ALUMINUM ALLOY SHEET FOR MOLDING

Abstract

The present invention relates to an Al—Mg—Si-based aluminum alloy plate for molding which contains, in mass %, Mg: 0.3-1.3%, Si: 0.5-1.5% and Sn: 0.005-0.2%, the remainder being Al and unavoidable impurities, wherein when the Sn content in the residue compound separated by a hot phenol residue extraction method and having a particle size exceeding 0.1 μm is subtracted from the Sn content in the aluminum alloy plate, the resulting difference is a 0.005 mass % or greater quantity of Sn. The aluminum alloy plate for molding combines moldability and BH properties after long-term aging at room temperature.

Description

TECHNICAL FIELD

The present invention relates to an Al—Mg—Si aluminum alloy sheet for forming. The aluminum alloy sheet described herein refers to a rolled sheet such as a hot-rolled sheet or a cold-rolled sheet, which has been subjected to tempering such as solution treatment and hardening treatment but has not been subjected to paint-bake hardening. Hereinafter, aluminum may be referred to as Al.

BACKGROUND ART

Recently, a social demand for weight saving of vehicles such as automobiles has increased more and more out of consideration for the global environment.

To meet such a social demand, lightweight aluminum alloy materials, which have good formability and good paint-bake hardenability, are increasingly used as materials for auto panels, particularly large body panels (outer panels and inner panels) such as a hood, a door, and a roof, in place of steel materials such as steel sheets.

In particular, use of Al—Mg—Si aluminum alloy sheets such as AA-series or JIS6000-series, which may be simply referred to as 6000-series hereinafter, aluminum alloy sheets are investigated as thin and high-strength aluminum alloy sheets for panels including an outer panel and an inner panel of a panel structure such as a hood, a fender, a door, a roof, and a trunk lid of an automobile.

The 6000-series aluminum alloy sheets essentially contain Si and Mg. In particular, an excessive-Si 6000-series aluminum alloy has a composition having a Si/Mg ratio by mass of 1 or more, and exhibits good age hardenability. The aluminum alloy sheets therefore maintain formability due to a low proof stress during press forming or bending, and have paint-bake hardenability (hereinafter, also referred to as bake hardenability (BH property) or baking hardenability), which provides strength necessary for a panel by a proof stress increased by age hardening through heating during artificial aging (hardening) at relatively low temperature such as paint baking treatment of a formed panel.

Moreover, the 6000-series aluminum alloy sheets have a relatively small amount of alloy elements compared with other aluminum alloys such as 5000-series aluminum alloys having a large alloy amount such as Mg amount. Hence, when scraps of such 6000-series aluminum alloy sheets are reused as an aluminum alloy melting material (melting source material), an original 6000-series aluminum alloy slab is easily reproduced, showing good recyclability of the 6000-series aluminum alloy sheets.

As well known, an outer panel of an automobile is fabricated through compositely performing various types of forming, such as stretch-expand forming and bending in press forming, on an aluminum alloy sheet. For example, for large outer panel such as a hood and a door, the aluminum alloy sheet is formed into a product shape of the outer panel by press forming such as stretch-expand forming, and then the formed product is joined to an inner panel through hemming of a flat hem of the outer panel periphery, or the like, so that a panel structure is formed.

The 6000-series aluminum alloy advantageously has a good BH property, but has room-temperature aging characteristics, which disadvantageously worsens the formability into a panel, particularly bendability, due to increased strength through age hardening after room-temperature holding for a few months after solution hardening. For example, when the 6000-series aluminum alloy sheet is used for an auto panel, the aluminum alloy sheet is subjected to solution-hardening (manufactured) by an aluminum manufacturer, and is then usually stood in room temperature (left at room temperature) for about one to four months while being considerably age-hardened (aged at room temperature), and is then formed into a panel by an automaker. In particular, the outer panel requiring hard bending can be formed without any difficulty after a lapse of one month from the fabrication. However, after a lapse of three months, the outer panel cannot be formed without a difficulty such as cracking during hemming. Hence, the 6000-series aluminum alloy sheet for the auto panel, particularly for the outer panel, must be limited in room-temperature aging over a relatively long term of about one to four months.

Furthermore, if such room-temperature aging is significant, the BH property is degraded, and proof stress is disadvantageously not increased to a strength necessary for the panel depending on heating during artificial aging (hardening) at relatively low temperature such as paint baking treatment of the formed panel.

There have been various reports on improvement in properties, such as improvement in formability or BH property, or restriction of room-temperature aging in light of a microstructure of the 6000-series aluminum alloy sheet, particularly compounds (crystallized grains, precipitates) produced by components. In particular, in a recently reported approach, a cluster (aggregate of atoms) affecting the BH property or the room-temperature aging characteristics of the 6000-series aluminum alloy sheet is directly measured and controlled.

In previous patents relating to addition of Sn of the present invention, there have been reported many methods for improving the paint-bake hardenability through actively adding Sn to the 6000-series aluminum alloy sheet to limit room-temperature aging. For example, PTL 1 reports a method for combining restriction of room-temperature aging and paint-bake hardening through adding an appropriate amount of Sn having an aging variation restriction effect and performing pre-aging after solution treatment. PTL 2 reports a method for improving formability, paint-bake performance, and corrosion resistance by adding Sn having an aging variation restriction effect and Cu that improves formability.

CITATION LIST

Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication No. Hei09(1997)-249950

PTL 2: Japanese Unexamined Patent Application Publication No. Hei10(1998)-226894

PTL 3: Japanese Unexamined Patent Application Publication No. 2000-144294

PTL 4: Japanese Unexamined Patent Application Publication No. 2003-221637

PTL 5: Japanese Unexamined Patent Application Publication No. 2003-268472

SUMMARY OF INVENTION

Technical Problem

However, the existing Al—Mg—Si aluminum alloy sheet containing additional Sn also has a room for improvement in light of combining good formability and a good BH property after long room-temperature aging.

In light of such a problem, an object of the invention is to provide a Sn-containing Al—Mg—Si aluminum alloy sheet for forming, which exhibits a good BH property and good formability even after vehicle-body paint baking after long room-temperature aging.

Solution to Problem

To achieve the object, an aluminum alloy sheet for forming according to the present invention is summarized by an Al—Mg—Si aluminum alloy sheet that contains, in mass %, Mg: 0.3 to 1.3%, Si: 0.5 to 1.5% and Sn: 0.005 to 0.2%, the remainder consisting of Al and unavoidable impurities, in which an amount of Sn is 0.005 mass % or more, the amount being obtained by subtracting the Sn content in a residue compound from the Sn content in the aluminum alloy sheet, the residue compound being separated by a hot-phenol residue extraction method and having a particle size of more than 0.1 μm.

Advantageous Effects of Invention

In a microstructure of the Al—Mg—Si aluminum alloy sheet, Sn captures (traps) atomic vacancies at room temperature and thus limits diffusion of Mg or Si at room temperature, and limits an increase in strength at room temperature and thus advantageously improves press formability including hemming performance, drawing performance, and stretch expanding performance (hereinafter, such press formability may be represented as hemming performance) during forming of the sheet into a panel. Sn releases the captured vacancies during artificial aging such as paint baking of the panel, which rather prompts diffusion of Mg and Si, leading to improvement in BH property.

According to the findings of the inventors, however, such addition of Sn is greatly limited due to the special properties of Sn. The effect of Sn, i.e., capture or release of atomic vacancies, is not exhibited until Sn is dissolved in a matrix. However, an extremely small amount of Sn is dissolved in a matrix. For a typical method of manufacturing the sheet, therefore, even if the adding amount of Sn is limited to a theoretical solid-solution amount or smaller, most of the added Sn is crystallized or precipitated as a compound instead of being dissolved. When Sn is thus crystallized or precipitated as compounds, it does not exhibit the effect of capture or release of atomic vacancies.

In the invention, therefore, the method of manufacturing the sheet is intentionally reviewed. In addition, as described later, a manufacturing condition including process annealing is designed to control an existing state of the contained Sn to limit precipitation of Sn as a compound, so that Sn is more dissolved in the matrix and thus the solid-solution amount of Sn is maintained. Thus, aging is restricted by the effect of Sn, i.e., capture or release of atomic vacancies, so that the effect of improving the hemming performance or the BH property is sufficiently exhibited.

Consequently, it is possible to provide a Sn-containing Al—Mg—Si aluminum alloy sheet capable of exhibiting better formability and a better BH property even after long room-temperature aging for, for example, 100 days after manufacturing of the sheet.

The existing Sn-containing Al—Mg—Si aluminum alloy sheets have not sufficiently exhibited the effect of Sn.

This is probably because while Mg and Si are main components and their solid solution and precipitation have been constantly noticed, Sn is merely one of selective additive elements and thus an existence form of dissolved or precipitated Sn has been less noticed. When the aluminum alloy sheet is manufactured by a common process, Sn in the sheet exists in a form of crystallized or precipitated, which may be simply referred to as precipitated hereinafter, a compound. In addition, Sn is difficult to be dissolved and rarely exists in a dissolved state; hence, the effect exhibited by dissolved Sn has been probably difficult to be found.

DESCRIPTION OF EMBODIMENTS

An embodiment of the invention is now specifically described on each of requirements.

(Chemical Composition)

The chemical composition of the Al—Mg—Si series, which may be referred to as 6000 series hereinafter, aluminum alloy sheet of the invention is now described. The 6000-series aluminum alloy sheet covered by the invention is required to have good properties, such as formability, a BH property, strength, weldability, and corrosion resistance, as a sheet for an auto panel.

To satisfy such a requirement, the composition of the aluminum alloy sheet includes, in mass %, Mg: 0.3 to 1.3%, Si: 0.5 to 1.5% and Sn: 0.005 to 0.2%, the remainder consisting of Al and unavoidable impurities. The percentage representing the content of each element refers to mass percent. In this description, the percentage based on mass (mass %) is equal to the percentage based on weight (weight %). For the content of each chemical component, “X % or less (excluding 0%)” may be represented as “more than 0% and X % or less”.

The 6000-series aluminum alloy sheet covered by the invention is preferably an excessive-Si 6000-series aluminum alloy sheet that is further good in BH property and has a mass ratio of Si to Mg, Si/Mg, of 1 or more. The 6000-series aluminum alloy sheet maintains formability during press forming or bending due to its low proof stress, and has good aging hardenability (BH property) that allows the sheet to be age-hardened and increased in proof stress by heating during artificial aging at relatively low temperature such as paint baking treatment of a formed panel, and thus to maintain a necessary strength. In particular, the excessive-Si 6000-series aluminum alloy sheet has a better BH property than a 6000-series aluminum alloy sheet having the mass ratio Si/Mg of less than 1.

In the invention, elements other than Mg and Si are each an impurity or a containable element within a range of the content (allowable amount) level of each element according to the AA standard or the JIS standard.

Specifically, from the viewpoint of resource recycle, the invention may also use, as melting materials for alloy, not only high-purity Al bullions but also a large amount of scraps of the 6000-series alloy or other aluminum alloys containing elements other than Mg and Si as additive elements (alloy elements) and/or a large amount of low-purity Al bullions. In such a case, an effective amount of each of the following elements is inevitably contaminated. Although the amounts of such elements can be decreased by refining, the refining leads to an increase in cost; hence, a certain amount of each element must be allowed to be contained. In a certain content range, even if an effective amount of the element is contained, the object and the effects of the invention are not substantially affected thereby.

Hence, the invention permits each of the elements to be contained within a content range equal to or lower than the upper limit defined according to the AA standard or the JIS standard as described below.

Specifically, the aluminum alloy sheet contains the above-described basic composition, and may further contain one or more of Mn: 1.0% or less (not including 0%), Cu: 1.0% or less (not including 0%), Fe: 1.0% or less (not including 0%), Cr: 0.3% or less (not including 0%), Zr: 0.3% or less (not including 0%), V: 0.3% or less (not including 0%), Ti: 0.05% or less (not including 0%), Zn: 1.0% or less (not including 0%), and Ag: 0.2% or less (not including 0%).

When such elements are contained, since a large content of Cu tends to degrade corrosion resistance, the content of Cu is preferably 0.7% or less and more preferably 0.3% or less. A large content of each of Mn, Fe, Cr, Zr, and V tends to produce a relatively coarse compound, which is likely to degrade hemming performance (hem bendability) as an issue of the invention. The content of Mn is therefore preferably 0.6% or less and more preferably 0.3% or less. The content of Cr, Zr, and V is preferably 0.2% or less and more preferably 0.1% or less. The content of Fe is preferably 0.8% or less.

Each of the elements of the 6000-series aluminum alloy is now described on its content range, significance, and allowable amount.

Si: 0.5 to 1.5%

Si is a main element essential for providing solution strengthening, and allowing age hardenability to be exhibited through formation of Mg—Si precipitates, which contributes to increasing strength, during artificial aging such as paint baking in order to provide a strength (proof stress) necessary for an outer panel of an automobile. To allow good age hardenability to be exhibited through paint baking after forming of the sheet into a panel, Si/Mg is preferably adjusted to 1.0 or more in mass ratio to produce a 6000-series aluminum alloy composition that contains a larger amount of Si relative to Mg than the typical excessive-Si 6000-series aluminum alloy. An excessively small content of Si results in insufficient production of Mg—Si precipitates, which greatly degrades the BH property. An excessively large content of Si results in formation of coarse crystallized grains and precipitates, which greatly degrades the bendability. Consequently, the Si content is within a range from 0.5 to 1.5%. A more preferred lower limit of Si content is 0.6%, and a more preferred upper limit thereof is 1.4%.

Mg: 0.3 to 1.3%

Mg is also a main element essential for providing solution strengthening, and allowing age hardenability to be exhibited through formation of Mg—Si precipitates, which contributes to increasing strength, during artificial aging such as paint baking in order to provide a proof stress necessary for the panel. An excessively small content of Mg results in insufficient production of Mg—Si precipitates, which greatly degrades the BH property. This prevents the panel from having the necessary proof stress. An excessively large content of Mg results in formation of coarse crystallized grains and precipitates, which greatly degrades the bendability. Consequently, the Mg content is within a range from 0.3 to 1.3%. A more preferred lower limit of Mg content is 0.4%, and a more preferred upper limit thereof is 1.2%.

Sn: 0.005 to 0.2%

Sn is an essential element that captures atomic vacancies at room temperature, and thereby suppresses diffusion of Mg or Si at room temperature, and limits an increase in strength at room temperature (room-temperature aging) for a long term, and thus has an effect of improving press formability, particularly hemming performance, in press forming of the room-temperature-aged sheet into a panel. In addition, Sn releases the captured vacancies during artificial aging such as paint baking of the formed panel, which prompts diffusion of Mg or Si, leading to improvement in BH property.

If the content of Sn is excessively small, Sn cannot limit the increase in strength at room temperature, resulting in an increase in proof stress. This degrades the hemming performance, and decreases production of the Mg—Si precipitates in BH treatment, so that the BH property tends to be degraded. Consequently, the Sn content is within a range from 0.005 to 0.2%. A more preferred lower limit of Sn content is 0.01%, and a more preferred upper limit thereof is 0.18%.

However, such effects of Sn are not exhibited until Sn is dissolved. In the invention, therefore, the necessary solid-solution amount of Sn is determined by the residue extraction method and maintained.

Hence, the Sn-containing Al—Mg—Si aluminum alloy sheet of the invention is, structurally and characteristically, greatly different from the Al—Mg—Si aluminum alloy sheet without containing Sn in terms of solid solution of Sn. Even an Al—Mg—Si aluminum alloy sheet containing Sn in the same way (by the same amount) is different in solid-solution amount of Sn if a manufacturing condition such as a process annealing condition is different. In a typical manufacturing condition (common process) of an aluminum alloy sheet, Sn is easily precipitated as a compound, and the solid-solution amount of Sn is extremely small, leading to a greatly different microstructure of the aluminum alloy sheet. Hence, even if Sn is contained in the same way (by the same amount), there is not necessarily produced a microstructure showing an effect of limiting the room-temperature aging at a high level and an effect of improving the BH property or the hemming performance as in the invention.

(Microstructure)

The microstructure of the 6000-series aluminum alloy sheet of the invention is now described.

Rough Guide of Solid-Solution Amount of Sn:

The invention characteristically maintains the solid-solution amount of Sn necessary for exhibition of the effect of Sn. In a rough guide (criterion) for maintaining the solid-solution amount of Sn, an amount of Sn is 0.005 mass % or more, the amount being obtained by subtracting the Sn content in a residue compound from the Sn content in the aluminum alloy sheet, the residue compound being separated by a hot-phenol residue extraction method and having a particle size of more than 0.1 μm. The insoluble residue compound, which is separated by the hot-phenol residue extraction method and has a particle size of more than 0.1 μm, is a precipitate. The Sn content in the residue compound refers to the amount of Sn separated as the precipitate other than the dissolved Sn in the Sn content in an alloy composition of the sheet. Hence, the amount of Sn, which is obtained by subtracting the Sn content in the residue compound from the Sn content in the alloy composition of the aluminum alloy sheet, refers to the solid-solution amount of Sn in the invention, the residue compound being separated by the hot-phenol residue extraction method and having a particle size of more than 0.1 μm.

Accordingly, it is shown that if the amount of Sn obtained from the subtraction is 0.005 mass % or more, a small amount of Sn is precipitated, and the solid-solution amount of Sn is enough for the added Sn to exhibit the effects. In addition, it is shown that if the amount of Sn obtained from the subtraction is less than 0.005 mass %, the solid-solution amount of Sn is too small for the added Sn to exhibit the effects.

The upper limit of the amount of the subtracted Sn (solid-solution amount of Sn) corresponds to the amount of Sn when the Sn content (the amount of precipitated Sn) in the residue compound is zero, the residue compound being separated by the residue extraction method and having a particle size of more than 0.1 μm. In other words, in such a case, the whole amount of Sn in the sheet is dissolved in a matrix, i.e., the Sn content in the sheet is equal to the amount of the subtracted Sn. In a common process, however, when at least a certain amount of Sn is added or contained, Sn is easily precipitated, and it is difficult to entirely dissolve Sn in light of an efficient (industrial) manufacturing limit. Hence, the actual upper limit of the amount of the subtracted Sn (solid-solution amount of Sn) has a value smaller than the value of the Sn content in the alloy composition of the sheet. Specifically, the upper limit of the amount of the subtracted Sn is about 0.15 mass %.

In the invention, it has been found that the amount of Sn precipitated as the residue compound (precipitate) having a particle size of more than 0.1 μm can be accurately and reproducibly determined using the residue extraction method as described later, and that the solid-solution amount of Sn can be alternatively (indirectly) determined by subtracting the amount of the precipitated Sn from the Sn content in the alloy composition. It has been further found that the determination of the solid-solution amount of Sn using the residue amount determined by the residue extraction method is well correlated with the effect exhibited by actual Sn (dissolved Sn).

In a generally used residue extraction method, solid and liquid are typically separated and classified through filtration separation with a filter having a mesh of 0.1 μm with reference to a size boundary of 0.1 μm for each particle (solid) to be separated from the liquid. While a residue compound having a size of more than 0.1 μm is assumed (handled) as a precipitate, a residue compound having a size of 0.1 μm or less is assumed (handled) as a solution with an alloy element dissolved therein (solid-solution state). As the separated residue compound having a size of 0.1 μm or less becomes further smaller, the residue compound is closer to a nano-level solid-solution state. It therefore becomes more difficult to determine whether the residue compound is dissolved or precipitated, resulting in a reduction in correlation with the solid-solution amount or the precipitate, or a reduction in correlation with the effect to be exhibited. The same is applied to a case where the content of an element in the filtrate is measured and determined as the solid-solution amount of that element. In the generally used residue extraction method, therefore, an amount of an element is often assumed as the solid-solution amount of that element, the amount being obtained by subtracting the content of the element in the separated residue compound from the content of that element in the alloy composition, the residue compound having a particle size of more than 0.1 μm.

The invention therefore follows such a method, in which the particle size is separated and classified with reference to 0.1 μm, and a residue compound having a size of more than 0.1 μm is assumed as the residue compound, (to be exact) a precipitate of Sn, and an amount of Sn obtained by subtracting the amount of the (assumably) precipitated Sn from the content of Sn in the alloy composition is assumably defined as (to be exact) the solid-solution amount of Sn.

As defined above, the effect of limiting diffusion of Mg or Si at room temperature due to capture of atomic vacancies at room temperature, and the effect of limiting the increase in strength at room temperature (room-temperature aging) for a long time are not exhibited until the solid-solution amount of Sn is provided. As a result, press formability, particularly hemming performance, is improved in press forming of the room-temperature-aged sheet into a panel. In addition, the effect of releasing the captured vacancies is exhibited during artificial aging such as paint baking of the formed panel, which prompts diffusion of Mg or Si, leading to improvement in BH property.

Extraction Residue Method:

The extraction residue method for determining the solid-solution amount of Sn is performed as follows. First, phenol is put into a decomposition flask and heated, and then each test sheet sample to be measured is transferred into the decomposition flask and thermally decomposed. Subsequently, benzyl alcohol is added, and then undissolved residue on a filter is collected by suction filtration. The collected residue is washed by benzyl alcohol and methanol, and is subjected to quantitative analysis of the Sn content. The atomic absorption analysis (AAS) or the inductively-coupled plasma emission spectrometry (ICP-OES) is appropriately used for the quantitative analysis. A 47 mm diameter membrane filter having a mesh (collection particle size) of 0.1 μm as described above is used for the suction filtration. The Sn content in the residue compound having a particle size of more than 0.1 μm is subtracted from the Sn content in the alloy composition to calculate the solid-solution amount (mass %) of Sn. Such measurement and calculation are performed for each of appropriate 10 points on the test sheet sample (10 specimens are taken), and the solid-solution amounts (mass %) of Sn in the specimens are averaged.

(Manufacturing Method)

The method of manufacturing the aluminum alloy sheet of the invention is now described. The manufacturing process of the aluminum alloy sheet of the invention is a common or known process, in which an aluminum alloy slab having the 6000-series composition is casted, and is then subjected to homogenization heat treatment, hot rolling, and cold rolling, and is thus formed into a sheet having a predetermined thickness. The sheet is then subjected to tempering such as solution hardening so as to be into the aluminum alloy sheet of the invention.

During such a manufacturing process, however, the average cooling rate for casting is controlled, and process annealing in the middle of cold rolling is performed under a preferred condition as described later in order to dissolve Sn. If such process annealing is not performed, Sn is less likely to be dissolved.

(Cooling Rate in Melting-and-Casting)

In a melting-and-casting step, molten metal of aluminum alloy, which is melted and adjusted to have a composition within the 6000-series composition range, is casted by an appropriately selected common melting-and-casting process such as a continuous casting process and a semi-continuous casting process (DC casting process). The average cooling rate during casting is preferably controlled to be as high (fast) as possible, i.e., 30° C./min or more from the liquidus temperature to the solidus temperature in order to dissolve Sn as defined in the invention.

When such temperature (cooling rate) control in a high temperature region during casting is not performed, the cooling rate in the high temperature region inevitably becomes lower. Such a lower average cooling rate in the high temperature region increases the amount of crystallized compounds that are coarsely produced within the temperature range in the high temperature region, and thus increases variations in size and amount of the crystallized grains in each of width and thickness directions of the slab. As a result, Sn is probably not dissolved within a range defined in the invention.

(Homogenization Heat Treatment).

Subsequently, the casted aluminum alloy slab is subjected to homogenization heat treatment prior to hot rolling. This homogenization heat treatment (soaking) is performed for homogenizing a microstructure, i.e., eliminating segregation in a crystal grain of a slab microstructure. Any heat treatment condition including typical onetime or one-stage treatment may be used without limitation as long as such a purpose is achieved.

Temperature of the homogenization heat treatment is appropriately selected from a range from 500° C. to lower than a melting point. Homogenization time is appropriately selected from a range of four hours or more. If the homogenization temperature is low, segregation in the crystal grain cannot be sufficiently eliminated and acts as an origin of fracture, resulting in degradation in stretch-flangeability and in bendability. Subsequently, hot rolling may be immediately started, or may be started after the slab is cooled to and held at an appropriate temperature.

After the homogenization heat treatment, the aluminum alloy slab is cooled to room temperature at an average cooling rate of 20 to 100° C./hr from 300 to 500° C. Subsequently, the slab may be reheated to a range from 350 to 450° C. at an average heating rate of 20 to 100° C./hr to start hot rolling in such a temperature range.

If the average cooling rate condition after the homogenization heat treatment and the subsequent reheating rate condition are not satisfied, coarse Mg—Si compounds are probably formed. This degrades basic mechanical properties such as strength and elongation of the 6000-series aluminum alloy sheet as a prerequisite for exhibition of the effects of Sn.

(Hot Rolling)

Hot rolling is configured of a rough rolling step and a finish rolling step of a slab depending on thickness of a sheet to be rolled. In the rough rolling step and the finish rolling step, a reverse-type or tandem-type rolling mill is appropriately used.

In a condition where the hot rolling (rough rolling) start temperature is over the solidus temperature, hot rolling itself is difficult due to burning. In the hot rolling start temperature of less than 350° C., hot rolling itself is difficult due to an excessively high load during hot rolling. Hence, the hot rolling start temperature is within a range from 350° C. to the solidus temperature, preferably from 400° C. to the solidus temperature.

(Annealing of Hot-Rolled Sheet)

Although annealing (rough annealing) of the hot-rolled sheet before cold rolling is not necessarily required, the annealing may be carried out to improve properties such as formability through refining of crystal grains or optimization of a texture.

(Cold Rolling)

In cold rolling, the hot-rolled sheet is rolled to be produced into a cold-rolled sheet (including a coil) having a desired final thickness. The total cold reduction is desirably 60% or more regardless of the number of passes in order to further refine the crystal grains.

(Process Annealing)

Preferably, the sheet is repeatedly subjected to process annealing at least two times to dissolve Sn in a form of compounds produced in a previous step such as the hot rolling step. In the process annealing, the sheet is held at a high temperature, 480° C. to the melting point, for 0.1 to 10 sec before the cold rolling (after the hot rolling) or during the cold rolling (between passes) and then forcedly cooled (rapidly cooled) to room temperature at an average cooling rate of 3° C./sec or more. Sn is easily precipitated in a common process, and the precipitated Sn is considerably difficult to be dissolved again. As defined in the invention, therefore, such short heat treatment at high temperature must be performed several times in order to dissolve Sn. The heat treatment condition may be varied between the several heat treatment steps as long as the condition is within the above-described condition range.

With the process annealing condition, if the sheet temperature is lower than 480° C., an insufficient solid-solution amount of Sn is given even after at least two times of process annealing. The same is applied to the case of onetime process annealing of which the annealing temperature and the rapid-cooling condition are each within the range. While the holding time may be a short time including momentary time such as 0.1 sec, if the holding time exceeds 10 sec, the mechanical properties of the sheet are extremely degraded. If the cooling after annealing is not forced cooling (rapid cooling) to room temperature by air cooling, mist cooling, or water cooling, which enables an average cooling rate of 3° C./sec or more, i.e., if the average cooling rate is less than 3° C./sec, the dissolved Sn is reprecipitated and formed into a compound.

The annealing under such a condition including the rapid cooling is difficult to be performed by a batch furnace, and requires a continuous heat treatment furnace, in which the sheet is threaded through a furnace and wound up while being rewound. According to the findings of the inventors, even if such continuous annealing enabling rapid cooling is performed, onetime continuous annealing inevitably leads to insufficient solid-solution amount of Sn. Hence, the process annealing by the continuous annealing is repeated at least two times. However, a larger number of repetition of the continuous annealing significantly reduces efficiency of the manufacturing process; hence, the number of repetition is preferably two.

(Solution and Hardening)

The cold-rolled sheet is subjected to solution hardening. The solution hardening may be performed through heating and cooling in a normal continuous heat treatment line without limitation. However, a sufficient solid-solution amount of each element and finer crystal grains of the sheet microstructure are desirable; hence, the solution hardening is performed under a condition where the cold-rolled sheet is heated to a temperature from 520° C. to the solution treatment temperature at a heating rate of 5° C./sec or more, and is held at the temperature for 0 to 10 sec. The average cooling rate from the solution temperature to the hardening stop temperature is 3° C./sec or more. If the cooling rate is low, Mg—Si compounds are easily precipitated during cooling, and are likely to act as crack origins during press forming or bending, leading to degradation in such formability. To achieve such a cooling rate, the hardening is conducted while cooling methods such as air cooling with a fan and water cooling with mist, spray, or immersion, and conditions thereof are selectively used.

The solution hardening condition and the rough annealing condition after the hot rolling are each similar to the process annealing condition in light of temperature or the like. However, if the process annealing is not performed or is performed while conditions such as the temperature of 520° C. or higher are not satisfied, and if the solution hardening or the rough annealing after the hot rolling is merely performed, Sn cannot be dissolved by the required amount or by the defined amount.

(Reheating)

Subsequently, pre-aging (reheating) is performed after the solution hardening in order to form aggregate of atoms (clusters) to be nuclei of the Mg—Si compounds produced during the BH treatment. The achieving temperature (actual temperature) of the sheet is desirably within a temperature range from 80 to 150° C. The holding time is desirably within a range from 3 to 50 hr. After the reheating, the sheet may be cooled to room temperature by natural cooling, or may be forcedly cooled using a cooling means for the hardening to promote production efficiency.

Although the invention is now described in detail with Example, the invention should not be limited thereto, and modifications or alterations thereof may be made within the scope without departing from the gist described before and later, all of which are included in the technical scope of the invention.

Example

Example of the invention is now described. 6000-series aluminum alloy sheets having different solid-solution amounts of Sn were appropriately manufactured depending on the process annealing conditions, and were each investigated in solid-solution amount of Sn defined in the invention. Each sheet was held for 100 days at room temperature, and was then evaluated in BH property (paint baking hardenability) and hemming performance. The results are shown in Table 2.

Specific manufacturing conditions of such aluminum alloy sheets are as follows. Aluminum alloy slabs having compositions listed in Table 1 were in common melted by a DC casting process. This melting is in common performed such that the average cooling rate during the casting was 50° C./min from the liquidus temperature to the solidus temperature. In representation of the content of each element in Table 1 showing the compositions of the 6000-series aluminum alloy sheet samples, representation with no numerical value for each element indicates that the content of the element is 0%, which refers to a content equal to or lower than the detection limit, namely, that element is not contained.

Subsequently, the slabs were in common soaked at 540° C. for four hr and then subjected to hot rough rolling. The slabs were in common hot-rolled into a thickness of 2.5 mm by subsequent finish rolling so as to be each formed into a hot-rolled sheet. The hot-rolled aluminum alloy sheets were in common subjected to rough annealing of 500° C.×1 min. Subsequently, the aluminum alloy sheets were subjected to process annealing during passes (between passes) of the cold rolling. The process annealing was performed using a continuous annealing furnace under various conditions between which the number of annealing steps, temperature, average cooling rate, and the like were each varied as listed in Table 2. Each aluminum alloy sheet was finally formed into a cold-rolled sheet 1.0 mm in thickness.

Such cold-rolled sheets were in common subjected to solution treatment in a salt bath at 560° C. and held for 10 sec after arriving at a target temperature, and were then hardened by water cooling. Each cold-rolled sheet was subjected to pre-aging at 100° C. for 5 hr immediately after the hardening (and then slowly cooled at a cooling rate of 0.6° C./hr).

Test sheets (blanks) were cut from each sheet immediately after such tempering, and a microstructure (solid-solution amount of Sn) of each test sheet was determined. Test sheets (blanks) were cut from each sheet that was left at room temperature for 100 days after such tempering, and a strength (AS proof stress) and a BH property of each test sheet were investigated. The results are shown in Table 2.

(Microstructure of Test Sheet)

The amount (mass %) of Sn was obtained by subtracting the Sn content in the residue compound from the Sn content in each test sheet immediately after the tempering, the residue compound being separated by a hot-phenol residue extraction method and having a particle size of more than 0.1 μm. The obtained amount of Sn was investigated while being assumed as the solid-solution amount of Sn in the test sheet.

(Tensile Test)

The tensile test was performed at room temperature while a JIS Z2201 No. 5 test piece (25 mm×50 mm gage length (GL)×thickness) was taken from each test sheet that had been left at room temperature for 100 days after the tempering. In the tensile test, the tensile direction of the test piece was perpendicular to the rolling direction. The tensile speed was 5 mm/min below the 0.2% proof stress and 20 mm/min over the 0.2% proof stress. Each mechanical property was measured five times, and the average of such measured values was calculated to determine the mechanical property. The test piece for proof stress measurement following the BH treatment was allowed to have a pre-strain of 2% as simulated press forming of a sheet, and was then subjected to the BH treatment.

(BH Property)

The test sheets were in common subjected to the room-temperature aging for 100 days and then subjected to artificial age hardening (BH) of 185° C.×20 min, and then 0.2% proof stress (proof stress after BH) of each test sheet was determined by the tensile test. The BH property of each test sheet was evaluated from a difference (an increase in proof stress) between values of such 0.2% proof stress. A test sheet having the increase in 0.2% proof stress of 100 MPa or more was determined to be acceptable.

(Hemming Performance)

Hemming performance was examined on each of the test sheets left at room temperature for 100 days. The examination was conducted using a strip specimen 30 mm wide through bending, pre-hemming, and flat hemming in this order. The bending was performed at an angle of 90° using a down flange with an inner bending radius R of 1.0 mm. In the pre-hemming, a folded portion was further folded inside about 130° with an inner strip 1.0 mm thick. In the flat hemming, an end portion of the folded portion was brought into tight contact with the inner strip through folding at an angle of 180°.

A bent portion (curled portion) of the flat hem was visually observed to see a surface state, including roughing, microcrack, and large crack, and the surface state was visually evaluated according to the following criteria, and the criteria 0 to 2 were determined to be acceptable.

0: no crack and no roughing; 1: slight roughing; 2: deep roughing; 3: surface microcrack; 4: linearly continued surface crack; and 5: breaking.

The inventive examples shown by Nos. 1 to 4 and 12 to 23 in Table 2 each have a composition within the composition range of the invention (alloy Nos. 1 to 13 in Table 1), and are each manufactured within the preferred range of each of the conditions including a process annealing condition. Hence, as shown in Table 2, each of the inventive examples satisfies the amount of Sn (mass %) as defined in the invention, the amount being obtained by subtracting the Sn content in the residue compound from the Sn content in the sheet, the residue compound being separated by the residue extraction method, and has a high solid-solution amount of Sn since the contained Sn is limitedly precipitated.

As a result, as shown in Table 2, each of the inventive examples has a good BH property, i.e., shows a post-BH (bake hardening) proof stress level of 190 MPa and the difference in proof stress of 100 MPa or more even at an As proof stress level of 90 to 110 MPa after long room-temperature aging for 100 days after the tempering. Even after long room-temperature aging after the tempering, the As proof stress is relatively low, showing good press formability into an auto panel and good hemming performance.

As seen in Table 2, even if the same alloy No. 1 in Table 1 is used, the solid solution state of Sn greatly varies depending on intermediate treatment conditions, resulting in greatly different properties. Specifically, among the inventive examples 1 to 4, the inventive examples 3 and 4 are each relatively high in intermediate annealing temperature and in average cooling rate compared with the inventive examples 1 and 2. In the inventive examples 3 and 4, therefore, a large amount of Sn (mass %) exists, the amount being obtained by subtracting the Sn content in the residue compound from the Sn content in the sheet, the residue compound being separated by the residue extraction method. That is, the contained Sn is limitedly precipitated, resulting in a high solid-solution amount of Sn. As a result, the inventive examples 3 and 4 are large in proof stress difference after BH, i.e., good in BH property compared with the inventive examples 1 and 2 even after long room-temperature aging for 100 days after the tempering.

In contrast, comparative examples 5 to 11 in Table 2 are examples in each of which the process annealing condition is out of the preferred range despite using the alloy No. 1 in Table 1 as with the above-described inventive examples. Hence, such comparative examples are each excessively small in the amount of Sn (mass %), the amount being obtained by subtracting the Sn content in the residue compound from the Sn content in the sheet as defined in the invention, the residue compound being separated by the residue extraction method, i.e., each comparative example has a small solid-solution amount of Sn since precipitation of the contained Sn is substantially not limited. Hence, compared with the above-described inventive examples having the same alloy composition, the comparative examples are each bad in press formability into an auto panel or in hemming performance, and each have a small proof stress difference of less than 100 MPa, showing bad BH property.

The comparative example 5 is not subjected to process annealing.

The comparative example 6 is subjected to one-time process annealing that satisfies the conditions of temperature, holding time, and average cooling rate.

In the comparative example 7, temperature of the first process annealing is low, 400° C., i.e., lower than 480° C., while the second process annealing satisfies the conditions of temperature, holding time, and average cooling rate.

In the comparative example 8, temperature of the second process annealing is low, 400° C., i.e., lower than 480° C., while the first process annealing satisfies the conditions of temperature, holding time, and average cooling rate.

In the comparative example 9, temperature of each of the first process annealing and the second process annealing is low, 460° C., i.e., lower than 480° C.

In the comparative examples 10 and 11, the average cooling rate of the first or second process annealing is too low while the first process annealing and the second process annealing each satisfy the conditions of temperature and holding time.

Although comparative examples 24 to 29 in Table 2 are each manufactured within the preferred range of each of the conditions including the process annealing condition, the comparative examples use the alloy Nos. 14 to 19, respectively, in Table 1, in each of which the content of at least one of Mg, Si, and Sn as essential elements is out of the range of the invention. Hence, as shown in Table 2, the comparative examples 24 to 29 are each high in As proof stress after room-temperature holding for 100 days compared with the inventive examples, resulting in bad press formability into an auto panel, bad hemming performance, or a bad BH property. In the comparative example 27, since an excessively large amount of Sn exists, cracking occurred during hot rolling, and thus a hot-rolled sheet was failed to be manufactured.

The comparative example 24 corresponds to the alloy 14 in Table 1, and has an excessively small amount of Si.

The comparative example 25 corresponds to the alloy 15 in Table 1, and has an excessively large amount of Si.

The comparative example 26 corresponds to the alloy 16 in Table 1, and has an excessively small amount of Sn.

The comparative example 27 corresponds to the alloy 17 in Table 1, and has an excessively large amount of Sn.

The comparative example 28 corresponds to the alloy 18 in Table 1, and has an excessively small amount of Mg.

The comparative example 29 corresponds to the alloy 19 in Table 1, and has an excessively large amount of Mg.

The above-described results of the Example support the critical meanings and advantageous effects of the composition and the solid-solution amount of Sn as defined in the invention and/or the preferred manufacturing condition such as the process annealing condition for improvement in hemming performance or BH property after long room-temperature aging of the 6000-series aluminum alloy sheet containing Sn.

TABLE 1
Alloy	Chemical composition 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	0.18	0.2	0.08				0.03
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	1.30	1.00	0.10	0.2						0.1		0.1
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
14	1.30	0.30	0.05	0.2
15	0.40	1.80	0.05	0.2
16	0.60	1.20	0.002	0.2
17	0.60	1.00	0.30	0.2
18	0.20	0.80	0.05	0.2
19	1.90	1.00	0.05	0.2

TABLE 2
	Microstructure
	of aluminum alloy
	sheet immediately
	Process annealing condition	after tempering
	between cold rolling passes	Amount of Sn	Properties of aluminum alloy sheet after
	First continuous	Second continuous	obtained by	holding at room temperature for 100 days
	annealing	annealing	subtracting		Post-BH
		Alloy	Tempera-	Average	Tempera-	Average	amount of Sn in	As proof	proof	Increase
		numbe	ture ×	cooling	ture ×	cooling	residue compound	stress	stress	in proof	Hemming
Classifi-		in	time	rate	time	rate	from Sn content	0.2%	0.2%	stress	perfor-
cation	No.	Table 1	° C. × 5 sec	° C./sec	° C. × 5 sec	° C./sec	mass %	MPa	MPa	MPa	mance
Inventive	1	1	480	5	480	5	0.030	87	188	101	1
example
Inventive	2	1	490	10	500	10	0.032	90	194	104	1
example
Inventive	3	1	510	100	520	50	0.039	105	230	125	1
example
Inventive	4	1	520	100	510	100	0.040	108	242	134	1
example
Comparative	5	1	—	—	—	—	0.001	79	149	70	3
example
Comparative	6	1	490	10	—	—	0.002	84	159	75	3
example
Comparative	7	1	400	50	510	100	0.003	102	194	92	2
example
Comparative	8	1	520	50	400	100	0.003	101	190	89	2
example
Comparative	9	1	460	50	460	100	0.002	96	183	87	3
example
Comparative	10	1	510	1	510	1	0.003	87	166	79	3
example
Comparative	11	1	510	50	510	1	0.004	93	173	80	3
example
Inventive	12	2	520	50	510	100	0.036	104	231	127	1
example
Inventive	13	3	520	50	510	100	0.033	91	207	116	1
example
Inventive	14	4	520	50	510	100	0.067	97	235	138	1
example
Inventive	15	5	520	50	510	100	0.041	101	201	100	1
example
Inventive	16	6	520	50	510	100	0.030	119	248	129	2
example
Inventive	17	7	520	50	510	100	0.095	90	218	128	1
example
Inventive	18	8	520	50	510	100	0.038	105	233	123	1
example
Inventive	19	9	520	50	510	100	0.018	108	242	134	1
example
Inventive	20	10	520	50	510	100	0.072	101	216	115	1
example
Inventive	21	11	520	50	510	100	0.034	105	223	118	1
example
Inventive	22	12	520	50	510	100	0.009	116	248	132	2
example
Inventive	23	13	520	50	510	100	0.038	107	236	129	1
example
Comparative	24	14	520	50	510	100	0.042	103	170	67	1
example
Comparative	25	15	520	50	510	100	0.025	120	228	108	3
example
Comparative	26	16	520	50	510	100	0.002	130	232	102	3
example
Comparative	27	17	520	50	510	100	Cracking during hot rolling
example
Comparative	28	18	520	50	510	100	0.039	83	155	73	1
example
Comparative	29	19	520	50	510	100	0.029	127	232	105	3
example

Although the invention has been described in detail with reference to a specific embodiment, it should be understood by those skilled in the art that various alterations and modifications thereof may be made without departing from the spirit and the scope of the invention.

The present application is based on Japanese patent application (JP-2013-267591) filed on Dec. 25, 2013, the entire contents of which are incorporated herein by reference.

INDUSTRIAL APPLICABILITY

According to the invention, there can be provided a 6000-series aluminum alloy sheet combining a good BH property and good formability after long room-temperature aging. As a result, the 6000-series aluminum alloy sheet can be extensively used for members or components of an automobile, a ship or a transport aircraft for vehicles, a household electric appliance, a building, and a structure, particularly for members of a transport aircraft for automobiles.

Claims

1. An aluminum alloy sheet for forming, comprising an Al—Mg—Si aluminum alloy that contains, in mass %, Mg: 0.3 to 1.3%, Si: 0.5 to 1.5% and Sn: 0.005 to 0.2%, the remainder consisting of Al and unavoidable impurities,

wherein an amount of Sn is 0.005 mass % or more, the amount being obtained by subtracting Sn content in a residue compound from Sn content in the aluminum alloy sheet, the residue compound being separated by a hot-phenol residue extraction method and having a particle size of more than 0.1 μm.

2. The aluminum alloy sheet for forming according to claim 1, wherein the aluminum alloy sheet further contains one or more of Mn: more than 0% and 1.0% or less, Cu: more than 0% and 1.0% or less, Fe: more than 0% and 1.0% or less, Cr: more than 0% and 0.3% or less, Zr: more than 0% and 0.3% or less, V: more than 0% and 0.3% or less, Ti: more than 0% and 0.05% or less, Zn: more than 0% and 1.0% or less, and Ag: more than 0% and 0.2% or less.

3. The aluminum alloy sheet for forming according to claim 1, wherein a mass ratio of Si to Mg, Si/Mg, is 1 or more.

4. An aluminum alloy comprising in mass %,

Mg: 0.3 to 1.3%,

Si: 0.5 to 1.5% and

Sn: 0.005 to 0.2%,

the remainder consisting of Al and unavoidable impurities;

wherein a solid-solution amount of Sn in the aluminum alloy is at least 0.005 mass %.

5. The aluminum alloy of claim 4, wherein the solid-solution amount of Sn in the aluminum alloy is about 0.15 mass %.

6. A sheet comprising the aluminum alloy according to claim 4.

7. The sheet of claim 6 that has been bent, folded or otherwise formed.

8. The sheet of claim 6 that has been formed into a member or component of an automobile, a ship, a transport aircraft for vehicles, a household electric appliance, a building, or a structure.

9. A method for making an aluminum sheet containing a solid-solution amount of Sn of at least 0.005 mass % comprising:

hot rolling a sheet of an aluminum alloy comprising in mass %, Mg: 0.3 to 1.3%, Si: 0.5 to 1.5% and Sn: 0.005 to 0.2%, the remainder consisting of Al and unavoidable impurities,

annealing the sheet at least two times by holding the sheet at a temperature ranging from 480° C. to the alloy melting point for 0.1 to 10 sec,

cold rolling the sheet, and

then cooling the sheet during or after said cold rolling at an average cooling rate of at least 3° C./sec to room temperature.