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United States Patent Application 20170349978
Kind Code A1
SHISHIDO; Hisao ;   et al. December 7, 2017

ALUMINUM ALLOY SHEET

Abstract

The present invention relates to an Al--Mg--Si sheet which contains, in terms of mass %, 0.3-1.0% Mg, 0.5-1.5% Si, 0.005-0.2% Sn, 0.02-1.0% Fe, and 0.02-0.6% Mn, with the remainder comprising Al and unavoidable impurities, characterized by having a structure wherein compounds that are detected with an SEM having a magnification of 500 diameters and are identified with an X-ray spectrometer include Sn compounds which contain Mn and Fe and which have an Sn content of 1.0 mass % or higher and a diameter of 0.3-20 .mu.m in terms of equivalent circular diameter, the average number density of the Sn compounds being 500-3,000 /mm.sup.2, and wherein the length of the boundary between each Sn compound and the aluminum matrix is in the range of 3-20/mm on average in terms of value obtained by dividing the total peripheral length of the Sn compound grain by the area thereof determined with the SEM. This aluminum alloy sheet satisfies the formability after natural aging at room temperature and bake hardenability which are required of automotive outer panels, and has excellent filiform rust resistance.


Inventors: SHISHIDO; Hisao; (Hyogo, JP) ; MATSUMOTO; Katsushi; (Hyogo, JP) ; ARUGA; Yasuhiro; (Hyogo, JP) ; TAKADA; Satoru; (Hyogo, JP) ; OZAWA; Takahiro; (Hyogo, JP) ; TATSUMI; Akihiko; (Hyogo, JP)
Applicant:
Name City State Country Type

KABUSHIKI KAISHA KOBE SEIKO SHO (KOBE STEEL, LTD.)

Kobe-shi

JP
Assignee: KABUSHIKI KAISHA KOBE SEIKO SHO (KOBE STEEL, LTD.)
Kobe-shi
JP

Family ID: 1000002822920
Appl. No.: 15/506595
Filed: August 27, 2015
PCT Filed: August 27, 2015
PCT NO: PCT/JP2015/074304
371 Date: February 24, 2017


Current U.S. Class: 1/1
Current CPC Class: C22F 1/05 20130101; C22C 21/02 20130101
International Class: C22C 21/02 20060101 C22C021/02; C22F 1/05 20060101 C22F001/05

Foreign Application Data

DateCodeApplication Number
Aug 27, 2014JP2014-173277

Claims



1. An aluminum alloy sheet which is an Al--Mg--Si alloy sheet comprising, in terms of mass %: 0.3-1.0% of Mg, 0.5-1.5% of Si, 0.005-0.2% of Sn, 0.02-1.0% of Fe, and 0.02-0.6% of Mn, with the remainder being Al and unavoidable impurities, the aluminum alloy sheet having a microstructure wherein: among compounds as examined with an SEM with a magnification of 500 times and identified with an X-ray spectrometer, an Sn compound comprising Mn and Fe, having an Sn content of 1.0 mass % or higher, and having an equivalent circular diameter in a range of 0.3-20 has an average number density in a range of 500-3,000 counts/mm.sup.2; and a boundary between the Sn compound and an aluminum matrix has a length in a range of 3-20 /mm on average in terms of a value obtained by dividing a total peripheral length of the Sn compound by an area as examined with the SEM.

2. The aluminum alloy sheet according to claim 1, further comprising at least one kind of ingredient selected from the group consisting of more than 0% and 0.4% 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 0.1% or less of Ti, more than 0% and 0.4% or less of Cu, more than 0% and 0.2% or less of Ag, and more than 0% and 1.0% or less of Zn, in terms of mass %.

3. An Al--Mg--Si alloy comprising, in terms of mass %, 0.3-1.0% of Mg, 0.5-1.5% of Si, 0.005-0.2% of Sn, 0.02-1.0% of Fe, and 0.02-0.6% of Mn, with the remainder being Al and unavoidable impurities.

4. The Al--Mg--Si alloy of claim 3, further comprising at least one kind of ingredient selected from the group consisting of more than 0% and 0.4% 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 0.1% or less of Ti, more than 0% and 0.4% or less of Cu, more than 0% and 0.2% or less of Ag, and more than 0% and 1.0% or less of Zn, in terms of mass %.

5. A sheet comprising the Al--Mg--Si alloy of claim 3.

6. A sheet comprising the Al--Mg--Si alloy of claim 4.

7. The sheet according to claim 5 that comprises an aluminum matrix containing a Sn compound comprising Sn, Mn and Fe, wherein the content of Sn in the compound is 1.0% by mass or higher, wherein the Sn compound in the aluminum matrix has an equivalent circular diameter ranging from 0.3 to 20 .mu.m and the Sn compound has an average density ranging from 500 to 3,000 counts/mm.sup.2, and wherein a boundary between the Sn compound and the aluminum matrix ranges in average length ranging from 3 to 20/mm and has a length in a range of 3-20 /mm on average in terms of a value obtained by dividing a total peripheral length of the Sn compound by an area as examined with the SEM.

8. The sheet according to claim 6 that comprises an aluminum matrix containing a Sn compound comprising Sn, Mn and Fe, wherein the content of Sn in the compound is 1.0% by mass or higher, wherein the Sn compound in the aluminum matrix has an equivalent circular diameter ranging from 0.3 to 20 .mu.m and the Sn compound has an average density ranging from 500 to 3,000 counts/mm.sup.2, and wherein a boundary between the Sn compound and the aluminum matrix ranges in average length ranging from 3 to 20/mm and has a length in a range of 3-20 /mm on average in terms of a value obtained by dividing a total peripheral length of the Sn compound by an area as examined with the SEM.
Description



TECHNICAL FIELD

[0001] The present invention relates to an Al--Mg--Si alloy sheet, and especially relates to an aluminum alloy sheet excellent in formability, BH response and corrosion resistance. 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

[0002] 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.

[0003] Among those large body panel structures of automobiles, as outer panels (outer sheets), such as hoods, fenders, doors, roofs, and trunk lids, use of Al--Mg--Si-based AA or

[0004] JIS 6000-series aluminum alloy sheets which are thin high-strength aluminum alloy sheets is being investigated.

[0005] As is known well, the automotive outer panel is produced by subjecting a 6000-series aluminum alloy sheet as a material 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.

[0006] The 6000-series aluminum alloy sheets have the advantage of having excellent BH response (bake hardenability) but have room-temperature aging properties. There has hence been a problem in that they, when held at room temperature after a solution quenching treatment, undergo age hardening and increase in strength, thereby deteriorating in formability into panels. Moreover, in the case where such room-temperature aging is great, there is also a problem 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.

[0007] A large number of metallurgical measures for overcoming those have been proposed for far. One of these is a method in which Sn is positively added to a 6000-series aluminum alloy sheet in order to inhibit room-temperature age hardening and improve the BH response. For example, Patent Document 1 proposes a method in which Sn 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 age hardening and BH response. Patent Document 2 proposes a method in which Sn and Cu, which improves formability, are added to a 6000-series aluminum alloy sheet to improve formability, bake hardenability and corrosion resistance.

PRIOR ART DOCUMENTS

Patent Documents

[0008] Patent Document 1: JP-A-09-249950

[0009] Patent Document 2: JP-A-10-226894

SUMMARY OF THE INVENTION

Problem that the Invention is to Solve

[0010] In those conventional 6000-series aluminum alloy sheets to which Sn has been positively added, there has still been room for improvement for achieving, as materials for automotive outer panels, combined satisfactory formability and high BH response after long-term room-temperature aging with various properties including excellent filiform corrosion resistance.

[0011] For example, improvements in filiform corrosion resistance are essential as automotive outer panels (panels for outside use). Although used after being painted, automotive outer panels are exposed to corrosive environments (under-paint-film corrosive environments) involving seawater, brine, or the like as environments in which the automobiles run. There is hence a problem in that threadlike rust called filiform rust generates from precipitates or inclusions as origins and grows on the surface of the aluminum alloy sheet beneath the paint film, resulting in a decrease in the strength of the member and an appearance failure. It is therefore necessary that Sn-added Al--Mg--Si alloy sheets for use as automotive outer panels should have excellent filiform corrosion resistance.

[0012] In 6000-series aluminum alloy sheets also, various techniques for improving the compositions or microstructures of the matrixes in order to improve the filiform corrosion resistance have been proposed so far. However, the case where Sn has been added shows a metallurgical behavior different from that in the case where no Sn has been added, and it has been uncertain as to whether the above-described conventional techniques for improving the matrixes are effective, as expected, also in the case where Sn has been added. Consequently, for improving the filiform corrosion resistance of a 6000-series aluminum alloy sheet to which Sn has been positively added, together with the other properties mentioned above, such as formability and BH response, it is necessary to pursue a distinctive measure for improving Sn-added 6000-series aluminum alloy sheets.

[0013] With respect to the formability also, the properties required of 6000-series aluminum alloy sheets as materials for automotive outer panels tend to become severer increasingly. Automotive outer panels are required to attain strain-free, beautiful curved-surface configurations and produce character lines just as designed. This is a problem attributable to the peculiar designs of outer panels. Recessed portions having given depths (protrudent portions, embossed portions) for attaching devices or members, such as knob mount bases, lamp mount bases, and license (number plate) mount bases, or for drawing wheel arches are partly provided to outer panels.

[0014] In the cases when such a recessed portion is press-formed together with consecutive curved surfaces which surround the recessed shape, face strains are prone to occur in 6000-series aluminum alloy sheets, which have poorer formability than steel sheets, and it is difficult to attain the strain-free, beautiful curved-surface configuration and character line. Consequently, it is essential for 6000-series aluminum alloy sheets that the occurrence of such face strains during forming thereof into automotive outer panels should be inhibited. The problem of such face strains is not a problem only for those recessed portions (protrudent portions) but a problem common to automotive panels which partly have a recessed portion (protrudent portion) that may suffer a face strain, such as the saddle-shaped portion of a door outer panel, the vertical wall portion of a front fender, the window corner portion of a rear fender, the character-line termination portions of a trunk lid or hood outer panel, and the root portion of a rear fender pillar.

[0015] From the standpoint of inhibiting the occurrence of the face strains to overcome the problem, it is desirable that a 6000-series aluminum alloy sheet (which has undergone room-temperature aging after production) should have a reduced 0.2% proof stress when press-formed. However, in the cases when the proof stress in press forming has been reduced, it is rather difficult to obtain a high proof stress after a bake hardening treatment (bake hardening).

[0016] The present invention has been achieved in order to overcome such problem. An object thereof is to provide an Sn-containing 6000-series aluminum alloy sheet which satisfies the requirements for use as automotive outer panels, concerning formability and BH response after room-temperature aging, and which further has improved filiform corrosion resistance.

Means for Solving the Problem

[0017] For achieving the object, the gist of the aluminum alloy sheet of the present invention is an Al--Mg--Si alloy sheet containing, in terms of mass %, 0.3-1.0% of Mg, 0.5-1.5% of Si, 0.005-0.2% of Sn, 0.02-1.0% of Fe, and 0.02-0.6% of Mn, with the remainder being Al and unavoidable impurities, the aluminum alloy sheet having a microstructure in which, among compounds examined with an SEM with a magnification of 500 times and identified with an X-ray spectrometer, an Sn compound containing Mn and Fe, having an Sn content of 1.0 mass % or higher, and having an equivalent circular diameter in a range of 0.3-20 .mu.m, has an average number density in a range of 500-3,000 counts/mm.sup.2, and a boundary between the Sn compound and an aluminum matrix has a length in a range of 3-20 /mm on average in terms of a value obtained by dividing a total peripheral length of the Sn compound by an area examined with the SEM.

Effects of the Invention

[0018] In the microstructure of the 6000-series aluminum alloy sheet, the Sn has an action to capture (trap) atomic holes in a room-temperature state. Due to this action of Sn, the room-temperature diffusion of Mg and Si is inhibited to suppress room-temperature aging (hardening) and inhibit the strength from increasing. Thus the effect of improving the press formability including hem workability, drawability, and punch stretch formability during the forming of the sheet into panels is brought about. Meanwhile, during an artificial aging treatment of the panels, such as a paint baking treatment, the Sn releases the captured holes and hence has the effect of in tern enhancing the diffusion of Mg and Si to heighten the BH response.

[0019] However, the present inventors have found that 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 or below a theoretical solute amount in ordinary 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, although it has the effect of improving the filiform corrosion resistance which will be described later.

[0020] Because of this, in the present invention, the present inventors have ventured to reconsider sheet production processes and contrived production conditions concerning, for example, soaking treatment to control the number density of Sn-containing compounds having a specific composition and a specific size, thereby controlling a balance between the formation of solid solution and precipitation of the Sn contained and ensuring a solute Sn amount, as will be described later. Thus, age hardening is suppressed by producing both the solute Sn's effect of capturing and releasing atomic holes and the effect of the presence of the Sn compounds having the specific composition and size, thereby improving the formability and the BH response. Specifically, the sheet produced is made to have the following properties after room-temperature aging: the proof stress during press forming into automotive outer panels (before bake finish) is 110 MPa or less; the hem workability is 2.0 or less in terms of the criteria which will be described later in Examples; and the artificial-aging hardening amount (BH response) as an automotive outer panel under bake finish conditions of 185.degree. C..times.20 min is 100 MPa or greater.

[0021] Meanwhile, in the present invention, precipitates or crystals are formed so that boundaries between the Sn compounds having the specific composition and size and the aluminum matrix are large (long) in order to improve the filiform corrosion resistance. Thus, boundaries between compounds containing no Sn and the matrix can be made small (short). Consequently, a 6000-series aluminum alloy sheet which combines satisfactory filiform corrosion resistance with formability and BH response can be provided.

MODES FOR CARRYING OUT THE INVENTION

[0022] Modes for carrying out the present invention will be explained below in detail with respect to each requirement.

(Chemical Component Composition)

[0023] 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 outer panels, 6000-series aluminum alloy sheets to which the present invention relates are required to have various properties including excellent formability and BH response after room-temperature aging and filiform corrosion resistance.

[0024] It is preferable that the sheets should have the following properties which are necessary for satisfying those requirements: as properties of the sheets which are produced and then have undergone refining, e.g., T6, and subsequently undergone 30-day room-temperature aging, a proof stress during press forming into automotive outer panels (before bake finish) is 110 MPa or less and a hem workability, in terms of the criteria which will be described later in Examples, is 2.0 or less; and as an automotive outer panel, an artificial-aging hardening amount (BH response) under bake finish conditions of 185.degree. C..times.20 min is 100 MPa or greater.

[0025] With respect to requirements concerning alloy composition for satisfying those preferred sheet properties, the aluminum alloy sheet has a specific composition among 6000-series, containing, in terms of mass %, 0.3-1.0% of Mg, 0.5-1.5% of Si, 0.005-0.2% of Sn, 0.02-1.0% of Fe, and 0.02-0.6% of Mn, 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".

[0026] Among ones having the above alloy composition, preferred is a 6000-series aluminum alloy sheet with excess Si in which the mass ratio of Si to Mg, Si/Mg, is 1 or greater and which has better BH response.

[0027] Elements other than Mg, Si, Sn, Fe, and Mn as the alloy composition are unavoidable impurities, and are regulated to contents (permissible amounts) on element 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, 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 other than Mg, Si, Sn, Fe, and Mn 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 so long as the inclusion does not inhibit the object or effects of the present invention.

[0028] Specifically, there may be contained one kind or two or more kinds selected from the group consisting of 0.4% 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, 0.1% or less (exclusive of 0%) of Ti, 0.4% or less (exclusive of 0%) of Cu, 0.2% or less (exclusive of 0%) of Ag, and 1.0% or less (exclusive of 0%) of Zn, in terms of mass %.

[0029] 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: 0.5-1.5%

[0030] 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.

[0031] Meanwhile, in the case where the content of Si is too high, coarse crystals and precipitates are formed within grains and at grain boundaries, resulting in considerable decreases in bendability and filiform corrosion resistance. Consequently, the Si is in the range of 0.5-1.5%. A more preferred lower limit thereof is 0.6%, and a more preferred upper limit thereof is 1.4%.

Mg: 0.3-1.0%

[0032] 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 0.3-1.0%. A more preferred lower limit thereof is 0.4%, and a more preferred upper limit thereof is 0.8%.

Fe: 0.02-1.0%

[0033] Fe is an element necessary for yielding, in a specific number density, Sn--containing compounds of a specific size which are specified in the present invention, together with Al and other elements including Si, Mn and Sn during a soaking treatment and hot rolling. In the case where the content thereof is too low, the specific Sn-containing compounds are yielded in so small an amount that the boundaries between the specific Sn-containing compounds and the matrix become small (short), resulting in a decrease in the effect of improving filiform corrosion resistance. Meanwhile, in the case where the Fe content is too high, the specific Sn-containing compounds are yielded in too large an amount within grains and at grain boundaries, resulting in deteriorations in formability such as hem workability and in filiform corrosion resistance.

Mn: 0.02-0.6%

[0034] Like the Fe, Mn is an element necessary for yielding, in a specific number density, Sn-containing compounds of a specific size which are specified in the present invention, together with Al and other elements including Si, Fe and Sn during a soaking treatment and hot rolling. In the case where the content thereof is too low, the specific Sn-containing compounds are yielded in so small an amount that the boundaries between the specific Sn-containing compounds and the matrix becomes small (short), resulting in a decrease in the effect of improving filiform corrosion resistance. Meanwhile, in the case where the Mn content is too high, the specific Sn-containing compounds are yielded in too large an amount within grains and at grain boundaries, resulting in deteriorations in formability such as hem workability and in filiform corrosion resistance.

Sn: 0.005-0.2%

[0035] Sn is an essential element and in a solid-solution state 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.

[0036] These effects of Sn are exhibited only when the Sn forms a solid solution. In the case where the content of Sn is too low, a decrease in solute Sn amount results and holes cannot be sufficiently trapped, making it impossible to produce the Sn's effect of inhibiting room-temperature age hardening. As a result, not only the room-temperature increase in strength cannot be inhibited, resulting in an increase in proof stress and a deterioration in hem workability, but also Mg--Si precipitates are prone to be yielded in a reduced amount during a BH treatment, resulting in a decrease in BH response.

[0037] In the present invention, other than the Sn being made to form a solid solution, Sn is caused, in a certain amount, to precipitate or crystallize out as Sn-containing compounds to improve filiform corrosion resistance. However, in the case where the content of Sn is too low, the amount of Sn-containing compounds also is decreased.

[0038] Consequently, among the compounds containing Mn and Fe, the average number density of compounds which have a content of Sn of 1.0 mass % or higher and an equivalent circular diameter in the range of 0.3-20 .mu.m is insufficient. As a result, the length of the boundaries between these compounds and the aluminum matrix also is insufficient, making it impossible to improve the filiform corrosion resistance.

[0039] It is, however, noted that even when the content of Sn is increased excessively, solute Sn amount does not increase since there is a limit on the solid-solution amount. In addition, in the case where the content of Sn is too high, Sn segregates at grain boundaries and this is causative of intergranular cracks. As a result, cracks are prone to occur during hot rolling in sheet production steps.

[0040] Consequently, the content of Sn is in the range of 0.005-0.2%. A more preferred lower limit thereof is 0.01%, and a more preferred upper limit thereof is 0.18%.

(Microstructure)

[0041] Next, the microstructure of the 6000-series aluminum alloy sheet of the present invention is explained below.

Sn Compounds:

[0042] In the present invention, the sheet after being produced (refined) has a microstructure in which the average number density of Sn compounds which have a specific composition and a specific size and which are examined with an SEM having a magnification of 500 times and are identified with an X-ray spectrometer is specified and the amount of the boundaries between the Sn compounds and the aluminum matrix is specified.

[0043] The Sn compounds having a specific composition and a specific size are Sn compounds (Sn-containing compounds) which contain both Mn and Fe or contain either Mn or Fe and which have a content of Sn of 1.0 mass % or higher and an equivalent circular diameter in the range of 0.3-20 .mu.m.

[0044] The average number density of Sn compounds which satisfy such requirements is regulated so as to be in the range of 500-3,000 counts/mm.sup.2, preferably in the range of 500-2,000 counts/mm.sup.2, thereby ensuring a solute Sn amount necessary for enabling the solute Sn to exhibit the effect of inhibiting room-temperature age hardening.

[0045] Furthermore, the length of the boundaries between the Sn compounds, which satisfy those requirements, and the aluminum matrix is regulated so as to be in the range of 3-20 /mm on average, preferably in the range of 3-10 /mm on average, in terms of a value obtained by dividing the total peripheral length of the Sn compounds by the area examined with the SEM. By precipitating or crystallizing Sn compounds having the specific composition and size so that the boundaries with the aluminum matrix are present in such a large amount, the boundaries between Sn-free compounds, which reduce filiform corrosion resistance, and the matrix are diminished to improve the filiform corrosion resistance.

Average Number Density of the Sn Compounds:

[0046] In the case where the average number density of the Sn compounds having the specific composition and size is too high beyond 3,000 counts/mm.sup.2, a reduced solute Sn amount results, making it impossible to produce the Sn's effect of inhibiting room-temperature age hardening. As a result, not only the room-temperature increase in strength cannot be inhibited, resulting in an increase in proof stress and a deterioration in hem workability, but also Mg--Si precipitates are prone to be yielded in a reduced amount during a BH treatment, resulting in a decrease in BH response.

[0047] Meanwhile, in the present invention, Sn is caused, to some degree, to precipitate or crystallize out as compounds having the specific composition and size so that boundaries between these Sn compounds and the matrix become large (long), in order to improve the filiform corrosion resistance.

[0048] The present inventors investigated relationships between the addition of Sn and filiform corrosion resistance. As a result, the inventors discovered that in the microstructure of an Al--Mg--Si alloy sheet, a peculiar phenomenon in which Sn added comes into coarse compounds to render them less apt to serve as filiform-corrosion starting points occurs under certain production conditions.

[0049] The term "coarse compounds" herein means intermetallic compounds, such as Al--Fe, Al--Fe--Mn, Al--Fe--Si, and Al--Fe--Mn--Si intermetallic compounds, which are relatively large and have an equivalent circular diameter of submicrometers to tens of micrometers and that are yielded during casting, soaking, and hot rolling. In the cases when such coarse compounds are present in an aluminum alloy, they have a nobler potential than the surrounding aluminum and serve as so-called cathode sites.

[0050] Consequently, the boundaries between these coarse compounds and the aluminum matrix have a large potential difference and are in the state of being highly susceptible to corrosion. This corrosion phenomenon occurs as filiform corrosion (rust extending in the form of threads) in the case where the surface of the aluminum alloy sheet (panel) is covered with a resinous coating film, as in the automotive panels.

[0051] In contrast, the inclusion of Sn in the coarse compounds reduces the potential difference with the surrounding aluminum to render the coarse compounds less apt to serve as cathode sites and less apt to serve as starting points for filiform corrosion. Namely, the length of the boundaries between the Sn compounds and the aluminum matrix is regulated so as to be not less than a certain value range and the boundaries between Sn-free compounds, which reduce filiform corrosion resistance, and the matrix are diminished. The filiform corrosion resistance can hence be improved.

[0052] Thus, formability and BH response, and satisfactory filiform corrosion resistance are combinedly exhibited.

[0053] Consequently, the specified average number density of the Sn compounds having the specific composition and size is a measure of the amount of Sn which has precipitated or crystallized out, for precipitating or crystallizing Sn just in a certain amount (certain number density and certain peripheral length) in order to improve the filiform corrosion resistance. In the case where the average number density of the specific Sn compounds is too low and below 500 counts/mm.sup.2, the specific Sn compounds themselves, which contain Mn and Fe, are not obtained and the filiform corrosion resistance cannot be improved.

Sn Compounds Containing Mn and Fe:

[0054] In the sheet alloy composition, together with the Mn and Fe contained therein, Sn forms Sn compounds having the specific composition and size. Hence, in the case where the sheet does not contain these Mn and Fe, Sn compounds themselves which have the specific composition and size are not yielded. It is, however, noted that so long as Mn and Fe are present in the Sn compounds in amounts on a level (range) detectable with the EDX which will be described later, the amounts thereof suffice, and there is no need of quantitatively specifying the contents thereof in the Sn compounds.

Sn Content and Size of the Sn Compounds:

[0055] Among Sn compounds, even when compounds containing Sn in a too small amount, in which an Sn content is less than 1.0 mass %, or compounds having too small an equivalent circular diameter less than 0.3 .mu.m are present so as to satisfy the average number density or the sufficient amount of the boundaries of the compounds, this cannot ensure a solute Sn amount, and the effect of improving formability, BH response, filiform corrosion resistance, etc. is low. Consequently, these compounds are excluded from the Sn compounds having the specific composition and size.

[0056] There is no particular upper limit on the Sn content of the specific Sn compounds. However, an upper limit thereof is about 10% by mass in view of limitations in production. Meanwhile, in the case where the specific Sn compounds are coarse compounds having an equivalent circular diameter exceeding 20 .mu.m, they are causative of cracks, and cracks are prone to occur during hot rolling, etc. in sheet production steps.

Length (Amount) of Boundaries of the Sn Compounds:

[0057] With respect to the state in which the Sn compounds having the specific composition and size are present in the sheet microstructure, in the cases when boundaries between these Sn compounds and the matrix are made longer (present in a larger amount), the filiform corrosion resistance is improved. In the case where the amount of the boundaries between these Sn-containing compounds and the matrix is too small, the effect of improving filiform corrosion resistance is lessened. Specifically, in the case where the length of the boundaries between these Sn compounds and the aluminum matrix is less than 3/mm in terms of a value obtained by dividing the total peripheral length of these compounds (total of the peripheral lengths of all the Sn compounds having the specific composition and size) by the area examined with the SEM, the boundaries between the Sn compounds and the matrix become short. Because of this, the boundaries between Sn-free compounds, which reduce the filiform corrosion resistance, and the matrix are longer (present in an increased amount) and the effect of improving filiform corrosion resistance is lessened.

[0058] Meanwhile, in the case where the amount of the boundaries between the Sn compounds and the matrix is made too large beyond 20/mm, the number density of the Sn-containing compounds is too high and a reduced solute Sn amount results, making it impossible to obtain a low proof stress and high BH response. Consequently, the amount of the boundaries between the Sn-containing compounds and the matrix is regulated to 3-20/mm on average in terms of a value obtained by dividing the total peripheral length of these compounds by the area examined with the SEM. More preferably, it is in the range of 3-10/mm on average.

Examination of the Sn Compounds:

[0059] A measurement for determining the number density of compounds which have an equivalent circular diameter in the range of 0.3-20 .mu.m and which contain 1.0 mass % or more Sn and further contain both Mn and Fe is made with an SEM (scanning electron microscope) having a magnification of 500 times. These Sn compounds having the specific composition and size are identified with an X-ray spectrometer belonging to the SEM and are distinguished from compounds which have an Sn content less than 1.0 mass % or which do not contain Mn or Fe. Furthermore, they are distinguished, with the SEM, also from compounds which do not satisfy the range of sizes.

[0060] The measurement with the SEM is made with respect to ten portions arbitrarily selected at a depth corresponding to 1/4 the sheet thickness direction from a surface of a test sheet (ten specimens are collected). The number densities of Sn compounds having the specific composition and size determined with respect to these specimens are averaged to obtain an average number density (counts/mm.sup.2). 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 1/4 the sheet thickness direction from a surface and which is parallel with the sheet surface, an examination is made with an SEM 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 .mu.m.times.180 .mu.m.

[0061] The X-ray spectrometer is well known as an analyzer based on energy dispersive X-ray spectroscopy, is usually called EDX, and belongs to the SEM and is used for quantitative analysis for determining the compositions of compounds each having an equivalent circular diameter within the above-described range. When determining the number of compounds each having an equivalent circular diameter within that range, the specific compounds are distinguished from other compounds by Sn content and by whether Mn and Fe are substantially contained or not. The Sn compounds having the specific composition and size only are identified. In the present invention, in the case where either Mn or Fe cannot be detected in a compound with the X-ray spectrometer, this is regarded as a compound not containing Mn or Fe and as a compound other than the Sn compounds having the specific composition and size, as in the case where the content of Sn is less than 1.0 mass %.

[0062] Furthermore, through analysis of reflected-electron images in the SEM, the total peripheral length (mm) of the Sn compounds having the specific composition and size is determined. This length is divided by the area examined with the SEM (field of view of the SEM; 240 .mu.m.times.180 .mu.m, converted to area in mm.sup.2), and the resultant values (mm/mm.sup.2) are averaged with respect to the number of the specimens to determine the length (/mm) of the boundaries with the aluminum matrix.

Differences With Conventional Art:

[0063] As described above, the Sn-containing 6000-series aluminum alloy sheet of the present invention differs from 6000-series aluminum alloy sheets into which Sn has been incorporated similarly (in the same amount), in both microstructure and property because of the feature concerning the solid-solution state of Sn and because of the feature in which the solid-solution state is balanced with the Sn compounds which have been precipitated or crystallized. Specifically, differences in production conditions regarding soaking treatment, etc. result in considerable differences in the present states, such as solute Sn amount, the compositions and number density of Sn compounds, the amount of boundaries with the matrix, etc.

[0064] In other words, under ordinary sheet production conditions (ordinary processes), Sn is prone to precipitate as compounds and a considerably small solute amount results. In addition, these Sn compounds differ in the composition and number density, and the boundaries with the matrix are present in a smaller amount. Because of this, even though Sn is contained similarly (in the same amount), a microstructure which is effective in inhibiting room-temperature age hardening on a high level and in improving the BH response and hem workability and which gives excellent filiform corrosion resistance as in the present invention cannot always be obtained.

[0065] In a conventional Sn-containing 6000-series aluminum 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 mainly 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.

(Production Process)

[0066] 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.

[0067] However, during the producing step, in order to make the sheet after being produced (refined) have a microstructure in which the average number density of Sn compounds having the specific Sn-containing composition and size is within the specified range and in which Sn has formed a solid solution and the formation of solid solution of Sn is balanced with the precipitation thereof, not only the average cooling rate during the casting is controlled but also use is made of preferred conditions specified for process annealing to be performed during the cold rolling, as will be described later. In the case where such process annealing conditions are not used, it is difficult to make the Sn form a solid solution.

[0068] In addition, a soaking treatment is conducted in two stages under specific conditions in order to make the sheet after being produced (refined) have the microstructure in which the amount of the boundaries between the Sn compounds having the specific Sn-containing composition and size and the aluminum matrix is within a specified range.

Melting and Casting Cooling Rate:

[0069] 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 causing the Sn to form a solid solution as specified in the present invention, it is preferable that the average rate of cooing from the liquidus temperature to the solidus temperature during the casting should be as high (quick) as possible at 30.degree. C./min or greater.

[0070] 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 Sn cannot be made to form a solid solution within the ranges specified in the present invention.

Homogenizing Heat Treatment:

[0071] 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.

[0072] In the present invention, however, the soaking treatment is conducted under the following specific conditions in order that the sheet after being produced (refined), after having undergone room-temperature aging after the refining treatment, may have a microstructure in which the amount of the boundaries between Sn compounds having the specific composition and size and the aluminum matrix is within the specified range.

[0073] In the first stage in the soaking treatment, holding is performed in the range of 400-500.degree. C. for 1-10 hours. Sn compounds having the specific composition and size are thereby finely dispersed to regulate the number density of these compounds and the amount of the boundaries with the aluminum matrix so as to be within the specified ranges. In the case where the soaking temperature is lower than 400.degree. C. or the holding time is less than 1 hour, it is difficult to finely disperse the Sn compounds having the specific Sn-containing composition and size to regulate the amount of the boundaries with the aluminum matrix to 3 /mm or larger on average in terms of a value obtained by dividing the total peripheral length of these compounds by the area examined with the SEM. Meanwhile, in the case where the holding time in the first stage exceeds 10 hours, the number density of the Sn compounds having the specific composition and size becomes too high beyond 3,000 counts/mm.sup.2, resulting in a shortage in the solute Sn amount which is necessary for inhibiting room-temperature age hardening.

[0074] Subsequently, in the second stage in the soaking treatment in which further heating is performed, holding is performed in the range of 520-560.degree. C. for 3 hours or longer. In this second stage in the soaking treatment, Mg-Si-Sn compounds present as crystals in the slab are caused to form a solid solution to increase the solute Sn amount. In the case where the temperature in this second stage in the soaking treatment is lower than 520.degree. C. or the holding time therein is less than 3 hours, the formation of solid solution of the Mg--Si--Sn compounds present as crystals in the slab is insufficient, resulting in a shortage in the solute Sn amount which is necessary for inhibiting room-temperature age hardening. Meanwhile, in the case where the soaking temperature in this second stage exceeds 560.degree. C., the slab suffers a fusion loss. Although the holding time in the second stage may be long, there is no need of prolonging it beyond 20 hours from the standpoints of production efficiency and profitability.

[0075] So long as the holding time in the temperature range of 400-500.degree. C. can be set to 1-10 hours, the soaking treatment including two stages may be one in which holding is performed at a constant temperature or may be a heat treatment in which the temperature is gradually changed by temperature raising, gradual cooling, etc., as described in the Examples which will be described later. In short, the temperature may be continuously changed by temperature raising, gradual cooling, etc. so long as the holding is performed in the temperature range of 400-500.degree. C. for 1 hour or more and 10 hours or less.

Hot Rolling:

[0076] 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.

[0077] 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.degree. 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.degree. C. to the solidus temperature, more preferably in the range of 400.degree. C. to the solidus temperature.

Annealing of Hot-Rolled Sheet:

[0078] 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:

[0079] 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:

[0080] It is preferable that before this cold rolling (after the hot rolling) or during the cold rolling (between passes), process annealing should be performed to bring the Sn which has formed 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.degree. 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.degree. 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. It is difficult to cause the Sn to form a solid solution, as specified in the present invention, by merely performing the solution treatment which will be described later, and it is necessary to perform a high-temperature heat treatment by process annealing.

[0081] With respect to the conditions for this process annealing, in the case where the sheet temperature is lower than 480.degree. C., an insufficient solute Sn amount results. 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.degree. 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.degree. C./sec, the Sn which has once formed a solid solution undesirably precipitates again to form compounds.

[0082] 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.

Solution and Quenching Treatments:

[0083] 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.degree. C./sec or higher to a solution treatment temperature of 520.degree. 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.degree. C./sec or higher. In the case where the cooling rate is too low, the number density of the Sn compounds becomes too high, resulting in too small a solute Sn amount. It hence becomes difficult to satisfy a 0.2% proof stress during forming as low as 110 MPa or less, a hem workability of 2.0 or less, and a BH response through 185.degree. C..times.20 min of 100 MPa or greater. In addition, Mg--Si compounds and the like are prone to precipitate during the cooling, and they prone to serve 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 forced 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.

[0084] The conditions for the solution and quenching treatments and for the rough annealing after the hot rolling are akin to the conditions for the process annealing in temperature, etc. However, in the case where the process annealing is omitted or where it is performed but the various conditions including a temperature of 520.degree. C. or higher are not satisfied, it is impossible to cause the Sn to form a solid solution just in the necessary amount or the specified amount by merely conducting the solution and quenching treatments and the rough annealing after the hot rolling.

Preliminary Aging Treatment (Reheating Treatment):

[0085] After such solution treatment, quenching and cooling to room temperature are performed. Thereafter, the sheet is subjected to a preliminary aging treatment (reheating treatment) as soon as possible in 1 hour (60 minutes).

[0086] In the case where the room-temperature holding time from the end of quenching to room temperature to initiation of the preliminary aging treatment (initiation of heating) is too long and exceeds 1 hour, room-temperature age hardening proceeds, resulting in a decrease in BH response. Consequently, the shorter the room-temperature holding time is, the better. The solution and quenching treatments and the reheating treatment may be consecutively performed so that there is substantially no pause therebetween, and there is no particular lower limit thereof.

[0087] With respect to the temperature and holding time in this preliminary aging treatment, holding is preferably performed at a temperature in the range of 80-150.degree. C. for 3 hours or more and 50 hours or less. In this treatment, the holding in the temperature range of 80-150.degree. C. may be a heat treatment in which temperature is constant within that temperature range or in which the temperature is gradually changed within that temperature range by temperature raising or gradual cooling. In short, the temperature may be continuously changed by gradual cooling, temperature raising, etc., so long as the holding is performed in the temperature range of 80-150.degree. C. for 3 hours or more and 50 hours or less. 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.

[0088] Unless the preliminary aging treatment is performed under conditions within those preferred ranges, it is difficult to provide a sheet which, in forming into automotive panels, has a 0.2% proof stress reduced to 110 MPa or less and which has a BH response of 100 MPa or greater.

[0089] 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

[0090] Examples of the present invention are explained. 6000-series aluminum alloy sheets were individually produced so as to differ in the average number density of Sn compounds having the specific composition and size and in the amount of the boundaries between the Sn compounds and the aluminum matrix, by changing the soaking treatment conditions or process annealing conditions. These sheets were held at room temperature for 30 days after the production, and then evaluated for strength, BH response (bake hardenability), hem workability, and filiform corrosion resistance. The results thereof are shown in Table 2.

[0091] 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.degree. 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%.

[0092] The slabs were each subjected to a soaking treatment under the conditions shown in Table 2, and hot rough rolling in each Example was then initiated at the temperature for the second stage. 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 sheets were subjected, in common with all the Examples, to process annealing with a continuous annealing furnace, during cold-rolling passes (between passes), under various conditions as shown in Table 2. Thus, cold-rolled sheets (product sheets) having a thickness of 1.0 mm were finally obtained.

[0093] Furthermore, these cold-rolled sheets were subjected to a solution heat treatment with a 560.degree. C. niter furnace, hold for 10 seconds after a target temperature had been reached, and then quenched by water cooling in which the average rate of cooling from the solution heat treatment temperature to the quenching stop temperature was 50.degree. C./sec, in common with all the Examples. Immediately after this quenching, a preliminary aging treatment was conducted in which holding is performed at 100.degree. C. for 5 hours (after the holding, gradually cooling is performed at a cooling rate of 0.6.degree. C./hr).

[0094] From the sheets which had just undergone these refining treatments, test sheets (blanks) were cut out. As the microstructure of the test sheets, the average number density of Sn compounds having the composition and size and the amount of the boundaries between the Sn compounds and the aluminum matrix were examined. Furthermore, test sheets (blanks) were cut out of the sheets which had been allowed to stand at room temperature for 30 days after the refining treatments, and examined for strength (AS proof stress; 0.2% proof stress measured after 30-day room-temperature aging after the sheet production) and BH response. The results thereof are shown in Table 2.

(Microstructure of each test sheet)

[0095] With respect to each test sheet which had just undergone the refining treatments, among compounds containing Mn and Fe, the average number density (counts/mm.sup.2) of compounds which had an Sn content of 1.0 mass % or higher and an equivalent circular diameter in the range of 0.3-20 .mu.m was determined by the measuring method in which an SEM having a magnification of 500 times and an X-ray spectrometer were used.

[0096] Furthermore, the lengths of the boundaries between the Sn compounds having the composition and size and the aluminum matrix were determined as a value (/mm) obtained by dividing the total peripheral length of the Sn compounds having the composition and size (total of the peripheral lengths of all the Sn compounds having the composition and size) by the area examined with the SEM, by the measuring method in which an SEM having a magnification of 500 times and an X-ray spectrometer were used.

(Tensile Test)

[0097] A tensile test was conducted in the following manner. No. 5 specimens (25 mm.times.50 mmGL.times.sheet thickness) according to JIS Z2201 were sampled from each test sheet which had been allowed to stand at room temperature for 30 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.

[0098] With respect to properties of the sheets during forming after the 30-day room-temperature aging, the sheets having an As 0.2% proof stress (0.2% proof stress during forming) shown in Table 2 of 110 MPa or less and a hem workability, according to the criteria shown later in the Examples, of 2 or less were rated as acceptable regarding the formability of sheets as materials for automotive outer panels.

(BH Response)

[0099] The test sheets were subjected to the 30-day room-temperature aging and then to an artificial age hardening treatment of 185.degree. C..times.20 min, and were thereafter examined for 0.2% proof stress (0.2% 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 increase amount in proof stress shown in Table 2 (difference between the 0.2% proof stress after BH and the As 0.2% proof stress). In the case where the increase amount in 0.2% proof stress was 100 MPa or greater, the BH response was regarded as acceptable.

(Hem Workability)

[0100] Hem workability was evaluated with respect to the test sheets which had undergone the 30-day room-temperature standing. In the test, strip-shaped specimens having a width of 30 mm were used and subjected to 90.degree. 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.degree. and flat-hem working in which the bent part was further bent inward to 180.degree. 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. [0101] 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.

(Filiform Corrosion Resistance)

[0102] The test sheets which had undergone the room-temperature aging were evaluated for filiform corrosion resistance. The test method used for the evaluation was as follows. A sheet of 80.times.150 mm was cut out of each test sheet which had undergone the 3-day room-temperature aging, and was immersed in a sodium-carbonate-containing degreasing bath at 40.degree. C. for 2 minutes (with stirring with a stirrer) to degrease the test sheet surfaces. Next, immersing was performed for 1 minute in a zinc-containing surface-regulating bath having room temperature (with stirring with a stirrer), subsequently immersing was performed in a 35.degree. C. zinc phosphate bath for 2 minutes to conduct a zinc phosphate treatment, and further electrodeposition coating (thickness, 20 .mu.m) was performed in accordance with an ordinary step for coating automotive members and then a 20-minutes baking treatment at 185.degree. C. was performed. Thereafter, a cross cut incision having a length of 50 mm was made in the coating film, and cycles each configured of 24-hour salt spray 120-hour wetting (humidity, 85%; 40.degree. C.).fwdarw.24-hour air drying (room temperature) were performed for eight cycles. The width of the rust on one side of the cross cut part was measured as the length of filiform corrosion.

[0103] The filiform corrosion resistance was evaluated in terms of the maximum width of the rust on one side of the cross cut part. The test sheet in which the maximum width was less than 1 mm was rated as .smallcircle..smallcircle., that in which the maximum width was 1 mm or larger but less than 2 mm was rated as .smallcircle., that in which the maximum length was 2 mm or larger but less than 3 mm was rated as .DELTA., and that in which the maximum length was 3 mm or larger was rated as .times.. The test sheets rated as .smallcircle..smallcircle. and .smallcircle. were regarded as excellent (acceptable) materials in terms of filiform corrosion resistance.

[0104] Invention Examples shown as Nos. 1 to 3, 9, 12, and 14 to 21 in Table 2 are within the component composition range according to the present invention (alloys Nos. 1 to 11 in Table 1), and have been produced under conditions within the preferred ranges including those for soaking treatment and process annealing. Because of this, these Invention Examples each satisfy both the average number density of Sn compounds having the composition and size specified in the present invention and the amount of the boundaries between the Sn compounds and the aluminum matrix specified in the present invention, as shown in Table 2, and has a satisfactory balance between the formation of solid solution of Sn and the precipitation thereof.

[0105] As a result, as shown in Table 2, the Invention Examples each have an excellent feature in which even after 30-day room-temperature aging after the refining treatments, the As 0.2% proof stress during press forming into automotive outer panels (before baking finish) is 110 MPa or less and the evaluation of hem workability is 0-2, and the automotive outer panels can have an artificial-aging hardening amount (BH response), as measured under the bake finish conditions of 185.degree. C..times.20 min, of 100 MPa or greater. They further have excellent filiform corrosion resistance.

[0106] Meanwhile, in Comparative Examples 4 to 8, 10, 11, 13, 28, and 29, in which the soaking treatment conditions or the process annealing conditions were outside the preferred ranges although they use alloys Nos. 1, 2, 3, 18, and 19 in Table 1, which are within the component composition range according to the present invention, either the average number density of Sn compounds having the composition and size specified in the present invention or the amount of the boundaries between the Sn compounds and the aluminum matrix specified in the present invention is outside the specified range, as shown in Table 2. The formation of solid solution of Sn has not been balanced with the precipitation thereof.

[0107] As a result, in each of these Comparative Examples, the proof stress during press forming into automotive outer panels after 30-day room-temperature aging after the refining treatments is too high beyond 110 MPa, the BH response is as low as below 100 MPa, or the filiform corrosion resistance is poor, as shown in Table 2.

[0108] In Comparative Examples 4, 6 and 13, the holding time in the first stage in the soaking treatment was too short or the first stage in the soaking treatment was not performed. Because of this, the average number density of Sn compounds having the composition and size described above is too low, the amount of the boundaries between the Sn compounds and the matrix is less than 3/mm, and the filiform corrosion resistance is poor.

[0109] In Comparative Examples 5, 7 and 10, the holding time in the first stage in the soaking treatment was too long or the soaking treatment temperature in the second stage was too low. Because of this, Sn compounds have been yielded in too large an amount and a sufficient solute Sn amount cannot be ensured. Consequently, AS proof stress is high and proof stress increase amount is small. In addition, the process annealing was not performed in Comparative Example 7, and the rate of the cooling after the process annealing in Comparative Example 10 was too low.

[0110] In Comparative Examples 8 and 11, the process annealing temperature was too low. Because of this, Sn compounds have been yielded in too large an amount and a sufficient solute Sn amount cannot be ensured. Consequently, AS proof stress is too high and proof stress increase amount is small.

[0111] In Comparative Examples 28 and 29, use was made of alloys Nos. 18 and 19 in Table 1, which are within the component composition range according to the present invention. However, the process annealing was not performed, or the rate of the cooling after the process annealing was too low. Because of this, Sn compounds have been yielded in too large an amount and a sufficient solute Sn amount cannot be ensured. Consequently, AS proof stress is too high and proof stress increase amount is small.

[0112] Comparative Examples 22 to 27 and 30 to 32 in Table 2 have been produced under the preferred condition ranges, but alloys Nos. 12 to 17 and 20 to 22 in Table 1 were used therefor. Hence, the content of any of Mg, Si and Sn, which are essential elements, is outside the range according to the present invention. Because of this, in each of Comparative Examples 22 to 27 and 30 to 32, the proof stress during press framing after 30-day room-temperature aging after the refining treatment is too high beyond 110 MPa, the BH response is as low as below 100 MPa, or the filiform corrosion resistance is poor, as shown in Table 2.

[0113] Comparative Example 22 is alloy 12 of Table 1, in which the Si content is too low.

[0114] Comparative Example 23 is alloy 13 of Table 1, in which the Si content is too high.

[0115] Comparative Example 24 is alloy 14 of Table 1, in which the Sn content is too low.

[0116] Comparative Example 25 is alloy 15 of Table 1, in which the content of Sn is too high. Because of this, cracks were generated during the hot rolling, making the production of a hot-rolled sheet itself impossible.

[0117] Comparative Example 26 is alloy 16 of Table 1, in which the Fe content is too high.

[0118] Comparative Example 27 is alloy 17 of Table 1, in which the Mn content is too high.

[0119] Comparative Example 30 is alloy 20 of Table 1, in which the Fe and Mn contents are too low. Comparative Example 31 is alloy 21 of Table 1, in which the Mg content is too low.

[0120] Comparative Example 32 is alloy 22 of Table 1, in which the Mg content is too high.

[0121] Those results of the Examples establish the critical significance or effects of the composition specified in the present invention and the feature of balancing the formation of solid solution of Sn with the precipitation thereof or of the preferred production conditions, with respect to combinedly achieving strength after room-temperature aging, formability, in particular, hem workability, BFI response, and filiform corrosion resistance of Sn-containing 6000-series aluminum alloy sheets.

TABLE-US-00001 TABLE 1 Alloy Chemical components of Al--Mg--Si alloy sheet (mass %; remainder, Al) No. Mg Si Sn Fe Mn Cr Zr V Ti Cu Zn Ag 1 0.45 1.02 0.036 0.19 ower 2 0.42 0.82 0.025 0.21 0.41 3 0.39 1.18 0.058 0.20 0.04 0.05 4 0.55 0.83 0.041 0.20 0.09 0.22 5 0.36 1.23 0.084 0.21 0.12 0.20 6 0.54 1.31 0.053 0.22 0.09 0.05 0.05 7 0.55 0.79 0.197 0.07 0.10 0.16 0.01 8 0.45 0.93 0.040 0.70 0.10 0.03 0.60 9 0.64 1.15 0.027 0.22 0.09 0.12 10 0.47 1.23 0.055 0.19 0.12 0.30 11 0.71 0.72 0.007 0.19 0.12 0.10 0.10 12 0.82 0.47 0.042 0.18 0.10 13 0.40 2.11 0.042 0.20 0.09 14 0.59 1.00 0.002 0.21 0.12 15 0.61 1.12 0.452 0.20 0.10 16 0.40 0.76 0.053 1.27 0.11 17 0.51 1.00 0.049 0.19 0.78 18 0.62 1.03 0.052 0.21 0.12 19 0.73 0.81 0.053 0.22 0.09 20 0.55 1.26 0.051 0.01 0.01 21 0.25 1.01 0.043 0.15 0.16 22 1.23 1.04 0.048 0.15 0.14

TABLE-US-00002 TABLE 2 Process annealing between Soaking treatment cold-rolling passes First Second (continuous annealing) stage First stage Second Average Alloy Temper- stage Temper- stage Temper- cooling No. in ature Time ature Time ature rate Classification No. Table 1 .degree. C. hr .degree. C. hr .degree. C. .times. 5 sec .degree. C./sec Inv. Ex 1 1 450 3 550 3 480 5 Inv. Ex. 2 1 420 8 550 3 490 10 Inv. Ex. 3 1 480 2 550 3 510 50 Comp. Ex. 4 1 450 0.5 550 3 520 50 Comp. Ex. 5 1 450 20 550 3 520 50 Comp. Ex. 6 1 -- -- 550 3 520 50 Comp. Ex. 7 1 450 3 450 3 -- -- Comp. Ex. 8 1 450 3 550 3 450 50 Inv. Ex. 9 2 450 5 550 3 520 50 Comp. Ex. 10 2 450 3 450 3 510 1 Comp. Ex. 11 2 450 3 550 3 450 50 Inv. Ex. 12 3 450 3 550 3 510 50 Comp. Ex. 13 3 -- -- 550 3 520 50 Inv. Ex 14 4 450 3 550 3 520 50 Inv. Ex. 15 5 450 3 550 3 520 50 Inv. Ex. 16 6 450 3 550 3 520 50 Inv. Ex 17 7 450 3 550 3 520 50 Inv. Ex. 18 8 450 3 550 3 520 50 Inv. Ex 19 9 450 3 550 3 520 50 Inv. Ex. 20 10 450 3 550 3 520 50 Inv. Ex. 21 11 450 3 550 3 520 50 Comp. Ex. 22 12 450 3 550 3 520 50 Comp. Ex. 23 13 450 3 550 3 520 50 Comp. Ex. 24 14 450 3 550 3 520 50 Comp. Ex. 25 15 450 3 550 3 cracks in hot rolling Comp. Ex. 26 16 450 3 550 3 520 50 Comp. Ex. 27 17 450 3 550 3 520 50 Comp. Ex. 28 18 450 3 550 3 -- -- Comp. Ex. 29 19 450 3 550 3 500 1 Comp. Ex. 30 20 450 3 550 3 520 50 Comp. Ex. 31 21 450 3 550 3 520 50 Comp. Ex. 32 22 450 3 550 3 520 50 Microstructure of aluminum alloy sheet after refining Properties of aluminum alloy sheet after Average Length of 30-day room-temperature holding number boundaries As Increase density of between Sn As 0.2% amount Sn compounds tensile proof in proof Hem Filiform Classifi- compounds and matrix/ strength stress stress work- corrosion cation No. counts/mm.sup.2 mm MPa MPa MPa ability resistance Inv. Ex 1 830 4.2 218 93 122 1 .smallcircle. Inv. Ex. 2 910 5.6 214 90 126 1 .smallcircle. Inv. Ex. 3 660 3.5 219 92 125 1 .smallcircle. Comp. Ex. 4 380 2.8 212 88 131 1 .DELTA. Comp. Ex. 5 3860 13.3 229 112 92 2 .smallcircle..smallcircle. Comp. Ex. 6 320 2.1 221 95 129 1 .DELTA. Comp. Ex. 7 4820 22.2 231 114 88 2 .smallcircle..smallcircle. Comp. Ex. 8 4010 10.6 239 118 93 2 .smallcircle..smallcircle. Inv. Ex. 9 2850 9.0 211 94 108 2 .smallcircle..smallcircle. Comp. Ex. 10 5470 23.9 224 106 73 2 .smallcircle..smallcircle. Comp. Ex. 11 3820 13.8 248 122 87 2 .smallcircle..smallcircle. Inv. Ex. 12 680 3.6 219 96 118 1 .smallcircle. Comp. Ex. 13 280 1.8 215 91 124 1 .DELTA. Inv. Ex 14 2630 10.3 226 102 112 2 .smallcircle..smallcircle. Inv. Ex. 15 2710 8.7 210 94 108 2 .smallcircle. Inv. Ex. 16 1020 5.0 231 106 117 1 .smallcircle. Inv. Ex 17 1090 5.5 195 80 129 1 .smallcircle. Inv. Ex. 18 2720 11.7 214 91 113 2 .smallcircle. Inv. Ex 19 830 4.3 230 105 133 1 .smallcircle. Inv. Ex. 20 1460 6.8 224 92 124 1 .smallcircle. Inv. Ex. 21 760 3.9 228 106 120 1 .smallcircle. Comp. Ex. 22 260 1.4 185 68 61 0 .smallcircle..smallcircle. Comp. Ex. 23 7570 30.6 227 108 103 4 x Comp. Ex. 24 80 0.9 253 128 86 3 .DELTA. Comp. Ex. 25 cracks in hot rolling Comp. Ex. 26 8240 22.5 214 95 83 4 x Comp. Ex. 27 8830 25.1 220 102 82 4 x Comp. Ex. 28 3250 13.4 231 117 91 2 .smallcircle. Comp. Ex. 29 3380 16.1 222 113 84 2 .smallcircle. Comp. Ex. 30 20 0.4 198 87 131 1 .DELTA. Comp. Ex. 31 760 3.3 166 65 53 1 .smallcircle..smallcircle. Comp. Ex. 32 1620 7.0 262 141 117 3 .smallcircle.

[0122] 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.

[0123] The present application is based on a Japanese patent application filed on Aug. 27, 2014 (Application No. 2014-173277), the whole thereof being incorporated herein by reference.

INDUSTRIAL APPLICABILITY

[0124] According to the present invention, it is possible to provide Sn-containing 6000-series aluminum alloy sheets which satisfy the requirements as automotive outer panels, concerning formability and BH response after room-temperature aging, and which further have improved filiform corrosion resistance. As a result, the 6000-series aluminum alloy sheets are usable in extended applications, especially as automotive outer panels.

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