U.S. patent application number 15/506595 was filed with the patent office on 2017-12-07 for aluminum alloy sheet.
This patent application is currently assigned to KABUSHIKI KAISHA KOBE SEIKO SHO (KOBE STEEL, LTD.). The applicant listed for this patent is KABUSHIKI KAISHA KOBE SEIKO SHO (KOBE STEEL, LTD.). Invention is credited to Yasuhiro ARUGA, Katsushi MATSUMOTO, Takahiro OZAWA, Hisao SHISHIDO, Satoru TAKADA, Akihiko TATSUMI.
Application Number | 20170349978 15/506595 |
Document ID | / |
Family ID | 55399824 |
Filed Date | 2017-12-07 |
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: |
55399824 |
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: |
C22C 21/06 20130101;
C22F 1/05 20130101; C22C 21/02 20130101; C22F 1/00 20130101 |
International
Class: |
C22C 21/02 20060101
C22C021/02; C22F 1/05 20060101 C22F001/05 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 27, 2014 |
JP |
2014-173277 |
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.
* * * * *