.	15	5	520	50	510	100
Inv. Ex.	16	6	520	50	510	100
Inv. Ex.	17	7	520	50	510	100
Inv. Ex.	18	8	520	50	510	100
Inv. Ex.	19	9	520	50	510	100
Inv. Ex.	20	10	520	50	510	100
Inv. Ex.	21	11	520	50	510	100
Inv. Ex.	22	12	520	50	510	100
Inv. Ex.	23	13	520	50	510	100
Comp. Ex.	24	14	520	50	510	100
Comp. Ex.	25	15	520	50	510	100
Comp. Ex.	26	16	520	50	510	100
Comp. Ex.	27	17	520	50	510	100
Comp. Ex.	28	18	520	50	510	100
Comp. Ex.	29	19	520	50	510	100
		Microstructure of aluminum
		alloy sheet just after refining
		treatments
		Compounds having
		equivalent circular diameter
		of 0.3-20 μm
			Average	Properties of aluminum alloy sheet
			proportion in	after 100-day room-temperature
			number of	holding
			compounds	As	proof	Proof
		Average	containing 0.5	proof	stress	stress
		number	mass % or more	stress	0.2%	increase
		density	Sn	0.2%	after BH	amount	Hem
Classification	No.	counts/mm2	%	MPa	MPa	MPa	workability
Inv. Ex.	1	4019	34	87	188	101	1
Inv. Ex.	2	3723	30	90	194	104	1
Inv. Ex.	3	3014	18	103	231	128	1
Inv. Ex.	4	2902	17	107	242	135	1
Comp. Ex.	5	6110	70	79	149	70	3
Comp. Ex.	6	5865	66	84	159	75	3
Comp. Ex.	7	4814	61	101	195	94	2
Comp. Ex.	8	4962	57	99	190	91	2
Comp. Ex.	9	5396	48	96	183	87	3
Comp. Ex.	10	5717	59	87	166	79	3
Comp. Ex.	11	5605	55	93	173	80	3
Inv. Ex.	12	3269	18	104	231	127	1
Inv. Ex.	13	2831	16	91	207	116	1
Inv. Ex.	14	3366	25	97	235	138	1
Inv. Ex.	15	2652	20	101	201	100	1
Inv. Ex.	16	4554	20	119	248	129	2
Inv. Ex.	17	2999	43	89	219	130	1
Inv. Ex.	18	3432	18	105	233	128	1
Inv. Ex.	19	3494	10	108	242	134	1
Inv. Ex.	20	3585	32	102	217	115	1
Inv. Ex.	21	3310	19	105	223	118	1
Inv. Ex.	22	4845	4	116	248	132	2
Inv. Ex.	23	3264	17	107	237	130	1
Comp. Ex.	24	2933	16	105	170	65	1
Comp. Ex.	25	4100	24	123	234	111	3
Comp. Ex.	26	3239	2	130	232	102	3
Comp. Ex.	27	cracking occurred during hot rolling
Comp. Ex.	28	3009	15	80	153	73	1
Comp. Ex.	29	3902	22	130	237	107	3

While the present invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope of the present invention.

The present application is based on a Japanese patent application filed on Aug. 27, 2014 (Application No. 2014-173278), the whole thereof being incorporated herein by reference.

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to provide 6000-series aluminum alloy sheets which combine BH response and formability after long-term room-temperature aging. As a result, the 6000-series aluminum alloy sheets are usable in extended applications as members or components for transportation machines such as automobiles, ships and other vehicles, domestic electrical appliances, buildings, and other structures, and especially as members for transportation machines including automobiles.

Claims

1. An aluminum alloy sheet for forming, which is an Al—Mg —Si alloy sheet comprising, in terms of mass %:

0.2-2.0% of Mg;

0.3-2.0% of Si;

0.005-0.3% of Sn; and

Al and unavoidable impurities,

wherein the aluminum alloy sheet has a microstructure in which: compounds each having an equivalent circular diameter in a range of 0.3-20 ∥m, as examined with an SEM having a magnification of 500 times, have an average number density of more than 0 count/mm2 and 5,000 counts/mm2 or less; and

among the compounds examined with the SEM, compounds comprising Sn in an amount of 0.5 mass % or more as identified with an X-ray spectrometer have an average proportion in number of 0% or more and less than 50%.

2. The aluminum alloy sheet for forming according to claim 1, further comprising at least one selected from the group consisting of

more than 0% and 1.0% or less of Mn,

more than 0% and 1.0% or less of Cu,

more than 0% and 1.0% or less of Fe,

more than 0% and 0.3% or less of Cr,

more than 0% and 0.3% or less of Zr,

more than 0% and 0.3% or less of V,

more than 0% and 0.05% or less of Ti,

more than 0% and 1.0% or less of Zn, and

more than 0% and 0.2% or less of Ag, in terms of mass %.